WO2014105407A1 - Electrode and method of making the same - Google Patents

Abstract

Electrode comprising a pure Pt having a first limiting current and a Pt-Co alloy having a second, lower limiting current, wherein collectively the pure Pt and Pt-Co alloy have a limiting current is higher than predicted by the rules of mixture, wherein the limiting currents are measured using the Limit Current Test, and wherein the first limiting current is higher than the second limiting current. In some embodiments, electrodes described herein are cathodes (e.g., for fuel cells).

Description

ELECTRODE AND METHOD OF MAKING THE SAME

Cross Reference To Related Application

This application claims the benefit of U.S. Provisional Patent Application Number 61/746,729, filed December 28, 2012, the disclosure of which is incorporated by reference herein in its entirety.

Background

[0001 ] A typical fuel cell membrane-electrode assembly (MEA) contains one ion conducting membrane. On each side of that membrane is bonded an electrode. The anode electrode reacts with fuel. The cathode electrode reacts with air. Attached to each electrode is a gas diffusion layer (GDL) containing a microporous layer (MPL) and a backing layer. The GDL promotes air and fuel (hydrogen) diffusion to the electrodes, and aids in product water vapor and product liquid water away from the electrode. Fuel cell electrodes typically comprise catalyst supported on carbon and an ion conducting polymer (ionomer) .

[0002] Fuel cell catalysts are typically platinum or a platinum-alloy (e.g., Pt-Co) dispersed on a support; typically a carbon support. The dispersed catalyst typically has a thickness in a range from 1 nanometer to 10 nanometers. The carbon support typically has a thickness or diameter in a range from 5 nanometer to 100 nanometers. Often both the catalyst and the support are spheroid in shape.

[0003] Typical catalyst-coated membranes have two catalyst electrodes, an anode and a cathode, separated by a proton-conducting membrane.

[0004] It is desirable to achieve high fuel cell performance, for example, in terms of high activity (e.g., above about 0.15 A/ mg of Pt measured at 80°C cell temperature with hydrogen at the anode and oxygen at the cathode, 51.7 kPa (7.5 psi) gauge at anode and cathode, measured 0.9 V)) and good voltage (i.e., at least about 0.6 V per cell) at high current densities (i.e., at least 1 A/cm²) at relatively low cost. In particular, it is desirable to do so with low platinum content as the amount of platinum present significantly affects the cost. It is also desirable to have good durability in terms, for example, of the fuel cells being capable of operating at the desired performance levels for thousands of hours.

[0005] Platinum alloy (e.g., Pt-Co) catalysts, while achieving better activity than pure Pt, are known to exhibit decreased performance (e.g., lower cell voltage) at high current densities (i.e., above about 1

A/cm²). In another aspect, relatively large particle catalysts (i.e., greater than 3.5 nm) and/or heat treated catalysts (typically above about 1000°C) also tend to exhibit decreased performance at high current densities. On the other hand, smaller particle catalysts (i.e., less than 3.5 nm) and alloy catalysts tend to operate well at high current densities, but have decreased durability, Pure Pt catalysts tend to exhibit high current densities, but show lower activities than alloy catalysts.

Summary

[0006] In one aspect, the present disclosure describes an electrode (e.g., a cathode) comprising a pure Pt having a first limiting current and a Pt-Co alloy having a second, lower limiting current, wherein surprisingly collectively the pure Pt and Pt-Co alloy have a limiting current is higher than predicted by the rules of mixture, wherein the limiting currents are measured using the Limit Current Test, and wherein the first limiting current is higher than the second limiting current. In some embodiments, electrodes described herein are cathodes (e.g., for fuel cells).

[0007] In another aspect, the present disclosure describes a method of making electrodes described herein, the method comprising:

milling a mixture comprising at least two different catalysts on carbon support, an ionomer, a dispersing liquid, and grinding media to provide an ink;

coating the ink onto a substrate;

at least partially drying the coating; and

if the substrate is not a membrane, transferring the at least partially dried coating onto a membrane.

[0008] Uses of electrodes described herein include an electrode(s) in a fuel cell (e.g., proton electrode membrane (PEM) fuel cells). Fuel cells are useful, for example, automobiles, stationary power, and backup power.

[0009] Examples of advantages of embodiments of electrodes described herein include performance improvement in terms of relatively higher current densities at equivalent voltages compared to rule of mixture prediction. This in turns allows for a reduction in materials costs by creating similar power from a fuel cell containing less platinum. Another advantage is good durability at relatively low overall platinum content.

Brief Description of the Drawings

[0010] FIG. 1 is a schematic of an exemplary fuel cell having an electrode described herein.

[001 1] FIG. 2 shows the mass activities of Example 1 and Comparative Examples A and B at 0.9 V, 80°C, 100% relative humidity, and 7.5 psig (51.7 kPa-gauge).

