Mixed-potential sensors

Fig. 2 Basic principles of solid electrolyte cells Left oxygen sensor (Nemst sensor), Right combustible sensor (mixed potential sensor) [2]...

The mixed potential accounts for a large portion of reported artifacts in the unorthodox potentiometric sensors, particularly biosensors, and can be rightfully called evil potential . The physical origin of such artifacts can be illustrated using a simple example. Let us assume that a multiple electron transfer takes place simultaneously at the interface of a lump of Zn immersed in dilute HC1. Because this metal is not externally connected the net current is zero. The redox reactions taking place are as follows. 

The ion-selective membrane is the key component of all potentiometric ion sensors. It establishes the preference with which the sensor responds to the ion of interest in the presence of various other ionic components of the sample. By definition, the ion-selective membrane forms a nonpolarized interface with the solution. If the interface is permeable to only one ion, the potential difference at that interface is expressed by the Nernst equation (6.6). If more than one ion can permeate, the interface can be anything between the liquid junction and the mixed potential. The key distinguishing feature is the absolute magnitude of the total exchange current density. 

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

Figure 13.2 Mixed potential sensor mechanism, (a) Mixed...

This mixed potential can be applied to a chemical sensor by using electrodes from two different materials with different polarization characteristics [20-23]. In Figure 13.2b, a second set of polarization curves are added (i.e., for Au) to demonstrate that the difference in polarization behavior leads to the establishment of a different mixed potential, (Au). The sensor response is based on the difference between these two mixed potentials, AE (Au). The effect of CO composition on this mixed potential, as illustrated in Figure 13.2c, is due to a shift in the CO polarization curves, which leads to a change in the two mixed potentials, and thus a change in the difference between the two mixed potentials. 

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 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]. 

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]. 

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. 

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. 

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. 

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. 

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. 

Elumalai, P Wang, J.. Zhuiykov. S., Terada, D, Hasei, M. and Mima. N. (2005) Sensing characteristics of YSZ-based mixed-potential-type planar NOy sensors using NiO sensirrg electrodes sintered at different temperatures, J. Electrochem. Soc., 152 (7), H95-101,... 

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. 

M. and Miura, N. (2006) Mixed-potential-type zircorria-based NO sensor using Rh-loaded NiO sensing electrode operating at high temperatures. Solid State Ionics, 177 (26-32), 2305-11. 

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

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. (2006) Influence of thickness of ITO sensing electrode film on sensing performance of planar mixed potential CO sensor. Sens. Actuators B, 120 (1), 150-5. 

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. 

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

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