Electrochemical Production of Formic Acid from CO2 with Cetyltrimethylammonium Bromide-Assisted Copper-Based Catalysts

Article abstract of DOI:10.1002/cssc.202100275

The electrochemical reduction of CO₂ (ERC) to valuable chemicals has attracted extensive attention. However, the relatively low selectivity and efficiency of the reaction remain challenges. In this study, Cu electrodes derived from Cu₂O with predominant (111) facets are synthesized by cetyltrimethylammonium bromide-assisted preparation. The optimized electrode shows a high faradaic efficiency of 90 % for HCOOH obtained by ERC at ?2.0 V (vs.SCE), which surpasses most reported Cu electrodes. Based on a comprehensive analysis of the relationship between the catalytic activity and the thickness of the Cu₂O layer, the catalytic activity of the unit active site on the Cu₂O-derived Cu electrodes is found to be higher than that on the blank Cu electrode. DFT calculations indicate that OCHO* would be produced preferentially over *COOH in the presence of cetyltrimethylammonium bromide (CTAB). This deduction is verified by testing of the effects of CTAB and KBr addition on HCOO^? selectivity.

Full text of DOI:10.1002/cssc.202100275

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Electrochemical Production of Formic Acid from CO with
2
Cetyltrimethylammonium Bromide-Assisted Copper-Based
Catalysts
+
[a]
+ [a]
[a, b]
[a]
[a]
Yanling Qiu , Wenbin Xu , Pengfei Yao,
Qiong Zheng, Huamin Zhang, and
[a]
Xianfeng Li*
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The electrochemical reduction of CO2 (ERC) to valuable
chemicals has attracted extensive attention. However, the
relatively low selectivity and efficiency of the reaction remain
the relationship between the catalytic activity and the thickness
of the Cu O layer, the catalytic activity of the unit active site on
2
the Cu O-derived Cu electrodes is found to be higher than that
2
challenges. In this study, Cu electrodes derived from Cu O with
on the blank Cu electrode. DFT calculations indicate that
OCHO* would be produced preferentially over *COOH in the
presence of cetyltrimethylammonium bromide (CTAB). This
deduction is verified by testing of the effects of CTAB and KBr
2
predominant (111) facets are synthesized by cetyltrimeth-
ylammonium bromide-assisted preparation. The optimized
electrode shows a high faradaic efficiency of 90% for HCOOH
obtained by ERC at À 2.0 V (vs.SCE), which surpasses most
reported Cu electrodes. Based on a comprehensive analysis of
À
addition on HCOO selectivity.
Introduction
tuned by the Cu surface modification or its crystal structure
[3]
regulation. More recently, sulfur-doped copper catalyst has
been reported to produce HCOOH as an almost exclusive ERC
In recent decades, a continuous increase of CO emissions into
2
[4]
the atmosphere has become a major contributor to the
greenhouse effect. The conversion of CO2 into valuable
products from renewable energy is a potential way to mitigate
the negative effect. Among various CO conversion techniques,
the electrochemical reduction of CO (ERC) is regarded as one
of the most attractive strategies. However, the ERC suffers from
serious kinetic barriers, intricate multistep reactions and com-
peting hydrogen evolution reaction (HER), leading to high
overpotential, low conversion rate and poor product selectivity
for the reaction.
Among different products generated from ERC reaction,
HCOOH is quite appealing due to its excellent characteristics of
nontoxicity, safety and transportability, which is also promis-
ing for hydrogen storage and fuel cells. To date, Pb, Pd, Sn, In,
product. Copper oxides have also been reported as electro-
catalysts for conversion of CO2 into HCOOH with moderate
[5]
selectivity (ca. 59%) and activity. Since ERC reaction occurs on
the interface of the electrode and the electrolyte, some specific
chemicals presenting in the electrolyte might alter the binding
energy of the key intermediates through adsorbing on the
electrode surface, leading to the change of the ERC activity and
2
2
[6]
product selectivity. For example, Cu electrodes modified with
amino acid have exhibited noticeably improved hydrocarbon
[7]
productivity and selectivity. In addition, adsorbed halide
anions on the Cu surface have been confirmed to be able to
facilitate the electron transfer at the interface by quickly
[
1]
[8]
stabilizing the adsorbed ERC intermediates.
In this study, Cu O layer was synthesized on copper
2
Co, Bi and SnO electrodes have been successfully employed to
substrate by a hydrothermal method and cetyltrimeth-
2
convert CO to HCOOH with high faradaic efficiency (FE) of over
ylammonium bromide (CTAB) was used to modulate the
morphology and the effective active sites of the copper
2
[2]
9
0%. Among the reported metals, it is worth to mention that
À
copper has been widely explored as an electrocatalyst for ERC
due to its appropriate binding strength to ERC intermediates
and diversified products. The desired product yielding can be
electrode. CTAB is selected since Br has been regarded as an
[
9]
efficient capping agent to manipulate the facet exposure and
+
the large cation (CTA ) has the ability to absorb on the
[10]
substrate surface to tune the crystal growth.
