Designed Synthesis Of Transition Metal/Carbon Compounds For Electrochemical Catalytic Hydrogen Evolution

The survival and sustainable development of human society are confronted with energy crisis,environmental pollution problems and so forth.Producing green energy carrier——hydrogen using electrolysis of water technology was considered an effective programme to alleviate the dilemma currently.Highly efficient catalysts play key role in ensuring reaction run smoothly.Noble metal platinum-based materials are the state-of-art catalyst materials in hydrogen evolution reaction,however,its practical application was hindered by the low reservoir abundance and high price.Developing new kinds of non-noble metal containing catalysts to replace platinum-based ones are keystone investigated today.Carbon has the advantage of high stability,good conductivity,low price and acid-alkali corrosion resistivity,making it suitable for working in abominable environment and solving energy conversion and stortage related problems and inspired high hopes.While the fact that carbon materials themselves have no（or faint）catalytic activity prompted researchers resort to chemical methods,such as compounding,dopping,loading ect.to activate carbon materials.In this thesis,we selected carbon-based electrocatalysts as study subjects,concentrated on optimizing material catalytic properties and have successfully designed and developed several transition metal compound/porous carbon composites and systematically studied the relationship between catalytic performance with composition and morphology structure.The thesis mainly includes the following aspects:1.We demonstrate a facile and efficient approach,referred to as a“sprout-like growth”strategy,for the fabrication of a new type of mesoporous Mo₂C/NC composite film with a mesoporous Mo₂C nanonetwork immobilized and embedded within an N-doped carbon sheet.Ethylenediamine,carbon tetrachloride and ammonium molybdate were selected as starting materials.Positively charged polymer dots assembled from the polymerization reaction of the precursor molecules ethylenediamine and carbon tetrachloride effectively adsorbed and fixed Mo₇O₂₄^6-through electrostatic interaction.After calcination in inert atmosphere at high temperature,mesoporous Mo₂C film with open framework were obtained.Mophorlogical structure can be tailored by controlling the amount of metal source.The as-prepared nanocomposites can act as excellen electrochemical water splitting catalysts,and the one owns uniform film strcture performs the best.It needed a small overpotential of 140 mV to drive a current density of 10 mA/cm² and stand for at least 120 h in an acidic environment.XPS,Raman,BET and TEM characterizations show the porous open structure was beneficial for keeping intimate contact with electrolyte,facilitate electron transfer and active sites exposure.2.CoN/Co Janus nanoparticles embedded in a porous nitrogen doped carbon（CoN/Co-NC）composite catalyst were obtained by heat treatment of a Co²⁺containing polymer in ammonia atmosphere at 800^oC.When heat treated in nitrogen atmosphere while keeping other conditions unchanged,we get Co nanoparticles embedded in porous nitrogen doped carbon（Co-NC）;When heat treated in nammonia atmosphere while keeping other conditions unchanged,we get Co nanoparticles embedded in porous nitrogen doped carbon（CoN/Co-NC）.The as-obtained hybrid catalyst showed excellent electrocatalytic activities for the hydrogen evolution reaction in both acidic and basic media,and it delivered a current density of 10 mA/cm² at an overpotential of 160 mV in 1 M KOH and190 mV in 0.5 M H₂SO₄ electrolyte.In addition,the catalyst could sustain potentiostatic electrolysis for at least 100 hours at 10 mA/cm² in both acidic and alkaline solutions.Obviously,CoN/Co Janus nanoparticles embedded in porous carbon performs better in catalysis than that of signle Co,further mechanistic study suggested that the high activity of the composite electrocatalyst originated from the Janus effects between Co and CoN.That is,contact potential difference would appear at the heterointerface of the two metallic phases with different work functions,making surface electrons transferred from Co to CoN until the Fermi level equilibrium between Co and CoN was reached.This effect lead to Co active sites in CoN/Co heterojunction electron-rich and are more preferable to adsorb hydrogen,thereby facilitate the transformation of proton to hydrogen kinetically.3.Uniform carbon coated FeS₂（FeS₂@C）nanochains were synthesized from a magnetic-field guided interface coassembly approach.In particular,the Fe₃O₄nanospheres as precursors are assembled into chains during polymer coating process in dynamic magnetic field environment,followed by the successive carbonization and sulfidation to generate FeS₂@C.Thickness of the coating carbon can be controlled in the range of 0-85 nm by tailoring the amount of phenolic resin in the synthesis.When the layer was too thick（i.e.85 nm）,the coated Fe₃O₄ nanospheres can not be sulphized completely;When the layer was too thin（i.e.0 nm）,the coated Fe₃O₄ nanospheres can not keep its morphology after sulfuration.Nanochains with a thickness of 40 nm gives the most excellent catalytic behavior.It delivers a current density of 10 mA/cm² at an overpotential of 195 mV to catalyze HER and kept almost unchanged for 150 hours.The remaekable performance can be interpretered by the fact that carbon coating not only protects FeS₂ from decaying during electrocatalysis but also reducing the size of materials and make the most of active sites.Besides,hierarchical structure of carbon coated FeS₂ nanochain bundles ensured electron and mass transfer continuity.Together with the enhanced electronic conductivity benefited from uniform carbon coating,these merits contribute to the remarkable catalytic behavior of the iron sulfide nanocomposite.Synthesis and design strategies here provide new ideas for preparing and stabilizing other transition metal sulphides with hiearchical structures.