Preparation Of Transition Metal Nitride Carbon Composites And Study On Their Oxygen Reduction Electrocatalytic Properties

In recent years,the problems of environmental pollution and energy crisis have been increasing day by day,fuel cell and metal-air battery have attracted great attention due to their advantages such as high energy conversion rate,high power density,green pollution and so on.The core of this type of battery technology is the oxygen reduction reaction(ORR)at the cathode,the performances of the electrocatalysts for ORR have great influence on the comprehensive performance of this type of battery.At present,the best known catalysts for ORR are still precious Pt-based metals catalysts.However,the high costs,scarcity,poor durability,and low poison resistance of these precious metals-based catalysts have been the main bottlenecks that hampered the widespread commercialization for the fuel cell and metal-air battery.Accordingly,it is very important to develop non-precious metal electrocatalysts with low cost,high activity,high stability and good selectivity.Among various non-precious metal electrocatalysts,transition metal nitride-carbon(M-N-C,M = Fe,Co,etc.)catalysts showed high ORR catalytic activity due to the metal-nitrogen synergistic effect.The thesis is based on the idea of modifying heteroatom-doped functional carbon materials on the surface of oxidized graphene sheets,the prepared transition metal nitrogen-carbon composites can maintain the high conductivity and large specific surface area of graphene,and introduce a large number of ORR active sites on the surface of graphene.The main contents of this thesis are as follows:(1)A reasonable method was designed to prepare Fe,N-graphene composites with sandwich structure by two-step reaction and subsequent annealing in high temperature inert atmosphere.At first,the GO@PPy precursor was synthesized in situ chemical polymerization of pyrrole with GO/FeCl3 complexes served as oxidant and template,and then the precursor was annealed at various temperatures to obtain Fe-N-C materials.This is a robust method to make Fe species coated between GO and PPy.The structural characterization showed that the sandwich-like Fe-N-graphite nanosheets has high degree of graphitization and strong electron transport ability;the structure with a large specific surface area is favorable for exposure to active sites and increased mass transfer capacity;the Fe species is coated with a thin carbon layer to enhance stability and methanol tolerance.The resulted Fe-N-C catalysts obtained from above precursors annealed at 825 ? with sandwich-like structure as the electrocatalysts for ORR showed that,comparable to the commercial Pt/C catalyst,positive onset potential,high ORR limiting current density,favoured an apparent 4 electron reduction mechanism,outstanding stability,excellent methanol tolerance.(2)Some reports have shown that ZIF-8 can be grown in situ on GO,as the strong interactions between the metal clusters from ZIF and oxygen-containing groups from GO.Based on this idea,the ZIF-8 and ZIF-67 composites are initially grown in a continuous fashion on the sueface of graphene oxide to obtain ZIF/GO.The uniform coating of ZIF nanocrystals on the GO layer can effectively inhibit the agglomeration of GO during high-temperature treatment.Graphene acts as a conductive layer to accelerate the electron transfer in the carbon matrix and readily reach to the active sites smoothly.Nanoporous carbon derived from ZIF acts as a catalytic layer,where the active sites(i.e.,N-doping carbon,Co-Nx-C)were located in the nanoporous carbon.Due to its high specific surface area and nanoporous structure,large numbers of active sites are easily accessible by the reactants.The Zn-Co-ZIF/GO-920 is demonstrated to be an effective ORR catalyst in the alkaline medium,the onset potential and the limiting current density transcend the commercial Pt/C catalyst.The current efficiency is high,followed by the 4 electron reduction mechanism,the stability and the methanol tolerance are both excellent.