Formic Acid Electro-Synthesis by Concurrent Cathodic CO2 Reduction and Anodic CH3OH Oxidation

Article abstract of DOI:10.1002/anie.202012066

The electrochemical conversion of carbon dioxide into energy-carrying compounds or value-added chemicals is of great significance for diminishing the greenhouse effect and the efficient utilization of carbon-dioxide emissions, but it suffers from the kinetically sluggish anodic oxygen evolution reaction (OER) and its less value-added production of O₂. We report a general strategy for efficient formic-acid synthesis by a concurrent cathodic CO₂ reduction and anodic partial methanol-oxidation reaction (MOR) using mesoporous SnO₂ grown on carbon cloth (mSnO₂/CC) and CuO nanosheets grown on copper foam (CuONS/CF) as cathodic and anodic catalysts, respectively. Anodic CuONS/CF enables an extremely lowered potential of 1.47 V vs. RHE (100 mA cm^?2), featuring a significantly enhanced electro-activity in comparison to the OER. The cathodic mSnO₂/CC shows a rather high Faraday efficiency of 81 % at 0.7 V vs. RHE for formic-acid production from CO₂. The established electrolyzer equipped with CuONS/CF at the anode and mSnO₂/CC at the cathode requires a considerably low cell voltage of 0.93 V at 10 mA cm^?2 for formic-acid production at both sides.

Full text of DOI:10.1002/anie.202012066

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International Edition Chemie
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Accepted Article
Title: Formic Acid Electro-synthesis by Concurrent Cathodic CO2
Reduction and Anodic CH3OH Oxidation
Authors: Xinfa Wei, Yan Li, Lisong Chen, and Jianlin Shi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202012066
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Formic Acid Electro-synthesis by Concurrent Cathodic CO
2
Reduction and Anodic CH
3
OH Oxidation
a
a
a
a,b
Xinfa Wei , Yan Li , Lisong Chen *, Jianlin Shi *
Xinfa Wei and Yan Li contributed equally.
[
[
a]
b]
Dr. X.Wei, Dr.Y.Li, Dr L. Chen and Prof. J. Shi
Shanghai Key Laboratory of Green Chemistry and Chemical Processes
School of Chemistry and Molecular Engineering, East China Normal University
North Zhongshan Road 3663, Shanghai 200062, P. R. China
E-mail: jlshi@mail.sic.ac.cn (J. Shi) E-mail: lschen@chem.ecnu.edu.cn (L. Chen)
Prof. J. Shi
State Key Laboratory of High Performance Ceramics and Superfine Microstructures
Shanghai Institute of Ceramics, Chinese Academy of Sciences
Ding-xi Road 1295, Shanghai 200050, P. R. China
Supporting information for this article is given via a link at the end of the document.
Abstract: Electrochemical conversion of carbon dioxide into energy
carrying compounds or value-added chemicals is of great
significance for diminishing the greenhouse effect and the efficient
utilization of carbon dioxide emissions, which, however, suffers from
the kinetic-sluggish anodic oxygen evolution reaction (OER) and its
oxygen evolution kinetics, resulting in a large overpotential and
high overall energy input. According to the Gibbs free energy
(via Hess’s law Table S1) analysis,[4] 94.5% of the overall energy
is paid to the OER at the anode, in a typical electrochemical CO
reduction reaction to HCOOH (equations 1-3). In addition, the
obtained O is a rather low value-added product and may lead to
the formation of reactive oxygen species (ROS), which will
reduction and anodic partial methanol oxidation reaction (MOR), shorten the life of cell membrane.[5] Therefore, it is highly
2
less value-added production of O
strategy for efficient formic acid synthesis by concurrent cathodic
CO
using mesoporous SnO
2
. Herein, we report a general
2
2
2
grown on carbon cloth (mSnO
2
/CC) and
desirable to find a more thermodynamically favorable reaction to
substituting for OER, which can even generate value-added
chemicals at much lower potential.
