General View on Electrochemical Gas Sensors

Most gas sensors that use solid electrolytes are operated potentiometrically. Configurations for potentiometric gas sensors have been reported (Yamazoe and Miura 1998 Miura et al. 2000). The simplest scheme for such sensors is represented in Table 6.1 as Type I. In this diagram illustrating the potentiometric sensor mechanism, A is the analyte with variable activity/concentration, is the constant activity of analyte A on the reference side of the solid electrolyte membrane, AB is a solid electrolyte membrane (A+ ion conductor), and the electrodes facilitate the reaction A +e =A when the activity of A is different on each side of the membrane, a potential, V, is observed. The voltage produced is from the concentration dependence of the chemical potential, which at equilibrium is represented by the Nemst equation  

Similar to liquid systems, solid-state electrochemical cells for gas sensing are typically constructed by combining a membrane of solid electrolyte (ion conductor) with a pair of electrodes (electronic conductors). As typical of all electrochemical systems, the interface of solid electrolyte and electrode 

Korotcenkov, Handbook of Gas Sensor Materials Properties, Advantages and Shortcomings for Applications Volume 1 Conventional Approaches, Integrated Analytical Systems, 

As shown in Table 6.1, at present three types of potentiometric gas sensors can be designed based on solid electrolytes (Yamazoe and Miura 1998). The well-known oxygen YSZ-based probe with an oxygen ion conductor is a sensor of Type I. This means that a gas sensor of Type I is constructed from a solid electrolyte for which the mobile ion is the same as that electrochemically derived from the gas phase. In this case, the interface potential can be obtained from the local equilibrium reaction, occurring at the electrode/electrolyte/gas three-phase boundary involving the mobile 0 ions in the electrolyte, electrons 

At the platinum working electrode, the equilibrium reaction at Eq. (6.3) occurs  

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