U.S. patent application number 16/078673 was filed with the patent office on 2019-02-21 for oxygen evolution electrocatalysts with carbon coated cobalt (ii, iii) oxide layers.
This patent application is currently assigned to SABIC Global Technologies B.V.. The applicant listed for this patent is SABIC Global Technologies B.V.. Invention is credited to Hicham Idriss, Lain-Jong Li, Xiulin Yang.
Application Number | 20190055657 16/078673 |
Document ID | / |
Family ID | 58670103 |
Filed Date | 2019-02-21 |
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United States Patent
Application |
20190055657 |
Kind Code |
A1 |
Li; Lain-Jong ; et
al. |
February 21, 2019 |
OXYGEN EVOLUTION ELECTROCATALYSTS WITH CARBON COATED COBALT (II,
III) OXIDE LAYERS
Abstract
Oxygen evolution reaction (OER) catalysts and uses thereof are
described. An OER catalyst can include a carbon support, a
discontinuous catalytic cobalt (II,III) oxide (Co.sub.3O.sub.4)
nanolayer in direct contact with the carbon support, and an
amorphous continuous carbon layer. The Co.sub.3O.sub.4 nanolayer is
positioned between the carbon support and an amorphous continuous
carbon layer.
Inventors: |
Li; Lain-Jong; (Thuwal,
SA) ; Yang; Xiulin; (Thuwal, SA) ; Idriss;
Hicham; (Thuwal, SA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SABIC Global Technologies B.V. |
Bergen op Zoom |
|
NL |
|
|
Assignee: |
SABIC Global Technologies
B.V.
Bergen op Zoom
NL
|
Family ID: |
58670103 |
Appl. No.: |
16/078673 |
Filed: |
April 13, 2017 |
PCT Filed: |
April 13, 2017 |
PCT NO: |
PCT/IB2017/052154 |
371 Date: |
August 22, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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62324093 |
Apr 18, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y02E 60/366 20130101;
B01J 23/75 20130101; C25B 11/0478 20130101; C25B 1/04 20130101;
B01J 35/0033 20130101; C25B 11/0415 20130101; B01J 33/00 20130101;
Y02E 60/36 20130101; B01J 35/002 20130101 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C25B 1/04 20060101 C25B001/04 |
Claims
1. An oxygen evolution reaction (OER) electrocatalyst comprising: a
carbon support; a discontinuous catalytic cobalt nanolayer
comprising a compound selected from the group consisting of CoP,
CoP.sub.2 and Co.sub.3O.sub.4 that is in direct contact with the
carbon support; and an amorphous continuous carbon layer, wherein
the discontinuous catalytic cobalt nanolayer is positioned between
the carbon support and the amorphous continuous carbon layer.
2. The OER electrocatalyst of claim 1, wherein the thickness of the
amorphous carbon layer is 0.5 to 15 nm.
3. The OER electrocatalyst of claim 1, wherein the discontinuous
catalytic cobalt nanolayer has a thickness of 1 to 1000 nm.
4. The OER electrocatalyst of claim 1, wherein the carbon support
is carbon fiber paper.
5. The OER electrocatalyst of claim 1, wherein the discontinuous
catalytic nanolayer comprises a Co.sub.3O.sub.4 compound and the
discontinuous layer further comprises Co oxide (CoO), cobalt
hydroxide (Co(OH).sub.2), or both.
6. The OER electrocatalyst of claim 1, wherein the discontinuous
catalytic nanolayer comprises the CoP, CoP.sub.2 or both.
7. The OER electrocatalyst of claim 4, wherein the carbon support
has been acid treated.
8. The OER electrocatalyst of claim 1, wherein the discontinuous
catalytic cobalt nanolayer comprises Co.sub.3O.sub.4.
9. An electrode comprising the OER electrocatalyst of claim 1.
10. An apparatus comprising the electrode of claim 9, wherein the
apparatus is for the electrolytic splitting of water into hydrogen
and oxygen, the apparatus further comprising a container for
holding an aqueous electrolyte, a counter electrode, and a power
source configured to apply a voltage across the electrodes.
11. A method of producing the electrocatalyst of claim 1 having a
nanolayer comprising Co.sub.3O.sub.4 compound, the method
comprising: (a) obtaining a carbon support comprising a
discontinuous cobalt precursor nanolayer deposited on the carbon
support; (b) thermally treating the carbon support from step (a)
under vacuum to convert the cobalt species to CoO; and (c)
thermally treating the carbon support from step (b) in an
oxygen-rich environment to convert the CoO Co.sub.3O.sub.4.
12. The method of claim 11, wherein the carbon support from step
(a) comprises a carbon-containing layer deposited on the
discontinuous cobalt precursor layer, and wherein the thermal
treating steps (b) and/or (c) convert the carbon-containing layer
into an amorphous carbon layer, wherein the amorphous carbon layer
is continuous.
13. The method of claim 11, wherein the carbon support material
comprises carbon paper.
14. The method of claim 11, wherein obtaining the carbon support
material of step (a) comprises: (i) acid treating the carbon
support; and (ii) contacting the acid treated carbon support with
the cobalt precursor under conditions sufficient to deposit the
discontinuous cobalt precursor nanolayer on the surface of the
carbon support.
15. The method of claim 14, wherein the deposition in step (ii) is
electrochemical deposition (ECD), atomic layer deposition (ALD) or
chemical vapor deposition (CVD).
16. The method of claim 12, wherein the carbon-containing layer
comprises a hydrocarbon, a sugar-based compound, a sulfonated
carbon compound, nitrogen-based carbon compound, carbon-based
monomer, aromatic compound, or any combination thereof, preferably
glucose.
17. The method of claim 11, wherein the amorphous carbon layer has
a thickness of 0.5 nm to 15 nm.
18. A method of producing the OER electrocatalyst of claim 1 having
a nanolayer comprising the CoP compound the CoP.sub.2 compound or
both, the method comprising: contacting a carbon support material
having a cobalt precursor deposited thereon; and thermally treating
the cobalt precursor and reacting with a phosphorous source in an
oxygen deficient atmosphere to produce the CoP and/or CoP.sub.2
nanolayer on the carbon support.
19. A method for the electrolytic splitting of water into hydrogen
and/or oxygen, the method comprising: electrolyzing an aqueous
solution comprising electrolyte and the OER catalysts of claim 1;
and producing hydrogen gas and oxygen gas.
20. The OER electrocatalyst of claim 19, wherein the aqueous
solution is an acidic solution or a basic solution.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority of U.S.
Provisional Patent Application No. 62/324,093 filed Apr. 18, 2016,
which is hereby incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
A. Field of the Invention
[0002] The invention generally concerns oxygen evolution reaction
(OER) electrocatalysts that includes a discontinuous catalytic
Co.sub.3O.sub.4 nanolayer in direct contact with a carbon support
and an amorphous continuous carbon layer. The discontinuous
Co.sub.3O.sub.4 catalytic nanolayer is between the carbon support
and the amorphous continuous carbon layer.
B. Description of Related Art
[0003] Use of hydrogen as an energy source has increased with the
needed minimize CO.sub.2 emission from fossil fuels. One source of
hydrogen can be from the splitting of water into hydrogen and
oxygen. Water splitting can be achieved on a large scale using an
electrochemical apparatus (e.g., electrolyzer). In an electrolyzer,
water is split into hydrogen (H.sub.2) and oxygen (O.sub.2) by
passing electricity through a water solution that includes an
electrolyte to split the water. The hydrogen production in an
electrolytic water splitting reaction can be limited by the
kinetics of oxygen evolution reaction (OER) at the anodes. Various
solutions have been attempted to improve the production of oxygen
at the anode. For example, electrocatalysts containing metal or
oxide forms of precious metals such as iridium (Ir), ruthenium
(Ru), and their alloys have been coated on the anode. However,
their scarce nature and associated high-cost considerably limit
large-scale implementation of industrial devices. Furthermore,
these catalysts can be unstable in corrosive acids. Other attempts
to improve the efficiency of the oxygen production include the use
of cobalt (Co), iron (Fe), nickel (Ni) and manganese (Mn)
oxides/hydroxides, phosphides, dichalcogenides and some
non-metallic compounds, as OER catalysts in an alkaline solution.
However, the development of alkaline water electrolysis has been
restricted by several issues including low current density and
cross-diffusion of the produced gases. By contrast, the proton
exchange membrane (PEM) electrolysis in acids has shown critical
advantages in current densities, voltage efficiency and purity of
produced gases. Various anode catalysts suitable in acids, such as
ruthenium oxide (RuO.sub.2) and iridium oxide (IrO.sub.2), and
their ternary oxides have been developed for use in these types of
applications. To reduce the usage of precious metals (e.g., Ru and
Ir), other metal elements including tin (Sn), antimony (Sb),
niobium (Nb), lead (Pb), nickel (Ni), copper (Cu), tantalum (Ta),
zirconium (Zr), and molybdenum (Mo) have been added to precious
metals to the form alloys or core-shell structures. However, anodes
made with these metals also suffered from corrosion during use.
[0004] Recent attempts to make OER electrocatalysts have centered
on the use of cobalt (II, III) oxides (Co.sub.3O.sub.4).
