A New Alkali‐Stable Phosphonium Cation Based on Fundamental Understanding of Degradation Mechanisms

Abstract 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 5000 h at 80 °C in a 1 m KOD in CD 3 OD/D 2 O (5:1 v / v ) solution.