Efficient Electrocatalytic CO2 Reduction Driven by Ionic Liquid Buffer-Like Solutions

Article abstract of DOI:10.1002/cssc.201901076

Electrocatalysis of CO₂ reduction in aqueous electrolytes containing the ionic liquid (IL) 1-n-butyl-2,3-dimethylimidazolium acetate ([BMMIm][OAc]) and DMSO proceeded at low overpotentials (?0.9 V vs. Ag/AgCl) at commercially-available Au electrodes, with high selectivity for CO production (58 % faradaic efficiency at ?1.6 V vs. Ag/AgCl). 0.43 mol CO₂ per mol IL could be absorbed into the electrolyte at atmospheric pressure, forming bicarbonate and providing a constant supply of dissolved CO₂ to the surface of the electrode. Electrocatalysis of CO₂ reduction in the electrolyte was facilitated by stabilization of CO₂ radical anions by the imidazolium cations of the IL and buffer-like effects with bicarbonate.

Full text of DOI:10.1002/cssc.201901076

Accepted Article
Title: Efficient electrocatalytic CO2 reduction driven by ionic liquid
buffer-like solutions
Authors: Jairton Dupont, Wellington D. G. Goncalves, Marcileia
Zanatta, Nathalia M Simon, Luciane M Rutzen, and Darren A.
Walsh
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To be cited as: ChemSusChem 10.1002/cssc.201901076
Link to VoR: http://dx.doi.org/10.1002/cssc.201901076
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10.1002/cssc.201901076
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Efficient electrocatalytic CO2 reduction driven by ionic liquid
buffer-like solutions
Wellington D. G. Gonçalves[a], Marcileia Zanatta[a], Nathalia M. Simon [a], Luciane M. Rutzen [a], Darren
A. Walsh[b] and Jairton Dupont *[a]
Abstract: We show here that electrocatalysis of CO2 reduction in
aqueous electrolytes containing the ionic liquid (IL) 1-n-butyl-2,3-
dimethylimidazolium acetate ([BMMIm][OAc]) and dimethyl sulfoxide
(DMSO) proceeds at low overpotentials (−0.9 V vs. Ag/AgCl) at
commercially-available Au electrodes, and with high selectivity for CO
production (58% faradaic efficiency at –1.6 V vs Ag/AgCl). 0.43 mol
of CO2 per mol of IL can be absorbed into the electrolyte at
atmospheric pressure, forming bicarbonate and providing a constant
supply of dissolved CO2 to the surface of the electrode. We also show
that electrocatalysis of CO2 reduction in the electrolyte is facilitated by
stabilization of CO2 radical anions by the imidazolium cations of the IL
and buffer-like effects with bicarbonate.
inherent conductivities, are attracting a lot of attention from
electrochemists.[11, 12]
.
Figure 1. Examples of ILs used as electrolytes for CO2 reduction.
