Synthesis And Electrocatalytic Oxygen Reduction Performance Of Nitrogen-Doped Hollow Carbon Spheres

The complex energy system built in the twentieth century gains the required energy by means of fossil fuel combustion,which not only results in serious energy waste due to the low conversion efficacy,but also produces a great deal of dust and poisonous/harmful gases such as carbon dioxide,nitrogen oxide and sulfur oxide severely threatening the living environment of human.As a new kind of efficient and environmentally friendly energy conversion device without burning process and limits of Carnot cycle,fuel cell has potential applications in vehicle power and domestic electricity and is the key goal of development for the future sustainable energy system.The oxygen reduction reaction?ORR?,due to its kinetically sluggish nature,is the pivotal step controlling the overall performance of fuel cells.To date,Pt-based catalysts are still the main ORR catalysts due to their superior activity.However,the low storage and high cost of Pt greatly hinder the commercial development of fuel cells.Therefore,the development of non-Pt electrocatalysts with high performance and low cost is the key issue to realize the commercialization of fuel cells.Aimed at the new energy system of fuel cell and with the purpose of acquiring low-cost and highly active ORR catalysts,the research in this thesis is to develop a simple,green and feasible way to synthesize nitrogen-doped hollow carbon spheres,together with the study of their ORR activity.The main works in this thesis are as follows:?1?The hollow polymer spheres?HPSs?were prepared by hydrothermal method with sodium oleate?SO?/P123 as the soft template and 2,4-dihyoxybenzoicacid?DA?/hexamethylentetramine?HMT?as the carbon source.Then,the nitrogen-doped hollow carbon spheres were prepared by using HPSs as the precursor and ammonia gas or aqua ammonia as the nitrogen source.Whatever the nitrogen source is,it is found that they both show higher surface area,hierarchical porosity and greatly enhanced ORR activity with with a close four-electron reaction pathway and a peroxide yield lower than 10%.However,NHCSs with ammonia gas as the nitrogen source exhibites higher surface area and better ORR activity.?2?Based on the mechanism of the synthesis of HPSs,in situ formation of nitrogen-doped carbon nanoparticles on hollow carbon spheres?NHCSs?were realised via in situ polymerization route with melamine as the nitrogen precursor,followed by high temperature pyrolysis.It is found that the melamine has great effect on the morphology,structure and electrocatalytic ORR performance of NHCSs.The addition of melamine produces a large amount of micropores,which provides microreaction sites for ORR,greatly increasing the surface area and ORR activity.When the molar ratio of melamine to HMT is 1,the as-synthesized NHCS-2 exhibites the highest surface area(837.7 m² g^-1),micropore surface area(620.1 m² g^-1)and the highest relative content of total pyridinic-N and graphitic-N(64.9 m² g^-1).The NHCS-2 exhibits the best ORR activity with a close four-electron reaction pathway.?3?The iron-nitrogen-doped hollow carbon spheres?FeNCSs?were prepared by using ammonia gas as the nitrogen source,anhydrous ferric chloride as the iron source and HPSs as the in situ carbon template.It is found that the amount of doping Fe and pyrolysis temperature both significantly affect the morphology,structure and electrocatalytic ORR performance of FeNCSs.FeNCS-1000 with 2 wt%Fe?mass ratio of Fe to C?prepared by the pyrolysis at 1000?has the highest surface area(613 m²g^-1),relative content of Fe-N_x?38.9 at%?and shows the best ORR activity both in alkaline and acidic media.FeNCS-1000 exhibits very superior ORR activity with a half-wave potential(E_1/2)of 0.886 V in alkaline media,15 mV more positive than that of commercial Pt/C catalyst.Both in alkaline and acidic media,FeNCS-1000 shows excellent ORR activity with a direct four-electron reaction pathway and a peroxide yield lower than 10%.FeNCS-1000 also showes much better long-term stability and resistance to methanol crossover effect.