Co.sub.3O.sub.4 has shown catalytic properties and chemical
stability in alkaline solutions, however, the application of
Co.sub.3O.sub.4 for OER in acidic medium has not been successful as
the Co.sub.3O.sub.4 suffers from corrosion at potentials higher
than 1.47 V (vs. RHE). At these potentials, cobalt (IV) oxide
(CoO.sub.2) can be formed from the Co.sub.3O.sub.4, which can then
decompose into soluble cobalt (II) oxide CoO with the simultaneous
liberation of the O.sub.2. Furthermore, the cobalt OER electrodes
suffer from poor adhesion of the Co.sub.3O.sub.4 to the electrode
substrate (e.g., titanium foil substrates). Various attempts to
improve electrodes containing cobalt have been disclosed. By way of
example, Leng et al. in "Carbon-encapsulated Co.sub.3O.sub.4
Nanoparticles as Anode Materials with Super Lithium Storage
Performance", Scientific Reports, November 2015, pp. 1-11 discloses
the use of carbon-encapsulated Co.sub.3O.sub.4 nanoparticles (i.e.,
Co.sub.3O.sub.4@C structures) embedded in a carbon support. These
electrodes suffer in that the charge flow to and from the cobalt
core is inefficient due to the cobalt being isolated from the
solution by two layers of carbon. Various attempts to make cobalt
OER catalysts include cobalt-carbon composites (See, Chinese Patent
No. 104056630 to Yiming et al.) and uniformly distributed metal
hydroxides in holes of porous carbon skeletons (See, Chinese Patent
Application Publication No. 10495582 to Cheng et al.). These OER
catalysts suffer from anodic corrosion at potentials higher than
1.47 V.
[0005] As discussed above, many of the OER catalysts currently
available suffer from anodic corrosion in acidic environments,
leaching of catalytic material from the support material, and/or
manufacture of the OER catalyst is not cost effective.
SUMMARY OF THE INVENTION
[0006] A discovery has been made that provides an elegant solution
to the problems associated with the use of cobalt species in OER
catalysts. The solution is premised on the idea of providing a
discontinuous catalytic cobalt (II, III) oxide (Co.sub.3O.sub.4)
nanolayer between a carbon support and an amorphous continuous
carbon layer. The discontinuous catalytic Co.sub.3O.sub.4 nanolayer
is in direct contact with the carbon support. Without wishing to be
bound by theory it is believed that the when OER catalyst is used
as in a set of electrodes in an electrolysis reaction, direct
contact of the cobalt with the carbon support provides for
efficient charge transfer between the cobalt species and the carbon
support, while the carbon layer inhibits exfoliation of the
catalytic Co.sub.3O.sub.4 from the surface of the carbon support.
Notably, the solution of the present invention does not require use
of precious metals to provide low over-potentials and acceptable
current density in acid or alkaline solutions. Furthermore, the OER
electrocatalyst is stable (e.g., doe not breakdown) over an
extended period of time in both acid and alkaline solutions.
[0007] The solution also includes a novel method to produce an OER
electrocatalyst. The method includes a 2-step calcination process
of a carbon support that includes a discontinuous cobalt precursor
layer deposited on the carbon support. This first step can include
heat treating the carbon support under vacuum conditions (oxygen
lean conditions). In this first step, the surface integrity of the
carbon support (e.g., carbon paper or carbon cloth) is reinforced,
and the cobalt precursor is converted CoO. The second calcination
step heat treats the carbon support from the first step in an
oxygen rich environment to convert the CoO to Co.sub.3O.sub.4
without degrading the mechanical strength of the
Co.sub.3O.sub.4-carbon support interface. The discontinuous
Co.sub.3O.sub.4 layer can be coated with an amorphous carbon layer.
Such a carbon layer can adhere the Co.sub.3O.sub.4 to the carbon
support.
[0008] In a specific aspect of the invention, an oxygen evolution
reaction (OER) electrocatalyst is described. The OER
electrocatalyst can include a carbon support; a discontinuous
catalytic Co.sub.3O.sub.4 nanolayer in direct contact with the
carbon support; and an amorphous continuous carbon layer (e.g.,
carbon fiber paper and/or acid treated carbon fiber paper). The
discontinuous catalytic Co.sub.3O.sub.4 nanolayer can be positioned
between the carbon support and the amorphous continuous carbon
layer. The thickness of the amorphous carbon layer can be 0.5 to 15
nm, preferably 1 nm to 10 nm, more preferably 3 nm to 5 nm and/or
the discontinuous catalytic Co.sub.3O.sub.4 nanolayer has a
thickness of 1 to 1000 nm, preferably 500 nm. In an instance, the
carbon support can be substantially coated with the discontinuous
catalytic Co.sub.3O.sub.4 nanolayer. In some instances, the
discontinuous catalytic Co.sub.3O.sub.4 nanolayer can also include
Co(II) oxide (CoO), cobalt hydroxide (Co(OH).sub.2), or both. By
way of example, the OER catalyst can include up to 25 wt. % of
Co(OH).sub.2 and 75 wt. % or more of Co.sub.3O.sub.4. In certain
instances, the OER electrocatalyst can have a ratio of the Disorder
(D)-Raman peak to the Graphite (G)-Raman peak (I.sub.D/I.sub.G)
from 0.2 up to 0.9, preferably 0.6, which can indicate minimal
defects on the surface of the carbon support. The OER
electrocatalyst of the present invention can be included in an
electrode. Such an electrode can be included in an apparatus (e.g.,
an electrolyzer) for the electrolytic splitting of water into
hydrogen and/or oxygen. The apparatus can also include a container
for holding an aqueous electrolyte, a counter electrode, and a
power source configured to apply a voltage across the
electrodes.
[0009] In yet another aspect of the invention, a method of
producing an OER electrocatalyst is described. The method can
include: (a) obtaining a carbon support comprising a discontinuous
cobalt precursor nanolayer deposited on the carbon support; (b)
thermally treating the carbon support from step (a) under vacuum to
convert the cobalt species to CoO; and (c) thermally treating the
carbon support from step (b) in an oxygen-rich environment to
convert the CoO to cobalt (KIM oxide (Co.sub.3O.sub.4). In some
instances, the carbon support from step (a) can include a
carbon-containing (e.g., a hydrocarbon, a sugar-based compound, a
sulfonated carbon compound, nitrogen-based carbon compound,
carbon-based monomer, aromatic compound, or any combination
thereof, preferably glucose) layer deposited on the discontinuous
cobalt precursor layer, and the thermal treating steps (b) and/or
(c) convert the carbon-containing layer into an amorphous carbon
layer. Carbon support material (e.g., carbon fiber paper or carbon
cloth) of step (a) can be obtained by (i) acid treating the carbon
support; and (ii) contacting the acid treated carbon support with
the cobalt species precursor under conditions sufficient to deposit
the discontinuous cobalt species precursor nanolayer on the surface
of the carbon support. By way of example, the cobalt precursor can
be deposited using electrochemical deposition (ECD), atomic layer
deposition (ALD) or chemical vapor deposition (CVD).
[0010] In some instances of the present invention, a method for the
electrolytic splitting of water into hydrogen and/or oxygen is
described. The method can include electrolyzing an aqueous solution
comprising electrolyte and any one of the OER or HER catalysts of
the present invention and producing hydrogen gas, oxygen gas, or
both. The hydrogen gas, the oxygen gas, or both can be
collected.
[0011] The following includes definitions of various terms and
phrases used throughout this specification.
[0012] The phrase "water splitting" or any variation of this phrase
describes the chemical reaction in which water is separated into
oxygen and hydrogen.
[0013] The terms "about" or "approximately" are defined as being
close to as understood by one of ordinary skill in the art. In one
non-limiting embodiment, the terms are defined to be within 10%,
preferably within 5%, more preferably within 1%, and most
preferably within 0.5%.
[0014] The terms "wt. %", "vol. %", or "mol. %" refers to a weight,
volume, or molar percentage of a component, respectively, based on
the total weight, the total volume of material, or total moles,
that includes the component. In a non-limiting example, 10 grams of
component in 100 grams of the material is 10 wt. % of
component.
[0015] The term "substantially" and its variations are defined to
include ranges within 10%, within 5%, within 1%, or within
0.5%.
[0016] The terms "inhibiting" or "reducing" or "preventing" or
"avoiding" or any variation of these terms, when used in the claims
and/or the specification includes any measurable decrease or
complete inhibition to achieve a desired result.
[0017] The term "effective," as that term is used in the
specification and/or claims, means adequate to accomplish a
desired, expected, or intended result.
[0018] The use of the words "a" or "an" when used in conjunction
with any of the terms "comprising," "including," "containing," or
"having" in the claims, or the specification, may mean "one," but
it is also consistent with the meaning of "one or more," "at least
one," and "one or more than one."
[0019] The words "comprising" (and any form of comprising, such as
"comprise" and "comprises"), "having" (and any form of having, such
as "have" and "has"), "including" (and any form of including, such
as "includes" and "include") or "containing" (and any form of
containing, such as "contains" and "contain") are inclusive or
open-ended and do not exclude additional, unrecited elements or
method steps.
[0020] The electrocatalysts of the present invention can
"comprise," "consist essentially of," or "consist of" particular
ingredients, components, compositions, etc. disclosed throughout
the specification. With respect to the transitional phase
"consisting essentially of," in one non-limiting aspect, a basic
and novel characteristic of the OER and/or HER electrocatalysts of
the present invention are their abilities to catalyze a
water-splitting reaction.
[0021] Other objects, features and advantages of the present
invention will become apparent from the following figures, detailed
description, and examples. It should be understood, however, that
the figures, detailed description, and examples, while indicating
specific embodiments of the invention, are given by way of
illustration only and are not meant to be limiting. Additionally,
it is contemplated that changes and modifications within the spirit
and scope of the invention will become apparent to those skilled in
the art from this detailed description. In further embodiments,
features from specific embodiments may be combined with features
from other embodiments. For example, features from one embodiment
may be combined with features from any of the other embodiments. In
further embodiments, additional features may be added to the
specific embodiments described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Advantages of the present invention may become apparent to
those skilled in the art with the benefit of the following detailed
description and upon reference to the accompanying drawings.