In 2011, it was shown that the CO2-reduction overpotential in
aqueous electrolytes could be lowered drastically by adding the
IL 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF4], to
the electrolyte, a phenomenon that was attributed to complexation
Introduction
−
of the reduced “CO2 ” by the IL.[13] Since that initial report, a
Concerns about escalating CO2 emissions are driving the
development of new methods for capturing and utilising
atmospheric CO2. Of the methods available for activating CO2,
electroreduction using renewable electricity is one of the most
promising.[1-5] The possible products of CO2 electroreduction
include CO, formate, methanol, methane, and ethanol.[6] CO is
particularly attractive as CO2 reduction to CO requires just two
electrons,[7] and CO can be used in the water−gas shift reaction
to produce H2.[8] Fischer−Tropsch (FT) methods can then be used
to convert mixtures of CO and H2 (syngas) into synthetic
petroleum and other liquid fuels.[9] Metals such as Ag and Au can
electrocatalyze reduction of CO2 to CO in aqueous media, but
relatively high reduction overpotentials and low selectivity are
often observed.[10] Moreover, CO2 electroreduction in aqueous
media is complicated by competitive co-electrogeneration of H2,
which reduces the faradaic efficiency for CO production.[2]
number of authors have demonstrated that ILs can promote CO2
electroreduction at low overpotentials and at different electrode
materials.[13-21] However, the mechanism by which ILs lower
reduction overpotentials has not yet been fully elucidated. It was
proposed that lowering of the pH upon hydrolysis of [BF4]−
increases the conversion of CO2 to CO, while adsorption of
[EMIM]+ cations onto the electrode hinders proton adsorption and
competitive H2 evolution.[18] Clear cation (as well as anion) effects
during CO2 electroreduction have been observed, indicating that
−
the formation of an IL imidazolium cation-CO2 complex after an
initial, rate-determining reduction step is unlikely.[22] While
reduction of CO2 in the presence of imidazolium-based ILs usually
yields CO, oxalate and HCOO− can also form as the nature of the
IL anions and the composition of the electrode are varied.[23]
The recent literature has also revealed remarkable insights
into the interactions of CO2 with ILs. ILs containing basic anions
(such as imidazolate and triazolate) or dialkylimidazolium cations
(usually associated with the acetate anion) can covalently interact
with CO2, yielding carbamates and carboxylates, respectively
(Scheme 1).[24, 25] It has been proposed that H+ ions released
during the formation of imidazolium carbonates (Scheme 1B) may
act as a source of H+ ions during electroreduction of CO2.[26-28] It
was also recently shown that in ILs containing basic anions, CO2
capture most likely proceeds primary via the formation of
bicarbonate followed by IL-CO2 adducts, and the IL (which
Ionic liquids (ILs – Figure 1) are salts that are liquid below
100 °C and which, due to their wide electrochemical windows and
[a]
[b]
Dr. W. D. G. Gonçalves, Dra. M. Zanatta, Dra. N. M. Simon, L. M.
Rutzen and Prof. Dr. J. Dupont
Institute of Chemistry
Universidade Federal do Rio Grande do Sul.
Av. Bento Gonçalves, 9500 Porto Alegre 91501-970 RS (Brazil)
E-mail: jairton.dupont@ufrgs.br
Prof. Dr. D. A. Walsh
invariably contains some water) behaves as
a buffer-like
School of Chemistry and GSK Carbon Neutral Laboratory for
Sustainable Chemistry
University of Nottingham
solution.[25] When dissolved in DMSO/H2O, 1-n-butyl-2,3-dimethyl
imidazolium acetate ([BMMIm][OAc], in which no reaction of the
cation with CO2 has been detected)[25] can stabilize up to 1.70 mol
dm−3 of free formic acid from the hydrogenation of CO2.[9, 29]
Jubilee Campus, Nottingham NG7 2TU, UK
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201901076
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Scheme 1. Examples of ILs used as electrolytes for CO2 reduction that involves
covalent interactions of the IL.
Figure 2. Graph of log of the bicarbonate concentration vs. log of the
[BMMIm][OAc] concentration in IL/DMSO/H2O mixtures. The IL was dissolved
in DMSO/H2O solution (5.2 vol% water) and CO2 was bubbled into the solution
for 30 minutes at RT. Bicarbonate concentrations were determined by
integrating the 13C resonances at ca. 159 ppm relative to that of C10 (CH3-N) at
Bicarbonate has also been implicated in the electroreduction
of CO2; while it had generally been thought that dissolved CO2 is
the only active species during electroreduction of CO2,[1],[1, 30, 31] it
was recently shown that dissolved bicarbonate formed upon
dissolution of CO2 in aqueous media increases the concentration
of CO2 near Au electrodes and increases the rate of CO2
electroreduction.[31]
-
34 ppm (See SI Figures S2 and S3). pH=pKa-log([IL]/[HCO3 ]).
We explored the effects of addition of H2O to DMSO solutions
containing the buffering IL on electroreduction of CO2 using linear-
sweep voltammetry in blank and CO2-saturated mixtures. In all
electrochemical studies the IL concentration were fixed at 0.1 mol
dm–3, changing only the concentration of water in DMSO. (Figure
3). Note that the conductivity of the solution did not change
significantly upon addition of varying amounts of water to the
electrolyte (conductivity from 1.598 mS at 0.5 vol% to 1.499 mS
at 5 vol% water).
