Performance and solubility studies of an iron-based complex for redox flow batteries

Although renewable energy, such as wind or solar panels, is advancing very fast nowadays, energy storage is still an unsolved problem to making renewable energy economically competitive with fossil fuel-based energy. Redox flow batteries (RFBs), which are emerging electrochemical technologies for large scale energy storage, are a promising solution. They have a simple design, high energy efficiency and are eco-friendly but are too expensive with currently used materials (typically vanadium). This research aims to synthesize and investigate the viability of inexpensive iron-based compounds for RFBs, in order to improve their output voltage, energy density, and lifetime. A major challenge with RFBs in general and Iron (Fe)-complexes specifically is achieving high solubility of the active compound to give a battery with high energy density. This work explores options such as additives and alternative supporting electrolytes to increase the aqueous solubility of Tris(bipyridyl) Iron complexes (Fe(bpy)3SO4) and related active RFB compounds. The solubility was studied based on the absorbance of Iron complexes solution combined with different additives. The results show that Isopropyl alcohol (IPA) can increase the solubility of Fe(bpy)3SO4 in RFB electrolytes from ~ 0.1M to 0.35M. Multiple instruments will be used to investigate the iron-based compound’s electrochemical characteristics and suitability for RFBs (including effects of the additive). The Rotating Disk Electrode will be used to study the iron-based compound’s redox behavior and kinetics study, such as: redox potentials via cyclic voltammetry, linear sweep voltammetry, diffusion coefficient, electrons transfer rate constants, etc. In addition, the compound’s battery performance in full, lab-scale redox flow batteries will be tested with the novel high – concentration solution of the iron compound to determine its capacity and check for degradation. Moreover, the side reaction of Fe(bpy)3SO4, which is dimerization that sacrifices battery voltage, also was studied in this project. It is important to minimize the dimerization in order to avoid voltage efficiency loss during cycling. Ultimately, a new RFB composed of 2,7- anthraquinone disulfonic acid disodium salt anolyte and the solubility-enhanced iron – complex catholyte was constructed and put through a variety of operational tests including long-term performance for over 200 cycles. This new battery design operates under mild conditions with all earth-abundant elements and is a promising candidate for grid-scale energy storage.