Selective production of methanol by the electrochemical reduction of CO2 on boron-doped diamond electrodes in aqueous ammonia solution

Article abstract of DOI:10.1039/c6ra20466j

The electrochemical reduction of CO₂ was investigated in an aqueous ammonia solution using boron-doped diamond electrodes. Methanol was mainly produced by reduction at a potential of -1.3 V (vs. Ag/AgCl) with a faradaic efficiency as high as 24.3%. Also, even in an aqueous ammonium bicarbonate solution (pH 7.9) without CO₂ bubbling, methanol was produced, while no methanol production was observed at higher (10.6) and lower pH (3.38). These observations suggest that the selectivity for methanol production in aqueous ammonia solutions is due to the electrochemical reduction of bicarbonate ions which are formed by the reaction between ammonia and CO₂. Moreover, we present the important role of ammonia as an electrolyte for the selective production of methanol by electrochemical reduction of CO₂.

Full text of DOI:10.1039/c6ra20466j

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: P. K. Jiwanti, K.
Natsui, K. Nakata and Y. Einaga, RSC Adv., 2016, DOI: 10.1039/C6RA20466J.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Please do not adjust margins
RSC Advances
Page 1 of 4
View Article Online
DOI: 10.1039/C6RA20466J
Journal Name
ARTICLE
Selective Production of Methanol by the Electrochemical
Reduction of CO2 on Boron-Doped Diamond Electrodes in
Aqueous Ammonia Solution
Received 00th January 20xx,
Accepted 00th January 20xx
Prastika K. Jiwanti,a Keisuke Natsui,a Kazuya Nakata,b and Yasuaki Einagaa,c
*
DOI: 10.1039/x0xx00000x
The electrochemical reduction of CO2 was investigated in an aqueous ammonia solution using boron-doped diamond
electrodes. Methanol was mainly produced by the reduction at a potential of −1.3 V (vs. Ag/AgCl) with a faradaic
efficiency as high as 24.3%. Also, even in an aqueous ammonium bicarbonate solution (pH 7.9) without CO2 bubbling,
methanol was produced, while no methanol production was observed at higher (10.6) and lower pH (3.38). These
observations suggest that the selectivity for methanol production in aqueous ammonia solution is due to the
electrochemical reduction of bicarbonate ions which are formed by the reaction between ammonia and CO2. Moreover,
we present the important role of ammonia as an electrolyte for the selective production of methanol by electrochemical
www.rsc.org/
reduction
of
CO2.
such as wide potential window, low background current,
chemical inertness, and mechanical durability, and it has been
used for many electrochemical applications.12-14 The wide
potential window and conductive behavior of BDD electrodes
are suitable for supporting CO2 reduction in aqueous solutions
and minimizing hydrogen evolution. In fact, we reported on
the electrochemical reduction of CO2 using BDD electrodes in
methanol, seawater, and NaCl solutions, producing
formaldehyde and formic acid with high faradaic efficiency.15
On the other hand, carbon dioxide capture and storage
(CCS) technology is one of the promising technologies for
Introduction
CO2 reduction is an important next-generation technology and
has been widely studied by researchers, since it emphasizes
the value of processes by producing chemicals or fuel.
Although the high thermodynamic stability of CO2 makes the
reduction of it difficult in general, efficient and suitable
processes must be achieved. One of the useful strategies is to
develop suitable catalysts, for example, the Ru(II)–Re(I)
supramolecular photocatalyst,1 Re(CO)3(bpy)Cl complexes,2
and the Ag(II) porphyrin complex.3 On the other hand, several
studies have been also reported on the direct electrochemical
reduction using metal electrodes,4 electrodes with some
modifications,5 and carbon-based electrodes.6-9 However, a
high overpotential is needed for direct electron transfer to the
CO2 molecule (Eq. 1).
reducing carbon emissions.