[0012] FIGS. 3 and 4 show voltage-current behavior of Example 1 and Comparative Examples A and B at 70°C, 100% relative humidity, S=1.7/2.5, and 0 psig (0 kPa-gauge). [0013] FIGS. 5 and 6 show the voltage-current behavior of Example 2 and Comparative Examples C and D at 70°C, 100% relative humidity, S= 1.7/2.5, and 0 psig (0 kPa-gauge).

[0014] FIG. 7 shows the voltage-current behavior of Example 3 and Comparative Examples E and F at 70°C, 100% relative humidity, S=l .7/2.5, and 0 psig (0 Pa).

[0015] FIGS. 8 and 9 show the voltage-current behavior of Examples 4 and 5 and Comparative Examples G and H at 70°C, 100% relative humidity, S=2.5/3.4, and 25 psig (152 kPa-gauge).

Detailed Description

[0016] Surprisingly, for electrodes described herein, collectively the pure Pt and Pt-Co alloy has a higher limiting current than predicted by the rules of mixture. In some embodiments, collectively the pure Pt and Pt-Co alloy have a limiting current at least 5% (in some embodiments, at least 10%, 15%, 20%, or even at least 25%; in some embodiments, in a range from 5% to 25%, or 10% to 25%) higher than predicted by the rules of mixture (i.e., Pic_tu_re, = (P_A*X + P_B*(1-X))/2, where Pic_tu_re is the property of the mixture, P_A is the property component A, X is the weight fraction of component A, and P_B is the property component B).

[0017] The limiting current of an electrode (hereafter the "Limiting Current Test") can be measured for a single cell at 0.4 V at a cell temperature of 80°C where the anode inlet gas is H₂ at 100% relative humidity (RH) at flow a rate of 1.5 times (the stoichiometric amount) that of the cell current at a gas gauge pressure of 51.7 kPa. The cathode inlet gas is air at 100 % relative humidity (RH) at a flow rate 1.5 times (the stoichiometric amount) that of the cell current at a gas-gauge pressure of 51.7 kPa. Data is generated for at least 60 seconds starting at 0.1 A/cm² and increasing at 0.1 A/cm² intervals until a minimum voltage (<0.3V) is reached. Once the minimum voltage is reached, data is generated at decreasing 0.1 A/cm² intervals until 0.1 A/cm² is again reached.

[0018] The same type of fuel cell performance test can be used to evaluate catalyst performance properties at a higher stoichiometric amount for which there is no limiting current where the voltage of various samples are compared at specific current densities. Although not wanting to be bound by theory, it is believed differences in catalyst performance may be attributed to activity, tendency to flood, and spatial changes down the flow field. The details of the test conditions used to obtain a voltage-current behavior curve are as follows. Within a single operating fuel cell, at a cell temperature of 70°C, the anode inlet gas is H₂ flowing at a rate 2.5 times (the stoichiometric amount) that of the cell current at 173 kPa-gauge pressure and the cathode inlet gas is air flowing at a rate 3.4 times (the stoichiometric amount) that of the cell current at 152 kPa-gauge pressure. Data is generated for at least 300 seconds starting at 0.1 A/cm² and containing 1.9 A/cm² and 2.1 A/cm².

[0019] Electrodes described herein are useful, for example, in fuel cell catalysts (i.e., an anode or cathode catalyst). Referring to FIG. 1, fuel cell 10 includes first gas diffusion layer (GDL) 12 adjacent anode 14. Adjacent the anode 14 includes electrolyte membrane 16. Cathode electrode described here 18 is adjacent electrolyte membrane 16, and second gas diffusion layer 19 is adjacent the cathode 18. GDLs 12 and 19 can be referred to as diffuse current collectors (DCCs) or fluid transport layers (FTLs). In operation, hydrogen fuel is introduced into the anode portion of fuel cell 10, passing through first gas diffusion layer 12 and over anode 14. At anode 14, the hydrogen fuel is separated into hydrogen ions (H⁺) and electrons (e^~).

[0020] Electrolyte membrane 16 permits only the hydrogen ions or protons to pass through electrolyte membrane 16 to the cathode portion of fuel cell 10. The electrons cannot pass through electrolyte membrane 16 and, instead, flow through an external electrical circuit in the form of electric current. This current can power, for example, electric load 17, such as an electric motor, or be directed to an energy storage device, such as a rechargeable battery.

[0021] Oxygen flows into the cathode side of fuel cell 10 via second gas diffusion layer 19. As the oxygen passes over cathode 18, oxygen, protons, and electrons combine to produce water and heat. In some embodiments, the fuel cell catalyst comprises no electrically conductive carbon-based material (i.e., perylene red, fluoropolymers, or polyolefines).

[0022] "Pure Pt" refers to Pt having a purity of at least 99.5 wt.% platinum.

