59. Inorganic Perovskite Oxides Pe 59 | E Part R Tatsumi Ishihara

1405 Inorganic59. Inorganic Perovskite Oxides Pe 59 | E Part r Tatsumi Ishihara

59.1 Typical Properties Crystal structure and important functions of of Perovskite Oxides ...... 1409 inorganic perovskite oxides are introduced. Pe- 59.1.1 Dielectric Properties...... 1409 rovskite oxides comprise large families among 59.1.2 Electrical Conductivity the structures of oxide compounds, and sev- and Superconductivity ...... 1409 eral perovskite-related structures are currently 59.1.3 Catalytic Activity ...... 1410 recognized. Typical structures (ABO3) consist of large-sized 12-coordinated cations at the A-site 59.2 Photocatalytic Activity...... 1411 and small-sized 6-coordinated cations at the 59.3 Application for Solid Oxide B-site. Several complex halides and sulfides and Fuel Cells (SOFCs) ...... 1413 many complex oxides have a perovskite structure. 59.3.1 Cathode...... 1413 From a variety of compositions and structures, 59.3.2 Anode ...... 1414 a variety of functions are observed in perovskite 59.3.3 Electrolyte...... 1415 oxides. In particular high electronic conductivity, 59.3.4 Interconnector ...... 1417 which is at a similar level as metal, and surface 59.4 Oxygen Separating Membrane ...... 1418 activity to oxygen dissociation, are highly attrac- tive in this oxide. Perovskite oxide is now widely 59.5 Summary...... 1419 used for solid oxide fuel cells. References...... 1420

Oxide groups consisting of two or more different two different cations. The ilmenite structure has the cations are called complex or mixed oxides, and many same composition as the perovskite one, i. e., ABO3; types of crystal structures are known. In some special however, A and B in this structure are cations of ap- cases, oxides consisting of single cations in different proximately the same size, which occupy an octahedral oxidation states are also classified as mixed oxides. site. Therefore, in spite of the fact that they share the For example, in Eu3O4, the mixed oxide consists of same general chemical formula, structures classified as Eu(III) and Eu(II) in 6- or 8-coordination respectively. ilumenite- or ilmenite-related structures (e.g., LiSbO3) However, the most typical structure of a mixed ox- are different from perovskite. ide consists simply of two or more different cations Perovskite oxides comprise large families among with different oxidation states, ionic radii, and coor- the structures of oxide compounds, and several dination numbers. This diversity, which comes from perovskite-related structures are currently recognized. the complexity of these structures, results in a larger Typical structures consist of large-sized 12-coordinated number of different properties as compared to those of cations at the A-site and small-sized 6-coordinated simple oxides. One of the most well known and im- cations at the B-site. Several complex halides and sul- portant complex oxide structures is the spinel structure fides and many complex oxides have a perovskite struc- (AB2O4), which shows important magnetic properties. ture. In particular, (Mg,Fe)SiO3 or CaSiO3 is thought to The structure of such oxides displays a most interesting be the predominant compound in the geosphere [59.1, complexity. Since the size of the A and B ions in this 2]. Perovskite compounds with different combinations structure is close, oxides of this type are typical exam- of charged cations in the A and B-sites, for example ples of the versatility of mixed oxides. 1 C 5, 2 C 4and3C 3, have been discovered. Even Another important structure of mixed oxide is pe- more complex combinations are observed, such as . 0 00 / 0 00 rovskite and a variety of related structures are classified Pb B1=2B1=2 O3,whereB D Sc, Fe and B D Nb, Ta, . 0 00 / 0 00 as this oxide. The typical chemical formula of the pe- or La B1=2B1=2 O3,whereB D Ni, Mg, etc. and B D rovskite structure is ABO3, where A and B denotes Ru(IV) or Ir(IV). In addition, many ABO3 compounds

© Springer International Publishing AG 2017 S. Kasap, P. Capper (Eds.), Springer Handbook of Electronic and Photonic Materials, DOI 10.1007/978-3-319-48933-9_59 3 La ]. As D 3 :668 :603 :706 :645 :676 :94 :13 :038 :153 :04 lies be- 7 7 7 7 7 c 7 8 4 4 5 t 0 0 0 0 06 05 06 06 ı ı ı ı Lu or Y if A 60 60 60 60 10. It is noted that the D D D D Ho :616 :592 :502 :443 :503 rovskite structure when ˛ ˛ ˛ ˛ 5 5 5 5 5 b combinations, which are D 3 Lattice parameter (Å) O shows the crystal groups for is close to 1 or at least higher C 3 80 and 1: values crystallize in the ilmenite t :357 :461 :632 :016 :346 :283 :426 :381 :363 :989 :929 :949 :040 :193 :904 :189 :121 :61 :86 :994 :899 :02 B t 5 5 5 4 5 5 5 5 5 a 3 3 3 4 4 3 4 4 7 7 3 3 5 shows chemical elements that can C 59.2 3 59.3 decreases, the structure of the unit lattice and A t 3 Typical peroskite compound O type type 3 3 3 3 3 3 89. Figure 3 3 C 3 3 3 3 3 3 3 : 3 3 3 3 3 4 3 3 3 3 B C 2 LaAlO LaNiO BiFeO KNbO GdFeO GdFeO YFeO NdGaO CaTiO NaMgF Compound Cubic structure KTaO NaTaO NaNbO BaMnO BaZrO SrTiO KMnF KFeF Tetragonal structure BiAlO PbSnO BaTiO PdTiO TlMnCl LaAlO the value of compounds crystallize in the pe In perovskite-type compounds, the value of A cation isstructure La with or 5- Ce–Dy, andspectively, whereas 7-coordination is formed of a when Mn A new and hexagonal A re- Table 59.1 tween approximately 0: related to deviation from the ideal structure [59. is shifted from cubic to triclinictortions. due Figure to the increased dis- A oxides with the lower than 0 structure, which is a polymorph ofture. the It perovskite seems struc- superfluous to saystructure, that the for value the ideal of cubic be accommodated within theevident perovskite that structure. almost It all is elementscan except for occupy noble either gases Aovskite or structure, including B dopants. latticethe The positions crystal stability in group and the isthe mainly per- ionic radii determined of the by Ature the and is B ratio cations. dependent of Indeed, not thenature struc- only of on the the size A but and also B on the atoms. For example, AMnO O B ion A ion oxides are first regarded as 3 ny compounds are classified /: O r C B / 59.1, where it is clear that a large r O / . r O 2 r C 59.1, is a cubic lattice. Although few p B C r D A . r 2 O . tolerance factor t Ideal cubic perovskite structure ions holds true r p C 2 D A r t The ideal structure of perovskite, which is illus- In order to understand the deviations from the ideal Therefore, the deviation from theovskite ideal oxides structure can in be per- expressedso-called through the following Fig. 59.1 crystallize in polymorphic structures, whicha show small only distortion fromthe perovskite the structure. most symmetrical form of trated in Fig. compounds have this ideal cubic structure, manyhave oxides slightly distorted(e.g., variants hexagonal with or lower orthorhombic).though symmetry Furthermore, some even compoundsmany have oxides ideal display cubiclower symmetry. slightly structure, Several examples distorted of perovskite variantsare oxides with listed in Table number of perovskite oxidestice. Additionally, have in many a compounds a rombohedral largeoxygen lat- extent or of cation deficiency hasthe been large observed. lattice Due energy, to ma as perovskite oxides in spiteoxygen of deficiencies. There the are large various cationtions types in and/or the of perovskite distor- structure that areto strongly their related properties, inferroelectricity. particular their ferromagnetic or cubic structure, these ABO purely ionic crystals. Inthe the following case relationship between of the the radiiand ideal of O the structure, A, B, Novel Materials and Selected Applications

Part E

Part E | 59 1406 Inorganic Perovskite Oxides Inorganic Perovskite Oxides 1407 atE|59 | E Part 2+ 4 3+ 3+ a) A B O3 b) A B O3 Ionic Mn4+ Ti4+ Hf4+ U4+ Ionic Al3+ Cr3+ Sc3+ Y3+ Nd3+ La3+ size (Å) V4+ Sn4+ Zr4+ Ce4+ Th4+ size (Å) Ga3+ Fe3+ Ti3+ In3+ Sm3+ Ce3+ 1.35 Ba2+ 1.20 Rhombo- La2O3 3+ Tetragonal type La 1.30 Cubic –c < 1 hedral a 1.10 Ce3+ c Perovskite Nd3+ 1.25 – > 1 a 1.00 Sm3+ 2+ 1.20 SrVO Pseudo Pb 3+ 3 cubic 0.90 Orthorhombic Y Eu2+ 1.15 3+ 2+ In Sr 0.80 3+ Tl O type Sc 1.10 Pseudo Rhombohedral 2 3 cubic 0.70 1.05 Corundum Fe3+ Cr3+ Orthorhombic 0.60 3+ 1.00 Ca2+ Ca 2+ Cd 3+ 0.95 0.50 Al 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 Ionic size (Å) Ionic size (Å)

