Fabrication And Electrochemical Performance Of Molybdenum And Tungsten Based Catalysts

The increasingly serious environmental pollution and energy crisis are the main problems facing the sustainable development of today’s society.Exploring advanced energy conversion technology is the key to fully utilizing renewable energy.The development of energy conversion technology represented by Li-CO₂batteries still faces many challenges,such as large overpotential,low efficiency,poor cycle performance,etc.,and the construction of efficient catalyst materials is the key to solving the above problems.Molybdenum and tungsten based compounds have broad prospects in the field of energy conversion due to their abundant valence and valence electron configurations,high electron density,abundant reserves,and adjustable band gap.However,the problems of low intrinsic catalytic activity,few active sites and poor electrical conductivity are faced by molybdenum and tungsten based compounds as catalysts.Therefore,a reasonable modification strategy is the key to improving the electrochemical activity of molybdenum and tungsten based catalysts.In this paper,a series of molybdenum and tungsten based transition metal compounds were designed and prepared based on nano-engineering,doping,and vacancy modification strategies,which realized the regulation of the structure and properties of the catalyst,and revealed the mechanism of enhancing the electrochemical performance of the catalyst.The main research contents are as follows:Given the problems of few active sites and low intrinsic catalytic activity on the WS₂base plane,phosphorus dopant was used as an electronic structure modulator,and phosphorus-doped tungsten sulfide（P-WS₂）nanowire array was designed and prepared by combining hydrothermal and chemical vapor deposition technology.Phosphorus doping gives WS₂excellent acid hydrogen evolution properties,low overpotential（88 m V）at 10 m A cm^-2,small Tafel slope(62 m V dec^-1),and excellent cyclic stability.Density functional theory calculation shows that phosphorus doping can redistribute the state density near the Fermi level of WS₂,promote electron transport,hydrogen adsorption and reduction on the P-WS₂base plane,and make the inert base plane have edge-like catalytic activity.To further demonstrate the feasibility of phosphorus doping in regulating the catalytic activity of transition metal sulfur compounds,a phosphorus-doped tungsten selenide（P-WSe₂）nanowire array was prepared as a hydrogen evolution catalyst.The results showed that phosphorus doping could effectively improve the catalytic activity of WSe₂,which provided an important reference for regulating the catalytic activity of transition metal sulfide compounds.Aiming at the problem that Li₂CO₃,the discharge product of Li-CO₂battery,is difficult to decompose,and the catalyst material has weak adsorption capacity for CO₂and Li,the vertical graphene supported nitrogen-doped Mo S₂nanosheet composite with the sulfur vacancy(N-Mo S_2-x/VG)was prepared by room temperature NH₃plasma technology and used as the cathode catalyst of Li-CO₂battery.The synergistic action of nitrogen doping and sulfur vacancy accelerates the adsorption of Li and CO₂and promotes the decomposition of Li₂CO₃.At the same time,the large specific surface area of VG can provide sufficient space for the deposition of Li₂CO₃,and the abundant array structure speeds up the transport of electrons and ions.Li-CO₂battery based on N-Mo S_2-x/VG cathode catalyst has a high full discharge capacity of 15.8 m Ah cm^-2at 0.02 m A cm^-2,low first-cycle overpotential（1.42 V）,and excellent cycle stability(1650 h stable operation at 0.06m A cm^-2).To further improve the energy conversion efficiency of Li-CO₂battery and accelerate CO₂evolution and reduction reaction kinetics,nitrogen-doped tungsten oxide(N-WO_3-x)nanosheet arrays rich in oxygen vacancies were fabricated by the N₂plasma technique.In the oxygen-deficient sub-stoichiometric oxides,oxygen gradually disappears along with the accumulation of free electrons,achieving the transition from semiconductor to quasi-metal.Meanwhile,nitrogen doping and oxygen vacancies synergistically optimize the electronic structure and generate new unsaturated coordination atoms,providing abundant electrochemically active sites for fast reaction kinetics.The Li-CO₂battery based on the N-WO_3-xcathode catalyst exhibited an ultrahigh full discharge capacity at 0.02 m A cm^-2(30.9 m Ah cm^-2),low first-cycle overpotential（1.24 V）,and excellent high current density cycle stability(stable cycle for 1200 h at 0.1 m A cm^-2).The growth mechanism of discharge products was further explored by in situ Raman and other techniques.Finally,the Li-CO₂battery based on gel electrolytes was assembled,exhibiting superior electrochemical performance compared to commercial separators.