Chapter 1

INTRODUCTION

1.1 SCOPE

Fuel cells are a radically different way of making electrical power from

a variety of fuels. A fuel cell is an energy conversion device that produces

electricity (and heat) directly from a gaseous fuel by electrochemical combination

of the fuel with an oxidant. Such a device bypasses the conversion of chemical

energy of fuel into thermal and mechanical energy, and thus achieves theoretical

efficiency significantly higher than that of conventional methods of power generation. In addition to the high conversion efficiency, fuel cells have the

characteristics of environmental compatibility, modularity, siting flexibility, and

multifuel capability.

(i) High conversion efficiency: The primary feature of a fuel cell is its

high fuel-to-electricity conversion efficiency (45 to 60%). A fuel cell converts

the chemical energy of fuel directly into electrical energy. Thus, the usual losses

involved in the conversion of fuel to heat, to mechanical energy, and then to

electrical energy are avoided. The efficiency of a fuel cell is further improved

when the byproduct heat is fully utilized (in cogeneration or bottoming cycles).

(ii) Environmental compatibility: Fuel cells are capable of using practical fuels as an energy source with insignificant environmental impact. Emissions of

key pollutants from fuel cells are several orders of magnitude lower than those

produced by conventional power generators. Production of undesirable materials

such as NOx, SOx, and particulates is either negligible or undetectable for fuel cell systems (for examples, see Figures 11.1 and 11.6, Chapter 11).

(iii) Modularity: Fuel cells have the characteristic of modularity, i.e.,

cells can be made in modular sizes. Thus, fuel cell size can be easily increased

or decreased. Since the efficiency of a fuel cell is relatively independent of size,

fuel cells can be designed to follow loads with fast response times without significant efficiency loss at part-load operation.

2 Chapter 1

(iv) Siting flexibility: Because fuel cells can be made in a variety of sizes, they can be placed at different locations with minimum siting restrictions.

Fuel cell operation is quiet because a fuel cell has no moving parts; the only noises are those from auxiliary equipment. Consequently, fuel cells can be easily

located near points of use such as urban residential areas.

(v) Multifuel capability: Certain types of fuel cells have multifuel capability. High-temperature fuel cells can process (reform) hydrocarbon fuels

internally and do not need expensive subsystems to process conventional fuels into simple forms.

Ceramic fuel cells having the attributes discussed above are among the

several fuel cell technologies being developed for a broad spectrum of electric

power generation applications. The key characteristic of this type of fuel cell is its ceramic electrolyte. The use of a solid electrolyte in ceramic fuel cells eliminates material corrosion and electrolyte management problems and permits unique cell designs with performance improvements. The conductivity requirement for the ceramic electrolyte necessitates high operating temperatures (600 ~ to 1000~ High operating temperatures promote rapid reaction kinetics, allow reforming of hydrocarbon fuels within the fuel cell, and produce high- quality byproduct heat suitable for use in cogeneration or bottoming cycles. On

the other hand, high operating temperatures impose stringent material and processing requirements. The present key technological challenge facing ceramic

fuel cells is the development of suitable materials and fabrication processes to incorporate materials into required structures.

To date, although ceramic fuel cell technology is still evolving, it has made excellent technical progress. Multikilowatt fuel cells incorporating various features of a practical power generation system have been operated for thousands of hours and have shown excellent performance. Recently, ceramic fuel cell

research and development has received much attention, reflecting widening interest in this technology.

The objectives of this book are to provide a comprehensive treatise on

both the fundamental and technological aspects of ceramic fuel cells and to serve

as a reference source on this emerging technology. This book consists of eleven chapters. Chapter 1 introduces the general characteristics of ceramic fuel cells and gives a brief historical perspective on the development of this type of fuel

cell. Chapter 2 provides an overview of the operating principles, with brief

discussions on thermodynamic aspects of cell operation, key features of the fuel cell, types of fuel and oxidant, and other elements of a fuel cell power system.

Introduction 3

Since the operation of a ceramic fuel cell is based fundamentally on electrical

processes in the ceramic components, it is instructive to outline some of the

relevant theoretical considerations on electrical conduction in ceramics. Chapter

3 thus includes discussion on the general principles of electrical conduction, the

relationships between conduction and defect structure, and transference numbers

of ions and electrons in ceramics. Chapters 4, 5, 6, and 7 cover the principal

components of a fuel cell stack: the electrolyte, the cathode, the anode, and the

interconnect, respectively. Emphasis is given to the discussion on the preparation

of each component material, its stability, and its chemical, electrical, and thermal

properties under cell fabrication and operation conditions. Chapter 8 reviews

various aspects of the electrode reactions, including reforming and contaminant

reactions, in a ceramic fuel cell. The discussion in this chapter focuses on the

reaction mechanisms of the hydrogen oxidation and the oxygen reduction.

