(Digital Presentation) Modern State and Prospects of Electrochemical CO2 Conversion in Molten Salts
Authors: Novoselova Inessa , Omel'chuk Anatoliy 
Published: November 2022

Over the last century, anthropogenic activity has led to a significant increase in the concentration of carbon dioxide in the Earth's atmosphere. Today, the annual increase in CO2 is 3200-3600 million tons. Carbon dioxide together with other greenhouse gases (water vapor H2O, methane CH4, nitrous oxide N2O, sulfur hexafluoride SF6, etc.) causes global warming, which leads to serious environmental degradation - increasing the acidity of sea water, melting glaciers, strengthening the water cycle, which in some regions leads to heavy rainfall and floods, and in others - to extreme droughts. Therefore, the effective carbon dioxide utilization is becoming an urgent scientific and environmental task.

One of the ways of carbon dioxide utilization is the electrochemical method. It consists in the electrochemical reduction of CO2 at the cathode. Depending on the conditions of electrolysis (composition of the electrolytic bath, temperature, current density, voltage in the bath, electrode materials), the chemical composition of cathode products changes dramatically.

Report is a review of the present state of research concerning the electrochemical conversion of carbon dioxide from conducting liquid and solid media of different chemical composition. They include solid, aqueous, non-aqueous (organic and ionic liquids), and molten salt electrolytes. It contains a comparative analysis of the effectiveness of using these electrolytes, as well as products obtained by the electrochemical splitting of CO2 and the prospects for their use. Special emphasis has been made on the electrochemical decomposition of carbon dioxide from molten salts, several variants of decomposition have been considered, the advantages and disadvantages of each variant have been analyzed. An analysis of literature data on this subject as well as research results of the author on the direct electrochemical conversion of CO2 into valuable solid-phase chemicals with added value is presented.

A feature of the electrochemical reduction of CO2 dissolved in molten salts, in contrast to aqueous, organic, solid electrolytes and ionic liquids is the possibility of forming a solid carbon phase at the cathode. Knowledge of the laws of electrode processes involving CO2 is necessary to create a scientific basis and control for the processes of electrochemical synthesis of nanosized carbon-containing inorganic compounds - nanotubes, nanofibers, graphene, amorphous carbon, carbides of refractory metals and numerous composites based on them.

The voltammetric study of carbon electrowinning mechanism from carbon dioxide under excessive pressure and lithium carbonate in different gas media (air, argon, carbon dioxide) dissolved in chloride melts at temperatures of 550–800 °С was conducted. The characterization of the chemical and phase compositions, morphological and structural features of nanosized carbon phases obtained by electrolysis in the studied melts has been carried out; a correlation between the structure of the products and the synthesis conditions is established.

Thermodynamic analysis of possible ways of electroreduction of CO2 and carbonates of alkali and alkaline earth metals in the temperature range 700 - 1000 K showed that the cathode product can be: (1) - solid carbon phase; (2) - carbon monoxide; (3) - carbide phase. At temperatures below 1000 K, the first path will be thermodynamically advantageous.

Electrochemical behavior of CO2 in molten salt mixtures NaCl–KCl (0.5: 0.5 mol%); NaCl–KCl–CsCl (0.3: 0.245: 0.455 mol%); NaCl–KCl–NaF (0.423: 0.423: 0.154 mol.%) under gas excess pressure (1-17 atm.) was studied by cyclic voltammetry at different potential scan rates (0.02-10 Vs-1) and reverse potentials at Pt, Au and GC electrodes. It was established that CO2 electroreduction takes place in two stages, the electrode process is controlled by the charge transfer rate and the rate of CO2 diffusion in the melts. The solubility of CO2 in the melts of Na,K|Cl and Na,K|Cl,F was determined and the implementation of Henry's law in the studied pressure range was shown. An ECE (electrochemical–chemical–electrochemical) mechanism of cathodic reaction has been proposed for the cathode discharge of CO2. Oxidation of oxide- and carbonate- anions takes place at the anode, with the release of oxygen.

Electrochemical reduction of Li2CO3 in molten equimolar mixture of Na,K|Cl in various gaseous media (air, argon, CO2) was studied by CV. Reduction of Li2CO3 to carbon in air occurs through the stage of preliminary chemical reaction of acid-base type with the formation of two electrochemically active particles of CO2 and {LixCO3}2-x at potentials -0.8 and -1.7 V, respectively, relative to Pb|PbCl2 reference electrode . Both processes are irreversible, and the limiting stage of reduction {LixCO3}2-x is the diffusion of the depolarizer to the electrode surface. In the atmosphere of argon and CO2, only {LixCO3}2-x participates in the electroreduction process.

Promising areas of application of electrolytic carbon are presented.