Zinc-Substituted Cobalt Phosphate [ZnCo₂(PO₄)₂] as a Bifunctional Electrocatalyst

Development of highly efficient, earth-abundant, and stable bifunctional electrocatalysts is pivotal for designing viable next-generation metal-air batteries. Cobalt-based phosphates provide a treasure house to design electrocatalysts, with a wide range of cation substitutions to further enhance their electrocatalytic activity. In particular, phosphates with distorted geometry show favorable binding efficiency toward water molecules with low overpotential. In the present work, zinc-substituted cobalt phosphate ZnCo₂(PO₄)₂ was investigated. Its crystal structure was solved to a monoclinic framework built with CoO₆ octahedra and distorted CoO₅/ZnO₅ trigonal bipyramid leading to efficient bifunctional electrocatalytic activity. It offers robust structural stability with onset potential values of 0.87 V (vs reversible hydrogen electrode (RHE)) and 1.50 V (vs RHE) for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes, respectively, comparable to the precious metal catalysts. The origin and stability of the bifunctional activity were probed by combining ex situ diffraction and electron microscopy corroborated by ab initio calculations. Overall, zinc-substituted cobalt phosphate [ZnCo₂(PO₄)₂] forms a potential bifunctional electrocatalyst with tunable local cobalt coordination that can be harnessed for metal-air batteries.