NiO-based sensor for in situ CO monitoring above 1000 °C: behavior and mechanism

An advanced smart sensor network is essential to a combustion system, which favors in situ, locally placed, and low-cost gas sensors. However, most chemical/electrochemical sensors fail to work in a combustion boiler, due to demanding operation temperatures (> 1000 degrees C). This work, for the first time, reports a well-functioning mixed-potential type CO sensor at 1000-1200 degrees C using nickel oxide (NiO) as the sensing material on the yttrium-stabilized zirconium (YSZ) oxide electrolyte. The influences of feed flow rate, electrode thickness, and porosity on the sensor behavior were investigated and the developed mixed potential gas sensor delivered notable and fast responses to 1000 ppm CO in 3% O-2: 109 mV @1000 degrees C, 40 mV @1100 degrees C, and 7 mV @1200 degrees C. The sensing mechanism was identified different from the common theory based on CO oxidation coupled with oxygen reduction. A new mechanism is attributed to the inverse reactions: CO reduction coupled with oxygen evolution. For the first time, an inversion temperature was found for the CO-NiO reaction. Below the inversion temperature, CO oxidation occurs as commonly presented. Above the inversion temperature, CO is electrochemically reduced over NiO into carbon and oxygen ions, evidenced by electrochemical and compositional characterizations. The results can complement CO sensing mechanisms for mixed potential sensors and further trigger more research efforts to expand the working temperature limit of mixed potential sensors for practical application in combustion control systems.

Automated summary

This study presents a well-functioning mixed-potential CO sensor using nickel oxide as the sensing material, capable of operating at high temperatures. The sensor exhibits notable and fast responses to 1000 ppm CO, and its sensing mechanism is attributed to the inverse reactions of CO reduction coupled with oxygen evolution. These findings enhance our understanding of CO sensing mechanisms for mixed potential sensors and suggest possibilities for expanding their working temperature limit in practical combustion control systems.