Vanadium oxide integrated on hierarchically nanoporous copper for efficient electroreduction of CO2to ethanol

Article abstract of DOI:10.1039/d0ta09522b

The electrochemical reduction of CO₂to an ethanol product is regarded as a highly promising route for CO₂utilization. However, the poor selectivity is still a critical challenge for increasing the yield of the specific ethanol. As a CO₂reduction catalyst, the hierarchically nanoporous copper integrated with vanadium oxide can achieve a 30.1% faradaic efficiency for CO₂-to-ethanol production and an ethanol partial current density of ?16 mA cm^?2at ?0.62 Vvs.RHE, corresponding to a 4-fold increase in activity compared to bare nanoporous Cu. It even delivers an ethanol partial current density that exceeds ?39 mA cm^?2at ?0.8 Vvs.RHE in a flow-cell reactor. The hierarchically nanoporous Cu skeleton not only facilitates both electron and electrolyte transport but also provides a large specific surface area for high active site density. Density functional theory reveals that the vanadium oxide decorated Cu surface can facilitate water dissociation and optimize the hydrogen adsorption energy on Cu, lowering the energy barrier for the protonation of carbon dioxide and C-C coupling. Meanwhile, it can increase hydrogen proton coverage on the catalyst surface and inhibit dehydration, which are beneficial for breaking the C=C bond of the *HCCOH intermediate, thus enhancing the faradaic efficiency of ethanol significantly. The highly efficient conversion of CO₂to ethanol demonstrates that the hybrid electrocatalyst is considered as a promising candidate for practical electrocatalytic CO₂RR applications.

Full text of DOI:10.1039/d0ta09522b

View Article Online
View Journal
Journal of
Materials Chemistry A
Materials for energy and sustainability
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Q. Yang, X. Liu, W.
Peng, Y. Zhao, Z. Liu, M. Peng, Y. Lu, T. Chan, X. Xu and Y. Tan, J. Mater. Chem. A, 2021, DOI:
10.1039/D0TA09522B.
Volume
Number 12
28 March 2018
Pages 4883-5230
6
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry A
Materials for energy and sustainability
rsc.li/materials-a
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2050-7488
COMMUNICATION
Zhenhai Wen et al.
An electrochemically neutralized energy-assisted low-cost
acid-alkaline electrolyzer for energy-saving electrolysis
hydrogen generation
rsc.li/materials-a

Please do not adjust margins
Journal of Materials Chemistry A
Page 1 of 8
View Article Online
DOI: 10.1039/D0TA09522B
ARTICLE
Vanadium Oxide Integrated on Hierarchically Nanoporous
Copper for Efficient Electroreduction of CO2 to Ethanol
Received 00th January 20xx,
Accepted 00th January 20xx
Qingcheng Yang,a Xunlin Liu,a Wei Peng,a Yang Zhao,a Zhixiao Liu,a* Ming Peng,a Ying-Rui
Lu,b Ting-Shan Chan,b Xiandong Xu,a Yongwen Tana*
DOI: 10.1039/x0xx00000x
Electrochemical reduction of CO2 into ethanol product is regarded as a highly promising route for CO2 utilization. However,
the poor selectivity is still a critical challenge for increasing the yield of the specific ethanol. As CO2 reduction catalyst, the
hierarchically nanoporous copper integrated with vanadium oxide can achieve a 30.1% Faradaic efficiency for CO2-to-ethanol
production and an ethanol partial current density of −16 mA cm−2 at −0.62 V vs. RHE, corresponding to a 4-fold increase in
activity compared to the bare nanoporous Cu. It even delivers ethanol partial current density that exceeds -39 mA cm-2 at -
0.8 V vs. RHE in a flow-cell reactor. The hierarchically nanoporous Cu skeleton not only facilitates both electron and
electrolyte transports but also provides large specific surface area for high active site density. Density functional theory
reveals that the vanadium oxide decorated Cu surface can facilitate water dissociation and optimize the hydrogen adsorption
energy on Cu, lowering the energy barrier of protonation of carbon dioxide and C-C coupling. Meanwhile, it can increase
hydrogen proton coverage on the catalyst surface and inhibit dehydration, which are beneficial for breaking the C=C bond
of the *HCCOH intermediate, thus enhancing the Faradaic efficiency of ethanol significantly. The highly efficient conversion
of CO2 to ethanol demonstrates that the hybrid electrocatalyst is considered as a promising candidate for practical
electrocatalytic CO2RR applications.
needed to reduce reaction overpotential and improve catalytic activity and
Introduction
selectivity.
Electrochemical conversion of carbon dioxide (CO2) into commercially
valuable fuels and chemicals utilizing renewable electricity is an appealing
way for reducing atmospherical CO2 emissions.1, 2 Recently, various valuable
products have been obtained through electrochemical CO2 reduction reaction
(CO2RR), while the C2+ products are more attractive owing to their ultra-high
energy densities and high economic value. Especially, ethanol is highly
desirable, which can be either used directly as a fuel or indirectly as fuel
precursors.3-6 However, producing ethanol via electrochemical CO2RR is far
away from commercialization because of the high overpotential, low activity,
selectivity and poor stability of catalysts.7 Therefore, efficient catalysts are
Copper is the most promising electrocatalysts for the reduction of CO2
to C2 products due to its appropriate binding energy to many intermediates.7-
13
To improve the selectivity of ethanol production, several strategies have
been developed to modify the copper catalysts surface, such as roughness
effect,14 interface engineering,15 dopant modification,16,17 bimetallic
catalysis,18-21 and tandem catalysis,22,23 These methods mainly control the
production of the key C1 intermediate (*CO) by another component to further
enhance the production of C2+ species, because the C2+ selectivity can be
tuned by CO coverage. However, most of the research indicate that methods
of modifying copper catalysts surface focus on improving selectivity of C2
mixed products rather than single target products,19-24 Currently, the
selectivity of ethanol still remains low, thus it is imperative to design catalysts
to improve ethanol selectivity.
