Dual Role of Alkyl-Substituted Phosphonic Acids on Oxygen Reduction Reaction on Platinum

High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are vital for heavy-duty vehicles. A key component is phosphoric acid (PA), which enables proton conduction at higher temperatures. However, PA also adsorbs onto platinum, impeding the oxygen reduction reaction (ORR) and causing substantial current losses in HT-PEMFCs. Phosphonated ionomers, featuring covalently bound phosphonic acid groups, have been proposed as alternative electrolytes, although their impact on the ORR remains underexplored. Herein, we investigate the influence of various acid anions, PA and alkyl-substituted phosphonic acids (R-PAs), on ORR using a combination of simulations and experiments. Our findings highlight the dual role of R-PAs: while blocking more active sites due to stronger binding and steric hindrance, they also promote intrinsic activity at neighboring sites by altering the electronic structure and disrupting the stabilizing surface water layer. These insights pave the way toward developing design strategies for phosphonated ionomers with enhanced ORR activity in HT-PEMFCs.