Oxygen evolution at La_1-xBa_xCoO₃ perovskite-type electrodes

Perovskite oxides of the type La_1-x-Ba_xCoO₃ (x = 0 1, 0.2, 0.4) and La_0.8Ca_0.2CoO₃ were synthesized and tested as oxygen anodes in 1 mol dm^-3 NaOH solution at room temperature. The electrodes were prepared by pressing and sintering the synthesized perovskite powders. The roughness factors of the electrodes were determined by Hg-porosimetry and by double layer capacitance measurements. The ones determined by Hg-porosimetry, r_s, were found to be in the range 42<-r_s<1500, while the ones determined from the double layer capacitance, r_DL, were found to be in the range 11<r_DL<470. No correlation between the composition of the oxides and the roughness factors was found, whereas the ratio r_DL/r_s remained approximately constant and amounted to around 0.3 (±10%) for all the electrodes. The behaviour of the electrodes in the oxygen evolution reaction (OER) was studied. All the electrodes showed Tafel slopes of about 60 mV dec^-1, whereas the activity in the OER, expressed using the corresponding roughness factors, increased with increasing x and was almost one order of magnitude higher for the Ba-than the Ca-substituted perovskite, for the same degree of substitution, x=0.2. The enhanced activity of the perovskites with higher Ba content was attributed to the higher Co⁴⁺ ions content, together with the higher specific activity of these active centres for the OER, determined by cyclic voltammetry.