[0023] FIGS. 1A-1C are cross-sectional illustrations of the
catalyst of the present invention. FIG. 1A depicts the
discontinuous Co.sub.3O.sub.4 nanolayer on two sides of a carbon
support. FIG. 1B depicts the discontinuous Co.sub.3O.sub.4
nanolayer on one side of a carbon support. FIG. 1C is a top view of
the catalyst depicting discontinuous Co.sub.3O.sub.4 nanolayer on
the carbon support.
[0024] FIG. 2 is a schematic of a method to make an OER
electrocatalyst using a 2-step thermal treating method of the
present invention.
[0025] FIG. 3 is a schematic of a method to make an OER
electrocatalyst having a carbon-layer using a 2-step thermal
treating method of the present invention.
[0026] FIG. 4 is a schematic of a water-splitting reaction using
the OER catalyst of the present invention.
[0027] FIG. 5 shows X-ray Diffraction (XRD) patterns of catalysts
of the present invention and comparative catalysts.
[0028] FIG. 6 shows X-ray Diffraction (XRD) patterns of catalysts
of the present invention and comparative catalysts before and after
acid-OER testing.
[0029] FIG. 7A is a scanning electron microscopy (SEM) image of
electrodeposited Co-species on CP.
[0030] FIG. 7B is an SEM image of and the catalyst of the present
invention.
[0031] FIG. 8A is a transmission electron microcopy (TEM) image of
a flake peeled off from an electrode that includes the catalyst of
the present invention
[0032] FIG. 8B is a high-resolution TEM image of the image of a
flake peeled off from an electrode that includes the catalyst of
the present invention The inset in FIG. 8B is the diffraction
pattern.
[0033] FIG. 8C is high-angle annular diffraction field scanning
transmission electron microscopy (HAADF-STEM) image of the catalyst
of the present invention.
[0034] FIG. 9 shows Raman spectra of catalysts of the present
invention and comparative catalysts.
[0035] FIG. 10 shows Raman spectra of carbon paper under various
heat treatments.
[0036] FIGS. 11A-D shows X-ray photoelectron spectroscopy (XPS)
spectra of Co 2p for (A) catalyst of the present invention, (B)
catalyst of the present invention without a carbon layer, (C)
comparative Co.sub.3O.sub.4/CP catalyst and (D) comparative CoO/CP
catalyst.
[0037] FIGS. 12A-12C depict graphs of electrochemical measurements
of catalysts of the present invention and comparative catalysts.
FIG. 12A are polarization curves of the catalysts of the present
invention and comparative catalysts measured in 0.5 M
H.sub.2SO.sub.4 with a scan rate of 5 mV/s, where the current is
normalized by the geometrical area of carbon fiber paper and the
potential is after internal resistance correction; FIG. 12B are
graphs of double-layer capacitance (C.sub.dl) for the catalysts of
the present invention and comparative catalysts; FIG. 12(C) are
Tafel slopes extracted from the polarization curves in FIG.
12A.
[0038] FIG. 13 shows SEM images of Co(OH).sub.2 precursor material,
cobalt oxide, and cobalt phosphide catalysts of the present
invention prepared at various temperatures.
[0039] FIG. 14 shows the electro-catalytic activity of a
comparative catalyst and the cobalt phosphide catalysts of the
present invention for OER in water.
[0040] FIG. 15 shows electro-catalytic activity of a comparative
catalyst and the cobalt phosphide catalysts of the present
invention for HER in water.
[0041] While the invention is susceptible to various modifications
and alternative forms, specific embodiments thereof are shown by
way of example in the drawings and may herein be described in
detail. The drawings may not be to scale.
DETAILED DESCRIPTION OF THE INVENTION
[0042] The solution to the problems associated with the
conventional OER catalysts has been discovered. The solution lies
in an OER catalyst that has a discontinuous catalytic
Co.sub.3O.sub.4 nanolayer between a carbon support and an amorphous
carbon layer, where the discontinuous catalytic Co.sub.3O.sub.4
nanolayer is in direct contact with the carbon support.
Furthermore, the methods of the present invention to make the OER
catalyst provide means to attach the Co.sub.3O.sub.4 to the carbon
support without degrading the mechanical strength of the
Co.sub.3O.sub.4-carbon support interface. In situ formation of a
layer of amorphous carbon on top of Co.sub.3O.sub.4 can attach the
catalytic Co.sub.3O.sub.4 discontinuous nanolayer to the carbon
support which inhibits detachment or dissolution of the cobalt
species from the carbon support, thereby providing a catalyst that
is stable in acidic or basic medium. Notably, the Co.sub.3O.sub.4
OER catalyst as shown in the Examples section is highly active and
has a longer lifetime than conventional catalysts (e.g., OER
catalyst made from RuO.sub.2 nanoparticles on the same carbon
support covered by an ionic polymer, and/or an OER catalyst made
from Co.sub.3O.sub.4 nanoparticles casted on carbon paper covered
by an ionic polymer (e.g., Nafion.RTM., DuPont, USA)). The coated
carbon layers effectively inhibit the direct degradation of CP
surface as well as provided a mechanical supporting layer to
further inhibit the exfoliation of the catalytic discontinuous
Co.sub.3O.sub.4 nanolayer from the substrate.
[0043] These and other non-limiting aspects of the present
invention are discussed in further detail in the following
sections.
A. OER Electrocatalyst
[0044] The OER electrocatalyst of the present invention includes a
carbon support (e.g., carbon fiber paper) having a discontinuous
catalytic CO.sub.3O.sub.4 nanolayer deposited (coated) on the
surface of the carbon support. The catalytic layer can be on one,
two, three, four, or all surfaces of the carbon support, preferably
all surfaces. An amorphous continuous carbon layer can be formed
around the carbon support/discontinuous catalytic CO.sub.3O.sub.4
nanolayer to provide stability to the catalyst.
[0045] 1. Structure of the OER Electrocatalyst
[0046] FIGS. 1A and 1B are cross-sectional view of non-limiting OER
catalysts 100 and 100' of the present invention. Catalyst 100
depicts the discontinuous CO.sub.3O.sub.4 nanolayer on all surfaces
of a carbon support and catalyst 100' depicts the discontinuous
CO.sub.3O.sub.4 nanolayer on one surface. FIG. 1C is a top view of
the OER catalyst 100 without an amorphous carbon coating. Each
catalyst includes a carbon support 102, a discontinuous
CO.sub.3O.sub.4 nanolayer 104 having regions 106 and an amorphous
continuous carbon layer 108. As shown in the FIGS. the
CO.sub.3O.sub.4 nanolayer 104 has spaces between the regions 106.
In some embodiments, amorphous continuous carbon layer 108 is not
necessary.
[0047] The OER electrocatalyst can have a significant number of
sp.sup.2 carbons in the skeleton of the carbon support. The
sp.sup.2 carbon in the carbon support can be determined using Raman
spectroscopy and determining a ratio (I.sub.D/I.sub.G) between the
disordered structures in sp.sup.2 hybridized carbon materials (D)
and the graphene (sp.sup.2 carbon). As the ratio increases in value
the less ordered sp.sup.2 carbon atoms are in the structure. In the
present invention, I.sub.D/I.sub.G of the OER catalyst can be 0.2
to 0.9, or 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65,
0.70, 0.75, 0.8, 0.85, 0.9 or any range or value there between. In
some instances, the I.sub.D/I.sub.G of the carbon support is 0.6.
Without wishing to be bound by theory, it is believed that the
method of making the OER electrocatalyst as described below and
throughout the specification provides a stable carbon support
surface structure.
[0048] a. Carbon Support
[0049] The carbon support can have a large surface area, good
electric conductivity, and excellent chemical stability in a wide
variety of liquid electrolytes. The carbon support can be any
conductive carbon material having a significant number of sp.sup.2
carbons in the skeleton of the carbon support. The I.sub.D/I.sub.G
of the carbon support can be 0.1 to 0.8, or 0.1, 0.15, 0.2, 0.25,
0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.70, 0.75, 0.8 or any
range or value there between. In some instances, the
I.sub.D/I.sub.G of the carbon support is 0.17. Non-limiting
examples of carbon supports include carbon cloth, carbon fiber
paper, reticulated glassy carbon, and graphene, Toray paper or
Bucky paper. In a preferred embodiment, the carbon support is
carbon fiber paper. Carbon support are available from various
commercial suppliers such as Shanghai Shenglongpan Electric Co.,
Ltd. (China) or Hobby Carbon CNC Ltd. (China).
[0050] b. Discontinuous Catalytic Nanolayer
[0051] The discontinuous catalytic cobalt nanolayer (nanolayer) can
be made as described in the Examples section and throughout the
specification. The nanolayer can include Co.sub.3O.sub.4, and
optionally, Co(II) oxide (CoO) and/or cobalt hydroxide
(Co(OH).sub.2). The catalytic cobalt species is capable of
promoting the formation of oxygen from water (e.g.,
2H.sub.2O+4e.sup.-.fwdarw.O.sub.2+4H.sup.+).