Chapter 9 is devoted to a thorough discussion of stack design and fabrication.

Detailed description of design characteristics, gas manifolding, and fabrication

processes, along with a summary of the technological status of each stack design,

are presented. Chapter 10 provides a treatment of modeling and analysis used

in ceramic fuel cell design, especially thermal stress analysis, electrical analysis,

and performance modeling of various cell and stack configurations. Finally, the

applications of ceramic fuel cells are discussed in Chapter 11.

1.2 GENERAL CHARACTERISTICS OF CERAMIC FUEL CELLS

A ceramic fuel cell is an all-solid-state energy conversion device that produces electricity by electrochemically combining fuel and oxidant gases across

an ionic conducting ceramic. A ceramic fuel cell consists of two electrodes (the anode and cathode) separated by a solid electrolyte. Fuel is fed to the anode,

undergoes an oxidation reaction, and releases electrons to the external circuit.

Oxidant is fed to the cathode, accepts electrons from the external circuit, and

undergoes a reduction reaction. The electron flow (from the anode to the

cathode) produces direct-current electricity (Figure 1.1) [1.1]. The solid

electrolyte conducts ions between the two electrodes.

Present ceramic fuel cells use exclusively hydrogen as fuel, and oxygen

as oxidant. In theory, any gases capable of being electrochemically oxidized and

reduced can be used as fuel and oxidant in a fuel cell. However, hydrogen is

currently the most common fuel, since it has high electrochemical reactivity and

can be derived from common fuels such as hydrocarbons, alcohols, or coal.

4 Chapter 1

FUEL~

, ox,o..

ANODE

ELECTROLYTE

CATHODE

EXTERNAL LOAD AND HEAT /

Figure 1.1. Schematic diagram of fuel cell operation [1.1]

Oxygen is the most common oxidant, since it is readily and economically

available from air. For the hydrogen/oxygenreaction, to date, only oxides are

being considered for use as ceramic fuel cell electrolytes. Since fuel cells are

commonly identified by the type of electrolyte used, ceramic fuel cells are

referred to as solid oxide fuel cells (SOFCs). Due to the conductivity require-

ment for the oxide electrolyte, current SOFCs operate in the temperature range

of 600 ~ to IO00~

1.2.1 Types of ceramic fuel cells

A fuel cell electrolyte must ionically conduct one of the elements present

in the fuel or oxidant. Thus, a solid electrolyte for SOFCs based on the

electrochemical reactions of hydrogen and oxygen must conduct either oxygen

ions or hydrogen ions (protons). (Although hydroxide-ion conduction is also

possible, it has been shown to be a proton conduction with oxygen-ion carrier

species. It is a special case and, for simplification,it will be considered as proton

conduction here.) The present generation of ceramic fuel cells can be classified

into two types [1.2]: (i) those based on oxygen-ion-conducting electrolytes and

(ii) those based on proton-conducting electrolytes. Figures 1.2 and 1.3 show the

reactions in an oxygen-ion-conductor SOFC and a proton-conductor SOFC,

respectively. An oxygen-ion-conductor SOFC can be considered as an oxygen

concentration cell, and a proton-conductor SOFC as a hydrogen concentration

Introduction 5

FUEL .... ~ / H2, CO. H20,C;2

0 H2'CO / 1, / ~2 / / ANODE H~O. H20 + C02

+H20~C02 + H2 ~ / e ' . ~ "~ .... C O ~ ~ e - ~ "

INTERFACE ,0 = ,,0: 0 = + H 2 ----~H20+ 2e-

~EXCESS FUEL~ "1 TO BURNE~/

e-

OUTER CIRCUIT

,,THO0, ~ 0 2 + 4e- )20 = 132-t 02/ | / e"

/ / /

AIR OR 02 " \ 02 CATHOD EXHAUST/

REACTION H2 + 1/202 ~- H2O CO + 1/202 = C02

Figure 1.2. Schematic diagram of reactions in SOFCs based on oxygen-ion conductors [1.1]

FUEL H2 " ~

ANODE H 2 ~ 2H + + 2e-

/

I / H2

H~ ..... -e~''~' !N_TERFACE_---~

2H + +I/202 + 2e--*H20

CATHOOE

OXIDANT AIR OR 02

,H +

, \r

02 H20

H2

H+~ j e 1 I H OUTER ~, _ [, CIRCUIT

, ! HHo ,,..-

OVERALL REACTION H2 + V202 ~ H20

EXCESS FUER~ "-I TO BUR.NE

J CATHOOE~ "-I EXHAUST

Figure 1.3. Schematic diagram of reactions in SOFCs based on proton conductors [1.1]

6 Chapter 1

cell. The major difference between the two SOFC types is the side in the fuel

cell in which water is produced (the fuel side in oxygen-ion conductor cells and

the oxidant side in proton-conductor cells). Also, certain gases, such as CO, can

be used as fuel in oxygen-ion conductor SOFCs but not in proton-conductor

SOFCs. To date, almost all of the development work on ceramic fuel cells has

focused on SOFCs with oxygen-ion-conducting ZrO2 electrolytes. Work on

proton-conductor SOFCs is limited to material studies, clarification of conduction

mechanisms, and testing of small, laboratory-scale cells.

