Oxygen electroreduction at catalysts PtM (M = Co,Ni, or Cr)

Bimetallic catalysts PtM (M = Co, Ni, or Cr) are synthesized. They exceed purely platinum commercial catalyst E-TEK (20 wt % Pt) in its mass activity (mA/mg_Pt) and specific activity (mA/c_Pt²) in the oxygen reduction reaction. According to XRD data, the high-temperature synthesis involving metal N₄-complexes, chloroplatinic acid, and XC72 carbon black as precursors, yields alloys (or solid solutions) of the metals. The higher activity of the bimetallic catalyst PtCo/C is likely to be caused by the practically entire formation of solid solutions (Pt₃Co and PtCo), unlike PtNi and PtCr where nickel and chromium exist also as oxides that decorate the electrode surface and partly block active centers. It is shown that the mechanism of the oxygen reduction reaction at the synthesized catalysts is similar to that of oxygen reduction at the purely platinum catalyst. The slow stage in the process is transfer of the 1st electron; at potentials more positive than 0.6 V the reaction mainly yields water. The higher electrocatalytic activity of the bimetallic systems is caused by the alloy formation, which leads to changes in the bond length between platinum atoms. The achieving of the optimal bond length, as a result of the alloy formation, provides appropriate conditions for dissociative adsorption of oxygen molecules; the surface coverage with oxygen-containing particles adsorbed from water (which block active centers for O₂ adsorption) decreased. The increase in the activity may also be caused by the formation of the “core-shell” structures whose surface is enriched with platinum whose surface properties are changed under the ligand action of the core formed by the metal alloy