Preparation Of Nitrogen-doped Porous Carbons And Their Performance In Oxygen Electroreduction

Platinum-based oxygen reduction reaction(ORR)catalysts in the cathodes of proton-exchange membrane fuel cells usually encounter a searies a problems in practical applications such as scarcity,high cost,loss of activity due to poisoining with CO and other carbon-derivates and so on,which hinder the wide spread utilization of fuel cell technology.Among the nonprecious alternative catalysts,heteroatom doped carbons represent one promising class of ORR catalysts because of many advantages such as high ORR catalytic activity,abundantly available source materials,facile preparation,tunable sturcture,low cost,and high operation stability.For carbon electrocatlaysts,besides the nature of active sites two key factors govern their performance,the content of active sites and the specific surface area.Normally,the specific surface area and associated porous structures,which are highly influeneced by synthetic methods and procedures,largely determine the number of accessible active sites and also affect the transfer of ORR-relevant species.Herein,a series of catalysts with high surface area as well as excellent performance in catalyzing oxygen reduction are synthesized with appropriate precursor materials and sophisticated synthetic methods.Site-probing control experiments are also conducted to shed light on the origin of excellent catalytic activity for present carbon catalysts.In this dissertation,the research efforts are mainly devoted to the following two aspects:(1)a hydrogen-bonded organic framework(HOF)comprising of melamine and trimesic acid was used as a highly porous carbonaceous precursor to prepare N-doped carbons through facile,direct pyrolysis.The resulting catalyst had a high content of nitrogen dopants(4.8 at%)and high surface area of 1321m2g-1 and exhibited excellent ORR catalytic activity in 0.1M KOH aqueous solution compared to commercial Pt/C catalyst.In addition,The results of active sites probing experiments showed that the efficient ORR activity of catalyst was ascribed to the synegisitic effects between the trace amount of Fe-containing species,graphitic N dopants and the abundant mesopores.(2)Secondly,we developed a facile,bottom-up route to the preparation of N-doped carbons with a high surface area as well as plenty of hierarchical pores by thermal annealing of the black bread-like sulphuric acid-dehydrated sucrose.This simple sulphuric acid treatment created a remarkable surface area of 645.7m2g-1 in the dehydrated sucrose,and the subsequent thermal annealing helped further generated more porous textures in the carbon matrix,which contributed to a high surface area of 2450m2g-1,a large number of hierarchical pores ranging from 2 nm to 150 nm in a highly porous N-doped carbon to sufficiently expose active sites and also boost the mass transfer of ORR-relevant species.Significantly,thus-synthesized catalysts showed a higher electron-transfer number,higher working stability,stronger tolerance to methanol crossover effect and a very comparable onset potential,diffusion-limited current density in alkaline electrolyte,compared with the bench-mark Pt/C catalyst.