Preparation Of Heteroatom Doped Carbon-Based Electrocatalytic Materials And Study On The Property Of Oxygen Reduction Reaction

New energy storage and conversion technologies such as renewable fuel cells,metal-air batteries,and water electrolysis have become increasingly prominent in the energy revolution.However,the oxygen reduction reaction（ORR）,as one of their core reactions,shows slow kinetics,which greatly limits their energy conversion efficiency,so it is necessary to synthesize highly efficient electrocatalysts to improve ORR activity.The key to improving ORR activity is to rationally optimize the interaction force between the electrocatalyst surface and oxygen or oxygen-containing intermediates,so that this force is not too strong or too weak,but“just right”.The precious metal represented by Pt has a high catalytic activity for ORR,because its surface structure is more suitable for the combination of oxygen or oxygen-containing intermediates.However,the shortcomings of high cost,low stability,and obvious effect of methanol cross-effect seriously hinder its large-scale application.Therefore,it is urgent to develop a low-cost,high-efficiency and strong-stability electrocatalyst to replace the noble metal-based electrocatalyst.Carbon material,as a material with great potential,has always been regarded as a high-quality substitute for noble metal catalysts,but the surface of the original carbon material is inert to the adsorption or activation of oxygen or oxygen-containing intermediates,which often requires surface modified.The chemical doping method can introduce heteroatoms with different atomic size,electronegativity and bonding state from carbon atoms,and then improve the ORR activity of the catalyst by modulating the local electronic state,which is a simple and efficient modification method.However,the actual doping process often faces many challenges,such as the problem that the doping and high graphitization cannot be coordinated and the catalytic mechanism is imperfect,which all need to be further resolved.Based on the above problems,in this paper,Co/N and N/P co-doped carbon-based electrocatalysts were synthesized by chemical doping with coal tar pitch（CTP）as the main raw material.Their electrocatalytic properties were characterized,and the effects of the multi-scale structure of the electrocatalysts on the mass transfer of boundary layer oxygen molecules in ORR process were investigated.The detailed research contents and results are as follows:1.The M-N-C carbon-based electrocatalyst was prepared by introducing the active site Co-N_x on the surface of the high-graphitization carbon support through the“two-step carbonization method”.By optimizing conditions such as temperature and Co loading,a“Point discharge”structure was constructed on the surface of the high-graphitization carbon support,so that the synthesized catalyst had a hierarchical pore structure,high graphitization and efficient Co-N_xactive sites simultaneously.Among them,the hierarchical pore structure accelerated the mass transfer process of the reaction medium;high graphitization was conducive to conductivity and stability;the active site Co-N_x improved the catalytic effect.The ORR performance of ACTP₅@Co,N-800 under alkaline conditions was significantly better than that of commercial Pt/C catalysts,with the initial potential of 1.00 V and half-wave potential of 0.891 V.It was little affected by the cross effect of methanol,and had almost no current loss within 12 000 s,showing excellent stability.Moreover,OER dual-function characteristics were demonstrated,and the overpotential at current density of 10 m A/cm²was only 370 m V.Based on this,with the typical M-N-C carbon-based catalyst as the research object,the effect of the catalyst structure on the mass transfer of oxygen molecules in the boundary layer during the ORR process was further explored.By using the chronocurrent method and the Cottrell equation,the diffusion coefficient（D₀）and the diffusion layer thickness（δ）of the research object were firstly calculated,and it was found that the sample with better catalytic performance had larger D₀ andδvalues.From the perspective of mass transfer kinetics,three cases of oxygen molecule transfer in the catalyst boundary layer were discussed in detail,and it was proved that the effective synergy between the active structure Co-N_x andπ-πinteractions is the key to promote the mass transfer in the catalyst boundary layer.2.Using melamine and phytic acid as the precursors of N and P,respectively,and modifying the porous carbon support by self-assembly of the two to prepare a non-metal doped carbon-based electrocatalyst.Firstly CTP was made into a porous carbon support by using a pore-forming agent at a high temperature,then the macromolecular formed by the reaction of phytic acid and melamine was attached to the surface of the support,and finally further pyrolysis was performed to obtain the N/P co-doped carbon-based electrocatalyst C-N-P-800.The sample C-N-P-800 with high graphitization,hierarchical pore structure and active structure N-C and P-C showed excellent bifunctional electrochemical characteristics under alkaline conditions.The initial potential and half-wave potential of the ORR test were respectively 0.976V and 0.841 V;the initial overpotential of OER was about 220 m V,and the corresponding overpotential was about 350 m V when the current density is 10 m A/cm².Its good catalytic activity was mainly due to two factors:the hierarchical pore structure promoted the transmission process of the reaction medium;the active sites P-C and N-C synergistically promote the ORR process.