A comparative study of the effects of different TiO2supports toward CO2electrochemical reduction on CuO/TiO2electrode

Article abstract of DOI:10.1039/d1ra02837e

CuO-based electrodes possess vast potential in the field of CO₂electrochemical reduction. Meantime, TiO₂supports show the advantages of being non-toxic, low-cost and having high chemical stability, which render it an ideal electrocatalytic support with CuO. However, different morphologies and structures of TiO₂supports can be obtained through various methods, leading to the discrepant electrocatalytic properties of CuO/TiO₂. In this paper, three supports, named dense TiO₂, TiO₂nanotube and TiO₂nanofiber, were applied to synthesize CuO/TiO₂electrodes by thermal decomposition, and the performances of the electrocatalysts were studied. Results show that the main product of the three electrocatalysts was ethanol, but the electrochemical efficiency and reaction characteristics are obviously different. The liquid product of CuO/Dense TiO₂is pure ethanol, however, the current efficiency is rather low owing to the higher resistance of the TiO₂film. CuO/TiO₂nanotube shows high conductivity and ethanol can be synthesized at low overpotential with high current efficiency, but the gas products cannot be restricted. CuO/TiO₂nanofiber has a larger specific surface area and more active sites, which is beneficial for CO₂reduction, and the hydrogen evolution reaction can be evidently restricted. The yield of ethanol reaches up to 6.4 μmol cm^?2at ?1.1 V (vs.SCE) after 5 h.

Full text of DOI:10.1039/d1ra02837e

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A comparative study of the effects of different TiO2
supports toward CO2 electrochemical reduction on
CuO/TiO2 electrode
Cite this: RSC Adv., 2021, 11, 21805
*
Shenghang Xu, Chenxi Jia
Yueheng Lu,
and Guoqu Zheng
Huazhen Cao,
*
CuO-based electrodes possess vast potential in the field of CO2 electrochemical reduction. Meantime, TiO2
supports show the advantages of being non-toxic, low-cost and having high chemical stability, which
render it an ideal electrocatalytic support with CuO. However, different morphologies and structures of
TiO2 supports can be obtained through various methods, leading to the discrepant electrocatalytic
properties of CuO/TiO2. In this paper, three supports, named dense TiO2, TiO2 nanotube and TiO2
nanofiber, were applied to synthesize CuO/TiO2 electrodes by thermal decomposition, and the
performances of the electrocatalysts were studied. Results show that the main product of the three
electrocatalysts was ethanol, but the electrochemical efficiency and reaction characteristics are
obviously different. The liquid product of CuO/Dense TiO2 is pure ethanol, however, the current
efficiency is rather low owing to the higher resistance of the TiO2 film. CuO/TiO2 nanotube shows high
conductivity and ethanol can be synthesized at low overpotential with high current efficiency, but the
gas products cannot be restricted. CuO/TiO2 nanofiber has a larger specific surface area and more active
sites, which is beneficial for CO2 reduction, and the hydrogen evolution reaction can be evidently
restricted. The yield of ethanol reaches up to 6.4 mmol cmꢀ2 at ꢀ1.1 V (vs. SCE) after 5 h.
Received 12th April 2021
Accepted 14th June 2021
DOI: 10.1039/d1ra02837e
rsc.li/rsc-advances
applied with a support.19 As an n-type semiconductor material,
TiO2 is one of the widely used photocatalyst and electrocatalyst
1. Introduction
supports due to its advantages of non-toxicity, low cost and high
chemical stability.20–22 Importantly, during the CO2 reduction
reaction, a TiO2 support can help to adsorb CO2 and thereby
facilitate the electrocatalytic reduction.23 Many researchers have
been focused on the preparation of CuO/TiO2 electrodes applied
in the eld of photocatalytic reduction of CO2 and hydrogen
production.24–27 The morphology and structure of TiO2 supports
are various through different preparation methods, which in
turn obviously inuences the electrocatalytic performance. The
combination of n-type TiO2 with the p-type CuO semi-
conductor28 can form a heterojunction, which has unidirec-
tional conductivity, and it can facilitate the intramolecular
electron transfer.29,30 However, the research about the
morphology and structure of the TiO2 support on the electro-
catalytic properties of CuO/TiO2 electrodes is rather scarce.
