Controlling the C2+ product selectivity of electrochemical CO2 reduction on an electrosprayed Cu catalyst

Article abstract of DOI:10.1039/c9ta13173f

Cu catalysts prepared by modifying bulk Cu foils have achieved high performance for value-added C2+ compounds from electrochemical CO₂ reduction (CO₂RR) but the transformation of active sites can be affected by the bulk substrate, which make it complex to design the catalyst. Herein, we newly introduce a simple electrospray pyrolysis method to take advantage of a facile wet-chemical synthesis applicable on non-copper substrates, such as a porous carbon paper, and demonstrate highly enhanced selectivity for C₂H₄ production from CO₂RR. The electrosprayed copper oxide on the carbon paper showed uniquely improved C2 selectivity compared with that on the copper substrate. The improved performance is proposed to be related to the presence of Cu mixed state and retention of morphology of the electrosprayed catalyst on the carbon paper, showing the importance of the substrate. In addition, the C2 product selectivity can be tuned by the electrospray synthesis time as it affects the size of the surface nanostructure as well as the porosity of the catalyst, which can provide an effective way to regulate the C2/C1 ratio.

Full text of DOI:10.1039/c9ta13173f

View Article Online
View Journal
Journal of
Materials Chemistry A
Materials for energy and sustainability
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. Y. Lee, S. Y.
Chae, H. Jung, C. W. Lee, D. L. T. Nguyen , H. Oh, B. K. Min and Y. J. Hwang, J. Mater. Chem. A, 2020,
DOI: 10.1039/C9TA13173F.
Volume
Number 12
28 March 2018
Pages 4883-5230
6
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry A
Materials for energy and sustainability
rsc.li/materials-a
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2050-7488
COMMUNICATION
Zhenhai Wen et al.
An electrochemically neutralized energy-assisted low-cost
acid-alkaline electrolyzer for energy-saving electrolysis
hydrogen generation
rsc.li/materials-a

Please do not adjust margins
Journal of Materials Chemistry A
Page 1 of 8
View Article Online
DOI: 10.1039/C9TA13173F
ARTICLE
Controlling C2+ Products Selectivity of Electrochemical CO2
Reduction on Electrosparyed Cu Catalyst
Si Young Leea, b, Sang Youn Chaea, Hyejin Junga, b, Chan Woo Leea, c, Dang Le Tri Nguyena, b, d, Hyung-
Received 00th January 20xx,
Accepted 00th January 20xx
Suk Oha, b, Byoung Koun Minb, e, Yun Jeong Hwang*a, b, f
DOI: 10.1039/x0xx00000x
Cu catalysts prepared by modifying bulk Cu foils have achieved high performance for value-added C2+ componds from
electrochemical CO2 reduction (CO2RR), but active sites transformation can be affected by the bulk substrate which make it
complex to design catalyst. Here, we newly introduce a simple electrospray pyrolysis synthesis to take advantage of facile
wet-chemical synthesis applicable on non-copper substrate such as porous carbon paper, and demonstrate highly enhanced
selectivity for C2H4 production from CO2RR. The electrosprayed copper oxide on the carbon paper was showed uniquely
improved C2 selectivity compared with that on the copper substrate. The improved performance is proposed to be related
with having Cu mixed state and keeping the morphology of the electrosprayed catalyst on the carbon paper showing the
importance of the substrate. In addition, the C2 production selectivity can be tuned by the electrospray synthesis time as
affecting the size of the surface nanostructure as well as the prososity of the catalyst, which can provide an effective way
regulate the C2/C1 ratio.
CO2RR can also produce value-added multi-carbon chemicals such as
1. Introduction
C2H4, which is widely used in polymer synthesis and the chemical
12
engineering industry.
To produce multi-carbon chemicals,
The steadily rising levels of atmospheric carbon dioxide (CO2) are of
growing concern to cause current global climate change.1, 2 Although
CO2 can be fixed via photosynthesis, the natural carbon cycle is not
enough to compensate the currently increasing excessive CO2
emissions from fossil fuels that humans heavily rely on.3 For
sustainable carbon cycle, additional carbon fixation processes such
as carbon capture and utilization (CCU) technology have been
proposed, and the electrochemical CO2 reduction reaction (CO2RR)
has gained interest as one of these promising carbon processes. CO
or HCOO- have been aimed for the promising chemicals from CO2RR
due to techno-economical analysis and life cycle assessment
consideration. They can be used for Fischer-Tropsch synthesis or
understanding C-C bond formation reaction is important, and
notably Cu is a unique single metal element to have promising
production of C2+ compounds.13, 14 The activity and stability of C2H4
production has been reported to be affected by sub-surface oxygen
or oxidized Cu species, as with the Cu-O catalyst.15-19 C2H4 production
activity can also be effectively increased by controlling the surface
structure of metallic Cu, or inducing a local pH gradient by mass
transfer.20-22 However, because of the complexity of the CO2RR
products as well as the variety of Cu-based catalyst preparation
methods, heavy researches are still underway to develop a coherent
descriptor for selective activity. Furthermore, beyond the nature of
the Cu surface, recent studies have demonstrated the importance of
catalyst loading systems, and electrochemical cell design to improve
CO2RR activity.23 The catalyst has been applied to a porous carbon
electrode as a gas diffusion layer (GDL) electrode, or a specially
designed flow cell system.17, 24, 25 However, to improve the design of
such catalyst systems, it is still necessary to better understand how
the selectivity of C2+ products is affected on the catalyst level.
