Microwave-assisted Efficient Synthesis And Electrochemical Properties Of Biomass-derived Hierarchical Porous Carbon

The energy density of supercapacitors depends on the charge storage capacity of the electrode material and the operating voltage of the electrolyte.The development of electrode materials with high charge storage capacity and electrolytes with high operating voltage is the key to significantl y increase the energy density of aqueous supercapacitors.Currently,porous carbon has become a mainstream supercapacitor electrode material due to its large specific surface area,high porosity and low cost advantages.The cost of porous carbon electrode materials can be further reduced by preparing porous carbon electrode materials from cheap and easily available and renewable biomass.Carbonization-activation under inert atmosphere protection is the usual method for the preparation of biomass-derived activated carbon,which requires a cumbersome and time-consuming method under an inert atmosphere.In this thesis,we developed a series of microwave carbonization methods using biomass as the carbon source to explore the conversion of biomass into hierarchic al porous carbon with high specific surface in a few minutes under a large atmosphere,while cooperating with the acidic and alkaline high-voltage aqueous electrolytes developed by the research group to substantially increase the energy density of aqueous supercapacitors without sacrificing cycle stability and excellent multiplier performance.（1）Phosphorus-doped hierarchical porous carbon（PHPC）was successfully obtained by microwave carbonization of waste passion fruit shell for 1～5 minutes in an ambient atmosphere using a mixed acid of polyphosphoric acid（PPA）and concentrated sulfuric acid（CSA）as a new composite microwave absorber,and trifluoroacetic acid（TFA）electrolyte developed by the research group was used to expand its operating voltage.The obtained PHPC material has a specific surface area of up to 1858.2 m² g^-1,can be stably charged and discharged in TFA electrolyte within a wide voltage window of-1.0 to 0.5 V,and exhibits a specific capacitance of up to297.1 F g^-1 at a current density of 1 A g^-1.The capacitance retention rate of the assembled supercapacitor devices reached 77.0%when the current density was increased from 1 A g^-1 to 20 A g^-1,and the capacitance retention rate reached 94.9%after 30,000 charge/discharge cycles,demons trating its excellent multiplicity and cycling performance.By using microwave carbonization method and TFA electrolyte,the energy density of acidic aqueous supercapacitor has been significantly increased while reducing the cost of preparation of activate d carbon electrode materials.（2）Oxygen-enriched hierarchical porous carbon（HPC-W_T）was successfully obtained by microwave carbonization of sycamore fruit for 2～4 minutes in an ambient atmosphere with KOH as microwave absorber and activator.While the working voltage of aqueous electrolyte was expanded by using the Ch OH/KOH mixed electrolyte（MAA）developed by the research group.The prepared HPC-W_T material has a specific surface area of up to 1523.0 m² g^-1,can be stably charged and discharged in MAA mixed electrolyte within a wide voltage window of-1.0 to 0.3 V,and exhibits a high specific capacitance of 318.8 F g^-1 at a current density of 1 A g^-1.The symmetric supercapacitor devices assembled from the prepared HPC-W_T material and MAA electrolyte can operate at voltages up to 1.3 V and provide energy densities up to 13.0Wh kg^-1 at a power density of 162.2 W kg^-1,which is nearly 1.5 times higher than that of the devices using conventional KOH electrolyte.The capacitance retention rate was as high as 77.7%when the current density was increased from 0.5 A g^-1 to 20 A g^-1,and the capacitance retention rate was up to 90.6%after 30,000 charge/discharge cycles,showing its excellent multiplicity and cycling performance.By using microwave carbonization method and MAA electrolyte,the energy density of alkaline aqueous supercapacitor was significantly increased while reducing the cost of preparation of activated carbon electrode material.（3）Semi-graphitized hierarchical porous carbon（Se G-HPC-W_T）was successfully obtained by microwave carbonization of sycamore fruit for 3～4 minutes in an ambient atmosphere using K₂Fe O₄ as microwave absorber,activator and catalyst.While the working voltage of aqueous electrolyte was expanded by using Ch OH/KOH mixed electrolyte（MAA）developed by the research group.The prepared Se G-HPC-W_Tmaterial has a specific surface area of up to 1322.2 m² g^-1,can be stably charged and discharged in MAA mixed electrolyte within a wide voltage window of-1.0 to 0.3 V,and exhibits a specific capacitance of up to 256.7 F g^-1 at a current density of 1 A g^-1.The symmetrical supercapacitor devices assembled from the prepared Se G-HPC-W_Tand MAA electrolytes can operate up to 1.3 V and provide an energy density of nearly9.2 Wh kg^-1 at a power density of 162.1 W kg^-1,which is nearly 1.4 times higher than that of the devices using conventional KOH electrolyte.The initial specific capacitance of nearly 72.8%is retained when the current density is increased from 0.5 A g^-1 to 20A g^-1,and the capacitance retention rate reaches 86.8%after 20,000 charge/discharge cycles,showing a more satisfactory multiplication and cycling performance.