Reference electrode yttria-stabilized zirconia

A typical electrochemical NOx sensor design involves the use of two electrodes on an oxygen-ion conducting ceramic, such as yttria-stabilized zirconia (YSZ), as shown in Fig. la. Both chemical and electrochemical reactivity at each electrode is critical to sensor performance [8, 18-20]. We have obtained optimal results with a Pt electrode covered with Pt-containing zeolite Y (PtY) as the reference electrode and WO3 as the sensing electrode [17, 21, 22]. These electrodes were identified by temperature programmed desorption of NO from NOx/02-exposed PtY and WO3, and the ability of PtY and WO3 to equilibrate a mixture of NO and O2. Significant reactivity differences were found between the PtY and WO3, with the latter... 

A unique proposed applieation for an yttria-stabilized zirconia is in carbon monoxide detection. A platinum electrode is attached on both sides of a zirconia electrolyte. One side is covered with a platinum eatalyst on a porous alxunina substrate and the Pt electrode is not in direct contact with the sample gas. Platinum on the substrate acts as a catalyst for CO oxidation. A cross-sectional view of the CO sensor is shown in fig. 15 (Okamoto et al. 1980) (the operating temperature is around 300°C). When carbon monoxide exists in the atmosphere, most will be eatalytically oxidized by the oxygen in air during difiusion through the porous substance. Therefore, the gas that reaches the Pt electrode is not CO but a CO2-O2 mixture. On the other hand, on the surface of the platinum electrode without the catalyst, carbon monoxide is oxidized to CO2 and causes an anomalous EMF. This potential shows a one-to-one correspondence to the CO concentration. The typical performance of the CO sensor in air at 300°C is shown in fig. 16 (Okamoto et al. 1980). The EMF output increases with the CO content, but the slope of the curve decreases gradually. This sensor can operate at temperatures between 260 and 350 C and no speeial O2 reference gas is necessary. 

Fig. 9.13 Typical configurations of potentiometric oxygen sensors with yttria-stabilized zirconia solid electrolyte, where the oxygen chemical potential over porous metallic reference electrode is fixed by supplying a gas mixture of known composition (a) and where a solid-state RE is used (b)...

The conventional oxygen sensor, shown Figure 1 a, is based on a closed yttria-stabilized zirconia tube [Kleitz et al., 1992]. The electrodes are made of platinum paste. The outer electrode, in contact with air, forms the reference electrode. The cell emf obeys the Nernst law ... 

With data on the activity of hydrogen ion in the standard solution available from model estimations, relatiorrships between the pH and cell potential, and hence a practical pH scale, for supercritical systems was developed. The potential of the cell comprising a pH sensor (yttria stabilized zirconia membrane) and a reference electrode (silver/silver chloride external pressure balanced electrode) (Emeas) can be written in the following form "... 

It consisted of a solid electrolyte disk (thickness 2 mm) made of yttria (8.5 wt. %) stabilized zirconia, which was coated with the same active component (cf. 2.3.) used in the reactor on the measuring side, and with a porous platinum electrode on the reference side. The measuring... 

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