The global energy transition demands large-scale solutions. Lithium Ion Batteries (LIBs), prevalent in our society, dominate energy storage for devices, EVs, and smart grids. In 2019, LIBs accounted for 8.8 GWh of stationary energy storage compared to 0.25 GWh for Redox Flow Batteries (RFBs). However, LIBs suffer from high maintenance costs, safety concerns, and lithium scarcity, prompting the search for efficient alternatives. Large-scale grid storage requires durable, low-cost batteries with good cyclability and round-trip efficiency. Installation and maintenance costs remain obstacles to grid storage adoption. The SET Plan has set clear 2030 targets for stationary energy storage in terms of cost (0.05 €/kWh-cycle) and durability (10,000 cycles per 20-year). RFBs have emerged as promising candidates for sustainable energy generation. Their unique architecture allows power and energy decoupling, offering benefits like flexible modular design, scalability, reasonable maintenance costs, and long lifespan. RFBs comprise three components: energy storage tanks, electrochemical cell stacks, and a flow system. Active species are stored in tanks as an energy-dense solution, pumped into the stack for electrochemical conversion, and returned to the tanks. Stack size determines system power, while tank electrolyte volume indicates total energy. RFBs excel in round-trip efficiency, depth of discharge, responsiveness, and environmental impact (e.g., aqueous RFBs), compensating for lower power and energy density compared to LIBs through cost-effective scalability. [1, 2].

In this study, we detail the synthesis of various viologen derivatives, specifically 4,4'-([4,4'-bipyridine]-1,1'-diium-1,1'-diyl)di/ethanoate/butanoate/pentanoate/heptanoate, with the intent of investigating their suitability as electrolytes in redox-flow batteries. All four distinct viologen compounds were synthesized utilizing identical reaction conditions in N,N´-dimethylformamide, conducted at 100 °C under an inert N₂ atmosphere for a duration of 48 hours. The synthesis process involved the reaction of 4,4'-bipyridine with ethyl 4-bromo/ethanoate/butyrate/pentanoate/heptanoate, followed by a subsequent acidic de-esterification step in HBr solution to yield the corresponding dicarboxylic acid as the final product. It is noteworthy that the reaction yield of the viologen compounds was influenced by various factors encompassing the synthetic routes employed. Detailed results of these synthetic procedures and their impact on the reaction yields will be comprehensively presented in the poster section of the forthcoming conference.