Grain-Boundary-Rich Copper for Efficient Solar-Driven Electrochemical CO2 Reduction to Ethylene and Ethanol

Article abstract of DOI:10.1021/jacs.0c00971

The grain boundary in copper-based electrocatalysts has been demonstrated to improve the selectivity of solar-driven electrochemical CO₂ reduction toward multicarbon products. However, the approach to form grain boundaries in copper is still limited. This paper describes a controllable grain growth of copper electrodeposition via poly(vinylpyrrolidone) used as an additive. A grain-boundary-rich metallic copper could be obtained to convert CO₂ into ethylene and ethanol with a high selectivity of 70% over a wide potential range. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy unveils that the existence of grain boundaries enhances the adsorption of the key intermediate (CO) on the copper surface to boost the further CO₂ reduction. When coupling with a commercially available Si solar cell, the device achieves a remarkable solar-to-C2-products conversion efficiency of 3.88% at a large current density of 52 mA·cm^-2. This low-cost and efficient device is promising for large-scale application of solar-driven CO₂ reduction.

Full text of DOI:10.1021/jacs.0c00971

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Communication
Grain-Boundary-Rich Copper for Efficient Solar-Driven
Electrochemical CO2 Reduction to Ethylene and Ethanol
ZHIQIANG CHEN, Tuo Wang, Bin Liu, Dongfang Cheng, Congling Hu, Gong
Zhang, Wenjin Zhu, Huaiyuan Wang, Zhi-Jian Zhao, and Jinlong Gong
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c00971 • Publication Date (Web): 27 Mar 2020
Downloaded from pubs.acs.org on March 27, 2020
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Grain-Boundary-Rich Copper for Efficient Solar-Driven
Electrochemical CO2 Reduction to Ethylene and Ethanol
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Zhiqiang Chen, Tuo Wang, Bin Liu, Dongfang Cheng, Congling Hu, Gong Zhang, Wenjin Zhu,
Huaiyuan Wang, Zhi-Jian Zhao, and Jinlong Gong*
School of Chemical Engineering and Technology, Tianjin University; Key Laboratory for Green Chemical
Technology of Ministry of Education, Tianjin University; Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin), Tianjin 300072, China
Supporting Information Placeholder
(PVP) is used as an additive. Unlike other additives such as
citric acid that forms complex with Cu ion to control the
ABSTRACT: The grain boundary in copper-based
electrocatalysts has been demonstrated to improve the
selectivity of solar-driven electrochemical CO2 reduction
towards multi-carbon products. However, the approach to
form grain boundaries in copper is still limited. This paper
deposition kinetics, PVP is
a physicochemically inert
polymer, which can adsorb on copper surface reversibly to
kinetically increase the rate of nucleation and reduce the size
of crystallite.15-18 Moreover, the inert nature of PVP avoids the
introduction of carbon contamination. With the existence of
GBs, GB-Cu shows a remarkable FE of about 70% for C2
products in the wide potential range, which achieves a solar-
to-C2 energy conversion efficiency of 3.88 % with a
remarkable current density of 52 mA∙cm−2 when coupled to a
Si solar cell. In-situ attenuated total reflection surface-
enhanced infrared absorption spectroscopy (ATR-SEIRAS)
investigations and theoretical calculations are adopted to
explore the role of GBs of copper during CO2RR.
describes
a
controllable grain growth of copper
electrodeposition via poly (vinylpyrrolidone) used as an
additive. A grain-boundary-rich metallic copper could be
obtained to convert CO2 into ethylene and ethanol with a
high selectivity of 70% over a wide potential range. In-situ
attenuated total reflection surface-enhanced infrared
absorption spectroscopy unveils that the existence of grain
boundaries enhances the adsorption of the key intermediate
(*CO) on the copper surface to boost the further CO2
reduction. When coupling with a commercially available Si
solar cell, the device achieves a remarkable solar-to-C2-
products conversion efficiency of 3.88% at a large current
density of 52 mA∙cm-2. This low-cost and efficient device is
promising for large-scale application of solar-driven CO2
reduction.
The copper catalysts were in-situ grown on gas diffusion
layer by potentiostatic electrodeposition with or without PVP
(details in Supporting Information). As shown in scanning
electron microscopy (SEM) images, the surface of GB-Cu is
piled up by tiny particles, which is different form ED-Cu
(Figure S1). Transmission electron microscopy (TEM) reveals
that the copper electrodeposited without PVP (denoted as
ED-Cu) shows a uniform lattice orientation without GBs
(Figure 1a). When adding PVP as the additive, a high density
of grain boundaries is introduced into GB-Cu nanoparticle
(Figure 1b, more images of different GB-Cu in Figure S2). The
lattice spacing of ED-Cu and GB-Cu nanoparticles (Figure 1a
and b) both correspond to (111) facet of copper, which is
consistent with the X-ray diffraction (XRD) result (Figure 1c).
