U.S. patent application number 15/371276 was filed with the patent office on 2017-04-06 for carbon supported catalyst.
The applicant listed for this patent is IMERYS GRAPHITE & CARBON SWITZERLAND LTD., JOHNSON MATTHEY FUEL CELLS LIMITED. Invention is credited to Sarah Caroline BALL, Graham Alan HARDS, Marlene RODLERT, Jonathan David Brereton SHARMAN, Michael E. SPAHR.
Application Number | 20170098833 15/371276 |
Document ID | / |
Family ID | 44994114 |
Filed Date | 2017-04-06 |
United States Patent
Application |
20170098833 |
Kind Code |
A1 |
BALL; Sarah Caroline ; et
al. |
April 6, 2017 |
CARBON SUPPORTED CATALYST
Abstract
A catalyst includes (i) a primary metal or alloy or mixture
including the primary metal, and (ii) an electrically conductive
carbon support material for the primary metal or alloy or mixture
including the primary metal, wherein the carbon support material
(a) has a specific surface area (BET) of 100-600 m2/g, and (b) has
a micropore area of 10-60 m2/g. Methods for producing the carbon
support material and methods for decreasing the corrosion rate of
the carbon support material are also provided.
Inventors: |
BALL; Sarah Caroline; (Oxon,
GB) ; HARDS; Graham Alan; (Berkshire, GB) ;
RODLERT; Marlene; (Breganzona, CH) ; SHARMAN;
Jonathan David Brereton; (Berkshire, GB) ; SPAHR;
Michael E.; (Bellinzona, CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JOHNSON MATTHEY FUEL CELLS LIMITED
IMERYS GRAPHITE & CARBON SWITZERLAND LTD. |
London
Bodio |
|
GB
CH |
|
|
Family ID: |
44994114 |
Appl. No.: |
15/371276 |
Filed: |
December 7, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14345754 |
Mar 19, 2014 |
9548500 |
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PCT/GB2012/052306 |
Sep 19, 2012 |
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15371276 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01B 32/336 20170801;
C01P 2006/12 20130101; H01M 4/9041 20130101; H01M 4/926 20130101;
Y02E 60/50 20130101; H01M 4/9083 20130101; H01M 2008/1095
20130101 |
International
Class: |
H01M 4/92 20060101
H01M004/92 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 28, 2011 |
GB |
1116713.7 |
Claims
1. A carbon support material having a reduced specific corrosion
rate for an electrode, comprising a carbon material having a
specific surface area (BET) of 100-600 m2/g and a micropore area of
10-60 m2/g.
2. The carbon support material of claim 1, wherein the carbon
material has a specific surface area (BET) of 200-600 m2/g and a
micropore area of 25-60 m2/g.
3. The carbon support material of claim 1, wherein the carbon
material has a specific surface area (BET) of 300-600 m2/g.
4. The carbon support material of claim 1, wherein the carbon
material has a specific surface area (BET) of 300-600 m2/g and a
micropore area of 25-60 m2/g.
5. The carbon support material of claim 1, wherein the carbon
support material loses 20% or less of its mass in an accelerated
test involving a 1.2V potential hold over a 24 hour period at
80.degree. C.
6. The carbon support material of claim 1, wherein the carbon
support material has a specific corrosion rate of less than
65%.
7. The carbon support material of claim 1, wherein the carbon
material is electrically conductive.
8. The carbon support material of claim 1, wherein the carbon
material comprises treated carbon black.
9. A method for producing a carbon support material having a
specific reduced corrosion rate for an electrode, the method
comprising treating a carbon black with at least one gas selected
from the group consisting of oxygen, ozone, hydrogen peroxide,
nitrogen dioxide, air, carbon dioxide and steam to produce a carbon
material having a specific surface area (BET) of 100-600 m2/g and a
micropore area of 10-60 m2/g.
10. The method of claim 9, wherein the carbon material has a
specific surface area (BET) of 200-600 m2/g and a micropore area of
25-60 m2/g.
