Quest for Organic Active Materials for Redox Flow Batteries: 2,3-Diaza-anthraquinones and Their Electrochemical Properties

To be competitive with other electrically rechargeable large scale energy storage, the range of active materials for redox flow batteries is currently expanded by organic compounds—this holds especially for the redox active material class of quinones that can be derived from naturally abundant resources at low cost. Here we propose the modified quinone 2,3-diaza-anthracenedione, and two of its derivatives, as a promising active material for aqueous redox flow batteries. We systematically study the electrochemical performance (redox potentials, rate constants, diffusion coefficients) for these three compounds at different pH values experimentally and complement the results with density functional calculations: A positive redox potential shift of about 300 mV is achieved by the incorporation of a diaza moiety into the anthraquinone base structure. Our experiments at low pH show 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—although the impact of the conjugate base is not clear yet. Furthermore, a functionalization with two hydroxyl groups evokes a negative redox potential shift of 54 mV in acidic and 264 mV in alkaline solution. This demonstrates that this novel class of compounds is very versatile and can be tailor-made for use as active material in redox flow batteries—either in alkaline or acidic media. The 2,3-diaza-anthracenediones presented in this study were used as anolyte active materials in a full redox flow cell as a proof of concept; best cycling stability was achieved with 2,3-diaza-anthracenediones functionalized with a methoxy group as active material. Transferring our findings to other quinone base structures, such as naphthoquinones, could lead to even better performing catholyte and anolyte active materials for future redox flow batteries with organic active material.