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

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The computational domain (Fig. 9.16) is prepared by generating a grid for the

entire stack. Due to diverse length scales along the axis of the stack, grid

optimization is done to make the numerical results independent of the mesh size.

At fuel manifold inlets, gaseous species concentra tions are specified as

equilibrium compositions of the town gas reformate at 6508C. Steam-to-carbon

ratio is kept as 3.06 for this particular steady-state analysis. Both fuel and air

gas manifo ld inlet conditions are summarized in Table 9.5. Mixed convective

and radiative heat transfer bounda ry conditions are applied to the side surfaces

of the stack to accurately model the heat exchange with the balance of plant

components. Top and bottom surfaces, on the other hand, are assigned with

Fuel Inlet

Air Inlet

Potential ta

Current tap

Fuel Inlet

Radiator Plate

The generic stack

Fig. 9.16 Computational domain prepared for the generic stack

Table 9.5 Independent variables of the generic stack simulation

Volume fraction

Inlet

temp.(8C)

Flow rate

(liter/min) O

CO H

Air 570 32.0 0.21 0.79

Fuel 650 6.93 0.0148 0.0840 0.0759 0.5347 0.2906

Table 9.6 Physical properties used in the generic stack simulation

Thermal

conductivity

(W/mK)

Electrical

resistivity

( m)

Viscous

resistance

(1/m

) Porosity Tortuosity

Separator 26.3 5.88  10

8

–––

Fuel-side current

collector

2.47 4.26  10

7

1  10

0.9 1.0

Anode 10.7 7.09  10

7

1  10

+12

0.4 1.5

Electrolyte 1.32 1.02  10

1

–––

Cathode 3.2 1.26  10

–5

1  10

+12

0.5 1.5

Air-side current

collector

5.67 4.76  10

8

1  10

0.9 1.0

9 Intermediate-Temperature Solid Oxide Fuel Cells Using LaGaO

199

known heat flux values. All external surfaces are kept at zero current, except

those labeled as ‘‘potential tap’’ and ‘‘current tap,’’ for which a fixed electrical

current value (29.95 A, corresponding to 72.5% fuel utilization) is assigned. The

potential tap is kept at the ground potential. Physical properties used in the

model are listed in Table 9.6.

Figure 9.17 shows the species concentration profi les along the radial axis of

cells numbered 1, 9, and 17. Although the maximum temperatures estimated at

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Radial Distance [m]

Partial Pressure [atm]

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Radial Distance [m]

Partial Pressure [atm]

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Radial Distance [m]

Partial Pressure [atm]

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Radial Distance [m]

Partial Pressure [atm]

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Radial Distance [m]

Partial Pressure [atm]

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.010.020.030.040.050.060.07

0 0.010.020.030.040.050.06

0.07

0 0.01 0.02 0.03 0.04 0.05 0.06

0.07

0 0.010.020.030.040.050.060.07

0 0.010.020.030.040.050.06

0.07

0 0.010.020.030.040.050.06

0.07

Radial Distance [m]

Partial Pressure [atm]

a) Anode side of cell #1. b) Cathode side of cell #1.

e) Anode side of cell #17. f) Cathode side of cell #17.

c) Anode side of cell #9. d) Cathode side of cell #9.

Fig. 9.17 Gaseous species concentrations on selected cells

200 T. Akbay

the center of these three cells are quite different, the gas concentrations remain

uniform, implying that the manifolds are capable of contributing to stable gas

dynamics in the stack. In addition, the model successfully captures the phenom-

enon of back-diffusion of air against the depleted fuel stream over the anode

side of the cells. It can be noticed that methane undergoes the in situ steam

reforming process over the cell and becomes 100% converted at a distance

corresponding to two-thirds of the cell radius.

The surface temperature distribution of the stack is shown in Fig. 9.18. The

cooling effect of the air is slightly more noticeable than that of the fuel. Because

the stack loses extra heat from the top and bottom surfaces as well as the

surfaces adjacent to the radiator plate, there are unavoidable hot spots on the

front and back surfaces. On the other hand, the maximum temperature reaches

about 8008C at the center of the separator plates adjacent to cells numbered 9

and 26 (Fig. 9.19).

a) Front view

b) Back view

Air manifold.

754.4

743.1

731.7

720.3

708.9

697.5

686.2

674.8

663.4

652.0

640.6

629.2

617.9

606.5

595.1

583.7

572.3

560.9

549.6

538.2

526.8

754.4

743.0

731.6

720.3

708.9

697.5

686.1

674.7

663.4

652.0

640.6

629.2

617.8

606.5

595.1

583.7

572.3

560.9

549.6

538.2

526.8

Fuel manifold.