Amine solutions, such as
monoethanolamine (MEA) and diethanolamine (DEA), are used
for CCS technology, since they are chemically strong CO2
absorbers. In addition, ammonia (NH3) solution is also known
as a strong absorber and has high loading capacity for CO2
scrubbing systems (3 times higher than MEA solutions).16-18
Along these lines, in this present work, the electrochemical
reduction of CO2 on a BDD electrode was performed in weakly
alkaline aqueous NH3 solution, resulting in the selective
production of methanol.
•−
CO2 + e−
→
CO2
E0 = −1.9 V (vs. NHE)
(1)
Thus, hydrogen evolution, which also occurs together with CO2
reduction, is competing reaction, resulting in high
production of hydrogen gas.10,11
a
a
Conductive boron-doped diamond (BDD) is a material of
great interest due to its superior electrochemical properties,
Experimental
Chemicals. NH4HCO3 was purchased from Sigma Aldrich, and
other reagents were purchased from Wako Pure Chemical
Industries. All reagents were used without any further
purification. Ultrapure water was obtained from a Symply-Lab
water system (Direct-Q UV3, Millipore).
a.Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Yokohama 223-8522,
Japan.
b.Photocatalysis International Research Center, Tokyo University of Science, 2641
Yamazaki, Noda, Chiba 278-8510, Japan.
c. ACCEL, Japan Science and Technology Agency, 5-3 Yonbancho, Chiyoda 102-8666,
Japan.
Electronic Supplementary Information (ESI) available: [The apparatus for
electrochemical reduction, pH measurements, CO2 concentration, and SEM
images]. See DOI: 10.1039/x0xx00000x
Preparation of the BDD Electrodes. BDD films were deposited
onto Si (111) wafers using a microwave plasma-assisted
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 2 of 4
View Article Online
DOI: 10.1039/C6RA20466J
ARTICLE
Journal Name
chemical vapor deposition system (Model AX 5400, CORNES efficiency: 19.7%). Here, the dependence of the faradaic
Technology Corp.). Details of the preparation are described efficiency on the applied potential is summarized in Table 1.
elsewhere.14 A mixture of trimethoxyborane and acetone was When the applied potential was −1.3 V (vs. Ag/AgCl), the
used as the carbon and boron sources to achieve a B/C ratio of efficiency of the methanol production was the highest. At the
1%.
same time, the efficiencies for the production of CO and CH4
were quite low. Thus, methanol can be selectively produced
Electrochemical
Measurements
.
The
electrochemical by CO2 reduction on a BDD electrode in an aqueous NH3
measurements were conducted in two-compartment cell solution. On the other hand, when the applied potential was
separated by a Nafion membrane with 100 ml of solution in more negative, the faradaic efficiency of the hydrogen
each cell (Figure S1). BDD, Pt mesh, and Ag/AgCl were used as evolution increased and that of the methanol production
a working electrode, a counter electrode, and a reference diminished.
electrode, respectively. BDD with an area of 4.9 cm2 and the
Ag/AgCl electrode were immersed on the catholyte side (1 M
NH3), and the Pt mesh electrode was on the anolyte side (0.1
M NaCl). The BDD and Pt electrodes were pretreated by
ultrasonication in ultrapure water. Ultrasonication in acetone
and ultrapure water was also performed after each reduction
process to clean the electrodes. Before reduction started, N2
gas was purged for 30 minutes to remove oxygen gas in the
solution, followed by CO2 gas bubbling for 2 hours with a flow
rate of 200 sccm. Cyclic voltammograms were taken before
and after gas bubbling with a scan rate of 100 mV/s.
Electrochemical reduction of CO2 was performed by
chronoamperometry for 2 hours at potentials ranging from
−1.2 V to −1.5 V (vs. Ag/AgCl) at the room temperature and the
atmospheric pressure. To study the mechanism, the reduction
of an aqueous ammonium bicarbonate (NH4HCO3) solution
was performed without CO2 bubbling. All electrochemical
measurements were recorded using a potentiostat (Autolab
PGSTAT204, Metrohm Autolab B.V.).
