inorganicper 59.inorganicperovskiteoxides...inorganicper 1405 parte|59 59.inorganicperovskiteoxides...

Inorganic Per1405

PartE|59

59. Inorganic Perovskite Oxides

Tatsumi Ishihara

Crystal structure and important functions ofinorganic perovskite oxides are introduced. Pe-rovskite oxides comprise large families amongthe structures of oxide compounds, and sev-eral perovskite-related structures are currentlyrecognized. Typical structures (ABO3) consist oflarge-sized 12-coordinated cations at the A-siteand small-sized 6-coordinated cations at theB-site. Several complex halides and sulfides andmany complex oxides have a perovskite structure.From a variety of compositions and structures,a variety of functions are observed in perovskiteoxides. In particular high electronic conductivity,which is at a similar level as metal, and surfaceactivity to oxygen dissociation, are highly attrac-tive in this oxide. Perovskite oxide is now widelyused for solid oxide fuel cells.

59.1 Typical Propertiesof Perovskite Oxides .......................... 1409

59.1.1 Dielectric Properties ........................... 140959.1.2 Electrical Conductivity

and Superconductivity ....................... 140959.1.3 Catalytic Activity ................................ 1410

59.2 Photocatalytic Activity ....................... 1411

59.3 Application for Solid OxideFuel Cells (SOFCs) ............................... 1413

59.3.1 Cathode ............................................ 141359.3.2 Anode .............................................. 141459.3.3 Electrolyte. ........................................ 141559.3.4 Interconnector .................................. 1417

59.4 Oxygen Separating Membrane ........... 1418

59.5 Summary .......................................... 1419

References ................................................... 1420

Oxide groups consisting of two or more differentcations are called complex or mixed oxides, and manytypes of crystal structures are known. In some specialcases, oxides consisting of single cations in differentoxidation states are also classified as mixed oxides.For example, in Eu3O4, the mixed oxide consists ofEu(III) and Eu(II) in 6- or 8-coordination respectively.However, the most typical structure of a mixed ox-ide consists simply of two or more different cationswith different oxidation states, ionic radii, and coor-dination numbers. This diversity, which comes fromthe complexity of these structures, results in a largernumber of different properties as compared to those ofsimple oxides. One of the most well known and im-portant complex oxide structures is the spinel structure(AB2O4), which shows important magnetic properties.The structure of such oxides displays a most interestingcomplexity. Since the size of the A and B ions in thisstructure is close, oxides of this type are typical exam-ples of the versatility of mixed oxides.

Another important structure of mixed oxide is pe-rovskite and a variety of related structures are classifiedas this oxide. The typical chemical formula of the pe-rovskite structure is ABO3, where A and B denotes

two different cations. The ilmenite structure has thesame composition as the perovskite one, i. e., ABO3;however, A and B in this structure are cations of ap-proximately the same size, which occupy an octahedralsite. Therefore, in spite of the fact that they share thesame general chemical formula, structures classified asilumenite- or ilmenite-related structures (e.g., LiSbO3)are different from perovskite.

Perovskite oxides comprise large families amongthe structures of oxide compounds, and severalperovskite-related structures are currently recognized.Typical structures consist of large-sized 12-coordinatedcations at the A-site and small-sized 6-coordinatedcations at the B-site. Several complex halides and sul-fides and many complex oxides have a perovskite struc-ture. In particular, (Mg,Fe)SiO3 or CaSiO3 is thought tobe the predominant compound in the geosphere [59.1,2]. Perovskite compounds with different combinationsof charged cations in the A and B-sites, for example1C 5, 2C 4 and 3C 3, have been discovered. Evenmore complex combinations are observed, such asPb.B0

1=2B00

1=2/O3, where B0 D Sc, Fe and B00 D Nb, Ta,or La.B0

1=2B00

1=2/O3, where B0 D Ni, Mg, etc. and B00 DRu(IV) or Ir(IV). In addition, many ABO3 compounds

© Springer International Publishing AG 2017S. Kasap, P. Capper (Eds.), Springer Handbook of Electronic and Photonic Materials, DOI 10.1007/978-3-319-48933-9_59

PartE|59

1406 Part E Novel Materials and Selected Applications

O

B ion

A ion

Fig. 59.1 Ideal cubic perovskite structure

crystallize in polymorphic structures, which show onlya small distortion from the most symmetrical form ofthe perovskite structure.

The ideal structure of perovskite, which is illus-trated in Fig. 59.1, is a cubic lattice. Although fewcompounds have this ideal cubic structure, many oxideshave slightly distorted variants with lower symmetry(e.g., hexagonal or orthorhombic). Furthermore, eventhough some compounds have ideal cubic structure,many oxides display slightly distorted variants withlower symmetry. Several examples of perovskite oxidesare listed in Table 59.1, where it is clear that a largenumber of perovskite oxides have a rombohedral lat-tice. Additionally, in many compounds a large extent ofoxygen or cation deficiency has been observed. Due tothe large lattice energy, many compounds are classifiedas perovskite oxides in spite of the large cation and/oroxygen deficiencies. There are various types of distor-tions in the perovskite structure that are strongly relatedto their properties, in particular their ferromagnetic orferroelectricity.

In order to understand the deviations from the idealcubic structure, these ABO3 oxides are first regarded aspurely ionic crystals. In the case of the ideal structure,the following relationship between the radii of the A, B,and O2� ions holds true

rA C rO Dp2.rB C rO/ :

Therefore, the deviation from the ideal structure in per-ovskite oxides can be expressed through the followingso-called tolerance factor t

t D .rA C rO/p2.rB C rO/

Table 59.1 Typical peroskite compound

Compound Lattice parameter (Å)

a b cCubic structureKTaO3 3:989NaTaO3 3:929NaNbO3 3:949BaMnO3 4:040BaZrO3 4:193SrTiO3 3:904KMnF3 4:189KFeF3 4:121

Tetragonal structureBiAlO3 7:61 7:94PbSnO3 7:86 8:13BaTiO3 3:994 4:038PdTiO3 3:899 4:153TlMnCl3 5:02 5:04

LaAlO3 typeLaAlO3 5:357 ˛ D 60ı060

LaNiO3 5:461 ˛ D 60ı050

BiFeO3 5:632 ˛ D 60ı060

KNbO3 4:016 ˛ D 60ı060

GdFeO3 typeGdFeO3 5:346 5:616 7:668YFeO3 5:283 5:592 7:603NdGaO3 5:426 5:502 7:706CaTiO3 5:381 5:443 7:645NaMgF3 5:363 5:503 7:676

In perovskite-type compounds, the value of t lies be-tween approximately 0:80 and 1:10. It is noted that theoxides with the lower t values crystallize in the ilmenitestructure, which is a polymorph of the perovskite struc-ture. It seems superfluous to say that for the ideal cubicstructure, the value of t is close to 1 or at least higherthan 0:89. Figure 59.2 shows the crystal groups forA2CB4CO3 and A3CB3CO3 combinations, which arerelated to deviation from the ideal structure [59.3]. Asthe value of t decreases, the structure of the unit latticeis shifted from cubic to triclinic due to the increased dis-tortions. Figure 59.3 shows chemical elements that canbe accommodated within the perovskite structure. It isevident that almost all elements except for noble gasescan occupy either A or B lattice positions in the per-ovskite structure, including dopants. The stability andthe crystal group is mainly determined by the ratio ofthe ionic radii of the A and B cations. Indeed, the struc-ture is dependent not only on the size but also on thenature of the A and B atoms. For example, AMnO3

compounds crystallize in the perovskite structure whenA cation is La or Ce–Dy, whereas a new hexagonalstructure with 5- and 7-coordination of Mn and A re-spectively, is formed when A D Ho�Lu or Y if A D La

