Switchable Product Selectivity in the Electrochemical Reduction of Carbon Dioxide Using Boron-Doped Diamond Electrodes

Article abstract of DOI:10.1021/jacs.9b01773

The main product obtained by electrochemical reduction of CO₂ depends on the electrode material, and in many cases the Faradaic efficiency for this is determined by the electrolyte. Only a few investigations in which attempts to produce different products from the same electrode material have been done so far. In this work, we focus on boron-doped diamond (BDD) electrodes with which plentiful amounts of formic acid and small amounts of carbon monoxide have been produced. By optimizing certain parameters and conditions used in the electrochemical process with BDD electrodes, such as the electrolyte, the boron concentration of the BDD electrode, and the applied potential, we were able to control the selectivity and efficiency with which carbon monoxide is produced. On one hand, with a BDD electrode with 1% boron used for the cathode and KClO₄ for the catholyte, the selectivity for producing carbon monoxide was high. On the other hand, with a BDD electrode with 0.1% boron used for the cathode and KCl for the catholyte, the production of formic acid was the most evident. In situ attenuated total reflectance-infrared (ATR-IR) measurements during electrolysis showed that CO₂^·- intermediates were adsorbed on the BDD surface in the KClO₄ aqueous solution. Here, switchable product selectivity was achieved when reducing CO₂ using BDD electrodes.

Full text of DOI:10.1021/jacs.9b01773

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Article
Switchable Product Selectivity in the Electrochemical Reduction
of Carbon Dioxide Using Boron-Doped Diamond Electrodes
Mai Tomisaki, Seiji Kasahara, Keisuke Natsui, Norihito Ikemiya, and Yasuaki Einaga
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019
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Journal of the American Chemical Society
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Switchable Product Selectivity in the Electrochemical Reduction of
Carbon Dioxide Using Boron-Doped Diamond Electrodes
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Mai Tomisaki,† Seiji Kasahara,† Keisuke Natsui,† Norihito Ikemiya,†Yasuaki Einaga*,†,‡
†Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Yokohama 223-8522, Japan.
‡
JST-ACCEL, 3-14-1 Hiyoshi, Yokohama 223-8522, Japan.
KEYWORDS: boron-doped diamond, carbon dioxide, carbon monoxide, electrochemistry, infrared spectroscopy
ABSTRACT: The main product obtained by electrochemical reduction of CO2 depends on the electrode material, and in many
cases the faradaic efficiency for this is determined by the electrolyte. Only a few investigations in which attempts to produce
different products from the same electrode material have been done so far. In this work, we focus on boron-doped diamond
(BDD) electrodes with which plentiful amounts of formic acid and small amounts of carbon monoxide have been produced.
By optimizing certain parameters and conditions used in the electrochemical process with BDD electrodes, such as the
electrolyte, the boron concentration of the BDD electrode, and the applied potential, we were able to control the selectivity
and efficiency with which carbon monoxide is produced. With a BDD electrode with 1% boron used for the cathode and KClO4
for the catholyte, the selectivity for producing carbon monoxide was high. On the other hand, with a BDD electrode with 0.1%
boron used for the cathode and KCl for the catholyte, the production of formic acid was the most evident. In-situ ATR-IR
measurements during electrolysis showed that CO2•– intermediates were adsorbed on the BDD surface in the KClO4 aqueous
solution. Here, switchable product selectivity was achieved when reducing CO2 using BDD electrodes.
1. Introduction
material, not by optimizing the electrode material, but by
optimizing the electrolysis conditions as in previous
The conversion of carbon dioxide (CO2) into value added
products has attracted the attention of many researchers.
Atmospheric CO2 is considered to be a greenhouse gas and
therefore must be reduced. On the other hand, CO2 is an
abundant source of carbon and conversion of CO2 into
useful carbon-based chemicals by organic, biochemical,
photochemical or electrochemical processes is desirable1,2.
Of these methods, electrochemical conversion has some
advantages, because the conditions used for this type of
process are moderate and the reactions can be easily
controlled by adjusting the applied potential or current.
studies12,13
.
Many metal electrodes used for the electroreduction of
CO2 have problems of toxicity or stability2,14. Boron-doped
diamond (BDD) is a promising electrode for CO2 reduction.
