## Materials and Methods  
### Fabrication of the Sensor  
YSZ powder with a nominal composition ZrO\({}_{2}\) + 8 mol% Y\({}_{2}\)O\({}_{3}\) was synthesized by the solid-state reaction from ZrO\({}_{2}\) and Y\({}_{2}\)O\({}_{3}\). The ceramic samples were prepared by grinding, compacting and sintering at 1650 \({}^{\circ}\)C for 5 h. Phase composition of the ceramics was studied by the X-ray diffraction (XRD) method with Cu K\(\alpha\) radiation (D-Max 2200, Rigaku, Tokyo, Japan). Diffraction was carried out in the range from 20\({}^{\circ}\) to 85\({}^{\circ}\) at a scanning speed of 0.02\({}^{\circ}\)/min and step of 0.1\({}^{\circ}\). The surface microstructure of the samples was studied using MIRA 3 LMU (Tescan, Brno, Czechia) scanning electron microscope (SEM).  
Three rectangular plates of equal size (1.1 \(\times\) 1.6 \(\times\) 0.1 cm\({}^{3}\)) were cut, and then 0.05 cm deep cavities were carved from two plates to form the inner sensor chambers as shown in Figure 1a. Pt slurry prepared by mixing a dispersed platinum powder and ethanol was symmetrically painted onto the opposite faces of the carved plates, and then sintered at 1160 \({}^{\circ}\)C for 1 h. The electrode area was 1 cm\({}^{2}\) and the Pt loading was about 5 mg cm\({}^{-2}\). Pt wires of 0.2 mm diameter were attached to each electrode by sintering at 1100 \({}^{\circ}\)C for 1 h and served as current collectors.  
Figure 1b shows an image of the manufactured dual chamber sensor. It consists of two identical electrically isolated cells made of YSZ ceramics. The inner chambers of the cells are connected with the outside atmosphere via alumina ceramic capillaries with a length of 20 mm and an inner diameter of 0.26 mm. Junctions of the ceramic plates were sealed with a hermetic glass, which ensured gas tightness and electrical isolation.  
The sensor was installed into a tube furnace, which was supplied with a gaseous mixture, N\({}_{2}\) + CO\({}_{2}\) + CO. Testing of the sensor was performed in the temperature range of  
Figure 1: Photo of (**a**) YSZ plate with a cavity and a groove for a capillary, (**b**) an image of the manufactured sensor.  
600-750 \({}^{\circ}\)C in the presence of up to 10% of CO and CO\({}_{2}\) in nitrogen. The gaseous mixtures were prepared by mixing definite amounts of N\({}_{2}\) (99.99% purity), CO (99.99% purity) and CO\({}_{2}\) (99.99% purity) using gas flow controllers F-201C-33-V (Bronkhorst, Germany). The prepared gases were dried with phosphorus pentoxide. When the effect of water presence in the gaseous mixture on the sensor signal was studied, the gas was bubbled through a water bath kept in a thermostat. The water vapor concentration in the gas mixture was controlled by a thermoyrrometer IVTM-7 M (Eksis, Moscow, Russia). The gaseous mixtures were supplied to the sensor at a rate of 30 mL/min and filled the inner chambers of the sensor through the capillaries, so that the inside and outside electrodes were under the same conditions.  
Figure 2 shows a schematic view of the manufactured dual chamber sensor. When direct current (DC) voltages (V1 and V2) were applied to cells 1 and 2 as shown in Figure 2, the oxide-ion current was generated in both cells. The currents were measured by a multimeter GDM-8246 (GW INSTEC, Taiwan, Taipei), and the voltages were regulated using a DC source GPS-18500 (GW INSTEC, Taiwan, Taipei). Cells 1 and 2 operated as CO\({}_{2}\) and CO sensors, respectively. The error in determination of the current density (\(\sim\)5%) was mainly caused by inaccuracy of the measurements of the capillary length and diameter and the electrode effective area.