Molten Carbonates Electro-synthesis Of Carbon-Based Materials And Their Electrochemical Properties

Energy and environmental crisis are the two major problems for the sustainable development of society.Therefore,it is not only necessary to develop new energy storage materials,but also to develop low-carbon-footprint chemical processes to reduce carbon emissions,respectively.Exploring new strategies for synthesizing energy storage materials and catalytical materials has become the focus of today’s research.Molten salt electrosynthesis of carbon materials is a promising method due to the advantages of low cost,high speed and wide reaction temperature window.According the limitations in molten salt electrolysis,such as most products are amorphous carbon,single electrolysis strategy.In this paper,new methods for synthesizing carbon-based materials are explored based on the electrolytic synthesis of carbon-based materials in molten carbonate.According to the structure and properties of the synthesized carbon-based materials,their applications in the fields of energy storage and catalysis are explored.The main research contents and conclusions are as follows:（1）Oxygen-rich spongy porous carbons（SPCs）were electrolytically synthesized in molten Li₂CO₃-Na₂CO₃-K₂CO₃ composite at 580°C using nickel anode and galvanized iron wire cathode at different current densities,and the formation mechanism of SPCs during electrodeposition and supercapacitor performance of SPCs were explored.The results show that SPCs grow in the form of"branch on a piece"and its specific surface area increases with the increase of current.The specific capacitance of the symmetric supercapacitor device（QSSD）assembled by SPC4-0.5 can reach 336.0 F/g.The capacity retention rate of SPC4-0.5（electrolysis at 4 A for 0.5 h）is as high as 91.1%after 15 000 cycles at 5 A/g.Furthermore,the potential application of SPCs as catalysts for two electron oxygen reduction（2e^-ORR）to hydrogen peroxide was also explored.（2）An“electrochemical reduction-reoxidation”strategy is proposed to construct defects on SPC4-0.5 to form defect-rich porous carbon materials（DPCs）due to the poor 2e^-ORR performance of SPCs.By regulating the time and current in the“reoxidation”stage,the evolution of the defective degree of DPCs was explored.The study showed that the“reoxidation”process improved the defective degree of DPCs.The H₂O₂ selectivity of the DPC0.5-5（Reoxidation stage:0.5 A,5 min）exceeded 90%in the voltage range of 0.35～0.7 V_RHE（vs.RHE）（0.1 mol/L KOH）.DPC0.5-5 also showed good stability.（3）Inspired by the construction of defects on carbon by“electrochemical reoxidation”in（2）,a one-step electrolysis method to regulate the defective degree of carbon materials was explored.The effects of electrolytic current on the defective degree of the CNTs in Li₂CO₃-based melts at 770℃was investigated.The results show that the weaker the cathodic polarization is,the more defective of the electro-synthetic CNTs become.CNTs-B₂O₃-300 synthesized in Li₂CO₃-B₂O₃ composite melt had high defective degree and showed hgih H₂O₂ selectivity of 83.42%（0.1 mol/L KOH）and good stability.（4）Because of the unavoidable anodic nickel corrosion during the electro-synthesis of SPCs and DPCs in（1）and（2）,an“in-situ electrochemical co-reduction”strategy was proposed to synthesize Ni-intercalated octahedral carbon（Ni@OC）by utilizing the anodic corrosion of Ni electrode during electrolysis.The results show that co-reduction of Ni O and CO₃^2-at the cathode(Ni²⁺+2e^-→Ni,CO₃^2-+4e^-→C+3O^2-)contributes to the formation of Ni@OC.When applied to Li-ion battery active materials,the specific capacity of Ni@OC can reach 433.6 m Ah/g（25 m A/g）,and the capacity retention rate is about 83.3%after 4 800 cycles（2 000 m A/g）.