Made from carbon and boron, BDD contains no metals, and
has a wide potential window compared to metal electrodes
in aqueous solutions, so the hydrogen evolution reaction,
which is a competitive reaction, occurs at high potential and
the CO2 reduction reaction can be made the dominant one.
Moreover, BDD has high chemical and physical
stability15,16,17, so the electrolysis can be continued for a long
time18. In our previous studies19,20,21,22,23, electrochemical
reduction of CO2 using BDD electrodes produced
formaldehyde, methanol, formic acid and carbon monoxide
depending on the conditions such as electrolyte, boron
content of BDD, and so on. Of these products, formic acid
and carbon monoxide were obtained easier than other
products because they are two-electron reductants. Both
formic acid and carbon monoxide are used in many
industrial fields. Formic acid is an excellent fuel for direct
fuel cells24 and can be used as a carrier for hydrogen25.
Carbon monoxide is an important raw material in C1
chemistry and is used as a ligand in inorganic chemistry.
However, currently, the faradaic efficiency for the
production of carbon monoxide from CO2 reduction using
BDD electrodes is much less than that for the production of
formic acid. In this work, we optimized the electrolysis
conditions, including the electrolyte, the boron
CO2 can be electrochemically reduced to produce formic
acid, carbon monoxide, hydrocarbons, or alcohols. It is well
known that the main product obtained by the
electrochemical reduction of CO2 depends on the electrode
material. Metal electrodes are classified into several groups
according to the adsorption strength of CO2•–, the CO2 anion
radical, which is an intermediate in the CO2 reduction
reaction2,3,4. Sn, Pb, In, and Hg, which do not adsorb CO2
,
•–
mainly produce formic acid or formate; Ag, Au, and Zn,
which do adsorb CO2•–, mainly produce carbon monoxide.
Cu, an electrode used specifically for the CO2 reduction,
produces hydrocarbons and alcohols. In addition,
electrolytes can affect the faradaic efficiencies of the
products, so in much previous research the effect of the
cations and anions of the electrolytes on CO2 reduction
using
several
metal
electrodes
has
been
investigated5,6,7,8,9,10,11. However, it is of great interest to
produce different products from the same electrode
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concentration in the electrode, and the applied potential, to
counter electrodes of this cell27 were both 9.62 cm2. 0.1 mol
L−1 KClO4 and KCl aqueous solutions were used for the
electrolytes. The volumes of the catholyte and anolyte were
each 50 mL. Unless noted otherwise, the electrolytes were
circulated at a flow rate of 100 mL min−1.
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produce carbon monoxide selectively and more efficiently
by the electrochemical reduction of CO2 using BDD
electrodes.
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N2 gas was bubbled into the catholyte for 30 min at a gas
flow rate of 200 mL min−1 to remove the dissolved oxygen
and then CO2 gas was bubbled for 15 min at a gas flow rate
of ca. 250 mL min−1. At the same time the electrolytes were
circulated. Electrolysis was carried out at constant potential
using a potentiostat/galvanostat system (PGSTAT204;
Metrohm Autolab) until the total charge had reached 69 C.
CO2 was bubbled into the solution at a flow rate of less than
25 mL min−1 during electrolysis. After electrolysis, N2 gas
(50 mL min−1) was bubbled for 10 min to collect the gas
products. All the experiments were conducted at room
temperature (20-25˚C) and atmospheric pressure (1 atm).
2. Experimental
2.1. Preparation and characterization of the BDD
electrodes
The BDD electrodes were deposited onto Si wafer
substrates in a microwave plasma-assisted chemical vapor
deposition (MPCVD) system (AX5250M, AX6500; Cornes
Technologies Ltd.). The boron-to-carbon ratio in the feed
gases was 1000 ppm (0.1%) and 10000 ppm (1%) (0.1%
BDD, 1% BDD) and deposition was carried out for 6 hours
at 6 kW and 5 kW, respectively, using conditions reported
in a previous paper26. For the 0.1% BDD electrode, the
carbon and boron sources were methane and
trimethylboron, respectively. On the other hand, for the 1%
BDD electrode, the carbon and boron sources were acetone
and trimethyl borate, respectively. The BDD electrodes
were characterized by Raman spectroscopy (excited
wavelength: 532 nm), scanning electron microscopy (SEM),
and glow discharge optical emission spectrometry
(GDOES). Raman spectra were recorded with an Acton
SP2500 (Instruments). The SEM images were obtained with
a JCM-6000Plus (JEOL Ltd.), and GDOES analysis was
conducted using a GD-Profiler2 (HORIBA Ltd.) to estimate
the boron concentration in the BDD films. The results of the
characterizations are shown in the Supporting Information.
