Study On The Preparation Of Transition Metal-based Catalysts And Their Water Splitting Performance

Hydrogen energy is considered as one of the most promising clean energy sources in the 21st century.The main carrier of hydrogen energy is hydrogen.Sustainable hydrogen generation via electrocatalytic/photocatalytic water splitting has been widely regarded as the most promising energy carrier and has attracted extensive attention.The reaction of water splitting includes two half reactions,oxygen evolution reaction(OER)and hydrogen evolution reaction(HER).The hydrogen evolution reaction is a relatively simple reaction.The water oxidation reaction is the key process involved in the generation of H2 owing to its sluggish dynamics and the involvement of four electrons.Therefore,the development of catalysts with high catalytic activity is very important for efficient water splitting.The transition metal element has advantages including low cost,high efficiency,and good stability,making the entire water splitting more practical and feasible.In this dissertation,the Fe-based and Ni-based catalysts were synthesized.by hydrothermal and sulfurization respectively.The structure and properties of the catalysts were tested by the X-ray diffraction(XRD),Scanning Electron Microscope(SEM),Transmission Electron Microscope(TEM),UV-Vis spectroscopy,fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS),and so on.Finally,the catalytic performance of the samples was evaluated by electrochemical workstation.The major achievements are as follows:1.This is the first time that the direct use of b-FeOOH samples has been reported in a water splitting system,in which ?-FeOOH has always been used as a co-catalyst.The ?-FeOOH doped with low content of Ni and Co were synthesized by hydrothermal,hexahydrate ferric chloride and anhydrous sodium nitrate as precursors,adding salts containing Ni2+ and Co2+ in the precursor solution.From a combination of DFT caculations and experimental results,(1)The band gap of ?-FeOOH is 2.26 eV.The band gaps became narrower,with values of 0.92 and 0.43 eV for Ni-FeOOH and Co-FeOOH.(2)After doping with Co/Ni,the photocurrent densities of Co-FeOOH and Ni-FeOOH were 4.71 mA cm-2 and 2.55 mA cm-2 at 1.0 V vs.RHE,which are much higher than pure(3-FeOOH 0.69 mA cm-2.The narrowing of the band gap and the difference in the Fermi level between FeOOH and MOOH(M =Ni or Co)can promote the separation of photogenerated carriers and thus enhance the photocatalytic activity;(3)The electrocatalytic oxygen evolution performance of Ni/Co-doped samples was better than that of pure ?-FeOOH.At the same time,the catalyst has a good stability.2.Most of the hydrogen evolution reactions(HER)always accomplished through using precious metals.In this dissertation,we report a new HER candidate,an Fe-doped NiS2 nanosheet,with the performance of high activity and electrochemical stability.We chose the sulfidation of NiFe2O4/a-Ni(OH)2 under mild calcinated temperature for Fe-NiS2 formation.The experimental results suggest that the Fe3+ doping in the surface lattice of the NiS2(002)plane reduces the activation energy of the hydrogen evolution reaction.The synthesized 2%Fe-NiS2 sample exhibits the best electrocatalytic HER performance with a low Tafel slope of 37 mV·dec-1 and a low overpotential of 121 mV at 10 mA·cm-2.In addition,The Fe-NiS-2 sample was in situ loaded onto the surface of CdS nanorods by the same sulfidation conditions as a cocatalyst for the photocatalytic hydrogen production.Among them,2%FeNiS/CdS exhibited the best hydrogen evolution effect and produced 3.2 mmol·h-1g-1 hydrogen was generated under visible light,which is 46 times higher than pure CdS.