Design Of MoS₂-based Electrocatalysts For Boosting Hydrogen Evolution From Water Splitting

Global warming or environmental pollution caused by excessive use of fossil fuels（i.e.,coal,oil,and natural gas）is increasingly becoming a human concern.It is urgent to seek alternative energy sources to replace fossil fuels.Hydrogen energy is considered as an ideal substitute for fossil fuels because of its high calorific value and“green”feature.Now,hydrogen gas is generally produced through methane reforming or coal gasification with the inevitable emission of pollutants,which is not an optimal way.Electrocatalytic hydrogen evolution from water splitting is considered to a“green”technique if the electricity for water splitting originates from renewable solar or wind energy.However,to commercialize the hydrogen evolution reaction（HER）,we have to address the problem of high overpotential of H⁺electroreduciton.To date,platinum is still the most effective catalyst for HER but impossible to be used on a large scale owing to its high cost and limited reserve.Therefore,developing efficient Pt-free electrocatalysts becomes a hot topic in the HER research.MoS₂,as a chemically stable,non-toxic and low-cost material,has been widely used as solid lubricant,transistor,and elecrocatalyst.Especially,MoS₂ edges show excellent HER elecrocatalytic activity.However,the basal plane of MoS₂ is completely inactive to HER,and the poor conductivity also limits the application of MoS₂ in electrochemistry.To address these issues,this thesis develops phase transition,defect and heterojunction engineering strategies to design MoS₂-based electrocatalysts for boosting HER.We hope these studies can narrow the gap between MoS₂ and Pt in the HER catalytic activity.Specific research contents are as follows:（1）Design and construction of Se-MoS₂ nanomaterials and electrocatalytic hydrogen evolution propertiesSe-and O-co-inserted MoS₂（Se-MoS₂）microspheres composed of intertwined nanosheets were fabricated by a hydrothermal strategy.Co-insertion of Se and O creates abundant S defects on the basal plane of MoS₂ and thus increases the number of active sites.Additionally,Co-insertion of Se and O is apt to induce the transition of MoS₂ from 2H to 1T phase（～60%1T MoS₂）,thereby exhibiting much high electric conductivity.Furthermore,optimized Se-MoS₂ not only shows electrocatalytic HER activity(an overpotential of 108 m V at 10 m A cm^-2and a Tafel slope of 47 m V dec^-1),also inherits the outstanding electrocatalytic durability of pristine MoS₂.Density function theory（DFT）calculations reveal that the excellent HER performance of optimized Se-MoS₂ can be well explained with Volmer-Heyrovsky mechanism,which is intrinsically attributed to the emergence of new band structures near the Fermi energy level of MoS₂,increasing the electrical conductivity and reducing the Gibbs free energy of hydrogen adsorption.（2）Design and construction of MoS_2-x nanomaterials and electrocatalytic hydrogen evolution propertiesA defect-pre-designed strategy was proposed to produce MoS₂ with single-atomic S vacancies（SV-MoS₂）simply by preparing Se-doped MoS₂（Se-MoS₂）and subsequent removing the Se of Se-MoS₂.S vacancies originate from the vaporization of the doped Se atoms.This strategy has a high selectivity and raises a good possibility for precisely modulating the concentration of S vacancies.The results show that the concentration of S vacancies can be controlled over the range from～7.46%to 13.54%.MoS_1.76 with～12.10%of S vacancies exhibits outstanding HER performance:an overpotential of100 m V at 10 m A cm^-2 and a Tafel slope of 49 m V dec^-1,corroborating the theoretical prediction about the optimum concentration of S vacancies.Density functional theory calculation further reveals that the activation of MoS₂ basal planes may intrinsically originate from the modification of S vacancies to band structure and density of state of MoS₂,optimizing the hydrogen adsorption energy.This defect-pre-designed strategy reduces the probability that the aggregates of S vacancies are formed,and will be more helpful for understanding how S vacancies affect the properties of MoS₂.（3）Design and construction of Au^δ+/MoS_1.76 nanomaterials and electrocatalytic hydrogen evolution propertiesMott-Schottky heterojunction Au^δ+/MoS_1.76 electrocatalyst was designed by integrating interface engineering,phase transition,and S-vacancy creation.Gold nanoparticles adhering on MoS₂ produce a Mott-Schottky hetero-junction.The existence of hetero-junctions can not only provide active sites for catalytic reaction,also is capable of stabilizing 1T-phase MoS₂（～64%1T MoS₂）.Meanwhile,the presence of S vacancies increases the number of active sites.Thus,Au^δ+/MoS_1.76exhibits a low overpotential（～90 m V）and Tafel slope(～47 m V dec^-1).DFT calculation shows that the formation of Mott-Schottky hetero-junctions can endow MoS₂ with more electrons,effectively stabilize the metal 1T phase of MoS₂,adjust the electronic structure of MoS₂,and optimize the Gibbs free energy of H*adsorption.（4）Design and construction of Mo₂C-MoS_1.76/G nanomaterials and electrocatalytic hydrogen evolution propertiesMo₂C-MoS_1.76/G heterojunction structures are successfully prepared by growing Mo₂C nanocrystals in one side of MoS₂ surface is proposed,and.The formation of Mo₂C nanocrystals can be ascribed to the activation of S vacancy to the surface of MoS₂.Mo₂C nanocrystals can not only provide abundant interfacial catalytic active sites for catalytic reactions,but also enhance the conductivity of materials.In addition,the addition of graphene substrate not only enhances the conductivity of the material,but also inhibits the formation of MoS₂ aggregates to a certain extent,ensuring the full exposure of active sites.The electrochemical results show that Mo₂C-MoS_1.76/G exhibits good catalytic performance(an overpotential of 82 m V at 10 m A cm^-2and a Tafel slope of 46 m V dec^-1)and cyclic stability.（5）Design and construction of P-MoS_1.76/G nanomaterials and electrocatalytic hydrogen evolution propertiesP-MoS_1.76/G catalyst with double active centers was prepared by filling sulfur vacancy with phosphorus atoms.Phosphorus atoms can act as the catalytic active site,and is capable of inducing the transformation of MoS₂ from 2H to 1T phase（～74%1T MoS₂）,making P-MoS_1.76/G with a higher conductivity than pristine MoS₂.Phosphorus atoms and sulfur vacancies together work as active centers,offering a strong possibility for improving the HER activity.The electrochemical results show that P-MoS_1.76/G exhibits excellent hydrogen evolution catalytic performance(an overpotential of 78 m V at 10 m A cm^-2and a Tafel slope of 45 m V dec^-1)and cyclic stability.The DFT calculation shows that the excellent catalytic performance of P-MoS_1.76/G is due to the fact that the phosphorus atom introduced into the sulfur vacancy effectively changes the electronic structure of MoS₂ and optimizes the Gibbs free energy of H*adsorption.