Preparation Of Lignocellulosic Hydrogel Electrolyte And Its Application In Supercapacitors

Wood is one of the most abundant renewable resources in nature.Cellulose extracted from wood has unique structure,renewability,sustainability,biocompatibility,high Young’s modulus,high aspect ratio,and excellent stability in different media,so it is considered to be a good candidate for preparing hydrogels.Cellulose hydrogels are degradable,safe,non-toxic and have excellent mechanical properties,which can be widely used in point chemical energy storage,sensing,biomedicine and other fields.The rise of flexible wearable electronic devices has triggered a research boom in flexible supercapacitors.Hydrogels are ideal electrolytes for flexible supercapacitors because of their advantages of simple preparation,good flexibility and easy adsorption of conductive ions.So far,a large number of hydrogel electrolytes have been extensively studied by researchers.However,the high cost of traditional hydrogel polymer materials and their difficulty in regeneration and degradation limit their large-scale application.In addition,the mechanical stability of hydrogel electrolyte is also a key factor restricting its development.Based on this,this paper designed and prepared a series of hydrogel electrolytes with lignocellulose as the main raw material,and assembled flexible supercapacitors with activated carbon flexible electrodes and tested their electrochemical performance,exploring the possibility of lignocellulosic hydrogels applied in the field of electrochemical energy storage.The main results of this paper are as follows:（1）In order to explore the feasibility of safe and degradable hydrogel electrolytes,cellulose hydrogel electrolytes were prepared by one-pot method.A series of transparent and flexible cellulose hydrogel electrolytes were obtained by cross-linking at room temperature by controlling the ratio of epichlorohydrin and cellulose after low temperature dissolution of cellulose in alkali-urea system.The optimal hydrogel electrolyte has a compressive strength of411 KP and an ionic conductivity of 109.7 m S cm^-1.By assembling with activated carbon electrode,a flexible symmetric supercapacitor was successfully obtained.When the current density is 2 m A cm^-2,the area-specific capacitance of the supercapacitor is as high as 909.37m F cm^-2,and the energy density is 0.12 m Wh cm^-2 at 0.967 m W cm^-2 power density.Because the cellulose hydrogel has certain adhesion and the electrolyte and electrode can be well combined,the device can be charged and discharged normally under different bending angles.Meanwhile,the excellent mechanical properties make the supercapacitor can be charged and discharged normally even when loaded with heavy objects.This indicates that the supercapacitor assembled by cellulose hydrogel electrolyte has excellent flexibility and electrochemical stability.（2）In order to solve the problems of low elasticity and low swelling performance of traditional polyvinyl alcohol hydrogel electrolyte,this part uses cellulose to modify polyvinyl alcohol hydrogel electrolyte.A flexible and transparent cellulose-polyvinyl alcohol composite hydrogel electrolyte was prepared by one pot method.Compared with polyvinyl alcohol hydrogel,the compressive strength of the composite hydrogel is increased by about ten times,and the ionic conductivity of the hydrogel electrolyte is up to 164 m S cm^-1.The symmetrical flexible supercapacitor assembled by the hydrogel electrolyte has excellent electrochemical performance:when the current density is 2 m A cm^-2,it has a high specific capacitance of 1037m F cm^-2,the electrochemical behavior is better than that of traditional polyvinyl alcohol hydrogel electrolytes.In addition,due to the excellent mechanical properties and flexibility of the composite hydrogel,the assembled flexible supercapacitor can be bent at will,and the electrochemical behavior of the bending Angle from 30°to 180°has no obvious attenuation.The cellulose-polyvinyl alcohol hydrogel electrolyte has excellent mechanical properties,high ionic conductivity and electrochemical stability,which indicates that the doping of cellulose can effectively improve the performance of polyvinyl alcohol hydrogel electrolyte.（3）In order to explore higher properties of cellulose-based hydrogel electrolytes,cellulose hydrogel as the first network and polyacrylamide hydrogel as the second network were used to prepare cellulose-polyacrylamide interpenetrating network hydrogel electrolytes under mild conditions.The combination of flexibility and strength gives the hydrogel electrolyte abundant ion channels and excellent mechanical stability,effectively reducing the interface contact resistance with the electrode,and the ionic conductivity up to 131.4 m S cm^-1.The flexible supercapacitor assembled with hydrogel electrolyte has excellent energy storage performance:when the current density is 2 m A cm^-2,it has a high specific capacitance of 989 m F cm^-2.When the power density is 0.98 m W cm^-2,the energy density is 0.13 m Wh cm^-2.Stable cycling performance（81%capacity retention after 5000 charges and discharges）,electrochemical behavior is no different from that of liquid electrolyte-assembled supercapacitors.In addition,due to the excellent adhesion and flexibility of hydrogels,supercapacitors can better resist delamination and deformation in the process of external forces,and supercapacitors can cope with deformation and compression from various angles while maintaining stable electrochemical behavior.In conclusion,cellulose hydrogel electrolytes,cellulose-polyvinyl alcohol hydrogel electrolytes and cellulose-polyacrylamide interpenetration network hydrogel electrolytes were successfully prepared using Wood-based Cellulose in this work,and were successfully applied to flexible supercapacitors,verifying the application potential of renewable lignocellulose in flexible energy storage devices.