11. The method of claim 9, wherein the carbon material has a
specific surface area (BET) of 300-600 m2/g.
12. The method of claim 9, wherein the carbon material has a
specific surface area (BET) of 300-600 m2/g and a micropore area of
25-60 m2/g.
13. The method of claim 9, wherein the carbon material is
electrically conductive.
14. The method of claim 9, wherein the carbon black is treated at a
temperature between 800.degree. C. and 1100.degree. C.
15. The method of claim 9, wherein the carbon black is treated for
a time ranging between 30 minutes and 4 hours.
16. A method of decreasing the specific corrosion rate of a carbon
support material for an electrode, the method comprising treating a
carbon black with at least one gas selected from the group
consisting of oxygen, ozone, hydrogen peroxide, nitrogen dioxide,
air, carbon dioxide and steam to produce a carbon support material
having a specific surface area (BET) of 100-600 m2/g and a
micropore area of 10-60 m2/g.
17. The method of claim 16, wherein the carbon support material has
a specific surface area (BET) of 200-600 m2/g and a micropore area
of 25-60 m2/g.
18. The method of claim 16, wherein the carbon support material has
a specific surface area (BET) of 300-600 m2/g.
19. The method of claim 16, wherein the carbon support material has
a specific surface area (BET) of 300-600 m2g and a micropore area
of 25-60 m2/g.
20. The method of claim 16, wherein the carbon support material
loses 20% or less of its mass in an accelerated test involving a
1.2V potential hold over a 24 hour period at 80.degree. C.
21. The method of claim 16, wherein the carbon support material has
a specific corrosion rate of less than 65%.
22. The method of claim 16, wherein the carbon material is
electrically conductive.
23. The method of claim 16, wherein the carbon black is treated at
a temperature between 800.degree. C. and 1100.degree. C.
24. The method of claim 16, wherein the carbon black is treated for
a time ranging between 30 minutes and 4 hours.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a novel corrosion resistant
catalyst, suitable for use as an electrocatalyst in fuel cells.
BACKGROUND OF THE INVENTION
[0002] A fuel cell is an electrochemical cell comprising two
electrodes separated by an electrolyte. A fuel, e.g. hydrogen, an
alcohol (such as methanol or ethanol), a hydride or formic acid, is
supplied to the anode and an oxidant, e.g. oxygen or air, or other
oxidant such as hydrogen peroxide 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 the oxidant at the cathode.
[0003] Fuel cells are usually classified according to their
electrolyte: proton exchange membrane (PEM) fuel cells including
hydrogen (including reformed hydrocarbon fuel) fuel cells, direct
methanol fuel cells (DMFC), direct ethanol fuel cells (DEFC),
formic acid fuel cells and hydride fuel cells; alkaline electrolyte
fuel cells; phosphoric acid fuel cells (including hydrogen or
reformed hydrocarbon fuel); solid oxide fuel cells (reformed or
unreformed hydrocarbon fuel); and molten carbonate fuel cells
(hydrogen and reformed hydrocarbon fuel).
[0004] In proton exchange membrane (PEM) fuel cells, the
electrolyte is a solid polymeric membrane. The membrane is
electronically insulating but ionically conducting.
Proton-conducting membranes are typically used, and protons,
produced at the anode, are transported across the membrane to the
cathode, where they combine with oxygen to create water.
[0005] The principle 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 solid polymeric membrane. On
either side of the membrane there is an electrocatalyst layer,
containing an electrocatalyst, which is tailored for the different
requirements at the anode and the cathode. 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, must allow products to be removed from 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.
