Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED CATHODE FOR SOLID POLYMER ELECTROLYTE FUEL CELL
BACKGROUND
Field of the Invention
The present invention pertains to solid polymer electrolyte fuel cells, and
particularly to cathodes for
obtaining improved cell performance over a range of current densities.
Description of the related art
Solid polymer electrolyte fuel cells electrochemically convert reactants,
namely fuel (such as
hydrogen) and oxidant (such as oxygen or air), to generate electric power.
These cells generally
employ a proton conducting polymer membrane electrolyte between two
electrodes, namely a cathode
and an anode. A structure comprising a proton conducting polymer membrane
sandwiched between
two electrodes is known as a membrane electrode assembly (MEA). MEAs in which
the electrodes
have been coated onto the membrane electrolyte to form a unitary structure are
commercially
available and are known as a catalyst coated membrane (CCM). In a typical fuel
cell, flow field
plates comprising numerous fluid distribution channels for the reactants are
provided on either side of
a MEA to distribute fuel and oxidant to the respective electrodes and to
remove by-products of the
electrochemical reactions taking place within the fuel cell. Water is the
primary by-product in a cell
operating on hydrogen and air reactants. Because the output voltage of a
single cell is of order of IV,
a plurality of cells is usually stacked together in series for commercial
applications. Fuel cell stacks
can be further connected in arrays of interconnected stacks in series and/or
parallel for use in
automotive applications and the like.
Catalysts are used to enhance the rate of the electrochemical reactions which
occur at the cell
electrodes. Catalysts based on noble metals such as platinum are typically
required in order to
achieve acceptable reaction rates, particularly at the cathode side of the
cell. To achieve the greatest
catalytic activity per unit weight, the noble metal is generally disposed on a
corrosion resistant
support with an extremely high surface area, e.g. high surface area carbon
particles. However, noble
metal catalyst materials are relatively quite expensive. In order to make fuel
cells economically
viable for automotive and other applications, there is a need to reduce the
amount of noble metal (the
loading) used in such cells, while still maintaining similar power densities
and efficiencies. This can
be quite challenging.
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One approach considered in the art is the use of certain noble metal alloys
which have demonstrated
enhanced activity over the noble metals per se. For instance, alloys of Pt
with base metals such as Co
have demonstrated circa two-fold activity increases for the oxygen reduction
reaction taking place at
the cathode in the kinetic operating region (amounting to about a 20-4OmV
gain). However, despite
this kinetic advantage, such catalyst compositions suffer from relatively poor
performance in the mass
transport operating regime (i.e. at high power or high current densities). For
instance, state-of-the-art
commercial CCMs comprising PtCo alloy cathode catalysts with Pt loadings in
the range of about
0.25-0.4 mg Pt/cm2) show good performance (about 2 times the mass activity) at
low current densities
but poor performance at high current densities (e.g. greater than about
1.5A/cm2) relative to Pt
catalysts on the same carbon support. Some of the advantages and disadvantages
of such alloys as
cathode catalysts are discussed for instance in "Effect of Particle Size of
Platinum and Platinum-
Cobalt Catalysts on Stability"; K. Matsutani et at., Platinum Metals Rev., 54
(4) 223-232 and
"Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen
reduction catalysts for
PEMFCs", H. Gasteiger et al., Applied Catalysis B: Environmental 56 (2005) 9-
35.
Thus, neither the common noble metal catalysts nor their alloys seemed able to
satisfy the desired
performance requirements of many applications at both low and high current
densities. Mixtures of
various kinds may be considered but with an expectation of a performance
compromise at both low
and high current densities. So instead, alloy catalyst compositions, such as
PtCo, are presently
considered predominantly for stationary applications and are less attractive
for automotive
applications which require higher power density.
Thus, in general there is a continuing need to obtain improved cathode
catalysts and/or structures,
particularly for those based on PtCo alloy catalysts, so as to provide
desirable performance at both
low and high current densities while further reducing the amount of expensive
noble metal required.
Many polymers have been suggested for use as binders in fuel cell electrodes.
For instance,
US20110223520 suggests the use of thermoplastic polyvinylidene fluoride (PVDF)
polymer as a
binder in fuel cell electrodes. PVDF polymer is also used as a binder of
choice in commercial lithium
ion cell electrodes, particularly in the anodes. PVDF has also been suggested
for use in binder blends
for solid polymer electrolyte fuel cells, e.g. in A hydrophobic blend binder
for anti-water flooding of
cathode catalyst layers in polymer electrolyte membrane fuel cells, K-H Oh et
al., International
Journal of Hydrogen Energy, Vol. 36, Issue 21, Oct. 2011, p 13695-13702.