In this paper, three TiO2 supports with different morphol-
ogies were fabricated, and CuO particles were deposited on the
surface of TiO2 support by thermal decomposition method.
Through linear sweep voltammetry (LSV) and cyclic voltamme-
try (CV) method, the product selectivity, stability and catalytic
efficiency of these CuO/TiO2 electrodes were studied in detail,
and the mechanism of TiO2 support morphology on catalytic
behavior was revealed.
CO2 can be converted to chemicals with high energy density
through reduction reaction, such as methanol and ethanol, and
the reaction can be achieved at room temperature and atmo-
spheric pressure. The electrocatalytic reduction reaction is ex-
pected to become a feasible way to sustainably transform the
waste CO2 stream into value-added low-carbon fuels, which is of
great signicance for energy conversion and CO2 emission
reduction.1,2 However, signicant competition between CO2
reduction and hydrogen evolution in aqueous solution occurs,
leading to the low efficiency and product selectivity.3,4 There-
fore, the application of CO2 reduction is rather limited.
In recent decades, numerous trials have been made to
explore electrodes with distinguished performance for CO2
electrochemical reduction, including metals,5–8 metal oxides9–11
and metal complexes.12–15 Herein, CuO, as a relatively low-cost
and earth abundant metal oxide, possesses a unique capacity
to produce hydrocarbons through a multiple protons and
electrons transfer pathway, which renders it an electrocatalytic
reduction electrode material with large-scale application
potential.16–18 However, CuO electrodes have a high over-
potential and poor selectivity to the products, and are usually
College of Materials Science and Engineering, Zhejiang University of Technology,
Hangzhou 310014, China. E-mail: caohz@zjut.edu.cn; zhenggq@zjut.edu.cn
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substrates were sealed by PTFE raw tape and sealant, and only
one side of substrate is exposed.
2. Experimental
2.1 Reagents
Dense TiO2 was obtained by anodizing in an ionic liquid (1-
butyl-3-methylimidazolium hexauorophosphate). The nega-
tive electrode is platinum and the anodic oxidation reaction is
Pure Ti sheet (99.99%) was purchased from Baoji LiTai Co., Ltd.
Hydrouoric acid (HF, 40%), absolute ethanol (CH3CH2OH,
99.7%), methanol (CH3OH, 99.5%), sodium hydroxide (NaOH,
96%), hydrochloric acid (HCl, 37%) and anhydrous sodium
sulfate (Na2SO4, 99%) were purchased from Sinopharm Reagent
Co., Ltd. Chromium trioxide (CrO3, 99%), formic acid (HCOOH,
99%), copper nitrate (Cu(NO3)2$3H2O, 99%) and sodium bicar-
bonate (NaHCO3, 98%) were purchased from Macklin Co., Ltd. 1-
Butyl-3-methylimidazolium hexauorophosphate (C8H15N2PF6,
99%) was purchased from Lanzhou Greenchem ILs.
ꢁ
applied at a voltage of 8 V and 10 C for 20 min.
TiO2 nanotubes (TNTs) were obtained by anodic oxidation in
1 wt% hydrouoric acid aqueous solution. The negative elec-
trode is graphite and the ꢁanodic oxidation reaction is applied at
a voltage of 20 V and 25 C for 20 min.
TiO2 nanobers (TiO2NFs) were fabricated by putting Ti
substrate in 10 M NaOH aqueous solution at 130 ꢁC for 4 h
through hydro-thermal method. Then, the Ti substrate was
transferred to 0.1 M HCl aqueous solution for ion exchange for
12 h. Aerwards, the above electrodes were calcined in air at
2.2 Preparation of TiO2 supports with different morphology
ꢁ
450 C for 2 h.
The pure titanium substrates were grinded and polished, and
then cleaned with deionized water and ethanol sequentially.
