Researches On The Surface Reconstruction And Electrocatalytic Water Splitting Of Molybdenum-dased Transition Metal Catalysts

Electrolysis of water to hydrogen and oxygen has become an effective way to solve the increasingly energy problem.To further improve the wide application of water splitting,reducing overpotential is critical because slow reaction kinetics cause most of the efficiency loss in water electrolyzers.However,compared with the high cost and scarcity of precious metals,transition metals are the best candidates for water electrolysis catalysts.Moreover,the reaction rate of electrocatalysis is closely related to the reactant adsorption and product desorption processes that take place on the catalyst surface.Then,it is crucial to explore the possible electrochemical reconstruction phenomena between the interfaces.Up to now,the metal electronic structure of the catalyst surface can be tuned to optimize the catalytic activity by doping,heterostructure construction,defect structure and synthesis of core-shell structure.In recent years,some studies have shown that the surface state of the catalyst may change in the process of electrocatalytic water splitting.But researches in this area mainly focus on the electrochemical oxygen evolution reaction（OER）catalyst.The electrochemical reconstruction in the electrochemical hydrogen evolution reaction（HER）is poorly understood.In this thesis,a molybdenum-based bimetallic catalyst is designed and synthesized as the research object.The purpose of this paper is to investigate the regulation mechanism and promotion effect of polyaniline coating and electrochemical cathodic activation on the electrocatalytic water splitting.Besides,the electrode surface reconstitution is caused by the activation process.The mechanism of electrode reconfiguration to promote the HER reaction is elucidated,which provides a new perspective for the development of efficient molybdenum-based bimetallic HER catalysts.The specific works are as follows:1.A high-efficiency OER catalyst with the core-shell heterostructure is synthesized by hydrothermal method and electrodeposition method on nickel foam.The active material Ni₃Mo₂P-Mo O₃ heterostructure is the core and the conductive polymer polyaniline acts as the shell.Among them,the coupling effect between nitrogen species of polyaniline shell and metals of core materials imparts improved intrinsic catalysis activity and more active sites.On the one hand,the interaction between the core-shell materials reduces the interfacial charge transfer resistance and thus enhances the intrinsic catalytic activity of the catalyst.On the other hand,the construction of the core-shell structure increases the active area of the catalyst and generates more active sites.The high conductivity of polyaniline accelerates charge transport and kinetics.Thus,the catalytic exhibits efficient OER catalytic activity with overpotential of 290 m V at 100 m A cm^-2,while the Tafel slope is only 60.58 m V dec^-1.Moreover,the PANI@NiMoOP/NF maintains the constant OER current density of 100 m A cm^-2 for over 30 h.The sample manifests excellent catalytic efficiency for alkali OER,only needing low overpotentials of290 and 412 m V at high current densities of 100 and 400 m A cm^-2,respectively.The high catalytic efficiency and good stability at large current density make the catalyst promising for industrial water splitting applications.Moreover,after the reaction,the PANI structure preserves well because of the strong structural stability.And after OER,the catalysts show the in-situ reconstitution to form the active species Ni OOH on the catalyst surface.2.NiMoO₄-Co O nanoarrays on carbon cloth are synthesized by hydrothermal method,and the surface of NiMoO₄-Co O is reconstructed by electrochemical activation.Mo species dissolute from NiMoO₄-Co O by activating the pristine catalyst at certain potential,which enables the formation of K₂Mo₃O₁₀ on the catalyst surface.The surface reconstruction not only brings large numbers of active sites and enhanced charge transfer,but also boost the intrinsic activity per catalysis site.At a current density of 10 m A cm^-2,the overpotential is only 55 m V,which is 105 m V less than that of the pristine catalyst.The Tafel slope of the activated catalyst is 41 m V dec^-1,while the Tafel slope of pristine catalyst is 65 m V dec^-1.The activation process greatly promotes the reaction kinetics of HER.The effects of activation at different potential（-0.2,-0.25,and-0.3 V）on the catalytic activity are also explored,and-0.25 V is the optimum activation potential which gives the best catalytic activity.This surface reconstruction phenomenon provides a new understanding of the active center in the HER catalytic process.3.To verify the universality of molybdenum-based bimetallic catalyst cathodic activation and reconstruction for promoting HER performance,the Co Mo-LDH/CC nanosheet arrays on carbon cloth are synthesized by hydrothermal method.The surface reconstruction of Co Mo-LDH/CC is promoted by electrochemical cathodic activation,and then improving the catalytic activity of alkaline HER.It is proved that the structure of Co Mo-LDH can not be stably maintained and transforms into Co（OH）₂ during the electroactivation process.With the dissolution of Mo species into the electrolyte,a new material of K₂Mo₃O₁₀ forms with the electrolyte KOH.On the surface of the nanosheets,the final product after electrochemical reconstruction is K₂Mo₃O₁₀@Co（OH）₂.After surface reconstruction,the overpotential is 153 m V at a current density of 10 m A cm^-2,which is lower 182 m V than that of the pristine catalyst.The Tafel slope of the activated catalyst is 67 m V dec^-1,and the pristine catalyst is 101 m V dec^-1.The activation process greatly promotes the reaction kinetics of HER.This cathodic activation provides unique insights into the surface reconstruction and catalytic activity enhancement of HER electrocatalysts.