Fig. 59.2a,b The effect of ionic size of A- and B-site cations on the observed distortions of the perovskite structure; 2C 4C 3C 3C (a) A B O3 case (b) A B O3 case

Fig. 59.3 Chemical elements that can IA 0 occupy cation sites in the perovskite A-site cation 1 H IIA IIIB IVB VB VIBVIIB He structure 2 Li Be B-site cation BCNOFNe VIII 3 Na Mg IIIA IVA VA VIA VIIA IB IIB Al Si P S Cl Ar 4 KCaScTiVCrMnFeCoNiCuZnGaGeAsSeBrKr

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

6 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

7 Fr Ra Ac 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 or Ce–Dy [59.4]. Here, the attention should be paid to portant index for the stability of perovskite structures; the nature of the B atom, where the nature of the bond is however, the contribution of the chemical nature, such highly covalent, and therefore the coordination number as coordinating number of the constituent elements, is lower than 6. The typical example of this type struc- needs to be taken into account. ture is BaGeO3. In spite of the t value close to t D 1, The formation of superstructures in the perovskites i. e., ideal ionic size combination, BaGeO3 crystallizes is discussed next. If a B-site cation is progressively re- not in the perovskite structure but in the silicate-related placed by a dopant, a large difference in ionic radii one. This is attributable to the fact that the preferred tends to lead to the formation of the superstructures coordination number of Ge is 4. On the other hand, rather than random arrangements of the two kinds of due to the progress in high pressure technology, the ions. The typical case of this is Ba2CaWO6,whichis synthesis of new Ge-based perovskite oxides has been regarded as Ba2.CaW/O6. Similarly, in the compounds reported [59.5]. Since the coordination number of Ge with the general formula Ba3MTa2O9, there is random increases with the pressure, the perovskite structures distribution of M and Ta ions in the octahedral posi- with higher coordination numbers are preferred, and tions when M is Fe, Co, Ni, Zn, or Ca, whereas the a typical example of this is CaGeO3. Another group formation of the superstructure with a hexagonal lattice of interesting perovskite compounds is the oxynitrides, is observed in Ba3SrTa2O9. Another interesting type of i. e., LaWO3xNx, LaTaO2N, etc. Therefore, the value superstructure observed in the perovskite is the order- of t, which is determined by the ionic size, is an im- ing of cation vacancies located on A-sites: for example . 9 La, O type 3 struc- D 3 4 (M Perovskite Rock salt Perovskite 9 NiF 2 O 3 units in an ordered 4 )andK 5 O ) is an oxygen-deficient 2 5 ) of the ideal perovskite as B O p 2 2 and BO a B 6 2 . Cu-based oxides or Ni-based p a shows the structure of LaNb 4 D c La, Ce, Pr, Nb) and MTa 59.4 , p D a 2 Ruddelsden–Popper structure, another type of (M p 9 D O 3 b Other typical polymorphs of the perovskite struc- A B O D arrangement. Due to thedination oxygen number deficiency, of the A-sitelattice coor- cations parameter decreases of to the Brownmillalite 8.to structure The cubic relates lattice parameter ( tures. Brownmillalite (A perovskite-related structure oxides tend to adoptbecause these of oxygen-deficient the large structures amount of oxygen defects. a with incomplete occupancy of theA-sites. 12-fold Figure coordinated The B-sites of the perovskiteNb structure ion are and occupied two-thirds by of the A-sites remain vacant. ture are Brownmillalite (A Fig. 59.6 MNb Ce,Pr,Nd,Sm,Gd,Dy,Ho,Y,Er).Intheseoxides, there is an octahedral framework of the ReO type of perovskite whereThe unit oxygen cell contains vacancy BO is ordered. Perovskite Rock salt Perovskite Rock salt Perovskite 1.98Å 1.89Å 2.07Å A ion B ion O ion , an A-site-deficient pe- 9 B O 3 163° A structure, a perovskite-related structure O 4 B NiF A 2 Structure of LaNb K A-site deficient rovskite oxide Fig. 59.5 Fig. 59.4 Novel Materials and Selected Applications

Part E

Part E | 59 1408 Inorganic Perovskite Oxides 59.1 Typical Propertiesof Perovskite Oxides 1409

A combination of ordered B-sites and oxygen de- the isostructural Sr2TiO4 or Ca2MnO4 with SrTiO3 or 59.1 | E Part fects is seen in K2NiF4 structures, which are well- CaMnO3, which crystallize in the perovskite structures. known as they show superconducting properties. The Two different A cations forming the perovskite and the K2NiF4 structures consist of two units; a KNiF3 per- rock salt units are also possible, and LaO nSrFeO3 is ovskite unit and a KF rock salt unit (Fig. 59.5), which the typical example of this arrangement. Another inter- are connected in series along the c axis. Since the rock esting variant of these K2NiF4 structures is when two salt structure is embedded into the c axis direction, different anions occupy the two building blocks exclu- the K2NiF4 compound shows strong two-dimensional sively, i. e., SrFeO3 SrF or KNbO3 KF. In any case, it properties. Based on the intergrowth of the different is evident that perovskite oxides comprise a large family number of KNiF3 and KF units, there are many struc- of oxides. As a result, a variety of crystal structures and tures called Ruddelsden–Popper compounds with the properties is expected in these compounds. For further general formula .ABO3/nAO (Fig. 59.6), i. e., Sr3Ti2O7 detailed discussion on the perovskite-related oxides, the (n D 2), Sr4Ti3O10 (n D 3). It is interesting to compare reader is referred to references [59.5–8].

