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 m(2) 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 degrees 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 degrees C. The 2-electron pathway of oxygen reduction, yielding hydrogen peroxides as intermediates, may however be favoured over the 4-electron O-2 reduction at higher temperatures. (C) 2009 Elsevier B.V. All rights reserved.