Cobalt tetraphosphate as an efficient bifunctional electrocatalyst for hybrid sodium-air batteries

Economic and efficient bifunctional electrocatalysts are pivotal in realization of rechargeable (hybrid) metal-air batteries. It is ideal to employ noble-metal free bifunctional electrocatalysts that are not selective towards oxygen evolution and reduction (OER and ORR) activities. This work unveils cobalt-based tetraphosphate K2Co(PO3)4 as an economic bifunctional electrocatalyst acting as cathode for rechargeable hybrid sodium-air batteries. Autocombustion route led to the development of homogeneous, carbon-coated, spherical K2Co(PO3)4 nanoparticles enabling active site exposure to incoming guest molecules (O2, OH-). This monoclinic compound exhibited superior oxygen evolution activity with low overpotential (ca. 0.32 V) surpassing the commercial RuO2 catalyst. Tetraphosphate K2Co(PO3)4 was successfully implemented in hybrid Na-air batteries delivering reversible cycling with roundtrip efficiency over 70%. DFT study revealed this catalytic activity stem from the most active and stable surface (001) and half-metallic nature of Co in K2Co(PO3)4. Cobalt tetraphosphates can be harnessed to design low cost electrocatalysts for hybrid sodium-air batteries.