Synthesis Of Molybdenum-based Nanomaterials For Electrocatalytic Hydrogen Evolution Reaction

Hydrogen is considered as the most promising energy due to its high energy density,environmental friendliness and reproducibility.Sustainable hydrogen production from electrochemical catalytic hydrogen evolution reaction(HER)is one of the most attractive methods for energy storage and conversion.However,traditional strategies are too expensive to realize HER due to its high cost resulting from high overpotential needed to operate the electrolysis cell.It is essential to introduce catalyst to reduce the overpotential.So far,platinum-based materials have proved to be the most effective HER electrocatalysts.Nevertheless,high cost and low earth abundance of these materials inhibit the practical application and commercial promotion.Development of highly efficient non-noble metal HER catalysts for hydrogen production is imperatively urgent.Recently,remarkable advances have been made regarding the use of transition metals and their compounds.In this thesis,molybdenum-based nanomaterials,including vertical MoS₂ nanosheets,porous?-Mo₂C nanostructure and porous Mo₂C-MoP heterostructure,were synthesized by chemical vapor deposition.A series of characterization methods,electrochemical test and density functional theory were used to investigate the microstructure morphology of the catalyst,explore the mechanism of synthesis and catalytic reaction,and improve the electrocatalytic performance for hydrogen evolution.Firstly,we synthesized vertical MoS₂ nanosheets on the surface of various carbon materials,and demonstrated the enhanced HER activity of vertical MoS₂ nanosheets with H₂-annealing.Dense vertical MoS₂ nanosheets were synthesized on various carbon materials,including glassy carbon,activated carbon,carbon paper,graphene and r GO films.However,on Si O₂/Si,glass,quartz and GO films,MoS₂ nanosheets were lying down.Density functional theory results showed that vertical MoS₂ nanosheets on carbon substrates had a lower energy barrier than horizontal ones,suggesting carbon atoms played an influential role in the vertical growth of MoS₂ nanosheets.For the enhanced HER activity,H₂-annealing roughened the edges and produced S-vacancies in the basal plane of vertical MoS₂ nanosheets,which increased the number of active sites.Besides,H₂-annealing enhanced the conductivity due to the production of elementary Mo.The samples of H₂-annealing at 780?temperature for 20 min showed the best performance of electrocatalytic hydrogen evolution.Secondly,we developed a simple CVD process and synthesized porous?-Mo₂C nanostructure using MoO₃ as Moprecursor and carbon electrode as carbon source and substrate.A variety of characterizations revealed that the amount of MoO₃ and growth temperature had significant influence on the composition and morphology of porous?-Mo₂C nanostructures and suggested a two-step synthetic mechanism.The dual role of carbon electrode as substrate and carbon source resulted into intimate adhesion of?-Mo₂C on carbon cloth,offering fast electron transfer at the interface.In particular,a thin film of porous?-Mo₂C nanostructures was obtained on carbon cloth with 50 mg MoO₃ at 850?.With the merits of unique porous nanostructures,a low?₁₀ of 106 m V and a small Tafel slope of 49.8 m V dec^-1 was achieved for porous?-Mo₂C nanostructure in 0.5 M H₂SO₄,and a low?₁₀ of 72 m V and a small Tafel slope of 52.8 m V dec^-1 was achieved in 1.0 M KOH.Besides,cyclic voltammetry treatment for 5000 cycles indicated excellent electrochemical stability.Finally,porous Mo₂C-MoP heterostructure was prepared by phosphating above porous?-Mo₂C nanostructures in a sealed tube.It is found that the growth temperature had significant influence on the composition and morphology of porous Mo₂C-MoP heterostructure.MoC/P-550,prepared at 550?,shows excellent catalytic hydrogen evolution performance and stability.A low?₁₀ of 82 m V and a small Tafel slope of 49.8m V dec^-1 was achieved for porous Mo₂C-MoP heterostructure in 0.5 M H₂SO₄,and a low?₁₀ of 51 m V and a small Tafel slope of 56.7 m V dec^-1 was achieved in 1.0 M KOH.Density functional theory calculation showed that Mo₂C-MoP has more suitable H adsorption free energy than Mo₂C and MoP.This work has enriched the studies on the controllable preparation of molybdenum-based nanomaterials and their application in electrocatalytic hydrogen evolution.These results provide reliable selection of materials and research basis for the application in related fields.