118 CHEMICAL SENSORS. VOLUME 5: ELECTROCHEMICAL AND OPTICAL SENSORS of the sensing electrode. Further improvements can be achieved by optimized doping of semiconduc- tor oxide SEs using noble materials or other oxides. Notwithstanding the recent progress in the sensor development, it has to be admitted that the vast majority of the reported zirconia-based sensors are still only prototypes used under laboratory conditions. They have been designed and developed to extend new possibilities in gas sensing, and the ability to sense various gases using mixed-potential sensors at the parts-per-billion level is evidence of that. However, regardless of the promising performance in con- trolled laboratory conditions, these sensors have yet to be tested in specific industrial environments. Lack of long-term stability tests reported so far shows that there is still a long way to go to achieve industrial acceptance. For example, one of the biggest and strictest industries is the automotive industry, which does require sensors to operate for more than 10 years in vehicle exhausts without failure. Therefore, new approaches and breakthroughs in solid-state electrochemistry and fabricating technologies are expected to meet such tough requirements. Mixed-potential and different electrode equilibria gas 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. These are comparatively simple in design, and so far they have shown high sensitivity and selectivity toward the measured gas. Since the measured gas is involved in complex interactions at the YSZ/SE/gas triplel-phase boundary and within the structure of the SE, it is vital to investigate the fabrication methods so as to determine their effects on the overall sensor output signal. The optimization of existing technologies, microstructure, and properties of