Cobalt Metaphosphates as Economic Bifunctional Electrocatalysts for Hybrid Sodium-Air Batteries

Bifunctional electrocatalysts are pre-eminent to achieve high capacity, cycling stability, and high Coulombic efficiency for rechargeable hybrid sodium-air batteries. The current work introduces metaphosphate (Na)KCo(PO₃)₃ nanostructures as noble metal-free bifunctional electrocatalysts suitable for the rechargeable aqueous sodium-air battery. Prepared by the scalable solution combustion method, the metaphosphate class of (Na)KCo(PO₃)₃ with spherical morphology exhibited robust oxygen reduction as well as evolution activity similar to the state-of-the-art catalysts. NaCo(PO₃)₃ metaphosphate, when employed as an air cathode in hybrid sodium-air batteries, delivered reasonably low overpotential along with excellent cycling stability with a round-trip energy efficiency of 78%. Cobalt metaphosphates thus form a new class of economical bifunctional catalysts to develop hybrid sodium-air batteries.