Inorganic Perovskite Oxides Inorganic Perovskite Oxides 1407Part

E|59

0.950.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05

1.00

1.05

1.10

1.15

1.20

1.25

1.30

1.35

0.500.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30

0.60

0.70

0.80

0.90

1.00

1.10

1.20

Cd2+Ca2+

Sr2+

Eu2+

Pb2+

Ba2+

Mn4+

V4+Ti4+

Sn4+Hf4+

Zr4+ Ce4+U4+

Th4+Al3+

Ga3+Cr3+

Fe3+ Ti3+Sc3+

In3+Y3+

Sm3+Nd3+

Ce3+La3+

Al3+

Ca3+ Cr3+Fe3+

Sc3+In3+

Y3+

Sm3+Nd3+Ce3+

La3+

Ionic size (Å)

Ionic size (Å) Ionic size (Å)

Ionic size (Å)

A2+B4O3 A3+B3+O3a) b)

Cubic Tetragonal

Pseudo cubic

Pseudo cubic

Orthorhombic

Rhombohedral

Rhombo-hedral

Perovskite

Orthorhombic

Tl2O3 type

La2O3type

Corundum

SrVO3

c–a > 1

c–a < 1

Fig. 59.2a,b The effect of ionic size of A- and B-site cations on the observed distortions of the perovskite structure;(a) A2CB4CO3 case (b) A3CB3CO3 case

H

Li Be

Na

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Al Si P S Cl Ar

B C N O F Ne

He

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba La

Fr Ra Ac

Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Mg

IA

IIA

IIIA IVA VA VIAVIIA IB IIB

IIIB IVB VB VIBVIIB

0

VIII

7

6

5

4

3

2

1 A-site cation

B-site cation

Fig. 59.3 Chemical elements that canoccupy cation sites in the perovskitestructure

or Ce–Dy [59.4]. Here, the attention should be paid tothe nature of the B atom, where the nature of the bond ishighly covalent, and therefore the coordination numberis lower than 6. The typical example of this type struc-ture is BaGeO3. In spite of the t value close to t D 1,i. e., ideal ionic size combination, BaGeO3 crystallizesnot in the perovskite structure but in the silicate-relatedone. This is attributable to the fact that the preferredcoordination number of Ge is 4. On the other hand,due to the progress in high pressure technology, thesynthesis of new Ge-based perovskite oxides has beenreported [59.5]. Since the coordination number of Geincreases with the pressure, the perovskite structureswith higher coordination numbers are preferred, anda typical example of this is CaGeO3. Another groupof interesting perovskite compounds is the oxynitrides,i. e., LaWO3�xNx, LaTaO2N, etc. Therefore, the valueof t, which is determined by the ionic size, is an im-

portant index for the stability of perovskite structures;however, the contribution of the chemical nature, suchas coordinating number of the constituent elements,needs to be taken into account.

The formation of superstructures in the perovskitesis discussed next. If a B-site cation is progressively re-placed by a dopant, a large difference in ionic radiitends to lead to the formation of the superstructuresrather than random arrangements of the two kinds ofions. The typical case of this is Ba2CaWO6, which isregarded as Ba2.CaW/O6. Similarly, in the compoundswith the general formula Ba3MTa2O9, there is randomdistribution of M and Ta ions in the octahedral posi-tions when M is Fe, Co, Ni, Zn, or Ca, whereas theformation of the superstructure with a hexagonal latticeis observed in Ba3SrTa2O9. Another interesting type ofsuperstructure observed in the perovskite is the order-ing of cation vacancies located on A-sites: for example

PartE|59


A-sitedeficient

O

B

163°

B

1.89Å

1.98Å

2.07Å

A

A

Fig. 59.4 Structure of LaNb3O9, an A-site-deficient pe-rovskite oxide

A ion

B ion

O ion

Perovskite

Rock salt

Perovskite

Rock salt

Perovskite

Fig. 59.5 K2NiF4 structure, a perovskite-related structure

A

B

O

Perovskite

Rock salt

Perovskite

Fig. 59.6 Ruddelsden–Popper structure, another type ofperovskite-related structure

MNb3O9 (M D La, Ce, Pr, Nb) and MTa3O9 (M D La,Ce, Pr, Nd, Sm, Gd, Dy, Ho, Y, Er). In these oxides,there is an octahedral framework of the ReO3 typewith incomplete occupancy of the 12-fold coordinatedA-sites. Figure 59.4 shows the structure of LaNb3O9.The B-sites of the perovskite structure are occupied byNb ion and two-thirds of the A-sites remain vacant.

Other typical polymorphs of the perovskite struc-ture are Brownmillalite (A2B2O5) and K2NiF4 struc-tures. Brownmillalite (A2B2O5) is an oxygen-deficienttype of perovskite where oxygen vacancy is ordered.The unit cell contains BO6 and BO4 units in an orderedarrangement. Due to the oxygen deficiency, the coor-dination number of A-site cations decreases to 8. Thelattice parameter of the Brownmillalite structure relatesto cubic lattice parameter (ap) of the ideal perovskite asa D b D p

2ap, c D 4ap. Cu-based oxides or Ni-basedoxides tend to adopt these oxygen-deficient structuresbecause of the large amount of oxygen defects.

Inorganic Perovskite Oxides 59.1 Typical Propertiesof Perovskite Oxides 1409Part

E|59.1

A combination of ordered B-sites and oxygen de-fects is seen in K2NiF4 structures, which are well-known as they show superconducting properties. TheK2NiF4 structures consist of two units; a KNiF3 per-ovskite unit and a KF rock salt unit (Fig. 59.5), whichare connected in series along the c axis. Since the rocksalt structure is embedded into the c axis direction,the K2NiF4 compound shows strong two-dimensionalproperties. Based on the intergrowth of the differentnumber of KNiF3 and KF units, there are many struc-tures called Ruddelsden–Popper compounds with thegeneral formula .ABO3/nAO (Fig. 59.6), i. e., Sr3Ti2O7

(n D 2), Sr4Ti3O10 (n D 3). It is interesting to compare

the isostructural Sr2TiO4 or Ca2MnO4 with SrTiO3 orCaMnO3, which crystallize in the perovskite structures.Two different A cations forming the perovskite and therock salt units are also possible, and LaO � nSrFeO3 isthe typical example of this arrangement. Another inter-esting variant of these K2NiF4 structures is when twodifferent anions occupy the two building blocks exclu-sively, i. e., SrFeO3 �SrF or KNbO3 �KF. In any case, itis evident that perovskite oxides comprise a large familyof oxides. As a result, a variety of crystal structures andproperties is expected in these compounds. For furtherdetailed discussion on the perovskite-related oxides, thereader is referred to references [59.5–8].

