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.