Mitigating the Phosphate Anion Poisoning Effect on Oxygen Reduction Reaction (ORR) with a Hydrophobic Side Group

High-temperature polymer electrolyte fuel cells (HT-PEMFCs) have been developed for heavy duty applications, utilizing phosphoric acid (H₃PO₄) to overcome the proton conductivity limit of Nafion at elevated temperatures (120-220 °C). However, the adsorption of phosphoric acid onto the Pt catalyst hampers the oxygen reduction reaction (ORR) activity, necessitating a high Pt loading in HT-PEMFCs. Understanding the interaction between Pt and phosphate anions, as well as the anion adsorption effect on ORR, is crucial for activity improvement. In this study, we conducted a comprehensive theoretical and experimental investigation to gain insights into the effect of phosphate adsorption on ORR. While electrolyte adsorption inevitably obstructed active sites, our study revealed a weakened binding between Pt and key ORR intermediates, namely the OH_ad, in the presence of adsorbed anions. This weakened Pt-OH_ad binding had a beneficial effect on boosting the ORR kinetics. By appropriately modifying the side group structure of a substituted phosphonic acid (R-PO(OH)₂), the activity loss caused by anion adsorption could be effectively compensated through enhanced ORR kinetics. Our findings highlight an alternative approach, alongside catalyst material modulation, to mitigate the adverse effects of electrolytes in HT-PEMFCs. The tuning of catalyst interfacial structure using a hydrophobic surface modifier can also serve as a strategy to optimize ORR kinetics in both low-temperature (LT-) and HT-PEMFCs.