U.S. patent application number 16/361974 was filed with the patent office on 2019-07-18 for catalyst layer.
The applicant listed for this patent is Johnson Matthey Fuel Cells Limited. Invention is credited to Janet Mary Fisher, Enrico Petrucco, David Thompsett, Edward Anthony Wright.
Application Number | 20190221857 16/361974 |
Document ID | / |
Family ID | 43567311 |
Filed Date | 2019-07-18 |
United States Patent
Application |
20190221857 |
Kind Code |
A1 |
Thompsett; David ; et
al. |
July 18, 2019 |
Catalyst Layer
Abstract
A catalyst layer comprising an electrocatalyst and an oxygen
evolution catalyst, wherein the oxygen evolution catalyst comprises
a crystalline metal oxide comprising: (i) one of more first metals
selected from the group consisting of yttrium, lanthanum, cerium,
praseodymium, neodymium, promethium, samarium, europium,
gadolinium, terbium, dysprosium, holmium, erbium, thulium,
ytterbium, lutetium, magnesium, calcium, strontium, barium, sodium,
potassium, indium, thallium, tin, lead, antimony and bismuth; (ii)
one or more second metals selected from the group consisting of Ru,
Ir, Os and Rh; and (iii) oxygen characterised in that: (a) the
atomic ratio of first metal(s):second metal(s) is from 1:1.5 to
1.5:1 (b) the atomic ratio of (first metal(s)+second
metal(s)):oxygen is from 1:1 to 1:2 is disclosed.
Inventors: |
Thompsett; David; (Reading,
GB) ; Wright; Edward Anthony; (Reading, GB) ;
Fisher; Janet Mary; (Reading, GB) ; Petrucco;
Enrico; (Reading, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Johnson Matthey Fuel Cells Limited |
London |
|
GB |
|
|
Family ID: |
43567311 |
Appl. No.: |
16/361974 |
Filed: |
March 22, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13994186 |
Aug 1, 2013 |
10297836 |
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|
PCT/GB2011/052472 |
Dec 14, 2011 |
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16361974 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 2008/1095 20130101;
H01M 4/8647 20130101; H01M 4/8657 20130101; H01M 4/9025 20130101;
H01M 4/9016 20130101; H01M 4/8652 20130101 |
International
Class: |
H01M 4/86 20060101
H01M004/86; H01M 4/90 20060101 H01M004/90 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 16, 2010 |
GB |
1021352.8 |
Claims
1. A catalyst layer comprising an electrocatalyst and an oxygen
evolution catalyst, wherein the oxygen evolution catalyst comprises
a crystalline metal oxide of formula (AA').sub.a(BB').sub.bO.sub.c.
wherein A and A' are the same or different and are selected from
the group consisting of yttrium, lanthanum, cerium, praseodymium,
neodymium, promethium, samarium, europium, gadolinium, terbium,
dysprosium, holmium, erbium, thulium, ytterbium, lutetium,
magnesium, calcium, strontium, barium, sodium, potassium, indium,
thallium, tin, lead, antimony and bismuth; B is selected from the
group consisting of Ru, Ir, Os, and Rh; B' is selected from the
group consisting of Ru, Ir, Os, Rh, Ca, Mg or RE (wherein RE is a
rare earth metal), indium, thallium, tin, lead, antimony and
bismuth; c is from 3-11; the atomic ratio of (a+b):c is from 1:1 to
1:2; the atomic ratio of a:b is from 1:1.5 to 1.5:1.
2. The catalyst layer according to claim 1, wherein a is 0.66 to
1.5; b is 1; and c is 3 to 5.
3. The catalyst layer according to claim 1, wherein a is 2 to 4.5;
b is 3; and c is 10 to 11.
4. The catalyst layer according to claim 1, wherein the oxygen
evolution catalyst and the electrocatalyst are in separate layers
in the membrane electrode assembly.
5. The catalyst layer according to claim 1, wherein the oxygen
evolution catalyst and the electrocatalyst are in a single layer in
the membrane electrode assembly.
6. The catalyst layer according to claim 5, wherein the oxygen
evolution catalyst acts as a support material for the
electrocatalyst.
7. An electrode comprising a gas diffusion layer and a catalyst
layer according to any one of claim 1.
8. A catalysed membrane comprising a proton conducting membrane and
a catalyst layer according to claim 1.
9. A catalysed transfer substrate comprising a catalyst layer
according to claim 1.
10. A membrane electrode assembly comprising a catalyst layer
according to claim 1.
11. A fuel cell comprising a catalyst layer according to claim 1.
Description
[0001] The present invention relates to a catalyst layer,
particularly a catalyst layer for use in a fuel cell that
experiences high electrochemical potentials.
[0002] A fuel cell is an electrochemical cell comprising two
electrodes separated by an electrolyte. A fuel, such as hydrogen or
an alcohol such as methanol or ethanol, is supplied to the anode
and an oxidant, such as oxygen or air, is supplied to the cathode.
Electrochemical reactions occur at the electrodes, and the chemical
energy of the fuel and the oxidant is converted to electrical
energy and heat. Electrocatalysts are used to promote the
electrochemical oxidation of the fuel at the anode and the
electrochemical reduction of oxygen at the cathode.
[0003] In proton exchange membrane (PEM) fuel cells, the
electrolyte is a solid polymeric membrane. The membrane is
electronically insulating but proton conducting, and protons,
produced at the anode, are transported across the membrane to the
cathode, where they combine with oxygen to form water.
