Hydrogen separation using proton-conducting perovskites

Article abstract of DOI:10.1016/j.jallcom.2004.12.093

Two methods of hydrogen separation using proton-conducting perovskites, electrochemical hydrogen pumping with proton-conducting electrolytes and hydrogen sieving through protonic-electronic mixed conductors, are stated. In the former case, high temperature proton conductors (HTPCs) enable the hydrogen transport by external application of electricity. In the latter case, it is a fundamental concern how the protonic and electronic charge carriers coexist in oxides to allow hydrogen permeation. Experimental results and the problems for future investigations are discussed.

Full text of DOI:10.1016/j.jallcom.2004.12.093

Journal of Alloys and Compounds 408–412 (2006) 456–462
Hydrogen separation using proton-conducting perovskites
a,∗
b
b
c
Hiroshige Matsumoto , Tetsuo Shimura , Hiroyasu Iwahara , Tohru Higuchi ,
a
a
a
a
Keiji Yashiro , Atsushi Kaimai , Tatsuya Kawada , Junichiro Mizusaki
a
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku,
Fukuoka 812-8581, Japan
Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
b
c
Department of Applied Physics, Tokyo University of Science, Tokyo 162-8601, Japan
d
IMRAM, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan
Received 30 July 2004; received in revised form 24 November 2004; accepted 15 December 2004
Available online 25 May 2005
Abstract
Two methods of hydrogen separation using proton-conducting perovskites, electrochemical hydrogen pumping with proton-conducting
electrolytes and hydrogen sieving through protonic–electronic mixed conductors, are stated. In the former case, high temperature proton
conductors (HTPCs) enable the hydrogen transport by external application of electricity. In the latter case, it is a fundamental concern how
the protonic and electronic charge carriers coexist in oxides to allow hydrogen permeation. Experimental results and the problems for future
investigations are discussed.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Proton conductors; Hydrogen separation; Hydrogen pumps; Mixed conductors
1
. Introduction
This paper deals with the following methods of hydrogen
separation using proton-conducting perovskites, i.e., electro-
chemical hydrogen pumping with proton-conducting elec-
trolytes and hydrogen sieving through protonic–electronic
mixed conductors. General understanding of the proton-
conducting perovskites is explained with the defect chemistry
for the generation of protonic charge carriers. The two kinds
of hydrogen separation are shown with experimental results
and the problems to be solved are discussed from both prac-
tical and fundamental view points.
Ion-conducting solid materials are useful for energy con-
version, gas separation, gas sensing and so on. Rare earth
metals play an important role for the ionic conduction in the
materials. Some perovskite-type oxides show protonic con-
duction at high temperatures [1–15]. Particularly, the choice
of tetravalent cerium combined with an alkaline earth metal
and a trivalent rare earth dopant, e.g., SrCe0.95Yb0.05O3 − α,
results in high protonic conductivity; various trivalent cations
with different ionic radii are conveniently chosen from the
lanthanoids to optimize the ion-conducting properties. Such
a protonic conduction in solids can be useful for various ap-
plications, as instructively summarized by Iwahara [16] and
Schober [17]. Namely, the proton conductors allow selective
transport of hydrogen, and thusare most attractive inapplying
to hydrogen separation.
2. High temperature proton conductors
Some oxides, typically having perovskite-type structure
with the general formula of ABO3, show protonic conduction
at high temperatures and are referred to as high temperature
proton conductors (HTPCs) [1–17]. Either Ce or Zr is avail-
able as the B-site cation, i.e., cerates and zirconates can be
chosen as HTPCs. In general, the cerates have high protonic
∗
Corresponding author. Tel.: +81 92 642 3552; fax: +81 92 642 3551.
E-mail address: matsumoto@cstf.kyushu-u.ac.jp (H. Matsumoto).
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.12.093

H. Matsumoto et al. / Journal of Alloys and Compounds 408–412 (2006) 456–462
457
conductivity, while the zirconates are characterized by high
chemical stability as well as good mechanical strength. Due
to such a trade-off relation between the protonic conductiv-
ity and chemical/mechanical stability, we have to select the
B-site element appropriately depending on the applications.
