Structure Design Of Molybdenum Carbide Catalyst And Study On High Current Density Electrolytic Water Hydrogen Evolution

Hydrogen energy has natural advantages such as cleanliness,environmental protection and recyclability,and has attracted wide attention.Electrolysis of water is an effective way to produce hydrogen on a large scale.At present,the most advanced electrocatalysts for hydrogen evolution are platinum（Pt）based materials with strong electrocatalytic activity.However,Pt materials are rare in nature,which seriously hindering their wide application.Therefore,it is necessary to find a cheap catalyst that can replace Pt materials.Transition metal carbide,such as molybdenum carbide,can be used as replacement Pt catalyst because of its Pt-like catalytic performance,low price,strong stability and excellent electrochemical activity.However,most of the transition metal electrocatalysts,especially the powdered ones,are difficult to work stably under high current density.In view of this,this paper focuses on the preparation of highly active molybdenum carbide electrocatalytic materials which can work stably in acidic electrolyte under high current density.Specifically,from the point of charge attraction,the coordination growth mode between molybdenum carbide precursor and carbon nitride nanotubes（NCNTs）was changed,and finally the strengthening combination of molybdenum carbide particles and NCNTs was realized,thus improving the mass transfer effect of the material and reducing the energy barrier of the electrolytic water hydrogen evolution reaction.By constructing high performance composite electrolytic water catalyst materials,combined with surface and interface structure characterization,electrochemical performance testing and density functional theory simulation calculation,the electrocatalytic performance and practical application of the materials were deeply investigated.In this paper,the self-made carbon nanotubes（CNTs）in the laboratory were doped with non-metallic nitrogen in situ,which was used as the template agent.Cetyltrimethyl ammonium bromide（CTAB）was used as a cationic surfactant to play a medium role.The charge attraction effect was used to make the charged positive CTAB and charged negative NCNTs and MoO₄^2- adsorbed each other.The close binding of molybdate ion(MoO₄^2-)to carbon nitride nanotubes（NCNTs）was further induced.After freeze-drying,methanol was used as a liquid carbon source,and the dried products were carburized and reduced by programmed warming method to study the hydrogen evolution reaction（HER）performance.Specific research contents are as follows:（1）The catalyst iron particles,free carbon and other impurities in CNTs newly prepared in the laboratory were purified by calcination and pickling.The purified CNTs were then placed in ammonia（NH₃）atmosphere at 850℃ for in-situ doping to obtain nitrogenous CNTs（NCNTs）,which were then dispersed in ethanol by ultrasound for use as template.NCNTs had better wettability to the electrolyte and improved the mass transfer performance of the material surface.Changes in the electronic structure of NCNTs accelerated electron transport and the reaction rate of hydrogen evolution by electrolytic water.（2）The templating agents NCNTs and CTAB were added to ammonium hepta4(（NH₄）Mo₇O₂₄)in a mixture of acid.Through hydrothermal reaction,the product molybdic acid（H₂MoO₄）particles grew along the surface of the NCNTs bundle.By adjusting the molar ratio of NCNTs and（NH₄）₆Mo₇O₂₄,the morphologic size of H₂MoO₄ on the surface of NCNTs bundles was regulated.Finally,the c-H₂MoO₄/NCNTS composite was obtained.The H₂MoO₄ particle was smaller than 2 nm and distributed uniformly on the surface of NCNTs bundles.This indicated that the charged negative NCNTs and MoO₄^2-were closely adsorbed by CTAB as a medium through charge attraction effect,which improved the effective binding rate between NCNTs and MoO₄^2-.（3）The c-H₂MoO₄/NCNTS composites were freeze-dried and placed in a tubular furnace.Methanol was used as a liquid carbon source,and c-H₂MoO₄/NCNTS were reduced and carbonized under nitrogen atmosphere.H₂ generated by methanol cracking at 800℃promoted the reaction as a reducing gas.At the same time,H₂ etched carbon deposits on the surface of molybdenum carbide and inhibited the formation of amorphous carbon.The H₂MoO₄ particles grown uniformly along the network of NCNTs were carbonized and reduced to Mo₂C nanoparticles,whose particle size inherited the characteristics of the small size（5-20 nm）precursor H₂MoO₄.（4）c-Mo₂C/NCNTS were tested by HER in 0.5 M H₂SO₄ solution.When the current density reached 10 mA cm^-2,the overpotential was 173 mV and the Tafel slope was 58.4 mV dec^-1.When the current density reached the industrial level of 500 mA cm^-2,the overpotential presented was 315 mV After 9 h time current test at the current density of 450 mA cm^-2,the electrolytic water catalytic stability was still maintained.It is because of the strong chemical bond between Mo₂C particles and NCNTs that c-Mo₂C/NCNTS have the ability of electrolytic water hydrogen evolution at a high current density of 500 mA cm^-2.