Alkaline Anion Exchange Membrane from Alkylated Polybenzimidazole

Abstract

Despite the myriad literature reports on the alkaline anion exchange membrane (AAEM) in recent times, the main bottleneck yet to be resolved adequately is the development of a polymer membrane with excellent alkaline stability and high hydroxide conductivity. In order to mitigate these, in this work we have studied the influence of ion yielding alkyl group structure on the improvement of the alkaline stability and hydroxide conductivity of the AAEM with dual ion exchange sites, namely, pyridinium and imidazolium. Three different types of polymers, polybenzimidazole (PBI), pyridine bridged PBI (PyPBI), and tertiary butyl PyPBI (tBut-PyPBI), were converted to their iodide forms by alkylation of the imidazole ring by reacting with various kinds of alkyl iodides such as methyl iodide, butyl iodide, and isobutyl iodide. All the iodide forms of polybenzimidazolium membranes were transformed into the hydroxide form so as to obtain an AAEM by immersing them in KOH solution. FT-IR and 1H NMR spectroscopic studies were employed to confirm the structure of polymer, iodide forms, and KOH-loaded AAEM of all three PBI structures studied here. Ion exchange capacity (IEC), hydroxide conductivity, and thermal, mechanical, and alkaline stability of all the membranes were studied, and we found that PyPBI alkylated with butyl iodide (PyPBI-BuI) has the highest IEC, 3.37 mequiv/g, and maximum hydroxide conductivity, 128.6 mS/cm, at 80 °C among all the membranes developed in this work. All the membranes, irrespective of the polymer structure, when alkylated with isobutyl and butyl chain displayed excellent alkaline stability even in 5 M KOH aqueous solution up to 60 °C, whereas when alkylated with methyl iodide all the membranes showed poor alkaline stability even in 1 M KOH at room temperature. This observation showed the importance of the alkylated moieties structure on the alkaline stability of the AAEM. This significant stability may be due to the bulky nature of the alkyl moieties, which prevented the hydroxide ion attack on both pyridinium and imidazolium groups. Further, our computational studies using DFT calculations confirmed that the electronic factors are the major driving forces rather than the steric hindrance for such high alkaline stability in the case of longer (isobutyl, butyl) alkylated chains, particularly in the case of PyPBI and tBut-PyPBI.