U.S. patent application number 16/648794 was filed with the patent office on 2020-10-01 for durable oxygen evolution electrocatalysts.
The applicant listed for this patent is King Abdullah University of Science and Technology. Invention is credited to Keisuke Obata, Kazuhiro Takanabe.
Application Number | 20200308720 16/648794 |
Document ID | / |
Family ID | 1000004913657 |
Filed Date | 2020-10-01 |
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United States Patent
Application |
20200308720 |
Kind Code |
A1 |
Takanabe; Kazuhiro ; et
al. |
October 1, 2020 |
DURABLE OXYGEN EVOLUTION ELECTROCATALYSTS
Abstract
Oxygen evolution electrocatalysts, electrodes using oxygen
evolution electrocatalysts, and methods of making oxygen evolution
electrocatalysts are provided. The oxygen evolution electrocatalyst
includes an oxide electrocatalyst and a permselective amorphous
layer deposited on the oxide electrocatalyst. In this regard, the
permselective amorphous layer may prevent diffusion of redox ions
but permit diffusion of hydroxide ions and evolved O2 through the
permselective amorphous layer.
Inventors: |
Takanabe; Kazuhiro; (Thuwal,
SA) ; Obata; Keisuke; (Thuwal, SA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
King Abdullah University of Science and Technology |
Thuwal |
|
SA |
|
|
Family ID: |
1000004913657 |
Appl. No.: |
16/648794 |
Filed: |
September 19, 2018 |
PCT Filed: |
September 19, 2018 |
PCT NO: |
PCT/IB2018/057212 |
371 Date: |
March 19, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62560438 |
Sep 19, 2017 |
|
|
|
62565732 |
Sep 29, 2017 |
|
|
|
62626963 |
Feb 6, 2018 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 11/0452 20130101;
C25B 1/04 20130101; C25B 11/0415 20130101; C25B 11/0478
20130101 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C25B 1/04 20060101 C25B001/04 |
Claims
1. An oxygen evolution electrocatalyst comprising: an oxide
electrocatalyst; and a permselective amorphous layer deposited on
the oxide electrocatalyst, wherein the permselective amorphous
layer prevents diffusion of redox ions but permits diffusion of
hydroxide ions to the oxide electrocatalyst.
2. The oxygen evolution electrocatalyst according to claim 1,
wherein the oxide electrocatalyst comprises an oxide of a
transition metal.
3. The oxygen evolution electrocatalyst according to claim 1,
wherein the oxide electrocatalyst comprises at least one of
NiO.sub.x CoO.sub.x, FeO.sub.x, MnO.sub.x, IrO.sub.x, RuO.sub.x, or
any combination thereof to form a mixed oxide.
4. The oxygen evolution electrocatalyst according to claim 1,
wherein the oxide electrocatalyst comprises NiFeO.sub.x or
CoFeO.sub.x.
5. (canceled)
6. The oxygen evolution electrocatalyst according to claim 1,
wherein the permselective amorphous layer comprises a catalytically
inactive material.
7. The oxygen evolution electrocatalyst according to claim 1,
wherein the permselective amorphous layer comprises at least one of
CeO.sub.x, TiO.sub.x, AlO.sub.x, ZnO.sub.x, ZrO.sub.x, SiO.sub.x,
CaO.sub.x, MgO.sub.x, or any combination thereof.
8. The oxygen evolution electrocatalyst according to claim 1,
wherein the permselective amorphous layer comprises CeO.sub.x;
wherein the permselective amorphous layer comprises a thickness
from about 100 nm to about 600 nm; and/or wherein the permselective
amorphous layer is deposited approximately uniformly on the oxide
electrocatalyst.
9. (canceled)
10. (canceled)
11. An electrode comprising: a substrate; and an oxygen evolution
electrocatalyst, the oxygen evolution electrocatalyst comprising:
an oxide electrocatalyst; and a permselective amorphous layer
disposed on the oxide electrocatalyst, wherein the permselective
amorphous layer prevents diffusion of redox ions but permits
diffusion of hydroxide ions to the oxide electrocatalyst.
12. The electrode according to claim 11, wherein the substrate
comprises a conductive substrate.
13. The electrode according to claim 11, wherein the substrate
comprises at least one of cobalt, gold, nickel, a fluorine-doped
tin oxide (FTO) substrate, a gold-coated FTO substrate or any
combination thereof and optionally, wherein the oxide
electrocatalyst comprises an oxide of a transition metal.
14. (canceled)
15. (canceled)
16. The electrode according to claim 11, wherein the oxide
electrocatalyst comprises at least one of NiO.sub.x, CoO.sub.x,
FeO.sub.x, MnO.sub.x, IrO.sub.x, RuO.sub.x, or any combination
thereof to form a mixed oxide.
17. The electrode according to claim 11, wherein the oxide
electrocatalyst comprises NiFeO.sub.x or CoFeO.sub.x.
18. (canceled)
19. The electrode according to claim 11, wherein the permselective
amorphous layer comprises a catalytically inactive material.
20. The electrode according to claim 11, wherein: (i) the
permselective amorphous layer comprises at least one of CeO.sub.x,
TiO.sub.x, AlO.sub.x, ZnO.sub.x, ZrO.sub.x, SiO.sub.x, CaO.sub.x,
MgO.sub.x, or any combination thereof; (ii) permselective amorphous
layer comprises a thickness from about 100 nm to about 600 nm
and/or the permselective amorphous layer is deposited approximately
uniformly on the oxide electrocatalyst.
21. The electrode according to claim 11, wherein the permselective
amorphous layer comprises CeO.sub.x.
22. (canceled)
23. (canceled)
24. A method of making an oxygen evolution electrocatalyst, the
method comprising: providing an oxide electrocatalyst; and
depositing a permselective amorphous layer on the oxide
electrocatalyst via anodic deposition, wherein the permselective
amorphous layer prevents diffusion of redox ions but permits
diffusion of hydroxide ions to the oxide electrocatalyst.
25. The method according to claim 24, wherein depositing the
permselective amorphous layer comprises depositing the
permselective amorphous layer approximately uniformly on the oxide
electrocatalyst.
26. The method according to claim 24, wherein the oxide
electrocatalyst comprises an oxide of a transition metal.
27. The method according to claim 24, wherein the oxide
electrocatalyst comprises at least one of NiO.sub.x, CoO.sub.x,
FeO.sub.x, MnO.sub.x, IrO.sub.x, RuO.sub.x, or any combination
thereof to form a mixed oxide.