[0006] The electrocatalyst layer is generally composed of a metal,
(such as a platinum group metal (platinum, palladium, rhodium,
ruthenium, iridium and osmium), gold or silver, or a base metal)
either unsupported in the form of a finely dispersed metal powder
(a metal black) or supported on an electrically conducting support,
such as a high surface area carbon material. Suitable carbons
typically include those from the carbon black family, such as oil
furnace blacks, extra-conductive blacks, acetylene blacks and
graphitised versions thereof. Exemplary carbons include Akzo Nobel
Ketjen EC300J, Cabot Vulcan XC72R and Denka Acetylene Black. The
electrocatalyst layers suitably comprise other components, such as
ion-conducting polymer, which is included to improve the ionic
conductivity within the layer. The electrocatalyst layers also
comprise a certain volume fraction of porosity, which allows
reactant ingress and product egress.
[0007] During normal PEM fuel cell operation, hydrogen-containing
gas is fed to the anode and air to the cathode; however during shut
down and start up conditions depletion of hydrogen and an ingress
of air to the anode electrode can occur and results in an increase
in potential at both electrodes. This so-called `reverse current
decay mechanism` can lead to high potentials in excess of 1.2V at
the cathode electrode, resulting in electrochemical oxidation
(corrosion) and loss of the carbon support. This process leads to a
collapse in the catalyst layer structure, a loss of active catalyst
metal surface area and irreversible fuel cell performance decay. An
operational system will experience repeated start/stops over the
lifetime of thousands of hours and therefore repeated excursions to
high voltages causing corrosion and associated performance decay.
Under normal operating conditions depletion of hydrogen fuel at the
anode electrode whilst under load can also lead to carbon
corrosion. Prolonged `idling` of the system results in exposure of
the cathode electrode to potentials around .about.0.9V which could
cause deterioration of the carbon support and catalyst. Operation
at higher temperatures up to 120.degree. C. is particularly
desirable for automotive PEM fuel cell systems; however increasing
temperature also promotes the carbon corrosion process and is
therefore likely to accelerate any of the decay mechanisms
described.
[0008] It is generally the case with the carbon materials used as
catalyst support materials for fuel cell applications, that an
increase in the total (BET) surface area results in an increase in
the catalyst surface metal area, due to the formation of smaller
catalyst particles, as measured by ex-situ gas phase chemisorption
metal area or also the in-situ electrochemical surface area under
fuel cell testing conditions. The increased catalyst surface area
is often associated with an increase in the activity of the
catalyst in a fuel cell environment. However, an increase in total
(BET) surface area of the carbon support invariably corresponds
with an increase in the corrosion of the support under fuel cell
operating conditions where high potentials occur.
[0009] It is possible to improve the corrosion resistance of carbon
supports through various processes, particularly high temperature
graphitising treatments, but the resultant catalysts have lower
active catalyst metal area and thus lower activity compared to the
comparable untreated carbon supported catalysts.
[0010] It is therefore an object of the present invention to
provide an improved catalyst which demonstrates both comparable
activity to conventional catalysts, but which is more resistant to
corrosion.
SUMMARY OF THE INVENTION
[0011] Accordingly, the present invention provides a catalyst
comprising (i) a primary metal or alloy or mixture comprising the
primary metal, and (ii) an electrically conductive carbon support
material for the primary metal or alloy or mixture comprising the
primary metal, characterized in that the carbon support
material:
[0012] (a) has a specific surface area (BET) of 100-600 m.sup.2/g,
and
[0013] (b) has a micropore area of 10-90 m.sup.2/g.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows single cell performance data for cathode
catalyst layers containing the electrocatalysts of Examples 4 and 5
and Comparative Examples 4 and 5 at 80.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The carbon support material has a specific surface area
(BET) of 100 m.sup.2/g to 600 m.sup.2/g, suitably 250 m.sup.2/g to
600 m.sup.2/g, preferably 300 m.sup.2/g to 600 m.sup.2/g. In an
alternative embodiment, the carbon support material has a specific
surface area (BET) of 100 m.sup.2/g to 500 m.sup.2/g, suitably 250
m.sup.2/g to 500 m.sup.2/g, preferably 300 m.sup.2/g to 500
m.sup.2/g. In a further alternative embodiment, the carbon support
material has a specific surface area (BET) of 100 m.sup.2/g to 400
m.sup.2/g, suitably 250 m.sup.2/g to 400 m.sup.2/g, preferably 300
m.sup.2/g to 400 m.sup.2/g, and most preferably 100 m.sup.2/g to
300 m.sup.2g. 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/P.sub.0 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)/V.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.
