Efficient conversion of CO2 to CO using tin and other inexpensive and easily prepared post-transition metal catalysts

Article abstract of DOI:10.1021/ja5121088

The development of affordable electrocatalysts that can drive the reduction of CO₂ to CO with high selectivity, efficiency, and large current densities is a critical step on the path to production of liquid carbon-based fuels. In this work, we show that inexpensive triflate salts of Sn²⁺, Pb²⁺, Bi³⁺, and Sb³⁺ can be used as precursors for the electrodeposition of CO₂ reduction cathode materials from MeCN solutions, providing a general and facile electrodeposition strategy, which streamlines catalyst synthesis. The ability of these four platforms to drive the formation of CO from CO₂ in the presence of [BMIM]OTf was probed. The electrochemically prepared Sn and Bi catalysts proved to be highly active, selective, and robust platforms for CO evolution, with partial current densities of j_CO = 5-8 mA/cm² at applied overpotentials of η < 250 mV. By contrast, the electrodeposited Pb and Sb catalysts do not promote rapid CO generation with the same level of selectivity. The Pb material is only ～10% as active as the Sn and Bi systems at an applied potential of E = -1.95 V and is rapidly passivated during catalysis. The Sb-comprised cathode material shows no activity for conversion of CO₂ to CO under analogous conditions. When taken together, this work demonstrates that 1,3-dialkylimidazoliums can promote CO production, but only when used in combination with an appropriately chosen electrocatalyst material. More broadly, these results suggest that the interactions between CO₂, the imidazolium promoter, and the cathode surface are all critical to the observed catalysis.

Full text of DOI:10.1021/ja5121088

Subscriber access provided by UMKC Libraries
Article
Efficient Conversion of CO2 to CO Using Tin and other
Inexpensive and Easily Prepared Post-Transition Metal Catalysts
Jonnathan Medina-Ramos, Rachel C. Pupillo, Thomas P. Keane, John L DiMeglio, and Joel Rosenthal
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja5121088 • Publication Date (Web): 19 Feb 2015
Downloaded from http://pubs.acs.org on February 23, 2015
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 9
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Efficient Conversion of CO2 to CO Using Tin and other
Inexpensive and Easily Prepared Post-Transition Metal
Catalysts
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Jonnathan Medina-Ramos, Rachel C. Pupillo, Thomas P. Keane, John L. DiMeglio and Joel
Rosenthal*
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE, 19716
Supporting Information Placeholder
efficient production of CO from non-petroleum resources is
a critical step toward the sustainable production of liquid
fuels.16 One promising strategy to accomplish this goal is the
marriage of renewable sources of electric energy with
cathode materials that are robust and can promote the
conversion of CO2 to CO with high selectivity (i.e., Faradaic
efficiency (FECO)) and activity (i.e., partial current density
(jCO) at low applied overpotentials (η).
ABSTRACT: The development of affordable electrocatalysts
that can drive the reduction of CO2 to CO with high
selectivity, efficiency and large current densities is a critical
step on the path to production of liquid carbon-based fuels.
In this work, we show that inexpensive triflate salts of Sn2+,
Pb2+, Bi3+ and Sb3+ can be used as precursors for the
electrodeposition of CO2 reduction cathode materials from
MeCN solutions, providing
a
general and facile
electrodeposition strategy, which streamlines catalyst
synthesis. The ability of these four platforms to drive the
formation of CO from CO2 in the presence of [BMIM]OTf
was probed. The electrochemically prepared Sn and Bi
catalysts proved to be highly active, selective and robust
platforms for CO evolution, with partial current densities of
jCO = 5 – 8 mA/cm2 at applied overpotentials of h < 250 mV.
By contrast, the electrodeposited Pb and Sb catalysts do not
promote rapid CO generation with the same level of
selectivity. The Pb-material is only ~10% as active as the Sn
and Bi systems at an applied potential of E = –1.95 V and is
rapidly passivated during catalysis. The Sb-comprised
cathode material shows no activity for conversion of CO2 to
CO under analogous conditions. When taken together, this
work demonstrates that 1,3-dialkylimidazoliums can promote
CO production, but only when used in combination with an
appropriately chosen electrocatalyst material. More broadly,
these results suggest that the interactions between CO2, the
imidazolium promoter and the cathode surface are all critical
to the observed catalysis.
Several cathode materials are known to catalyze the
conversion of CO2 to CO,17,18 however, until recently, only
precious metals such as Ag19–21 and Au22–24 could promote this
process with Faradaic Efficiencies (FEs) above 80%, while
displaying high current density (competent rate) at modest
overpotentials.25 Since the exorbitant price of these precious
metals is incompatible with schemes to produce renewable
fuels on a commercial scale, the search for new inexpensive
materials that drive CO evolution with reasonable activity at
low overpotential is a critical step on the path to the
sustainable production of carbon-based fuels from renewable
energy sources.
Although most inexpensive metallic cathodes typically
require very large overpotentials in order to activate CO2,26
our laboratory recently demonstrated that an economical
bismuth-based material could promote the electrochemical
conversion of CO2 to CO with high efficiency and
selectivity.27 This Bismuth-Carbon Monoxide Evolving
Catalyst (Bi-CMEC) was prepared using a straightforward
electrodeposition method to produce
a
robust Bi0/BiIII
containing material on conducting carbon substrates from
strongly acidic aqueous solutions containing low
concentrations of Bi3+ salts.