Air manifold.

Fuel manifold.

Fig. 9.18 Temperature distribution (8C) on the generic stack surface

Cell #1

805.8

799.1

788.3

777.4

766.6

755.7

744.8

734.0

724.1

712.3

701.4

690.5

679.7

668.8

658.0

647.1

636.2

625.4

615.5

603.7

592.8

581.9

800.7

795.7

790.7

785.6

780.6

775.6

770.6

765.5

760.5

755.5

750.4

745.4

740.4

735.3

730.3

725.3

720.3

715.2

710.2

705.2

Cell # 9

Cell #17

Cell #18

Cell #26

Cell #34

)b )a

Fig. 9.19 Temperature distribution (8C) on (a) selected cells and (b) selected separators

9 Intermediate-Temperature Solid Oxide Fuel Cells Using LaGaO

201

Figure 9.20 shows the current density distribution on the cells numbered 1, 9,

17, 18, 26, and 34, starting from the top of the stack. The current density

assumes a bell-shaped profile on the surface of each cell. The maximum current

density reaches 0.6 A/cm

at the center of the cells that observe the highest

temperature. The average cell potential, on the other hand, is calculated as

0.785 V.

References

1. B.C.H. Steele, J. Power Sources 49, 1 (1994)

2. N.Q. Minh, T. Takahashi, Science and Technology of Ceramic Fuel Cells, Elsevier,

Amsterdam (1995)

3. N.Q. Minh, Solid State Ionics 174, 271 (2004)

4. S.C. Singhal, Solid State Ionics 152–153, 405 (2002)

5. T. Ishihara, H. Matsuda, Y. Takita, J. Am. Chem. Soc. 116, 3801 (1994)

6. T. Ishihara, H. Matsuda, Y. Takita, Solid State Ionics 79, 147 (1995)

7. M. Feng, J.B. Goodenough, Eur. J. Solid State Inorg. Chem. 31, 663 (1994)

8. K. Huang, R.S. Tichy, J.B. Goodenough, J. Am. Ceram. Soc. 81, 2565 (1998)

9. P. Huang, A. Petric, J. Electrochem. Soc. 142, 1644 (1996)

10. T. Ishihara, T. Akbay, H. Furutani, Y. Takita, Solid State Ionics 113–115, 585 (1998)

11. T. Ishihara, S. Ishikawa, C. Yu, T. Akbay, K. Hosoi, H. Nishiguchi, Y. Takita, Phys.

Chem. Chem. Phys. 5, 2257 (2003)

12. T. Inagaki, H. Yoshida, K. Miura, S. Ohara, R. Maric, X. Zhang, K. Mukai, T. Fukui,

Proc. Electrochem. Soc. Vol. 2001–16, 963 (2001)

13. X. Zhang, S. Ohara, R. Maric, KI. Mukai, T. Fukui, H. Yoshida, M. Nishimura,

T. Inagaki, K. Miura, J. Power Sources 83, 170 (1999)

14. S. Ohara, R. Maric, X. Zhang, K. Mukai, T. Fukui, H. Yoshida, T. Inagaki, K. Miura,

J. Power Sources 86, 455 (2000)

15. S. Awatsu, H. Iwasaki, K. Isono, N. Chitose, G. Uozumi, T. Inagaki, T, Ishihara, ECS

Trans. 7(1), 1583 (2007)

Cell #1

6042.71

5740.57

5438.44

5136.30

4834.17

4532.03

4229.90

3927.76

3625.63

3323.49

3021.36

2719.22

2417.08

2114.95

1812.81

1510.68

1208.54

906.41

604.27

302.14

0.00

Cell # 9

Cell #17

Cell #18

Cell #26

Cell #34

Fig. 9.20 Current density

distribution (A/m

)on

selected cells in the generic

stack

202 T. Akbay

16. T. Ishihara, M. Honda, T. Shibayama, H. Minami, H. Nishiguchi, Y. Takita, J. Electro-

chem. Soc. 145, 3177 (1998)

17. M. Kawasaki, N. Chitose, J. Akikusa, T. Akbay, H. Eto, T. Inagaki, T. Ishihara, ECS

Trans. 7(1), 1229 (2007)

18. J. Akikusa, K. Adachi, K. Hoshino, T. Ishihara, Y. Takita, J. Electrochem. Soc. 148,

1275, (2001)

19. F. Nishiwaki, T. Inagaki, J. Kano, J. Akikusa, N. Murakami, K. Hosoi, J. Power Sources

155, 809, (2006)

