High Performance Fe Porphyrin/Ionic Liquid Co-catalyst for Electrochemical CO2Reduction

Article abstract of DOI:10.1002/chem.201603359

The efficient and selective catalytic reduction of CO₂is a highly promising process for both of the storage of renewable energy as well as the production of valuable chemical feedstocks. In this work, we show that the addition of an ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate, in an aprotic electrolyte containing a proton source and FeTPP, promotes the in situ formation of the [Fe⁰TPP]^2?homogeneous catalyst at a less negative potential, resulting in lower overpotentials for the CO₂reduction (670 mV) and increased kinetics of electron transfer. This co-catalysis exhibits high Faradaic efficiency for CO production (93 %) and turnover number (2 740 000 after 4 hour electrolysis), with a four-fold increase in turnover frequency (TOF) when compared with the standard system without the ionic liquid.

Full text of DOI:10.1002/chem.201603359

A Journal of
Accepted Article
Title: High Performance Fe Porphyrin/Ionic Liquid Co-catalyst for Electrochemical
CO2 Reduction
Authors: Jaecheol Choi; Tania M Benedetti; Rouhollah Jalili; Ashley Walker; Gordon
G. Wallace; David L. Officer
This manuscript has been accepted after peer review and the authors have elected
to post their 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 pu-
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To be cited as: Chem. Eur. J. 10.1002/chem.201603359
Link to VoR: http://dx.doi.org/10.1002/chem.201603359
Supported by

