INTRODUCTION
[0001] There is considerable commercial interest in the production of "green" hydrogen by
electrolysis of water. Hydrogen has great potential as a fuel for a wide variety of
applications. Further, green hydrogen can be produced using electricity from renewable
sources, and so green hydrogen is a desirable replacement for fossil fuels. Accordingly,
the optimisation of the production of green hydrogen is of great importance.
[0002] Hydrogen production is performed in an electrolyser, comprising an anode and a cathode.
In use, a potential difference is applied across the anode and the cathode. The cathode,
which is negatively charged, releases electrons which are accepted by hydrogen, allowing
hydrogen gas to evolve at the cathode. Oxygen gas is evolved at the anode.
[0003] In order to increase the efficiency of production of hydrogen (and oxygen), various
strategies may be adopted. For instance, the aqueous solution in the electrolyser
may be modified by varying pH, ionic strength, and so on. One such strategy to increase
efficiency of hydrogen production is to provide the electrodes in the electrolyser
with a catalytic coating. This can reduce the amount of energy needed to produce hydrogen.
[0004] Transition metals are often desirable components in such coatings, as they are able
to catalyse redox reactions. Platinum is known to have excellent qualities in this
regard, but it is extremely expensive and so it is not commercially viable to use
platinum-based catalyst materials on a very large scale.
[0005] Accordingly, there remains a need for improved catalytic coatings for electrolysers,
which are highly active and cost-effective, allowing them to be used for the production
of hydrogen on a large scale.
[0007] Cobalt coatings are highly susceptible to corrosion when contacted with the aqueous
solutions present in an electrolyser (which are generally acidic or alkaline solutions).
Consequently, coatings containing high levels of cobalt cannot be realistically used
for large-scale electrolysis of water. The problem of corrosion is particularly pronounced
on the high surface-area materials which are desirable as electrodes in electrolysers.
[0008] There are strategies available to improve the corrosion resistance of a coating.
One strategy which can be used to increase corrosion resistance of a metallic coating
is to alloy the reactive metal with another element. However, this strategy poses
considerable difficulties in connection with cobalt alloys, as these are difficult
to produce by electrodeposition or electrochemical deposition. Cobalt alloys can be
produced by sintering combinations of compounds including cobalt oxides, hydrothermal
synthesis, and plasma spraying. However, these are typically energy-intensive batch
processes and are not commercially desirable.
[0009] It is particularly desirable to produce transition metal coatings by electrodeposition
processes, because these processes are typically low-energy (they do not require high
temperatures, unlike sintering processes); they can coat complex substrate surface
morphologies, and they can produce very thin coatings (which is useful as transition
metals are generally expensive). However, electrodeposition of two metals simultaneously
is problematic as each metal has a different electrochemical potential, and will start
to be deposited at different reduction potentials. Consequently, it is difficult to
ensure that sufficient quantities of two different metals are deposited, as the metal
which is easier to reduce will naturally predominate.
[0010] It is particularly difficult to co-deposit cobalt with a corrosion-resistant metal
such as chromium. The reduction potential at which chromium begins to electrodeposit
upon an electrode is close to the reduction potential at which hydrogen begins to
form. The difficulty of depositing chromium has been noted by previous workers (see
"
Stainless steel-like FeCrNi nanostructures via electrodeposition into AAO templates
using a mixed-solvent Cr(III)-based electrolyte", Bertero et al., Materials and Design
190, 108559, 2020). Accordingly, in addition to the problems mentioned above, attempts to deposit cobalt
together with chromium on an electrode are also subject to the additional difficulty
of interference by the production of hydrogen at the electrode, which further reduces
(or entirely prevents) chromium deposition. The inventors have found that, using known
electrodeposition conditions, they have been unable to produce a satisfactory electrode
coating comprising enough cobalt to be highly catalytically active, and enough chromium
to provide good corrosion resistance. The problem is especially pronounced on rough
surfaces, which have uneven current density during electrodeposition. This can lead
to very little deposition of chromium in regions of the electrode where current is
low.
SUMMARY OF THE INVENTION
[0011] It has now been found that, by selection of appropriate production conditions, it
is possible to produce a coating layer on a substrate, wherein the coating layer contains
a significant amount of chromium in addition to cobalt. This has been achieved by
codepositing both chromium and one or more further transition metals and/or one or
more non-metallic elements with cobalt, by a carefully selected electrodeposition
process. Accordingly, the invention provides an electroplated electrode comprising
a substrate and an electroplate coating, wherein the coating comprises 5 to 35 wt%
chromium, 10 to 75 wt% cobalt and 10 to 60 wt% one or more additional transition metals
and/or one or more non-metallic elements. By "wt%" is meant "weight expressed as a
percentage of total weight of the coating". Typically the percentage weight of the
components is such that the total weight of chromium, cobalt and one or more additional
transition metals and/or one or more non-metallic elements is 100%. However, it may
also be the case that further components, such as impurities discussed herein, are
present.
[0012] In order to achieve this advantageous coated electrode, a new electrolyte has been
used. The electrolyte comprises precursor salts which act as sources of chromium,
cobalt and the one or more additional transition metals and/or one or more non-metallic
elements; the electrolyte also comprises a suppressing agent which acts to suppress
cobalt deposition and/or hydrogen evolution. Accordingly, the invention also provides
an electrolyte comprising water, a Cr(III) salt, a cobalt salt, one or more further
transition metal salts and/or one or more non-metallic element salts, and a suppressing
agent.
[0013] Also provided herein is a process for producing an electroplated electrode as described
herein. Accordingly, the invention provides an electroplating process for electroplating
an electrode of an electrochemical cell, the process comprising:
- providing an electrode, which comprises a substrate;
- providing an electrolyte, which comprises water, a Cr(III) salt, a cobalt salt, one
or more further transition metal salts and/or one or more non-metallic element salts,
and a suppressing agent;
- contacting the electrode with the electrolyte; and
- depositing a coating on the substrate by applying an electrical current to the electrode
wherein the coating comprises 5 to 35 wt% chromium, 10 to 75 wt% cobalt and 10 to
60 wt% one or more transition metals and/or one or more non-metallic elements.
[0014] The electroplated electrode described herein is envisaged for a variety of applications.
For instance, the electroplated electrode is able to efficiently catalyse the electrolysis
of water. Accordingly, also provided herein is a method of electrolysing water, the
method comprising:
- providing an electrolyser comprising an anode and a cathode, and optionally a separator;
- contacting the anode and/or the cathode with an aqueous solution comprising water;
and
- electrolysing the water by applying a potential difference from the anode to the cathode,
wherein the cathode comprises an electroplated electrode as defined in herein. Thus,
the electroplated electrode may be provided in an electrolyser.
[0015] The electroplated electrode described herein may also be used to catalyse the reaction
of hydrogen and oxygen. Accordingly, the electroplated electrode may be provided in
a fuel cell. In general terms, the electroplated electrode may be contacted with an
electrolyte (such as a neutral electrolyte, salt water, or acidic or alkaline solution)
in order to perform an electrolysis process.
BRIEF DESCRIPTION OF THE FIGURES
[0016]
Figure 1 is an SEM image of nickel foam, coated with a coating as described herein,
containing 10.79 wt% chromium, after exposure to an accelerated degradation test.
Figure 2 is an SEM image of a nickel foam coated with a coating produced using a method
as described herein except that the suppressing agent was omitted from the electrolyte.
Consequently, the coating produced included only 1.29 wt% chromium. The image is taken
after exposure of the coated electrode to an accelerated degradation test, as in Figure
1. The corrosion of the coating is clearly visible.
Figure 3 is a map sum spectrum obtained by summing EDS spectra obtained over the surface
of a nickel foam substrate coating produced using a method as described herein except
that the nickel precursor salt was omitted from the electrolyte. The spectrum does
include nickel, confirming that in the absence of the nickel salt, the process described
herein does not lead to complete coverage of the substrate and the nickel substrate
remains exposed.
Figure 4 shows an original EDS image of a nickel foam substrate produced using a method
as described herein except that the nickel precursor salt and the suppressing agent
were omitted from the electrolyte. The density of Cr, Ni and Co is represented by
colour density; Cr in yellow, Ni in blue and Co in green. It can be clearly seen that
some areas are more yellow, while other areas are more blue, demonstrating that in
the absence of all components of the electrolyte, the element coverage produced is
not homogenous.
Figure 5 is an SEM image of a nickel foam coated with a coating according to the invention,
except that the suppressing agent was omitted from the electrolyte. The uneven nature
of the coating is clearly visible. The foam cavities are 100-200 µm in size.
Figure 6 is an SEM image of an etched nickel mesh, coating according to the invention,
except that the suppressing agent was omitted from the electrolyte. The mesh is 1-30
µm in size. The uneven nature of the coating is somewhat visible and was confirmed
by EDS.
Figure 7 is a photograph of a basic electrolyser setup used to test the ability of
coated electrodes to catalyse the electrolysis of water to produce hydrogen.
Figures 8 and 9 are current vs potential plots obtained by electrolysing water in
the setup of figure 7, using a number of coated electrodes according to the invention
and a number of comparative uncoated surfaces.
Figure 10 is SEM images of a nickel foam substrate coated with a coating according
to the invention. These images were taken before treatment with KOH.
Figure 11 is SEM images of a nickel foam substrate coated with a coating according
to the invention, except in that a suppressing agent was not used in the production.
These images were taken before treatment with KOH. As can be seen, the coating is
unevenly distributed showing that the coating is not homogenous.
Figure 12 is SEM images of a nickel foam substrate coated with a coating according
to the invention which has been treated under 50% KOH at 125°C for 5 hours. As can
be seen the coating skill has a smooth surface and has not been subject to high levels
of degradation.
Figure 13 is SEM images of a nickel foam substrate coated with a coating prepared
without the use of a suppressing agent which has been treated under 50% KOH at 125°C
for 5 hours.
Figure 14 shows current vs potential plots obtained by electrolysing water in the
setup of Figure 7, using a number of coated electrodes according to the invention
and electrodes coated with only chromium and only cobalt.
