Electrochemical hydrogen production from carbon monoxide and steam with a cell employing CsH2 PO4 / SiP2 O7 composite electrolyte

Article abstract of DOI:10.1149/1.3232302

Electrochemical hydrogen pumping was investigated with a cell consisting of a CsH₂ PO₄ / SiP₂ O₇ composite electrolyte at 200°C. When humidified hydrogen was fed to the anode, the evolution rate of hydrogen at t

Full text of DOI:10.1149/1.3232302

Journal of The Electrochemical Society, 156 ͑12͒ B1389-B1393 ͑2009͒
B1389
0013-4651/2009/156͑12͒/B1389/5/$25.00 © The Electrochemical Society
Electrochemical Hydrogen Production from Carbon Monoxide
and Steam with a Cell Employing CsH2PO4ÕSiP2O7
Composite Electrolyte
b,
a,
Hiroki Muroyama,a, ,z Toshiaki Matsui,a, Ryuji Kikuchi, and Koichi Eguchi
*
*
*
*
aDepartment of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University,
Kyoto 615-8510, Japan
bDepartment of Chemical System Engineering, Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8656, Japan
Electrochemical hydrogen pumping was investigated with a cell consisting of a CsH2PO4/SiP2O7 composite electrolyte at 200°C.
When humidified hydrogen was fed to the anode, the evolution rate of hydrogen at the cathode obeyed Faraday’s law up to a
current density of 1000 mA cm−2. In this system, the water–gas shift reaction ͑WGSR͒ was promoted electrochemically at the
anode during polarization with a feed of humidified carbon monoxide. The overpotential was much higher in humidified carbon
monoxide than in humidified hydrogen. With the reactor for the catalytic WGSR located upstream of the anode gas, the overpo-
tential significantly depended on the temperature and space velocity of the reactor. The bilayered electrode composed of a Cu
spinel oxide and a Pt catalyst exhibited a low overpotential at low current densities due to the formation of hydrogen over the Cu
spinel catalyst.
© 2009 The Electrochemical Society. ͓DOI: 10.1149/1.3232302͔ All rights reserved.
Manuscript submitted April 21, 2009; revised manuscript received August 26, 2009. Published October 2, 2009.
Hydrogen has attracted much attention as a new energy carrier
for the near future and is generally produced by partial oxidation,
steam reforming, and autothermal reforming of hydrocarbons. Be-
cause the production of hydrogen from fossil fuels is accompanied
with the formation of carbon monoxide and carbon dioxide, the
purification of hydrogen is important for the effective use of hydro-
gen. The separation of hydrogen from synthesis gases is conducted
with pressure swing adsorption, cryogenic distillation, or membrane
separation.
Electrochemical hydrogen pumping is one of the effective meth-
ods for the purification of hydrogen from the reformed gases. In this
system, proton conductors are employed as electrolytes. Hydrogen
and proton are oxidized and reduced at the anode and cathode, re-
spectively, with the current supplied from an external power source
We
have
recently
reported
the
proton-conductive
CsH2PO4/SiP2O7 composite at 110–280°C. The composite with a
CsH2PO4/SiP2O7 molar ratio of 1/2 achieved 44 mS cm−1 at
266°C under 30% H2O/Ar atmosphere.10-12 In this composite, the
chemical reaction at the contacting interface between the two com-
ponents resulted in the formation of CsH5͑PO4͒2, which served as a
proton-conductive phase. This electrolyte was stable in the solid
state under fuel cell operating conditions. The fuel cell composed of
this composite was operated successfully at around 200°C.13 In this
study, electrochemical hydrogen pumping employing the
CsH2PO4/SiP2O7 composite was investigated at 200°C. Initially, the
fundamental property of the electrochemical cell was examined by