Chemistry - A European Journal
10.1002/chem.201603359
COMMUNICATION
High Performance Fe Porphyrin/Ionic Liquid Co-catalyst for
Electrochemical CO Reduction
2
‡
‡
Jaecheol Choi , Tania M. Benedetti , Rouhollah Jalili, Ashley Walker, Gordon G. Wallace* and David
L. Officer*`
‡These authors contributed equally.
Abstract: The efficient and selective catalytic reduction of CO
2
is a
have been developed to avoid this high energy intermediate with
subsequent electron transfer done by adding a source of protons
(Equation 2).
highly promising process for both of the storage of renewable energy
as well as the production of valuable chemical feedstocks. In this
work, we show that the addition of an ionic liquid, 1-butyl-3-
methylimidazolium tetrafluoroborate, in an aprotic electrolyte
containing a proton source and FeTPP, promotes the in situ
[
10]
ꢀꢁ ꢂꢃꢄ
ꢅ
ꢀ ꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢈꢉꢊ
ꢁ
ꢆꢄ
0
2-
ꢀꢁ ꢂꢋꢃꢄ ꢂꢋꢌꢍ ꢎꢏꢐꢂꢌ ꢀꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢈꢋꢊ
formation of the [Fe TPP] homogeneous catalyst at a less negative
potential, resulting in lower overpotentials for the CO reduction (670
ꢁ
2
mV) and increased kinetics of electron transfer. This co-catalysis
exhibits high Faradaic efficiency for CO production (93%) and
turnover number (2,740,000 after 4 hour electrolysis), with a 4 times
increase in turnover frequency (TOF) when compared with the
standard system without the ionic liquid.
A variety of catalysts have been investigated. Noble metal
catalysts, whilst effective in lowering the overpotential, are of
limited practical utility due to constraints such as low selectivity
[
11]
and
high
cost.
Metalloporphyrins,
with
iron
Ⅲ
tetraphenylporphyrin chloride (Fe TPPCl) being the most
efficient, are promising catalysts due to their high selectivity for
production of CO, low cost, and ease of preparation on a large
The increasing global production of carbon dioxide (CO
2
)
[12]
scale. The active catalyst is generated in situ upon reduction
from power generation, transport and other industrial processes
threatens our very existence and challenges researchers to
Ⅲ
0
2-
of Fe TPPCl to [Fe TPP] , which is the species responsible for
catalysis of the reduction of CO . The decrease in overpotential
for the CO reduction reaction is limited, however, by the
2
develop carbon-neutral processes for the amelioration of CO
One approach to this is to utilize CO for fuel production and as
a source of other basic organic feedstocks through its reductive
conversion to a variety of gaseous (e.g. CO, CH or C ) or
OH) products. In particular, CO, as a
reduction, is an essential component of synthesis
gas (syngas) utilized to produce liquid hydrocarbons via
2
.
2
2
Ⅲ
potential required to reduce the Fe TPPCl pre-catalyst into its
active form. Therefore, the development of a better approach for
4
2
H
4
0
2-
the in situ generation of the [Fe TPP] at a lower energetic cost
is a challenge to be addressed.
[1]
liquid (e.g. HCO
product of CO
2 3
H or CH
2
Ⅲ
Pioneering reports on Fe TPPCl as catalyst have shown
that very efficient performance is obtained when TFE is
[
2]
[3]
hydroformylation and Fischer-Tropsch synthesis.
Reduction of the thermodynamically inert CO
[10]
employed as the proton source.
Ionic liquids (ILs) have also
2
, however,
been employed as proton sources in the presence of either
demands the development of catalytic systems to reduce the
energy costs and different reduction methods have been
[13]
heterogeneous or homogeneous catalysts
as well as to
[14]
improve selectivity in aqueous electrolytes. In the latter case,
the improved selectivity is attributed to suppressed proton
adsorption due to the adsorption of the IL cation at the surface of
the electrode, combined with the formation of a complex with the
[
4]
[5]
[6]
investigated, including enzymatic, thermo-, photo-, and
electro-chemical
[
7]
methods. Among these, electrochemical
reduction that can be driven by renewable energy sources is a
particularly attractive option due to its environmental and
CO
2
reduction intermediate, facilitating the reaction.
[
7a, 8]
economic advantages.
Any CO reduction catalyst must reduce the energy
required for the transfer of the first electron to the CO (Equation
), whose equilibrium potential is as negative as -1.97 V (in DMF
Herein, we show increased catalytic activity when an
2
imidazolium-based
IL,
1-butyl-3-methylimidazolium
2
tetrafluoroborate ([BMIM]BF
Fe TPPCl as the catalyst. In contrast to previous work, the IL
does not act as the proton source and the improved activity is
4
) was added to the electrolyte using
1
Ⅲ
[
9]
vs. normal hydrogen electrode (NHE)). Consequently, catalysts
not related to the interaction of the IL cation with CO
2
. We show
that the IL cation acts as a co-catalyst by shifting the reduction
0
2-
J. Choi, Dr. T. M. Benedetti, Dr. R. Jalili, Dr. A. Walker, Prof. G. G. Wallace,
Prof. D. L. Officer
ARC Centre of Excellence for Electromaterials Science and the Intelligent
Polymer Research Institute
University of Wollongong
Wollongong, NSW 2522, Australia
potential for production of the [Fe TPP] towards less negative
values. This leads to around 200 mV decrease in
a
overpotential and 4 times higher current densities, resulting in
the production of CO with high efficiency and selectivity.
Ⅲ
The cyclic voltammograms (CVs) of Fe TPPCl in a DMF-
E-mail: davido@uow.edu.au, gwallace@uow.edu.au
based electrolyte containing either TFE and/or [BMIM]BF
an atmosphere of Ar (1a) or CO (1b) are shown in Figure 1.
Concentrations of [BMIM]BF and TFE were systematically
4
under
Supporting information for this article is given via a link at the end of the
document.
2
(
4