However, a substantial amount of binder is typically required in order to
adequately bind fuel cell
electrodes together for mechanical reasons. Typically, greater than about 10%
by weight is required.
Since such a significant amount is required, a binder type may be selected to
achieve additional
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desired functions or at least is selected to minimize adverse effects on
performance or the like. Often,
polymer electrolyte material (which may be the same as that used in the
membrane electrolyte) is used
as binder in fuel cell electrodes.
SUMMARY
The performance at high current densities can surprisingly be improved in
solid polymer electrolyte
fuel cells using platinum cobalt alloy cathode catalysts by incorporating a
small amount of PVDF
additive in the cathode formulation. The performance of such cells is
competitive with cells using Pt
cathode catalysts at both low and high current densities.
Specifically, a solid polymer electrolyte fuel cell of the invention comprises
a solid polymer
electrolyte, an anode, and a cathode in which the cathode comprises a platinum
cobalt alloy catalyst
composition and an amount of polyvinylidene fluoride additive less than about
10% by weight (e.g.
about 2% by weight). Incorporating this amount of PVDF additive in the cathode
can result in an
increase in the current density capability of the solid polymer electrolyte
fuel cell at high rate. The
PVDF amount can be incorporated by introducing it into the cathode catalyst
ink during preparation
of the fuel cell cathodes.
Because the mechanisms for the observed improvements with PtCo alloy catalyst
and PVDF additive
may relate to hydrophobicity and pore structure of the catalyst layer and the
oxygen solubility and
melt processable nature of the PVDF, it is reasonable to believe that other
polymer additives (e.g.
polyethylene, polypropylene) with similar related characteristics (e.g. melt
processability) may work
in the same manner. In a like manner, cathodes based on catalyst compositions
other than supported
PtCo but which suffer from similar high current density performance losses
(e.g. unsupported PtCo or
PtCo on other supports, PtNi alloy, oxide and/or carbon-oxide hybrid supported
catalysts) may benefit
from the addition of a small amount of PVDF-like additive to the catalyst
formulation.
The present invention addresses the low performance problems of noble metal
alloy cathode catalysts
at high current densities while still maintaining their performance at low
current densities. Superior
cell performance can thus be obtained over the range of current densities
while minimizing the total
amount of noble metal used.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 compares a plot of cell voltage versus current density for a cell of
the invention to those of
different comparative cells operating under normal automotive conditions.
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Fig. 2 compares a plot of cell voltage versus current density for a cell of
the invention to those of
different comparative cells operating under dry automotive conditions.
Fig. 3 compares a plot of cell voltage versus current density for a cell of
the invention to those of
different comparative cells operating under hot automotive conditions.
Fig. 4 compares a plot of cell voltage versus current density for a cell of
the invention to those of
different comparative cells operating under warm-up automotive conditions.
DETAILED DESCRIPTION
Fuel cells of the invention can be made in many conventional manners. For
instances, the
components employed include a solid polymer membrane electrolyte, anode, and a
PtCo alloy catalyst
cathode comprising a small amount of PVDF additive. The amount of PVDF
additive is less than
about 10% by weight and based on the Examples to date may preferably be about
2% by weight. In
any given embodiment, the optimal amount may differ somewhat from this and can
be readily
determined by those skilled in the art. Adjacent the two cathode and anode
electrodes may be anode
gas diffusion layer (GDL) and cathode GDL respectively. Adjacent the two GDLs
can be an anode
flow field plate and a cathode flow field plate.
The PtCo alloy catalyst cathode may be prepared in a number of conventional
ways. A preferred
method starts with a solid-liquid ink dispersion of suitable ingredients in
which the PVDF is itself
provided in the form of a component dispersion. Using a suitable coating
technique, the complete
cathode ink dispersion can be applied to a decal transfer sheet. After
appropriately drying the coating,
the cathode can be decal transferred under heat and pressure to the membrane
side of a conventional
membrane-anode assembly to create a complete catalyst coated membrane (CCM).
Dispersions for applying coatings in this manner will typically comprise an
amount of the desired
catalyst particles, one or more liquids in which the particles are dispersed,
and optionally other
ingredients such as binders (e.g. ionomer) and/or materials for engineering
porosity or other desired
characteristics in the cathode layer. Water is a preferred dispersing liquid
but alcohols and other
liquids such as methyl ethyl ketone may be used to reduce foaming, to promote
wetting, to adjust
viscosity, to dissolve binders, and so forth.