[0016] The carbon support material also has a micropore area of 10
m.sup.2/g to 90 m.sup.2/g, suitably 25 m.sup.2/g to 90 m.sup.2/g,
more suitably 40 m.sup.2/g to 90 m.sup.2/g when determined by the
method described below. Alternatively, the carbon support material
has a micropore area of 10 m.sup.2/g to 80 m.sup.2/g, suitably 25
m.sup.2/g to 80 m.sup.2/g, more suitably 40 m.sup.2/g to 80
m.sup.2/g when determined by the method described below. In a
further alternative embodiment, the carbon support material has a
micropore area of 10 m.sup.2/g to 75 m.sup.2/g, suitably 25
m.sup.2/g to 75 m.sup.2/g, more suitably 40 m.sup.2/g to 75
m.sup.2/g when determined by the method described below. In a
further alternative embodiment, the carbon support material has a
micropore area of 10 m.sup.2/g to 60 m.sup.2/g, suitably 25
m.sup.2/g to 60 m.sup.2/g, more suitably 40 m.sup.2/g to 60
m.sup.2/g when determined by the method described below. In a
further alternative embodiment, the carbon support material has a
micropore area of 10 m.sup.2/g to 50 m.sup.2/g, suitably 25
m.sup.2/g to 50 m.sup.2/g, more suitably 40 m.sup.2/g to 50
m.sup.2/g when determined by the method described below. In a
further alternative embodiment, the carbon support material has a
micropore area of 10 m.sup.2/g to 45 m.sup.2/g, suitably 25
m.sup.2/g to 45 m.sup.2/g, more suitably 40 m.sup.2/g to 45 m2/g
when determined by the method described below. The micropore area
refers to the surface area associated with the micropores, where a
micropore is defined as a pore of internal width less than 2 nm.
The micropore area is determined by use of a t-plot, generated from
the nitrogen adsorption isotherm as described above. The t-plot has
the volume of gas adsorbed plotted as a function of the standard
multilayer thickness, t, where the t values are calculated using
the pressure values from the adsorption isotherm in a thickness
equation; in this case the Harkins-Jura equation. The slope of the
linear portion of the t-plot at thickness values between 0.35 and
0.5 nm is used to calculate the external surface area, that is, the
surface area associated with all pores except the micropores. The
micropore surface area is then calculated by subtraction of the
external surface area from the BET surface area. More details can
be found in `Analytical Methods in Fine Particle Technology`, by
Paul A. Webb and Clyde Orr, Micromeritics Instruments Corporation
1997.
[0017] The carbon support material also loses 20% or less, suitably
18% or less, more suitably 11% or less of its mass in an
accelerated test involving a 1.2V potential hold over a 24 hour
period at 80.degree. C. The loss of carbon can be determined by the
following commonly accepted test used by those skilled in the art
and as described in more detail in Journal of Power Sources, Volume
171, Issue 1, 19 Sep. 2007, Pages 18-25: an electrode of the chosen
catalyst or carbon is held at 1.2V in 1M H.sub.2SO.sub.4 liquid
electrolyte vs. Reversible Hydrogen Electrode (RHE) and 80.degree.
C. and the corrosion current monitored over 24 hrs. Charge passed
during the experiment is then integrated and used to calculate the
carbon removed, assuming a 4 electron process converting carbon to
CO.sub.2 gas; the first 1 min of the test is not included as the
charge passed during this time is attributed to the charging of the
electrochemical double layer and therefore not due to corrosion
processes. The mass of carbon lost during the 24 hr test is then
expressed as a percentage of the initial carbon content of the
electrode.
