U.S. patent application number 15/847643 was filed with the patent office on 2018-06-21 for photoelectrochemical cell for carbon dioxide conversion.
The applicant listed for this patent is Board of Trustees of the University of Illinois. Invention is credited to Mohammad Asadi, Bijanda Kumar, Alessandro Monticelli, Amin Salehi-Khojin, Poya Yasaei.
Application Number | 20180171492 15/847643 |
Document ID | / |
Family ID | 62557315 |
Filed Date | 2018-06-21 |
United States Patent
Application |
20180171492 |
Kind Code |
A1 |
Salehi-Khojin; Amin ; et
al. |
June 21, 2018 |
PHOTOELECTROCHEMICAL CELL FOR CARBON DIOXIDE CONVERSION
Abstract
the present disclosure relates to photoelectrochemical cells and
methods for using such for reduction of carbon dioxide and
oxidation of water. In one aspect, the disclosure provides a method
of electrochemically reducing carbon dioxide in an electrochemical
cell, comprising contacting the carbon dioxide with at least one
transition metal dichalcogenide in the electrochemical cell and at
least one helper catalyst and applying a potential to the
electrochemical cell.
Inventors: |
Salehi-Khojin; Amin;
(Chicago, IL) ; Asadi; Mohammad; (Chicago, IL)
; Monticelli; Alessandro; (Chicago, IL) ; Yasaei;
Poya; (Chicago, IL) ; Kumar; Bijanda;
(Chicago, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Board of Trustees of the University of Illinois |
Urbana |
IL |
US |
|
|
Family ID: |
62557315 |
Appl. No.: |
15/847643 |
Filed: |
December 19, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62436870 |
Dec 20, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y02E 60/36 20130101;
C25B 9/10 20130101; Y02P 20/133 20151101; C25B 9/04 20130101; C25B
1/00 20130101; C25B 11/0447 20130101; Y02E 60/366 20130101; Y02P
20/135 20151101; C25B 11/0442 20130101; C25B 1/04 20130101; C25B
3/04 20130101 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C25B 1/04 20060101 C25B001/04; C25B 9/04 20060101
C25B009/04; C25B 9/10 20060101 C25B009/10 |
Claims
1. A method of electrochemically reducing carbon dioxide and
oxidizing water in an electrochemical device, the method comprising
providing an electrochemical device, the device including a first
and second compartment and at least one photovoltaic cell, wherein
the first compartment includes a cathode in electrical contact with
at least one transition metal dichalcogenide, a first electrolyte,
and carbon dioxide, carbonic acid, or a carbonic acid salt; the
second compartment includes an anode in electrical contact with at
least one water oxidizing catalyst, a second electrolyte, and
water; the at least one photovoltaic cell is in electrical contact
with the anode and the cathode; and the first compartment is in
ionic contact with the second compartment; and exposing the
photovoltaic cell to light irradiation sufficient to create a
potential difference between the anode and the cathode sufficient
to reduce carbon dioxide at the cathode and to oxidize water at the
cathode.
2. A method according to claim 1, wherein the transition metal
dichalcogenide is selected from the group consisting of TiS.sub.2,
TiSe.sub.2, MoS.sub.2, MoSe.sub.2, WS.sub.2 and WSe.sub.2.
3. A method according to claim 1, wherein the transition metal
dichalcogenide is MoS.sub.2.
4. A method according to claim 1, wherein the transition metal
dichalcogenide is in nanoparticle form, wherein the transition
metal dichalcogenide nanoparticles have an average size between
about 1 nm and about 400 nm.
5. A method according to claim 1, wherein the transition metal
dichalcogenide is in nanoflake, nanosheet, or nanoribbon form,
wherein the transition metal dichalcogenide nanoflakes, nanosheets,
or nanoribbons have an average size between about 1 nm and about
400 nm.
6. A method according to claim 1, wherein the first electrolyte
comprises at least one helper catalyst.
7. A method according claim 6, wherein the helper catalyst is an
imidazolium, pyridinium, pyrrolidinium, phosphonium, ammonium,
choline, sulfonium, prolinate, or methioninate salt.
8. A method according to claim 6, wherein wherein the helper
catalyst is an imidazolium, pyridinium, pyrrolidinium, phosphonium,
ammonium, choline or sulfonium salt having a counterion selected
from the group consisting of C.sub.1-C.sub.5 alkylsulfate,
tosylate, methanesulfonate, bis(trifluoromethylsulfonyl)imide,
hexafluorophosphate, tetrafluoroborate, triflate, halide,
carbamate, and sulfamate.
9. A method according to claim 6, wherein in the first electrolyte
the helper catalyst is present in the aqueous solution in a
concentration within the range of about 25 vol. % to about 75 vol.
%.
10. A method according to claim 1, wherein the first electrolyte is
an aqueous solution.
11. A method according to claim 1, wherein reducing carbon dioxide
provides CO or a mixture of CO and H.sub.2.
12. A method according to claim 1, wherein the reduction of carbon
dioxide is initiated at an overpotential of less than about 100 mV,
and the reduction of the carbon dioxide has a Faradaic efficiency
of at least 70%.
13. A method according to claim 1, wherein the second electrolyte
and the water comprise an aqueous solution.
14. A method according to claim 1, wherein the water oxidizing
catalyst comprises a cobalt-comprising film disposed on the
anode.
15. A method according to claim 1, wherein oxidizing water produces
a mixture of O.sub.2 and H.sup.+.
16. A method according to claim 1, wherein the first compartment is
in ionic contact with the second compartment through a
proton-conductive membrane.
17. A method according to claim 1, wherein the cathode and the
anode are disposed on opposite surfaces of the photovoltaic cell
such that the photovoltaic cell is sandwiched between the cathode
and the anode.
18. An electrochemical device having a first and second compartment
and at least one photovoltaic cell, wherein the first compartment
includes a cathode in electrical contact with at least one
transition metal dichalcogenide, a first electrolyte, and carbon
dioxide, carbonic acid, or a carbonic acid salt; the second
compartment includes an anode in electrical contact with at least
one water oxidizing catalyst, a second electrolyte, and water; the
at least one photovoltaic cell is in electrical contact with the
anode and the cathode; and the first compartment is in ionic
contact with the second compartment.
19. A method of electrochemically reducing carbon dioxide in an
electrochemical cell, comprising contacting the carbon dioxide with
at least one transition metal dichalcogenide in the electrochemical
cell and at least one helper catalyst and applying a potential to
the electrochemical cell, wherein the at least one transition metal
dichalcogenide is MoSe.sub.2, MoSe.sub.2, WSe.sub.2 or
WS.sub.2.
20. A method of electrochemically reducing carbon dioxide according
to claim 19 comprising providing an electrochemical cell having a
cathode in contact with at least one transition metal
dichalcogenide, and an electrolyte comprising at least one helper
catalyst in contact with the cathode and the at least one
transition metal dichalcogenide, wherein the at least one
transition metal dichalcogenide is MoSe.sub.2, MoSe.sub.2,
WSe.sub.2 or WS.sub.2; providing carbon dioxide to the
electrochemical cell; and applying a potential to the
electrochemical cell.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority of U.S.
Provisional Patent Application No. 62/436,870, filed Dec. 20, 2017,
which is hereby incorporated herein by reference in its
entirety.
BACKGROUND OF THE DISCLOSURE
Field of the Disclosure
[0002] This disclosure relates generally to photoelectrochemical
cells. More particularly, the present disclosure relates to
photoelectrochemical cells and methods for using such for reduction
of carbon dioxide and oxidation of water.
Technical Background
[0003] In 2013, the global concentration of carbon dioxide in the
atmosphere reached 400 parts per million (ppm) for the first time
in recorded history. Such levels will cause radical and largely
unpredictable changes in the environment. Recent efforts have shown
that CO.sub.2 can be converted by electrochemical reduction
processes driven by renewable energy sources into energy-rich fuels
(e.g., syngas, methanol), offering an efficient path for both
CO.sub.2 remediation and an alternative energy source.
[0004] The chemical inertness of CO.sub.2, however, renders most
conversion processes highly inefficient. Current catalysts are
plagued by weak binding interactions between the reaction
intermediates and the catalyst (giving rise to high
overpotentials), or by slow electron transfer kinetics (giving rise
to low exchange current densities).
[0005] A photoelectrochemical cell capable of carbon dioxide
reduction and water oxidation may generate, e.g., CO, O.sub.2,
and/or H.sub.2 by irradiating a photovoltaic cell with light,
generating spatially separated electron hole pairs. The generated
pairs may be captured by catalysts capable of reducing carbon
dioxide or oxidizing water. Current attempts at such systems have
been limited by expensive light-absorbing materials and/or
catalysts, and by the requirement for strongly acidic or basic
reaction media, which are corrosive and difficult to manage or a
large scale.
[0006] Accordingly, there remains a need for photoelectrochemical
systems capable of reducing CO.sub.2 and/or oxidizing water using
robust, relatively inexpensive catalysts and manageable reaction
media.
SUMMARY OF THE DISCLOSURE
[0007] One aspect of the disclosure is a method of
electrochemically reducing carbon dioxide and oxidizing water in an
electrochemical device, the method comprising providing an
electrochemical device, the device including a first and second
compartment and at least one photovoltaic cell, wherein
[0008] the first compartment includes [0009] a cathode in
electrical contact with at least one transition metal
dichalcogenide, [0010] a first electrolyte, and [0011] carbon
dioxide, carbonic acid, or a carbonic acid salt;
[0012] the second compartment includes [0013] an anode in
electrical contact with at least one water oxidizing catalyst,
[0014] a second electrolyte, and [0015] water;
[0016] the at least one photovoltaic cell is in electrical contact
with the anode and the cathode; and
[0017] the first compartment is in ionic contact with the second
compartment; and
exposing the photovoltaic cell to light irradiation sufficient to
create a potential difference between the anode and the cathode
sufficient to reduce carbon dioxide at the cathode and to oxidize
water at the cathode.
[0018] Another aspect of the disclosure is an electrochemical
device having a first and second compartment and at least one
photovoltaic cell, wherein
[0019] the first compartment includes [0020] a cathode in
electrical contact with at least one transition metal
dichalcogenide, [0021] a first electrolyte, and [0022] carbon
dioxide, carbonic acid, or a carbonic acid salt;
[0023] the second compartment includes [0024] an anode in
electrical contact with at least one water oxidizing catalyst,
[0025] a second electrolyte, and [0026] water;
[0027] the at least one photovoltaic cell is in electrical contact
with the anode and the cathode; and
[0028] the first compartment is in ionic contact with the second
compartment.
[0029] Another aspect of the disclosure is a method of
electrochemically reducing carbon dioxide in an electrochemical
cell, comprising contacting the carbon dioxide with at least one
transition metal dichalcogenide in the electrochemical cell and at
least one helper catalyst and applying a potential to the
electrochemical cell, wherein the at least one transition metal
dichalcogenide is WSe.sub.2 or WS.sub.2.
[0030] Another aspect of the disclosure is a method of
electrochemically reducing carbon dioxide comprising
[0031] providing an electrochemical cell having [0032] a cathode in
contact with at least one transition metal dichalcogenide, and
[0033] an electrolyte comprising at least one helper catalyst in
contact with the cathode and the at least one transition metal
dichalcogenide, [0034] wherein the at least one transition metal
dichalcogenide is WSe.sub.2 or WS.sub.2;
[0035] providing carbon dioxide to the electrochemical cell;
and
[0036] applying a potential to the electrochemical cell.
[0037] Another aspect of the disclosure is an electrochemical cell
having a cathode in contact with at least one transition metal
dichalcogenide and a first electrolyte comprising at least one
helper catalyst, wherein the at least one transition metal
dichalcogenide is WSe.sub.2 or WS.sub.2.
[0038] Another aspect of the disclosure is an electrochemical cell
having a cathode in contact with at least one transition metal
dichalcogenide and a first electrolyte comprising at least one
helper catalyst, wherein the at least one transition metal
dichalcogenide is WSe.sub.2 or WS.sub.2.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 is a schematic cross-sectional view of an
electrochemical device 100 comprising a first compartment 120
including at least one transition metal dichalcogenide 122 disposed
on a cathode 121, which cathode is disposed on at least one
photovoltaic cell 130. Device 100 also comprises a second
compartment 140 including at least one water oxidizing catalyst 142
disposed on an anode 141, which anode is disposed on cell 130.
Compartments 120 and 140 include a first electrolyte 123 and a
second electrolyte 143, respectively, and are in ionic contact
through an ion-conductive membrane 150.
[0040] FIG. 2 is a schematic cross-sectional view of an
electrochemical device 200 comprising a first compartment 220
including at least one transition metal dichalcogenide 222 disposed
on a cathode 221. Device 200 also comprises a second compartment
240 including at least one water oxidizing catalyst 242 disposed on
an anode 241. Compartments 220 and 240 include a first electrolyte
223 and a second electrolyte 243, respectively, and a separated by,
and in ionic contact through, an ion-conductive membrane 250.
