Research On Wide Temperature Range Near-neutral Aqueous Supercapacitors Based On Potassium Formate And Potassium Acetate Electrolyte

As a kind of electrochemical energy storage device,supercapacitor has attracted much attention and research because of its advantages such as fast charging and discharging speed,high power density and excellent cycle stability,and has been applied in many fields of life.According to the different electrolyte,supercapacitors can be generalized into organic and aqueous,their wide temperature performance（especially low temperature performance）is not satisfactory.In addition,the problems of spontaneous combustion of organic electrolyte,high requirements on the assembly environment（moisture content must be very low）,volatile and possibly toxic,and high price still need to be solved,while the aqueous electrolyte does not have these problems,but the common water system electrolyte（such as sodium sulfate aqueous solution,etc.）operating temperature range is relatively narrow,The practical application of supercapacitor based on water electrolyte is restricted to a large extent.Potassium formate and potassium acetate are two common potassium salts,which have high solubility in water.Their aqueous solutions are nearly neutral,low toxicity and low corrosion,and their aqueous solutions can have a wide liquid temperature range.However,the drainage electrolyte based on potassium formate and potassium acetate has been reported only at room temperature for the study of widening voltage window.Considering the outstanding advantages of water electrolyte,the actual needs of intercalated pseudocapacitor and double-layer capacitor,as well as the characteristics of wide liquid temperature zone of aqueous solutions of potassium formate and potassium acetate,this paper focuses on the study of the near-neutral aqueous supercapacitor based on potassium formate and potassium acetate electrolyte.By adjusting the concentration of two potassium salts and matching suitable electrode materials,the supercapacitor is constructed.In this paper,the operating temperature range of near-neutral aqueous supercapacitors is widened and the temperature dependence of their electrochemical capacitance performance is reduced within the operating temperature range.The main points of this paper are as follows:（1）Near-neutral aqueous symmetric supercapacitors AC//AC and asymmetric supercapacitors Mn O₂//AC were constructed using aqueous potassium formate solutions with concentrations of 50 wt%,55 wt%and 60 wt%as electrolytes and activated carbon（AC）and manganese dioxide（Mn O₂）as electrode materials,respectively.The temperature dependence of electrochemical energy storage performance of capacitors over a wide operating temperature range was studied using cyclic voltammetry（CV）,constant current charging（GCD）,and electrochemical impedance spectroscopy（EIS）.The temperature dependence of electrolyte viscosity was also studied,and a possible mechanism for the temperature dependence of electrochemical energy storage performance of capacitors based on potassium formate electrolyte was proposed.It was found that the operating voltage of AC//AC gradually becomes higher as the concentration of potassium formate electrolyte increases.Importantly,the operating voltage of AC//AC based on different concentrations of potassium formate electrolyte becomes gradually higher as the operating temperature decreases,and can meet a certain linear relationship with temperature,so,using a suitable concentration of potassium formate aqueous solution as electrolyte can not only guarantee the continued operation of the capacitor at low temperature,but also improve some of the performance such as energy density to a certain extent.When the potassium formate concentration is 55 wt%,AC//AC has the widest operating temperature range of-55～80℃.Under the charge/discharge current density condition of 1 A/g,the specific capacitance of the capacitor is 24.6 F/g,50.6 F/g,65.8 F/g,and the specific capacitance retention(C_T/C₂₅℃)is 49%,100%,and 130%,respectively,at operating temperatures of-55℃,25℃,and 80℃.The energy density retention(E_T/E₂₅℃)of this capacitor at the corresponding operating temperature conditions are 98%,100%,and 58%,respectively.The power density retention(P_T/P₂₅℃)of this capacitor is 176%,100%,68%,respectively.The capacitance retention of the capacitor was 96.8%after 10,000 GCD cycles at a current density of 10 A/g at 25℃.In addition,the asymmetric supercapacitor Mn O₂//AC using an electrolyte with a potassium formate concentration of 55 wt%can also operate in the temperature range of-55 to 80°C,and can obtain higher operating voltage and energy density than the symmetric supercapacitor AC//AC.（2）Near-neutral aqueous symmetric double-layer supercapacitors were constructed using aqueous potassium acetate solutions with concentrations of 45 wt%,47.5 wt%,and50 wt%,respectively,as the electrolyte and AC as the electrode material.The temperature dependence of the electrochemical energy storage performance of the device was investigated using CV,GCD,and EIS,and the temperature dependence of the viscosity of the electrolyte was studied,and the possible mechanism of the temperature dependence of the electrochemical energy storage performance of the capacitor based on potassium acetate electrolyte was proposed.The higher the potassium acetate concentration,the higher the operating voltage of the supercapacitor.In addition,the operating voltage of the device at the same time becomes higher with decreasing temperature and is linearly related to temperature.When the concentration of potassium acetate is 47.5 wt%,the corresponding supercapacitor AC//AC has the widest operating temperature range of-55～80℃,and within this temperature range,the temperature dependence of the electrochemical energy storage performance of the capacitor is the smallest.At a current density of 1 A/g,the specific capacitance of the capacitor is 24.1 F/g,60.5 F/g,65.5 F/g,and 75.8 F/g at-55℃,25℃,and 80℃,respectively.The specific capacitance retention（C_T/C25℃）are 38%,100%,and 121%.In addition,at 0℃,the energy density of the capacitor reached its maximum of 46.7 Wh/kg,while the power density increased throughout the entire operating temperature range as the temperature decreased.The capacitance retention of this capacitor reached 81.9%after 8000 GCD cycles tested at25°C with a current density of 10 A/g.