59.1 Typical Properties of Perovskite Oxides

Due to the variety of structures and chemical com-positions, perovskite oxides exhibit a large varietyof properties. The well-known properties of the per-ovskite oxides are ferroelectricity in BaTiO3-based ox-ides and superconductivity in Ba2YCu3O7 etc. In addi-tion to these well-known properties, several perovskiteoxides exhibit good electrical conductivity, which isclose to those of metals, ionic conductivity as wellas mixed ionic and electronic conductivity. Based onthese variations in the electrical conducting property,perovskite oxides are chosen as the components forsolid oxide fuel cells (SOFCs). It is also well knownthat several perovskite oxides exhibit high catalyticactivity with respect to various reactions, in partic-ular oxidation reactions [59.9]. Table 59.2 providesexamples of the typical properties of perovskite ox-ides. In this section, several typical properties of theperovskite oxides, namely ferroelectricity, magnetism,superconductivity, and catalytic activity, are brieflyoverviewed.

59.1.1 Dielectric Properties

Ferroelectricity, piezoelectricity, electrostriction, andpyroelectricity are special properties inherent to dielec-tric materials, and are important properties of elec-troceramics. The most well-known property of per-ovskite oxides is ferroelectric behavior, where BaTiO3,PdZrO3, and their doped compounds are representativeexamples. The study of the ferroelectricity in BaTiO3

has long history, and many detailed reviews have beenpublished. Furthermore, since the ferroelectric behaviorof BaTiO3 has a strong relationship with the crys-tal structure, detailed studies of the crystal structurehave been reported for BaTiO3. BaTiO3 undergoesmainly three phase transformations from, i. e., mono-

clinic to tetragonal and cubic as the temperature in-creases. Above 303K, BaTiO3 crystallizes in the cubicperovskite structure, which does not show ferroelec-tric behavior. The high dielectric constant observed inBaTiO3 can be explained by the basis of the anisotropyof the crystal structure.

59.1.2 Electrical Conductivityand Superconductivity

One of the most well-known properties of perovskiteoxides is superconductivity. In 1984, superconductiv-ity was first reported by Bednorz and Müller in La–Ba–Cu–O perovskite oxide [59.10]. After their report,much attention has been paid to new types of high-temperature oxide superconductors, mainly Cu-basedoxides. As a result, several superconducting oxides withdifferent A-site cations have been discovered. However,the presence of Cu on the B-site is found to be essential

Table 59.2 Typical properties of perovskite oxides

Typical property Typical compoundFerromagnetism BaTiO3, PdTiO3

Piezoelectricity Pb.Zr;Ti/O3, .Bi;Na/TiO3

Electrical conductivity ReO3, SrFeO3, LaCoO3,LaNiO3, LaCrO3

Superconductivity La0:9Sr0:1CuO3,YBa2Cu3O7,HgBa2Ca2Cu2O8

Ion conductivity La.Ca/AlO3, CaTiO3,La.Sr/Ga.Mg/O3, BaZrO3,SrZrO3, BaCeO3

Magnetism LaMnO3, LaFeO3,La2NiMnO6

Catalytic properties LaCoO3, LaMnO3, BaCuO3

Electrode materials La0:6Sr0:4CoO3,La0:8Ca0:2MnO3

PartE|59.1


for superconductivity to occur. High-temperature ox-ide superconductors of the YBa2Cu3O7 system [59.11]and the Bi2Sr2Ca2Cu3O10 system [59.12] were re-ported in 1987 and 1988 respectively, and currentlythe critical temperature of the superconducting tran-sition (Tc) has been further increased to 130�155Kin the HgBa2Ca2Cu3O8Cı system [59.13]. Since allhigh-temperature superconducting oxides are cuprites(Cu-based oxides), superconductivity is clearly relatedto the Cu–O layers. The critical temperature for su-perconductivity, Tc, is related to the number of Cu–Olayers in the crystal structure:

1. Cu–O layer Tc � 30K2. Cu–O layers Tc � 90K3. Cu–O layers Tc � 110K4. Cu–O layers Tc � 120K.

It is expected that further increase in the number ofCu–O layers may result in the higher Tc values. How-ever, due to the low chemical stability, synthesis of fiveor more Cu–O layered compounds has not been suc-cessful so far. YBa2Cu3O7 is one of the most importantsuperconductor systems with high Tc, and detailed stud-ies of its crystal structure have been performed. Also,the content of oxygen nonstoichiometry is an impor-tant factor for high Tc. When the value of d is smallerthan 0:5, YBa2Cu3O7 crystallizes in an orthorhombicstructure, which is superconductive, while for d > 0:5,YBa2Cu3O7 has a tetragonal structure, which does notexhibit superconductivity.

In addition to superconductivity, there are many per-ovskite oxides showing high electronic conductivity,which is close to those of metals like Cu. The typical ex-amples of such perovskite oxides are LaCoO3, LaFeO3,and LaMnO3, which are now commonly used as cath-odes in SOFCs. These perovskite oxides show superiorhole conductivity, which is as high as � D 100S=cmoriginated from excess oxygen [59.14, 15]. Doping ofaliovalent cations on the A-site is also highly effectivein enhancing the electrical conductivity due to the in-creased number of mobile charge carriers generated bythe charge compensation.

59.1.3 Catalytic Activity

Because of the variety of component elements and thehigh chemical stability, perovskite oxides have alsobeen extensively studied as catalysts for various reac-tions. Two types of research trends clearly emergedfrom the above reasons. The objective of the firstone is the development of the oxidation catalysts oroxygen-activated catalysts as an alternative to catalystscontaining precious metals. The second trend regards

perovskite as a model for active sites. The stabilityof the perovskite structure allows preparation of com-pounds with unusual valence states of elements ora high extent of oxygen deficiency. It is also noted thatthe high catalytic activity of perovskite oxides is basedpartially on the high surface activity to oxygen reduc-tion or oxygen activation due to the large number ofoxygen vacancies presented.

Among the various catalytic reactions studied, theones applicable to environmental catalysis (e.g., auto-mobile exhaust gas cleaning catalysts) attract particularattention. Initially, it was reported that perovskite ox-ide consisting of Cu, Co, Mn or Fe exhibited superioractivity to NO direct decomposition at higher tempera-ture [59.16–18]. The direct NO decomposition reaction(2NO D N2 CO2) is one of the dream reactions in thecatalysis field. In this reaction, the ease in the removalof surface oxygen as a product of the reaction playsan important role, and due to the facility of oxygendeficiency present, perovskite oxides are active with re-spect to this reaction at high temperatures. It is pointedout that the doping is highly effective in enhancingNO decomposition activity. Under an oxygen enrichedatmosphere (up to 5%), a relatively high NO decom-position activity was reported for Ba.La/Mn.Mg/O3

perovskite [59.19].Recently, another interesting application of per-

ovskite oxides as an automobile catalyst has been re-ported, namely the so-called intelligent catalyst [59.20].Up to now, three-way Pd–Rh–Pt catalysts have beenwidely used for the purpose of removal of NO, CO,and uncombusted hydrocarbons. In order to decreasethe amount of precious metals, the catalyst consist-ing of fine particles with high surface-to-volume ratiois required. However, these fine particles are not sta-ble under the operating conditions and easily sinter,resulting in the deactivation of the catalyst. In orderto maintain a high dispersion state, the redox prop-erty of perovskite oxides has been proposed, i. e., underoxidation conditions, palladium is oxidized and existsas LaFe0:57Co0:38Pd0:05O3, and under reducing condi-tions, palladium is deposited as fine metallic particleswith a radius of 1�3 nm. This cycling of the catalystthrough oxidizing and reducing conditions results in thepartial substitution of Pd into, and deposits from, theperovskite framework, thus maintaining a high disper-sion state of Pd. This was found to be highly effective inimproving the long-term stability of Pd during removalof NOx, CO, and hydrocarbons from the exhaust gas.The high dispersion state of Pd can be recovered byexposing the catalyst to oxidation and reduction envi-ronments. As a result this catalyst is called an intelligentcatalyst. This unique property also originates from thehigh stability of the perovskite crystal structure and

Inorganic Perovskite Oxides 59.2 Photocatalytic Activity 1411Part

E|59.2

charge compensation is automatically done by redoxcouples of another cation in the lattice.