28. The method according to claim 24, wherein: (i) the oxide
electrocatalyst comprises NiFeO.sub.x or CoFeO.sub.x; ii) the
permselective amorphous layer comprises at least one of CeO.sub.x,
TiO.sub.x, AlO.sub.x, ZnO.sub.x, ZrO.sub.x, SiO.sub.x, CaO.sub.x,
MgO.sub.x, or any combination thereof; and/or (iii) the
permselective amorphous layer comprises CeO.sub.x.
29. (canceled)
30. (canceled)
31. (canceled)
Description
CROSS-REFERENCED TO RELATED APPLICATIONS
[0001] This application claims the benefit of and priority to U.S.
Provisional Application No. 62/560,438, filed Sep. 19, 2017,
62/565,732, filed Sep. 29, 2017, and 62/626,963 filed Feb. 6, 2018,
which are hereby incorporated herein by reference in their
entirety.
TECHNICAL FIELD
[0002] The presently-disclosed invention relates generally to
providing stable and selective electrocatalysts for use in the
oxygen evolution reaction, and more particularly to oxygen
evolution electrocatalysts, electrodes using oxygen evolution
electrocatalysts, and methods of making oxygen evolution
electrocatalysts.
BACKGROUND
[0003] Hydrogen is one of the energy carriers that can effectively
store intermittent energy from renewables, such as solar and wind
power. A lot of techniques are studied to generate hydrogen and
oxygen from water via, for example, water electrolysis and
photocatalytic water splitting. Since the oxygen evolution reaction
(OER) is a kinetically sluggish reaction compared with the hydrogen
evolution reaction (HER) in water splitting, the development of
highly active, durable and cost-effective electrocatalysts for OER
is desired. NiFeO.sub.x is one of the most active electrocatalysts
towards OER in alkaline conditions and it is known that Fe has a
critical role to improve activity while the reason why doping of Fe
boosts the kinetics of OER is still under discussion. Although a
lot of research has been devoted to decrease overpotential via
development of layered double hydroxide (LDH) structure,
compositional control, or deposition on conductive support with
high surface area, the stability of NiFeO.sub.x under harsh
oxidative conditions has not been considered. Indeed, the efforts
relating to decreasing overpotential to date have resulted in
either increased overpotential or decreased iron content in the
electrocatalyst during a stability test.
[0004] Accordingly, there still exists a need for an
electrocatalyst that provides improved stability and selectivity in
the oxygen evolution reaction (OER).
BRIEF SUMMARY OF THE INVENTION
[0005] One or more embodiments of the invention may address one or
more of the aforementioned problems. Certain embodiments provide
oxygen evolution electrocatalysts, electrodes using oxygen
evolution electrocatalysts, and methods of making oxygen evolution
electrocatalysts. In one aspect, an oxygen evolution
electrocatalyst is provided. The oxygen evolution electrocatalyst
may include an oxide electrocatalyst, and a permselective amorphous
layer deposited on the oxide electrocatalyst. The permselective
amorphous layer may prevent diffusion of redox ions but permit
diffusion of hydroxide ions to the oxide electrocatalyst.
[0006] In another aspect, an electrode using an oxygen evolution
electrocatalyst is provided. The electrode may include a substrate
and an oxygen evolution electrocatalyst. The oxygen evolution
electrocatalyst may include an oxide electrocatalyst, and a
permselective amorphous layer deposited on the oxide
electrocatalyst. The permselective amorphous layer may prevent
diffusion of redox ions but permit diffusion of hydroxide ions to
the oxide electrocatalyst.
[0007] In yet another aspect, a method of making an oxygen
evolution electrocatalyst is provided. The method may include
providing an oxide electrocatalyst, and depositing a permselective
amorphous layer on the oxide electrocatalyst via anodic deposition.
The permselective amorphous layer may prevent diffusion of redox
ions but permit diffusion of hydroxide ions to the oxide
electrocatalyst.
BRIEF DESCRIPTION OF THE DRAWING(S)
[0008] Having thus described the invention in general terms,
reference will now be made to the accompanying drawings, which are
not necessarily drawn to scale, and wherein:
[0009] FIG. 1a illustrates results from electrocatalytic stability
tests on electrocatalysts in accordance with certain embodiments of
the invention.
[0010] FIGS. 1b and 1c are cyclic voltammograms of electrodes in
accordance with certain embodiments of the invention.
[0011] FIG. 1d is a Tafel plot taken from the cyclic voltammograms
of FIGS. 1b and 1c.
[0012] FIGS. 2a and 2b are scanning electron microscope (SEM)
images of electrocatalysts in accordance with certain embodiments
of the invention.
[0013] FIG. 2c is an SEM image of an electrode in accordance with
certain embodiments of the invention.
[0014] FIG. 2d is a Raman spectra of electrodes in accordance with
certain embodiments of the invention.
[0015] FIG. 3a illustrates Nyquist plots of electrocatalysts in
accordance with certain embodiments of the invention.
[0016] FIG. 3b illustrates Bode plots of electrocatalysts in
accordance with certain embodiments of the invention.
[0017] FIG. 4a illustrates cyclic voltammograms for electrodes in
accordance with certain embodiments of the invention.
[0018] FIG. 4b illustrates Faradaic efficiency of oxygen during
controlled current electrolysis for electrocatalysts in accordance
with certain embodiments of the invention.
[0019] FIGS. 5a and 5b are cyclic voltammograms for substrates and
electrodes in accordance with certain embodiments of the
invention.
[0020] FIG. 5c is a chronoamperogram of an electrode in accordance
with certain embodiments of the invention.
[0021] FIG. 6 illustrates peak position of Ni reduction peaks taken
from FIGS. 1b and 1c.
[0022] FIG. 7 illustrates XRD diffractograms of various substrates
in accordance with certain embodiments of the invention.
[0023] FIGS. 8a and 8b are XPS spectra of the CeOx layer in
accordance with certain embodiments of the invention.
[0024] FIGS. 9a and 9b illustrate equivalent circuits for (a)
Randles circuit and (b) Voigt circuit in accordance with certain
embodiments of the invention.
[0025] FIG. 10 is a cyclic voltammogram of electrodes in accordance
with certain embodiments of the invention.
[0026] FIG. 11 is a chronoamperogram of electrocatalysts in
accordance with certain embodiments of the invention.
[0027] FIG. 12a is a chronoamperogram of electrocatalysts in
accordance with certain embodiments of the invention.
[0028] FIG. 12b illustrates electrocatalytic stability test results
for electrodes in accordance with certain embodiments of the
invention.
