Electrochemical reduction of carbon dioxide to hydrocarbons with high faradaic efficiency in LiOH/methanol

Article abstract of DOI:10.1021/jp990021j

The electrochemical reduction of CO₂ with a Cu electrode in LiOH/methanol-based electrolyte was investigated. A divided H-type cell was employed, the supporting electrolytes were 80 mmol dm^-3 lithium hydroxide in methanol (catholyte) and 300 mmol dm^-3 potassium hydroxide in methanol (anolyte). The main products from CO2 were methane, ethylene, carbon monoxide, and formic acid. The maximum current efficiency for hydrocarbons (methane and ethylene) was of 78%, at -4.0 V vs Ag/AgCl, saturated KCl. The ratio of current efficiency for methane/ethylene, r_f(CH₄)/r_f(C₂H₄), was in the range from 2.2 to 4.3. In LiOH/methanol, the efficiency of hydrogen formation, a competing reaction of CO₂ reduction, was depressed to below 2% at relatively negative potentials. On the basis of this work, the high efficiency electrochemical CO₂ to hydrocarbon conversion method appears to be achieved. Future work to advance this technology may include the use of solar energy as the electric energy source. This research can contribute to the large-scale manufacturing of fuel gases from readily available and inexpensive raw materials, CO₂-saturated methanol from industrial absorbers (the Rectisol process).

Full text of DOI:10.1021/jp990021j

7456
J. Phys. Chem. B 1999, 103, 7456-7460
Electrochemical Reduction of Carbon Dioxide to Hydrocarbons with High Faradaic
Efficiency in LiOH/Methanol
Satoshi Kaneco,* Kenji Iiba, Syo-ko Suzuki, Kiyohisa Ohta, and Takayuki Mizuno
Department of Chemistry for Materials, Faculty of Engineering, Mie UniVersity, Tsu, Mie 514-8507, Japan
ReceiVed: January 4, 1999; In Final Form: June 25, 1999
The electrochemical reduction of CO2 with a Cu electrode in LiOH/methanol-based electrolyte was investigated.
A divided H-type cell was employed, the supporting electrolytes were 80 mmol dm-3 lithium hydroxide in
methanol (catholyte) and 300 mmol dm-3 potassium hydroxide in methanol (anolyte). The main products
from CO2 were methane, ethylene, carbon monoxide, and formic acid. The maximum current efficiency for
hydrocarbons (methane and ethylene) was of 78%, at -4.0 V vs Ag/AgCl, saturated KCl. The ratio of current
efficiency for methane/ethylene, rf(CH4)/rf(C2H4), was in the range from 2.2 to 4.3. In LiOH/methanol, the
efficiency of hydrogen formation, a competing reaction of CO2 reduction, was depressed to below 2% at
relatively negative potentials. On the basis of this work, the high efficiency electrochemical CO2 to hydrocarbon
conversion method appears to be achieved. Future work to advance this technology may include the use of
solar energy as the electric energy source. This research can contribute to the large-scale manufacturing of
fuel gases from readily available and inexpensive raw materials, CO2-saturated methanol from industrial
absorbers (the Rectisol process).
Introduction
However, even at a copper electrode, no hydrocarbons were
obtained in these organic solvents.13
The chemical fixation of carbon dioxide (CO2), by radio-
chemical, chemical, thermochemical, photochemical, electro-
chemical, and biochemical procedures, has been of significant
interest form both fundamental and practical viewpoints.1,2 The
electrochemical method appears to be a very suitable method
for the conversion and reduction of CO2.2,3
Methanol is a better solvent of CO2 than water, particularly
at low temperature. The solubility of CO2 in methanol is
approximately 5 times that in water, at ambient temperature,
and 8-15 times that in water, at temperatures below 273 K.14-16
Therefore, methanol has been industrially used as a physical
absorbent of CO2 in the Rectisol method, at 243-263 K.16
Currently, over 70 large-scale plants apply the Rectisol process.
In addition, compared to water, methanol is a poor solvent of
acidic gases such as SOx and NOx. Due to these two properties
of methanol, the direct electrochemical reduction of CO2 in
methanol-based electrolyte is an advantageous choice. Thus, we
have investigated the electrochemical reduction of CO2 on
copper electrodes by using methanol as the solvent, at 243
K.17-21 In methanol-based catholyte, the formation of methane
and ethylene was observed. The maximum current efficiency
for hydrocarbons was of 43%, with benzalkonium chloride salt.17
However, the achieved efficiency was inadequate for applying
the process on the industrial scale.
