Synthesis Of Molybdenum Diselenide Electrocatalyst And Study On Its Hydrogen Evolution Performance

As a renewable energy source,hydrogen has the advantages of green and high combustion calorific value,which is regarded as the“ultimate energy”for mankind.At present,hydrogen energy has been widely applied in hydrogen fuel cell vehicles,hydrogen fuel trams and other fields.The future application prospects of the hydrogen energy will become more and more broad.Currently,although electrolysis of water is not the main way of hydrogen production,such a method has the superiority of environment-friendly and high purity of hydrogen obtained,and it is expected to become the mainstream method of hydrogen production in the future.Electrolysis of water is usually divided into two half reactions,namely hydrogen evolution reaction and oxygen evolution reaction.Both steps require high efficient catalysts to reduce the overpotentials during the reactions,thereby improving reaction efficiency and reducing costs.At present,noble metal catalysts represented by Pt and Ru have been commercialized.Although these noble metal electrocatalysts have excellent catalytic performances,their shortcomings such as deficit and high cost extremely limit their large-scale industrial application.Therefore,it is necessary to find cheap and efficient catalysts to replace noble metal catalysts and realize the electrolysis of water to produce hydrogen at a lower cost.In the past decades,the emerging transition-metal dichalcogenides have attracted extensive attention and interest,due to their advantages of adjustable band gap,low price,and earth-abundant resource.Among them,molybdenum diselenide（MoSe₂）,as one of the representatives of transition metal dichalcogenides has a great application potential in the study of electrocatalytic water splitting,due to its low hydrogen adsorption free energy（ΔG_H～0.05 e V）,good intrinsic conductivity and other advantages.However,its hydrogen evolution reaction（HER）performance has a large gap compared with precious metal-based catalysts.So,there are still many opportunities and challenges to improve its HER performances.In this thesis,through morphology and structure control strategies,MoSe₂ catalysts were designed and prepared,and the synthesis process was introduced in detail.The structures,morphologies,and HER performances of the materials were tested and characterized.And we made a related analysis and explanation of its HER performance mechanism.The thesis mainly includes three parts:（1）The multilayer Ti₃C₂ MXene was successfully prepared by etching MAX phase Ti₃AlC₂through hydrofluoric acid（HF）.In the process of hydrothermal synthesis of MoSe₂ nanosheets,certain amounts of Ti₃C₂ MXene were added to prepare and construct the MoSe₂/Ti₃C₂ MXene heterostructure successfully.And the influence of different mass ratios（MoSe₂:Ti₃C₂ MXene）on the HER performance of MoSe₂ was explored in detail.The experimental results show that the Ti₃C₂ MXene can act as the growth framework of MoSe₂,which is beneficial to the dispersion of MoSe₂ nanosheets,thereby exposing more active sites.The electrochemical test results show that the HER performance of MoSe₂ is improved most obviously when the added mass of Ti₃C₂ MXene is 50 mg and the corresponding mass ratio of MoSe₂ to Ti₃C₂ MXene is2.5.The corresponding overpotential of MoSe₂/Ti₃C₂ MXene（50 mg）heterostructure is 238m V at a current density of 10 m A cm^-2,and the Tafel slope is 73 m V dec^-1.In addition,stability tests show that the heterostructure can maintain long-term catalytic activity.（2）MoSe₂ nanosheets were synthesized by hydrothermal method,and then etched with the environmentally friendly hydrogen peroxide（H₂O₂）solutions.By controlling hydrogen peroxide concentration,etching time and etching temperature,MoSe₂ nanosheets were obtained with different HER properties,morphologies and sizes.The related structure and morphology characterizations show that the nanosheets after etching become more dispersed and fluffy.Via the electrochemical HER performance test,it is found that the best HER performance is obtained for MoSe₂ when the etching concentration is 0.35 mol/L,the temperature is 35^°C,and the etching time is 6 min.The corresponding overpotential of the best MoSe₂ nanosheets is 197m V at a current density of 10 m A cm^-2 and the Tafel slope is 70 m V dec^-1.The i-t test proves that the material also has a good stability.（3）The MoSe₂ nanosheets were synthesized through hydrothermal method,and different surfactants of hexadecyl trimethyl ammonium bromide（denoted as CTAB）and polyethylene-polypropylene glycol（denoted as F68）were added into the precursors,respectively.MoSe₂-CTAB and MoSe₂-F68 were successfully prepared under the hydrothermal reaction conditions of 200^°C for 20 hours.The electrochemical test results show that,their overpotentials are 237and 225 m V at a current density of 10 m A cm^-2,and Tafel slopes are 74 and 73 m V dec^-1,respectively,which are better than the HER performance of the parent MoSe₂.When dual surfactants（CTAB and F68）were used in the precursors to assist the synthesis of MoSe₂,the final product MoSe₂-CTAB@F68 catalyst obtained has the best HER performance.The overpotential of MoSe₂-CTAB@F68 at a current density of 10 m A cm^-2 is 189 m V and the Tafel slope is 62 m V dec^-1.This is mainly due to the synergistic effect of the two surfactants.The final product has the largest specific surface area,which is conducive to exposing more active sites.