Preparation And Performance Study Of High-valence Metal Doped Transition Metal-based Water-electrolysis Catalysts

To alleviate the serious energy crisis,exploring renewable energy has become a priority for researchers.Hydrogen energy has attracted great attention due to its high calorific value,pollution-free and renewable features.Electrocatalytic water splitting is regarded as an important strategy for the production of hydrogen.In this regard,it is crucial to develop efficient and stable electrocatalysts.Compared with precious metals,transition metal materials have attracted tons of attentions due to their highly adjustable electronic structure and low cost.However,transition metal catalysts exhibit poor catalytic activity and high energy barriers during the reaction process.Therefore,it is necessary to enhance the intrinsic activity of transition metal catalysts via reasonable regulation strategy.Doping with high-valence metals is considered as one of the important strategies to improve the performance of electrocatalysts.The higher the valence state of the doped element,the stronger its electronegativity,thus resulting in a more significant effect on the catalysts.Doping with high-valence metals can effectively optimize the charge distribution of the catalysts,improve the adsorption energy of the intermediate,and reduce the reaction energy barrier.This paper mainly focuses on improving the catalytic activity of transition metal-based water electrolysis catalysts（including transition metal phosphides,selenides,and sulfides）through doping with high-valence metals.The relationship between the structure and performance of the catalysts is clarified,and their reaction mechanisms are explored.The main contents are as follows:1.It is of great significant to develop highly active catalysts via introducing high-valence metals into transition metal phosphides for the large-scale industrial hydrogen production.This is mainly due to the weak water-adsorption capacity of phosphide in the HER reaction,as well as its excessively strong adsorption effect on reactive hydrogen species.For the OER reaction,it is necessary to overcome high energy barriers during the reaction process.These are the challenges need to be solved for transition metal phosphide.Therefore,the Zr_xNi_2-xP catalyst is engineered by introducing high-valence Zr into Ni₂P.Theoretical results suggest that the introduction of high-valence Zr tunes the electronic sates of Ni₂P,enhances the water-adsorption ability,accelerates the process of water dissociation,and optimizes the adsorption energy of the OER reaction intermediate.The as-prepared catalyst exhibits excellent performances in alkline media with the low overpotentials of only 68 m V and 239m V at 10 m A cm^-2(j₁₀)for HER and OER,respectively.2.When designing highly active catalysts for water electrolysis,the advantage of heteroatom incorporation is to use theoretical calculations to predict the impact of doped atoms on the material,thus providing guidance on the doping content of the experiment at the atomic scale.To this end,through DFT calculations,we found that appropriate incorporation of Zr can effectively expedites the sluggish H₂O dissociation kinetics and optimizes the adsorption energy of reaction intermediates of Co P,thereby enhancing the intrinsic activity of Co P.Experimental results also show that the as-prepared catalyst exhibits superior HER and OER catalytic performances under alkaline media,with low overpotentials of 62 m V and 240 m V to afford the current density of j₁₀,respectively.3.It is necessary to develop highly active catalyst via introducing high-valence metals into transition metal selenide to enhance its catalytic activity.And it is crucial to explore how doped high-valence metals promote the reaction process,especially for the OER reaction.As we all know,transition metal selenides have sluggish HER kinetics and poor adsorption capacity for active H intermediates.For the OER reaction,there exists a high reaction energy barrier during the catalytic process.To address these challenges,the catalyst is synthesized by introducing high-valence Cr into Ni Se₂.Theoretical calculations indicate that the incorporation of high-valence Cr effectively accelerates water dissociation kinetics and improves H*adsorption in HER process,with reducing the energy barrier of OER and the adsorption energy of oxygen-containing intermediates.These regulations kinetically and thermodynamically enhance the intrinsic catalytic activity of both HER and OER.In-situ Raman spectroscopy suggests that the incorporation of Cr promotes the formation of active species Ni OOH during the OER reaction.Moreover,experimental results show that the final catalyst requires low overpotentials of 89 m V（HER）and272 m V（OER）to drive a current density of j₁₀ in alkaline medium.4.As we known,the performance of transition metal sulfides is inferior to that of precious metal materials.In order to better apply them in the energy field,incorporating high-valence metals into transition metal sulfides is an effective strategy.During the reaction process,transition metal sulfides exhibit a high energy barrier in the Volmer step.For the OER reaction,sulfides will undergo surface reconstruction under oxidizing conditions,so it is necessary to clarify the important role played by the doped high-valence metals during the reaction process.At the same time,we need to set higher requirements for the testing conditions of materials with a practical industrial goal.Based on the above considerations,the highly active electrocatalyst is synthesized via introducing high-valence Ce into NiS₂.Theoretical calculation results show that the incorporation of high-valence Ce regulates the electronic structure of NiS₂,accelerates the reaction kinetics of water dissociation,promotes the Volmer reaction step of NiS₂,optimizes the adsorption and desorption of active H*,and reduces the adsorption energy of the rate-determining step of OER.In-situ Raman testing shows that high-valence metal Ce promotes the reconstruction process of NiS₂.Electrochemical analyses present that the overpotentials of the as-obtained sample at j₁₀ for HER and OER is only 82 m V and 232 m V in alkaline solution,respectively.In addition,under simulated industrial environment conditions,the electrocatalyst exhibits good overall water splitting performance and stability.