Functional Design And Study Of Cellulose-based Materials In Zinc-ion Hybrid Supercapacitors

As industries such as new energy and portable electronic devices continue to develop,higher requirements are being put forward for energy storage devices in terms of capacity,energy/power density,service life,and device functions.Aqueous zinc-ion hybrid supercapacitors,as a hybrid energy storage device that integrates the advantages of batteries and capacitors with guaranteed economy and safety,are expected to play a major role in transportation and wearable devices.Zinc-ion hybrid supercapacitors typically comprise a capacitive cathode electrode,a zinc salt electrolyte,and a metallic zinc anode electrode.Among these components,the electrolyte and capacitive electrode are key to achieving high capacity,high energy density,and high power density in the device.For the electrolyte,hydrogels containing zinc salts can maintain the high performance of energy storage devices while further enhancing their safety and supporting flexible and wearable applications.However,most of the current hydrogel electrolytes use petroleum-based polymers,which are difficult to biodegrade and pose a potential threat to the ecological environment.For the electrode,carbon materials offer a capacitance advantage unmatched by other materials,ensuring the stability of energy storage devices during high-power and long-cycle usage.In zinc-ion hybrid supercapacitors,carbon materials need to consider the pore and doping structure suitable for the storage of zinc salt ions.In addition,the voltage window of aqueous energy storage devices is usually narrow,which is not conducive to the energy and power density.Furthermore,the metal anode electrode may meet by-products,dendritic crystal etc.,which will affect the stability of the device.Biomass is a renewable resource with wide global distribution and abundant stocks.Developing biomass is essential for promoting sustainable human development.Biomass materials are environmentally friendly and cost-effective,but their functionalization and efficient preparation present challenges in actual production.In this thesis,the hydrogel electrolytes（cellulose hydrogel）and electrode materials（biomass-derived porous carbon）suitable for zinc-ion hybrid supercapacitors are designed based on cellulosic biomass,realizing a high-performance and multifunctional energy storage device.Cellulose hydrogels overcome the limitations of natural polymer hydrogels in zinc-salt adaptability,ionic conductivity,and mechanical strength.Biomass-derived porous carbons achieve efficient storage of zinc salt ions.The thesis proposes a highly efficient strategy with full-component utilization of biomass for the preparation of functional materials.Further exploration of the device’s functions and energy storage mechanism in zinc-ion hybrid supercapacitors has been conducted.The specific researches are carried out from four aspects:electrolyte,electrode material,preparation technology and exploitation of device function.In terms of the electrolyte,a synthesis strategy of hydrogel electrolyte is designed with the use of cellulose and a highly concentrated Zn Cl₂.High concentration of Zn Cl₂ can dissolve cellulose,and the dissolved cellulose can be cross-linked in solution to form a mechanically stable hydrogel.Using the cellulose hydrogel as the electrolyte,the zinc-ion hybrid supercapacitor achieves a wide stable voltage window of 2.0 V.Combined with microporous activated carbon,the device reaches the capacity of 193 m Ah g^-1,the energy density of 192 Wh kg^-1 and the power density of 16976 W kg^-1.The Zn²⁺provided by the cellulose hydrogel achieves efficient deposition and exfoliation on the Zn metal anode,avoiding the formation of dendrites and by-products.The cellulose hydrogel has a high ionic conductivity of 47.7 m S cm^-1 at a low temperature of-20 ^oC,endowing the device with cold-resistant and flexible energy storage properties.In terms of the electrode material,the porous carbon with abundant mesopores,macropores,and oxygen doping can be obtained by using cotton pulp cellulose as carbon source and Zn Cl₂molten salt as both template and catalytic agent.The performance of the porous carbon in zinc-ion hybrid supercapacitors is investigated by using different aqueous zinc salt electrolytes（Zn Cl₂,Zn SO₄ and Zn（CF₃SO₃）₂）.The cellulose-derived porous carbon demonstrates excellent properties for the storage of Zn Cl₂ salt ions,including low impedance,high reversibility and a high specific capacitance of 357 F g^-1.The integration of cellulose-derived porous carbon with the cellulose hydrogel electrolyte（containing highly concentrated Zn Cl₂）has resulted in a significant breakthrough in the electrochemical performance of the device.The zinc-ion hybrid supercapacitor exhibits a specific capacity of 247 m Ah g^-1 and an energy density of 243 Wh kg^-1,which is comparable to battery.The device also has a stable cycle capacity of up to 20,000 times.Lignocellulosic biomass has a high proportion in cellulosic biomass.The thesis proposes a strategy for simultaneously preparing porous carbon and hydrogel electrolyte from lignocellulosic biomass.When lignocellulosic biomass is hydrothermally treated in a Zn Cl₂ solution,its components,mainly cellulose,lignin,and hemicellulose,are separated into residues and dissolved substances.The solid residues can be carbonized directly to produce porous carbon,while the dissolved substances,which includes cellulose,undergoes cross-linking in the filtrate to form a hydrogel.Zn Cl₂ can serve two purposes in biomass processing:it can dissolve biomass components,and it can act as a molten salt agent to carbonize and activate biomass sources.The thesis has systematically investigated the effects of hydrothermal processes on the physical and chemical properties of porous carbons and hydrogel electrolytes.Both porous carbons and hydrogels prepared by this strategy exhibit outstanding electrochemical performance in zinc-ion hybrid supercapacitors.The device assembled with these materials shows a wide voltage window and long cycle life,and achieves a high capacity of 229 m Ah g^-1 and an energy density of 226 Wh kg^-1 at a power density of 429 W kg^-1,while retaining low-temperature energy storage characteristics.This strategy enables complete utilization of all components of lignocellulosic biomass with the specific functional transformations,and is widely applicable.The cellulose hydrogel has exceptional ion transport ability and high concentration of Zn Cl₂salt ions,and thus the assembled carbon-based zinc ion hybrid supercapacitor is capable of continuous and repeated discharging in off-grid conditions.After the device is discharged to a low potential,the voltage and capacity can be significantly and spontaneously recovered.The device can obtain a discharge capacity of 827.9 m Ah g^-1 with 20"self-charging"cycles,and the discharge process maintain a discharge power matching the capacitive process.The"self-charging"characteristic is related to the storage mechanism of Zn²⁺in the cathode of zinc-ion hybrid supercapacitor.