supplying hydrogen to the anode. Furthermore, we evaluated the
feasibility of this system for the production and separation of hydro-
gen from the carbon monoxide and steam mixture. The water–gas
shift reaction ͑WGSR͒ of carbon monoxide and steam to form car-
bon dioxide and hydrogen ͑Reaction 3͒ was conducted with the use
of catalysts at ca. 200°C
Anode: H2 → 2H+ + 2e−
Cathode: 2H+ + 2e− → H2
͓1͔
͓2͔
CO + H2O → CO2 + H2
͓3͔
The electrochemical pumping cells with proton exchange mem-
branes were demonstrated.1-3 Purification of hydrogen from the re-
formed gas, however, is unfavorable at low temperatures because of
the deactivation of the platinum electrocatalyst ascribable to poison-
ing by carbon monoxide, as in polymer electrolyte fuel cells. Iwa-
hara et al.,4,5 and Matsumoto et al.,6-9 reported the electrochemical
hydrogen extraction from various gases, with the cell consisting of
proton-conductive ceramics based on cerates and zirconates as elec-
trolytes at 650–900°C.4-9 In cerate electrolytes, the hydrogen evolu-
tion rate did not attain the theoretical value expected from Faraday’s
law above a specific current density. This low current efficiency
resulted from partial electron or hole conduction in the electrolyte
under a reducing or oxidative atmosphere. Water management in
feeding gas to both electrodes was required for suitable oxygen par-
tial pressure. Furthermore, the inclusion of oxide-ionic conduction
in the electrolyte led to a decrease in the current efficiency of hy-
drogen pumping from anode to cathode. The cerate-based ceramics
are decomposed by carbon dioxide, which is contained in the re-
formed gases. Meanwhile, the zirconates with superior chemical and
mechanical stability exhibited a lower conductivity than cerates. Ac-
cordingly, there are few stable electrochemical cells available to the
electrochemical hydrogen pumping in the presence of carbon mon-
oxide and carbon dioxide.
The present investigation aimed at the coupling of electrochemical
pumping and WGSR because either of the reactions is operative at
similar temperature ranges ͑ca. 200°C͒. Although CO tolerance of
the Pt electrocatalyst was significantly improved at elevated tem-
peratures, the catalyst was inactive for WGSR at 200°C.14-17 In
contrast, Cu-based mixed oxides are excellent catalysts for WGSR.
Tanaka et al. reported the relatively high CO conversion of the
spinel-type oxides such as CuMn2O4 and CuFe2O4 at 200°C.18,19
Then, the series of hydrogen production processes consisting of the
catalytic reaction and the electrochemical separation was demon-
strated by locating a Cu-based mixed oxide catalyst at the upstream
part of the gas flow.
Experimental
A commercial Pt/C supported on carbon paper ͑BASF Fuel Cell,
Inc., phosphoric acid fuel cell, Pt loading 1.0 mg cm−2͒ was used
as an electrode to examine the property of the cell by feeding hy-
drogen to the anode. With a supply of carbon monoxide and steam,
some catalysts were applied on carbon paper for the anode. A com-
mercial Pt catalyst supported on carbon black ͑40 wt % Pt/C, Alfa
Aesar͒ was employed. The copper-based spinel oxide
CuFe1.5Mn0.5O4 was prepared by the citric acid complex method
and was calcined at 900°C for 10 h.19 This spinel catalyst was
mixed with carbon black ͑Vulcan XC-72͒ to be a 40 wt % spinel
catalyst/C ͑hereafter abbreviated as Cu spinel/C͒. The anode was
*
Electrochemical Society Active Member.
z E-mail: hiroki-m@t01.mbox.media.kyoto-u.ac.jp
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B1390
Journal of The Electrochemical Society, 156 ͑12͒ B1389-B1393 ͑2009͒
Table I. Pt and CuFe1.5Mn0.5O4 loading in anode.
