Turning the Surface Electronic Effect Over Core‐Shell CoS₂─Fe_xCo_1‐xS₂ Nanooctahedra Toward Electrochemical Water Splitting in the Alkaline Medium

The long-term challenge in overall water splitting is the conflict in the pH condition of electrolytes for achieving efficient hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at the same time, in addition to the typical cost issue in catalysts. It hence raises an intense research interest in seeking cost-efficient non-noble metal electrocatalysts as well as compromising electrolyte conditions for electrocatalytic HER and OER. To tackle the problems, various approaches are demonstrated to engineer the electronic effect on the active sites of catalysts for enhancing the activities. In this work, the core-shell CoS₂─Fe_xCo_1-xS₂ nanooctahedra is fabricated with a tunable Fe content over the surface and took them as the model catalyst for systematic studies in alkaline OER and HER. By various X-ray spectroscopies as well as electron microscopy, the results showed that the shells of CoS₂─Fe_xCo_1-xS₂ nanooctahedra formed the {111} surfaces of Fe_0.9Co_1.0S₂ and Fe_0.25Co_0.75S₂ with and without the promotion by OH^- anions during the syntheses. Catalyzed by the CoS₂, Fe_0.25Co_0.75S₂, and Fe_0.9Co_1.0S₂ {111} surfaces, the results of alkaline OER and HER indicated the Fe_0.9Co_1.0S₂ the most superior activities by virtue of the optimized Fe─Co electronic effect. From the predictions by density functional theory (DFT) calculations in reaction thermodynamics, the energy barriers in OER and HER both follow the order of Fe_0.9Co_0.1S₂(111) < Fe_0.25Co_0.75S₂(111) < CoS₂(111). However, FeS₂(111) is worse than Fe_0.9Co_0.1S₂(111). From the confirmations by in-situ X-ray spectroscopies in reaction kinetics, the Co sites of Fe_0.9Co_0.1S₂(111) on the core-shell nanooctahedra exhibited much higher activities than those of CoS₂(111) under the applied potentials for OER and HER, which reflected the electronic benefits from the existing Fe neighbors.