Highly ion conductive ion solvating membranes for durable alkaline water electrolysis at low temperature and voltage

Abstract

High ion conductivity is required for polybenzimidazole ion solvating membranes (ISM) to decrease the operational KOH concentration, and thus, improve the alkaline stability of ISM in an electrolyzer. Herein, gel-state ISM with a sponge-like inner structure and a dense surface was developed based on poly (2,2′-(1,4-naphthalene)-5,5′-bibenzimidazole) (NPBI), yielding more volume for KOH uptake. As a result, the Gel-NPBI membrane exhibited excellent mechanical properties, outstanding alkaline stability (2 M KOH at 80 °C for 285 days), and impressive ion conductivity even in 2 M KOH solutions (190 mS cm⁻¹ at 80 °C) as compared to the ISMs prepared with dense membranes with/without phosphoric acid pretreatment (PK-NPBI and K-NPBI). Thus, the resulting Gel-NPBI membrane achieved superior electrolyzer performance with a high current density of 2.5 A cm⁻² at 1.8 V (80 °C, 2 M KOH) using platinum-group-metal catalysts. Impressively, we identified that the high operation voltage resulted in the thinning of ISMs during the working of in situ electrolyzer devices probably due to the oxidative degradation near the anodic side of ISMs that cannot have been gained from the previous reports. Following up on these findings, the cell with Gel-NPBI membrane in a KOH solution of low concentration demonstrated a lifetime of 1780 h under 0.5 A cm⁻² using PGM-free electrodes.