| With the great specific surface area and ultra-high electrical conductivity,graphene has attracted much attention for their applications and research in the field of flexible devices.However,during the assembly of graphene into macroscopic film,the tightly packed graphene will severely reduce the specific surface area of materials,resulting in electrodes failing to exhibit the expected electrochemical performance.Compounding with pseudocapacitive materials not only increases the specific surface area of material,but also provides abundant pseudocapacitive energy storage sites,which is one of the most common and effective methods to enhance the electrochemical performance of graphene-based films.However,this strategy tends to make the electrodes exhibit a low gravimetric capacitance,making it difficult to meet the lightweight requirements of wearable devices in practical applications.Moreover,most pseudocapacitive materials always shows undesirable cyclic stability,which further limits its large-scale application.Functionalizing graphene with oxygen not only provides abundant redox reaction energy storage sites,enhancing the pseudocapacitive contribution of film,but also effectively improves its wettability and increases the electrolyte-accessible surface area.More importantly,the introduction of oxygen groups will not significantly increase the quality of macroscopic graphene materials,which is conducive to achieving both high gravimetric and volumetric performance.Therefore,the oxygen-functionalized graphene is expected to be an ideal electrode material used in flexible supercapacitors.On the basis of that,a porous graphene film with abundant reactive oxygen functional groups were designed and prepared in this paper.The electrochemical properties of the film as a flexible supercapacitor electrode were investigated.In addition,the effects of oxide level of precursors on the structure,morphology and energy storage properties of film materials were also studied.The major study contents are as follows:(1)The Hummers'method for the preparation of highly oxidized graphene was improved in this work,and the oxygen-enriched graphene oxides(ORGO)with an oxygen content of 33.7 at.%was synthesized.Furthermore,by vacuum filtration and low-temperature thermal reduction process,the highly oxidized graphene was used as the precursor to obtained reduced oxygen-enriched graphene film(rOGF),which shows excellent mechanical performance and is enriched with abundant active oxygen groups.The results show that the synergistic effect of high oxidation and low-temperature thermal reduction promotes the conversion of C-O-C to electrochemically active groups,such as C-OH and C=O,which effectively increases the pseudocapacitive contribution of film.In addition,the strong deformation of graphene oxide caused by a large number of oxygen groups hinders the self-stacking of graphene during the self-assembly process,which not only provides a path for the diffusion of ions,but also facilitates the rapid release of gases during thermal reduction,thus avoiding the over-expansion and obtaining a high density(1.5 g cm-3)for film;meanwhile,the capillary force generated from the rapid flow of gas causes the films to shrink and forming"W"shaped wrinkles,which help enriching the pore structure,increasing the electrolyte accessible surface area,and exposing more energy storage sites.Based on these characteristics,rOGF,as a flexible electrode for supercapacitors,achieves a high volumetric capacitance(825 F cm-3),high gravimetric capacitance of 550 F g-1 and excellent rate performance(20 A g-1,503 F cm-3).The assembled symmetrical supercapacitor based on rOGF shows an ideal power density(1350 W L-1)at high energy density(55.2 Wh L-1)as well as superior stability with a capacity retention of144.7%after 4,000 cycles.(2)In this part,the rOGF electrodes prepared by the above work were connected with metal tabs via the action of conductive silver adhesive,and the reliability of tabs was evaluated through electrochemical testing.Subsequently,the electrodes were used to assemble the flexible supercapacitor and its electrochemical performance were tested.The results show that the silver glue achieves a firm connection between the metal and carbon-based flexible material.The efficient transport of electrons from tabs to the graphene films allows the rOGF to exhibit ideal electrochemical properties.When large size rOGF were used to assemble flexible device,the undesirable conductivity and the absence of a vertical hole structure of the film result in a capacitance of the device far below its theoretical value.Based on the above research,the design principles about constructing free-standing graphene-based film electrode are submitted,namely,the idea electrical conductivity for fast electron transport and abundant vertical pore channels for shortening the electrolyte transport path. |
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