350
300
250
200
150
100
50
Theoretical value
1st
2nd
Pt loading
CuFe1.5Mn0.5O4 loading
Anode
͑mg cm−2
͒
͑mg cm−2
͒
Pt/C
1.0, 8.1, 10.6
1.5
—
12.1
Pt/C + Cu spinel/C
fabricated with these resultant powders. The catalyst was ultrasoni-
cally dispersed in a mixture of deionized water and ethanol ͑Wako
Pure Chemical Industries͒. The mixture was applied on a Teflon-
coated carbon paper ͑Toray͒ and dried at ca. 100°C. With the Cu
spinel/C catalyst, the carbon paper was initially coated with the
Cu-based catalyst, followed by the Pt catalyst to be a bilayered
electrode. The Pt and CuFe1.5Mn0.5O4 loadings in these electrodes
are summarized in Table I. For the cathode and reference electrode,
the commercial Pt/C supported on carbon paper was used.
0
Cesium dihydrogen phosphate, CsH2PO4, was synthesized by
drying an aqueous solution of Cs2CO3 ͑Aldrich͒ and H3PO4 ͑Wako
Pure Chemical Industries͒ overnight at ca. 100°C. The X-ray dif-
fraction pattern of the resultant powder was identical to that of
CsH2PO4 in the literature.20 The silicon pyrophosphate, SiP2O7, was
obtained from SiO2 ͑Nippon Silica͒ and H3PO4, which consisted of
forms II ͑monoclinic͒ and III ͑pseudotetragonal͒.21,22 The composite
electrolyte of CsH2PO4/SiP2O7 was prepared by mixing the result-
ant powders in a molar ratio of 1:2. The membrane electrode assem-
bly ͑MEA͒ was fabricated by uniaxial pressing of the composite
powder with electrodes ͑13 mm diameter, ca. 1.3 mm thickness, and
0.283 cm2 electrode area͒. The reference electrode was set on the
cathode side. Then, the resulting MEA was heat-treated at 220°C for
1 h and placed in the apparatus, as shown in Fig. 1. The shift reactor
was located at the upstream part of the gas flow to evaluate the
combined system of WGSR and electrochemical hydrogen pumping.
A physically mixed catalyst of CuFe1.5Mn0.5O4 spinel-type oxide
and ␥-alumina ͑JRC-ALO-8, The Catalysis Society of Japan͒ with a
weight ratio of 2:1 was used. Before the measurement, a catalyst of
1.0 g in the reactor was reduced at 250°C for 1 h in 30% H2O/H2.
The hydrogen pumping capacity was examined by applying the
dc to the cell at 200°C. Hydrogen or carbon monoxide was fed to
the anode. The gas supplied to the cathode consisted of argon and a
small amount of hydrogen so as to decrease the concentration over-
potential of hydrogen.9 The feeding gas was humidified at 30% by
passing through water at ca. 70°C. The total flow rate for both
electrodes was maintained at 150 mL min−1, unless otherwise
noted. The concentrations of hydrogen and carbon dioxide in the
outlet gas after the water trap were analyzed by a thermal conduc-
tivity detector ͑Varian, CP-4900͒. The evolution rate was estimated
by measuring the increase in concentration with passing currents.
For the evaluation of the overpotential, the reactant gas and humidi-
fied hydrogen were supplied to the anode and cathode sides, respec-
tively. The potential and ohmic resistances of the anode were mea-
sured against the reference electrode by galvanostatic steady state
0
200
400
600
800 1000
Current density / mA cm-2
Figure 2. Evolution rates of hydrogen gas at the cathode as a function of
current density with a supply of 30% H2O/H2 to the anode at 200°C.
polarization and ac impedance spectroscopy techniques, respectively
͑Solartron 1287 potentiostat and Solartron 1260 frequency response
analyzer͒. The overpotential at the anode was calculated from the
ohmic loss and potentials under an open-circuit condition and
polarization.8
Results and Discussion
Hydrogen pumping from humidified hydrogen fed to anode.—
The evolution rates of hydrogen gas at the cathode as a function of
current density are shown in Fig. 2. The broken line represents the
theoretical evolution rates calculated from Faraday’s law. The ob-
tained results of two consecutive measurements obeyed Faraday’s
law and were highly reproducible. At 1000 mA cm−2, the differ-
ence in voltage between the anode and cathode increased by ca. 5 V
in 30 min. The impedance spectra of a cell with a two-electrode
system measured under an open-circuit condition before and after
the hydrogen pumping tests are shown in Fig. 3. The ohmic resis-
tance remained unchanged during the first measurement. In contrast,
the impedance spectrum after the second measurement exhibited a
remarkable increase in ohmic and polarization resistances. This im-
pedance increase indicated that the large reaction overpotential at
1000 mA cm−2 should give rise to the partial decomposition of the
electrolyte accompanied with a decrease in its conductivity. This
degradation, however, did not affect the current efficiency of hydro-
gen pumping, judging from the hydrogen evolution rate. Conse-
quently, this system was sufficiently operable as a hydrogen pump.
