Reconstructing two-dimensional defects in CuO nanowires for efficient CO2electroreduction to ethylene

Article abstract of DOI:10.1039/d1cc03171f

Here we report that in situ reconstructed Cu two-dimensional (2D) defects in CuO nanowires during CO2RR lead to significantly enhanced activity and selectivity of C2H4 compared to the CuO nanoplatelets. Specifically, the CuO nanowires achieve high faradaic efficiency of 62% for C2H4 and a partial current density of 324 mA cm-2 yet at a low potential of -0.56 V versus a reversible hydrogen electrode. Structural evolution characterization and in situ Raman spectra reveal that the high yield of C2H4 on CuO nanowires is attributed to the in situ reduction of CuO to Cu followed by structural reconstruction to form 2D defects, e.g., stacking faults and twin boundaries, which improve the CO production rate and ?CO adsorption strength. This finding may provide a paradigm for the rational design of nanostructured catalysts for efficient CO2 electroreduction to C2H4.

Full text of DOI:10.1039/d1cc03171f

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Reconstructing two-dimensional defects in CuO
nanowires for efficient CO2 electroreduction to
ethylene†
Cite this: DOI: 10.1039/d1cc03171f
Received 16th June 2021,
Accepted 27th July 2021
ab
cd
Jianfang Zhang,
Zhengyuan Li,b Shuai Xia,c Tianyu Zhang,b Yan Wang,
*
Yucheng Wu*cd and Jingjie Wu
*
b
DOI: 10.1039/d1cc03171f
rsc.li/chemcomm
Here we report that in situ reconstructed Cu two-dimensional (2D)
Copper and copper oxides have been widely studied as
defects in CuO nanowires during CO2RR lead to significantly efficient catalysts for electrochemical CO2RR towards hydro-
enhanced activity and selectivity of C2H4 compared to the CuO carbon products.6 Despite the state-of-the-art Cu-based cata-
nanoplatelets. Specifically, the CuO nanowires achieve high fara- lysts achieved a high C2H4 formation rate (4100 mA cmÀ2) with
daic efficiency of 62% for C2H4 and a partial current density of faradaic efficiency (FE) 4 60%, the high overpotential of C2H4
324 mA cmÀ2 yet at a low potential of À0.56 V versus a reversible (oÀ0.80 V) on Cu catalysts requires massive energy input for
hydrogen electrode. Structural evolution characterization and practical and economic C2H4 production.4,7–9 Thus, reducing
in situ Raman spectra reveal that the high yield of C2H4 on CuO the overpotential of CO2RR catalysis remains an important
nanowires is attributed to the in situ reduction of CuO to Cu objective that realizes industrial production of C2H4 under
followed by structural reconstruction to form 2D defects, e.g., steady state conditions at a high current density (4200 mA
stacking faults and twin boundaries, which improve the CO produc- cmÀ2) and an appreciable selectivity (460%).
tion rate and *CO adsorption strength. This finding may provide a
The evolution of oxide-derived Cu nanostructures present
paradigm for the rational design of nanostructured catalysts for various chemical state, residual oxygen, and crystalline defects
efficient CO2 electroreduction to C2H4.
when Cu oxides undergo electrochemical CO2RR, which leads
to an altered activity and selectivity of C2H4 relative to metallic
Electrochemical CO2 reduction reaction (CO2RR) provides a Cu catalysts.10,11 In particular, the crystalline defects in oxide-
promising technology to convert CO2 into fuels and value- derived Cu catalysts have been proved as highly active sites for
added chemicals using renewable electricity.1 Great progress electrochemical CO2RR towards C2H4 at low overpotentials.12–14
has been made on the electrochemical CO2RR since the semi- Recently, the electrochemical reconstruction of Cu oxides has
nal work by Hori et al.2,3 To date, the mainstream research received increasing attention, which induced abundant grain
focus is still on the design of highly active and selective boundary interfaces that lead to efficient electrochemical CO2RR
catalysts to produce a specific commodity product with high towards C2H4 production.15–18 Apart from regular grain bound-
market price and size.4 In particular, the electrochemical aries, stacking faults and twin boundaries in some metal nanos-
CO2RR to C2+ products, including ethylene (C2H4) and ethanol tructures (e.g., Ni-Co bimetal phosphide and Ag metal) have been
(C2H5OH), has attracted increasing attention in the energy demonstrated to be active for electrocatalysis of water splitting
storage and chemical industry.5
and CO2-to-CO conversion.19–21 Such two-dimensional (2D)
defects involving stacking faults and twin boundaries in Cu are
presumed to increase the yield of CO2-to-C2H4 conversion while
lowering overpotentials. The key to exploiting these particular 2D
defects lies in developing a facile synthesis method to introduce
stacking faults and twin boundaries. Controlling the morphology
of Cu oxides can potentially induce stacking faults and twin
boundaries during in situ electrochemical reconstructing process.
