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Title: Electrode modification and in-situ studies to enhance the performance of vanadium redox flow batteries
Authors: Ghimire, Purna Chandra
Keywords: Engineering::Materials::Energy materials
Issue Date: 2019
Source: Ghimire, P. C. (2019). Electrode modification and in-situ studies to enhance the performance of vanadium redox flow batteries. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: The demand for renewable energy has increased in the recent decades due to the environmental benefits and their better availability. However, electricity generated from renewable energy sources is inherently intermittent and hence fluctuating. Suitable energy storage systems are required to stabilize the intermittency and to fill the deficit between demand and supply. Redox flow batteries (RFB) hold the capacity to meet these demands, as it offers unlimited capacity. Among different redox flow batteries, the vanadium redox flow battery (VRFB) is considered as one of the promising technologies for large-scale energy storage. The benefits of the VRFB are associated with its flexible power rating and storage time, low-maintenance cost, excellent load levelling capacity and long lifetime. The unique feature of the VRFB is the separation of the battery stack and electrolyte, which allows the decoupling of power output and energy capacity. Unlike other redox couples, due to the use of same redox species, undesirable performance fading and ion crossover can be mitigated during operation in VRFB. Simple remixing of the electrolytes in two half-cell periodically can reverse the capacity fade. Despite the ongoing commercial deployment of VRFB, there are research needs with respect to the improvement of battery performance and reducing the cost of VRFB. The research work in VRFB are mainly on membrane, electrode, electrolyte and the design of the stack, as role of these components determines the overall performance. The electrode is one of the major components of VRFB, which largely determines the efficiency and durability of the battery. In particular, as key component, electrode provides the site for redox reaction in each half-cell, which determines the cell performance. PAN-based graphite felts are widely used as electrodes in VRFB, however, pristine graphite felts from the manufacturer are inherently hydrophobic and have poor electrocatalytic properties towards vanadium species. Therefore, various modification methods are proposed to activate the surface of the graphite felt. Among these, thermal oxidation in the air is the most widely used technique owing to its simplicity and cost-effectiveness. Despite being widely used, there is a lack of scientific investigation on electrode treatment at various temperature and duration range. Most importantly, the correlation between the change in electrode physicochemical and electrochemical properties need to be investigated for achieving the optimum modification conditions. The alternative electrode modification method, which prevents the performance loss due to degradation of the electrode, also needs to be investigated. There are also no suitable methods to precisely estimate the electrolyte utilization across the porous media after the modification of the electrode by various techniques. Thus, this PhD research aimed to investigate the correlation between the physiochemical and electrochemical properties of PAN-based graphite felt electrodes. The study will use such correlation to prepare the optimized electrode with best VRFB performance and demonstrate in a single VRFB cell. With respect to the battery performance, catalyst decoration and surface modification were explored to fulfil the major objective of this study. A novel synthesis method has been carried out to dope binder-free titanium carbide (TiC) particles onto the surface of carbon fibres using titanium tetrafluoride (TiF4) as the titanium source. The method of preparation was inexpensive and yielded uniformly distributed TiC particles on the carbon fibre surface, which enhance the charge transfer kinetics of the sluggish negative redox couple (V2+/ V3+). Additionally, a systematic investigation of the effect on thermal oxidation of PAN-based graphite felts on its performance at the negative electrode of the VRFB was performed. The results from ex-situ characterization techniques were employed to identify crucial material properties, such as surface composition, specific surface area, defects and electrochemical parameters including wetting properties, mass loss, surface morphology etc. and relate with the cell performance of the cell at the specific condition. Locally resolved segmented cell study was performed to investigate the in-situ behaviour of a flow cell. A single cell with an electrode area of 100 cm2 was divided into sixteen segments to investigate the local voltage and open circuit voltage (OCV). The spatially resolved OCV was then converted to the corresponding state of charge (SOC) of the electrolyte, which was used to predict the flow behaviour and electrolyte utilization across the electrode. For the first time, a rationale to estimate the degree of electrolyte utilization is proposed based on the differences between experimental and theoretical changes in SOC. The concept was then verified with the experiment from the optimized electrode compression study of the single non-segmented cell. In addition, the study also concluded that 25% of electrode compression with respect to its initial thickness gave optimum conversion efficiency. The investigation was further extended to study the electrolyte utilization at various operating conditions, such as differently modified electrodes, various flow rates, various current densities etc. Furthermore, overpotential at each half-cell at various operating conditions was studied using the half-cell potential measurement method by inserting reference electrodes at the outlet tube of the VRFB single cell. In summary, this PhD study has successfully demonstrated the use of a low-cost novel catalyst to enhance the negative half-cell of VRFB. The study has also successfully investigated the correlation between the physiochemical and electrochemical properties of PAN-based graphite felt electrode by studying the effect of thermal oxidation on the electrode physical and electrochemical properties under various treatment temperatures and durations. By doing so, it can gain an in-depth understanding with respect to treatment temperature and duration on VRFB performance. The research has also successfully carried out first ever in-situ study based on OCV mapping to estimate the degree of electrolyte utilization.
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