Preparation Of Iron-based Composite Materials And Nonmetal-doped Carbon Materials And Their Electrocatalytic Performance In Oxygen Reduction/oxygen Evolution Reactions

Developing highly active and stable electrocatalysts for oxygen reduction reaction（ORR）/oxygen evolution reaction（OER）to replace precious metal Pt and Ru O₂benchmarking electrocatalysts for rechargeable zinc air batteries remains a challenge.Fe-based transition metal catalysts have become a hot topic in electrocatalyst design due to their superior catalytic activity and low cost.Carbon-based metal-free catalysts also demonstrate their potential as ORR/OER catalysts due to their low price,good conductivity,and facile regulation of physical and chemical properties.In this thesis,composite electrocatalyst（Fe/NC@Nb₂C）and N,P co-doped metal-free carbon-based catalyst（NPC-950）were created by stepwise pyrolysis and heteroatom doping strategies.The microstructure and electronic properties of the electrocatalysts were revealed by a series of characterization methods.,and the ORR/OER activity and stability of the catalysts were tested.The main research contents of this paper are as follows:1.A stepwise pyrolysis strategy was employed with iron acetylacetone as Fe source,2-methylimidazole,and zinc nitrate hexahydrate for producing nitrogen-doped carbon support by high-temperature calcination at 950℃.Subsequently,another 500℃treatment was performed to obtain Fe/NC@Nb₂C catalyst.X-ray analysis（XRD）,scanning transmission electron microscopy（SEM）,transmission electron microscopy（TEM）,high-resolution transmission electron microscopy（HR-TEM）,and X-ray photoelectron spectroscopy（XPS）confirmed the successful fabrication of Nb₂C and the large number of carbon nanotubes produced by Fe nanoparticles grow in situ on carbon nanotubes.The synergic effects of them promote the catalytic activity.Fe/NC@Nb₂C showed excellent ORR activity and stability in0.1 M KOH:the onset potential and half-wave potential were 0.99 V vs.RHE and 0.90 V vs.RHE,respectively.Tafel slope was determined to be 64.0 m V dec^-1;The kinetic current density was 19.08 m A cm^-2at 0.85V vs.RHE.The reaction catalysed by this catalyst follows a four-electron transfer mechanism.After 10,000 cycles,the half-wave potential only attenuates by 5 m V.This work provides the experimental basis for the synthesis of efficient electrocatalysts.2.A heteroatom doping strategy was used to fabricate NPC-950 by polymerization of phytic acid as the P source,dicyandiamide as the N and C source,and zinc nitrate hexahydrate,followed by freeze-drying and calcination.The characterization of SEM,TEM,STEM,XPS,and NEXAFS confirmed that the catalyst is amorphous and has a layered porous nanosheet structure with a large specific surface area.These properties are favorable for promoting the transport of reactants and electrons and providing a high number of active sites.The nitrogen and phosphorus elements in the catalyst mainly exist in the dominant pyridine nitrogen,graphite nitrogen,and P-C structure.The P doping can induce the charge space redistribution around the NC center and modulate the catalyst’s electronic properties.NPC-950 exhibits excellent bifunctional ORR/OER electrocatalytic activity in 0.1 M KOH,with ORR half-wave potential(E_1/2)of 0.88 V vs.RHE,OER potential at 10 m A cm^-2(E_j=10)of 1.60 V vs.RHE,and potential difference of△E=E_j=10-E_1/2=0.72 V.It also showed exceptional ORR activity in 0.5 M H₂SO₄with E_1/2of 0.72 V.Moreover,NPC-950 exhibits superior stability in acidic and alkaline environments.In addition,this catalyst has strong potential in zinc-air batteries and achieved a high peak power density of 184.5 m W cm^-2and excellent long-term cycle stability within 85 h.This work provides theoretical guidance for the design and development of highly efficient metal-free electrocatalysts.