| Electrocatalytic water splitting to generate high-purity hydrogen has been considered as one of the cleanest sustainable energy strategies,which is divided into two half reactions:the hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）.Currently,the noble metal Pt based materials are the most effective catalysts for hydrogen evolution reaction,while the noble metal Ru/Ir based materials are the best catalysts for oxygen evolution reaction.However,high costs and less natural reserves limit their extensive applications.Due to the unique d-orbital electronic structure,adjustable atomic structure and abundant reserves,transition metal materials have become one of the most promising electrocatalysts to replace noble metal materials,which have a good application prospect in the field of energy catalysis.Single atoms catalysts have highly exposed active sites and close to 100%atom utilization,which have attracted widespread attention.Consequently,this thesis aims to prepare transition metal single atoms based on the electrospinning technology,and improve their electrocatalytic activity and stability by rational designing the structure of carbon fiber,different metal species and adjustment of the coordination environment.1.The N-doped porous carbon fiber supported Ni single atom/nanoparticle catalyst（Ni SA/NP-NCF）were prepared by electrospinning technology.The effect of carbonization temperatures on fiber morphology has been studied,and the effects of HF etching concentrations and time on fibers porosity and Ni metal type were further investigated.The Ni element in porous carbon fiber coexisted in the form of single atoms and nano particles,which synergistically promoted the HER activity and stability of catalyst materials.The Ni NP-NCF-800 catalyst was prepared by NaOH etching.The hydrogen evolution performances of Ni SA/NP-NCF and Ni NP-NCF-800 in alkaline condition were tested.The Ni SA/NP-NCF-800shows the best performance,which only requires overpotential of 137.2 mV to approach current density of 10 mA·cm-2.The local coordination condition of Ni single atoms was experimentally and theoretically demonstrated to be Ni-N4 coordination,which could induce the rearranged electron structure of Ni nanoparticles.The cooperation effect of Ni single atoms and nanoparticles contributes to enhance water adsorption ability,reduce water dissociation and H adsorption energy barriers in alkaline electrolyte.2.Ni-N/CNFs,Ni-N,B/CNFs,Ni-N,P/CNFs and Ni-N,S/CNFs catalysts were prepared by co-doping with different heteroatoms（N,N-B,N-P,N-S）.The effect of the coordination microenvironment around Ni single atoms on the geometric and electronic structure of metals were studied.The experiments and calculations results show that the d-band center position of Ni atom are adjusted appropriately by coordination with different species,which optimizes the adsorption and dissociation capabilities of intermediates.The coordination of 3 N and 1 P（Ni-N,P/CNFs）exhibits the best electrocatalytic performance for hydrogen evolution reaction,oxygen evolution reaction and full water splitting.For HER in acidic condition and HER and OER in alkaline condition,Ni-N,P/CNFs only requires overpotentials of 38,84and 330 mV to approach a current density of 10 mA·cm-2.Moreover,the Ni-N,P/CNFs was used as both cathode and anode to assemble a two-electrode overall water splitting device,achieving current density of 10 mA·cm-2 by 1.67 V in alkaline condition.Ni-N,P/CNFs also exhibites robust stability and durability in acidic and alkaline conditions.3.In order to further explore the universality of single atoms preparation method based on electrostatic spinning technology,N and P co-doped transition metal Co and Fe single atoms catalysts(Co SA/CNFs and Fe SA/CNFs)were prepared,and the effects of N and P heteroatoms on the electronic and geometric structures of transition metal single atoms were studied.The chemical configuration and local coordination of metal single atoms were unraveled by X-ray absorption fine spectroscopy,which demonstrated the local atomic architectures around metal were coordinated by N and O atoms to form the M-N（O）configuration.For HER in acidic condition,CoSA/CNFs and Fe SA/CNFs only require overpotentials of 61 and 64 mV to approach a current density of 10 mA·cm-2.For HER and OER in alkaline condition,Co SA/CNFs and Fe SA/CNFs only require overpotentiasl of 89 and 390 mV,125 and 420 mV to approach current density of 10 mA·cm-2,respectively.Furthermore,the cell voltage required to achieve a current density of 10 mA·cm-2 for overall water splitting is 1.68 and 1.74 V,respectively. |
