A strategy to control the grain boundary density and Cu+/Cu0 ratio of Cu-based catalysts for efficient electroreduction of CO2 to C2 products

Article abstract of DOI:10.1039/d0gc00247j

Cu(OH)₂/CuO nanocomposite-derived Cu₂O/Cu was highly efficient for CO₂ electroreduction to C2 products. The highest faradaic efficiency and current density could reach 64.5% and 26.2 mA cm^-2, respectively. By changing the calcination time, moderate grain boundary density and the Cu⁺/Cu⁰ ratio in catalysts can be controlled, leading to a large number of active sites and low interfacial charge transfer resistance.

Full text of DOI:10.1039/d0gc00247j

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DOI: 10.1039/D0GC00247J
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A strategy to control grain boundary density and Cu+/Cu0 ratio of
Cu-based catalysts for efficient electroreduction of CO2 to C2
products
Received 00th January 20xx,
Accepted 00th January 20xx
Chunjun Chen,a,b Xiaofu Sun,a,b* Xupeng Yan,a,b Yahui Wu,a,b Mingyang Liu,a,b Shuaishuai Liu,a,b
Zhijuan Zhao,a Buxing Hana,b,c,d*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cu(OH)2/CuO nanocomposites-derived Cu2O/Cu were highly facilitate CO2 molecules activation and C-C bond formation
efficient for CO2 electroreduction to C2 products. The highest efficiently.[28]
faradaic efficiency and current density could reach 64.5% and 26.2
It was discovered that the preparation method of OD-Cu
mA cm-2, respectively. By changing calcination time, moderate catalysts played a crucial role for the distribution of the
grain boundary density and Cu+/Cu0 ratio in catalysts can be products. Generally, the methods can be divided into two
controlled, leading to large number of active sites and low categories,[29-32] i.e. annealing and unannealing methods. The
interfacial charge transfer resistance.
annealed Cu2O exhibited high selectivity for C1 products, but
low selectivity for C2 products at low overpotential. The total
C1 faradic efficiency (FE) reached up 70% (40% CO and 30%
formate).[29] Meanwhile, it was reported that electroreduction-
annealed OD-Cu could be used to produce CO with FE up to
60%.[30] Although annealing could decrease the overpotential
for CO2 reduction, the major product was CO and the FE of C2
products were low. Interestingly, OD-Cu prepared via the
unannealed method was shown to be more inclined to
produce C2 products. An electrodeposited Cu2O exhibited high
selectivity for ethylene, and the FE could reach up 40%.[31] In
another case, O-plasma-treated Cu electrode achieved high
ethylene selectivities, and the FE was about 60%.[32] Also, the
total FE of ethanol and acetate reached up to 80% over Cu
electrode derived from Cu-based complexes via in situ
electrosynthesis.[33] Recently, it was reported that a branched
Cu2O nanoparticles obtained in aqueous ammonia solution can
effectively electroreduction CO2 to ethylene, and the FE was
about 70%.[34] Further studies showed that Cu+ promoted CO2
reduction to C2 products over the unannealed OD-Cu due to
the enhancement of CO dimerization.[35] However, they usually
suffer from high overpotentials.
Electrochemical CO2 reduction has received wide attention,
which not only reduces the greenhouse gases (CO2) into
chemical fuels and feedstock, but also provides an energy
storage solution to the renewable energy sources.[1-5] The
activity of CO2 reduction has been boosted by various
methods, such as controlling morphology of nanostructured
catalysts,[6-9] manipulation of oxidation states,[10-12] combing
with other component,[13-16] and choosing appropriate
electrolyte.[17-18] Among C-based products, C2 products have
higher energy density and economic value, which are much
more attractive and significant than C1 counterparts. Up to
date, Cu-based materials have proved to be the most
promising electrocatalysts for converting CO2 to C2
products.[19-27] In particular, previous research showed that
oxide-derived copper (OD-Cu) could be prepared using a
simple synthesized method. The surface Cu+ sites were
suggested to act as the active sites for CO2 reduction. They can
a. 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
Bulk defects in metals, such as grain boundaries (GBs), can
create regions of increased strain at the surface, which have
previously been correlated with catalytic activity in some
reactions.[26, 36] It was demonstrated that the high densities of
E-mail: sunxiaofu@iccas.ac.cn; hanbx@iccas.ac.cn
b. School of Chemistry and Chemical Engineering, University of Chinese Academy of
Sciences; Beijing 100049, China.
GBs in OD-Cu electrocatalysts could improve CO2 reduction
c. Physical Science Laboratory, Huairou National Comprehensive Science Center, No.
