Studies On Heterogeneous Nano-tungsten-based Electrocatalysts And Catalytic Performance For Water Splitting

In the new era,the consumption of traditional fossil fuels continues to increase,the gas and dust produced gradually disturb the global temperature,climate,and marine ecology,hindering the development of human civilization.The development of green,safe and renewable energy is a major challenge to meet the sustainable development of mankind.In the renewable energy system,hydrogen energy,which has the absolute advantages of zero carbon,ultra-high energy density and renewability,is undoubtedly one of the most potential clean energy sources.Electrochemical water decomposition is considered as the greenest and most convenient method to produce hydrogen,which can be divided into two reactions:hydrogen evolution（HER）and oxygen evolution（OER）.Precious metals（Pt,Ir,Ru O₂,etc.）are considered as the most advanced catalysts for HER or OER,but their high cost and low abundance limit their large-scale application.It is of great significance to develop low-cost and high-reserve transition metal catalysts（Ni,Fe,Mo,W,etc.）to replace precious metal catalysts.Based on this,this paper prepared a hydrogen evolution electrode with excellent performance based on tungsten-based oxides,and successfully introduced Mo atoms to further optimize its HER performance and stability.By exploring different materials and introducing Fe element which can enhance water oxidation ability,a tungsten-based catalyst with the same excellent oxygen evolution performance was prepared.The main work of this thesis is as follows:（1）With nickel foam（NF）as the substrate material,the precursor of nickel tungsten hydroxide tetrahydrate(Ni₄W₆O₂₁（OH）₂·4H₂O)was grown on the substrate by a simple hydrothermal method,then hydrogen reduction reaction was carried out at a certain temperature,and the multiphase electrode with rich interfaces was successfully prepared.A comparative electrode was prepared by changing the reduction temperature and adding reactants.The function of each component in the electrode was explored by combining the structure,morphology and electrochemical test of the material.The results show that the catalyst is a composite material composed of WO₂,Ni₁₇W₃ and W,and its overall morphology is rhombic dodecahedron.When the WO₂@Ni₁₇W₃@W/Ni electrode reaches the current density of 10 and300 m A cm^-2 in the environment of 1M KOH,the overpotential is 30 and 242 m V respectively.The constant current stability test shows that the voltage remains unchanged at a low current density of 10 m A cm^-2,but when the current density reaches 50 m A cm^-2,the voltage increases by 27 m V after 12 h.The WO₂@Ni₁₇W₃@W/Ni electrode shows good HER catalytic activity,but its stability still has a lot of room for improvement.（2）In order to further improve the catalytic activity of the electrode,doping was chosen to optimize the electronic characteristics of the catalyst.Mo atoms were introduced into it to adjust its Fermi level,at the same time,Mo can also be used as an active site to accelerate the production of H₂.For stability,the combination degree between catalyst and substrate is enhanced by changing process parameters and introducing stabilizers.The results showed that the prepared Mo-WO₂@Ni₁₇W₃@W/Ni catalyst showed excellent HER activity,when the electrode reaches the current density of 10 and 300 m A cm^-2,the overpotential is only 14 and72 m V,respectively,and the Tafel slope is 24.26 m V dec^-1.The constant current stability test showed that the overpotential only increased by 32 m V after continuous electrolysis for 24 h at a high current density of 500 m A cm^-2,showing excellent stability.（3）In order to realize the application of tungsten-based catalyst in the field of total water hydrolysis,Fe WO₄with certain water oxidation activity was selected as the research object,and WO₂ and Ni₁₇W₃ were introduced to construct heterostructure,which resulted in new interface structure.The integration of OER active components and HER active components is helpful to greatly reduce the overpotential of water decomposition,then realize the function of highly efficient catalytic water decomposition.The results show that Fe WO₄-WO₂@Ni₁₇W₃/Ni is a good catalyst material for electrolytic water.The overpotential of HER and OER only needs 77and 203 m V to generate a current density of 10 m A cm^-2,and the slope of Tafel is only 49.37m V dec^-1 and 38.32 m V dec^-1,respectively.The OER performance far exceeds that of commercial Ru O₂.Using this electrode as cathode and anode,the electrolytic cell needs a voltage of 1.52 V to obtain a current density of 10 m A cm^-2,and it shows excellent catalytic stability in simulated industrial environment.