Highly Efficient Electroreduction of CO2 to C2+ Alcohols on Heterogeneous Dual Active Sites

Article abstract of DOI:10.1002/anie.202006847

Electroreduction of CO₂ to liquid fuels such as ethanol and n-propanol, powered by renewable electricity, offers a promising strategy for controlling the global carbon balance and addressing the need for the storage of intermittent renewable energy. In this work, we discovered that the composite composed of nitrogen-doped graphene quantum dots (NGQ) on CuO-derived Cu nanorods (NGQ/Cu-nr) was an outstanding electrocatalyst for the reduction of CO₂ to ethanol and n-propanol. The Faradaic efficiency (FE) of C2+ alcohols could reach 52.4 % with a total current density of 282.1 mA cm^?2. This is the highest FE for C2+ alcohols with a commercial current density to date. Control experiments and DFT studies show that the NGQ/Cu-nr could provide dual catalytic active sites and could stabilize the CH₂CHO intermediate to enhance the FE of alcohols significantly through further carbon protonation. The NGQ and Cu-nr had excellent synergistic effects for accelerating the reduction of CO₂ to alcohols.

Full text of DOI:10.1002/anie.202006847

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Highly Efficient Electroreduction of CO2 to C2+ Alcohols on
Heterogeneous Dual Active Sites
Authors: Chunjun Chen, Xupeng Yan, Shoujie Liu, Yahui Wu, Qiang
Wan, Xiaofu Sun, Qinggong Zhu, Huizhen Liu, Jun Ma, Lirong
Zheng, Haihong Wu, and Buxing Han
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202006847
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10.1002/anie.202006847
Angewandte Chemie International Edition
COMMUNICATION
Highly Efficient Electroreduction of CO2 to C2+ Alcohols on
Heterogeneous Dual Active Sites
Chunjun Chen,[a,b] Xupeng Yan,[a,b] Shoujie Liu,[c] Yahui Wu,[a,b] Qiang Wan,[a,b] Xiaofu Sun,[a,b] Qinggong
Zhu,[a] Huizhen Liu, [a,b] Jun Ma, [a] Lirong Zheng,[f] Haihong Wu, [e] and Buxing Han,[a,b,d,e ]
*
Abstract: Electroreduction of CO2 to liquid fuels such as ethanol and
n-propanol, powered by renewable electricity, offers a promising
strategy for controlling the global carbon balance and addressing the
need for the storage of intermittent renewable energy. In this work, we
discovered that the composite composed of nitrogen-doped graphene
quantum dots (NGQ) on CuO-derived Cu nanorods (NGQ/Cu-nr) was
outstanding electrocatalyst for CO2 reduction to ethanol and n-
propanol. The Faradaic efficiency (FE) of C2+ alcohols could reach
52.4% with a total current density of 282.1 mA•cm-2. This is the
highest FE for C2+ alcohol with commercial current density to date.
Control experiments and density functional theory (DFT) studies
demonstrated that the NGQ/Cu-nr could provide dual catalytic active
sites, and could stabilize the CH2CHO intermediate to enhance the
FE of alcohols significantly through further carbon protonation. The
NGQ and Cu-nr had excellent synergistic effect for accelerating the
CO2 reduction reaction to alcohols.
with gasoline to give an overall cleaner-burning fuel.[11-12]
However, the selectivity for C2+ alcohols was too low at high
current density,[13-15] and designing of highly efficient and robust
electrocatalysts is crucial to solve the problem.
The Cu-based catalysts are the most promising
electrocatalysts for converting CO2 to C2+ products.[16-19]
However, the most Cu-based catalysts generally favor the
production of ethylene during CO2RR.[20-23] In order to improve the
selectivity of C2+ alcohols, different methods have been applied
to modify the Cu-based catalysts, including modifying
morphology,[24] using two metals,[25] doping heteroatom,[26] and
modifying with other molecules.[27] These methods mainly
increase the production of the key C1 intermediate (CO) by
another component to further enhance the production of alcohols,
due to the selectivity of C2+ products can be tuned by the
coverage of CO.[28] In spite of these impressive efforts, the
Faradaic efficiency (FE) of C2+ alcohols remains below 43% at
commercial current density (≥100 mA•cm-2). Moreover, it is
unclear how the electrocatalytic performance of Cu is influenced
by another component, mainly because microstructural features
such as interfaces and defects influence CO2RR and are difficult
to control.
