Effect of pH and temperature on carbon-supported manganese oxide oxygen reduction electrocatalysts

Manganese oxides nanoparticles were chemically deposited on a high area (ca. 300 m2 g−1) carbon black substrate to act as electrocatalysts for oxygen reduction. The morphology and chemistry of the carbon-supported MnOx nanoparticles was characterised by Transmission Electron Microscopy), X-ray Diffraction, and chemical analysis. The oxygen reduction reaction (ORR) catalytic activity was studied in the 7–10 pH range using a rotating disk electrode (RDE). High activity towards oxygen reduction and very good stability in neutral and slightly basic solution were obtained. At low current densities, at 25 °C, MnOx/C displayed a reaction order with respect to OH− ions of −0.5 and Tafel slopes of −0.153 and −0.167 V dec−1 at pH 7 and 10 respectively; showing that the ORR mechanism on MnOx/C is unchanged in the 7–10 pH range. From the data, we propose that the first electrochemical step of the 4-electron ORR mechanism, in the 7–10 pH range, is the quasi equilibrium proton insertion process in MnO2 yielding MnOOH (insoluble in neutral or slightly basic solution). The ORR activity of the MnOx/C materials increased with increasing temperatures from 5 to 40 °C. The 2-electron pathway of oxygen reduction, yielding hydrogen peroxides as intermediates, may however be favoured over the 4-electron O2 reduction at higher temperatures.