Phase-Induced\nStrain Effect to Synthesize an Iron-Doped\nOrthogonal Cobalt Selenide Electrocatalyst for the Oxygen Evolution\nReaction

The etching effect has the capability to control atom\ndoping and\ntrigger phase transformation, thereby enhancing the electrocatalytic\nreaction. Herein, iron-doped cobalt selenide (Fe-CoSe<sub>2</sub>)\nnanoparticle-decorated carbon nanofibers (Fe-CoSe<sub>2</sub>/CNFs)\nare synthesized by assembling an FeCo-Prussian blue analogue (FeCo-PBA)\ncube precursor with polyacrylonitrile fibers and then treating with\nhydrochloric acid, followed by gas phase selenization. The Fe-CoSe<sub>2</sub>/CNFs catalyst exhibits a large surface area and a porous\nstructure, facilitating the permeation of electrolytes. Moreover,\northorhombic CoSe<sub>2</sub> is obtained, which is in favor of improving\nthe oxygen evolution reaction (OER). By modulating the etching time,\nthe ideal crystal phase and the optimal amount of the dopant (Fe)\ncan be achieved, thus showing favorable OER activity. Specifically,\nthe Fe-CoSe<sub>2</sub>/CNFs electrocatalyst enables high electrocatalytic\nactivity for the OER with a low overpotential of 263 mV to drive a\ncurrent density of 10 mA cm<sup>–2</sup> in 1 M KOH. A small\nTafel slope of 51 mV dec<sup>–1</sup> shows fast charge transfer\nkinetics. Density functional theory (DFT) calculations reveal that\nFe-doped orthorhombic CoSe<sub>2</sub>(111) can modulate the electron\nstructure, contributing to OH<sup>–</sup> adsorption ability.\nGiven this, a strategy for phase transformation induced by etching\ntechnology is proposed to improve the intrinsic activity of the catalyst.