Polybenzimidazoles crosslinked by alkyne-azide cycloaddition for anion exchange membrane fuel cell application

Abstract

In this study, we report the synthesis of polybenzimicazole (PBI)-based anion exchange membranes (AEMs) crosslinked by alkyne-azide cycloaddition. Three types of side chains, long alkyl quaternary ammonium groups, ethyl groups, and alkyne, were grafted onto poly[2,2’-(m-phenylene)-5,5’-bis(N, N’-dimethylbenzimidazole) (m-PBI). 1,3-Diazidopropane was added as the crosslinking agent. The ion exchange capacity (IEC) values and crosslinking density can be controlled by adjusting the ratios of the side groups. Due to the poor solubility of m-PBI, the cationic m-PBI without ethyl side chains exhibited limited solubility in organic solvents when the IEC was lower than 2.85 mmol/g. After ethyl side chains were grafted onto the polymer backbones, the solubility was enhanced, resulting in a broader range of IEC values from 0.76 to 2.65 mmol/g. We also prepared another PBI variant containing biphenyls with methyl groups at the 2 and 2’ positions. Crosslinked membranes prepared by alkyne-azide cycloaddition (AAC) exhibited low swelling ratios, minimal water uptakes, and excellent mechanical properties. After immersing the membranes in 1.0 M KOH at 60 ℃ for 700 h, the hydroxide conductivity (at 80 ℃) of m-1.15Q0.5Et0.15Py (IEC = 2.32 mmol/g) and m-1.55Q0.25Py (IEC = 2.85 mmol/g) was maintained at 82% and 71% of their original values, respectively. Furthermore, the hydroxide conductivity (at 80 ℃) of Me-1.32Q0.24Py (IEC = 2.13 mmol/g) and Me-1.68Q0.24Py (IEC = 2.48 mmol/g) remained at 80% and 82%, respectively. A single cell based on an AEM (m-1.0Q0.85Et0.15Py) achieved a peak power density of 226.3 mW/cm². In comparison, the single cell using the commercial Sustainion X37-50 grade T exhibited a peak power density of 303.6 mW/cm² under the same testing conditions.