[0052] The discontinuous CO.sub.3O.sub.4 nanolayer can a thickness
of 1 to 1000 nm, preferably 500 nm, or 1 nm, 25 nm, 50 nm, 75 nm,
100 nm, 125 nm, 150 nm, 175 nm, 200 nm, 225 nm, 250 nm, 300 nm, 325
nm, 350 nm, 375 nm, 400 nm, 425 nm, 450 nm, 475 nm, 500 nm, 525 nm,
550 nm, 575 nm, 600 nm, 625 nm, 650 nm, 675 nm, 700 nm, 725 nm, 750
nm 775 nm, 800 nm, 825 nm, 850 nm, 825 nm, 900 nm, 925 nm, 950 nm,
975 nm, 1000 nm or any value or range there between. In some
embodiments, the discontinuous CO.sub.3O.sub.4 nanolayer can be a
CO.sub.3O.sub.4 particle, a combination of CO.sub.3O.sub.4
particles, or a plurality of CO.sub.3O.sub.4 particles that are
arranged in multiple layers (e.g., a stack of particles). The
diameter of these particle(s) and/or height of the stack can
determine the thickness of the nanolayer. By way of example, each
particle(s) diameter and/or stack height can be 1 to 1000 nm,
preferably 500 nm, or 1 nm, 25 nm, 50 nm, 75 nm, 100 nm, 125 nm,
150 nm, 175 nm, 200 nm, 225 nm, 250 nm, 300 nm, 325 nm, 350 nm, 375
nm, 400 nm, 425 nm, 450 nm, 475 nm, 500 nm, 525 nm, 550 nm, 575 nm,
600 nm, 625 nm, 650 nm, 675 nm, 700 nm, 725 nm, 750 nm 775 nm, 800
nm, 825 nm, 850 nm, 825 nm, 900 nm, 925 nm, 950 nm, 975 nm, 1000 nm
or any value or range there between. The catalytic nanolayer can be
adhered to the surface of the carbon support. For example, the
Co.sub.3O.sub.4 can be adhered to the substrate. Without wishing to
be bound by theory, it is believed that the chemical stability of
the catalyst is due to the stability of the Co.sub.3O.sub.4 and/or
the adhesion between the Co.sub.3O.sub.4 and the carbon
substrate.
[0053] The catalytic cobalt nanolayer can include 75 wt. % or more,
or 75 wt. %, 76 wt. %, 77 wt. %, 78 wt. %, 79 wt. %, 80 wt. %, 81
wt. %, 82 wt. %, 83 wt. %, 84 wt. %, 85 wt. %, 86 wt. %, 87 wt. %,
88 wt. %, 89 wt. %, 90 wt. %, 91 wt. %, 92 wt. %, 93 wt. %, 94 wt.
%, 95 wt. %, 96 wt. %, 97 wt. %, 98 wt. %, 99 wt. %, 100 wt. % or
any range or value there between of Co (II, III) oxide
(Co.sub.3O.sub.4). Co(OH).sub.2 can be present in up to 25 wt. %,
or 25 wt. %, 24 wt. %, 23 wt. %, 22 wt. %, 20 wt. %, 19 wt. %, 18
wt. %, 17 wt. %, 16 wt. %, 15 wt. %, 14 wt. %, 13 wt. %, 12 wt. %,
11 wt. %, 10 wt. %, 9 wt. %, 8 wt. %, 7 wt. %, 6 wt. %, 5 wt. %, 4
wt. %, 3 wt. %, 2 wt. %, 1 wt. %, 0 wt. % or any range or value
there between. CoO can be present in amounts up to 1 wt. %, 0.5 wt.
%, 0.25 wt. %, 0.1 wt. %, 0 wt. % or any value or range there
between. The catalytic nanolayer can have a composition of 95 wt. %
Co.sub.3O.sub.4 and 5 wt. % of Co(OH).sub.2, 90.2 wt. % of
Co.sub.3O.sub.4 and 9.8 wt. % of Co(OH).sub.2, or 79.0 wt. % of
Co.sub.3O.sub.4 and 21.0 wt. % of Co(OH).sub.2.
[0054] c. Amorphous Continuous Carbon Layer
[0055] The amorphous continuous carbon layer can be made as
described in the Examples and throughout the specification. The
amorphous continuous carbon layer can be have significantly little
to no crystalline structure and/or be significantly porous to allow
transport of reactants and products to and from the discontinuous
catalytic nanolayer (e.g., water, molecular oxygen, and hydronium).
The amorphous continuous layer can have a thickness of 1 carbon
layer, 0.5 nm to 15 nm, or preferably 1 nm to 10 nm, more
preferably 3 nm to 5 nm, or 0.5 nm, 0.6 nm, 0.7 nm, 0.8 nm, 0.9 nm,
10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm or any value or range
there between. In some embodiments, the amorphous carbon layer can
be conductive.
B. Preparation of OER Electrocatalysts
[0056] The OER electrocatalyst can be made as described in the
Examples and throughout the specification. Notably, the method
includes a 2-step thermal treatment, which substantially reduces
the degradation of the carbon support surface structure, while
converting a cobalt precursor material into Co.sub.3O.sub.4. The
first thermal treatment can be performed under an oxygen deficient
conditions (e.g., under vacuum) and convert the cobalt precursor to
CoO to form a CoO/carbon support material. The CoO/carbon support
material can be heated under oxygen rich conditions (e.g., in the
presence of air, oxygen, or oxygen enriched air) to convert the CoO
to Co.sub.3O.sub.4. The use of the step wise thermal treatment
provides a chemically stable OER electrocatalyst and/or adhered the
Co.sub.3O.sub.4 to the carbon support. For example, the OER
electrocatalyst is stable in corrosive acidic environments and does
not form soluble CoO and O.sub.2 from the acid decomposition of
Co.sub.3O.sub.4.
[0057] 1. 2-Step Thermal Treatment
[0058] FIG. 2 is a schematic of a method 200 to prepare an OER
electrocatalyst. In step one of the method a cobalt
precursor/carbon support material 202 can be obtained. The cobalt
precursor/carbon support material 202 includes support 102 having
cobalt precursor 204 deposited upon the surface of the carbon
support. As shown, cobalt precursor 204 is coated on two sides of
the support material 202, however, it should be understood that all
sides can include cobalt precursor 204.
[0059] In step two, the cobalt precursor/carbon support material
202 can be heated under vacuum (oxygen deficient atmosphere) to
produce CoO/carbon support material 206. During thermal treatment
of the cobalt precursor/carbon support material 202, the cobalt
precursor 204 can be converted to Co oxide regions 208 to produce
CoO/carbon support material 206. CoO regions 208 make up
discontinuous CoO layer. Without wishing to be bound by theory, it
is believed that thermally treating the cobalt precursor/carbon
support material 202 in the presence of a minimal amount of oxygen
maintains the surface integrity of the carbon support while
converting the cobalt precursor to Co(II)O. The first thermal
treating conditions can include a temperature of 300.degree. C. to
550.degree. C., 350.degree. C. to 500.degree. C., or 300.degree.
C., 325.degree. C., 350.degree. C., 375.degree. C., 400.degree. C.,
425.degree. C., 450.degree. C., 475.degree. C., 500.degree. C.,
525.degree. C., 550.degree. C., or any range or value there between
at a reduced pressure (vacuum) of 1 to 10 mTorr (0.14 to 1.3
pascal), 3 to 8 mTorr, 4 to 7 mTorr, or 1 mTorr, 2 mTorr, 3 mTorr,
4 mTorr, 5 mTorr, 6 mTorr, 7 mTorr, 8 mTorr, 9 mTorr, 10 mTorr, or
less than 500 mTorr, or any value or range there between until the
cobalt precursor has been substantially converted to CoO (e.g., 1
hour, 2 hours, 3 hours, 4 hours, 5 hours, or 10 hours). Under these
conditions, substantially no, or no, Co metal is produced.
[0060] In step 3, the CoO/carbon support material 206 can be
thermally treated in an oxygen rich atmosphere to convert the CoO
to Co.sub.3O.sub.4 and produce Co.sub.3O.sub.4/carbon support 210.
Co.sub.3O.sub.4/carbon support 210 includes discontinuous layer 204
that includes Co.sub.3O.sub.4 regions 106. Co.sub.3O.sub.4/carbon
support 210 can be used as an OER electrocatalyst. The second
thermal treating conditions can include a temperature of 20.degree.
C. up to 300.degree. C., 25.degree. C. to 200.degree. C., or
30.degree. C. to 100.degree. C., or 20.degree. C., 25.degree. C.,
30.degree. C., 35.degree. C., 40.degree. C., 45.degree. C.,
50.degree. C., 100.degree. C., 150.degree. C., 200.degree. C., or
any range or value there between at a reduced pressure (vacuum) of
500 to 1500 mTorr (66 to 200 pascal), 600 to 1000 mTorr, 800 to 700
mTorr, or 500 mTorr, 600 mTorr, 700 mTorr, 800 mTorr, 900 mTorr,
1000 mTorr, 1200 mTorr, 1300 mTorr, 1400 mTorr, 1500 mTorr or any
value or range there between until the CoO has been substantially
converted to Co.sub.3O.sub.4 (e.g., 1 hour, 2 hours, 3 hours, 4
hours, 5 hours, or 10 hours). Under these conditions only traces of
Co metal is produced and the mechanical strength of the
Co.sub.3O.sub.4-carbon support interface is not degraded.
