Preparation And Electrochemical Performances Of Hydrochar-based Porous Carbons Derived From Biomass

Porous carbon has the advantages of high specific surface area,excellent conductivity and low cost,which has broad application prospects in the field of energy storage.Using biomass as the carbon source to prepare porous carbons for electrode materials is an efficient method However,most of the biochars reported so far have irregular morphology and single structure,leading to poor performance and application effects.Therefore,there are still challenges to explore new strategies to control the morphology,structure,and physicochemical properties of biomass-derived porous carbons.In this study,the original biomass(rice straw,moso bamboo,and corncob)were used as the carbon sources,and the preparation and performances of hydrochar-based porous carbons were studied.This work proposed a green synthesis route under mild conditions and a new way of morphology and structure control for porous carbons.This work also realized the application of biomass-derived porous carbons as electrode materials for supercapacitors,clarified the influence mechanism of metal ions on the morphology of hydrothermal carbon-spheres,and revealed the effect mechanism of porous carbon-spheres on the improvement of electrochemical performances.The specific research contents,methods and results are as follows(1)The preparation and study on electrochemical performances of hydrochar-based porous carbons by KHCO3-activation method.Using rice straw as carbon source and KHCO3 as activator,high-performance porous carbons were prepared through "HTC+KHCO3-activation" method Experiments were carried out to study the effect of activator content on the structure and electrochemical performances of porous carbons.The structure and energy storage performance of porous carbons were controlled by adjusting the activation ratio of KHCO3.Results showed that the more KHCO3 added,the higher the specific surface area and specific capacitance of porous carbons,and the best performance was obtained when the activation ratio was 6:1.Through pore structure analysis,the high efficiency of the KHCO3-activation was verified:when the proportion of K was the same,the specific surface area of porous carbon prepared by KHCO3-activation was close to that of KOH-activation,with the highest value of 1965.6 m2/g.In addition,the yield of porous carbon prepared by KHCO3-activation was significantly increased(about 50%),and the morphology of hydrochar was retained in the porous carbon.By introducing melamine into KHCO3-activation process,nitrogen-doped porous carbon was prepared with ultra-high specific surface area(2786.5 m2/g),developed pore structure,and good hydrophilicity.In the three-electrode system,the obtained porous carbon exhibted a high capacitance of 317 F/g at 1 A/g.In the two-electrode system,the KOH electrolyte-based device holded 99.4%capacity retention after 5000 GCD cycles at 10 A/g,and the Na2SO4 electrolyte-based capacitor exhibited a high energy density of 18.4 W h/kg.This work proposed an ecological friendly,universal,less corrosive and effecitive activation strategy,which provided a new method for the preparation of biomass-derived porous carbons(2)The preparation and study on electrochemical performances of Fe-decorated porous carbon sphere/nanosheet.Fe2(SO4)3 was introduced into hydrothermal process both as catalyst and dopant to prepare Fe-doped hydrochar,and then the Fe-decorated porous carbon sphere/nanosheet was synthesized by KHCO3-activation.Experiments were carried out to study the effect of Fe2(SO4)3 content on the morphology and physicochemical properties of hydrochar Results showed that with the increase of Fe2(SO4)3 concentration,the number of carbon spheres on the surface of the hydrochar increased first and then decreased.When the mass ratio of Fe2(SO4)3/bamboo was 1:10,the carbon-spheres with good dispersibility were produced,and the hydrochar showed carbon sphere/nanosheet architecture.After KHCO3-activation,the porous carbon maintained the morphology of hydrochar,and the effect mechanism of carbon-spheres on the improvement of electrochemical performance was discussed:the function of carbon-spheres was to prevent the aggregation of carbon nanosheets,and provide more space for electrolyte storing/diffusion to guarantee sufficient contact between electrolytes and porous materials,thereby making efficient use of the specific surface area.In addition,Fe-oxides were detected on the surface of porous carbon sphere/nanosheet.The introduced Fe-oxides provided pseudo-capacitance by the reversible faradaic reaction between Fe3+and Fe3+,then improving the capacitance.In the three-electrode system,the Fe-decorated porous carbon sphere/nanosheet had an ultra-high capacitance of 467 F/g at 0.5 A/g.The self-assembled symmetrical capacitor(KOH electrolyte)exhibited high capacitance(284 F/g),excellent cycling stability(only 0.2%capacitance loss after 5000 cycles)and excellent conductivity.The Na2SO4 electrolyte-based capacitor achieved the maximum energy density of 20.31 W h/kg at 225 W/kg.This work demonstrated that introducing metal ions into hydrothermal carbonization process to modify the hydrochar is an effective way to convert biomass into high-performance electrode materials(3)The preparation and study on electrochemical performances of Fe-N co-doped porous carbon spheres.Using corncob as carbon source and FeCl3 both as catalyst and dopant,independent,dispersed,and Fe-doped hydrothermal carbon spheres(particle size 2-5,um)with smooth surface were prepared by one-pot hydrothermal method.Subsequently,the Fe-N co-doped porous carbon spheres were synthesized by "KHCO3+melamine" activation.Experiments were carried out to study the influence and effect mechanism of the melamine content on the pore structure of porous carbon spheres.The structure and energy storage performance of porous carbon spheres were controlled by adjusting the amount of melamine.Results showed that the more melamine added,the higher the specific surface area and specific capacitance of porous spheres.When the mass ratio of melamine/hydrothermal carbon-spheres was 50%,porous carbon spheres exhibited the best performance,the specific surface area was up to 2305.7 m2/g.The effect mechanism of porous carbon-spheres on the improvement of electrochemical performance was revealed:porous carbon-spheres possess short ion diffusion distance and space-efficient packing structure,resulting in superior ion transfer kinetic and rate performance(87.4%retention up to 20 A/g).In addition,the Fe-oxides and nitrogen-containing functional groups on the surface of porous carbon-spheres could provide pseudo-capacitance and further increased the capacitance In the three-electrode system,the Fe-N co-doped porous carbon spheres illustrated 338 F/g at 1 A/g.In the two-electrode system,The KOH electrolyte-based capacitor exhibited excellent stability(102.7%capacity retention after 5000 cycles),extremely low Rat(0.59)and RS(4.54 Q).The Na2SO4 electrolyte-based capacitor presented a superb energy density of 18.60 W h/kg at 455 W/kg.This work proposed a novel method for preparing porous carbon-spheres derived from original biomass,which could provide new ideas for the research and development of high-performance biochar-based electrode materials.