U.S. patent application number 12/576026 was filed with the patent office on 2010-06-03 for catalytic materials, photoanodes, and photoelectrochemical cells for water electrolysis and other, electrochemical techniques.
This patent application is currently assigned to Massachusetts Institute of Technology. Invention is credited to Arthur J. Esswein, Matthew W. Kanan, Thomas A. Moore, Daniel G. Nocera, Steven Y. Reece, Yogesh Surendranath.
Application Number | 20100133110 12/576026 |
Document ID | / |
Family ID | 41417441 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100133110 |
Kind Code |
A1 |
Nocera; Daniel G. ; et
al. |
June 3, 2010 |
CATALYTIC MATERIALS, PHOTOANODES, AND PHOTOELECTROCHEMICAL CELLS
FOR WATER ELECTROLYSIS AND OTHER, ELECTROCHEMICAL TECHNIQUES
Abstract
Catalytic materials, photoanodes, and systems for electrolysis
and/or formation of water are provided which can be used for energy
storage, particularly in the area of solar energy conversion,
and/or production of oxygen and/or hydrogen. Compositions and
methods for forming photoanodes and other devices are also
provided.
Inventors: |
Nocera; Daniel G.;
(Winchester, MA) ; Kanan; Matthew W.; (Palo Alto,
CA) ; Moore; Thomas A.; (Scottsdale, AZ) ;
Surendranath; Yogesh; (Cambridge, MA) ; Reece; Steven
Y.; (Cambridge, MA) ; Esswein; Arthur J.;
(Boston, MA) |
Correspondence
Address: |
WOLF GREENFIELD & SACKS, P.C.
600 ATLANTIC AVENUE
BOSTON
MA
02210-2206
US
|
Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
Arizona Board of Regents
Scottsdale
AZ
Sun Catalytix Corporation
Cambridge
MA
|
Family ID: |
41417441 |
Appl. No.: |
12/576026 |
Filed: |
October 8, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61103905 |
Oct 8, 2008 |
|
|
|
61187995 |
Jun 17, 2009 |
|
|
|
Current U.S.
Class: |
205/340 ;
252/182.1; 502/101 |
Current CPC
Class: |
H01M 14/005 20130101;
Y02E 60/36 20130101; B01J 35/004 20130101; Y02P 20/133 20151101;
C25B 1/55 20210101 |
Class at
Publication: |
205/340 ;
502/101; 252/182.1 |
International
Class: |
C25B 1/04 20060101
C25B001/04; C25B 11/04 20060101 C25B011/04 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with the support under the following
government contract F32GM07782903 awarded by the National
Institutes of Health and CHE-0533150 awarded by the National
Science Foundation. The government has certain rights in the
invention.
Claims
1. A method for forming a photoanode for the catalytic production
of oxygen from water, comprising: providing a solution comprising
metal ionic species and anionic species; providing a photoactive
electrode comprising a photoactive composition and a
photosensitizing agent; and causing the metal ionic species and the
anionic species to form a catalytic material associated with the
photoactive electrode by application of a voltage to the
photoactive electrode.
2. A method for producing oxygen from water, comprising the steps
of: providing a photoelectrochemical cell comprising: a photoactive
electrode comprising a photoactive composition and a
photosensitizing agent; an electrolyte; and a catalytic material
integrally connected to the photosensitizing agent, the catalytic
material comprising metal ionic species and anionic species, and
wherein the catalytic material does not consist essentially of a
metal oxide or metal hydroxide; and illuminating the
photoelectrochemical cell with light to thereby produce oxygen gas
from water.
3-9. (canceled)
10. The method of claim 1, wherein the voltage is applied to the
photoactive electrode by a power source.
11. (canceled)
12. The method of claim 1, wherein the voltage is applied to the
photoactive electrode by exposing the photoactive electrode to
electromagnetic radiation.
13-33. (canceled)
34. The method of claim 1, wherein the anionic species comprises
phosphorus.
35. The method of claim 1, wherein the metal ionic species
comprises cobalt ions.
36. (canceled)
37. The method of claim 1, wherein the metal ionic species with an
oxidation state of (n+x) and the anionic species define a
substantially non-crystalline composition and have a K.sub.sp value
which is less, by a factor of at least 10.sup.3, than the K.sub.sp
value of a composition comprising the metal ionic species with an
oxidation state of (n) and the anionic species.
38. (canceled)
39. The method of claim 2, wherein the photoelectrochemical cell
further comprises a second electrode.
40-41. (canceled)
42. The method of claim 39, wherein the second electrode is a
photoactive electrode.
43-49. (canceled)
50. The method of claim 2, wherein the water contains at least one
impurity
51-89. (canceled)
90. The method of claim 1, wherein the photosensitizing agent
comprises a metal complex dye.
91. The method of claim 1, wherein the photosensitizing agent
comprises an organic dye.
92-107. (canceled)
108. A photoanode for the catalytic production of oxygen from
water, comprising: a photoactive electrode comprising a
photosensitizing agent and a photoactive composition; and a
catalytic material comprising metal ionic species and anionic
species, wherein the catalytic material is formed by application of
a voltage to a photoactive electrode.
109. The photoanode of claim 108, wherein the metal ionic species
comprises cobalt ions.
110. The photoanode of claim 108, wherein the anionic species
comprises phosphorus.
111-162. (canceled)
163. The photoanode of claim 108, wherein the metal ionic species
comprise at least a first and a second type of metal ionic
species.
164-169. (canceled)
170. The photoanode of claim 108, wherein the photosensitizing
agent comprises a metal complex dye.
171. The photoanode of claim 108, wherein the photosensitizing
agent comprises an organic dye.
172-234. (canceled)
235. The method of claim 2, wherein the anionic species comprises
phosphorus.
236. The method of claim 2, wherein the metal ionic species
comprises cobalt ions.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 61/103,905, filed Oct. 8, 2008,
entitled "Catalyst Compositions and Photoanodes for Photosynthesis
Replication and Other Photoelectrochemical Techniques," by Nocera,
et al., and U.S. Provisional Patent Application Ser. No.
61/187,995, filed Jun. 17, 2009, entitled "Catalytic Materials,
Photoanodes, and Systems for Water Electrolysis and Other
Electrochemical Techniques," by Nocera, et al., each herein
incorporated by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to photoanodes for
electrolysis of water which can be used for energy storage. The
invention also relates to compositions and methods for forming a
photoanode. In some embodiments, electrochemical devices such as
photoelectrochemical devices are provided for the catalytic
formation of oxygen gas and/or hydrogen gas from water.
BACKGROUND OF THE INVENTION
[0004] Solar energy can be considered to be a carbon-neutral energy
source of sufficient scale to meet future global energy demand. The
diurnal variation in local insolation, however, requires a
cost-effective storage of solar energy for its large scale
deployment as a primary energy source. In nature, photosynthesis
captures sunlight and converts it into a wireless current which is
stored. An approach to duplicating natural photosynthesis outside
of a leaf is to capture and convert solar light into spatially
separated electron/hole pairs within a photoelectrochemical cell.
Photoelectrochemical devices may be used to produce hydrogen and
oxygen gases from water. Photoelectrochemical devices utilize solar
energy for the electrolysis of water, and generally employ a
photoactive electrode, which, upon exposure to sunlight, produces
electron/hole pairs that may be used for the electrolysis of water
to produce hydrogen and/or oxygen gases. The net result is the
storage of solar energy in the chemical bonds of H.sub.2 and
O.sub.2.
[0005] In order to store energy via electrolysis, catalysts are
required which efficiently mediate the bond rearranging "water
splitting" reaction. The standard reduction potentials for the
O.sub.2/H.sub.2O and H.sup.+/H.sub.2 half-cells are given by
Equation 1 and Equation 2.
O 2 + 4 H + 4 e - H 2 O E 0 = + 1.23 - 0.059 ( pH ) V ( 1 ) 2 H 2 4
H + + 4 e - E 0 = 0.00 - 0.059 ( pH ) V ( 2 ) 2 H 2 + O 2 2 H 2 O
##EQU00001##
For a catalyst to be efficient for this conversion, the catalyst
should operate at voltages close to the thermodynamic value of each
half reaction, which are defined by half-cell potentials, E.sup.o.
Voltage in addition to E.sup.o that is required to attain a given
catalytic activity, referred to as overpotential, limits the energy
conversion efficiency. Considerable effort has been expended by
many researchers in efforts to reduce overpotential in this
reaction. It may be considered that oxygen gas production from
water at low overpotential and under benign conditions using
catalytic materials composed of earth-abundant materials presents
the greatest challenge to water electrolysis. The oxidation of
water to form oxygen gas requires the coupled transfer of four
electrons and four protons to avoid high-energy intermediates. In
addition to controlling multi-proton-coupled electron transfer
reactions, a catalyst, in some cases, should also be able to
tolerate prolonged exposure to oxidizing conditions.
[0006] While photoelectrochemical devices and photoanodes exist for
the electrolysis of water, these devices are generally composed of
expensive materials and/or operate with low energy conversion
efficiencies. Therefore, a need remains for the development of
improved materials and devices that operate with increased energy
conversion efficiency.
SUMMARY OF THE INVENTION
[0007] The present invention relates to catalytic materials for
electrolysis of water that can be used for energy storage,
particularly in the area of solar energy conversion. The invention
also relates to compositions and methods for forming a photoanode.
In some embodiments, photoelectrochemical devices are provided for
the catalytic formation of oxygen gas from water. The subject
matter of the present invention involves, in some cases,
interrelated products, alternative solutions to a particular
problem, and/or a plurality of different uses of one or more
systems and/or articles.
[0008] In one aspect, the invention is directed to a method.
According to a first embodiment, a method for forming a photoanode
for the catalytic production of oxygen from water comprises
providing a solution comprising metal ionic species and anionic
species, providing a photoactive electrode comprising a photoactive
composition and a photosensitizing agent, and causing the metal
ionic species and the anionic species to form a catalytic material
associated with the photoactive electrode by application of a
voltage to the photoactive electrode.
[0009] According to another embodiment, a method for producing
oxygen from water, comprises the steps of providing a
photoelectrochemical cell comprising a photoactive electrode
comprising a photoactive composition and a photosensitizing agent,
an electrolyte, and a catalytic material integrally connected to
the photosensitizing agent, the catalytic material comprising metal
ionic species and anionic species, and wherein the catalytic
material does not consist essentially of a metal oxide or metal
hydroxide, and illuminating the photoelectrochemical cell with
light to thereby produce oxygen gas from water.
[0010] In another aspect, the invention is directed towards
photoanodes. According to a first embodiment, a photoanode for the
catalytic production of oxygen from water, comprises a photoactive
electrode comprising a photoactive composition and a
photosensitizing agent, and a catalytic material associated with
the photoactive electrode, comprising cobalt ions and anionic
species comprising phosphorus.
[0011] According to another embodiment, a photoanode for the
catalytic production of oxygen from water comprises a photoactive
electrode comprising a photoactive composition and a
photosensitizing agent, and a catalytic material comprising metal
ionic species and anionic species, wherein the metal ionic species
with an oxidation state of (n+x) and the anionic species define a
substantially non-crystalline composition and have a K.sub.sp value
which is less, by a factor of at least 10.sup.3, than the K.sub.sp
value of a composition comprising the metal ionic species with an
oxidation state of (n) and the anionic species.
[0012] According to yet another embodiment, a photoanode for the
catalytic production of oxygen from water comprises a photoactive
electrode comprising a photosensitizing agent and a photoactive
composition, and a catalytic material comprising metal ionic
species and anionic species, wherein the catalytic material is
formed by application of a voltage to a photoactive electrode.
[0013] In yet another aspect, the invention relates to
photoelectrochemical cells. According to a first embodiment, a
photoelectrochemical cell comprises a photoanode comprising a
photoactive electrode comprising and photoactive composition and a
photosensitizing agent, and a catalytic material comprising metal
ionic species and anionic species, wherein the catalytic material
does not consist essentially of a metal oxide or metal hydroxide,
at least one second electrode, and an electrolyte, wherein the
photoelectrochemical cell is capable of producing oxygen gas from
water.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Non-limiting embodiments of the present invention will be
described by way of example with reference to the accompanying
figures, which are schematic and are not intended to be drawn to
scale. Unless indicated as representing the prior art, the figures
represent aspects of the invention. In the figures, each identical
or nearly identical component illustrated is typically represented
by a single numeral. For purposes of clarity, not every component
is labeled in every figure, nor is every component of each
embodiment of the invention shown where illustration is not
necessary to allow those of ordinary skill in the art to understand
the invention. In the figures:
[0015] FIG. 1 illustrates a non-limiting example of a
photoelectrochemical cell, according to one embodiment.
[0016] FIG. 2 illustrates an energy diagram of a
photoelectrochemical device comprising a photoactive electrode and
an electrode, wherein the photoactive electrode is biased
positively with respect to the electrode, according to one
embodiment.
[0017] FIG. 3 illustrates an energy diagram of a
photoelectrochemical cell comprising a first photoactive electrode
and a second photoactive electrode, wherein the first photoactive
electrode is biased positively with respect to the second
photoactive electrode, according to one embodiment.
[0018] FIGS. 4A-4B illustrate the formation of a photoanode,
according to one embodiment.
[0019] FIGS. 5A-5D illustrate examples of how a composition may
associate with a photoactive electrode upon application of a
voltage (e.g., a photovoltage) to the photoactive electrode,
according to some embodiments.
[0020] FIG. 6 illustrates an energy diagram for a
photoelectrochemical device comprising a photoactive electrode and
an electrode, wherein the photoactive electrode is associated with
a dye, according to one embodiment.
[0021] FIGS. 7A-7E illustrate the formation of a catalytic material
on a photoactive electrode, according to one embodiment.
[0022] FIGS. 8A-8C illustrate a non-limiting example of a dynamic
equilibrium of a catalytic material, according to one
embodiment.
[0023] FIGS. 9A-9C represent an illustrative example of changes in
oxidation state that may occur for a single metal ionic species
during a dynamic equilibrium of an electrode, according to one
embodiment, during use.
[0024] FIG. 10 shows a non-limiting embodiment of a hybrid
photoelectrochemical cell.
[0025] FIG. 11 shows a non-limiting example of an electrochemical
device.
[0026] FIG. 12 shows a non-limiting embodiment of a
bi-photoelectrochemical cell.
[0027] FIG. 13 shows another non-limiting embodiment of a
bi-photoelectrochemical cell.
[0028] FIG. 14 shows a SEM image of a catalytic material comprising
cobalt electrodeposited onto a thin film of CdS, according to a
non-limiting embodiment.
[0029] FIG. 15 shows SEM images of a thin film CdS electrode
treated with a solution of 0.1M MePi (pH 8.5) and 2 mM Co.sup.2+ in
the (A) presence and (B) absence of irradiation with visible light,
according to a non-limiting embodiment.
[0030] FIG. 16 shows the band edge positions of various forms of
TiO.sub.2 along with the standard reduction potential of the
hydroxyl radical and the potential for operation of the CoPi
catalyst, according to some embodiments.
[0031] FIGS. 17A-17F illustrate non-limiting examples of
photoelectrochemical cells.
[0032] Other aspects, embodiments, and features of the invention
will become apparent from the following detailed description when
considered in conjunction with the accompanying drawings. The
accompanying figures are schematic and are not intended to be drawn
to scale. For purposes of clarity, not every component is labeled
in every figure, nor is every component of each embodiment of the
invention shown where illustration is not necessary to allow those
of ordinary skill in the art to understand the invention. All
patent applications and patents incorporated herein by reference
are incorporated by reference in their entirety. In case of
conflict, the present specification, including definitions, will
control.
DETAILED DESCRIPTION
[0033] The present invention relates to photoanodes for
electrolysis of water which can be used for energy storage. The
invention also relates to compositions and methods for forming a
photoanode. In some embodiments, photoelectrochemical devices are
provided for the catalytic formation of oxygen gas and/or hydrogen
gas from water. The invention allows for the facile, low-energy
conversion of water to hydrogen gas and/or oxygen gas using a
photoactive electrode. In some cases, the conversion may be driven
by exposing the photoactive electrode to electromagnetic radiation
(e.g., sunlight). Energy can be stored, via the catalytic material
of the invention, in the form of oxygen gas and hydrogen gas. The
hydrogen and oxygen gases can be recombined at any time, for
example, later as a stored source of energy, whereby they form
water and release significant energy that can be captured in the
form of mechanical energy, electricity, or the like. In other
cases, the hydrogen and/or oxygen gases may be used in another
process.
[0034] According to some embodiments, compositions and methods for
forming a photoanode are provided. In some cases, a photoanode may
catalytically produce oxygen gas from water. As shown in Equation
1, water may be split to form oxygen gas, electrons, and hydrogen
ions. Although it need not be, a photoanode of the invention can be
operated in benign conditions (e.g., neutral or near-neutral pH,
ambient temperature, ambient pressure, etc.). A photoanode may
comprise a photoactive electrode, metal ionic species and anionic
species, wherein the metal ionic species and anionic species are
associated with the photoactive electrode. The metal ionic species
and anionic species may be selected such that, when exposed to an
aqueous solution (e.g., an electrolyte), the metal ionic species
and anionic species are in dynamic equilibrium with the aqueous
solution, as described herein.
[0035] Photoanodes of the present invention, in some cases,
comprise a catalytic material. Many species of the class of
catalytic material provided by the invention are made of
readily-available, low-cost material, and are easy to make.
Accordingly, the invention has the potential to dramatically change
the field of solar energy capture, storage, and use. The subject
matter of the present invention involves, in some cases,
interrelated products, alternative solutions to a particular
problem, and/or a plurality of different uses of one or more
systems and/or articles.
[0036] In all descriptions of the use of water (e.g., for the
production of oxygen gas) for catalysis herein, it is to be
understood that the water may be provided in a liquid and/or
gaseous state. The water used may be relatively pure, but need not
be, and it is one advantage of the invention that relatively impure
water can be used. The water provided can contain, for example, at
least one impurity (e.g., halide ions such as chloride ions). In
some cases, the device may be used for desalination of water. It
should be understood that while much of the application herein
focuses on the catalytic formation of oxygen gas from water, this
is by no means limiting, and the compositions, photoanode, methods,
and/or systems described herein may be used for other catalytic
purposes, as described herein.
[0037] In some embodiments, photoanodes are provided which may
produce oxygen gas from water. As shown in Equation 1, water may be
split to form oxygen gas, electrons, and hydrogen ions. Although it
need not be, an electrode may be operated in benign conditions
(e.g., neutral or near-neutral pH, ambient temperature, ambient
pressure, etc.). In some cases, the electrodes described herein
operate catalytically. That is, an electrode may be able to
catalytically produce oxygen gas from water, but the electrode may
not necessarily participate in the related chemical reactions such
that it is consumed to any appreciable degree. Those of ordinary
skill in the art will understand the meaning of "catalytically" in
this context. An electrode may also be used for the catalytic
production of other gases and/or materials.
[0038] As used herein, a photoanode is a photoactive electrode, in
addition to any catalytic material adsorbed thereto. In some
embodiments, the photoactive electrode may comprise a photoactive
composition and a photosensitizing agent. The catalytic material
may comprise metal ionic species and anionic species, wherein the
metal ionic species and anionic species are associated with the
photoactive electrode. The metal ionic species and anionic species
may be selected such that, when exposed to an aqueous solution
(e.g., an electrolyte or water source), the metal ionic species and
anionic species are in dynamic equilibrium with the aqueous
solution, as described herein.
[0039] In some embodiments, a photoanode of the present invention
comprises a photoactive electrode and a catalytic material
associated with the photoactive electrode. A "catalytic material"
as used herein, means a material that is involved in and increases
the rate of a chemical electrolysis reaction (or other
electrochemical reaction) and which, itself, undergoes reaction as
part of the electrolysis, but is largely unconsumed by the reaction
itself, and may participate in multiple chemical transformations. A
catalytic material may also be referred to as a catalyst and/or a
catalyst composition. A catalytic material is not simply a bulk
photoactive electrode material which provides and/or receives
electrons from an electrolysis reaction, but a material which
undergoes a change in chemical state of at least one ion during the
catalytic process. For example, a catalytic material might involve
a metal center which undergoes a change from one oxidation state to
another during the catalytic process. In another example, the
catalytic material might involve metal ionic species which bind to
one or more oxygen atoms from water and release the oxygen atoms as
dioxygen (i.e., O.sub.2). Thus, catalytic material is given its
ordinary meaning in the field in connection with this invention. As
will be understood from other descriptions herein, a catalytic
material of the invention that may be consumed in slight quantities
during some uses and may be, in many embodiments, regenerated to
its original chemical state.
[0040] "Electrolysis," as used herein, refers to the use of an
electric current to drive an otherwise non-spontaneous chemical
reaction. For example, in some cases, electrolysis may involve a
change in redox state of at least one species and/or formation
and/or breaking of at least one chemical bond, by the application
of an electric current. Electrolysis of water, as provided by the
invention, can involve splitting water into oxygen gas and hydrogen
gas, or oxygen gas and another hydrogen-containing species, or
hydrogen gas and another oxygen-containing species, or a
combination.
[0041] In some embodiments, methods are provided for forming a
photoanode comprising a photoactive electrode (e.g., an n-type
semiconductor photoactive material), metal ionic species, and
anionic species. The photoanode may be formed by exposing a
photoactive electrode to a solution comprising metal ionic species
and anionic species, followed by application of a voltage to the
photoactive electrode. The term "application of a voltage," as used
herein, in some embodiments, is synonymous with the term formation
of a photovoltage (e.g., formation of electron/hole pairs in a
material by exposing the material to electromagnetic radiation).
For example, the voltage may be applied to a photoactive electrode
by an external power source (e.g., a battery) or by exposing a
photoactive electrode to electromagnetic radiation (e.g., sunlight,
to produce a photovoltage), as described herein. The metal ionic
species and anionic species may associate with the photoactive
electrode and form a composition (e.g., a catalytic material)
associated with the photoactive electrode. In some cases, when
associating with the photoactive electrode, the metal ionic species
may be oxidized or reduced as compared to the metal ionic species
in solution, as described herein.
[0042] In some embodiments, photoelectrochemical devices are
provided which comprise a photoanode as described herein. In some
embodiments, photoelectrochemical devices comprising a photoanode
as described herein are capable of catalytically producing oxygen
gas from water. In some cases, the device may additionally produce
hydrogen gas. Devices are described herein.
[0043] Without wishing to be bound by theory, the devices and
methods as described herein may be used for the photoelectrolysis
of water and conversion of light to electrical energy, and in some
cases, solely use solar energy (e.g., sunlight) as the power
source. For example, a photoanode as described herein may comprise
a catalytic material associated with an n-type semiconductor
photoactive electrode. When the photoanode is exposed to light,
electrons are excited from the valence band to the conduction band
of the n-type semiconductor, thereby creating holes in the valence
band and free electrons in the conduction band. In some
embodiments, the excited electron and corresponding electron-hole
may separate spatially within the semiconductor material from the
point of generation. Such charge separation may give rise to a
photovoltage within the semiconductor. The electron-holes may be
transported to the semiconductor-electrolyte interface, where they
may react with a water molecule (e.g., via a catalytic material),
resulting in the formation of oxygen gas and/or hydrogen ions. The
electrons produced at the photoanode may be conducted by means of
an external electrical connection to the counter electrode where
they may combine with hydrogen ions of water molecules (or another
source such as an acid) in the electrolytic solution to produce
hydrogen gas. In instances where the conduction band level of the
semiconductor is more negative than the H.sub.2O/H.sub.2 energy
level and the valence band level of the semiconductor is more
positive than the O.sub.2/H.sub.2O energy level, the electrolysis
of water may be accomplished solely through the use of solar energy
(e.g., without the use of an external power source). In some cases,
association of a catalytic material with a photoactive electrode
(e.g., a photoanode as described herein) may cause the efficiency
and/or yield of the formation of oxygen to increase as compared to
the photoactive electrode itself, when operated under essentially
identical conditions, as described herein.
[0044] There are many benefits to photoanodes and to the methods
for producing photoanodes as described herein. For example, the
method for forming a photoanode is easily adaptable and may be used
to produce photoanodes of varying sizes and shapes, as described
herein. In addition, the photoanodes produced by the provided
methods can be robust and long-lived, and can be resistant to
poisoning by acidic and/or basic conditions and/or environmental
conditions (e.g., the presence of carbon monoxide). Photoanode
poisoning may be thought of as any chemical or physical change in
the status of the photoanode that may diminish or limit the use of
a photoanode in a photoelectrochemical device and/or lead to
erroneous measurements. Photoanode poisoning may manifest itself as
the development of unwanted coatings, and/or precipitates, on the
photoanode, or dissolution and/or erosion of the photoanode. For
example, some photoactive electrodes (e.g., CdS, CdSe, GaAs, GaP)
which may be employed in a photoanode as described herein, may be
subject to surface reactions in aqueous, acidic, and/or basic
conditions. In some embodiments, the presence of the catalytic
material associated with the photoactive electrode may help prevent
and/or reduce unwanted surface reactions as opposed to a
photoactive electrode which does not comprise the catalytic
material.
[0045] Methods of making a photoanode as described herein also
represent a significant development. In some embodiments,
photoanodes are provided which are made from materials which are
easily, reproductively, and inexpensively made. In some
embodiments, a photoanode may be formed by immersing a photoactive
electrode (e.g., hematite, TiO.sub.2, etc.) in a solution
comprising metal ionic species and anionic species. Application of
a voltage to the photoactive electrode may cause the metal ionic
species and the anionic species to associate with the photoactive
electrode to form a catalytic material associated with the
photoactive electrode, thereby forming the photoanode. In some
cases, association of the metal ionic species with the photoactive
electrode may comprise a change in oxidation state of the metal
ionic species from (n) to (n+x), wherein x may be 1, 2, 3, and the
like.
[0046] The invention can also be characterized in terms of
performance of the catalytic material of the invention (and/or
photoanode comprising the catalytic material). One way of doing
this, among many, is to compare the energy conversion efficiency
and/or the current density of the photoanode versus the photoactive
electrode alone. Typical photoactive electrodes are described more
fully herein and may comprise Fe.sub.2O.sub.3, TiO.sub.2, and the
like. The photoactive electrode may be able to function, itself, as
a catalytic photoanode for water electrolysis, and may have been
used in the past to do so. So, comparison of the energy conversion
efficiency and/or the current density during catalytic water
electrolysis (where the photoanode catalytically produces oxygen
gas from water), using the photoactive electrode, as compared to
essentially identical conditions (with the same counter electrode,
same electrolyte, same circuitry, same water source, etc.), using
the photoanode of the invention including both photoactive
electrode and catalytic material, can be compared. In most cases,
the energy conversion efficiency and/or the current density of the
photoanode is greater than the energy conversion efficiency and/or
the current density of the photoactive electrode alone, where each
is tested independently under essentially identical conditions.
[0047] In some cases, the energy conversion efficiency of the
photoanode comprising the composition may be at least about 5%, at
least about 10%, at least about 15%, at least about 20%, at least
about 30%, at least about 40%, at least about 50%, at least about
60% at least about 70%, at least about 80%, at least about 90%, at
least about 100%, at least about 150%, at least about 200%, or
greater, than the efficiency of the photoactive electrode alone,
operated under essentially identical conditions. The increase in
energy conversion efficiency may be determined operating a
photoanode as described herein (e.g., comprising a photoactive
electrode and catalytic material) and the photoactive electrode
under essentially identical conditions and comparing the results.
Energy conversion efficiency is the ratio between the useful output
of an energy conversion device and the input, in energy terms and
techniques for measuring the efficiency, as will be known to those
of ordinary skill in the art. In some cases, the current density of
the photoanode may be greater than the current density of the
photoactive electrode by a factor of at least about 10, about 100,
about 1000, about 10.sup.4, about 10.sup.5, about 10.sup.6, about
10.sup.8, about 10.sup.10, and the like. In some embodiments, the
current density of the photoanode may exceed the current density of
the photoactive electrode by a factor between about 10.sup.4 and
about 10.sup.10, between about 10.sup.5 and about 10.sup.9, or
between about 10.sup.4 and about 10.sup.8. The current density may
either be the geometric current density or the total current
density, as described herein.
[0048] In some embodiments, an electrochemical cell comprising at
least a first electrode, a second electrode, and an electrolyte,
wherein at least one electrode is capable of converting solar
energy into an electrochemical potential and used for water
electrolysis is provided. In a first non-limiting embodiment, the
first electrode may comprise an n-type semiconductor electrode
(e.g., comprising a catalytic material) and a second electrode
comprises a conductor (e.g., a metal), wherein the first electrode
may be biased positively with respect to the second electrode. In
some cases, the bias may be supplied by a photovoltaic cell. In a
second non-limiting embodiment, the first electrode comprises an
n-type semiconductor and the second electrode comprises a p-type
semiconductor (e.g., a tandem configuration), wherein the first
electrode is biased positively with respect to the second
electrode. In a third non-limiting embodiment, the first electrode
comprises a conductor (e.g., a metal) and the second electrode
comprises a p-type semiconductor, wherein the first electrode may
be biased positively with respect to the second electrode.
