U.S. patent application number 14/739239 was filed with the patent office on 2015-12-24 for highly active mixed-metal catalysts made by pulsed-laser ablation in liquids.
The applicant listed for this patent is CALIFORNIA INSTITUTE OF TECHNOLOGY. Invention is credited to HARRY B. GRAY, BRYAN M. HUNTER, ASTRID M. MULLER, JAY R. WINKLER.
Application Number | 20150368811 14/739239 |
Document ID | / |
Family ID | 54869129 |
Filed Date | 2015-12-24 |
United States Patent
Application |
20150368811 |
Kind Code |
A1 |
GRAY; HARRY B. ; et
al. |
December 24, 2015 |
HIGHLY ACTIVE MIXED-METAL CATALYSTS MADE BY PULSED-LASER ABLATION
IN LIQUIDS
Abstract
The invention is directed to mixed-metal nanocatalysts,
particularly nano-dimensioned layered double-hydroxide nanostacks,
methods of making nanocatalysts using laser ablation techniques,
and the electrochemical devices comprising and using these
nanocatalysts, for example in the electrochemical oxidation of
water oxidation.
Inventors: |
GRAY; HARRY B.; (PASADENA,
CA) ; WINKLER; JAY R.; (PASADENA, CA) ;
MULLER; ASTRID M.; (PASADENA, CA) ; HUNTER; BRYAN
M.; (PASADENA, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CALIFORNIA INSTITUTE OF TECHNOLOGY |
PASADENA |
CA |
US |
|
|
Family ID: |
54869129 |
Appl. No.: |
14/739239 |
Filed: |
June 15, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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62013976 |
Jun 18, 2014 |
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Current U.S.
Class: |
205/630 ;
204/290.01; 204/290.14; 204/290.15; 502/201; 502/5 |
Current CPC
Class: |
B01J 35/0033 20130101;
B01J 2523/00 20130101; B01J 23/755 20130101; B01J 37/349 20130101;
C25B 11/0484 20130101; B01J 23/83 20130101; Y02E 60/366 20130101;
Y02E 60/36 20130101; C25B 1/04 20130101; B01J 2523/00 20130101;
B01J 2523/842 20130101; B01J 2523/847 20130101; B01J 2523/00
20130101; B01J 2523/3706 20130101; B01J 2523/842 20130101; B01J
2523/847 20130101; B01J 2523/00 20130101; B01J 2523/47 20130101;
B01J 2523/842 20130101; B01J 2523/847 20130101 |
International
Class: |
C25B 1/04 20060101
C25B001/04; C25B 11/12 20060101 C25B011/12; C25B 11/04 20060101
C25B011/04 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] This invention was made with government support under
CHE1305124 awarded by the National Science Foundation. The
government has certain rights in the invention.
Claims
1. A layered double hydroxide nanostack comprising a plurality of
nanosheets represented by the general formula: [M.sub.(1-x)M'.sub.x
(OH).sub.2].sup.x+, wherein: M is a metal cation in a formal +2
oxidation state; M' a metal cation in a formal +3 oxidation state;
wherein the nanosheets are associated with or intercalate
[A.sup.m-.sub.x/m], where A is a displaceable anion; m is an
integer; x is a positive number less than 1; the nanostack being
optionally hydrated with a stoichiometric amount or a
non-stoichiometric amount of water; and the nanosheet having at
least one lateral edge dimension in a range of from about 5 nm to
about 100 nm.
2. The layered double hydroxide nanostack of claim 1, wherein M is
at least one of Ba.sup.2+, Be.sup.2+, Ca.sup.2+, Cd.sup.2+,
Cu.sup.2+, Co.sup.2+, Fe.sup.2+, Mg.sup.2+, Mn.sup.2+, Ni.sup.2+,
Sr.sup.2+, and Zn.sup.2+, and M' is at least of Al.sup.3+,
Ce.sup.3+, Co.sup.3+, Cr.sup.3+, Fe.sup.3+, Ga.sup.3+, In.sup.3+,
La.sup.3+, Mn.sup.3+, V.sup.3+, Y.sup.3+, Ce.sup.3+.
3. The layered double hydroxide nanostack of claim 1, wherein A
comprises an organic or inorganic anion, or a combination
thereof.
4. The layered double hydroxide nanostack of claim 3, wherein A
comprises F.sup.-, Cl.sup.-, Br.sup.-, I.sup.-, BF.sub.4.sup.-,
PF.sub.6.sup.-, CO.sub.3.sup.2-, HCO.sub.3.sup.-, CrO.sub.4.sup.2-,
NO.sub.2, NO.sub.3, ONO.sub.2.sup.-, ClO.sub.2.sup.-,
ClO.sub.3.sup.-, ClO.sub.4..sup.-, H.sub.2PO.sub.4.sup.-,
HPO.sub.4.sup.2-, PO.sub.4.sup.3-, IO.sub.3.sup.-, OH.sup.-,
S.sup.2-, SO.sub.3.sup.2-, S.sub.2O.sub.3.sup.2-, SO.sub.4.sup.2-,
WO.sub.4.sup.2-, or a combination thereof.
5. The layered double hydroxide nanostack of claim 3, wherein A
comprises acetate, propionate, lactate, terephthalate, adipate,
succinate, dodecyl sulfonate, p-hydroxybenzoate, and benzoate, or a
combination thereof.
6. The layered double hydroxide nanostack of claim 3, wherein A
comprises a complex anion comprising a transition metal
compound.
7. The layered double hydroxide nanostack of claim 1, wherein M is
or comprises Ni.sup.2+.
8. The layered double hydroxide nanostack of claim 1, wherein M' is
or comprises Fe.sup.3+.
9. The layered double hydroxide nanostack of claim 1, wherein x is
in a range of from 0.05 to 0.95.
10. The layered double hydroxide nanostack of claim 1, wherein A is
or comprises hydroxide.
11. The layered double hydroxide nanostack of claim 1, wherein the
nanosheet has at least one lateral edge dimension in a range of
from about 7 nm to about 22 nm.
12. The layered double hydroxide nanostack of claim 1, wherein at
least one of the nanosheets is further doped with at least one
Lewis acid ion of a transition metal, lanthanide, or actinide metal
ion.
13. The layered double hydroxide nanostack of claim 1, wherein at
least one of the nanosheets is further doped with titanium or
lanthanum.
14. A method of preparing a layered double hydroxide nanostack of
claim 1, comprising subjecting a solid ablation target to an energy
source, the solid ablation target comprising a first metal capable
of oxidizing to a positive oxidation state, the energy source
impinging on the first metal in the presence of an aqueous ablation
solution containing a second metal ion in a positive oxidation
state for a time and energy sufficient to ionize at least a portion
of the first metal to a positive oxidation state.
15. The method of claim 14, wherein the aqueous ablation solution
is substantially free of surfactants.
16. The method of claim 14, wherein the first metal is capable of
achieving a +2 oxidation state, and the second metal is in a +3
oxidation state.
17. The method of claim 14, wherein the first metal is capable of
achieving a +3 oxidation state, and the second metal is in a +2
oxidation state.
18. The method of claim 16, wherein the first metal comprises
beryllium, calcium, cadmium, copper, cobalt, iron, magnesium,
manganese, nickel, strontium, zinc, or an alloy or mixture thereof
and the second metal ion comprises at least one of Al.sup.3+,
Ce.sup.3+, Co.sup.3+, Cr.sup.3+, Fe.sup.3+, Ga.sup.3+, In.sup.3+,
La.sup.3+, Mn.sup.3+, V.sup.3+, and Y.sup.3+.
19. The method of claim 17, wherein the first metal comprises
aluminum, cerium, cobalt, chromium, iron, gallium, indium,
lanthanum, manganese, vanadium, yttrium, or an alloy or mixture
thereof, and the second metal comprises at least one of Be.sup.2+,
Ca.sup.2+, Cd.sup.2+, Cu.sup.2+, Co.sup.2+, Fe.sup.2+, Mg.sup.2+,
Mn.sup.2+, Ni.sup.2+, Sr.sup.2+, and Zn.sup.2+.
20. The method of claim 14, wherein the energy is a laser.
21. The method of claim 20, wherein the laser energy is provided in
pulses
22. The method of claim 21, wherein the laser pulse delivers an
energy in a range of from about 90 mJ/pulse to about 210
mJ/pulse.
23. The method of claim 14, wherein the displaceable ion A of the
layered double hydroxide nanostack comprises a counterion
associated with the second metal ion in the aqueous ablation
solution.
24. The method of claim 23, further comprising exchanging the
displaceable ion A of the layered double hydroxide nanostack with a
different anion.
25. The method of claim 14, wherein the initially-formed layered
double hydroxide nanostack is further sintered.
26. An electrode comprising a coating comprising the layered double
hydroxide nanostack of claim 1.
27. The electrode of claim 26, wherein the electrode comprises
gold, nickel, platinum, or an allotrope of carbon.
28. The electrode of claim 26, wherein the electrode comprises
graphite, graphene, glassy (or vitreous) carbon, diamond, or a
combination thereof.
29. The electrode of claim 27, that exhibits an overpotential for
the oxidation of water to oxygen of less than 300 mV at 10
mA/cm.sup.2 on a flat supporting electrode.
30. An electrochemical cell comprising an electrode of claim
26.
31. A method for oxidizing water comprising applying a potential to
an electrode of claim 26, and passing sufficient current to oxidize
water to form oxygen.
32. The method of claim 31, wherein the potential is within 300 mV
of the thermodynamically determined potential for the oxidation of
water to oxygen at 10 mA/cm.sup.2.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This applications claims priority to U.S. Patent Application
No. 62/013,976, filed Jun. 18, 2014, the contents of which are
incorporated by reference in their entirety herein.
TECHNICAL FIELD
[0003] The invention relates to mixed-metal nanocatalysts,
particularly nano-dimensioned layered double-hydroxide nanostacks,
methods of making nanocatalysts using laser ablation techniques,
and the electrochemical devices comprising and using these
nanocatalysts, for example in the electrochemical oxidation of
water oxidation.
BACKGROUND
[0004] Conversion of solar energy into storable fuels in a clean
and sustainable way will be essential to meet future global energy
demand. Worldwide scalability requires materials to be made from
earth-abundant elements, Splitting water into oxygen and hydrogen
with only sunlight as energy input is seen as a particularly
attractive route. Rut such systems for the production of solar
fuels will require robust, highly active catalysts.
[0005] Most widely used water oxidation catalysts are based on rare
metals such as ruthenium and iridium. First-row transition metal
oxides and hydroxides continue to attract attention because of
their low cost and stability in base. The overpotentials of
earth-abundant catalysts at 10 mA cm.sup.-2 typically range from
350 to 430 mV in pH 14 aqueous electrolytes. Researchers have shown
that hollow spheres of .alpha.-Ni(OH).sub.2 catalyzed water
oxidation in base with an overpotential of 331 mV at 10 mA
cm.sup.-2 on glassy carbon working electrodes. But such
overpotentials are unsuitably high for large scale applicability of
such systems.
SUMMARY
[0006] The present invention is directed to nanocatalysts and novel
methods of making and using such nanocatalysts that overcome some
of the deficiencies of the prior art. Certain embodiments of the
present invention provide nanocatalysts, especially layered double
hydroxide nanostacks comprising a plurality of nanosheets
represented by the general formula: [M.sub.(1-x)M'.sub.x
(OH).sub.2].sup.x+, wherein: M is a metal cation in a formal +2
oxidation state; M' a metal cation in a formal +3 oxidation state;
wherein
[0007] the nanosheets are associated with or intercalate
[A.sup.m-.sub.x/m], where A is a displaceable anion;
[0008] m is an integer;
[0009] x is a positive number less than 1 (preferably, 0.5 or
less);
[0010] the nanostack being optionally hydrated with a
stoichiometric amount or a non-stoichiometric amount of water;
and
[0011] the nanosheet having at least one lateral edge dimension in
a range of from about 5 nm to about 500 nm.
