A proton exchange membrane fuel cell (PEMFC) includes an anode, a
cathode and a proton exchange membrane (PEM) between the anode and cathode. In one
example, hydrogen gas is fed to the anode and air or pure oxygen is fed to the cathode.
However, it is recognized that other types of fuels and oxidants can be used. At the anode,
an anode catalyst causes the hydrogen molecules to split into protons (H+) and electrons (e ) .
The protons pass through the PEM to the cathode while the electrons travel through an
external circuit to the cathode, resulting in production of electricity. At the cathode, a
cathode catalyst causes the oxygen molecules to react with the protons and electrons from
the anode to form water, which is removed from the system. The anode catalyst and
cathode catalyst are commonly formed of platinum supported on carbon. The platinum
catalyst is only active when it is accessible to protons, electrons and the reactant (i.e.,
hydrogen or oxygen).
Platinum and other suitable noble metal catalysts are expensive. In order to
reduce costs, it is desirable to use electrodes with low platinum loads. Much work has been
conducted to reduce the platinum loading in the cathode in order to reduce manufacturing
costs. Low platinum loadings, however, result in high power performance losses that
exceed that predicted for kinetic activation losses alone. High-performing low platinum
load electrodes cannot be formed by simply reducing the platinum loading of an electrode.
Work has been conducted to improve the kinetics of catalyst layers in order to improve the
efficiency of the fuel cell.
SUMMARY
An electrode for an electrochemical cell includes platinum catalysts, carbon
support particles and an ionomer. The carbon support particles support the platinum
catalysts, and the ionomer connects the platinum catalysts. The electrode has a platinum
loading less than about 0.2 mg/cirT and an ionomer-to-carbon ratio between about 0.5 and
about 0.9.
A membrane electrode assembly includes a proton exchange membrane, a
cathode layer and an anode layer. The cathode layer includes platinum catalysts, carbon
support particles for supporting the platinum catalysts and an ionomer connecting the
platinum catalysts. The cathode layer has a platinum loading less than about 0.2 mg/cm2
and an ionomer-to-carbon ratio between about 0.5 and about 0.9. The anode layer includes
platinum catalysts, carbon support particles for supporting the platinum catalysts and an
ionomer connecting the platinum catalysts.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a fuel cell repeat unit.
FIG. 2 is an enlarged perspective view of the ionomer cathode catalyst layer
of the fuel cell repeat unit of FIG. 1.
DETAILED DESCRIPTION
Fuel cells convert chemical energy to electrical energy using one or more
fuel cell repeat units. The fuel cell repeat unit described herein includes an electrode with a
low platinum load. The electrode has a platinum loading less than about 0.2 mg/cm2. The
electrode includes platinum catalysts, carbon support particles and an ionomer. The
ionomer-to-carbon ratio is tailored to provide an efficient electrode with a low platinum
load.
FIG. 1 illustrates a perspective view of one example fuel cell repeat unit 10,
which includes membrane electrode assembly (MEA) 12 (having anode catalyst layer (CL)
14, membrane 6 and cathode catalyst layer (CL) 18), anode gas diffusion layer (GDL) 20,
cathode gas diffusion layer (GDL) 22, anode flow field 24 and cathode flow field 26. Fuel
cell repeat unit 10 can have coolant flow fields adjacent to anode flow field 24 and cathode
flow field 26. Coolant flow fields are not illustrated in FIG. 1.
Anode GDL 20 faces anode flow field 24 and cathode GDL 22 faces cathode
flow field 26. Anode CL 14 is positioned between anode GDL 20, and membrane 16 and
cathode CL 18 is positioned between cathode GDL 22 and membrane 6. In one example,
fuel cell repeat unit 10 can be a polymer electrolyte membrane (PEM) fuel cell that uses
hydrogen fuel (e.g., hydrogen gas) and oxygen oxidant (e.g., oxygen gas or air). It is
recognized that fuel cell repeat unit 10 can use alternative fuels and/or oxidants.
In operation, anode GDL 20 receives hydrogen gas (H2) by way of anode
flow field 24. Anode CL 14, which contains a catalyst such as platinum, causes the
hydrogen molecules to split into protons (H+) and electrons (e ) . The protons and electrons
travel to cathode CL 18; the protons pass through membrane 16 to cathode CL 18, while the
electrons travel through external circuit 28, resulting in a production of electrical power.
