Thanks to the potential size of the market for electric vehicles, battery research in recent years has tended to focus on innovations in lithium-ion and other related chemistries that promise to serve this market.
For the large-scale energy storage that is increasingly required to balance the intermittency of wind and solar energy, however, redox-flow batteries present an attractive opportunity. The batteries are inherently scalable, and avoid many of the issues with long-term performance, safety, and material availability associated with lithium-ion batteries. Many of the early commercial projects for redox flow batteries (RFBs) rely on vanadium, which comes with some toxicity concerns. There is, however, a long list of materials worth investigating as flow battery components, including abundant organic materials.
Among other issues, organic redox flow batteries are held back by low specific capacity. Finding new materials (and combinations of materials) with better characteristics is the simplest way to overcome this challenge, and has been the focus for a group of scientists led by Russian’s Skolkovo Institute of Science and Technology (Skoltech). “We are working with organic redox-active materials solubilized in organic solvents (non-aqueous organic RFBs),” says Skoltech Ph.D. student Elena Romadina. “The main advantages for non-aqueous organic RFB are high cell voltage (up to 5V, versus around 1.6 V for water-based systems), a huge variety of organic redox-active molecules which could be applied, and potential operability at low temperatures, without any concern for freezing below 0 degrees C,” she stated
Romadina is the lead author of two new papers exploring new organic materials for RFBs, published in the Journal of Materials Chemistry A and in Chemical Communications. The first evaluated a series of seven promising catholyte materials, and the second describes the synthesis of a phenazine-based anolyte material.
Combining the two resulted in a flow battery that achieved a high cell voltage of 2.3 V and better than 95% coulombic efficiency, as well as high capacity and good stability over 50 cycles. This battery is described further in the Chemical Communications paper.
The group notes that the poly-triarylamine materials it demonstrated as catholytes have previously shown promise as cathodes in metal-ion batteries, but had not previously been investigated in flow battery chemistries. “A new and very promising core structure was opened up for us and other scientists. Triarylamines have a stable and fully reversible redox potential, and could be easily modified, providing different redox potentials and physical properties,” explains Romadina. “Moreover, we found that triarylamines-based compounds could retain their electrochemical properties even in the presence of water in organic solvent, which lowered the requirements for solvent preparation and cost.”
And while these are promising developments, Skoltech notes that more work is still needed in other areas for organic RFBs to achieve commercial interest. “To make organic RFBs commercially viable, we also need research in areas such as low-cost scalable synthesis of highly soluble redox-active molecules; the development of high-performance membranes that are good ionic conductors, but inhibit cross-over of anolytes and catholytes upon charge and discharge; and the scaling of larger cell and stack level device configurations to enable grid scale energy storage,” noted Skoltech Professor Keith Stevenson.
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