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CA 02934237 2016-06-28
Docket No.: P830851/CA/1
ANODE FOR IMPROVED REVERSAL TOLERANCE IN FUEL CELL STACK
BACKGROUND
Field of the Invention
The present invention relates to designs of the anode for solid polymer
electrolyte fuel cells and
particularly to fuel cells intended for use in stacks for automotive
applications. The anode designs
improve the voltage reversal tolerance of the fuel cells.
Description of the Related Art
Solid polymer electrolyte fuel cells electrochemically convert fuel and
oxidant reactants, such as
hydrogen and oxygen or air respectively, to generate electric power. These
cells generally employ a
proton conducting, solid polymer membrane electrolyte between cathode and
anode electrodes. Gas
diffusion layers are typically employed adjacent each of the cathode and the
anode electrodes to
improve the distribution of gases to and from the electrodes. In a typical
fuel cell, flow field plates
comprising numerous fluid distribution channels for the reactants are provided
adjacent the gas
diffusion layers 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
1 V, 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.
If for some reason a cell (or cells) in a series stack is not capable of
delivering the same current being
delivered by the other cells in the stack, that cell or cells may undergo
voltage reversal. Depending on
the severity and duration of the voltage reversal, the cell may be
irreversibly damaged and there may
be an associated loss in cell and stack performance. Thus, it can be very
important in practical
applications for the cells in large series stacks to either be protected
against voltage reversal or
alternatively to have a high tolerance to voltage reversal.
The application of fuel cell technology in the automotive industry has
accelerated in recent years.
Many car makers have recently launched or will start mass production of fuel
cell cars in the near
future. The success of these activities will strongly depend on the technical
progress with regards to
fuel cell performance, durability and cost reduction. Several technical
hurdles remain challenging.
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One of these pertains to the possibility for the occurrence of a fuel
starvation condition on the anode
(i.e. where the anode receives insufficient fuel for intended operation). A
fuel starvation condition
can happen during start up from below freezing temperatures as a result of ice
blockages in the anode,
or during operation at normal operating temperatures as a result of anode
"flooding" (where liquid
water blocks passageways in the anode). It is well recognized that anode fuel
starvation conditions
can lead to cell voltage reversal due to the associated rise of anode
potential, and further can lead to
corrosion of the carbon supports which are typically used to support the anode
catalyst (typically
platinum). As a consequence of this corrosion, a loss in effective platinum
surface area occurs at the
anode and cell function is degraded. Therefore, a voltage reversal tolerant
anode is a critical design
requirement for the anodes in commercial fuel cell stacks.
There are several ways to improve fuel cell anodes for purposes of voltage
reversal tolerance. For
example, material approaches were described in US6517962 and US6936370 in
which voltage
reversal tolerance was improved by incorporating materials, namely catalysts
for promoting the
oxygen evolution reaction (OER) such as ruthenium, iridium, and/or their
oxides into the anode. An
approach involving a structural change was also described by US6517962 in
which the anode porosity
and/or the anode water intake was modified appropriately by employing a
hydrophobic layer (e.g. a
polytetrafluoroethylene layer) between the anode and an adjacent anode gas
diffusion layer.
There remains a desire for improvement in fuel cells with regards to tolerance
to voltage reversal.
The present invention fulfills this and other needs.
SUMMARY
The present invention relates to solid polymer electrolyte fuel cells and
particularly to anode designs
for such cells. Such fuel cells comprise a cathode, a solid polymer
electrolyte, and an anode, and the
anode comprises a mixture of an amount of hydrogen oxidation reaction (HOR)
catalyst supported on
an amount of catalyst support and an amount of an oxygen evolution reaction
(OER) catalyst. The
anode also typically comprises ionomer and other optional additives.
Surprisingly it has been found
that the voltage reversal tolerance of such fuel cells can be improved by
decreasing the amount of
catalyst support relative to the amount of OER catalyst. This results in an
anode which is both thinner
and in which the volumetric density of the OER catalyst is increased.
Depending on the approach
used, the amount of HOR catalyst in the anode may remain unchanged or may be
varied.
The HOR catalyst typically may be Pt catalyst but is not limited thereto. The
catalyst support may be
carbon. The OER catalyst may be iridium, ruthenium, oxides thereof and may be
supported or non-
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supported. An exemplary anode of the invention comprises a mixture of carbon
supported Pt and
iridium oxide.
In one approach, the decreasing step is accomplished by increasing the loading
of hydrogen oxidation
reaction catalyst on the catalyst support. In this way, the total amount of
catalyst support in the anode
can be reduced while keeping the amount of hydrogen oxidation reaction
catalyst in the anode
essentially constant.
