## 3 Results and Discussion
The slope of the limiting current-concentration lines determines the sensitivity of the sensor; as can be seen in Figure 6, the sensitivity increases with temperature and reaches the values of 25 mA cm\({}^{-2}\) per 1 vol.% CO\({}_{2}\) and 36 mA cm\({}^{-2}\) per 1 vol.% CO at 750 \({}^{\circ}\)C.  
The next stage of testing was aimed at the study of the sensing properties in dry ternary mixtures N\({}_{2}\) + CO + CO\({}_{2}\). The current-voltage curves of the CO\({}_{2}\) and CO cells measured in the N\({}_{2}\) + 4.9 vol.% CO + 4.9 vol.% CO\({}_{2}\) gas at the temperatures of 600-750 \({}^{\circ}\)C are shown in Figure 7. For more comfortable visual perception, it was taken into account that the DC voltage was applied to the CO\({}_{2}\) and CO cells in the opposite directions. For both cells, the slope of the initial step of the current-voltage dependences decreases with decreasing temperature, which is definitely related to lowering the ionic conductivity of YSZ and the slowing down of the electrochemical reactions on the electrodes. The values of limiting the current of the CO\({}_{2}\) and CO cells in the ternary gaseous mixtures N\({}_{2}\) + CO + CO\({}_{2}\) are similar to those measured in the binary mixtures N\({}_{2}\) + CO and N\({}_{2}\) + CO\({}_{2}\) at the corresponding concentration of CO or CO\({}_{2}\) (see Figure 5), indicating that there is no perceptible cross  
Figure 6: Limiting current vs. (**a**) CO\({}_{2}\) concentration in N\({}_{2}\) + CO\({}_{2}\) gaseous mixture, (**b**) CO concentration in N\({}_{2}\) + CO gaseous mixture. Error bars are smaller than the symbol size. sensing effect toward the counterpart. The fact that the designed amperometric sensor is capable of simultaneous measuring of the concentrations of the two oxides of carbon is of high importance, as these gases coexist in many practical applications. It is important to evaluate the role of water vapor in the gas-sensing properties as some amount of water vapor may exist in the analyzed gas. For this purpose, the binary mixtures N\({}_{2}\) + CO and N\({}_{2}\) + CO\({}_{2}\) were bubbled through a water bath kept in a thermostat. The measured dependences of the sensor current on the applied voltage in dry and wet atmospheres are shown in Figure 8. As can be seen, water vapor strongly affects the CO-sensing properties at low concentrations of CO (Figure 8a). This can be explained by the interaction of CO and H\({}_{2}\)O at the temperatures above 500 \({}^{\circ}\)C according to the following reaction:  
Figure 3:0  
Figure 3:1 Current-voltage dependences of (**a**) CO cell in dry and humid mixtures N\({}_{2}\) + 0.5 vol.% CO and N\({}_{2}\) + 5 vol.% CO, (**b**) CO\({}_{2}\) cell in dry and humid mixtures N\({}_{2}\) + 5 vol.% CO\({}_{2}\), at 600 \({}^{\circ}\)C. Error bars are smaller than the symbol size. Figure 3:2 Current-voltage dependences of CO\({}_{2}\) cell (solid symbols) and CO cells (empty symbols) in dry N\({}_{2}\) + 4.9 vol.% CO + 4.9 vol.% CO\({}_{2}\) atmospheres. Error bars are smaller than the symbol size. As a result, the hot gaseous mixture contains not only the carbon oxides and water vapor, but also hydrogen, which is oxidized together with CO on electrode 7. As far as the diffusion coefficient for H\({}_{2}\) being almost four times higher than that for CO, the limiting current of the CO cell significantly increases in the presence of water vapor. When the CO concentration is high compared to that of H\({}_{2}\)O, the cross-sensing effect toward water is reduced due to a small concentration of the produced hydrogen. The limiting current in the CO\({}_{2}\) cell also changes in the presence of water vapor in the analyzed gas (Figure 8b).