[0004] The principal component of a PEM fuel cell is known as a
membrane electrode assembly (MEA) and is essentially composed of
five layers. The central layer is the polymer ion-conducting
membrane. On either side of the ion-conducting membrane there is an
electrocatalyst layer, containing an electrocatalyst designed for
the specific electrochemical reaction. Finally, adjacent to each
electrocatalyst layer there is a gas diffusion layer. The gas
diffusion layer must allow the reactants to reach the
electrocatalyst layer and must conduct the electric current that is
generated by the electrochemical reactions. Therefore the gas
diffusion layer must be porous and electrically conducting.
[0005] Electrocatalysts for fuel oxidation and oxygen reduction are
typically based on platinum or platinum alloyed with one or more
other metals. The platinum or platinum alloy catalyst can be in the
fbrm of unsupported nanometre sized particles (such as metal blacks
or other unsupported particulate metal powders) or can be deposited
as even higher surface area particles onto a conductive carbon
substrate, or other conductive material (a supported catalyst).
[0006] The MEA can be constructed by several methods. The
electrocatalyst layer may be applied to the gas diffusion layer to
form a gas diffusion electrode. Two gas diffusion electrodes can be
placed either side of an ion-conducting membrane and laminated
together to form the five-layer MEA. Alternatively, the
electrocatalyst layer may be applied to both faces of the
ion-conducting membrane to form a catalyst coated ion-conducting
membrane. Subsequently, gas diffusion layers are applied to both
faces of the catalyst coated ion-conducting membrane. Finally, an
MEA can be formed from an ion-conducting membrane coated on one
side with an electrocatalyst layer, a gas diffusion layer adjacent
to that electrocatalyst layer, and a gas diffusion electrode on the
other side of the ion-conducting membrane.
[0007] Typically tens or hundreds of MEAs are required to provide
enough power for most applications, so multiple MEAs are assembled
to make up a fuel cell stack. Field flow plates are used to
separate the MEAs. The plates perform several functions: supplying
the reactants to the MEAs, removing products, providing electrical
connections and providing physical support.
[0008] High electrochemical potentials can occur in a number of
real-life operational situations and in certain circumstances can
cause damage to the catalyst layer/electrode structure. Further
descriptions of a number of situations where high electrochemical
potentials are seen are described below:
[0009] (a) Cell Reversal
[0010] Electrochemical cells occasionally are subjected to a
voltage reversal condition, which is a situation where the cell is
forced to the opposite polarity. Fuel cells in series are
potentially subject to these unwanted voltage reversals, such as
when one of the cells is forced to the opposite polarity by the
other cells in the series. In fuel cell stacks, this can occur when
a cell is unable to produce, from the desired fuel cell reactions,
the current being forced through it by the rest of the cells.
Groups of cells within a stack can also undergo voltage reversal
and even entire stacks can be driven into voltage reversal by other
stacks in an array. Aside from the loss of power associated with
one or more cells going into voltage reversal, this situation poses
reliability concerns. Undesirable electrochemical reactions may
occur, which may detrimentally affect fuel cell components.
Component degradation reduces the reliability and performance of
the fuel cell, and in turn, its associated stack and array.
[0011] A number of approaches have been utilised to address the
problem of voltage reversal, for example employing diodes capable
of carrying the current across each individual fuel cell or
monitoring the voltage of each individual cell and shutting down an
affected cell if a low voltage is detected. However, given that
stacks typically employ numerous fuel cells, such approaches can be
quite complex and expensive to implement.
[0012] Alternatively, other conditions associated with voltage
reversal may be monitored instead, and appropriate corrective
action can be taken if reversal conditions are detected. For
instance, a specially constructed sensor cell may be employed that
is more sensitive than other fuel cells in the stack to certain
conditions leading to voltage reversal (for example, fuel
starvation of the stack). Thus, instead of monitoring every cell in
a stack, only the sensor cell need be monitored and used to prevent
widespread cell voltage reversal under such conditions. However,
other conditions leading to voltage reversal may exist that a
sensor cell cannot detect (for example, a defective individual cell
in the stack). Another approach is to employ exhaust gas monitors
that detect voltage reversal by detecting the presence of or
abnormal amounts of species in an exhaust gas of a fuel cell stack
that originate from reactions that occur during reversal. While
exhaust gas monitors can detect a reversal condition occurring
within any cell in a stack and they may suggest the cause of
reversal, such monitors do not identify specific problem cells and
they do not generally provide any warning of an impending voltage
reversal.
[0013] Instead of, or in combination with the preceding, a passive
approach may be preferred such that, in the event that reversal
does occur, the fuel cells are either more tolerant to the reversal
or are controlled in such a way that degradation of any critical
cell components is reduced. A passive approach may be particularly
preferred if the conditions leading to reversal are temporary. If
the cells can be made more tolerant to voltage reversal, it may not
be necessary to detect for reversal and/or shut down the fuel cell
system during a temporary reversal period. Thus, one method that
has been identified for increasing tolerance to cell reversal is to
employ a catalyst that is more resistant to oxidative corrosion
than conventional catalysts (see WO01/059859).
[0014] A second method that has been identified for increasing
tolerance to cell reversal is to incorporate an additional or
second catalyst composition at the anode for purposes of
electrolysing water (see WO01/15247). During voltage reversal,
electrochemical reactions may occur that result in the degradation
of certain components in the affected fuel cell. Depending on the
reason for the voltage reversal, there can be a significant rise in
the absolute potential of the fuel cell anode to a higher potential
than that of the cathode. This occurs, for instance, when there is
an inadequate supply of fuel (i.e. fuel starvation) to the anode.
