Imidazolium structural isomer pyrazolium: A better alkali-stable anion conductor for anion exchange membranes

Abstract

Anion exchange membranes (AEMs) are a core component for energy conversion technologies such as fuel cells and water electrolyzers. Unfortunately, the functional cations in AEM often suffer from inferior durability in caustic media, especially at elevated temperatures or highly concentrated alkaline solutions. Therefore, the exploitation of novel cations that can endure highly alkaline environments while with synthetic feasibility and good ion conducting property is desired. Herein, a simple and easily available pyrazolium cation was evaluated and was alkaline stable in 5 M NaOH (aq) at 80 °C for at least 240 h, which is substantially stable than its structural isomer, imidazolium. The alkaline stability of pyrazolium was rationalized through analyses of the degradation energy barriers calculated by density functional theory. The excellent alkaline stability of pyrazolium cation encouraged the fabrication of AEMs functionalizing with pyrazolium head group anchored to poly(arylene alkylene) architecture. In contrast to imidazolium-based counterpart, pyrazolium cation enable the AEM with higher base stability, retaining ～80% of its initial conductivity after a 1000-h treatment in 1 M NaOH (aq) at 80 °C. Moreover, the AEM based on pyrazolium showed a high hydroxide ion conductivity up to 120 mS cm−1 at 80 °C and a high peak power density of 502 mW cm−2 after assembled into a H2–O2 fuel cell device.