[0049] In one embodiment, the processes that may occur in a
photoelectrochemical cell are as follows. The first electrode may
be exposed to electromagnetic radiation, wherein the first
electrode comprises an n-type semiconductor and may be biased
positively with respect to a second electrode. The light may excite
the semiconducting material of the first electrode, and result in
the formation of electronic charged carriers (e.g., electron/hole
pairs). Water may be oxidized by the electron holes produced at the
first electrode. The hydrogen ions produced at the first electrode
may be transported (e.g., through the electrolyte) to the second
electrode, and the electrons produced at the first electrode may be
transferred to the second electrode through an external circuit.
The transported hydrogen ions (e.g., H.sup.+ or another form such
as H.sub.2PO.sub.4.sup.-) may be reduced with transported electrons
at the second electrode, thereby forming hydrogen gas. FIG. 1
illustrates one possible arrangement of a photoelectrochemical cell
and is described herein.
[0050] The photovoltage of the n-type semiconducting material may
be related to the energy of the electromagnetic radiation as well
as to the band gap of the material. The band gap of a material is
the energy difference between the top of the valence band and the
bottom of the conduction band, as will be known to those of
ordinary skill in the art. If a photon has energy greater than or
equal to the band gap of the material, then electrons can form in
the conduction band and holes can form in the valence band, related
by the following Equation 3:
hv.fwdarw.e'+h. (3)
where h is Planck's constant, v is the frequency of the photon, e'
is an electron, and h. is an electron hole. Generally, an electric
field or bias (e.g., provided through doping of the semiconductor
material and/or through the application of an external voltage) may
be required at the photoactive electrode/electrolyte interference
to avoid recombination of the electron and the hole.
[0051] In some embodiments, the process that takes place at the
first electrode which is biased positively with respect to the
second electrode is shown in Equation 4.
4h.+H.sub.2O (liquid or gas).fwdarw.O.sub.2 (gas)+4H.sup.+ (4)
The process shown in Equation 4, in some cases, may take place at
the first electrode/electrolyte interface. This process produces
oxygen gas which may be released, stored, and/or used in various
devices/methods. The electrons and the hydrogen ions may combine at
the second electrode to form hydrogen gas, as shown in Equation
5.
2H.sup.++2e'.fwdarw.H.sub.2 (gas) (5)
The overall reaction that takes place is shown in Equation 6.
4hv+H.sub.2O (liquid or gas) O.sub.2.fwdarw.(gas)+2H.sub.2 (gas)
(6)
The overall reaction can occur if the energy of the photons
absorbed by the first electrode is equal to or greater than the
threshold energy, E.sub.t, which are related by Equation 7:
E t = .DELTA. G H 2 O o 4 ( 7 ) ##EQU00002##
where .DELTA.G.sub.H.sub.2.sub.O.sup.o is the standard free energy
of the reaction shown in Equation 6 (4.92 eV). E.sub.t is equal to
1.23 eV and the electrolysis of water is possible when the
electromotive force of the photoelectrochemical device is equal to
or greater than 1.23 eV. It should be understood, however, that in
some embodiments, the proton may be associated with a species and
may be transported via association with a species in solution
(e.g., a buffer species). The thermodynamics discussed above, in
most cases, would be applicable in such embodiments.
[0052] It should be understood, that photoanodes as described
herein may be formed prior to incorporation into a device or may be
formed during operation of a device. For example, in some cases, a
photoanode may be formed using methods described herein (e.g.,
exposing a photoactive electrode to a solution comprising metal
ionic species and anionic species, followed by application of a
voltage to the photoactive electrode and association of catalytic
material comprising the metal ionic species and anionic species
with the photoactive electrode). The photoanode may then be
incorporated into a device. As another example, in some cases, a
device may comprise a photoactive electrode, and a solution (e.g.,
electrolyte) comprising metal ionic species and anionic species.
Upon operation of the device (e.g., application of a voltage to the
photoactive electrode), a catalytic material (e.g., comprising the
metal ionic species and anionic species from the solution) may
associate with the photoactive electrode, thereby forming the
photoanode in the device. After formation of the photoanode, the
photoanode can be used for purposes described herein with or
without change in environment (e.g., change in solution or other
medium to which the electrode is exposed), depending upon the
desired formation and/or use medium, which would be apparent to
those of ordinary skill in the art.
[0053] FIG. 2 shows an energy diagram of a photoelectrochemical
cell comprising a photoactive electrode and an electrode, wherein
the photoactive electrode is biased positively with respect to the
electrode and comprises an n-type semiconductor (e.g., a
photoanode). In this figure, E.sub.F is the Fermi energy, E.sub.C
and E.sub.V are the energies of the bottom of the conduction band
and the top of the valence band, respectively, of the photoanode,
and E.sub.g is the band gap. For the cell to operate, the oxygen
energy levels (O.sub.2/H.sub.2O) should be above the valence band
of the photoanode for electron-hole transfer to occur, and for the
same reason, the hydrogen energy level (H.sup.+/H.sub.2) should be
below the Fermi level of the electrode (e.g., when the electrode is
a conductor). In some cases, a photoelectrochemical device may
require an external bias (e.g., voltage) in order for the
photoelectrochemical device to operate. Application of an external
bias may aid in creating increased charge separation between the
electron/hole pairs at the photoanode as compared to a photoanode
without a charge bias. In some embodiments, a charge bias of at
least about 0.1 V, at least about 0.3 V, at least about 0.5 V, at
least about 1.0 V, at least about 2.0 V, or greater, may be
provided to the photoelectrochemical device. The charge bias may
aid in reducing the probability of recombination of an electron in
the conduction band and a hole created in the valence band upon the
absorption of light energy. Some possible arrangements of
photoelectrochemical devices are described herein.
[0054] FIG. 3 shows the energy diagram of a photoelectrochemical
device comprising a first photoactive electrode (e.g., an n-type
semiconductor) and a second photoactive electrode (e.g., a p-type
semiconductor). The intrinsic nature of the band position leads to
the first photoactive electrode being "biased" positively with
respect to the second photoactive electrode (although no external
bias is provided, e.g., via a power source). This type of
photoelectrochemical device may be referred to as a
bi-photoelectrochemical cell or a tandem photoelectrochemical cell.
In the figure, E.sub.F is the Fermi energy, E.sub.C and E.sub.V are
the energies of the bottom of the conduction band and the top of
the valence band, respectively, of the photoactive electrodes, and
E.sub.g is the band gap for each photoactive electrode. A
bi-photoelectrochemical cell may be able to operate using only
solar energy without the need for an external bias, e.g., as may be
generally required for a photoelectrochemical cell comprising a
single photoactive electrode. Various possible arrangements for a
bi-photoelectrochemical cell are described herein. Additional
devices that may be used in combination with a photoanode are also
discussed in more detail below.
[0055] In some embodiments, a method of forming a photoanode
comprises causing metal ionic species and anionic species to
associate with a photoactive electrode by application of a voltage
to the photoactive electrode. In some embodiments, the method may
comprise providing a solution containing metal ionic species and
anionic species and immersing a photoactive electrode in the
solution, followed by application of voltage to the photoactive
electrode. A non-limiting example of formation of a photoanode is
shown in FIG. 4. FIG. 4A shows a container 110 comprising a
photoactive electrode 112 and a solution 114 in which are
suspended, but more typically dissolved, metal ionic species 116
and anionic species 118. In some cases, the photoactive electrode
is in electrical communication 120 with a power source (not shown).
FIG. 4B shows the same experimental set-up upon application of
voltage to the photoactive electrode by the power source. In some
cases, however, voltage may be applied to the photoactive electrode
by exposing the photoactive electrode to electromagnetic radiation
or by an external power source (e.g., a battery). Metal ionic
species 122 and anionic species 124 associate with the photoactive
electrode 126 to form a composition (e.g., a catalytic material)
128 associated with the photoactive electrode. The catalytic
material may be transparent, substantially transparent,
substantially opaque, and/or opaque. In a particular embodiment,
the catalytic material is transparent and/or substantially
transparent.
[0056] In some cases, voltage may be applied to a photoactive
electrode by a power source. For example, voltage may be applied to
the photoactive electrode by batteries, power grids, regenerative
power supplies (e.g., wind power generators, photovoltaic cells,
tidal energy generators, etc.), generators, and the like. The power
source may comprise one or more of such power supplies (e.g.,
batteries and a photovoltaic cell). The voltage applied may be AC
or DC. In such embodiments, the voltage applied to the photoactive
electrode may be substantially similar to all surfaces of the area.
In some cases, the thickness of the composition formed on the
photoactive electrode is substantially uniform across the areas
where the composition is present. For example, as shown in FIG. 5A,
application of voltage to photoactive electrode 2 through wire 4
connected to an outside power source which is immersed in solution
6 comprising metal ionic species and anionic species causes
composition 8 to associate with photoactive electrode 2.
[0057] In other cases, voltage (e.g., photovoltage) may be applied
to the photoactive electrode by exposing the photoactive electrode
to electromagnetic radiation (e.g., sunlight). As described herein,
application of electromagnetic radiation to a photoactive electrode
may cause the production of electron/hole pairs to form (e.g.,
formation of a photovoltage). In some instances, the photoactive
electrode may be exposed to varying levels of electromagnetic
radiation. For example, some surfaces of the photoactive electrode
may be exposed to a differing electromagnetic radiation (e.g.,
wavelength or range of wavelengths, time of exposure, power (e.g.,
wattage, etc.)), than other surfaces of the photoactive electrode.
In some cases, at least a portion of all the surfaces of the
photoactive electrode are exposed to substantially similar
electromagnetic radiation. In some cases, surfaces which are
exposed to a solution comprising metal ionic species and anionic
species are exposed to electromagnetic radiation, while in other
cases, surfaces which are not exposed to a solution comprising
metal ionic species and anionic species are exposed to
electromagnetic radiation. In some instances, the thickness of the
composition associated with the photoanode may or may not be
substantially similar in the areas of the photoactive electrode
which are exposed to electromagnetic radiation. In some cases, the
areas of the photoactive electrode which are more active (e.g.,
produce more electron/hole pairs in that area), may comprise a
catalytic material which is thicker than the composition in areas
which are less active (e.g., produce less electron/hole pairs in
that area). For example, as shown in FIGS. 5B and 5C, exposing
photoactive electrode 2 to electromagnetic radiation 10 causes
composition 8 to associate with photoanode. In some cases, the
composition may associate with only the areas which were directly
exposed to light as shown in FIG. 5B (e.g., surface 12 was exposed
to light and is associated with composition 8). In other cases,
substantially all surfaces may be associated with the composition
upon exposure to electromagnetic radiation, as shown in FIG. 5C
(e.g., surface 12 was exposed to light and both surface 12 and 14
are associated with composition 8), for example, in instances where
the photoactive electrode is substantially transparent.
[0058] In some cases, some areas of the photoactive electrode may
be disproportionately exposed to electromagnetic radiation at a
level greater than exposure at other areas of the photoactive
electrode, such that the formation of a composition is greater in
areas that were exposed to greater levels of electromagnetic
radiation than areas that received less electromagnetic radiation.
For example, in some embodiments, the photoactive electrode may be
exposed to patterned electromagnetic radiation which may allow for
the formation of the composition in a pattern. Various techniques
may be employed, for example, passing electromagnetic radiation
through a mask (e.g., lithographical techniques). For example, as
shown in FIG. 5D, photoactive electrode 2 is exposed to
electromagnetic radiation through mask 16 such that selected areas
of the photoactive electrode are exposed to electromagnetic
radiation. The areas of the photoactive electrode that were exposed
to light comprise composition 8. The boundaries between areas which
comprise the composition and areas which do not comprise the
composition may be sharp (e.g., the thickness of a composition in
an area is substantially uniform throughout) or gradual (e.g., the
thickness of the composition in an area is not substantially
uniform and/or the thickness of the composition decreases away from
the center of the area).
[0059] Electromagnetic radiation (e.g., in the formation of the
composition associated with the photoactive electrode or during
operation of a device as described herein) may be provided by any
suitable source. For example, electromagnetic radiation may be
provided by sunlight and/or an artificial light source. In an
exemplary embodiment, the electromagnetic radiation is provided by
sunlight. In some embodiments, light may be provided by sunlight at
certain times of operation of a device (e.g., during daytime, on
sunny days, etc.) and artificial light may be used at other times
of operation of the device (e.g., during nighttime, on cloudy days,
etc.). Non-limiting examples of artificial light sources include a
lamp (mercury-arc lamp, a xenon-arc lamp, a quartz tungsten
filament lamp, etc.), a laser (e.g., argon ion), and/or a solar
simulator. The spectra of the artificial light source may be
substantially similar or substantially different than the spectra
of natural sunlight. The light provided may be infrared
(wavelengths between about 1 mm and about 750 nm), visible
(wavelengths between about 380 nm and about 750 nm), and/or
ultraviolet (wavelengths between about 10 nm and about 380 nm). In
some cases, the electromagnetic radiation may be provided at a
specific wavelength, or specific ranges of wavelengths, for
example, through use of a monochromatic light source or through the
use of filters. The power of the electromagnetic radiation may also
be varied. For example, the light source provided may have a power
of at least about 100 W, at least about 200 W, at least about 300
W, at least about 500 W, at least about 1000 W, or greater. The
formation and properties of the composition are described
herein.
[0060] In some cases, the photoactive electrode associated with a
catalytic material as described herein may comprise a photoactive
composition, such as an n-type semiconductor. The photoactive
composition may be selected such that the band gap is between about
1.0 and about 2.0 eV, between about 1.2 and about 1.8 eV, between
about 1.4 and about 1.8 eV, between about 1.5 and about 1.7 eV, is
about 2.0 eV, or the like. The photoactive composition may also
have a Fermi level which is compatible with the electrolyte and/or
a small work function (e.g., such that electrons may diffuse into
the water to attain thermal equilibrium). This may cause the energy
bands of the photoactive composition to bend up near the interface
of the electrolyte. In some cases, the photoactive electrode may be
transparent, substantially transparent, substantially opaque, or
opaque. In an exemplary embodiment, the photoactive electrode and
the composition associated with the photoactive electrode are
transparent and/or substantially transparent. The photoactive
electrode may be a solid, semi-porous or porous. Non-limiting
examples of photoactive compositions (or, in some cases, n-type
semiconducting materials) include TiO.sub.2, WO.sub.3, SrTiO.sub.3,
TiO.sub.2--Si, BaTiO.sub.3, LaCrO.sub.3--TiO.sub.2,
LaCrO.sub.3--RuO.sub.2, TiO.sub.2--In.sub.2O.sub.3, GaAs, GaP,
p-GaAs/n-GaAs/pGa.sub.0.2In.sub.0.48P, AlGaAs/SiRuO.sub.2, PbO,
FeTiO.sub.3, KTaO.sub.3, MnTiO.sub.3, SnO.sub.2, Bi.sub.2O.sub.3,
Fe.sub.2O.sub.3 (including hematite), ZnO, CdS, MoS.sub.2, CdTe,
CdSe, CdZnTe, ZnTe, HgTe, HgZnTe, HgSe, ZnTe, ZnS, HgCdTe, HgZnSe,
etc., or composites thereof. In some cases, the photoactive
composition may be doped. For example, TiO.sub.2 may be doped with
Y, V, Mo, Cr, Cu, Al, Ta, B, Ru, Mn, Fe, Li, Nb, In, Pb, Ge, C, N,
S, etc., and SrTiO.sub.3 may be doped with Zr. The photoactive
composition may be provided in any suitable morphology or
arrangement, for example, including single crystal wafers, coatings
(e.g., thin films), nanostructured arrays, nanowires, etc. Those of
ordinary skill in the art will be aware of methods and techniques
for preparing a photoactive composition in a chosen form. For
example, doped TiO.sub.2 may be prepared by sputtering, sol-gel,
and/or anodization of Ti.
[0061] In an exemplary embodiment, the photoactive composition may
comprise alpha-Fe.sub.2O.sub.3, also known as hematite. In some
embodiments, hematite may be doped, for example, with Nb, Si, or
In. Hematite has a band gap of about 2 eV and in some cases, has
been found to absorb about 40% of the solar flux at ground level.
Hematite may be provided in any suitable arrangement, for example,
as a single crystal, as a coating (e.g., film) on a surface of a
material (e.g., SnO.sub.2 glass, Ti, etc.), as nanowires (e.g., on
a material), etc.
[0062] In some cases, the photoactive electrode may consist
essentially of the photoactive composition (e.g., the photoactive
composition forms the photoactive electrode). In such cases, the
photoactive composition may be a single crystal or polycrystalline.
The photoactive composition may or may not comprise interfaces
(e.g., grain boundaries, surface linear defects, etc.). In some
cases, the macroscale (e.g., properties representative of the
composition as a continuum such as electronic structure, Fermi
energy, etc.) and microscale properties (e.g., properties of
specific sites on the surface such as surface-active centers formed
by surface defects) of the photoactive composition may be found to
effect the reactivity and photoreactivity of the photoactive
composition.
[0063] In other cases, the photoactive electrode may not consist
essentially of the photoactive composition. For example, the
photoactive electrode may comprise the photoactive composition and
a second material. The second material, in some embodiments, may
form a core and the photoactive composition may substantially cover
the core. In other embodiments, the photoactive composition may be
formed on only a portion of the second material (e.g., one side of
the material). In some embodiments, the second material may be
non-conductive (e.g.,) inorganic substrates, (e.g., quartz, glass,
etc.) and polymeric substrates (e.g., polyethylene terephthalate,
polyethylene naphthalate, polycarbonate, polystyrene,
polypropylene, etc.) or conductive (e.g., metal, metal oxides,
etc.). For example, the photoactive composition may be formed on
the surface (e.g., as a film, as particles, as nanotubes) of a
material (e.g., Ti, stainless steel, fluorine-doped SnO.sub.2
coated glass (e.g., FTO), etc.). The photoactive composition may be
formed on a second material using techniques known to those of
ordinary skill in the art (e.g., solution techniques, sputtering,
ultrasonic spray coating, chemical vapor deposition, etc.). The
thickness of the photoactive composition may be at least about 10
nm, at least about 100 nm, at least about 1 um, at least about 10
um, at least about 100 um, at least about 1 mm, or greater. Methods
for determining the thickness of a material is described
herein.
[0064] In some cases, a photoactive electrode may comprise a
photoactive composition and a photosensitizing agent. For example,
the photoactive composition may be associated with a
photosensitizing agent (e.g., an organic dye). The photosensitizing
agent may increase the conversion efficiency of the reactions. As
an illustrative embodiment, electromagnetic radiation absorbed by
the dye causes dye molecules to be transferred from a ground-state
(Dye) to an excited state (Dye*) (e.g., see Equation 8). The
excited state dyes may transfer electrons to the photoactive
composition, resulting in the formation of a higher oxidation state
dye (Dye.sup.+) and a reduced photoactive composition (e) (e.g.,
see Equation 9). The oxidized dye molecules may react with water,
thereby resulting in the formation of oxygen at a photoactive
electrode (e.g., see Equation 10). The electrons may be transferred
from the photoactive composition to an electrode (e.g., through a
circuit) where they may react with protons to produce hydrogen gas
(e.g., see Equation 11), wherein the photoactive electrode is
biased positively with respect to the electrode.
Dye+hv.fwdarw.Dye* (8)
Dye*.fwdarw.Dye.sup.++e.sup.- (9)
Dye.sup.++1/2H.sub.2O.fwdarw.Dye+1/4O.sub.2+H.sup.+ (10)
H.sup.++e.sup.-.fwdarw.1/2H.sub.2 (11)
FIG. 6 shows an energy diagram for a photoelectrochemical device
comprising a photoactive electrode, and an electrode, wherein the
photoactive electrode comprises a photoactive composition and a
dye, wherein the electrons and electrons holes are transferred as
discussed above.
[0065] A wide variety of photosensitizing agents may be applied to
and/or associated with a photoactive composition. In some cases,
the photoactive electrode may consist essentially of the
photoactive composition and the photosensitizing agent (for
example, in instances where the photosensitizing agent is formed on
a surface of a photoactive material). In other cases, the
photoactive electrode may not consist essentially of the
photoactive composition and the photosensitizing agent (for
example, in instances where the photoactive composition is formed
on a substrate (e.g., as a film) and the photosensitizing agent is
formed (e.g., as a film) on the photoactive composition film). The
photosensitizing agent may have a single, a narrow range (e.g.,
less than about 100 nm range), a plurality, and/or a wide range
(e.g., greater than about 100 nm range) of light absorption peaks.
In some cases, the absorption may occur at a wavelength(s) between
about 300 nm and about 1000 nm. In some cases, the photosensitizing
agent may comprise a metal complex dye, an organic dye, quantum
dots, etc. Quantum dots will be known to those of ordinary skill in
the art and may comprise ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, PbS,
Bi.sub.2S.sub.3, HgS, HgSe, HgTe, MgTe, GaN, GaP, GaAs, GaSb, InN,
InP, InAs, InSb, AlAs, AlP, AlSb, AlS, and the like, or
combinations thereof (e.g., CdTe/CdSe(core/shell),
CdSe/ZnTe(core/shell)). Quantum dots may allow for improved
stability as compared to some metal or organic dyes, tailoring of
the band gap of the quantum dots (e.g., by size quantification),
and/or tailoring of the optical absorption of the quantum dots.
[0066] In some cases a metal complex dye may comprise a metal such
as ruthenium, platinum, or any other suitable metal and an organic
component (e.g., a ligand) such as biquinoline, bipyridyl,
phenanthroline, thiocyanic acid or derivatives thereof. In some
instances, an organic dye may comprise an organic component such as
a porphyrin-based system. The organic dyes may or might not
comprise at least one metal (e.g., Zn, Mg, etc.). In some cases,
the sensitizing agent may comprise a composition of the formula
ML.sub.x(L').sub.y(SCN).sub.z where M is a metal (e.g., Ru), L and
L' may be the same or different and are polypyridyl ligands (e.g.,
4,4''-(CO.sub.2H)-2,2''-bipyridine), and x, y, and z can be the
same or different and are any whole number 0, 1, 2, 3, etc.
[0067] In some cases, the photosensitizing agent comprises a
porphyrin-based system, for example:
##STR00001##
wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4 can be the same or
different and are H, an alkyl, an alkenyl, an alkynyl, a
heteroalkyl (e.g., CF.sub.2CF.sub.2CF.sub.3), a heteroalkenyl, a
heteroalkynyl, an aryl, or a heteroaryl, all optionally
substituted, or are optionally absent (e.g., such that the compound
is an anion, dianion, etc.). In some cases, additional carbons on
the porphyrin may be optionally substituted. In some instances, the
porphyrin may be an anion, dianion, etc. (e.g., such that at least
one center nitrogen atom is an anion). In some embodiments, the
porphyrin-based system may comprise a metal ion (e.g., such that
the porphyrin is an anion or a dianion, etc., and the metal ion is
coordinated in the center of the porphyrin by the nitrogen atoms).
Non-limiting examples of such metals include Ru, Rh, Fe, Co, Mg,
Al, Ag, Au, Zn, Sn, etc., as known to those of ordinary skill in
the art. In a particular case, at least one of R.sup.1 through
R.sup.4 is an aryl, for example, --C.sub.6H.sub.5,
--C.sub.6F.sub.5, --C.sub.6H.sub.4(COOH), --C.sub.6H.sub.4OH,
--C.sub.6H.sub.4(CH.sub.3), --C.sub.6H.sub.4(C(.dbd.O)OCH.sub.3),
(ortho, meta, or para)-C.sub.6H.sub.3X.sub.2 where X is a halide
(e.g., F, Cl, Br, I), etc. Non-limiting examples of porphyrins
include, but are not limited to:
##STR00002## ##STR00003## ##STR00004##
[0068] Additional suitable photosensitizing agents may include, for
example, dyes that include functional groups, such as carboxyl
and/or hydroxyl groups that can chelate to the nanoparticles, e.g.,
to Ti(IV) sites on a TiO.sub.2 surface. Examples of suitable dyes
include, but are not limited to, anthocyanins, phthalocyanines,
merocyanines, cyanines, squarates, eosins, and metal-containing
dyes. In some cases, a metal-containing dye may be a polypyridyl
complex of ruthenium(II) (e.g.,
cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(I-
I),
tris(isothiocyanato)-ruthenium(II)-2,2':6',2''-terpyridine-4,4',4''-tr-
icarboxylic acid,
cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(I-
I) bis-tetrabutylammonium,
cis-bis(isocyanato)(2,2'-bipyridyl-4,4'-dicarboxylato)ruthenium
(II), and tris(2,2'-bipyridyl-4,4'-dicarboxylato)ruthenium(II)
dichloride).
[0069] The porosity of a photoactive electrode (or other component,
for example, a photoanode) may be measured as a percentage or
fraction of the void spaces in the photoactive electrode. The
percent porosity of a photoactive electrode may be measured using
techniques known to those of ordinary skill in the art, for
example, using volume/density methods, water saturation methods,
water evaporation methods, mercury intrusion porosimetry methods,
and nitrogen gas adsorption methods. In some embodiments, the
photoactive electrode may be at least about 10% porous, at least
about 20% porous, at least about 30% porous, at least about 40%
porous, at least about 50% porous, at least about 60% porous, or
greater. The pores may be open pores (e.g., have at least one part
of the pore open to an outer surface of the electrode and/or
another pore) and/or closed pores (e.g., the pore does not comprise
an opening to an outer surface of the electrode or another pore).
In some cases, the pores of a photoactive electrode may consist
essentially of open pores (e.g., the pores of the photoactive
electrode are greater than at least 70%, greater than at least 80%,
greater than at least 90%, greater than at least 95%, or greater,
of the pores are open pores). In some cases, only a portion of the
photoactive electrode may be substantially porous. For example, in
some cases, only a single surface of the photoactive electrode may
be substantially porous. As another example, in some cases, the
outer surface of the photoactive electrode may be substantially
porous and the inner core of the photoactive electrode may be
substantially non-porous. In a particular embodiment, the entire
photoactive electrode is substantially porous.
[0070] In some embodiments, the photoactive electrode may have a
high surface area (e.g., geometric or total surface area). In some
cases, the surface area of the photoactive electrode may be greater
than about 0.01 m.sup.2/g, greater than about 0.05 m.sup.2/g,
greater than about 0.1 m.sup.2/g, greater than about 0.5 m.sup.2/g,
greater than about 1 m.sup.2/g, greater than about m.sup.2/g,
greater than about 10 m.sup.2/g, greater than about 20 m.sup.2/g,
greater than about 30 m.sup.2/g, greater than about 50 m.sup.2/g,
greater than about 100 m.sup.2/g, greater than about 150 m.sup.2/g,
greater than about 200 m.sup.2/g, greater than about 250 m.sup.2/g,
greater than about 300 m.sup.2/g, or the like. In other cases, the
surface area of the photoactive electrode may be between about 0.01
m.sup.2/g and about 300 m.sup.2/g, between about 0.1 m.sup.2/g and
about 300 m.sup.2/g, between about 1 m.sup.2/g and about 300
m.sup.2/g, between about 10 m.sup.2/g and about 300 m.sup.2/g
between about 0.1 m.sup.2/g and about 250 m.sup.2/g, between about
50 m.sup.2/g and about 250 m.sup.2/g, or the like. In some cases,
the surface area of the photoactive electrode may be due to the
photoactive electrode comprising a highly porous material. The
surface area of a photoactive electrode may be measured using
various techniques, for example, optical techniques (e.g., optical
profiling, light scattering, etc.), electron beam techniques,
mechanical techniques (e.g., atomic force microscopy, surface
profiling, etc.), electrochemical techniques (e.g., cyclic
voltammetry, etc.), etc., as known to those of ordinary skill in
the art.
[0071] The photoactive electrode may be of any size or shape.
Non-limiting examples of shapes include sheets, cubes, cylinders,
hollow tubes, spheres, and the like. The photoactive electrode may
be of any size, provided that at least a portion of the photoactive
electrode may be immersed in the solution comprising the metal
ionic species and the anionic species. The methods described herein
are particularly amenable to forming the catalytic material on any
shape and/or size of photoactive electrode. In some cases, the
maximum dimension of the photoactive electrode in one dimension may
be at least about 1 mm, at least about 1 cm, at least about 5 cm,
at least abut 10 cm, at least about 1 m, at least about 2 m, or
greater. In some cases, the minimum dimension of the photoactive
electrode in one dimension may be less than about 50 cm, less than
about 10 cm, less than about 5 cm, less than about 1 cm, less than
about 10 mm, less than about 1 mm, less than about 1 um, less than
about 100 nm, less than about 10 nm, less than about 1 nm, or less.
Additionally, the photoactive electrode may comprise a means to
connect the photoactive electrode to a power source and/or other
electrical devices. In some cases, the photoactive electrode may be
at least about 10%, at least about 30%, at least about 50%, at
least about 60%, at least about 70%, at least about 80%, at least
about 90%, at least about 95%, or at least about 100% immersed in
the solution.