Another interesting application of perovskite oxideis as a photocatalyst for water splitting. Among the cat-

alysts for water splitting into H2 and O2, it is reportedthat Ta- or Nb-based perovskite oxide shows high activ-ity by using ultraviolet light. This will be introduced indetail in the next section.

59.2 Photocatalytic Activity

Photo-excited electrons and holes can be used for split-ting water into H2 and O2 and this reaction is attractingmuch interest for converting solar energy to hydro-gen. Various inorganic catalysts have been studied asphotocatalysts for water splitting, in particular, Pt/TiO2

is a well-known inorganic semiconductor for photo-catalysis. Among the various catalysts reported, in thissection, photocatalysts based on perovskite structureare briefly introduced. The Ta-based oxide is gen-erally active in the photocatalytic water splitting re-action [59.22]. In particular, the Ta-based perovskiteoxide, ATaO3 (A D alkaline cation) shows high activ-ity in water splitting [59.23]. The activity is stronglyaffected by the A cation and this is because crys-tal structure is related to the electronic configurationof the oxide. The bond angles of Ta–O–Ta are 143ı

(LiTaO3), 163ı (NaTaO3), and 180ı (KTaO3). As thebond angle is close to 180ı, migration of excited en-ergy in the crystal occurs more easily and the band gapbecomes smaller. Therefore, the order of the delocaliza-tion of excited energy is LiTaO3 < NaTaO3 < KTaO3,while that of the band gap is reversed in the order asshown in Fig. 59.7. Table 59.3 shows photocatalyticactivities for water splitting into H2 and O2 in purewater on alkali tantalite photocatalysts with and with-out NiO cocatalysts. NaTaO3 photocatalysts showed thehighest photocatalytic activity when NiO cocatalystswere loaded. In this case, excess sodium in the startingmaterial was indispensable for showing the high activ-

Potential(eV vs. NHE)

Conductionband

Valenceband

LiTaO3 NaTaO3 KTaO3

4.7 eV 4.0 eV 3.6 eV 3.6 eV

NiO

3210

–1

Fig. 59.7 Band structures of alkali tantalates ATaO3

(A W Li, Na and K) with perovskite-type structures incomparison to a normal hydrogen electrode (NHE). (Af-ter [59.21])

ity [59.23]. The conduction band level of the NaTaO3

photocatalyst was higher than that of NiO (� 0:96 eV)as shown in Fig. 59.7 [59.23]. Moreover, the excited en-ergy was delocalized in the NaTaO3 crystal. Therefore,the photogenerated electrons in the conduction band ofthe NaTaO3 photocatalyst were able to transfer to theconduction band of the NiO cocatalyst of an active sitefor H2 evolution, resulting in the enhancement of thecharge separation. Therefore, NiO loading was effec-tive for the NaTaO3 photocatalyst even without specialpretreatment.

It is reported that the activity of photocatalytic wa-ter splitting is also much increased by additives. For

Table 59.3 Photocatalytic activities for water splitting intoH2 and O2 in pure water on alkali tantalite photocatalystswith and without NiO cocatalysts. (After [59.21])

Catalysta Ratioofalkalito Tab

Bandgapc

(eV)

Surfacearea(m2 g�1)

Activity(�mol h�1)

H2 O2

LiTaO3 1:05 4:7 0:3 430 220NiO(0:10wt%)/LiTaO3

1:05 4:7 – 98 52

NaTaO3 1:00 4:0 0:5 11 4:4NiO(0:05wt%)/NaTaO3

1:00 4:0 – 480 240

NaTaO3 1:05 4:0 0:4 160 86NiO(0:05wt%)/NaTaO3

1:05 4:0 – 2180 1100

KTaO3 1:10 3:6 1:6 29 13NiO(0:10wt%)/KTaO3

1:10 3:6 – 7:4 2:9

a Catalyst: 1 g, pure water: 350ml, 400W high-pressure mer-cury lamp, inner irradiation cell made of quartzb In starting materialsc Estimated from the onset of absorption

PartE|59.2


example, Table 59.4 shows band gaps, surface areasand photocatalytic activities for water splitting intoH2 and O2 in pure water on various lanthanide-dopedNaTaO3 (denoted as NaTaO3:Ln hereafter) with NiOcocatalysts [59.21]. NaTaO3:Ln powders had larger

20

15

10

5

00 2 4 6 8 10

Time (h)

Amounts of products (mmol)

1st 2nd 3rd 4th

Evac.

Fig. 59.8 Photocatalytic water splitting overNiO(0:05wt%)/NaTaO3:La(1mol%). The cell wasevacuated (evac.) after each experimental run. Catalyst:1 g, pure water: 390ml, 400W high-pressure mercurylamp, inner irradiation cell made of quartz. Open marks:H2, closed marks: O2. (After [59.23])

Table 59.4 Band gap, surface areas and photocatalytic ac-tivities for water splitting into H2 and O2 in pure wateron various lanthanidedoped NaTaO3 with NiO cocatalysts.(After [59.21])

Ln-doped Bandgap(eV)

Surfacearea(m2 g�1)

Activity (mmol h�1)a

H2 O2

None 4:01 0:44 2:18b 1:10b

La 4:07 2:5 5:90 2:90Pr 4:09 3:1 5:29 2:58Nd 4:07 3:0 5:19 2:51Sm 4:08 2:6 5:29 2:63Eu 4:08 2:5 0:254 0:122Gd 4:08 1:9 4:29 2:11Tb 4:07 1:4 4:30 2:19Dy 4:07 1:7 4:46 2:23Yb 4:05 1:3 1:72 0:820

a Catalyst: 1 g, pure water: 390ml, inner irradiation cell madeof quartz, 400W high-pressure mercury lampb Initial activity

surface areas than that of nondoped NaTaO3. Bandgaps of NaTaO3:Ln are slightly higher than that ofnondoped NaTaO3. The activity of the NiO/NaTaO3

photocatalyst is remarkably improved by doping ofLn, except for Eu and Yb. In particular, obviously,NiO/NaTaO3:La is the most active: H2 and O2 evolvedsteadily and efficiently as shown in Fig. 59.8. Theoptimized NiO(0:2wt%)/NaTaO3:La(1:5%) photocat-alyst evolves H2 and O2 with the rates of 14:6and 7:2mmol h�1 respectively. The apparent quan-tum yield is approximately 50% at 270 nm. Thus, it isdemonstrated that the photocatalytic water splitting isable to proceed efficiently using a photocatalyst pow-der system.

