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This chapter begins with a brief history of fuel cell development and introduces solid oxide fuel cells (SOFCs) as high efficiency energy conversion devices. Following this the fundamentals of SOFC performance and cell design are explored, with special focus given to the significance of operating temperature and microstructure. Next the current commercial status of SOFCs is outlined in brief. Finally, SOFC degradation, the major theme of this book, is introduced; the various mechanisms are split into the two broad categories of physical and chemical degradation.

The widely used Bruggeman equations correlate tortuosity factors of porous media with their porosity. Finding diverse application from optics to bubble formation, it received considerable attention in fuel cell and battery research, recently. The ability to estimate tortuous mass transport resistance based on porosity alone is attractive, because direct access to the tortuosity factors is notoriously difficult. The correlation, however, has limitations, which are not widely appreciated owing to the limited accessibility of the original manuscript. We retrace Bruggemans derivation, together with its initial assumptions, and comment on validity and limitations apparent from the original work to offer some guidance on its use.