Improving the Properties of Anion Exchange Blend Membranes via Optimizing the NPBI Content and Deprotonation Degree: A Theoretical Study

Blendingpoly-[2,2 '-(1,4-naphthalene)-5,5 '-benzimidazole](NPBI) with main chain-type N-spirocyclic quaternary ammonium ionenes(SI) could effectively improve the dimensional stability of anionexchange blend membranes (AEBMs), while the mechanism of the effecton OH- transport has not been elucidated. In thiswork, the effect of the NPBI content and deprotonation degree on OH- transport was revealed by molecular dynamics simulation.The simulation results showed that the free water content increasedwith increasing NPBI content at the same NPBI deprotonation degree,which facilitated the OH- transport. However, thedecreased distance between adjacent N-spirocyclic quaternary ammoniumcations (NSQAs) in the interchain and the increased correlation betweenOH(-) were detrimental to OH- transport.These resulted in AEBMs with 20 wt % NPBI content exhibiting the bestOH(-) transport. When the NPBI content was 20 wt %,the distance between adjacent NSQAs in the interchain increased andOH(-) was more dispersed in the aqueous phase withdecreasing NPBI deprotonation degrees, leading to the increased OH- transport. Because of the "trade-off"effect between the OH- transport and dimensionalstability of AEBMs, we suggest that AEBMs with 20 wt % NPBI contentand 50% NPBI deprotonation degree (SI/NP-50-20) can be selected tobalance OH- transport and dimensional stability.This work will provide further guidance for the design of blend modificationof AEBMs.