Electrolysis of Lithium-Free Molten Carbonates

Abstract

Electrochemically converting CO₂/carbonate to functional carbon materials and oxygen in molten carbonates has been viewed as a promising approach to tackling the environmental and energy challenges. Until now, most molten carbonate electrolyzers have focused on lithium-based molten salts due to their relatively low eutectic temperature and thermodynamic feasibility. However, the use of lithium-based molten salts will be constrained by its limited resource. Herein, the electrolysis of lithium-free molten carbonates (MCO₃–Na₂CO₃–K₂CO₃, M = Mg, Ca, Sr, and Ba) is systematically studied. Using the low-cost Ni-based inert anode, high value-added carbon and oxygen are prepared in the molten carbonates containing CaCO₃, SrCO₃, and BaCO₃. In addition, the solubilities of alkaline-earth metal oxides (MO) in molten carbonates are measured and the results show that MgO, CaO, and SrO are nearly insoluble and BaO is soluble (8.62 wt % BaO in 10 wt % BaCO₃–Na₂CO₃–K₂CO₃, 750 °C) in the corresponding alkaline-earth metal carbonates. Based on the solubility difference of oxides, two different electrolysis mechanisms are proposed—first, electrochemical reduction of CO₂ if MO is soluble in molten carbonate with the production of C and oxygen; second, electrolysis of carbonate with the production of MO, C, and oxygen.