Synthesis And Electrochemical Energy Storage Of Transition Metal Compound-Carbon Composites

With the increasing demand of green energy,highly efficient energy storage devices have drawn significant attention.Li-ion batteries?LIBs?and Na-ion batteries?NIBs?are two of the most important types of energy storage systems that have been investigated extensively owing to their advantages of environmental friendliness,high energy/power density and long cycle life.LIBs have already made great commercial success in portable electronics,and recently application of LIBs as power sources for electric vehicles?EVs?and hybrid electric vehicles?HEVs?has raised intense attention in many countries,especially in China.Nevertheless,owing to the limit reserve of Li resources,NIBs which shares similar operation mechanism with LIBs have recently re-gained great interest as a promising alternative to LIBs because of the rich abundance of Na in Earth.To achieve high-energy density with LIBs and NIBs,it is crucial to develop high-capacity electrode materials,especially anode materials.Various strategies have been proposed to address the above undesired issues,including incorporating them with conductive nanocarbon,tuning the morphology and structure at the nanoscale,and increasing the interlayer distance.Hydrogen has emerged as a promising clean and affordable energy fuel compared to other fossil fuels owing to its high gravimetric energy density and renewability.Among various hydrogen production methods,electrolysis of water is considered to be promising for the future hydrogen economy because it provides a green and sustainable approach to store electric energy which is converted from solar and wind renewable sources.Designing high-performance catalysts towards hydrogen evolution reaction?HER?are necessary to ensure the energy efficiency of water electrolysis.Although noble metals such as Pt-based materials are considered as the excellent electrocatalyst for efficient hydrogen production,the cost and scarcity hamper their large scale application.Therefore,it is highly desirable to develop inexpensive and earth-abundant elements composed active and stable electrocatalysts in realizing hydrogen production in future.During the past years,to pursue inexpensive HER electrocatalyst alternatives for Pt-based noble metals,a great number of non-precious transition metals based inorganic materials,e.g.,transition metal alloys,sulfides,phosphides,carbides and nitrides have been explored.Improving the quantity of active sites as well as the conductivity of these transition metals based electrocatalysts is critical for the enhancement of their HER activity.On the other hand,doping with metal?V,Co?and non-metal elements?O,N,P?has recently been demonstrated as a promising way to improve catalyst performance.In this thesis,according to the characteristic of sulfur transition metal and the existing defects,and combining the advantages of graphene materials,the properties of electrochemical lithium/sodium storage and electrocatalytic hydrogen evolution were studied from the aspects of material composition,morphology and electrode structure.The main contents are summaried as following:1.MoS₂ shows promising potential in the application of lithium ion batteries?LIBs?or sodium ion batteries?SIBs?,however,suffering from the sluggish kinetics and terrible volume expansion.Here we fabricate interlayer-expanded VMo₂S₄nanosheets onto RGO and exhibit excellent electrochemical properties for lithium and sodium storage.The 2D nature and interlayer-expanded feature assure the shortened ion transfer path and improved diffusion kinetics.RGO substrates enhance the electric conductivity and preserve the material integrity.Based on such benefits,the present sample serves as advanced anode for both LIBs and SIBs,delivering reversible capacities of 1081 mAh g^-1 for LIBs and 254 mAh g^-1 for SIBs after 200 cycles at 1 A g^-1.2.In this work,vanadium and nitrogen co-doped MoS₂ on reduced graphene oxide with new defect sites on the basal/edge planes and expanded interlayer spacing was successfully synthesized by hydrothermal method,which shows remarkable catalytic merits with an extremely low overpotential of 68 mV at 10 mA cm^-2 and a Tafel slope of 41 mV dec^-1.Its performance is superior to most current MoS₂electrocatalysts.In addition to the charge transfer benefits of the RGO substrate,the vanadium and nitrogen dopants trigger defect sites on the basal/edge planes,and an optimized electronic structure and expanded interlayer distance are also responsible for the enhancement in catalysis.3.Electrochemical water splitting,allowing energy conversion from renewable resources into non-polluting chemical fuels,is vital for future sustainable energy systems,and great efforts have been made for developing efficient and cheap bifunctional electrocatalysts.Herein we report a bifunctional vanadium-doped Ni₃S₂nanorod array electrode for overall water splitting in alkaline media.To afford a catalytic current of 10 mA cm^-2,the designed V-Ni₃S₂ electrode only requires overpotentials of 133 mV for hydrogen evolution and 148 mV for oxygen generation,meanwhile showing high long-term stability.The excellent catalytic properties are attributed to the V dopant and geometric advantages of the nanorod array.The V-Ni₃S₂electrodes are simultaneously utilized as cathode and anode in one two electrode cell for overall water splitting,exhibiting a cell voltage of 1.421 V at 10 mA cm^-2.The water splitting in this cell can also be feasibly driven by a single-cell AA battery?1.5V?.Our report shows substantial advancement in the exploration of efficient bifunctional electrocatalysts for water splitting.