On the other hand, effects of substitution of Ta-sitesof KTaO3 are also reported [59.24]. Not only dopantsat A-sites, but also those at B-sites are strongly influ-enced by photocatalytic activity. Since Pyrex glass wasused for the reactor, the catalytic activity is differentfrom those in Table 59.4 because of negligible ultra-violet light, and it is reported that KTaO3 is more activethan that of NaTaO3, which corresponds with the mostnarrow band gap among ATaO3. The effects of substi-tution of various elements on mainly Ta-sites in KTaO3

on photocatalytic activity to water splitting are shownin Table 59.5. Obviously, H2 and O2 formation rates are

Table 59.5 Effect of substitution of various elements onmainly Ta-sites in KTaO3 on photocatalytic activity to wa-ter splitting. (After [59.24])

Catalysta Formation rate (�mol=h)H2 O2

LiTaO3 0:0 0:0NaTaO3 0:8 0:0Rb4Ta6O17 0:0 0:0KTaO3 4:5 0:0

KT0:9M0:1O3

M D Zn2C 29:1 0:0Y3C 4:4 0:0Al3C 9:7 2:7Ga3C 67:7 22:3In3C 21:3 8:1Ce4C trace 0:0Ti4C 50:6 12:6Zr4C 93:5 42:1Hf4C 98:5 39:8Si4C 17:2 4:1Ge4C 8:3 0:0Nb5C 3:7 0:0Sb5C trace 0:0W6C 1:0 0:0Pt/TiO2 (0:3wt%) 106:1 0:0

a 1wt% NiO loaded

Inorganic Perovskite Oxides 59.3 Application for Solid Oxide Fuel Cells (SOFCs) 1413Part

E|59.3

significantly increased by doping Ta-sites, in particu-lar, the highest H2 formation rate can be achieved bysubstituting Ta partially with Zr or HF. Since positiveeffects are tended to be obtained by doping tetravalentcations, it seems that a decrease in carrier density is ef-fective for increasing photocatalytic activity for watersplitting.

In addition to Ta-based perovskite, another impor-tant oxide based on the perovskite structure is SrTiO3,which has narrower band gap than that of Ta-based

perovskite oxide [59.25]. Since the valence band isnot low enough for the formation of O2, complete de-composition of water can be achieved by combiningwith oxygen-formation catalysts such as BiVO4 withPt/SrTiO3 doped with Rh [59.22]. Among the variouscatalysts reported, it is obvious that perovskite oxide isan important family for photocatalysts from a uniquesemiconducting property viewpoint, and it is expectedthat research is expanded to the oxygen-deficient-typeperovskite oxides such as K2NiF4-type structures.

59.3 Application for Solid Oxide Fuel Cells (SOFCs)

An important application area of inorganic perovskiteoxide is solid oxide fuel cells and air electrodes ofmetal-air batteries because of high catalytic activityto oxygen reduction and superior mixed conductivityachieved simultaneously. In this section, the applicationof perovskite oxide for SOFCs is briefly mentioned.Further details are available in the another book [59.26].

Table 59.6 summarizes the important applicationsof perovskite oxides for solid oxide fuel cell technol-ogy. As shown in Table 59.6, LaCoO3 or LaMnO3 arepromising candidates for SOFC cathodes and LaGaO3-based oxides for the electrolyte. In addition, recentlythere have been several reports on the application ofCr-based perovskites as anodes. Therefore, the con-cept of SOFCs based entirely on perovskite com-ponents, all-perovskite SOFCs, is also proposed andsome preliminary results have been reported [59.27].In contrast to the SOFCs using oxide-ion-conductingelectrolytes, the development of SOFCs using high-temperature proton-conducting electrolytes is slightlydelayed, particularly when compared with develop-ment of polymer electrolyte-type fuel cells. However,the Toyota group has quite successfully demonstrateda high-power SOFC using a BaCeO3-based electrolytefilm on Pd foil [59.28]. Their data suggest that theproton-conducting perovskite oxides might also be animportant component in real SOFCs in the near future.

59.3.1 Cathode

The principal requirement for a cathode of a SOFC is toelectrochemically reduce oxygen molecules into oxide

Table 59.6 Important materials for perovskite oxide for solid oxide fuel cell applications

Component Typical MaterialsCathode La.Sr/MnO3, La.Sr/CoO3, Sm0:5Sr0:5CoO3, La.Sr/Fe.Co/O3

Electrolyte La.Sr/Ga.Mg/O3.O2�/, BaCeO3.HC/, BaZrO3.HC/, SrZrO3.HC/, Ba2In2O5.O2�/

Anode La1�xSrxCr1�yMyO3 (M = Mn, Fe, Co, Ni), SrTiO3

Interconnector La.Ca/CrO3

ions, and there are several requirements for oxide ap-plied for cathodes of SOFCs i. e., catalytic activity, ther-modynamic stability and compatibility in mechanicaland chemical properties. Perovskite oxide is the mostsuitable material satisfying these requirements for cath-odes. Figure 59.9 shows the reaction route consideredfor cathodes of SOFCs. Oxygen reduction proceeds onthe electrode surface or at the electrode/electrolyte/gas-phase interface; the so-called triple phase boundary(TPB). The electrode material catalyzes the oxygenmolecules to be dissociated into atoms, charged andincorporated into the electrolyte (Fig. 59.9). For thecathode material, the electrocatalytic activity is an im-portant parameter to be considered. The surface reac-tion rate constant in oxygen isotope exchange is a goodmeasure for the catalytic activity. Kilner et al. [59.29]compared various oxides in isotope diffusion coefficientand found a positive correlation between those param-eters. A highly mixed electronic and ionic conductormay be a promising candidate in terms of the elec-trode performance. At the early stage of SOFC de-velopment, La0:8Ca0:2MnO3 or La0:6Sr0:4MnO3 werewidely used, however, because of low oxide ion con-ductivity in Mn-based perovskite oxide, the reactionis limited to a three-phase boundary resulting in largecathodic overpotential. Recently, several oxides havebeen reported to show extremely high surface exchangerate for oxygen activation. Baumann et al. [59.30]compared several Co- and Fe-based perovskites ina controlled shape and found Ba0:5Sr0:5Co0:8Fe0:2O3

shows 100 times smaller electrochemical resistancethan that of La0:6Sr0:4Co0:2Fe0:8O3 (LSCF) which is

PartE|59.3


Adsorption

Charge transfer

Charge transfer /incorporation

Surfacediffusion

Surface pathwayBulk pathway

O2–

O2–

O2–

On

O2 (ad)

O2 (g)

O (s)

e–

e–Diffusion

½ O2(g) + VO°° + 2e– ↔ OOx

Fig. 59.9 Reaction route considered for a cathode ofSOFC

often used for the intermediate-temperature SOFC cath-ode. One reason for such high cathodic activity isassigned to the high mixed conductivity and largereaction area available, i. e., the two phase bound-ary (route 2 in Fig. 59.9) is also contributed to theoxygen dissociation reaction. Another active com-position to the cathode is Sm0:5Sr0:5CoO3, whichalso has high oxide ion conductivity [59.31]. How-ever, the most popular composition used for SOFCis La0:6Sr0:4Co0:2Fe0:8O3 (LSCF) due to surface ac-tivity and stability. Recently, research has shifted tothe more oxygen-deficient perovskites such as double(A2BaO6), Rudrusden–Poppered (AxByOz), or K2NiF4structure. Sase et al. [59.32] reported that existence ofa .La; Sr/2CoO4 phase on the .La;Sr/CoO3 electrodeenhances the oxygen exchange reaction rate. The dou-ble perovskite phase of PrBaCo2O6 is also reportedas an active phase for oxygen dissociation [59.33]and so not only perovskite but also perovskite-relatedphases, in particular the oxygen-deficient perovskitephase, is now attracting as much attention as the activecathode catalyst for intermediate temperature opera-tions.

