The Study On Optical Characterization And Electro/Photoelectrochemical Water Splitting For Hydrogen Evolution In Nickel-Based Compounds

Renewable energy sources like solar,wind and others,always exhibit a property of intermittency,requiring a battery or other energy storage technology to compensate for that.The hydrogen,H₂,is a carbon-free fuel with high specific energy,being highly regarded as an ideal material to store the energy.Electrochemical water splitting is an efficient technology for hydrogen production with the advantages of environmental protection and high yield.However,it consumes a large amount of electric power due to the high overpotential required.In particular,the most representative catalyst for electrochemical hydrogen evolution is still platinum-based material,so obtaining a new alternative that is also highly efficient,but without using noble metals,is the ultimate goal to reduce the cost of electrochemical water splitting for hydrogen evolution.Another way for reducing the electricity demand is to use solar energy to make up some of the energy required during water splitting.Among all the nickel-based compounds,nickel phosphide and nickel selenide have attracted the attention of electrocatalytic hydrogen evolution.However,compared with platinum-based catalysts,their electrocatalytic activity for electrocatalytic hydrogen evolution still needs to be further enhanced.In this dissertation,by means of optimizing nickel phosphide and nickel selenide through several simple designs such as doping and modification,their performance of electrochemical and photoelectrochemical hydrogen evolution is improved.In addition,both nickel phosphide and nickel selenide have several different phases,therefore,a series of optical characterization are conducted to measure the morphology and composition.Finally,photoelectrocatalytic hydrogen evolution could be carried out by one electrocatalyst,which is significant to the research of photoelectrochemical water splitting.Firstly,a new composite nickel phosphide material consisting of foam,nanosheet arrays,nanoparticles is carried out according to the design of a layered structure.This structure makes nickel phosphides possess a large surface area offering abundant active sites to promote hydrogen generation.With such large active sites,the as-synthesized nickel phosphide requires overpotentials of only 35 and 65 m V to achieve current densities of 10 m A cm^-2 in acidic and alkaline conditions,respectively.Moreover,although this nickel phosphide is worse than platinum wire at low current density,it becomes much better than platinum at high current densities in base media.It should be emphasized that the as-synthesized sample exhibits excellent stability at a current density of 1200 m A cm^-2 after two calcines,indicating that it plays a potential role in large-scale hydrogen production.Secondly,for pursuing efficient HER electrocatalysis,besides morphology control and suitable structural design to achieve a large specific surface area catalyst,improving the intrinsic activity of nickel phosphide are also effective.Therefore,two methods of doping and modification are used to optimize the nickel phosphide at the same time in this work.With the manganese doping and the modification of carbon nanodots,the electrochemical effective surface area and intrinsic activity of the composite nickel phosphide are significantly higher than that of the untreated nickel phosphide.The carbon nanodots decorated nickel-manganese phosphide catalyst requires the overpotentials of only 31 and 56 m V to achieve a current density of 10 m A cm^-2 in acidic and alkaline solutions,respectively.The performance of the carbon nanodots decorated nickel-manganese phosphide for hydrogen evolution has caught up with platinum nanowires at both low and high current density.By analyzing the mechanism,it is found that the modification with carbon nanodots could not only increase the active sites on the surface of nickel phosphide,but also enhance the charge transfer between the electrode and the electrolyte,which proves that carbon nanodots are beneficial on the catalytic activity of electrocatalyst for hydrogen evolution.Finally,as nickel phosphide has proved that it could surpass platinum in catalytic performance through a simple design,this work tries to dope heterometal into nickel selenide to improve its catalytic activity,in order to further illustrate that nickel-based compounds may replace platinum-based materials.Ni Se and heterometal?Co,Fe,or Mn?-doped Ni Se are prepared by a simple method of chemical vapor deposition.It is concluded that the doping of a heterogeneous metal results in small changes in the morphology and effective catalytic area of nickel selenide but has a significant impact on its charge transfer performance,leading to the change of electrochemical transfer impedance.Furthermore,Co or Fe doping results in better performance for the Ni Se-based catalysts,and the cobalt doped nickel selenide exhibits the best electrocatalytic activity for hydrogen evolution.Ni Se shows a unique electronic structure as a semiconductor with good electrocatalytic activity,therefore,the photo-induced electrons will directly reduce the energy demand during the electrocatalytic hydrogen evolution by illuminating the Ni Se catalyst with sunlight.The overpotential needed for Co-doped Ni Se to achieve a current density of 10 m A cm^-2 is reduced to 88 m V when exposed to sunlight.Therefore,the introduction of sunlight on the semiconductor electrocatalysts,such as Ni Se,can greatly promote the efficiency of electrochemical reaction on the surface and then improve the catalytic activity of the electrocatalyst,thus realizing the photoelectrochemical hydrogen evolution by a single material.