There is an increasing need to develop energy storage materials to enable further utilization of renewable energy production. Energy storages are required due to the intermittency of solar and wind power, to add flexibility in the consumption. Flow battery (FB) is a promising energy storage technology to be integrated into renewable energy plants due to its many suitable characteristics. Abundant, environmentally friendly, and stable energy storage materials are needed for a cost-effective and competitive energy storage solution.
In this thesis, FB candidate materials were studied for their possible usage in flow battery applications. The material selection is abundant iron and titanium combined with easily tunable organic ligands to produce water-soluble metal complexes.
Metal complexes of iron with 2,2´-bipyridine, 1,10-phenanthroline, and 2,2´:6´,2´´-terpyridine, all with various functional groups, and titanium(IV) with 2,3-dihydroxynaphthalene were synthesized, characterized electrochemically via cyclic voltammetry and in lab-scale FB tests when applicable. Simulations were performed to obtain information on the kinetics of the redox pair from the experimental cyclic voltammograms. A commercial asymmetric [Fe(II)(bpy)(CN)4]2- complex was also studied for flow battery applications.
Obtained redox potentials are sufficiently high (0.57 to 1.29V vs. SHE) for iron complexes to be utilized as posolytes for aqueous FB applications, while the titanium complex had a sufficiently low redox potential (–1.18 V vs. SHE) for usage as negolyte. In lab-scale flow battery studies, the symmetric iron complexes undergo side reactions leading to voltage drop and decreased energy efficiency, which is not detected for [Fe(II)(bpy)(CN)4]2-. Analysis of the voltammetry of [Fe(II)(bpy)3]2+ complexes by simulations revealed a counterion effect on the stability, while additional information on the kinetics of the redox process was obtained.
The complex decomposition mechanism of [Fe(II)(bpy)3]2+ led to the introduction of an in situ tool to monitor the battery electrolytes and their side reactions via the formation of other redox-active species during FB operation. The tool was also used to monitor the SOC of the redox active species during flow battery operation via half-cell OCP and simulations of CVs.