[0029] FIGS. 13a-13e illustrate Faradaic efficiency of oxygen
during controlled current electrolysis for electrodes in accordance
with certain embodiments of the invention.
[0030] FIG. 14 illustrates electrocatalytic stability test results
for electrodes in accordance with certain embodiments of the
invention.
[0031] FIGS. 15A and 15B illustrates cyclic voltammograms of
electrodes in accordance with certain embodiments of the
invention.
[0032] FIGS. 16a and 16b are SEM images of electrodes in accordance
with certain embodiments of the invention.
[0033] FIGS. 17a and 17b are cyclic voltammograms of electrodes in
accordance with certain embodiments of the invention.
[0034] FIG. 18 is a schematic block diagram illustrating a method
of making an oxygen evolution electrocatalyst in accordance with
certain embodiments of the invention.
[0035] FIG. 19 illustrates a cross-section of an electrode in
accordance with certain embodiments of the invention.
DETAILED DESCRIPTION
[0036] The invention now will be described more fully hereinafter
with reference to the accompanying drawings, in which some, but not
all embodiments of the inventions are shown. Indeed, this
inventions may be embodied in many different forms and should not
be construed as limited to the embodiments set forth herein;
rather, these embodiments are provided so that this disclosure will
satisfy applicable legal requirements. Like numbers refer to like
elements throughout. As used in the specification, and in the
appended claims, the singular forms "a", "an", "the", include
plural referents unless the context clearly dictates otherwise.
[0037] Through combined effort and ingenuity, the inventors have
developed a highly durable OER electrocatalyst by deposition of a
permselective amorphous layer on an oxide electrocatalyst. The
permselective amorphous layer includes a mixed oxide and hydroxide
layer which prevents diffusion of redox ions while allowing
OH.sup.- and evolved O.sub.2 to permeate through. Improved
stability of the oxide electrocatalyst can be attributed to the
permselectivity of the amorphous layer, which can regulate
diffusion of redox anions, such as iodide, ferrocyanide and
ferrate, between the electrolyte and oxide electrocatalyst. In this
regard, the oxygen evolution electrocatalyst is a highly active and
durable OER catalyst that undergoes harsh oxidative condition.
I. Oxygen Evolution Electrocatalyst
[0038] In accordance with certain embodiments, oxygen evolution
electrocatalysts are provided. The oxygen evolution electrocatalyst
includes an oxide electrocatalyst and a permselective amorphous
layer deposited on the oxide electrocatalyst. In this regard, the
permselective amorphous layer may prevent diffusion of redox ions
(e.g., iodide, ferrocyanide, ferrate, etc.) but permit diffusion of
hydroxide ions and evolved O.sub.2 through the permselective
amorphous layer. Without intending to be bound by theory, the
diffusion of hydroxide ions to the oxide electrocatalyst under the
permselective amorphous layer allows the evolution of oxygen in the
OER while preventing diffusion and further dissolution of
easily-dissolved iron species from the underlying oxide to the
electrolyte. Accordingly, the resulting oxygen evolution
electrocatalyst described herein may achieve significantly improved
stability over bare electrocatalysts (i.e. electrocatalysts without
the permselective amorphous layer).
[0039] In accordance with certain embodiments, for example, the
oxide electrocatalyst may comprise an oxide of a transition metal.
In some embodiments, for instance, the oxide electrocatalyst may
comprise at least one of NiO.sub.x, CoO.sub.x, FeO.sub.x, MnO.sub.x
IrO.sub.x, RuO.sub.x, or any combination thereof to form a mixed
oxide, where "x" is the number of oxygen atoms in the oxide
electrocatalyst and may depend on the number of metal cations
present, the number of oxidation states, and/or the like as
understood by one of ordinary skill in the art. For example, in
some embodiments, "x" may be from 2-8, and in further embodiments,
"x" may be from 2-4. In certain embodiments, the oxide
electrocatalyst may comprise NiFeO.sub.x. In other embodiments, for
instance, the oxide electrocatalyst may comprise CoFeO.sub.x. In
some embodiments, for example, the oxide electrocatalyst may
comprise a thickness from about 10 nm to about 300 nm. By way of
example only, the thickness of the CoO.sub.x electrocatalyst of
FIG. 16A is about 10 nm, while the thickness of the NiFeO.sub.x
electrocatalyst of FIG. 2C is about 300 nm.
[0040] According to certain embodiments, for instance, the oxide
electrocatalyst may comprise an LDH structure or an amorphous
structure. In further embodiments, for example, the oxide
electrocatalyst may comprise a spinel composition. Spinels are a
mineral oxide having the general formula of AB.sub.2O.sub.4 and may
be supported on a plurality of support oxides. As such, the A
component is tetrahedrally coordinated with the oxygens and the B
component is octahedrally coordinated with the oxygens. Spinels may
include a transition metal (e.g., iron (Fe), manganese (Mn), nickel
(Ni), cobalt (Co), copper (Cu), vanadium (V), silver (Ag), titanium
(Ti), etc.) and an "other metal" (i.e., aluminum (Al), magnesium
(Mg), gallium (Ga), tin (Sn), thallium (Tl), lead (Pb), bismuth
(Bi), or indium (In)).
[0041] In some embodiments, for example, a spinel composition may
include copper, nickel, cobalt, iron, manganese, or chromium at any
concentration, including quaternary, ternary, and binary
combinations thereof. Ternary combinations may include Cu--Mn--Fe,
Cu--Mn--Co, Cu--Fe--Ni, Cu--Co--Fe, Cu--Mn--Ni, Cu--Mn--Co, and
Cu--Co--Ni. Binary combinations may include Cu--Mn, Cu--Fe, Cu--Co,
Cu--Ni, Cu--Ni, Mn--Fe, Mn--Co, Mn--Ni, Co--Ni, and Co--Fe. In
further embodiments, for instance, the oxide electrocatalyst may
comprise one or more of CoFe.sub.2O.sub.4 and
NiFe.sub.2O.sub.4.