Enhanced
+
+
HCOOH formation is obtained with the faradaic efficiency as
high as 90% under À 2.0 V (vs. SCE), which is the highest value
based on copper electrodes. As a comparison of morphology
regulator, KBr was also used in the hydrothermal reaction to
[
a] Y. Qiu, W. Xu, P. Yao, Q. Zheng, Prof. H. Zhang, Prof. X. Li
Division of Energy Storage
Dalian Institute of Chemical Physics
Chinese Academy of Sciences
57 Zhongshan Road, Dalian, 116023 (P.R. China)
E-mail: lixianfeng@dicp.ac.cn
b] P. Yao
University of Chinese Academy of Sciences
Beijing 100039 (P.R. China)
4
À
explore the specific adsorption effect of Br on HCOOH
[
[
formation. Cu-based electrodes prepared in the basic precursor
without any additions and with different amounts of KBr or
CTAB are designated as CuÀ Blank, CuÀ KBr(x), and CuÀ CTAB(x),
respectively, where x mmol is the additive concentration.
+
] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cssc.202100275
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Results and Discussion
obtained on CuÀ CTAB(2), and the result keeps consistent with
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XRD results. On the other hand, the decreased reduction
current on the CuÀ CTAB(2) (15 mA vs. 18 mA on the CuÀ Blank)
implies the active site number on CuÀ CTAB(2) is smaller than
that on CuÀ Blank. Furthermore, compared with the CuÀ Blank,
the dynamic current has been increased by 71% on CuÀ CTAB
(2) under À 1.9 V (vs. SCE; Figure S2b), suggesting the catalytic
Composition and morphology of the Cu O electrodes
x
X-ray diffractogram reveals that Cu O was successfully gener-
2
ated on the electrodes (Figure 1a). The distinctly weakened
diffraction intensity of the Cu(111) and Cu(200) observed for
the underlying Cu sheet (see the Supporting Information,
Figure S1) implies that the obtained Cu O layer on Cu-Blank is
much thicker than the other two electrodes. According to the
activity of CuÀ CTAB(2) has been greatly improved. H À TPR
2
testing results (Figure S2c and Table S1) demonstrated that the
2
peak area of H reduction for the three electrodes is: S
>
Cu-Blank
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electrode order of CuÀ Blank, CuÀ KBr(2) and CuÀ CTAB(2), the
SCu-KBr(2) >S
, implying that the number of the active sites
CuÀ CTAB(2)
intensity ratios I
/I
are 0.72, 0.81, and 1.57,
sequence for the three electrodes also follow the same rules.
Cu2Oð111Þ Cu2Oð220Þ
respectively, indicating that CTAB incorporation is conducive to
On the other hand, the H À TPR spectra are deconvoluted to
2
the priority generation of Cu O(111). This is due to the
identify the Cu O species on the three electrodes (Figure S3).
2
x
À
contribution of Br capping effect, which can absorb on
The deconvolution results demonstrate the almost unique
[
9b,10a]
Cu O(111) facet to retard its growth rate.
composition of Cu O on CuÀ CTAB(2), whereas trace amounts of
2
2
SEM images show that electrode morphology can change
with different amounts of added CTAB (Figure 1b–g). A Cu O
layer with dark red color can be observed on the CuÀ Blank, and
the surface is composed of polyhedral particles (size >1.5 μm)
with well-defined boundaries (Figure 1b). After adding CTAB
into the basic precursor, the surface color gradually approaches
CuBr and CuO might be present on CuÀ KBr(2) and CuÀ Blank,
respectively. SEM observations confirm the thickness estimation
of the three electrodes with electrochemical spans (Figure S4),
2
in which the Cu O thickness sequences on the substrate is:
x
δ
>δ
>δ . The Cu O layer on the surface of
CuÀ CTAB(2) x
CuÀ Blank
CuÀ KBr(2)
the Cu-Blank electrode is relatively loose, whereas that on the
the intrinsic color of Cu O (brick red), and the boundaries of the
surface of CuÀ CTAB (2) electrode is thin and dense. Considering
2
crystal facets are smoother with pits or bumps appearing on
specific facets (Figure 1c–f). With higher CTAB concentration,
particles with discontinuous rough surfaces (pits smaller than
the thinner Cu O layer and lower number of active sites on
x
CuÀ CTAB(2), it can be readily inferred that the catalytic activity
of the unit active site on the Cu electrodes with CTAB-assisted
preparation has been improved.
1
00 nm) can be observed (Figure 1e,f), which finally evolves to
be continuous with a size of small particles less than 100 nm.
The rough surface and small particles are expected to extend
the surface area and improve the reaction rate.
The distribution of the catalytic products on the Cu O-
x
derived electrodes for ERC reaction has been distinctly different
from that on the underlying Cu sheet (Figure 2 and Figure S5).
