High-yield electrochemical production of formaldehyde from CO2 and seawater

Article abstract of DOI:10.1002/anie.201308657

The catalytic, electrocatalytic, or photocatalytic conversion of CO ₂ into useful chemicals in high yield for industrial applications has so far proven difficult. Herein, we present our work on the electrochemical reduction of CO₂ in seawater using a boron-doped diamond (BDD) electrode under ambient conditions to produce formaldehyde. This method overcomes the usual limitation of the low yield of higher-order products, and also reduces the generation of H₂. In comparison with other electrode materials, BDD electrodes have a wide potential window and high electrochemical stability, and, moreover, exhibit very high Faradaic efficiency (74 %) for the production of formaldehyde, using either methanol, aqueous NaCl, or seawater as the electrolyte. The high Faradaic efficiency is attributed to the sp ³-bonded carbon of the BDD. Our results have wide ranging implications for the efficient and cost-effective conversion of CO₂. Boron is a diamond's best friend: A boron-doped diamond (BDD) electrode exhibited very high Faradaic efficiency (74 %) for the production of formaldehyde using either methanol, aqueous NaCl, or seawater as the electrolyte at room temperature and ambient pressure. Copyright

Full text of DOI:10.1002/anie.201308657

Angewandte
Chemie
DOI: 10.1002/anie.201308657
CO2 Reduction
High-Yield Electrochemical Production of Formaldehyde from CO2
and Seawater**
Kazuya Nakata,* Takuya Ozaki, Chiaki Terashima, Akira Fujishima, and Yasuaki Einaga*
Abstract: The catalytic, electrocatalytic, or photocatalytic
conversion of CO2 into useful chemicals in high yield for
industrial applications has so far proven difficult. Herein, we
present our work on the electrochemical reduction of CO2 in
seawater using a boron-doped diamond (BDD) electrode
under ambient conditions to produce formaldehyde. This
method overcomes the usual limitation of the low yield of
higher-order products, and also reduces the generation of H2.
In comparison with other electrode materials, BDD electrodes
have a wide potential window and high electrochemical
stability, and, moreover, exhibit very high Faradaic efficiency
(74%) for the production of formaldehyde, using either
methanol, aqueous NaCl, or seawater as the electrolyte. The
high Faradaic efficiency is attributed to the sp3-bonded carbon
of the BDD. Our results have wide ranging implications for the
efficient and cost-effective conversion of CO2.
naturally occurring carbon fixation process as a model for
manufacturing synthetic chemicals.
The CO2 molecule is thermodynamically stable. To
efficiently convert CO2 into a desired product, suitable
activation mechanisms and reaction conditions must be
found. Among the various possible approaches, electrochem-
ical reduction is a promising one, mainly because it has the
advantages that the products from the electrochemical
reduction of CO2 can be tuned by the reaction conditions,
and that water can be used as both a source of electrons and
a source of protons to produce hydrocarbons.[4–8] In most of
the studies made using metal electrodes, the main products
obtained through CO2 reduction have been CO or formic
acid, although higher reduction products, such as formalde-
hyde, methanol, and methane, have also been obtained with
semiconductor or other metal electrodes[9,10] under atmos-
pheric or high-pressure conditions.[11–16] However, direct
electron transfer to a CO2 molecule requires a high over-
potential,[17] which means that the evolution of hydrogen is
a competitive process. This can reduce the efficiency of the
CO2 reduction. Thus, efficient catalysis is needed for the
successful reduction of CO2.
Boron-doped diamond (BDD) electrodes with p-type
surfaces exhibit interesting properties, such as a wide poten-
tial window, a low background capacitive current, and a very
high stability, which renders them chemically inert and
mechanically durable. These properties are significantly
different from those of other electrodes, such as glassy
carbon (GC),[18–24] and make BDD an attractive candidate for
the electrochemical reduction of CO2. In particular, the wide
potential window promotes the reduction of CO2, which may
be masked by the decomposition of solvents produced with
other electrodes. Other desirable properties, such as the
intrinsically inert nature of BDD, are expected to lead to
practical applications.
