Study On Electronic Structure Regulation And Water Splitting Of Cobalt Based Materials

The excessive consumption of global fossil energy has not only caused serious environmental pollution,but also brought about an energy crisis.In order to achieve the goal of“carbon neutrality”and“emission peak”,the development of sustainable clean energy has become the primary choice to achieve this goal.Hydrogen energy is a very promising energy carrier with the characteristics of high energy density,clean and pollution-free,green and sustainable,and many available forms.Water electrolysis technology can produce hydrogen directly and quickly.Two reactions are involved in this technique:the cathodic hydrogen evolution reaction（HER）and the anodic oxygen evolution reaction（OER）.The efficiency of two reactions is critical for total water splittingIn order to improve the reaction efficiency,researchers have developed a variety of HER and OER catalysts with excellent performance.The current high-performance electrocatalysts represented by noble metals cannot be widely used due to their high cost and poor stability.For the non-precious metal electrocatalyst cobalt-based catalysts with low cost and high catalytic activity,the unique 3d electronic structure enables them to have suitable HER and OER activities,and has broad application prospects.However,there is still a big gap between the activity of pure cobalt-based materials and noble metals,and their electrical conductivity,electronic structure,and intrinsic activity need to be improved.In this thesis,cobalt-based materials with unique 3d electronic structures are designed and prepared,aiming to improve the adsorption capacity of intermediates,realize fast charge transfer,and improve the performance of electrocatalysts through the internal and interfacial electronic modification of cobalt-based material catalysts.The research contents of this paper are as follows:First,a bimetallic Co_xNi_3-xS₂/NF catalyst with a large specific surface area was prepared.Using the different electronegativity of cobalt and nickel metals,short-range electron transfer can be achieved to optimize the electronic structure.Compared with Ni₃S₂,cobalt’s effect on the material structure and catalytic activity was analysized.The morphology of the material is found to be strongly dependent on the cobalt content.When Ni²⁺was coupled with Co²⁺,Co²⁺received partial charge transfer from Ni²⁺by bridging S^2-viaπelectron donor,which changed the electronic structure around Ni.This strong electron coupling and synergistic effect resulted in abundant active sites on the surface and optimized the electronic structure of the material.Density functional theory calculations showed that the introduction of Co atom could reduce the adsorption of H⁺and thus optimized the adsorption free energy of hydrogen.When the x value was 1.4,Co_1.4Ni_1.6S₂/NF showed superior catalytic activity as an electrode for HER under an alkaline solution,with overpotentials of 68 mV,160 mV,and 226 mV at current densities of 10 m A cm^-2,50 m A cm^-2,and 100 m A cm^-2,respectively.Co_1.4Ni_1.6S₂/NF as an electrode for OER,achieved overpotential of 300 mV at current density of 10 m A cm^-2.Consider of the insufficient OER activity of cobalt-based sulfides,the second part selected highly active cobalt-based layered double hydroxides（LDHs）as the research object tuning the electronic structure of materials for efficient bifunctional electrocatalysts.Photoelectron spectroscopy and orbital electron coupling analysis show that due to the different filling degrees of the 3d orbitals of Co,Ni,and Fe ions,the bridges between them and O^2-are different,and a"pull-push phenomenon"of electrons is formed between different metals.The tuning of the internal electronic structure of CoNiFe-LDH is achieved.The strong electronic coupling and abundant oxygen vacancies optimize the electron cloud density of the active site and the adsorption of reaction intermediates,thereby enhancing the OER and HER activities of the catalyst.Thus,the trimetallic CoNiFe-LDH catalyst in 1.0 M KOH required a low overpotential of 143 mV for HER at a current density of 10 m A cm^-2,and 219 mV for OER at 20 m A cm^-2,enabling the alkaline electrolyzer to provide the low cell voltage of 1.56 V at 10 m A cm^-2.As the electrocatalytic reaction is the adsorption and desorption reaction at the interface,the third part uses the semiconductor band theory to establish a p-p type heterojunction to change the electron distribution at the interface to enhance the adsorption capacity of the target intermediate.The analytical results showed that the formation of p-p heterojunctions could drive the electron flow from CoO to CoP.This electron modulation contributed to the formation of positively charged regions on CoO,which enhanced the adsorption of OH^-during the OER.Other exposed negatively charged CoP sites were easily covered by adsorbed H⁺,facilitating the water absorption and dissociation steps that determine HER activity.The self-supported CoP-CoO p-p type heterojunction catalysts exhibited excellent OER activity(210 mV overpotential at 10m A cm^-2),HER activity(104 mV overpotential at 10 m A cm^-2),and water splitting activity(1.65 V at 10 m A cm^-2).In this paper,the strategy of improving the water splitting performance of electrocatalysts by regulating the electronic structure of the catalyst interior and interface lays the experimental foundation and provides a new idea for the design of highly active bifunctional transition metal electrocatalysts in the future.