59.1 Typical Properties of Perovskite Oxides

Due to the variety of structures and chemical com- clinic to tetragonal and cubic as the temperature in- positions, perovskite oxides exhibit a large variety creases. Above 303 K, BaTiO3 crystallizes in the cubic of properties. The well-known properties of the per- perovskite structure, which does not show ferroelec- ovskite oxides are ferroelectricity in BaTiO3-based ox- tric behavior. The high dielectric constant observed in ides and superconductivity in Ba2YCu3O7 etc. In addi- BaTiO3 can be explained by the basis of the anisotropy tion to these well-known properties, several perovskite of the crystal structure. oxides exhibit good electrical conductivity, which is close to those of metals, ionic conductivity as well 59.1.2 Electrical Conductivity as mixed ionic and electronic conductivity. Based on and Superconductivity these variations in the electrical conducting property, perovskite oxides are chosen as the components for One of the most well-known properties of perovskite solid oxide fuel cells (SOFCs). It is also well known oxides is superconductivity. In 1984, superconductiv- that several perovskite oxides exhibit high catalytic ity was first reported by Bednorz and Müller in La– activity with respect to various reactions, in partic- Ba–Cu–O perovskite oxide [59.10]. After their report, ular oxidation reactions [59.9]. Table 59.2 provides much attention has been paid to new types of high- examples of the typical properties of perovskite ox- temperature oxide superconductors, mainly Cu-based ides. In this section, several typical properties of the oxides. As a result, several superconducting oxides with perovskite oxides, namely ferroelectricity, magnetism, different A-site cations have been discovered. However, superconductivity, and catalytic activity, are briefly the presence of Cu on the B-site is found to be essential overviewed. Table 59.2 Typical properties of perovskite oxides 59.1.1 Dielectric Properties Typical property Typical compound Ferromagnetism BaTiO3,PdTiO3 . ; / . ; / Ferroelectricity, piezoelectricity, electrostriction, and Piezoelectricity Pb Zr Ti O3, Bi Na TiO3 pyroelectricity are special properties inherent to dielec- Electrical conductivity ReO3,SrFeO3,LaCoO3, tric materials, and are important properties of elec- LaNiO3,LaCrO3 troceramics. The most well-known property of per- Superconductivity La0:9Sr0:1CuO3, YBa2Cu3O7, ovskite oxides is ferroelectric behavior, where BaTiO3, HgBa2Ca2Cu2O8 PdZrO3, and their doped compounds are representative Ion conductivity La.Ca/AlO3,CaTiO3, examples. The study of the ferroelectricity in BaTiO3 La.Sr/Ga.Mg/O3,BaZrO3, has long history, and many detailed reviews have been SrZrO3, BaCeO3 published. Furthermore, since the ferroelectric behavior Magnetism LaMnO3, LaFeO3, of BaTiO3 has a strong relationship with the crys- La2NiMnO6 tal structure, detailed studies of the crystal structure Catalytic properties LaCoO3,LaMnO3,BaCuO3 have been reported for BaTiO3.BaTiO3 undergoes Electrode materials La0:6Sr0:4CoO3, mainly three phase transformations from, i. e., mono- La0:8Ca0:2MnO3 3 ]. O / 20 in the Mg [59. . intelligent Mn / La . dream reactions intelligent catalyst ing conditions results in the , and under reducing condi- 3 O 05 : 3 nm. This cycling of the catalyst 0 Pd ) is one of the ]. 2 38 : 19 ]. The direct NO decomposition reaction O 0 18 C Co – 2 57 : N 16 0 , CO, and hydrocarbons from the exhaust gas. x D Recently, another interesting application of per- Among the various catalytic reactions studied, the The high dispersionexposing state the of catalyst Pd toronments. oxidation can As and a be result reduction this recovered catalyst envi- catalyst. is by called This an unique property alsohigh originates stability from the of the perovskite crystal structure and catalysis field. In thisof reaction, the surface ease oxygen inan as the removal a important product role,deficiency of and present, perovskite the oxides due are reaction active to withspect plays re- the to this facility reaction at ofout high oxygen temperatures. that It the isNO pointed doping decomposition activity. is Under an highlyatmosphere oxygen (up effective enriched to in 5%),position enhancing a activity relatively was high NO reported decom- for Ba tions, palladium is depositedwith as a fine radius metallic of particles 1 ovskite oxides as anported, namely automobile the so-called catalyst has been re- perovskite [59. Up to now, three-waywidely Pd–Rh–Pt used catalysts for haveand the been uncombusted purpose hydrocarbons. of Inthe removal order of amount to NO, of decrease ing CO, precious of metals, fine particles theis with required. catalyst high However, consist- surface-to-volume theseble ratio fine under particles the areresulting not operating in sta- conditions the andto deactivation easily maintain of sinter, a theerty of catalyst. high perovskite oxides In dispersion has been order oxidation state, proposed, i. conditions, e., the palladium under is redoxas oxidized prop- LaFe and exists perovskite as aof model the perovskite for structurepounds active allows preparation with sites. of The unusuala com- high stability valence extent of states oxygenthe deficiency. of It high is catalytic elements also activity noted ofpartially that or perovskite on oxides the is high based tion surface activity or to oxygen oxygen activationoxygen reduc- vacancies due presented. to the large number of ones applicable to environmentalmobile catalysis exhaust gas (e.g., cleaning auto- catalysts)attention. attract particular Initially, it waside reported consisting of that Cu, perovskiteactivity Co, ox- to Mn NO or direct Fe decompositionture exhibited at [59. superior higher tempera- (2NO through oxidizing and reduc partial substitution of Pdperovskite framework, into, thus and maintaining a depositssion high state from, of disper- Pd. the This was foundimproving to the be long-term highly stability effective in ofof Pd during NO removal , ] 3 5, : cm 11 0 = 155 K > d , LaFeO is smaller 100 S 3 ] were re- ]. Since all d D ]. Doping of values. How- 12 13 system [59. c 15 , T 7 O , and detailed stud- 14 3 c T Cu 2 system [59. system [59. mponent elements and the is one of the most important ı 10 7 C O O 8 crystallizes in an orthorhombic 3 3 90 K 110 K 120 K. . When the value of 30 K O c 7 3 T Cu Cu O , is related to the number of Cu–O 2 2 3 c c c c c Cu T 2 T T T T Ca Cu 2 2 Ca , which are now commonly used as cath- 2 has a tetragonal structure, which does not Sr 3 2 7 ) has been further increased to 130 O c 3 T 5, YBa : Cu 2 In addition to superconductivity, there are many per- It is expected that further increase in the number of odes in SOFCs. These perovskitehole oxides show conductivity, superior which is as high as aliovalent cations on thein A-site is enhancing also the highly electricalcreased effective conductivity number due of to mobile the chargethe in- carriers charge compensation. generated by originated from excess oxygen [59. exhibit superconductivity. ovskite oxides showingwhich high is close electronic to those conductivity, ofamples metals of like Cu. such The perovskite oxides typical are ex- LaCoO Cu–O layers may result in the higher 3. Cu–O layers 4. Cu–O layers and LaMnO layers in the crystal structure: 1. Cu–O layer ever, due to the lowor chemical stability, more synthesis of Cu–O five layeredcessful so compounds far. has YBa not been suc- superconductor systems with high 59.1.3 Catalytic Activity Because of the variety of co high chemical stability,been perovskite extensively studied oxides astions. have catalysts Two for also types variousfrom reac- of the research above trendsone is clearly reasons. the emerged Theoxygen-activated development catalysts objective as of an of the alternative tocontaining oxidation catalysts the precious catalysts metals. first or The second trend regards for superconductivity to occur.ide superconductors High-temperature of ox- the YBa high-temperature superconducting oxides are(Cu-based cuprites oxides), superconductivity is clearlyto related the Cu–Operconductivity, layers. The critical temperature for su- and the Bi structure, which is superconductive, while for ported in 1987the and critical 1988 temperature respectively,sition of and ( the currently superconducting tran- ies of its crystalthe structure content have of been oxygentant performed. factor nonstoichiometry Also, for is high an impor- than 0 2. Cu–O layers in the HgBa YBa Novel Materials and Selected Applications

Part E

Part E | 59.1 1410 Inorganic Perovskite Oxides 59.2 Photocatalytic Activity 1411

charge compensation is automatically done by redox alysts for water splitting into H2 and O2, it is reported 59.2 | E Part couples of another cation in the lattice. that Ta- or Nb-based perovskite oxide shows high activ- Another interesting application of perovskite oxide ity by using ultraviolet light. This will be introduced in is as a photocatalyst for water splitting. Among the cat- detail in the next section.

59.2 Photocatalytic Activity

Photo-excited electrons and holes can be used for split- ity [59.23]. The conduction band level of the NaTaO3 ting water into H2 and O2 and this reaction is attracting photocatalyst was higher than that of NiO ( 0:96 eV) much interest for converting solar energy to hydro- as shown in Fig. 59.7 [59.23]. Moreover, the excited en- gen. Various inorganic catalysts have been studied as ergy was delocalized in the NaTaO3 crystal. Therefore, photocatalysts for water splitting, in particular, Pt/TiO2 the photogenerated electrons in the conduction band of is a well-known inorganic semiconductor for photo- the NaTaO3 photocatalyst were able to transfer to the catalysis. Among the various catalysts reported, in this conduction band of the NiO cocatalyst of an active site section, photocatalysts based on perovskite structure for H2 evolution, resulting in the enhancement of the are brieﬂy introduced. The Ta-based oxide is gen- charge separation. Therefore, NiO loading was effec- erally active in the photocatalytic water splitting re- tive for the NaTaO3 photocatalyst even without special action [59.22]. In particular, the Ta-based perovskite pretreatment. oxide, ATaO3 (A D alkaline cation) shows high activ- It is reported that the activity of photocatalytic wa- ity in water splitting [59.23]. The activity is strongly ter splitting is also much increased by additives. For affected by the A cation and this is because crystal structure is related to the electronic conﬁguration Table 59.3 Photocatalytic activities for water splitting into ı of the oxide. The bond angles of Ta–O–Ta are 143 H2 and O2 in pure water on alkali tantalite photocatalysts ı ı (LiTaO3), 163 (NaTaO3), and 180 (KTaO3). As the with and without NiO cocatalysts. (After [59.21]) ı bond angle is close to 180 , migration of excited en- Catalysta Ratio Band Surface Activity ergy in the crystal occurs more easily and the band gap of gapc area (mol h1) becomes smaller. Therefore, the order of the delocaliza- alkali (eV) (m2 g1) b tion of excited energy is LiTaO3 < NaTaO3 < KTaO3, to Ta while that of the band gap is reversed in the order as H2 O2 : : : shown in Fig. 59.7.Table59.3 shows photocatalytic LiTaO3 1 05 4 7 0 3 430 220 activities for water splitting into H and O in pure NiO 1:05 4:7 – 98 52 2 2 : water on alkali tantalite photocatalysts with and with- (0 10 wt%)/ LiTaO3 out NiO cocatalysts. NaTaO3 photocatalysts showed the highest photocatalytic activity when NiO cocatalysts NaTaO3 1:00 4:0 0:5 11 4:4 were loaded. In this case, excess sodium in the starting NiO 1:00 4:0 – 480 240 material was indispensable for showing the high activ- (0:05 wt%)/ NaTaO3