In this contribution, we show that the use of basic ILs that form
buffer-like solutions in aqueous media allow electrochemical
reduction of CO2 at remarkably low overpotentials and with high
selectivity and faradaic efficiency for CO production. Specifically,
we show that mixtures of [BMMIm][OAc], DMSO and water can
dissolve up to 43 mol% of CO2 (27 mol% via formation of
bicarbonate and 16 mol% via solvated CO2) at atmospheric
pressure (see SI Table S2). This solution supports selective
electroreduction of CO2 to CO at Au electrodes at potentials as
low as −1.40 V vs. Ag/AgCl. At −1.80 V vs. Ag/AgCl, faradaic
efficiencies for CO formation above 98% are observed. Addition
of large amounts of water to the electrolyte favours the formation
of free bicarbonate, which reduces the energy required for the
electrochemical reduction of CO2, but this is accompanied by the
co-electrogeneration of H2, yielding syngas that could be used for
the formation of liquid fuel using follow-on processing.
Results and Discussion
The solubility of CO2 in [BMMIm][OAc]/DMSO/H2O solutions
was quantified using 13C-[1H] NMR spectroscopy (see SI Table
S2).[32] By varying the water content in DMSO from 0.2 vol% to
100% water (without DMSO) at constant IL concentration (1.0 mol
−
dm–3) we determined that the maximum concentration of HCO3
Figure 3. Onset potential for electroreduction of CO2 at Au electrodes in 0.1 mol
(0.41 mol dm−3) was obtained when the water content was 5.2
vol% (see SI Table S2 entries 1-4 and Figure S2). When the
water content was kept at 5.2% and the concentration of IL was
dm−3 [BMMIm][OAc] in DMSO containing increasing concentrations of H2O.
–
varied from 0.4 to 3.0 mol dm−3, the concentration of HCO3
The reduction onset potential (defined at the intersection point
of the tangent at maximum slope of cathodic peak and the
capacitive current curve) shifted anodically as the water content
increased, until it reached a limit of −0.9 V at 5.2 vol% water
(Figure 3). This value is close to the theoretical value of −0.7 V vs
Ag/AgCl for reduction of CO2 to CO at pH 7.[2, 33] Note that the
irreversible electrochemical reduction of CO2 to the radical anion
increased (Figure 2). That the log of the bicarbonate
concentration increased as the log of the IL concentration
increased can be explained by the Henderson-Hasselbalch
equation, typically employed to describe buffer systems.
Spectroscopy of all IL solutions revealed that the major signal was
due to bicarbonate (ca. 159 ppm) and a minor signal appeared at
ca. 124 ppm due to the presence of dissolved CO2. It has recently
been shown that the formation of HCO3− near electrodes can lead
to high CO2-reduction efficiencies in aqueous media.[31]
occurs
at
−1.3
V
vs.
the
potential
of
the
colbatocene/colbatocinium couple in [BMIm][OAc], and −2.0 V vs.
Ag/Ag+ in 0.2 mol dm−3 tetraethylammonium perchlorate in
DMSO.[27, 34]
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All electrochemical reactions were carried out in mixtures
comprising (a) 0.1 mol dm−3 [BMMIm][OAc] in DMSO containing
0.2 vol% H2O, (b) 0.1 mol dm−3 [BMMIm][OAc] in DMSO
containing 5.2 vol% H2O, and (c) 0.1 mol dm−3 [BMMIm][OAc] in
H2O. The faradaic efficiency for CO formation during electrolysis
of CO2 IL solutions was determined by applying various potentials
in solutions a, b and c (the potentials are shown by the vertical
lines in Figure 4).
Figure 4. Linear sweep voltammetry of Au electrodes in Ar-saturated (dashed line) and CO2-saturated (solid line) in [BMMIm][OAc] 0.1 mol dm−3 in a) DMSO
containing 0.2 vol% H2O, b) DMSO containing 5.2 vol% H2O and c) H2O. The vertical lines are the applied potentials during chronoamperometry.
