Aromatic Ester-Functionalized Ionic Liquid for Highly Efficient CO2 Electrochemical Reduction to Oxalic Acid

Article abstract of DOI:10.1002/cssc.202001194

Electrochemical reduction of CO₂ into valuable chemicals is a significant route to utilize CO₂ resources. Among various electroreduction products, oxalic acid (H₂C₂O₄) is an important chemical for pharmaceuticals, rare earth extraction, and metal processing. Here, an aprotic aromatic ester-functionalized ionic liquid (IL), 4-(methoxycarbonyl) phenol tetraethylammonium ([TEA][4-MF-PhO]), was designed as an electrolyte for CO₂ electroreduction into oxalic acid. It exhibited a large oxalic acid partial current density of 9.03 mA cm^?2 with a faradaic efficiency (FE) of 86 % at ?2.6 V (vs. Ag/Ag⁺), and the oxalic acid formation rate was as high as 168.4 μmol cm^?2 h^?1, which is the highest reported value to date. Moreover, the results of density functional theory calculations demonstrated that CO₂ was efficiently activated to a ?COOH intermediate by bis-active sites of the aromatic ester anion via the formation of a [4-MF-PhO-COOH]^? adduct, which finally dimerized into oxalic acid.

Full text of DOI:10.1002/cssc.202001194

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Novel Aromatic Ester-Functionalized Ionic Liquid for Highly
Efficient CO2 Electrochemical Reduction to Oxalic Acid
Authors: Yingliang Yang, Hongshuai Gao, Jiaqi Feng, Shaojuan Zeng,
Lei Liu, Licheng Liu, Baozeng Ren, Tao Li, Suojiang Zhang,
and Xiangping Zhang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.202001194
Link to VoR: https://doi.org/10.1002/cssc.202001194
01/2020

10.1002/cssc.202001194
ChemSusChem
COMMUNICATION
Novel Aromatic Ester-Functionalized Ionic Liquid for Highly
Efficient CO2 Electrochemical Reduction to Oxalic Acid
Yingliang Yang,[a,b] Hongshuai Gao,[a] Jiaqi Feng,[a,c] Shaojuan Zeng,[a] Lei Liu,[a] Licheng Liu,[d,e]
Baozeng Ren,[b] Tao Li,[b] Suojiang Zhang,*[a,c] and Xiangping Zhang*[a,c,e]
[a]
Y. Yang, Dr. H. Gao, J. Feng, Dr. S. Zeng, Dr. L. Liu, Prof. S. Zhang, Prof. X. Zhang
Beijing Key Laboratory of ionic liquids Clean Process, State Key Laboratory of Multiphase Complex Systems
Institute of Process Engineering, Chinese Academy of Sciences
Beijing, 100190, China
E-mail: xpzhang@ipe.ac.cn sjzhang@ipe.ac.cn
Y. Yang, Prof. B. Ren, Prof. T. Li
College of Chemical Engineering
Zhengzhou University
Zhengzhou 450001, China
J. Feng, Prof. S. Zhang, Prof. X. Zhang
College of Chemical Engineering
University of Chinese Academy of Sciences
Beijing 100049, China
[b]
[c]
[d]
[e]
Prof. L. Liu
CAS Key Laboratory of Bio-based Materials
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences
Qingdao 266101, Shandong, China
Prof. L. Liu, Prof. X. Zhang
Dalian National Laboratory for Clean Energy,
Dalian 116023, China
Supporting information for this article is given via a link at the end of the document.
Abstract: Electrochemical reduction of CO2 into valuable chemicals
is a significant route to utilize CO2 resources. Among various
electroreduction products, oxalic acid (H2C2O4) is an important
chemical for pharmaceutical, rare earth extraction and metal
processing. Here, we designed a novel aprotic aromatic ester-
functionalized ionic liquid (IL) of [TEA][4-MF-PhO] as an electrolyte
for CO2 electroreduction into oxalic acid. It exhibited a large oxalic
acid partial current density of 9.03 mA cm-2 with a Faradaic efficiency
(FE) of 86 % at -2.6 V (vs. Ag/Ag+), and the oxalic acid formation
rate is as high as 168.4 μmol cm-2 h-1, which is the highest reported
value to date. Moreover, the results with density functional theory
(DFT) calculations demonstrated that CO2 was efficiently activated
to -COOH intermediate by bis-active sites of the aromatic ester
anion via the formation of [4-MF-PhO - COOH]- adduct, which finally
dimerized into oxalic acid.
the development of high efficient electrochemical CO2 reduction
system for oxalic acid is of great significance.