[0061] 2. 2-Step Thermal Treatment with a Carbon-Containing
Compound
[0062] In some embodiments, the OER electrocatalyst can include
amorphous continuous carbon layer. FIG. 3 is a schematic of a
method 300 for the preparation of OER electrocatalysts 100, which
have amorphous continuous layer 108. The OER electrocatalysts 100
or 100' can be prepared using the 2 step thermal treatment method
described in FIG. 2, with the following additions. The cobalt
precursor/carbon support material 202 can include a
carbon-containing layer that can be converted into the amorphous
carbon layer 108 during the first and/or second thermal treating
steps. Referring to FIG. 3, in step 1, cobalt precursor/carbon
support material 202 can be contacted with a carbon-containing
compound to form coated cobalt precursor/carbon support material
302. Coated cobalt precursor/carbon support material 302 can
include carbon-containing coating 304, support 102, and cobalt
precursor 206. The carbon-containing compound can be any carbon
containing compound that can be carbonized upon heating.
Non-limiting examples of carbon-containing compounds include a
hydrocarbon, a sugar-based compound, a sulfonated carbon compound,
nitrogen-based carbon compound, carbon-based monomer, aromatic
compound, or any combination thereof. In a particular instance,
glucose is used. Contacting the carbon-containing compound to form
coated cobalt precursor/carbon support material 302 can include
immersing the cobalt precursor/carbon support material 202 into a
carbon-containing solution (e.g., an alcoholic solution of the
carbon-containing solution), spraying or atomizing the
carbon-containing compound on the cobalt precursor/carbon support
material, or other known methods of coating a substrate. In some
embodiments, the carbon-containing compound is only applied to the
portions of the electrocatalyst that include the cobalt precursor.
The amount of carbon-containing compound can be varied to adjust
the thickness of the amorphous carbon layer. By way of example,
cobalt precursor/carbon support material 202 can be immersed in a 1
mg/L to 10 mg/L or 1 mg/mL, 2 mg/L, 3 mg/L, 4 mg/L, 5 mg/L, 6 mg/L,
7 mg/L, 8 mg/L, 9 mg/L, 10 mg/L, preferably 5 mg/L of glucose
solution. In steps 2 and 3, the coated cobalt precursor/carbon
support material 302 can be subjected to the two-step thermal
treatment described in the Examples, FIG. 2, and throughout the
specification to form OER electrocatalyst 100. During the 2-step
thermal treatment the carbon-containing compound is converted to
the amorphous carbon layer and the cobalt nanolayer maintains its
morphologies. Without wishing to be bound by theory it is believed
that the conversion of the carbon-containing compound to the
amorphous carbon helps increase the binding of the cobalt species
to the carbon support. OER electrocatalyst 100' can be made in a
similar manner except that the cobalt precursor 206 is only applied
to one side of the carbon support.
[0063] 3. Preparation of Cobalt Precursor/Carbon Support
Material
[0064] The cobalt precursor/carbon support material can be prepared
by providing a cobalt precursor to one or more surfaces of the
carbon support. In a non-limiting example, the cobalt precursor can
be deposited on the carbon support by electrochemical deposition
(ECD), atomic layer deposition (ALD) or chemical vapor deposition
(CVD) methods. The cobalt precursor can be any suitable salt of
cobalt, for example, cobalt(II) nitrate hexahydrate. Cobalt salts
are available from various commercial sources, for example,
Sigma-Aldrich.RTM. (USA). The amount of cobalt precursor can be
based on the total amount of elemental cobalt to be provided to a
given weight of carbon support.
[0065] In some embodiments, the carbon support is acid treated.
Acid treatment of the carbon support can promote bonding of the
carbon surface to the cobalt species. Acid treatment can include
treating the carbon support with an acid under oxidizing
conditions. Non-limiting examples of acids include sulfuric acid
(H.sub.2SO.sub.4), hydrochloric acid (HCl), a hydrophilic organic
acid, or a combination thereof) to form acid treated carbon support
(e.g., acid treated carbon support 102). Without wishing to be
bound by theory, it is believed that the acid oxidizes portions of
the surface of the carbon support, which can then bond to the
cobalt species. By way of example, the carbon support can be soaked
with alcohol (e.g., methanol or ethanol), added to in an acid
solution containing an electrolyte (e.g., an aqueous sulfuric acid
solution with potassium chloride), and then oxidized with cyclic
voltammetry between 1.5 and 2.3 V.
[0066] 4. Preparation of Cobalt Phosphorous/Carbon Support
Material
[0067] A CoP and/or CoP.sub.2 electrode can be prepared by
obtaining a cobalt precursor/carbon support material. The cobalt
precursor/carbon support material can include a support having the
cobalt precursor deposited upon the surfaces of the carbon support.
In some embodiments, the cobalt precursor/support material is
Co(OH).sub.2 electrodeposited on carbon cloth.
[0068] The cobalt precursor/carbon support material can be
contacted with a phosphorous source (e.g., red phosphorous) under
vacuum (oxygen deficient atmosphere) to produce CoP/carbon support
material and CoP.sub.2/support material. During thermal treatment
of the cobalt precursor/carbon support material, the phosphorous
precursor can be react with cobalt precursor to CoP and/or
CoP.sub.2 regions to produce CoP and/or CoP.sub.2/carbon support
material. Without wishing to be bound by theory, it is believed
that treating the cobalt precursor/carbon support material in the
presence of a phosphorous source and a minimal amount of oxygen
maintains the surface integrity of the carbon support while
converting the cobalt precursor to Co(III)P or Co(IV)P.sub.2. The
thermal treating conditions can include a temperature of
300.degree. C. to 850.degree. C., 350.degree. C. to 550.degree. C.,
or 300.degree. C., 325.degree. C., 350.degree. C., 375.degree. C.,
400.degree. C., 425.degree. C., 450.degree. C., 475.degree. C.,
500.degree. C., 525.degree. C., 550.degree. C., 600.degree. C.,
650.degree. C., 700.degree. C., 750.degree. C., 800.degree. C.,
850.degree. C., or any range or value there between at a reduced
pressure (vacuum) of 1 to 10 mTorr (0.14 to 1.3 pascal), 3 to 8
mTorr, 4 to 7 mTorr, or 1 mTorr, 2 mTorr, 3 mTorr, 4 mTorr, 5
mTorr, 6 mTorr, 7 mTorr, 8 mTorr, 9 mTorr, 10 mTorr, or less than
500 mTorr, or any value or range there between until the cobalt
precursor has been substantially converted (e.g., 1 hour, 2 hours,
3 hours, 4 hours, 5 hours, or 10 hours) to a cobalt phosphide
compounds (e.g., CoP, CoP.sub.2, Co.sub.2P.sub.3, or mixtures
thereof). Under these conditions, substantially no, or no, Co metal
is produced. The cobalt phosphide compounds can be used as HER
electrocatalysts and/or OER electrocatalysts.
C. Use of the OER Electrocatalyst
[0069] The OER electrocatalysts of the present invention can be
used to produce hydrogen and water from water. For example, the
catalysts of the present invention can be integrated to PEM-based
hydrolyzers for high-rate production of hydrogen and oxygen from
water. Referring to FIG. 4, a non-limiting representation of a
water-splitting system 30 of the present invention is provided. The
system 400 can include container 402, the OER electrocatalyst 100,
counter electrode 404 (e.g., HER electrocatalyst of the present
invention), porous barrier for ion transport 406, power source 408,
and aqueous, alcoholic or organic conductive solution 410. In some
embodiments, aqueous conductive solution 410 can include
electrolyte material. Non-limiting examples, of electrolyte
material include Li.sup.+, Rb.sup.+, K.sup.+, Cs.sup.+, Ba.sup.2+,
Sr.sup.2+, Ca.sup.2+, Na.sup.+, and Mg.sup.2+ hydroxides sulfuric
acid, methanesulfonic acid, nitric acid, mixtures of HCl, organic
acids like acetic acid, and the like. The electrolyte can also be a
gel and/or a solid. OER electrocatalyst 100 can serve as an anode
and counter electrode 404 can be a cathode. Any appropriate
cathode, such as platinum or platinum/graphene cathodes can be
used. Power source 408 can provide voltage across the electrodes
such sufficient electrical current passes through the conductive
solution to split water into hydrogen at the cathode and oxygen at
the anode. Oxygen is generated in at the anode by contact of the
water with the OER electrocatalyst 100. Due to the discontinuous
Co.sub.3O.sub.4 nanolayer deposited on the surface of the carbon
support and the amorphous carbon layer surrounding on the
discontinuous Co.sub.3O.sub.4 nanolayer, the water splitting
activity of the catalytic cobalt species is extended. When, the
cobalt phosphide nanolayer is used the cobalt phosphide species can
generate cobalt oxides (e.g., Co.sub.3O.sub.4), resulting in a
mixture of cobalt phosphides and cobalt oxides being present on the
surface of the support (e.g., carbon cloth or carbon paper).
EXAMPLES
[0070] The present invention will be described in greater detail by
way of specific examples. The following examples are offered for
illustrative purposes only, and are not intended to limit the
invention in any manner. Those of skill in the art will readily
recognize a variety of noncritical parameters which can be changed
or modified to yield essentially the same results.
Example 1
Preparation of OER Electrocatalyst of the Present Invention
[0071] Materials:
[0072] All chemical reagents including cobalt(II) nitrate
hexahydrate, glucose, potassium hydroxide (KOH), sulfuric acid
(H.sub.2SO.sub.4) and ethanol were purchased from Sigma
Aldrich.RTM. (U.S.A.). Ultrapure water was obtained from a
Millipore filtration system.
