Cross-Linked Polybenzimidazoles as Alkaline Stable Anion Exchange Membranes

Abstract

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.

Highly Conducting Poly(arylene-imidazolium) Anion Exchange Membranes Containing Thin-Span Channels Constructed by Cation–Dipole Interaction

“…The 1 H NMR spectra of N,N′-bis(4-hydroxyphenyl)-1,4-diazabutadiene precursor (BHPD), imidazolium monomers (AIm1,2), poly(aryl-imidazolium)1,2 (PAIm1,2), and PAIm1/ PVP cross-linked membrane (Cross-PAIm1-PVP) were recorded on an Avance 400 MHz Bruker spectrometer. 13 C NMR spectra of imidazolium monomers (AIm1,2) were recorded on an Avance 100 MHz Bruker spectrometer. The Fourier transform infrared spectroscopy (FT-IR) was measured on a Tensor II Bruker FT-IR spectrophotometer.…”

Synergy between Ionic Capacity and Intrinsic Porosity in Imidazolium-Based Cationic Organic Polymers and Its Effect on Anionic Dye Adsorption

“…The ionic capacity (IC, mmol g –1 ) of the imidazolium-based COPs is the ion content per unit mass which can be calculated from the number of ions per repeat unit of the imidazolium-based COPs according to the feeding proportion by the following equation where M (g mol –1 ) is the molar mass of the repeat unit of the imidazolium-based COPs and n (mmol mol –1 ) is the molar cation content of the repeat unit of the imidazolium-based COPs.…”

Insight into the Alkaline Stability of Arylene-Linked Bis-Benzimidazoles and Polybenzimidazoles

“…22 As mentioned above, the N,N′-dialkylbenzimidazolium cations are highly electrophilic around the benzimidazolium C2 position, and steric hindrance has proven very effective in preventing nucleophilic attack by hydroxide ions as demonstrated at the polymer level 16,17 and supported by small-molecule model system studies and simulations. 23 The degradation can be further mitigated by cross-linking, 24,25 and computational studies show that inductive effects from electron-rich adjacent groups effectively contribute to suppressing the degradation of sterically protected N,N′dialkylbenzimidazolium. 26 T h i s c o n t e n t i s The benzimidazolide salts (iii), on the other hand, carry a negative charge, which intuitively should result in reduced electrophilicity around the C2 position.…”