Gore-tex® Hyper-sheet® gasket
SUS310S
Before 1st
-10
-5
0
After 1st (Before 2nd)
After 2nd
Reference (Pt wire)
0
5
10
15
20
25
30
Z' / Ω cm2
Cathode side
Anode side
Figure 1. Schematic drawing of cell configuration.
Electrolyte
Figure 3. Impedance spectra of a single cell at 200°C before and after the
hydrogen pumping tests.
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Journal of The Electrochemical Society, 156 ͑12͒ B1389-B1393 ͑2009͒
B1391
(a) Electrochemically-promoted WGSR
(b) Electrolysis of water
Current density / mA cm-2
e-
e-
1
10
e-
e-
2H+ + 2e-
H2
H2O
1/2O2 + 2H+ + 2e-
Anode
2H+ + 2e-
H2
CO + H2O
CO2 + 2H+ + 2e-
Anode
1.0
0.8
0.6
0.4
0.2
0.0
30% H O/CO
30% H22O/Ar
H+
H+
Cathode
Cathode
Figure 4. Schematic drawing of the electrochemical hydrogen extraction
from a gas mixture of carbon monoxide and steam.
Hydrogen extraction from humidified carbon monoxide fed to
anode.— The electrochemical production and separation of hydro-
gen from carbon monoxide and steam were examined. The fabri-
cated Pt/C electrode ͑Pt loading 1.0 mg cm−2͒ was used for the
anode. When carbon monoxide and steam were fed to the anode
under an open-circuit condition, hydrogen was hardly detected at the
anode. Thus, the catalytic activity against WGSR for the Pt/C cata-
lyst was negligibly small at 200°C. With the passing current, two
reactions have to be considered: electrochemically-promoted WGSR
͑Fig. 4a͒ and electrolysis of water ͑Fig. 4b͒.
0.0
0.5
1.0
1.5
log(I / mA cm-2)
Electrochemically-promoted WGSR
Anode: CO + H2O → CO2 + 2H+ + 2e−
Cathode: See Reaction 2.
͓4͔
Figure 5. Anodic overpotential as a function of current density with a supply
of 30% H2O/CO or 30% H2O/Ar at 200°C.
Electrolysis of water
Anode: H2O → 1/2O2 + 2H+ + 2e−
͓5͔
odic overpotential of Pt/C electrodes decreased with an increase in
the Pt loading. The slope of overpotential vs logarithm of current
density was almost the same for different Pt loadings in the range of
1.0–10.6 mg cm−2, suggesting that the reaction process was un-
changed for Pt/C electrodes in this range of Pt loading.
The electrochemically-promoted WGSR over Pt/C electrode did
not readily proceed, while this electrode was significantly active for
hydrogen oxidation ͑Reaction 1͒. It is expected that the reduction in
overpotential for hydrogen extraction and separation from carbon
monoxide and steam is achieved by the combination of catalytic
WGSR ͑Reaction 3͒ and the subsequently electrochemical separa-
tion of hydrogen from the post-WGSR gas ͑Reactions 1 and 2͒.
Then, the shift reactor with a mixed catalyst of CuFe1.5Mn0.5O4
Cathode: See Reaction 2.
