Decoupling Site-Selective CO Poisoning From Hydrogen Oxidation Activity on Pt/C in PEMFCs

Trace CO impurities in reformed hydrogen severely impair the performance of proton exchange membrane fuel cells (PEMFCs), yet the site-specific mechanism of CO poisoning on hydrogen oxidation reaction (HOR) activity at Pt anodes under realistic operating conditions remains poorly understood. Herein, we reveal the site-selective relationship between CO poisoning and HOR activity on Pt/C under practical fuel cell conditions by combining operando infrared spectroscopy with molecular probe experiments that quantitatively distinguish Pt surface sites during operation. Spectroscopic descriptors derived from the evolution of CO vibrational frequency and full width at half maximum (FWHM) reveal concentration-dependent spatial distribution of adsorbed CO: at comparable coverages, low CO concentrations favor dispersed adsorption, whereas higher concentrations promote locally compact CO adlayers. Operando infrared measurements show that CO initially adsorbs on low-coordinated Pt sites and progressively extends to high-coordinated sites as coverage increases, while molecular probe experiments demonstrate that the initially poisoned low-coordinated sites contribute negligibly to HOR activity, with high-coordinated Pt sites dominating the HOR current. This work provides a molecular-level understanding of site-selective HOR activity on Pt/C and a mechanistically guided approach to improve anodic CO tolerance in PEMFCs.