Structural Evolution And Stabilization Strategy Of Nickel/Cobalt-Based Electrocatalysts For Alkaline Hydrogen Evolution Reaction

The increasingly prominent energy crisis and environmental pollution need to build a clean,low-carbon,safe and efficient energy system,vigorously develop new renewable energy.Hydrogen energy,as a zero-carbon clean energy source,has attracted much attention.Hydrogen production through water electrolysis technology has unique advantages such as simple equipment,high hydrogen purity,and environmentally friendly production process.It is a promising way to achieve green hydrogen production,and can also convert unstable electric energy generated by renewable energy into stable chemical energy to realize efficient utilization of renewable energy.However,the large-scale application of alkaline water electrolysis currently faces problems such as low efficiency and high energy consumption.This is because the alkaline hydrogen evolution reaction（HER）involves not only the adsorption and desorption of hydrogen intermediates but also the overcoming of the energy barrier of water dissociation,which reduces the catalytic kinetics of the electrocatalyst.In addition,the combined effect of strong alkaline electrolyte and external voltage puts higher demands on the structural stability of the alkaline HER electrocatalyst.In particular,for transition metal-based alkaline HER electrocatalysts,the crystal structure is susceptible to erosion by a large number of hydroxyl ions in the alkaline electrolyte,and the nanoscale structure faces the challenge of local super-high current density.Therefore,in order to design and prepare highly active and stable electrocatalysts,it is necessary to have an in-depth understanding of the structural evolution behavior of transition metal-based electrocatalysts during alkaline HER,and explore the influence of structural evolution on catalytic activity and stability.Based on this,this paper studied the evolution process of nickel/cobalt-based electrocatalysts in terms of element composition,crystal structure and nanomorphology during alkaline HER,deeply understood the mechanism of structural evolution,revealed the structure-performance relationship between structural evolution and electrochemical behavior.Meanwhile,efficient and stable electrocatalysts for alkaline HER were prepared by utilizing structure evolution behavior.Further,a simple and effective strategy was developed to regulate the structural evolution behavior of electrocatalysts and optimize their catalytic activity and stability.The specific research contents are as follows:（1）Nickel selenide with core-shell structure by in situ phase transformation for alkaline hydrogen evolution reaction:A Ni_0.85Se nanofilms were prepared on carbon cloth through electrodeposition and hydrothermal methods.Se leaching of Ni_0.85Se occurs during the alkaline HER process,which leads to the formation of more Se vacancies and induces in situ phase transformation,accompanied by enhanced electrochemical activity of the electrocatalysts.The initial hexagonal phase Ni_0.85Se preferentially transforms into hexagonal phase NiSe,then into rhombohedral phase Ni₃Se₂.Finally,a core-shell structure of NiSe/Ni₃Se₂ is formed.The short-range ordered and Se vacancy-rich Ni₃Se₂ grains in the shell not only protect the NiSe core with low crystallinity by inhibiting the continuous leaching of Se,enhancing the structural stability of the electrocatalyst,but also act as a real catalytic active species,exhibiting superior catalytic activity.This is because the metallic Ni₃Se₂ has high conductivity,which can accelerate charge transfer and improve reaction kinetics.Se vacancies and defects can adjust the electronic structure of catalytic sites and enhance catalytic activity.Further expansion revealed that phase transition is a universal behavior applicable to other nickel selenide electrocatalysts.