In this situation the cathode reaction and thus the cathode
potential remains unchanged as the oxygen reduction reaction
(ORR):
1/2O.sub.2+2H.sup.++2e.sup.-.fwdarw.H.sub.2O
whereas the normal fuel cell reaction at the anode--the hydrogen
oxidation reaction (HOR):
H.sub.2.fwdarw.2H.sup.++2e.sup.-
can no longer be sustained and other electrochemical reactions then
take place at the anode to maintain the current. These reactions
can typically be either water electrolysis--the oxygen evolution
reaction (OER):
H.sub.2O.fwdarw.1/2O.sub.2+2H.sup.++2e.sup.-
or carbon electrochemical oxidation:
1/2C+H.sub.2O.fwdarw.1/2CO.sub.2+2H.sup.+2e.sup.-
Both these reactions occur at a higher absolute potential than the
oxygen reduction reaction at the cathode (hence the cell voltage
reverses).
[0015] During such a reversal in a PEM fuel cell, water present at
the anode enables the electrolysis reaction to proceed and the
carbon support materials used to support the anode catalyst and
other cell components enables the carbon oxidation reaction also to
proceed. It is much more preferable to have water electrolysis
occur rather than the carbon oxidation reaction. When water
electrolysis reactions at the anode cannot consume the current
forced through the cell, the rate of oxidation of the carbonaceous
anode components increases, thereby tending to irreversibly degrade
certain anode components at a greater rate. Thus, by incorporating
a catalyst composition that promotes the electrolysis of water,
more of the current forced through the cell may be consumed in the
electrolysis of water than in the oxidative corrosion of anode
components, such as carbon.
[0016] A reversal condition can also be experienced due to oxidant
starvation on the cathode. However, this is much less detrimental
to the cell, because the reaction likely to occur instead of the
reduction of the oxidant is that the protons produced at the anode
cross the electrolyte and combine with electrons directly at the
cathode to produce hydrogen via the hydrogen evolution reaction
(HER):
2H.sup.+.fwdarw.2e.sup.-+H.sub.2
In this reversal situation the anode reaction and thus the anode
potential remain unchanged, but the absolute potential of the
cathode drops to below that of the anode (hence the cell voltage
reverses). These reactions do not involve potentials and reactions
at which significant component degradation is caused.
[0017] (b) Start-Up Shut-Down
[0018] For many fuel cells it is also not practical or economic to
provide purging of hydrogen from the anode gas space with an inert
gas such as nitrogen during shut down. This means that there may
arise a mixed composition of hydrogen and air on the anode whilst
air is present on the cathode. Similarly, when a cell is re-started
after being idle for some time, air may have displaced hydrogen
from the anode and as hydrogen is re-introduced to the anode, again
a mixed air/hydrogen composition will exist whilst air is present
at the cathode. Under these circumstances an internal cell can
exist, as described by Tang et al (Journal of Power Sources 158
(2006) 1306-1312), which leads to high potentials on the cathode.
The high potentials can cause carbon to oxidise according to the
electrochemical carbon oxidation reaction indicated previously:
1/2C+H.sub.2O.fwdarw.1/2CO.sub.2+2H.sup.++2e.sup.-
and this is highly damaging to the structure of the catalyst layer
where the catalyst layer contains carbon. If the cathode layer is
able to support oxygen evolution by the water electrolysis reaction
(OER) however, the high potentials can be used to drive water
electrolysis rather than carbon corrosion.
[0019] (c) Regenerative Fuel Cells
[0020] In regenerative fuel cells the electrodes are bi-functional
and both electrodes must support two electrochemical reaction types
at different times. When operating as a fuel cell the oxygen
electrode must reduce oxygen (ORR) and the hydrogen electrode must
oxidise hydrogen (HOR); when operating as an electrolyser the
hydrogen electrode must evolve hydrogen (HER) and the oxygen
electrode must evolve oxygen (OER).
[0021] Electrocatalysts for the water electrolysis reaction are
generally based on iridium oxide or iridium oxide mixed with at
least one other metal oxide. However, iridium-based catalysts are
not sufficiently active at the loadings required in a fuel
cell.
[0022] It is therefore an object of the present invention to
provide a catalyst layer comprising alternative water electrolysis
catalysts, which have superior activity to state of the art water
electrolysis catalysts for the oxygen evolution reaction, and which
demonstrate superior performance when incorporated into a MEA and
operated under practical real-life fuel cell operating
conditions.
[0023] Accordingly, the present invention provides a catalyst layer
comprising an clectrocatalyst and an oxygen evolution catalyst,
wherein the oxygen evolution catalyst comprises a crystalline metal
oxide comprising: [0024] (i) one of more first metals selected from
the group consisting of yttrium, lanthanum, cerium, praseodymium,
neodymium, promethium, samarium, europium, gadolinium, terbium,
dysprosium, holmium, erbium, thulium, ytterbium, lutetium,
magnesium, calcium, strontium, barium, sodium, potassium, indium,
thallium, tin, lead, antimony and bismuth; [0025] (ii) one or more
second metals selected from the group consisting of Ru, Ir, Os and
Rh; and [0026] (iii) oxygen characterised in that: [0027] (a) the
atomic ratio of first metal(s):second metal(s) is from 1:1.5 to
1.5:1 [0028] (b) the atomic ratio of (first metal(s)+second
metal(s)):oxygen is from 1:1 to 1:2.
[0029] Suitably, the first metal is one or more metals selected
from the group consisting of: sodium, potassium, calcium,
strontium, barium, lead and cerium.
