Of mixed-potential sensors

One of the advantages of mixed potential sensors is that it is possible for both electrodes to be exposed to the same gas. The elimination of a need to separate the two electrodes simplifies the sensor design, which in turn reduces fabrication costs. Although this simpler planar design is often used, the electrodes are sometimes separated to provide a more stable reference potential. As with equilibrium potentiometric sensors, the minimum operating temperature is often limited by electrolyte conductivity. However, the maximum operation temperatures for nonequilibrium sensors are typically lower than those of equilibrium sensors, because the electrode reactions tend towards equilibrium as the temperature increases. This operating temperature window depends on the electrode materials, as will be discussed later in the chapter. 

Lalauze, R., Visconte, E., Montanaro, L. and Pijolat, C. (1993) A new type of mixed potential sensor using a thick film of beta alumina. Sens. Actuators B, 13-14, 241-3. 

Szabo, N.F. and Dutta, P.K. (2004) Correlation of sensing behavior of mixed potential sensors with chemical and... 

Gas sensors — (b) Gas sensors with solid electrolytes — Figure 5. Schematic drawing of the layer structure of a mixed potential sensor... 

The mixed potential mechanism was described above, using CO as an example. However, the mechanism can be applied to any pair of oxidation and reduction reactions. Thus, mixed potential sensors have been reported for other reducing gases, such as hydrocarbons. Figure 13.20 shows that gold and platinum electrodes can be used to measure the amount of propylene (CsHe) [228, 229, 231, 233, 236-238]. Mixed potential hydrocarbon sensors have also been reported using proton-conducting electrolytes [239-242]. 

Figure 13.19 Outputs of mixed potential CO sensors with gold and platinum electrodes and yttria-stabilized zirconia (YSZ), ceria gadolinium oxide, and p alumina electrolytes [228-235],...

Figure 13.20 Outputs of mixed potential C3H5 sensors with gold electrodes and various ceria- and zirconia-based electrolytes [228, 229, 231, 233, 236-238],...

Among mixed oxides employed in mixed potential sensors is ITO, this having been used for both NO [291] and CO [292-294] sensors. A further example of a doped oxide being used as an electrode is TiO2, which has been doped with tantalum for hydrocarbon sensors [295] or vanadium for SO2 sensors [296]. 

Two-phase mixtures of oxides have also been used in mixed potential sensors. Such examples include Cr2O3 + NiO [297] for NO sensors, CuO + ZnO [298, 299] or SnO2 + CdO [300] for CO sensors, and In2O3 + MnO2 [301, 302] for hydrocarbon sensors. Some examples ofthe outputs of NO -and CO sensors with two-phase oxide mixtures as electrodes are shown in Figure 13.24 [270, 297, 298, 300]. 

Garzon, R., Mukundan, R. and Brosha, E.L. (2001) Modeling the response of mixed potential electrochemical sensors. Proceedings of the Electrochemical Society, 2000-32 Solid-State Ionic Devices II Ceramic Sensors, The Electrochemical Society, Pennington, New Jersey, pp. 305-13. 

Mukundan, R.. Brosha, E.L., Brown, D.R. and Garzon, F.H. (1999) Ceria-electrolyte-based mixed potential sensors for the detection of hydrocarbons and carbon monoxide. Electrochem. Solid-State Lett.. 2 (12), 412-14. 

Miura, N., Warrg, J, Nakatou, M., Elumalai, P. and Hasei, M. (2005) NO sensirrg characteristics of mixed-potential-type zirconia sensor using NiO sensirrg electrode at high temperatures. Electrochem. Solid-State Lett., 8 (2), H9-11. 

Elumalai, P., Plashnitsa, V.V., Ueda, T. and Miura, N. (2008) Sensing characteristics of mixed potential- type zirconia-based sensor using laminated-oxide sensing electrode. Electrochem. Commun., 10 (5), 745-8. 

In 2005, it was experimentally found that the slow recovery rate to NO2 of the YSZ-based mixed-potential sensor with the NiO-SE can be significantly improved when the sample gas was humidified with 5 vol. % water vapor. These results were published in 2006 [13]. Figure 2.12 [41] illustrates numerical and experimental values for the response/recovery transients to 400 ppm NO2 in 5 vol. % O2 with a N2 balance in the absence (a) and presence (b) of 5 vol. % H2O at 850°C. Sample... 

Brosha, E.L. et al.. Development of ceramic mixed potential sensors for automotive... 

Miura, N. et al.. High-temperature operating characteristics of mixed-potential-type NO2 sensor based on stabilized-zirconia tube and NiO sensing electrode. Sensors and Actuators B, Chem. 114 (2006) 903-909. 

The mixed potential developed is a function of various electrode parameters including, morphology, adsorption, catalytic, and electrocatalytic properties [54], To get a measurable potential difference between two electrodes, there must be asymmetry between them. Therefore, in most of the mixed-potential sensors the RE is usually Pt and the SE is oxide and/or an oxide mixture [13]. As a result, depending on the nature of the SE, it is possible that both reducible and oxidizable gases can be analyzed by the single sensor having a simple design. 

Consequently, it can be concluded that the enhancement of the NO2 sensitivity and selectivity for the zirconia mixed-potential sensors with the oxide-SE will be focused on the technological improvements in fabrication of the SE. Eurther development of the sintering technology for the SE will be based on predicted calculation of how the initial binary oxide mixtures will be transferred into solid solutions with domination of one oxide phase and trace of another. In these solid solutions, the electrode kinetic of the bulk reactions within the SE will be responsible for sensitivity and selectivity of the sensor. 

The comparison of mixed-potential emf from perovskite, fluorite, and spinel metal-oxide electrodes used in the mixed-potential HC sensors was presented [95] for justification of using the precatalyst to mitigate cross-reference. The thermodynamic, chemical and mechanical stability in the exhaust gases, sufficient electron conductivity to control device impedance, as well as the ability to generate stable... 

Potentiometric, non-Nemstian, zirconia-based, mixed-potential sensors offer several advantages. The recent shift from random to carefully selected properties for SEs of both single oxides and spinels has increased the working temperature of these sensors to 700°C, which is compatible with the working temperature at the vehicle exhausts. These devices are comparatively simple in design, and they exhibit... 

Ono, T. et al.. Sensing performances of mixed potential type NO sensor attached with oxidation-catalyst electrode. Electrochemistry 71 (2003) 405 07. 

Wang, J. et al.. Improvement of NO2 sensitivity of mixed-potential-type zirconia-based sensor by the addition of WO3 to NiO sensing-electrode, in Proc. llth Int. Meeting on Chemical Sensors, Brescia, Italy, 11-17 July 2006, 32. 

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