At −1.8 V, the faradaic efficiency was 98%, 91% and 2% in
solutions a), b), and c), respectively (Figure 5).
polarisation curves in Figure 4c). The benefits of the buffer effect
(keeping a constant concentration of HCO3 ) is evident when
−
Addition of water modified of the reaction path, causing co-
electrogeneration of H2 (and hence syngas, CO/H2, at a ratio of
approximately 1:4, Figure 5c). Figure 5c) shows that 40% of the
current at −1.6 V led to H2 in the solution containing no DMSO
(co-generation of H2 and CO is shown by the overlapping
comparing with the electrolysis performed using solution
containing the non-buffering IL [BMIm][BF4] instead of
[BMMIm][OAc], in which CO was produced at very low faradaic
efficiency (15% at −1.6 V vs Ag/AgCl, see Figure S5).
Figure 5. Chronoamperometry performance in [BMMIm][OAc] 0.1 mol dm−3 in a) DMSO containing 0.2 vol% H2O, b) DMSO containing 5.2 vol% H2O, and c) H2O.
FE is the faradaic efficiency.
The high faradaic efficiency for CO electrogeneration in 0.1
mol dm−3 [BMMIm][OAc] in DMSO containing 0.2 vol% H2O at a
relatively low overpotential is among the most efficient
electrochemical CO2 reductions reported so far, without the need
for sophisticated electrodes (see Table S1). Tafel slopes
extracted from CO2-reduction polarisation curves are shown in
Figure 6. Multiple Tafel slopes are visible in each case.
slope increased dramatically (bdif), most likely due to the reaction
36]
coming under mass-transport limitations.[35,
In the solution
containing no DMSO, a Tafel slope of 110 mV dec−1, which we
attribute to water reduction (see also Figure 5c), was also
observed (bH2O). That the CO2 reduction Tafel slope is Solution c
is close to 118 mV dec−1 indicates that the first electron transfer
to CO2 is rate limiting. In contrast, the lower CO2 reduction Tafel
slopes observed when the solutions contained DMSO indicate
that fast initial electron transfer to CO2 occurred in these media.
The Tafel slopes for CO2 reduction at low overpotentials (bCO2
)
are 79 mV dec−1, 64 mV dec −1, and 148 mV dec−1 in Figures 6A-
C, respectively. At high overpotentials in each solution, the Tafel
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Figure 6. Polarisation curves of CO2-saturated in [BMMIm][OAc] 0.1 mol dm−3 in (a) DMSO containing 0.2 vol% H2O, (b) DMSO containing 5.2 vol% H2O and (c)
H2O. The symbols: bCO2, bH2O and bdif represent the Tafel slopes for CO2 reduction, water reduction and diffusion step respectively.
We hypothesise that the CO2•− moiety is more stabilized in the
buffer solution, resulting in a consequent decrease in the onset
potential, as shown in Figure 3.[37-39] Carbonate (167.2 ppm) and
bicarbonate (158.5 ppm) was detected in the 13C NMR spectra of
the reaction mixture containing 13C-enriched CO2 (See SI Figure
S6) after 24 h electrolysis, indicating that the reduction proceeds
preferentially via the 2e− pathway (Scheme 2).[1]
Simple IL/DMSO/H2O solutions dissolve CO2 at atmospheric
pressure yielding, bicarbonate buffer-like solution. The
dissolved CO2 can be selectively electrochemically reduced either
to carbon monoxide or a syngas mixture at simple commercial Au
electrodes at low overpotentials and relatively high current
a
densities.
The
1-n-butyl-2,3-dimethylimidazolium
acetate/DMSO/H2O solution plays multiple roles in the process,
acting as CO2 sorbent and producing bicarbonate that provides
•-
the stabilization of the CO2 . This work provides a clear evidence
of the role of bicarbonate for the effective CO2 reduction. This
approach, employing basic ILs in solution that can produce buffer-
like solutions, open a new window of opportunities for the ambient
pressure CO2 capture and transformation.