Figure 15 shows current vs potential plots obtained by electrolysing water in the
setup of Figure 7, wherein the substrate is a nickel foam substrate, and wherein one
plot shows activity of a chromium cobalt coating according to the invention, and one
plot shows activity of cobalt alone.
Figure 16 shows current vs potential plots obtained by electrolysing water in the
setup of Figure 7, wherein the substrate is a nickel foam substrate, and wherein one
plot shows activity of a chromium cobalt coating according to the invention, and one
plot shows activity of a Raney Nickel catalyst.
Figure 17 shows current vs potential plots obtained by electrolysing water in the
setup of Figure 7, wherein the substrate is a flat surface, and wherein one plot shows
activity of a chromium cobalt coating, and one plots shows activity of a platinum
sheet.
Figure 18 shows current vs potential plots obtained by electrolysing water in the
setup of Figure 7, wherein the substrate is a nickel foam substrate, and wherein one
plot shows activity of a chromium cobalt coating according to the invention; one plot
shows activity of a Raney Nickel catalyst; and a third plot shows activity of a platinum
carbon catalyst.
Figure 19 shows a plot of cell voltage against time, wherein the cell undergoes regular
shutdown to provide the conditions similar to an intermittent current. As can be seen
the activity of the catalyst (shown by the non-zero voltage) does not decrease throughout
the harsh treatment.
Figure 20 shows assembly process of the cell with photographs of (a) the unassembled
Elyflow gaskatel cell with its three parts and counter electrode (CE) connections
marked ;(b)different view of the Elyflow gaskatel cell with its three parts labelled
as 1,2 and 3 ; (c) graphite plate (counter electrode) mounted on Part 1 of the cell;
(d) Part 2 is placed on the Part 1; (e) Nickel plate (working electrode (WE)) mounted
on part 3; (f) assembled cell after placing part 3 and tightened the screws and connected
to flow chamber; (g) inset shows the reference electrode (RE) compartment and RE is
placed into the compartment after electrolyte is introduced.
DETAILED DESCRIPTION
[0017] Unless defined otherwise, all technical and scientific terms used herein have the
same meanings as commonly understood by one of skill in the relevant art.
[0018] The invention is described hereafter with reference to particular embodiments and
drawings. However, the invention is not limited to any specific embodiment or aspect
of the following description.
[0019] It should be noted that the following descriptions of various aspects of the invention
are applicable to each of the differing embodiments of the invention. For instance,
the description of the substrate concerns the substrate as present in the coated electrode
of the invention, and as utilised in the process of the invention. Similarly, the
description of the electrolyte concerns the electrolyte of the invention, and the
electrolyte utilised in the process of the invention.
Definitions
[0020] As used in this specification and the appended claims, the singular forms "a", "an",
and "the" include plural referents unless the content clearly dictates otherwise.
Thus, for example, reference to "an anode" includes two or more anodes.
[0021] Where the term "comprising" is used in the present description and claims, it does
not exclude other elements or steps. Furthermore, the terms first, second, third and
the like in the description and in the claims, are used for distinguishing between
similar elements and not necessarily for describing a sequential or chronological
order. It is to be understood that the terms so used are interchangeable under appropriate
circumstances and that the embodiments of the invention described herein are capable
of operation in other sequences than described or illustrated herein.
[0022] "About" as used herein when referring to a measurable value such as an amount, a
is meant to encompass variations of ± 20 % or ± 10 %, more preferably ± 5 %, even
more preferably ± 1 %, and still more preferably ±0.1 % from the specified value,
as such variations are appropriate to perform the disclosed methods.
[0023] The term "amino acid" is used herein to refer to molecules comprising a carboxylic
acid group and an amino group. An amino acid may be charged or uncharged. In particular,
the term "amino acid" encompasses species carrying a neutral -COOH group, or the ionised
-COO
- group. Typical amino acids include arginine, histidine, lysine, aspartic acid, glutamic
acid, serine, threonine, asparagine, glutamine, cysteine, glycine, proline, alanine,
valine, isoleucine, methionine, phenylalanine, tyrosine and tryptophan. Exemplary
amino acids include alanine, valine, glycine, proline, serine and threonine. Particularly
preferred amino acids include alanine, valine and glycine; the most preferred amino
acid described herein is glycine.
[0024] The term "carboxylic acid" is used herein to refer to molecules comprising a neutral
-COOH group, or a charged -COO
- group. Typically, a carboxylic acid as described herein comprises an alkyl group
and a -COOH group, or is formic acid (HCOOH). Preferred carboxylic acids are formic
acid and acetic acid.
[0025] "Alkyl" as used herein refers to monovalent straight-chained and branched alkyl groups.
Typically, the alkyl group is a straight-chained alkyl group. An alkyl group usually
has from 1 to 10 carbon atoms (i.e. is a C
1-10 alkyl group). Preferred alkyl groups include C
1-6 alkyl groups, for example C
1-4 alkyl groups. Examples of alkyl groups include methyl and ethyl groups.
[0026] As used herein, the term "ceramic" refers to an inorganic, non-metallic solid. Typically,
"ceramic" refers to an oxide, boride or nitride or a metal or metalloid element. Exemplary
ceramics include oxides of tellurium, aluminium, zirconium, titanium, magnesium and
silicon. Particularly preferred is tellurium dioxide.
[0027] As used herein, the term "nanoparticle" refers to a particle having at least one
dimension which is in the range of 1 nm - 1000 nm in length. Preferably a nanoparticle
has a diameter of 1 nm - 1000 nm, the diameter being the largest dimension of the
particle in question.
[0028] As used herein, the term "salt" encompasses both a solid salt and a salt which is
dissociated in solution (typically aqueous solution). Generally, unless the context
indicates otherwise, the term "salt" as used herein refers to a dissociated salt in
solution (typically aqueous solution).
[0029] As used herein, the term "metallic" is used to refer to a solid substance which consists
essentially of, or consists entirely of, one or more metals. For instance, a metallic
substance may comprise at least 90% metal by weight, or at least 95% metal by weight,
preferably at least 99% metal by weight. In practice, a metallic substance may contain
unavoidable impurities and so a metallic substance which "consists entirely of" one
or more metals contains only metal atoms, together with unavoidable impurities. By
way of example, a metal may comprise or consist of elements appearing in groups 1-12
of the periodic table. Preferably, a metallic substance as described herein comprises
or consists of a d-block metal. By this is meant that the metallic substance comprises
or consists of one or more metals appearing in any of groups 3-12 of the periodic
table.
[0030] A metallic substance may be an alloy, containing more than one type of metal atom,
or may contain only one type of metal atom.
[0031] As used herein, the term "non-metallic element" has its usual definition in the art.
Typically, a non-metallic element is an element selected from hydrogen, helium, boron,
carbon, nitrogen, oxygen, fluorine, neon, silicon, phosphorus, sulfur, chlorine, argon,
germanium, arsenic, selenium, bromine, krypton, antimony, tellurium, iodine, xenon
and radon.
[0032] The term "electrolyser" is used herein to refer specifically to an electrolytic cell
which uses an external electric current to drive the electrolysis of water, ultimately
producing hydrogen and oxygen. An electrolyser may utilise any known water electrolysis
technology, for instance alkaline water electrolysis; acidic water electrolysis; proton-exchange
membrane water electrolysis; anion-exchange membrane water electrolysis; and high-temperature
water electrolysis. The electrolyte included within the electrolyser may be salt water,
or a neutral electrolyte, or an acidic or alkaline solution. Preferably, the electrolyser
is an electrolyser configured for alkaline water electrolysis.
[0033] The term "fuel cell" or "hydrogen fuel cell" is used herein to refer to an electrochemical
cell which produces an electric current from the reaction between hydrogen and oxygen.
Preferably the fuel cell is an alkaline fuel cell.
[0034] An "electrodeposition process" may also be referred to as an electroplating process
or galvanic deposition process in the art, and is a process in which a layer comprising
or consisting of a metal is deposited onto a substrate under the influence of an electric
field.
[0035] The term "electroplated" is used herein to refer to a surface or object which has
been coated using an electrodeposition process. Therefore an "electroplate coating"
is used herein to refer to a coating on a surface which has been applied using electrodeposition.
When a "coated" electrode or article is referred to, this typically refers to an electroplated
electrode or article.
Electroplated electrode
[0036] The invention concerns an electroplated electrode, comprising a substrate, typically
a metallic substrate, and an electroplate coating. The electroplated electrode may
be obtained or obtainable by the electrodeposition process as described herein.
[0037] The electroplated electrode is a solid object. The precise form of the electroplated
electrode will depend on its intended use.
[0038] The electroplated electrode is capable of conducting electricity, and of catalysing
reactions. Accordingly, the electroplated electrode may be used as an electrode; or
may be an electrode. The electroplated electrode may be in any form suitable for use
as an electrode; merely by way of example, the electroplated electrode may be in the
form of a plate, foam or rod. The electroplated electrode may comprise one or more
electrical terminals to enable an electrical connection to be made to the electrode.
[0039] In particular, the electroplated electrode is able to catalyse the electrolysis of
water. In consequence, the invention also relates to the use of an electroplated electrode
as described herein to electrolyse water. For example, the invention relates to the
use of an electroplated electrode as described herein in a method of electrolysing
water as described herein. The invention also relates to an electrolyser, comprising
one or more electroplated electrodes as described herein; particularly to an electrolyser
comprising an electrode which is an electroplated electrode as described herein.
[0040] The electroplated electrode is particularly useful as an electrode in alkaline water
hydrolysis. Accordingly, the invention particularly relates to an electrolyser, comprising:
- a cathode which comprises an electroplated electrode as described herein;
- an anode; and
- optionally an aqueous alkaline solution, typically comprising aqueous potassium hydroxide.
[0041] The skilled person is readily aware of electrolyser constructions in which the electroplated
electrode may be used.
[0042] The electrolyser typically also comprises a separator. A separator may be a membrane
or diaphragm. The separator is permeable to ions or ionic charge transfer, and separates
the gasses produced at the anode and the cathode respectively. The skilled person
will be familiar with suitable membranes and diaphragms for use in an electrolyser.
