Preparation And Electrochemical Properties Investigation Of Supported POM-Based Composites

Nowadays,the rapid growth of the demand for energy leads to more and more serious problems of energy exhaustion and environmental pollution.As a result,one of the most important tasks is to make energy consumption more efficient,clean and sustainable in the development of economy and society.In many fields of energy related researches,lithium-ion batteries?LIBs?and electrocatalytic hydrogen production are considered as the key energy storage and conversion pathways,which have become the focus of study.It is of great significance to provide strong support for the transition to clean energy.Polyoxometalates?POMs?are a class of anion-type metal-oxygen clusters formed by bridging oxygen atoms with multiple transition metal-oxygen octahedrons.Due to their unique structural characteristics and excellent electrochemical properties,POMs have immeasurable application value in the fields of electrochemical energy conversion and storage.On the one hand,POMs show a variety of molecular structures and compositions,adjustable redox and designability.On the other hand,POMs have reversibly multi-electrons redox property.Therefore,POMs have attracted widespread attention in the field of electrochemistry.However,due to the limitation of their properties,the application of POMs faces many challenges in the fields of the lithium ion batteries?LIBs?and electrocatalytic hydrogen production.At first,the conductivity of the POMs is poor,which leads to unsatisfactory electrochemical performance.Second,it's difficult to disperse the nano-sized POMs evenly.Third,POM clusters are easy to run away during the electrode reaction.Although high-performance POM-based electrode materials have been reported nowadays,how to prepare efficient and stable POM-based electrochemical functional materials via reasonable design and controllable method still faces many challenges.In this work,we have synthesized high-performance and stable supported POM-based electrode materials by in-situ synthesis,electrostatic interaction and confinement strategy.The studies expand the application of POM-based composites in the field of electrochemistry.The main research contents are as follows:?1?We have successfully constructed a series of Dexter-Silverton POM/Ni foam composites?denoted as NiM-POM/Ni;M=Co,Zn,Mn?using in-situ hydrothermal synthesis method,in which the Keggin type PW₁₂ clusters as the building blocks and the Ni foam as the conductive substrates.The as-prepared NiM-POM/Ni composites showed high activity as the electrocatalytic materials for hydrogen evolution reaction?HER?in alkaline condition?pH=14?.Among them,the highest HER performance can be observed in the NiCo-POM/Ni,featuring an overpotential of 64 mV(at 10 mA·cm^-2,vs.RHE),and a Tafel slope of 75 mV·dec¹ in 1.0 M aqueous KOH.By tuning the hydrothermal reaction time,it can be found that the NiCo-POM/Ni with 2?m nanoparticle size exhibited the highest number of electrochemically active surface sites and electrocatalytic activity.The excellent HER activity of NiCo-POM/Ni can be attributed to the in-situ synthesis strategy led to the stable linkage between the POM clusters and the Ni foam electrode.Besides,the Ni foam conductive substrates can be improved the dispersibility of POMs and promoted the electrical conductivity,and thus improved the transfer of ions and electrons.?2?The 3D carbon foam supported N-doped MoS₂ nanoflakes were successfully fabricated by using the commercially available polyurethane foam?PU?as 3D carbon template and PMo₁₂ clusters as Mo source via redox polymerization and followed by sulfurization.The method can effectively inhibited aggregation and improved the conductivity of MoS₂.The as-prepared MoS₂-CF composite possessed well-exposed active edge sites and the N-doped sites enhanced intrinsic catalytic activity of MoS₂ during the high temperature conversion process.Moreover,the 3D carbon substrate facilitated the transfer of electrons and ions.Hence,the N-doped MoS₂-CF composite showed high HER performance and stability with a low overpotential of 92 mV in 1.0 M of KOH and 155 mV in 0.5 M of H₂SO₄ at a current density of 10 mA·cm^-2,respectively.?3?The 3D N-doped Carbon substrate with high specific surface area and microporous structure was obtained by ZIF-8 as precursor.Then,the Carbon-PMo₁₂ composite was successfully fabricated by electrostatic interaction between PMo₁₂ cluster and the active protonation of N-doped sites in Carbon substrate.The as-prepared novel 3D Carbon-PMo₁₂ composite displayed a significantly improved Li-ion storage performance as the LIBs anode with excellent reversible specific capacity and rate capacity,as well as high cycling performance.The Carbon-PMo₁₂ composite showed the capacity of 985 mAh g^-1 after 200 cycles under the current density of 1 A g^-1,and had the 62.8%capacity retention ratio even in the large current density of 10 A g^-1.Charging and discharging mechanism investigations indicated that the uniform distributed PMo₁₂ clusters remained good stability mainly by reversible redox reaction with 24 electrons transfer,and the 3D microporous Carbon-PMo₁₂ gave more opportunities to contact with electrolyte and enhanced electrical conductivity,and thereby improve the Li-ion conduction and electron transfer accordingly.The Carbon-PMo₁₂ composite exhibited highly surface capacitive contributions for anode material to improve the rate capacity.?4?Single walled carbon nanotube?SWNTs?-encapsulated PMo₁₂(PMo₁₂@SWNTs)composite were prepared by the spontaneous electron transfer and electrostatic interactionbetween SWNTs and PMo₁₂.PMo₁₂@SWNTs composite exhibited excellent electrochemical lithium storage performance and stability.After 250 cycles at 1 A g^-1,the reversible capacity of PMo₁₂@SWNTs composite was 1075 mAh g^-1.In addition,the PMo₁₂@SWNTs composite showed good rate performance.Through the in-depth analysis of the charging and discharging process,it was found that the SWNTs with suitable inner diameter size can be conducive to the stable loading and uniform dispersion of PMo₁₂,and thus avoided the loss and aggregation of active components.SWNTs can enhanced the conductivity of composite and shorten the Li+ transmission path.Besides,the resulted P-N type heterostructure of SWNTs and PMo₁₂ spontaneously developed a built-in electric field,which accelerated charge transport significantly.Benefiting from the confinement strategy,the PMo₁₂@SWNTs composite showed high capacity of lithium storage and exhibited high capacitive contribution to improve the rate performance.