Synthesis And Electrochemical Properties Of Fe-Co-Ni Composites Based On MOFs

Metal-organic frameworks（MOFs）are widely used to prepare Fe-Co-Ni based composites because of their large surface area and diversity of composition,morphology and structure.In this paper,a typical cobalt-based MOFs（ZIF-67）was used as a precursor to prepare a cobalt-based electrocatalyst,a nickel-cobalt bimetallic MOFs（Ni Co-ZIFs）was used as the material to synthesize a Ni Co-based bimetallic composite,and an iron-nickel-based composite was synthesized with MIL-53（Fe）.The evolution of composition,structure,morphology and electrochemical properties of the composite was studied,and the formation of microstructure and electrocatalytic mechanism were discussed.The specific research contents are as follows:Graphite carbon/Co S₂/Mo S₂ and reduced graphene oxide（r GO）/Co Se₂composites were synthesized based on ZIF-67.First,the Co S₂/Mo S₂ heterostructure was constructed on nitrogen-doped graphite carbon by calcination and hydrothermal sulfide using ZIF-67 as the precursor.Nitrogen-doped graphite carbon is beneficial for the improvement of conductivity,while the formation of Co S₂/Mo S₂ heterostructure enhances the interfacial electron transfer ability and increases the number of active sites.When the loading of Mo S₂ is 25 wt%,the prepared electrocatalyst shows excellent electrocatalytic hydrogen evolution activity in 0.5 M H₂SO₄ solution（overpotential is 215 m V at 10 m A/cm²）.Secondly,ZIF-67 was grown in situ using graphene oxide as the substrate.After hydrothermal treatment,ZIF-67 was transformed into Co Se₂,and graphene oxide was transformed into reduced graphene oxide（r GO）,thus obtaining Co Se₂/r GO composites.The results showed that the introduction of r GO increased the conductivity of the samples,while the unique three-dimensional structure of ZIF-67 reduced the coverage of active sites caused by stacking of r GO during hydrothermal treatment.Co Se₂/r GO composites showed high electrocatalytic oxygen evolution activity,which showed an overpotential of 296 m V（10 m A/cm²）.In view of the similar ionic radii of Ni²⁺and Co²⁺,bimetallic Ni Co-ZIFs was successfully synthesized.After further hydrothermal vulcanization,Ni²⁺-doped hollow Co₃S₄was obtained.The experimental results show that both the hollow structure and Ni²⁺doping contribute to the improvement of electrocatalytic hydrogen evolution properties,and the prepared samples show good activity and stability in both acidic and alkaline conditions.In order to further study the influence of material morphology on the properties of bimetallic catalysts,Ni/Co carbon nanotubes derived from Ni Co-ZIFs were prepared by grinding bimetallic Ni Co-ZIFs and graphite phase carbon nitride and thermal polymerization at a certain temperature,and then Ni Fe-LDH modified Ni/Co carbon nanotube composites were successfully synthesized.The formation of bimetallic carbon nanotubes improves the conductivity of the sample and promotes the transfer of electrons at the same time.The electronic interaction between bimetallic carbon substrate and Ni Fe-LDH optimizes the bonding strength between metal ions and oxygen-containing intermediates,and promotes the electrocatalytic oxygen evolution activity of the sample,with an overpotential of 267 m V（10 m A/cm²）.Using MIL-53（Fe）as the precursor,a layer of metal（Ni）phenol network was formed on the surface of MIL-53（Fe）by means of the chelating ability of tannic acid to various metal ions.The results show that the metal phenol（Ni）network can promote electron transfer in this Fe/Ni composite,while MIL-53（Fe）is beneficial to the OH*intermediate.In order to further improve the properties of the composites,MIL-53（Fe/Ni）was modified with layered Ni Co-LDH after hydrothermal treatment.Due to the difference of Fermi energy levels between Ni Co-LDH and MIL-53（Fe/Ni）,the electrons at the interface were redistributed,and the oxygen-containing intermediates were directionally regulated.Among them,Ni Co-LDH showed enhanced OH*adsorption capacity,MIL-53（Fe/Ni）would promote the deprotonation of OOH*intermediate.The sample showed better electrocatalytic oxygen evolution activity than MIL-53（Fe）modified by Ni Co-LDH,and its overpotential was 244 m V at a current density of 10 m A/cm².