Materials: Electrolytes, Anodes, Cathodes, Interconnects, and Sealants

Overview of Solid Oxide Fuel Cell (SOFC) Materials

SOFCs have been developed worldwide for many years. A very high technical refinement has been achieved by continuous improvement in materials, cell design, and manufacturing processing for SOFCs. To commercialize SOFC systems and make them more economically viable, multiple factors are involved, including high cost of the cell materials, susceptibility of the cell materials failure because of mechanical shock, oxidation, and rapid thermal transients, unreliable cell sealing, and large production issues with ceramic parts. Therefore, due to the importance of materials selection and development on SOFC performance, many noteworthy companies and centers all around the world such as Bloom Energy (USA), Ris0 National Labratoary (Denmark), Ceramic Fuel Cells Limited (Australia), Tokyo Gas (Japan), Mitsui Engineering & Shipbuilding Corporation Limited (Japan), Mitsubishi Heavy Industries & Electric Power Development Company (Japan), Energy Research Center/Innovation Dutch Electroceramics (Netherlands), Forschungszentrum Jiilich (Germany), Versa Power (Canada), and Space Weather Prediction Center (USA) have been working on design and processing of SOFC component materials [1-4]. The materials development for the five key components of SOFC, i.e., electrolyte, anode, cathode, interconnect, and sealant, along with the related issues are presented in Figure 2.1. The general outline of the materials requirements for cell components is given below: ^•

In order to prevent the mixing of reducing and oxidizing gases, electrolyte and interconnect must be fully dense, whereas the electrodes must be porous to maximize gas mobility for the reaction.

• • Both anode and cathode electrodes and interconnect must have high electrical conductivity, whereas electrolytes must have high ionic conductivity. The electrodes could also be mixed conductor (electronic and ionic) in which oxygen-ion diffusivity is very high.
• • During cell operation, chemical stability is necessary for the adjoining cell components to withstand oxidizing and/or reducing environments at cell operating temperatures.
• • During cell long-term operation, each component must have chemical, dimensional, morphological, and phase stabilities.

FIGURE 2.1 Materials and related issues for SOFC. (Reprinted with permission from Ref. [2].)

• • Each component must have high thermal and mechanical shock resistance.
• • The supporting cell components, i.e., electrolyte- and electrode-supported, must have high toughness and mechanical strength during cell operation.
• • In order to avoid delamination and crack formation and to reduce internal stresses during fabrication and cell operation, all cell components must have a close thermal expansion coefficient.
• • The low fabrication cost and easily available technologies for each cell component must be addressed.

A comprehensive list of different materials, having been used for each component, is shown in Figure 2.2. Table 2.1 presents an overview of the SOFCs’ diversification, along w'ith some of the main developers involved. This chapter will discuss technical achievements to date for the SOFC’s five key component materials.

Electrolytes

Oxygen-Ion-Conducting Electrolytes

The electrolyte of SOFC mainly conducts oxygen ions (0²~) from cathode to anode where it reacts with fuel (hydrogen or hydrocarbons) to form H₂0 and C0₂, thereby completing the overall electrochemical reaction and producing electron, passing through an external circuit. The oxygen-ion conduction occurs where the oxygen ion in a thermally activated process moves from one crystal lattice site to its neighbor site. To achieve a high ion-conducting electrolyte, its crystal structure must contain a high level of oxygen vacancy sites, and low migration energy barrier, certainly less than leV. Since the oxygen ion with an ionic radius of 0.14nm is the largest

FIGURE 2.2 A list of different materials used for each of five key components in SOFC. (Reprinted with permission from Ref. [3].)

TABLE 2.1

Developers of SOFC Materials for Planar and Tubular Cell Designs and Corresponding Fabrication and Design Details

(Continued)

TABLE 2.1 (Continued)

Developers of SOFC Materials for Planar and Tubular Cell Designs and Corresponding Fabrication and Design Details

Source: Data adapted and modified from Ref. [4].

component in the crystal lattice, it seems that it is difficult to attain a small barrier for oxygen-ion migration.

Thus, it would be expected that the smaller metal cations in such a metal oxide structure would be more likely to possess an appreciable movement in the lattice and thereby carry the current. However, metal cations are not capable of free movement in the lattice due to their large charge valence. In addition, in a certain metal oxide crystal structure, the number of oxygen vacancies is predominant; thus, oxygen ions move in the electric field. Some noteworthy examples of these metal oxides with partially occupied oxygen sites in the crystal structure are ZrO_r, Ce0₂-, and Bi₂0,-based oxides with the fluorite structure, LaGaO,-based perovskites, Bi₄V₂0|_r and La₂Mo₂0₉-based derivatives, Ba₂ln₂0₅-derived perovskite- and brownmillerite-like phases, pyrochlores-Gd₂Zr₂0₇, Gd₂Ti₂0₇, and rare-earth-based apatite-La₉SrSi₆0_{26 5} [5,6].

Among ion-conducting oxides, only a few selected oxides have mostly been developed to be used as the electrolyte for SOFCs due to the variety of essential requirements for the electrolyte component, including high oxygen-ion conductivity (~0.1 S cm^-1 at operating temperature), low electronic conductivity, and electronic transfer number (<10^-3), chemical and thermodynamic stabilities over a wide range of temperatures and oxygen partial pressures, negligible volatilization, thermal expansion compatibility with adjoining components, sufficient mechanical properties, and negligible interaction with the electrode material during processing and service. Figure 2.3 shows the oxygen-ion conductivity of the selected oxide electrolytes.