INTRODUCTION
Efficient CO production was realized with this Bi-modified
carbon electrode in CO2 saturated MeCN containing
The generation of CO via the 2e–/2H+ reduction of CO2 is a
reaction of fundamental importance to the field of molecular
energy conversion.1,2 This electrochemical half reaction
produces an energy rich commodity chemical that is a key
millimolar concentrations of
a
1,3-dialkyl substituted
imidazolium based ionic liquid (IL) promoter such as 1-butyl-
3-methylimidazolium triflate ([BMIM]OTf).27 Under these
conditions, the Bi-CMEC/IL system was found to drive CO
production with high selectivity (FECO > 80%) and impressive
kinetics (jCO = 5 - 25 mA/cm2) at an applied cathode potential
of –1.95 to –2.0 V versus SCE. As has been observed for other
–
5
starting material for the synthesis of methanol,3 as well as
–
8
organic acids,6 aldehydes9 and alcohols.10 Moreover, carbon
monoxide is the critical C1 feedstock for Fischer-Tropsch
(FT)11–13 and electrolytic methods14,15 that deliver synthetic
petroleum, hydrocarbons and oxygenates. As such, the
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 9
1
2
3
4
5
6
7
8
9
Figure 1. SEM images of electrodeposited cathode materials derived from (a) Bi(OTf)3, (b) Sn(OTf)2, (c) Pb(OTf)2, and (d) Sb(OTf)3.
heterogeneous28–30 and homogenous31 systems for CO2
electrocatalysis, the presence of the imidazolium IL promoter
in the electrolyte solution significantly improves the activity
for CO evolution using Bi-CMEC. The promotion effect was
observed using [BMIM]+ ILs comprised of a number of
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
–
–
common anions including OTf–, PF6 , BF4 , and halides.32
In addition to being able to form Bi-CMEC modified
electrodes from acidic aqueous solutions, this catalyst
platform can also be prepared via in-situ electrodeposition
methods from MeCN/[BMIM]+ electrolyte solutions that
contain low concentrations of Bi(OTf)3.32 The ability to
electrodeposit Bi-CMEC from non-caustic organic solutions
streamlines preparation and study of the material while
enabling the use of conducting supports that normally would
not tolerate the strongly acidic media required for aqueous
electrodeposition. Building on this previous work, we
wondered whether bismuth, as well as tin and other
inexpensive post-transition metals could be prepared ex-situ
from an organic catholyte and promote CO evolution using
the same MeCN/[BMIM]+ electrolyte system that had been
successfully employed with the Bi-CMEC platform. We were
also curious to see if the imidazolium additive would ensure
efficient CO2 activation irrespective of the post-transition
metal cathode material employed, or if cathode materials
other than bismuth would display distinct activities and
reactivity profiles for CO2 reduction.
Figure 2. High-resolution XPS spectra recorded for (a) Bi 4f7/2
region of the Bi-modified electrode, (b) Sn 3d5/2 region of the Sn-
modified electrode, (c) Pb 4f7/2 region of the Pb-modified elec-
trode, and (d) Sb 3d5/2 region of the Sb-modified electrode.
The elemental composition of each of the materials of
Figure 1 was analyzed using energy-dispersive X-ray (EDX)
spectroscopy. EDX analysis was performed on 40 × 40 μm2
regions of multiple independently prepared samples of each
post-transition metal modified electrode. (Figure S1). This
analysis revealed that each electrodeposited material
contained the respective post-transition metal along with
oxygen, fluorine and phosphorus (presumably from the
CF3SO3– and PF6– in the electrodeposition baths). The surface
of each material was also analyzed by X-ray photoelectron
RESULTS AND DISCUSSION
Thin film materials based upon bismuth, tin, lead and
antimony were prepared using a facile electrodeposition
method. Polarization of an inert glassy carbon or nickel
working electrode at potentials between −1.25 and −1.55 V
versus SCE (all potentials are reported with respect to this
reference) in an MeCN solution containing 0.1 M TBAPF6 and
20 mM of either Bi(OTf)3, Sn(OTf)2, Pb(OTf)2 or Sb(OTf)3
lead to electrodeposition of dark-colored films onto the inert
electrode substrate. Thin films for each of the above p-block
metal triflate salts were electrodeposited to a total thickness
of 1.0 C/cm2 and were analyzed by a combination of physical
methods.
spectroscopy (XPS), which identified O,
F and the
corresponding post-transition metal as the principal
elemental components of the deposited materials, along with
–
–
trace amounts of S, N and P from CF3SO3 , Bu4N+ and PF6
ions that were incorporated into the electrodeposited films
(Figures S2 – S5).
The morphology of each of the electrodeposited materials
was probed by scanning electron microscopy (SEM), as
shown in Figure 1. The electrodeposited Bi based material is
visualized as an amorphous granular material (Figure 1a),
which bears some resemblance to the electrochemically
prepared Sb-based deposits (Figure 1d). The other two
electrodeposited materials have a unique appearance under
magnification. The Sn-based electrodeposits appear as
micrometer sized crystallites that bear resemblance to the
fronds of a fern, while the electrodeposited Pb-material is
visualized as an array of striated clusters.
Closer inspection of XPS spectra for the Bi, Sn, Pb and Sb
regions of the four different modified electrodes showed that
the major post-transition metal component for each material
does not exist in the metallic state (Figure 2). For example,
the high-resolution Bi 4f7/2 spectrum recorded for the Bi-
modified electrode was fitted to two components at 159.4 and
157.6 eV that correspond to Bi3+ and Bi0, respectively.33 The
ratio of these two components was 94:6, indicating that that
the overwhelming majority of the surface of the
electrodeposited Bi-material exists in an oxidized state
(Figure S2). A similar situation was observed for the other
2
ACS Paragon Plus Environment

Page 3 of 9
Journal of the American Chemical Society
Table 1. Faradaic efficiencies (FE) and current densities for electrocatalytic reduction of CO2 to CO at an applied poten-
tial of –1.95 V vs. SCE in MeCN containing 100 mM [BMIM]OTf.
