Study On High-performance Supercapacitors Based On Biomass-derived Carbon Materials And Their CoSe₂ Composites

Resulting from the high power density,long cycling life and high safety,supercapacitors exhibit great application prospects in the medical,new energy and military fields.However,the low energy density restricts their further development.Therefore,the key of current study on supercapacitors is to enhance the energy density.As the core components of supercapacitor,electrode materials plays a crucial role in their capacitive performance.Since electric double layer capacitors（EDLCs）are the mainstream capacitor products in the market and their electrode active materials are porous carbons,it is of importance to prepare carbon electrode with the high capacitance performance for enhancing the energy density of supercapacitors.Furthermore,it is also effective for improving the energy density of supercapacitors by constructing asymmetric supercapacitors（ASCs）with a porous carbon electrode and redox-active material-based electrode.Although among various redox-active electrode materials,CoSe₂ attracts extensive attention due to the high theoretical specific capacitance and reactivity,its relatively poor conductivity limits the power density of ASCs to some extent.Therefore,in order to further enhance the energy density of supercapacitors,it is important to compound carbonaceous materials with high conductivity and construct structures conducive to the rapid transport of electrons and ions for CoSe₂.Among various precursors,biomass is considered to be one of ideal precursors for developing various functional carbonaceous materials due to its abundant sources,environmental friendliness,diverse internal structures and strong controllability.Therefore,this thesis first adopt walnut shell as a precursor to prepare a sub-1 nm pore-rich carbon with the high capacitance performance combining the low-temperature pre-treatment and high-temperature activation.Meanwhile this thesis uses silk fibroin protein-derived carbon fibers to optimize the capacitance performance of CoSe₂,thereby constructing a supercapacitors with high energy density and power density.The detail research contents are as follows:（1）Sub-1 nm pores can lead to an anomalous increase in the capacitive performance for carbon electrode,but the current approaches of preparing carbonaceous materials with sub-1 nm pores still suffer from some problems including the relatively low sub-1 nm pore content,complicated preparation process,low yield and high cost.In this regard,this work adopt walnut shell as a precursor to prepare porous carbon with different sub-1 nm pore contents(PC_Ar4,SNPC_Air3,SNPC_Air4 and SNPC_Air5)by controlling the functional groups and defects of the intermediate depending on adjusting the low-temperature pre-treatment condition followed by activated at a high temperature.results reveal that among all the materials,SNPC_Air4 material delivers the highest sub-1nm pore content with the volume and volume percentage up to 0.57 cm^-3 g^-1 and 58.4%,respectively.Benefiting from its abundant sub-1 nm pores,a supercapacitor made from the electrode material exhibits a high initial specific capacitance of 298.7 F g^-1 at 1 A g^-1and a long-cyclic life with 94%capacitance retention over 10000 cycles at 10 A g^-1 in KOH.Meanwhile it also accomplishes a high energy density of 12.6 Wh kg^-1 at a power density of 36 kW kg^-1.（2）Because ASCs possess higher energy density than that of EDLCs,it is of importance to construct a redox-active material with high capacitance performance matching SNPC_Air4 carbon electrode for further enhancing the energy density of supercapacitors.In light of this,this work first prepare a material of Mn-doped CoSe₂by a hydrothermal reaction followed by subsequently selenized,and then adopts silk fibroin protein as the carbon source to synthesize a fibrous composite of Mn-CoSe₂@CNFs by catalyzing at a high temperature.Because the fibrous structure of Mn-CoSe₂@CNFs material can inhibit the agglomeration of CoSe₂ nanoparticles and shorten the diffusion path of ions and electrons within the electrode,it exhibits a high specific capacitance of 1.81 F cm^-2 at 4 mA cm^-2.Even at as high as 20 mA cm^-2,it still demonstrates a specific capacitance of 1.352 F cm^-2.Furthermore,an ASC made from it and above SNPC_Air4 carbon electrode exhibits a high energy density of 210.76μWh cm^-2 at a power density of 1550μW cm^-2 and long cycling life with a high capacitance retention of 90%over 10000 cycles at a current density up to 20 mA cm^-2,illustrating its excellent capacitance performance.（3）MXenes possesses good conductivity and unique layered structure,and embedding carbon fiber between its layers can further accelerate the transport of electrons and ions.In light of this,this work first combine hydrothermal reaction and selenization treatment to embed Mn-CoSe₂ between the interlayers of MXene,and subsequently implant carbon fibers between their interlayers using different amounts silk protein as the carbon source by a high-temperature catalysis strategy,thereby preparing composite electrode materials of MX/Mn-CoSe₂/CNFs-1,MX/Mn-CoSe₂/CNFs-2 and MX/Mn-CoSe₂/CNFs-3 for ASCs.Results reveal that among all the electrodes,MX/Mn-CoSe₂/CNFs-2 electrode with the best capacitance performance delivers specific capacitances up to 3.428 F cm^-2 and 2.907 F cm^-2at 4 mA cm^-2 and 20 mA cm^-2,respectively.Meanwhile,an ASC made from it and above SNPC_Air4 carbon electrode exhibits a high energy density of 306.76μWh cm^-2 at a power density up to 3874.9μW cm^-2 and long cycling life with a high capacitance retention of 90%over 10000 cycles at a high current density of 20 mA cm^-2.In conclusion,this thesis aims to design supercapacitors with high power density and energy density.First,the waste biomass walnut shells are used as precursor to synthesize sub-1 nm-rich carbonaceous material which is applied to EDLCs showing the excellent capacitance performance.And then silk fibroin is used as precursor to design the structures of carbon fibers and carbon fibers supporting the MXene layers,respectively for gradually optimizing the pseudocapacitance performance of CoSe₂,thereby further improving the energy density for supercapacitor.This thesis provides theoretical guidance for the preparation of advanced supercapacitor electrode materials based on biomass-derived carbon materials.