Multi-Interfacial Composite Catalysts Based On Molybdenum And Tungsten:Design,Preparation And Hydrogen Evolution Properties By Water Electrolysis

The environmental pollution and energy crisis caused by the over-exploitation of traditional fossil energy make it an urgent issue to seek a new alternative energy system.Hydrogen energy is considered to be one of the most promising energy carriers to substitute fossil fuel energy because of its higher energy density and environmental friendliness.It is one of the ideal ways to obtain hydrogen energy with renewable electricity to drive the water splitting.Platinum-based material is the best hydrogen evolution reaction?HER?electrocatalyst at present,but its high price and low abundance seriously restrict the large-scale industrial application.Therefore,it is of great significance for the development of hydrogen energy and the transformation of traditional energy structure to develop cheap,stable and efficient HER catalysts for real industrial applications.Based on the above problems,in this thesis,novel molybdenum and tungsten-based multi-interface composite electrocatalysts were designed and synthesized by means of surface and interface control such as doping,morphology control and the construction of heterojunctions.These composite catalysts were used to study the properties of hydrogen evolution by water electrolysis.The specific work is as follows:1.Using polyoxometalate?POM?with multiple quasi-interfaces as the molecular pre-assembly platform,a few-layer N-doped carbon coated molybdenum phosphide/molybdenum carbide multi-interface composite nanomaterial MoP/Mo₂C@C was designed and synthesized.The composite nanomaterial was obtained by using P₄Mo₆ POM with appropriate P/Mo ratio as the precursor,and dicyandiamide as the carbon source and nitrogen source,after a simple one-step calcination to achieve simultaneous carbonization and phosphating in the nanoscale confined space.The MoP/Mo₂C@C exhibits better electrocatalytic hydrogen evolution activity than single component catalysts MoP@C and Mo₂C@C,and shows favorable activity and stability over the entire pH range.In addition,MoP/Mo₂C@C possesses better transition metal ions-tolerance(Fe²⁺,Co²⁺and Ni²⁺)than that of commercial 20%Pt/C due to the protective effect of carbon shells.2.Using ultrafine W₁₈O₄₉ nanowire as morphometric guiding agent and precursor,the cable-like WC/W₂C multi-interface composite nanomaterial WC/W₂C@C NWs coated with few-layer carbon shells was designed and synthesized.The composite nanomaterial was prepared by two-step pyrolysis treatment of the mixture of W₁₈O₄₉ nanowires and urea.The WC/W₂C@C NWs contains abundant WC/W₂C interface and adjustable phase composition,and the optimal catalyst of WC/W₂C@C NWs?WC:W₂C=1:1.3?with appropriate phase composition only needs the overpotentials of 69 and 56 mV in 0.5 M H₂SO₄ and 1M KOH to reach the current density of 10 mA cm^-2,respectively.The results of experimental data and DFT calculation demonstrate that the excellent HER performance should be attributed to the abundant WC/W₂C interfaces,the close connection between WC and W₂C enables the composite catalyst to possess both high electrocatalytic activity of W₂C phase and excellent stability and conductivity of WC phase.In addition,the synergistic effect at the interface between WC and W₂C further improves the electrocatalytic performance of WC/W₂C@C NWs.3.Using the rich anchoring sites of tungsten oxynitride nanowires,a multi-interface composite nanomaterial with few-layer carbon coated,ultra-small ruthenium nanoclusters loaded on the WNO nanowires was designed and synthesized?denoted as Ru/WNO@C?.The composite nanomaterial was synthesized by a one-step pyrolysis of the mixture of W₁₈O₄₉ nanowires,ruthenium trichloride and molten urea in an inert atmosphere.The overpotential of Ru/WNO@C?Ru wt%=3.37%?is only 2 mV at 10 mA cm^-2 in 1M KOH,representing the best alkaline HER catalyst at present.DFT calculations reveal that the electron transfer at the interfaces between ultra-small Ru nanoclusters and WNO nanowires can not only effectively optimize the adsorbed free energy of hydrogen??G_H^*?,but also significantly reduce the barrier of water dissociation of catalyst,thus promoting the HER performance in alkaline electrolyte.It is worth mentioning that the Ru/WNO@C?Ru wt%=3.37%?shows more remarkable HER performance than that of low carbon steel?the cathode used in chlor-alkali industry at present?under a simulated chlor-alkali condition.In general,this thesis aims to use a variety of means of surface and interface control,from the composition,morphology and the electronic structure three aspects to optimize hydrogen evolution properties of molybdenum and tungsten-based catalysts,providing a reference to find more efficient and practical HER electrocatalysts.