[0043] The electroplated electrode is also capable of catalysing the reaction of hydrogen
and oxygen, ultimately producing water. Consequently, the invention also relates to
the use of an electroplated electrode as described herein to catalyse the reaction
of hydrogen and oxygen. The invention also relates to a fuel cell (which may more
completely be referred to herein as a hydrogen fuel cell), comprising one or more
electroplated electrodes as described herein. The invention particularly relates to
a fuel cell comprising:
- an anode which comprises an electroplated electrode as described herein;
- a cathode;
- a source configured to provide hydrogen;
- a source configured to provide oxygen; and
- a medium capable of conveying protons from the anode to the cathode.
[0044] The fuel cell typically comprises a separator which is permeable to hydrogen ions
but impermeable to electrons, which enables protons to flow through the above-mentioned
medium from the anode to the cathode whilst preventing a flow of electrons across
the said medium. The skilled person is readily aware of fuel cell constructions in
which the electroplated electrode may be used as an anode.
Coating
[0045] The electroplated electrode comprises a coating, which is typically an electroplate
coating. The electroplate coating may comprise 5 to 35 wt% chromium and 10 to 75 wt%
cobalt, typically 9.5 to 35 wt% chromium and 10 to 75 wt% cobalt. The remainder of
the composition may comprise, for example, one or more further transition metals and/or
one or more non-metallic elements. Typically, the electroplate coating comprises 5
to 35 wt% chromium, 10 to 75 wt% cobalt and 10 to 60 wt% one or more additional transition
metals and/or one or more non-metallic elements.
[0046] When the one or more transition metals and/or one or more non-metallic elements comprise
one more transition metals, the one or more additional transition metals are preferably
selected from titanium, vanadium, manganese, iron, nickel, copper and zinc. Typically
the one or more additional transition metals are selected from nickel and iron and
mixtures thereof. Often the one or more additional transition metals and/or one or
more non-metallic elements is one additional transition metal. Typically when the
one or more additional transition metals and/or one or more non-metallic elements
is one additional transition metal, it is selected from iron and nickel, preferably
nickel. In these cases, the electroplate coating comprises 5 to 35 wt% chromium, 10
to 75 wt% cobalt and 10 to 60 wt% nickel and/or iron, or 5 to 35 wt% chromium, 10
to 75 wt% cobalt and 10 to 60 wt% nickel.
[0047] When the one or more transition metals and/or one or more non-metallic elements comprises
one or more non-metallic elements the one or more non-metallic elements are typically
selected from hydrogen, helium, boron, carbon, nitrogen, oxygen, fluorine, neon, silicon,
phosphorus, sulfur, chlorine, argon, germanium, arsenic, selenium, bromine, krypton,
antimony, tellurium, iodine, xenon and radon. Preferably the one or more non-metallic
elements are selected from boron, nitrogen, carbon and phosphorus, more preferably
boron and phosphorus. The one or more transition metals and/or one or more non-metallic
elements may comprise one non-metallic element selected from boron or phosphorus.
In these cases, the electroplate coating typically comprises 5 to 35 wt% chromium,
10 to 75 wt% cobalt and 10 to 60 wt% boron, phosphorus or a mixture thereof, such
as 5 to 35 wt% chromium, 10 to 75 wt% cobalt and 10 to 60 wt% phosphorus.
[0048] Often the one or more transition metals and/or the one or more non-metallic elements
comprise carbon. In these cases, the electroplate coating typically comprises 5 to
35 wt% chromium, 10 to 75 wt% cobalt and 10 to 60 wt% carbon, nickel and mixtures
thereof, such as 5 to 35 wt% chromium, 10 to 75 wt% cobalt and 10 to 60 wt% carbon.
[0049] When the one or more transition metals and/or one or more non-metallic elements comprise
both transition metals and non-metallic elements, the electroplate coating typically
comprises 5 to 35 wt% chromium, 10 to 75 wt% cobalt and one or more transition metals
and one or more non-metallic elements wherein the total weight of the one or more
transition metals and one or more metallic elements is 10 to 60 wt%, such as 5 to
35 wt% chromium, 10 to 75 wt% cobalt and 10 to 60 wt% nickel, phosphorus and combinations
thereof.
[0050] If the amount of chromium is less than 5 wt%, the ability of the coating to resist
corrosion is insufficient. Accordingly, the coating comprises at least 5 wt% chromium.
However, the amount of chromium should not be so large that the quantity of other
transition metals (and any additives present) is reduced to a low amount. Accordingly,
the coating may comprise up to 35 wt% chromium, or up to 20 wt% chromium; similarly,
the coating generally comprises at least 9.5 wt% chromium. In particular, the coating
may preferably comprise from 9 to 25 wt% chromium, from 9.5 to 35 wt% chromium, more
preferably from 9.5 to 20 wt% chromium. Often, the coating may comprise from 5 to
20 wt% chromium, preferably from 7 to 20 wt% chromium, more preferably from 9 to 20
wt% chromium. For instance, the coating may comprise about 10, 11, 12, 13, 14, 15,
16 or 17 wt% chromium, i.e. from about 10 to 17 wt% chromium.
[0051] If the amount of cobalt is less than 10 wt%, the ability of the coating to catalyse
reactions (particularly the electrolysis of water) is less than optimal. However,
if the amount of cobalt is above 75 wt%, the susceptibility of the coating to corrosion
increases, as the amount of nickel and chromium present is restricted. Typically,
the coating comprises at least 25 wt% cobalt, or at least 40 wt% cobalt; similarly,
the coating typically comprises no more than 50 wt% cobalt. Accordingly, it is preferred
that the coating comprises from 20 to 75 wt% cobalt, more preferably 25 to 60 wt%
cobalt, for instance from 20 to 30 wt% cobalt or from 45 to 55 wt% cobalt. Typically,
the coating comprises from 35 to 60 wt% cobalt.
[0052] To enable the effective co-deposition of cobalt and chromium, it has been found that
one or more additional transition metals and/or one or more non-metallic elements
should be present in the electrodeposited coating. Preferably, the coating comprises
no more than 60 wt% additional transition metals and/or non-metallic elements or no
more than 55 wt% additional transition metals and/or non-metallic elements, and in
some cases no more than 40 wt% additional transition metals and/or non-metallic elements
(although the coating always comprises at least 10% additional transition metals and/or
non-metallic elements). For instance, the coating may preferably comprise 20 to 55
wt% additional transition metals and/or non-metallic elements. For example, the coating
may comprise about 20 to 40 wt% additional transition metals and/or non-metallic elements
or 45 to 55 wt% additional transition metals and/or non-metallic elements.
[0053] Thus, in a preferred example, the coating may comprise 9 to 20 wt% chromium, 20 to
60 wt% cobalt, and 20 to 60 wt% additional transition metals and/or non-metallic elements.
For instance, the coating may comprise 9 to 15 wt% chromium, 45 to 55 wt% cobalt and
20 to 35 wt% additional transition metals and/or non-metallic elements. In this aspect,
the substrate typically comprises nickel. In another example, the coating may comprise
15 to 20 wt% chromium, 20 to 30 wt% cobalt and 45 to 55 wt% additional transition
metals and/or non-metallic elements. In this aspect, the substrate typically comprises
copper.
[0054] Thus, in a preferred example, the coating may comprise 9 to 20 wt% chromium, 20 to
60 wt% cobalt, and 20 to 60 wt% nickel. For instance, the coating may comprise 9 to
15 wt% chromium, 45 to 55 wt% cobalt and 20 to 35 wt% nickel. In another example,
the coating may comprise 15 to 20 wt% chromium, 20 to 30 wt% cobalt and 45 to 55 wt%
nickel. In this aspect, the substrate typically additionally comprises copper.
[0055] It is possible to determine the elemental composition of the coating described herein
by energy-dispersive X-ray spectroscopy, which is a technique well-known in the art.
The technique may be referred to variously herein as EDXS, EDX or EDS. The elemental
composition of the coating described herein is preferably determined by this technique.
Accordingly, the percentage by weight (wt%) of each of Cr, Co and additional transition
metals and/or non-metallic elements in the coating described herein is preferably
determined by energy-dispersive X-ray spectroscopy. Similarly, the percentage by weight
(wt%) of any other element or compound present in the coating described herein is
preferably determined by energy-dispersive X-ray spectroscopy.
[0056] Generally, the coating directly contacts the substrate (typically a metallic substrate).
That is, the coating is generally electrodeposited directly onto the metallic substrate.
For completeness, though, it should be borne in mind that the substrate may itself
comprise a metallic coating (which is not a coating according to the invention). For
instance, a metallic substrate may consist primarily of a first metal but may also
comprise a metallic coating, onto which coating the coating of the invention is electrodeposited.
[0057] The coating described herein may or may not be exposed to the environment.
[0058] In some cases, one or more additional coatings may optionally be applied to the coating
described herein. In such cases, the coating described herein is not exposed to the
environment. The one or more additional coatings may include, by way of example, a
further corrosion-resistant coating. One example of a further corrosion-resistant
coating is a layered double hydroxide coating. Another example is a carbon coating,
which may be produced by carbon vapour deposition (CVD).
[0059] Preferably, however, further coating(s) are not applied to the coating described
herein. In such cases, the coating described herein is exposed to the environment.
The environment will vary depending on the location of the coated electrode. For instance,
if the coated electrode is located within an electrolyser, the environment directly
adjacent to the coated electrode acting as a cathode will typically include an aqueous
solution. For instance, where the electrolyser is an alkaline water electrolyser,
the environment surrounding the coated electrode will typically include an aqueous
alkaline solution, generally comprising aqueous potassium hydroxide. If the coated
electrode is located within a fuel cell, the environment directly adjacent to the
coated electrode acting as an anode will typically include hydrogen and/or a medium
capable of conveying protons from the anode to the cathode. Alternatively, the environment
may simply include air and/or or packaging material(s) (for instance where the coated
electrode is stored after being prepared).