Previous studies about CO2 electroreduction to oxalic acid
were focused on the introduction of ordinary quaternary
ammonium salts auxiliary electrolyte[11] and the development of
metal complexes catalysts[12], which improved the Faradaic
efficiency (FE) of oxalic acid to more than 90 %, but the partial
current density (< 6 mA cm-2) and formation rate of oxalic acid (<
120 μmol cm-2 h-1) are very low. The reaction process of CO2
electroreduction to oxalic acid generally involves the reduction of
CO2 to CO2·- radical anion intermediate and the dimerization
2- [13]
process for CO2·- into C2O4
.
When the electrolyte contains
active proton H, the CO2·- intermediate easily combines with H
to form formic acid preferentially,[14] so aprotic electrolytic
environment is important to produce oxalic acid.[15] Gennaro et
al.[16] added the aromatic ester or aromatic nitrile compound into
0.1 M n-Bu4NClO4-DMF aprotic electrolyte solution, resulting in
higher FE of oxalate product as 99 %. However, the current
density was only 1.6 mA cm-2, and the oxalate formation rate
was less than 30 μmol cm-2 h-1.
The unprecedented emissions of carbon dioxide (CO2) has led
to an undesirable impact on global climate and environment.[1]
However, with regards to CO2 as a natural, abundant and
inexpensive C1 feedstock,[2] the conversion of CO2 to high
value-added chemicals is benefit both for environment and
economy.[3] Compared with thermal reduction[4] and
As a new type of medium composed entirely of anion and
cation, ionic liquids (ILs) are used in CO2 electroreduction as an
excellent electrolyte because of the unique properties for high
efficient CO2 dissolution and activation, precise control products,
and high conductivity, etc.[17] At present, there were almost no
photocatalytic reduction,[5] electrochemical reduction[6] is
a
promising method because of simple operation and easy
industrial amplification under mild conditions.[7] Among various
CO2 electrochemical reduction products, oxalic acid is an
important chemical widely used in pharmaceutical, rare earth
extraction, metal processing and so on.[8] The traditional oxalic
acid production processes include sodium formate method,
coupling gas phase method from CO and polysaccharide
oxidation method,[9] which suffer from the problems of high raw
material cost, low yield, and serious pollution, etc.[10] Therefore,
any reports of CO2 electroreduction to oxalic acid in IL
16,
electrolyte systems. Inspired by these reports[15,
18], we
designed an aprotic aromatic ester-functionalized IL for CO2
electroreduction to oxalic acid in order to achieve high current
density and FE by enhancing CO2 dissolution and activation
under the effect of IL with bis-active sites.
Herein, we conducted firstly CO2 electrochemical reduction to
oxalic acid using a novel aromatic ester anion functionalized IL
of 4-(methoxycarbonyl) phenol tetraethylammonium ([TEA][4-
1
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10.1002/cssc.202001194
ChemSusChem
COMMUNICATION
MF-PhO]). The principle of designing [TEA][4-MF-PhO] is based
on the catalysis of aromatic esters and the stability of the
quaternary ammonium salt in electrolyte solution. More
importantly, the increase of multiple active sites with phenol and
ester groups in IL could cause the large current density. In
addition to the design the structure of the IL, screening a
suitable electrolyte system is also a nodus. The combination of
[TEA][4-MF-PhO] and aprotic acetonitrile solvent not only
effectively improves the CO2 mass transfer rate, but also
provides aprotic electroreduction environment for CO2, which is
conducive to the generation of oxalic acid. Our results showed
that the partial current density reached 9.03 mA cm-2 with an
oxalic acid formation rate of 168.4 μmol cm-2 h-1, which is the
highest value in the literature to date. To investigate the
interaction between [4-MF-PhO]- and CO2, density functional
theory (DFT) calculations were performed. Subsequently, a
possible reaction mechanism was proposed.