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The products obtained by CO2 reduction were quantified
by a high-performance liquid chromatography (HPLC)
system equipped with an electroconductivity detector or a
UV-visible detector (Prominence; Shimadzu Corp.), a gas
chromatography (GC) system equipped with a thermal
conductivity detector or a flame ionization detector (GC-
2014; Shimadzu Corp.), and a gas chromatography-mass
spectrometer (GCMS-QP2010 Ultra; Shimadzu Corp.).
Details of the analysis are given in our previous paper23. The
faradaic efficiency was calculated using equation (1):
Faradaic efficiency (%) = nFc/Q × 100
(1)
where n is the number of electrons used in producing the
products from CO2, F is the Faraday constant (96,485 C
mol−1), c is the amount of the product (mol), and Q is the
total charge passed in the reduction process (69 C). All
electrolysis experiments were conducted once at a time.
2.2. Chemicals
KClO4, KCl, HCl, HNO3, and HClO4 were purchased from
FUJIFILM Wako Pure Chemical Corporation. Formic acid
(FUJIFILM Wako Pure Chemical Corp.), formaldehyde
solution (37 wt.% in H2O, contains 10-15% Methanol;
Sigma-Aldrich), methanol (Tokyo Chemical Industry Co.,
Ltd.), and ethanol (Kanto Chemical Co., Inc.) were used to
make standard solutions for quantitative analysis. All the
chemicals were used without any purification. All the
aqueous solutions were prepared from ultrapure water
with a resistivity of 18.2 MΩ cm at 25°C obtained from a
DIRECT-Q UV3 system (Millipore Corp.).
2.4. In-situ ATR-IR measurements
A submicrometer thick BDD film was deposited onto a Si
ATR-IR prism using the MPCVD system described above.
The carbon and boron sources were methane and
trimethylboron, respectively. In order to enable isotope
labeling, BDD films were prepared with both 12CH4 (12C-
BDD) and 13CH4 (13C-BDD). The BDD prism was
characterized by Raman spectroscopy, SEM (JSM-7600F;
JEOL Ltd.), GDOES, and a stylus profiler (DektakTM; Bruker
Corp.). Data are shown in Figure S2. The BDD prism, a glassy
carbon rod, and Ag/AgCl (3 mol L−1 NaCl) were used as the
working, counter, and reference electrodes, respectively.
Electrical contact to the BDD film was made using Ag paste
and placing Au tape at the edge of the prism.
Electrochemical measurements were carried out with a
PGSTAT204 (Metrohm Autolab). ATR-IR spectra were
measured with a FT/IR-6600 (JASCO Corp.) using a liquid
nitrogen cooled MCT detector. Each spectrum was collected
at a resolution of 4 cm−1 and had 256 scans.
2.3. Procedure for the electrolysis
The BDD electrodes were pretreated by immersion in
aqua regia for 30 min and UV- ozone oxidation using a low-
pressure mercury lamp (UVB40; Sen Lights Corp.) for 30
min, as described in our previous work23. For the purposes
of comparison, a Sn (99.9%; Nilaco Corp.) electrode was
also prepared. This was pretreated by polishing with
sandpaper (Ultra fine #800-1000; 3M) to get a clean
surface.
A two-compartment flow cell made of PTFE27 was used
for the electrolysis of CO2. The compartments were
separated by a Nafion membrane (Nafion® NRE-212; Sigma-
Aldrich). 0.1% BDD, 1% BDD, or Sn was used for the
working electrode. A Pt plate and Ag/AgCl (3 mol L−1 NaCl)
were used for the counter and reference electrodes,
respectively. The geometric areas of the working and
3. Results and discussion
The electrochemical reduction of CO2 was performed
using BDD and Sn electrodes in KClO4 and KCl aqueous
solutions. The main products obtained by CO2 reduction
were carbon monoxide and formic acid. Methanol and
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methane were hardly obtained. In addition, hydrogen was
monoxide was a maximum in KClO4 aqueous solution (–2.1
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V vs. Ag/AgCl). The results for the two electrolysis are
shown in Figure 1b. When KCl was used for the electrolyte,
the faradaic efficiency for producing formic acid was high,
and very little carbon monoxide was obtained. From these
experiments, it can be concluded that carbon monoxide is
produced more easily when KClO4 is used as the electrolyte
rather than when KCl is used.
