A stable ion-solvating PBI electrolyte enabled by sterically bulky naphthalene for alkaline water electrolysis

Abstract Ion-solvating membranes based on polybenzimidazoles (PBI) have attracted increasing attention in alkaline water electrolysers due to their robust mechanical strength and excellent gas-tightness. However, they suffer from significant chemical degradation under alkaline conditions, especially at elevated temperature. Herein, we present an alkaline-stable ion-solvating membrane based on poly (2,2’-(1,4-naphthalene)-5,5′-bibenzimidazole) (NPBI) for alkaline water electrolysis application. The chemical composition and electrochemical properties of NPBI based ion-solvating membrane with bulky naphthyl groups around benzimidazolide C2 position was investigated when doping with different concentrations of KOH. More importantly, compared to poly (2,2’-(m-phenylene) -5,5′-bibenzimidazole) (m-PBI) membrane with sole phenyl next to the benzimidazolide moieties, ex-situ alkali stability test in 6 M KOH at 80 °C for 228 days demonstrated that introducing bulky naphthyl moieties around benzimidazolide C2 position in the backbones could increase the alkaline resistance for NPBI membrane, which agree well with model compounds investigation. In alkaline water electrolysis tests circulating 6 M KOH solution at 90 °C, membrane electrode assembly (MEA) with NPBI membrane demonstrated a current density of 2.5 A/cm2 at 2.01 V. Preliminary durability study of alkaline electrolysers was undertaken at 0.5 A/cm2 and 90 °C, demonstrating the excellent durability for NPBI-based MEA that stably operated for 298 hours, while gradual voltage increase was observed after 148 h for the electrolyser with m-PBI membrane attributable to the significant chemical degradation of the membrane.