[TEA][4-MF-PhO] was synthesized by tetraethylammonium
hydroxide and methyl 4-hydroxybenzoate via one-step reaction.
The structure of [TEA][4-MF-PhO] was shown in Figure 1, and
the result of nuclear magnetic resonance (NMR) confirmed
[TEA][4-MF-PhO] was synthesized successfully (Figure S1). The
viscosity is fundamental physical parameters of ILs. Since the
electrolyte system is a mixture of IL and acetonitrile (AcN,
aprotic solvent), the viscosities of [TEA][4-MF-PhO] and [TEA][4-
MF-PhO]-AcN solution with different IL mass fractions were
tested. Figure S2 shows that viscosities of various mass fraction
IL decrease with increasing temperature, and the viscosity of
pure IL is greatly affected by temperature. In addition, as the
mass fraction of IL decreasing from 100 wt% to 30 wt%, the
viscosity decreases from 1406.7 to 0.5 mPa s. The lower
viscosity of electrolyte is beneficial to mass transfer in the
reaction process, so we adopt a mixture system with IL-AcN for
the electrolysis experiment. The CO2 absorption experimental
was conducted by the apparatus represented in Figure S3, and
the CO2 absorption capacity in [TEA][4-MF-PhO] was plotted in
Figure S4. 0.58 mol CO2 per mol IL was obtained at 323.15 K
under ambient pressure. Adequate CO2 in the electrolyte can
effectively facilitate the electrochemical conversion of CO2
according to previous reports.[3a]
The electrochemical experiments were carried out with the H-
type cell[2] and using Pb as working electrode (Figure S5). As
revealed by LSV results (Figure 2a), the system with N2-
saturated electrolyte exhibited much lower current density than
that with CO2-saturated electrolyte, which testifying the
occurrence of CO2 electroreduction. The current density as a
function of time at different potentials (vs. Ag/Ag+, all potentials
in this paper are vs. Ag/Ag+) of -2.5 V to -2.9 V in the [TEA][4-
MF-PhO] (0.9 M)-AcN mixture were shown in Figure 2b. The i-t
curves show that the current density is stable in the overall
reaction time of 2 h, indicating that the [TEA][4-MF-PhO] exhibits
a good stability.
100
80
60
40
20
0
(b)
(a)
80
60
40
20
0
-2.5 V
-2.6 V
-2.7 V
-2.8 V
-2.9 V
0.9M IL-AcN, N2
0.9M IL-AcN, CO2
0
1000 2000 3000 4000 5000 6000 7000
Time (s)
-2.1
-2.4
-2.7
-3.0
E (V vs. Ag/Ag+)
Figure 2. The CO2 electrochemical reduction performance at Pb electrode. (a)
The LSV curves in N2 and CO2 saturated electrolytes and (b) the i-t curves in
[TEA][4-MF-PhO] (0.9 M)-AcN electrolyte.
The potentiostatic electrolysis experiments were performed to
investigate the products distribution of CO2 electroreduction in
[TEA][4-MF-PhO] system, and [TEA][BF4], [TEA][PF6] and
[Bmim][BF4] were also tested for comparison. Due to the low
solubility of [TEA][PF6] in AcN, 0.1 M [TEA][PF6]-AcN electrolyte
was selected. After 2 h potentiostatic electrolysis, the partial
current density and FE for oxalic acid in various ILs-AcN
electrolytes were measured. As shown in Figure 3a and Figure
3b, [TEA][4-MF-PhO] (0.9 M)-AcN exhibits an excellent catalytic
activity. For instance, the highest FE of 86 % with partial current
density for H2C2O4 (9.03 mA cm-2) at -2.6 V are obtained, and
the relatively high oxalic acid FE (>70%) can be kept in a wide
range of potentials (-2.5 ~ -2.8 V). However, the FE for oxalic
acid in [TEA][BF4], [TEA][PF6] and [Bmim][BF4] electrolyte
systems are less than 35% entirely, which are much lower than
that of [TEA][4-MF-PhO].
