Electrocatalytic Reduction of CO2 to Ethylene by Molecular Cu-Complex Immobilized on Graphitized Mesoporous Carbon

Article abstract of DOI:10.1002/smll.202000955

The electrochemical reduction of carbon dioxide (CO₂) to hydrocarbons is a challenging task because of the issues in controlling the efficiency and selectivity of the products. Among the various transition metals, copper has attracted attention as it yields more reduced and C2 products even while using mononuclear copper center as catalysts. In addition, it is found that reversible formation of copper nanoparticle acts as the real catalytically active site for the conversion of CO₂ to reduced products. Here, it is demonstrated that the dinuclear molecular copper complex immobilized over graphitized mesoporous carbon can act as catalysts for the conversion of CO₂ to hydrocarbons (methane and ethylene) up to 60%. Interestingly, high selectivity toward C2 product (40% faradaic efficiency) is achieved by a molecular complex based hybrid material from CO₂ in 0.1 m KCl. In addition, the role of local pH, porous structure, and carbon support in limiting the mass transport to achieve the highly reduced products is demonstrated. Although the spectroscopic analysis of the catalysts exhibits molecular nature of the complex after 2 h bulk electrolysis, morphological study reveals that the newly generated copper cluster is the real active site during the catalytic reactions.

Full text of DOI:10.1002/smll.202000955

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Electrocatalytic Reduction of CO2 to Ethylene by Molecular
Cu-Complex Immobilized on Graphitized Mesoporous Carbon
Mani Balamurugan, Hui-Yun Jeong, Venkata Surya Kumar Choutipalli, Jung Sug Hong,
Hongmin Seo, Natarajan Saravanan, Jun Ho Jang, Kang-Gyu Lee, Yoon Ho Lee,
Sang Won Im, Venkatesan Subramanian, Sun Hee Kim, and Ki Tae Nam*
However, electrochemical CO2 reduction
reaction (CO2RR) is kinetically sluggish,
The electrochemical reduction of carbon dioxide (CO2) to hydrocarbons is
a challenging task because of the issues in controlling the efficiency and
selectivity of the products. Among the various transition metals, copper has
attracted attention as it yields more reduced and C2 products even while
using mononuclear copper center as catalysts. In addition, it is found that
reversible formation of copper nanoparticle acts as the real catalytically active
site for the conversion of CO2 to reduced products. Here, it is demonstrated
that the dinuclear molecular copper complex immobilized over graphitized
mesoporous carbon can act as catalysts for the conversion of CO2 to hydro-
carbons (methane and ethylene) up to 60%. Interestingly, high selectivity
toward C2 product (40% faradaic efficiency) is achieved by a molecular com-
plex based hybrid material from CO2 in 0.1 m KCl. In addition, the role of local
pH, porous structure, and carbon support in limiting the mass transport to
achieve the highly reduced products is demonstrated. Although the spectro-
scopic analysis of the catalysts exhibits molecular nature of the complex after
2 h bulk electrolysis, morphological study reveals that the newly generated
copper cluster is the real active site during the catalytic reactions.
which always requires large overpotential
and also usually generates multiple prod-
ucts.[5,8–10] Interestingly, plants and algae
efficiently utilize the photosynthetic pro-
cess to convert CO2 and water to various
organic compounds and several attempts
have been made to mimic the CO2 reduc-
tion process.[11,12] Various kinds of elec-
trocatalysts such as homogeneous,[13–15]
heterogeneous,[3,16,17] and single-atom cata-
lysts are explored for the electrochemical
CO2 conversions.[18–22] Among the elec-
trocatalytic materials studied so far, the
metal complexes have attracted significant
importance, because they possess well-
defined structures that can be easily tuned
in molecular level.[15,23–26] However, the
key issues that remain with the molecular
systems are stability and current den-
sity. Therefore, significant interest in the
development of electrocatalytic materials
by immobilizing over nanocarbon support has been estab-
lished to enhance catalytic activity, stability, and selectivity for
electrochemical CO2 reduction.[27–29] Several molecular com-
plexes based on various transition metals are used as catalysts
for CO2RR and most of them produced CO selectively. Among
them, bipyridine, cyclam, porphyrins, and phthlocyanine-based
catalysts are well studied with different metals. For example,
Re(bpy)(CO)3Cl (bpy = 2,2′-bipyridine),[30] Ru(bpy)(CO)2Cl2,[31,32]
[Mn(bpy)(CO)3Br],[33] and Ir pincer complexes[34] produced CO
with high selectivity. The first row transition metal complexes
such as [Ni(cyclam)]2+ (cyclam = 1,4,8,11-tetraazacyclotetrade-
cane),[35–37] iron and cobalt porphyrins and phthalocyanines
also exhibited high stability and selectivity for CO produc-
tion.[23,38–41] Similarly, a few copper complexes also exhibited
electrochemical CO2 reduction activity.[42,43] However, most
of the above-mentioned catalysts displayed the production of
simple C1 product predominantly, and hydrocarbon-producing
molecular based catalysts are scarce.
