Frontiers in Ceramic Science

Fuel cells are electrochemical devices that convert chemical energy into electrical energy with high potential for commercial power generation applications. Among various types of existing fuel cells, solid oxide fuel cell (SOFC) is one of the most promising types of fuel cells, due mainly to its ability to utilize several types of fuels such as hydrogen, CO, hydrocarbon fuels, and ethanol. This chapter introduces the reader into the fundamentals of SOFCs, including its working principle and the main components used as electrodes.

It is well recognized that the development of low-temperature solid oxide fuel cells (LT-SOFCs) replies on the exploration of new functional materials and optimized microstructures with facilitated oxygen reduction reaction (ORR) that involves complicated electrochemical processes occurring at triple-phase boundaries (TPB). This urgent and critical demand promotes great research efforts on pursuing superior catalysts as electrodes owing comprehensive electrochemical and physicochemical properties, and relevant catalyst optimization on materials and microstructures. The material development is mostly based on perovskite with extensive doping strategies to maximize the catalytic activity while other properties such as stability, thermal and chemical compatibility, etc. are well compromised. Other types of materials such as K2NiF4, double perovskite were also studied as potential candidates, owing to the excellence of catalytic activity resulting from the special features of crystal structures. In this chapter, the fundamental knowledge of cathode is briefly introduced, such as reaction processes of ORR, catalysis mechanism and defect transport. Several typical perovskites are reviewed to better understand the required specific material properties for an excellent ORR catalyst as cathode material that can be operated at practical low temperatures (350~500 °C). Particularly, recent development of the layered perovskites is specifically introduced because they show very promising performance at low temperatures due to the fast oxygen exchange and oxygen diffusion yielded by the ordered cation distribution in crystal.

This chapter presents a state-of-art brief review on anode materials for SOFC. Materials, processing and synthesis routes to attain porous anodes are highlighted. Especial attention is given to Ni-ceramic phase (especially fluorite-type structure ceramics) cermets. Some aspects about prospects and problems of the currently developed electrodes materials are elucidated. Electrodes for intermediate temperature SOFCs (IT-SOFCs) are discussed in relation to other conventional electrodes. The electrochemical characterization of anodes, as mixed ionic-electronic conductors, is briefly outlined.

YSZ (yttrium stabilized zirconia) with fluorite structure is the traditional electrolyte used in SOFC (solid oxide fuel cell), where the operating temperatures are above 900 oC. In those high temperatures, reactions between the components may occur, in addition to the thermal expansion and contraction, causing the diminution of the cell life. Therefore, the reducing of operating temperature to development of the intermediate temperature SOFs (IT-SOFCs) is one of important task to the power production area. Find an alternative electrolyte for IT-SOFCs became the interest of researchers. Recently, rare earth silicate-based compositions materials with apatite-type structure, with general formula Ln10-aSi6O26+b, (where Ln is La, Sm, Nd, Dy or Gd, and a = 8 to 11), have attracted significant attention as electrolyte. This is because of the structure allows high ionic conductivity with low activation energy at intermediate temperatures. For example, the lanthanum silicate apatite (LSA) solid electrolyte, with the composition La10Si6O27 has exhibited high oxygen ionic conductivity at 500 oC. However, several problems in obtaining the pure apatite single phase and the low sinterability of LSA are disadvantageous for its application as electrolyte. Therefore, the development of a viable synthesis process to attain LSA apatite crystalline powder, with high sinterability for applying in the production of IT-SOFCs electrolytes, turned out to be a challenge for SOFC researchers. Efforts have focused to reach the pure single phase of apatite with the reduction of temperature and time of the sintering process. In this review, different methods, of LSA synthesis, are summarized and discussed.

Many properties of ceramic materials depend largely on its composition and structure, being also affected by the synthesis method. Intense research activity has been carried out for the development of materials and processes to components of solid oxide fuel cells (SOFCs). Regarding ceramic powders preparation to be used as SOFC components, chemical approaches such as polymeric precursor method, sol-gel, combustion, and co-precipitation have been mentioned in literature. This chapter aims to present some methods that have been used to prepare functional SOFC materials, highlighting its main advantages and disadvantages in the performance of these materials.

Micro-tubular solid oxide fuel cells (MT-SOFC) have recently attracted increasing attention due to their advantages, such as quick start-up/shut-down, high volumetric power density and better mobile/portable characteristics comparing favorably with planar and tubular SOFCs. Basically, there are four fabrication techniques to prepare MT-SOFCs: extrusion, cold isostatic pressing, slip-casting and phase-inversion. Among these techniques, the phase inversion method is a recently emerging technology to prepare the MT-SOFCs, which can be easily scaled up to realize an automatic and continuous preparation process. Therefore, this chapter gives a detailed account of the fabrication skills of micro-tubular SOFCs based on the phaseinversion techniques and the brief review of their performance test results.

This chapter focuses to discuss about the recently introduced phaseinversion based extrusion technique for fabrication of electrolyte hollow fiber support and multi-layer of the micro tubular solid oxide fuel cell (MT-SOFC). The effects of different fabrication parameters on the morphologies and electrochemical performances of developed electrolytes via the technique were critically discussed. The future direction of this advanced phase-inversion-based extrusion technique in the MT-SOFCs fabrication was also being discussed at the end of this book chapter.

Solid oxide fuel cells (SOFCs) offer a large potential as a future green energy technology, as they show high fuel conversion efficiency with limited pollution and fuel flexibility for a wide range of applications [1]. These devices operate in the high temperature range (800-1000 °C), where cost and longevity related problems have slowed their commercialization. Nonetheless, the high operating temperature hurdle of SOFCs can be crossed by using proton conducting ceramic oxides as solid electrolytes, producing so called protonic ceramic fuel cells (PCFCs). The perovskite AB1-xMxO3-δ, A=Ca, Ba, Sr; B=Ce, Zr; M=Y, Gd, Yb) materials have been suggested as electrolyte materials for PCFCs, since these materials show high protonic conductivity and lower activation energy (0.4-0.6 eV) in the intermediate temperature range 500-750 °C [2]. The alkaline earth doped cerate family possess high proton conductivity (about 10-2 S cm-1 at 600 °C) with low activation energy, but suffer from poor chemical stability due to degradation in the presence of acidic gases (e.g., CO2) and steam, precluding their practical use as electrolytes in PCFCs. On the contrary, the alkaline earth doped zirconates exhibit good chemical stability, although their overall proton conductivity is limited (about 10-3 S·cm-1 at 600 °C) due to a low grain boundary conduction, compounded by poor sinterability and limited grain growth [3]. By tailoring the chemical composition of these two material families, a compromise between good proton conductivity, good sinterability and chemical stability can be achieved [4]. Due to the novelty and potential of this technology, this chapter will be dedicated to recent developments in PCFCs, highlighting potential electrolyte and electrode materials, their microstructure and property relationships.