a. College of Materials Science and Engineering, Hunan University, Changsha,
Hunan 410082, China
b. National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
it is well known that ethanol production is a typical twelve-electrons
transfer process, which means more hydrogenation steps in comparison to
other products.25,26 H coverage plays an important role in the hydrogenation
*Corresponding Author: zxliu@hnu.edu.cn; tanyw@hnu.edu.cn
Q. Yang and X. Liu contributed equally to this work
†
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 2 of 8
ARTICLE
Journal Name
process.27,28 Thus, enhancing H adsorption on the catalyst surface is a Material Characterizations
View Article Online
DOI: 10.1039/D0TA09522B
potential strategy to promote the formation of key intermediate and ethanol. X-ray diffraction (XRD) patterns were obtained using a Rigaku MiniFlex X-ray
Actually, the elements near the sixth subfamilies have good affinity for polycrystalline powder diffractometer with Cu K α ray ( λ =0.15418 nm).
hydrogen, such as Cr, Mo, V, W and so on. Previous reports have shown that Observation and Analysis of Microstructure and Morphology were probed
the doping of chromium oxide or vanadium oxide can accelerate the with a HitachiS-4800 HD SEM (at 10 kV and 8.5 mm work distance). The
hydrolysis kinetics and provide hydrogen protons.29,30 Previous works have transmission electron microscopy (TEM) was conducted by a Tecnai G2 F20 S-
indicated that Cu surfaces decorated by oxides or hydroxides can accelerate TWIN whose limit point resolution is 0.17 nm. High-angle annular dark field-
H2O dissociation and provide strong affinity to H atoms, which can boost scanning TEM (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS)
hydrogenation reaction in CO2RR.31-34 Herein, we construct a vanadium oxide mapping were conducted on a JEM-ARM 200F with acceleration voltage of
integrated on hierarchically nanoporous copper hybrid electrocatalysts with 200 kV. Chemical states of surface atoms were studied by X-ray photoelectron
a three-dimensional (3D) bicontinuous nanoporous architecture for high- spectroscopy with a Thermo Scientific Escalab 250Xi. The X-ray absorption
performance CO2RR. These hybrid electrocatalysts are composed of spectroscopy (XAS) were obtained on beamline BL01C1 at National
amorphous vanadium oxide, which are in situ grown on 3D bicontinuous Synchrotron Radiation Research Center in the fluorescence mode using a
nanoporous Cu skeleton (denoted as np-Cu@VO2) by one-step chemical Lytle detector with a step-size of 0.25 eV at room temperature. The specific
dealloying. Such hierarchical architecture offers suitable pathways for both surface area was obtained from the results of N2 physisorption at 77 K
electron and reactants transports. In addition, nano-sized interconnective Cu (Micromeritics ASAP 2020) by using the BET (Brunauer-Emmet-Teller).
ligaments and bimodal nanopore channels remarkably increase the number
Electrochemical measurements
density of highly active sites for promoting the reactant accessibility.35,36
A CHI660 electrochemical workstation with three-electrode cell was used to
Therefore, the np-Cu@VO2-5% hybrid electrocatalyst enables
a 30.1%
control the external reaction conditions. Electrochemical performance of the
samples was tested by electrochemical workstation in a three-electrode
system which had a working electrode (catalysis), a Ag/AgCl reference
electrode filled with saturated KCl solution and a Pt counter electrode. All
potentials applied by electrochemical workstation were transformed to the
potentials relative to reversible hydrogen electrode (RHE) as a criterion with
a formula (ERHE = EAg/AgCl + 0.0591 pH + 0.197 V). The electrolyte, 0.1 M KHCO3
solution, was bubbled with CO2 at least 30 min to ensure formation of CO2-
saturated solution before experiments. 1.0 M KOH solution and Hg/HgO
reference electrode was used in flow cell. Linear sweep voltammetry (LSV)
measurements in CO2-saturated or Ar-saturated electrolytes were taken at a
given potential range with the sweep rate of 50 mV/s. Carbon dioxide
electrochemical reduction experiments were measured in a H-type cell
(separated by Nafion 117 proton exchange membrane) and flow cell (anion
Faradaic efficiency (FE) for CO2-to-ethonal production and an ethanol partial
current density of −16 mA cm−2 at −0.62 V versus reversible hydrogen
electrode (RHE), corresponding to a 4-fold increase in activity compared to
the bare nanoporous Cu. It even delivers ethanol partial current density that
exceeds -39 mA cm-2 much higher than that of np-Cu at -0.8 V vs. RHE in a
flow-cell reactor. Density functional theory (DFT) results reveal that the
vanadium oxide integrated on surface of nanoporous Cu can promote the
water dissociation and change the adsorption energy of hydrogen on Cu, thus
reducing the pivotal energy barriers of carbon dioxide protonation and C-C
coupling steps in the ethanol formation pathway. Meanwhile, it can also
increase the surface hydrogen proton (*H) coverage of the catalyst and inhibit
dehydration, which are beneficial for breaking the double bond of the
*HCCOH intermediate, leading to the enhancement of ethanol FE
significantly.
exchange membrane) with
a CHI660 electrochemical workstation,
respectively. Before testing, ten cycles cyclic voltammetry were performed to
activate the catalyst and remove the residual impurities. The open circuit
voltage was tested for half an hour with fresh solution as the blank group,
which eliminated the interference of impurities. Gas products (hydrogen and
small molecular hydrocarbons) are detected by gas chromatograph (GC2014
Shimadzu) equipped with TCD and FID detector with argon as carrier gas
which connected with the cathodic compartment. Liquid products were
characterized by 1H NMR on Bruker Avance III 400MHz HD spectrometer.
