Synthesis And Applications Of Amorphous Transition Metal Oxides Nanosheets

The world’s energy consumption largely depends on the fossil fuels combustion,which results in energy and environmental crisis,this makes energy structure transition an important topic and challenge faced by human society in the 21st century.At the same time,in order to meet the power supply reeds of electric vehicles,mobile phones,flexible electronics and other new equipments,the development of low-cost efficient and clean and renewable novel energy conversion and storage devices(such as lithium battery and water electrolysis equipment,etc.)is of crucial importance.However,the electrode materials of these energy devices are still difficult to meet the practical performance or heavily depend on the noble metal based materials,hindering the largescale development and commercial applications of new energy devices.3d transition metal(such as vanadium,manganese,iron,cobalt,nickel,etc.)oxides have the characteristics of low cost,easy preparation,special physical properties,good chemical stability and catalytic activity,which show great application potential in the field of lithium battery,electrocatalysis and other new energy technologies.Reasonable design of cheap transition metal oxides to further improve their performance is expected to solve the difficulties in large-scale applications of new energy systems.In this doctoral dissertation,based on cheap transition metal oxides,we have developed a general synthesis method to control the formation of two-dimensional(2D)ultrathin nanostructures,and at the same time,use amorphization strategy to fine-tune their atomic arrangements at the atomic scale,thereby significantly improving their availabilities in specific applications.In addition,we use advanced characterization and simulation techniques to accurately analyze and model the atomic structure of amorphous transition metal oxides.Thus,the mechanism of the influence of the shortrange order and electronic structures of amorphous materials on the charge storage process and catalytic conversion reaction was studied,and the structure-performance relationship of 2D amorphous transition metal oxides applied in lithium batteries and electrocatalysis was established.The main content of the paper is summarized as follows:1.The morphology,size and amorphization regulations of cheap transition metal oxide nanomaterials are briefly summarized,and the importance and challenge of these regulations and the progress of applied research are expounded.On this basis,the topic selection basis and main research contents of this paper are discussed.2.Lithium-sulfur batteries show great potential in realizing novel high-energydensity storages.However,the polysulfides produced in the discharge process of lithium-sulfur batteries will dissolve into the electrolyte,leading to the decrease of cycle stability,which limits their practical applications.In order to inhibit the dissolution of polysulfides,metal oxides are often used as cathode material additives to adsorb polysulfides,but their adsorption capabilities still need to be improved urgently.Herein,2D ultra-thin amorphous cobaltous oxide(CoO)nanosheets with thickness of about 10.4 nm were synthesized by one-step pyrolysis of metal precursor Cobalt(Ⅱ)acetylacetonate using inorganic salt as template,and selected as cathode material additive for lithium-sulfur batteries.X-ray absorption spectra showed that the the ligand field symmetry around Co atoms in amorphous CoO nanosheets have changed.In combination with charge transfer multiplet calculations and density functional theory calculations,we have revealed that compared with that of crystalline CoO nanosheets,the ligand field symmetry around Co atoms in amorphous CoO nanosheets partially changes from octahedral(Oh)configuration to tetrahedral(Td)configuration.Due to the special atomic configuration of amorphous CoO nanosheets,when it is used as the cathode material additive for lithium-sulfur battery,it can significantly improve the capacity,rate,and stability of lithium-sulfur battery.The initial capacity of 1248.2 mAh g-1 can be achieved by testing the charge-discharge stability at 1 C rate,and the capacity of 1037.3 mAh g-1 can still be achieved after 500 cycles.Density functional theory(DFT)calculations reveal that the amorphization-induced change of the symmetry of the ligand field around Co atom leads to more electrons occupying the high energy level,resulting in strong binding energy between the amorphous CoO nanosheet and polysulfides,thus achieving high-performance lithium-sulfur battery.3.The traditional industrial anthraquinone method for H2O2 production has many problems,such as high-energy consumption,environmental unfriendly,etc.Therefore,the electrocatalysis H2O2 production through 2-electron oxygen reduction reaction(ORR)is expected to become an alternative method.The competitive relationship exists between 2-electron ORR and 4-electron ORR,noble-metal Pt and Pd based alloys nanomaterials showed high selectivity of 2 electronic oxygen reduction,but its high cost limits the practical application.Herein,2D ultra-thin amorphous nickel oxide(NiO)nanosheets with thickness of about 12.3 nm were synthesized by salt-templated method,which provides a cheap and excellent catalyst for electrocatalytic H2O2 production.Xray absorption spectroscopy analysis reveal that the short-range orders of a-NiO NSs and c-NiO NSs mainly take the NiOs pyramid and NiO6 octahedron respectively The a-NiO NSs for electrochemical oxygen reduction reaction(ORR)to H2O2 production in 0.1 M KOH exhibits both high selectivity over 90%and high activity(1 mA cm-2 at 0.66 V vs.RHE),while c-NiO NSs tends to catalyze ORR through 4-electron pathway to generate H2O.Theoretical calculations indicate that the changed short-range order of a-NiO NSs leads to the alteration of Ni d orbital states,which can regulate the adsorption orientation and strength of*OOH intermediate to achieve high selectivity and activity of 2-electron ORR.4.The water electrolysis is the most promising green hydrogen production scheme.The oxygen evolution reaction(OER)on the anode largely determines the efficiency of the water electrolysis device.At present,the most efficient catalysts for OER are precious metal based RuO2 and IrO2.Therefore,it is very important to develop cheap and efficient OER electrocatalysts.In this chapter,we synthesized highly crumpled 2D ultrathin amorphous boron-doped cobaltous oxide nanosheets(B/a-CoO NSs)with the thickness of only 2.2 nm by simple salt-templated method.On the one hand,the high specific surface area of B/a-CoO NSs caused by the ultrathin 2D structure can provide abundant active sites for electrocatalytic OER.On the other hand,X-ray absorption spectra analysis showed that boron doping changed the short-range order of Co coordination,which improved the intrinsic catalytic activity.Therefore,the B/a-CoO NSs catalyst showed excellent electrocatalysis OER activity,where the overpotential was as low as 292 mV at 10 mA cm-2.In addition,the B/a-CoO NSs also showed excellent OER stability.