Design And Synthesis Of Transition Metal Phosphides And Hydroxides Composite Nanomaterials For Oxygen Evolution Reaction

In the context of“dual carbon”,the use of traditional energy sources is increasingly restricted.Hydrogen energy has become the most ideal energy to replace fossil energy due to its high energy density,environmental cleanliness and wide sources.The water splitting to produce"green hydrogen"has become the most popular hydrogen production technology.The half-reaction anodic oxygen evolution reaction（OER）in the water electrolysis reaction is a four-electron reaction process,which leads to slow reaction kinetics and requires a high overpotential to overcome the energy barrier in the reaction process.Thus,it is particularly important to find high-performance OER catalysts.At present,most OER catalysts used in industry are noble-metal based catalysts,such as Ir O₂and Ru O₂,but they cannot be applied on a large scale due to high cost and scarce materials.Now many researchers focus on the development of non-noble-metal based catalysts with high activity and stability for OER.Transition metal phosphates and hydroxides are potential non-noble metal catalysts due to their suitable electronic structure and controllable morphology.However,their OER activity is greatly restricted by their poor electrical conductivity.In this paper,transition metal phosphates and hydroxides were studied to improve their OER catalytic activity by constructing heterojunction structure,anionic doping and morphology optimization.And the Density Functional Theory（DFT）is used to make a theoretical analysis on the improvement of the catalytic performance of the material.The main research contents of this paper are as follows:Design and synthesis of Co P-Ni₂P/NF heterostructure and their oxygen evolution performance:Transition metal phosphates are potential OER catalysts,but their OER catalytic activity is restricted by poor electrical conductivity.A large number of literatures have proved that the heterojunction structure can improve the conductivity of the material,and the valence electron structures of Ni²⁺and Co²⁺are 3d⁸(t_2g⁶ e_g²)和3d⁷(t_2g⁵ e_g²),respectively,and part of the electrons can be transferred from Ni²⁺to Co²⁺.In this chapter,we use nickel foam as the metal substrate to synthesize Co P-Ni₂P nanosheet heterojunction composites（Co P-Ni₂P/NF）by hydrothermal and phosphating methods.The experimental results and DFT calculations show that there was a strong electronic interaction between Co P and Ni₂P,and charge was transferred from Ni₂P to Co P,and the combination of Co P and Ni₂P can improve the electron transfer rate and conductivity of the overall phosphide,thereby improving the catalytic activity of OER.The optimal Co P-Ni₂P/NF displays excellent OER performance with low overpotential of 255 m V and 310m V to achieve the current density of 20 m A cm^-2 and 100 m A cm^-2,low Tafel slope of68.1 m V dec^-1 and robust stability in alkaline electrolyte.Design and synthesis of Ni Se@S-Co（OH）₂/NF composites and their oxygen evolution performance:Co（OH）₂ nanosheets are considered as OER catalysts with high intrinsic activity.However,due to the poor conductivity of hydroxides,the nanosheet structure is easy to pile up and mask the active sites,and the OER activity of Co（OH）₂ is still limited.In this study,Ni Se@S-Co（OH）₂/NF composites with core-shell structure were synthesized by electrodeposition method using Ni Se nanowires（Ni Se/NF）grown on nickel foam as a substrate.Due to the unique core-shell structure exposing more active surface area and increasing the ionic diffusion rate of the catalyst as well as the strong interaction between S^2-and OH^-,the Ni Se@S-Co（OH）₂/NF exhibits excellent OER properties under alkaline conditions.It can achieve 285 m V and 300 m Vat the current density of 50 m A cm^-2 and 100 m A cm^-2with Tafel slope of 101.8 m V dec^-1,maintaining100 h at the current density of 100 m A cm^-2.Design and synthesis of heterostructured Ni Se@Co Fe LDH/NF composites and their oxygen evolution performance:In order to further investigate the interaction between Ni Se and hydroxide,in this chapter,Ni Se@Co Fe LDH/NF composites with obvious heterojunction structure were synthesized.The optimal Ni Se@Co Fe LDH/NF possesses superior OER performance with ultralow overpotential of 203 and 236 m V to achieve 10 and 100 m A cm^-2 with low Tafel slope of 90.3 m V dec^-1 and the OER activity is better than most catalysts reported at present.DFT calculations unveil that the interfacial synergism on Ni Se@Co Fe LDH can induce electrons redistribution by charge transfer from Co Fe LDH to Ni Se,and reduce the energy barrier of the ratelimiting step（*OH→*O+H⁺+e^-）by the stronger chemical bond of Fe-O in OER,and thus significantly increase the intrinsic OER activity.