T
he demand for fossil fuels has continued to increase
because of our reliance on it as a source of energy by
combustion and as a resource for the production of plastics
and industrial chemicals. As CO2 is a final product of the
combustion process, the back conversion and utilization of
CO2 are important areas of research from the viewpoints of
conservation of resources and the development of a sustain-
able society.[1–3] The use of CO2 as an alternative to fossil fuels
is also inherently promising, both from ecological and
economical standpoints, and has received much attention
owing to the fact that the reaction of CO2 with H2O using an
external energy source is a simplified model of artificial
photosynthesis. It has been an aim of chemists to harness this
[*] Prof. K. Nakata
Department of Applied Biological Science, Faculty of Science and
Technology, Tokyo University of Science
2641 Yamazaki, Noda, Chiba 278-8510 (Japan)
E-mail: nakata@rs.tus.ac.jp
In the present work, we report on the results of an
investigation into the reduction of CO2 using BDD electrodes
in various electrolytes, including seawater, which acts as
a source for both electrons and protons.
Prof. K. Nakata, Prof. C. Terashima, Prof. A. Fujishima
Research Institute for Science and Technology
Photocatalysis International Research Center
Tokyo University of Science
To confirm the reduction of CO2 using the BDD
electrode, we first performed cyclic voltammetry in a meth-
anol solution containing tetrabutylammonium perchlorate
(TBAP). Methanol was chosen as the electrolyte because the
solubility of CO2 in MeOH is about five times that in water,
thus allowing easier detection of the reduction of CO2. Cyclic
voltammograms for the BDD electrode are presented in
Figure 1. A cathodic current with an onset potential of ꢀ1.8 V
vs. Ag/Ag+ was observed at a pressure of 1 atm in a solution
through which N2 was bubbled. The N2 was then replaced with
CO2 at a pressure of 1 atm, and after bubbling with CO2 for
2641 Yamazaki, Noda, Chiba 278-8510 (Japan)
T. Ozaki, Prof. Y. Einaga
Department of Chemistry, Faculty of Science and Technology
Keio University, 3-14-1 Hiyoshi, Yokohama 223-8522 (Japan)
E-mail: einaga@chem.keio.ac.jp
Prof. Y. Einaga
JST, CREST, 3-14-1 Hiyoshi, Yokohama 223-8522 (Japan)
[**] This work was supported by JST, CREST.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308657.
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Ag/Ag+. At higher potentials, above ꢀ1.9 V vs/Ag/Ag+, the
Faradaic efficiency for formaldehyde and formic acid
decreased, whereas that for hydrogen increased, because of
the electrochemical decomposition of the electrolyte, which
suppressed the direct reduction of CO2, as observed in the
cyclic voltammogram. Note that longer electrolysis (20 h)
produced formaldehyde with a Faradaic efficiency of 74%,
which is the same as that for a shorter electrolysis time (1 h).
Although there have been reports regarding the forma-
tion of formaldehyde during the electrochemical reduction of
CO2, the production of formaldehyde with such a high yield is
quite difficult.[10] Formaldehyde is an industrially important
material used for the production of plastics, such as resins, and
of industrial chemicals for adhesives, paint, and preservatives.
Presently, formaldehyde for industrial use is typically
obtained from the catalytic oxidation of methanol, so an
alternative high-yielding method for the production of
formaldehyde, such as from CO2, is desirable. Figure 3
Figure 1. Cyclic voltammograms for a BDD electrode with CO2. The
cyclic voltammograms were measured in a MeOH solution (0.1m)
containing TBAP at a scan rate of 100 mVsꢀ1. First, cyclic voltammetry
was performed at a pressure of 1 atm, with N2 bubbled through the
solution. After the measurement, the N2 was replaced by CO2 at
a pressure of 1 atm. Cyclic voltammograms were measured after
bubbling with CO2 for 1 h, 2 h, and 2.5 h.
1 h, a large peak was observed at ꢀ1.3 V vs. Ag/Ag+. The
current density of the peak increased and became saturated to
97.5 mAcmꢀ2 after 2.5 h bubbling CO2, thus indicating that the
CO2 concentration in the methanol electrolyte had saturated.
After saturation of the MeOH electrolyte with CO2,
electrolysis was performed with the BDD electrode at various
potentials for 1 h at room temperature and atmospheric
pressure. The reaction products were formaldehyde, formic
acid, and hydrogen (Figure 2). A maximum Faradaic effi-
ciency for formaldehyde (74%) was observed at ꢀ1.7 V vs.
Ag/Ag+. Formic acid was also generated with a best Faradaic
efficiency of 15% at ꢀ1.5 V vs. Ag/Ag+. The efficiency for the
formation of hydrogen was less than 1.1% below ꢀ1.7 V vs.
Figure 3. Faradaic efficiency of the products generated by the electro-
chemical reduction of CO2 using various electrodes in a MeOH
electrolyte. The electrochemical reduction was performed at ꢀ1.5 V vs.
