Preparation Of Super High Specific Surface Area Porous Carbon Based On Biomass And Study Of Supercapacitance Performance

With the development of social economy and life style,the demand for energy grows rapidly,which makes energy storage devices with high energy density and power density to become the focus of research.Supercapacitors have been attracting a lot of attention from researchers due to their high power density and reasonable energy density.Electrode material is the key part of supercapacitors,which directly affects the power density and energy density.Carbon materials are the most widely used electrode materials,and biomass has become an important source of electrode carbon materials as it is abundant,renewable,inexpensive,and so on.In this thesis,eggplant and lotus leaf are selected as carbon sources to prepare carbon materials with high specific surface area.The carbons were prepared by using high temperature carbonization and KOH activation.The physicochemical properties of biomass based carbon materials were investigated by elemental analyzer,X-ray photoelectron spectroscopy,X-ray diffraction,Raman spectra,scanning electron microscopy,transmission electron microscopy,nitrogen adsorption-desorption isotherms,solid state nuclear magnetic resonance.The electrochemical performances of the as-prepared biomass carbons materials were investigated.The details are as follows:（1）Eggplant was used as the precursor of biomass-based carbide,which is prepared through pyrolysis at different temperatures（600 ℃ 1000 ℃）.Hoever,the specific surface areas of these directly-pyrolysised carbons without activation are relatively low,and their capacitive performances are rather poor.The three kinds of eggplant-based activated carbon are prepared from KOH activation by adjusting the mass ratio of KOH/C.The three products were denoted as EK-1,EK-2,EK-4.The specific surface areas of EK-1,EK-2,EK-4,were 1596 m2/g,2454 m2/g,3667 m2/g,respectively.EK-4 showed the best electrochemical performance in both alkaline electrolyte and acidic electrolyte.In alkaline electrolyte,the specific capacitance of EK-4 was 249.7 F/g,the power density is 124.9 W/kg and the energy density is 8.6 Wh/kg,at the current density of 0.5 A/g.It also showed excellent rate and cyclic performances,i.e.the capacitance retention was 94.6% after the 5000 cycles.The as-prepared carbons showed similar electrochemical performances in the acidic electrolyte,the specific capacitance of EK-4 is 228.4 F/g,the power density is 124.9 W/kg,the energy density is 8.6 Wh/kg,at the current density of 0.5 A/g;the specific capacitance of EK-4 is 207.6 F/g（capacitance retention rate of 90.9%）,the power density is 2467.6 W/kg,the energy density is 7.0 Wh/kg,under the current density of 10 A/g;under the constant current density of 5 A/g,the capacitance retention rate was 99.2% after the 5000 cycles.（2）Lotus leaf was also used as the carbon source.Activated carbon with ultra-high surface area was prepared by pre-carbonization and KOH activation of lotus leaf.Four kinds of lotus leaf based activated carbon were prepared from KOH activation by adjusting the mass ratio of KOH/C.The specific surface areas of CLL-800,LK-1,LK-2,LK-4,were 78 m2/g,1941 m2/g,3537 m2/g,and 4482 m2/g respectively.The as-prepaed carbons were used as electrodes to prepare coin-cell supercapacitors using KOH,H₂SO₄ and an ionic liquid as the electrolyte.In the alkaline electrolyte,the electrochemical performance of LK-4 was the best.The specific capacitance is 288.7 F/g,the power density is 124.9 W/kg and the energy density is 10.0 Wh/kg at current density of 0.5 A/g.LK-4 also showed outstanding rate and cyclic performances,retening 102.9% of its initical capacity after the 5000 cycles of charge/discharge at 5 A/g.The performance of the electrode in the acidic electrolyte is comparable with that in KOH.Meanwhile,LK-4 also showed the highest performance among the four samples in the ionic liquids,showing the energy density is of 111.9 Wh/kg which is comparable with the best values reported for carbon-based electrode materials in literatures.