Role of Ionic Concentration and Distribution in Anionic Conductivity: Case Study on a Series of Cobaltocenium-Containing Anion Exchange Membranes with Precise Structure Control

Abstract

Three new anion exchange membranes (AEMs) (P1, P2, P3) with varied numbers (1, 2, 3) of cobaltocenium substituents were synthesized by ring-opening metathesis polymerization (ROMP). The living polymerization characteristic of ROMP endows the resulting AEMs with tunable ionic concentration and precise structure control, which provide an ideal model to investigate the influence of ionic concentration and substituent on the ion-conducting property of AEMs. According to Mohr titration and the alternating-current impedance method, the as-prepared AEMs demonstrated an obvious ionic concentration and substituent influence on ionic conductivity in an ordering of P2 > P3 > P1. In addition, the hydrophilic/hydrophobic microphase separation structures of the three AEMs were observed by atomic force microscopy (AFM), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). To further verify the relationship between ionic concentration, substituent, and ionic conductivity, molecular simulation was also used to calculate the ionic diffusivity of the three AEMs, and the result of simulation computation was consistent with the experimental result. Moreover, a tentative anion exchange membrane fuel cell (AEMFC) performance test also indicates the corresponding sequence of P2 > P3 > P1. The current work may provide an effective method for the rational design and preparation of AEMs, as well as guidance in investigating the structure–property relationship.