Effect of MnO Content on the Oxygen Reduction Activity of MnO/C Nanostructures

Manganese oxide based materials are considered as alternate non-noble metal electrocatalysts for oxygen reduction reaction (ORR). These materials possess rich redox chemistry and can decompose hydrogen peroxide disproportionately to drive the oxygen reduction towards efficient 4-electron pathway. In this work, a set of MnO nanostructures supported on activated charcoal (MnO/C) with varying MnO loadings are prepared by ball milling followed by in-situ pyrolysis. The MnO/C composites are tested for ORR activity by employing cyclic voltammetry and linear sweep voltammetry using rotating-ring disk electrode (RRDE) in 0.1 M KOH. The results indicate that the ORR activity as well as catalytic pathways are sensitive to MnO loading. The ORR activities of the composites follow volcano type relationship with the quantity of MnO loadings. The role of MnO loading on surface morphology, hydrophilicity, electrochemical double layer capacitance (C_dl) and electrochemical active surface area (ECSA) of the composites has been investigated and correlated with ORR activity. Among the MnO electroctalysts studied, 18 wt% MnO loaded sample showed the highest activity, close to that of standard Pt/C, with onset potential of 1.02 V vs. RHE and 3.48 mA cm^-2 limiting disk current in RRDE at 0.2 V. This electrocatalyst also preferred 4-electron reduction pathway in ORR and produced least amount of hydrogen peroxide. No hydrophilicity effect is found on the ORR activity of MnO/C electrocatalysts.