（2）Modulation of selenium leaching by selenite ions to enhance the electrocatalytic stability of Co₃Se₄ for alkaline hydrogen evolution reaction:Co₃Se₄ electrocatalysts with three dimensional nanosheet arrays were loaded on carbon cloth by a two-step hydrothermal method.Co₃Se₄ electrocatalyst exhibits an initial activation followed by decay behavior during the HER process,accompanied by Se leaching and hydroxylation.Moderate Se leaching and hydroxylation can promote partial Co₃Se₄ to transform into Co（OH）₂,forming a two-component electrocatalyst,which can enhance the catalytic activity.However,excessive Se leaching and hydroxylation can severely damage the crystal structure and nanomorphology of Co₃Se₄ electrocatalyst and significantly reduce the content of highly active cobalt selenide,resulting in the decline of catalytic activity.The addition of SeO₃^2-ions to the electrolyte can effectively inhibit the Se leaching of Co₃Se₄ electrocatalyst and maintain the appropriate degree of hydroxylation,which can stabilize the catalytic activity during alkaline HER.Further,optimizing the electrolytic cell device by using a double compartment electrolytic cell with anion exchange membrane can significantly reduce the concentration of SeO₃^2-ions in the electrolyte and enhance the stability of alkaline HER,which can continuously operate at a high current density of 100 mA cm^-2 for 270 h.Mechanism studies have shown that SeO₃^2- ions in the electrolyte can regulate Se leaching and hydroxylation by affecting the ion concentration in the electric double layer,thus stabilizing the catalytic activity of Co₃Se₄.（3）Electrochemically induced Co（OH）₂/CoP binary synergistic electrocatalyst for alkaline hydrogen evolution reaction:A uniform and dense nanowire array of cobalt carbonate hydroxide on carbon cloth was obtained by hydrothermal method.After partially phosphorization,a two-component CoOx/CoP was formed by controlling the phosphorization degree.Under alkaline electrochemical conditions,the amorphous CoOx was dissolved and redeposited,in situ transforming into Co（OH）₂ nanosheets,which combined with porous CoP nanowires to form a nanotree-like Co（OH）₂/CoP binary electrocatalyst.Co（OH）₂ can lower the energy barrer of water dissociation,accelerate the Volmer step,and promote the generation of hydrogen intermediates.The electrochemical induced in-situ transformation allows for rapid charge-proton transfer between Co（OH）₂ nanosheets and CoP nanowires,facilitating the overflow and transfer of hydrogen intermediates and accelerating reaction kinetics.The unique three-dimensional nanotree structure provides more active sites for water molecule dissociation and hydrogen molecule formation,promoting electron transfer at the interface between the electrode surface and the electrolyte.Finally,the Co（OH）₂/CoP binary electrocatalyst exhibits excellent catalytic activity and stability for alkaline HER.Finally,the optimized electrode（Co（OH）₂/CoP@NF）only requires a smaller overpotential of 56 mV to drive the current density of 10 mA cm-2,which is comparable to the activity of the advanced alkaline HER electrocatalysts.（4）Coordination polyhedral ions enhance the catalytic activity and stability of a-Co（OH）₂ for alkaline hydrogen evolution reaction:A α-Co（OH）₂ nanosheets array on carbon cloth was obtained by hydrothermal method.The transformation of α-Co（OH）₂ to cobalt metal occurs during the alkaline HER process,accompanied by the formation of soluble ions.This reconstruction behavior can temporarily enhance the catalytic activity of theα-Co（OH）₂ electrode,but it can also destroy the crystal structure and nanomorphology and ultimately decrease the stability for alkaline HER,leading to an electrochemical behavior of activation followed by decay.Therefore,a simple and effective strategy to stabilize the crystal structure and nanomorphology of α-Co（OH）₂ electrode by adding appropriate concentrations of MoO₄^2-and WO₄^2-ions into the electrolyte during alkaline HER was developed,which can improve the catalytic activation and stability of the electrocatalyst.The experimental and theoretical results demonstrate that MoO₄^2-ions in the electrolyte can rivet on the surface ofα-Co（OH）₂ to form coordination polyhedrons,which enhances the stability of oxygen vacancies and hinders the erosion of hydroxyl groups in the alkaline electrolyte on oxygen vacancies,improving the structural stability of α-Co（OH）₂.The modification of MoO₄^2-not only enhances the conductivity of α-Co（OH）₂,promoting charge transfer,but also optimizes the electronic structure of nearby cobalt sites,accelerating the dissociation of water molecules and optimizing the hydrogen adsorption energy,which can lead to enhanced catalytic activity for alkaline HER.In addition,MoO₄^2-ions in the electrolyte can form electrostatic repulsion with hydroxyl ions,regulating the chemical environment in the electric double layer,reducing the possibility of hydroxyl ions entering the electrocatalyst surface,thus improving the stability of the α-Co（OH）₂ electrode.