Preparation And Performance Study Of Core-shell Structure ZnO@ZIF Derivatized Oxygen Reduction Electrocatalyst

In recent years,with the rapid development of industry,various resources have been exploited and utilized in large quantities,which leads to more and more serious environmental pollution,and thus threatens human life and health.The ways of environmental governance are relatively limited,while the development and utilization of new energy technologies have broad prospects.Electrochemical energy conversion and storage technologies such as metal-air batteries and fuel cells play an important role in today's society.The wide application of these technologies is inseparable from the electrocatalysts with high-efficiency,low-toxicity,low-cost and green-sustainability.In order to achieve the desired electrochemical performance,precious metal based matrials such as Pt,IrO₂,RuO₂ are the most efficient catalysts.For example,Pt-based nanomaterials are the most excellent electrocatalysts for oxygen reduction reaction?ORR?.However,the scarcity and high-price of Pt greatly limits the widespread appliacation in energy storage,in which the ORR process plays a most crucial role.In recent years,metal-organic frameworks?MOFs?composed of various metal ions and organic ligands have been widely used as precursors for the preparation of porous carbon-based electrocatalysts due to their excellent properties such as large specific surface area.In this thesis,we mainly focus on the synthesis and electrocatalytic application of carbon-based catalysts derived from MOFs.The morphology and component of the catalysts are controlled by a template synthesis method,and the zinc-air battery is constructed to determine the electrochemical activity of the catalysts.Our synthetic strategy?ZnO as the template?has unique advantages:1)precise control of the morphology and dimensions of carbon-based catalysts?0 D particles to 1 D carbon nanotubes?;2)precise control of the hollow structures and hierarchical pore systems?in-situ evaporation of ZnO template?;3)uniform distribution and full exposure of active sites.The structure and active component characteristics make the catalysts exhibit excellent catalytic activity.1.Design and synthesis of zero dimensional hollow carbon-based polyhedra catalyst.A synthetic method for the preparation of hollow Co and N co-doped carbon nanopolyhedra?H-CoNC?was developed by a one-step pyrolysis of core-shell structural ZnO@Zn/Co-ZIF?Zeolitic imidazolate framework,ZIF?precursor,in which ZnO nanosphere serves as a template and provides zinc source for the growth of the shell ZIF crystals.We also synthesized solid Co,N-co-doped carbon nanopolyhedra?S-CoNC?without inner macro-cavity from bimetallic Zn/Co-ZIF precursors.As compared to S-CoNC,H-CoNC had several advantages:1)enlarged pores spanning from micro-to macro-range,2)higher N contents,and 3)more homogenous Co-N_x distribution throughout the structure.These advantages make H-CoNC exhibit good electrocatalyitic performance in ORR,showing enhanced kinetic current in 0.1 M KOH electrolyte.2.Design and synthesis of one dimensional functionalized carbon nanotube catalyst.A similar synthetic strategy was used for the synthesis of the catalyst.ZnO nanowires were used as a template for the synthesis of core-shell structural ZnO@Zn/Co-ZIF nanowires precursor.One-dimensional core-shell Co@CoO_x-embedded nitrogen-doped carbon nanotubes?Co@CoO_x/NCNT?was prepared by pyrolysis of the precursor.The fusion and aggregation of MOF derived carbon nanoparticles,accompanied by the collapse of the carbon framework during the high temperature pyrolysis process,decrease the accessible surface area of the electrocatalysts.The synthetic strategy of Co@CoO_x/NCNT effectively solved the aggregation problem.The resulting Co@CoO_x/NCNT showed comparable ORR activity and higher kinetic current density compared with the 20 wt.%Pt/C.3.In order to evaluate the practical application of the catalysts,we assembled zinc-air batteries based on the above catalysts for electrocatalytic performance testing.The results show that in 6 M KOH,the assembled Zn-air batteries with H-CoNC yielded a higher open circuit potential?1.59 V vs.Zn,stabilized at 1.52 V?,high power density(331.0 mW cm^-2),and promising rate performance.At the same time,the constructed Zn-air battery based on Co@CoOx/NCNT exhibited good rate performance with a galvanostatic discharge current density ranging from 1 to 500 mA cm^-2,while existing batteries have a galvanostatic discharge current density less than 300 mA cm^-2.