CuO nanosheets grown on copper foam (CuONS/CF) as cathodic
and anodic catalysts, respectively. Anodic CuONS/CF enables an
extremely lowered potential of 1.47 V versus reversible hydrogen
Anode: CO
+2[H]→HCOOH
(1)
(2)
(3)
2
solution
solution
-
2
0
-1
electrode (RHE) to obtain 100 mA cm , featuring a significantly
enhanced electro-activity in comparison to OER. Meanwhile, the
(∆퐺
= 13.7 KJ mol ; 5.5% of total energy)
푟푒푎푐푡푖표푛
0
E cell (V) = 0.07 V
cathodic mSnO
at 0.7 V vs RHE for formic acid production from CO
established electrolyzer equipped with CuONS/CF at the anode and
mSnO /CC at the cathode requires a considerably low cell voltage of
.93 V at 10 mA cm for formic acid production at both sides.
2
/CC shows a rather high Faraday efficiency of 81%
Cathode: H O(l) → ꢀ[H] + 1/ꢀ푂
2
2
0
-1
2
. Moreover, the
∆퐺
0
= 237.1 KJ mol ; 94.5% of total energy
푟푒푎푐푡푖표푛
E cell (V) = 1.229 V
2
Overall: CO
2
푠표푙푢푡푖표푛
+ H O(l) → HCOOH
2
+ 1/ꢀ푂2
푠표푙푢푡푖표푛
−
2
0
= 250.8 KJ mol-1 E cell (V) = 1.299 V
0
0
∆퐺
푟푒푎푐푡푖표푛
In recent years, strategies of substituting anodic OER with
thermodynamically flexible and kinetically favorable oxidation
reactions have been developed for electrocatalytic water splitting
for hydrogen generation.[6] Following this strategy, anodic
Introduction
potentials can be significantly reduced by the introduction of
various reductive molecules, such as N H , NH , methanol,
2 4 3
ethanol, furfuryl alcohol, benzyl alcohol and urea, into the
electrolyte.[7] More impressively, important chemicals could be
obtained at anode via the partial oxidation of small organic
molecules at much lowered electricity consumption.[8] Among
The increasingly serious energy shortage and global warming
have brought a series of environmental and social issues which
have aroused widespread concerns. Carbon dioxide (CO ), a
2
major cause of greenhouse effects, is a massive and readily
available C1 resource which can be converted to various fuel
chemicals to meet the requirement of green chemistry and
those organic molecules, methanol is the simplest monohydric
alcohol mainly derived from coal chemical industry.[9] Owing to
[
1]
sustainable developments.
Electrochemical conversion of
carbon dioxide to useful fuel chemicals with renewable energy
its huge quantity of massive production and low cost, methanol
can be utilized as a general molecule platform for various
input, such as wind, solar and tidal energy, has been attracting
great attentions.[
2]
In general, an electrochemical CO
2
[10]
chemical syntheses. For instance, one of its partial oxidation
conversion system includes carbon dioxide reduction at the
cathode and oxygen evolution reaction (OER) at the anode. The
products, formic acid, is
a kind of important industrial
intermediate that can be used in diverse chemical industry fields
electrochemical CO
process in which electrons are transferred to CO
solution or CO
2
reduction (ECR) is an energy-efficient
molecules in
solvated ions from the surface of cathodic
such as rubber, pharmaceutical, dye, direct formate fuel cell and
2
[11]
hydrogen storage.
However, traditional industrial formic acid
2
production usually employs complicated multi-step processes
and requires large amounts of energy consumption, resulting in
its high cost.[12] Considering the industrial price, a ton of
electrode.[3] In contrast, however, the anodic OER, which is
responsible for donating electrons to ECR, has extremely slow
1
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RESEARCH ARTICLE
on carbon cloth (mSnO
2
/CC ) was synthesized and employed as
an efficient cathodic catalyst for the electrochemical reduction of
CO , which also produces formic acid. In this study, the
2
substitution of OER with kinetically favorable partial MOR at
anode significantly lowers the anodic and cell potential input.