5
hydrogen storage.4, Various metals (Ag, Au, Zn, Bi, and Pd) and
modified carbon (N-doped carbon, Fe-N-C and Ni-N-C etc) materials
have been developed which have high performance, product
selectivity, and up to ~ 95% Faradaic efficiency (FE).6-11
a. Clean Energy Research Center, Korea Institute of Science and Technology, 5,
Hwarang-ro 14-gil, Seongbuk-gu, Seoul, 02792, Republic of Korea.
b. Division of Energy and Environmental Technology, KIST School, Korea University
of Science and Technology, Hwarang-ro 14 gil 5, Seongbuk-gu, Seoul, 02792,
Republic of Korea
Cu-based electrocatalysts for CO2RR have been wet-chemically
synthesized from metal precursor solutions to control their
morphologies. A separate process is then typically employed to load
the catalyst onto the electrode substrate, and binder chemicals are
often added, which complicates understanding of catalytic activity as
well as preparation of the catalyst electrode. Meanwhile, many of
the high performing Cu catalysts have been directly prepared by
modifying the bulk Cu polycrystalline foil surface19, 26, 27, but those
catalysts on the foil are difficult to directly transfer to a porous
c. Department of Chemistry, Kookmin University, Seoul 02707, Republic of Korea
d. Institute of Research and Development, Duy Tan University, Da Nang 550000,
Vietnam
e. Green School, Korea University, 145, Anam-ro, Seonbuk-gu, Seoul, 02842,
Republic of Korea
f. Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul
03722, Republic of Korea
†
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 2 of 8
ARTICLE
Journal Name
electrode application. In addition, the morphology of the copper
catalyst is known to be transformed chemically or electrochemically
during CO2RR18, 28, 29, and the effect of the underlying copper foil
cannot be ignored. Given these limitations, a new and simple wet-
chemical synthetic approach can allow scalable preparation of the
catalyst electrode with wide interests.
2. Experimental Section
View Article Online
DOI: 10.1039/C9TA13173F
One-step Synthesis of CuO catalyst-substrate by Electrospray
Pyrolysis : The CuO spray nanoparticle (CSNP) catalyst was
synthesized using an electrospray system (Nano NC, NNC-ESP
200T; Korea) on a porous carbon paper substrate (Toray, TGP-
H-120). The precursor solution for the electrospray deposition
was prepared using 0.03 M of copper (II) nitrate trihydrate (77 -
80% Cu(NO3)2 3H2O assay, Shinyo pure chemical) dissolved in
ethanol (99.8%, Sigma-Aldrich), and was sprayed through a
stainless steel needle tip (27 G; 13 mm) using a syringe pump
system (NORM-JECT®; Luer Lock; Hheke Sass Wolf Germany).
During the electrospray pyrolysis, the carbon paper substrate (4
In this study, we developed a one-step synthesis method to prepare
oxidized Cu catalysts, using electrospray pyrolysis on porous carbon
papers (Scheme 1). This method has the advantages of simultaneous
catalyst synthesis from precursor solution and electrode fabrication.
It also eliminates the need for a separate process to spray the
catalyst onto the substrate after synthesis. Compared with a Cu foil
catalyst, the prepared CuO catalyst-porous carbon support showed
threefold enhanced CO2RR performance, especially for C2H4
production. These results suggest the interaction of both the catalyst
and the substrate is important in CO2RR. We also observed that the
synthesized nanoparticles enlarged and shape changed with
increasing reaction time, and this enlargement was related to a
decrease in C2H4 selectivity. The ratio of C2H4 / CH4 could be
controlled by adjusting the amount of sprayed catalyst. With this
structural improvement and chemical state changes, our catalyst
showed a FE of C2H4 up to 52%, and also showed higher than 70%
selectivity for C2+ compounds. In addition, the FE for the hydrogen
evolution reaction (HER) was significantly suppressed, down to 10%.
Our synthesis method can be widely applied to other conductive
substrates and CO2RR reactor systems, including the fabrication of
porous electrodes.
×
cm
4 cm) was heated to 350 ℃ on a temperature control
plate (TZ4M, Autonics) and a high direct current (DC) voltage
between the nozzle and the carbon paper substrate was
adjusted to 20 kV to change the solution droplets to an aerosol,
and then sprayed. The distance from the needle tip to the
substrate was fixed at 9 cm, and the spray precursor solution
was pumped at a rate of 15 μl/min. To ensure uniformity of
electrospray on the substrate, the substrate was placed
×
between PTFE masks with a hole of 5.5 cm
5.5 cm. For a
control sample, CSNP was prepared on a Cu foil (Alfa Aesar,
99.9999 %) under the same conditions, except Cu foil was used
as the substrate.
Electrochemical Measurement of the CO2 Reduction Reaction :
The performance of the prepared catalysts for CO2RR was
measured in an electrochemical cell divided into two sections
by an anion exchange membrane (Selemion AMV) to avoid the
cross-over of gaseous products as well as liquid products. This
electrochemical cell was made of polyether ether ketone (PEEK)
which has strong chemical resistance. A CSNP sample working
electrode, and an Ag/AgCl reference electrode (Basi, 3 M NaCl)
were placed in the cathode section, and a platinum counter
electrode was put in the other part. The geometrical areas of all
samples were fixed to be 0.503 cm2 for better comparison. All
of the electrochemical potentials and currents were measured
by potentiostat (Iviumtechnology). To prevent contamination,
all of the apparatus were washed prior to CO2RR, and the 0.1 M
KHCO3 (Sigma-Aldrich, 99.95%) electrolyte solution was
prepared with deionized water (18.2 MΩ). At every applied
potential, the working electrode sample was replaced with a
fresh one to prevent unpredictable contamination and surface
state change. For long-term stability, CO2 RR was conducted at
one fixed potential. The measured potential values were
converted to reversible hydrogen electrode (RHE) using the
equation below.
