Exploring Alkaline Stable Organic Cations for Polymer Hydroxide Exchange Membranes
COLORADO SCHOOL OF MINES GOLDEN
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Hydroxide exchange membranes HEMs are important polymer electrolytes for electrochemical energy conversion devices. One major concern with the practical application of HEMs is their poor alkaline stabilities which stems from the hydroxide attack on the cationic group. Accordingly, the nature of the cationic group is the top priority in terms of alkaline stability. The most used cation system, which is quaternary ammonium QA based, has insufficient alkaline stability for practical applications. In this study, the tertiary sulfoniums TS and the quaternary phosphonium QP were selected as two candidates to overcome the intrinsic limitation of the QA. The triaryl-substituted TS TAS, for the first time, was introduced as the cationic group for HEMs. The methoxysubstituted TAS based HEM exhibits reasonable alkaline stability and hydroxide conductivity. The alkaline stabilities of a series of TAS model compounds showed that more electron density on the central sulfur atom results in enhanced TAS alkaline stability. However, due to the susceptibility of the central sulfur atom to hydroxide attack, the most stable TAS cation in this study is still inferior to the benchmark cation, benzyltrimethylammonium BTMA. Through degradation kinetics study, Benzyl tris2,4,6- trimethoxyphenylphosphonium BTPP-2,4,6-MeO was determined to have higher alkaline stability than the benchmark, BTMA. A new multi-step degradation mechanism related to the degradation of the methoxy groups for BTPP-2,4,6-MeOwas proposed and verified. It suggested that the elimination of the substituent degradation can further enhance the QP s alkaline stability.
- Physical Chemistry