Geyu Lu

Highly selective CO sensor using stabilized zirconia and a couple of oxide electrodes

Abstract The authors have designed a new solid-state electrochemical device based on stabilized zirconia for selective detection of CO at high temperatures by using two oxide electrodes. Of the oxides tested, the combination of CdO and SnO 2 was found to be best suited for the electrode couple, giving a rapid and selective response to CO in air at 600°C. The 90% response time to 100 ppm CO of the sensor obtained was as short as ca. 10 s at 600°C. The EMF value varied logarithmically with the CO concentration in the range of 20–4000 ppm. In addition, the EMF was not significantly affected by the other gases, such as H 2 , NO, NO 2 , CO 2 , O 2 and H 2 O. Based on the measurements of anodic and cathodic polarization curves, the authors found that the sensing mechanism of the above sensor involved a mixed potential at both oxide electrodes.

High-temperature hydrogen sensor based on stabilized zirconia and a metal oxide electrode

A zirconia-based electrochemical device attached with an oxide electrode was developed for the detection of H2 in air at elevated temperature. Among the oxides tested, ZnO was found to be best suited for the sensing electrode. The device attached with ZnO showed fairly good sensing characteristics to 50–500 ppm H2 in air at 450–600°C. The 90% response and 90% recovery times to 200 ppm H2 at 600°C for the ZnO-attached device were 5 s and 10 s, respectively. The EMF value of the device was almost linear with the logarithm of H2 concentration. It was also observed that the device exhibited excellent selectivity to H2 in the coexistence of NO, NO2, CH4, CO2 and H2O, as well as a good long-term stability. On the basis of the measurements of anodic and cathodic polarization curves, the sensing mechanism was confirmed to involve a mixed potential at the oxide sensing electrode.

High-temperature potentiometric/amperometric NOx sensors combining stabilized zirconia with mixed-metal oxide electrode

Electrochemical sensors using stabilized zirconia and oxide electrodes were tested for detecting NO or NO2 potentiometrically and amperometrically at high temperature. For a mixed-potential-type NOx sensor of two-electrode structure, CdCr2O4 was found to give fairly excellent NOx sensing characteristics among the perovskite-type and spinel-type oxides tested. The CdCr2O4-attached device gave a linear correlation between EMF and the logarithm of NO or NO2 concentration at 500–600°C. The mixed potential sensing mechanism was also verified. In order to improve the sensitivity and selectivity to NO, the above device was modified to three-electrode structure. With a fixed bias voltage being applied between the sensing (oxide) and counter (Pt) electrodes, the voltage between the sensing and reference (Pt) electrodes was measured as a sensing signal. The responses to NO and NO2 were found to be enhanced and suppressed with a positive bias, respectively. The tubular device with two-electrode structure was tested for amperometric detection of NOx. A CdCr2O4-attached tubular device was found to show quick and selective response to NO over NO2 at 500 and 550°C, if the oxide sensing electrode was polarized at +100 mV versus the reference Pt electrode. The NOx sensing mechanisms of the three-electrode device were proposed and discussed on the basis of the sensing characteristics obtained.

Stabilized zirconia-based sensor using oxide electrode for detection of NOx in high-temperature combustion-exhausts

Abstract New type NOx sensors were newly designed by using stabilized zirconia and an oxide sensing electrode. Among the various oxides examined for the sensing electrode, CdMn2O4 was found to be the most excellent material. Both planar and tubular devices attached with a sputtered CdMn2O4 layer could respond well to NO2 as well as NO in air at higher temperature, such as 500 °C. The EMF values of the devices were almost linear to the logarithm of NOx concentration with a positive slope for NO2 and a negative slope for NO. A NOx sensing mechanism involving mixed potential was proposed based on the measurements of polarization curves.

Progress in mixed-potential type devices based on solid electrolyte for sensing redox gases

Abstract Gas sensors based on solid electrolytes have been explored extensively in recent years, especially from growing concerns to environmental issues. Among the various solid electrolyte sensors, the group of devices based on the mixed-potential gas-sensing principle has been paid special attention due to its attractive performances as well as unique working mechanism. These devices, if designed properly, can exhibit high sensitivity and selectivity to redox gases in oxygen containing atmospheres even at high temperature, showing promise of being applied for combustion exhaust monitoring. This article aims at over-viewing recent progress in the mixed-potential type gas sensors. Particular attention is focused on our original work about the zirconia-based sensors using oxide electrodes for detecting various gases such as H 2 , CO and NO x at high temperatures.