On the sample of CuÀ Blank (Figure 2a), diversified products
distribution can be observed with a maximum FE
of 40%
HCOOH
11]
[
Catalytic activity and product selectivity
at À 1.7 V, which is close to the reported results. Conversely,
the FE
is up to 90% under À 2.0 V for CuÀ CTAB(2)
HCOOH
The thickness of the Cu O layer can be estimated based on the
(Figure 2b), which is more than 2 times of that on the CuÀ Blank
and has surpassed most of the reported Cu-based electrodes
(Table S2). The high selectivity for HCOOH can be mainly
x
time span required to reduce Cu O under a specific reduction
x
[2j]
potential. For CuÀ CTAB(2), about 1600 s is required before a
stable current is obtained (Figure S2a), whereas the time for Cu-
attributed to the developed Cu O-derived layer on the Cu
2
Blank is more than 3000 s, indicating a much thinner Cu O layer
electrode, as the addition of CTAB can only contribute less than
x
Figure 1. Crystal structure (a) and the particle morphology as well as the surface appearance (b–g) of the prepared materials: b) CuÀ Blank; c) CuÀ CTAB(0.5); d)
CuÀ CTAB(1); e) CuÀ CTAB(2); f) CuÀ CTAB(4); g) CuÀ KBr(2).
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Figure 2. Faradaic efficiency of ERC products on CuÀ Blank (a), CuÀ CTAB(2) (b), TGP-H-060 (c), and CuÀ KBr(2) (d) under different applied potentials. For (c),
.5 mM CTAB was added into 0.2 M NaHCO solution.
0
3
1
0% FE for the HCOOH production (Figure 2c). Furthermore,
great potential for extending the application of Cu-based
catalysts.
the Cu surface is negatively charged during the ERC process,
which is favor to the competitive adsorption of the large cation
Under all applied potentials, the liquid resistance of the
CuÀ CTAB (2) electrode is lower than that of the CuÀ Blank
electrode (Figure S6 and Table S3), which may be related to the
residual CTAB on the surface. When the ERC reaction is
performed at low overpotential, the charge transfer resistance
+
(CTA ) on Cu active sites, as a result, active sites for H*
adsorption is decreased and the H* coverage is reduced. The
inhibited H2 generation on CuÀ CTAB(2) confirms the above
assumptions.
In addition, hydrocarbons were not detected on CuÀ CTAB
(R ) of the CuÀ CTAB(2) electrode is far lower than that of the
ct
(
2), indicating a relatively simple product composition. Based on
CuÀ Blank electrode, indicating a rapid kinetic reaction rate of
the comparison, it can be speculated that the pure Cu O phase
ERC for the CuÀ CTAB(2). At medium overpotential, the R is
2
ct
prepared by the CTAB-assisted hydrothermal reaction is
beneficial to selectivity towards HCOOH as the main product of
the ERC reaction.
essentially similar for all the electrodes. While mass transfer
polarization of CuÀ CTAB(2) electrode can be observed at higher
overpotential, which may be attributed to the violent ERC
The thickness of the prepared Cu O layer gradually
reaction and insufficient supply of CO concentration.
x
2
decreases with increasing CTAB concentration, which can be
expressed by the time span required to reduce the Cu O layer,
x
which is more than 3000 s for the CuÀ Blank and less than 600 s
for CuÀ CTAB(4) (Figure 3a). The activity of the CuÀ CTAB electro-
des demonstrates a non-monotonic variation trend (Figure 3b).
The optimum content of CTAB in the hydrothermal precursor is
Study on the improvement mechanism of catalytic activity
and selectivity toward HCOOH on Cu O electrode
x
To unveil the possible reasons behind the significant selectivity
to HCOOH on CuÀ CTAB electrodes, the composition in the
electrolyte after ERC reaction is analyzed by ion chromatog-
raphy and the surface functional groups are detected through
2
mM. Under À 1.75 V (vs. SCE), the produced HCOOH concen-
tration on the CuÀ CTAB(2) is 300% more than that on CuÀ Blank
with a high FE of 80% (Figure 3c,d). This selectivity
HCOOH
À
variation trend implies that CTAB addition is capable of tuning
the product distribution of Cu-based electrodes, which offers
FTIR. Br is detected in the electrolyte after ERC reaction
(Table 1), indicating a trace amount of residual CTAB on the
CuÀ CTAB electrode surface. FTIR spectroscopy further confirms
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Figure 3. Effect of CTAB addition on Cu
Cu O-derived electrodes.
x
x
O layer thickness (a), activity (b), HCOOH concentration (c) and product selectivity (d) of ERC reaction catalyzed by
Table 1. Br detection by ICS-1100.
