safe limit 350 ppm), [7] which increased to 414 ppm in 2020, [3,8] with an annual growth rate of ≈1.57 ppm. This value will probably reach 570 ppm by 2100 without proper regulation. [9] Such high concentration of CO 2 in the atmosphere have triggered a series of ecological and environmental issues, including climate change, glacier melting, and sea levels rising, [7,8,10,11] which threaten the sustainable development of humans. Significant endeavors have been devoted to impede the increase of CO 2 concentration, which includes the exploitation of renewable green energy to alleviate the dependence on fossil fuels, [7,12,13] CO 2 sequestration, [14] and mimicking the natural photosynthesis process by converting CO 2 and H 2 O into valuable chemical products to close the carbon cycle. [15,16] Artificial photosynthesis in particular has attracted considerable research interest over the past few decades, since it offers the possibility of producing chemical products that traditionally rely on fossil fuels using waste CO 2 . [15,17] Various technologies have been implemented in artificial photosynthesis process, such as biocatalysis, [18,19] thermocatalysis, [20,21] photocatalysis, [22][23][24][25][26] electrocatalysis, [27][28][29] as well as photoelectrocatalysis. [30] Electrochemical CO 2 reduction (ECO 2 R) has attracted considerable attention due to its unique advantages, which include the relatively mild operating conditions; the tunable product selectivity and reaction path by regulating reaction Excessive anthropogenic CO 2 emission has caused a series of ecological and environmental issues, which threatens mankind's sustainable development. Mimicking the natural photosynthesis process (i.e., artificial photosynthesis) by electrochemically converting CO 2 into value-added products is a promising way to alleviate CO 2 emission and relieve the dependence on fossil fuels. Recently, Sn-based catalysts have attracted increasing research attentions due to the merits of low price, abundance, non-toxicity, and environmental benignancy. In this review, the paradigm of nanostructure engineering for efficient electrochemical CO 2 reduction (ECO 2 R) on Sn-based catalysts is systematically summarized. First, the nanostructure engineering of size, composition, atomic structure, morphology, defect, surficial modification, catalyst/ substrate interface, and single-atom structure, are systematically discussed. The influence of nanostructure engineering on the electronic structure and adsorption property of intermediates, as well as the performance of Sn-based catalysts for ECO 2 R are highlighted. Second, the potential chemical state changes and the role of surface hydroxides on Sn-based catalysts during ECO 2 R are introduced. Third, the challenges and opportunities of Sn-based catalysts for ECO 2 R are proposed. It is expected that this review inspires the further development of highly efficient Sn-based catalysts, meanwhile offer protocols for the investigation of Sn-based catalysts.
