Development of Hydrogen Separation Ceramics Membranes for Ultra Pure Hydrogen
Production and Evaluation of Their Transport Properties
超高純度水素製造のための水素分離セラミクス膜の開発と輸送特性評価

Atsushi Unemoto, Assistant Professor, Institute of Multidisciplinary Research for Advanced
Materials, Tohoku University

1. Introduction

Hydrogen enables us to generate electricity cleanly by the combination with polymer
electrolyte fuel cells (PEFCs). Thus, hydrogen is considered as an energy carrier to the next
generation. However, CO poisoning on Pt electro-catalysts of PEFC invites a serious
irreversible degradation of the electricity generation performance. Thus, ultra pure hydrogenwith CO concentrations below 10 ppm is requested.
Possible hydrogen sources are such as the waste gas, e.g. coke oven gas (COG) etc.
and reformed gas of methane and methanol. In these cases, hydrogen needs to be recovered
selectively from the gas mixtures. Pressure swing adsorption (PSA) method is employed
conventionally for the above purpose. On the other hand, hydrogen separation membranes allow
simplified and compact system because those purify hydrogen in only one step. In order tomake use of this advantage, hydrogen separation membrane with a cost effective materials andhigh stability at higher temperatures is one of the important technological challenges to realize
commercial use of the system because it directly determines the hydrogen production/recovery
efficiencies.
This study aims to develop ceramic membranes based on the protonic conductors,
which pumps hydrogen by the current flow via the external circuit. This is because a class of themembranes possibly meets the above requirements for hydrogen separation. In the case ofhigh-temperature proton conducting zirconates, the interstitial protonic defects contribute to theprotonic conductivity. Substitution of lower valent cation into the Zr-site leads to the formationof oxygen vacancy. For example, the reaction between SrZrO3 with the metal oxides having
lower valent cation than Zr-ion such as M2O3 is expressed as,

1
3
1
M 2O 3 → M ′Zr + O ×O + V O••222－ 159 －

(1)where Kröger – Vink notation [1] was employed to expressed the defects. M ′Zr , O ×O and V O••
are M3+-ion in Zr-site, oxide-ion in their site and oxygen vacancy, respectively. Then, theoxygen vacancy accepts ambient water vapor, which leads to the formation of protonic defectsas,[ ][ ] [H 2O + V O•• + O ×O = 2OH •O and K1 p H 2O VO•• O ×O = OH •O

]

2

(2)

where OH •O and K1 represents protonic defect and the equilibrium constant.
Since the first discovery of high temperature proton conducting cerates, Sr(Ce,Y)O3,
by Iwahara et al. [2], the protonic conductivities of rare earth doped zirconates have been
investigated so far because zirconates have superior stability to cerates against carbon dioxide
and water vapor [3]. However, the ceramics suffer from the insufficient protonic conductivities.
In this study, we chose the pseudo-ternary system, LaFeO3 – SrFeO3-δ – SrZrO3. In a reducing
environment, the concentration of protonic defects is expected to increase by the increase ofoxygen vacancy concentration accompanied by the partial reduction of Fe-ion. To date,formation diagrams [4] and electrical conductivity [4-6] were investigated. However, theknowledge on the conductivity is limited to comparatively higher temperatures such as above
1273 K, where the concentration of protonic defect is expected to be small due to the desorptionof water vapor from the ceramics.
In this study, the electrical conductivities of {La0.05(1-x)Sr0.95+0.05x}(Zr0.95Fe0.05)O3-δ [or(La1-xSrxFeO3-δ)0.05(SrZrO3)0.95] (x = 0, 0.1, 0.25 and 0.5) was evaluated at temperatures below
1273 K. Electromotive force (EMF) measurements of hydrogen concentration cells inhumidified hydrogen were carried out to investigate the transport number of ionic species.
X-ray absorption spectroscopic (XAS) measurements were conducted to investigate theoxidation state of Fe in the oxide.