[0018] Furthermore, the carbon support material has a specific
corrosion rate of less than 65%, suitably less than 60%, preferably
such as less than 50%. The specific corrosion rate is determined by
expressing the amount of carbon corroded as a percentage of the
number of surface carbon atoms. Assuming 3.79.times.10.sup.19 atoms
m.sup.-2 of carbon and a four-electron process, the maximum charge
required to remove one monolayer of the carbon is determined. The
experimentally determined charge associated with carbon corrosion
is then expressed as a percentage of a monolayer, giving the
specific corrosion rate.
[0019] The electrically conductive carbon support material can be
obtained by functionalization of a pre-existing carbon material.
Functionalization or activation of carbon has been described in the
literature and is understood in the case of physical activation as
a post treatment of carbon with gases like oxygen or air, carbon
dioxide, steam, ozone, or nitrogen oxide or in the case of a
chemical activation as a reaction of the carbon pre-cursor with
solid or liquid reagents like KOH, ZnCl.sub.2 or H.sub.3PO.sub.4 at
elevated temperatures. Examples of such functionalization or
activation are described by H. Marsch and F. Rodriguez-Reinoso in
`Activated Carbon`, Elsevier Chapter 5 (2006). During the
activation process parts of the carbon is lost by the chemical
reaction or burn-off.
[0020] The activation of carbon black is typically performed with
oxidizing gases such as oxygen, ozone, hydrogen peroxide, or
nitrogen dioxide which, as well as leading to an increase of the
specific surface area, also leads to an increasing amount of
surface groups. Activation can also be performed by air, carbon
dioxide or steam treatment, which mainly affects the carbon black
porosity, for example as described in `Carbon Black` (J-B. Donnet,
R. C. Bansal and M-J Wang (eds.), Taylor & Francis, 62-65
(1993)).
[0021] Suitably, the primary metal is selected from [0022] (i) the
platinum group metals (platinum, palladium, rhodium, ruthenium,
iridium and osmium), or [0023] (ii) gold or silver, or [0024] (iii)
a base metal or an oxide thereof.
[0025] The primary metal may be alloyed or mixed with one or more
other precious metals such as ruthenium, or base metals such as
molybdenum, tungsten, cobalt, chromium, nickel, iron, copper or an
oxide of a precious metal or base metal. In a preferred embodiment,
the primary metal is platinum.
[0026] The primary metal, in the catalyst of the invention, has a
gas phase metal area, determined using gas phase adsorption of
carbon monoxide (CO), of at least 30 m.sup.2/g, suitably at least
45 m.sup.2/g, more preferably at least 60 m.sup.2/g, The gas phase
CO metal area is determined by reducing the catalyst in hydrogen,
then titrating aliquots of CO gas until there is no more uptake.
The moles of CO absorbed can then be converted into a metal surface
area, by assuming 1.25.times.10.sup.19 atoms/m.sup.2 for Pt as
defined in `Catalysis-Science and Technology, Vol 6, p257, Eds J.
R. Anderson and M. Boudart. A high Pt surface area determined by
this method is well known to translate to high electrochemical
surface area under fuel cell testing conditions
[0027] The loading of primary metal particles on the carbon support
material is suitably in the range 5-95 wt %, preferably 5-75 wt %.
The actual loading of the primary metal particles on the carbon
support material will be dependent on the ultimate use of the
catalyst. For example, for use in a proton exchange membrane fuel
cell, the loading will ideally be in the rage of 30-95 wt %,
preferably 35-75 wt %; for use in a phosphoric acid fuel cell, the
loading will ideally be in the range of 5-25 wt %. The skilled
person would know what a suitably loading would be for the given
application.
[0028] The catalysts of the invention have utility in fuel cells
and other electrochemical devices. Accordingly, a further aspect of
the invention provides an electrode, either anode or cathode,
comprising a gas diffusion layer (GDL) and a catalyst according to
the invention. In one embodiment, the electrode is the cathode and
may provide improved stability; in a second embodiment, the
electrode is the anode, and may show particular benefit under cell
reversal conditions. 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. Typical GDLs
are fabricated from substrates based on carbon paper (e.g.
