Study On High-performance Microsupercapacitors

In this paper,the key problem of microsupercapacitors,that is the high-performance electrode material,is studied.The important factors to improve the energy density of microsupercapacitor are further clarified.It is shown that the high specific surface area of graphene material can improve the electric double layer capacitors.To further obtain higher specific capacitance,pseudocapacitive materials such as transition metal oxide like MnFe2O4 and transition metal sulfide like Ni Co2S4 were used.Besides,this paper presents some preliminary tests results of microsupercapacitors for wearable electronic applications by designing the electrode material as a flexible electrode.Specifically,the research contents and conclusions of this paper are summarized as following:(1)This paper presents thin film-like microsupercapacitors based on MnFe2O4/graphene.The MnFe2O4/graphene electrode prepared in this study has a specific capacitance up to 300 F g-1 at a current density of 0.3 A g-1,and the assembled solid-state film-like flexible microdevice was tested and the maximum specific capacitance was found to be 120 F g-1 at a current density of 0.1 A g-1.After 5000 times charged and discharged experiments,the microsupercapacitors can maintained the initial capacitance of 105%,indicating that the microdevice has a very excellent cycle ability.The energy density of the MnFe2O4/graphene microsupercapacitor fabricated in this paper was calculated to be 5.0 Wh kg-1 and the power density is 0.4 kW kg-1.In order to verify the practical application of the device,two microsupercapacitors were connected in series and successfully drived a 1.5 V LCD monitor.The MnFe2O4/graphene microsupercapacitors in this study have excellent electrochemical performance.(2)This paper creatively developed an efficient method for the continuous preparation of graphene fibers with nanoscale surface porosity by using metal needles of medical syringes as spinneret.In this paper,it was found that the porous graphene fibers had a very high specific surface area of 839 m2 g-1.The porous graphene fibers had a volume specific capacitance up to 228 mF cm-2 at a current density of 39.7 ?A cm-2.In addition,the microsupercapacitors based on porous graphene fibers show long-life cycle stability and still maintain 98.7% of the initial capacity after 10,000 cycles of charge and discharge.Furthermore,it shows excellent areal energy density up to 7.9 ?W h cm-2 and an impressive volumetric energy density up to 4.0 mWh cm-3,which is comparable to that of thin-film lithium batteries.In addition,the porous graphene fiber microsupercapacitors can be charged by a commercially available flexible solar cell.This study has developed a method for preparing porous graphene fibers,and the porous fibers are expected to serve as electrode materials for high performance microsupercapacitors.(3)This paper reported the fabrication of NiCo2S4 nanoparticles coated graphene fiber(GF/NiCo2S4)and the asymmetric microsupercapacitors with voltage output of 1.5 V.The graphene fibers were synthesized by the reduction of graphene oxide at 80 ?.The graphene fibers are quite light,with density of 0.24 g cm-3.Furthermore,the graphene fibers are also very conductive,with conductivity of 39 S cm-1.Besides,the graphene fibers have mechanical tensile strength of 221 MPa.NiCo2S4 nanoparticles were further deposited on the surface of graphene fibers by solvothermal method.The asymmetric microsupercapacitor with graphene fiber as the negative electrode and GF/NiCo2S4 as the positive electrode showed excellent electrochemical performance.When the three-electrode test method was used,the volumetric capacitance of the GF/NiCo2S4 electrode reached 388 F cm-3 at a scan rate of 2 mV s-1.When using the two-electrode test method,the volumetric capacitance of the GF/NiCo2S4 electrode was 300 F cm-3 at a current density of 175.7 mA cm-3.The energy density of the microsupercapacitors is 12.3 mWh cm-3 and the maximum power density is 1.6 W cm-3(calculation based on the electrode material active material).If the electrode material,seperator and electrolyte are included,then the energy density was calculated to be 3.0 mWh cm-3,which was comparable to the thin-film lithium battery.This study also shows that there was no significant degrade of the electrochemical performance of asymmetric microcapacitors at different folding angles from 0 to 180 °.To further demonstrate the potential application in wearable electronics,an asymmetric microsupercapacitor was woven into a cotton glove.The microsupercapacitor also exhibited excellent flexibility and were expected to be used in the field of wearables.