La0.6Ca0.4CoO3, La0.1Ca0.9MnO3 and LaNiO3 as bifunctional oxygen electrodes

A series of perovskite catalysts was investigated for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline electrolyte and at room temperature, supplied by oxygen or air. A meniscus cell was used to screen-test candidate catalysts for their bifunctionality and assess their activity for ORR at 3 mm depth of immersion (DOI) in the electrolyte. Based on the meniscus data LaNiO₃, La_0.1Ca_0.9MnO₃ and La_0.6Ca_0.4CoO₃ were selected for further assessment in microelectrode and half-cell studies. Activity tests for the ORR and OER, Tafel slopes at high current densities and apparent activation energies for the ORR were determined using a microelectrode technique on samples of the selected perovskites, La_0.1Ca_0.9MnO₃, La_0.6Ca_0.4CoO₃ and LaNiO₃ with and without graphite support. Tafel slopes of ca. 120 mV per decade and apparent activation energies of approximately 18 kcal mol⁻¹ were measured at high cathodic current densities. Cycle-life and performance of La_0.1Ca_0.9MnO₃, La_0.6Ca_0.4CoO₃ and LaNiO₃-based gas-diffusion electrodes in half-cell configurations were tested at a constant current density of 25 mA cm⁻² with subsequent and intermittent polarizations. Similar activities resulted in the ORR, while increased numbers of cycles were observed for the La_0.1Ca_0.9MnO₃-based electrode. Furthermore, electrode material compositions, especially PTFE contents were optimized to conform to the establishment of the three phase interactions of the electrode structure. Transmission Electron microscopy (TEM) and BET-surface area analyses were carried out in order to find out the morphological and surface properties of the perovskite materials.