Design Of Highly Efficient Metal-organic-framework Based Electrocatalysts

In recent years,metal organic frameworks(MOFs)have been widely used in the preparation of various electrocatalysts due to the large specific surface area and highly tunable structures.Although a series of MOF derived carbon materials with high electrocatalytic activity,such as metal(M)-N-C,metal/metal compounds coated with porous carbon layers,single metal atom/C,etc.,have been developed by pyrolysis of MOFs,it is still a great challenge to regulate the morphology and understand the structure-activity relationship of these materials.In addition,the general sintering problem in MOF derived carbon materials,which limits the exposure of active sites,has always been a great challenge.To solve these problems,researchers have made a lot of efforts,such as coating MOF precursors with Si O₂,then carbonizing the MOF@Si O₂ followed by etching the product with HF to avoid the aggregation of carbon materials.However,the complex synthesis process is of high cost and HF is of great toxicity,which is not conducive to its production and application.Searching for more efficient strategies to prevent the aggregation of MOF derived carbon materials can further enhance the catalytic activity.Up to now,MOF derived carbon catalysts have made a great progress in electrocatalysis,however,they have lost the unique chemical structure and pore-environment of MOF in the pyrolysis,which has greatly sacrificed the structural advantages of MOF.If MOF crystal is directly used as electrocatalyst,it will face the problems of low conductivity and poor stability.On the other hand,the traditional MOF crystal powder is not conducive to the mass transfer in the electrocatalytic water splitting process due to the steric hindrance and size effect.Therefore,developing new synthesis methods to regulate the size and morphology of MOF can furtherly take advantage of MOF's unique structure.In view of above challenges,a series of structure-regulation of MOF and its derivatives have been designed in this dissertation,which have successfully solved the general problems that MOF based electrocatalysts have faced for a long time.The electrocatalytic activities,mechanisms and structure-activity relationship of MOF based electrocatalysts in oxygen reduction reaction(ORR),oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)have been systematically studied through combining theory with experiments,which is of great values for the research of MOF based energy materials.Firstly,the structure of Zn/Fe-ZIF's derivatives are regulated and the structure-activity relationship is studied.By controlling the proportion of metal atoms in Zn/Fe-ZIF and tuning the target-pyrolysis-temperature,the morphology and chemical composition of the Fe-N-C have been effectively optimized.It is identified that the formation of carbon nanotubes(CNTs)and the size of Fe-N active species are the decisive elements for the high-activity of ORR electrocatalysts.Next,to avoid the sintering of MOF derived carbon materials,Co Fe-ZIF is coated with graphene oxide(GO),and then pyrolyzed to prepare the reduced graphene(r GO)-coated carbon based electrocatalyst,which has not only effectively avoided the aggregation,but also enhanced the electrical conductivity.In addition,highly efficient bifunctional electrocatalysts with dual active sites for ORR and OER,respectively,are prepared through an incomplete sulfurization followed by pyrolysis process.Then,in order to make better use of MOF's unique chemical structure and pore environment,Co P is successfully doped on the surface of MOF by controlling the phosphating of Co-MOF,which has created a huge exposed area for the active Co P as well as tuned the electronic structure of Co P through the N-containing groups in MOF,determined by X-ray photoelectron spectroscopy combining with the density functional theory calculation,so as to enhance the water adsorption and optimize the adsorption Gibbs free energy to hydrogen,which account for the excellent catalytic performance of Co P/Co-MOF toward HER in the whole p H range.Finally,a general vapor phase synthesis method is developed to regulate the morphology of MOF,thus furtherly utilizing the unique chemical structure of MOF.The morphology of MOF obtained by reacting the organic-ligand vapor with metal precursors is remarkably consistent with that of the metal precursors.As a result,MOF's morphology can be tuned by changing the morphology of the metal precursors.By constructing metal sulfides on the surface of MOF,the unique structure of MOF has been furtherly utilized to promote the catalytic efficiency,which shows comparable activity and better durability than the commercial noble metal catalysts in the electrocatalytic overall water splitting test.