Synthesis Of Cobalt/Nitrogen Co-doped Hierarchical Porous Carbon Nanospheres And Their Electrocatalytic Performance For ORR

Oxygen reduction reaction（ORR）is one of the crucial electrode reactions of new energy conversion and storage devices such as fuel cells and metal-air batteries,and it heavily depends on electrocatalysts because of its sluggish kinetics.Up to now,commercial Pt-based catalysts are regarded as the most efficient catalysts for ORR.However,owing to its prohibitive cost,scarcity,poor durability and methanol tolerance,it is extremely imperative to explore cost-effective ORR catalysts with superior performance to substitute Pt-based catalysts.Transition metal and nitrogen co-doped carbon（M/N-C,M=Fe,Co）catalysts are considered to be one of the promising candidates because of their earth-abundant resource as well as their excellent catalytic performance.In this context,a novel method for constructing nitrogen-doped porous carbon nanospheres based on host-guest self-assembly and template method was developed.Furthermore,a single atomic transition metal and nitrogen co-doped carbon nanospheres with excellent ORR performance was prepared.First of all,γ-cyclodextrin（γ-CD）,a natural macrocyclic compounds with specific structure was selected as the carbon source,amphiphilic block copolymer（F127）and p-phenylenediamine（PPDA）were respectively used as the template and the nitrogen source.Nitrogrn doped porous carbon nanospheres catalysts with tunable nitrogen content and morphology were successfully synthesized via self-assembly,hytrothermal polymerization-carbonization and high temperature pyrolysis.Due to the coordination of nitrogen and metal elements,Co element was adsorbed on the surface of hierarchical porous carbon nanospheres and anchored by high-temperature treatment,successfully preparing sigle-atomic cobalt and nitrogen co-doped porous carbon nanospheres（Co/N-PCNS）catalyst.This method is simple,controllable,repeatable,can be mass-produced,and has an excellent application prospects.The specific research contents and major conclusions are as follows:（1）The structures of self-assembly host-guest complexs derived fromγ-CD and F127,as well asγ-CD,F127 and PPDA,were systematically studied.The influence of the proportion betweenγ-CD and PPDA on nitrogen cntent and morphology of products was also investigated.The mass ratio ofγ-CD and F127 remained unchanged,the molar ratios of PPDA andγ-CD were adjusted to 0:1,1:1 and 2:1 respectively.After hydrothermal polymerization at 220°C and further pyrolysis activation at900°C,three kinds of nitrogen-doped porous carbon spheres were prepared and were denoted as PCNS-900,N-PCNS-900 and DN-PCNS-900,respectively.The experimental results show that:with the increasing of p-phenylenediamine concentration,the size of hierarchical porous carbon nanospheres increased firstly and then decreased,while the nitrogen content increased gradually（0～7.7 at.%）.And the order of catalytic performance of catalysts with different nitrogen content is DN-PCNS-900>N-PCNS-900>PCNS-900.What’s more,the effects of pyrolysis temperature（T=800,900 and 1000°C）on the microstructure,element content and ORR electrocatalytic performance of hierarchical porous carbon nanoparticles were systematically researched.With the increase of carbonization temperature,it can be seen that the nitrogen content of porous carbon nanospheres decrease and the specific surface area increase gradually.Sample obtained by pyrolysis at 900°C（DN-PCNS-900）have the highest pyridine nitrogen content（31%）,large specific surface area（1086 m²/g）,which can offer more effective active sites.Hence it shows the optimal ORR electrocatalytic performance(E_onset=0.8 V vs.RHE).（2）In this paper,the transition metal cobalt element was adsorbed and coordinated by nitrogen of nitrogen-doped hierarchical porous carbon nanosphere,and then anchored through high temperature pyrolysis to obtain atomic cobalt and nitrogen co-doped porous carbon nanosphers（Co/N-PCNS）.HAADF-STEM combined with XAFS have been selected to characterize the distribution,valence and chemical coordination of cobalt in Co/N-PCNS.The results directly prove that there is about 0.2 at.%cobalt in CoN-PCNS,and most of them are atomically dispersed and form Co-N₄ structure coordinated with four N atoms.The advantage of this material is that the metal-nitrogen coordination effectively avoided the aggregation and oxidation of metal.The catalytic performance of the transition metal active sites was improved by atomizing metal.The ORR performance of CoN-PCNS in alkaline environment is superior to that of commercial Pt/C catalyst:the half-wave potential reaches 0.85 V vs.RHE in 0.1M KOH alkaline electrolyte,37 m V more positive than that of Pt/C.Within potential range of 0.2～0.6v,the number of transferred electrons is as high as3.99,and the corresponding H₂O₂ yields is only 1.7～3.8%.In addition,CoN-PCNS catalyst exhibits excellent cyclic stability and methanol resistance,the current density of the catalyst could maintain 98%of the initial current after 10000 s’cyclic stability test.（3）Based on the Density functional theory（DFT）,the relationships between the microstructure,the distribution of elements,the active sites and the ORR electrocatalytic performance of catalysts were systematically analysed.The results show that during the catalytic process of CoN-CPNS,O₂ molecules are adsorbed in the Co atoms in the Co-N₄ structure with end-on model,which is beneficial to improve stability of the active site.Besides,by comparing the catalytic process on microporous defective carbon,nitrogen-doped carbon and atomic cobalt-nitrogen co-doped carbon,we found that the activation energy of each step of oxygen reduction reaction show the trend as follow:microporous defective carbon<nitrogen-doped carbon<monoatomic cobalt-nitrogen co-doped carbon.It is consistent with the gradual improvement of ORR catalytic performance of the porous carbon spheres（PCNS）,nitrogen-doped porous carbon spheres（N-PCNS）and single-atomic cobalt and nitrogen co-doped porous carbon spheres（Co/N-PCNS）,which were prepared by host-guest self-assembly and template method.