Preparation And Electrocatalytic Properties Of Transition Metal Phosphides/Carbon-Based Composites

The excessive use of fossil fuels has led to an increasing level of carbon dioxide in the atmosphere year by year.The development of efficient green energy conversion devices is of great significance for achieving the goal of“double carbon”.Among many energy conversion and storage technologies,rechargeable zinc-air batteries（ZABs）have shown promising applications due to its high energy density,low cost,safety,portability and environmental friendliness.Oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）are the core reactions for rechargeable ZABs to achieve energy conversion,and noble metal Ru（Ir）and Pt-based materials are usually required as catalysts to effectively improve energy conversion efficiency.However,the expensive price,limited reserves and poor stability of noble metals greatly limit the commercial application of rechargeable ZABs.Therefore,people have tried to develop non-noble metal catalysts to replace noble metal catalysts for more than half a century.Metal-organic framework（MOF）-derived transition metal phosphides（TMPs）such as Co P,Ni₂P,Fe P,etc.It maintain the original structural characteristics of MOF,which not only possess uniformly dispersed catalytic active sites and high specific surface area,but also have adjustable component structures,which makes them show great potential in replacing noble metal catalysts.In addition,in the field of OER/ORR,carbon-based materials have attracted widespread attention because of their wide sources,good electrical conductivity,large specific surface area and excellent thermodynamic properties.This thesis aims to develop inexpensive and efficient non-noble metal catalysts.Through rational design,a series of highly active transition metal phosphide/carbon-based composites have been prepared as electrode materials for electrocatalysis applications by using strategies such as elemental doping,morphology modulation and compounding with carbon materials.The main research contents are as follows:（1）A three-dimensional La₂O₃/Ni_xP_ynanoparticle embedded nitrogen-doped porous carbon material（La₂O₃/Ni_xP_y@NC）derived from La Ni-MOF was synthesized by solvothermal,pyrolysis and low-temperature solid-phase phosphating strategy.The highly conductive nitrogen-doped porous carbon material and the synergistic effect between La₂O₃and Ni_xP_yprovide abundant active catalytic sites and enhance electron mass transfer capability,which endows the La₂O₃/Ni_xP_y@NC catalyst with excellent oxygen evolution reaction activity.Moreover,La₂O₃/Ni_xP_y@NC catalyst provides a low overpotential(η₁₀₀=384 m V),which is superior to that of commercial Ru O₂(η₁₀₀=451m V).More importantly,the introduction of La₂O₃can not only effectively adjust the electronic structure and morphology of the catalyst,but also significantly improve the long-term stability and durability of the La₂O₃/Ni_xP_y@NC electrocatalyst.（2）The N,P co-doped Nd₂O₃/Co P@NC nanocomposite was designed and synthesized by element doping and morphology modulation strategies.The introduction of dicyandiamide during pyrolysis provides abundant conditions for the self-growth of carbon nanotubes.The higher nitrogen content,conductivity,graphitization and large electrochemical active area greatly promoted the electrocatalytic activity of Nd₂O₃/Co P@NC.It exhibited excellent OER(η₁₀=271 m V)and ORR(E_1/2=0.845 V)activities in alkaline conditions.When assembled into ZABs,it exhibits high open-circuit voltage（1.422 V）and excellent cycling stability（207 h）.（3）A Ni-Co bimetallic phosphide material（Ni₂P/Co P/GCN）composited with graphite-like carbon nitride（g-C₃N₄）nanosheets was prepared by a simple ultrasonic method,annealing treatment and low temperature solid phase phosphating method.The synergistic effect between the self-grown carbon nanotubes and the Ni-Co bimetallic increases the number of active sites in Ni₂P/Co P/GCN and enhances the charge transfer rate,resulting in excellent bifunctional activity(η₁₀=286 m V,E_1/2=0.854 V).In the practical application of rechargeable ZABs,it shows high peak power density(147.4m W cm^-2)and good cycle stability（141 h）,which is expected to be a cathode candidate catalyst for rechargeable ZABs.The three types of catalysts La₂O₃/Ni_xP_y@NC,Nd₂O₃/Co P@NC and Ni₂P/Co P/GCN prepared in this thesis all inherit many advantages of MOF derivatives,showing excellent OER/ORR activity and cycle stability,which can effectively improve the slow kinetics of redox reactions.In addition,the rechargeable ZABs assembled based on the above catalysts exhibit high open circuit voltage,high peak power density and excellent cycle stability,which have potential application in rechargeable ZABs.