Cross-Linked Polybenzimidazoles as Alkaline Stable Anion Exchange Membranes

Despite the large volume of literature on alkaline anion exchange membranes (AAEMs), the development of efficient AAEMs with excellent alkaline stability and superior hydroxide conductivity is the major challenge. To mitigate this problem, cross-linked poly(butylated pyridinium benzimidazolium) iodide (CPBPBI) membranes were synthesized from pyridine-bridged polybenzimidazole (PyPBI) utilizing butyl iodide as the alkylating agent and 1,4-diiodobutane as a cross-linker. The AAEMs were fabricated by dipping the CPBPBI membranes in KOH to obtain CPBPBI-OH. Among the several structural variations, the CPBPBI-OBA-OH membrane displayed the highest hydroxide conductivities of 39.4 and 111 mS/cm at 30 and 80 °C, respectively. The ion exchange capacity (IEC) of CPBPBI-OH membranes was found to be altered in the range of 2.30–3.97 mequiv·g–1 at an ambient temperature depending on the polymer structure. The membranes studied in 5 M KOH solution at 80 °C for 16 days showed excellent chemical and dimensional stability. The IEC, Fourier transform infrared (FT-IR) and 1H nuclear magnetic resonance (NMR) spectra, and hydroxide ionic conductivity data of the AAEMs were compared before and after the chemical stability test, and all of these investigations proved that the membranes were highly stable in concentrated alkaline solution and almost zero or very negligible degradation was observed after harsh alkali treatment. The negligible loss of hydroxide ion conductivity of membranes even after 16 days in 5 M KOH treatment at 80 °C is attributed to the cross-linking formation that prevented the attack of OH– ions on C2 imidazolium moieties. Furthermore, the thermal and mechanical properties of the AAEMs showed excellent thermomechanical stability. Overall, the cross-linking of the PyPBI chains immensely helped in improving various physical properties, particularly hydroxide conductivity, alkaline stability, and mechanical robustness, which are important for the development of efficient AAEMs.