[0072] The photoactive electrode may or may not be substantially
planar. For example, the photoactive electrode may comprise
ripples, waves, dendrimers, spheres (e.g., nanospheres), rods
(e.g., nanorods), a powder, a precipitate, a plurality of
particles, and the like. In some embodiments, the surface of the
photoactive electrode may be undulating, wherein the distance
between the undulations and/or the height of the undulations are on
a scale of nanometers, micrometers, millimeters, centimeters, or
the like. In some instances, the planarity of the photoactive
electrode may be determined by determining the roughness of the
photoactive electrode. As used herein, the term "roughness" refers
to a measure of the texture of a surface (e.g., photoactive
electrode), as will be known to those of ordinary skill in the art.
The roughness of the photoactive electrode may be quantified, for
example, by determining the vertical deviations of the surface of
the photoactive electrode from planarity. Roughness may be measured
using contact (e.g., dragging a measurement stylus across the
surface such as a profilometers) or non-contact methods (e.g.,
interferometry, confocal microscopy, electrical capacitance,
electron microscopy, etc.). In some cases, the surface roughness,
R.sub.a, may be determined, wherein R.sub.a is the arithmetic
average deviations of the surface valleys and peaks, expressed in
micrometers. The R.sub.a of a non-planar surface may be greater
than about 0.1 um, greater than about 1 um, greater than about 5
um, greater than about 10 um, greater than about 50 um, greater
than about 100 um, greater than about 500 um, greater than about
1000 um, or the like.
[0073] Without wishing to be bound by theory, the formation of a
catalytic material on a photoactive electrode may proceed according
to the following example. A photoactive electrode may be immersed
in a solution comprising metal ionic species (M) with an oxidation
state of (n) (e.g., M.sup.n) and anionic species (e.g., A.sup.-y).
As voltage is applied to the photoactive electrode, metal ionic
species near to the photoactive electrode may be oxidized to an
oxidation state of (n+x) (e.g., M.sup.(n+x)). The oxidized metal
ionic species may interact with an anionic species near the
electrode to form a substantially insoluble complex, thereby
forming a catalytic material. In some cases, the catalytic material
may be in electrical communication with the photoactive electrode.
A non-limiting example of this process is depicted in FIG. 7. FIG.
7A shows a single metal ionic species 40 with an oxidation state of
(n) in solution 42. Metal ionic species 44 may be near photoactive
electrode 46, as depicted in FIG. 7B. As shown in FIG. 7C, metal
ionic species may be oxidized to an oxidized metal ionic species 48
with an oxidation state of (n+x) and (x) electrons 50 may be
transferred to photoactive electrode 52 or to another species near
or associated with the metal ionic species and/or the photoactive
electrode. FIG. 7D depicts a single anionic species 54 nearing
oxidized metal ionic species 56. In some instances, as depicted in
FIG. 7E, anionic species 58 and oxidized metal ionic species 60 may
associate with photoactive electrode 62 to form a catalytic
material. In some instances, the oxidized metal ionic species and
the anionic species may interact and form a complex (e.g., a salt)
before associating with the electrode. In other instances, the
metal ionic species and anionic species may associate with each
other prior to oxidation of the metal ionic species. In other
instances, the oxidized metal ionic species and/or anionic species
may associate directly with the photoactive electrode and/or with
another species already associated with the photoactive electrode.
In these instances, the metal ionic species and/or anionic species
may associate with the photoactive electrode (either directly, or
via formation of a complex) to form the catalytic material (e.g., a
composition associated with the photoactive electrode).
[0074] In some embodiments, a photoanode may be formed by immersing
a photoactive electrode associated with a material comprising a
metal ionic species (e.g., a layer of a metal such as metallic
cobalt associated with a photoactive electrode) in a solution
comprising ionic species (e.g., phosphate). The metal ionic species
(e.g., in an oxidation state of M.sup.n) may be oxidized and/or
dissociated from the photoactive electrode into solution. The metal
ionic species that are oxidized and/or dissociated from the
photoactive electrode may interact with anionic species and/or
other species, and may re-associate with the photoactive electrode,
thereby re-forming a catalytic material.
[0075] As noted above, one aspect of the invention involves an
efficient and robust catalytic material (for electrolysis of water
and/or other electrochemical reactions) that is primarily
photoactive electrode-associated, rather than functioning largely
as a homogeneous solution-based catalytic materials. Such a
catalytic material "associated with" a photoactive electrode will
now be described with reference to a metal ionic species and/or
anionic species which can define a catalytic material of the
invention. In some cases, the anionic species and the metal ionic
species may interact with each other prior to, simultaneously to,
and/or after the association of the species with the photoactive
electrode, and result in a catalytic material with a high degree of
solid content resident on, or otherwise immobilized with respect
to, the photoactive electrode. In this arrangement, the catalytic
material can be solid including various degrees of electrolyte or
solution (e.g., the material can be hydrated with various amounts
of water), and/or other species, fillers, or the like, but a
unifying feature among such catalytic material associated with
photoactive electrodes is that they can be observed, visually or
through other techniques described more fully below, as largely
resident on or immobilized with respect to the photoactive
electrode, either in electrolyte solution or after removal of the
photoactive electrode from solution.
[0076] In some cases, the catalytic material may associate with the
photoactive electrode via formation of a bond, such as an ionic
bond, a covalent bond (e.g., carbon-carbon, carbon-oxygen,
oxygen-silicon, sulfur-sulfur, phosphorus-nitrogen,
carbon-nitrogen, metal-oxygen, or other covalent bonds), a hydrogen
bond (e.g., between hydroxyl, amine, carboxyl, thiol, and/or
similar functional groups), a dative bond (e.g., complexation or
chelation between metal ions and monodentate or multidentate
ligands), Van der Waals interactions, and the like. "Association"
of the composition (e.g., catalytic material) with the photoactive
electrode would be understood by those of ordinary skill in the art
based on this description. In some embodiments, the interaction
between a metal ionic species and an anionic species may comprise
an ionic interaction, wherein the metal ionic species is directly
bound to other species and the anionic species is a counterion not
directly bound to the metal ionic species. In a specific
embodiment, an anionic species and a metal ionic species form an
ionic bond and the complex formed is a salt.
[0077] A catalytic material associated with a photoactive electrode
may be most often arranged with respect to the photoactive
electrode so that it is in sufficient electrical communication with
the photoactive electrode to carry out purposes of the invention as
described herein. "Electrical communication," as used herein, is
given its ordinary meaning as would be understood by those of
ordinary skill in the art whereby electrons can flow between the
photoactive electrode and the catalytic material in a facile enough
manner for the photoanode to operate as described herein. That is,
charge may be transferred between the photoactive electrode and the
catalytic material (e.g., the metal ionic species and/or anionic
species present in the catalytic material). In one arrangement, the
composition is in direct contact with the photoactive electrode. In
another arrangement, a material may be present between the
composition and the photoactive electrode (e.g., a photosensitizing
agent, an insulator, a conducting material, a semiconducting
material, etc.).
[0078] In some cases, the composition may be in "direct electrical
communication" with the photoactive electrode. "Direct electrical
communication," as used herein, is given its ordinary meaning as
defined above with respect to electrical communication, but in this
instance, the photoactive electrode and the composition are in
direct contact with one another (e.g., as opposed to through a
secondary material, through use of circuitry, etc.). In some
embodiments, the composition and the photoactive electrode may be
integrally connected. The term "integrally connected," when
referring to two or more objects, means objects that do not become
separated from each other during the course of normal use, e.g.,
separation requires at least the use of tools, and/or by causing
damage to at least one of the components, for example, by breaking,
peeling, dissolving, etc. A composition may be considered to be in
direct electrical communication with a photoactive composition
during operation of a photoanode even in instances where a portion
of the composition may dissociate from the photoactive composition
when taking part in a dynamic equilibrium.
[0079] In some embodiments, a composition (e.g., catalytic
material) may be in "indirect electrical communication" with a
photoactive electrode. That is, a material and/or circuitry may be
interposed between the composition and the photoactive electrode.
In some cases, the material may be a "hole-tunneling barrier." That
is, a material through which electron-holes generated in the
photoactive electrode may tunnel through to access the composition
(e.g., catalytic material). The hole-tunneling barrier may aid in
protecting the photoactive electrode from corrosion. In some
instances, the material may be a conducting material, thereby
allowing electrons to flow between the photoactive electrode and
the composition. The electrons may be used for the production of
oxygen gas from water via the composition. Without wishing to be
bound by theory, a material disposed between the composition and
the photoactive electrode may act as a membrane and may allow for
the transmission of electron holes generated at the photoactive
electrode to the composition. This arrangement may be advantageous
in devices where the separation of oxygen gas and hydrogen gas
formed from the oxidation of water is important. The presence of
the material may prevent oxygen gas formed where the composition is
present from transversing the device and entering the area where
hydrogen gas is produced. In some cases, the material may be
selected such that no oxygen gas is produced in the material (e.g.,
in instances where the overpotential for production of oxygen gas
is high).
[0080] In instances where the photoanode comprises a catalytic
material and a photoactive electrode (e.g., comprising a
photoactive composition and a photosensitizing agent), the
catalytic material, the photoactive composition and the
photosensitizing agent may be in electrical communication with one
another. In some cases, the photoactive composition and the
photosensitizing agent and/or the photosensitizing agent and the
catalytic material may be in direct electrical communication with
one another and/or integrally connected. For example, in some
cases, a photoanode may comprise a photoactive composition in
direct electrical communication with a photosensitizing agent,
wherein the photosensitizing agent is in direct electrical
communication with a catalytic material (e.g., the photoactive
composition comprises a coating of a photosensitizing agent
followed by a coating of catalytic material).
[0081] It should be understood that while much of the discussion
herein focuses on a photoanode comprising a catalytic material
associated with a photoactive electrode (e.g., the photoactive
electrode and the catalytic material are in direct electrical
communication), this is by no means limiting, and the photoanode
may comprise one or more materials between the photoactive
electrode and the catalytic material (e.g., such that the
photoactive electrode and the catalytic material are in indirect
electrical communication).
[0082] One aspect of the invention involves the development of a
regenerative catalytic photoanode. As used herein, a "regenerative
photoanode" refers to a photoanode which is capable of being
compositionally regenerated as it is used in a catalytic process,
and/or over the course of a change between catalytic use settings.
Thus, a regenerative photoanode of the invention is one that
includes one or more species associated with the photoanode (e.g.,
adsorbed on the photoanode) which, under certain conditions,
dissociate from the photoanode, and then a significant portion or
substantially all of those species re-associate with the photoanode
at a later point in the photoanode's life or use cycle. For
example, at least a portion of the catalytic material may
dissociate from the photoanode and become solvated or suspended in
a fluid to which the photoanode is exposed, and then become
re-associated (e.g., adsorbed) at the photoanode. The
disassociation/re-association may take place as a part of the
catalytic process itself, as catalytic species cycle between
various states (e.g., oxidation states), in which they are more or
less soluble in the fluid. This phenomenon during use, for example,
nearly or essentially steady-state use of the electrode, can be
defined as a dynamic equilibrium. "Dynamic equilibrium," as used
herein, refers to an equilibrium comprising metal ionic species and
anionic species, wherein at least a portion of the metal ionic
species are cyclically oxidized and reduced (as discussed elsewhere
herein). Regeneration over the course of a change between catalytic
use settings can be defined by a dynamic equilibrium which
experiences a significant delay in its cyclical nature.
[0083] In some embodiments, at least a portion of the catalytic
material may dissociate from the photoanode and become solvated or
suspended in the fluid (or solution and/or to other medium) as a
result of a significant reaction setting change, and then become
re-associated at a later stage. A significant reaction setting
change, in this context, can be a significant change in potential
applied to the electrode, significantly different current density
at the photoanode, significantly different properties of a fluid to
which the photoanode is exposed (or removal and/or changing of the
fluid), or the like. In one embodiment, the photoanode is exposed
to catalytic conditions under which the catalytic material
catalyzes a reaction, then the circuit of which the photoanode is a
part is changed so that the catalytic reaction is significantly
slowed or even essentially stopped (e.g., the process is turned
off), and then the system can be returned to the original catalytic
conditions (or similar conditions that promote the catalysis), and
at least a portion, or essentially all of the catalytic material,
can re-associate with the photoanode. Re-association of some or
essentially all of the catalytic material with the photoanode can
occur during use and/or upon change in conditions as noted above,
and/or can occur upon exposure of the catalytic material, the
electrode, or both to a regenerative stimulus, such as a
regenerative electrical potential, current, temperature,
electromagnetic radiation, or the like. In some cases, the
regeneration may comprise a dynamic equilibrium mechanism involving
oxidation and/or reduction processes, as described elsewhere
herein.
[0084] Regenerative photoanodes of the invention can exhibit
disassociation and re-association of catalytic species at various
levels. In one set of embodiments, at least about 0.1% by weight of
catalytic material associated with the photoanode disassociates as
described herein, and in other embodiments as much as about 0.25%,
about 0.5%, about 0.6%, about 0.8%, about 1.0%, about 1.25%, about
1.5%, about 1.75%, about 2.0%, about 2.5%, about 3%, about 4%,
about 5%, or more of the catalytic material disassociates, and some
or all re-associates as discussed. In various embodiments, of the
amount of material that disassociates, at least about 50%, at least
about 60%, at least about 70%, at least about 80%, at least about
85%, at least about 90%, at least about 95%, at least about 97%, at
least about 98%, at least about 99%, or essentially all material
re-associates. Those of ordinary skill in the art will understand
the meaning of disassociation and re-association of material in
this regard, and will know of techniques for measuring these
factors (for example, scanning electron microscopy and/or elemental
analyses of the electrode, chemical analysis of the fluid,
photoanode performance, or any combination). Further, those of
ordinary skill in the art will quickly be able to select catalytic
materials which meet these parameters with knowledge of
solubilities and/or catalytic reaction screening, or combinations.
As a specific example, in some cases, during use of a catalytic
material comprising cobalt ions and anionic species comprising
phosphorus, at least a portion of the cobalt ions and the anionic
species comprising phosphorus periodically associate and dissociate
from the electrode.
[0085] Catalytic materials of the invention may also exhibit
significant robustness through varying levels of use in a way that
is a significant improvement over the general state of the art.
Through a mechanism that may be related to regeneration as
described herein, systems and/or photoanodes employing catalytic
materials of the invention may be operated at varying rates of
applied energy, as would result from being driven by power sources
that vary (e.g., wind power, solar power which generally varies
over the daily cycle and weather patterns, etc.), and/or go through
full on/off energy cycles. In particular, systems and/or
photoanodes of the invention may be cycled such that potential
and/or current supplied to the system and/or photoanode is reduced
by at least about 20%, at least about 40%, at least about 60%, at
least about 80%, at least about 90%, at least about 95%, or
essentially 100% from peak use current, for at least from a period
of about 2 minutes, at least about 5 minutes, at least about 10
minutes, at least about 20 minutes, at least about 30 minutes, at
least about 1 hour, at least about 2 hours, at least about 3 hours,
at least about 5 hours, at least about 8 hours, at least about 12
hours, at least about 24 hours or greater, and cycled at least
about five times, at least about 10 times, at least about 20 times,
at least about 50 times, or more, while overall performance (e.g.,
overpotential at a selected current density, production of oxygen
gas, production of water, etc.) of the system and/or photoanode,
decreases by no more than about 20%, no more than about 10%, no
more than about 8%, no more than about 6%, no more than about 4%,
no more than about 3%, no more than about 2%, no more than about
1%, or the like. In some cases, the performance measurement may be
taken at about the same period of time after reapplication of the
voltage/current to the photoanode/system (e.g., after
voltage/current has been reapplied to the photoanode/system for
about 1 minute, about 5 minutes, about 10 minutes, about 30
minutes, about 60 minutes, etc.).
[0086] It should be understood, however, that not every metal ionic
species and/or anionic species which exhibits a change in oxidation
state can dissociate and re-associate with a photoactive electrode.
In some cases, only a small portion (e.g., less than about 20%,
less than about 15%, less than about 10%, less than about 5%, less
than about 2%, less than about 1%, or less) of the oxidized/reduced
metal ionic species may dissociate/associate with the photoactive
electrode during operation or between uses.
[0087] Those of ordinary skill in the art also will quickly
recognize the significance of the contribution of this aspect
(e.g., regeneration mechanism) of the invention to the field. It is
known that degradation of catalytic materials and photoanodes can
be problematic during their use, or especially when they are shut
off between uses, especially in the case of metal organic,
inorganic, and/or organometallic catalytic materials exposed to
conditions previously assumed necessary for standard catalytic
processes, and/or conditions described in accordance with catalysis
according to the present invention (e.g., metal oxides and/or
hydroxides or other catalytic materials used in processes at high
pH). Without wishing to be bound by any theory, the inventors
believe their development of regenerative photoanodes relates to
selection of species with high enough stability under catalytic
conditions described herein, and/or combination of this feature
with the process of some amount of catalytic material loss from the
photoanode followed by re-association of the material with the
photoanode, which is believed to involve a material cleansing
process. The regeneration mechanism may also inhibit unwanted
coating or other accumulation of auxiliary species, which do not
play a role in the catalytic process and which may inhibit
catalysis and/or other performance characteristics.
[0088] Regenerative photoanodes of the invention also exhibit
strong and surprising performance associated with their
regenerative properties. Thus, in various embodiments, a
regenerative photoanode of the invention not only has good
long-term robustness, but exhibits surprisingly good stability even
upon significant variations in its use. Significant use variations
can involve the photoanode and its corresponding catalysis system
being switched from on to off states, or other significant changes
in use profile. This can be particularly important where the
photoanode is driven by solar power, where variation in the sun
intensity can vary dramatically. In such a situation, a photoanode
of the invention may be operating at essentially full capacity at
times, and be switched off at times (e.g., where an electrical
circuit in which the photoanode exists is in an "open" position).
The photoanode of the invention exhibits robustness such that, when
it is operated at or close to its highest capacity for catalysis,
i.e., at its highest rate of catalysis, and then switched off
("open circuit"), and this is repeated at least ten times, the
photoanode exhibits less than about 10%, less than about 5%, less
than about 4%, less than about 3%, less than about 2%, less than
about 1%, less than about 0.5%, or less than about 0.25% loss in
performance. In this case, performance can be measured as current
density at a particular set overpotential, with all other
conditions being essentially identical between all tests. Of
course, the photoanode need not necessarily be switched between
essentially full capacity and off in this way, but a photoanode of
the invention, when treated in this way, will exhibit a level of
robustness.
[0089] In some cases, the photoanode may be capable of
regeneration, as described herein, in a closed system. That is, the
photoanode may be capable of regeneration without removal and/or
addition of any material(s) that aids and/or assists in the
regeneration of the photoanode. Alternatively, removal of and/or
addition of such material in only small amounts in various
embodiments, such as, for example, no more than about 1% by weight,
or no more than about 2%, 4%, 6%, 10%, or more, by weight of such
material. For example, in instances where the photoanode comprises
a regenerative catalytic material, the catalytic material may be
capable of regeneration without addition of any of the components
comprised in the catalytic material (e.g., metal ionic species
and/or anionic species where the catalytic material is composed of
these materials) in such a closed system, or addition of one or
such components in amounts no more than those described above in
various embodiments. It should be understood, however, that a
"closed system" as used herein does not exclude addition or removal
of species that do not define, or can not react within the system
to define, the catalytic material. For example, additional fuel
and/or water may be provided to such a system.
[0090] In some embodiments, a dynamic equilibrium may comprise at
least a portion of the metal ionic species being cyclically
oxidized and reduced, wherein the metal ionic species are thereby
associated and disassociated, respectively, from the photoactive
electrode. An example of a dynamic equilibrium (or regenerative
mechanism) which can, but need not necessarily, take place in
accordance with the invention is depicted in FIG. 8. FIG. 8A
depicts a photoanode comprising photoactive electrode 80 and
catalytic material 82 comprising metal ionic species 84 and anionic
species 86. The dynamic equilibrium is depicted in FIGS. 8B-8C.
FIG. 8B shows the same photoanode, wherein a portion of metal ionic
species 88 and anionic species 90 have disassociated from
photoactive electrode 92. FIG. 8C shows the same photoanode at some
point later in time where a portion of the metal ionic species and
anionic species (e.g., 94) which disassociated from the photoactive
electrode have re-associated with photoactive electrode 96.
Additionally, different metal ionic species and anionic species
(e.g., 98) may have disassociated from the photoactive electrode.
Metal ionic species and anionic species can repeatedly disassociate
and associate with the photoactive electrode. For example, the same
metal ionic species and anionic species may disassociate and
associate with the photoactive electrode. In other instances, the
metal ionic species and/or anionic species may only disassociate
and/or associate with the photoactive electrode once. A single
metal ionic species may associate with the photoactive electrode
simultaneously as a second single metal ionic species disassociates
from the photoanode. The number of single metal ionic species
and/or single anionic species that may disassociate and/or
associate simultaneously and/or within the lifetime of the
photoanode has no numerical limit.
[0091] It should be understood that a solution in which metal ionic
species and/or anionic species may be solubilized may be
transiently present (e.g., the solution might not necessarily be in
contact with the photoactive electrode during the entire operation
and/or formation of the photoanode). For example, in instances
where water is provided to the photoanode in a gaseous state, in
some embodiments, the solution may be comprised of transiently
formed aqueous molecules and/or droplets on the surface of the
photoanode and/or electrolyte. In other instances, where the
electrolyte is a solid, the solution may be present in addition to
the electrolyte (e.g., as water droplets on the surface of the
photoanode and/or solid electrolyte) or in combination with the
fuel (e.g., water). The photoanode may be operated with a
combination of solid electrolyte/gaseous fuel, fluid
electrolyte/gaseous fuel, solid electrolyte/fluid fuel, fluid
electrolyte/fluid fuel, or any combination thereof.
[0092] In some embodiments, during the dynamic equilibrium, at
least a portion of the metal ionic species are cyclically oxidized
and reduced. That is, the oxidation state of at least a portion of
the metal ionic species involved in the dynamic equilibrium is
repeatedly changed during the dynamic equilibrium. A change in the
oxidation state of a metal ionic species may also correlate to the
association or dissociation of the metal ionic species with the
photoactive electrode.
[0093] In some embodiments, the metal ionic species in solution may
have an oxidation state of (n), while the metal ionic species
associated with the photoactive electrode may have an oxidation
state of (n+x), wherein x is any whole number. The change in
oxidation state may facilitate the association of the metal ionic
species on the photoactive electrode. It may also facilitate the
oxidation of water to form oxygen gas or other electrochemical
reactions. The cyclically oxidized and reduced oxidation states for
a single metal ionic species in dynamic equilibrium may be
expressed according to Equation 12:
M.sup.nM.sup.n+x+x(e.sup.-) (12)
where M is a metal ionic species, n is the oxidation state of the
metal ionic species, x is the change in the oxidation state, and
x(e.sup.-) is the number of electrons, where x may be any whole
number. In some cases, the metal ionic species may be further
oxidized and/or reduced, (e.g., the metal ionic species may access
oxidation states of M.sup.(n+1), M.sup.(n+2), etc.)
[0094] An illustrative example of changes in oxidation state that
may occur for a single metal ionic species during a dynamic
equilibrium is shown in FIG. 9. FIG. 9A depicts a photoactive
electrode 100 and a single metal ionic species 102 in oxidation
state of (n), (e.g., M.sup.n). The metal ionic species 102 may be
oxidized to a metal ionic species 104 with an oxidation state of
(n+1) (e.g., M.sup.(n+1)) and/or associate with the photoactive
electrode 106, as shown in FIG. 9B. At this point, the metal ionic
species (e.g., M.sup.(n+1)) may disassociate from the photoactive
electrode 106 or may undergo a further change in oxidation state.
In some cases, as shown in FIG. 9C, the metal ionic species may be
further oxidized to a single metal ionic species 108 with an
oxidation state of (n+2), (e.g., M.sup.(n+2)) and may remain
associated with the photoactive electrode (or may disassociate from
the photoactive electrode). At this point, metal ionic species 108
(e.g., M.sup.(n+2)) may accept electrons (e.g., from water or
another reaction component) and may be reduced to form metal ionic
species with a reduced oxidation state of (n) or (n+1) (e.g.,
M.sup.(n+1), 106 or M.sup.n, 102). In other cases, the metal ionic
species 106 (e.g., M.sup.(n+1)) may be reduced and reform metal
ionic species in oxidation state (n) (e.g., M.sup.n, 102). The
metal ionic species in oxidation state (n) may remain associated
with the photoactive electrode or may disassociate from the
photoactive electrode (e.g., dissociate into solution).
[0095] Those of ordinary skill in the art will be able to use
suitable screening tests to determine whether a metal ionic species
and/or anionic species are in dynamic equilibrium and/or whether a
photoactive electrode is regenerative. For example, in some cases,
the dynamic equilibrium may be determined using radioisotopes of
the metal ionic species and/or anionic species. In such cases, a
photoanode comprising a photoactive electrode and a catalytic
material comprising radioisotopes may be prepared. The photoanode
may be placed in an electrolyte which comprises non-radioactive
ionic species. The catalytic material may dissociate from the
photoactive electrode and therefore, the solution may comprise
radioactive isotopes of the anionic species and/or metal ionic
species. This may be determined by analyzing an aliquot of the
electrolyte for the radioisotopes. Upon application of the voltage
to the photoactive electrode, in instances where the metal ionic
species and anionic species are in dynamic equilibrium, the
radioisotopes of the metal ionic species may re-associate with the
photoactive electrode. Aliquots of the electrolyte may be analyzed
to determine the amount of radioisotope present in the electrolyte
at various time points after application of the voltage. If the
metal ionic species and anionic species are in dynamic equilibrium,
the percentage of radioisotopes in solution may decrease with time
as the radioisotopes re-associate with the photoactive electrode.
This screening technique may be used both to determine how a
catalytic material may be functioning, and to select materials
which can be used as catalytic materials suitable for the
invention.
[0096] Further techniques useful for selecting suitable catalytic
material follow. Without wishing to be bound by theory, the
solubility of a material comprising anionic species and oxidized
metal ionic species may influence the association of the metal
ionic species and/or anionic species with the photoactive
electrode. For example, if a material formed by (c) number of
anionic species and (b) number of oxidized metal ionic species is
substantially insoluble in the solution, the material may be
influenced to associate with the photoactive electrode. This
non-limiting example may be expressed according to Equation 13:
b(M.sup.(n+x))+c(A.sup.-y){[M].sub.b[A].sub.c}.sup.(b(n+x)-c(y))(s)
(13)
where M.sup.(n+x) is the oxidized metal ionic species, A.sup.-y is
the anionic species, and {[M].sub.b[A].sub.c}.sup.(b(n+x)-c(y)) is
at least a portion of catalytic material formed, where b and c are
the number of metal ionic species and anionic species,
respectively. Therefore, the equilibrium may be driven towards the
formation of the catalytic material by the presence of an increased
amount of anionic species. In some cases, the solution surrounding
the photoactive electrode may comprise an excess of anionic
species, as described herein, to drive the equilibrium towards the
formation of the catalytic material associated with the photoactive
electrode. It should be understood, however, that the catalytic
material does not necessarily consist essentially of a material
defined by the formula {[M].sub.b[A].sub.c}.sup.n+x-y), as, in most
cases, additional components can be present in the catalytic
material (e.g., a second type of anionic species). However, the
guidelines described herein (e.g., regarding K.sub.sp) provide
information to select complimentary anionic species and metal ionic
species that may aid in the formation and/or stabilization of the
catalytic material. In some cases, the catalytic material may
comprise at least one bond between a metal ionic species and an
anionic species (e.g., a bond between a cobalt ion and an anionic
species comprising phosphorus).
[0097] Selection of metal ionic species and anionic species for use
in the invention will now be described in greater detail. It is to
be understood that any of a wide variety of such species meeting
the criteria described herein can be used and, so long as they
participate in catalytic reactions described herein, they need not
necessarily behave, in terms of their oxidation/reduction
reactions, cyclical association/disassociation from the photoactive
electrode etc., in the manner described in the application. But in
many cases, metal ionic and anionic species selected as described
herein, do behave according to one or more of the
oxidations/reduction and solubility theories described herein. In
some embodiments, the metal ionic species (M.sup.n) and the anionic
species (A.sup.-y) may be selected such that they exhibit the
following properties. In most cases, the metal ionic species and
the anionic species are soluble in an aqueous solution. In
addition, the metal ionic species may be provided in an oxidized
form, for example with an oxidation state of (n), where (n) is one,
two, three, or greater, i.e., in some cases, the metal ionic
species have access to an oxidation state greater than (n), for
example, (n+1) and/or (n+2).
[0098] The solubility product constant, K.sub.sp, as will be known
to those of ordinary skill in the art, is a simplified equilibrium
constant defined for the equilibria between a composition
comprising the species and their respective ions in solution and
may be defined according to Equation 15, based on the equilibrium
shown in Equation 14.
