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Aqueous redox flow batteries for sustainable storage of intermittent energy – Energy-4S

Aqueous batteries for sustainable storage of intermittent energy

Energy transition supported by renewable energies will succeed thanks to the emergence of storage solutions. In this context, redox flow batteries can play an important role. One of their advantages is that the active material made up of organic redox molecules is dissolved in an electrolyte solution. Depending on the volume of the tanks, this technology becomes relevant to achieve an electrical capacity ranging from kilowatt hours to Megawatt hours.

Solubilization of organic redox molecules

Flow redox batteries developed by the start-up Kemiwatt do not contain any noble or toxic metals. Only metal cations from iron are present in the electrolytic solutions for the operation of the battery. This technology has very little impact on mining resources, which gives them a prominent place in supporting renewable energies. The operation of redox flow batteries is based on an essential feature which is the decoupling between power and electrical capacity. The power depends on the nature of the redox couples and the electrical capacity is linked to their concentration and to the volume of the reservoir in which the redox molecules are dissolved. Given this fact, to position themselves in the energy storage market, redox batteries must achieve a minimum energy value of around 10 Wh.L-1. This order of magnitude is obtained for an exchange of one mole of electron per liter of solution. However, the redox molecules studied belong to the anthraquinone family, which exchange two electrons. In conclusion, the objective is to achieve an anthraquinone solubility of 0.5 mol.L-1. Solubility of organic molecules is difficult, and more particularly in solutions highly charged with ions, the purpose of which is to ensure excellent ionic conductivity. To increase the solubility, the main strategy is to chemically modify a commercially available anthraquinone by adding to it one or more solubilizing functions (-COOH, -OH, -SO3H) which can be easily ionized in a basic medium. But this strategy is costly and often alters the electrochemical behavior of the molecule. The strategy proposed in this project is to study the solubility of a poorly soluble anthraquinone by taking into account its ionic environment by adding a solubilizing adjuvant to the molecule.

Anthraquinones studied in the Energy_4S project carry a variety of acid functions (carboxylic, sulfonic, phenolic). Depending on the pH value, the degree of ionization of anthraquinones varies and modifies the solubility of the molecule. To evaluate the behavior of each anthraquinone in an aqueous solution, a predictive model will be developed to model the potential vs pH diagrams (Pourbaix diagram) of each molecule. The accuracy of the predictions will be refined based on experimental results from the literature or obtained as part of this ANR (electrochemical analysis, experimental measurement of accessible pKa). Based on these models, two solubilization strategies are considered. The first is to study the influence of the nature of the cations surrounding the anthraquinone. The solubility of an ionized anionic species such as an anthraquinone is related to Coulombic interaction between charges (anions / cations), which weaken with the use of large cations (low charge density). Taking this theoretical fact into account, the use of large alkaline cations (example of cesium in an academic approach) or the use of cations as secondary tertiary or quaternary ammonium should increase the solubility of anthraquinones in ionizing (basic) media. The second strategy is to use highly soluble adjuvants capable of interacting specifically with anthraquinone. By relying on a fine-grained modeling approach (COSMO-RS) and data from the literature, the objective will be to define the molecular structure most compatible with anthraquinones.

Energy_4S project started with the establishment of a predective model of the potential vs pH diagrams. This model is built by comparison with known values of acidity constants (Ka) and standard potentials (E °). At the end of this work, the proposed model predicts with excellent precision the standard potential and the pKa belonging to an unreferenced anthraquinone. At the midpoint of the project, substantial work highlighted the interactions between anthraquinone and the cations surrounding it. In a basic environment (use in battery mode) anthraquinones carry several negative charges. Anthraquinone / cation interactions engage Coulombic interaction which, depending on the nature of the cations, lead to the formation of ion pairs and limit solubilization. To weaken these interactions, the idea is to use bulky cations in order to decrease their charge density. This approach is effective. For example, 1,2-dihydroxy-anthraquinone, whose solubility is of the order of 0.1 mol.L-1 in a K + environment, reaches a solubility of 1 mol.L-1 in a tetramethylammonium environment. In parallel to this work, experimental studies have shown a dependence of the mobility of ammonium ions with the structure of the cationic membrane (Nafion, Fumatech). Following these results, laboratory tests of these concentrated solutions in battery mode began. A second solubilization strategy supported by modeling work (COSMO-RS) highlights the use of soluble adjuvant capable of interacting with anthraquinones. The first results guide the choice towards polarized adjuvants caused by the presence of a hydrophobic group located on the molecular structure opposite to a hydrophilic group. Experiments with this adjuvant family will be evaluated over the remaining eighteen months.

