Surface-Engineered Ni₂P: An Efficient Oxygen Electrocatalyst for Zinc-Air Battery

The surface engineering of electrocatalysts is one of the promising strategies to increase the intrinsic activity of electrocatalysts. It generates anion/cation vacancy defects and increases the electrochemically active surface area. We describe the surface engineering of Ni₂P to favorably tune the bifunctional oxygen electrocatalytic activity and the development of a rechargeable zinc-air battery (ZAB). Ni₂P encapsulated with N and P-dual doped carbon (Ni₂P@NPC) is synthesized using a single-source precursor complex tris-(2,2'-bipyridine)nickel(II) bis(hexafluorophosphate). The surface engineering of the as-synthesized Ni₂P@NPC catalyst is achieved by the controlled acid treatment at room temperature. The surface engineering removes the carbon debris and opens the pores, exfoliates the encapsulating carbon layer, increases the P-vacancy in the crystal lattice, and boosts the electrochemically active surface area. The surface-engineered catalyst exhibits enhanced bifunctional activity towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The electrocatalytically active sites of engineered catalysts are highly accessible for facilitated electron transfer kinetics. P-vacancy favors the facile formation of defect-rich OER active metal oxyhydroxide species. The rechargeable ZAB based on the engineered catalyst delivers a specific capacity of 770.25 mA h g_Zn ^-1, energy density of 692 Wh kg_Zn ^-1, and excellent charge-discharge cycling performance with negligible voltaic efficiency loss (0.6 %) after 100 h.