In another approach, the decreasing step is accomplished simply by using less
of the same supported
HOR catalyst in the anode. That is, the loading of hydrogen oxidation reaction
catalyst on the catalyst
support is kept essentially constant and less of this supported HOR catalyst
is used in the anode. Both
the amount of catalyst support and the amount of hydrogen oxidation reaction
catalyst are reduced in
this approach. This approach may be adopted if the expected reduction in
performance and durability
is not too significant and is considered acceptable. An advantage of this
approach is that the cost of
the HOR catalyst used is reduced accordingly.
Desirably, the method of the invention results in an anode which is thinner
and in which the
volumetric density of the OER catalyst is increased. The anode thickness can
be further controlled to
a certain extent not only by the material amounts, but also by the specific
materials chosen and their
associated properties (e.g. particle shape). Further, anode thickness can also
be controlled by the
processes used to produce it (including catalyst ink mixing and coating
processes).
BRIEF DESCRIPTION OF THE DRAWINGS
Figures la and lb show schematics of fuel cell anodes comprising mixtures of
supported HOR
catalyst and OER catalyst in which the volumetric density of the OER catalyst
is relatively dilute and
relatively dense respectively.
Figures 2a and 2b show SEM images of fuel cell anodes in the Examples in which
the volumetric
density of the OER catalyst is relatively dilute and relatively dense
respectively. In these Figures, the
white areas are OER catalyst.
Figure 3 plots reversal tolerance time versus Ir02/carbon support weight ratio
for cells tested in the
Examples and illustrates the effect of decreasing the amount of carbon support
relative to OER
catalyst.
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Figure 4 plots reversal tolerance time versus amount of Pt loading in the
anode for cells tested in the
Examples. The amount of C support here was proportional to the amount of Pt.
Figure 5 compares reversal tolerance time versus amount of Ir02 in the anode
for cells tested in the
Examples where the anodes comprised carbon supported Pt catalysts with two
different Pt loadings on
the supports.
Figure 6 plots reversal tolerance time versus 1r02/carbon support weight ratio
for cells tested in the
Examples where the anodes comprised carbon supported Pt catalysts with two
different Pt loadings on
the supports.
DETAILED DESCRIPTION
It is well known that corrosion of the typical carbon supports used in fuel
cell anodes occurs when the
anode is exposed to the high potentials experienced (e.g. >1.6V) during a
voltage reversal event. The
carbon corrosion will lead to decreased electrical conductivity in the anode
and eventually the OER
reaction will be inhibited when electron transport becomes more difficult.
Therefore, if the anode can
be made in such a way that electron transport is less affected by the
structural changes experienced
during voltage reversal (e.g. due to carbon corrosion), the OER reaction can
be extended and the
length of time that the fuel cell can be exposed to reversal can be
substantially increased before
failure.
The present invention is applicable to solid polymer electrolyte fuel cells
whose anodes comprise a
mixture of supported HOR catalysts and OER catalysts. Applicable HOR catalysts
include platinum
and other noble metals and alloys. Catalyst supports for these HOR catalysts
are typically high surface
area materials and include high surface area carbons but also other materials
such as niobium oxide,
tungsten oxide, titanium oxide, and combinations thereof. Applicable OER
catalysts include but are
not limited to Ir02 supported on Ti02. An exemplary anode of the invention
comprises a mixture of
carbon supported Pt and iridium oxide.
In the present invention, the voltage reversal tolerance of the fuel cells is
improved by decreasing the
amount of catalyst support relative to the amount of OER catalyst. This
desirably results in an anode
which is both thinner and in which the volumetric density of the OER catalyst
is increased.
In a preferred embodiment, the ratio between the amount or loading of the OER
catalyst to the
thickness of the anode is greater than 20 fig/cm3. Such a ratio can be
achieved by increasing OER
catalyst loading. Alternatively such a ratio can be achieved by reducing the
amount of supported
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HOR catalyst in the anode. In a yet further alternative, a different supported
HOR catalyst material
can be used which has a different (e.g. greater) loading of HOR catalyst on
the support. In this case,
the higher loaded, supported HOR catalyst can allow for a thinner catalyst
layer to be produced with
the same total amount of HOR catalyst present.
Without being bound by theory, it is believed that the present method results
in an anode with a
denser OER catalyst structure in which there is improved electrical contact
between the OER and
HOR catalyst materials. The improved electrical contact renders the anode less
susceptible to the
adverse effects of corrosion of the catalyst support which can occur during
reversal events. Figures la
and lb illustrate the expected structural effects resulting from the inventive
method. Figures la and lb
show schematics of fuel cell anodes comprising mixtures of supported HOR
catalyst and OER
catalyst in which the volumetric density of the OER catalyst is relatively
dilute and relatively dense
respectively.
The following Examples have been included to illustrate certain aspects of the
invention but should
not be construed as limiting in any way.
EXAMPLES
In the following, various experimental fuel cell anodes were prepared. Where
indicated, experimental
fuel cells were prepared with these anodes and their voltage reversal
tolerance was determined at their
beginning of life.
