Study On Structure Control And Electrocatalytic Performance Of Transition Metal Compounds

In recent years,global warming,environmental damage caused by fossil fuels and energy shortage have slowed down the global economic development.In this regard,both at china and overseas have realized that only actively seeking clean,efficient,and sustainable new energy sources is the most effective way to solve the above problems.Among them,it is an effective strategy to convert renewable electric energy into new energy by electrocatalysis.The core factor of this strategy is electrocatalyst,whose role is to start and maintain the efficient and continuous operation of electrocatalysis reaction.Precious metal catalyst is one of the best commercial catalysts for most electrocatalytic reactions,but its high cost and limited output have caused some obstacles for its large-scale application.To solve this problem,the raw materials of transition metal compounds are cheaper,the reserves are richer and the sources are wider.Therefore,the academic circles at home and abroad have invested a lot of energy to develop efficient transition metal compound electrocatalysts,and reached a consensus that in order to make the transition metal compound electrocatalysts play a more efficient catalytic effect,it is necessary to carry out a series of regulation and control on their structures.In view of this,this paper successfully realized a series of structural adjustments of electrocatalysts based on tin disulfide(SnS₂)and molybdenum oxide(MoO_x),and then optimized their electrocatalytic performance,as follows:(1)SnS₂ with semimetal phase structure was synthesized for the first time by phase transition engineering,and it was found that phase transition engineering can be used to control the selectivity of its electrocatalytic carbon dioxide reduction products.The specific method is to successfully transform the stable semiconductor phase SnS₂(1T-SnS₂)into the metastable semimetal phase SnS₂(1H-SnS₂)for the first time by a simple and easy hydrogen-assisted low-temperature calcination method,which is the phase transformation project of SnS₂ from semiconductor phase to semimetal phase.Then the atomic configurations of different phases of SnS₂ were studied by aberration-corrected scanning transmission electron microscope.At last,the novel 1H-SnS₂ was used for electrocatalytic carbon dioxide reduction(CO₂RR),and it was found that the main product was CO,and the Faraday efficiency of CO was as high as 98.2%under the overpotential of-0.8 V,indicating that the selectivity of CO generation was close to100%.It is worth noting that,in the CO₂RR reaction,the main catalytic product of 1T-SnS₂ is formate(corresponding Faraday efficiency is not high,it is 20-60%).Unlike this,the main product of 1H-SnS₂ synthesized for the first time in this paper is CO,which is accompanied by ultrahigh selectivity.This work points out a new strategy to control products,that is,phase transition engineering of two-dimensional materials can be used to control the selectivity of CO₂RR products.(2)Ultra-thin SnS₂ with different structural phases were synthesized,and their hydrogen evolution(HER)performance was measured by micro-nano electrochemical reaction device.It was found that semimetal SnS₂ showed excellent hydrogen evolution performance,and then the root cause of excellent hydrogen evolution performance of ultra-thin 1H-SnS₂ was deeply analyzed.In this process,the differential phase contrast(DPC)imaging technique is used for the first time to obtain atomic resolution results of the electric field distribution on the surface of ultra-thin 1H-SnS₂ and 1T-SnS₂,and it is clearly found that the electric field distribution of Sn atoms in 1H-SnS₂ deviates.The electric field region with this kind of shift is more likely to exchange electrons with the outside world in the electrocatalysis process,which is the reason why 1H-SnS₂ has better catalytic performance for HER.The above DPC method for analyzing the electric field distribution of catalytic active sites provides a new method for deep understanding of electrocatalytic activity.(3)Amorphous MoO_x was synthesized by wet chemistry-calcination method,and Pt single-atom was supported on it.Then,it was found that the amorphous oxide supported single-atom catalyst(Pt-SA/?-MoO_x)has excellent hydrogen evolution performance.In acidic electrolyte(0.5 M H₂SO₄),Pt-SA/?-MoO_x shows Pt mass activity up to 52.0 A mg _Pt^–1 at an overpotential of-50 mV,which is not weaker than all reported electrocatalysts containing Pt for hydrogen evolution.This is the first time that amorphous oxide and metal monatomic combination have been used to catalyze HER efficiently.Further experimental results and theoretical calculations show that there are many oxygen defects in amorphous MoO_x,which make it easier for Pt single atoms to adsorb hydrogen ions in solution.This synergistic effect reduces the energy barrier of adsorption of hydrogen ions in the process of HER reaction,thus improving the electrocatalytic performance of HER.In addition,the synthesis method of Pt-SA/?-MoO_x was successfully extended to support Ir,Au and Pb single-atom on amorphous MoO_x substrate.The above results successfully used phase transition engineering and amorphization to control the structure of SnS₂ and MoO_x,and further improved their electrocatalytic performance.These series of work show that it is a feasible strategy to improve the electrocatalytic performance of transition metal compounds by phase transition engineering and amorphization,which provides a new idea for the optimization of transition metal electrocatalysts.