Sysnthesis And Energy Storage Performance Of Biomass-Derived Nano Porous Carbon Materials

Inseparable are the energy storage and materials.From the very first Leyden jar to dry cells and to the widely used lithium-ion batteries,every jump for material techniques inevitably leads to revolution of energy storage and each storage revolution will eventually bring our life qualitative leap.With enormous utilization of electric vehicles and mobile phones,energy storage devices today are of highly importance.An electric source with great performance needs not only ultra-high specific energy and power,but also longer cycle life,shorter charge-discharge time and better thermo-stability.Supercapacitors,with their higher power density as well as longer cycle life,show prospect in fields where quick charge-discharge with long cycle life is needed.Activated carbons?AC?,due to its low cost,high specific surface area?SSA?and electrochemical stability,are the common material for the electrode of commercial supercapacitors.Coconut husk carbonization and activation is a popular way to get activated carbons.The essence of this method is to carbonize and activate the fiber,where the carbon chain of the cellulose as well as the pore structure can be largely in-situ maintained while micro-and meso-pores with higher specific surface area can grow.There are two kinds of biomass fibers,the plant fiber and the bacterial cellulose.In this thesis,rice-straw and acetobacter xylinum cellulose were exemplified as the precursors,respectively,to prepare activated carbons.The supercapacitive properties in acid,basic and neutral aqueous electrolyes and the structural influence of processing conditions were studied,including activation time,temperature and agent dosage,so as to reveal the optimum.The main work and results are as follows:Rice-straw was carbonized in vacuum at 500 ^oC and activated at750⁹00 ^oC for 1² h,respectively.X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?and Fourier Translation Infrared spectroscopy?FTIR?were used to characterize the phase,elemental content and surface functional groups.Scanning electron microscopy?SEM?was used to see the morphology of samples.Nitrogen sorption measurement was used to quantitatively examine the pore structure.Carbon powders were made into electrode coins and tested in acidic,basic and neutral aqueous electrolytes,respectively,using two-electrode system.Results show that with the temperature platform rising,SSA of ACs increases and peaks at 800^oC,while the mesopore rate inclines and the micropore rate declines.Compared with the other samples,the one prepared at 800 ^oC for 1 h reveals an ultra-high SSA of 3239 m²/g and a better supercapacitive performance.Because of a large number of micropores smaller than 1.4 nm and mesopores between 2⁴ nm,it reaches a specific capacitance of 259 F/g?1 mol/L H₂SO₄,0.1 A/g?,a specific energy of 16.6 Wh/kg?1 mol/L Na₂SO₄,0.1 A/g?and a rate retention of 14.5%?6 mol/L KOH,0.1⁵.0 A/g?.Acetobacter xylinum cellulose was soaked in KOH solution with different concentrations from 0.1 to 1.4 wt%for 24 h and then freeze-dried.After activated at 700⁹00 ^oC,respectively,the original 3D amorphous network structure vanished and formed the activated carbon foams containing micropores smaller than 1.4 nm and mesopores between 2³.5nm in quantity.Further investigation indicates that when concentration of KOH solution and temperature platform rises both SSA and mesopore rate are increasing and peak at 0.9 wt%and 900 ^oC,respectively.Relatively speaking,samples prepared under these two conditions are also electrochemically performing well.It reveals a specific capacitance of 184F/g in 1 mol/L H₂SO₄ at 0.1 A/g,a specific power of 10.7 kW/kg in 6 mol/L KOH in 5.0 A/g and a rate retention of only 3.0%in 6 mol/L KOH when current density rises from 0.1 to 5.0 A/g.The effects of processing conditions on both plant fiber and bacterial cellulose activated carbon were studied in this thesis.Novel trials were for the first time executed to make the 3D network shrunk into bulks of carbon foams with even and adjustable pores.It's an innovative idea for late-model supercapacitor electrode materials preparation using biomass.