Micro-Electrode with Fast Mass Transport for Enhancing Selectivity of Carbonaceous Products in Electrochemical CO2 Reduction

Article abstract of DOI:10.1002/adfm.202103966

During electrochemical carbon dioxide (CO₂) reduction on copper electrodes in an aqueous electrolyte, one of the key challenges is the competition between hydrogen evolution and CO₂ reduction, especially under large current density. Here, micro-electrodes are designed with a copper wire as the substrate, which shows improved mass transport compared to the planar electrode. The Faradaic efficiency for C₂₊ products reaches 79% with a partial geometric current density ?77.7?mA?cm^?2 on Cu₂O nanowire/micro-electrode, which is 3.7?times higher than Cu₂O nanowire/planar-electrode. The authors also designed CuO and metallic Cu with micro-electrode as substrate and observed enhanced selectivity for carbonaceous products, proving the universality of the concept. The improved activity is attributed to the fast mass transport of CO₂ to the catalytic interface and thus the suppression of hydrogen production.

Full text of DOI:10.1002/adfm.202103966

ReseaRch aRticle
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Micro-Electrode with Fast Mass Transport for Enhancing
Selectivity of Carbonaceous Products in Electrochemical
CO Reduction
2
Qixing Zhang, Dan Ren, Sanjiang Pan, Manjing Wang, Jingshan Luo, Ying Zhao,
Michael Grätzel, and Xiaodan Zhang*
with appreciable amounts.[11–13] However,
achieving high selectivity of carbon-based
products on Cu remains challenging and
During electrochemical carbon dioxide (CO ) reduction on copper electrodes
in an aqueous electrolyte, one of the key challenges is the competition between
2
the competition with H formation is ubiq-
hydrogen evolution and CO reduction, especially under large current density.
2
2
[
14]
uitous in aqueous media. Besides, the
partial current density for carbon-based
products in H-cells still has a large gap
Here, micro-electrodes are designed with a copper wire as the substrate,
which shows improved mass transport compared to the planar electrode.
The Faradaic efficiency for C products reaches 79% with a partial geometric
2
+
with the practical target, which is at least
current density −77.7 mA cm− on Cu O nanowire/micro-electrode, which
2
−2 [15–17]
2
100 mA cm .
Many efforts have been
is 3.7 times higher than Cu O nanowire/planar-electrode. The authors also
designed CuO and metallic Cu with micro-electrode as substrate and observed
enhanced selectivity for carbonaceous products, proving the universality of the
concept. The improved activity is attributed to the fast mass transport of CO to
the catalytic interface and thus the suppression of hydrogen production.
devoted to improving the selectivity of Cu-
based catalysts to hydrocarbons or oxy-
2
[
18–22]
genates, such as morphology design,
[
23–25]
interface engineering,
doping,
alloy, or
2
[26–29]
etc. However, achieving both
high selectivity and large partial current
density of multi-carbon products remains
challenging, especially in H-cells.
1
. Introduction
This is believed to be caused by the low solubility and lim-
ited diffusion of CO from the bulk electrolyte to the inner
2
[
30,31]
Converting carbon dioxide (CO ) to fuels or chemicals using
renewable energy is one of the promising ways to reduce our
dependence on fossil fuels and mitigate the environmental
Helmholtz plane,
production with CO reduction, especially at higher current
leading to the competition of hydrogen
2
2
[
30]
[32–34]
density. The mass transport rate can be estimated by:
[1–4]
impact of CO emission.
Electrochemical reduction of CO2
2
powered by renewable electricity has attracted much attention
since appreciable reaction rates have been achieved, and it
promises a sustainable carbon-neutral economy.[
D
M =
(1)
δ
N
5–10]
Copper (Cu) is considered the only metal capable of cata-
Where D is the diffusion coefficient of CO , and δ is the
2 N
lyzing the reduction of CO to hydrocarbons and oxygenates
Nernst diffusion layer thickness. The thinner diffusion layer
will lead to faster mass transport based on Equation (1) and
Figure 1a. Besides, compared to the planar-electrode with linear
diffusion (Figure 1b), the diffusion pattern of cylindrical micro-
electrode is nonlinear (Figure 1c), which will enhance the mass
2
Dr. Q. Zhang, S. Pan, M. Wang, Prof. J. Luo, Prof. Y. Zhao, Prof. X. Zhang
Institute of Photoelectronic Thin Film Devices and Technology
Key Laboratory of Photoelectronic Thin Film Devices and
Technology of Tianjin
[
33,34]
transport rate.