reduction current are obtained on CuÀ KBr(2) than on Cu<C-
À
Precursor
C
Br [ppm] in 0.2 M NaHCO
3
(aq)
À
Blank (Figure S2a,b), indicating the specific adsorption of Br
0
.2 M NaHCO
3
(aq)
–
–
can significantly increase the catalytic activity of the Cu O-
x
CuÀ Blank
derived electrodes. HER is significantly inhibited with the lowest
FEH2 close to 20% on CuÀ KBr(2) (Figure 2d). However, the ERC
products are diversified and the best selectivity to HCOOH
occurs at À 1.75 V with a FE of 63%. Significant amounts of CO
can also be detected with FE from 20% to 40%, which is
CuÀ CTAB(0.5)
CuÀ CTAB(1)
CuÀ CTAB(2)
CuÀ CTAB(4)
0.2
0.2
12
18
partially attributed to the impure Cu O phase (small amount of
2
this assertion (Figure 4a). Energy-dispersive X-ray spectroscopy
CuBr; Figure 1a and Figure S3c). The results suggest that the
À
(
EDX) results also show CTAB retention on the CuÀ CTAB
specific adsorption of Br on the Cu O electrode surface can
x
surfaces (Figure S7 and Table S4). The mass spectrometry test of
alleviate the occurrence of the competitive HER, but the ERC
products, especially HCOOH, were not selectively formed as the
most preferable component. In addition, the well-defined
morphology of CuÀ KBr(2), which is totally different from those
H À TPR effluent (Figure S8) demonstrate that the CuÀ CTAB(2)
2
electrode produces CO when the temperature is lower than
2
2
20°C, which can be caused by the thermal decomposition of
CTAB remaining on the electrode surface (the thermal decom-
position temperature of CTAB is ca. 150°C). The MS results
further indicate that organic species exist on the surface of
CuÀ CTAB(2) electrode.
of the CuÀ CTAB(x) electrodes (Figure 1f), further confirms that
À
specific adsorption of Br did not change the Cu O growth
x
mechanism. Therefore, the steric hindrance effect of CTAB
might be the main reason for the growth of smooth particles
[10]
Based on the detected CTAB on the CuÀ CTAB electrodes,
one can firstly doubt whether the improved selectivity of
HCOOH on the CuÀ CTAB electrode is ascribed to the specific
and the crystal orientation of the Cu O layer. In addition, the
x
surface composition of the CuÀ CTAB(2) electrode after the ERC
reaction is different from the other two electrodes (Figure S9).
The ICu(111)/ICu(200) ratio is close to 1, whereas the value is much
higher for the other two electrodes. These results reflect that
the surface composition of the CuÀ CTAB (2) electrode is
predominantly Cu(110) after potential reduction.
À
adsorption of Br . The second question is whether the residual
CTAB has a coverage effect over the active sites on the surface
of the CuÀ CTAB electrodes. The selectivity improvement of
HCOOH on the referenced CuÀ KBr(2) electrode can partially
answer the first query. A thinner layer and a much higher
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x
Figure 4. a) FTIR spectroscopic detection of additives on Cu O-derived electrodes. b) Validation of the beneficial effect of CTAB on ERC product selectivity. The
working electrode is CuÀ Blank. c) Tafel slope comparison of Cu O electrode-catalyzed ERC reactions. d) Electrochemical measurement of surface roughness for
x
CuÀ CTAB(2) and CuÀ Blank.
To answer the second question, i.e., whether the CTAB
coverage on the surface of CuÀ CTAB electrodes can affect the
catalytic properties of the active sites, 0.5 mM CTAB was directly
CuÀ CTAB(2) catalyzing the ERC reaction, which is 17% lower
than that on the CuÀ Blank. This means the electrochemical
resistance of the ERC reaction has been decreased, which is
consistent with the catalytic activity improvement of the unit
active site.
Another probability leading to the smaller electrochemical
resistance might be a rougher surface on the prepared
CuÀ CTAB layer than that of the CuÀ Blank, thus extending the
electrochemical reaction areas. However, the lower surface
roughness of the CuÀ CTAB (2) than that of the CuÀ blank
(Figure 4d and Table S5) and the lower surface active sites
(Figure S2c) seem to be deviated from the expectation. The
smaller surface area participating in the ERC reaction on the
CuÀ CTAB and the higher catalytic activity (Figure S2b) confirms
once more the catalytic activity of the unit active site on
CuÀ CTAB(2) is higher than that on CuÀ Blank.
added to 0.2 M NaHCO aqueous solution, and the variations of
3
FE for each product are shown in Figure 4b. By comparison of
the curves in Figures 4b and 2b, it can be easily deduced that
the variation trends of the three products (CO, HCOOH, and H2)
are very similar. Especially, the tendency is almost the same
when the potential is lower than À 1.4 V (vs. SCE), suggesting
that CTAB addition into the electrolyte can significantly improve
HCOOH selectivity along with the distinct inhibition of the
[
12]
HER.
This result implies that the selectivity of HCOOH can be
significantly improved by simply covering CTAB on the surface
of CuÀ Blank electrode. On the other side, CTAB might cover on
those active sites favoring the competitive HER when compar-
ing the FEH2 values in Figure 2a and 4b. As CTAB addition in the
basic precursor is beneficial to the generation of Cu O(111)
2
(
Figure 1a), we can reasonably deduce the improved HCOOH
DFT calculations on the role of CTAB involvement at the
electrode-electrolyte interface
selectivity to the Cu O(111) preference. Considering the ERC
2
intermediates, it is speculated that Cu O(111)-derived Cu facet
2
might facilitate the first proton-electron transfer of the ERC
Interfacial properties at the boundary between the electrode
and electrolyte have important effects on the surface reactivity
in electrocatalysis. To fairly reveal the role of CTAB involvement
at the electrode-electrolyte interface, an in-depth thermody-
À
reaction, and reduce the energy barrier from CO to OCHO*
2
À
(
(
HCOO* ). Tafel slope testing confirmed the above assumption
Figure 4c). A Tafel slope of 152 mVdec is obtained with
À 1
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namic analysis on top of DFT calculation has been carried out.