Liquid products were also analyzed by GC whose FID detector equipped with
Bond-Q column. The Faradaic efficiency of the products was calculated
through GC and NMR data. The Faradaic efficiency (FE) and energy efficiency
(EE) for the products was calculated as following formula: FE = x*F*n/Q
=x*F*n/(I*t), where x is the number of electrons transferred, F is the Faraday
Experimental
Fabrication of hierarchical np-Cu and np-Cu@VO2
All catalysts with nanoporous structures were fabricated by
a facile
alloying/dealloying procedure.37,38 The Al80Cu20-xVx (x=0, 0.2, 1, 2) precursor
ribbon with ~50 m thickness and ~5 mm wide were first produced by a
melting-spinning method in Ar gas from their ingots, which were made by arc
melting using pure Cu, V, and Al in Ar atmosphere. The as-prepared alloy
ribbons were chemically dealloying in Ar-purged 2.0 M NaOH aqueous
electrolyte for 2 h. These produced catalysts were rinsed in pure water for
multiple times to remove chemical substance in nanopore channels. After
vacuum drying, the catalysts obtained by chemical corrosion were self-
supporting materials, which could be used directly as cathodes in H-type cell
without ink preparation process.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 8
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
constant, Q is the total transferred charge, I is the current, t is the running the precursor alloy, the products are defined as the np-Cu@VViOew2-1Ar%tic,l5e%Onalinnde
DOI: 10.1039/D0TA09522B
time and n is the total amount of product (in mole); EEproduct = (1.23- 10%, respectively. Scanning electron microscope (SEM) images of the np-
Eproduct)*FEproduct /(1.23-Eapplied), Eproduct represents the equilibrium potential of Cu@VO2-5% hybrid electrocatalyst fabricated by chemically dealloying
CO2 electroreduction to each products, which is 0.09 V for ethanol.
Cu19V1Al80 precursor exhibit a uniform bicontinuous nanoporous architecture
consisting of interconnective metallic ligaments and bimodal nanopore
channels with characteristic lengths of ~120 nm first ordered pore and ~15
nm secondary pore (Figure 1a, 1b and Figure S2-3). High-angle annular dark
field scanning transmission electron microscopy (HAADF-STEM) further
confirms the hierarchically porous structure of np-Cu@VO2-5% (Figure 1c).
The representative high resolution HAADF-STEM image of np-Cu@VO2-5%
shows well-defined lattice fringes (Figure 1d and 1e), further confirming the
crystalline structure of np-Cu. The lattice fringe of np-Cu with a d-spacing of
~0.21 nm could be assigned to be (111) plane of cubic Cu (JCPDS 04-0836).
The fast Fourier transform (FFT) pattern indicates that the tetragonal CuAl2
transforms to the cubic Cu (insets of Figure 1e). These are confirmed by X-ray
diffraction (XRD) patterns (Figure S4), wherein the characteristic diffraction
peaks correspond to the cubic Cu (JCPDS 04-0836). In addition, it is observed
that crystalline np-Cu is surrounded by amorphous-like vanadium oxide
nanoparticle (Figure 1d). The elemental
Working introduction of flow cell
To prepare the electrode in flow cell, 5 mg of ground precatalyst was
dispersed in 0.48 ml methanol, and then 20 μl Nafion solution was added.
Next, the ink was ultrasonically treated for 10 min. 100 uL as-prepared
suspension was then dripped evenly onto hydrophobic carbon paper (0.5 cm
x 2 cm), which ensured that catalyst loading should be around 1.0 mg/cm2.
Finally, the catalysts dried for at least 6 h in vacuum before use.
The hydrophobic carbon paper consisted of mesoporous layer and
microporous layer which enhanced the gas diffusion, and the high
conductivity also improved the current density. In the process of electrolysis,
CO2 gas first diffused through the mesoporous layer and then entered into
the microporous layer. At the junction of the microporous layer and the
catalyst layer, the catalyst formed interface contact and directly participates
in the reaction.
Density functional theory calculations
All density functional theory (DFT) calculations were performed by Vienna Ab-
initio Simulation Package (VASP).39,40 The exchange-correlation functional was
generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof
(PBE).41-43 The cutoff energy was set as 450 eV. Minimum force convergence
was -0.02 eV. Minimum energy iterative convergence was set as 0.0001 eV.
The bared Cu (111) surface was modeled by a four-layers (4X4) supercell with
20 Å vacuum. The bottom two layers were fixed as the bulk phase and the
upper two layers were allowed to relax. For VO2 modified Cu surface, a VO2
molecule was placed on Cu (111)-(4X4) surface supercell , and the vanadium
content was about 3 at%. Cu-O bond was found after the structure
optimization. In the CO2RR process, the adsorption of *COOH resulted in the
reconstruction of VO2. Monkhorst-Pack method was used to generate 3×3×1
k-point mesh.44
Results and Discussion
The np-Cu@VO2 hybrid electrocatalysts are fabricated through facile
alloying/dealloying processes (see Methods), during which the chemical
compositions are tuned by controlling precursor alloy ribbons of Cu20−xVxAl80
(x = 0.2, 1, and 2 at%). These precursor ribbons are first produced by melt-
spinning in vacuum from their alloy ingots made of pure Cu and Al with
different amounts of V, which are consisted of a α-Al and CuAl2 phase (Figure
S1). This precursor alloy is expected to produce a bimodal nanoporous
architecture of hybrid electrocatalysts when selectively etching less-noble Al
component from the -Al and CuAl2 phase in 2.0 M NaOH solution. With the
formation of np-Cu skeleton via fully chemical dealloying processes, the
metallic V is exposed to alkaline solution, and then oxidized on the surface of
np-Cu to form vanadium oxide species (VO2). According to the V atom ratio in
Figure 1. Structural and composition characterizations of np-Cu@VO2-5%. (a)
SEM image of catalytic surface. (b) High magnification SEM images. (c) low-
magnification HAADF-STEM image. (d) HAADF-STEM image. (e) Fast Fourier
transformation filtered HAADF-STEM image. (f) EDS element mappings of np-
Cu@VO2-5%. (g) V 2p XPS spectra for np-Cu@VO2 with different vanadium
components. (h) Vanadium K-edge XANES spectra before and after CO2RR and
the edge position-amplification view. (i) FT of the EXAFS spectra before and
after 12h CO2RR testing at a constant potential of -0.62 V vs. RHE. Scale bars:
(a) 400 nm, (b) 100 nm, (c) 100 nm, (d) 2 nm, (e) 1 nm, (f) 40 nm.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 4 of 8
ARTICLE
Journal Name
distribution of the np-Cu@VO2-5% is examined by energy-dispersive X-ray area, which indicates that the enhancement of CO2RR performance of
View Article Online
DOI: 10.1039/D0TA09522B
spectroscopy (EDS). Element mappings show that vanadium is npCu@VO2 is not caused by specific surface area. Thus, the BET results
homogeneously distributed in the surface of copper skeleton (Figure 1f).