Air or pure oxygen (0 2) is supplied to cathode GDL 22 through cathode flow field 26. At
cathode CL 18, oxygen molecules react with the protons and electrons from anode CL 14 to
form water (H20), which then exits fuel cell 10, along with excess heat.
Membrane 16 is a semi-permeable membrane located between anode CL 14
and cathode CL 18. Membrane 16 allows movement of protons and water but does not
conduct electrons. Protons and water from anode CL 1 can move through membrane 16 to
cathode CL 18. Membrane 6 can be formed of an ionomer. An ionomer is a polymer with
ionic properties. In one example, membrane 16 is formed of a perfluorosulfonic acid
(PFSA)-containing ionomer, such as Nafion® by E.I. DuPont, USA. PFSA polymers are
composed of fluorocarbon backbones with sulfonate groups attached to short fluorocarbon
side chains.
In another example, membrane 16 is formed of a hydrocarbon ionomer. In
general, the main chains of hydrocarbon ionomers do not contain large amounts of fluorine,
unlike PFSA ionomers which have highly fluorinated backbones. A hydrocarbon ionomer
is an ionomer having a main chain which contains hydrogen and carbon, and which may
also contain a small mole fraction of hetero atoms such as oxygen, nitrogen, sulfur, and/or
phosphorus. These hydrocarbon ionomers primarily include aromatic and aliphatic
ionomers. Examples of suitable aromatic ionomers include but are not limited to sulfonated
polyimides, sulfoalkylated polysulfones, poly( -phenylene) substituted with sulfophenoxy
benzyl groups, and polybenzimidazole ionomers. Non-limiting examples of suitable
aliphatic ionomers are those based upon vinyl polymers, such as cross-linked poly(styrene
sulfonic acid), poly(acrylic acid), poly(vinylsulfonic acid), poly(2-acrylamide-2-
methylpropanesulfonic acid) and their copolymers.
The composition of membrane 16 affects the operating temperature of fuel
cell repeat unit 10. For example, hydrocarbon ionomers typically have a higher glass
transition temperature than PFSA ionomers, which enables a hydrocarbon ionomer
membrane 6 to be operated at a higher temperature than a PFSA ionomer membrane 16.
Cathode CL 18 is adjacent to the cathode side of membrane 16. Cathode CL
18 includes an ionomer and a catalyst, as described further below. The catalyst of cathode
CL 8 promotes electrochemical reduction of oxidant (i.e., oxygen). Example catalysts for
cathode CL 18 include carbon-supported platinum particles, carbon-supported alloys of
platinum and carbon-supported platinum intermetallics. Cathode CL 18 has a low platinum
loading, such as less than about 0.2 milligrams platinum per square centimeter of cathode
CL 18. In one embodiment, cathode CL 18 has a platinum loading between about 0.05
mg/cm2 and about 0.15 mg/cm2. In another embodiment, cathode CL18 has a platinum
loading of about 0.1 mg/cm2. Low platinum loading reduces the fuel cell costs. However,
it has been observed that some low platinum loading electrodes experience higher than
expected oxygen transport losses.
Anode CL 14 is adjacent to the anode side of membrane 16, and opposite
cathode CL 18. Anode CL 14 includes a catalyst. The catalyst of anode CL 14 promotes
electrochemical oxidation of fuel (i.e., hydrogen). Example catalysts for anode CL 14
include carbon-supported platinum particles. Anode CL 14 can also include an ionomer.
Anode CL 14 can have a structure similar to that described above for cathode CL 18,
although anode CL 14 and cathode CL 18 can have different compositions.
Fuel cell performance losses prevent a fuel cell from operating at its
theoretical efficiency. As discussed above, low catalyst loading electrodes generally have
performance losses that are higher than those predicted based on kinetic activation losses
alone. This suggests that at least one source of overpotential other than kinetics increases
with decreasing catalyst loading. For example, it has been found that oxygen transfer losses
are greater in low catalyst loading electrodes, while ohmic losses are lower in thin, low
catalyst loading electrodes. More specifically, it has been found that oxygen transport
losses, as measured by oxygen gain, are greater for low catalyst loading electrodes. The
oxygen gain is defined as the difference in performance measured on oxygen and air on the
cathode and an increase in the oxygen gain usually results from higher oxygen transport
losses. The oxygen gain has been observed to increase with decreasing relative humidity,
indicating that the oxygen transport losses are primarily associated with oxygen transport
through the ionomer. Typically, in a hydrogen-air fuel cell, the hydrogen oxidation reaction
(HOR) occurring at anode CL 14 has a significantly lower overpotential at a given current
than the oxidation reduction reaction (ORR) of cathode CL 18.
