Oxygen evolution electrocatalysis in alkaline medium at thin MnxCo3-xO4 (0<= x <= 1) spinel films on glass/SnO2 : F prepared by spray pyrolysis

Films of a series of manganese-cobalt mixed valency spinel oxides of theore tical formula MnxCo3-xO4, (x = 0; 0.25; 0.5; 0.75 and 1), have been prepare d by spray pyrolysis at 150 degrees C on conductive glass in order to relat e solid state and surface chemistry to electrocatalytical activity towards the oxygen evolution reaction (OER). X-ray diffraction (XRD) and IR spectro scopy characterized their bulk crystalline properties. Both techniques have confirmed that all oxides belong to the spinel family. Their surface prope rties were characterized by XPS yielding the Co3+/Co2+ surface ratio. The k inetics of the OER was investigated in 1 M KOH by means of steady state cur rent-potential curves, obtained in the potentiostatic mode and by reaction order determination. The order of reaction ranging from 1.0 to 1.2, and the Tafel slopes ranging from 67 to 69 mV dec(-1) can be tentatively interpret ed by a mechanism involving a Temkin type of electrosorption of OH intermed iates associated with an electrochemical rate-determining step. Replacing p rogressively Co by Mn, the electrodes were increasingly electrocatalyticall y less active, surface Co3+ ions being the active sites, and making Co3O4 t he most active in the series.