Electrochemical storage of energy is a valuable asset for the integration of intermittent renewable energy sources such as wind and solar power. Redox flow batteries (RFBs) are the only type of battery in which the energy content and the power output can be scaled independently, offering flexibility for applications from frequency regulation and load levelling. The energy in a RFB is stored in dissolved redox shuttles of its electrolytes which offers many intrinsic advantages for grid scale energy storage: (1) Independent scalability of energy and power content because the volume of the tanks and the size of the electrodes can be chosen independently. (2) Degradation of RFBs is be limited because the electrodes do not undergo conversion or intercalation reactions, instead they only serve as electron sink or reservoir. (3) To a certain degree RFBs are chemistry-agnostic, one battery cell can be run with various redox electrochemistries.

However, while the advantages of the flow battery concept are undisputed, there is no agreement on the best (electro-)chemistry yet. The champion of the metal chemistries is the all-vanadium redox flow battery (VRFB)¹. Utilizing four oxidation states of vanadium (V²⁺,V³⁺,VO²⁺,VO₂⁺) this cell chemistry has the main advantage that cross-over of species from one half-cell through the separator into the other half-cell does not lead to a chemical contamination and can be rebalanced electrochemically. The main drawbacks of the VRFB are the sluggish kinetics of the V²⁺/V³⁺ and the VO²⁺/VO₂⁺ redox reactions which limit the current density and therefore the power density². Organic redox couples can be low cost and made from abundant elements, and they offer greater variability than metallic redox couples due to their tuneable structure³. A great number of organic redox couples were presented in recent years, with capital cost of metallic RFB chemistries being the main driver for their development. As most studies have been restricted to laboratory cell operation, insights into scale-up with larger cell areas and bigger electrolyte volumes and long-term cycling are currently not available³.

We have proposed a new class of redox electrolyte which combines the tuneability of organic molecules with the stability of metal ions: Polyoxometalates (POMs) functioning as nanostructured electron carriers 4. Employing electrochemistry and in-situ ⁵¹V NMR, we show that POMs exhibit four main advantages for the use as electrolyte in a RFB: (1) POMs do not permeate cation exchange membranes because they are large anions. This prevents cross-over and thereby self-discharge and capacity fade. (2) POMs exhibit facile redox kinetics with electron transfer constants four orders of magnitude faster than V²⁺/V³⁺ and VO²⁺/VO₂⁺. This can enable high current densities. (3) POMs undergo multi-electron redox reactions which increases the capacity per molecule. (4) The investigated POMs are soluble and stable. The catholyte species even spontaneously reassembles when destroyed by adverse solvent conditions.
In flow battery studies the theoretical capacity (10.7 Ah L^-1) could be achieved under operating conditions. The cell showed a capacity fade of 0.16% per cycle when the cell was cycled for 14 days with current densities from 30 to 60mA cm^-2. Avenues to improve the redox electrochemistry of the POMs and the battery will be discussed.

References
(1) Rychcik, M.; Skyllas-Kazacos, S. J. Power Sources 1987, 19, 45–54.
(2) Friedl, J.; Stimming, U. Electrochim. Acta 2017, 227, 235–245.
(3) Leung, P.; Shah, A. A.; Sanz, L.; Flox, C.; Morante, J. R.; Xu, Q.; Mohamed, M. R.; Ponce de León, C.; Walsh, F. C. J. Power Sources 2017, 360, 243–283.
(4) Friedl, J.; Bauer, C.; Al-Oweini, R.; Yu, D.; Kortz, U.; Hoster, H.; Stimming, U. In 222nd MEeting of the ECS; 2012.