{M.sub.yA.sub.n}.sub.(s)py(M).sup.n.sub.(aq)+pn(A).sup.-y.sub.(aq)
(14)
K.sub.sp=[M].sup.y[A].sup.n (15)
In Equations 14 and 15, M is the positively charged metal ionic
species and A is the anionic species and y is not equal to n. In
embodiments where y is equal to n, the equation may be simplified
as shown in Equation 16.
{M.sub.yA.sub.n}.sub.(s)(M).sup.n.sub.(aq)+(A).sup.-y.sub.(aq)
(16)
The solid complex M.sub.yA.sub.n may disassociate into solubilized
metal ionic species and anionic species. Equation 15 shows the
solubility product constant expression. As will be known to those
of ordinary skill in the art, the solubility product constant value
may change depending on the temperature of the aqueous solution.
Therefore, when choosing metal ionic species and anionic species
for the formation of a photoanode, the solubility product constant
should be determined at the temperature at which the photoanode is
to be formed and/or operated in. In addition, the solubility of a
solid complex may change depending on the pH. This effect should be
taken into account when applying the solubility product constant to
the selection of a metal ionic species and an anionic species.
[0099] In many cases, the metal ionic species and anionic species
are selected together, for example, such that a composition
comprising the metal ionic species with an oxidation state of (n)
and the anionic species is soluble in an aqueous solution, the
composition having a solubility product constant which is greater
than the solubility product constant of a composition comprising
the metal ionic species with an oxidation state of (n+x) and the
anionic species. That is, the composition comprising the metal
ionic species with an oxidation state of (n) and the anionic
species may have a K.sub.sp value substantially greater than the
K.sub.sp for the composition comprising the metal ionic species
with an oxidation state of (n+x) and the anionic species. For
example, the metal ionic species and anionic species may be
selected such that the K.sub.sp value of a composition comprising
the anionic species and the metal ionic species with an oxidation
state of (n) (e.g., M.sup.n) is greater than the K.sub.sp value of
the composition comprising the anionic species and the metal ionic
species with an oxidation state of (n+x) (e.g., M.sup.(n+x)) by a
factor of at least about 10, at least about 10.sup.2, at least
about 10.sup.3, at least about 10.sup.4, at least about 10.sup.5,
at least about 10.sup.6, at least about 10.sup.8, at least about
10.sup.10, at least about 10.sup.15, at least about 10.sup.20, at
least about 10.sup.30, at least about 10.sup.40, at least about
10.sup.50, and the like. Where these K.sub.sp values are realized,
a catalytic material may be more likely to serve as a photoanode or
photoactive electrode-associated material.
[0100] In some instances, a catalytic material, such as a
composition comprising a metal ionic species with an oxidation
state of (n+x) and an anionic species may have a K.sub.sp between
about 10.sup.-3 and about 10.sup.-50. In some cases, the solubility
constant of this composition may be between about 10.sup.-4 and
about 10.sup.-50, between about 10.sup.-5 and about 10.sup.-40,
between about 10.sup.-6 and about 10.sup.-30, between about
10.sup.-3 and about 10.sup.-30, between about 10.sup.-3 and about
10.sup.-20, and the like. In some cases, the solubility constant
may be less than about 10.sup.-3, less than about 10.sup.-4, less
than about 10.sup.-6, less than about 10.sup.-8, less than about
10.sup.-10, less than about 10.sup.-15, less than about 10.sup.-20,
less than about 10.sup.-25, less than about 10.sup.-30, less than
about 10.sup.-40, less than about 10.sup.-50, and the like. In some
cases, the composition comprising metal ionic species with an
oxidation state of (n) and the anionic species may have a
solubility product constant greater than about 10.sup.-3, greater
than about 10.sup.-4, greater than about 10.sup.-5, greater than
about 10.sup.-6, greater than about 10.sup.-8, greater than about
10.sup.-12, greater than about 10.sup.-15, greater than about
10.sup.-18, greater than about 10.sup.-20, and the like. In a
particular embodiment, the composition comprising metal ionic
species and the anionic species may be selected such that the
composition comprising the metal ionic species with an oxidation
state of (n) and the anionic species have a K.sub.sp value between
about 10.sup.-3 and about 10.sup.-10 and the composition comprising
the metal ionic species with an oxidation state of (n+x) and the
anionic species have a K.sub.sp value less than 10.sup.-10.
Non-limiting examples of metal ionic species and anionic species
that can be soluble in an aqueous solution and have a K.sub.sp
value in a suitable range includes Co(II)/HPO.sub.4.sup.-2,
Co(II)/H.sub.2BO.sub.3.sup.-, Co(II)/HAsO.sub.4.sup.-2,
Fe(II)/CO.sub.3.sup.-2, Mn(II)/CO.sub.3.sup.-2, and
Ni(II)/H.sub.2BO.sub.3.sup.-. In some cases, these combinations may
additionally comprise at least a second type of anionic species,
for example, oxide and/or hydroxide ions. The composition that
forms on the photoactive electrode may comprise the metal ionic
species and anionic species selected, as well as additional
components (e.g., oxygen, water, hydroxide, counter cations,
counter anions, etc.).
[0101] As noted, a photoanode can be formed by deposition of a
catalytic material from solution. Whether the photoanode has been
properly formed, with proper association of the catalytic material
with the photoactive electrode, may be important to monitor, both
for selecting proper metal ionic species and/or anionic species
and, of course, determining whether an appropriate photoanode has
been formed. The photoanode may be determined to have been formed
using various procedures. In some instances, the formation of a
catalytic material on the photoactive electrode may be observed.
The formation of the material may be observed by a human eye, or
with use of magnifying devices such as a microscope or via other
instrumentation. In one case, application of a voltage to the
photoanode, in conjunction with an appropriate counter electrode
(or photocathode) and other components (e.g., circuitry, power
source, electrolyte) may be carried out to determine whether the
system produces oxygen gas at the photoanode when the photoanode is
exposed to water.
[0102] In some cases, the onset potential (and/or minimum
overpotential) that is required by the photoanode to produce oxygen
gas may be different than the onset potential (and/or
overpotential) required by the photoactive electrode alone. The
term, "onset potential," as used herein, refers to the potential at
which the photocurrent of the photoanode becomes positive as the
potential applied to the photoanode is swept from negative to
positive values. In some cases, the onset potential required for
the photoanode is less positive than the onset potential required
for the photoactive electrode alone (i.e., the onset potential is
less positive for the photoanode that includes both the photoactive
electrode and catalytic material, than for the photoactive
electrode alone). In some embodiments, the onset potential of a
photoanode comprising a photoactive electrode and a catalytic
material is at least about 100 mV, at least about 200 mV, at least
about 250 mV, at least about 300 mV, at least about 350 mV, at
least about 400 mV, at least about 450 mV, at least about 500 mV,
or more, less positive than the onset potential of the photoactive
electrode alone. Or, in some cases, the onset potential is about
100 mV, about 200 mV, about 250 mV, about 300 mV, about 350 mV,
about 400 mV, about 450 mV, about 500 mV less positive.
[0103] In some cases, the incident photon-to-current conversion
efficiency (or IPCE, also known as energy quantum efficiency) that
is required by the photoanode to produce oxygen gas may be
different than the IPCE required by the photoactive electrode
alone. The term "incident photon-to-current conversion efficiency,"
as used herein, refers to a measure of the photon to electron
conversion efficiency at a specific wavelength. As will be known to
those of ordinary skill in the art, IPCE may be determined from
measuring the monochromatic light power density, and may be
calculated as a function of short circuit current density, incident
light power density, and wavelength. In some cases, the IPCE for
the photoanode is greater than the IPCE for the photoactive
electrode alone. In some embodiments, the IPCE of a photoanode
comprising a photoactive electrode and a catalytic material is
about 1%, about 2%, about 5%, about 10%, about 20%, about 25%,
about 30%, about 40%, about 50%, about 75%, about 100%, or more,
greater than the IPCE of the photoactive electrode alone. In some
cases, the IPCE is measured with solar simulated light (e.g.,
AM-1.5 illumination).
[0104] In some cases, a device (e.g., photoelectrochemical cell)
comprising the photoanode may be characterized by its overall
efficiency for conversion of solar energy to chemical energy. In
such embodiments, a photoelectrochemical cell may be illuminated
with light (e.g. solar simulated AM 1.5 radiation) to generate a
photocurrent. The overall energy conversion efficiency of the
device may be determined by Equation 17:
.eta.(%)=100(E-V.sub.bias)(i.sub.t)/(P.sub.hvA) (17)
wherein .eta. is the overall energy conversion efficiency of the
device, E is the Nernstian value for electrolysis of the solution
redox species (e.g., conversion of water to hydrogen and oxygen
gas), V.sub.bias is the voltage across the cell, i.sub.t is the
total current flowing in the device, P.sub.hv is the power of the
incident light radiation, and A is irradiated surface area.
V.sub.bias is generally defined to be negative if the cell can
simultaneously produce electrical power and stored chemical energy,
and is generally defined to be positive if an additional power
input is needed for the cell to perform the desired electrolysis
reaction. In some embodiments, the overall energy conversion
efficiency may be less than about 0.1%, less than about 1%, less
than about 2%, less than about 5%, less than about 10%, less than
about 15%, less than about 18%, less than about 20%, less than
about 25%, less than about 30%, less than about 50%, or the like.
In some cases, the overall energy conversion efficiency is about
0.1%, about 0.5%, about 1%, about 5%, about 10%, about 15%, about
18%, about 20%, about 25%, about 30%, about 35%, about 40%, about
50%, or the like, or between about 0.1% and about 30%, between
about 1% and about 30%, between about 10% and about 50%, between
about 10% and about 30%, or any range therein. Those of ordinary
skill in the art will be aware of techniques for determining the
overall energy conversion efficiency, for example, see Parkinson et
al., Acc. Chem. Res. 1984, 17, 431-437.
[0105] The catalytic material (and/or the photoanode comprising the
catalytic material) may also be characterized in terms of
performance. One way of doing this, among many, is to compare the
current density of the photoanode versus the photoactive electrode
alone. Typical photoactive electrodes are described more fully
below and can include titanium dioxide (e.g., TiO.sub.2), and the
like. The photoactive electrode may be able to function, itself, as
a photoanode in water electrolysis, and may have been used in the
past to do so. So, the current density during catalytic water
electrolysis (where the photoanode catalytically produces oxygen
gas from water), using the photoactive electrode, as compared to
essentially identical conditions (with the same counter electrode
or photocathode, same electrolyte, same external circuit, same
water source, etc.), using the photoanode including both
photoactive electrode and catalytic material, can be compared. In
most cases, the current density of the photoanode is greater than
the current density of the photoactive electrode alone, where each
is tested independently under essentially identical conditions. For
example, the current density of the photoanode may exceed the
current density of the photoactive electrode by a factor of at
least about 10, about 100, about 1000, about 10.sup.4, about
10.sup.5, about 10.sup.6, about 10.sup.8, about 10.sup.10, and the
like. In a particular case, the difference in the current density
is at least about 10.sup.5. In some embodiments, the current
density of the photoanode may exceed the current density of the
photoactive electrode by a factor between about 10.sup.4 and about
10.sup.10, between about 10.sup.5 and about 10.sup.9, or between
about 10.sup.4 and about 10.sup.8. The current density may either
be the geometric current density or the total current density, as
described herein.
[0106] This characteristic, namely, significantly increased
catalytic activity of the photoanode (comprising a photoactive
electrode and catalytic material associated with the photoactive
electrode) as compared to the photoactive electrode alone, may be
used to monitor formation of a catalytic photoanode. That is, the
formation of the catalytic material on the photoactive electrode
may be observed by monitoring the current density over a period of
time. The current density, in most cases, increases during
application of a voltage to the photoactive electrode. In some
instances, the current density may reach a plateau after a period
of time (e.g., about 2 hours, about 4 hours, about 6 hours, about 8
hours, about 10 hours, about 12 hours, about 24 hours, and the
like).
[0107] Metal ionic species useful as one portion of a catalytic
material of the invention may be any metal ion selected according
to the guidelines described herein. In most embodiments, the metal
ionic species have access to oxidation states of at least (n) and
(n+x). In some cases, the metal ionic species have access to
oxidation states of (n), (n+1) and (n+2). (n) may be any whole
number, and includes, but is not limited to, 0, 1, 2, 3, 4, 5, 6,
7, 8, and the like. In some cases, (n) is not zero. In particular
embodiments, (n) is 1, 2, 3 or 4. (x) may be any whole number and
includes, but is not limited to 0, 1, 2, 3, 4, and the like. In
particular embodiments, (x) is 1, 2, or 3. Non-limiting examples of
metal ionic species include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y,
Zr, Nb, Mo, Tc, Rh, Ru, Ag, Cd, Pt, Pd, Ir, Hf, Ta, W, Re, Os, Hg,
and the like. In some cases, the metal ionic species may be a
lanthanide or actinide (e.g., Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,
Er, Tm, Yb, Lu, Th, Pa, U, etc.). In a particular embodiment, the
metal ionic species comprises cobalt ions, which may be provided as
a catalytic material in the form of Co(II), Co(III) or the like. In
some embodiments, the metal ionic species is not Mn. The metal
ionic species may be provided (e.g., to the solution) as a metal
compound, wherein the metal compound comprises metal ionic species
and counter anions. For example, the metal compound may be an
oxide, a nitrate, a hydroxide, a carbonate, a phosphite, a
phosphate, a sulphite, a sulphate, a triflate, and the like.
[0108] An anionic species selected for use as a catalytic material
of the invention may be any anionic species that is able to
interact with the metal ionic species as described herein and to
meet threshold catalytic requirements as described. In some cases,
the anionic compound may be able to accept and/or donate hydrogen
ions, for example, H.sub.2PO.sub.4.sup.- or HPO.sub.4.sup.-2.
Non-limiting examples of anionic species include forms of phosphate
(H.sub.3PO.sub.4 or HPO.sub.4.sup.-2, H.sub.2PO.sub.4.sup.-2 or
PO.sub.4.sup.-3), forms of sulphate (H.sub.2SO.sub.4 or
HSO.sub.4.sup.-, SO.sub.4.sup.-2), forms of carbonate
(H.sub.2CO.sub.3 or HCO.sub.3.sup.-, CO.sub.3.sup.-2), forms of
arsenate (H.sub.3AsO.sub.4 or HAsO.sub.4.sup.-2,
H.sub.2AsO.sub.4.sup.-2 or AsO.sub.4.sup.-3), forms of phosphite
(H.sub.3PO.sub.3 or HPO.sub.3.sup.-2, H.sub.2PO.sub.3.sup.-2 or
PO.sub.3.sup.-3), forms of sulphite (H.sub.2SO.sub.3 or
HSO.sub.3.sup.-, SO.sub.3.sup.-2), forms of silicate, forms of
borate (e.g., H.sub.3BO.sub.3, H.sub.2BO.sub.3.sup.-,
HBO.sub.3.sup.-2, etc.), forms of nitrates, forms of nitrites, and
the like.
[0109] In some cases, the anionic species may be a form of
phosphonate. A phosphonate is a compound comprising the structure
PO(OR.sup.1)(OR.sup.2)(R.sup.3) wherein R.sup.1, R.sup.2, and
R.sup.3 can be the same or different and are H, an alkyl, an
alkenyl, an alkynyl, a heteroalkyl, a heteroalkenyl, a
heteroalkynyl, an aryl, or a heteroaryl, all optionally
substituted, or are optionally absent (e.g., such that the compound
is an anion, dianion, etc.). In a particular embodiment, R.sup.1,
R.sup.2, and R.sup.3 can be the same or different and are H, alkyl,
or aryl, all optionally substituted. A non-limiting example of a
phosphonate is a form of PO(OH).sub.2R.sup.1 (e.g.,
PO.sub.2(OH)(R.sup.1).sup.-, PO.sub.3(R.sup.1).sup.-2), wherein
R.sup.1 is as defined above (e.g., alkyl such as methyl, ethyl,
propyl, etc.; aryl such as phenol, etc.). In a particular
embodiment, the phosphonate may be a form of methyl phosphonate
(PO(OH).sub.2Me), or phenyl phosphonate (PO(OH).sub.2Ph). Other
non-limiting examples of phosphorus-containing anionic species
include forms of phosphinites (e.g., P(OR.sup.1)R.sup.2R.sup.3) and
phosphonites (e.g., P(OR.sup.1)(OR.sup.2)R.sup.3) wherein R.sup.1,
R.sup.2, and R.sup.3 are as described above. In other cases, the
anionic species may comprise one any form of the following
compounds: R.sup.1SO.sub.2(OR.sup.2)), SO(OR.sup.1)(OR.sup.2),
CO(OR.sup.1)(OR.sup.2), PO(OR.sup.1)(OR.sup.2),
AsO(OR.sup.1)(OR.sup.2)(R.sup.3), wherein R.sup.1, R.sup.2, and
R.sup.3 are as described above. With respect to the anionic species
discussed above, those of ordinary skill in the art will be able to
determine appropriate substituents for the anionic species. The
substituents may be chosen to tune the properties of the catalytic
material and reactions associated with the catalytic material. For
example, the substituent may be selected to alter the solubility
constant of a composition comprising the anionic species and the
metal ionic species.
[0110] In some embodiments, the anionic species may be good
proton-accepting species. As used herein, a "good proton-accepting
species" is a species which acts as a good base at a specified pH
level. For example, a species may be a good proton-accepting
species at a first pH and a poor proton-accepting species at a
second pH. Those of ordinary skill in the art can identify a good
base in this context. In some cases, a good base may be a compound
in which the pK.sub.a of the conjugate acid is greater than the
pK.sub.a of the proton donor in solution. As a specific example,
SO.sub.4.sup.-2 may be a good proton-accepting species at about pH
2.0 and a poor proton-accepting species at about pH 7.0. A species
may act as a good base around the pK.sub.a value of the conjugate
acid. For example, the conjugate acid of HPO.sub.4.sup.-2 is
H.sub.2PO.sub.4.sup.-, which has a pK.sub.a value of about 7.2.
Therefore, HPO.sub.4.sup.-2 may act as a good base around pH 7.2.
In some cases, a species may act as a good base in solutions with a
pH level at least about 4 pH units, about 3 pH units, about 2 pH
units, or about 1 pH unit, above and/or below the pK.sub.a value of
the conjugate acid. Those of ordinary skill in the art will be able
to determine at which pH levels an anionic species is a good
proton-accepting species.
[0111] The anionic species may be provided as a compound comprising
the anionic species and a counter cation. The counter cation may be
any cationic species, for example, a metal ion (e.g., K.sup.+,
Na.sup.+, Li.sup.+, Mg.sup.+2, Ca.sup.+2, Sr.sup.+2),
NR.sub.4.sup.+ (e.g., NH.sub.4.sup.+), H.sup.+, and the like. In a
specific embodiment, the compound employed may be
K.sub.2HPO.sub.4.
[0112] The catalytic material may comprise the metal ionic species
and anionic species in a variety of ratios (amounts relative to
each other). In some cases, the catalytic material comprises the
metal ionic species and the anionic species in a ratio of less than
about 20:1, less than about 15:1, less than about 10:1, less than
about 7:1, less than about 6:1, less than about 5:1, less than
about 4:1, less than about 3:1, less than about 2:1, greater than
about 1:1, greater than about 1:2, greater than about 1:3, greater
than about 1:4, greater than about 1:5, greater than about 1:10,
and the like. In some cases, the catalytic material may comprise
additional components, such as counter cations and/or counter
anions from the metallic compound and/or anionic compound provided
to the solution. For example, in some instances, the catalytic
material may comprise the metal ionic species, the anionic species,
and a counter cation and/or anion in a ratio of about 2:1:1, about
3:1:1, about 3:2:1, about 2:2:1, about 2:1:2, about 1:1:1, and the
like. The ratio of the species in the catalytic material will
depend on the species selected. In some instances, a counter cation
may be present in a very small amount and serve as a dopant to, for
example, to improve the conductivity or other properties of the
material. In these instances, the ratio may be about X:1:0.1, about
X:1:0.005, about X:1:0.001, about X:1:0.0005, etc., where X is 1,
1.5, 2, 2.5, 3, and the like. In some instances, the catalytic
material may additionally comprise at least one of water, oxygen
gas, hydrogen gas, oxygen ions (e.g., O.sup.-2), peroxide, hydrogen
ion (e.g., H.sup.+), and/or the like.
[0113] In some embodiments, a catalytic material of the invention
may comprise more than one type of metal ionic species and/or
anionic species (e.g., at least about 2 types, at least about 3
types, at least about 4 types, at least about 5 types, or more, of
metal ionic species and/or anionic species). For example, more than
one type of metal ionic species and/or anionic species may be
provided to the solution in which the photoactive electrode is
immersed. In such instances, the catalytic material may comprise
more than one type of metal ionic species and/or anionic species.
Without wishing to be bound by theory, the presence of more than
one type of metal ionic species and/or anionic species may allow
for the properties of the photoanode to be tuned, such that the
performance of the photoanode may be altered by using combinations
of species in different ratios. In a particular embodiment, a first
type of metal ionic species (e.g., Co(II)) and second type of metal
ionic species (e.g., Ni(II)) may be provided in the solution in
which the photoactive electrode is immersed, such that the
catalytic material comprises the first type of metal ionic species
and the second type of metal ionic species (e.g., Co(II) and
Ni(II)). Where a first and second type of metal ionic species are
used together, each can be selected from among metal ionic species
described as suitable for use herein.
[0114] Where both first type and a second type of metal ionic
and/or anionic species are used, both the first and second species
need not both be catalytically active, or if both are catalytically
active they need not be active to the same level or degree. The
ratio of the first type of metal ionic and/or anionic species to
the second type of metal ionic and/or anionic species may be varied
and may be about 1:1, about 1:2, about 1:3, about 1:4, about 1:5,
about 1:6, about 1:7, about 1:8, about 1:9, about 1:10, about 1:20,
or greater. In some instances, the second type of species may be
present in a very small amount and serve as a dopant to, for
example, to improve the conductivity or other properties of the
material. In these instances, the ratio of the first type of
species to the second type of metal ionic species may be about
1:0.1, about 1:0.005, about 1:0.001, about 1:0.0005, etc. In some
embodiments, a catalytic material comprising more than one metal
ionic species and/or anionic species may be formed by first forming
a catalytic material comprising a first type of metal ionic species
and a first type of anionic species, followed by exposing the
photoanode comprising the catalytic material to a solution
comprising a second type of metal ionic species and/or second type
of anionic species and applying a voltage to the photoanode (e.g.,
via an external power source or by exposing the photoanode to
electromagnetic radiation). This may cause the second type of metal
ionic species and/or second type of anionic species to be comprised
in the catalytic material. In other embodiments, the catalytic
material may be formed by exposing a photoactive electrode to a
solution comprising the components (e.g., first and second type of
metal ionic species, and anionic species) and applying a voltage to
the photoactive electrode, thereby forming a catalytic material
comprising the components.
[0115] In some cases, a first type of anionic species and a second
type of anionic species (e.g., a form of borate and a form of
phosphate) may be provided to the solution and/or otherwise used in
combination in a catalytic material of the invention. Where both
first and second catalytically active anionic species are used,
they can be selected from among anionic species described as
suitable for use herein.
[0116] In some instances, the first type of anionic species is
hydroxide and/or oxide ions, and the second type of anionic species
is not hydroxide and/or oxide ions. It should be understood,
however, that when at least type of anionic species is an oxide or
hydroxide, the species might not be provided to the solution but
instead, may be present in the water or solution the species is
provided in and/or may be formed during a reaction (e.g., between
the first type of anionic species and the metal ionic species).
[0117] In some embodiments, the catalytic metal ionic
species/anionic species do not consist essentially of metal ionic
species/O.sup.-2 and/or metal ionic species/OH.sup.-. A material
"consists essentially of" a species if it is made of that species
and no other species that significantly alters the characteristics
of the material, for purposes of the invention, as compared to the
original species in pure form. Accordingly, where a catalytic
material does not consist essentially of metal ionic
species/O.sup.-2 and/or metal ionic species/OH.sup.-, the catalytic
materials has characteristics significantly different than a pure
metal ionic species/O.sup.-2 and/or metal ionic species/OH.sup.-,
or a mixture. In some cases, a composition that does not consist
essentially of metal ionic species/O.sup.-2 and/or metal ionic
species/OH.sup.- comprises less than about 90%, less than about
80%, less than about 70%, less than about 60%, less than about 50%,
less than about 40%, less than about 30%, less than about 20%, less
than about 10%, less than about 5%, less than about 1%, and the
like, weight percent of O.sup.-2 and/or OH.sup.- ions/molecules. In
some instances, the composition that does not consist essentially
of metal ionic species/O.sup.-2 and/or metal ionic species/OH.sup.-
comprises between about 1% and about 99%, between about 1% and
about 90%, between about 1% and about 80%, between about 1% and
about 70%, between about 1% and about 60%, between about 1% and
about 50%, between about 1% and about 25%, etc., weight percent
O.sup.-2 and/or OH.sup.- ions/molecules. The weight percent of
O.sup.-2 and/or OH.sup.- ions/molecules may be determined using
methods known to those of ordinary skill in the art. For example,
the weight percent may be determined by determining the approximate
structure of the material comprise in the composition. The weight
percentage of the O.sup.-2 and/or OH.sup.- ions/molecules may be
determined by dividing the weight of O.sup.-2 and/or OH.sup.-
ions/molecules over the total weight of the composition multiplied
by 100%. As another example, in some cases, the weight percentage
may be approximately determined based upon the ratio of metal ionic
species to anionic species in a composition and knowledge regarding
the general coordination chemistry of the metal ionic species.
[0118] In a specific embodiment, the composition (e.g., catalytic
material) associated with the photoactive electrode may comprise
cobalt ions and anionic species comprising phosphorus (e.g.,
HPO.sub.4.sup.-2). In some cases, the composition may additionally
comprise cationic species (e.g., K.sup.+). In some cases, the
photoactive electrode with which the composition is associated does
not consist essentially of platinum. An anionic species comprising
phosphorus may be any molecule that comprises phosphorus and is
associated with a negative charge. Non-limiting examples of anionic
species comprising phosphorus include H.sub.3PO.sub.4,
H.sub.2PO.sub.4.sup.-, HPO.sub.4.sup.-2, PO.sub.4.sup.-3,
H.sub.3PO.sub.3, H.sub.2PO.sub.3.sup.-, HPO.sub.3.sup.-2,
PO.sub.3.sup.-3, R.sup.1PO(OH).sub.2, R.sup.1PO.sub.2(OH).sup.-,
R.sup.1PO.sub.3.sup.-2, or the like, wherein R.sup.1 is H, an
alkyl, an alkenyl, an alkynyl, a heteroalkyl, a heteroalkenyl, a
heteroalkynyl, an aryl, or a heteroaryl, all optionally
substituted.
[0119] In some embodiments, a catalytic material of the invention,
especially when associated with the photoactive electrode, may be
substantially non-crystalline. Without wishing to be bound by
theory, a substantially non-crystalline material may aid in the
transport of protons and/or electrons, which may improve the
function of the photoanode in certain electrochemical devices. For
example, improved transport of protons (e.g., increase proton flux)
during electrolysis may improve the overall efficiency of an
electrochemical device comprising a photoanode as described herein.
A photoanode comprising a substantially non-crystalline catalytic
material may allow for a conductivity of protons of at least about
10.sup.-1 S cm.sup.-1, at least about 20.sup.-1 S cm.sup.-1, at
least about 30.sup.-1 S cm.sup.-1, at least about 40.sup.-1 S
cm.sup.-1, at least about 50.sup.-1 S cm.sup.-1, at least about
60.sup.-1 S cm.sup.-1, at least about 80.sup.-1 S cm.sup.-1, at
least about 100.sup.-1 S cm.sup.-1, and the like. In other
embodiments, the catalytic material may be amorphous, substantially
crystalline, or crystalline. Where substantially non-crystalline
material is used, this would be readily understood by those of
ordinary skill in the art and easily determined using various
spectroscopic techniques.
[0120] The above and other characteristics of the metal ionic
species and anionic species can serve as selective screening tests
for identification of particular metal ionic and anionic species
useful for particular applications. Those of ordinary skill in the
art can, through simple bench-top testing, reference to scientific
literature, simple diffractive instrumentation, simple
electrochemical testing, and the like, select metal ionic and
anionic species based upon the present disclosure, without undue
experimentation.
[0121] In some cases, the catalytic material associated with the
photoactive electrode may be porous, substantially porous,
non-porous, and/or substantially non-porous. The pores may comprise
a range of sizes and/or be substantially uniform in size. In some
cases, the pores may or may not be visible using imaging techniques
(e.g., scanning electron microscope). The pores may be open and/or
closed pores. In some cases, the pores may provide pathways between
the bulk electrolyte surface and the surface of the photoactive
electrode.
[0122] In some instances, the catalytic material may be hydrated.