1.2.2 Cell components

A SOFC single cell consists of an oxide electrolyte sandwiched between

an anode and a cathode. Under typical operating conditions (with hydrogen fuel

and oxygen oxidant), a single cell produces less than 1 V. Thus, practical

SOFCs are not operated as single units; rather, they are connected in electrical

series to build voltage. A series of cells is referred to as a stack. A component,

variously called an interconnect or a bipolar separator, connects the anode of one

cell to the cathode of the next in a stack (Figure 1.4). SOFC stacks can be

configured in series, parallel, both series and parallel, or as single units,

depending on the particular application.

REPEATING ELEMENTS

INTERCONNECT

ANODE

ELECTROLYTE

CATHODE

Figure 1.4. Fuel cell component

Introduction 7

The principal components of a SOFC stack are the electrolyte, the anode,

the cathode, and the interconnect. Each component serves several functions in the fuel cell and must meet certain requirements. Each component must have the

proper stability (chemical, phase, morphological, and dimensional) in oxidizing

and/or reducing environments, chemical compatibility with other components,

and proper conductivity. The components for ceramic fuel cells must, in

addition, have similar coefficients of thermal expansion to avoid separation or

cracking during fabrication and operation. The electrolyte and interconnect must

be dense to prevent gas mixing, while the anode and cathode must be porous to

allow gas transport to the reaction sites. The requirements for the various cell

component are summarized in Table 1.1.

In addition to the requirements listed in Table 1.1, other desirable

properties for the cell components from practical viewpoints are high strength

and toughness, fabricability, and low cost. Also, for certain cell designs, the

components for a ceramic fuel cell must be amenable to limited fabrication

conditions since the process conditions cannot be selected independently for each

component. For example, if the components are built up one by one, the

temperature of sintering for each successive component must be lower than that

of the preceding component to avoid altering the microstructure of the preceding

component. If the components are formed in the green state, then all components

must be sintered under the same firing conditions. Furthermore, the components

of a ceramic fuel cell must be compatible not only at the operating temperature

but also at the much higher temperatures at which the ceramic structures are fabricated.

Cell components are connected (in electrical series) in proper order in a

stack. The height or number of single cells (thus, voltage) and footprint or active

area (thus, current) of a stack can vary, depending on the particular design and

power output required. Because all the components are solid, the SOFC stack

can be configured into unique shapes unachievable in other types of fuel cells.

At present, four common stack configurations have been proposed and fabricated

for SOFCs: the sealless tubular design, the segmented-cell-in-series design, the

monolithic design, and the flat-plate design (for more details, see Chapter 9).

Each design may have several different versions and is presently at a different

stage of technology development. Figure 1.5 shows, as an example, the

schematic diagrams of the various SOFC stack designs [1.3].

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Introduction 9

INTERCONNECTION

ELECTRODE

, ~ . I POROUS S ~

AIR FLOW ~ - ~ ~ FUEL ELECTRODE

ELECTROLYTE

ELECTROLYTE CATHODE INTERCONNECT'~~

/ ~ i l i \i : : Po.ous s0PPo.T i : :::i: : : : : i : /

~U )) ) ) ' ~ /) /)_ ) --" OXIDANT -----e,

Seal-less Tubular Design Segmented-Cell-in-Series Design

ELECTROLYTE

Monolithic Design Flat-plate Design

Figure 1.5. SOFC stack designs [1.3]

10 Chapter 1

1.2.3 Comparison with other types of fuel cells

The SOFC is one of several types of fuel cells currently under develop-

ment for clean and efficient electric power generation from a variety of fuels.

Besides the SOFC, the other major types of fuel cells are polymer membrane,

alkaline, phosphoric acid, and molten carbonate fuel cells. Among these fuel

cells, the phosphoric acid fuel cell is presently at the initial stage of commer-

cialization for electric utility and cogeneration uses. The molten carbonate is the

next most likely candidate for commercialization, whereas the SOFC is

considered as the third-generation technology. The polymer membrane fuel cell

is being developed mainly for space and transportation applications, and the

alkaline fuel cell is an important power source for space flights. Typical features

and operational characteristics of the SOFC and other types of fuel cells are listed

in Table 1.2.

1.3 HISTORICAL BACKGROUND OF CERAMIC FUEL CELLS

The principles of fuel cell operation were first reported by Sir William

Grove in 1839 [1.4]. His fuel cell used dilute sulfuric acid as the electrolyte and

operated at room temperature. Ceramic fuel cells came much later and began

with Nernst's discovery of solid-oxide electrolyte in 1899 [1.5] and the operation

of the first ceramic fuel cell at 1000~ by Baur and Preis in 1937 [1.6].

Nernst discovered solid oxygen-ion conductors when he invented the so-

called glower in the end of the 19th century [1.5]. Nernst proposed to use solid

compositions such as ZrO2 with 15 wt% Y203 addition (called the Nernst mass)

as a glower to replace carbon filaments in electric lamps. The Nernst glower

was operated for hundreds of hours on direct current, though electrolysis was

found to occur. It was explained that any loss of oxygen liberated at the anode

was balanced by an equal amount of oxygen taken into the glower at the cathode.

This phenomenon was the reverse of fuel cell operation. In 1935, Schottky

published a paper suggesting that the Nernst mass could be used as a fuel cell

solid electrolyte [1.7].

In 1937 Baur and Preis demonstrated the operation of the first ceramic

fuel cells [1.6]. They used mainly ZrO2-based ionic conductors (e.g., ZrO2 with

10 wt % MgO or 15 wt % Y203 addition) in the form of a tubular crucible as the

electrolyte, with iron or carbon as the anode and Fe304 as the cathode. Observed

open-circuit voltages were between 1.1 and 1.2 V at 1000 ~ to 1050~ Baur and

Introdu

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12 Chapter 1

Preis constructed a ceramic fuel cell battery consisting of eight ZrO2-Y203 crucibles filled with coke and immersed in a common magnetite bath. With

hydrogen, CO, or town gas as fuel, the open-circuit voltage was 0.8 V per cell

(0.2 V lower than the theoretical value). At a current density of approximately

0.3 mA/cm 2, the cell voltage was 0.65 V, corresponding to an imernal resistance

of 1.8 to 2.6 ~. Although operation was demonstrated, the current outputs of

these cells were too low to be practical.

Initial development work on practical ceramic fuel cells began in the

early 1960s. The cell configuration in this time period was either a flat-plate

design using the electrolyte in the form of a disk, or a segmented-cell-in-series

design (bell-and-spigot configuration) using short tubular segments of the

electrolyte joined together with conducting seals. These designs used very thick

electrolytes, thus suffering significant internal resistance losses. This led to the

development of the thin-wall concept to improve cell performance. In 1970s the banded configuration (a segmented-cell-in-series design) was proposed, which

made use of the thin-wall concept in which a number of thin-film cells were deposited on a porous support. Development of fuel cells based on this

configuration is still going on; kilowatt-size stacks of banded SOFC cells have

been tested. In 1980 the sealless tubular design was proposed, with several

advantages over the segmented-cell-in-series design. The key features of the

sealless tubular design include individual thin cells formed on a tubular support

and electrically connected into a bundle in a fuel-reducing atmosphere. This

design is presently the most advanced; multikilowatt sealless tubular SOFC

generators have been fabricated and operated for thousands of hours. In 1982

the monolithic design, in which cells are configured in a honeycomb structure

(resulting in extraordinarily high power density), was advanced. At the same

time, interest in the fiat-plate design has been renewed, and due to many

advances in ceramic forming and processing technologies, various advanced fiat-

plate concepts have been proposed.

Early SOFC stacks used noble metals (e.g., platinum) as electrode and

interconnect materials. In the early 1970s nickel/YSZ, doped In203, and

CoCr204 were used as anode, cathode, and interconnect, respectively. CoCr204

was later replaced by LaCrO3, and in 1980 LaMnO3 and LaCoO 3 were proposed

for cathode use. Recently, high-temperature alloys have been tested as

interconnect material for flat-plate SOFCs. Figure 1.6 summarizes key

historical events in the development of the SOFC technology.

Introduction 1

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14 Chapter 1

References

1.1

1.2 1.3

1.4 1.5 1.6 1.7

Morgantown Energy Technology Center, Fuel Cells -- Technology Status Report,

Report No. DOE/METC-87/0257, Morgantown Energy Technology Center, Morgantown, WV, 1986. N.Q. Minh, J. Am. Ceram. Soc., 76 (1993) 563. N.Q. Minh, in Science and Technology of Zirconia V, S.P.S. Badwal, M.J. Bannister, and R.H.J. Hannink (eds.), Technomic Publishing Company, Lancaster, PA, 1993, p. 652. W.R. Grove, Philos. Mag., 14 (1839) 127. W. Nernst, Z. Elektrochem., 6 (1899) 41. E. Baur and H. Preis, Z. Elektrochem.,43 (1937) 727. W. Schottky, Wiss. VerOff. Siemens Werken, 14 (1935) 1.