The Study On The Design And Synthesis Of Fe-based Metal Phosphide Nanostructures And Their Performances In Electrocatalysis

As a new type of clean energy that can be stored,powered,and burned,hydrogen energy has become a major strategic direction in the world’s energy transition because of its abundant sources,friendly use environment,high conversion efficiency,and high energy density.The hydrogen energy chain consists of three main links:hydrogen production,hydrogen transportation,and hydrogen use.Among the hydrogen production technologies,electrolysis of water is considered one of the most promising green hydrogen production routes due to its simple and reliable process,high purity of the produced hydrogen,and lack of pollution.The electrode reactions for hydrogen production from electrolytic water include hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）,but the process kinetics of the anodic four-electron oxygen evolution reaction（OER）is slow and requires a large overpotential to drive it.Therefore,replacing the OER process by oxidizing small organic molecules at low voltages is an effective way to reduce the electrolytic cell voltage.Among these,urea oxidation（UOR）has a low theoretical potential（0.37 V vs.RHE）and can replace the anodic OER reaction,while also cleaning up the environment for efficient hydrogen production.However,current HER,OER,and UOR electrode materials are still dominated by noble metal-based catalysts,whose low abundance,high cost,and poor stability have become bottlenecks limiting their large-scale application.Therefore,the development of non-precious metal-based catalysts with high abundance,low cost,high activity,and durability is imminent.The endowed advantages of transition metal phosphides and phosphates（TMPs）,such as their noble metal-like electronic structure and high reactivity,make them strong candidates for non-precious metal-based catalysts for electrolytic water.However,the development of low-content,high-conductivity,and durable TMPs catalysts is still a goal pursued in this research field.Based on this,this paper takes the electrocatalytic decomposition of water as a functional orientation and modulates the structural properties of transition metal phosphides and phosphates through strategies such as elemental doping,morphological design,and construction of heterostructures,and obtains some regular conclusions that provide valuable information for the in-depth study of such TMPs materials.Details are as follows:（1）1D@2D coral-like V-Co₂P₄O₁₂/CC hierarchical nanostructures were constructed on the surface of smooth conductive carbon fibers by hydrothermal and low-temperature chemical vapor deposition（CVD）phosphorylation,and their HER,OER,and UOR overpotentials as self-supported electrodes were 109 m V,310 m V,and 1.33 V,respectively,at 1 M KOH alkaline electrolyte and 10 m A cm^-2 current density,1.33 V vs.RHE（1 M KOH+0.5 M Urea）;the overpotential for the overall water splitting systems HER-UOR and HER-OER were 1.42 V vs.RHE（1 M KOH+0.5 M Urea）and 1.60 V（1 M KOH）,respectively.The experimental results show that the introduction of the heterogeneous spin state V is an effective pathway to enhance the electrocatalytic performance of the material:1)optimizing the morphology of the catalyst,increasing the contact area between the catalyst and the electrolyte,2)the local unbalanced forces generated at the lattice interface induce lattice distortion of Co₂P₄O₁₂,resulting in the formation of more reactive sites,and 3)modulation of the local electronic states,changing the charge distribution and leading to a stronger catalytic center metal Co-O covalency is stronger,contributing to the cleavage of adsorbed water molecules and lowering the reaction potential barrier.（2）Hydrothermal methods and chemical vapor deposition（fluorination and phosphorylation）were used to construct 1D@1D staggered growth bimetallic MF₂-M₂P₄O₁₂/CC（M=Ni,Co）heterogeneous nanostructures on the surface of smooth conducting carbon fibers.The same was obtained for HER,OER,UOR as well as overall water splitting.Their overpotentials as self-supported electrodes for HER,OER,and UOR at 1 M KOH alkaline electrolyte and 10 m A cm^-2 current density were 176 m V,210 m V,and 1.33 V vs.RHE（1 M KOH+0.5 M Urea）in that order;the overvoltages for the HER-UOR and HER-OER systems for total water solution were 1.61 V vs.RHE（1 M KOH+0.5 M Urea）and 1.68V（1 M KOH）.It can be seen that the bimetallic synergy with the modulation of the electronic structure by the heterostructure improves the performance of single-phase metaphosphates and fluorides for a wide range of electrolytic water-catalyzed reaction systems.（3）Using the unique advantages of ZIF-67 nanoarrays,porous Co/NC precursors with large specific surface areas and excellent electrical conductivity were prepared by one-step pyrolysis.On this basis,Ru-doped Co@Co₂P heterostructured catalysts were synthesized by partial phosphorylation and elemental doping strategies.The overpotentials of the heterostructured HER and OER were 35 m V and 217 m V,respectively,at a current density of10 m A cm^-2 in a 1.0 M KOH electrolyte,and 1.53 V vs.RHE for the fully dissolved water system HER-OER assembled as both cathode and anode.In addition,the overpotentials of the heterostructured HER and OER in 0.5 M H₂SO₄,1.0 M PBS electrolytes up to a current density of 10 m A cm^-2 current densities,the overpotential of the heterogeneous material HER was 119m V and 150 m V,respectively,indicating that the material can be used as an efficient catalyst for hydrogen precipitation and alkaline total hydrolysis over a wide p H range.Finally,a summary and outlook on the work of this thesis are provided.