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

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connection of the short B–O bond may supply a percolation path for charge

transfer, which may lead to high electronic conductivity.

Among the various (Ba,Sr,Ca,La)

0.6

1–x

(M ¼Ni, Mg, Mn, Fe, Cr,

In, Sn) compositions, Sr

0.2

0.4

0.2

0.8

exhibits the highest conductivity

(10 S/cm at P

¼10

20

atm at 9308C) [60, 61]. These materials exhibit rather

low conductiv ity in air ( 10

3

S/cm at 9308C) because there is limited oxygen

reduction under this condition. The conductivity increases with decreasing P

and reaches 1–10 S/cm at a P

lower than 10

17

atm at 9308C when Ti

4þ

5þ

was partially reduced, which releases electrons for charge transfer; how-

ever, the performance of the Sr

0.2

0.4

0.2

0.8

tungsten bronze as an

SOFC anode is not ideal [61]. Introduction of Mn into the bronze was observed

to significantly reduce polarization losses; however, performance was still

inferior to better mixed conducting oxides such as titania-doped YSZ [62, 63].

8.8 Anode Materials for All-Perovskite Fuel Cells

The all-perovskite solid oxide fuel cell concept is highly attractive, offering structu-

rally coherent interfaces with good physical and thermal matching. Perovskite

electrolytes, normally based upon lanthanum gallate, and perovskite cathodes are

already widely utilized in SOFCs; however, perovskite anodes are not so available.

A number of possible perovskite anodes have been described in this chapter,

and there have been some early successes in implementing these in all-perovskite

SOFCs. (La

0.75

0.25

)Cr

0.5

(LSCM) has been utilized successfully in an all-

perovskite SOFC with Co-doped LSGM and La

0.6

0.4

CoO

cathode, achieving a

power density of 0.3 Wcm

2

at 8508C using a 0.6-mm-thick electrolyte [64]. Using

an LSGM electrolyte prepared by tape casting with a thickness of about 120 mm

0.8

0.2

MnO

3–d

(LSM) and La

0.75

0.25

0.5

3–d

(LSCM) as cathode and

anode material, respectively, good values of power output in a conventional

Fig. 8.8 Comparison between perovskite and tetragonal tungsten bronze lattices

8 Perovskite Oxide Anodes for SOFCs 179

electrolyte-supported cell were achieved, 570 mW/cm

using wet hydrogen

as fuel and pure O

asoxidantat8008C [65]. An all-perovskite solid oxide

fuel cell incorporating (La,Sr)(Ga,Mg)O

(LSGM) electrolyte, (La,Sr)

(Ga,Mn)O

(LSGMn) anode, and (La,Sr)CoO

(LSC) cathode fabricated by

a combination of dry pressing and screen printing, yielded a maximum power

density of 350 mW/cm

2

8008C under test conditions with H

and O

the relevant electrodes [66]. Even higher performance was achieved with

MgMoO

6–d

, but using a ceria interlayer between the perovskite anode

and electrolyte [58].

8.9 Conclusions

Some materials can match some requirements, but it is very hard to find a

material that can match all the stringent requirements for SOFC anodes,

particularly redox stability and conductivity. Materials with perovskite struc-

ture are promising, such as the redox stable anode (La

0.75

0.25

)Cr

0.5

[54] and Sr

MgMoO

6–d

[58]. These materials exhibit performance approaching

that of the traditional Ni–YSZ anode. Therefore, we can use a nickel-free

redox-stable anode for SOFCs. With further optimization of the composition

and microstructure, the performance of these materials may be further

improved and hopefully replace the traditional Ni–YSZ anode in the future.

There have been a number of recent reviews including the topic of oxide anodes

where further information may be obtained [67, 68]

Acknowledgments The author thanks his colleagues, especially S.W. Tao and T.D. McColm,

for their assistance in developing this manuscript, and thanks ESF, EPSRC, and NEDO

(Japan) for financial support for research on perovskite anodes.

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182 J.T.S. Irvine

Chapter 9

Intermediate-Temperature Solid Oxide Fuel

Cells Using LaGaO

Taner Akbay

9.1 Introduction

Among other types of fuel cells, deployment of solid oxide fuel cells (SOFCs)

into the distributed energy sector of the consumer market is fast becoming a

reality, predominantly because technological advancements are making them

more reliable and relatively cost competitive. In addition, there has been a

concerted effort in designing and manufacturing novel ceramic materials for

high-performance cell components [1–4]. Over the decades, a large collection of

materials have been systematically investigated for possible application of

SOFC electrolyte and electrodes. In addition, reduced temperature operation

of SOFCs has been attracting attention due to numerous advantages offered

by this mode of operation. Some of these advantages can be listed as follows:

(1) less expensive metallic materials can be used as interconnects, (2) less

degradable components imply higher stability and durability, and (3) responses

to start-up and shut-down procedures are faster. Even internal reforming of

hydrocarbonaceous fuels remains possible at intermediate temperatures. Over-

all, the common denominator impetus is to achieve higher area (or volume)-

specific power densities at lower temperatures.

