## Materials and Methods
### Sensor Operation  
The designed sensor possesses two isolated electrochemical cells, which allows conducting simultaneous measurement of CO and CO\({}_{2}\) as follows. A target gaseous mixture of N\({}_{2}\) + CO\({}_{2}\) + CO is supplied into the furnace tube, and the sensor is installed in and fills the inner chambers of the sensor via the capillaries.  
Application of a DC voltage to the electrodes as it is shown in Figure 2 produces the oxide-ion current across the YSZ membranes, which is accompanied by the electrochemical reactions at the YSZ-Pt interfaces. At the negatively charged electrodes 4 and 6, electrolysis of CO\({}_{2}\) occurs:  
\[\text{CO}_{2}+2\text{e}^{-}\rightarrow\text{CO}+\text{O}^{2-}, \tag{1}\]  
while at the positively charged electrodes 5 and 7, carbon monoxide is oxidized by oxide-ions:  
\[\text{CO}+\text{O}^{2-}\rightarrow\text{CO}_{2}+2\text{e}^{-}. \tag{2}\]  
At small voltages, the sensor current, which is equivalent to the oxide-ion current across the electrolyte, increases with the applied voltage. Consumption of CO\({}_{2}\) at electrode 4 leads to an increase in the diffusive flux of CO\({}_{2}\) from the outside toward the electrode via the diffusion channel. According to Fick's law, the diffusive flux is proportional to the  
Figure 2: Schematic view of the sensor: 1, 2, 3–YSZ ceramic plates; 4–7–Pt electrodes; 8, 9–ceramic capillaries; 10–sealing glass; 11, 12–inner chambers of the cells 1 and 2; V1 and V2–DC voltages applied to the cells 1 and 2; A1 and A2-ammeters.  
concentration gradient; so, the higher the applied voltage and the related ionic current across the electrolyte, the greater the concentration gradient and the diffusive flux of the gas species. Decrease in CO concentration at the electrode 7 during the reaction 2 also results in an increase in the diffusive flux of CO. At some voltage value, the diffusive supply of the gas component, which is restricted by the diffusion channel parameters, reaches its limit; as a result, the sensor current attains a stationary value (it is the so-called limiting current), corresponding to the conditions of equality between the rates of consumption and diffusive supply of the active gas species. The limiting current is determined by the diffusion channel parameters, concentration and diffusion coefficient of the considered component of the gaseous mixture as follows:  
\[\text{I}_{\text{lim}}=\frac{2\text{FSP D}}{\text{RTL}}\ln(1-\text{X}), \tag{3}\]  
where D denotes the diffusion coefficient of the given component in the gaseous mixture, S and L are the cross section and the length of the diffusion channel, P is the total gas pressure, and X is the molar fraction of the component in the gas [18].  
At small concentrations of the active component in the gas (below \(\sim\)10%), a linear relation provides a good approximation:  
\[\text{I}_{\text{lim}}=\frac{2\text{FSP D}}{\text{RTL}}\,\text{X} \tag{4}\]  
Thus, the sensor limiting current is directly proportional to the concentration of the gas component participating in the electrochemical reaction caused by the applied voltage. A linear relationship between the limiting current and the concentration of a target gaseous species makes calibration of the amperometric-type sensors simple but accurate, which is an important advantage.