Influential factors on oxygen reduction at La1−xCaxCoO3 electrodes in alkaline electrolyte

Reduction of oxygen was investigated on porous electrodes made of La1−xCaxCoO3 (with 0 ≤ x ≤ 0.6) perovskite-structured oxides, prepared by a sol–gel process. It was found that both the reaction rate and the electrode active surface area (determined by cyclic voltammetry in a narrow capacitive potential range and by impedance spectroscopy) depend on the partial substitution ratio x, both displaying a maximum value for x nearly equal to 0.4. However, in spite of the parallel trend exhibited by the reduction current and the electroactive surface area parameter, the current varies much more with x, that is, while the change in the surface area amounts to approximately 40% over the explored x range, the current increases fivefold. Subsequent investigation of the electrode surface composition revealed that surface cobalt concentration (estimated by XPS analysis) deviates significantly from the nominal bulk composition (determined by EDX analysis). It follows a similar dependence on x, showing equally a maximum for x near to 0.4. Such a behaviour seems to have a greater effect on the reaction rate, since Co cations are the surface active sites for oxygen electro-reduction.