[0041] FIG. 3 is an optical image of a crystalline WSe.sub.2
structure grown by the chemical vapor transport technique of
Example 1, described in more detail below. The scale bar is 10
.mu.m.
[0042] FIG. 4 is an image of transition metal dichalcogenide
nanoflakes prepared according to Example 2, described in more
detail below.
[0043] FIG. 5 is a set of plots showing the normal size
distributions of the transition metal dichalcogenide nanoflakes
synthesized according to Example 2, described in more detail in
Example 3, below.
[0044] FIG. 6 is a set of Raman spectra of the transition metal
dichalcogenide nanoflakes synthesized according to Example 2,
described in more detail in Example 3, below.
[0045] FIG. 7 is a scanning electron microscopy (SEM) image of
WSe.sub.2 nanoflakes, described in more detail in Example 3,
below.
[0046] FIG. 8 is a schematic view of the two-compartment
three-electrode electrochemical cell of Example 4, described in
more detail below.
[0047] FIG. 9 is a set of cyclic voltammetry (CV) curves for
WSe.sub.2 nanoflakes, bulk MoS.sub.2, Ag nanoparticles, and bulk Ag
in a CO.sub.2 environment, as described in more detail in Example
4, below. Inset highlights the current densities under low
overpotentials.
[0048] FIG. 10 is a set of CV curves for MoS.sub.2, WS.sub.2,
MoSe.sub.2, and WSe.sub.2 nanoflakes in a CO.sub.2 environment, as
described in more detail in Example 4, below.
[0049] FIG. 11 is a set of two chronoamperometry (CA) experiments
carried out for an hour at different applied potentials, as
described in more detail in Example 4, below. FIG. 11 (A) shows the
results at -0.164 V and -0.264 V, and (B) shows the results at
-0.764 V, -0.564 V, and -0.364 V.
[0050] FIG. 12 is a plot of the current density of CO.sub.2
reduction by the WSe.sub.2 catalyst obtained through CA and pH of
the electrolyte as a function of the water volume fraction of the
electrolyte, as described in more detail in Example 5, below.
[0051] FIG. 13 is a set of CV curves for (A) Ag nanoparticles, (B)
bulk MoS.sub.2, and (C) WSe.sub.2 nanoflakes at different scan
rates. Curves were obtained in 0.5 M H.sub.2SO.sub.4 by sweeping
from 0 to +0.3V vs RHE, as described in more detail in Example 6,
below.
[0052] FIG. 14 is a set of plots showing the current density of the
CV experiments shown in FIG. 13 at +0.2 V vs RHE as a function of
scan rate. The slope of the linear fit provides the double layer
capacitance for each material, as described in more detail in
Example 6, below.
[0053] FIG. 15 is a plot of the CO formation turnover frequency
(TOF) of WSe.sub.2 nanoflakes, bulk MoS.sub.2, and Ag nanoparticles
at overpotentials of 54 to 650 mV, as described in more detail in
Example 6, below.
[0054] FIG. 16 is a calibration curve for CO production analysis by
the gas chromatography (GC) setup of Example 7, described in more
detail below.
[0055] FIG. 17 is a calibration curve for H.sub.2 production
analysis by the GC setup of Example 7, described in more detail
below.
[0056] FIG. 18 is a differential electrochemistry mass spectrometry
(DEMS) spectrum of the product of the .sup.13CO.sub.2 reduction
experiment described in more detail in Example 7, below.
[0057] FIG. 19 is a plot of the Faradaic efficiency (FE) of CO and
H.sub.2 production by WSe.sub.2 nanoflakes as a function of applied
potential, as described in more detail in Example 8, below.
[0058] FIG. 20 is a plot of the FE of CO and H.sub.2 production by
MoS.sub.2 nanoflakes as a function of applied potential, as
described in more detail in Example 8, below.
[0059] FIG. 21 is a plot of the FE of CO and H.sub.2 production by
MoSe.sub.2 nanoflakes as a function of applied potential, as
described in more detail in Example 8, below.
[0060] FIG. 22 is a plot of the FE of CO and H.sub.2 production by
WS.sub.2 nanoflakes as a function of applied potential, as
described in more detail in Example 8, below.
[0061] FIG. 23 is a plot of the performance (the product of the
current density and faradaic efficiency) of several catalytic
materials as a function of overpotential, as described in more
detail in Example 8, below.
[0062] FIG. 24 is a plot of the current density of WSe.sub.2
nanoflakes as a function of time over a 27-hour stability test, as
described in more detail in Example 9, below.
[0063] FIG. 25 is a schematic cross-sectional view of the
electrochemical device of Example 10, as described in more detail
below.
[0064] FIG. 26 is a schematic representation of the (A) transient
and (B) steady state operation regimes of the electrochemical
device of Example 10, as described in more detail in Example 11,
below.
[0065] FIG. 27 is a plot of the rate of CO and H.sub.2 formation of
the electrochemical device of Example 10, as described in more
detail in Example 12, below.
[0066] FIG. 28 is a set of optical images of the indium tin oxide
(ITO) layer of the photovoltaic (PV) cell of the electrochemical
device of Example 10 (A) before and (B) after 5 hours of continuous
operation, as described in more detail in Example 13, below. (C)
shows a selected region of the corrosion in more detail. Scale bars
are 250 .mu.m.
[0067] FIG. 29 is a set of plots of (A) the rate of product
formation of the electrochemical device of Example 10 under
different illumination levels and (B) the solar-to-fuel efficiency
(SFE) of the device, as described in more detail in Example 15,
below.
[0068] FIG. 30 is a plot of the SFE of the device of Example 10 as
a function of time, as described in more detail in Example 15,
below.
[0069] FIG. 31 is a representative electrochemical impedance
spectroscopy (EIS) spectrum for WSe.sub.2 nanoflakes, bulk
MoS.sub.2, and Ag nanoparticles at various overpotentials, as
described in more detail in Example 17, below. The smallest curve
is for WS.sub.2 NFs and the largest is for Ag NPs.
[0070] FIG. 32 is a plot of the work functions of various materials
calculated using ultraviolet photoelectron spectroscopy, as
described in more detail in Example 18, below.
[0071] FIG. 33 is a set of images and corresponding intensity
profiles of a WSe.sub.2 nanoflake (A-B) before and (C-D) after a
27-hour CA experiment, as described in more detail in Example 19,
below. Scale bars are 2 nm.
[0072] FIG. 34 is a set of representative X-ray photoelectron
spectroscopy (XPS) spectra of a WSe.sub.2 nanoflake (A-B) before
and (C-D) after a 27-hour CA experiment, as described in more
detail in Example 20, below.
[0073] FIG. 35 is a set of plots of the partial density of states
of the d band (spin up) of (A-C) the surface bare metal edge atom
(Mo and W) of the MoS.sub.2, MoS.sub.2, and WSe.sub.2 nanoflakes,
respectively, and (D) the surface Ag atom of bulk Ag(111), as
described in more detail in Example 21, below.
[0074] FIG. 36 is a set of plots of (A) the calculated partial
density of states of the d band (spin up) of the surface Ag atom of
Ag.sub.55 and (B) the surface bare metal edge atom (W) of WSe.sub.2
nanoflakes, as described in more detail in Example 21, below.
[0075] FIG. 37 is a set of the calculated free energy diagrams for
CO.sub.2 electroreduction to CO an Ag(111), Ag55 nanoparticles, and
MoS.sub.2, WS.sub.2, MoSe.sub.2, and MoS.sub.2 nanoflakes at 0 V vs
RHE, as described in more detail in Example 21, below. The traces,
top-to-bottom, are for Ag (111), Ag.sub.55, MoSe.sub.2, MoS.sub.2,
WSe.sub.2 and WS.sub.2, respectively.
[0076] FIG. 38 is a plot of the theoretical work functions
calculated for transition metal dichalcogenide monolayers, as
described in more detail in Example 21, below.
DETAILED DESCRIPTION
[0077] The particulars shown herein are by way of example and for
purposes of illustrative discussion of the preferred embodiments of
the present disclosure only and are presented in the cause of
providing what is believed to be the most useful and readily
understood description of the principles and conceptual aspects of
various embodiments of the disclosure. In this regard, no attempt
is made to show structural details of the devices and methods
described herein more detail than is necessary for the fundamental
understanding of the devices and methods described herein, the
description taken with the drawings and/or examples making apparent
to those skilled in the art how the several forms of the devices
and methods described herein may be embodied in practice. Thus,
before the disclosed processes and devices are described, it is to
be understood that the aspects described herein are not limited to
specific embodiments, apparati, or configurations, and as such can,
of course, vary. It is also to be understood that the terminology
used herein is for the purpose of describing particular aspects
only and, unless specifically defined herein, is not intended to be
limiting.
[0078] The terms "a," "an," "the" and similar referents used in the
context of describing the methods and devices of the disclosure
(especially in the context of the following claims) are to be
construed to cover both the singular and the plural, unless
otherwise indicated herein or clearly contradicted by context.
Recitation of ranges of values herein is merely intended to serve
as a shorthand method of referring individually to each separate
value falling within the range. Unless otherwise indicated herein,
each individual value is incorporated into the specification as if
it were individually recited herein. Ranges can be expressed herein
as from "about" one particular value, and/or to "about" another
particular value. When such a range is expressed, another aspect
includes from the one particular value and/or to the other
particular value. Similarly, when values are expressed as
approximations, by use of the antecedent "about," it will be
understood that the particular value forms another aspect. It will
be further understood that the endpoints of each of the ranges are
significant both in relation to the other endpoint, and
independently of the other endpoint.
[0079] All methods described herein can be performed in any
suitable order of steps unless otherwise indicated herein or
otherwise clearly contradicted by context. The use of any and all
examples, or exemplary language (e.g., "such as") provided herein
is intended merely to better illuminate the methods and devices of
the disclosure and does not pose a limitation on the scope of the
disclosure otherwise claimed. No language in the specification
should be construed as indicating any non-claimed element essential
to the practice of the methods and devices of the disclosure.
[0080] Unless the context clearly requires otherwise, throughout
the description and the claims, the words `comprise`, `comprising`,
and the like are to be construed in an inclusive sense as opposed
to an exclusive or exhaustive sense; that is to say, in the sense
of "including, but not limited to". Words using the singular or
plural number also include the plural and singular number,
respectively. Additionally, the words "herein," "above," and
"below" and words of similar import, when used in this application,
shall refer to this application as a whole and not to any
particular portions of the application.
[0081] As will be understood by one of ordinary skill in the art,
each embodiment disclosed herein can comprise, consist essentially
of or consist of its particular stated element, step, ingredient or
component. As used herein, the transition term "comprise" or
"comprises" means includes, but is not limited to, and allows for
the inclusion of unspecified elements, steps, ingredients, or
components, even in major amounts. The transitional phrase
"consisting of" excludes any element, step, ingredient or component
not specified. The transition phrase "consisting essentially of"
limits the scope of the embodiment to the specified elements,
steps, ingredients or components and to those that do not
materially affect the embodiment.
[0082] Unless otherwise indicated, all numbers expressing
quantities of ingredients, properties such as molecular weight,
reaction conditions, and so forth used in the specification and
claims are to be understood as being modified in all instances by
the term "about." Accordingly, unless indicated to the contrary,
the numerical parameters set forth in the specification and
attached claims are approximations that may vary depending upon the
desired properties sought to be obtained by methods and devices of
the present disclosure. At the very least, and not as an attempt to
limit the application of the doctrine of equivalents to the scope
of the claims, each numerical parameter should at least be
construed in light of the number of reported significant digits and
by applying ordinary rounding techniques. When further clarity is
required, the term "about" has the meaning reasonably ascribed to
it by a person skilled in the art when used in conjunction with a
stated numerical value or range, i.e. denoting somewhat more or
somewhat less than the stated value or range, to within a range of
.+-.20% of the stated value; .+-.19% of the stated value; .+-.18%
of the stated value; .+-.17% of the stated value; .+-.16% of the
stated value; .+-.15% of the stated value; .+-.14% of the stated
value; .+-.13% of the stated value; .+-.12% of the stated value;
.+-.11% of the stated value; .+-.10% of the stated value; .+-.9% of
the stated value; .+-.8% of the stated value; .+-.7% of the stated
value; .+-.6% of the stated value; .+-.5% of the stated value;
.+-.4% of the stated value; .+-.3% of the stated value; .+-.2% of
the stated value; or .+-.1% of the stated value.
[0083] Notwithstanding that the numerical ranges and parameters
setting forth the broad scope of the disclosure are approximations,
the numerical values set forth in the specific examples are
reported as precisely as possible. Any numerical value, however,
inherently contains certain errors necessarily resulting from the
standard deviation found in their respective testing
measurements.
[0084] Groupings of alternative elements or embodiments of the
disclosure disclosed herein are not to be construed as limitations.
