The high oxygen-ion conductivity over wide ranges of temperature and oxygen pressure in stabilized cubic zirconia has led to its use as a solid-oxide electrolyte in a variety of electrochemical applications. Zirconia-based oxygen sensors are widely used for combustion control, especially in automobiles, for atmosphere control in furnaces, and as monitors of oxygen concentration in molten metals. Other applications include electrochemical pumps for control of oxygen potential, steam electrolyzers, and high-temperature solidoxide fuel cells (SOFCs). High-temperature SOFCs offer a clean, pollution-free technology to electrochemically generate electricity at high efficiencies. These fuel cells provide many advantages over traditional energy-conversion systems, including high efficiency, reliability, modularity, fuel adaptability, and very low levels of NOx and SOx emissions. The quiet, vibrationfree operation of SOFCs also eliminates the noise usually associated with conventional power-generation systems. Furthermore, because of the high temperature of operation (~1000°C) of SOFCs, naturalgas fuel can be reformed within the cell stack, eliminating the need for an expensive external reformer system. Also, pressurized SOFCs can be successfully used as replacements for combustors in gas turbines; such hybrid SOFC/gas-turbine power systems are expected to reach efficiencies approaching 70%.