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Preparation of New Titanium Nitride-Carbon Nanocomposites and Evaluation of their Electrocatalytic Behavior

Yousefi, Elahe | 2016

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 49331 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Ghorbani, Mohammad; Dolati, Abolghasem
  7. Abstract:
  8. Titanium nitride-carbon nanocomposites are synthesized by the reaction of TiCl4 and NaN3 in supercritical benzene medium that also serves as a carbon source. In order to improve the crystallinity of the as-prepared precursor (SI), it is further heat-treated at 1000 ˚C for 3-10 h using anhydrous ammonia and UHP nitrogen atmospheres at 1000 ˚C (SIII-SV). Moreover, to improve electrochemical behavior, the synthesized nanocomposite (SIV) is modified with Pt nanoparticles using a polyol process. For better understanding of synthesized catalyst nature and justifying their variant ORR activity several analyses are done. X-ray diffraction (XRD), Raman spectrum, field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) are used to analyze phase evolution and morphology of the powders. The surface structure and chemical state of samples are analysized using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Meanwhile, ToF–SIMS and CHN analysis are carried out to analyze the surface chemicals and to determine the amount of carbon in the samples, respectively. These combined techniques gain a comprehension of the molecular processes occurring in the precursor during the reaction in supercritical benzene and its subsequent phase evolution after heat-treatment. Applying NH3-treatment at 1000 ˚C for 10 h leads to carbon phase transforms to graphene-layered structure. The synthesis of TiN-G nanocomposite is one of the most interesting achievements of this study. The samples are tested as electrocatalyst for oxygen reduction (ORR) and methanol oxidation (MOR) reactions and the related mechanisms are investigated by electrochemical methods. Moreover, to investigate the effect of pH on electrochemical performance, these reactions are studied in alkaline and acidic media. It is shown that the electrocatalytic properties of the synthesized nanoparticles are highly dependent on the heat treatment atmosphere and duration. The heat treatment under ammonia atmosphere at 1000 ˚C for 10 h increases the ORR mass activity from -3.26 (SI) to -6.52 A cm-2 g-1 (SIV) at -0.6 V vs. SCE. Moreover, this sample shows almost twice ORR mass activity as commercial TiN. The high ORR activity and stability of the sample SIV are mainly due to (i) random layer structure of carbon that combines through a hybrid state with TiN nanoparticles, (ii) unstoichiometric nitrogen and oxygen doped into TiN lattice, and (iii) higher electrochemical surface area.
    The 7 wt.% Pt/TiN-G nanocomposite is a bi-functional electrocatalyst for both MOR and ORR, which possesses significant electrochemical activity and stability in both acidic and alkaline media in such a way that it shows higher electrocatalytic activity for both ORR and MOR in acidic media than smooth Pt electrode via improved electrochemical mechanisms. The main reasons for such significant electrochemical activity can be due to finely dispersed Pt nanoparticles on TiN-G support, Pt–Ti alloy formation indicating the existence of strong metal-support interaction (SMSI) between Pt NPs and TiN-G support, higher content of Pt(0) and higher stability due to greater corrosion resistance of Pt(0).
    The highly efficient mixed conducting network and the internal defects between G layers induced by nitrogen doping lead to specific capacity as large as 381 mAh g-1 at charge/discharge (C/D) rates of 0.2 C. By contrast, when TiN-G nanocomposite is cycled at higher rate (1.6 C), it shows specific capacity retention 1.6 times better than when is cycled initially at lower rate (0.2 C) and subsequently subjected to higher rate (1.6 C). This unusual behavior of TiN/G nanocomposite can be due to (i) affecting lithiation/delithiation mechanism and consequently stabilizing carbon layered structure, and (ii) reducing solid electrolyte interface (SEI) formation. The discovered rate-dependent behavior of TiN-G anode provides a novel pathway towards the development of high capacity Li-ion anodes by anchoring of TiN nanoparticles on G that promotes energy storage applications in high performance LIBs.
  9. Keywords:
  10. Oxygen Reduction Reaction ; Titanium Nitride Nanoparticles ; Lithium Ion Batteries ; Titanium Nitride-Carbon-Graphene Nanocomposite ; Methanol Oxidation ; High Rate Capability

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