[0030] The second metal is one or more of Ru, Ir, Os, Rh (suitably
Ru and/or Ir) having an oxidation state of from 3' to 6', including
intermediate partial oxidation states. In certain crystalline metal
oxides included in the invention, some of the one or more second
metal is replaced by a third metal; the atomic ratio of first
metal:(second metal+third metal) is from 1:1.5 to 1.5:1 and the
atomic ratio of (first metal+second metal+third metal):oxygen is
from 1:1 to 1:2. The third metal is suitably selected from the
group consisting of: calcium, magnesium or a rare earth metal (RE,
wherein RE is as hereinafter defined), indium, thallium, tin, lead,
antimony and bismuth.
[0031] Alternatively, there is provided a catalyst layer comprising
an electrocatalyst and an oxygen evolution catalyst, wherein the
oxygen evolution catalyst comprises a crystalline metal oxide of
formula
(AA').sub.a(BB').sub.bO.sub.c. [0032] wherein A and A' are the same
or different and are selected from the group consisting of yttrium,
lanthanum, cerium, praseodymium, neodymium, promethium, samarium,
europium, gadolinium, terbium, dysprosium, holmium, erbium,
thulium, ytterbium, lutetium, magnesium, calcium, strontium,
barium, sodium, potassium, indium, thallium, tin, lead, antimony
and bismuth; B is selected from the group consisting of Ru, Ir, Os,
and Rh; B' is selected from the group consisting of Ru, Ir, Os, Rh,
Ca, Mg, RE (wherein RE is as hereinafter defined), indium,
thallium, tin, lead, antimony and bismuth; c is from 3-11; the
atomic ratio of (a+b):c is from 1:1 to 1:2; the atomic ratio of a:b
is from 1:1.5 to 1.5:1.
[0033] Suitably, A and A' are selected from the group consisting
of: sodium, potassium, calcium, strontium, barium, lead and
cerium.
[0034] Suitably, B is selected from the group consisting of Ru, Ir,
Os, Rh (suitably Ru and Ir) having an oxidation state of from
3.sup.+ to 6', including intermediate partial oxidation states.
[0035] Suitably, B' is selected from the group consisting of Ru,
Ir, Os, Rh (suitably Ru and Ir) having an oxidation state of from
3.sup.+ to 6.sup.+, including intermediate partial oxidation
states, Ca, Mg, RE (wherein RE is as hereinafter defined), indium,
thallium, tin, lead, antimony and bismuth.
[0036] c is from 3-11. Since the atomic ratio of (a+b):c is known,
the value of (a+b) can be determined. Similarly, since the atomic
ratio of a:b and the value of (a+b) is known, the values of a and b
can be determined.
[0037] Specific examples of crystalline metal oxides which may be
used as the oxygen evolution catalyst include, but are not limited
to: RERuO.sub.3; SrRuO.sub.3; PbRuO.sub.3; REIrO.sub.3;
CaIrO.sub.3; BaIrO.sub.3; PbIrO.sub.3; SrIrO.sub.3; KIrO.sub.3;
SrM.sub.0.5Ir.sub.0.5O.sub.3; Ba.sub.3LiIr.sub.2O.sub.9;
Sm.sub.2NaIrO.sub.6; La.sub.1.2Sr.sub.2.7IrO.sub.7.33;
Sr.sub.3Ir.sub.2O.sub.7; Sr.sub.2Ir.sub.3O.sub.9;
SrIr.sub.2O.sub.6; Ba.sub.2Ir.sub.3O.sub.9; BaIr.sub.2O.sub.6;
La.sub.3Ir.sub.3O.sub.11; RE.sub.2Ru.sub.2O.sub.7;
RE.sub.2Ir.sub.2O.sub.7; Bi.sub.2Ir.sub.2O.sub.7;
Pb.sub.2Ir.sub.2O.sub.7; Ca.sub.2Ir.sub.2O.sub.7;
(NaCa).sub.2Ir.sub.2O.sub.6; (NaSr).sub.3Ir.sub.3O.sub.11;
(NaCe).sub.2Ir.sub.2O.sub.7; (NaCe).sub.2Ru.sub.2O.sub.7;
(NaCe).sub.2(RuIr).sub.2O.sub.7.
[0038] In the above specific examples: RE is one or more rare earth
metals selected from the group consisting of: yttrium, lanthanum,
cerium, praseodymium, neodymium, promethium, samarium, europium,
gadolinium, terbium, dysprosium, holmium, erbium, thulium,
ytterbium, lutetium; M is Ca, Mg or RE (where RE is as defined
before).
[0039] In one specific embodiment, crystalline metal oxides of the
formula (AA').sub.a(BB')O.sub.c. are used. In this formula: A, A',
B and B' are as hereinbefore defined; a is 0.66 to 1.5, b is 1 and
c is 3 to 5. These crystalline metal oxides have a perovskite type
crystalline structure, as described in Structural Inorganic
Chemistry: Fifth Edition, Wells, A. F., Oxford University Press,
1984 (1991 reprint). Specific examples of crystalline metal oxides
with a perovskite type crystalline structure include, but are not
limited to, RERuO.sub.3; SrRuO.sub.3; PbRuO.sub.3; REIrO.sub.3;
CaIrO.sub.3; BaIrO.sub.3; PbIrO.sub.3; SrIrO.sub.3; KIrO.sub.3;
SrM.sub.0.5Ir.sub.0.5O.sub.3 (wherein RE and M are as hereinbefore
defined).