GB-Cu and ED-Cu both present a sharp Cu 2p peak at 932.4
eV in X-ray photoelectron spectroscopy (XPS) (Figure 1d),
corresponding to Cu0 or Cu+ species.19 Auger electron
spectroscopy (AES) of Cu LMM (Figure S3) illustrates that
GB-Cu mainly consists of Cu0 (918.6 eV), with a very limited
amount of Cu+ (916.6 eV), which could be caused by the
rapid oxidation in the air.20-21 These results indicate that PVP
makes no noticeable influence on the exposed crystal planes
and chemical states of copper during the electrodeposition
while inducing the formation of GBs in copper.
The electrochemical transformation of CO2 to value-added
multi-carbon (C2+) products is a potential solution to close
the anthropogenic carbon cycle, which could be driven by
renewable energies such as solar power.1-2 Copper is a unique
metal that can reduce CO2 to C2+ products.3-6 However, the
moderate binding energy of copper leads to poor product
selectivities.7 It is believed that grain boundary (GB) can
control the product selectivity of CO2 reduction reaction
(CO2RR).8-9 The density of GBs of copper exhibits a
quantitative correlation with CO reduction reaction
activity.10 Therefore, GBs are considered as one of the reasons
for the increased selectivity of oxide-derived copper towards
C2+ products.11-13 But the reduction of the copper oxide
14
would lead to the change of copper valence states, making
it difficult to identify the role of GBs. Thus, new approaches
are desired to introduce GBs into metallic copper without
changing the valence states of copper.
This communication describes the fabrication of a grain-
boundary-rich copper catalyst (GB-Cu) by the additive-
control electrodeposition, in which poly(vinylpyrrolidone)
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Figure 1. HRTEM images of (a) ED-Cu catalysts and (b) GB-
Figure 2. Faradaic efficiencies of gas products on (a) ED-Cu
and (b) GB-Cu as a function of the electrode potential; (c)
Faradaic efficiencies of C2+ products on GB-Cu; (d)
Comparison of Faradaic efficiencies of C2 products on ED-Cu
and GB-Cu.
Cu catalysts. Different lattice orientation regions divided by
orange dotted lines; (c) XRD and (d) Cu 2p XPS spectra of as-
prepared ED-Cu and GB-Cu catalysts.
For
electrochemical
measurement,
the
a
CO2
three-
electroreduction reaction was conducted in
To explore the impact of the GB formation on the CO2
reduction, in-situ ATR-SEIRAS was employed to monitor the
CO adsorption behavior in CO-saturated KOH electrolyte.
The absorption band at 1650 cm−1 is corresponding to the
H−O−H bending vibration of water (Figure S11).25 On the
surface of ED-Cu, there is a stretching band at 2070 cm−1 at
low potential (Figure 3a), corresponding to the stretching
band of linearly atop-bound CO (COad) (1920 ~ 2100 cm−1) on
Cu surface.26-27 As for GB-Cu, the COad band presents a
distinct red-shift from 2070 cm−1 to 2060 cm−1 at −0.1 V vs.
RHE (Figure 3b). The lower wavenumber of the COad band
indicates the stronger adsorption of CO on the GB-Cu
surface.28-31 As shown above, both of ED-Cu and GB-Cu
expose the same (111) plane of metallic copper with similar
nanoparticle morphology. The main difference between ED-
Cu and GB-Cu is the formation of GBs on GB-Cu, which
should be attributed to explain the enhancement of CO
adsorption.
compartment flow cell (Figure S4). Linear sweep
voltammetry (LSV) shows that ED-Cu and GB-Cu exhibit
similar electrocatalytic activity (Figure S5). However, GB-Cu
shows a higher selectivity for ethylene (~38 % at −1.2 V vs.
RHE) and much less hydrogen production than ED-Cu
(Figure 2a and b). Combined with liquid-phase products, the
Faradaic efficiency for C2+ products of GB-Cu (Figure 2c)
including ethylene, ethanol, and propanol reaches 73 % in a
wide potential range from −1.0 to −1.3 V vs. RHE, which is
superior to most of the metallic copper electrocatalysts
(Table S1). Compared with ED-Cu, GB-Cu shows a much
higher selectivity for C2 products at each applied potential
(Figure 4d).
Moreover, the ethanol-to-ethylene ratio is enhanced from
0.49 on ED-Cu to 0.85 on GB-Cu (Figure S6). The ethanol FE
of GB-Cu achieves 31.74 % with a current density of 45
mA∙cm−2 at −1.3 V vs. RHE (Figure S7), which is superior to
most previously reported Cu-based electrocatalysts (Table
S2). It has been identified that ethylene and ethanol are
produced at different active sites of copper by the isotope-
labeling method,22 while the undercoordinated sites, such as
GBs, are more favorable for the formation of oxygenates.23 In
this respect, the higher ethanol-to-ethylene ratio on GB-Cu
compared with ED-Cu could be a good indication that GBs
play a significant role in the product distribution of CO2RR.
