Exploring alkaline stable organic cations for polymer hydroxide exchange membranes

Zhang, Bingzi
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University of Delaware
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 stem 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 methoxy-substituted 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 (tris(2,4,6-trimethoxyphenyl)phosphonium (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-MeO) was proposed and verified. It suggested that the elimination of the substituent degradation can further enhance the QP's alkaline stability. By replacing methoxy substituents with methyl groups, a superior QP cation, methyl tris(2,4,6-trimethyphenyl)phosphonium (MTPP-(2,4,6-Me)) was developed. MTPP-(2,4,6-Me) demonstrated a level of stability that has not been achieved by any other known HEM cations. The alkaline degradation kinetics and mechanisms of a wide range of QPs determined that the electronic effect and steric effect combined determine the alkaline stabilities of QP cations. To take advantage of the outstanding alkaline stability of MTPP-(2,4,6-Me), a feasible synthesis route was developed to synthesize MTPP-(2,4,6-Me) functionalized polymers for constructing HEMs.