1
2
3
4
5
6
7
jtot (mA/cm2)
10.1 ± 2.1
7.2 ± 1.9
0.5 ± 0.1
1.0 ± 0.4
5.0 ± 1.0
FECO
jco (mA/cm2)
8.4 ± 1.7
5.0 ± 1.8
–––––
0.4 ± 0.2
4.1 ± 0.9
FEH2
FEHCO
H
2
Electrode
Eappl (η)
Bi
Sn
Sb
Pb
Pb
–1.95 V (~200 mV)
–1.95 V (~200 mV)
–1.95 V (~200 mV)
–1.95 V (~200 mV)
–2.05 V (~300 mV)
78 ± 5%
77 ± 5%
–––––
40 ± 12%
81 ± 5%
–––––
–––––
30 ± 20%
11 ± 6%
–––––
–––––
5 ± 1%
29 ± 21%
31 ± 17%
–––––
8
9
significant amount of oxygen and fluoride to balance the
charge of the metal ions.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The ability of each of the electrodeposited materials to
electrochemically activate CO2 was assessed in MeCN
containing 100 mM [BMIM]OTf. This solvent/IL combination
supports a large electrochemical window and was previously
employed for CO2 conversion studies involving Bi-CMEC.27,32
As shown in Figure 3, linear sweep voltammograms (LSVs)
recorded in CO2 saturated MeCN containing 100 mM
[BMIM]OTf using glassy carbon or nickel electrodes
modified with the four materials described above, results in
polarization curves with onset potentials that vary from
approximately –1.8 to –1.95 V.
Of the four electrodeposited films, the Bi-based material
shows the earliest onset potential of ~ –1.8 V, followed by Sn
and Sb at approximately –1.85 V and Pb at –1.95 V. LSVs
recorded under the same conditions but in the absence of
CO2 do not show any appreciable rise in current (Figures S6 –
S9). This observation is consistent with the large current
responses shown in Figure
3
being due to CO2
activation/conversion at each of the modified cathode
assemblies.
Although the ability of Bi-CMEC to drive the
electrocatalytic conversion of CO2 to CO has been
demonstrated, the effect that oxygen and fluorine
incorporation into the Bi-based cathode material might have
on the ability of this platform to promote CO2 conversion
was not clear. Moreover, the ability of Sn and Sb-based
materials to promote the electrochemical reduction of CO2
to CO in MeCN/IL electrolytes had not been previously
ascertained. In order to establish that the electrochemical
responses shown for the cathode materials of Figure 1
correspond to conversion of CO2 to CO, controlled potential
electrolysis (CPE) experiments were carried out for CO2
saturated solutions of MeCN containing 100 mM
[BMIM]OTf. Given that both the electrodeposited Bi and Sn
materials showed similar onset potentials in the presence of
the IL promoter at approximately –1.8 V, we initially probed
the ability of these two cathode materials to promote CO2
reduction at –1.95 V. Since these electrochemical
experiments were conducted under conditions of low proton
Figure 3. (a) Linear sweep voltammograms (LSVs) and (b) total
current density profiles recorded for electrodeposited cathode
materials in CO2 saturated MeCN containing 100 mM
[BMIM]OTf. Current density plots for which no potential is
listed were recorded at –1.95 V versus SCE.
three materials surveyed. The respective high-resolution XPS
spectra recorded for the Sn-modified electrode showed two
peaks in the Sn 3d5/2 region at 487.0 and 484.9 eV (Figure S3),
which correspond to Sn2+/4+ and Sn0 in a ratio of 86:14.33 This
composition is similar to that observed to electrodeposited
Sn/SnOx films prepared from aqueous solutions.34
High-resolution XPS spectra recorded for the Sb-modified
electrode showed two peaks in the Sb 3d5/2 region at 530.5
and 528.1 eV (Figure S4), which correspond to Sb3+ and Sb0 in
a ratio of 81:19.33 Lastly, high-resolution XPS spectra recorded
for the Pb-modified electrode showed only a single peak in
the Pb 4f7/2 region at 139.0 eV (Figure S5), which corresponds
availability, the equilibrium potential (E°CO /CO
)
for
2
conversion of CO2 to CO under the MeCN/[BMIM]+
catholyte conditions employed in this study can be
computed as is described in the Supporting Information35
to Pb2+/4+ 33
. XPS did not show any evidence for metallic lead
Bookmark not defined.,36
and previous studies.27,32Error!
We note
on the surface of the electrodeposited Pb-material. When
taken together, the EDX and XPS analyses indicate that the
surface of the the post-transition metal materials shown in
Figure 1 that are electrodeposited from solutions of the
appropriate M(OTf)n salts in MeCN containing TBAPF6,
contain little to none of the metallic (M0) p-block element.
Instead the modified electrodes are coated in films that are
largely comprised of Mn+ ions that have incorporated a
that the value of E°CO /CO under the conditions employed in
2
this study, is within 100 mV of the catalytic onset potentials
observed for CO2 reduction by the Bi- and Sn-based
materials.