When CuONS/CF was used as the anodic electrocatalyst in
methanol containing electrolyte, the potential to achieve a
current density of 10 mV cm-2 is 250 mV lower than that of OER
and a Faraday efficiency of 95% can be achieved. Besides, the
mSnO
efficiency of 81% for CO
equipped with CuONS/CF at the anode and mSnO
cathode, the established electrolyzer demands as low as 0.93 V
of cell voltage to achieve 10 mA cm , which is 500 mV lower
than that of conventional overall ECR under the same catalysis
2
/CC shows a high catalytic activity and a Faraday
reduction reaction. Finally, when
/CC at the
2
Figure 1: Schematic illustration for the concurrent electrochemical conversions
of cathodic carbon dioxide and anodic methanol to formic acid in one electrolysis
system.
2
-2
methanol might be sold at about $350, while the price of formic
acid, a methanol oxidation product, could be as high as
approximately $1300 due to the high-cost productions and the
increasing market demand. In view of the above discussion, it is
highly potential for formic acid or formate production by
electrochemical methanol oxidation at low energy consumption.
The partial MOR delivers 4 electrons per molecule at equilibrium
2
by CuONS/CF at anode and mSnO /CC at cathode without the
addition of methanol. The present strategy provides an
innovative and highly promising way for the efficient production
of formic acid with high efficiency.
potential of 0.103
equation 4), significantly lower than that of OER (1.229 V vs
standard hydrogen electrode as shown in equation 2).
V versus standard hydrogen electrode
Results and Discussion
(
Partial MOR to formic acid at anode
CH OH
+ H O(l) → 4[H] + HCOOH
(4)
3
0
푠표푙푢푡푖표푛
2
푠표푙푢푡푖표푛
= 40.1 KJ mol-1 E cell (V) = 0.103 V
0
CuONS/CF was synthesized via a facile one-step oxidation of
copper foam by ammonium persulfate under strong alkaline
conditions. Typically, the cleaned copper foam was put into the
solution of 0.2 mol L-1 ammonium persulfate with 3 mol L-1 NaOH
at 60 ℃ for twenty minutes as shown in Figure 2 (see the
Methods for detailed synthetic approaches). The crystal
structure of the obtained CuONS/CF was measured by X-ray
diffraction (XRD). As shown in Figure 3a, in addition to copper
∆퐺
푟푒푎푐푡푖표푛
The strategy of utilizing upgraded anodic reaction with lower
energy requirement than OER for largely reducing the energy
input for hydrogen evolution reaction has achieved considerable
progresses.[13] Excitingly, this strategy is also applicable for
cathodic CO
designed an electrochemical CO
2
reduction reaction. Sumit Verma et al have
conversion electrolyzer by
2
coupling ECR with the partial oxidation of small organic
molecules, and claimed that up to 53% of overall electricity can
(
JCPDS no: 49-0652) and copper oxide (JCPDS no: 28-0666),
no other phase can be observed. The morphology of the as-
prepared CuONS/CF was observed by scanning electron
microscopy (SEM). As shown in Figure 3b, two-dimensional
CuO has been obtained covering on the surface of the copper
foam. The CuO layer is composed of strip nanosheets as shown
in Figure 3c, which is consistent with the transmission electron
microscopy (TEM) imaging (Figure 3d) showing a thickness of
about 6 nm. In addition, the layer thickness of the CuO material
is about 1.8 microns as shown in the side view SEM image and
its corresponding linear-scanning EDX results (Figure S1a,b). As
shown in the high-resolution TEM (HRTEM) image, the visible
lattice d-spacings of the nanosheets are around 0.252 nm, which
can be indexed to the (002) lattice plane of CuO (Figure 3e).