The chemical polish process was conducted in an aqueous
2.3 Preparation of TiO2-supported CuO electrodes
solution of chromium trioxide hydrouoric acid (4% HF and The copper nitrate aqueous solution of 1.66 M was spread on
7.5% CrO3) at 50 ꢁC for 20 min. Aerwards, the titanium the surface of TiO2 carriers. Then, the samples were placed in
Fig. 1 SEM images of (a) dense TiO2, (b) CuO/Dense TiO2, (c) TNTs, (d) CuO/TNTs, (e) TiO2NFs and (f) CuO/TiO2NFs.
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a vacuum bottle and the excess solution on the surface was
removed through centrifugation. Subsequently, all samples
were calcined in air at 450 ꢁC for 20 min. The above operations
were repeated until the coating weight reaches 3 mg cmꢀ2
.
2.4 Characterization and electrochemical reduction tests
The composition and crystal structure of those electrodes were
conducted by an X-ray diffraction (XRD, Panalytical X'Pert PRO)
at 40 kV and 40 mA with a scan speed of 10ꢁ minꢀ1. The
morphology and microstructure of electrodes were character-
ized by a eld emission scanning electron microscopy (SEM,
Vega3) and a transmission electron microscopy (TEM, Nano
Nova 450) equipped with an energy dispersive X-ray (EDX)
spectrometer.
The electrochemical experiments were investigated by
ꢁ
a CHI660C electrochemical workstation at 25 C water bath. A
platinum foil (2 ꢂ 2 cm2) and a saturated calomel electrode
(SCE) were used as counter electrode and reference electrode,
respectively. Linear sweep voltammetry (LSV) was researched in
0.1 M Na2SO4 solution with a scanning rate of 50 mV sꢀ1 in the
potential range between 0 and ꢀ1.3 V (vs. SCE). The electrical
double-layer capacitance (Cdl) was measured in 0.5 M Na2SO4
solution to characterize electrochemical surface area (ECSA).
Prior to tests, CO2 or N2 was bubbled into aqueous solution for
20 minutes. And the electrodes were sealed with sealant to make
the exposed area 1 ꢂ 1 cm2.
Fig.
TiO2NFs.
3
XRD patterns of CuO/Dense TiO2, CuO/TNTs and CuO/
temperature of 25 ꢁC. Liquid phase products were quantitatively
characterized by headspace gas chromatograph (GC-G5, Pur-
kinje, China) equipped with DB-WAX column and ame ioni-
zation detector (FID).
3. Results and discussion
2.5 Products analysis
3.1 Microstructure of CuO/TiO2 electrodes
The reduction experiment was carried out in an 80 mL 0.1 M Fig. 1 shows the SEM morphologies of three CuO/TiO2 elec-
NaHCO3 solution for 5 h. The process is conducted at a constant trodes. The surface of dense TiO2 (Fig. 1a) support is gully-like.
Fig. 2 Cross-sectional SEM images of electrode: (a) dense TiO2, (b) CuO/TNTs, (c) TiO2NFs, (d) HRTEM image of CuO/TiO2NFs.
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network-like structure (Fig. 1e) and CuO particles are tightly
attached on the surface of TiO2 nanober with the size of about
40 nm (Fig. 1f).
Fig. 2a shows the SEM image of cross-section of dense TiO2
support. It can be seen that the connection between TiO2 and
matrix is well. However, obvious cracks can be seen inside the
TiO2 support, which are attributed to the occurrence of interior
stress during thermal decomposition process. The cross-section
of CuO/TiO2 nanotube electrode demonstrates the well
connection between nanotube and matrix, as shown in Fig. 2b.
Besides, the length of nanotube is about 300 nm and CuO
particles can be deposited inside the nanotube. The cross-
section of TiO2 network-like structure is presented in Fig. 2c,
and the thickness of TiO2 network is about 500 nm. In Fig. 2d,
the HRTEM image shows the lattice fringes of TiO2 and CuO.