On the other hand, one of another important issuesfor SOFC development is long-term stability and de-crease in cathode performance is pointed out. Thereare several reasons decreasing cathodic performance isreported, i. e., chemical poisoning with Cr, S and Band sintering, and phase separation. Among them, sur-face segregation with Sr on the perovskite cathode hasbeen suggested recently [59.34]. Figure 59.10 showsthe change in surface composition of LSCF by low-energy ion scattering techniques, which are sensitive to

1

0.8

0.6

0.4

0.2

00 2 4 6 8

1

0.8

0.6

0.4

0.2

0

Annealing time (h)

B/A cations ratioSr/(Sr + La)

Sr/(La + Sr)B:A

Fig. 59.10 Change in surface composition ofLa0:6Sr0:4Fe0:8Co0:2O3 (LSCF) analyzed with low-energyion scattering spectroscopy

the elements in the outermost surface layer. Obviously,the surface of LSCF is immediately enriched with Srand it is considered that an Sr-enriched surface is highlyreactive with Cr or S, resulting in the decrease in sur-face activity. Apparently, the surface composition ofperovskite is slightly different from that of bulk, how-ever, reactivity for oxygen dissociation is more stronglyaffected by B-site cations. Therefore, there is still someunclear points on parameters determining the activityfor oxygen dissociation and this will be discussed moreintensively in the future.

59.3.2 Anode

For the anode of SOFC, metal-oxide ion-conducting ox-ide composites named cermet have been widely used,in particular, Ni-Y2O3-stabilized ZrO2 (Y0:16Zr0:84O2,YSZ) cermet has been widely used. However, Ni iswell known to be deactivated easily by aggregation andcoarsening. In addition, re-oxidation of Ni is also oc-curs easily, resulting in the permanent failure of the cell.Therefore, recently, the application of oxide for the an-ode has also been proposed and among the proposedoxide anodes, perovskite oxides such as SrTiO3 dopedwith La, Nd, etc. [59.35] or La0:75Sr0:25Mn0:5Cr0:5O3

(LSCrM) [59.36] show interesting performance and arepromising as oxide anodes.

In particular, improved performance has beenobtained with complex perovskites based uponCr and Mn at the B-sites forming compositions(La,Sr)Cr1�xMxO3�ı . Tao and Irvine have focusedupon doped lanthanum chromite doped with Sr andMn up to 20% dopant on the B-site, usually 5 or


E|59.3

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.0

0.1

0.2

0.3

0.4

0.5

0.6

Current density (A cm–2)

Voltage (V) Power density (W cm–2)

Wet H2 @ 900 °CWet H2 @ 850 °C

Wet CH4 @ 950 °CWet 5% H2 @ 900 °C

Fig. 59.11 Power generation property of the cell usingLa0:75Sr0:25Cr0:5Mn0:5O3 for the anode

10%. .La0:75Sr0:25/Cr0:5Mn0:5O3 (LSCrM) exhibitscomparable electrochemical performance to Ni/YSZcermets [59.27, 36]. Figure 59.11 shows the I–V,I–P curves of the cell using LSCrM for the anodeat 1173 and 1123K. Apparently, the open circuitpotential (OCV) is achieved to a theoretical level andreasonable power density of 0:4W=cm2 was achievedat 1173K. The electrode polarization resistance ap-proaches 0:2� cm2 at 1173K in 97%H2/3%H2O.Good performance is achieved for methane oxidationwithout using excess steam. The anode is stable inboth fuel and air conditions and shows stable electrodeperformance in methane. Thus both redox stabilityand operation in low steam hydrocarbons have beendemonstrated. Catalytic studies of LSCM demonstratethat it is primarily a direct-oxidation catalyst formethane oxidation as opposed to a reforming catalyst,with the redox chemistry involving the Mn–O–Mnbonds. Although oxygen ion mobility is low in theoxidized state, the diffusion coefficient for oxide ionsin reduced LSCrM is comparable to YSZ.

Another important double perovskite isSr2MgMoO6�ı , which has recently been shownto offer good performance, with power densities of0:84W=cm2 in H2 and 0:44W=cm2 in CH4 at 1073K,and good sulfur tolerance [59.37]. The molybdenum-containing double perovskite was initially preparedat 1473K in flowing 5% H2 and then deposited ontop of a lanthanum ceria buffer layer before testing.Although there is some decomposition recognized, it isalso reported that La0:5Sr0:5MnO3 is active to not onlythe cathode but also the anode. The maximum powerdensity of 1W=cm2 was reported at 1273K whenLa0:9Sr0:1Ga0:8Mg0:2O3 (LSGM, 0:5mm thickness) isused for the electrolyte [59.38].

–4

–3

–2

–1

0

0.7 0.8

1000 900 800 700 600

0.9 1.0 1.1 1.2 1.3

Temperature (°C)log σ (S cm–1)

1000/T (K–1)

ZrO2–7:5mol%Sc2O3

ZrO2–15mol%CaO

ZrO2–8mol%Y2O3

La0.8Sr0.2Ga0.8Mg0.115Co0.085O3

La0.8Sr0.2Ga0.8Mg0.2O3

Bi2O3–25mol%Y2O3

CeO2–10mol%Gd2O3

CeO2–5mol%Y2O3

SrCe0.95Yb0.05O3

ThO2–15mol%Y2O3

[H+]

Fig. 59.12 Comparison of the oxide ion conductivity ofLSGM with typical fluorite oxide

59.3.3 Electrolyte

An oxide ion conductor is widely used for electrolytesof SOFCs. In the case of SOFCs, solid proton con-ducting ceramics such as BaZrO3 or SrCeO3 dopedwith various rare earth cations to B-sites have beenalso studied [59.39], in particular for low-temperatureSOFCs; however, considering the chemical stability andpower density achieved, SOFCs using oxide ion con-ducting electrolytes are more promising. At present,almost all SOFCs under development for commer-cialization use YSZ for the electrolyte, with only afew exceptions. However, for increasing power den-sity, there is still strong demand for replacing YSZwith an alternative electrolyte showing higher oxide ionconductivity. CeO2 with fluorite structure and LaGaO3

with perovskite structure are showing promise as elec-trolytes for this purpose. In particular, LaGaO3 dopedwith Sr and Mg is the first pure oxide ion conductorwith a perovskite structure [59.40]. Figure 59.12 showsa comparison of the oxide ion conductivity of LSGMwith typical fluorite oxide. Apparently, oxide ion con-ductivity in LSGM is much higher than that of YSZ andalmost comparable with Gd-doped CeO2. In the case ofCeO2, partial electron conductivity appears in reducingatmospheres, however, LSGM exhibits the pure oxideion conductivity across a wide pO2 range, i. e., from pureoxygen to pure hydrogen.