Scheme 2. Possible reaction pathways involved in the CO2 reduction to CO in
IL buffer-like solutions
Experimental Section
Moreover, the typical signals of the imidazolium ring are intact
and non-duplicated, indicating that there was no IL decomposition
(compare Figures S2, S3 with S6, before and after
electrochemical reaction, respectively). The origin of CO from
electroreduction of CO2 was verified by per- forming the reaction
with 13CO2 and an aqueous solution of BMMIm.OAc IL produced
13CO which was detected by the appearance of a signal at mass
to charge ratio (m/z) of 29 in the GC–mass spectroscopy (GC-
General methods
1,2-dimethylimidazolium and DMSO were purchased from
Sigma Aldrich. n-Chlorobutane and acetic acid were purchased
from Merck and used as received. Carbon dioxide was supplied
by Air Liquide with purity above 99.998%. 13CO2 was supplied by
Cambridge Isotope Laboratories, Inc. (13C, 99%). 1-n-butyl-2,3-
dimethylimidazolium acetate ([BMMIm][OAc]) was prepared
according to literature procedures [40] from ([BMMIm][Cl]).[41]
All NMR experiments were performed using a Bruker Avance
400 spectrometer, equipped with a BBO 5 mm probe with z-
gradient operating at 400 MHz for 1H and 100 MHz for 13C, or an
Agilent 400 DD2 equipped with a NMR 5 mm probe with z-
MS) (Figure S7).
Electroreduction of CO2 in electrolytes
containing non-buffering ILs occurred with higher onset potentials
and current densities lower, than those obtained in this work
(Table S1, entries 3-11) and even in the presence of sodium
bicarbonate (see Figure S4). This suggests that the formation of
bicarbonate by dissolution in the IL/DMSO/water mixture, is
essential for the CO2 electrochemical reduction; stabilizing the
CO2•−, by the buffered system, formed by the activation of water
and CO2 forming bicarbonate in solution. We also note that
electroreduction of CO2 in the non-buffering IL [BMIm][BF4], gives
a faraday efficiency of only 15% for CO production under the
same reaction conditions.
gradient running at 400 MHz for H and 100 MHz for 13C. The
1
spectra were obtained at 298 K, unless otherwise specified.
Chemical shifts are reported in parts per million (ppm, δ)
referenced to [D6]DMSO as an external reference (capillary).
Electrochemistry measurements were performed using an
Autolab/PGSTAT101 potentiostats/galvanostats. The gold
working electrodes used were the RDE.AU 2 mm diameter model
and Pt wire were purchased from Metrohm.
For construction of the reference electrode, a silver wire
previously cleaned in HNO3 was inserted into an electrochemical
cell containing 0.1 M aqueous HCl solution using a platinum
counter electrode. A constant current of 10 mA/cm2 for 1min was
Conclusions
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applied. During the electrodeposition of Ag / AgCl a black layer is
formed. The electrodeposited electrode was immersed in KCl
solution for one hour. After, the wire electrodeposited was
inserted in a glass tube molten into ceramic frit molten, containing
saturated solution of KCl.
The reference electrode (Ag/AgCl) were calibrated with a
commercial master electrode purchased from Gamry, by
chronopotentiometry analysis for 10 min once the potential
maintained a difference of +/- 3 mV. The other experiments were
carried out.
Keywords: CO2 reduction, ionic liquid, electroreduction, buffer-
like solutions, CO2 sequestration.
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10.1002/cssc.201901076
ChemSusChem
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A simple ionic liquid/DMSO/H2O
solution is able to decrease the
overpotential in a commercial
electrode by in situ formation of
bicarbonate providing an alternative
way to produce syngas from CO2.
Wellington D. G. Gonçalves, Marcileia
Zanatta, Nathalia M. Simon, Luciane
M. Rutzen, Darren A. Walsh and
Jairton Dupont*
Page No. – Page No.
Efficient electrocatalytic CO2
reduction driven by ionic liquid
buffer-like solutions
This article is protected by copyright. All rights reserved.