Electrochemical CO2 reduction reaction (CO2RR) has attracted
increasing attention, which can not only reduce the CO2
concentration in the environment, but also can facilitate the
storage of renewable energy at large scale.[1-5] In recent years,
various valuable products have been obtained, and the C2+
products are more attractive due to their high energy densities
and high economic value per unit mass.[6-10] Among the various
C2+ products, alcohols (e.g. ethanol and n-propanol) are highly
desirable, which can be either used directly as a fuel or blended
It is known that the ethylene and ethanol usually share a
penultimate reaction intermediate: *C2H3O. Thus, it is crucial to
modify the catalyst structure to stabilize and hydrogenate this
intermediate to promote alcohol production.[29] Nitrogen (N)-
doped nanostructured carbon materials are of interest in
catalyzing CO2 reduction into CO and C2 products (especially for
31]
ethanol).[30,
For example, the N-doped mesoporous carbon
[a]
Dr. C. Chen, X. Yan, Y. Wu, Q. Wan, Prof. Dr. X. Sun, Dr Q. Zhu,
Prof. Dr. H. Liu, Dr. J. Ma, Prof. Dr. B. Han
Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Colloid and Interface and Thermodynamics, CAS
Research/Education Center for Excellence in Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190, China
E-mail: hanbx@iccas.ac.cn
Dr. C. Chen, X. Yan, Y. Wu, Q. Wan, Prof. Dr. X. Sun, Prof. Dr. H.
Liu, Prof. Dr. B. Han
could efficiently catalyze electroreduction CO2 to ethanol with very
high FE of 77%, but the reaction rate was very slow with current
density of less than 1 mA•cm-2.[31] The density functional theory
(DFT) calculations demonstrated that N-doped carbon catalysts
could enhance the adsorption of oxygen-containing reaction
intermediates (*OCHx, *OH and *O) during the CO2RR due to its
high oxophilicity, which prefer to the generation of deep reduction
products.[32] Furthermore, the FE of ethanol reached up to 63%
over the composite composed of Cu nanoparticles on N-doped
carbon nanospike film, although the current density was less than
2 mA•cm-2.[33] Thus, we can assume that the N-doped carbon
species may be effective active sites for modifying the Cu-based
catalysts to improve the selectivity of alcohols.
[b]
School of Chemistry and Chemical Engineering, University of
Chinese Academy of Sciences
Beijing 100049, China
[c]
[d]
Dr. S. Liu
Chemistry and Chemical Engineering of Guangdong Laboratory,
Shantou 515063, China.
Prof. Dr. B. X. Han
Physical Science Laboratory, Huairou National Comprehensive
Science Center, No. 5 Yanqi East Second Street, Beijing 101400,
China
Herein, we used N-doped graphene quantum dot (NGQ) as
the second component to modify CuO-derived Cu nanorod (Cu-
nr), due to its high electroactivity, excellent chemical stability and,
very importantly, its high oxophilicity. As a result of this strategy,
outstanding efficiency of CO2 to C2+ alcohols was achieved with
a FE of 52.4% at a current density of 282.1 mA • cm-2.
Experimental and DFT studies indicate that the dual active sites
and the oxophilicity of the NGQ/Cu-nr played a key role for highly
efficient C2+ alcohols production.
