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 OH−adsorption.