High-temperature electrochemical heat pump using a water gas shift reaction. Electrochemical reduction of CO2 to CO

Article abstract of DOI:10.1246/bcsj.74.1517

The electrochemical reduction of CO₂ to CO, which is considered to be the key reaction in a new electrochemical heat pump using the water gas shift reaction, was theoretically and experimentally investigated. Thermochemical predictions show that CO₂ can be reduced to CO below -0.8 V at 973 K. It was found that the overpotential of the electrochemical reduction of CO₂ to CO on a gold electrode was about 0.3 V at 0.01 mA cm^-2. Below - 1.4 V, in the range where carbon deposition took place, CO could be produced by a chemical reaction between CO₂ and carbon. X-ray diffraction showed that the deposited carbon was not amorphous, but graphitized carbon.

Full text of DOI:10.1246/bcsj.74.1517

c. Jpn.
© 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 1517–1523 (2001) 1517
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High-Temperature Electrochemical Heat Pump Using a Water Gas Shift
Reaction. Electrochemical Reduction of CO2 to CO
Electrochemical Reduction of CO2 to CO
A. Ishihara et al.
Akimitsu Ishihara, Tomotaka Fujimori, Naobumi Motohira, Ken-ichiro Ota,* and Nobuyuki Kamiya
01
17
23
SJA8
09-2673
027
Department of Energy & Safety Engineering, Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai,
Hodogaya-ku, Yokohama-shi, Kanagawa 240-8501
(Received January 29, 2001)
The electrochemical reduction of CO2 to CO, which is considered to be the key reaction in a new electrochemical
heat pump using the water gas shift reaction, was theoretically and experimentally investigated. Thermochemical predic-
tions show that CO2 can be reduced to CO below −0.8 V at 973 K. It was found that the overpotential of the electro-
chemical reduction of CO2 to CO on a gold electrode was about 0.3 V at 0.01 mA cm−2. Below −1.4 V, in the range
where carbon deposition took place, CO could be produced by a chemical reaction between CO2 and carbon. X-ray dif-
fraction showed that the deposited carbon was not amorphous, but graphitized carbon.
01
01
.2
1
A new concept for an electrochemical heat pump was pro-
posed in order to upgrade the quality of thermal energy.1,2 The
new electrochemical heat pump uses a combination of an elec-
trolytic reaction at a lower temperature, absorbing low-grade
thermal energy, and a thermochemical reaction at a higher tem-
perature, producing high-grade thermal energy (Fig. 1).
The following water gas shift reaction is a possible candi-
date applicable to this new electrochemical heat pump:
CO(g) + H2O(g) ꢀ CO2(g) + H2(g).
(1)
The water gas shift reaction (1) can thermochemically proceed
to the right with heat release while using an appropriate cata-
lyst.3 When molten carbonate is used as the electrolyte, the re-
verse reaction of the water gas shift reaction occurs, involving
the following two half-cell reactions:
Fig. 1. Principle of a new electrochemical heat pump system.
H2(g) + CO32− (l) → H2O(g) + CO2(g) + 2e,
2CO2(g) + 2e → CO(g) + CO32− (l).
(2)
(3)
structure of the carbon. Borucka et al.7,8 have investigated the
behavior of the CO2/CO/Au gas electrode in the Li–Na–K ter-
nary eutectic; they found that the equilibrium potential of this
electrode accurately follows the Nernst equation over the en-
tire range of gas compositions at temperatures of 700–800 °C.