[0060] Thus, in a preferred aspect, the coating directly contacts the substrate, and the
coating is exposed to the environment.
[0061] The thickness of the coating is not particularly limited. However, if the coating
is excessively thin, it may not sufficiently promote catalytic activity. If the coating
is too thick, the cost of producing the coating may be unnecessarily high. Accordingly,
the thickness of the coating is generally at least about 1 micron. The thickness of
the coating is the thickness as measured in a direction perpendicular to the surface
of the substrate. Preferably the thickness of the coating is in the region of 2 microns
to 10 microns.
[0062] A particular advantage of the coating described herein is that it can be highly homogenously
distributed across the coated surface of the substrate. That is, not only is the coating
typically distributed across the entirety of the substrate's coated surface, but also
the chemical composition of the coating is homogeneous throughout the coating. Thus,
in a preferred aspect, the coating described herein is present on the entire surface
(or coated surface) of the substrate. In other words, the coating is formed without
holes. The coated surface of the substrate is preferably not exposed to the environment,
at any point.
[0063] In a particularly preferred aspect, the Cr content of the coating varies by no more
than 20wt% across the coated surface of the substrate. That is, it is preferred that
the maximum local wt% of Cr at any point on the coated surface is no more than 10%
above the mean wt% of Cr, and the minimum local wt% of Cr at any point on the coated
surface is no more than 10% below the mean wt% of Cr. For instance, any local 10x10
micron region of the coating may have a wt% concentration of Cr deviating from the
mean wt% Cr in the coating by no more than 10%. Still further preferably, the Cr content
of the coating varies by no more than 10 wt% across the coated surface, measured as
described above.
[0064] Similarly, in another preferred aspect, the Co content of the coating varies by no
more than 20% or no more than 10% across the surface of the substrate. In yet another
preferred aspect, the additional transition metal and/or non-metallic element content
of the coating varies by no more than 20% or no more than 10% across the surface of
the subject. These values may be measured as described in connection with Cr, in the
preceding paragraph.
[0065] The electroplate coating described herein may be provided on all or part of the surface
of the substrate. It is possible to avoid providing the coating on any part of the
substrate's surface, for instance by masking that part (or those parts) of the substrate's
surface. Accordingly, it should be noted in connection with the above discussion of
the homogeneity of the coating, the homogenous properties discussed refer to the properties
of the coating observed on the part(s) of the substrate's surface actually intended
to be coated with the coating described herein. This may be the portion of the substrate's
surface exposed to a process for producing an electroplated electrode, as described
herein. That portion of the surface may be referred to as the "coated portion". Thus,
the coating is highly homogenous as discussed above across the
coated portion of the substrate's surface.
[0066] The coating may or may not be metallic, as described herein. The coating typically
comprises at least 60, 70 or 80 wt% metal, or at least 85 wt% metal; preferably at
least 90 wt% metal. For instance, the coating may consist of cobalt, chromium and
one or more further transition metals and/or one or more non-metallic elements, together
with unavoidable impurities.
[0067] However, the coating may also comprise other chemical components. These other components
discussed in the following section.
Additives
[0068] In addition to the essential elements Cr, Co and one or more further transition metals
and/or one or more non-metallic elements, the coating may optionally comprise one
or more other chemical components, referred to herein as additives. An additive is
a species which is deliberately included in the coating.
[0069] The additive(s) may each individually be an element, a compound or a mixture. The
additive(s) may be introduced by different routes. In one example, the additive(s)
may be included in the coating by co-deposition with Cr, Co and the one or more further
transition metals and/or one or more non-metallic elements, during production of the
coating by electrodeposition. Such additive(s) may include one or more metals. In
another example, particles of an additive may be included in the coating. For instance,
particles of an additive may be present during production of the coating by electrodeposition,
and may become embedded in the coating as it forms. One or more additives may be introduced
by each of these routes, or not at all.
[0070] Generally, where an additive is present, the additive is in the form of particles.
Typically, the additive is present in the form of nanoparticles.
[0071] These additive materials increase the conductivity of the coating and improve performance
as an electrolyser. These additives, where present, are preferably in the form of
nanoparticles.
[0072] The coating may comprise 9.5 to 35 wt% chromium, 10 to 75 wt% cobalt and 10 to 60
wt% one or more additional transition metals and/or one or more non-metallic elements
selected from P, B, N and C. In this case, exemplary additive materials include tellurium
dioxide and selenium dioxide.
[0073] The coating may comprise 9.5 to 35 wt% chromium, 10 to 75 wt% cobalt and 10 to 60
wt% phosphorus (P). In this case, exemplary additive materials include ceramic (especially
tellurium dioxide), selenium dioxide, graphene, doped graphene (particularly graphene
doped with one or more non-metals, such as nitrogen-doped graphene), and graphene
oxide, in particular graphene oxide. These additives, where present, are preferably
in the form of nanoparticles.
[0074] The coating may comprise 9.5 to 35 wt% chromium, 10 to 75 wt% cobalt and 10 to 60
wt% nickel. In this case, exemplary additive materials include ceramic (especially
tellurium dioxide), selenium dioxide, graphene, doped graphene (particularly graphene
doped with one or more non-metals, such as nitrogen-doped graphene), graphene oxide
iron and molybdenum, in particular ceramic (especially tellurium dioxide), selenium
dioxide, graphene, doped graphene (particularly graphene doped with one or more non-metals,
such as nitrogen-doped graphene), graphene oxide. Graphene oxide is preferred. These
additives, where present, are preferably in the form of nanoparticles.
[0075] Where one or more additives are present, the coating typically comprises up to 20
wt% of the additive(s) (wt% being percentage by total weight of the coating). Preferably,
where one or more additives are present, the coating comprises up to 10 wt% of the
additive(s). For instance, the coating may comprise about 1, 2, 3, 4, 5, 6, 7, 8,
9 or 10 wt% of additive(s).
[0076] For completeness, it should be noted that the coating may also contain one or more
unavoidable impurities, which are not deliberately added. Unavoidable impurities are
typically elements, but may be compounds or mixtures. Specific examples of impurities
include oxygen. The coating may comprise oxygen as an unavoidable impurity. For instance,
the coating may comprise up to 20wt% oxygen. Manipulation of electrodeposition conditions
may increase or decrease the amount of oxygen present; if the conditions are adjusted
to deliberately increase the amount of oxygen present, oxygen may be present as an
additive in addition to being an unavoidable impurity. Other examples of elements
which may be present as unavoidable impurities include counter-ions present when the
coating is prepared by electrodeposition (including, for instance, Cl or K).
Substrate
[0077] The electroplate coating comprises a substrate. The substrate is typically a metallic
substrate. A metallic substrate is a solid, metallic object capable of conducting
electricity. The precise form of the substrate will depend on any intended ultimate
use of the coated electrode. As explained above, in connection with the coated electrode,
the substrate may typically be an electrode suitable for use in a fuel cell or an
electrolyser. The substrate may for instance be in the form of a plate, fin, foam
or rod. The substrate may comprise one or more electrical terminals to enable an electrical
connection to be made to the substrate.
[0078] The substrate may alternatively comprise a non-metallic material which conducts electricity,
for example graphene.
[0079] The substrate may consist of a single material, or may contain two or more different
materials. Simply by way of illustration, it is noted that the substrate may comprise
a metallic object coated with a metallic coating (other than the coating described
herein).
[0080] The substrate may comprise or consist of steel. Any grade of steel may be used, in
particular stainless steel. Stainless steel is a highly durable material desirable
for many industrial processes. Alternatively or additionally, the substrate may comprise
or consist of nickel. Nickel is a chemically resistant material which strongly binds
to the coating described herein. For instance, the substrate may comprise or consist
of steel (in particular stainless steel) and/or nickel. Preferably, the substrate
consists of steel (in particular stainless steel) and/or nickel.
[0081] It is preferred that the substrate has a rough surface. The coated electrode produced
therefrom is often intended for use as a catalyst, and the efficiency of the catalyst
is typically greatly enhanced by the presence of a large surface area to maximise
the contact between catalyst and any reactant(s). However, even with the highly homogeneous
coating described herein, it can be difficult to deposit the coating homogeneously
on an excessively rough surface.
[0082] Preferably, therefore, the substrate may have a surface roughness of up to 100 microns,
or up to 50 microns. Typically, the substrate may have a surface roughness of at least
0.1 microns or at least 1 micron. For instance, the substrate may have a surface roughness
of from 0.1 to 100 microns, particularly from 1 to 50 microns. A suitably rough surface
can be achieved by, for instance, pickling the substrate in acid (such as HCl) before
the coating is electrodeposited thereon.
[0083] Surface roughness is usually measured with a scanning electron microscope (SEM).
Techniques for determining surface roughness by this method are well known. Typically,
the size of surface features within a sample area is determined (usually by determining
their height from the surface) and a mean value of feature height can then be calculated,
which provides the surface roughness. The surface roughness described is typically
a mean surface roughness as determined by SEM.
[0084] In order to achieve very high surface area, the substrate may comprise a microstructured
surface. That is, the substrate may comprise regular or irregular forms, having at
least one dimension in the size range 1-1000 microns, preferably 2-500 microns. These
regular or irregular forms typically have no dimension larger than 1-1000 microns,
preferably no larger than 2-500 microns.
[0085] Exemplary microstructures which may be present on the surface of the substrate include
nanowires, microchannels and high aspect ratio structures. Other such microstructures
include cavities or holes (for instance, where the substrate comprises or consist
of a foam or a mesh). Thus, in one aspect, the substrate may comprise or consist of
a foam comprising cavities having at least one dimension in the size range 1-1000
microns, preferably 2-500 microns. In another aspect, the substrate may comprise or
consist of a mesh wherein the mesh hole size is in the size range 1-1000 microns,
preferably 2-500 microns.