[0012] In some of these embodiments, M is at least one of
Ba.sup.2+, Be.sup.2+, Ca.sup.2+, Cd.sup.2+, Cu.sup.2+, Co.sup.2+,
Fe.sup.2+, Mg.sup.2+, Mn.sup.2+, Ni.sup.2+, Pb.sup.2+, Sr.sup.2+,
or Zn.sup.2+, and M' is at least one of Al.sup.3+, Ce.sup.3+,
Co.sup.3+, Cr.sup.3+, Fe.sup.3+, Ga.sup.3+, In.sup.3+, La.sup.3+,
Mn.sup.3+, V.sup.3+, or Y.sup.3+. In certain of these embodiments,
M is or comprises Ni.sup.2+ and/or M' is or comprises
Fe.sup.3+.
[0013] In some of these embodiments, A independently comprises one
or more organic or inorganic anion, for example F.sup.-, Cl.sup.-,
Br.sup.-, I.sup.-, BF.sub.4.sup.-, PF.sub.6.sup.-, CO.sub.3.sup.2-,
HCO.sub.3.sup.-, CrO.sub.4.sup.2-, NO.sub.2.sup.-, NO.sub.3.sup.-,
ONO.sub.2.sup.-, ClO.sub.2.sup.-, ClO.sub.3.sup.-,
ClO.sub.4..sup.-, H.sub.2PO.sub.4.sup.-, HPO.sub.4.sup.2-,
PO.sub.4.sup.3-, IO.sub.3.sup.-, OH.sup.-, S.sup.2-,
SO.sub.3.sup.2-, S.sub.2O.sub.3.sup.2-, SO.sub.4.sup.2-,
WO.sub.4.sup.2-, acetate, propionate, lactate, terephthalate,
adipate, succinate, dodecyl sulfonate, p-hydroxybenzoate, and
benzoate, or a combination thereof. In some preferred embodiments,
A is or comprises hydroxide.
[0014] In other embodiments, x is in a range of from 0.05 to
0.95.
[0015] The nanocatalysts are nanodimensioned and where present as
layered double hydroxide nanostacks comprising a plurality of
nanosheets, nanosheets can have at least one lateral edge dimension
in a range of from about 5 nm to about 50 nm, for example from
about 7 nm to about 22 nm.
[0016] Where the nanocatalysts are present as layered double
hydroxide nanostacks, at least one of the nanosheets may be further
doped with at least one Lewis acid ion of a transition metal,
lanthanide, or actinide metal ion, for example doped with titanium
or lanthanum.
[0017] Additional embodiments include those of methods of
subjecting a solid ablation target to an energy source, the solid
ablation target comprising first metal capable of oxidizing to a
positive oxidation state, the energy source impinging on the first
metal in the presence of a solution, preferably an aqueous
solution, containing a second metal ion in a positive oxidation
state for a time and energy sufficient to ionize at least a portion
of the first metal to a positive oxidation state. When the solution
is aqueous, the methods provide for the inventive layered
double-hydroxide nanostacks described herein. Preferably, these
aqueous ablation solutions are substantially free of surfactants.
The energy for the ablation may be applied using a laser, for
example, a pulsed laser, the pulsed laser optionally capable of
delivering or delivering energy in a range of from about 90
mJ/pulse to about 210 mJ/pulse.
[0018] The nanocatalysts described herein may also be incorporated
into electronic or electrochemical devices (e.g., electrochemical
cells or sensors). Some embodiments provide for electrodes having
coating comprising the nanocatalysts. In some of these embodiments,
the layered double hydroxide nanostacks described herein are water
oxidation catalysts. Associated embodiments include those where the
inventive electrodes are used to pass sufficient current to oxidize
water to form oxygen. These electrodes may comprise gold, nickel,
platinum, or an allotrope of carbon, such as graphite, graphene,
glassy (or vitreous) carbon, diamond, or a combination thereof.
These nanostacks coatings may allow for the electrodes to exhibit
water oxidation overpotentials of less than 300 mV at 10
mA/cm.sup.2.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The present application is further understood when read in
conjunction with the appended drawings. For the purpose of
illustrating the subject matter, exemplary embodiments of the
subject matter are shown in the drawings; however, the presently
disclosed subject matter is not limited to the specific methods,
devices, and systems disclosed. In addition, the drawings are not
necessarily drawn to scale. In the drawings:
[0020] FIGS. 1A and 1B provide schematic structural representations
of the [Ni--Fe]-LDH
[Ni.sub.1-xFe.sub.x(OH).sub.2](NO.sub.3).sub.y(OH).sub.x-y.nH.sub.2O.
[0021] FIG. 2 shows overpotential .eta. for water oxidation at 10
mA cm.sup.-2 vs. Ni content for catalysts 1 to 5. Depicted in the
photos are catalysts 3 to 5 in aqueous suspension to visualize
their different colors.
[0022] FIG. 3 shows XRD data of catalyst 5 after anodization, (a)
on Si, (b) on carbon cloth after 30 min anodization in 1.0 M pH
14.0 aqueous KOH at 0.807 V vs NHE, (c) on carbon cloth before
anodization; bare carbon cloth (d).
[0023] FIG. 4A shows Tafel plots of current density (j) as a
function of electrode polarization potential (E.sub.p) (circles, 5;
squares, 6; gray squares, Ni oxide electrodeposited according to
Dinec{hacek over (a)}, M.; Surendranath, Y; Nocera, D. G Proc.
Natl. Acad. Sci. U.S.A. 2010, 107, 10337; gray circles, bare
electrode), and a photograph of 5 and 6; FIG. 4B shows XRD data of
catalysts 5 (bottom trace) and 6 (top trace), where
*[Ni.sub.1-xFe.sub.x(OH).sub.2](NO.sub.3).sub.y(OH).sub.x-y.nH.sub.-
2O, |NiFe.sub.2O.sub.4 spinel; FIG. 4C shows far-IR spectra of
catalysts 5 (bottom trace) and catalyst 6 (top trace).
[0024] FIG. 5A shows TOF vs. N.sub.405.1 eV/N.sub.407.3 eV (for
neat Fe--Ni-based catalysts; and catalysts, 7 and 8). FIG. 5B shows
XPS data of catalysts 4 to 8 (the vertical dashed lines mark the N
1s binding energies (405.1 and 407.3 eV)).
[0025] FIG. 6 shows determination of R.sub.u for post-measurement
iR drop correction; circles, measured data; line, linear fit.
[0026] FIG. 7 shows XPS data of catalysts 1 to 8 in the Fe 2p, Ni
2p, O 1s, and N 1s regions. The gray dashed lines are at the N 1s
binding energies of interstitial (405.1 eV) and surface-adsorbed
(407.3 eV) nitrate.
[0027] FIG. 8 shows XPS data of catalysts 7 and 8 in the Ti 2p and
La 3d regions.
[0028] FIG. 9 shows XRD data of catalysts 1 to 8. Normalized fixed
slit intensities of known minerals are displayed as vertical lines:
black, maghemite; cyan, magnetite; red, jamborite; gray, goethite;
blue, trevorite; green, TiO.sub.2; purple, ulvospinel; yellow,
Ni.sub.3TiO.sub.5; dark blue, La(Ni,Fe)O.sub.3.
[0029] FIG. 10 shows TEM images of water oxidation catalysts 1 to
8. The insets show particles that imaged with a higher contrast.
All scale bars are 20 nm.
[0030] FIG. 11 shows BET data of catalysts 5 to 8; P/P.sub.0
denotes the relative pressure, and W is the weight of the adsorbed
argon,
[0031] FIG. 12 shows Raman spectra of catalysts 1 to 8 (black). The
sharp spikes in the spectrum of 2 are from cosmic ray events. Also
depicted is a reference spectrum of the .alpha.-Ni(OH).sub.2
mineral jamborite (red, RRUFF ID R070619, collected with 532 nm
excitation).
[0032] FIG. 13 shows infrared spectra of catalysts 5 (bottom trace)
and 6 (top trace) with band assignments. The inset shows a
magnification of the adsorbed and interstitial nitrate (.nu.3)
region: open circles, data; thick lines, overall fits; thin lines
Gaussian peak fits. The band was best fit by two Gaussian
distributions, indicating the presence of two distinct nitrate
species.
[0033] FIG. 14 shows infrared spectra (solid lines) of catalysts 5
and 6 with spectral deconvolutions (dotted lines).
[0034] FIG. 15 shows cyclic voltammograms of catalysts 1 to 8; j,
current density, E.sub.p, polarization potential. The disjointed
segments in the measured data occurred due to bubble release from
the electrode surface.
[0035] FIG. 16 shows Tafel data of catalysts 1 to 8 (marked black
squares); j, current density, Ep, polarization potential. For
comparison, Tafel data of electrodeposited nickel oxide (unlabeled
gray squares, same mass loading as catalysts) and bare HOPG (gray
circles) are also plotted. The solid lines are fits. Plotting the
overpotential at 10 mA cm.sup.-2 vs. the Ni content in the catalyst
(from XPS data) shows that the highest water oxidation activity was
obtained with the highest Ni content (78%) in the material.
[0036] FIG. 17 shows current density j as a function of time data
of catalysts 5, 6 and 8.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0037] The present invention is directed to nanocatalysts,
especially layered double-hydroxide nanostacks, methods employing
novel laser ablation techniques to make such nanocatalysts, and
methods and devices for using the inventive nanocatalysts.
[0038] The present invention may be understood more readily by
reference to the following description taken in connection with the
accompanying Figures and Examples, all of which form a part of this
disclosure. It is to be understood that this invention is not
limited to the specific products, methods, conditions or parameters
described or shown herein, and that the terminology used herein is
for the purpose of describing particular embodiments by way of
example only and is not intended to be limiting of any claimed
invention. Similarly, unless specifically otherwise stated, any
description as to a possible mechanism or mode of action or reason
for improvement is meant to be illustrative only, and the invention
herein is not to be constrained by the correctness or incorrectness
of any such suggested mechanism or mode of action or reason for
improvement. Throughout this text, it is recognized that the
descriptions refer to compositions and methods of making and using
said compositions. That is, where the disclosure describes or
claims a feature or embodiment associated with a composition or a
method of making or using a composition, it is appreciated that
such a description or claim is intended to extend these features or
embodiment to embodiments in each of these contexts (i.e.,
compositions, methods of making, and methods of using).
[0039] In the present disclosure the singular forms "a," "an," and
"the" include the plural reference, and reference to a particular
numerical value includes at least that particular value, unless the
context clearly indicates otherwise. Thus, for example, a reference
to "a material" is a reference to at least one of such materials
and equivalents thereof known to those skilled in the art, and so
forth.
[0040] When a value is expressed as an approximation by use of the
descriptor "about," it will be understood that the particular value
forms another embodiment. In general, use of the term "about"
indicates approximations that can vary depending on the desired
properties sought to be obtained by the disclosed subject matter
and is to be interpreted in the specific context in which it is
used, based on its function. The person skilled in the art will be
able to interpret this as a matter of routine. In some cases, the
number of significant figures used for a particular value may be
one non-limiting method of determining the extent of the word
"about." In other cases, the gradations used in a series of values
may be used to determine the intended range available to the term
"about" for each value. Where present, all ranges are inclusive and
combinable. That is, references to values stated in ranges include
every value within that range.
[0041] It is to be appreciated that certain features of the
invention which are, for clarity, described herein in the context
of separate embodiments, may also be provided in combination in a
single embodiment. That is, unless obviously incompatible or
specifically excluded, each individual embodiment is deemed to be
combinable with any other embodiment(s) and such a combination is
considered to be another embodiment. Conversely, various features
of the invention that are, for brevity, described in the context of
a single embodiment, may also be provided separately or in any
sub-combination. Finally, while an embodiment may be described as
part of a series of steps or part of a more general structure, each
said step may also be considered an independent embodiment in
itself, combinable with others.
