On the stability of imidazolium and benzimidazolium salts in phosphoric acid based fuel cell electrolytes

Polymers containing organic cations are considered as promising materials for phosphoric acid (PA) doped high temperature polymer electrolyte membranes (HT-PEMs) due to higher PA retention and wider operation temperature range. However, the thermal/acidic stability of the organic cations remains to be determined. Herein, a series of model cations based on various (benz)imidazoliums are designed and prepared to assess the stability on molecular level under simulated operating conditions for HT-PEM fuel cell. NMR analysis indicates that C2, C4 and C5 substitutions show little effect on the stability while N1 and N2 substitutions show opposite effect. Among the N-functionalized derivatives, N, N-dimethyl (benz)imidazolium cations are fully stable while N-benzyl imidazoliums degraded severely due nucleophilic substitution with H₂PO₄⁻. On this background, blend membranes based on fully methylated polybenzimidazole (PBI) are prepared, and the swelling is balanced by introducing pristine PBI. The proton conductivity of the PA doped blending membrane with IEC of 1.2 mmol g⁻¹ reaches 165 mS cm⁻¹ and a peak power density of 863 mW cm⁻² is demonstrated in H₂/O₂ fuel cell test. These results lay a solid foundation for designing and developing durable high-performance HT-PEMs and fuel cell devices for green energy conversion applications.