Orthorhombic\nCr<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>PO<sub>4</sub> (0 ≤ <i>x</i> ≤ 0.2): An Efficient\nOxygen Evolution Reaction\nElectrocatalyst in Alkaline Medium

In\nthe current scenario of the energy crisis and environmental\nconcerns, oxygen evolution reaction (OER) electrocatalysts are of\nvital importance for their application in metal–air batteries\nand efficient water splitting to produce hydrogen. The best possible\nelectrocatalysts for the OER are considered to be noble metal-based\nmaterials like RuO<sub>2</sub> and IrO<sub>2</sub>, but their high\ncost and instability have prevented their wider use for application\nin large-scale water splitting. Here, we envisaged CrPO<sub>4</sub> as a cost-efficient host due to the formation of CrO<sub>6</sub> octahedra interconnected through PO<sub>4</sub> linkage, making\nit a stable framework structure to harness e<sub>g</sub> electrons\nthat are pinned over the O(2p) orbital, required for superior OER\nactivity. The presence of more electronegative Fe<sup>3+</sup> or\nCr<sup>3+</sup> sites in the framework structure, through the inductive\neffect, enhances the iconicity of the Cr–O bond in the framework,\nand creation of CrO<sub>4</sub><sup>2–</sup> groups at the\nsurface of the catalyst directly facilitates the adsorption/desorption\nof OH<sup>–</sup> in the form of –Cr–O–OH,\nwhich reduces the overpotential of the OER with higher structural\nstability of the electrocatalyst. In turn, Cr<sub>0.9</sub>Fe<sub>0.1</sub>PO<sub>4</sub> demonstrated superior electrocatalytic activity\ntoward the OER in the alkaline electrolyte, together with a low overpotential\nof 292 mV at 10 mA cm<sup>–2</sup> current density with a Tafel\nslope of 49 mV dec<sup>–1</sup>, better than that of the well-known\nelectrocatalyst RuO<sub>2</sub>. Enhanced OER activity was observed\ndue to the feasibility of 3-electron transfer with Cr<sup>3+/6+</sup> redox as there was a significant increase at the redox peak current\ndensity and the presence of Cr<sup>6+</sup> also facilitates oxygen\nevolution due to the strong overlap between Cr(3d) and O(2p) orbitals,\nwhich generates superior charge transfer from O(2p) bands to the Cr(3d)\norbital, resulting in the reduction of oxygen or evolution of oxygen\nat a lower overpotential.