Construction,Structure And Performance Of Flexible Supercapacitors Based On Morphology Of Biomass Materials

Supercapacitor（SC）is considered as one of the potential energy storage devices because of its high power density,long cycle life and low cost,and is widely used in the field of portable/wearable electronic equipment.As we all know,electrode material is one of the key factors affecting the performance of SC.Common electrodes are generally made of synthetic materials,which are often plagued by complex synthesis process,low yield,high cost,unstable structure and incompatible environment.Biomass materials have low cost,wide source,environmental friendliness and microstructure,which are beneficial to the design of flexible and wearable SCs.In this paper,a series of biomass-based flexible SC were prepared by making full use of the microstructure and performance characteristics of biomass materials,including porosity,structural stability,flexibility,shape design and shape recovery,and the practicability of biomass-based flexible wearable SC was explored.The main contents of this paper are as follows:1.Cotton cloth has the characteristics of low cost,environmental protection and stable structure.The micro-nano porous structure woven by micro-sized cellulose fibers makes it has excellent flexibility and can be used to prepare flexible SC electrodes.How to improve the conductivity of cotton cloth while maintaining its structure and flexibility is a problem to be solved in order to realize its application in SC.In this study,cotton cloth was successfully transformed into flexible conductive material（CCF）by simple high-temperature heat treatment,and nickel hydroxide（Ni（OH）₂）was loaded on its surface by electrochemical deposition method,which improved the electrochemical performance of CCF.The symmetric SCs of Ni（OH）₂@CCF//Ni（OH）₂@CCF system prepared and assembled in this paper can run stably at 1.4 V,and its energy density is as high as 35.78 Wh kg^-1 under the power density of 0.35 k W kg^-1.The SCs also shows excellent flexibility,and can still maintain its electrochemical performance even when it is bent at 180°,which can still maintain 102.61%of the initial specific capacitance.In this work,flexible SC based on cotton cloth was successfully prepared.2.Flexible SC fabric has been widely studied because of its great application prospect in wearable field.However,with the gradual popularization of flexible wearable devices,people put forward more requirements for the aesthetics and shape diversity of devices.Therefore,the structural invariance of SC electrode in the manufacturing process limits its further development in the field of flexible wearable.Single micro-sized silk has high strength,while silk fabrics woven from silk have higher strength,flexibility and abundant micro-nano porous structure.In this study,silk was transformed into conductive carbon fabric（CS）by carbonization at high temperature,and the structure,flexibility and strength of silk fabric were retained.A biomass-based SC electrode material with shape design was successfully prepared,which can meet the different shape requirements of most flexible wearable devices.Manganese dioxide nanoparticles were grown on the surface of CS by high temperature hydrothermal method to improve the electrochemical performance of CS.The CS-Mn O₂-10//CS-Mn O₂-10 aqueous symmetric SC assembled shows excellent energy storage performance,and can maintain 101.54%of the initial capacitance after 10,000 cycles,showing excellent electrochemical stability.In addition,the SCs prepared in this work can be shape-designed.SCs with different shapes and patterns are prepared and assembled by simple cutting.After connecting SCs with different shapes in series,it can supply power for simple watch calculator.This work provides a new method for shape design of SC.3.In recent years,portable and wearable electronic devices have become more and more popular.However,they are inevitably damaged to varying degrees due to various local stresses,and even lead to permanent mechanical deformation of the equipment over time.Juncus effuses（JE）is a perennial herbaceous flowering plant with unique natural three-dimensional reticular hollow tubular structure and fiber-like appearance.In this work,it is found for the first time that the pretreated JE has excellent shape recovery performance,which enables JE to be used to prepare SC electrodes which can resist various external deformations at the same time.In order to improve the electrochemical properties of JE,carbon nanotubes（CNTs）and polyaniline（PANI）were loaded on JE surface in turn,and JE-CNT-PANI electrode was obtained,eventually.Furthermore,JE loaded by CNTs and PANI still maintains its three-dimensional hollow tubular structure and excellent shape recovery performance.Even after severe deformation,the JE-CNT-PANI electrode can return to its original shape very quickly once it is immersed in water/electrolyte