Preparation And Performance Study Of Carbon/Carbon Composite Flexible Electrode

In recent years,flexible supercapacitors have been rapidly developed in portable/wearable electronic devices due to their high energy density,fast charging/discharging,long lifetime,and their excellent mechanical flexibility（including bending,twisting,and folding）,and have become a hot research topic in sensing.Biomass carbon materials have been widely used as flexible electrodes for supercapacitors due to their structural stability,high specific surface area,wide source,low price and environmental friendliness.However,the practical application of pure carbon materials is hindered by their low hydrophilicity,low pore structure utilization and low electrical conductivity.Among them,although the conductivity of nanocellulose is poor,it can be used as a flexible backbone compounded with conductive substances with excellent electrochemical properties to obtain flexible energy storage device electrode materials with high specific capacitance and high electrochemical stability.Graphene oxide is widely used in electrochemistry because of its high electrical conductivity and electron mobility,and its unique two-dimensional structure makes graphene electrodes mechanically flexible.In this paper,we introduced biomass charcoal material as a pseudocapacitive material and graphene as a conductive matrix,which was combined with biomass charcoal material to prepare electrode materials with flexibility and high specific capacitance.They were assembled into flexible solid-state supercapacitors,and the energy density of the flexible supercapacitors was further improved by structural modulation.the research contents are as follows:（1）Graphene oxide/cellulose nanofiber composites（GO/CNF_n）were prepared by microwave-assisted hydrothermal method.The nanocellulose surface is rich in hydrophilic groups,which not only can generate hydrogen bonding interaction with the oxygen-containing functional groups of GO to form a unified whole,but also makes a good contact between the carbon composite and the gel electrolyte,providing a diffusion channel for the electrolyte ions.Meanwhile,nanocellulose（CNF）acts as a dispersant and spacer,and graphene oxide（GO）nanosheets are uniformly dispersed by its surface effect,which is less likely to agglomerate and repack.The GO/CNF₁ composite with a GO to CNF mass ratio of 3:1 exhibited excellent overall electrochemical properties with a specific capacitance of 295 F/g at a current density of 0.5 A/g.The effect of pressure on the GO/CNF₁ electrode material was investigated,and the specific surface area and porosity of the material decreased with increasing pressure,and the structure became increasingly dense,forming a lamellar striped structure.The composite has the best overall electrochemical performance at 6 MPa pressure with a specific capacitance of493 F/g（0.5 A/g）in the three-electrode system.It was assembled into a flexible solid-state supercapacitor with a specific capacitance of 148 F/g（0.5 A/g）and a mass energy density of 20.6 Wh/kg at a power density of 250 W/kg.When used as a multifunctional sensor,it shows high sensitivity to mechanical stimuli（compressive strain,folding and bending,and human motion）and various physiological signals（sound,pulse,and respiration）.（2）Based on the low energy density of solid-state supercapacitors assembled from graphene oxide/cellulose nanofiber composite electrode materials,the porous structure of activated carbon is introduced to increase the specific surface area of the composite material to obtain energy storage devices with both high energy density and flexibility.Graphene oxide/activated carbon/nanocellulose composite hydrogels（GO/AC/CNF_n）with 3D porous network structure were prepared by one-step microwave hydrothermal synthesis.GO nanosheets were assembled into conductive backbone during the heating process.AC particles with multi-level pore structure are trapped into the graphene network as dopants and spacers to inhibit the re-stacking of GO nanosheets.However,the interface between the two is poorly bonded,and the 3D porous structure formed is collapsed.The hydroxyl groups exposed on the CNF molecular chains form hydrogen bonds with the oxygen-containing groups on the GO lamellae,and then the GO lamellae act as"cross-linkers"to form a cross-linked network by connecting multiple CNF molecular chains with hydrogen bonds.The CNF surface also presents a highly hydrophilic and charged surface with abundant carboxyl groups,which enables it to disperse and stabilize the carbon nanomaterials by surface effects,tightly immobilize the AC particles on the reduced graphene oxide nanosheets,improve the poor interfacial bonding between AC and graphene and enhance the wettability of the composite.The composite flexible electrodes exhibit ultra-high mass specific capacitance（672 F/g at0.5 A/g）and volume specific capacitance（618 F/cm3 at 0.5 A/g）in a three-electrode system.When assembled into flexible supercapacitors,they exhibit excellent energy density（32.2 Wh/kg at 150 W/kg）and show high sensitivity to compressive strain,folding and bending,sound and pulse.（3）S-doping contribution to pseudocapacitance by using lignin as S source.Compared with the above biomass-based carbon materials,the full utilization of lignin in black liquor of paper waste is not only more economically sustainable but also can solve the environmental problems caused by it.The flexible composites（GO/BLs/CDs）were prepared by self-assembling the lignin in the black liquor with graphene oxide and bio-based carbon dots using microwave-assisted hydrothermal method and used as electrode materials for the preparation of flexible supercapacitors.The black sap lignin effectively inhibited the aggregation of GO,increased the ordered structure,and introduced sulfur-containing groups to the composite.The introduction of CDs provided electron-rich regions,which improved the electrical conductivity and reduced the charge transfer resistance.In addition,the formation of pyridine-N-oxide groups changed the surface properties of the composites and improved the wettability of the composites.The solid-state supercapacitor composed of GO/BLs/CDs electrodes and lignin hydrogel electrolyte achieves mass specific capacitance of 94 F/g at current density of 0.5 A/g and energy density of 13.4 Wh/kg at power density of 150 W/kg.