| Supercapacitor is a new type of energy storage device which is between traditional capacitor and battery and has both characteristics.In recent years,the rapid development of miniaturized wearable devices has greatly promoted the research and development of ultra-thin,flexible and lightweight energy storage devices.As one of the most promising power supply equipments,the supercapacitor has attracted more and more attention because of its long service life,high power density and fast charging and discharging speed.The commercial lens wiping paper has a porous network structure composed of cellulose fibers.It is ultra-thin(40-50μm thick)while tough enough,and the price is low.Its porous network structure can effectively promote the penetration of the electrode active material and diffusion of the electrolyte,and its toughness lays a foundation for the preparation of flexible supercapacitors.Therefore,in this work,the lens wiping paper was used as the substrate to fabricate the graphene-fiber composite conductive material by soaking the paper in the graphene ink.The graphene-fiber 3D electrodes with different thickness(layers)were prepared,and their microstructures,electrical properties,flexibility and foldability were characterized and analyzed.After that,sandwich-structured supercapacitors were prepared based on the 3D graphene-fiber composite electrodes,followed by electrochemical performance characterizations.The following main results were obtained:(1)In the graphene fiber-composite conductive material prepared by the impregnation method,graphene is wrapped on the fibers to form a continuous and conductive network;3D electrodes with different thickness(layers)were prepared,and the relationship between the electrode resistance and the number of layers was measured.It was found that with more layers,the electrode resistances decreased gradually and the electrical conductive properties were improved.(2)The conductance of the electrodes was measured at different bending angles and folding degrees under control of the stepping motor and the flexibility and foldability of the electrodes were analyzed.It was found that the conductance of the graphene-fiber composite electrodes remained nearly unchanged(with the conductance retention rate of about 100%)under different bending angles.Besides,after 1000 folding cycles,the conductance retention rate of the electrode was still as high as 96.3%,indicating that the graphene-fiber composite electrode has excellent flexibility and foldability.(3)Based on the 3D graphene-fiber composite electrodes and the sulfuric acid-PVA hydrogel electrolyte,the sandwich structured supercapacitors were prepared and the electrochemical performance was analyzed.The results show that the specific areal capacitance of the capacitor reaches 30.55 mF/cm~2 at the scanning rate of 10 mV/s,and the capacitance is 36.8 mF/cm~2 at the current density of 0.1 mA/cm~2.At the current density of 2.0mA/cm~2,the specific areal capacitance is still as high as 29.3 mF/cm~2,and the retention rate of capacitance is as high as 80%,which shows that the supercapacitor has high energy storage ability and high rate performance.(4)The sandwich structured supercapacitors were prepared based on the 3D electrodes with different thickness(layers)and their performances were compared.The results showed that with the increase of the electrode thickness,the internal resistance of the supercapacitor decreased,the specific areal capacitance increased,and the rate performance was improved.(5)The sandwich structured supercapacitors with different electrode configurations,such as square,hollow square,triangle,ring and circle,were prepared,and their performances were preliminarily studied and compared.The results have verified the feasibility of preparing the random-shaped supercapacitors with the proposed strategy in this work,and also indicate that the configuration of the electrode also influences the energy storage performance of the supercapacitor. |
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