Improving Properties of Anion Exchange Blend Membranes Via Optimizing Npbi Content and Deprotonation Degree: A Theoretical Study

Abstract

Blending poly [2,2'-(1,4-naphthalene)-5,5'-benzimidazole] (NPBI) with main-chain types N-spirocyclic quaternary (SI) could effectively improve the dimensional stability of anion exchange blended membranes (AEBMs), while the mechanism of the effect on OH- transport has not been elucidated. It was essential for membrane modification design to further to explore the effect of NPBI content and deprotonation degrees on OH- transport to obtain AEBMs with both OH- transport and dimensional stability. In this work, the effect of NPBI content and deprotonation degrees on OH- transport was revealed from a microscopic perspective by molecular dynamics simulation. The simulation results showed that when NPBI deprotonation degree was certain, with the increase of NPBI content, increasing free water content was favorable to OH- transport, but enhanced interaction between SI and NPBI made the distance between adjacent N-spirocyclic quaternary ammonium cations (NSQAs) in interchain decrease, and interaction between Na+ and OH- destroyed solvation structure of OH- and promoted aggregation of OH-, resulting in OH- transport increased and then decreased, in which AEBM with 20 wt% NPBI content showed the best OH- transport. When NPBI content was 20 wt%, with the decrease of NPBI deprotonation degrees, increasing distance between NSQAs in interchain leads to wider ion transport channels, and decreasing correlation between OH- makes OH- more dispersed in the aqueous phase, leading to an increased in OH- transport. Because of the "trade-off" effect between OH- transport and dimensional stability of AEBMs, we suggest that AEBM with 20 wt% NPBI content and 50% NPBI deprotonation degree (SI/NP-50-20) can be selected to balance OH- transport and dimensional stability. This work will provide further guidance for design of blend modification of AEBMs.

Keywords: Anion exchange blend membrane, Content, Deprotonation degree, OH- transport, Molecular dynamics simulation