Ishihara T. (Ed.) Perovskite Oxide for Solid Oxide Fuel Cells (Fuel Cells and Hydrogen Energy)

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covalent, and therefore the coordination numbers is lower than 6. The typical

example of this type is BaGeO

. In spite of a t value close to 1, i.e., ideal ionic

size combination, BaGeO

crystallizes not in the perovskite structure but in the

silicate-related one. This difference occurs because the preferred coordination

number of Ge is 4. On the other hand, due to the progress in high-pressure

technology, the synthesis of new Ge-based perovskite oxides has been reported

[5]. As the coordination number of Ge increases with the pressure, perovskite

structures with higher coordination numbers are preferred, and a typical exam-

ple of this is CaGeO

. Another group of interest ing perovskite compounds is

oxynitrides, i.e., LaWO

3–x

, LaTiO

N, etc. Therefore, the value of t, which is

determined by the ionic size, is an important index for the stability of perovskite

structures; however, the contribution of the chemical nature, such as the coor-

dinating number of the constituent elements, needs to be taken into account.

The formation of superstructures in the perovskites is discussed next. If a

B-site cation is progressively replaced by a dopant, a large difference in ionic

radii tends to lead to the form ation of the superstructures rather than random

arrangements of the two kinds of ions. The typical case of this is Ba

CaWO

which is regarded as Ba

(CaW)O

. Similarly, in compounds with the general

formula Ba

MTa

, there is random distribution of M and Ta ion in the

octahedral positions when M is Fe, Co, Ni, Zn, or Ca, whereas formation of

a superstructure with hexagonal lattice is observed in Ba

SrTa

. Another

interesting type of superstructure observed in the perovskite is the ordering of

cation vacancies located on A sites: e.g., MNb

(M ¼La, Ce, Pr, Nb) and

MTa

(M ¼La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Y, Er). In these oxides, there

is an octahedral framework of the ReO

type with incomplete occupancy of

the 12-fold-coordinated A sites. Figure 1.4 shows the structure of LaNb

A site

deficient

Fig. 1.4 Structure of

LaNb

, A site-deficient

perovskite oxide

1 Structure and Properties of Perovskite Oxides 5

The B sites of the perovskite structure are occupied by Nb ion, and two-thirds of

the A sites remain vacant.

Other typical polymorphs of the perovskite structure are Brownmillerite

)andK

NiF

structures. Brownmillerite (A

) is an oxygen-

deficient type of perovskite in which the oxygen vacancy is ordered. The unit

cell contains BO

and BO

units in an ordered arrangement. Because of the

oxygen deficiency, the coordination number of A-site cations decreases to 8.

The lattice parameter of the Brownmillerite structure relates to the cubic lattice

parameter (a

) of the ideal perovskite as a ¼b ¼

,c¼4a

. Cu-based oxides

or Ni-based oxides tend to adopt these oxygen-deficient structures because of

the large amou nt of oxygen defects.

A combination of ordered B sites and oxygen defects is seen in K

NiF

structures, which is well known as it shows superconducting properties. The

NiF

structures consist of two units, the KNiF

perovskite unit and the KF

rock salt unit (Fig. 1.5), which are connected in series along the c-axis. As the

rock salt structure is embedded into the c-axis direction, the K

NiF

compound

shows strong two-dimensional properties. Based on the intergrowth of the

different numbers of KNiF

and KF units, there are many structures called

Ruddelsden-Popper compounds with the general formula (ABO

)

(Fig. 1.6); i.e., Sr

(n ¼2), Sr

(n ¼3). It is interesting to compare

the isostructural Sr

TiO

or Ca

MnO

with SrTiO

or CaMnO

, which crystal-

lize in the perovskite structures. Two different A cations forming the perovskite

and the rock salt units are also possible, and LaO



nSrFeO

is the typical

example of this arrangement. Another interesting variant of these K

NiF

structures occurs when two different anions occupy the two building blocks

A ion

B ion

O ion

Perovskite

Rock salt

Perovskite

Rock salt

Perovskite

Fig. 1.5 K

NiF

structure, a

perovskite-related structure

6 T. Ishihara

exclusively, i.e., SrFeO



SrF or KNbO



KF. In any case, it is evident that per-

ovskite oxides comprise a large family of oxides. As a result, a variety of crystal

structures and properties is expected in these compound s. For further detailed

discussion on the perovskite-related oxides, the reader is referred to references [6–9]