Ministry of Education Engineering Research Center of Thin Film
Photoelectronic Technology
Renewable Energy Conversion and Storage Center
Solar Energy Conversion Center
Nankai University
Tianjin 300350, China
E-mail: xdzhang@nankai.edu.cn
Dr. D. Ren, Prof. M. Grätzel
Laboratory of Photonics and Interfaces
Institute of Chemical Sciences and Engineering
École Polytechnique Fédérale de Lausanne (EPFL)
Lausanne 1015, Switzerland
In this study, we synthesized Cu O nanowire onto a micro-
2
sized Cu wire electrode with a radius of 50 µm, named
Cu O-ME, to enhance the mass transport of CO . As a con-
2
2
trol sample, Cu O nanowire onto planar Cu mesh substrate,
2
named Cu O-PE, was also used for CO reduction. Cu O-ME
2
2
2
exhibits enhanced selectivity of C2+ products and a remarkable
partial current density, which are higher than those observed
on Cu O-PE. In addition, CuO nanowire/micro-electrode
2
(
Hereafter referred to as a CuO-ME) and Cu/micro-electrode
(Hereafter referred to as a Cu-ME) are also synthesized simi-
larly. Interestingly, both micro-electrodes showed suppressed
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202103966.
hydrogen evolution and enhanced CO reduction process. The
2
improved performance of all micro-electrodes is attributed to
improved mass transport, leading to higher concentration of
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Figure 1. Schematic illustration of active substance diffusion on electrodes of different geometric shapes. a) The curve of the mass transport rate versus
−
5
2
−1 [31]
the diffusion layer thickness, the diffusion coefficient D = 1.91 × 10 cm s . Insert images is the schematic diagram of the concertation of CO on
2
the surface of the electrode. The diffusion path of the active substance on b) planar-electrode and c) cylindrical micro-electrode (Radius = 50 µm).
CO on the surface of the electrode and the suppression of H2
X-ray diffraction (XRD) patterns of Cu O-PE demonstrated
2
2
o
production. The conclusion achieved from the studied single
micro-electrode with fast mass transport in electrocatalysis can
be implemented to achieve more efficient commercially avail-
able electrodes in future and provides a new strategy to design
the presence of Cu O and Cu (Figure 2g). The peaks at 36.9
2
and 42.6° could be assigned to Cu O (111) and Cu O (200),
2
2
respectively, consistent with HRTEM. The peak at 43.3° could
be assigned to Cu (111), originating from Cu mesh substrate.
X-ray photoelectron spectroscopy (XPS) illustrated the features
electrodes for efficient CO reduction.
2
of Cu( at 932 and 952 eV (Figure 2h). The peaks at 934.4
0/I)
[36]
(II)
[37]
and 954.5 eV indicated Cu existing.
After the electrochemical prereduction process, the sur-
face morphology of Cu O-ME and Cu O-PE still maintained
2
. Results
2
2
2
.1. Synthesis and Characterization of Electrodes
a nanowire structure, and the grains on the nanowire showed
no noticeable change (Figure S2a,b, Supporting Information).
Besides the peaks at 38.8o and 42.6o assigned to CuO and
Cu2O respectively, the main peaks represent the Cu phase
(Figure S2c, Supporting Information). Besides, the XPS result
Figure 2a illustrates the synthetic process for preparing
Cu O nanowire electrodes (Cu O-NW). First, Cu(OH) NWs
2
2
2
(Figure S1, Supporting Information) were obtained by anodic
−2
(0/I)
(II)
oxidation of Cu mesh at 10 mA cm current density for
min in 3 M KOH and then transferred to a tube furnace for
annealing at 500 °C for 3 h under argon environment to form
also verifies the Cu
porting Information).
and Cu features (Figure S2d, Sup-
[
36,37]
5
After CO reduction testing, the sur-
2
face morphology of Cu O-PE and Cu O-ME was characterized
2
2
[35]
Cu O planar electrode (Cu O-PE).