The fringes with a spacing of 0.251 nm match the (ꢀ1 1 1)
crystal plane of CuO, and the fringes with a spacing of 0.229 nm
match the (1 1 2) crystal plane of anatase TiO2.
Fig. 4 LSV results of CuO/Dense TiO2, CuO/TNTs and CuO/TiO2NFs
in 0.1 M Na2SO4 saturated with N2 or CO2.
Phase composition of different CuO/TiO2 electrodes were
analyzed by XRD, as shown in Fig. 3. Due to the thin lm of
CuO/TiO2, the peaks of Ti matrix can be obviously observed
(JCPDS card 01-1198) at 2q ¼ 40.17, 53.00 and 70.66ꢁ. The peaks
of anatase TiO2 (JCPDS card 75-1537) can be detected at 2q ¼
25.7ꢁ in CuO/TNTs and CuO/TiO2NFs, corresponding to (1 0 1)
crystal plane. However, the peaks of anatase TiO2 cannot be
detected in CuO/Dense TiO2 electrode, which is attributed to
the limited thickness of TiO2 layer. The peaks of CuO (JCPDS
card 65-2309) can be seen in all electrodes at 32.6, 35.6, 38.8,
Aer thermal decomposition process, CuO particles of about
100 nm are gathered on the surface of TiO2, as shown in Fig. 1b.
Fig. 1c shows the regularly arranged TiO2 nanotube with an
inner diameter of about 80 nm and a wall thickness of about
10 nm. The CuO particles are tightly packed inside and
inbetween nanotubes, as shown in Fig. 1d. The particle sizes of
CuO inside and inbetween nanotubes is about 40 and 100 nm,
respectively. The TiO2 nanober support shows a typical
Fig. 5 CV curves of (a) CuO/Dense TiO2, (b) CuO/TNTs, (c) CuO/TiO2NFs in 0.5 M Na2SO4 aqueous solution. (d) Average value of anode and
cathode current density (Dj ¼ (ja ꢀ jc)/2) around the open circuit potential against the scan rate for CuO/Dense TiO2, CuO/TNTs and CuO/
TiO2NFs, respectively.
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48.9, 66.6 and 68.1ꢁ, corresponding to (1 1 0) (ꢀ1 1 1), (1 1 1) (ꢀ2 speeds. It can be considered that only non-Faraday charge and
0 2), (ꢀ3 1 1) and (1 1 3) planes, respectively.
discharge occur in this range. The average value of anode and
cathode current under the open circuit potential is selected as
the electric double layer capacitance current iC. It can be
demonstrated that the double layer capacitance of CuO/Dense
TiO2, CuO/TNTs and CuO/TiO2NFs electrode is 74, 110 and
127 mF cmꢀ2 (Fig. 5d), respectively, which shows that the CuO/
TiO2NFs electrode possesses the highest ESCA and largest
number of active sites.34,35
3.2 Electrocatalytic properties
Fig. 4 reveals the LSV curves of CuO/Dense TiO2, CuO/TNTs and
CuO/TiO2NFs in 0.1 M Na2SO4. The dotted line represents the
current density under N2 saturated atmosphere while the solid
line shows the current density under CO2 saturated atmo-
sphere. The deviation between the solid line and the dashed
line can be regarded as the reaction current of CO2 reduction
3.3 Electrochemical reduction of CO2
(ICO ) approximately. The three electrodes all showed catalytic
2
abilities for CO2 reduction, but the overpotentials on different
Fig. 6a–c shows the liquid product yield and Faraday efficiency
of CuO/Dense TiO2, CuO/TNTs and CuO/TiO2NFs aer electro-
chemical reduction of CO2 for 5 h. Faraday efficiency (EF) can be
calculated by the following formula:
electrodes are much different. Specically, the ICO on CuO/
2
TNTs is nearly 0.2 mA cmꢀ2 at ꢀ0.5 V (vs. SCE). While for the
case of CuO/Dense TiO2 and CuO/TiO2NFs electrodes, the CO2
reduction begins at ꢀ0.6 V and ꢀ0.7 V, respectively. The
different overpotentials on different electrodes are mainly
related with the distinctive lm resistances. For example, the
X
znF
EF ¼
(1)
Q
dense TiO2 layer is only about 100 nm, while the TiO2NFs layer where z is the number of necessary electrons for product, 2 for
is nearly 1 mm. As a result, the current of CuO/Dense TiO2 is formic acid, 6 for methanol, and 12 for ethanol; n is the molar
larger than that of CuO/TiO2NFs. It is also observed that the number of products; F is the Faraday constant and Q is the total
hydrogen evolution reaction is evidently inhibited on CuO/ charge.
