Thermally Induced Strains on the Catalytic Activity and Stability of Pt-M2O3/C (M=Y or Gd) Catalysts towards Oxygen Reduction Reaction

Yttrium oxide and gadolinium oxide modified platinum nanoparticles supported on carbon (Vulcan XC-72), noted as Pt-M2O3/C (M=Y and Gd), were synthesized by water-in-oil nanoemulsion chemical route, followed by heat treatment at 100 and 300 degrees C under hydrogen/nitrogen (H-2/N-2) atmosphere and tested as electrocatalysts for the oxygen reduction reaction (ORR). As revealed by powder X-ray diffraction analysis, all obtained catalysts showed solely Pt face-centered-cubic structure, and absence of a secondary phase before and after heat treatments, indicating that Y2O3 and Gd2O3 are highly disordered (amorphous) and dispersed clusters. The surface Pt:M ratio (M=Y and Gd), as revealed by X-ray photoelectron spectroscopy, increased after heat treatment with respect to the value of as-prepared samples, for which the ratio was 1:1. Microstrain data extracted from Williamson-Hall plots for Pt-M2O3/C (M=Y and Gd) catalysts surprisingly increased after heat treatment at 100 degrees C, remaining nearly constant after heat treatment at 300 degrees C, whereas the value of pure Pt nanoparticles, noted as Pt/C, decreased after heat treatments. The mean particle size derived from TEM images for Pt-M2O3/C (M=Y and Gd) was almost unchanged after heat treatments, at variance with the Pt/C case where a clear increase is observed. Surface specific activity and mass activity towards ORR obtained with as-prepared Pt-M2O3/C (M=Y and Gd) catalysts were higher than those of as-prepared Pt/C catalyst. After heat treatment, the ORR activity of Pt-M2O3/C (M=Y and Gd) augmented, whereas that of Pt/C diminished. Moreover, after 6000cycles between 0.5 and 0.95V versus reversible hydrogen electrode (vs. RHE), Pt-M2O3/C (M=Y and Gd) catalysts retained a large active surface area and a high kinetic current density at 0.9V vs. RHE in comparison with Pt/C samples. These facts assess a positive effect of the interaction between M2O3 (M=Y and Gd) and Pt catalytic centers both on the catalytic activity of the material towards ORR and on its durability.