Recently, among the workers engaged in MCFC improvement,
some researchers have shown interest in the reduction of CO2
in molten carbonate. Malinowska et al.9 observed the reduc-
tion wave of CO2 to CO in the Li/Na/K eutectic under 100%
CO2 at 650 °C during their study concerning the behavior of
nickel species. Peelen et al.10 investigated CO2 reduction in or-
der to elucidate the role of CO2 in the oxygen reduction mech-
anism of MCFC cathodes. They made an electrochemical
study about CO2 reduction in the 62/38 mol% Li/K eutectic
under 100% CO2 with the temperature range between 575–650
°C; they obtained the product between the square root of the
diffusion constant and the solubility rate of CO2, and conclud-
ed that CO2 reduction to CO was reversible in that melt. How-
ever, in their paper, the chemical species were specified only
In the electrolytic process, the electrical energy input enables
reactions (2) and (3) to proceed to the right while absorbing
heat. It has been reported that in molten carbonate the oxida-
tion of H2 to H2O (reaction (2)) proceeds very easily as the an-
ode reaction of a molten carbonate fuel cell (MCFC).4,5 There-
fore, the reduction of CO2 to CO (reaction (3)) is considered to
be the key reaction in an electrochemical heat pump using the
water gas shift reaction.
Regarding CO2 reduction in molten carbonate, several stud-
ies have been made so far. For example, Ingram et al.6 experi-
mentally investigated the formation of carbon from molten car-
bonate. An analysis involving X-ray powder diffraction
showed that the electrodeposited carbon was amorphous. The
electrochemical feature of the electrodeposited carbon was dif-
ferent from that of the commercial, graphitized carbon. Such
differences are considered to be attributable to the porous

1518 Bull. Chem. Soc. Jpn., 74, No. 8 (2001)
Electrochemical Reduction of CO2 to CO
by the value of the standard electrode potential, e.g., CO was
not detected by analytical instruments. Furthermore, there was
no consideration of carbon deposition in their study. A com-
plete understanding of CO2 reduction involving carbon deposi-
tion has not yet been achieved. The purpose of this paper is to
investigate the reactivity of CO2 reduction to CO and to evalu-
ate the effect of carbon deposition.
Thermochemical Considerations
A theoretical study was performed in order to predict the
stability region of carbon species in various electrode poten-
tials.
The following electrode reactions involving carbon species
in molten carbonate have been considered:
2CO2(g) + 2e ꢀ CO(g) + CO32− (l),
3CO2(g) + 4e ꢀ C(s) + 2 CO32− (l),
3CO(g) + 2e ꢀ 2C(s) + CO32− (l).
(3)
(4)
(5)
The Nernst equations of these reactions are as follows, respec-
tively:
E(CO2/CO) = E0(CO2/CO)-(RT/2F) ln (P(CO2)2/P(CO)), (6)
E(CO2/C) = E0(CO2/C)-(RT/4F) ln P(CO2)3,
E(CO/C) = E0(CO/C)-(RT/2F) ln P(CO)3,
(7)
(8)
Fig. 2. Dependence of CO and CO2 pressures on the elec-
trode potential at 973 K.
2−
(a) —ꢀ— 2CO2 + 2e = CO + CO3
,
2−
– -✸-– 3CO + 2e = 2C + CO3
.
2−
where the activity of a carbonate ion is assumed to be unity. E
and P represent the electrode potential and the gas pressure, re-
spectively. E0 is the standard electrode potential.
The following standard oxygen electrode is used as a refer-
ence electrode:
(b) —ꢀ— 2CO2 + 2e = CO + CO3
,
2−
– -✸-– 3CO2 + 4e = C + 2CO3
.
The degree of freedom of the system characterized by Eq. 10 is
two. If the temperature and total pressure are fixed, the CO
and CO2 pressures are specified. Below a potential of −1.048
V, CO is unstable, because the CO pressure governed by the
equilibrium between carbon and CO is always lower than the
CO pressure governed by the equilibrium between CO2 and
CO. Therefore, it can be said that carbon and CO2 are stable
below a potential of −1.048 V. The equilibrium CO2 pressure
is given by Eq. 7, as shown in Fig. 2b. As described above, the
maximum value of the CO pressure theoretically reaches about
0.6 atm when the total pressure is 1 atm. The maximum
P(CO) value depends on the temperature. The dependence of
the maximum P(CO) and P(CO2) on the temperature is depict-
ed schematically in Fig. 3. Fig. 3 shows that at a temperature
higher than 800 K, P(CO) increases rapidly in accordance with
the temperature increase, whereas at a temperature below 750
K, only a micro amount of CO is produced by CO2 reduction.
