Mixed-Potential Gas Sensors

When the zirconia sensor with dissimilar electrodes is exposed to the combustion exhaust, the presence of at least two independent nonequilibrium gases of different 

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. 

The vast majority of researchers investigating mixed-potential-type zirconia-based sensors in recent years discovered that the sensing properties of these devices 

3 Concepts of the Total-NO Sensor Based on Mixed Potential 

High-temperature exhaust gas enters the first half cavity through the gas inlet attached with a newly developed oxidation catalyst layer and an electrode for oxidation of all HCs and CO in the nonequilibrium gas 

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Garzon, F.H., Munkundan, R. and Brosha, E.L. (2000) Solid-state mixed potential gas sensors Theory, experiments and challenges. Solid State Ionics, 136-137. 633-8. 

Hibino, T, Hashimoto, A., Mori, K.-t. and Sano, M. (2001) A mixed-potential gas sensor using a SrCeo.gsYbodsO, electrolyte with a platinum electrode for detection of hydrocarbons. Electrochem. Solid-State Lett., 4 (5), H9-11. 

Zosel, J., Westphal, D., Jakobs, S., Muller, R. and Guth, U. (2002) Au-oxide composites as HC-sensitive electrode materials for mixed potential gas sensors. Solid State Ionics, 152—153, 525—9. 

Li, X. and Kale, G.M. (2007) Influence of sensing electrode and electrolyte on performance of potentiometric mixed-potential gas sensors. Sens. Actuators B, 123 (1). 254-61. 

Li, X., Xiong, W., and Kale, G.M., Novel nanosized ITO electrode for mixed potential gas sensor, Electrochem. Solid-State Lett. 8 (2005) H27-H30. 

Development of ionic conductors based on stabilized zirconia has reached a level of maturity, where most of the research on such materials concentrates mainly on obtaining incremental empirical improvements in conductivity by better processing control and refinement of the microstructure of the solid electrolyte and SE. Further increases in the conductivity are important in terms of enhancing the efficiency of systems such as O2 sensors, zirconia-based mixed-potential gas sensors, electrochemical oxygen pumps, heating elements, and fuel cells [4-7]. The systematic errors, as have been considered before, are errors with a known determined functional connection with the source of their cause, and the conformity of their appearance can be definitely described. 

Zosel J, Tuchtenhagen D, Ahlborn K, Gith U (2008) Mixed potential gas sensor with short response time. Sens Actuators... 

Satsuma A, Katagiri M, Kakimoto S, Sugaya S, Shimizu K (2011) Effects of calcination temperature and acid-base properties on mixed potential ammonia sensors modified by metal oxides. Sensors 11 2155-2165 SbervegUeri G (1992) Qassical and novel techniques for the preparation of SnO thin-film gas sensors. Sens Actuators B 6 239-247... 

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

These examples are based on both electrodes operating in the activation polarization regime, in which the logarithm of the current is proportional to the overpotential. However, there are situations - particularly at low concentrations - in which the electrochemical reaction is limited by mass transport to the electrode surface. This is referred to as concentration polarization, and is illustrated in Figure 13.2d. In this case, above a critical overpotential the current becomes constant, which appears as a vertical line in the plot. A new mixed potential is established at the intersection of this vertical line and the cathode polarization for the oxygen reduction. This potential depends on the gas concentration, and thus can be used for the chemical sensor signal. 

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. 

The outputs of some mixed potential-type chemical sensors correlate with the type of electronic defect (i.e., n-type versus p-type), so the response has been attributed to the semiconducting behavior of the electrode material [314]. LaFeO3, which has been used as a semiconductor-type gas sensor ] 315, 316], has also been used as an electrode with YSZ [255, 263, 317] or NASICON ]317, 318] electrolytes for potentiometric NO, sensors. Strontium (i.e., (La,Sr)FeO3 ]255, 256, 284]) or strontium and cobalt (i.e., (La, Sr)(Co,Fe)O3 ]275, 280, 309]) have been added to LaFeO3 to improve electrode performance. (La,Ca)MnO3 doped with either cobalt or nickel on the manganese site has been used as the electrode for N O, sensors ]319]. The outputs of some NO, sensors with perovskite electrodes are shown in Figure 13.26 ]255, 256, 264, 275, 309, 312]. 

F. (2003) The role of heterogeneous catalysis in the gas-sensing selectivity of high-temperature mixed potential sensors. Proceedings of the Electrochemical Society, 2002—26 Solid-State Ionic Devices 111, The Electrochemical Society, Pennington, New Jersey, pp. 261—71. 

Using the assumptions given above, it was possible to determine the binary metal compositions with unUmited solubility, which is illustrated in Figure 1.15. These binary metal compositions are vital for the selection of materials for a sensing electrode of the mixed-potential zirconia-based gas sensors [23, 24]. This is because... 

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... 

Bartolomeo, E.D., GriUi, M.L., and Traversa, E., Sensing mechanism of potentiomet-ric gas sensors based on stabilized zirconia with oxide electrodes Is it always mixed potential J. Electrochem. Soc. 151 (2004) H133-H139. 

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