Synergistically Modulating Electronic Structure Of One-dimensional Fe-based Chalcogenides With Enhanced Catalytic Activity For Electrochemical Water Splitting

Hydrogen energy is a potential energy source because of its many outstanding properties,which is considered an effective method for industrial production.Among many hydrogen production methods,electrochemical water splitting for producing hydrogen is an essential breakthrough for the future large-scale production of hydrogen energy,and the main of this technology lies in the design of efficient catalysts.At present,precious metals such as Pt,Ru,and Ir are widely used as catalysts for water electrolysis,but their scarcity and high price restrict their commercial application.Therefore,it is particularly necessary to design and develop catalytic materials that are inexpensive,have high electrocatalytic activity,and have excellent stability.The transition metal chalcogenide has a plentiful chemical composition and electronic structure,which can be controlled by doping,constructing heterojunctions,and creating defects to manage its electronic structure and physicochemical,which could exhibit catalytic activity similar to that of precious metals.Besides,these compounds have abundant reserves,low prices,and excellent chemical stability as a superior ideal candidate material to replace precious metal catalysts.Herein,the one-dimensional iron-based chalcogenide is used as a research model to control its electronic structure through morphological design,construction of heterojunctions and heteroatom doping,etc.In-depth study of different control methods on electrochemical hydrogen evolution reaction and oxygen evolution reaction impact to explore the relationship between changes in the electronic structure of materials and charge transport during the reaction of electrolyzed water.The specific work content of this article is as follows:1.Iron sulfide is an excellent and low-cost catalyst for catalytic hydrogen evolution,but its catalytic activity still has excellent potential for research.The control of the one-dimensional nanostructure of the material can further improve the catalytic activity of the hydrogen evolution of iron sulfide.However,the controllable synthesis of one-dimensional iron sulfide nanomaterials is facing enormous challenges.In this chapter,ferrous chloride was used as the iron source,sulfur power was used as the sulfur source,ethylenediamine was used as the surfactant and solvent.The organic-inorganic precursor material(S-ethylenediamine/Fe-ethylenediamine,abbreviated as SEFE)with a one-dimensional nanostructure was synthesized by the hydrothermal reaction,and then the Fe₃S₄@C one-dimensional porous nanowire was successfully prepared by high-temperature annealing treatment.During the annealing process,the organic precursor decomposes and transforms into a carbon material for generating porous structures.It improved the specific surface area of the catalyst,enhanced catalytically active sites,and facilitated the infiltration of electrolyte and the escape of bubbles,speeding up a mass transfer.Besides,the coating of the carbon layer could effectively improve the conductivity of the catalyst,and also effectively hinder the aggregation of Fe₃S₄ nanoparticles.Therefore,the composite material showed good catalytic activity for hydrogen evolution and oxygen evolution in an alkaline solution.It showed a overpotential of 186 m V for HER at a current density of 10 m A cm^-2 and a overpotential of 258 m V for OER at a current density of 20 m A cm^-2.2.Constructing a multi-component heterostructure catalyst is an effective way to improve the catalytic performance of HER and OER.Due to pure Fe₃S₄ still exhibited unsatisfactory electrocatalytic performance.This chapter used the above SEFE one-dimensional nanowires as a precursor to prepare carbon-coated Fe₃S₄-Fe₇Se₈heterogeneous nanocomposites by vapor deposition.After selenization,the material still maintained a relatively complete one-dimensional nanowire structure and had a large specific surface area,which can provide a large number of active sites.At the same time,the carbon layer was coated on the surface of the material to enhanced conductivity,thereby improving the catalyst's catalytic performance.More importantly,the formation of the Fe₃S₄-Fe₇Se₈ heterojunction regulates the charged active centers,allowing the electrons redistributed at the interface,and realizing the adjustment of electronic structure and Fermi energy level,and then optimized the adsorption free energy of hydrogen(H*)and oxygenated intermediates(OH*,O*and OOH*).As a result,the as-synthesized porous nanowires enabled the enhancement of both HER and OER in alkaline solution,which exhibited a low overpotential of 124 m V to reach a current density of 10 m A cm^-2 with a Tafel slope of 111.2 m V dec^-1 for HER.Besides,an outstanding overpotential of 219 m V to reach a current density of 20 m A cm^-2 with a Tafel slope of 45.4 m V dec^-1 for OER.On the other hand,the assembled electrolyzer delivers a current density of 10 m A cm^-2 at a low voltage of 1.67 V with excellent stability for at least 12 h.The above results showed that the construction of heterojunctions provided a new method for the development of iron-based electrocatalysts and provides further references and inspiration for other researchers.3.Heteroatom doping could change the charge density distribution on the surface of the catalyst,thereby optimizing the hydrogen adsorption capacity and electrocatalytic performance of the surface.In this chapter,P-Fe₇S₈@C nanomaterials were prepared by a chemical vapor deposition method,using SEFE as a precursor and sodium hypophosphite as a phosphorus source.The effective control of the phosphorus doping could be achieved by adjusting the ratio of sodium hypophosphite.The study had shown that the doping of phosphorus changed the charge density distribution on the surface of the catalyst and reduced the free energy barrier of adsorption on the active site,thereby providing less charged transfer resistance and rapid electrode kinetics for an increase in the rate of electrocatalytic water-splitting reaction.As a result,the as-synthesized catalytic exhibited outstanding bi-functional catalysts in alkaline solution.It showed a low overpotential of 136 m V for HER to reach a current density of 10 m A cm^-2 with a Tafel slope of 113.9 m V dec^-1.Besides,an untrallow overpotential of 210m V for OER to reach a current density of 20 m A cm^-2 with a Tafel slope of 42.5 m V dec^-1,and excellent stability was at least 12 h.