Understanding the Highly Electrocatalytic Active Mixed Triple Conducting Na_xCa_(3–x)Co_4O_(9–δ) Oxygen Electrode Materials

摘要

Despite their high electrocatalytic activity for oxygen electrode reactions,the low phase stability and high thermal expansion of perovskitestructured materials have created difficulties in cell fabrication scale-upand long-term operational stability of reversible ceramic cells. Herein, anexceptionally high-performance electrocatalyst is presented based on amisfit-layered structure, Na_(0.15)Ca_(2.85)Co_4O_(9–δ) (NCCO). NCCO cells enableexceptional fuel cell performance down to 400 ℃, with peak power densitiesof 0.18–5.15 W cm~(?2) at 400–800 ℃, as well as electrolysis performanceof minus current density 5.96–15.07 A·cm~(?2) (at 1.4 V) at 600–750 ℃,exceeding the values of all previously described reversible (oxygen andproton) ceramic cells. Furthermore, the durability of NCCO cells isdemonstrated 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 loadcycle and constant current reversible operation, respectively. Doping withbasic monovalent Na~+ ions in the Ca-site in Ca_3Co_4O_(9+δ) generates a highdensity of extra charge carrier species with the increased Co oxidationstate and facilitates the proton uptake and diffusion properties of misfitlayeredmaterials. This finding can deliver a new opportunity to developinnovative bifunctional oxygen electrode catalysts, while providing morefavorable reaction pathways for the diffusion of charged species.