[0040] In a second specific embodiment, crystalline metal oxides of
the formula (AA').sub.a(BB').sub.2O.sub.c. are used. In this
formula: A, A', B and B' are as hereinbefore defined; a is 1.33 to
3, b is 2 and c is 3 to 10, preferably 6-7. These crystalline metal
oxides have a pyrochlore type crystalline structure, as described
in Structural Inorganic Chemistry: Fifth Edition, Wells, A. F.,
Oxford University Press, 1984 (1991 reprint). Specific examples of
crystalline metal oxides with a pyrochlore type crystalline
structure include, but are not limited to, RE.sub.2Ru.sub.2O.sub.7;
RE.sub.2Ir.sub.2O.sub.7; Bi.sub.2Ir.sub.2O.sub.7;
Pb.sub.2Ir.sub.2O.sub.7; Ca.sub.2Ir.sub.2O.sub.7 (wherein RE is as
hereinbefore defined).
[0041] In a third specific embodiment, crystalline metal oxides of
the formula (AA').sub.a(BB').sub.3O.sub.c. are used. In this
formula: A, A', B and B' are as hereinbefore defined; a is 2 to
4.5, b is 3 and c is 10 to 11. These crystalline metal oxides have
a KSbO.sub.3 type crystalline structure, as described as a cubic
form with space group Pn3 in Structural Inorganic Chemistry: Fifth
Edition, Wells, A. F., Oxford University Press, 1984 (1991
reprint). Specific examples of crystalline metal oxides with a
KSbO.sub.3 type crystalline structure include, but are not limited
to, K.sub.3Ir.sub.3O; Sr.sub.2Ir.sub.3O.sub.9;
Ba.sub.2Ir.sub.3O.sub.9; La.sub.3Ir.sub.3O.sub.11.
[0042] In some of these compositions listed above, there may be
oxygen vacancies which will reduce the oxygen stoichiometry in the
crystalline structure. Similarly, some of the one or more first
metal sites (or A, A' sites) may be left vacant, reducing the
stoichiometry of the first metal (or A, A' metal) in the
crystalline structure. Furthermore, in some instances, water
molecules are known to occupy some vacant sites to provide a
hydrated or partially hydrated crystalline metal oxide.
[0043] Preferably, the specific surface area (BET) of the
crystalline metal oxide is greater than 20 m.sup.2/g, preferably
greater than 50 m.sup.2/g. The determination of the specific
surface area by the BET method is carried out by the following
process: after degassing to form a clean, solid surface, a nitrogen
adsorption isotherm is obtained, whereby the quantity of gas
adsorbed is measured as a function of gas pressure, at a constant
temperature (usually that of liquid nitrogen at its boiling point
at one atmosphere pressure). A plot of 1/[V.sub.a((P.sub.0/P)-1)]
vs P/P.sub.0 is then constructed for P/Po values in the range 0.05
to 0.3 (or sometimes as low as 0.2), where V.sub.a is the quantity
of gas adsorbed at pressure P, and P.sub.0 is the saturation
pressure of the gas. A straight line is fitted to the plot to yield
the monolayer volume (V.sub.m), from the intercept 1/V.sub.mC and
slope (C-1)N.sub.mC, where C is a constant. The surface area of the
sample can be determined from the monolayer volume by correcting
for the area occupied by a single adsorbate molecule. More details
can be found in `Analytical Methods in Fine Particle Technology`,
by Paul A. Webb and Clyde Orr, Micromeritics Instruments
Corporation 1997.
[0044] The crystalline metal oxide can be made by a variety of
routes, including solid state synthesis, hydrothermal synthesis,
spray pyrolysis and in some cases co-precipitation. The direct
solid state synthesis route involves heating stoichiometric
mixtures of oxides and/or carbonates in air to high temperature,
typically >800.degree. C. Hydrothermal synthesis involves
heating mixtures of appropriate starting salts and if necessary an
oxidising agent at a more modest temperature (typically
200-250.degree. C.) in a suitable scaled vessel. This method
generally gives materials with much higher surface area (i.e.
smaller crystallite size) than those prepared by solid state
routes.
[0045] The electrocatalyst comprises a metal (the primary metal),
which is suitably selected from [0046] (i) the platinum group
metals (PGM) (platinum, palladium, rhodium, ruthenium, iridium and
osmium), or [0047] (ii) gold or silver, or [0048] (iii) a base
metal [0049] or an oxide thereof.
[0050] The primary metal may be alloyed or mixed with one or more
other precious metals, or base metals or an oxide of a precious
metal or base metal. The metal, alloy or mixture of metals may be
unsupported or supported on a suitable inert support. In one
embodiment, if the electrocatalyst is supported, the support is
non-carbonaceous. Examples of such a support include titania,
niobia, tantala, tungsten carbide, hafnium oxide or tungsten oxide.
Such oxides and carbides may also be doped with other metals to
increase their electrical conductivity, for example niobium doped
titania.
[0051] The electrocatalyst and oxygen evolution catalyst may be
present in the catalyst layer either as separate layers or as a
mixed layer or as a combination of the two. If present as separate
layers, the layers are suitably arranged such that the oxygen
evolution catalyst layer is next to the membrane in the MEA. In a
preferred embodiment, the electrocatalyst and the oxygen evolution
catalyst are present in the catalyst layer as a mixed layer.
[0052] In an alternative embodiment of the invention, the
electrocatalyst and the oxygen evolution catalyst are present in
the catalyst layer as a mixed layer and the oxygen evolution
catalyst acts as the support material for the electrocatalyst.
[0053] Suitably, the ratio (by weight) of the oxygen evolution
catalyst to total electrocatalyst in the catalyst layer is from
20:1 to 1:20, preferably from 1:1 to 1:10. The actual ratio will
depend on whether the catalyst layer is employed at the anode or
cathode and whether the oxygen evolution catalyst is used as a
support for the electrocatalyst.
