J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Electrochemical Hydrogen Pump using a High Temperature Type
Proton Conductor under Reduced Pressure
Masahiro TANAKA
National Institute for Fusion Science, Oroshi, Toki, Gifu, 509-5292, Japan(Received: 30 October 2009 / Accepted: 13 April 2010)

The electrochemical hydrogen pump properties of the CaZr0.9In0.1O3-α proton conductor were observed under
reduced pressure condition in the cathode compartment for the recovery of hydrogen isotopes. The applied voltage
between electrodes under reduced pressure condition reduced than that under atmospheric pressure condition, when
the hydrogen pump performance was evaluated by direct current method. It is likely that the total pressure in thecathode compartment affects the overall electric resistance. From the impedance spectroscopy analysis, it wassuggested that diffusion process in porous electrode was affected by reduced pressure condition. The hydrogen
pump performance could be improved under the reduced pressure condition of around few kPa and the reducedpressure condition had an effect on the mass transfer process in the electrode.
Keywords: proton conductor, hydrogen pump, tritium recovery, reduced pressure, electrochemical impedance
spectroscopy, charge transfer process, mass transfer process.
.

chosen CaZr0.9In0.1O3-α, which was used for the one-end
closed tube of the test pump, as the proton-conducting
oxide, because CaZr0.9In0.1O3-α is excellent in the chemical
stability against carbon dioxide attack at elevatedtemperature and in mechanical strength. Then, thehydrogen pump performances under various conditions
have been reported and shown the possibility of realization[9,10]. On the other hands, in order to recovery purehydrogen isotope gases for fuel cycle, hydrogen pump intoa vacuum would be required to avoid from being mixed in
impurity gaseous as purge gas. However, the hydrogen
pump characteristics into vacuum are not well-known.
Kato et al. have proposed a small tritium purification andrecovery system for muon-catalyzed fusion (µCF) andhave evaluated hydrogen pump performance using a hightemperature type proton conductor under a high vacuum
condition [11]. As the results, although they showed thathydrogen could be extracted into vacuum, the dependenceof total pressure in the cathode compartment has not beenstudied in detail.
As to the applications of proton conductor in vacuum
technology, Schober has demonstrated a novel water vaporand hydrogen gas source into vacuum, and then proposed
as a leak detector for vacuum purposes [12, 13]. These
reports, however, were also not considered the dependenceof total pressure in the cathode compartment on hydrogenpump.
In this research, we report the preliminary results of
the hydrogen pump characteristics under reduced pressure
condition in the pressure range from 10 Pa to 105 Pa.

1. Introduction

In a future nuclear fusion plant, hydrogen isotope gasof deuterium (D) and tritium (T) is used as fuel. The
developments of hydrogen process technologies for fuelcycle are one of the important issues. As for the hydrogenisotope recovery process for chemical purification ofhydrogen, the candidate material is a palladium membranediffuser [1]. Hydrogen permeates through the membraneby providing pressure difference as a driving force andhigh purity hydrogen gas is obtained. However, thepalladium membrane cannot directly recovery hydrogen in
the hydrogen compounds such as water vapor andhydrocarbons. Thus, it has been suggested to decomposehydrogen compounds to hydrogen gas by a cracking
catalysts or electrochemical reactor, and the membranereactor systems combined with cracking catalyst have been
researched and developed [2-4].
An electrochemical hydrogen pump using a
proton-conducting oxide is one of the candidate materials
for hydrogen recovery. Some perovskite-type oxides suchas SrCeO3, CaZrO3, and SrZrO3 which are doped with
Yb3+, Y3+ and In3+ etc exhibit proton conduction inhydrogen-containing atmospheres at high temperature [5].
It has attractive advantages such as: hydrogen extraction
from both hydrogen molecules and hydrogen compounds;
control by electric current; no pressurization, etc. Thus, theapplication of electrochemical hydrogen pump by a proton
conducting oxide instead of the palladium membranediffuser have been proposed by Asakura [6] andKawamura et al. [7,8]. In our previous research, we haveauthor’s e-mail: tanaka.masahiro@nifs.ac.jp352©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