exclude the influence of specific surface area on performance, which proves
the contribution of VO2 modification in enhancing performance (Figure S14).
The partial current density for ethanol of the np-Cu@VO2-5% and np-Cu
product are further measured (Figure 2c and 2d). For the np-Cu@VO2-5%
hybrid catalyst, the FE for ethanol increase to 30.1%. Accordingly, the partial
current densities for ethanol reaches the maximum value of ~ -16 mA cm−2 at
−0.62 V vs. RHE (Figure 2d), indicating a 4-fold increase in CO2RR activity
compared to the bare nanoporous Cu. The lowest charge-transfer resistance
(RCT) of np-Cu@VO2-5% among the catalysts results in the fastest charge-
transfer process, which may contribute to the increase of current density
(Figure S15). Therefore, the np-Cu@VO2-5% hybrid catalyst shows one of the
best selectivities among the reported electrocatalysts for ethanol production
(Figure S16 and Table S2). The better performance of np-Cu@VO2-5% can be
attributed to the stronger hydrogen affinity of vanadium oxide, which results
in a high H coverage on the catalyst surface.7,28,34 It also shows that the
increase of ethanol FE is not at all potential, which indicates that there is a
competition between the formation of H2 and C2H5OH (Figure 2c). In addition,
the sharp increase of the partial current density of H2 at low potential further
illustrate the better formation of H2 andthe higher H coverage of np-Cu@VO2-
5%, which is caused by the strong affinity of vanadium oxide to hydrogen
(Figure S17). More interestingly, the ethanol FE of np-Cu@VO2 gradually
increases when the V ratio increases from 1% up to
The X-ray photoelectron spectroscopy (XPS) of np-Cu@VO2-5% further
verifies the existence of metallic Cu states (Figure S5). In contrast, the V 2p
spectrum of np-Cu@VO2-5% shows the co-existence of V4+ and V5+, implying
that the vanadium species is indeed formed (Figure 1g). The O 1s spectra
confirms that the vanadium species exist as oxide (Figure S6). Further, X-ray
adsorption spectroscopy (XAS) is performed to probe the electronic structure
of np-Cu@VO2-5% (Figure 1h, 1i and S7). The V K-edge XANES and EXAFS
spectra of np-Cu@VO2-5% confirm that vanadium specie exists as an oxide in
the surface of nanoporous copper. Moreover, once a potential of −0.62 V vs.
RHE is applied, the chemical state of vanadium remains unchanged (Figure
1i). We found that, in agreement with the XPS results, Cu species are slightly
oxidized before the reaction (Figure S7). However, once a negative potential
is applied during CO2RR, only peaks corresponding to metallic Cu are observed
on the hybrid electrocatalyst, which excludes the presence of copper oxide
on the surface. Thus, all results enable us to tentatively describe the surface
of the np-Cu as an amorphous-like vanadium oxide species.
The electrocatalytic CO2RR performance of np-Cu@VO2 is investigated
by using a customized H-type cell with a standard three-electrode system. The
linear sweep voltammetry (LSV) curves of np-Cu@VO2-5% and monometallic
np-Cu are shown in Figure S8. Compared with monometallic np-Cu, the
current densities of np-Cu@VO2-5% obviously increase under Ar and CO2
conditions, implying a higher intrinsic activity toward CO2RR than that of
monometallic np-Cu. The gas chromatographic (GC) analysis for gas-phase
products and nuclear magnetic resonance (NMR) analysis for liquid products
are shown in Figure S9-11. Figure 2a and 2b depict the CO2RR products of np-
Cu and np-Cu@VO2-5%, respectively. It is found that the np-Cu exhibits a
similar product distribution to that of metallic copper reported previously.45-
47 Excluding H2, np-Cu shows six reduction products (CO, HCOOH, CH3OH, C2H4,
C2H6, C2H5OH). Hydrocarbon products that include C2H4 and C2H6 exist
extremely low faradaic efficiency (FE). Meanwhile, C1 products of CO, HCOOH
and CH3OH possess the Faraday efficiency within ten at a potential range from
-0.52 V vs. RHE (Reversible Hydrogen Electrode) to -0.92 V vs. RHE. The
diversity of products leads to low FE of C2H5OH. Obviously, vanadium doping
nanoporous copper not only reduces the type of products, but also reduces
the FE of other CO2 products (Figure 2b). Hydrocarbon products directly
disappear and CO product is close to none. while the np-Cu@VO2-5% shows
higher faradaic efficiency (FE) for H2 and C2H5OH with a proper potential. The
H2 and ethanol product profiles as a function of applied potential for each
catalyst are shown in Figure 2c, revealing that vanadium oxide could promote
the H2 and C2H5OH selectivity of eletrochemical CO2RR and restrain the
formation of other reduction products at a proper potential condition (Figure
S12 and S13). Meanwhile, the difference of specific surface area between np-
Cu and np-Cu@VO2-5% is small, even the np-Cu has higher specific surface
Figure 2. Electrochemical CO2RR performance for np-Cu@VO2 in a typical H-
type cell. (a) The Faradaic efficiency of main products for np-Cu at different
specific potentials. (b) The Faradaic efficiency of main products for np-
Cu@VO2-5% at different specific potentials. (c) H2 and ethanol Faraday
efficiency distribution of np-Cu@VO2-5% and np-Cu at the critical potential.
