Highly efficient bifunctional catalytic activity of bismuth rhodium oxide pyrochlore through tuning the covalent character for rechargeable aqueous Na-air batteries

Na-air batteries have received significant attention as possible candidates for alternative battery systems due to their high specific energy density (1683 W h kg(-1)). However, the undesirable sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) limit the practicality of the production of rechargeable Na-air batteries. Recently, pyrochlore oxides (A(2)B(2)O(7)) have received great attention as effective bifunctional electrocatalysts. However, the comprehensive understanding of catalytic activity with the development of new pyrochlore catalysts is unsatisfactory due to the limited use of B-site cations. Here, we report the use of a novel nanocrystalline bismuth rhodium oxide (Bi2Rh2O6.8) crystal with a pyrochlore structure as a bifunctional electrocatalyst. Moreover, the surface of Bi2Rh2O6.8 was modified via phosphate-ion functionalization (P-Bi2Rh2O6.8) to enhance its surface chemical reactivity, resulting in fast and efficient charge transfer with high ORR and OER activities. During electrocatalysis, the functionalized H2PO4- ion can not only significantly enhance the surface reactivity for a fast and efficient electron/charge-transfer reaction but also facilitate the oxidation of Bi and Rh ions and boost the electron donation by improving the electron transport. Finally, the first successful translation of the bifunctional electrocatalytic activities of P-Bi2Rh2O6.8 to a practical device, an aqueous Na-air battery, was demonstrated.