Research On Aqueous System Supercapacitors And Broadening Operating Potential Window

In recent years,with the rapid development of the energy storage industries,the industries have gradually matured,especially in the supercapacitors industry which is dominated by organic system.However,with the further development nowadays,the problems and harms caused by organic system have been gradually inspected.Among all of the issues,toxicity of the organic electrolytes severely harms the environment and ecology.Therefore,the aqueous system has been widely researched recently.In those studies,how to broaden the operating voltage window in aqueous system to compensate the energy density loss which caused by the replacement of organic system is the main issue in those papers.In this study,there are two experimental section,respectively,describing two ways to broaden the operating voltage window of aqueous system:assemble asymmetric supercapacitors and add redox-active additives into the aqueous electrolyte.The asymmetric supercapacitors uses the different operating potential window of positive and negative electrode materials respectively,then integrates them to a whole to reach a higher operating voltage window than 1.23 volts which is the water decomposition voltage.In this dissertation,we used electroplated manganese dioxide as active material for positive electrode.Tuning the electroplating time could significantly change the morphology of manganese dioxide,and then affect its electrochemical performance.In this experiment,when the electroplating time reached 30 minutes,the manganese dioxide can form a unique three-dimensional honeycomb structure.It enables more manganese dioxide surface being penetrated through by electrolyte,also facilitating the faraday reaction,and thus obtains the best electrochemical performance.This positive electrode material is assembled with a conventional activated carbon as negative electrode to obtain a working potential window of 1.8 volts,and can deliver a maximum energy density of up to 43.84 Wh kg^-1 and a power density of 6.62 kW kg^-1.The redox-active additives utilize its redox reaction which takes place near the oxygen evolution reaction（OER）potential（or hydrogen evolution reaction（HER）potential）and undergoes faster than the OER or HER to suppress the oxygen and hydrogen production.In this experiment,potassium ferricyanide/potassium ferrocyanide were used as redox-active additives,and the Nicandra physaloides was used as carbon precursor for further carbonization and activation.The Nicandra physaloides is a common ingredient in the beauty salons and it consumes a large quantity daily.As a result,it is cost-effective to choose Nicandra physaloides as carbon source.Besides,the yield of activated carbon obtained b y activating the raw material with a 1:2 ratio of carbon to alkali is as high as 23.14%and in the meantime reaches 2250 m² g^-1 high specific surface area.The assemble symmetric supercapacitor using this activated carbon shows only an energy density of 7.98Wh kg^-1 and a working voltage window of 1 volt under the conventional neutral electrolyte.With the addition of redox-active additives,the energy density can be increased up to 22.73 Wh kg^-1 due to the widening of the operating potential window to 1.6 volts.