[0042] The transitional terms "comprising," "consisting essentially
of," and "consisting" are intended to connote their generally in
accepted meanings in the patent vernacular; that is, (i)
"comprising," which is synonymous with "including," "containing,"
or "characterized by," is inclusive or open-ended and does not
exclude additional, unrecited elements or method steps; (ii)
"consisting of" excludes any element, step, or ingredient not
specified in the claim; and (iii) "consisting essentially of"
limits the scope of a claim to the specified materials or steps
"and those that do not materially affect the basic and novel
characteristic(s)" of the claimed invention. Embodiments described
in terms of the phrase "comprising" (or its equivalents), also
provide, as embodiments, those which are independently described in
terms of "consisting of" and "consisting essentially of" For those
embodiments provided in terms of "consisting essentially of," the
basic and novel characteristic(s) is the facile operability of the
methods (and the systems used in such methods and the compositions
derived therefrom) to prepare and use the inventive materials, and
the materials themselves, where the methods and materials are
capable of delivering the highlighted properties using only the
elements provided in the claims. That is, while other materials may
also be present in the inventive compositions, the presence of
these extra materials is not necessary to provide the described
benefits of those compositions or devices (i.e., the effects may be
additive) and/or these additional materials do not compromise the
performance of the product compositions or devices. Similarly,
where additional steps may also be employed in the methods, their
presence is not necessary to achieve the described effects or
benefits and/or they do not compromise the stated effect or
benefit.
[0043] When a list is presented, unless stated otherwise, it is to
be understood that each individual element of that list, and every
combination of that list, is a separate embodiment. For example, a
list of embodiments presented as "A, B, or C" is to be interpreted
as including the embodiments, "A," "B," "C," "A or B," "A or C," "B
or C," or "A, B, or C."
[0044] Throughout this specification, words are to be afforded
their normal meaning, as would be understood by those skilled in
the relevant art. However, so as to avoid misunderstanding, the
meanings of certain terms will be specifically defined or
clarified.
[0045] For example, while generally understood by those skilled in
the electrochemical arts, the term "overpotential" refers to a
potential more positive than the thermodynamic onset voltage for
oxygen evolution from water at a positive electrode and more
negative than the thermodynamic onset voltage for hydrogen
evolution from water at a negative electrode.
[0046] The term "plurality" connote a number of two or more.
[0047] Layered double hydroxides (LDHs), which belong to a typical
family of anionic layered materials, can generally be represented
by the general formula
[M.sub.1-xM'.sub.x(OH).sub.2].sup.x+(A.sup.n-).sub.x/n. mH.sub.2O,
where M is a bivalent metal cation, M' is a trivalent metal cation,
A.sup.n- denotes an interlayer anions with negative charge n, m is
the number of interlayer water molecules, and x is the molar ratio
of the M' to the sum of the M and M'. The term "double," then, in
this context refers to the presence of two oxidation states, not to
the presence of only two metals. The identity and ratio of the
layer elements as well as the interlayer guest anions can be
adjusted over a wide range in order to obtain materials with
specific structures and properties. Because of their flexible
composition and versatility, LDHs have been widely investigated for
their potential applications in the fields of catalysis,
adsorption, ion exchange, cosmetics, biocidal and drug delivery,
and functional materials.
[0048] LDHs are traditionally synthesized by coprecipitation
methods, hydrothermal methods, ion-exchange methods or
calcination-rehydration methods. A large amount of sodium salt is
produced as by-product during the preparation of LDHs by
traditional methods. The sodium salt mother liquor is always
discharged directly due to the high energy costs of evaporation,
and thus leads to environmental pollution. In addition, the use of
strong alkali in the synthesis process means that the product must
be well washed with water (tens or even hundred times of the
product's mass), which leads to significant waste of water and
problems with treatment of the alkaline effluent. Thus it is
necessary to develop an environmentally friendly technology for
preparation of LDHs.
[0049] The coprecipitation method is the most popular method used
to prepare LDHs. A mixed salt solution containing the metal ions
which constitute the layers are coprecipitated with alkali in order
to obtain the LDHs. Either the mixed salt solution or the alkali
solution can contain the corresponding interlayer anionic group.
Additionally, LDHs may be prepared by coprecipitation of a mixture
of soluble salts of bivalent and trivalent metal ions with
Na.sub.2CO.sub.3 and NaOH. However, in this method, large quantity
of water is required to wash the product after the reaction, due to
the large amount of sodium salts produced in the reaction and the
strongly alkaline solution, and a significant waste of water is
thus caused.
[0050] The hydrothermal method for preparation of LDHs is another
method in which insoluble oxides or hydroxides containing the metal
ions to be incorporated in the layers are treated with water at a
high temperature under a high pressure. In this method,
Na.sub.2CO.sub.3 or NaHCO.sub.3 may generally used as main starting
materials, and the sodium salt formed as a co-product needs be
removed by washing which causes a lot of water waste.
[0051] The ion-exchange method is used when M and M' are not stable
in alkaline medium or no suitable soluble salt of the anion AI' can
be found. An LDHs precursor is first synthesized and the
ion-exchange reaction is then carried out in the presence of the
required interlayer anions under appropriate conditions in order to
prepare the target LDHs. In this method, the washing process cannot
be omitted due to the formation of salt by-products in the
production of the precursor.
[0052] In the calcination-rehydration method, complex metal oxides
(LDO) are obtained by calcination of an LDHs precursor, and the LDO
is added into a solution containing the desired anions to restore
or partly restore the ordered layered structure of LDHs. Generally,
it is possible to restore the ordered layered structure when the
calcination temperature is below 500.degree. C. When the
calcination temperature exceeds 600.degree. C., a spinel phase is
formed from which the layer structure of the LDHs cannot be
restored. An LDHs precursor must also be synthesized for use in the
calcination-rehydration method and therefore the washing process
cannot be omitted due to the formation of salt by-products.
[0053] Certain embodiments of the present invention include layered
double hydroxide nanostacks comprising at least one but preferably
a plurality of nanosheets represented by the general formula:
[M.sub.(1-x)M'.sub.x(OH).sub.2].sup.x+, wherein: M is a metal
cation in a formal +2 oxidation state; M' a metal cation in a
formal +3 oxidation state; wherein
[0054] the nanosheets are associated with or intercalate
[A.sup.m-.sub.x/m], where A is a displaceable anion;
[0055] m is an integer (for example 1, 2, 3, or 4);
[0056] x is a positive number less than 1;
[0057] the nanostack being optionally hydrated with a
stoichiometric amount or a non-stoichiometric amount of water;
and
[0058] the nanosheet having at least one lateral edge dimension in
a range of from about 5 nm to about 500 nm.
[0059] In other embodiments, the nanostacks may comprise dehydrated
mixed metal oxides or hydrated mixed metal oxides, for example,
equivalent to sintered layered double hydroxide, and comprise the
corresponding mixed metal oxide or hydrated oxide
nanocatalysts.
[0060] M can be any metal cation, derived from a transition or main
group metal, that exists in a formal +2 oxidation state. In
separate independent embodiments, M is or comprises at least one of
Ba.sup.2+, Be.sup.2+, Ca.sup.2+, Cd.sup.2+, Cu.sup.2+, Co.sup.2+,
Fe.sup.2+, Mg.sup.2+, Mn.sup.2+, Ni.sup.2+, Pb.sup.2+, Sr.sup.2+,
Zn.sup.2+, or any combination thereof. In some embodiments, M is
independently Cu.sup.2+, Co.sup.2+, Fe.sup.2+, Ni.sup.2+,
Pb.sup.2+, Sr.sup.2+, or Zn.sup.2+. In other embodiments, the mixed
oxide or double-hydroxide nanostacks (i.e., excluding the dopants
described herein) comprise a single M metal, for example
Ni.sup.2+.
[0061] Similarly, M' can be any metal cation that exists in a
formal +3 oxidation state. Non-limiting examples of M' include at
least of Al.sup.3+, Ce.sup.3+, Co.sup.3+, Cr.sup.3+, Fe.sup.3+,
Ga.sup.3+, In.sup.3+, La.sup.3+, Mn.sup.3+, V.sup.3+, Y.sup.3+,
Ce.sup.3+. In separate independent embodiments, M' is or comprises
at least one of Al.sup.3+, Ce.sup.3+, Co.sup.3+, Cr.sup.3+,
Fe.sup.3+, La.sup.3+, Mn.sup.3+, V.sup.3+, Y.sup.3+, or Ce.sup.3+.
Again, in some embodiments, the nanostacks (i.e., excluding the
dopants described herein) comprise a single M' metal, for example
Fe.sup.3+. While M and M' can comprise the same metal in different
oxidation states (e.g., Co.sup.2+/Co.sup.3+, Fe.sup.2+/Fe.sup.3+,
or Mn.sup.2+/Mn.sup.3+), in most embodiments, the two metals M and
M' are different.
[0062] In addition to the template M and M' cations, in some
embodiments, at least one of the nanosheets is further doped with
at least one other Lewis acid ion of another transition metal,
lanthanide, or actinide metal ion. For example, [Ni--Fe]LDH (where
x<0.5, <0.4, or <0.3) may be doped with titanium or
lanthanum cations. Typically, such doping is done at a level of 10
atomic %, 5 atomic %, or less, based on the total metal content of
the layered double hydroxide.
[0063] A may comprise any organic or inorganic anion, or a
combination thereof. Again, these anions, at least in the double
hydroxide nanostacks, are exchangeable, and that such exchanges are
known and documented in the art. Such interchangeability of the
intercalations anions may occur either as by deliberate synthesis
(i.e., deliberate exchange reactions) or in situ during their use
in certain electrochemical environments. Such independent inorganic
exemplars for A include F.sup.-, Cl.sup.-, Br.sup.-, I.sup.-,
BF.sub.4.sup.-, PF.sub.6.sup.-, CO.sub.3.sup.2-, HCO.sub.3.sup.-,
CrO.sub.4.sup.2-, NO.sub.2.sup.-, NO.sub.3.sup.-, ONO.sub.2.sup.-,
ClO.sub.2.sup.-, ClO.sub.3.sup.-, ClO.sub.4..sup.-,
H.sub.2PO.sub.4.sup.-, HPO.sub.4.sup.2-, PO.sub.4.sup.3-,
IO.sub.3.sup.-, OH.sup.-, S.sup.2-, SO.sub.3.sup.2-,
S.sub.2O.sub.3.sup.2-, SO.sub.4.sup.2-, WO.sub.4.sup.2-, or any
combination thereof. Independent inorganic exemplars for A include
acetate, propionate, lactate, terephthalate, adipate, succinate,
dodecyl sulfonate, p-hydroxybenzoate, and benzoate, or a
combination thereof. A may comprise a mixed organic or inorganic
anion. A may also comprise a complex anion comprising a transition
metal compound (e.g.,
Mo.sub.7O.sub.24.sup.6-V.sub.10O.sub.28.sup.6-,
PW.sub.11CuO.sub.39.sup.6-, or SiW.sub.9V.sub.3O.sub.40.sup.7-).
When the double hydroxide nanostacks are derived from ablation in
the presence of an aqueous solution, A typically comprises
hydroxide, as well as the anion associated with the aqueous metal
ion (vide infra).
[0064] Additionally, these inventive double hydroxide nanostacks
may also comprise other neutral or anionic payloads. For example,
analogous LDHs have been used as delivery systems for intercalated
biocides (fungicides, fungicide, an algicide or a bactericide, as
described in U.S. Pat. No. 8,986,445); pharmaceuticals (e.g.,
4-biphenylacetic acid, Diclofenac, Gemfibrozil, Ibuprofen,
Naproxen, 2-propylpentanoic acid and Tolfenamic acid as described
in U.S. Pat. No. 8,709,500 or 8,747,912); pyrithione or a
polyvalent metal salt of pyrithione (anti-dandruff; as described in
U.S. Pat. No. 8,673,274); or anionic organic or organometallic
pigments or colorants as described in U.S. Pat. No. 7,799,126).
Such intercalation products are considered within the scope of the
present invention.
[0065] As described above, x has been defined in terms of a
non-zero, positive number less than 1. In other independent
embodiments, x is 0.9 or less, 0.8 or less, 0.7 or less, 0.5 or
less, 0.5 or less, 0.4 or less, 0.3 or less, 0.2 or less, or 0.1 or
less. In other embodiments, the layered double hydroxide nanostack
may be described in terms of x, wherein x is in a range bounded at
the lower end by a value of about 0.1, about 0.05, about 0.1, about
0.15, about 0.2, about 0.25, about 0.3, about 0.35, about 0.4,
about 0.45, or about 0.5 and at the upper end by a value of about
0.95, about 0.9, about 0.85, about 0.8, about 0.75, about 0.7,
about 0.65, about 0.6, or about 0.5, with non-limiting exemplary
ranges including those from about 0.05 to about 0.95, from about
0.20 to about 0.95, from about 0.2 to about 0.35, or from about
0.22 to about 0.95.
