Trace Sulfur-Incorporation of Palladium Nanosheets to Boost the Oxygen Reduction Reaction

With a S: Pd feeding ratio of 1: 100, Pd nanosheets (denoted as S_0.01-Pd NSs) with wrinkled nanosheet morphology and polycrystalline structure that are almost indistinguishable with pristine Pd NSs are synthesized via a facile one-pot solvothermal process. Notably, the trace amount of S-incorporation (0.83 atom%) distinctively altered the surface electronic structure of the Pd NSs and boosted the performance of the S_0.01-Pd NSs as oxygen reduction reaction (ORR) electrocatalyst. The S_0.01-Pd NSs exhibited a half-wave potential of 0.940 V, 12 mV more positive than that of the Pd NSs, and improved durability. Theoretical calculations with density functional theory indicated that the d-band center of the Pd(111) surface would shift increasingly negative with the amount of S incorporated. Furthermore, the adsorption, activation and hydrogenation of O₂, would change drastically when the S-incorporation increased from 3S to 4S on Pd(111) surface. The free energy changes in sequential ORR steps on the 3S-Pd(111) surface are optimized, leading to a lower estimated overpotential compared to pristine Pd(111); while on 4S-Pd(111) surface, ORR process is suppressed. Such a trend is consistent with the outstanding catalytical performance of the S_0.01-Pd NSs, demonstrating that the trace-amount incorporation is the optimized strategy for doping S in Pd-based catalysts.