Structure Control And Electrochemical Performances Of Biomass-Based Carbon Materials

The development of electrochemical energy storage systems plays an important role in reducing the consumption of fossil fuel and the widespread use of intermittent renewable energy.Among various electrochemical energy storage systems,supercapacitors have many advantages,such as high power density,long cycle life,and safety.And they have been widely used in uninterrupted power systems,hybrid electric vehicles,and consumer electronics.Carbon materials are regarded as the leading electrode materials for supercapacitors owing to their high specific surface area,tunable pore structure and good conductivity.These properties directly influence their electrochemical performances.However,the cost,preparation method,and environmental friendliness are also vital for the development of carbon-based supercapacitors.Among various carbon materials,biomass-based carbon has naturally selected microstructure and adjustable physical/chemical properties.So it is expected to offer more fascinating characteristics compared with the artificially synthesized ones.Moreover,biomass-derived carbon is generally environmentally friendly and low-cost.Inspired by above mentioned,we have prepared various porous carbon materials by utilizing natural tubular structure of biomass.Meanwhile,to improve the specific surface area and conductivity of biomass-based carbon,a series of activating-graphitizing agents have been provided.We have analyzed the effect of different catalysts on the structure of carbon materials,and their electrochemical performances were also studied.This paper provided simple and environmentally friendly strategies for preparing porous graphitic carbon by utilizing the unique structure of biomass waste.The main researches and results are as following:?1?Carbon microtube bundle was simply prepared by direct carbonization sawdust using its natural tubular porous structure as template.The obtained CMB was composed of numerous parallel tubes with a diameter of 2.96¹9.74?m,which is beneficial to electrolyte ions transportation and storage.Through electrodeposition method,nano-scale MnO₂ was uniformly deposited on the surface of CMB.With appropriate amount of MnO₂,there formed a three-dimensional porous network structure,which is beneficial to provide enough active sites.Increasing the mass loading of MnO₂ increased the thickness and leaded to a low conductivity.When tested as an electrode,MnO₂/CMB-0.6C exhibited a high specific capacitance of 617.6 F/g at 1 A/g.Moreover,it also had a good rate performance and an excellent cycling stability with 80%capacitance retention after 1000 cycles at 10 A/g.?2?Willow catkins with single-walled microtubular structure were carbonized with different catalysts?such as K₄Fe?CN?₆,FeCl₂,or NiCl₂?to improve conductivity of biomass-derived carbon.The obtained carbon materials inherited the natural tubular structure of willow catkins and the graphitization degree had been significantly improved.Among various carbon materials,PGCMT?with K₄Fe?CN?₆ as catalyst?had the largest specific surface area?1066.6 m²/g?,which was significantly higher than that of others.The results indicated that K₄Fe?CN?₆ not only acts as a graphitizating catalyst but also is anactivating agent.That is to say,simultaneous activation-graphitization can be realized by a simple catalyst K₄Fe?CN?₆.PGCMT with microtubule porous structure and good conductivity can act as an ideal substrate of MnO₂ to alleviate MnO₂ accumulation and to facilitate ion/electron transfer.The MnO₂/PGCMB composite electrode expressed excellent electrochemical performances,including a significantly enhanced specific capacitance,a high capacitance retention,and an excellent cycling stability.?3?The mixtures of Fe?NO₃?₃ and KCl were used as catalysts to prepare willow catkins-derived porous graphitic carbon.For comparison,Fe?NO₃?₃was replayed by Fe₂?SO₄?₃ and FeCl₃ in synthesis to evaluate the effects of the mixture of other ferric salts and KCl on the structure of the resultant carbon materials.The results of nitrogen adsorption-desorption,XRD,and Raman indicated that the obtained PGC-N?treated with Fe?NO₃?₃ and KCl?had improved large specific surface area and graphitization degree.The presence of KCl was crucial for developing a porous structure.In addition,the specific surface area and pore volume of PGC-N were much higher than those of PGC-Cl?treated with FeCl₃ and KCl?and PGC-S?treated with Fe₂?SO₄?₃ and KCl?,indicating that using the mixture of Fe?NO₃?₃ and KCl can realize simultaneous activation and graphitization.The electrochemical performance of MnO₂/PGC-N had been significantly improved owing to good electrical conductivity and high specific surface area of PGC-N.At a current density of2 A/g,the specific capacitance of MnO₂/PGC-N reached 571.1 F/g.Even at a high current density of 50 A/g,it still maintained at 382.1 F/g.Furthermore,the electrode exhibited an outstanding cycling stability with 85.2%capability retention after 3000 cycles.?4?An ideal carbon material for supercapacitors should simultaneously satisfy large specific surface area and good conductivity and avoid using corrosive activating agents.Based on the above consideration,we provided a novel strategy to convert disposable chopstick into high-value-added porous graphitic carbon by one-step carbonization using K₂C₂O₄ and Fe?NO₃?₃ as activating agent and graphitic catalyst,respectively.The porous structure and graphitization degree of carbon materials can be easily controlled by tuning the amount of K₂C₂O₄ and the carbonization temperature.When the mass ratio of K₂C₂O₄/Fe?NO₃?₃/SD was 0.4:0.3:1 and the carbonization temperature was850°C,the obtained sample PGC-4-850 had a high specific surface area of1651.9 m²/g and good graphitization degree?g=0.92,I_G/I_D=1.05?.By analyzing the relationship among the pore structure,graphitization degree,and electrochemical performances of PGC-10c-T samples,it can be concluded that the high proportion of micropores/mesoporous is beneficial to the improvement of specific capacitance.When the carbon materials have same micropores surface area,increasing the content of mesoporous or macroporous will lead to low conductivity and decrease the specific capacitance.?5?A new type of porous carbon like“mushroom–branch”with numerous graphitic carbon sheets grown on cellulose fiber skeleton was fabricated by carbonization glucose and cellulose fiber with the aid of K₃[Fe?C₂O₄?₃].K₃[Fe?C₂O₄?₃]was selected as both activating agent?K₂C₂O₄?and graphitizing catalyst?Fe?to promote the development of porous graphitic structure.In addition to having sheet-like structure and good electrical conductivity,the as-obtained CS@CF-KFe also had a large specific surface area?1515.6 m²/g?and a high content of ion-accessible micropores.When tested as an electrode for supercapacitor,the sample exhibited a high specific capacitance of313.0 F/g at 1 A/g and a superb cycling stability with 100.2%capacitance retention after 10000 cycles at 100 mV/s.Furthermore,the assembled symmetric supercapacitor displayed a remarkable energy density of21.5 Wh/kg at a power density of 456.5 W/kg in 1 M Na₂SO₄ electrolyte.The superior electrochemical performances of CS@CF-KFe are attributed to the multiple synergistic effects:?i?thin 2D carbon sheets offer a mass transport on a large electrode/electrolyte interface and shorten the diffusion pathways;?ii?partially graphitic carbon is more effective for electron transfer during the charge/discharge process;?iii?the high density of ion-accessible micropores brings micropore effect that increases charge accommodation as well as the specific capacitance.