electrodes showed high selectivity toward HCOOH. The elec-
trode synthesized with 2 mM CTAB addition exhibited the
highest FEHCOOH of 90%, which is approximately twice that on
the CuÀ Blank electrode and surpasses most reported Cu-based
electrodes. The CuÀ CTAB(2) electrode maintained approxi-
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By concentrating on Cu O-derived Cu(110) surface, we inves-
2
tigate the Gibbs free energy diagram along two established
reaction pathways proposed in related literatures of ERC on
Cu(110) surface with and without CTAB involvement (Figure 5
[13]
and Table S6). In addition, both more stabilized *COOH and
OCHO* intermediates rationalize the overall increment of the
unit active site in the presence of CTAB, despite the lower
surface roughness. On other hand, the Gibbs free energy of the
mately 80% of its initial faradaic efficiency(FE
) over 8 h,
HCOOH
showing excellent stability for the ERC reaction. In comparison
of the activity, the thickness of Cu O layer, and HCOOH
x
concentrations, the unit active site on the CuÀ CTAB electrode
elementary reaction CO *!OCHO* is lowered by 0.34 eV (from
derived from Cu O, which was produced by the CTAB-assisted
2
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À 0.42 eV to À 0.76 eV), while the value of CO *!*COOH was
hydrothermal method, was deduced to have higher catalytic
activity than the CuÀ Blank electrode. By combining experimen-
tal verification of the effects of CTAB and KBr addition on
HCOOH selectivity with DFT calculations, we deduce that
2
lowered by only 0.2 eV, indicating OCHO* would be yielded
preferentially over *COOH resulting in higher selectivity of
HCOOH. The more negative potential further promotes this
preferential more obviously (Figure S10). The decreased FECO in
Figure 2b is consistent with the DFT calculation results.
+
competitive CTA adsorption on the active sites, which inhibits
H2 formation, and the pure Cu O phase with predominant
2
Cu O(111) orientation reduce the energy barrier from CO to
2
2
OCHO*, which contributes to the high selectivity to HCOOH on
the CuÀ CTAB electrode. These findings provide mechanistic
insight into an earth-abundant ERC catalyst, and we expect that
the selectivity toward HCOOH can be further enhanced on
Stability of the CuÀ CTAB electrode during ERC reaction
It is well known that the Cu electrodes are less stable in the
[14]
prolonged ERC reaction.
HCOOH selectivity catalyzed by the CuÀ CTAB electrode, a nearly
h durability test was conducted (Figure S11). The reaction
current reaches its maximum within 1 h and then gradually
To examine the stability of the
CuÀ CTAB electrodes by increasing the CO concentration in the
2
electrolyte.
8
decreases. At the end of the reaction, the current decrement is Experimental Section
ca.11% while the FEHCOOH exhibits ca. 16% reduction (from 87%
to 73%), implying an excellent stability for ERC reaction and the
superior catalytic activity maintenance. The SEM observations
Preparation of the Cu O-derived electrodes
x
1 mM of CuAc
(Aladdin Industrial Corp., 99.5%) aqueous solution
2
(Figure S11c,d) reveal that slight morphology changes have
was used as the basic precursor of the hydrothermal reaction. Cu O
x
been occurred which has not detrimental influence on the high
selectivity of HCOOH.
electrodes were prepared with or without the addition of KBr or
CTAB under the hydrothermal environment. For the additive-
assisted hydrothermal reactions, KBr or CTAB powder (Aladdin
Industrial Corp., 99.0%) was firstly introduced into the basic
precursor solution under vigorous stirring to obtain the Cu
precursor of the hydrothermal reaction. The KBr concentration is
controlled as 2 mM, and that of CTAB is 0.5 mM, 1 mM, 2 mM and
4 mM, respectively. Then, 60 mL Cu precursor solution and Cu sheet
(99.7 wt%) were transferred into a 100 mL Teflon-lined stainless-
steel autoclave. The autoclave was treated under 180°C for 12 h
and then cooled down naturally. Thirdly, the Cu sheet covered with
Conclusions
In summary, a facile method to prepare oxide-derived (OD) Cu
electrodes was developed by using a CTAB-assisted strategy
during the hydrothermal reaction process, which can afford
pure Cu O phase with (111) preference. The prepared Cu
2
Cu
O layer was taken out from the autoclave, rinsed thoroughly
x
Figure 5. Free energy diagrams of ERC on the surface of Clean_Cu (a) and CTAB_Cu (b).
