The chemistry of carbon deposition in molten salts has been studied by a number of researchers with quite different interpretations of the reactions involved. This paper therefore reviews and discusses available literature on the cathodic and anodic reaction mechanisms of carbon electrodeposition in molten carbonate salts, particularly considering new voltammetric studies. The effects of process variables (e.g. salt composition, temperature, voltage, electrode material etc.) on the electrochemical process are also addressed. Description of the properties of the deposited carbon and how these properties transfer to potential applications in energy storage and electricity production (carbon re-oxidation) are discussed.

The electrochemical deposition and re-oxidation of solid carbon were studied in CO3(2-) ion-containing molten salts (e.g. CaCl2-CaCO3-LiCl-KCl and Li2CO3-K2CO3) at temperatures between 500 and 800 °C under Ar, CO2 or N2-CO2 atmospheres. The electrode reactions were investigated by thermodynamic analysis, cyclic voltammetry and chronopotentiometry in a three-electrode cell under various conditions. The findings suggest that the electro-reduction of CO3(2-) is dominated by carbon deposition on all three tested working electrodes (Ni, Pt and mild steel), but partial reduction to CO can also occur. Electro-re-oxidation of the deposited carbon in the same molten salts was investigated for potential applications in, for example, direct carbon fuel cells. A brief energy and cost analysis is given based on results from constant voltage electrolysis in a two-electrode cell.