Sensing Electrodes in Solid Electrolyte-Based Gas Sensors

2 Sensing Electrodes in Solid Electrolyte-Based Gas Sensors 

Source Data extracted from SaktMvel and Weppner (2008) and Miura and Yamazoe (1998) 

In Table 9.2, the EMF response is shown to be clearly dependent on the oxides used. In particular it was found that TiO, with many transition metal oxides such as Fe Oj, Cr Oj, NiO, MUjO, and CO3O4, shows insignificant or no response to CO and H. These results suggest that semiconducting oxides with modest catalytic activity tend to make the best sensing electrodes. It is seen that the maximum response was obtained with ZnO electrodes. Moreover, these oxide electrode sensors exhibited an excellent selectivity to in the presence of NO, NO, CH, CO, and H O. In the device with a Pt-sensing electrode, for comparison, the EMF response was not observed above 500 C. The maximum response to CO with good selectivity was obtained with CdO electrodes (see Fig. 9.6b). 

The transition to composites improves the characteristics of NiO-based sensing electrodes as well. Research has been conducted on composites such as NiO-Rh (3 wt%) (Wang et al. 2006), NiO-WOj (10 wt%) (Miura et al. 2007), NiO-CuO (10 wt%) (Plashnitsa et al. 2006, 2007), and NiO-(Pd, Pt, Ru, or Rh) (Miura et al. 2007). It has been shown that among the noble metals such as palladium, platinum, ruthenium, and rhodium, rhodium (3 wt%) provided substantial improvement in NO sensitivity (Miura et al. 2007). 

It was also found that this approach based on use of composites has apparently contributed to sensor stability. For example, the use of nanocomposites of Au-MeO seems to resolve the instability of gold electrodes (Westphal et al. 2001). It was established that electrodes made from Au-Ga Oj, containing 20 wt% Ga Oj nanoparticles, in a potentiometric YSZ sensor, kept working and remained stable up to 850 °C. These Au-Ga Oj nanocomposite electrodes were prepared by using thick film technology and were sintered at between 900 and 950 °C. 

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