The application of LSGM for electrolytes of SOFCsis also studied and the power generation property ofthe cell using LSGM is shown in Fig. 59.13. Since

PartE|59.3


50.0

0.5

1.0

1.5

2.0

43210

0.2

0.4

0.6

0.8

1.0

0.0

1.2Terminal voltage (V)

Current density (A cm–2)

Power density (W cm–2)

1073K

873K

0.183 mm thickness

Fig. 59.13 Power generation property of the cell usingLSGM with 0:182 mm thickness

transport number of the oxide ion in LSGM is closeto unity from pure O2 to pure H2, the open circuitpotential is 1:15V, which is almost the same as thatof theoretical open circuit potential. In addition, inspite of the thick electrolyte (0:5mm), high maximumpower density is achieved compared with that of thecell using YSZ. Therefore, it is expected that the op-erating temperature of SOFCs could be decreased byusing LSGM for the electrolytes. Making thin filmsof LSGM is another important subject, however, it isreported that LSGM has some reactivity with Ni inthe anode substrate and so the power density is not ashigh as expected from the film thickness and ionic con-ductivity. Therefore, for LSGM film cells, preventingreaction with Ni in the anode substrate is an impor-tant issue. Huang et al. reported that La-doped CeO2 iseffective for preventing Ni diffusion [59.41], however,electrical conductivity of La-doped CeO2 is insufficient.Recently, Hong et al. reported that Ti- and La-codopedCeO2 is promising as a buffer layer for preventing Nidiffusion with reasonable electrical conductivity andsintering properties [59.42]. In any case, LSGM per-ovskite oxide has a high potential as a fast oxide ionconductor for intermediate-temperature SOFCs and asan alternative to YSZ. Another interesting oxide ionconductor is Ba2In2O5, which is a Brownmillerite struc-ture and although oxide ion conductivity is slightlylower than that of YSZ, this new family of oxide ionconductors is also considered as a promising electrolytefor SOFCs [59.43].

10–4

10–3

10–2

10–1

1.21.11.00.90.80.7

1000 900 800 700 600Temperature (°C)Conductivity (S cm–1)

1000/T (K–1)

BaCe0.8Y0.2O3–a

BaCe0.4Zr0.5Y0.1O3

BaCe0.9Nd0.1O3–a

SrCe0.95Yb0.06O3–a

SrZr0.95Y0.06O3–a

CaZr0.9In0.1O3–a

Fig. 59.14 Comparison of proton conductivityin perovskite oxide. (After [59.44].) Plots forBaCe0:4Zr0:5Y0:1O3 were added from [59.45]

Another important family for electrolytes of SOFCis oxide proton conductor and perovskite oxide ofACeO3 or BZrO3(A D Ba and Sr), which shows fastproton conductivity. Protons in perovskite oxide wereformed by the following equation and the intersti-tial protons are mobile at the interstitial positionof the perovskite lattice. Figure 59.14 shows thecomparison of proton conductivity in perovskite ox-ide [59.44]. BaCeO3 doped with Y shows reason-ably high proton conductivity at low temperature.It is reported that extremely large power density isachieved by using BaCeO3 doped with a Y thin filmfor the electrolyte of SOFC; however, these proton-conducting perovskites show high reactivity with CO2

resulting in decomposition by formation of carbon-ate. Therefore, an increase in chemical stability isrequired for these oxide perovskite. Recently, therehas been high interest on ACe1�xZrxO3-based ox-ides doped with Y from chemical stability; in par-ticular it is reported that BaCe0:4Zr0:5Y0:1O3 showsreasonably high proton conductivity and reasonablechemical stability to CO2 and hydrolysis with wa-ter [59.45].


E|59.3

Interconnection

Electrolyte

Airelectrode

Air flow Fuel electrode

Fuel flow

Fig. 59.15 Schematic view of the SOFC stack

59.3.4 Interconnector

The interconnector is an important component forstacking SOFCs and requires high stability in reduc-ing and oxidizing atmospheres. The interconnector isused for connecting the SOFC single cell as shown inFig. 59.15 and so high electrical conductivity and nooxide ion conductivity are also required for the highperformance of the SOFC stack. Perovskite oxides ofLaCrO3 have been also widely used for interconnectors.Although LaCrO3 is stable under reducing and oxi-dizing atmospheres, the electrical conductivity is stillinsufficient and also sintering is rather difficult. There-fore, doping Ca2C for La-site in LaCrO3 is generallyperformed for increasing conductivity as well as sinter-ing properties.

Figure 59.16 shows electronic conductivity ofdoped LaCrO3 as a function of temperature [59.46,47]. The electronic conductivity increases with in-creasing temperature, suggesting the semiconductortemperature dependence. An increasing of the Ca con-centration in La1�xCaxCrO3�ı enhanced the electronicconductivity due to the increase of Cr4C concen-tration. There are some deviations of the electricalconductivity among the examined alkaline earth ele-ments: Ca-doped LaCrO3 shows larger electrical con-ductivity than Sr-doped LaCrO3. This difference wasreported to be due to the difference of lattice dis-tortion and phase stability. The activation energy forconductivity was 0:12�0:14 eV and the mobility was0:066�0:075 cm2=.Vs/ at 1173�1323K.

The electronic conductivity decreases with a reduc-tion of oxygen partial pressure because of the decreaseof Cr4C concentration in a reducing atmosphere. The

12

11

10

9

8

7

6

53.53.02.52.01.51.00.5

1000/T (K–1)

lnσT (S·cm–1K)

La1–xCaxCrO3–δ

x = 0.1x = 0.2x = 0.3

Fig. 59.16 Electrical conductivity of La1�xCaxCrO3�ı

(x D 0:1�0:3) in air as a function of inverse temperature.(After [59.47])

electrical conductivity decreases with a reduction ofoxygen partial pressures. The electrical conductivityis proportional to p1=4O2

, which is consistent with thedefect chemistry of La1�xCaxCrO3�ı [59.47]. A dop-ing of the B-site has been also considered by severalauthors [59.48]. A typical dopant cation is Mg2C, re-placed into Cr3C sites. This substitution also increasesthe concentration of Cr4C, and eventually increasesthe electrical conduction. Because of low sinteringproperty, SrTiO3 doped with La for Sr sites or Ndfor Ti sites is also studied as an interconnector. Pe-rovskite oxide is also important for interconnectors ofSOFCs.

In summary, perovskite oxide is widely used forcurrent SOFCs and becomes important compounds forSOFC components. Therefore, the all-perovskite con-cept is also proposed for SOFC and the device con-sisting of materials with the same structure is highlyinteresting.

PartE|59.4


59.4 Oxygen Separating Membrane

Perovskite oxide shows high electronic and oxide ionicconductivity and these conductors are called mixed con-ductors. Since charge compensation by ion transportcan be automatically achieved with electronic conduc-tivity in mixed conductors, so ions can be transported inmixed conductors without outside circuits. Therefore,an important application of such mixed conductors is asa separation membrane for oxygen from air. The oxy-gen permeation rate is shown as the following equationif the bulk diffusion is a rate-limiting step

JO2 D RT�el�ion16F2.�el C �ion/t

Inphp1

(59.1)

Here, JO2 ; oxygen flux, �el; electron conductivity, �ion;oxide ion conductivity, F; Faraday constant, R; gas con-stant, t; membrane thickness, T; temperature and ph,pl means high and low oxygen partial pressure respec-tively.