Therefore, it leads to the conclusion that higher temperatures
are more favorable for CO2 reduction to CO. Because carbon
deposition takes place only above 1200 K, almost all of the
CO2 can be reduced to CO under this condition. However, for
the new electrochemical heat pump, the electrolytic reaction
should proceed as an endothermic reaction at a lower tempera-
ture. Therefore, it is not practical to assume the reaction tem-
perature to be above 1000 K. A new high-temperature electro-
CO2(g; 2/3 atm) + 1/2 O2 (g; 1/3 atm) + 2e ꢀ CO32− (l). (9)
The relationship between the equilibrium electrode potential
and P(CO) as well as the relationship between the equilibrium
electrode potential and P(CO2) can be obtained when the total
pressure, P(CO) + P(CO2), is 1 atm. The Gibbs energies of
the carbon species are quoted from thermochemical data.11
The dependence of P(CO) and P(CO2) on the electrode po-
tential at 973 K is shown in Figs. 2a and b, respectively. Fig.
2a indicates that in the range of −0.9 V–−1.048 V for reaction
(3), CO2 reduction to CO proceeds gradually to the right in ac-
cordance with the decrease in the electrode potential. In this
potential range, no carbon deposition takes place, as described
in Eq. 5, because the CO pressure produced through CO2 re-
duction to CO (Eq. 3) is insufficient to enable the carbon depo-
sition process shown in Eq. 5.
At −1.048 V, the electrode reactions expressed by Eqs. 3
and 5 achieve a simultaneously electrochemical equilibrium
state in which the carbon, CO and CO2 can coexist. Therefore,
the Boudouard reaction (10) is in chemical equilibrium,
C(s) + CO2(g) ꢀ 2CO(g).
(10)

A. Ishihara et al.
Bull. Chem. Soc. Jpn., 74, No. 8 (2001) 1519
Fig. 3. Dependence of maximum CO and CO2 pressures on
temperature.
Fig. 5. Experimental cell.
Experimental
Lithium and potassium carbonates were of reagent grade from
Junsei Chemicals, which were stored and mixed (in a proportion
of 62 + 38 mol%) in a nitrogen-filled dry box. The amounts of
the carbonates needed to produce a 150 g mixture were weighed.
The salt mixture was dried at 350 °C for 12 h in a vacuum and
melted at 700 °C under a CO2 atmosphere. High-purity grade CO2
was bubbled through the molten phase at a constant flow rate of 6
dm3 min−1 under atmospheric pressure. When the potential-
sweep method was performed, the bubbling was stopped. In this
study, the potential sweep and the constant-potential method were
carried out using a Nikko Keisoku potentiostat (NPGS-305). In
the potential-sweep method, a Toho Technical Research function
generator (FG-02) was used as the wave generator. The cell was a
compact single-compartment cylindrical crucible (300 mm × 45
mmI.D. × 55 mmO.D.) hermetically sealed by a silicon stopper,
as shown in Fig. 5. An alumina tube (5 mmI.D. × 7 mmO.D.) was
used as the gas inlet. The working electrode used in the potential-
sweep method was a 99.98% pure gold flag of 7 × 7 × 0.2 mm (1
cm2 area) spot-welded to a gold wire (φ = 0.3 mm). For the con-
stant-potential method, large coils made of 99.98% pure gold wire
(φ= 1 mm) with a 20 cm length were used. Before use in the mol-
ten carbonate, the electrode surface was etched with hot concen-
trated hydrochloric acid, and cleaned with acetone. The reference
electrode was a coiled gold wire (φ = 1 mm, length = 20 cm)
placed in the same atmosphere. The potential of the reference
electrode was corrected by the standard oxygen electrode (SOE)
in the melt. The potential of our reference electrode was 119 mV
vs SOE at 973 K. This potential was in good agreement with the
O2 concentration (6.8 × 10−4 atm) in the CO2 gas detected by gas
chromatography. These results show that our reference electrode
could act as a reversible oxygen electrode (ROE) in the melt. In
this study, the electrode potential was expressed in vs SOE. The
product gases in the constant potential method experiment were
analyzed using a Shimadzu gas chromatograph (GC-8A) with the
C-R5A software package. 5A molecular sieves were used as the
column packing to separate O2, N2 and CO. The electrode surfac-
Fig. 4. Relationship between the electrode potential and gas
pressures at 973 K.