Cu O micro-electrode
(Figure S3, Supporting Information). The nanowires become
2
2
2
(
Cu O-ME) is consisted of only one mesh wire of Cu O-PE by
rough, which could be attributed to the reduction of Cu O to
Cu during the electrocatalytic process.
2
2
2
[
27]
simply removing one wire from the mesh.
The structure of Cu O-ME with micron size is illustrated
2
in Figure 2b. They are composed of multiple grains with
distinct grain boundaries. High-resolution TEM (HRTEM)
image showed the lattice of Cu O (111) and Cu O (200) facets
2.2. Enhanced Selectivity of Cu O-ME for CO2
Reduction to C2+ Products
2
2
2
with lattice spacing values of 0.242 and 0.214 nm, respectively
Figure 2c). In addition, elemental Cu and O were uniformly
distributed throughout the nanowire as identified by EDX map-
ping images (Figure 2d–f).
(
The working electrode was electrochemically tested in an
H-type reactor of a three-electrode setup (Figure S4, Supporting
Information). Potentiostatic measurements from −0.7 to −1.15 V
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Figure 2. Preparation and chemical characterizations. a) Schematic diagram of preparation of Cu O-PE and Cu O-ME. b) SEM and c) HRTEM images
2
2
of Cu O NWs. d) TEM, e) Cu, and f) O EDX mapping images of Cu O NWs. g) XRD patterns of Cu O NWs. h) High-resolution Cu 2p XPS spectra of
2
2
2
Cu O NWs. Scale bars: 1 µm and 5 nm for (b) and (c) respectively; 500 nm for (d–f).
2
versus RHE in CO -saturated 0.1 M KHCO solution were car-
50 µm size, the FE of the C2+ on the larger micro-electrode with
2
3
ried out on Cu O-ME with 50 µm and Cu O-PE with 100-mesh
90 µm decreased at −1.05 V, and the FE of C2+ on the Cu O-ME
2
2
2
electrode. The current density remained stable during the
000 s measurement (Figure S5, Supporting Information). Gas
chromatography (GC) and high-performance liquid chroma-
tography (HPLC) was used to analyze gas and liquid products,
respectively.
with 25µm was also slightly reduced at −1.1 V (Figure S6 and
Tables S5,S6, Supporting Information). The above results illus-
trated that the size of micro-electrode influences the selectivity
of C2+ under different applied potential. Still, all of the micro-
electrode successfully suppressed hydrogen generation and
promoted the selectivity of multi-carbon products. The perfor-
2
The geometric current density jtotal and partial geometric cur-
rent density for CO reduction (jCO2RR), C2+ products (jC2+), and
mance of the Cu O-PE with different mesh numbers was also
2
2
H (jH2) were calculated on each electrode in the potential range
2
studied. Similar selectivity of C2+ products was achieved on
from −0.7 to −1.15 V (Figure 3a,b). jtotal and jCO2RR on Cu O were
Cu O-PE with 60 mesh, 100 mesh, and 150 mesh at −1.05 V
2
2
improved compared to the value on Cu O-PE (Figure 3a). The
(Figure S7 and Tables S7,S8, Supporting Information), but
2
−
2
Cu O-ME also achieved an impressive j
Figure 3b), while jC2+ only reached −20.8 mA cm for the
of −77.7 mA cm
the Cu O-PE with 100 mesh has the best selectivity for multi-
2
C2+
2
−
2
(
carbon products at −1.15 V. Faradaic efficiency of C2+ products
at the optimal current density was summarized in Figure S8,
Cu O-PE at −1.15 V.
2
The FE values for hydrogen (H ), ethylene (C H ), eth-
Supporting Information, showing our Cu O-ME with a diam-
2
2
4
2
anol (C H OH), and other C products on the Cu O-ME
eter of 50 µm shows the highest current density for multi-
carbon products.
