Researches On The Preperations And Electrocatalytic Performance Of Nickel-based Transtion Metal Heterostructures Catalysts

Hydrogen production from electrolytic water is favored by scientific researchers because of its green,sustainable and efficient advantages.The development of high-performance catalysts is an important way to achieve high-efficiency water decomposition,especially for the slow anodic oxygen evolution reaction（OER）.At present,noble metal based materials are considered to be the most effective electrocatalysts,among which Ru O₂and Ir O₂are OER catalysts and Pt is hydrogen evolution reaction（HER）catalyst.However,its high price and scarcity limit the large-scale use of electrolytic water system.Transition metal based materials,such as transition metal hydroxides,selenides and sulfides,can still maintain good electrochemical and mechanical stability and excellent electrocatalytic properties in long-term electrocatalytic reactions due to their cheap and abundant,highly dispersed active sites and unique d-orbitals.Among transition metal based materials,nickel based materials have the best adsorption adaptability to intermediates,and nickel based hydroxide is the best choice of OER catalyst.In this thesis,a series of nickel based hydroxides and their derivatives were designed and synthesized for electrocatalytic water decomposition.By adjusting the structure and morphology of each component of the material,the catalytic performance of the catalyst material in HER and OER was strengthened.The specific research contents are as follows:（1）A dual phase electrocatalyst（Ni Se₂/Ni₃Se₄/NF）rich in Ni Se₂and Ni₃Se₄phase boundaries was prepared by selenoconversion of Ni（OH）₂grown on nickel foam in a tubular furnace by hydrothermal method combined with calcination.Experimental results show that electrocatalysts with higher Ni state(Ni³⁺)and more Ni₃Se₄phases promote OER,while electrocatalysts with lower Ni state(Ni²⁺)and more Ni Se₂phases promote HER.For HER,the optimized Ni Se₂/Ni₃Se₄/NF-4 catalyst in 1.0 M KOH has an overpotential of 228 m V at a current density of 100 m A cm^-2and a Tafel slope of 69.7 m V dec^-1.For OER,the optimized Ni Se₂/Ni₃Se₄/NF-1 catalyst has an overpotential of 309 m V at a current density of 100 m A cm^-2and a Tafel slope of 104.3 m V dec^-1.The electrolyzer equipped with Ni Se₂/Ni₃Se₄/NF-1 and Ni Se₂/Ni₃Se₄/NF-4 catalysts has a low potential of1.56 V at the current density of 10 m A cm^-2,and good durability.Experiments and calculations of density functional theory（DFT）show that negatively charged Ni Se₂can promote the adsorption of hydrogen radicals（H*）and positively charged Ni₃Se₄hydroxyl（OH）and oxygen-containing intermediates（OH*,O*and OOH*）.In addition,interfacial charge transfer also confirmed that Ni Se₂/Ni₃Se₄/NF-4 containing more Ni Se₂phases had better HER activity,and Ni Se₂/Ni₃Se₄/NF-1 containing more Ni₃Se₄phases had better OER activity.This study provides a new way to prepare nickel selenide electrocatalyst with high catalytic activity.（2）A two-step hydrothermal process was used to produce nickel-iron layered double hydroxide and its sulfide heterostructure catalyst（Ni₂Fe-LDH/Fe Ni₂S₄/NF）by adjusting the hydrothermal duration.The heterostructures afford abundant interfaces.In the 1.0 M KOH solution,Ni₂Fe-LDH/Fe Ni₂S₄/NF exhibits an excellent OER catalytic activity with a much smaller overpotential（240 m V）to reach the current density of 100 m A cm^-2and a loewe tafel slope(29.2 m V dec^-1)due to the synergistic effect.DFT calculation also proved that the heterostructure had smaller Gibbs free energy and conductivity than that of single phase.More impressively,2000 cycles of cyclic voltammetry scan for water oxidation results in the formation of a sulfate layer over the catalyst.The corresponding post-catalyst demonstrates better OER activity and durability than the initial one in the alkaline simulated seawater electrolyte.The post-Ni₂Fe-LDH/Fe Ni₂S₄/NF delivers smaller overpotential（250 m V）at 100 m A cm^-2and longer stability time than the original form（260 m V）,which is better than the OER catalytic performance of most transition metal catalysts in alkaline salt solution.The post-formed sulfate passivating layer is responsible for the outstanding corrosion resistance of the salty-water oxidation anode since it can effectively repel chloride.