Structure Construction And Electrocatalytic Performance Of Simple Compounds Of Transition Metals

As the major reactions,Oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?are of vital importance for new energy storage and conversion equipments?such as hydrogen-oxygen fuel cells,reversible zinc-air batteries?.Commercially,in order to avoid the slow reaction kinetics and high overpotentials for ORR and OER,the precious metals with excellent performances for ORR and OER,were used as electrocatalysts.However,there are still severe challenges such as high cost,scarcity of resources and low stability hindering the large-scale applications of the commercial precious metal catalysts.On the other hands,the transition metals also exhibit excellent performances for ORR and OER.Therefore,the low-cost simple transition metal compounds were researched as electrocatalysts for ORR and OER in this paper.Firstly,the structures of the electrocatalysts were built.Subsequently,the electrocatalytic properties were studied,and the following results are obtained in conclusion:?1?The hollow nano-carbon spheres were synthesized as support by glucose hydrothermal method,and the Ag nanoparticles,which are low-cost relatively to noble metals such as Pt,Pd,were used as active components.The Ag nanoparticles were loaded on hollow nano-carbon spheres by in situ reduction to construct the composite material?Ag@HCNS?.The cyclic voltammetry curves,the linear scanning voltammetry curves,the Tafel curves,the EIS curves,current-time chronoamperometric tests and rotating disk electrode and rotating ring disk electrode were used to study the electrocatalytic activity and mechanism.Results show that the onset potential of the Ag@HCNS catalyst is 0.82 V vs.RHE in alkaline electrolyte for ORR,and the Ag@HCNS exhibits excellent long-term running stability.It is the synergistic effect of Ag nano-particles and hollow nano-carbon spheres that enhances the performances for ORR.?2?The ZIF-8 was synthesized in water system,and the Co element was introduced.Then the catalyst with the active site for Co-N-C was prepared by annealing at 1000°C,which maintained the regular morphology and uniform particle size of ZIF-8.In alkaline electrolyte,the cyclic voltammetry,the linear scanning voltammetry,the Tafel curves,etc.were used to test the electrocatalytic performances of Co-N-C for ORR.Results show that the onset potential of Co-N-C is 0.94 V vs.RHE,and the peak current density is 5.2 mA cm^-2 for ORR.And the catalytic mechanism of Co-N-C for ORR was tested by rotating disk electrode and rotating ring disk electrode.It shows that the ORR occurs by the 4-electron reaction pathway.In addition,the catalyst shows superb activity for OER,and the overpotential is 470 m V at 10 mA cm^-2.?3?At room temperature,the MOFs precursor of FeCo was synthesized by a coprecipitation method(Fe²⁺,Co²⁺and trimesic acid were used as the metal centers and the organic ligand respectively).Then the precursor further annealed in an NH₃ atmosphere.Finally,a core-shell structure of Co₃Fe₇@Fe₂N was constructed on reduced graphene oxide?r GO?nanosheets?Co₃Fe₇@Fe₂N/rGO?.Electrochemical tests show that the Co₃Fe₇@Fe₂N/rGO exhibits an onset potential of 0.98 V v.s.RHE,peak current intensity of 5.5 mA cm^-2 and the long-term running stability for ORR,which is comparable to the performance of commercial Pt/C catalysts.What's more,the Co₃Fe₇@Fe₂N/rGO also shows an overpotential of 371 mV(at10 mA cm^-2)for OER.The robust ORR and OER catalytic activity of Co₃Fe₇@Fe₂N/rGO can be attributed to the core-shell structure and the large specific surface area,because such special structure provides high density of active sites?Fe-N-C?and high electron transfer capability.It also shows superb cycle stability and power density in reversible zinc-air batteries.