Redox flow batteries (RFB) are a well-established and elegant means of storing and delivering electricity. Yet, despite decades of research the front-runner in technology remains the all-vanadium RFB. The four stable oxidation states and relatively high concentrations in acidic media, have left a difficult act to follow in flow battery development. Yet, as we scale up and deploy flow batteries further, the all vanadium system becomes increasingly unsustainable. It’s still a costly solution to energy storage, energy density is low and bigger capacity means much bigger footprint.
Redox active species with properties ideal for RFBs may exist in the vast range of options proffered by metal organic and organic molecules. Ascertaining the best candidates is not a fast process however, as a change in redox species requires considerable amount of study of each element of the whole system, including identifying suitable membranes and electrolyte, solubility properties, stability in the long term, to name but a few. This necessitates researchers embarking on this path to be Jacks-of-all-trades in order to master what is considered a single research problem. Furthermore, mass pre-emptive patenting of “chemistry” in the context of flow batteries undoubtedly deters researchers, business, and funding bodies from properly investing in routine yet necessary studies of possible alternative redox complexes.
At Lancaster University we are striving in the direction of new chemistries for flow batteries. We are looking to new metal organic complexes to offer multiple oxidation state species, high solubility, low cost membrane options, and long-term stability and sustainability, primarily with the objective of being a lower cost and commercially viable storage system. Here recent studies of cobalt complexes with a range of variants of the terpyridine ligand are presented. The complexes are limited to organic electrolytes, but our initial results regarding membrane materials, electrode materials and long term cycling of the 4-oxidation state species are discussed.