(d) The comparison of ethanol partial current density between np-Cu and np-
Cu@VO2-5%. (e) Ethanol Faradaic efficiency of np-Cu@VO2 with different V
component. (f) Chronoamperometry result and ethanol FE of np-Cu@VO2-5%
in CO2-saturated 0.1 M KHCO3 solution at -0.62 V vs. RHE.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 8
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
5% (Figure 2e). Further increase of the V content leads to the decline of
ethanol FE because of the more H2 formation. Long-term stability is confirmed
by 12 h electrolysis of np-Cu@VO2-5% at −0.62 V vs. RHE (Figure 2f). Stable
current density and average FE reached 30.1% for the overall run are
achieved. Furthermore, structural features and chemical compositions of the
np-Cu@VO2-5% are maintained as well (Figure S18-20, Table S1).
View Article Online
DOI: 10.1039/D0TA09522B
To achieve practical applications, the use of flow cell reactors could be
an effective solution.48-50 Therefore, the CO2RR performance of np-Cu and np-
Cu@VO2-5% is further measured in a self-designed flow cell reactor (Figure
3a). The current densities measured in 1.0 M KOH continuously increased
with decreasing the applied potential (Figure S21). Figure 3b and 3c show the
Faraday efficiency of main products at different specific potential for np-Cu
and np-Cu@VO2-5%, respectively. Obviously, it has similar phenomenon that
vanadium doping nanoporous copper reduces the type of products and the
FE of other CO2 products. It is easy to see that the ethanol selectivity of np-
Cu@VO2-5% is greatly improved at the same potential compared to np-Cu,
which further proves above view that V doping has a positive effect on
ethanol production (Figure 3d). The ethanol FE of the flow cell reactor can
even reach the highest level of 38.2% at -0.8 V vs. RHE with a high partial
current density of -39 mA cm-2. It is worth noting that the ethanol partial
current density of np-Cu@VO2-5% is all significantly higher than that of np-Cu
(Figure 3e). This is attributed to unique gas-liquid interface and porous
conductive framework for mass transfer in the electrode. Meanwhile, energy
conversion efficiency could achieve 21.3% in the flow cell (Figure 3f). Notably,
np-Cu@VO2-5% is capable of delivering stable current densities of -102 mA Figure 3. CO2RR performance of np-Cu and np-Cu@VO2-5% catalyst in the
cm-2 at low applied potentials of -0.80 V vs. RHE, with ethanol FE close to flow cell. (a) Schematic diagram of gas diffusion electrode in flow cell (red and
38.2% (Figure 3g), indicating that the commercial application could be white balls stand for carbon, oxygen, respectively. Orange atomic layer is
realized close. Consequently, the extraordinary electrocatalytic performance catalyst layer). (b) The Faraday efficiency of main products for np-Cu at
of the as-prepared np-Cu@VO2-5% makes it is comparable with recently different specific potential. (c) The Faraday efficiency of main products for np-
reported Cu-based electrocatalysts for CO2-to-ethonal production in flow cell Cu@VO2-5% at different specific potential. (d) The ethanol Faraday efficiency
(Table S3).
To understand the origin of such high electrocatalytic performances, the
distribution of np-Cu and np-Cu@VO2-5% at different specific potential. (e)
The comparison of ethanol partial current density between two catalysts at
different specific potential (f) The comparison of energy conversion efficiency
between two catalysts at the critical potential. (g) Chronoamperometry result
and ethanol FE in 1.0 M KOH solution at -0.8 V vs. RHE.
CO2RR mechanisms of np-Cu@VO2 are investigated by first-principles
calculations. We first examine the water dissociation energy and the H
adsorption energy on both bare Cu (111) and Cu (111)@VO2 (Table S4). As
shown in Figure 4a, there is a supramaximal difference of 1.33 eV in the
dissociation energy of hydrolysis between the Cu and Cu@VO2, which
indicates that water dissociation is more favorable on the vanadium oxide
modified-Cu surface in comparison with that on the bare Cu. While, H2O
adsorption of vanadium oxide modified-Cu is stronger than that of
monometallic Cu. This validates that the surface of oxide modified-Cu is
surrounded by more protons, which are from water dissociation in
comparison to bare Cu. The more negative adsorption energy of H on
Cu@VO2 hybrid catalyst indicates that the oxide decoration can increase the
*H coverage. These findings indicate that building such hybrid catalyst could
enable us to investigate and exploit *H in CO2RR.
Moreover, adsorptions of intermediate products on catalyst surfaces are
also theoretically investigated by means of the first-principles approach.
(Figure 4b, Figure S22 and S23). It is observed that CO2 forms C2 intermediate
on the copper surface through protonation firstly to form *COOH, then
dehydrate to *CO, and finally copolymerize to *OCCO.2,48,51 In the initial CO2
protonation process, compared with bare Cu (0.5 eV), Cu@VO2 exhibits lower
energy barrier (0.29 eV), which is considered as the rate-determining step
(Figure 4c, Table S5). In further reaction process for Cu@VO2, *COOH
intermediate hydrogenation and dehydration to *CO intermediate
abnormally appears footsteps. This is basically certain that dehydration is
inhibited by strong water adsorption for Cu@VO2, which caused small energy
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 6 of 8
ARTICLE
Journal Name
barrier to the formation of *CO. It is worth noting that Cu@VO2 has lower concentration caused by bimodal nanopore can reduce the VrieeawcAtirotinclebOarnrliineer
D57O-59I: 10.1039/D0TA09522B
*CO coupling energy barrier (0.94 eV) in comparison to Cu (1.39 eV, Table S4). of C-C coupling and promote the reaction kinetics.
