Stable and Highly Efficient Electrochemical Production of Formic Acid from Carbon Dioxide Using Diamond Electrodes

Article abstract of DOI:10.1002/anie.201712271

High faradaic efficiencies can be achieved in the production of formic acid (HCOOH) by metal electrodes, such as Sn or Pb, in the electrochemical reduction of carbon dioxide (CO₂). However, the stability and environmental load in using them are problematic. The electrochemical reduction of CO₂ to HCOOH was investigated in a flow cell using boron-doped diamond (BDD) electrodes. BDD electrodes have superior electrochemical properties to metal electrodes, and, moreover, are highly durable. The faradaic efficiency for the production of HCOOH was as high as 94.7 %. Furthermore, the selectivity for the production of HCOOH was more than 99 %. The rate of the production was increased to 473 μmol m^?2 s^?1 at a current density of 15 mA cm^?2 with a faradaic efficiency of 61 %. The faradaic efficiency and the production rate are almost the same as or larger than those achieved using Sn and Pb electrodes. Furthermore, the stability of the BDD electrodes was confirmed by 24 h operation.

Full text of DOI:10.1002/anie.201712271

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Stable and Highly Efficient Electrochemical Production of Formic
Acid from Carbon Dioxide Using Diamond Electrodes
Authors: Keisuke Natsui, Hitomi Iwakawa, Norihito Ikemiya, Kazuya
Nakata, and Yasuaki Einaga
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201712271
Angew. Chem. 10.1002/ange.201712271
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10.1002/anie.201712271
Angewandte Chemie International Edition
COMMUNICATION
Stable and Highly Efficient Electrochemical Production of Formic
Acid from Carbon Dioxide Using Diamond Electrodes
Keisuke Natsui,[a] Hitomi Iwakawa,[a] Norihito Ikemiya,[a] Kazuya Nakata,[b] and Yasuaki Einaga*[a,c]
Abstract: High faradaic efficiencies can be achieved in the
production of formic acid (HCOOH) by metal electrodes, such as Sn
or Pb, in the electrochemical reduction of carbon dioxide (CO2).
However, the stability and environmental load in using them are
problematic. Here, we investigated the electrochemical reduction of
CO2 to HCOOH in a flow cell using boron-doped diamond (BDD)
rubber, medicine, fiber, etc.). In addition, HCOOH has recently
become regarded as an excellent fuel for direct fuel cells[13] and
a useful hydrogen-storage material.[14] There are many reports
on the production of HCOOH by the electrochemical reduction of
CO2 using various cathode materials such as Sn, Pb, Cd, Hg, In,
Pd-Pt nanoparticles, and so on.[15-19] Especially, Sn and Pb
electrodes have been frequently used, because they are cheap
and high faradaic efficiencies for producing HCOOH can be
achieved (e.g. 88.4% and 97.4%, respectively).[15] However,
their environmental load is very high, and their stability also
seems to be problematic.[20]
Boron-doped diamond (BDD) is the most promising
alternative to these electrodes, because BDD is metal-free and,
as such, is environmentally friendly. It also has excellent
electrochemical properties such as a wide potential window in
aqueous solutions and high durability derived from intrinsic
diamond.[21] It is expected that the wide potential window will
enable the suppression of hydrogen evolution, which is a
competing reaction in the reduction of CO2, leading to more
efficient production of the desirable products. In fact, using BDD
for the electrochemical reduction of CO2, we were able to
produce formaldehyde[22], methanol[23], and HCOOH[24] with high
faradaic efficiencies. However, in our previous paper, the
maximum faradaic efficiency for HCOOH was only 71% using a
two-compartment batch cell.[24]
electrodes.
BDD electrodes have superior electrochemical
properties to metal electrodes, and, moreover, are highly durable.
The faradaic efficiency for the production of HCOOH was as high as
94.7%. Furthermore, the selectivity for the production of HCOOH
was more than 99%. The rate of the production was increased to
473 μmol m−2 s−1 at a current density of 15 mA cm−2 with a faradaic
efficiency of 61%. The faradaic efficiency and the production rate
are almost the same as or larger than those achieved using Sn and
Pb electrodes. In addition, the stability of the BDD electrodes was
confirmed by 24 hours operation.
