Growth of Vanadium Compounds Nanosheets and Their Applications in Energy Storage and Conversion

含釩化合物納米片的合成及其在能源存儲與轉換領域的應用

Student thesis: Doctoral Thesis

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Award date17 Jul 2017

Abstract

Vanadium, a typical transition metal element with a great deal of reserves in the nature, has been investigated by large numbers of researchers worldwide due to its multiple valence states and low cost. Combining with other metal and non-metal elements, a series of compounds can be produced based on vanadium, especially vanadium oxide, vanadium carbide, vanadium nitride, and vanadate, which are considered as fascinating functional materials and widely applied in energy related fields because of their unique physical, chemical, and electronic properties.
In this thesis, the vanadium compounds including vanadium oxide, vanadium carbide, and vanadium nitride based nanosheets are synthesized by simple hydrothermal reaction and the following annealing methods. The morphology, structure, composition and formation mechanism of these compounds have been investigated in detail. The results show that the excellent performance in energy storage and conversion applications of these compounds suggesting large potential in clean and renewable energy related schemes.
Firstly, the overview of vanadium based compounds is provided. Especially the advantages, preparation methodologies, modification strategies, and applications of vanadium compounds based nanomaterials in energy storage and conversion related fields.
Secondly, oxygen deficient V2O5 nanosheets prepared by traditional hydrogenation and followed by low temperature hydrogenation are described. V2O5 is a promising cathode material for lithium ion batteries boasting a large energy density due to its high capacity as well as abundant source and low cost. However, the poor chemical diffusion of Li+, low conductivity, and poor cycling stability limit its practical application. In this chapter, the hydrogenated V2O5 (H-V2O5) nanosheets with oxygen defects mainly at bridging O(II) sites improves the conductivity and accelerates the Li+ diffusion and therefore performs excellent overall electrochemical lithium storage performance. The H-V2O5 nanosheets deliver an initial discharge capacity as high as 259 mAh g-1 and it remains 55% when the current density is increased 20 times from 0.1 to 2 A g-1, and also excellent cycling stability with only 0.05% capacity decay per cycle after stabilization. The results reveal here a simple and effective strategy to improve the capacity, rate capability, and cycling stability of V2O5 materials which have large potential in high efficiency energy storage applications.
Thirdly, porous vanadium nitride (VN) nanosheets are synthesized hydrothermally followed by an ammonia treatment. Generally, VN is promising in lithium ion battery (LIB) anodes due to its high energy density, chemical stability, and corrosion resistivity. In this chapter, the porous VN nanosheets offer a large interfacial area between the electrode and electrolyte as well as short Li+ diffusion path and consequently, the VN nanosheet electrode has high capacity and rate capability as an anode in LIB. The VN anode delivers a high reversible capacity of 455 mAh g-1 at a current density of 100 mA g-1 and it remains at 341 mAh g-1 when the current density is increased to 1 A g-1. The charge transfer and Li+ diffusion kinetics during the lithiation process is studied systematically. A highly stable SEI film is formed during the initial discharging-charging cycles to achieve a long cycle life and sustained capacity at a high level for 250 discharging-charging cycles without deterioration. This chapter demonstrates the preparation of high-performance LIB anode materials by a simple method and elucidates the lithiation kinetics.
Fourthly, a hierarchical nanosheet structure comprising isolated vanadium carbide nanoparticles encapsulated in a highly conductive mesoporous graphitic carbon network (VC-NS) is synthesized by a hydrothermal reaction and subsequent low-temperature magnesium thermic reaction. It has a large specific surface area and boasts highly efficient HER (hydrogen evolution reaction) activity such as very small overpotential, fast proton discharge kinetics, and excellent durability. The small Tafel slope of 56 mV dec-1 with a low overpotential of only 98 mV at 10 mA cm-2 is quite close to that of the commercial 20% Pt/C catalyst. The excellent durability is indicated by the overpotential shift of only 10 mV after 10,000 cyclic voltammetric scans at a current density of 80 mA cm-2. The high-performance precious-metal-free electrocatalyst is promising in HER and related energy generation applications.
Lastly, metallic cobalt (Co) nanoparticles segregated in situ on conductive vanadium nitride (Co/VN) nanosheets synthesized by ammonia nitridation of hydrothermally prepared Co2V2O7 nanosheets are investigated as high-performance oxygen evolution reaction (OER) electrocatalysts. VN has been commonly used as the support for precious metal electrocatalysts due to the metallic properties, resistance to most chemical solvents, and stability under a wide variety of conditions. The metallic Co nanoparticles with a large number of exposed active sites are distributed uniformly and adhere firmly to the VN substrate to enhance the OER efficiency, facilitate fast charge transfer, and improve the stability. As a result, a small overpotential of 320 mV is required to achieve a current density of 10 mA cm-2 with a small Tafel slope of 55 mV dec-1. The excellent stability is indicated by an overpotential shift of only 34 mV after 2,000 cyclic voltammetry cycles at a large current density of 200 mA cm-2. The precious-metal-free Co/VN nanosheets deliver outstanding OER performance and are promising as electrocatalysts in water splitting and related energy conversion applications.