[0073] Electrochemical Deposition of Co-Species on Carbon Fiber
Paper:
[0074] The carbon paper, 1 cm.times.2.5 cm) was first soaked with
ethanol, and then oxidized in 0.5 M H.sub.2SO.sub.4 solution with
cyclic voltammetry for 10 cycles between 1.5 to 2.3 V (vs. Ag/AgCl,
in saturation KCl solution). The oxidized carbon paper (1
cm.times.1 cm) was then immersed into a 0.1 M Co(NO.sub.3).sub.2
solution for the electrodeposition of Co-precursor. A Pt foil and
an Ag/AgCl (in saturation KCl solution) electrode were used as the
counter and reference electrodes respectively. Electrodeposition
was performed at a constant current mode (-10 mA/cm.sup.2) from 10
to 60 min in a PGSTAT 302N Autolab workstation. The as-deposited
sample was then exposed to air to form oxide and hydroxide surface
layers for further treatment (Co-precursor/carbon paper).
[0075] Preparation of Carbon-Containing Compound Coated
Co.sub.3O.sub.4 on Carbon Paper:
[0076] The prepared Co-precursor/carbon paper was immersed into 5
mg/mL glucose solution for 4 h under slow agitation, removed from
the solution, and then dried at room temperature.
[0077] Two-Step Thermal Treating.
[0078] The glucose coated Co-precursor/carbon paper was put into a
tube furnace and then pumped under vacuum (<5 mTorr). The
furnace was then heated to 350.degree. C. in 2 h and kept at this
temperature for another 1 h. After that, the vacuum pressure was
adjusted to 1000 mTorr by passing air into the furnace chamber and
kept for 4 h, where the glucose was thermally decomposed to
amorphous carbon and uniformly covered on the formed
Co.sub.3O.sub.4/carbon paper [Co.sub.3O.sub.4 coated with C/carbon
paper (vacuum 1 h+air 4 h)]. The Co.sub.3O.sub.4 catalyst loading
amount on carbon paper was determined to be 12.6 mg using a high
precision weighing balance.
Example 2
Preparation of a Co.sub.3O.sub.4/Carbon Paper OER
Electrocatalyst
[0079] Co.sub.3O.sub.4/carbon paper was prepared under the same
experimental conditions used in Example 1 with the exception that
carbon-containing layer was omitted. The Co.sub.3O.sub.4 catalyst
loading amount on carbon paper (50 min) was determined to be 12.6
mg using a high precision weighing balance.
Example 3
Preparation of OER Electrocatalyst Comparative Samples
CoO/Carbon Paper (Example 3A)
[0080] The cobalt precursor was deposited on carbon paper using the
procedure in Example 1 and then heated under vacuum treatment for 1
hour to yield CoO/carbon paper.
Co.sub.3O.sub.4/Carbon Paper (Example 3B)
[0081] The cobalt precursor was deposited on carbon paper using the
procedure in Example 1, and then heated in air at 350.degree. C.
for 5 h to yield Co.sub.3O.sub.4/carbon paper. The Co.sub.3O.sub.4
catalyst loading amount on carbon paper for Examples 3A and 3B was
determined to be 12.6 mg.+-.2 mg using a high precision weighing
balance.
Preparation of Nafion Coated Co.sub.3O.sub.4/Carbon Paper (Example
3C) and RuO.sub.2/Carbon Paper (Example 3D)
[0082] The Co.sub.3O.sub.4 and RuO.sub.2 powder were prepared by
directly annealing Co(NO.sub.3).sub.2.6H.sub.2O and RuCl.sub.3
precursors in a porcelain boat and placed in a muffle furnace, and
then heated to 350.degree. C. with a ramp of 2.5.degree. C./min and
maintained for 5 h in air. After that, the furnace was allowed to
cool to room temperature. Nafion (DuPont, USA) is a polymer
commonly used as a capping layer to protect the catalysts from
exfoliation during OER.
[0083] Co.sub.3O.sub.4 Coated with Nafion/Carbon Paper
Catalyst.
[0084] Co.sub.3O.sub.4 powder (62.5 mg) was first dispersed in a
mixed solvent consisting of equal volume amounts of 2-propanol (0.5
mL) and water (0.5 mL), and the mixture was ultrasonicated for 30
min by using ultrasonic oscillators. Then, 200 .mu.L of the
well-dispersed mixture was drop-coated on the acid-oxidized carbon
paper, and 70 .mu.L of 1.0 wt. % Nafion solution in 2-propanol was
added to fix the catalyst onto the carbon paper surface, and
further dried at 40.degree. C. in air for electrochemical
measurements. In addition, RuO.sub.2 on acid-oxidized carbon paper
(RuO.sub.2@nafion/carbon paper) was prepared using similar
procedures as described above.
Example 4
Characterization
Methods
[0085] The catalysts samples were characterized before and after
electrochemical measurement using X-ray diffraction (XRD),
field-emission scanning electron microscope (ESEM), transmission
electron microcopy (TEM), Raman spectroscopy, and X-ray
photoelectron spectroscopy (XPS).
[0086] XRD Analysis.
[0087] The crystalline structure of the samples was analyzed by
X-ray diffraction (XRD, Bruker D8 Discover diffractometer, using Cu
K.alpha. radiation, .lamda.=1.540598 .ANG.). FESEM. FESEM (FEI
Quanta 600) was used to observe the surface morphology of the
catalysts and electron energy loss spectroscopy (EELS) mapping.
TEM. The nanoscale crystal structure was revealed by a transmission
electron microscopy (FEI Titan ST, operated at 300 KV).
[0088] Raman Spectroscopy.
[0089] Raman spectrometer LabRAMAramis (HoribaJobinYvon) was
employed and the range of 100-3500 cm.sup.-1 was explored. A
Diode-pumped solid-state (DPSS) laser with wavelength of 473 nm was
used as the excitation source. The laser power on the sample
surface was adjusted using different filters to avoid the heating
effects on the sample. Fourier transform infrared spectroscopy
(Nicolet iS10 FT-IR spectrometer, Thermo Scientific) was used to
characterize the functionalized groups and catalysts on carbon
fibers.
[0090] XPS.
[0091] XPS studies were carried out in a Kratos Axis Ultra DLD
spectrometer equipped with a monochomatic Al K.alpha. x-ray source
(h.nu.=1486.6 eV) operating at 150 W, a multichannel plate and
delay line detector under a vacuum of 1.times.10-9 mbar. The survey
and high-resolution spectra were collected at fixed analyzer pass
energies of 160 eV and 20 eV, respectively. Binding energies were
referenced to the C 1s peak (set at 284.4 eV) of the sp.sup.2
hybridized (C.dbd.C) carbon from the sample.
Characterization
[0092] XRD Analysis.
[0093] The samples from Examples 1, 2, 3A and 3B were analyzed
using XRD. FIG. 5 shows XRD patterns of the cobalt electrodes from
Examples 1, 2, 3A and 3B. Data line 500 is the XRD pattern for
Example 3B (CoO/carbon paper). Data line 502 is the XRD pattern for
Example 3B (Co.sub.3O.sub.4/carbon paper). Data line 504 is the XRD
pattern for Example 1 catalyst of the present invention. The broad
peaks at 20=26.2 and 53.9 are associated with the (002) and (004)
planes of the graphite-like structure of the carbon paper. The
peaks for the Example 3A comparative catalyst can be attributed to
the cubic structure of CoO (JCPDS No. 65-2902), and the XRD peaks
for Example 3B comparative catalyst and Example 1 catalyst of the
present invention are similar, which are ascribed to the cubic
structure of the Co.sub.3O.sub.4 (JCPDS No. 42-1467).
[0094] FIG. 6 shows the XRD patterns before and after OER testing
in acid. Data line 600 is the XRD pattern for the cobalt
electroplated on carbon paper (no heating), data line 602 is the
XRD pattern for Example 2 catalyst of the present invention
(Co.sub.3O.sub.4/carbon paper), data line 604 is the XRD pattern
for Example 3B comparative catalyst (Co.sub.3O.sub.4/carbon paper),
data line 606 is the XRD of Example 1 catalyst of the present
invention after OER, and data line 608 is the XRD of Example 3A
comparative catalyst (CoO/carbon paper) after OER. From the XRD
pattern 600 it was determined electrodeposited Co-species on carbon
paper (Co-species/CP) was a mixture of Co(OH).sub.2, CoO and
disordered Co.sub.3O.sub.4.
[0095] SEM.
[0096] The electrodeposited Co/carbon paper catalysts were further
analyzed by scanning electron microscopy (SEM). FIG. 7A is an SEM
image of the cobalt precursor on carbon paper prior to heating.
From this image, it was determined that the carbon paper surface
was fully covered with discontinuous cobalt precursor nanolayer.
FIG. 7B is an SEM image of the Example 1 catalyst of the present
invention. From this image it was determined that the high surface
area nanolayer still maintained their morphologies after
glucose-soaking and then heating to convert the carbon-containing
compound/Co-precursor/CP the carbon coated Co.sub.3O.sub.4/carbon
paper. Without wishing to be bound by theory, it is believed that
with the arising heating temperature in oxygen-deficient condition,
the adsorbed glucose molecules start to dehydrates and cross-links,
and as the reaction continues, aromatization and carbonization will
further take place, resulting in formation carbonized shell
covering on the surfaces of Co.sub.3O.sub.4 sheet-like structures
(i.e., nanolayer).
[0097] TEM.
[0098] Example 1 catalyst of the present invention was analyzed
using TEM, high-resolution TEM, and high-angle annular diffraction
field scanning transmission electron microscopy (HAADF-STEM).
