Construction And Electrochemical Performance Of Graphene-based Planar Flexible Supercapacitors

With the rapid rise of the Internet of Things and artificial intelligence,the development and integration of miniaturized,flexible and wearable electronic products has been greatly accelerated,which has put forward higher requirements for electrochemical energy storage devices,especially in terms of flexibility of energy storage devices.Flexible electronic products and flexible energy storage devices are the relationship between demand and supply.In order to adapt to the development of flexible wearable electronic products,energy storage devices not only need high energy density,power density and long lifetime,but also need to have better compatibility and cooperation with electronic products in terms of miniaturization,flexibility,integration and customization.Among the flexible energy storage devices of many configurations,the planar flexible supercapacitor has been widely concerned and studied by researchers due to its advantages of high power density,long cycle life,and easy integration.Graphene is regarded as the electrode material for planar micro-supercapacitors with great advantages due to its ultra-thin two-dimensional structure,easy film formation,large specific surface area,good electrical conductivity and good mechanical properties.In general,the method to achieve device flexibility is to place the active layer on a flexible substrate（such as polyethylene terephthalate（PET）,polydimethylsiloxane（PDMS）,polyurethane（PU）,polyimide（PI）,etc.）.However,the rigid active layer with high Young’s modulus and the flexible base layer with low Young’s modulus have a modulus mismatch,bringing the development bottlenecks for planar flexible supercapacitors.During the deformation of the device,the active layer cannot deform with the same amount as the base layer’s,which leads to separation of the active layer from the base layer or even the active layer fracture,and the device fails.The problem of slip separation or fracture,thereby improving the stability and effectiveness of the device.The specific research content is as follows:（1）The good structural stability of the ultra-thin active layer in planar flexible supercapacitors is good for maintaining durability of the devices.Here,the planar flexible supercapacitors with ultra-thin active layer were manufactured by ultrasonic spraying and laser etching with graphene as electrode material.The devices were verified to have excellent durability because of the structure stability of the ultra-thin active layer.The planar flexible supercapacitors maintain the stable electrochemical performance at different bending states,even after 10000 times bending cycle,there is no attenuation of areal capacitance.Laser etching can be used for pattern design,and can be used for device series and parallel design quickly and effectively.After simple series and parallel connection,the electrochemical performance of the unit device can still be exerted stably,and high voltage or high current output can be realized.Due to the excellent structural stability of the ultra-thin active layer,the areal capacitance of the planar flexible devices made with different number spraying（15,30,60）have no attenuation after numerous bending cycles.（2）The use of one-dimensional nanomaterials with high aspect ratio as the active materials of planar flexible supercapacitors,and its intertwined structure improves the strength of the film electrode,thereby improving the stability of the device.The strength of film electrodes is improved by introducing high aspect ratio graphene nanorolls,and the active layer can maintain decent structure stability at deformation states.The lateral size of graphene oxide sheet（GO）is reorganized by controlling the ultrasonic crushing time to prepare large,medium and small size GO（L,M,S-GO）.The graphene nanoscrolls with similar dimensions and different initial dimensions（L,M,S-GNSs）are prepared with L,M,S-GO as the precursor by the quenching method.L,M,S-GNSs as active materials,are mixed with highly conductive graphene sheets and dispersed in a mixed solution of water and alcohol to prepare ink.The interdigital planar electrodes are obtained by vacuum filtrating the ink to form a film,and transferring the film to a PET or a pre-stretched nitrile substrate.After coating the polyvinyl alcohol/sulfuric acid（PVA/H₂SO₄）aqueous gel electrolyte,bendable and stretchable planar flexible supercapacitors（L,M,S-MSCs）are manufactured.The as-prepared bendable MSC can maintain nearly 100%of initial capacitance when bent for 1000 cycles,and the stretchable MSC can maintain 88%of initial capacitance when stretched at a high stress ratio of 200%for 1000 cycles.（3）In order to increase the energy density of supercapacitors,ultra-long manganese dioxide nanowires（Mn O₂ NWs）and L-GNS are used as the positive and negative active materials to prepare stable asymmetric planar stretchable supercapacitors（MNG//GNS-MSCs）.Ultra-long Mn O₂ NWs are prepared by hydrothermal method and coated with graphene on the surface of 1D Mn O₂ NWs to improve the electrical conductivity of Mn O₂ NWs by hydrothermal treatment of composite materials of Mn O₂ and GO.In the hydrothermal process,urea is added as a nitrogen source,and doping N on graphene shell which is coated on the surface of Mn O₂NWs.MNG//GNS-AMSCs in the PVA/Li Cl aqueous gel electrolyte exhibits a high voltage of 1.6 V and a volume energy density of 5.9 m Wh cm^-3,which is higher than the traditional SMSCs(<1 m Wh cm^-3)or AMSCs(<5 m W h cm^-3).In addition,there is almost no capacitance attenuation of the stretchable MNG//GNS-AMSCs at the stretching deformation of 100%strain ratio.What’s more,the stretchable MNG//GNS-AMSCs show the excellent stability after cycling for 2000 times at different stretching states,indicating that the devices have decent mechanical properties.（4）In order to solve the problem of modulus mismatch between the rigid active layer and the flexible substrate layer,and eliminate the instability of the microwrinkle structure,a low modulus and high tenacity gel active layer without microwrinkle structure was prepared,and PDMS was used as flexible substrate to manufacture planar stretchable supercapacitor which achieve that the modulus match between the active layer and the substrate layer.The embedded elastic conductor（PDMS-Au）was prepared by embedding gold（Au）nanoparticles into PDMS by sputtering method.PDMS-Au was used as the current collector of planar stretchable supercapacitor and the conductivity of PDMS-Au was maintained well after reciprocating stretching-releasing.Here,PVA/Li Cl gel filled with graphene is used as the active layer（PVA/Li Cl-G）.After removing water and solidifying of PVA/Li Cl-G,Li Cl can absorb water in the air and change from solid state to gel state,so it has good tenacity.PVA/Li Cl-G was used as the active layer to drip coating on the elastic conductor to prepare wrinkle-free planar electrode.After water absorption and gelation,PVA/Li Cl gel electrolyte was coated to prepare stretchable supercapacitor.The device exhibits a stable electrochemcal performance.At different stretching states with the strain ratio of10%,30%and 50%,the device can also maintain stable electrochemcal performance.