Cationic Catalysis during the Discharge of Carbonate Anions in Molten Salts
Authors: Novoselova Inessa , Kuleshov Serhii Volodymyrovych , Omel'chuk Anatolii A , Soloviev Veniamin Vasilevich , Solovyova Natalia Veniaminovna 
Published: December 2020

In the recent years, much attention has been focused on the synthesis of various carbon nanomaterials (CNMs) because of their wide range of potential applications. These materials have been fabricated by different methods such as the laser and arc evaporation of graphite, catalytic pyrolysis of hydrocarbons, disproportionation of CO on metal-catalysts etc. Promising method for synthesis of CNMs is high-temperature electrochemical synthesis (HTES) in molten salts. Although much research has been devoted to this subject, little is known about the mechanism of HTES and which fundamental reactions taking place during this process.

In this work an attempt to confirm the mechanism of cation-anion interaction between the CO3
2- anion and the strongly polarizing cations (Li+, Ca2+, Mg2+) is made. The mechanism of these interactions has a complex nature. Cation-anion interaction can result either in the formation of cationized anions (metal complexes) or in anion dissociation both under the direct influence of cations and through the intermediate stage of formation of "short-lived" metal complexes.

To test this hypothesis, the quantum chemistry modeling and the cyclic voltammetry experiments were done. It has been shown by the quantum chemistry method that by changing the cationic composition of the electrolyte, one can transform anionic carbonate complexes into a new active state – cationized carbonate complexes. Experimental confirmation of cation-anion interaction was studied in the chloride melts Na,K,Cs|Cl, Na,K,Rb|Cl, Na,K|Cl by the cyclic voltammetry method at temperature ranges of 550–580 °C and of 700–800 °C, which are much lower than the thermal decomposition temperature. The electrolyses was carried out in electrolytes under potentiostatic conditions at a potential of -1.1 V at the concentrations of lithium, calcium and magnesium chlorides, corresponding to the clear observation of cathodic waves. According to the data of a chemical and an X-ray phase analysis a black powder of the cathodic product was carbon. The morphology of electrolytical carbon is presented in the figure below.

Based on the obtained results the following conclusions can be done:

In the Na,K,Rb|Cl melt containing weakly polarizing Na+, K+, Rb+ cations, the CO3
2- anion shows no electrochemical activity in the temperature range of 570–700 °C. This is accounted for by the large values of the activation barriers to the two- and four-electron reduction of CO3
2-.

The addition of strongly polarizing cations (cations with high specific charge) results in the activation of the carbonate ion, which is caused by a large excess of Li+ and Ca2+ cations. In the case of Mg2+ cations, the carbonate ion shows electroactivity at much lower concentrations.

Independent of the polarizing power of the cation (melt acidity), the CO3
2- reduction process in molten chlorides occurs in the same potential range of -0.7 to -0.9 V versus a silver reference electrode.

The CO3
2- electroreduction process under the action of strongly polarizing cations occurs at temperatures much lower than the thermal decomposition temperature of the corresponding carbonate. This suggests that the formation of CO2 is not the result of thermal decomposition, but is a consequence of the polarizing action of cations.

Figure 1