(b)
(a)
100
80
60
40
20
0
[TEA][4-MF-PhO]
[TEA][BF4]
30
25
20
15
10
5
[TEA][PF6]
[Bmim][BF4]
0
-2.5
-2.6
-2.7
-2.8
-2.9
-2.5
-2.6
-2.7
-2.8
-2.9
E (V vs. Ag/Ag+)
E (V vs. Ag/Ag+)
Figure 3. The effect of various ILs on the CO2 electroreduction to oxalic acid.
(a) FE and (b) partial current density of oxalic acid at applied potential of -2.5
V to -2.9 V (vs. Ag/Ag+) in various ILs-AcN electrolytes.
Figure 1. Molecular structures of (a) [TEA][4-MF-PhO], (b) [TEA][BF4], (c)
[TEA][PF6] and (d) [Bmim][BF4].
In particular, various electrolysis products and FE at a
controlled potential of -2.6 V in different ILs-AcN system was
exhibited in Figure 4, which were quantified based on the
2
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10.1002/cssc.202001194
ChemSusChem
COMMUNICATION
standard curves of oxalic acid, formic acid, CO and H2 (Figure
S6). It is clear that the FE for oxalic acid followed the order of
The effect of [TEA][4-MF-PhO] content in the IL-AcN
electrolyte on CO2 electroreduction performance was
[TEA][4-MF-PhO]>[TEA][BF4]>[TEA][PF6]>[Bmim][BF4]. Typically, investigated, and the LSV curves at different [TEA][4-MF-PhO]
there was no oxalic acid generated in [Bmim][BF4] electrolyte,
and most of CO2 was converted to HCOOH. According to the
performance of [TEA][4-MF-PhO], [TEA][BF4] and [TEA][PF6], it
was found that anions play a significant role in the reaction of
CO2 electroreduction to H2C2O4. The reason is probably
attributed to the difference of the interaction between CO2 and
the anions of the ILs. On the other hand, a large amount of
HCOOH is obtained instead of H2C2O4 in [Bmim][BF4]-AcN
system, which is attributed to the active proton H exist on the
imidazole, and it easily combines with the activated CO2·- to
form HCOOH. However, [TEA]+ provides an aprotic
electroreduction environment for CO2, which is more conducive
to generating oxalic acid. Moreover, large current density and
high FE of oxalic acid result in the remarkable oxalic acid
formation rate.[19] The oxalic acid formation rate in [TEA][4-MF-
PhO] reaches as high as 168 μmol cm-2 h-1 at the optimal
potential, which is the highest value among the reported values
of CO2 electroreduction to oxalic acid up to now (Figure 5).
contents were shown in Figure S7a. The current density
increases and the onset potential decreases as the IL content
increase (from 0.6 to 0.9 M). The potentiostatic electrolysis were
performed at -2.6 V to investigate the influence of IL content on
oxalic acid partial current density and FE. As shown in Figure
S7b, the partial current density and FE of oxalic acid significantly
increase until IL content is up to 0.9 M. The phenomenon could
be explained that the number of ions increase with the IL content
increase, resulting in an increasing conductivity of the electrolyte.
Nevertheless, the oxalic acid partial current density and FE
decrease distinctly above 0.9 M IL. This attributed to the reason
that the electrostatic attraction between the IL cations and
anions is enhanced and hindering the movement of the ionic
species. Therefore, 0.9 M [TEA][4-MF-PhO] content is suitable
for CO2 electroreduction to oxalic acid.
The influence of H2O content in the electrolyte solution was
also investigated. The LSV curves in IL (0.9 M)-AcN-H2O
electrolyte with various H2O content (1.0 wt%, 2.5 wt%, 5.0 wt%
and 10.0 wt%) were shown in Figure S8. It shows that the onset
potential decreases with the increase of H2O content in ternary
electrolyte. Furthermore, Figure S9 exhibits the partial current
density of H2C2O4 and FE of all reduction products in IL-AcN-
H2O electrolyte with different H2O content at -2.6 V. The FE and
partial current density of oxalic acid drop sharply with the
increase of H2O content from 0 to 10.0 wt%. Under anhydrous
condition, the highest FE for oxalic acid with 86 % is obtained,
and the by-products are mainly CO with a bit of H2. When H2O
content exceeds 10.0 wt%, oxalic acid is hardly produced. The
reason is that the presence of water in the electrolyte provides
proton H and breaks the aprotic electroreduction environment,
which easily combines with the activated CO2·- to form HCOOH.
