Orthorhombic Cr_1–<i>x</i>Fe_<i>x</i>PO₄ (0 ≤ <i>x</i> ≤ 0.2): An Efficient Oxygen Evolution Reaction Electrocatalyst in Alkaline Medium

In the current scenario of the energy crisis and environmental concerns, oxygen evolution reaction (OER) electrocatalysts are of vital importance for their application in metal–air batteries and efficient water splitting to produce hydrogen. The best possible electrocatalysts for the OER are considered to be noble metal-based materials like RuO2 and IrO2, but their high cost and instability have prevented their wider use for application in large-scale water splitting. Here, we envisaged CrPO4 as a cost-efficient host due to the formation of CrO6 octahedra interconnected through PO4 linkage, making it a stable framework structure to harness eg electrons that are pinned over the O(2p) orbital, required for superior OER activity. The presence of more electronegative Fe3+ or Cr3+ sites in the framework structure, through the inductive effect, enhances the iconicity of the Cr–O bond in the framework, and creation of CrO42– groups at the surface of the catalyst directly facilitates the adsorption/desorption of OH– in the form of –Cr–O–OH, which reduces the overpotential of the OER with higher structural stability of the electrocatalyst. In turn, Cr0.9Fe0.1PO4 demonstrated superior electrocatalytic activity toward the OER in the alkaline electrolyte, together with a low overpotential of 292 mV at 10 mA cm–2 current density with a Tafel slope of 49 mV dec–1, better than that of the well-known electrocatalyst RuO2. Enhanced OER activity was observed due to the feasibility of 3-electron transfer with Cr3+/6+ redox as there was a significant increase at the redox peak current density and the presence of Cr6+ also facilitates oxygen evolution due to the strong overlap between Cr(3d) and O(2p) orbitals, which generates superior charge transfer from O(2p) bands to the Cr(3d) orbital, resulting in the reduction of oxygen or evolution of oxygen at a lower overpotential.