2
5
2+
2
and Cu O-PE at different applied potentials are shown in
2
Figure 3c,d. By comparing the product distributions at
The stability of the Cu O-ME was evaluated at −1.15 V in
2
optimal potentials, we found that the total C2+ FE on Cu O-
0.1 M KHCO electrolyte for 10 h. The performance was main-
2
3
ME added up to 79% at −1.15 V, while the FE of 49% was
tained during long-term electrolysis, and the average FE of
70% for C2+ products was achieved (Figure S9, Supporting
Information). After the stability test, the surface morphology
achieved on Cu O-PE. An ethylene FE of 42% and ethanol
2
FE of 34% on Cu O-ME was achieved with a current density
2
−
2
–2
of −41.6 mA cm and −34.0 mA cm at −1.15 V, respectively
Figure 3c, Tables S1–S4, Supporting Information). A distinc-
of Cu O-ME becomes rough (Figure S10, Supporting Infor-
2
(
mation) due to the reduction of Cu O to Cu during the CO
2
2
tively different selectivity of H2 was observed on Cu O-ME
reduction reaction. XPS results demonstrated that peaks at
2
932.2 eV and 952.1 eV were attributed to Cu(
0)/(I)
(Figure S10b,
and Cu O-PE (Figure 3d). The C products-to-hydrogen ratio
2
2+
increased from 1.13 on Cu O-PE to 4.34 on Cu O-ME at −1.15 V.
This result indicates that increasing the mass transport of CO2
helps suppress hydrogen evolution effectively.
Supporting Information). The small peaks at 934.8 and
2
2
954.7 eV assigned to Cu( were attributed to quick oxidation
II)
when exposed to air. HRTEM further verified that Cu O was
2
The diameter of ME was also studied by using Cu O-ME
reduced to Cu after a long-term reaction (Figure S10d, Sup-
porting Information).
2
from different mesh substrates. Comparing the Cu O-ME with
2
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Figure 3. CO performance comparisons on Cu O-ME and Cu O-PE. a) Total current density (j ) and partial current density of CO reduction
2
2
2
total
2
(
jCO2RR). b) The partial current density of H2 (jH2) and C2+ products (jC2+). c) Faradaic efficiency of ethylene, ethanol, and other C2+ products.
d) Faradaic efficiency of all C2+ products and hydrogen, and the Faradaic efficiency ratio of C2+ products to hydrogen. Orange stands for Cu O-ME
2
and red for Cu O-PE.
2
2
.3. Effect of the Electrochemical Active Surface Area on the
rate of *CO (TOFCO), which is composed of gas (TOFCO(g)) and
be further reduced CO (TOF*CO) (Figure 4c). TOF*CO is enhanced
Rate of Formation for C2+ Products
−1
−2
−1
−2
from 410.3 nmol s cm on Cu O-PE to 1431.2 nmol s cm on
2
−1
−2
To evaluate the intrinsic catalytic activity, the electrochemi-
cally active surface area (ECSA) of both electrodes was cal-
culated (Figure S11 and Table S17, Supporting Information).
The specific current densities are shown in Figure 4a,b.
Cu O-ME still has a more significant specific current den-
sity than Cu O-PE at the applied potential (Figure 4a,b),
Cu O-ME at −1.15 V, where ΔTOF
production rate of CO gas increased from 29.2 nmol s cm on
Cu-PE to 38.4 nmol s cm on Cu O-ME, where ΔTOF
9.2 nmol s cm . That is to say, only 9.2 nmol s cm of the
increased CO was released to form CO gas, while 99% of CO
was further reduced to C2+ products, which proved that a higher
ratio of CO intermediate was reduced to multi-carbon products
= 1020.9 nmol s cm . The
2
*CO
−1
−2
−1
−2
=
2
CO(g)
−1
−2
−1
−2
2
2
demonstrating enhanced activity of the micro-electrode. The
smaller specific current density of hydrogen was observed on
on Cu O-ME.
2
Cu O-ME, which indicated that hydrogen evolution was sup-
Operando Raman spectroscopy was used to identify key
2
pressed at potential <−1.05 V. These results demonstrated
that the micro-electrode with fast mass transport increased
the concentration of CO2 on the surface, contributing to
enhanced selectivity of multi-carbons products was achieved
by suppressing the hydrogen.
reaction intermediates on Cu O-ME (Figure S12, Supporting
2
Information) in CO -saturated 0.1 M KHCO . Three bands at
2
3
−
1
275–285, 354–362, and 2070–2080 cm were recorded. They
can be attributed to the adsorbed CO intermediates on the sur-
The bonds at 2844 and 2912 cm could be assigned
to CH containing intermediates.
trate that these intermediates were vital to the formation of
[
27,29,38]
−1
face.