Furthermore, scanning transmission electron microscopic high-
angle annular dark field (STEM-HAADF) imaging and
corresponding elemental mapping were carried out to confirm
the composition and uniform spatial distribution of Cu and O as
be saved when the glycerol oxidation reaction is used to replace
OER.[4] However, this study is limited to reducing the anode
potential. Our concern is, is it possible to integrate cathodic ECR
with anodic upgraded reaction to produce value-added
chemicals concurrently in an electrocatalytic system? Especially,
it will be much more desirable to produce the same chemical
from both anode and cathode in the same electrochemical
reactor simultaneously by finding applicable electrolytes and
active catalysts.
Herein, we report
a general and effective strategy for
concurrent electrochemical conversions of carbon dioxide at the
cathode and methanol at the anode to an identical value-added
product of formic acid, at rather low electricity consumption
using non-precious metal catalysts (Figure 1). Cu-based
catalysts of rather low costs, abundance and ease of handling
are promising for MOR.[14] Particularly, Cu-based materials
directly grown in situ on a current collector enables fast electron
transfer owing to the firm conjunction between the substrates
and active compositions, which can also avoid the catalysts
deterioration in the electrolysis process.[15] The copper oxide
nanosheets grown on copper foam (CuONS/CF) which was
prepared via facile one-step oxidation of copper foam in sodium
hydroxide solution using ammonium persulfate as the oxidizing
agent, were utilized to electro-catalyze the partial MOR to formic
acid at the anode. Mesoporous tin dioxide is a popular catalyst
for electrocatalytic CO
2
reduction with high Faraday efficiency[16]
and exhibited significantly higher performance when grown on
carbon cloth.[17] In this work, the mesoporous tin dioxide grown
Figure 2: Schematic diagram for the synthesis of CuONS/CF.
2
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RESEARCH ARTICLE
Figure 3: Structural and morphological characterizations of CuONS/CF. (a) XRD pattern, (b, c) SEM, (d) TEM, (e) HRTEM images and (f) HAADF image
and corresponding EDS elemental mapping of CuONS/CF. (g) XPS survey spectrum of CuONS/CF. (h) High resolution Cu 2P XPS spectra of CuONS/CF and
copper foam. (i) High resolution O 1s XPS spectra of CuONS/CF.
shown in Figure 3f. Additionally, the SEM elemental mapping
images (Figure S1c) also show the uniform spatial distribution of
Cu and O in CuONS/CF. To further investigate the surface
chemical composition and valence states, the X-ray
photoelectron spectroscopy (XPS) was carried out. The Figure
the current density increases markedly, and the anodic potential
-2
strikingly decreased to 1.47 V vs RHE at 100 mA cm . As
shown in Figure 4b, the anodic potentials in the methanol
solution are reduced by at least 200 mV at the current densities
-2
of 20, 50, 100 and 150 mA cm , compared with those of OER.
The Tafel plots (Figure 4c), which are derived from
corresponding LSV curves, have been involved to evaluate the
kinetics of anodic reaction catalyzed by CuNS/CF. The partial
3g shows XPS survey spectrum of CuONS/NF, in which
characteristic peaks of Cu, O and C elements can be clearly
observed. The quantitative analysis of XPS reveals that the ratio
of oxygen to copper is about 1.18:1, which is in line with the ratio
of 1.23:1 measured by inductively coupled plasma optical
emission spectrometry (ICP-OES). Both peaks of Cu 2p1/2 and
Cu 2p3/2 respectively located at 952.6 eV and 932.8 eV in the
high-resolution spectrum (Figure 3h, bottom) can be assigned to
elemental copper. While as exhibited in the high-resolution Cu
-1
MOR shows a much lower Tafel slop (137 mV dec ) than that of
-1
OER (168 mV dec ), indicating a more favorable catalytic
kinetics of methanol oxidation. Moreover, the electrochemical
impedance spectroscopy (EIS) (Figure S3 and electrochemical
surface area (ECSA) results (Figure S4) show a relatively low
-2
impedance and large ECSA (0.0207 F cm ) of CuONS/CF,
which boost the mass transfer and offer more catalytic sites on
the electrode surface.
