Efficient hydrogen evolution reaction with platinum stannide PtSn₄: Via surface oxidation

By means of surface-science experiments, electrochemical tests and density functional theory, we unveil the mechanisms ruling the catalytic activity of PtSn₄. Specifically, through an investigation of the surface chemical reactivity toward CO, H₂O, and O₂ molecules at room temperature and, moreover, of surface stability in air, we show that the catalytic activity of PtSn₄ is determined by the atomic tin layer constituting its surface termination. The PtSn₄ surface is not affected by CO poisoning, although it evolves into a tin-oxide skin in the ambient atmosphere. We demonstrate that the hydrogen evolution reaction on PtSn₄ can be modelled by the combination of two steps, i.e. Volmer and Tafel reactions. Surprisingly, surface oxidation induces a reduction of the energy barrier for the Tafel reaction, so that oxidized PtSn₄ behaves similarly to Pt(111), in spite of the reduced amount of Pt in the alloy and without available over-surface Pt sites. Correspondingly, we observe in electrochemical experiments a Tafel slope of 86 mV dec^-1 and an onset potential similar to that of pure Pt. However, PtSn₄ contains only 29% wt of Pt, with a reduction of the economic cost by 70%. Our results indicate PtSn₄ as a promising novel material for electrocatalytic reactions, whose performance could be further tuned by surface treatments.