Synthesis And Electrochemical Properties Of Two-dimensional Transition Metal Nanoarrays

The growing energy crisis and environmental pollution have forcing global efforts to develop clean and renewable energy resources to replace traditional fossil fuels.Hydrogen energy,as a sustainable energy,which has the advantages of high resources,high calorific value and cleanliness,has become one of the most promising candidates.Currently,about 90%of hydrogen is produced by hydrocarbon reforming,which is neither environmentally friendly nor energy efficient.As an clean and efficient method for hydrogen production and storage,electrochemical water splitting?2H₂O?O₂+2H₂?has been extensive exploration in recent years.Therefore,it is of great significance to exploring an efficient,economical and durable electrocatalyst for water splitting.At present,noble-metal based catalysts is the state of the art electrocatalyst?Pt for HER,and IrO₂ or RuO₂ for OER?.However,their large-scale application for water splitting is limited by the high cost and extreme scarcity.In recent years,transition metals?TM?,which is inexpensive and earthabundant,have demonstrated excellent electrochemical properties due to the unique electron configuration,showing the potential to replace noble-metal based catalysts for water splitting.This thesis aims at the characteristic and defects of transition metal compounds?such as sulfides,phosphides,hydroxides?,and combines the advantages of some conductive substrate materials?such as nickel foam?NF??,and discussed electrochemical water-splitting performance from the aspects of material composition,morphology,electrode structure and etc.The main work is summarized as follows:1.Integrating heterogeneous hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?electrocatalysts into compatible composite catalysts with specific interfaces and structures holds great promise for the development of efficient bifunctional catalysts for overall water splitting.Herein,heterogeneous MoS₂-Ni?OH?₂ catalyst supported by Ni foam?MoS₂@Ni?OH?₂/NF?is developed,in which the Ni?OH?₂ nanorod array is grafted with MoS₂ nanosheets.The hierarchical1D/2D structures benefit not only the exposure of catalytic site but also the facilitated charge transfer.The heterointerfaces allow the synergism of electric structure modulation for improved catalytic activity and kinetics towards HER,OER,and overall water splitting.MoS₂@Ni?OH?₂/NF achieves a current density of 10 mA cm^-2 at low overpotentials of 134 and 233 m V for HER and OER in 1 M KOH,respectively.Moreover,a cell voltage as low as 1.46 V is obtained to afford 10 mA cm^-2 in a tow-electrode electrolyzer using MoS₂@Ni?OH?₂/NF as bifunctional catalysts,assuring it superior to most of the reported noble-metal-free bifunctional electrocatalysts.Such heterogeneous design of composite catalyst should shed light on the exploration of functional materials for energy applications.2.Efficiently generating hydrogen in neutral solution is essentially important for the electrocatalyst toward practical water splitting application,however,most of the catalysts show their promising activity only in acidic or alkaline media.Herein,the surface modulation method is developed via doping Ru onto the surface of Ni₂P nanosheet array supported on nickel foam?Ru-Ni₂P/NF?to activate the robust hydrogen evolution activity.The Ru species achieve optimized surface chemistry for catalysis,including boosted intrinsic activity,improved center density,and enhanced kinetics.The as-obtained electrode exhibits Pt-like catalysis activity,as well as good stability in both neutral and alkaline solution.Exploring efficient and wide-pH electrocatalyst by such surface chemistry modulation would be of great value for the development of advanced materials for energy conversion.3.Oxygen evolution reaction?OER?is a key process in water splitting systems,fuel cells and metal-air batteries,but it is a huge challenge to develop a cheap and efficient OER catalyst in a simple way.Herein,Ni₃S₂/Co₃S₄ nanosheets grow directly on nickel foam,a three-dimensional conductive substrate,with two-step hydrothermal method,and the two-dimensional nanosheets were staggered into a three-dimensional nanoflower microstructure.The incorporation of Co atoms in Ni-Co-S enhances the intrinsic catalytic performance of the material by creating substantial defect sites at atomic scale and modifying the electronic structure of Ni-Co.As well as,three-dimensional nanoflower microstructure increases the conductivity and stability of the electrode.Notably,the overpotentials are as low as 242 mV at current density of 10 mA cm^-2,with a low Tafel slope(45.3 mV dec^-1)and it can still maintain good morphology and catalytic activity after testing of 30 hours in 1 M KOH.This work provides an effective strategy for manufacturing efficient,stable,and low-cost OER electrocatalysts,which is expected to facilitate the development of energy storage and conversion technologies related to OER.