[0023] The Pt-Co alloy can be, for example, Pt-Co in nano-particle form and Pt-Co as nano-particles dispersed on a support (typically carbon). In some embodiments, the weight ratio of pure Pt to the Pt-Co alloy is in a range from 10:90 to 90: 10 (in some embodiments, in a range from 10:90 to 40:60; or even 20:80 to 30:70), although other values may also be useful.

[0024] In some embodiments of electrodes described herein, the pure Pt and Pt-Co alloy are in a single layer or respectively in separate, contacting layers. In some embodiments, for electrodes described herein. In some embodiments of the former, the pure Pt and Pt-Co alloy uniformly blended together.

[0025] In some embodiments of electrodes described herein, the first limiting current is at least 1.0 A/cm² (in some embodiments, at least 1.5 A/cm², 2.0 A/cm², or even at least 2.5 A/cm²; in some embodiments range from 1.0 A/cm² to 2.5 A/cm²), although other values may also be useful. In some embodiments, the second limiting current is not greater than 2.5 A/cm² (in some embodiments, not greater than 2.0 A/cm², 1.5 A/cm², 1.0 A/cm², or even not greater than 0.5 A/cm²; in some embodiments, in a range from 0.5 A/cm² to 2.5 A/cm²), although other values may also be useful.

[0026] In some embodiments of electrodes described herein, the electrodes have a total Pt loading of at least 0.05 mg Pt/cm² (in some embodiments, at least 0.1 mg Pt/cm², 0.15 mg Pt/cm², 0.2 mg Pt/cm², 0.25 mg Pt/cm², or even at least 0.5 mg Pt/cm²; in some embodiments range from 0.05 Pt/cm² to 0.5 Pt/cm²), although other values may also be useful. [0027] In some embodiments of electrodes described herein, the electrodes have a thickness in a range from 0.5 micrometer to 100 micrometers (in some embodiments, in a range from 1 micrometer to 25 micrometers, or even 3 micrometer to 15 micrometers), although other values may also be useful.

[0028] In another aspect, the present disclosure describes a method of making electrodes described herein, the method comprising:

milling (e.g., ball milling) a mixture comprising at least two different catalysts on carbon support, an ionomer, a dispersing liquid, and grinding media to provide an ink;

coating the ink onto a substrate (e.g., a liner, a membrane, or a gas diffusing layer (GDL)); at least partially drying the coating; and

if the substrate is not a membrane, transferring the at least partially dried coating onto a membrane.

[0029] Suitable grinding media for mixing at least the two different catalysts on carbon support, the ionomer, and the dispersing liquid include zirconia and tungsten grinding media.

[0030] For fuel cell applications, for example, the platinum metal is typically dispersed on a support (typically a carbon support). Carbon supports typically range from 5 nm to 100 nm in diameter, although sizes outside this range may also be useful. Platinum metal dispersed on these supports typically ranges from 1 nm to 5 nm (in some embodiments as much as 10 nm), although sizes outside this range may also be useful. Although not wanting to be bound by theory, it is believed that advantages to dispersing Pt on carbon include increasing exposed Pt metal surface area vs. Pt catalysts without support, improving fuel cell mass transport within the electrode. Typically carbon supports are spheroid in shape, although other forms include carbon fibers (including nanotubes) and non-carbon supports (metal oxides including Sn0₂, Zr0₂, and Ti0₂). Exemplary carbon fibers have aspect ratios from 1.5: 1 to 100: 1, although sizes outside this range may also be useful. Mass activities of Pt electrodes as measured in a fuel cell with H₂ gas in the anode and 0₂ gas in the cathode, a cell temperature of 80°C and gas pressure of 7.5 psi (51.7 kPa)-gauge can range from 0.05 A/mg Pt to 0.3 A/mg Pt measured at 0.9 V.

[0031 ] For fuel cell applications, for example, platinum metal is frequently alloyed with other materials to improve fuel cell properties such as activity and/or durability. One common alloying metal is cobalt. Typical atomic ratios of platinum to cobalt are in a range from 2: 1 to 5: 1 (in some

embodiments, in a range from 2:3 to 20: 1), although sizes outside this range may also be useful. For fuel cell applications, for example, platinum:cobalt alloys (Pt:Co) are typically dispersed on a support (typically a carbon support). Carbon supports typically range from 5 nm to 100 nm in diameter in some embodiments, 1 nm to 5 nm, and can be as much as 10 nm), although sizes outside this range may also be useful. Although not wanting to be bound by theory, it is believed that advantages to dispersing Pt:Co on carbon include increasing exposed P:Co metal surface area vs. P:Co catalysts without support, improving fuel cell mass transport within the electrode. Typically carbon supports are spheroid in shape, although other forms include carbon fibers (including nanotubes) and non-carbon supports (metal oxides including SnC>₂, ZrC>₂, and T1O₂). Exemplary carbon fibers have aspect ratios from 1.5 to 100, although sizes outside this range may also be useful. Mass activities of Pt:Co electrodes as measured in a fuel cell with H₂ gas in the anode and 0₂ gas in the cathode, a cell temperature of 80°C and gas pressure of 7.5 psi (51.7 kPa)-gauge can range from 0.20 A/mg Pt to 0.70 A/mg Pt measured at 0.9 V.

