Ion solvating membranes: Quaternary ammonium-free alternatives to AEM

Abstract

Ion solvating membranes (ISM), especially KOH doped polybenzimidazole (PBI), are investigated for use as separator in alkaline water electrolysers.[1] Because conductivity of state-of-the-art meta-PBI membranes decreases strongly below concentrations of 3 M KOH, ISM until recently were not considered for use in AEMWE. In this work,[2] an ISM based on sulfonated para-polybenzimidazole was fabricated. Owing to the presence of sulfonic acid groups, this ISM exhibits significant swelling in 1 M KOH, attaining a conductivity exceeding 100 mS cm-1. Furthermore, the absence of quaternary ammonium groups and crosslinking with dibromoxylene resulted in excellent alkaline stability. Conductivity, tensile strength, and Young's Modulus of membrane with 5wt% DBX (5MS-PBI) remained high after 6 months of stability test in 1 M KOH at 80 °C. In comparison, four commercial AEMs lost the majority of their conductivity after 30 days. The electrolyser performance test operating with 1 M KOH at 80 °C was stable without failure for over 500 hours, suggesting that the high alkaline stability observed in ex-situ tests is also achieved in the electrolyser. In conclusion, the use of ISM potentially opens a pathway towards durable AEMWE systems.