Controlled Synthesis Of Transition Metal Materials At Low Temperature For Tuning Electrocatalytic Performance

The problems of global warming and environmental pollution are becoming increasingly serious due to the overuse of fossil fuels,so increasing the development and efficient utilization of clean and sustainable energy is required.The development of sustainable and renewable energy sources as well as efficient energy storage and conversion technologies is extremely important to address environmental and energy challenges.Therefore,it is necessary to accelerate the research on the technology of hydrogen evolution from the electrocatalytic water splitting and use"surplus electricity"to produce high-purity clean energy hydrogen,which are of great significant to continue the research and development of fuel cells and metal-air batteries.However,the environmentally friendly and efficient energy conversion technology need to rely on the ORR and HER catalysts with low cost,high activity,and high stability.In this thesis,transition metal oxide and transition metal selenide nanomaterials for were successfully controlled and synthesized by simple preparation mehod at low temperature.Simple low-temperature controlled synthesis not only saves resources,but also maintains the micro-morphology and structure of the original material itself.Simultaneously,introduction of oxygen vacancies,exposure of high-energy crystal planes and establishment of heterostructures on the original materials,thereby optimizing the nanostructure for the materials and obtaining better electrochemical catalytic performance.（1）The micromorphology and structure of transition metal oxide catalysts have great influence on catalytic activity.In this paper,Co₃O₄ nanomesh was calcined in the Ar gas mosphere under low-temperature to obtain Co O nanomesh.Low temperature calcination makes Co O nanomesh maintain these advantages of ultra-thin,exposed[1^—12]high-energy crystal plane and large surface area from Co₃O₄ nanomesh.In addition,plenty of oxygen vacancies are generated during calcination process,which also indicates that Co₃O₄ nanomesh transforms into Co O nanomesh is only deoxidized process.The ultra-thin nanosheet makes Co O has the ability of strong electron conduction and rich active sites,making its ORR catalytic activity better than Co₃O₄nanomesh,Co O obtained from commercial or cubic Co₃O₄ conversion,and it also has better stability.（2）The unique three-dimensional flower-like Ni Se₂/Mn_0.8Ni_0.2Se heterostructure with Ni Se₂ nanoparticles embedded in Mn_0.8Ni_0.2Se nanosheets was successfully controlled and synthesized using two steps of hydrothermal and low-temperature calcination.Under the low-temperature calcination at 450℃,the micromorphology of precursor nanosheet is well kept and the Ni Se₂ particles are uniformly dispersed on the surface of Mn_0.8Ni_0.2Se,which are conductive to generating a large surface area for the material.As a result,the Ni Se₂/Mn_0.8Ni_0.2Se heterostructure exhibited excellent HER catalytic activity and strong stability.The overpotential was 71 m V at a current density of 10 m A cm^-2,where possessed a lower Tafel slope of 56 m V dec^-1,meaning that the catalytic activity of calcined synthesis products at 400℃and 500℃was preferable.This work provides ideas and basis for the transition metal heterostructures with excellent catalytic performance via low-temperature controlled synthesis,which is expected to bring new possibilities for better and more stable catalytic performance of transition metals and the replacement of precious metal catalysts in the future.