HTPCs are designed to have oxide ion vacancy by a partial
substitution of trivalent cations for the B-site tetravalent ones.
In a moist atmosphere, protons are formed in these oxides in
accordance with the following defect equilibriums [3]:
•
O
•
•
1
2
K1
×
V + O2 ꢀ O + 2h
(1)
O
K2
•
•
1
2
H2O + 2h ꢀ 2H + O2
(2)
K3
•
O
•
•
×
H2O + V ꢀ 2H + O
(3)
(4)
O
K3 = K1K2
•
•
•
•
×
O
where V , O , H and h denote oxide ion vacancy, oxide
O
ion at the regular site, proton and electron hole, expressed,
respectively, in the Kr o¨ ger–Vink notation, and Kn is the equi-
librium constant.
Fig. 1. Schematic structure of the electrochemical test cell.
Eq. (3) suggests that the incorporation of ambient water
molecules into the oxide ion vacancies is essential for the
generation of protons that are mobile in the oxide. There-
fore, the HTPCs need ambient moisture; the oxygen partial
pressure, p(O2), has only a little effect to the protonic conduc-
tivity. Since this equilibrium moves from the right hand side
to the left as temperature increases, an oxide ionic conduc-
products were ground with planetary ball mill, isostatically
pressed in disk shape at 300 MPa, and sintered for 10 h in air
to obtain dense ceramics; typical sintering temperatures were
◦
◦
1550–1600 C for strontium cerates, 1650 C for barium cer-
◦
ates and 1650–1700 C for strontium zirconates, respectively.
Fig. 1 shows the schematic illustration of the electro-
chemical test cell. A disk specimen (0.5 mm thickness and
13–14 mm diameter), on which platinum paste was painted
•
•
tion via oxide ion vacancies, V , also arises with increasing
O
2
temperature. The protonic and oxide-ionic transport numbers
depend not only on the temperature but also on the materials
and will be indicated individually for each HTPC present in
the following sections.
One can find from Eq. (1) that the hole concentration in-
creases with p(O2). Consequently, the HTPCs usually have
a large hole conductivity in an oxidative atmosphere, like in
air. The equilibrium goes to the left as p(O2) decreases, and
the protonic conduction (with a contribution of oxide-ionic
conduction) dominates in moist hydrogen. This feature al-
lows the use of HTPCs as electrolytes for hydrogen pumps
(projected area: 0.5 cm ) as the anode and cathode, was
placed between ceramic tubes with glass gaskets; the ref-
erence electrode was also prepared on the side of the elec-
trolyte disk in the case of hydrogen-pumping experiments.
Gases, usually hydrogen, argon or their mixtures, were intro-
duced to the cell at given flow rates; the gases were moistened
3
with water vapor typically at pW = 1.9 × 10 Pa. Hydrogen
and water vapor in the outlet gases were determined by gas
chromatography and with a chilled mirror hygrometer, re-
spectively.
(Section 4). In the case of hydrogen-permeating membrane
via protonic–electronic mixed conduction, the main subject
is to manage a kind of electronic charge carriers surviving
even in such reducing atmospheres (Section 5).
4. Hydrogen pumps
4.1. Principle and experimental procedure
The HTPCs can be used as the electrolytes for electro-
chemical hydrogen pumps as schematically shown in Fig. 2.
When an electrochemical cell of a proton conductor is con-
structed and a direct current is sent to the cell, hydrogen in the
anode compartment can be electrochemically pumped to the
cathode, as shown in the figure, by the following electrode
reactions:
3
. Experimental
Here, general experimental procedures and conditions for
the following sections are given.
