Single crystalline Bi2Ru2O7 pyrochlore oxide nanoparticles as efficient bifunctional oxygen electrocatalyst for hybrid Na-air batteries

The sodium-air (Na-air) batteries are spotlighted as state-of-the-art electrical energy storage system, because of their high sodium-ion conductivity and specific energy density performance. However, the undesirable sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) kinetics limit the practical production of rechargeable Na-air batteries. Therefore, it is essential to develop highly effective bifunctional electrocatalysts for OER and ORR. Although the pyrochlore oxides (A2B2O7) exhibits great potential for highly-active bifunctional electrocatalyst, the lack of studies regarding to A-site cations have hindered the development of new pyrochlore catalysts with comprehensive understanding of catalytic activity. In this work, we report the use of a novel nanocrystalline bismuth ruthenate pyrochlore oxide (Bi2Ru2O7) as an effective oxygen electrocatalyst by using the favorable oxidation nature of Bi and Ru ions in Bi2Ru2O7. Further, the oxidized cations can donate the electrons to the surface and inner layers, providing the low-resistance pathway during OER and ORR. Finally, the bifunctional electrocatalytic activities of Bi2Ru2O7 are successfully translated to a practical device, an aqueous Na-air battery, for the first time.