[
27,39]
To understand the effect of the micro-electrode on enhanced
C2+ products, we further quantitatively analyzed the production
Above results illus-
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Figure 4. The intrinsic activity of Cu O-ME and Cu O-PE. a) Values for the total current density (j ) and CO reduction partial current density
2
2
total
2
(
jCO2RR) on Cu O-PE and Cu O-ME within the potential range −0.7 to −1.15 V, respectively. b) hydrogen partial current density (jH2) and C2+ partial
2 2
current density (jC2+) on Cu O-PE and Cu O-ME within the potential range −0.7 to −1.15 V, respectively. c) Quantitative analysis of the turnover
2
2
frequency for producing (TOFCO):gas (TOFCO(g)) and for forming reduced CO (TOF*CO) on Cu O-PE and Cu O-ME within the potential range −0.7
2
2
to −1.15 V, respectively. d) Schematic illustration of the suppression of hydrogen formation and improved the concertation of CO intermediate at
Cu O-PE and Cu O-ME.
2
2
hydrocarbons or oxygenates. In addition, bands at 503–586 and
transfer rate is inversely proportional to the thickness of the
diffusion layer. Therefore, the above results quantitatively
confirmed that the micro-electrode with fast mass trans-
port decreased the thickness of the diffusion layer, contrib-
−
1
7
03–752 cm were proposed to be assigned to adsorbed OH
−
−
group and in-plane bending δ(CO ) vibrations of (C, O)-CO ,
2
2
respectively.[ The formed OH group is due to the existed
40]
nucleation site for hydroxylation on the subsurface oxygen of
uting to increased concentration of CO on the surface of the
electrode.
2
−
1
Cu O-ME in an aqueous solution. The band at 1054 cm was
2
2−
considered to adsorbed CO3 , which formed by bicarbonate
reacts with hydroxide during CO reduction reaction.
The schematic concentration of CO near the surface was
2
[40–42]
illustrated in Figure 4d, illustrating more CO near the surface
2
2
Electrochemical impedance spectroscopy can be used to
measure the thickness of the diffusion layer (Figure S13, Sup-
porting Information).[43] The diffusion layer thickness δ was
estimated to be 123 and 40.3 µm on Cu O-PE and Cu O-ME
of micro-electrode. A higher concentration of CO intermediate
formed covers the micro-electrode surface which attributing to
the fast mass transport, increased the concentration of CO2,
leading to enhanced further reduced CO to multi-carbon prod-
ucts and inhibited the formation of hydrogen.
2
2
electrodes, respectively. As shown in Figure 1c, the mass
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Figure 5. CO performance on CuO-ME and Cu-ME. a) Faradaic efficiency of all C products and H of CuO-PE and CuO-ME. b) Faradaic efficiency
2
2+
2
of CH and H of Cu-PE and Cu-ME.
4
2
2
.4. Universality of the Concept
and the surface of Cu O-ME is increased, favoring CC cou-
2
pling to C2 products. CuO-ME and Cu-ME were also prepared,
+
CuO-ME and Cu-ME were prepared to verify that different
materials with different structures also fit into this design
strategy. The CuO NWs exhibited ribbed structure (Figure S14,
Supporting Information) and Cu-ME shows a plane surface
with some folds (Figure S15, Supporting Information). Com-
paring with the planar electrode, similar results were achieved,
where the FE of H2 was suppressed on both CuO-ME and
Cu-ME (Figure 5, Tables S9–S12, Supporting Information).
and the improved performance further verified the universality
of this strategy. In general, faster mass transport on micro-
electrode leads to a thinner diffusion layer that does not expand
with time, contributing to a higher concentration of CO on the
surface of the electrode and improving the selectivity of hydro-
carbons or oxygenates. This work will provide a novel idea and
general way for catalyst design.
Formation of C2+ products and CH were maximized on the
4
CuO-ME and Cu-ME with FE of 75% at −1.0 V and 63% −1.25 V, 4. Experimental Section
respectively. The CuO-PE and Cu-PE with different mesh num-
bers were also prepared. As increased the mesh number, the
Chemicals: All reagents were used without further purification unless
noted. All solutions were prepared with 18.2 MΩ∙cm deionized water.
FE of H increased. And the CuO-PE and Cu-PE with different
2
Potassium bicarbonate (KHCO , ≥99.95%, Sigma-Aldrich), potassium
3
mesh numbers shown similar selectivity of CO reduction to
hydroxide (KOH, 99.999%, Sigma-Aldrich), Hydrochloric acid (HCl,
analysis pure). Cu mesh (99.99%, 60, 100, and 150 mesh) was purchased
from Hebei Zhanmo metal materials.