It is supposed that the anions in the electrolytes have
some effect on the selectivity of the products. The effect of
anions has been investigated by many research groups, and
it has been suggested that the CO2 reduction reaction can be
influenced by the buffering effect of the anions, or by the
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specific adsorption of anions on the electrode surface2,9,10,11
.
It has also been reported that the anions have little effect on
CO2 reduction28. However, BDD is a carbon-based electrode,
so these assertions cannot be applied directly to this
research. Generally, carbon monoxide is produced using
electrodes at which the intermediate in the CO2 reduction
reaction (CO2•–) is easily adsorbed, whereas formic acid is
produced using electrodes for which the adsorption of the
•–
CO2 intermediate is difficult. When KClO4 is used as the
•–
electrolyte, adsorption of the CO2 intermediate on the
electrodes can be promoted, so carbon monoxide
production is high. On the other hand, when KCl is used as
the electrolyte, adsorption of the CO2•– intermediate on the
electrodes is more difficult, so the production of formic acid
is promoted. From the results of electrolysis obtained by
CO2 reduction using KClO4, KCl, and other electrolytes
(Table S1), perchlorate ions play an important role in the
electroreduction of CO2 using BDD electrodes.
Figure 1. (a) Faradaic efficiencies for producing carbon
monoxide (red triangles), formic acid (blue squares), and
hydrogen (black circles) by the electrochemical reduction of
CO2 with a 0.1% BDD electrode in 0.1 mol L−1 KClO4 aqueous
solution. (b) Comparison between the electrochemical
reduction of CO2 using a 0.1% BDD electrode at –2.1 V (vs.
Ag/AgCl) in KClO4 and in KCl aqueous solutions.
3.1.2. In-situ ATR-IR measurements during CO2 reduction
In order to obtain an insight into the effect of the anions,
ATR-IR measurements were conducted during electrolysis
in 0.1 mol L−1 KClO4 and KCl aqueous solutions.
detected. The faradaic efficiencies for the products are
summarized in Table S1. From the experiments, the
conditions to produce carbon monoxide efficiently were
optimized.
Figure 2 shows time-dependent ATR-IR spectra during
CO2 reduction in 0.1 mol L−1 KClO4 and KCl aqueous
solutions, respectively. After anodic oxidation (+3 V vs.
Ag/AgCl, 0.1 mol L−1 HClO4 aqueous solution, 5min), a
background spectrum was taken just after the reduction
potential (−2.1 V vs. Ag/AgCl) was applied. Then, each ATR-
IR spectrum was taken at 5 minute intervals. The ATR-IR
spectrum with the KClO4 aqueous solution (Figure 2a) has
two strong peaks at 1360, and 1632 cm−1. On the other hand,
the ATR-IR spectrum with the KCl aqueous solution (Figure
2b) has three strong peaks at 1360, 1390, and 1616 cm−1.
Some chemical species relevant to CO2 show absorption
peaks between 1300 and 1400 cm−1 (the spectra for these
are shown in Figure S3), so it is difficult to assign the peaks
at around 1300-1400 cm−1. In contrast, based on previous
studies29,30, the peaks at 1632 cm−1 (Figure 2a) and 1616
cm−1 (Figure 2b) can be assigned to asymmetric OCO
stretching, suggesting the presence of CO2•–. In order to
distinguish this peak from H2O vibrations, the same
measurements were conducted in D2O. The peaks were also
observed in D2O, suggesting that the peaks can be assigned
to CO2•– intermediates. Figure 2c shows a comparison of the
peaks observed in KClO4 and KCl aqueous solutions after 30
minutes of electrolysis.
3.1. The electrochemical reduction of CO2 using BDD
electrodes
3.1.1. The effect of the electrolyte on the production of
carbon monoxide
First, we investigated how the electrolyte can affect the
selectivity of the products. Figure 1a shows the faradaic
efficiencies for producing carbon monoxide, formic acid,
and hydrogen using a 0.1% BDD electrode in KClO4 aqueous
solution. The faradaic efficiency for the production of
carbon monoxide increased when the applied potential
became less negative, and reached a maximum at –2.1 V (vs.