20. M. Shibata, N. Murakami, T. Akbay, H. Eto, K. Hosoi, H. Nakajima, J. Kano, F.

Nishiwaki, T. Inagaki, S. Yamasaki, ECS Trans. 7(1), 77 (2007)

21. Fluent, 2006, FLUENT 6.3 User’s Guide, Fluent Incorporated, Canterra Resource Park,

Cavendish Court, Lebanon, NH, USA

22. T. Akbay, N. Chitose, T. Miyazawa, N. Murakami, K. Hosoi, F. Nishiwaki, T. Inagaki,

Proc. The Fourth Int. Conf. Fuel Cell Sci. Eng. Technol. June 19–21 (2006)

23. T. Akbay, N. Chitose, T. Miyazawa, M. Shibata, F. Nishiwaki, and T. Inagaki, Proc. of

The Fifth Int. Conf. on Fuel Cell Sci. Eng. and Tech., June 18–20 (2007)

9 Intermediate-Temperature Solid Oxide Fuel Cells Using LaGaO

203

Chapter 10

Quick-Start-Up Type SOFC Using LaGaO

Based New Electrolyte

Akira Kawakami

10.1 Introduction

TOTO is the top sanitary ware manufacturer in Japan. The company also has

much experience in making both traditional and advanced ceramic products.

Solid oxide fuel cells (SOFCs) are mainly composed of ceramics, and our

fabrication technology has been utilized to pro duce high-performance SOFCs

at a low cost. TOTO commenced research and development of the tubular-type

cell (whose diameter is 16 mm) using a zirconia-based electrolyte for stationary

power applications in 1989 (Table 10.1). We have demonstrated more than

10,000 h of continuous operation in single-cell tests, and our modules have

accumulated more than 3,000 h of operation as of 2004 with electrical efficiency

(Lowe heating value, LHV) of 55% [1]. Since 2004, TOTO has been

collaborating with Hitachi, Ltd. on a new NEDO (New Energy and Industrial

Technology Development Organization) project. We are developing a 20-kW

class co-generation system by integration with TOTO’s stacks (Fig. 10.1).

Meanwhile, TOTO also started developing micro-tubular SOFCs using an

LaGaO

-based new electrolyte for portabl e power applications in 2002.

A micro-tubular SOFC has potential for portable and transportation appli-

cations because of its advantages, including high volumetric power density and

high thermal shock resistance, which enable rapid start-up [2]. We started

developing the micro-SOFCs using micro-tubular cells (whose diameter is less

than 5 mm) in 2002 under the sponsorship of the NEDO project [3]. The

objectives of this project are to lower the operating temperature to

5008–7008C and to develop a compact stack that will enable both downsizing

of the system and rapid start-up (in minutes or less) for the SOFC system. To

achieve high power density at lower temperatures, the anode-supported design

with a thin film of lanthanum gallate with strontium and magnesium doping

was selected. The single cells and cell stacks were evaluated using various

A. Kawakami (*)

TOTO Ltd., Chigasaki, Kanagawa 253-8577 Japan

e-mail: akira.kawakami@toto.co.jp

T. Ishihara (ed.), Perovskite Oxide for Solid Oxide Fuel Cells,

Fuel Cells and Hydrogen Energy, DOI 10.1007/978-0-387-77708-5_10,

Ó Springer ScienceþBusiness Media, LLC 2009

205

practical fuels, i.e., hydrogen, simulated reformate gas of liquid petroleum gas

(LPG), and direct fueling of dimethyl ether (DME).

In this chapter, we summarize the current status of micro-tubular SOFC for

portable power applications at TOTO.

10.2 Micro-Tubular Cell Development

Table 10.2 shows materials and fabrication processes of the TOTO micro-tubular

cell. The anode substrate tube made of NiO/(ZrO

)

0.9

)

0.1

(NiO/YSZ) was

formed by extrusion molding. The anode interlayer and the electrolyte were sub-

sequently coated onto the anode substrate by slurry coating and co-fired. These

techniques allow us to significantly reduce the cost of SOFCs. An NiO/

(Ce

0.9

0.1

1.95

(NiO/GDC10) anode interlayer was inserted between the sub-

strate and the electrolyte to enhance the anode performance at lower temperatures.