Flakes were peeled off from the electrode containing the Example 1
catalyst and structural analysis was performed. FIGS. 8A-8C are TEM
(FIG. 8A) and HRTEM (FIG. 8B) and HAADF-STEM (FIG. 8C) images of
the OER catalyst from Example 1 catalyst of the present invention.
From the analysis of the TEM in FIG. 8, it was determined that a
layer of amorphous carbon with the thickness of approximately
3.6.+-.0.5 nm was found uniformly coated on the Co.sub.3O.sub.4
crystals. The HRTEM image for the selected area and the
corresponding electron diffraction (FFT--Fast Fourier Transform)
pattern are shown in FIG. 8B. This image showed two lattice
spacings of 0.28 and 0.23 nm that corresponded to the
Co.sub.3O.sub.4 crystal planes (220) and (222), respectively. From
analysis of the HAADF-STEM image (FIG. 8C, the elemental mappings
revealed that Co and O were homogeneously distributed in the
selected areas, and the amorphous carbon was coated on the surface
of Co.sub.3O.sub.4 crystals.
[0099] Raman Spectroscopy.
[0100] Raman spectroscopy was used to characterize cobalt catalysts
from Examples 1, 2, 3A and 3B. FIG. 9 shows the Raman spectra for
the catalysts. From the Ramon measurements, it was determined that
a structural transformation of Co-precursor, consistent with XRD
and TEM resulted, under various thermal treating conditions. Data
line 900 is the Raman spectra for the Co-precursor/carbon paper (no
heating), data line 902 is the Raman spectra for Example 3A
comparative catalyst, data line 904 is the Raman spectra for
Example 3B comparative catalyst, data line 906 is the Raman spectra
of Example 2 catalyst of the present invention, and data line 908
is the Raman spectra of Example 1 catalyst of the present
invention.
[0101] Raman spectroscopy was also used to probe into the integrity
of the CP, in particular the interfacial area between carbon paper
and cobalt species. The G-band at .about.1585 cm.sup.-1 was
associated with the sp.sup.2 carbon atom vibrations; and the 2D
band at .about.2725 cm.sup.-1 originated from a double resonance
process: photon-electron band structure. The D-band peak of raw-CP
at about 1370 cm.sup.-1 originated from the disordered structures
in sp.sup.2 hybridized carbon materials. FIG. 10 shows the Ramon
spectroscopy of the catalysts of carbon paper. Data line 1000 is
the Raman spectra for raw carbon paper, data line 1002 is the Raman
spectra for acid oxidized carbon paper, data line 1004 is the Raman
spectra for carbon paper heated for 5 h in air at 350.degree. C.,
data line 1006 is the Raman spectra for carbon paper heating in a
vacuum for 1 h at 350.degree. C., and data line 1008 is the Raman
spectra of carbon paper heated in a vacuum for 1 h and then in air
for 4 h (2-step thermal treatment). As shown in FIG. 10, the ratio
of I.sub.D/I.sub.G was 0.91 for the carbon paper after annealing in
air at 350.degree. C. for 5 h, which was higher than the 0.62 for
that annealed in vacuum (350.degree. C. for 1 h). Without wishing
to be bound by theory, it is believed that the surface structure of
carbon paper was significantly degraded after calcination in air.
However, from the XRD analysis of Example 3A, vacuum thermal
treatment at 350.degree. C. for 1 hour only produces CoO/CP and not
the Co.sub.3O.sub.4 catalyst. Surprisingly, the two-step thermal
treatment process (1 h vacuum+4 h in air) gave the ratio of
I.sub.D/I.sub.G 0.60. Without wishing to be bound by theory, it is
believed that the 1.sup.st step treatment in vacuum was critical
for stabilizing the carbon paper surface structure, thereby
providing OER stability as shown in the electrochemical measurement
section.
[0102] XPS measurements. XPS measurements were used to determine
the atomic composition and the chemical state of Examples 1, 2, 3A
and 3B. From the XPS spectra, it was determined that all the
prepared samples contained carbon, oxygen and cobalt elements with
no other impurities. FIGS. 11A-11D show high resolution Co 2p
spectra of Examples 1, 2, 3A and 3B. FIG. 11A shows high resolution
Co 2p spectra obtained for Example 1 catalyst of the present
invention, which consisted of two main broad peaks at 779.6 and
794.7 eV corresponding to 2p.sub.3/2, 2p.sub.1/2 spin orbit lines
respectively. The spectrum also contained weak satellite structures
at the high binding energy side of 2p.sub.1/2 and 2p.sub.3/2 main
peaks, which indicated the existence of cobalt in the oxide form.
In order to identify the oxidation state of Co, peak fitting of Co
2p.sub.3/2 was conducted. The approach used for the peak fitting is
similar to the one used by Biesinger et al. (Appl. Surf Sci., 2011,
257, pp. 2717-2730), of fitting of a broad main peak combined with
the satellite structure. A Shirley background was applied across
the Co 2p.sub.3/2 peak of the spectrum. The Co 2p.sub.3/2 from
Example 1 catalyst was well fitted using a combination of the
parameters derived from both Co.sub.3O.sub.4 and Co(OH).sub.2
standard samples. The results indicated that the sample contained
90.2% of Co.sub.3O.sub.4 and 9.8% of Co(OH).sub.2. Similar fitting
parameters were applied to FIG. 11B (Example 2 catalyst) and FIG.
11C (Example 3B catalyst). The composition of the Example 2
catalyst (Co.sub.3O.sub.4/carbon paper prepared by two-step thermal
treatment) was 79.0% of Co.sub.3O.sub.4 and 21.0% of Co(OH).sub.2.
The composition of Example 3B catalyst (Co.sub.3O.sub.4/CP, air 5
h) was 84.0% of Co.sub.3O.sub.4 and 16.0% of Co(OH).sub.2. The Co
2p.sub.3/2 in FIG. 11D of Example 3A (CoO/CP) was well fitted using
a combination of the parameters derived from Co metallic, CoO and
Co(OH).sub.2 standard samples. From these analysis, it was
determined that the CoO/carbon paper contains 8.8% of Co, 4.3% of
Co(OH).sub.2 and 86.9% of CoO. From the results, it was determined
that most of Co-species/carbon paper transformed first into
CoO/carbon paper under vacuum heating condition, and then further
oxidized into Co.sub.3O.sub.4/CP with some Co(OH).sub.2/CP in air
gas flow, which was similar with surface composition of the
Co-species/carbon paper calcined in air directly. However, they
were greatly different from the Example 1 catalyst of the present
invention (carbon coated Co.sub.3O.sub.4/carbon paper). The carbon
coating reduced the content of Co(OH).sub.2/CP species, which
attributed to the stability of the Example 1 catalyst of the
present invention under OER conditions.
Example 5
Electrochemical Measurements
Measurement Conditions
[0103] Reference Electrode Calibration:
[0104] The electrochemical measurements were performed in a PGSTAT
302N Autolab Potentiostat/Galvanostat (Metrohm). A graphite rod and
an Ag/AgCl (in saturation KCl solution) electrodes were used as the
counter and reference electrodes respectively. The solutions used
for reference electrode calibration were 0.5 M H.sub.2SO.sub.4 and
1.0 M KOH solutions purged with H.sub.2 for 30 min prior to
measurements. The reference electrode calibration was performed in
a high purity hydrogen saturated electrolyte solution with a Pt
wire as the working and counter electrodes, respectively. The
current-voltage curves were scanned at a scan rate of 5 mV/s, and
the average of the two potentials at which the current crossed zero
was taken to be the thermodynamic potential for the hydrogen
electrode reactions. The E(Ag/AgCl) was lower than E(RHE) by 0.215
V in 0.5 M H.sub.2SO.sub.4 and by 1.022 V in 1 M KOH.
[0105] Electrochemical Measurements:
[0106] The OER activity of the catalysts was evaluated by measuring
polarization curves with linear sweep voltammetry (LSV) at a scan
rate of 5 mV/s in 0.5 M H.sub.2SO.sub.4 and 1.0 M KOH solutions.
The stability test for the Examples was performed with the time
dependent potential measurement, where a constant current density
(100 mA/cm.sup.2) was provided. All data were corrected for a small
ohmic drop based on impedance spectroscopy.
[0107] Electrochemical Measurements Results.
[0108] Electrochemical measurements (e.g., overpotential in acid,
electrocatalytic activity and stability) for the catalysts of
Examples 1, 2, 3A, and 3B were determined.
[0109] Overpotential Measurements.
[0110] FIG. 12A shows the polarization curves in 0.5 M sulfuric
acid with a scan rate of 5 mV/s. The current was normalized by the
geometrical area of carbon fiber paper and the potential was
recorded after internal resistance correction of the samples. Data
line 1200 is the Example 3C comparative catalyst, data line 1202 is
the graph for Example 3B comparative catalyst, data line 1204 is
Example 1 catalyst of the present invention, data line 1206 is
Example 2 catalyst of the present invention, data line 1208 is the
Example 3A comparative catalyst, data line 1210 is the Example 3D
comparative catalyst. The onset potentials of the electroplated
catalysts of Examples 1, 2, and 3A, were all similar (ca. 1.54 V),
where they achieved current densities of 10, 20, and 100
mA/cm.sup.2 at overpotentials of 370, 390 and 460 mV, respectively.
The overpotential at the current density of 10 mA/cm.sup.2 was
typically used for evaluating the electrochemical activity of an
OER catalyst. Although the overpotential 370 mV for Example 1
catalyst of the present invention (at 10 mA/cm.sup.2) was higher
than the state-of-art Example 3C comparative catalyst (220 mV).
[0111] Electrocatalytic Activity.
