Efficiency enhancement in Electrical CO2 reduction with 3-dimensional nanostructures copper catalyst

Abstract

Conversion of carbon dioxide (CO2) to fuel can mitigate the global climate crisis and the depletion of fossil fuel sources. Electrochemical reduction of CO2 at a metal surface in aqueous solution is a simple process that enables control of the types of products such as carbon monoxide (CO), formic acid, and hydrocarbons by selecting an appropriate metal catalyst. In particular, CO is an intermediate resource for the production of methanol, synthetic hydrocarbons, and synthetic petroleum by the Fischer-Tropsch process. Noble-metal catalysts such as Au, Ag, and Pd can convert CO2 electrochemically to CO at Faradaïc efficiencies (FECO) > 90%. However, these metals are expensive and are therefore not appropriate for limited use in scalable production for electrochemical CO2 reduction. To overcome these problems, earth abundant and inexpensive Cu has been assessed as catalysts for the CO production. Cu nanowires (NWs) known as the oxide-derived Cu catalysts prepared on Cu mesh showed a low potential (-0.4 VRHE) for the CO2 reduction, but it also suffered from low FECO (61.8%). To increase the selectivity of the CO2 reduction to CO, bimetallic Cu catalysts such as Cu-In, Cu-Sn, and Cu-Pd have been developed because the binding strength of both CO and absorbed hydrogen (H*) can be weakened. By decreasing the binding strength of H* on Cu surface, the hydrogen evolution reaction (HER) is suppressed due to lack of available sites at which H* can recombine with an electron. Also, decreasing the binding strength of CO on the surface of Cu inhibits further reduction of CO to alcohols or hydrocarbons. As a result, high selectivity up to 90% of FECO at -0.7 ~ -0.8 VRHE could be obtained. Although the FECO was remarkably increased, these bimetallic catalysts still have problems of low current density (< -3 mA cm-2) and relatively high potential (> -0.7 VRHE). Such problems could be solved by implementing three-dimensional hierarchical (3D-h) nanostructure on bimetallic Cu catalysts. The 3D-h nanostructures could allow the increase of surface area acting as active sites and their roughened surface could induce a strong electric field in electrocatalytic surface leading to the enhancement of activity and reduction potential. Here we report a way to fabricate 3D hierarchical Cu (3D-h Cu) coated with Sn NPs (3D-h Cu/Sn NPs) by template-based pattern transfer, thermal oxidation, electrochemical reduction, and electroless deposition of Sn NPs. Presence of Sn NPs along with 3D-h Cu showed the maximum FECO of 98.6 % at -0.45 VRHE and high partial current density of -9.6 mA cm-2 at -1.0 VRHE, thereby resulting in the highest CO production rate of 179.0 μmol h-1 cm-2. The 3D nanostructure increased surface area, facilitating transfer of electrons to interface between catalyst and electrolyte. Based on the experimental results, the active sites of bimetallic Cu/Sn system were discussed with coverage of Sn on Cu surface as a triple phase boundary at Cu/Sn/electrolyte. Electrical simulation suggests that the highly intensive electric field is localized at the surface of 3D-h Cu, thereby resulting in an increased local CO2 concentration at the interface between bimetallic catalyst and electrolyte. These results clearly show that it can be effectively reduced from CO2 to CO at the surface of 3D-h Cu/Sn NP catalyst.