A potentiometric thin-film sensor to detect CO2 in a wide range (2-100%) has been developed. The system has been fabricated depositing a reference electrode of Pt, a solid electrolyte of YSZ (Yttria-stabilized Zirconia), a sensing phase made of Li2CO3 and a working electrode of Au via Physical Vapor Deposition (PVD). Characterization of the different elements has provided the optimal fabrication parameters and the system response for CO2 concentrations can be measured from 2 to 100% at 450°C. The sensor behaves as a non-Nerstian system and slightly deviates from a linear response with the logarithm of CO2 until the CO2 concentration reaches the 30%. Higher CO2 amounts make the response divert more from the Nernst law but give a stable and reproducible response to CO2 in a wide range of concentrations. Based on these promising results the recovery time, stability, repeatability and selectivity of the sensor have been measured. The performance showed by the thin film sensor proves the feasibility of the use of this system for biogas and natural gas applications owing to its very good consistency at low temperature in a wide concentration range.

Li2CO3 sputtered films of 300 nm have been subjected to physical and electrochemical characterization methods to analyze the influence of annealing treatments at 600 degrees C for 2 h, 6 h, 12 h and 18 h on the microstructure, surface and conductivity. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS) have been used for this purpose. XRD and FT-IR have illustrated the evolution of the microstructure with annealing time. AFM analysis has shown the growth of new Li2CO3 particles that increases with annealing time presenting a maximum diameter of 16.8 mu m without compromising the continuity of the films. EIS measurements have described a fall in the activation energy of the Li2CO3 thin films presenting a minimum around 1.18 eV. The results concerning the activation energy of the films have shown an improvement compared to the results obtained previously for Li2CO3. These results serve to understand and optimize the behaviour of the Li2CO3 thin films in gas sensors, fuel cells or Li+ ion batteries.

The physical and electrochemical characterization of Li2CO3 thin films allows for an improved understanding of their behaviour in electrochemical devices. Firstly, the Li2CO3 deposition process by RF magnetron sputtering was described. Afterwards, X-ray diffraction (XRD), atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS) characterization techniques were employed to study the influence of temperature on the material. This way, the optimal annealing temperature as well as the optimal operating temperature of Li2CO3 were determined. In light of the obtained results, it was concluded that a fabrication annealing temperature of 600 degrees C and an optimal operating temperature of 350 degrees C were set.