5 Yanqi East Second Street, Beijing 101400, China.
30]
activity, including CO2-to-CO[29,
and CO-to-fuels.[37] By
d. Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University, Shanghai
200062, China.
annealing, larger GBs with appropriate types and distribution
can be created. However, the ratio of Cu+/Cu0 active sites is
difficult to tune, leading to a low stability and C2 product
selectivity. Thus further modifying the local electronic
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI:10.1039/x0xx00000x
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images for R-Cu-2, R-Cu-5, R-Cu-10 and R-Cu-20, respectively. N) XRD
structure of Cu with positive valence sites can keep the high
Cu+ content on the OD-Cu surface. Combined with the high
density of GBs, it probably enables control over CO generation,
adsorption and dimerization, and makes preference for stable
CO2 reduction to C2 products possible.
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patterns of Composites-x. O) XRD patterns of R-Cu-x.
DOI: 10.1039/D0GC00247J
The procedures to synthesize R-Cu-x composites are shown
schematically in Figure 1A. A polished Cu foil was used as the
substrate, and Cu(OH)2 nanosheets were firstly synthesized
through modified chemical immersion method.[15] Then, it was
calcined in the muffle at 500 °C for desired time. Cu(OH)2 was
partially transformed to CuO, and Cu(OH)2/CuO composites
were formed. For clarity, Composites-x refers to different
Cu(OH)2/CuO composites, and x represents the calcination
time (min). Finally, Cu(OH)2/CuO-x were electrochemically
reduced at -2.0 V vs. RHE for 500 s, leading to the formation of
R-Cu-x. During the electrochemically reducing, we can observe
that the current density decreased before 500 s, while the
current density was stable after 500 s, indicating that
Cu(OH)2/CuO-x was completely reduced. Figure S1 shows
optical pictures of the samples, displaying an observable color
change and uniformity during the process.
In this work, we tried to combine the advantages to achieve
high selectivity for C2 products with low overpotentials. We
successfully prepared five reduced Cu(OH)2/CuO composites
(R-Cu). By adjusting the calcination time, Cu(OH)2/CuO
composites with different ratios of Cu(OH)2 and CuO were first
prepared. After electrochemically reducing, a series of R-Cu
composites with different GB densities and Cu+/Cu0 ratios was
obtained. The catalytic performance of these composites for
electrochemical CO2 reduction was studied. It was discovered
that the highest FE for C2 products could reach 64.5% with a
current density of 26.2 mA cm-2 at -0.9 V vs. RHE. This is one of
the highest FE for C2 products at such low applied potential.
The high catalytic activity resulted mainly from the synergistic
effect of GB densities and Cu+ species, which could be tuned
easily by changing the calcination time.
Fig. 2 Electrocatalytic results in CO2-saturated 0.1 M KHCO3
electrolyte. A) LSV curves on different electrodes with a scan
speed of 20 mV s-1. B) FE for C2 products over different
electrodes at different potentials. C) The distribution of C2
products at different potentials over R-Cu-5. D) The long-term
stability of R-Cu-5 at -0.9 vs. RHE during 24 h electrolysis.
The morphologies of Composites-x and R-Cu-x were first
characterized by scanning electron microscopy (SEM). The
obtained Cu(OH)2 had a sheet-like morphology (Figure S2).
After calcination, Composites-2 remained to have the sheet-
like morphology (Figure 1B). However, with the increased
calcination time, a three-dimensional (3D) structure was
formed for Composites-5 and Composites-10 (Figure 1E and
1H). Then the 3D structure collapsed to the dense films when
the calcination time extended to 20 minutes (Figure 1K). After
Composites-x was electrochemically reduced, the structure of
R-Cu-x became more porous compared with Composites-x
(Figure 1C, 1F, 1I and 1L). Transmission electron microscopy
(TEM) images of R-Cu-x are presented in Figure 1D, 1G, 1J and
1M, demonstrating that GBs existed in the bulk of the
Fig. 1 A) Schematic illustration of the process to prepare R-Cu-x. B, E,
H and K) SEM images for Composites-2, Composites-5, Composites-10
and Composites-20, respectively. C, F, I and L) SEM images for R-Cu-2,
R-Cu-5, R-Cu-10 and R-Cu-20, respectively. D, G, J and M) HR-TEM
2 | J. Name., 2012, 00, 1-3
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composites. The as-obtained samples were then characterized surface areas (ECSA) were determined by measVuierwinAgrticdleoOunblinlee
DOI: 10.1039/D0GC00247J
by X-ray diffraction (XRD). For the Composites-x (Figure 1N), layer capacitance (Cdl; Table S4). The ECSA for R-Cu-5 was
with increasing calcination time, the characteristic peaks of higher than that of other catalysts (Figure 3A). The increased
CuO were strengthened (36.0° and 41.8°) and the ECSA indicated larger number of active sites and increased
characteristic peaks of Cu(OH)2 were weakened (35.5° and apparent concentration of reactant at the surface of R-Cu-5,
38.6°). It indicated that Cu(OH)2 transformed to CuO gradually. leading to the higher efficiency for CO2 reduction. For R-Cu-x,
For R-Cu-x, the characteristic peaks of Cu(OH)2 and CuO the partial current density for C2 products were also
disappeared. A new peak around 36.5° was observed, which normalized by ECSA (Figure 3B). Similar to the geometric
can be assigned to Cu2O. It demonstrated that Cu+ species was partial current density, the normalized partial current density
generated after the electroreduction of Composites-x (Figure for C2 products of R-Cu-5 was also highest among the five
1O). The strong peaks at about 50° were corresponding to the catalysts. Therefore, the increased activity of R-Cu-5 was not
Cu foil, which were not changed after electrochemical only attributed to the increasing number of active sites.
reduction.