Here, we report that the CuO nanowires, distinguished from
CuO nanoplatelets, tend to undergo in situ reconstruction to
form 2D defects, including stacking faults and twin boundaries
during electrochemical CO2RR. Such abundant planar defects
direct high-efficiency CO2-to-C2H4 conversion. As a result, the
a School of Instrument Science and Opto-electronics Engineering,
Hefei University of Technology, Hefei 230009, China
b Department of Chemical and Environmental Engineering, University of Cincinnati,
Cincinnati, OH 45221, USA. E-mail: jingjie.wu@uc.edu
c School of Materials Science and Engineering, Hefei University of Technology,
Hefei 230009, China. E-mail: stone@hfut.edu.cn, ycwu@hfut.edu.cn
d China International S&T Cooperation Base for Advanced Energy and
Environmental Materials & Anhui Provincial International S&T Cooperation Base
for Advanced Energy Materials, Hefei University of Technology, Hefei 230009,
China
† Electronic supplementary information (ESI) available: Experimental section
and supplementary figures. See DOI: 10.1039/d1cc03171f
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spacing of 0.275 nm for both CuO-1 and CuO-4, which was
referred to the CuO(110) facets. The selective electron diffrac-
tion pattern suggests the single-crystal structure of CuO
(Fig. S3, ESI†). The CuO-1 nanowires and CuO-4 nanoplatelets
also exhibited a similar surface chemical state of exclusive Cu2+
as reflected by the XPS analysis of Cu 2p (Fig. S4a, ESI†), O 1s
(Fig. S4b, ESI†), and Cu LMM (Fig. S4c, ESI†).12,22 No K element
was found in the CuO samples (Fig. S5, ESI†).
The electrocatalytic CO2RR performance of as-prepared four
CuO samples was studied in a customized flow cell using 1.0 M
KOH as the catholyte. The electrolysis products were analyzed
using gas chromatography (GC) and 1H nuclear magnetic
resonance (NMR). The CO2RR current densities and CO2 con-
version progressively increased in order of CuO-1 4 CuO-2 4
CuO-3 4 CuO-4 (Fig. S6 and S7, ESI†). It is also worth noting
that the onset potential of CO2RR rose from CuO-1 to CuO-4
electrodes. Regarding the product selectivity, CuO-1 electrode
containing pure nanowires achieved the best FEs of C2+ pro-
ducts and the specific product of C2H4 (Fig. 2a, b and Fig. S8,
ESI†). The CuO-1 electrode could reach the overall FE of C2+
products up to 81% at À0.56 V, where the FE of C2H4 achieved a
maximum of 62% (Fig. 2a and b). The FEs of both C2+ products
and C2H4 decreased when the fraction of nanoplatelets in CuO
electrodes increased (Fig. 2b). The CuO-4 electrode with pure
nanoplatelets exhibited the lowest FE of C2H4, which was only
38% at the same potential of À0.56 V. Moreover, the partial
current densities of C2H4 ( jC H ) showed the same tendency as
Fig. 1 (a) Schematic illustration for the synthetic process of CuO nano-
structures; (b–e) SEM and HRTEM images of (b and d) CuO-1 and (c and e)
CuO-4.