1.3 Typical Properties of Perovskite Oxides

Because of the variety of structures and chemical compositions, perovskite

oxides exhibit a large variety of properties. Well-known pro perties of the

perovskite oxides are ferroelectricity in BaTiO

-based oxides and superconduc-

tivity in Ba

YCu

, etc. In a ddition to these well-known properties, several

perovskite oxides exhibi t good electrical conductivity, which is are close to that

of metals, and ionic conductivity, as well as mixed ionic and electronic con-

ductivity. Based on these variations in electrical cond ucting property, perovs-

kite ox ides are chosen as the components for SOFC. It is also well known that

several perovskite oxides exhibit high catalytic activity with respect to various

reactions, in particular, oxidation reactions [10]. Table 1.2 provides examples of

the typical propert ies of perovskite oxides. In this section, several typical

properties of the perovskite oxides, namely, ferroelectricity, magnetism, super-

conductivity, and catalytic activity, are briefly discussed.

Perovskite

Rock salt

Fig. 1.6 Ruddelsden-

Popper structure, another

type of perovskite-related

structure

1 Structure and Properties of Perovskite Oxides 7

Dielectric properties: Ferroelectricity, piezoelectricity, electrostriction, and

pyroelectricity are special properties inherent to diel ectric materials and are

important properties of electroceramics. The best known property of perovs-

kite oxides is ferroelectric behavior, where BaTiO

,PdZrO

, and their doped

compounds are representative example s. The study of ferroelectricity in

BaTiO

has a long history, and many detailed reviews have been published.

Furthermore, because the ferroelectric behavior of BaTiO

has a strong

relationship with the crystal structure, detailed studies of crystal structure

have been reported for BaTiO

.BaTiO

undergoes mainly three-phase trans-

formation, that is, from monoclinic, to tetragonal, and to cubic, as the

temperature increases. Above 303 K, BaTiO

crystallizes in the cubic perovs-

kite structure, which does not show ferroelectric behavior. The high dielectric

constant observed in BaTiO

can b e explained on the basis of the anisotropy

of the crystal structure. Figure 1.7 shows the crystal structure of BaTiO

using

Table 1.2 Typical properties of perovskite oxides

Typical property Typical compound

Ferromagnetic property BaTiO

, PdTiO

Piezoelectricity Pb(Zr, Ti)O

, (Bi, Na)TiO

Electrical conductivity ReO

, SrFeO

, LaCoO

, LaNiO

, LaCrO

Superconductivity La

0.9

0.1

CuO

, YBa

, HgBa

Ion conductivity La(Ca)AIO

,CaTiO

, La(Sr)Ga(Mg)O

,BaZrO

, SrZrO

,BaCeO

Magnetic property LaMnO

, LaFeO

,La

NiMnO

Catalytic property LaCoO

, LaMnO

, BaCuO

Electrode La

0.6

0.4

CoO

,La

0.8

0.2

MnO

= 4.6 × 10

= –7.5 × 10

= 1.1 × 10

=4.1×10

δz = 0.003Å

δz = –0.05Å

δz = 0.06Å

δz = –0.09Å

BaTiO

O (3)

O (2)

Fig. 1.7 Crystal structure of

BaTiO

using Ewald method

and local density of charge

8 T. Ishihara

the Ewald method as well as local charge density [11]. It is seen that the large

negative potential is localized on the O

oxygen atom. When the electric field is

applied, B a

2þ

and Ti

4þ

cations move to the direction opposite to that of the

oxygen atom. Thus, a net dipole moment is created in the unit cell. According

to the Slater theory [11], the electrostatic field is strongly affected by the atoms

locatedinO

sites; thus, a large dipole moment is generated in BaTiO

Electrical conductivity and superconductivity: One of the most well known

properties of perovskite oxides is superconductivity. In 1984, superconductivity

was first reported by Bednorz and Mu

ller in La-Ba-Cu-O perovskite oxide [12].

