Effect of surface modification on the structure and electrochemical properties of electrocatalysts for polymer electrolyte membrane fuel cells

Abstract

Due to strong dependence of electrocatalytic reactions on the surface structure of the catalysts, surface-modified nanoparticles have been studied widely to achieve highly efficient and economical electrocatalysts. There have been various strategies to achieve the surface-modified nanoparticles. The effect of surface-modification on the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) activity was investigated using carbon supported nanoparticle catalysts which were subjected to various surface modification methods and discussed based on the effect of heterogeneous atoms in the catalyst surfaces. The surface modifications were conducted three different ways: 1) deposition of Pt-sub-monolayer-shell on Pd-core nanoparticles, 2) thermal treatment of Pt-Au alloy nanoparticles under CO or Ar gas which led to different surface Pt-Au compositions but identical bulk compositions, and 3) selective adsorption of phosphoric acid on Se in RuSey/C catalyst. Tht HOR activity of Pd nanoparticles modified by sub-monolayer of Pt was investigated at a Pt surface concentration of 0 ~ 5.7 %. The combination of electrochemical measurements and high resolution-X-ray photoelectron spectroscopy revealed a significant decrease in oxidized Pd atoms (23.4 %) with a Pt surface concentration of 1.7% compared with that of Pd/C. X-ray absorption spectroscopy of the Pt LIII suggested preferred Pt deposition, which led to more oxidized Pt atoms during Pt shell growth. The exchange current density of the hydrogen oxidation reaction on the electrocatalyst with a Pt surface concentration of 4.9 % was 3.5 times higher than that on Pd/C and was comparable with that on Pt surfaces. The changes in hydrogen oxidation reaction apparent enthalpy due to Pt shell growth suggested a rate determining step (RDS) change (Volmer reaction → Tafel reaction) at a Pt surface concentration of 1.7 %. In order to investigate the effect of CO induced surface segregation on the ORR activity of PtAu nanoparticles, the catalysts were subject to heat treatment at 423 K under a CO or Ar atmosphere. The surface composition was analyzed by XPS and the composition was shown to increase from 66 ± 2 % (PtAu-AP) to 74 ± 2 % (PtAu-CO) after heat treatment under a CO atmosphere, which was confirmed by electrochemical techniques, while the bulk composition was invariant at 55 %. For the oxygen reduction reaction (ORR), the mass activity of PtAu-CO increased by 75.6 % (33.2 A/gPt) when compared to that of PtAu-AP (18.9 A/gPt). Since the increase in EAS was only 15.8 %, it was concluded that the specific activity was enhanced by 52.6 % due to surface Pt segregation after heat treatment under a CO atmosphere. The enhanced specific activity was attributed to the reduced OH adsorption energy which was characterized by measuring the potential of total zero charge. The weaker OH adsorption was resulted from the higher Pt/Au ratio at the surface layers. The effect of phosphate adsorption on the ORR activity of Se-modified-Ru catalysts (RuSey/C) was examined. The ORR activity of unmodified Ru/C catalyst decreased by 26.8% as the active site was blocked by electrochemical adsorption of phosphoric acid. However, for RuSey/C, the ORR activity was enhanced with phosphoric acid (RuSe1.56/C: 63.8%), which indicates that the kinetics at each site increased to compensate for the site blocking effect. The XAS results demonstrated that, for RuSey/C, phosphoric acid molecules or phosphate anions primarily interacted with Se atoms, and the oxidation state of Ru atoms decreased. Therefore, it was concluded that the enhanced ORR kinetics originated from the decreased oxygen binding energy with larger electrostatic repulsion.