Structural Engineering And Performance Study Of Hierarchical Porous Co/N/C-based Oxygen Elecrocatalysts Via Self-template Method

Zinc-air batteries are regarded as the most promising next-generation energy conversion devices due to their high energy densities and zero carbon emissions.Unfortunately,the oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?suffer from sluggish dynamics;thus,high efficient catalysts are required to drive these reactions.Currently,precious metal-based catalysts are generally regarded as the best ORR and OER electrocatalysts.However,the limited supply,high cost and inferior durability of these materials impede their large-scale commercialization.In order to alleviate the dependency on precious metals and further enhance electrocatalytic performance,it is of great importance to engineer advanced electrocatalysts with well-defined nanostructures and optimized compositions based on non-precious materials.In this paper,two kinds of catalysts with unique structure and excellent electrochemical performance were prepared by the ingenious use of organic and inorganic templates.A variety of physical methods?SEM,TEM,XRD,BET,etc.?were used to characterize the materials.The effects of temperature and acid removal time on the structure,the effects of morphology and chemical composition on the electrochemical properties were investigated.Specific research contents are as follows:?1?Structural engineering of S-doped Co/N/C mesoporous nanorods?S-Co/N/C MNR?for oxygen reduction reaction and zinc-air batteriesAimed at the problems of difficult control of pore structure and easy agglomeration of metal species in the synthesis of M/N/C materials,a facile Ostwald ripening-assisted template method is carried out to controllably synthesize one-dimensional S-doped Co/N/C mesoporous nanorods.The results show that S-Co/N/C MNR has highly dispersed Co/N/C active sites,which effectively solves the problem of agglomeration and uneven distribution of metal particles during heat treatment.Introducing the S doping into the Co/N/C structure increases the intrinsic activity and electrochemically active surface area.In addition,the structural characteristics?e.g.,the surface area,pore structure?of mesoporous nanorods are optimized on the basis of the Ostwald ripening effect for sufficiently exposing active centers and facilitating fast ion transport.Therefore,S-Co/N/C MNR demonstrates excellent catalytic performance for both oxygen reduction and zinc-air batteries.On the one hand,it exhibits high oxygen reduction activity in 0.1 M KOH with onset potential of 0.970 V vs.RHE and half-wave potential of 0.890 V vs.RHE,and excellent long-term durability with only 2.6%reduction after 40000 s.On the other hand,when used as a catalyst for air electrode,the assembled zinc-air battery exhibits remarkable long-term cycling performance.It can be cycled over 1000 cycles at 5 mA cm^-2,and after 200 h?600 cycles?,the voltage gap only slightly increased 16 mV?1.5%?.?2?Structural engineering of plum-like Co/Fe3C mesoporous nitrogen-doped carbon spheres?Co/Fe3C/PMNCS?for oxygen reduction reaction and zinc-air batteriesIn order to further improve the catalytic performance,a dual-site coupling strategy was used to design materials by adding Fe₃C active sites to the catalyst with Co/N/C active sites.Using melamine-formaldehyde microspheres as templates and chemical vapor deposition?CVD?method,nitrogen-doped mesoporous carbon spheres with unique plum-like morphology and Co/Fe₃C inlay are synthesized.During the synthesis process,a novel method of direct initiation of dopamine polymerization by Fe³⁺is used,and it is the key to the formation of plum-like structure.This unique plum-like structure contains sufficient mesoporous structure,which can effectively expose a large number of active sites,and has a larger electrochemical active area to ensure the rapid mass transfer process.In addition,Co/Fe₃C/PMNCS catalyst materials have highly active Fe₃C sites,and Co doping can form additional Co/N/C active sites.The synergistic effect between these two sites is helpful to further enhance the catalytic activity.Thus,the ORR catalytic performance of Co/Fe3C/PMNCS is better than the reported materials ever before.The half-wave potential of Co/Fe3C/PMNCS in 0.1 M KOH solution is 0.928 V vs.RHE,which is58 mV higher than the state-of-the-art Pt/C catalyst.At the same time,the zinc-air battery assembled with Co/Fe3C/PMNCS exhibits a peak power density of 44.2 mW cm^-2,which is 49.8%higher than that of Pt/C.