[0032] An ionomer, or ion-conducting polymer, is a polymeric substance capable of conducting protons. These are frequently used in fuel cells and especially proton-exchange membrane (PEM) fuel cells to conduct protons to and from catalyst reaction sites. For example, cathode electrodes consume electrons, protons and oxygen as part of the fuel cell's overall series of reactions that create current and power. Ionomers in electrodes also can act, for example, as a binder and adhesive, binding the fuel cell catalyst locally to additional carbon catalyst to create the larger electrode network (typically a blend of ionomer, catalyst and gas pores), and also binding the electrode or electrodes to other layers of the fuel cell. These additional layers can be, for example, gas diffusion layers (GDLs), membranes, microporous layers and additional fuel cell electrode layers.

[0033] Exemplary ionomers are known, for example in the art for fuel cells.. One exemplary ionomer can be made, for example, making a tetrafluoroethylene (TFE) and FS02-CF2CF2CF2CF2-0-CF=CF2 co-monomer A as described in U.S. Pat. Pub. No. US 2004-01 16742 Al (Guerra)) and U.S. Pat. No. 6,624,328 (Guerra), the disclosures of which are incorporated herein by reference, and polymerizing the co-monomer by aqueous emulsion polymerization as described in U.S. Pat. No. 7,348,088 (Hamrock et al.), the disclosure of which is incorporated herein by reference. Typically the ionomer has an equivalent weight in a range from 600 to 1200, although equivalent weights outside of this range may also be useful.

[0034] Fuel cell electrodes are typically created from an ink containing electrode components such as catalyst and ionomer. To facilitate good mixing, and good coatability, these inks contain dispersing liquids. Exemplary dispersing liquids include water, various alcohols (e.g., N-Propanol, ethanol, methanol, and iso-propanol), and other dispersing agents such as surfactants.

[0035] In some embodiments, the collective ratio of Pt on carbon catalyst and Pt-Co on carbon catalyst milled together is in a range from 25:75 to 75:25 (in some embodiments, is in a range from 10:90 to 90: 10).

[0036] The catalysts on carbon support, ionomer, and dispersing liquid are milled (e.g., ball milled) until the mixture suitably well mixed providing an ink.

[0037] The ink is typically coated on a substrate and then at least partly dried to open up pores for gas transport to catalyst sites within the electrode. The ink can be coated by any of a variety of techniques known in the art, including dye coating, transfer coating, spraying, and brushing. [0038] Exemplary substrates include gas diffusing layers (GDLs), fuel cell membranes, and liners known in the art. When coating on a liner, the electrode, once partially dried, is then typically transferred from the liner to either a fuel cell membrane or GDL.

[0039] Typically the GDL is comprised of sheet material comprising carbon fibers. Typically the GDL is a carbon fiber construction selected from woven and non-woven carbon fiber constructions.

Suitable commercially carbon fiber constructions are available, for example, under the trade designation "TORAY CARBON PAPER" from Toray Inc., Tokyo Japan; "SPECTRACARB CARBON PAPER" from Spectracorp, Shelton, CT; and "ZOLTEK CARBON CLOTH" from Zoltek Corp., St. Louis, MO. The GDL may be coated or impregnated with various materials, including carbon particle coatings, hydrophilizing treatments, and hydrophobizing treatments (e.g., a coating with polytetrafluoroethylene (PTFE)).

[0040] The PEM according to the present disclosure may comprise any suitable polymer electrolyte. The polymer electrolytes useful in the present disclosure typically bear anionic functional groups bound to a common backbone, which are typically sulfonic acid groups but may also include carboxylic acid groups, imide groups, amide groups, or other acidic functional groups. The polymer electrolytes useful in the present disclosure are highly fluorinated and most typically perfluorinated. The polymer electrolytes useful in the present disclosure are typically copolymers of tetrafluoroethylene and at least one fluorinated, acid-functional co-monomers. Typical polymer electrolytes (available, for example, under the trade designation "NAFION" from DuPont Chemicals, Wilmington, DE, and "FLEMION" from Asahi Glass Co. Ltd., Tokyo, Japan). The polymer electrolyte may be a copolymer of tetrafluoroethylene (TFE) and FS02-CF₂CF2CF₂CF2-0-CF=CF2, described in U.S. Pat. Pub. No. US 2004-01 16742 Al (Guerra) and U.S. Pat. Nos. 6,624,328 (Guerra) and 7,348,088 (Hamrock et al.), the disclosures of which are incorporated herein by reference. The polymer typically has an equivalent weight (EW) of no greater than 1200 (in some embodiments, no greater than 1 100, 1000, 900, or even not greater than 800), which is often observed to exhibit improved performance in comparison to the use of higher EW polymer.

