ABSTRACT
Despite of superior performance of the oxide-derived copper (OD-Cu) in producing valuable hydrocarbons during CO2RR, its fabrication process is still ambiguous and complicated. In this work, we develop a simple microwave-assisted method to synthesize the oxide-derived Cu nanosheet (OD-Cu NS) and reveal that the oxidation state of Cu species is controlled by varying the Cu precursor amount. Notably, the simultaneous formation of nano-sized Cu domains influence the surface roughness of OD-Cu NS. The partially oxidized Cu surface exhibits a superior faradaic efficiency (FE) of C2+products up to 72%, along with a partial current density of 55 mA cm−2 in a neutral KHCO3 solution. More importantly, the as-obtained OD-Cu NS shows a synergetic effect on dissociating of CO2 molecules by the strong binding energy and promoting of C2+ compounds productivity by the enlarged electrochemical surface area. This work provides a new insight for designing efficient OD-Cu catalysts towards CO2RR.
Introduction
Fast industrialization and economic growth have aroused huge consumption of fossil fuels and released a large amount of carbon dioxide (CO2) into the atmosphere,1,2 which has caused global warming issues and serious climate change. The electrochemical CO2reduction reaction (CO2RR), which converts CO2 to various value-added chemicals, has drawn wide interest as a promising approach for the management of CO2 emission. Particularly, C2+molecules are regarded as highly valuable products due to their applications in wide industrial field.3-6 Since Hori et al. firstly reported that Cu foil has unique properties to generate various hydrocarbons products beyond 2e reduction pathway,7,8 Cu-based catalysts have been explored extensively. The constraints in the bulk Cu foils,9such as low selectivity, high overpotentials, insufficient stability, and strong competition from the hydrogen evolution reaction (HER), were addressed by Cu-based nanocatalysts with modified structures,10,11 tuned crystal facets,12 regulated the compositions with other metal elements and/or organic molecules, etc.13-15
Among various types of Cu-based nanocatalysts, the oxide-derived copper (OD-Cu) has shown great potential for producing the value-added multi-carbon compounds.16 Notably, the chemical state of Cu+, i.e., the partially oxidized Cu on the surface of OD-Cu, could modify the electronic structure of active sites, thereby lead to a strong binding energy with one of the most crucial intermediates *CO, and further promote those dimerization to produce multicarbon compounds.17,18 Therefore, various methods have been developed to fabricate the partially oxidized Cu+ state for enhancing the productivity of C2+ hydrocarbons. For instance, a thin CuOx sheath on the surface of OD-Cu nanowires was formed by slowly oxidized in air atmosphere and reached up to 78% of FE for C2+ products.19 In addition, by using the O2 plasma treated Cu nanocubes, the selectivity of C2H4 and the current density in CO2RR were enhanced compared to that under Ar plasma treatment.20 Besides the oxidation state, the role of surface roughening on the OD-Cu was also examined to enrich the catalytic active sites.21-25 The roughened surface possesses abundant interfaces, grain boundaries, and low coordinated sites, which are generally regarded as the active sites to trap the key intermediate *CO and initiate the dimerization step.19 However, the existing post-treatments on the pristine Cu such as plasma irradiation,10 air-oxidation,26 and electropolishing27 are still difficult to well-controllably synthesize OD-Cu. A facile method to efficiently fabricate the OD-Cu nanocatalysts is highly desired.18,28
Herein, we explored a simple microwave heating procedure to synthesize OD-Cu nanosheets (OD-Cu NSs) and revealed how adding extra Cu precursor affected the development of the surface Cu+ state by increasing the surface roughness. Impressively, the optimized OD-Cu NS with 30 wt% of extra-added Cu precursor, denoting as Cu-30, achieved the faradaic efficiency (FE) of 72% and the geometric current density of 31 mA cm−2 for C2+ products at −1.1 VRHE. Furthermore, the electrochemical surface area (ECSA) of the as-obtained Cu-30 with a rough surface is about five times over the pristine CuO NS. This works provides insights about the relationship between the binding energy of *CO intermediate on the surface of Cu-30 with the specific chemical state of the partially oxidized Cu by the means of inspection of OHadsorption.