Meanwhile, the higher *CO adsorption energy can inhibit the desorption of
*CO, leading to the *CO coupling,52-54 which is consistent with experimental
result that the FE of CO sharply declines after introducing vanadium oxide.
Based on the above analysis, the possible CO2RR mechanism of np-Cu@VO2
is proposed (Figure 4d). The strong hydrolytic ability of the vanadium oxide
grown on the Cu surface leads to the great increase of the *H coverage. The
high *H coverage reduces the hydrogenation reaction energy barrier of
protonation of carbon dioxide. Furthermore, the higher *CO adsorption
energy of Cu@VO2 are accompanied with a distinct decrease of free energy
for the *COCO formation, which is deemed as the rate-determining step in
the ethanol formation path. In addition, the *HCCOH hydrogenates into
*HCCHOH is a key process in the ethanol pathway *HCCOH,34,55,56 The
accelerated water dissociation and strong hydrogen adsorption of vanadium
oxide increase the surface *H coverage of the hybrid catalyst so as to break
the double bond of the *HCCOH intermediate to form *HCCHOH, which
results in the improvement of ethanol selectivity.
Conclusion
In summary, we have successfully developed a new np-Cu@VO2 hybrid
catalyst toward high-efficiency CO2RR by one-step dealloying method. The
hierarchically bicontinuous nanoporosity, produced by chemical dealloying,
offers a large specific surface area, bimodal porous structure and excellent
electrochemical stability, leading to remarkably CO2RR performance and long-
term catalytic lifetime. The resultant np-Cu@VO2-5% exhibits a 30.1%
Faradaic efficiency for CO2-to-ethonal production and an ethanol partial
current density of −16 mA cm−2 at −0.62 V versus reversible hydrogen
electrode (RHE), corresponding to a 4-fold increase in activity compared to
the bare nanoporous Cu. It even delivers ethanol partial current density that
exceeds -39 mA cm-2 at -0.8 V vs. RHE in a flow-cell reactor, which is much
higher than that of np-Cu. DFT studies reveal that VO2 modified Cu surface
can facilitates the dissociation of water and adsorption of hydrogen on Cu,
which would decrease energy barrier in two rate-determining steps reaction
and promote the key *HCCHOH intermediate species forming on copper
surfaces in the ethanol production pathway. The present results hold a great
promise for the design of multi-step CO2 electroreduction catalysts with great
potential for practical application.
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(Grant No. 51771072, 51901076), the Youth 1000 Talent Program of China,
the Outstanding Youth Scientist Foundation of Hunan Province (Grant No.
2020JJ2006), Fundamental Research Funds for the Central Universities, and
Hunan University State Key Laboratory of Advanced Design and
Manufacturing for Vehicle Body Independent Research Project (Grant No.
71860007).
Figure 4. DFT calculation. (a) Calculated H2O adsorption energies, H2O
dissociation reaction energies, hydrogen adsorption energies on Cu and
Cu@VO2 surfaces. (b) DFT calculated reaction state on Cu@VO2 model, from
left to right is *COOH adsorbed state, *CO adsorbed state and *OCCO
adsorbed state. (c) The free energy diagram for electrochemical CO2RR path.
(d) schematic diagram of key steps in the CO2RR process. red, white, green,
gray and blue spheres stand for copper, oxygen, vanadium, carbon, and
hydrogen atoms, respectively.
Notes and references
1.
2.
D. T. Whipple, P. J. A. Kenis, J. Phys. Chem. Lett., 2010, 1, 3451-3458.
J. Hansen, P. Kharecha, M. Sato, V. Masson-Delmotte, F. Ackerman, D.
J. Beerling, P. J. Hearty, O. Hoegh-Guldberg, S. L. Hsu, C. Parmesan, J.
Rockstrom, E. J. Rohling, J. Sachs, P. Smith, K. Steffen, L. V. Susteren,
K. V. Schuckmann, J. C. Zachos, PLoS One., 2013, 8, e81648.
W. Ju, A. Bagger, G. Hao, A. S. Varela, I. Sinev, V. Bon, B. R. Cuenya, S.
Kaskel, J. Rossmeisl, P. Strasser, Nat. Commun., 2017, 8, 944.
W. Zhu, B. M. Tackett, J. G. Chen, J. Jiao, Top. Curr. Chem., 2018, 376,
41.
Besides the intrinsically catalysis of the VO2 modified np-Cu, the
hierarchically nanoporous structure fabricated by the chemical dealloying
plays important roles in electrochemical CO2RR of np-Cu@VO2. The large
surface area and bimodal bicontinuous porous structure, together with nano-
size Cu ligaments and thus reduced diffusion length, promote timely fast
diffusion of the both electrons and reactants.35-37 The bimodal pore has a
limiting effect on the diffusion of C1 intermediate and hydroxyl radical on the
catalyst surface. Thereby, the increase of local pH and C1 intermediate
3.
4.
5.
6.
D. Zhu, J. L. Liu , S. Qiao, Adv. Mater., 2016, 28, 3423-3452.
G. G. Morales-Guio, E. R. Cave, S. A. Nitopi, J. T. Feaster, L. Wang, K. P.
Kuhl, A. Jackson, N. C. Johnson, D. N. Abram, T. Hatsukade, C. Hahn, T.
