Electrocatalytic activity of co-redox center in phosphate-based oxyanionic sodium battery materials

Rechargeable metal-air batteries have come up with high theoretical energy density as compared to existing lithium and sodium ion batteries. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the two processes, related to water splitting, over which metal-air batteries run. Since both processes are sluggish, hence, they require a catalyst and since the existing precious-metal based catalysts are selective and expensive, there is a need to find a suitable bifunctional catalyst for the same. Motivated by enhanced structural stability imparted by the polyanionic groups along with many reports on electrocatalytic activity of Cobalt phosphate based materials, we tried to look into the electrocatalytic behavior of Na₂CoPO₄F and Na₄Co₃(PO₄)₂P₂O₇. Both materials were synthesized via solution combustion route resulting in carbon coated and amorphous particles. The ORR/OER properties of the materials were studied in alkaline solution showing bifunctional behavior. An onset potential of 0.854 V and 0.771 V vs. RHE was observed for Na₂CoPO₄F and Na₄Co₃(PO₄)₂P₂O₇ with a current density of 5.84 mA cm⁻² and 3.4 mA cm⁻² respectively. The onset potential during OER was observed to be 1.591 V and 1.596 V vs. RHE for Na₂CoPO₄F and Na₄Co₃(PO₄)₂P₂O₇ respectively. As an application, aqueous Na-air battery was fabricated using Na₂CoPO₄F as an air-cathode showing good (dis)charge behavior.