[0086] A microstructured surface as described above may also be roughened by the presence
of smaller structures in addition to the microstructures discussed above. The surface
may be described as having dual-order roughness, or double roughness. For instance,
where the surface comprises microstructures with dimensions of 100-1000 microns, these
structures may be further roughened by substructures having dimensions of 0.001 to
10 microns. In another example, where the surface comprises microstructures with dimensions
of 10-100 nm, these structures may be further roughened by substructures having dimensions
of 0.001 to 1 microns. In yet another example, where the surface comprises microstructures
with dimensions of 1-10 nm, these structures may be further roughened by substructures
having dimensions of 0.001 to 0.1 microns.
[0087] It should be noted that the discussion of surface roughness above relates to the
surface(s) of the substrate to which the coating described herein is applied. The
substrate may comprise one or more other surfaces, or parts of a surface, which are
not to be coated and which may or may not have a roughness as discussed above.
Electrolyte
[0088] Also provided herein is an electrolyte. The electrolyte may be used in the process
of the invention, and may be used to produce the electroplated electrode of the invention.
[0089] The electrolyte is a liquid, specifically an aqueous solution. The electrolyte comprises
the ingredients necessary to produce an electroplated electrode as described herein
in an electrodeposition process as described herein. Thus, the electrolyte comprises
water, a Cr(III) salt, a cobalt salt, a salt of one or more further transition metals
and/or a salt of one or more non-metallic elements, and a suppressing agent. Typically,
the electrolyte comprises water, a Cr(III) salt, a cobalt salt, a salt of one or more
further transition metals, and a suppressing agent.
[0090] Each salt typically comprises a metal ion and at least one counter-ion. However,
all salts are in solution, and consequently in dissociated form. Thus, in solution
the counter-ion(s) are not typically associated with the metal ion(s). These salts
may be referred to as "precursor salts", as they act as a source of Cr, Co, and the
one or more transition metals and/or one or more non-metallic elements, to be deposited
on a substrate during an electrodeposition process. Thus, the Cr(III) salt may be
referred to as, for instance, a Cr(III) precursor salt or a Cr precursor salt; the
Co salt may be referred to as a Co precursor salt, the salt(s) of the one or more
transitions metals may be referred to as a transition metal precursor salt, the salt(s)
of the one or more non-metallic elements may be referred to as non-metal salts.
[0091] The metal ions present in the Cr(III) salt, the cobalt salt and the one or more transition
metal salts and/or non-metal salts may or may not be present in the form of a complex
containing one or more ligands. By "ligands" here is meant complexing ligands other
than H
2O. Preferably, the Cr(III) salt comprises a Cr(III) ion in the form of a complex with
one or more ligands.
[0092] Although the cobalt salt and one or more further transition metal salts and/or non-metal
salts are typically not added in the form of a complex form when the electrolyte is
created, in practice they may form complex ions with any excess ligand present in
solution capable of forming such a complex (for instance an amino acid such as glycine).
Thus, by way of example the cobalt ion may be present in the form of a glycine complex,
and/or the further transition metal ion(s) may be present in the form of a glycine
complex. Similarly, by example, the one or more non-metallic ion(s) may be present
in the form of a glycine complex. Alternatively or additionally, one or more further
complexing agents may be included in the electrolyte in order to form a complex with
the cobalt and/or the further transition metal ion(s) and/or non-metal ions. A suitable
complexing agent for this purpose is an amino acid or a carboxylic acid. Particular
examples of such a complexing agent include citric acid or malic acid, or salts thereof.
[0093] It is preferred to provide the Cr(III) salt containing the Cr
3+ ion (also referred to as the Cr(III) ion) in complex with one or more ligands as
this can reduce the tendency of the Cr
3+ ion to polymerise (which undesirably reduces its effective concentration and hence
its availability for electrodeposition). For this reason, the ligand is typically
a strong complexing agent, which binds strongly to the Cr
3+ ion. In a preferred aspect, the Cr(III) salt comprises a complex ion containing Cr(III)
and an amino acid or a carboxylic acid, preferably wherein the Cr(III) salt comprises
a complex ion containing Cr(III) and glycine or formic acid. It is particularly preferred
that the Cr(III) salt comprises a complex ion containing Cr(III) and glycine, as although
both glycine and formic acid are highly effective, formic acid is toxic and so may
be avoided for convenience. The complex ion may be, for example, [Cr(Gly)
3]
3+, Gly being glycine. In practice, the complex ion may be associated additionally with
one or more water molecules.
[0094] The Cr(III) salt comprising a Cr(III) ion in complex with one or more ligands is
typically produced by heating a Cr(III) salt (wherein the chromium ion is not complexed)
with one or more complexing agents. For instance, the Cr(III) salt comprising a Cr(III)
ion in complex may be produced by heating another Cr(III) salt in an aqueous solution
comprising an amino acid or a carboxylic acid (preferably glycine or formic acid).
Processes for producing such complexes of Cr(III) ions are known in the art. The other
Cr(III) salt, wherein the chromium ion is not complexed, is not particularly limited.
It may be, for instance, a Cr(III) nitrate; a Cr(III) halide, a Cr(III) sulphate,
a Cr(III) sulphite, a Cr(III) sulphonate, and so on. Particular examples of such other
Cr(III) salts include Cr(III) chloride and Cr(III) sulphate.
[0095] Regardless of whether or not the Cr
3+ ion is in complex form, the Cr(III) salt comprises a counter-ion in addition to the
Cr(III) species. The counter-ion is not particularly limited. Typical counter-ions
include nitrate, halide, carboxylate, sulphate, sulphide or sulphonate. Particular
examples of counter-ions include chloride and sulphate. Typical examples of the Cr(III)
salt include a Cr(III) chloride or a Cr(III) sulphate salt, optionally wherein the
Cr(III) ion is complexed with a complexing agent.
[0096] The electrolyte also comprises one more further transition metal salts and/or non-metal
salts. The oxidation state of the one or more further transition metal ions within
the one or more further transition metal salt(s) is not particularly significant.
The oxidation state of the one or more further transition metal ions within the one
or more non-metal salt(s) is not particularly significant. In addition to a transition
metal ion, the further transition metal salt(s) comprise a counter-ion. The counter-ion
is not particularly limited and may be, for instance, nitrate, halide, carboxylate,
sulphate, sulphide or sulphonate. In addition to a non-metal ion, the further transition
metal salt(s) comprise a counter-ion. The counter-ion is not particularly limited
and may be, for instance, H
+ or ammonium. In some embodiments the one or more further transition metal salts include
a nickel salt, wherein the transition metal ion is typically a nickel (II) ion. For
such embodiments, a preferred example of a counter-ion within the nickel (II) salt
is a chloride ion, as nickel (II) and chloride ions dissociate easily and effectively
in solution. Thus, in a preferred aspect, the nickel salt may be NiCl
2 (optionally including water molecules of hydration).
[0097] The electrolyte also comprises a cobalt salt. The oxidation salt of the cobalt ion
within the cobalt salt is not particularly significant, but the salt is typically
a cobalt (II) salt. In addition to a cobalt ion, the cobalt salt comprises a counter-ion.
The counter-ion is not particularly limited and may be, for instance, nitrate, halide,
carboxylate, sulphate, sulphide or sulphonate. A preferred example of a counter-ion
within the cobalt (II) salt is an acetate or sulphate ion. Thus, in a preferred aspect,
the cobalt salt may be CoAc
2 (Ac being an acetate ion) or CoSO
4 (optionally including water molecules of hydration).
[0098] In a preferred example of the electrolyte, the nickel salt is nickel chloride and
the cobalt salt is cobalt acetate. In a particularly preferred example of the electrolyte,
the Cr(III) salt comprises a Cr(III) ion complexed with glycine and either a chloride
or sulphate counter-ion; the nickel salt is NiCl
2 and the cobalt salt is CoAc
2 (Ac being an acetate ion).
[0099] The electrolyte may comprise components in addition to those mentioned above. For
example, the electrolyte may comprise one or more additives as described herein.
[0100] The electrolyte may include one or more supporting salts. A supporting salt may improve
the ability of the electrolyte to deposit a coating as described herein during the
method of the invention. Without wishing to be bound by theory, it is speculated that
the presence of supporting salts may make the current flow through the electrolyte
more uniform, and hence the electrodeposition process more efficient. Supporting salts
(particularly chloride salts) may assist the activity of the suppressing agent. A
supporting salt may be, for instance, a buffer species. A typical example of a supporting
salt is a chloride salt.
[0101] Supporting salts, if they contain metal ions, generally contain metal ions which
are difficult to reduce in order to minimise the likelihood that additional metals
are deposited in the coating. For instance, additional metal salts may be salts of
Group (I) or Group (II) metals. Typical examples of supporting salts may therefore
include lithium, sodium, potassium, magnesium or calcium salts.
[0102] The agent may comprise a complexing agent, as described herein. The complexing agent
may be an amino acid or a carboxylic acid. Preferred examples of the complexing agent
include as glycine and formic acid.
[0103] The pH of the electrolyte is typically acidic. An acidic pH is advantageous in that
it can reduce the tendency of the Cr(III) ions in solution to polymerise and thus
have reduced availability for an electrodeposition process. Accordingly, the pH of
the electrolyte is generally less than 7. Preferably, the pH is from 1.5 to 4.5; for
example, the pH of the electrolyte may be about 3.
Suppressing agent
[0104] The function of the suppressing agent is to suppress the process or processes which
compete with electrodeposition of chromium during the process described herein. The
suppressing agent may suppress electrodeposition of cobalt, and/or evolution of hydrogen
during the method.
[0105] Examples of the suppressing agent are polyethylene glycol (PEG), ethylene glycol
and N, N-dimethylformamide (DMF). In some preferred embodiments, the suppressing agent
does not comprise DMF as this is a relatively toxic agent, and thus can lead to difficulty
in handling or disposing of electrolytes comprising the suppressing agent. Preferred
examples are polyethylene glycol (PEG) and ethylene glycol. These species have been
found to suppress both cobalt deposition and hydrogen evolution during the electrodeposition
process for producing an electroplated electrode described herein. PEG and ethylene
glycol may be used as the sole suppressing agent, or may be combined with each other
and/or one or more other suppressing agents. Ethylene glycol is particularly preferred
as a suppressing agent. In an aspect, the suppressing agent consists of ethylene glycol
or polyethylene glycol; preferably ethylene glycol.
