| Summary: | Copper-based bimetallic catalysts have been recently showing promising performance for the selective electrochemical reduction of CO<sub>2</sub>. In this work, we successfully fabricated the partially reduced oxides SnOx, CuOx modified Cu foam electrode (A-Cu/SnO<sub>2</sub>) through an electrodeposition-annealing-electroreduction approach. Notably, in comparison with the control electrode (Cu/SnO<sub>2</sub>) without undergoing annealing step, A-Cu/SnO<sub>2</sub> exhibits a significant enhancement in terms of CO<sub>2</sub> reduction activity and CO selectivity. By investigating the effect of the amount of the electrodeposited SnO<sub>2</sub>, it is found that A-Cu/SnO<sub>2</sub> electrodes present the characteristic Sn-Cu synergistic catalysis with a feature of dominant CO formation (CO faradaic efficiency, 70~75%), the least HCOOH formation (HCOOH faradaic efficiency, <5%) and the remarkable inhibition of hydrogen evolution reaction. In contrast, Cu/SnO<sub>2</sub> electrodes exhibit a SnO<sub>2</sub> coverage-dependent catalysis—a shift from CO selectivity to HCOOH selectivity with the increasing deposited SnO<sub>2</sub> on Cu foam. The different catalytic performance between Cu/SnO<sub>2</sub> and A-Cu/SnO<sub>2</sub> might be attributed to the different content of Cu atoms in SnO<sub>2</sub> layer, which may affect the density of Cu-Sn interface on the surface. Our work provides a facile annealing-electroreduction strategy to modify the surface composition for understanding the metal effect towards CO<sub>2</sub> reduction activity and selectivity for bimetallic Cu-based electrocatalysts.
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