Chemistry - A European Journal
10.1002/chem.201603359
COMMUNICATION
(Ⅰ⁄0)
optimized by measuring Fe
potential and current density by
containing only TFE, was observed. In short, a combination of
cyclic voltammetry as shown in Figure S1).
the features of both [BMIM]BF
Similar results were obtained when 1-butyl-2,3-
dimethylimidazolium tetrafluoroborate, with methyl group
instead of a proton at the C position of the imidazolium ring,
was used (Figure S2), confirming that the acid proton from the
BMIM] cation is not acting as a proton source for CO reduction
in this case.
4
and TFE was observed.
a
2
[
2
In order to investigate if the shifted potential is not an effect
of increasing ionic strength due to the additional [BMIM]BF in
the electrolyte, a CV of Fe TPPCl in DMF containing 0.4 M
TBAPF was performed and is shown in Figure S3. In the
presence of 0.4 M TBAPF as supporting electrolyte (which is the
4
Ⅲ
6
6
sum of supporting electrolyte and IL concentrations), there is
Ⅲ
little positive shift in the redox potentials of Fe TPPCl as
6 4
compared to 0.1 M TBAPF . By adding 0.3 M [BMIM]BF ,
I
0
however, the reduction potential of Fe to Fe was noticeably
shifted toward positive potential. These results support that the
potential shift is an effect of the interaction of the IL with the
negatively charged species rather than increased ionic strength.
Ⅲ
During the reduction of Fe TPPCl, the three characteristic
cathodic peaks are observed and the peak potential values are
disclosed in Table 1. In the presence of TFE, the first reduction
potential – at which the chloride ligand is dissociated – is
negatively shifted from -0.14 V to -0.28 V, because the Lewis
Ⅲ
acidity of the Fe cation is increased due to coordination with
Ⅲ
Ⅲ
Figure 1. Cyclic voltammogram of 0.5 mM Fe TPPCl; 0.5 mM Fe TPPCl – 1
the alcohol instead of chloride, making the dissociation more
Ⅲ
Ⅲ
M TFE; 0.5 mM Fe TPPCl – 0.3 M [BMIM]BF
.3 M [BMIM]BF in DMF + 0.1 M TBAPF at glassy carbon electrode (a) under
Ar (Inset shows an expansion of the active area); (b) under CO (Scan rate: 50
4
; 0.5 mM Fe TPPCl – 1 M TFE –
[15]
difficult.
However, the first reduction potential is positively
possibly due
interaction of the IL anion with the Fe . More interestingly, the
0
4
6
shifted back to -0.23 V by adding [BMIM]BF
4
2
-
1
III
mV s ).
Ⅰ
0
potential for the reduction of Fe to Fe , which influences the
overpotential of CO reduction, is remarkably shifted by around
+190 mV in the presence of the [BMIM]BF . A similar but smaller
2
Under an Ar atmosphere, the reduction current increases
when either TFE or [BMIM]BF is added, possibly due increased
porphyrin reduction rate. Interestingly, the potential required for
the reduction of Fe to Fe was shifted from -1.59 V to -1.40 V
Table 1) when the IL was present, suggesting that this is
4
Ⅱ
Ⅰ
4
effect was also noted for the reduction of Fe to Fe , with a +20
mV shift in the potential. These results suggest an electrostatic
I
0
+
interaction between the [BMIM] cation and the negatively
Ⅰ
-
0
2-
(
charged species, [Fe TPP] and [Fe TPP] .
somehow interacting with the porphyrin, favoring its reduction.
Experimental onset potential and faradaic efficiencies were
obtained by gas chromatography (GC) analysis of the product
gases at different applied potentials. Experiments were
Table 1. T Reduction potentials (E1/2) of each sample determined by
Ⅲ
-
1
conducted with Fe TPPCl + TFE with and without the addition
cyclic voltammetry (V vs. NHE, Scan rate: 50 mV s ).
4
of [BMIM]BF . The results of these experiments are summarized
in the graphs in Figure 2.
Ⅲ/Ⅱ
Ⅱ/Ⅰ
Ⅰ
⁄ꢓ
Fe
Fe
ꢑꢒ
From these results, we could estimate the onset potentials.
In the absence of IL, CO is initially detected at -1.51 V; in
contrast, it is first detected at -1.36 V when [BMIM]BF is added
4
Ⅲ
Fe TPPCl
TFE
[BMIM]BF
-0.14
-0.28
-0.22
-1.06
-1.00
-1.00
-1.63
-1.59
-1.44
+
to the electrolyte. This corresponds to a decrease of the
overpotential from 820 mV to 670 mV (Equilibrium potential in
+
4
0
[16]
+
TFE
2
DMF: E for CO /CO is 0.69 V in DMF) . In addition, the rate of
-0.23
-0.98
-1.40
+
[BMIM]BF
4
CO production is significantly higher with increasing applied
overpotential when the ionic liquid is present in the electrolyte.
However, under a CO
in the reduction current was observed in the presence of the
BMIM]BF , suggesting that, in contrast to TFE, this compound is
not acting as a proton source. By combining both TFE and
BMIM]BF , a significant increase in the cathodic current and a
decreased onset potential, as compared with the system
2
atmosphere, no significant increase
[
4
[
4