In the electrochemical reduction of CO2, in water, at most
metal electrodes the major reaction products were carbon
monoxide and formic acid.4,5 However, only copper has proven
a suitable electrode for the formation of hydrocarbons such as
methane and ethylene, which can be used as fuel gases.5-10
Azuma et al.5 investigated the electrochemical reduction of CO2
at a Cu electrode in 50 mmol dm-3 KHCO3 aqueous solution,
at 293 K, and obtained methane, ethylene, and ethane with
Faradaic efficiencies of 17.8%, 12.7%, and 0.039%, respectively.
Moreover, at 273 K, the current efficiency was 24.7% for
methane, 6.5% for ethylene, and 0.015% for ethane. Murata et
al.6 reported 32.2% Faradaic efficiency for methane and 5.2%
for ethylene in the electrochemical reduction of CO2 on Cu
electrode, in 100 mmol dm-3 aqueous LiHCO3 solution.
Kyriacou et al.7 described the formation efficiencies in the
electrochemical reduction of CO2 on Cu, in 500 mmol dm-3
LiHCO3 solution, at 298 K, as follows: 26% for methane; 4%
for ethylene; etc.
This study deals mainly with the electrochemical reduction
of CO2 to hydrocarbons, at copper electrodes, in LiOH/
methanol-based electrolyte, a process that can be performed with
high Faradaic efficiency.
Recently, many investigators have actively studied the
electrochemical reduction of CO2 using various metal electrodes
in organic solvents, given that organic aprotic solvents dissolve
much more CO2 than water.11-13 Reduced products containing
carbon monoxide, oxalic acid, and formic acid were produced
by the electroreduction of CO2 in dimethyl sulfoxide, N,N-
dimethylformamide, propylene carbonate, and acetonitrile.
Experimental Section
The apparatus and experimental conditions for the electro-
chemical reduction of CO2 are shown in Table 1. The
electrochemical reduction of CO2 was performed in a home-
made, divided H-type cell. An Aldrich Nafion 117-type ion
exchange membrane (0.18 mm thickness) was used as the
diaphragm. The cathode potential was measured with respect
to an Ag/AgCl, sat. KCl electrode that was connected with the
catholyte through an agar salt bridge.
* Corresponding author. Phone: +81-59-231-9427. Fax: +81-59-231-
9442, 9471, or 9427. E-mail: kaneco@chem.mie-u.ac.jp.
10.1021/jp990021j CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/13/1999

Reduction of Carbon Dioxide to Hydrocarbons
J. Phys. Chem. B, Vol. 103, No. 35, 1999 7457
TABLE 1: Apparatus and Experimental Conditions
Electrochemical Reduction
cell
H-type cell
potentiostat/galvanostat Hokuto HA-105
coulometer
Integrator 1109
(Fusou Seisakujyo, Inc., Japan)
Hokuto HB-111 function generator
Graphtec WX1100
potential sweep
XY recorder
thermostat
NES Lab., Instruments, Inc., RTE-110
Cu foil (30 mm × 20 mm,
working electrode
0.1 mm thickness, 99.98% purity)
Pt foil (30 mm × 20 mm,
counter electrode
0.1 mm thickness, 99.98% purity)
Ag/AgCl sat. KCl (Horiba, 2060A-10T)
reference electrode
electrolyte
catholyte
anolyte
carbon dioxide
potential
80 mmol dm-3 LiOH in methanol
300 mmol dm-3 KOH in methanol
99.9999% purity
-2.0 to -5.0 V vs Ag/AgCl sat. KCl
243 ( 0.5 K
temperature
Figure 1. Cyclic voltammograms on Cu electrode in methanol at 243
K. Solid line, CO2 atmosphere; broken line, N2 atmosphere. Catholyte,
80 mmol dm-3 LiOH/methanol; anolyte, 300 mmol dm-3 KOH/
methanol.
Product Analysis
Gas-chromatography
gas products
TCD (GL Science GC-320, Molecular
Sieve 5A; 13X-S, Ar and He carrier gas)
FID (Shimadzu GC-14B, Porapak Q, N2
and H2 carrier gas)
HPLC with UV detector (Hitachi L4000)
TCD and FID gas-chromatography
since one cannot observe any precipitate. Therefore, CO2 can
be considered to be physically dissolved, i.e., under intact form
in the methanol catholyte. The increased solubility of carbon
dioxide in our system relative to water appears to be very
advantageous.