FIGURE 2.3 Oxygen ionic conductivity of various solid-state electrolytes. (Reprinted with permission from Ref. [5].)

Proton-Conducting Electrolytes

Proton-conducting electrolytes, carrying proton (H⁺) from anode to cathode where it reacts with oxygen, thus produce water and electrons, find applications for low- temperature SOFCs since most of them decompose at 300°C [7]. It has demonstrated that the SOFC systems working with the proton-conducting electrolytes possess higher thermodynamic efficiency than the SOFC systems working with the oxygen-ion-conducting electrolytes. The proton-conducting materials derive their conductivity from the protonic defects in their crystal structure which have high mobility. Some examples of proton-conducting perovskites used as the SOFC electrolyte are BaCeO,, BaZrO, and SrCeO,, SrZrO,, CaZrO,, BaCe₀₈Y₀₂O₃, and BaZr₀₄Ce₀₄In₀₂O₃. The perovskite materials usually have a general formula of АВ|__л.М_л0₃, where M is a trivalent rare-earth element dopant. At elevated temperatures and in the presence of water, these perovskites show proton conductivity. Thus, the level of humidity is important for a proton conductor. Since the size of protons is small, they cannot occupy the interstitial sites in the perovskites structure. Therefore, protons are mainly embedded in the electron cloud of an oxygen ion.

The Kroger-Vink notation for protonic conductors can be expressed as:

where the water molecule dissociates to form two hydroxide ions, forming two protonic defects by replacing the oxide ions in the crystal lattice [7]. Figure 2.4 shows the proton conduction for different materials.

FIGURE 2.4 Conductivity of different perovskite-based proton-conducting electrolytes. (Reprinted with permission from Ref. [7].)

Zirconia-Based Electrolytes

The zirconia-based oxides are the most common fast oxygen-ion-conducting electrolytes with the crystal structure of the fluorite type, operating either at high temperatures (800°C-1000°C) or intermediate temperatures (600°C-800°C). Pure ZrO, has the monoclinic structure at room temperature with relatively low electronic conductivity and not a good ionic conductivity. Upon heating, monoclinic to tetragonal and tetragonal to cubic phase transformations occur at 1170°C and 2370°C, respectively. These martensitic phase transformations are reversible on cooling, however, tetragonal to monoclinic phase transformation occurs at lower temperatures (900°C-1000°C) during cooling. In addition, a large volume change leading to the disintegration of the ceramic body occurs during cooling for the monoclinic and tetragonal phase transformations. In order to increase the concentration of oxygen vacancies within the crystal structure required for ion conduction and to stabilize the cubic structure at lower temperatures, dopants are introduced into the cation sublattice [8-17]. The three crystal structures adopted by ZrO, are shown in Figure 2.5.

The cubic ZrO, structure can be stabilized to room temperature by adding dopants such as CaO, MgO, Y,0„ Yb,0„ and Sc,0,. This can be described by the Kroger-Vink notation:

where M is a divalent cation, R is a trivalent cation, and Vq is a compensating oxygen vacancy in the crystal structure [17]. For instance, 8% mol Y,0, is believed as the lowest concentration to stabilize the ZrO, cubic phase at room temperature. The composition range over which the cubic phase exists is narrow and temperature dependent, and is affected by the type of dopant. A complex phase assemblage consisting of two or more phases may occur if the composition is below the required amount of dopant for stabilization of the cubic structure at low temperatures. The two-phase mixture in partially stabilized ZrO, can be beneficial since it improves the mechanical properties of the ceramic.

FIGURE 2.5 Crystal structures of ZrO,: (a) cubic, (b) tetragonal, and (c) monoclinic. Dark spheres represent oxygen atoms and bright spheres are zirconium atoms. (Reprinted with permission from Ref. [16].)

Yttria-stabilized zirconia (YSZ) and scandia-stablized zirconia (ScSZ) with dopant concentrations in the range of 8-10 mol% are mainly used and developed as the effective electrolyte for the high-performance SOFC systems due to their high ionic conductivity, low cost, high mechanical and chemical stability, thermal shock resistance, and desire compatibility with adjoining components used in the SOFC system. Zirconia ceramics also show low electronic conductivity in a wide range of oxygen partial pressure (from 100 to 200 atm down to 10~²⁵ to 1()~²⁰ atm) [5,6]. This range covers 1-10"¹⁸ atm pressure which an SOFC electrolyte has to tolerate at the anode side during cell operation at high temperatures [18]. There are various studies over a decade that tried to improve the performance of the YSZ and ScSZ electrolytes toward lowering the operating temperature of SOFC. The investigations have mainly been focused on:

• • Developing the fabrication method of the electrolyte to enhance its properties and performance.
• • Developing the thin-film structure electrolyte for a simple design structure of SOFC to expand the usage in transportation and portable applications.
• • Modification of the electrolyte with co-dopants and bilayer structures in order to increase the amount of oxygen vacancies and the ionic conductivity.
• • Improvement of mechanical and thermal properties of the electrolyte to enhance the durability of the SOFC.