[0106] Where the suppressing agent comprises or consists of polyethylene glycol, the polyethylene
glycol is generally a low molecular weight polyethylene glycol. For instance, the
molecular weight of the polyethylene glycol may be up to 10000 Da, preferably up to
5000 Da, more preferably up to 2000 Da. The molecular weight of the polyethylene glycol
may be, for instance, in the range of 200-10000 Da, or 200-5000 Da, or 200-2000 Da.
Suitable examples of PEG which have shown efficacy as a suppressing agent include
PEG 200, PEG 400 and PEG 600.
[0107] The electrolyte is typically prepared by mixing aqueous solutions of the various
precursor salts (and any other species present, such as a complexing agent, a supporting
salt, or an additive) with the suppressing agent (which is typically a liquid). It
is therefore convenient to define the amount of suppressing agent present with reference
to the volume of the electrolyte.
[0108] Generally, the electrolyte comprises up to 50% or 25% of the suppressing agent, expressed
as a volume of the suppressing agent by total volume of the electrolyte (
i.e. 50 or 25 v.v% of the electrolyte). For example, the electrolyte may comprise up to
15 v/v% of the suppressing agent. The electrolyte typically comprises at least 1 v/v%
of the suppressing agent, and preferably at least 5 v/v% of the suppressing agent.
Thus, the electrolyte usually comprises from 1 to 25 v/v% of the suppressing agent,
and preferably comprises from 5 to 20 or 5 to 15 v/v% of the suppressing agent. By
way of example the electrolyte may comprise about 8-12 v/v% of the suppressing agent,
particularly about 10 v/v% of the suppressing agent.
[0109] Often, the only liquids present in the electrolyte are the suppressing agent (where
the suppressing agent is a liquid) and water.
[0110] If the suppressing agent used is a heavier PEG, the suppressing agent may be a solid
rather than a liquid. In such cases, it can be more convenient to express the quantity
of suppressing agent present as a mass per unit volume of the electrolyte. The electrolyte
may comprise, for instance, up to 100 g/L, more typically up to 50 g/L or up to 10
g/L or up to 1 g/L of suppressing agent. Generally, the electrolyte comprises at least
0.01 g/L of suppressing agent and more usually at least 0.1 g/L, for example at least
0.5 g/L or at least 2 g/L of suppressing agent. The electrolyte may therefore comprise
from 0.01-100 g/L of suppressing agent, for instance 0.1-50 g/L or 1 to 50 g/L, or
from 0.1 to 10 g/L or 0.1 to 1 g/L of suppressing agent.
Process for producing an electroplated electrode
[0111] Also described herein is an electroplating process for electroplating an electrode
of an electrochemical cell, the process comprising:
- providing an electrode, which comprises a substrate;
- providing an electrolyte, which comprises water, a Cr(III) salt, a cobalt salt, one
or more further transition metal salts; and/or one or more non-metal salts, and a
suppressing agent;
- contacting the electrode with the electrolyte; and
- depositing a coating on the substrate by applying an electrical current to the electrode
wherein the coating comprises 5 to 35 wt% chromium, 10 to 75 wt% cobalt and 10 to
60 wt% one or more further transition metals and/or one or more non-metallic elements.
[0112] The substrate is preferably a metallic substrate.
[0113] Also described herein is a process for producing a coated electrode comprising a
substrate and a coating, the process comprising:
- providing an electrode, which comprises a substrate;
- providing an electrolyte, which comprises water, a Cr(III) salt, a cobalt salt, one
or more further transition metal salts, and a suppressing agent;
- contacting the electrode with the electrolyte; and
- depositing a coating on the substrate by applying an electrical current to the electrode
wherein the coating comprises 5 to 35 wt% chromium; 10 to 75 wt% cobalt; and 10 to
60 wt% one or more further transition metals.
[0114] The substrate is preferably a metallic substrate.
[0115] The process may be described as an electrodeposition process, or a galvanic deposition
process. The applied electrical current reduces metal ions in solution and causes
them to become deposited on the electrode. As explained elsewhere, a particular advantage
of the present method is that it enables Cr, Co, and further transition metals and/or
non-metallic elements to be deposited in high quantities, simultaneously and homogeneously,
despite their differing reduction potentials.
[0116] The "electroplated electrode" directly produced by the above process comprises the
electrode and the electroplate coating deposited thereon.
[0117] Typically, the electroplated electrode is as described herein (in particular, the
substrate utilised in the method is typically a metallic substrate as described herein
and the electroplate coating deposited thereon is typically a coating as described
herein in connection with the electroplated electrode). Similarly, the electrolyte
and its components are typically as described herein.
[0118] Thus, the electroplated electrode as described herein may be obtained or obtainable
by this method.
[0119] The invention relates to an electrode obtained or obtainable by this method. Additionally,
the above method may optionally be followed by one or more further steps. Thus, the
invention also relates to a process comprising one or more such further steps, and
to an electrode obtained or obtainable by this method, followed by one or more additional
steps.
[0120] By way of example, the method may comprise one or more of any of the following steps,
after the process described above:
- repeating the process one or more times;
- washing the electroplated electrode;
- cleaning the electroplated electrode (for instance by treatment with an acid or alkali);
- heat-treating the electroplated electrode;
- assembling an electrolyser wherein the cathode comprises the coated electrode;
- assembling a fuel cell wherein the anode comprises the coated electrode.
[0121] The process may alternatively or additionally comprise one or more preliminary steps
preceding the electrodeposition process described above. For instance, the process
may initially involve preparing the electrolyte. In that case, the process may involve:
- (a) providing an aqueous solution comprising a Cr(III) salt wherein the Cr(III) ion
is not complexed;
- (b) heating said aqueous solution with a complexing agent to produce a solution comprising
a Cr(III) salt wherein the Cr(III) ion is in the form of a complex with the complexing
agent; and
- (c) mixing the solution comprising the Cr(III) ion in the form of a complex with a
suppressing agent.
[0122] The process may involve allowing the solution to cool between step (b) and step (c),
or after step (c) and prior to the deposition process.
[0123] One or more further transition metal salt(s) and/or non-metal salts and/or the cobalt
salt may be present in the aqueous solution provided in step (a). If not, one, some
or all of these salts may be added later (either in solution or as solids) between
steps (b) and (c) or after step (c).
[0124] During step (b), the aqueous solution is typically heated to a temperature between
50 °C and 100 °C; preferably between 60 °C and 90 °C; for example about 80 °C.
[0125] The Cr(III) salt, complexing agent, cobalt salt and one or more transition metal
salts and/or non-metal salts are as described herein.
[0126] It has been found that, during the process, the particular electrical conditions
used have little effect on the quantity of Cr deposited, and on the percentage of
Cr deposited during the process. Accordingly, the current and/or voltage to be applied
during the process are not particularly limited. Higher voltages and current may achieve
quicker deposition but are likely to be more costly in terms of energy.
[0127] The process is typically performed at a temperature between room temperature and
50 °C, for instance between 15 °C and 50 °C. Preferably, the process is performed
at a temperature of 30 °C to 45 °C, for example about 30 °C.
[0128] During the electrodeposition process described herein, a metallic substrate is typically
used as the cathode. The nature of the anode is not particularly limited. The anode
could comprise a metallic substrate, but could equally be a graphite anode or an anode
of any other conductive material. Typically, a graphite anode is used. The system
may optionally also comprise one or more additional electrodes, such as a reference
electrode; as with the anode, the material used for the reference electrode (where
present) is not particularly limited. A reference electrode may for instance comprise
a metallic substance or graphite.
Method of electrolysing water
[0129] As is explained herein, the electroplated electrode provided has excellent catalytic
properties. The electroplated electrode may, for instance, be used as an electrode
in an electrolyser to produce hydrogen, or in a fuel cell, to burn hydrogen. Particularly
described herein is a method of electrolysing water, the method comprising:
- providing an electrolyser comprising an anode and a cathode;
- contacting the anode and/or the cathode with an aqueous solution comprising water;
and
- electrolysing the water by applying a potential difference from the anode to the cathode,
wherein the cathode or anode comprises a coated electrode as described herein, preferably
wherein the cathode comprises a coated article as described herein.
[0130] The skilled person is aware of typical electrolyser constructions, in which the coated
electrode of the present invention may be used as a cathode. Common examples of electrolysers
include electrolysers which perform alkaline or acidic water hydrolysis. Preferably,
the method is a method of alkaline water hydrolysis meaning that, in the above, the
step of contacting the anode and/or the cathode with an aqueous solution involves:
- contacting the anode and/or the cathode with an aqueous alkaline solution, typically
comprising potassium hydroxide.
Further aspects
[0131] In some aspects the invention provides an electroplated electrode comprising a substrate
and a coating wherein the coating may comprise 5 to 35 wt% chromium and 10 to 75 wt%
cobalt, typically 9.5 to 35 wt% chromium and 10 to 75 wt% cobalt. The remainder of
the composition may comprise, for example, one or more further transition metals and/or
one or more non-metallic elements.
[0132] Also provided is an electrolyte, a coating, a process of manufacture and a method
of electrolysing water with features corresponding to said coated article.
EXAMPLES
1. Preparation of coated electrodes
[0133] An electrolyte was prepared as follows. 0.15 moles of as-received Ni(Cl)
2·6H
2O (purity <99.5%, Sigma-Aldrich), 0.4 moles Cr(Cl)
3·6 H
2O (purity >98%, Sigma-Aldrich), 0.025 moles CoSO
4·6 H
2O (purity >98%, Sigma-Aldrich) and 1.5 moles of Glycine (purity >98%, Sigma-Aldrich)
were dissolved in 900mL of deionized (DI) water under stirring and heated to 60 °C
for 2 hours. The solution was then cooled to 30 °C. 100 ml ethylene glycol was added.
The pH of the solution was adjusted to 2.8.
