Construction Of Bimetallic Nanoclusters By Core-shell MOF And Their Electrocatalytic Properties

At present,the massive burning of fossil fuels has caused serious pollution to the environment,and as people’s living standards improve,the demand for renewable and clean energy is increasing day by day.Fuel cells and metal-air cells with high energy density values,low prices as well as no pollutant emissions are important devices to alleviate the current energy shortage and environmental degradation problems.However,the key in fuel cells is the catalyst for the oxygen reduction reaction（ORR）,and an efficient catalyst can reduce the energy barrier of the ORR.Therefore,the development of efficient,clean and low cost catalysts is of great importance for fuel cell applications.Currently,noble metal catalysts such as Pt-based have high activity for ORR.However,the high cost,low storage capacity and poor stability of noble metal catalysts limit their large-scale application,so it is important to work on non-metallic catalysts with low cost,high efficiency and high stability.Transition metal and nitrogen co-doped carbon catalysts（M-N-C,M=Co,Fe,Mn,Ni,etc.）,which have similar catalytic activity to Pt-based noble metals and are abundant and readily available,are considered to be the most promising alternatives to Pt-based noble metal catalysts and have attracted much attention.Metal organic skeletons（MOFs）are a new class of porous crystalline materials composed of metal ions/clusters and organic ligands.MOFs have unique advantages such as ultra-high specific surface area and well-defined pore structure,and show great potential in addition to catalysis and energy storage.In the past decades,MOFs have been widely used as precursors for the preparation of functional nanomaterials,such as metal oxides,nitrides and porous carbon.MOFs are ideal precursors for anchoring metallic substances due to their various organic ligands,unsaturated metal sites for coordination and the ordered pore structure of adsorbed small molecules.Monometallic atoms loaded on carbon can be obtained from MOFs or MOFs composites by a simple pyrolysis process.Typically,the use of MOFs as precursors for MOF-derived single-atom catalysts（SACs）not only improves the metal loading of the catalyst but also the stability of the SACs,which is of great interest in the field of electrochemical energy conversion.However,MOFs also suffer from poor electrical conductivity and poor stability,which also severely hinders their application in electrocatalysis.In this context,combining MOFs with other types of MOFs is a promising way to make individual MOFs functional.By preparing core-shell MOFs by assembling one MOF in another,not only can the physical,chemical and structural advantages of the original MOF be retained,but this core-shell structure may lead to some unexpected synergistic effects.Therefore,a series of bimetallic core-shell MOFs were synthesised using MOFs as feedstock by constructing multi-stage core-shell MOFs with different metals,and a series of single-atom catalysts with different metals were prepared by a simple pyrolysis method.The details of the study are listed below.1.ZIF-8 was used as a template for the preparation of monolayer Fe-ZIF by a simple solvothermal method,and then a series of Ni-ZIFs with different Ni contents were synthesised in a second step using monolayer Fe-ZIF as the crystalline species.The materials have relatively good ORR performance due to the organic ligand being used as a carbon source without the need for an additional carbon source.The specific surface area of Ni₃₀-NC@Fe-NC catalyst synthesized by the above method reached 1106.1 m²/g,possessing a large number of microporous structures,the catalyst reached a half-wave potential of 0.86 V in 0.1 M KOH solution and an ultimate current of 3.16 m A cm^-2,superior to other catalysts including commercial Pt/C catalysts.The high stability of the catalyst Ni₃₀-NC@Fe-NC showed only a slight current decay of 7%for 20,000 s,still maintaining 93%of the current.2.In order to further improve the ORR performance of the material based on the adjusted metal content of the shell layer,we then investigated the effect of different metals in the shell layer on the ORR performance.Again,we used ZIF-8 as a template to prepare a monolayer of Fe-ZIF by a simple solvothermal method,and then used the monolayer of Fe-ZIF as a crystalline species to carry out a second step to synthesise a series of M-ZIF@Fe-ZIF with different metals.A series of M-NC@Fe-NC core-shell materials were obtained by a simple pyrolysis process,which resulted in a series of M-NC@Fe-NC core-shell materials due to the organic ligand as a carbon source without the need for an additional carbon source,thus giving the The materials have a relatively good ORR performance due to the organic ligand as a carbon source without the need for an additional carbon source.The specific surface area of Pt-NC@Fe-NC catalyst synthesized by the above method reached 1121.903 m²/g,possessing a large number of microporous structures.Because of the synergistic interaction between the bimetals,the synthesised Pt-NC@Fe-NC catalysts exhibited good ORR activity,reaching a half-wave potential of0.93 V in 0.1 M KOH solution,which is superior to other catalysts including the commercial Pt/C catalysts.In addition,Pt-NC@Fe-NC has good stability,with an initial current density retention of up to 95%compared to commercial Pt/C for 20,000s of continuous testing.