Preparation Of Soy Protein Isolate Polymer Electrolytes And Study On Performance Of Supercapacitors

Supercapacitor has attracted much attention due to their high energy density,fast charge-discharge rate,and long cycle life.Electrolyte is the main component of supercapacitor,and the state and performance directly affect the electrochemical performance and safety of supercapacitors.The gel polymer electrolyte(GPE)can serve as both a separator and an electrolyte for a supercapacitor because it has similar ionic conductivity to a liquid electrolyte and has similar flexibility to a solid electrolyte.GPE mainly consists of polymer matrix,plasticizer and supporting salt.The polymer matrix used for preparing GPEs should have good electrolyte affinity and can form a membrane to provide suitable mechanical strength and stability for GPE.As a kind of renewable,environmentally friendly biomass material,soy protein isolate(SPI)has good hydrophilicity and can form a membrane,which can be used as a polymer matrix for the preparation of GPE.However,SPI is difficult to be dissolved,and the brittleness and poor flexibility of the SPI membrane limits its application in electronic devices.Therefore,in this thesis,a series of SPI-based membranes were prepared by plastic modification,crosslinking modification and constructing semi-interpenetrating networks.The effects of different modification methods on the performance of electric double layer supercapacitors with modified SPI membranes were systematically investigated.Firstly,a series of flexible SPI membranes were prepared with triethanolamine(TEA)as plasticizer.Gel polymer electrolyte(GPE)base on SPI could exhibit an ionic conductivity of up to 8.55×10-3 S cm-1.The electric double layer supercapacitor(EDLC-X)was then fabricated with activated carbon as the electrodes and GPEs prepared with modified SPI membranes saturated by 1 mol L-1 Li2SO4.The EDLC-X could be operating stably over a potential window of 1.5 V and exhibited excellent electric double layer capacitance characteristics.The supercapacitor EDLC-1.6 delivered a single electrode gravimetric capacitance of 113.44 F g-1,and an energy density of 8.86 W h kg-1,demonstrating an excellent electrochemical performance.However,the plasticized SPI membrane showed too soft with the liquid electrolyte and could not maintain a stable morphology in water for a long period of time.In order to improve the water resistance and mechanical strength of SPI membranes,a series of crosslinked SPI membranes were prepared with ethylene glycol diglycidyl ether(EGDE)as a crosslinker.The mechanical and water-resistance of crosslinked SPI membranes were greatly improved compared to those of the pure SPI membrane.The optimal crosslinked membrane with 20%EGDE showed a good comprehensive performance.A solid-state supercapacitor(S-EDLC-X)was fabricated with the GPE-E-X based on crosslinked SPI membrane and activated carbon electrodes.The electrochemical performance of S-EDLC-X decreased with the increasing content of crosslinked structure,but S-EDLC-20%still delivered a good capacitance retention rate with a small amount of crosslinked structure.The capacitance retention rate can be maintained at 72%when the current density increased from 1.0 A g-1 to 10.0 A g-1.Although mechanical properties and stability of GPEs based on SPI membranes were elevated due to the introduction of a crosslinked structure,the electrochemical performance of S-EDLC-X shows a downward trend.To enhance the electrochemical performance of the GPE,a series of modified SPI membranes with semi-interpenetrating network(Semi-IPN)were prepared by introduced a hydrophilic linear macromolecule(HEC)into crosslinked SPI system.Semi-interpenetrating network gel polymer electrolytes(Semi-IPN GPEs)based on SPI and HEC were prepared and the solid-state supercapacitors(SSC-X)were assembled by using Semi-IPN GPEs and activated carbon electrodes.The electrolyte affinity of the membranes based on EGDE crosslinked SPI is greatly improved when a small amount of HEC is added,which led to a significant increase in the ionic conductivity of the Semi-IPN GPE based SPI and HEC.The ionic conductivity of Semi-IPN GPE based SPI and HEC at room temperature can reach 1.125×10-2 S cm-1,which is far superior to that of the GPE based on crosslinked SPI.The flexible solid-state supercapacitor SSC-10%exhibited an optimal cycle stability and charge and discharge reversibility,and the overall electrochemical performance was superior to that of a supercapacitor assembled from a commercial aqueous PP/PE separator.The bending test shows that the SSC-10%exhibits excellent electrochemical performance and good flexibility at any bending angle.