[e]
[f]
Prof. Dr. B. Han, Prof. Dr. H. Wu
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, School of Chemistry and Molecular Engineering, East
China Normal University, Shanghai 200062, China
Dr. L. Zheng
Institute of High Energy Physics, Chinese Academy of Sciences,
Beijing 100049, China.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.202006847
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The NGQ/CuO-nr was first prepared blending NGQ and CuO
nanorod (CuO-nr), and the NGQ/Cu-nr was then formed in-situ by
electroreduction of the CuO-nr in the NGQ/CuO-nr. The CuO-nr
was obtained from Cu(OH)2 nanorod by calcination under N2
atmosphere, and the NGQ was synthesized through in-situ N
doping of graphene oxide in the dimethylformamide (DMF)
solvent.[30] The obtained CuO-nr had a diameter about 40 nm
(Figure S1). The NGQ had a narrow lateral size distribution of 2-
5 nm (Figure S2). The NGQ/CuO-nr hybrid was prepared by
blending NGQ and CuO-nr in DMF with the assistance of
sonication (see Methods for experimental details). Scanning
electron microscope (SEM) and transmission electron microscopy
(TEM) revealed that the NGQ/CuO-nr also exhibited the nanorod
morphology (Figure 1A, 1B), which was similar to the original
CuO-nr. High-resolution transmission electron microscopy (HR-
TEM) suggests direct interfacial contact between NGQ and CuO-
nr (Figure 1C), and the corresponding lattice distance of NGQ and
CuO were observed in the HR-TEM image. The energy dispersive
X-ray spectroscopy maps show that the elements of Cu and N
uniformly dispersed over the catalyst (Figure 1D, 1E and 1F),
which confirm that the NGQ was successfully dispersed on the
CuO-nr.
(Figure 1H). It is noted that the NGQ vibrational features were not
observed for the NGQ/CuO-nr, which confirms that the small NGQ
was uniformly dispersed on the CuO-nr. Raman spectroscopy
was further used to characterize the NGQ/CuO-nr hybrid (Figure
1I). Signature vibrational peaks of NGQ can be discerned in the
spectrum, which indicates that the NGQ/CuO-nr hybrid was
successfully prepared.
Due to the electronic structure of Cu can influence the CO2
reduction, we carried out operando X-ray adsorption
spectroscopy (XAS) to monitor the local structure of Cu during
CO2RR. For both the NGQ/CuO-nr and CuO-nr, only peaks
corresponding to metallic Cu were observed once a negative
potential had been applied during CO2RR (Figure 2 and Figure
S4-S5), indicating that the CuO was rapidly reduced to metallic
Cu. Thus, NGQ/Cu-nr or Cu-nr was the catalyst in the reaction.
We also characterized NGQ/Cu-nr after 100 h electrolysis by SEM,
TEM and EDS (Figure S6). No obvious change in morphology and
structure was observed, showing the excellent stability of
NGQ/Cu-nr in the reaction. Moreover, we used the ARTEMIS
programs of IFEFFIT to fit the Cu-Cu coordination number of the
NGQ/Cu-nr and Cu-nr during CO2RR (Figure S7-S8 and Table
S1). No obvious difference of Cu-Cu coordination number and
bond distance was observed for the NGQ/Cu-nr and Cu-nr
throughout CO2RR process. These results indicate that the NGQ
did not change the coordination property of Cu during CO2RR.
The electrocatalytic activity and selectivity of catalysts towards
CO2 reduction were evaluated in a flow cell (Figure S9) employing
1 M aqueous KOH as an electrolyte. The polytetrafluoroethylene
(PTFE) membrane with average pore size of 220 nm was used as
the gas diffusion electrode (Figure S10).[1] The gaseous and liquid
products were analyzed by gas chromatography (GC) and
nuclear magnetic resonance (NMR) spectroscopy (Figure S11),
respectively.
Figure 1. (A) The SEM image of the NGQ/CuO-nr. (B) The TEM image of the
NGQ/CuO-nr. (C) HR-TEM image of the NGQ/CuO-nr. (D, E and F) The energy
dispersive X-ray spectroscopy (EDS) maps of the NGQ/CuO-nr. (G) XPS
spectra of N1s orbits of the NGQ/CuO-nr. (H) The XRD patterns of different
catalysts. (I) The Raman spectra of different catalysts.
X-ray photoelectron spectroscopy (XPS) was employed to
study the chemical states and composition of the catalysts. The N
1s spectra of NGQ was fitted with three components, including
pyridinic N (398.8 eV), pyrrolic N (399.9 eV) and graphitic N
(401.1 eV),[34, 35] and the content of N in the NGQ was about 5.2
mol%, in which the amounts of pyridinic N, pyridonic N, and
graphitic N in the NGQ were 63%, 12%, and 25%(Figure S2E).
For the NGQ/CuO-nr, the content pyridinic N decreased to 41%
and pyridonic N increased to 26% (Figure 1G), due to the
interaction between the NGQ and CuO. According to previous
literature, most of the pyridinic N would locate at the edge sites in
NGQ, due to the lower formation energy of N doping at the edge
sites than the basal planes.[36, 37] The Cu valence state in the
NGQ/CuO-nr was similar to that in the CuO-nr (Figure S3).
Besides, XRD was also used to characterize the NGQ/CuO-nr
Figure 2. (A) XANES spectra at the Cu K-edge for NGQ/Cu-nr at different
potentials during CO2RR. (B) The corresponding Fourier transforms FT(k3w(k))
for NGQ/Cu-nr at different potentials during CO2RR.
It can be clearly observed that NGQ/Cu-nr obtained
outstanding efficiency for CO2 reduction to C2+ products (Figure
3A). The FE of C2+ products (FEC2+) could reach up to 80.4% with
a current density of 282.1 mA•cm-2 at -0.9 V vs RHE. Especially,
the FE of C2+ alcohols could reach 52.4%, which is significantly
higher than that of ethylene. The FEC2+ for the Cu-nr was 64.6%,
and the FE of C2+ alcohols was only 28.1% at -0.9 V vs. RHE
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(Figure 3B). The partial current density (current density times FE)
of C2+ alcohols over NGQ/OD-Cu-nr could reach 147.8 mA•cm-2
at -0.9 V vs. RHE, which is about 2 times that of over Cu-nr (Figure
S12). The FEs of all products generated in the reaction are given
in Figure S13, and the total FE was around 100 %. Compared
with the state of the art catalysts, the FE for C2+ alcohol over
NGQ/Cu-nr was the highest with commercial current density
(Figure 3C and Table S2). In addition, we assembled a full cell
system with NGQ/Cu-nr as the cathode catalyst for the CO2RR
and commercial IrO2 catalyst (Figure S14) as the anode for
surface areas (ECSAs) and Nyquist plots were measured (Figure
S17-S18), we can observe that the ECSAs and charge transfer
resistance (Rct) for the NGQ/Cu-nr were similar to that of the Cu-
nr, which suggested that the improved C2+ alcohols generation
resulted not mainly from the slight change of the ECSAs and
electronic conductivity.
Our experiment showed that the bare NGQ could produce CO
and C2 products (Figure S19), and the coverage of CO on
catalysts could alter the selectivity of C2+ products. In order to
explore the role of the local CO concentration on the activity of the
oxygen evolution. The system delivered C2+ alcohols FE of 51.6% NGQ/Cu-nr and Cu-nr, CO elctroreduction reaction (CORR) in 1
with a current density of 132 mA• cm-2 at a cell voltage of 3.6 V in
1.0 M KOH, and the C2+ alcohols full-cell energy conversion
efficiency (EE) was 16.4% (Figure S15). These results indicate
that NGQ/Cu-nr can significantly enhance the selectivity of C2+
alcohols. In the meantime, the alcohol-to-ethylene ratio was
enhanced from 0.81 on the Cu-nr to 2.18 on the NGQ/Cu-nr
(Figure 3D), while overall C2+ selectivity only increased 1.2 times.
It indicates that the selectivity mainly shifts from producing
ethylene to alcohols. Moreover, using labelled 13CO2, we further
confirmed that the carbon source of products was only from CO2
(Figure S16). Furthermore, continuous CO2 reduction was
performed at -0.9 V vs. RHE for 100 h to elucidate the electrode
stability of the NGQ/Cu-nr. As shown in Figure 3E, there was no
obvious change in both current density and FE of the products.
M KOH was conducted in the flow cell. For the Cu-nr (Figure S20),
we can observe that the FE of oxygenic products (ethanol, acetate
and n-propanol) was higher than that of in CO2RR, and the FE of
C2+ alcohols can reach up to 35.1% at -0.7 V vs. RHE. However,
for the NGQ/Cu-nr (Figure 4A), the FE of C2+ alcohols was 75.3%
at -0.7 V vs. RHE, which is significantly higher than that of Cu-nr,
and the FE of n-propanol could reach up to 27.2% at -0.7 V vs.
RHE over NGQ/Cu-nr, which is one of the highest FE of C3
alcohols (Table S3). In the meantime, the alcohol-to-ethylene
ratio was also compared in CORR (Figure 4B), we can observe
that the NGQ/Cu-nr exhibited distinct high alcohol-to-ethylene
ratio (about 9.8), which is 10.6 times than that of Cu-nr. These
results indicate that the significant difference of C2+ alcohols
selectivity between Cu-nr and NGQ/Cu-nr was not due to the
different CO concentration. Thus, we can assume that the
increasing FE of C2+ alcohols over NGQ/Cu-nr was mainly from
the improved stability of the oxygenic C2 intermediates. The
hypothesis can also be verified by the enhancement of n-propanol,
since the n-propanol was produced by the stable oxygenic C2
intermediates and CO.[13] Therefore, the introduced-NGQ can
significantly increase the stability of oxygenic C2 intermediates.
Figure 3. (A, B) The distribution of C2+ products at different potentials over
NGQ/Cu-nr and Cu-nr. (C) Plot of C2+ alcohol partial current density versus
maximum C2+ alcohol FE for various catalysts, and the source of literatures are
listed in supporting information (Table S2). (D) FE of C2+ alcohols (ethanol and
propanol) and ethylene on different catalysts at the potential of -0.9ꢀV versus
RHE. The five-pointed star show the corresponding FE ratio of alcohols to
alkenes. (E) The current density and FE of C2+ products on NGQ/Cu-nr at -0.9
V vs. RHE with 100-hour potentiostatic electrolysis tests.
Figure 4. (A) The distribution of C2+ products at different potentials over
NGQ/Cu-nr in CORR. (B) FE of C2+ alcohols (ethanol and propanol) and
ethylene on different catalysts at the potential of -0.9ꢀV versus RHE in CORR.
The five-pointed star shows the corresponding FE ratio of alcohols to alkenes.
The intrinsic reason for the enhanced CO2 conversion to
alcohols was further investigated. The electrochemical active
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(C) The in-stiu Raman spectra for NGQ/Cu-nr at different potentials during
CO2RR.
We further investigated the role of the NGQs by varying its
content in the NGQ/Cu-nr (Figure S21). The ratio of alcohols to
ethylene gradually increased with the loading of NGQ on the CuO-
nr and reached the maximum when the content was 5 wt%. A
further increase of the NGQ loading led to decreasing the
selectivity of both C2+ products and alcohols, which may result
from the fact that too many Cu sites were covered by the NGQs.
Thus, we can conclude that the selectivity of alcohols was
affected by both NGQ and Cu-nr.
To gain mechanistic insight into the cooperative effect of the
NGQ and Cu-nr substrate, the in situ Raman spectroscopy was
carried out to reveal the reaction intermediates during CO2
reduction. A custom-built Raman setup was used to probe the
surface of NGQ/Cu-nr and Cu-nr electrodes during CO2 reduction
at different potentials (Figure S22).
For the Cu-nr, the presence of adsorbed *CO on Cu was
demonstrated by the appearance of Raman peaks located at 282-
288, 364-368, and 2024-2044 cm-1, which correspond to the
restricted rotation of adsorbed *CO on Cu, Cu-CO stretching, and
C≡O stretching, respectively (Figure S23).[38] These peaks were
also observed on NGQ/Cu-nr, showing the vibration of adsorbed
*CO on Cu sites (Figure 4C). There was no obvious difference in
these peaks on Cu-nr and NGQ/Cu-nr, indicating that the
adsorbed *CO on Cu was not influenced by the added NGQ.
Combining with the results of CORR, it can be deduced that the
enhancement of alcohols was not from the high local CO via
spillover or tandem effects. We can observe that the signature
vibrational peaks of NGQ can be discerned at every potential,
indicating that the NGQ were stable on NGQ/Cu-nr during CO2RR.
From the above results, we can propose that the high FE of
C2+ alcohols over the NGQ/Cu-nr was mainly from the synergistic
effect of NGQ and Cu-nr, not the high local CO via spillover or
tandem effects and electronic structure of Cu. Both NGQ and Cu-
nr can produce C2 products, suggesting that they both have
active sites for producing C2+ products. Thus, we can assume
that the NGQ/Cu-nr can provide dual active sites (NGQ and Cu)
for the CO2 reduction to C2+ products. Previous study also
showed that the synergistic effect of dual active sites can promote
generation of C2 products, although the selectivity of C2 products
was low.[32]
In order to further verify the hypothesis, the N-doped carbon
quantum dots (NCQ) was prepared (Figure S24), the size of NCQ
was similar to that of the NGQ. The NCQ could promote
transformation of CO2 to CO, and the FE was 62% at -1.3 V vs.
RHE (Figure S25), but there were no C2+ products or CH4,
indicating that desorption of CO was easy and the C-C coupling
cannot be occurred on the NCQs. For the NCQ/Cu-nr, the FE of
C2H4 was higher than that of Cu-nr at high potential (Figure
S26).Thus we believe that the increase of C2H4 over NCQ/Cu-nr
is because that CO produced on NCQs transferred to the Cu
surface, which further transformed into C2H4. However, the
alcohol-to-ethylene ratio is similar to that on the Cu-nr. Therefore,
the ability of C-C coupling over NGQ played a vital role for the
enhancement of producing alcohols, and the NGQ and Cu
showed a synergistic effect.
Figure 5. (A) Proposed reaction paths for CO2 reduction to C2H5OH and C2H4
over NG/Cu(111). (B) Reaction Gibbs free energy diagram from the adsorbed
C2H3O intermediate to ethylene (blue lines) and ethanol (red lines) on Cu(111).
(C) Reaction Gibbs free energy diagram from the adsorbed C2H3O intermediate
to ethylene (blue lines) and ethanol (red lines) on NG/Cu(111). (D) Reaction
Gibbs free energy diagram from the adsorbed C2H3O intermediate to ethylene
(blue lines) and ethanol (red lines) on Cu(111) after applying a -0.5ꢀV bias
potential. (E) Reaction Gibbs free energy diagram from the adsorbed C2H3O
intermediate to ethylene (blue lines) and ethanol (red lines) on NG/Cu(111) after
applying a -0.5ꢀV bias potential.
DFT calculations were then performed to gain insight into the
outstanding performance of the NGQ/Cu-nr. From the results of
CO2RR of NGQ/Cu-nr, we can observe that n-propanol (C3H7OH)
and ethanol (C2H5OH) follow a similar selectivity trend at the
applied potentials, indicating that ethanol and n-propanol share
common intermediates along their reaction pathways. Thus, we
used ethanol as a proxy for the alcohols in the DFT study.
From the results of CO2RR, it is known that the increase of
C2+ alcohols was mainly from the decrease of ethylene.
According to previous literature,[29] ethylene and ethanol share a
penultimate reaction intermediate: CH2CHO intermediate
(*C2H3O). Thus, the alteration of selectivity for C2 products was
attributed mainly to the stability of intermediate on catalyst surface.
As shown in Figure 5A, the adsorbed *C2H3O may change to
ethanol through further carbon protonation, or to ethylene by
leaving an oxygen atom adsorbed on the surface. Therefore, we
used DFT to study the effect of surface modifications on the
thermodynamics of the adsorbed CH2CHO intermediate, and in
turn on the selectivity for ethylene versus ethanol production.
The Cu(111) surface was selected as the model of Cu-nr
(Figure S27), due to the main crystal plane is (111) (Figure S28).
A layer of N-doped graphene (NG) was covered on the surface of
Cu (111) to represent the model of the NGQ/Cu-nr. Then, the
energy barriers of every step were characterized over Cu (111)
and NG/Cu(111), starting with the bound *CH2CHO intermediate
(Figure S29-32 and Table S4).
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COMMUNICATION
[2]
[3]
H. Wang, Y. K. Tzeng, Y. Ji, Y. Li, J. Li, X. Zheng, A. Yang, Y. Liu, Y.
Gong, L. Cai, Y. Li, X. Zhang, W. Chen, B. Liu, H. Lu, N. A. Melosh, Z. X.
Shen, K. Chan, T. Tan, S. Chu, Y. Cui, Nat. Nanotech. 2020, 15, 131-
137.
For the Cu (111) surface, the formation of *C2H4O is
endergonic, and the free energy barrier is 1.22 eV (Figure 5B).
This indicates that transform of the adsorbed *C2H3O to ethanol
through further carbon protonation is unfavourable. In contrast,
the formation of C2H4 is exergonic, indicating that the C-O bond
of *C2H3O is prone to break and leave an oxygen atom adsorbed
on the Cu (111) surface. Furthermore, the Gibbs free energy
diagrams were charaterized at -0.5 V applied potential (Figure 5D),
which is the requirement to overcome the C-C coupling step. The
ethanol pathway remains endergonic, and the ethylene pathway
becomes more favorable, due to the energy barriers of *OH
formation becomes lower. These results show that the ethylene is
more favorable to be produced on Cu (111) surface, which is
consistent with the experimental results and previous literature.[39]
Interestingly, for the NG/Cu(111), the O atom of *C2H3O is
preferentially attached to the NG (Figure 5A and Figure S31). This
is mainly attributed to the strong oxophilicity properties of NG.
Meanwhile, the formation of *C2H4O become exergonic, and the
free energy barrier is -0.26 eV (Figure 5C), which is lower than
that of the formation of C2H4 (-0.10 eV). Furthermore, the free
energy barrier of the *C2H4O formation is still lower than that of
the C2H4 formation at -0.5 V applied potential, and the desorption
of C2H5OH becomes easier (Figure 5E). These results indicated
that the ethanol pathway is more favorable on NG/Cu (111)
surface, which is in agreement with the experimental results.
In summary, we designed NGQ/Cu-nr catalyst for
electroreduction CO2 to C2+ alcohols. The FE of C2+ alcohols
could reach up to 52.4% with a current density of 282.1 mA cm-2.
Based on the detailed study, the dual active sites mechanism was
verified, which can significantly enhance CO2 reduction to
alcohols. DFT calculations suggested that the combination of the
NGQ and Cu-nr can enhance the stabilization of oxygenic C2
intermediate, and the formation of ethanol is favorable on the
NGQ/Cu-nr. We believe that the efficient catalyst has potential of
application in CO2 reduction to C2+ alcohols, and that design of
dual active sites catalysts is a promising way for preparing other
highly efficient electrocatalysts.
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Keywords: carbon dioxide • electrocatalysis • dual active sitesl •
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
The electrocatalyst composed of
nitrogen-doped graphene quantum
dots (NGQ) on CuO-derived-Cu
nanorod (NGQ/Cu-nr) is reported,
which is outstanding electrocatalyst
for CO2 reduction to ethanol and n-
propanol. The Faradaic efficiency of
C2+ alcohols could reach 52.4% with
a total current density of 282.1
mA•cm-2. Control experiments and
DFT studies indicate that the
Chunjun Chen, Xupeng Yan, Shoujie
Liu, Yahui Wu, Qiang Wan, Xiaofu Sun,
Qinggong Zhu, Huizhen Liu, Lirong
Zheng, Haihong Wu, Buxing Han *
Page No. – Page No.
Highly Efficient Electroreduction of
CO2 to C2+ Alcohols on
Heterogeneous Dual Active Sites
NGQ/Cu-nr can provide dual active
sites and stabilize the oxygenic C2
intermediates, thus resulting in a great
improvement of CO2 reduction to
alcohols.
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