Mitsubishi Materials Corporation, The Kansai Electric Power Co., Inc., and

Kyushu University have been worki ng together on multiple projects for devel-

oping intermediate-temperature SOFCs. Substantial technical advancement

has been made in manufacturing SOFC units ranging from a 1-kW class module

to a scalable 10-kW class combined heat and power (CHP) system. The core

technology is based on high-performance cells operable in a temperatur e win-

dow between 6008 and 8008C.

Lanthanum gallate-based perovskite oxide is selected as the electrolyte

material for high-performance cells. Strontium is used as a dopan t to the A

site, and magnesium together with cobalt are doped to the B site of the

T. Akbay (*)

Mitsubishi Materials Corporation, Tokyo, Japan

e-mail: akbay@mmc.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_9,

Ó Springer ScienceþBusiness Media, LLC 2009

183

lanthanum gallate. Double doping the B site with cobalt in addition to strontium

actually causes a decrease in oxide ion transport number at high temperatures.

However, this property is regarded as not detrimental for the intermediate-

temperature operation of the cells. In addition, a unique cell stacking technol-

ogy is developed by using metallic separator plates that eliminate the use of

seals.

In this chapter, the key competences of our technology toward the develop-

ment of commercial SOFC power generation units utilizing cation-doped

lanthanum gallate-based electrolyte are summarized.

9.2 Cell Development

9.2.1 Electrolyte

The primary requirement of a fully dense (i.e., impermeable) electrolyte is to

possess sufficient levels of ionic conduction under SOFC operating conditions.

In addition to the chemical compatibility with the electrode materials, electro-

lyte mate rials are also expected to be stable in dual atmospheres of considerably

different oxygen partial pressures. On the other hand, mechanical strength is a

dominant criterion in deciding the cell geometries (i.e., electrode or self-

support).

9.2.1.1 Doped Lanthanum Gallate

High oxide ion conductivity of doped lanthanum gallate compounds at intermedi-

ate temperatures makes them favorable materials as electrolytes for self-supported

SOFC designs [5–10]. In our manufacturing facilities, the self-supported cells are

exclusively fabricated in disk-type planar geometries. Typical dimensions of the

electrolyte support are 120 mm in diameter and 200 mm in thickness.

A conventional solid-state reaction technique is employed for the synthesis

of the electrolyte materials. Commercially available powders of La

, SrCO

, MgO, and CoO are mixed, ball-milled. and calcined in air to obtain the

final composition of La

0.8

0.2

0.8

0.15

0.05

3–d

(LSGMC). The calcined

mixture is then reground and mixed with an organic binder to produce the

LSGMC slurry. As the most prevalent method for fabricating relatively thick

layers of cell components, tape casting is employed for the preparation of green

sheets. Hence, the LSGMC slurry is tape cast into green sheets of appro priate

thickness by using a doctor blade. Green densities are measured as almos t 50%.

Disks are then cut out of the green sheet and fired in air at 14008–15008C for 6 h.

As for the relative densities of the sintered disks, 98%–99% of the theoretical

values are confirmed.

Because of the inclusion of cobalt as an additional B-site dopant, LSGMC

possesses a certain amount of electronic (electron or hole) conductivity, which,

184 T. Akbay

in turn, causes a decreas e in the open circuit voltage (OCV) of the cell. Reducing

the electrolyte thickness is generally thought to be the most favorable way

to increase the cell performance via decreasing the ohmic losses. However,

one needs to be careful in reducing the thickness of mixed conducting electro-

lytes, such as LSGMC, because the electronic leak current would cause

considerable decrease in the cell potential. Our estimations suggests that, at

intermediate temperatures, the optimum LSGMC electrolyte thickness is

around 100 mm [11].

For self-supported cells, the other factor that indirectly dictates the electro-

lyte thickness is the mechanical strength of the electrolyte. As the electrolyte

layer gets st ronger, the possibility of manufacturing thinner self-supported

cell s becomes higher. As powd er characteristics and processing conditions

determine the final microstructure, densely packed small-grained electrolytes

can be manufactured by using fine-size d spherical powders together with lower

sintering temperatures. Figure 9.1 shows the scanning electron microscopy

(SEM) images of the dense LSGMC microstructures obtained by sintering at

diff erent temperatures. The electrochemically a ctive cell can then be manufac-

tured by attaching air- and fuel-side electrodes to the electrolyte, as shown in

Fig. 9.2.