Ag/AgCl). The faradaic efficiency for formic acid production
decreased and then increased when the applied potential
became less negative. The selectivity for the production of
carbon monoxide is maximum at –2.2 V (vs. Ag/AgCl).
In previous research27, formic acid was produced with
high efficiency by electrolysis in an aqueous solution of KCl.
Electrolysis in KCl was also conducted in this study, but at
the same potential at which the production of carbon
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Figure 2. Time-dependent ATR-IR spectra during electrochemical reduction of CO2 in 0.1 mol L−1 (a) KClO4 and (b) KCl aqueous
solutions at −2.1 V (vs. Ag/AgCl). Each spectrum was taken after every 5 minutes of electrolysis. The arrows in the figures show the
direction of time. (c) Comparison between the spectra in KClO4 (red) and KCl (blue) aqueous solutions taken after 30 minutes of
electrolysis.
The wavenumbers of the peaks that can be assigned to
asymmetric OCO stretching are different for the KClO4 and
KCl aqueous solutions. It is plausible that this peak shift is
due to a difference in the nature of the adsorption of the
ATR-IR spectra with the 13C-BDD electrode were measured
using the same procedure used to take the spectra shown in
Figure 2.
Figure 3a shows a comparison of the ATR-IR spectra
taken during CO2 reduction in different electrolytes using
13C- and 12C-BDD electrodes. The most obvious feature is the
•–
CO2 intermediates on the BDD electrodes in the two
•–
different solutions. The adsorption characteristics of CO2
intermediates are an important determinant for the main
•–
peak assigned to CO2 intermediates, which has shifted
product obtained by CO2 electrochemical reduction4,31
.
from 1632 cm−1 (on 12C-BDD) to 1622 cm−1 (on 13C-BDD),
only in the 0.1 mol L−1 KClO4 aqueous solution. This is
possibly because CO2•– intermediates are directly adsorbed
on BDD in KClO4 aqueous solutions, and the vibrations of
these are directly affected by the different carbon isotopes.
In contrast, the spectra taken in 0.1 mol L−1 KCl aqueous
solution do not show any shift due to the different isotopes.
This indicates that the CO2•– intermediates are not adsorbed
on BDD in KCl aqueous solution. From the ATR-IR analyses
and the electrolysis results, it is inferred that, in KClO4
Thus, it is anticipated that the electrolyte has an effect on
the adsorption behavior of CO2•– intermediates when using
BDD electrodes.
In order to confirm whether the electrolyte does affect the
adsorption characteristics of the CO2•– intermediates, ATR-
IR measurements were also conducted when using a BDD
electrode prepared with a 13C carbon source (13C-BDD).
•–
aqueous solution, CO2 intermediates are adsorbed on the
BDD electrode, thereby promoting the production of carbon
monoxide, but in the case of KCl aqueous solution many
intermediates exist as free intermediates, so the production
of formic acid is promoted (Figure 3b). It is of great interest
•–
that the strength of the adsorption of the CO2
intermediates on BDD can be controlled by the electrolyte.
3.1.3. Optimizing the conditions to produce carbon
monoxide
With a 0.1% BDD cathode and KClO4 catholyte, more or
equal amounts of formic acid was produced compared to
carbon monoxide, so it is necessary to increase the
selectivity and faradaic efficiency for the production of
carbon monoxide. Research has been done focusing on the
effect of the boron concentration in the BDD electrode on
the electrochemical reduction of CO2. In that study, the
carbon monoxide production was somewhat higher when
the boron concentration in the electrode was higher22. Here,
in our next experiment, we used a 1% BDD electrode for the
cathode.
Figure 3. (a) Comparison of the in-situ ATR-IR spectra for CO2
reduction using 13C-BDD and 12C-BDD electrodes in 0.1 mol L−1
KClO4 and KCl aqueous solutions. Each spectrum was taken 30
minutes after applying −2.1 V (vs. Ag/AgCl). (b) The possible
reaction pathway of CO2 reduction using BDD electrodes in
KClO4 and KCl aqueous solutions.
Figure 4 shows the faradaic efficiencies for producing
carbon monoxide, formic acid, and hydrogen using a 1%
BDD electrode in KClO4 aqueous solution at –2.1 V (vs.
