Polybenzimidazole films doped with phosphoric acid are being investigated as potential polymer electrolytes for use in hydrogen/air and direct methanol fuel cells. In this paper, we present experimental findings on the proton conductivity, water content, and methanol vapor permeability of this material, as well as preliminary fuel cell results. The low methanol vapor permeability of these electrolytes significantly reduces the adverse effects of methanol crossover typically observed in direct methanol polymer electrolyte membrane fuel cells.

Polybenzimidazole ͑PBI͒ film, a candidate polymer electrolyte membrane ͑PEM͒ for high-temperature ͑120-200°C͒ fuel cells, was cast from PBI/trifluoacetyl/H 3 PO 4 solution with constant molecular weight PBI powder and various acid doping levels. Conductivity measurements on these membranes were performed using an ac method under controlled temperature and relative humidity ͑RH͒. A complete set of conductivity data for H 3 PO 4 acid-doped PBI is presented as a function of temperature ͑60-200°C͒, RH ͑5-30%͒, and acid doping level ͑300-600 mol %͒. A mechanism of conductivity is proposed for the proton migration in this PBI/acid system based on this and previous work. Proton transfer in this system appears to occur along different paths for different doping levels, RHs, and temperatures. Hydrogen bonds immobilize the anions and form a network for proton transfer by a Grotthuss mechanism. The rate of proton transfer involving H 2 O is faster, leading to higher conductivity at higher RH. The order of the rate of proton transfer between various species is H 3 PO 4 (H 2 PO 4 Ϫ

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In developing advanced fuel cells and other electrochemical reactors, it is desirable to combine the advantages of solid polymer electrolytes with the enhanced catalytic activity associated with temperatures above 100~ This will require polymer electrolytes which retain high ionic conductivity at temperatures above the boiling point of water. One possibility is to equilibrate standard perfluorosulfonic acid polymer electrolytes such as Nafion TM, with a high boiling point Bronsted base such as phosphoric acid. The Nafion/H3P04 electrolyte has been evaluated with respect to water content, ionic conductivity and transport of oxygen, and methanol vapor. The results show that at elevated temperatures reasonably high conductivity (>0.05 ~Q-1 cm-1) can be obtained. Methanol permeability is shown to be proportional to the methanol vapor activity and thus decreases with increasing temperature for a given methanol partial pressure. Comparisons and distinctions between this electrolyte and pure phosphoric acid are also considered.

This review article summarizes the development of flow fields and stacks, and design considerations for next-generation flow batteries.