Understanding the Highly Electrocatalytic Active Mixed Triple Conducting NaxCa3–xCo4O9–δ Oxygen Electrode Materials

Abstract

Despite their high electrocatalytic activity for oxygen electrode reactions, the low phase stability and high thermal expansion of perovskite structured materials have created difficulties in cell fabrication scale-up and long-term operational stability of reversible ceramic cells. Herein, an exceptionally high-performance electrocatalyst is presented based on a misfit-layered structure, Na0.15Ca2.85Co4O9–δ (NCCO). NCCO cells enable exceptional fuel cell performance down to 400 °C, with peak power densities of 0.18–5.15 W cm−2 at 400–800 °C, as well as electrolysis performance of minus current density 5.96–15.07 A·cm−2 (at 1.4 V) at 600–750 °C, exceeding the values of all previously described reversible (oxygen and proton) ceramic cells. Furthermore, the durability of NCCO cells is demonstrated for over 900 h at high current densities of 1 and 2 A cm−2 in fuel cells and –0.5 and –4 A cm−2 electrolysis cell modes under load cycle and constant current reversible operation, respectively. Doping with basic monovalent Na+ ions in the Ca-site in Ca3Co4O9+δ generates a high density of extra charge carrier species with the increased Co oxidation state and facilitates the proton uptake and diffusion properties of misfit-layered materials. This finding can deliver a new opportunity to develop innovative bifunctional oxygen electrode catalysts, while providing more favorable reaction pathways for the diffusion of charged species.