Charge Transfer Characteristics of Diaza-Anthraquinones in Different Solvents and Their Application As Organic Active Material in Redox Flow Batteries

Abstract

Redox flow batteries (RFBs), with their independent scalability of power and capacity and their versatility concerning applicable active materials and solvents, pose a promising technology to meet the challenges of cost efficient large scale stationary energy storage. This holds particularly for RFBs based on the redox active material class of quinones that can be derived from naturally abundant resources at low cost. In this work, we electrochemically investigate the modified quinone 2,3-diaza-anthracenedione, and two of its derivatives, as promising active material for aqueous redox flow batteries. Interestingly, these anthraquinone derivatives exhibit varying charge transfer properties depending on their functionalization and the applied solvent and pH value. In general, a positive redox potential shift of about 300 mV is achieved by the incorporation of a diaza moiety into the anthraquinone base structure. Moreover, our experiments at low pH prove that the addition of a methoxy group to the base structure of the 2,3-diaza-anthracenedione strongly increases the electrochemical stability in aqueous acidic media. A functionalization with two hydroxyl groups in close vicinity to the carbonyl group evokes a negative redox potential shift of 54 mV in acidic and 264 mV in alkaline solution. For this study the influence of protonation and hydrogen bonding on the charge transfer characteristics of diaza-anthraquinones was investigated specifically by comparison of measurements taken in non-aqueous solvents and in aqueous solutions at high and low pH. Whereas at low pH values solely one voltammetric anodic and cathodic wave can be observed for the occurring two-electron transfer, peak splitting takes place at high pH values for two of the derivatives, which is highly unusual for quinonoid compounds. Further, measurements in non-aqueous solvents reveal a greater potential difference between the two subsequent electron transfers compared to the pristine anthraquinone. This indicates a discrepancy in charge transfer kinetics evoked by the incorporation of a diaza-moiety into the anthraquinone base structure. Our findings allow for a deeper understanding of the redox characteristics of diaza-functionalized quinonoid compounds that could lead up to efficient active materials for future redox flow batteries with organic active material. Figure 1