Anodes were made using the same iridium (Ir) oxide powder as the OER catalyst
in all cases. Two
supported HOR catalysts were used in the experimental anodes. Both were carbon
black supported Pt
catalyst powders but comprised different Pt loading on the carbon black
supports, namely 50 and 30
weight %.
Using these materials, fuel cell anodes having different total amounts of OER
catalyst, HOR catalyst,
and carbon black supports, and having certain different ratios of OER
catalyst/C support were
prepared.
Experimental fuel cells using these anodes were also made in a conventional
manner. The cells were
then conditioned by operating at a current density of 1.5 A/cm2, with hydrogen
and air as the supplied
reactants at 100 %RH, and at a temperature of 60 C for at least 16 hours to
obtain a stable steady-
state performance. Voltage reversal testing was then carried out which
involved operating the cells at
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a current density of 0.2 A/cm2 with nitrogen on the anode and air on the
cathode. Typically, the cell
voltage would roughly plateau at a value between 0 and about ¨ 1.5V for a
variable amount of time
and then drop off suddenly to a value much less than ¨2.5V, at which point
testing ended. The length
of time to this sudden drop off point is representative of the cell's ability
to tolerate voltage reversal
and is denoted in the following as the reversal tolerance time.
Table 1 below summarizes the structure and certain properties of each of the
anodes made using the
catalyst with 50 wt% Pt loading on the carbon black supports (denoted as HOR
Pt 50 wt%), and also
tabulates the reversal time for each at the beginning of life (BOL) for the
experimental fuel cells made
with these anodes.
In a like manner, Table 2 below summarizes the structure and certain
properties of each of the anodes
made using the catalyst with 30 wt% Pt loading on the carbon black supports
(denoted as HOR Pt 30
wt%), and also tabulates the reversal time for each at the beginning of life
(BOL) for the experimental
fuel cells made with these anodes.
Table 1. HOR Pt 50 wt% anode properties and cell results
Anode # Pt loading OER loading Reversal tolerance
0ER/carbon
( g/cm 2) (pg/cm 2) time (min)
i 30 15 0.5 137
ii 30 50 1.7 543
iii 30 85 2.8 1342
iv 65 15 0.2 95
v 65 50 0.8 341
vi 65 85 1.3 552
vii 100 15 0.2 97
viii 100 50 0.5 271
ix 100 85 0.9 505
x 65 50 0.8 284
xi 65 50 0.8 350
xii 50 50 1.0 394
xiii 40 50 1.3 463
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Table 2. HOR Pt 30 wt% anode properties and cell results
Anode # Pt loading OER loading Reversal tolerance
OER/carbon
(ftg/cm 2) (ftg/cm 2) time (mm)
xiv 30 15 0.2 104
xv 30 50 0.7 324
xvi 30 85 1.2 676
As is evident from the data in the tables above, for each OER loading, the
thinner the anode is, the
longer the reversal time.
Figures 2a and 2b show SEM images of fuel cell anode #s iv and vi respectively
in which the
volumetric density of the OER catalyst is relatively dilute and relatively
dense respectively. In these
Figures, the white areas are OER catalyst. The anode of Figure 2b appears to
have a denser OER
structure and much better electrical connectivity of OER catalyst throughout
than that of Figure 2a.
Therefore, it is expected that the anode of Figure 2b will have a more stable
electrical conductivity
than that of Figure 2a during voltage reversal.
Figure 3 plots reversal tolerance time versus 1i-02/carbon support weight
ratio for the anodes and
associated cells in Table I. Figure 3 illustrates the effect of decreasing the
amount of carbon support
relative to OER catalyst. As shown here, the reversal time increases as the
1r02/carbon support ratio
increases.
Figure 4 plots reversal tolerance time in selected cells from Table 1 which
had the same amount of
Ir02 in the anodes (i.e. 50 tig/cm2) but where the amount of C supported
catalyst was varied. Here,
reversal tolerance time is plotted versus amount of Pt loading in the anodes.
The amount of C support
here was proportional to the amount of Pt. The results here suggest that
reversal tolerance can be
improved by decreasing the relative amount of carbon supported Pt catalyst
relative to a fixed amount
of OER catalyst in the anode.
The results obtained in cells using the two different C supported Pt catalyst
materials (i.e. the HOR Pt
wt% and the HOR Pt 50 wt%) are compared in Figures 5 and 6 for three different
OER loadings
(i.e. 15, 50, and 85 p,g/cm2). Figure 5 compares reversal tolerance time
versus amount of Ir02 in the
anode. In cells having greater amounts of Ir02, the results for reversal
tolerance time were quite
different even though the amount of Ir02 present in the anodes was the same.
On the other hand,
30 Figure 6 compares reversal tolerance time versus 1r02/carbon support
weight ratio. The reversal
tolerance times here are quite similar as long as the 1r02/carbon support
weight ratio is the same.
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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
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. Such
modifications are to be
considered within the purview and scope of the claims appended hereto.
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