Cross-Linked Alkaline Anion Exchange Membrane from N-Spirocyclic Quaternary Ammonium and Polybenzimidazole

Abstract

Two major challenges, namely, hydroxide conductivity and alkaline stability of the polymer membrane, are yet to be resolved adequately in spite of significant research outcomes on alkaline anion exchange membrane (AAEM) in the recent past. To address these challenges, in this work, the development of ionically cross-linked AAEMs has been achieved by blending pyridine-bridged polybenzimidazole (PyPBI) and N-spirocyclic quaternary ammonium spiro ionene polymer (SP). Further, membranes were converted to porous membranes by adding different weight percentages of porogen in the membrane matrix. Membranes were converted to hydroxide-conducting AAEMs by dipping into 1 M KOH solution, and under this condition, a part of the -NH- groups of PyPBI was deprotonated to form ammonium-imidazolate complexes with SP, which resulted in ionic cross-linking in the AAEM. Hydroxide ion conductivity of 129 mS/cm at 90 °C was obtained in the case of the S70P30-OH membrane, which was a hydroxide-form membrane obtained from the blend of 70 wt % SP and 30 wt % PyPBI, and this membrane showed the highest KOH uptake among all other AAEMs prepared in this study. On the other hand, among the porous ionically cross-linked membranes studied here, the S50P50-P25-OH (blends of 50 wt % SP and 50 wt % PyPBI with 25% porogen) membrane showed the highest hydroxide ion conductivity (117 mS/cm at 90 °C). All the ionically cross-linked AAEMs displayed excellent alkaline stability and remained unaffected during alkaline stability test in 1 M KOH at 80 °C for as long as the test was carried out (960 h). Observing the exceptional stability in 1 M KOH of S50P50-OH and S50P50-P25-OH membranes, OH- conductivity analysis and alkaline stability tests of these samples were carried out even in 2 M KOH, and we found that these membranes retained ∼80% of their OH- conductivity value even after 500 h of alkaline treatment in 2 M KOH at 60 °C. Furthermore, membranes were found to be useful in alkaline water electrolysis, and the best performance was shown by the S70P30-OH membrane, which displayed a current density of 100 mA cm-2 at 2.6 V. Overall, these recently developed membranes retained hydroxide conductivity, structural and thermal stability even after harsh alkaline treatment for a longer period of time.