Development And Application Of High-voltage Aqueous Electrolyte For Carbon-based Supercapacitors

The development of environmentally friendly and cost-effective sustainable energy storage devices has raised higher demands for the energy density of supercapacitors.High-performance supercapacitors require faster charging and discharging rates,higher energy and power densities,and longer service life.Another strategy to optimize energy density and increase capacitance is to expand the voltage window.The aqueous electrolytes have many advantages,such as smaller ion radii,low viscosity,and high conductivity,which can meet the rapid charging and discharging requirements of supercapacitors.In this paper,biomaterials with a wide range of raw materials are used as carbon sources to prepare hierarchical porous carbon electrode materials,and their morphology,pore size distribution,and surface elements are finely optimized to improve the charge storage capacity of carbon electrode materials.At the same time,a new high-voltage water-based electrolyte is developed,which can significantly increase the energy density of water-based supercapacitors while maintaining excellent rate performance and cycle stability.（1）A novel trifluoroacetic acid（TFA）aqueous electrolyte was proposed and hierarchical porous carbon（HPC）material derived from passion fruit shells.The HPC material exhibits an interconnected hierarchical pore structure with a specific surface area of up to 3649.4 m² g^-1 and excellent electrical conductivity of up to 4.5 S cm^-1.The electrochemical properties of HPC materials with different pore size distributions in the TFA electrolyte were investigated,and the obtained carbon material achieved a high operating voltage of 1.4 V and a specific capacitance of 506 F g^-1 at a current density of 1 A g^-1.The supercapacitor device was assembled using the TFA electrolyte and prepared carbon electrode material,which exhibited a specific capacitance of 84 F g^-1 at a current density of 1 A g^-1 and an energy density of 19.54 Wh kg^-1 at a power density of 347 W kg^-1.Despite the high power density,the energy density remained high at 12.69 Wh kg^-1 after 30,000 charge-discharge cycles at a current density of 5 A g^-1.Additionally,the assembled TFA/PVA gel solid-state supercapacitor（SSC）also achieved a high operating voltage of 1.4 V,an energy density of 19.54 Wh kg^-1 at a power density of 347 W kg^-1,and a capacitance retention rate of more than 75.7%after10,000 charge-discharge cycles.（2）A porous carbon material（FHPC）with hierarchical pore size,large specific surface area(2081 m² g^-1),and large total pore volume(1.28 cm³ g^-1)was prepared from passion fruit shells using K₂FeO₄ as an activator.The FHPC material prepared in a three-electrode system exhibits a high specific capacitance of 434.4 F g^-1 at a current density of 1 A g^-1 in the TFA electrolyte.Supercapacitors and SSC were then assembled with FHPC material as working electrodes and TFA aqueous electrolyte,and their electrochemical properties were studied.The symmetric supercapacitor device can work at a voltage of 1.4 V and has a specific capacitance of 58.1 F g^-1 at a current density of 1 A g^-1,retaining 72%of the initial specific capacitance at a high current density of 50 A g^-1.The power density of the device is 348.8 W kg^-1,providing a high energy density of 15.8 Wh kg^-1 at a power density of 16.4 k W kg^-1.The capacitance retention rate of the device is 99.6%after 100,000 charge and discharge cycles.The assembled solid-state supercapacitor also achieves a high operating voltage of 1.4 V,an energy density of 13.12 Wh kg^-1 at a power density of 348 W kg^-1,and a capacitance retention rate of more than 88.9%after 10,000 charge and discharge cycles.Using trifluoroacetic acid/hydroquinone（TFA/HQ）electrolyte,the FHPC material prepared at a current density of 5 A g^-1 in a three-electrode system exhibits a specific capacitance of 927 F g^-1.The assembled device can provide a high energy density of 32.0 Wh kg^-1at a power density of 693.7 W kg^-1,and the capacitance retention rate reaches more than 80.4%after 5,000 cycles of charge and discharge.（3）A porous carbon material（GHPC）with a specific surface area of 3622 m² g^-1was obtained by utilizing mangosteen shells as a carbon source.At the same time,a zinc trifluoroacetate（Zn（CF₃COO）₂）aqueous electrolyte was developed to achieve stable charge and discharge over a wide operating voltage window of 1.8 V.The specific capacitance of the prepared GHPC material in Zn（CF₃COO）₂ electrolyte is as high as 466 F g^-1.The symmetric supercapacitor device was assembled using the mangosteen-derived porous carbon and Zn（CF₃COO）₂ electrolyte.At a current density of 1 A g^-1,the specific capacitance reached 52 F g^-1 and the device exhibited good electrochemical stability.The power density of 227 W kg^-1 provided a high energy density of 25.0 Wh kg^-1,while at a higher power density of 11.9 k W kg^-1,the energy density could still reach 17.3 Wh kg^-1.After 30,000 charge and discharge cycles,the capacity retention rate was as high as 86.5%.An aqueous zinc ion hybrid capacitor was assembled using the prepared GHPC material as the capacitive positive electrode,zinc foils as the negative electrode,and Zn（CF₃COO）₂ as the electrolyte.The device can operate stably in a wide operating voltage window of 1.9 V,and its energy density is as high as 180.9 Wh kg^-1 at a power density of 634.3 W kg^-1.It can be stably cycled for 10,000 cycles at a current density of 10 A g^-1,and the capacity retention rate is as high as 96.2%,indicating that the assembled device has excellent cycle stability.