Mixed-Potential Hydrocarbon Sensors

At the end of the twentieth century, a number of HC-sensitive electrode materials for solid electrolyte sensors have been investigated. These materials include terbium 

FIGURE 3.14 A cross-sectional view output of a HC sensor attached with a Prf 0,-SE. (Reprinted from Inaba, T., Saji, K., and Sakata, J., Characteristics of an HC sensor using a Pr Oii electrode. Sens. Actuators B Chem. 108 (2005) 374-378, with permission from Elsevier Science.) 

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 

FIGURE 3.16 Sensitivity to the different gases of the Tb-doped YSZ-SE deposited on a sapphire substrate versus a Pt counterelectrode at 600°C in flowing air. (Reprinted from Brosha, E.L. et al., Mixed potential NO sensors using thin film electrodes and electrolytes for stationary reciprocating engine type applications, Sens. Actuators B Chem. 119 (2006) 398-408, with permission from Elsevier Science.) 

---

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]. 

F. (2003) Mixed potential hydrocarbon sensors based on a YSZ electrolyte and oxide electrodes. J. Electrochem. Soc., 150 (1-2), H279-84. 

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. 

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]. 

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. 

On gold surface this reaction does not occur. This electrode material is preferred for so called mixed potential sensors which can be applied for the measurements of hydrocarbons and nitric oxides. Depending on electrode material, the thermodynamic window can be opened in a broad range of temperatures (4(X)-1,500 °C) and oxygen partial pressures (10-10 ° bar). The thermod5mamic expressions for different gas and solid state electrodes are given in detail by Mobius [2, 3]. 

Moseley P, Williams DE (1989) Gas sensors based on oxides of early transition meteds. Polyhedron 8 1615-1618 Mosley PT, Norris J, Williams DE (eds) (1991) Techniques and mechanisms in gas sensing. Adam Hilger, New York Moulson AJ, Herbert JM (1990) Electroceramics materieils, properties, applications. Chapman and Hall, London Mukundan R, Brosha E, Brown D, Garzon F (1999) Ceria-electrolyte-based mixed potential sensors for the detection of hydrocarbons and carbon monoxide. Electrochem Soc Lett 2(8) 412-414 MuUa IS, Ramgir NS, Hwang YK, Chang J-S (2004) Semiconductor tin oxide gas sensors from bulk to thin films. J Ind Eng Chem 10(7) 1242-1256... 

For the measurement of gas components like hydrocarbons (HC) or nitric oxides (NOx) in non-equilibrated gas phases kinetically determined sensors are used (Fig. 19.2 middle).Depending on the electrode material, the gas components do not equilibrate on the measuring electrode at temperatures <700 °C. Thus gas components which are not thermodynamically stable are electrochemically active. In an HC- and 02-containing gas, for example, at least two electrode reactions can take place the electrochemical reduction of oxygen and the electrochemical oxidation of hydrocarbons. The measured open-circuit voltage does not obey the Nemst equation. Therefore such electrode behaviour is often referred to non-Nernstian electrodes (or mixed potential sensors). The cell voltage depends logarithmically on the concentrations of the hydrocarbons ... 

At low operation temperatures, polarisation losses and the importance of catalysis of the electrode reactions Increase. At the cathode, mixed potentials can arise when traces of combustible substances determine the electrode potential in competition with oxygen, an effect, mentioned near the end of Section 2.3, whose cause was recognised by Hartung in 1981 [143], today the basis of the development of hydrocarbon sensors. For the anodes growing interest is directed to materials which accelerate the electrochemical oxidation of CO and hydrocarbons and is stable against fuel impurities. 

---