Solid-state electrochemical gas sensors

The solid-state electrochemical principle has been a selective and accurate way of sensing chemical components in various environments, including liquid metal, for an extended period of time. Since after Carl Wagner's interpretation of zirconia, there appeared many advances in chemical sensor applications. The electrochemical techniques for the chemical measurements have, in general, several major advantages compared to other methods. The information of interest is directly converted into electrical signal which may be employed in electronic circuits. Electrochemical measurements are always selective for the quantities that undergo the electrochemical redox reaction. In most cases, reactions at equilibrium are considered, but techniques have also been developed to be able to use kinetic limit. Furthermore, the signal is independent of materials properties, such as the ionic conductivity or impurity as long as it is a predominant ionic conductor. Depending on the type of application, voltage or current measurements are employed. While potentiometric method commonly allows measuring chemical species over a wide range of concentration, amperometric sensors generally cover a quite limited range but have a much higher resolution. In this paper, various principles of electrochemical techniques to measure the chemical quantities are introduced. And there are many examples of the status of researches on electrochemical sensors, such as oxygen sensor, carbon dioxide sensor, NO (x) sensor, SO (x) sensor, and hydrogen sensor.