Design And Preparation Of MOF Derived Carbon Nanocomposite Bifunctional Catalyst And Its Application In Zinc-Air Batteries

Due to the consumption of traditional fossil fuels and energy,as well as the increasingly serious environmental pollution,the development of efficient clean energy technologies has attracted extensive attention,including fuel cells,supercapacitors,solar cells and metal air cells.Among them,rechargeable zinc air battery has become an ideal candidate for energy storage and conversion system because of its high theoretical energy density,high safety,rich zinc storage and environmental-friendly.Oxygen reduction reaction and oxygen evolution reaction（ORR and OER）are considered to be the two most critical reaction steps in rechargeable zinc air batteries.However,the slow kinetic processes of ORR and OER greatly limit the commercialization of zinc air batteries.So far,although Pt/C,Ir O₂ and Ru O₂ have good ORR and OER electrocatalytic activities as noble metal oxygen catalysts,their large-scale application is seriously hindered by high cost,low durability and scarcity.Therefore,designing low-cost,efficient and stable non noble metal bifunctional oxygen catalysts is very urgent.In this paper,nitrogen doped porous carbon materials were obtained by calcining MOF precursors at high temperature:Firstly,porous carbon materials with core-shell structure were prepared by simple coprecipitation/hard template/chemical etching/calcination method Co Ni@NPC nano catalyst;Secondly,Micro/mesoporous Co Ni/N-CNN was formed by simple coprecipitation/calcination method and Zn ion as pore forming agent at high temperature calcination stage;Thirdly,using the prepared Co Ni/N-CNN as cathode catalyst,zinc air batteries were assembled,including semi-solid zinc air batteries and all solid flexible zinc air batteries.（1）We designed a core-shell structure catalyst embedded in nitrogen doped porous carbon（NPC）and prepared different metal salts,named Co Ni,Co and Ni Co Ni@NPC,Co@NPC and Ni@NPC,they were obtained by pyrolysis of Co Ni-MOF,CO-MOF and Ni-MOF.The prepared Co Ni-MOF catalyst has a mesoporous carbon shell,which can effectively inhibit the aggregation of metal active sites.XPS analysis shows that Co and Ni may combine with N to form Co Nx and Ni Nx.At the same time,Co Ni alloy is also successfully synthesized instead of pure physical mixing of Co and Ni.Therefore,the prepared Co Ni@NPC catalyst showed good ORR activity in0.1 M KOH,and the half wave potential was as high as 0.77V,which was better than that of commercial Pt/C-Ru O₂.In 6 M KOH solution,The OER overpotential for Co Ni@NPC is very low,only 101m V.The unique core-shell structure and the synergistic effect between Co Ni bimetallic active center and nitrogen doped carbon can well improve the bifunctional activity of ORR and OER for Co Ni@NPC.（2）On the basis of（1）,we still take MOF as precursor and take the Zn ion as pore forming agent after high-temperature calcination.Zn Co Ni-MOF,Co Ni-ZIF,Co-ZIF and Ni-ZIF can be obtained by simple coprecipitation method.After pyrolysis,Co Ni/N-CNN,Co Ni/N-CNN-x,Co/N-CNN and Ni/N-CNN can be obtained.Through TEM images,it can be observed that only the Co Ni/N-CNN catalyst with Zn ion added to the precursor has successfully grown carbon nanotubes in-situ;The nitrogen adsorption/desorption isotherm shows that the Co Ni/N-CNN catalyst has micro/mesoporous structure.This shows that Zn ion not only successfully creates pores,but also induces the in-situ growth of carbon nanotubes.Thanks to the unique micro/mesoporous and carbon nanotube structures,The catalyst can expose both ORR and OER to the maximum extent at the same time.Specifically,the ORR onset potential and the half wave potential of Co Ni/N-CNN catalyst are 1.183V and 0.819V;at the same time,it shows a low voltage of 1.718 V at a current density of 10 m A cm^-2.The strong coupling effect between the Co Ni center site and the defect rich N-anchored porous carbon and nanotube structure can enhance the bifunctional electrocatalytic activity of the catalyst.In addition,the excellent structure of the catalyst is also conducive to the formation of uniformly dispersed active centers,adjustable electronic configuration,excellent conductivity and convenient charge transfer process.（3）On the basis of（2）,the prepared catalyst was sprayed on carbon paper/carbon cloth to assemble semi-solid zinc air battery and all solid zinc air battery,and a series of tests were carried out on the battery,including open circuit voltage,power density,charge-discharge stability,charge transfer resistance and practical application.The test of semi-solid battery shows that the open circuit voltage,power density and specific capacity of battery assembled with Co Ni/N-CNN as cathode catalyst can reach 1.43V,209 m W cm^-2 and 810 m A h g^-1,which are higher than those of commercial Pt/C-Ru O₂（1.42V）,power density(122 m W cm^-2)and specific capacity(710 m A h g^-1).Meanwhile,when semi-solid zinc air battery is charged and discharged for a long time,Co Ni/N-CNN also shows better performance than commercial Pt/C-Ru O₂.The battery stack composed of 4 zinc air batteries has also successfully generated electricity for LED bulbs.After testing the assembled flexible zinc air battery,it is found that Co Ni/N-CNN still maintains higher power density and stability than commercial Pt/C-Ru O₂.When the flexible battery is bent into different angles（0^o,60^o,120^o）,the gap between charge and discharge of Co Ni/N-CNN catalyst does not change much,which further shows that Co Ni/N-CNN catalyst has stable performance.