That is, the catalytic material may comprise water and/or other
liquid and/or gas components. Upon removal of the photoactive
electrode comprising the catalytic material from solution, the
catalytic material may be dehydrated (e.g., the water and/or other
liquid and/or gas components may be removed from the catalytic
material). In some cases, the catalytic material may be dehydrated
by removing the material from solution and leaving the material to
sit under ambient conditions (e.g., room temperature, air, etc.)
for at least about 1 hour, at least about 2 hours, at least about 4
hours, at least about 8 hours, at least about 12 hours, at least
about 24 hours, at least about 2 days, at least about 1 week, or
more. In some cases, the catalytic material may be dehydrated under
non-ambient conditions. For example, the catalytic material be
dehydrated at elevated temperature and/or under vacuum. In some
instances, the catalytic material may change composition and/or
morphology upon dehydration. For example, in instances where the
catalytic material forms a film, the film may comprise cracks upon
dehydration.
[0123] Without wishing to be bound by theory, in some cases, the
catalytic material may reach a maximum performance (e.g., rate of
O.sub.2 production, overpotential at a specific current density,
onset potential, Faradaic efficiency, etc.) based upon the
thickness of the catalytic material. Where a porous photoactive
electrode is used, the thickness of the deposited catalytic
material and the pore size of photoactive electrode may
advantageously be selected in combination so that pores are not
substantially filled with the catalytic material. For example, the
surface of the pores may comprise a layer of the catalytic material
that is thinner than the average radius of the pores, thereby
allowing for sufficient porosity to remain, even after catalytic
material is deposited, so that the high surface area provided by
the porous photoactive electrode is substantially maintained. In
some cases, the average thickness of the catalytic material may be
less than about 90%, less than about 80%, less than about 70%, less
than about 60%, less than about 50%, less than about 40%, less than
about 30%, less than about 20%, less than about 10%, or less, the
average radius of the pores of the photoactive electrode. In some
cases, the average thickness of the catalytic material may be
between about 40% and about 60%, between about 30% and about 70%,
between about 20% and about 80%, etc., the average radius of the
pores of the photoactive electrode. In other embodiments, the
performance of the catalytic material might not reach a maximum
performance based upon the thickness of the catalytic material. In
other embodiments, the performance of the catalytic material might
not reach a maximum performance based upon the thickness of the
catalytic material. In some cases, the performance (e.g.,
overpotential at a certain current density may decrease) of the
catalytic material may increase with increasing thickness of the
catalytic material. Without wishing to be bound by theory, this may
indicate greater than just the outside layer of the catalytic
material is catalytically active.
[0124] The physical structure of the catalytic material may vary.
For example, the catalytic material may be a film and/or particles
associated with at least a portion of the photoactive electrode
(e.g., surface and/or pores) that is immersed in the solution. In
some embodiments, the catalytic material might not form a film
associated with the photoactive electrode. Alternatively or in
addition, the catalytic material may be deposited on a photoactive
electrode as patches, islands, or some other pattern (e.g., lines,
spots, rectangles), or may take the form of dendrimers,
nanospheres, nanorods, or the like. A pattern in some cases can
form spontaneously upon deposition of catalytic material onto the
photoactive electrode and/or can be patterned onto a photoactive
electrode by a variety of techniques known to those of ordinary
skill in the art (lithographically, via microcontact printing,
etc.) and as discussed herein. Further, a photoactive electrode may
be patterned itself such that certain areas facilitate association
of the catalytic material while other areas do not, or do so to a
lesser degree, thereby creating a patterned arrangement of
catalytic material on the photoactive electrode as the photoanode
is formed. Where a catalytic material is patterned onto a
photoanode, the pattern might define areas of catalytic material
and areas completely free of catalytic material, or areas with a
particular amount of catalytic material and other areas with a
different amount of catalytic material deposition. The catalytic
material may have an appearance of being smooth and/or bumpy. In
some cases, the catalytic material may comprise cracks, as can be
the case when the material dehydrated.
[0125] In some cases, the thickness of catalytic material may be of
substantially the same throughout the material. In other cases, the
thickness of the catalytic material may vary throughout the
material (e.g., a film does not necessarily have uniform
thickness). The thickness of the catalytic material may be
determined by determining the thickness of the material at a
plurality of areas (e.g., at least 2, at least 4, at least 6, at
least 10, at least 20, at least 40, at least 50, at least 100, or
more areas) and calculating the average thickness. Where thickness
of a catalytic material is determined via probing at a plurality of
areas, the areas should be selected so as not to specifically
represent areas of more or less catalytic material presence based
upon a pattern. Those of ordinary skill in the art will easily be
able to establish a thickness-determining protocol that accounts
for any non-uniformity or patterning of catalytic material on the
surface. For example, the technique might include a sufficiently
large number of area determinations, randomly selected, to provide
overall average thickness. The average thickness of the catalytic
material may be at least about 10 nm, at least about 100 nm, at
least about 300 nm, at least about 500 nm, at least about 700 nm,
at least about 1 um (micrometer), at least about 2 um, at least
about 5 um, at least about 1 mm, at least about 1 cm, and the like.
In some cases, the average thickness of the catalytic material may
be less than about 1 mm, less than about 500 um, less than about
100 um, less than about 10 um, less than about 1 um, less than
about 100 nm, less than about 10 nm, less than about 1 nm, less
than about 0.1 nm, or the like. In some instances, the average
thickness of the catalytic material may be between about 1 mm and
about 0.1 nm, between about 500 um and about 1 nm, between about
100 um and about 1 nm, between about 100 um and about 0.1 nm,
between about 0.2 um and about 2 um, between about 200 um and about
0.1 um, or the like. In particular embodiments, the catalytic
material may have an average thickness of less than about 0.2 um.
In another embodiment, the catalytic material may have an average
thickness between about 0.2 um and about 2 um. The average
thickness of the catalytic material may be varied by altering the
amount and length of time a voltage is applied to the photoactive
electrode, the concentration of the metal ionic species and the
anionic species in solution, the surface area of the photoactive
electrode, the surface area density of the photoactive electrode,
and the like.
[0126] In some cases, the average thickness of the catalytic
material may be determined according to the following method. A
photoanode comprising a photoactive electrode and a catalytic
material may be removed from solution (e.g., the solution the
photoanode was formed in and/or the electrolyte). The photoanode
may be left to dry for about 1 hour, about 2 hours, about 4 hours,
about 6 hours, about 8 hours, about 12 hours, about 24 hours, or
more. In some cases, the photoanode may be dried under ambient
conditions (e.g., in air at room temperature). In some embodiments,
during drying, the catalytic material may crack. The thickness of
the catalytic material may be determined using techniques known to
those of ordinary skill in the art (e.g., scanning electron
microscope (SEM)) to determine the depth of the cracks (e.g., the
thickness of the dehydrated catalytic material).
[0127] In other embodiments, the thickness of the catalytic
material may be determined without dehydration (e.g., in situ)
using techniques known to those of ordinary skill in the art, for
example, SEM. In such embodiments, a mark (e.g., scratch, hole) may
be made in the catalytic material to expose at least a portion of
the underlying substrate (e.g., the photoactive electrode). The
thickness of the catalytic material may be determined by measuring
the depth of the mark.
[0128] In some embodiments, a film of the catalytic material may be
formed by the coalescing of a plurality of particles formed on the
photoactive electrode. In some cases, the material may be observed
to have the physical appearance of a base layer of material
comprising a plurality of groups of protruding particles. The
thickness of the film may be determined by the thickness of the
base layer, although it should be understood that the thickness
would be substantially greater if measured by determining the
thickness of the areas comprising protruding particles.
[0129] Without wishing to be bound by theory, the formation of
groups of protruding particles on the surface of the film may aid
in increasing the surface area and thus increase the production of
oxygen gas. That is, the surface area of the catalytic material
comprising a plurality of groups of protruding particles may be
substantially greater than the surface area of a catalytic material
which does not comprise a plurality of groups of protruding
particles.
[0130] In some embodiments, the catalytic material may be described
as a function of mass of catalytic material per unit area of the
photoactive electrode. In some cases, the mass of catalytic
material per area of the photoactive electrode may be about 0.01
mg/cm.sup.2, about 0.05 mg/cm.sup.2, about 0.1 mg/cm.sup.2, about
0.5 mg/cm.sup.2, about 1.0 mg/cm.sup.2, about 1.5 mg/cm.sup.2,
about 2.5 mg/cm.sup.2, about 3.0 mg/cm.sup.2, about 4.0
mg/cm.sup.2, about 5.0 mg/cm.sup.2, or the like. In some cases, the
mass of catalytic material per unit area of the photoactive
electrode may be between about 0.1 mg/cm.sup.2 and about 5.0
mg/cm.sup.2, between about 0.5 mg/cm.sup.2 and about 3.0
mg/cm.sup.2, between about 1.0 mg/cm.sup.2 and about 2.0
mg/cm.sup.2, and the like. Where the amount of catalytic material
associated with a photoactive electrode is defined or investigated
in terms of mass per unit area, and the material is present
non-uniformly relative to the photoactive electrode surface
(whether through patterning or natural variations in amount over
the surface), the mass per unit area may be averaged across the
entire surface area within which catalytic material is found (e.g.,
the geometric surface area). In some cases, the mass of the
catalytic material per unit area may be a function of the thickness
of the catalytic material.
[0131] The formation of the catalytic material may proceed until
the voltage applied to the photoactive electrode is turned off
(e.g., the power source or the light source is turn off/removed),
until there is a limiting quantity of materials (e.g., metal ionic
species and/or anionic species) and/or the catalytic material has
reached a critical thickness beyond which additional film formation
does not occur or is very slow. Voltage may be applied to the
photoactive electrode for minimums of about 1 minute, about 5
minutes, about 10 minutes, about 20 minutes, about 30 minutes,
about 60 minutes, about 2 hours, about 4 hours, about 8 hours,
about 12 hours, about 24 hours, and the like. In some cases, a
potential may be applied to the photoactive electrode between 24
hours and about 30 seconds, between about 12 hours and about 1
minute, between about 8 hours and about 5 minutes, between about 4
hours and about 10 minutes, and the like. The voltages provided
herein, in some cases, are supplied with reference to a `normal
hydrogen electrode` (NHE). Those of ordinary skill in the art will
be able to determine the corresponding voltages with respect to an
alternative reference electrode by knowing the voltage difference
between the specified reference electrode and NHE or by referring
to an appropriate textbook or reference. The formation of the
catalytic material may proceed until about 0.1%, about 1%, about
5%, about 10%, about 20%, about 30%, about 40%, about 50%, about
60%, about 70%, about 80%, about 90%, about 99%, about 100% of the
metal ionic species and/or anionic species initially added to the
solution have associated with the photoactive electrode to form the
catalytic material.
[0132] The voltage applied to the photoactive electrode (e.g., via
an external power source or by exposing the photoactive electrode
to electromagnetic radiation) may be held steady, may be linearly
increased or decreased, and/or may be linearly increased and
decreased (e.g., cyclic). In some cases, the voltage applied to the
photoactive electrode may be substantially similar throughout the
application of the voltage. That is, the voltage applied to the
photoactive electrode might not be varied significantly during the
time that the voltage is applied to the photoactive electrode. In
some instances, the voltage applied to the current collect by an
external power source may be at least about 0.1 V, at least about
0.2 V, at least about 0.4 V, at least about 0.5 V, at least about
0.7 V, at least about 0.8 V, at least about 0.9 V, at least about
1.0 V, at least about 1.2 V, at least about 1.4 V, at least about
1.6 V, at least about 1.8 V, at least about 2.0 V, at least about 3
V, at least about 4 V, at least about 5 V, at least about 10 V, and
the like. In some cases, the voltage applied is between about 1.0 V
and about 1.5 V, about 1.1 V and about 1.4 V, or is about 1.1 V. In
some instances, the voltage applied to the photoactive electrode
may be a linear range of voltages, and/or cyclic range of voltages.
Application of a linear voltage refers to instances where the
voltage applied to the photoanode (and/or photoactive electrode) is
swept linearly in time between a first voltage and a second
voltage. Application of a cyclic voltage refers to application of
linear voltage, followed by a second application of linear voltage
wherein the sweep direction has been reversed. For example,
application of a cyclic voltage is commonly used in cyclic
voltammetry studies. In some cases, the first voltage and the
second voltage may differ by about 0.1 V, about 0.2 V, about 0.3 V,
about 0.5 V, about 0.8 V, about 1.0 V, about 1.5 V, about 2.0 V, or
the like. In some cases, the voltage may be swept between the first
voltage and the second voltage at a rate of about 0.1 mV/sec, about
0.2 mV/sec, about 0.3 mV/sec, about 0.4 mV/sec, about 0.5 mV/sec,
about 1.0 mV/sec, about 10 mV/sec, about 100 mV/sec, about 1 V/sec,
or the like. The potential applied may or might not be such that
oxygen gas is being formed during the formation of the photoanode.
In some cases, the morphology of the catalytic material may differ
depending on the potential applied to the photoactive electrode
during formation of the photoanode.
[0133] In some embodiments, wherein the catalytic material is a
regenerative material, between application of a voltage (e.g.,
during periods when the photoanode is not in use), at least about
1%, at least about 2%, at least about 5%, at least about 10%, at
least about 20%, or more, by weight of the catalytic material may
dissociate from the photoactive electrode over a period of about 10
minutes, about 30 minutes, about 1 hour, about 2 hours, about 6
hours, about 12 hours, about 24 hours, or more. Upon reapplication
of the voltage, at least about 50%, at least about 60%, at least
about 70%, at least about 80%, at least about 90%, at least about
95%, at least about 99%, or more, by weight of the dissociate
material may re-associate with the photoanode. In some cases,
substantially all of the metal ionic species may re-associate with
the photoanode and only a portion of the anionic species may
re-associate with the photoanode (e.g., in instances where the
electrolyte comprises anionic species and there may be an exchange
of the anionic species which dissociate and those which
re-associate).
[0134] In another embodiment, a photoanode of system comprising a
catalytic material may be prepared as follows. A catalytic material
may be associated with a photoactive electrode as described above
in any manner described herein. For example, at relatively low
potentials at which oxygen gas is not evolved, and/or at a higher
potentials at which oxygen gas is evolved and a higher rate of
deposition of material on the photoanode occurs, and/or at any
other rate or under any conditions suitable for production of a
catalytic material associated with the photoactive electrode. The
catalytic material can be removed from the photoactive electrode
(and, optionally, the process can be cyclically repeated with
additional catalytic material associated with the photoanode,
removed, etc.) and the catalytic material can be optionally dried,
stored, and/or mixed with an additive (e.g., a binder) or the like.
The catalytic material may be packaged for distribution and used as
a catalytic material. In some cases, the catalytic material can
later be applied to a photoactive electrode, can simply be added to
a solution of water and associated with a different photoactive
electrode as described above, e.g., in an end-use setting, or used
otherwise as would be recognized by those of ordinary skill in the
art. Those of ordinary skill in the art can readily select binders
that would be useful for addition to such catalytic material, for
example, poly tetrafluoroethylene (Teflon.TM.), Nafion.TM., or the
like. For eventual use in an electrolyzer, photoelectrochemical
cell, or other electrolysis system, non-conductive binders may be
most suitable. Conductive binders may be used where they are stable
to photoelectrochemical conditions.
[0135] In some embodiments, after application of the voltage and
formation of a photoanode comprising a photoactive electrode, metal
ionic species, and anionic species, the photoanode may be removed
from the solution and stored. The photoanode may be stored for any
period of time or used immediately in one of the applications
discussed herein. In some cases, the catalytic material associated
with the photoactive electrode may dehydrate during storage. The
photoanode may be stored for at least about 1 day, at least about 2
days, at least about 5 days, at least about 10 days, at least about
1 month, at least about 3 months, at least about 6 months or at
least about 1 year, with no more than 10% loss in photoanode
performance per month of storage, or no more than 5%, or even 2%,
loss in performance per month of storage. Photoanodes as described
herein may be stored under varying conditions. In some instances,
the photoanode may be stored in ambient conditions and/or under an
atmosphere of air. In other instances, the photoanode may be stored
under vacuum. In yet another instance, the photoanode may be stored
in solution. In this case, the catalytic material may disassociate
from the photoactive electrode over a period of time (e.g., 1 day,
1 week, 1 month, and the like) to form metal ionic species and
anionic species in solution. Application of a voltage and/or a
photovoltage to the photoactive electrode, in most cases, may cause
the metal ionic species and anionic species to re-associate with
the photoactive electrode to reform the catalytic material.
[0136] In some embodiments, a photoanode comprising a photoactive
electrode and a catalytic material may be used for an extended
period of time as compared to the photoactive electrode alone,
under essentially identical conditions. Without wishing to be bound
by theory, the dynamic equilibrium of the catalytic material may
cause the photoanode to be robust and provides a self-repair
mechanism. In some cases, a photoanode may be used to catalytically
produce oxygen gas from water for at least about 1 month, at least
about 2 months, at least about 3 months, at least about 6 months,
at least about 1 year, at least about 18 months, at least about 2
years, at least about 3 years, at least about 5 years, at least
about 10 years, or greater, with less than 50%, less than 40%, less
than 30%, less than 20%, less than 10%, less than 5%, less than 3%,
less than 2%, less than 1%, or less, change in a selected
performance measure (e.g., overpotential, rate of production of
oxygen, etc.).
[0137] In some cases, the composition of the catalytic material
associated with the photoactive electrode after storage may be
substantially similar to the catalytic material immediately after
formation. In other cases, the composition of the catalytic
material associated with the photoactive electrode after storage
may be substantially different than the catalytic material
immediately after formation. In some instances, the metal ionic
species in the catalytic material may be oxidized as compared to
the metal ionic species in solution. For example, the metal ionic
species immediately after deposition may have an oxidation state of
(n+x), and after storage, at least a portion of the metal ionic
species may have an oxidation state of (n). The ratio of metal
ionic species to anionic species in the catalytic material after
storage may or might not be substantially similar to the ratio
present immediately after formation.
[0138] The solution in which the photoactive electrode is immersed
may be formed from any suitable material. In most cases, the
solution may be a liquid and may comprise water. In some
embodiments the solution may consist of or consist essentially of
water, i.e. be essentially pure water or an aqueous solution that
behaves essentially identical to pure water, in each case, with the
minimum electrical conductivity necessary for an electrochemical
device to function. In some embodiments, the solution may be
selected such that the metal ionic species and the anionic species
are substantially soluble. In some cases, when the photoanode is to
be used in a device immediately after formation, the solution may
be selected such that it comprises water (or other fuel) to be
oxidized by a device and/or method as described herein. For
example, in instances where oxygen gas is to be catalytically
produced from water, the solution may comprise water (e.g.,
provided from a water source). In some cases, the solution may be
contained within a container which is substantially transparent to
visible light (e.g., such that the photoactive electrode may be
exposed to electromagnetic radiation through the container).
[0139] The metal ionic species and the anionic species may be
provided to the solution by substantially dissolving compounds
comprising the metal ionic species and the anionic species. In some
instances, this may comprise substantially dissolving a metal
compound comprising the metal ionic species and anionic compound
comprising the anionic species. In other instances, a single
compound may be dissolved that comprises both the metal ionic
species and the anionic species. The metal compound and/or the
anionic compound may be of any composition, such as a solid, a
liquid, a gas, a gel, a crystalline material, and the like. The
dissolution of the metal compound and anionic compound may be
facilitated by agitation of the solution (e.g., stirring) and/or
heating of the solution. In some cases, the solution may be
sonicated. The metal species and/or anionic species may be provided
in an amount such that the concentration of the metal ionic species
and/or anionic species is at least about 0.1 mM, at least about 0.5
mM, at least about 1 mM, at least about 10 mM, at least about 0.1
M, at least about 0.5 M, at least about 1 M, at least about 2 M, at
least about 5M, and the like. In some cases, the concentration of
the anionic species may be greater than the concentration of the
metal ionic species, so as to facilitate the formation of the
catalytic material, as described herein. As non-limiting examples,
the concentration of the anionic species may be about 2 times
greater, about 5 times greater, about 10 times greater, about 25
times greater, about 50 times greater, about 100 times greater,
about 500 times greater, about 1000 times greater, and the like, of
the concentration of the metal ionic species. In some instances,
the concentration of the metal ionic species will be greater than
the concentration of the anionic species.
[0140] In some cases, the pH of the solution may be about neutral.
That is, the pH of the solution may be between about 6.0 and about
8.0, between about 6.5 and about 7.5, and/or the pH is about 7.0.
In other cases, the pH of the solution is about neutral or acidic.
In these cases, the pH may be between about 0 and about 8, between
about 1 and about 8, between about 2 and about 8, between about 3
and about 8, between about 4 and about 8, between about 5 and about
8, between about 0 and about 7.5, between about 1 and about 7.5,
between about 2 and about 7.5, between about 3 and about 7.5,
between about 4 and about 7.5, or between about 5 and about 7.5. In
yet other cases, the pH may be between about 6 and about 10,
between about 6 and about 11, between about 7 and about 14, between
about 2 and about 12, and the like. In some embodiments, the pH of
the solution may be about neutral and/or basic, for example,
between about 7 and about 14, between about 8 and about 14, between
about 8 and about 13, between about 10 and about 14, greater than
14, or the like. The pH of the solution may be selected such that
the anionic species and the metal ionic species are in the desired
state. For example, some anionic species may be affected by a
change in pH level, for example, phosphate. If the solution is
basic (greater than about pH 12), the majority of the phosphate is
the form PO.sub.4.sup.-3. If the solution is approximately neutral,
the phosphate is in approximately equal amounts of the form
HPO.sub.4.sup.-2 and the form H.sub.2PO.sub.4.sup.-1. If the
solution is slightly acidic (less than about pH 6), the phosphate
is mostly in the form H.sub.2PO.sub.4.sup.-. The pH level may also
affect the solubility constant for the anionic species and the
metal ionic species.
[0141] In one embodiment, a photoanode as described herein may
comprise a photoactive electrode and a composition comprising metal
ionic species and anionic species in electrical communication with
the photoactive electrode. The composition, in some cases, may be
formed by self-assembly of the metal ionic species and anionic
species on the photoactive electrode and be sufficiently
non-crystalline such that the composition allows for the conduction
of protons. In some embodiments, a photoanode may allow for a
conductivity of protons of at least 10.sup.-1 S cm.sup.-1, at least
about 20.sup.-1 S cm.sup.-1, at least about 30.sup.-1 S cm.sup.-1,
at least about 40.sup.-1 S cm.sup.-1, at least about 50.sup.-1
S.sup.-1 cm.sup.-1, at least about 60 S cm.sup.-1, at least about
80 S cm.sup.-1, at least about 100 S cm.sup.-1, and the like.
[0142] In some embodiments, a photoanode as described herein may be
capable of producing oxygen gas from water at a low overpotential.
Voltage in addition to a thermodynamically determined reduction or
oxidation potential that is required to attain a given catalytic
activity is herein referred to as "overpotential," and may limit
the efficiency of the electrochemical device (e.g.,
photoelectrochemical device). Overpotential is therefore given its
ordinary meaning in the art, that is, it is the potential that must
be applied to a component of a system such as a photoanode to bring
about a electrochemical reaction (e.g., formation of oxygen gas
from water) minus the thermodynamic potential required for the
reaction. Those of ordinary skill in the art understand that the
total potential that must be applied to a particular system in
order to drive a reaction is typically the total of the potentials
that must be applied to the various components of the system. For
example, the potential for an entire system is typically higher
than the potential as measured at, e.g., a photoanode at which
oxygen gas is produced from the electrolysis of water. Those of
ordinary skill in the art will recognize that where overpotential
for oxygen production from water electrolysis is discussed herein,
this applies to the voltage required for the conversion of water to
oxygen itself, and does not include voltage drop at the counter
electrode.
[0143] The thermodynamic potential for the production of oxygen gas
from water varies depending on the conditions of the reaction
(e.g., pH, temperature, pressure, etc.). Those of ordinary skill in
the art will be able to determine the required thermodynamic
potential for the production of oxygen gas from water depending on
the experimental conditions. For example, the pH dependence of
water oxidation may be determined from a simplified form of the
Nernst equation to give Equation 18:
E.sub.pH=E.sup.o-0.059V.times.(pH) (18)
where E.sub.pH is the potential at a given pH, E.sup.o is the
potential under standard conditions (e.g., 1 atm, about 25.degree.
C.) and pH is the pH of the solution. For example, at pH 0, E=1.229
V, at pH 7, E=0.816 V, and at pH 14, E=0.403 V.
[0144] The thermodynamic potential for the production of oxygen gas
from water at a specific temperature (E.sub.T) may be determined
using Equation 19:
E.sub.T=[1.5184-(1.5421.times.10.sup.-3)(T)]+[(9.523.times.10.sup.-5)(T)-
(ln(T))]+[(9.84.times.10.sup.-8)T.sup.2] (19)
where T is given in Kelvin. For example, at 25.degree. C.,
E.sub.T=1.229 V, and at 80.degree. C., E.sub.T=1.18 V.
[0145] The thermodynamic potential for the production of oxygen gas
from water at a given pressure (E.sub.p) may be determined using
Equation 20:
E P = E T + ( RT 2 F ) ln { [ ( P - P w ) 1.5 ] / ( P w P wo ) } (
20 ) ##EQU00003##
where T is in Kelvin, F is Faraday's constant, R is the universal
gas constant, P is the operating pressure of the electrolyzer,
P.sub.w is the partial pressure of water vapor over the chosen
electrolyte, and P.sub.wo is the partial pressure of water vapor
over pure water. By this equation, at a 25.degree. C., the E.sub.P
increases by 43 mV for a tenfold increase in pressure.
[0146] In some instances, a photoanode as described herein may be
capable of catalytically producing oxygen gas from water (e.g.,
gaseous and/or liquid water) with an overpotential of less than
about 1 volt, less than about 0.75 volts, less than about 0.6
volts, less than about 0.5 volts, less than about 0.4 volts, less
than about 0.35 volts, less than about 0.325 volts, less than about
0.3 volts, less than about 0.25 volts, less than about 0.2 volts,
less than about 0.1 volts, or the like. In some embodiments, the
overpotential is between about 0.1 volts and about 0.4 volts,
between about 0.2 volts and about 0.4 volts, between about 0.25
volts and about 0.4 volts, between about 0.3 volts and about 0.4
volts, between about 0.25 volts and about 0.35 volts, or the like.
In another embodiment, the overpotential is about 0.325 volts. In
some cases, the overpotential of a photoanode is determined under
standardized conditions of an electrolyte with a neutral pH (e.g.,
about pH 7.0), ambient temperature (e.g., about 25.degree. C.),
ambient pressure (e.g., about 1 atm), a photoactive electrode that
is non-porous and planar, and at a geometric current density (as
described herein) of about 1 mA/cm.sup.2. It is to be understood
that systems of the invention can be used under conditions other
than those described immediately above and in fact those of
ordinary skill in the art will recognize that a very wide variety
of conditions can exist in use of the invention. But the conditions
noted above are provided only for the purpose of specifying how
features such as overpotential, amount of oxygen and/or hydrogen
produced, and other performance characteristics defined herein are
measured for purposes of clarity of the present invention. In a
specific embodiment, a catalytic material may produce oxygen gas
from water at an overpotential of less than 0.4 volt at an
electrode current density of at least 1 mA/cm.sup.2. As described
herein, the water which is oxidized may contain at least one
impurity (e.g., NaCl), or be provided from an impure water
source.
[0147] In some embodiment, a photoanode may be capable of
catalytically producing oxygen gas from water (e.g., gaseous and/or
liquid water) with a Faradaic efficiency of about 100%, greater
than about 99.8%, greater than about 99.5%, greater than about 99%,
greater than about 98%, greater than about 97%, greater than about
96%, greater than about 95%, greater than about 90%, greater than
about 85%, greater than about 80%, greater than about 70%, greater
than about 60%, greater than about 50%, etc. The term, "Faradaic
efficiency," as used herein, is given its ordinary meaning in the
art and refers to the efficacy with which charge (e.g., electrons)
are transferred in a system facilitating an electrochemical
reaction. Loss in Faradaic efficiency of a system may be caused,
for example, by the misdirection of electrons which may participate
in unproductive reactions, product recombination, short circuit the
system, and other diversions of electrons and may result in the
production of heat and/or chemical byproducts.
[0148] Faradaic efficiency may determined, in some cases, through
bulk electrolysis where a known quantity of reagent is
stoichiometrically converted to product as measured by the current
passed and this quantity may be compared to the observed quantity
of product measured through another analytical method. For example,
a device or photoanode may be used to catalytically produce oxygen
gas from water. The total amount of oxygen produced may be measured
using techniques know to those of ordinary skill in the art (e.g.,
using an oxygen sensor, a zirconia sensor, electrochemical methods,
etc.). The total amount of oxygen that is expected to be produced
may be determined using simple calculations from the amount of
charge passed. The Faradaic efficiency may be determined by
measuring the percentage of oxygen gas produced and comparing that
value with the expected amount of oxygen gas produced based on the
charge passed during photo-assisted electrolysis. In some cases,
the Faradaic efficiency of a photoanode changes by less than about
0.1%, less than about 0.2%, less than about 0.3%, less than about
0.4%, less than about 0.5%, less than about 1.0%, less than about
2.0%, less than about 3.0%, less than about 4.0%, less than about
5.0%, etc., over a period of operation of the photoanode of about 1
day, about 2 days, about 3 days, about 5 days, about 15 days, about
1 month, about 2 months, about 3 months, about 6 months, about 12
months, about 18 months, about 2 years, etc.
