Research On Flexible Asymmetric Supercapacitors With Redox-active Gel Electrolyte

Nowadays,human beings are facing energy crisis and environmental pollution.In order to solve this two issues,there is increasing demand for developingclean and renewable energy sources and high-efficiencyenergy storage devices.Supercapacitors,also known as electrochemical capacitors,have been widely used in the field of electronic devices,electric vehicles and military,for its merits of high power density,high efficiency of charging-discharging,wide operating temperature range,non-pollution and long cycling life.However,supercapacitors possess a lower energy density compared withLi-ion-batteries and lead acid batteries,whichhinders itswider application.According to the formula of energy density: E=1/2CV2,we can propose two efficient ways to improve the energy density of supercapacitors,that is,improving the capacitance?C?of electrode material andbroadening the working voltage window?V?of supercapacitor devices.In this paper,MnO₂/rGO composite and MnO₂ nanosheets arrays were prepared on carbon fiber cloth?CFC?by electro-deposition andhydrothermal method,respectively.And asandwich-like flexible asymmetric supercapacitor?ASC?was assembled by using as-prepared electrode and PVA-KOH gel electrolyte which was introduced redox-active species.Themorphologies and structures of electrode materials and their electrochemical properties were characterized and investigated through relevant tests.The main research contents are listed as follows:MnO₂/rGO composite electrode was successfully fabricated through one-step CV electro-deposition.Then the structure and morphology of MnO₂/rGO were characterized by X-ray powder diffraction?XRD?,scanning electron microscope?SEM?and transmission electron microscope?TEM?.The electrochemical performance of MnO₂/rGO electrode was tested in three-electrode system.The electrode showed a relatively high area capacitance of 385.9 mF cm-2,much higher than theMnO₂ electrode without co-depositing rGO?116.1 mF cm-2?.A flexible ASC device was assembled by using the MnO₂/rGO electrode as positive electrode,electro-activated carbon fiber cloth?EACC?as negative electrode,and PVA-KOH gel as electrolyte,respectively.For improving the energy density of supercapacitors,K3Fe?CN?6,redox-active species,was introduced into gel electrolyte.At the current density of 1 mA cm-2,the flexible ASCMnO₂/rGO//PVA-KOH-K3Fe?CN?6//EACC demonstrated an outstanding specific area capacitance of 144.2mF cm-2 and energy density of 0.177 mWh cm-2,respectively,which is 2.1 times of MnO₂/rGO//PVA-KOH//EACC.In order to investigate the practicability of flexible devices,two ASC devices in serial could light a LED for more than 45 seconds.Additionally,the device exhibited a stable cycle lifespan?87.3% retained after 5000 cycles?.MnO₂ nanosheets arrays electrode was fabricated by simple hydrothermal process.The redox reaction occurred between potassium permanganate and carbon fiber cloth.MnO₂ nanosheets wareself-assembled on the surface of carbon fibers and formed nanosheets arrays.Then the structures and morphologies of the material were characterized byXRD,SEMand TEM.The morphology and structure of MnO₂ shows that well-ordered nanosheets arrays covered the surface of carbon fibers,and the nanosheets arrays formed a 3D porous structure.N2 adsorption-desorption isotherms were employed to characterize the specific surface area and pore size distribution of MnO₂ nanosheets arrays.It's showed that the specific surface area of nanosheets arrays was 111.7 m2 g-1,and the average pore size distributionwasbetween 2 and 50 nm,meanwhile,the electrochemical performances of MnO₂ nanosheets arrays electrode was tested in a three-electrode system.The as-prepared MnO₂ nanosheets arrays electrode displayed an area specific capacitance of 1950 mF cm-2 at the current density of 1mA cm-2.A flexible asymmetric supercapacitor were fabricated by using MnO₂/CFC and activated carbon as positive electrode and negativeelectrode,respectively,and PVA-KOH gel as electrolyte.For improving the energy density of supercapacitors,we introduced redox-active speciesKI into gel electrolyte.At the powder density of 0.58 mW cm-2,the flexible supercapacitors MnO₂//PVA-KOH-KI//ACdemonstrated an outstanding specific area capacitance of930 mF cm-2 and energy density of 0.177 mWh cm-2,respectively,which is 2.2 times of MnO₂//PVA-KOH//AC.It's worth noting that two devices in serial could light a red LED for more than 3 minutes.