Study On Controllable Preparation Of Iron-group-element-based Electrode Materials And Evaluation Of Their Electrochemical Performance

Echoing to the ever-increasingly severe energy crisis and environmental issues,the development and utilization of renewable energy sources is imminent.Nevertheless,typical clean renewable energy,such as solar energy and wind energy,is unevenly distributed in time and space,which consequently has to rely on energy conversion and storage devices for further utilization.The two representative technologies in this regard,water electrolysis and lithium secondary batteries,have been attracting much attention over the past three decades.It has been well recognized that the overall electrochemical performance of these systems depends significantly on the quality of the electrode materials.Aiming at the rational design of the optimal electrode materials in the above two systems,this thesis has developed a highly active oxygen evolution reaction(OER)electrocatalyst and a high-performance lithium-sulfur(Li-S)battery cathode material,through compositional and structural engineering of a series of the iron-group-element relevant solid compounds and their composites.The major findings are summarized below:(1)This thesis explores the physiochemistry of a series of important iron-group-element-based advanced functional materials and their applications in electrochemical energy storage and conversion systems,which leads to the successful synthesis of a variety of relevant(oxy)hydroxides(e.g.,FeOOH nanoparticles,Ni-Fe LDH,FeOOH/Ni-Fe LDH),carbon-supported composites(e.g.,single-layered hollow C-Ni2Co shell,single-layered hollow C-Ni2CoP shell,double-layered hollow C-Ni2CoP@C-Ni2Co shell).The evolutions of the structure,size,and composition of these solid compounds or composites were regulated and investigated in different physical and chemical processes,based on which a range of synthetic strategies have been established for efficient manipulation of the process kinetics to achieve control over the structure,the composition and the size of the above iron-group-element relevant solid compounds and their hybrids;their excellent catalytic activity both in water oxidation and polysulfide decomposition in lithium-sulfur battery cathode are revealed.These results enriches our knowledge in iron-group-element relevant chemistry.(2)For the OER electrocatalysis,by controlling the synthetic process kinetics,FeOOH nanoparticles with different sizes supported on Ni-Fe LDH were prepared;particularly,the sample FeOOH2nm/LDH was found to outperform the state-of-the-art noble or non-noble metal OER electrocatalysts reported so far.Its overpotential at a current density of 10 mA cm-2 in 1.0 M KOH is as low as 174 mV and the Tafel slope is 27 mV dec-1.It also shows excellent structural and catalytic stability.XANES,EXAFS,DC voltammetry and large-amplitude Fourier transform AC voltammetry analysis show that there exists strong solid-solid interface interaction between Ni-Fe LDH nanosheets and FeOOH nanoparticles;more importantly,the existence of highly unsaturated Fe(3+δ)+ on the ultrafine FeOOH nanoparticles could affect the redox behavior of Ni2+in Ni-Fe LDH through interfacial oxygen bridges,which thus significantly enhance the OER activity.Such an interfacial synergistic effect was found to become increasingly significant along with decreasing size of FeOOH nanoparticles.(3)For Li-S battery cathode materials,a series of single-layered or double-layered hollow shells of Ni2+/Co2+-based tannate or phytate with controlled morphology and size were prepared by taking the advantage of the differentiated solubilities of the metal-organic complex solids.The thermal derivatives of these metal-organic precursors are a series of porous hollow carbon nanoshells supported with various Ni2Co and/or Ni2CoP nanoparticles,including the single layered C-Ni2Co,single layered C-Ni2CoP and the double-layered C-Ni2CoP@C-Ni2Co.Thus functionalized hollow carbon nanoshells could not only serve as a host to contain the S active material and tolerate its volume expansion,but also the metal moieties were found capable of chemically adsorbing and catalyzing the conversion of the polysulfide species.In particular,the double-layered S@C-Ni2CoP@C-Ni2Co composite cathode yields a discharges 1236.8 mAh g-1 in the first cycle with a loading of 3.5 mg cm-2 at a current density of 0.1 C,and after 100 cycles of charge and discharge,it still maintains a capacity of 1010.1 mAh g-1.At 0.5 C,the first cycle discharge has a specific capacity of 1038 mAh g-1,and after 500 cycles of charge and discharge,a specific capacity of 893 mAh g-1 could still be retained,spelling a capacity retention rate as high as 86%.At a large current density of 5 C,the discharge specific capacity is 740.1 mAh g-1,proving its excellent rate performance.Our control experiments show that,in such a double-layered structure,the outer shell C-Ni2Co could improve the local conductivity while the inner shell C-Ni2CoP could accelerate the conversion of polysulfides through chemical adsorption and catalysis.Overall,by designing and controlling the composition,structure,and size of iron-group-element-based solid compounds and their composites,we have obtained high-performance electrode materials for OER electrocatalysis and lithium-sulfur batteries.Our work demonstrated controlled synthesis of a series of iron-group-element-based micro-nano functional materials by manipulating their physicochemical properties,and also put an emphasis on their great catalytic potential in improving the performances of electrochemical energy storage and conversion electrode materials.