1. Introduction
Electrochemical conversion of CO2 to chemicals and fuels using
electricity generated from renewable energy sources could be
the alternate for the development of carbon-neutral fuels.[1–7]
Dr. M. Balamurugan, Dr. H.-Y. Jeong, J. S. Hong, H. Seo, Dr. N. Saravanan,
J. H. Jang, K.-G. Lee, Y. H. Lee, S. W. Im, Prof. K. T. Nam
Department of Materials Science and Engineering
Seoul National University
1 Gwanak-ro, Seoul 08826, Republic of Korea
E-mail: nkitae@snu.ac.kr
V. S. K. Choutipalli, Prof. V. Subramanian
Inorganic and Physical Chemistry Laboratory
CSIR-Central Leather Research Institute, Adyar, Chennai 600 020, India
V. S. K. Choutipalli, Prof. V. Subramanian
Academy of Scientific and Innovative Research (AcSIR)
CSIR-CLRI Campus, Chennai 600020, India
Dr. S. H. Kim
Western Seoul Center
Korea Basic Science Institute (KBSI)
150, Bukahyeon-ro, Seodaemun-gu, Seoul 120-140, Korea
In this context, among the various metal complexes reported
so far Cu-based metal complexes displayed hydrocarbon pro-
duction and C2 products during the electroreduction of CO2
similar to the heterogeneous catalysts (Figure 1).[25,29,44–46] First,
Bouwman and coworkers identified a dinuclear copper com-
plex coordinated with pyridine and sulfur-containing ligand can
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202000955.
DOI: 10.1002/smll.202000955
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Figure 1. a) Cu complex employed as CO2 reduction catalysts by immobilization on graphitized mesoporous carbon in CO2 saturated 0.1 m KCl.
b) The molecular complexes exhibiting C2 product formation during electrocatalytic reduction of CO2.
electrochemically reduce CO2 to oxalate. Based on the mecha-
nistic and crystallographic study, formation of tetranuclear
Cu cluster with two oxalate formed from the reaction of four
Cu(I) sites with CO2 was suggested as crucial reaction inter-
mediate.[25] Recently, Wang group deposited the Cu-porphyrin
complex over carbon nanoparticles and used as catalysts for
the electrochemical CO2 reduction to methane and ethylene
with a faradaic efficiency (FE) of 25% and 17%, respectively.[29]
Later, crystalline Cu(II) phthalocyanine (CuPc) immobilized
on the carbon support was used as catalyst for CO2 reduction
and generated ethylene up to 25% FE. Based on X-ray photo-
electron spectroscopy (XPS) analysis, it has been suggested that
the molecular nature remains after bulk electrolysis in both
porphyrin and phthalocyanine-based copper catalysts.[44] Very
recently, restructuring of Cu complexes during electrocatalytic
CO2RR has been revealed by in situ and operando X-ray absorp-
tion spectroscopy (XAS) characterization of various Cu systems
such as CuPc, HKUST-1, and [Cu(cyclam)]Cl2. In particular, the
reversible formation of ≈2 nm metallic Cu nanoclusters is iden-
tified as the active sites during the electrocatalytic reactions.[45]
Similarly, the transient formation of metallic copper nanopar-
ticle during the catalytic reduction of CO2 was identified by
hydrogen evolution reaction (HER) (Figure 1). At −1.278 V
versus reversible hydrogen electrode (RHE) the maximum
FE (42%) was achieved for ethylene and total hydrocarbons
(FE > 60%). Also, we identified that the C2 product selectivity
is dictated by other factors such as nuclearity of the com-
plex system and nature of the carbon support and stirring of
electrolyte. Based on the experimental observation, control-
ling the closeness of copper center, local pH, and limiting
the mass transport during CO2RR essential to enhance the
C2 product selectivity was suggested. The XPS, electron para-
magnetic resonance (EPR), and X-ray absorption near-edge
structure (XANES) spectroscopic characterization revealed
that the molecular nature of the complex system remains
even after 2 h bulk electrolysis of the composite material.
Moreover, the selectivity of the catalyst surpasses all the
other molecular system for ethylene production, though the
selectivity is lower than that of other heterogeneous copper
materials.[47–52]
2. Results and Discussion
 
XAS studies on the Cu N C material with single-site copper
2.1. Synthesis and Characterization
center. Although the postcatalytic analysis of the copper-based
molecular systems reveals that the molecular nature remains
unchanged after electrolysis, the in situ XAS analysis revealed
the involvement of real active site under operating conditions.
