Preparation Of Manganese-doped Molybdenum-based Compounds Towards Electrocatalytical Hydrogen Evolution

Hydrogen,because of its green,sustainable and zero carbon emission characteristics,has become a promising alternative to fossil fuels.Hydrogen production from water electrolysis is a feasible way to produce hydrogen on a large scale in the future.At present,Pt noble metal catalysts are still the best catalysts for hydrogen evolution,but its scarcity and high cost greatly impedes its practical applications.Therefore,the development of high efficiency and low-cost non-noble metal catalyst is the core demand of industrial water electrolysis hydrogen production.In this thesis,molybdenum-based compounds（Mo₂N,Mo P and Mo₂C）with abundant resources,low cost,similar electronic structure to Pt and good hydrogen evolution activity were selected as the research objects.Based on the structural transformation of Mo₂N→Mo₂C→Mo P in thermodynamics,Mo₂N/Mo P and Mo₂C/Mo P heterostructures with better hydrogen evolution performance than monomer catalysts（Mo₂N,Mo P and Mo₂C）were prepared by two-step controllable phosphating.Specifically,Mn doping,DFT calculation and heterostructure construction were carried out to provide experimental guideline for the development of Pt substitutes.The main research results are summarized as follows:（1）The effect of Mn doping on hydrogen evolution of Mo₂N was studied.Mn-doped Mo₂N nanosheets with rough surface were prepared by high temperature nitriding.It can be found that the introduction of Mn into Mo₂N reduces the grain size,lowers crystallinity and cell parameters of Mo₂N.Mn doping results in higher electrochemical active area of Mo₂N nanosheets.The theoretical calculation shows that electrons of Mn are partially transferred to Mo and N,the electronic structure of Mo active site is optimized,the hydrogen adsorption free energy(△G_H*)is decreased,and the intrinsic activity and electron transfer rate is also enhanced.0.05Mn-Mo₂N nanosheets exhibit the best hydrogen evolution performance,and the Tafel slopes in acidic and alkaline solution are 65 m V dec-1 and 66 m V dec-1,respectively.（2）Since Mn-doping significantly optimizes the hydrogen evolution activity of Mo₂N and Mn-doped Mo₂N/Mo P heterostructure is the final preparation target,the effect of Mn-doping on the hydrogen evolution performance of Mo P was investigated.Mn-doped porous Mo P nanosheets were prepared by Na Cl template+high temperature phosphorization,and manganese doped Mo P nanowires were prepared by hydrothermal method+high temperature phosphorization.Mn doping decreases the grain size of Mo P,changes the lattice parameters,and increases the number of active sites.Theoretical calculation shows that electron transfer of Mn optimizes△G_H*on the active site.The 0.5Mn-Mo P nanowires present the best hydrogen evolution performance with the Tafel slope of 47 m V dec-1.（3）Since Mn optimizes△G_H*at the active site of Mo P hydrogen evolution,improves its hydrogen evolution performance significantly and the Mo₂C/Mo P heterostructure is also the final preparation target,therefore,the effect of Mn doping on the hydrogen evolution performance of Mo₂C was researched.Mn and P co-doped Mo₂C nanosheets anchored on porous carbon were prepared using KCl as hard-template.It could be found that Mn is successfully doped into Mo₂C crystal structure.Theoretical calculation shows that Mn and P jointly regulate the electronic structure of Mo atom.And the△G_H*（-0.06439 e V）on Mo site in Mn and P co-doped Mo₂C is close to 0,which enhances the intrinsic hydrogen evolution activity.The optimized 0.08Mn-Mo₂C NS has the best hydrogen evolution performance with the Tafel slope of 50 m V dec-1.The effect of Mn on the hydrogen evolution performance of N and P co-doped molybdenum carbide was studied.The 0.08Mn NP-Mo₂C co-doped with Mn,N and P possesses the best hydrogen evolution performance with the Tafel slope of49 m V dec-1.Theoretical calculation manifests that Mn regulates the electronic structure of N,P and Mo atoms,and optimizes△G_H*of Mo active site in NP-Mo₂C.（4）Mn-doped Mo₂C（Mo₂N）/Mo P heterostructure was prepared by a two-step controllable phosphating process.0.05Mn-Mo₂N and 0.08Mn NP-Mo₂C were successfully transformed into 0.05Mn-Mo₂N/Mo P heterostructure and 0.08Mn NP-Mo₂C/Mo P heterostructure,respectively,by phosphine gas.The heterostructure interface increases the intrinsic hydrogen evolution activity and reduces the charge transfer impedance.The Tafel slope of 0.05Mn-Mo₂N/Mo P heterostructure is 53 m V dec-1,and0.08Mn NP-Mo₂C/Mo P heterostructure owns the lowest Tafel slope of 44m V dec-1 in this thesis.