## 1 Introduction  
Carbon oxides, CO\({}_{2}\) and CO (CO\({}_{\rm x}\)), are components of the atmosphere that play an important role in producing the so-called greenhouse effect. The carbon oxides are poisonous when accumulated to dangerous levels. At small concentrations, both gases are colorless, odorless and tasteless and can only be detected with a measuring instrument. Thus, reliable and fast control of the carbon oxides level in the atmosphere and process gases is of high importance. Therefore, there is considerable interest in developing proper analytical techniques to measure CO\({}_{\rm x}\) concentration. Nowadays, different approaches for sensing CO\({}_{\rm x}\) are known, among them spectroscopic, catalytic, acoustic, electrochemical, metal oxide semiconductor (MOS)-based methods, etc. The spectroscopic sensors are based on the ability of gas molecules to absorb a specific wavelength of the light; the absorption level is related to the target gas concentration. Spectroscopic gas sensors exhibit high sensitivity and low cross-sensitivity to other gases. An optical CO sensor based on tunable diode laser spectroscopy was reported to possess the sensing resolution of 3 ppm for 1 s, averaging [1]. A ppm-level CO sensor based on a 2\(f\) wavelength modulation spectroscopy technique was described in [2]. However, the spectroscopic methods are bulky, expensive and time consuming, which hinders their use for in situ monitoring. The detection principle of the semiconducting type sensors is based on the change in electrical resistance of MOS (e.g., SnO\({}_{2}\), and WO\({}_{3}\)) depending on the composition of the gaseous atmosphere [3, 4, 5]. The MOS-based sensors were shown to possess good sensitivity, fast response time and performance stability; however, the sensor signal can be affected by the adsorption of different components of the gaseous atmosphere onto the semiconductor surface, which negatively influences the selectivity. The operation mechanism of the catalytic-type sensors is based on using catalytic materials which are sensitive to combustible gases (CO, hydrocarbons, etc.) [6; 7; 8]. The oxidation of combustible gas on the catalyst leads to an increase in temperature and, as a consequence, in resistance of the active element. The resistance change is proportional to the temperature change, and therefore, to the amount of the oxidized gas. A CO\({}_{2}\) sensor based on SnO\({}_{2}\) and reduced graphene oxide composite showed an excellent detection limit of 5 ppm at room temperature [6]. A combustion-type CO gas sensor using a catalyst composed of 11 wt% Pt supported on the apatite-like oxide La\({}_{9,0}\)Si\({}_{5,8}\)Mn\({}_{0,2}\)O\({}_{27-\delta}\) was reported to demonstrate high sensitivity (the relative resistance change reached a value of 0.0053 for 500 ppm CO) and a rapid response time of 10-20 s at 130 \({}^{\circ}\)C [8]. The main drawbacks of the catalytic-type sensors are a need for oxygen for the oxidation process and low selectivity related with the possible oxidation of different combustible components of the gaseous atmosphere [8]. Electrochemical sensors based on solid-oxide electrolytes are considered the most reliable way of sensing CO\({}_{\rm x}\) for industrial applications. These sensors operate at temperatures of 450 \({}^{\circ}\)C and above to ensure sufficient ionic conductivity in a solid-oxide electrolyte [9; 10; 11; 12; 13; 14; 15]. Yttria-stabilized zirconia (YSZ) is typically used as an electrolyte. YSZ-based electrochemical sensors have such important advantages as good corrosion resistance in aggressive gaseous atmospheres, high stability and simplicity in design. Mixed potential, amperometric and impedance-electric electrochemical sensors were reported to be applicable for measuring CO. Impedance-electric sensors use the sensitivity of the sensor impedance to concentration of a target component in the gaseous atmosphere [9; 10; 11; 12].