Cu²⁺-based Redox Additive Aqueous Electrolyte For Supercapacitors

In recent years,compared with lithium-ion batteries and other energy storage devices,supercapacitors have higher power density and good cycle life,which has maintained good research value.In the application process,the low energy density is the main reason that restricts its development.Compared with the methods of modifying electrode materials,electrolyte modification with simple process and obvious performance improvement has attracted more and more attention.Compared with the organic and the ionic electrolyte,the aqueous electrolyte has the advantages of high ionic conductivity,low impedance value,high ion concentration,no pollution and environmental protection,and water is a good solvent and it can dissolve polyvalent ions to store energy by redox reaction at the interface.At present,most of the redox additive system in aqueous electrolyte can effectively increase the specific capacitance but there is a cross-diffusion problem in the process of charge and discharge,which causes serious self-discharge behavior.Cu2+ can provide the faraday capacitance and the redox product Cu will deposit on the surface of electrode during the reaction.The deposited Cu avoids the cross-diffusion of the whole system and solves the problem of self-discharge.However,the problem of low cycle life caused by large Cu deposition still needs to be solved,and on this basis,the capacitance still needs to be further improved.Based on this,the paper solves the problem of large Cu deposition in a new design,and further improves the electrochemical performance of the system,and expands its application in gel electrolytes.(1)The surfactant tetrabutylammonium bromide(TBAB)was introduced into the Na2S04/CuCl2 electrolyte.Through the interaction between TBAB and Cu2+,the energy storage on electrode interface and the cycle stability of the supercapacitor have been improved.The experimental results show that the supercapacitor exhibits a specific capacitance of 441.8 F g-1 at a current density of 1 A g-1,and the specific capacitance increase of 164%compared with the Na2SO4/CuCl2 system.After 2,500 cycles,the capacitance retention rate of the entire system was 81.8%.(2)A flexible gel electrolyte was prepared by complexing a Na2SO4/CuCl2/TBAB electrolyte with a polyvinyl alcohol(PVA)matrix and a bacterial cellulose matrix(BC),respectively.The results show that the specific capacitance of PVA and Na2SO4/CuCl2/TBAB assembled gel electrolyte supercapacitors is 398.5 F g-1 at a current density of 0.5 A g-1,which is improved to 79%of the comparative Na2SO4/CuCl2 system.The PVA-based gel electrolyte has an elongation at break of 107.10%.In addition,bacterial cellulose(BC)with excellent liquid absorption and mechanical properties is also widely used in supercapacitor gel electrolytes.The strength of the BC matrix gel electrolyte prepared by conventional hot pressing reaches 2.93 MPa,and the specific capacitance of the gel electrolyte is further increased with the assembly of Na2SO4/CuCl2/TBAB.(3)PVA film exhibits comprehensive performance and advantages.On this basis,we introduced the fast ion conductor Cu2-xSe into the gel electrolyte assembled by PVA and Na2SO4/CuCl2 to promote the transport of Cu2+in the gel system.Throughout the system we explored the optimum concentration of Cu2-xSe mixed in PVA is 2 wt%and bulk impedance of the PVA composite gel electrolyte is decreased from 4.55 ? to 1.16 ?.At a current density of 0.5 A g-1,the specific capacitance of the PVA-2 wt%Cu2-xSe sample is 359.28 F g-1,which is 61.5%higher than pure one.