FIG. 2 is an enlarged schematic view of a portion of membrane 16 and
cathode CL 8 which includes catalysts 30 (having catalyst particles 32 and catalyst support
34) and ionomer 36. Cathode CL 18 is a matrix of catalyst supports 34, ionomer 36 and
platinum catalyst particles 32. Ionomer 36 of cathode CL 18 contacts catalysts 30 to form a
layer having catalyst particles 32 dispersed throughout. As illustrated, cathode CL 18 has a
porous structure which enables water to be removed from the system and gas to move
through cathode CL 18. In this example, cathode CL 18 is formed of one discrete catalyst
layer. In one example, cathode CL 18 is between about 2 and about 1 microns thick.
Catalyst supports 34 support catalyst particles 32. In one example, catalyst
supports 34 are formed from activated carbon or carbon black, such as etj en Black (KB).
Catalyst supports 34 generally have a diameter between about 10 nanometers and about 100
nanometers.
Catalyst particles 32 are deposited on catalyst supports 34. Catalyst particles
32 promote the oxidation reduction reaction (ORR) according to the oxidation reduction
reaction: 0 2 + 4H+ + 4e~® 2H 0 . For example, catalyst particles 32 can be platinum and
alloys thereof. To maximize the activated catalyst surface area, catalysts 30 can be formed
with a plurality of catalyst particles 32 on a single catalyst support 34. Alternatively,
catalysts 30 can be formed of a plurality of catalyst supports 34 supporting finely dispersed
catalyst particles 32.
Ionomer 36 in cathode CL 18 connects electrolyte 16 to catalyst particles 32
on an ionic conductor level. As illustrated in FIG. 2, ionomer 36 creates a scaffolding
structure between catalyst supports 34 of catalysts 30. Ionomer 36 creates a porous
structure that enables gas to travel through cathode CL 18 and water to be removed from
cathode CL 18. Ionomer 36 also transfers protons from electrolyte 16 to active catalyst sites
on catalyst particles 32.
Due to the high cost of platinum, electrodes with low platinum loads are
desirable. However, merely reducing the amount of platinum in the catalyst layer of an
electrode yields an inefficient electrode and fuel cell. Reducing the amount of platinum in
an electrode increases the kinetic overpotential consistent with the Tafel equation.
Additionally, oxygen transport losses also increase as the platinum loading is reduced. This
additional transport loss appears to be inversely proportional to the platinum surface area.
Applicants discovered that reducing the ionomer-to-carbon ratio of the
electrode minimizes some of the drawbacks of reduced platinum loading and provides a low
platinum load electrode with acceptable performance and efficiency. Typically, lowering
the ionomer-to-carbon ratio leads to an increase in ohmic losses at the electrode due to the
decreased ionomer concentration. However, Applicants unexpectedly found that reducing
the ionomer-to-carbon ratio in a low platinum load electrode did not produce the expected
ohmic losses.
Decreasing the ionomer-to-carbon ratio decreases the thickness of ionomer
film 36 surrounding catalysts 30. This decreased ionomer film thickness generally reduces
oxygen transport losses within the catalyst layer. At the same time, the reduced ionomer-tocarbon
ratio normally yields increased ohmic losses in the catalyst layer. However,
electrodes with low platinum loads generally have thinner catalyst layers than normal. For
example, in an electrode having a low platinum load (e.g., 0.1 mg/cm ), the catalyst layer is
usually very thin (about 2 microns to about 5 microns). A catalyst layer with 0.4 mg/cm of
platinum is about four times thicker than a catalyst layer with 0.1 mg/cm of platinum.