However, to enhance the selectivity issues and to understand
more about the real active species responsible for the conver-
sion of CO2 to hydrocarbon, more molecular systems need to
be studied.
In this study, we compared the activity of mononuclear
Cu complex with dinuclear system with tris(2-benzimida-
zolylmethyl)amine (NTB) as common ligand, immobilized
on graphitized mesoporous carbon for CO2RR. In 0.1 m KCl,
the composite displayed higher selectivity for ethylene pro-
duction along with methane and CO from CO2 rather than
The carbon-complex composite has been prepared by mixing
the copper complexes with graphitized mesoporous carbon
(GMC) followed by stirring for 6 h. The strong interaction
between the copper complexes and GMC was observed due to
the π–π stacking of the ligand with carbon support, which is
confirmed by the decrease in absorption intensity in the UV–vis
spectroscopy (Figure S1, Supporting Information) of the com-
plex solution with and without GMC. The intensity of the d-d
band of dimer [Cu2(NTB)2Cl2]2+ at 340 nm was decreased when
GMC was added suggesting the adsorption of the complex onto
the surface of the GMC. Also, it has been previously reported
that the NTB ligand can strongly interact with graphene oxide
sheets.[19]
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The excess or unabsorbed complex was removed by washing
with methanol followed by water, and the composite GMC-
[Cu2(NTB)2] was lyophilized and used as catalyst material. The
similar procedure was used to prepare the GMC-[Cu(NTB)]
using monomeric complex [Cu(NTB)NO3]+ with GMC in
methanol. The structure and activity of the composite GMC-
[Cu2(NTB)2] were investigated by various characterization
methods and discussed next.
has been enhanced after immobilization of the complex over
GMC. Initially, the redox behavior of the GMC-[Cu(NTB)] has
been performed using CV measurements in 0.1 m aqueous
KHCO3 solution saturated with argon or CO2. In argon atmos-
phere the catalytic current for the hydrogen production started
to evolve around −1.0 V versus RHE. After saturated with
CO2, the current observed was negatively shifted due to the
change in pH and reduction of CO2. Only one reduction peak
was observed for the Cu(II) to Cu(I) redox in the CV, whereas
during the oxidation more than two oxidation peaks was
observed resulting from the metallic copper dissolution peak,
which is generated due to the highly applied negative poten-
tial. Also, bulk electrolysis (BE) was conducted at various poten-
tial in the range from −1.27 to −1.57 V versus RHE (Figure S5,
Supporting Information). Interestingly, at −1.37 V versus RHE,
C2H4 production reached the maximum FE of 22% along
with CH4 and CO and H2 were also generated as coproducts
(Figure 2c). Also, the NMR analysis of the electrolyte does not
show any liquid products. Previously, it has been reported that
the crystalline nature of the copper complex enhanced the C2
product selectivity.[44] Based on the C2 selectivity of the mono-
nuclear CuNTB and previous studies on copper molecular com-
plex systems, we anticipated that instead of the mononuclear
complex, copper dimer may enhance the ethylene production
as a result of two nearby copper center for C−C coupling during
the reduction reactions.
Further, CO2 reduction activity of GMC-[Cu2(NTB)2] was
investigated and similar to the GMC-[Cu(NTB)] composite, the
GMC-[Cu2(NTB)2] composite also shows multiple peaks on the
oxidation region of the complex system in the cyclic voltam-
metry (Figure 2b). This kind of multiple redox behavior may be
attributed to the restructuring of the molecular system and dis-
solution of the metallic copper during applied potential. Inter-
estingly, the bulk electrolysis at −1.278 V versus RHE leading
to the formation of ethylene with 35% FE and 12% methane
and the total hydrocarbon selectivity reached 47% (Figure 2d).
Compared to the GMC-[Cu(NTB)], the GMC-[Cu2(NTB)2] shows
enhanced C2 product as well as hydrocarbon selectivity. Previ-
ously, it has been reported that the crystallinity of CuPc plays
an important role in the formation C2H4 with 25% FE, in which
the nearness of copper center was suggested as the origin of
catalytic activity.[44] Similarly, the GMC-[Cu2(NTB)2] composite
displayed higher selectivity compared to the GMC-[Cu(NTB)].
The effect of electrolyte concentration was also studied by
varying the concentration of KHCO3 and observed that the C2
product selectivity has been decreased upon increasing the con-
centration of the electrolyte. Whereas upon changing the elec-
trolyte from 0.1 m KHCO3 to 0.1 m KCl the ethylene selectivity
has been enhanced from 35% to 42% and methane selectivity
from 12% to 18% and overall hydrocarbon selectivity reached
over 60%.
