Aqueous redox flow batteries: How ‘green’ are the redox active materials?

Energy storage using aqueous organic redox flow batteries (ORFBs) is gaining momentum in recent years parallel with the renewable energy industry, principally to store energy from solar, wind and hydro generation. The safety and environmental effects of active materials utilised in aqueous ORFB systems are vital to stay true to the ultimate aim of producing a 'green' technology. In this review, we evaluate the 'greenness' of the synthetic methodology employed to prepare the anolyte or catholyte active materials which have been developed for aqueous ORBFs using the 12 principles of green chemistry. Some of the most common synthetic strategies that will be discussed involve acid catalysis, catalyst free procedures, and oxidative polymerisation. Additionally, the 'greenness' of several key commercial materials with limited synthetic procedures published will be considered.