Mixed Copper States in Anodized Cu Electrocatalyst for Stable and Selective Ethylene Production from CO2 Reduction

Article abstract of DOI:10.1021/jacs.8b02173

Oxygen-Cu (O-Cu) combination catalysts have recently achieved highly improved selectivity for ethylene production from the electrochemical CO₂ reduction reaction (CO₂RR). In this study, we developed anodized copper (AN-Cu) Cu(OH)₂ catalysts by a simple electrochemical synthesis method and achieved ～40% Faradaic efficiency for ethylene production, and high stability over 40 h. Notably, the initial reduction conditions applied to AN-Cu were critical to achieving selective and stable ethylene production activity from the CO₂RR, as the initial reduction condition affects the structures and chemical states, crucial for highly selective and stable ethylene production over methane. A highly negative reduction potential produced a catalyst maintaining long-term stability for the selective production of ethylene over methane, and a small amount of Cu(OH)₂ was still observed on the catalyst surface. Meanwhile, when a mild reduction condition was applied to the AN-Cu, the Cu(OH)₂ crystal structure and mixed states disappeared on the catalyst, becoming more favorable to methane production after few hours. These results show the selectivity of ethylene to methane in O-Cu combination catalysts is influenced by the electrochemical reduction environment related to the mixed valences. This will provide new strategies to improve durability of O-Cu combination catalysts for C-C coupling products from electrochemical CO₂ conversion.

Full text of DOI:10.1021/jacs.8b02173

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Article
Mixed Copper States in Anodized Cu Electrocatalyst for
Stable and Selective Ethylene Production from CO2 Reduction
Si Young Lee, Hyejin Jung, Nak-Kyoon Kim, Hyung-Suk Oh, Byoung Koun Min, and Yun Jeong Hwang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02173 • Publication Date (Web): 18 Jun 2018
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Mixed Copper States in Anodized Cu Electrocatalyst for Sta-
ble and Selective Ethylene Production from CO2 Reduction
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Si Young Lee†, ‡, Hyejin Jung†, ‡, Nak-Kyoon Kim§, Hyung-Suk Oh†, Byoung Koun Min†, ∥, Yun
Jeong Hwang*†, ‡
†Clean Energy Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu,
Seoul, 02792, Republic of Korea
‡ 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
§ Advanced Analysis Center, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul,
02792, Republic of Korea
∥Green School, Korea University, 145, Anam-ro, Seonbuk-gu, Seoul, 02842, Republic of Korea
KEYWORDS: CO2 reduction, Cu electrode, ethylene production, stability, electrocatalyst
ABSTRACT: Oxygen-Cu (O-Cu) combination catalysts have recently achieved highly improved selectivity for ethylene
production from the electrochemical CO2 reduction reaction (CO2RR). In this study, we developed anodized copper (AN-
Cu) Cu(OH)2 catalysts by a simple electrochemical synthesis method and achieved ~40% Faradaic efficiency for ethylene
production, and high stability over 40 hours. Notably, the initial reduction conditions applied to AN-Cu were critical to
achieving selective and stable ethylene production activity from the CO2RR, as the initial reduction condition affects the
structures and chemical states, crucial for highly selective and stable ethylene production over methane. A highly nega-
tive reduction potential produced a catalyst maintaining long term stability for the selective production of ethylene over
methane, and a small amount of Cu(OH)2 was still observed on the catalyst surface. Meanwhile, when a mild reduction
condition was applied to the AN-Cu, the Cu(OH)2 crystal structure and mixed states disappeared on the catalyst, becom-
ing more favorable to methane production after few hours. These results show the selectivity of ethylene to methane in
O-Cu combination catalysts is influenced by the electrochemical reduction environment related to the mixed valences.
This will provide new strategies to improve durability of O-Cu combination catalysts for C-C coupling products from elec-
trochemical CO2 conversion.
