Template Synthesis And Regulation Of Bowl-like Powder Materials

In recent years,asymmetric nano-powder materials and their preparation techniques are attracting significant scientific and technological interests.Hollow bowl powder has many distinct characteristics,such as weak symmetry,high specific surface area,higher packing density of particles than that of hollow spheres,controllable cavity,similarity to erythrocyte,etc.,endowing them with outstanding performances in many different applications.However,there are many shortcomings in the preparation technique of hollow bowl-shaped materials,such as complicated processes,difficult to achieve precise control of products’morphology and quality.In this paper,the hollow bowl-like carbon(carbon HB)was prepared by combination of hydrothermal carbonization(HTC)and emulsion template method.Then the hollow bowl-like iron oxide was prepared by the cation adsorption and the two-step calcination method with the carbon HB as the template.By adjusting the reaction time,temperature,ratios of raw material and calcination atmosphere,precisely control of the composition,morphology and size of the products can be achieved and the mechanism of products is further studied,leading to obtain in g the optim al product with good appl ication prospects in the field of lithium-ion batteries,potassium-ion batteries,supercapacitors.The main research work of the paper focus on the following aspects:(1)The combining of emulsion templating and HTC was developed to synthesize hollow bowl-like carbon(HBCs).This method utilizes the trioctylamine(TOA)/water emulsion as the suitable soft template and ascorbic acid(VC)as the carbon predecessor.The HBCs have a matrix of hydrothermal carbon,a diameter of about 1.5 μm,a shell of about 100nm,complete shape and relatively uniform size.TOA is essential for getting a hollow structure.In this work,TOA/water emulsion is formed under sonication.Subsequently,VC transformed into carbon at the interface of the templates forming hollow carbon spheres under the hydrothermal environment.The emulsion is unstable at high temperature and the templates escape from the carbon shell into water,resulting in forming capillary force.The carbon shell collapses and transforms into the unique shape of a hollow bowl-like sphere.The conductivity and porosity are increased by the simple carbonization treatment of the HBCs,leading them performing very well as electrodes of electrochemical double layer capacitors.Compared with solid structural electrode,HBCs have a higher capacitance,better capacitance retention and very high cycle stability(96%)at a current density of 2 A/g over 5,000 cycles.(2)Carbon hollow bowls(carbon HBs)are prepared under hydrothermal conditions by coupling emulsification and hydrothermal carbonization and using sodium oleate and P123 double surfactants as soft template and inexpensive natural biomass(sugar cane)as a carbon source.Carbon HBs have well-defined bowl-like shape with uniform and controllable size(0.5-2μm)and shell thickness of～50nm.The formation mechanism and morphology of carbon HBs are studied by adjusting the reaction time and ratio of raw material.Nitrogen/oxygen co-doped carbon porous hollow bows(CPHBs)are successfully prepared by calcination and further activation in N2 atmosphere.When used as anode of KIB,the CPHBs exhibit a highly reversible capacity of 304 mAh·g-1 at the current density of 0.1 A·g-1 after 150 cycles,superior stability of 133 mAh·g-1 over 1000 cycles at 1 A·g-1.Notably,the volumetric specific capacity of the CPHBs is 56%greater than that of hollow spheres,because of the high packing density of bowl-like particles.Finite-element simulation results reveal that CPHBs can effectively release the stress induced by discharge/charge processes and make them have high cycling stability.(3)Nitrogen-doped carbon-coated hollow bowl-shaped iron oxide(α-Fe2O3 HBs@NC)was prepared by combination of cation adsorption and two-step calcination method(two different atmospheres:N2 and air),followed by coating N-doped carbon layer.As anodes for LIBs,the α-Fe2O3 HBs@NC exhibit a reversible gravimetric capacity above the theoretical Fe2O3 value(1452 mAh·g-1 at 0.1 A·g-1),an enhanced rate performance(950 mAh·g-1 at 2A g-1,609 mAh·g-1 at 8 A·g-1),a cycling stability without capacity fading even after 1600 cycles.Moreover,the volumetric specific capacity of the α-Fe2O3 HBs@NC 42%larger than that of the hollow sphere structure.The materials show good cycling stability as anode electrose for KIBs(214 mAh·g-1 after 500 cycles at 0.05 A·g-1).We simulate the von Mises stress distribution on mesoporous hollow bowl(HB)by finite element method.The simulations results indicate the porous hollow bowl structure can effectively alleviate the volume expansion caused by long-term charge and discharge,thereby achieving high cycling stability.