Capture And Electrochemical Conversion Of CO₂ In Lithium Chloride-based Molten Salts

The Global warming and environmental deterioration compel scientists and engineers to take some effective technologies to develop clean energy and explore much greener industrial processes, search for new methods for capture and utilization of CO₂ and protect the environment by reducing greenhouse gases emissions. In recent years, high temperature molten salts, which has good ionic conductance, wide liquid temperature range, huge heat capacity, wide electrochemical window and fast kinetics for reactions, was gradually used for CO₂ capture and electrochemical conversion. In this study, the kinetic processes of CO₂ capture were firstly investigated in molten chlorides and then the carbon depositon were conducted on different kinds of cathode substrates. The electrochemical mechanism for carbon deposition in the molten salt was investigated by cyclic voltammetry and Short-term electrolysis has been applied to study the formation and growth of the varied carbon microstructures. The influence of parameters on the cathodic products, such as melt composition, voltage, working temperature and the concerntration of CO₃^2-were also considered during the electrolysis. The main work and results are summarized as follows:?1? The solubility of oxide ions in LiCl-based and CaCl₂-based melts was estimated by double-tracer technique as part of the CO₂ capture and conversion process. The solubility of oxide ions in pure LiCl and CaCl₂ is 10.6 mol% at 600? and 12.5 mol% at 800?, respectively. The solubility of oxide ions encounters serious decrease when NaCl or KCl is added to the melts. The kinetic processes of CO₂ capture in molten chlorides demonstrates that the melts containing Li₂O has a faster absorption rate than the melts containing CaO. The absorption rate of the melts containing Li₂O was enhanced by higher temperature, O₂- concentration and the addition of the other chloride ?NaCl or KCl?. In the case of the melts containing CaO, high temperature will suppress the absorption rate. The absorption currents switched by the absorption rate were much larger than the electrolytic currents during the electrolysis, so for the whole capture and electrochemical conversion process, the rate-limiting step is the electrolysis process.?2? The cyclic voltammetry results in LiCl demonstrated that there are one step in reduction of CO₃^2-?carbon deposition?. No CO was found during the electrolysis and the cathodic products were amorphous carbon. Short-term electrolysis has been applied to study the formation and growth of the varied carbon microstructures. Quasi-spherical structure is found on the Mo cathode substrate and W. And two primary morphologies, quasi-spherical and nanofiber structures are observed at the nickel and copper plate cathodes during the electrolysis. For nickel, the formation and growth of carbon nanofibers are enhanced by using a high cell voltage.?3? The cyclic voltammetry results in LiCl-NaCl demonstrated that there are one step in reduction of CO₃^2-?carbon deposition?. There are no CO even in 800? during the electrolysis. The minimum energy consumption is 20.1 kWh for 1 kg of carbon. The carbon products deposited at different electrolysis conditions were composed of amorphous carbon and mainly comprised of quasi-spherical carbon particles. Besides, the electrodeposition of carbon films with a MoC interlayer was obtained at 900? during the electrolysis, carbon diffusion in Mo ?or Mo₂C? leads to the formation and growthof Mo₂C interlayer.?4? The reduction mechanism of CO₃^2-was investigated by cyclic voltammetry and square wave voltammetry. Results demonstrated that there are two steps in electrochemical reduction of CO₃^2-?carbon deposition and CO evolution?. When Cu and Ni were used cathode, carbonnanotubes were found after the electrolysis, but the main product is still amorphous carbon. Besides, a series of ?1-x?CaTiO₃-xNi anodes were tested.0.5CaTiO₃0.5Ni and 0.4CaTiO₃-0.6Ni anodes exhibited superior performances in the anodic polarization, Tafeland constant-current tests. Oxygen gas was generated successfully and continuously on the surface. And the corrosion rate of 0.5CaTiO₃-0.5Ni anode at the current density of 0.36 A/cm² for 16 h electrolysis was calculated to be 4.958×10^-3gcm-h^-1weight loss), show an excellent inert performance. The formation of protective NiO enhanced by dispersed CaTiO₃ ensured the inertness and stability of the anodes.