Study On The Preparation Of Transition Metal Hydroxides/Oxides Nanoarrays And Their Energy Conversion And Storage Applications

With the exhaustion of the fossil energy and increasing severity of environmental pollution,the development and application of new energy materials and technologies are imminent.Advanced energy conversion and storage technology can convert intermittent energy,such as solar energy and wind energy,into new fuels or stored in second batteries,which is the key of the popularization and application of new energy technologies.Water splitting and LIBs play critical roles in energy conversion and storage sector,respectively.At present,the anode catalysts of electrolyzing water rely on noble metal,which is difficult to realize large-scale promotion.Meanwhile the low specific capacity of conventional graphite Lithium anode material hinders the further development of LIBs.Therefore,the design and development of new electrode materials with low price,high activity,and good stability are of great significance for energy conversion and storage technology.This dissertation is based on a comprehensive overview of the transition metal nanomaterials mainly used in OER and lithium ion batteries.From the aspects of crystal structure,nanostructure and metal doping,a series of novel hydroxides and oxides nanoarrays based on Co and Ni elements were prepared.According to their different characteristics,nanoarrays were used in OER or anode of lithium ion batteries.The main conclusions are as follows:?1?Amorphous Co?OH?₂ nanoarrays?A-Co?OH?₂ NS As?Amorphous metal-oxides?hydroxides?tend to be excellent OER catalysts by providing a higher concentration of defect sites.Co?OH?₂ and A-Co?OH?₂ nanosheet arrays were achieved by dealloying CoAl alloy in NaOH and NaOH with additional NaBH4,respectively.The Co?OH?₂ and A-Co?OH?₂ have same nanoarrays structure constructed of nanosheets with a thickness of about 100nm and a diameter of 1-3?m.Both of the Co?OH?₂ and A-Co?OH?₂ had the superior activities compared to that of commercial IrOx and Pt/C catalysts.Among them,A-Co?OH?₂ posses a higher OER catalytic activity than Co?OH?₂,which exhibits an onset potential of 280 mV,an overpotential of 350 mV for a current density of 10 mA cm-2,and a Tafel slope as low as 43.2 mV dec¹.The outstanding OER performance of A-Co?OH?₂ can be attributed to the special nanoarray structure and amorphous crystal structure caused by the special dealloying method.?2?Ni-based hydroxide hierarchical nanoarrays?NiyM?OH?_x HNAs?The catalytic performance of metal-oxide?hydroxide?can be enhanced by modulating their 3d orbital electron state through heteroatom doping.The morphology and oxygen evolution reaction?OER?activity of NiyM?OH?_x HNAs were modified using two non-noble transition metals?Fe and Zn?as dopants.These novel hierarchical nanostructures contain small secondary nanosheets that are grown on the primary nanowire arrays,providing a higher surface area and more efficient mass transport for electrochemical reactions.According to both experimental and DFT-based theoretical analyses,Fe and Zn had a different effect on the catalysts'activity.Fe was an effective dopant,while Zn decreased the OER activity.Ni_2.2Fe?OH?_x HNAs had a low onset ? of 234 mV,a small Tafel slope of 64.3 mV dec¹ and an excellent long-term stability for over 20 h.These values are also superior to those of a commercial IrOx electrocatalyst.The specific activity,which is normalized to the BET surface area of the catalyst,of the Ni_2.2Fe?OH?_x HNAs is 1.15 mA cm¹ BET at an overpotential of 350 mV,which is?4-times higher than that of Ni?OH?₂ HNAs.The excellent electrocatalytic performance of Ni_2.2Fe?OH?_x HNAs can be attributed to the hierarchical nanostructures,the appropriate elemental composition of the catalyst,and the presence of a multifunctional 3D conductive substrate.?3?Co-Fe hydroxide nanosheet arrays?CoyFe1-y?OH?_x NSAs?The positive efforts of Fe-doping on OER activity of Co-base oxide/hydroxide are roughly the same as that of Ni-base oxide/hydroxide.The catalytic performance of metal-oxide?hydroxide?can be enhanced by modulating their 3d orbital electron state through heterogeneous doping.We have succeeded in synthesizing a series of CoyFe1-y?OH?_x NSAs which integrated on a Cu foam three-dimensional electrode.The Fe-doping helps to increase both the geometric roughness and density of active sites.Ultrathin secondary nanosheets with different Co to Fe ratios that are subsequently grown on these primary nanoarrays are found to exhibit high OIER activity.The optimal composition of CoyFe1-y?OH?_x NSAs turns out to be Co0.7Fe0.3?OH?_x NSAs,which allows for an OER onset overpotential as low as 220 mV and a small Tafel slope of 62.4 mV dec¹,while also provids excellent long-term durability?>100 hrs?and a high TOF of 0.1 72 s¹ at an overpotential of 380 mV.The specific activity of Fe-doped Co0.7Fe0.3?OH?_x NSAs at an overpotential of 350 mV?0.37 mA cm-2BET?is also twice as high as that of undoped Co?OH?₂ NSAs.The outstanding electrocatalytic performance of co0.7Fe0.3?OH?_x NSAs can be attributed to an optimization of the effects of Fe-doping and an open 3D electrode design.?4?CoO/Cu₂O hierarchical porous nanoarrays?CoO/Cu₂O HPAs?Hydroxide nanomaterils can form porous nanoarrays after sintering.Employed in a LIBs anode,this structure is good at accommodating the volume expansion during cycling.We have successfully fabricated a unique CoO/Cu₂O HPAs on the three-dimensional Cu foam substrate.Owing to the hierarchical porous nanostructure,free-standing electrode structure and synergistic bi-active components,the as-prepared composite displays some superior electrochemical performances with superior capacity and rate capability.Moreover,the CoO/Cu₂O HPAs exhibited a high capacity of 832 mAh g¹ even after 50 cycles at a discharge/charge current density of 200 mA g¹.More importantly,the CoO/Cu₂O HPAs electrode exhibits a capacity of 267mAh g¹ even at a discharge/charge current density of 2000 mA g¹.Besides,the porous structure of CoO/Cu₂O HPAs electrode was still retaieds very well after 50 cycles.