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Tin oxide(SnOx)nanocatalysts with a special coralline-like structure are developed via a simple one-step hydrothermal synthesis and then used to prepare a SnOx/GDL(gas diffusion layer)electrode for CO2 electroreduction to formate in 0.5 M KHCO3 aqueous solution.The effects of synthesis conditions such as hydrothermal synthesis temperature(T)and time(△t)on catalysts' morphologies,and valence state of Sn in SnOx nanocatalysts on Faradaic efficiency for formate production are investigated systematically.Using a SnOx(100-8)/GDL(i.e.T and △t are 100℃ C and 8 hours,respectively)as cathode electrode,the maximum Faradaic efficiency as high as 87.1%was obtained at controlled potential of-1.6 V vs.SHE(standard hydrogen electrode),superior to all reported SnOx and Sn/SnOx catalysts in literature.Combined XPS and XRD analysis,it is found that SnOx are composed of SnO and SnO2,where the SnO nanoparticles are covered by about 1~2 nm thickness SnO2 film,contributing to the catalytic active sites for CO2 electroreduction.Such a SnOx(100-8)nanocatalyst(corallined structure)exhibits the better durability,compared with a SnOx(100-4)(non-corallined structure),as evaluated by a stable catalytic current density of approximately 10 mA cm-2 over 20 hours continuous operation.