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

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4.5 Basic Properties of the LSGM Electrolyte System

4.5.1 Phase Diagram of La-Sr-Ga-Mg-O

The phase diagram of the quaternary system LaO

1.5

–SrO–GaO

1.5

–MgO has

been reported for LaGaO

-based oxides (Fig. 4.12) [18, 25, 26]. Several impur-

ity phases, i.e., LaSrGa

(237), LaSrGaO

(214) phases, are reported for this

LaGaO

perovskite oxide, and the single, two-phase, and three-phase regions

appeared in phase diagrams. However, no phase containing Mg was found in

the compositional range of Fig. 4.12, which implies a higher so lubility of Mg in

the perovskite phase and related compound. As already discussed in a previous

section, doping with Mg is also effective for expanding the solubility of Sr in the

La site and expands the perovskite regions compared to the non-doped La

–

binary phase diagrams; thus, doping Mg is effective not only for the

introduction of vacancies but also in expanding the perovskite regions.

4.5.2 Reactivity with SOFC Component

The reactivity of this LaGaO

oxide has been also investigated by several

groups. The reactivity of LaGaO

-based oxide [27] with La(Sr)CoO

perovskite

oxide or a Pt electrode [28] is important. In particular , platinum seems to react

Fig. 4.12 Phase diagram of pseudo-quaternary LaO

1.5

–SrO–GaO

1.5

–MgO system

4 Oxide Ion Conductivity in Perovskite Oxide for SOFC Electrolyte 77

easily with gallium oxide to redu ce Ga

3þ

to Ga

to form Ga

O, which is volatile

[28]. Therefore, for the practical application of this material to the SOFC, one

should pay attention to the choice of the electrode mate rial and/or its condi-

tions of use, such as temperature or atmosphere. Another undesirable reaction

of LSGM for the electrolyte of a SOFC is that with an Ni-based anode [27].

Because LaNiO

is one of the typical perovskite oxides, Ni is easily substituted

with Ga on the B site of the perovskite phase to form a highly resistive phase

between the LSGM electrolyte and NiO anode during cell preparation [29]. To

prevent the reaction between the components, man y buffer layers are used for

the current SOFC, even for the case of YSZ electrolyte. In the case of the LSGM

perovskite electrolyte, it is reported that CeO

doped with La (LDC) shows low

reactivity when the amount of La is in a narrow range, around 40 mol%.

Therefore, by insertion of an LDC layer between an NiO anode and an

LSGM electrolyte, a SOFC with high power density can be achieved [30]. It is

also reported that insertion of an LDC layer is effective for preparing an LSGM

thin film by a conventional method such as slip casting. However, sintering

LDC is rather difficult, and also the electrical conductivity of this compound is

low. Therefore, this LDC buffer layer makes a significant contribution to the

total resistance of the cell. Further suitable buffer layer compound for LSGM

electrolyte is still needed to prevent the reaction between NiO and LSGM. One

of the useful compounds is Sm-doped CeO

(SDM), which also exhibits high

oxide ion conduction.

In contrast, the reactivity of the LSGM electrolyte with the cathode perovs-

kite oxide is not extensive. When YSZ is used for the electrolyte, Co-based

perovskite oxides such as LaCoO

cannot be used, in spite of the high surface

activity to oxygen dissociation; this is because the reaction between YSZ and

LaCoO

forms the La

pyrochlore phase with high resistivity [31]. How-

ever, in case of LSGM electrolyte, compatibility with LaCoO

is high enough to

use it as a cathode of SOFCs. Horita et al. report ed that no reaction products

were observed after a reasonably long period of operation [32]. As a result, Co-

based perovskites can be used as cathodes with low values of cathodic over-

potentials. High compatibility with Co-based perovskites is one of the main

advantages of LaGaO

perovskites as the electrolyte of SOFCs. It is also noted

that La

NiO

has low reactivity toward LSGM; however, in the case of

LaMnO

, which is the most popular cathode material, some interactions

between La

NiO

and LaMnO

were observed with the formation of the

insulator layer.

