With the increasing utilization of renewable energy, large-scale energy storage technologies are in urgent need. Because of several technical virtues, redox flow batteries (RFBs) have been realized as one of the most promising energy storage technologies for scalable and dispatchable renewable energy storage such as solar and wind. First, benefiting from the unique cell architecture, RFBs are featured with decoupled energy (the volume of the electrolyte reservoirs) and power (electrode surface area), which is favorable for facile power-supplying operation but unrealizable for static rechargeable batteries. Second, RFBs can operate at high current (> 50 mA/cm²) and high-power densities (in the order of 102 mW/cm²) because of fast electrochemical kinetics and high conductivities of supporting electrolytes. Third, aqueous RFBs are safe energy storage technologies by using non-flammable aqueous electrolytes. Recently, aqueous organic redox flow batteries (AORFBs) have experienced rapid research development under acidic, pH neutral, and alkaline conditions employing redox active molecules including viologen, ferrocene, quinone, TEMPO, and pyrazine derivatives [1, 2].

Here we report the synthesis of viologen (4,4'-([4,4'-bipyridine]-1,1'-diium-1,1'-diyl)dibutanoate), which will be studied as an electrolyte in redox-flow batteries. Since it is necessary to prepare an organic electrolyte in the highest possible purity and quantity during commercialization with minimal costs associated with energy and chemicals, the synthesis reaction was studied under different synthetic conditions. The preparation of viologen consists of 4,4'-bipyridine with ethyl 4-bromobutyrate reaction at elevated temperature, in an inert atmosphere, and subsequent acidic de-esterification to form a dicarboxylic acid as the final product. The first mentioned synthetic step was studied in detail with the aim of minimizing the production costs with maximum reaction yield. From the energy cost minimization point of view, the reaction was carried out at different temperatures (100, 80, 60, and 40 °C), and reaction times (48, 24, 12, 6, and 3h), or using microwave synthesis. In order to reduce the finances associated with the purchase of chemicals, the reaction was carried out at different molar ratios (3: 1, 2.5: 1, and 2:1), atmosphere (argon, nitrogen, air), or the use of solvents (DMF, DMSO, MEG or solvent-free). All mentioned factors influenced the resulting reaction yield, and the achieved results will be presented in detail at the conference.