Ag/Ag+ using a platinum counter electrode in a two-compartment cell
(100 mL) separated by Nafion for 1 h at room temperature and
atmospheric pressure.
shows a comparison of the Faradaic efficiency of products
obtained through the electrochemical reduction of CO2 in
a methanol electrolyte at ꢀ1.5 V vs Ag/Ag+ using different
electrodes. No formaldehyde was observed with metallic Cu,
Sn, Ag, or W electrodes. With a glassy carbon electrode, a low
Faradaic efficiency for formaldehyde (19%) was observed.
Thus, the high-yield production of formaldehyde is an
inherent property of BDD electrodes.
The mechanism for the production of formaldehyde in this
case may be due to the presence of sp3-bonded carbon on the
BDD. Glassy carbon electrodes have a large amount of sp2-
bonded carbon, which results in a low Faradaic efficiency for
formaldehyde. To compare the role of sp3- and sp2-bonded
carbon in the production of formaldehyde, we fabricated
a BDD electrode containing a large amount of sp2-bonded
carbon by controlling the preparation conditions. Character-
ization was done by Raman spectroscopy (Supporting Infor-
mation, Figure S1). The BDD electrode with a large amount
of sp2-bonded carbon gave a low Faradaic efficiency (15%)
Figure 2. Faradaic efficiency of the various products obtained through
the electrochemical reduction of CO2 in a MeOH electrolyte, with
respect to potential. The electrochemical reduction was performed in
the range from ꢀ1.0 V to ꢀ1.9 V vs. Ag/Ag+ using the BDD working
electrode and platinum counter electrode in a two-compartment cell
(100 mL) separated by Nafion for 1 h at room temperature and
atmospheric pressure. After electrolysis, the gaseous products were
collected in aluminium Tedlar bags and analyzed by gas chromatog-
raphy. Products soluble in the electrolyte were analyzed by high-
performance liquid chromatography.
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during electrochemical reduction of CO2 in MeOH at ꢀ1.5 V
vs. Ag/Ag+. Thus, sp3-bonded carbon may have specific
properties beneficial for the production of formaldehyde.
Formaldehyde can be formed by two electron reduction of
formic acid:[10]
Finally, as durability is an important issue for industrial
applications, we analyzed the electrochemical durability of
a BDD electrode used in the electrochemical reduction of
CO2. The morphology of the BDD was examined by FE-
SEM, which showed that it had not changed after electro-
chemical reduction, even after 20 h (Figure 4). The results
were compared with those obtained for a glassy carbon
electrode, whose surface was dramatically changed after 20 h
of electrolysis, thus demonstrating another significant advant-
age that BDD has over other electrode types.
CO2 þ 2 Hþ þ 2 eꢀ ! HCOOH
ð1Þ
ð2Þ
HCOOH þ 2 Hþ þ 2 eꢀ ! HCHO þ H2O
Thus, CO2 is reduced to form formic acid, which is then
continuously reduced to produce formaldehyde. To test this
hypothesis, we performed the electrochemical reduction of
formic acid instead of CO2 in MeOH at ꢀ1.5 V vs. Ag/Ag+,
for which a high Faradaic efficiency of formaldehyde pro-
duction was obtained (85%). For comparison, a glassy carbon
electrode and some metal electrodes (Cu, Sn, Ag, and W)
were also used for the electrochemical reduction of formic
acid. These exhibited quite low Faradaic efficiency, 6.4% and
less than 0.1% for the glassy carbon and all of the metal
electrodes, respectively. Thus, the BDD electrode is partic-
ularly efficient in the process of reducing formic acid to
formaldehyde. Furthermore, it has been reported that BDD
electrodes have an electrocatalytically inactive surface.[18,20] It
has been reported in previous papers that electrocatalytically
inactive surfaces preferentially produced formic acid.[25,26]
Thus, the production of formaldehyde with the BDD
electrode can be attributed to the reduction of formic acid
produced from the reduction of CO2. The formation of
hydrogen competes with the electrochemical reduction of
CO2; therefore, it is very important to suppress the formation
of hydrogen in order to avoid wasting energy on hydrogen
evolution instead of using it for reducing CO2. In this
experiment, the wide potential window of BDD reduced the
Faradaic efficiency for the formation of hydrogen.
Figure 4. SEM images before and after the electrolysis for 20 h.
a,b) BDD electrode before (a) and after (b) electrolysis. c,d) Glassy
carbon electrode before (c) and after (d) electrolysis. Scale bars are
10 mm.