[0041 ] The polymer can be formed into a membrane by any suitable method. The polymer is typically cast from a suspension. Exemplary casting methods include bar coating, spray coating, slit coating, and brush coating. The membrane may also be formed, for example, from neat polymer in a melt process (e.g., extrusion). After forming, the membrane may be annealed, typically at a temperature of at least 120 C (in some embodiments, at least 130 C, 140 C, or even at least 150 C). In some embodiments, additives are added to the membrane only after annealing and not before, and therefore annealing conditions are not impacted by their presence, which may, for example, raise membrane glass transition, Tg, thus necessitating higher annealing temperatures.

[0042] The PEM typically has a thickness of not greater than 50 micrometers (in some embodiments, not greater than 40 micrometers, 30 micrometers, or even not greater than 25 micrometers. [0043] In some embodiments, at least one manganese salt is added to the polymer electrolyte of the PEM prior to, during or after membrane formation. Exemplary manganese salts may comprise any suitable anion, including chloride, bromide, nitrate, or carbonate. Once cation exchange occurs between the transition metal salt and the acid form polymer, it may be desirable for the acid formed by combination of the liberated proton and the original salt anion to be removed. Thus, it may be desirable to use anions that generate volatile or soluble acids (e.g., chloride or nitrate). Manganese cations may be in any suitable oxidation state, but are most typically Mn^⁺. Although not wanting to be bound by theory, it is believed that the manganese cations persist in the polymer electrolyte because they are exchanged with H⁺ ions from the anion groups of the polymer electrolyte and become associated with those anion groups. Furthermore, it is believed that polyvalent manganese cations may form crosslinks between anion groups of the polymer electrolyte, further adding to the stability of the polymer.

Typically, the amount of salt added is in a range from 0.001 to 0.5 (in some embodiments, 0.005 to 0.2,

0.01 to 0.1, or even 0.02 to 0.05) charge equivalents, based on the molar amount of acid functional groups present in the polymer electrolyte.

[0044] In some embodiments, the PEM may further comprises a porous support (e.g., a layer of expanded polytetrofluoroethylene (PTFE)), where the pores of the porous support contain the polymer electrolyte. In some embodiments, the PEM comprises no porous support. In some embodiments, the PEM comprises a crosslinked polymer.

Exemplary Embodiments

1. An electrode (e.g., a cathode) comprising a pure Pt having a first limiting current and a Pt-Co alloy having a second, lower limiting current, wherein collectively the pure Pt and Pt-Co alloy have a limiting current is higher than predicted by the rules of mixture, wherein the limiting currents are measured using the Limit Current Test, and wherein the first limiting current is higher than the second limiting current.

2. The electrode of Exemplary Embodiment 1, wherein collectively the pure Pt and Pt-Co alloy have a limiting current at least 5% (in some embodiments, at least 10%, 15%, 20%, or even at least 25%; in some embodiments, in a range from 5% to 25%, or 10% to 25%) higher than predicted by the rules of mixture.

3. The electrode of either Exemplary Embodiment 1 or 2, wherein the weight ratio of the pure Pt to the Pt-Co alloy is in a range from 10:90 to 90: 10 (in some embodiments, in a range from 10:90 to 40:60; or even 20:80 to 30:70). 4. The electrode of any preceding Exemplary Embodiment, wherein the pure Pt and Pt-Co alloy are in a single layer.

5. The electrode of any of Exemplary Embodiments 1 to 3, wherein the pure Pt and Pt-Co alloy are respectively in separate, contacting layers.

6. The electrode of any preceding Exemplary Embodiment, wherein the first limiting current is at least 1.0 A/cm² (in some embodiments, at least 1.5 A/cm², 2.0 A/cm², or even at least 2.5 A/cm²; in some embodiments range from 1.0 A/cm² to 2.5 A/cm²).

7. The electrode of any preceding Exemplary Embodiment, wherein the second limiting current is not greater than 2.5 A/cm² (in some embodiments, not greater than 2.0 A/cm², 1.5 A/cm², 1.0 A/cm², or even not greater than 0.5 A/cm²; in some embodiments, in a range from 0.5 A/cm² to 2.5 A/cm²).

8. The electrode of any preceding Exemplary Embodiment having a total Pt loading of at least 0.05 mg Pt/cm² (in some embodiments, at least 0.1 mg Pt/cm², 0.15 mg Pt/cm², 0.2 mg Pt/cm², 0.25 mg Pt/cm², or even at least 0.5 mg Pt/cm²; in some embodiments range from 0.05 Pt/cm² to 0.5 Pt/cm²).

9. The electrode of any preceding Exemplary Embodiment having a thickness in a range from 0.5 micrometer to 100 micrometers (in some embodiments, in a range from 1 micrometer to 25 micrometers, or even 3 micrometer to 15 micrometers).

10. The electrode of any preceding Exemplary Embodiment which is a cathode.

1 1. A fuel cell comprising an electrode of any preceding Exemplary Embodiment.

