Controllable Preparation And Performance Study Of Biomass-based Heteroatom Porous Carbon

Biomass resource is one of the most widespread resources in nature,which has been used by people through direct combustion for a long time,resulting in low utilization and air pollution.Because biomass contains more carbon elements and can be used as the main raw material of biomass-based carbon,biomass-based carbon has a wide application prospect,so the preparation of carbon materials from biomass conversion is one of the ways to make effective use of biomass resources.This paper mainly carries out the following three aspects of research:?1?Four different heteroatom porous carbons?BPCn-T?were synthesized by using nitrogen-containing biomass chitosan as the carbon source and four activation methods:nitrogen activation,carbon dioxide activation,ammonia activation,potassium hydroxide activation.The influence law and mechanism among preparation technology,physical and chemical properties of materials,heteroatom composition,electrochemical performance are analyzed.The specific surface area of BPC_N2-900activated by nitrogen is 25 m²g^-1,the nitrogen content is 4.5%,mainly exists in the form of graphite nitrogen that can improve conductivity,and the oxygen content is 5.6%.Both nitrogen and oxygen mainly come from the raw materials themselves.At a current density of 0.1 A g^-1,the specific capacitances in alkaline and acid electrolytes are 149 F g^-1 and 121 F g^-1,respectively,but their rate performance is poor.Carbon dioxide activated BPC_CO2-800 has a specific surface area of 654 m²g^-1 and a mesoporous ratio of 8%.The nitrogen content is 6.3%,mainly pyridine nitrogen and graphite nitrogen.The oxygen content is 7.4%.At a current density of 0.1 A g^-1,the specific capacitances in alkaline and acid electrolytes are 164 F g^-1 and 216 F g^-1,respectively.The ammonia-activated BPC_NH3-800 has a specific surface area of 508 m²g^-1 and a mesopore ratio of10%.The nitrogen content is as high as 8.4%,of which pyridine nitrogen with high electrochemical activity accounts for 43.4%.The oxygen content is 5.6%.At a current density of 0.1 A g^-1,the specific capacitances in alkaline and acid electrolytes are 143 F g^-1 and 208 F g^-1,respectively.The specific surface area of BPC_KOH-700 activated by potassium hydroxide is up to 2566 m²g^-1,mesopore ratio is 34%,the nitrogen content is4.2%,mainly graphite nitrogen and pyrrole nitrogen,the content is 10.7%.At a current density of 0.1 A g^-1,the specific capacitances in alkaline and acid electrolytes are 235 F g^-1 and 389 F g^-1,respectively.It can be seen that the specific surface area of the sample after nitrogen activation is small,and BPC_N2-T as a supercapacitor electrode material cannot exert good performance.Carbon dioxide activation has a good pore-forming and hole-expanding effect.By adjusting the activation temperature,the pore size distribution can be effectively changed.Besides,carbon dioxide activation can reduce the loss of nitrogen and oxygen elements during the activation process.The nitrogen element mainly exists in the form of stable pyridine nitrogen and graphite nitrogen.The higher nitrogen content and the considerable specific surface area make BPC_CO2-T as an electrode material with better performance.Ammonia gas activation has a better pore-forming effect.The activation effect can be enhanced by increasing the activation temperature.The pore structure is mainly micropores.Ammonia gas activation has a better pore-forming effect.The activation effect can be enhanced by increasing the activation temperature,and the pore structure is mainly micropores.Ammonia gas activation is also an effective nitrogen doping method.And nitrogen mainly exists in the form of pyridine nitrogen and pyrrole nitrogen with higher activity,which improves the surface activity of carbon materials.The considerable specific surface area and high surface activity make BPC_NH3-800 have better electrochemical performance.The effect of pore-forming and hole expansion activated by potassium hydroxide is much higher than that of the other three activation methods,and it has a certain oxygen doping effect.The large specific surface area and considerable mesopore ratio make BPC_KOH-700 as a supercapacitor electrode material to play a higher mass specific capacitance and better rate performance.?2?It can be seen that nitrogen doping is a very effective modification method,but the cost of nitrogen doping using ammonia activation is higher,and it causes environmental pollution,and directly using raw materials with higher nitrogen content is also a kind of modification.An effective method for preparing nitrogen-doped porous carbon.HYC-T and FDC-T are synthesized from colloid prepared by dissolving chitosan in glycine hydrochloride aqueous solution as carbon source.Both glycine hydrochloride and chitosan have higher nitrogen content.The colloid is pre-treated by hydrothermal and freeze-drying methods,and then a carbon dioxide activation method that can reduce the loss of nitrogen is used.After hydrothermal pretreatment,HYC-900 is irregularly spherical,with a specific surface area of 1231 m²g^-1 and a mesopore ratio of 19%.However,a higher activation temperature?900??is required.The nitrogen content is6.89%,mainly in the form of pyridine nitrogen and pyrrole nitrogen.The oxygen content is 7.98%.At a current density of 0.2 A g^-1,the mass specific capacitances in alkaline and acid electrolytes are 199 F g^-1 and 269 F g^-1,respectively,and the reasonable mesopore ratio makes it have better rate performance.After freeze-drying pretreatment,FDC-800has a fluffy and porous layered structure with a specific surface area of 760 m²g^-1 and a mesoporous ratio of 13%.The higher nitrogen content is 9.43%,which is also mainly pyridine nitrogen and pyrrole nitrogen.The oxygen content is 7.98%.At a current density of 0.2 A g^-1,the mass specific capacitances in alkaline and acid electrolytes are199 F g^-1 and 269 F g^-1,respectively.It can be seen from the above data that the nitrogen in HYC-T and FDC-T are mainly in the form of pyridine nitrogen and pyrrole nitrogen.Hydrothermal pretreatment combined with carbon dioxide activation can make HYC-900 have a larger specific surface area,but the nitrogen content is slightly lower.The structure of FDC-800 after freeze-drying pretreatment is fluffy and porous,making it able to reach a relatively low calcination temperature With good activation effect and high nitrogen content,FDC-800 still exerts better electrochemical performance when the specific surface area is lower than that of HYC-900.?3?In order to continue to explore the effect of doping other elements on the performance of porous carbon,the pinewood powder of agricultural and forestry waste was used as the carbon source,and microwave-assisted phosphoric acid activation was used.?Pn-T?.The P4-500 process parameters with better performance:microwave reaction time 11 min,calcination temperature 500?,optimal phosphoric acid impregnation ratio 4:1.The specific surface area of P4-500 is 929 m²g^-1,the proportion of mesopores is 63%,and the amount of phosphorus-doped is 0.94%.Phosphorus is mainly connected to the carbon skeleton in the form of P-O-C bridge oxygen bonds,which improves the electrochemical activity of P4-500 surface.The oxygen doping amount is 7.85%.the mass specific capacitance is 174.8 F g^--1at 0.2 A g,¹,and it has a high rate performance.It is an excellent electrode material for supercapacitors.Compared with the traditional phosphoric acid activation,the activation time is shortened effectively,and the damage to equipment corrosion is reduced.Lower calcination temperature is beneficial to reduce energy consumption.