Introduction
be produced via a two-electron (2e-) reduction reaction,
relatively simple compared with the reduction pathway of
other products such as methane (CH4) and ethylene
(C2H4), which are 8e- and 12e- reduced products, respec-
tively.7 Even in the case of two-electron mediated reac-
tions, selective CO2 reduction is still challenging because
of the highly competitive hydrogen evolution reaction
(HER) from water splitting in the aqueous solution. Re-
cent progress with nanostructured electrocatalysts based
on metals such as Au, Ag, and Zn etc. have included re-
ports that defective surface structures such as grain
boundaries, or lower coordinated sites, can achieve high
product selectivity for CO of over 95%, with decreased
overpotential. This consequentially improves the energy
conversion efficiency and opens up industrial applica-
tions.8-12
Recently, electrochemical CO2 conversion has received
a great deal of attention because of its potential to pro-
duce value-added chemicals while utilizing CO2. In addi-
tion, the electrochemical conversion pathway is applica-
ble as a future energy storage system (ESS), which is in-
creasingly in demand due to the imbalanced supply of
renewable energy sources.1-2 To achieve economic feasibil-
ity of the electrochemical CO2 conversion technique, it is
required to produce market valuable chemicals with effi-
cient energy conversion from electricity to chemical ener-
gy, and the electrochemical system is strongly depends on
catalyst performance.3 In the last few years, among vari-
ous possible chemical products from the electrochemical
CO2 reduction reaction (CO2 RR), focus has been concen-
trated on the production of C1 chemicals such as carbon
monoxide (CO) or formate (HCOO-) due to their poten-
tially large global usage.4-6 In addition, CO or formate can
Meanwhile, to improve the economic feasibility of the
electrochemical CO2 reduction, diversifying the product
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toward more valuable chemicals has been proposed. One derived copper electrocatalyst is required to design selec-
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of the promising candidates is ethylene, which is widely
used in the production of polymer plastics. However, un-
like CO or formate production, direct CO2 conversion to
ethylene has still low product selectivity, near 30~40 % of
Faradaic efficiency (FE). Also, while copper is almost the
only heterogeneous catalyst for producing ethylene, it can
also catalyze other multiple-electron transfer reactions
beyond CO, formate, hydrogen, and methane.4, 7 There-
fore, understanding the copper catalyst states related with
selective ethylene production13 becomes important to de-
sign efficient electrocatalysts for CO2 reduction reaction.
tive and stable electrocatalysts.
In addition, stability is as important as selectivity, be-
cause the selectivity of CO2 reduction in the Cu cathode
can be easily deteriorated by long-term electrolysis. Hori
et al. reported the deactivation phenomena on Cu elec-
trodes due to temperature or impurity deposition,28 while
durability study of a Cu catalyst has been performed by
utilizing foreign metal atoms.29 However, more studies
are still required to increase the durability of the CO2
reduction catalysts. In our study, we investigated ano-
dized-Cu (AN-Cu) electrocatalysts for the CO2RR, and the
AN-Cu catalysts exhibited a two times improvement in
ethylene selectivity compared to Cu-foil. Its selective eth-
ylene production remained stable for 40 hours, one of the
longest reported to date. In addition, the highly perform-
ing electrocatalyst was prepared using a simple electro-
chemical synthesis method which did not require a com-
plex Cu precursor solution, as is the case with an electro-
deposited copper oxide (Cu2O) catalyst, or a lengthy an-
nealing, as is the case with heat-treated oxide-derived Cu.
We also found that the durability of the selective C2H4
production over CH4 was critically sensitive to the initial
reduction environments applied to the AN-Cu electrode
possibly affecting Cu oxidation states and structural
changes. These changes in selectivity and surface state
will be very important information for studying ethylene
selectivity in future O-Cu combined catalyst studies.
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Researchers have tried to increase the number of active
sites on Cu-based electrocatalysts that have a high eth-
ylene/methane production ratio,14-15 and great improve-
ments in selective ethylene production has been accom-
plished using copper and oxygen combined electrocata-
lysts, such as oxide-derived Cu electrodes, and mixtures
of Cu and copper oxides.14, 16 In addition, various strate-
gies, such as using halide ions in the electrolyte, or induc-
ing local pH deviation via control of surface morphology,
have been also demonstrated to increase ethylene pro-
duction activity on Cu electrodes.17-19
However, we find that the previously reported product
distributions largely deviate among CO, formate, CH4,
C2H4, and H2 depending on the preparation methods of
the oxides or oxide-derived Cu catalysts. For example,
annealed Cu2O electrodes exhibit high selectivity for C1
products, CO with ~ 40 % FE and formate with ~ 30 %
FE.20 Its high CO2 reduction activity is attributed to grain
boundary formation during the oxidation/reduction pro-
cess.21-22 Meanwhile, D. Raciti et al. reported that the an-
nealed copper oxide electrocatalysts have different prod-
uct selectivity depending on the reduction conditions23; a
thermally reduced Cu electrode primarily produces H2
hydrogen (60 ~ 80% FE) and formate (~ 20% FE), whereas
an electrochemically reduced one produces CO at up to
60 % FE. Unlike the annealed Cu-based electrocatalysts,
electrodeposited Cu2O has been reported to have high
ethylene production up to ~ 40 % FE, and suppressed C1
chemical production have been reported.16
Experimental
Cu Electrocatalyst Preparation
All Cu samples were prepared from 0.1 mm thick poly
crystalline Cu foil (Alfa Aesar, 99.9999%). Anodized Cu
electrodes were prepared by modifying a previous meth-
od.30-31 Cu foil was electrochemically oxidized in 3 M KOH
(Sigma-Aldrich) aqueous electrolyte at a constant current
condition for 60 sec at 10 mA/cm2 which formed Cu(OH)2
nanowire arrays.
