Sulfonate-functionalized polybenzimidazole as ion-solvating membrane toward high-performance alkaline water electrolysis

Polybenzimidazole (PBI)-based ion-solvating membranes (ISM) have gained increasing attention for alkaline water electrolysis (AWE). Herein, we designed a series of sulfonate-grafted poly (2,2’-(1,4-naphthalene)-5,5′-benzimidazole) (NPBI) as ISM materials for AWE application. Through the reaction of NPBI and 1,4-butane sultone, butyl sulfonate side chains with different contents were attached to NPBI backbone to afford NPBI-BS-X polymer. Compared to the pristine NPBI membrane, the introduction of hydrophilic sulfonate side chains showed more KOH liquid absorption (59% for NPBI-BS-47 membrane vs. 45% for NPBI), higher hydroxide conductivity (133 vs. 103 mS/cm), and reduced H2 permeability (0.51 vs. 2.05 barrer). Ex situ alkaline stability in 8 M KOH at 80 °C demonstrated that 89% of initial conductivity was retained after 1000 h of testing. As a result, the AWE performance of NPBI-BS-47 was evaluated with 6 M KOH at 80 °C, reaching a current density of 1900 mA cm−2 at 2.0 V, which is much higher than that of the cell with the unmodified NPBI membrane. In the durability test under dynamic operating conditions, the cell with NPBI-BS-47 membrane delivered stable operation for 100 h at a constant current density switching from 0.8 A cm−2 to 1.6 A cm−2 after 20 h of operation at each point. The presented design concept of PBI-based polymer offers new opportunities to develop high-performance ISM for alkaline water electrolysis.