[0149] As will be known to those of ordinary skill in the art, an
example of a side reaction that may occur during the catalytic
formation of oxygen gas from water is the production of hydrogen
peroxide. The production of hydrogen peroxide may decrease the
Faradaic efficiency of a photoanode. In some cases, a photoanode,
in use, may produce oxygen that is in the form of hydrogen peroxide
of less than about 0.01%, less than about 0.05%, less than about
0.1%, less than about 0.2%, less than about 0.3%, less than about
0.4%, less than about 0.5%, less than about 0.6%, less than about
0.7%, less than about 0.8%, less than about 0.9%, less than about
1%, less than about 1.5%, less than about 2%, less than about 3%,
less than about 4%, less than about 5%, less than about 10%, etc.
That is, less than this percentage of the molecules of oxygen
produced is in the form of hydrogen peroxide. Those of ordinary
skill in the art will be aware of methods for determining the
production of hydrogen peroxide at a photoanode and/or methods to
determine the percentage of hydrogen peroxide produced. For
example, hydrogen peroxide may be determined using a rotating
ring-disc electrode. Any products generated at the disk electrode
are swept past the ring electrode. The potential of the ring
electrode may be poised to detect hydrogen peroxide that may have
been generated at the ring.
[0150] In some cases, the performance of a photoanode may also be
expressed, in some embodiments, as a turnover frequency. The
turnover frequency refers to the number of oxygen molecules
produced per second per catalytic site. In some cases, a catalytic
site may be a metal ionic species (e.g., a cobalt ion). The
turnover frequency of a photoanode (e.g., comprising a photoactive
electrode and a catalytic material) may be less than about 0.01,
less than about 0.005, less than about 0.001, less than about
0.0007, less than about 0.0005, less than about 0.00001, less than
about 0.000005, or less, moles of oxygen gas per second per
catalytic site. In some cases, the turnover frequency may be
determined under standardized conditions (e.g., ambient temperature
and pressure, 1 mA/cm.sup.2, planar photoactive electrode, etc.).
Those of ordinary skill in the art will be aware of methods to
determine the turnover frequency.
[0151] In one set of embodiments, the invention provides a
photoanode and/or catalytic system which can facilitate
photo-assisted electrolysis (or other electrochemical reactions)
wherein a significant portion, or essentially all of the electrons
provided to or withdrawn from a solution or material undergoing
electrolysis are provided through reaction of catalytic material.
For example, where essentially all the electrons provided to or
withdrawn from a system undergoing electrolysis are involved in a
catalytic reaction, essentially each electron added or withdrawn
participates in a reaction involving change of a chemical state of
at least one element of a catalytic material. In other embodiments,
the invention provides a system where at least about 98%, at least
about 95%, at least about 90%, at least about 80%, at least about
70%, at least about 60%, at least about 50%, at least about 40%, or
at least about 30% of all electrons added to or withdrawn from a
system undergoing electrolysis (e.g., water being split) are
involved in a catalytic reaction. Where less than essentially all
electrons added or withdrawn are involved in a catalytic reaction
some electrons can simply be provided to and withdrawn from the
electrolysis solution or material (e.g., water) directly to and
from a photoactive electrode and/or photoanode which does
participate in a catalytic reaction.
[0152] In some embodiments, systems and/or devices may be provided
that comprise at least one photoanode as described herein and/or
prepared using the methods described herein may be provided. In
particular, a device may be a photoelectrochemical device.
Non-limiting examples of photoelectrochemical devices includes
photoelectrochemical cells, bi-photoelectrochemical cells, hybrid
photoelectrochemical cells, and the like. A photoelectrochemical
device in some cases, may function as an oxygen gas and/or hydrogen
gas generator by photoelectrochemically decomposing water (e.g.,
liquid and/or gaseous water) to produce oxygen and/or hydrogen
gases. Fuel (e.g., water) may be provided to a device in a solid,
liquid, gel, and/or gaseous state. In some cases, as described
herein, the oxygen gas and/or hydrogen gas produced may be
converted to water using a secondary device, for example, an energy
conversion device such as a fuel cell. An energy conversion device,
in some embodiments, may be used to provide at least a portion of
the energy required to operate an automobile, a house, a village, a
cooling device (e.g., a refrigerator), etc. In some cases, more
than one device may be employed to provide the energy.
[0153] In some embodiments, a device may be used to produce O.sub.2
and/or H.sub.2. The O.sub.2 and/or H.sub.2 may be converted back
into electricity and water (e.g., through use of a fuel cell). In
some cases, however, the O.sub.2 and/or H.sub.2 may be used for
other purposes. For example, the O.sub.2 and/or H.sub.2 may be
burned to provide a source of heat. In some cases, O.sub.2 may be
used in combustion processes (e.g., burning of the hydrocarbon
fuels such as oil, coal, petrol, natural gas) which may be used to
heat homes, power cars, as rocket fuel, etc. In some instances,
O.sub.2 may be used in a chemical plant for the production and/or
purification of a chemical (e.g., production of ethylene oxide,
production of polymers, purification of molten ore). In some cases,
the H.sub.2 may be used to power a device (e.g., in a hydrogen fuel
cell), wherein the O.sub.2 may be released into the atmosphere
and/or used for another purpose. In other cases, H.sub.2 may be
used for the production of a chemical or in a chemical plant (e.g.,
for hydrocracking, hydrodealkylation, hydrodesulfurization,
hydrogenation (e.g., of fats, oils, etc.), etc.; for the production
of methanol, acids (e.g., hydrochloric acid), ammonia, etc.).
H.sub.2 and O.sub.2 may also be used for medical, industrial,
and/or other scientific processes (e.g., as medical grade oxygen,
combustion with acetylene in an oxy-acetylene torch for welding and
cutting metals, etc.). Those of ordinary skill in the art will be
aware of uses for O.sub.2 and/or H.sub.2. Other non-limiting
examples of device uses include O.sub.2 production (e.g., gaseous
oxygen), H.sub.2 production (e.g., gaseous hydrogen),
H.sub.2O.sub.2 production, ammonia oxidation, hydrocarbon (e.g.,
methanol, methane, ethanol, and the like) oxidation, exhaust
treatment, etc.
[0154] In some embodiments, a photoelectrochemical cell is provided
that allows for electrochemically producing oxygen and/or hydrogen
gases from water and systems and/or methods associated with the
same. In some embodiments, the photoelectrochemical cell may
comprise a photoanode (e.g., comprising a photoactive electrode and
a catalytic material, wherein a catalytic material is integrally
connected with the photoactive electrode (or photosensitizing
agent)) and an electrode (or photocathode). The catalytic material
may comprise metal ionic species and anionic species and/or may not
consist essentially of metal oxide or metal hydroxide. Illumination
of the device (e.g., by exposure to electromagnetic radiation) may
produce oxygen gas. In some instances, hydrogen gas may also be
produced at the electrode. As shown in FIG. 1, in a non-limiting
configuration, a device comprises a chamber 128, a photoactive
electrode 130, an electrode (or second photoactive electrode) 134,
wherein the photoactive electrode is biased positively with respect
to the electrode, means for connecting the photoactive electrode
and the electrode 131, an electrolyte 132, wherein the photoactive
electrode and the electrode are in fluid contact with the
electrolyte, and in most cases, a power source 138 in electrical
communication with the photoactive electrode and the electrode. In
some cases, the device may also comprise a resistor 136.
[0155] A photoactive electrode biased negatively or positively
towards an electrode (or second photoactive electrode) means that
the potential of the photoactive electrode is negative or positive
with respect to the potential of the electrode (or second
photoactive electrode). The electrode may be biased negatively or
positively with respect to the photoactive electrode by less than
about 1.23 V (e.g., the minimum defined by the thermodynamics of
transforming water into oxygen and hydrogen gas), less than about
1.3 V, less than about 1.4 V, less than about 1.5 V, less than
about 1.6 V, less than about 1.7 V, less than about 1.8 V, less
than about 2 V, less than about 2.5 V, and the like. In some cases,
the bias may be between about 1.5 V and about 2.0 V, about 1.6 V
and about 1.9 V, or is about 1.6 V, between about 1 V and about 2.5
V, between about 1.5 and about 2.5 V, and the like. Voltage may be
applied to the photoactive electrode (e.g., via an external power
source and/or by exposing the photoactive electrode to light) to
generate electron-hole pairs. The electron-hole pairs may be
separated between the photoactive electrode and the electrode, to
produce photoelectrochemical reduction and photoelectrochemical
oxidation reactions at the electrode and photoactive electrode,
respectively, thereby producing oxygen gas. In the case of the
photoactive electrode, holes combine with water molecules
(H.sub.2O) to produce an oxidation reaction, thereby producing
oxygen gas. The reverse reaction may occur at the electrode, where
electrons combine with protons (e.g., H.sup.+, or a proton source),
to produce a reduction reaction, thereby producing hydrogen gas.
The net effect is a flow of electrons from the first photoactive
electrode to the second electrode, resulting in reduction at the
latter (hydrogen gas formation), and oxidation at the former
(oxygen gas formation). In some cases, the hydrogen and/or oxygen
gases produced may be stored and used in further reactions.
[0156] As another non-limiting embodiment, in some cases, the
photoelectrochemical cell may comprise a hybrid
photovoltaic/photoelectrode. A hybrid photovoltaic/photoelectrode
generally comprises a photoelectrode that is electrolytically
active (e.g., an electrode where water oxidation takes place), a
photovoltaic cell, which acts to provide a voltage bias to the
photoelectrode, and a electrode (e.g., where the corresponding
reduction of protons may occur to fulfill the second half-reaction
in overall water splitting for the device.). A non-limiting
illustration of a photoelectrochemical cell is shown in FIG. 10. In
this figure, the hybrid photovoltaic/photoelectrode comprises
photoanode 202 in electrical connection with p-n junction solar
cell 204 (e.g., comprising silicon), electrode 206, and in some
cases, coating 200 to protect the solar cell and electrode from
outside exposure (e.g., to the electrolyte, etc.). Upon exposure to
light, the photoanode absorbs photons having energy equal to or
greater than its band gap while the rest of the light is
transmitted to the solar cell. The solar cell provides the
additional energy required to bias the device for water
electrolysis.
[0157] A non-limiting example of a photoelectrochemical cell is
depicted in FIG. 11. The photoelectrochemical cell comprises
housing 298, in which at least one section or side of the housing
is substantially transparent to light (e.g., wall 298a and walls
298). During operation, the photoelectrochemical cell may be
illuminated on the wall(s) which are substantially transparent. The
housing may comprise at least first outlet 320 and second outlet
322 for the collection of O.sub.2 and H.sub.2 gases, respectively,
produced during the photoelectrochemical reaction. The housing may
comprise at least one photovoltaic cell comprising first electrode
(or photoanode) 306, and second electrode (or photocathode) 302. In
some cases, material 304 may be present between the first electrode
and the second electrode (e.g., a non-doped semiconductor). The
cell also comprises an electrolyte (e.g., 300, 318). The cell also
may comprise material 316. Material 316 may be a porous
electrically conductive material (e.g., valve metal, metallic
compound) wherein the electrolyte (e.g., 318) fills the pores of
the material. In some embodiments, a catalytic material 308 may
associate with material 316 (e.g., indirect association) as
compared to direct association with the photoactive electrode (or
electrode). Without wishing to be bound by theory, material 316 may
act as a membrane and may allow for the transmission of electrons
generated at first electrode (or photoactive electrode) 306 to
outer surface 324 of material 316. Material 316 may also be
selected such that no oxygen gas is produced in the pores of
material 316, for example, if the overpotential for production of
oxygen gas is high. Oxygen gas may form on or near surface 324 of
material 316 (e.g., via the composition associated with outer
surface 324 or material 316). Non-limiting examples of materials
which may be suitable for use as material 316 includes titanium,
zirconium, vanadium, hafnium, niobium, tantalum, tungsten, or
alloys thereof. In some cases, the material may be a valve metal
nitride, carbide, borides, etc., for example, titanium nitride,
titanium carbide, or titanium boride. In some cases, the material
may be titanium oxide, or doped titanium oxide (e.g., with niobium,
tantalum, tungsten, fluorine, etc.).
[0158] In some cases, a photoelectrochemical cell may be a
bi-photoelectrochemical device or tandem photoelectrochemical cell
and may comprise a first and a second photoelectrode. The first and
second photoelectrodes may work in tandem to split water to produce
hydrogen and oxygen gases using electromagnetic radiation (e.g.,
visible light, solar energy). The first and the second
photoelectrodes may be in electrical communication with one
another. A non-limiting arrangement of a bi-photoelectrochemical
cell is shown in FIG. 12. In this figure, 150-1 and 151 are
transparent materials (e.g., glass) through which light can pass.
The light may pass through material 150-1 and through electrolyte
152 (e.g., aqueous electrolyte) and impinge on a photoelectrode
comprising components 153 (e.g., light absorbing material,
catalytic material, etc.) and 154-1 (e.g., material which may
collect electrons produced by light absorbing material, catalytic
material, etc.). In some cases, in this device, photoelectrode
153/154-1 may absorb only a part of the visible light spectrum
(e.g., blue and green light) and the remainder of the spectrum
(e.g., red and yellow light) may pass through another transparent
material (e.g., glass, 150-2) to a second cell. Oxygen gas may be
produced at photoelectrode 153/154-1. The second cell may comprise
material 154-2 (e.g., a conducting oxide material) and material 156
(e.g., a dye-derivatized metal oxide material), which may function
as a light-driven electric bias and may increase the
electrochemical potential of the electrons which emerge from
photoelectrode 153/154-1. The second cell may also comprise
electrolyte 157 (e.g., organic redox electrolyte) and counter
electrode 158. Behind counter electrode 158, there may also be a
compartment comprising electrolyte 159, in which hydrogen gas may
be produced at cathode 160. Electrolytes 152 and 159 may be
substantially similar, in some embodiments, and may be connected by
a ion-conducting membrane or glass frit 161.
[0159] As another example, as shown in FIG. 13, a
bi-photoelectrochemical cell may comprise first photoelectrode 180
(e.g., comprising a photoanode as described herein), second
photoelectrode 182 biased negatively with respect to the first
photoelectrode (e.g., photocathode such as p-type GaP), electrolyte
190 (e.g., an aqueous electrolyte), and means for connecting 184
the first and the second photoelectrode. In some cases, the
bi-photoelectrochemical cell may optionally comprise power source
188 (e.g., especially in cases where the photoanode and the
photocathode comprise similar materials but are differently doped
such as p-type and n-type TiO.sub.2) and/or a resistor 186.
[0160] Yet another embodiment for a photoelectrochemical cell for
the electrolysis of water, may comprise a container, an aqueous
electrolyte in the container, wherein the pH of the electrolyte is
neutral or below, a photoanode mounted in the container and in
contact with the electrolyte, wherein the first electrode comprises
a photoactive electrode, metal ionic species and anionic species,
the metal ionic species and the anionic species defining a
substantially non-crystalline composition and have an equilibrium
constant, K.sub.sp, between about 10.sup.-3 and 10.sup.-10 when the
metal ionic species is in an oxidation state of (n) and have a
K.sub.sp less than about 10.sup.-10 when the metal ionic species is
in an oxidation state of (n+x), an electrode (or second
photoactive) mounted in the container and in contact with the
electrolyte, wherein the electrode is biased negatively with
respect to the photoanode, and means for connecting the photoanode
and the electrode. In this embodiment, when a voltage is applied
between the photoanode and the electrode, gaseous hydrogen may be
evolved at the electrode and gaseous oxygen may be produced at the
photoanode.
[0161] The performance of a photoanode of a device may be measured
by current density (e.g., geometric and/or total current density),
wherein the current density is a measure of the density of flow of
a conserved charge. For example, the current density is the
electric current per unit area of cross section. In some cases, the
current density (e.g., geometric current density and/or total
current density, as described herein) of a photoanode as described
herein is greater than about 0.1 mA/cm.sup.2, greater than about 1
mA/cm.sup.2, greater than about 5 mA/cm.sup.2, greater than about
10 mA/cm.sup.2, greater than about 20 mA/cm.sup.2, greater than
about 25 mA/cm.sup.2, greater than about 30 mA/cm.sup.2, greater
than about 50 mA/cm.sup.2, greater than about 100 mA/cm.sup.2,
greater than about 200 mA/cm.sup.2, and the like.
[0162] In some embodiments, the current density can be described as
the geometric current density. The geometric current density, as
used herein, is current divided by the external surface area of the
photoanode. The external surface area of a photoanode will be
understood by those of ordinary skill in the art and refers to the
surface defining the outer boundary of the photoanode, for example,
the area that may be measured by a macroscopic measuring tool
(e.g., a ruler) and does not include the internal surface area
(e.g., area within pores of a porous material such as a foam, or
surface area of those fibers of a mesh that are contained within
the mesh and do not define the outer boundary, etc.).
[0163] In some cases, the current density can be described as the
total current density. Total current density, as used herein, is
the current density divided by essentially the total surface area
(e.g., the total surface area including all pores, fibers etc.) of
the photoanode. In some cases, the total current density may be
approximately equal to the geometric current density (e.g., in
cases where the photoanode is not porous and the total surface area
is approximately equal to the geometric surface area).
[0164] In some embodiments, a device and/or photoanode as described
herein is capable of producing at least about 1 umol (micromole),
at least about 5 umol, at least about 10 umol, at least about 20
umol, at least about 50 umol, at least about 100 umol, at least
about 200 umol, at least about 500 umol, at least about 1000 umol
oxygen and/or hydrogen, or more, per cm.sup.2 at the photoanode at
which oxygen production and/or hydrogen production occurs,
respectively, per hour. The area of the photoanode may be the
geometric surface area or the total surface area, as described
herein.
[0165] The devices and methods as described herein, in some cases,
may proceed at about ambient conditions. Ambient conditions define
the temperature and pressure relating to the device and/or method.
For example, ambient conditions may be defined by a temperature of
about 25.degree. C. and a pressure of about 1.0 atmosphere (e.g., 1
atm, 14 psi). In some cases, the conditions may be essentially
ambient. Non-limiting examples of essentially ambient temperature
ranges include between about 0.degree. C. and about 40.degree. C.,
between about 5.degree. C. and about 35.degree. C., between about
10.degree. C. and about 30.degree. C., between about 15.degree. C.
and about 25.degree. C., at about 20.degree. C., at about
25.degree. C., and the like. Non-limiting examples of essentially
ambient pressure ranges include between about 0.5 atm and about 1.5
atm, between about 0.7 atm and about 1.3 atm, between about 0.8 and
about 1.2 atm, between about 0.9 atm and about 1.1 atm, and the
like. In a particular case, the pressure may be about 1.0 atm.
Ambient or essentially ambient conditions can be used in
conjunction with any of the devices, compositions, catalytic
materials, and/or methods described herein, in conjunction with any
conditions (for example, conditions of pH, etc.).
[0166] In some cases, the devices and/or methods as described
herein may proceed at temperatures above ambient temperature. For
example, a device and/or method may be operated at temperatures
greater than about 30.degree. C., greater than about 40.degree. C.,
greater than about 50.degree. C., greater than about 60.degree. C.,
greater than about 70.degree. C., greater than about 80.degree. C.,
greater than about 90.degree. C., greater than about 100.degree.
C., greater than about 120.degree. C., greater than about
150.degree. C., greater than about 200.degree. C., or greater.
Efficiencies can be increased, in some instances, at temperatures
higher than ambient. The temperature of the device may be selected
such that the water provided and/or formed is in a gaseous state
(e.g., at temperatures greater than about 100.degree. C.). In other
cases, devices and/or methods as described herein may proceed at
temperatures below ambient temperature. For example, a device
and/or method may be operated at temperatures less than about
20.degree. C., less than about 10.degree. C., less than about
0.degree. C., less than about -10.degree. C., less than about
-20.degree. C., less than about -30.degree. C., less than about
-40.degree. C., less than about -50.degree. C., less than about
-60.degree. C., less than about -70.degree. C. or the like. In some
instances, the temperature of the device and/or method may be
affected by an external temperature source (e.g., a heating and/or
cooling coil, infrared light, refrigeration, etc.). In other
instances, however, the temperature of the device and/or method may
be affected by internal processes, for example, exothermic and/or
endothermic reactions, etc. In some cases, the device and/or method
may be operated at approximately the same temperature throughout
the use of the device and/or method. In other cases, the
temperature may be changed at least once and/or gradually during
the use of the device and/or method. In a particular embodiment,
the temperature of the device may be elevated during times when the
device is used in conjugation with sunlight or other radiative
power sources.
[0167] In some embodiments, the water provided and/or formed during
use of a method and/or device as described herein may be in a
gaseous state (e.g., steam). Those of ordinary skill in the art can
apply known electrochemical techniques carried out with steam, in
some cases, without undue experimentation. As an exemplary
embodiment, water may be provided in a gaseous state to an
electrochemical device (e.g., high-temperature electrolysis or
steam electrolysis) comprising a photoanode. In some cases, the
gaseous water may be produced by a device or system which
inherently produces steam (e.g., a nuclear power plant). Without
wishing to be bound by theory, in some cases, providing water in a
gaseous state may allow for the electrolysis to proceed more
efficiently as compared to a similar device when provided water in
a liquid state. This may be due to the higher input energy of the
water vapor. In some instances, the gaseous water provided may
comprise other gases (e.g., hydrogen gas, nitrogen gas, etc.).
[0168] Individual aspects of the overall electrochemistry and/or
chemistry involved in electrochemical devices such as those
described herein are generally known, and not all will be described
in detail herein. It is to be understood that the specific
electrochemical devices described herein are exemplary only, and
the components, connections, and techniques as described herein can
be applied to virtually any suitable electrochemical device
including those with a variety of solid, liquid, and/or gaseous
fuels, and a variety of photoanodes, electrodes, photocathodes,
and/or electrolytes, which may be liquid or solid under operating
conditions (where feasible; generally, for adjacent components one
will be solid and one will be liquid if any are liquids). It is
also to be understood that photoelectrochemical device unit
arrangements discussed are merely examples of photoelectrochemical
devices that can make use of photoanodes as described herein. Many
structural arrangements other than those disclosed herein, which
make use of and are enabled by the present invention, will be
apparent to those of ordinary skill in the art.
[0169] A photoelectrochemical device accordingly may be combined
with additional electrochemical devices (e.g., a fuel cell, an
electrolytic device, etc.) to form a larger device or system. In
some embodiments, this may take the form of a stack of units or
devices. Where more than one electrochemical device is combined,
the devices may all be devices according to an embodiment the
present invention, or one or more devices according to an
embodiment the present invention may be combined with other
photoelectrochemical devices, such as a fuel cell. It is to be
understood that where this terminology is used, any suitable
electrochemical device, which those of ordinary skill in the art
would recognize could function in accordance with the systems and
techniques as described herein can be substituted.
[0170] Water may be provided to the systems, devices, photoanodes,
and/or for the methods provided herein, using any suitable source.
In some cases, the water is provided from a substantially pure
water source (e.g., distilled water, deionized water, chemical
grade water, etc.). In some cases, the water may be bottled water.
In some cases, the water is provided from a natural and/or impure
water source (e.g., tap water, lake water, ocean water, rain water,
lake water, pond water, sea water, potable water, brackish water,
industrial process water, etc.). In some cases, although it need
not be, the water is not purified prior to use (e.g., before being
provided to the system/photoanode for electrolysis). In some
instances, the water may be filtered to remove particulates and/or
other impurities prior to use. In some embodiments, the water that
is electrolyzed to produce oxygen gas (e.g., using a photoanode
and/or device as described here) may be substantially pure. The
purity of the water may be determined using one or more methods
known to those of ordinary skill in the art, for example,
resistivity, carbon content (e.g., through use of a total organic
carbon analyzer), UV absorbance, oxygen-absorbance test, limulus
ameobocyte lysate test, etc. In some embodiments, the water may
contain at least one impurity. In some embodiments, the at least
one impurity may be substantially non-participative in the
catalytic reaction. That is, the at least one impurity does not
participate in aspects of the catalytic cycle and/or regeneration
mechanism. The at least one impurity may be solid (e.g.,
particulate matter), a liquid, and/or a gas. In some cases, the
impurity may be solubilized and/or dissolved. For example, an
impurity may comprise ionic species. In some cases, an impurity may
be an impurity which may generally be present in a water source
(e.g., tap water, non-potable water, potable water, sea water,
etc.). In a particular embodiment, the water source may be sea
water and one of the impurities may be chloride ions, as described
herein. In some cases, an impurity may comprise a metal such as a
metal element (including heavy metals), a metal ion, a compound
comprising at least one metal, an ionic species comprising a metal,
etc. For example, an impurity comprising metal may comprise an
alkaline earth metal, an alkali metal, a transition metal, or the
like. Specific non-limiting examples of metals include lithium,
sodium, magnesium, titanium, vanadium, chromium, manganese, iron,
cobalt, nickel, copper, zinc, potassium, mercury, lead, barium,
etc. In some instances, an impurity comprising a metal may be the
same or different than the metal comprised in the metal ionic
species of a catalytic material as described herein. In some cases,
the impurity may comprise organic materials, for example, small
organic molecules (e.g., bisphenol A, trimethylbenzene, dioxane,
nitrophenol, etc.), microorganisms (such as bacteria (e.g., E.
coli, coliform, etc.), microbes, fungi, algae, etc.), other
biological materials, pharmaceutical compounds (e.g., drugs,
decomposition products from drugs), herbicides, pyrogens,
pesticides, proteins, radioactive compounds, inorganic compounds
(e.g., compounds comprising boron, silicon, sulfur, nitrogen,
cyanide, phosphorus, arsenic, sodium, etc.; carbon dioxide,
silicates (e.g., H.sub.4SiO.sub.4), ferrous and ferric iron
compounds, chlorides, aluminum, phosphates, nitrates, etc.),
dissolved gases, suspended particles (e.g., colloids), or the like.
In some cases, an impurity may be a gas, for example, carbon
monoxide, ammonia, carbon dioxide, oxygen gas, and/or hydrogen gas.
In some cases, the gas impurity may be dissolved in the water. In
some cases, a photoanode may be capable of operating at
approximately the same, at greater than about 95%, at greater than
about 90%, at greater than about 80%, at greater than about 70%, at
greater than about 60%, at greater than about 50%, or the like, of
the activity level using water containing at least one impurity
versus the activity using water that does not substantially contain
the impurity under essentially identical conditions. In some cases,
a photoanode may catalytically produce oxygen from water containing
at least one impurity such that less than about 5 mol %, less than
about 3 mol %, less than about 2 mol %, less than about 1 mol %,
less than about 0.5 mol %, less than about 0.1 mol %, less than
about 0.01 mol % of the products produced comprise any portion of
the at least one impurity.
[0171] In some cases, an impurity may be present in the water in an
amount greater than about 1 ppt, greater than about 10 ppt, greater
than about 100 ppt, greater than about 1 ppb, greater than about 10
ppb, greater than about 100 ppb, greater than about 1 ppm, greater
than about 10 ppm, greater than about 100 ppm, greater than about
1000 ppm, or greater. In other cases, an impurity may be present in
the water in an amount less than about 1000 ppm, less than about
100 ppm, less than about 10 ppm, less than about 1 ppm, less than
about 100 ppb, less than about 10 ppb, less than about 1 ppb, less
than about 100 ppt, less than about 10 ppt, less than about 1 ppt,
or the like. In some cases, the water may contain at least one
impurity, at least two impurities, at least three impurities, at
least five impurities, at least ten impurities, at least fifteen
impurities, at least twenty impurities, or greater. In some cases,
the amount of impurity may increase or decrease during operation of
the photoanode and/or device. That is, an impurity may be formed
during use of the photoanode and/or device. For example, in some
cases, the impurity may be a gas (e.g., oxygen gas and/or hydrogen
gas) formed during the electrolysis of water. Thus, in some cases,
the water may contain less than about 1000 ppm, less than about 100
ppm, less than about 10 ppm, less than about 1 ppm, less than about
100 ppb, less than about 10 ppb, less than about 1 ppb, less than
about 100 ppt, less than about 10 ppt, less than about 1 ppt, or
the like, prior to operation of the photoanode and/or device.
