Preparation Of Asphalt-based Activated Carbons As Well As Their Capacitive Energy Storage Characteristics And Electromagnetic Wave Absorption Performances

As a common porous carbon material,the activated carbon has been widely used in the fields of capacitive energy storage and electromagnetic wave absorption,due to its advantages of high specific surface area,well-developed pore structure,good electrical conductivity,light weight and low cost.The capacitive performance of activated carbon can be further improved through pore structure regulation and heteroatom doping modification of activated carbon.By introducing manganese oxide into activated carbon,the dielectric constant of the material can be effectively adjusted and then the impedance matching characteristic can be optimized,leading to the improvement of electromagnetic wave absorption performance.In this paper,porous activated carbons were prepared respectively by gradual activation process and template-activation coupled with thermal calcination method,using low-cost asphalt as carbon source.S-doped activated carbons were prepared by the post-treatment doping method.The above materials were used as electrode materials for supercapacitors and lithium-ion capacitors,and their capacitive energy storage characteristics were systematically studied.At the same time,activated carbon/manganese oxide composites were synthesized by one-step calcination method.They were used as wave-absorbing materials,and their electromagnetic wave absorption performances were also researched.The main results are as follows:（1）Study on the preparation of porous activated carbons by gradual activation process and their capacitive energy storage characteristics.Novel sponge-like porous activated carbons（PACs）were prepared by gradual activation process with asphalt as carbon source and KOH as activating agent.In a certain range,with the increase of tableting pressure and tablet thickness,the pore size distribution range of PACs becomes wider,the average pore size increases,and the content of mesopores and macropores increases continuously.Compared with traditional KOH activation process,the gradual activation process improves the activation efficiency,and the meso-macroporous structures of porous activated carbons can be regulated directionally by adjusting process parameters of the gradual activation process（tableting pressure and tablet thickness）.The regulation mechanism is as follows:Increasing the tableting pressure can strengthen the sealing effect of activated gas inside the tablet,and increasing the tablet thickness can prolong the axial escape distance of activated gas inside the tablet,which can both increase the residence time of potassium vapor and CO₂,and thus enhance the gradual activation effect and promote the formation of more abundant meso-macroporous structure for porous activated carbons.PAC-3.0-10,which was prepared under the condition of alkali/carbon ratio of 3.0 and tableting pressure of 10 MPa,has ultrahigh specific surface area(3211 m² g^-1),abundant mesoporous structure(V_meso=0.77 cm³ g^-1)and lower oxygen content（4.04 at%）.When used as the electrode material for supercapacitors,the PAC-3.0-10 has the highest specific capacitance(up to 356 m A h g^-1at 0.5 A g^-1).Meanwhile,it exhibits superior rate performance(275 m A h g^-1 at 30 A g^-1)and cycling stability(no decay in capacitance after 10000 cycles at 6 A g^-1).When used as the cathode material for lithium-ion capacitors,the PAC-3.0-10 also exhibits the best capacitive performance.The specific capacity is up to 251 m A h g^-1 at 0.5 A g^-1.Even at a high current density of 15 A g^-1,the capacity is still as high as 158 m A h g^-1（capacity retention rate is 63.1%）,showing extremely high specific capacity and excellent rate capability,which is obviously superior to previously reported activated carbons and other control groups.The above results illustrate that PAC-3.0-10 presents superior capacitive energy storage characteristics in both aqueous and organic electrolytes.（2）Study on the preparation of porous activated carbon by template-activation coupled with high temperature calcination method and its application in lithium-ion capacitors.The porous activated carbon（HPAC）was prepared by template-activation method with asphalt as carbon source,KOH as activator and porous Mg O as template,and then modified porous activated carbon（HPAC1000）was obtained after high temperature calcination of HPAC.HPAC1000 provides ultrahigh specific surface area(3331 m² g^-1),numerous mesoporous structure(V_meso=0.86 cm³ g^-1),low oxygen content（3.63 at%）and good electrical conductivity.When used as the cathode materials for lithium-ion capacitors,in comparison with HPAC,HPAC1000 exhibits higher specific capacity(231 m A h g^-1 at 0.5 A g^-1)as well as better rate performance(capacity retention is up to 80.1%at 10 A g^-1)and cycling stability(capacity retention of 98.1%after 500cycles at 1 A g^-1).Asphalt-based boron-doped porous carbon（BPC）was synthesized by one-step carbonization template method.When used as the anode material for lithium-ion capacitors,the BPC exhibits high specific capacity(372 m A h g^-1 at 0.5 A g^-1),superior rate performance(250 m A h g^-1 at 5 A g^-1)and outstanding cycling stability(capacity retention of 90.0%after 1000 cycles at 1 A g^-1).The full lithium-ion capacitor HPAC1000//BPC was assembled with HPAC1000 as the cathode material and BPC as the anode material.The specific capacitances of HPAC1000//BPC deliver 92.2 and 70.4 F g^-1at 1 and 10 A g^-1,showing high specific capacitance and excellent rate capability.Besides,it also exhibits ultrahigh energy density(204 Wh kg^-1)and power density(11193 W kg^-1)as well as outstanding cycling performance(capacitance retention of 82.6%after 2000cycles at 1 A g^-1).Both cathode and anode materials suitable for lithium-ion capacitors can be prepared with low-cost asphalt as carbon source,which provides a promising prospect for the development of high value-added utilization of asphalt.（3）Study on the preparation of S-doped activated carbons by post-treatment doping method and their application in supercapacitors.S-doped activated carbons（SACs）were prepared by post-treatment S-doping modification of the pre-synthesized asphalt-based activated carbon（AC）with Mn SO₄ as S source.The S content and pore structure of the product can be controlled by adjusting the mass ratio of Mn SO₄/AC and calcination temperature.The maximum S content is 18.40 wt%,and the maximum mesoporous volume is 0.93 cm³ g^-1.When used as the electrode material for supercapacitors,the SAC3-T700 exhibits the best capacitive performance.The specific capacitance at 1 A g^-1reaches 297 F g^-1 and the specific capacitance maintains 264 F g^-1 at a high current density of 30 A g^-1（capacitance retention rate is up to 89.0%）,showing extremely superior rate capability,which is superior to the initial undoped AC and other SACs.This could be attributed to its highly well-developed mesoporous structure(V_meso=0.93 cm³ g^-1)and significantly enhanced conductivity due to a large amount of S doping（14.91 wt%）.In addition,the capacitance retention rate is up to 98.4%after 10000 cycles at 6 A g^-1,showing superior cycling stability.（4）Study on the preparation and absorption properties of activated carbon/manganese oxide composites.Activated carbon/manganese oxide composites（AC/MnO）were synthesized by one-step calcination method with asphalt as carbon source,KOH as activator,and KMnO₄ as metal salt precursor.Compared with pure activated carbon,the introduction of a proper amount of manganese oxide nanoparticles in activated carbon can not only effectively improve the impedance matching characteristic,but also possess the strong attenuation loss ability.AC/MnO-0.8 exhibits the best electromagnetic wave absorption performance under the synergistic effect of good impedance matching and multiple loss mechanism（multiple reflection/scattering,conductive loss,interface polarization and dipole polarization）.When the filling amount is only 15 wt%,the RL_min value can reach 50.9 d B,and the effective absorption bandwidth is 4.2 GHz at a thin thickness of 1.8 mm,which conforms to the development trend of"thin,light,wide and strong"for new absorbing materials.