12. A method of making an electrode of any of Exemplary Embodiments 1 to 10, the method comprising:

milling (e.g., ball milling) a mixture comprising Pt on carbon, Pt-Co on carbon, an ionomer, a dispersing liquid (e.g., an organic solvent (e.g., N-propanol)), and grinding media to provide an ink; coating the ink onto a substrate (e.g., a liner, a membrane, or a gas diffusing layer); at least partially drying the coating; and

if the substrate is not a membrane, transferring the at least partially dried coating onto a membrane.

13. The method of Exemplary Embodiment 12, wherein the collective carbon content of the Pt on carbon and Pt-Co on carbon is in a range from 25:75 to 75:25 (in some embodiments, is in a range from 10:90 to 90: 10).

[0045] Advantages and embodiments of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention. All parts and percentages are by weight unless otherwise indicated. Materials

[0046] Unless otherwise noted, all reagents were obtained or are available from Aldrich Chemical Co., Milwaukee, WI, or may be synthesized by known methods.

Ionomer

[0047] The ionomer used in catalyst inks and membranes in each of the following Examples and

Comparative Examples was a copolymer of tetrafluoroethylene (TFE) and FSO2-CF2CF2CF2CF2-O-

CF=CF2 (co-monomer). The co-monomer was made according to the procedures disclosed in U.S. Pat.

Appl. having Serial. No. 12/834,531, filed, July 12, 2010, the disclosure of which is incorporated herein by reference. Polymerization was performed by aqueous emulsion polymerization as described in U.S. Pat. No. 7,348,088 (Hamrock et al.), the disclosures of which are incorporated herein by reference. The equivalent weight (EW) of the ionomer was 800 or 1000 as indicated.

Catalyst Ink Preparation

[0048] Catalyst inks were made by ball milling desired carbon-supported platinum or platinum-alloy catalyst(s) with ionomer (prepared as described above) and water in to the ionomer/catalyst ratio by weight specified in each Example and a solids content of about 20% by weight. The catalyst ink was mixed via balling milling with 6 mm ceramic beads until a uniform mixture having a viscosity of about 100- 10000 centipoise was obtained. Polymer Electrolyte Membranes (PEM) Preparation

[0049] Polymer electrolyte membranes (PEM's) were made from the 800 EW ionomer. The ionomer was diluted with 70:30 by weight N-propanol/water to provide in a casting solution containing 22.3 wt % solids. Manganese nitrate Mn(NC>3)2 was added to the casting solution in an amount equal to 0.035 charge equivalents based on the molar amount of anionic functional groups present in the polymer electrolyte in accord with the procedures disclosed in U.S. Pat. No. 7,572,534 (Frey et al.), the disclosure of which is incorporated herein by reference. Membranes were cast at a wet thickness of about 400 to 500 micrometers, onto a substrate of PET (polyethylene terephthalate) or polyimide (obtained under the trade designation "KAPTON" from E. I. du Pont de Nemours and Company, Wilmington, DE) as specified in the Example. The castings were dried at 80°C-100°C, and then annealed at 160°C-200°C for about 3 to 5 minutes. After cooling, the membranes were peeled form the liner and used without further purification. The final membrane thickness was 0.8 mil (20 micrometers).

Membrane Electrode Assemblies (MEA) Preparation

[0050] Membrane electrode assemblies (MEA's) having 50 cm^ of active area were made by addition of a catalyst coated backing (CCB), which was a gas diffusion layer (GDL) coated with catalyst ink (prepared as described above), to opposite faces of the PEM followed by addition of a gasket to each face, as detailed below.

[0051] GDL's were made by applying a microporous polytetrofluoroethylene (PTFE) suspension to a non-woven carbon fiber paper followed by application of a carbon particle-polytetrofluoroethylene microporous layer (MPL), as disclosed in U.S. Pat. No. 7,608,334 (Frisk et al.), the disclosure of which is incorporated herein by reference.

[0052] Anode catalyst inks were prepared as described above using the 1000 EW co-monomer and carbon-supported platinum catalyst (obtained under the trade designation "10V30E"; 30 wt.% Pt supported on a high surface area carbon "VULCAN XC72" from Tanaka Kikinzoku, Tokyo, Japan) at an ionomer to catalyst weight ratio of about 0.8. Anode catalyst inks were hand-painted on one face of a liner at a loading of 0.1 mg Pt/crn^ and then were annealed in a vacuum oven for 30 minutes at 150°C and 7 psi (48.2 kPa) pressure (absolute) before transferring onto a face of the GDL to make anode CCB's.

[0053] Cathode CCB's were prepared similarly to the anode CCB's except that the catalyst inks for Examples and Comparative Examples were varied as described below at a loading of 0.1 mg Pt/crn^ or 0.1 mg Pt-alloy/cm^.