9.2.2 Anode

The level of catalytic activity of anode determines how efficiently the fuel can be

electrochemically oxidized in a fuel cell. In anode conditions, i.e., reducing

atmospheres, most noble and transition metals may provide the desired activity.

However, at intermediate temperatures only a selected set of metals can meet

the whole set of criteria, such as high morphological and dimensional stability

and low thermal expansion mismatch. Currently, nickel is the metal most

commonly used as an anode material in SOFCs. The total electrical conductiv-

ity of a desired anode material should possess electronic as well as ionic

components. Ionic conduction is widely accepted as the necessary conduction

mechanism to enlarge the reaction zone.

9.2.2.1 Nickel/Rare Earth Metal-Doped Ceria Cermet

It is well known that the ionic conductivity of aliovalent-doped ceria solutions

show a maximum at a certain dopant concentration and cation radius. Com-

pared to divalent cation doping, however, trivalent dopants are observed to

contribute higher conductivity values in ceria. Among trivalent rare earth

metals, samarium and gadolinium are accepted as the most effective dopants.

Therefore, we have selected a cermet made up of nickel and 20 at% samarium-

doped ceria (SDC) as the anode material in our standard cells [12–14].

9 Intermediate-Temperature Solid Oxide Fuel Cells Using LaGaO

185

For preparation of the anode, a slurry composed of NiO and Ce

0.8

0.2

2–d

(SDC) mixture is screen printed onto the electrolyte. The firing process involves

treatment of the screen-printed anode layer on the electrolyte at 11008–13008C

for 3 h in air. The relative amounts and particle size ratios of the mixture of NiO

and SDC are optimized in such a way that the resulting microstructure is

resistant to coarsening of reduced nickel particles under the operating

6µ

(a)

(b)

(d) (c)

Fig. 9.1 Scan ning electron microscopy (SEM) images of La

0.8

0.2

0.8

0.15

0.05

3–d

(LSGMC) sintered at (a) 13508C, (b) 14008C, (c) 14508C, and (d)15008C

186 T. Akbay

conditions of the fuel cell. Figure 9.3(a) shows the typical air-fired microstruc-

ture of the Ni-SDC anode. The average thickness of the porous anode is about

30–50 mm.

Apart from nickel coarsening, additional causes of the inadmissible increase

in anodic overpotentials can be thought of as blockage of the nickel network by

the presence of SDC particles, poor adhesion between the ano de and the

electrolyte, slow in situ steam reforming reaction of methane on nickel, insuffi-

cient active length of triple-phase boundaries, and so on. To overcome these

Electrolyte

200

Anode

30 – 50 µm

Cathode

– 50 µm

Fig. 9.2 Cross section of the

standard cell

(b)(a)

Fig. 9.3 SEM images of (a) anode and (b) cathode

9 Intermediate-Temperature Solid Oxide Fuel Cells Using LaGaO

187

problems, nano-sized particles of certain transition metals may be incorporated

in the anode to modify its morphology. For this, we have selected ruthenium to

be dispersed in the cermet of nickel and SDC.

The Ni-Ru-SDC anode is prepared by using a modified batch of SDC

powder. Rel atively coarse particles of SDC are mixed with ruthenium chloride

to form a suspension. Following the reduction of ruthenium cation, the result-

ing powder is separated and dried. Figure 9.4 shows the transmission electron

microscopy (TEM) images of 1 wt% (a) and 10 wt% (b) Ru-dispersed SDC

particles obtained by the aforementioned preparation method. The optimum

value, on the other hand, is determined in such a way that when mixed with NiO

to form the anode, the concentration of ruthenium should be around 1 wt%, as

estimated from the co mpositions of widely used precious metal catalysts [15].

Among trivalent rare earth metals, gadolinium is also known to make a solid

solution with ceria to increase its ionic co nductivity. Catalytic activity of the

anode, on the other hand, can be further improved by embedding highly

dispersed nickel particles in the ceramic matrix [15]; this should improve the

long-term stability of the anode through delayed sintering of nickel particles.

9.2.3 Cathode

Similar to the basic requirements for anodes, a suitable cathode material should

also possess high catalytic activity to electroch emically reduce oxygen at oper-

ating conditions of SOFCs. At intermediate temperatur es, howeve r, mixe d

conducting oxides and certain noble metals may be utilized as cathode materi-

als. Although most of the candidates have intrinsic thermal expansion

mismatch and incompatibility with other cell components, a subset of doped

(b)(a)

SDC

Fig. 9.4 Ru-dispersed SDC (NiO and Ce

0.8

0.2

2–d

) particles with the concentration of (a)

1 wt% (b) and 10 wt%

188 T. Akbay