Molecular Design for Organic Redox Flow Batteries
Authors: Pakarinen Darius Aryan , Lundberg Helena , Lindstrom Rakel , Khataee Amirreza 
Published: February 2024

Redox flow batteries have in recent years become an increasingly researched alternative to lithium-ion batteries for stationary energy storage, stabilizing fluctuations between demand and the intermittent electricity production from solar and wind. The most common vanadium redox flow battery technology suffers from high market volatility for vanadium metal. As a potentially more sustainable alternative to the electrolytes based on metal-ions used in e.g. the vanadium RFB, organic redox active species are being developed to store charge. The organic electrolytes used in aqueous organic redox flow batteries (AORFBs) are reaching closer to economic viability in terms of stability and electrochemical properties but is still inferior to its vanadium counterpart in long-term performance.[1]
The degrees of freedom when designing organic electrolytes are endless. However, to improve strategies to designing new electrolytes more systematic studies on the working principles behind what makes organic compounds accept and store charge has to be made. Using less electronegative organo-metallic compounds, radical stabilization and multiple electron reactions, are some of these strategies that can be utilized. Figure 1 shows how examples of current parent molecules with functional groups modifications to tailor stability, solubility and electrochemical properties and in various pH regions.
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Molecular design can be divided into three important factors that will impact the performance of the electrolyte; electrochemical properties, solubility and stability. The electrochemical properties of the redox active species are essential for the application towards RFBs, regarding potential, electrochemical reversibility, coulombic- and voltaic efficiencies. The second criteria to maintain a reasonable operating energy density of the RFB is to optimise the water solubility, being proportional to capacity and energy density. The organic compounds solubility is highly pH dependent, thus pairings of posolyte and negolyte will need to operate within the same pH range. By substituting different functional groups to the parent molecule, the operating pH of the species can be tailored. Electrolyte conductivity is also an important factor when altering pH or co-electrolytes in terms of efficiency. Stability or capacity decay is the main focal point for AORFBs and limitation of the redox active species. These decay mechanisms can be further categorized into three different types of degradation, all which result in loss of active electrolyte. First, radical-induced reactions given rise to disproportionation or dimerization reactions, second, pH dependent chemical reactions such as hydrolysis, addition, substitution and elimination reactions and third, physical reactions dependent on molecular size and charge, that can supress or promote crossover through the membrane.
As of designing structurally novel molecules, the electrochemical properties are the first property that must be assessed followed by solubility and stability. Since there are already some well explored compounds such as anthraquinones and viologens, much research has been put into increasing the solubility and stability for these.[2] The poster aims to highlight the interconnection between the parameters that make these compounds viable for use in AORFBs. Issues such as what has been explored in terms of posolytes, negolytes, alkaline, neutral and acidic electrolytes and what important components are missing for development of AORFB will be discussed.
[1] J. Luo, A. P. Wang, M. Hu, and T. L. Liu, “Materials challenges of aqueous redox flow batteries,” MRS Energy and Sustainability, 9, 1–12, 2022, doi: 10.1557/s43581-022-00023-1.
[2] Z. Li, T. Jiang, M. Ali, C. Wu, and W. Chen, “Recent Progress in Organic Species for Redox Flow Batteries,” Energy Storage Materials, 50, 105–138, 2022. doi: 10.1016/j.ensm.2022.04.038.

Figure 1