2
4
the FE of C2H4 among the four CuO electrodes (Fig. 2c and
Fig. S9, ESI†). The CuO-1 electrode achieved a high jC H of
2
4
324 and 418 mA cmÀ2 at À0.56 and À0.65 V, respectively, which
reduced CuO nanowires achieve a FE of 62% for C2H4 and a were about 2.4 and 1.8 times as those of CuO-4 electrode at the
partial current density of 324 mA cmÀ2 at a low potential of equal potentials (Fig. 2c). Note that CuO-1 electrode achieved
À0.56 V (versus RHE, hereafter). In situ Raman spectra reveal higher partial current density and FE of C2H4 under lower
that the 2D defects act as highly active sites that improve the
adsorption of CO intermediates and consequently promote the
*CO surface coverage and C–C coupling rate.
The CuO nanostructures as the Cu catalyst precursor were
facilely synthesized by microwave heating CuCl2 dissolved in
3 M KOH solution as illustrated in Fig. 1a. The microwave
power determines the morphology of CuO nanostructures, i.e.,
nanowires versus nanoplatelets (Fig. 1b, c and Fig. S1, ESI†). At
a low power of 200 W, the obtained CuO-1 sample exhibited a
uniform structure of nanowires with an average diameter of
20 nm (Fig. 1b). The nanowires became wider, accompanied by
the formation of some nanoplatelets when the microwave
power was increased to 300 W (CuO-2, Fig. S1a, ESI†). CuO
nanoplatelets prevailed over nanowires as the power was
increased to 400 W (CuO-3, Fig. S1b, ESI†). Further increasing
the power to 500 W, pure CuO nanoplatelets were obtained,
which had a lateral size of 150–300 nm and thickness of
30–50 nm (CuO-4, Fig. 1c). The crystalline structures of these
four CuO samples were confirmed by XRD patterns (Fig. S2,
ESI†). TEM images in the inset of Fig. 1d and e demonstrated
the pure nanowires and nanoplatelets structures of CuO-1 and
CuO-4, respectively. The corresponding high-resolution TEM
Fig. 2 Comparison of the CO2 electro-reduction performance among
different CuO electrodes. (a) Faradaic efficiencies of CuO-1 electrode as a
function of potential; (b) the distribution of C2+ products at À0.56 V;
(c) partial current density of C2H4 as a function of potential; and (D) Tafel
(HRTEM) images showed clear lattice fringes with a lattice plots.
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overpotential than most reported Cu-based catalysts (Table S1, boundaries in nanowires, contribute to the highly active and
ESI†). Tafel plots for electrocatalytic CO2-to-C2H4 conversion selective CO2-to-C2H4 conversion at low overpotentials.
were investigated to explore the reaction kinetics of these CuO
electrodes (Fig. 2d). Obviously, a significant difference in Tafel towards C2H4 conversion on the reconstructed stacking faults
slopes towards C2H4 was observed on CuO-1 (126 mV decÀ1
and twin boundaries in the nanowires electrode, the effect of
To uncover the origin of the improved activity and selectivity
)
and CuO-4 (160 mV decÀ1) electrodes, indicating the different surface roughness should be considered first. The higher sur-
rate-determining steps for CO2-to-C2H4 conversion. We envi- face roughness was previously discovered to favor C2H4
sioned that such a significant difference in CO2RR performance formation.23 The double-layer capacitances (Cdl) of the four
is attributed to the different morphologies of these CuO elec- CuO electrodes were calculated from the cyclic voltammetry
trodes, leading to distinct behaviour of in situ structural evolu- (CV) curves in the non-faradaic reaction potential window at
tion during CO2RR.
