Atomic-Scale Mechanisms of Electrochemical Pt Dissolution

Pt dissolution under potential cycling has been identified as the dominant process that causes cathode losses in proton-exchange membrane fuel cells. In recent years, significant insights on the Pt dissolution process have been obtained from in situ Pt dissolution detection enabled by voltammetry coupled to inductively coupled plasma mass spectrometry. Despite extensive experimental research, theoretical studies continue to lag in the understanding of the atomic-scale mechanism of the Pt dissolution process due to the complicated subprocesses involved, including Pt oxidation, surface reconstruction, Pt oxide reduction, chemical corrosion, etc. Here, we employ global optimization and constant-potential density functional theory to simulate the complete process of Pt dissolution. We show that a two-dimensional Pt surface oxide consisting of interconnected square planar PtO4 units forms at applied potentials higher than 1.1 VRHE. The structural signatures and oxidation states of the Pt surface oxide are close to that of bulk Pt3O4 oxide. The PtO4 units can be reduced to [PtOH­(H2O)3]+ in the cathodic scan and dissolve into the electrolyte. The dissolved [PtOH­(H2O)3]+ species favorably accepts a proton and becomes [Pt­(H2O)4]2+. We also find that the dissolution of one Pt atom leads to the decomposition of the connected Pt­(OH)4 units because of ligand losses, which then renders them susceptible to be reduced to Pt0. On the basis of our findings, we propose a cathodic Pt dissolution mechanism: Pt3O4 s + 8H+ + 6e– → [Pt­(H2O)4]2+ + 2Pt. An anodic Pt dissolution mechanism is also proposed. Our work provides a fundamental understanding of Pt dissolution under potential cycling, which is needed for the rational design of durable Pt-based cathodes for fuel cells.