(329c) Self-Crosslinking of Quaternary Phosphonium-Based Polymers: Highly Dimension-Stable Hydroxide Exchange Membranes

Hydroxide (OH-) exchange membrane fuel cells (HEMFCs) have been demonstrated to have the potential to solve the proton exchange membrane fuel cells' (PEMFC's) heavy dependence on precious metals as electrode catalysts [1]. Also, HEMFCs can completely avoid the electrolyte leakage and eliminate the metal (bi)carbonate formation problems of the traditional liquid alkaline fuel cells (AFCs).

As one of the most crucial parts of HEMFCs, the hydroxide exchange membranes (HEMs) are required to serve not only as the efficient hydroxide conductor, but also as the complete separator between fuel and oxidant. With a unique tertiary phosphine [2], the quaternary phosphonium-based HEMs have been demonstrated to possess high hydroxide conductivity, considerably improved stability, and the best single cell performance (e.g. 258 mW/cm2) reported, and are superior to those of conventional quaternary ammonium-based HEMs [3]. However, the good solubility and high hydrophilicity arising from nature of quaternary phosphonium group make the quaternary phosphonium-based HEMs suffer excessive swelling and too much water-uptake, then reducing the possibility of long-term usage. The development of highly dimension-stable HEMs is now considered as the most important challenge for the full realization of the benefits of quaternary phosphonium-based polymers.

In this work, we explored and developed an easy covalent crosslinking technique: self-crosslinking. The prepared self-crosslinked quaternary phosphonium-based HEMs (SCL-TPQPOHs) have very high dimension-stability: the swelling ratios are substantially smaller (more than one order of magnitude) for SCL-TPQPOHs than uncrosslinked TPQPOH; In addition, SCL-TPQPOH exhibits extraordinary solvent-resistance, enhanced thermal stability, and also high hydroxide conductivity. Those properties confirm that the self-crosslinking technique of quaternary phosphonium functionalized HEM is highly promising for use in HEMFCs.

References

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

[2] S. Gu, R. Cai, T. Luo, Z. Chen, M. Sun, Y. Liu, G. He, Y. Yan, Angewandte Chemie International Edition 2009, 48, 6499.

[3] S. Gu, R. Cai, T. Luo, K. Jensen, C. Christain, Y. Yan. ChemSusChem 2010, In press.