Synthesis Of Nickel-based Composites And Study Of Their Electrochemical Performance

The crisis of traditional fossil resources has driven public attention to the development of new green energy sources.Hydrogen energy stands out among the many new energy sources with its high energy density,wide range of sources,zero pollution and zero emissions.Compared to the various industrial hydrogen production methods,electrolytic water is one of the most efficient and viable methods because of its greater yield and the simplicity and stability of the process.However,the oxygen evolution reaction（OER）during electrolysis of water involves a slow four-electron transfer process with a high overpotential and high power consumption,resulting in a high total overpotential of electrolytic water,which is key to limiting the rate of hydrogen production.Thus,the anode reaction with a lower theoretical oxidation potential is chosen to replace the OER.The urea oxidation reaction（UOR）not only has a low overpotential,but also solves the problem of water pollution caused by urea.However,the UOR is still subject to slow kinetics due to its complex six-electron transfer process,so the development of efficient and stable electrocatalysts is a top priority to improve the efficiency of hydrogen production from electrolytic water.In this thesis,a series of nickel-based transition metal compounds were designed and developed as electrocatalysts,and the electrocatalytic performance of the nickel-based complexes was optimized by using heteroatom doping,constructing heterogeneous interfaces and modulating the electronic structure,and the main research is as follows:（1）NiWO₄@NiSe₂/NF heterostructured bifunctional electrocatalysts loaded on nickel foam were designed and synthesized by a two-step hydrothermal method,which showed excellent electrochemical performance.Only 258 m V low overpotential(Tafel slope:38.93 m V dec^-1)is required to drive a current density of 10 m A cm^-2 as an OER electrocatalyst and only132 m V low overpotential(Tafel slope:70.52 m V dec^-1)as a HER electrocatalyst.In addition,the NiWO₄@NiSe₂/NF//NiWO₄@NiSe₂/NF electrolyzer was assembled for over water splitting driving a current density of 10 m A cm^-2 with a cell voltage of only 1.60 V.The reaction was stable for fifteen hours,demonstrating good stability.The excellent catalytic performance is mainly attributed to the unique nanosphere structure and heterogeneous interface,which effectively improves the conductivity and catalytic activity of the catalyst.The density functional theory calculation shows that the introduction of the NiSe₂ material greatly reduces the Gibbs free energy of hydrogen adsorption,and the introduction of the NiWO₄material greatly enhances the conductivity of the material.This work provides a viable strategy for the construction of heterostructured of nickel-based materials.（2）Nitrogen and iron co-doped Ni₃S₂@NiP₂ heterostructured materials with efficient catalytic properties for oxygen precipitation and urea oxidation reactions were successfully prepared on nickel foam by hydrothermal and high temperature calcination methods.Benefiting from their hierarchical structure,the exposure of more active sites and the doping effect of nitrogen and iron atoms,the N-Fe-Ni₃S₂@NiP₂/NF materials show excellent electrocatalytic activity towards OER and UOR.A low overpotential of 251 m V is sufficient to drive a current density of 100 m A cm^-2 as an oxygen evolution reaction catalyst and only 1.353 V is required to drive the same current density as a catalyst for the urea oxidation reaction.It is noteworthy that the SEM results show a rough N-Fe-Ni₃S₂@NiP₂/NF surface with some nanopores,which may account for the increase in active sites during catalysis.The N-Fe-Ni₃S₂@NiP₂/NF electrode also has relatively long-term durability in alkaline solutions.Density functional theory calculations indicate that the in situ generated Fe-doped nano-oxides have a strong water adsorption energy,which may be one of the reasons for their good catalytic activity.This work contributes to the rational design of electrocatalysts for efficient electrolysis of water for hydrogen production and urea-containing wastewater treatment.（3）Synthesis of cobalt and manganese co-doped bimetallic sulphide Fe₉S₁₁@Ni₉S₈electrocatalysts by a one-step hydrothermal method.The prepared Co-Mn-Fe₉S₁₁@Ni₉S₈electrocatalysts showed excellent electrocatalytic activity for the oxygen evolution reaction and the urea oxidation reaction.An overpotential of only 193 m V is required to provide a current density of 10 m A cm^-2 for the oxygen evolution reaction and a potential of only 1.33 V for the urea oxidation reaction,which is far superior to most reported electrocatalysts.Notably,the unique nano-flower structure of Co-Mn-Fe₉S₁₁@Ni₉S₈ increases the specific surface area of the material,and the introduction of Co and Mn elements promotes the formation of higher valence Ni and Fe,increasing the charge transfer rate.Density functional theory calculations show that Co-Mn-Fe-Ni OOH materials derived from Co-Mn-Fe₉S₁₁@Ni₉S₈ generated in situ have outstanding water adsorption energy and exceptional electrical conductivity,thus enhancing the catalytic performance of the materials.The present work provides new ideas for the development of efficient and low-cost bimetallic cation-doped electrocatalysts.