Study On The Improvement Of Electrochemical Performance Of Tea Residue Biomass Porous Carbon By Redox Electrolyte

How to prepare supercapacitors with high energy density to compensate for the defects of batteries and traditional capacitors is a research hotspot in this field.Electrode materials and electrolytes,as important components of supercapacitors,have always been the main directions of researchers.Waste biomass activated carbon has become a popular electrode material due to its high specific surface area,abundant pore structure,and low cost.Redox electrolytes have an undeniable contribution to the energy density enhancement of carbon-based supercapacitors due to their ability to provide additional pseudocapacitance and broaden the voltage window.This paper uses biomass porous carbon TAC from waste tea residues as raw materials.Redox active substances,such as sodium molybdate（Na₂Mo O₄）,potassium bromide（KBr）,and sodium anthraquinone-2-sulfonate（AQS）were introduced into the system in two ways.Electrochemical performance changes of TAC electrodes assisted by redox electrolytes were explored,with particular attention paid to the influence of the ion shuttle effect on Coulombic efficiency and rate capability.Specific research contents and conclusions are as following:（1）Using waste tea residue as the carbon source,biomass porous carbon TAC with 2521.8m²?g^-1 high specific surface area and rich pore structure was prepared through pre-carbonization and KOH high-temperature activation.The electrochemical properties were tested by cyclic voltammetry（CV）,galvanostatic charge-discharge（GCD）,and alternating current impedance（EIS）methods.In the 1 M H₂SO₄ electrolyte,TAC has a specific capacitance of 358.7 F?g^-1 at the current density of 1 A?g^-1.In the study of introducing a single redox electrolyte into the system,it was found that when the concentration of Na₂Mo O₄ was 0.1 M,the maximum specific capacitance was 2804.3 F?g^-1,but the Coulomb efficiency was 359.5%.When the concentration of KBr was 0.3 M,the maximum specific capacitance is 1442.3 F?g^-1,and the Coulomb efficiency was only 79%.Although addition of a single Na₂Mo O₄ or KBr can effectively improve the specific capacitance,the low reversibility of the Na₂Mo O₄ reaction and the influence of Br₃^-appear unreasonable phenomena of high or low Coulombic efficiency.Therefore,by adjusting the concentration ratio of Na₂Mo O₄ and KBr,a dual redox electrolyte system was constructed,and the gap between the discharge capacity and the charge capacity was regulated to obtain a reasonable Coulombic efficiency.The result showed that when the concentration of Na₂Mo O₄ was 0.05 M and the concentration of KBr was 0.3 M,the specific capacitance of TAC was 1901.4 F?g^-1 at 1 A?g^-1,and the Coulombic efficiency reached 100.7%,but the capacitance retention rate is only 42.5%at the current density of 1～5 A?g^-1.In addition,the symmetric supercapacitor（TAC-RE-1）assembled with TAC as the electrode and 0.3 M KBr+0.05 M Na₂Mo O₄+1 M H₂SO₄ as the electrolyte had an inhibitory effect on the decomposition of water due to the introduction of redox substances,making its the voltage window was increased to 0～1.4 V.At the current density of 1 A?g^-1,the maximum energy density was 57.52 Wh?kg^-1,the power density was 980 W?kg^-1,and the capacitance remained 85%after5000 charge-discharge cycles,which highlight the good electrochemical performance.As three light-emitting diodes（LEDs,2.2 V,0.06 W）were lit up by two TAC-RE-1 devices in series,the material’s value for practical applications was demonstrated.（2）Although reasonable Coulombic efficiency was obtained by adjusting the concentration ratio of Na₂Mo O₄ and KBr in the work of the previous chapter,the rate performance was not satisfactory.This chapter compared the effects of redox electrolyte introduction methods on the electrochemical performance of TAC.Firstly,different concentrations of AQS were added to the 1 M H₂SO₄ electrolyte to construct a redox electrolyte.The result showed that when the AQS concentration was 7 m M,the maximum specific capacitance of 1 A?g^-1 was 616.9 F?g^-1,however,the capacitance retention rate was only 50.57%at 10 A?g^-1,because the electrode reaction was controlled by the diffusion of AQS and H⁺at the same time,and the ion migration rate of AQS was slow.Secondly,the AQS in the electrolyte was stored in the pores of TAC under the action of continuous applied electric field force by cyclic voltammetry,and the electrochemical performance was tested in 1 M H₂SO₄ electrolyte with TAC/AQS as the electrode material.The result showed that,when the AQS concentration was 5 m M,the specific capacitance of TAC/AQS-5 in 1 M H₂SO₄ electrolyte was 733.3 F?g^-1at 1 A?g^-1,and the capacitance retention rate could reach 93%at 10 A?g^-1.This indicated that the AQS was confined in the TAC pores,and only the H⁺in the electrolyte was involved in the reaction,which made the electrode have excellent rate performance.In addition,the specific surface area test result showed that the specific surface area of TAC/AQS-5 decreased from2521.8 m²?g^-1 to 503.6 m²?g^-1 due to AQS occupying the small pores of TAC.To investigate whether the remaining specific surface area is still effective,potassium ferricyanide（K₃Fe（CN）₆）and Carmine（AR18）were added to the 1 M H₂SO₄ electrolyte to test the electrochemical performance of TAC/AQS-5.And the result showed that the specific capacitance of TAC/AQS-5 was further improved to 1320.9 F?g^-1（50 m M K₃Fe（CN）₆）and 1118.8 F?g^-1（10 m M AR18）,but the capacitance retention droped to 58.37%and 63.5%at 10 A?g^-1,respectively.It showed that although the specific surface area was reduced,other redox species could still react on the remaining surface to provide pseudocapacitance.However,the shuttle effect of electrolyte ions at the two poles reduced the rate performance again,which in turn proved the superiority of this method of confinement storage of electrolyte ions in nanopores.Finally,TAC/AQS-5|1 M H₂SO₄|TAC/AQS-5（device WSC）and TAC/AQS-5|polyvinyl alcohol（PVA）+1 M H₂SO₄|TAC/AQS-5（device SSC）were assembled supercapacitors.The maximum energy density of the WSC could reach 41.48 Wh?kg^-1.After 5000 charge-discharge cycles,the capacitance retention rate of the WSC is 86.2%,and two WSCs in series successfully light a2.2 V LED.The maximum energy density of the SSC was 53.68 Wh?kg^-1,and the corresponding power density was 750.02 W?kg^-1.It was again demonstrated that the nanoporous carbon electrode exhibited high energy density and good cycling stability after storing AQS.