[0066] The nanostacks have been described as comprising at least
one but preferably a plurality of nanosheets, wherein the nanosheet
has at least one lateral edge dimension in a range of from about 5
nm to about 500 nm. One of the key features of the inventive laser
ablation methods described is the ability to provide such
nanodimensioned sheets. In additional embodiments, these nanosheets
may have at least one, and preferably two, lateral dimensions
ranging from about 5 nm to about 500 nm, 400 nm, 300 nm, 250 nm,
200 nm, 150 nm, 100 nm, 50 nm, or about 25 nm. As shown in the
Examples, such nanostacks are conveniently prepared having
nanosheets having at least one lateral edge dimension in a range of
from about 7 nm to about 22 nm.
[0067] Also, the nanostacks have been described in terms of
"preferably a plurality of nanosheets." While such nanostacks
includes those embodiments comprising a single nanosheet with its
associated anion set, as used herein, the term "plurality" is
intended to connote two or more, with specific embodiments
providing that this number is at least two, at least three, or at
least four.
[0068] Independent embodiments include methods of preparing any of
the layered double hydroxide nanostacks or related nanoparticle
catalysts described herein comprising subjecting a solid ablation
target to an energy source, the solid ablation target comprising a
first metal capable of oxidizing to a positive oxidation state, the
energy source impinging on the first metal in the presence of an
aqueous ablation solution containing a second metal ion in a
positive oxidation state for a time and energy sufficient to ionize
at least a portion of the first metal to a positive oxidation
state.
[0069] Preferably, but not necessarily, the aqueous ablation
solution is substantially free of surfactants. As used herein, the
term "substantially free" of surfactants means that there is no
added surfactant. This absence of the surfactants within the
aqueous ablation solution translates into the absence of
surfactants within the layered double-hydroxide nanostacks, which
may in part contribute to their superior electrochemical oxidation
performance.
[0070] These methods are operable whether M is derived the solid
ablation target or the second metal metal ion. Likewise M' may be
derived from the solid ablation target or the second metal metal
ion. That is, in some embodiments, the first metal is capable of
achieving a +2 oxidation state, and the second metal is in a +3
oxidation state. In other embodiments, the first metal is capable
of achieving a +3 oxidation state, and the second metal is in a +2
oxidation state. The first metal may comprise beryllium, calcium,
cadmium, copper, cobalt, iron, magnesium, manganese, nickel,
strontium, zinc, or an alloy or mixture thereof and the second
metal ion comprises at least one of Al.sup.3+, Ce.sup.3+,
Co.sup.3+, Cr.sup.3+, Fe.sup.3+, Ga.sup.3+, In.sup.3+, La.sup.3+,
Mn.sup.3+, V.sup.3+, and Y.sup.3+. In other embodiments, the first
metal comprises aluminum, cerium, cobalt, chromium, iron, gallium,
indium, lanthanum, manganese, vanadium, yttrium, or an alloy or
mixture thereof, and the second metal comprises at least one of
Be.sup.2+, Ca.sup.2+, Cd.sup.2+, Cu.sup.2+, Co.sup.2+, Fe.sup.2+,
Mg.sup.2+, Mn.sup.2+, Ni.sup.2+, Sr.sup.2+, and Zn.sup.2+.
[0071] In certain specific embodiments, M is Ni and M' is
Fe.sup.3+.
[0072] The energy used for the ablation is conveniently produced by
a laser, and in some embodiments a pulsed laser (or a conventional
laser adapted to provide the energy in pulses). In certain
embodiments, the laser pulse delivers energy at a sufficient flux
and to ionize the metal of the laser ablation target. Without
intending to be bound by the correctness of any particular theory,
it is believed that the energy of the laser causes this
transformation by establishing a plasma environment at or near the
surface of the laser ablation target, confined by the boundaries
imposed by the aqueous environment. In this case, the energy is
defined by both the absolute energy and frequency of each pulse and
the duration of the treatment. Such conditions are conveniently
established when the pulse energy is in a range bounded at the
lower end by a value of about 50 mJ/pulse, 70 mJ/pulse, 90
mJ/pulse, 110 mJ/pulse, 130 mJ/pulse, 150 mJ/pulse, 170 mJ/pulse,
190 mJ/pulse, or 210 mJ/pulse and at the upper end by a value of
about 500 mJ/pulse, 450 mJ/pulse, 400 mJ/pulse, 350 mJ/pulse, 300
mJ/pulse, 250 mJ/pulse, 230 mJ/pulse, 210 mJ/pulse, or 190
mJ/pulse. Under these conditions, the frequency of the pulse can be
provided in a range of from about 10, 20, 30, 40, or 50 Hz for
periods of time ranging from minutes to hours. In the Examples
provided, pulse energies in a range of from about 90 mJ/pulse to
about 210 mJ/pulse were applied at 10 Hz for 60 minutes,
corresponding to about 90/210.times.10.sup.-3 J.times.10
sec.sup.-1.times.3600 seconds or about 3.2-7.6 kJ. These parameters
are tunable, depending on the nature of materials and the desired
quantity and type of materials, and so should not be considered
limiting as to the nature of the invention.
[0073] Generally, the formed nanostacks incorporate hydroxide (in
the case of aqueous ablation media) and the counterions associated
with the second metal ion present in the ablation solutions.
However, these counterions are displaceable with other counterions,
and further processing steps may comprise exchanging these
displaceable anions with others by treatments known in the art.
Again, these exchanges may be accomplished deliberately (i.e.,
deliberate exchange reactions) or may be affected in situ during
their use in certain electrochemical environments.
[0074] Still further embodiments provide that the initially-formed
layered double hydroxide nanostack is subjected to further
processing. In some of these embodiments, the initially-formed
layered double hydroxide nanostack sintered. In certain aspects of
this embodiment, the sintering may done in an oxidizing
environment, producing mixed oxide or hydrated oxide
nanostructures. In other aspects, the sintering is done in an
oxidatively inert environment (e.g., under argon or nitrogen), in
some cases, again providing mixed oxide or hydrated oxide
nanostructures. In still other aspects, the sintering may be done
in reducing environment (e.g., under hydrogen), in some cases
providing mixed metallic nanostructures.
[0075] To this point, the various embodiments or aspects of this
invention have been described in terms of the nanostacks and their
methods of making, but the invention also comprises those
structures which incorporate these nanomaterials. For example,
these nanocatalysts or nanostructures may be incorporated into
electrochemical devices, for example as coatings used in such
devices, and these devices or coatings are considered to be within
the scope of the present invention. For example, certain
embodiments of the invention provide for the incorporation of these
nanomaterials into templates, polymers, or coatings. For example,
these embodiments may include electrodes, each comprising a coating
comprising one or more of one or more of the inventive layered
double hydroxide nanostacks, or mixed oxide or hydrated oxide
nanostacks. When the layered double hydroxide nanostack are
incorporated into a coating of an electrode, especially the
[NiFe]LDH structures described herein, that electrode shows
excellent properties as an electrochemicals water oxidation
catalyst.
[0076] Such coatings may be applied to electrode comprises gold,
nickel, platinum, or an allotrope of carbon. In some embodiments,
the electrodes comprise graphite, graphene, glassy (or vitreous)
carbon, diamond, or a combination thereof.
[0077] In certain of these embodiments, the coated electrodes show
superior performance as water oxidation catalysts, for example
being capable of exhibiting or actually exhibiting an overpotential
for the oxidation of water to oxygen of less than 300 mV at 10
mA/cm.sup.2 in use. In certain embodiments, these coated electrodes
provide overpotentials of within 290, 280, and 270 mV, perhaps to
as low as 250 or 200 mV, of the thermodynamically determined
potential for the oxidation of water to oxygen, at 10 mA/cm.sup.2.
It is not clear that the lower boundary of these overpotentials
have been realized. These coated electrodes operable and can
achieve these low overpotentials across a broad pH range (e.g.,
from 0 to about 2, from about 2 to about 4, from about 4 to about
6, from about 6 to about 8, from about 8 to about 10, from about 10
to about 12, from about 12 to about 14, or any combination of these
ranges), but given the nature of chemical nature of the LDHs, and
their ability to retain their crystallinity, longer lasting
performance is expected in alkaline conditions; i.e., at pHs
ranging from about 7, 7.5, 8, 8.5, 9, 9.5, or 10 to about 14, 13.5,
13, 12.5, 12, 11.5, 11, 10.5, or 10. Again, the longevity of these
electrodes will depend on the specific nature of the chosen LDHs
and the operating conditions of the electrochemical reactions
(e.g., pH and temperature).
[0078] Additional embodiments include the electrochemical cell or
cells comprising these inventive electrode, as well as power
systems incorporating the electrochemical cell or cells.
[0079] Also within the scope of the present invention are those
methods for operating these electrodes or electrochemical cells in
electrocatalysis. For example, certain exemplary embodiments
include those methods for oxidizing water comprising applying a
potential to any of the inventive electrodes and passing sufficient
current to oxidize water to form oxygen. The person of ordinary
skill in the art would appreciate how to operate such devices and
methods without undue experimentation. These methods include
operating the cell or device such that the potential is within 300
mV (or any of the ranges cited above) of the thermodynamically
determined potential for the oxidation of water to oxygen at 10
mA/cm.sup.2. It should be recognized that reference to 10
mA/cm.sup.2 is intended only to refer one basis for claims to the
superior overpotential performance, and that any reasonable current
density may be used in conjunction with these cells and
devices.
[0080] The following listing of embodiments is intended to
complement, rather than displace or supersede, the previous
descriptions.
Embodiment 1
[0081] A layered double hydroxide nanostack comprising a plurality
of nanosheets represented by the general formula:
[M.sub.(1-x)M'.sub.x(OH).sub.2].sup.x+, wherein: M is a metal
cation in a formal +2 oxidation state; M' a metal cation in a
formal +3 oxidation state; wherein
[0082] the nanosheets are associated with or intercalate
[A.sup.m-.sub.x/m], where A is a displaceable anion;
[0083] m is an integer; [e.g., 1, 2, 3, or 4]
[0084] x is a positive number less than 1; [preferably, but not
necessarily <0.5]
[0085] the nanostack being optionally hydrated with a
stoichiometric amount or a non-stoichiometric amount of water;
and
[0086] the nanosheet having at least one lateral edge dimension in
a range of from about 5 nm to about 100 nm.
Embodiment 2
[0087] The layered double hydroxide nanostack of Embodiment 1,
wherein M is at least one of Ba.sup.2+, Be.sup.2+, Ca.sup.2+,
Cd.sup.2+, Cu.sup.2+, Co.sup.2+, Fe.sup.2+, Mg.sup.2+, Mn.sup.2+,
Ni.sup.2+, Sr.sup.2+, and Zn.sup.2+, and M' is at least of
Al.sup.3+, Ce.sup.3+, Co.sup.3+, Cr.sup.3+, Fe.sup.3+, Ga.sup.3+,
In.sup.3+, La.sup.3+, Mn.sup.3+, V.sup.3+, Y.sup.3+, Ce.sup.3+
Embodiment 3
[0088] The layered double hydroxide nanostack of Embodiment 1 or 2,
wherein A comprises an organic or inorganic anion, or a combination
thereof.
Embodiment 4
[0089] The layered double hydroxide nanostack of Embodiment 3,
wherein A comprises F.sup.-, Cl.sup.-, Br.sup.-, I.sup.-,
BF.sub.4.sup.-, PF.sub.6.sup.-, CO.sub.3.sup.2-, HCO.sub.3.sup.-,
CrO.sub.4.sup.2-, NO.sub.2.sup.-, NO.sub.3.sup.-, ONO.sub.2.sup.-,
ClO.sub.2.sup.-, ClO.sub.3.sup.-, ClO.sub.4..sup.-,
H.sub.2PO.sub.4.sup.-, HPO.sub.4.sup.2-, PO.sub.4.sup.3-,
IO.sub.3.sup.-, OH.sup.-, S.sup.2-, SO.sub.3.sup.2-,
S.sub.2O.sub.3.sup.2-, SO.sub.4.sup.2-, WO.sub.4.sup.2-, or a
combination thereof.
