Controlled Preparation Of Transition Metal Phosphides For Electrocatalysis Water Splitting

The green and low-carbon energy structure is crucial for sustainable human development.Hydrogen energy,due to its high energy density and zero carbon emissions,is widely regarded as the green energy of the 21st century.Hydrogen production via water electrolysis,utilizing water as a natural resource and renewable electricity as a carbon-free energy source,significantly facilitates the transition of energy structure and the implementation of carbon neutrality strategies.In the electrolysis of water industry,catalysts are commonly employed to lower the reaction barrier for water splitting,thus achieving efficient hydrogen gas production.Transition metal phosphides have garnered widespread attention from scholars in the field due to their excellent electrical conductivity,thermal stability and chemical corrosion resistance.On the other hand,too strong hydrogen adsorption as well as high water decomposition barriers prevent them from exhibiting high activity for water splitting.This thesis aims to enhance the electrocatalytic performance of transition metal phosphide catalysts by employing various strategies such as strain engineering,thermodynamic trend regulation,coordination optimization,and in-situ construction of surface electric fields to develop highly active new structures.These efforts significantly improve their electrolysis performance and broaden the optimization and modification strategies for electrolysis catalysts.The specific research contents are outlined as follows:1.We constructed a phosphide heterostructure with"Similar Stacking"characteristics by stacking Co₂P with a similar atomic arrangement on Ni₂P nanosheets to achieve interface strain engineering,thereby promoting efficient water splitting.The underlying Ni₂P lattice influenced Co₂P to undergo compressive strain,and the stacking layers of Co₂P further adjusted the degree of strain and the optimization range of hydrogen adsorption.Under strain engineering,the degree of orbital hybridization between the active sites of Ni₂P/Co₂P and H atoms was reduced,optimizing hydrogen adsorption energy.Successive X-ray diffraction,transmission electron microscopy,and X-ray absorption near-edge structure spectroscopy confirmed significant compressive strain of Co₂P and shortened Co-P bond lengths under the similar stacking strategy.Under strain engineering,Ni₂P/Co₂P achieved a HER overpotential of only 66 m V at10 m A cm^-2 in alkaline media,surpassing Pt/C performance at high current densities.The overpotential for OER at 100 m A cm^-2 current density was 272 m V,requiring only1.46 V cell voltage to achieve overall water splitting at 10 m A cm^-2 current density with long-term stability.The material design strategy of similar stacking broadens the scope of strain engineering for electrocatalysts,providing new avenues for developing high-performance heterogeneous electrocatalysts.2.Utilizing bromine atom doping to alter the thermodynamic stability trend of nickel phosphide,we synthesized P-poor deficient nickel phosphide nanosheets,achieving efficient water splitting.The introduction of Br atoms reversed the relative thermodynamic stability relationship between the original Ni₂P and Ni₁₂P₅ structures and introduced P vacancies.During synthesis,Br induced the formation of P-poor deficient nickel phosphide nanoparticles on the surface of Ni₂P nanosheets.The P-poor deficient phase Ni₁₂P_5-xBr_x reduced the reaction barrier of water decomposition transition states and optimized the hydrogen adsorption free energy.Further adjustment of vacancy concentration through heat treatment temperature enabled depth modulation of hydrogen adsorption,displaying a volcano-type relationship.In alkaline media,at a current density of 10 m A cm^-2,the overpotential for HER was only 18 m V,the best among reported non-noble metal phosphides at that time.Combined with outstanding OER performance,overall water splitting could be driven at extremely low cell voltages.The novel halogen-induced structural transformation strategy provides an innovative perspective for optimizing and improving energy conversion electrocatalysts.3.Based on the influence of iodine atoms on the formation energy of nickel phosphide,we propose an active center coordination optimization strategy to synthesize nickel phosphide nanocoral structures for efficient water splittng.The coordination of iodine with Ni leads to increased thermodynamic stability of the Ni₅P₄ structure.During the phosphorization process with iodine vapor atmosphere coordination,the precursor nanosheets transform into nanocoral morphology,and precise control of the coordination structure is achieved through temperature adjustment.The construction of Ni₅P_4-xI_x/Ni₂P nanocoral structures optimizes the coordination environment of active centers from multiple angles,including crystal structure,doping,and vacancies,which is fully validated through electron paramagnetic resonance and extended X-ray absorption fine structure spectroscopy.The coordination environment optimization adjusts the hydrogen adsorption energy and water dissociation energy,exhibiting outstanding bifunctional catalytic activity.The overpotentials for HER and OER at 10m A cm^-2 are 46 m V and 163 m V,respectively.We also summarize a four-dimensional comprehensive material design strategy for electrolysis catalysts,termed"thermodynamic stability-electronic properties-charge transfer-adsorption energy"（TECA）.This coordination optimization concept and material design method broaden the path for optimizing and upgrading electrocatalytic materials.4.Built-in opposite electric field to promote efficient water splitting by in situ reconstruction of boron-doped nickel phosphide surface.Based on density functional theory simulations,we attempted to enhance catalytic performance by promoting a balance between hydrogen adsorption and desorption through localized built-in opposite electric field（OEF）.Planar average potential calculations demonstrated that the new structure of upper-layer Ni₃（BO₃）₂ and bottom-layer Ni₅P₄ possessed built-in opposite electric field,which could be utilized to modulate electron transfer between catalytic sites and hydrogen atoms by altering orbital hybridization,thereby significantly optimizing the hydrogen adsorption/desorption equilibrium on the catalyst surface after water dissociation.The experimental synthesis of this innovative structure was achieved through in-situ reconstruction during the electrochemical process of B-doped Ni₅P₄.The surface reconstruction of Ni₃（BO₃）₂,directly related to the B content,was further validated through grazing incidence wide-angle X-ray scattering and in-situ Raman spectroscopy.With the adjustment of hydrogen adsorption equilibrium by the opposite electric field,HER only required an overpotential of 33 m V to achieve a current density of 10 m A cm^-2 in alkaline media.Simultaneously,it exhibited excellent OER performance,providing superior water electrolysis advantages compared to commercial electrodes.The concept of built-in opposite electric field and the in-situ construction of surface heterostructures provide a new pathway for the development of water splitting catalysts.