In consequence of the increasing relevance of intermittent sustainable energy sources such as solar and wind energy, there is a growing demand for highly efficient and at the same time economic stationary storage concepts. In this context, redox flow batteries (RFBs) gained a lot of scientific interest in recent years. This especially applies to aqueous RFBs based on aromatic organic active materials such as quinones, which are characterized by a high natural abundance and favorable charge transfer characteristics. However, current concepts are restricted by severe drawbacks such as fast capacity fading due to active material degradation as well as insufficient cell voltages and voltage efficiencies. These issues primarily originate from a lack of suitable high-potential active materials, which are commonly prone to nucleophilic side reactions as a function of their redox potential. As current state-of-the-art active materials are not capable of providing high redox potentials together with stable cycling performances, new structural motifs are required. In this context, this work provides novel design strategies for the development of capable quinonoid active materials towards efficient concepts of stationary energy storage. This thesis presents a systematic investigation of the structure-property relationships of nitrogen-functionalized quinone compounds. The class of diaza-quinones was thoroughly investigated according to different analytical techniques, in order to evaluate the benefit of this structural modification and to expand upon the existing knowledge of their homocyclic base structures. Performance-relevant key quantities such as diffusion coefficients, redox potentials and kinetic rate constants were determined by means of varying voltammetric techniques. The electrochemical stability of the compounds was investigated according to a combination of exhaustive electrolysis and a subsequent structural elucidation based on 1H NMR spectroscopy. Based on the gained results, vulnerable sites could be identified thus shedding light on the mechanism of potential degradation reactions. Experiments were conducted in varying solvents, comprising aprotic as well as acidic and alkaline aqueous solution, in order to elucidate the impact of solvent-related effects such as hydrogen bonding and protonation on the charge transfer behavior of diaza-quinones. With these results, the structural motif of diaza-quinones was refined to obtain capable catholyte active materials for aqueous RFBs based on organic active materials (ORFBs). In addition, the experimental approach was complemented by theoretical considerations: experimental trends were successfully reconstructed by means of density functional theory (DFT) calculations. Based on the developed model, desirable diaza-quinone structures were defined to propel future research in this field. Conclusively, this work provides useful guidance regarding the development of stable and capable quinonoid active materials for ORFBs and hence can contribute to meet the requirements for a commercial application of this promising technology.
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