Comprehensively speaking, the addition of water to electrolyte is
capable of increasing reduction current and lowing onset
potential, but reducing the selectivity to H2C2O4 and inhibiting the
yield of H2C2O4. That is, the FE of oxalic acid can be easily
controlled by adjusting H2O content in the catholyte.
H2
CO
0.4%
HCOOH
0.6%
H2C2O4
2%
1%
100
80
60
40
20
0
12%
12%
17%
21%
66%
80%
67%
20%
86%
11%
[TEA][4-MF-PhO][TEA][BF4] [TEA][PF6] [Bmim][BF4]
Figure 4. FE of electrolysis products for H2C2O4, HCOOH, CO and H2 in
various ILs-AcN electrolytes at -2.6 V (vs. Ag/Ag+) on Pb electrode.
In order to evaluate the interaction between aromatic ester
anion of [4-MF-PhO]- and CO2, DFT calculations were performed.
Figure S10a and Figure S10b illustrate that the [4-MF-PhO]- with
phenoxy and ester double active sites can effectively bend a
stable linear CO2 molecule to CO2·- with O-C-O bond angle of
140° and 145° respectively, which is consistent with previous
reported.[16, 20] After that, Figure S10c and Figure S10d show the
activated [4-MF-PhO]-CO2·- combines with H+ provided by
anodic electrolyte to form the [4-MF-PhO - COOH]-. Importantly,
the O-C-O bond angle of -COOH intermediate with 126° is close
to oxalic acid (125°). The DFT calculations indicate that the
aromatic ester IL of [TEA][4-MF-PhO] is indeed beneficial to CO2
electroreduction to oxalic acid.
Bu4NClO4-DMF/Hg (Gennaro, 1996)[16]
LiClO4-AcN/C (Angamuthu, 2010)[12a]
TEAP-AcN/Pb (Sun, 2014)[12e]
KCl-aqueous/Cr-Ga (Paris, 2019)[3c]
TEA BF4-AcN/Pb (This work)
200
150
100
50
TEA PF6-AcN/Pb (This work)
TEA 4-MF-PhO-AcN/Pb (This work)
For CO2 electroreduction into oxalic acid, Gennaro et al.[16]
indicated that the catalytic effect of esters on CO2 is conducive
to the formation of CO2 anion radical. On the basis of the above
viewpoint, and the results of further study on the O-C-O bond
angle, we proposed a possible mechanism as shown in Figure 6.
During the electroreduction process, CO2 is activated by [4-MF-
PhO]- to form a new C-O single bond with the O atom, and the
[4-MF-PhO]-CO2·- is easily combined with H+ to generate [4-MF-
PhO - COOH]- adduct at the same time. The C-O bond newly
0
0
20
40
60
80
100
FEH2C2O4 (%)
Figure 5. Comparison of oxalic acid partial current density and formation rate
in [TEA][4-MF-PhO]-AcN electrolyte at Pb electrode with other electrolytes and
electrodes system in literature.
3
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10.1002/cssc.202001194
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formed is successively broken, generating the parent aromatic
ester IL of [TEA][4-MF-PhO] and -COOH intermediate, which
eventually dimerize to oxalic acid.
Synthesis and characterization of ionic liquid.
[TEA][4-MF-PhO] is synthesized by a one-step method of a
neutralization reaction. A certain amount of methyl 4-
hydroxybenzoate is dissolved into 50 mL ethanol in a 250 mL single
round-bottom flask under magnetic stirring. An equimolar amount of
tetraethylammonium hydroxide aqueous solution is added dropwise
into the flask using a constant pressure funnel. The mixture is kept
stirring at 303.15 K for 12 h to allow complete reaction.
