Rational Design And Preparation Of Transition Metal-based Oxygen Reduction Reaction Catalysts

In modern society,plenty of consumption of fossil fuels leads to energy crisis and severe environmental pollution.To solve these issues,looking for clean and pollution-free new energy sources has become a very important choice currently.Among all kinds of new energy and new technologies,zinc-air batteries show great advantages because of their merits such as high energy density,low cost,cleanliness,free pollution,and safety.Oxygen reduction reaction（ORR）plays an important role in electrochemical energy conversion devices such as polymer electrolyte membrane fuel cells（PEMFCs）and metal air batteries.ORR is a complex reaction involving four-electron transfer,and its kinetics is quite sluggish,which greatly affects the overall efficiency of the system.Therefore,it is paramount to develop efficient ORR catalysts.At present,platinum and its alloys are the most effective ORR catalysts.However,their disadvantages such as high cost,low storage and poor stability,the problem have greatly hindered the large-scale commercial application of these precious metal catalysts.Therefore,the development of inexpensive non-platinum-based catalysts is very important for the world today.In this dissertation,three kinds of transition metal-based hybrid materials were designed and successfully,and they were used as non-precious metal electrocatalysts for oxygen reduction reaction.The compositions,morphological and structural properties of the as-prepared transition metal-based catalysts were analyzed by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and Raman spectroscopy.The electrochemical testing techniques such as cyclic voltammetry（CV）,linear sweep voltammetry（LSV）,chronoamperometric current-time（i-t）method and accelerated durability test（ADT）were applied to characterize the electrocatalytic performance of the as-obtained transition metal-based catalysts.The test results were provided as follows:First,using melamine as nitrogen source,polyvinylpyrrolidone（PVP）as carbon source and iron acetylacetone as metal source,Fe C-loaded nitrogen-doped graphitized carbon materials,named Fe/N-GC-T（where T represents the pyrolysis temperature）,were successfully synthesized by a facile hydrothermal method with combination of pyrolysis.The effect of reaction conditions on electrocatalytic performance of the as-synthesized materials for ORR was studied.The optimized Fe/N-GC-900 showed excellent ORR electrocatalytic activity and good stability in alkaline solution.The zinc-air battery assembled with Fe/N-GC-900 as the zinc-air cathode showed higher open-circuit voltage,larger peak power density,higher specific capacity and better rate performance than the commercial Pt/C catalyst-based zinc-air battery.Second,the polymerization reaction of 2,6-diaminopyridine and pyrrole was initiated by using iron and zinc as metal sources,ammonium persulfate as oxidant and silicon oxide as a hard template.The subsequent pyrolysis of the above composite and the NH₄HF leaching of the obtained product gave rise to new bimetallic Fe/Zn-N-C-T（where T denotes the pyrolysis temperature）materials.The influencing rule of different components and pyrolysis temperature on electrocatalytic activity of the as-prepared catalysts was summarized.The kinetics and reaction mechanism of the optimized Fe/N-GC-900 catalyst for ORR,as well as its stability and durability in catalyzing the ORR were systematically researched.In addition,the battery performance of the assembled zinc-air battery by using Fe/Zn-N-C-900 as the air cathode was tested.Third,the Fe₂C-loaded nitrogen-doped carbon materials with porous structure,named Fe-NC-T（where T represents the pyrolysis temperature）,were prepared by using polyvinylpyrrolidone（PVP）,ferric nitrate and melamine as carbon,metal and nitrogen sources and NaCl as a template.NaCl template could effectively adjust the porous structure of the as-obtained catalysts,making the optimized Fe-NC-800 possess good electrocatalytic performance for ORR.The Fe-NC-800 based zinc air battery assembled also exhibited excellent battery performance,superior to commercial Pt/C based battery.