The electrolysis was performed at 243 K because temperature
in the Rectisol process is customarily in the range of 243 to
263 K.16
liquid products
Methanol (99%, Nacalai Tesque, Inc., Japan) was purified
by double distillation from metallic magnesium. Water content
in the pure methanol was less than 1 ppm (confirmed by the
Karl Fischer test). Lithium hydroxide (95%, Nacalai Tesque,
Inc.) was used as the ionophore in the methanol-based catholyte.
The pH of catholyte was measured with a glass electrode for
nonaqueous solvent (Horiba, 6377-10D), calibrated in water.
Other details for the pH measurement were described else-
where.22,23 The pH scale for the methanol was in the range of
1.8 to 17.2.24 The pH of 80 mmol dm-3 LiOH/methanol was
∼14 and after the saturation of the electrolyte with CO2 the pH
was 6.2. Mechanical processing of the Cu and Pt electrodes
required polishing each surface with successively finer grades
of alumina powder (Baikalox emulsion, manufactured by
Baikowski International Co.) down to 0.05 µm, followed by
the removal of grease with acetone. Both electrodes were
activated electrochemically at 500 mA, for 100 s, in 14.7 mol
dm-3 phosphoric acid. Following the above treatment, the
electrodes were rinsed with both water and ethanol.
A discontinuous electroreduction procedure was used. First,
CO2 gas was bubbled into the methanol catholyte for 1 h at a
rate of 30 mL min-1. Then, the CO2-saturated solution was
reduced electrolytically at cathodic polarizations in the range
from -2.0 to -5.0 V vs Ag/AgCl, sat. KCl. The catholyte was
stirred magnetically. The Faradaic efficiency of the main
products was calculated assuming that a total of 50 coulombs
of charge passed through the cell. Gaseous products obtained
during electroreduction were collected in a gas collector and
were analyzed by GC. Products soluble in the catholyte were
analyzed by using HPLC and GC.
Cyclic Voltammetry. First, cyclic voltammograms (CV) at
the Cu electrode in LiOH/methanol-based catholyte were
recorded at 243 K. The potential was scanned at a sweep rate
of 50 mV s-1. Figure 1 shows the current-potential curves with
the Cu electrode in CO2-saturated methanol and in N2-purged
methanol. CO2 reduction is evident on the voltammogram
recorded in CO2-saturated methanol. No voltammetric peak was
observed in the potential range down to -5.2 V because further
CO2 reduction may proceed with increasingly negative poten-
tials. Under nitrogen atmosphere, hydrogen formation efficiency
was approximately 98% in the electrolysis, with no CO2 being
reduced. Hence, we conclude that the cathodic currents can be
attributed solely to the reduction of residual water, present
slightly in the methanol catholyte. The onset (starting) potential
of the cathodic current, i.e., those potential value at which a
current density of 0.1 mA cm-2 is observed, in CO2-saturated
methanol was around -0.5 V. From this potential value,
scanning in the negative direction, the current density gradually
increased until reaching the potential of -2.0 V in which the
current abruptly increased. At cathodic polarizations below -2.0
V, the current density recorded in CO2-saturated electrolyte
appears to be similar to that in N2-purged electrolyte. The
current-potential characteristics can be attributed to a number
of processes such as kinetics, adsorption, desorption, and
diffusion. Ortiz et al.25 have reported a cyclic voltammogram
with Cu electrode in CO2-saturated NaClO4/methanol electro-
lyte, at room temperature. In the voltammetric measurements,
the onset potential was about -0.6 V vs Ag/Ag+ and the
polarization curve slowly dropped scanning at values more
negative than -0.6 up to -1.3 V. More negative than this latter
value, the polarization curve significantly diminished. Thus, the
shape of our CVs in LiOH/MeOH was nearly the same as that
reported for NaClO4/MeOH. In LiOH/methanol-based electro-
lyte, the onset potential recorded in N2-purged methanol was
shifted to more negative potential values relative to the onset
Results and Discussion
The solubility of CO2 in a solution of 80 mmol dm-3 LiOH/
methanol, at 243 K, was 18 cm3 cm-3 (∼800 µmol CO2/cm3
methanol). Literature data14,15 for the solubilities of CO2 in pure
methanol and water, at 288 K, were of 4.6 and 0.821 cm3 cm-3
,
respectively. Although LiHCO3 may be formed in the methanol
while bubbling CO2 through the solution for several minutes,
the amount of hydrogen carbonate is assumed to be negligible,

7458 J. Phys. Chem. B, Vol. 103, No. 35, 1999
Kaneco et al.
