CA2802623A1 - Improved cathode for solid polymer electrolyte fuel cell - Google Patents

Abstract

Use of noble metal alloy catalysts, such as PtCo, as the cathode catalyst in solid polymer electrolyte fuel cells can provide enhanced performance at low current densities over that obtained from the noble metal itself. Unfortunately, the performance at high current densities has been relatively poor.
However, incorporating a small amount of polyvinylidene fluoride additive in the cathode formulation can provide superior performance at high current densities without detriment to performance at low current densities.

Description

Docket No.: P822619/CA/1 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.

Docket No.: P822619/CA/1 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

2 Docket No.: P822619/CA/1 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.

3 Docket No.: P822619/CA/1 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.

4 Docket No.: P822619/CA/i 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.

5 Docket No.: P822619/CA/1 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

6 Docket No.: P822619/CA/1 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

7 Docket No.: P822619/CA/l 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

8 Docket No.: P822619/CA/1 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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Claims (2)

WHAT IS CLAIMED IS:

1. A solid polymer electrolyte fuel cell comprising a solid polymer electrolyte, an anode, and a cathode wherein the cathode comprises a platinum cobalt alloy catalyst composition and an amount of polyvinylidene fluoride additive wherein the amount is less than 10% by weight.

2. A method of increasing the current density capability of a solid polymer electrolyte fuel cell at high rate, the fuel cell comprising a solid polymer electrolyte, an anode, and a cathode, the cathode comprising a platinum cobalt alloy catalyst composition, and the method comprising:
incorporating an amount of polyvinylidene fluoride additive in the cathode wherein the amount is less than 10% by weight.