Preparation Of Three-dimensional Graphene And MXene-based Materials And Their Applications In Electrocatalysis And Electrosorption

Energy and environment are two major themes related to human survival.The increasing consumption of traditional fossil fuels and the pollution of water environment are two major issues that human society needs to be solved at present.In recent decades,various techniques have been developed to meet the growth of requirement in fuel and the critical issue for the treatment of the water pollution.Among these techniques,electrochemistry technique exhibits a key role,such as using efficient electrocatalysts to fabricate fuel cells and designing freestanding electrodes to construct electrosorption systems.Hence,it is of great significance for the development of efficient and practical electrode materials,promoting the rapid development of fuel cells and the effective treatment of environmental water pollution.Up to date,various new materials have been developed for the fabrication of various electrodes in various application areas such as solar cell,fuel cells and electrocatalytic degradation and electrosoprtion system.Graphene and MXene,two kinds of two-dimensional(2D)structure materials with their excellent conductivities and various mechanical properties,are the most promising two candidates.However,the disadvantages of 2D nanomaterials are that they are easy to agglomerate and stack,and their performance in different areas are reduced and limited.To overcome these obstacles,scientists have developed various methods,and converting 2D materials to three-dimensional(3D)structures is an ideal strategy.The aim of this thesis is to use graphene oxide(GO)and MXene as the based 2D materials to fabricate 3D materials with various structure.Then the as-prepared materials are used as the electrode materials to develop fuel cell catalysts and as electrode directly for the electrosorption of pollutant,specifically,as fuel cell catalsysts supports to load Pt and Pd nanoparticles for the electrocatalytic oxidation of methanol and ethanol,and as electrode in the electrosorption of organic dyes.The main content in this research is listed in the following three aspects:(1)Preparation of three-dimensional hollow graphene microspheres supported Pt nanoparticles and their electrocatalytic oxidation of methanolA novel polydopamine(PDA)coating graphene oxide(CG)composite microsphere(named as PDA/CG)is successfully synthesized through spray-drying technology.Graphene oxide(GO)is used as a structural material and carbon black nanoparticles(CB)act as spacers to form the 3D hollow microsphere,which can also prevent the re-stacking of graphene oxide during the reduced process and facilitate the formation and stability of spherical structure.Then,a PDA layer was coated on the surface of CG to obtain the PDA/CG.The as-prepared PDA/CG provides high electrical conductivity,good hydrophilic and functional nitrogen contained groups,which are beneficial to anchor platinum nanoparticles(Pt NPs),stabilize Pt NPs and enhance the electrocatalytic performance of Pt-based catalysts in fuel cells.As for the methanol oxidation reaction(MOR),the microstructure of Pt/PDA/CG provides a suitable microspace to facilitate the reactor agent transport,electron transport and electrolyte fluidity.Pt/PDA/CG exhibits excellent electrocatalytic activity(2120 mA mgpt-1)toward MOR,which is superior than that of Pt/CG(779 mA mgpt-1)and Pt/C(218 mA mgpt-1).(2)Preparation of three-dimensional hollow nitrogen-doped graphene microspheres supported Pd nanoparticles and their electrocatalytic oxidation of ethanolA new type of microspheres of graphene oxide(GO)encapsulated zeolitic imidazolate framework-8(ZIF-8)were prepared through spray-drying methodology,and hollow N-doped graphene microspheres(ZGC)were then obtained by the calcination of the ZIF-GO(ZG)composite microspheres.The ZGC can be used as a supporting material anchoring Pd nanoparticles for ethanol oxidation reaction(EOR).This novel ZGC precursor offers several advantages over other materials.The nitrogen doped structure offer more activity sites for support catalysts.The 3D hollow structure contains highly order nanoporous and provide more space for catalysis reaction.The unique 3D structure can avoid the aggregation and deactivation of catalysts.The conversion of nanostructure of graphene and nanocarbon to microsphere structure offers easy handle process.The optimized ZGC has a high orderly hierarchical pore structure and high specific surface area.The Pd/ZGC electrocatalyst shows higher electrocatalytic activity(2490 mA mgpd-1)than that of Pd/ZIF(1494 mA mgpd-1),Pd/rGO(1232 mA mgpPd-1)and Pd/C(467 mA mgPd-1).The advanced electrocatalytic performance of Pd/ZGC can be ascribed to the particular architecture of ZGC,which will have a wide application prospect in the direct ethanol fuel cells(DEFCs).(3)Preparation of 3D porous MXene-based flexible and freestanding film and its application in electrosorption of organic dyesA 3D porous MXene/single-walled carbon nanotubes(p-MXene/SWCNTs)film was synthesized through vacuum filtration technology and used as a freestanding electrode for the electrosorption of the organic dyes in wastewater.At present,dye adsorption of wastewaters is a crucial issue in the water treatment.Nevertheless,there are still several obstacles in the adsorption process hindering its practical application,such as the low efficiency and hard recycle.Electrosorption is an electrochemically accelerate adsorption technique with high removal efficiency,low cost and easy control.As a result,the as-prepared p-MXene/SWCNTs electrode shows spectacular adsorption capacity for methylene blue(MB)with 1068.8 mg g-1 when the applied voltage is-1.2 V,which is much higher than that of open circuit with 55.8 mg g-1.Notably,the asprepared electrode exhibits excellent reusability without secondary pollution.After repeating for five times,the removal efficiency of MB can still up to 95.3%.The pMXene/SWCNTs electrode also possesses outstanding selective adsorption for MB and methyl orange(MO)by adjusting the pH values.Furthermore,the as-synthesized pMXene/SWCNTs film is expected to be acted as flexible electrode applicating in supercapacitors,solar cell,catalysts,and biosensors.