[0054] Suitably, the loading of the primary metal of the
electrocatalyst in the catalyst layer is less than 0.4 mg/cm.sup.2,
and is preferably from 0.01 mg/cm.sup.2 to 0.35 mg/cm.sup.2, most
preferably 0.02 mg/cm.sup.2 to 0.25 mg/cm.sup.2.
[0055] The catalyst layer may comprise additional components, for
example a polymer binder, such as an ionomer, suitably a proton
conducting ionomer. Examples of suitable proton conducting ionomers
will be known to those skilled in the art, but include
perfluorosulphonic acid ionomers, such as Nafion.RTM. and ionomers
made from hydrocarbon polymers.
[0056] The catalyst layer of the invention has utility in
electrochemical cells, and in particular in PEM fuel cells.
Accordingly, a further aspect of the invention provides an
electrode comprising a gas diffusion layer (GDL) and a catalyst
layer according to the invention. In one embodiment, the electrode
is an anode of a conventional fuel cell. In a second embodiment,
the electrode is a cathode of a conventional fuel cell.
[0057] The catalyst layer can be deposited onto a GDL using well
known techniques, such as those disclosed in EP 0 731 520. The
catalyst layer components may be formulated into an ink, comprising
an aqueous and/or organic solvent, optional polymeric binders and
optional proton-conducting polymer. The ink may be deposited onto
an electronically conducting GDL using techniques such as spraying,
printing and doctor blade methods. The anode and cathode gas
diffusion layers are suitably based on conventional non-woven
carbon fibre gas diffusion substrates such as rigid sheet carbon
fibre papers (e.g. the TGP-H series of carbon fibre papers
available from Toray Industries Inc., Japan) or roll-good carbon
fibre papers (e.g. the H2315 based series available from
Freudenberg FCCT KG, Germany; the Sigracet.RTM. series available
from SGL Technologies GmbH, Germany; the AvCarb.RTM. series
available from Ballard Material Products, United States of America;
or the NOS series available from CeTech Co., Ltd. Taiwan), or on
woven carbon fibre cloth substrates (e.g. the SCCG series of carbon
cloths available from the SAATI Group, S.p.A., Italy; or the WOS
series available from CeTech Co., Ltd, Taiwan). For many PEMFC and
DMFC applications the non-woven carbon fibre paper, or woven carbon
fibre cloth substrates are typically modified with a hydrophobic
polymer treatment and/or application of a microporous layer
comprising particulate material either embedded within the
substrate or coated onto the planar faces, or a combination of both
to fobrm the gas diffusion layer. The particulate material is
typically a mixture of carbon black and a polymer such as
polytetrafluoroethylene (PTFE). Suitably the gas diffusion layers
are between 100 and 300 .mu.m thick. Preferably there is a layer of
particulate material such as carbon black and PTFE on the faces of
the gas diffusion layers that contact the electrocatalyst
layers.
[0058] In PEM fuel cells, the electrolyte is a proton conducting
membrane. The catalyst layer of the invention may be deposited onto
one or both faces of the proton conducting membrane to form a
catalysed membrane. In a further aspect the present invention
provides a catalysed membrane comprising a proton conducting
membrane and a catalyst layer of the invention. The catalyst layer
can be deposited onto the membrane using well-known techniques. The
catalyst layer components may be formulated into an ink and
deposited onto the membrane either directly or indirectly via a
transfer substrate.
[0059] The membrane may be any membrane suitable for use in a PEM
fuel cell, for example the membrane may be based on a
perfluorinated sulphonic acid material such as Nafion.RTM.
(DuPont), Flemion.RTM. (Asahi Glass) and Aciplex.RTM. (Asahi
Kasei); these membranes may be used unmodified, or may be modified
to improve the high temperature performance, for example by
incorporating an additive. Alternatively, the membrane may be based
on a sulphonated hydrocarbon membrane such as those available from
FuMA-Tech GmbH as the Fumapem.RTM. P, E or K series of products,
JSR Corporation. Toyobo Corporation, and others. The membrane may
be a composite membrane, containing the proton-conducting material
and other materials that confer properties such as mechanical
strength. For example, the membrane may comprise an expanded PTFE
substrate. Alternatively, the membrane may be based on
polybenzimidazole doped with phosphoric acid and include membranes
from developers such as BASF Fuel Cell GmbH, for example the
Celtec.RTM.-P membrane which will operate in the range 120.degree.
C. to 180.degree. C.
[0060] In a further embodiment of the invention, the substrate onto
which the catalyst of the invention is applied is a transfer
substrate. Accordingly, a further aspect of the present invention
provides a catalysed transfer substrate comprising a catalyst layer
of the invention. The transfer substrate may be any suitable
transfer substrate known to those skilled in the art but is
preferably a polymeric material such as polytetrafluoroethylene
(PTFE), polyimide, polyvinylidene difluoride (PVDF), or
polypropylene (especially biaxially-oriented polypropylene, BOPP)
or a polymer-coated paper such as polyurethane coated paper. The
transfer substrate could also be a silicone release paper or a
metal foil such as aluminium foil. The catalyst layer of the
invention may then be transferred to a GDL or membrane by
techniques known to those skilled in the art.