F. Jaramillo, Nat. Catal., 2018, 1, 764-771.
7.
A. Vasileff, C. Xu, Y. Jiao, Y. Zheng, S. Qiao, Chem., 2018, 4, 1-23.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 8
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
8.
9.
E. Ren, Y. Deng, A. D. Handoko, C. Chen, S. Malkhandi, B. S. Yeo, ACS 44. H. J. Monkhorst, J. D. Pack, Phys. Rev. B., 1976, 13, 5188.
View Article Online
Catal., 2015, 5, 2814-2821.
A. D. Handoko, K. W. Chan, B. S. Yeo, ACS Energy Lett., 2017, 2, 2103- 46. A. Loiudice, P. Lobaccaro, E. A. Kamali, T. Thao, B. H. Huang, J. W. Ager,
2109. R. Buonsanti, Angew. Chem. Int. Ed., 2016, 55, 5789-5792.
10. Y. Hori, I. Takahashi, O. Koga, N. Hoshi, J. Phys. Chem. B., 2002, 106, 15- 47. D. Higgins, A. T. Landers, Y. Ji, S. Nitopi, C. G. Morales-Guio, L. Wang, K.
45. S. Sen, D. Liu, G. T. R. Palmore, ACS Catal., 2014, 4, 3091-3095.
DOI: 10.1039/D0TA09522B
17.
Chan, C. Hahn, T. F. Jaramillo, ACS Energy Lett., 2018, 3, 2947-2955.
48. M. G. Kibria, J. P. Edwards, C. M. Gabardo, C. Dinh, A. Seifitokaldani, D.
Sinton, E. H. Sargent, Adv. Mater., 2019, 1807166.
49. M. Jouny, W. Luc, F. Jiao, Nat. Catal., 2018, 1, 748-755.
50. D. M. Weekes, D. A. Salvatore, A. Reyes, A. Huang, C. P. Berlinguette,
Acc. Chem. Res., 2018, 51, 910-918.
11. S. Lee, G. Park, J. Lee, ACS Catal., 2017, 7, 8594-8604.
12. A. Bagger, W. Ju, A. S. Varela, P. Strasser, J. Rossmeisl, ChemPhysChem.,
2017, 18, 3266-3273.
13. A. A. Peterson, J. K. Nørskov, J. Phys. Chem. Lett., 2012, 3, 251-258.
14. K. Jiang, Y. Huang, G. Zeng, F. M. Toma, W. A. Goddard, A. T. Bell, ACS
Energy Lett., 2020, 5, 1206-1214.
51. Q. Fan, M. Zhang, M. Jia, S. Liu, J. Qiu, Z. Sun, Mater. Today Energy,
2018, 10, 280-301.
15. F. Pan, Y. Yang, Energy Environ. Sci., 2020, 13, 2275-2309.
16. Y. Zhou, F. Che, M. Liu, C. Zou, Z. Liang, P. D. Luna, H. Yuan, J. Li, Z. Wang, 52. G. Iijima, T. Inomata, H. Yamaguchi, M. Ito, H. Masuda, ACS Catal., 2019,
H. Xie, H. Li, P. Chen, E. Bladt, R. Quintero-Bermudez, T. Sham, S. Bals, 9, 6305-6319.
J. Hofkens, D. Sinton, G. Chen, E. H. Sargent, Nat. Chem., 2018, 10, 974- 53. Y. Yang, S. Ajmal, Y. Feng, K. Li, X. Zheng, and L. Zhang, Chem. Eur. J.,
980. 2019, 25, 1-11.
17. T. Zhuang, D. Nam, Z. Wang, H. Li, C. M. Gabardo, Y. Li, Z. Liang, J. Li, X. 54. H. Wang, Y. Tzeng, Y. Ji, Y. Li, J. Li, X. Zheng, A. Yang, Y. Liu, Y. Gong, L.
Liu, B. Chen, W. R. Leow, R. Wu, X. Wang, F. Li, Y. Lum, J. Wicks, C. P.
O’Brien, T. Peng, A. H. Ip, T. Sham, S. Yu , D. Sinton, E. H. Sargent, Nat.
Commun., 2019, 10, 4807.
Cai, Y. Li, X. Zhang, W. Chen, B. Liu, H. Lu, N. A. Melosh, Z. Shen, K. Chan,
T. Tan, S. Chu, Y. Cui, Nat. Nanotech., 2020, 15, 131-137.
55. H. Xiao, T. Cheng, W. A. Goddard, J. Am. Chem. Soc., 2017, 139, 130-
136.
56. T. Cheng, H. Xiao, W. A. Goddard, Proc. Natl. Acad. Sci., 2017, 114,
1795-1800.
57. C. Dinh, T. Burdyny, M. G. Kibria, A. Seifitokaldani, C. M. Gabardo, F. P.
G. Arquer, A. Kiani, J. P. Edwards, P. D. Luna, O. S. Bushuyev, C. Zou, R.
Quintero-Bermudez, Y. Pang, D. Sinton, E. H. Sargent, Science, 2018,
360, 783-787.
58. T. Zhuang, Y. Pang, Z. Liang, Z. Wang, Y. Li, C. Tan, J. Li, C. T. Dinh, P. D.
Luna, P. Hsieh, T. Burdyny, H. Li, M. Liu, Y. Wang, F. Li, A. Proppe, A.
Johnston, D. Nam, Z. Wu, Y. Zheng, A. H. Ip, H. Tan, L. Chen, S. Yu, S. O.
Kelley, D. Sinton, E. H. Sargent, Nat. Catal., 2018, 1, 946-951.
59. P. Yang, X. Zhang, F. Gao, Y. Zheng, Z. Niu, X. Yu, R. Liu, Z. Wu, S. Qin, L.
Chi, Y. Duan, T. Ma, X. Zheng, J. Zhu, H. Wang, M. Gao, S. Yu, J. Am.