Two solubilization strategies are developed in the project. The first strategy uses large cations to exacerbate the solubility of anthraquinones. Two routes are preferably explored. The first way (to be finalized) consists in using quaternary ammoniums obtained from a strong base (eg: tetramethyl hydroxide). The pH of the electrolyte solution will be adjusted naturally at a value of around 13 to 14 depending on the amount of strong base added. The second way (to be developed) will consist in using secondary or tertiary ammoniums and using them in a secondary role as a buffer. For example an R-NH3+ / R-NH2 mixture will buffer the solution to pH range 9 to 10 depending on the nature of the R group (methyl, ethyl ..). This mixture will enhance the solubility of anthraquinone while imposing a less basic environment. Regarding these two routes, special attention will be paid to the evolution of solubility when exacerbating cations (ammonium, Cs+ ..) are mixed with conventional, non-exacerbating alkaline cations (K+, Na+ and Li+). The second strategy based on the use of adjuvants started showing some positive results. A first approach will consist in modeling the operating mode of adjuvants. The solubilization can be obtained thanks to an adjuvant / anthraquinone interaction which can be carried out by pi-stacking or hydrogen bonds or interactions between positive and negative charge. A second path will consist in studying the formation of oligomers corresponding to the assembly of the «adjuvant« unit. Ideally this route would result in the formation of a crown around the anthraquinone. These two pathways are supported by modeling work (COSMO-RS) and are also based on experimental data on the solubilization of organic molecules / organic solvent.

The results concerning the increase in the solubilization of anthraquinones depending on the nature of the surrounding cations were very positive. An efficient methodology has been developed to apply these results to redox batteries. All of this work bringing together the three parts (ISCR / UCCS / Kemiwatt) was the subject of a first invention declaration intended to be patented. The proposed patent blocks the communication of the results concerning the methods of solubilization of anthraquinones. Nevertheless, concerning the modeling work, the following communications could be made:
4. JTMS 2020 : Journées « Théorie, modification et Simulation », T.Gaudin (poster)
5. Current Trends in Electrochemistry, Paris, T.Gaudin (2 posters)
6. Atelier redox flow, T. Gaudin (communication orale / visio)

In 2018, renewable energies overcame fossil fuels in terms of electrical capacity investments for the 9th consecutive year. This raise of the renewable energy production capacity naturally leads to the question of the energy storage. The Energy-4S project (Safety, Sustainability, Solubility, Storage) concerns the sustainable storage of intermittent energy in aqueous organic redox flow batteries. In this active research field, the project combines original fundamental and applied aspects for the development of such particularly promising batteries. The strategy is based on the judicious choice of hydrotropes and electrolytes in order to significantly increase the solubility of poorly water-soluble but highly electro-reversible quinones, while lowering the pH, increasing the sustainability of the systems and minimizing the costs of the compounds. Energy-4S involves two highly complementary teams, one specialized in electrochemistry (ISCR - MaCSE - University of Rennes), the other one in physico-chemistry of formulation (UCCS - CISCO- University of Lille) as well as the company KEMIWATT (A french start-up), which will carry out the tests on a larger scale to validate the industrial viability of the new systems developed within the frame of the Energy-4S project. Thus, this project will allow scientists of different scientific fields to work together to solve a challenging societal issue for energy storage.

Project coordination

DIDIER FLONER (INSTITUT DES SCIENCES CHIMIQUES DE RENNES)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

KEMIWATT
ISCR INSTITUT DES SCIENCES CHIMIQUES DE RENNES
UCCS Unité de Catalyse et de Chimie du Solide

Help of the ANR 549,439 euros
Beginning and duration of the scientific project: January 2020 - 36 Months

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