[0054] CCB's and polytetrafluoroethylene/glass composite gaskets were applied to the PEM by pressing using a press obtained from Fred Carver Co., Wabash, IN, with 13.4 kN of force at 132°C for 10 minutes.

MEA Performance Evaluation

[0055] MEA's of Examples and Comparative Examples were mounted in a test cell station (obtained from Fuel Cell Technologies, Inc., Albuquerque, NM). The test station included a variable electronic load with separate anode and cathode gas handling systems to control gas flow, pressure, and humidity. The electronic load and gas flows were computer controlled. Fuel cell polarization curves were obtained after 3-4 hours of the following cell conditioning/break-in: electrode area of 50 crn^; anode gas pressure of 0 psig (0 Pa); cathode gas pressure of 0 psig (0 Pa), anode stoichiometric flow rate 1.7 (i.e., 1.7 times the moles of gas required to produce the desired electric current) and cathode stoichiometric flow rate 2.5 (i.e., S=1.7/2.5). Humidification of the cathode and anode was provided by steam injection (injector temperature of 120°C). Operating temperature and relative humidity (RH) were controlled as indicated in the Example or Comparative Example. The Limiting Current Test was used as indicated in the Examples and Comparative Examples.

Example 1 and Comparative Examples A and B

[0056] Example 1 and Comparative Examples A and B MEA's were prepared using the processes described above and varying the cathode catalyst ink used.

[0057] For Comparative Example A the catalyst ink was prepared using the 1000 EW ionomer and carbon-supported platinum catalyst (obtained under the trade designation "10F50E-HT"; 50 wt.% Pt supported on a F carbon having a surface area of 800 m²/g) from Tanaka Kikinzoku) at an ionomer to catalyst weight ratio of about 1.2.

[0058] For Comparative Example B the catalyst ink was prepared using the 1000 EW ionomer and carbon-supported platinum-cobalt catalyst (obtained under the trade designation "36F32-HT2"; 30 wt.% Pt-Co supported on a F carbon having a surface area of 800 m²/g obtained from Tanaka Kikinzoku) at an ionomer to catalyst weight ratio of about 1.2. Comparative Example B was replicated as well (i.e., prepared and test twice).

[0059] For Example 1 the catalyst ink was prepared using the 1000 EW ionomer and a 50:50 by weight blend of carbon-supported platinum catalyst ("10F50E-HT") and carbon-supported platinum-cobalt ("36F32-HT2") at an ionomer to catalyst weight ratio of about 1.2.

[0060] Example 1 and Comparative Examples A and B MEA performances were evaluated using methods described above under indicated test conditions, including the Limiting Current Test. Results are presented in FIGS. 2-4. Example 2 and Comparative Examples C and D

[0061] Example 2 and Comparative Examples C and D MEA's were prepared using the processes described above and varying the cathode catalyst ink used.

[0062] For Comparative Example C the catalyst ink was prepared using the 1000 EW ionomer and carbon-supported platinum catalyst (obtained under the trade designation "SA50BK"; 50 wt.% Pt supported on carbon ("KETJIN") having a surface area of 800 m²/g from Catalysts BASF Corporation, Iselin, NJ) at an ionomer to catalyst weight ratio of about 1.0.

[0063] For Comparative Example D the catalyst ink was prepared using 1000 EW ionomer and carbon- supported platinum-cobalt catalyst (obtained under the trade designation "36E32"; 30 wt.% Pt-Co supported on carbon ("KETJIN") having a surface area of 800 m²/g from Tanaka Kikinzoku) at an ionomer to catalyst weight ratio of about 1.0.

[0064] For Example 2 the catalyst ink was prepared using the 1000 EW ionomer and a 50:50 by weight blend of carbon-supported platinum catalyst ("SA50BK") and carbon-supported platinum-cobalt catalyst ("36E32") at an ionomer to catalyst weight ratio of about 1.0.

[0065] Example 2 and Comparative Examples C and D MEA performances were evaluated using methods described above under indicated test conditions, including the Limiting Current Test. Results are presented in FIGS. 5 and 6.

Example 3 and Comparative Examples E and F

[0066] Example 3 and Comparative Examples E and F MEA's were prepared using the processes described above and varying the cathode catalyst ink used.

[0067] For Comparative Example E the catalyst ink was prepared using the 800 EW ionomer and carbon-supported platinum-cobalt catalyst (obtained under the trade designation "36F32"; 30 wt.% Pt- Co supported on F carbon having a surface area of 800 m²/g from Tanaka Kikinzoku) at an ionomer to catalyst weight ratio of about 1.2.

[0068] For Comparative Example F the catalyst ink was prepared using the 800 EW ionomer and carbon-supported platinum-cobalt catalyst ("36F32-HT2") at an ionomer to catalyst weight ratio of about 1.2.

[0069] For Example 3 the catalyst ink was prepared using the 800 EW ionomer and a 50:50 by weight blend of platinum-cobalt catalyst ("36F32") and carbon-supported platinum-cobalt catalyst ("36F32- HT2") at an ionomer to catalyst weight ratio of about 1.2.

