Porous polybenzimidazole membranes doped with KOH for alkaline water electrolysis

The porosity of polybenzimidazole (PBI) based ion-solvating membranes (ISM) with thin skin layer is controlled and utilized to enhance its capacity for electrolyte absorption for the use of alkaline water electrolysis (AWE). The solution of poly(2,2'-(1,4-naphthalene)-5,5′-benzimidazole) (NPBI) containing porogen additive in DMSO solvent was casted and then transformed into a porous membrane by using selective solvents to leaching out the low-molecular-weight additive. Compared with the pristine NPBI membrane, the introduction of porosity significantly enhanced electrolyte absorption from 70 % to 190 %, and reduced the areal resistance from 0.4 Ω cm2 to 0.15 Ω cm2. Besides, the lower H2 permeability (7.3 Barrer) and higher bubble point pressure (>10 Bar) exhibited by the KOH doped porous NPBI membrane (NPBI-75) depicted its potential to work efficiently and safely. In an alkaline water electrolysis test using a PGM-free catalyst in 6 M KOH at 80 °C, the NPBI-75 exhibited a current density of 2500 mA/cm2 at 2.06 V, which outperformed the pristine membrane and commercial Zirfon separator. Moreover, the NPBI-75 exhibited excellent durability up to 700 h at 2000 mA/cm2, which is the highest current density reported so far. The excellent ionic conductivity of NPBI-75 enabled it to work at a relatively lower temperature for 700 h without voltage increase. The improved strategy based on PBI polymer proposed in this paper provided a new strategy for the development of high-performance ion-solvating membranes for AWE.