[0134] Various metallic substrates were provided. In one case, the substrate was a nickel
foam comprising cavities of 100-200 microns in diameter. In another case, the substrate
was a nickel mesh having a mesh hole size of 1-30 microns. In both cases, the nickel
surface was initially further roughened by pickling in hydrochloric acid. In the following
tables and spectra, reference is made to the experiment performed on the nickel foam.
[0135] An electrochemical cell was prepared comprising one of the aforementioned metallic
substrates as the cathode; the electrolyte; and two graphite anodes. An electrodeposition
process was performed by applying a current density of 0. 1A/cm
2 for 20 minutes.
[0136] The surface was analysed by EDS, using an Oxford Instruments EDS system. EDS imaging
occurs over an area of the surface, calculating the average weight % across that area.
Thus, EDS measurements provide a representative sample of the average weight % of
the coating. EDS measurements provide a MapSum spectrum. The following elemental composition
at the surface was observed, based on a MapSum spectrum:
Table 1: coating of the example
Map Sum Spectrum |
|
|
|
|
Element |
Line Type |
Weight % |
Weight % Sigma |
Atomic % |
Co |
K series |
57.69 |
0.21 |
48.96 |
O |
K series |
5.92 |
0.10 |
18.52 |
Cr |
K series |
10.79 |
0.10 |
10.38 |
Ni |
K series |
24.88 |
0.21 |
21.20 |
Cl |
K series |
0.29 |
0.03 |
0.42 |
K |
K series |
0.42 |
0.03 |
0.53 |
Total |
|
100.00 |
|
100.00 |
[0137] Evidently, a large amount (>10 wt%) of Cr had been successfully deposited.
[0138] The experiments were repeated using PEG 200, PEG 400 and PEG 600 respectively instead
of ethylene glycol. Similar results were observed. For instance, the following table
describes the elemental composition achieved using a process as described in connection
with table 1, except that PEG 400 was used in place of ethylene glycol. PEG 400 was
added to a quantity of 20 ml/L.
Table 2: elemental composition using PEG rather than EG
Spectrum 3 |
|
|
|
|
Element |
Line Type |
Weight % |
Weight % Sigma |
Atomic % |
O |
K series |
7.51 |
0.25 |
22.80 |
Cr |
K series |
9.93 |
0.17 |
9.28 |
Co |
K series |
58.23 |
0.38 |
48.01 |
Ni |
K series |
23.76 |
0.35 |
19.66 |
In |
L series |
0.58 |
0.14 |
0.24 |
Total |
|
100.00 |
|
100.00 |
[0139] It was still possible to deposit >9 wt% chromium and >20 wt% nickel, together with
nearly 60 wt% cobalt, demonstrating that PEG is a suitable alternative to ethylene
glycol for this procedure.
[0140] 2. Variation of substrate A further electroplated electrode was produced using a similar process to that described
in (1), except that the substrate utilised was a copper mesh rather than a nickel
mesh or nickel foam. The following elemental composition at the surface was observed:
Table 3: coating of the copper mesh
Element |
Weight % |
Ni |
53.64 |
Co |
25.39 |
Cr |
16.00 |
Cu |
2.77 |
O |
2.20 |
3. Comparative examples
[0141] All SEM images discussed were obtained using a Hitachi TM2000 or TM4000.
[0142] The process of example 1 above was repeated in the absence of a further transition
metal salt or non-metal salt in the electrolyte. The observed elemental composition
of the coating is shown in table 4, and the underlying Map Sum spectrum is shown in
figure 3.
Table 4: coating produced without further transition metal salt or non-metal salt
Map Sum Spectrum |
|
|
|
|
Element |
Line Type |
Weight % |
Weight % Sigma |
Atomic % |
O |
K series |
4.48 |
0.10 |
14.42 |
Cr |
K series |
12.28 |
0.12 |
12.16 |
Co |
K series |
78.03 |
0.23 |
68.14 |
Cl |
K series |
0.83 |
0.04 |
1.21 |
K |
K series |
0.51 |
0.04 |
0.67 |
Ni |
K series |
3.44 |
0.19 |
3.02 |
Fe |
K series |
0.42 |
0.09 |
0.39 |
Total |
|
100.00 |
|
100.00 |
[0143] The total amount of further transition metals and/or non-metal salts observed was
less than 5 wt%, but nickel was still present (as is seen in table 2 and figure 3).
This indicates that in the absence of the further transition metal salt or non-metal
salt, the process described herein does not lead to complete coverage of the substrate
and the nickel substrate remains exposed.
[0144] In addition, the amount of cobalt was high (>75 wt%), indicating a coating susceptible
to corrosion.
[0145] The process of example 1 was also repeated in the absence of the suppressing agent
ethylene glycol. The observed elemental composition of the coating is shown in table
5.
Table 5: coating produced without suppressing agent.
Map Sum Spectrum |
|
|
|
|
Element |
Line Type |
Weight % |
Weight % Sigma |
Atomic % |
Co |
K series |
72.85 |
0.30 |
69.27 |
Ni |
K series |
23.72 |
0.29 |
22.64 |
O |
K series |
1.74 |
0.08 |
6.10 |
Cr |
K series |
1.29 |
0.07 |
1.39 |
K |
K series |
0.23 |
0.03 |
0.33 |
Cl |
K series |
0.17 |
0.03 |
0.27 |
Total |
|
100.00 |
|
100.00 |
[0146] It is clear that the deposition of chromium was unable to compete with the deposition
of cobalt and nickel, and the amount of chromium present is less than 1 wt%. This
indicates a coating highly susceptible to corrosion. Further, an SEM image of the
coating is shown in figure 5. The uneven distribution of the coating is clearly visible,
showing that the coating is not homogenous. A similar SEM image of the above-discussed
coated nickel mesh (prepared as discussed in example 1 except that the suppressing
agent was omitted) is shown in figure 6. The uneven nature of the coating is somewhat
visible and was confirmed by EDS.
[0147] The process of example 1 above was further repeated in the absence of a further transition
metal salt or non-metal salt and in the absence of the suppressing agent ethylene
glycol. The observed elemental composition of the coating is shown in table 6, and
an EDS image of the surface is shown in figure 4.
Table 6: coating produced without nickel salt or suppressing agent
Map Sum Spectrum |
|
|
|
|
Element |
Line Type |
Weight % |
Weight % Sigma |
Atomic % |
Cr |
K series |
4.89 |
0.06 |
3.91 |
Co |
K series |
70.55 |
0.29 |
49.78 |
O |
K series |
2.35 |
0.06 |
6.10 |
Ni |
K series |
12.44 |
0.16 |
8.81 |
Cl |
K series |
0.24 |
0.02 |
0.28 |
K |
K series |
0.23 |
0.02 |
0.24 |
Fe |
K series |
0.44 |
0.06 |
0.33 |
C |
K series |
8.79 |
0.32 |
30.42 |
Si |
K series |
0.09 |
0.02 |
0.13 |
Total |
|
100.00 |
|
100.00 |
[0148] The table shows that the amount of Cr is less than 5 wt%, indicating that the coating
is subject to corrosion. The table also shows that the amount of Ni is low but not
non-zero, meaning that the coating was not homogenous and that patches of substrate
were exposed. In figure 4, the density of Cr, Ni and Co is represented by colour density;
Cr in yellow, Ni in blue and Co in green. It can be clearly seen that some areas are
more yellow, while other areas are more blue, demonstrating that in the absence of
all components of the electrolyte, the element coverage produced is not homogenous
4. Accelerated degradation test
[0149] To assess the efficacy of the coating described herein in preventing corrosion, an
accelerated degradation test was performed. A nickel foam produced as in example 1,
and a nickel foam as produced in the comparative example wherein the suppressing agent
was omitted, were utilised in the test. Under the test conditions, the sample was
placed in a beaker with 50% KOH at 125°C for 5 hours. This simulates the conditions
which occur in an electrolyser under alkaline conditions, during shutdowns.
[0150] The results are shown in figure 1 and figure 2. The sample produced as described
herein (as in figure 1) clearly survived the conditions well, with little unevenness
appearing in the coating. By contrast, the sample of the comparative example was visibly
highly degraded; not only the coating but also the underlying substrate appear to
have significantly degraded.
5. Efficiency of hydrogen production and effect of additives
[0151] A basic electrolyser setup was produced to test the efficacy of the coatings described
in the production of hydrogen. An image of the apparatus is shown in figure 7. The
apparatus contained a cathode; an AgCl reference electrode, and a platinised titanium
mesh. The electrolyte was an aqueous solution of 1M KOH at room temperature. The potentiostat
used was a Metrohm Vionic.
[0152] The cathodes used were as follows.
- (a) nickel mesh
- (b) nickel mesh coated with pure cobalt
- (c) nickel mesh coated with a coating comprising 5 to 35 wt% chromium, 10 to 75 wt%
cobalt and 10 to 60 wt% nickel;
- (d) nickel mesh coated with a coating comprising 5 to 35 wt% chromium, 10 to 75 wt%
cobalt and 10 to 60 wt% nickel, together with graphene oxide nanoparticles;
- (e) nickel mesh coated with a coating comprising 5 to 35 wt% chromium, 10 to 75 wt%
cobalt and 10 to 60 wt% nickel, together with molybdenum nanoparticles;
- (f) nickel mesh coated with a coating comprising 5 to 35 wt% chromium, 10 to 75 wt%
cobalt and 10 to 60 wt% nickel, together with iron nanoparticles.
[0153] Where the graphene oxide nanoparticles were included in the coating, this was achieved
by sonicating a mixture of powdered graphene oxide mixed with deionised water and
including that in the electrolyte during electrodeposition of the coating to include
the additive.
[0154] Molybdenum and iron were incorporated instead by codeposition of the metal. In order
to include molybedenum, NaMoO
4 was dissolved and included in the electrolyte used to produce the coated electrode.
In order to include iron, iron sulphate was dissolved and included in the electrolyte.
[0155] The cathodes were used to electrolyse the electrolyte, over a range of cathode potentials
varying from -1.8V to -0.8 V. The current was measured as the potential varied. A
larger magnitude of current indicated more effective hydrolysis of water, and greater
production of hydrogen.