Upon initiating CPE experiments at –1.95 V for electrodes
modified with the Bi, Sn, Pb or Sb materials described above,
the reaction headspace was periodically analyzed by gas
chromatography. This analysis showed that for the Bi and Sn
3
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 9
modified electrodes, CO was the only gaseous product
from CO2 with competent kinetics at both Bi- and Sn-based
cathodes.
Upon establishing that the Bi- and Sn-modified electrodes
1
2
3
4
5
6
7
8
formed during the electrolysis experiment, with no detected
coproduction of H2, CH4 or other small hydrocarbons.
Quantification of the gas produced showed that polarization
of the Bi and Sn materials at –1.95 V leads to the evolution of
CO with FEs of 78% and 77%, respectively. Under these CPE
conditions, these materials were found to promote the 2e–
/2H+ conversion of CO2 to CO with appreciable partial
current densities that were measured to be jCO = 8.4 ± 1.7
mA/cm2 for Bi and jCO = 5.0 ± 1.8 mA/cm2 for Sn (Table 1).
could promote the rapid conversion of CO2 to CO in the
presence of [BMIM]OTf, we moved to ascertain how the Sb
and Pb electrodeposited materials would compare.
Repetition of the CPE experiment described above at –1.95 V
using the Sb-modified electrode, did not generate CO and
resulted in very little charge being passed over the course of
a 1 hour experiment (Figure 3b, Table 1). The poor activity of
the electrodeposited Sb-material for CO2 reduction is also
reflected by the LSV recorded for this material under the CPE
conditions (Figure 3a), which displays a level of current
enhancement that is only slightly higher than that observed
for the inert Ni and GCE substrates. Moreover, the current
response obtained with the Sb-modified electrode is not
significantly attenuated in the absence of CO2 (Figure S8).
9
Analysis of the catholyte solutions at the end of the CPE
experiments by NMR showed coproduction of a small
amount of formate (FE ~5%) using the Sn-modified
electrode, however, no oxalate or glyoxalate was detected.
Non-volatile CO2 reduction products were not observed in
the catholyte solutions following CPE experiments that
employed the Bi-modified electrodes. That these carboxylate
containing products are not formed to any significant extent
using the Bi- and Sn-modified electrodes is noteworthy, as
these species are often the dominant products observed upon
reduction of CO2 in organic catholytes.37,38 Moreover,
although formic acid is often the major product obtained
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CPE experiments employing the Pb modified electrode
resulted in a distribution of products that was dramatically
different from those observed for Bi and Sn under the same
conditions, with FEs for CO, H2 and HCO2H of 40%, 11% and
31%, respectively (Table 1). In addition to the fragmented
product distribution, under these CPE conditions Pb
operates with appreciably slower kinetics for CO evolution as
compared to the Bi and Sn catalyst systems. Displaying a
measured partial current density for CO production at –1.95
V of jCO ~0.4 mA/cm2, the Pb catalyst is less than 10% as
active as Sn and 5% as active as the Bi homologue (Table 1).
The lower activity of Pb is also reflected by the later onset
potential for CO2 reduction displayed by this cathode
material, as compared to the Bi and Sn systems.
upon reduction of CO2 with cathodes based upon Bi39 or
–
Sn,40 formation of this competing 2e–/2H+ CO2 reduction
44
side product is overwhelmingly suppressed upon CPE of CO2
using the electrodeposited Bi- and Sn-materials in
MeCN/[BMIM]+ catholyte solutions (Table 1).
When considering the product distribution outlined for
the Bi- and Sn-materials in Table 1, roughly 20% of the
charge passed during the CPE experiments is unaccounted
for. This disparity may be due to formation of non-volatile
CO2 reduction products at levels that are below our detection
limits. For example, recent work has demonstrated that
copper electrodes can promote the conversion of CO2 to a
slew of highly reduced C2 and C3 products, such as
glycoaldehyde, acetaldehyde, acetate and propionaldehyde,
among others.45 For the case of copper, and possibly for the
cathode materials described here, each of these products are
formed with current efficiencies below 1%. However, since
formation of such highly reduced compounds requires the
consumption of anywhere from 6-20 electron equivalents, a
modest fraction of the overall catalytic current may be
diverted to drive the formation of negligible amounts of C2 or
C3 products. Additional unaccounted charge may be coopted
to drive reduction of the oxidized Bi and Sn-materials during
the CPE experiments to generate catalyst films with greater
proportions of the post-transition metal elements in the
metallic state under in-situ conditions.
Although the Pb modified electrode displays poor
selectivity and kinetics for CO generation at –1.95 V, the
general shape of the LSV recorded for this catalyst in the
presence of CO2 and [BMIM]OTf suggested that this system
might display a more pronounced CO2 conversion efficiency
at higher applied overpotentials (Figure 2). When the CPE
experiment described above is repeated with a Pb-modified
electrode at an applied potential of –2.05 V, the selectivity of
CO production is markedly improved as the recorded FE for
this product is above 80% at these more negative potentials
(Table 1). As expected, the kinetics of CO evolution at the Pb
catalyst are also enhanced at –2.05 V to jCO = 4.1 ± 0.9
mA/cm2. Although it is not readily apparent why the
selectivity for CO production at the Pb-cathode is markedly
improved as the applied overpotential increases, one
potential explanation for this behavior is that formic acid and
CO are formed at Pb via distinct mechanistic pathways with
differing Tafel profiles.46 We note that many cathodes for
CO2 reduction display varied and complex product
distribution profiles as a function of applied potential.26 As
was the case for the Bi and Sn-modified electrodes, no
current enhancement was observed for the Pb-material in
the absence of CO2 or [BMIM]+ (Figures S9).