푬푹 (퐯퐬 퐑퐇퐄) = 푬푨 (풗풔 푨품/푨품푪풍) + ퟎ.ퟐퟎퟗ 푽 + ퟎ.ퟎퟓퟗퟏퟔ × 풑푯
To compensate the resistance loss, solution resistance (Rs) was
analyzed by electrochemical impedance spectroscopy (EIS)
from 1000 to 0.1 Hz, and general values of Rs were fitted, and
found to be 75~80 Ω in CO2 saturated 0.1 M KHCO3 whose pH
was measured to be 6.8.
Scheme 1. One-step Cu based electrode preparation by using electrospray pyrolysis
method
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 8
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
Analysis of CO2 Reduction Reaction Products : The gas products the chemical state of the catalyst were analyzed by high-
View Article Online
DOI: 10.1039/C9TA13173F
at each fixed potential were quantitatively detected using a gas performance X-ray photoelectron spectroscopy (HP-XPS,
chromatograph (GC; Agilent 6890N), which was direct linked to Thermo-Fisher Scientific, K-Alpha XPS system) equipped with an
the top of the cathode. The sample injection was controlled by Al-Ka monochromator (1486.6 eV).
a six-port valve. During CO2 RR, high purity CO2 gas (≥99.999%) Measurement of In-situ/operando XAS : Cu K-edge X-ray
was continuously flowed without interruption in each absorption spectroscopy (XAS) was performed using hard X-rays
compartment of the electrochemical cell. The CO2 gas flux was at the 1D beamline of the Pohang Accelerator Laboratory (PAL)
regulated with a mass flow meter to maintain 20 mL/min, which to characterize the Cu status in-situ/operando CO2RR condition.
was measured at an outlet by a universal flow meter (Agilent, X-ray adsorption near-edge structure (XANES) and extended X-
AMD 2000 and ADM Flow meter). A carbonxene 1000 (15ft, ray absorption fine structure (EXAFS) were employed to analyze
Supleco) packed column was installed in the GC to separate H2, standard samples, and the catalyst was also analyzed through
CO, CH4, C2H4, and C2H6. The GC had two detectors, a thermal the measured spectra.
conductivity detector (TCD) and a flame ionization detector The CSNP/CP working electrode was masked with Kapton
(FID) with a methanizer for measuring H2 and others carbon polyamide tape and installed in the electrochemical cell facing
based gases (CH4, CO, C2H4, C2H6), respectively. The carrier gas the backside of the electrode in the window of the
was Ar (≥99.999%).
푪풙 × 풒 × 풑
electrochemical cell, to prevent interruption by the aqueous
electrolyte. The synchrotron radiation beam was positioned to
illuminate the backside of the working electrode and XAS was
measured using a fluorescence detector. Identical CO2RR
conditions were applied during the XAS measurements. High
purity (99.999%) CO2 gas was continuously flowed into the 0.1
M KHCO3 electrolyte. A Pt coil and an Ag/AgCl electrode were
used as the counter and reference electrodes, respectively. A
potentiostat (Ivium Vertex) was used to generate a potential
difference of -1.0 V vs RHE at the working electrode. Under the
CO2RR conditions, X-ray absorption spectra were measured at
5, 35, 65 and 105 min. Each spectrum measurement took 30
min.
풊풙 =
× 풏풙푭
(ퟏ)
(ퟐ)
푹푻
풊풙
퐅퐄 (%) =
× ퟏퟎퟎ %
풊풕풐풕풂풍
Equation (1) was used to calculate the partial current (ix) of each
of the products (x= H2, CH4, CO, C2H4, or C2H6). Cx is the
volumetric concentration of products, q is the gas flux, p is the
pressure, R is the ideal gas constant (8.314 m3∙Pa/K∙mol), and T
is the temperature. nx is the equivalent number of electrons
required by CO2 RR for each product x, F is the Faraday constant
(96485 s∙A/mol). Then, the Faradaic efficiency (FE) was obtained
from Equation (2) where itotal is the total current density
measured by chronoamperometry during CO2 RR. The 13CO2
(Cambridge Isotope Laboratories, Inc.; 99%) isotope
experiments were performed using gas chromatograph and
mass spectroscopy (GC-MS; Agilent), composed of 6890 GC
system and 5973 mass selective detector.
The liquid products were collected at the catholyte after bulk
electrolysis and quantitated by nuclear magnetic resonance
(NMR; DD2, Agilent, 600 MHz). The formate (HCOO-) was
quantitatively analyzed using the phenol internal standard, and
the other liquid products were analyzed with the DMSO internal
standard. The FE of each liquid product was calculated using
3. Results and discussion
The catalyst synthesized with electrospray pyrolysis in carbon paper
was the shape of a cluster of nanoparticles that grew well on the
carbon fiber of carbon paper and had a size of 30 to 40 nm (Fig. 1a
and b). An XRD and Cu K-edge XANES analysis was conducted to
confirm the crystal structure and chemical composition of copper
Equation (3).
푳풄 × 푽풄풂풕풉풐풍풚풕풆 × 풏풚푭
푭.푬.풍풊풒풖풊풅 (%) =
(ퟑ)
푪풕풐풕풂풍
Ctotal is the total charge during bulk electrolysis, Lc is the
detected liquid product concentration. The catholyte volume is
expressed as Vcatholyte, and the electron demand of each liquid
product (y= HCOO-, C2H5OH, C3H7OH and etc.) is expressed as
ny.
