Study of a method for utilization of solar energy by means of high temperature electrolysis of CO2

The present study is an investigation of a one-step, high temperature electrolysis of CO2. The long term goal is implementation of this method for conversion of solar energy to chemical potential, namely CO fuel.;I. High-Temperature One-Step CO2 Electrolysis.;The one-step electrolysis of CO2 in gas phase was studied at temperatures of 600°C-1400°C. For a first time such measurements were conducted and studied at temperatures higher then 1000°C. 8% YSZ (8% mol Yttria stabilized Zirconia) was used as oxygen ions conductor (solid electrolyte) in this study.;II. Process Efficiency Analysis and Characterization.;Direct measurements of the CO2 electrolysis efficiency were performed at temperatures of up to 1400°C, also for the first time. The electrolysis was performed over a wide range of CO concentrations. The maximal CO volumetric concentration that was reached during electrolysis is 98%. It is important to note that this CO concentration was reached during experiments at 1400°C. The efficiency of the electrolysis as a function of temperature, current density and CO concentration was measured and evaluated. Up to 89% electrolysis efficiency was observed, even though non-optimal, relatively simple, hand-made Pt-YSZ cermet electrodes were used. Under the experimental conditions of the present study, the highest electrolysis efficiency was reached at 950°C. The factors influencing the efficiency measurement were detected and analyzed, and adjustment for oxygen penetration into the chamber and electrode sintering was performed. After the adjustment for oxygen penetration into experimental chamber the highest electrolysis efficiency appears to be at 1250°C.;III. Simultaneous Dissociation of Water during the CO2 Electrolysis.;H2 at volumetric concentration of up to 0.25% was consistently measured in the reaction chamber during the CO2 electrolysis tests. This concentration was many times higher than the concentration of N 2, created due to ambient air leak into the chamber. A possible explanation for this observation is electrolysis of moisture from the air surrounding the anode in parallel with the electrolysis of CO2 on the cathode. Such co electrolysis was observed in first time and might have significant consequences for direct generation of syngas.;IV. Analysis of Possible Utilization of the Present CO2 Electrolysis Process.;A synthetic fuel production process based on high temperature CO 2 electrolysis and powered only with solar energy was analyzed. The dependence of process efficiency on various parameters was analyzed. The efficiencies of this process and of conventional water electrolysis were compared. If the power source of the process is a PV plant with efficiency of 20%, then a fuel production efficiency of about 25% could be reached. In comparison, the solar to fuel conversion efficiency of the water electrolysis utilizing the same PV plant is 16%. The reason for the higher efficiency of the present high temperature CO2 electrolysis process is partial substitution of electric energy by heat.;The present work shows for first time a possibility of performing electrolysis of CO2, and possibly also, simultaneously, of steam, at temperatures higher than 1000°C (and at least up to 1400°C). This research provides better understanding of processes, occurred during the electrochemical reaction of CO2 reduction and suggests possible ways to improve efficiency of the process when only direct solar radiation is used. (Abstract shortened by UMI.).