12 - Solid-state electrochemical gas sensors for emission control

Abstract

This chapter reviews the recent trends in the development of solid-sate electrochemical gas sensors for environmental monitoring of gases, such as CO, CO 2 , C x H y , NO x , SO x , and H 2 . The monitoring of exhaust gases, particularly in vehicles and environmentally important pollutants are the two main areas of application for these sensors. Although Yttria Stabilized Zirconia (YSZ)-based gas sensors show excellent sensing performances for emissions control and environmental monitoring, they generally are prototypes used in laboratory studies to explore new possibilities in gas sensing. Despite the promising performance in the controlled conditions of the laboratory, these sensors have yet to be tested for long-term stability, which is a prerequisite to industry acceptance. Potentiometric, non-Nernstian, mixed-potential sensors offer several advantages. A recent shift from random to carefully selected thermal histories for SEs of both single oxides and spinels has increased the working temperatures of these sensors to 700 °C, which is compatible with the working temperature of vehicle exhausts. These devices are comparatively simple in design and they exhibit high sensitivities and good selectivities. The fabrication of several electrodes on a single solid electrolyte is a realistic goal, which gives rise to the possibility of sensors for C x H y and NO x . Since the requirements for catalytic activity at high temperatures of Sensing Electrodes (SEs) are relatively easy to meet, there are many possibilities for SEs for non-Nernstian potentiometric gas sensors. From the practical point of view, the main problems are related to the non-ideal selectivity of single electrodes and the lack of long-term stability of their interfaces.