Repetition of the above CPE experiments under an
atmosphere of N2 leads to negligible current densities for
both the Bi- and Sn-materials (Figures S6 and S7). Further,
CO was not formed under these conditions, indicating that
the product obtained under CO2 is not derived from the
electrochemical decomposition of the [BMIM]+ electrolyte.
Similarly, repeating the above CPE experiments under an
atmosphere of CO2 but while using a more conventional
organic electrolyte such as TBAPF6 in place of [BMIM]OTf,
results in little charge being passed and an overwhelming
decrease in the FE for CO production (Figures S6 – S7, Table
S1). When taken together, these experiments establish that
the imidazolium ([BMIM]+) is critical to the observed
electrocatalysis and promotes the selective production of CO
In addition to displaying competent kinetics for CO
evolution, the electrodeposited Pb-material is also highly
selective, as no coproduction of oxalate, glyoxalate or
formate was detected in the catholyte following the CPE
experiment at –2.05 V. The potential at which the
electrodeposited Pb-material evolves CO at a current density
of 4 – 5 mA/cm2 is more positive than that required to effect
the same process using a piece of lead foil under similar CPE
conditions by ~300 mV.47 The improved performance of the
4
ACS Paragon Plus Environment

Page 5 of 9
Journal of the American Chemical Society
to the other two post-transition metal materials. As shown in
1
2
3
4
5
6
7
8
Figure 3, over the course of a 60 min CPE experiment at –2.05
V, the current density obtained with a Pb-based cathode is
not steady, and slowly decays below 4-5 mA/cm2. This drop
in the rate of CO production continues upon prolonged
electrolysis, and is accompanied by a dramatic decrease in
the FE of CO generation, which plateaus at ~10% (Figure
S10). These observations suggest that the electrodeposited
Pb-material either degrades or becomes passivated during
catalysis. The same loss in activity is not observed for the
electrodeposited Bi and Sn-materials for which stable current
densities and FEs for CO evolution are maintained for at
least 6 – 8 hours (Figure S10), suggesting that these latter two
materials are neither passivated nor degraded during
electrocatalysis.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Of the four electrodeposit ed materials, only the Bi and Sn-
based cathodes proved to be robust, selective and kinetically
competent catalysts for the cathodic half reaction that
generates CO. We thus turned our attention to optimizing
these two platforms. Increasing the concentration of
[BMIM]OTf in the catholyte solution leads to an enhanced
current response. The current response corresponding to
CO2 reduction plateaus at a [BMIM]+ concentration of ~300
mM for both the electrodeposited Bi and Sn materials. CPE
experiments were conducted at –1.95 V using these cathode
materials suspended in CO2 saturated MeCN containing 300
mM [BMIM]OTf. Under these conditions, the Bi-modified
electrode displays impressive enhancements in selectivity
and activity, with partial current density for CO production
of jCO = 14 ± 2 mA/cm2 and FECO = 80%. An even more
striking improvement in CO evolution activity and catalytic
efficiency is realized for the Sn-modified electrode, which
operated with a partial current density of jCO = 13 ± 3 mA/cm2
and FECO = 91%. These metrics are extremely noteworthy, as
they demonstrate that the Sn-modified electrode can
catalyze the conversion of CO2 to CO with a level of
efficiency and selectivity that is typically only accessible with
precious metal catalysts that are both rare and expensive. As
such, the Sn-modified electrode described herein, is to the best
of our knowledge, the only example to date of a heterogeneous
electrocatalyst platform consisting only of inexpensive and
earth abundant elements that can efficiently promote the
production of CO from CO2 with current densities above 10
mA/cm2 at low overpotentials.
Figure 4. (a) CV traces recorded at a Ni disk electrode in MeCN
solutions containing 1.0 mM Sn(OTf)2 under an atmosphere of
either CO2 or N2. (b) Total current density trace recorded for a Ni
disk electrode at –1.95 V in MeCN containing 100 mM
[BMIM]OTf before an after addition of 1 mM Sn(OTf)2. Inset:
partial current density profiles for CO production (jCO) at a Ni
disk electrode in MeCN containing 100 mM [BMIM]OTf in the
presence (red) and absence (black) of 1.0 mM Sn(OTf)2.
electrodeposited Pb catalyst may originate from the porous
surface structure of the material (Figure 1c), as compared to
the flatter mesoscopic structure of more uniform bulk Pb
substrates.
NMR analysis of the catholyte solutions following
prolonged CPEs using either the Bi or Sn-modified electrodes
(at –1.95 V) or the Pb-modified assembly (at –2.05 V) did not
reveal significant levels of [BMIM]+ decomposition or
formation of the imidazolium-carboxylate adduct [BMIM–
CO2]. Previous work has shown that such imidazolium-
carboxylate adducts can be formed upon polarization of
platinum48 or lead foil electrodes47 in the presence of [BMIM]
and CO2 at very negative potentials. The observation of rapid
and selective conversion of CO2 to CO with the Bi, Sn and
Pb-modified electrodes that we describe herein, suggests that
formation of [BMIM–CO2] and related species may occur via
a pathway that is distinct from the electrolytic process that
results in CO evolution. As such, the electrodeposited Bi, Sn
and Pb catalyst materials that are described above offer
distinct advantages over other cathode/[BMIM]+ systems,
since the electrodeposited materials can activate CO2 at
lower potentials, which circumvents the formation of
imidazolium-carboxylate side products.
