ABSTRACT
The objective of this chapter is to provide a comprehensive review of critical issues and recent progress on materials for both high-temperature solid oxide fuel cells (SOFCs) (600°C–1000°C), which are currently undergoing commercialization, and low-temperature SOFCs (400°C–600°C), widely known as the next-generation technology. The widespread commercialization of SOFC is strongly related to the cost reduction of SOFC and operating temperature below 600°C is essential for that, but might also disclose new applications. Thus, a new generation of ceramics, high-temperature proton-conducting oxides have gained widespread interest as electrolyte materials alternative to oxygen-ion conductors. In this chapter, the commercial electrolyte materials will be discussed, followed by recent efforts to improve the electrolyte materials’ performance with higher conductivity at the low operating temperature of SOFC than the most commonly used yttria-stabilized zirconia (YSZ). Anode and cathode materials are covered in two separate sections since there is a difference between the gas compositions at these two electrodes. The commercial anode material used in high-temperature SOFC is operating on H2 and CO, and generally, a porous Ni/electrolyte cermet is used that supports a thin, dense electrolyte. The role of the anode is to oxidize the fuel with O2− ions received from the electrolyte and to deliver electrons of the fuel chemisorption reaction to a current collector. The role of Ni is to act as both the electronic conductor and the catalyst for splitting the H2 bond where the oxidation of H2 to H2O occurs at the Ni/electrolyte/H2 triple-phase boundary (TPB). The CO is oxidized at the oxide component of the cermet, which may be the electrolyte, YSZ, or a mixed oxide-ion/electron conductor such as Gd-doped ceria. The Ni cermet anode operates well under H2. However, with natural gas, Ni is fouled by carbon deposition unless a large amount of steam is added to the fuel and it has a low tolerance to sulfur due to the formation of NiS. Commercial SOFCs must be operated at high temperatures of around 800°C, primarily in order to overcome the slow kinetics of the oxygen reduction reaction at the cathode. The cathode in SOFC must perform three functions: (1) breaking the covalent bond of the O2 oxidant molecule, (2) accepting electrons from the external circuit and distributing them to the reaction sites for the reduction of the oxidant, and (3) allowing passage of the reduced O2− ions to the electrolyte for transfer to the anode where they combine with the fuel to form H2O and CO2 with the release of electrons to the external circuit. These three functions can be realized in a transition-metal oxide that is both an electronic and an oxide-ion conductor. A representative electronic conductor that is not an oxide-ion conductor such as La1− xSrxMnO3 (LSM) and La1− xSrxCo1− yFeyO3 (LSCF) appearing widely as a cathode material in commercial SOFCs. LSM is the most common cathode material used in SOFCs, but improvements in electrochemical properties and resistance to chromium poisoning are still needed for the cathode materials. Another issue with the LSM cathode is that the thermal expansion mismatch between the LSM electrode and the electrolyte and interconnect makes this system unsuitable as a cathode material for a SOFC subject to repeated thermal cycling. Later in this chapter, two supporting components of the SOFC stack, interconnects and sealants, will be discussed. For interconnects, conducting electrical current between the anode and the cathode through an external circuit, the stability of material used is very crucial since it is exposed to both low oxygen partial pressure (anode) and high oxygen partial pressure (cathode). So far, different ceramic interconnects have been used in the SOFC stack; however, metallic interconnects significantly reduce the production cost of SOFC. The most common approach for the sealants, separating the two gases, is to use a glass or glass-ceramic material in order to provide a gas-tight seal, which can increase the risk of failure due to thermal and mechanical stresses. Alternatively, compressive seals are less susceptible to thermal expansion, but they need the application of a load during cell operation.