Figure 1. CVs on a BDD electrode in 1 M NH3 aqueous solution after
nitrogen gas purging (a: solid line) and after CO2 bubbling for 2 hours
Products Analysis. The liquid products were analyzed using a (b: dashed line) with a scan rate of 100 mV/s.
gas chromatography-mass spectrometry (GCMS-QP2010 Ultra,
Shimadzu Corp.) by the headspace method using ethanol as
Table 1. The faradaic efficiencies of the products for 2-hours reduction
of CO2 on a BDD electrode in 1 M NH3 aqueous solution at the
potentials from −1.2 V to −1.5 V (vs. Ag/AgCl).
the internal standard. Meanwhile, the gas products were
analyzed by gas chromatography with a flame ionization
detector and a thermal conductivity detector (GC-2014,
Shimadzu Corp.).
Faradaic Efficiency (%)
Potential
(V vs. Ag/AgCl)
CO
CH4
H2
CH3OH
Results and Discussion
- 1.2
- 1.3
- 1.4
- 1.5
0.24
0.13
0.03
0
2.61
24.3
15.1
2.02
0
0
Figure 1 shows cyclic voltammograms (CVs) on a BDD
electrode in 1 M NH3 aqueous solution. After nitrogen gas
purging, no reduction peak was observed for potentials
ranging from 0 V to −1.8 V (vs. Ag/AgCl) (Figure 1a). Aꢀer CO2
gas was bubbled into the solution at a flow rate of 200 sccm
for 2 hours, still no specific reduction peak was observed.
However, a cathodic current was increased from around −1.0 V
(vs. Ag/AgCl) (Figure 1b).
0.05
0.12
0.20
19.7
25.8
57.2
The pH was monitored during CO2 bubbling in the 1 M NH3
aqueous solution. The initial pH was 11.7. During CO2
bubbling for 2 hours, the pH decreased to 7.7 (Figure 2). In
aqueous solutions, NH3 and CO2 react as follows (Eq. 2-4)16:
After CO2 saturation (2-hours bubbling) in 1 M NH3
aqueous solution, the electrochemical reduction was
performed for
2 hours at various potentials at room
temperature and atmospheric pressure. The products after
electrolysis at −1.3 V (vs. Ag/AgCl) were methanol (0.25 mg/L,
faradaic efficiency: 24.3%), carbon monoxide (0.002 mg/L,
faradaic efficiency: 0.05%), methane (0.0006 mg/L, faradaic
efficiency: 0.13%), and hydrogen (0.04 mg/L, faradaic
−
NH3 + CO2 + H2O
⇄
NH4+ + HCO3 Keq(293) = 1.02 × 103 (2)
−
NH3 + HCO3
NH3 + HCO3
⇄
⇄
NH2CO2− + H2O Keq(293) = 3.61
(3)
−
+
CO32− + NH4
Keq(293) = 1.04 × 10−1 (4)
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Page 3 of 4
RSC Advances
View Article Online
DOI: 10.1039/C6RA20466J
Journal Name
ARTICLE
12
11
10
9
8
7
0
20
40
60
80
100
120
Time (min)
Figure 3. Faradaic efficiencies of methanol production by the 2-hours
reduction of 0.1 M NH4HCO3 aqueous solution (pH 7.9) on a BDD
−1.2 V to −1.5 V (vs. Ag/AgCl).The
Figure 2. pH of the aqueous ammonia solution during CO2 gas bubbling.
The measurement was conducted while stirring the solution at ~25oC. After electrode at the potentials from
2-hours bubbling, pH decreased to 7.7.
faradaic efficiencies of the products for 2-hours reduction of CO2 on a
BDD electrode in 1 M NH3 aqueous solution at the potentials from −1.2
V to −1.5 V (vs. Ag/AgCl).