Each group member may be referred to and claimed individually or in
any combination with other members of the group or other elements
found herein. It is anticipated that one or more members of a group
may be included in, or deleted from, a group for reasons of
convenience and/or patentability. When any such inclusion or
deletion occurs, the specification is deemed to contain the group
as modified thus fulfilling the written description of all Markush
groups used in the appended claims.
[0085] Some embodiments of this disclosure are described herein,
including the best mode known to the inventors for carrying out the
devices and methods of the disclosure. Of course, variations on
these described embodiments will become apparent to those of
ordinary skill in the art upon reading the foregoing description.
Skilled artisans will employ such variations as appropriate, and
the it is intended for the devices and methods of the disclosure to
be practiced otherwise than specifically described herein.
Accordingly, this disclosure includes all modifications and
equivalents of the subject matter recited in the claims appended
hereto as permitted by applicable law. Moreover, any combination of
the above-described elements in all possible variations thereof is
encompassed by the disclosure unless otherwise indicated herein or
otherwise clearly contradicted by context.
[0086] Furthermore, numerous references have been made to patents
and printed publications throughout this specification. Each of the
cited references and printed publications are individually
incorporated herein by reference in their entirety.
[0087] In closing, it is to be understood that the embodiments of
the disclosure disclosed herein are illustrative of the principles
of the methods and devices of the present disclosure. Other
modifications that may be employed are within the scope of the
disclosure. Thus, by way of example, but not of limitation,
alternative configurations of the methods and devices of the
present disclosure may be utilized in accordance with the teachings
herein. Accordingly, the methods and devices of the present
disclosure are not limited to that precisely as shown and
described.
[0088] In various aspects and embodiments, the disclosure relates
to the electrochemical or photoelectrochemical reduction of carbon
dioxide and, optionally, the oxidation of water in a device
including a cathode comprising at least one transition metal
dichalcogenide and, optionally, an anode comprising a metal and a
photovoltaic cell. The disclosure demonstrates such methods and
devices to efficiently reduce carbon dioxide, and to involve
robust, relatively inexpensive catalysts and manageable reaction
media.
[0089] One aspect of the disclosure is a method of
electrochemically reducing carbon dioxide and oxidizing water. The
method includes providing an electrochemical cell comprising a
first compartment including a cathode in contact with at least one
transition metal dichalcogenide, carbon dioxide, and a first
electrolyte, the first compartment being in ionic contact with a
second compartment including an anode in contact with at least one
water oxidizing catalyst, water, and a second electrolyte, and a
photovoltaic cell in electrical contact with the anode and the
cathode. The method also includes exposing the photovoltaic cell to
light irradiation, for example, sufficient to create a potential
difference between the anode and the cathode sufficient to reduce
carbon dioxide at the cathode and to oxidize water at the cathode.
The light irradiation can be, for example, at an average intensity
of at least 0.5 sun, for example at least 0.75 sun or at least 0.9
sun. For example, the light irradiation can be, in various example
embodiments of the devices and methods as described herein, in the
range of 0.5 sun to 3 sun, or 0.75 sun to 3 sun, or 0.9 sun to 3
sun, or 0.5 sun to 2 sun, or 0.75 sun to 2 sun, or 0.9 sun to 2
sun.
[0090] In the methods and devices of the disclosure, the cathode of
the first compartment is in contact with at least one transition
metal dichalcogenide. In some embodiments of the methods and
devices as otherwise described herein, the transition metal
dichalcogenide is, e.g., TiX.sub.2, VX.sub.2, CrX.sub.2, ZrX.sub.2,
NbX.sub.2, MoX.sub.2, HfX.sub.2, WX.sub.2, TaX.sub.2, TcX.sub.2, or
ReX.sub.2, wherein X is independently S, Se, or Te. In some
embodiments, the transition metal dichalcogenide is TiX.sub.2,
MoX.sub.2, or WX.sub.2, wherein X is independently S, Se, or Te. In
some embodiments, the transition metal dichalcogenide is TiS.sub.2,
TiSe.sub.2, MoS.sub.2, MoSe.sub.2, WS.sub.2, or WSe.sub.2. In one
embodiment, the transition metal dichalcogenide is MoS.sub.2 or
WS.sub.2. In another embodiment, the transition metal
dichalcogenide is MoSe.sub.2 or WSe.sub.2. In yet another
embodiment, the transition metal dichalcogenide is MoSe.sub.2 or
WSe.sub.2. In one example, the transition metal dichalcogenide is
MoS.sub.2. In another example, the transition metal dichalcogenide
is WSe.sub.2.
[0091] The at least one transition metal dichalcogenide can be
provided in a variety of forms, for example, as a bulk material, in
nanostructure form, as a collection of particles, and/or as a
collection of supported particles. As the person of ordinary skill
in the art will appreciate, the transition metal dichalcogenide in
bulk form may have a layered structure as is typical for such
compounds. The transition metal dichalcogenide may have a
nanostructure morphology, including but not limited to monolayers,
nanotubes, nanoparticles, nanoflakes (e.g., multilayer nanoflakes),
nanosheets, nanoribbons, nanoporous solids etc. As used herein, the
term "nanostructure" refers to a material with a dimension (e.g.,
of a pore, a thickness, a diameter, as appropriate for the
structure) in the nanometer range (i.e., greater than 1 nm and less
than 1 .mu.m). In some embodiments, the transition metal
dichalcogenide is a layer-stacked bulk transition metal
dichalcogenide with metal atom-terminated edges (e.g., MoS.sub.2
with molybdenum-terminated edges). In other embodiments, transition
metal dichalcogenide nanoparticles (e.g., MoS.sub.2 nanoparticles)
may be used in the devices and methods of the disclosure. In other
embodiments, transition metal dichalcogenide nanoflakes (e.g.,
nanoflakes of MoS.sub.2) may be used in the devices and methods of
the disclosure. Nanoflakes can be made, for example, via liquid
exfoliation, as described in Coleman, J. N. et al.,
"Two-dimensional nanosheets produced by liquid exfoliation of
layered materials." Science 331, 568-71 (2011) and Yasaei, P. et
al., "High-Quality Black Phosphorus Atomic Layers by Liquid-Phase
Exfoliation." Adv. Mater. (2015) (doi:10.1002/adma.201405150), each
of which is hereby incorporated herein by reference in its
entirety. In other embodiments, transition metal dichalcogenide
nanoribbons (e.g., nanoribbons of MoS.sub.2) may be used in the
devices and methods of the disclosure. In other embodiments,
transition metal dichalcogenide nanosheets (e.g., nanosheets of
MoS.sub.2) may be used in the devices and methods of the
disclosure. The person of ordinary skill in the art can select the
appropriate morphology for a particular device.
[0092] In certain embodiments of the methods and devices as
otherwise described herein, the transition metal dichalcogenide
nanostructures (e.g., nanoflakes, nanoparticles, nanoribbons, etc.)
have an average size between about 1 nm and 1000 nm. The relevant
size for a nanoparticle is its largest diameter. The relevant size
for a nanoflake is its largest width along its major surface. The
relevant size for a nanoribbon is its width across the ribbon. The
relevant size for a nanosheet is its thickness. In some
embodiments, the transition metal dichalcogenide nanostructures
have an average size between from about 1 nm to about 400 nm, or
about 1 nm to about 350 nm, or about 1 nm to about 300 nm, or about
1 nm to about 250 nm, or about 1 nm to about 200 nm, or about 1 nm
to about 150 nm, or about 1 nm to about 100 nm, or about 1 nm to
about 80 nm, or about 1 nm to about 70 nm, or about 1 nm to about
50 nm, or 50 nm to about 400 nm, or about 50 nm to about 350 nm, or
about 50 nm to about 300 nm, or about 50 nm to about 250 nm, or
about 50 nm to about 200 nm, or about 50 nm to about 150 nm, or
about 50 nm to about 100 nm, or about 10 nm to about 70 nm, or
about 10 nm to about 80 nm, or about 10 nm to about 100 nm, or
about 100 nm to about 500 nm, or about 100 nm to about 600 nm, or
about 100 nm to about 700 nm, or about 100 nm to about 800 nm, or
about 100 nm to about 900 nm, or about 100 nm to about 1000 nm, or
about 400 nm to about 500 nm, or about 400 nm to about 600 nm, or
about 400 nm to about 700 nm, or about 400 nm to about 800 nm, or
about 400 nm to about 900 nm, or about 400 nm to about 1000 nm. In
certain embodiments, the transition metal dichalcogenide
nanostructures have an average size between from about 1 nm to
about 200 nm. In certain other embodiments, the transition metal
dichalcogenide nanostructures have an average size between from
about 1 nm to about 400 nm. In certain other embodiments, the
transition metal dichalcogenide nanostructures have an average size
between from about 400 nm to about 1000 nm. In certain embodiments,
the transition metal dichalcogenide nanostructures are nanoflakes
having an average size between from about 1 nm to about 200 nm. In
certain other embodiments, the transition metal dichalcogenide
nanoflakes have an average size between from about 1 nm to about
400 nm. In certain other embodiments, the transition metal
dichalcogenide nanoflakes have an average size between from about
400 nm to about 1000 nm.
[0093] In certain embodiments of the methods and devices as
otherwise described herein, transition metal dichalcogenide
nanoflakes have an average thickness between about 1 nm and about
100 .mu.m (e.g., about 1 nm to about 10 .mu.m, or about 1 nm to
about 1 .mu.m, or about 1 nm to about 1000 nm, or about 1 nm to
about 400 nm, or about 1 nm to about 350 nm, or about 1 nm to about
300 nm, or about 1 nm to about 250 nm, or about 1 nm to about 200
nm, or about 1 nm to about 150 nm, or about 1 nm to about 100 nm,
or about 1 nm to about 80 nm, or about 1 nm to about 70 nm, or
about 1 nm to about 50 nm, or about 50 nm to about 400 nm, or about
50 nm to about 350 nm, or about 50 nm to about 300 nm, or about 50
nm to about 250 nm, or about 50 nm to about 200 nm, or about 50 nm
to about 150 nm, or about 50 nm to about 100 nm, or about 10 nm to
about 70 nm, or about 10 nm to about 80 nm, or about 10 nm to about
100 nm, or about 100 nm to about 500 nm, or about 100 nm to about
600 nm, or about 100 nm to about 700 nm, or about 100 nm to about
800 nm, or about 100 nm to about 900 nm, or about 100 nm to about
1000 nm, or about 400 nm to about 500 nm, or about 400 nm to about
600 nm, or about 400 nm to about 700 nm, or about 400 nm to about
800 nm, or about 400 nm to about 900 nm, or about 400 nm to about
1000 nm); and average dimensions along the major surface of about
20 nm to about 100 .mu.m (e.g., about 20 nm to about 50 .mu.m, or
about 20 nm to about 10 .mu.m, or about 20 nm to about 1 .mu.m, or
about 50 nm to about 100 .mu.m, or about 50 nm to about 50 .mu.m,
or about 50 nm to about 10 .mu.m, or about 50 nm to about 1 .mu.m,
or about 100 nm to about 100 .mu.m, or about 100 nm to about 50
.mu.m, or about 100 nm to about 10 .mu.m, or about 100 nm to about
1 .mu.m), The aspect ratio (largest major dimension:thickness) of
the nanoflakes can be on average, for example, at least about 5:1,
at least about 10:1 or at least about 20:1. For example, in certain
embodiments the transition metal dichalcogenide nanoflakes have an
average thickness in the range of about 1 nm to about 1000 nm
(e.g., about 1 nm to about 100 nm), average dimensions along the
major surface of about 50 nm to about 10 .mu.m, and an aspect ratio
of at least about 5:1.
[0094] In some embodiments of the methods and devices as otherwise
described herein, the first electrolyte comprises at least one
helper catalyst. The person of ordinary skill in the art will
appreciate that the term "helper catalyst" refers to an organic
molecule or mixture of organic molecules that does at least one of
the following: (a) speeds up the carbon dioxide reduction reaction,
or (b) lowers the overpotential of the carbon dioxide reduction
reaction, without being substantially consumed in the process. The
helper catalysts useful in the methods and the compositions of the
disclosure are described in detail in International Application
Nos. PCT/US2011/030098 (published as WO 2011/120021) and
PCT/US2011/042809 (published as WO 2012/006240) and in U.S.
Publication No. 2013/0157174, each of which is hereby incorporated
herein by reference in its entirety. In certain embodiments, the
helper catalyst is a compound comprising at least one positively
charged nitrogen, sulfur, or phosphorus group (for example, a
phosphonium or a quaternary amine). Aqueous solutions including one
or more of: ionic liquids, deep eutectic solvents, amines, and
phosphines; including specifically imidazoliums (also called
imidazoniums), pyridiniums, pyrrolidiniums, phosphoniums,
ammoniums, choline sulfoniums, prolinates, and methioninates can
form complexes with (CO.sub.2).sup.-, and as a result, can serve as
the helper catalysts. Specific examples of helper catalysts
include, but are not limited to, one or more of acetylcholines,
alanines, aminoacetonitriles, methylammoniums, arginines, aspartic
acids, threonines, chloroformamidiniums, thiouroniums,
quinoliniums, pyrrolidinols, serinols, benzamidines, sulfamates,
acetates, carbamates, inflates, and cyanides. These examples are
meant for illustrative purposes only, and are not meant to limit
the scope of the present disclosure. Aqueous solutions including
the helper catalysts described herein can be used as the
electrolyte. Such aqueous solutions can include other species, such
as acids, bases and salts, to provide the desired electrochemical
and physicochemical properties to the electrolyte as would be
evident to the person of ordinary skill in the art.