2
p spectrum of CuONS/CF (Figure 3h, top), the strong shake-up
2+
peaks at 940.5 eV and 961.3 eV suggest the presence of Cu ,
which cannot be found in the spectrum of copper foam.[18] The
main band of Cu 2p3/2 can be further divided into two
contributions at 933.7 eV assigned to Cu from CuO and 932.7
eV assigned to Cu for inevitable minor flaws of materials from
To analyze the products of anodic reaction, 13C NMR has
been adopted and the results are shown in Figure 4d, from
which formic acid are detected at the chemical shift of 171.1.
This result can also be confirmed by ion chromatography (IC)
measurement (Figure 4d inset) from which an obvious peak of
formic acid at the retention time of 3.48 min can be seen (details
can be seen in experiment part). The Faraday Efficiency of the
partial MOR to formic acid can be calculated from the
quantitative results of IC. As shown in Figure 4e and table inset,
partial MOR Faraday Efficiency is quite high at above 85% in a
broad potential range (from 1.27 to 1.59 V vs RHE) and reaches
a maximum value of 97% at 1.36V vs RHE, indicating a
significantly high selectivity for the conversion of methanol to
formic acid. To know more about the MOR products in addition
to formic acid, further characterizations have been made. Ion
chromatographic spectrum (Figure S5a) shows an increasing
concentration of carbonate after MOR, besides the much higher
2
+
0
the process of synthesis (Figure S2). Besides, the O 1s
spectrum (Figure 3i) can be deconvolved to two peaks belonging
to Cu–O bond of CuO at 529.4 eV and adsorbed oxygen on the
surface of CuO at 530.6 eV, respectively.
To investigate the electrocatalytic performance of as-prepared
CuONS/CF for the partial methanol oxidation to formic acid at
the anode, a series of electrochemical measurements have
been carried out in a three-electrode setup. Figure 4a presents
the linear sweep voltammetry (LSV) curves of CuONS/CF in 1
-
1
-1
mol L KOH with and without 1 mol L methanol. The obtained
CuONS/CF shows a moderate OER activity, which reaches the
-
2
anodic current density of 100 mA cm at the potential of 1.69 V
vs RHE. While in 1 mol L-1 KOH containing 1 mol L-1 methanol,
3
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RESEARCH ARTICLE
-
1
Figure 4: Electrocatalytic performances of CuONS/CF towards methanol oxidation at the anode. (a) LSV curves of CuONS/CF at anode in 1.0 mol L
-
1
-1
KOH with and without the addition of 1 mol L methanol at the scan rate of 5 mV s and (b) anodic potentials to achieve varied current densities (20, 50, 100,
and 150 mA cm ) by CuONS/CF in 1.0 mol L KOH with and without the addition of 1 mol L methanol. (c) Tafel plots for the anodic partial MOR and OER
derived from the LSV results. (d) C NMR spectrum and ion chromatographic spectrum (inset) of the electrolyte after electrochemical test. (e) Faraday
efficiencies of partial methanol oxidation to formic acid at varied potentials. (f) Chronoamperometry responses at constant potential of 1.32 V for 42 h.
-
2
-1
-1
13
amount of formic acid. In this case, formic acid and carbonate
are the only products from MOR that can be detected. To
exhibited in Figure 4f, there has been no more than 5%
decrease of the initial current density after the electrolysis for as
long as 42 hours (Figure 4f). Moreover, the crystal structure,
morphology, composition as well as the element distribution of
CuONS/CF in 42 hours of electrolysis have been measured by
XRD, SEM, and EDS-mapping, respectively. The almost
identical XRD patterns (Figure S7) and SEM images (Figure S8)
before and after long-time electrolysis suggest that the
unchanged lattice phase and morphologies of CuONS/CF.
