Dynamic reconstruction of metal sulphides during electrocatalytic oxygen evolution reaction (OER) has hampered the acquisition of legible evidence for comprehensively understanding the phase-transition mechanism and electrocatalytic activity origin. Herein, modelling on a series of cobalt-nickel bimetallic sulphides, we for the first time establish an explicit and comprehensive picture of their dynamic phase evaluation pathway at the pre-catalytic stage before OER process. By utilizing the in-situ electrochemical transmission electron microscopy and electron energy loss spectroscopy, the lattice sulphur atoms of (NiCo)S1.33 particles are revealed to be partially substituted by oxygen from electrolyte to form a lattice oxygen-sulphur coexisting shell surface before the generation of reconstituted active species. Such S-O exchange process is benefitted from the subtle modulation of metal-sulphur coordination form caused by the specific Ni and Co occupation. This unique oxygen-substitution behaviour produces an (NiCo)OxS1.33-x surface to reduce the energy barrier of surface reconstruction for converting sulphides into active oxy/hydroxide derivative, therefore significantly increasing the proportion of lattice oxygen-mediated mechanism compared to the pure sulphide surface. We anticipate this direct observation can provide an explicit picture of catalysts’ structural and compositional evolution during the electrocatalytic process.

Nanostructured metal-nitrides have attracted tremendous interest as a new generation of catalysts for electroreduction of CO2, but these structures have limited activity and stability in the reduction condition. Herein, we report a method of fabricating FeN/Fe3N nanoparticles with FeN/Fe3N interface exposed on the NP surface for efficient electrochemical CO2 reduction reaction (CO2RR). The FeN/Fe3N interface is populated with Fe−N4 and Fe−N2 coordination sites respectively that show the desired catalysis synergy to enhance the reduction of CO2 to CO. The CO Faraday efficiency reaches 98% at −0.4 V vs. reversible hydrogen electrode, and the FE stays stable from −0.4 to −0.9 V during the 100 h electrolysis time period. This FeN/Fe3N synergy arises from electron transfer from Fe3N to FeN and the preferred CO2 adsorption and reduction to *COOH on FeN. Our study demonstrates a reliable interface control strategy to improve catalytic efficiency of the Fe–N structure for CO2RR.