Proton-conducting specimens were prepared by the solid
state reactions in the following manner. Powders of either
carbonates (Ba and Sr) or oxides (Zr, Ru and rare earths)
were used as starting materials. Their appropriately weighed
amounts were mixed and calcined for 10 h in air typically
+
−
H2 → 2H + 2e (anode)
(5)
(6)
+
−
◦
◦
2
H + 2e → H2 (cathode)
at 1250 C for cerates and at 1350 C for zirconates. The

4
58
H. Matsumoto et al. / Journal of Alloys and Compounds 408–412 (2006) 456–462
Fig. 2. Working principle of a hydrogen pump.
Experiments of hydrogen pumps were conducted with the
following gas atmospheres:
Moist H2, Pt|electrolyte|Pt, moist Ar
(7)
The gases both in the anode and cathode compartments
were moistened with water vapor because HTPCs need am-
bient water vapor to work. In addition, the water has a role
to avoid too reducing atmosphere resulting in the occurrence
of electronic conduction in the electrolyte as explained later.
In order to evaluate the energy efficiency of the hydrogen
pumps, electrode overpotentials were evaluated as follows.
The electric potentials of the anode and cathode against the
reference electrode, VRA(j) and VRC(j), were measured as
functions of the current density, j. The overpotentials, de-
fined as the changes in the potentials from those under open
circuit condition, consist of electrode reaction polarization,
ηRA and ηRC (the second subscript means either anode or
cathode), overpotentials due to the changes in hydrogen con-
centrations, ηCA and ηCC, and ohmic losses, ηOA and ηOC, as
written in Eqs. (8) and (9).
Fig. 3. (a) Evolution rate of hydrogen at the cathode and (b) elec-
trode overpotentials in the ionic current region during hydrogen pump-
ing at 800 C as functions of current density using SrZr0.9Y0.1O3 − α and
◦
SrCe0.95Yb0.05O3 − α electrolytes; the dotted line in (a) indicates the theo-
retical evolution rate of hydrogen calculated from Faraday’s law. The left
upper graph in (a) is shown in smaller scale for SrZr0.9Y0.1O3 − α and con-
centration polarizations were excluded in (b). Porous platinum electrodes
VRA(j) − VRA(0) = ηRA + ηCA + ηOA
VRC(j) − VRC(0) = −(ηRC + ηCC + ηOC)
(8)
(9)
3
−1
were used; anode gas = moist H2 (pw = 2.3 × 10 Pa, fed at 30 mL min );
3
−1
cathode gas = moist Ar (pw = 2.3 × 10 Pa, fed at 30 mL min ).
The ohmic loss and the changes in the hydrogen pres-
sure can be evaluated by the current interrupt method and
gas chromatography, respectively, to obtain the rest reaction
polarization, which directly leads to the energy dissipation at
the electrodes.
(enthalpy change of water formation divided by 2F; F being
Faraday’s constant), so that the energy loss (reaction overpo-
tentials) of a few hundred mV is not a high value from a view
point of hydrogen separation.
In contrast, the hydrogen pump using SrZr0.9Y0.1
O
3 − α
has quite poor performance. As shown in Fig. 3(a) the
hydrogen-pumping rate deviates from the Faradaic one at
4
.2. Hydrogen-pumping performance of a cerate and a
2
zirconate
a current density as low as 20 mA/cm . Fig. 3(b) indicates
quite high overpotentials (>1 V) both at the anode and cath-
ode needed for such a low current density. Therefore, the hy-
drogen pump using SrCe0.95Yb0.05O3 − α electrolyte is much
superior in the performance of both energy efficiency and
hydrogen-pumping rate.
Fig. 3 compares hydrogen-pumping characteristics when
a cerate and a zirconate are used as the proton-conducting
electrolytes. SrCe0.95Yb0.05O3 − α [18] and SrZr0.9Y0.1O3 − α
◦
were chosen and the pumps were operated at 800 C.