Potential NaTaO 1:05 4:0 0:4 160 86 (eV vs. NHE) 3 NiO 1:05 4:0 – 2180 1100 Conduction (0:05 wt%)/ –1 band NaTaO3 0 1 4.7 eV 4.0 eV 3.6 eV 3.6 eV KTaO3 1:10 3:6 1:6 29 13 2 NiO 1:10 3:6 – 7:4 2:9 Valence (0:10 wt%)/ 3 band KTaO3 LiTaO3 NaTaO3 KTaO3 NiO

a Catalyst: 1 g, pure water: 350 ml, 400 W high-pressure mer- Fig. 59.7 Band structures of alkali tantalates ATaO3 (A W Li, Na and K) with perovskite-type structures in cury lamp, inner irradiation cell made of quartz b In starting materials comparison to a normal hydrogen electrode (NHE). (Af- c Estimated from the onset of absorption ter [59.21]) 3 3 6 : h) = .Band evolved 3 mol 2 59.8.The :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :7 :1 :0 :1 2 0 0 0 0 0 0 0 0 0 0 0 2 8 0 4 O 22:3 12:6 42:1 39:8 is more active 5%) photocat- 3 : and O 2 formation rates are ]. Not only dopants 2 :La(1 3 24 :3 :7 :0 :0 :8 :0 :5 :4 :7 . The effects of substi- 8 3 1 0 0 0 4 4 9 3 2 29:1 67:7 21:3 50:6 93:5 98:5 17:2 Formation rate ( with the rates of 14 trace 106:1 H trace water splitting are shown and O catalytic water splitting is using a photocatalyst pow- 2 2 because of negligible ultra- on photocatalytic activity to wa- ]) 3 24 59.4 respectively. The apparent quan- , which corresponds with the most and O 3 . The activity of the NiO/NaTaO 2wt%)/NaTaO 1 3 : 2 :Ln are slightly higher than that of 3 :La is the most active: H 3 3 Effect of substitution of various elements on are also reported [59. O :3wt%) 1 : C 17 3 59.5. Obviously, H a 0 (0 2 O 3 2mmolh 2 3 6 M : 3 Zn 9 : C Ta C C C 0 C C C C C On the other hand, effects of substitution of Ta-sites C 5 C C 4 4 3 D C 4 5 4 3 6 4 4 3 4 1 wt% NiO loaded 3 a Ge Nb Sb W Pt/TiO Catalyst LiTaO NaTaO Rb KTaO KT M Y Al Ga In Ce Ti Zr Hf Si than that of NaTaO gaps of NaTaO steadily and efficientlyoptimized as NiO(0 shown in Fig. narrow band gap among ATaO tution of various elements on mainly Ta-sites in KTaO and 7 ter splitting. (After [59. on photocatalytic activity to in Table Table 59.5 surface areas than that of nondoped NaTaO photocatalyst is remarkablyLn, improved except by forNiO/NaTaO doping Eu of and Yb. In particular, obviously, mainly Ta-sites in KTaO at A-sites, but alsoenced those by at photocatalytic activity. B-sites Since Pyrex areused glass strongly was for influ- thefrom reactor, those the in catalytic Table activity is different violet light, and it is reported that KTaO tum yield is approximately 50%demonstrated at that 270 the nm. photo Thus,able it to is proceed efficiently der system. of KTaO nondoped NaTaO alyst evolves H a ) 1 b Time (h) 2 :10 :90 :58 :51 :63 :122 :11 :19 :23 :820 O 1 2 2 2 2 0 2 2 2 0 in pure water 2 b 2 :18 :90 :29 :19 :29 :254 :29 :30 :46 :72 with NiO cocatalysts. H 2 5 5 5 5 0 4 4 4 1 Activity (m mol h and O ]) 3 ) 2 23 1 :Ln hereafter) with NiO :Ln powders had larger 3 g 3 2 :44 :5 :1 :0 :6 :5 :9 :4 :7 :3 0 2 3 3 2 2 1 1 1 1 Surface area (m :La(1 mol%). The cell was shows band gaps, surface areas 3 Evac. . (After [59. 2 ]. NaTaO 59.4 :01 :07 :09 :07 :08 :08 :08 :07 :07 :05 21 4 4 4 4 4 4 4 Band gap (eV) 4 4 4 ]) in pure water on various lanthanide-doped Band gap, surface areas and photocatalytic ac- 21 Photocatalytic water splitting over 2 1st 2nd 3rd 4th (denoted as NaTaO 3 05 wt%)/NaTaO : 0246810 and O Catalyst: 1 g, pure water: 390 ml, inner irradiationInitial cell activity made ,closedmarks:O 0 5 2 2 10 15 Amounts of products (mmol) products of Amounts 20 of quartz, 400 W high-pressure mercury lamp a b Nd Sm Eu Gd Tb Dy Yb Ln-doped None La Pr cocatalysts [59. NiO(0 tivities for water splitting into H NaTaO and photocatalytics activitie H for water splitting into on various lanthanidedoped NaTaO Table 59.4 Fig. 59.8 example, Table evacuated (evac.) after1 each g, experimental pure run.lamp, water: Catalyst: inner 390 irradiation ml, cellH 400 made W of high-pressure quartz. Open mercury marks: (After [59. Novel Materials and Selected Applications Part E

1412 Part E | 59.2 Inorganic Perovskite Oxides 59.3 Application for Solid Oxide Fuel Cells (SOFCs) 1413

signiﬁcantly increased by doping Ta-sites, in particu- perovskite oxide [59.25]. Since the valence band is 59.3 | E Part lar, the highest H2 formation rate can be achieved by not low enough for the formation of O2, complete de- substituting Ta partially with Zr or HF. Since positive composition of water can be achieved by combining effects are tended to be obtained by doping tetravalent with oxygen-formation catalysts such as BiVO4 with cations, it seems that a decrease in carrier density is ef- Pt/SrTiO3 doped with Rh [59.22]. Among the various fective for increasing photocatalytic activity for water catalysts reported, it is obvious that perovskite oxide is splitting. an important family for photocatalysts from a unique In addition to Ta-based perovskite, another impor- semiconducting property viewpoint, and it is expected tant oxide based on the perovskite structure is SrTiO3, that research is expanded to the oxygen-deﬁcient-type which has narrower band gap than that of Ta-based perovskite oxides such as K2NiF4-type structures.

