(697a) A Soluble and Highly Conductive Ionomer for High Performance Hydroxide Exchange Membrane Fuel Cells

Hydroxide (OH-) exchange membrane fuel cells (HEMFCs) have the potential to solve the catalyst cost and durability problems of proton exchange membrane fuel cells (PEMFCs) while achieving high power and energy density. Further, HEMFCs can offer fuel flexibility (e.g., methanol, ethanol, ethylene glycol, etc.) because of their low over-potential for hydrocarbon fuel oxidation and reduced fuel crossover [1]. One of the most significant problems for HEMFCs is the lack of a soluble ionomer that can be used in the catalyst layer to build an efficient three-phase-boundary and thus drastically improve the utilization of the catalyst particles and reduce the internal resistance. However, the general quaternary ammonium contained polymers are difficult to be used as the ionomer due to its poor solubility in low-boiling-point water-soluble solvents.

Quaternary phosphonium contained polymers showed excellent solubility in methanol [2]. Also strong basicity of the tertiary phosphine [3] suggests that quaternary phosphonium hydroxides are very strong bases. Therefore in this work, we synthesized for the first time a new quaternary phosphonium based ionomer that is soluble in some low-boiling-point water-soluble solvents and highly hydroxide-conductive: tris(2,4,6-trimethoxyphenyl) polysulfone-methylene quaternary phosphonium hydroxide (TPQPOH). The TPQPOH exhibited excellent solubility in such solvents, high hydroxide-conductivity, and outstanding alkaline stability. The TPQPOH ionomer adopted H2/O2 HEMFC also exhibits substantial increase (2.5 times) of peak power density and significant reduction (50%) of internal resistance.

References

[1] J. R. Varcoe, R. C. T. Slade, Fuel Cells 2005, 5, 187.

[2] T. Nishikubo, J. Uchida, K. Matsui, T. Iizawa, Macromolecules 1988, 21, 1583-1589.

[3] W. A. Henderson, C. A. Streuli, J. Am. Chem. Soc. 1960, 82, 5791-5794.