Chemistry - A European Journal
10.1002/chem.201603359
COMMUNICATION
After the bulk electrolysis had been allowed to proceed for
h (Figure 3), 3.4 times more CO was produced, even at a
4
lower overpotential, with an average faradaic efficiency of 93%,
-
2
for the system containing [BMIM]BF
4
(198 μmol cm ) as
-2
compared to the IL-free system (59 μmol cm , with 89%
efficiency) as summarized in Table 2. In both cases, the
production of H was observed to increase over time, possibly
2
due the increasing amount of produced water (Equation 2),
however it remained relatively low as compared to CO
production.
-1
The TOF (s ) and TON were calculated using Equations 3
and 4, respectively (See Supporting Information for more
[16-17]
details).
ꢔꢐꢑꢕꢖꢗꢘꢙꢚꢛꢉꢂꢒꢜꢝ ꢇꢞ ꢈꢑꢚꢟꢔꢈꢠꢡꢢꢣꢚꢁꢊꢊꢤꢥꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢈꢦꢊ
ꢔꢐꢧꢕꢔꢐꢑꢇꢨꢇꢩꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢈꢪꢊ
-1
From these equations, a TOF of 190 s and a TON of 2.74
6
×
4
10 were calculated for the system containing the [BMIM]BF ,
-1
5
whereas
values 4 times lower (47 s and 6.77 x 10
respectively) were calculated for the system without the IL
(Table 2). The small discrepancies in the ratio between the TON
and TOF values, and the product amount ratio (4 vs 3.4) arises
Figure 2. Faradaic efficiencies for CO (red bar)/H
2
(black bar) and CO (blue
atmosphere. (a)
.5 mM Fe TPPCl + 1 M TFE; (b) 0.5 mM Fe TPPCl + 1 M TFE + 0.3 M
BMIM]BF
line) evolution rates at various applied potentials under a CO
2
Ⅲ
Ⅲ
0
[
4
.
2
from the non-included produced H as well as the fact that the
average current over the four hours of electrolysis was
employed for the TOF calculation while the amount of product
was obtained at specific times and currents. The TOF and TON
values are significantly higher in the system containing the
We have also examined the long-term stability of the
catalytic systems with and without [BMIM]BF . Bulk electrolysis
analyses were done at a -160 mV potential difference from the
half-wave potential (E1/2) of generation of [Fe TPP] for each
catalytic system. Therefore, -1.56 V was applied to the system
4
[
4
BMIM]BF even with the both systems at the same potential
0
2-
difference from their respective E1/2 of catalyst formation (i.e. the
overpotential for system containing the ionic liquid is 190 mV
more positive than the IL free system). This difference arises
because of the higher current density in the presence of the
4
with [BMIM]BF and -1.75 V was applied to the one without the
IL in order to verify whether the ionic liquid was playing a role
other than decreasing the potential for generation of the
[
4 4
BMIM]BF . It suggests that the [BMIM]BF is positively affecting
0
2-
[
Fe TPP] .
the kinetics of the electron transfer reactions in addition to
lowering the potential for in situ generation of the catalyst .
Table 2. Results of bulk electrolyses held at -1.56 V and -1.75 V for each
catalytic system.
Gas evolution
Current
density
mA cm )
Faradaic
efficiency
(%)
-2
TOF
(s )
TON
(ꢨꢇꢉꢫ ꢊ
(μmol cm )
-1
ꢬ
-
2
(
CO
59
H
2
1
.14
.26
89
93
47
6.77
27.4
2
TFE
TFE /
3
190
198
16
[
BMIM]BF
4
The improved kinetics can be seen in Tafel plots as
presented in Figure 4, showing that the slope (representing the
-
1
rate limiting electron transfer from the electrode) is 197 mV dec
for the system containing the ionic liquid compared with 286 mV
dec for Fe TPPCl/TFE only. The smaller slope indicates that
the electron transfer is faster, resulting in enhanced catalytic
Figure 3. Comparison of gas products (CO(■,□) and H
2
(▲,△)) by bulk
Ⅲ
-1
Ⅲ
electrolysis at -1.75V (open symbols) for 0.5 mM Fe TPPCl + 1 M TFE and at
Ⅲ
-
4
1.56V (closed symbols) for 0.5 mM Fe TPPCl + 1 M TFE + 0.3 M [BMIM]BF .
2
activity for CO reduction in the presence of the IL.