Embodiment 5
[0090] The layered double hydroxide nanostack of Embodiment 3,
wherein A comprises acetate, propionate, lactate, terephthalate,
adipate, succinate, dodecyl sulfonate, p-hydroxybenzoate, and
benzoate, or a combination thereof.
Embodiment 6
[0091] The layered double hydroxide nanostack of Embodiment 3,
wherein A comprises a complex anion comprising a transition metal
compound.
Embodiment 7
[0092] The layered double hydroxide nanostack of any one of
Embodiments 1 to 6, wherein M is or comprises Ni.sup.2+.
Embodiment 8
[0093] The layered double hydroxide nanostack of any one of
Embodiments 1 to 7, wherein M' is or comprises Fe.sup.3+.
Embodiment 9
[0094] The layered double hydroxide nanostack of any one of
Embodiments 1 to 8, wherein x is in a range of from 0.05 to
0.95.
Embodiment 10
[0095] The layered double hydroxide nanostack of any one of
Embodiments 1 to 9, wherein A is or comprises hydroxide.
Embodiment 11
[0096] The layered double hydroxide nanostack of any one of
Embodiments 1 to 10, wherein the nanosheet has at least one lateral
edge dimension in a range of of from about 5 nm to about 25 nm.
Embodiment 12
[0097] The layered double hydroxide nanostack of any one of
Embodiments 1 to 11, wherein at least one of the nanosheets is
further doped with at least one Lewis acid ion of a transition
metal, lanthanide, or actinide metal ion.
Embodiment 13
[0098] The layered double hydroxide nanostack of any one of
Embodiments 1 to 12, wherein at least one of the nanosheets is
further doped with titanium or lanthanum.
Embodiment 14
[0099] A method (of preparing a layered double hydroxide nanostack
of any one of Embodiments 1 to 13) comprising subjecting a solid
ablation target to an energy source, the solid ablation target
comprising a first metal capable of oxidizing to a positive
oxidation state, the energy source impinging on the first metal in
the presence of an aqueous ablation solution containing a second
metal ion in a positive oxidation state for a time and energy
sufficient to ionize at least a portion of the first metal to a
positive oxidation state.
Embodiment 15
[0100] The method of Embodiment 14, wherein the aqueous ablation
solution is substantially free of surfactants.
Embodiment 16
[0101] The method of Embodiment 14, wherein the first metal is
capable of achieving a +2 oxidation state, and the second metal is
in a +3 oxidation state.
Embodiment 17
[0102] The method of Embodiment 14, wherein the first metal is
capable of achieving a +3 oxidation state, and the second metal is
in a +2 oxidation state.
Embodiment 18
[0103] The method of Embodiment 16, wherein the first metal
comprises beryllium, calcium, cadmium, copper, cobalt, iron,
magnesium, manganese, nickel, lead, strontium, zinc, or an alloy or
mixture thereof and the second metal ion comprises at least one of
Al.sup.3+, Ce.sup.3+, Co.sup.3+, Cr.sup.3+, Fe.sup.3+, Ga.sup.3+,
In.sup.3+, La.sup.3+, Mn.sup.3+, V.sup.3+, or Y.sup.3+.
Embodiment 19
[0104] The method of Embodiment 17, wherein the first metal
comprises aluminum, cerium, cobalt, chromium, iron, gallium,
indium, lanthanum, manganese, vanadium, yttrium, or an alloy or
mixture thereof, and the second metal comprises at least one of
Ba.sup.2+, Be.sup.2+, Ca.sup.2+, Cd.sup.2+, Cu.sup.2+, Co.sup.2+,
Fe.sup.2+, Mg.sup.2+, Mn.sup.2+, Ni.sup.2+, Sr.sup.2+, or
Zn.sup.2+. In certain of aspects of this Embodiment, M is Fe and M'
is Ni.sup.2+.
Embodiment 20
[0105] The method of any one of Embodiments 14 to 19, wherein the
energy is a laser.
Embodiment 21
[0106] The method of Embodiment 20, wherein the laser energy is
provided in pulses
Embodiment 22
[0107] The method of Embodiment 21, wherein the laser pulse
delivers an energy in a range of from about 90 mJ/pulse to about
210 mJ/pulse.
Embodiment 23
[0108] The method of any one of Embodiments 14 to 22, wherein the
displaceable ion A of the layered double hydroxide nanostack
comprises a counterion associated with the second metal ion in the
aqueous ablation solution.
Embodiment 24
[0109] The method of Embodiment 23, further comprising exchanging
the displaceable ion A of the layered double hydroxide nanostack
with a different anion.
Embodiment 25
[0110] The method of any one of Embodiments 14 to 24, wherein the
initially-formed layered double hydroxide nanostack is further
sintered. In certain aspects of this Embodiment, the sintering is
done in an oxidizing environment. In other aspects of this
Embodiment, the sintering is done in an oxidatively inert
environment. In still other aspects of this Embodiment, the
sintering is done in reducing environment.
Embodiment 26
[0111] A composition prepared by any one of Embodiments 14 to
25.
Embodiment 27
[0112] An electrode comprising a coating comprising the layered
double hydroxide nanostack of any one of Embodiments 1 to 13.
Embodiment 28
[0113] The electrode of Embodiment 27, wherein the electrode
comprises gold, nickel, platinum, or an allotrope of carbon.
Embodiment 29
[0114] The electrode of Embodiment 27, wherein the electrode
comprises graphite, graphene, glassy (or vitreous) carbon, diamond,
or a combination thereof.
Embodiment 30
[0115] The electrode of Embodiment 27 or 29, that exhibits an
overpotential for the oxidation of water to oxygen of less than 300
mV at 10 mA/cm.sup.2 on a flat supporting electrode.
Embodiment 31
[0116] An electrochemical cell comprising an electrode of any one
of Embodiments 27 to 30.
Embodiment 32
[0117] A method for oxidizing water comprising applying a potential
to an electrode of any one of Embodiments 27 to 30 and passing
sufficient current to oxidize water to form oxygen.
Embodiment 33
[0118] The method of Embodiment 32, wherein the potential is within
300 mV of the thermodynamically determined potential for the
oxidation of water to oxygen at 10 mA/cm.sup.2.
EXAMPLES
[0119] The following Examples are provided to illustrate some of
the concepts described within this disclosure. While each Example
is considered to provide specific individual embodiments of
composition, methods of preparation and use, none of the Examples
should be considered to limit the more general embodiments
described herein.
[0120] In the following examples, efforts have been made to ensure
accuracy with respect to numbers used (e.g. amounts, temperature,
etc.) but some experimental error and deviation should be accounted
for. Unless indicated otherwise, temperature is in degrees C.,
pressure is at or near atmospheric.
Example 1
Overview
[0121] In certain exemplary, non-limiting, embodiments,
surfactant-free mixed-metal hydroxide water oxidation nanocatalysts
can be synthesized by pulsed-laser ablation in liquids. In a series
of [Ni--Fe]-layered double hydroxides with intercalated nitrate and
water,
[Ni.sub.1-xFe.sub.x(OH).sub.2](NO.sub.3).sub.y(OH).sub.x-y.nH.sub.2O,
higher activity was observed as the amount of Fe decreased to 22%.
Addition of Ti.sup.4+ and La.sup.3+ ions further enhanced
electrocatalysis, with a lowest overpotential of 260 mV at 10 mA
cm.sup.-2. Electrocatalytic water oxidation activity increased with
the relative proportion of a 405.1 eV N 1s (XPS binding energy)
species in the nanosheets.
[0122] Specific embodiments include surfactant-free, highly active
[Ni.sub.1-xFe.sub.x(OH).sub.2](NO.sub.3).sub.y(OH).sub.x-y.nH.sub.2O
nanosheet water oxidation catalysts with admixed ions. The best
catalyst had an overpotential of 260 mV at 10 mA cm.sup.-2 on flat
highly-ordered pyrolytic graphite working electrodes. The higher
activity may be attributable to unique morphological and structural
properties, which were synthetically accessible by the use of
pulsed-laser ablation in liquids (PLAL). In PLAL, nanoparticles are
formed by very rapid cooling of a plasma comprised of elements from
the solid ablation target and the surrounding liquid. This
condensation process, which is kinetically controlled, produces
predominantly crystalline nanomaterials. PLAL offers size and
composition control through a wide range of tunable experimental
parameters.
[0123] With PLAL, mixed-metal nanomaterials with tailored
compositions can be prepared readily by adding metal ions into the
aqueous ablation liquid. Different amounts of Fe were intentionally
incorporated into .alpha.-Ni(OH).sub.2 nanocatalysts, as variable
concentrations of Fe in electrodeposited nickel (oxy)hydroxides
have been shown to improve electrocatalytic activity. Ti.sup.4+ and
La.sup.3+ ions were also added to the ablation liquid and screened
the resulting materials for water oxidation activity.
[0124] Eight mixed-metal catalysts were synthesized using PLAL by
varying ablation targets, metal ion type and concentrations, and
laser pulse energies (see SI for experimental details, all ablation
solutions contained nitrate). The nanomaterials were prepared with
Fe concentrations ranging from 22 to 95% of the total metal content
(Table 1). We identified their compositions spectroscopically; and,
notably, they all exhibited high electrocatalytic oxygen-evolution
activities in basic electrolytes.
[0125] Powder X-ray diffraction (XRD) measurements indicated that
the Fe-rich nanoparticles 1-3 were poorly crystalline; the Ni-rich
nanoparticles 4-8 displayed diffraction patterns consistent with
layered double hydroxide (LDH) structures. XRD data indicated minor
contributions from Fe(O)OH; 6 also contained the crystalline spinel
NiFe.sub.2O.sub.4, and Ti-based oxides were present in 7 and 8.
LDHs have the general formula
[M.sub.1-xM'.sub.x(OH).sub.2](A.sup.m-).sub.x/m.nH.sub.2O; the
structures are comprised of sheets of
[M.sub.1-xM'.sub.x(OH).sub.2].sup.x+ edge-shared octahedra.
Cationic charges arising from M'.sup.3+ in the sheets are balanced
by intercalated hydrated anions (A.sup.m-).
TABLE-US-00001 TABLE 1 Preparation conditions of catalysts 1 to 8
and concentrations of Fe with respect to total metal content. Ion
con- Pulse Fe Solid Added centration energy (% metal Catalyst
target ions (M) (mJ) content).sup.a 1 Ni Fe 0.1 90 95 2 Ni Fe 0.01
90 86 3 Fe Ni 0.1 90 70 4 Fe Ni 1.0 90 36 5 Fe Ni 3.0 90 22 6 Fe Ni
3.0 210 30 7 Fe Ni 3.0 210 23 Ti 0.015 8 Fe Ni 3.0 210 29 Ti 0.015
La 0.023 .sup.aDetermined by XPS
[0126] X-ray photoelectron spectroscopy (XPS) was employed to
obtain binding energies of Ni 2p and Fe 2p core levels in 1-8;
these energies were indicative of Ni(OH).sub.2 and (hydrous) iron
oxides. In addition, Mossbauer and x-ray absorption spectroscopic
data indicated that Fe was incorporated as Fe.sup.3+ in place of
Ni.sup.2+ in [Ni--Fe]-LDHs. Two well-resolved N 1s peaks appeared
in the XP spectra of nanoparticles 4-8, with binding energies of
407.3 and 405.1 eV. The higher binding-energy feature (407.3 eV)
was assigned to nitrate. The 2.2 eV reduction in N 1s binding
energy for the second feature could have arisen from nitrate in an
unusual electronic environment, although nitrogen in a lower
oxidation state (e.g., NO.sub.2, NO.sub.2.sup.-) could not be ruled
out. Infrared spectra were consistent with the presence of a second
type of NO.sub.x species. Infrared and Raman data supported the
presence of intercalated nitrate anions in the LDH structure. On
the basis of these data, the predominant crystalline material in
4-8 was assigned to the [Ni-- Fe]-LDH
[Ni.sub.1-xFe.sub.x(OH).sub.2](NO.sub.3).sub.y(OH).sub.x-y.nH.sub.2O
(FIG. 1).