2
multi-carbons and CH products, respectively (Figure S16 and
4
Tables S13–S16, Supporting Information). To demonstrate the
intrinsic activity of each electrode, the ECSA was also estimated
Cu O-PE and Cu O-ME: Cu mesh was cleaned with ethanol and HCl
2
2
to remove the surface organics and oxides. Then Cu mesh was oxidized
(Figures S17,S18 and Tables S18,S19, Supporting Information).
−
2
in 3 M KOH at 10 mA cm current density to form Cu(OH) nanowires/
2
The conversion efficiency of CO being further reduced based
on specific TOFCO was calculated to be ≈100% on CuO-ME at
Cu mesh. Deionized water was used to remove residual KOH. Cu(OH)2
nanowires/Cu mesh was transferred to a tube furnace under an Ar
environment for heat treatment. The furnace was programmed to heat
at a rate of 5 °C per min to 500 °C and was maintained at 500 °C for
−
1.0 V and ≈96% on Cu-ME at −1.20 V, respectively (Figures S19
and S20, Supporting Information).
3
h. The furnace was cooled down to room temperature to form Cu O
This work is different from previously published works
2
using nanowire substrates,[
44–45]
nanowire/Cu mesh (Cu2O-PE). Cu2O micro-electrode was prepared by
taking one wire from Cu O nanowire/Cu mesh (Cu O-ME).
where researchers employed
2
2
substrates with multiple nanowires. These electrodes experi-
ence serious mass transport issues, especially at large overpo-
tentials. In contrast, this work uses a single nanowire which
has improved mass transport. The current density and faradaic
efficiency of C2+ products have been improved significantly.
CuO-PE and CuO-ME: Cu(OH) nanowires/Cu mesh was synthesized
2
by a similar process as Cu O-PE except for the heat treatment step.
2
Cu(OH) nanowires/Cu mesh were annealed in a muffle furnace under
2
an air environment at 200 °C for 2 h.
Cu-PE and Cu-ME: Cu mesh as the Cu-PE after cleaned by ethanol
and HCl in turn. Cu micro-electrode was prepared by taking one wire
from the Cu mesh, as Cu-ME.
Material Characterization: SEM images were taken using an Apreo
S LoVac microscope. Structural characterization of electrodes was
obtained using XRD (Rigaku) with Cu Kα radiation. XPS data were
acquired by Thermo Scientific ESCALAB 250Xi with an Al Kα source.
TEM characterizations were performed using a JEM-2800 with a field
emission gun operated at 200 kV.
Operando Raman spectroscopy: Horiba XPloRA confocal Raman
microscope, equipped with a diode laser of 638 nm with 30 mW power,
was used for oerpando Raman spectroscopy. A water immersion
objective (100×, Olympus) was used for all the measurements. A Teflon
FEP parafilm (Dupon, 25 µm thickness) was used to cover the objective
3
. Conclusions
In summary, we designed the Cu O-ME for CO reduction and
achieved Faradaic efficiency of 79% to C products with a par-
tial current density of −77.7 mA cm . The Faradaic efficiencies
of the products demonstrate that this micro-electrode can effec-
tively suppress hydrogen production, yielding enhanced C2
products. The results of the production rate of CO further illus-
trate that the chance of the contact between CO intermediates
2
2
2+
−2
+
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with a water drop between parafilm and objective. 0.1 M KHCO3 was
The partial current density of the product ƒ was calculated by the
used as the electrolyte, and CO continuously flowed into the electrolyte
equation:
jf = jtotal × FE
Faradaic efficiency of gas products ƒ:
2
during measurement. Ag/AgCl (saturated KCl) and electrodeposited
IrO2 were used as the reference and counter electrode, respectively.
Spectra were presented as they were without any background smoothing
or subtraction.
Electrocatalytic Measurements: Electrochemical tests were performed
using an Autolab Pgstat204 potentiostat in an H-cell. The H-cell includes
two compartments, separated by an anion exchange membrane under
ambient conditions. The measurements were carried out in a three-
electrode system. Ag/AgCl, as the reference electrode, was placed in
(
f
)
(7)
(8)
I × t
I0 × t
0
Ntotal
=
=
1.602 × 10−19C /e
e
the cathodic compartment. IrO /Ti foil was used as a counter electrode.
