Synthesis Of Hybrid Electrocatalysts Derived From Metal-organic Frameworks For Water Splitting

Metal-organic frameworks?MOFs?are a class of long-ranged and well-alligned porous crystalline materials,constructed by various metal ions and organic ligands.MOFs usually possess large surface area,high porosity and adjustable structures.Meantime,different guest molecules can also be loaded into the cages or channels of MOFs,to make the hybirds more functionale.Thus,MOFs can be used as ideal precursors and sacrificial templates to fabricate functional nanomaterials,which have aroused widely attention in the past few years.The multicomponent hybrids drived from MOFs usually exhibit uniform distribution,high surface area and excellent performance,resulting in huge potential applications in energy storage and conversion.Electrocatalytic water splitting is generally considered as a clean,highly-efficient and secure way to produce hydrogen.Currently,the widespread parctical applications of electrocatalysts are constricted by the Pt-based noble catalysts duo to their high-cost and low abundance.Therefore,it is crucial to develop highly-efficicnt and cheap electrocatalysts to substitue Pt-based nanomaterials.Four MOFs derived nanohybrids have been designed and synthesized based on energy storage and conversion in mind.Three bimetallic MOFs with ordered heterogeneity were synthesized by hydrothermal reaction using common metal ions and organic ligands.A binary hybrid coated with graphene shells was acquired via a direct carburization process.At the same time,two ternary heterostructures was fabricated by a two-step method,a facile carbonization process and followed phosphorization process.Moreover,POM@ZIF-8 hybrid was synthesized using a mechanochemical method under room temperature.Futhermore,molybedenum carbides embedded in N-doped porous graphene carbon layers was synthesized using POM@ZIF-8 as precursor based on a facile pyrolysis process.A variety of physical characterization methods were used to characterize the structure of the nanomaterials in detail and the properties of the water splitting were studied.Electrochemical methods were used to characterize the kinetics of reaction on the electrode.At last,the synergistic effect of different components on the catalytic performance of these materials was investigated.1.A bimetallic MOF?NiMo-MOF?were synthesized under hydrothermal conditions using Ni ions,Mo ions and bpp ligands?1,3-bis?4-pyridyl?propane?.NiMo₂C@C,constructed by Ni/Mo₂C nanoparticles coated with graphitized carbon,was synthesized by a direct carbonization method under nitrogen atmosphere using NiMo-MOF as precursor.This is the first example that the hybrids of Ni and Mo₂C were synthesized derived from MOFs.This hybrid showed excellent electrocatalytic hydrogen generation performance in both acidic and alkaline electrolytes with overpotentials of 169 mV and 181 mV,respectively,to reach a current density of 10 mA cm^-2.In addition,NiMo₂C@C catalyst also showed good stability that can be stable for 10 h under work conditions.2.Two bimetallic MOFs?CoMo-MOF and NiMo-MOF?were synthesized under hydrothermal conditions using Ni ions,Co ions,Mo ions and bimbp ligands?4,4'-bis?imidazolyl?biphenyl?.Using CoMo-MOF and NiMo-MOF as precursors,two ternary hybrids were prepared by a two-step method,a carbonization process and a following phosphating process,terned as Co₂P/Mo₂C/Mo₃Co₃C@C and Ni/Ni₂P/Mo₂C@C.The two ternary complexes are composed of Co₂P/Mo₂C/Mo₃Co₃C@C and Ni/Ni₂P/Mo₂C@C nanoparticles uniformly embedded into the porous graphene layers.BothCo₂P/Mo₂C/Mo₃Co₃C@CandNi/Ni₂P/Mo₂C@Cexhibitedexcellent electrocatalytic activity and stability.In acidic solutions,the overpotentials for reaching a current density of 10 mA cm^-2 were 154 mV and 183 mV,respectively.More importantly,both Co₂P/Mo₂C/Mo₃Co₃C@C and Ni/Ni₂P/Mo₂C@C can be used as bifunctional electrocatalysts to achieve complete water splitting reaction in alkaline solutions.The overpotentials of Co₂P/Mo₂C/Mo₃Co₃C@C for affording a current density of 10 mA cm^-2 in HER and OER were 182 m V and 362 mV,respectively.At the same time,Ni/Ni₂P/Mo₂C@C requires overpotentials of 223 mV and 368 mV to reach a 10 m A cm^-2 for HER and OER.In addition,Co₂P/Mo₂C/Mo₃Co₃C@C and Ni/Ni₂P/Mo₂C@C were loaded on carbon cloth and can be used as cathode and anode,which only needed 1.74 and 1.78 V to achieve a 10 mA cm^-2 during the overall water splitting.3.The PMo₁₂@ZIF-8 complex was prepared by mechanochemical solid-state ball milling of a mixture of ZnO,2-methylimidazole and PMo₁₂ at room temperature.Using this composite as a precursor,a MoC@NC electrocatalyst was synthesized by a facile one-step carbonization process under Ar atmosphere.The structure of the electrocatalyst was that MoC nanoparticles were coated with N-doped porous graphitic carbon.This is a simple way to synthesize highly efficient MoC electrocatalysts in large quantities.MoC@NC possesses excellent electrocatalytic HER activity in both acidic and alkaline solutions with overpotentials of 132 mV and 122 mV at 10 mA cm^-2,respectively.Moreover,MoC@NC presents very good stability long as 24 h under operating conditions.