2.1.1. Density Functional Theory (DFT) Calculations
To understand the interaction of the monomer versus dimer
with the GMC, DFT calculation has been performed. From
the DFT results, it is clear that the dimer (interaction energy
−3.11 eV) has strong affinity with the carbon surface compared
to the monomer (−0.70 eV) (Figures S2 and S3, Supporting
Information). Moreover, structural reorganization of the dimer
during the immobilization on the carbon surface has been
observed due to the strong electrostatic interaction of the com-
plex dimer with the GMC. The higher interaction energy may
be resulting from the large size of the dimer and availability
of high surface area to interact with graphene. From the calcu-
lated geometries of the dimeric copper complex with graphene
(the Cu···Cu distance = 3.77 Å), interestingly, the two copper
individual monomers were slightly moved apart leading to the
formation of two monomers at short distance (the Cu···Cu dis-
tance = 4.49 Å). This may be due to the strong interaction of the
copper dimer with graphene and weak bridging ability of the
coordinated chlorines. The charge density analysis reveals that
no significant variation of the charge on Cu atom in monomer
is observed before and after the interaction with carbon sheet
(Table S1, Supporting Information). Whereas, in case of dimer,
one of the Cu atoms gains electrons from the graphene sheet.
This observation is well supported by the frontier molecular
orbital (FMO) analysis (Figures S3 and S4, Supporting Infor-
mation), where highest occupied molecular orbital (HOMO) of
the graphene-[Cu2(NTB)2Cl2] hybrid is predominantly located
on graphene sheet and the lowest unoccupied molecular orbital
(LUMO) is resided on one of the Cu-NTB fragments, which
indicates the charge transfer from graphene sheet to Cu-NTB
fragments of the dimer. On the other hand, in the case of gra-
phene-[Cu(NTB)] hybrid, both HOMO and LUMO are located
on the graphene sheet indicating the absence of charge transfer.
The variation in charge on Cu (oxidation state) atoms in all the
complexes is given in Table S1 (Supporting Information) and
the FMO contours of the same are depicted in Figure S3 (Sup-
porting Information). The difference in charge density on the
copper atoms of the hybrid from the dimeric copper complex
may play some key role during the catalytic reactions because
it has been previously reported that mixed oxidation state of the
[48]

catalytic center can boost the C C coupling.
To evaluate the importance of the carbon support, the dimer
is immobilized on multiwalled carbon nanotube (MWCNT)
and graphene oxide (GO) and studied the catalytic ability of
the composites toward CO2RR (Figures S6 and S7, Supporting
Information). Both MWCNT-[Cu2(NTB)2] and GO-[Cu2(NTB)2]
exhibited lower C2 product selectivity and higher methane
production compared to GMC-[Cu2(NTB)2] (Figure 3a). More-
over, the CO2 reduction product selectivity is also decreased
2.2. Electrochemical CO2 Reduction Activity
The electrochemical CO2 reduction activity of the mono- and
dimeric copper complexes was evaluated by cyclic voltammetry
(CV) (Figure 2). Although the complexes show little activity, it
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Figure 2. a) Cyclic voltammogram of GMC, [Cu(NTB)NO3]+, and GMC-[Cu(NTB)] measured in 0.1 m KCl under N2 (black line) and CO2 (red line)
atmosphere at a scan rate of 10 mV s−1. b) Cyclic voltammagram of GMC, [Cu2(NTB)2Cl2]2+, and GMC-[Cu2(NTB)2] composites. c) faradaic efficiencies
of CO2 reduction products catalyzed by GMC-[Cu(NTB)] at various applied electrode potentials. d) faradaic efficiencies of CO2 reduction products
catalyzed by GMC-[Cu2(NTB)2] at different electrolytes and concentrations at −1.278 V versus RHE.
and favored HER. These results clearly indicate the impor-
can enhance the further reduction of CO to methane as well
as coupling of CO to produce C2 products. Also, possibly
the local pH may be tuned by stirring, which affects the C2
product selectivity. So, the porous nature of the carbon sup-
port also plays a critical role in enhancing the C2 product
selectivity by limiting the mass transport and by changing the
local pH.
