Ultrathin Nitrogenated Carbon Nanosheets with Single-Atom Nickel as an Efficient Catalyst for Electrochemical CO<sub>2</sub> Reduction

The gradual increase of CO2 concentration in the atmosphere is believed to have a profound impact on the global climate and environment.To address this issue, strategies toward effective CO2 conversion have been developed.As one of the most available strategies, the CO2 electrochemical reduction approach is particularly attractive because the required energy can be supplied from renewable sources such as solar energy.Electrochemical reduction of CO2 to chemical feedstocks offers a promising strategy for mitigating CO2 emissions from anthropogenic activities; however, a critical challenge for this approach is to develop effective electrocatalysts with ultrahigh activity and selectivity.Herein, we report the facile synthesis of a highly efficient and stable atomically isolated nickel catalyst supported by ultrathin nitrogenated carbon nanosheets (Ni-N-C) for CO2 reduction through pyrolysis of Ni-doped metal-organic frameworks (MOFs) and dicyandiamide (DCDA).MOFs are crystalline and assembled by metalcontaining nodes and organic linkers, which have a large specific surface area, tunable pore size and porosity, and highly dispersed unsaturated metal centers.Thus, Ni-doped MOFs were chosen as the precursors to endow Ni-N-C with a porous carbon structure and nickel ions.The nitrogen in Ni-N-C came from DCDA, which acts as the active site to anchor nickel ions.Because of the porous structure and numerous nitrogen sites, the Ni content of Ni-N-C was as high as 7.77% (w).There were two types of nickel ion-containing structures, including Ni + -N-C and Ni 2+ -N-C.The structure transformation of the Ni + -N-C species from the initial Ni 2+ (Ni-MOF) was achieved by pyrolysis, and the ratio of Ni + and Ni 2+ varied with the pyrolysis temperature.Compared to other Ni-N-C prepared at other temperatures, Ni-N-C-800 contained more Ni + -N-C species that possessed the optimum *CO binding energy and thus boosted the CO desorption and evolution rate, thereby exhibiting higher CO Faradaic efficiency (FE) up to 94.6% at -0.9 V (vs. the reversible hydrogen electrode, RHE) in 0.1 mol•L -1 KHCO3.In addition, it has been found that the rate of CO formation on the Ni-N-C-800 electrode relies on the electrolyte concentration.With the optimal electrolyte concentration, the Ni-N-C-800 electrode achieved a superior Faraday efficiency of > 90% for CO over a wide potential range of -0.77 to -1.07 V (vs.RHE) and displayed a CO FE as high as 97.9% with a current density of 12.6 mA•cm -2 at -0.77 V (vs.RHE) in 0.5 mol•L -1 KHCO3.After testing at -0.77 V for 12 h, the Ni-N-C-800 electrode maintained a CO FE of approximately 95%, indicating superior long-term stability.We believe that this study will contribute to the design and synthesis of highly catalytically active atomically dispersed monovalent metal sites for metal-N-C catalysts.