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

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272 K.D. Kreuer

Chapter 14

Intermediate-Temperature SOFCs Using

Proton-Conducting Perovskite

Naoki Ito

14.1 Introduction

Today, most vehicles are driven by internal combustion engines with liquid fuel.

The efficiency of internal combustion engines is only 15%–25%, and a power

source with higher efficie ncy is needed from the aspec ts of energy conservation

and CO

emissions. A fuel cell is one leading candidate for this alternative

power source. There are several types of fuel cells, but the polymer electrolyte

membrane fuel cell (PEFC) is the top runner now, and auto makers are putting

most of their research and development resources into a PEFC + on-board

hydrogen stora ge system [1]. Regarding the PEFC, it is well known that several

problems remain, such as high cost due to the use of platinum, liquid water

management, and the durability of the polymer electrolyte. However, the two

major issues of this system lie on the fuel side, hydrogen infrastructure and

hydrogen storage. Although hydrogen itself has very high power density, it is

very difficult to store hydrogen with high physical density. Many researchers

are working on the search for better hydrogen storage technology, such as a

high-pressure tank, hydrogen storage metal, carbon nano-tubes, and liquid

hydrogen. However, so far the driving range of fuel cell (FC) vehicles is much

less than that of vehicles with internal combustion engines. The issue regarding

hydrogen infrastructure is more a political and economic one. ‘‘The hydrogen

economy’’ needs a huge expendit ure to build up the network of hydrogen

distribution and refueling stations, but no one has a vision of this huge cost yet.

As discussed above, there are two major issues on the hydrogen fuel side. If a

FC vehicle propelled with liquid fuel is realized, that can be the main trend of a

future vehicle because liquid fuel does not have such issues (Table 14 .1). The

solid oxide fuel cell (SOFC) is the only fuel cell that can be directly driven by

liquid fuel; other fuel cells need fuel processors that produce hydrogen from

N. Ito (*)

Fuel Cell System Development Division, Toyota Motor Corporation, 1200,

Mishuku, Susono, Shizuoka 410-1193, Japan

e-mail: naoki@ito.tec.toyota.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_14,

Ó Springer ScienceþBusiness Media, LLC 2009

273

liquid fuel. So, in the case of liquid fuel, an on-board fuel processor is required,

and that is a major challenge. ‘‘Reforming’’ is a build-up technology in the area

of petroleum industry, but the challenge here is to make the system so compact

as to be on board. Toyota mad e a prototype vehicle with a system that has a

conventional fuel processor and PEFC in 2002, named the FCHV-5. The

system configuration is shown in Fig. 14.1(a). The high -temperature shift

reactor, low-temperature shift reactor, and a PReferential OXidation reactor

should be placed afte r a reformer to minimize CO content in fuel gas because

PEFC ca n be easily poisoned by less than 100 ppm of CO. These components

and the heat exchangers to connect them made the system very complicated and

huge. As a result, it was proved that this complicated system is not practical.

As shown in Fig. 14.1(b), if the operating temperature of the fuel ce ll is close

to the reforming temperature of liquid fuel, a simple and attractive system is

possible. Another advantage of this alternative system is high efficiency because

the exhaust heat from the fuel cell can be used in the endothermic reforming

reaction. On the other hand, a very high power density fuel cell is needed for the

main power source of a vehicle because the maximum power required for a

passenger vehicle is as high as 50–100 kW. These requirements are summarized

Table 14.1 Fuel options for FC vehicle

Hydrogen Liquid fuels

Fuel Requires new infrastructure

and storage technology

Easy handling and

infrastructure

Vehicle Simple system Complex system

and low efficiency

Reformer

Fuel

Water

Air

High temperature

Shift reactor

400°C

Low temperature

Shift reactor

250°C

PROX

150°C

PEFC

80°C600°C

Air

Fuel

Anode off-gas

Air

Combustion exhaust

Cathode off-gas

HMFC

Reformer

Fig. 14.1 (a) PEMFC+conventional fuel processor system; (b) Alternative system with

intermediate Temperature FC

274 N. Ito

in Fig. 14.2. An on-board reforming FC vehicle can be realized only with a fuel

cell in this target range. PEFCs have high power density, but they cannot be

operated above 1008C because the polymer electrolyte needs liquid water.

SOFCs also have high power density above 8008C. However, below 8008C

power density decreases rapidly, even with a thin-film electrolyte system.