[0172] In some embodiments, the at least one impurity may be an
ionic species. In some cases, when the water contains at least one
ionic species, the water purity may be determined, at least in
part, by measuring the resistivity of the water. The theoretical
resistivity of water at 25.degree. C. is about 18.2 M.OMEGA.cm. The
resistivity of water that is not substantially pure may be less
than about 18 M.OMEGA.cm, less than about 17 M.OMEGA.cm, less than
about 16 M.OMEGA.cm, less than about 15 M.OMEGA.cm, less than about
12 M.OMEGA.cm, less than about 10 M.OMEGA.cm, less than about 5
M.OMEGA.cm, less than about 3 M.OMEGA.cm, less than about 2
M.OMEGA.cm, less than about 1 M.OMEGA.cm, less than about 0.5
M.OMEGA.cm, less than about 0.1 M.OMEGA.cm, less than about 0.01
M.OMEGA.cm, less than about 1000 .OMEGA.cm, less than about 500
.OMEGA.cm, less than about 100 .OMEGA.cm, less than about 10
.OMEGA.cm, or less. In some cases, the resistivity of the water may
be between about 10 M.OMEGA.cm and about 1 .OMEGA.cm, between about
1 M.OMEGA.cm and about 10 .OMEGA.cm, between about 0.1 M.OMEGA.cm
and about 100 .OMEGA.cm, between about 0.01 M.OMEGA.cm and about
1000 .OMEGA.cm, between about 10,000 .OMEGA.cm and about 1,000
.OMEGA.cm, between about 10,000 .OMEGA.cm and about 100 .OMEGA.cm,
between about 1,000 and about 1 .OMEGA.cm, between about 1,000 and
about 10 .OMEGA.cm, and the like. In some cases, when the water
source is tap water, the resistivity of the water may be between
about 10,000 .OMEGA.cm and about 1,000 .OMEGA.cm. In some cases,
when the water source is sea water, the resistivity of the water
may be between about 1,000 .OMEGA.cm and about 10 .OMEGA.cm. In
some instances, where the water may be taken from an impure source
and purified prior to use, the water may be purified in a manner
which does not resistivity of the water by a factor of more than
about 5%, about 10%, about 20%, about 25%, about 30%, about 50%, or
the like. Those of ordinary skill in the art will be aware of
methods to determine the resistivity of water.
[0173] In some cases, where the water is obtained from an impure
water source and/or has a resistivity of less than about 16
M.OMEGA.cm the water may be purified (e.g., filtered) in a manner
that changes its resistivity by a factor of less than about 50%,
less than about 30%, less than about 25%, less than about 20%, less
than about 15%, less than about 10%, less than about 5%, or less,
after being drawn from the source prior to use in the
electrolysis.
[0174] In some embodiments, the water may contain halide ions
(e.g., fluoride, chloride, bromide, iodide), for example, such that
a photoanode may be used for the desalination of sea water. In some
cases, the halide ions might not be oxidized (e.g., to form halogen
gas such as Cl.sub.2) during the catalytic production of oxygen
from water. Without wishing to be bound by theory, halide ions (or
other anionic species) that might not be incorporated in the
catalytic material (e.g., within the lattice of the catalytic
material) might not be oxidized during the catalytic formation of
oxygen from water. This may be because the halide ions might not
readily form bonds with the metal ionic species, and therefore, may
only have access to outer sphere mechanism for oxidation. In some
instances, oxidation of halide ions by an outer sphere mechanism
may be not kinetically favorable. In some cases, a photoanode may
catalytically produce oxygen from water comprising halide ions such
that less than about 5 mol %, less than about 3 mol %, less than
about 2 mol %, less than about 1 mol %, less than about 0.5 mol %,
less than about 0.1 mol %, less than about 0.01 mol % of the gases
evolved comprise oxidized halide species. In some embodiments, the
impurity is sodium chloride.
[0175] In some cases, under catalytic condition, halide ions (or
other impurities) might not associate with a catalytic material
and/or with metal ionic species. In some instances, a complex
comprising a halide ion and a metal ionic species may be
substantially soluble such that the complex does not form a
catalytic material and/or associate with the photoactive electrode
and/or photoanode. In some cases, the catalytic material may
comprise less than about 5 mol %, less than about 3 mol %, less
than about 2 mol %, less than about 1 mol %, less than about 0.5
mol %, less than about 0.1 mol %, less than about 0.01 mol % of the
halide ion impurities.
[0176] In some cases, the rate of oxidation of water may dominate
over the rate of oxidation of halide ions (or other impurities) due
to various factors including thermodynamics, solubility, and the
like. For example, the binding affinity of a metal ionic species
for an anionic species may be substantially greater than the
binding affinity of the metal ionic species for a halide ion, such
that the coordination sphere of the metal ionic species may be
substantially occupied by the anionic species. In other cases, the
halide ions might not be incorporated into the lattice of a
catalytic material (e.g., as part of the lattice or within the
interstitial holes of the lattice) due to the size of the halide
ion (e.g., the halide is too large or too small to be incorporated
into the lattice of the catalytic material). Those of ordinary
skill in the art will be able to determine if a photoanode as
described herein is able to catalytically produce oxygen using
water containing halide ions, for example, by monitoring the
production of halogen gas (or species comprising oxidized halide
ions) using suitable techniques, for example, mass
spectrometry.
[0177] Various components of the invention, such as the photoanode,
electrode, photocathode, power source, electrolyte, separator,
container, circuitry, insulating material, gate electrode, etc. can
be fabricated by those of ordinary skill in the art from any of a
variety of components, as well as those described in any of those
patent applications described herein. Components of the invention
can be molded, machined, extruded, pressed, isopressed,
infiltrated, coated, in green or fired states, or formed by any
other suitable technique. Those of ordinary skill in the art are
readily aware of techniques for forming components of devices
herein. In some cases, components (e.g., photoanodes, electrodes,
electrolyte, electrical connectors, wires, etc.) of a device may be
selected as to minimize the ohmic resistances of the device. This
may aid in achieving the maximum energy conversion efficiency
possible for a selected device.
[0178] While electromagnetic radiation sources are described
herein, it should be understood that electromagnetic radiation may
be provided in any suitable arrangement or using any suitable
source, and may depend on the arrangement and components of the
photoelectrochemical device. In some cases, electromagnetic
radiation may be provided to one or more surface and/or components
of a photoelectrochemical device. For example, electromagnetic
radiation may be provided directly to a catalytic material (e.g.,
light is shone on the catalytic material), or may be provided
indirectly, for example, through the backside of the catalytic
material (e.g., light is shone through one or more other materials,
including, but not limited to, the photoactive electrode). Those of
ordinary skill in the art will be able to determine the portions of
a device to be exposed to electromagnetic radiation.
[0179] In some cases, the device may comprise a light management
system and/or solar concentrator, which are capable of focusing
electromagnetic radiation and/or solar energy. Generally, light
management systems or solar concentrators may receive
electromagnetic radiation and/or solar energy over a first surface
area and direct the received radiation to a second, smaller,
surface area. Light management systems and solar concentrators will
be known to those of ordinary skill in the art and may comprise,
for example, magnifying lenses, parabolic mirrors, and/or Fresnel
lenses for focusing incoming light and/or solar energy. In some
cases, the light management system or solar collector may collect
and waveguide the light to an area or surface of the
photoelectrochemical device, for example, a surface associated with
the catalytic material, a photoactive electrode, a photoanode, a
photocathode, etc.
[0180] In some cases, a device may be portable. That is, the device
may be of such size that it is small enough that it is movable. In
some embodiments, a device of the present invention is portable and
can be employed at or near a desired location (e.g., water supply
location, field location, etc.). For example, the device may be
transported and/or stored at a specific location. In some case, the
device may be equipped with straps or other components (e.g.,
wheels) such that the device may be carried or transported from a
first location to a second location. Those of ordinary skill in the
art will be able to identify a portable device. For instance, the
portable device may have a weight less than about 25 kg, less than
about 20 kg, less than about 15 kg, less than about 1 kg, less than
about 8 kg, less than about 7 kg, less than about 6 kg, less than
about 5 kg, less than about 4 kg, less than about 3 kg, less than
about 2 kg, less than about 1 kg, and the like, and/or have a
largest dimension that is no more than 50 cm, less than about 40
cm, less than about 30 cm, less than about 20 cm, less than about
10 cm, and the like. The weight and/or dimensions of the device
typically may or might not include components associated with the
device (e.g., water source, water source reservoir, oxygen and/or
hydrogen storage containers, etc.).
[0181] An electrolyte, as known to those of ordinary skill in the
art is any substance containing free ions that is capable of
functioning as an ionically conductive medium. In some cases, an
electrolyte may comprise water, which may act as the water source.
The electrolyte may be a liquid, a gel, and/or solid. The
electrolyte may also comprise methanol, ethanol, sulfuric acid,
methanesulfonic acid, nitric acid, mixtures of HCl, organic acids
like acetic acid, etc. In some cases, the electrolyte comprises
mixtures of solvents, such as water, organic solvents, amines and
the like. In some cases, the pH of the electrolyte may be about
neutral. That is, the pH of the electrolyte may be between about
5.5 and about 8.5, between about 6.0 and about 8.0, about 6.5 about
7.5, and/or the pH is about 7.0. In a particular case, the pH is
about 7.0. In other cases, the pH of the electrolyte is about
neutral or acidic. In these cases, the pH may range from about 0 to
about 8, about 1 to about 8, about 2 to about 8, about 3 to about
8, about 4 to about 8, about 5 to about 8, about 0 to about 7.5,
about 1 to about 7.5, about 2 to about 7.5, about 3 to about 7.5,
about 4 to about 7.5, about 5 to about 7.5. In yet other cases, the
pH may be between about 6 and about 10, about 6 and about 11, about
7 and about 14, about 2 and about 12, and the like. In a specific
embodiment, the pH is between about 6 and about 8, between about
5.5 and about 8.5, between about 5.5 and about 9.5, between about 5
and about 9, between about 3 and about 11, between about 4 and
about 10, or any other combination thereof. In some cases, when the
electrolyte is a solid, the electrolyte may comprise a solid
polymer electrolyte. The solid polymer electrolyte may serve as a
solid electrolyte that conducts protons and separate the gases
produces and or utilized in the electrochemical cell. Non-limiting
examples of a solid polymer electrolyte are polyethylene oxide,
polyacrylonitrile, and commercially available NAFION.
[0182] In some cases, the electrolyte is used to selectively
transport one or more ionic species. In some embodiments, the
electrolyte(s) are at least one of oxygen ion conducting membranes,
proton conductors, carbonate (CO.sub.3.sup.-2) conductors, OH.sup.-
conductors, and/or mixtures thereof. In some cases, the
electrolyte(s) are at least one of cubic fluorite structures, doped
cubic fluorites, proton-exchange polymers, proton-exchange
ceramics, and mixtures thereof. Further, oxygen-ion conducting
oxides that may be used as the electrolyte(s) include doped ceria
compounds such as gadolinium-doped ceria
(Gd.sub.1-xCe.sub.xCe.sub.xO.sub.2-d) or samarium-doped ceria
(Sm.sub.1-xCe.sub.xO.sub.2-d), doped zirconia compounds such as
yttrium-doped zirconia (Y.sub.1-xZr.sub.xO.sub.2-d) or
scandium-doped zirconia (Sc.sub.1-xZr.sub.xO.sub.2-d), perovskite
materials such as La.sub.1-xSr.sub.xGa.sub.1-yMg.sub.yO.sub.3-d,
yttria-stabilized bismuth oxide, and/or mixtures thereof. Examples
of proton conducting oxides that may be used as electrolyte(s)
include, but are not limited to, undoped and yttrium-doped
BaZrO.sub.3-d, BaCeO.sub.3-d, and SrCeO.sub.3-d as well as
La.sub.1-xSr.sub.xNbO.sub.3-d.
[0183] In some embodiments, the electrolyte may comprise an
ionically conductive material. In some embodiments, the ionically
conductive material may comprise the anionic species comprised in
the catalytic material on at least one photoanode. The presence of
the anionic species in the electrolyte, during use of the
photoanode comprising a catalytic material, may shift the dynamic
equilibrium towards the association of the anionic species and/or
metal ionic species with the photoanode, as described herein.
Non-limiting examples of other ionically conductive materials
include metal oxy-compounds, soluble inorganic and/or organic salts
(e.g., sodium or potassium chloride, sodium sulfate, quaternary
ammonium hydroxides, etc.).
[0184] In some cases, the electrolyte may comprise additives. For
example, the additive may be an anionic species (e.g., as comprised
in the catalytic material associated with a photoactive electrode).
For example, a photoanode used in a device may comprise a
photoactive electrode and a catalytic material comprising at least
one anionic species and at least one metal ionic species. The
electrolyte may comprise the at least one anionic species. In some
cases, the electrolyte can comprise an anionic species which is
different from the at least one anionic species comprised in the
catalytic material. For example, the catalytic material may
comprise phosphate anions and the electrolyte may comprise borate
anions. In some cases, when the additive is an anionic species, the
electrolyte may comprise counter cations (e.g., when the anionic
species is added as a complex, a salt, etc.). The anionic species
may be good proton-accepting species. In some cases, the additive
may be a good proton-accepting species which is not anionic (e.g.,
is a neutral base). Non-limiting examples of good proton-accepting
species which are neutral include pyridine, imidazole, and the
like.
[0185] In some cases, the electrolyte may be recirculated in the
electrochemical device. That is, a device may be provided which is
able to move the electrolyte in the electrochemical device.
Movement of the electrolyte in the electrochemical device may help
decrease the boundary layer of the electrolyte. The boundary layer
is the layer of fluid in the immediate vicinity of an electrode
and/or photoanode. In general, the extent to which a boundary layer
exists is a function of the flow velocity of the liquid in a
solution. Therefore, if the fluid is stagnant, the boundary layer
may be much larger than if the fluid was flowing. Therefore,
movement of the electrolyte in the photoelectrochemical device may
decrease the boundary layer and improve the efficiency of the
device.
[0186] In most embodiments, a device may comprise at least one
photoanode as described herein (e.g., comprising a photoactive
electrode and a catalytic material). In some instances, the device
can additionally comprise at least one electrode and/or
photocathode. In general, an electrode may be any material that is
substantially electrically conductive. The electrode may be
transparent, semi-transparent, semi-opaque, and/or opaque. The
electrode may be a solid, semi-porous or porous. Non-limiting
examples of electrodes include indium tin oxide (ITO), fluorine tin
oxide (FTO), glassy carbon, metals, lithium-containing compounds,
metal oxides (e.g., platinum oxide, nickel oxide), graphite, nickel
mesh, carbon mesh, and the like. Non-limiting examples of suitable
metals include gold, copper, silver, platinum, nickel, cadmium,
tin, and the like. In some instances, the electrode may comprise
nickel (e.g., nickel foam or nickel mesh). Nickel foam and nickel
mesh materials will be known to those of ordinary skill in the art
and may be purchase from commercial sources. Nickel mesh usually
refers to woven nickel fibers. Nickel foam generally refers to a
material of non-trivial thickness (e.g., about 2 mm) comprising a
plurality of holes and/or pores. In some cases, nickel foam may be
an open-cell, metallic structure based on the structure of an
open-cell polymer foam, wherein nickel metal is coated onto the
polymer foam. The electrodes may also be any other metals and/or
non-metals known to those of ordinary skill in the art as
conductive (e.g., ceramics). The electrodes may also be photoactive
electrodes used in photoelectrochemical cells. The electrode may be
of any size or shape. Non-limiting examples of shapes include
sheets, cubes, cylinders, hollow tubes, spheres, and the like. The
electrode may be of any size. Additionally, the electrode may
comprise a means to connect the electrode and to another electrode,
a power source and/or another electrical device.
[0187] Various electrical components of device may be in electrical
communication with at least one other electrical component by a
means for connecting. A means for connecting may be any material
that allows the flow of electricity to occur between a first
component and a second component. A non-limiting example of a means
for connecting two electrical components is a wire comprising a
conductive material (e.g., copper, silver, etc.). In some cases,
the device may also comprise electrical connectors between two or
more components (e.g., a wire and an electrode and/or photoanode).
In some cases, a wire, electrical connector, or other means for
connecting may be selected such that the resistance of the material
is low. In some cases, the resistances may be substantially less
than the resistance of the electrodes, photoanodes, and/or
electrolyte of the device.
[0188] In some embodiments, a power source may be provided to
supply DC or AC voltage to an electrochemical device. Non-limiting
examples include batteries, power grids, regenerative power
supplies (e.g., wind power generators, photovoltaic cells, tidal
energy generators), generators, and the like. The power source may
comprise one or more of such power supplies (e.g., batteries and a
photovoltaic cell).
[0189] In some embodiment, a device may comprise a power management
system, which may be any suitable controller device, such as a
computer or microprocessor, and may contain logic circuitry which
decides how to route the power streams. The power management system
may be able to direct the energy provided from a power source or
the energy produced by the electrochemical device to the end point,
for example, another device. It is also possible to feed electrical
energy to a power source and/or to consumer devices (e.g., cellular
phone, television).
[0190] In some cases, electrochemical devices may comprise a
separating membrane. The separating membranes or separators for the
photoelectrochemical device may be made of suitable material, for
example, a plastic film. Non-limiting examples of plastic films
included include polyamide, polyolefin resins, polyester resins,
polyurethane resin, or acrylic resin and containing lithium
carbonate, or potassium hydroxide, or sodium-potassium peroxide
dispersed therein.
[0191] A container may be any receptacle, such as a carton, can, or
jar, in which components of an electrochemical device may be held
or carried. A container may be fabricated using any known
techniques or materials, as will be known to those of ordinary
skill in the art. For example, in some instances, the container may
be fabricated from gas, polymer, metal, and the like. The container
may have any shape or size, providing it can contain the components
of the electrochemical device. Components of the electrochemical
device may be mounted in the container. That is, a component (e.g.,
an electrode) may be associated with the container such that it is
immobilized with respect to the container, and in some cases, is
supported by the container. A component may be mounted to the
container using any common method and/or material known to those
skilled in the art (e.g., screws, wires, adhesive, etc). The
component may or might not physically contact the container. In
some cases, an electrode may be mounted in the container such that
the electrode is not in contact with the container, but is mounted
in the container such that it is suspended in the container.
[0192] Where the catalytic material, photoanode, and/or electrode
of the invention is used in connection with an electrochemical
device such as a fuel cell, any suitable fuels, oxidizers, and/or
reactant product may be provided to and/or produced by
electrochemical devices. In some embodiments, the
photoelectrochemical device may produce a fuel (e.g., hydrogen). In
a particular embodiment, in addition to oxygen, hydrogen is
produced by the photoelectrochemical device. In other embodiments,
the photoelectrochemical device may produce fuel such as a
hydrocarbon (e.g., methane, ethane, propane) and/or a product from
the reduction of carbon monoxide or carbon dioxide. Other fuels and
oxidants can be used to produce oxygen and a second product, as
will be known to those of ordinary skill in the art.
[0193] Protons may be provided to the devices described herein
using any suitable proton source, as will be known to those of
ordinary skill in the art. The proton source may be any molecule or
chemical which is capable of supplying a proton, for example,
H.sup.+, H.sub.30.sup.+, NH.sub.4.sup.+, etc. A hydrogen source
(e.g., for use as a fuel in a fuel cell) may be any substance,
compound, or solution including hydrogen such as, for example,
hydrogen gas, a hydrogen rich gas, natural gas, etc. The oxygen gas
provided to a device may or may not be substantially pure. For
example, in some cases, any substance, compound or solution
including oxygen may be provided, such as, an oxygen rich gas, air,
etc.
[0194] The fuel may be supplied to and/or removed from a device
and/or system using a fuel transport device. The nature of the fuel
delivery may vary with the type of fuel and/or the type of device.
For example, solid, liquid, and gaseous fuels may all be introduced
in different manners. The fuel transport device may be a gas or
liquid conduit such as a pipe or hose which delivers or removes
fuel, such as hydrogen gas or methane, from the electrochemical
device and/or from the fuel storage device. Alternatively, the
device may comprise a movable gas or liquid storage container, such
as a gas or liquid tank, which may be physically removed from the
device after the container is filled with fuel. If the device
comprises a container, then the device may be used as both the fuel
storage device while it remains attached to the electrochemical
device, and as a container to remove fuel from the
photoelectrochemical device. Those of ordinary skill in the art
will be aware of systems, methods, and/or techniques for supplying
and/or removing fuel from a device or system.
[0195] A variety of definitions are now provided which may aid in
understanding various aspects of the invention.
[0196] In general, the term "aliphatic," as used herein, includes
both saturated and unsaturated, straight chain (i.e., unbranched)
or branched aliphatic hydrocarbons, which are optionally
substituted with one or more functional groups, as defined below.
As will be appreciated by one of ordinary skill in the art,
"aliphatic" is intended herein to include, but is not limited to,
alkyl, alkenyl, alkynyl moieties. Illustrative aliphatic groups
thus include, but are not limited to, for example, methyl, ethyl,
n-propyl, isopropyl, allyl, n-butyl, sec-butyl, isobutyl,
tert-butyl, n-pentyl, sec-pentyl, isopentyl, tert-pentyl, n-hexyl,
sec-hexyl, moieties and the like, which again, may bear one or more
substituents, as previously defined.
[0197] As used herein, the term "alkyl" is given its ordinary
meaning in the art and may include saturated aliphatic groups,
including straight-chain alkyl groups, branched-chain alkyl groups,
cycloalkyl (alicyclic) groups, alkyl substituted cycloalkyl groups,
and cycloalkyl substituted alkyl groups. An analogous convention
applies to other generic terms such as "alkenyl," "alkynyl," and
the like. Furthermore, as used herein, the terms "alkyl,"
"alkenyl," "alkynyl," and the like encompass both substituted and
unsubstituted groups.
[0198] In some embodiments, a straight chain or branched chain
alkyl may have 30 or fewer carbon atoms in its backbone, and, in
some cases, 20 or fewer. In some embodiments, a straight chain or
branched chain alkyl has 12 or fewer carbon atoms in its backbone
(e.g., C.sub.1-C.sub.12 for straight chain, C.sub.3-C.sub.12 for
branched chain), has 6 or fewer, or has 4 or fewer. Likewise,
cycloalkyls have from 3-10 carbon atoms in their ring structure or
from 5, 6 or 7 carbons in the ring structure. Examples of alkyl
groups include, but are not limited to, methyl, ethyl, propyl,
isopropyl, cyclopropyl, butyl, isobutyl, tert-butyl, cyclobutyl,
hexyl, cyclochexyl, and the like. In some cases, the alkyl group
might not be cyclic. Examples of non-cyclic alkyl include, but are
not limited to, methyl, ethyl, propyl, isopropyl, n-butyl,
tert-butyl, n-pentyl, neopentyl, n-hexyl, n-heptyl, n-octyl,
n-decyl, n-undecyl, and dodecyl.
[0199] The terms "alkenyl" and "alkynyl" refer to unsaturated
aliphatic groups analogous in length and possible substitution to
the alkyls described above, but that contain at least one double or
triple bond respectively. Alkenyl groups include, but are not
limited to, for example, ethenyl, propenyl, butenyl,
1-methyl-2-buten-1-yl, and the like. Non-limiting examples of
alkynyl groups include ethynyl, 2-propynyl (propargyl), 1-propynyl,
and the like.
[0200] The terms "heteroalkenyl" and "heteroalkynyl" refer to
unsaturated aliphatic groups analogous in length and possible
substitution to the heteroalkyls described above, but that contain
at least one double or triple bond respectively.
[0201] As used herein, the term "halogen" or "halide" designates
--F, --Cl, --Br, or I.
[0202] The term "aryl" refers to aromatic carbocyclic groups,
optionally substituted, having a single ring (e.g., phenyl),
multiple rings (e.g., biphenyl), or multiple fused rings in which
at least one is aromatic (e.g., 1,2,3,4-tetrahydronaphthyl,
naphthyl, anthryl, or phenanthryl). That is, at least one ring may
have a conjugated Pi electron system, while other, adjoining rings
can be cycloalkyls, cycloalkenyls, cycloalkynyls, aryls, and/or
heterocycyls. The aryl group may be optionally substituted, as
described herein. "Carbocyclic aryl groups" refer to aryl groups
wherein the ring atoms on the aromatic ring are carbon atoms.
Carbocyclic aryl groups include monocyclic carbocyclic aryl groups
and polycyclic or fused compounds (e.g., two or more adjacent ring
atoms are common to two adjoining rings) such as naphthyl group.
Non-limiting examples of aryl groups include phenyl, naphthyl,
tetrahydronaphthyl, indanyl, indenyl and the like.
[0203] The terms "heteroaryl" refers to aryl groups comprising at
least one heteroatom as a ring atom, such as a heterocycle.
Non-limiting examples of heteroaryl groups include pyridyl,
pyrazinyl, pyrimidinyl, pyrrolyl, pyrazolyl, imidazolyl, thiazolyl,
oxazolyl, isooxazolyl, thiadiazolyl, oxadiazolyl, thiophenyl,
furanyl, quinolinyl, isoquinolinyl, and the like.
[0204] It will also be appreciated that aryl and heteroaryl
moieties, as defined herein, may be attached via an aliphatic,
alicyclic, heteroaliphatic, heteroalicyclic, alkyl or heteroalkyl
moiety and thus also include -(aliphatic)aryl,
-(heteroaliphatic)aryl, -(aliphatic)heteroaryl,
-(heteroaliphatic)heteroaryl, -(alkyl)aryl, -(heteroalkyl)aryl,
-(heteroalkyl)aryl, and -(heteroalkyl)-heteroaryl moieties. Thus,
as used herein, the phrases "aryl or heteroaryl" and "aryl,
heteroaryl, (aliphatic)aryl, -(heteroaliphatic)aryl,
-(aliphatic)heteroaryl, -(heteroaliphatic)heteroaryl, -(alkyl)aryl,
-(heteroalkyl)aryl, -(heteroalkyl)aryl, and
-(heteroalkyl)heteroaryl" are interchangeable.
[0205] Any of the above groups may be optionally substituted. As
used herein, the term "substituted" is contemplated to include all
permissible substituents of organic compounds, "permissible" being
in the context of the chemical rules of valence known to those of
ordinary skill in the art. It will be understood that "substituted"
also includes that the substitution results in a stable compound,
e.g., which does not spontaneously undergo transformation such as
by rearrangement, cyclization, elimination, etc. In some cases,
"substituted" may generally refer to replacement of a hydrogen with
a substituent as described herein. However, "substituted," as used
herein, does not encompass replacement and/or alteration of a key
functional group by which a molecule is identified, e.g., such that
the "substituted" functional group becomes, through substitution, a
different functional group. For example, a "substituted phenyl
group" must still comprise the phenyl moiety and can not be
modified by substitution, in this definition, to become, e.g., a
pyridine ring. In a broad aspect, the permissible substituents
include acyclic and cyclic, branched and unbranched, carbocyclic
and heterocyclic, aromatic and nonaromatic substituents of organic
compounds. Illustrative substituents include, for example, those
described herein. The permissible substituents can be one or more
and the same or different for appropriate organic compounds. For
purposes of this invention, the heteroatoms such as nitrogen may
have hydrogen substituents and/or any permissible substituents of
organic compounds described herein which satisfy the valencies of
the heteroatoms.
[0206] Examples of substituents include, but are not limited to,
aliphatic, alicyclic, heteroaliphatic, heteroalicyclic, halogen,
azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl,
alkoxyl, amino, nitro, sulfhydryl, imino, amido, phosphonate,
phosphinate, carbonyl, carboxyl, silyl, ether, alkylthio,
heteroalkylthio, heteroarylthio, sulfonyl, sulfonamido, ketone,
aldehyde, ester, heterocyclyl, aromatic or heteroaromatic moieties,
--CF.sub.3, --CN, aryl, aryloxy, perhaloalkoxy, aralkoxy,
heteroaryl, heteroaryloxy, heteroarylalkyl, heteroaralkoxy, azido,
amino, halide, alkylthio, oxo, acylalkyl, carboxy esters,
-carboxamido, acyloxy, aminoalkyl, alkylaminoaryl, alkylaryl,
alkylaminoalkyl, alkoxyaryl, arylamino, aralkylamino,
alkylsulfonyl, -carboxamidoalkylaryl, -carboxamidoaryl,
hydroxyalkyl, haloalkyl, alkylaminoalkylcarboxy-,
aminocarboxamidoalkyl-, cyano, alkoxyalkyl, perhaloalkyl,
arylalkyloxyalkyl, (e.g., SO.sub.4(R').sub.2), a phosphate (e.g.,
PO.sub.4(R').sub.3), a silane (e.g., Si(R').sub.4), a urethane
(e.g., R'O(CO)NHR'), and the like. Additionally, the substituents
may be selected from F, Cl, Br, I, --OH, --NO.sub.2, --CN, --NCO,
--CF.sub.3, --CH.sub.2CF.sub.3, --CHCl.sub.2, --CH.sub.2OR.sub.x,
--CH.sub.2CH.sub.2OR.sub.x, --CH.sub.2N(R.sub.x).sub.2,
--CH.sub.2SO.sub.2CH.sub.3, --C(O)R.sub.x, --CO.sub.2(R.sub.x),
--CON(R.sub.x).sub.2, --OC(O)R.sub.x, --C(O)OC(O)R.sub.x,
--OCO.sub.2R.sub.x, --OCON(R.sub.x).sub.2, --N(R.sub.x).sub.2,
--S(O).sub.2R.sub.x, --OCO.sub.2R.sub.x, --NR.sub.X(CO)R.sub.X,
--NR.sub.x(CO)N(R.sub.x).sub.2, wherein each occurrence of R.sub.x
independently includes, but is not limited to, H, aliphatic,
alicyclic, heteroaliphatic, heteroalicyclic, aryl, heteroaryl,
alkylaryl, or alkylheteroaryl, wherein any of the aliphatic,
alicyclic, heteroaliphatic, heteroalicyclic, alkylaryl, or
alkylheteroaryl substituents described above and herein may be
substituted or unsubstituted, branched or unbranched, cyclic or
acyclic, and wherein any of the aryl or heteroaryl substituents
described above and herein may be substituted or unsubstituted.