M. Tanaka, Electrochemical Hydrogen Pump Using a High Temperature Type Proton Conductor under Reduced Pressure

φ15 mm

φ13.5 mm

155 mm(Cathode)

Fig.1 A photograph of one end closed proton
conductor; CaZr0.9In0.1O3-α.
Fig.3

2. Experimental
2.1 Proton Conducting Oxide

From the viewpoint of the practical use, we carriedout the performance tests by use of the one end closed tube
made of CaZr0.9In0.1O3-α, which was prepared by TYK Co.
Ltd. The shape of the test tube was 12 mm inner diameter,
0.75 mm thickness and 340 mm length. The appearance oftest tube is shown in Fig. 1. Platinum electrodes wereattached on both sides of the test tube by paste-baking on

Flow diagram of hydrogen pump apparatus andvacuum control system.

the inner surface. The electrode on the outer surface was
attached by electroless-plating method. The each electrodeof inner and outer surfaces was used as anode and cathode,respectively. In addition, a platinum paste 5 mm in width
as a collector electrode was also calcined from top tobottom on the outer surface. The effective electrode length
along the axial direction at the outer surface was about 155
mm and that at the inner surface was 340 mm. The
effective outer electrode was approximately 62 cm2.

2.2 Hydrogen Pump Apparatus and VacuumControl System

Schematic diagrams of the hydrogen pump apparatus
and the vacuum control system are shown in Fig. 2 and 3.
The test tube was heated from 873 K to 1073 K by anelectric furnace. The gas was humidified by using abubbler immersed in a constant-temperature water bath
controlled at 283K. Wet argon gas was fed to the anodeside at 100 cm3/min and the flow rate was controlled by amass flow controller. The electrochemical hydrogen pumpthrough the specimens was examined by sending direct
current with a potentio/galvanostat (HOKUTO DENKO,
HA-151) to the following electrolytic cell:( +) Ar+H2O, Pt/CaZr0.9In0.1O3-α/Pt, Ar ( - ),

Fig.2

(1)

The accuracy of the voltage measurement was ± 3 mV. An
impedance analysis measurement was conducted by use of
two LCR meters with different frequency range (HIOKI,
3522-50 and 3532-80). These frequency ranges are 1 mHz
to 100 kHz and 4 Hz to 1 MHz, respectively. The pressurein the cathode was measured by a capacitance vacuum
gauge (MKS, Baratoron 626A) in the measuring pressure
range of 10 Pa to 105 Pa.
In this experimental system, the total pressure in thecathode compartment could be reduced until around 10 Pa
by a scroll type vacuum pump (Sato Vacuum Inc,
OFS-M-300). Then, the pressure in the cathode

Schematic of the hydrogen pump apparatus.

353

M. Tanaka, Electrochemical Hydrogen Pump Using a High Temperature Type Proton Conductor under Reduced Pressurecompartment was up to 105 Pa by introducing argon gas.
The pressure was controlled by a gas dosing system control
unit (Pfeiffer vacuum, RVC-300 and RME 005 valve) witha pirani/capacitive transmitter (Pfeiffer vacuum, PCR-260).
The dew point was measured with a chilled-mirror dew
point recorder (General eastern, 1311DR). The accuracy ofdew point measurement is ±0.2 oC.