2CO2(g) + 2e = CO(g) + CO32− (l)
3CO2(g) + 4e = C(s) + 2CO32− (l)
chemical heat pump can be applied to the following processes,
which exhaust industrial high-temperature waste heat: 1. Steel
industry ((i) Coke oven 900–1050 K, (ii) Heating oven 850–
1050 K); 2. Ammonia oxidation 1000–1100 K; 3. Ethylene
heating 1050–1400 K and so on. According to this consider-
ation, 973 K was selected as the reaction temperature, where
the CO pressure was expected to be about 0.6 atm. The rela-
tionship between the electrode potential and the gas pressures
at 973 K is shown in Fig. 4. This figure indicates that CO2 can
be reduced to CO below about −0.8 V, and that the P(CO) in-
creases up to 0.6 atm in accordance with decreasing potential.
It can also be seen that the disproportionation reaction of CO
to CO2 and carbon occurs below −1.048 V. Needless to say, if
the carbon deposition is restrained, i.e. the overpotential of the
carbon deposition is large, the CO pressure can increase even
at a potential below −1.048 V.

1520 Bull. Chem. Soc. Jpn., 74, No. 8 (2001)
Electrochemical Reduction of CO2 to CO
Fig. 7. Cyclic voltammograms at lower limit of scanning po-
Fig. 6. Cyclic voltammograms at a gold flag electrode at
lower limit of scanning potential of −1.0, −1.1, −1.2 and
−1.3 V.
tential of −1.4 and −1.5 V.
P(CO2) = 1 atm; start potential −0.119 V; v = 0.4 V s−1
.
P(CO2) = 1 atm; start potential −0.119 V; v = 0.4 V s−1
.
es after electrolysis were analyzed by XRD (Shimadzu XRD-
6000) with a Cu target. Temperature control was achieved with
an Ohkura RTC-1030 and calibrated with a chromel-alumel ther-
mocouple.
Results and Discussion
Potential-Sweep Method. The cyclic voltammograms at
several lower limits of scanning potential are shown in Fig. 6.
As expressed by the figure, the reduction current started to
flow below −0.8 V, the reduction wave (ꢁc) appeared around −
1.1 V and the oxidation peak (ꢁa) was observed around −0.9 V.
According to thermochemical predictions, the reduction wave
(ꢁc) was attributed to the reduction of CO2 to CO. Therefore,
CO2 reduction to CO seemed to proceed below −0.8 V. How-
ever, in this study, we could find no apparent peak of CO2 re-
duction to CO around this potential, such as that observed by
Peelen et al.10 Furthermore, a linear relationship between the
reduction current at −1.1 V and the square root of the scanning
rate could also not be obtained. It is considered that such dif-
ferences were attributed to the reaction temperature. Because
the reaction temperature of this study, i.e. 700 °C, was higher
than that of Peelen’s, i.e. 575–600°C, the following possible
causes were suggested: 1. Other reductions such as carbon
deposition proceeded simultaneously; 2. carbonate ion as the
electrolyte participated in the reduction. These assumptions
should be verified by other experiments. The oxidation peak
(ꢁa) was presumed to have a certain relation with the oxidation
of CO to CO2, because the peak (ꢁa) appeared only when the
scanning potential was sufficiently negative below −1.1 V, the
point at which CO2 reduction to CO took place.
Fig. 8. Positive potential sweep voltammogram after the po-
tential was maintained at a constant for 30 s.
CO corresponded to peak (ꢁa) and there was no chemical spe-
cies which could be oxidized, except carbon. Therefore, it can
be assumed that CO2 and/or carbonate ion was reduced to car-
bon below −1.4 V.