Toray.RTM. paper available from Toray Industries, Japan or U105 or
U107 paper available from Mitsubishi Rayon, Japan), woven carbon
cloths (e.g. the MK series of carbon cloths available from
Mitsubishi Chemicals, Japan) or non-woven carbon fibre webs (e.g.
AvCarb series available from Ballard Power Systems Inc, Canada;
H2315 series available from Freudenberg FCCT KG, Germany; or
Sigracet.RTM. series available from SGL Technologies GmbH,
Germany). The carbon paper, cloth or web is typically modified with
a particulate material either embedded within the layer or coated
onto the planar faces, or a combination of both to produce the
final GDL. The particulate material is typically a mixture of
carbon black and a polymer such as polytetrafluoroethylene (PTFE).
Suitably the GDLs are between 100 and 400 .mu.m thick. Preferably
there is a layer of particulate material such as carbon black and
PTFE on the face of the GDL that contacts the catalyst layer.
[0029] Alternatively, the substrate onto which the catalyst of the
invention is applied may be a preformed gas diffusion electrode,
either an anode or a cathode i.e. a GDL (which may be as described
above) which already has applied thereto a catalyst layer. The
catalyst layer in the preformed gas diffusion electrode may
comprise a catalyst according to the invention or may comprise a
conventional catalyst as applied by conventional techniques, for
example screen printing.
[0030] In PEM fuel cells, the electrolyte is a solid polymeric
membrane. Electrocatalysts may be deposited onto one or both faces
of the solid polymeric membrane to form a catalyzed membrane. In a
further aspect the present invention provides a catalyzed membrane
comprising a solid polymeric membrane and a catalyst of the
invention. The catalyst can be deposited onto the membrane using
well known techniques. The catalyst may be formulated into an ink
and either directly deposited onto the membrane or deposited onto a
decal blank for subsequent transfer to a membrane. One or more
additional catalyst (for example Pt, PtRu) may subsequently be
applied to the catalyzed membrane to form a catalyzed membrane
having two or more catalyst layers. The one or more additional
catalyst layers may comprise a catalyst according to the invention
or may comprise a conventional catalyst as applied by conventional
techniques, for example screen printing.
[0031] The membrane may be any membrane suitable for use in a 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 performance
and durability, for example by incorporating an additive.
Alternatively, the membrane may be based on a sulphonated
hydrocarbon membrane such as those available from Polyfuel, JSR
Corporation, FuMA-Tech GmbH 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 a proton-conducting membrane
and a matrix of silica fibres, as described in EP 0 875 524 or 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. The catalyst
layer of the invention is also suitable for use with membranes that
use charge carriers other than proton, for example OH.sup.-
conducting membranes such as those available from Solvay Solexis
S.p.A., FuMA-Tech GmbH.
[0032] 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 catalyzed transfer substrate comprising a transfer
substrate and a catalyst 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) 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 of the invention may then be transferred to a
GDL, gas diffusion electrode, membrane or catalyzed membrane by
techniques known to those skilled in the art.
[0033] In PEM fuel cells, the solid polymeric membrane is
interposed between two catalyst layers, and each catalyst layer is
in contact with an electronically conducting substrate. This
five-layer assembly is known as a membrane electrode assembly. A
further embodiment of the invention provides a membrane electrode
assembly (MEA) comprising a catalyst of the invention. The MEA may
be made up in a number of ways including, but not limited to:
[0034] (i) a solid polymeric membrane may be sandwiched between two
gas diffusion electrodes (one anode and one cathode), at least one
of which is an electrode according to the present invention;
[0035] (ii) a catalyzed membrane coated on one face only by a
catalyst may be sandwiched between (i) a GDL and a gas diffusion
electrode, the GDL contacting the catalyzed face of the catalyzed
membrane, or (ii) two gas diffusion electrodes, and wherein at
least one of the catalyzed membrane and the gas diffusion electrode
adjacent to the uncataloged face of the catalyzed membrane is
according to the present invention;
[0036] (iii) a catalyzed membrane coated on both faces with a
catalyst may be sandwiched between (i) two GDLs, (ii) a GDL and a
gas diffusion electrode or (iii) two gas diffusion electrodes, and
wherein the catalyst coating on at least one face of the catalyzed
membrane is according to the invention.
