Controllable Preparation Of Hollow Nano-carbon Materials And Their Electrocatalytic Performance

Oxygen reduction reaction(ORR)is one of the important electrochemical reactions in fuel cells.However,due to its slow kinetic process,catalyst participation is required to accelerate the reaction process,so the development of high-performance catalysts has become the key controlling factors to the commercial application of fuel cell.Hollow nano-carbon materials usually have the characteristics of high specific surface area,rich pore structure,and adjustable active sites.They are currently recognized as one of the preferred materials for preparing low-cost and high-activity oxygen reduction electrocatalyst.In this paper,firstly,polyaniline was used as the carbon source and nitrogen source,and the nitrogen-doped carbon nanotube catalyst was successfully prepared by controlled carbonization and ammonia activation.The nitrogen-doped carbon nanotube have a top opening with a length of about 1-3 ?m and a diameter of about 130-170 nm.According to the hollow structure characteristics of polyaniline precursors,the transition group metal iron was introduced in the preparation process to prepare a hierarchical porous carbon nanotube catalyst doped with iron-nitrogen species,which has a high specific surface area(1038.8 m2/g),rich micropore(0.37 cm3/g)and mesoporous(0.13 cm3/g)structures,and shows excellent ORR activity(Half-wave potential in alkaline system:0.91 V vs RHE;Half-wave potential in acid system:0.78 V vs RHE).Secondly,in order to further increase the density of the active sites,the self-made nitrogen-doped polyaniline-based carbon nanotube and ferrocene were used as raw materials and modifier,using solid-phase thermal migration method successfully prepared activity enhanced catalyst.Compared with the liquid-phase reaction system,the catalyst prepared by the solid-phase synthesis system can better maintain the nanotube morphology,and can effectively avoid the agglomeration of metal particles.The catalyst has a half-wave potential of 0.87 V(vs RHE)in an alkaline system,and has durability and resistance to methanol poisoning that exceed the commercial Pt/C.XPS results indicate that the catalyst is rich in pyridinic-N,graphitic-N and Fe-Nx active sites,which is the reason for its enhanced catalytic performance.The proposed solid-phase synthesis method is simple to operate,does not require post-treatment processes such as pickling,and can achieve batch preparation.This method can guide the design of transition metal-nitrogen-carbon catalysts.Finally,in order to explore the effect of iron-nitrogen species and nano-carbon hollow structure on ORR activity,the commercial carbon black was selected as the raw material,and iron-nitrogen co-doped hollow carbon microspheres were successfully prepared using a hydrothermal-ammonia activation two-step method.The obtained catalyst has a half-wave potential of 0.9 V(vs RHE)in an alkaline system,and has excellent durability and resistance to methanol poisoning.The zinc-air battery assembled with the catalyst has a maximum power density(173.7 mW cm-2)which exceeding Pt/C,and has a high specific capacity(787.5 mAh g-1)and specific energy density(945 Wh kg-1).The XAS results confirm that the catalyst has one Fe atom connected to four N atoms and anchored on the curved carbon shell surface to form Fe-N4-C8 structure.Density functional theory calculations indicate that the positively moving d-band center of the Fe-N4 active site anchored on the curved carbon surface and the increase of the surface charge of the active Fe are the reasons for the significant improvement of the catalyst's ORR performance.