In order to determine the exact potential at which carbon
was deposited, a positive potential sweep was applied after
maintaining a constant potential for 30 s. If carbon was depos-
ited on the electrode surface, the anodic current of carbon oxi-
dation could be observed. Figure 8 shows the results. When
the maintained potential was higher than −1.3 V, anodic peak
(ꢁa) appeared, which corresponded to CO oxidation around −
0.9 V. On the other hand, when the maintained potential was
lower than −1.4 V, the second anodic peak (ꢂa) was observed.
This meant that carbon started to be deposited at −1.4 V. Un-
der decreasing potential, peak (ꢂa) grew faster than peak (ꢁa).
This implied that the rate of carbon deposition became faster
than that of CO formation when the potential was maintained
at less than −1.5 V. Furthermore, the third anodic peak (ꢃa),
which could not be observed in cyclic voltammogram, ap-
When the scanning potential became more negative, a large
reduction current (ꢂc) started to flow, as shown in Fig. 7. The
limiting diffusion current could not be observed, which indi-
cated that in a lower potential region the electrolyte, carbonate
ion took part in the reduction. When the lower limit of the
scanning potential reached about −1.4 V, the oxidation peak
(ꢂa) appeared. Peak (ꢂa) was thought to correspond to the oxi-
dation current of deposited carbon, because the oxidation of

A. Ishihara et al.
Bull. Chem. Soc. Jpn., 74, No. 8 (2001) 1521
peared around −0.6 V. Peak (ꢃa) was assumed to be caused
by the oxidation of carbon, which had different electrochemi-
cal features from the carbon oxidized at peak (ꢂa). This, how-
ever, should be confirmed by other methods. The X-ray dif-
fraction of deposited carbon is described later.
Constant-Potential Method. Based on thermochemical
predictions and the results of the potential-sweep voltammo-
gram, it was considered that CO2 could be reduced to CO be-
low −0.8 V. Therefore, the constant-potential method was
performed in order to confirm CO formation below −0.8 V
and to investigate the effect of deposited carbon.
Figure 9 shows the time-variations in the current density and
the volume percent of CO measured at −1.3 V (no carbon dep-
osition observed) as well as those at −1.4 V (black carbon was
deposited on the surface of the electrode after electrolysis). In
both cases the current rapidly reached a constant value after
the double-layer capacitance was electrically charged. On the
contrary, the volume percent of formed CO slowly became
steady state because the cell volume was so large that it took a
longer time for the CO to obtain a certain concentration. Fig-
ure 10 shows the whole current actually observed at each po-
tential. It also shows the estimated CO formation current,
whose value was calculated (assuming 2 electrons reaction)
from the CO amount detected by gas chromatography. As
shown in Fig. 10, the formed CO could be detected below −
1.1 V. This was lower by ca. 0.3 V than the expected value of
−0.8 V. Because the CO detection limit of the gas chromato-
graph was calculated to be about 10−4 mA cm−2, it could be
thought that above −1.1 V no CO was formed from the CO2
reduction. Therefore, the overpotential of CO2 reduction to
CO in the constant-potential method was estimated to be ca.
0.3 V at 0.01 mA cm−2. On the other hand, under a potential
lower than −1.4 V, the carbon was observed as black color
deposition on the surface of the electrode after electrolysis was
completed. This result coincided with that obtained by the po-
tential-sweep method. Thermochemically considered, the car-
bon should be deposited at −1.048 V. Therefore, the overpo-
tential of carbon deposition was estimated to be about 0.35 V.
According to thermochemical considerations, in a closed
system CO becomes unstable below −1.048 V due to a dispro-
portionation reaction. Regarding the reason why CO was ob-
served even below −1.048 V in this experiment, we consid-
ered the following: above −1.4 V, the direct process of CO2 re-
duction could form CO because carbon deposition was re-
strained in this potential range. Below −1.4 V, deposited car-
bon could react with CO2 because in an open system the CO2
pressure at the electrode surface covered with deposited carbon
was higher than the equilibrium value. Therefore, the Boud-
ouard reaction (10) could proceed to the left. As a result, even
though carbon was deposited, the rate of CO production could
increase in accordance with the decreasing potential.
