The Preparation And Supercapacitive Properties Study Of Carbon Materials By Salt Assisted Method

The depletion of fossil-fuel and ever-increasing demands for clean and renewable energy sources have accelerated the design and development of energy conversion and storage devices to meet the urgent energy requirements all over the world.Supercapacitors have been considered as one of the most promising energy storage devices due to their high power density,long cycling life and fast charge/discharge rates.In general,electrode material is the key that affected the electrochemical performance of supercapacitors.Among various electrode materials,carbon materials are the most widely used as electrode materials for commercial supercapacitors due to their high specific surface area,excellent electrical conductivity,low cost and high chemical stability.However,their low energy density limits its application and further development of high-performance carbon based material has become research hotspot in recent years.Therefore,the aim of this thesis focuses on structure design and surface modification of carbon electrode materials for supercapacitors and using simple and novel methods to prepare high performance carbon materials,realizing the improvement of the energy density of supercapacitors.The main research contents are as follows:Inspired by flour food making process,a chemical foaming strategy is adopted to prepare the three-dimensional hierarchical porous carbon?HPC?by introducing NaHCO₃ as activation agent and flour as carbon precursor.Benefitting from the stepwise decomposition of NaHCO₃,the CO₂ gas produced during the decomposition of NaHCO₃ at a low temperature will result in the producers of the macro-pores and meso-pores,and the subsequent decomposition of Na₂CO₃ will activate the obtained carbon fully to form the micro-pores,finally formation of HPC with a three-dimensional hierarchical porous structure and high specific surface area(1376 m²g^-1).HPC has a high discharge specific capacitance of 350 F g^-1at a current density of 1 A g^-1.Even at an ultrahigh discharge rate of 50 A g^-1,the specific capacitance still has215 F g^-1,which demonstrates superior rate performance.Moreover,the as-assembled symmetric supercapacitor based on HPC exhibits a high energy density of 23 Wh kg^-1in the neutral electrolyte and the specific capacitance retention keeps 95.6%after 10000 cycles at a current density of 4 A g^-1.A three-dimensional,densely nitrogen-doped porous carbon?NPC?material derived from nitrogen-rich tofu precursor is successfully synthesized by using the low melting point molten salts LiCl/KCl as reaction media and LiNO₃ introduced into the molten salts as an activating agent based on the principle of black powder.During the preparation process of NPC,the low melting point molten salts act as in-built templates to prevent the adjacent tofu from agglomeration during carbonization.At the same time,they are used as the solvent to dilute a LiNO₃,slowing the rate of activation.The as-obtained NPC has a high surface area(1202 m² g^-1),high bulk density(about 0.84 g cm^-3)and high nitrogen doping?4.72%?.According to the electrochemical tests,it is found that NPC shows a maximum specific volumetric capacitance of 360 F cm^-3and gravimetric capacitance of 418 F·g^-1with excellent cycling stability without capacitance loss after 10 000 cycles at a high charge current of 10 A g^-1.Moreover,the as-fabricated symmetric supercapacitor delivers a maximum gravimetric energy of 32.95 Wh kg^-1and volumetric energy of 27.68 Wh L^-1in 1 mol L^-1Na₂SO₄ aqueous solution.The capacitance retention of the symmetric supercapacitor keeps 97%after cycling10,000 cycles,exhibiting an excellent cycling stability.A“soaking-recrystallization-calcination”method is designed to enhance the commercialized carbon fiber cloth?CFC?capacitance performance through increasing its surface area and introducing pseudo-capacitance via heteroatom doping simultaneously.The CFC-fresh is first soaking in different inorganic salt solution and further making the salt recrystallized on the surface of CFC through the water evaluation.Up on the calcined process,the decomposition of the inorganic salt will cause an in-situ etching on the fiber surface and synchronous heteroatom doping into the carbon matrix.The CFC with a binary element doping of N and S?CFC-750-N-S?shows the highest capacitive property compared with other CFC samples,and exhibits a high areal capacitance of 362 mF cm^-2,is about 1448 fold enhancement than that of fresh carbon fiber cloth calcined at 750°C?CFC-750?.Meanwhile,the assembled flexible solid-state supercapacitor based on CFC-750-N-S has a volumetric energy density of 0.35 mWh cm^-33 and gravimetric energy density of 0.84 Wh kg^-11 with 97%capacity retention after 10,000 cycles at a current density of 5 mA cm^-2.Carbon nanobelts?CNB?from tofu precursor are prepared using molten salts as reaction media by a stripping and cutting strategy of Zn²⁺ions.The unique crosslinked carbon nanobelt networks can facilitate the increase of contact area with electrolyte and the rapid transmission of electrons.CNB electrode has a high discharge specific capacitance of 262 F g^-11 at a current density of 0.5 A g^-1.Even at an ultrahigh discharge rate of 50 A g^-1,the specific capacitance still has 145 F g^-1,which demonstrates superior rate performance.Additionally,the electrode has outstanding electrochemical stability with capacitance retention up to 102%after 10 000 cycles at a current density of 3 A g^-1.The assembled symmetric supercapacitor based on carbon nanobelts has a high energy density of 18.19 Wh kg^-11 at a power density of178 W kg^-1.