In an electrochemical cell, the Cu foil working elec-
trode was kept positioned 2 cm away from a platinum
plate counter electrode. The AN-Cu sample was rinsed
with deionized water (18.2 MΩ) and consecutively dried
with nitrogen gas flow. Then, the AN-Cu electrocatalysts
were either used as-prepared, or after electrochemical
reduction. The AN-Cu samples underwent electrochemi-
cal reduction in CO2 (≥99.999%) saturated 0.1M KHCO3
(Sigma-Aldrich, 99.95%) electrolyte, and instantly pro-
ceeded to CO2RR.
However, the origin of the large differences in product
selectivity has not been fully understood yet, and the ac-
tive species of the Cu-based electrocatalysts are also diffi-
cult to identify during the reduction reaction. Based on
in-situ and theoretical studies with electrochemically
prepared Cu-based samples, it has been proposed that
defect structures, oxygen moieties in the copper, or cop-
per oxidation states contribute to stabilize the intermedi-
ate and increase ethylene production by the CO2 reduc-
tion reaction (CO2 RR).24-27 Other researchers have re-
ported that an oxygen-plasma-treated Cu electrode
achieved one of the highest ethylene production selectivi-
ties (~60 %), and the residual Cu+ phase under a cathodic
potential was proposed to play an important role in selec-
tive ethylene production.14 Therefore, better understand-
ing of the changes of the Cu and O states in the oxide-
Two different types of Cu electrodes were prepared by
changing the condition of this electrochemical reduction
step prior to the CO2RR. A highly negative bias potential
(- 4.0 V versus Ag/AgCl reference electrode) was applied
for 10 min to reduce the anodized Cu electrode for a short
time (denoted as HPR-Cu). For comparison, a lower re-
duction potential (-1.15 V versus an Ag/AgCl reference
electrode) was applied for 100 min (denoted as LPR-Cu)
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for slow reduction. Except for the applied potential and
ity of CO by FID. High purity Argon (Ar; 99.999%) was
used as a carrier gas.
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the period of the time, other sample preparation envi-
ronments and CO2RR activity measurements were identi-
cal. In order to compare the catalytic activities of bare Cu
foil electrodes, Cu foil samples were cleaned by electro-
polishing in phosphoric acid (Sigma-Aldrich, 85% in H2O)
at 4 V versus a platinum plater counter electrode for 300
sec.
ꢂꢁ × ꢃ × ꢄ
ꢀꢁ =
× ꢇꢁꢈ
× 100
(1)
ꢅꢆ
ꢀꢁ
F. E. (%) =
(2)
ꢀꢉꢊꢉꢋꢌ
The partial current (ix) needed to produce each product
(x=H2, CO, CH4, C2H4, or C2H6) was obtained from equa-
tion (1) in which ꢂꢁ is the volumetric concentration of
product x extracted from the GC calibration curve, and q
is the gas flow rate. Here, p is the pressure, R is the ideal
gas constant (8.314 m3∙Pa/K∙mol), and T is the tempera-
ture. ꢇꢁ is the number of reduced electrons needed to
produce x from CO2 molecules, F is the Faradaic constant
(96485 s∙A/mol). Then, the Faradaic efficiency (F.E.) was
derived from the following equation (2) in which ꢀꢉꢊꢉꢋꢌ is
the current density measured by the potentiostat during
CO2RR .
Electrochemical CO2 Reduction Reaction Meas-
urements
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All electrochemical measurements were conducted us-
ing an Ivium potentiostat (Iviumtechnology) in a two-
compartment electrochemical cell made of polyether
ether ketone (PEEK). A Selemion AMV anion exchange
membrane was sandwiched between the separated cath-
ode and anode compartment. A Cu sample working elec-
trode, and an Ag/AgCl reference electrode (Basi, 3M
NaCl) were placed in the cathode side, and a platinum
counter electrode was placed in the other anode side. The
geometrical surface areas of the Cu cathodes were con-
trolled to be 0.5 cm2 for easy comparison of their catalytic
activities.
The accumulated liquid products in electrolyte was
analyzed by proton nuclear magnetic resonance (NMR)
spectroscopy (DD2 600 MHz FT NMR, Agilent). In case of
the HCOO- , phenol was used for internal standard. But
the other products were compared by internal standard,
dimethyl sulfoxide (DMSO) chemical shift at 2.6 ppm.
To compare CO2RR activity, a fresh working electrode
was replaced at each applied potential. For the long-term
activity, CO2RR measurement was also performed for 40
hours at a fixed potential. To compensate iR loss, solution
resistance (Rs) was measured by electrochemical imped-
ance spectroscopy (EIS) from 1000 to 0.1 Hz, and normal
values of Rs were determined to be 75~80 Ω in CO2 satu-
rated 0.1M KHCO3 whose pH was measured to be 6.8. The
measured potential values were converted to reversible
hydrogen electrode (RHE) by using the equation below.