[0042] In accordance with certain embodiments, for example, the
permselective amorphous layer may comprise a catalytically inactive
material. In some embodiments, for instance, the permselective
amorphous layer may comprise at least one of CeO.sub.x, TiO.sub.x,
AlO.sub.x, ZnO.sub.x, ZrO.sub.x, SiO.sub.x, CaO.sub.x, MgO.sub.x,
or any combination thereof, where "x" is the number of oxygen atoms
in the oxide electrocatalyst and may depend on the number of metal
cations present, the number of oxidation states, and/or the like as
understood by one of ordinary skill in the art. For example, in
some embodiments, "x" may be from 2-8, and in further embodiments,
"x" may be from 2-4. In further embodiments, for example, the
permselective amorphous layer may comprise a thickness from about
100 nm to about 600 nm. In some embodiments, for instance, the
permselective amorphous layer may comprise a thickness from about
100 nm to about 200 nm. If the thickness of the permselective
amorphous layer is less than 100 nm, for example, the permselective
amorphous layer begins to lose its selectivity. In certain
embodiments, for instance, the permselective amorphous layer may be
deposited approximately uniformly (i.e. homogenously) on the oxide
electrocatalyst.
II. Electrode
[0043] In another aspect, electrodes using oxygen evolution
electrocatalysts are provided. As shown in FIG. 19, the electrode
200 includes a substrate 202 and an oxygen evolution
electrocatalyst 204. The oxygen evolution electrocatalyst 204
includes an oxide electrocatalyst 206 and a permselective amorphous
layer 208 deposited on the oxide electrocatalyst 206. In this
regard, the permselective amorphous layer may prevent diffusion of
redox ions but permit diffusion of hydroxide ions to the oxide
electrocatalyst, as previously discussed herein.
[0044] In accordance with certain embodiments, for example, the
substrate may comprise any conductive substrate suitable for use
with the oxygen evolution electrocatalyst as understood by one of
ordinary skill in the art. In some embodiments, for instance, the
substrate may comprise at least one of cobalt, gold, nickel, a
fluorine-doped tin oxide (FTO) substrate, or any combination
thereof. In further embodiments, for example, the substrate may
comprise a gold-coated FTO substrate. By way of example only,
approximately 200 nm of gold may be deposited on approximately 500
nm of FTO substrate; however, the substrate may comprise any
thickness as long as it remains conductive.
III. Method of Making an Oxygen Evolution Electrocatalyst
[0045] In yet another aspect, methods of making oxygen evolution
electrocatalysts are provided. As shown in FIG. 18, the method 100
includes providing an oxide electrocatalyst at block 101, and
depositing a permselective amorphous layer on the oxide
electrocatalyst via anodic deposition at block 102. In this regard,
the permselective amorphous layer may prevent diffusion of redox
ions but permit diffusion of hydroxide ions to the oxide
electrocatalyst, as previously discussed herein.
[0046] In accordance with certain embodiments, for example,
depositing the permselective amorphous layer may comprise
depositing the permselective amorphous layer approximately
uniformly on the oxide electrocatalyst. As mentioned herein, anodic
deposition may be used to deposit the permselective amorphous layer
on the oxide electrocatalyst. By way of example only, the anodic
deposition may utilize an electrolyte (i.e. deposition solution)
including from about 0.1 M to about 0.5 M
Ce(NO.sub.3).sub.3.6H.sub.2O at room temperature. A constant anodic
potential may be applied to the deposition solution, and the
deposition solution may be bubbled with argon before adding metal
salts, and the argon atmosphere may be maintained during anodic
deposition.
EXAMPLES
[0047] The following examples are provided for illustrating one or
more embodiments of the present invention and should not be
construed as limiting the invention.
[0048] Material Preparation
[0049] Gold-coated fluorine-doped tin oxide (FTO) substrate was
prepared by sputter-coating 20 nm of Ti followed by 190 nm of gold.
Electrodeposition of NiFeO.sub.x on the gold-coated FTO substrate
was performed by cathodic electrochemical deposition. -20 mA
cm.sup.-2 was applied for 2 mM in an electrolyte solution
containing 50 mM NH.sub.4OH, 25 mM H.sub.2SO.sub.4, 9 mM
NiSO.sub.4.7H.sub.2O and 9 mM FeSO.sub.4.7H.sub.2O with a carbon
paper as a counter electrode. The pH of the deposition solution was
adjusted to 2.5. When NiFeCeO.sub.x was prepared, 3, 6 or 9 mM of
Ce(NO.sub.3).sub.3.6H.sub.2O was also added to the deposition
solution. A CeO.sub.x layer was formed on the NiFeO.sub.x electrode
by anodic deposition in the electrolyte containing 0.4 M
Ce(NO.sub.3).sub.3.6H.sub.2O and 0.4 M CH.sub.3COONH.sub.4. A
constant anodic potential of 1.1 V vs Ag/AgCl was applied for 6 h
with resting period of 1 mM in every 30 mM Deposition solution was
stirred by a magnetic stir during deposition. pH of 0.4 M
CH.sub.3COONH.sub.4 was adjusted to 8 by 4 M NaOH solution and it
turned to pH 7 after adding Ce(NO).sub.3.6H.sub.2O. Deposition
solutions were bubbled with Ar (99.9999%) for at least 30 mM before
adding metal salts and Ar atmosphere was maintained during
deposition.
[0050] Physical Characterization
[0051] X-ray diffraction (XRD) was collected with a Bruker D8
Advanced A25 diffractometer in the Bragg-Brentano geometry (with Cu
K.alpha. radiation at 40 kV and 40 mA). Data sets were acquired in
continuous scanning mode over a 20 range of 0-80.degree.. Raman
spectra was obtained by an Olympus BXFMILHS microscope with a He/Ne
laser, which has excitation at 633 nm. X-ray photoelectron
spectroscopy (XPS) spectra were obtained with an AMICUL KRATOS
using Al anode at 10 kV and 15 mA. A peak maximum of C 1S at 284.8
eV was used as an internal standard to correct the binding
energies. To obtain loading of catalysts on the substrates,
catalysts were dissolved in 1 mL of aqua regia for 12 h and then
diluted in 9 mL of Milli-Q water. Induced coupled plasma (ICP)
measurements were performed for the solutions using an ICP-OES
Varian 72 ES. Scanning electron microscopy (SEM) images were
obtained with Nova Nano 630 scanning electron microscope from FEI
Company. Cross-section views were taken using an FBI Helios NanoLab
400S FIB/SEM dual-beam system equipped with a Ga.sup.+ ion source.
The surfaces of the electrode were covered by C and Pt layers by
electron and ion beam to protect the sample from the milling
[0052] Electrochemical Measurement
[0053] 1 M KOH solution (pH=14) and 0.5 M
K.sub.0.6H.sub.2.4BO.sub.3 solution (pH=9.4) were prepared from
KOH, H.sub.3BO.sub.4, and Milli-Q water (18 M.OMEGA. cm).
