Facile Production of Ion-Conductive Anion Exchange Membranes through In Situ Quaternization

Abstract

Efficient and scalable water electrolyzers for green hydrogen production can be improved through more efficient and environmentally safe methods of casting ion exchange membranes. Anion exchange membrane (AEM) water electrolysis is potentially the best technology for achieving low-cost hydrogen. The processability of AEMs is limited by a lengthy postpolymerization conversion of the prepolymer film because the ion-conductive form is cross-linked and not soluble. The current method involves performing the quaternation reaction by soaking the prepolymer film in a tertiary amine for >24 h, which is not compatible with roll-to-roll processing. Herein, we disclose a pathway toward rapid quaternization of the cast AEM prepolymer film. The poly(norbornene) prepolymer was reacted with a tertiary amine in solution before film casting as opposed to quaternization in the solid state to reduce fabrication times. Quaternization was then completed in the solid state. The effect of the tertiary amine and reaction solvent on the solubility of the reaction mixture and the properties of the ion-conducting film was investigated. Increasing the alkyl chain length of the tertiary amine improved the solubility of the quaternized product in the reaction mixture. Quaternization with a moderately hydrophobic amine, dimethyldecylamine, was found to give high conversion efficiency in solution, resulting in high-conductivity films. Robust films with a conductivity of 37 mS cm^–1 at room temperature were achieved while completely avoiding the traditional lengthy and dangerous postpolymerization quaternization step. Short soak times of the prefunctionalized films in trimethyl amine further boosted the conductivity while circumventing the slow kinetics of functionalization in the solid state.