Preparation Of Soybean Protein-reinforced Hydrogel Electrolytes And Application In Flexible Supercapacitors

Compressible solid-state supercapacitors are emerging as promising power sources for next-generation flexible electronics with enhanced safety and mechanical integrity.In recent years,many efforts have been devoted to designing compressible electrodes,but the use of liquid electrolytes or non-elastic solid electrolytes still largely limits the compressibility and capacitive performance of these devices.Thus,it is highly sought to explore novel solid electrolytes with excellent conductivity,high elasticity and fatigue resistance that can be utilized in solid-state supercapacitors with reversible compressibility and capacitive stability.Herein,a highly elastic and fatigue-resistant hydrogel with a honeycomb-like cellular structure is firstly reported,in which polyacrylamide（PAAm）chains are cross-linked to form a cellular structure,and soybean protein isolate（SPI）nanoparticles are coagulated around the polymer chains to form cell walls.The synergistic effect of SPI nanoparticles and PAAm chains endows the hydrogel with high elasticity,compressibility and fatigue resistance simultaneously.Subsequently,by introducing electrolytic salts into the hydrogel matrices,ion-conducting hydrogel electrolytes are obtained.Compressible quasi-solid-state supercapacitors are then constructed by using the hydrogel electrolytes and polypyrrole-coated carbon nanotubes（PPy-coated CNTs）electrodes.More significantly,the resultant device demonstrates reversible compressibility and high capacitance retention under multiple cyclic compressions,even working well under 80%strain for 1 000compression cycles without structural damage and electrochemical failure.The demonstrated hydrogel electrolytes make it possible for the conventional electrode materials,such as porous carbon,conducting polymers,metal oxides and their composites,etc.to meet the requirements of compressible supercapacitors.This work is a milestone for the design and application of hydrogel electrolytes towards advanced energy storage devices,and provides an effective way for high-value utilization of soybean protein.（1）A highly elastic and fatigue-resistant SPI-reinforced hydrogel.A SPI-PAAm hydrogel with a honeycomb-like cellular structure was fabricated by integrating SPI nanoparticles and PAAm chains,in which PAAm chains were cross-linked to form a cellular structure,and SPI nanoparticles were coagulated around the polymer chains to form cell walls.The as-fabricated hydrogel exhibited high compressibility and elasticity,being rapidly recovering to its original shape upon 90%compressive strain without structural collapse or damage during 10 successive uniaxial compression cycles.To investigate the relationship between structure and mechanical property of the hybrid hydrogel,a series of hydrogels were prepared by adjusting the mass ratios of AAm to SPI,and the effect of SPI content on hydrogels was analyzed.Specifically,SEM images show that SPI contents mainly determined the thickness of cellular walls,due to the coagulation of SPI nanoparticles around the PAAm chains;Cyclic compression tests at 90%strain reveal that the sliding friction between SPI nanoparticles and plastic deformation of SPI nanoparticles could effectively disperse the applied stress and dissipate energy,while the PAAm network served as permanent crosslinks to maintain the shape and elasticity of the hydrogel.The synergistic effect of SPI nanoparticles and PAAm chains endows the hydrogel with remarkable elasticity and toughness.Moreover,the as-prepared hydrogel possessed outstanding fatigue resistance,withstanding 100 compression cycles without fatigue damage.After 100 compression cycles at 20%,50%and 80%strain,the hydrogel maintained over 90%of its maximum stress,only experienced below 10%plastic deformation and suffered less than 0.3 energy loss coefficient,showing excellent cyclic compressible performance.These results indicate that the introduction of SPI nanoparticles significantly reinforced the mechanical property of the hydrogel,and the synergistic effect of SPI nanoparticles and PAAm chains endowed the hydrogel with high elasticity,resilience and fatigue resistance simultaneously.