Structural Control Of CoSe₂ Nanosheets And Their Electrocatalytic Performance

Reducing carbon emissions and developing green renewable energy is one of the global goals.At present,a series of electrochemical reactions such as hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）and oxygen redox reaction（ORR）can convert H₂O,O₂and other molecules in the environment into H₂,H₂O₂and other high value-added products.However,to optimize the slow kinetics of the electrochemical reaction process,it is highly desirable to use catalysts to improve the reaction efficiency and reduce the reaction potential.Due to the abundance,diverse bonding properties,and potentially good catalytic activity,Cobalt-based compounds are regarded as one of the most promising inexpensive catalysts.Designing and optimizing the structure of Cobalt-based compounds is a promising strategy to obtain highly efficient electrocatalysts suitable for electrochemical reactions such as HER,OER,and ORR.In this paper,the morphology and structure of CoSe₂（a typical Co-based electrocatalytic material）are regulated by various methods such as hydrothermal method,chemical vapor deposition（CVD）,sacrificial template method,etc.This work explores and reveals the key factors that affect catalytic performance by the systematical structure characterization and electrochemical performance test.The first work of this thesis is the preparation of ultrathin and defect-rich CoSe₂nanosheets as electrocatalysts for the ORR reaction with high selectivity and high efficiency.The hydrophilicity and hydrophobicity of the carbon cloth substrate were found that would affect the morphology of the samples grown on its surface.In detail,this work successfully prepared CoSe₂nanosheets with a thickness of only 3 nm as well as abundant defects on the hydrophilic carbon cloth.It is found that Co sites in defect-rich ultrathin nanosheets can accelerate the adsorption of the intermediate OOH,and effectively prevent the breakage of O-O bonds.Therefore,not only the overall ORR reaction efficiency is promoted,but the selectivity of the reaction products is also effectively improved.The selectivity for obtaining H₂O₂in alkaline solution is as high as about 92%,and the yield of H₂O₂is1227.7 mg L^-1h^-1.This work provides a promising catalyst for the electrochemical synthesis of H₂O₂and a strategy to grow atomically thin 2D nanosheets by adjusting the hydrophilicity and hydrophobicity of the substrate.In the second work,hollow CoSe₂nanospheres were successfully designed and synthesised.Their electrochemical OER performance was then studied.Here,Ga Zn alloy nanoparticles were used as a template for reaction,and a Ga-Co based core-shell structure can be obtained by sacrificing Zn atoms in Ga Zn nanoalloy particles.A Co-based hollow structure can be formed by removing the inner Ga template,and then CoSe₂can be finally synthesized by conducting a post selenization step.This three-dimensional hollow structure can expose more active sites and facilitate gas transort,which shows a good OER activity with an overpotential of about 340 m V and a Tafel slope of 80.0m V dec^-1at 10 m A cm^-2.This work reports a method for constructing hollow-structured CoSe₂catalysts,which provides a new idea for designing electrocatalysts with high specific surface area and more active sites.In summary,the morphology and structure of CoSe₂are effectively regulated,exposing more active sites and optimising the catalytic activity.In this paper,a series of CoSe₂based catalysts with high electrocatalytic performance were developed,which provided new strategy and schemes for the rational design of catalysts.