4.5.3 Thermal Expansion Behavior and Other Properties

Thermal expansion is another important property for the application of mate-

rials to SOFC. The thermal expansion increased with the increa se in the dopant

concentration. Anomalies in thermal expansion behavior for LaGaO

and

78 T. Ishihara

0.9

0.1

GaO

were observed around 400 K and were assigned to the phase

transition from orthorhombic to rhombohedral structure. On the other hand,

Sr- and Mg-doped materials show a monotonic expansion; the average thermal

expansion coefficient measured was around 11.5  10

6

1

within the tem-

perature range from 298 to 1273 K. Therefore, the average thermal expansion

coefficient is slightly larger than but close to that of Y

-stabilized ZrO

Thermal conductivity of this material has also been studied. Table 4.1

summarizes the thermal conductivity, specific heat, and fracture strength of

LaGaO

perovskite. The average thermal conductivity of this LSGM is slightly

smaller than that of YSZ. Therefore, SOFCs using LaGaO

-based perovskites

are more desirable from the point of view of uniform temperature distribution.

4.5.4 Behavior of Minor Carrier

Since the concentration of the minor carriers (electrons and/or holes) deter-

mines the chemical leakage of oxygen when oxide ion conductor is used as the

electrolyte in SOFCs [33], analysis of the performance of electron and hole

conduction is an important subject for the electrolyte materials. Partial electro-

nic conduction is commonly analyzed by the ion blocking method, the so-called

Wagner polarization method. Partial electronic conductivity is the sum of

electronic and hole contribution to the total conductivity, and each conductiv-

ity is proportional to a carrier density. Therefore, the total electronic conduc-

tivity can be expressed as follows:

s ¼ ILF=RT ¼ s

þ s

¼ n

f1  expð FEðLÞ=RTÞg

þsp

fexpðFEðLÞ=RTÞ1g (4:3)

Here, I, L, E(L),F, R, and T are current, length of the sample, applied

voltage, the Faraday constant, the gas constant, and temperature, respectively.

When the hole cond uction is dominant, the second term in the above equation is

dominant and the current increases exponentially with applied potential. Since

the predominant charge carriers change from holes to electrons with decreasing

and p–n transition occurs at intermediate P

, current, I, shows a typical

‘‘S’’-shaped curve against potential, E(L). Figure 4.13 shows the typical I–E(L)

curve observed in Ni-doped LSGM by the ion blocking method [34].

Table 4.1 Thermal properties of LaGaO

oxide

Temperature

(K)

Specific heat

(J/g k)

Thermal conductivity

(W/m K)

Fracture

strength (MPa)

298 0.410 1.55 220

673 0.464 1.55 180

1073 0.556 1.77 136

4 Oxide Ion Conductivity in Perovskite Oxide for SOFC Electrolyte 79

Differential of the observed current with respect to potential, which corre-

sponds to the P

in the sample, gives the dependence of the partial electronic

conductivity on P

Determination of hole and electron conductivities and transport numbers of

oxide ion in LaGaO

-based oxides were performed by the polarization method

by Baker et al. [21], Yamaji et al. [35], and Kim and Yoo [36]. Kim et al. reported

that P

dependence of hole and electron conductivity is proportional to P

1/4

and P

1/4

, respectively, and well obeys the Hebb–Wagner theory. The results

of the polarization method clearly indicate that LaGaO

-based oxides exhibit

almost pure oxide ion conductivity over a wide oxygen partial pressure range

(10

> P

> 10

25

atm). Compared with CeO

-based oxides or Bi

oxide,

this is a major advantage of the LaGaO

-based oxides as well as the redox

stability comparable to that of ZrO

-based oxide. Consequently, LaGaO

based oxides are highly promising as an electrolyte for SOFCs, particularly

when compared with ceria-based oxides. Kim et al. [36] also investigated the

temperature dependence of hole and electronic conductivity in Mg-doped

gallate, La

0.9

0.1

0.8

0.2

, with the polarization method. Figure 4.14

shows the evaluated boundaries of the electrolytic domain in La

0.9

0.1-

0.8

0.2

plotted in the axis of log (P

/atm) versus reciprocal tempera-

ture. The lower boundary of the electrolytic domain (defined as t

ion

> 0.99) for

LSGM is 10

23

atm at 1273 K. This pressure is even lower than that of CaO-

stabilized ZrO

and that of YSZ, which is also plotted in Fig. 4.14. Conse-

quently, it is clear that the electrolytic domain covers the P

range required for

the operation of SOFCs and that the LSGM can be successfully used as

electrolyte in SOFCs.