Figure 2. Effect of potential on Faradaic efficiencies for the products
by electrochemical reduction of CO2 at Cu electrode in methanol at
243 K: CH4 (.); C2H4 (3), CO (O), HCOOH (0), H2 (b). Catholyte,
80 mmol dm-3 LiOH/methanol; anolyte, 300 mmol dm-3 KOH/
methanol.
Figure 3. Tafel plots of the products by electrochemical reduction of
CO2 at Cu electrode in methanol at 243 K. CH4 formation (.); C2H4
formation (3); CO formation (O); HCOOH formation (0); H2 evolution
(b). Catholyte, 80 mmol dm-3 LiOH/methanol; anolyte, 300 mmol
dm-3 KOH/methanol.
solution, respectively, and 52.0% in 50 mmol dm-3 KHCO3
solution. It has been reported that low temperature was effective
for the depression of hydrogen formation in the methanol
electrolyte.17-21 These effects may be due to the poor electro-
activity for hydrogen evolution reaction at low temperature
because of the stabilization of adsorbed hydrogen on the
electrode. Therefore, we were able to confirm that methanol-
based catholyte, at low temperature, was suitable to suppress
hydrogen formation.
Tafel Plots. The effect of the potential on partial current
densities for CO2 reduction and hydrogen evolution at Cu
electrode in LiOH/methanol-based catholyte was evaluated. The
Tafel plots are shown in Figure 3. Methane and ethylene
formations and hydrogen evolution became diffusion-controlled
at around -3.0 V. A maximum partial current density for
hydrocarbons was about 10 mA cm-2 at -5.0 V. At ap-
proximately -3.5 V, CO and formic acid formations became
diffusion-controlled. Again, the reaction rate on the electrode
is governed by various processes such as kinetics, adsorption,
desorption, and diffusion. By considering this Tafel plot study,
a limitation of CO2 diffusion to the electrode may be a dominant
process for the rate-determining step. The partial current density
ratio of CO2 reduction and hydrogen evolution, i(CO2)/i(H2),
was >47 at potentials more nagative than -3.0 V.
Mechanism of the Electrochemical Reduction of CO2. The
mechanism of the electrochemical reduction of CO2 in LiOH/
methanol-based electrolyte was investigated for a copper
electrode. A GC-MS study with deuterated methanol catho-
lyte demonstrated that no reduction product was produced
from methanol.26 When the electrolysis was conducted under
nitrogen atmosphere, electrolysis yielded exclusively hydro-
gen. Consequently, the targeted products were produced by
the electrochemical reduction of CO2. These experimental
data and literature reports7,17-21,26 suggest the pathway by
which methane, ethylene, carbon monoxide, and formic acid
are formed.
potential recorded in CO2-saturated methanol. The pH of
catholyte after saturation with CO2 was 6.2 and the pH of N2-
purged methanol was ∼14. Both the reduction and onset
potentials may be affected by the pH of catholyte. These effects
are still being examined.
Once the onset potentials were determined from polarization
experiments, we attempted to investigate the effect of potential
on the Faradaic efficiency of the products.
Effect of Potential of the Product Faradaic Efficiency. The
results dealing with the effect of the potential on the current
efficiencies for the products by the electrochemical reduction
of CO2 on Cu in LiOH/methanol at 243 K are illustrated in
Figure 2. Methane, ethylene, CO, and formic acid were detected
as reduction products from CO2.
