Electrocatalytic properties of new active ternary ferrite film anodes for O2 evolution in alkaline medium

Some ternary ferrites with molecular formula, CoFe2-xCrxO4 (0 less than or equal to x less than or equal to 1.0) have been synthesized at 70 degreesC by a precipitation method and were transformed into the film form at the pretreated Ni support (1.5 x 1.0 cm(2)) using an oxide-slurry painting technique. The study showed that Cr-substitution from 0.2 to 1.0 mol increased the electrocatalytic activity of the oxide towards the oxygen evolution reaction (OER), the optimum improvement in apparent electrocatalytic activity being with 0.8 mol Cr. At E = 600 mV versus Hg/HgO in 1 M KOH (25 degreesC), the apparent oxygen evolution current density (j(d)) with the catalyst, CoFe1.2Cr0.8O4, was similar to 80 times greater than that observed with the base oxide (i.e. CoFe2O4). The OER on Cr-substituted oxides showed two Tarel slopes, one (b = 42 +/- 1 mV per decade) at low overpotential and the other (b = 66 +/- 6 mV per decade) at higher potential. The reaction order with respect to OH- concentration was similar to 1.3 +/- 0.1 for each electrocatalyst. The thermodynamic parameters for the OER, namely, standard apparent electrochemical enthalpy of activation (DeltaH(el)(o#)), standard enthalpy of activation (DeltaH(o#)) and standard entropy of activation (DeltaS(o#)) have also been determined. It was observed that values of the DeltaH(el)(o#) and DeltaH(o#) decreased with Cr-substitution in the CoFe2O4 lattice; the decrement, however, being the greatest with 0.8 mol Cr. The DeltaS(o#) values were largely negative varying between similar to - 61 and - 126 J deg(-1) mol(-1). (C) 2002 Published by Elsevier Science Ltd.