Facile Synthesis of Hollow-Structured Fe, F Dual-Doped CoS₂ Electrocatalyst for Oxygen Evolution Reaction

Due to the serious environmental problems caused by excessive consumption of fossil fuels, developing renewable energy sources and sustainable storage process have attracted significant attention from many researchers. The hydrogen is considered as one of the leading options for alternative energy carrier owing to its high gravimetric energy density. Water electrolysis is an efficient way to produce ultra-high purity hydrogen without environmental pollution. However, sluggish reaction dynamics and high overpotential of oxygen evolution reaction (OER) at anode greatly hinders the overall efficiency of water electrolysis. Although precious metal-based materials, such as IrO₂ and RuO₂, are considered as state-of-the-art OER electrocatalysts, their high cost, scarcity and low stability during long-term operation have limited their large-scale applications. Therefore, enormous efforts have been performed for development of highly efficient and stable noble metal-free electrocatalysts. Recently, metal-organic frameworks (MOFs), a class of inorganic-organic hybrid crystalline materials fabricated via the self-assembly of metal cations and organic building blocks, have aroused considerable interest owing to their high porosity, multifunctional properties, large specific surface area. Prussian-blue-analogues (PBAs) with the formula M^II₃[M^III(CN)₆]₂ (M = Fe, Co, Ni, etc) are a class of MOFs with coordinated metal sites linked by the cyano groups. PBA materials can be simply converted to transition-metal based chalcogenides, which are widely explored as efficient OER electrocatalysts. Therefore, they can be utilized as an appropriate precursor for transition metal-based electrocatalysts.<br/>In this study, Fe, F dual-doped CoS₂ nanospheres with abundant active sites using bimetallic CoFe-PBA were fabricated for OER via facile self-templating co-precipitation strategy and subsequent annealing processes with sulfur and fluorine source. First, the monometallic Co-glycerate nanospheres were employed as a self-sacrificial template because of its facile synthetic procedure and low cost. After that, the Co-glycerate was converted to CoFe PBA via simultaneous dissolution and precipitation process at room temperature for 24 h. The self-templating strategy provides a simple synthetic route to prepare hollow-structured bimetallic nanospheres without additional steps of template elimination. The CoFe PBA hollow nanospheres were calcined with sulfur powder to prepare Fe-CoS₂. Subsequently, the prepared samples were pyrolyzed with NH₄F in Ar flow to obtain F, Fe dual-doped CoS₂ hollow nanospheres (Fe-CoS₂-F). The high electronegativity of F (4.0) compared with those of Fe (1.83), Co (1.88), and S (2.58) can partially withdraw electrons from other elements, thereby modulating the surface electronic properties and adsorption energy of oxygen-containing reaction intermediates. Furthermore, simultaneous cation and anion dual-doping can maximize the surface polarity, promoting the infiltration of aqueous electrolytes during OER procedure. As expected, the as-prepared Fe, F dual-doped CoS₂ nanospheres exhibited superior electrochemical performance toward OER with an overpotential of 298 mV to achieve the current density of 10mA cm^-2, and a Tafel slope of 46 mV dec^-1in 1.0 M KOH, which was much better than the commercial RuO₂ catalysts.