Molybdenum-based Nanostructured Materials: Design And Investigation Of Electrochemical Hydrogen Evolution Performances

In order to solve the global energy crisis and environmental problems,a lot of researches have been devoted to exploring sustainable and clean energy.Hydrogen（H₂）is an eco-friendly,renewable and green energy.As we all know,the water of electrolysis is considered to be the best strategy for hydrogen production.However,due to the slow kinetics of water cracking,efficient electrocatalysts are usually needed to reduce the high overpotential.Noble metal-based catalysts,especially platinum（Pt）,is considered to be the most efficient catalysts for hydrogen evolution（HER）,but the scarcity and high cost restrict their extensive application.Therefore,it is necessary to exploit and design non-noble metal HER catalysts.In recent years,transition metal molybdenum-based nanostructured materials have been widely researched and applied due to abundant reserves,low cost,and outstanding catalytic performance.In this research,three kinds of molybdenum-based nanostructured materials are designed and studied as HER catalysts.The main research works are as follows:（1）In the present work,Cu-doped molybdenum carbide encapsulated within two-dimension nanosheets assembled hierarchical tubular nitrogen-doped carbon（Cu/Mo₂C@HTNC）have been successfully prepared through the coordination reaction between Mo O₃ nanorod as template and dopamine in alkaline medium followed by two-step consecutive pyrolysis.The resultant hierarchical structure can efficiently prevent the aggregation of catalytically active materials during the pyrolysis process and thus is beneficial for exposing more catalytic active sites.In addition,single-atom Cu could be directly formed from Cu foil during the pyrolysis process,and is trapped as dopant by the resultant Mo₂C,which can efficiently regulate the electron density around Mo and catalytic active sites density for optimizing the strength of Mo-H bond.Benefited from the unique morphology and electron structure features,the optimal Cu/Mo₂C@HTNC exhibited excellent HER performance,that is,the overpotential corresponding to 10 m A/cm² current density is 113 m V,Tafel slope is 55 m V/dec and excellent stability.（2）Mo₂C/Co@NC nanobox hybrids constructed by two-dimension ultrathin nanosheets have been fabricated using ZIF-67 as reactive self-degraded template as well as cobalt source,ammonium molybdate as molybdenum source and etchant,dopamine hydrochloride as coordination ligand,ammonia solution as the polymerization initiator.The result shows that Co nanoparticles are encapsulated in N-doped porous carbon and Mo₂C nanoparticles are uniformly embedded in the whole Co@NC nano-frame.The introduction of Mo₂C nanoparticles into the Mo₂C/Co@NC nanobox hybrids can effectively reduce the overpotential and enhance the HER activity.In 1 mol/L KOH electrolyte,the optimal catalyst exhibits the low overpotental of 89m V at the current density of 10 m A/cm²,small Tafel slope of 82 m V/dec,and excellent stability.（3）Using Mo O₃ and thiourea as the raw materials,Mo_xS_y/Mo P₂ heterostructures with the rich sulfur vacancies were constructed by phosphating-hydrothermal-annealing.The results showed that the resultant P-Mo_xS_y/Mo P₂-Ar/H₂-900 catalyst exhibited the best HER activity in 1 mol/L KOH electrolyte,which is related to the successful construction of Mo_xS_y/Mo P₂ heterostructure and the formation of sulfur vacancies on the basal plane of Mo S₂.The heterojunction of Mo_xS_y/Mo P₂ could modulate electronic structure of Mo S₂,which leads to lower hydrogen adsorption free energy.Meanwhile,the sulfur vacancies on the basal plane of Mo S₂ can expose more active sites,which can improve the mass transfer rate and boost the catalytic performance of HER.