Fuel Cells Operating in the “Gap” Temperature Regime

Eugene S. Smotkin

Department of Chemistry University of Puerto Rico @ Rio Piedras

San Juan, Puerto Rico 00931 Department of Chemical Engineering

Illinois Institute of Technology Chicago, IL 60616

Introduction

Low temperature fuel cells are based on solid polymer electrolyte membranes (PEMs), e.g. Nafion™. The thickness of Nafion™ membranes (25–175µm) permits compact series stacking of cells enabling high power and energy density systems. PEM electrolytes require sufficient hydration of the anchored sulfonate groups to support the Grottus hopping proton conduction mechanism (1). The maintenance of adequate hydration (e.g. about 3 water molecules per proton exchange site) requires precise water management, which sets the upper limit of practical PEM operation at about 90°C. Pt based electrocatalysts are poisoned by CO, which is an intermediate in the formation of hydrogen prepared by steam reforming of commodity fuels such as gasoline or methane. Although CO can be removed by the addition of a water-gas-shift (WGS) reactor and a preferential oxidation (PROX) unit to the reformer, the overall power density of the fuel cell system is substantially reduced and, more significantly, the interfacing of a 1000°C reformer to a 90°C fuel cell is daunting process control task. Finally, the kinetics of the four-electron oxygen reduction reaction (ORR) at the cathode of PEM fuel cells demands about 400 mV of polarization to obtain 1 mA/cm2 of Pt. The surface area of Pt catalysts is typically about 25 cm2 per mg. Thus low temperature fuel cell membrane electrode assemblies (MEAs) require substantial Pt loadings. A 50 kW traction power fuel cell requires over 125 g of Pt.

An intermediate temperature electrolyte system enabling fuel cell operation between 250°C and 400°C—i.e. in the gap between the PEM fuel cell and the molten carbonate fuel cell (MCFC)—has the benefits of enhanced ORR kinetics, CO tolerance, and a simplified fuel processor without the materials thermal instability problems of the high temperature systems. Additionally the gap temperature region enables downward scalability for portable power, a power regime not accessible by MCFCs. A “gap” electrolyte would radically change the paradigm of fuel cell engineering. Inorganic electronically insulating proton conductors (EIPCs) are the most likely candidates for gap electrolytes and are widely studied.(2, 3, 4, 5, 6, 7) The best inorganic EIPCs in the gap regime have conductivities ranging from 10-3 to 10-2 S/cm. For comparison the conductivity of hydrated Nafion 117 (175 µm) at 100ºC is approximately 10-1 S/cm.

Thus a membrane based on an inorganic proton conductor must be about 5–20 µm thick in order to achieve the same conductivity as Nafion™. Until now, there have been no reports of fuel cell performance using inorganic EIPCs in the gap range. Researchers have failed to prepare freestanding films of inorganic EIPCs that can withstand the mechanical stresses associated with fuel cell assembly. The EIPC must simultaneously serve as an electrolyte and as a separator of the fuel and oxidant streams.

We report the first demonstration of a fuel cell operating in the gap regime. The innovation is a support structure that enables the use of very thin inorganic EIPCs. Figure 1 shows a schematic of a fuel cell assembly using a supported EIPC. The composite electrolyte system is based on an EIPC supported on a thin metal hydride membrane—e.g. Pd (8), a Pd alloy (9), or a group Vb alloy coated

with a thin layer of Pd (10, 11, 12, 13)—that is strong, flexible, and has excellent hydrogen transport properties. The metal hydride is coated on one or both sides with a thin film of the inorganic EIPC. By themselves, neither the metal hydride foil nor the EIPC can make an acceptable fuel cell membrane. The metal alone, being an electronic conductor, would short circuit the cathode and anode; the EIPC alone has poor mechanical properties and may also be fuel-permeable. Together, the two components form an electronically insulating, mechanically strong, fuel impermeable thin membrane that is ideally suited to the intermediate temperature regime.

We exemplify the support structure using ammonium polyphosphate/silica as the EIPC and Pd as the metal hydride support structure. Ammonium polyphosphate, which has the maximum conductivity of 5 × 10-3 S/cm at 300ºC, is made proton-conductive by the thermal decomposition to polyphosphoric acid (2). Silica is added to the framework material to keep the composite material in the solid state up to 400ºC. While this EIPC has been studied in pellet form, until now, freestanding films have never been studied because the EIPC is brittle and difficult to work with.