Moreover, the total current densities normalized by ECSA were
The CO2 electroreduction activities of R-Cu-x were tested in similar (Figure 3C), indicating a nearly identical consumption
a typical H-type cell[38] and CO2-saturated 0.1 M KHCO3 rate of local protons during CO2 reduction. We can conclude
solution (pH 6.8) was used as electrolyte. As revealed by linear that the high C2 selectivity for R-Cu-5 cannot ascribed to the
sweep voltammetry (LSV) in Figure 2A, R-Cu-5 exhibited a local pH.[39-40]
more positive onset potential and higher total current density
The electrochemical impedance spectroscopy (EIS) was also
than other catalysts, suggesting that R-Cu-5 was most active conducted, and the Nyquist plots were obtained by running
catalyst for CO2 reduction. Controlled potential electrolysis of the experiment at an open circuit potential (Figure 3D). R-Cu-5
CO2 at each given potential was then performed. Solution- showed the lowest interfacial charge transfer resistance,
phase and gas-phase products were quantified by nuclear suggesting that electron transfer on R-Cu-5 electrode surface
magnetic resonance (NMR) spectroscopy and gas was more facile than others. Therefore, R-Cu-5 showed
chromatography (GC), respectively. Under these conditions, significant advantages in all the key steps for CO2
various products, including ethylene, ethane, ethanol, CO, electroreduction to C2 products.
formate and H2, were detected with a combined FE of around
100 % for all the studied catalysts (Figure S3 and S4). From
Figure 2B, we can observe that R-Cu-5 could yield the highest
total FE of 64.5 % for C2 products at -0.9 V vs. RHE, and a FE of
22.5 % at -0.6 V vs. RHE. As shown in Figure S5, the onset
potential of C2 products for R-Cu-5 was -0.45V (ethane), -
0.50V (ethanol) and -0.51V (ethylene), respectively. The onset
potential of C2 products over other R-Cu-x catalysts were also
calculated (Table S1), and we can observe that R-Cu-5
exhibited lowest onset potential in the five catalysts. As far as
we known, this is one of the highest FE for C2 products with
such low applied potential (Table S2 and Table S3). However,
the maximum FE for C2 products over R-Cu-10 and R-Cu-20
were 55.3% and 35.2%, respectively. Interestingly, for R-Cu-2
and R-Cu(OH)2, C2 products could not be detected under low
potentials, and the maximum FE for C2 products were only
32.1% and 23.6% at -1.1 V vs. RHE, respectively. In detail, the
whole distribution of C2 products were presented in Figure 2C
and Figure S4. The results indicate that the appropriate
Fig. 3 A) Charging current density differences plotted against the scan
rates: R-Cu-5 (a), R-Cu-10 (b), R-Cu-20 (c), R-Cu-2 (d) and R-Cu(OH)2 (e).
calcination time could not only increase the selectivity of C2
B) The partial current density for C2 products normalized by ECSA. C)
products, but also reduced the overpotential. 500 oC is an
The total current density normalized by ECSA. D) Nyquist plots for
optimal calcination temperature. Catalysts would fall off from
the Cu foil when the temperature was 600 oC, while the
catalytic activity became lower when the temperature was 400
oC (Figure S6). Moreover, R-Cu-5 also had higher current
density and partial current density for C2 products than those
of other catalysts (Figure S7-S8). The long-term stability of R-
Cu-5 was tested at -0.9 V vs. RHE for 24 h (Figure 2D). It can be
seen that both the current density and the FE for C2 products
kept stable with time during the entire period.
different electrodes in CO2-saturated 0.1 M KHCO3 electrolyte.
It is important to investigate the contributors for the out-
standing catalytic activity of R-Cu-5 in CO2 reduction to C2
products. Firstly, R-Cu-5 possesses a high density of GBs, which
can enhance the activity of CO2 reduction.[29, 41] In our work,
GB density was analyzed by imaging ~30 particles of R-Cu-x
(Figure S10) and the detailed calculation method is provided in
the supporting information. From the HR-TEM images (Figure
S10C), we can observe that there was an obvious interface
According to the cyclic voltammograms (CV) curves under
different scan rates (Figure S9), the electrochemical active
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