Ag/AgCl). Comparison with the results using the 0.1% BDD
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and formic acid increase, and that for hydrogen production
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decreases sharply. This infers that the increased flow rate
increases the supply of CO2 to the electrode surface, thus
promoting the CO2 reduction reaction27. In addition, the
high flow rate would promote removal of the carbon
monoxide produced by the CO2 reduction and adsorbed on
the BDD electrode, and that new CO2 would be able to reach
the electrode surface. Promotion of the CO2 reduction
reaction decreases the number of electrons available for the
hydrogen evolution reaction, thereby reducing hydrogen
production. The flow rate dependency was also investigated
at –1.8 V (vs. Ag/AgCl), the potential at which the maximum
faradaic efficiency for carbon monoxide production was
obtained (Table S1). Increases in carbon monoxide and
formic acid production and a decrease in hydrogen
production were observed, but the selectivity for carbon
monoxide production was lower than the results at –2.1 V
(vs. Ag/AgCl).
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Figure 4. Faradaic efficiencies for producing carbon monoxide
(red), formic acid (blue) and hydrogen (black) by the
electrochemical reduction of CO2 using a 1% BDD electrode at
–2.1 V (vs. Ag/AgCl) in 0.1 mol L−1 KClO4 and KCl aqueous
solutions.
electrode as the cathode shows that the hydrogen
production has increased due to the low hydrogen
overpotential compared to 0.1% BDD26,32, whereas the
selectivity for producing carbon monoxide has increased.
As mentioned before, carbon monoxide is produced on
3.2. The electrochemical reduction of CO2 using a Sn
electrode
To compare the performance with a metal electrode, Sn
was chosen for the electrode material. It is known that BDD
electrodes are inert,16 so it is considered that reactants are
•–
electrodes such as Au, Ag, and Zn, on which CO2
intermediates can be adsorbed4,3. It has been reported that
BDD electrodes with high boron content have more
acceptor states and holes33. Thus, it is assumed that more
intermediates can be adsorbed on 1% BDD than on 0.1%
BDD, and that the production of carbon monoxide is
promoted on 1% BDD. When 1% BDD was used as the
cathode, the selectivity for carbon monoxide production
was higher than when 0.1% BDD was used as the cathode.
Moreover, when BDD electrode with much higher boron
content (5% BDD) was used as a cathode (Table S1), the
amounts of carbon monoxide produced were as much as or
less than with the 1% BDD electrode.
KCl was also used as a catholyte (Figure 4). Formic acid
production was lower compared to the results using a 0.1%
BDD electrode, but carbon monoxide production was still
less than formic acid production, which is much different to
the results obtained using KClO4 as the catholyte. The
experimental results so far have shown us that to produce
carbon monoxide selectively from CO2 using BDD
electrodes, it is important to use KClO4 as the catholyte and
1% BDD as the cathode.
Use of 1% BDD as the cathode and KClO4 as the catholyte
promotes the production of carbon monoxide. With these,
electrolysis was conducted at several applied potentials
(Figure 5a). As shown in Figure 5a, the faradaic efficiency
for the production of carbon monoxide is higher than that
for the production of formic acid at all potentials. The
faradaic efficiency for producing carbon monoxide
increases as the applied potential becomes less negative,
and the maximum faradaic efficiency is at –1.8 V (vs.
Ag/AgCl). In addition, the selectivity for producing carbon
monoxide has a maximum at –2.1 V (vs. Ag/AgCl).
Figure 5. Dependency of (a) the applied potential and (b) the
flow rate of the electrolyte on the faradaic efficiencies for
carbon monoxide (red triangles), formic acid (blue squares)
and hydrogen (black circles) production by the
electroreduction of CO2 using a 1% BDD electrode in 0.1 mol L−1
KClO4 aqueous solution.
