Covalent\nEntrapment of Cobalt–Iron Sulfides\nin N‑Doped Mesoporous Carbon: Extraordinary Bifunctional Electrocatalysts\nfor Oxygen Reduction and Evolution Reactions

Abstract

To alleviate the kinetic barriers\nassociated with ORR (oxygen reduction\nreaction) and OER (oxygen evolution reaction) in electrochemical systems,\nefficient nonprecious electrocatalysts are urgently required. Here\nwe report a facile soft-template mediated approach for fabrication\nof nanostructured cobalt–iron double sulfides that are covalently\nentrapped in nitrogen-doped mesoporous graphitic carbon (Co_0.5Fe_0.5S@N-MC). Notably, with a positive half-wave potential\n(0.808 V) and a high diffusion-limiting current density, the composite\nmaterial delivers unprecedentedly striking ORR electrocatalytic activity\namong recently reported nonprecious late transition metal chalcogenide\nmaterials in alkaline medium. Various characterization techniques,\nincluding X-ray absorption spectroscopy, X-ray photoelectron spectroscopy,\nand X-ray diffraction, are conducted to elucidate the correlation\nbetween structural features and catalytic activities of the composite.\nModerate substitution and well-dispersion of iron in bimetallic sulfide\ncomposites are believed to have positive effect on the adsorption\nand activation of oxygen-containing species, thus leading to conspicuous\nORR and OER catalytic enhancement compared to their monometallic counterparts.\nBesides, the covalent bridge between active sulfide particles and\nmesoporous carbon shells provides facile pathways for electron and\nmass transport. Beneficially, the intimate coupling interaction renders\nprolonged electrocatalytic performances to the composite. Our results\nmay possibly lend a new impetus to the rational design of bi- or multimetallic\nsulfides encapsulated in porous carbon with improved performance for\nelectrocatalysis and energy storage applications.