Indigo Carmine/TEMPO Combined Bifunctional Molecule for a Total Organic Symmetric Aqueous Redox Flow Battery
Authors: Nambafu Gabriel Sikukuu , Shao Minhua 
Published: May 2020

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 .
References
[1] T. Shibata, T. Kumamoto, Y. Nagaoka, K. Kawase, K. Yano, SEI Tech.Rev. 76 (2013) 14-22.
[2] P. Leung, X. Li, C.P. De León, L. Berlouis, C.J. Low, F.C. Walsh, Rsc Advances. 2 (2012) 10125-10156.
[3] M. Skyllas-Kazacos, M.H. Chakrabarti, S.A. Hajimolana, F.S. Mjalli, M. Saleem, J. Electrochem. Soc. 158 (2011) R55-R79.
[4] A.Z. Weber, M.M. Mench, J.P. Meyers, P.N. Ross, J.T. Gostick, Q. Liu, J. Appl. Electrochem. 41 (2011) 1137.
[5] J. Winsberg, T. Hagemann, T. Janoschka, M.D. Hager, U.S. Schubert, Angewandte Chemie International Edition. 56 (2017) 686-711.
[6] K. Wedege, E. Dražević, D. Konya, A. Bentien, Scientific reports. 6 (2016) 39101.
[7] S. Er, C. Suh, M.P. Marshak, A. Aspuru-Guzik, Chemical science. 6 (2015) 885-893.

Figure 1