Preparation Of Transition Metal（Ni,Co,Fe）Based Nanomaterials And Their Electrocatalytic Water Splitting Performance

The current society is facing with two thorny problems,the increasing energy requirement and the increasingly serious environmental contamination resulted from the use of non-renewable energy.The renewable,eco-friendly H₂ obtained through water electrolysis technology can solve the above problems.However,the electrolysis of water includes two half-reactions,namely,the hydrogen evolution reaction that occurs at the cathode and the oxygen evolution reaction（OER）that occurs at the anode,with a high activation energy barrier,which bring about the tangible that the actual potential of water splitting is larger than the theoretical value.Thence,it is indispensable to explore and prepare efficient and stable electrocatalysts to accelerate the reaction kinetics.Although noble metal-based（Pt,Ru,and Ir）materials have been regarded as the most electrochemically active catalysts,their high prices hinder their large-scale applications.So,a lot of effort has been devoted to developing low-cost transition metal-based（such as Ni,Co,Fe,etc.）electrocatalysts,and their performance is continuously optimized by strategies such as morphology control,construction of heterojunctions,and bonding with conductive substrates.Based on this,the research contents of this paper are as follows:（1）Using hydrothermal and annealing processes,multi-layered Ni Co₂O₄/Co₃S₄/NF nanocomposites were directly built on nickel foam.The electrode is a multi-layered,highly conductive and high specific surface area network without a binder,which facilitates electron transport and exposure of active sites,enhances full contact of the electrolyte/electrode interface,and accelerates gas diffusion during water electrolysis.It exhibits the expected electrocatalytic activity in 1M KOH electrolyte,achieving a current density of 10 m A cm^-2 at ultralow overpotentials of 71（HER）and170（OER）m V,respectively,and there is no obvious decay after up to 36 h of durability test.This strategy provides an economical,efficient,and durable bifunctional electrocatalyst.（2）Using foamed nickel as a substrate,a vertically grown Ni₃N/Co₄N nanosheet array was constructed by a facile two-step strategy of hydrothermal and nitridation.Benefitting from the synergistic effect between Ni₃N and Co₄N,nanosheet arrays and porous nickel foams form a three-dimensional cross-linked network,which exposes sufficient active sites and overall interactions,Ni₃N/Co₄N showed excellent HER activity Specifically,in alkaline fresh water（1 M KOH）and simulated seawater（1 M KOH+0.5 M Na Cl）,Ni₃N/Co₄N needed 58 and 85 m V to drive a current density of10 m A cm^-2,respectively;Ni₃N/Co₄N//Ni₃N/Co₄N electrolytic cells required an applied voltage of 1.59 and 1.63 V,respectively,to achieve a response current density of 10 m A cm^-2.（3）A novel Ni Co-OH/Fe（OH）₃-D_x（D_x represents the hydrolysis time,X=0.5,1,2 days）core-shell heterojunctions was synthesized by the hydrolysis reaction of Fe³⁺on the surface of Ni Co-OH at room temperature with nickel foam as the substrate.Electrocatalytic oxygen evolution tests showed that the prepared Ni Co-OH/Fe（OH）₃-D_x composites had better catalytic activity than pure Ni Co-OH in 1.0 M KOH.Especially Ni Co-OH/Fe（OH）₃-D₁,which only needed an ultra low overpotential of 270m V(η₁₀)and Tafel slope of 85.7 m V dec^-1.SEM and TEM showed that Ni Co-OH/Fe（OH）₃-D₁ is a perfect all-encapsulated core-shell structure,which has certain advantages for the exposure of active sites and the rapid transport of species,and the nickel foam base increases the overall conductivity and stability of the catalyst,in turn,significantly improve the oxygen evolution kinetics.