Synthesis Of Polyoxometalate Derivatives And Their Electrocatalytic Performance For Oxygen Evolution Reaction

Hydrogen energy has the advantages of high calorific value of combustion,clean and pollution-free,various utilization forms,energy storage safety and so on.It is a clean energy for research.Therefore,exploring energy conversion systems and researching renewable energy storage are important subjects at present.The electrocatalytic decomposition of water into hydrogen（HER）at the cathode and oxygen evolution（OER）at the anode is considered to be the most important and simplest method for the preparation of hydrogen.The target efficiency is still mainly limited by the performance of the oxygen evolution reaction because the reaction exhibits unusually slow kinetics.Many precious metals are used as electrocatalysts,such as Ru,Pt and Ir.But the disadvantage of noble metal composites is that precious metal sources are scarce,which limits the development of noble metal composites.Many efforts have been made to explore non-precious metal and rare earth compounds as electrocatalysts for OER.In order to reduce the overpotential in the reaction process,it is necessary to develop high activity and stable OER electrocatalyst with practical application value.Although great progress has been made,further improvements in the synthesis route and structural optimization are needed to develop highly active and stable OER electrocatalysts with practical applications.Polyoxomtalates（POMs）are inorganic nanoscale metal oxide clusters mainly composed of early transition metals（TMs）such as Mo,W and V.Due to its abundant storage and low cost,as well as its unique chemical composition and structural diversity,POMs can be widely used in the preparation of highly efficient molybdenum or tungsten based electrocatalysts.Therefore,based on the above problems,this paper uses POMs as the molecular element regulation platform to design and prepare a new composite OER electrocatalyst of metal phosphide and metal oxide.（1）Synthesis of highly active and stable electrocatalysts is the key to electrocatalytic decomposition.In particular,the introduction of secondary transition metal into the single metal phosphating system can adjust the electronic structure and surface properties,so that the phosphating based composites have better performance and higher activity.In this study,we synthesized ZIF-67@POM nanomaterials by doping H₃PMo₁₂O₄₀ in ZIF-67.Molybdenum-doped carbon matrix can effectively reduce CoP agglomeration,which was conducive to improving the specific surface area and active sites of the catalyst.The doping of Mo also optimizes the electronic structure of Mo-CoP-100 and improves its electrocatalytic activity.Compared with CoP derived from pristine ZIF-67,the optimal Mo-CoP-100 exhibits significantly enhanced OER catalytic activity with an almost constant overpotential of 290 m V to maintain a stable current density of 10 m A cm^-2 in alkaline media for at least 100 h.Furthermore,compared with traditional precious metal-free OER electrocatalysts,the Mo-CoP catalysts prepared in this work has high specific surface area,rich surface-active sites,good cycle stability and high diffusion efficiency.In addition,a considerable number of POMs contain a variety of transition metals,which have problems such as few reactive sites and poor electrical conductivity.（2）Synthesis of highly active and stable electrocatalysts is the key to electrocatalytic decomposition of water.A series of unique POM@ZIF-67 hybrids were synthesized by co-precipitation of strong Br?nsted acid H₃PMo₁₂O₄ with ZIF-67.The composite was calcined in air to produce Co₃O₄/Mo O₂ nanoparticles with ultra-fine grains,which provide rich active sites for surface redox.Compared with the CO₃O₄prepared by the original ZIF-67,the optimal Co₃O₄/Mo O₂-32 exhibited significantly enhanced OER catalytic activity,and the overpotential was almost constant at 295 m V at the current density of 10 m A cm^-2.The Co₃O₄/Mo O₂ can maintain a stable current density of 10ma cm^-2 for at least 100 hours in alkaline media.These electrochemical properties are superior to most of the reported non-noble metal-based OER catalysts.（3）An efficient and cheap electrocatalyst for OER was prepared in order to realize efficient electrocatalytic decomposition of water.ZIF-67@POM nanoparticles were successfully synthesized by hydrothermal method,and then the composite was calcined in air,a novel Co₃O₄/WO₃catalyst with nanoparticles structure was synthesized.This unique nanoparticles structure and the potential synergy between POM and ZIF-67 lead to superior electrocatalytic activity of OER.When the current density is 10 m A cm^-2,the overpotential is only 285 m V and the slope of Tafel is 85 m V dec^-1.Compared with the traditional non-noble metal OER electrocatalysts,the nanoparticles Co₃O₄/WO₃catalysts have good long-term stability,large surface area,many active regions and high diffusion efficiency.