Dependences of the Oxygen Reduction Reaction Activity of Pd-Co/C and Pd-Ni/C Alloy Electrocatalysts on the Nanoparticle Size and Lattice Constant

Carbon-supported Pd-based binary alloy electrocatalysts (Pd-Co and Pd-Ni) with different particle sizes for polymer electrolyte fuel cells were prepared by a NaBH4 reduction method and investigated to examine effects of the size and lattice constant of the Pd alloy nanoparticles on the oxygen reduction reaction (ORR) activity. The particle size and lattice constant were controlled in the wide ranges 4.2-12.1 and 0.3802-0.3948 nm, respectively by heating the catalysts in specific atmospheres. The alloy structures were characterized by X-ray diffraction, transmission electron microscopy and X-ray absorption fine structure. The electrochemical tests of the Pd-Co/C and Pd-Ni/C catalysts were performed by cyclic voltammetry and rotating disk electrode in 0.1 M HClO4. Nearly linear relationship between the lattice constant and nanoparticle size was observed with the Pd-Co and Pd-Ni nanoparticles. The nanoparticle sizes and lattice constants of the Pd-Co/C and Pd-Ni/C electrocatalysts, which influence the Pd d-band center, showed positive and inverse relations with the ORR specific activities, respectively. The mass activities of the Pd-Co/C and Pd-Ni/C electrocatalysts showed an increasing trend with the lattice expansion.