The Study Of High-voltage Aqueous Supercapacitors Based On Functional Regulation Of Electrolytes

In recent years,the environmental crisis and potential energy crisis have been emerging because of the large-scale use of traditional fossil fuels.Therefore,the development and utilization of new energy sources are receiving increasing attentions,where efficient energy storage devices are indispensable.Among all kinds of devices,supercapacitors?also known as electrochemical capacitors?are receiving more and more attentions because of their characters of high current charging/discharging capability,high output power and long cycle life.At present,most of the commercial supercapacitors are based on organic electrolytes,whose volatile and flammable properties shall bring safety and environmental concerns,along with the harsh requirements for the operation environment.In contrast,aqueous electrolytes have the advantages of high safety,high ion conductivity,and low cost of fabrication and use.However,the cell voltages of aqueous supercapacitors are usually no higher than 1.8V,leading to the low energy densities.According to the equation of energy density?E?=?dQ,it is necessary to increase the voltage?U?and charge capacity?Q?to elevate the energy density.Due to the interfacial charge storage characteristics of supercapacitors,the properties of the electrolyte/electrode interface shall have a decisive influence on its voltage and charge capacity.At present,most researches are focused on developing pseudo-capacitive electrodes to increase the voltage or charge capacity of aqueous supercapacitors,which shall require complicated preparation processes and be not conducive to large-scale preparation.In contrast,regulating the ion hydration and redox functions of the electrolytes for increasing the voltages and capacities of the supercapacitors has the advantages of easy operation,easy large-scale preparation and simplified electrode preparation process.Therefore,it is of great significance to study the high voltage and capacity aqueous supercapacitors based on functional regulation of electrolytes.In this paper,we regulated the compositions and functions of aqueous electrolytes to inhibit the decomposition of water molecules on the surface of the charged electrodes for broadening the electrochemical stability window.In addition,redox-active additives were introduced to functionalize the electrolytes for further increasing the charge storage at the electrode/electrolyte interfaces.Based on the in-situ electrochemical monitoring technology,we have studied the influence of the properties change of the electrolytes,especially the changes of concentration and reaction potential of the introduced redox-active additivities on the voltage,capacity and energy density of the supercapacitors.This study can provide reference and guidance for subsequent researches on high voltage and high capacity aqueous supercapacitors.The main work and results are summarized as follows:Regulating the ion hydration of the electrolyte and electrode structure for increasing the cell voltage.The changing of electrochemical stability window of activated carbon?AC?based symmetric supercapacitor as the concentration of lithium bistrifluoromethanesulfonimide?LiTFSI?aqueous electrolyte increases and the ion hydration enhances was studied based on the in-situ electrochemical monitoring technique.The results show that as the electrolyte concentration increases,the stable potential of the positive electrode rises and the size of the hydrated cation changes,leading to the more effectively utilization of negative electrode potential window.As a result,when the electrolyte concentration reaches 21 m,the voltage of the supercapacitor can reach 2.4 V.In addition,replacing the AC electrodes with freestanding reduced graphene oxide hydrogel?OGH?which shows higher positive electrode stability,the voltage of the supercapacitor can be further increased to 2.6 V and the energy density can reach 27 Wh/kg.Importantly,the open and connected pore structure of the OGH electrode is beneficial to the infiltration of the electrolyte and the rapid mass transfer and diffusion of the electrolyte ions inner the electrode,thus alleviating the unfavorable factors such as high viscosity and low ion conductivity of the high concentration electrolyte,making the supercapacitor show a good rate performance.Regulating the redox function of the electrolytes for enhancing charge storage at high voltages.Based on the aforementioned research,the 21 m LiTFSI electrolyte was further functionalized by introducing redox-active additives to achieve the wide electrochemical stability window and redox activity simultaneously.Then the effects of introduced different redox reactions on the voltages and capacities of the supercapacitors were studied.The results show that this kind of functionalized electrolyte has a wide electrochemical window,making the redox pairs with a high reaction potential?for example,Cl₂/2Cl^-?can be used.When the redox reactions were only introduced at one electrode,limited by the low capacity of the counter electrode,the introduced excessive redox reactions shall lead to a decrease in the voltage of the supercapacitor.In contrast,the redox-active additivities,which can generate redox reactions on both electrodes simultaneously,will be convinced to increase the voltage and capacity simultaneously.In addition,larger potential difference of redox reactions between the two electrodes is preferred to boost the energy density.Benefiting from the use of high potential redox pairs,the bi-redox activity of methyl viologen?MVCl₂?was explored.As a result,the supercapacitor shows a high discharge voltage plateau of 1.8 V and energy density of 102 Wh/kg.Jointly enhancing the energy density of aqueous lithium-ion-capacitors by regulating the redox activity of the electrodes and electrolytes.Studies have shown that a high-voltage 2.7 V aqueous lithium-ion-capacitorcan be obtained by using Li⁺in the 21 m LiTFSI electrolyte to intercalate/extract the TiO₂ based electrode at a low potential.By introducing high reaction potential redox-pair of Cl₂/2Cl^-into the 21 m LiTFSI electrolyte,a high discharing voltage-plateau of 2.1 V can be obtained,and the energy density of the lithium-ion-capacitor can be effectively elevated from 43.4to 69.4 Wh/kg.The redox reaction of Cl₂/2Cl^-on the surface of OGH based positive electrode and the intercalation/de-intercalation of Li⁺in the TiO₂-based negative electrode were further studied.The result shows that regulating the concentration of LiTFSI can regulate the activity of electrolyte ions in the electrolyte,thus changing their reaction potentials.As a result,as the concentration of LiTFSI decreases from 21to 10 m,the discharge voltage-plateau of the lithium-ion capacitor can be increased from 2.1 to 2.2 V,which shall lower the cost and solution resistance and further increase the energy density to 83.8 Wh/kg.Regulating local hydrophobic electrolyte interface for increasing the cell voltage and volumetric energy storage.Studies have shown that introducing a hydrophobic ionic liquid?[Bmim]PF₆?interphase at the electrolyte/electrode interface through displacement processes can effectively expand the electrochemical stability window of the low concentration aqueous electrolytes without changing the composition of the electrolyte bulk solutions.In addition,this strategy has a broad applicability to aqueous electrolytes of different pH,and the operating voltage of the symmetric supercapacitor in 1 M H₂SO₄,LiOH,and Li₂SO₄ electrolytes can reach 1.8,1.8,and2.5 V,respectively,which are the highest values based on such low-concentration electrolytes currently.In addition,by adjusting the concentration of the[Bmim]PF₆ in the nitrogen methylpyrrolidone?NMP?used in the displacement processes,the electrode structures can be controlled.The electrode density can be increased while maintaining the gravimetric specific energy and rate performance of the electrode.As a result,the volumetric energy density of the supercapacitor was effectively elevated to 21 Wh/L.