This is the objective of the HIGREEW project coordinated by CIC energiGUNE, which has allowed the integration of an "Aqueous organic redox flow batteries" prototype in a renewable energy plant. Molecular engineering and computational tools are fundamental elements in this quest to probe the vast chemical space comprising these batteries.

In previous articles we have already given a glimpse of how these batteries work and the advantages that their particular design implies. These advantages include the decoupling of power and energy, the widespread use of aqueous solutions as safe non-flammable electrolytes, and the excellent stability and durability of their components, which also contributes to a lower environmental impact.

The sustainability of these batteries was also addressed in another of our articles and is a key aspect in the massive implementation of energy storage systems, as envisaged in the European Commission´s agenda.

Aqueous Organic flow batteries

Beyond the durability of the materials, the impact of the extraction of the materials or the manufacture of the cell components must be considered. With the electrolyte being the most abundant component of the battery and the one with the greatest environmental impact, the focus is on the chemistry of these batteries.

In this sense, it could be said that batteries based on organic materials have the irrefutable argument of the high availability of raw materials; not in vain, these materials consist mainly of Carbon, Hydrogen, Nitrogen and Oxygen.

The design of materials has been based on the knowledge of organic chemistry and nature as a source of inspiration. Thus, compounds of great relevance in the chemistry of living beings, such as alloxazines or quinones, which participate in metabolic and respiration mechanisms, respectively, have been modified for use in batteries.

Molecular engineering has made it possible to introduce structural changes to preserve the activity of these compounds while defining their properties. Stability, redox potential and water solubility are the key aspects that define the efficiency and energy density of flow batteries. Thus, salt formation is a widespread strategy to improve solubility.

Today, families of compounds such as quinones, viologens, pyrazines or nitroxyl radicals are the most explored. Each family of materials has its particularities, and while some materials are suitable for operating at neutral pH, a mild and less harmful medium, other more robust materials allow working in the whole pH range, from acidic to alkaline.

The quinone family is an example of the versatility of these compounds that can operate over the entire pH range and act as both anode and cathode materials. However, these examples are but a notch on the surface of this vast spectrum of possibilities. It is the countless combinations of the above-mentioned bioelements that would give rise to active materials with high performance, i.e. potential ideal materials.

Computational calculations

Even restricting the possibilities considering aspects such as solubility, diffusion of the materials and the complexity of the structures, which mainly translates into limiting the size of the molecules used, the possible resulting combinations exceed hundreds of billions. It has become clear that there is an urgent need to introduce computational calculation tools for the selection of candidates to be studied.

At CIC energiGUNE we have gone further in the use of these tools, typically used for the prediction of intrinsic properties of materials. This first approach helps in the identification of battery performance in terms of energy density. However, durability, a key aspect of these batteries, requires studies aimed at understanding and predicting the behavior of these materials in the long term and in operation.

A study combining computational calculations and experimental tests in collaboration with UAM has allowed us to relate chemical structure and stability. By means of DFT calculations it has been possible to evaluate the performance of the organic compounds both in their loaded and unloaded form, the former being the most critical and less studied. Calculations have been combined with cyclization tests and experimental characterizations for viologen derivatives with different structural motifs. In this case, the study of Mulliken charges and natural bonding orbitals allowed the identification of the most stable viologen among the compounds designed and synthesized. The compound pointed out by prediction presented a four times higher stability.

Viologens and current challenges

Viologens are compounds of special interest since they can be obtained by direct synthesis from commercial precursors in easily scalable processes. It is worth mentioning the interest of these compounds to give rise to high voltage aqueous batteries operating at neutral pH. Aqueous batteries operating with non-harmful, non-flammable electrolytes and based on abundant organic compounds. This is the prospect offered by these batteries.

However, the stability of these organic compounds transcends as the main challenge of the technology, although generalities cannot be established in a concept as broad as that of organic flow batteries. This is where CIC energiGUNE is putting its efforts working on new material designs guided by computational tools.

Recent advances have allowed us to take steps towards stable composites with no obvious loss of capacity and that allow operating with cell voltages higher than 1.2 V. This is a sign that new generations of redox flow batteries are on the way.