Preparation And Catalytic Performance Of Three-dimensional Porous Nitrogen-doped Carbon-based Catalysts

The electrochemical oxygen reduction reaction（ORR）plays a key roles in energy storage and conversion devices.As a new type of environmentally friendly energy conversion device,fuel cells have attracted wide attention due to their good cycle stability and high energy density.Among them,the proton exchange membrane fuel cells（PEMFCs）are considered to be ideal clean energy conversion devices because of their high energy conversion efficiency,low operating temperature,and low environmental pollution.They are very suitable as the power source for green new energy vehicles.At present,the proton exchange membrane fuel cells are mostly based on platinum-based noble metal catalysts because of their excellent ORR performance.However,due to the high price of Pt and the low storage capacity in nature,the cost of Pt-based catalysts remains high.In addition,Pt-based electrocatalysts tend to aggregate and deactivate in the presence of CO or methanol.And the slow kinetics of the oxygen reduction reaction（ORR）limits the development of these energy conversion devices,making the fuel cell catalyst an important barrier to large-scale commercialization of proton exchange membrane fuel cells.Therefore,it is urgent to seek a non-precious metal catalyst to replace Pt-based catalysts.As is well known,among the many non-precious metal catalysts,carbon-based catalysts have received extensive attention due to their high catalytic activity,good chemical stability,high specific surface area,and low cost.Three-dimensional nitrogen-doped porous carbon materials are widely used in energy storage materials,catalytic materials and adsorption materials because of their special morphology and excellent three-dimensional structure.Therefore,the three-dimensional porous carbon material has been widely studied as a fuel cell cathode catalyst for oxygen reduction reaction.Based on the above discussion,chemical oxidation polymerization was used to add transition metal salts and strong oxidizing protonic acids containing halogen elements in the polymerization of nitrogen-containing aromatic small monomers to obtain halogen-modified transition metal-containing aromatic macromolecule precursors.Precursor was prepared through one-step gradient heat treatment and carbonized in one step to prepare a three-dimensional nitrogen-doped porous carbon material that is ultra-light and cross-linked and mesh-like.The results of the research are as follows.（1）The modified nitrogen-containing aromatic mascromolecular precursor was obtained by chemical oxidation method,after modifying the Fe/N molar ratio of doped transition metal iron salt and aromatic small molecule（1:4,1:6,1:8,1:10）.Then heated to a high temperature gradient and the next step of heat treatment gives a nitrogen-doped porous carbon material.（2）The morphology,chemical composition,specific surface area and pore size distribution of the three-dimensional nitrogen-doped porous carbon materials were analyzed by XRD,TEM,SEM,Raman,BET and XPS.SEM shows that the morphology of the porous structure is a three-dimensional cross-linked interconnected porous network.TEM shows that it is porous and thin;XRD shows that the carbon is mainly in the form of disordered carbon.Raman spectroscopy shows that this carbon material is graphitized,and it has a good degree of graphitization.The BET test shows that it has a high specific surface area（1080m²/g）and a good pore size distribution,and the pores have a hierarchical structure.XPS measurements characterize this porous carbon material with higher nitrogen doping of 3.04%.The existence of the hierarchical pore structure is more conducive to the mass transfer and the generation of active sites,leading to improved electrocatalytic performance.As a cathode catalyst for fuel cells,the oxygen reduction catalytic activity of this three-dimensional nitrogen-doped porous carbon-based catalyst was investigated by means of electrochemical analysis.（3）As a fuel cell cathode catalyst,this nitrogen-doped porous carbon material was used to investigate the influence of the molar ratio of the transition metal-doped iron salt and the nitrogen-containing aromatic small molecule on the ORR performance of the carbon-based catalyst.When the mass ratio of Fe/N is 1:6,this nitrogen-doped carbon-based catalyst has the highest ORR activity,its half-wave potential exceeds the standard commercial platinum carbon 5 mV in an alkaline medium,the half-wave potential is 0.857V,the initial reduction potential was 0.968 V and the limiting current density was 4.153 mA/cm^-2.After test of this nitrogen-doped carbon-based catalyst under 5000 cycles,the obtained half-wave potential was 0.830V,and its decay rate was3.2%,indicating that this nitrogen-doped catalyst has better cycle stability.（4）The precursors of modified nitrogen-containing aromatic macromolecules with ratio of 1:6 have been heat-treated at different temperatures（900℃,1000℃,1100℃）.The nitrogen-doped carbon-based catalyst has the highest ORR activity and good cycle stability at a heat treatment temperature of 1000°C.In the alkaline medium,the half-wave potential is 0.857 V,the initial reduction potential is 0.968 V,the limiting current density is 4.153 mA?cm^-2,and after 5000 cycles,the half-wave potential is 0.830 V,and the decay rate is 3%.The nitrogen content was 3.04%,the contribution of graphite nitrogen was obvious,and the degree of graphitization was good.