Controllable Synthesis Of Molybdenum-based Non-and Low-platinum Mesoporous Materials And Their Applications In Electrocatalytic Hydrogen Evolution Reaction

Water electrolysis is a green and sustainable technology for hydrogen production,which can accelerate the rapid development of green energy economy.The high efficiency of hydrogen production relies on the development of high-performance electrocatalysts.Molybdenum-based materials are regarded as a class of promising electrocatalysts for hydrogen evolution,owing to their platinum-like catalytic behavior.This thesis demonstrates a detailed study on mesoporous molybdenum-based nanomaterials,including the structural design,interfacial optimization,surface electronic modification,and their performances toward the electrochemical hydrogen evolution reaction(HER).The main contents of the thesis are as follows.In Chapter 3,in view of the fact that molybdenum carbide nanoparticles tend to be easily aggregated and oxidized,leading to the gradual deactivation,we proposed the synthesis of ordered mesoporous Mo2C@graphitic carbon(Mo2C@GC)core-shell nanowire arrays.Ultrathin graphitic carbon layers were uniformly coated on the surfaces of Mo2C nanowire arrays,which can not only inhibit the aggregation and oxidation of Mo2C nanoparticles,but also can improve the specific surface area and electrical conductivity of the material.A hard-templating approach combining chemical vapor deposition method has been introduced for the successful synthesis of ordered mesoporous Mo2C@GC core-shell nanowires with the mesoporous SBA-15 as the hard template,methane as the C source,and phosphomolybdic acid as the Mo source.The effects of template,calcining temperature and Mo2C/GC structure configuration on the HER performance were investigated via a series of control experiments.The optimized sample possesses a high surface area of 37 m2/g,a uniform mesopore size distribution centered at ~4.2 nm,and presents excellent HER performance in the acidic electrolyte with a low overpotential of 125 mV at a current density of 10 mA/cm2,a small Tafel slope of 66 mV/dec,a small electrochemical impedance and a good cyclic stability.In Chapter 4,taking account of the facts that two-dimensional molybdenum sulfide nanosheets are prone to be stacked causing the shield of the edge active sites,and they possess low electrical conductivity and low specific surface area,we proposed to use nitrogen-doped ordered mesoporous carbon as a support to highly disperse ultra-thin MoS2 nanosheets in order to expose the edge active sites of MoS2 as many as possible,and to increase the wettability and specific surface area of the material,leading to the expanded contact area between the material and the electrolyte.We proposed a novel solvent-free nanocasting for successful synthesis of ordered mesoporous MoS2@N-doped carbon(OM-MoS2@NC)nanomaterials with SBA-15 as the hard template,methionine as the S source,C and N sources,molybdenum chloride as a metal precursor.In acidic electrolyte,OM-MoS2@NC exhibits a good HER performance with an overpotential of 21 8 mV at a current density of 10 mA/cm2.In addition,by changing the type of metal precursor,a series of ordered mesoporous metal sulfide@NCs,including OM-Fe7S8@NC,OM-Co9S8@NC,OM-NiS@NC,OM-ZnS@NC and OM-Cu7S4@NC,have been prepared successfully,proving the universality of this method.All of these materials possess high specific surface area(430?754 m2/g),ordered mesoporous structure,uniform pore size distribution(?4.0 nm)and high metal sulfide content,and have promising potential for energy storage and conversion.In Chapter 5,given the deficient stability of supported Pt electrocatalysts and the strong Mo-H bonding energy of single Mo2C catalysts,we explored the introduction of a low amount of Pt on molybdenum carbide materials to construct Pt/Mo2C intimate interfaces.By utilizing the strong forces between Pt and Mo2C,ultrahigh dispersion of Pt with increased density of the active sites and regulation of its surface electronic structure to weaken the Mo-H bonding energy are achieved.A wet impregnation approach combined with the H2 thermal reduction method has been adopted for successful preparation of ultra-dispersed Pt@N-doped mesoporous Mo2C nanowires with most of Pt existing in the form of single atom or ultrafine nanoclusters.The material shows excellent HER performance in an acid electrolyte with an overpotential of 129 mV at the current density of 10 mA/cm2,a small slope of 67mV/dec,a low electrochemical impedance and a remarkable cycling stability.