From (59.1), the oxygen permeation rate is limitedby bulk diffusivity of oxygen. Figure 59.17 shows thediffusivity of oxide ions in several perovskite oxidesconsidered for oxygen permeation membranes [59.49].Apparently, Fe- or Co-based perovskite oxides showfast oxide ion conductivity and so large oxygen perme-ation rate is expected on these perovskite oxides. In fact,a large oxygen permeation rate of 1:05ml=.cm2 min/ at1173K is reported for Ba0:5Sr0:5Fe0:8Co0:2O3 (BSCF)(2mm thickness) [59.50]. According to (59.1), the oxy-gen permeation rate is determined by the diffusivity ofoxide ions and the thickness of the membrane whenthe oxygen partial pressure differential across the mem-brane is the same. Therefore, the oxygen permeationrate should be increased with decreasing membranethickness; however, it is reported that the oxygen per-meation rate is dependent on membrane thickness whenthickness is large, but with decreasing thickness, theoxygen permeation rate becomes independent of oxy-gen permeation rate because of the limitation by surfacereactions of oxygen dissociation. To achieve the largeoxygen permeation rate, not only high diffusivity of ox-ide ion in bulk but also high surface activity to oxygendissociation is required.

Recently, perovskite-related oxides are also attract-ing much interest as mixed conductors, and applieduse for oxygen permeation membranes. Among theperovskite-related oxides, there is strong interest for theK2NiF4-type oxide because of the large amount of oxy-gen deficiency. As explained in Figure 59.5, K2NiF4 ox-ides consist of a series of connected perovskite and rocksalt blocks, and a large free volume exists in the rocksalt block. Therefore, interstitial oxygen can be eas-ily introduced into the rock salt block. For the K2NiF4

oxides, the oxide ion permeation property in La2NiO4-based oxide is now studied in detail and it is reportedthat doping Cu and Ga is effective for increasingoxygen permeation rate and Pr2Ni0:71Cu0:24Ga0:05O3

(denoted as PNCG) is the optimized composition foroxygen permeation and permeation rate of approxi-mately 3 cc=.min cm2/ was reported on PNCG with0:5mm thickness from air to He [59.51]. The oxygentransport route is also studied by neutron diffractionanalysis and as shown in Fig. 59.18, oxide ion trans-ports through interstitial positions in the rock salt blockis clearly demonstrated. Therefore, in this PNCG, oxideions are mainly transported through rock salt but not inthe perovskite block. On the other hand, PNCG showshigh hole conduction and holes are mainly transportedin the perovskite block; and so in PNCG, it is interestingthat two different routes for oxide ion and hole conduc-tion are expected [59.52].

The most interesting use of mixed conductors is tocombine the catalytic reaction and the so-called mem-brane reactor system; and several catalytic reactionssuch as NOx decomposition and partial oxidation ofalkanes are reported by using mixed conductors for re-moving the reactant from or into reaction systems. Topermeate oxygen throughmixed conductor membranes,a gradient in oxygen partial pressure is required asa driving force for oxygen transport; however, in mem-brane reactor system, differences in oxygen partial pres-

–6.0

–5.5

–5.0

–4.5

10987104 T –1 (K–1)

log Dv (cm2s–1)

La0.75Sr0.25FeO3(D*)

LaFeO3(D*)

LaCoO3(D*)

La0.9Sr0.1FeO3(D*)

La0.9Sr0.1CoO3(D*)

La0.9Sr0.1CoO3(Dchem)

LaCoO3(Dchem)

La1–xCaxAlO3–x/2

Fig. 59.17 Diffusivity of oxide ion in several perovskiteoxides considered for an oxygen permeation membrane.(After [59.49])

Inorganic Perovskite Oxides 59.5 Summary 1419Part

E|59.5

(Ni, Cu, Ga)O6 octahedron layer

ba

cO2

O2O2

O2

O2 O2O2

O2 O2 O2

O3O3

O3 O3 O3 O3

(Pr, La)2(Ni, Cu, Ga)O4–δ mixed conductor


O2-O3-O2 oxide-ion diffusion pathsin (Pr, La)–O layer


O2-O3-O2 oxide-ion diffusion pathsin (Pr, La)–O layer

Fig. 59.18 Oxygen transport route in Pr2Ni0:81Cu0:24Ga0:05O4 estimated by neutron diffraction analysis. (After [59.52])

Mixed conductorCH4 CO, H2

O2 O2 O2

Fig. 59.19 Schematic image of the catalytic membrane re-actor using mixed conductors

sure are automatically achieved. Therefore, the combi-nation of mixed conductors with catalytic reactions isthe most ideal usage. From this aspect, partial oxida-tion of CH4 has been studied with perovskite mixedconductors for oxygen separation from air [59.53]. Theschematic image of this catalytic reactor system wasshown in Fig. 59.19. Since oxygen partial pressure in

CH4 is as low as approximately 10�20 atm, a largeoxygen permeation rate is achieved under CH4 partialoxidation conditions and it is reported that an oxygenpermeation rate of 5 cc=.min cm2/ was achieved by us-ing SrFe0:8Co0:2O3 perovskite [59.54]. However, theoxygen permeation rate is decreased by phase changesin a reducing atmosphere of CH4. Therefore, althoughFe- or Co-based perovskites have been mainly studied,the most important issues for application of perovskiteoxides to catalytic membrane reactors are stability ina wide oxygen partial pressure range and it is reportedthat Ni- or Fe-doped LaGaO3 shows hole and oxide ionconductivity stably over wide pO2 ranges and the oxy-gen permeation rate of 12 cc=.min cm2/ was exhibitedon La0:7Sr0:3Ga0:6Fe0:4O3 of 0:2mm thick at 1273Kin CH4 partial oxidation [59.55]. As a result, obvi-ously, perovskites are important materials for oxygenseparation membranes and the application of catalyticmembrane reactors is an important area; albeit thechemical stability is required to be much improved.

59.5 Summary

In this chapter, crystal structures of perovskites andrelated oxides were explained. Perovskite oxide hasa variety of composition and component elements. Inaddition, there are many isomorphs in crystal struc-ture. Therefore, based on variety of crystal struc-tures, there are many functions and rich applica-tion areas. In particular, in this chapter, the applica-tion of perovskite oxides for photocatalytic propertiesand solid oxide fuel cells were briefly overviewed.

Since high electric conductivity and surface activ-ity to oxygen dissociation are achieved simultane-ously, perovskite oxides, mainly Co-, Fe- and Mn-based oxides are widely used for SOFCs and oxy-gen permeation membranes. On the other hand, forthe application to photocatalysts, Ta- or Ti-based per-ovskites are highly active. Therefore, perovskite ox-ide is a highly important compound for these ar-eas.

PartE|59


References

59.1 R.M. Hazen: Sci. Amer. 258(6), 74 (1988)59.2 T. Yagi, H.K. Mao, P.M. Bell: Phys. Chem. Minerals

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