2. Experimental

Powders of {La0.05(1-x)Sr0.95+0.05x}(Zr0.95Fe0.05)O3-δ (x = 0, 0.1, 0.25 and 0.5) wereprepared by a conventional solid state reaction route. The starting powders, La2O3 (99.99 %,Rare Metallic Co., Ltd.), SrCO3 (99.99 %, Rare Metallic Co., Ltd.) and Fe2O3 (99.99 %,Kojundo Chemical Laboratory Co., Ltd.) and ZrO2 (TZ-0, Tosoh Corp.), were precisely
weighed in appropriate ratios and mixed in a zirconia mortar with a zirconia pestle in ethanol.
The mixtures were then calcined at 1473 – 1623 K for 5 – 10 hours in air. The calcined powders－ 160 －

were crushed in a planetary ball mill (Frisch, P-7) at 300 rpm for 3 hours. The calcination andthe crushing were repeated three times. X-ray diffraction (XRD, Brukar AXS, M18X-CE)
measurements were conducted to determine if the resultant products of the solid state reaction
were a single phase. After sieving, the powders were formed into disks, hydrostatically pressed
at 200 MPa, and sintered at 1873 – 2023 K for 5 hours for the electrical conductivity
measurements.
Platinum paste was painted on both surfaces of the disks as electrodes for theelectrical conductivity measurements with a two-probe ac technique in hydrogen and oxygen
containing environments. In order to identify if the proton is a carrier, the isotope effect ofhydrogen and deuteron on the electrical conductivity was examined. The individual disk wasconnected to the sets of and electroanalytical systems consisting of Electrochemical Interface
SI1287 and Impedance/Gain-Phase Analyzer SI1260 (Solartron Analytical). EMF (electron
motive force) measurements of hydrogen concentration cells under constant partial pressure ofwater vapor were carried out to determine the transport number of ionic species.
The oxidation states of the transition metal, Fe, of the specimens were investigated
by

X-ray

absorption

spectroscopic(XAS)

measurements.

The

powderofthe{La0.045Sr0.955}(Zr0.95Fe0.05)O3-δ was annealed in log( p O2 / atm) = -0.68 (in air), -15.73 (in1.0%H2 – 1.9%H2O) and -19.73 (in 1.0%H2 – 1.9%H2O) at 1173 K for 5 hours, then, quenched
to room temperature. The powders were mixed with boron nitride (BN, > 99 %, Kojundo
Chemical Laboratory Co., Ltd.) and then pressed into disks. A pressed body of a mixture of
Fe2O3 and BN was also prepared as a reference. Spectra of the X-ray absorption near-edge
structure (XANES) of Fe K-edge were collected via a transmission mode at the beam line,
BL-7C, of the synchrotron radiation facility, Photon Factory (High Energy Accelerator Research
Organization, Institute of Materials Structure Science of Japan).

3. Results and Discussion
3.1 Specimens

XRD patterns of the resultant powders and the annealed powders in hydrogencontaining environments well matched with the patterns of the literature of SrZrO3 with an
orthorhombic structure [7]. No extra peak was observed for all powders. This suggests that thepowders for the following measurements surely consisted of a single phase.

－ 161 －

3.2 Electrical conductivity in oxygen
Figure

1

(a)

shows

the

{La0.05(1-x)Sr0.95+0.05x}(Zr0.95Fe0.05)O3-δ (x = 0, 0.1,
0.25 and 0.5) in unhumidified 1.0%O2, 1.0%O2– 1.9%H2O and 1.0%O2 – 1.9%D2O as afunction of inverse temperature. For x = 0.1,
0.25 and 0.5, the electrical conductivities exhibit
higher values than non-doped SrZrO3 [4, 6].
This suggests that the electrical conductivity issurely enhanced by co-doping of La and Fe in
the Sr-site and Zr-site, respectively. However,
the electrical conductivity for x = 0 was slightly
smaller than that of non-doped SrZrO3 obtainedin air [4, 6]. This may be caused by p-typeconduction in which the defect may be inducedby the different valence cation impurity [6].
Differences between the electricalconductivities in unhumidifed 1.0%O2 and thosein 1.0%O2 – 1.9%H2O were not remarkable for x= 0 and 0.1 in the entire temperature ranges andfor x = 0.25 at higher temperatures. The
electrical conductivity in humidified oxygen was
higher for x = 0.25 at lower temperatures and x= 0.5 than those in unhumidified oxygen. In

Fig. 1 (a) Electrical conductivities of{La0.05(1-x)Sr0.95+0.05x}(Zr0.95Fe0.05)O3-δ (x =0, 0.1, 0.25 and 0.5), SrZr0.99Fe0.01O3-δ [9]and SrZr0.9Y0.1O3-δ [12]. (b) Isotope effectof H/D for x = 0.25 and 0.5 as functionsof inverse temperature.

