One step synthesis of carbon-supported Ag/MnyOx composites for oxygen reduction reaction in alkaline media

Abstract

Carbon-supported Ag/MnyOx composites were prepared by a one step synthesis method, i.e., silver permanganate pyrolysis at different temperatures. The physical and electrochemical properties of the obtained composites were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and cyclic voltammetry (CV). The electrochemical activity, selectivity and stability for the Ag/MnyOx/C composites toward oxygen reduction reaction (ORR) in alkaline media were investigated by rotating (ring)-disk electrode (R(R)DE) technique. The XRD results show that the manganese oxides were converted from MnO2 to Mn3O4 and then to MnO gradually with increasing temperature. Among all the Ag/MnyOx/C composites, Ag/Mn3O4/C obtained at 400°C exhibits the highest activity toward the ORR. Compared with the Ag/C, Mn3O4/C and the physical mixture of Ag/C and Mn3O4/C with the same metal loadings, the ORR activity of the Ag/Mn3O4/C is improved significantly, suggesting a synergic effect between Ag and Mn3O4. At an overpotential of 0.35V, the calculated apparent activation energy for the ORR on the Ag/Mn3O4/C catalyst is 56kJmol−1, which is apparently higher than that on a Pt/C catalyst (36kJmol−1). The results from both Koutecky–Levich plots of the ORR and the RRDE measurements suggest that the ORR proceeds via a 4-electron pathway on the Ag/Mn3O4/C composite. Moreover, the presence of methanol has little influence on the ORR activity toward the Ag/Mn3O4/C catalyst. The accelerated aging tests indicate that the Ag/Mn3O4/C is more stable than the Pt/C in 0.1M NaOH solution.