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© 2021 Wiley-VCH GmbH

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ChemSusChem
with ultrapure water and dried under Ar atmosphere. Finally, the
was carried out using (4×4) grid. The cutoff energy was set to
500 eV and 5000 eV for the orbitals, and the charge density,
respectively, and a Fermi level smearing of 0.1 eV was used. Spin
polarization and dipole correction have been involved for all the
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electrode was electrochemically reduced in CO -bubbled 0.2 M
2
NaHCO at À 0.9 V (vs. SCE) for 2 h to obtain the Cu O-derived Cu
3
x
electrode. Cu-based electrodes prepared in the basic precursor
without any additions, and with different amount of KBr or CTAB
are designated as CuÀ Blank, CuÀ KBr(x), CuÀ CTAB(x), respectively,
where x represents the additive concentration.
calculations. To ensure computationally tractable, we use
a
simplified model of CTAB, C6H16N, with a shorter carbon chain
considered since the carbon chain has a negligible influence on the
ERC reaction. Potential asymmetric adsorption sites of each
intermediate were calculated and only the most favorable was
used.
Surface roughness determination
Electrochemical cyclic voltammetry was employed to test the
The Gibbs free energies of typical proton-coupled electron transfer
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[18]
double layer charging capacitance of Cu O electrode to characterize
steps are estimated using the CHE approach, where the free
x
the electrochemical reaction surface area participating in the ERC
energy of the solvated proton and electron pair can be by
definition equal to one-half the chemical potential of molecular H2
at 101325 Pa, further as a function of applied potential at all
temperature and PH values. Within the CHE approach, the free
energy of intermediate is calculated by G=E+ZPE–TΔS, in which E
is the DFT-optimized total energy, ZPE is the zero-point vibrational
energy, T is the absolute temperature, and ΔS is the entropy
change. For the adsorbed intermediates, the ZPE and entropy were
calculated using the harmonic approximation with the ASE module
reaction. The test solution is N saturated 0.1 M HClO and the
2
4
potential window is À 0.24 V to À 0.34 V(vs. SCE).
Characterization
Powder X-ray diffraction (XRD) spectra were measured on a Rigaku
Ultima IV diffractometer with CuKα radiation (l=0.15418 nm)
operating at 40 kV and 40 mA over the two theta range of 5–100°
[19]
À 1
vibrations. The value of entropy of gas phase was taken from
[20]
with a scan speed of 10° min . Scanning electron microscopy
NIST,
while the ZPE of gas phase were also obtained from
(SEM) was recorded on JEOL JSM-6360 operating at an acceleration
voltage of 20 kV. Br element quantities were detected by EDX. The
IR spectra were recorded with a Fourier Transform Infra spectrom-
vibrational frequencies from ASE.
To simulate the chemical reaction in electrocatalysis, the effect of
solvent stabilization should be taken into account for adsorbed
species, as well as gas-phase error corrections for the standard free
energy deduced from the limitation of exchange-correlation func-
tional. It is noticed that the gas-phase error corrections are
functional dependence (BEEF-vdW functional in our case), where
the values are taken from reference [21], in which they implement
systematic investigation gas-phase error on few various exchange-
correlation functional used prevalently (PBE, PW91, RPBE, and BEEF-
vdW). Regarding the independent solvation correction of adsorbed
species involved in ERC, we use the result of previous work
obtained from the sensitivity analysis, which is 0 eV, À 0.1 eV,
À 1
eter (Nicolet iS50, Thermo Fisher, USA) in the 2000–400 cm
wavenumber region with ATR technique. H À TPR was carried out
2
on Micromeritics Chemi 2920 II. 10%H /90%Ar stream with flow
2
À 1
rate of 30 mLmin was used to determine the Cu O species and
x
the number of the surface active sites, the temperature rise rate of
À 1
10
(
°
Cmin . The effluent of the H À TPR was detected by the MS
2
MKS104-J0316005) to detect the reduction product.
Electrochemical testing
The Electrochemical measurements were performed on a potentio-
stat (EG&G 2273, Princeton Applied Research) in a self-made H cell
[22]
À 0.25 eV for OCHO*, *CO and *COOH.
(
(
separated by Nafion115). Pt sheet and saturated calomel electrode
SCE) were used as the counter electrode and the reference
electrode, respectively. The prepared Cu-based electrode was used
as the working electrode. CO -saturated 0.2 M NaHCO aqueous
Acknowledgements
2
3
solution was used as the electrolyte. The generated gas products
were analyzed online by a gas chromatograph (GC, Shimadzu GC-
The authors thank the financial grants from the National Natural
Science Foundation of China (No. 21577141).
2014).The hydrogen and CO were quantified by a thermal
conductivity detector (TCD), and hydrocarbons were detected and
quantified by flame ionization detector (FID), respectively. The
À
liquid products including Br were quantified by ion chromatog-
Conflict of Interest
raphy (ICS-1100, Dionex Corporation).
The authors declare no conflict of interest.
Computational details
All DFT calculations were performed with the Quantum ESPRESSO
Keywords: carbon dioxide
·
copper
·
electrocatalysis
·
[15]
code employing the state-of-art Bayesian error estimation func-
hydrothermal synthesis · reduction
[16]
tional with van der Waals correlation (BEEF-vdW), which has been
successfully used for surface chemistry studies. We used soft
pseudopotentials and a vacuum region of 30 Å perpendicular to
the surface. Pseudopotentials for C, O, H, Cu, and N were generated
by using the “atomic” code reported by Dal Corso (v.5.0.2 svn rev.