[0127] Nanoparticle sizes were obtained from transmission electron
micrographs (TEM), and crystalline domain sizes were determined by
Scherrer analysis of XRD data. Lateral sizes ranged from .about.7
to 22 nm (Table 2). Catalysts 1 to 5 consisted of nanosheets, as
expected for layered structures. Analysis of TEM and XRD data for 6
revealed that two types of nanoparticles were formed; smaller, more
spherical (6.5.+-.0.8) nm particles are attributed to the spinel
NiFe.sub.2O.sub.4, and larger (13.+-.1) nm sheets are assigned to
the LDH
[Ni.sub.1-xFe.sub.x(OH).sub.2](NO.sub.3).sub.y(OH).sub.x-y.nH.sub.2O.
Also, differences in TEM contrast, shape, and size were found for 7
and 8. Specific surface areas of catalysts 5 to 8 determined by
Brunauer-Emmett-Teller (BET) measurements are in agreement with
particle sizes derived from TEM data. Catalysts 6 to 8, which were
synthesized at 210 mJ pulse energy, had similar BET surface areas
(193.+-.1 m.sup.2 g.sup.-1), whereas 5, prepared at 90 mJ/pulse,
exhibited a slightly higher surface area (220 m.sup.2
g.sup.-1).
[0128] The electrocatalytic oxygen-evolution activity was assessed
in 1 M aqueous KOH. Faradaic yields of oxygen evolution for 5, 6
and 8 were all essentially 100%. Steady-state Tafel data were
measured to obtain overpotentials; virtually identical mass
loadings were used in all electrochemical experiments (all current
densities are reported per geometric area). Importantly,
chronoamperometry data showed that the catalytic activity of
nanoparticles 5-8 was maintained for more than 5 hours.
[0129] The electrocatalytic activities of materials 1 to 5,
synthesized at virtually the same pulse energy, steadily increased
with decreasing Fe content (FIG. 2). Catalyst 5 (22% Fe relative to
total metal content) performed best in the [Ni--Fe]-LDH materials,
with an overpotential of 280 mV at 10 mA cm.sup.-2. Incorporation
of less than 22% Fe relative to total metal content was limited by
the solubility of Ni nitrate in the aqueous ablation liquid. XRD
data for 5, collected before and after 30 min of anodic
polarization, confirmed that the crystallinity of the [Ni--Fe]-LDH
material was retained (FIG. 3). The Fe content of our best
performing catalyst is in agreement with previous reports. It
differed, however, from findings for amorphous materials, which
performed best with 40% Fe.
[0130] Catalyst 6 was made employing virtually the same precursor
conditions as for 5, but with a pulse energy of 210 instead of 90
mJ. It has been shown previously with cobalt oxide that pulse
energy can be used to control particle size. Varying pulse energy
in the synthesis of more complex mixed-metal materials led to
particles with different compositions (FIG. 4A-C). While 5
consisted mainly of crystalline [Ni--Fe]-LDH, 6 was mixed
crystalline [Ni--Fe]-LDH/NiFe.sub.2O.sub.4. Catalyst 6 showed
inferior activity for water oxidation relative to 5, presumably
because the active [Ni--Fe]-LDH was diluted by the spinel oxide.
This finding suggested that crystalline
[Ni.sub.1-xFe.sub.x(OH).sub.2](NO.sub.3).sub.y(OH).sub.x-y.nH.sub.2O
is the more active species in our materials for catalytic water
oxidation. IR spectra of 5 and 6 were consistent with
[Ni.sub.1-xFe.sub.x(OH).sub.2](NO.sub.3).sub.y(OH).sub.x-y.nH.sub.2O
with high interstitial water and nitrate content. The positions of
peaks in the IR spectrum of catalyst 5 indicated the incorporation
of Fe into the .alpha.-Ni(OH).sub.2 lattice.
[0131] The addition of Lewis-acidic Ti.sup.4+ and La.sup.3+ ions to
the ablation liquid was found to improve catalytic activity
relative to the most active [Fe--Ni]-LDH catalyst (5). Catalysts 7
and 8 were synthesized using virtually the same precursor
conditions as for 5, but with Ti.sup.+ (7) or Ti.sup.4+ and
La.sup.3+ (8) added to the ablation solution (Table 1). XRD data
revealed that both catalysts were primarily [Ni--Fe]-LDH materials.
Oxides containing added elements were also present; TiO.sub.2 and
Fe.sub.2TiO.sub.4 were found in 7, whereas crystalline
Ni.sub.3TiO.sub.5 and La(Ni,Fe)O.sub.3 were detected in 8. The
spinel oxide NiFe.sub.2O.sub.4 was absent from both 7 and 8. XPS
data showed that 8 contained 1% La relative to total metal content.
Both catalysts were more active than LDHs 5 and 6, with 7 and 8
exhibiting the lowest overpotentials at 10 mA cm.sup.-2 of 270 and
260 mV, respectively.
[0132] Highly active, surfactant-free, mixed transition metal
hydroxide water oxidation nanoparticle catalysts can be made by
PLAL. Acrystalline [Ni--Fe]-LDH was spectroscopically identified as
the catalytically most active material. The turnover frequency
(TOF) was found to correlate with the ratio of two nitrogen species
detected by XPS in the as-synthesized catalysts (FIG. 5A-B).
Addition of Ti.sup.4+ and La.sup.3+ ions further enhanced activity
(reaching 10 mA cm.sup.-2 at an overpotential of 260 mV). On a flat
electrode, this is the lowest overpotential reported to date for
mixed metal oxide catalysts.
Example 2
General Experimental Conditions and Apparatus
Example 2.1
Materials and Methods
[0133] Nanomaterial synthesis by pulsed laser ablation in liquids
was performed in the Beckman Institute Laser Resource Center at
California Institute of Technology. X-ray photoelectron
spectroscopy was carried out at the Molecular Materials Research
Center (Beckman Institute at California Institute of Technology).
Transmission electron micrographs were collected at the Beckman
Resource Center for Transmission Electron Microscopy (California
Institute of Technology).
[0134] All chemicals were used as received. Deionized water was
obtained from a Barnstead Diamond Nanopure system and had a
resistivity of .gtoreq.16 M.OMEGA. cm.sup.-1.
Example 2.2
Synthesis
[0135] Mixed metal nanomaterials were synthesized using the method
of pulsed laser ablation in liquids (PLAL). Suspensions of iron
(Alfa, -200 mesh, 99+%) or nickel (Alfa, -150+200 mesh, 99.8%)
powders were stirred in 10 mL aqueous metal nitrate solutions using
a magnetic stirrer in a 30 mL glass beaker at room temperature in
ambient air. For ablation, 0.5 g iron powder or 2.0 g nickel powder
were used. With iron as ablation target, the liquid consisted of 10
mL pH 10.0 water (adjusted with potassium hydroxide, Mallinckrodt)
with nickel nitrate (Ni(NO.sub.3).sub.2.6H.sub.2O, Alfa, 98%)
concentrations of 0.1 M, 1.0 M, and 3.0 M. With nickel as ablation
target, the liquid was 10 mL water with iron nitrate
(Fe(NO.sub.3).sub.3.9H.sub.2O, Alfa, 98.0-101.0%) concentrations of
0.01 M and 0.1 M. Nanomaterials with more than two metals were made
from 0.5 g iron powder suspended in 10 mL of a solution of 3.0 M
nickel nitrate and 0.015 M titanium(IV) oxide bis(acetylacetonate)
(Strem, >95%) in 10 mL pH 10.0 water (adjusted with KOH). Some
solutions also contained 0.023 M lanthanum nitrate
(La(NO.sub.3).sub.3.6H.sub.2O, Sigma-Aldrich, .gtoreq.99%). Beakers
and stir bars were thoroughly cleaned with aqua regia before
use.
[0136] A 355 nm, 8 ns pulse laser beam, provided by the third
harmonic of a 10 Hz Q-switched Nd:YAG laser (Spectra-Physics
Quanta-Ray PRO-Series), was focused 0.5 mm below the surface of the
liquid with a 100 mm focal length plano-convex quartz lens. Each
sample was irradiated for 60 min. Laser pulse energies were either
90 or 210 mJ/pulse.
[0137] After synthesis, nanoparticle suspensions were separated
from the metallic ablation targets using a strong magnet. Solid
nanoparticulate powders were obtained by centrifugation and washing
with water until the supernatant did not show any metal nitrate
absorption. The nanoparticles were then washed twice with acetone
(EMD, OmniSolv.RTM.) and dried under vacuum. A high precision
balance (Sartorius CPA225D) was used to weigh the nanoparticle
powders. Around 5 mg material were suspended in water to make 2 mg
mL-1 suspensions; 20 .mu.L of these were drop cast on
freshly-cleaved highly-ordered pyrolytic graphite (HOPG) working
electrodes and dried in ambient air under a heat lamp at 50.degree.
C., resulting in a catalyst loading of 40 .mu.g.
[0138] Electrodeposited nickel oxide catalyst was prepared
according to the procedure published by Dinc{hacek over (a)}, M. et
al., Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 10337. In detail,
2.18 g Ni(NO.sub.3).sub.2.6H.sub.2O was dissolved in 5 mL water and
added to 75 mL rapidly stirred 0.1 M pH 9.20 aqueous sodium borate
buffer, which immediately became turbid. The sodium borate buffer
was made from sodium tetraborate
(Na.sub.2B.sub.4O.sub.7.10H.sub.2O, Baker, 101.4%) and its pH was
adjusted by adding boric acid (H.sub.3BO.sub.3, Mallinckrodt,
99.9%). The filtrate of the suspension was used as the electrolyte;
the working electrode was a freshly cleaved HOPG electrode. An
Ag/AgCl/3.0 M NaCl reference electrode (Bioanalytical Systems,
Inc.; measured to be +0.212 V vs NHE, and a Ni gauze (Alfa) counter
electrode were used. A 51 mC charge was passed with an applied
voltage of 1.312 V vs. NHE; faradaically, we deposited 530 nmol Ni,
which corresponds to 40 .mu.g NiO. Before catalytic activity
testing the electrodeposited films were thoroughly washed with
water.
Example 2.3
Physical Characterization
[0139] X-ray photoelectron spectra (XPS) were taken using a Surface
Science Instruments M-probe surface spectrometer. Monochromatic Al
K.alpha. radiation (1486.6 eV) was used to excite electrons from
the samples, which had been drop-cast on clean Cu foil and dried in
ambient air at room temperature. The sample chamber was maintained
at <5.times.10.sup.-9 Torr. Survey scans from 0 to 1000 eV were
carried out to identify the elements present in the nanoparticles.
Binding energies were referenced to the C 1s peak arising from
adventitious carbon, taken to have a binding energy of 284.8 eV.
See Barr, T. L. et al., J. Vac. Sci. Technol. A 1995, 13, 1239.
High-resolution spectra were collected for the Fe 2p, Ni 2p, Ti 2p,
La 3d, N 1s, and O 1s regions. Quantitative peak areas were derived
after Shirley background subtraction (see Shirley, D. A. Phys. Rev.
B 1972, 5, 4709) and using relative sensitivity factors. Binding
energies were obtained from the same peak fits. Quantitative XPS
analysis was performed with CasaXPS (Version 2.3.16 PR 1.6).
[0140] X-ray diffraction (XRD) data were collected with a Bruker D2
PHASER diffractometer. Monochromatic Cu K.alpha. radiation (1.5418
.ANG.; tube power 30 kV, 10 mA) was used; the instrument was
equipped with 0.1.degree. divergence, 1.5.degree. Soller, and 0.6
mm detector slits, and had a 3-mm secondary anti-scatter screen.
Diffracted radiation was collected with a Lynxeye detector. The
instrument resolution was 0.05.degree. in 2.theta., and the
counting time was 3 seconds per step, resulting in a total scan
time of about 75 min for each sample. Solid samples were deposited
with vaseline (X-Alliance GmbH) on a zero-diffraction silicon plate
(MTI Corporation). XRD background subtraction, Scherrer and pattern
analysis were performed with the Bruker DIFFRAC.SUITE software
coupled to the International Centre for Diffraction Data powder
diffraction file database (ICDD, PDF-2 Release 2012).
