Materials for high temperature electrochemical NOx gas sensors

Solid electrolytes are well-suited for high-temperature electrochemical devices and thus have been used in sensors for monitoring exhaust gas concentrations. Oxygen content can be measured directly using a potentiometric sensor with an oxygen-ion conductor, such as yttria-stabilized zirconia, but sensors for other exhaust gases, including NOx, require more complicated approaches, such as auxiliary electrodes for equilibrium measurements or electrocatalytic electrodes for mixed potential non-equilibrium measurements. The successful development of these sensors depends critically on the identification of electrode materials with the appropriate thermodynamic and catalytic properties. In this paper, the materials used for electrochemical NOx gas sensors are reviewed with an emphasis on potentiometric sensors, but the materials used in other approaches, such as impedancemetric and amperometric sensors, are also discussed.