Enhancing Proton Delocalization in Non-Sulfonated Membranes through Coupled Electron-Withdrawing Groups for High-Temperature Fuel Cells

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) have significant practical potential due to their high tolerance to CO impurities and efficient heat and water management. However, the performance of HT-PEMFCs is limited by low proton conductivity at low humidity and the inadequate high-temperature stability of polymer electrolytes. In this study, we prepared a nonsulfonated perfluorosulfonimide-phenylphosphonic acid (PFSI-BPA) membrane that exhibits high performance in HT-PEMFCs. Our strategy is to enhance the acidity of protogenic groups (sulfonimide and phosphonic acid groups) in the membrane by incorporating electron-withdrawing groups (sulfone and phenyl groups). These electron-withdrawing groups promote proton delocalization by lowering the energy barrier, thereby improving the nonsulfonated membrane's proton conductivity and thermal stability, confirmed by combining in situ measurements and theoretical calculations. The proton conductivity of the PFSI-BPA membrane is 3.2 times higher than that of a control sample with electron-donating groups at 40% relative humidity (RH). Moreover, the power density of the PFSI-BPA-based fuel cell reaches a peak of 2.44 W cm^-2 under the conditions of 105 °C/40% RH, outperforming most previously reported values. This work provides valuable insights into the development of advanced polymer electrolytes for practical HT-PEMFCs. This study explores the enhancement of proton conductivity by increasing the acidity of protogenic groups through the inductive effect of electron-withdrawing groups, which lowers the energy barrier for proton delocalization. The synthesized perfluorosulfonyl imide-phenylphosphonic acid (PFSI-BPA) demonstrates outstanding water transport and proton conductivity even at low humidity, making it a promising candidate for next-generation high-temperature fuel cells.