Studies On Preparation And Electrochemical Performance Of Porous Carbon Materials In Li₂CO₃-Na₂CO₃ Molten Salt

Porous carbon materials have advantages in specific surface area,pore volume,electrical conductivity,and thermal conductivity.They are widely used in water purification,gas separation,electric double layer capacitors,lithium ion batteries,fuel cells,gas storage,and adsorbents fields.In this paper,porous carbon materials were prepared by molten salt electrolysis and molten salt oxidation methods,and their electrochemical properties were tested.The feasibility of Li₂CO₃-Na₂CO₃ as electrolyte was analyzed by comparing the theoretical deposition potential and experimental verification.At 600℃,the reduction process of CO₃^2-to carbon of tungsten cathode as working electrode was investigated by cyclic voltammetry,square wave voltammetry and chronopotentiometry.The cyclic voltammetry and square wave voltammetry at different sweep speeds were used to study and explore the diffusion coefficient of CO₃^2-in molten salt systems and the number of transferred electrons were calculated when CO₃^2-was reduced to carbon.Cyclic voltammetry and square wave voltammetry were used to investigate the electrochemical behavior of nickel wires as working electrodes,and different potential intervals were used to prove that more reduction peaks on nickel electrodes were attributed to carbon generation.The effect of electrolytic potential on micro-morphology and current efficiency was also studied.Firstly,Li₂CO₃-Na₂CO₃ molten salt system was used,nickel sheet as the working electrode,and galvanostatic electrolysis was performed at temperatures of 550,600,650 and 700℃ to obtain honeycomb,sheet and nanowire deposits.EDS analysis showed that the sediment was composed of elements C and O.The XRD pattern indicated that the product was amorphous carbon.A porous carbon material with higher specific surface area,better thermal stability and more regular morphology was synthesized at 550℃.Secondly,the used waste ion exchange resin resin（containing C,H,O,and S）was selected as a carbon source to be completely oxidized in the molten salt,and a porous carbon material was prepared by constant current electrolysis at550℃.Its microscopic morphology has a honeycomb and nanosheet stack structure,and the morphology does not change much compared with no carbon source introduced.Moreover,the Raman analysis has a reduced degree of graphitization.It was found from the HRTEM electron microscope lattice image that in addition to the amorphous carbon structure,there was an ordered graphite layer.The EDS elemental spectrum found that in addition to the C and O elements,a small amount of S elements existed.The surface porosity indicates that the pore size is still mainly mesopore.The electrochemical properties of carbon materials under two preparation conditions were tested,and it was found that the electrolytic temperature has an important effect on the capacitance performance of carbon materials.The carbon material prepared at 550℃had the largest specific discharge capacity and better electrochemical performance.At a current density of 0.5 A·g^-1,the specific capacitance value is 212 F·g^-1.The carbon material prepared by introducing carbon source electrolysis had a specific capacitance of231 F·g^-1 at a current density of 0.5 A·g^-1.A cation exchange resin and（Na,Li）₂CO₃ were mixed uniformly at a mass ratio of 1:2,and the porous carbon material was prepared after being maintained at 550℃ for 0.5,1,2 h.XRD analysis showed that the products were all amorphous carbons.Raman spectroscopy analysis showed that as the reaction time increased,the degree of graphitization of the material increased.The porosity analysis showed that there were many micropores,which have a higher specific surface area than the electrolytic method mainly distributed in the mesopores.The microstructure and micro-zone chemical analysis of the material were analyzed by SEM and EDS.The thermogravimetric test analyzed the thermal stability of materials prepared by different reaction times.The capacitance performances were explored by cyclic voltammetry,constant current charge and discharge,AC impedance.The specific capacitance of the carbon material prepared after 2 hours of reaction at a current density of 0.5 A·g^-1 is 260 F·g^-1.Carbon materials prepared by the molten salt method have higher specific surface area and more sulfur elements to provide rubidium capacitance.