Heteroatom-doped Porous Carbon Materials Derived From Metal-organic Frameworks For Electrocatalytic Oxygen Reduction Reaction

Oxygen Reduction Reaction(ORR)plays a crucial role as a cathode reaction for clean energy conversion devices such as fuel cells.However,the large-scale commercial applications of fuel cells are limited by the slow reaction kinetic,expensive Pt/C catalyst,and easy poisoning.Therefore,it is especially important to develop electrocatalytic catalysts with low-cost,high-efficiency and stability for ORR.Non-precious metal materials have attracted wide attention from material chemists.In recent years,Metal-Organic Frameworks(MOFs)have the characteristics of adjustable chemical composition,pore structure,morphology and uniform heteroatom distribution,so they are used as precursors or templates to prepare heteroatom-doped porous carbon materials.These carbon materials have received increasing attention as they have inherited the merits of MOFs to achieve excellent catalytic activities in oxygen reduction reaction.Based on this,we have used a series of different MOF materials as precursors to prepare different heteroatom-doped porous carbon materials,and studied their performance in oxygen reduction reaction.The main research contents of this dissertation are as follows:1.Although the morphologies of MOF precursors have significant effect on the properties of resultant materials,up to now,there is no systematic study on the relationship between the morphologies of MOFs and the properties of their pyrolized carbonaceous materials.Herein,three isomorphous imidazolate-based ZIF-7 with different morphologies(sphere,polyhedron and rod shape)have been selected as precursors and carbonized to obtain porous N-doped carbon materials with tunable morphology,pore features and surface areas.Their electrocatalytic performance have been investigated.The NC-D-800 with the highest Brunauer-Emmett-Teller(BET)of 538m~2 g~-11 derived from polyhedral ZIF-7-D have showed better performance than nanocarbon sphere and micron-sized carbon rod in oxygen reduction reaction.This indicates that the morphologies of MOF precursors could affect the morphologies,physical and catalytic properties of their corresponding carbon materials.2.Despite that heteroatom co-doped carbon materials have been investigated in ORR,halogen involved porous carbon materials are rarely studied.In order to investigate the halogen doping effect on the catalytic performance,nitrogen/halogen dual-doped porous nanocarbons catalysts have been prepared using imidazolium functionalized cationic MOFs as precursors and templates.Among these carbon materials,the nitrogen/bromine dual-doped catalyst,BrNC-800,exhibits the best ORR performance,excellent stability and methanol tolerance in alkaline conditions.Its excellent electrocatalytic activity is attributed to a few factors:(1)the carbons adjacent to high contents of pyridinic-N or graphitic-N species are the active sites for ORR,and the high graphitization degree is beneficial for improving electric conductivity to enhance ORR activity;(2)the bromine can change the chemical composition and structural characteristics of the catalyst,and activates adjacent carbon to generate additional active sites;(3)the high specific surface areas and hierarchical porous structure can provide more active sites and promote mass transfer to improve the catalytic efficiency.