Moreover, there is no significant change in the composition and
elemental distribution of CuONS/CF (Figure S9) and even the
binding energy shift of Cu2p (Figure S10) after the long-time
electrocatalysis. All these results above demonstrate the
significantly high stability of CuONS/CF which is critically
important for future industrial applications.
enhance the signal intensity of carbonaceous product in 13
C
NMR spectrum, 13CH
electrolyte. In addition to the main product HCOO- at about
3
3
OH was adopted instead of CH OH in the
1
71.1 ppm, peaks at 175.2 ppm and 168.3 ppm, corresponding
2
-
to HCHO and CO
S5b). Therefore, HCHO and CO
3
respectively, can also be found (Figure
2
-
3
are the by-products of MOR.
Besides, continuous chronopotentiometry experiment was
conducted at a potential of 1.32 V vs RHE to record the changes
of methanol and formate concentrations over time. As shown in
Figure S6, the concentration of formate could be as high as 0.67
-1
mol L , which can be further enhanced via replenishing KOH
and methanol to reduce the energy consumption of product
purification. Meanwhile, the stability of CuONS/CF as anodic
electrocatalyst for partial MOR was significantly high. As
2
Electrochemical CO reduction to formic acid at cathode
Figure 5: Structural and morphological characterizations of mSnO
mapping of mSnO /CC.
2
/CC (a) XRD patterns, (b, c) SEM, (d) TEM, (e) HRTEM images and (f) SEM-EDS
2
4
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RESEARCH ARTICLE
Figure 6: Electrocatalytic performances of mSnO
2
/CC in 1 mol L-1 KHCO
/CC, SnS
-saturated 1 mol L- KHCO solution. (d) 1H NMR spectra of the electrolyte after long-term electrochemical test, the inset in
2 3
3
solution at cathode for ECR. (a) Polarization curves of mSnO
2
/CC in CO
2
- or
-
1
-1
Ar-saturated 1 mol L KHCO
mSnO /CC, SnS /CC and CC in CO
d is the ion chromatographic results. (e) The i-t curve and (f) Faraday efficiencies of mSnO
3
solutions. (b) Polarization curves of mSnO
2
/CC and CC in CO -saturated 1 mol L KHCO solution. (c) ECSAs of
2 2 3
1
2
2
2
/CC for ECR at different potentials (from 0.2 V to 0.9 V vs RHE).
-
1
In parallel with the CuONS/CF catalysts used for the rapid
anodic oxidation reaction with ultra-high current density,
mSnO
2
2 3
/CC in Ar or CO saturated 1 mol L KHCO aqueous
-1
solution at a scan rate of 2 mV s (see details in methods
section), which show a remarkable ECR performance. Glaringly,
2
/CC reach a current density value of 50
mA cm-2 at the potential of 0.87 V vs RHE, which is much higher
mesoporous SnO
has been adopted as the efficient cathodic catalyst for ECR to
match the anodic reaction. The mSnO /CC electrode was
synthesized first by growing SnS on a pre-treated carbon cloth
through a typical solvent thermal method (Figure S11a, b),
followed by in-situ convertion to mesoporous SnO by thermal
treatment at 550 ℃ for 120 min. The SnS conversion to SnO
and the formation of mSnO /CC have been demonstrated by
XRD patterns, as shown in Figure 5a. SEM images (Figure 5b,
c and Figure S11c, d) show that SnO grown on the carbon
2 2
in situ grown on carbon cloth (mSnO /CC)
the synthesized mSnO
2
-2
2
than the current densities of SnS
2
(34 mA cm ) and blank
carbon (19 mA cm ) at the same potential as shown in Figure
6b. Such an excellent performance of mSnO /CC for ECR
benefits from its ultrahigh ECSA and BET surface area. The
-2
2
2
2
2
2
-1
2
BET surface area of mSnO
adsorption-desorption isotherm measuremens is much larger
than that of SnS
/CC (43.2 m2 g-1) (Figure S12b), and Figure
S15 and Figure 6c further indicate that mSnO /CC has an