[0095] In certain embodiments, the helper catalysts of the
disclosure include, but are not limited to imidazoliums,
pyridiniums, pyrrolidiniums, phosphoniums, ammoniums, sulfoniums,
prolinates, and methioninates salts. The anions suitable to form
salts with the cations of the helper catalysts include, but are not
limited to C.sub.1-C.sub.6 alkylsulfate, tosylate,
methanesulfonate, bis(trifluoromethylsulfonyl)imide,
hexafluorophosphate, tetrafluoroborate, triflate, halide,
carbamate, and sulfamate. In particular embodiments, the helper
catalysts may be a salt of the cations selected from those in Table
1.
TABLE-US-00001 TABLE 1 Cationic Helper Catalysts ##STR00001##
##STR00002## ##STR00003## ##STR00004## ##STR00005## ##STR00006##
##STR00007## ##STR00008## ##STR00009## ##STR00010## ##STR00011##
##STR00012## ##STR00013## ##STR00014## ##STR00015## ##STR00016##
##STR00017## ##STR00018## ##STR00019## ##STR00020##
##STR00021##
wherein R.sub.1-R.sub.12 are independently selected from the group
consisting of hydrogen, --OH, linear aliphatic C.sub.1-C.sub.6
group, branched aliphatic C.sub.1-C.sub.6 group, cyclic aliphatic
C.sub.1-C.sub.6 group, --CH.sub.2OH, --CH.sub.2CH.sub.2OH,
--CH.sub.2CH.sub.2CH.sub.2OH, --CH.sub.2CHOHCH.sub.3,
--CH.sub.2COH, --CH.sub.2CH.sub.2COH, and --CH.sub.2COCH.sub.3.
[0096] In certain embodiments, the helper catalyst of the methods
and compositions of the disclosure is imidazolium salt of
formula:
##STR00022##
wherein R.sub.1, R.sub.2, and R.sub.3 are independently selected
from the group consisting of hydrogen, linear aliphatic
C.sub.1-C.sub.6 group, branched aliphatic C.sub.1-C.sub.6 group,
and cyclic aliphatic C.sub.1-C.sub.6 group. In other embodiments,
R.sub.2 is hydrogen, and R.sub.1 and R.sub.3 are independently
selected from linear or branched C.sub.1-C.sub.4 alkyl. In
particular embodiments, the helper catalyst of the disclosure is
1-ethyl-3-methylimidazolium salt. In other embodiments, the helper
catalyst of the disclosure is 1-ethyl-3-methylimidazolium
tetrafluoroborate (EMIM-BF.sub.4).
[0097] In some embodiments, the helper catalyst may be neutral
organics, such as 2-amino alcohol derivatives, isoetarine
derivatives, and norepinepherine derivatives. These examples are
meant for illustrative purposes only, and are not meant to limit
the scope of the present disclosure.
[0098] Of course, not every substance that forms a complex with
(CO.sub.2).sup.- will act as a helper catalyst. When an
intermediate binds to a catalyst, the reactivity of the
intermediate decreases. If the intermediate bonds too strongly to
the catalyst, the intermediate will become unreactive, so the
substance will not be effective. The person of ordinary skill in
the art will understand that this can provides a key limitation on
substances that act as helper catalysts, and will select the helper
catalyst accordingly.
[0099] In general, a person of skill in the art can determine
whether a given ionic liquid is a co-catalyst for a reaction (R)
catalyzed by TM DC as follows: [0100] (a) fill a standard 3
electrode electrochemical cell with the electrolyte commonly used
for reaction R. Common electrolytes include such as 0.1 M sulfuric
acid or 0.1 M KOH in water can also be used; [0101] (b) mount the
TMDC into the 3 electrode electrochemical cell and an appropriate
counter electrode; [0102] (c) run several CV cycles to clean the
cell; [0103] (d) measure the reversible hydrogen electrode (RHE)
potential in the electrolyte; [0104] (e) load the reactants for the
reaction R into the cell, and measure a CV of the reaction R,
noting the potential of the peak associated with the reaction R;
[0105] (f) calculate VI, which is the difference between the onset
potential of the peak associated with reaction and RHE; [0106] (g)
calculate VIA, which is the difference between the maximum
potential of the peak associated with reaction and RHE; [0107] (h)
add 0.0001 to 99.9999 weight % of the ionic liquid to the
electrolyte; [0108] (i) measure RHE in the reaction with ionic
liquid; [0109] (j) measure the CV of reaction R again, noting the
potential of the peak associated with the reaction R; [0110] (k)
calculate V2, which is the difference between the onset potential
of the peak associated with reaction and RHE; and [0111] (l)
calculate V2A, which is the difference between the maximum
potential of the peak associated with reaction and RHE.
[0112] If V2<V1 or V2A<VIA at any concentration of the ionic
liquid (e.g., between 0.0001 and 99.9999 weight %), the ionic
liquid is a co-catalyst for the reaction.
[0113] The person of skill in the art will also recognize that the
benefits of the helper catalyst may be realized at small amount of
the helper catalyst relative to the transition metal
dichalcogenide. One can obtain an estimate of the helper catalyst
amount needed to change the reaction from a Pease study ("The
Catalytic Combination of Ethylene and Hydrogen in the Presence of
Metallic Copper III. Carbon Monoxide as a Catalyst Poison" J. Am.
Chem. Soc., 1925, 47(5), pp 1235-1240), which is incorporated into
this disclosure by reference in its entirety) of the effect of
carbon monoxide (CO) on the rate of ethylene hydrogenation on
copper. Pease found that 0.05 cc (62 micrograms) of carbon monoxide
(CO) was sufficient to almost completely poison a 100 gram catalyst
towards ethylene hydrogenation. This corresponds to a poison
concentration of 0.0000062% by weight of CO in the catalyst. Those
familiar with the technology involved here know that if 0.0000062%
by weight of the poison in a catalytically active element-poison
mixture could effectively suppress a reaction, then as little as
0.0000062% by weight of the helper catalyst relative to the amount
of the transition metal dichalcogenide could enhance a reaction.
This provides an example of a lower limit to the helper catalyst
concentration relative to the transition metal dichalcogenide.
Thus, in certain embodiments, the helper catalyst is present from
about 0.000005 weight % to about 50 weight % relative to the weight
of transition metal dichalcogenide. In some other embodiments, the
amount of the helper catalyst is between about 0.000005 weight % to
about 20 weight %, or about 0.000005 weight % to about 10 weight %,
or about 0.000005 weight % to about 1 weight %, or about 0.000005
weight % to about 0.5 weight %, or about 0.000005 weight % to about
0.05 weight %, or about 0.00001 weight % to about 20 weight %, or
about 0.00001 weight % to about 10 weight %, or about 0.00001
weight % to about 1 weight %, or about 0.00001 weight % to about
0.5 weight %, or about 0.00001 weight % to about 0.05 weight %, or
about 0.0001 weight % to about 20 weight %, or about 0.0001 weight
% to about 10 weight %, or about 0.0001 weight % to about 1 weight
%, or about 0.0001 weight % to about 0.5 weight %, or about 0.0001
weight % to about 0.05 weight %.
[0114] Further, the helper catalyst may be dissolved in water or
other aqueous solution, a solvent for the reaction, an electrolyte,
an acidic electrolyte, a buffer solution, an ionic liquid, an
additive to a component of the system, or a solution that is bound
to at least one of the catalysts in a system. These examples are
meant for illustrative purposes only, and are not meant to limit
the scope of the present disclosure.
[0115] In some embodiments of the methods and devices as otherwise
described herein, the first electrolyte is an aqueous solution. In
certain embodiments, the first electrolyte is an aqueous solution
comprising the at least one helper catalyst. In some embodiments,
the helper catalyst is present in the aqueous solution in a
concentration within the range of about 5 vol. % to about 75 vol.
%, e.g., about 10 vol. % to about 75 vol. %, or about 15 vol. % to
about 75 vol. %, or about 20 vol. % to about 75 vol. %, or about 25
vol. % to about 75 vol. %, or about 30 vol. % to about 75 vol. %,
or about 35 vol. % to about 75 vol. %, or about 40 vol. % to about
75 vol. %, or about 45 vol. % to about 75 vol. %, or about 30 vol.
% to about 70 vol. %, or about 35 vol. % to about 65 vol. %, or
about 40 vol. % to about 60 vol. %, or about 45 vol. % to about 55
vol. %, or about 5 vol. %, or about 10 vol. %, or about 15 vol. %,
or about 20 vol. %, or about 25 vol. %, or about 30 vol. %, or
about 35 vol. %, or about 40 vol. %, or about 45 vol. %, or about
50 vol. %, or about 55 vol. %, or about 60 vol. %, or about 65 vol.
%, or about 70 vol. %, or about 75 vol. %.
[0116] The person of ordinary skill in the art will appreciate that
the first electrolyte of the methods and devices as otherwise
described herein may further include, e.g., nonaqueous solvents, a
buffer solution, an additive to a component of the system, or a
solution that is bound to a catalyst included in the first
compartment. In certain embodiments of the methods and devices as
otherwise described herein, the first electrolyte may further
comprise other species, such as acids, bases, and salts. The
inclusion of such other species would be evident to the person of
ordinary skill in the art depending on the desired electrochemical
and physicochemical properties of the first electrolyte, and are
not meant to limit the scope of the present disclosure.
[0117] The devices and methods of the disclosure involve reducing
CO.sub.2. The person of ordinary skill in the art will appreciate
that, e.g., in water, CO.sub.2 may form chemical derivatives such
as carbonic acid, bicarbonate, or carbonate. As used herein,
CO.sub.2 and such derivatives may be referred to interchangeably,
i.e., reference to CO.sub.2 reduction in an aqueous solution may
also refer to carbonate, bicarbonate, or carbonic acid reduction in
an aqueous solution. In some embodiments of the methods and devices
as otherwise described herein, a reactant comprising CO.sub.2,
carbonate, or bicarbonate is fed into the first compartment of the
electrochemical device. For example, gaseous CO.sub.2 may be
continuously bubbled through the first compartment. A voltage is
applied to the first compartment, i.e., upon exposure of the
photovoltaic cell to light irradiation, and the CO.sub.2 reacts to
form new chemical compounds. As the person of ordinary skill in the
art will recognize, CO.sub.2 (as well as carbonate or bicarbonate)
may be reduced into various useful chemical products, including but
not limited to CO, syngas (mixture of CO and H.sub.2), OH.sup.-,
HCO.sup.-, H.sub.2CO, (HCO.sub.2).sup.-, H.sub.2CO.sub.2,
CH.sub.3OH, CH.sub.4, C.sub.2H.sub.4, CH.sub.3CH.sub.2OH,
CH.sub.3CO.sup.-, CH.sub.3COOH, C.sub.2H.sub.6, O.sub.2, H.sub.2,
(COOH).sub.2, and (COO.sup.-).sub.2. In certain embodiments,
CO.sub.2 may be reduced to form CO, H.sub.2, or a mixture of CO and
H.sub.2.
[0118] Advantageously, the carbon dioxide used in the embodiments
of the disclosure can be obtained from any source, e.g., an exhaust
stream from fossil-fuel burning power or industrial plants, from
geothermal or natural gas wells or the atmosphere itself. In
certain embodiments, carbon dioxide is anaerobic. In other
embodiments, carbon dioxide is obtained from concentrated point
sources of its generation prior to its release into the atmosphere.
For example, high concentration carbon dioxide sources are those
frequently accompanying natural gas in amounts of 5 to 50%, those
from flue gases of fossil fuel (coal, natural gas, oil, etc.)
burning power plants, and nearly pure CO.sub.2 exhaust of cement
factories and from fermenters used for industrial fermentation of
ethanol. Certain geothermal steams also contain significant amounts
of CO.sub.2. In other words, CO.sub.2 emissions from varied
industries, including geothermal wells, can be captured on-site.
Separation of CO.sub.2 from such exhausts is well-known. Thus, the
capture and use of existing atmospheric CO.sub.2 in accordance with
embodiments of the disclosure allows CO.sub.2 to be a renewable and
unlimited source of carbon.