order to determine if proton conductivity
contributes to the total in the oxides for x = 0.25 and 0.5, the isotope effects of hydrogen anddeuteron on the electrical conductivities were examined. Figure 1 (b) shows the ratio of theelectrical conductivities obtained in 1.0%O2 – 1.9%H2O and that in 1.0%O2 – 1.9%D2O. As
expected in the case which the contribution of protonic conduction is significant, a remarkableisotope effect was observed for x = 0.5 at entire temperatures and slightly for x = 0.25 at lower
temperatures below 973 K. It was found that the difference between the electrical conductivities
in 1.0%O2 – 1.9%H2O and 1.0%O2 – 1.9%D2O is 1.77 – 1.03 at 1223 – 622 K for x = 0.5. At
temperatures below 873 K, the ratio exceeded

2 , which is expected by the classical theory－ 162 －

[8]. A possible explanation is because of the difference of zero-point energy, namely,
semi-classical theory, which resulted in the difference in activation energies [8]. In the case of(La0.5Sr0.5FeO3-δ)0.05(SrZrO3)0.95, the activation energies of protonic and deuteron ion conduction
are 0.79 and 0.82 eV, respectively. This difference, 0.30 eV, is in the range of the expected value
by semi-classical theory. This suggests that proton is carrier for x is above 0.25 in oxygen
containing environment.

3.3 Electrical conductivity of (La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ as a function of oxygen partialpressure

Figure 2 shows the electrical conductivities of (La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ (x = 0.1)
and SrZr0.99Fe0.01O3-δ [9] as a function of oxygen partial pressure in H2 – 1.9%H2O, in D2 –
1.9%D2O and in O2 – 1.9%H2O at 973 – 1273 K. For (La0.075Sr0.925)(Zr0.9Fe0.1)O3-δ (x = 0.25), theelectrical conductivity degraded gradually in humidified hydrogen. Phase separation mightoccur with the reduction of Fe in the reducing atmosphere for x is above 0.1. Thus, thefollowing evaluation of the electrical conductivity in humidified hydrogen was demonstrated
only for (La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ. As seen in Fig. 2, the electrical conductivity increased
by increasing oxygen partialpressure. Among the possible
carriers, such as electron,
electron hole, oxide ion andproton, only electron holecanbeincreased

increasing

oxygenbypartialpressure as will be discussed
below.

Thus,

carrier

of

the

the

major

oxide

is

considered to be electron
hole in oxygen containing
environment.
Inhydrogen,ahumidified

Fig. 2 Total conductivity of (La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ

significant

and SrZr0.99Fe0.01O3-δ [9].

－ 163 －

isotope effect of hydrogen and deuteron on theelectricalconductivity

was(La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ

observed

for

as

as

well

SrZr0.99Fe0.01O3-δ [9]. This suggests that thecontribution of the protonic conductivity issignificant for both oxides in humidifiedhydrogen.

Theelectricalconductivityof(La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ was smaller thanthat of SrZr0.99Fe0.01O3-δ by 1 – 1.5 orders ofmagnitude at 1173 K. Oxygen partial pressure
dependence of the electrical conductivities
between the above oxides is different between
the above oxides. The electrical conductivity of(La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ was increased bydecreasing oxygen partial pressure while that ofFig. 3 Normalized Fe K-edge XANES
spectra of Fe2O3, SrZr0.99Fe0.01O3-δ [9]and (La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ.

SrZr0.99Fe0.01O3-δ was independent of oxygen
partial pressure. These results suggest that the oxidation state of Fe in the above oxides aredifferent.
In order to examine the difference of the oxidation state, XANES spectra were takenusing the powders annealed in various oxygen partial pressures at 1173 K for(La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ and SrZr0.99Fe0.01O3-δ [9] because XANES spectra sensitively
reflect the electronic state of the components in oxides. The spectra shift toward higher andlower energies when Fe is oxidized and reduced respectively. Figure 3 compares the Fe K-edge
XANES spectra of (La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ and SrZr0.99Fe0.01O3-δ [9], which were annealed
in log( p O2 / atm) = -0.68, -15.73 and -19.73 at 1173 K and then quenched to room temperature.
The spectrum of Fe2O3 powder is also shown in the figure. It was found that the spectra of(La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ obviously shifted toward lower energy as the annealing oxygen
partial pressure decreased contrary to SrZr0.99Fe0.01O3-δ. Considering the above results, it can beconcluded that Fe in the former oxide is in the mixed valence state of Fe2+ and Fe3+ while theone in the latter oxide is fixed to be Fe3+ in the oxygen partial pressure range investigated. Thus,
oxygen vacancy in SrZr0.99Fe0.01O3-δ is introduced via the eq. (1). On the other hand, for(La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ, the oxygen vacancy is introduced by the partial reduction ofiron-ion. The equilibrium of the oxide with oxygen is expressed as,－ 164 －