Experimental

The composite membrane electrolyte was prepared as follows: ammonium polyphosphate and silica spheres were stirred in methanol for two days. The slurry was sprayed onto one side of a 25 µm palladium foil. The coated foil was sintered in an ammonium atmosphere to melt and mix the ammonium polyphosphate with the silica spheres. The thickness of the EIPC layer can be varied by repetitive spraying and sintering. In this study, the EIPC thickness is 20 µm. The hybrid membrane, with the EIPC facing the cathode oxygen flow fields, was assembled into a fuel cell with Pt electrodes (4 mg/cm2 Pt black with 10 wt% ammonium polyphosphate) contacting both faces of the ITEC system. The fuel cell assembly was placed in a press equipped with electrically insulated heating platens.

Figure 1. Schematic of an intermediate temperature hybrid electrolyte composite incorporated into a fuel cell. The metal hydride foil could be Pd, a Pd alloy, or a group Vb alloy coated with Pd. An EIPC layer covers the foil on one or both sides. The catalytic area is prepared in a “solid state ink” incorporated into the EIPC.

Results and Discussion Figure 2 shows the fuel cell performance at 250˚C using

humidified fuel. The fuel was either hydrogen or a reformate simulant containing 5% CO balanced with hydrogen. Both fuel compositions were studied with oxygen at the cathode. In both cases, the CO had no effect on the performance curve: The ITEC shows

Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2003, 48(2), 887

complete tolerance to CO. The use of air instead of pure O2 results in a decrease of OCV and cell performance due to the lower partial pressure. Increasing the air flow-rate from 100 sccm to 200 sccm causes an increase in the OCV from 0.66 V to 0.71 V.

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100 sccm H2/100 sccm O2100 sccm H2-CO (5%)/100 sccm O2100 sccm H2-CO (5%)/100 sccm Air100 sccm H2-CO (5%)/200 sccm Air

Figure 2. I-V curves obtained at 250°C for a hydrogen/oxygen fuel cell containing a poly-phosphate-silica (4:1) 20 µm thick EIPC on the cathode side of a 25µm thick Pd foil. Pt catalysts were used on both the anode and cathode sides of the cell. Conclusions We have demonstrated the first example of a fuel cell operating in the “gap” regime. Within the gap regime, the water-gas-shift reactor and the PROX reactor will not be needed. Further, the intermediate temperature operation will permit downward scalability for portable power, not available to higher temperature fuel cell systems such as the MCFC or solid oxide fuel cell (SOFC).

Acknowledgement. Funding for this work was provided by NuVant Systems Inc. and the Army Research Office through the STTR program. References 1. P. Atkins, Physical Chemistry, 6th ed., Freeman, New York, 1999, p. 741. 2. T. Kenjo, Y. Ogawa, Solid State Ionics, 76, 29 (1995). 3. Y. Du, A. S. Nowick, Solid State Ionics, 91, 85 (1996). 4. K. D. Kreuer, Solid State Ionics, 97, 1 (1997). 5. A. S. Nowick, Y. Du, Solid State Ionics , 77, 137 (1995). 6. B. Gross, et al, Solid State Ionics, 125, 107 (1999). 7. S. M. Haile, D. A. Boysen, C. R. I. Chisholm, R. B. Merle, Nature, 410, 910 (2001). 8. T. Graham, Phil. Trans. Roy. Soc., 156, 399 (1866). 9. J. B. Hunter, U. S. Patent, 2,773,561 (1956). 10. A. C. Makrides, M. A. Wright, D. N. Jewett, U. S. Patent 3,350,846 (1967). 11. C. Nishimura, M. Komaki, S. Hwang, M. Amano, Journal of Alloys and Compounds, 330-332, 902 (2002). 12. R. E. Bexbaum, A. B. Kinney, Ind. Eng. Chem. Res., 35, 530 (1996). 13. N. M. Peachey, R. C. Snow, R. C. Dye, J. Membrane Sci., 111, 123 (1996).

Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2003, 48(2), 888