（2）A compressible quasi-solid-state supercapacitor based on the proton-conducting hydrogel electrolyte.A proton-conducting SPI-PAAm/H₃PO₄ hydrogel electrolyte was fabricated by dissolving phosphoric acid in SPI-PAAm hydrogel matrix with replacement method.The obtained hydrogel electrolyte exhibited favorable compressibility and elasticity,which could undergo cyclic compressions at 80%strain and display mechanical stability during10 compression cycles.Due to the fact that the replacement treatment did not change the internal structure of the original polymer matrix,the obtained hydrogel electrolyte possessed satisfied fatigue resistance,enduring 100 compression cycles without fatigue failure.The quasi-solid-state supercapacitor based on this hydrogel electrolyte demonstrated excellent electrochemical performance,delivering a specific capacitance of 229.85 F/g at 0.7 A/g with a high rate capability of 86.1%（from 0.7 to 11.2 A/g）,a maximum energy density of 18.31 Wh/kg and a maximal power density of 2 634.55 W/kg.In addition,the device could be intrinsically compressed to 80%strain without structural fracture and electrochemical failure,and showed the compression-induced capacitance enhancement property,achieving compressible integrity at a device level.These results indicate that an excellent compressible and conductive hydrogel electrolyte was obtained by introducing phosphoric acid in SPI-PAAm hydrogel matrix with replacement method;the compressibility of the hydrogel electrolyte could enable the supercapacitor to achieve compressible integrity and capacitive stability.（3）A reversible-compressible quasi-solid-state supercapacitor based on the lithium ion-conducting hydrogel electrolyte.A lithium ion-conducting SPI-PAAm/Li Cl hydrogel electrolyte was fabricated by dissolving lithium chloride salts in the precursor.Since lithium chloride salts exist in aqueous solution in the form of hydrated lithium ion and hydrated chloride ion,the introduction of lithium chloride did not affect the formation mechanism and internal structure of the original polymer matrix.The obtained hydrogel electrolyte exhibited high compressibility and elasticity,which could undergo cyclic compressions at 90%strain and remained more than 90%elastic recovery and 70%resilience during 10 compression cycles.This hydrogel electrolyte possessed remarkable fatigue resistance,maintaining the structure stability without fatigue damage after 1 000 compression cycles at 20%,50%and 80%strains.The quasi-solid-state supercapacitor based on this hydrogel electrolyte demonstrated excellent electrochemical performance,delivering a specific capacitance of 246.8 F/g at 0.3 A/g with a high rate capability of 62.5%（from 0.3 to 12 A/g）,a maximum energy density of 21.4 Wh/kg and a maximal power density of 2 580 W/kg,as well as remarkable cycling stability（～80%capacitance retention after 5 000 charge/discharge cycles）.The resultant device could be intrinsically compressed to 80%strain without structural fracture and capacitive failure,displaying the compression-induced capacitance enhancement behavior.More importantly,the device exhibited reversible compressibility under multiple cyclic compressions,respectively working well under 20%,50%and 80%strains for 1 000 compression cycles without sacrificing its capacitive performance.Furthermore,four serial devices could be compressed reversibly as one integrated unit and still powered a LED well under cyclic compression,achieving reversible compressibility at the device level.These results indicate that a highly resilient and ion conductive hydrogel electrolyte was obtained by dissolving lithium chloride salts in the precursor;the high resilience of the hydrogel electrolyte could enable the supercapacitor to achieve remarkable reversible compressibility and capacitive stability under cyclic compressions at a device level.（4）A compressible quasi-solid-state supercapacitor based on the ionic liquid-conducting gel electrolyte.An ionic liquid-conducting SPI-PAAm/EMIMCl gel electrolyte was fabricated by replacing water with EMIMCl ionic liquid.Mechanical properties of the obtained gel electrolytes were systematically investigated by adjusting the EMIMCl ionic liquid contents,MBAA crosslinker contents and SPI contents.The obtained gel electrolyte could undergo 100compression cycles at 20%,50%and 80%strain without fatigue damage,retaining over 80%maximum stress,experiencing below 15%plastic deformation and suffering less than 0.1 energy loss coefficient.The quasi-solid-state supercapacitor based on this gel electrolyte demonstrated excellent electrochemical performance,delivering a specific capacitance of 110.83 F/g at 1.8 A/g with a high rate capability of 74.5%（from 0.3 to 12 A/g）,a maximum energy density of 25.99Wh/kg and a maximal power density of 3 600 W/kg.Moreover,the device could be intrinsically compressed to 80%strain without structural fracture and electrochemical failure,and showed the compression-induced capacitance enhancement property.These results indicate that a highly conductive gel electrolyte with excellent mechanical property and widen potential window was obtained by replacing water with ionic liquid;the compressibility of the gel electrolyte could enable the supercapacitor to achieve compressibility and high capacitance retention.