0 500 1000 1500 2000

600°C

700°C

800°C

900°C

le(mA)

V(mV)

Fig. 4.13 Typical I–E (L)

curves observed in Ni-doped

LSGM by the ion blocking

method

80 T. Ishihara

On the other hand, doping by a small amount of Co is effective in increasing

the electrical conductivity, as discussed in Section 4.2. However, with the increas-

ing Co concentration, the partial hole conductivity also increases. As a result,

hole, electron, and oxide ion conductivities in Co-doped LSGM have also been

studied by the polarization method [34], and the estimated conductivities at

1073 K as a function of Co content are shown in Fig. 4.15. It is seen that both

the electronic and hole conductivities become significant with the increasing Co

concentration; however, at Co amount less than 5 mol% on the Ga site, the oxide

ion conductivity is much higher than partial hole conductivity. Furthermore, the

oxide ion conductivity also increases as Co amount increases. Although the hole

conduction becomes significant and the oxide becomes a mixed electronic and

ionic conductor when the amount of Co is excess, doping Co is preferable for

improving the oxide ion conductivity in LaGaO

-based oxide.

(7)

CaO-ZrO

2) Y

-ThO

3) (Y

)

0.05

(CeO

)

0.95

4) (CaO)

0.15

(La

)

0.85

5) (Y

)

0.27

(Bi

)

0.73

6) (Y

)

0.5

(TiO

)

0.5

7) La

0.9

0.1

0.8

0.2

Temperature /°C

Fig. 4.14 Boundaries of the

electrolytic domain of

0.9

0.1

0.8

0.2

the plane of log (P

/atm)

versus reciprocal

temperature

–0.55

0.00 0.02 0.04 0.06 0.08 0.10

– 0.90

–0.85

–0.80

–0.75

–0.70

–0.65

–0.60

log(σ

/S cm

–1

)

–11

–10

–9

–8

–7

–6

–5

–4

–3

–2

–1

1073K

=1atm

log(σ

/Scm

–1

)

Fig. 4.15 Estimated ionic

and electronic conductivity

in LSGM at 1073 K as a

function of Co content

under P

¼ 10

5

atm

4 Oxide Ion Conductivity in Perovskite Oxide for SOFC Electrolyte 81

4.5.5 Diffusivity of Oxide Ion

The diffusiv ity of ox ide ions in LSGM was further studied by

O tracer

diffusion measurements [37]. Diffusion of oxide ion in perovski te oxide is

explained in detail in Chapter 5. LSGM exhibits large values of diffusion

coefficient, and the observed fast diffusion in LSGM originates from the higher

mobility of oxide ions in the perovskite structure as compared with the fluorite

structure (Table 4.2), presumably due to a large free volume in the lattice.

Recently, atomic simulation of the oxygen transport in the perovskites, in

particular, LaGaO

, were performed based on quantum chemistry [38, 39]. In the

case of perovskite oxides, the migrating oxygen ion must pass through the

triangular orifice defined by two A-site (La

3þ

) ions and one B-site ion. As a result

of lattice relaxation during oxygen ion migration, it was suggested that there is a

small deviation from the direct path for oxygen migration, as illustrated schema-

tically in Fig. 4.16. Indeed the calculations predicted a curved path around the

Table 4.2 Comparison of mobility of oxide ion in selected fluorite and LSGM oxide at