After their report, much attention was paid to new types of high-temperature

oxide superconductors, mainly Cu-based oxides. As a result, several supercon-

ducting oxides with different A-site cations have been discovered. However, the

presence of Cu on the B site was found to be essential for superconductivity to

occur. High-temperature oxide superconductors of the YBa

system [13]

and the Bi

system [14] were reported in 1987 and 1988, respec-

tively, and currently the critical temperature of the superconducting transition

) has been further increased to 130–155 K in the HgBa

8+d

system

[15]. As all high-temperature superconducting oxides are cuprites (Cu-based

oxides), superconductivity is clearly related to the Cu-O layers. The critical

temperature for superconductivity, T

, is related to the number of Cu-O layers

in the crystal structure:

One Cu-O layer: T

 30 K

Two Cu-O layers: T

 90 K

Three Cu-O layers: T

 110 K

Four Cu-O layers: T

 120 K

It is expected that further increase in the number of Cu-O layers may result

in higher values of T

. However, because of the low chemical stability, synthesis

of five or more Cu- O layered compounds has not been successful so far.

YBa

is one of the most important superconductor systems with high

, and detailed studies of its crystal structure have been performed. Also,

the content of oxygen nonstoichiometry is an important factor for high T

When the value of d is smaller than 0.5, YBa

7–d

crystallizes in an orthor-

hombic structure, which is superconductive, whereas for d > 0.5, YBa

7–d

has a tetragonal structure, which does not exhibit superconductivity. Figure 1.8

shows the crystal structures of both oxygen-deficient phases in YBa

7–d

The main difference between the tw o structures is that the incorporation of

oxygen in the lattice expands the b lattice parameter to a greater extend than the

a lattice parameter. Those changes in crystal structure are related to the oxygen

content, which is determined by the annealing temperature and oxygen partial

pressure during postannea ling treatment. As discussed, superconductivity in

high T

oxides is also dependent on the crystal structure; thus, the high chemical

stability of the perovskite crystal structure could be effective in achieving high

values of T

1 Structure and Properties of Perovskite Oxides 9

In addition to supercond uctivity, there are many perovskite oxides showing

high electronic conductivity, which is close to those of metals such as Cu. The

typical examples of such perovskite oxides are LaCoO

and LaMnO

, which is

now commonly used as a cathode in SOFC. These perovskite oxides shows

superior hole conductivity, which is as high as s ¼100/S/cm. Doping of alio-

valent cation on the A site is also highly effective in enhancing the electrical

conductivity because of the increased number of mobile charge carriers gener-

ated by the charge compensation.

Catalytic activity: Because of the variety of component elements and their

high chemical stability, perovskite oxides have be en also extensively studied as

catalysts for various reactions. Two types of research trends clearly emerged

from these characteristics. The objective of the first trend is the development of

oxidation catalysts or oxygen-activated catalysts as an alternative to catalyst

containing precious metals, whereas the second trend regards perovskite as a

model for active sites. The stability of the perovski te structure allows prepara-

tion of compounds with an unusual valence state of elements or a high extent

of oxygen deficiency. Table 1.3 summarizes the reactions studied by using

perovskite oxides as catalysts. Evidently, the high catalytic activity of perovs-

kite oxides is based partially on the high surface activity to oxygen reduction

ratio or oxygen activation resulting from the large number of oxygen vacancies

present.