Purification of 1 M KOH solution was conducted. Electrochemical
measurements were performed using a BioLogic VMP3 potentiostat. Pt
wire and Ni foam were used as counter electrodes in purified KOH
and other solutions, respectively. Hg/HgO (1 M NaOH) (ALS CO., Ltd)
and Ag/AgCl (Saturated KCl) (ALS CO., Ltd) were used as reference
electrodes in KOH solution and KBi solution, respectively. All
potentials are reported with respect to the reversible hydrogen
electrode (RHE). Potentials were reported with iR-correction unless
otherwise specified. Solution resistance R.sub.s was measured by
impedance spectroscopy (100 mHz-10 kHz, 10 mV amplitude). Before
and during all the measurements, Ar (99.9999%) or O.sub.2 (99.999%)
gas was continuously supplied to the electrochemical cell. Product
gas from gas tight cell was quantified with a gas chromatography
(GC-8A; Shimadzu Co. Ltd.) equipped with a TCD detector and a
Molecular sieve 5A column using Ar (99.999%) as a carrier gas. Ar
was flowed at 22 SCCM in the electrochemical cell and outlet gas is
connected to a sampling loop in the GC.
Example 1
[0054] NiFeO.sub.x was prepared on a Au/Ti/FTO substrate by
cathodic deposition and used as a base electrode. The amorphous
CeO.sub.x layer was formed on the NiFeO.sub.x by applying a
constant anodic potential of 1.7 V vs. RHE in the deposition
solution containing 0.4 M cerium nitrate and 0.4 M ammonium acetate
(pH=7) for 6 h at room temperature. Without intending to be bound
by theory, in the deposition solution, Ce.sup.3+ is oxidized and
precipitates on the anode because of the lower solubility of
Ce.sup.4+ compared to Ce.sup.3+. As shown in FIG. 5A, which is a
cyclic voltammogram for Au/Ti/FTO in 0.4 M Ce(NO).sub.3+0.4 M
CH.sub.3COONH.sub.4 (pH=7) under Ar atmosphere (condition: 20 mV
s.sup.-1 and 298 K), oxidation of Ce.sup.3+ appeared around 1.0 V
vs. RHE in the 1.sup.st cycle and disappeared from the 2.sup.nd
cycle. Depending on the potential of the anode, Ce.sup.3+ can be
also oxidized by O.sub.2 evolved from the anode and deposited on
it. Because Ni(OH).sub.2 is not conductive but becomes conductive
once it is oxidized to NiOOH, in order to deposit CeO.sub.x on top
of the NiFeO.sub.x, more anodic potential was applied than the
potential for oxidization of Ce.sup.3+ where oxygen can evolve, as
shown in FIG. 5B, which is a cyclic voltammogram for an NiFeO.sub.x
electrode in 0.4 M Ce(NO).sub.3+0.4 M CH.sub.3COONH.sub.4 (pH=7)
under Ar atmosphere (condition: 20 mV s.sup.-1 and 298 K).
[0055] The bulk composition of electrodes were quantified by
inductively coupled plasma (ICP), shown in Tables 1 and 2 below.
Table 1 shows the composition of bare NiFeO.sub.x and
CeO.sub.x/NiFeO.sub.x electrodes. Table 2 shows the composition of
NiFeCeO.sub.x catalysts prepared by cathodic deposition in
deposition solution containing 25 mM NH.sub.4SO.sub.4, 9 mM
NiSO.sub.4, 9 mM FeSO.sub.4 and x mM Ce(NO.sub.3).sub.3.
TABLE-US-00001 TABLE 1 Ni/.mu.mol Fe/.mu.mol Ce/.mu.mol Fe/
cm.sup.-2 cm.sup.-2 cm.sup.-2 (Ni + Fe) Bare NiFeO.sub.x 1.3 0.64
0.33 CeO.sub.x/NiFeO.sub.x 1.3 0.66 1.6 0.34
TABLE-US-00002 TABLE 2 Ni/.mu.mol Fe/.mu.mol Ce/.mu.mol x/mM
cm.sup.-2 cm.sup.-2 cm.sup.-2 Composition of deposit 3 1.3 0.22
0.025 Ni.sub.0.84Fe.sub.0.14Ce.sub.0.02O.sub.x 6 0.47 0.10 0.038
Ni.sub.0.77Fe.sub.0.16Ce.sub.0.07O.sub.x 9 0.62 0.098 0.070
Ni.sub.0.79Fe.sub.0.12Ce.sub.0.09O.sub.x
[0056] The composition of Ni and Fe was maintained during
deposition of CeO.sub.x. The total amount of Ce deposited was 1.6
.mu.mol cm.sup.-2 quantified by ICP while total charge passed
during deposition was 14 C cm.sup.-2 which corresponds to 150
.mu.mol cm.sup.-2 of electrons, as shown in FIG. 5C, which is a
chronoamperogram of a NiFeO.sub.x electrode at 1.7 V vs. RHE during
the deposition process of CeO.sub.x in 0.4 M Ce(NO.sub.3).sub.3+0.4
M CH.sub.3COONH.sub.4 (pH=7) under Ar atmosphere. Therefore the
efficiency of deposition was not high in this anodic deposition
process.
[0057] FIG. 1A shows an electrocatalytic stability test by
controlled current electrolysis of uncoated and CeO.sub.x-coated
NiFeO.sub.x at 20 mA cm.sup.-2 in 1 M KOH. Uncoated NiFeO.sub.x
reached 20 mA cm.sup.-2 at 1.53 V vs. RHE in the beginning of the
stability test, but the overpotential increased more than 60 mV in
96 h. On the other hand CeO.sub.x coated NiFeO.sub.x maintained its
overpotential for 96 h while the potential to achieve 20 mA
cm.sup.-2 was similar to the uncoated NiFeO.sub.x at the initial
stage. In the cyclic voltammetry (CV) measured immediately after
stability test (96 h), slight recovery of current towards OER was
observed at the 2.sup.nd cycle, without going back completely to
the original current density measured before stability test on the
bare NiFeO.sub.x electrode, as shown in the cyclic voltammogram of
FIG. 1B. Cathodic sweep or resting potential may help to recover
the current of deactivated NiFeO.sub.x to some extent, but the
reason still remains unclear. In contrast, although a slight
increase of overpotential (.about.10 mV) was observed during the
stability test for the CeO.sub.x coated NiFeO.sub.x, by potential
sweeping, current for the OER recovered completely to the original
one, which was measured before the stability test, as shown in the
cyclic voltammogram of FIG. 1C. This full recovery was in marked
contrast relative to that for the bare NiFeO.sub.x electrode.
