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Angewandte
Communications
Ionic Liquids
Hot Paper
Reduction of Carbon Dioxide to Formate at Low Overpotential Using
a Superbase Ionic Liquid
Nathan Hollingsworth, S. F. Rebecca Taylor, Miguel T. Galante, Johan Jacquemin,
Claudia Longo, Katherine B. Holt, Nora H. de Leeuw, and Christopher Hardacre*
Abstract: A new low-energy pathway is reported for the
electrochemical reduction of CO2 to formate and syngas at
low overpotentials, utilizing a reactive ionic liquid as the
solvent. The superbasic tetraalkyl phosphonium ionic liquid
[P66614][124Triz] is able to chemisorb CO2 through equimolar
binding of CO2 with the 1,2,4-triazole anion. This chemisorbed
CO2 can be reduced at silver electrodes at overpotentials as low
as 0.17 V, forming formate. In contrast, physically absorbed
CO2 within the same ionic liquid or in ionic liquids where
chemisorption is impossible (such as [P66614][NTf2]) undergoes
reduction at significantly increased overpotentials, producing
only CO as the product.
ical reduction of CO2. High CO2 solubility, intrinsic ionic
conductivity, and wide potential windows of RTILs make
them attractive solvents for CO2 electroreduction.[6] Initial
reports on CO2 reduction in RTILs formed dialkyl carbonates
through generation of CCO2 radicals, which were reacted with
alcohols using 1-alkyl-3-methylimidazolium ([Cnmim]+)
based RTILs with a range of non-coordinating anions.[7]
Further RTIL studies focused on CO2 reduction to
products other than dialkyl carbonates. Rosen et al. reported
the use of Ag electrodes in [C2mim][BF4], which was found to
decrease the energy of formation of the [CCO2]À radical anion
through the complexation of CO2 with the [C2mim]+ cation.[8]
This significantly reduced the overpotential for CO2 reduction
to CO to < 0.2 V. Furthermore, the cation was shown to
suppress the competing H2 production reaction by forming
a monolayer on the electrode.[9] Further decreases of the
applied potential have been achieved by substitution of the
Ag working electrode with MoS2, giving overpotentials as low
as 0.054 V.[10] Brennecke and co-workers also showed that the
anion influenced the product selectivity, with oxalate forma-
tion favored over CO in [C2mim][NTf2].[11] This change in
product selectivity was also shown by Watkins and Bocarsly,
where formate was produced in [C2mim][TFA] using Pb and
Sn working electrodes.[12] Therein, no evidence was found for
a [C2mim]+-CO2 complex;[13] however, the RTIL was thought
to stabilize intermediates in formate production.
A
lthough CO2 is a greenhouse gas thought to be involved in
climate change,[1] it can also be considered as an abundant
carbon building block for carbon neutral fuels and chem-
icals.[2] Electrochemical reduction is one route to achieve this
goal. Indeed, the reduction of CO2 at low applied over-
potentials with high efficiencies is a significant current
challenge owing to its thermodynamic stability and kinetic
inertness.[3] The high overpotential for CO2 reduction is
related to the large reorganization energy associated with
reduction of linear CO2 to the bent [CCO2]À radical anion.
Thus, a very negative reduction potential is required for the
first electron reduction, that is, À1.9 V vs. NHE,[4] rendering
reduction highly energy inefficient. Materials that form
complexes with CO2 in a non-linear geometry can decrease
this reorganization energy and catalyze the electrochemical
CO2 reduction.[5]
Although interesting, CO2 reduction in non-coordinating
[Cnmim]+ based RTILs is of limited applicability, owing to low
CO2 solubility. For example, CO2 solubility in [C4mim][NTf2]
is < 0.04 CO2 mole fraction at 108C and 0.1 MPa.[14] The only
coordinating IL used to date for CO2 reduction studies is
[C4mim][OAc],[15] which has a CO2 solubility of 0.274 CO2
mole fraction at 258C and 0.1 MPa.[16] The low solubility
affects the rate of product formation and limits the industrial
significance of these systems.
Recently, promising results have been reported utilizing
room-temperature ionic liquids (RTILs) for the electrochem-
[*] Dr. N. Hollingsworth, Dr. K. B. Holt, Prof. N. H. de Leeuw
Department of Chemistry, University College London
20 Gordon Street, London, WC1H 0AJ (UK)
These low solubilities have been overcome by the use of
superbasic ionic liquids. In these systems, [P66614][124Triz], for
example, has been shown to absorb equimolar quantities of
CO2 through the chemical interaction of CO2 with the anion
and physical absorption of CO2 in the solution free space
(Scheme 1).[6b,17] This set of ILs have been studied extensively
for CO2 capture but, to date, no reports of their use in CO2
conversion have been published. A key feature of the anion–
CO2 interaction is that the CO2 chemically binds without prior
reduction to [CCO2]À. Notably, since CO2 is transformed from
a linear to bent geometry on binding to the anion, this can
significantly lower the CO2 reduction potential.
Dr. S. F. R. Taylor, Dr. J. Jacquemin, Prof. C. Hardacre
School of Chemistry and Chemical Engineering
Queen’s University Belfast
David Keir Building, Belfast, BT9 5AG (UK)
E-mail: c.hardacre@qub.ac.uk
M. T. Galante, Dr. C. Longo
Institute of Chemistry, University of Campinas—UNICAMP
Campinas, SP (Brazil)
Supporting information for this article is available on the WWW
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
In this study, we report the first electrochemical reduction
of CO2 captured within the superbasic RTIL [P66614][124Triz],
14164
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 14164 –14168