The potential dependent analysis was conducted in the range
of −1.025 to 1.575 V versus RHE to find out the peak potential
for ethylene FE (Figure 3c,d). Among the various applied poten-
tial at −1.27 V versus RHE, the highest ethylene production
was observed and upon increasing the potential over −1.27 V,
initially the methane production is slightly enhanced and eth-
ylene production is decreased. Upon further increase of applied
potential, the hydrogen evolution is also increased gradually
and reached around 60% at −1.575 V versus RHE. The dura-
bility of the catalysts was monitored for 2 h and found that the
FE of ethylene production is maintained for 2 h (Figure S8,
Supporting Information). Also, the overall current density is
increased gradually and reached the maximum total current

tance of porous structure of the support material for the C C
coupling during the CO2RR. We assumed that the local pH
may be enhanced during the reduction process inside the

pores, which might enhance the C C coupling to produce
more C2 product compared to the more flat surface carbon
materials such as MWCNT and GO. This kind of porous effect
on the C2 selectivity has been already verified using nanopo-
rous copper material and found that the flow field and local
pH are the responsible factors for the C2 product selectivity.[53]
Moreover, theoretical calculation is also suggesting that at
high pH, the CO–CO coupling pathway can be enhanced to
produce ethylene.[54] To verify the local pH effect during the
CO2RR catalyzed by the GMC-[Cu2(NTB)2], the effect of stir-
ring of the electrolyte on the C2 product selectivity has been
studied (Figure 3b). Interestingly, gradual increase of the rota-
tion speed from 100 to 300 rotation per minute (RPM) leads
to decreased product selectivity of both ethylene and methane
and enhances the CO release and HER. We assumed that
controlling the mass transport inside the pores by stirring
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Figure 3. a) Effect of carbon support on product selectivity catalyzed by GMC-[Cu2(NTB)2] at −1.278 V versus RHE. b) Effect of stirring on product
selectivity catalyzed by GMC-[Cu2(NTB)2] at −1.278 V versus RHE. c) faradaic efficiencies of CO2 reduction products catalyzed by GMC-[Cu2(NTB)2] at
various applied electrode potentials. d) CO2 reduction current density plot at various applied potential catalyzed by GMC-[Cu2(NTB)2].
GMC-[Cu2(NTB)2] show strong broad peaks at 934 and 953.6 eV
density of ≈13 mA cm−2 at −1.278 V versus RHE with 40%
along with satellite peaks of +2 oxidation state of the copper,
which is strongly supporting the immobilization of the com-
plex system over the GMC. After 2 h bulk electrolysis, the Cu
2p XPS spectra of GMC-[Cu2(NTB)2]-BE exhibited less intense
peaks (934.3 eV and satellite peaks) corresponding to +2 oxida-
tion state. Although, partially, the Cu(II) is reduced to Cu(I), the
XPS signal clearly suggests the coexistence of both Cu(II) and
Cu(I) after 2 h BE. Moreover, small peaks observed at 938 and
958 eV have been assigned to the multimeric copper system
formed during the bulk electrolysis.
Further, the high-resolution N 1s spectrum of all those mate-
rials has been performed and the data are shown in Figure S9
(Supporting Information). The deconvoluted N 1s spectra
of [Cu2(NTB)2Cl2]2+ are fitted to three different nitrogens of
NTB ligand. The peak at lowest binding energy of 399.0 eV is
assigned to imine nitrogen of benzimidazolyl moiety coordi-
nated with copper center and the peak at 400.3 eV is assigned
to the uncoordinated amine nitrogen of the benzimidazolyl
moiety. Also, another peak at 399.8 eV is deconvoluted and
assigned to the tertiary nitrogen coordinated with copper
ethylene selectivity.
2.3. Spectroscopic Characterization
Due to the high applied negative potential, local structure
modification is possible during the catalytic reaction. To inves-
tigate the chemical nature of the samples [Cu2(NTB)2Cl2]2+,
GMC-[Cu2(NTB)2], and GMC-[Cu2(NTB)2]-BE, XPS has
been performed and the Cu 2p XPS spectra of all the mate-
rials are shown in Figure 4a–c. The Cu 2p XPS spectra of
[Cu2(NTB)2Cl2]2+ exhibited 2p3/2 and 2p1/2 peaks at 934.3 and
954.2 eV, respectively, along with two strong satellite peaks at
942 and 962 eV corresponding to the +2 oxidation state of the
copper center. The low energy peak is deconvoluted to both
copper(II) and Cu(I) oxidation states and usually the dimeric
copper complexes exhibit both Cu(II) and Cu(I) signals due
to the reduction of some of the copper by intensity of the
X-ray during the measurements. The Cu 2p XPS spectra of
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Figure 4. a) Cu-2p XPS spectrum of [Cu2(NTB)2Cl2]2+. b) Cu-2p XPS spectrum of GMC-[Cu2(NTB)2]. c) Cu-2p XPS spectrum of GMC-[Cu2(NTB)2]-BE.
d) Comparative XANES spectra of [Cu2(NTB)2Cl2]2+ and GMC-[Cu2(NTB)2] and GMC-[Cu2(NTB)2]-BE with Cu reference materials. e) EPR spectra of
[Cu2(NTB)2Cl2]2+, GMC-[Cu2(NTB)2], and GMC-[Cu2(NTB)2]-BE. f) Dark-field HR-TEM image of GMC-[Cu2(NTB)2]-BE. g,h) Bright-field HR-TEM image
of GMC-[Cu2(NTB)2]-BE.
