Tuning The Structure Of Molybdenum Disuifide For Hydrogen Evolution Reaction

Escalating concerns about the shortage of energy and worsening environment have directed the spotlight to development of clean energy,among which hydrogen energy is expected to become the most potential energy alternative.Sustained and effective hydrogen production is the main prerequisite for the realization of the future hydrogen economy.In the past few decades,electrocatalytic hydrogen evolution has been envisioned as an efficient hydrogen production technology due to its environmental friendliness.However,only expensive Pt-based catalysts can provide effective hydrogen evolution power,which undoubtedly poses a major challenge to the development of electrocatalytic hydrogen evolution.Theoretical calculations show that MoS₂ displays a HER activity comparable to that of Pt,making it one of the most potential hydrogen evolution candidates.However,MoS₂ still faces the problems of poor conductivity and inert basal plane.In this regard,researchers have developed many effective strategies to solve the above problems,including increasing edge sites,introducing sulfur vacancies,interlayer expansion,phase transformation,heteroatom doping,and construction of heterojunctions.These tuning structures have be demonstrated to be effective for activating the inert basal plane of MoS₂,increasing the number of active sites,optimizing the free energy of hydrogen adsorption,and improving its electrical conductivity.With the rapid development of sodium ions batteries,utilizing sodium ions batteries to tune the structure of MoS₂ has been widely investigated.Nevertheless,seldom work has been devoted to examine the relationship between the tuned MoS₂ and its corresponding hydrogen evolution activity.In addition,due to the controllable nature of the MoS₂ layer spacing,improving the HER activity of MoS₂through tuning the layer spacing has become a special method.However,the mechanism of interlayer expansion have not been resolved so far.Therefore,this thesis will study the above mentioned problems,and the specific contents are as follows:（1）The MoS₂/CC grown on carbon cloth was obtained by a hydrothermal method.The sodium-ion half battery was assembled by employing the as-obtained MoS₂/CC as the working electrode and Na foil as both the reference and counter electrode,which was then discharged at a current density of 50μA cm^-2 to different potentials to obtain MoS₂-X/CC.The characterization techniques of SEM,XRD,HRTEM,In situ Raman,and XPS proved that the interlayer expansion and phase transition of MoS₂ during the sodium insertion stage,and the MoS₂ was sheared into fragmentation after triggering conversion reaction.We found that the HER activity of MoS₂ would gradually increase at the intercalation stage and further increase at the early stage of conversion reaction.Based on the above analysis,the first increase in activity was ascribed to interlayer expansion and phase transformation,and the second increase in activity was mainly ascribed to increase of active sites due to the fragmentation of MoS₂ after triggering conversion reaction.Among them,the optimal MoS₂-0.1 only needs an overpotential of 161 m V to achieve a current density of 10 m A/cm².（2）Using（NH₄）₆Mo₇O₂₄·4H₂O and CH₃CSNH₂ as raw materials,MoS₂ with enlarged interlayer is obtained by a hydrothermal method.The interlayer spacing of this material return to its normal state in acidic solution and can be enlarged again while the MoS₂ with shrinked interlayer is put in alkaline solution.XPS characterization reveals the reason for the reversible change of the interlayer spacing:the doped O promotes the insertion of external OH^-and the electrostatic repulsive force caused by the embedded OH^-is greater than the van der Waals force in the MoS₂layer,thereby expanding the interlayer spacing;When MoS₂ with enlarged interlayer encounters H⁺,a large amount of OH^-is combined with H⁺to form H₂O molecules with a size of 2.8（?）.MoS₂ with normal interlayer distance can accommodate its existence,resulting in a narrowing of the interlayer spacing.Based on the nature of the reversible change of the interlayer spacing,we controllably prepare MoS₂with different interlayer spacing,and discuss the relationship between the changed interlayer spacing and the hydrogen evolution activity.Among them,MoS₂ with shrinked interlayer（N-IR-250-MoS₂）has the best HER performance,and MoS₂ with enlarged interlayer expansion（E-R-MoS₂）has poor HER performance.Through the analysis of HER kinetics and stability test,we find that N-IR-250-MoS₂ has the smallest Tafel slope and excellent stability.