Electrolysis of Lithium-Free Molten Carbonates

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.