0.8

0.2

0.8

0.2

2.8

(LSGM) was selected for the electrolyte material, which is

being considered as a promising material for the intermediate-temperature operat-

ing SOFC [4–5]. For the cathode material, (La

0.6

0.4

)(Co

0.8

0.2

(LSCF) was

coated on the LSGM layer by slurry coating and then fired. An important point in

cell fabrication is the insertion of a dense (Ce

0.6

0.4

1.8

(LDC40) layer between

the LSGM layer and NiO/GDC10 layer to prevent undesirable nickel diffusion

from the anode to the LSGM during the co-firing procedure [6]. However, LDC40

is a material of extremely low sintering property, so that it is difficult to densify the

Fig. 10.1 Appearance of

TOTO stacks for stationary

power applications

Table 10.1 Materials used in TOTO tubular cell with zirconia-based electrolyte

Components Materials

Cathode tube (La,Sr)MnO

Electrolyte ScSZ

Anode Ni/YSZ

Interconnector (La,Ca)CrO

206 A. Kawakami

LDC40 layer. Therefore, we investigated the various additives for the sintering

promotion of LDC40 powder. As a result, it was found that the addition of a small

amount of Ga

to LDC40 was effective in obtaining a fully dense LDC40 layer

below a sintering temperature of 13008C (Fig. 10.2), and it also improved electrical

conductivity of the LDC40 itself.

According to electron probe micro-analyzer (EPMA) observations of the

cross sections of micro-tubular cells, it was seen that the reaction between the

LSGM layer and the anode was avoided by the introduction of the LDC40 layer,

and there was no obvious element diffusion between the LSGM layer and the

LDC40 layer. Furthermore, it was found that LDC40 is a preferable material as

the insertion layer between the LSGM layer and the anode because it is not very

reactive with LSGM. Shown in Fig. 10.3 are the crystal phases identified by the

X-ray diffraction (XRD) method after heat-treating the mixture at 14008C,

which is obtained by uniformly mixing equivalent weights of the LSGM powder

and each of the cerium-containing oxide powders. In the case of LDC40, no other

phase was observed in a matrix, whereas in the case of GDC10, a large amount of

reaction generating phase SrLaGa

was observed. The reaction-generating

phases of SrLaGa

are electrically highly resistant, which suggests that when

these reaction-generating phases are developed between the LSGM layer and the

cerium-containing layer, the electric power-generating capacity is lowered.

Table 10.2 Materials and fabrication process of micro-tubular cell

Components Material Fabrication Firing

Anode tube NiO/YSZ Extrude molding

Anode interlayer NiO/GDC10 Co-firing

Electrolyte LDC40(Ga

)-LSGM

(double layered)

Slurry coating

Cathode LSCF Firing

100

Relative Density (%)

1300ºC

No additive

1400ºC

No additive

1600ºC

No additive

1300ºC

addition

Fig. 10.2 Effect of small

addition of Ga

LDC40

10 Quick-Start-Up Type SOFC 207

Figure 10.4 is a picture of a micro-tubular single cell. The diameter of the cell

is 5 mm, and the active length is 50 mm. The single cell was joined to the current

collector cap with silver-brazed metal, and its performance was tested in a

furnace. Figure 10.5 shows the evaluation method for single-cell performance.

A fuel gas was supplied inside the cell, and air was supplied to the outside of the

cell. The current voltage and impedance of the single cells were measured using

a potentiostat and a frequency response analyzer in the temperature range from

5008 to 7008C.

GDC10/LSGM

LDC40/LSGM

LaGaO

SrLaGa

CeO

Intensity

Fig. 10.3 XRD pattern of powder mixture consisting of LSGM and cerium-containing oxide

after firing at 14008C

Fig. 10.4 TOTO micro-

tubular cell

208 A. Kawakami

Figure 10.6 shows the typical I-V curves of a micro-tubular single cell using

dry H

in N

as fuel. H

flow was fixed at 0.12 L/min. The open circuit voltage

(OCV) was close to the theoretical value. The results indicated that the

electrolyte has a good gas tightness, and the chemical reaction between

LSGM and Ni is effectively avoided by the layer of LDC40 with Ga

. (In

the case of LDC40 without Ga

, the OCV was 0.85 V.) We obtained the

maximum power densities of 0.85, 0.70, and 0.24 W/cm

at 7008, 6008, and

5008C, respectively. Figure 10.7 shows the impedance spectra of a micro-

tubular cell measured under 0.125 A/cm

at various temperatures. It has

Fuel

Air

Ag wire

wire

Glass tube

Current Collector

Furnace

Micro tube cell

Fig. 10.5 Evaluation

method for single-cell

performance

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.5 1.0 1.5 2.0

Current Density (A/cm

)

Voltage (V)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Power Density (W/cm

)

Fuel :H2 in N2

Oxidant :Air

700°C

600°C

500°C

Fig. 10.6 I–V characteristic of micro-tubular single cell

10 Quick-Start-Up Type SOFC 209