A novel ion-solvating polymer electrolyte based on imidazole-containing polymers for alkaline water electrolysis

Ion-solvating membranes (ISMs) have been considered as one of the promising candidates for alkaline water electrolysis due to their ability to uptake the alkaline electrolyte, and most studies of ISMs are focused on poly(benzimidazole) derivatives. Herein, a class of imidazole-containing polymers with different positions of imidazole were reported as promising ISMs for alkaline water electrolysis application. By rational selection of monomers, the pendant imidazole groups were tethered onto aryl-ether-free backbones at different positions, to yield poly(biphenyl 2-imidazolecarboxaldehyde) (PB2Im) and poly(biphenyl 4-imidazolecarboxaldehyde) (PB4Im) polymers. The PB4Im membrane with pendant imidazole at C4 position showed higher alkaline uptake than PB2Im membrane as a result of the steric hindrance effect, leading to its high hydroxide conductivity in alkaline solution (82 mS/cm). Alkaline stability tests in a liquid electrolyte concentration of 6 M KOH at 80 °C demonstrated that more than 55% of initial conductivity was lost after 120 days of immersion, and the degradation of imidazole groups was found via post-mortem analysis. Alkaline water electrolysers based on PB4Im membrane demonstrated a current density of 2500 mA cm−2 at a cell voltage of 2.08 V in 6 M KOH at 90 °C using platinum-group-metal (PGM) catalysts (IrO2 in anode; Pt/C in cathode). In situ durability tests showed that the cell is still able to deliver stable operation before 80 h at 500 mA cm−2, after which a sudden failure of the cell was found due to the mechanical failure of the PB4Im membranes. This work offers a new direction and opportunity to exploit high-performing ISMs for alkaline water electrolysis.