3

2.5

Voltage [V]2

3. Experimental Results and Discussion
3.1 Hydrogen Pump
Reduced Pressure

Characteristics

under

2 mA/cm2

1.5 mA/cm2

Fig.5

1 mA/cm2

2

Voltage [V]

1.4
1

10

100

Total pressure [kPa]

Fig.4

0.5

1

1.5

2

2.5

3

3.5

Current-voltage relationship on different totalpressure condition in the cathode compartment.

hydrogen evolution rate under reduced pressure condition
was enhanced or not.
Figure 5 shows the relationship between current
density and voltage under the different total pressure
condition of 11.05 kPa and 98.56 kPa in the cathodecompartment at 973 K. The voltage at 11.05 kPa was lower
than that at 98.56 kPa. Although the voltage of eachpressure condition increased with increase of current
density, the difference in voltage between 11.05 kPa and98.56 kPa decreased with increasing current density. Whencurrent density increases, more hydrogen gas is extracted
to the cathode electrode. From the viewpoint of the masstransfer process, the mass transfer resistance would mainly
consist of three phenomena: proton migration in theelectrolyte, the electrode reaction on both the anode andthe cathode and gas diffusion in the porous electrode. The
gas diffusion in the porous electrode would be affected byboth the porous diameter of electrode and mean free path.
If the pore diameter of the electrode is larger than the mean
free path which is the function of the reciprocal of gas
pressure, the diffusion phenomenon is considered bulk
diffusion. On the other hands, when the pore diameter ofthe electrode is less than the mean free path, the diffusion
phenomenon is close to Knudsen diffusion. It is not clearthat ether pore diffusion effects can be applied, because thehydrogen partial pressure in the pore electrode and the pore
diameter of electrode are not determined with accuracy.
These gas diffusion phenomena in the porous electrode areappeared as electrochemical polarization and theimpedance would be increased. Then, to separate the masstransfer phenomena, the impedance spectroscopy analysis
was carried out in the next section.

Anode gas flow :100 cm3/min
Anode : 1.1 kPa H O, Ar

0.1

0Current density [mA/cm2]

Temperature : 973 K

0.01

Anode gas flow :100 cm3/min
Anode : 1.1 kPa H O, Ar
2

0

1.8

0

Temperature : 973 K

0.5

2

1.6

1.5

1

Figure 4 shows the dependence of voltage on totalpressure in the cathode compartment for different current
density at 973K. Each pressure condition was maintained
for about 10 minutes. As seen in the figure, a very smallchange of voltage was observed with variation of totalpressure. All the voltage value under reduced pressure wassmaller than the voltage of atmospheric pressure in spite ofcurrent density. The voltage was minimized around 10 kPa
and the voltage drop was the range of 0.04 V to 0.07 V
each current density. It is probable that the appearance
electric resistance decreased slightly under reducedpressure condition. However, the voltage slightly increased
under reduced pressure less than 1 kPa. It is likely that the
total pressure in the cathode compartment affects theoverall electric resistance.
The dew point at anode compartment decreased withincrease of current density. However, the dependency ofdew point on the total pressure at the cathode compartment
was not observed clearly due to the resolution of the dew
point recorder. Therefore, it was not clear whether

2.2

98.56 kPa11.05 kPa

Dependence of voltage on total pressure in thecathode compartment for different current
density at 973K.

354

M. Tanaka, Electrochemical Hydrogen Pump Using a High Temperature Type Proton Conductor under Reduced Pressure

3.2. Impedance Spectroscopy Analysis

Impedances spectroscopy analysis is powerful methodof characterizing of the electrical properties of materials
and the interface of electrode [14]. It may be used toanalysis the charge and mass transfer processes [15]. In
this report, we discuss qualitatively the impedance
behavior using a simple model incorporated the charge andmass transfer processes as the first step.

