▎ 摘　　要

Copper oxides have been used as efficient electrocatalysts for electrocatalytic reduction of carbon dioxide (CO2). However, understanding the catalytic mechanism of copper oxides electrocatalysts based on identifying their active species is difficult due to the presence of multivalent Cu, such as Cu(0), Cu(I) and Cu(II) species, during the reaction. Also, developing copper oxides electrocatalysts with high selectivity and long durability for carbon dioxide reduction reaction (CO2RR) is highly desired. Herein, we report the preparation of CuxO decorated graphene oxides (G-CuxO-T) electrocatalyst by a controllable chemical reduction method. The G-CuxO-2 h electrocatalyst exhibits high selectivity towards HCOOH (81%) with a current density of 19.3 mA cm(-2) at -0.8 V vs RHE, as well as good durability (retaining 87.2% of initial activity after 9 hours continuous operation). Our study reveals that the observed high performance of G-CuxO-2 h electrocatalyst should not only benefit from the stabilized CO2 center dot- intermediate, but also contribute from the facilitated rate-limiting step of HCOO- desorption, which are both closely related to an optimized Cu(I) content in the electrocatalyst. Moreover, a "buffering effect" is proposed to explain the promising durability of G-CuxO-2 h, where Cu(II) species should serve as sacrificial sources to supply Cu(I) from the thick subsurface layers, thereby balancing the content of Cu(I) at the surface and maintaining the activity of the electrocatalyst during the reaction. Our work provides crucial insights into the role of multivalent Cu in CO2 reduction reaction, which are important for designing and preparing copper oxides based electrocatalysts with high selectivity and durability for electrochemical reduction of CO2 into liquid fuels.