Material Characterizations : The surface morphology of each
working electrode was analyzed by field emission scanning
electron microscopy (FE-SEM, FEI, Inspect F) and the
morphology was analyzed in more detail by transmission
electron microscopy (TEM, TitanTM and TechnaiTM). The crystal
structure was confirmed by grazing incidence X-ray diffraction
(GI-XRD, Rigaku corporation, D/Max 2500). The differences in
Fig. 1 SEM image, XRD pattern and ex-situ Cu K-edge XANES spectra of the; a) and b)
CSNP/CP; c) XRD pattern of CSNP/CP and carbon paper; d) Cu K-edge XANES spectra
of CSNP/CP and reference material
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 4 of 8
ARTICLE
Journal Name
View Article Online
DOI: 10.1039/C9TA13173F
Fig. 2 Characterization of CSNP and Cu foil sample; a) comparison of each sample’s performance; b) gas products stability of CSNP Cu; c) CO2RR 2 hours post sample of CSNP/CP
SEM image; d) CO2RR 30 hours post sample of CSNP/CP
electrospray nanoparticle (CSNP) catalyst synthesized on carbon produced and no CO2RR production was observed indicating CSNP
paper. The crystal structure of CSNP catalyst could be estimated as catalyses reduction reaction of CO2 (Fig.4b).
the CuO crystal structure (JCPDS #05-0661) through CuO main peak In addition, CSNP/CP was able to maintain this high C2H4 selectivity
at 35.6 and 38.8 degrees, excluding peak from carbon paper using and suppressed HER activity for 20h, but after that time C2H4
XRD analysis (Fig. 1c). In addition, XANES spectra of CSNP was similar selectivity slowly decreased and HER was increased (Fig. 2b). In this
to XANES fingerprint of CuO when compared with Cu K-edge XANES stability experiment, the CSNP/CP showed that the particle shape
spectra of Cu-foil, Cu2O and CuO reference materials (Fig. 1d). These and size changed with reaction time. After 2 hours of CO2RR, it is not
results confirm that the initial state of the synthesized CSNP catalyst much different from initial morphology (Fig. 2c). However, after 30
was CuO.
hours of reaction, the structure of the catalyst was electrochemically
When the CuO spray-nanoparticles were prepared on carbon paper reconstruction, and small nanoparticles of 30 to 40 nm were almost
(CSNP/CP) and carried out linear sweep voltammetry (LSV) in CO2 - or disappeared and changed into large particle of various shapes (Fig.
Ar- saturated electrolyte condition, current density became larger in 2d).
CO2-saturated electrolyte under the high overpotential ranges (Fig. To understand why C2 selectivity was increased and HER was
S1). This might be because CSNP catalyze HER in the absence of CO2 suppressed in CSNP/CP compared to Cu foil, CSNP was also
condition, but when CO2 is present, CO2 or CO2RR intermediates synthesized on Cu foil (CSNP/Cu) to compare CO2RR performance
adsorb on the catalyst surface which block HER active sites. So, it (Fig. S3). Our expectation was that CSNP/Cu would have high C2H4
significantly increased C2H4 production selectivity and suppressed selectivity and HER suppressed effects. The CSNP/Cu, however, had
HER, compared with bare Cu-foil at high overpetential region (Fig. high selectivity for C2H4 in CO2RR products, but still high hydrogen
2a). In Cu foil case, the selectivity for C2H4 was poor (FEC2H4 = 13.6%), selectivity (FEH2 = 56.6%) (Fig. 3a). When electrospraying the same
but a further disadvantage was that almost half of the total reaction volume of precursor solution (2mL), the catalyst amount of CSNP/Cu
current was used for HER (FEH2 = 46.7%). In contrast, the CSNP/CP (59.2 μg/cm2) was higher than that of CSNP/CP (38.4 μg/cm2), but
reduced HER selectivity by 19.8%, but C2H4 generation selectivity CSNP/CP showed better performance. The difference HER
increased to 48.7%. We performed 13CO2 isotope experiment to performance in CSNP according to these substrates was presumed to
confirm carbon origin of CO2RR products is indeed CO2 molecule (Fig. be because the HER activity of carbon paper was much lower than
S2). When normal CO2 was used, the major signal was observed at that of Cu foil (Fig. S4). In the previous study, when copper oxide
MW=28 and no signal at 30, a typical 12C2H4 spectrum. Meanwhile, catalysts were electrodeposited on the Cu-foil substrate30, 31, it was
with 13CO2 experiment, mass spectrum was shifted by two having ambiguous whether the bottom Cu foil affected CO2RR. In our
signal at MW = 30 (i.e. 13C2H4). In addition, we also confirmed that comparison, CO2RR to C2H4 production was greatly enhanced when
when only carbon paper was used as a cathode, only H2 was the CSNP was loaded on carbon paper, suggesting Cu foil is inferior
to carbon paper as a CO2RR support.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 8
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
View Article Online
DOI: 10.1039/C9TA13173F
Fig. 3 The CSNP/Cu and CSNP/CP performance compared and Cu 2p3/2 XPS peak and in the CSNP/CP catalyst, Cu K-edge in situ/operando X-ray absorption spectroscopy analysis; a)
FE compared with CSNP/Cu and CSNP/CP; b) Cu 2p3/2 XPS peak deconvolution for pre CO2RR CSNP sample c) Cu K-edge EXAFS spectra of Cu foil and in situ/operando CSNP/CP
data; d) Cu K-edge XANES spectra of Cu foil and Cu2O and in situ/operando CSNP/CP data
In addition, the morphology change of CSNP/CP by reaction time was morphology transformation and form defective sites on the carbon
observed through SEM (Fig. S5), the particle size increased to more support32, 33, which contribute to enhancement of CO2RR-to-C2H4
than 100 nm after 15 h CO2RR and at that moment C2 selectivity conversion.
began to decrease (Fig. S5b and Fig. 2b). After 30 hours, particle of Interestingly, the total current density of CSNP/Cu and CSNP/CP was
200 nm and cube of 300 ~ 400 nm were formed (Fig. S5c), and after similar, compared to the significant difference in HER performance
60 hours only 300 ~ 400 nm cubes remained (Fig. S5d). Therefore, of substrate (Fig. S8). We conducted further analysis to determine
maintaining the initial morphology of the catalyst may be an the difference the porous carbon substrate and the Cu-foil substrate.