[0119] In some embodiments of the methods and devices as otherwise
described herein, the reduction of carbon dioxide may be initiated
at high current densities. For example, in certain embodiments, the
current density of carbon dioxide reduction is at least 30
mA/cm.sup.2, or at least 40 mA/cm.sup.2, or at least 50
mA/cm.sup.2, or at least 55 mA/cm.sup.2, or at least 60
mA/cm.sup.2, or at least 65 mA/cm.sup.2. In one embodiment, the
current density of carbon dioxide reduction is between about 30
mA/cm.sup.2 and about 130 mA/cm.sup.2, or about 30 mA/cm.sup.2 and
about 100 mA/cm.sup.2, or about 30 mA/cm.sup.2 and about 80
mA/cm.sup.2, or about 40 mA/cm.sup.2 and about 130 mA/cm.sup.2, or
about 40 mA/cm.sup.2 and about 100 mA/cm.sup.2,or about 40
mA/cm.sup.2 and about 80 mA/cm.sup.2, or about 50 mA/cm.sup.2 and
about 70 mA/cm.sup.2, or about 60 mA/cm.sup.2 and about 70
mA/cm.sup.2, or about 63 mA/cm.sup.2 and about 67 mA/cm.sup.2, or
about 60 mA/cm.sup.2, or about 65 mA/cm.sup.2, or about 70
mA/cm.sup.2.
[0120] In some embodiments of the methods and devices as otherwise
described herein, the reduction of carbon dioxide may be initiated
at low overpotential. For example, in certain embodiments, the
initiation overpotential is less than about 200 mV. In other
embodiments, the initiation overpotential is less than about 100
mV, or less than about 90 mV, or less than about 80 mV, or less
than about 75 mV, or less than about 70 mV, or less than about 65
mV, or less than about 60 mV, or less than about 57 mV, or less
than about 55 mV, or the initiation overpotential is within the
range of about 50 mV to about 100 mV, or about 50 mV to about 90
mV, or about 50 mV to about 80 mV, or about 50 mV to about 75 mV,
or about 50 mV to about 70 mV, or about 50 mV to about 65 mV, or
about 50 mV to about 60 mV. In some embodiments, the reduction of
carbon dioxide is initiated at overpotential of about 50 mV to
about 57 mV, or about 51 mV to about 57 mV, or about 52 mV to about
57 mV, or about 52 mV to about 55 mV, or about 53 mV to about 55
mV, or about 53 mV, or about 54 mV, or about 55 mV.
[0121] The methods described herein can be performed at a variety
of pressures and temperatures, and a person of skill in the art
would be able to optimize these conditions to achieve the desired
performance. For example, in certain embodiments, the methods of
the disclosure are performed at a pressure in the range of about
0.1 atm to about 2 atm, or about 0.2 atm to about 2 atm, or about
0.5 atm to about 2 atm, or about 0.5 atm to about 1.5 atm, or about
0.8 atm to about 2 atm, or about 0.9 atm to about 2 atm, about 0.1
atm to about 1 atm, or about 0.2 atm to about 1 atm, or about 0.3
atm to about 1 atm, or about 0.4 atm to about 1 atm, or about 0.5
atm to about 1 atm, or about 0.6 atm to about 1 atm, or about 0.7
atm to about 1 atm, or about 0.8 atm to about 1 atm, or about 1 atm
to about 1.5 atm, or about 1 atm to about 2 atm. In one particular
embodiment, the methods of the disclosure are carried at a pressure
of about 1 atm. In other embodiments, the methods of the disclosure
are carried out at a temperature within the range of about
0.degree. C. to about 50.degree. C., or of about 10.degree. C. to
about 50.degree. C., or of about 10.degree. C. to about 40.degree.
C., or of about 15.degree. C. to about 35.degree. C., or of about
20.degree. C. to about 30.degree. C., or of about 20.degree. C. to
about 25.degree. C., or at about 20.degree. C., or at about
21.degree. C., or at about 22.degree. C., or at about 23.degree.
C., or at about 24.degree. C., or at about 25.degree. C. In one
particular embodiment, the methods of the disclosure are carried
out at a temperature of about 20.degree. C. to about 25.degree. C.
The methods of the disclosure may last, for example, for a time
within the range of about several minutes to several days and
months.
[0122] Advantageously, in certain embodiments the methods described
herein can be operated at a Faradaic efficiency (FE) within the
range of 0 to 100% for the reduction of carbon dioxide to CO. In
some embodiments, the Faradaic efficiency of the carbon
dioxide-to-CO reduction is at least about 3%, or at least about 5%,
or at least about 8%, or at least about 10%, or at least about 20%,
or at least about 25%, or at least about 50%, or at least about
60%, or at least about 70%, or at least about 75%, or at least
about 80%, or at least about 85%.
[0123] The person of ordinary skill in the art will appreciate that
the cathode of the first compartment may comprise any of a number
of conductive materials known in the art. In some embodiments, the
cathode comprises, e.g., copper, aluminum, carbon black, or
stainless steel. In some embodiments, the cathode comprises
stainless steel. The person of ordinary skill in the art will
further appreciate that the at least one transition metal
dichalcogenide may be contacted with the cathode by a variety of
means. For example, in some embodiments, the transition metal
dichalcogenide may be disposed on the cathode. In some embodiments,
the transition metal dichalcogenide is disposed on a porous member.
The porous member may be electrically conductive, in which case the
porous member may be in electrical contact with the cathode. In
cases where the porous member is not electrically conductive, the
person of ordinary skill in the art can arrange for the electrical
connection of the cathode to be made to some other part of the at
least one transition metal dichalcogenide.
[0124] In some embodiments of the methods as otherwise described
herein, the at least one transition metal dichalcogenide is coated
onto the cathode at a thickness of, e.g., up to 1000 .mu.m. The
person of ordinary skill in the art will appreciate that the
thickness of the at least one transition metal dichalcogenide may
be any convenient thickness, provided CO.sub.2 can be reduced in
the electrochemical device.
[0125] In the methods and devices of the disclosure, the second
compartment includes an anode in contact with at least one water
oxidizing catalyst. As used herein, the term "water oxidizing
catalyst" refers to a compound capable of catalyzing the
reaction:
2H.sub.2O.fwdarw.O.sub.2+4H.sup.+,
and may be used interchangeably with "oxygen evolving catalyst." In
some embodiments of the methods and devices as otherwise described
herein, the water oxidizing catalyst comprises cobalt, e.g.,
Co.sup.3+.
[0126] In the methods and devices of the disclosure, the second
compartment includes water and a second electrolyte. In some
embodiments of the methods and devices as otherwise described
herein, the second electrolyte and the water comprise an aqueous
solution. In some embodiments, the aqueous solution comprises
phosphate. In some embodiments, the phosphate comprises potassium
phosphate, e.g., KH.sub.2PO.sub.4. In some embodiments, the
phosphate is present in the aqueous solution in a concentration
within the range of about 0.01 mM to about 100 mM, e.g., about 0.05
mM to about 50 mM, or about 0.05 mM to about 10 mM, or about 0.05
mM to about 5 mM, or about 0.05 mM to about 1 mM, or about 0.05 mM
to about 0.9 mM, or about 0.05 mM to about 0.8 mM, or about 0.05 mM
to about 0.7 mM, or about 0.05 mM to about 0.6 mM, or about 0.05 mM
to about 0.5 mM, or about 0.05 mM to about 0.45 mM, or about 0.1 mM
to about 0.4 mM, or about 0.15 mM to about 0.35 mM, or about 0.2 mM
to about 0.3 mM.
[0127] The person of ordinary skill in the art will appreciate that
the second electrolyte of the methods and devices as otherwise
described herein may further include, e.g., nonaqueous solvents, a
buffer solution, an additive to a component of the system, or a
solution that is bound to a catalyst included in the second
compartment. In certain embodiments of the methods and devices as
otherwise described herein, the second electrolyte may further
comprise other species, such as acids, bases, and salts. The
inclusion of such other species would be evident to the person of
ordinary skill in the art depending on the desired electrochemical
and physicochemical properties of the second electrolyte, and are
not meant to limit the scope of the present disclosure.
[0128] The person of ordinary skill in the art will appreciate that
the anode of the second compartment may comprise any of a number of
conductive materials known in the art. In some embodiments, the
anode comprises, e.g., copper, aluminum, carbon black, or stainless
steel. In some embodiments, the anode comprises indium tin oxide
(ITO). The person of ordinary skill in the art will further
appreciate that the at least one water oxidation catalyst may be
contacted with the anode by a variety of means. For example, in
some embodiments, the water oxidation catalyst may be disposed on
the cathode. In some embodiments, the water oxidation catalyst is
disposed on a porous member. The porous member may be electrically
conductive, in which case the porous member may be in electrical
contact with the anode. In cases where the porous member is not
electrically conductive, the person of ordinary skill in the art
can arrange for the electrical connection of the anode to be made
to some other part of the at least one water oxidation
catalyst.
[0129] In some embodiments of the methods as otherwise described
herein, the at least one water oxidation catalyst is coated onto
the cathode at a thickness of, e.g., up to 1000 .mu.m. The person
of ordinary skill in the art will appreciate that the thickness of
the at least one water oxidation catalyst may be any convenient
thickness, provided water can be oxidized in the electrochemical
device.
[0130] In the methods and devices of the disclosure, the
electrochemical device includes at least one photovoltaic cell. The
person of ordinary skill in the art will appreciate that the
photovoltaic cell may provide the electrical energy for the
electrochemical reduction of carbon dioxide and the oxidation of
water. The person of ordinary skill in the art will appreciate that
the photovoltaic cell may be any of a variety of types and/or
arrangements of photovoltaic cells, provided the potential supplied
to the cathode and anode is sufficient to drive carbon dioxide
reduction and water oxidation in the electrochemical device.
[0131] In some embodiments of the methods and devices as otherwise
described herein, the at least one photovoltaic cell is a
multi-junction photovoltaic cell. In some embodiments, the
electrochemical device includes two or more photovoltaic cells
connected in series. For example, in certain embodiments, the
electrochemical device comprises two multi-junction photovoltaic
cells connected in series (See, e.g., FIG. 25).
[0132] In some embodiments of the methods and devices as otherwise
described herein, the at least one photovoltaic cell comprises Si
or Ge. In some embodiments, the at least one photovoltaic cell
comprises a layer comprising amorphous Si. In some embodiments, the
at least one photovoltaic cell comprises a layer comprising
amorphous SiGe. In some embodiments, the at least one photovoltaic
cell is a multi-junction cell photovoltaic cell comprising one or
more layers comprising amorphous Si, and one or more layers
comprising amorphous SiGe. For example, in certain embodiments, the
electrochemical device includes two identical multi-junction
photovoltaic cells connected in series, each cell comprising one
layer comprising amorphous Si and two layers comprising amorphous
SiGe.
[0133] In some embodiments of the methods and devices as otherwise
described herein, the at least one photovoltaic cell is capable of
providing at least about 2.5 V across the cell, e.g., at least
about 2.6 V, or at least about 2.7 V, or at least about 2.8 V, or
at least about 2.9 V, or at least about 3 V. In some embodiments,
the at least one photovoltaic cell is capable of operating with an
efficiency of at least about 4%, e.g. at least about 4.5%, or at
least about 5%, or at least about 5.5%, or at least about 6%.
[0134] In the methods and devices of the disclosure, the first
compartment and the second compartment are in ionic contact. As
used herein, the term "ionic contact" refers to the ability to
transport ions from one area to a second area. For example, ions
may be transported from a first compartment to a second compartment
in ionic contact therewith. Ionic contact may be selective for
ions, e.g., by limiting transport of neutral molecules. Ionic
contact may be selective for a specific charge, e.g., selective for
positive ions, or for a specific ion, e.g., selective for protons.
The person of ordinary skill in the art will appreciate that there
exists in the art a variety of means for providing ionic contact,
e.g., between two compartments, such as, for example,
ion-conductive membranes, proton-conductive membranes, etc.
[0135] In some embodiments of the methods and devices of the
disclosure, the first and second compartments are in general
contained and physically separated, e.g., by glass, steel, indium
tin oxide, etc., but in part are separated by an ion-conductive
(i.e., ion-exchange) membrane, e.g., a proton-conductive membrane.
In some embodiments, the first and second compartments are
physically contained, e.g., by glass, steel, indium tin oxide,
etc., but are separated only by an ion-conductive membrane, e.g., a
proton-conductive membrane. The person of ordinary skill in the art
will appreciate that the area of ionic contact (e.g., the area of a
proton-conductive membrane) may be optimized to achieve a desired
effect in the electrochemical device.
[0136] In some embodiments of the methods and devices as otherwise
described herein, the first and second compartments are in ionic
contact through a proton-conductive membrane. In some embodiments,
the proton-conductive membrane is a polymer electrolyte (i.e., an
ionomer) membrane comprising, e.g., a sulfonated fluoropolymer. In
some embodiments, the first and second compartments are in ionic
contact through a
tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfon- ic
acid copolymer (i.e., Nafion).
