| Owing to the decreasing fossil fuel resources and the ever-increasing concerns on the environment,the development of green sustainable energy has been a globally urgent issue.Zinc-air batteries are characterized by high energy conversion efficiency and environmental friendliness;however,the kinetics of its cathode reaction,the oxygen reduction reaction（ORR）,is more sluggish than that of the anode,as a result of which electrocatalysts are often required to accelerate the reaction.In this regard,the precious metal Pt,which is rare and expensive,is often used as electrocatalyst commercially.To pursue high-perforamnce non-noble ORR electrocatalysts,over the past decade,transition metal-nitrogen-carbon catalysts（M-N-C）have been identified as potential candidates with relatively high ORR electrocatalytic performance;among them,Fe-N-C materials have received the most attention due to their outstanding catalytic activity.In general,the synthesis of Fe-N-C electrocatalyst involves simultaneous pyrolysis of a specific iron precursor,carbon precursor and nitrogen precursor in a certain atmosphere at high temperature.In the resulting Fe-N-C materials,different kinds of N are doped within the carbon skeleton with a certain degree of graphitization,and the metal moiety Fe exists in its ionic state,which is atomically dispersed by coordinating with the N dopants.The resulting metal-nitrogen coordination active site（Fe-Nx）is considered to be the genuine ORR active site.At the same time,the structural features of the Fe-N-C materials is important for their ORR electrocatalytic performance;large specific surface area and desirable pore structure are required to obtain high mass density of active sites and support the rapid transport of oxygen,proton and water in the ORR reaction process,respectively.Additionally,the electrical conductivity of FeN-C material is also very important.In this thesis,a series of hollow Fe-N-C spheres with different compositions（in terms of the species and the relevant fraction of the nitrogen dopants and the metal moeity）and structures（in terms of size,specific surface area,pore volume and pore structure）were prepared using different carbon sources and templates;the influence of the calcination parameters including the atomosphere and the temperature on the composition and the structure of the resulting Fe-N-C materials were investigated;the physicochemical properties and the ORR electrocatalytic activity and stability under the alkaline conditions were evaluated,and the structure-relationship between the apparent ORR catalytic activity（an index of which is the so-called half-wave potential E1/2）and the composition as well as the structure-relevant features of the Fe-N-C electrocatalysts were also attempted.The main achievements of this work are summarized below.（1）A type of commercial chain-like hollow carbon nanospheres,ECP-600JD,was employed as carbon source and template,which was pyrolyzed straightforwardly with the iron source and 2-amino-4-hydroxy-6-methylpyrimidine under Ar,leading to the formation of the Fe203@Fe-N-C nanocomposite.It has a large specific surface area of 631.1 m2g1(with an area of 467.8 m2g-1 contributed by mesopores),high electronic conductivity and abundant active sites of Fe-Nx;moreover,the Fe2O3 nanoparticles being encapsulated within the carbon framework were believed to further enhance its activity.Due to the compositional and structural merits,the ORR electrocatalytic performance of the best Fe2O3@Fe-N-C catalyst in 0.1 M KOH solution is comparable to that of the commercial Pt-C,which shows a half-wave potential of 0.84 V;when applied in zinc-air battery,it yields a power density of 88.3mW cm-2.It also shows better rate performance and electrochemical stability than the Pt-C electrocatalyst.It shall also be mentioned that the synthesis of such Fe2O3@Fe-N-C is facile and easily scalable.（2）Hollow benzoxazine resin nanoshells were prepared by a dual-template-assisted polymerization process,which then were calcined under different atmospheres and temperatures,yeilding a series of porous hollow Fe-N/C thin nanoshells with different structural characteristics and different N species content.The calcination atmosphere and temperature were found to cast significant impact on the composition of nitrogen species doped in the carbon framework and pore structure.The best Fe-N/C electrocatalyst has a specific surface area of 998.0 m2 g-1,a pore volume of 0.63 cm3 gi,and a half-wave potential of 0.919 V in 0.1M KOH,which is 37 mV higher than that of the commercial Pt/C.When used as a cathode in a zinc-air cell,it shows a power density and specific capacity of 93.5 mW cm-2 and 779.7 mAh/gzn-1,respectively.In addition,based on these Fe-N/C electrocatalysts,the correlation coefficients between half-wave potential E1/2 and the fraction of the various N species or structural characteristics were calculated to clarify the influence of the meso-scale structural features and the composition characteristics to the apprent electrocatalytic activity of Fe-N/C ORR from a statistical point of view,which could provide guidance for the design of high performance Fe-N/C ORR and contribute to the study on the structureactivity relationship electrocatalysts study. |