important factor for C-C coupling. However, in CSNP/Cu, the shape Therefore, we additionally performed XPS analysis to understand the
of the catalyst changed rapidly CO2RR after 2 hours (Fig. S6). Also, the Cu states. High performance XPS analysis before and after the CO2RR
typical oxygen-Cu combination catalysts exhibited a relatively rapid of CSNP/Cu and CSNP/CP was compared. The CSNP/CP had a Cu 2p3/2
surface reconstruction within few hours (~ 2 hour) due to the peak at binding energy of 934.5 eV before the CO2RR, which was 0.2
reductive potential of CO2 RR.18, 28, 29 On the other hand, CSNP/CP eV higher than CSNP/Cu (Fig. 3b). Even after the CO2RR, these
well maintained the initial morphology after 2 hours (Fig. 2c). This binding energy differences were observed to be maintained (Fig. S9).
result suggests that carbon paper may be a better support than Cu These binding energy differences can be induced because carbon has
foil in terms of catalyst stability and performance.
higher electronegativity than Cu. Similarly, in other electrocatalyst
Nevertheless, both CSNP/CP and CSNP/Cu showed good C-C coupling cases, the chemical states of the metal element were reported to
ability at CO2RR (Fig. 3a). HR-TEM images of pre-/post CO2RR sample have changed as a result of the interaction between the catalyst and
simply showed that overall dimension of the nanostructures were support34-36. These results suggest that each substrate influences the
similar and post-CO2RR sample had defective structures (Fig. S7) both chemical state of the CSNP catalyst. It is also assumed that the CSNP
on the porous carbon substrate and on the Cu-foil. Although we also catalyst have a more electron deficient condition on carbon paper
do not believe the defect sites were identically maintained, the than on Cu-foil.
nanostructure and defective structure of as-sprayed CSNP sample Using in-situ/operando Cu K-edge extended X-ray absorption fine
would contribute to the transformation of the catalyst having structure (EXAFS) analysis, it was confirmed that the defect site in
defects. Because only TEM images are not enough to characterize the CSNP/CP mentioned above was maintained during CO2RR (Fig. 3c).
detail of the defective sites and compare their nature on the porous The ex-situ EXAFS of the reference material was assigned to the Cu-
carbon and Cu-foil, it is difficult to clarify the defect sites are the only
origin of showing high C2H4 production activity of CSNP/CP. However,
we believe that the interaction between CSNP and carbon support
can affect to form defective structures. Recent studies have
O characteristic at 1.5 Å and the Cu-Cu feature at 2.2 Å, respectively,
like the previous reports (Fig. S10)26, 37. The in-situ/operando EXAFS
results showed the Cu-O characteristics at 1.5 Å were retained after
the initial 5 min reaction, but the subsequent EXAFS measurement
demonstrated that the initial copper oxide nanoparticles can have
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 6 of 8
ARTICLE
Journal Name
View Article Online
DOI: 10.1039/C9TA13173F
Fig. 4 The catalyst spray amount control results: a) total current density for each sample, b) product selectivity change by spray amount, c) C2H4 and CH4 selectivity change
showed a decrease in Cu-O, and only the Cu-Cu feature remained. reconstruction phenomenon into the metallic Cu cube by reaction
These results are consistent with previously reported studies which time.
showed the Cu metallic state under CO2RR conditions38, 39. However, In order to investigate the effect of selectivity on the surface
structure of these catalysts, the surface structure was controlled by
CSNP/CP maintained a lower intensity of Cu-Cu feature at 2.2 Å than
adjusting the amount of catalyst sprayed on the carbon paper
that of Cu foil during CO2RR. This result mean that the average
coordination number of CSNP/CP during the CO2RR was less than 12
of Cu foil. If there are many defect sites in the catalyst, the
substrate. The amount of catalyst spray was adjusted from 4 μg/cm2
to 60.8 μg/cm2. SEM images confirmed that the surface morphology
greatly changed depending on the spray amount (Fig. S12). The
coordination number of Cu-Cu will decrease. Therefore, it could be
product distribution during CO2RR varied greatly even at the same
assumed that CSNP/CP maintained the defect site well for 105
biased potential, depending on the amount of catalyst sprayed, but
minutes during reaction.
total current densities had similar values except for the sample
However, in the in situ/operando Cu K-edge X-ray absorption near
edge structure (XANES) analysis, the chemical state of the CSNP/CP
sprayed by 4 μg/cm2 (Fig. 4a). The 4 μg/cm2 sample was exceptional
presumably because the amount of CSNP was too small to cover
during the reaction was different than the conventional Cu metal
most of the carbon surface compared with other samples.
reference (Fig. 3d). The CSNP/CP showed a different XANES
The FE of each product changed significantly, especially between CH4
fingerprint pattern from Cu foil even in the strong reduction
and C2H4, depending on the transition of surface morphology of the
environment (-1.0 V vs RHE) where CO2RR occurs. The XANES spectra
of Cu foil reference material showed characteristic peak at 8988 eV,
4, 8, 12.8 and 19.2 μg/cm2 sprayed samples (Fig. 4b). As the amount
of applied catalyst decreased, CH4 formation increased and C2H4
8998 eV and 9019 eV. These characteristic peak was also observed in
production decreased (Fig. 4c). The FE ratio of the C2H4 / CH4 was
CSNP/CP, but compared to Cu foil spectra, the position and shape
adjustable from 1.0 to 21 (Fig. S13a).
were slightly different. When analyzed with the XANES spectra of the
Several studies have reported that the production selectivity of CH4
bulk Cu2O reference material, we could estimate the peaks of
and C2H4 is affected by the local pH due to the pH dependence of
CSNP/CP overlap with the characteristic peaks of Cu2O (red arrow in
each reaction.20-22 similarly, our catalysts presumably had different
Fig. 3d).
