Sulfur Doping in PtNi: Not a Poison But a Promoter for Oxygen Reduction Reaction

Improving the activity and durability of cathode catalysts for the oxygen reduction reaction (ORR) is crucial for large-scale commercialization of proton exchange membrane fuel cells (PEMFCs). Pt-based alloy nanocatalysts often suffer from rapid degradation, primarily due to particle sintering and leaching. Sulfur species is a notorious poison for a Pt-based catalyst due to its strong adsorption on Pt surface sites. Interestingly, in this work, we found that incorporating sulfur into the PtNi lattice significantly boosts the ORR performance, as sulfur doping induces lattice strain and suppresses nanoparticle sintering and phase segregation. Half-cell tests show that the S-doped PtNi catalyst delivers a mass activity of 1.25 A mg Pt −1 at 0.9 V vs RHE, nearly double that of PtNi (0.65 A mg Pt −1 ), and with only 16% activity loss after an accelerated durability test. H 2 −air PEMFC evaluations yield a peak power density of 1.43 W cm −2, and just 12.6% degradation after 30,000 potential cycles in an accelerated stress test. Theoretical calculations reveal that sulfur doping modulates the electronic structure of Pt sites, weakens the Pt−O adsorption, and lowers the OH desorption barrier, thereby accelerating the ORR kinetics.