In the CO2 feed gas, 6.8 × 10−4 atm of O2 existed as an im-
purity. O2 was more easily reduced than CO2. At a higher po-
tential in the −1.1–−1.2 V range, O2 reduction predominantly
took place and the efficiency of the CO formation current be-
came smaller. Below −1.4 V, a cathodic current was formed
by the reduction of CO2 into CO and carbon. After both the re-
duction current and the volume percent of the formed CO
reached a constant value at −1.5 V, carbon deposition and CO
production determined by the chemical reaction between CO2
and carbon proceeded at the same rate. This also meant that
the reduction current associated with the carbon deposition in-
directly resulted in CO production. This led to the almost
100% current efficiency, measured at −1.5 V. However, in
spite of the observed reduction current and the rate of CO pro-
duction, which seemed to reach a steady state, the current effi-
ciency significantly decreased below −1.6 V. This can be ex-
plained in terms of the deposited carbon; the amount of depos-
ited carbon continuously increased because the rate of carbon
deposition was higher than that of CO production from CO2
and carbon. This resulted in a decrease in the current efficien-
cy. In fact, below −1.6 V, a large amount of carbon deposition
covered, indeed, the electrode surface after electrolysis was
finished. As described above, the rate of chemical CO produc-
tion from CO2 and carbon could be regarded as a factor which
governs the current efficiency as well as the electrochemical
reduction of CO2 to CO.
Fig. 9. Time-variations in the observed current density and
the volume percent of CO formed at −1.3 and −1.4 V.
Observed current density: ꢀ −1.3 V, ✸ −1.4 V.
Volume percent of formed CO: ꢁ −1.3 V, ꢂ −1.4 V.
Fig. 10. Plots of the total current density and CO formation
current density against the potential.
X-ray Diffraction. X-ray diffraction of the electrode sur-

1522 Bull. Chem. Soc. Jpn., 74, No. 8 (2001)
Electrochemical Reduction of CO2 to CO
Fig. 11. X-ray diffraction pattern of the gold electrode surface after the potential was maintained at several values.
ꢃ Au, ꢄ Carbon.
face was performed in order to characterize the deposited car-
bon. Fig. 11 shows the X-ray diffraction pattern of the elec-
trode surface after the potential was maintained at several val-
ues for a long time. As shown in Fig. 11, no peak was ob-
served at −1.3 V, except for gold. At −1.4 V, a new peak ap-
peared at 32 degrees. This peak corresponded to the graphi-
tized carbon formed on the nickel at high pressures and high
temperatures.12 Ingram et al. used X-ray powder diffraction,6
and found out that the electrodeposited carbon was amor-
phous. However, in this study, a larger cathodic current could
be observed even in the potential range where carbon deposi-
tion should take place. This result suggested that the deposited
carbon obtained in this experiment possessed higher electrical
conduction. As we assumed, the deposited carbon in this ex-
periment was not amorphous, but graphitized carbon. It can be
considered that these results were caused by a difference in the
Fig. 12. Dependence of COP on η.
reaction temperature and/or the different composition of the
investigated melt (this study; 700 °C, Li/K 62/38 mol%: In-
TH = 1023 K, TL = 973 K.
gram et al.; 600 °C, Li/Na/K 43.5/31.5/25 mol%). On the oth-
ally required to be more than 3. This means that η must be
lower than 50 mV under these conditions, as shown in Fig. 12.
The maximum η increases in accordance with the decrease of
temperature difference between TH and TL.
er hand, there was no observation of carbon which related to
peak (ꢃa). Therefore, the cause of peak (ꢃa) could not be clar-
ified.
New investigations are in progress in order to obtain further
experimental information on what kind of role the deposited
carbon plays in the process of CO2 reduction to CO.
