Synthesis And Electrocatalytic Properties Of Novel Transition Metal-N-C Materials

In recent years,the decrease of renewable fossil fuel resources and the growing environmental problems led to a paradigm shift in the utilization of resources.Zinc-air batteries（ZABs）,as a new energy device,show huge advantages such as high energy density,low cost and environmental friendliness.However,ZABs currently face the crucial challenges,namely the sluggish kinetics on the air cathode side during the ORR and OER processes result in a high overpotentials.Usually,noble metal catalysts such as platinum,ruthenium,iridium and their oxides are used to reduce the overpotential of oxygen reaction.However,these catalysts are expensive and have poor cycling performance in alkaline solution.Therefore,it is necessary to develop alternatives to noble metal catalysts.Biomass valorization has recently gained considerable attention in an attempt to produce value-added fine chemicals as well as carbon-based functional materials.Chitosan is a nitrogen rich（～7 wt%）renewable biomass resource derived from seafood waste.Nitrogen-containing function groups（amido and acetmido）in it make chitosan a suitable precursor used for synthesis of nitrogen-doped carbon materials.In this dissertation,novel methods were adopted to prepare a series of transition metal based electrocatalysts by using chitosan as the nitrogen-containing carbon source.The morphology and structural properties of the as-obtained hybrid materials were characterized by scanning electron microscope（SEM）,transmission electron microscope（TEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and Raman spectroscopy techniques.The electrocatalytic properties of the as-prepared catalysts were studied by electrochemical methods such as cyclic voltammetry（CV）,chronoamperometry（i-t）and accelerated durability test（ADT）,and many meaningful results were obtained.The results are summarized as follows:1.Using the mixture of pyridine-coordinated Fe MIL-101-NH₂ and chitosan as the precursor,novel Fe C nanoparticles-loaded N-doped carbon nanosheets,named Fe-NCSs-T（where T stands for pyrolysis temperature）,were successfully prepared.The optimized Fe-NCSs-900 exhibits excellent catalytic activity and good stability for oxygen reduction reaction（ORR）.The zinc-air battery assembled using Fe-NCSs-900as an air electrode has high peak power density and good durability,even better than the commercial Pt/C catalyst.2.Using chitosan as a carbon source,the g-C₃N₄ the nitrogen source and a ligand coordinated with Fe³⁺and Co²⁺,novel Fe₇Co₃ alloy nanoparticles-embedded nitrogen-doped carbon nanosheets bifunctional catalysts,labeled as Fe Co@NCSs-T（where T stands for pyrolysis temperature）,were obtained.The optimized Fe Co@NCSs-T displayed excellent electrocatalytic performance for both oxygen reduction and evolution reactions in alkaline media,comparable to precious metals.The zinc-air battery assembled with Fe Co@NCSs-800 air cathode showed higher peak power density and better long-term charge-discharge cycle stability than the Pt/C+Ru O₂air electrode.3.In order to further improve the ORR activity of the chitosan-derived catalyst,a three-dimensional honeycomb briquet-like Fe-N_x-C catalyst was synthesized by using chitosan as the carbon source,three-dimensional cyanate-melamine salt as the nitrogen source and structural guiding agent,and Fe salt as the metal source.The as-obtained material showed large electrochemically active surface area and small charge transfer resistance,making it highly active and durable in catalyzing the ORR,comparable to the benchmark Pt/C catalyst.