In the case of SrCe0.95Yb0.05O3 − α, the evolution rate of
The above experimental results suggest that the electro-
catalytic activity of Pt for the cerate will be much higher
than that for the zirconate, and thus is responsible for the dif-
ference in their hydrogen-pumping performance. We usually
assume a triple phase boundary of gas/electrode/electrolyte
as a place where electrode reactions take place. Actually,
the electrode reaction zone will not be restricted to the
hydrogen at the cathode well agrees with Faraday’s law up to
2
around 600 mA/cm as shown in Fig. 3(a) (the reduction in
Faradaic efficiency at higher current densities is discussed in
Section 4.3). In addition, overpotentials due to the electrode
reactions are fairly low as seen in Fig. 3(b). Hydrogen has
the chemical energy of 1.35 V to form water in voltage scale

H. Matsumoto et al. / Journal of Alloys and Compounds 408–412 (2006) 456–462
459
triple phase boundary, which is mathematically one dimen-
sional, but should spread to the gas/electrode interface by
surface diffusion of ionic species on the electrode and/or
to the gas/electrolyte interface by an electronic species lo-
cally present in the electrolyte. If one assumes the similar
microstructure of porous Pt electrodes for the two cases, the
latter gas/electrolyte interface might be a reason for the large
difference in the electrode overpotentials between the combi-
nations of Pt/SrCe0.95Yb0.05O3 − α and Pt/SrZr0.9Y0.1O3 − α.
How the interfaces differ with the kind of electrolyte is not
clear. It should be investigated further how the defect equi-
libriums differ in the electrolyte facing the metal electrode
from those in the bulk property.
4
.3. Reduction in current efficiency
Fig. 4. XRD patterns of SrCe0.95Yb0.05O3 − α (upper) and SrZr0.9Y0.1O3 − α
◦
(
lower) as prepared and after treated in CO2 at 800 C for 3 h. Peaks indicated
Hydrogen-pumping rate with SrCe0.95Yb
0.05
O3 − α shown
by (ꢁ)and (×) are assigned to CeO and SrCO , respectively.
2
3
in Fig. 3(a) deviates lower than the Faradaic rate at a current
density higher than 600 mA/cm . For this phenomenon, the
partial oxide ionic conduction in the oxide electrolyte, ex-
plained in the former section, will be responsible [18,19]. By
the oxide ionic charge carrier, the following electrode reac-
tions can be assumed:
2
action in Eq. (12) to decompose into SrCO3 and CeO2.
SrCeO + CO → SrCO + CeO
3
2
3
2
(12)
From thermodynamic data [20], this reaction has a nega-
◦
tive free energy change below 1050 C at the unique activity
−
2−
of CO2. In the case of SrZr0.9Y0.1O3 − α, the XRD patters
have no changes after the CO2 treatment. If we aim to sep-
arate hydrogen from the reformed gases of hydrocarbons,
which is the most demanded kind of hydrogen separation for
mass production of hydrogen, CO2 will inevitably generate
in the H2–CO mixture due to the reaction equilibrium in Eq.
H2O + 2e → H2 + O (cathode)
H2 + O − → H2O + 2e (anode)
(10)
(11)
2
−
Water vapor is decomposed to form hydrogen and oxide
ion at the cathode. These reactions have the same number of
electrons (two electrons) per molecular hydrogen as the elec-
trode reactions in Eqs. (5) and (6) have, so that the Faraday’s
law for the evolution of hydrogen will still be maintained.
These reactions due to the oxide ionic charge carriers con-
sume water vapor at the cathode; from the change in water
vapor pressure at the cathode outlet, the oxide-ionic transport
number was estimated to be 0.1 (the rest 0.9 is attributable
to the protonic transport number) [18,19]. The decrease in
the current efficiency from unity occurs on the exhaustion of
water at the cathode. Therefore, the water vapor at the cath-
ode has a role in avoiding too reducing an atmosphere which
can cause the electronic conduction in the electrolyte. By sup-
plying sufficient water vapor to the cathode compartment, the
(13).
CO + H2O ꢀ CO2 + H2
(13)
◦
This equilibrium is even at around 800 C [20].