Material Characterization
To observe the surface morphology of each electrode,
field emission scanning electron microscopy (FE-SEM,
FEI, Inspect F) was used. Each AN-Cu sample was charac-
terized in detail by transmission electron microscopy
(TEM, FEI Talos). The crystal structure information of the
Cu electrode was obtained using high resolution TEM
(HR-TEM) images and by grazing incidence X-ray diffrac-
tion (GI-XRD, Rigaku corporation, D/Max 2500), which
had a Cu Kα radiation wavelength of 0.15406 nm.
E (vs RHE) = E (vs Ag/AgCl) + 0.209 V + 0.05916 × pH
Products Analysis of CO2 Reduction Reaction
The produced gases at each fixed potential were quanti-
tatively analyzed by Gas Chromatograph (GC; Younglin
6500 GC) which was on-line connected with the head
space of the cathode side of the electrochemical cell, and
the gas products were injected to the GC using a six-port
valve system. High purity CO2 gas (≥99.999%) was con-
tinuously flowed into the electrochemical cell during
CO2RR. The CO2 gas flow rate was controlled with a mass
flow meter, and its flow rate was adjusted to be 20 cc/min,
as measured by a universal flow meter (Agilent AMD
2000) at the outlet of the electrochemical cell.
X-ray Spectroscopic Analysis of the Cu electrocata-
lysts
The element oxidation state changes were measured by
X-ray photoelectron spectroscopy (XPS) pre- and post-
CO2RR. Immediately after the Cu samples were taken out
of the electrolyte, they were thoroughly rinsed with DI
water and then dried with N2 gas flow. In order to prevent
further contamination, the Cu electrodes were vacuum-
packed and stored until X-ray measurement. The control
experiments storing the post-CO2RR catalyst to the air
ambient indicated the vacuum-package was effective to
decrease the copper oxide formation on the surface dur-
ing the sample transfer. High resolution XPS measure-
ment was performed at the 8A2 beam-line of the Pohang
accelerator laboratory (PAL) with an incident photon en-
ergy of 630 eV for higher surface sensitivity. The energy of
the incident X-ray photons at 8A2 were calibrated with
Au foil. In addition, we used an X-ray spectrometer
equipped with a K-alpha model of Thermo U.K. that had
an Al Kα monochromater (Ephoton = 1486.7 eV) to obtain
Cu LMM Auger spectra, which can distinguish Cu0, Cu+,
and Cu2+ states better than Cu 2p spectra.
The GC system was equipped with two columns and
two detectors to separate gas products from the CO2RR.
A HayeSep D (11ft, Agilent) packed column separated
CO2, C2H4, C2H6¸ etc., while a Molecular Sieve 13X (6 ft,
Restek®) packed column separated small gaseous mole-
cules such as H2, N2, O2, CO, and CH4. A thermal conduc-
tivity detector (TCD) and a flame ionization detector
(FID) with a methanizer were installed. The former device
detects hydrogen gas and the later one detects small hy-
drocarbon products (CO, CH4, C2H4, and C2H6). A
methanizer was utilized to increase the detection sensitiv-
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Additionally, the chemical state changes of each Cu C2H4 production selectivity was achieved (Figure 2a).
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electrode were further observed by using soft X-ray ab-
sorption near edge structure (XANES) in the 10D XAS
KIST beam-line of PAL. Cu L-edge XANES was measured
in the energy regions of 920~980 eV, respectively.
Consistent with the previous study, flat Cu-foil preferen-
tially resulted in CH4 production. However, the AN-Cu
achieved up to 38.1 % of Faradaic efficiency for C2H4 pro-
duction, twice that of the Cu-foil, while CH4 generation
was notably suppressed to 1.3 % FE. This result means
selective C2H4 production accounted for 50% of the total
gaseous products. The other liquid products faradaic effi-
ciency was shown in Figure S1, the total faradaic efficiency
was closed to 100 %.
Results and Discussion
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More notably, the AN-Cu catalysts showed significantly
enhanced stability for C2H4 production compared to the
Cu foil (Figure 2b-d). Short-term (i.e. 100 min) stability
test showed that the Cu-foil started to have a significant
drop in current density and C2H4 production selectivity
after 60 min. For the much longer period of 40 hours, the
AN-Cu maintained stable total current density and C2H4
selectivity, with close to 40 % of Faradaic efficiency (Fig-
ure 2d and Figure S2). There have been several Cu-based
catalysts which reported C2H4 selectivity similar to our
AN-Cu catalyst, but their reported stabilities were shorter
than 10 hours (Table 1).