[0141] Raman spectra of neat solid catalysts were collected at room
temperature in ambient air with a Renishaw M1000 micro-Raman
spectrometer. A 50.times. magnification objective and a 50-.mu.m
slit, resulting in 4 cm-1 resolution, were used. The laser
excitation wavelength was 514.3 nm (Cobolt Fandango.TM. 100 laser),
the power at the sample was 213 .mu.W (1% laser power, measured
with a Thorlabs PM100USB power meter), and depolarized scattered
light was detected. The excitation intensity was chosen as to
prevent radiation damage of the nanoparticulate powders; collected
spectra did not change during repeated scans. The radiation damage
threshold was approximated to be at a laser intensity that was
three times higher than that applied. Application of 10% laser
power through a 50.times. magnification objective led to immediate
radiation damage, and a dark spot was visible on the sample when
viewed through the microscope. Focusing the 10% power laser beam
through a 20.times. magnification objective led to gradual sample
degradation over multiple scans, which was also observed by visual
inspection with the microscope. The instrument's autofocus function
was used to maximize the signal-to-noise ratio. The accumulation
time was 10 s, and 8 scans were averaged for each sample. The
measured Raman shifts were calibrated against a Si standard.
Spectra were compared to reference spectra from the RRUFF database
(Downs, R. T. The RRUFF Project: an integrated study of the
chemistry, crystallography, Raman and infrared spectroscopy of
minerals. Program and Abstracts of the 19th General Meeting of the
International Mineralogical Association in Kobe, Japan. O03-13,
2006), which were collected with 532 nm excitation and depolarized
detection.
[0142] Attenuated total reflectance infrared spectra of neat
nanoparticulate powders were collected with a Thermo Nicolet iS50
FT-IR spectrometer, equipped with a Pike Technologies GladiATR
accessory plate and an uncooled pyroelectric deuterated triglycine
sulfate (DTGS) detector. In the 50 to 700 cm-1 range, a
far-infrared multilayer beamsplitter was used and a measured water
vapor spectrum was subtracted from the data; in the 400 to 4000
cm-1 range, a KBr beamsplitter was used. Spectra of the solid
nanoparticulate powders were collected at room temperature in
ambient air, and 132 scans were averaged for each sample.
Transmission electron microscopy (TEM) measurements were performed
with an FEI Tecnai T-12. For each material, 2 .mu.L of a suspension
of 2 mg mL-1 nanoparticles in water were drop cast on a 200 mesh Cu
grid coated with Formvar carbon (Ted Pella), which was placed on a
Kimwipe. The nanoparticles were dispersed on the hydrophobic grid
surface by adding 10 .mu.L isopropanol. The average diameter of the
nanoparticles was determined using the ImageJ software.
[0143] Specific surface areas were determined by
Brunauer-Emmett-Teller (BET) measurements, using a Quantachrome
Autosorb iQ instrument. Adventitious adsorbates were removed under
vacuum by heating approximately 40 mg of each catalyst powder at a
rate of 10 K min-1 from room temperature to 423 K, holding it there
for 1 hour, followed by heating to 573 K at a rate of 10 K min-1,
where it remained for 6 hours, and subsequent cooling to room
temperature. Multipoint argon adsorption-desorption isotherms were
collected at 87.45 K, and the specific surface areas were
calculated with the instrument's built-in software, based on the
BET equation.
Example 2.4
Electrochemical Characterization
[0144] Cyclic voltammetry, Tafel, and chronoamperometry data were
collected at room temperature. For all electrochemical
measurements, the electrolyte was aqueous 1.0 M pH 14.0 KOH
(Mallinckrodt); an Hg/HgO reference electrode (CH Instruments), a
Ni gauze (Alfa) counter electrode, and HOPG working electrodes with
40 .mu.g catalyst on them were used. Working electrodes for cyclic
voltammetry, faradaic oxygen yield, and chronoamperometry data
consisted of upward-facing HOPG (GraphiteStore, surface area: 0.09
cm.sup.2) electrodes. Their preparation is described in Blakemore,
J. D. et al., ACS Catal. 2013, 3, 2497; the only difference was
that the glass tubes were u-shaped at one end to make the HOPG
electrode surface face upwards, which facilitated measurements with
extensive oxygen evolution because it allowed the generated gas to
bubble up. Working electrodes were cleaned by sonication for 10 min
in concentrated hydrochloric acid, washed with water, and their
surfaces were polished using 400 and 600 grit sandpaper, after
which the graphite was cleaved with adhesive tape to obtain a fresh
HOPG surface for each catalyst.
[0145] Cyclic voltammograms were measured at 10 mV s.sup.-1 scan
rate with a Gamry Reference 600 potentiostat. Tafel data were
recorded using a rotating disk electrode (RDE) apparatus.
Measurements were carried out in a 100 mL three-neck round-bottom
flask with a Pine MSR variable speed rotator used at 1,500 rpm and
a Princeton Applied Research Parstat 4000 potentiostat. The dwell
time at each applied potential point was 5 min to reach
steady-state conditions. The disk electrode was made of HOPG with
stabilizing epoxy around its side (surface area: 0.13 cm.sup.2).
The current density versus potential data were post-measurement
corrected for uncompensated resistance losses (see below). All
potentials reported here are relative to the normal hydrogen
electrode (NHE), and current densities are per geometric area.
[0146] The ohmic drop (uncompensated resistance, Ru) was
experimentally determined for an HOPG working electrode, either
blank or with 40 .mu.g nanoparticulate catalyst loading, using a
Gamry Reference 600 potentiostat and its built-in "measure Ru"
utility that uses the current interrupt method. The working
electrode was swept between 0.107 and 0.907 V vs. NHE and Ru values
were collected. The averages of 3 Ru values were plotted as a
function of applied potential, and the data were fit with a line
(FIG. 6).
[0147] Post-measurement iR drop correction was performed according
to Oel.beta.ner, W., et al., Mater. Corros. 2006, 57, 455. This
method was chosen, where R.sub.u for the nanoparticulate catalysts
was experimentally determined under the same conditions as all
other electrochemical measurements, because automatic iRcorrection
is inherently problematic for high-surface-area materials. In
detail, the true polarization potential E.sub.p was calculated from
the applied potential E.sub.a, the measured current i, and the
uncompensated resistance R.sub.u as E.sub.p=E.sub.a-iR.sub.u.
[0148] Faradaic yields of oxygen evolution data were collected with
an apparatus described in Blakemore, J. D. et al., ACS Catal. 2013,
3, 2497. A glass cell was filled with 65 mL electrolyte, leaving 59
mL headspace, in which the O.sub.2 concentration was measured. A
potential of 0.857 V vs. NHE was applied for 30 min, using a Gamry
600 potentiostat. The electrolysis chamber was water-jacketed and
kept at a constant temperature of (22.0.+-.0.5.degree.) C. to
ensure a stable response from the O.sub.2 sensor. In a typical
experiment, based on the charge transferred, we expected 284 .mu.L
of O2 evolved and detected 297 .mu.L. This confirmed essentially
100% oxygen evolution within the error (10%) of our method.
[0149] Long-term stability measurements were performed using a
Gamry 600 potentiostat and a working electrode, onto which 40 .mu.g
catalyst had been drop cast from a 2 mg mL.sup.-1 suspension that
also contained 80 .mu.g mL.sup.-1 Nafion 117 (Aldrich). Nafion was
added for chronoamperometry experiments to improve the mechanical
stability of catalyst films on HOPG during oxygen evolution. A
voltage of 0.654 V vs. NHE was applied for 5.5 hours and the
current was recorded. Data analysis and graphing was performed with
Igor Pro 6.34 (Wavemetrics).
Example 3
Physical Characterization
Example 3.1
X-Ray Photoelectron Spectra
[0150] XPS data were collected to identify nanoparticle
compositions by peak integrations of high-resolution spectra of the
Fe 2p, Ni 2p, O 1s, N 1s, Ti 2p, and La 3d regions, where
applicable. The regions were chosen as to collect data on
transitions with the highest x-ray ionization cross-sections. Since
the x-ray ionization cross-section of Ti 2p is a factor of 5.4
lower that that of La 3d, and 1.5 times less Ti.sup.4+ than
La.sup.3+ was added to the ablation liquid, no Ti photoelectrons
were detected. We deliberately did not attempt to quantify oxygen
content from XPS data because the amount of this element is
regularly overestimated; oxygen also occurs in other sources, such
as adventitious carbon species and oxides of the underlying copper
substrate. See FIG. 7 and FIG. 8.
[0151] The Fe 2p core level spectra of catalysts 1 to 8 showed
peaks consistent with iron oxides and oxyhydroxides, with Fe
2p.sub.3/2 binding energies close to 710.9 eV. It is not possible
to distinguish the different Fe phases in our materials from Fe 2p
XPS data, as various iron oxides and oxyhydroxides, such as FeO,
Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, and FeOOH, have similar Fe
core-level binding energies and spectral shapes. The Ni 2p
core-level binding energies of catalysts 1 to 8 were indicative of
Ni(OH).sub.2 or NiOOH, with Ni 2p.sub.3/2 binding energies close to
855.7 eV. The O 1s spectra of 1 to 8 exhibited, among contributions
from adventitious oxygen species, two peaks centered around 528.8
eV and 531.4 eV, as expected for Fe or Ni oxide and hydroxide
species, respectively. 10 The N 1s core level spectra of catalysts
1 to 8 showed peaks with binding energies above 405 eV, ascribable
to nitrate. The N 1s peaks at 407.3 eV were assigned to
surface-adsorbed nitrate, in accord with a report on nitrate
adsorbed on hematite, and the peaks centered at 405.1 eV were
attributed to interstitial nitrate.
Example 3.2
X-Ray Diffraction Data
[0152] XRD data were collected to determine crystalline phases and
crystallite sizes by Scherrer analysis. Note that peak widths were
determined by factoring in multiple diffraction lines from the
corresponding PDF, where applicable. Overlapping diffraction lines
may give rise to peaks that appear broader in the total intensity
spectra. As a result, peak broadness in the total intensity
spectrum does not necessarily correlate to the actual crystalline
domain size. Crystalline phases were assigned using the automatic
search/match function of the Bruker software DIFFRAC.SUITE. The
Fe-rich catalysts were amorphous, 1 and 3 completely so, and 2
predominantly so, with some broad peaks that were assigned to
poorly crystallized magnetite and maghemite. XRD data of the more
Ni-rich catalysts 4 to 8 showed mainly crystalline
.alpha.-Ni(OH).sub.2 (jamborite) and a minor contribution from
crystalline FeOOH (goethite). We could not observe any
.beta.-Ni(OH).sub.2 (theophrastite) in our catalysts. Catalyst 6
additionally contained crystalline NiFe.sub.2O.sub.4 (trevorite).
In 7 and 8, minerals containing added elements were also present;
TiO.sub.2 and ulvospinel (Fe.sub.2TiO.sub.4) were detected in 7,
while crystalline Ni.sub.3TiO.sub.5 and La(Ni,Fe)O.sub.3 were found
in 8. The characterization of these crystalline material phases
were made by comparison with literature data. See FIG. 9 and FIG.
3.
Example 3.3
Transmission Electron Micrographs
[0153] TEM images were taken to obtain nanoparticle sizes. FIG. 10.
The intention was to avoid blocking catalytically active surface
sites; therefore the nanoparticles were synthesized by PLAL without
any surfactants. They naturally aggregated in aqueous suspensions.
Very dilute samples were prepared on TEM grids, resulting in only a
few (aggregated) nanoparticles being imaged per frame. Note that
frame-filling nanoparticle patterns will only form by self-assembly
of surfactant-capped nanoparticles due to repulsive or attractive
forces between surfactant molecules.
[0154] Nanocatalyst compositions and sizes are summarized in Table
2. Compositions were derived from XPS peak area quantification.
Scherrer analysis of XRD data for catalysts 4 to 8 was used to
obtain crystalline domain sizes (materials 1 to 3 were poorly
crystallized); the corresponding crystalline phases are given in
parentheses. Nanoparticle sizes were determined by TEM image
analysis.