product distributions due to the difference in local pH produced by
This result indicated that the CSNP/CP may have an oxidized state
the porous catalyst surfaces. Through SEM analysis, we found that
during the reaction. In order to accurately estimate the degree of
the nanoparticles of the 4 μg/cm2 deposited sample were well
oxidation, linear combination fitting (LCF) was carried out on the in
dispersed, and showed a morphology of relatively low porosity. On
situ/operando data of CSNP/CP. The LCF results for each in
the other hand, the nanoparticles of the samples sprayed with more
situ/operando data were fitted with a mixed state of Cu0, Cu2O and
than 19.2 μg/cm2 were well clustered with each other, and the
CuO (Fig. S11). In the 5 min data, the oxidized species exceeds 40%
(Cu2O : 28.7% and CuO 12.9%), but after 35 min, CuO ratio slowly
porosity was increased (Fig. S12).
We compared the partial current density of C2H4 and CH4 production
decreased while maintained about 70% metallic Cu and 25% Cu2O
depending on the spray amount which showing distinct trend (Fig.
(Table S1). This data was related to our previous report that C2
S13b). The partial current density for CH4 production decreased as
selectivity and stability were improved when Cu0, Cu+ and Cu2+ exist
the spray amount increased, while that of C2H4 was more or less
in metastable mixed state19. Because of this previous report, the Cu
same in the same ranges. The previous studies have reported C2H4
mixed state in CSNP/CP may be the cause of improved C2H4
and CH4 production ratio are sensitively affected by the electrolyte
selectivity. In other studies, the association between Cu+ species and
pH because of their different pH dependence, and porous catalyst
C2 selectivity has also been suggested15-18, 40 However, the oxidized
can tune CO2RR product distribution by inducing local pH increase at
species seen during the reaction may be due to the oxide domain
high current density condition due to unbalanced proton supply 20-22,
being isolated from the reaction surface of the catalyst. Alternatively,
41. It is proposed that protonation process is involved in a rate
the origin of the high selectivity in CSNP/CP may be due to the
determining step of CH4 production to form *CHO intermediate, and
formation of a defect site in the oxidized phase and the reduced Cu
CH4 production rate decreases at high pH condition42. On the other
domain, and deactivation factor of the catalyst may be
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 8
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
hand, in the case of C2H4, C-C coupling step is proposed as rate
determining step in which proton is not involved41, and thus have
constant C2H4 production rate at different pH electrolyte. Thus, CH4
production depends on the pH of the electrolyte while C2H4
production shows pH independence. Taken together, increasing pH
near the catalyst surface can induce decreasing CH4 production rate
while constant C2H4 production rate. Therefore, we proposed that
the higher spraying amount form the more porous surface and the
higher local pH increase near the catalyst surface which decrease CH4
production.
We developed a high performance catalyst-substrate system by
electrospraying copper-based catalyst on the porous carbon
View Article Online
DOI: 10.1039/C9TA13173F
substrate and demonstrated enhanced C2H4 production
compared to Cu-foil, with 3.8 times greater selectivity and 20-
fold higher production rate. Notably, this catalyst-substrates
system was synthesized by a very simple one-step electrospray
pyrolysis method, unlike the conventional method of loading
onto the substrate after catalyst synthesis. Also, it was
confirmed that the activity of the C2 product was different
depending on the type of substrate, even for catalyst
synthesized by the same method. Also, it was confirmed that
CO2RR activity was affected by the type of the support even the
same method was applied for Cu-catalyst synthesis, indicating
the importance of proper substrate material because it can
affect multiple parameter such as morphology, defect sites and
electronic structures.. The morphology of the catalyst was
retained longer on the carbon substrate than on the Cu-foil,
which contribute to long-term C2H4 selectivity and stability. In
addition, in situ/operando XAS experiments showed that the Cu
mixed state (Cu0, Cu2O and CuO) coexisted during CO2RR. This
mixed state was also suggested to play an important role in C-C
coupling. The nanostructure porosity of this catalyst was very
easily controlled by the amount of catalyst sprayed, and a local
pH gradient was induced to precisely control the C2H4 / CH4
ratio. As a result of this study, we are able to propose effective
methods for synthesizing C2 products using Cu catalysts, and to
explain some of the causes of selectivity of some products. Our
study can also be applied in the future to easily synthesize
catalyst on gas diffusion porous electrode, and the enhanced
performance produced by the catalyst-substrate interaction
suggests insights for catalyst-substrate system research.
In addition, we observed that the catalytic activity of CSNP/CP is
affected by the cation type (Fig. S15 and Fig S16), consistent with the
previous study. It suggests that as the cation size increases, the
hydrated alkali metal cation forms a dipole field in the outer
Helmholtz plane, which stabilizes the adsorbed intermediates (i.e.
*CO2, *CO and *OCCO), effectively increasing C-C coupling and
CO2RR on flat Cu electrodes. It is also proposed that large size cation
reduces *CHO formation by stabilizing *CO, which reduces the
formation of *CHO, an important rate determining intermediate for
CH4 formation43. Thus, we tested the performance of CSNP in Na+, K+
and Cs+ electrolytes (Fig. S14). As reported in the paper mentioned,
HER increased and CO2RR decreased in Na+ electrolytes with small
cation sizes. As the size of cation increased, *CO and *OCCO
stabilized and C2H4 production increased. The observed Tafel slope is
98.1 mV/dec in 0.1 M KHCO3, and similar values were obtained both
in NaHCO3 and CsHCO3 electrolytes as shown in Fig. S17. These values
are also similar with the recently reported values (75 ~ 97 mV/dec
depending on the Cu-based catalysts)44. Although it is difficult to
determine the electrokinetics based on the simple Tafel slope
measurement, it suggests that electrokinetics of C2H4 production on
our catalysts are similar to others. Turn over frequency (TOF) of CSNP
catalyst is estimated in 0.1 M KHCO3. The deposited Cu amount was
calculated by measuring the mass of deposited catalyst amount, and
the lower bound is calculated by assuming the all Cu atoms are
involved on the active sites. Our estimated TOF values for C2H4
production is 0.05 S-1 at -1.05 V vs RHE (Fig. S18) which is comparable
to previously reported TOF of 0.01 s-1 (32.6 h-1) at the optimum
potential in half cell45.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
This work was by Korea Institute of Science and Technology
(KIST) institutional program and the YU-KIST convergence
program and partially by National Research Foundation (NRF)
funded by the Korean government. (No. 2019R1A2C2005521).
