Understanding the degradation and redox mechanism of all-vanadium redox flow batteries with In-situ analysis and their improvement

Abstract

All-vanadium redox flow battery (VRFB), which is one of the most promising Energy Storage System (EES) in research of next-generation batteries, has been widely studied in many industrial and educational research group due to long life cycle, fast response time, environmentally friendly and the modularity of their energy and power capacity which are independent of each other. However, because of several problems including stability of electrolyte, inaccurate electrochemical analysis method and energy efficiency, there have been numerous efforts to overcome limitation that VRFB needs to operate. The thesis is classified in 5 chapters. In chapter 1, introduction of the components of VRFB such as electrode, membrane and electrolyte and their researches will be discussed. The chapter 2 presents the optimization of Dynamic Hydrogen Electrode (DHE) as a reference electrode for in-situ analysis of VRFB system. For in-situ electrochemical analysis without extra reference cell, a DHE is inserted into the membrane of a unit cell. This chapter provide the information on design process of a DHE with adjusting resistance and describe operating mechanism of DHE employed in VRFB. With a DHE, the electrochemical properties including potential profile of the positive and negative electrode are able to be observed separately. The chapter 3 focus on the in-situ observation of the degradation of VRFB with a designed DHE as described in chapter 2. Because the degradation of cell components can affect the performance decay, the exact degradation mechanism of VRFB have to be established. In this chapter, the ohmic/charge transfer resistance of a VRFB with in-situ measurement using a DHE according to cycle number will be presented. In addition, a postmortem analysis such as XPS, SEM and Raman are conducted to understand the surface damage of the carbon felt during operating. This results can provide the key factor of durable electrode design for preventing performance loss of the VRFB. The chapter 4 discuss about the redox reaction mechanism between vanadium electrolyte and electrode of VRFB. Electrochemical impedance analysis is conducted using positive and negative symmetric cells with untreated (not functionalized) and treated (functionalized) carbon felt electrode to compare the ohmic/charge transfer resistance of carbon felts. A molecular dynamics (MD) simulation indicates difference in the structure of the hydration shell for the vanadium ions. As a result, the negative electrode reaction follows inner-sphere mechanism, while the positive electrode reaction follows outer-sphere mechanism. The chapter 5 introduce the synthesis of titanium nitride coated carbon felt (TiN-CF) electrode for VRFB to suppress the degradation of electrode by sulfuric acid. The urea-route method can facilitate one-step synthesis of TiN-CF with heat-treatment method in N2 condition. Compare to heat-treated CF, the TiN-CF can show the enhanced electrochemical performance for cyclic voltammetry and charge-discharge test and suppressed the overpotential caused by deterioration of electrode.