Although Sn-based catalysts have recently achieved considerable improvement in selective electro-catalyzing CO2 into HCOOH,the role of various valence Sn species is not fully understood due to the complexity and uncertainty of their evolution during the reaction process.Here,inspired by the theoretical simulations that the concomitant multivalent Sn (Sn0,SnⅡ and SnⅣv) can significantly motivate the intrinsic activity of Sn-based catalyst,the Sn/SnO/SnO2 nanoshsets were proposed to experimentally vedfy the synergistic effect of multivalent Sn species on the CO2-into-HCOOH conversion.During CO2 reduction reaction,the Sn/SnO/SnO2 nanosheats,which are prepared by the sequential hydrothermal reaction,calcined crystallization and low-temperature H2 treatment,exhibit a high FEHCOOH of 89.6％ at-0.9 VRHE as well as a large cathodic currant density.Systematic experimental and theoretical results corroborate that multivalent Sn species synergistically energize the CO2 activation,the HCOO* adsorption,and the electron transfer,which make Sn/SnO/SnO2 favour the conversion from CO=into HCOOH in both thermodynamics and kinetics.This proof-of-concept study establishes a relationship between the enhanced performance and the multivalent Sn species,and also provides a practicable and scalable avenue for rational engineering high-powered electrocatalysts.