[0207] The following references are herein incorporated by
reference: U.S. Provisional Patent Application Ser. No. 61/103,898,
filed Oct. 8, 2008, entitled "Catalyst Compositions and Photoanodes
for Photosynthesis Replication and Other Photoelectrochemical
Techniques," by Nocera, et al., U.S. Provisional Patent Application
Ser. No. 61/218,006, filed Jun. 17, 2009, entitled "Catalytic
Materials, Photoanodes, and Systems for Water Electrolysis and
Other Electrochemical Techniques," by Nocera, et al., U.S.
Provisional Patent Application Ser. No. 61/103,905, filed Oct. 8,
2008, entitled "Catalyst Compositions and Photoanodes for
Photosynthesis Replication and Other Photoelectrochemical
Techniques," by Nocera, et al., U.S. Provisional Patent Application
Ser. No. 61/187,995, filed Jun. 17, 2009, entitled "Catalytic
Materials, Photoanodes, and Systems for Water Electrolysis and
Other Electrochemical Techniques," by Nocera, et al., U.S.
Provisional Patent Application Ser. No. 61/073,701, filed Jun. 18,
2008, entitled "Catalyst Compositions and Electrodes for
Photosynthesis Replication and Other Electrochemical Techniques,"
by Nocera, et al., U.S. Provisional Patent Application Ser. No.
61/084,948, filed Jul. 30, 2008, entitled "Catalyst Compositions
and Electrodes for Photosynthesis Replication and Other
Electrochemical Techniques," by Nocera, et al., U.S. Provisional
Patent Application Ser. No. 61/103,879, filed Oct. 8, 2008,
entitled "Catalyst Compositions and Electrodes for Photosynthesis
Replication and Other Electrochemical Techniques," by Nocera, et
al., U.S. Provisional Patent Application Ser. No. 61/146,484, filed
Jan. 22, 2009, entitled "Catalyst Compositions and Electrodes for
Photosynthesis Replication and Other Electrochemical Techniques,"
by Nocera, et al., U.S. Provisional Patent Application Ser. No.
61/179,581, filed May 19, 2009, entitled "Catalyst Compositions and
Electrodes for Photosynthesis Replication and Other Electrochemical
Techniques," by Nocera, et al., and U.S. patent application Ser.
No. 12/486,694, filed Jun. 17, 2009, entitled "Catalytic Materials,
Electrodes, and Systems for Water Electrolysis and Other
Electrochemical Techniques."
[0208] The following examples are intended to illustrate certain
embodiments of the present invention, but are not to be construed
as limiting and do not exemplify the full scope of the
invention.
Example 1
[0209] The following example describes non-limiting examples of
methods for deposition of a catalytic material comprising cobalt
(Co-OEC) onto a photoactive material (e.g., a semiconductor, CdS).
The method comprises, in this embodiment, providing a solution
comprising metal ionic species and anionic species, providing a
photoactive electrode, and causing the metal ionic species and the
anionic species to form a catalytic material associated with the
photoactive electrode by application of a voltage (e.g., by an
external power source or by exposure to a light source) to the
photoactive electrode.
[0210] Materials. Cadmium sulfate, thiourea, ammonium acetate,
ammonium hydroxide solution (28% NH.sub.3), cobalt nitrate,
methylphosphonic acid (Aldrich) and fluorine-doped tin oxide (FTO)
coated glass substrate (Solaronix) were used as received.
[0211] CdS Film Preparation. Thin films of CdS were prepared on
FTO-coated glass substrates by the chemical bath deposition
technique. An Erlenmeyer flask containing 100 mL of deionized,
distilled water (ddH.sub.2O) was placed in water bath and heated to
88.degree. C. Two 2.5.times.5 cm FTO-coated glass substrates were
placed in the bottom of the flask with the FTO face up. Cadmium
sulfate (0.5 mM), ammonium acetate (10 mM), and ammonium hydroxide
(0.4 M) were then added to the flask. After 10 minutes, four
aliquots of thiourea were added to the flask to a final
concentration of 0.975 mM with 10 minutes between aliquot
additions. Ten minutes after the addition of the final thiourea
aliquot, the substrates were removed from the bath and rinsed with
ddH.sub.2O. The entire procedure was repeated four times to yield
substantially thick CdS films.
[0212] Electrodeposition of Co-OEC on CdS films. The CdS film
prepared on FTO-coated glass substrate (e.g., a photoactive
electrode) was connected to a potentiostat (CH Instruments 760C)
via an alligator clip and immersed in water containing 2 mM cobalt
nitrate and 0.1 M methylphosphonic acid (pH 8.5) (e.g., a solution
comprising metal ionic species and anionic species). An Ag/AgCl
reference electrode (BASi) and platinum wire counter electrode were
connected to the potentiostat and immersed in the solution. The CdS
electrode was biased at 1.5 V vs. Ag/AgCl for one hour (application
of a voltage to the photoactive electrode using an external power
source). The CdS electrode was then removed from solution and
rinsed with deionized water. FIG. 14 shows a scanning electron
micrograph (SEM) of the resulting electrode. The dark material on
top is the resulting Co-OEC catalytic material (e.g., catalytic
material associated with the photoactive electrode comprising metal
ionic species and anionic species) that has been electrodeposited
onto the CdS semiconductor underneath. As shown in FIG. 14, a large
portion of the Co-OEC overlayer has flaked away during drying of
the electrode for SEM, revealing the CdS film underneath. Electron
dispersive x-ray (EDX) analysis confirms the presence of Co, Cd, S,
and P.
[0213] Photodeposition of Co-OEC on CdS. The CdS film prepared on
FTO-coated glass substrate was connected to a potentiostat (CH
Instruments 760C) via an alligator clip and immersed in water
containing 2 mM cobalt nitrate and 0.1 M methylphosphonic acid (pH
8.5). An Ag/AgCl reference electrode (BASi) and platinum wire
counter electrode were connected to the potentiostat and immersed
in the solution. The electrode was held at 0.5 V vs. Ag/AgCl and
illuminated for one hour with light (e.g., application of a voltage
using an external light source) from a 300 W Xe arc lamp equipped
with a 495 nm long pass filter (.lamda.>495 nm) and a 0.8 AU
neutral density filter. The electrode was then removed from
solution and rinsed with ddH.sub.2O. FIG. 15 shows an SEM of the
resulting electrode, both for regions that were exposed to light
(FIG. 15A) and maintained in the dark (FIG. 15B). The illuminated
portion of the film (FIG. 15A) exhibits a cracked morphology, which
may be attribute to drying of the resulting Co-OEC overlayer
coating. The region of film that was not exposed to light did not
exhibit this cracking morphology and, instead, shows a uniform CdS
film. EDX analysis confirms the presence of Co, Cd, S, and P for
the film exposed to light, while films kept in the dark lacked
measurable diffraction peaks for Co and P.
Example 2
[0214] The following prophetic example describes methods for
formation of a Co-OEC functionalized photoanode and
characterization of the enhanced photoassisted water oxidation
reaction rate.
[0215] Nanostructured iron oxide semiconductor
(.alpha.-Fe.sub.2O.sub.3) may be grown on electrically conductive
FTO-coated glass substrates by the atmospheric chemical vapor
deposition (CVD) technique as described previously (e.g., See Kay
et al., J. Am. Chem. Soc, 2006, 128, 15714-15721). The substrate
may then be attached to a potentiostat as the working electrode and
immersed in a solution of 0.1 M KPi (pH 7) and 0.5 mM
Co(NO.sub.3).sub.2. The electrode may then be biased at 1.1 V vs.
Ag/AgCl reference for the electrodeposition of the Co-OEC catalyst
as described in Example 1 and as done previously on ITO electrodes
(e.g., see Kanan et al., Science, 2008, 321, 1072). The resulting
.alpha.-Fe.sub.2O.sub.3/Co-OEC electrode may then serve as a
photoanode.
[0216] The .alpha.-Fe.sub.2O.sub.3/Co-OEC photoanode may exhibit an
enhanced rate for photoassisted water oxidation, compared to the
.alpha.-Fe.sub.2O.sub.3 photoanode alone. Photoanodes may be
immersed in 1 M NaOH aqueous electrolyte, along with an Ag/AgCl
reference and Pt wire counter electrode. The photoanode may then be
illuminated with AM 1.5 simulated solar irradiation and a bias
applied and swept from -0.2 to 0.6 V vs. Ag/AgCl reference. In this
experiment, the photocurrent onset potential is the applied bias
potential at which the photoanode exhibits a measurable anodic
(oxidative) current and is familiar to those skilled in the art.
The onset potential may be observed to shift to less positive
values for the .alpha.-Fe.sub.2O.sub.3/Co-OEC photoanode compared
to .alpha.-Fe.sub.2O.sub.3 alone, owing to the catalytic effect of
the Co-OEC water oxidation catalyst. Additionally, the overall
magnitude of the anodic photocurrent may be larger for the
.alpha.-Fe.sub.2O.sub.3/Co-OEC photoanode compared to
.alpha.-Fe.sub.2O.sub.3 alone. An incident photon-to-current
efficiency (IPCE) measurement may then be carried out, in which the
photon-to-current conversion efficiency is measured as a function
of excitation wavelength. Devices and methods for measuring the
IPCE will be known to those of ordinary skilled in the art. The
IPCE may be shown to increase by some value (e.g., at least about
50%, at least about 100%, at least about 200%, etc.) as a function
of excitation wavelength for the .alpha.-Fe.sub.2O.sub.3/Co-OEC
photoanode compared to .alpha.-Fe.sub.2O.sub.3 alone.
Example 3
[0217] The following prophetic example describes non-limiting
methods for water oxidation, O.sub.2 gas evolution, and detection
using Co-OEC functionalized photoanodes.
[0218] A Co-OEC functionalized photoanode (e.g., as prepared
according to Example 1 or 2, or otherwise as described herein) may
be attached to a potentiostat and serves as the working electrode
for this experiment. The working electrode may be immersed in a
buffered aqueous solution (e.g., 1 M KPi, pH 7) along with a
reference electrode (e.g., Ag/AgCl) and an auxiliary electrode
(e.g., Pt wire). The entire experiment may then be sealed from the
environment (e.g., using rubber septa in ground glass joints
attached to the electrochemical cell housing) and purged of air by
bubbling with He gas (or other inert gas, e.g., N.sub.2, Ar). The
photoanode may then be biased at some potential relative to the
reference electrode (e.g., 0<E<1.5 V). The photoanode may
then be illuminated with light (e.g., from a Xe arc lamp that may
or may not be filtered to produce solar AM 1.5 radiation) through a
transparent (e.g., quartz) window in the reaction vessel. The light
may or may not pass through the back side of the photoanode, such
that the semiconductor is illuminated first prior to illumination
of the Co-OEC film. Anodic photocurrents may be measured with the
potentiostat. Bubbles may or may not be visible at the photoanode.
Gaseous products of the photoelectrochemical reaction may then be
analyzed by withdrawing samples of the reaction headspace using a
gas-tight syringe and injecting the sample into a gas
chromatograph/mass spectrometer. The detection of a peak with
m/z=32 should indicate the production of O.sub.2. This may be
confirmed by operation of the photoelectrochemical cell in water
containing some fraction of H.sub.2.sup.18O and with the detection
of peaks with m/z=.sub.34 (.sup.18,16O.sub.2) and m/z=36
(18,18O.sub.2). Gaseous oxygen may also be detected and quantified
using a phosphorescence-based O.sub.2 sensing probe (e.g., FOXY,
Ocean Optics).
[0219] Control experiments may also be performed. The same
semiconductor photoanode minus the Co-OEC catalytic material may be
tested for photoelectrochemical water oxidation, the photocurrents
measured, and the O.sub.2 quantified with similar methods. The
catalytic effect of Co-OEC should yield higher photocurrents and
larger amounts of O.sub.2 produced per unit time for the
Co-OEC/semiconductor photoanode compared to the semiconductor
alone.
Example 4
[0220] The following prophetic example describes non-limiting
methods for fabricating a Co-OEC functionalized tandem
photoelectrochemical cell (e.g., n-CdS/n-TiO.sub.2).
[0221] A tandem photoelectrochemical cell composed of
n-CdS/n-TiO.sub.2 may be fabricated as previously described (e.g.,
See Nakato et al., Nature, 1982, 295, 312-313). The cell is
composed of a sandwich of one n-TiO.sub.2 wafer and one n-CdS
wafer. The sandwich houses a solution of 1 M NaOH, 1 M Na.sub.2S,
and 1.5 gram atom 1.sup.-1 S, which is maintained by epoxy sealant
on the edges of the wafers. The outer face of the CdS wafer is
attached to a copper wire via an indium metal contact. The outer
face of the TiO.sub.2 wafer is exposed to solution as the surface
for water oxidation.
[0222] The tandem PEC thus described may then be functionalized
with Co-OEC using a photodeposition procedure, for example, as
described in Example 1. In particular, the n-CdS/n-TiO.sub.2
photoelectrode may be immersed in the anode compartment of a
two-compartment photoelectrochemical cell containing an aqueous
solution of 0.5 mM Co(NO.sub.3).sub.2 and 0.1 M KPi (pH 7). The
copper wire from the CdS wafer may be attached to a platinum gauze
electrode immersed in the cathode compartment of the cell, which
may contain 0.1 M KPi (pH 7). The anode and cathode compartments
may be separated by a glass frit or a membrane. Excitation of the
anode with light of wavelength less than or equal to 400 nm may
then effect the deposition of the Co-OEC catalyst on the outer
surface of the TiO.sub.2 wafer. The photolysis time may correlate
with the thickness of the Co-OEC film. The photocurrent may or may
not be observed to rise with photolysis time.
[0223] The Co-OEC functionalized tandem PEC may then be tested for
enhanced photoassisted water oxidation as described in Example 2
and for O.sub.2 evolution, for example, as described in Example
3.
Example 5
[0224] The following prophetic examples described a non-limiting
method for fabricating and testing a Co-OEC functionalized tandem
photovoltaic-photoelectrochemical device (e.g.,
p,n-GaAs/p-GaInP.sub.2).
[0225] A device may be fabricated in which a p-GaInP.sub.2
photocathode is grown on top of a p,n-GaAs photovoltaic, as
previously described (e.g., See Khaselev et al., Science, 1998,
280, 425-427). The device may be electrically connected to
conductive anode support (e.g., Pt, FTO, Ni). The device may be
immersed in electrolyte within a two-compartment cell. The anode
compartment may contain the conductive anode support, 0.1 M KPi
buffer (pH 7), and 0.5 mM Co(NO.sub.3).sub.2. The cathode
compartment may contain the p,n-GaAs/p-GaInP.sub.2 device and 0.1 M
KPi buffer (pH 7). The two compartments may be separated by a glass
frit. Illumination of the p,n-GaAs/p-GaInP.sub.2 device may produce
a current and initiate deposition of the Co-OEC catalyst on the
anode. The photocurrents may be observed to increase with Co-OEC
layer thickness due to the catalytic effect of Co-OEC. The Co-OEC
functionalized tandem photovoltaic-photoelectrochemical device may
then be tested for O.sub.2 evolution, for example, as described in
Example 3 with the exception that in this case the cathode is
illuminated with light, rather than the anode.
Example 6
[0226] The following prophetic example describes a non-limiting
method for fabricating and testing of a Co-OEC functionalized
dye-sensitized photoanode. In this embodiment, the method comprises
providing a solution comprising metal ionic species and anionic
species, providing a photoactive electrode comprising a photoactive
composition and a photosensitizing agent, and causing the metal
ionic species and the anionic species to form a composition
associated with the photoactive electrode by application of a
voltage to the photoactive electrode.
[0227] Mesoporous titanium dioxide (TiO.sub.2) films (e.g.,
photoactive composition) may be prepared on a conducting glass
substrate (e.g., FTO glass) and RuL.sub.3
(L=2,2'-bipyridine-4,4'-dicarboxylic acid) dye (e.g.,
photosensitizing agent) may be adsorbed to the TiO.sub.2 film as
described previously (e.g., see O'Regan et al., J. Phys. Chem.,
1990, 94, 8720-8726). Thus formed, the dye-sensitized photoanode
(e.g., photoactive electrode) may then be attached to the working
electrode of a potentiostat and immersed in a two-compartment cell.
The anode compartment may contain the dye-sensitized photoanode, an
Ag/AgCl reference electrode, 0.5 mM Co(NO.sub.3).sub.2, and 0.1 M
KPi buffer (pH 7) (e.g., solution comprising metal ionic species
and anionic species). The cathode compartment may contain hydrogen
evolution electrode (e.g., a Pt-wire). The Co-OEC catalyst may then
be electro- or photodeposited at the anode (e.g., composition
associated with the photoactive electrode), for example, as
described in Example 1 (e.g., by application of voltage to the
photoactive electrode, via an external power source or via a light
source). The device may then be tested for enhanced photoassisted
water oxidation, (e.g., as described in Example 2) and
photochemical O.sub.2 evolution (e.g., as described in Example
3).
Example 7
[0228] The following prophetic example describes a non-limiting
example of a photoanode comprising a band-gap engineered titanium
dioxide semiconductor.
[0229] Titanium dioxide (TiO.sub.2) has a band-gap of 3.0 eV, thus
limiting its absorption to UV light (<2% of the solar spectrum).
Significant research has focused on the engineering of this metal
oxide semiconductor to lower its band-gap and allow for the
absorption of visible light. For example, TiO.sub.2 has been doped
with nitrogen, carbon, and sulfur atoms, to raise the energy of the
valence band (e.g., see Asahi et al., Science, 2001, 293, 269-271).
In most embodiments, absorbed red photons do not contribute to
substantial photocurrents, thus the overall efficiency of these
materials for solar powered water oxidation remains low. The
mechanism of water oxidation at TiO.sub.2 surfaces may involve the
formation of 1-electron oxidized, high energy intermediates (i.e.
hydroxyl radicals, .OH), thus owing to the inability of TiO.sub.2
to support the 4-electron hole (4 h.sup.+) catalysis of water
oxidation. Upon doping, the valence band electron-holes generally
lack the oxidative power to produce .OH (FIG. 16), thus shutting
down water oxidation chemistry. In contrast, the Co-OEC catalyst
may be found to operate near the 4-electron/hole potential for
water oxidation with low overpotential. FIG. 16 shows the band edge
positions of various forms of TiO.sub.2 along with the standard
reduction potential of the hydroxyl radical and the potential for
operation of the Co-OECcatalyst. Photogenerated electron-holes in
band-gap engineered TiO.sub.2 may thus possess sufficient energy to
oxidize any Co-OEC adsorbed at the surface. Thus, deposition of a
thin film of Co-OEC on band-gap engineered TiO.sub.2 may engender
photochemical water oxidation activity in this otherwise inactive
material.
[0230] Thin films of nitrogen-doped TiO.sub.2 may be prepared by
sputtering, sol-gel, and solution anodization of Ti. Co-OEC films
may then be formed by electro- and photodeposition methods, as
described herein. Incident photon-to-current efficiency (IPCE) may
be measured (e.g., as described herein) to test for enhanced
photochemical response of the doped TiO.sub.2 upon adsorption of
the Co. Undoped TiO.sub.2 films, which show no IPCE response for
visible wavelength excitation, may be used as a control.
Example 8
[0231] The following prophetic example describes the stabilization
of soft photoactive semiconductors towards photocorrosion in
aqueous media.
[0232] Electron-holes photogenerated in the valence band of soft
n-type semiconductors may diffuse to the semiconductor-electrolyte
interface where they initiate corrosion of the material. The
formation of a hole-tunneling layer on the surface of the soft
n-type semiconductor may prevent oxidation of the semiconductor
lattice and photocorrosion. The Co-OEC catalyst may then be
deposited on the hole-tunneling layer. Tunneling of the
electron-hole out of the semiconductor valence band, through the
hole-tunneling layer, and into surface-adsorbed Co-OEC may provide
a mechanism for water oxidation that circumvents photoanode
corrosion. The hole tunneling layer material may be another
semiconductor material (e.g., TiO.sub.2) and may be chosen such
that the valence band of the hole tunneling layer is more positive
in potential energy with respect to the valence band of underlying
soft, photoactive semiconductor (e.g., CdS).
[0233] CdS films may be prepared by the chemical bath technique
described in Example 1. TiO.sub.2 films may then be prepared over
the CdS film by standard sputtering, sol-gel, or electrodeposition
techniques. Co-OEC catalyst films may then be photo- and/or
electrodeposited from aqueous solutions of Co(II) ions as described
in Example 1. The TiO.sub.2 films may be prepared such that they
are thin enough (e.g., .about.10 nm thick) to allow for efficient
tunneling of the electron hole from the underlying CdS layer into
the Co-OEC film. Electrode photostability may be characterized as a
function of light intensity, catalyst deposition conditions,
thickness, and/or substrate morphologies. Materials that are
photoactive toward oxygen evolution and produce stable
photocurrents on long timescales may be further optimized.
Example 9
[0234] The following prophetic example describes non-limiting
designs of photoelectrochemical cells.
[0235] A single band gap device (FIG. 17A) contains a single
light-absorbing semiconductor material. The conduction band
electrons and valence band holes produced upon excitation have
suitable energy for proton reduction and water oxidation,
respectively. The valence band holes are transported into the
adsorbed CoPi catalyst, while the electrons may be directed to the
cathode catalyst either coated onto a separate electrode (as
shown), or deposited onto the back of the ohmic contact of the
semiconductor. The first method allows for collection of electrons
and generation of hydrogen at a remote site, while the second
method affords hydrogen evolution over the surface area of the
photoanode. A range of cathode catalysts may be employed, including
thin monolayers of Pt (e.g., deposited from solutions of
H.sub.2PtCl.sub.6) or Pt on C and alloys of Cu, Mo, and Ni.
Materials may be selected based on overall device performance and
cost.
[0236] If the conduction band electrons do not have sufficient
energy for proton reduction, a bias voltage may be required to
perform the water splitting reaction. In FIG. 17B, this voltage is
supplied by a p:n-junction PV stacked in series with the photoanode
semiconductor. In this tandem configuration, blue photons are
absorbed by the photoanode and red photons are transmitted to the
PV. The current collected by the ohmic contact may then be
channeled to the cathode electrode for hydrogen evolution.
[0237] FIG. 17C depicts another type of tandem PEC, in which the PV
has been replaced by a p-type semiconductor photocathode.
Electron-holes are directed to the semiconductor-electrolyte
interface for n-type semiconductors, and, similarly, electrons are
direct to the surface of p-type semiconductors where they may be
collected by an adsorbed layer of cathode catalyst for hydrogen
evolution. Photocathodes may be comprised of p-CdS, p-Si, and
p-Fe.sub.2O.sub.3, among others.
[0238] In some cases, PEC modules may also be produced on
dimensions similar to that for commercial PVs (.about.1 m.sup.2).
In a non-limiting design, as shown in FIG. 17D, a PEC system is
housed within glass or plexiglass domes 218 that focus the light
onto a semiconducting material coated with CoPi (e.g., 212) below.
Oxygen (white circles) may evolve from the semiconductor/CoPi 212,
while hydrogen (black circles) may evolve from a stainless steel or
conducting plastic cathode (e.g., 208) coated with a cathode
catalyst. The gases may be collected at the top of the anode and
cathode compartments, which can be separated by gas-impermeable,
ion-conductive membranes (e.g., 210). The device may comprise one
or more electrolytes (e.g., 214, 216, generally aqueous) in the
anode and cathode compartments. This device may require that the
cost per unit area of membrane is lower than for the
semiconductor/CoPi photoanode.
[0239] Alternatively, the membrane may be incorporated into the PEC
semiconductor. FIG. 17E shows a tandem PEC in which holes have been
drilled and filled with the membrane material. Oxygen may be
generated from the photoanode and released into the atmosphere,
while hydrogen may be generated at the photocathode underneath and
trapped at the apex of the device. The gases may also be produced
by collecting the electrodes the photocathode and concentrating
them at a separate electrode compartment as shown in FIG. 17F. An
ion-permeable membrane may provide contact between the anode and
cathode compartment while keeping the hydrogen and oxygen gases
separate.
[0240] While several embodiments of the present invention have been
described and illustrated herein, those of ordinary skill in the
art will readily envision a variety of other means and/or
structures for performing the functions and/or obtaining the
results and/or one or more of the advantages described herein, and
each of such variations and/or modifications is deemed to be within
the scope of the present invention. More generally, those skilled
in the art will readily appreciate that all parameters, dimensions,
materials, and configurations described herein are meant to be
exemplary and that the actual parameters, dimensions, materials,
and/or configurations will depend upon the specific application or
applications for which the teachings of the present invention
is/are used.
[0241] Those skilled in the art will recognize, or be able to
ascertain using no more than routine experimentation, many
equivalents to the specific embodiments of the invention described
herein. It is, therefore, to be understood that the foregoing
embodiments are presented by way of example only and that, within
the scope of the appended claims and equivalents thereto, the
invention may be practiced otherwise than as specifically described
and claimed. The present invention is directed to each individual
feature, system, article, material, kit, and/or method described
herein. In addition, any combination of two or more such features,
systems, articles, materials, kits, and/or methods, if such
features, systems, articles, materials, kits, and/or methods are
not mutually inconsistent, is included within the scope of the
present invention.
[0242] The indefinite articles "a" and "an," as used herein in the
specification and in the claims, unless clearly indicated to the
contrary, should be understood to mean "at least one."
[0243] The phrase "and/or," as used herein in the specification and
in the claims, should be understood to mean "either or both" of the
elements so conjoined, i.e., elements that are conjunctively
present in some cases and disjunctively present in other cases.
Other elements may optionally be present other than the elements
specifically identified by the "and/or" clause, whether related or
unrelated to those elements specifically identified unless clearly
indicated to the contrary. Thus, as a non-limiting example, a
reference to "A and/or B," when used in conjunction with open-ended
language such as "comprising" can refer, in one embodiment, to A
without B (optionally including elements other than B); in another
embodiment, to B without A (optionally including elements other
than A); in yet another embodiment, to both A and B (optionally
including other elements); etc.
[0244] As used herein in the specification and in the claims, "or"
should be understood to have the same meaning as "and/or" as
defined above. For example, when separating items in a list, "or"
or "and/or" shall be interpreted as being inclusive, i.e., the
inclusion of at least one, but also including more than one, of a
number or list of elements, and, optionally, additional unlisted
items. Only terms clearly indicated to the contrary, such as "only
one of" or "exactly one of," or, when used in the claims,
"consisting of," will refer to the inclusion of exactly one element
of a number or list of elements. In general, the term "or" as used
herein shall only be interpreted as indicating exclusive
alternatives (i.e. "one or the other but not both") when preceded
by terms of exclusivity, such as "either," "one of," "only one of,"
or "exactly one of." "Consisting essentially of," when used in the
claims, shall have its ordinary meaning as used in the field of
patent law.
[0245] As used herein in the specification and in the claims, the
phrase "at least one," in reference to a list of one or more
elements, should be understood to mean at least one element
selected from any one or more of the elements in the list of
elements, but not necessarily including at least one of each and
every element specifically listed within the list of elements and
not excluding any combinations of elements in the list of elements.
This definition also allows that elements may optionally be present
other than the elements specifically identified within the list of
elements to which the phrase "at least one" refers, whether related
or unrelated to those elements specifically identified. Thus, as a
non-limiting example, "at least one of A and B" (or, equivalently,
"at least one of A or B," or, equivalently "at least one of A
and/or B") can refer, in one embodiment, to at least one,
optionally including more than one, A, with no B present (and
optionally including elements other than B); in another embodiment,
to at least one, optionally including more than one, B, with no A
present (and optionally including elements other than A); in yet
another embodiment, to at least one, optionally including more than
one, A, and at least one, optionally including more than one, B
(and optionally including other elements); etc.
[0246] In the claims, as well as in the specification above, all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving," "holding," and the like are to
be understood to be open-ended, i.e., to mean including but not
limited to. Only the transitional phrases "consisting of" and
"consisting essentially of" shall be closed or semi-closed
transitional phrases, respectively, as set forth in the United
States Patent Office Manual of Patent Examining Procedures, Section
2111.03.
* * * * *