Construction Of Three-Dimensional Oxygen Electrode Materials For Electrocatalysis

The sluggish kinetics in the oxygen electrode catalytic reaction is the one of the main obstacles for the development of polymer electrolyte fuel cells and zinc-air batteries.In order to get rid of dependence on precious metals,cost-effect 3d-transition metal based catalysts and non-metal catalysts with abundant-reserves are widely investigated in energy conversion.In this thesis,three-dimensional bifunctional electrocatalysts for oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)were investigated and constructed,including single and binary hollow structured transition metal oxides,multi-shelled transition metal composites and heteroatoms co-doped graphene based nanocomposites oxygen electrodes.The formation mechanisms,micro morphology and electrochemical performance were in-depth studied,initially revealed the relationship between the physical structure and electrochemical performances,provides theoretical basis and new research ideas for the development of oxygen electrode catalysts for fuel cells and zinc-air batteries.The main achievements of the dissertation were as follows.(1)In terms of "Kirkendall" effect,carbon supported three dimensional NiO/C,Co3O4/C and Fe2O3/C with hollow structure were successfully prepared.The sizes of the obtained nanoparticles were ranging from 50 to 100 nm with wall thickness of-20 nm.Due to the existence of porous structure,the active sites were effectively enhanced.Among the three catalysts,Co3O4/C exhibited the highest ORR activity,meanwhile,Fe2O3/C and NiO/C exhibited excellent OER performance.(2)On the basis of single hollow structured transition metal oxides,Co2FeO4/C and NiCo2O4/C binary metal oxides with three-dimensional structure were prepared by using the same method.The reversibility of the redox couple between two metal species was significantly enhanced compared with the single metal oxides catalysts,which effectively facilitated the bifunctional electrocatalytic performance.In addition,compared with Pt/C,the binary catalysts exhibited higher stability and methanol tolerance.When served as oxygen electrode in zinc-air battery,the as-prepared catalysts showed excellent rechargeable stability.(3)In order to further promote the electrocatalytic activity,nitrogen doped carbon supported three-dimensional transition metal based composites were obtained via post-annealing process by using EDTA as nitrogen source and chelating agent.Co-EDTA and Ni-EDTA with a relative high complexation constant resulted in double-shelled Co@CoO/NDC and Ni@NiO/NDC nanomaterials after annealing process,while Mn-EDTA with a relative low complexation constant were covered by thick carbon layer after carbonization,and the carbon layer become more dense as the increasing of annealing temperature.The synergistic effect between Co and N in Co@COO/NDC-700 resulted in the best bifunctional catalytic performance compared with Ni@NiO/NDC and MnO/NDC.When served as oxygen electrode in rechargeable zinc-air battery,CO@CoO/NDC-700 exhibited excellent peak power density(192.1 mW cm-2),current density,discharge potential and rechargeable stability.(4)Compared with transition metal oxides,carbon based materials possess higher electronic-conductivity.Three dimensional intercalated composite was obtained by inserting Vulcan XC-72 carbon nanospheres into the graphene layers and then co-doped with N/S heteroatoms.The obtained intercalation structure effectively prevented graphene from re-stacking,increased the specific surface area and enhanced the active sites.The process of N/S co-doping facilitated the ORR catalytic performance.When served as oxygen electrode material in zinc-air battery,excellent battery performance was obtained.(5)Three-dimensional graphene based composites were successfully constructed via partially exfoliation of one dimensional multi-walled carbon nanotubes.The composites were experienced N/S or N/P co-doping to facilitate the electrocatalytic performance.Result showed that the exfoliation degree plays an important role in catalytic activity.Partially exfoliated nanocomposites,with unzipped graphene nanoribbons surrounded the unzipped nanotubes to form a three dimensional structure which effectively prevented the graphene from re-stacking and enhanced the electrocatalytic activity and zinc-air battery performance.