Subsequently, ethanol and water are removed via rotary evaporation
at 333.15 K and 348.15K under reduced pressure, respectively. At
last, the IL obtained is dried in a vacuum oven at 333.15 K for 48 h
to eliminate trace water and organic solvent. The water content in
the ILs is determined by means of Mettler Toledo Coulometric KF
titrator C20 detection (Karl Fischer titration). In this work, the water
content in ILs is less than 100 ppm. The molecular structure of
[TEA][4-MF-PhO] is confirmed by 1H NMR and 13C NMR spectra.
(Supporting Information, Figure S1). The viscosities of the IL and IL-
AcN complex solution are measured by an Anton Paar Lovis 2000
M/ME viscometer with a temperature range from 293.15 K to 343.15
K, at intervals of 10 K (Supporting Information, Figure S2).
Figure 6. Mechanism of CO2 electroreduction to oxalic acid on Pb electrode in
the [TEA][4-MF-PhO] (0.9 M)-AcN electrolyte.
CO2 absorption experiment
The CO2 absorption experimental setup was schematically
represented in Figure S3 (Supporting Information), and the
procedures was based on our previous work.[1] Typically, the CO2
absorption experiment in IL was implemented at the temperature of
323.15 K and atmospheric pressure. CO2 gas is bubbled into the IL
(about 5 g) through a glass container with an inner diameter of 2 cm.
The flow rate of CO2 is about 200 ml min-1, and the glass container is
partially immersed in a water bath to ensure the system temperature.
The amount of CO2 absorbed in the IL is determined at intervals by
an electronic balance with an accuracy of ±0.1 mg.
In summary, we have designed and applied a novel aromatic
ester-functionalized IL to enhance the CO2 electrochemical
reduction into oxalic acid for the first time. It was found that the
exceptionally high partial current density (9.03 mA cm-2) for
oxalic acid with appreciable FE (86 %) on Pb electrode were
reached in anhydrous [TEA][4-MF-PhO] (0.9 M)-AcN cathodic
electrolyte, which is much higher than other reported values to
date. Furthermore, the formation rate of oxalic acid is as high as
168.4 μmol cm-2 h-1. Subsequently, a related mechanism was
proposed, which reveals the unique role of IL, especially the
aromatic ester anion with bis-active sites on the CO2 dissolution,
activation, and selectivity conversion to oxalic acid. This work
proved that aprotic IL with active site can enhance the efficiency
of CO2 electroreduction to oxalic acid significantly, and open up
a potential path for the production of oxalic acid with CO2.
Electrochemical Test
Electrochemical measurements were performed using a commonly
used H-type cell at ambient temperature and pressure. The anode
and cathode compartments are separated by Alfa Nafion 117 proton
exchange membrane. The volume of the anode chamber and
cathode chamber are both 40 mL with a 30 mm diameter of each
chamber. In a typical electrochemical reduction, IL-AcN solution and
0.1 M H2SO4 aqueous solution are served as cathodic and anodic
electrolytes, respectively. The amount of electrolyte used is 27 mL in
all the experiments. Prior to the beginning of measurement, the
catholyte is bubbled with CO2 at a gas flow rate of 30 mL min-1 for 30
min under magnetic stirring to insure the solution was saturated with
CO2, a N2-saturated solution is also used for comparison.
Experimental Section
Materials
Gases used in absorption and electrochemical experiments including
CO2, CO, N2, and H2 were purchased from Beijing Beiwen Gas
Factory. Acetonitrile (HPLC Grade) and ethanol (C2H5OH, AR,
≥99.7%) were purchased from Fisher Chemical and Tianjin Damao
Chemical Reagent Factory, respectively. Oxalic acid Standard was
obtained from Dr. Ehrenstorfer GmbH. Silver perchlorate
(anhydrous) was purchased from Alfa Aesar Chemical Co., Ltd.
