Preparation Of Carbon-Supported Transition Metals Cathode Catalyst Materials Derived From Metal-Organic Frameworks And Investigation Of Their Electrochemical Performances

Due to the high theoretical energy density,low cost,and improved safety,the rechargeable zinc-air batteries are considered as one of the most promising energy storage systems.Design and synthesis a cheap material with high electrocatalytic activity,reducing the overpotential,improving the electrode reaction efficiency and enhancing the cycle stability are essential to solve the problems encountered in the development and application of zinc-air batteries.Studies have shown that coating transition metal nanoparticles in graphene layers or carbon nanotubes can bring extraordinary activity and stability to ORR and OER,making it possible for such non-precious metal nanocatalysts to replace precious metal catalysts.On the surface of the material,the transition metal nanoparticles coated with a carbon layer as the active site can not only prevent the agglomeration of the nanoparticles and directly contact the reactants and the electrolyte solution,but also effectively promote the electron transport in the composite material.Metal organic frameworks(MOFs),as a new class of porous materials,whose catalytically active nanoparticles(NPs)are confined in the cavity / channel of MOFs or surrounded by MOFs,can effectively promote the mass transfer and diffusion of substrates and products.It is one of the ideal precursors for preparing transition metal catalytic materials.In this paper,the research focus is mainly on carbon-coated transition metal materials derived from MOFs.The morphology,structure,and active sites of MOFs-derived materials are designed and regulated through confinement,pyrolysis,and hydrothermal reactions to prepare high-performance Long-life electrocatalyst.The main research structure is as follows:(1)Herein,a bottom-up method,i.e.,a surfactant-assisted synthetic method to synthesize a novel dual-metal(Co/Zn)zeolitic-imidazolate framework(ZIF)that could maintain its positive-hexagon-shaped morphology after high-temperature thermal treatment,has been developed.As a bifunctional electrocatalyst,the Co nanoparticles encapsulated into both nitrogen-doped positive-hexagon-shaped carbon nanosheets and carbon nanotubes distributed over the nanosheet surface(Co-N-PHCNTs)show excellent electrocatalytic performance,superior to that of state-of-the-art benchmark noble-metal electrocatalysts.The well-designed sheet-derived carbon nanotube structure enables Co nanoparticles to be evenly distributed on the carbon nanosheet as well as in the carbon nanotube tip,exposing the active sites,enhancing electrical conductivity and exhibiting a large specific surface area and effective electrochemical active surface area.In addition,the robust covalent bonds between Co and N also enhance the electrocatalyst durability.Remarkably,in a practical demonstration,the Co-N-PHCNTs serve as bifunctional air electrodes for Zn–air batteries,exhibiting a high peak power density of ≈125.41 m W cm-2 at a current density of ≈130 m A cm-2 with an extraordinary charge–discharge cyclability of over 673 h at 5 m A cm-2.This strategy not only provides guidance for the synthesis of 2D ZIF-derived materials but also for other 2D materials in cross-cutting applications.(2)Bottom-up construction of high-performance and long-term durability transition copper-nitrogen-carbon(Cu-N-C)electrocatalysts for oxygen reduction reaction(ORR)still remains great challenge.Herein,we propose a temperature-controlled with confinement effect synthesis strategy for fabrication of a novel two-dimension dual-metal(Cu/Zn)zeolitic imidazolate frameworks material,which presents an ultrathin nanosheet morphology after high-temperature thermal treatment(denoted as Cu-N-UNS).By controlling the reaction temperature as well as regulating the ratio of metal ions and taking advantages of the confinement effect of surfactants,the rational-designed ultra-thin carbon layer not only prevents aggregation of the transition Cu particles and avoids direct contact with reactants and electrolyte solutions to enhance the durability of electrocatalyst,but also shortens the electronic transmission path between active transition metal species and carbon surface.Therefore,the electrocatalyst exhibits excellent electrocatalytic performance for ORR(E1/2≈0.898 V),which is superior to those of state-of-the-art benchmark noble-metal electrocatalysts.Moreover,the evenly distribution of Cu-N-C and existence of N-Cu2+-Cu0 active sites makes a great contribution to the electrocatalyst activity.Notably,the implementation of Cu-N-UNS serves as air electrodes for Zn-air batteries also performs a high peak power density of ≈134.7 m W cm-2 at current density of ≈231.9 m A cm-2 with a remarkable durability(over 90 h at 10 m A cm-2).(3)The method of using the hydrothermal method to grow ultra-thin MoS2 nanosheets on a two-dimensional metal-organic frame(MOF)makes reasonable use of the synergy between the two-dimensional MOF-derived carbon-cobalt nanoparticles and MoS2 nanosheets.The morphology fully exposed the edges of MoS2 and increased its electrochemically active area with reactants.A new catalytic material(Co MoS @ CNS)with Co MoS structure was prepared.When it is used as a zinc air electrode cathode,it exhibits high electric power(≈247.5 m W cm-2 at current density of ≈350 m A cm-2)and remarkable durability(over 148 hat 5 mA cm-2)).