Mixed valency spinel oxides of transition metals and electrocatalysis: case of the MnxCo3-xO4 system

Correlations between solid state chemistry, surface properties and electroc atalytical reactivities towards the reaction of oxygen are investigated on powder electrodes of cobalt and manganese spinel type oxides MnxCo3-xO4 (0 less than or equal to x less than or equal to 1). The cationic distribution s have been computed from X-ray diffractometry, magnetism and oxidation pow er:data, showing the occurrence of the Mn4+/Mn3+ solid state redox couple i n octahedral sites. The change of the Mn4+/Mn3+ content as a function of x have been correlated to pH of zero charge, pH(z), rest potential, E-i=0, ac tivation-energy of conductance, E-a, and to the electrocatalytical paramete rs of the oxygen reduction reaction (ORR) and evolution reaction (OER). Man ganese catalyzes the ORR but inhibits the OER. (C) 1998 Published by Elsevi er Science Ltd. All rights reserved.