[0155] Analysis of TEM and XRD data of 6 suggested that smaller,
(6.5.+-.0.8) nm particles could be attributed to trevorite, and
larger (13.+-.1) nm nanosheets could be assigned to jamborite. It
became evident from inspection of TEM images of 6 that the smaller
(trevorite) nanoparticles exhibited more contrast, consistent with
more spherical shape, than the larger jamborite sheets. Trevorite
is a spinel that crystallizes in the cubic system, rendering the
formation of nanoparticles with radial symmetry likely. Jamborite,
however, crystallizes as a layered structure, leading to axially
elongated nanosheets. Likewise, differences in TEM contrast, shape,
and size were found for catalysts 7 and 8.
TABLE-US-00002 TABLE 2 Catalyst metal contents, concentrations of
interstitial and surface-adsorbed nitrate with respect to total
metal content, crystalline domain sizes, and nanoparticle sizes. %
% Nitrogen Nitrogen (405.1 (407.3 Nano- eV eV Crystalline particle
% Metal binding binding Domain Size Size Catalyst Fe Ni La energy)
energy) (nm) (nm) 1 95 5 -- 0 0 -- 22 .+-. 3 2 86 14 -- 0 8 -- 10
.+-. 2 3 70 30 -- 1 6 -- 7.7 .+-. 2 4 36 64 -- 6 10 12 .+-. 3 (LDH)
14 .+-. 2 5 22 78 -- 5 5 9 .+-. 2 (LDH) 12 .+-. 2 6 30 70 -- 5 5 13
.+-. 3 (LDH) 13 .+-. 2 6.1 .+-. 0.5 (spinel) 6.5 .+-. 0.8 7 23 77
-- 3 5 12 .+-. 3 (LDH) 13 .+-. 2 19 .+-. 2 8 29 70 1 8 4 14 .+-. 4
(LDH) 14 .+-. 2 8.7 .+-. 1
Example 3.4
Brunauer-Emmett-Teller Data
[0156] BET data were collected to obtain surface areas of the more
active water oxidation catalysts 5 to 8. See FIG. 11.
Example 3.5
Raman Spectra (FIG. 12)
[0157] The Raman spectra of 1 to 3 showed a broad feature centered
at around 650 cm.sup.-1. In this region, Raman shifts of
ferrous-ferric oxides, such as magnetite or ferrihydrite, occur.
The broadness observed for 1 to 3, however, strongly suggests the
presence of structurally ill-defined, poorly crystallized
materials. The Raman spectra of 4 to 8 were compared to a reference
spectrum of mineralogical jamborite and showed good agreement. The
strong peaks in the spectra of 4 to 8 at .about.1050 cm.sup.-1 were
assigned to inter-layer nitrate ions, consistent with peaks that
have previously been observed in electrochemically deposited
.alpha.-Ni(OH).sub.2 thin films. It has been reported that only
.alpha.-Ni(OH).sub.2 contained measurable nitrate, as formation of
crystalline .beta.-Ni(OH).sub.2 occurred with the concurrent loss
of interstitial layering; the .beta.-polymorph did not accommodate
interstitial ions because of tighter crystal packing.
[0158] Which Ni(OH).sub.2 phase is catalytically most active is
still subject of intense debate. During water oxidation,
.alpha.-Ni(OH).sub.2 is oxidized to .gamma.-NiOOH, whereas
.beta.-Ni(OH).sub.2 is transformed into .beta.-NiOOH; both
oxyhydroxides are reduced back to their starting hydroxides during
electrochemical cycling. It has been a long-held view that
.beta.-Ni(OH).sub.2 is more active for oxygen evolution. Studies of
electrodeposited amorphous .alpha.-Ni(OH).sub.2 and its ageing to
.beta.-Ni(OH).sub.2 in basic electrolytes suggested that oxygen
evolution occurred at lower onset potential for
.beta.-Ni(OH).sub.2/.beta.-NiOOH. .alpha.-Ni(OH).sub.2 is known to
be highly active for water oxidation.
Example 3.6
Infrared Spectra (FIG. 9)
[0159] Infrared (IR) spectra were collected to shed more light on
the compositions of catalysts 5 and 6. The IR spectra of 5 and 6
showed broad peaks with maxima at 340, 500, and 640 cm.sup.-1. The
.delta.(OH) band at 640 cm.sup.-1 is very sensitive to the amount
of water intercalated between the .alpha.-Ni(OH)2 layers. Bands,
attributed to OH-bending motions, typically appear at .about.650
cm.sup.-1 for Ni(OH).sub.2 with high water content and thus
indicate the presence of the .alpha.-polymorph. In contrast, for
the .beta.-polymorph, the band is shifted to .about.520 cm.sup.-1.
Additionally, the .alpha.-polymorph shows broad absorption in the
.nu.(OH) region (3400-3600 cm.sup.-1), whereas the .beta.-polymorph
features a sharp band at 3640 cm.sup.-1. The location and broadness
of the .delta.(OH) and .nu.(OH) bands in our catalysts 5 and 6 led
us to conclude that .alpha.-Ni(OH).sub.2 was the predominant
material. The band at 1340 cm.sup.-1 was further evidence of
interstitial nitrates.
[0160] The spectrum of Ni(OH).sub.2 with iron incorporation was
qualitatively determined from published transmission-mode IR
spectra. Two materials were used in this analysis, (1) almost
exclusively Ni(OH).sub.2 and (2) one of mixed (Ni,Fe) composition,
due to aging in KOH for 72 hours. The compositions of these
materials were determined by XRD and Mossbauer spectroscopy in the
original study.
[0161] The IR spectra were digitized from an electronic (PDF) copy
of the original manuscript using UN-SCAN-IT v.5.2 software.
Transmission values (digitized .gamma.-values) were aligned with
the wavelength (digitized x-values) for both spectra, omitting
points where digitization was not complete for both.
[0162] The spectrum of (2) was shifted down vertically by assuming
that the common feature at 495 nm is isosbestic in transmission.
The spectrum of (1) was scaled by a factor consistent with a second
isosbestic point at 670 nm. The absorbance spectra of the two
samples was then calculated using A(x)=2 log [T(x)], where A(x) is
the absorbance and T(x) is the decimal transmission at the
wavelength x.
[0163] Finally, the spectrum of mixed (Ni,Fe) `oxyhydroxide` was
approximated by subtracting the absorbance spectrum of (1) from
(2). It is plotted as a red dotted line in FIG. 14, alongside the
normalized absorbance spectrum of (1), graphed as a blue dotted
line.
[0164] It is important to note that, in the absence of an absolute
transmission value, these spectra are only qualitative. They do,
however, clearly indicate the direction that the peaks shift upon
incorporation of iron into the nickel phase. The growth of features
at .about.400 cm.sup.-1 and .about.600 cm.sup.-1 relative to the
features at .about.350 cm-1 and .about.650 cm.sup.-1 is indicative
of iron incorporation into the nickel phase. This trend has been
observed previously.
Example 4
Electrochemical Characterization
[0165] Electrochemical activity of the nanoparticulate catalysts
was assessed by cyclic voltammetry (FIG. 15) and Tafel data (FIG.
16), long-term stability as measured by chronoamperometry. See also
FIG. 2.
[0166] Chronoamperometry data (FIG. 17) showed that catalytic
activity of catalysts 5, 6 and 8 was maintained for more than 5
hours. The current fluctuations were due to formation and release
of oxygen bubbles from the electrode surface.
[0167] A summary of catalytic activity data is provided in Table
3.
TABLE-US-00003 TABLE 3 Overpotentials .eta. at current densities of
0.5 and 10 mA cm.sup.-2, Tafel slopes A, and turnover frequencies
(TOF) per gram catalyst at 250 mV and 300 mV overpotential of
catalysts 1 to 8 and electrodeposited Ni oxide for comparison. TOF
TOF @ .eta. = 250 @ .eta. = 300 .eta. (@ 0.5 .eta. (@ 10 mV mV mA
cm.sup.-2) mA cm.sup.-2) A (.mu.mol/O.sub.2 (.mu.mol/O.sub.2
Catalyst (mV) (mV) (mV/dec) s.sup.-1 g.sup.-1) s.sup.-1 g.sup.-1) 1
360 520 84.7 .+-. 2.1 0.23 0.89 2 300 470 73.3 .+-. 1.0 0.94 4.6 3
240 300 48.7 .+-. 0.7 7.1 78 4 230 290 47.5 .+-. 1.3 11 130 5 220
280 47.6 .+-. 0.6 21 220 6 220 350 42.0 .+-. 0.9 19 42 190 .+-.
11.6 7 210 270 45.2 .+-. 0.7 33 290 139 .+-. 35.6 8 200 260 44.7
.+-. 2.0 53 290 294 .+-. 90.6 Ni oxide 280 370 41.5 .+-. 0.6 0.63
10 170 .+-. 52.0
[0168] A comparison with published Fe--Ni-based water oxidation
catalysts is provided in Table 4. Direct comparability of catalytic
activity is in general problematic because of variations in mass
loading, film thickness, intricate details of the electrochemical
measurements, such as electrode substrate, rotation speed and dwell
time to reach steady-state conditions or scan rates; also,
overpotentials were recorded at different current densities.
Nevertheless, we compiled published data and compared them with our
catalysts made by PLAL. When measured at a current density of 10 mA
cm.sup.-2 on a flat electrode substrate, our best catalyst had the
lowest overpotential.
TABLE-US-00004 TABLE 4 Comparison of overpotentials .eta. (at given
nominal current densities) of this work with reported catalysts.
Electrode substrate materials are also given because only flat
working electrode substrates allow for a meaningful comparison of
electrocatalyst performance. Electrode Current density .eta.
Reference Catalyst substrate (mA cm.sup.-2) (mV) (below) 8 Flat
HOPG 10 260 this work 5 Flat HOPG 10 280 this work Thin-film
solution- Au/Ti- 10 336 (i) cast Ni.sub.0.9Fe.sub.0.1O.sub.x coated
quartz crystal Nanostructured .alpha.- Glassy 10 331 (ii)
Ni(OH).sub.2 carbon Electrodeposited Glassy 10 360 (iii) NiFeOx
carbon Thin-film Gold 10 280 (iv) electrodeposited Ni-Fe (40% Fe)
Graphene FeNi Ni foam, 10 220 (v) double hydroxide unspecified
hybrid pore size* Thin film nickel oxide Nickel foil 8 230 (vi)
with iron impurities Ni-Fe layered double Carbon 5 290 (vii)
hydroxide nanoplates fiber paper .beta.-NiOOH Nickel, 5 500 (viii)
polished with .mu.m- sized alumina powders Mixed Fe-Ni oxides
Carbon 1 375 (ix) paper Nickel-borate Glassy 1 425 (x) carbon
Amorphous .alpha.- FTO glass 0.5 210 (xi) Fe.sub.20Ni.sub.80O.sub.x
High surface-area Nickel 0.5 265 (xii) nickel metal oxides or steel
microdiscs NiFeAlO.sub.4 inverse Glassy 0.1 380 (xiii) spinel
carbon NiO.sub.x deposited from Glassy 0.1 390 (xiv) molecular
[Ni(en).sub.3].sup.2+ carbon *The high porosity of nickel foam
leads to an enlargement of the electrode substrate surface area
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[0229] As those skilled in the art will appreciate, numerous
modifications and variations of the present invention are possible
in light of these teachings, and all such are contemplated hereby.
For example, in addition to the embodiments described herein, the
present invention contemplates and claims those inventions
resulting from the combination of features of the invention cited
herein and those of the cited prior art references which complement
the features of the present invention. Similarly, it will be
appreciated that any described material, feature, or article may be
used in combination with any other material, feature, or article,
and such combinations are considered within the scope of this
invention.
[0230] The disclosures of each patent, patent application, and
publication cited or described in this document are hereby
incorporated herein by reference, each in its entirety, for all
purposes. The Appendices to this description are likewise
considered part of this application.
[0231] The file of this patent or application contains at least one
drawing/photograph executed in color. Copies of this patent or
patent application publication with color drawing(s)/photograph(s)
will be provided by the office upon request and payment of the
necessary fee.
* * * * *
References