2
CO -saturated 0.1 M KHCO solution was used as the electrolyte, which
Where I is the average current obtained from the chronoamperogram.
2
3
0
was stirred at a rate of 1500 rpm during electrolysis. Besides, CO gas
V0
2
The time t is taken to fill the sample loop is: t = . V is the volume of
0
flow was controlled by a mass flow controller at a specified flow rate
of 10 sccm. The applied potentials were IR-compensated and were
converted to the RHE scale.
v
the sample loop for hydrocarbons in the GC. v is the flow rate of the CO2
gas. The final equation of Ntotal is:
During electrolysis, gas-phase products from the H-cell were
quantified by a GC (Thermo scientific, Trace 1310) equipped with N as
the carrier gas. Liquid products were analyzed using HPLC (Agilent 1260
2
I0 × V
e × v
0
I0 × V0
Ntotal
=
=
(9)
v × 1.602 × 10−19C /e
Infinity II) with 0.5 mM H SO as the mobile phase. Typically, 900 µL of
2
4
the post-electrolysis catholyte was mixed with 100 µL of 4.5 M H SO to
2
4
neutralize the electrolyte before sampling.
The equation of the number of electrons required to produce ƒ (Nf) is:
N = x0 × n × NA × m1e
ECSA Measurement: ECSAs of the electrodes were quantified
by measuring double layer capacitances. Cyclic voltammetry
measurements were carried out at a potential window with no faradaic
(10)
f
−
1
process at scan rates of 20, 40, 60, 80, and 100 mV s (Figures S7, S11,
and S12, Supporting Information). The curve of the non-faradaic
current densities versus the scan rate was plotted, and the slope
of this curve was calculated. The rough factors were obtained by
the following equation: RF = C /C , where specific capacitance
Where x0 is the achieved ppm of the ƒ. m1 is the number of electrons
required to form 1 molecule of ƒ.
According to the ideal gas law, n is the amount of gas in each sample
loop V0 under ambient temperature:
DL
S
−
2 [46]
CS = 0.03 mF cm
.
Calculation of the Turnover Efficiency (TOF): The production rate of CO
P × V0
R × T
n =
intermediate (TOFCO) consisted of CO gas (TOFCO(g)) and CO (TOF*CO
)
(11)
(12)
that was being further reduced:
The final equation of N is:
f
TOFCO = TOFCO + TOF∗CO
(2)
( )
g
x0 ×NA × m e × P ×V0
1
N =
f
Where TOF*CO was calculated as:
R × T
TOF∗CO = TOFCH4 + 2TOFC2H4 + 2TOFC2H6 + 2TOFCH3COOH
The equation of the Faradaic efficiency of the product ƒ is:
(3)
+
2TOFC2H5OH + 3TOFC3H6O + 3TOFC3H8O + 3TOFC3H7OH
x0 × NA × m × P × v × 1.602 × 10−
19
Nf
Ntotal
FE
(
f
)
=
=
× 100%
(13)
(14)
In the case of the CO products, the equation of TOFCO was calculated
I0 × R × T
by:
Faradaic Efficiency of Liquid Products L:
jspecific
TOFCO
=
(4)
nF
NL
FE
( )
L =
Ntotal
Where F is the Faradaic constant, n is the transfer electron number
and jspecific is the specific partial current density.
The conversion efficiency of further reduced CO was estimated by:
The equation of Ntotal is:
Q0
e
Q0
∆
CO
TOF∗CO
TOF∗ + ∆TOFCO g
( )
Ntotal =
=
(15)
1.602 × 10−19C /e
Efficiency =
×100%
(5)
∆
Where Q is the total charge during the CO reduction process.
0
2
The difference of TOFCO between Cu O-PE and Cu O-ME was
2
2
The equation of N is:
L
calculated as the amount of CO produced on active sites, defined here
as ΔTOFCO
.
N = C × V × N × m2e
(16)
Calculation of the Faradaic efficiency of CO2 Reduction: The Faradaic
efficiency of the product ƒ was calculated by the equation:
L
L
A
Where V is the volume of catholyte. m is the number of electrons
2
Number of electronsrequired to produce f
required from 1 molecule of L.CL is the concentration of L in the
catholyte.