Applicability of the Water Gas Shift Reaction to the New
Electrochemical Heat Pump. The coefficient of perfor-
mance (COP) of a heat pump is defined as the ratio of the heat
output (QH) at a higher temperature (TH) to the electrical ener-
gy input (We) at a lower temperature (TL). As described in a
previous paper,1 the COP of a new electrochemical heat pump
system was expressed as
The overpotential can be decreased by means of increasing
the roughness factor of the working electrode. When the
roughness factor of the electrode was 100-times larger than
that used in this experiment, the reduction current was also
100-times larger than that of this experiment. The standard
electrode potential of the electrochemical reduction of CO2 to
CO was calculated to be −0.984 V. When a 50 mV overpoten-
tial was added, the electrode potential became ca. −1.03 V. In
the case of extrapolation, the reduction current at −1.03 V was
calculated to be ca. 1 mA cm−2 according to the experimental
results shown in Fig. 10. This reduction current is related to
the power of an electrochemical heat pump.
COP = QH/We = TH/(TH − TL) × (1 − η/(η + Uth)), (11)
Our electrochemical heat pump shows a feature in its steady
state that the reduction rate at a lower temperature equals the
rate of the thermochemical reaction at a higher temperature,
because the reaction substances are continuously circulated be-
tween lower and higher temperature phase. The power of a
where Uth is the theoretical electrolysis voltage and η the elec-
trolysis overvoltage. When TH and TL are fixed at 1023 K and
973 K, respectively, the COP dependence on η is like Fig. 12.
The COP decreased rapidly with increasing η. Considering
the loss of transmission of electricity and so on, COP is gener-

A. Ishihara et al.
Bull. Chem. Soc. Jpn., 74, No. 8 (2001) 1523
heat pump system means the rate of the thermal energy pro-
duction at a higher temperature. Therefore, the product of the
reduction rate at a lower temperature (5.2 × 10−5 mol sec−1
m2) and the enthalpy change of the water gas shift reaction at a
higher temperature (−35 kJ mol−1) represents the power of the
electrochemical heat pump. According to the experimental re-
sults, the maximum power of the electrochemical heat pump is
estimated to be about 2 W m−2 when the roughness factor of
the electrode exceeds the experimented electrode by 100-
times.
If the roughness factor of the working electrode was 100
times larger than that used in this experiment and at the same
time TH and TL were fixed at 1023 and 973 K, respectively, the
maximum power of the electrochemical heat pump could be
estimated to be about 2 W m−2
.
This study was supported by the Original Industrial Tech-
nology R & D Promotion Program from the New Energy & In-
dustrial Technology Development Organization (NEDO).
Regarding the catalysts, we should develop another material
that can work more effectively for CO formation.
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Conclusions
2
The electrochemical reduction of CO2 to CO, which was
considered to be the key reaction in the new electrochemical
heat pump using the water gas shift reaction, was theoretically
and experimentally investigated. Thermochemical predictions
show that CO2 can be reduced to CO below −0.8 V at 973 K
and carbon can be deposited at −1.048 V. Furthermore, CO
becomes unstable below −1.048 V due to a disproportionation
reaction.
3
D. C. Ford, in “Electrochemical and Electrocatalytic Reac-
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T. Nishina, M. Takahashi, and I. Uchida, J. Electrochem.
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The electrochemical reduction was experimentally exam-
ined using a potential-sweep and constant-potential methods.
The overpotential of the electrolytic reduction of CO2 to CO
on a gold electrode in the constant potential method was found
to be about 0.3 V at 0.01 mA cm−2, while the overpotential of
carbon deposition was estimated to be about 0.35 V. Below −
1.4 V in the range where carbon deposition took place, CO
could be produced by the chemical reaction between CO2 and
carbon. It could be thought that the rate of chemical CO pro-
duction from CO2 and carbon governed the electrochemical re-
duction of the CO2 to CO. In addition, X-ray diffraction
showed that the deposited carbon was not amorphous, but
graphitized carbon.
6
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7
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