59.3 Application for Solid Oxide Fuel Cells (SOFCs)

An important application area of inorganic perovskite ions, and there are several requirements for oxide ap- oxide is solid oxide fuel cells and air electrodes of plied for cathodes of SOFCs i. e., catalytic activity, ther- metal-air batteries because of high catalytic activity modynamic stability and compatibility in mechanical to oxygen reduction and superior mixed conductivity and chemical properties. Perovskite oxide is the most achieved simultaneously. In this section, the application suitable material satisfying these requirements for cath- of perovskite oxide for SOFCs is briefly mentioned. odes. Figure 59.9 shows the reaction route considered Further details are available in the another book [59.26]. for cathodes of SOFCs. Oxygen reduction proceeds on Table 59.6 summarizes the important applications the electrode surface or at the electrode/electrolyte/gas- of perovskite oxides for solid oxide fuel cell technol- phase interface; the so-called triple phase boundary ogy. As shown in Table 59.6,LaCoO3 or LaMnO3 are (TPB). The electrode material catalyzes the oxygen promising candidates for SOFC cathodes and LaGaO3- molecules to be dissociated into atoms, charged and based oxides for the electrolyte. In addition, recently incorporated into the electrolyte (Fig. 59.9). For the there have been several reports on the application of cathode material, the electrocatalytic activity is an im- Cr-based perovskites as anodes. Therefore, the con- portant parameter to be considered. The surface reac- cept of SOFCs based entirely on perovskite com- tion rate constant in oxygen isotope exchange is a good ponents, all-perovskite SOFCs, is also proposed and measure for the catalytic activity. Kilner et al. [59.29] some preliminary results have been reported [59.27]. compared various oxides in isotope diffusion coefficient In contrast to the SOFCs using oxide-ion-conducting and found a positive correlation between those param- electrolytes, the development of SOFCs using high- eters. A highly mixed electronic and ionic conductor temperature proton-conducting electrolytes is slightly may be a promising candidate in terms of the elec- delayed, particularly when compared with develop- trode performance. At the early stage of SOFC de- ment of polymer electrolyte-type fuel cells. However, velopment, La0:8Ca0:2MnO3 or La0:6Sr0:4MnO3 were the Toyota group has quite successfully demonstrated widely used, however, because of low oxide ion con- a high-power SOFC using a BaCeO3-based electrolyte ductivity in Mn-based perovskite oxide, the reaction film on Pd foil [59.28]. Their data suggest that the is limited to a three-phase boundary resulting in large proton-conducting perovskite oxides might also be an cathodic overpotential. Recently, several oxides have important component in real SOFCs in the near future. been reported to show extremely high surface exchange rate for oxygen activation. Baumann et al. [59.30] 59.3.1 Cathode compared several Co- and Fe-based perovskites in a controlled shape and found Ba0:5Sr0:5Co0:8Fe0:2O3 The principal requirement for a cathode of a SOFC is to shows 100 times smaller electrochemical resistance electrochemically reduce oxygen molecules into oxide than that of La0:6Sr0:4Co0:2Fe0:8O3 (LSCF) which is

Table 59.6 Important materials for perovskite oxide for solid oxide fuel cell applications Component Typical Materials Cathode La.Sr/MnO3,La.Sr/CoO3,Sm0:5Sr0:5CoO3,La.Sr/Fe.Co/O3 2 C C C 2 Electrolyte La.Sr/Ga.Mg/O3.O /, BaCeO3.H /,BaZrO3.H /, SrZrO3.H /,Ba2In2O5.O / Anode La1xSrxCr1yMyO3 (M = Mn, Fe, Co, Ni), SrTiO3 Interconnector La.Ca/CrO3 , 3 2 O O 5 1 0.8 0.6 0.4 0.2 0 : 0 84 : doped 0 Cr 3 5 Zr : 0 16 : 0 Mn (Y 25 B/A cationsB/A ratio : 2 0 Sr Annealing time (h) 75 : 0 have been widely used, ]orLa cermet -stabilized ZrO (LSCF) analyzed with low-energy 35 . Tao and Irvine have focused 3 3 ı O O 2 2 3 : 0 ] show interesting performance and are O x Co 36 8 M : Sr/(La + Sr) B:A x 0 Change in surface composition of 1 Fe 4 : 0 Sr 02468 6 In particular, improved performance has been : 0 1 0 Sr/(Sr + La) Sr/(Sr 0.6 0.4 0.8 0.2 ion scattering spectroscopy with La, Nd, etc. [59. YSZ) cermet haswell been known to widely be deactivated used.coarsening. easily by In However, aggregation addition, and Ni re-oxidationcurs of is easily, resulting Ni in the is permanent alsoTherefore, failure of recently, oc- the the cell. application ofode oxide has for the also an- beenoxide anodes, proposed perovskite and oxides among such as the SrTiO proposed in particular, Ni-Y the elements in the outermostthe surface layer. surface Obviously, of LSCFand it is is immediately considered that enriched an Sr-enrichedreactive with surface is with Sr highly Cr orface S, activity. resulting in Apparently, theperovskite the is decrease surface slightly in different composition sur- fromever, reactivity that of for of oxygen dissociation bulk, is how- affected more strongly by B-site cations. Therefore,unclear there is points still on some for parameters oxygen determining dissociation and the this willintensively activity be in discussed the more future. 59.3.2 Anode For the anode of SOFC, metal-oxide ion-conducting oxide composites named (LSCrM) [59. Fig. 59.10 promising as oxide anodes. obtained withCr complex and(La,Sr)Cr Mn perovskites at based the upon B-sites forming compositions La upon doped lanthanumMn chromite up doped with to Sr 20% and dopant on the B-site, usually 5 or ] 4 33 NiF n 2– 2 shows O O ,which ]. How- electrode 3 Surface diffusion 31 3 ), or K 59.10 – z CoO x e 5 O O (s) : CoO y is also reported 0 / O B Surface pathway 6 Sr x Sr 5 ↔ O ; O : – 0 2 2 (g) La ]. Figure . O 34 on. Another active com- ] reported that existence of nge reaction rate. The dou- (LSCF) due to surface ac- 32 3 2 (ad) – O O e 8 59.9) is also contributed to the : (g) + VO°° + 2e + VO°° (g) 0 2 Fe Charge transfer /incorporation transfer Charge phase on the 2 : ½ O 0 4 et al. [59. 2– 2– O O Adsorption Diffusion Co 4 : CoO 0 Sase Reaction route considered for a cathode of 2 ), Rudrusden–Poppered (A / 6 Sr 6 Sr : ; 0 Charge transfer Charge Bulk pathway On the other hand, one of another important issues BaO La 2 . also has high oxide ion conductivity [59. ever, the mostis popular La composition used for SOFC as an active phase for oxygen dissociation [59. and so not onlyphases, perovskite in but also particularphase, perovskite-related the is now oxygen-deﬁcient attracting as perovskite cathode much catalyst attention as for thetions. active intermediate temperature opera- for SOFC development iscrease long-term in stability cathode andare performance several de- reasons is decreasing pointed cathodicreported, performance out. is i. e., There chemicaland poisoning sintering, and with phase separation. Cr,face Among segregation S them, with sur- Sr and onbeen B the suggested perovskite cathode recently has [59. the change inenergy surface ion scattering composition techniques, of which are LSCF sensitive to by low- structure. Fig. 59.9 often used for the intermediate-temperature SOFCode. cath- One reasonassigned for to such thereaction high high area cathodic mixed available,ary activity i. conductivity (route e., is and 2 theoxygen in large two dissociation Fig. phase reacti bound- position to the cathode is Sm enhances the oxygen excha ble perovskite phase of PrBaCo SOFC tivity and stability.the Recently, more research oxygen-deﬁcient perovskites has(A such shifted as double to a Novel Materials and Selected Applications

Part E

Part E | 59.3 1414 Inorganic Perovskite Oxides 59.3 Application for Solid Oxide Fuel Cells (SOFCs) 1415

–2 59.3 | E Part Voltage (V) Power density (W cm ) log σ (S cm–1) Temperature (°C) 1.2 0.6 1000 900 800 700 600 Wet H2 @ 900 °C Wet CH4 @ 950 °C 0 Wet H @ 850 °C Wet 5 % H @ 900 °C 1.0 2 2 0.5 ZrO2–7:5mol%Sc2O3 La0.8Sr0.2Ga0.8Mg0.115Co0.085O3 Bi2O3–25mol%Y2O3 CeO2–5mol%Y2O3 0.8 0.4 –1 CeO2–10mol%Gd2O3 0.6 0.3

