Electro-deposition and re-oxidation of carbon in carbonate-containing molten salts

Abstract

The electrochemical deposition and re-oxidation of solid carbon were studied in CO₃²⁻ 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 CO₃²⁻ 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.