Preparation And Morphology Control Of Micro-nano Carbon Materials By Electrochemical Conversion Of CO₂ In Li Ca-Na/K High Temperature Molten Salts

With the rapid development of modern economy,human society’s demand for fossil fuels is increasing day by day,because its excessive use of CO₂releases serious greenhouse effect.Therefore,efficient conversion and resource utilization of CO₂through low-carbon means has become a hot spot in the field of dual-carbon research.High-temperature molten salt is a liquid-phase medium with a wide electrochemical window that can immobilize and electrochemically reduce CO₂to zero-valent carbon materials.In this paper,a high-temperature molten salt electrochemical carbon reduction system for CO₂production was successfully constructed by using a multifaceted high-temperature carbonate as the reaction medium and a non-precious metal electrode;Analysis of the reaction course of tetravalent to zero-valent carbon in high temperature molten carbonate media;To reveal the correlation laws between key control factors such as molten salt composition,electrolysis temperature,current density and microscopic morphology of carbon spheres;To investigate the activation effect of the corresponding alkali metal hydroxide on porous carbon materials,and finally to achieve the controlled conversion of CO₂to carbon spheres and porous carbon materials.The main study contents and results are as follows:1.Based on the thermodynamic theory analysis,the theoretical electrolytic potential of carbon generation reaction between alkaline earth metal carbonates and divalent cationic carbonates into carbon generation at different temperatures was calculated.The results showed that the absolute value of the theoretical electrolytic potential of carbon generation by electrolysis decreases with the increase of temperature.Compared to Na₂CO₃and K₂CO₃,the energy barrier of the carbon production reaction between Li₂CO₃and Ca CO₃was lower.Therefore,the experimental preference was to use Li₂CO₃-Ca CO₃as the main electrolyte,and Na₂CO₃and K₂CO₃as electrolyte supplements,Compose a ternary alkali/alkaline earth metal molten salt electrolyte system separately.2.The possible electrochemical redox reactions of the composite electrolyte system were analyzed by cyclic voltammetric curves,and the key control factors for the electrochemical conversion of CO₂from molten salts to carbon spheres were examined by means of SEM,Raman,XRD,TEM and other characterization.It was shown that the carbon spheres with smooth surface and regular morphology of about 200-300 nm in diameter were obtained by electrolysis at 750℃in the Li-Ca-Na system with gradient current densities of 6-10-20-200m A/cm²for 15-15-5-60 min.On this basis,the carbon spheres were modified by adding 5wt%Mn O₂molten salt additive to reduce the diameter to about 100 nm and increase the graphitization of the carbon material is greatly increased.3.The corresponding alkali hydroxide activator Na OH/KOH was introduced into the molten carbonate system to produce activated gas（H₂and CO）and carbonized products M₂O and M（Na/K）,which promoted the transformation of carbon materials from carbon spheres to porous carbon materials.In the Li-Ca-Na-Na OH（6:3:3:1 wt）system at 750°C,a gradient current density of 6-10-20-200 m A/cm²was used to electrolyze 15-15-5-60 min,respectively,to prepare honeycomb porous carbon materials with large specific surface area and pore volume,which were 203.9 m²/g and 0.54 cm³/g,respectively.