Effects of diverse organic phosphonic acids on the catalytic performance of platinum toward oxygen reduction reaction

While organic phosphonic acids (OPAs) recently show potential as secondary proton conductors in high-temperature proton exchange membrane fuel cells to solve the poisoning effect of phosphoric acid, their structure-specific effects on the ORR performance of Pt cathodic catalyst are poorly understood and limit their application in advancing electrochemical devices for energy conversion and storage. This study systematically investigated the impact of a series of OPAs with varying structure and elements on the catalytic performance of Pt through a combined approach of electrochemical experimentation and molecular simulation. This work revealed that amino trimethylene phosphonic acid exhibited the lowest poisoning effect compared to other OPAs, attributed to its weaker adsorption and modulation of Pt electronic structure. Correspondingly, the half-wave potential experienced only a negative shift of 15 mV. In contrast, phenylphosphonic acid induced the most severe poisoning, with the half-wave potential negatively shifting by nearly 150 mV due to its strong adsorption on Pt surface. Otherwise, it was found that the molecular electrostatic potential energy (MEP) of OPAs could serve as a descriptor for their poisoning to Pt. Nitrogenated OPAs exhibited MEP-dependent activity variations mediated by N-coordination modes, oxygenated OPAs with moderate MEP generated medium poisoning, while benzenic OPA owning minimum MEP caused near-total activity loss. In conclusion, MEP-directed optimization of OPAs accounting for heteroatom-induced electronic effects is critical to suppress Pt deactivation while preserving proton conduction efficiency.