Indigo Carmine/TEMPO Combined Bifunctional Molecule for a Total Organic Symmetric Aqueous Redox Flow Battery

Abstract

Redox flow batteries (RFBs) have demonstrated great potentials to assist the renewable energy systems cope with the problem of electricity generation-demand discrepancies[1] . 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 redox active materials[2-4] .Organic RFBs technology has shown great ability of overcoming these challenges and be a good alternative to traditional inorganic based RFBs[5] . Organic molecules are synthetically tuneable allowing design and modification of the molecules to have a combination of all the required properties of a suitable redox active materials for RFBs which is essential towards advancing and developing this competitive energy storage system[6, 7] .In this work, a total organic aqueous redox flow battery that employs indigo carmine/TEMPO combined molecule as a bifunctional redox active material is designed, synthesized and investigated. This design mimics all vanadium RFB chemistry to prevent capacity fade as a result of crossover encountered in RFBs based on two different materials as positive and negative electrolyte. Cyclic voltammetry studies of the combined molecule indicate a reversible redox reaction of the leucoindigo carmine/indigo carmine redox couple at -0.62V and the TEMPO (nitroxide radical/oxoammonium cation) redox couple at 0.52V versus Hg/Hg2SO4, leading to a theoretical cell voltage of 1.14V. The combined molecule demonstrated high diffusion coefficients in comparison to the individual separate molecules as investigated by rotating disc electrode experiments. 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 mAcm-2 as shown in Figure 1.This carbonyl based combined molecule synthesized from inexpensive starting materials eliminates cross contamination issues and facilitates an RFB system that uses cheap anion exchange membrane and a pH neutral supporting electrolyte resulting to a safe and low-cost system.Figure 1: Cycling performance - Charging and discharging capacities and coulombic efficiencies for each cycle in battery cycling of a pumped RFB, 0.01M combined molecule in 0.5 M NaCl/ supporting electrolyte, charge−discharge cycles at current density of 25 mA .