Insights into carbon production by CO2 reduction in molten salt electrolysis in coaxial-type reactor

Abstract

With carbon capture and utilization (CCU) technologies, it is possible to take CO2 from air and transform it into valuable carbon products, such as graphene and nanotubes. In this study, CO2 was reduced to solid carbon by molten lithium carbonate salt electrolysis. The reactor used for the electrolysis was a coaxial-type unit, where a cylindrical nickel cathode was placed inside a cylindrical stainless steel vessel; such a coaxial-type reactor has not been used before for molten salt electrochemical reduction of CO2. This reactor design allows studies of voltage-current characteristics, which have previously not been conducted for this specific process. As a result, voltage efficiency of this process could be determined along with effect of temperature to cell voltage. Based on these results it is evident that accurate temperature control is crucial in terms of energy efficiency of the process. Data monitoring revealed a high accuracy of temporal temperature control. Great temperature differences were observed spatially. Thermoneutral voltage from the process was determined to be 1.02 V. For this electrochemical process studied, carbon nano-onions (CNOs) were obtained as a main product. Identification of the product was carried out based on Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD) analyses. Results also showed that iron, chromium and nickel metals are released during electrolysis. Iron and chromium can only be released from the stainless steel anode, as nickel can come from anode or cathode.