[0137] The person of ordinary skill in the art will appreciate that
the compartments and at least one photovoltaic cell of the
electrochemical device may be arranged in a variety of different
ways such that the at least one photovoltaic cell is in electrical
contact with the anode and cathode and the first and second
compartments are in ionic contact. In some embodiments, the first
and second compartments are in general physically separated by the
photovoltaic cell and, in part separated by an ion-conductive
membrane, e.g., a proton-conductive membrane, wherein the anode and
cathode are disposed on and in electrical contact with the
photovoltaic cell. One such embodiment is shown in schematic view
in FIG. 1. Electrochemical device 100 comprises a first compartment
120 including at least one transition metal dichalcogenide 122
disposed on a cathode 121, which cathode is disposed on at least
one photovoltaic cell 130. Device 100 also comprises a second
compartment 140 including at least one water oxidizing catalyst 142
disposed on an anode 141, which anode is disposed on cell 130.
Compartments 120 and 140 include a first electrolyte 123 and a
second electrolyte 143, respectively, and are in ionic contact
through an ion-conductive membrane 150. The person of ordinary
skill in the art will appreciate that, in such a configuration, the
substrate of the anode or cathode may also function as the
substrate of the photovoltaic cell.
[0138] In some embodiments, the first and second compartments may
be separated only by an ion-conductive membrane, e.g., a
proton-conductive membrane, wherein the cathode and anode of the
first and second compartments, respectively, are electrically
connected to the at least one photovoltaic cell by conductive
wires. One such embodiment is shown in schematic view in FIG. 2.
Electrochemical device 200 comprises a first compartment 220
including at least one transition metal dichalcogenide 222 disposed
on a cathode 221. Device 200 also comprises a second compartment
240 including at least one water oxidizing catalyst 242 disposed on
an anode 241. Compartments 220 and 240 include a first electrolyte
223 and a second electrolyte 243, respectively, and are separated
by, and in ionic contact through, an ion-conductive membrane 250.
Anode 241 and cathode 221 are in electrical contact with
photovoltaic cell 230 through wires 261 and 262.
[0139] Another aspect of the disclosure is an electrochemical
device having a first and second compartment at least one
photovoltaic cell, wherein the first compartment includes a cathode
in electrical contact with the at least one transition metal
dichalcogenide, a first electrolyte, and carbon dioxide, carbonic
acid, or a carbonic acid salt; the second compartment includes an
anode in electrical contact with at least one water oxidizing
catalyst, a second electrolyte, and water; and wherein the at least
one photovoltaic cell is in electrical contact with the anode and
the cathode, and the first compartment is in ionic contact with the
second compartment.
[0140] In some embodiments of the electrochemical device, the first
compartment, cathode, transition metal dichalcogenide, first
electrolyte, second compartment, anode, water oxidizing catalyst,
second electrolyte, and photovoltaic cell are as otherwise
described herein. In some embodiments of the electrochemical device
as otherwise described herein, the first electrolyte comprises at
least one helper catalyst. In some embodiments, the at least one
transition metal dichalcogenide is MoS.sub.2 or WSe.sub.2. In some
embodiments, the at least one transition metal dichalcogenide is in
nanoflake, nanosheet, or nanoribbon form. In some embodiments, the
helper catalyst is 1-ethyl-3-methylimidazolium tetrafluoroborate.
In some embodiments, the first electrolyte is an aqueous solution.
In some embodiments, the helper catalyst is present in the aqueous
solution in a concentration within the range of about 25 vol. % to
about 75 vol. %. In some embodiments, the electrochemical device as
otherwise described herein is for use in reducing carbon dioxide
and oxidizing water.
[0141] Another aspect of the disclosure is a method of
electrochemically reducing carbon dioxide in an electrochemical
cell, comprising contacting the carbon dioxide with at least one
transition metal dichalcogenide in the electrochemical cell and at
least one helper catalyst and applying a potential to the
electrochemical cell, wherein the at least one transition metal
dichalcogenide is WSe.sub.2 or WS.sub.2. In some embodiments of the
method, the helper catalyst is as otherwise described herein. In
some embodiments, the electrochemical cell comprises a cathode as
otherwise described herein, wherein the cathode is in contact with
the at least one transition metal dichalcogenide. In some
embodiments, the electrochemical cell comprises a first electrolyte
as otherwise described herein, wherein the first electrolyte
comprises the at least one helper catalyst.
[0142] Another aspect of the disclosure is a method of
electrochemically reducing carbon dioxide comprising providing an
electrochemical cell having a cathode in contact with at least one
transition metal dichalcogenide, and a first electrolyte comprising
at least one helper catalyst in contact with the cathode and the at
least one transition metal dichalcogenide, wherein the at least one
transition metal dichalcogenide is WSe.sub.2 or WS.sub.2; providing
carbon dioxide to the electrochemical cell; and applying a
potential to the electrochemical cell. In some embodiments, the
cathode, first electrolyte, and helper catalyst are as otherwise
described herein. In some embodiments, the transition metal
dichalcogenide is in bulk form. In some embodiments, the transition
metal dichalcogenide is in nanoparticle form, as otherwise
described herein. In some embodiments, the transition metal
dichalcogenide nanoparticles have an average size between about 1
nm and 400 nm. In some embodiments, the transition metal
dichalcogenide is in nanoflake, nanosheet, or nanoribbon form, as
otherwise described herein. In some embodiments, the transition
metal dichalcogenide nanoflakes, nanosheets, or nanoribbons have an
average size between about 1 nm and 400 nm. In some embodiments,
the helper catalyst is as otherwise described herein. In some
embodiments, the helper catalyst is 1-ethyl-3-methylimidazolium
tetrafluoroborate.
[0143] Another aspect of the disclosure is an electrochemical cell
having a cathode in contact with at least one transition metal
dichalcogenide and a first electrolyte comprising at least one
helper catalyst, wherein the at least one transition metal
dichalcogenide is WSe.sub.2 or WS.sub.2. In some embodiments of the
cell, the cathode, first electrolyte, and cathode are as otherwise
described herein. In some embodiments, the first electrolyte is an
aqueous solution of the helper catalyst. In some embodiments, the
helper catalyst is 1-ethyl-3-methylimidazolium tetrafluoroborate.
In some embodiments the cell as otherwise described herein, the
cell is for use in reducing carbon dioxide.
EXAMPLES
[0144] The Examples that follow are illustrative of specific
embodiments of the disclosure, and various uses thereof. They are
set forth for explanatory purposes only, and are not to be taken as
limiting the disclosure.
Example 1
Transition Metal Dichalcogenide Preparation
[0145] Transition metal dichalcogenides (e.g., MoS.sub.2,
MoSe.sub.2, WS.sub.2, and WSe.sub.2) were synthesized through
direct reaction of pure forms of the relevant elements followed by
a vapor transport process in an evacuated ampule at elevated
temperatures. In this method, powders of the transition metals and
chalcogens (>99.99% trace metal basis purity) were mixed in the
desired stoichiometric ratio and loaded into a quartz ampule. The
total loaded weight was about one gram. Each quartz ampule had a 1
cm internal diameter and a 20 cm length. The ampule was then
evacuated with a turbo molecule pump (<10.sup.-6 mbar) and
sealed with a hydrogen torch. The ampule was placed into a two-zone
CVD furnace and the temperature of both zones was raised to
1080.degree. C. over 1 day. The temperature of the empty end of the
ampule (the cold zone) was then gradually cooled to 950.degree. C.
over 4 days, while the other end was maintained at 1080.degree. C.,
providing single crystalline grains with pristine structure via
direct vapor transport. The system was slowly cooled to room
temperature over 1 day, after which the crystalline material (See,
e.g., FIG. 3) was removed for characterization.
Example 2
Transition Metal Dichalcogenide Nanoflake Preparation
[0146] The crystalline grains produced according to Example 1 were
ground to a powder. Nanoflakes were formed by sonicating a
dispersion of 300 mg of ground transition metal dichalcogenide
powder in 60 mL of isopropanol. The dispersion was sonicated for 30
hours, using a sonication probe (Vibra Cell Sonics 130 W). The
sonicated dispersions were then centrifuged for 60 minutes at 2000
rpm, after which the supernatant (the top two thirds of the
centrifuged dispersion) was collected by pipette and stored in a
glass vial. FIG. 4 shows nanoflakes dispersed in isopropanol, after
centrifugation.
Example 3
Nanoflake Characterization
[0147] Dynamic light scattering (DLS) experiments were performed to
measure nanoflake sizes using a NiComp ZLS 380 system configured
with a 35 mW semiconductor laser (670 nm emission) and a
thermoelectric temperature control for samples (held at 25.degree.
C.). Nanoflakes dispersed in isopropanol were measured, providing
the normal distributions shown in FIG. 5.
[0148] The nanoflakes were also characterized by Raman
spectroscopy, using a HORIBA LabRAM HR Evolution confocal Raman
microscope configured with a 532 nm laser source, a 1200 g/mm
grating, a Horiba Andor detector, and a 100x objective. Laser
powers at the sample were held between 1-15 mW. Integration times
and averaging parameters were chosen to maximize the
signal-to-noise ratio. Results are shown in FIG. 6.
[0149] Finally, WSe.sub.2 nanoflakes were imaged with scanning
electron microscopy (SEM) to understand the microscale morphology
of the nanoflakes. Samples were imaged with a Carl Zeiss SEM
instrument integrated in a Rait e-LiNE plus ultra-high resolution
electron beam lithography system. Samples were kept at a distance
of 10 mm from the electron source, held at 10 kV. Results are shown
in FIG. 7.
Example 4
Three-Electrode Electrochemical Characterization
[0150] A two-compartment three-electrode electrochemical cell was
used to perform CO.sub.2 reduction reactions (See, FIG. 8).
Transition metal dichalcogenide nanoflakes prepared according to
Example 2 were drop-cast onto a glassy carbon substrate to form the
working electrode. Platinum gauze 52 mesh (Alfa Aesar) and Ag/AgCl
(BASi) were used as the counter and reference electrodes,
respectively. The working electrode, reference electrode, and
counter electrode (CE) were immersed in an aqueous solution of 50
vol. % 1-ethyl-3-methylimidazolium tetrafluoroborate
(EMIM-BF.sub.4). All potentials are presented with respect to a
reversible hydrogen electrode (RHE), using the following
equation:
Potential vs. RHE=Applied Potential vs. Ag/AgCl+0.197
V+(0.0592.times.pH)
[0151] The cathode and anode were separated by an ion-conductive
membrane to eliminate potential product oxidation at the anode
surface. All experiments were performed using a rotating disk
electrode (RDE) submerged in the cell. To eliminate any effect of
mass transport during the reactions, the working electrode was
rotated at 1000 rpm. The cell was connected to a potentiostat (CH
Instruments) connected to a computer through CH Instruments
software. A 6 mm polyethylene tube bubbled CO.sub.2 (99.9% UHP,
Praxair) through the electrolyte solution for 30 minutes prior to
experiments. Results of cyclic voltammetry and chronoamperometry
experiments, shown in FIGS. 9-11 and 13, show the performance of
transition metal dichalcogenide nanoflakes in comparison with
catalysts comprising bulk transition metal dichalcogenides, bulk
silver, or silver nanoparticles.
Example 5
pH Characterization of Electrolyte Composition
[0152] Table 2, below, shows the pH of the aqueous electrolyte
solution of the three-electrode cell as a function of different
concentrations of EMIM-BF.sub.4.
TABLE-US-00002 TABLE 2 pH Value of EMIM-BF.sub.4 Concentrations
Water Volume Fraction pH 0% H2O 6.54 5% H2O 4.87 10% H2O 4.54 25%
H2O 4.14 50% H2O 3.20 65% H2O 3.78 75% H2O 3.98 85% H2O 4.82 90%
H2O 5.30 95% H2O 5.98
[0153] A working electrode coated with WSe.sub.2 nanoflakes was
tested at a potential of -0.764 V vs. RHE in a chronoamperometry
experiment carried out according to Example 4, at various pHs
(i.e., different concentration of EMIM-BF.sub.4). The results,
shown in FIG. 12, indicate that the acidity associated with 50 vol
% EMIM-BF.sub.4 provided for maximum catalytic activity.
