Controllable Preparation Of Three-dimensional Graphene Based Hybrid Materials And Their Energy Storage Related Applications

Graphene,as a new two-dimensional?2D?material with excellent electrical,thermal,mechanical and chemical properties,has shown wide application prospects in the fields such as supercapacitor,lithium ion battery?LIB?,and photocatalysis.However,2D graphene nanosheets cannot meet the mass and volume requirements of energy storage devices electrodes.The three-dimensional?3D?graphene architecture,with high specific surface area,electrical conductivity,and plentiful cavity construction,is expected to be an ideal electrode material for high-performance electrochemical energy storage devices.The development of effective and efficient synthesis methods for high-quality free-standing 3D graphene foam?3D GF?and its composites with high specific surface area,electrical conductivity,mechanical strength,and structural stability,is recently a challenge in graphene research.In this work,a new method,namely powder metallurgy templates method,for preparing 3D GF and its composites was developed for the first time by combining chemical vapor depositon?CVD?method and traditional powder metallurgy technique.3D GF was prepared through cold-pressing and high temperature sintering processes followed by chemical etching with Ni or Cu powder as templates and catalysts,and solid organics like sucrose as the solid carbon sources.The influences of catalyst components,carbon source to template ratio,cold-pressing pressure,and heating method on structures and performance of as-prepared products,as well as the growth mechanism of graphene,were systematically investigated.Based on these,carbon nanotubes?CNTs?reinforced 3D GFs?3D rebar GFs?were developed by in-situ growing graphene and introducing CNT as the reinforcing bar?rebar?for further improving the mechanical strength of 3D GF.The growth mechanism of 3D rebar GFs and its strengthening mechanism by using CNTs were studied.The electrical conductivity,compressive properties,structural stability,and storage modulus of 3D rebar GFs with different CNTs contents were investigated,and the electrochemical performance of 3D rebar GFs as electrodes of lithium ion capacitor?LIC?was demonstrated.The results show that the high-quality 3D GF with high specific surface area,conductivity,strength,and mechanical stability can be achieved with Ni as the templates and catalysts,sucrose as the solid carbon source,through slow heating and fast cooling processes during graphene growth.When introducing CNTs into 3D GF,the strength,mechanical stability and conductivity of 3D GF can be significantly improved by CNTs through constructing a 3D network of CNTs.The storage modulus of 3D rebar GF with 18 wt%CNTs can reach to 290 kPa,the specific strength is 128.5 Nm kg^-1,and the maximum electrical conductivity is 34.1 S cm^-1.Without any current collectors and binder employed,the LIC was assembled by 3D rebar GFs with 10 wt%CNTs contents,which can achieve a high active materials mass loading per area(60 mg cm^-2),a large operating voltage window?0.01-4.2 V?,current density per area(>3 mA cm^-2)and comparable energy density per mass(32 Wh kg^-1),as well as excellent long cycling performance.Therefore,the 3D rebar GFs are expected to be ideal electrode materials in high-performance electrochemical energy storage devices.For further improving the preparation efficiency of the 3D GF,a 3D printing technique was employed for the first time in the synthesis of 3D GF?3DP GF?,the influences of laser wavelength,duty cycle and rastering speed on 3DP GF were systematically investigated,and the mechanical and electrical properties of the 3DP GF were also demonstrated.The results show that with Ni/sucrose composite powder as the precursor and a CO₂ laser,higher duty cycle?power?and lower rastering speed are beneficial to the formation of high-quality graphene.The 3DP GF prepared by 3D printing possesses high damping capacity?⁰.06?and electrical conductivity(⁸.7 S cm^-1).Compared to the traditional preparation processes of 3D GF and its composites,no long-time growth or high-temperature annealing processes is required in the 3D printing,and the shapes and structures of products are controllable and designable.Consequently,the 3D printing has a wide application prospects in the synthesis of 3D GF and its composites,as well as the development of new type 3D foam materials.Unlike the in-plane connection between CNTs and graphene in 3D rebar GF,growing CNTs carpets on graphene film substrates can further increase the space utilization of electrode materials in energy storage devices.Therefore,a new type of Fe₃O₄/AlO_x binary composite nanoparticles catalyst was developed in this work.The catalyst was dip-or spin-coated on 2D and 3D graphene substrates,and graphene/CNTs carpet?GCNTs?composites were in-situ controllably synthesized by CVD method.The influences of refluxing temperature,time and Al element content on Fe₃O₄/AlO_x binary catalyst were systematically investigated,and the structure and growth mechanism of binary catalysts were demonstrated.The growth mechanism of GCNTs was analysized as well.Based on these,the energy storage performance of GCNTs as electrodes of LIC was also studied.As a result,in the binary catalysts,Fe₃O₄ nanoparticles were coated by the oleic acid decorated AlO_x amorphous atomic clusters by sharing oleic acid passivation layer.GCNTs were grown on 2D and 3D carbon substrates by CVD method with binary nanoparticles as catalyst.The CNTs carpets show a tip-growth mechanism,consisting of single-walled and few-walled CNTs.The CNTs were covalently bonded with sp²carbon substrates like graphene,resulting in a seamlessly ohmic connection.The LIC with GCNTs as electrodes possesses excellent rate performance and long cycling stability,a large operating voltage window of 0.01-4.3 V,and the specific capacity of 70 mAh g_T^-1 under a current density of 0.05 A g_T^-1.Compared with the traditional e-beam evaporation method,the solution based binary catalyst in this work is more suitable for 3D substrates with complex structures,and is not limited by the size of samples as well.This general method is easy to scale-up.For exploring the application of graphene and its composites in fields of LIB and photocatalysis,in this work,TiO₂/graphene composites were prepared by using a traditional hydrothermal method.The TiO₂/graphene composites with different morphology were controllably prepared by adjusting the use of HF,TiCl₄,and graphene oxide?GO?.The lithium storage performance and photocatalytic properties of TiO₂/graphene composites were demonstrated.The results show that the thickness of TiO₂ can be significantly reduced by increasing the content of TiCl₄ with HF as the morphology controlling agent.TiO₂ nanosheets can be well dispersed by adding GO.The TiO₂/graphene composites show excellent cycling and rate performance as anode of LIB.A high specific capacity of 174.2 mAh g^-1 was obtained after 200cycles under a rate of 1 C.A good capacity of 112.9 mAh g^-1 was also achieved after260 cycles under 10 C with a coulombic efficiency of¹00%.As a photocatalyst,the introduction of GO and reduction of the TiO₂ nanosheets thickness can significantly increase the specific surface area,catalytic activity and adsorption performance.The step-structures on the surface of TiO₂ can also provide more active sites during photocatalytic reaction.The connection of TiO₂ and graphene by Ti-O-C bonds can prevent the recombination of electron-hole pairs,resulting in the increase of electron life,as well as the catalytic activity of the composites.Therefore,TiO₂/graphene composites are expected to be used in multi-fields such as LIB,photocatalytic degradation organic contaminants.