A review of recent progress in sensing of gas concentration by impedance change

The intent of this paper is to establish the state of the art of impedance-based gas sensors. This sensor type holds promise for accurate detection of gaseous species at single parts per million and below. Impedance-based sensors do not require reference air to function, but do require calibration. Progress in the development of impedancemetric sensors for the detection of NOx, H2O, hydrocarbons, and CO is reviewed. Sensing electrodes typically consist of a noble metal or a metal oxide. YSZ is the preferred electrolyte material. Counter electrodes of Pt were common in asymmetric cells. These sensors typically operate at 500-700 A degrees C and are interrogated at 10 Hz or less. Selectivity of these sensors remains a challenge especially in lean environments. Stability is an infrequently discussed yet important concern. Equivalent circuit analysis has shed light on various detection mechanisms. The impedance changes due to analyte gases are exhibited in parameters that represent the low frequency behavior of the electrochemical system. Although the search for a detailed mechanism continues, the change in impedance due to a specific gas is generally attributed to transport processes such as adsorption and charge transfer.