center. The similar N 1s spectra obtained for the before bulk
electrolyzed sample GMC-[Cu2(NTB)2] suggest the immobili-
zation of the complex over the GMC support. However, after
2 h bulk electrolysis, the sample GMC-[Cu2(NTB)2]-BE exhib-
ited a different peak shape and the N 1s peak is deconvoluted
in a similar way to the before bulk electrolyzed sample. In addi-
tion to both coordinated and uncoordinated nitrogens, another
peak is deconvoluted for the uncoordinated imine nitrogen
of the benzimidazolyl moiety as the intensity of the peak is
slightly enhanced around 399 eV. We anticipated that structural
reorganization of the complex can happen at the applied nega-
tive potential and possibly the trigonal bipyramidal geometry
cannot be retained and one of the benzimidazolyl moiety may
be detached to attain the planarity and to bind strongly with
the GMC surface. Overall, the XPS spectral results clearly sug-
gest both the oxidation changes of the copper center as well as
the structural reorganization of the complex system at negative
applied potential.
To further investigate the changes in the oxidation state of
the copper center after bulk electrolysis, XAS analyses were
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conducted. The copper K-edge XANES spectrum of GMC-
[Cu2(NTB)2]-BE along with original complex and after immobi-
lization are shown in Figure 4d. The absorption edge energy
of XANES can be very useful for determining the oxidation
state and geometry of the metal center in the samples. Because
higher X-ray energy is required to eject a core electron from a
metal center with high charge, the higher oxidation state of the
X-ray-absorbing metal can increase the absorption edge energy
and linear relationships between the absorption edge energy
and the oxidation state can be observed experimentally.[55,56]
To understand the spectral pattern of the samples, the XAS of
reference materials such as metallic Cu, Cu2O, and CuO have
been performed and presented in Figure 4d. In the XANES
spectrum of the [Cu2(NTB)2Cl2]2+ and GMC-[Cu2(NTB)2], the
absorption edge energy is located close to that of CuO, indi-
cating that the valence state of copper in the dimer complex and
GMC-[Cu2(NTB)2] is close to +2. The Cu K-edge XANES spec-
trum of [Cu2(NTB)2Cl2]2+ and GMC-[Cu2(NTB)2] also shows
very weak pre-edge peak at 8978 eV which corresponds to the
characteristic 1s→3d transition. The bulk electrolyzed sample
GMC-[Cu2(NTB)2]-BE also shows similar characteristic 1s→3d
transition, which is assigned to the Cu(II) center. The copper K
pre-edge is not observable for Cu(I) species because of the com-
pletely filled d10 system. Whereas, the electric dipole allowed
1s to metal 4p orbital transition can be useful in predicting the
geometry of the system. Previously, it has been observed that
the energy and intensity of the signal depends on the ligand
environment of the metal center.[57] So, the new rising edge
observed at 8982.6 eV is assigned to the 1s→4p transition of
the Cu(I) center with three coordinate geometries.[57–61] The
rising edge observed at 8981 and 8981.5 eV, respectively, were
also different from the XANES spectra of the metallic copper
and Cu2O. The observed spectral behavior also suggests that
because of the applied potential the immobilized complex
undergoes structural reorganization. Similar to the XPS results,
possibly one of the coordinated nitrogen atom may be detached
from the coordination sphere to form more planar structure
due to the strong π–π interaction of the ligand with GMC. Both
XPS and XANES results clearly support the structural modifi-
cation and change in oxidation state of the samples after bulk
electrolysis.
EPR analysis has been performed to further investigate
the change in the local structure of the metal center during
electrolysis and the EPR spectra of [Cu2(NTB)2Cl2]2+, GMC-
[Cu2(NTB)2], and GMC-[Cu2(NTB)2]-BE are shown in Figure 4e.
The glass formed from the acetonitrile solution of the dinuclear
[Cu2(NTB)2Cl2]2+ complex exhibited the rhombic EPR signals
with three g values at g = 2.261, 2.153, and 2.004 corresponding
to the dimeric nature of the complex system. Previously, it has
been reported that several chloride bridged dimeric copper
complexes are exhibiting rhombic character and usually the
hyperfine splitting is not well resolved.[62] After immobilization
of the complex on GMC, the isotropic signal is observed indi-
cating that the dimeric nature is modified during the immo-
bilization of the complex over GMC. This result is in agree-
ment with the theoretical calculation, which is also supported
by the fact that each copper monomer is moved slightly apart
over the GMC surface. The bulk electrolyzed sample GMC-
[Cu2(NTB)2]-BE also exhibits isotropic broad signal at g⊥ of
2.071 corresponding to Cu(II) center, due to the restructuration
of the complex system after bulk electrolysis. Different from the
GMC-[Cu2(NTB)2]-BE, the monomeric complex sample GMC-
[Cu(NTB)]-BE exhibited tetragonal distortion and hyperfine
splitting and the monomeric character is retained after bulk
electrolysis (Figure S10, Supporting Information).
