| Clean and renewable energy sources,such as hydrogen,solar,wind and tidal energy,have played important roles in the sustainable development strategy,thanks to their pollution-free nature.At present,water electrolysis and supercapacitor are reliable candidates for the development of new energy utilization.Water electrolysis technology is to prepare hydrogen by converting electric energy into hydrogen energy.The reaction process includes cathodic hydrogen evolution reaction(HER)and anodic oxygen evolution reaction(OER).In addition,as a new type of energy storage device,supercapacitor could realize the effective energy utilization by converting discontinuous energy into continuous one.It is worth noting that the key factor determining the performance of hydrogen evolution reaction(HER),oxygen evolution reaction(OER)and supercapacitor is electrode material.These materials are usually required to achieve the common characteristics,such as good conductivity,large specific surface area,full contact with electrolyte,more active sites and structural stability,et al.The chalcogenides or phosphides based on nickel/cobalt have attracted great attention in the field of energy conversion/storage due to their rich reserves and good environmental compatibility.But in recent years,such single-phase materials are much more difficult to meet the increasing requirements of activity and stability.Constructing heterogeneous composites,by introducing foreign components into active materials can take the advantages of the synergistic effect and interface engineering among various components,thus could result in a better electrochemical performance.However,the preparation of high-performance composites often demands fine or complex technology,and the rational design strategies still need to be improved.In this case,this thesis designed and optimized a series of Ni/Co based composite materials by means of synthetic strategies,such as structural composite,phase control and interface engineering.The main works are as follow:(1)In order to rationalize the structure and composition of catalyst,Mo-based materials are introduced for the composites,then a series of Ni S/Mo S2/C powders with different Ni/Mo molar ratios are prepared as HER electrocatalysts through calcination/sulfurization process.When the ratio of Ni/Mo in the material is optimized to 1:4,the catalyst material of sample/active carbon(AC)shows a three-dimensional(3D)nanorods/sheets composite structure.In the alkaline medium,it shows the best hydrogen evolution reaction activity with an overpotential of 117 m V at 10 m A cm-2,a small Tafel slope of 58 m V dec-1 and excellent electrochemical durability,which is attributed to the synergistic effect of the different components,abundant exposed active sites and excellent conductivity.This kind of heterostructure possess good conductivity,rich exposed active sites and synergistic effect,which can improve its activity and stability in alkaline medium.(2)It is synthesized the unique Mo-O linkage(MOL)enhanced CoP nanosheet arrays supported on Ni foam(NF)and systematically investigate their performances for overall water splitting as a function of the composition.The fine-tuning of Co/Mo ratios in the Co-Mo precursors realized by controlling the dosages of capping agent(urea),which enables Co O to grow along with the incorporation of Mo-O.As a result,the optimized CoP/MOL/NF-10 exhibits the best hydrogen evolution reaction(HER)activity with low overpotentials(?)of 87 and 210 m V at 10 and 100 m A cm-2,respectively.Density functional theory(DFT)calculation confirms the electron density distribution on the interface is tuned when proper amounts of Mo-O linkages are introduced into CoP,thus gives rise to a decent?GH value.When the CoP/MOL/NF-10employed as anode for OER,the current density of 20 m A cm-2 can be achieved at a low overpotential of 270 m V,which is comparable to Ru O2.It is found that abundant crystalline defects induced by the combination and interaction between metallic CoP and Mo-O linkage can increase the number of exposed edge sites,which are important participants during the electrolysis process,and this material is superior to single component catalyst in HER and OER.In the end,a symmetric electrolyzer built by CoP/MOL/NF-10 deliveries a current density of 10 m A cm-2 at an operating voltage of1.52 V with excellent stability,it is comparable to the Pt/C(-)//Ru O2(+)electrolyzer.(3)Co-Mo-O/Ni3S2 nanosheet arrays are grown on nickel foam(NF)via two-step hydrothermal reaction.Thanks to the synergistic effect between components and the unique coating array structure,the obtained Co-Mo-O/Ni3S2/NF exhibits excellent OER electrocatalytic activity in alkaline medium,the OER current density reaches 50 m A cm-2 at overpotential of 290 m V.When the Co-Mo-O/Ni3S2/NF is employed as supercapacitor electrode,its specific capacity can achieve as high as 340 m Ah g-1 at 2m A cm-2and the rate performance is 82.1%(2 to 32 m A cm-2),which is much more better than that of single-phase material.In addition,Co-Mo-O/Ni3S2/NF as OER electrode has good durability in 12-hour electrolysis experiment.As supercapacitor electrode,it can maintain 74.5%initial capacity after 10000 galvanostatic charge-discharge(GCD)cycles at 170 m A cm-2.Moreover,the fabricated Co-Mo-O/Ni3S2/NF//activated carbon(AC)asymmetrical supercapacitor shows an ultra-high energy density of 57.9 Wh kg-1 at a power density of 1.1 k W kg-1.(4)Ni Se,Ni3S2 and Ni Se/Ni3S2 heterostructures with different Se/S ratios are grown on Ni foam(NF)via one-step solvothermal reaction,and they exhibited various morphologies and electrochemical behaviors.Among them,the Ni Se/Ni3S2/NF-1:1heterojunction(Se/S molar ratio=1:1)shows the best electrochemical performance,which benefits from the unique morphology of self-supporting nanofiber arrays vertically grown on Ni foam.The epitaxial growth mechanism of the free-standing Ni Se/Ni3S2 nanofibers is related to their matching d spacings between Ni Se and Ni3S2.After further phosphorization treatment,Ni12P5 nanoparticles can grow on the surface of Ni Se/Ni3S2 nanofibers due to their matched exposed facets.The obtained Ni Se/Ni3S2/Ni12P5/NF-1:1 hierarchical nanostructure with a large mass-loading of 8.5mg cm-2 can provide an ultrahigh areal capacity of 2.04 m Ah cm-2 at 10 m A cm-2(272m Ah g-1 at 1 A g-1)and an excellent cycling stability.Its capacity remains 69.6%when current density is increased from 5 to 60 m A cm-2.The outstanding electrochemical behavior of Ni Se/Ni3S2/Ni12P5/NF-1:1 could be associated with the re-distribution of electron densities within the heterostructure,and the good electron conductivity of the protective Ni12P5 nanoparticles on the surface.In the end,the as-prepared Ni Se/Ni3S2/Ni12P5/NF-1:1//AC hybrid supercapacitor's volume capacity can achieve to7.26 m Ah cm-3,and provide 9.23 m Wh cm-3 energy density at the power density of 84.8m W cm-3 with decent stability. |
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