Syntheses And Electrocatalytic Properties Of Metal Organic Frameworks Derived Nanostructured Materials

Metal-Organic Frameworks（MOFs）are often emerged as functional materials or precursors to synthesize inorganic materials due to their porosities,high specific surface area and rich composition.In recent years,MOFs based materials are widely used in electrochemical energy-storage and conversion devices.The usage of MOFs s-based materials often categorized into two kinds:（1）MOFs were directly used as functional materials;（2）MOFs were converted into functional materials by pyrolyzation or modification.On basis of reviewing the types of MOFs derived materials and their applications,we systematically studied the preparation methods and formation mechanism of MOFs-based materials including N-doped carbon encapsulated Fe/FeC polyhedrons（Fe/FeC@N-C）,reduced graphene oxide（RGO）supported N-doped carbon encapsulated NiFe alloy composite（NiFe@NC/RGO）,RGO encapsulated core-shell-structured CoFe alloy@N-doped carbon composite（CoFe@NC@RGO）and S-doped NiFe（CN）₅NO nanoparticles.The components,structures and morphology of as-synthesized products were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,Raman spectroscopy and X-ray photoelectron spectroscopy（XPS）.The applications in the field of electrocatalysts were systematically investigated.The main points are as follows:1.The Fe/FeC@N-C composite with Fe/FeC encapsulated by N-doped carbon was synthesized by a two-step pyrolyzation of Zn₃[Fe（CN）₆]₂@MB.The Zn₃[Fe（CN）₆]₂@MB was formed by the strong adsorption of Zn₃[Fe（CN）₆]₂ towards methylene blue（MB）.It was revealed that the Fe/FeC@N-C product could well maintain the polyhedral morphology of the Zn₃[Fe（CN）₆]₂@MB precursor,accompanying with some carbon nanotubes grown on the surface of the polyhedral particles.This material exhibits a large specific surface area of 141.4 m² g^-1.Electrochemical tests indicate that the Fe/FeC@N-C composite has decent HER activity with a relatively low overpotential of 219 mV at a current density of 10 mA cm^-2 and a small Tafel slope of 91.3 mV dec^-1.Moreover,the Fe/FeC@N-C composite also exhibits an excellent catalytic activity toward ORR with an onset potential（0.969V vs.RHE）and half-wave potential（0.810 V vs.RHE）,which is comparable to the commercial 20 wt%Pt/C catalyst（0.975 V and 0.820 V）.Therefore,as a bifunctional catalyst,Fe/FeC@N-C promises a potential application in energy-storage and conversion fields.2.The composite of NiFe alloy encapsulated in N-doped carbon（NiFe@NC）supported on reduced graphene oxide（NiFe@NC/RGO）were synthesized by in-situ growth of NiFe（CN）₅NO onto graphene oxide（GO）,followed with a subsequent pyrolyzation in Ar atmosphere.It was revealed that the NiFe@NC nanocrystals with a size of ca.30-50 nm were uniformly dispersed on RGO sheets.The composite exhibits an excellent ferromagnetic property and outstanding electrocatalytic activities towards oxygen evolution reaction（OER）withη₁₀ of 273 mV and Tafel slope of 53.2mV dec^-1 in 1 M KOH medium.The OER electrocatalytic performance of NiFe@NC/RGO can be optimized through adjusting the content of RGO in NiFe@NC/RGO composites.The simple preparation and excellent properties make the NiFe@NC/RGO composites a promising candidate in magnetic and electrocatalytic applications.3.The RGO encapsulated core-shell-structured CoFe alloy@N-doped carbon composite（CoFe@NC@RGO）was synthesized by in-situ growth of CoFe（CN）₅NO onto graphene oxide（CoFe（CN）₅NO@GO）,followed with a subsequent pyrolyzation in Ar atmosphere.The as-synthesized CoFe@NC@RGO composite is composed of microcubes,which are wrapped by RGO sheets and contain a great number of core-shell-structured CoFe@NC nanocrystals.The CoFe@NC@RGO exhibits superior electrocatalytic activities towards OER withη₁₀ of 336 mV and Tafel slope of 54.3 mV dec^-1 in 1M KOH.Moreover,the CoFe@NC@RGO composite shows a high electrochemical stability.After 1000 cycles,the OER catalytic activity shows no obvious degradation.The electrocatalytic performance of CoFe@NC@RGO is better than CoFe@NC and even the commercial RuO₂,indicating that RGO plays an important role in enhancing the OER performance of CoFe@NC@RGO.This study provides a simple and effective route for synthesizing complicated graphene-based composites with excellent electrochemical properties.4.The S-doped NiFe（CN）₅NO nanoparticles（S-NiFe（CN）₅NO）were prepared by in-situ doping sulfur into NiFe（CN）₅NO.The as-synthesized nanoparticles can well maintain the morphology and size of pristine NiFe（CN）₅NO nanocrystals,and show a large specific surface area of 288.7 m² g^-1.Electrochemical study demonstrates that the S-NiFe（CN）₅NO nanoparticles have excellent electrocatalytic activity towards OER withη₁₀ of 276 mV and Tafel slope of 34.5 mV dec^-1 in 1M KOH,which are much better than those of pristine NiFe（CN）₅NO.Moreover,after 1000 cyclic voltammetry cycles,the OER catalytic activity of S-NiFe（CN）₅NO was well maintained,indicating that S-NiFe（CN）₅NO owns an excellent electrochemical stability.It was revealed that the enhanced performance of S-NiFe（CN）₅NO benefits from S-doping modification and the porosity of the material.Our findings provide an alternative OER electrocatalyst and introduce an advanced strategy to improve MOFs performance through elemental doping.