The XPS, XANES, and EPR spectral data suggest the struc-
tural reorganization and partial reduction of Cu(II) to Cu(I)
after the bulk electrolysis of the immobilized complex. In
addition, the molecular nature of the system is retained as the
spectral behavior still remains even after 2 h bulk electrolysis.
However, it may be due to the reversible restructuring of the
catalytic systems and the real active site under operating condi-
tions may be different from this observation. Also, previously it
has been observed that reversible formation of the copper nano-
particle is the real active site under operating conditions.
So, we further conducted the morphological studies using
high-resolution transmission electron microscopy (HR-TEM)
and the results are shown in Figure 4f–h. The dark-field HR-TEM
analysis of the sample GMC-[Cu2(NTB)2]-BE does not show any
appreciable difference between the samples before and after bulk
electrolysis (Figure S11, Supporting Information). However, the
bright-field HRTEM analysis shows the formation of thin copper
clusters during catalytic reactions. Although the spectral analysis
of the bulk electrolyzed sample shows molecular nature, the
morphological analysis shows the formation of copper cluster
during catalytic reactions. So it is hypothesized that the copper
cluster possibly stabilized by the ligands might be the real active
site during the catalytic reduction of CO2.
3. Conclusion
In summary, we used the simple immobilization strategy to
disperse the copper(II) complexes over graphitized mesoporous
carbon and used as catalysts for CO2RR. Unlike metal com-
plex as such, immobilization over GMC enhanced the catalytic
activity for the electrochemical reduction of CO2 to ethylene,
methane, and CO with high C2 product selectivity. Compared
to previously reported copper molecular systems, the GMC-
[Cu2(NTB)2] composite displayed high selectivity for ethylene
(>40%) during electrolysis. The spectroscopic analyses such as
XPS, EPR, and XANES suggest that the molecular nature of the
system was not completely lost even after 2 h bulk electrolysis
possibly due to restructuring. Compared to the composite with
mononuclear copper center, the dimer composite exhibiting
high selectivity for C2 product may be due to the nearness of
the copper center. Although the nuclearity of the system is not

the general criteria for C2H4 selectivity, it enhances the C C
bond-forming step in our composite system. Also, the mor-
phological study clearly suggests the newly generated copper
cluster might be the real active site under operating conditions.
The importance of diatomic copper sites for C2 product selec-
tivity on the carbon support is under progress.
4. Experimental Section
Materials: Carbon mesoporous with average pore size of 6 and
13 nm, Nafion 117 solution (Sigma Aldrich), nitrilotriacetic acid,
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1,2-phenylenediamine, copper(II) nitrate trihydrate, and carbon paper
(Toray) (Alfa Aesar) were purchased and used as received. Organic
solvents such as methanol, ethanol, and acetonitrile (Daejung,
Korea) were used without further purification. The ligand tris(2-
benzimidazolylmethyl)amine (NTB) was synthesized as reported
earlier.[63]
Synthesis of Copper(II) Complexes: Cu complexes [Cu(NTB)NO3]NO3
and [Cu2(NTB)2Cl2]Cl2 were prepared as reported previously.[64] Briefly,
[Cu(NTB)NO3]NO3 was prepared by mixing copper(II) nitrate dissolved
in ethanol with NTB in ethanol and stirred for 30 min. The greenish blue
precipitate was filtered and washed with ethanol and then dried under
vacuum. [Cu2(NTB)2Cl2]Cl2 was prepared from CuCl2 and NTB by mixing
with a molar ratio of 1:1 in methanol/acetonitrile mixture and stirred for
1 h. Afterward, the complex was dissolved by adding more methanol and
recrystallized by slow evaporation of the solvent. The resultant yellow
crystals were filtered and washed with cold methanol and dried in air
for 1 d.
GMC-Cu Complex Composite Preparation: First, the mesoporous
carbon (4 mg) was dispersed in 1 mL of acetonitrile:ethanol mixture
(90:10) by sonication for 5 min. Then (1.25 wt%) Nafion solution
was introduced and the stirring was continued for another 5 min
to completely disperse the carbon. Followed by (2 mg) copper(II)
complexes were added in to the carbon-Nafion composite and shaken
well for 6 h to attain the immobilization of the complex and equilibrium
condition. Later, the composite was centrifuged and washed with
methanol (1 mL) twice and centrifuged. The final composite material
was freeze-dried to get dry powder.
