Preparation And Performance Of Molybdenum Disulfide And Cobalt Disulfide Composite Catalysts For Electrocatalytic Nitrogen Fixation

Ammonia（NH₃）is one of the most vital industrial chemicals in the world,which is widely used in fertilizer production,clean fuel and other fields.At present,the industrial production of NH₃ mainly depends on the energy-intensive Haber-Bosch process,which does not meet the requirements of sustainable development.Therefore,researchers have been devoted to studying environment-friendly ammonia production methods.Electrocatalytic nitrogen reduction to ammonia（NRR）is a clean,efficient and sustainable method.At present,precious metals such as Ru are considered to be the most effective electrocatalysts.However,it is unsuitable for large-scale production because of their scarce reserves and high prices.Therefore,the development of cheap,efficient and environment-friendly electrocatalysts is of great significance for the large-scale application of electrocatalytic NRR.MoS₂ is considered to be a promising electrocatalyst due to its abundant reserves,low price and tunable electronic structure.However,2H-MoS₂ is a typical semiconductor material with poor conductivity,decreasing charge transfer rate in the electrochemical reaction,which is not conducive to NRR.Moreover,the interaction between MoS₂ and N₂ is too weak for efficient N₂ adsorption.In addition,MoS₂displays a poor capacity to break the stable N-N triple bonds,which shows such a high bonding energy.Therefore,MoS₂ usually exhibits poor catalytic activity in NRR.In order to improve the NRR catalytic activity of MoS₂,this article regards MoS₂ as the main research object,successfully realized modulation of MoS₂ electronic structure by combining MoS₂ and CoS₂ to prepare two different MoS₂-based electrocatalysts with different structures and phases to study its NRR activity.1.CoS₂/2H-MoS₂ heterostructure was constructed the by a simple one-step hydrothermal method,realizing the modulation of the electronic structure of MoS₂,which greatly improves the intrinsic catalytic activity of 2H-MoS₂.CoS₂nanoparticles with uniform sizes grow in-situ on the surface of 2H-MoS₂nanosheets,and two phases are in close contact with each other with strong interaction.And the charge transfer rate of the catalyst is greatly improved after the formation of heterostructure.It is worth mentioning that the electrons flow from CoS₂ to 2H-MoS₂at the interface,thus forming electrophilic and nucleophilic regions,which are conducive to promoting N₂ adsorption and the breakage of the N-N triple bond,respectively.In order to further reveal the mechanism of NRR,we performed density functional theory（DFT）calculations:the energy required by the potential determined step（PDS）of CoS₂/2H-MoS₂ heterostructure is less than that of pure MoS₂,and in the important step of N₂ adsorption,N₂ can spontaneously adsorb on the surface of CoS₂/2H-MoS₂.In contrast,the adsorption of N₂ on the MoS₂ surface requires a large energy barrier.The modulated electronic structure greatly promoted the adsorption of N₂ and the breaking of the strong N-N triple bond.The results show that CoS₂/2H-MoS₂ shows excellent FE（20.8%）and NH₃ yield(54.7μg h^-1 mg^-1),and a splendid electrochemical stability.2.Compared with 2H-MoS₂,1T-MoS₂ displays higher conductivity and more basal surface active sites,which is conducive to promoting the electrocatalytic reaction.Therefore,in order to further optimize the NRR performance of MoS₂-based catalysts,we prepared the CoS₂/1T-MoS₂ heterostructure with CoS₂ nanoparticles growing uniformly on 1T-MoS₂ nanosheets.Consistent with our assumption,the CoS₂/1T-MoS₂heterostructure showed higher catalytic activity than CoS₂/2H-MoS₂,with FE/NH₃ yields of 26.6%/59.3μg h^-1 mg^-1,respectively.Similar to the catalytic mechanism of CoS₂/2H-MoS₂,charge reforming on the surface of CoS₂/1T-MoS₂ also produces electron-deficient regions which are beneficial to N₂ adsorption and electron-rich regions which are conducive to N-N triple bond breakage,thus greatly improving the intrinsic catalytic activity of CoS₂/1T-MoS₂.Density functional theory（DFT）calculations were used to reveal the mechanism of NRR process.The results of DFT show that the vital step of N₂ adsorption occurs spontaneously on the surface of CoS₂/1T-MoS₂ heterostructure.In contrast,1T-MoS₂ has to overcome a large energy barrier,so the introduction of CoS₂ promotes the N₂ adsorption on the surface of CoS₂/1T-MoS₂ catalyst.In addition,the energy barrier needed to overcome in the PDS（*N₂→*N₂H）step of CoS₂/1T-MoS₂heterostructure is much smaller than that of pure 1T-MoS₂,so the modulated electronic structure is conducive to promoting the strong N-N triple bond fracture.Moreover,CoS₂/1T-MoS₂ greatly inhibits HER,thus improving the selectivity of NRR.It is worth mentioning that,compared with CoS₂/2H-MoS₂,there is a wider charge distribution at the interface of CoS₂/1T-MoS₂,and the electron density at the Fermi level is higher,which greatly promotes the NRR reaction kinetics.Most importantly,the energy barriers needed to overcome by CoS₂/1T-MoS₂ and CoS₂/2H-MoS₂ heterostructures in PDS are 0.74 e V and 0.93 e V,respectively.Compared with CoS₂/2H-MoS₂,the energy barriers needed to overcome by CoS₂/1T-MoS₂ heterostructures in PDS are smaller,thus further improving the NRR catalytic activity.In conclusion,in this paper,two kinds of MoS₂ electrocatalysts with different structures were prepared.The modulation of the electronic structure of MoS₂ was also realized,which solves the problems of poor catalytic activity and improves the the intrinsic catalytic activity of NRR,providing a novel idea for people to develop new NRR catalysts.