In conclusion, we have demonstrated the electrochemical
reduction of CO2 using a BDD electrode, which produces
formaldehyde in high yield. Furthermore, the production of
formaldehyde was performed using seawater as a source of
both electrons and protons, which suggests that formaldehyde
can be simply prepared from CO2 and seawater under
ambient conditions. If the electrical energy required for the
reduction could be obtained from solar cells, we might
thereby achieve artificial photosynthesis through the produc-
tion of CO2, water, and solar energy.
As mentioned previously, the electrochemical reduction
of CO2 requires electrons and protons. Seawater is a better
option as a source of both electrons and protons because of its
abundance and conductivity. Thus, we next used seawater as
an electrolyte. The use of seawater as the electrolyte yielded
formaldehyde (36%) at room temperature and atmospheric
pressure (Table S1). The poorer Faradaic efficiency, com-
pared with the MeOH electrolyte, is due to the narrow
potential window in water and the inorganic and organic
impurities in seawater, which lower the electrochemical
reduction of CO2. For reference, we used water containing
NaCl (0.1m) as the electrolyte, which gave a Faradaic
efficiency of 62%. The higher Faradaic efficiency versus
that of seawater can be attributed to the absence of
impurities. In practice, the high conductivity of the BDD
electrode is beneficial for the high production rate of
folmaldehyde. Indeed, the amount of formaldehyde produced
was 7.5 ꢀ 10ꢀ3 m per hour for electrolysis in seawater (ꢀ1.5 V
vs. Ag/AgCl), which is greater than that previously reported
(for example, 1.4 ꢀ 10ꢀ3 m per hour for TiO2).[10] These results
suggest that formaldehyde can be effectively produced from
CO2 and seawater. Note that the BDD electrode has
extremely good corrosion resistance compared to other
electrodes, which is also beneficial for practical applications.
Experimental Section
BDD thin films were grown using a microwave plasma assisted
chemical vapor deposition (MPVCD) system (ASTeX Corp. Woburn,
MA). The boron source, B(OCH3), was dissolved in acetone (as
a carbon source) with a B/C atomic ratio of 1.0 w/w. BDD was
deposited on Si(111) wafers in an MPCVD chamber at 5 kW using
high-purity hydrogen as a carrier gas. BDD thin films with a high
amount of sp2-bonded carbon were prepared on Si(111) wafers in the
MPCVD chamber (AX6500X, Seki Technotron Corp.) at 6 kW using
methane and B(OCH3) as sources.
The surface morphology and crystalline structures were charac-
terized using scanning electron microscopy. The typical size of the
diamond crystals on the BDD electrodes was about 5 mm. To confirm
the ratio of boron atoms in the BDD, glow discharge-optical emission
spectroscopy (GDOES) carried out on the obtained BDD and a BDD
electrode with a known atomic ratio measured by secondary ion mass
spectrometry (SIMS). The boron atomic ratio of the BDD was 1.45%.
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To confirm the film quality of the BDD, Raman spectroscopy was
carried out with Ar laser illumination (Acton-SP2500). Raman
spectra were measured for the BDD electrodes. The presence of
a narrow peak at 1324 cmꢀ1 is attributed to the D band of sp3 bonded
carbon in a BDD with a boron atomic ratio of 1.45%, which was
shifted from 1332 cmꢀ1 in natural diamond (Figure S1). This peak
shift, which is usually observed in heavily doped diamond, is
attributed to the quantum interference between this phonon and
a continuum of electronic transitions. The large bands observed at
around 477 cmꢀ1 and 1204 cmꢀ1 in the spectra of all BDD films are
usually observed in metallic heavily doped BDD. In the spectrum of
BDD with a high amount of sp2-bonded carbon, an additional peak
appeared at 1538 cmꢀ1, which may be attributed to the G band of sp2-
bonded carbon. Further detail of morphology of the surface of BDD
is shown in Figure S2. The apparatus for the electrochemical
reduction of CO2 are shown in Figure S3. All of the electrochemical
experiments were performed in a two-compartment cell (100 mL),
using a platinum counter electrode and a Ag/Ag+ electrode as
a reference. Methanol (99%, Nacalai Tesque, Japan) was used as
received. Water was obtained from a Millipore Milli-Q Plus system.
