Preparation Of Transition Metal Sulfides By Molten Salt Method And Their Electrocatalytic Properties

Hydrogen energy is one of the important directions for future development,and electrocatalytic water splitting is the cleanest and most sustainable way to produce hydrogen.At present,the most effective electrocatalysts for electrocatalytic water splitting are still precious metal-based catalysts which are expensive and have limited reserves,so they are not conducive to large-scale applications.In contrast,non-precious metal-based electrocatalysts are inexpensive,resourceful and environmentally-friendly.They are expected to become potential replacements for precious metal materials to achieve large-scale industrial production of hydrogen from electrocatalytic water splitting.However,the activity of non-precious metal-based electrocatalysts is not high,so the design and research of high-efficiency non-precious metal-based electrocatalysts is still a challenging work.In this thesis,abundant and inexpensive transition metal sulfide（tungsten sulfide,cobalt sulfide and nickel sulfide）are prepared by an innovative molten salt method.Based on them,the catalyst suitable for electrolysis of water and electrocatalytic oxidation of pollutants are constructed,and the electronic structure and catalytic active sites of electrocatalysts are optimized through strategies such as vacancy engineering and heterojunction modulation.The mechanism of electrocatalytic processes of transition metal sulfide is also investigated by the combination of theoretical calculations and practical tests.The main contents of this thesis are as follows:（1）WS₂ are synthesized by a simple molten salt method,and the sulfur vacancy concentration is regulated by changing the degree of mixing,showing ideal performance for oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）in alkaline electrolytes.The introduction of sulfur vacancy modulates the electronic structure of WS₂ and improves the electrocatalytic performance.Moreover,the production of hydrogen is improved by the norfloxacin oxidation reaction instead of the OER.To achieve the same current density,the cell voltage required for the norfloxacin-based electrolyser is almost identical to that of a typical water-based electrolyser.The use of norfloxacin-based electrolytic cells can degrade norfloxacin while producing hydrogen.Compared to water-based electrolyser,there is no increase in power consumption.（2）Multiphase cobalt sulfide composites are synthesized by a simple molten salt method,and mono-,binary-and ternary-cobalt sulfides are constructed by changing the spatial positions of precursor,showing ideal OER and HER performance in alkaline electrolytes.The ternary heterojunction modulates the electronic structure and promotes the adsorption and dissociation of water.To achieve a current density of 50 m A cm^-2,the norfloxacin-based electrolyser requires a cell voltage of only 1.47 V,which is 0.80 V（or 35.2%）lower than that of the conventional water-based electrolyser.（3）A series of NiS/NiS₂ composites are synthesized by a simple molten salt method,in which the ratio of Ni S to Ni S₂ can be easily tuned merely by switching calcination temperature.Ni S/Ni S₂ composites show ideal performance for OER and HER in both acidic and alkaline electrolytes.Ni S/Ni S₂ heterogeneous structure presents a small Gibbs free energy for hydrogen adsorption,suggesting that a thermodynamically favorable adsorption/desorption of hydrogen.To achieve a current density of 10 mA cm^-2 in a neutral electrolyte,the formaldehyde-based electrolyser requires a cell voltage of 2.52 V,which is0.33 V lower than that of the conventional water-based electrolyser.In addition,2 mg L^-1 of formaldehyde is degraded by 90.4%after 2 h at a cell voltage of 3 V.The dual-function electrolysis cell in this thesis can not only produce hydrogen,but also simultaneously efficiently degrade pollutants,which is highly energy efficient and environmentally-friendly.It can provide valuable reference for further development of electrolysis systems that simultaneously produce hydrogen and degrade contaminants.Moreover,it can provide a certain basis for the development of excellent electrocatalysts,which is important for hydrogen production by electrocatalytic water splitting.