Large-scale energy storage technology such as vanadium redox flow batteries (VRFBs), offering a well-established ability to improve grid reliability and utilization with inherent safety, has been attracted increasing attention over the past decades. Although appealing, this technology has yet to meet the stringent cost requirements for widespread commercialization. A major reason responsible for the high capital cost is its relatively low-power density power pack, which is mainly induced by the use of thick graphite felt electrodes in conventionally flow-through structured VRFBs. Recently, a new cell architecture with a zero-gap flow-by mode has shown significant performance improvements, but its power density is still limited by conventional electrodes. The electrodes fitting...[ Read more ]

Large-scale energy storage technology such as vanadium redox flow batteries (VRFBs), offering a well-established ability to improve grid reliability and utilization with inherent safety, has been attracted increasing attention over the past decades. Although appealing, this technology has yet to meet the stringent cost requirements for widespread commercialization. A major reason responsible for the high capital cost is its relatively low-power density power pack, which is mainly induced by the use of thick graphite felt electrodes in conventionally flow-through structured VRFBs. Recently, a new cell architecture with a zero-gap flow-by mode has shown significant performance improvements, but its power density is still limited by conventional electrodes. The electrodes fitting the flow-by architecture need to be much thinner than conventional ones, but thin electrodes will render insufficient active areas for vanadium redox reactions. The primary objective of this thesis is to design, fabricate and characterize high-performance electrodes with high specific area and catalytic activity, fitting the need of the flow cell architecture with the zero-gap flow-by mode.

This work begins with the design, fabrication, and test of a carbon nanoparticle-decorated thin graphite felt electrode. The electrode is prepared by uniformly depositing 2 mg cm^-2 acid-treated carbon nanoparticles on a 1.5 mm graphite felt and then equipped with a zero-gap flow-by battery. The battery, possessing a significantly reduced ohmic loss through reducing electrode thickness, an improved electrocatalytic activity by coating carbon nanoparticles, and an enhanced mass transport of active species via the addition of the serpentine flow field, demonstrates 71% of power density increase compared to the conventional battery.

To further enhance the battery performance, three kinds of nanostructured metal-based electrocatalysts decorated electrodes are proposed to catalyze the vanadium redox reactions for the first time. Firstly, 2 mg cm^-2 of titanium carbide nanoparticles are deposited onto the surface of the carbon paper electrode via an immersion method. The electrode exhibits high catalytic activity towards V³⁺/V²⁺ redox reaction in acid solution and enables the oxidation peak potential to be negatively shifted by 100 and 183 mV relative to those of pure carbon nanoparticles and pristine carbon paper, respectively. Next, a highly catalytic, stabilized and binder-free titanium nitride (TiN) nanowire array-decorated graphite felt electrode is prepared for VRFBs via a two-step process. TiO₂ nanowires are grown on the graphite felt using a hydrothermal method and then converted to TiN nanowires under a NH₃ atmosphere. Moreover, a tiny amount of Cu salts (0.005 M) is introduced into the electrolyte, and Cu nanoparticles are synchronously electrodeposited onto the electrode surface during charge process without any pretreatment procedures or binder. It is confirmed that Cu nanoparticles facilitate the electrochemical activity and reversibility of V³⁺/V²⁺ redox reaction. The battery equipped with this electrode enables the electrolyte utilization and energy efficiency to be 83.7% and 80.1% at 300 mA cm^-2, which are 53.1% and 17.8% higher than those of battery assembled with a pristine electrode. Most impressively, batteries equipped with the aforementioned electrocatalysts decorated electrodes exhibit excellent stability and high capacity retention rate during the long-term cycle tests, indicating that these high-performance electrodes without tedious preparation process offer great promises for VRFB applications.

All of these above-mentioned methods are imperative and can effectively decrease the stack cost to some extent. On the other hand, it is well known that hydrogen evolution reaction (HER) is easy to occur under high current density operation. Under these circumstances, a three-electrode electrochemical cell and transparent VRFB are conceived and fabricated to in-situ investigate the hydrogen evolution reaction during battery operation. It is found that HER is sensitive to temperature variation and occurs at the early of the charge process, suggesting that attention to the hydrogen formation at the negative electrode in the early charge process should also be paid to during long-term battery operations.

Keywords: Vanadium redox flow battery; electrodes; power density; catalysts; costs; hydrogen evolution reaction