Using the catalyst synthesis method proposed in this study, a
difference in porosity which can induce a local pH gradient can be
easily achieved, just by controlling the amount of catalyst spray,
without a complicated catalyst design process. The optimal C2H4 FE
was 51% when a 12.8 μg/cm2 spray amount was applied (Fig. 4 c),
but the CH4 selectivity was still about 10%. When K+ was replaced
with Cs+, 12.8 μg/cm2 CSNP/CP decreased HER at lower
overpotentials and decreased C1 gas products at all potential ranges,
and a maximum of 52 % of FEC2H4 was achieved (Fig. S15 and S16). In
particular, the partial current density of C2H4 production was greatly
increased, up to ~ 21 mA/cm2 (Fig. S16b).
References
1 T.R. Karl, K.E. Trenberth, Science 2003, 302, 1719-1723.
2 J.P. Huang, H.P. Yu, X.D. Guan, G.Y. Wang, R.X. Guo, Nat Clim
Change 2016, 6, 166.
3 D.S. Reay, F. Dentener, P. Smith, J. Grace, R.A. Feely, Nat Geosci
2008, 1, 430-437.
4 W.C. Sheng, S. Kattel, S.Y. Yao, B.H. Yan, Z.X. Liang, C.J.
Hawxhurst, Q.Y. Wu, J.G.G. Chen, Energy Environ. Sci. 2017, 10,
1180-1185.
5 D. Mellmann, P. Sponholz, H. Junge, M. Beller, Chem. Soc. Rev.
2016, 45, 3954-3988.
6 Y.H. Chen, C.W. Li, M.W. Kanan, J. Am. Chem. Soc. 2012, 134,
19969-19972.
4. Conclusions
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry A
Page 8 of 8
ARTICLE
Journal Name
7 Y.C. Hsieh, S.D. Senanayake, Y. Zhang, W.Q. Xu, D.E. Polyansky, 35 B.S. Yeo, A.T. Bell, J. Phys. Chem. C 2012, 116, 83V9ie4w-8A4rti0cl0e Online
ACS Catal. 2015, 5, 5349-5356. 36 S.Y. Lee, H. Jung, S.Y. Chae, H.S. Oh, B.K. Min, Y.J. Hwang,
DOI: 10.1039/C9TA13173F
8 D.H. Won, H. Shin, J. Koh, J. Chung, H.S. Lee, H. Kim, S.I. Woo, Electrochim. Acta 2018, 281, 684-691
Angew. Chem., Int. Ed. 2016, 55, 9297-9300.
37 Z. Weng, Y.S. Wu, M.Y. Wang, J.B. Jiang, K. Yang, S.J. Huo, X.F.
9 Z.Y. Zhang, M.F. Chi, G.M. Veith, P.F. Zhang, D.A. Lutterman, J. Wang, Q. Ma, G.W. Brudvig, V.S. Batista, Y.Y. Liang, Z.X. Feng, H.L.
Rosenthal, S.H. Overbury, S. Dai, H.Y. Zhu, ACS Catal. 2016, 6, Wang, Nat. Commun. 2018, 9, 415.
6255-6264.
38 A. Eilert, F.S. Roberts, D. Friebel, A. Nilsson, J. Phys. Chem. Lett.
2016, 7, 1466-1470.
10 C.H. Lee, M.W. Kanan, ACS Catal. 2015, 5, 465-469.
11 W. Ren, X. Tan, W. Yang, C. Jia, S. Xu, K. Wang, S.C. Smith, C.
Zhao, Angew. Chem., Int. Ed. 2019, 58, 6972-6976.
12 M. Tejeda-Serrano, M. Mon, B. Ross, F. Gonell, J. Ferrando-
Soria, A. Corma, A. Leyva-Perez, D. Armentano, E. Pardo, J. Am.
Chem. Soc. 2018, 140, 8827-8832.
39 S.B. Scott, T.V. Hogg, A.T. Landers, T. Maagaard, E.
Bertheussen, J.C. Lin, R.C. Davis, J.W. Beeman, D. Higgins, W.S.
Drisdell, C. Hahn, A. Mehta, B. Seger, T.F. Jaramillo, I.
Chorkendorff, ACS Energy Lett. 2019, 4, 803-804
40 D. Gao, I. Sinev, F. Scholten, R.M. Arán-Ais, N.J. Divins, K.
Kvashnina, J. Timoshenko and B.R. Cuenya, Angew. Chem. Int. Ed.
