Preparation Of Hierarchical Porous Carbon Derived From Characteristic Plant Materials And Its Performance In Supercapacitor

Supercapacitor is a promising energy storage device as it has many peerless advantages such as quick charging/discharging,high power density and long cycle life.Porous carbon has recieved much attention as it has stable physical-chemical properties,highly developed pore structures and the precursors are widely distributed.China has huge storage of biomass,and the price is cheap,which makes it an ideal precursor for porous carbon.However,there are wide varieties of biomass.The difference of strcuture and component of biomass makes the huge difference of the product properties.Besides,the low energy density of the electrode material is also one of the resitrcitions of the wide application of the supercapacitors.Based on the above problems,a selection method for precursor of hierarchical porous carbon is proposed.the selection of activation methods based on the component of plant precursor is proposed.The plant tissue was differenciated and selected by its structure and functional characterisitics.The effect of fabrication method on the structure and electrochemical performance of the product was explored,and the the preparation theory of biomass based hierarchical porous carbon was further improved.On this basis,a synchronous structural nitrogen-doping method based on hierarchical porous carbon was studied,and the mechanism was revealed.Four different woody plant-based porous carbon were prepared by phosphoric acid activation method,and the biomass precursors were three wood materials(pinewood,candlenut,cedarwood)and seaweed(gulfweed).The different pore-making results as well as the electrochemical perfromance of different precursor under same preparation technology were investigated.The results show that the phosphoric acid activation is suitable for the wood materials which is rich in lignin and cellulose.However,seaweeds have great difference with terrestrial plant.It has little lignin and cellulose,therefore,some part of the reaction doesn't work at low temperatures,which makes bad pore-forming effect.The low preparation temperature of phosphoric aicd activation and the introduction of phosphoric acid makes some functional groups retained in the porous carbon,which endowed the material with a certain pseudocapacitance.After phosphoric acid activation,three wood materials based porous carbon already have the electrochemical properties close to those commercial activated carbon.However,their specific surface area is still underdeveloped,which means the untilization rate of these precursors are low.Therefore,a certain means should be used to optimized the pore structures and the electrochemical performance.Secondary activation modification based on the four kind of porous carbon made by phosphoric acid acitvation was carried out.FeCl3 and KOH were used to reactivated the four porous carbons at high temperature,the changes of pore structure evolution and electrochemical properties of the four material under different impregnation ratios were investigated.The study shows that FeCl3 is a rather milder activator compared with KOH,and it shows relatively weak pore-making ability and weak improvement effect on the electrochemical performance.There is no obvious linear relationship between the specific capacitance of the material and the specific surface area of the material.However,the samples with higher specifc capacitance after reactivation shows significantly improved interal surface area,indicating that except for the developed pore structure,the interconnected channels are also curcial to the energy storage effect of the material.After etching by high temperature and the activate agent,most of the functional groups are destroyed,and the pseudocapacitance was greatly weakened.In order to establish a hierarchical porous structure with interconnected channels,four plant materials rich in parenchyma and vascular bundle are selected.KOH was used as activate agent,and carbonization-high temperature avtivation process was carried out to explore the change trend of the pore structure and electrochemical performance of four plant based porous carbon under differnet impregnation ratio.The emchanism of the tissue evolution during the preparation process is revealed.The dispersed arrangement of parenchyma cells makes the prcursor materials have certain pores,ant its low structural strength makes it form less layer graphite in the activation process,which is conductive to the improvement of electrical conductivity.Vascular bundles are the natura transport tissues of higher plants,which have certain structural strength,and the morphology of the vascular bundles can be preserved after activation.On this basis,the activation process enables the materal with highly developed channeals and pores,which greatly optimized the hierarchical pore structure.Meanwhile,a two-step aicd pickling process was introduced to reduce the resistance of the materialIn order to further improve the energy density of the materials,a new process about structural nitrogen-doping on the carbon skeleton with hierarchical pores was proposed.For conventional surface doping,the excessive functional groups can provide pseudocapacitance,but it also leads to the bad cycle performance.To overcome this problem,a method to dope nitrogen atoms deep into the carbon skeleton was proposed.KOH and melamine were used at high temperature for doping and pore-forming simultaneously.The influence of mixing method and mixing ratio of melamine on the pore structure and electrochemical performance were studied.The study shows that the melamine has the synergistic effect with KOH during the pore-forming process.In addition,melamine has cyclic structure,the molecule has similar structure to the carbon ring after deamination,which is easy to combine with carbon skeleton to form N-6 or N-Q structure.Both the two structure is conducive to the energy storage property.For the direct carbonization product of biomass,it has abundant N-5 structure,which is unfavorable to energy storage.However,the proportion of this structure shows obvious decrease after activation and doping.During the carbonization process,some defective structures in the carbon skeleton can combine with the amidogen removed from melamine and converted into N-6 structure at high temperature.The specific capacitance of the product has been effectively improved.It reaches 715 F g"1 at the current density of 1 A g-1,and it can even keep 526 F g-1 at the current density of 100 A g-1.The specific capacitance is mainly provided by electrical double layer capacitance.When the power density reaches 200 W kg-1,the energy density can be 118 W h kg-1.The capacitance retention reaches 98.28%after 5000 cycles of charge-discharge at the current density of 5 A g-1,showing good cycle stability.