Example 6
Turn Over Frequency (TOF) Measurement
[0154] To further characterize the catalytic activity of the
materials of Examples 1-5, a roughness factor (RF) technique was
employed to determine the number of active edge sites of the
materials. All experiments were performed using the same surface
area, (the catalyst loadings for each material were different,
however). The RF number of WSe.sub.2 nanoflakes were estimated by
comparing its double layer capacitance (D.sub.dl) with a flat
standard capacitor of MoS.sub.2 (See, Table 3). Cyclic voltammetry
experiments were performed at different scan rates in 0.5M
H.sub.2SO.sub.4 to calculate the C.sub.dl of each material (See,
FIG. 13). FIG. 14 shows the extracted C.sub.dl values of 2.6, 2.23,
and 3.71 mF/cm.sup.2 at +0.2 V vs. RHE for WSe.sub.2 nanoflakes,
bulk MoS.sub.2, and Ag nanoparticles, respectively. As shown in
Table 3, RF values of 44, 37, and 148 were obtained for WSe.sub.2
nanoflakes, bulk MoS.sub.2, and Ag nanoparticles, respectively. The
calculated number of active sites for each catalyst was obtained
using the following equation:
Density of active sites(sites/cm.sup.2)=(Density of active sites of
standard sample).times.RF
TABLE-US-00003 TABLE 3 Active Sites for Example Materials Flat
Standard Double Layer Capacitance Capacitance Roughness Active
Sites Catalyst (.mu.F/cm.sup.2) (mF/cm.sup.2) Factor (#) WSe2 NFs
60 2.6 44 5.1 .times. 1016 Bulk MoS2 60 2.23 37 4.3 .times. 1016 Ag
NPs 25 3.71 148 4.44 .times. 1017
[0155] Additionally, the CO formation TOF of active sites for
CO.sub.2 reduction by WSe.sub.2 nanoflakes, bulk MoS.sub.2, and Ag
nanoparticles in the aqueous EMIM-BF.sub.4 electrolyte was
calculated at various overpotentials using the following
equation:
CO formation TOF(s.sup.-1)=i.sub.0(A/cm.sup.2).times.CO formation
FE/{[active site
density(sites/cm.sup.2)].times.[1.602.times.10.sup.-19(C/e.sup.-).times.[-
2e.sup.-/CO.sub.2]}
Results, provided in FIG. 15, show that the CO formation TOF for
WSe.sub.2 was approximately three orders of magnitude higher than
that of Ag nanoparticles in the overpotential range of 150 to 650
mV.
Example 7
Gas Chromatography (GC) Analysis
[0156] The products of the electrochemical experiments of Example 4
were analyzed with gas chromatography (GC) using an SRI 8610C GC
system equipped with a 72 in..times.18 in. stainless steel column
packed with molecular sieves. A thermal conductivity detector was
used to analyze and differentiate the injected samples.
Ultra-high-purity helium and nitrogen (Praxair) were used as
carrier gases for CO and H.sub.2 detection, respectively.
[0157] The GC apparatus was calibrated to determine the moles of
products (i.e., CO and H.sub.2) using 2, 5, 10 and 20 vol % of CO
and H.sub.2 in He and N (99.99% research grade, Praxair),
respectively. The known volume of standard samples (1 mL) were
injected at a constant pressure (10 psi) and temperature
(25.degree. C.), using helium and nitrogen as carrier gases for CO
and H.sub.2 detection, respectively. A distinct CO peak was
apparent at 3.83 min., and H.sub.2 was detected at 0.98 min.
Calibration curves (See, FIGS. 16-17) were calculated using the
integrated peak area and the known number of moles of CO or H.sub.2
that were injected.
[0158] 1 mL of the gas products of the electrochemical experiments
of Example 4, carried out for a desired time (e.g., 10 min.), was
injected into the GC instrument using a sample lock syringe. Only
CO and H.sub.2 were detected.
[0159] In order to identify any other potential carbon-based
products, .sup.13CO.sub.2 was reduced in the electrochemical
experiment of Example 4, the products of which were analyzed using
differential electrochemical mass spectrometry (DEMS) with a
quadrupole detector (HPR-20, purchased from Hiden Analytical Inc.).
The DEMS instrument was operated at ultra-high-vacuum pressure
(1.times.10.sup.-6 torr) through the analysis. The product stream
was injected to the DEMS instrument at a flow rate of 0.8-1 mL/min
using a quartz coated very-low-flow capillary line. Analysis of the
product stream, shown in FIG. 18, indicates that CO was the only
detectable carbon-based product of the reaction.
Example 8
Faradaic Efficiency (FE) of Transition Metal Dichalcogenides
[0160] The Faradaic efficiency (FE) of the transition metal
dichalcogenides materials produced according to Example 2 were
calculated using the following equation:
FE=100.times.(moles of product)/[{j(mA/cm.sup.2).times.t(s)}/nF
]
wherein the number of moles of product is determined according to
Example 7, j is the curve area of the plot of current density vs.
time, provided in Example 4, n is the number of electrons required
for the reduction of CO.sub.2 to CO (i.e., 2), and F is the Faradic
number.
[0161] The resultant FE values (See, FIGS. 19-22) indicated that CO
and H.sub.2 were dominant products of the materials and systems
described in the preceding Examples, at a potential window of 0 to
-0.764 V, with an overall FE of 90.+-.5%. Accordingly, the
formation efficiency of other products, e.g., HCOOH, methanol, and
other liquid phase products is -10%. These results indicate that at
very low potentials, e.g., 0 to -0.2 V, H.sub.2 is the major
product, while at higher potentials, e.g., -0.2 to -0.764 V, CO
becomes the major product. Without being bound by a particular
theory, this difference is believed to be attributable to the
differences in the CO.sub.2 reduction and hydrogen evolution
reaction (HER) mechanisms. In principle, the thermodynamic
potential for H.sub.2 evolution is lower than that for CO.sub.2
reduction. As the applied potential exceeds the onset potential of
CO.sub.2 reduction (-0.164 V), the reaction is activated, and
catalyst sites become occupied by CO.sub.2 reduction
intermediates.
[0162] The product of the current density and FE of the materials
of the preceding Examples was plotted as a function of
overpotential to provide an overview of catalytic performance.
Results, provided in FIG. 23, show that the performance of
WSe.sub.2 nanoflakes at 100 mV overpotential exceeded that of bulk
MoS.sub.2 and Ag nanoparticles under identical conditions by a
factor of nearly 60.
Example 9
WSe.sub.2 Nanoflake Stability Investigation
[0163] The long-term stability of WSe.sub.2 nanoflakes were
investigated in an electrochemical experiment configured and
carried out similarly to Example 4. A magnetic stirrer was placed
in the electrolyte solution to eliminate any potential
complications due to mass transport. The stability of the
WSe.sub.2-coated electrode was recorded for 27 hours at a potential
of -0.364 V (0.254 V overpotential) using a Voltalab PGZ100
potentiostat (Radiometer Analytical SAS) calibrated with a RCB200
resistor capacitor box. The potentiostat was connected to a PC
using Volta Master (Version 4) software. The results, shown in FIG.
24, indicate that the material is highly stable. The observed
spikes are due primarily to fluctuations in the flow rate of the
CO.sub.2 bubbled through the electrolyte solution.
Example 10
Photoelectrochemical Device Configuration
[0164] All chemicals were used as received, without any
purification, unless required. Cobalt nitrate hexahydrate (Alfa
Aesar), potassium based buffer solution (0.071 M KPi, pH=7,
Sigma-Aldrich), Nafion 117 (10.0 cm.times.10.0 cm, FuelCellsEtc)
were used in the following configuration. Triple-junction
amorphous-Si solar cells were purchased from Xun-light Corp.
(Toledo, Ohio). The acrylic used for the chambers was purchased
from Total Plastics Inc.
[0165] A photoelectrochemical chamber was machined from acrylic
plastic and assembled with acrylic glue. The transparent chamber
was separated into two compartments by two tandem
amorphous-Si-based triple-junction (a-Si/A-SiGe/A-SiGe)
photovoltaic (PV) cell comprising an indium tin oxide (ITO) anode
layer disposed on the exposed a-Si layer and a stainless steel
cathode layer disposed on the exposed a-SiGe layer, connected in
series through copper tape and separated by a piece of nafion
membrane (See, FIG. 25).
[0166] A cobalt oxygen-evolving catalyst was electrodeposited onto
the ITO surface of the PV cells from a cobalt (II) nitrate
hexahydrate solution. The electrodeposition was carried out using a
solution prepared by mixing 73 mg of cobalt nitrate hexahydrate in
500 mL of potassium phosphate (2.6.times.10.sup.-4 M K.sup.+, pH=7)
using a three-electrode cell configuration comprising a platinum
mesh counter electrode and a Ag/AgCl reference electrode, wherein
the ITO layer of the PV cell served as the working electrode.
Electrodeposition was carried out at a potential of 1.5 V vs
Ag/AgCl for 5 minutes, without stirring and without any i-R
compensation. The stainless steel layer was covered throughout the
electrodeposition.
[0167] WSe.sub.2 nanoflakes were prepared according to Example 2
and suspended in isopropanol. The suspension was drop cast onto the
stainless steel anode of the PV cells, which were allowed to dry
completely.
[0168] The catalyst-coated anode/PV cell/cathode unit and a section
of nafion membrane (activated by treatment with 5 wt. % KOH) were
configured to separate the two compartments of the device while
allowing for ionic contact between the compartments (See, FIG. 25).
The compartment exposed to the cathode and WSe.sub.2 nanoflakes was
filled with 100 mL of an aqueous solution of 50 vol % EMIM-BF.sub.4
(pH=3.23). CO.sub.2 (99.9% UHP, Praxair) was bubbled through the
solution at 1 mL/min for 30 minutes to saturate the solution. The
compartment exposed to the anode and Co catalyst was filled with
100 mL of an aqueous potassium phosphate solution
(2.6.times.10.sup.-4 M K.sup.+, pH=7).
Example 11
Photoelectrochemical Device Operation
[0169] Upon exposure to light irradiation, the photoelectrochemical
device configured according to Example 10 operates first in a
transient regime, after which the device operates in a
"steady-state." Initially, the H.sup.+ concentration in the
solution of the anodic compartment is much lower than that of the
cathodic compartment. Upon exposure to light irradiation, H.sup.+
is produced at the anode (i.e., through water oxidation), lowering
the pH of the anodic solution, while the reduction of CO.sub.2 at
the cathode consumes available H.sup.+, increasing the pH of the
cathodic solution. During this time, K.sup.+ ions diffuse through
the proton-conductive membrane to compensate for charge imbalance.
After approximately 5 minutes, the pH of the solutions of both
compartments equilibrate at 3.35, at which point the
electrochemical device reaches steady-state operation, wherein
diffusion of H.sup.+ from the anodic compartment to the cathodic
compartment overtakes that of K.sup.+ (See, FIG. 26).
[0170] K.sup.+ crossover in the device was quantified using a
PerkinElmer Inductively Coupled Plasma--Optical Emission
Spectroscopy (ICP-OES, Optima 5300DV) instrument. Solution samples
were collected at various time intervals and diluted by a factor of
5 or 20 using 2% HNO.sub.3, based on sample volume. Diluted samples
were analyzed using an ESI Fast auto-sampler coupled with the
ICP-OES device. Measurements demonstrated that the K.sup.+
concentration of the aqueous solution of EMIM-BF.sub.4 (i.e., the
cathodic solution) reached 1.43.times.10.sup.-4 M after five
minutes of exposure of the photovoltaic cell to irradiation, after
which the concentration remained constant. This result is
consistent with the slightly increased pH of the cathodic solution
after the same period of operation, which corresponds to a change
in H.sup.+ concentration of 1.52.times.10.sup.-4 M. The performance
of the device decreases after about 5 hours of continuous
operation.
Example 12
Photoelectrochemical Device Product Analysis
[0171] The product stream of the device operated according to
Example 11 was analyzed according to the method of Example 7.
Results, shown in FIG. 27, indicated that H.sub.2 and CO were the
main products of the photoelectrochemical reaction. No other
detectable carbon based products were observed during
operation.
Example 13
Photoelectrochemical Device Stability Analysis
[0172] As evidenced by FIG. 28, the drop in performance of the
device operated according to Example 11 after 5 hours of continuous
operation is believed to be due to the corrosion of the ITO layer
disposed on the PV cells. Device performance is restored upon
replacement of the catalyst-coated anode/PV cell/cathode unit. To
test the stability of the anodic and cathodic solutions, the
catalyst-coated anode/PV cell/cathode unit was replaced every 4
hours throughout a period of continuous operation lasting 100
hours. Results, shown in Table 4, indicate that the same quantities
of CO and H.sub.2 are produced throughout the 100 hour period of
operation, suggesting that the anodic and cathodic solutions are
highly robust. No significant change to the pH of either solution
was observed.
TABLE-US-00004 TABLE 4 Device Performance Time CO H2 (hours) (mmol)
(mmol) 4 3.9588 0.4468 8 3.9469 0.4069 12 4.0263 0.3969 16 3.9985
0.4348 20 3.9548 0.3985 24 3.9311 0.4039 28 4.0858 0.3981 32 4.0580
0.3965 36 4.0520 0.4348 40 4.0719 0.3973 44 3.9628 0.3981 48 4.0144
0.4269 52 3.9349 0.3881 56 3.9690 0.4098 60 3.9471 0.4315 64 3.9006
0.4237 68 4.0302 0.4189 72 3.9982 0.4205 76 3.9487 0.4349 80 3.8882
0.4017 84 4.0302 0.4251 88 4.0501 0.4234 92 4.0144 0.4191 96 3.8859
0.4278 100 3.9714 0.4338
Example 14
Cathodic Water Production Calculation
[0173] The volume of water produced through the device operation of
Example 11 was calculated. Because water and CO are be produced in
a stoichiometric ratio of 1:1 at the cathode of the device, the
rate of water production, under 1 sun of illumination is known to
be 2.75.times.10.sup.-7 mol/s, or 9.9.times.10.sup.-4 mole of water
generated over 1 hour of operation. After 100 hours of operation,
only 1.8 mL water will have been produced, which will have a
negligible effect on the pH or composition of the cathodic
solution. Table 5 shows the amount of produced water at different
levels of illumination of the device configured according to
Example 10.