Conventional coating techniques, such as Mayer rod coating, knife coating,
decal transfer, or other
methods known to those skilled in the art, may be employed to apply
dispersions appropriately.
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Without being bound by theory, it is hypothesized that the addition of the
PVDF fluoropolymer resin
may result in advantageous differences in the cathode catalyst layer structure
and processability by
modifying hydrophobicity and pore structure of the cathode catalyst layer and
thereby enhancing
water management and improving gas diffusion.
The following Examples have been included to illustrate certain aspects of the
invention but should
not be construed as limiting in any way.
EXAMPLES
A series of experimental fuel cells was made with varied cathode constructions
in which the catalyst
was either Pt or PtCo alloy and the binder was the perfluorosulfonic acid
polymer Nafion DE2021
from Du Pont. Cells of the invention contained varied amounts of PVDF additive
while comparative
cells had no additive. Each cell comprised a catalyst coated membrane (CCM) in
which the electrolyte
was an 18 m thick ionomer membrane and a standard anode catalyst layer (both
from W. L. Gore).
The cathode catalysts used were commercially available Pt or PtCo alloy from
Tanaka Kikinzoku
Kogyo supported on a high surface area carbon support. The former comprised
46.3 % Pt by weight
and the latter comprised 47.4% Pt and 6.4% Co by weight.
Cathode layers were made by casting them from suitable cathode ink
dispersions. The ink dispersions
comprised approximately 20 wt% (% by weight) solids and were prepared by
mixing the selected
catalyst, an ionomer solution (e.g. -41 % by weight Nafion DE2021 from Du
Pont), water, and methyl
ethyl ketone solvents in a weight ratio of about 2.1/8.6/1.3/0.65. The ionomer
to carbon weight ratios
were adjusted to 1:1. For cathodes comprising PVDF additive, an appropriate
amount of a 20 wt%
aqueous dispersion of PVDF polymer Latex 32 (from Arkema) was added to this
dispersion in order
to obtain the desired target amount of PVDF solids in the cast catalyst layer.
The cathode catalyst ink dispersions were mixed by probe sonification and were
cast onto Teflon
sheet substrates using metering rods and then allowing them to dry. The
cathodes were then dried at
ambient temperature and 80 C respectively. The total target Pt loading in
each case was 0.25 mg
Pt/cm2. The actual Pt loading was calculated using a gravimetric method and by
referencing to the
geometric electrode area. The average catalyst layer thicknesses were about 10
gm. CCMs having an
active area of about 48cm2 were prepared using a decal-transfer process in
which a given catalyst
coated PTFE substrate was hot pressed (at 150 C and 15 bar for several
minutes) against a
commercially obtained membrane and anode.
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Individual fuel cells were prepared by hot press bonding carbon fibre gas
diffusion layers in a similar
manner onto each side of each CCM. Then, cell assembly was completed by
providing carbon flow
field plates having straight flow field channels adjacent each gas diffusion
layer.
The experimental cells included two comparative cells and three inventive
cells. One comparative
cell was made with a conventional cathode comprising carbon supported Pt
catalyst and no PVDF
additive (denoted "Pt" in the results below). The other comparative cell was
made with a
conventional cathode comprising carbon supported PtCo alloy catalyst and no
PVDF additive
(denoted "PtCo" in the results below). The cathodes in the inventive cells
comprised carbon
supported PtCo alloy catalyst and varied amounts of PVDF additive,
specifically 1 %, 2 %, and 4 % by
weight (denoted "PtCo +1% PVDF', "PtCo +2% PVDF', and "PtCo +4% PVDF'
respectively in
the results below).
These experimental cells were operated and tested in a common experimental
fuel cell stack in which
the cells were stacked in a series stack separated by bus plates. In this way,
the cells could be
simultaneously operated and tested under identical conditions. The
experimental fuel cell stack was
then run under various sets of operating conditions and the results reported
as indicated .
The stack was supplied with hydrogen and air reactants at flow rates of 10 and
60 slpm respectively.
Initially, the stack was run at a high humidity condition (60 C and both
reactants at 100% RH) and
then afterwards, the stack was run at a relatively low humidity condition (80
C and 30% RH (RH is
relative humidity)). Polarization results (voltage output versus current
density) were obtained for the
cells at representative low and high current densities and are tabulated in
Tables 1 and 2 below.