[0070] Example 3 and Comparative Examples E and F MEA performances were evaluated using methods described above under indicated test conditions, including the Limiting Current Test. Results are presented in FIG. 7.

Examples 4 and 5 and Comparative Examples G and H

[0071] Examples 4 and 5 and Comparative Examples G and H MEA's were prepared using the processes described above and varying the cathode catalyst ink used.

[0072] For Comparative Example G (which was the same as Comparative Example F above) the catalyst ink was prepared using the 800 EW ionomer and carbon-supported platinum-cobalt catalyst ("36F32-HT2") at an ionomer to catalyst weight ratio of about 1.2.

[0073] For Comparative Example H the catalyst ink was prepared using the 800 EW ionomer and carbon-supported platinum catalyst ("10F50E") at an ionomer to catalyst weight ratio of about 1.2.

[0074] For Example 4 the catalyst ink was prepared using the 800 EW ionomer and a 50:50 by weight blend of carbon-supported platinum-cobalt catalyst ("36F32-HT2") and carbon-supported platinum catalyst ("10F50E") at an ionomer to catalyst weight ratio of about 1.2.

[0075] For Example 5 the catalyst ink was prepared using the 800 EW ionomer and a 75:25 by weight blend of carbon-supported platinum-cobalt catalyst ("36F32-HT2") and carbon-supported platinum catalyst ("10F50E") at an ionomer to catalyst weight ratio of about 1.2.

[0076] Examples 4 and 5 and Comparative Examples G and H MEA performances were evaluated using methods described above under indicated test conditions, including the Limiting Current Test. Results are presented in FIGS. 8 and 9.

Examples 6-8

[0077] Examples 6 and 8 MEA's were prepared using the processes described above and varying the cathode catalyst ink used. Examples 6-8 were not tested.

[0078] For Example 6 the catalyst ink was prepared using the 800 EW ionomer and a 50:50 by weight blend of carbon-supported platinum-cobalt catalyst ("36F32") and carbon-supported platinum catalyst ("10F50E") carbon-supported catalyst at an ionomer to catalyst weight ratio of about 1.2.

[0079] For Example 7 the catalyst ink was prepared using 800 EW ionomer and a 50:50 by weight blend of carbon-supported platinum catalyst ("10F50E-HT") and carbon-supported platinum catalyst ("10F50E") at an ionomer to catalyst weight ratio of about 1.2.

[0080] For Example 8 the catalyst ink was prepared using the 800 EW ionomer and a 50:50 by weight blend of carbon-supported platinum-cobalt catalyst ("36E32") and carbon-supported platinum catalyst (" 10F50E") at an ionomer to catalyst weight ratio of about 1.2. [0081] Foreseeable modifications and alterations of this disclosure will be apparent to those skilled in the art without departing from the scope and spirit of this invention. This invention should not be restricted to the embodiments that are set forth in this application for illustrative purposes.

Claims

What is claimed is:

1. An electrode comprising a pure Pt having a first limiting current and a Pt-Co alloy having a second, lower limiting current, wherein collectively the pure Pt and Pt-Co alloy have a limiting current is higher than predicted by the rules of mixture, wherein the limiting currents are measured using the Limit Current Test, and wherein the first limiting current is higher than the second limiting current.

2. The electrode of claim 1, wherein collectively the pure Pt and Pt-Co alloy have a limiting current at least 5% higher than predicted by the rules of mixture.

3. The electrode of either claim 1 or 2, wherein the weight ratio of the pure Pt to the Pt-Co alloy is in a range from 10:90 to 90: 10.

4. The electrode of any preceding claim, wherein the pure Pt and Pt-Co alloy are in a single layer.

5. The electrode of any of claims 1 to 3, wherein the pure Pt and Pt-Co alloy are respectively in separate, contacting layers.

6. The electrode of any preceding claim, wherein the first limiting current is at least 1.0 A/cm².

7. The electrode of any preceding claim, wherein the second limiting current is not greater than 2.5 A/cm².

8. The electrode of any preceding claim having a total Pt loading of at least 0.05 mg Pt/cm².

9. The electrode of any preceding claim having a thickness in a range from 0.5 micrometer to 100 micrometers.

The electrode of any preceding claim which is a cathode.

1 1. A fuel cell comprising an electrode of any preceding claim.

12. A method of making an electrode of any of claims 1 to 10, the method comprising:

milling a mixture comprising Pt on carbon, Pt-Co on carbon, an ionomer, a dispersing liquid, and grinding media to provide an ink;

coating the ink onto a substrate;

at least partially drying the coating; and

if the substrate is not a membrane, transferring the at least partially dried coating onto a membrane.

13. The method of claim 12, wherein the collective carbon content of the Pt on carbon and Pt-Co on carbon is in a range from 25:75 to 75:25.