Performance Study Of Micro-supercapacitors Based On Sodium Alginate/polyacrylamide Hydrogel Electrolyte

In recent years,as miniature wearable and portable electronic products gradually enter our lives,people have put forward higher requirements for miniature power supplies.Due to advantages such as long cycle life and high power density,micro-supercapacitors(MSC)have become one of the energy storage power sources with great development potential.However,in order to adapt to the development of electronic devices,micro-supercapacitors not only need to be compatible with wearable electronic devices in terms of miniaturization and integration,but also need to have high flexibility,high energy density,and the ability to work normally in harsh environments.The main factors that determine the high performance of micro-supercapacitors are electrode materials,electrolytes,current collectors and their optimal matching.Graphene has been widely used as electrode material for supercapacitors due to its unique properties such as high surface area,high conductivity,and high electrochemical stability.However,the integration of micro graphene electrodes often involves complex and time-consuming technologies such as inkjet printing,screen printing and lithography,which limits its large-scale application.Hydrogel electrolyte can endow supercapacitors with excellent flexibility,which has developed rapidly in recent years.However,due to the fact that hydrogel electrolyte contains a large amount of water,it is easy to freeze at low temperatures and will lose water and dry at high temperatures,which will greatly reduce its ionic conductivity and flexibility affect its energy storage characteristics.In addition,the lower energy density of micro-supercapacitors also hinders its development in the field of flexible electronic devices,and widening the operating voltage window of the electrolyte is an effective means to increase the energy density of energy storage devices.Therefore,how to broaden the operating temperature range of supercapacitors based on hydrogel electrolytes and increase the operating voltage window of the device has become a key issue to be solved in this field.In this paper,the preparation of micro interdigital electrode based on graphene is realized by laser direct writing technology,and then the device assembly process of direct bonding and integration of hydrogel electrolyte and interdigital electrode is put forward,and the influence of interface wetting between hydrogel electrolyte and micro interdigital electrode on the device performance is discussed.On this basis,the strategy of using hydrogel skeleton,N,N-dimethylformamide(DMF),NaClO₄salt and water to cooperate to reduce the content of free water molecules in hydrogel and improve the operating voltage of the device is put forward as follows:1.Firstly,interdigital laser induced graphene(LIG)electrode with 3D porous structure was prepared in situ on the surface of polyimide sheet by laser direct writing technology,and then sulfuric acid(H₂SO₄)was introduced into sodium alginate/polyacrylamide(SA/PAAM)double network hydrogel by soaking method,thus obtaining high ionic conductivity(256.5 m S cm^-1)and water retention capacity of sodium alginate/polyacrylamide-sulfuric acid(SA/PAAM-H₂SO₄)hydrogel electrolyte.The introduction of H₂SO₄improved the interfacial wetting characteristics between electrolyte and electrode and accelerated the ion transfer rate.The micro-supercapacitors assembled by directly pasting LIG microelectrodes and SA/PAAM-H₂SO₄hydrogel electrolyte shows a high area specific capacitance of 7.12 mFcm^-2,which is 31.4 times that of SA/PAAM hydrogel electrolyte devices.And obtain an energy density of 0.63?Wh cm^-2and excellent cycle stability.In addition,LIG-MSC has excellent flexibility.Under any bending angle,its electrochemical performance remains stable.Even if it is hit by a heavy hammer,its dynamic energy output performance is not significantly attenuated.Without the use of external metal wires for connection,the integration of micro-supercapacitors is realized through series and parallel connection,so as to adjust the working voltage and current.2.Introduce high concentration of DMF and high content of NaClO₄into the SA/PAAM hydrogel system through the immersion strategy to obtain SA/PAAM/DMF/NaClO₄hydrogel.The effects of the ratio of DMF to NaClO₄on the mechanical strength,ionic conductivity and voltage window of hydrogel were discussed by mechanical properties and electrochemical tests.The results show that the synergistic effect of the hydrogel framework,DMF,NaClO₄salt and water molecules played a key role in increasing the voltage window.On the basis of flexibility and electrochemical characteristics,the best preparation process of hydrogel electrolyte was determined.The resulting SA/PAAM/DMF/NaClO₄hydrogel has a high ionic conductivity of 82.1 m S cm^-1,a breaking strength of 563.2KPa,and a wide voltage stability window of 3.5 V.Assembling it with LIG electrodes into a micro-supercapacitors can obtain a wide voltage stability window of 2.0 V,and exhibit a specific capacitance of 2.41 mF cm^-2and a high energy density of 1.34?Wh cm^-2at this voltage,and also have a high cycle stability,good flexibility and integration performance.At the same time,micro-supercapacitors also have excellent anti-freezing and anti-drying properties,with a specific capacitance of 2.21 mF cm^-2and 2.08 mF cm^-2at temperatures of-20°C and 45°C,respectively.