Importantly, as shown in FIG. 1D, the Tafel slope remained 40 mV
dec.sup.-1 after CeO.sub.x deposition, which suggests that the
CeO.sub.x layer did not affect the fundamental reaction mechanism
of NiFeO.sub.x towards OER. When the redox peaks of Ni.sup.2+/3+ in
the CVs measured before and right after stability test are
compared, as shown in FIG. 6, the redox peaks showed a similar
negative shift after anodic polarization on bare and CeO.sub.x
coated NiFeO.sub.x, although they had different stability. This
indicates that the peak position of Ni.sup.2+/3+ does not reflect
the activity for OER. Surface SEM images show almost all the
NiFeO.sub.x was uniformly covered by CeO.sub.x layer, as shown in
comparing FIG. 2A, which shows a bare NiFeO.sub.x electrode, with
FIG. 2B, which shows a CeO.sub.x coated NiFeO.sub.x electrode.
Although cathodically deposited NiFeO.sub.x had a coarse structure
on the substrate, the CeO.sub.x layer was successfully formed with
thickness ranged 100-200 nm on top of the NiFeO.sub.x, as shown in
the cross-section SEM image of a CeO.sub.x coated NiFeO.sub.x
electrode milled by focused ion beam (FIB) of FIG. 2C. As shown in
FIG. 7, the XRD diffraction pattern did not show any peaks from
crystalline CeO.sub.2 and NiFeO.sub.x, while it shows only a broad
peak around 25.degree. indicating that the electrochemically
deposited oxide or hydroxide consists of an amorphous phase. Raman
spectra show O--Ce--O vibration around 450 cm.sup.-1 with broad
peak of Ni--O vibration from Ni(OH).sub.2 around 560 cm.sup.-1, as
shown in FIG. 2D. When compared with commercial CeO.sub.2, the
vibration of O--Ce--O was shifted negatively and became broader.
Without intending to be bound by theory, this change of peak can be
caused by particle size or coexistence of Ce.sup.3+. XPS spectra of
Ce 3d showed multiplet splitting composed of Ce.sup.3+ and
Ce.sup.4+,.sup.26-28 which indicates that Ce.sup.3+ was
precipitated together with Ce.sup.4+, as shown in FIG. 8A. As shown
in FIG. 8B, O 1s spectra showed two main peaks at 529 eV and 532
eV, which are characteristic of oxide and hydroxide, respectively.
Since XPS is a surface sensitive technique, peaks from Ni and Fe
underneath the CeO.sub.x layer were not detected.
[0058] Impedance spectra shown in FIG. 3A of bare and CeO.sub.x
coated NiFeO.sub.x electrodes show a single semicircle which arises
from the parallel connection of charge transfer resistance
(R.sub.CT) towards OER and double layer capacitance (C.sub.dl), as
shown in the Randles circuit shown in FIG. 9A. The corresponding
peak appeared around 100 Hz in the phase of the Bode plot shown in
FIG. 3B. A thick oxide film is often reported to have an additional
semicircle in the Nyquist plot and corresponding peak around 1 kHz
in the Bode plot. This additional semicircle is assigned to the
parallel connection of resistance (R.sub.f) and capacitance
(C.sub.f) from oxide film, as shown in the Voigt circuit in FIG.
9B, and induces potential drop through the oxide film. CeO.sub.x
coated NiFeO.sub.x did not show any other semicircle, which also
implies CeO.sub.x layer itself did not cause potential loss through
the layer in alkaline condition.
[0059] Although the CeO.sub.x layer improved the stability of
NiFeO.sub.i CeO.sub.x itself is not active for OER because
deposition of CeO.sub.x on Au substrate did not show any current
around 1.5 V vs. RHE, as shown in the cyclic voltammogram of FIG.
10. As shown in the chronoamperogram of FIG. 11, when Ce.sup.3+
precursor was not present in the deposition process, stability of
the NiFeO.sub.x was not improved, which confirms that deposition of
CeO.sub.x layer is essential to obtain stable OER performance. In
other words, acetate ion or ammonium ion in the deposition solution
did not contribute to the improved stability. To confirm the
importance of CeO.sub.x layer, different compositions of
NiFeCeO.sub.x electrodes were also prepared by adding Ce(NO).sub.3
in the solution for cathodic deposition of oxide catalysts. The
composition of deposition solutions and electrocatalysts deposited
are shown in Table 2 above. Doping of Ce to the Ni-based catalyst
improved the kinetics of OER, and 3-5 nm of segregated CeO.sub.2
was observed after electrochemical measurements were taken,
although the contribution of CeO.sub.2 to the improved kinetics is
still under investigation. As shown in FIGS. 12A and 12B,
Ni.sub.0.84Fe.sub.0.14Ce.sub.0.02O.sub.x showed improved current
compared with NiFeO.sub.x without Ce. However, none of the
NiFeCeO.sub.x electrodes improved the stability as much as
CeO.sub.x coated NiFeO.sub.x Ce-doped NiFeO.sub.x can be also
stabilized by deposition of CeO.sub.x layer showing a slightly
smaller overpotential at 20 mA cm.sup.-2 compared with
CeO.sub.x/NiFeO.sub.x This universal improvement of OER stability
with the CeO.sub.x layer is directed to the permselectivity of the
layer, which prevents dissolution of iron species from the
catalysts.
Example 2
[0060] Permselectivity of the amorphous CeO.sub.x layer was
investigated by electrochemical measurements in the presence of
various kinds of reducing agents which can be oxidized on
NiFeO.sub.x. FIG. 4A shows representative cyclic voltammograms of
bare and CeO.sub.x coated NiFeO.sub.x electrodes in 1 M KOH with
and without 0.2 M K.sub.4Fe(CN).sub.6. In the alkaline solution
with the ferrocyanide redox ion, bare NiFeO.sub.x displays
oxidation and reduction peaks of ferrocyanide and ferricyanide,
which has redox formal potential at 1.33 V vs. RHE in pH 14,
followed by redox peaks of Ni.sup.2+/3+. On the other hand,
CeO.sub.x coated NiFeO.sub.x did not show any redox peaks of
ferrocyanide ion and only redox peaks from Ni.sup.2+/3+ were
observed, which suggests the CeO.sub.x layer blocked the permeation
and charge transfer of ferrocyanide. A slight difference was
observed between the solution with and without ferrocyanide in the
potential range from 1.4 to 1.5 V vs. RHE, which suggests that
there was still leak current for redox reactions.