Carbon dioxide (CO2) is well known as the most significant
greenhouse gas, and the continual increase in the atmospheric
concentration of CO2 have lead to climate change. It is a
serious problem for the world. Consequently, many researchers
have made great efforts to develop solutions for reducing CO2 in
the atmosphere, such as the development of renewable energy
sources[1] and carbon capture and storage (CCS) technology.[2]
From another point of view, CO2 is a cheap and abundant
source of carbon, so the utilization of CO2 is of great interest. In
the past few decades, various methods to convert CO2 into
useful chemicals and fuels have been tried, such as
photochemical,[3-5] electrochemical,[6-9] and photoelectrochemical
methods.[10-12] Of these, a great deal of attention has been
devoted to electrochemical methods, since these can be
performed under ambient temperature and pressure conditions,
and, moreover, these methods are relatively easy to operate on
a large scale for practical applications. Various chemicals can
be produced by the electrochemical reduction of CO2, such as
formic acid/formate (HCOOH/HCOO−), carbon monoxide (CO),
methane (CH4), alcohol, and hydrocarbons.[9]
In this paper, in an effort to achieve greater efficiency in the
production of HCOOH, we investigated the electrochemical
reduction of CO2 with BDD electrodes using a circulation flow
cell (Figure 1). With this cell the mass-transport of CO2 onto the
electrode is improved, and we were able to obtain a high
faradaic efficiency for the production of HCOOH (94.7%) at a
high production rate.
HCOOH is an important chemical for industry (leather,
[a]
Dr. K. Natsui, H. Iwakawa, Dr. N. Ikemiya, and Prof. Dr. Y.Einaga
Department of Chemistry
Keio University
3-14-1 Hiyoshi, Yokohama 223-8522 (Japan)
*E-mail: einaga@chem.keio.ac.jp
Prof. Dr. K. Nakata
Photocatalysis International Research Center
Tokyo University of Science
[b]
[c]
2641 Yamazaki, Noda, Chiba 278-8510 (Japan)
Prof. Dr. Y. Einaga
JST-ACCEL
3-14-1 Hiyoshi, Yokohama 223-8522 (Japan)
Figure 1. Schematic diagram of two-compartment flow cell.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201712271
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First, we investigated the effect of the flow rate of the
electrolyte on the electrochemical reduction of CO2 using a BDD
electrode. The electroreduction of CO2 was performed with a
BDD electrode at a current density of 2 mA cm−2 for 60 min with
various flow rates. The main product was HCOOH, plus a small
amount of CO. Moreover, H2 was also obtained, which is a
competing reaction. The amounts of other products were
negligible. The production rates of HCOOH and the faradaic
efficiencies for the products are summarized in Table 1. Note
that the measurements were repeated three times (n = 3) for
each condition, and we calculated the means and standard
deviations of the faradaic efficiencies and production rates. The
production rate is defined as the quantity of HCOOH (mol)
produced per unit cathode area (m2) and unit time (s). The
faradaic efficiency for the production of HCOOH increased to
94.7% with increasing flow rate to 200 mL min−1, whereas the
faradaic efficiency for the production of H2 decreased. In
addition, the selectivity for the production of HCOOH by CO2
reduction was more than 99%. The selectivity for end products
by CO2 reduction depends on the binding ability of electrode
sweep voltammograms after CO2 saturation on a BDD electrode
with a scan rate of 100 mV s−1 at various flow rates of the
electrolytes showed that the reduction current increased and the
onset potential shifted to more positive values with increasing
flow rate (Figure S2). These suggest that the reduction reaction
can proceed at a lower potential because fresh reactants are
brought more frequently to the electrode surface due to the
increased flow rate. Therefore, it is assumed that as the mass-
transport of the CO2 increases, so also does the faradaic
efficiency for the production of HCOOH compared to the H2
evolution reaction. In addition, the highly efficient CO2 reduction
is probably related to not only the use of a flow cell, but also the
property of the BDD electrode. Referring to the previous
report,[26] the positively charged B atoms, which caused by
polarization of B-C bonds due to low electronegativity of B, and
•−
defects on BDD may stabilize the produced CO2 on the BDD
surface by the electronic interaction, and thus the energy barrier
is reduced, resulting in the acceleration of CO2 reduction.