0.4 0.2 –2

0.2 0.1

0.0 0.0 –3 ThO2–15mol%Y2O3 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 SrCe0.95Yb0.05O3 –2 ZrO2–15mol%CaO Current density (A cm ) [H+] ZrO2–8mol%Y2O3 Fig. 59.11 Power generation property of the cell using La0.8Sr0.2Ga0.8Mg0.2O3 –4 La0:75Sr0:25Cr0:5Mn0:5O3 for the anode 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1000/T (K–1) 10%. .La0:75Sr0:25/Cr0:5Mn0:5O3 (LSCrM) exhibits comparable electrochemical performance to Ni/YSZ Fig. 59.12 Comparison of the oxide ion conductivity of cermets [59.27, 36]. Figure 59.11 shows the I–V, LSGM with typical fluorite oxide I–P curves of the cell using LSCrM for the anode at 1173 and 1123 K. Apparently, the open circuit 59.3.3 Electrolyte potential (OCV) is achieved to a theoretical level and reasonable power density of 0:4W=cm2 was achieved An oxide ion conductor is widely used for electrolytes at 1173 K. The electrode polarization resistance ap- of SOFCs. In the case of SOFCs, solid proton con- 2 proaches 0:2 cm at 1173 K in 97%H2/3%H2O. ducting ceramics such as BaZrO3 or SrCeO3 doped Good performance is achieved for methane oxidation with various rare earth cations to B-sites have been without using excess steam. The anode is stable in also studied [59.39], in particular for low-temperature both fuel and air conditions and shows stable electrode SOFCs; however, considering the chemical stability and performance in methane. Thus both redox stability power density achieved, SOFCs using oxide ion con- and operation in low steam hydrocarbons have been ducting electrolytes are more promising. At present, demonstrated. Catalytic studies of LSCM demonstrate almost all SOFCs under development for commer- that it is primarily a direct-oxidation catalyst for cialization use YSZ for the electrolyte, with only a methane oxidation as opposed to a reforming catalyst, few exceptions. However, for increasing power den- with the redox chemistry involving the Mn–O–Mn sity, there is still strong demand for replacing YSZ bonds. Although oxygen ion mobility is low in the with an alternative electrolyte showing higher oxide ion oxidized state, the diffusion coefficient for oxide ions conductivity. CeO2 with fluorite structure and LaGaO3 in reduced LSCrM is comparable to YSZ. with perovskite structure are showing promise as elec- Another important double perovskite is trolytes for this purpose. In particular, LaGaO3 doped Sr2MgMoO6ı, which has recently been shown with Sr and Mg is the first pure oxide ion conductor to offer good performance, with power densities of with a perovskite structure [59.40]. Figure 59.12 shows 2 2 0:84 W=cm in H2 and 0:44 W=cm in CH4 at 1073 K, a comparison of the oxide ion conductivity of LSGM and good sulfur tolerance [59.37]. The molybdenum- with typical fluorite oxide. Apparently, oxide ion con- containing double perovskite was initially prepared ductivity in LSGM is much higher than that of YSZ and at 1473 K in flowing 5% H2 and then deposited on almost comparable with Gd-doped CeO2. In the case of top of a lanthanum ceria buffer layer before testing. CeO2, partial electron conductivity appears in reducing Although there is some decomposition recognized, it is atmospheres, however, LSGM exhibits the pure oxide also reported that La0:5Sr0:5MnO3 is active to not only ion conductivity across a wide pO2 range, i. e., from pure the cathode but also the anode. The maximum power oxygen to pure hydrogen. density of 1 W=cm2 was reported at 1273 K when The application of LSGM for electrolytes of SOFCs La0:9Sr0:1Ga0:8Mg0:2O3 (LSGM, 0:5 mm thickness) is is also studied and the power generation property of used for the electrolyte [59.38]. the cell using LSGM is shown in Fig. 59.13.Since 2 ) –1 1.2 3 shows O 3 0.1 O Y shows the 1 3–a -based ox- 3–a : 3 0.5 1000/T (K 1000/T 0 ] O 3–a O 1.1 3–a O Y Zr O 0.1 ].) Plots for x O 45 0.06 5 : 0.4 Temperature (°C) Y In 0.2 0 0.06 44 Zr 59.14 Y x 0.9 0.95 Zr Yb 0.8 4 1 : BaCe 3–a 0 0.95 1.0 O SrZr CaZr 0.1 BaCe and hydrolysis with wa- doped with a Y thin film SrCe Nd w high reactivity with CO 3 2 0.9 BaandSr),whichshowsfast doped with Y shows reason- 0.9 D BaCe 3 ) were added from [59. –1 (A 3 3 O 1 : 0 0.8 Y 1000 900 800 700 600 5 ]. BaCeO Comparison of proton conductivity : 0 ]. 44 or BZrO Zr 45 4 3 : 0 0.7 Another important family for electrolytes of SOFC –4 –1 –2 –3 Conductivity (S cm (S Conductivity 10 10 10 10 proton conductivity. Protons informed perovskite oxide by were thetial following protons equationof are and the mobile the perovskite at intersti- lattice. the Figure interstitial position comparison of protonide conductivity in [59. perovskite ox- ably highIt proton is conductivity reportedachievedbyusingBaCeO at that low extremely temperature. large power density is ides doped withticular Y it from is chemical reported stability; that in BaCe par- ter [59. for the electrolyteconducting of perovskites sho SOFC; however, these proton- is oxide protonACeO conductor and perovskite oxide of Fig. 59.14 resulting in decompositionate. by Therefore, formationrequired an of for increase carbon- these inhas oxide chemical been perovskite. stability high Recently, is interest there on ACe in perovskite oxide. (After [59. reasonably high protonchemical conductivity stability and to reasonable CO BaCe is ) 2 –2 2.0 1.5 1.0 0.5 0.0 ) 5 –2 ], however, is insufficient. 43210 2 , the open circuit 2 Power density Power (W cm 5 mm), high maximum : 1073K Current density cm (A ]. In any case, LSGM per- to pure H 42 2 , which is a Brownmillerite struc- 873K 5 O 2 et al. reported that La-doped CeO ]. In et al. reported that Ti- and La-codoped 2 43 15 V, which is almost the same as that 182 mm thickness : Huang Power generation property of the cell using Hong 0.183 mm thickness0.183 is promising as a buffer layer for preventing Ni 2 1.0 1.2 Terminal voltage (V) 0.0 0.6 0.4 0.8 0.2 ovskite oxide has aconductor high for potential intermediate-temperature SOFCs as andan a as alternative fast to oxideconductor ion YSZ. is Ba Another interesting oxide ion effective for preventing Ni diffusionelectrical41 [59. conductivity of La-doped CeO transport number of theto oxide unity ion from in pure LSGM is O close Fig. 59.13 LSGM with 0 power density iscell achieved using compared YSZ. with Therefore,erating that it temperature of is of the expectedusing SOFCs that could LSGM the be for op- of decreased the LSGM by electrolytes. is another Makingreported important thin that subject, films however, LSGMthe it has anode is substrate some andhigh reactivity so as the expected with from power the density Ni filmductivity. is thickness in Therefore, not and ionic as for con- LSGMreaction film with cells, Ni preventing tant in issue. the anode substrate is an impor- potential is 1: of theoretical openspite of circuit the thick potential. electrolyte In (0 addition, in diffusion with reasonablesintering electrical properties conductivity [59. and Recently, CeO ture and althoughlower oxide than that ion ofconductors conductivity is YSZ, also is this considered as new slightly afor promising family electrolyte SOFCs of [59. oxide ion Novel Materials and Selected Applications

Part E

Part E | 59.3 1416 Inorganic Perovskite Oxides 59.3 Application for Solid Oxide Fuel Cells (SOFCs) 1417 atE|59.3 | E Part Interconnection lnσT (S·cm–1K) 12 Electrolyte x = 0.1 x = 0.2 x = 0.3 Air 11 electrode Fuel flow