extraordinarily high double-layer capacitance (Cdl) value of 1.5 F
2
/CC (740.3 m g ) from the nitrogen
5
2
2
cloth is in the morphology of two-dimensional nanoplates of 207
nm in diameter and 5 nm in thickness, respectively, inheriting
2
-2
the morphology of SnS
structure of SnO can be clearly observed in SEM images as
shown in Figure 5c and the Brunauer-Emmett-Teller (BET) N
2
(Figure S11a, b). Besides, the porous
2
cm , which is remarkably larger than those of SnS /CC (0.0027
-2
-2
2
F cm ) and blank carbon cloth (0.0013 F cm ). To exclude the
influence of charging and discharging on the ECR performance,
three independent uninterrupted LSV curves have been
recorded as shown in Figure S16, which are in good agreement
with each other (the fluctuations might be due to the slightly
varied ventilation conditions). The liquid products of ECR were
qualitatively and quantiatively determined by the 1H NMR and
ion chromatography, respectively (Figure 6d and inset), and
formic acid is found to be the only carbon-containing product
that can be detected. Figure 6e and 6f show the FE value of
ECR to formic acid at different potentials (from 0.2 to 0.9 V vs
RHE) quantified by ion chromatography, which exhibit the
highest FE value of 81.3% at the potential of 0.7 V. Moreover,
the crystalline structure, morphology, composition as well as the
2
sorption measurement (Figure S12a) shown a IV type of
isotherms which means the formation of mesoporore. TEM
(
Figure 5d) and high-resolution TEM image show that the leaf-
like nanoplate is composed of SnO nanoparticles with a lattice
d-spacing of 0.28 nm corresponding to the (111) lattice plane
Figure 5e). To obtain the chemical states of elements at the
surface of mSnO /CC, XPS data are provided in Figure S13. As
2
(
2
shown in the high-resolution Sn 2d spectrum (Figure S13a), the
peaks at 487.3 eV and 495.7eV can be assigned to Sn 3d3/2 and
Sn 3d5/2 ， respectively, which confirm again the successful
synthesis of SnO2.[2a] As shown in the O1s spectrum of
2
mSnO /CC (Figure S13b), the peak at 531.1 eV can be assigned
to Sn-O, while the shoulder peak at 532.3 eV comes from the
adsorbed oxygen. Furthermore, the elemental composition
measured by SEM-EDS-mapping shows the homogeneous
distribution of Sn and O (Figure 5f) with a Sn/O mole rate of 1:
2
distribution of element of mSnO /CC after 12 hours electrolysis
have been measured by XRD, SEM, TEM and EDS-mapping.
The XRD patterns show the presence of elemental tin, implying
the partial reduction of SnO
cathode (Figure S17). In addition, TEM image (Figure S18a)
shows the degradation of SnO nanoparticles to a certain degree
after long-time electrolysis, while the morphology of two-
dimensional nanoplates of SnO remains unchanged and the
metallic Sn has also been observed in HRTEM image (Figure
2
to metallic Sn during ECR at
1
.95, which is in good agreement with the TEM-EDS-mapping
1: 1.92, Figure S14) results.
The CO electroreduction reaction activities of the as-prepared
mSnO /CC electrode were evaluated in a three-electrode setup
at ambient atmosphere. Figure 6a gives the LSV curves of
(
2
2
2
2
5
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RESEARCH ARTICLE
S18b). Besides, XPS was used to characterize the chemical Conclusion
state of mSnO /CC after ECR. The Sn3d spectrum (Figure
2
S19a) is composed of two contributions from Sn-O (peaks at
In summary, a general strategy has been demonstrated for
the concurrent and efficient formic acid productions at both
anode and cathode by the partial methanol oxidation and carbon
dioxide reduction, respectively. In the current case, non-noble
4
eV).