[
1] A. S. Agarwal, Y. Zhai, D. Hill, N. Sridhar, ChemSusChem 2011, 4, 1301–
310.
1
[17]
[2] a) X. Min, M. W. Kanan, J. Am. Chem. Soc. 2015, 137, 4701–4708; b) S.
9
415). In terms of our experimental observation, Cu O-derived
2
Gao, Y. Lin, X. Jiao, Y. Sun, Q. Luo, W. Zhang, D. Li, J. Yang, Y. Xie, Nature
the typical (110) facet of Cu with and without CTAB involvement
was modeled using periodic slabs, where (3×3) unit cell and 4
layers were used. The bottom-most layer was fixed in their bulk
truncated positions, while the layers above were relaxed until
reaching a maximum force threshold of 0.03 eV/Å. k-point sampling
2
2
016, 529, 68–71; c) S. Zhang, P. Kang, T. J. Meyer, J. Am. Chem. Soc.
014, 136, 1734–1737; d) F. Li, L. Chen, M. Xue, T. Williams, Y. Zhang,
D. R. MacFarlane, J. Zhang, Nano Energy 2017, 31, 270–277; e) K. Gupta,
M. Bersani, J. A. Darr, J. Mater. Chem. A 2016, 4, 13786–13794; f) S.
Zhang, P. Kang, S. Ubnoske, M. K. Brennaman, N. Song, R. L. House, J. T.
Glass, T. J. Meyer, J. Am. Chem. Soc. 2014, 136, 7845–7848; g) P. Su, W.
ChemSusChem 2021, 14, 1962–1969
www.chemsuschem.org
1968
© 2021 Wiley-VCH GmbH

Full Papers
doi.org/10.1002/cssc.202100275
ChemSusChem
Xu, Y. Qiu, T. Zhang, X. Li, H. Zhang, ChemSusChem 2018, 11, 848–853;
[12] S. Banerjee, X. Han, V. S. Thoi, ACS Catal. 2019, 9, 5631–5637.
[13] a) Q. Li, J. Fu, W. Zhu, Z. Chen, B. Shen, L. Wu, Z. Xi, T. Wang, G. Lu, J.
Zhu, S. Sun, J. Am. Chem. Soc. 2017, 139, 4290–4293; b) X. Zheng, Y. Ji, J.
Tang, J. Wang, B. Liu, H.-G. Steinruck, K. Lim, Y. Li, M. F. Toney, K. Chan,
Y. Cui, Nat. Catal. 2019, 2, 55–61.
[14] Y. Hori, H. Konishi, T. Futamura, A. Murata, O. Koga, H. Sakurai, K.
Oguma, Electrochim. Acta 2005, 50, 5354–5369.
[15] P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D.
Ceresoli, G. L. Chiarotti, M. Cococcioni, I. Dabo, A. D. Corso, S. Gironcoli,
S. Fabri, G. Fratesi, R. Gebauer, U. Gerstmann, C. Gougoussis, A. Kokalj,
M. Lazzeri, L. Martin-Samos, N. Marzari, F. Mauri, R. Mazzarello, S. Paolini,
A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A. P.
Seitsonen, A. Smogunov, P. Umari, R. M. Wentzcovitch, J. Phys. Condens.
Matter. 2009, 21, 395502.
[16] J. Wellendorff, K. T. Lundgaard, A. Møgelhøj, V. Petzold, D. D. Landis,
J. K. Nørskov, T. Bligaard, K. W. Jacobsen, Phys. Rev. B 2012, 85, 235149–
235154.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
h) L. Lu, X. Sun, J. Ma, Q. Zhu, C. Wu, D. Yang, B. Han, Sci. China Chem.
2018, 61, 228–235; i) W. Lv, R. Zhang, P. Gao, C. Gong, L. Lei, J. Solid
State Electrochem. 2013, 17, 2789–2794; j) H. Zhang, Y. Ma, F. Quan, J.
Huang, F. Jia, L. Zhang, Electrochem. Commun. 2014, 46, 63–66; k) S.
Gao, X. Jiao, Z. Sun, W. Zhang, Y. Sun, C. Wang, Q. Hu, X. Zu, F. Yang, S.
Yang, L. Liang, J. Wu, Y. Xie, Angew. Chem. Int. Ed. 2016, 55, 698–702;
Angew. Chem. 2016, 128, 708–712.
[
3] a) Y. Hori, I. Takahashi, O. Koga, N. Hoshi, J. Phys. Chem. B 2002, 106, 15–
1
7; b) Y. Hori, I. Takahashi, O. Koga, N. Hoshi, J. Mol. Catal. A 2003, 199,
39–47; c) C.-T. Dinh, T. Burdyny, M. G. Kibria, A. Seifitokaldani, C. M.
Gabardo, F. Arquer, A. Kiani, O. P. Edwards, P. D. Luna, O. S. Bushuyev, C.
