The ever-growing energy demand of the world demands for high energy density storage systems. In this pursuit, metal-air batteries with high-energy density (~5200 Wh kg^-1) based on O₂-H₂O based chemistry have been studied extensively. Though they are marked with high safety, low cost and enhanced ionic conductivity, the performance is still largely affected due to the formation of insoluble discharge product. Hence, hybrid aqueous air batteries can be a good alternative as the discharge product is soluble. Air batteries run on two key processes namely oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), kinetics of which are too slow hence, they require a catalyst to increase the rate of reaction. Existing catalysts are costly and lacks bifunctionality. Motivated by recent reports of various phosphate based materials having bifunctional activity, we studied the electrocatalytic activity of Na₂MPO₄F (M: Fe, Mn, Co) family. Solution-combustion method was used for the synthesis of all materials giving rise to carbon-coated grains. Presence of carbon not only helps in enhancing the electrical conductivity of the material but also improves the catalytic properties. Catalytic properties of all three materials were studied in alkaline medium and Na₂CoPO₄F was found to give the best results. The onset potential of Na₂CoPO₄F was found to be 0.854 V vs. RHE for ORR process, which is comparable to 20% Pt/C while an onset potential of 1.591 V vs. RHE was recorded for OER process. A hybrid Na-air battery was fabricated using Na₂CoPO₄F as an air-cathode and a roundtrip efficiency of 88% was recorded for over 30 cycles. The catalytic properties of all three materials were also studied by means of density functional theory (DFT) analysis. A detailed structural and catalytic analysis of the family will be presented.

Figure 1: Linear sweep voltammetry plots of Fe, Mn and Co-based fluorophosphtes recorded at 1600 rpm in 0.1 M NaOH electrolyte showing their bifunctional nature.