Effect of Electrolyte Ions on Crystalline/Amorphous -PtO₂ Formation in the Electrocatalytic Oxidation of Pt(100) Preferentially Oriented Nanoparticles

Degradation of the Pt electrode in fuel cells and water electrolysis is closely related to the position-exchange process, in which a Pt surface atom exchanges with an oxygen species and the subsequent formation of PtO2. With in situ electrochemical Raman spectroscopy, the effect of solvent electrolyte ions in the double-layer microenvironment on the morphology of alpha-PtO2 during the electro-oxidation of Pt(100) preferentially oriented nanoparticles was investigated. The formation of crystalline alpha-PtO2 depends on the generation and lattice transport of surface-adsorbed oxygen (O-ads) during the electro-oxidation of Pt as proposed by the crystal-growth theory; this theory suggests that a low diffusion rate benefits perfect crystal growth. This study examined electrolyte ions with different adsorption free energies on the Pt surface, as calculated with the density functional theory. Amorphous alpha-PtO2 is generated in acidic solutions containing fluoride, perchlorate, and phosphorate with high adsorption free energies. By contrast, nitrate and sulfate ions exhibit low adsorption free energy and induce strong adsorption on the Pt surface, which competes with the adsorption of surface H2O and causes slow production and reduced diffusion of the oxygen atom, thereby favoring the crystallinity of alpha-PtO2. The Pt complex (PtCl62-) is formed owing to strong complexation of chloride ions with Pt species, leading to the dissolution of Pt and production of amorphous PtO2. Moreover, a high concentration of hydrogen ions inhibits the oxidation of OHads (adsorbed hydroxyl) to O-ads, favoring the crystalline formation of PtO2 during electro-oxidation. This study provides valuable insight that can be extended to synthesize other metal oxides, enabling control over the structure and morphology of materials and catalysts.