Repetition of the above CPE experiments under an
atmosphere of N2 lead to little passed charge and no CO
production. Accordingly, it is highly unlikely that the
improved selectivity and kinetics for CO generation realized
upon use of higher [BMIM]+ concentrations is due to
imidazolium decomposition. Moreover, the metrics observed
for conversion of CO2 to CO using the electrodeposited Sn-
material are much better than those obtained using other tin
based systems, which typically promote the reduction of CO2
to formate and additional products other than CO. Since the
Sn-modified electrode is intimately involved in the
electrocatalytic conversion of CO2 to CO and promotes half-
reaction with excellent current densities at low
overpotentials, this system represents a Sn-Carbon Monoxide
Evolving Catalyst (Sn-CMEC).
The CPE experiments described above demonstrate that
the electrodeposited Bi, Sn and Pb materials can all promote
rapid conversion of CO2 to CO with good selectivity at low
overpotential when in the presence of [BMIM]OTf. However,
the Pb-material is not as robust during the CPE as compared
Since Sn-CMEC is a proficient system for CO2 reduction,
we pursued the in-situ formation of this platform from the
CO2 saturated MeCN/[BMIM]+ electrolyte, as has been done
previously for Bi-CMEC. Such in-situ electrodeposition
5
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 9
strategies not only streamline preparation of the catalyst
key milestone toward the storage of renewable energy and
1
2
3
4
5
6
7
8
material, but can also allow for catalyst electrodeposition on
substrates that are too delicate or reactive to survive
excessive manipulation. Since the Sn(OTf)2 salt is soluble in
CO2 saturated MeCN containing [BMIM]OTf and is reduced
at a potential that is more positive than the onset potential
for CO2 reduction using Sn-CMEC, we suspected that this
sustainable production of liquid fuels. Although cathodes
based upon silver and gold can efficiently promote this
chemistry, the demand for and scarcity of these materials has
limited their use on the scale required for commercial fuel
production. By contrast, tin, bismuth, lead and other post-
transition metals are significantly more affordable, and are in
some cases earth abundant. As such, the development of CO
evolving catalyst platforms that utilize these elements is of
great potential interest for electrochemical fuel production.
system would be
a fine candidate for in-situ catalyst
electrodeposition.
Electrochemical reduction of CO2 saturated solutions of
MeCN containing 1.0 mM Sn(OTf)2 and 100 mM [BMIM]OTf
show that this system promotes electrocatalysis. Cyclic
voltammograms recorded for the above solution show a
9
In the present study, we have shown that triflate salts of
Sn2+, Pb2+, Bi3+ and Sb3+ are excellent precursors for the
electrodeposition of post-transition metal based cathode
materials from organic electrolyte solutions. In the presence
of a 1,3-dialkyl substituted imidazolium based IL, such as
[BMIM]OTf, these platforms show varied activities and
efficiencies for the electrolytic conversion of CO2 to CO. Both
the Sn- and Bi-based cathodes selectively catalyze the
reduction of CO2 to CO with partial current densities that
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
cathodic feature at appoximately
E = –1.5 V, which
corresponds to reduction of Sn(OTf)2. This initial redox
event is followed by an intense catalytic wave at E = –1.85 V
(Figure 4a). This sharp current enhancement is nearly
identical to the polarization curve recorded for CO2
reduction using the ex-situ generated Sn-CMEC (Figure 2a).
This result suggests that cathodic polarization of the
MeCN/[BMIM]OTf solution containing Sn(OTf)2 leads to
electrodeposition of a Sn-based material that can activate
CO2 at potentials that are more negative than –1.8 V. CV
experiments conducted for MeCN solutions containing 1.0
mM Sn(OTf)2 and 100 mM [BMIM]OTf but under an
atmosphere of N2 are consistent with this analysis; under
these conditions (i.e., in the absence of CO2), virtually no
current enhancement is observed from –1.8 to –2.2 V.
Similarly, CVs recorded in the absence of [BMIM]OTf show
no current enhancement at these negative potentials
indicating that Sn, [BMIM]+ and CO2 are all integral to the
observed catalysis.
range from approximately
5 –
10 mA/cm2 under the
MeCN/[BMIM]+ electrolyte conditions employed for these
studies. Moreover, both the Bi- and Sn-based electrocatalysts
are robust and selectively promote CO evolution for at least
6 – 8 hours. These metrics compare favorably to those
obtained using precious metal CO2 reduction cathodes and
distinguish the electrodeposited Sn and Bi-containing
materials detailed in this work as proficient Carbon
Monoxide Evolving Catalysts (CMECs).
In contrast to the electrodeposited Sn-CMEC and Bi-
CMEC platforms, similarly prepared materials derived from
Pb(OTf)2 or Sb(OTf)3 in MeCN do not catalyze CO evolution
with the same level of activity. While the Sn and Bi-based
materials drive CO evolution at an applied potential of at E =
–1.95 V, the electrodeposited Pb-material is less than 10% as
active as these catalysts at the same applied potential. The
electrodeposited Pb-material can selectively promote CO
evolution when polarized at more negative potentials, but is
rapidly passivated during catalysis, which limits the utility of
this platform for electrolytic CO evolution. Although also
easily prepared, the Sb-modified electrodes described in this
work are not competent platforms for CO2 conversion.