The changes in gas products for the flat Cu-foil and AN-
Cu were also compared depending on the applied poten-
tials, and the surface Cu states were analyzed pre- and
post-CO2RR (Figure 3). First, in terms of product varia-
tion, the Cu-foil always preferred CH4 for all applied po-
tential regions (-0.75 ~ -1.15 V vs. RHE), and the Faradaic
efficiency changes are almost synchronized during CH4
and C2H4 production, indicating the bare Cu foil has
poorer activity for C-C bond coupling regardless of the
applied potentials. On the other hand, the AN-Cu sam-
ples suppressed CH4 and CO generation and selectively
generated C2H4 suggesting C-C bond formation was acti-
vated even at low applied potentials (-0.9 V vs. RHE). In
particular, extremely low CH4 production for all applied
potentials was noted.
Figure 1. Cu-foil SEM image and AN-Cu synthesis confirmed
a) Cu-foil SEM image, c) AN-Cu HR-TEM image, d) Cu-foil
and AN-Cu GI-XRD pattern.
The nanowire arrays were obtained on the Cu foil after
a short-time anodization process in a KOH condition as
shown in the SEM images (Figure 1). The SEM image
shows the polycrystalline Cu-foil has a flat surface (Figure
1a), while the AN-Cu is composed of bundles of nanowire
arrays (Figure 1b), similar to other reports.30-31 The length
and the coverage of the nanowire arrays were varied by
controlling the anodizing time, and we optimized the
anodization condition to avoid delamination issues. In
addition, the HR-TEM of a single nanowire and GI-XRD
of the ensemble AN-Cu sample were measured, and both
of them confirmed the prepared nanowire had a Cu(OH)2
crystal structure (Figure 1c and d). The obtained XRD pat-
terns of the AN-Cu samples were all matched with
Cu(OH)2 (JCPDS # 80-1916) and metallic Cu (JCPDS # 04-
0836) because our anodization process synthesized
Cu(OH)2 nanowire arrays on the Cu foil substrate.
The surface of the AN-Cu was characterized by XPS be-
fore and after CO2RR (Figures 3c and d). Cu 2p XPS clear-
ly showed that the as-prepared AN-Cu surface has Cu2+
states which were distinctively reduced to the lower bind-
ing energy associated with the Cu+ or Cu0 state after 100
minutes of reaction. Because the binding energies of the
Cu+ and Cu0 states in the Cu 2p peak are similar,32-33 it is
difficult to distinguish them, as seen in Figure 3c. There-
fore, further surface state analysis was performed with a
Cu LMM auger peak (Figure 3d). Previous studies have
shown a Cu2O (or Cu+) auger peak at 570.0 eV, Cu0 peaks
at 567.7 to 567.9 eV, and Cu(OH)2 auger peaks at 570.4
eV.34-37 Again, the feature around 570.4 eV confirms the
formation of Cu(OH)2 in the as-prepared AN-Cu sample.
After the CO2RR of the AN-Cu sample, both the Cu0 and
Cu+ states were present.
Next, the electrochemical activity of the prepared AN-
Cu was measured for CO2 RR. Highly improved selectivi-
ty for C2H4 over CH4 was achieved compared to the poly-
crystalline Cu flat surface (Figure 2). Our GC measure-
ment can analyze H2, and gas products up to C3 chemicals
such as CO, CH4, C2H4, C2H6, C3H8, etc.. The main detect-
ed gas products were H2, CH4, C2H4, and negligible
amounts of others at -1.08 V vs. RHE, where the highest
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Table 1. C2H4 selectivity and stability comparison of reported Cu based CO2 RR electrocatalysts.
Catalyst
electrolyte
Experiment
conditions
C2H4 F.E. (%)
C2H4 Partial J (mA/cm2)
Stability (h)
4
5
6
7
8
AN-Cu (this work)
Cu2O film16
0.1 M KHCO3
0.1 M KHCO3
0.1 M KHCO3
0.1 M KHCO3
0.1 M KHCO3
1M KOH(GDE)
0.3 M KHCO3
0.1 M KHCO3
0.1 M KHCO3
0.1 M KHCO3
0.25 M KHCO3
0.1 M KHCO3
0.1 M KHCO3
-1.08 V vs RHE
-0.99 V vs RHE
-1.1 V vs RHE
38.1
7.3
~12.9
~ 12.5
~ 6.8
~ 6.6
~ 90*
~ 0.5
~ 7.2
40
37.5
1
9
3C-Cu2O film44
Cu-mesocrystal45
Plasma-Cu*14
Nanoporous-Cu*46
Cu-foil27
32.5
5
10
11
12
13
14
15
16
17
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20
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22
23
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25
26
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60
-0.99 V vs RHE
-0.9 V vs RHE
-0.60 V vs RHE
-1.4 V vs SHE
-1.7 V vs NHE
-1.15 V vs RHE
-1.9 V vs Ag/AgCl
0.96 V vs RHE
1.0 V vs RHE
27.2
6
60*
5
38.2
8*(only current)
10
1
Mesopore-Cu*18
Prism-Cu47
38
4*(only current)
30
~ 12
1.5
Amino-Cu48
13
32.5
~ 1.4
12
Cu nanocube49
Cu Cube50
~21
2.5
Not available
~ 38
10 (Total J)
~ 22
Not available
1
ERD Cu27
1.2 V vs RHE
Plasma-Cu*: Faradaic selectivity (target product amount/total detected product amount) instead of faradaic efficiency.