Meanwhile, the adsorption of CO on GB-Cu at different
potentials is different from ED-Cu. Until the applied
potential reaches −0.4 V vs. RHE, the peak areas of the COad
band region of GB-Cu is increasing indicating the increased
coverage of COad on GB-Cu surface (Figure S12), which is
proportional to the surface coverage of CO.32-33 Moreover, the
distinct COad bands are still observed on the GB-Cu surface
under much more negative potentials until −1.0 V vs. RHE,
while the COad bands on the ED-Cu are absent after −0.5 V
vs. RHE. Under the same cathodic potentials, the
relationship between applied potential and CO adsorption,
such as the Stark effect and π-interaction, should be
similar.29 Hence, it can be proposed that GBs of GB-Cu could
enhance and stabilize CO adsorption at much more negative
potentials.
To ensure that the improved CO2RR performance for C2
products is indeed originated from the introduction of GBs,
the effect of PVP and surface area should be excluded. The
characteristic Fourier transform infrared spectroscopy (FTIR)
bands of PVP, such as C−N (stretching at 1292 cm−1) and C−O
(vibrating at 1679 cm−1) as shown in PVP reagent,24 cannot be
observed for GB-Cu (Figure S8), revealing the absence of PVP
on GB-Cu. Though PVP will not participate in CO2RR, it can
alter the C2 products selectivity by inducing the formation of
GB (details in Figure S9). In addition, the similar ECSA
(Figure S10 and Table S3) of GB-Cu and ED-Cu indicates that
the higher catalytic activity of GB-Cu is not caused by the
increase of surface area, but the improvement related to
intrinsic active sites.
Density functional theory (DFT) calculations are employed
to further reveal the role of GB on the high selectivity
towards C2 production. The Σ3 twin boundary, which
presents the highest density of coincident lattice sites at the
GB plane, is chosen for its stable atomic structure34. The
calculation of CO binding energies in different sites (Figure
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3c-3d) suggests that CO is binding stronger in both convex
density could be reached in the flow cell compared with the
traditional H-cell, where a stable current density of 52.1
mA∙cm−2 (Figure 4d) is obtained at a voltage of ~2.8 V in EC
cell (Figure S15) without external bias under AM 1.5G
illumination. The FE of C2 products reaches up to 68 %
during the 3h operation. According to the operating current
and FE of C2 products, the solar-to-C2-products efficiency
achieves 3.88% (details in Supporting Information). Few PV-
EC devices can convert solar energy into chemical energy in
C2 products under such a large current density and high
efficiency (Table S4).
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GB sites (GB1) and concave GB sites (GB2) than terrace sites
(t1~t7), consisting with the results of in-situ ATR-SEIRAS. It
is worth noting that the GBs also alter the coordination of
their surrounding atoms, therefore slightly increasing the CO
binding energies of t1, t2 and t7 sites than other terrace sites.
The increasing CO binding energy at the GBs is beneficial to
the kinetics of CO dimerization.
As for the C−C coupling step, many studies have shown
that CO dimerization is the first step towards C2 products,
followed by the hydrogenation of *COCO to form
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*COCHO35-37. Bell et al found that *COCHO was a critical
38
intermediate towards C2 products.
The pathway to
ethylene or ethanol is downhill thermodynamically from
*COCHO intermediate.39 Hence, the formation energy of
*COCHO is an appropriate descriptor to describe the C−C
coupling step. From our calculations, the formation energy of
*COCHO intermediate is 0.46 eV on GB1 site and 0.58 eV on
GB2 site, 0.24 eV and 0.12 eV lower than terrace site like t4
(0.70 eV), respectively (Figure S13), suggesting that the
presence of GB can promote the C−C coupling.
Figure 4. (a) LSV of GB-Cu cathode and Ni foam anode; (b)
LSV of GB-Cu cathode and Se-(NiCo)Sx/(OH)x anode; (c)
Current–potential characteristic of the SHJ solar cell under 1
sun ( AM 1.5G) illumination (blue line) and measured
operating current of the electrolyzer cell (brown line); (d)
The stability test of PV-EC device.
In summary, GBs are introduced into metallic copper in a
facile approach in which PVP is applied as the additive.
Compared with normal copper electrocatalyst, GB-Cu
exhibits a high selectivity (FE =~70%) to C2 products in a
wide potential range of −1.0 to −1.3 V vs. RHE. In-situ ATR-
SEIRAS and DFT reveal that the existence of GBs on metallic
copper enhances the adsorption of key intermediate (*CO) to
promote the C−C coupling reaction. Subsequently, a low-cost
PV-EC system with all-earth-abundant materials is designed
to convert CO2 into C2 products at a remarkable solar-to-fuel
conversion of 3.88%. This grain-boundary-rich copper
electrocatalyst and its use in the PV-EC device show new
possibilities in applications for the solar-driven CO2 reaction.
