Study On Microstructure Adjustment Of Molybdenum Carbide And Hydrogen Evolution Reaction Of Water Splitting

Development of effective technologies for clean and sustainable hydrogen energy has been attracting great attention.Toward this end,an effective and promising approach is based on the electrolysis of water for hydrogen production.Platinum（Pt）-based materials have excellent electrocatalytic activity and stability.Due to their low content in nature,however,the high cost and scarcity severely limit their broad utilization.Nanostructured transition metal catalysts have low cost,good electrocatalytic activity and good stability,and thus have been widely studied as the most promising alternatives to precious metal platinum.However,thus far,it remained a great challenge to develop highly active hydrogen evolution reaction（HER）catalysts with a low overpotential based on earth-abundant and cost-effective materials.So,the main research content of this project is to synthesize an ultra-thin two-dimensional molybdenum carbide electrocatalytic material,regulate the microstructure of molybdenum carbide,study its catalytic mechanism of HER in electrolytic water,and improve the catalytic activity of hydrogen evolution in electrolyzed water.Design and construct an ultra-thin two-dimensional interface structure of molybdenum carbide on the nano level,regulate the catalytic activity of HER in electrolyzed water,reduce the HER potential in water splitting,increase the HER current density and cycling stability of electrolyzed water,and adopt various surface and nano-preparation methods,combined with surface interface structure characterization,electrochemical testing and simulation calculation of density functional theory（DFT）,provide insights into the catalysis of two-dimensional catalytically active components.The specific work and research results are as follows:Firstly,we provide a simple cost-efficient fabrication process to prepare macroscopic metal Mo sheets via the controlled dynamic equilibrium between sublimation of MoO₃ and reduction of H₂.Porous MoP sheets were synthesized from the obtained Mo sheets as the Mo source and template which exhibited notable activity in the HER with a low onset potential of-88 mV,small Tafel value of 54.5 mV dec^-1 and strong catalytic stability.With Mo sheets as the universal Mo source and template,MoS₂ and Mo₂C sheets were synthesized by a similar process,and the corresponding catalytic activities were simulated and calculated by DFT.Secondly,to further analyze the influence of nitrogen-doped and molybdenum carbide components on the hydrogen evolution activity of electrolyzed water,herein,N-doped carbon-wrapped molybdenum carbide heterophase（γ-MoC andβ-Mo₂C）sheets（N-Mo_xC@C HSs）were prepared by a facile chemical vapor reduction（CVR）procedure and following calcination at desired temperatures.The best HER electrocatalytic activity of N-Mo_xC@C HSs was detailedly examined in 0.5 M H₂SO₄,which exhibited a small overpotential of 172 mV(10 mA cm^-2)with a Tafel slope of only 60 mV dec^-1 and durability.The improved HER activities and catalytic stability were due to heterocrystal Mo_xC,nitrogen atoms doping,and carbon layers coating.Particularly,this study effectively combined the respective advantages of bothγ-MoC andβ-Mo₂C via the interface effect and matched polarity of hydrogen adsorption.Thirdly,to further study the effect of nitrogen doping and two-dimensional structure on the activity of hydrogen evolution in electrolyzed water,we highlight ultrathin N-doped Mo₂C nanosheets（N-Mo₂C NSs）in the role of greatly efficient platinum-free-based electrocatalysts for HER.Structural analyses make clear that the surfaces of the N-Mo₂C NSs are absolutely encompassed of apical Mo atoms,hence affording an ideal catalyst prototype to expose the role of Mo atoms for the duration of HER catalysis.Through detailed electrochemical investigations,N-Mo₂C NSs possess the HER activity with onset potential of-48.3 mV,Tafel slope of 44.5m V dec^-1,and overpotential of 99 mV at the cathodic current density of 10 mA cm^-2 with excellent long-term stability.Theoretical calculations demonstrate that the nanosheet structure,nitrogen atoms doping and particular crystalline phase of Mo₂C produce more exposed Mo active sites.This synthesis mechanism will facilitate the understanding and optimization of Mo-based electrocatalysts for energy conversion field.Fourthly,in order to further study the effect of pure phase molybdenum carbide component,the carbon support and hierarchical porous structure on the hydrogen evolution of electrolyzed water,Mo₂C on hierarchical porous carbon rods composed of cross-linked carbon networks（Mo₂C/CLCN）was synthesized by using Cu-MoO₂ rods as Mo source and Cu template.The Cu plays key roles in protecting Mo₂C from excess covering carbon to enhance the HER performance.When evaluated for HER activity,Mo₂C/CLCN exhibited a low onset potential of-85 mV with a Tafel slope of 48.2 mV dec^-1,an operating overpotential of 145 mV at the cathodic current density of 10 m A cm^-2 and outstanding long-term cycling stability in acidic electrolyte,which superior to most of non-doping Mo₂C.This work suggests that the in-situ template method by multi-element compounds is an inspiring strategy for synthesizing the efficient water splitting electrocatalysts with high electrochemical active area and more catalytically active sites.