New Organic Molecules for Energy Storage Applications

Abstract

A range of structurally variable redox active molecules that are suitable for electrical energy storage were investigated for their potential applications in redox flow batteries. The molecules were selected based on their electron acceptor and/or donor properties, their solubility in aqueous and non-aqueous solvents, their lifetimes in reduced and oxidized states, their formal potentials, their abilities to coexist together in the catholyte and anolyte cell compartments, their resistance to degradation reactions with molecular oxygen and water, their behavior in the presence of acids and bases, and their ease of synthesis. The molecules included a number of redox active quinones and flavins, vitamins (B2, E and K1) and modified forms of the vitamins, aromatic esters, 1,4-phenylenediamines and benzidines. The performance of the molecules were evaluated using a combination of voltammetry, electrolysis under constant potential conditions and in situ spectroelectrochemical analysis (UV-vis and electron paramagnetic resonance). It was found that systems with high redox potentials, although allowing for large voltages (3 – 5 V), suffered from relatively short lifetimes in their charged states due to undesirable reactions with trace impurities including molecular oxygen and trace water (in non-aqueous solutions). Therefore, an optimal potential difference range for the redox couples was found to be between 1 – 2 V, which decreased the likelihood of decomposition reactions of the redox active components and often allowed seemingly indefinite lifetimes. Two systems that provided excellent characteristics were, (i) a modified form of vitamin K1 (as the electron acceptor) and a modified form of vitamin E (as the electron donor), and, (ii) a substituted anthraquinone (as the electron acceptor) and o-tolidine (as the electron donor). Figure 1