Study On Oxygen Reduction Reaction (ORR) Electrocatalysts Constructing Hierarchical Structure By Hemin

Proton exchange membrane fuel cells（PEMFCs）,which have the advantages of high energy density and low pollution,are a typical representative of green energy conversion systems.However,the slow oxygen reduction reaction（ORR）kinetics causes continuous loss of electrode voltage,which makes its energy efficiency continuous reduce.Usually platinum（Pt）based precious metal catalysts are used to accelerate the ORR process,but the use of Pt is restricted by problems such as high cost,low toxicity and durability.Therefore,the research goal of this paper is to explore a low-cost,high-efficiency oxygen reduction catalysts.In numerous material studies,non-noble metal nitrogen-doped carbon-based（M-N-C）catalysts have shown excellent catalytic activity and stability,and have always been considered as the substitutes for Pt-based catalysts.Therefore,based on this original intention,this paper uses Hemin as the active precursor to prepare zero-dimensional and two-dimensional catalysts with hierarchical active sites and synergistic catalytic effects.The research contents are as follows:The first is to construct a zero-dimensional spherical Fe-N-C catalyst with hierarchical distribution of active sites.In the experiment,a sol-gel method was used to prepare monodisperse 300 nm Si O₂spheres.Using this as a template,dopamine（DA）was modified on the surface of the Si O₂sphere as the first layer of N-rich carbon-based framework layer,and then Hemin was polymerized by olefin polymerization.The second layer of Fe-N-HCS-X（X=600～900）hollow carbon ball electrocatalyst containing Fe-N-C active sites was coated on the surface of the carbon framework and prepared by pyrolysis and template removal.The physical and chemical characterization results show that the high content of pyrrdinic N,Fe-N and graphitic N species in the hierarchical structure is the main factor to improve the catalytic activity.At the same time,in-depth exploration of the sensitivity of the catalyst performance to the pyrolysis temperature found that the Fe-N-HCS-800sample prepared by 800℃pretreatment has comparable ORR catalytic activity to Pt/C,and shows a high initial level under alkaline conditions.Potential（0.97 V）and half-wave potential（0.84 V）.At the same time,the limiting current density can reach-5.82 m A cm^-2,and the activity attenuation is only 3.3%after the endurance test of20000 s,and the 15000 cycle cyclic voltammetry test The potential drop in the second half wave is only 9.3 m V.In view of the above-mentioned research characteristics and high-efficiency M-N4 active sites,this section uses a non-pyrolysis strategy to construct a two-dimensional hierarchical Fe-N-C catalyst.The experimental idea is to use graphene oxide（GO）as the carbon substrate,and use theπ-πconjugation between Hemin molecules and the two-dimensional graphene structure to load Hemin during the hydrothermal reduction process to prepare Fe/N doping ORR electrocatalyst with layered structure（r GO-hemin）.The physical and chemical characterization results show that the polymerized Hemin successfully links the graphene sheets,and the Fe-N₄macrocyclic compound structure in the r GO-hemin sample is retained intact.Its active center is Fe-N species with a content of up to 52%.The initial potential under the stable conditions is 0.92 V,the half-wave potential is 0.79 V,the limiting current density is-4.98 m A cm^-2,and the activity is only attenuated by 4.7%after the endurance test of 20000 s,5000 cycles of cyclic voltammetry After the test,the half-wave potential is only reduced by 3.6 m V.Based on the structural characteristics of natural heme,this paper provides two ways to construct an efficient ORR catalyst strategy,which provides important theoretical data for the application of ring-mounted tetrapyrrole compounds in the ORR direction.