[0061] This permselectivity was also confirmed by gas
quantification of O.sub.2 during controlled current electrolysis at
10 mA cm.sup.-2, as shown in FIG. 13A. In the case of bare
NiFeO.sub.x oxygen did not evolve in the initial 3 h, which means
redox oxidation dominated on the bare NiFeO.sub.x surface rather
than OER. After 3 h of electrolysis, oxygen started to evolve,
which comes from decreasing concentration of redox species during
the electrolysis. Because this choronopotentiometry is done in a
batch electrochemical cell, redox concentration kept decreasing,
and oxygen evolution could start when redox diffusion limiting
current density became lower than applied current density. On the
other hand, 90% of the faradaic efficiency was maintained on
CeO.sub.x/NiFeO.sub.x after 7 h.
Example 3
[0062] To investigate the permselectivity of the amorphous
CeO.sub.x layer further, Faradaic efficiencies of O.sub.2 in the
presence of different kinds of reducing agents, such as iodide,
methanol, ethanol and iso-propanol, were further evaluated, as
shown in FIGS. 13B-13E, and summarized in FIG. 4B. In alkaline
solution, iodide can be oxidized to iodate (eq. 1, shown below).
Methanol and ethanol are oxidized to corresponding carboxylic acids
(eq. 2 and 3, shown below), and iso-propanol is oxidized to acetone
(eq. 4, shown below).
I.sup.-+6OH.sup.-.fwdarw.IO.sub.3.sup.-+H.sub.2O+6e.sup.- (1)
CH.sub.3H.sub.5OH+5OH.sup.-.fwdarw.HCOO.sup.-+4H.sub.2O+4e.sup.-
(2)
C.sub.2H.sub.5OH+5OH.sup.-.fwdarw.CH.sub.3COO.sup.-+4H.sub.2O+4e.sup.-
(3)
CH.sub.3CH(OH)CH.sub.3+2OH.sup.-CH.sub.3COCH.sub.3+2H.sub.2O+2e.sup.-
(4)
Clear improvement of selectivity toward O.sub.2 was observed in the
solution with redox anions rather than neutral alcohols. This trend
suggests that diffusion of reducing agent is impacted by their
charges. Since the isoelectronic point of CeO.sub.2 is around 7,
CeO.sub.x layer should be negatively charged and repulse anions
which resulted in suppression of diffusion of anion through the
layer in alkaline condition. Although the OH.sup.- ion is also
negatively charged, the CeO.sub.x layer electrodeposited by anodic
polarization is reported to have hydrous disordered structure,
which could contribute the diffusion of OH.sup.- to the NiFeO.sub.x
catalyst underneath the layer.
Example 4
[0063] Coating of the CeO.sub.x layer can be applied to seawater
splitting to improve the stability of NiFeO.sub.x catalysts. The
overpotential of NiFeO.sub.x increased more than 60 mV in 6 h in
the solution with 1 M KCl, while it increased 30 mV in the solution
without KCl, as shown in FIG. 14. There is no oxidation of (eq. 5,
shown below) in alkaline conditions because the redox potential is
reported to be 1.72 V vs. RHE, which is higher than the potential
observed.
Cl.sup.-+2OH.sup.-.fwdarw.ClO.sup.-+H.sub.2O+2e.sup.- (5)
[0064] Cl.sup.- seems to facilitate the deactivation of
NiFeO.sub.x, although the reason of promoted degradation is not
well understood. On the other hand, the overpotential of CeO.sub.x
coated NiFeO.sub.x increased 15 mV, while the potential was
comparable to the bare NiFeO.sub.x at the beginning of stability
test. This result suggests that CeO.sub.x/NiFeO.sub.x can be an
attractive candidate for seawater splitting which does not require
purification process.
Example 5
[0065] Suppressed diffusion of anion was also observed in the
reaction condition of OER. In near neutral 0.5 M
K.sub.0.6H.sub.2.4BO.sub.4 solution (pH=9.4), the overpotential
towards OER drastically increased by deposition of CeO.sub.x, while
those in 1 M KOH were quite similar between bare and CeO.sub.x,
coated NiFeO.sub.x, as shown in FIGS. 15A and 15B. In neutral pH
(<11), buffer anion is required to remove protons produced from
water during OER because insufficiency of OH.sup.- induces reactant
switching from OH.sup.- (eq. 6, shown below) to water (eq. 7, shown
below). Increased overpotential by the CeO.sub.x layer also
indicates that the layer prevented diffusion of the buffer
anion.
4OH.sup.-.fwdarw.O.sub.2+2H.sub.2O+4e.sup.- (6)
2H.sub.2O+4B(OH).sub.4.sup.-.fwdarw.O.sub.2+4H.sub.3BO.sub.3+4e.sup.-
(7)
[0066] From these results, it can be seen that the CeO.sub.x layer
has permselectivity which prevents redox anions from diffusing
through while it allows OH.sup.- ion to evolve O.sub.2 from OER
catalysts underneath the layer. This permselectivity may contribute
to the improved stability of NiFeO.sub.x in the long term current
electrolysis in alkaline solution. The permselective layer may have
also suppressed FeO.sub.4.sup.2- anion to diffuse to the
electrolyte, which resulted in maintaining the active sites in
NiFeO.sub.x during anodic polarization.
Example 6
[0067] A thin film of cobalt phthalocyanine was deposited by
thermal evaporation for 10 min at room temperature. The deposited
cobalt phthalocyanine was transformed to CoO.sub.x by annealing in
air at 400.degree. C. for 30 min CeO.sub.x deposition was conducted
on the CoO.sub.x electrode following the deposition procedure
mentioned above. A constant anodic current (10 .rho.A cm.sup.-2)
was applied for 1 h under an Ar atmosphere. Surface SEM images show
that almost all the CoO.sub.x was uniformly covered by the
CeO.sub.x layer, as shown in comparing FIG. 16A, which shows a bare
CoO.sub.x electrode, with FIG. 16B, which shows a CeO.sub.x coated
CoO.sub.x electrode. Although deposited CoO.sub.x had a
nanoparticle structure on the substrate, the CeO.sub.x layer was
successfully formed on top of the CoO.sub.x.
[0068] As shown in the cyclic voltammogram of a bare CoO.sub.x
electrode in FIG. 17A, bare CoO.sub.x showed a drastic decrease of
the current and a positive shift of the onset potential with
potential cycles that were attributed to the gradual loss of the Co
species from the electrode surface. In contrast, the CeO.sub.x
coating did not alter the initial onset, i.e., the electrocatalytic
activity of CoO.sub.x. These observations are attributed to the
confinement introduced by the layer where the dissolved Co species
is maintained within the layer, assisting self-healing, as shown in
the cyclic voltammogram of FIG. 17B. Thus, the essential role of
the additional CeO.sub.x layer for the stability was confirmed to
be universal regardless of the OER catalyst.