To clarify the reaction at the anode, the evolution rate of carbon
dioxide in the outlet gas with passing current was evaluated, as
summarized in Table II. The calculated values are based on the
assumption that the passed current is completely consumed for the
electrochemically-promoted WGSR, in conformity with Faraday’s
law. The measured value agreed well with the calculated one. Thus,
it can be concluded that hydrogen was extracted to the cathode via
the electrochemically-promoted WGSR under current loading. Fur-
thermore, the anodic overpotential was remarkably lower for the
supply of humidified carbon monoxide than for humidified argon
͑Fig. 5͒.
In this condition, the hydrogen production was confirmed at the
cathode during polarization, as shown in Fig. 6. Then, considering
the low activity against catalytic WGSR for the Pt/C catalyst at
200°C, the operation of the cell gave rise to the formation of hydro-
gen from a gas mixture of carbon monoxide and steam. In Fig. 6, the
electrochemically-promoted WGSR ͑Fig. 4a͒ is regarded as a fara-
daic reaction to calculate theoretical values. Hydrogen was extracted
in the theoretically estimated amount, as in humidified hydrogen
͑Fig. 2͒. Accordingly, the electrochemical extraction and separation
of hydrogen from humidified carbon monoxide were successfully
verified at 200°C.
The anodic overpotential of Pt/C electrodes with various Pt load-
ings was evaluated in a gas mixture of carbon monoxide and steam,
as shown in Fig. 7. These electrodes exhibited extremely higher
overpotential in humidified carbon monoxide than in humidified hy-
drogen ͑cf. 1–2 mV at 2–10 mA cm−2 in 30% H2O/H2͒. The an-
Theoretical value
Measured value
6
4
2
0
Table II. Evolution rate of carbon dioxide in the outlet gas of
anode with passing current.
Evolution rate of CO2
͑␮mol min−1 cm−2
͒
0
5
10
15
20
Current density
Current density / mA cm-2
͑mA cm−2
͒
Measured value
Calculated value
5
10
1.28
2.98
1.55
3.11
Figure 6. Evolution rates of hydrogen gas at the cathode as a function of
current density with a supply of 30% H2O/CO to the anode at 200°C.
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B1392
Journal of The Electrochemical Society, 156 ͑12͒ B1389-B1393 ͑2009͒
Current density / mA cm-2
Current density / mA cm-2
1
10
1
10
Pt/C (Pt: 1.0 mg cm-2)
Pt/C (Pt: 8.1 mg cm-2)
Pt/C (Pt: 10.6 mg cm-2)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Reactor temp., S.V.
250oC, 9000 l kg-1 h-1
250oC, 3000 l kg-1 h-1
250oC, 1500 l kg-1 h-1
0.5
0.4
0.3
0.2
0.1
0.0
0.0
0.5
1.0
1.5
log(I / mA cm-2)
0.0
0.5
1.0
log(I / mA cm-2)
Figure 7. Overpotential of the anode with various Pt loadings as a function
of current density with a supply of 30% H2O/CO at 200°C.
Figure 9. Anodic overpotential as a function of current density with a supply
of post-WGSR gas at 200°C. This measurement was conducted with chang-
ing the SV in the shift reactor.
spinel-type oxide and ␥-alumina was located at the upstream part of
the gas flow, as shown in Fig. 8. This spinel-type oxide was reported
to be active for the catalytic WGSR above 175°C in the hydrogen-
rich gas.19 The influence of the space velocity ͑SV͒ in the shift
reactor on the anodic overpotential of the Pt/C electrode ͑Pt loading
1.0 mg cm−2͒ was analyzed at 200°C by changing the flow rate of
the supply gas. Figure 9 shows the anodic overpotential as a func-
tion of current density with a supply of post-WGSR gas. Clearly, the
formation of hydrogen over the mixed catalyst in the reactor reduced
duction in the shift reactor. The high conversion of carbon monoxide
was responsible for the low concentration of carbon monoxide and
steam in the post-WGSR gas at 250°C. Thus, the electrochemically-
promoted WGSR did not readily proceed, leading to a higher over-
potential in the high current density region of 7–20 mA cm−2 at
250°C than at the other reactor temperatures.