BDD electrodes were sonicated in ultrapure water prior to use. All
cyclic voltammograms were taken at a scan rate of 100 mVsꢀ1, unless
otherwise stated. The electroreduction procedure was as follows:
First, N2 gas was bubbled into the electrolytes for 1 h at a rate of
100 mLminꢀ1 to remove oxygen, after which CO2 gas was bubbled
into the methanol for 2.5 h at a rate of 100 mLminꢀ1. The obtained
CO2-saturated solution was electrolytically reduced at cathodic
polarization. Stirring of the electrolyte was done with a magnetic
bar. The Faradaic efficiency of formation for the products was
calculated assuming that a total charge passed through the cell.
[4] T. Saeki, K. Hashimoto, A. Fujishima, N. Kimura, K. Omata, J.
Phys. Chem. 1995, 99, 8446.
[5] M. Jitaru, D. A. Lowy, M. Toma, B. C. Toma, L. Oniciu, J. Appl.
Electrochem. 1997, 27, 889.
[6] R. Angamuthu, P. Byers, M. Lutz, A. L. Spek, E. Bouwman,
Science 2010, 327, 315.
[7] H. Li, P. H. Opgenorth, D. G. Wernick, S. Rogers, T. Y. Wu, W.
Higashide, P. Malati, Y. X. Huo, K. M. Cho, J. C. Liao, Science
2012, 335, 1596.
[8] B. A. Rosen, A. Salehi-Khojin, M. R. Thorson, W. Zhu, D. T.
Whipple, P. J. A. Kenis, R. I. Masel, Science 2011, 334, 644.
[9] T. Saeki, K. Hashimoto, N. Kimura, K. Omata, A. Fujishima, J.
Electroanal. Chem. 1996, 404, 302.
[10] T. Inoue, A. Fujishima, S. Konishi, K. Honda, Nature 1979, 277,
638.
[11] T. Mizuno, K. Ohta, A. Sasaki, T. Akai, M. Hirano, A. Kawabe,
Energy Sources 1995, 17, 508.
[12] K. Hara, A. Kudo, T. Sakata, J. Electroanal. Chem. 1997, 421, 4.
[13] K. Hara, A. Tsuneto, A. Kudo, T. Sakata, J. Electrochem. Soc.
1994, 141, 2103.
[14] S. Kaneco, K. Iiba, H. Katsumata, T. Suzuki, K. Ohta, Electro-
chim. Acta 2006, 51, 4885.
[15] M. Todoroki, K. Hara, A. Kudo, T. Sakata, J. Electroanal. Chem.
1995, 394, 203.
[16] K. Hara, T. Sakata, Bull. Chem. Soc. Jpn. 1997, 70, 576.
[17] D. A. Tryk, A. Fujishima, Electrochem. Soc. Interface 2001, 10,
32 – 36.
[18] A. Fujishima, Y. Einaga, T. N. Rao, D. A. Tryk, Diamond
Electrochemistry, BKC Inc and Elsevier, Tokyo, 2005.
[19] E. Brillas, C. A. Martinez-Huitle, Synthetic Diamond Films,
Wiley, Hoboken, 2011.
[20] A. Fujishima, T. N. Rao, D. A. Tryk, Electrochim. Acta 2000, 45,
4683 – 4690.
Received: October 4, 2013
Revised: October 23, 2013
[21] S. R. Waldvogel, B. Elsler, Electrochim. Acta 2012, 82, 434 – 443.
[22] S. Waldvogel, S. Mentizi, A. Kirste in Radicals in Synthesis III,
Vol. 320 (Eds.: M. Heinrich, A. Gansꢁuer), Springer, Berlin,
2012, pp. 1 – 31.
[23] S. A. Yao, R. E. Ruther, L. Zhang, R. A. Franking, R. J. Hamers,
J. F. Berry, J. Am. Chem. Soc. 2012, 134, 15632 – 15635.
[24] N. Yang, F. Gao, C. E. Nebel, Anal. Chem. 2013, 85, 5764 – 5769.
[25] S. Ikeda, M. Yoshida, K. Ito, Bull. Chem. Soc. Jpn. 1985, 58,
1353 – 1357.
Published online: November 26, 2013
Keywords: boron · CO2 reduction · electrochemistry ·
.
formaldehyde · seawater
[1] M. M. Halmann, Chemical fixation of carbon dioxide, CRC,
Florida, 1993.
[2] M. Aresta, Carbon dioxide recovery and utilization, Springer,
Dordrecht, 2003.
[3] C. Song, A. M. Gaffney, K. Fujimoto, CO2 conversion and
utilization, American Chemical Society, Washington, DC, 2002.
[26] M. Aresta, Carbon dioxide as chemical feedstock, Wiley-VCH,
Weinheim, 2010.
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