SrCe0.95Yb0.05O3 − α electrolyte is reactive with CO2 at its
◦
working temperature, e.g., 800 C, and thus the electrolyte
is not adequate for hydrogen separation from the reformed
gases. Zirconate-based electrolyte is only available for this
purpose. Several studies suggest that cerate–zirconate solid
solutions have durability to CO2 [21–23]. In our prelimi-
nary experiments, however, the partial substitution of Zr will
also reduce the catalytic activity of Pt electrodes and is not
hopeful solution. We are investigating the improvement of
electrode performance for zirconate-based electrolytes and
some candidate electrodes have been found; the results will
be presented elsewhere (to be submitted).
2
pumping current density higher than 1 A/cm is achievable
[19].
4
.4. Chemical stability
As explained above, SrCe0.95Yb0.05O3 − α electrolyte has
5. Hydrogen permeation membrane
excellent applicability in hydrogen pumps. However, the
electrolyte is reactive with CO2, so that the electrolyte can
be used only under the condition that CO2 can be excluded.
Fig. 4 shows the XRD patterns of SrCe0.95Yb0.05O3 − α and
5.1. Protonic–electronic mixed conductors
The hydrogen pumps described in the former section em-
ploy the HTPCs as proton-conducting electrolytes that have
negligibly low electronic conductivity under operation. Con-
versely, if we can add electronic conductivity in the proton-
SrZr0.9Y0.1O3 − α as prepared and after being exposed in
◦
1
00% CO2 at 800 C for 3 h. After the CO2 treatment,
SrCe0.95Yb0.05O3 − α obviously reacted with CO2 by the re-

4
60
H. Matsumoto et al. / Journal of Alloys and Compounds 408–412 (2006) 456–462
Fig. 5. Hydrogen permeation via protonic–electronic mixed-conducting
membrane.
conducting solids, hydrogen permeation will occur through
the materials; such mixed conductors may not usually be
counted as HTPCs since the term is referred to electrolytes,
i.e., the dominant ionic conductors. The schematic principle
of the hydrogen-permeating membrane is presented in Fig. 5.
No external electric power supply is necessary, so that the
electrodes and external electrical circuit are excluded and a
simple hydrogen separator can be constructed [24]; the mem-
braneusesthehydrogenpotentialgradientasthedrivingforce
for the hydrogen transport.
Fig. 6. Hydrogen permeation flux of BaCe0.9 − xY0.1RuxO3 − α (x = 0.05,
◦
0
.075) at 800 C as a function of the logarithm of the ratio of hydrogen
partial pressure at the hydrogen-supplying side, p(H2, supp), to that at the
permeating side, p(H , perm).
2
3
vapor at 1.9 × 10 Pa and were fed at 30 mL/min. The partial
pressures of hydrogen in the Ar sweep gas, p(H2, perm) was
determined by gas chromatography.
X-ray absorption spectroscopy (XAS) [26,27] was em-
ployed to estimate the electronic charge carrier. Since this
technique elucidates unoccupied density of states of electron
holes, unoccupied states in the valence band, can be detected.
The measurements were conducted at the revolver undulator
beamline BL-19B at the Photon Factory of the High Energy
Accelerator Organization, Tsukuba, Japan. Synchrotron radi-
ation was monochromatized using a varied-line spacing plain
grating. The XAS spectra were measured using a Si photodi-
ode. The samples were scraped in situ with a diamond file in
vacuum to obtain a clean surface. Prior to the measurements,
the specimen were either as sintered in air or annealed in
It seems there have been no accepted materials so far as
the protonic–electronic mixed conductors that allow high hy-
drogen drain. Therefore, in contrast to the conventional HT-
PCs that already have long history, the protonic–electronic
mixed conductors are not established but in quest now. We re-
port here the protonic–electronic mixed conduction occurring
in ruthenium-doped perovskites, BaCe0.9 − xY0.1RuxO3 − α
(x = 0–0.1) [25].
5
.2. Theoretical backgrounds and experimental
procedures
◦
moist 1% H2 at 800 C for 10 h to compare the electronic
The hydrogen permeation through the specimens was ex-
structure in the oxidative and reducing atmospheres.