CPE experiments also demonstrate that the in-situ
generated Sn-material is active for the conversion of CO2 to
CO. As shown by the green trace in Figure 4b, electrolysis of
a CO2 saturated solution of MeCN containing 100 mM
[BMIM]OTf at –1.95 V using a Ni working electrodes
exhibited low current densities of jtot ~ 1.5 mA/cm2. No CO
was detected under these conditions. Addition of Sn(OTf)2 to
the CPE solution led to a dramatic rise in current, which
ultimately plateaued at jtot ~ 9 mA/cm2. This rise in current
was accompanied by the formation of a dark film on the Ni
cathode and the formation of CO as judged by periodic GC
analysis of the reaction headspace. As was the case for the ex-
situ Sn-CMEC experiments, CO is the only gaseous product
formed during the CPE. Quantification of the CO generated
revealed that the in-situ generated Sn-CMEC operates with
FECO ~ 75% and an average partial current density of jCO = 9.8
± 2 mA/cm2. Repetition of this CPE experiment under N2
leads to little passed charge and does not generate CO.
Similarly, repeating this CPE experiment under CO2 but in
the absence of [Sn(OTf)2] leads to low current densities (~0.6
mA/cm2) and virtually no CO production. These results are
consistent with in-situ generation of the Sn-CMEC catalyst
and represent a promising strategy toward formation of a
CO2 reduction electrocatalyst that can be easily formed in-
situ from an inexpensive and earth abundant metal
precursor, and that operates with competent kinetics and
efficiency.
The facility with which the cathode materials detailed here
have been prepared using a combination of ex-situ and in-
situ electrodeposition methods, suggests that other
inexpensive and easily prepared cathode materials may also
be quickly prepared and screened for CO2 reduction activity
using an analogous approach. Extension of the M(OTf)n
reduction strategy highlighted in this work to additional
transition and main group metals triflates may streamline
the preparation of new electrodeposited materials comprised
of one or multiple metallic elements.
While it is clear that the [BMIM]+ component of these
catalyst systems plays a pivotal role in promoting the
efficient activation and reduction of CO2 at the
electrodeposited Sn-CMEC and Bi-CMEC platforms,
utilization of this imidazolium with an arbitrarily chosen
cathode material does not guarantee an efficient
electrocatalysis with CO2. For example, CPE of CO2 saturated
solutions of MeCN containing 300 mM [BMIM]OTf at –1.95 V
using inert GCE or nickel foil electrodes does not result in an
appreciable current response or the formation of CO2
reduction products (Table S1). Similarly, CPEs conducted
using an Sb-modified glassy carbon electrode do not
CONCLUSIONS AND FUTURE DIRECTIONS
The efficient and rapid electrocatalytic conversion of CO2
to CO using robust and inexpensive catalyst materials is a
6
ACS Paragon Plus Environment

Page 7 of 9
Journal of the American Chemical Society
generate CO and result in very little charge being passed over
the course of a one hour experiment (Table 1). Since the
1
2
3
4
5
6
7
8
electrolytic reduction of CO2 in the presence of imidazolium-
based promoters is sensitive to the identity of the cathode
material, it is clear that the observed CO evolution
electrocatalysis that we observe, cannot simply be attributed
to homogenous reduction of the [BMIM]+ or CO2 in solution.
Instead, the results we have presented suggest that the
intimate interaction of either CO2, imidazolium or both with
the polarized Bi-CMEC and Sn-CMEC architectures is critical
to the catalysis observed for these systems. Future work from
our lab will be aimed at probing the nature of these complex
interactions in an effort to optimize existing and innovate
new CO2 reduction platforms.
12) Vennestrøm, P. N. R.; Osmundsen, C. M.; Christensen, C. H.;
Taarning, E. Angew. Chem. Int. Ed. 2011, 50, 10502–10509.
13) Takeshita, T.; Yamaji, K. Energy Policy, 2008, 36, 2773–2784.
14) Li, C. W., Ciston, J.; Kanan, M. W. Nature, 2014, 508, 504–
507.
15) Gattrell, M.; Gupta, N.; Co, AJ. Electroanal. Chem. 2006, 594,
1–19.
16) Rosenthal, J Prog. Inorg. Chem. 2014, 59 299–338.
17) Azuma, M.; Hashimoto, K.;, Hiramoto, M.; Watanabe, M.;
Sakata, T. J. Electrochem. Soc. 1990, 137, 1772–1778.
18) Hori, Y.; Kikuchi, K; Suzuki, S; Chem. Lett. 1985, 14, 1695–
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1698.
19) Hoshi, N.; Kato, M.; Hori, Y. J. Electroanal. Chem. 1997,
440, 283–286.
20) Salehi-Khojin, A. Jhong, H.-R. M.; Rosen, B. A.; Zhu, W.; Ma,
S.; Kenis, P. J. A.; & Masel, R. I. J. Phys. Chem. C. 2013, 117,
1627–1632.
ASSOCIATED CONTENT
Supporting Information
High-resolution XPS spectra, control electrochemistry
experiments and experimental details can be found in the
Supporting Information section. This material is available
free of charge via the Internet at http://pubs.acs.org.
21) Tornow, C. E.; Thorson, M. R.; Ma, S.; Gewirth, A. A.; Kenis,
P. J. A. J. Am. Chem. Soc, 2012, 134, 19520–19523.
22) Hori, Y.; Murata, A.; Kikuchi, K.; Suzuki, S. J. Chem. Soc.,
Chem. Commun. 1987, 10, 728-729.
23) Chen, Y.; Li, C. W.; Kanan, M. WJ. Am. Chem. Soc. 2012, 134,
AUTHOR INFORMATION
Corresponding Author
19969–19972.
24) Zhu, W.; Michalsky, R.; Metin, Ö.; Lv, H.; Guo, S.; Wright, C.
J.; Sun, X.; Peterson, A.; Sun, S. J. Am. Chem. Soc. 2013, 135,
16833-16836.