9 Air flow Fuel electrode

Fig. 59.15 Schematic view of the SOFC stack 8 59.3.4 Interconnector La1–xCaxCrO3–δ The interconnector is an important component for 7 stacking SOFCs and requires high stability in reducing and oxidizing atmospheres. The interconnector is used for connecting the SOFC single cell as shown in 6 Fig. 59.15 and so high electrical conductivity and no oxide ion conductivity are also required for the high performance of the SOFC stack. Perovskite oxides of 5 0.5 1.51.0 2.0 2.5 3.0 3.5 LaCrO3 have been also widely used for interconnectors. 1000/T (K–1) Although LaCrO3 is stable under reducing and oxidizing atmospheres, the electrical conductivity is still Fig. 59.16 Electrical conductivity of La1xCaxCrO3ı insufficient and also sintering is rather difficult. There- (x D 0:10:3) in air as a function of inverse temperature. 2C fore, doping Ca for La-site in LaCrO3 is generally (After [59.47]) performed for increasing conductivity as well as sintering properties. electrical conductivity decreases with a reduction of Figure 59.16 shows electronic conductivity of oxygen partial pressures. The electrical conductivity doped LaCrO as a function of temperature [59.46, p1=4 3 is proportional to O2 , which is consistent with the 47]. The electronic conductivity increases with in- defect chemistry of La1xCaxCrO3ı [59.47]. A dop- creasing temperature, suggesting the semiconductor ing of the B-site has been also considered by several temperature dependence. An increasing of the Ca con- authors [59.48]. A typical dopant cation is Mg2C,re- 3C centration in La1xCaxCrO3ı enhanced the electronic placed into Cr sites. This substitution also increases conductivity due to the increase of Cr4C concen- the concentration of Cr4C, and eventually increases tration. There are some deviations of the electrical the electrical conduction. Because of low sintering conductivity among the examined alkaline earth ele- property, SrTiO3 doped with La for Sr sites or Nd ments: Ca-doped LaCrO3 shows larger electrical con- for Ti sites is also studied as an interconnector. Pe- ductivity than Sr-doped LaCrO3. This difference was rovskite oxide is also important for interconnectors of reported to be due to the difference of lattice dis- SOFCs. tortion and phase stability. The activation energy for In summary, perovskite oxide is widely used for conductivity was 0:120:14 eV and the mobility was current SOFCs and becomes important compounds for 0:0660:075 cm2=.Vs/ at 11731323 K. SOFC components. Therefore, the all-perovskite con- The electronic conductivity decreases with a reduc- cept is also proposed for SOFC and the device con- tion of oxygen partial pressure because of the decrease sisting of materials with the same structure is highly of Cr4C concentration in a reducing atmosphere. The interesting. - 3 4 ) O 10 –1 ) ) 05 : NiO 0 (K chem chem 2 (D*) –1 3 Ga (D T (D 3 3 4 24 : 10 CoO 0 CoO 0.1 ]. The oxygen Cu 0.1 Sr LaCoO 51 71 Sr 0.9 : (D*) 9 0 3 0.9 La 3–x/2 Ni La 2 59.18, oxide ion trans- AlO x LaCoO Ca 1–x was reported on PNCG with ]. La / 52 2 (D*) 8 3 (D*) 3 FeO FeO decomposition and partial oxidation of 0.25 (D*) ) 0.1 3 ]) x Sr –1 =.min cm Sr s 2 Diffusivity of oxide ion in several perovskite 49 0.75 0.9 LaFeO La La (cm v 7 The most interesting use of mixed conductors is to 5 mm thickness from air to He [59. –5.0 –5.5 log D –6.0 –4.5 : alkanes are reported by usingmoving mixed the conductors for reactant re- frompermeate oxygen or through mixed into conductor membranes, reactiona systems. To gradient ina driving oxygen force for partial oxygenbrane transport; pressure reactor however, in system, is differences mem- in oxygen required partial pres- as oxides considered for(After an [59. oxygen permeation membrane. transport route isanalysis also and studied as by shownports neutron through in interstitial diffraction positions Fig. in theis rock clearly demonstrated. salt Therefore, block in this PNCG,ions oxide are mainly transported through rockthe salt perovskite but block. not On in thehigh other hole hand, conduction PNCG and shows holesin are the perovskite mainly block; transported and sothat in PNCG, two it different is routes interesting for oxidetion ion are and expected hole conduc- [59. 0 combine the catalytic reactionbrane and the reactor so-called system; mem- such and as several NO catalytic reactions Fig. 59.17 oxides, the oxide ion permeation property in La (denoted as PNCG)oxygen is the permeation optimized andmately composition permeation for 3 cc rate of approxi- based oxide is nowthat studied in doping detail and Cuoxygen it permeation is and reported rate Ga and is Pr effective for increasing , ; 4 ]. h at p ion ox- 49 4 NiF (59.1) 2 / min (BSCF) 2 NiF 3 2 shows the O 2 : =.cm 0 n can be eas- 59.1), the oxy- 59.17 Co 59.5,K 8 : 0 05 ml Fe ; temperature and 5 : T h 1 0 p p Sr In 5 ; electron conductivity, : t 0 / el se of the large amount of oxy- terstitial oxyge ]. According to ( ion 50 ion C el el T . 2 R 59.1), the oxygen permeation rate is limited 16F ; oxygen flux, 2 -type oxide becau D O ; membrane thickness, 4 t J 2 O J From ( Recently, perovskite-related oxides are also attract- NiF means high and low oxygen partial pressure respec- 2 l p oxide ion conductivity, F; Faraday constant, R;stant, gas con- gen deficiency. As explained in Figure ides consist of a series of connectedsalt perovskite and blocks, rock and asalt large free block. volume Therefore, exists in in the rock ily introduced into the rock salt block. For the K Here, Perovskite oxide shows high electronic andconductivity and oxide these ionic conductors are called mixed conductors. Since chargecan compensation be automatically by achieved ion withtivity in electronic transport mixed conduc- conductors, so ions canmixed be transported conductors in without outsidean circuits. important application Therefore, of such mixed conductorsa is separation as membrane forgen oxygen permeation from rate air. is The shownif oxy- as the the bulk following diffusion equation is a rate-limiting step 59.4 Oxygen Separating Membrane 1173 K is reported for Ba diffusivity of oxide ionsconsidered for in oxygen permeation several membranes perovskite [59. oxides tively. Apparently, Fe- or Co-basedfast oxide perovskite ion conductivity oxides and soation show rate large is oxygen expected perme- on these perovskitea oxides. large In oxygen fact, permeation rate of 1: (2 mm thickness) [59. gen permeation rate is determinedoxide by ions the diffusivity and of the the oxygen partial thickness pressure differential of across thebrane the mem- is membrane the when same.rate Therefore, should the be oxygenthickness; increased however, permeation it with is reported decreasingmeation rate that is membrane dependent the on oxygen membrane thicknessthickness per- when is large,oxygen but permeation with rate decreasing becomesgen independent thickness, permeation of rate the because oxy- of thereactions limitation of by surface oxygen dissociation.oxygen To permeation achieve rate, not the only large high diffusivityide of ion ox- in bulk butdissociation also is high required. surface activity to oxygen ing much interestuse as for mixed oxygen conductors,perovskite-related oxides, and permeation there is membranes. applied strong interestK Among for the the by bulk diffusivity of oxygen. Figure Novel Materials and Selected Applications

Part E

Part E | 59.4 1418 Inorganic Perovskite Oxides 59.5 Summary 1419 atE|59.5 | E Part

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

(Ni, Cu, Ga)O6 octahedron layer

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

(Ni, Cu, Ga)O6 octahedron layer

O3 O2 O2 O3 O2 O2 c O2 O3 O2-O3-O2 oxide-ion diffusion paths O3 O3 O3 in (Pr, La)–O layer O2 O2 O2 O2 O2 a (Ni, Cu, Ga)O6 octahedron layer b

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

20 CH4 is as low as approximately 10 atm, a large O2 O2 O2 oxygen permeation rate is achieved under CH4 partial oxidation conditions and it is reported that an oxygen permeation rate of 5 cc=.min cm2/ was achieved by using SrFe0:8Co0:2O3 perovskite [59.54]. However, the oxygen permeation rate is decreased by phase changes CH4 Mixed conductor CO, H2 in a reducing atmosphere of CH4. Therefore, although Fe- or Co-based perovskites have been mainly studied, the most important issues for application of perovskite oxides to catalytic membrane reactors are stability in Fig. 59.19 Schematic image of the catalytic membrane re- a wide oxygen partial pressure range and it is reported actor using mixed conductors that Ni- or Fe-doped LaGaO3 shows hole and oxide ion

conductivity stably over wide pO2 ranges and the oxy- sure are automatically achieved. Therefore, the combi- gen permeation rate of 12 cc=.min cm2/ was exhibited nation of mixed conductors with catalytic reactions is on La0:7Sr0:3Ga0:6Fe0:4O3 of 0:2 mm thick at 1273 K the most ideal usage. From this aspect, partial oxida- in CH4 partial oxidation [59.55]. As a result, obvi- tion of CH4 has been studied with perovskite mixed ously, perovskites are important materials for oxygen conductors for oxygen separation from air [59.53]. The separation membranes and the application of catalytic schematic image of this catalytic reactor system was membrane reactors is an important area; albeit the shown in Fig. 59.19. Since oxygen partial pressure in chemical stability is required to be much improved.