◦
amined at 800 C using the following cell:
H2 at p(H2, supp), Pt|specimen|Pt, Ar.
(14)
5.3. Hydrogen permeability
+
2−
If one assumes proton (H ), oxide ion (O ), conduc-
tion electron (e) and electron hole (h) as ionic and electronic
charge carriers, hydrogen evolution at the permeating side
will obey the ambipolar diffusion mechanism, expressed by
Eq. (15) [25].
Fig. 6 shows the hydrogen flux through BaCe0.9−x
◦
Y0.1RuxO3 − α (x = 0.075, 0.1) at 800 C measured with the
cell in Eq. (14); hydrogen permeation was not experimentally
observed for the Ru-undoped sample (x = 0). It is notable that
the hydrogen flux is proportional to ln(p(H2, supp)/p(H2,
perm)), suggesting that the hydrogen transport occurred
in accordance with the ambipolar diffusion mechanism in
Eq. (15). It is evident that the hydrogen permeation occurs
via the ionic-electronic mixed conduction in the Ru-doped
perovskites. The maximum hydrogen flux obtained is
RT (σH
+
+ σO2− )(σe + σh)
σT
p(H2, supp)
p(H2, perm)
JH2 ≈
ln
2
4
F L
(15)
in which σj and σT are the partial conductivity of j species
and the total conductivity, respectively, and L is the thickness
of the mixed conductor; the equation assumes the equal water
vapor pressure in the two compartments and constant partial
conductivities. The ambipolar hydrogen flux is proportional
to ln(p(H2, supp)/p(H2, perm)). The test cell construction was
the same as that used for the hydrogen concentration cell mea-
surements. H2/Ar mixtures were fed to the left compartment
of cell (14) and argon gas was passed to sweep the hydrogen
evolved in the right compartment. The gases contained water
−
8
2
6.5 × 10 mol/s cm (x = 0.1), equivalent to the internally
2
short-circuited current density of 12.5 mA/cm .
As explained in previous sections, protonic and oxide-
ionic change carriers can be assumed. By observing the
change in water vapor pressure in the permeating compart-
ment, a half of the ionic conduction was estimated to be due
to the oxide ions and the other half could be attributed to pro-
◦
tons in the case of BaCe0.8Y0.1Ru0.1O3 − α at 800 C. This
fraction is quite similar to that of the base HTPC of the ac-

H. Matsumoto et al. / Journal of Alloys and Compounds 408–412 (2006) 456–462
461
tion of the valence of Ru and the conductivity dependences
on p(O2) and p(H2O), will provide a definite model for the
protonic–electronic mixed conduction in the Ru-doped per-
ovskites.
The hydrogen permeability of the materials is still low
compared to the conventional methods. For example, Pd–Ag
membranes have 10–100 times higher permeability in very
rough estimation; the difference in the mechanisms (driv-
ing forces) of hydrogen diffusion is neglected. However,
this report provides the possible mechanism for the mixed-
protonic–electronic conduction and will lead to the next find-
ing of new materials.
Acknowledgements
This study was supported by CREST of Japan Science
andTechnology(JST), IndustrialTechnologyResearchGrant
Program in 02B65001c from New Energy and Industrial
Technology Development Organization (NEDO) of Japan
and a 21st Century COE Program Grant of the International
COE of Flow Dynamics from the Ministry of Education, Cul-
ture, Sports, Science and Technology.
Fig. 7. XAS spectra of BaCe0.9 − xY0.1RuxO3 − α (x = 0, 0.075); spectra
overviews and 10 times magnified spectra around the band gap regions are
shown. The enlarged spectra in dashed lines are of the specimens as sintered
in air and those in solid lines are of the specimens after annealed in moist
◦
% H2 at 800 C for 10 h.
1
ceptor doped barium cerate [11,12]. Therefore, the doping
of Ru will provide an electronic conduction without chang-
ing the protonic/oxide-ionic fraction of conductivity in the
acceptor doped BaCeO3.
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