[0061] A yet further aspect of the invention provides a membrane
electrode assembly comprising a catalyst layer, electrode or
catalysed membrane according to the invention. The MEA may be made
up in a number of ways including, but not limited to: [0062] (i) a
proton conducting membrane may be sandwiched between two electrodes
(one anode and one cathode), at least one of which is an electrode
according to the present invention; [0063] (ii) a catalysed
membrane coated on one side only by a catalyst layer may be
sandwiched between (a) a gas diffusion layer and an electrode, the
gas diffusion layer contacting the side of the membrane coated with
the catalyst layer, or (b) two electrodes, and wherein at least one
of the catalyst layer and the electrode(s) is according to the
present invention; [0064] (iii) a catalysed membrane coated on both
sides with a catalyst layer may be sandwiched between (a) two gas
diffusion layers, (b) a gas diffusion layer and an electrode or (c)
two electrodes, and wherein at least one of the catalyst layer and
the electrode(s) is according to the present invention.
[0065] The MEA may further comprise components that seal and/or
reinforce the edge regions of the MEA for example as described in
WO2005/020356. The MEA is assembled by conventional methods known
to those skilled in the art.
[0066] Electrochemical devices in which the catalyst layer,
electrode, catalysed membrane and MEA of the invention may be used
include fuel cells, in particular proton exchange membrane (PEM)
fuel cells. The PEM fuel cell could be operating on hydrogen or a
hydrogen-rich fuel at the anode or could be fuelled with a
hydrocarbon fuel such as methanol. The catalyst layer, electrode,
catalysed membrane and MEA of the invention may also be used in
fuel cells in which the membranes use charge carriers other than
protons, for example OH.sup.- conducting membranes such as those
available from Solvay Solexis S.p.A., FuMA-Tech GmbH. The catalyst
layer and electrode of the invention may also be used in other low
temperature fuel cells that employ liquid ion conducting
electrolytes, such as aqueous acids and alkaline solutions or
concentrated phosphoric acid. Other electrochemical devices in
which the catalyst layer, electrode, catalysed membrane and MEA of
the invention may be used are as the oxygen electrode of
regenerative fuel cells, and as the anode of an electrolyser where
oxygen evolution is performed by the water electrolysis catalyst
and contaminant hydrogen is recombined with oxygen by the
electrocatalyst.
[0067] Accordingly, a further aspect of the invention provides a
fuel cell, preferably a proton exchange membrane fuel cell,
comprising a catalyst layer, an electrode, a catalysed membrane or
an MEA of the invention.
[0068] The present invention will now be described further with
reference to the following examples which are illustrative, but not
limiting, of the invention.
EXAMPLE 1 (NA.sub.0.54CA.sub.1.18IR.sub.2O.sub.6.0.66H.sub.2O)
[0069] To a 22 ml volume autoclave, 8 ml of 10M NaOH solution, 0.5
ml of de-ionised water, 0.250 g (1.06.times.10.sup.-3 mole)
Ca(NO.sub.3).sub.2 and 0.411 g (1.06.times.10.sup.-3 mole)
IrCl.sub.3 was added and stirred for 1 hour. 0.174 g
(2.23.times.10.sup.-3 mole) Na.sub.2O.sub.2 was added to the
reaction solution and stirred for another 10 minutes; then the same
weight Na.sub.2O.sub.2 was added again before closing the
autoclave. The autoclave was heated at 240.degree. C. for 96 hr in
an oven. The autoclave was cooled to room temperature. The reaction
mixture was transferred to a beaker and left to settle. The
solution was decanted leaving the precipitate, rinsed with
de-ionised water and repeated several times. The precipitate was
then similarly washed with excess 1M H.sub.2SO.sub.4 then with
de-ionised water and dried to yield a black powder.
[0070] At 240.degree. C., maximum pressure generated inside the
autoclave was not more than 51 bar. (H.sub.2O vapour pressure of
Water=34 bar+decomposition of all Na.sub.2O.sub.2=17 bar
maximum).
[0071] An alternative preparation may add concentrated
H.sub.2O.sub.2 dropwise while stirring instead of Na.sub.2O.sub.2
addition and/or may use solid NaOH in place of prepared NaOH
solution. Before any peroxide compound is added all other reagents
are well-mixed.
[0072] The following Examples were prepared by a similar
method:
TABLE-US-00001 Example Autoclave No. Composition Reagents
conditions Example
Na.sub.0.54Ca.sub.1.18Ir.sub.2O.sub.6.cndot.0.66H.sub.2O
1Ca(NO.sub.3).sub.2.cndot.4H.sub.2O + 240.degree. C. 1
1IrCl.sub.3.cndot.7H.sub.2O + 8 ml 10M 96 hours NaOH + 4.2
Na.sub.2O.sub.2 + 0.5 ml H.sub.2O Example Bi.sub.2Ir.sub.2O.sub.7 1
NaBiO.sub.3 + 1.25 IrCl.sub.3.cndot.7H.sub.2O + 240.degree. C. 2 8
ml 5M NaOH + 8Na.sub.2O.sub.2 120 hours Example
Pb.sub.2Ir.sub.2O.sub.7 1 Pb(NO.sub.3).sub.2 +
1IrCl.sub.3.cndot.7H.sub.2O + 6 240.degree. C. 3 ml H.sub.2O + 2
Na.sub.2O.sub.2 + NaOH 112 hours Example
Na.sub.0.8Sr.sub.2.2Ir.sub.3O.sub.10.1 0.75 Sr(NO.sub.3).sub.2 +
1IrCl.sub.3.cndot.7H.sub.2O + 240.degree. C. 4 100NaOH + 3 ml
H.sub.2O + 10 .mu.l 72 hours conc HF + 6 ml conc H.sub.2O.sub.2
Example Na.sub.0.66Ce.sub.1.34Ru.sub.2O.sub.7 0.66
CeCl.sub.3.cndot.7H.sub.2O + 1 225.degree. C. 5
RuCl.sub.3.cndot.nH.sub.2O + 5M NaOH + 120 hours 10 Na.sub.2O.sub.2
Example Na.sub.0.66Ce.sub.1.34Ir.sub.2O.sub.7 0.66
CeCl.sub.3.cndot.7H.sub.2O + 1 240.degree. C. 6
IrCl.sub.3.cndot.7H.sub.2O + 5M NaOH + 10 120 hours Na.sub.2O.sub.2
Example Na.sub.0.66Ce.sub.1.34Ru.sub.0.6Ir.sub.1.4O.sub.7 0.66
CeCl.sub.3.cndot.7H.sub.2O + 0.3 225.degree. C. 7
RuCl.sub.3.cndot.nH.sub.2O + 0.7 IrCl.sub.3.cndot.7H.sub.2O + 120
hours 5M NaOH + 10 Na.sub.2O.sub.2 Example
Na.sub.0.54Ca.sub.1.18Ir.sub.2O.sub.6.cndot.0.66H.sub.2O
1Ca(NO.sub.3).sub.2.cndot.4H.sub.2O + 240.degree. C. 8
1IrCl.sub.3.cndot.7H.sub.2O + 22 ml 10M 70 hours NaOH + 4.2
Na.sub.2O.sub.2 + 0.5 ml H.sub.2O
COMPARATIVE EXAMPLE 1
[0073] An unsupported RuO.sub.2/IrO.sub.2 mixed oxide with a
nominal Ru:Ir atomic ratio of 90:10.