Nanoporous-Cu*: Gas diffusion electrode (GDE) flow cell was used and did not report C2H4 production stability.
Mesopore-Cu*: Stability of C2H4 production and selectivity of HER and hydrocarbon were not accessible.
Here, it was confirmed that the initial Cu(OH)2 phase was
also reduced under the CO2RR condition, and the reduced
copper states irreversibly changed to the mixed states of
metallic and oxidized coppers post CO2RR. Although the
oxidized copper states were evolved to reduced forms by
the CO2RR, the improved selectivity for C2H4 was stable
for as long as 40 hours.
understand the relation of CO2RR activity to the surface
states. First, a highly negative potential reduction-Cu
(HPR-Cu) was prepared by applying a very negative po-
tential (-4.0 V vs. Ag/AgCl) to the as-prepared AN-Cu
sample for 10 minutes. In the other case, a milder reduc-
tion condition was applied to AN-Cu with a small nega-
tive potential (-1.15 V vs Ag/AgCl) for 100 minutes to make
a low negative potential reduction-Cu (LPR-Cu). The total
charge passing through the AN-Cu during the HPR-Cu
preparation was almost an order magnitude higher than
that of LPR-Cu (Table S1), but the initial morphology of
the Cu(OH)2 nanowire arrays on the LPR-Cu were more
severely collapsed during the pre-reduction steps (insert-
ed images in Figures 4a and 4c).
To understand the high stability and activity of
the AN-Cu, additional surface spectroscopic analyses and
catalytic activity measurements were performed with the
AN-Cu and its modified samples (Figure 4). Two different
types of electrocatalysts were prepared from the AN-Cu
(Figure 4), by introducing an electrochemical reducing
pre-treatment step prior to the CO2RR. Their surface Cu
states were scrutinized pre- and post-CO2RR to further
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Figure 2. AN-Cu and Cu-foil feature comparison a) HER and hydrocarbon selectivity, b) current density and stability,
c) ethylene production stability, d) AN-Cu C2H4 production stability.
Figure 3. AN-Cu and Cu-foil feature comparison a) Cu-foil CO2RR selectivity, b) AN-Cu CO2 RR selectivity, c)
AN-Cu pre-post Cu 2p XPS peak, d) AN-Cu pre-post Cu LMM auger peak.
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Figure 4. Selectivity and stability of HPR-Cu and LPR-Cu a) HPR-Cu CO2 RR product selectivity; the insert SEM is HPR-Cu as
prepared, b) HPR-Cu 10h stability, c) LPR-Cu CO2RR product selectivity; the insert SEM is LPR-Cu as prepared, d) LPR-Cu 10h
stability.
Comparing the current density (i.e. conversion rate) of
as well as at a fixed potential (-1.05 V vs. RHE) for a 10
hour reaction (also see Figure S5). The HPR-Cu showed
almost similar selectivity for C2H4 (38 % of Faradaic effi-
ciency) over CH4 (< 1.0 % of Faradaic efficiency) com-
pared to AN-Cu, and it also had highly stable C2H4 pro-
duction activity for 10 hours at – 1.05 V vs. RHE (Figures
4a and 4b). On the other hand, the LPR-Cu showed in-
creased amounts of CH4 production (~ 7 % of Faradaic
efficiency at – 1.05 V vs. RHE; Figure 4c). This became
more pronounced with the longer reaction time, and larg-
er than the C2H4 production after 6 hours (Figure 4d).
the AN-Cu and two other Cu samples derived therefrom
(HPR-Cu and LPR-Cu), they had similar total current
densities, which were two times higher than that of the
Cu-foil (Figure S3). In addition, the partial current densi-
ties of CH4 and C2H4 depending on the applied potentials
showed that the HPR-Cu had higher activity for C2H4
production than the other Cu samples, while the LPR-Cu
showed exceptionally enhanced CH4 production activity,
which caused a higher C2H4 selectivity for the HPR-Cu.
All of the Cu-based catalysts reached their highest C2H4
production selectivity near -1.05 ~ -1.1 V vs. RHE where
hydrogen production was the smallest, suggesting that
the surface active sites might be competing between HER
and CO2 to C2H4.
