Design,Synthesis And Properties Of Metal-Organic Frameworks-Based Non-noble Electrocatalysts

With the graveness of energy shortage and environment pollution,it is particularly urgent to develop a clean and renewable new energy source to reduce dependence on traditional fossil fuels.Theoretically,water splitting to produce hydrogen has great advantages and potential in new energy development.However,the process of electrochemical water spilitting usually requires the participation of precious metals,such as platinum(Pt),palladium(Pd),ruthenium(Ru)and iridium(Ir),it is difficult to realize large-scale application due to low reserves and high cost.In the studies of non-expensive electrocatalysts,the electrocatalytic activity is mainly affected by.the number of active sites and the intrinsic activity.Therefore,the high efficiency electrocatalyst requires the accurate control of the component,structure,size and morphology of the materials,which also poses a great challenge to the preparation method of the electrocatalsts.Metal-organic frameworks(MOFs),as a new type of porous material,have diverse structures and adjustable components.Besides,the inner metals and organic ligands are arranged in order and periodically at the atomic level,which provides a new idea for the design and synthesis of functional nanomaterials.At present,much work has been done on the structural design of MOFs,but relatively little has been done on the control of size and morphology.On the basis of MOFs,this paper mainly explored the design and synthesis methods of a series of MOFs-based nanomateials,including nanoscale MOFs materials,MOFs composites and MOFs derivatives,non-noble electrocatalysts with high catalytic activity were obtained by the construction and optimization of electrochemical system.Furthermore,We have studied the reaction mechanism of electrocatalytic processes in depth through a series of spectrum test and theoretical calculation methods,which provided a theoretical basis for the design and synthesis of such electrocatalysts.The specific research content is as follows:(1)A series of nano-scale MOFs with different dimensions were synthesized by solvothermal method.The experimental results show that the type of metal salts,solvents,temperature,even the order of feeding all have an important influence on the size and self-assembly behavior of MOFs materials.A multifunctional MOFs composite with different load of MOFs was synthesized by controlling the growth of MOFs on the suitable substrate materials.In addition,the influence of pyrolysis atmosphere on the morphology of MOFs derivatives was also discussed thoroughly with MOFs as the precursor,which provided a new idea for the synthesis of MOFs derivatives.(2)Based on the classical MOFs material(Fe-MIL-53),a series of bimetallic Fe/Ni-MOFs nanorods with uniform size were synthesized through solvothermal method.It is found that they can be directly used as an electrocatalyst for oxygen precipitation reaction(OER),and their electrocatalytic performance is affected by the metal proportion in MOFs.When introducing a third metal(Mn and Co)into the best performing Fe/Ni2.4-MOFs nanorods,a series of trimetallic MOFs(Fe/Ni/Co(Mn)-MIL-53)nanorods can be synthesized,and the OER performance is further improved.The optimized Fe/Ni2.4/Co0.4-MIL-53 nanorod can reach a current density of 20 mA cm-2at a low overpotential of 236mV with a small Tafel slope of 52.2 mV·dec-1,which is superior to commercial Ir/C materials.In addition,with nickel foam(NF)as self-consuming template,we can also realize the synthesis of MOFs composites(Fe/Ni/Co(Mn)-MIL-53/NF)and the improvement of OER electrocatalytic performance.(3)A series of bimetallic ultrathin MOFs nanosheets(Ni-M-MOFs,M=Fe,Al,Mn,Co,Zn and Cd)were synthesized through bottom-up wet chemical method.It was found that the mixed solvent of N,N-dimethylacetamide and water played an important role in the formation of two-dimensional nanosheets.Ni-Fe-MOFs nanosheets demonstrated an excellent catalytic performance when directly used as OER electrocatalysts,which deliver a current density of 10 mA·cm-2 at a low overpotential of 221 mV with a small Tafel slope of 56.0 mV dec-'.In addition,the structure-activity relationship and reaction mechanism of electrocatalysts were further studied,and we proposed the possible reaction process at different metal sites based on the results of density functional theory,which proved that Fe in Ni-Fe-MOFs nanosheets,as the active center,is the key to improve OER performance.(4)A kind of inorganic composite material(Ni2.75Co/CNTs)assembled by NiCo alloy and carbon nanotubes was synthesized by pyrolysis of hollow structure of MOFs nanosphere(Ni/Co-MOFs)in a reducing atmosphere,and we investigated the effects of temperature and pyrolysis atmosphere on MOFs derivatives.Compared with Ni/Co-MOFs,Ni2.75Co/CNTs significantly improved the performance of electrocatalytic hydrogen evolution under alkaline condition due to its unique hollow structure and the advantages of conductive carbon nanotubes.The experimental results showed that the catalyst can deliver a current density of 10 mA·cm-2 at a low overpotential of 177 mV in the absence of an auxiliary catalyst(carbon black).