A redox flow battery using two polyoxometalate electrolytes for anolyte and catholyte is described and investigated.

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.

While redox flow batteries carry a large potential for electricity storage, specifically for regenerative energies, the current technology-prone system—the all-vanadium redox flow battery—exhibits two major disadvantages: low energy and low power densities. Polyoxometalates have the potential to mitigate both effects. In this publication, the operation of a polyoxometalate redox flow battery was demonstrated for the polyoxoanions [SiW12O40]4– (SiW12) in the anolyte and [PV14O42]9– (PV14) in the catholyte. Emphasis was laid on comparing to which extent an upscale from 25 to 1400 cm2 membrane area may impede efficiency and operational parameters. Results demonstrated that the operation of the large cell for close to 3 months did not diminish operation and the stability of polyoxometalates was unaltered.

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.

Lithium-Ion Batteries (LIBs) are widely discussed and investigated as versatile energy storage systems. However, LIBs may pose safety risks, such as flammability of the electrodes and the electrolytes, which is a problem that may be amplified when transitioning from small-scale to medium- or large-scale energy storage. Yet, large-scale low-cost energy storage is crucial for the transition from a fossil fuel-based energy economy to renewable energy sources and in order to utilise the already existing energy sources like wind and solar power more efficiently. A promising technology for this task are Redox-Flow-Batteries (RFBs). The RFB is the only type of battery where power output and energy capacity can be scaled independently, allowing it to be specifically tailored to a variety of applications. However, the most mature RFB so far, the All-Vanadium RFB, suffers from low energy density and low power density. In order to overcome these challenges while maintaining the advantages of an aqueous RFB, like non-flammability and minimal self-discharge, we are investigating new redox chemistries. As redox systems, polyoxometalates (POMs) and specifically [SiW12O40]4- and [PV14O42]9- as nano-sized electron shuttles were investigated.1 These POMs exhibit fast redox kinetics (electron transfer constant k 0 ≈ 10-2 cm s-1 for [SiW12O40]4-) which together with their high solubility in water and multiple redox-centres per molecule provides the potential for for high power densities and high energy densities. The POMs that were used also exhibit high electrochemical and chemical stability, thus providing long cycle lifetimes. Other POMs were investigated as well. The system was scaled up from a lab-sized cell of 25 cm2 membrane area to a cell of 1400 cm2 in order to assess the implications on efficiency and operational parameters.2 The cell was operated for 1400 cycles over a time of nearly three months, providing some very promising results; the Coulombic efficiency was nearly 100% with the energy efficiency dropping only from 86.1% to 85.1% during the whole period, indicating a highly stable system. The observed capacity loss of 0.011% per cycle could be attributed to ambient air leakage leading to oxidation of SiW12. Post-cycling analysis showed no sign of degradation of the electrolytes. Acknowledgement We acknowledge funding and the fruitful cooperation with SIEMENS AG. Also, funding from NECEM, the North East Centre for Energy Materials (EP/R021503/10) is thankfully acknowledged.