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

Some ternary ferrites with molecular formula, CoFe_2−xCr_xO₄ (0≤x≤1.0) have been synthesized at 70 °C by a precipitation method and were transformed into the film form at the pretreated Ni support (1.5×1.0 cm²) 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 °C), the apparent oxygen evolution current density (j_a) with the catalyst, CoFe_1.2Cr_0.8O₄, was ∼80 times greater than that observed with the base oxide (i.e. CoFe₂O₄). The OER on Cr-substituted oxides showed two Tafel 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 ∼1.3±0.1 for each electrocatalyst. The thermodynamic parameters for the OER, namely, standard apparent electrochemical enthalpy of activation (ΔH°_el^#), standard enthalpy of activation (ΔH°^#) and standard entropy of activation (ΔS°^#) have also been determined. It was observed that values of the ΔH°_el^# and ΔH°^# decreased with Cr-substitution in the CoFe₂O₄ lattice; the decrement, however, being the greatest with 0.8 mol Cr. The ΔS°^# values were largely negative varying between ∼−61 and −126 J deg⁻¹ mol⁻¹.