-Z"(-Im(Z))

Rs

ZD

Rct

Mass transfer control(Diffusion process)Cdl

Charge transfer
controlZ'

3.2.1. Electrochemical Impedance Spectroscopy(Re(Z))

Electrochemical impedance spectroscopy is atechnique for characterization of electrochemical system.
In general, an electrochemical cell can be consideredsimply impedance circuit to a small sinusoidal excitation;
hence the electrochemical cell is able to be represented byan equivalent circuit of resistors, capacitors, inductor,
constant phase element and impedance as a model ofdiffusion at the electrode interface, etc. These elements are
combined with series and/or parallel to make complex
equivalent circuit. A simple and common use
semi-empirical model for an electrochemical cell is theRandles circuit which describes the response of asingle-step charge-transfer process with diffusion [14, 16].
It consists of an electrolyte resistance Rs in series with theparallel combination of the double-layer capacitance Cdl
and the charge transfer resistance Rct and the combined
impedance Z is expressed as follow:

Z = RS +R ct,
jωR ct Cdl + 1

Rs

Fig.6


K
jω 
tanh − δ

D
jωD


s

ct

R +R
low frequency: ω

s

p

L

Illustrating of the impedance spectrum of theRandles circuit with the faraday impedance offinite-length diffusion.

Z F, ω→∞ = R ct ,ZF, ω→0 = R ct +(4)Kδ
= Rp ,
D(5)

where, Rp is polarization resistance. To identify andseparate the different processes, it is useful to represent
the measured impedance spectra as Nyquist plot. Figure 6
shows a calculated illustration of the impedance spectrumof the Randles circuit with faraday impedance as Nyquist
plot. The impedance spectrum has two semicircles. The left
semicircle, which is high frequency region, corresponds tothe parallel circuit of Cdl and Rct and represents the chargetransfer process. The right semicircle corresponding to low
frequency region represents the impedance spectrum of
faraday impedance of Eq. (3) as diffusion process. The
shape of right semicircle depends on several parameters ofgas effective diffusion coefficient D, constant K anddiffusion distance δ.
In the experimental system, the compartments of theanode and the cathode have different conditions such asatmosphere, the electrode area and porosity. For theapplication of impedance analysis, these complicatedeffects should be taken into consideration. Therefore, in a
general way, more complex equivalent circuits
corresponding to the equivalent circuits of both conditionsof anode and cathode than the simple Randle circuit asshown in Fig. 6 would be required to simulate theimpedance spectrum. The impedance related to the
diffusion is usually found in the lowest frequency range,
down to 10 mHz or even lower [14]. Therefore, in thepresent study, since the operational parameter is only totalpressure in the cathode compartment, the diffusion process
in the cathode electrode would be discussed qualitatively
from the experimental results of the impedance spectrum innext section.

(2)

These elements are modified depending on the chargetransfer and mass transfer processes such as chemical
reaction, gas diffusion and adsorption etc. Taking into
consideration of the elementary processes in/on proton
conductor, the faraday impedance ZF, which is due tochemical process, mass transfer process and adsorption
process etc around the electrode, instead of the chargetransfer impedance Rct may be applied as the diffusion
impedance. The ZF with finite-length diffusion impedanceis expressed by following equation [14, 17]:Z F = R ct +R +Rhigh frequency: ωH(3)= R ct + Z D,
where, j is imaginary unit, ω is angular frequency, K isconstant of diffusion impedance, D is effective diffusivityof gas through porous layer, δ is diffusion distance. The
first and second terms express the charge transfer
resistance Rct and the diffusion impedance ZD,
respectively. The limiting value of ZF under high and low
frequency are express as follows:355

M. Tanaka, Electrochemical Hydrogen Pump Using a High Temperature Type Proton Conductor under Reduced Pressure

Temperature: 973 K
Anode: 1.2 kPa H2O, Ar

1.19 Hz20000.007 Hz0.113 Hz

50

100

150

Z' [Ω]

200

20
0

0

0.001 Hz150

Z' [Ω]

100 66-Z" [Ω]

40

Z' [Ω]

150

200(e). Cathode pressure: 99.07 kPa

40

0.007 Hz1.19 Hz00.113 Hz50

0.001 Hz100

Z' [Ω]

150

200

Fig.7 Comparison of the Nyquist plots fordifferent total pressure in the cathodecompartment:(a). 0.04 kPa, (b). 0.15 kPa, (c). 1.14 kPa,(d). 9.13 kPa, (e). 99.07 kPa