Ohmic losses scale with electrode thickness, thus a thinner catalyst layer provides an
electrode subject to diminished ohmic penalties. Applicants suspect that ohmic losses due
to the thickness of the catalyst layer play a smaller role in low platinum load (i.e. thinner)
electrodes and the primary contributor to ohmic losses occurs in the ionomer film around
catalysts 30. Additionally, gas transport losses are typically the limiting process in low
platinum load electrodes.
In electrodes having a normal platinum load (0.4 mg/cm ), the ionomer-tocarbon
ratio is usually greater than 1.2. While the optimum amount of ionomer 36 for a low
platinum load electrode depends on the operating conditions of the fuel cell, a particular
ionomer-to-carbon ratio range provides an efficient low platinum electrode. In one
embodiment of the present invention, the ionomer-to-carbon ratio of a catalyst layer is
between about 0.5 and about 0.9. In another embodiment, the ionomer-to-carbon ratio of a
catalyst layer is between about 0.6 and 0.8. In still another embodiment, the ionomer-tocarbon
ratio of a catalyst layer is between about 0.6 and 0.7. These values represent
commercially practicable ionomer-to-carbon ratios. While lower ionomer-to-carbon ratios
could prove more optimal theoretically, catalyst supports 34 must be fully coated with
ionomer 36. The minimum film thickness for ionomer 36 is about 3 nanometers (nm). This
minimum thickness essentially prevents workable ionomer-to-carbon ratios below about
0.5.
In addition to finding exemplary ionomer-to-carbon ratios for low platinum
load electrodes, Applicants examined ionomer equivalent weights. The equivalent weight
(EW) of an ionomer is the molecular weight that contains 1 mol of ionic groups and
indicates the ionic content of the polymer. A low EW ionomer has a high ionic content
relative to a high EW ionomer, and is therefore more conductive. Decreasing EW lowers
ionomer phase resistances, but may increase gas phase transport resistance due to flooding
or ionomer expansion into pores of the catalyst layer. Applicants experimental results
showed that ionomers with an EW less than 1100 had superior transport properties
compared with higher EW ionomers.
In one embodiment of the present invention, ionomer 36 of a catalyst layer is
a PFSA or hydrocarbon polymer having an EW between about 700 and about 1100. In
another embodiment, ionomer 36 is a PFSA or hydrocarbon polymer having an EW
between about 800 and about 900. In still another embodiment, ionomer 36 is a PFSA or
hydrocarbon polymer having an EW between about 820 and about 840. PFSA ionomers
have a high gas permeability, but also a high affinity for platinum. Thus, although oxygen
can permeate easily through the PFSA ionomer to catalyst particles 32, catalyst particles 32
also dissolve and move through the PFSA ionomer as ionic species. In contrast to PFSA
ionomers, hydrocarbon ionomers generally have a low gas permeability and a low solubility
for platinum. Additionally, hydrocarbon ionomers are thermally and dimensionally stable
ionomers, functioning as an effective scaffolding in porous cathode CL 18. Thus, a mixture
of PFSA and hydrocarbon ionomer with the PFSA preferentially coating catalyst particles
32 may be beneficial.
In addition to the above mentioned ionomer characteristics, Applicants also
studied the effects of varying the metal weight percent of the electrode (i.e. the percentage
of platinum in the electrode). Increasing the metal weight percent of the electrode (i.e.
increasing the relative amount of platinum present in the electrode) provides a thinner
electrode, which reduces the gas phase transport losses and ohmic losses. Gas phase
transport losses through the thickness of the electrode are relatively minor at low platinum
loading. Changing the metal weight percent of an electrode has no effect on oxygen
transport losses in the ionomer phase in contrast to generally accepted models of oxygen
transport in fuel cell catalyst layers.
In one embodiment of the present invention, the metal weight percent of a
carbon-supported catalyst layer is between about 20% and about 70%. For example,
cathode CL 18 contains carbon-supported platinum particles having between about 20
weight percent platinum with about 80 weight percent carbon and about 70 weight percent
platinum with about 30 weight percent carbon. The amount of ionomer added to the
carbon-supported platinum catalyst is adjusted proportionally to the amount of carbon to
achieve the desired ionomer-to-carbon ratio. In another embodiment, the metal weight
percent is between about 40% and about 60%. In still another embodiment, the metal
weight percent is about 50%. Increasing the metal weight percent above about 70% results
in a thinner catalyst layer, but may cause manufacturing difficulties or result in an uneven
loading of platinum in the catalyst layer. Decreasing the metal weight percent below about
20% results in a thicker catalyst layer that increases mass transport resistance and ohmic
losses.