Figure 3. In-situ ATR-SEIRAS spectras of (a) ED-Cu and (b)
GB-Cu; (c) Different binding sites on the schemed atomic
structure of GB-Cu; (d) *CO binding energies in different
sites of the schemed structure of GB-Cu.
As mentioned above, GB-Cu keeps a high selectivity for C2
products in a wide potential range, showing a tremendous
potential to be the cathode for
a
photovoltaic-
electrochemical (PV-EC) system. As for the anode, a low-cost
OER anode electrocatalyst, Se-(NiCo)Sx/(OH)x nanosheets,
was adopted as reported before.40 The onset voltage of the
cell, defined by the difference between the onset potentials
(defined as the potential required to achieve an absolute
current density of 1 mA∙cm−2) of cathode and anode, is 1.8 V
when the Ni foam is used as the anode (Figure 4a). When
using the Se-(NiCo)Sx/(OH)x nanosheets as the anode, the
onset voltage is reduced to 1.5 V (Figure 4b).
ASSOCIATED CONTENT
Supporting Information
Experimental details and supporting data. This material is
available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
According to the high selectivity (70%) for C2 products of
GB-Cu at −1.0 V vs. RHE, a 6-cell a-Si/c-Si heterojunction
(SHJ) module is adopted (Figure S14, details in Supporting
Information) to couple with the flow cell directly. The
operation point determined by the intersection of two LSV
curves is very close to the maximum power point (MPP) of
the solar cell module (Figure 4c), meaning that two devices
can match well with each other. Meanwhile, a higher current
Corresponding Author
*Email: jlgong@tju.edu.cn
ORCID
Jinlong Gong: 0000-0001-7263-318X
Tuo Wang: 0000-0002-9862-5038
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towards Control of Particle Size and Stability. J. Nanopart. Res. 2016, 18,
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Zhi-Jian Zhao: 0000-0002-8856-5078
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Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENT
(19) Jung, H.; Lee, S. Y.; Lee, C. W.; Cho, M. K.; Won, D. H.; Kim, C.;
Oh, H. S.; Min, B. K.; Hwang, Y. J., Electrochemical Fragmentation of
Cu2O Nanoparticles Enhancing Selective C–C Coupling from CO2
Reduction Reaction. J. Am. Chem. Soc. 2019, 141, 4624-4633.
(20) Grosse, P.; Gao, D.; Scholten, F.; Sinev, I.; Mistry, H.; Roldan
Cuenya, B., Dynamic Changes in the Structure, Chemical State and
Catalytic Selectivity of Cu Nanocubes during CO2 Electroreduction: Size
and Support Effects. Angew. Chem. Int. Ed. 2018, 57, 6192-6197.
(21) Zhuang, T. T.; Pang, Y. J.; Liang, Z. Q.; Wang, Z. Y.; Li, Y.; Tan, C.
S.; Li, J.; Dinh, C. T.; De Luna, P.; Hsieh, P. L.; Burdyny, T.; Li, H. H.;
Liu, M. X.; Wang, Y. H.; Li, F. W.; Proppe, A.; Johnston, A.; Nam, D. H.;
Wu, Z. Y.; Zheng, Y. R.; Ip, A. H.; Tan, H. R.; Chen, L. J.; Yu, S. H.;
Kelley, S. O.; Sinton, D.; Sargent, E. H., Copper Nanocavities Confine
Intermediates for Efficient Electrosynthesis of C3 Alcohol Fuels from
Carbon Monoxide. Nat. Catal. 2018, 1, 946-951.
(22) Lum, Y.; Ager, J. W., Evidence for Product-Specific Active Sites on
Oxide-Derived Cu Catalysts for Electrochemical CO2 Reduction. Nat.
Catal. 2018, 2, 86-93.
(23) Hahn, C.; Hatsukade, T.; Kim, Y. G.; Vailionis, A.; Baricuatro, J. H.;
Higgins, D. C.; Nitopi, S. A.; Soriaga, M. P.; Jaramillo, T. F., Engineering
Cu Surfaces for the Electrocatalytic Conversion of CO2: Controlling
Selectivity toward Oxygenates and Hydrocarbons. Proc. Natl. Acad.
Sci.U. S. A. 2017, 114, 5918-5923.
We acknowledge the National Key R&D Program of China
(2016YFB0600901), the National Natural Science Foundation
of China (21525626, U1662111, 21722608, 51861125104), the
Natural Science Foundation of Tianjin City (18JCJQJC47500)
and the Program of Introducing Talents of Discipline to
Universities (B06006) for financial support.
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