0.001 Hz1.19 Hz0.113 Hz

0100(c). Cathode pressure: 1.14 kPa

00
10
12
14
60 00 22 44 66 88 10
14
12

0

50

44
80 22
00 0 2 4 6 8 10 12 14
60 0 2 4 6 8 10 12 14

0

200

44
80 22

20

0

20

100

0.001 Hz0.007 Hz6

0.113 Hz 0.007 Hz

50

0.113 Hz100 640
1.19 Hz400(b). Cathode pressure: 0.15 kPa

44
80 22
00
10
12
14
60 00 22 44 66 88 10
12 14

00 0 2 4 6 8 10 12 14
0 2 4 6 8 10 12 1460

200.001 Hz100 66-Z" [Ω]

44
80 22-Z" [Ω]

40(d). Cathode pressure: 9.13 kPa

100 66(a). Cathode pressure: 0.04 kPa

44
80 22
0
10
12
14
60 000 22 44 66 88 10
1214-Z" [Ω]-Z" [Ω]

100 6650

0.007 Hz100

Z' [Ω]

3.2.2. Impedance Behavior
Pressure Condition

150

200

under

ReducedP-1 according to the Chapman-Enskog theory [18].
However, the semicircle became larger at the total pressure
below 1 kPa. The pressure dependence of impedance
spectra is corresponding to that of the voltage variations onhydrogen pump as shown in Fig. 4. The diffusion effects
as described in section 3.1 might be being shifted
depending on the total pressure below 1 kPa. Although
the mechanism of the increase in the impedance spectra
less than 1 kPa could not be determined in this report, it
might be suggested that the hydrogen pump performance
could be improved under the reduced pressure conditionof around few kPa and the reduced pressure condition has
an effect on the mass transfer process in the electrode.

Figure 7 shows comparison of the Nyquist plots fordifferent total pressure in the cathode compartment at 973
K. The Nyquist plots of lower impedance region measured
by LCR meter 3532-80 were shown in the higher left handcorner of each figure. It seems that the impedance spectraof this system had two or three semicircles. The left
semicircle seems not to be almost affected the totalpressure variations in the cathode compartment, while theshape of right semicircle is much changed by thevariation of total pressure. As shown in Fig. 6, the left
semicircle would represent the charge transfer control in
the electrolyte-electrode interface and the left endpoint ofsemicircle corresponds to electrolyte resistance. Therefore,
the left semicircle would be almost independent of the totalpressure.
On the other hand, the right semicircle would
represent the mass transfer control with diffusion processof faraday impedance, ZF. According to Eq. (3) and (5), thesemicircle of diffusion process should become smaller withdecreasing total pressure until few kPa, because thepressure dependence of diffusivity of gas is correlated to

4. Conclusion

The electrochemical hydrogen pump properties of
the CaZr0.9In0.1O3-α were observed under reduced pressure
condition in the cathode compartment. From the behaviorof hydrogen pumping under reduced pressure atmosphere,
it is suggested that the total pressure in the cathodecompartment affects the overall electric resistance. Fromthe impedance spectroscopy analysis, it was suggested
that diffusion process in porous electrode was affected by356

M. Tanaka, Electrochemical Hydrogen Pump Using a High Temperature Type Proton Conductor under Reduced Pressure

reduced pressure condition. It might be suggested that thehydrogen pump performance could be improved under
the reduced pressure condition of around few kPa and the
reduced pressure condition had an effect on the masstransfer process in the electrode.

Acknowledgements

The author is grateful to Dr. T. Sugiyama of Nagoya
University and Ms. T. Ohshima of TYK Co. Ltd. for theirsupport. This study was supported by NIFS budget(NIFS07ULSS505, NIFS08ULSS505), the cooperative
research with TYK Corporation and Grant-in-Aid forYoung Scientist (B) (No. 21760693) from the Ministry of
Education, Science, Sports and Culture of Japan.
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