Chapter 7Chapter 7

Materials for Solid-Oxide Materials for Solid-Oxide Fuel Cells (SOFCs)Fuel Cells (SOFCs)

Fuel Cell Background In principle, fuel cells can be considered as batteries that convert chemical

energy to electricity without combustion, but instead through electrochemical reactions.

The difference between fuel cells and conventional batteries is that fuel cells can run continuously as long as fuels are available for electrochemical reactions, whereas a battery needs to be recharged periodically.

The concept of fuel cells was conceived by Sir William Robert Grove, known as the father of the fuel cell, who develop a ”gas voltaic battery” in 1839, later named the Grove cell.

Accelerated research and engineering for fuel cell development started in the 1960s due to the unique needs for the long-endurance manned space exploration missions.

For space applications, in addition to less toxcity, fuel cells have the advantage over traditional batteries, as they offer significantly higher energy density (energy per equivalent unit of weight) than conventional and advanced batteries.

Fuel cells are classified primarily according to the nature of the

eletrolyte.

Moreover, the nature of the eletrolyte governs the choices of the

electrodes and the operation temperature.

Shown in Table 10.1 are the fuel cell technologies currently under

development.

Technologies attracting attention toward development and

commercialization include direct methanol (DMFC), polymer

electrolyte membrane (PEMFC), solid-acid (SAFC), phosphoric acid

(PAFC), alkaline (AFC), molten carbonate (MCFC), and solid-oxide

(SOFC) fuel cells.

This chapter is aimed at ten solid-oxide fuel cells (SOFCs) and

related eletrolytes used for the fabrication of cells.

Scheme of a solid-oxide fuel cell

Cross section of the three ceramic layers of an SOFC. From left to right: porous cathode, dense electrolyte, porous anode

The Electrolyte for SOFCs The mobile species within a fuel cell are ions, which necessitate the

electrolyte being an ionic conductor and electronic insulator. If the oxygen ions are the only charge carriers, the electron motive force (EMF) of the cell is determined from the chemical potential of oxygen (i.e., oxygen activity), which is expressed by the Nernst equation as

where Γ is the ionic transference number (ionic conductivity/total conductivity), Γ is the cell operation temperature, F is the Faraday constant, (pO2)a is the oxygen partial pressure (fugacity/activity) at the

cathode side, and (pO2)b is similarly the oxygen partial pressure

(fugacity/activity) at the cathode side. In the case of an open circuit (without external current flow), the EMF of the cell corresponds to the open-circuit voltage (OCV).

b

a

pO

pO

F

RTEMF

2

2ln4

Requirements Long-term successful operation of SOFCs requires that the

electrolyte possess adequate chemical and structural stability

over a wide renge of oxygen partial pressures, form air or

oxygen to humidfied hydrogen or hydrocarbons.

The requirements for the electrolyte used in the intermediate-

temperature SOFCs (IT SOFCs) include:

1. Conductivity: The materials must have an ionic transference

number close to unity; i.e., the electronic conductivity in the

electrolyte must be sufficiently low in order to minimize internal

shorting and provide high energy conversion efficiency. The

electrolyte materials should also possess high oxygen ion

conductivity to minimize the ohmic losses in the cell.

2. Density: In order to minimize molecular transport of gases across the

electrolyte membrane and prevent combustion (to produce maximum

power density), the electrolyte must remain gas tight during the life of the

cell. This indicates that when we consider the SOFC electrolyte, the main

challenge has to be related to the processing of dense, thin electrolyte

layers using either the anode or cathode as the supporting structure.

3. Stability: The operation of SOFCs requires the cathode and the anode to be

porous for gas transport; therefore, the electrolyte is exposed to both the air

and the fuel at elevated temperature. The electrolyte must remain

chemically phase stable in these environments, along with thermal and

mechanical stability during thermal cycling. This requires matching of the

thermal expansion coefficients of adjoining layers. Chemical interactions

and formation of interfacial compounds between adjoining components

must also be minimized or prevented to mitigate increase in cell resistance

and polarization losses. It should be kept in mind that the SOFCs, currently

designed for stationary applications, have a target life of 40,000 h, and

hence the long-term chemical and structural stability of the electrolyte plays

a crucial role.

Materials The most commonly used electrolyte materials under development and

deployment in SOFCs are the oxides with low-valence element

substitutions, sometimes named acceptor dopant, which create oxygen

vacancies through charge compensation.

It is straightforward to design the oxygen ion conductors by increasing

the oxygen vacancy concentration.

This, however, may not be valid in many cases, as other factors, such

as vacany ordering charge mobility, and compatibility with other cell

components, must also be taken into consideration.

As a result, the most commonly utilized electrolyte materials that can

satisfy these requirements are Y-stabilized ZrO2 (YSZ), with acceptor-

substituted CeO2 and (La,Sr)(Mg,Ga)O3.

Other interesting electrolyte materials include pseudo-perovskite

structures, Bi4V2O11 (MIMEVOX) and La2Mo2O2; pyrochlore structures,

(Gd,Ca)2Ti2O7 ; and apatites, La10-XSr6O26.

Fluorites: ZRO2, CEO2, and Bi2O3

The fluorite class of oxides is most studied as solid-oxide electrolyte

materials because of their chemical and structural stability.