Mixed potential type NO{sub x} sensor based on stabilized zirconia and oxide electrode

A new solid-state NO{sub x} sensor operative at high temperature was designed using stabilized zirconia and an oxide sensing electrode. Among the various oxides examined, CdMn{sub 2}O{sub 4} was found to be the most excellent material for NO{sub x} sensing at 500 C. The EMF values of the sensor were almost linear to the logarithm of NO{sub x} concentration with a positive slope for NO{sub 2} and a negative slope for NO. A nitrogen oxide sensing mechanism involving mixed potential was proposed based on the measurements of polarization curves.

High-temperature sensors for NO and NO2 based onstabilized zirconiaand spinel-type oxide electrodes

Zirconia-based electrochemical devices attached with an oxide sensing nelectrode have been examined for gas sensing properties towards NO and nNO n 2 n at high temperature. Among the twelve kinds of spinel-type noxides tested, CdCr n 2 nO n 4 n was the most sensitive nelectrode material to both NO and NO n 2 n in air at 500–600 n°C, also being endowed with quick response and recovery ncharacteristics. Values of E n EMF n of the nCdCr n 2 nO n 4 n-attached device decreased or increased nlinearly with an increase in the logarithm of NO or NO n 2 n nconcentration, respectively. The same device gave small or insignificant ncross-sensitivities to H n 2 n , CO, CH n 4 n , CO n 2 n nand H n 2 nO. Sensing mechanisms involving mixed potential were nconfirmed from the measurements of polarization curves.

Stabilized zirconia-based sensors using WO3 electrode for detection of NO or NO2

Abstract A stabilized zirconia-based electrochemical device, the sensing electrode of which was provided with a single-metal oxide, was tested for detection of NO and NO 2 at high temperature, Among the many single-metal oxides examined, WO 3 was found to give the best sensing properties to NO and NO 2 at 500–700°C. The EMF response of the WO 3 -attached device was almost linear to the logarithm of NO or NO 2 concentration. It also gave speedy response and recovery kinetics to NO or NO 2 . The device gave very small cross-sensitivities to H 2 , CO, CH 4 , CO 2 and water vapor.

Sensing characteristics and mechanisms of hydrogen sulfide sensor using stabilized zirconia and oxide sensing electrode

A new type of electrochemical sensors for H2S was developed by combining a Y 2 O 3 -stabilized zirconia (YSZ) tube with a sensing oxide layer of WO 3 . The sensor device composed was an electrochemical cell of the form, air, Pt|YSZ|WO 3 , Pt, H 2 S (+air). This device was found to respond well to 0.2-25 ppm H2S in air at 400°C, with acceptable response rates. The EMF of the sensor was linearly related to the logarithm of the H2S concentration with a slope of -74 mV/decade. In addition, the EMF was hardly affected by the coexistence of CO 2 and water vapor. Based on the measurements of anodic and cathodic polarization curves, the H 2 S sensing signal was suggested to reflect the mixed potential at the zirconia/WO 3 interface. The possibility of amperometric detection by using the present device was also suggested.

Selective detection of NO by using an amperometric sensor based on stabilized zirconia and oxide electrode

Abstract Stabilized zirconia-based devices using oxide electrode (CdCr2O4) were fabricated and examined for selective amperometric detection of NO at high temperature. The CdCr2O4-attached tubular device showed quick and selective response to NO over NO2 at 500 and 550°C, if the potential of sensing electrode was polarized properly at +100 mV vs the reference Pt electrode. The 90% response time of the device to 100 ppm NO was as short as ≈20 s at 500°C. The current response was well correlated to the NO concentration in the range of 0–200 ppm. Furthermore, the current response was hardly affected by the presence of the other gases, such as NO2, H2, CO, CH4, CO2, and water vapor. A compact, plate-type device which required no reference gas was also fabricated and its sensing properties were confirmed to be also rather good. A plausible NO sensing mechanism of the present devices was proposed on the basis of the sensing characteristics obtained.