A local difference in pH induced by the surface mor-
phology has been demonstrated to cause different prefer-
ences, to C2H4 versus CH4 production. When SEM images
were taken after 100 min of CO2RR, however, the mor-
phology of the HPR-Cu and LPR-Cu appeared to be simi-
lar, unlike their initial morphologies (Figure S6). This
suggests their different activities in C2H4 and CH4 selec-
tivity may not be due to a difference in macro morpholo-
gy. During the electrochemical modification, the different
reduction potential conditions can affect the structural
changes forming under-coordinated defective atoms
known as active sites for ethylene production. Consider-
ing the significant
Interestingly, both the AN-Cu and HPR-Cu, the two ac-
tive catalysts for selective C2H4 production, produced a
small amount of C2H6 (Figures S4b and c). According to
the proposed mechanism,38-39 C2H6 is produced through
different reaction pathways than CH4. It is presumed that
the AN-Cu and HPR-Cu catalysts are capable of reducing
C2H4 to a more reduced product, C2H6.Next, as shown in
Figure 4, the CO2RR activities of these two types of Cu
samples were measured under various applied potentials,
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Figure 5. GI-XRD analysis a) pretreated sample XRD comparison, b) HPR-Cu time dependent ex-situ analysis, c)LPR-Cu time
dependent ex-situ analysis.
difference in activity between HPR-Cu and LPR-Cu, it is
hypothesized that the initial states of the catalysts before
CO2RR measurement are important for determining the
long-term activity, although the surface oxidized copper
reduces to metallic form during the CO2RR.14-15
the pre-treatment step at low applied potential (Figure
5c).
Soft X-ray XANES spectra was measured to further dis-
tinguish the copper states on the top layer of HPR-Cu and
LPR-Cu surfaces (Figure 6). The catalyst samples were
stored and transferred under the vacuum package prior to
the spectroscopy measurements in order to minimize the
changes of the surface (Figure S8). Figure 6a shows the Cu
L-edge XANES spectra of the reference Cu(OH)2, CuO,
Cu2O, and Cu-foil as well as the HPR-Cu and LPR-Cu
samples. The Cu2+ state of Cu(OH)2 or CuO has a single
L3-edge peak distinctively at around 930 ~ 940 eV, be-
cause Cu2+ has 9 electrons in 3d orbitals which allows ex-
citation of the 2p electron.40-41 Meanwhile, the Cu2O and
Cu-foil have two features in the 930 ~ 940 eV region. Fig-
ure 6b shows that the Cu2+ states were observed on the
HPR-Cu after 100 minutes of CO2 RR, and the Cu2+ peak
decreased after 10 hours, consistent with the XRD pat-
terns. The LPR-Cu results in Figure 6c also show similar
results to previous XRD results, where the Cu2+ peak de-
creased a lot at the initial state, and its spectrum shape
was similar to that of Cu2O, or Cu0 did not change signifi-
cantly with the reaction time.
Next, various surface analysis measurements were per-
formed on the HPR-Cu and LPR-Cu to characterize their
surface states over the course of the CO2 reduction reac-
tion at – 1.05 V vs. RHE, initial, 100 min, and 10 hours lat-
er. First, GI-XRD patterns were compared, as shown in
Figure 5. Again, our synthesized AN-Cu had a crystalline
Cu(OH)2 structure on a metallic Cu substrate. After elec-
trochemical pre-treatment was applied to the AN-Cu,
both the HPR-Cu and LPR-Cu had an evolution of new
patterns at 36.5°, 42.6° and 61.5° (JCPS # 65-3288) associ-
ated with crystalline Cu2O. However, interestingly, the
HPR-Cu sample still had a small intensity of Cu(OH)2
patterns at 16.7°, 23.9° and 34.2°, implying mixed states
with Cu2O , Cu, and Cu(OH)2 were present on the HPR-
Cu surface. Also, AN-Cu had a small amount of Cu(OH)2
signal even after 40 h bulk electrolysis (Figure S7). In Fig-
ure 5b, Cu(OH)2 can be noticeably observed on the HPR-
Cu even after 100 min of CO2RR, and the Cu2O peaks be-
came stronger after 10 hour of reaction. On the other
hand, more apparent Cu2O patterns were observed on the
LPR-Cu, but Cu(OH)2 features almost disappeared after
In addition, the XPS elementary analysis results indi-
cated that the AN-Cu and HPR-Cu surfaces have relative-
ly higher oxygen concentrations than the LPR-Cu (Table
Figure 6. Cu L-edge XANES spectra a) reference sample and An-Cu modified samples Cu L-edge, b) HPR-Cu ex-situ time de-
pendent Cu L-edge spectra, c) LPR-Cu ex-situ time dependent Cu L-edge.
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production activity with expanded durability, and it can
S2). Also, according to detail analysis of XPS O 1s peak,
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LPR-Cu samples had more Cu2O phase than HPR-Cu
samples (Figure S9). These results also indicated that the
LPR-Cu samples was more reduced than the HPR-Cu
samples and that the oxidation state of the surface may
affected the ethylene selectivity. Before CO2RR, the atom-
ic concentration ratio of O to Cu (O/Cu) on the HPR-Cu
was 1.15, whereas LPR-Cu had an O/Cu ratio of 0.10, an
almost one order of magnitude lower oxygen amount.
affect the surface properties of the oxygen containing on
the copper surface.