The fluorite lattice structure is basically face center cubic (FCC; space

group, Fm3m) with a general formula of MO2, in which M typically is Zr

or Ce, as in ZrO2 or CeO2.

There are four MO2 formulas in unit cell, in which cations are in cubic

closest packing with oxygen ions in all tetrahedral holes.

The CeO2 lattice is known to possess a large open-volume fraction

(>35 %) and, as a result, is capable of tolerating oxygen

nonstoichiometry and forming solid solution with various low-valence

elements.

This gives the materials scientist an opportunity to alter the properties

of a given base oxide by substituting different cations or varying

oxygen content.

Perovskites LaGaO3

The perovskite structure is basically cubic with the genertral formula of ABO3, in whick A, the large cation site, is an alkaline earth, or rare

earth ion, and B, the small cation site, is a transition metal cation. The large cations are in 12-fold coordination with oxygen, while the

small cations fit into octahedral positions. The occupancy of these sites by different cations is determined

primarily by ionic radius rather than the valence. The opens the door for the materials scientists to substitute selectively

for either the A or B ion by introducing isovalent or aliovalent cations. An oxygen ion conductor can be tailored because of the geometrical

and chemical flexibility of the perovskite structure.

This is borne out by(La,Sr)(Mg,Ga)O3 (LSMG), which has attracted

great attention since its discovery.

There exist, however, two drawbacks for LSMG electrolytes: (1) the uncertainty in the cost of Ga sources and (2) the chemical and mechanical stability of LSMG.

It is apparent that ordering occurs (sometimes at specific temperatures) that significantly decreases the oxygen ionic conductivity because of lower defect mobility and reduced effective vacancy concentration.

Stevenson et al. studied the role of microstructure and nonstoichiometry on ionic conductivity of LSMG.

The electrical conductivity of sintered LSGM tends to decrease with increasing A/B cation nonstoichiometry.

The flexural strength of LSGM with an A/B cation ratio of 1.00 was also measured and found to be closer to 150 MPa at room temperature, and the strength decreased to 100 MPa at higher temperatures (600 to 1,000 ). ℃

The fracture toughness, as measured by notched beam analysis, was closer to 2.0 to 2.2 MPa at room temperature, with similar reduction to 1.0 MPa at 1,000 .℃

Other phases There are many other solid-state oxide ion conductors, primarily

derived from either fluoite or perovskite structures. The perovskite-related oxide ion conductors include (1)

Ln(Al,In,Sc,Y)O3-based materials, (2) the doped and undoped

brownmillerite Ba2In2O5, and (3) La2Mo2O9.

The transference number of doped La2Mo2O9 can be higher than

0.99 in an oxidant environment.

The drawbacks of La2Mo2O9-based materials are instability in

reducing conditions, a relatively large thermal expansion coefficient (>16 ppm/K for La1.7Bi0.3Mo2O9), and the order of the

anion sublattice.

Doped LaAlO3 has reasonable ionic conductivity (~0.006 S/cm at

800 ) and excellent thermal expansion coefficient (TEC) ℃match with other components (~11 ppm/K); however, it is rather challenging to sinter LaAlO3-based oxides and to prevent the

formation of highly insulating grain boundary phases (Al2O3).

Corrosion and Protection of Metallic Interconnects

Among the various types of fuel cells, solid-oxide fuel cells (SOFCs) operate at high temperature (typically 650 to 1,000 ) and have ℃advantages in term of high conversion efficiency and the flexibility of using hydrocarbon fuels, in addition to hydrogen.

The high-temperature operation, however, can lead to increased mass transport and interactions between the surrounding environment and components that are required to be stable during a lifetime of thousands of hours and up to hundreds of thermal cycles.

For stacks with relatively low operating temperatures (< 800 ), the ℃interconnects that are used to electrically connect a number of cell in series are typically made from cost-effective metals or alloys.

The metallic interconnects must demonstrate excellent stability in a very challenging environment during SOFC operation.

Until recently, the leading candidate material for the interconnect

was doped lanthanum chromite, LaXSr1-XCrO3, a ceramic that could

easily withstand traditional 900 to 1,000 operating temperature.℃

The recent trend in developing lower-temperature (650 to 800 ), ℃more cost-effective cells that utilize anode-supported, thin

electrolytes or new electrolytes with improved conductivity makes it

feasible for lanthanum chromite to be supplanted by cost-effective

metals or alloys as the interconnect materials.

The metals are typically those alloys that contain “active”

constituents, mainly Cr, Al, or Si, which are preferentially oxidized at

the surface to form an oxide scale that minimizes further

environmental attack during high-temperature exposure.

Surface Modification for Improved Stability

For satisfactory long-term stability against oxidation and

corrosion, metallic interconnects are often coated with a

protection layer that helps minimize electrical contact

resistance and mitigate or prevent potential cell

degradation due to chromia scale evaporation.

Functionally, the protection layer is intended first to serve

as a mass barrier to both chromium cation outward and

oxygen inward transport (via solid-stare diffusion if the

barrier contains no open porosity).

ReferencesReferences

Materials for the Hydrogen Economy, Jones, R. H. and Materials for the Hydrogen Economy, Jones, R. H. and Thomas, G. J., ed., CRC Press, Boca Raton, 2008.Thomas, G. J., ed., CRC Press, Boca Raton, 2008.

http://en.wikipedia.org/wiki/Solid_oxide_fuel_cell