Template-assisted Pyrolysis For The Construction Of Efficient Oxygen Reduction Catalysts For Zinc-air Batteries

With the growing demand for high energy density batteries,metal-air batteries are receiving increasing attention and research.Among the many metal-air batteries,zinc-air batteries have gained widespread attention because of their high theoretical specific energy(1086 Wh kg^-1)and the use of inexpensive and environmentally friendly zinc as the anode active material.Zinc-air batteries have been successfully used in low-power devices in medical and telecommunication industries,and are also gradually expanding their practical applications to energy storage devices and power sources such as electric vehicles.However,Zn-Air batteries are still facing problems such as slow cathode reaction rate,poor cycling stability and heat generation,which restrict their further development.Therefore,optimizing the design and preparing an efficient oxygen reduction catalyst that can accelerate the reaction rate is an important way to enhance the battery performance.At present,ORR catalysts represented by noble metals are constrained by their high price and poor stability,and the development of cheap and stable non-precious metal catalytic materials has become a popular research topic.This thesis describes the preparation of several different non-precious metal heteroatom doped catalysts by using template agents and presents a detailed characterization of the morphology,structure and composition of the catalysts as well as their electrochemical properties and practical applications,with the following main studies and results:1.Iron-nitrogen co-doped porous carbon（Fe-N-C）electrocatalysts with rich mesoporous structures were prepared by pyrolysis and acid etching of precursor mixtures using alkali magnesium carbonate（Mg₂（OH）₂CO₃）as the autogenous template source,ferrocene（Fe Cp）as the metal source,disodium ethylenediaminetetraacetic acid zinc salt（EDTA-Na₂Zn）as the carbon source,and o-diazofi（Phen）as the nitrogen source.catalysts with rich mesoporous structures.The optimized R-Fe-N-C catalysts exhibited excellent oxygen reduction performance due to their high specific surface area and well-developed pore structure,which facilitated the exposure of a large number of active sites and improved catalyst utilization.In addition,the zinc air cell assembled with R-Fe-N-C as the air cathode has a maximum power density of 122.9 m W cm^-2,a specific capacity of 785.8 m Ah g^-1,and a long-term stability of more than 110 h,which is better than commercial Pt/C.This work provides a new strategy for increasing the M-N_x active site and utilization on M-N-C catalysts.2.Using ZIF-8 as the template,polyacrylonitrile（PAN）as the carbon and nitrogen sources,4,4’-diaminodiphenyl disulfide as the sulfur source,and cobalt nitrate as the metal source,cobalt-modified nitrogen-sulfur co-doped carbon nanofibers（Co-NSC-R）were obtained by electrostatic spinning technique and subsequent pre-oxidation and carbonization treatments.The introduction of ZIF-8 template assisted pyrolysis led to the formation of a graded pore structure of Co-NSC-R material,which increased the specific surface area and fully exposed the high density of Co-N_x active sites,effectively promoting the ORR electrocatalytic activity of the material.catalyst(E_1/2=0.821 V).In addition,the zinc-air battery assembled with Co-NSC-R catalyst as the air cathode has a maximum power density of 73.1 m W cm^-2,a high specific capacity of 752.7 m Ah g^-1,and a long-term durability of 94 h.This work provides new ideas for developing the design and application of non-precious metal modified heteroatom-doped porous carbon nanocatalysts.3.Fe Pc-NSC-R electrolytic catalysts with rich mesoporous structures were prepared by mixing and grinding of alkaline magnesium carbonate（Mg₂（OH）₂CO₃）and disodium ethylenediaminetetraacetic acid zinc salt（EDTA-Na₂Zn）as mixed template sources,iron phthalocyanine（Fe Pc）as metal source,and thiourea as sulfur and nitrogen sources.Mg₂（OH）₂CO₃ was decomposed at high temperature to form Mg O particles as a self-generated template,which formed a large number of porous cavities after subsequent acid etching.Thanks to the effect of self-generated templates of mixed metal salts,the resulting Fe Pc-NSC-R materials have large specific surface area and graded pore structure,which provide abundant active sites for electrochemical reactions.Therefore,the Fe Pc-NSC-R catalyst exhibits excellent ORR catalytic performance with a half-wave potential of 0.856 V and good stability,which is superior to that of commercial Pt/C.In addition,the zinc-air cell assembled with Fe Pc-NSC-R catalyst as the air cathode has good stability with a maximum power density of 93.4 m W cm^-2,which is much better than most of the current non-precious metal catalysts and Pt/C.This study provides a new strategy for constructing transition metal heteroatom-doped carbon nanocatalysts with porous structures and enhancing the catalytic activity of the materials.