Intermediate-temperature fuel cells with high power density have not been

realized because there is not a ‘‘good’’ electrolyte with high ion condu ctivity in

this temperature region. So, research has been focused on decreasing electrolyte

resistance because a major part of fuel cell resistance is derived from the

electrolyte resistance. There have been two main research approaches for

intermediate-temp erature fuel cells: (1) the search for a new electrolyte with

high ionic conductivity, and (2) search for a thin-film electrolyte.

One approach is the search for a new electrolyte material with high ion

conductivity. Ishihara et al. found that LaGaO

-based perovskite oxides are

good oxygen ion conductors at intermediate temperatures, and they are work-

ing on SOFCs based on these electrolyte mate rials [2, 3]. The major challenge of

this approach is a trade-off between conductivity and chemical stability. Haile

et al. proposed proton-conducting solid acids, such as CsHSO

and CsH

for intermediate-temperature fuel cells [4, 5]. Poor physical properties, chemical

stability, and temperature limit (around 2508C) are three major issues for solid

acids.

The other approach is an attempt for a thinner electrolyte supported on a

porous electrode. Recently, many researchers have been working on a thin-film-

type SOFC to realize an intermediate-temperature fuel cell [6–9]. In this

approach, the electrolyte must be supported on either the anode or cathode.

The main difficulty regarding this approach lies in making a solid thin film on a

porous electrode. To get a dense electrolyte layer, the electrolyte thickness should

be several times larger than the maximum pore size of the support electrode.

Thus, the preparation method of the support electrode with good pore-size

distribution is a key technology of this approach; not only it is difficult, but

0 200 400

600

Temperature/°C

PEFC

MCFC

PAFC

1.5

1.0

0.5

SOFC

Hydrocarbon Reforming

Easy for Module Design

Power Density/W cm

–2

Target

800

Fig. 14.2 Target of hydrogen membrance fuel cell

14 Intermediate-Temperature SOFCs 275

also it needs expensive techniques to commercialize. If the pore size of the support

electrode is too small, concentration polarization can be a problem.

There is a group of metals that is known to be used as a hydrogen permeation

membrane. Their mechanism is as follows. First, hydrogen dissolves into metal

as hydrogen atoms at the surface of one side of the film. Second, hydrogen

atoms move in the metal film by diffusion. Finally, hydrogen atoms recombine

into hydrogen at the surface of the other side of the film. The driving force of the

hydrogen permeation is the hydrogen partial pressure gradient, and these

metals are a ‘‘short-circuited proton conductor’’ in reality. The hydrogen per-

meation ability of this group of metals is shown in Fig. 14.3 [10]. Their hydrogen

permeability is several orders of magnitude high er than perovskite proton

conductors. So, these metals can be used as a dense anode.

The essence of our new concept is an ultrathin proton conductor electrolyte

supported on solid hydrogen membrane. We named this new type of fuel cell a

hydrogen membrane fuel cell (HMFC). The schematic diagram of the HMFC

structure is shown in Fig. 14.4. A much thinner electrolyte can be easily realized

because it is formed on a solid, nonporous membrane. Another advantage of

this approach is an ease of high-density stacking because the physical base of

this fuel cell is a metal film, not ceramics as in the case of SOFC s. The HMFC

can use only proton conductors as an electrolyte because the hydrogen mem-

brane layer only allows hydrogen to permeate.

1.E-12

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

0.001 0.0011 0.0012 0.0013 0.0014 0.0015 0.0016

1/T [K–1]

Hydrogen Permeability

[mol/m/s/Pa1/2]

Fig. 14.3 Hydrogen

permeability of metals

400–600°C

–

⇒

1/2 O

–

⇒ H

–

Electrolyte ~1 µm

Cathode

Hydrogen Membrane

40~80 µm

Anode

Fig. 14.4 Schematic

diagram of hydrogen

membrane fuel cell structure

276 N. Ito

14.2 Preparation of Fuel Cells

BaCe

0.8

0.2

was chosen as the electrolyte material because it has the highest

proton conductivity in the intermediate-temperature region. BaCe

0.8

0.2

is a

pure proton conductor under 6008C, so it is a good electrolyte material to prove

the HMFC concept [11]. Palladium was chosen as the hydrogen membrane

material because it has enough hydrogen permeation capability and chemical

stability. Some other metals have higher hydrogen permeation capability, but

they have problems regarding hydrogen embrittlement or oxidation. Pd is a very

expensive metal, and we should consider switching to cheaper metals before the

stage of commercialization. The pulse laser deposition method is chosen to make

the electrolyte film. This method has an advantage regarding crystal quality and

film composition and is a good technique to prove the HMFC concept.

