Polymeric Membranes for Fuel Cells: Overview and Future Outlook
Peter N. Pintauro and Ryszard Wycisk
Department of Chemical Engineering
Case Western Reserve University Cleveland, OH 44106
VOICE: 216.368.4150; FAX: 216.368.3016; E-MAIL: pnp3@case.edu
Polymeric membranes play a crucial role during electricity generation in hydrogen/air and direct methanol proton-exchange membrane (PEM) fuel cells. The membrane in such devices performs two roles: It separates the positive and negative electrodes and provides a conduit for ion (proton) movement between the electrodes. Consequently, there is considerable research and development around the world to develop new membrane materials with tailored physical and transport properties. For hydrogen/air proton-exchange membrane fuel cells, the membrane must exhibit low gas permeability and high proton conductivity. For a direct liquid methanol PEM fuel cell, the ion-exchange membrane must conduct protons and be a good methanol barrier. DuPont’s Nafion TM (a perfluoro sulfonic acid polymer) is the membrane of choice for H 2/air PEM fuel cells that operate at a temperature of about 80 oC. At temperatures above 100 oC, Nafion looses water and conductivity unless the water activity in the gas feed is near unity. Nafion has also been used in direct methanol fuel cells; it works well with diluted methanol, but methanol crossover is high at methanol feed concentrations > 0.5 M. Current membrane fuel cell work is focused on: (i) proton conductors that operate at a temperature of about 120 oC under low humidity conditions (25% relative humidity) for hydrogen/air fuel cells, (ii) polymeric membranes that conduct protons in the dry state during H 2/air fuel cell operation at high temperatures (160-200 oC), and (iii) proton conducting ion-exchange membranes with very low methanol permeability for operating temperatures in the range of 60-140 oC and high methanol feed concentrations.
Schematic diagrams of H 2/air and direct methanol PEM fuel cells are shown in Figures 1 and 2. The key component of these devices is a membrane-electrode-assembly (MEA), which consists of an ion-exchange membrane with catalytic powder electrodes attached to the two membrane faces.
Figure 1 – Schematic diagram of a H 2/air Figure 2 – Schematic diagram of a PEM fuel cell. direct methanol PEM fuel cell.
The general required properties of an ion-exchange membrane for use in a PEM fuel cell are: low electrical resistance under cell operating conditions, long-term chemical stability at operating temperature in oxidizing and reducing environments, good mechanical strength (preferably with resistance to solvent/water swelling), low gas (oxidant and fuel) cross-over, interfacial compatibility with catalyst layers, and low cost.
Work that has led to some decrease in methanol crossover for a DMFC include: investigations of new polymers, incorporating inorganic nanoparticles into the polymer, interpolymerization, blending with vinylidene fluoride-hexafluoropropylene copolymer, and surface treatments (see, for example references 1-4). Unfortunately, with these approaches, there was always a significant loss in proton conductivity when the methanol permeability was reduced. The only membrane that works well in a direct methanol fuel cell is phosphoric acid doped polybenzimidazole [5], but this membrane requires high temperature (140 oC) fuel cell operation (with a vaporized methanol feed) and suffers from slow oxygen reduction kinetics.
Research on high temperature H 2/air fuel cells includes: (i) the addition of hydrophilic, non-conducting inorganic particles (e.g., SiO 2 or TiO 2) or hydrophilic conducting compounds (e.g., silicotungstic acid, phosphotungstic acid, zirconium phosphate/phosphonate) [6] and (ii) replacing water in an ion-exchange membrane with a high boiling point, proton-dissociating solvent [7,8], e.g., polybenzimidazole (PBI) doped with concentrated phosphoric acid. As with the case of a direct methanol fuel cell, these investigations have resulted in some improvement in fuel cell operation, but further advances are needed.
For all fuel cell membrane development, a better understanding of the interrelationship between polymer/membrane micro-morphology and the membrane’s macroscopic behavior (transport/mechanical/swelling) in a fuel cell is needed. The use of various x-ray and neutron scattering techniques may be useful here. In this presentation, the basic operation of PEM fuel cells will be described, including the role and requirements of the polymeric membrane. An overview of polymeric membrane research for such devices will be reviewed and the possible use of x-ray techniques will be discussed.
References
1. Carter, R.; Wycisk, R.; Yoo, H.; Pintauro, P. N.. Blended Polyphosphazene/Polyacrylonitrile Membranes for Direct Methanol Fuel Cells. Electrochemical and Solid-State Letters (2002), 5(9), A195-A197.
2. Pivovar, Bryan S.; Hickner, Michael; Wang, Feng; McGrath, James; Zelenay, Piotr; Zawodzinski, Thomas A., Jr. Direct methanol fuel cell performance using sulfonated poly(arylene ether sulfone) random copolymers as electrolytes. Pre-Print Archive - American Institute of Chemical Engineers, [Spring National Meeting], New Orleans, LA, United States, Mar. 11-14, 2002 (2002), 2433-2440.
3. Jung, D. H.; Cho, S. Y.; Peck, D. H.; Shin, D. R.; Kim, J. S. Performance evaluation of a Nafion/silicon oxide hybrid membrane for direct methanol fuel cell. Journal of Power Sources (2002), 106(1-2), 173-177.
4. Si, Yongchao; Lin, Jung-Chou; Kunz, H. Russell; Fenton, James M. Trilayer Membranes with a Methanol-Barrier Layer for DMFCs. Journal of the Electrochemical Society (2004), 151(3), A463-A469.
5. Wainright, J. S.; Savinell, R. F.; Litt, M. H. Acid doped polybenzimidazole as a polymer electrolyte for methanol fuel cells. New Materials for Fuel Cell and Modern Battery Systems II, Proceedings of the International Symposium on New Materials for Fuel Cell and Modern Battery Systems, 2nd, Montreal, July 6-10, 1997 (1997),
6. Shao, Zhi-Gang; Joghee, Prabhuram; Hsing, I-Ming. Preparation and characterization of hybrid Nafion-silica membrane doped with phosphotungstic acid for high temperature operation of proton exchange membrane fuel cells. Journal of Membrane Science (2004), 229(1-2), 43-51.
7. Li, Qingfeng; Hjuler, H. A.; Bjerrum, N. J. Phosphoric acid doped polybenzimidazole membranes: physiochemical characterization and fuel cell applications. Journal of Applied Electrochemistry (2001), 31(7), 773-779.
8. Schuster, M. F. H.; Meyer, W. H.; Schuster, M.; Kreuer, K. D.. Toward a New Type of Anhydrous Organic Proton Conductor Based on Immobilized Imidazole. Chemistry of Materials (2004), 16(2), 329-337