different scan rates (Fig. S15 and S16, ESI†). A capacitance of
Next, we probed the evolution of the structures and chemical 3.36 mF cmÀ2 was obtained for the CuO-1 electrode, which was
composition of CuO electrodes after CO2RR. The bulk structure 1.40 times that of the CuO-4 electrode (2.38 mF cmÀ2). The
remained intact for CuO-1 and CuO-4 electrodes after CO2RR ECSA-normalized jC H of the CuO-1 electrode was still higher
2
4
(Fig. 3). However, CuO species in these electrodes were fully than that of the CuO-4 electrode (Fig. 4a and ESI†). Of note, the
reduced to metallic Cu after CO2RR as shown by XRD results ratios of ECSA-normalized jC H between CuO-1 and CuO-4
2
4
(Fig. S10, ESI†). The results of Cu 2p and Cu LMM revealed that electrodes were only 1.7 and 1.3 at potentials of À0.56 and
the same fraction of Cu+ species on the surfaces of nanowires À0.65 V, respectively. The ratios of C2H4 formation rate
and nanoplatelets after CO2RR (Fig. S11, ESI†). The EDS map- between these two electrodes were 2.4 and 1.8 at À0.56 and
ping and spectrum of post CuO-1 electrode after CO2RR also À0.65 V, respectively (Fig. S17, ESI†), much larger than the
confirmed the coexistence of Cu and O elements (Fig. S12, ratios of ECSA-normalized jC H , indicating that the surface
2
4
ESI†), which is mainly ascribed to the easy re-oxidation of roughness cannot fully account for the improvement of C2H4
metallic Cu exposed to the air. Interestingly, lots of stacking yield on the CuO-1 electrode. Further, we calculated the gross
faults were observed on CuO-1 nanowires together with some CO formation rates at different potentials over the different
distinct twin boundaries of Cu (111) after CO2RR as shown in CuO electrodes (Fig. S18, ESI†). The gross CO molecule is the
HRTEM images (Fig. 3c and Fig. S13a, S14, ESI†). However, the sum of coupled CO to generate C2+ products, hydrogenated CO
CuO-4 nanoplatelets showed the integrity of lattice fringes of to CH4 and the remaining CO in the gas product stream (ESI†).
Cu (111) facet in HRTEM images (Fig. 3f and Fig. S13b, ESI†). As shown in Fig. 4b, the CuO-1 electrode achieved an ESCA-
The distinctive structural evolution of the CuO-1 nanowires and normalized CO formation rate of 1.9 mA cmÀ2 at À0.56 V, while
CuO-4 nanoplatelets during CO2RR can be assigned to the the CuO-4 electrode only possessed 1.2 mA cmÀ2, indicating an
different morphology and size between nanowires and nano- intrinsic faster CO generation rate on the CuO-1 electrode. The
platelets. The CuO-1 nanowires with smaller size at least in one intrinsic faster CO generation rate will translate to a higher *CO
dimension may be quickly reduced to metallic Cu, during
which the surface undergoes reconstruction to form 2D defects.
Based on the structural and chemical composition analysis of
CuO electrodes after CO2RR, we hypothesized that the recon-
structed 2D defects, such as stacking faults and twin
Fig. 4 (a) ECSA-normalized partial current density of C2H4 for CuO-1 and
CuO-4 electrodes; (b) ECSA-normalized gross formation rate of CO on
Fig. 3 SEM, TEM, and HRTEM images of (a–c) CuO-1 and (d–f) CuO-4
CuO-1 and CuO-4 electrodes; in situ Raman spectra of (c) CuO-1 and (d)
after the CO2RR. SF: stacking faults; TB: twin boundary.
CuO-4 electrodes at different applied potentials.
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In summary, our research shows the morphology depen-
dence of Cu-based catalysts reconstruction during CO2RR,
which clarifies the significant difference in CO2RR activity
and selectivity toward C2+ products among many nanostruc-
tured Cu catalysts. CuO nanowires tend toward in situ recon-
struction that forms enriched stacking faults and twin
boundaries compared to nanoplatelets probably because CuO
nanowires possess a smaller dimension than nanoplatelets.
These 2D defects offer a higher intrinsic production rate of CO,
leading to increased *CO surface coverage, along with higher
adsorption energy of *CO, further increasing *CO surface
coverage while promoting CO dimerization rate. As a result,
CuO nanowires exhibit a high faradaic efficiency of 62% for
C2H4 and a partial current density of 324 mA cmÀ2 yet at a low
potential of À0.56 V.
This work was partially financially supported by NSF CBET-
2033343. J. Z. thanks the support from National Natural Science
Foundation of China (51772072, 51672065, U1810204). Y. W.
and Y. W. would like to thank the financial support from the
111 Project (B18018).
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Conflicts of interest
There are no conflicts to declare.
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Chem. Commun.
This journal is © The Royal Society of Chemistry 2021