Characterization: The UV–visible spectra were measured using Agilent
8453 photodiode array spectrophotometer. The surface characteristics
of the catalysts were examined by X-ray photoelectron spectroscopy
(K-Alpha, Thermo Scientific Inc., UK). The XPS samples were prepared
by embedding the catalyst powders in the surface of indium foils. The
obtained XPS spectra were calibrated on the basis of the C1s peak by

correcting the C C peak position to 284.5 eV. The chemical structures
of the metal sites in the catalysts were further investigated by X-ray
absorption spectroscopy. XANES analyses were conducted using
the BL10C beam line at the Pohang light source (PLS-II). The storage
ring was operated with a ring current of 350 mA at 3.0 GeV operated
in top-up mode. A monochromatic X-ray beam was obtained using a
liquid-nitrogen-cooled Si double-crystal monochromator. XAS spectra
were acquired in fluorescence mode under ambient conditions using a
passivated implanted planner silicon (PIPS) detector. Energy calibration
was carried out for each metal atom with reference metal foils. The
obtained XANES spectra were analyzed with the ATHENA software
packages following standard procedures.[66] EPR was performed using
a Bruker EMX/Plus spectrometer equipped with a dual mode cavity
(ER 4116DM). Low temperatures were achieved and controlled using a
liquid He quartz cryostat (Oxford Instruments ESR900) with gas flow
controller (Oxford Instruments ITC503). The experimental conditions
are as follows: Microwave frequency 9.64 GHz (perpendicular mode),
modulation amplitude 10G, modulation frequency 100 kHz, microwave
power 0.94 mW (perpendicular mode), and temperature 5.7 K. Ten scans
were added for each spectrum.
Computational Details: All the geometry optimizations were carried
out with DFT-based B3LYP hybrid functional in conjunction with 6-31G
(d,p) Pople basis set.[65] The calculations were done with G16 suite of
programme. Due to the complexity of the system, the vibrational analysis
was not carried out for the complex systems. However, vibrational
analysis was done for monomer and dimer forms of the substrate (Cu-
NTB) to ensure that they are global minima on the potential energy
surface.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Electrochemical Measurements: The electrochemical measurements
were carried out using H-cell with a conventional three-electrode
configuration. An Ag/AgCl electrode was used for the reference, and
Pt wire was used as the counter electrode. The working and counter
electrodes were separated by a Nafion 117 membrane. Carbon paper
was used as the working electrode. Electrical contact to the carbon
paper was made by fixing a copper wire at the edge of the carbon paper
using silver paste, and the electrical contact was sealed with epoxy resin
exposing a 0.5 × 0.5 cm2 area of the carbon paper as the working area.
To immobilize the catalyst on the carbon paper, the catalyst (4 mg) was
dispersed in ethanol (1 mL) by sonication, and then, a sample of the
as-prepared ink (60 µL) was drop-casted onto the carbon paper and
Acknowledgements
This research was supported by Creative Materials Discovery Program
through the National Research Foundation of Korea (NRF) funded by
Ministry of Science and ICT (NRF-2017M3D1A1039377), the Global
Frontier R&D Program of the Center for Multiscale Energy System
funded by the National Research Foundation under the Ministry of
Science and ICT, Korea (2012M3A6A7054855), the National Research
Foundation of Korea (NRF) grant funded by the Korea government
(MSIT) (NRF-2017R1A2B3012003), the KIST-SNU Joint Research Lab
project (2V06170) under the KIST Institutional Program by the Korea
government (Ministry of Science and ICT), BK21PLUS SNU Materials
Division for Educating Creative Global Leaders (21A20131912052). K.T.N.
appreciates the support from Institute of Engineering Research and
Research Institute of Advanced Materials (RIAM) and Soft Foundry at
Seoul National University.
dried with a heat gun, resulting in a catalyst loading of 0.24 mgcm−2
.
Electrochemical measurements were conducted with a potentiostat
(CHI600D, CH Instruments, USA). The electrolyte solutions were
purged and saturated with Ar or CO2 for 30 min before measuring the
CV or bulk electrolysis. During the CV measurements, the electrolyte
was stirred to effectively supply dissolved CO2 to the electrode surface
and detach the produced bubbles from the electrode. In addition, the
electrode potentials measured with an Ag/AgCl reference electrode were
converted to potentials versus the RHE.
Conflict of Interest
The authors declare no conflict of interest.
To quantify the product amounts, a gas chromatograph equipped
with a thermal conductivity detector (TCD) and a flame ionization
detector (FID) (PerkinElmer, NARL8502 model 4003) was used. The
gas products were separated with a Hayesep N and a Molesieve 13X
column. Hydrogen, oxygen, and nitrogen were detected by TCD, and all
other products were detected by FID. After the bulk electrolysis, 0.1 mL
of gas was sampled from the headspace of the electrochemical cell and
injected into the GC (Figure S12, Supporting Information). The number
of moles was calculated from the area of the peaks and head space
volume, and the area was calibrated using a standard gas (0.1 mol%
of each gas species). All the FE values given in the manuscript are an
average of three runs.
Keywords
CO2 reduction, ethylene production, immobilization, mesoporous
carbon, molecular copper complex
Received: February 15, 2020
Revised: April 17, 2020
Published online:
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