Stable, Active, and Methanol-Tolerant PGM-Free Surfaces in an Acidic Medium: Electron Tunneling at Play in Pt/FeNC Hybrid Catalysts for Direct Methanol Fuel Cell Cathodes
Tomasz Kosmala, Nicolas Bibent, Moulay Tahar Sougrati, Goran Dražić, Stefano Agnoli, Frédéric Jaouen, Gaetano Granozzi
Abstract
PGM-free catalysts have high initial activity for O2 reduction reaction, but they suffer from low stability in acid medium in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). Here, we shed light on the atomic-scale structure of hybrid Pt/FeNC catalysts (1–2 wt % of Pt), revealing, via scanning tunnelling electron microscopy and energy-dispersive X-ray spectroscopy, the presence of Pt@FeOx particles. The absence of exposed Pt on the surface is confirmed by the suppression of methanol oxidation reaction and CO stripping experiments. The promising application of such Pt/FeNC catalysts, comprising FeNx sites and Pt@FeOx particles, is demonstrated at the cathode of DMFC. To gain fundamental understanding on the stability in acid medium and on the intrinsic ORR activity of Pt@FeOx, we constructed model surfaces by depositing FeOx films with controlled thickness (from 1.0 nm to 6.4 nm), fully covering the Pt(111) surface, which resulted stable in acid medium in the potential range of 0.45–1.05 V vs RHE. The specific ORR activity of Fe2O3/Pt(111) increases exponentially with decreasing overlayer thickness, which is explained by the tunneling of Pt electrons through Fe2O3. This special phenomenon sheds light onto recently reported excellent durability of Pt/FeNC composites in PEMFC and identify a promising core@shell strategy leading to stable PGM-free surfaces in acid medium, and tolerant to methanol.