Chem. Soc., 2020, 142, 6400-6408.
18. C. Kim, F. Dionigi, V. Beermann, X. Wang, T. Möller, P. Strasser, Adv.
Mater., 2018, 1805617.
19. C. W. Lee, K. D. Yang, D. Nam, J. H. Jang, N. H. Cho, S. W. Im, K. T. Nam,
Adv. Mater., 2018, 1704717.
20.
J. Wanga, Y. Jib, Q. Shaoa, R. Yina, J. Guoc, Y. Lib, X. Huang, Nano
Energy, 2019, 59, 138-145.
21. S. Ma, M. Sadakiyo, M. Heima, R. Luo, R. T. Haasch, J. I. Gold, M.
Yamauchi, P. J. A. Kenis, J. Am. Chem. Soc., 2017, 139, 47-50.
22. D. Ren, B. S.-H. Ang, B. S. Yeo, ACS Catal., 2016, 6, 8239-8247.
23. C. G. Morales-Guio, E. R. Cave, S. A. Nitopi, J. T. Feaster, L. Wang, K. P.
Kuhl, A. Jackson, N. C. Johnson, D. N. Abram, T. Hatsukade, C. Hahn, T.
F. Jaramillo, Nat. Catal., 2018, 1, 764-771.
24. T. T. H. Hoang, S. Verma, S. Ma, T. T. Fister, J. Timoshenko, A. I. Frenkel,
P. J. A. Kenis, A. A. Gewirth, J. Am. Chem. Soc., 2018, 140, 5791-5797.
25. R. Kortlever, J. Shen, K. J. P. Schouten, F. Calle-Vallejo, M. T. M. Koper,
J. Phys. Chem. Lett., 2015, 6, 4073-4082.
26. Y. Wang, P. Han, X. Lv, L. Zhang, G. Zheng, Joule., 2018, 2, 2551-2582.
27. M. G. Kibria, J. P. Edwards, C. M. Gabardo, C. T. Dinh, A. Seifitokaldani,
D. Sinton, E. H. Sargent, Adv. Mater., 2019, 1807166.
28. W. Ma, S. Xie, T. Liu, Q. Fan, J. Ye, F. Sun, Z. Jiang, Q. Zhang, J. Cheng, Y.
Wang, Nat. Catal., 2020, 3, 478-487.
29. C. Dinh, A. Jain, F. P. G. de Arquer, P. D. Luna, J. Li, N. Wang, X. Zheng,
J. Cai, B. Z. Gregory, O. Voznyy, B. Zhang, M. Liu, D. Sinton, E. J. Crumlin,
E. H. Sargent, Nat. Energy., 2019, 4, 107-114.
30. D. Wang, Q. Li, C. Han, Q. Lu, Z. Xing, X. Yang, Nat. Commun., 2019, 10,
3899.
31. C. Dinh, A. Jain, F. P. G. Arquer, P. D. Luna, J. Li, N. Wang, X. Zheng, J.
Cai, B. Z. Gregory, O. Voznyy, B. Zhang, M. Liu, D. Sinton, E. J. Crumlin,
E. H. Sargent, Nat. Energy., 2019, 4, 107-114.
32. R. Subbaraman, D. Tripkovic, K. Chang, D. Strmcnik, A. P. Paulikas, P.
Hirunsit, M. Chan, J. Greeley, V. Stamenkovic, N. M. Markovic, Nat.
Mater., 2012, 11, 550-557.
33. W. Ma, S. Xie, X. Zhang, F. Sun, J. Kang, Z. Jiang, Q. Zhang, D. Wu, Y.
Wang, Nat. Commun., 2019, 10, 892.
34. M. Luo, Z. Wang, Y. C. Li, J. Li, F. Li, Y. Lum, D. Nam, B. Chen, J. Wicks,
A. Xu, T. Zhuang, W. R. Leow, X. Wang, C. Dinh, Y. Wang, Y. Wang, D.
Sinton, E. H. Sargent, Nat. Commun., 2019, 10, 5814.
35. Y. Zhao, M. Luo, S. Chu, M. Peng, B. Liu, Q. Wu, P. Liu, F. M. F. Groote,
Y. Tan, Nano Energy, 2019, 59, 146-153.
36. Q. Wu, M. Luo, J. Han, W. Peng, Y. Zhao, D. Chen, M. Peng, J. Liu, F. M.
F. Groot, Y. Tan, ACS Energy Lett., 2020, 5, 192-199.
37. K. Jiang, B. Liu, M. Luo, S. Ning, M. Peng, Y. Zhao, Y. Lu, T. Chan, F. M.
F. Groot, Y. Tan, Nat. Commun., 2019, 10, 1743.
38. W. Luc, F. Jiao, ACS Catal., 2017, 7, 5856-5861.
39. G. Kresse, J. Furthmüller, Phys. Rev. B., 1996, 54, 11169
40. G. Kresse, J. Furthmüller, Comput. Mater. Sci., 1996, 6, 15-50.
41. P. E. Blöchl, Phys. Rev. B., 1994, 50, 17953.
42. G. Kresse, D. Joubert, Phys. Rev. B., 1999, 59, 1758.
43. J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 8 of 8
ARTICLE
Journal Name
View Article Online
DOI: 10.1039/D0TA09522B
TOC Figure
A vanadium oxide integrated on hierarchically nanoporous copper hybrid electrocatalyst with three-dimensional bicontinuous nanoporous architecture is
exploited for high-performance electrochemical CO2 reduction. The obtained np-Cu@VO2-5% hybrid electrocatalyst enables a 30.1% Faradaic efficiency for
CO2-to-ethonal production and an ethanol partial current density of −16 mA cm−2 at −0.62 V vs. RHE, corresponding to a 4-fold increase in activity compared
to the bare nanoporous Cu.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins