Boron-doped CuO nanobundles for electroreduction of carbon dioxide to ethylene

Article abstract of DOI:10.1039/d0gc00730g

Novel boron-doped CuO nanobundles are designed for CO₂ reduction to the single multi-carbon product of ethylene, and their faradaic efficiency can reach 58.4% with a current density of 18.2 mA cm^-2. This active, selective and simply prepared electrocatalyst provides a promising electrocatalyst candidate for CO₂ reduction to ethylene.

Full text of DOI:10.1039/d0gc00730g

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DOI: 10.1039/D0GC00730G
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B-doped CuO nanobundles for electroreduction of carbon dioxide
to ethylene
Qiang Wan,a Jianling Zhang,*a, b Bingxing Zhang,a Dongxing Tan,a Lei Yao,c Lirong Zheng,c Fanyu
Received 00th January 20xx,
Accepted 00th January 20xx
Zhang,a Lifei Liu,a Xiuyan Chenga and Buxing Hana, b
DOI: 10.1039/x0xx00000x
convert CO2 into ethylene with a maximum FE of 51.3% in 0.1 M
K2SO4 solution27. It still remains a great challenge to selectively
obtain a single multi-carbon product from electrocatalytic
CO2RR.
A novel boron-doped CuO nanobundles is designed for CO2
reduction to the only multi-carbon product of ethylene and its
faradaic efficiency can reach 58.4% with a current density of 18.2
mA cm-2. The active, selective and simply prepared electrocatalyst
provides a promising electrocatalyst candidate for CO2 reduction to
ethylene.
Herein, we propose for the first time the B-doped CuO
nanobundles for highly efficient production of ethylene from
electrocatalytic CO2RR. Ethylene is the only multi-carbon
product and its FE can reach 58.4% at -1.1 V (versus the
reversible hydrogen electrode, vs. RHE) with a current density
of 18.2 mA cm-2. The B-doped CuO nanobundles have a
nanowire stack structure and abundant oxygen vacancies,
which increase the exposure of accessible active sites, CO2
adsorption capacity and facilitate the diffusion of CO2 and
electrocatalytic product.
In recent years, the emission of carbon dioxide (CO2) has
brought huge impacts on global environment, such as melting
of icebergs, rise of sea level, forest fires, extremely hot and cold
weather and so on.1-5 Therefore, how to utilize CO2 and turn it
into renewable carbon resources have become into a new
trend.6 Particularly, the electroreduction of CO2 expands the
range of chemical applications of CO2 because it can run the
reaction with Gibbs free energy both less than and higher than
zero.7 So far, the faradaic efficiency (FE) of low energy C1
products, represented by carbon monoxide and formic acid, is
close to 100%,8 while the FE values of multi-carbon products
with higher values and energy are not high enough. Up to now,
copper is one of the most efficient metals that can catalyze CO2
into multi-carbon compounds owing to its appropriate
adsorption and desorption energy for intermediates in CO2
reduction reaction (CO2RR) process.8-13 Great efforts have been
devoted to the efficient acquisition of multi-carbon products on
copper catalysts, such as using noble metals to form alloys with
copper,14-18 adjusting crystal surface of copper catalyst8,19-21 and
heteroatom doping.22-25 Among these studies, the
incorporation of light element boron (B) into Cu catalyst has
been reported to boost CO2 conversion.24,26 Even though the
total FE of C2 hydrocarbons could reach ~80%24 or 48.2%26 at
applied potentials, other C2 products (ethylene, ethanol,
ethane) are generated simultaneously and the FE of ethylene
were 52%24 and 15.2%.26 Recently, we reported the
construction of multi-shelled CuO microboxes, which can
The catalyst was prepared by a simple method (Scheme S1).
Typically, the desired amounts of copper chloride dihydrate,
polyethylene glycol (PEG, average MW: 20000) and boric acid
were dissolved in water, followed by addition of a certain
amount of sodium hydroxide. PEG plays as a directing reagent
for nanowire formation. After reaction for 2 h, the product was
obtained by centrifugation, washing and drying. It is worth
noting that such a wet chemical route for catalyst synthesis is
simple, convenient, rapid and involves no calcination procedure
that is usually requisite for the preparation of copper
electrocatalysts24,27
.
Fig. 1 shows the characterizations of the catalyst with B
content of 0.49 wt%, which was determined by inductively
coupled plasma-atomic emission spectrometry (ICP-AES). It
presents a spindle structure assembled by nanowires in length
of 1 μm and width of 10 nm, as revealed by scanning electron
microscopy (SEM) and transmission electron microscopy (TEM)
(Fig. 1a-d). High-resolution TEM (HRTEM) image reveals an
interplanar distance of 0.198 nm (Fig. 1e), corresponding to (11-
1) plane of face centered Tenorite. Energy-dispersive X-ray
spectroscopy (EDS) images (Fig. 1f) show the uniform
distributions of copper and oxygen elements on nanowire, as
well as a small amount of B. X-ray diffraction (XRD) pattern (Fig.
1g) presents two sharp diffractions at 35.4o, 38.7o and other
diffractions at 48.7o, 58.2o, 61.5o, 65.8o and 66.2o,
corresponding to (002), (11-1), (20-2), (202), (11-3), (022) and
(31-1) planes of Tenorite, respectively (PDF #48-1548). For
comparison, the B-free CuO was prepared by the method for B-
doped CuO without boric acid addition, which shows a similar
morphology with B-doped CuO (Fig. S1 † ). The B-doped CuO
a. Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid,
Interface and Chemical Thermodynamics, CAS Research/Education Center for
Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, P.R.China.
b. Physical Science Laboratory, Huairou National Comprehensive Science Center,
Beijing 101400, P.R.China.
c. Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics,
Chinese Academy of Sciences, Beijing 100049, P.R.China.
†Electronic Supplementary Information (ESI) available: See DOI: 10.1039/
x0xx00000x
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nanobundles and B-free CuO nanobundles are defined as B-CuO those of CuO and commercial CuO (7.2 and 8 respectively, Table
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and CuO, respectively. Additionally, the commercial CuO was S1 † ).40 The results show that the introduction of B in CuO
DOI: 10.1039/D0GC00730G
used for comparison, which presents microparticles in 0.5-4 μm reduces the coordination number of copper.
(Fig. S2†).
Fig. 2 FT-IR spectrum (a) and O1s XPS spectrum of B-CuO (b), Cu K-edge
XANES spectra (c) and k3-weighted  (k) functions of the EXAFS spectra (d) of
B-CuO, CuO, commercial CuO, Cu2O and Cu foil.
Fig. 1 SEM images (a, b), TEM images (c, d), HRTEM image (e) and EDS image
(f) of B-CuO and XRD patterns of different catalysts (g). Scale bars: 800 nm in
a, 200 nm in b, 400 nm in c, 50 nm in d, 1 nm in e, 120 nm in f.
The electrocatalytic performance of B-CuO for CO2RR was
detected. For comparison, the electrocatalytic performances of
CuO and commercial CuO were also determined. Firstly, the CO2
electrolysis of the three catalysts were determined by linear
Fourier transform infrared spectroscopy (FT-IR) was
sweep voltammetry (LSV) acquired in CO2-saturated 0.1 M
employed to probe the interaction between B and CuO in B-CuO
KHCO3 solution with a scan rate of 10 mV s-1. As shown in Fig.
(Fig. 2a and Fig. S3†). There are two absorptions at 1372 cm-1
3a, B-CuO exhibits earlier onset potential in CO2 saturated
and 1500 cm-1 for B-CuO (Fig. 2a), which are absent in CuO. It
solutions than CuO and commercial CuO, implying its higher
can be attributed to the symmetrical stretching vibration and
CO2RR performance. The current density of B-CuO increases
asymmetric stretching vibration of B-O bond.28-31 For O1s X-ray
more sharply than the other two catalysts, indicating that it
photoelectron spectroscopy (XPS), there are four peaks at 529.8
performs the best electrocatalytic activity for CO2RR. B-CuO,
eV (red line), 530.5 eV (blue line), 531.5 eV (plum line) and 533.4
CuO and commercial CuO show current densities of 18.2, 15.2
eV (green line) corresponding to Cu-O-Cu, B-O,32-34 O-O and Cu-
and 14.1 mA cm-2 (normalized by the geometrical surface area)
OH in B-CuO, respectively (Fig. 2b).35-37 All these results prove
at potential of -1.1 V (vs. RHE), respectively. The activities of the
the formation of B-O bond in B-CuO. For Cu2p XPS spectrum,
three catalysts for CO2RR were evaluated in an H-type
there are two peaks corresponding to Cu2+ at 934.2 eV and
electrochemical cell separated by a Nafion 117 membrane. The
954.1 eV (Fig. S4†). To further detect the local structure of B-
gaseous and liquid products were analyzed by gas
CuO at atomic level, the synchrotron X-ray absorption fine
structure (XAFS) was determined. For comparison, the XAFS
1
chromatography (GC) and H nuclear magnetic resonance (1H
NMR) spectroscopy, respectively. There were only gaseous
spectra of four contrast samples of CuO, commercial CuO,
products (H2, CO, ethylene) and no liquid product was detected
commercial cuprous oxide (Cu2O) and Cu foil were determined.
in the potential range from -0.8 to -1.2 V (vs. RHE) (Fig. S5†). The
For Cu K-edge X-ray absorption near-edge structure (XANES, Fig.
FE of C2H4 reaches a maximum of 58.4% over B-CuO at -1.1 V
2c), the absorption edge positions of B-CuO and CuO are located
(vs. RHE), which is much higher than the maximum FE of
behind commercial Cu2O and Cu foil and near that of
ethylene produced by CuO (17.2%) and commercial CuO (6.1%)
commercial CuO, implying the valence of +2.24 The radial
(Fig. 3b). The CO produced by B-CuO is very small (FECO = 3.6%),
distribution function (RDF) obtained from the fourier transform
while the FECO of CuO and commercial CuO are 24.8% and
(FT) k3-weighted normalized extended X-ray absorption fine
17.3%, respectively (Fig. S6†). The selectivity to ethylene of the
structure (EXAFS) data (Fig. 2d) of commercial CuO shows a
B-CuO synthesized in this work is higher than those of the
peak at 1.96 Å, corresponding to Cu-O coordination.38-39 As for
reported B-doped Cu catalysts, which produced multiple C2
B-CuO, the intensity of peak for Cu-O bond increases, indicating
hydrocarbons such as ethanol.24,26 The possible reason will be
the increased oxygen defect. Meanwhile, the peak at ~2.89 Å
corresponding to Cu-Cu bond for B-CuO greatly reduced
discussed in following.
compared with commercial CuO. The coordination number of
copper in B-CuO was determined to be 6.8, which is lower than
2 | J. Name., 2012, 00, 1-3
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rapidly to form Cu(OH)2 nanowires. SubsequentlVyi,ewCAurt(icOleHO)n2linies
DOI: 10.1039/D0GC00730G
dehydrated to form CuO nanowires, which assemble to
nanobundles. During this process, boric acid is hydrolysed to
form [B(OH)4]-, which could interact with copper ions and form
Cu-O-B bond after dehydration and thus resulting in oxygen
vacancies.38-39,44,45 The performances of these samples
synthesized at different reaction time for CO2RR were also
detected. It reveals that the B-CuO nanobundles with oxygen
vacancies is the most selective for ethylene production (Fig. 4f).
Fig. 3 LSV curves (a), FE of ethylene (b), and EIS spectra (c) of different
catalysts, FE of ethylene of B-CuO with time at potential of -1.1 V vs. RHE (d).
Fig. 4 TEM images of the products synthesized at reaction time of 15 min (a),
30 min (b), 60 min (c) and 90 min (d), corresponding XRD patterns (e) and
maximum FE of C2H4. Scale bar: 400 nm.
Electrochemical impedance spectroscopy (EIS) measurement
was performed to gain insight of reaction kinetics of CO2RR on
B-CuO, CuO and commercial CuO. A single fluid cell with 0.1 M
KHCO3 electrolyte was used and the frequencies are in the
range of 10-1-105 HZ. The Nyquist plots confirm that B-CuO
The above results show that the B-doped CuO nanobundles
possesses the smallest interfacial charge-transfer resistance have much improved activity and selectivity than the B-free CuO
and the fastest electron transfer among the three catalysts (Fig.
3c), suggesting the most favorable faradaic process.41 The
stability of the B-CuO for electrocatalysis was investigated.
There is no decline for the FE of ethylene of B-CuO after 5 h of
continuous electrolysis (Fig. 3d), indicating its stability can be
well kept during catalysis.
nanobundles. The effect of B content on catalytic effect was
investigated. A series of CuO with different B contents were
synthesized by varying the dosage of boric acid, while the other
experimental conditions were the same with those for the
above B-CuO. The B contents in those samples were determined
by ICP-AES to be 0.04 wt%, 0.22 wt% and 0.54 wt%. The samples
present similar morphologies (Fig. S8-S10 †) with that of the
above B-CuO with B content of 0.49 wt%. All these samples can
catalyze CO2RR to produce ethylene. The maximum FE of
ethylene on the B-CuO with B content of 0.04 wt%, 0.22 wt%,
0.49 wt% and 0.54 wt% are 22.9%, 52.8%, 58.4% and 43.3%,
respectively (Fig. S11†), which are 3.7, 8.7, 9.6 and 7.1 times
higher than that of commercial CuO. Obviously, the FE of
ethylene of B-CuO increases with increasing B content until 0.49
On the basis of the above results, the possible reason for the
high activity of the as-synthesized B-CuO is proposed. First, B-
CuO has a nanowire structure, which favors the exposure of
accessible active sites and facilitates the diffusion of CO2 and
electrocatalytic products42. Second, EXAFS results reveal the
existence of abundant oxygen vacancies in B-CuO, which
increase the binding affinities to the key intermediates that are
favourable for electrocatalytic CO2RR to ethylene43. Also, the
existence of abundant oxygen vacancies would promote the
adsorption of CO243. Our experiment shows that B-CuO has a wt% and the increase of B content to 0.54 wt% cannot further
much higher CO2 adsorption capacity than CuO and commercial
CuO (Fig. S7†). Owing to these unique features, the as-
synthesized B-CuO shows a high activity for CO2RR to ethylene.
The formation mechanism for the B-CuO electrocatalyst is
very interesting. Its evolution process with reaction time was
studied. At the early stage of 15 min, the sample presents as
nanowires, with thickness of 10 nm and length of 1-3 μm (Fig.
4a). With the increasing reaction time, the nanowires begin to
assemble to form nanobundles (Fig. 4b-d). XRD patterns (Fig.
4e) show that the sample formed at 15 min is copper hydroxide
(Cu(OH)2). At 30 min, the diffractions corresponding to CuO
occur with the co-existence of a small amount of Cu(OH)2, which
completely converts to CuO at reaction time of 90 min. It is
proposed that the copper ions in the solution interact with PEG
to form a linear chain of ions, which can react with hydroxide
improve the FE of ethylene. As evidenced by EXAFS results
(Table S1), the B-CuO catalyst with B content of 0.49 wt% has
the maximum oxygen vacancies. Thus, it exhibits the highest FE
of ethylene for electrocatalytic CO2RR.
Conclusions
In conclusion, we develop a novel electrocatalyst of B-doped
CuO nanobundles for CO2RR. The catalyst has a nanowire stack
structure and abundant oxygen vacancies, which favor the
exposure of accessible active sites, CO2 adsorption and diffusion
of CO2 and electrocatalytic product. Owing to these unique
features, the B-doped CuO nanobundles exhibit a highly
selective performance for CO2 conversion into ethylene.
This journal is © The Royal Society of Chemistry 20xx
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19 W. Zhu, L. Zhang, P. Yang, C. Hu, H. Dong, Z. J. ZhVaioew, RAr.tiMcleuOanlninde
Ethylene is the only multi-carbon product and its FE can reach
58.4% at -1.1 V (vs. RHE) with a current density of 18.2 mA cm-
2. This study provides a new strategy for producing single multi-
carbon product from electrocatalytic CO2RR by combining
nano-engineering and hetero-atom doping.
J. Gong, ACS. Energy. Lett., 2018, 3, 2144.
DOI: 10.1039/D0GC00730G
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Acknowledgements
The authors thank the financial supports from Ministry of
Science and Technology of China (2017YFA0403003), National
Natural Science Foundation of China (21525316, 21673254),
Chinese Academy of Sciences (QYZDY-SSW-SLH013) and Beijing
Municipal
Science
&
Technology
Commission
(Z191100007219009).
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