1

1VO•• + O 2 +Fe′Zr =O×O +Fe×Zr +h• and K 2 p O2 2 [V O•• ][Fe′Zr ] = [O ×O ][Fe ×Zr ][h • ] (3)2•where h , Fe′Zr and Fe×Zr are electron hole, Fe3+ and Fe4+ in Zr-site, respectively. K2
represents the equilibrium constant. The Fe3+ may experience disproportionation into Fe2+ andFe4+ [10,11]. Thus, we have,

2Fe′Zr = Fe′Zr′ + Fe ×Zr and K 3 [Fe′Zr ] = [Fe′Zr′ ][Fe ×Zr ]2(4)

where K3 is the equilibrium constant. When the concentration of Fe ions is small such as 0.05,
the hopping distance is too long and band conduction becomes predominant. The internalvalence of Fe changes due to the trapping of electron hole. One expects,

Fe×Zr = Fe′Zr + h• and K 4 [Fe ×Zr ] = [Fe′Zr ][h • ](5)where K4 is the equilibrium constant. The equilibrium of electronic defects,

e′ + h • = null and K 5 = [e′][h • ](6)where K5 is the equilibrium constant. The charge
neutrality condition provides,[La ] + 2[V ] + [h ]
•
Sr

••
O

•

(7)

= [Fe′Zr ] + 2[Fe′Zr′ ] + [e′]where

[La ] is La
•
Sr

3+

in Sr2+-site. The sitebalances of O-site and Fe-site give,[O ]+ [V ] = 3
×
O

••
O

(8)

[Fe′Zr′ ] + [Fe′Zr ] + [Fe ] = 0.05
×
Zr

(9)

Figure 4 (a) shows the Brouwer diagram, whichenable to understand the defect concentrations as
a function of oxygen partial pressure, of(La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ assuming K5 <<K3 << K4. In addition, it was also assumed thatthe concentration of protonic defect is smallenough compared to the other positive defects,
i.e.

[OH ] << [La ],[V ],[h ]
•
O

•
Sr

••
O

•

for

Fig. 4 (a) Brouwer diagram and (b)simulated

simplicity.
The partial conductivity of species i,

conductivitiesasfunction of oxygen partial pressurefor (La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ.

－ 165 －aσ i (i = e, h, O2- and H+), is expressed by,
σ i = z i em i ci

(10)where zi, mi and ci are charge number, mobility and concentration of species i and e is theelementary charge. When the partial conductivities are assumed to be linear functions of thecarrier concentrations, those can be predicted based on the Brouwer diagram constructed as inFig. 4 (b). Since the contribution of electronic conductivity in the oxide is negligibly small as
will be discussed in Section 3.4, thus, the electronic conductivity was not drawn in the figure.
Figure 4 (b) shows the simulated conductivities as a function of oxygen partial pressure,assuming m O2− << m H+ and σ e << σ h , σ O2− ,σ H+ .

The simulated oxygen partial pressure

dependence of the total conductivity is similar to the experimental one as shown in Fig. 2. Thus,
the ionic conductivity is predominant in hydrogen containing environment while the electron
hole is in oxygen. In this gap of these oxygen partial pressures, the carriers are the mixture ofelectron hole, oxide ion and protons.