TABLE-US-00005 TABLE 5 Water Production at the Cathode #sun Water
Illumination CO (mol/s) (mol/s) Water (mL/h) 0.5 1.32 .times.
10.sup.-7 1.32 .times. 10.sup.-7 0.00855 1 2.75 .times. 10.sup.-7
2.75 .times. 10.sup.-7 0.0178 1.5 4.08 .times. 10.sup.-7 4.08
.times. 10.sup.-7 0.0264 2 5.21 .times. 10.sup.-7 5.21 .times.
10.sup.-7 0.0338
Example 15
Solar to Fuel Conversion Efficiency (SFE) Calculation
[0174] The solar to fuel conversion efficiency (SFE) of the device
operation of Example 11 was calculated using the following
equation:
.eta. = N 1 E 1 + N 2 E 2 U g A cat ##EQU00001##
wherein N.sub.1 and N.sub.2 are the molar quantities of produced
gas per unit time (mol/s) provided by the GC analysis of Example
12, E.sub.1 and E.sub.2 are the energy densities of the
corresponding gas (kJ/mol), which are 283.24 and 140 kJ/mol for CO
and H.sub.2, respectively, A.sub.cat is the overall catalystic
surface area available for the reaction (cm.sup.2), which is 18
cm.sup.2, and U.sub.g is the total solar irradiance (mW/cm.sup.2),
which is 100 mW/cm.sup.2 for 1 sun of illumination. FIG. 29 shows
the SFE and rate of product formation of the electrochemical device
at various levels of illumination.
[0175] To estimate the uncertainty of the calculations, the partial
derivative method is used to calculate the sensitivity of the SFE
values to different input parameters. For this purpose, each
parameter of the above equation was perturbed by a small amount
(.differential.x.sub.i) around its typical value (x.sub.i) to
provide a corresponding change in the extracted SFE
(.differential..eta.). The dimensionless sensitivities were then
calculated using the following equation:
s i = x i .eta. .differential. .eta. .differential. x i
##EQU00002##
[0176] The overall uncertainty (u.sub..eta.) was also calculated,
using the following equation:
u .eta. .eta. = i ( s i .times. u x i x i ) 2 ##EQU00003##
where u.sub.x.sub.i is the overall uncertainty of the i.sup.th
parameter around its typical value (x.sub.i), s.sub.i is the
sensitivity to that particular input, and .eta. is the SFE
value.
u x i x i ##EQU00004##
for N.sub.1 and N.sub.2 was calculated based on the standard
deviations of the values from three different experiments, which
were 0.07 and 0.09, respectively. Because E.sub.1 and E.sub.2
values were literature values,
u x i x i ##EQU00005##
for these values were considered to be zero. The value of
u x i x i ##EQU00006##
for U.sub.g was based on the fluctuation in the response of the
photodiode during device operation. The uncertainty in A.sub.cat
was 0.05 (based on measurement with a Vernier caliper). A summary
of the uncertainty analysis is provided in Table 6. The error bars
shown in FIG. 29 represent the calculated overall uncertainty
values (u.sub..eta.). The uncertainty values for 0.5, 1, 1.5 and 2
suns of illumination were 0.39058, 0.40904, 0.40154, and 0.38885,
respectively.
TABLE-US-00006 TABLE 6 SFE Uncertainty Analysis c.sub.i = |S.|
.times. Input Units x.sub.i (values) u.sub.xi/x.sub.i Sensitivity
u.sub.xi/x.sub.i (ci).sup.2/.SIGMA.(ci).sup.2 N.sub.1 mol/s 2.75
.times. 10-7 0.07 0.952643 0.0667 0.00445 N.sub.2 mol/s 2.78
.times. 10-8 0.09 0.047357 0.0043 1.8 .times. 10-5 E.sub.1 kJ/
283.24 0 0.952643 0 0 mol E.sub.2 kJ/ 140 0 0.047357 0 0 mol
U.sub.g W/ 100 0.035 0.98039 0.0343 0.0012 cm.sup.2 A.sub.cat.
cm.sup.2 18 0.05 0.98039 0.049 0.0024
[0177] The SFE values over 5 hours and 100 hours of continuous
operation according to Example 11 were also calculated, and are
provided in FIG. 30 and Table 7.
TABLE-US-00007 TABLE 7 SFE Values of 100-hour Continuous Operation
Time (hours) SFE % 4 4.570 8 4.535 12 4.617 16 4.607 20 4.539 24
4.516 28 4.682 32 4.651 36 4.665 40 4.667 44 4.548 48 4.620 52
4.512 56 4.561 60 4.549 64 4.494 68 4.633 72 4.599 76 4.553 80
4.468 84 4.636 88 4.657 92 4.616 96 4.480 100 4.577
Example 16
PV Cell Efficiency Measurement
[0178] The solar-to-electricity conversion efficiency of a
triple-junction photovoltaic cell comprising an ITO layer and a
cobalt oxygen evolving catalyst disposed thereon was measured under
one sun of illumination. The voltage was measured directly with a
multi-meter while the resistance of the circuit was changed using
variable resistors. The open circuit voltage (V.sub.OC), short
circuit current, and average fill factor of the cell were 2.12 V,
6.1 mA/cm.sup.2 and 0.55, respectively. Accordingly, a dry cell
efficiency of 7.1% was calculated by dividing the product of the
three aforementioned parameters by the energy of one sun of
illumination (100 mW/cm.sup.2). The V.sub.OC of two such cells
connected in series was 3.6 V, which decreased to 3V when submerged
in the electrolyte solutions of Example 10 (the short circuit
current remained constant). The fill factor was assumed to remain
relatively constant under the configuration and operation
parameters of Examples 10 and 11. Accordingly, the efficiency of
the PV cell was estimated to be about 6% when submerged in the
electrolyte.
Example 17
Electrochemical Impedance Spectroscopy (EIS)
[0179] Electrochemical impedance spectroscopy (EIS) experiments
were performed using an electrochemical cell and electrodes
configured similarly to Example 4. The Nyquist plot for different
CO.sub.2 reduction over-potentials, e.g., 150, 200, 300, 400, and
500 mV, were recorded at a small (10 mV) AC voltage amplitude (to
avoid nonlinearity) and over a frequency range of 10 to 10.sup.5
Hz, using a Voltalab PGZ100 potentiostat. An equivalent Randles
circuit model was fit to the data to calculate R.sub.ct for each
catalyst system. FIG. 31 shows the recorded Nyquist plots and
fitted curve at an overpotential of 150 mV for WSe.sub.2
nanoflakes, bulk MoS.sub.2, and Ag nanoparticles disposed on glassy
carbon. FIG. 32 shows the recorded Nyquist plots and fitted curve
at overpotentials of 150-500 mV for WSe.sub.2 nanoflakes disposed
on glassy carbon.
Example 18
Ultraviolet Photoelectron Spectroscopy (UPS)
[0180] The work function for four transition metal dichalcogenides
and Ag nanoparticles was measured by ultraviolet photoelectron
spectroscopy (UPS) (See, FIG. 32). UPS data were acquired with a
Physical Electronics PHI 5400 photoelectron spectrometer using He I
(21.2 eV) UV radiation and a pass energy of 8.95 eV.
Example 19
Scanning Transmission Electron Microscopy (STEM) images of
WSe.sub.2
[0181] Scanning transmission electron microscopy (STEM) images were
acquired on a JEAL JEM-ARM200CF instrument operated at 200 kV.
Images were acquired in either high/low angle annular dark field
(H/LAADF) or annular bright field (ABF) mode. FIG. 33 shows STEM
images of WSe.sub.2 before and after a 27 hour chronoamperometry
experiment carried out according to Example 4.
Example 20
X-Ray Photoelectron Spectroscopy (XPS)
[0182] X-ray photoelectron spectroscopy (XPS) experiments were
carried out on a Thermo Scientific ESCALAB 250Xi instrument
equipped with an electron flood and scanning ion gun. Spectra were
calibrated to the C1s binding energy of 284.8 eV. As shown in FIG.
34, WSe.sub.2 nanoflakes, after a 27 hour chronoamperometry
experiment carried out according to Example 4, showed a negligible
(0.2 eV) change in the W 4f and Se 3d spectra relative to that of
fresh nanoflakes prepared according to Example 2, indicating that
the nanoflakes are highly stable, even over prolonged periods of
use.
Example 21
Density Functional Theory (DFT) Calculations
[0183] Density functional theory (DFT) calculations were carried
out to investigate the catalytic properties of transition metal
dichalcogenide nanoflakes (e.g., nanoflakes prepared according to
Example 2). Periodic DFT calculations were performed with plane
wave basis sets in the VASP package. Reaction free energies and
density of states (DOS) calculations were performed on single-layer
nanoribbons of the transition metal dichalcogenides truncated with
zig-zag edges. FIGS. 35-36 show the calculated partial density of
states (PDOS) of the d band (spin up) of the surface bare metal
edge atom (Me and W) of MoSe.sub.2, MoS.sub.2, WS.sub.2, and
WSe.sub.2 nanoflakes, respectively, in addition to the surface Ag
atom of bulk Ag(111) and Ag.sub.55 nanoparticles. FIG. 37 shows the
calculated free energy diagrams for CO.sub.2 electroreduction to CO
on Ag(111), Ag.sub.55 nanoparticles, MoS.sub.2, WS.sub.2,
MoSe.sub.2, and WSe.sub.2 nanoflakes at 0 V RHE.
[0184] Both monolayer slabs and nanoribbons of the transition metal
dichalcogenides were used to calculate the work functions. For the
nanoribbons, each unit well included 4.times.4 (16 total) metal
atoms and 32 S or Se atoms (for low CO coverage calculations, the
unit cell included 6.times.4 metal atoms and 48 S or Se atoms),
containing both the metal and the S/Se edges. A 10 A vacuum space
was set both on top of the metal edge and between two nanoribbon
periodic images. For the single-layer slabs for work function
calculations, only the minimum atoms to construct a unit cell were
used. The Ag(111) surface was constructed by a 4.times.4.times.4
slab in a unit cell, with 10 .ANG. vacuum space. A kinetic energy
cutoff of 400 eV was used for all calculations. All atoms in the
system were allowed to relax, while the cell shape and volume were
kept fixed. K-point grids of 3.times.1.times.1 and
3.times.3.times.1 were used for energy calculations of the
nanoribbons and Ag(111), respectively. K-point grids of
6.times.1.times.1 and 6.times.6.times.1 were used for DOS
calculations of the nanoribbons and Ag(111), respectively. r-point
was used for gas-phase molecules. For work function calculations of
the monolayer transition metal dichalcogenide slabs, a
10.times.1.times.1 K-point grid was used. All calculations were
spin-polarized calculations.
[0185] The effect of the CO coverage on the CO binding energies on
the metal edges of the transition metal dichalcogenides was
investigated. The DFT results show that each metal atom on the
transition metal dichalcogenide nanoflake edge can bind up to two
CO molecules (.theta..sub.co=2 ML). As shown in Table 8, the
binding energies of CO on the metal edge decrease as the coverage
increases. At the highest coverage (.theta..sub.co=2 ML), the
average binding energy per second CO on the metal atom becomes
smaller than 0.5 eV. This suggests that during the catalytic
reaction, CO is likely to have a high coverage (.theta..sub.co>1
ML) on the metal edges of the transition metal dichalcogenides, and
second CO molecule on the metal atom can easily desorb. These
results indicate that the catalyst site may have at least one CO
molecule binding the metal atom during most of the catalytic
cycle.
TABLE-US-00008 TABLE 8 Calculated Binding Energies CO Coverage 1/6
ML 1 ML 1.25 ML 2 ML MoS.sub.2 1.27 0.85 0.80 0.27 MoSe.sub.2 1.20
0.81 0.82 0.31 WS.sub.2 1.55 1.14 0.88 0.28 WSe.sub.2 1.42 1.05
0.90 0.48 For .theta..sub.CO = 1/6 ML and 1 ML, the values are
average binding energies per CO molecule; for .theta..sub.CO = 1.25
ML and 2 ML, the values are the average binding energies per second
CO on the metal atom.
[0186] FIG. 38 shows the calculated work functions for the
transition dichalcogenide monolayers. A clear trend was observed
among the work functions of the four transition metal
dichalcogenides, wherein
MoS.sub.2>WS.sub.2>MoSe.sub.2>WSe.sub.2. The calculated
work functions of the nanoribbons were consistently around 0.3 eV
lower than those of the monolayers.
* * * * *