Tetrabutylammonium perchlorate (for electrochemical analysis,
≥99.0%) was obtained from Sigma-Aldrich. AR reagents including
Tetraethylammonium hydroxide, Methyl 4-hydroxybenzoate,
Tetraethylammonium tetrafluoroborate, Tetraethylammonium
hexafluorophosphate, and 1-Butyl-3-methylimidazolium
In the reaction process, an electrochemical workstation (CHI 660E,
Shanghai CH Instruments Co., China) was applied to monitor the
potential of the experiments. Linear Sweep Voltammetry (LSV)
measurements were recorded to determine the appropriate
potentials for the reduction with three-electrode configuration. The
working electrode is a lead sheet with a 1 cm2 surface area, which is
polished by #1000 sandpaper and then is sonicated in ethanol and
acetone for 5 minutes, followed by rinsing with acetonitrile, and
finally dried in N2 atmosphere before each set of experiment. The
reference electrode is 0.01 M Ag/Ag+ constituted by dissolving 0.01
M AgClO4 in 0.1 M TBAP-AcN and segregates from the catholyte by
a glass casing. The counter electrode is a 1 × 1 cm platinum gauze
(Pt, purity >99.99%). LSV tests are recorded from -2.1 V to -3.0 V
(vs. Ag/Ag+) at a scan rate of 50 mV s-1 in gas-saturated electrolyte.
Afterward, the potentiostat electrolysis were performed at controlled
tetrafluoroborate were purchased from Aladdin Biochemical
Technology Co., Ltd. Potassium phosphate monobasic (for HPLC,
≥99.5%) and Phosphoric acid (for HPLC) were obtained from
Macklin Biochemical Technology Co., Ltd.
4
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potentials with light magnetic stirring during the process for better
mixing. After a desired electrolysis time, the gaseous product and
cathodic electrolyte were collected up for analysis, respectively.
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Products analysis
H2C2O4 and HCOOH were the liquid products quantified by high-
performance liquid chromatography (HPLC) and nuclear magnetic
resonance (NMR) spectroscopy. CO and H2 were the gaseous
products according to the analysis of chromatography (GC) data.
The gaseous products after electrochemical reduction of CO2 were
analyzed by gas chromatography (GC, Agilent 7890B) equipped with
thermal conductivity detector (TCD) and flame ionization detector
(FID). Separation of CO, H2 and CO2 is performed on a double
column system with an analytical column of molecular sieve 5A and
haye Sep Q column. During the detection, High purity nitrogen is
used as a carrier gas. The H2 is detected and determined directly
when the gas stream passs through the TCD detector, and the CO is
quantitatively measured when the gas stream passes through the
FID detector.
The liquid phase products were mainly analyzed by 1H NMR
spectroscopy and high-performance liquid chromatography (HPLC).
The electrolyte sample is dissolved in deuterated dimethyl sulfoxide
(d6-DMSO) solvent with phenol as an internal standard, and the
formic acid product is determined and quantified by using a 600 MHz
liquid NMR spectrometer. On the other hand, the amount of oxalic
acid product is detected by HPLC, performed on Shimadzu LC-20AT
HPLC instrument equipped with a UV detector using an InertSustain
AQ-C18 reverse column (5 μm, 4.6 × 250 mm). The mobile phases
are acetonitrile and 20 mM KH2PO4 aqueous solution with a pH of
2.5, the flow ratio is 5: 95 at a total flow rate of 0.5 mL min-1.
[4]
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Acknowledgements
This work is financially supported by the National Key R&D
Program of China (2018YFB0605802), the National Natural
Science Foundation of China (21838010, 51574215), the Major
Program of National Natural Science Foundation of China
(21890760, 21890762), the program of Beijing Municipal Natural
Science Foundation (2182072, 2182071), the DNL Cooperation
Fund, CAS (DNL 180406), and the Zhengzhou High Level
Talent (No. 20180200029).
Keywords: aromatic ester-functionalized ionic liquid • CO2
reduction • electrocatalyst • electrolysis conversion • oxalic acid
5
This article is protected by copyright. All rights reserved.

10.1002/cssc.202001194
ChemSusChem
COMMUNICATION
Entry for the Table of Contents
C (●)，H (●)， O (●) ，Pb (●)
A novel aprotic aromatic ester-functionalized IL of [TEA][4-MF-PhO] was designed for CO2 electroreduction to oxalic acid. The
multiple active sites exhibit excellent catalytic performance of high current density and Faradaic efficiency for oxalic acid via
interacting with CO2. This study provides a promising prospect for using IL as electrolyte to develop efficient electrocatalysts.
6
This article is protected by copyright. All rights reserved.