Interface Design And Capacitive Performance Of Nitrogen-doped Graphene/Carbon Nanotube Composites

Supercapacitors have attracted much attention due to their high power density and good cycle stability.Supercapacitors are divided into electric double layer capacitors(EDLC)and pseudocapacitors depending on the capacitance mechanism.EDLC stores charge through the reversible electrostatic adsorption of the electrode-electrolyte surface.Carbon-based materials are considered as ideal candidates for electrode materials for EDLC because of their high electrical conductivity and extraordinary electrochemical stability,while carbon nanotubes(CNTs)and graphene are hotspots in the research filed of carbon materials.However,graphene sheets are easily stacked and CNTs are easily aggregated,which will reduce their specific surface area and cause irreversible capacity loss.Combining graphene and CNTs can make up for their respective shortcomings.That is,graphene can provide a support platform for CNTs and a channel that facilitates electron transport;CNTs can not only reduce the stacking of graphene layers,increase the specific surface area and specific volume density of the material,but also reduce the effects of defects in graphene on its conductive properties.Besides,graphene/CNTs composites generally exhibit hydrophobicity to some certain degree.Nitrogen doping can enhance its wettability,which is more conducive to sufficient contact between the electrode material and the electrolyte.Addtionally,graphene/CNTs composites also undergo the limiation of efficient interfacial connection between carbon structures of different dimensions and the problem of low bulk density of the composites.To address the above issues,this research aims to construct interface chemical bonding in different dimensions and to densify the nitrogen-doped graphene/CNTs through a novel interface engineering method.The preparation and capacitance performance of nitrogen-doped graphene/CNTs composites are systematically investigated in the aspects of following contents:(1)Nitrogen-doped graphene/CNTs(N-G/CNTs)composites were grown on a NiMgAl layered ternary metal hydroxide(LDHs)catalyst by one-step chemical vapor deposition(CVD).The effects of ion ratio in ternary metal catalyst,growth temperature,methane flowrate,growth time,and nitrogen source type on the capacitance performance of N-G/CNTs composites were investigated.It is confirmed that the graphene and CNTs in the N-G/CNTs composites are connected by covalent bonds,which promotes the construction of effective interfaces between different dimensions of materials.And nitrogen doping improves the wettability of N-G/CNTs composites,which is beneficial to the improvement of the electrochemical performance of the composites.(2)Using graphene/CNTs(G/CNTs)composites grown from NiMgAl layered ternary hydroxides(LDHs)as the matrix,a novel interface engineering method was designed to open‘branches and leaves'on the matrix to prepare the nitrogen-doped graphene/CNTs(N-G@CNTs)composites with fractal tree structure.This method first uses CO₂ etching to generate defect sites in the G/CNTs matrix to anchor metal nanoparticles and graft tiny graphene oxide(GO)flakes.Then,CNTs as‘branches'were grown on metal nanoparticles by chemical vapor deposition(CVD),and GO as‘leaves'were covalently bonded to the matrix by CVD,thereby achieving the functional integration of the components of the N-G@CNTs composites with fractal tree structure.The effects of etching sequence,the weight percentage of impregnated Ni,the weight ratio of G/CNT to GO,and the times of impregnation on the electrochemical performance of N-G@CNTs composites were investigated.The results show that the optimized N-G@CNTs composites have a high specific surface area of 190.66 m²/g,a narrow pore size distribution of 4-6 nm,a uniform microstructure and composition distribution,and good electronic and ion conductivity.The N-G@CNTs electrodes exhibit good capacitance performance,with a high specific capacitance of 198.1 F/g at a current density of 0.2 A/g and cycle stability of 91.1%after 2000 cycles at a current density of 1 A/g.In addition,three quasi-solid-state symmetrical supercapacitors assembled from optimized N-G@CNTs composites can illuminate LED lights,indicating that the materials have great application potential.