Solid Oxide Fuel Cells: Principles, Designs and State-of-the-Art in Industries

Abstract

A solid oxide fuel cell (SOFC) is composed of two porous ceramic electrodes and a solid state electrolyte, made of solid metal oxides. For this reason, the SOFC is also referred to as ‘ceramic fuel cell’. The idea of using a stabilized zirconia material as an electrolyte is derived from the experiments conducted by Nernst in 1899. Further studies, including those of Bauer and Preis (1937), showed that the so-called “Nernst Mass” (85% zirconia and 15% yttria), and other zirconia-based materials, at high temperature (600–1000°C), present an ionic conduction that meets the SOFC requirements. The ‘typical modern’ SOFC is composed of an electrolyte made of yttria-stabilized zirconia (YSZ), a porous anode made of nickel and ytrria stabilized zirconia (Ni/YSZ) cermet and a porous cathode composed of doped LaMnO3 (LSM). In order to obtain acceptable ionic conductivity, YSZ needs to operate at a relatively high temperature, typically above 700°C. Historically, the first SOFC operated at 1000°C. At this temperature, the protonic conductivity of the YSZ is negligible, while the ionic conductivity is about 15 S/m (Bossel). Due to the poor mechanical characteristics of the electrodes manufactured until the late 90s, SOFC had to be electrolyte supported. This is the main factor that constrained the operating temperature at typically 1000°C. Because of the minimum required thickness of the supporting structure (electrolyte), in fact, a lower temperature would lead to unacceptable ohmic resistance. Alternative configurations, like for example the previous design of the tubular Westinghouse cell (Bessette and Wepfer 1996) were based on an external (porous) support. However, this solution limits the mass transport through the cell and makes the manufacturing process more complex. By the end of the 90s and the early 2000s, the good mechanical characteristics, mostly due to the development of nano-powders for the manufacturing process, enabled the realization of electrode supported cells. Although some developers, like for example Siemens-Westinghouse, based their design on a cathode supported structure (Minh and Takahashi, 1995) many SOFC developers are now focusing on anode supported cells (Patel et al. 2004, Minh et al. 2004, Patel-Maru et al. 2004, Minh, Amaha et al. 2004).