Palladium-coated manganese dioxide catalysts for oxygen reduction reaction in alkaline media

Pd-coated manganese dioxide catalysts (Pd@MnO2) were synthesized by depositing Pd on the surface of β-MnO2 nanorod particles in aqueous solutions at room temperature. TEM, XRD and electrochemical characterizations indicated that the MnO2 nanorods were successfully coated with Pd particles when the Pd weight percentage was more than 4.6%. The activities of the Pd@MnO2 catalysts for oxygen reduction reaction (ORR) were investigated using a rotating disk electrode (RDE) and a rotating ring-disk electrode (RRDE). The ORR onset potentials on the Pd@MnO2 catalyst shifted positively for more than 250 mV compared with the MnO2 catalyst without Pd coatings. Both the ORR onset potentials and the limiting current density obtained by the RDE measurements on the Pd@MnO2 catalysts were close to those on the Pd black catalyst. The mass activity of the Pd@MnO2 catalysts (normalized by Pd mass) was 2.5 times higher than that of the Pd black catalyst. Based on the Tafel slopes of the Pd@MnO2 catalysts (which is about 60 mV dec−1 at low overpotentials), and based on the fact that the activation energies of the Pd@MnO2 catalysts are very close to the activation energies of the Pd catalysts, one may conclude that the small amount of Pd coating provides the primary ORR activity of the Pd@MnO2 catalysts.