COMPARATIVE EXAMPLE 2
[0074] A TaIr mixed oxide was prepared in accordance with the
preparation of Example 2 of WO2011/021034.
[0075] Typical Powder Characterisation/Analysis
[0076] Samples were analysed by BET to determine surface area. A
sample typically was degassed at 200.degree. C. for 15 hrs under
N.sub.2 flow before N.sub.2 adsorption BET surface area measurement
was determined. Moisture content and thermal stability was
determined by DSC. Elemental composition was determined via ICPES.
Samples were analysed by XRD to identify crystallographic
parameters.
[0077] Sample chemical composition was refined based on modelling
powder neutron diffraction data obtained using the POLARIS
diffractometer at ISIS (R. Walton et al. Chem. Sci., 2011, 2,
1573). The XRD data was used to obtain a starting crystal structure
from which peak intensities were matched in order to identify the
fractional A and A' content in an (AA').sub.a(BB').sub.bO.sub.c
structure. A crystal structure with inclusive water accounted for
evident moisture from DSC data. The refined chemical composition
was compared against ICPES elemental data.
[0078] Ink, Catalyst Layer and MEA Preparation
[0079] 65 mg of the crystalline metal oxide example of the
invention was added to a 5 ml vial with 1.7 mL H.sub.2O. The
mixture was processed with a high intensity cm microtip ultrasonic
probe for 2 minutes at 3 W. The mixture was added to 0.65 g of
HiSPEC.RTM. 18600 (Johnson Matthey PLC) catalyst in a separate
container. The vial was rinsed three times with 350 .mu.L
de-ionised H.sub.2O and added to the container with the catalyst.
The catalyst slurry was mixed manually with a spatula to wet all
material, then mixed at 3000 rpm in a planetary mixer for 3
minutes. The mixed catalyst was dried at 80.degree. C. in a
fan-assisted oven.
[0080] The dried catalyst was broken into a powder, aqueous
Nafion.RTM. solution (available from DuPont) was added to dried
mixed-catalyst and the ink was shear-mixed in a planetary mixer
using 5 mm YSZ ceramic beads. After having mixed for 3 minutes at
3000 rpm the ink was stirred manually with a spatula to break up
any sediment. The ink was further milled for 5 minutes.
[0081] The ink was screen-printed onto a PTFE sheet to give a layer
having a targeted PGM loading of 0.1 mg/cm.sup.2. The layer was
transferred from the PTFE (polytetrafluoroethylene) sheet onto a
Nafion.RTM. N112 membrane (available from DuPont) at 150.degree. C.
with pressure. A Pt/C layer was transferred to the opposite side of
the N112 membrane simultaneously in order to produce a catalyst
coated membrane (CCM).
[0082] Fuel Cell Testing
[0083] The CCM was assembled in the fuel cell hardware using Toray
TGP-H-060 as the gas diffusion substrate, coated with a PTFE/carbon
coating to form the gas diffusion layer. The fuel cell was tested
at 80.degree. C. and 10 psig with humidified H.sub.2/N.sub.2 gas
reactants. The oxygen evolution mass activity of the mixed catalyst
layer was determined at 1.5V vs RHE by scanning the potential from
20 mV to 1.6V at 5 mV/s. The results are shown in Table 1.
TABLE-US-00002 TABLE 1 O.sub.2 Evolution Catalyst PGM Apparent
Loading M.sub.act (1.5 V) BET Example No. .mu.g/cm.sup.2 A/g PGM
m.sup.2/g Comparative Example 1 18.3 334 8 Comparative Example 2
13.9 291 45 Example 1 11.8 3051 68 Example 2 8.1 4717 42 Example 3
8.8 2500 18 Example 4 10.6 1322 38 Example 5 9.9 17458 50 Example 6
10.5 2279 87 Example 7 8.3 4530 91.5 Example 8 10.7 1186 28
[0084] From the data it can be seen that the MEAs having the
catalyst layers of the invention have a far higher oxygen evolution
mass activity than the Comparative Examples.
* * * * *