[0037] 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.
[0038] The MEA may be used in a fuel cell stack, for example a PEM
fuel cell, a direct methanol fuel cell (DMFC), a high temperature
fuel cell (for use at temperatures in the range of 100.degree. C.
to 250.degree. C.) or an alkali fuel cell. Accordingly, a further
aspect of the invention provides a fuel cell comprising a MEA of
the invention. The MEA may be incorporated into the fuel cell using
conventional methods.
[0039] Alternatively, an electrode of the invention may be used
directly in a fuel cell, for example a phosphoric acid fuel cell
wherein the electrolyte is liquid phosphoric acid in a supporting
matrix, for example silicon carbide. Accordingly, a further aspect
of the invention provides a fuel cell, in particular a phosphoric
acid fuel cell, which comprises an electrode of the invention. Such
fuel cells may be operated in the range of from 150.degree. C. to
210.degree. C.
[0040] The invention will now be further described with reference
to the following examples, which are illustrative and not limiting
of the invention.
[0041] Carbon Support Materials
[0042] The carbon support materials used in the Examples are as
described below: [0043] Comparative Example 1: Ensaco.TM. 250G
available from Timcal Ltd [0044] Comparative Example 2: Ensaco.TM.
350G available from Timcal Ltd [0045] Comparative Example 3: Vulcan
XC-72R available from Cabot Corporation [0046] Comparative Example
4: Ketjen EC 300J available from Akzo Nobel [0047] Comparative
Example 5: Ketjen EC 300J graphitised at high temperature
>2000.degree. C.
[0048] Carbons for Examples 1 to 7 were prepared by physical
functionalization of granulated highly structured conductive carbon
black Ensaco.RTM. 250G (Comparative Example 1) in a fluidized bed
reactor. The carbon material (800-1200 g) was introduced in the
reaction chamber at room temperature. A flow of inert gas
(nitrogen) was introduced in order to fluidize the carbon material.
The chamber was slowly heated up to 800.degree.-1100.degree. C.,
where it was kept at constant temperature with a flow of reacting
gas for a time ranging between 30 minutes and 4 hours. The reacting
gas used was air, carbon dioxide, or steam. The reaction time
controlled the degree of the post treatment with the individual gas
at a given gas flow and reactor design. Thereafter the reaction
chamber with the post treated carbon material was left to cool down
to room temperature under a flow of inert gas.
[0049] Preparation of Catalyst
[0050] General Method of Preparation
[0051] The carbon support material (1 g) was dispersed in water
(150 ml) using a shear mixer. The slurry was transferred to a
beaker (if required with 50 ml additional water), fitted with
temperature and pH probes and two feed inlet tubes connected to a
pH control unit. The Pt salt (Pt nitrate or K.sub.2PtCl.sub.4) was
added in an amount sufficient to give a nominal loading of 60 wt %
Pt (Examples 1 to 5) and a nominal loading of 50 wt % Pt (Examples
6 and 7). NaOH was added to maintain the pH between 5.0 and 7.0
(final pH). The slurry was stirred and once hydrolysis was
complete, formaldehyde was added to reduce the Pt. Once the
reaction was complete, the catalyst was recovered by filtration and
washed on the filter bed. The material was dried overnight at
105.degree. C. Properties of the carbon and catalysts prepared
therefrom are given in Table 1.