Although adjusting each electrode characteristic studied by Applicants
(ionomer-to-carbon ratio, ionomer EW and catalyst layer metal weight percent) has
advantages and drawbacks, when these characteristics are controlled within the prescribed
ranges, the effect of any individual drawbacks is mitigated while amplifying the benefits of
the characteristics as a whole. Accordingly, by balancing each of the electrode
characteristics, drawbacks can be minimized to provide an efficient low platinum electrode.
For example, lowering the ionomer-to-carbon ratio reduces oxygen transport losses while
increasing ohmic losses. The increase in ohmic losses can be mitigated by using a high
platinum-to-carbon ratio to make the electrode thinner.
To summarize, the present invention provides an efficient low platinum load
electrode by utilizing a reduced ionomer-to-carbon ratio in the catalyst layer. Additionally,
the ionomer equivalent weight and the metal weight percent of the catalyst layer can be
modified to further increase the efficiency of a low platinum load electrode.
Although the present invention has been described with reference to
preferred embodiments, workers skilled in the art will recognize that changes may be made
in form and detail without departing from the spirit and scope of the invention.

CLAIMS:
1. An electrode for an electrochemical cell, the electrode comprising:
platinum catalysts;
carbon support particles for supporting the platinum catalysts; and
an ionomer connecting the platinum catalysts,
wherein the electrode has a platinum loading less than about 0.2 mg/cm and
an ionomer-to-carbon ratio between about 0.5 and about 0.9.
2. The electrode of claim 1, wherein the ionomer-to-carbon ratio is between
about 0.6 and about 0.8.
3. The electrode of claim 2, wherein the ionomer-to-carbon ratio is between
about 0.6 and about 0.7.
4. The electrode of claim 1, wherein the electrode has a metal weight percent
between about 20% and about 70%.
5. The electrode of claim 4, wherein the electrode has a metal weight percent of
between about 40% and about 60%.
6. The electrode of claim 5, wherein the electrode has a metal weight percent of
about 50%.
7. The electrode of claim 1, wherein the ionomer has an equivalent weight
between about 700 and about 1100.
8. The electrode of claim 7, wherein the ionomer has an equivalent weight
between about 800 and about 900.
9. The electrode of claim 8, wherein the ionomer has an equivalent weight
between about 820 and about 840.
10. The electrode of claim 1, wherein the electrode has a platinum loading
between about 0.05 mg/cm 2 and about 0.15 mg/cm2
. The electrode of claim 10, wherein the electrode has a platinum loading of
about 0.1 mg/cm .
12. The electrode of claim 1, wherein the electrode has a thickness between
about 2 microns and about 5 microns.
13. A membrane electrode assembly comprising:
a proton exchange membrane;
a cathode layer comprising:
platinum catalysts;
carbon support particles for supporting the platinum catalysts; and
an ionomer connecting the platinum catalysts,
wherein the cathode layer has a platinum loading less than about 0.2
mg/cm2 and an ionomer-to-carbon ratio between about 0.5
and about 0.9; and
an anode layer comprising:
platinum catalysts; and
carbon support particles for supporting the platinum catalysts; and
an ionomer connecting the platinum catalysts.
14. The membrane electrode assembly of claim 13, wherein the anode layer has
a platinum loading less than about 0.2 mg/cm and an ionomer-to-carbon ratio between
about 0.5 and about 0.9.
15. The membrane electrode assembly of claim 13, wherein the ionomer-tocarbon
ratio is between about 0.6 and about 0.8.
16. The membrane electrode assembly of claim 15, wherein the ionomer-tocarbon
ratio is between about 0.6 and about 0.7.
. The membrane electrode assembly of claim 13, wherein the electrode has a
metal weight percent of between about 40% and about 60%.
8. The membrane electrode assembly of claim 1 , wherein the electrode has a
metal weight percent of about 50%.
9. The membrane electrode assembly of claim 13, wherein the ionomer has an
equivalent weight between about 800 and about 900.
20. The membrane electrode assembly of claim 19, wherein the ionomer has an
equivalent weight between about 820 and about 840.