With the applied potential at –2.1 V (vs. Ag/AgCl), the flow
rate of the electrolyte was varied from 100 mL min–1 to 500
mL min–1 (Figure 5b). As the flow rate increases, the
faradaic efficiencies for the production of carbon monoxide
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ATR-IR study showed that the adsorption peak for the CO2
Page 6 of 8
∙−
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intermediates only appeared when KClO4 aqueous solution
was used, supporting the results of the electrolysis. The
selectivity for producing carbon monoxide using KClO4 as
the electrolyte increased when a 1% BDD electrode was
used rather than a 0.1% BDD electrode. Increasing the flow
rate of the electrolytes, increased the faradaic efficiency for
producing carbon monoxide and formic acid, and that for
hydrogen dropped significantly. This is attributed to the
increase in the supply of CO2 and promotion of carbon
monoxide removal from the electrode surface. CO2
reduction using Sn electrodes was also conducted in KClO4
and KCl aqueous solutions. In both cases, formic acid was
produced with high efficiency, while only small amounts of
carbon monoxide were obtained. Unlike BDD electrodes,
the product selectivity could not be switched when using Sn
electrodes. With BDD electrodes, the product selectivity
could be switched by changing the electrolyte from KClO4 to
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Figure 6. Faradaic efficiencies for producing carbon monoxide
(red), formic acid (blue) and hydrogen (black) by CO2 reduction
using a Sn electrode at –2.0 V (vs. Ag/AgCl) in 0.1 mol L−1 KClO4
and KCl aqueous solutions.
∙−
KCl. Here, we found that more CO2 intermediates are
difficult to adsorb on the electrode surface. The trend of the
properties and the products by CO2 reduction of BDD
electrodes27 are similar to those of Sn. In previous
researches, Sn, Pb, and Hg are known as the electrodes
which do not adsorb CO2∙− intermediates on their surfaces.3,4
The products by galvanostatic electrolysis are mainly
formic acid and small amounts of carbon monoxide.34
adsorbed on BDD in KClO4 aqueous solution compared to
∙−
KCl aqueous solution; however, it is still unclear why CO2
intermediates are more favorably adsorbed on BDD
electrodes in KClO4 aqueous solution rather than in KCl
aqueous solution. In addition, on which site of the BDD
∙−
surface CO2 intermediates are adsorbed should be
investigated. To obtain the answers for these, DFT
calculations are in progress.
Electrolysis with the Sn electrode was conducted at –2.0
V (vs. Ag/AgCl) in KClO4 and KCl aqueous solutions (Figure
6). In both cases, the faradaic efficiency for the production
of formic acid was high, and little carbon monoxide was
obtained. Unlike the BDD electrode, switching the product
selectivity could not be done when Sn was used as the
cathode. In addition, there were some adsorbed species on
the surface of the Sn electrode after electrolysis. From
previous research14, this is due to corrosion and the
formation of alkali metal deposits on the Sn electrode. It is
inferred that changing the electrolyte does not affect the
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org.
Characterization
of
BDD
electrodes
used
for
electrochemical reduction of CO2, characterization of a BDD
films used for in-situ ATR-IR measurements, the faradaic
efficiencies of products obtained by CO2 reduction in
various electrolytes using BDD or Sn electrodes,
representative ATR-IR peaks observed during CO2
reduction, potential-dependent ATR-IR spectra during CO2
reduction in 0.1 mol L−1 KClO4 aqueous solution, ATR-IR
spectra during electrochemical reduction of 13CO2, ATR-IR
spectra during cathodic reduction without CO2, and linear
sweep voltammograms using BDD electrodes in KClO4 and
KCl aqueous solutions
∙−
adsorption strength of the CO2 intermediates on Sn
electrodes, so highly efficient formic acid production was
observed. On the other hand, with BDD electrodes the
adsorption strength changes with the electrolyte, so the
product selectivity can be switched by changing the
composition of the electrolyte. In addition, unlike metal
electrodes, the surfaces of BDD electrodes are covered with
∙−
some functional groups35,36, so the situation for the CO2
intermediates is different to that of metals.
AUTHOR INFORMATION
Corresponding Author
*Tel.: +81-45-566-1704. Fax: +81-45-566-1697. E-mail:
einaga@chem.keio.ac.jp
4. Conclusions
In this work, we investigated the electrochemical
reduction of CO2 using BDD electrodes and examined the
conditions that would enable us to control the selectively
for producing carbon monoxide or formic acid. The
electrolyte, the boron concentration of the BDD electrodes,
and the applied potential were optimized. We found a
correlation between the product selectivity and the
electrolyte. When KClO4 was used as the electrolyte, the
selectivity for the production of carbon monoxide tended to
increase, whereas when KCl was used as the electrolyte, the
selectivity for the production of formic acid tended to
increase. It was deduced that when KClO4 was used as the
electrolyte, more CO2∙− intermediates were adsorbed on the
electrode than when KCl was used as the electrolyte. An
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by Tohoku Electric Power Co., Inc.,
Japan.
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