PhD Thesis Presentation - Organic Active Materials for Redox Flow Batteries

Redox flow batteries (RFBs) have demonstrated great potentials to store the renewable energies, coping with the problem of electricity generation-demand discrepancies. However, further development of the traditional metal based RFBs have been hindered by various techno-economic challenges, such as expensive redox active materials, low solubility, slow reaction kinetics, hazardous and corrosive electrolyte. On the other hand, organic RFBs technology has shown great ability of overcoming these challenges and be a good alternative to traditional inorganic based RFBs. Organic molecules are synthetically tunable, allowing design of molecules with a combination of all the required properties of suitable redox active materials.

This thesis focuses on advancing and developing a competitive RFB technology through design, synthesis, and modification of organic materials. The designs involve mimicking all vanadium RFB chemistry to engineer bipolar organic molecules for applications in symmetric RFBs. This strategy helps mitigate crossover/cross contamination issues which causes capacity loss in RFBs based on different active materials as positive and negative electrolyte. First, an aqueous RFB that employs indigo carmine/2,2,6,6-Tetramethylpiperidinyl-N-oxyl (TEMPO) bipolar redox active material is designed and investigated. Cyclic voltammetry (CV) studies of the combined molecule indicate a reversible redox reaction of the leucoindigo carmine/indigo carmine redox couple at -0.62 V and the TEMPO (nitroxide radical/oxoammonium cation) redox couple at 0.52 V versus Hg/Hg2SO4, leading to a theoretical cell voltage of 1.14 V. A pumped cell test exhibits a charge/discharge cycle performance of over 70 consecutive cycles with nearly 100% coulombic efficiency at current density of 25 mA cm-2. Secondly, a low-cost riboflavin organic molecule is coupled with TEMPO radical molecule to form a single Riboflavin-TEMPO (RIBOTEMPO) bifunctional electroactive material, which is used in both aqueous anolyte and catholyte. CV analysis of the RIBOTEMPO molecule displays electrochemically reversible reactions for the redox couples at -0.72 V and 0.51 V versus Hg/Hg2SO4, leading to a theoretical cell voltage of 1.23 V. A symmetric cell testing demonstrated over 100 consecutive charge/discharge cycles with nearly 80% coulombic efficiency and capacity retention of 44.7% at a current density of 2.5 mA cm-2. Also, a diimide based molecule is designed, analyzed, and employed as active material in a symmetric total-organic RFB. CV studies show three redox couples, two anodic one electron transfer and one cathodic two electrons transfer redox transitions that are electrochemically reversible. The potential difference between the cathodic peak and the inner and outer a nodic peaks led to a promising cell voltage of up to 1.62 V and 2.22 V respectively. The symmetric battery can be operated at a current density of 20 mA cm-2, and with a coulombic efficiency of 90% for over 100 cycles.