The current efficiency of methane increased from 20% to 60%
from -2.0 to -3.0 V. At potentials more nagative -3.0 V, the
high efficiency was maintained. The current efficiency of
ethylene gradually increased to 18%, as the potential was
lowered. The current efficiency of methane was larger than those
of ethylene in the entire investigated potential range. A
maximum Faradaic efficiency for hydrocarbons (methane and
ethlene, 78%) was observed at -4.0 V. The formation efficiency
curves of CO and formic acid gradually increased with the
positive direction at less than -3.0 V. At the polarizations above
-3.0 V, both curves rose abruptly. In LiOH/methanol electro-
lyte, ethane was not produced but the efficiency of methane
was much better than those obtained in water (24.7%, 32.2%,
and 26%)5-7 and the methanol containing benzalkonium chloride
(43%).17
Generally, in the electrochemical reduction of CO2 in water,
hydrogen formation is simultaneous to CO2 reduction. Therefore,
the suppression of hydrogen formation is very important because
the applied energy is wasted on hydrogen evolution instead of
being used for the reduction of CO2. In LiOH/methanol-based
electrolyte, the Faradaic efficiency for hydrogen formation on
the Cu electrode, at 243 K, was suppressed to less than 2%, at
polarizations below -3.0 V. In the electrochemical reduction
of CO2 on Cu in water,5-7 the hydrogen formation efficiencies
were 60.5% and 68% in 100 and 500 mmol dm-3 LiHCO3
Hydrocarbons form by a series of simultaneous or consecutive
electronation/protonation steps, according to the following
simplified scheme:

Reduction of Carbon Dioxide to Hydrocarbons
J. Phys. Chem. B, Vol. 103, No. 35, 1999 7459
•
-
The adsorbed CO2 radical anion formed in the first elec-
tronation step undergoes a second electronation/protonation,
to yield adsorbed CO as the key intermediate. By a succession
of four electronation/protonation steps, an adsorbed reactive
methylene group forms, and this may either stabilize as a
methane molecule by a subsequent double electronation/
protonation sequence or dimerize to form ethylene.
the major route to loss of electrode activity is via the formation
of formic acid which is then reduced to graphitic carbon.9 In
the present study, the Faradaic efficiency for formic acid was
relatively low (less than 13%) and the black deposit on the
surface of cathode was not formed during the electrolysis.
Therefore, the deactivating process may not become appreciable
in the electrochemical reduction of CO2 on Cu in LiOH/
methanol-based electrolyte until 50 coulombs.
For the formation of formic acid and CO, we assume the
usual pathway, which involves a one-electron reduction followed
by a second electronation/protonation to yield formic acid, and
by the disproportionation of •CO2- radical anions to neutral CO
molecules and dinegative carbonate ions.
Conclusion
The electrochemical reduction of CO2 with Cu electrode in
LiOH/methanol-based electrolyte at low temperature (243 K)
was studied. The best current efficiency for methane was 63%
at -4.0 V vs Ag/AgCl. A predominant formation of methane
was observed in the potential range of -2.5 to -5.0 V vs Ag/
AgCl. The formation efficiency for hydrogen was suppressed
to less than 2% at relatively negative potentials. Since methanol
is widely used in industry as a CO2 absorbent at 243 to 263 K
in the Rectisol process,16 this research may contribute to
applications in the conversion of CO2-saturated methanol into
fuel products. Thus, the synthesis of hydrocarbons by the
electrochemical reduction of CO2 might be of practical interest
in fuel production, storage of solar energy, and the production
of intermediate materials for the petrochemical industry.
In the electrochemical reduction of CO2, at a copper electrode
in LiHCO3 aqueous solution,6,7 the ratios of the current
efficiency of methane and ethylene, rf(CH4)/rf(C2H4), were 6.2
and 6.5. In LiOH/methanol-based electrolyte, the ratio was in
the range 2.2-4.3. On the other hand, the ratio was relatively
low (0.13 to 0.34) in CsOH/methanol catholyte.21 Several
researchers6,7 have presented that the methane/ethylene Faradaic
efficiency ratio increased as the radius of the cation increased,
i.e., Li+ < Na+ < K+ < Cs+, in the electrochemical reduction
of CO2 on Cu in water. Hence, a similar tendency may be
obtained in the electrochemical reduction of CO2 at Cu electrode
in methanol. Small cations such as Li+ and Na+ were not
adsorbed at the electrode surface, due to their strong hydration.
In addition, small cations carry to the cathode a large number
of water molecules and thus supply protons for the electrore-
duction. The conversion of intermediate CudCH2 to methane
requires the presence of adsorbed hydrogen. Thus, this reaction
may be favored at larger surface hydrogen coverage, as is the
case of Li+.
Acknowledgment. This work was partially supported by the
Ministry of Education, Science and Culture of Japan, as well
as the Chubu Electric Power Foundation.
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