Synthesis And Electrocatalytic Performance Of The Cobalt Diselenide Based Nanomaterials

Hydrogen has become the most promising substitute among the new energy sources because of its high calorific value,wide source and no pollution.However,the traditional hydrogen energy synthetic methods have the disadvantages of high energy consumption,high initial investment cost and environmental pollution,which can not meet the current demand for hydrogen energy.Producing hydrogen by water electrolysis is a pollution-free and facile method,but the low conversion efficiency restricts the wide application of this technology in industry.Therefore,developing highly active electrode materials is significant to reduce the energy consumption of hydrogen production in water splitting and improve the energy conversion efficiency.In this thesis is used as substrate and modified by doping and combining with conductive materials.The catalytic performance of nanomaterials is tuned by carefully adjusting the morphology and structure.The mechanism of different modification methods was analyzed,and a new type of low-cost and efficient bifunctional electrocatalyst was developed.CoSe₂ was doped with S element and Ni element,respectively.The structure and morphology are characterized and the electrocatalytic activity are tested.When the current density of the electrocatalytic hydrogen evolution reaction reaches 10 mA/cm²in 0.5 M H₂SO₄ electrolyte,the required overpotential is–157 mV and the Tafel slope is only 28.2 mV/dec.For oxygen evolution when the current density of Ni-doped CoSe₂ in 1 M KOH electrolyte reaches 10 mA/cm²,the overpotential is 389 mV and the Tafel slope is as low as 68.6 mV/dec.The physical and chemical test results indicate that doping with non-metal can effectively regulate the d-band electronic structure of Co and make the filling degree of e_g orbitals more beneficial for hydrogen evolution.However,incorporating metal in CoSe₂ could facilitate the formation of more metal oxides and hydroxides as active sites for catalytic oxygen evolution reaction.Co-Ni-S-Se quaternary system was prepared and it was found that a three-dimensional rose-like structure composed of two-dimensional nanosheets is obtained when the atomic ratio of Ni to Co is 3:7 in CoS_0.5Se_1.5.The quaternary material exhibits excellent catalytic activity for hydrogen evolution and oxygen evolution.The test results show that the synergistic effects between components modulate the electronic structure of the material close to t⁶_2ge_g¹,which is favorable to hydrogen evolution.Furthermore,the metallic nature of Co-based and Ni-based materials can form more active centers in alkaline electrolyte.As a result,the catalytic oxygen evolution activity of quaternary materials has also been improved.Bifunctional RGO/Co Ni SSe materials were obtained by changing the reaction temperature and the atomic ratio of source materials.When the current density reaches 10 mA/cm²,the overpotentials of RGO/Co Ni SSe for catalytic hydrogen evolution and oxygen evolution in acidic and alkaline electrolyte are–147 and 347mV,respectively.The Tafel slopes for catalytic hydrogen evolution and oxygen evolution are 33.5 and 58.8 mV/dec,respectively.The test results show that nitrogen is incorporated into RGO by small-molecule-ammonia-assisted hydrothermal method,which can not only increase the electron density and conductivity of RGO surface,but also provide nucleation sites for CoSe₂ nanosheets.The improvement also could be ascribed to the fact that the doping of S and Ni in CoSe₂ increases the number of defects and activates the inert basal surface,correspondingly increases the number of active sites and thus improves the catalytic activity of the composites.Furthermore,the electrical and chemical coupling between RGO and CoSe₂ promote the migration of electrons from the surface of RGO to CoSe₂ materials and improve the electrical conductivity of the composites.Therefore,RGO/Co_0.7Ni_0.3S_0.5Se_1.5shows the best catalytic activity and stability,which has not been reported in other literatures.In this thesis,the non-noble metal catalyst CoSe₂ was modified by doping and composite with conductive substrate,and the relationship among composition,structure and performance was studied.We hope that the ideas and strategies of non-precious metal catalysts can be used as a reference for the preparation of bifunctional water splitting catalysts with superior performance and stability.