Moreover, the high H2 overpotential of the BDD electrode can
suppress the H2 evolution reaction, leading to the improvement
of the efficiency for CO2 reduction. On the other hand, the
faradaic efficiency for the production of HCOOH decreases
slightly from 94.7% at 200 mL min−1 to 88.4% at 500mL min−1.
This may be because the rate is so fast that it inhibits electron
transfer from the electrode to the CO2 at a current density of 2
mA cm−2, and H2O, of which there is a greater amount than CO2,
is preferentially reduced to H2 gas. Thus far, the maximum
faradaic efficiency for the production of HCOOH, 94.7%, is the
best value achieved for CO2 reduction using a BDD electrode.
Recently, we reported on the production of HCOOH by CO2
reduction using a BDD electrode in an H-type cell.[24] In that
case, the maximum faradaic efficiency was 71% using Rb+
cations, which are relatively high cost. Also, the production rate
of HCOOH was lower (76.5 μmol m−2 s−1) than the present case
operating at the same current density (2 mA cm−2). The
improvements here are due to the increased mass-transport of
CO2 using the flow cell.
surface
for
the
produced
intermediates
(CO2•−).[7]
HCOOH/HCOO− is formed on the electrodes which can hardly
bind the CO2•−. As the BDD surface is inert,[25] the CO2 could
•−
be hardly bound. Thus, the highly selective production of
HCOOH could be achieved on the BDD electrode. In a previous
work,[26] nitrogen-doped diamond also can produce HCOO−,
though acetate is the main product. This implies that diamond
surface seems to have a tendency for HCOOH production. At
20 mL min−1, the combined faradaic efficiency was less than
100%, possibly due to H2 gas partially leaking from the
electrolysis cell.
Table 1. Electrochemical reduction of CO2 using a BDD electrode in a flow
cell at 2 mA cm−2 for 60 min with various flow rates of the electrolytes.
Flow rate
Production rate
of HCOOH
Faradaic efficiency (%)
(ml min−1
)
(μmol m−2 s−1
)
HCOOH
CO
H2
Total
20
50
36.6±9.8
35.4±
9.4
0.4±
0.1
40.8±
9.6
76.6±5.1
87.7±2.0
94.3±0.8
97.9±2.3
91.4±1.9
84.7±
2.0
0.9±
0.6
8.6±
2.5
94.2±0.7
95.2±0.9
99.4±0.6
99.2±0.8
100
200
500
91.1±
0.7
0.5±
0.4
3.6±
1.3
94.7±
2.3
0.6±
0.5
4.1±
1.9
88.4±
1.8
2.7±
2.6
8.1±
3.4
Figure 2. Chronopotentiograms with a BDD electrode at 2 mA cm−2 for 60 min
with flow rates of 20 mL min−1 (purple), 50 mL min−1 (blue), 100 mL min−1
(green), 200 mL min−1 (orange), and 500 mL min−1 (red).
Figure 2 shows chronopotentiograms during CO2 reduction
using a BDD electrode at various flow rates of the electrolyte.
As can be seen in Figure 2, the reduction potential shifts to more
positive values with increasing flow rate. In addition, linear
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10.1002/anie.201712271
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Next, we investigated the dependence of the faradaic
efficiency and the production rate of HCOOH on current density.
In order to obtain increased amounts of HCOOH, the current
density was increased. Electroreduction of CO2 was performed
using a BDD electrode at current densities of 2, 5, 10, 15, and
20 mA cm−2 for 60 min with flow rates of 200 mL min−1 and 500
mL min−1. Figure 3 shows the faradaic efficiency and the
production rate of HCOOH as functions of the applied current
density. At 20 mA cm−2, the faradaic efficiency for HCOOH has
dropped to just 19%, and the production rate has also been
significantly reduced, because H2 evolution occurs much easily
compared to CO2 reduction.
electron transfer as the current density increases, so that the
faradaic efficiency for the production of HCOOH decreases. In
addition, the faradaic efficiencies at 500 mL min−1 are larger than
those at 200 mL min−1 when a high current density is applied.
This is also because the mass transport of CO2 at 500 mL min−1
is higher than at 200 mL min−1. On the other hand, the
production rate of HCOOH is 473 μmol m−2 s−1 at 15 mA cm−2
and 500 mL min−1 (Figure 3B). This value is larger than the
rates for Sn and Pb electrodes using a similar flow cell as this
work in a previous work (e.g. 440 μmol m−2 s−1 at a faradaic
efficiency of around 70% with Sn).[27]
Finally, we investigated the electrochemical reduction of CO2
at 2 mA cm−2 with a flow rate of 200 mL min−1 for 24 hours to
confirm the stability of the production of HCOOH and the
durability of the BDD electrode. These conditions are those that
gave the highest faradaic efficiency for the production of
HCOOH achieved in this work. During the electrolysis, CO2 gas
was bubbled to maintain saturation. In Figure 4A, the faradaic
efficiency remained at around 90% for 24 hours. Moreover, the
concentration of HCOOH increased linearly with the electrolysis
time, as shown in Figure 4B. Therefore, we confirmed the
stability of the production of HCOOH by CO2 reduction using a
BDD electrode in a flow cell under the above conditions for at
least 24 hours. In addition, no degradation of the BDD electrode
was observed by SEM after 24 hours of electrolysis (data not
shown).
Figure 3. Faradaic efficiency (A) and production rate (B) of HCOOH by
electrochemical reduction of CO2 with a BDD electrode at various current
densities with flow rates of 200 mL min−1 (black) and 500 mL min−1 (red). The
dashed lines in the figures are guides for eye.
In Figure 3A, the faradaic efficiency for HCOOH decreases
with increasing applied current density. Note that no other
products were obtained and H2 evolution increases with
increasing current density. Increasing the current density means
that the rate of electron transfer between the electrode and the
reactants is increased. Thus, it is assumed that the transport
rate of CO2 onto the cathode becomes slower than the rate of
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Figure 4. Faradaic efficiency (A) and concentration (B) of HCOOH by the
Electrochemical measurements were performed under atmospheric
electrochemical reduction of CO2 with a BDD electrode at 2 mA cm−2 and a
pressure and temperature in
a two-compartment PTFE flow cell
flow rate of 200 mL min−1
.
separated with Nafion NRE-212 (Sigma-Aldrich) membrane using a
potentiostat/galvanostat system (PGSTAT204, Metrohm Autolab), as
shown in Figure 1. The construction details of the flow cell are shown in
Figure S1. BDD, a Pt plate, and Ag/AgCl (saturated KCl) were used as
the working, counter, and reference electrodes, respectively. The
geometric area of the BDD electrode in the contact with electolyte was
9.62 cm2 (diameter: 3.5 cm). The interelectrode gap between the BDD
electrode and the Pt plate was 3 cm. Electrical contacts were made
between Cu plates and the back sides of the BDD electrode and the Pt
plate. Before each electrolysis, the BDD electrode was cleaned using an
electrochemical pretreatment (10 cycles between potentials of −3.5 V
and 3.5 V and then 20 cycles between potentials of 0 V and 3.5 V with a
scan rate of 1 V s−1 in a 0.1 M H2SO4 aqueous solution). 0.5 M KCl
aqueous solution (50 mL) was used as the catholyte, and 1.0 M KOH (50
mL) was used as the anolyte. The catholyte was bubbled with N2 for 30
min to remove dissolved oxygen, and then bubbled with CO2 for 60 min.
After saturation with CO2, the pH of the catholyte was around 3.9. Linear
sweep voltammetry (LSV) after CO2 bubbling was performed with a scan
rate of 100 mV s−1 from −0.5 to −2.5 V (vs. Ag/AgCl), while circulating the
electrolytes using peristaltic pumps with the flow rates of 20, 50, 100, 200,
and 500 mL min−1 (Figure S2). Electrolysis of the CO2 was performed at
current densities of 2, 5, 10, 15, and 20 mA cm−2 for 60 min. CO2
bubbling was maintained during electrolysis. The electrolytes were
circulated during electrolysis using peristaltic pumps, and the flow rates
In this work, we investigated the electrochemical reduction of
CO2 using a boron-doped diamond (BDD) electrode in a two-
compartment flow cell to obtain HCOOH efficiently. The faradaic
efficiency for the production of HCOOH increased with
increasing flow rate of the electrolyte, reaching a maximum of
94.7%, due to the greater mass transport of CO2 onto the BDD
surface. This value is almost equivalent to that achieved with
other metal electrodes, such as Sn (88.4%) and Pb (97.4%).[15]
In addition, the selectivity for the production of HCOOH by CO2
reduction was more than 99%. On the other hand, although the
faradaic efficiency for the production of HCOOH decreased with
increasing applied current density, the production rate of
HCOOH was as much as 473 μmol m−2 s−1 at 15 mA cm−2 and
500 mL min−1 with a faradaic efficiency of 61%. This production
rate is also comparable to the values for Sn and Pb electrodes
using a similar flow cell (440 μmol m−2 s−1 with a faradaic
efficiency of around 70%).[27] Moreover, we confirmed the
stability of the BDD electrode after electrochemical reduction of
CO2 for 24 hours at 2 mA cm−2. Consequently, we found that
the use of BDD electrodes is a viable alternative to other metal
electrodes with which HCOOH can be produced, such as Sn
and Pb. However, so far, the detailed mechanisms about highly
selective and efficient production of HCOOH on the BDD
electrodes using the flow cell are unclear. To elucidate the
mechanisms, the investigations into the concentration of CO2
near the electrode surface and the intermediates of CO2
reduction using in situ infrared attenuated total reflection (IR-
ATR) technique are now in progress. In the next phase, it
should be possible to further improve the faradaic efficiency and
the production rate by making a gas diffusion electrode
system[28] using BDD.
of the electrolytes were 20, 50, 100, 200, and 500 mL min−1
.
After
electrolysis, N2 gas was bubbled through the catholyte for 15 min to
collect the gas products into an aluminum bag. The liquid products were
analyzed using a high-performance liquid chromatograph equipped with
an electroconductivity detector (CDD-10A, Shimadzu Corp.), and the gas
products were analyzed using a gas chromatograph equipped with a
flame ionization detector and a thermal conductivity detector (GC-2014,
Shimadzu Corp.). The faradaic efficiency was estimated using the
following equation:
Faradaic efficiency (%) = nFc/Q×100
(1)
where n is the number of electrons involved in reducing the CO2 to the
product, F is the Faraday constant (96,485 C mol−1), c is the amount of
product (mol), and Q is the total charge passed through the reduction
process (C).
Experimental Section
BDD electrodes were prepared on Si wafer substrates using a
microwave plasma-assisted chemical vapor deposition system (AX5400,
Corns Technologies Ltd.) following a procedure described in our previous
paper.[29] The boron-to-carbon ratio in the feed gases was 1000 ppm,
and the deposition time was 6 hours. Characterization of the BDD was
performed by Raman spectroscopy (excited wavelength: 532 nm) and a
scanning electron microscope (SEM). Raman spectra were recorded
using an Acton SP2500 (Prinston Instruments), and SEM images were
taken with a JCM-6000 Plus (JEOL). The Raman spectra of the obtained
BDD showed an asymmetrical sharp peak at 1332 cm−1 due to the zone-
center optical phonon of diamond.[30] This asymmetrical feature is
attributed to the Fano-effect,[31] which is typically observed in
semimetallic BDD (boron concentration ≥ 1020 cm−3).[32] SEM images of
the BDD showed that it had a polycrystalline nature with a grain size of
about 10~20 μm.
Acknowledgement
This work was supported by Tohoku Electric Power Co., Inc., Japan.
Conflict of interest
The authors declare no conflict of interest.
Keywords: boron • diamond • electrochemistry • carbon dioxide
• formic acid
KCl, KOH, and H2SO4 were purchased from Wako Pure Chemical
Industries Ltd., and used without any further purification. Deionized
water with a resistivity of 18.2 MΩ cm at 25 °C was obtained from a
Simply-Lab water system (DIRECT-Q 3 UV, Millipore).
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Entry for the Table of Contents (Please choose one layout)
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K. Natsui, H. Iwakawa, N. Ikemiya, K.
Nakata, Y. Einaga*
Page No. – Page No.
Stable and Highly Efficient
Electrochemical Production of Formic
Acid from Carbon Dioxide Using
Diamond Electrodes
Efficient CO2 conversion: Formic acid could be obtained with high faradaic
efficiency, as high as 95%, by the electrochemical reduction of CO2 on boron-doped
diamond (BDD) electrodes using a circulation flow cell. Moreover, the stability of
the BDD electrodes was confirmed by operating the system for 24 hours. We found
that the use of BDD electrodes is a viable alternative to other metal electrodes with
which formic acid can be produced.
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