Table 1. Polarization results at 60 C and 100% RH
Cell Cell voltage Cell voltage
(mV) at 0.1 (mV) at 2.0
A/cm2 A/cm2
Pt 841 571
PtCo 887 516
PtCo +1 % PVDF 885 533
PtCo +2% PVDF 886 578
PtCo +4% PVDF 882 544
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Table 2. Polarization results at 80 C and 30% RH
Cell Cell voltage Cell voltage
(mV) at 0.1 (mV) at 2.0
A/cm2 A/cm2
Pt 848 460
PtCo 858 399
PtCo+1%PVDF 856 396
PtCo +2% PVDF 849 422
PtCo +4% PVDF 855 403
As is evident from Table 1, all the cells with PtCo cathode catalyst and PVDF
additive outperformed
the conventional PtCo based cell at a higher current density under this high
humidity testing
condition. In particular, the PtCo +2% PVDF cell showed the greatest
improvement. And, Table 2
shows that the presence of PVDF additive does not have any adverse effects
under this hotter and
lower humidity condition where liquid water management is not such an issue in
the fuel cell.
The stack was then operated under conditions considered normal for automotive
applications, namely
at 68 C, a relative humidity range varying between about 50-70%, and with both
hydrogen and air
stoichiometries of 1.65. Again, polarization results were obtained for each
cell. Figure 1 plots the
complete polarization data for the Pt, PtCo, and PtCo + 2% PVDF cells under
these normal
automotive conditions. (For clarity, the plots for the other PVDF based cells
have not been included.)
The limiting current density that could be obtained (defined as that current
density obtained when the
cell is under sufficient load to output a mere 100 mV) was determined for each
cell using
electrochemical limiting current measurements taken from mass transport free
polarization curves.
Table 3 tabulates the results for the limiting current density. In both Figure
1 and Table 3, the PtCo +
2% PVDF cell shows a definite improvement over the conventional PtCo cell with
no additive at
higher current densities and shows similar performance to that of the
conventional Pt cell. The cells
with 1 % or with 4% PVDF additive did not show as significant an improvement.
Table 3. Limiting current density (at a cell voltage of 100 mV)
Cell Current
density
(A/cm)
Pt 5.70
PtCo 4.91
PtCo +I% PVDF 5.54
PtCo +2% PVDF 5.70
PtCo +4% PVDF 5.17
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In a next test, the stack was operated under a relatively dry set of operating
conditions. This involved
operating the cell at 68 C, a relative humidity of about 50%, and with both
hydrogen and air
stoichiometries of 1.65. Figure 2 compares polarization plots for the same
cells under these dry
automotive conditions. Again, in this regime where liquid water management in
the fuel cell is not
such an issue, the presence of PVDF additive does not adversely affect
performance. The PtCo +2%
PVDF cell still appears to show an improvement over the conventional PtCo cell
with no additive at
the higher current densities and seems similar to the conventional Pt cell.
In a further test, the stack was operated under a relatively hot set of
operating conditions. This
involved operating the cell at 85 C, about 40-50% relative humidity, and with
both hydrogen and air
stoichiometries of 1.65. Figure 3 compares polarization plots for the same
cells under these hot
automotive conditions. Again here, the differences in high current density
performance are not so
great and are very similar at the highest current densities tested.
Finally, the stack was operated under a set of conditions that might typically
be used in starting or
warm-up of an automotive stack. This involved operating the cell at 40 C,
about 50% relative
humidity and with both hydrogen and air stoichiometries of 1.65. Fig. 4
compares polarization plots
for the same cells under these warm-up automotive conditions. Here, the
performance of the PtCo
based cells was strikingly different at higher current densities. The
conventional PtCo cell with no
additive showed poor performance at current densities as low as 1 A/cm2, while
the PtCo +2 % PVDF
cell showed a marked improvement and was only modestly worse than the
conventional Pt cell at the
highest current density tested.
These Examples demonstrate that fuel cells of the invention can provide the
same or superior
performance to that of prior art PtCo cathode catalyst based cells,
particularly at high current
densities, and represent a sufficient improvement so as to be comparable to
conventional Pt cathode
catalyst based cells. The improvements obtained vary depending on the
operating conditions. For
instance, generally more of an improvement is seen under wet, cool conditions
as opposed to hot and
dry conditions.
All of the above U.S. patents, U.S. patent application publications, U.S.
patent applications, foreign
patents, foreign patent applications and non-patent publications referred to
in this specification, are
incorporated herein by reference in their entirety.
While particular elements, embodiments and applications of the present
invention have been shown
and described, it will be understood, of course, that the invention is not
limited thereto since
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modifications may be made by those skilled in the art without departing from
the spirit and scope of
the present disclosure, particularly in light of the foregoing teachings.
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