Hydrophobic Association Hydrogels And The Applications Of Gel Electrolytes In Flexible Supercapacitors

Hydrogels,due to their unique hydrophilic 3d network structure,which can be modified and functionalized on demand,have become a major research hotspot in the field of biology and materials.the improvement of the mechanical properties of hydrogels and the application of functionalization are the focus of current scholars.At present,there are many ways to improve the mechanical properties of hydrogels,such as protein enhancement,nanocomposite,multi-cross-linking mechanism,molecular slip ring design and so on.Recently years,our laboratory has synthesized a novel hydrophobic association hydrogel?HA-gels?with high strength physical cross-linking.the hydrogel exhibits excellent mechanical properties,excellent flexibility,light transmittance,self-recovery and secondary processing properties.A typical HA-gel preparation method is as follows.Acrylamide as the main monomer.The hydrophobic monomers form micelle structure under the action of small molecule surfactant sodium dodecyl sulfate?SDS?.The HA-gel is prepared by free radical micelle polymerization.Nevertheless,hydrophobic monomers must be solubilized by SDS to form micelles,which largely limits the development and application of HA-gel.This paper adopts a simpler method:the surfactant fatty alcohol?dodecyl alcohol?polyoxyethylene ether?AEO-n?with amphiphilic properties is modified to make it a polymerizable hydrophobic monomer?AEO-n-AC?.Then,a novel hydrophobic association hydrogel was prepared without SDS,and its mechanical properties,the mechanism of gel formation and so on were discussed.AEO-n-AC with different hydrophilic EO fragment lengths?n=3,7,9,15,23?were used to synthesize HA-gels.The effects of hydrophilic EO fragment length and hydrophobic monomer concentration on the mechanical properties of gels have also been discussed.HA-gel has the best mechanical properties when n=7 or 9.Furthermore,transmission electron microscopy?TEM?of the micelles confirmed that the morphology of the micelles was the most uniform and the size was moderate.When the concentration of hydrophobic monomer is 2%or 4%,the mechanical properties of the gel are the best.When the two monomers of n=3 and n=23 were mixed,we examined the mechanical properties of the gels prepared under different compounding ratios.When the compounding ratio is 1:1,the obtained gel has the best mechanical properties.Similarly,TEM tests also proved that the micelle structure is most stable at this time.The compression test results of the HA-gel show that the length of the hydrophilic EO fragment has the most significant effect on the compressive properties of the gel,and the longer the EO fragment,the greater the compressive strength of the gel.The swelling behavior of this series of gels has also been investigated.At room temperature,AEO_n-gel showed a long swelling lifetime.The AEO₃-gel with short EO chains and the AEO₂₃-gel of long EO chains exhibit longer swelling life than the AEO₇-gel and AEO₁₅-gel because of different hydrophilic chain lengths on association micelle dissolution and re-association.At 40?,the maximum swelling degree and swelling life of the gel increased significantly with the increase of hydrophobic monomer concentration,and the gel life of the medium EO chain was lower.At 60?the swelling behavior of the gel changed.The swelling lifetime of the gel increases with the increased hydrophobic monomer content.While,the maximum swelling degree of the gel does not increase,even some gels begin to become smaller.This indicates that an appropriate increase in temperature will strengthen the hydrophobic interaction and promote the swelling of hydrogels.However,when the temperature is too high,the dissolution of the hydrogel will be accelerated due to the fast movement of polymer molecular chains and water molecules.Subsequently,we fabricated flexible supercapacitors with this hydrophobic association hydrogel.AEO-9-AC was used to prepare hydrophobic association hydrogel electrolytes.The hydrophobic association hydrogel electrolyte?HA-GPE?was obtained by facile recombination with LiClO₄ salts.The special network structure of the hydrogel endowed the HA-GPE ionic conductivity 31.1mS/cm,which was three times that of the LiClO4 solution.Moreover,flexible supercapacitors with HA-GPE as electrolyte and activated carbon as electrode also show excellent electrochemical performance.At current density of 0.5 A/g,the specific capacitance is 130.3 F/g.After5000 cycles,the capacitance retention is 90.1%,which is significantly better than that of liquid electrolyte and commercial separator supercapacitors.The tensile strength of the HA-GPE is only about 0.4 MPa,which is not enough to meet the actual requirements.to develop gel electrolytes that have better mechanical strength and can satisfy more flexible wearable devices,we introduced ion complexation of Fe³⁺and carboxyl groups as secondary cross-linking based on hydrophobic association networks to prepare double cross-linked hydrogel electrolytes?DC-GPE?.The tensile strength of the DC-GPE is up to 3.1 MPa.The ionic conductivity can reach 41.2 mS/cm,which is superior to the HA-GPE.Moreover,flexible supercapacitors prepared by DC-GPE have a specific capacitance of 178.5 F/g,energy density of 11.3 Wh/kg at current density of 0.5 A/g.After 2000 cycles,the capacitance retention rate can still reach 92.2%.Bending experiments also demonstrated that the electrolyte maintained almost unchanged electrochemical behavior in repeated bending experiments.