## 1 Introduction
The sensor based on the Pt/YSZ/Au-Ga\({}_{2}\)O\({}_{3}\) cell was reported to exhibit strong dependence of the impedance on CO concentration in the range of 100-800 ppm and small response time of 10 s at 550 \({}^{\circ}\)C [12]. However, the impedance-type sensor signal can be affected by non-target gaseous species; further, these sensors require using a high-frequency AC signal source and a frequency response analyzer that leads to an increase in cost [11]. The YSZ-based mixed potential sensor with NiO sensing electrode was proved to have good sensitivity to CO, showing a signal of 36 mV to 1000 ppm CO in the presence of 0.5\(-\)3% O\({}_{2}\) at 1000 \({}^{\circ}\)C and excellent selectivity, being insensitive to the presence of CO\({}_{2}\), CH\({}_{4}\) and H\({}_{2}\)O in the gaseous atmosphere; however, the sensor signal and the response time were affected by the microstructure of the sensing electrode [16]. Using ZnO as a sensing electrode was also shown to be justified; the sensor demonstrated good sensitivity, reaching a response of 68 mV to 400 ppm of CO at 700 \({}^{\circ}\)C [17]. However, the mixed potential sensors, as a kind of potentiometric sensors, need a reference gas, which complicates their design; besides, they show low sensitivity to small changes of the target component concentration because of their logarithmic relation with the sensor signal [17]. In the amperometric sensors, a signal is generated by a voltage applied across the oxide-ion conducting electrolyte. The generated oxide-ion current is determined by the availability of the oxygen-containing species in the gaseous atmosphere. Gas supply to the electrolyte/electrode interface is limited by a diffusion barrier that can be either a capillary connecting the outer and inner sensor compartments, or a porous electrolyte. An important advantage of the amperometric sensors over the mixed potential ones is that there is no need of a reference atmosphere that allows using a simple design. Further, these sensors do not require the presence of free oxygen for operation as the catalytic-type sensors do. A linear relationship between the sensor signal (the limiting current) and concentration of a target gaseous specie is another serious advantage of this type of sensors. Recently, we reported the possibility of measuring the concentrations of carbon oxides by using a solid oxide amperometric sensor [13; 15]. A YSZ-based amperometric sensor with a couple of symmetrical Pt electrodes was shown to possess good sensitivity to CO in the concentration range of 1-6 vol.%, which was about 2 mA cm\({}^{-2}\) per 1 vol% CO at 450 \({}^{\circ}\)C in nitrogen atmosphere [13]. A CO\({}_{2}\) sensor based on the proton-conducting La\({}_{9,0}\)Sr\({}_{0,1}\)VO\({}_{3-d}\) electrolyte showed the linear dependence of the limiting current on CO\({}_{2}\) concentration in the range of 2-14 vol.% with a slope of \(\sim\)0.7 mA cm\({}^{-2}\) per 1 vol% CO\({}_{2}\) at 500-600 \({}^{\circ}\)C in nitrogen atmosphere, which characterizes high sensor sensitivity [15]. However, the sensor exhibited a rather slow response time of \(\sim\)30 min under the stepwise change of CO\({}_{2}\) concentration in the range of 4.6-13.7 vol.%, which can be explained by insufficient proton conductivity of the electrolyte or poor electrode performance. In the present study, we designed a dual-chamber amperometric sensor based on YSZ (ZrO\({}_{2}\) + 8 mol% Y\({}_{2}\)O\({}_{3}\)) electrolyte and Pt electrodes for simultaneous measurement of CO and CO\({}_{2}\) concentrations in inert gases. The simultaneous sensing of several gaseous species is relevant for many practical applications, e.g., in the medical and pharmaceutical industry, food preservation and storage industry, chemical manufacturing, etc. The use of solid-oxide amperometric sensors for simultaneous measurement of CO\({}_{2}\) and CO in inert gases has not been reported to date, as far as we know. Nitrogen was used as a chemically inert gas for sensor testing.