59.5 Summary

In this chapter, crystal structures of perovskites and Since high electric conductivity and surface activ- related oxides were explained. Perovskite oxide has ity to oxygen dissociation are achieved simultane- a variety of composition and component elements. In ously, perovskite oxides, mainly Co-, Fe- and Mn- addition, there are many isomorphs in crystal struc- based oxides are widely used for SOFCs and oxy- ture. Therefore, based on variety of crystal struc- gen permeation membranes. On the other hand, for tures, there are many functions and rich applica- the application to photocatalysts, Ta- or Ti-based per- tion areas. In particular, in this chapter, the applica- ovskites are highly active. Therefore, perovskite ox- tion of perovskite oxides for photocatalytic properties ide is a highly important compound for these ar- and solid oxide fuel cells were brieﬂy overviewed. eas. , 154 ,213 2 ,1699 140 (39/40), 2963 , 320 (2003) ,9(1996) 176 2 (9), 3177 (1998) 7,3593(2014) , 709 (2002) 177, 3087 (2006) , 13 (2000) 145 171, 1 (2004) 86–88 129 212, 157 (2003) Goodenough, C. Milliken: 152/153 , A2085 (2006) ,1736(1996) , 2581 (1998) 153 143 81 , 4100 (2012) 24 , 1843 (2008) (11), 3588 (2012) , 703 (1996) , 254 (2006) 3–4,359(1981) 44, 21 (2000) , 200 (2005) 178 95 312 152 ,265(1987) 86–88 70 ,3801(1994) , 613 (2012) em. Mater. Science ram. Soc. State Ion. 116 (1996) J. Electrochem. Soc. J. Am. Ceram. Soc. Source State Ion. B931 (2007) Y. Takita: J. Electrichem. Soc. Ion. H.U. Habermeier, J. Maier: J. Electrochem. Soc. S. Cook, D. McPhail,J.A. T. Kilner: Ishihara, Energy H.H. Environ. Sci. Brongersma, A.V. Berenov, S.J. Skinner,24 J.A. Kilner: Chem. Mat. S. Shin, I. Hattori: Solid State Ion. N. Sakai, K. Yamaji,State Ion. T. Horita, H. Yokokawa: Solid (2005) Soc. Y. Takita: Solid Sate Ion. J. Electrochem. Soc. N.P. Xu: J. Membrane Sci. (1993) H. Matsumoto: Solid State Ion. Ch 59.38 Y.H. Huang, R.I. Dass, Z.L. Xing, J.B. Goodenough: 59.42 K. Huang, R. Tichy, J.B. 59.43 J.E. Hong, T. Inagaki, S. Ida, T. Ishihara: J. Am. Ce- 59.40 H. Iwahara, T. Esaka, H. Uchida, N. Maeda: Solid 59.41 T. Ishihara, H. Matsuda, Y. Takita: J. Am. Chem. Soc. 59.36 J.T.S. Irvine, P.R. Slater, P.A. Wright: Ionics 59.37 S.W. Tao, J.T.S. Irvine: Nat. Mater. 59.39 T. Ishihara, S. Fukui, M. Enoki, H. Matsumoto: 59.29 N. Ito, M. Iijima, K. Kimura, S. Iguchi: J. Power 59.44 J.B. Goodenough, J.E. Ruiz-Diaz, Y.S. Zhen: Solid 59.45 H. Iwahara: Solid State Ion. 59.32 T. Ishihara, M. Honda, T. Shibayama, H. Nishiguchi, 59.30 J.A. Kilner, R.A. De Souza, I.C.59.31 Fullarton: Solid State F.S. Baumann, J. Fleig, G. Cristiani, B. Stuhlhofer, 59.35 J. Druce, H. Tellez, M. Burriel, M. Sharp, L. Fawcett, 59.34 M. Burriel, J. Pena-Martinez, R.J. Chater, S. Fearn, 59.46 I. Higuchi, T. Tsukamoto, N. Sata, S. Yamaguchi, 59.48 I. Yasuda, T. Hikita: J. Electrochem. Soc. 59.33 M. Sase, K. Yashiro, K. Sato, J. Mizusaki, T. Kawada, 59.47 W.J. Weber, C.W. Griffin, J.L.Bates: J. Ceram. Am. 59.55 T. Ishihara, Y. Tsuruta, T. Todaka, H. Nishiguchi, 59.54 B. Ma, U. Balachandran, J.H. Park, C.U. Segre: 59.51 L.A. Tan, X.H. Gu, L. Yang, W.Q. Jin, L.X. Zhang, 59.49 J.W.59.50 Fergus: Solid State Ion. J.A. Kilner: Solid State Ion. 59.52 T. Ishihara, S. Miyoshi, T. Furuno, O. Sanguanruang, 59.53 M. Yashima, H. Yamada, S. Nuansaeng, T. Ishihara: , 58 17,1734 , 5th edn. ,189(1986) , 373 (2000) 64 331 ,633(1979) , 467 (1993) ,543(1980) , 561 (2003) 14 12 78 Advanced Inorganic 140 ,74(2007) , ed. by Japanese So- (6), 74 (1988) 299 ese Society of Chemistry, 258 Y. Hatakeyama, K. Ogawa, ,257(1985) Perovskites Modern and An- ,165(1977) rovskite Oxide for Solid Oxide 58 , 1 (1999) 14 , 3492 (1993) Pe 103 66 ,L209(1988) , 94 (1887) 27 Perovskites and High Tc Superconduc- ,265(1986) 1998 (Wiley, New York 1988) Structural Inorganic Chemistry (Springer, New York 2009) ,4260(1994) 26 50 , 237 (2011) (Almaz, Thunder Bay 2002) 12 (Gordon Breach, New York 1990) , 164 (2002) ,2770(1975) ,97(1978) ishi, M. Kimura,418 T. Okamoto, N. Hamada: Nature Chem.Soc.Jpn. Rev. day Trans. T. Ishihara: Appl. Catal. B K. Shimomura: Mater. Res. Bull. cient Catalysis J. Chem. Soc. Chem. Commun. K. Fueki: J. Electrochem. Soc. J. Phys. Chem. B J. Solid State Chem. tors D. Ramirez, C.W. Chu,Rev. J.H. B Eggert, H.K. Mao: Phys. 1891 (1987) J. Appl. Phys. Chemistry J. Bechtold, K. Forster, C.W. Chu: Phys. Rev. Lett. Fuel Cells (Oxford Univ. Press, Oxford 1984) Kikan Kigaku Sasetsu,ciety No.32 of Chemistry (Japan Tokyo 1997) p. 9 31 Earth Planet. Inter. 3 (2005) 59.22 Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M. Uen- 59.19 H. Yasuda, T. Nitadori, N. Mizuno, M. Misono: Bull. 59.21 H. Kato, A. Kudo: Catal. Today 59.23 K. Maeda: J. Photochem. Photobiol. C Photochem. 59.24 A. Kudo, H. Kato: Chem. Phys. Lett. 59.18 Y. Teraoka, T. Harada, S. Kagawa: J. Chem. Soc. Fara- 59.20 H. Iwakuni, Y. Shinmyou, H. Yano, H. Matsumoto, 59.17 S. Shin, H. Arakawa, 59.11 J.B. Bednorz, K.A. Müller: Z. Phys. B 59.27 T. Ishihara (Ed.): 59.10 H. Arai, T. Yamada, K. Eguchi, T. Seiyama: Appl. 59.26 K. Domen, S. Naito, M. Soma, T. Onishi, K. Tamaru: 59.16 J. Mizusaki, I. Yasuda, J. Shimoyama, S. Yamaguchi, 59.25 T. Ishihara, H. Nishiguchi, K. Fukamachi, Y. Takita: 59.15 J. Mizusaki, M. Yoshihiro, S. Yamauchi, K. Fueki: 59.14L.Gao,Y.Y.Xue,F.Chen,Q.Xiong,R.L.Meng, 59.9 R.H. Mitchell, T. Bay: 59.13 H. Maeda, Y. Tanaka, M. Fukutomi, T. Asano: Jpn. 59.8 F.S. Galasso: 59.6 A.F. Well: 59.12 P.H. Hor, R.L. Meng, Y.Q. Wang, L. Gao, Z.J. Huang, 59.28 S. Tao, J.T.S. Irvine, J.A. Kilner: Adv. Mater. 59.7 A.F. Cotton, G. Wilkinson: 59.3 F. Kanamura: Perovskite related compound. In: 59.4 S. Geller, J.B. Jeffries, P.J. Curlander: Acta Cryst. B 59.5 R.C. Liebermann, L.E.A. Jones, A.E. Ringwood: Phys. 59.2 T. Yagi, H.K. Mao, P.M. Bell: Phys. Chem. Minerals 59.1 R.M. Hazen: Sci. Amer. References Novel Materials and Selected Applications

Part E

Part E | 59 1420