## 3 Results and Discussion  
Figure 3 shows the XRD pattern of the sintered YSZ sample. The observed diffraction peaks are related to the YSZ phase, which is confirmed by the ICDD file (030-1468) for the 8 mol.% Y\({}_{2}\)O\({}_{3}\)-doped ZrO\({}_{2}\) powder with a cubic fluorite structure. SEM image of the surface of the YSZ ceramic sample is shown in Figure 4. The micrograph reveals a dense grained microstructure of the sintered ceramics, which is needed to avoid gas leakage through the electrolyte membrane. Figure 3: XRD patterns of YSZ sample. At the first stage of testing, the sensor response in binary mixtures, N\({}_{2}\) + CO and N\({}_{2}\) + CO\({}_{2}\) was studied. To avoid the interference with water, the gaseous mixtures were dried with phosphorus pentoxide. The current-voltage dependences measured on the CO\({}_{2}\) and CO cells (cell 1 and cell 2, respectively) at 700 \({}^{\circ}\)C are presented in Figure 5. It can be seen that the obtained curves look similar: the current increases with the applied voltage at low potential differences, then a plateau is observed, which corresponds to the stationary state described in Section 2.2. The difference in the voltage values at which the limiting current is attained in the CO\({}_{2}\) and CO cells can be caused by different catalytic activities of Pt electrodes to reactions 1 and 2. With further increasing voltage, the limiting current plateau is followed by the next step of rising the sensor current (Figure 5). We assume that this can be explained by the partial reduction of YSZ in harsh conditions of a large DC voltage applied across the electrolyte under a low oxygen activity in the surrounding atmosphere. The reduction is accompanied by the generation of electrons, according to the following reaction [19]:  
\[\mathrm{O}_{\mathrm{O}}^{\times}\rightarrow\mathrm{V}_{\mathrm{O}}+\frac{1}{ 2}\mathrm{O}_{2}+2\mathrm{e}^{-}. \tag{5}\]  
Figure 4: SEM image of the surface of sintered YSZ sample. Figure 5: Current–voltage dependences of (**a**) cell 1 in N\({}_{2}\) + CO\({}_{2}\) gaseous mixtures, (**b**) cell 2 in N\({}_{2}\) + CO gaseous mixtures, at 700 \({}^{\circ}\)C. Error bars are smaller than the symbol size. Concentration of electrons n increases with decreasing the oxygen partial pressure in line with the mass action law for the reaction 5 as follows:  
\[\text{n }\sim\text{ pO}_{2}^{-1/4}. \tag{6}\]  
That means that with decreasing the concentration of CO\({}_{\times}\), the concentration of electrons rises, and consequently, the electronic current increases. The above assumption is supported by previously reported data on the blackening of YSZ from the cathode side related to electron association with point defects under a current load, and even the metallization of YSZ from the cathodic side [20; 21; 22]. However, the reoxidation of the electrolyte occurs after removal of the DC load so that the traces of metallicity, including the black color, disappear [21]. Appearance of the electronic charge carriers in YSZ should result (i) in a decrease in the oxide-ion flux across the electrolyte at a given applied DC voltage because of short circuiting, and (ii) in an extension of the effective area of the electrode reactions. As a consequence, the sensor current rises upon increasing the voltage at high DC loads. As can be seen in Figure 5, the limiting currents in the CO\({}_{2}\) and CO cells increase with a rise in the carbon oxides content. Figure 6 displays the sensor signal vs. CO\({}_{\times}\) concentration. The dependences are linear in agreement with Equation (4). The limiting current increases with temperature, which can be explained by faster diffusion of the corresponding gaseous species.