A new alkali-stable phosphonium cation based on fundamental understanding of degradation mechanisms
Kaspar, Robert B.
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B. Zhang, R. B. Kaspar, S. Gu, J. Wang, Z. Zhuang, Y. Yan, ChemSusChem 2016, 9, 2374
Highly alkali-stable cationic groups are a critical component of hydroxide exchange membranes (HEMs). To search for such cations, we studied the degradation kinetics and mechanisms of a series of quaternary phosphonium (QP) cations. Benzyl tris(2,4,6-trimethoxyphenyl)phosphonium [BTPP-(2,4,6-MeO)] was determined to have higher alkaline stability than the benchmark cation, benzyl trimethylammonium (BTMA). A multi-step methoxy-triggered degradation mechanism for BTPP-(2,4,6-MeO) was proposed and verified. By replacing methoxy substituents with methyl groups, a superior QP cation, methyl tris(2,4,6-trimethylphenyl)phosphonium [MTPP-(2,4,6-Me)] was developed. MTPP-(2,4,6-Me) is one of the most stable cations reported to date, with <20% degradation after 5000h at 80 degrees C in a 1m KOD in CD3OD/D2O (5:1 v/v) solution.
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