| Ammonia is a carbon-free energy transporter as well as an essential raw element in the manufacturing of agricultural fertilizers and industrial goods such as dyes and explosives.In recent years,the synthesis of ammonia through electrocatalytic nitrogen reduction reaction（eNRR）using water and nitrogen as feedstock has received a lot of attention from scientists for its low energy consumption and environmental protection.However,the development of eNRR is facing great challenges due to the chemical inertia of N2 and the strong competitive hydrogen evolution.In order to obtain high eNRR activity,it is necessary to develop electrocatalysts that can effectively adsorb and activate N2 molecules.At present,both theoretical and experimental results have confirmed that noble metal-based materials are efficient eNRR electrocatalysts.However,the high cost and low crustal abundance make them unsuitable for large-scale industrial applications.Therefore,the development of low-cost and efficient electrocatalysts is critical for eNRR.Mo atom is the main active site in nitrogenase Mo Fe cofactor for the synthesis of ammonia by nitrogen reduction.Recently,experiments and theoretical calculations have confirmed that Mo S2 exhibit potential eNRR activity.It is well known that in nature,Mo S2 exists as a thermodynamically stable 2H phase（2H-Mo S2）.However,2H-Mo S2 is a semiconductor material with low electrical conductivity and its poor electron transport ability limits the the efficiency of proton-coupled electron in the eNRR process,leading to slow reaction kinetics.In addition,the active sites in Mo S2 are limited to a small number of edge sites and each active site is weak to N2 activation.As a result,Mo S2 typically exhibits poor intrinsic performance for eNRR.The ammonia yield and faraday efficiency are far from adequate for production.In summary,it is necessary to adopt effective strategies to solve the above problems to further improve the eNRR catalytic performance of Mo S2.Doping and constructing heterostructures are two major strategies to optimize the electronic structure and improve electrical conductivity of catalysts.This paper takes Mo S2 as the research object,and successfully realize the modulation of Mo S2 electronic structure through two strategies of doping-induced phase transition and constructing heterogeneous interface,thereby enhancing its catalytic activity and improving eNRR catalytic performance.The main research contents and results are as follows:（1）Compared with 2H phase Mo S2,1T phase Mo S2 is of higher intrinsic conductivity,which is conducive to electron transfer during the electrocatalytic reaction process,thus accelerating the electrocatalytic reaction.Based on the above considerations,we prepared Sn-doped Mo S2（Sn-Mo S2）by simple one-step hydrothermal method and applied it to eNRR studies.On this basis,we combined structural characterisation and electrochemical tests to reveal the mechanism of the effect of Sn doping on the catalytic activity of eNRR:the doping of Sn heteroatom can introduce lattice distortion,thereby changing the coordination structure between Mo-S and inducing Mo S2 to transform from 2H phase to 1T phase,which is beneficial to improve the electrical conductivity and accelerate eNRR.The ammonia yield and Faraday efficiency of the prepared Sn-Mo S2 catalysts in 0.1 M Li2SO4 electrolyte were25.9μg h-1 mg-1 and 9.6%,respectively,with significantly improved performance compared to pure Mo S2(8.3μg h-1 mg-1 and 3.7%).（2）Compared with single-component catalysts,the two-phase heterostructure can effectively tune the electronic structure and enhance the electrical conductivity of catalysts through the surface/interface effect.We employed simple hydrothermal method to construct Sn S2/Mo S2 heterostructures,in which Sn S2 nanoparticles were uniformly loaded on Mo S2 nanosheets.Mo S2 is a p-type semiconductor material,while Sn S2 is an n-type semiconductor.Electrons are transferred from Sn S2 to Mo S2 at the heterointerface.On the one hand,the electronic interaction can increase the electrical conductivity of Mo S2,thereby accelerating the reaction kinetics of eNRR;on the other hand,it can increase the electron density at the Mo active site,which helps to inject electrons into the empty N2 antibonding orbitals to promote N2 molecular polarisation,which is conducive to enhancing the Mo-N bond strength.Compared with pure Mo S2or Sn S2,the eNRR catalytic activity of Sn S2/Mo S2 is significantly improved with NH3yield rate and faradaic efficiency of 34.3μg h-1 mg-1 and 13.8%,respectively.Moreover,it showed good electrochemical and structural stability.（3）Compared with Mo S2,SnO2 shows weaker hydrogen adsorption capacity and can inhibit the hydrogen precipitation reaction.Based on this,we prepared SnO2/Mo S2heterostructure using one-step hydrothermal method by homogeneously loading SnO2nanoparticles onto Mo S2 nanosheets.The heterointerface formed by the two can not only improve the catalytic activity of the active sites in Mo S2,but also construct N2active sites with weak H adsorption capacity on the inert Mo S2 basal plane,avoiding unnecessary competitive hydrogen evolution reaction and thus improving the Faraday efficiency of the catalyst.The SnO2/Mo S2 catalyst exhibited excellent eNRR catalytic performance with NH3 yield rate of 47.1 ug h-1 mg-1 and FE of 19.3%in 0.1 M Li2SO4solution,which is much higher than that of pure Mo S2. |
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