Electrochemical Processes in Molten Alkaline Metal Carbonates under Carbon Dioxide Overpressure

The laws of the electrochemical behavior of molten alkaline metal carbonate mixtures K2CO3–Na2CO3 and K2CO3–Na2CO3–Li2CO3 at platinum and gold electrodes in air and under carbon dioxide overpressure up to 10 × 105 Pa are found. The peculiarities of the voltammetric characteristics of the molten alkaline metal carbonates are explained taking into account the following acid–base equilibria involving the carbonate ion: $${\text{CO}}_{3}^{{2 - }}$$ ↔ CO2 + O2–. The carbonate ion is stable in the K2CO3–Na2CO3 eutectic melt; i.e., Na+ and K+ cations are characterized by weakly pronounced oxoacidic properties. The carbonate ion exhibits no electrochemical activity to the discharge potentials of the alkaline metal cations. Under these conditions, the electroreduction of $${\text{CO}}_{3}^{{2 - }}$$ is accompanied by the electroreduction of K+ and Na+ cations or the secondary reduction of the carbonate ion by the formed alkaline metal. The addition of lithium carbonate containing the cation with a higher polarization strength (higher oxoacidity) to the K2CO3–Na2CO3 melt shifts the acid–base equilibrium to CO2 formation and affects the electrochemical behavior of the carbonate melts, leading to the appearance of a cathodic wave of CO2 reduction to elemental carbon. The carbon dioxide overpressure above the molten K2CO3–Na2CO3 and K2CO3–Na2CO3–Li2CO3 mixtures up to 10 × 105 Pa results in the saturation of the melts with carbon dioxide, and the polarization of platinum and gold electrodes in these systems leads to CO2 reduction to elemental carbon. The product of the galvanostatic electrolysis of the K2CO3–Na2CO3 equimolar mixture and the K2CO3–Na2CO3–Li2CO3 eutectic mixture in a wide current density range of 0.25–2.0 A/cm2 at 873 K consists of graphite, fullerenes C60 and C70, and carbon nanotube phases.