The shift catalyst installed upstream of the gas flow was effective
for the promotion of hydrogen extraction from humidified carbon
monoxide during passing the low current densities. Then, the over-
potential of the bilayered electrode composed of Cu spinel/C and
the overpotential of the electrode at
a current density of
2–10 mA cm−2. As the SV in the reactor increased, the drastic
increase in the overpotential was confirmed at a lower current den-
sity. This indicated that the hydrogen formed by the catalytic reac-
tion is preferentially pumped into the cathode. Moreover, the anodic
overpotential of the Pt/C electrode ͑Pt loading 1.0 mg cm−2͒ also
depended on the temperature of the shift reactor, as shown in Fig.
10. At 100 and 200°C, the overpotential in the low current density
region was substantially higher than that at 250°C. The conversion
of carbon monoxide over the mixed catalyst was low, and hydrogen
was deficient below 200°C. At 250°C, the overpotential signifi-
cantly increased with the current density, though hydrogen was rap-
idly pumped into the cathode at a low current density. As the current
density increases, the supplied current should initiate
electrochemically-promoted WGSR as well as oxidation of hydro-
gen from the catalytic WGSR because of the limited hydrogen pro-
Current density / mA cm-2
1
10
Reactor temp., S.V.
100oC, 9000 l kg-1 h-1
200oC, 9000 l kg-1 h-1
250oC, 9000 l kg-1 h-1
1.0
0.8
0.6
0.4
0.2
0.0
outlet
outlet
Electric furnace
Catalyst
bed
Post-WGSR gas
CO + H2O
0.0
0.5
1.0
1.5
log(I / mA cm-2)
Shift reactor
CO + H2O → CO2 + H2
Anode
Cathode
H
2 → 2H+ + 2e-
2H+ + 2e- → H2
Figure 10. Anodic overpotential as a function of current density with a
supply of post-WGSR gas at 200°C. This measurement was conducted with
changing the shift reactor temperature.
Figure 8. Schematic drawing of the apparatus for electrochemical hydrogen
pumping with the shift reactor.
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Journal of The Electrochemical Society, 156 ͑12͒ B1389-B1393 ͑2009͒
B1393
Current density / mA cm-2
10
drogen evolution rate at the cathode agreed well with that calculated
from Faraday’s law in the whole range of current density investi-
gated regardless of the feeding gas. With the supply of humidified
carbon monoxide to this electrochemical cell, the electrochemically-
promoted WGSR proceeded. The electrode in humidified carbon
monoxide, however, exhibited higher overpotential by 2 orders of
magnitude than that in humidified hydrogen. The reactor with a shift
catalyst was installed at the upstream part of the gas flow for the
decrease in overpotential. The overpotential of the Pt/C electrode in
post-WGSR gas was reduced by hydrogen formed in the shift reac-
tor. The bilayered electrode composed of a Cu spinel and Pt cata-
lysts was evaluated for the series of hydrogen production processes
consisting of the catalytic WGSR and the electrochemical pumping
in the anode chamber. This electrode was effective for smooth hy-
drogen extraction from carbon monoxide and steam at low current
densities. Consequently, it is important not only to select the suitable
catalyst material but also to optimize the location of the shift cata-
lyst and the electrode structure for further investigation of hydrogen
production and separation processes.
1
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Pt/C + Cu spinel/C
Pt/C (Pt: 1.0 mg cm-2)
Pt/C (Pt: 10.6 mg cm-2)
Acknowledgment
This work was supported by a Grant-in-Aid for Scientific Re-
search from the Ministry of Education, Culture, Sports, Science and
Technology of Japan.
0.0
0.5
1.0
1.5
log(I / mA cm-2)
Kyoto University assisted in meeting the publication costs of this article.
Figure 11. Overpotential of the anode consisting of Pt/C with and without
Cu-based catalyst as a function of current density with a supply of 30%
H2O/CO at 200°C.
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Conclusions
The cell consisting of a CsH2PO4/SiP2O7 composite electrolyte
was investigated for the current efficiency for hydrogen pumping
and the anodic overpotential of an electrode with a feed of humidi-
fied hydrogen and humidified carbon monoxide at 200°C. The hy-
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