Among the various catalytic react ions studied, those applicable to envir-

onmental catalysis (e.g., automobile exhaust gas cleaning catalyst) attract

particular at tention. Ini tially, it was reported that perovskite oxide consist-

ing of Cu, Co, Mn, or Fe exhibited superior activity to NO direct decom-

position at higher temperatures [16–18]. The direct NO decomposition

reaction (2NO ¼N

þO

) is one of the ‘‘dream reacti ons’’ in the catalysis

field. In this reaction, the ease of removal of surface oxygen as a product of

Orthorhombic Tetragonal

Oxygen

deficient

layer

Oxygen

deficient

layer

Fig. 1.8 Orthorhombic and

tetragonal crystal structures

of BaY

, an oxygen-

deficient perovskite

10 T. Ishihara

the reaction plays an important role, and due to the facil ity of oxygen

deficiency present, per ovskite oxides are active with res pect to this reaction

at high temperatures. It is pointed out that doping is highly effective in

enhancing N O decomposition activity. Under an oxygen-enri ched atmo-

sphere (up to 5%), a relatively high NO decomposition activity was reported

for Ba(La)Mn(Mg)O

perovskite [19].

Recently, another interes ting application of perovskite oxides as autom o-

bile catalysts has been reported, namely, the so-called int ellig ent cata lysts [20].

Up to now, three- way Pd-Rh-Pt catalysts have been widely used for the

removal of NO, CO, and uncombus ted hydrocarbons. T o decrease the amount

of precio us metals, a catalyst consistin g of fine particles with high surface-to-

volume ratio is required. Howeve r, these fine particles are not stable under

oper ating conditions and easily sinter, resulting in deactivati on of the catalyst.

To maintain a high dispersion state, the redox property of perovskite oxides

has been proposed; i.e., under oxidat ion conditions, palladium is oxidized and

exists as LaFe

0.57

0.38

0.05

, and under reducing conditions, palladium is

deposited as fine metallic partic les with a radi us of 1–3 nm. This cycling of the

catalyst t hrough oxidizing and reducing conditions results in the partial sub-

stitution of Pd into and deposition fr om the perovskite framework, thus

maintaining a high dispersion state of Pd. This method was found to be highly

effective in improving the long-term stability of Pd during removal of pollu-

tants from exhaust gas (Fig. 1.9). The high dispersion state of Pd can be

recovered by exposing the catalys t to an oxidation and reduction environment.

As a result, this catalyst is called an intelligent catalyst. This unique property

also originates from the high stability of the perovskite crystal structure in

complex oxides.

Table 1.3 Main catalytic reactions studied by using perovskite oxides

Catalytic reaction Example

Oxidation CO, lower hydrocarbon, Methanol

Catalytic combustion

LaCoO

, LaMnO

deNOx Selective reduction LaAIO

, SrTiO

NO decomposition BaMnO

, SrFeO

YBa

NO absorption LaAIO

, BaCeO

, BaFeO

Hydrogenation C

hydrogenation LaCoO

coupling Oxidative CH

coupling BaTiO

0.5

0.2

0.8

Oxygen electrode Oxygen reduction (alkaline solution)

Oxygen generation (alkaline solution)

Cathode for Solid Oxide Fuel Cell

Oxygen sensor

LaCoO

, LaMnO

LaCoO

, LaFeO

LaCoO

, LaMnO

LaCoO

, LaMnO

Gas sensor Oxygen sensor, Humidity sensor,

Alcohol Sensor

SrTiO

, BaSnO

LaCr(Ti)O

, GdCoO

1 Structure and Properties of Perovskite Oxides 11

1.4 Preparation of Perovskite Oxide

Because the perovskite structure is stable at high temperatures and also stable in

terms of thermodynamic equilibrium, the perovskite oxides form only at a

temperature typically higher than 1273 K. The most simple and popular

method for preparat ion of perovskite oxides is the so-called solid-state reaction

method, when the starting compounds (often simple oxides and carbonates) are

calcined at temperatures higher than 1273 K. However, because of the high

temperature of the calcination, the Burumauer-Emmott-Teller (BET) surface

area of the resulting perovskite powders is generally small, usually less than 10

/g. The preparation of perovskite oxide powders with a large surface area,

namely, fine particles, is strongly demanded in various fields, in particular, for

catalyst and electrode application not only for solid oxide fuel cells (SOFC) but

also for batteries and/or electrolysis. To obtain fine particles of perovskite

oxides, some advanced synthetic methods that general ly involve the use of

organic compounds have been developed. However, the preparation of perov s-

kite oxide powders with a large surface area is quite a difficult subject, and the

