| Hydrogen evolution reaction(HER)is the key reaction of hydrogen production by electrochemical water splitting,in which the cathode catalyst is the core to improve the efficiency of hydrogen production.Platinum(Pt)cathode catalysts are the most ideal HER catalysts for hydrogen evolution thanks to their excellent catalytic performance.However,the lack of Pt resources leads to its high cost and poor stability in alkaline catalytic environment,which hinders the industrialization process of hydrogen production by electrochemical water splitting.Therefore,it is urgent to develop novel HER catalysts with low cost,high efficiency and catalytic stability.Metal-organic frameworks(MOFs)are considered as one of the most promising hydrogen evolution catalysts thanks to their ultrahigh porosity(about 90%free volume)and surface area.At present,there are two interrelated scientific problems in the research of MOFs catalysts:1.The chemical and thermal instability of pure MOFs under harsh reaction conditions;2.The poor electronic conductivity due to the inherent insulating properties of organic ligands and the weak overlap between the p-orbitals of organic ligands and the d-orbitals of metal ions,which hinders the fast response of MOFs to electrochemistry.Metal nanoparticles(MNP)encapsulated on the surface of MOFs can not only improve the electronic conductivity of MOFs,but also form a synergistic effect between MNP and MOFs,which is conducive to the HER performance of the catalysts.Therefore,in this paper,we focuse on the design and synthesis of metal-organic frameworks encapsulated metal nanoparticles derived alloys(MOFs/MNP),and studied their HER performance.The following work has been carried out:(1)A series of Ni-based metal-organic frameworks(Ni-MOF)are synthesized by using nickel chloride hexahydrate as the metal source and trimellitic acid as the organic ligand.The samples are characterized by SEM,which shows as-synthesized Ni-MOF with different morphologies,such as two-dimensional layered,hexagonal,fibrous and spherical.Then HER performances of the samples are tested by Chenhua CHI 760E electrochemical workstation.The results show that the spherical Ni-MOF catalyst has the best hydrogen evolution performance.In 1.0 M KOH,only 559 m V overpotentials are required to reach a current density 10 m A/cm~2,and the Tafel slope is 121 m V/dec.(2)In order to improve the conductivity and catalytic stability of spherical Ni-MOF,Ni-MOF/Pd catalysts are prepared by reduction of palladium chloride into spherical Ni-MOF by solution impregnation and wet chemical reduction method.The results of physical structure characterization and electrochemical tests show that Ni-MOF/Pd exists in the form of Pd/Ni alloy,and there is electron transfer between Pd and Ni,which can form bimetallic synergistic effect.The results show that Ni-MOF/Pd only needs 50 m V overpotential to achieve the current density of 10 m A/cm~2,and the corresponding Tafel slope is 102 m V/dec.In addition,after 5000 cycles of CVs accelerated degradation test,the HER performance of the catalyst showed no obvious degradation.(3)With the help of surfactant functionalized guest silver ions,not only the silver nanoparticles can be evenly distributed when the guest silver ions are reduced in-situ,but also the Ni-MOF-core and Ag-shell can be bridged tightly.The results show that Ni-MOF/Ag catalysts are core-shell structure and have a large number of mesopores and micropores.When the current density is 10 m A/cm~2,the over potential of the 5%Ni-MOF/Ag catalyst is 47 m V and Taffel slope is 85 m V/dec,and after 12 hours’stability tests,the catalyst can still maintain excellent catalytic performance.In addition,the experiment is also extended to other transition metal based MOFs and characterized by physicochemical methods.The results show that Ag NPs doping can greatly improve the catalytic performance of MOFs for hydrogen evolution. |
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