1073 K (Cited from Ref. 54)

Dt /em

/s Ea /eV d ½V



 /cm

Dcm

/s m cm

/Vs

0.81

0.19

2d

6.2x10

8

1.0 0.10 2.95x10

1.31x10

6

1.41x10

5

0.858

0.142

2d

7.54x10

9

1.53 0.142 4.19x10

1.06x10

7

1.15x10

6

0.85

0.15

2d

1.87x10

8

1.22 0.15 4.43x10

2.49x10

7

2.69x10

6

0.9

0.1

2d

2.70x10

8

0.9 0.05 1.26x10

1.08x10

6

1.17x10

5

0.9

0.1

0.8

0.2

3d

3.24x10

7

0.74 0.15 2.53x10

6.4x10

6

6.93x10

5

0.8

0.2

0.8

0.2

3d

4.13x10

7

0.63 0.20 3.34x10

6.12x10

6

6.62x10

5

0.8

0.2

0.8

0.125

4.50x10

7

0.42 0.1645 2.78x10

8.21x10

6

8.89x10

5

0.085

3d

: Tracer diffusion coefficient, D: Self diffusion coefficient

Fig. 4.16 Calculated path

for oxygen vacancy

migration. h: Vacancy

82 T. Ishihara

octahedron edge with the saddle point located away from the adjacent B-site

cation. Similar experimental results are discussed in Chapter 6. Further detailed

molecular dynamic calculations were also done for the LSGM system. Figure 4.17

shows the calculated mean square displacement of constituent atoms as a func-

tion of time. It is evident that the mean displacement between O–O ions expanded

with time; however, the positions of other ions remain essentially constant, which

suggests only diffusion of oxygen but not of cations in LSGM. The diffusion

coefficients were calculated according to the random walk theory by using the

slope in Fig. 4.17, and the results are compared in Fig. 4.18 with the diffusion

O-O

Sr-Sr

La-La

Ga-Ga

Mg-Mg

0.0

1.0

2.0

3.0

4.0

5.0

0.40

0.30

0.20

0.10

0.0

Time /

Mean square displacement /

Fig. 4.17 Mean

displacement of constituent

atoms as a function of time

in LSGM

0.8 0.9 1.0 1.1 1.2 1.3

–6

–7

–8

from Conductivity

from Tracer diffusion

from MD Calculation

Diffusion constant /cm

–1

1000/T /K

–1

Fig. 4.18 Comparison of diffusion coefficient in LSGM estimated from ionic conductivity

and tracer diffusion measurements and calculated by molecular dynamic calculations

4 Oxide Ion Conductivity in Perovskite Oxide for SOFC Electrolyte 83

coefficients measured during the tracer diffusion experiments. A good agreement

of the values of oxygen diffusion coefficients estimated from the results of the

computer simulation and experimentally measured in tracer diffusion measure-

ments is observed, suggesting that the mobile oxygen vacancies and substituted

ions in this oxide behave as ideal noninteracting point defects.

Furthermore, the defect binding energy in LaGaO

was also calculated by

Islam et al. [38, 39 ], and Table 4.3 summarizes the calculated cluster binding

energies (E

) for some selected dopants on the La and Ga sites. The low binding

energy calculated for the Sr substitution on the La site could be assigned to the

similarity in ionic size of Sr with that of La, leading to smaller local stresses in

the lattice. In contrast, rather large cluster binding energies of about 0.9 eV are

estimated for B-site dopants, suggesting that the oxygen vacancies tend to be

trapped around dopants in the Ga site. Although dopants on the Ga sites are

effective for expanding the unit cell volume and improving the solubility of Sr

on the La site, observed binding of oxygen vacancies may de crease the ox ygen

vacancy diffusivity. The results of computer calculations presented in Table 4.3

suggest that doping of Cu on the Ga site has the smallest binding energy;

however, the electrical conductivity of LSGM decreased when doped with

2þ

, presumably because of the low chemical stability of Cu

2þ

. Consequently,

from the aspect of chemical stability, Co

2þ

/Co

3þ

doping is more desirable

experimentally. In any case, due to the high crystal symmetry, it is evident

that the perovskites are interesting subjects for a computer simulation of ion

conductivity, in particular, oxygen ion diffusivity.

