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The construction of efficient and stable non-noble metal bifunctional electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) presents a challenge in electrocatalytic water splitting. CoS 2 is a promising electrocatalyst that could replace precious metals. In this work, a series of Co-MOF precursors having average particle sizes ranging from the nanometer to the micron scale (86–1084 nm) were synthesized in an unprecedented attempt; Co/CoS 2 heterojunction embedded in nitrogen doped carbon framework (Co/CoS 2 @NC) was prepared through further carbonization and sulfurization. The samples were characterized and evaluated by X-ray powder diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electrochemical techniques. The results indicate that when the size of Co-MOF is less than 300 nm, the cubic structure framework easily collapses to form the spherical structure during the carbonization process. The catalytic activity of Co@NC is size-dependent; Co/CoS 2 @NC exhibited the best performance among the series, with an overpotential of 188 mV for HER and 349 mV for OER at a current density of 10 mA cm−2. This work provides experimental guidance for the design and synthesis of low-cost, efficient, and robust Co-based electrocatalysts. • With Co-MOF cubic nanoparticles as the precursor, Co/CoS 2 @NC was prepared through carbonization and sulfurization. • Co/CoS 2 @NC exhibited the best performance among the series. • The catalytic activity of Co@NC is size-dependent. • This work provides a guidance for the design and synthesis of low-cost, efficient, and robust Co-based electrocatalysts. [ABSTRACT FROM AUTHOR]

Copper‐based materials are efficient electrocatalysts for the conversion of CO2 to C2+ products, and most these materials are reconstructed in situ to regenerate active species. It is a challenge to precisely design precatalysts to obtain active sites for the CO2 reduction reaction (CO2RR). Herein, we develop a strategy based on local sulfur doping of a Cu‐based metal–organic framework precatalyst, in which the stable Cu−S motif is dispersed in the framework of HKUST‐1 (S‐HKUST‐1). The precatalyst exhibits a high ethylene selectivity in an H‐type cell with a maximum faradaic efficiency (FE) of 60.0 %, and delivers a current density of 400 mA cm−2 with an ethylene FE up to 57.2 % in a flow cell. Operando X‐ray absorption results demonstrate that Cuδ+ species stabilized by the Cu−S motif exist in S‐HKUST‐1 during CO2RR. Density functional theory calculations indicate the partially oxidized Cuδ+ at the Cu/CuxSy interface is favorable for coupling of the *CO intermediate due to the modest distance between coupling sites and optimized adsorption energy. [ABSTRACT FROM AUTHOR]

Copper‐based materials are efficient electrocatalysts for the conversion of CO2 to C2+ products, and most these materials are reconstructed in situ to regenerate active species. It is a challenge to precisely design precatalysts to obtain active sites for the CO2 reduction reaction (CO2RR). Herein, we develop a strategy based on local sulfur doping of a Cu‐based metal–organic framework precatalyst, in which the stable Cu−S motif is dispersed in the framework of HKUST‐1 (S‐HKUST‐1). The precatalyst exhibits a high ethylene selectivity in an H‐type cell with a maximum faradaic efficiency (FE) of 60.0 %, and delivers a current density of 400 mA cm−2 with an ethylene FE up to 57.2 % in a flow cell. Operando X‐ray absorption results demonstrate that Cuδ+ species stabilized by the Cu−S motif exist in S‐HKUST‐1 during CO2RR. Density functional theory calculations indicate the partially oxidized Cuδ+ at the Cu/CuxSy interface is favorable for coupling of the *CO intermediate due to the modest distance between coupling sites and optimized adsorption energy. [ABSTRACT FROM AUTHOR]