Large Scale Continuous Fabrication Of Vertically Oriented Graphenes For Electrochemical Energy Storage Application

Vertically oriented graphenes（VGs）are a kind of graphene material formed by vertically aligned few-layer graphene nanosheets on the surface of substrates.In addition to characteristics of graphene（e.g.high electrical and thermal conductivity）,VGs have several unique properties such as ultra-fast electrochemical response and ultra-high light absorption,showing excellent performances in several energy or environmental applications（e.g.electrochemical energy storage）.The development of equipments and processes for large-scale,low energy consumption VG preparation is the key to the industrialization of VGs.Recently,the most widely used VG preparation method is plasma enhanced chemical vapor deposition（PECVD）.The preparation technology of VGs still have two technical problems:（1）The preparation scale is relatively small and it is difficult to continuously synthesis;（2）The growth temperature during PECVD process is relatively high（usually>650 ^oC）,leading to high energy consumption.In order to enlarge VG preparation scale,this work builds an arrayed linear antenna microwave plasma device.Since the energy of microwave is confined in the thin layer on the surface of quartz tubes,this device can excite large area,high density plasma,achieving large growth area（225 cm²）and high growth rate(160 nm min^-1)of VGs.Continuous preparation of VGs is achieved by installing a roll-to-roll driving system on the linear antenna microwave plasma device,which can further expand the preparation scale of VGs.In order to decrease the growth temperature of VGs,we tried the following two solutions for promoting the generation of C₂ radicals（the main carbon source of VG growth）:increasing the electron temperature and reducing the decomposition energy barrier of CH₄.In this work,the growth temperature of VGs is reduced from 700 ^oC to 400 ^oC by adding argon into methane plasma to increase the electron temperature.On the other hand,oxygen-containing reactive substances are proved to have the ablity to reduce the energy barrier of CH₄ dessociation and reduce the growth temperature of VGs from 700 ^oC（pure CH₄ Plasma）to 300 ^oC（CH₄:CO₂=1:2）.Based on these,large-area highly branched VGs and high-value syngas（the main components are H₂and CO）are fabricated using two of the most widespread greenhouse gases（CO₂ and CH₄）.The conversion of CH₄ and CO₂ can reach about80%.Due to the oxygen-containing active substances provided by CO₂ can prompt the formation of VG branches,VGs prepared by CH₄+CO₂ plasma have high specific surface area and high edge density.By adjusting the technological parameters such as growth time,substrate temperature and gas flow ratio during PECVD process,the directional regulation of VG morphology is realized.The performance of large scale VGs is verified through the application experiments of AC line filter capacitor and bridging activated carbon supercapacitor.VGs have an ultra-fast electrochemical response which can meet the need of 60 Hz AC filter circuits.A 60 Hz AC voltage signal can be converted into a ripple-free DC voltage signal with an RC filter circuit composed of a VG supercapacitor.The high conductivity and vertical orientation make VGs can be used as"conductive bridge"to reduce the contact resistance between activated carbon and current collector.