Bifunctional Electrocatalytic Behavior of Sodium Cobalt Phosphates in Alkaline Solution

Noble-metal-free, bifunctional catalysts are essential to develop high-performance, cost-effective water-splitting systems. The current work explores the bifunctional electrocatalytic behavior of two sodium cobalt phosphate systems, namely, NaCoPO4 and Na2CoP2O7, prepared by solution combustion synthesis (SCS) for the first time. Formation of phase-pure orthorhombic maricite NaCoPO4 (space group: Pnma) and orthorhombic Na2CoP2O7 (space group: Pna2(1)) was confirmed by Rietveld refinement. The electrocatalytic activity of maricite NaCoPO4 and Na2CoP2O7 was investigated by using linear sweep voltammetry with a rotating disk electrode (RDE). The oxygen reduction reaction (ORR) activities of these sodium cobalt phosphates are comparable to Vulcan carbon black and Pt/C. Oxygen evolution reaction (OER) activity of Na2CoP2O7 is dominant compared to NaCoPO4 and Pt/C. The Tafel slope, electron-transfer number, and stability of the sodium-metal phosphates were calculated in different concentrations of Na+-containing aqueous electrolyte. The bifunctional activity and good stability of the sodium cobalt phosphates stem from cobalt ions and stabilization of the catalytic centers by the phosphate frameworks. The present work demonstrates sodium cobalt phosphates as alternate cost-effective, novel electrocatalysts for efficient OER/ORR activity in alkaline solution.