3.4 Evaluation of emf in humidified hydrogen

In order to determine the contribution of partial ionic conductivities in(La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ, emf measurements were conducted by using hydrogenconcentration cells. The theoretical emf value is obtained by,t O2− + t H+ = −4F ∂ERT ∂ ln p O2

(11)

where t O2− and t H+ , F, R, T, E and p O2 are transport numbers of oxide ion and proton, theFaraday constant, the gas constant, temperature, emf and oxygen partial pressure, respectively.
From

the

neighboring

points

of

the

(

E

as

a

function

of

p O2

by

setting)∂E ∂ ln p O = ΔE Δ ln p O = (E n +1 − E n ) ln p O ,n +1 − ln p O ,n , the transport number of
2

2

2

2

ionic species is evaluated. In humidified environment, it was found that the transport number ofionic species exceeded 0.94 in the temperature range of 973 – 1273 K. The results suggest thatthe contribution of electronic conductivity is small enough compared to the other species.

3.5 Separation of partial protonic conductivity from the total conductivity

Total conductivity of the oxide is expressed by the sum of partial conductivity－ 166 －

contributions,σ = σh +σO +σH
2-

(12)

+

When the concentration of protonic defectis sufficiently smaller than those of
positive defects, i.e. oxygen vacancy and
La in Sr-site, and the partial conductivitiesarefunctions

of

only

carrier

concentrations, eq. (12) is rewritten as eq.
(13) based on eqs. (2) and (10),1σ = σ h + σ O + σ Ho p H2
2-

+

2

Fig.

(13)

where σ Ho+ and p H2 are a constant and

5

Total

conductivityof(La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ as a function ofsquare root of water vapor pressure at 1173 K.

hydrogen partial pressure, respectively.
Figure 5 shows the total conductivity of (La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ typically at
1173 K as a function of square root of water vapor pressure in log( p O2 / atm) = -13.67 ~ -19.70.
It was found that the electrical conductivity is a linear function of the square root of water vapor
pressure at temperature shown in Fig. 5. Subtracting the segment values from the total
conductivities separates the partial protonic
conductivity based on eq. (13). The segment
represents the sum of partial conductivity
contributions of oxide ion and electron hole,which was shown in Fig. 2.
By the same analysis, the partial
protonic conductivity could be separated at 1073
– 1273 K. On the other hand, at 973 K, theelectrical conductivity was almost independentof water vapor pressure, suggesting that the
separation of the protonic conductivity using eq.
(13) is inappropriate at lower temperatures.
Fig. 6 Transport numbers of protons in
3.6 Transport numbers of protons in humidifiedhydrogenvariousSrZrO3-basedoxides at 1173 K.

－ 167 －

perovskite-type

Figure 6 compares the transport numbers of protons in various SrZrO3-based
provskite-type oxides, evaluated by Eq. (13), at 1173 K. For (La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ, it
was found that the transport number of proton depended on the oxygen partial pressure. The
transport number of proton increased as the oxygen partial pressure decreased. On the other
hand, the transport numbers of protons in SrZr0.99Fe0.01O3-δ [9] and SrZr0.9Y0.1O3-δ [12,13] areindependent of the oxygen partial pressure. Thus, it can be concluded that the cause of theoxygen partial pressure dependence of the transport numbers of protons essentially comes fromthe variation of the oxidation state of Fe-ion. For (La0.045Sr0.955)(Zr0.95Fe0.05)O3-δ, in a lower
oxygen partial pressure such as log( p O2 / atm) = -19.70, the transport numbers of protons were
found to be 0.7 – 0.75, which is comparable with those of SrZr0.99Fe0.01O3-δ [9] and is double of
SrZr0.9Y0.1O3-δ [12,13].

4. Conclusions

The defect structure of {La0.05(1-x)Sr0.95+0.05x}(Zr0.95Fe0.05)O3-δ (x = 0, 0.1, 0.25 and 0.5)
was analyzed based on the electrical conductivity, emf and XAS measurements. In oxygen gas,carrier is electron hole for x is below 0.1 while mixture of electron hole, oxide ion and proton
for x is above 0.25. In humidified hydrogen for x = 0.1, the total conductivity increased bydecreasing the oxygen partial pressure due to a variation of oxygen nonstoichiometry caused by
the partial reduction of Fe from 3+ to 2+. The transport number of proton was found to increaseby decreasing the oxygen partial pressure. This study revealed that the oxides exhibit
considerable protonic conductivity. Thus, the ceramic membrane might be applicable to use inhydrogen separation membranes with electrochemical hydrogen pumping.

Acknowledgements

This study was supported by the Technical Research Aid Projects 2010 from the JFE
21st Century Foundation.

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－ 169 －