The BaCe

0.8

0.2

electrolyte was deposited on a Pd film by pulse laser deposi-

tion. Then, the perovskite cathode paste was screen printed on the coated Pd

structure and dried by a heat gun. This procedu re was repeated three times to

make a complete cathode layer that is 30 mmthick.ThesizeofthePdfilmwas15

15 mm. The electrolyte and electrode were coated on the Pd film as a circle 6 mm in

diameter to make an effective fuel cell area of 0.283 cm

. The pulse laser deposition

apparatusisshowninFig.14.5.Twotypesoftestcells,cellAandcellB,were

prepared for comparison. Their characteristics are summarized in Table 14.2.

14.3 Characterization of Fuel Cells

Figure 14.6a shows the cross-section scanning electron microscopy (SEM)

image of the electrolyte and hydrogen membrane. Before preparation of the

cross-section sample, the tungsten protection layer was coated on the electrolyte

layer to avoid damage. As seen, a solid and uniform electrolyte layer without

Vacuum

gauge

Target

Substrate

Plume

KrF

excimer

laser

Lens

Quartz glass window

Target

rotator

TMP+RP

Fig. 14.5 Schematic

diagram of pulse laser

deposition apparatus

14 Intermediate-Temperature SOFCs 277

any pores, peeling, or flaking was formed on the Pd substrate. Figure 14.6(b)

shows the X-ray diffraction pattern of the elect rolyte film. All the peaks of the

electrolyte sample match the BaCe

0.8

0.2

target peaks when the Pd pe aks are

excluded. From these data, it is confirmed that a good perovskite layer can be

formed on Pd film by pulse laser de position. Figure 14.7 shows the cross-section

transmission electron microscopy (TEM) image around the electrolyte and

hydrogen interface. It is shown that long columnar crystals are made over the

microcrystal around 100 nm from the interface. This columnar crystal structure

is typic al for the vapor deposition methods.

020406080

XRD Intensity

Targ et

Pd Substrate

BaCe

0.8

0.2

Diffraction angle

Coated Layer

Pd Hydrogen Membrane

Perovskite Layer

Protection Layer

Fig. 14.6 (a) SEM cross section image of the perovskite thin film on Pd substrage; (b) X-ray

diffraction pattern of the perovskite thin film on Pd substrate

Table 14.2 Characteristics of the HMFC test cells

Test cell A Test cell B

Electrolyte BaCe

0.8

0.2

2 m m BaCe

0.8

0.2

0.7 m m

Hydrogen

Membrane

Pd 80 m mPd40m m

Cathode La

0.5

MnO

+Pt(0.5 wt%)

Screen printing

0.6

0.4

CoO

Ceramic

coating

100nm

Electrolyte

Micro crystals

Columnar crystals

Fig. 14.7 TEM cross section

image of the Pd and

electrolyte interface

278 N. Ito

14.4 Operation and Evalu ation of Fuel Cells

The V–I characteristic of a single cell A is shown in Fig. 14.8(a). Although the

power density has not reached the target, it is proved that the HMFC concept

works without any serious pro blems. The cell resi stance was divided into an IR

resistance and a polarization resistance by the AC impedance method. The

composition of cell resistance is shown in Fig. 14.8(b). The polarization resis-

tance is further analyzed by an AC impedance method because it is difficult to

introduce a reference electrode to a fuel cell with a thin-film electrolyte [12]. AC

impedance spectra were measured for various P

of anode side and P

of the

cathode side to determine the polarization (Fig. 14.9). In all conditions, two

polarization semicircles were seen. One semicircle has a peak around 3 kHz

(Ra), and the other has a peak around 30 kHz (Rb). Bot h of these are affected

by P

of cathode gas, and none of these is affected by P

of anode gas. So,

both Ra and Rb correspond to the cathode polarization, and the anode polar-

ization is very small compared to the cathode polarization. As mentioned, Pd

has very high hydrogen permeation capability, and it is reasonable that Pd has

high activity as a fuel cell anode. The P

dependence of Ra and Rb is analyzed

0.2

0.3

0.1

0.4

0.6

0.8

1.0

1.2

I [A/cm2]