Conclusion
In this study, we found that a Cu(OH)2 nanowire cata-
lyst stably retained an average of 38.1% Faradaic efficiency
for ethylene production over 40 hours from the CO2 re-
duction reaction, and high selectivity of ethylene over
methane was achieved. We identified that an electro-
chemical treatment condition on the as-prepared
Cu(OH)2 catalyst causes the highly selective and stable
ethylene generation. When the electrochemical reduction
pre-treatment was applied to Cu(OH)2 at mild biased po-
tential, the copper species were vulnerable to reducing
environment, which quickly changed products from eth-
ylene to methane. Meanwhile, when the pre-treatment
was applied under a highly biased harsh condition, the
catalyst can have much extended durability for selective
C-C coupling. From the ex-situ surface analysis by XRD,
XPS, and XANES, it was found that C2H4 production was
favored on the catalysts containing mixed oxidized copper
species along with high oxygen content, while sharp in-
crease of CH4 production was observed when Cu2+ disap-
peared on the surface. These results show the relation
between the durability of ethylene production and Cu-O
containing surface states. This will provide new strategies
to control the selectivity and durability of O-Cu combina-
tion catalysts for electrochemical CO2 conversion to val-
ue-added chemicals.
9
These results also imply that the more abundant oxy-
gen (Cu2+ and Cu+) state of the Cu-based catalysts result-
ed in their better C2H4 selectivity. High resolution XPS of
the Cu 3p spectra also showed that the HPR-Cu had a
higher intensity Cu2+ peak than the LPR-Cu (Figure S10
and S11). The Cu XPS spectra of the as-prepared AN-Cu
had only Cu0 and Cu2+ states on the surface, but after the
reduction pre-treatment, Cu+, Cu0, and Cu2+ peaks were
observed on the HPR-Cu or LPR-Cu surfaces. Because the
Cu(OH)2 related signal was marginal on the LPR-Cu in
the XRD measurements, the LPR-Cu might have Cu2+
peaks due to native oxide formation even with the vacu-
um package during sample transfer.42-43 Again, the Cu2+
peak was smaller on the LPR-Cu compared with the HPR-
Cu. Meanwhile, between AN-Cu and HPR-Cu, the HPR-
Cu achieved a lower overpotential to C2H4 production
than just anodized copper.
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However, all surface analysis measurement were per-
formed in ex-situ manners which have innate limitations
to estimate the copper oxidation states in a real opera-
tional condition of CO2RR. In other words, the surface
state can be distorted due to the ex-situ condition alt-
hough cares have been taken during sample storage and
transfer. For instance, it is possible that the HPR-Cu cata-
lyst surface are preferentially re-oxidized to have the
mixed oxide states after the CO2RR compared to the LPR-
Cu or Cu foil, then the mixed oxide states would be an
indirect observation of the catalyst surface not the origin
of the enhanced activity for ethylene production.
ASSOCIATED CONTENT
Supporting Information.
This Supporting Information is available free of charge via
the Internet at http://pubs.acs.org.
Additional gas product quantitative data and CO2 RR
post SEM and additional XPS data
AUTHOR INFORMATION
Based on the observation of the higher oxygen content
and the mixed valences of Cu-O on the active catalysts, it
is proposed that mixed valences are related to stable and
selective C2H4 production, while simple Cu2O/Cu cata-
lysts are vulnerable to product changes from C2H4 to CH4.
The mixed valences of Cu-O can be related with the active
species of the ethylene production or can be the conse-
quence of the structural changes. It is still controversial
whether the CO2 RR active species in the O-Cu combina-
tion catalysts are Cu+ or Cu0. Recent in-situ spectroscopy
studies have suggested that the oxidized states of copper
reduces to metallic Cu0 under the negatively biased con-
dition during the CO2 RR although there can still be a
contribution from the oxygen species.14-15 The role of sub-
surface oxygen in ethylene formation has been proposed,
but little study has been taken to keep those oxygen spe-
cies stable in the copper matrix.24-26 Our study suggests
that a harsh electrochemical reduction treatment applied
to the anodized copper contributes to the high ethylene
Corresponding Author
*yjhwang@kist.re.kr
ACKNOWLEDGMENT
This work was supported by Korea Institute of Science and
Technology (KIST) institutional program and the KIST
Young Fellow programs, and also supported by “Next Gener-
ation Carbon Upcycling Project
”
(Project No.
2017M1A2A2046713) through the National Research Founda-
tion (NRF) funded by the Ministry of Science and ICT, Re-
public of Korea.
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