On the other hand, according to the distribution of
carbonaceous species in the aqueous solution, the pH
determines which species are dominant in the solution. It is
known that dissolved CO2 (aq) is the dominant species at pH <
On the other hand, we considered the effect of NH3 on
methanol production by the electrochemical reduction of
−
5, while at pH levels from 7.5 to 9, HCO3 (bicarbonate ion) is
dominant, and CO32− (carbonate ion) is dominant above pH 12.
The reaction is generally denoted as follows (Eq. 5-6)19:
bicarbonate ions.
The electrochemical reduction was
performed in 0.1 M KOH solution saturated with CO2 (pH 7.0)
and in 0.1 M NaOH solution saturated with CO2 (pH 7.2) at a
potential of −1.3 V (vs. Ag/AgCl) for 2 hours on a BDD
electrode, resulting in no methanol production. It can be
explained by the buffering effect of the solution. During the
electrochemical reduction, the hydroxide ions (OH−) were
produced by the reduction of bicarbonate ions and also by the
hydrogen evolution. Therefore, the local pH will become
slightly higher. However, in the aqueous NH3 solution, OH−
reacts with ammonium ions to form ammonia and water by
the reaction (Eq. 8):
CO2 (aq) + H2O (l)
H+(aq) + HCO3− (aq)
⇄
H+(aq) + HCO3− (aq)
2H+(aq) + CO32− (aq) pK2 = 10.33 (6)
pK1 = 6.35 (5)
⇄
Since the reaction of aqueous NH3 with CO2 rapidly
produces bicarbonate (pH 7.6~8.0) after CO2 saturation, it is
assumed that bicarbonate ions are reduced to methanol in our
system.
In order to confirm that bicarbonate ions are reducible
species, the electrochemical reduction of a 0.1 M NH4HCO3
aqueous solution (pH 7.9) was performed without CO2
bubbling. As the result, methanol was mainly produced in
common with the reduction in an aqueous NH3 solution
saturated with CO2 (Figure 3).
NH4+ + OH−
⇄
NH3 + H2O
(8)
On the other hand, in the KOH and NaOH solution, the
bicarbonate ions will react with OH− to form carbonate ions by
the following reaction (Eq. 9):
In addition, we investigated into the influence of pH by
adding HCl for low pH (<4) and NaOH for high pH (>10) to the
NH4HCO3 solution. As the results, methanol was not detected
in either condition at the same reduction potential of −1.3 V
(vs. Ag/AgCl). These results suggest that bicarbonate ions are
reducible species, not CO2 and CO32−. That is, we propose the
mechanism of methanol production in our system as follows
(Eq. 7):
HCO3− + OH−
⇄
CO32− + H2O
(9)
As mentioned previously, the reducible species are
bicarbonate ions, and carbonate ions could not be reduced to
methanol. Therefore, the presence of NH3 is important for
methanol production.
In addition, ammonia has the
advantage of the higher loading capacity of CO2 than other
aqueous solutions (Table S1 and Table S2). However, a clear
and complete mechanism will need further study.
−
HCO3 + 5H2O + 6e−
→
CH3OH + 7OH−
(7)
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
RSC Advances
Page 4 of 4
View Article Online
DOI: 10.1039/C6RA20466J
ARTICLE
Journal Name
5
6
7
8
M. Ma, K. Djanashvili, W. A. Smith, Phys. Chem. Chem. Phys.,
2015, 17, 20861.
B. R. Eggins, E. M. Brown, E. A. McNeill, J. Grimshaw,
Tetrahedron Lett., 1988, 29, 945.
P. A Christensen, A. Hamnett, A. V G. Muir, N. A. Freeman, J.
Electroanal. Chem. Interfacial Electrochem., 1990, 288, 197.
K. Hara, A. Kudo, T. Sakata, J. Electroanal. Chem., 1997, 421
1.
,
9. N. Yang, S. R. Waldvogel, X. Jiang, ACS. Appl. Mater. Interfaces,
2015, DOI: 10.1021/acsami.5b09825.
