Structure-Controlled Synthesis And Electrochemical Performance Of Carbon Based And Metal Oxide Nanomaterials

In recent years,carbon-based and metal oxide nanomaterials have made great progress in their preparation method and performance research.Various multifunctional carbon-based and metal oxide nanomaterials have been prepared,which show a good application prospect in fuel cells and lithium ion batteries.At the same time,the fuel cell catalysts themselves suffer from the drawbacks of high cost,low activity,poor stability and low poison resistance,and the anode materials for lithium ion batteries are also limited by their low conductivity and big volume expansion during charge/discharge processes,which lead to unsatisfactory performance.This dissertation focuses on solving these problems and a series of novel,low energy consumption and environmentally friendly synthetic methods were developed including electrostatic spinning,hydrothermal method,and sol-gel-assisted template method.Using these synthetic methods,we synthesized a series of new carbon based and metal oxide nanomaterials and realized the control on their microstructures.These materals show excellent electrochemical performance in fuel cells and lithium ion batteries.This dissertation mainly includes following several aspects:?1?We have developed a simple and green approach to heteroatom?N and S?co-doped hierarchically porous carbons?N-S-HC?by using ionic liquid?IL?as nitrogen,sulfur and carbon source and the KCl-Zn Cl2 eutectic salt as“salt templating”.The resultant N-S-HC can be used as metal-free oxgen reduction reaction?ORR?electrocatalyst and it exhibits excellent activity and stability,which is superior to the commercial Pt/C electrocatalyst in alkaline solution.To the best of our knowledge,such excellent electrochemical performance towards ORR has rarely been observed in metal-free catalysts.The N-S-HC shows high specific surface area?1056 m2 g-1?and a relatively high heteroatom loading?N: 2.80%;S: 5.16%?.The heteroatom loading and specific surface area can be easily controlled by simply changing the reaction temperature.The high heteroatom loading for N-S-HC can provide more active sites,whereas the high specific surface area ensures an excellent electron transfer and reactant transport rate towards ORRs.This newly developed method for the dual heteroatom doping can thus provide simple “green” route to low-cost mass production of N-S-HC as efficient metal-free ORR electrocatalyst for fuel cells and other applications.?2?We have constructed Mn₃Co₇ nanoparticles supported on hierarchically porous carbon nanofibers by an electrospinning method followed by a fine annealing treatment.As a potential non-noble metal electro-catalyst for ORR in fuel cells,the current density of Mn₃Co₇/PCF is higher than that of the commercial Pt/C catalyst.The unique microstructures of the Mn₃Co₇/PCF,such as high BET surface area?301.95 m2 g-1?,one-dimensional fiber nanostructure,the enhanced degree of graphitization and the well dispersed Mn₃Co₇ nanoparticles,are responsible for their significantly improved electrocatalytic activity for ORR.Thus Mn₃Co₇/PCF could be a highly active electrocatalyst,for potential replacement of the current commercially available Pt/C as a cathode catalyst in fuel cells.?3?We have constructed NiO/NiCo₂O₄ hybrid with a porous tubular structure by coaxial electrospinning method followed by a fine annealing treatment.As a potential non-noble metal ORR electro-catalyst for fuel cells,the onset potential was about-0.05 V and the current density of NiO/NiCo₂O₄ is about 115% of that of Pt/C.The heterogeneous hybrid structure,the open porous tubular structure,and the well dispersion of the two components,are responsible for their significantly improved electrocatalytic activity for ORR.Moreover,the promising and straightforward coaxial electrospinning has proved itself to be an efficient pathway for the preparation of nanomaterials with tubular architectures and it can be used for large-scale production of catalysts in fuel cells.?4?Using a dual hydrothermal self-assembly followed by a thermal treatment process,ultrafine NiCo₂O₄ NPs supported on 3DNG?denoted as 3DNG-NiCo?were prepared,which can be used as a synergistic electrocatalyst for ORR.The resultant 3DNG-NiCo exhibits significantly enhanced electrocatalytic activity in alkaline environment.The excellent electrocatalytic performance may be attributed to the unique interconnected porous structure with uniform dispersion of ultrafine NiCo₂O₄ nanoparticles and the strong synergistic effect of 3DNG and NiCo₂O₄ towards ORR.Furthermore,this simple and low-cost approach can be further extended to develop other metal or metal oxide supported on 3DNG for fuel cells and other applications.?5?Porous Co₃O₄ nanotubes?denoted as P-Co₃O₄-NTs?were prepared by a coaxial electrospinning technique followed by an annealing treatment.The resultant P-Co₃O₄-NTs display a unique porous tubular structure,and that the morphologies of the P-Co₃O₄-NTs could befinely tuned by adjusting the reactant concentrations.Furthermore,when used an anode in LIBs,the as-prepared P-Co₃O₄-NTs exhibit an ultrahigh reversibly capacity of about 1830.5 m Ah g-1 at a current density of 0.3 A g-1 for 100 cycles and an excellent rate performance?about 1000 m Ah g-1 at a current density of 5 A g-1?.To the best of our knowledge,this is the first report using coaxial electrospinning P-Co₃O₄-NTs as an anode electrode for LIBs.This kind of porous tubular structure has higher surface-to-volume ratio and can facilitate the transportation of ion and/or electron as well as the alleviation of volume expansion during cycling processes.Moreover,the promising and straightforward coaxial electrospinning proves itself to be an efficient pathway for the preparation of structure-tunable metal oxides with enhanced electrochemical behavior.?6?Three-dimensional?3D?ordered porous vanadium nitride@carbon composite?NV@C?was synthesized via a sol-gel method followed by an annealing treatment by using SiO₂ spheres as a hard template and PVP as carbon source.The resultant hybrid has a relatively large specific surface area of 153.14 m2 g-1 and the 3D ordered porous structure is very beneficial for the penetration of the electrolyte and mass transmission.As a fuel cell cathode catalyst,the material shows very excellent catalytic properties for oxygen reduction.