Non-Limiting Exemplary Embodiments
[0069] Having described various aspects and embodiments of the
invention herein, further specific embodiments of the invention
include those set forth in the following paragraphs.
[0070] Certain embodiments provide oxygen evolution
electrocatalysts, electrodes using oxygen evolution
electrocatalysts, and methods of making oxygen evolution
electrocatalysts. In one aspect, an oxygen evolution
electrocatalyst is provided. The oxygen evolution electrocatalyst
may include an oxide electrocatalyst, and a permselective amorphous
layer deposited on the oxide electrocatalyst. The permselective
amorphous layer may prevent diffusion of redox ions but permit
diffusion of hydroxide ions to the oxide electrocatalyst.
[0071] In accordance with certain embodiments, for example, the
oxide electrocatalyst may comprise an oxide of a transition metal.
In some embodiments, for instance, the oxide electrocatalyst may
comprise at least one of NiO.sub.x, CoO.sub.x, FeO.sub.x,
MnO.sub.x, IrO.sub.x, RuO.sub.x, or any combination thereof to form
a mixed oxide. In further embodiments, for example, the oxide
electrocatalyst may comprise NiFeO.sub.x or CoFeO.sub.x. In certain
embodiments, for instance, the oxide electrocatalyst may comprise
NiFeO.sub.x.
[0072] In accordance with certain embodiments, for example, the
permselective amorphous layer may comprise a catalytically inactive
material. In some embodiments, for instance, the permselective
amorphous layer may comprise at least one of CeO.sub.x, TiO.sub.x,
AlO.sub.x, ZnO.sub.x, ZrO.sub.x, SiO.sub.x, CaO.sub.x, MgO.sub.x,
or any combination thereof. In further embodiments, for example,
the permselective amorphous layer may comprise CeO.sub.x.
[0073] According to certain embodiments, for instance, the
permselective amorphous layer may comprise a thickness from about
100 nm to about 600 nm. In some embodiments, for example, the
permselective amorphous layer may be deposited approximately
uniformly on the oxide electrocatalyst.
[0074] In another aspect, an electrode using an oxygen evolution
electrocatalyst is provided. The electrode may include a substrate
and an oxygen evolution electrocatalyst. The oxygen evolution
electrocatalyst may include an oxide electrocatalyst, and a
permselective amorphous layer deposited on the oxide
electrocatalyst. The permselective amorphous layer may prevent
diffusion of redox ions but permit diffusion of hydroxide ions to
the oxide electrocatalyst.
[0075] In accordance with certain embodiments, for example, the
substrate may comprise a conductive substrate. In some embodiments,
for instance, the substrate may comprise at least one of cobalt,
gold, nickel, a fluorine-doped tin oxide (FTO) substrate, or any
combination thereof. In further embodiments, for example, the
substrate may comprise a gold-coated FTO substrate.
[0076] In accordance with certain embodiments, for example, the
oxide electrocatalyst may comprise an oxide of a transition metal.
In some embodiments, for instance, the oxide electrocatalyst may
comprise at least one of NiO.sub.x, CoO.sub.x, FeO.sub.x,
MnO.sub.x, IrO.sub.x, RuO.sub.x, or any combination thereof to form
a mixed oxide. In further embodiments, for example, the oxide
electrocatalyst may comprise NiFeO.sub.x or CoFeO.sub.x. In certain
embodiments, for instance, the oxide electrocatalyst may comprise
NiFeO.sub.x.
[0077] In accordance with certain embodiments, for example, the
permselective amorphous layer may comprise a catalytically inactive
material. In some embodiments, for instance, the permselective
amorphous layer may comprise at least one of CeO.sub.x, TiO.sub.x,
AlO.sub.x, ZnO.sub.x, ZrO.sub.x, SiO.sub.x, CaO.sub.x, MgO.sub.x,
or any combination thereof. In further embodiments, for example,
the permselective amorphous layer may comprise CeO.sub.x.
[0078] According to certain embodiments, for instance, the
permselective amorphous layer may comprise a thickness from about
100 nm to about 600 nm. In some embodiments, for example, the
permselective amorphous layer may be deposited approximately
uniformly on the oxide electrocatalyst.
[0079] In yet another aspect, a method of making an oxygen
evolution electrocatalyst is provided. The method may include
providing an oxide electrocatalyst, and depositing a permselective
amorphous layer on the oxide electrocatalyst via anodic deposition.
The permselective amorphous layer may prevent diffusion of redox
ions but permit diffusion of hydroxide ions to the oxide
electrocatalyst.
[0080] In accordance with certain embodiments, for example,
depositing the permselective amorphous layer comprises depositing
the permselective amorphous layer approximately uniformly on the
oxide electrocatalyst.
[0081] In accordance with certain embodiments, for example, the
oxide electrocatalyst may comprise an oxide of a transition metal.
In some embodiments, for instance, the oxide electrocatalyst may
comprise at least one of NiO.sub.x, CoO.sub.x, FeO.sub.x,
MnO.sub.x, IrO.sub.x, RuO.sub.x, or any combination thereof to form
a mixed oxide. In further embodiments, for example, the oxide
electrocatalyst may comprise NiFeO.sub.x or CoFeO.sub.x. In certain
embodiments, for instance, the oxide electrocatalyst may comprise
NiFeO.sub.x.
[0082] In accordance with certain embodiments, for example, the
permselective amorphous layer may comprise a catalytically inactive
material. In some embodiments, for instance, the permselective
amorphous layer may comprise at least one of CeO.sub.x, TiO.sub.x,
AlO.sub.x, ZnO.sub.x, ZrO.sub.x, SiO.sub.x, CaO.sub.x, MgO.sub.x,
or any combination thereof. In further embodiments, for example,
the permselective amorphous layer may comprise CeO.sub.x.
[0083] Many modifications and other embodiments of the inventions
set forth herein will come to mind to one skilled in the art to
which the inventions pertain having the benefit of the teachings
presented in the foregoing descriptions and the associated
drawings. Therefore, it is to be understood that the inventions are
not to be limited to the specific embodiments disclosed and that
modifications and other embodiments are intended to be included
within the scope of the appended claims. Although specific terms
are employed herein, they are used in a generic and descriptive
sense only and not for purposes of limitation.
* * * * *