10. T. Yamamoto, D. A. Tryk, K. Hashimoto, A. Fujishima, M. Okawa,
J. Electrochem. Soc., 2000, 147, 3393.
11 R. M. Hernández, J. Márquez, O. P. Márquez, M. Choy, C.
Ovalles, J. J. Garcia, B. Scharifker, J. Electrochem. Soc., 1999,
146, 4131.
Figure 4. SEM images of the BDD surface (a) before and (b) after more than
30 hours reduction.
12 W. T. Wahyuni, T. A. Ivandini, P. K. Jiwanti, E. Saepudin, J.
Gunlazuardi, Y. Einaga, Electrochemistry, 2015, 83, 357.
13 Y. Einaga, J. Appl. Electrochem., 2010, 40, 1807.
14 T. Yano, D. A. Tryk, K. Hashimoto, A. Fujishima, J.
Electrochem. Soc., 1998, 145, 1870.
15 K. Nakata, T. Ozaki, C. Terashima, A. Fujishima, Y. Einaga,
Angew. Chem. Int. Ed., 2014, 53, 871.
16 F. Mani, M. Peruzzini, P. Stoppioni, Green Chem., 2006, 8,
995.
17 S. Y. Park, K. B. Yi, C. H. Ko, J.-H. Park, J.-N. Kim, W. H. Hong,
Energy Fuels, 2010, 24, 3704.
18 H. Bai, A. C. Yeh, Ind. Eng. Chem. Res., 1997, 36, 2490.
19 H. Zhong, K. Fujii, Y. Nakano, F. Jin, J. Phys. Chem. C, 2015,
119, 55.
For comparison, the electrochemical reduction of CO2 on
glassy carbon electrode was conducted at a potential of −1.3 V
(vs. Ag/AgCl) for 2 hours. However, no methanol was
detected, and most of the product was H2 reaching an amount
of 0.26 mg/L (on the BDD electrode, it was 0.04 mg/L).
Therefore, methanol can be selectively produced by CO2
reduction on a BDD electrode, as long as the hydrogen
evolution can be suppressed. This kind of continuous
electrochemical reduction at quite high potential may cause
surface corrosion of the glassy carbon electrode (emphasizing
roughness, changing pores volume, etc).19,20 On the other
hand, BDD has high durability. In order to show evidence for
the durability of BDD, the surface morphology of a BDD
electrode was examined by SEM after using the BDD electrode
for more than 30 hours in a reduction process. The SEM
images revealed no difference as compared to the as-grown
BDD (Figure 4). This is consistent with the results in our
previous report. 15
20 G. M. Swain, J. Electrochem. Soc., 1994, 141, 3382.
Conclusions
We found that methanol can be selectively produced by
the electrochemical reduction of CO2 in an aqueous ammonia
solution on BDD electrodes at the potential of −1.3 V (vs.
Ag/AgCl) with the faradaic efficiency as high as 24.3%. The
methanol production is most likely due to the electrochemical
reduction of bicarbonate ions which are formed by the
reaction between ammonia and CO2. In addition, BDD
electrodes show the high durability and the suppression of
hydrogen evolution for the electrochemical reduction of CO2,
compared with a glassy carbon electrode. This study shows
the possibility for the practical applications on reducing
−
industrial stored-CO2 (HCO3 ), which usually use amine-based
solutions for CO2 scrubbing systems.
Notes and references
1
A. Nakada, K. Koike, K. Maeda, O. Ishitani, Green Chem.,
2016, 18, 139.
2
3
J. M. Smieja, C. P. Kubiak, Inorg. Chem., 2010, 49, 9283.
J. Y. Becker, B. Vainas, R. Eger, L. Kaufman, J. Chem. Soc.,
Chem. Commun., 1985, 1471.
4
S. Kaneco, K. Iiba, K. Ohta, T. Mizuno, A. Saji, Electrochim.
Acta, 1998, 44, 573.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins