Impedance-Based Zirconia Gas Sensors

The mixed-potential type of potentiometric sensors, described in the previous sections, appears to be advantageous for combustion applications and for on-board NOx, CO, and HC sensors owing to their high sensitivities, especially in the measurement of a lower concentration (less than 100 ppm). 

It was hrst reported in 2001 [32] that the evaluation by impedance spectroscopy of a biased zirconia sensor with an attached LaFeOa-SE has shown that only the electrode resistance is a function of NO2 content. Consequently, impedance spectroscopy offers a method for directly probing the electrode reactions that are the basis for mixed-potential-type gas sensors [67]. As a result of further development by using impedance spectroscopy in zirconia-based gas sensors with oxide-SEs, a new type of YSZ-based sensor for detecting total NO and HCs at high temperatures has been proposed recently [2, 14, 21, 62, 74, 96-100]. In this case, the change in the complex impedance of the device attached with a specific oxide-SE was measured as a sensing signal. 

FIGURE 3.17 Complex impedance plots in base air and the sample gas with each of the various concentrations of (a) NO and(b) NO2 at 700°C for the YSZ-based sensor attached with a ZnCr204-SE. (Reprinted from Miura, N., Nakaton M., and Zhuiykov, S., Impedanee-metric gas sensor based on zirconia solid electrolyte and oxide sensing electrode for detecting total NOj at high temperature, Sens. Actuators B, Chem. 93 (2003) 221-228, with permission from Elsevier Science.) 

Experience with the mixed-potential type of sensors employing spinel SEs suggests that the change in O2 concentration may affect the performance of impedance-based sensors. In order to check this point, the O2 concentration in the sample 

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