Fabrication and Modification of Carbon Electrode Materials for Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) are amongst the most promising energy storage systems for storing renewable energies on a large scale. However, the system still has some challenges which limit its widespread application in industry. For instance, the unsatisfactory activity together with insufficient long-term stability of the state-of-the-art commercial activated carbon electrode (i.e., carbon felts) are the most known issues referenced in literature. This thesis introduces two novel solutions for the aforementioned problems: (i) fabrication of non-woven carbon nanofiber networks as alternative, efficient and stable electrodes and (ii) improving the performance of the commercial carbon felt electrodes via coating their surfaces with metal oxide nanostructures. Additionally, the hydrogen evolution reaction, a parasitic side reaction, which is believed to have detrimental impact on the performance of VRFBs, will be investigated at different operating temperatures. Non-woven polyacrylonitrile-based carbon nanofiber networks with a very high electrochemically active surface area were first produced by the scalable electrospinning approach and then directly used as an electrode in a vanadium redox flow battery. Using five sheets of polyacrylonitrile-based electrospun nanofibers achieved ~10% increase in the energy efficiency compared to the commercial carbon felt at current densities of 15 mAcm-2. Furthermore, low-cost highly active carbon-carbon composite freestanding nanofibers were produced by electrospinning a mixture of polyacrylonitrile and carbon black powder using poly acrylic acid (PAA) as binder. PAA enables the loading with higher amounts of the relatively cheap carbon black material. This results in an increase of the productivity of the electrospun carbon nanofibers at lower cost together with simultaneously enhancing the performance of the battery. Battery test results demonstrated a promising performance for the newly designed electrospun carbon fibers as negative and positive electrodes of vanadium redox flow batteries at current densities below 60 mAcm-2. The damaging role of the parasitic hydrogen evolution reaction (HER) in the negative half-cell of the vanadium redox flow battery on the performance of the commercial carbon felt electrodes was studied at different temperatures. Increasing the temperature resulted in a better catalytic performance for both the positive and negative half-cell reactions. Nevertheless, higher temperature significantly enhanced also the undesired HER at the negative side. This led to a decrease in the coulombic efficiency attributed to the higher amount of generated hydrogen causing faster fading of the overall VRFBs performance. Due II to rapid degradation of the commercial carbon felts resulting from the corrosion of their fibers. To minimize the degradation of the commercial carbon felt electrodes and therefore extend the lifetime of the battery, neodymium oxide (Nd2O3) nanoparticles were chemically deposited onto the fibers of the carbon felt by a precipitation method in non-aqueous solution. Nd2O3 modified carbon felts showed 4 times higher stability compared to the unmodified thermally activated carbon felt after 50 consecutive charge/discharge cycles. Moreover, Nd2O3 modified felts retained their original performance after exchanging the electrolyte, indicating less degradation occurred. Additionally, they could maintain their oxygen donating functionalities compared to the thermally activated commercial carbon felt.