Experimental and theoretical studies of palladium-based alloys for oxygen reduction reaction

Co-electrodeposited polycrystalline Pd-Ni alloy was studied for oxygen reduction reaction (ORR) in both acidic and alkaline aqueous media using rotating ring disk electrode (RRDE), and it showed excellent ORR activity in aqueous alkaline media. The number of electrons was calculated to be close to four, and the obtained Tafel slope was 62 mV/dec. Carbon supported Pd-based alloys (Pd-M, M = Ni, Co, Fe, Ag) catalysts with various compositions and annealing temperatures, were synthesized by chemical reduction of the Pd-M hydroxide co-precipitation on the carbon. A comparative study of Pd-Ni/C with different compositions in acidic and alkaline media was carried out using rotating disk electrode (RDE). The difference in ORR activity in two media was attributed to the good stability of surface Ni element in alkaline media. Heat-treated (300°C) Pd3Co and Pd3Fe samples showed higher ORR activity than pure Pd in aqueous acidic media due to surface area, surface roughness, and electronic effect. The electronic effect was thoroughly discussed on Pd-Co and Pd-Ni with different compositions and surface atomic configurations using density functional theory (DFT) calculations. The oxygen (*O) binding energy, as an ORR activity descriptor, was evaluated on different alloy surfaces. It was found that Pd-Co and Pd-Ni with Pd skin surface were more active than pure Pd. The binding energies of *OOH and *OH were also evaluated, and the detailed ORR free energy scheme was plotted for further understanding of the ORR mechanism on different alloy surfaces. It was also found that Pd 3Co with bulk composition surface was slightly higher active than pure Pd due to the much stronger *OH binding energy, which reduced the energy barrier of the rate-determining-step (*O + H+ + e- => *OH). The carbon supported Pd and Pd-based alloys were also tested as cathode ORR catalysts in a proton exchange membrane (PEM) fuel cell at 25, 45, 60 and 80°C. The property of Pd and Pd-base alloys surfaces were found different from that of Pt at 80°C.