TiO2NFs or CuO/TNTs.
Fig. 7 is the gas chromatography analysis of liquid product
The electrical double layer capacitance is the most obtained by three electrodes through electrochemical reduction
commonly used method to determine the electrochemically of CO2 at different potential (vs. SCE). The retention times of
surface area (ECSA).31–33 Fig. 5a–c shows the CV curves of the methanol and ethanol are at 3.03 min and 3.25 min, respec-
three electrodes near open circuit potential by different sweep tively. Besides, the retention time of formic appears at 2.52 min
Fig. 6 The yield of liquid products and faradaic efficiency at different external voltage of (a) CuO/Dense TiO2, (b) CuO/TNTs and (c) CuO/
TiO2NFs for 5 h. (d) The optical image of gas products of the three electrodes at ꢀ0.9 V (vs. SCE) for 5 h.
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Fig. 7 Liquid products produced by different electrodes: (a) CuO/Dense TiO2, (b) CuO/TNTs and (c) CuO/TiO2NFs.
while that of air is at 2.12 min. It can be demonstrated that only Cu2O and potentially Cu.36 Our previous studies also conrmed
gas and ethanol can be generated by using CuO/Dense TiO2 the existence of Cu aer CuO/TNTs catalyst was applied in CO2
electrode. A very small amount of methanol is generated by reduction.37 So a modest increase in current was observed
CuO/TNTs electrode while a little formic can be detected by during the initial stage of CO2 reduction process. However, the
CuO/TiO2NFs electrode. By contrast, the yield of ethanol is Cu lm exhibited a lower activity compared with the oxide lm,
pretty higher by CuO/TiO2NFs electrode than those by CuO/ and the gas bubbled at the interface of electrode blocks the
Dense TiO2 and CuO/TNTs electrodes.
Fig. 6 and 7 indicate that the liquid products by CuO/Dense
TiO2, CuO/TNTs and CuO/TiO2NFs have high selectivity, and
the main product is ethanol. With increasing the bias voltage,
the yield of liquid products by CuO/Dense TiO2 electrode
gradually improves. The maximum ethanol production of 3.2
mmol cmꢀ2 can be obtained when the bias voltage is around
ꢀ1.1 V. As for the CuO/TNTs electrode, the Faraday efficiency
reaches up to 74.2% at ꢀ0.5 V, and the yield of ethanol is 1.6
mmol cmꢀ2 with small amount of methanol. With increasing the
bias voltage, the yield of liquid products and Faraday efficiency
decrease evidently. As regards the CuO/TiO2NFs electrode, the
improvement of bias voltage leads to the increased formation of
liquid products. The yield of ethanol and formic acid reaches up
to 6.4 mmol cmꢀ2 and 1.0 mmol cmꢀ2 at ꢀ1.1 V, respectively.
Fig. 8 shows the time–current curves of three electrodes for
electrochemical reduction of CO2 at ꢀ0.9 V, reaches
a maximum value and then decreases to a stable value. Many
literatures has proved that during CO2 electroreduction process,
less of the charge could be attributed to the reduction of CuO to
Fig. 8 Current–time curve of CuO/Dense TiO2, CuO/TNTs and CuO/
TiO2NFs in 0.1 M NaHCO3 saturated with CO2 at ꢀ0.9 V (vs. SCE) for
5 h, and the inset optical image shows the surface condition after
electrochemical reduction.