BET surface area is generally smaller than 50 m

/g. This restriction is easily

understood by considering a sim ple relationship between the specific surface

area (S) and the diameter of a spherical particle (D) [21]:

S ¼ 6=ðrDÞ (1:1)

where r is the density of the sample. Figure 1.10 shows the relationship between

the geometrical surface area (S) of a spherical body and radius (D): the density

Removal of pollutant/%

Operation at 900°C/h

Conventional catalyst

(Pd/Al

)

Intelligent catalyst

LaFe

0.57

0.37

0.05

oxidation

reduction

100

Fig. 1.9 Structure of ‘‘intelligent catalyst’’ and comparison of the catalytical activity of the

intelligent catalyst and the conventional Al

-supported one for the removal of pollutants in

exhaust gas

12 T. Ishihara

of LaCoO

perovskite oxide is much lower than that of a general single oxide

such as MgO or Al

. Therefore, for the purpose of obtaining a high surface

area, such as 100 m

/g, the required particle size of the perovskite oxide must be

smaller than 10 nm, which is quite difficult to achieve.

Figure 1.11 summarizes the general procedure of the liquid-phase synth-

esis method used in the preparation of perovskite oxides with a large surface

Fig. 1.10 Relationship between geometrical surface area (S) of a spherical body and radii (D)

Starting material

(Metal salt, metal

Alkoxide,

metal organic compound)

Solvent

(Water, organic one)

Solution

Precipitating agent

Gel formation agent

Complex formation agent

Evaporation

Precursor

(precipitate, gel, etc.)

heating

Final Oxide

Unique reaction

condition

(Hydrothermal,

Supercrytical etc.)

Fig. 1.11 General procedure of the liquid-phase synthesis method

1 Structure and Properties of Perovskite Oxides 13

area. In this method, atomic-level dispersi on of the c omponent elemen ts in

the precursor solution is essential. Based on the dispersion method, the

proposed liquid-phase preparation method could be classified into three

groups (Table 1.4). The techniques classified into group I use energy such as

ultrasonic vibration or supercritical conditions to achieve a high dispersion

state. The application of microwave heating to a precursor containing

BaCl

, Ti isopropoxide, and KOH has been employed during the synthesis

of BaTiO

fine pa rticles. It has been reported that BaTiO

perovskite

powder with a particle size of 20–30 nm was successfully prepared [22]. On

the other hand, group II focuses on the usage of micelles, which limit the

space for the perovskite precursor. LaMnO

prepared by using reverse

micelles has been repor ted to possess high electrode a ctivity when used at

the anode of a metal-air battery. Finally, techniques in group III involve the

usage of organic compounds for achieving atomic-level dispersion in the

precursor solutions. In the most popular cases, the addition of a mmonia is

used to obtain uniform precipitates of perovskite precur sors. However,

because of the difference in the precipitation rates, it is difficult to obtain

a precursor with uniform distribution of constituent elem ents at the atomic

level.

Teraoka et al. reported the use of organic coordination compounds for the

preparation of perovskites [23]. They found that addition of acetic acid or

maleic acid is useful for obtaining finely powdered perovskite oxides by decreas-

ing the crystallization temperature. Figure 1.12 shows the C

oxidation rate

of LaMnO

perovskite oxide prep ared by various methods and compositions

plotted against the BET surface area. It is evident that the C

oxidation rate

increases monotonically with increasing the BET surface area of LaMnO

, and

it can be easily understood how the preparation method is important for

improving the surface activity of perovski tes.

Table 1.4 Proposed liquid phase synthesis method for perovskite oxides

Category Method

Group I

(Controlled evaporation or

reactant decomposition rate)

Spray pyrosis, Spray (mist, aerosol) thermal

decomposition, Freeze dry, Combustion synthesis,

Microwave assisted method, Supercritical water

Group II

(Usage of designed micro pore) Antimicelle

Group III

(Designed precursor) Hy droxide precursor; Uniform precipitation, Sol gel

method another precursor; Cyanide decomposition,

Oxalic Acid method, EDTA-citrate complexing

method, Pechini method

14 T. Ishihara