25) Hori, Y.; Wakebe, H.; Tsukamoto, T.; Koga, O. Electrochim.
Acta 1994, 39, 1833–1839.
Correspondence and requests for materials should be
addressed to J.R. at joelr@udel.edu.
Notes
26) Hori, Y. Mod. Aspect. Electroc. 2008, 42, 89–189.
27) DiMeglio, J. L.; Rosenthal, J. J. Am. Chem. Soc. 2013, 135,
The authors declare no competing financial interests.
8798–8801.
ACKNOWLEDGMENT
28) Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. R.; Zhu, W.;
Whipple, D. T.; Kenis, P. J. A.; Masel, R. I. Science 2011, 334,
643–644.
29) Rosen, B. A.; Haan, J. L.; Mukherjee, P.; Braunschweig, B.;
Zhu, W.; Salehi-Khojin, A.; Dlott, D. D.; Masel, R. I. J. Phys.
Chem. C, 2012, 116, 15307–15312.
30) Rosen, B. A.; Zhu, W.; Kaul, G.; Salehi-Khojin, A.; Masel, R. I.
J. Electrochem. Soc. 2012, 160, H138–H141.
31) Grills, D. C.; Matsubara, Y.; Kuwahara, Y.; Golisz, S. R.; Kurtz,
D. A.; Mello, B. A. J. Phys. Chem. Lett. 2014, 5, 2033–2038.
32) Medina-Ramos, J.; DiMeglio, J. L.; Rosenthal, J. J. Am. Chem.
Soc. 2014, 136, 8361–8367.
33) Handbook of Photoelectron Spectroscopy; Chastain, J., King,
R. C., Jr., Eds.; Physical Electronics: Eden Prairie, MN, 1995.
34) Chen, Y; Kanan, M. W. J. Am. Chem. Soc. 2012, 134, 1986-
We thank Prof. Thomas P. Beebe for providing access to the
XPS spectrometer used in this study and Tian Qiu for
assistance in the preparation of lead and antimony triflate
salts. J.R. was supported through a DuPont Young Professor
Award. Additional financial support for this work was
provided by the University of Delaware.
REFERENCES
1)
Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M.
Chem. Soc. Rev. 2008, 38, 89–99.
2) Whipple, D. T.; Kenis, P. J. A. J. Phys. Chem. Lett. 2010, 1,
3451–3458.
1989.
35) Determination of E°CO
in [BMIM]+/MeCN electrolyte is
3) Lee, S. Methanol Synthesis Technology; CRC Press: Boca Ra-
ton, Fl, 1989.
/CO
2
presented in the Supporting Information.
36) Costentin, C.; Drouet, S.; Robert, M.; Savéant, J. M. Science,
2012, 338, 90–94.
4) Klier, K. Methanol Synthesis. Adv.Catal. 1982, 31, 243–313.
5) Burch, R.; Golunski, S. E.; Spencer, M. S. Faraday Trans.
1990, 86, 2683–2691.
37) Ikeda, S.; Takagi, T.; Ito, K. Bull. Chem. Soc. Jap. 1987, 60,
6) Singh, A. D.; Krase, N. W. Ind. Eng. Chem. 1935, 27, 909–914.
7) Jones, J. H. Platinum Metals Rev. 2000, 44, 94–105.
8) Cheung, H.; Tanke, R. S.; Torrence, G. P. Acetic Acid. In
Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH,
Weinheim Germany, 2000.
2517–2522.
38) Amatore, C.; Savéant, J. M. J. Am. Chem. Soc. 1981, 103, 5021–
5023.
39) Komatsu, S.; Yanagihara, T.; Hiraga, Y.; Tanaka, M.; Kunugi,
A. Denki Kagaku 1995, 63, 217−224.
40) Köleli, F.; Atilan, T.; Palamut, N.; Gizir, A. M.; Aydin, R.;
Hamann, C. H. J. of Appl. Electrochem. 2003, 33, 447–450.
41) Whipple, D. T.; Finke, E. C.; Kenis, P. J. A. Solid-State Lett.
2010, 13, B109–B111.
9) Ojima, I.; Tsai, C.-Y.; Tzamarioudaki, M.; Bonafoux, D. Org.
React. 2000, 56, 1
10) Henrici-Olivé, G; Olivé, S. Springer-Verlag, Berlin, 1984.
11) Rofer-DePoorter, C. K. Chem. Rev. 1981, 447–474.
7
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 9
1
2
42) Agarwal, A. S.; Zhai, Y.; Hill, D.; Sridhar, N. ChemSusChem
2011, 4, 1301–1310.
3
4
43) Oloman, C.; Li, H. ChemSusChem 2008, 1, 385–391.
44) Zhang, S.; Kang, P;, Meyer, T. J.. J. Am. Chem. Soc. 2014, 136,
5
6
7
8
1734–1737.
45) Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. Energy
& Environ. Sci. 2012, 5, 7050–7059.
46) Gileadi, E. Electrode Kinetics for Chemists, Chemical Engi-
neers and Materials Scientists. Wiley-VCH, Weinheim Ger-
many, 1993.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
47) Sun, L.; Ramesha, G. K.; Kamat, P. V.; Brennecke, J. F. Lang-
muir 2014, 30, 6302–6308.
48) de Robillard, G.; Devillers, C. H.; Kunz, D.; Cattey, H.;
Digard, E.; Andrieu, J. Org. Lett. 2013, 15, 4410–4413.
8
ACS Paragon Plus Environment

Page 9 of 9
Journal of the American Chemical Society
Table of Contents Graphic
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
9
ACS Paragon Plus Environment