[0112] The electrocatalytic activity of a given material is
proportional to its active surface area, and, thus can be
correlated to the capacitance of the double layer at the
solid-liquid interface with cyclic voltammetry.
[0113] Double Layer Capacitance Analysis.
[0114] To obtain the double layer capacitance, the potential was
scanned from 1.10 V to 1.24 V at varying scan rates in a
non-Faradaic potential window and the resulting current density was
plotted against the scan rate at 1.17 V and is shown in FIG. 12B.
Data line 1212 is Example 1 catalyst of the present invention, data
line 1214 is Example 3D comparative catalyst, and data line 1216 is
acid oxidized carbon paper. The capacitance of Example 1 catalyst
of the present invention was 113.3 mF/cm.sup.2, which was
approximately 3 times higher than that of Example 3D comparative
catalyst having Nafion-coated Co.sub.3O.sub.4 nanoparticles on
carbon paper (38.4 mF/cm.sup.2) under the same catalyst loading
amount. In FIG. 12B, the capacitance for the acid-treated carbon
paper is shown (25.0 mF/cm.sup.2) as a reference. From these
results, it was determined that the electrochemically
Co.sub.3O.sub.4 nanolayers possessed a higher active surface area
than Co.sub.3O.sub.4 nanoparticles on carbon paper. FIG. 12C shows
the Tafel plots for various OER comparative catalysts (Examples 3A,
3B, and 3D) and catalysts of the present invention (Examples 1 and
2). Data line 1218 is Example 3D, data line 1220 is Example 1
catalyst of the present invention, data line 1222 is Example 2
catalyst of the present invention, data line 1224 is Example 3A
comparative catalyst, and data line 1226 is Example 3D comparative
catalyst. The Tafel slope of carbon coated CO.sub.3O.sub.4/carbon
paper catalyst of the present invention (Example 1) was
approximately 82 mV/dec, which was similar to the value for the
Nafion-coated RuO.sub.2/carbon paper (Example 3C, not shown) and
less than that of comparative Nafion-coated Co.sub.3O.sub.4/carbon
paper (ca. 112 mV/dec) or comparative CoO/carbon paper (106
mV/dec). From these results, it was determined that the Example 1
catalyst of the present invention was an efficient catalyst for
OER.
[0115] Electrochemical Stability.
[0116] The electrochemical stability at a constant current density
100 mA/cm.sup.2 were determined for Example 1, 2, and 3A-3D
catalysts. The actual electrode potential gradually increased with
time for all the electrocatalysts. The time for the potential to
sharply rise to 2.0 V was determined. Table 1 lists the catalyst,
and hours to reach 2.0 V.
TABLE-US-00001 TABLE 1 Catalyst Time to 2.0 V No.
Composition/Conditions (hours) 1 Continuous carbon layer on
discontinuous 86.8 Co.sub.3O.sub.4 nanolayer on a carbon paper
formed by heating using the 2-step thermal treatment method of the
present invention 2 Co.sub.3O.sub.4 on carbon paper formed 68.9 by
heating using the 2-step thermal treatment method of the present
invention 3B Co.sub.3O.sub.4 on carbon paper heated in air 5 h 56.4
3C RuO.sub.2 particles on CP covered with Nafion 46 3A CoO on
carbon paper 8.8 3D Co.sub.3O.sub.4 particles on carbon paper 2
covered with Nafion
[0117] From the results, it was determined that the Example 3A
comparative catalyst was the least stable, which was attributed to
its instability in the acidic pH. Examples 1 and 2 catalysts of the
present invention showed longer catalyst lifetime compared with the
Example 3A-3C, with the Example 1 catalyst of the present invention
showing the longest catalyst lifetime. In addition to the chemical
stability of the catalysts, several factors were identified as the
causes for OER electrode failure. These included low conductivity
of the catalysts and weak adhesion between catalysts and
substrates. The least OER stability found for the Example 3B
comparative catalyst was attributed to a weak interface interaction
between the catalyst and substrate, where the oxygen-rich
environment (during heat treatment) can degrade the surface of the
carbon paper substrates. From the data, it was determined that the
addition of vacuum heat treatment before oxidation in air
considerably improved the OER stability of the Co.sub.3O.sub.4.
From these data, it was determined that the OER stability was not
only determined by the chemical stability of the catalysts, but
also by the adhesion between Co.sub.3O.sub.4 and the substrate.
Hence, Example 1 catalyst of the present invention showed the best
stability owing to the protection by amorphous carbon layers on
discontinuous Co.sub.3O.sub.4 nanolayer.
[0118] Stability in Basic Medium. The stability of the
electrocatalysts from Example 3A and Example 1 were evaluated in
basic medium (1.0 M KOH). The lifetime (reaching potential 2.0 V at
a constant current density of 100 mA/cm.sup.2) is 292.7 h (Example
3A comparative catalyst) and 413.8 h (Example 1 catalyst of the
present invention. The Example 1 catalyst of the present invention
demonstrated superior activity in an alkaline solution. FIG. 12A
shows polarization curve and Tafel slope data of the Example 1
catalyst of the present invention. The Tafel slope was determined
to be (68.8 mV/dec) for the Example 1 catalyst of the present
invention. It was observed that the overpotential to generate 10
mA/cm.sup.2 was only 310 mV in 1.0 M KOH, which was lower than most
of the reported non-precious alkaline OER electrocatalysts. From
this data, it was determined that the stability of Example 1
catalyst of the present invention was the highest among the
comparative OER catalysts in basic medium.
[0119] In sum, the catalyst of the present invention having an
amorphous continuous carbon layer and a discontinuous catalytic
Co.sub.3O.sub.4 nanolayer deposited on a carbon support had a
better electrochemical stability than the commercial
RuO.sub.2/carbon paper at high current densities in both acidic and
basic medium. Furthermore, the two-step thermal treatment process
of the current invention inhibited degradation of the carbon paper
surface, and, thus enhanced the interfacial strength between the
Co.sub.3O.sub.4 and substrates, which attributed to the high OER
stability. Without wishing to be bound by theory, it is believed
that the thin layer of carbon coating inhibited exfoliation of the
catalyst from the substrate. Thus, catalysts of the present
invention and the methods of preparing electrocatalyst provide
solutions to the problems and costs associated with conventional
OER electrocatalysts.
Example 6
Preparation of a CoP/CC and CoP.sub.2 Electrocatalyst of the
Present Invention
[0120] CoP on carbon cloth and CoP.sub.2 on carbon cloth was
prepared by reacting red phosphorous (0.1) with Co(OH).sub.2
electrodeposited on carbon cloth at 450.degree. C., 500.degree. C.,
550.degree. C., 650.degree. C., 750.degree. C., 850.degree. C. for
30 minutes under vacuum. A control of red phosphorous and carbon
cloth was also prepared at 450.degree. C. The temperatures and
loading amounts are listed in Table 2. Table 3 lists the
electrocatalyst material, crystal structure and particle size of
the crystals. FIG. 13 depicts SEM images of the electrocatalysts
prepared at the above temperatures, Co(OH).sub.2 on carbon cloth,
and CoO prepared at 350.degree. C. The upper SEM images are at a
scale of 40 microns and the lower images are at a scale of 10
microns. From the images, the cobalt phosphides are attached to the
amorphous carbon layer in a discontinuous manner.
TABLE-US-00002 TABLE 2 CoP CoP/CoP.sub.2 CoP.sub.2 CoP.sub.2 CoP
CoP Carbon Cloth 450.degree. C. 500.degree. C. 550.degree. C.
650.degree. C. 750.degree. C. 850.degree. C. 450.degree. C. mass
before 2.18 2.17 2.23 2.14 2.19 2.17 2.36 phosphidation (mg) mass
after 3.31 3.64 3.58 3.56 3.42 3.52 2.37 phosphidation (mg) loading
amount 5.65 7.35 7.5 5.68 6.15 7.5 no change (mg/cm.sup.2)
TABLE-US-00003 TABLE 3 Treatment Electrocatalyst Crystal Particle
size Temp (.degree. C.) Material Structure (nm) 450 CoP
Orthorhombic 16.7 500 CoP Monoclinic 17.92 CoP.sub.2 Orthorhombic
26.96 550 CoP.sub.2 Monoclinic 28.53 650 CoP.sub.2 Monoclinic 39.32
750 CoP Orthorhombic 44.49 850 CoP Orthorhombic 44.31
Example 7
Electrochemical Measurements of Electrocatalysts of Example 6
[0121] The OER and HER activity of the cobalt phosphide catalysts
prepared in Example 6, and a comparative CoO catalyst were
evaluated by measuring polarization curves with linear sweep
voltammetry (LSV) at a scan rate of 5 mV/s in 1.0 M KOH solutions.
The stability test for the Examples was performed with the time
dependent potential measurement, where a constant current density
(100 mA/cm.sup.2) was provided. All data were corrected for a small
ohmic drop based on impedance spectroscopy. FIG. 14 shows the
polarization curves in 1 M KOH with a scan rate of 5 mV/s of
comparative cobalt oxide (CoO) and the cobalt phosphide samples of
the present invention. The overpotential of the samples prepared at
450 to 650.degree. C. (269 to 234 mV) was higher than samples
prepared at 750.degree. C. and 850.degree. C. FIG. 15 shows the
polarization curves for hydrogen generation comparative cobalt
oxide (CoO) and cobalt phosphide samples of the present invention.
The cobalt phosphide samples prepared at 450 to 650.degree. C. had
the lowest over potential and are therefore the most active in as
an HER electrocatalyst.
* * * * *