[0156] The results are shown in figures 8 and 9. It can be seen from figure 8 that a cobalt
surface (b) is slightly more effective at generating hydrogen than an etched nickel
surface (a), but that the coating of the invention (c) is much more effective than
either, particularly at larger potentials. Moreover, it can be seen from line (d)
that graphene oxide nanoparticles present in the coating produced a significant further
improvement in the efficacy of hydrogen production.
[0157] Figure 9 compares the effect of various different additives on the hydrogen production
efficiency of the coating (c) comprising nickel, chromium and cobalt. All additives
(Mo, Fe and graphene oxide "rGO") improved hydrogen production efficiency, but graphene
oxide was most effective.
5. Further degradation test
[0158] To further assess the efficacy of the coating described herein in preventing corrosion,
an accelerated degradation test was performed. A coated nickel foam was produced as
in example 1 using around 10% by volume ethylene glycol based on the total volume
of the electrolyte. A further coated nickel foam was produced as a comparative example
wherein the suppressing agent (ethylene glycol) was omitted. The resulting coated
electrodes were utilised in the test.
[0159] The observed elemental composition of the coating
with the additive was measured and is displayed in Table 6 below. SEM images of the surface
are shown in Figure 10. As can be seen in Figure 10, a coating prepared using an additive
as described herein is evenly distributed across the surface of the substrate.
Table 6
Map Sum Spectrum |
|
Element |
Weight % |
Atomic % |
|
C |
9.74 |
34.16 |
|
Cr |
12.99 |
10.52 |
|
Co |
47.87 |
34.22 |
|
Ni |
29.40 |
21.10 |
[0160] The observed elemental composition of the coating
without the additive was measured and is displayed in Table 7 below. SEM images of the surface
are shown in Figure 11. As can be seen from the table below chromium deposition was
unable to complete with the deposition of cobalt and nickel, and the amount of chromium
present is less than 1 wt%. The uneven distribution of the coating is clearly visible
in Figure 11, showing that the coating is not homogenous.
Table 7
Map Sum Spectrum |
|
Element |
Weight % |
Atomic % |
C |
17.07 |
50.11 |
Cr |
2.70 |
1.83 |
Co |
55.46 |
33.18 |
Ni |
24.77 |
14.87 |
[0161] Under the test conditions, each sample was placed in a beaker with 50% KOH at 125°C
for 5 hours. This simulates the conditions which occur in an electrolyser under alkaline
conditions, during shutdowns.
[0162] The results are shown in Figure 12 (coating of the invention) and Figure 13 (comparative
example). The sample produced as described herein (as in figure 10) clearly survived
the conditions well, with little unevenness appearing in the coating. By contrast,
the sample of the comparative example was visibly highly degraded; not only the coating
but also the underlying substrate appear to have significantly degraded.
[0163] Furthermore, the percentage weight of the coating of each coating was assessed with
a Map Sum Spectrum. The results are displayed in Tables 8 and 9 below.
Table 8: Coating of the invention
Map Sum Spectrum |
|
Element |
Weight % |
Atomic % |
C |
5.25 |
21.12 |
Cr |
9.52 |
8.85 |
Co |
46.59 |
38.22 |
Ni |
38.64 |
31.81 |
Table 9: comparative example
Map Sum Spectrum |
|
Element |
Weight % |
Atomic % |
C |
12.26 |
40.58 |
Cr |
0 |
0 |
Co |
6.89 |
4.65 |
Ni |
80.85 |
54.77 |
[0164] The percentage difference between the elemental composition at each site pre- and
post-KOH treatment was calculated, and is displayed in Tables 10 (with additive) and
11 (without additive) below. While there is some minor variation at all sites (due
to difficulty in measure the exact same site at different times), it is clear that
the change in the composition is significantly greater for the coating without the
additive. Due to the lack of chromium present it is subject to significant degradation.
In particular, it can be seen that after KOH treatment the amount of chromium and
cobalt present on the surface of the substrate decreases significantly in the comparative
example where the additive was not used. In contrast little variation is seen in the
coating prepared according to the invention, even after KOH treatment.
With additive
[0165]
Table 10
Map Sum Spectrum |
Pre KOH w/w% |
POST KOH w/w% |
% difference |
Element |
|
|
|
C |
9.74 |
5.25 |
-46.10% |
Cr |
12.99 |
9.52 |
-11.32% |
Co |
47.87 |
46.59 |
-2.67% |
Ni |
29.40 |
38.64 |
31.43% |
Without additive
[0166]
Table 11
Map Sum Spectrum |
Pre KOH w/w% |
POST KOH w/w% |
% difference |
Element |
|
|
|
C |
17.07 |
12.26 |
-28.18% |
Cr |
2.70 |
0 |
-100.00% |
Co |
55.46 |
6.89 |
-87.58% |
Ni |
24.77 |
80.85 |
226.40% |
7. Activity of catalytic coating
[0167] A basic electrolyser setup was produced to test the efficacy of the coatings described
in the production of hydrogen. An image of the apparatus is shown in Figure 20. The
analysis was performed in an ElyFlow cell obtained from Gaskatel. The apparatus contained
a cathode; a hydrogen reference electrode, and a counter electrode comprising a 99.99%
nickel sheet. The electrolyte was an aqueous solution of 8M KOH at room temperature.
The scan rate was 50 mVs
-1. iR compensation was developed to correct for the voltage loss (i.e. iR drop) caused
by the electrolyte solution between the working electrode and the reference electrode,
where R stands for the resistance of the electrolyte solution.
[0168] The cathodes used were low surface area catalysts comprising the following coatings:
- (a) pure chromium
- (b) pure cobalt
- (c) coating as detailed in Table 12 below.
Table 12
Map Sum Spectrum |
Site 1 |
Element |
Weight % |
Atomic % |
C |
12.89 |
41.57 |
Cr |
13.70 |
10.20 |
Co |
73.41 |
48.23 |
[0169] The cathodes were used to electrolyse the electrolyte, over a range of cathode potentials
varying from -0.3V to 0.0 V. The current was measured as the potential varied. A larger
magnitude of current indicated more effective hydrolysis of water, and greater production
of hydrogen.
[0170] The results are shown in Figures 14. It can be seen from Figure 14 that a chromium
surface has no activity alone, and a cobalt surface has some hydrogen generating activity,
but that the comprising both cobalt and chromium is much more effective than either,
particularly at larger potentials.
[0171] Further tests were performed using a nickel foam substrate as the substrate in the
electrode, wherein the substrate was coating either with pure cobalt, or with a chromium-cobalt
coating as described in Table 12. The results are shown in Figure 15. As can be seen
in Figure 15, the cobalt chromium combination has significantly higher activity, in
particular at larger potentials.
[0172] Using the same technique the activity of the catalyst of the Table 12 was also assessed
in relation to a Raney Nickel catalyst, which is a commonly used catalyst in the field
of hydrogen generation, and is known for its activity. The results are shown in Figure
16, and show that the catalyst of the invention has much higher activity particularly
at larger potentials. Furthermore, the catalyst of the invention has a lower overpotential
than the nickel catalyst (i.e. starts producing hydrogen at a lower voltage).
8 - Comparison of activity on planar vs high surface area substrates
[0173] A basic electrolyser setup was produced to test the efficacy of the coatings described
in the production of hydrogen. An image of the apparatus is shown in Figure 20. The
analysis was performed in an ElyFlow cell obtained from Gaskatel. The apparatus contained
a cathode; a hydrogen reference electrode, and a counter electrode comprising a 99.99%
nickel sheet. The electrolyte was an aqueous solution of 8M KOH at room temperature.
The scan rate was 50 mVs
-1. iR compensation was developed to correct for the voltage loss (i.e. iR drop) caused
by the electrolyte solution between the working electrode and the reference electrode,
where R stands for the resistance of the electrolyte solution. The potentiostat used
was a Metrohm Vionic.
[0174] The cathodes used were as follows.
(a) a planar cathode comprising a coating according to Table 13
Table 13
Elements |
Percentage weight based on total weight of coating |
Ni |
37.68 |
Co |
34.9925 |
Cr |
16.5725 |
C |
10.7575 |
(b) a sheet of platinum
(c) a nickel foam cathode comprising a coating according to Table 14
Table 14
|
Percentage weight based on total weight of the coating |
Elements |
|
Ni |
40.92 |
Co |
35.54 |
Cr |
10.34 |
C |
13.17 |
(d) Raney nickel catalyst on a high surface area substrate
(e) platinum carbon catalyst on a high surface area substrate.
[0175] The cathodes were used to electrolyse the electrolyte, over a range of cathode potentials
varying from -0.3V to 0.0 V. The current was measured as the potential varied. A larger
magnitude of current indicated more effective hydrolysis of water, and greater production
of hydrogen.
[0176] The results comparing (a) and (b) are shown in Figure 17. As can be seen, the cobalt
chromium catalyst has much higher activity particularly at larger potentials, than
platinum on a flat surface. This can be useful as it means that the catalytic coating
of the invention is effective even for simpler lower surface area electrodes, which
are easier to construct.
[0177] Similarly, the results comparing (c), (d) and (e) are shown in Figure 18. It can
be seen from Figure 18 that a cobalt chromium electrode is much more effective in
hydrogen generation than either a Raney Nickel catalyst or a platinum carbon catalyst,
particularly at larger potentials.
9 - Durability of the catalyst
[0178] A basic electrolyser setup was produced to test the durability of the coatings of
the present invention. The apparatus contained a cathode and a nickel anode. The electrolyte
was an aqueous solution of 8M KOH at 85 °C. A current density of 0.5 A/cm
2 was applied over 36 hours. After the initial steady current period, the system was
periodically shut down for one hour then turned on for an hour to simulate intermittent
current. As shown in Figure 19, the catalyst was able to maintain its activity for
over a hundred hours in this harsh environment.
[0179] The test was carried out on a nickel foam cathode comprising a coating according
to Table 14 above.