MOF Intramolecular Steric Hindrance And Lattice Defect Engineering Modification Of Cobalt-Based Sulfide To Enhance Electrolytic Water And The Mechanism Of Hydrogen And Oxygen Evolution

Hydrogen production from electrolytic water is a key measure to solve the energy problem.The preparation of electrocatalysts with high efficiency and low cost is the key to large-scale hydrogen production.Transition metal sulfides have been widely studied because of their good electrocatalytic activity and high earth reserves,but there are still some problems,such as easy agglomeration of crystals,small specific surface area,insufficient exposure of active sites,poor electrical conductivity and poor mechanical stability.Metal organic framework（MOF）forms a long-range ordered structure through the coordination of metal ions with a variety of organic ligands.Different functional groups,such as electron-donating group amino group（-NH₂）and electron-absorbing group carboxyl group（-COOH）on benzene ring,have different intramolecular steric hindrance effects by affecting the electronic symmetry of organic ligands,π-conjugation effect of benzene ring structure and coordination with metal ions,which further affect the particle size of the catalyst produced by subsequent high temperature pyrolysis.In the first part of this paper,the particle size of the catalyst was controlled by the intramolecular steric hindrance effect of MOF,and a good alkaline electrocatalytic hydrogen production（HER）performance was obtained by introducing a metal-S-C/N highly conductive channel,and further actively introducing defects into the Co₃S₄ lattice to trigger stress engineering to significantly improve its electrocatalytic oxygen production（OER）performance.Finally,Co₃S₄-Ni₃S₂-C/N and Co₃S₄/Ni₃S₂-10%Mo@NC were established as cathode and anode catalysts for overall water splitting（OWS）,respectively.The research results are summarized as follows:（1）Co₃S₄-Ni₃S₂-C/N with small particle size and high catalytic activity was prepared by introducing-NH₂ and-COOH at the same time,and the Metal-S-C/N high conductivity channel formed by-NH₂ and C skeleton carbonized and vulcanized at high temperature greatly improved the conductivity of the material.In 1M KOH,when the current density is 20 m A cm^-2,Co₃S₄-Ni₃S₂-C/N only needs a very low HER overpotential of 101 m V.In situ Raman testing reveals the existence of Co（OH）₂ active intermediates on the Co₃S₄ surface in alkaline environments,the Co sites are tightly bound to the O in the-OH group,and the characterization after durability testing proves that the catalysts are stable in alkaline environments a large amount of transformation into Co₃O₄ after HER.（2）Due to the dense lattice structure of Co₃S₄-Ni₃S₂-C/N,its alkaline electrochemical oxygen production performance is poor,so in further research,combined with the intramolecular space limitation effect of NH₂-MOF,Co₃S₄/Ni₃S₂-10%Mo@NC catalysts with lattice expansion and stacking faults were prepared by medium-temperature-short-time hydrothermal method.Using nickel foam（NF）as the substrate,Mo O₄^2-was used as etching agent during the growth of Co（OH）F precursor.The slow dissolution of Ni substrate by Mo⁶⁺caused Ni²⁺to deposit in the growth process of Co（OH）F precursor,resulting in a large number of defects,and further formed Co₃S₄ crystals rich in lattice defects after vulcanization.The catalyst only needs 164 m V and 223 m V overpotential to reach 10 m A cm^-2 and 100m A cm^-2 current density in 1M KOH.It is proved by a variety of physical characterization that the lattice defects in the catalyst trigger stress engineering,increase the exposure of the reaction center,increase the total charge transfer rate,adjust the distribution of valence band electrons in the active metal center,optimize the adsorption-desorption performance of the reactants on the catalyst surface,and improve the catalytic activity.The existence of active intermediates of metal hydroxides in OER reaction in alkaline environment was studied by in situ Raman spectroscopy.（3）Co₃S₄-Ni₃S₂-C/N and Co₃S₄/Ni₃S₂-10%Mo@NC were used as cathode and anodic catalyst,when OWS reaction is carried out in 1M KOH,the overpotential of 1.48 V and 1.58 V is needed to reach the current density of 10m Acm^-2and 100 m A cm^-2.To sum up,through MOF intramolecular steric hindrance effect and defect engineering,cobalt-based sulfide electrocatalysts with low cost,high activity and high stability were successfully prepared for synergistic electrocatalysis of hydrogen evolution and oxygen evolution,respectively,and their catalytic mechanism was elucidated.