E [V]

0.5

1.5

Cell Resistance/m

Ωcm

Porlarization

IR resistance

2.7

–4

1.5

–2

Proton

Conductivity

[S/cm]

LiteratureHMFC

(Estimated

from IR)

Fig. 14.8 (a) I–V characteristics of test cell A at 5008 C. Anode gas was moist H

, and Cathode

gas was moist air (Both 408C humidified.); (b) Composition of cell resistance of test cell A

14 Intermediate-Temperature SOFCs 279

in Fig. 14.10. Log Ra showed 1/4 slope to log P

and log Rb showed 1/2 slope

to P

. Below is the cathode reaction mechanism proposed for an LSM-based

electrode.

Step 1 1/2 O

! O

(equilibrium)

Step 2 O

+2e

! O

2–

Step 3 O

2–

! O

2–

TPB

Step 4 O

2–

TPB

+2H

! H

Either step 2 or 3 is most likely to be the rate-determining step [13]. In the

case of HMFC, the polarization Ra can correspond to step 3 and the polariza-

tion Rb can correspond to step 2 from P

dependence analysis.

Although the cathode material used in our fuel cell is popular in SOFCs, the

cathode polarization is much smaller than that reported in SOFC literature

using bulk electrolyte [14]. Hibino et al. also reported anode and cathode

polarization of fuel cells using a perovskite proton conductor as a bulk electro-

lyte, and the polarization was much larger than our result [15]. These facts

suggest that electrolyte thickness may affe ct the polarization.

–6.0

–4.0

–2.0

0.0

6.04.02.00.0

Z' [cm

]

Z'' [cm

]

100% O2

21%

50% O2

21%

10% O2

21%

100% O2=100%

100% O2

50%

100% O2

10%

Fig. 14.9 AC Impedance

spectra of test cell A

–0.2

0.2

0.4

0.6

–1 –0.5 0

log P(O

) [atm]

log Ra, Rb [Ωcm

]

Fig. 14.10 P(O2)

dependance of cathode

polarization

280 N. Ito

Performance of cell B was measured in the next step. Cell B has a thinner

electrolyte, hydrogen membrane, and more active cathode than cell A for better

performance. The open circuit voltage (OCV) of a single cell was measured at

various temperatures and hydrogen concentrations. The results of OCV measure-

ment are shown in Table 14.3. Proton transport numbers were calculated as

measured OCVs divided by theoretical voltages. Measured OCVs are close to

theoretical voltages and much higher than typical PEFCs. The difference between

the measured OCVs and theoretical values is probably the result of hydrogen leak

through cracks of the electrolyte because measured transport numbers are almost

independent from operating conditions. The high OCV of the HMFC is a

substantial advantage over PEFCs, especially in the low current density region.

The V-I characteristic of the cell B is shown in Fig. 14.11. The power densities

were 0.9 W/cm

and 1.4 W/cm

at the operating temperatures of 4008 and

6008C, respectively. The temperature dependence of the HMFC performance is

smaller than that of the SOFCs [16], which can be explained by the fact that the

proton conductivity has smaller temperature dependence than oxygen ion con-

ductivity. It is shown that the HMFC has a relatively wide operation temperature

window and that further decrease of the operating temperature is possible if

system design or fuel choice requires. The composition of the cell resistance of cell

B is shown in Fig. 14.12. In Fig. 14.13, the conductivity of the film electrolytes is

calculated from measured IR resistances and compared with the bulk

Table 14.3 Open circuit voltage of test cell B in various conditions

Temperature 8C Anode gas

Cathode

gas

Measured

OCV

Theoretical

voltage

Transport

number

mV mV

440 H2 ¼ 100% Air 1103 1140 0.968

440 H2 ¼ 50% Air 1082 1120 0.966

440 H2 ¼ 10% Air 1036 1073 0.966

530 H2 ¼ 100% Air 1081 1120 0.965

610 H2 ¼ 100% Air 1051 1100 0.955

0.0

3.02.52.01.51.00.50.0

0.2

0.4

0.6

0.8

1.0

1.2

400°C

600°C

Current density/A

–2

Voltage/V

Fig. 14.11 I–V characteristics

of test cell B. Anode gas was

moist H

, and Cathode gas

was moist air (Both 408 C

humidified.)

14 Intermediate-Temperature SOFCs 281