86.6 eV and 494.9 eV) and Sn-Sn (peaks at 487 eV and 494.3
[
2c, 19]
The phenomenon that the partial reduction of the SnO
2
into metallic Sn has been observed at very negative potentials
by a number of researchers[16b, 20] which is not desired to
metal mSnO
developed as the cathodic and anodic electrocatalysts,
respectively. Both CuONS/CF and mSnO /CC show significantly
2
/CC and CuONS/CF catalysts have been
occur.[21] Fortunately, only a small fraction of SnO
has been
reduced to Sn even after long time electrolysis, therefore the
obtained mSnO /CC shows relatively high stability. The O1s
2
2
2
high electrocatalytic activities, excellent stabilities as well as high
Faradaic efficiencies for both reactions (91.3% and 80.5% at the
anode and cathode, respectively). In addition to high value-
added formic acid production at both sides, the assembled
electrolyzer requires a quite low cell voltage of 0.93V at 10 mA
spectrum (Figure S19b) shows the presences of Sn-O bonds
located at 531 eV and adsorbed oxygen located at 532.3 eV.
Electrochemical performance of CuONS/CF‖mSnO
Based on the excellent electrocatalytic performance of
CuONS/CF for the anodic partial MOR and mSnO /CC for
2
/CC
−
2
2
cm for overall reaction, which is over 500 mV lower than that of
conversional overall ECR under the same condition. The distinct
advantages of this strategy, e.g., low cost, reduced energy input,
high value-added chemicals obtained at both sides, endow the
strategy with an appealing future for formic acid production from
cathodic ECR, a two-electrode electrolyzer using CuONS/CF as
the anode in 1 mol L-1 KOH electrolyte containing 1 mol L-1
methanol, and mSnO
electrolyte (denoted as CuONS/CF‖mSnO
2
/CC as the cathode in 1 mol L-1 KHCO
/CC) has been set
3
2
up and operated at room temperature for the concurrent
electrochemical formic acid productions at both anode and
cathode. The assembled electrolyzer needs a potential as low
both electrochemical CO
oxidation.
2
reduction and partial methanol
−
2
as 0.93 V to obtain the current density of 10 mA cm (Figure
a), exhibiting excellent formic acid synthesis performances by
Supporting Information
7
simultaneous cathodic ECR and anodic partial MOR. It is worth
noting that the required cell voltages of 0.93 V at 10 mV cm-2 in
the present case is over 500 mV lower than that of conventional
overall ECR under the same catalysis by CuONS/CF at anode
Supporting Information is available from the Wiley Online Library
or from the author.
2
and mSnO /CC at cathode without the anodic addition of
methanol. In fact, the voltages are considerably lower than that
of theoretical voltage (1.299 V in equation 3) for overall ECR
Acknowledgements
This work was supported by National Natural Science
Foundation of China (51702099), , China Postdoctoral Science
Foundation funded project (2020T130193). The authors would
like to thank ECNU Multifunctional Platform for Innovation for
support of SEM and TEM characterizations. We gratefully
acknowledge Dr. Guirong Zhang at the East China Normal
University for his help with the IC analysis.
2
Keywords: electrocatalysts, CO reduction, methanol oxidation,
concurrent formic acid production.
2
Figure 7: LSV curves for the CuONS/CF || mSnO /CC cell: (a): Anode: 1.0
-1
-
1
mol L KOH with and without the addition of 1.0 mol L methanol; Cathode:
-
1
-1
Reference
1
.0 mol L KHCO
3
solution saturated with CO
2
). (b): Anode: 1.0 mol L KOH
-
1
-1
added with 1.0 mol L methanol; Cathode: 1.0 mol L KHCO
saturated with CO and Ar). Scan rate, 2 mV s .
2
3
solution
-
1
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7
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RESEARCH ARTICLE
Entry for the Table of Contents
This work reports a strategy of substituting anodic OER with thermodynamic flexible and kinetic favorable oxidation reactions for
efficient electrochemical conversion of CO . Same product of formic acid can be obtained with low cell voltage from both anodic
2
2 2
partial methanol oxidation reaction catalyzed by CuONS/CF and cathodic CO reduction catalyzed by mSnO /CC.
8
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