Chemistry - A European Journal
10.1002/chem.201603359
COMMUNICATION
Development (CNPq – T.M.B.; 229862/2013-6) is gratefully
acknowledged. The authors thank the Materials Node of the
Australian National Fabrication Facility (ANFF) for their facilities
and research support.
Keywords: Carbon dioxide reduction
• Electrocatalysis •
Electrochemistry • Ionic liquid • Iron tetraphenylporphyrin
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[
4]
5]
Ⅲ
Figure 4. Tafel plots of CO formation for 0.5 mM Fe TPPCl – 1 M TFE (black)
6
Ⅲ
4
and 0.5 mM Fe TPPCl – 1 M TFE - 0.3 M [BMIM]BF (red).
1
[
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As demonstrated above, the rationale for this increased
electron transfer rate as well as lower overpotential of the CO
reduction by the [BMIM]BF cannot be found in an increased
2
4
[
6]
7]
ionic strength or the IL acting as a proton source. Previous
reports in terms of the influence of the mixed IL/molecular
solvent on redox potentials of electroactive species such as
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[
18]
[19]
nickel(II) octaethylporphyrin
and fullerene have shown that
[
IL nanodomains are formed in the molecular solvent and it
stabilizes the anionic species produced during their
electrochemical reduction due to interaction with cationic
species of IL. These results suggest that the generation of the
3
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0
2-
[
[
Fe TPP] at a less negative potential in the presence of the
BMIM]BF is related to the interaction of the positively charged
4
[
8]
BMIM cation with the negatively charged reduced porphyrin.
II
This is also supported by the decrease in potential of the Fe to
I
Fe reduction, indicative of the IL stabilization of the negatively
Ⅰ
-
[9]
Y. Hori, in Mod. Aspects Electrochem. (Eds.: C. G. Vayenas, R. E.
White, M. E. Gamboa-Aldeco), Springer New York, New York, NY,
charged [Fe TPP] .
In conclusion, the ionic liquid [BMIM]BF
4
acts as a co-
catalyst in the homogeneous Fe TPPCl-catalyzed reduction of
CO to CO. This results in the production of CO with high
efficiency and selectivity, times increase in turnover
frequency at +190 mV difference in overpotential and
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2
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2
act as a proton source but has a positive effect on the reduction
in the presence of TFE as the source of protons. The
improvement observed is rationalized on the basis of the
stabilization of negatively charge catalyst formed in situ opening
up the possibility that the use of ionic liquids could be expanded
to a variety other molecular catalytic systems.
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Funding from the Australian Research Council (ARC) Centre of
Excellence Scheme (J.C., R.J., A.W.; CE 140100012) and
Brazilian National Council for Scientific and Technological

Chemistry - A European Journal
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COMMUNICATION
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Chemistry - A European Journal
10.1002/chem.201603359
COMMUNICATION
Entry for the Table of Contents
Layout 2:
COMMUNICATION
Jaecheol Choi, Tania M. Benedetti,
Rouhollah Jalili, Ashley Walker, Gordon
G. Wallace* and David L. Officer*
Page No. – Page No.
High Performance Fe Porphyrin/Ionic
Liquid Co-catalyst for
Electrochemical CO Reduction
2
The potential of attraction: The presence of an imidazolium ionic liquid has a
significant effect on the Fe porphyrin-catalyzed reduction of CO to CO in the
2
presence of a proton source. The ionic liquid cation interacts with the reduced
porphyrin leading to a significantly lowered overpotential and substantially higher
catalytic activity in the reduction of CO2.