TABLE-US-00001 TABLE 1 Properties of carbon supports and catalysts
Corrosion Test (1.2 V, 24 hours, Gas Carbon surface 80.degree. C.)
phase area (m.sup.2/g) Absolute Specific metal Surface corrosion
corrosion % area Total Area in wt % monolayer (CO) Example (BET)
Micropores carbon loss corroded (m.sup.2/g) Comparative 65 5 2.5 52
29 Example 1 Comparative 751 117 24 42 70 Example 2 Comparative 226
96 12 67 65 Example 3 Comparative 846 169 32 51 90 Example 4
Comparative 124 7 1 10 28 Example 5 Example 1 110 28 5.3 64 34
Example 2 196 40 7.2 49 36 Example 3 262 41 9.7 49 45 Example 4 337
42 9.1 37 43 Example 5 396 33 9 26 60 Example 6 541 74 16.6 41 71
Example 7 466 65 17.8 51 62
[0052] Examples 1-7 were prepared by application of the carbon
treatment process to Comparative Example 1. The total BET surface
area of Examples 1 to 7 was increased by the application of the
carbon treatment process and this resulted in an associated
increase in Pt surface area on catalyzing the carbon supports.
[0053] Preparation of Example 1 from Comparative Example 1 via the
treatment process resulted in an increase in total BET area,
micropore area and slight increase in absolute and specific
corrosion rates. Thereafter, through Examples 1-7 on application of
the treatment process, the overall carbon BET surface area
increased, the proportion of area in micropores decreased
accompanied by a decrease in the specific corrosion rate. This
resulted in a plateauing of the absolute corrosion determined by wt
% carbon loss. Thus the application of the treatment process
creates a support surface that is less intrinsically corrodible
(exhibiting a lower specific corrosion rate), such that a carbon
with greater overall BET surface area (Examples 3, 4 and 5) can
show lower absolute corrosion than a commercial carbon with lower
BET surface area (such as Comparative Example 3).
[0054] Comparative Example 5 is representative of a carbon support
prepared by graphitisation of a high surface area carbon support by
heat treatment at high temperature >2000.degree. C. These
typically have low BET areas and low surface area in
micropores;
[0055] however exact properties are dependent on the graphitisation
temperature. Typically catalysation of such carbon supports results
in low Pt dispersion due to the lower surface functionality of the
graphitised carbon support.
[0056] Performance Data
[0057] The catalysts prepared in Comparative Examples 4 and 5 and
Examples 4 and 5 were used to prepare electrodes. The catalysts
(Comparative Examples and catalysts of the invention) were
formulated into inks using the techniques outlined in EP 0 731 520
and used to prepare cathode electrocatalyst layers at a total metal
loading of 0.4 mgPt/cm.sup.2. The anode comprised a conventional
Pt/C catalyst at a loading of 0.4 mgPt/cm.sup.2. MEAs were
fabricated by hot pressing the anode and cathode either side of a
30 micron PFSA membrane. Samples were tested as 50 cm.sup.2 MEAs
and initially conditioned at 100% relative humidity for several
hours. The cathode and anode relative humidities were then reduced
to 30% and the sample reconditioned for up to 8 hrs, at 50 kPa
gauge, 80.degree. C., 0.5 A/cm.sup.2 on H.sub.2/Air, stoichiometry
2:2 H.sub.2/O.sub.2 until stable performance was achieved. Air
polarization curves were then measured. FIG. 1 shows single cell
performance data for cathode catalyst layers containing the
electrocatalysts of Examples 4 and 5 and Comparative Examples 4 and
5 at 80.degree. C. The performance of the electrodes containing the
Examples of the invention is comparable to that of Comparative
Example 4 and yet the catalysts in the electrodes of the invention
are considerably more corrosion resistant. In addition samples of
the invention show significantly enhanced performance under the 30
% RH dry conditions used compared to Pt on graphitised carbon
Comparative Example 5, associated with only a moderate increase in
corrosion rate from 1 to 10% wt C (Comparative Example 5 compared
to Comparative Examples 4 and 5).
* * * * *