Study On The Preparation And Electrocatalytic Water Splitting Performance Of Transition Metal-based Carbon Nanofiber Composite Materials

With the development of human civilization and the rapid consumption of traditional fossil energy,the search and development of alternative green energy devices have attracted widespread attention.Hydrogen energy has the characteristics of high energy density,green environmental protection,and zero carbon emissions,and is regarded as a form of energy with strong sustainable development to achieve the goal of carbon neutrality.Compared with other forms of hydrogen production,electrolyzed water technology is a green,efficient,and recyclable effective method for large-scale production of hydrogen energy,while using renewable natural energy forms,such as solar energy,wind energy,tidal energy,etc.Electricity has good environmental adaptability.The electrolyzed water comes from two half-reactions,namely the hydrogen evolution reaction at the cathode and the oxygen evolution reaction at the anode.The theoretical decomposition voltage is 1.23 V.However,the decomposition of water often requires a higher voltage（1.8-2.0 V）to overcome the energy barrier of the reaction,which is mainly attributed to the existence of the over-potential in the reaction,the resistance of the solution,and the internal resistance of the external circuit.Therefore,the use of high-efficiency electrocatalysts to reduce the overpotential in the reaction and increase the reaction kinetics rate is the focus of current research.Numerous studies have shown that noble metal-based electrocatalysts,such as Ru O₂/Ir O₂ and Pt/C catalysts,are considered to be the most effective electrocatalysts for oxygen evolution and hydrogen evolution due to their good electrical conductivity and catalytic activity.However,due to high cost,scarcity of reserves,and poor stability in operation,they limit their large-scale commercial use.Therefore,in recent years,domestic and foreign scientific researchers have devoted themselves to the development of electrocatalysts with abundant crustal reserves and cost-effectiveness to improve the overall conversion efficiency of electrolyzed water,and reduce the cost of catalysts to meet practical applications.One-dimensional carbon nanofibers（CNFs）have special structural advantages,high electrical conductivity,high thermal conductivity,and good electrochemical stability.They are a type of catalyst carrier with broad application prospects.Therefore,it is of great significance to prepare carbon nanofiber materials and transition metal composites through the electrospinning strategy to construct excellent electrocatalytic materials with uniform active site exposure.In this paper,starting from the design of constructing a nano-electrocatalyst with a one-dimensional carbon nanostructure and uniformly distributed active sites,the electrospinning technology combined with the pyrolysis process is used to control the morphology and structure of transition metal-based nanomaterials,and the preparation has excellent catalytic performance.And the high-efficiency electrocatalytic material with controllable morphology and size,and its electrocatalytic performance and related reaction mechanism are explored.Investigate the influence of different electrocatalytic materials on the performance of electrolyzed water.Different characterization methods,such as field emission scanning electron microscope,transmission electron microscope,X-ray photoelectron spectroscopy and other characterization results,are used to analyze the morphological characteristics of composite materials,the internal correspondence between metal and carbon carrier sizes and electrocatalytic performance.The internal relationship between the performance and structure of the material in the electrocatalytic reaction is studied,and the possible electrocatalytic mechanism is explored.The specific research results are as follows:（1）Hollow Co₃O₄/Ce O₂ nanoparticle composite material coated with nitrogen-doped carbon nanofibers and its oxygen evolution performanceUsing electrospinning technology,transition metal cobalt nitrate and cerium nitrate as metal sources,high polymer polyvinylpyrrolidone as carbon and nitrogen sources,through simple electrospinning and high temperature pyrolysis strategies to prepare a unique hollow Co₃O₄/Ce O₂ heterostructure nanoparticles are coated in situ in a composite material of nitrogen-doped carbon nanofibers（denoted as h-Co₃O₄/Ce O₂@N-CNFs）.Different from previous studies,Co₃O₄/Ce O₂ heterostructure nanoparticles exhibit a hollow and porous structure,which is mainly due to the Kirkendall effect during the pyrolysis process.Nitrogen-doped carbon nanofibers can effectively fix and disperse Co₃O₄/Ce O₂ nanoparticles,acting as a medium bridge for highly directional material transport and electron transfer.Electrochemical tests demonstrate that the material has high oxygen evolution activity and good stability,which is specifically characterized by its low overpotential(the corresponding overpotential is 310 m V at a current density of 10 m A cm^-2),and it can operate stably40000 s and the current density is not significantly attenuated,the performance of the material is better than the commercial Ru O₂ catalyst.The excellent electrocatalytic performance is attributed to the synergistic effect of combining the hollow Co₃O₄ and Ce O₂ heterostructure with the embedded three-dimensional porous carbon nanofiber network.