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available catalytic sites, which both caused the current decay (sub)surface oxygen play a role.43,44 And CuO/TiO2NFs shows
with prolonging time. As a result, reduction peaks were more grain boundaries compared with the other two electrodes.
observed in the chronoamperometry curves of CuO catalysts It is generally believed that during the CO2 reduction process,
during the initial stage of CO2 reduction. This result was the copper oxide on the surface of these electrodes should be
consistent with the studies by Yeo et al.38 Besides, it is noted completely converted to metallic Cu. However, a residual oxide
that the maximum current of CuO/Dense TiO2, CuO/TNTs layer was shown to remain present on the surface during the
occurs at 3000 s while that of CuO/TiO2NFs is 7000 s. The catalysis process, and it was proposed that the surface oxide
stable current density of CuO/Dense TiO2 and CuO/TNTs is 0.22 layer still serves as key reaction sites for catalysis.45
and 0.20 mA cmꢀ2, respectively, and CuO/TiO2NFs shows
a higher stable current density of 0.33 mA cmꢀ2. The longer
peak time and the higher stable current both indicate that CuO/
4. Conclusions
TiO2NFs may have a higher efficiency for the generation of CO2
reduction products. Aer CO2 electrochemical reduction for
18 000 s, the surface of CuO/Dense TiO2 electrode cracks and
strips while those of CuO/TiO2TNTs and CuO/TiO2NFs are
decomposition. The CuO/Dense TiO2, CuO/TNTs and CuO/
relatively stable.
It is well-accepted that the electrochemical reduction effi-
ciency is tightly related to the surface roughness of CuO/TiO2
heterojunction, the stability of TiO2 supports, ESCA and the
activity of hydrogen evolution reaction. The current density of
dense TiO2 support is high, however, the electric resistance of
TiO2 layer is rather tremendous, leading to the low current
ojunction, representing best property of CO2 reduction reaction.
efficiency. Besides, owing to the relatively low specic surface
area, the contact area between CO2 and electrode is limited,
leading to the inhibition of electrochemical reduction of CO2
and the promotion of hydrogen evolution reaction. Meanwhile,
Three TiO2 supports with different morphologies are success-
fully fabricated through anodic oxidation and hydrothermally
methods, and the CuO particles are deposited by thermal
TiO2NFs show relatively good selectivity and the main product
is ethanol. The liquid product of the CuO/Dense TiO2 electrode
is pure ethanol, but the current efficiency is poor. Ethanol can
be synthesized by CuO/TNTs at low overpotential with high
current efficiency, but the yield of gaseous products is redun-
dant. CuO/TiO2NFs shows rougher surface of CuO/TiO2 heter-
When the bias voltage is at ꢀ1.1 V, the yield of ethanol of CuO/
TNTs electrode reaches 6.4 mmol cm2 aer 5 h.
Conflicts of interest
as mentioned above, some cracks generate during the fabri-
cating process, and the bubbles produced by hydrogen evolu-
tion reaction will accelerate the generation and peeling of
There are no conicts to declare.
cracks, which leads to the instability of CuO/Dense TiO2. In the
CuO/TNTs electrode, the TiO2 nanotubes combine well with Ti
matrix and CuO particles. The nanotubes have excellent elec-
trical conductivity, so the current density is extremely high
Zhejiang Province (No. LY17B030009 and No. LQ16E020002).
during electrochemical reduction process. Besides, the CuO/
Acknowledgements
This work was supported by the Natural Science Foundation of
TNTs electrode is more favorable for reduction reaction of
CO2, and the synthesized ethanol shows excellent purity and
high current efficiency. However, with the increase of bias
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21812 | RSC Adv., 2021, 11, 21805–21812
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