Understanding the alkaline stability of imidazolium and benzimidazolium functionalized poly(phenylene oxide) based hydroxide exchange membranes

Abstract

The imidazolium and benzimidazolium groups containing poly(phenylene oxide) (PPO) hydroxide exchange membrane were synthesized and characterized. The membranes were cast by solution casting method yielding transparent, flexible, and rigid membranes. The physical properties of the membrane, such as ion exchange capacity (IEC), water absorption, and swelling behavior, were investigated. A comparative study of imidazolium and benzimidazolium functionalized PPO membrane focusing on hydroxide conductivity, and alkaline stability was comprehensively examined. The C2 substituted methyl imidazolium membrane displayed unique morphology and conductivity of 15 mSCm−1 at 25 °C. Due to the sterically protected methyl group at the C2 position, imidazolium functionalized PPO membrane showed higher alkaline stability for 700 h. Furthermore, benzimidazolium cations would be more easily attacked by a nucleophile (OH−) than imidazolium cations, resulting in ring-opening of the heterocyclic ring.