Derek Fray

Abstract:

Carbon has been used in contact with molten salts for over a century since Hall and Heroult, independently, electrolysed aluminium from molten cryolite. In molten salt nuclear reactors, carbon is purely used as a container material, but in the electrolysis of alumina, dissolved in cryolite, or the electro-deoxidation of nuclear waste, the evolved oxygen reacts with the carbon to form carbon dioxide. In other cases where the electrolyte is a chloride, as in magnesium, lithium and sodium production, the anodic reaction is the evolution of chlorine and provided there are no oxides in the electrolyte, there is no reaction with the carbon anodes. If graphite is made the cathode in lithium chloride, lithium intercalates into the structure and pushes out graphene sheets which can roll up to form either nanoscrolls or nanoparticles, depending on the temperature and the grain size of the graphite. If tin or silicon chlorides are present in the salt, these metals will deposit preferentially on the graphite and subsequently form lithium silicon or lithium tin alloys which will be incorporated into carbon nanotubes and nanoparticles. These materials may find applications in lithium ion batteries. Simply heating graphite in lithium chloride will lead to a breakdown in the structure of the graphite and, under certain conditions, can lead to the production of graphene sheets, a method which may be more appealing than laborious mechanical methods. Furthermore, intercalation of ionic liquids into graphite has also led to the formation of graphene. Carbon can also be extracted cathodically from carbonate containing melts below about 700oC to form nanosized carbon particles which are ideal as the anode in lithium ion batteries. Above about 700oC, the cathodic product is carbon monoxide, which is the stable phase at this temperature. In addition, it is known that carbon dioxide has a high solubility in lithium oxide containing melts and this may offer a way of both extracting carbon dioxide from polluting gases or even the environment and, at the same time, converting it to high value nanosized carbon particles for lithium ion batteries. These varied and diverse applications of the interaction of carbon with molten salts will be discussed in this paper.