The atomically dispersed Fe³⁺ sites of Fe-N-C single-site catalysts (SSCs) are demonstrated as the active sites for CO₂ electroreduction (CO₂RR) to CO but suffer from the reduction to Fe²⁺ at ~ −0.5 V, accompanied by the drop of CO faradaic efficiency (FE_CO) and deterioration of partial current (J_CO). Herein, we report the construction of F-doped Fe-N-C SSCs and the electron-withdrawing character of fluorine could stabilize Fe³⁺ sites, which promotes the FE_CO from the volcano-like highest value (88.2%@−0.40 V) to the high plateau (> 88.5%@−0.40–−0.60 V), with a much-increasedJ_CO (from 3.24 to 11.23 mA·cm⁻²). The enhancement is ascribed to the thermodynamically facilitated CO₂RR and suppressed competing hydrogen evolution reaction, as well as the kinetically increased electroactive surface area and improved charge transfer, due to the stabilized Fe³⁺ sites and enriched defects by fluorine doping. This finding provides an efficient strategy to enhance the CO₂RR performance of Fe-N-C SSCs by stabilizing Fe³⁺.

The investigation of earth-abundant electrocatalysts for efficient water electrolysis is of central importance in renewable energy system, which is currently impeded by the large overpotential of oxygen evolution reaction (OER). NiFe sulfides show promising OER activity but are troubled by their low intrinsic conductivities. Herein, we demonstrate the construction of the porous core-shell heterojunctions of FeNi₃@(Fe, Ni)S₂ with tunable shell thickness via the reduction of hierarchical NiFe(OH)_x nanosheets followed by a partial sulfidization. The conductive FeNi₃ core provides the highway for electron transport, and the (Fe, Ni)S₂ shell offers the exposed surface for in situ generation of S-doped NiFe-oxyhydroxides with high intrinsic OER activity, which is supported by the combined experimental and theoretical studies. In addition, the porous hierarchical morphology favors the electrolyte access and O₂ liberation. Consequently, the optimized catalyst achieves an excellent OER performance with a low overpotential of 288 mV at 100 mA·cm^-2, a small Tafel slope of 48 mV·dec^-1, and a high OER durability for at least 1, 200 h at 200 mA·cm^-2. This study provides an effective way to explore the advanced earth-abundant OER electrocatalysts by constructing the heterojunctions between metal and corresponding metal-compounds via the convenient post treatment, such as nitridation and sulfidization.

Metal-nitrogen-carbon materials are promising catalysts for CO₂ electroreduction to CO. Herein, by taking the unique hierarchical carbon nanocages as the support, an advanced nickel-nitrogen-carbon single-site catalyst is conveniently prepared by pyrolyzing the mixture of NiCl₂ and phenanthroline, which exhibits a Faradaic efficiency plateau of > 87% in a wide potential window of -0.6 - -1.0 V. Further S-doping by adding KSCN into the precursor much enhances the CO specific current density by 68%, up to 37.5 A·g^-1 at -0.8 V, along with an improved CO Faradaic efficiency plateau of > 90%. Such an enhancement can be ascribed to the facilitated CO pathway and suppressed hydrogen evolution from thermodynamic viewpoint as well as the increased electroactive surface area and improved charge transfer fromkinetic viewpoint due to the S-doping. This study demonstrates a simple and effective approach to advanced electrocatalysts by synergetic modification of the porous carbon-based support and electronic structure of the active sites.