again,and has strong resistance to external stress.In addition,JE-CNT-PANI also shows excellent stability,and can still maintain 112.4%of the initial capacitance even after100 shape recovery in electrolyte.This work provides a method for preparing flexible wearable SC which can resist various external deformations at the same time.4.One-dimensional SC has attracted wide attention because of its small size,light weight,knittability,strong deformability and potential application in flexible wearable electronic devices.Based on the limitations of one-dimensional SCs in raw materials,the design idea of one-dimensional strip flexible wearable SC is put forward.CNTs/CNF-PANI（cellulose nanofibers,carbon nanotubes and polyaniline mixed membranes）electrode was prepared by vacuum filtration and rapid in-situ polymerization.The excellent mechanical properties of nanocellulose and carbon nanotubes and the strong force between them give the electrode excellent flexibility and strength.CNTs/CNF-PANI//CNTs/CNF-PANI water-system symmetric SC（SASC）assembled by CNTs/CNF-PANI directly as electrode has a specific capacitance of 244.8 F g^-1 at a current density of 2.0 A g^-1 and an energy density of34.0 Wh kg^-1 at a power density of 2.0 KW kg^-1.When PVA-H₂SO₄ gel is used as electrolyte,the SASCs shows a high specific capacitance of 160.0 F g^-1 at a current density of 5.0 A g^-1 and excellent cycle stability（the initial capacity retention rate can reach 92.86%after 1000 cycles）.In addition,in this work,CNTs/CNF-PANI all-fiber mixed film is cut into strips with certain size,and directly used as electrode to assemble all-solid-state strip SC（ASSC）and the ASSC shows the same flexibility,knittability and wearability as the yarn type SC.Four ASSCs are connected in series and woven in fabric to supply power for simple electronic watches.Finally,a self-powered system is designed by connecting four series ASSCs with solar cells.This self-powered system can obtain clean and sustainable energy from the sun,and power watches,showing great potential in the actual possession of flexible wearable electronic devices.This work provides a green strategy for the rational design and preparation of 1D SCs with excellent electrochemical performance.5.Most flexible SCs are usually assembled by two independent flexible electrodes with a separator in the middle.Components of these SCs are easy to slip during the application,which leads to performance degradation or even failure of equipment.Commercial water-based cellulose mixed membrane has a porous structure composed of three-dimensional skeleton,and its skeleton is also rich in nanopores.This microstructure make it show excellent ionic conductivity,electrochemical stability,thermal stability,mechanical properties,hydrophilicity and moisture retention,uniform pore structure and high porosity,so it is an ideal material for supercapacitor separator.In this work,the commercial water-based cellulose mixed membrane was used as the membrane,and PANI anode material and Fe₃O₄ cathode material were integrated on both sides of the porous surface by vacuum filtration method without using adhesive,additive and metal current collector,so that the all-in-one asymmetric SC（ASC）was prepared.This all-in-one design avoided the risk of slippage among the components.In this work,cellulose nanocrystals（CNC）were used as dispersants for anode and cathode material dispersions.The working potential window of the finally prepared integrated ASC can reach 1.8 V.In a non-corrosive lithium chloride electrolyte,when the current density is 2.5 m A cm^-2,the compact integrated ASC exhibits a high surface capacitance of 13.61 m F cm^-2.It is worth noting that two integrated ASCs can be integrated and connected in series to obtain high output voltage（3.6 V）.Meanwhile,two ASCs connected in series can be knitted on the wrist and still able to power a watch,indicating their excellent application prospect in wearable electronic devices.The compact all-in-one structural design of thin film flexible energy storage device can meet the structural stability requirements of flexible wearable energy storage devices.To sum up,a series of high-performance flexible wearable SC devices,including flexible,shape-recoverable and knitted SCs,were prepared by using low-cost and environment-friendly natural materials combined with their own structural characteristics and performance advantages.At the same time,the integrated ASC of design and preparation improves the structural stability of SC in use.In this paper,the properties of biomass-based electrode materials and the electrochemical properties and flexible wearable properties of SC assembled from electrode materials were prepared and studied by making full use of the structural characteristics of biomass materials,providing new ideas and technologies for sustainable biological resource design and manufacturing of new flexible wearable electronic devices.