Activation/Deactivation Phenomena’s in the Electrochemical Reduction of O₂ and NO on La_1−xSr_xFeO_3−δ

Abstract

Three ferrite based perovskites were investigated as cathodes for the electrochemical reduction of oxygen and nitric oxide, using cone-shaped electrodes and cyclic voltammetry. The ferrites investigated were LaFeO₃, La_0.85Sr_0.15FeO_3−δ and La_0.5Sr_0.5FeO_3−δ. Especially the effect of anodic pre-treatment of the electrodes was investigated. It was shown that the ferrite based electrodes are better at reducing nitric oxide than oxygen in the temperature range 200 to 400°C. The ability of the ferrites to reduce oxygen depends mainly on the amount of Fe(III) and oxide anion vacancies, leading to the highest activity towards the reduction of oxygen for the intermediate compound La_0.85Sr_0.15FeO_3−δ in the investigated temperature range. All the electrodes were either activated or de-activated when polarized at 0.4 vs. air, before recording the voltammograms, depending on temperature and gas atmosphere. This was attributed to annihilation of oxide ion vacancies, strontium segregation to the surface of the electrodes and perhaps formation of Fe(IV). The effect of water vapor was also studied. It had a slightly deactivating effect on the activity in the nitric oxide containing atmosphere, the effect being highest at the lowest temperatures.