A robust and reliable gas sensing device is considered as a convenient and practical solution for gas concentration monitoring that has become a mandatory requirement in different field of applications. For in situ hazardous gases detection, a mixed-potential type gas sensor has been regarded as a promising solid-state gas sensor. For the past three decades, there has been a significant progress in achieving high performance in mixed-potential type sensors. Therefore, this review is focused on reporting the development of mixed-potential type gas sensors with combined yttria-stabilized zirconia (YSZ) as the base solid electrolyte material and various classes of electrode materials for their potential utilization as a high-performance sensing electrode. The underlying sensing mechanism of a mixed-potential type YSZ-based sensor is elaborated here in detail. Transformation in design and configuration of this type of sensor is also covered in this report. In addition, recent progresses on mixed-potential type gas sensors development for detection of several target gases, such as carbon monoxide, hydrocarbons, nitrogen oxides, hydrogen, and ammonia, are reviewed. Strategies to improve the sensing characteristic, particularly gas sensitivity and selectivity, are also reported. Based on the understanding of the fundamental sensing mechanism and the requirements for high-performance gas sensors, challenges and future trends for this type of gas sensor development are discussed.

A planar mixed-potential-type sensor using a yttria-stabilized zirconia and oxide-based sensing electrode (SE) was fabricated and the preliminary results on sensing characteristics were reported here. It was found that the sensor attached with exhibited good sensitivity and excellent selectivity to at under the wet condition ( ). While the cross sensitivities to the other examined gases were about or negligible, the sensitivity to was about . The of the sensor varied almost linearly with the concentration in the examined range of .

A planar <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>NO2 sensor was fabricated by using an yttria-stabilized zirconia plate and NiO-based sensing electrode (SE). The improvement in sensing characteristic of mixed-potential-type planar <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>NO2 sensor was examined by the addition of various metal oxides to NiO-SE. Among the metal-oxide additives tested, <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">W</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>WO3 was found to give a significant enhancement in <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>NO2 sensitivity. In addition, this enhancement was maximum when the initial <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">W</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>WO3 content in the NiO-based paste before sintering was <mml:math display="inline"><mml:mrow><mml:mn>10</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi>wt</mml:mi><mml:mspace width="0.3em"></mml:mspace><mml:mo>%</mml:mo></mml:mrow></mml:math>10wt% . The sensitivity (Δ electromotive force) to <mml:math display="inline"><mml:mrow><mml:mn>50</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi>ppm</mml:mi></mml:mrow></mml:math>50ppm <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>NO2 was as high as about <mml:math display="inline"><mml:mrow><mml:mn>70</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi>mV</mml:mi></mml:mrow></mml:math>70mV for the sensor using the <mml:math display="inline"><mml:mrow><mml:mn>10</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi>wt</mml:mi><mml:mspace width="0.3em"></mml:mspace><mml:mo>%</mml:mo></mml:mrow></mml:math>10wt% <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">W</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>WO3 -added NiO-SE at <mml:math display="inline"><mml:mrow><mml:mn>800</mml:mn><mml:mo>°</mml:mo><mml:mi mathvariant="normal">C</mml:mi></mml:mrow></mml:math>800°C under the wet condition (containing <mml:math display="inline"><mml:mrow><mml:mn>5</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi>vol</mml:mi><mml:mspace width="0.3em"></mml:mspace><mml:mo>%</mml:mo></mml:mrow></mml:math>5vol% <mml:math display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>H2O ). The sensitivity of this sensor was still <mml:math display="inline"><mml:mrow><mml:mn>8</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi>mV</mml:mi></mml:mrow></mml:math>8mV even down to <mml:math display="inline"><mml:mrow><mml:mn>1</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi>ppm</mml:mi></mml:mrow></mml:math>1ppm <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>NO2 at <mml:math display="inline"><mml:mrow><mml:mn>800</mml:mn><mml:mo>°</mml:mo><mml:mi mathvariant="normal">C</mml:mi></mml:mrow></mml:math>800°C . The <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>NO2 sensitivity was found to be invariant to different concentrations of <mml:math display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>H2O and <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">C</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>CO2 in the examined range of 5–15 and 5–20 <mml:math display="inline"><mml:mrow><mml:mi>vol</mml:mi><mml:mspace width="0.3em"></mml:mspace><mml:mo>%</mml:mo></mml:mrow></mml:math>vol% , respectively. It is speculated that the modification of morphology of the NiO-SE layer by the addition of <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">W</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>WO3 is the main contributing factor for the improved <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>NO2 sensitivity of the sensor.