FE
(
f
)
=
×100%
(6)
Totalnumber of electrons for CO reduction
2
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The equation of the Faradaic efficiency of the product L is:
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
C × V ×N × m ×1.602 × 10−
19
L
A
2
Q0
FE
(
L
)
=
× 100%
(17)
Details of Faradaic Efficiency Calculation: In order to verify the accuracy
of the FE calculation formula, the CO reduction results of Cu O-ME at Acknowledgements
2
2
−
1.15 V were used as an example to calculate the value of FE (Table S20,
Supporting Information).
This work was supported by the following projects: International
Cooperation Projects of the Ministry of Science and Technology
(2014DFE60170), the Strategic Japanese–Swiss Science and Technology
Program from the Swiss National Science Foundation (project No.
IZJSZ2_180176), the Sino-Swiss Science and Technology Cooperation
Faradaic efficiency of C H gas product: V is the volume of the sample
2
4
0
3
3
−1
loop in GC is 1 cm , the flow rate of CO is v = 10 cm min . Therefore,
to fill the sample loop, the time t is:
2
(
SSSTC) 2016 project from the Swiss National Science Foundation
1 cm3
V0
v
(project No. IZLCZ2_170294), the National Natural Science Foundation of
China (Grant No. 61674084), the Overseas Expertise Introduction
Project for Discipline Innovation of Higher Education of China (Grant
No. B16027), Tianjin Science and Technology Project (Grant No.
18ZXJMTG00220), and the Fundamental Research Fund for the Central
Universities of China.
t =
=
= 0.1 min = 6 s
1
(18)
10 min−
n is the amount of gas in each sample loop V0 under ambient
temperature:
1.013 × 10 Pa × 1× 10 m3
5
−6
P ×V
0
4.073 × 10−5 mol
n =
=
(19)
R ×T 8.314 J·K− ·mol × 299.150K
1
−1
Conflict of Interest
C H of vial 4 as an example. The number of electrons required to
2
4
form 1 molecule of C H is 12. The number of electrons (NC2H4) needed
The authors declare no conflict of interest.
2
4
to get x ppm of ethylene is:
0
−
6
−5
N = x × n × N × m e = 156.66 × 10 × 4.073 × 10 mol
f
0
A
3
1
−1
(20) Author Contributions
2
16
×
6.02 × 10 mol ×12e = 4.6094 × 10
e
Q.Z. and D.R. contributed equally to this work. X.Z. supervised the
According to the injection time to fill up vial 4, the recorded current
project. Q.Z. designed and executed the experiments. D.R. conducted
the operando Raman step and data processing. J. L., S.P., and M.W.
aided in the course of the experiment. J.L. and X.Z. provided the
experiment platform. J.L., M.G., and Y.Z. provided constructive advice.
X.Z., M.G., Q.Z., and D.R. contributed to the writing of the manuscript.
All authors discussed the results.
is I = 2.9999 mA(this data was obtained from the chronoamperogram).
0
Total number of electrons (Ntotal) measured during this sampling period:
I × t 2.9999 × 10− × 6 s
3
0
= 1.1236 × 1017
Ntotal
=
=
e
(21)
(22)
−19
e
1.602 × 10 C /e
Hence,
Data Availability Statement
Research data are not shared.
4.6094 ×10 6e
1
NC2H4
Ntotal
FE
(
C2H4
)
=
× 100% =
× 100% = 41.02%
17
1.1236 ×10 e
Faradaic Efficiency of C H OH Liquid Product: C H OH as an example
2
5
2
5
Keywords
for calculation. 12 electrons were required to form one ethanol molecule
3
from two CO2 molecules. The volume of catholyte is V = 10 cm . The
CO2 reduction, diffusion layer, increased concentration of CO ., mass
transport, micro-electrodes
2
equation of the produced ethanol NC2H5OH is:
× V × NA × 12e = 0.191× 10 molL−1
−
3
Received: April 26, 2021
Revised: June 10, 2021
Published online:
NC2H5OH = C
C2H5OH
10 × 10−3 L × 6.02 × 10 mol × 12e = 1.38 × 1019e
23
−1
(23)
×
From the chronoamperogram, the total charge is 6.24 C (Q0).
Hence, the equation of the Ntotal is:
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