2019, 58, 2-9
41 A.S. Varela, M. Kroschel, T. Reier, P. Strasser, Catal. Today
2016, 8-13
42 M. Schreier, Y. Yoon, M.N. Jackson, Y. Surendranath, Angew.
Chem. Int. Ed. 2018, 57, 10221-10225
43 J. Resasco, L.D. Chen, E. Clark, C. Tsai, C. Hahn, T.F. Jaramillo,
K. Chan, A.T. Bell, J. Am. Chem. Soc. 2017, 139, 11277-11287.
44 Y. Gao, Q. Wu, X. Liang, Z. Wang, Z. Zheng, P. Wang, Y. Liu, Y.
Dai, M.H. Whangbo, B. Huang, Adv. Sci. 2020, 1902820
45 Q. Zhu, X. Sun, D. Yang, J. Ma, X. Kang, L. Zheng, J. Zhang, Z.
Wu, B. Han, Nat. Commun. 2019, 10, 3851
13 Y. Hori, H. Wakebe, T. Tsukamoto, O. Koga, Electrochim. Acta
1994, 39, 1833-1839.
14 K.P. Kuhl, E.R. Cave, D.N. Abram, T.F. Jaramillo, Energy Environ.
Sci. 2012, 5, 7050-7059.
15 A. Eilert, F. Cavalca, F.S. Roberts, J. Osterwalder, C. Liu, M.
Favaro, E.J. Crumlin, H. Ogasawara, D. Friebel, L.G.M. Pettersson,
A. Nilsson, J. Phys. Chem. Lett. 2017, 8, 285-290.
16 M. Favaro, H. Xiao, T. Cheng, W.A. Goddard, J. Yano, E.J.
Crumlin, Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 6706-6711.
17 M.G. Kibria, C.-T. Dinh, A. Seifitokaldani, P. De Luna, T.
Burdyny, R. Quintero-Bermudez, M.B. Ross, O.S. Bushuyev, F.P.
García de Arquer, P. Yang, D. Sinton, E.H. Sargent, Adv. Mater.
2018, 30, 1804867.
18 P. De Luna, R. Quintero-Bermudez, C.T. Dinh, M.B. Ross, O.S.
Bushuyev, P. Todorovic, T. Regier, S.O. Kelley, P.D. Yang, E.H.
Sargent, Nat. Catal. 2018, 1, 103-110.
19 S.Y. Lee, H. Jung, N.K. Kim, H.S. Oh, B.K. Min, Y.J. Hwang, J. Am.
Chem. Soc. 2018, 140, 8681-8689.
20 K.D. Yang, W.R. Ko, J.H. Lee, S.J. Kim, H. Lee, M.H. Lee, K.T.
Nam, Angew. Chem., Int. Ed. 2017, 56, 796-800.
21 A. Dutta, M. Rahaman, N.C. Luedi, P. Broekmann, ACS Catal.
2016, 6, 3804-3814.
22 H. Song, M. Im, J.T. Song, J.A. Lim, B.S. Kim, Y. Kwon, S. Ryu, J.
Oh, Appl. Catal., B 2018, 232, 391-396.
23 J. Li, G.X. Chen, Y.Y. Zhu, Z. Liang, A. Pei, C.L. Wu, H.X. Wang,
H.R. Lee, K. Liu, S. Chu, Y. Cui, Nat. Catal. 2018, 1, 592-600.
24 C.T. Dinh, T. Burdyny, M.G. Kibria, A. Seifitokaldani, C.M.
Gabardo, F.P.G. de Arquer, A. Kiani, J.P. Edwards, P. De Luna, O.S.
Bushuyev, C.Q. Zou, R. Quintero-Bermudez, Y.J. Pang, D. Sinton,
E.H. Sargent, Science 2018, 360, 783-787.
25 T.T.H. Hoang, S. Verma, S.C. Ma, T.T. Fister, J. Timoshenko, A.I.
Frenkel, P.J.A. Kenis, A.A. Gewirth, J. Am. Chem. Soc. 2018, 140,
5791-5797.
26 H. Mistry, A.S. Varela, C.S. Bonifacio, I. Zegkinoglou, I. Sinev,
Y.W. Choi, K. Kisslinger, E.A. Stach, J.C. Yang, P. Strasser, B. Roldan
Cuenya, Nat. Commun. 2016, 7, 12123.
27 Q. Zhu, X. Sun, D. Yang, J. Ma, X. Kang, L. Zheng, J. Zhang, Z.
Wu, B. Han, Nat. Commun. 2019, 10, 3851
28 K. Jiang, R.B. Sandberg, A.J. Akey, X.Y. Liu, D.C. Bell, J.K.
Norskov, K.R. Chan, H.T. Wang, Nat. Catal. 2018, 1, 111-119
29 T.T. Zhuang, Y.J. Pang, Z.Q. Liang, Z.Y. Wang, Y. Li, C.S. Tan, J.
Li, C.T. Dinh, P. De Luna, P.L. Hsieh, T. Burdyny, H.H. Li, M.X. Liu,
Y.H. Wang, F.W. Li, A. Proppe, A. Johnston, D.H. Nam, Z.Y. Wu,
Y.R. Zheng, A.H. Ip, H.R. Tan, L.J. Chen, S.H. Yu, S.O. Kelley, D.
Sinton, E.H. Sargent, Nat. Catal. 2018, 1, 946-951.
30 D. Ren, Y.L. Deng, A.D. Handoko, C.S. Chen, S. Malkhandi, B.S.
Yeo, ACS Catal. 2015, 5, 2814-2821.
31 P. Grosse, D.F. Gao, F. Scholten, I. Sinev, H. Mistry, B. Roldan
Cuenya, Angew. Chem., Int. Ed. 2018, 57, 6192-6197.
32 H. Jung, S.Y. Lee, C.W. Lee, M.K. Cho, D.H. Won, C. Kim, H.S.
Oh, B.K. Min, Y.J. Hwang, J. Am. Chem. Soc. 2019, 141, 4624-4633.
33 J. Kim, W. Choi, J.W. Park, C. Kim, M. Kim, H. Song, , J. Am.
Chem. Soc. 2019, 141, 6986-6994
34 B.S. Yeo, A.T. Bell, J. Am. Chem. Soc. 2011, 133, 5587-5593
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins