Electrochemical Processes in Molten Alkaline Metal Carbonates under Carbon Dioxide Overpressure

• Published in: Russian metallurgy Metally 2021-02, Vol.2021 (2), p.141-150
• Main Authors: Kushkhov, Kh. B., Ligidova, M. N., Ali, J. Z., Khotov, A. A., Tlenkopachev, M. R., Karatsukova, R. Kh
• Format: Article
• Language: English
• Subjects: Carbon dioxide; Carbon nanotubes; Carbonates; Cations; Chemistry and Materials Science; Electrochemical analysis; Electrode polarization; Electrolysis; Electrowinning; Fullerenes; Gold; Lithium carbonate; Materials Science; Melts; Metallic Materials; Overpressure; Platinum; Potassium carbonate; Sodium carbonate
• ISSN: 0036-0295; 1555-6255; 1531-8648
• DOI: 10.1134/S0036029521020142

The laws of the electrochemical behavior of molten alkaline metal carbonate mixtures K 2 CO 3 –Na 2 CO 3 and K 2 CO 3 –Na 2 CO 3 –Li 2 CO 3 at platinum and gold electrodes in air and under carbon dioxide overpressure up to 10 × 10 5 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: ↔ CO 2 + O 2– . The carbonate ion is stable in the K 2 CO 3 –Na 2 CO 3 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 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 K 2 CO 3 –Na 2 CO 3 melt shifts the acid–base equilibrium to CO 2 formation and affects the electrochemical behavior of the carbonate melts, leading to the appearance of a cathodic wave of CO 2 reduction to elemental carbon. The carbon dioxide overpressure above the molten K 2 CO 3 –Na 2 CO 3 and K 2 CO 3 –Na 2 CO 3 –Li 2 CO 3 mixtures up to 10 × 10 5 Pa results in the saturation of the melts with carbon dioxide, and the polarization of platinum and gold electrodes in these systems leads to CO 2 reduction to elemental carbon. The product of the galvanostatic electrolysis of the K 2 CO 3 –Na 2 CO 3 equimolar mixture and the K 2 CO 3 –Na 2 CO 3 –Li 2 CO 3 eutectic mixture in a wide current density range of 0.25–2.0 A/cm 2 at 873 K consists of graphite, fullerenes C 60 and C 70 , and carbon nanotube phases.