4.6 Performance of a Single Cell Using LSGM Electrolyte

The applications of LSGM as the electrolyte in fuel cells are now commonly

investigated. Figure 4.19 shows the temperature dependence of the maximum

power density and the open circuit potential (OCV) of the cell with Sm

0.5

CoO

cathode and Ni anode [40]. It is seen that open circuit potential (OCV)

increased with the decrease in the operating temperature; the results are in good

Table 4.3 Calculated cluster binding energies (E

) for some selected dopant on the La and Ga

sites in LaGaO

Location Cluster pair Cluster binding energy (E

/eV/defect)

La site Sr

2þ

-oxygen vacancy –0.01

2þ

-oxygen vacancy 0.10

Ga site Mg

2þ

-oxygen vacancy 0.90

2þ

-oxygen vacancy 0.87

2þ

-oxygen vacancy 0.91

2þ

-oxygen vacancy 0.65

84 T. Ishihara

agreement with the theoretical values estimated from the Nernst equation.

Furthermore, the power density was improved by using Sm

0.5

CoO

cathode at all temperatures studied, comparing with that of LaCoO

-based

conventional cathode. The maximum power density was higher than 1.0 and

0.1 W/cm

at 1273 and 873 K, respectively, in spite of the 0.5-mm thickness of the

electrolyte [40]. In comparison with the power densities of the cell utilizing YSZ

electrolyte, a reasonably high power density was achieved at 873 K. Goodenough

and coworkers also investigated the application of LaGaO

-based oxides as the

electrolyte in fuel cells [41]. Similar large values of power density were reported at

intermediate temperatures with La

0.6

0.4

CoO

cathode and Ni-La-doped CeO

cermet anode [42]. Due to high power density, LSGM has been attracting much

interest as a promising SOFC electrolyte for intermediate temperature SOFC.

Oxide ion conductivity in LSGM is further improved by doping Co to Ga

site, albeit the hole conductivity increases. Figure 4.20 shows the open circuit

potential as well as the maximum power density at 1073 K as a function of Co

content in LaGaO

-based oxide electrolyte [40]. The open circuit potential (OCV)

decreases monotonically with increasing Co concentration. In particular, the

decrease in OCV is significant when the Co content on the Ga site is higher

than 10 mol%; this is caused by the increased hole conductivity due to electronic

compensation of doped Co ions. The dependence of OCV on the amount of

doped Co is in good agreement with that of the transport number of oxide ion

(see Fig. 4.10). On the other hand, the power density increases with increasing Co

content and attains a maximum value when 8.5 mol% Co is doped on the Ga

sites. Improvement in the power density is simply explained by the enhanced

oxide ion conductivity resulting from Co doping. When the amount of doped Co

further increases, the hole conduction becomes significant, short-circuiting the

900 1000 1100 1200

Operating temperature /K

2 2 0.9 0.1 0.8 0.2 3 0.5 0.5 3

H +3vol%H O,Ni/La Sr Ga Mg O /Sm Sr CoO , O

Fig. 4.19 Temperature

dependence of the maximum

power density and the open

circuit potential (OCV) of

the cell with Sm

0.5

CoO

cathode o and Ni anode.

Thickness of electrolyte was

0.5 mm

4 Oxide Ion Conductivity in Perovskite Oxide for SOFC Electrolyte 85

cell and decreasing the power density. Consequently, the maximum power den-

sity is obtained when 8.5 mol% Co-doped LSGM electrolyte is used.

On the other hand, it is expected that the power density of the cell i ncreased

with decreasing thickness of the electrolyte, since the main internal resistance

is due to IR losses. Figure 4.21 shows the power density of a n H

-O

cell with

0.18-mm-thick LSGMC-8.5 electrolyte at 1073 and 873 K. As expected, the

power density of the cell increases with decreasing the thickness of the

LSGMC electrolyte. However, the open circuit potential exhibited a tendency

1073K

+3vol%H

O,Ni/La

0.8

0.2

0.8

0.2-X

/Sm

0.5

CoO

, O

(0.5mm thickness)

Fig. 4.20 Open circuit

potential as well as the

maximum power density at

1073 K as a function of Co

content in LaGaO

-based

oxide electrolyte

54321410

0.0

0.5

1.0

1.5

2.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Terminal voltage /V

Current density /A cm

–2

Power density /W cm

–2

1073K

873K

0.183mm thickness

Fig. 4.21 Power-generating

property of H

-O

cell at

1073 and 873 K using 0.18-

mm thickness LSGMC-8.5

as the electrolyte

86 T. Ishihara