Zou, R. Quintero-Bermudez, Y. Pang, D. Sinton, E. H. Sargent, Science
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
2
018, 360, 783–787; d) H. Mistry, A. S. Varela, C. S. Bonifacio, I.
Zegkinoglou, I. Sinev, Y.-W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P.
Strasser, B. R. Cuenya, Nat. Commun. 2016, 7, 12123–12130; e) D. Gao, I.
Zegkinoglou, N. J. Divins, F. Scholten, I. Sinev, P. Grosse, B. R. Cuenya,
ACS Nano 2017, 11, 4825–4831.
[17] A. D. Corso, Comput. Mater. Sci. 2014, 95, 338–350.
[4] a) K. R. Phillips, Y. Katayama, J. Hwang, Y. Shao-Horn, J. Phys. Chem. Lett.
018, 4407–4412; b) Y. Huang, Y. Deng, A. D. Handoko, G. K. L. Goh, B. S.
[18] J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T.
Bligaard, H. Jonsson, J. Phys. Chem. B 2004, 108, 17886–17892.
[19] A. H. Larsen, J. J. Mortensen, J. Blomqvist, I. E. Castelli, R. Christensen, M.
Dułak, J. Friis, M. N. Groves, B. Hammer, C. Hargus, E. D. Hermes, P. C.
Jennings, P. B. Jensen, J. Kermode, J. R. Kitchin, E. L. Kolsbjerg, J. Kubal,
K. Kaasbjerg, S. Lysgaard, J. B. Maronsson, T. Maxson, T. Olsen, L.
Pastewka, A. Peterson, C. Rostgaard, J. Schiøtz, O. Schütt, M. Strange,
K. S. Thygesen, T. Vegge, L. Vilhelmsen, M. Walter, Z. Zeng, K. W.
Jacobsen, J. Phys. Condens. Matter. 2017, 29, 273002.
[20] NIST Chemistry WebBook, https://webbook.nist.gov/chemistry (accessed
Oct 16, 2020).
[21] L. P. Granda-Marulanda, A. Rendón-Calle, S. Builes, F. Illas, M. T. M.
Koper, F. Calle-Vallejo, ACS Catal. 2020, 10, 6900–6907.
2
Yeo, ChemSusChem 2018, 11, 320–326; c) T. Shinagawa, G. O. Larrazabal,
A. J. Martin, F. Krumeich, J. Perez-Ramirez, ACS Catal. 2018, 8, 837–844.
[5] a) S. Min, X. Yang, A.-Y. Lu, C.-C. Tseng, M. N. Hedhili, L.-J. Li, K.-W.
Huang, Nano Energy 2016, 27, 121–129; b) J. Chung, D. H. Won, J. Koh,
E.-H. Kim, S. I. Woo, Phys. Chem. Chem. Phys. 2016, 18, 6252–6258.
[
6] M. Cho, J. T. Song, S. Back, Y. Jung, J. Oh, ACS Catal. 2018, 8, 1178–1185.
[7] M. S. Xie, B. Y. Xia, Y. Li, Y. Yan, Y. Yang, Q. Sun, S. H. Chan, A. Fisher, X.
Wang, Energy Environ. Sci. 2016, 9, 1687–1695.
[
[
8] a) A. S. Varela, W. Ju, T. Reier, P. Strasser, ACS Catal. 2016, 6, 2136–2144;
b) K. Ogura, R. Oohara, Y. Kudo, J. Electrochem. Soc. 2005, 152, D213-
D219.
9] a) S. A. Akhade, I. T. McCrum, M. J. Janik, J. Electrochem. Soc. 2016, 163,
F477-F484; b) H. Zhang, M. Jin, J. Wang, W. Li, P. H. C. Camargo, M. J.
Kim, D. Yang, Z. Xie, Y. Xia, J. Am. Chem. Soc. 2011, 133, 6078–6089;
c) X.-Q. M. A.-X. Yin, Y.-W. Zhang, C.-H. Yan, J. Am. Chem. Soc. 2011, 133,
[22] A. A. Peterson, F. Abild-Pedersen, F. Studt, J. Rossmeisl, J. K. Nørskov,
Energy Environ. Sci. 2010, 3, 1311–1315.
3
816–3819.
[
[
10] a) S. Yang, F. Hong, L. Wang, S. Guo, X. Song, B. Ding, Z. Yang, J. Phys.
Chem. C 2010, 114, 203–207; b) C. Zhu, S. Guo, S. Dong, Adv. Mater.
2012, 24, 2326–2331.
11] a) D. Kim, S. Lee, J. D. Ocon, B. Jeong, J. K. Lee, J. Lee, Phys. Chem. Chem.
Phys. 2015, 17, 824–830; b) M. Ma, K. Djanashvili, W. A. Smith, Phys.
Chem. Chem. Phys. 2015, 17, 20861–20867.
Manuscript received: February 9, 2021
Revised manuscript received: March 4, 2021
Accepted manuscript online: March 9, 2021
Version of record online: March 18, 2021
ChemSusChem 2021, 14, 1962–1969
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© 2021 Wiley-VCH GmbH