（2）Nitrogen-doped carbon nanofibers anchor Mott-Schottky Ni-Ce O₂nanoparticles for electrocatalytic overall water splittingThrough a feasible electrospinning-pyrolysis technology,Ni/Ce O₂ hybrid nanoparticles are fixed on nitrogen-doped carbon nanofibers（hereinafter referred to as Ni/Ce O₂@N-CNFs）to prepare a special Mott-Schottky electrocatalyst.Due to the built-in electric field in the special structure of Mott-Schottky material,the metal/semiconductor interface effect can achieve good electrocatalytic performance.Experimental results and theoretical calculations prove that the Ni/Ce O₂ heterojunction structure that we constructed effectively triggers spontaneous charge transfer on the heterogeneous interface,thereby increasing the charge transfer rate,optimizing the chemical adsorption energy of reaction intermediates,and ultimately accelerating reaction kinetic rate.Ni/Ce O₂@N-CNFs exhibits excellent catalytic activity for hydrogen evolution and oxygen evolution in alkaline media.At a current density of10.0 m A cm^-2,the overpotentials for hydrogen evolution and oxygen evolution are 100m V and 230 m V,respectively.In addition,Ni/Ce O₂@N-CNFs is used as both the cathode and anode material of electrolyzed water.When it is loaded in the electrolytic cell,it exhibits a lower decomposition voltage of 1.56 V when the current density reaches 10.0 m A cm^-2,and at the same time exhibits more than 55 h long-term cycle stability.By using the Mott-Schottky effect to establish a special design for electronic adjustment of electrocatalyst materials,it may provide ideas for the preparation and research of efficient and inexpensive electrocatalysts for the development of various sustainable energy systems in the future.（3）Ni₃Co nanoparticles coated with nitrogen-doped carbon nanotubes/nanofibers for efficient overall water splittingA simple electrospinning-pyrolysis strategy is used to directly encase uniform Ni₃Co nanoparticles in-situ to grow nitrogen-doped carbon nanotubes and graft carbon nanofibers(denoted as Ni_1.5Co_0.5@NC NT/NFs)in a hierarchical branch structure.The cleverly designed construction of this composite one-dimensional multilevel structure can effectively adjust the electronic structure of the active site,expand the active exposure site,promote electron transfer and substance diffusion,and is beneficial to the increased kinetics of the catalyst in hydrogen evolution and oxygen evolution.Therefore,Ni_1.5Co_0.5@NC NT/NFs catalyst exhibits excellent electrocatalytic activity for both hydrogen evolution reaction and oxygen evolution reaction under alkaline conditions.When the current density is 10 m A cm^-2,the overpotentials are 114 and 243m V,respectively.When used as a cathode and anode electrode material for overall water splitting,the two-electrode electrolyzer only needs a voltage of 1.57 V to reach a current density of 10 m A cm^-2,and after 27 h of continuous operation,there is still no significant degradation in performance.（4）Preparation of nitrogen-doped carbon nanotube/nanofiber supported Ni S/Ni O heterojunction and its oxygen evolution performanceElectronic structure adjustment through interface engineering is a general strategy to improve the conversion efficiency of non-noble metal electrocatalysts.In this chapter,a simple electrospinning-pyrolysis-vulcanization strategy is firstly used to prepare Ni nanoparticles coated in a one-dimensional multilevel structure of nitrogen-doped carbon nanotubes grafted carbon nanofibers,and then through low-temperature oxidation-sulfuration vapor deposition process,in-situ topological conversion into Ni S/Ni O heterojunction material（denoted as Ni S/Ni O@NC NT/NFs）.Under alkaline conditions,the Ni S/Ni O@NC NT/NFs catalyst exhibits excellent electrocatalytic activity in the oxygen evolution reaction.When the current density is 10 m A cm^-2,it has a low overpotential of269 m V,a small Tafel slope of 48.4 m V dec^-1,and excellent electrochemical stability,presenting an economical and potential electrocatalyst for many sustainable energy devices.The research results show that the simultaneous adjustment of interface engineering and nanostructure makes Ni S/Ni O@N-C NT/NFs have optimized electronic configuration,increased oxygen vacancies,promoted mass diffusion,and significant structural stability.Density functional theory further verified the formation of Ni S/Ni O heterojunction,which can effectively adjust the chemical adsorption energy of oxygen-containing intermediates,lower the reaction energy barrier,and greatly promote the kinetic rate of the oxygen evolution reaction.（5）Nitrogen-doped porous carbon nanofibers anchoring molybdenum single atoms and their hydrogen evolution propertiesThe development of cost-effective non-precious metal electrocatalysts,as a substitute for precious metal platinum-based catalysts,for hydrogen evolution reactions,plays a vital role in the substantial progress of sustainable hydrogen energy.The current research shows that the dual regulation of the coordination chemistry and geometric configuration of single-atom catalysts is a powerful method to overcome the thermodynamic and kinetic problems of electrocatalysis.In this chapter,we use the strategy of an electrospinning-hard template to synthesize Mo monoatomic materials anchored by nitrogen-doped porous carbon nanofibers,in which Mo active sites are coordinated with heteroatoms of carbon,nitrogen,and oxygen（labeled as Mo@NMCNFs）,as an electrocatalyst for hydrogen evolution.A series of system characterizations showed that a part of the local coordination microenvironment of molybdenum atoms was determined to be Mo-O₁N₁C₂.Through density functional theory（DFT）calculations,it was theoretically discussed that this part of Mo-O₁N₁C₂ is a favorable structure for H intermediate adsorption.Structurally,the open multi-channel porous carbon nanofibers can effectively expand the exposure of active centers,promote mass diffusion and charge transfer,and accelerate the release of H₂,thereby promoting the kinetics of the reaction.Therefore,the optimized Mo@NMCNFs exhibits excellent Pt-like hydrogen evolution performance in 0.5 M H₂SO₄ electrolyte,the overpotential is 66 m V at 10 m A cm^-2,the Tafel slope is 48.9 m V dec^-1,and it has excellent stability,and is superior to most non-noble metal hydrogen evolution electrocatalysts previously reported.