Abstract
Driven by environmental concerns, strong demands, supportive policies and technological innovation, renewable energy has seen increased penetration into electrical grids. However, the stability and reliability of such grids are confronted by the intermittent nature of renewable sources and variable atmospheric conditions. This poses a major roadblock in transitioning energy utilization toward carbon-neutral sources. Development of efficient, low-cost energy storage systems facilitates transition from fossil fuels. Among many other energy storage technologies, redox flow batteries stand out, due to their ability to accommodate various needs of renewable-powered grids. Development of robust RFB based on non-aqueous systems has the potential to greatly improve energy density, approaching that of lithium-ion batteries, while maintaining the advantages of flow systems. However, major setbacks in NRFB implementation arise from active material instability and low solubility of active materials. The following work systematically probes the above-mentioned challenges by utilizing a bio-inspired vanadium complex, vanadium-bis-hydroxyiminodiacetate (VBH) as a molecular scaffold. First, the excellent stability and cyclability of the active material was demonstrated by employing in-situ UV-vis characterization in tandem with electrochemistry. Second, we investigated an experimental-theoretical approach to improve solubility. This was accomplished by tuning the key thermodynamic quantities of free energy of solvation and free energy of the lattice via cation modifications. We present evidence that the lattice free energy, which has been largely ignored in theoretical solubility models, is vital to obtain a meaningful prediction of solubility, and cannot be overlooked. Based on these findings, a design strategy for a new generation of VBH compounds that exhibit greatly improved solubilities is presented. Finally, a full cell performance characterization of VBH catholyte with anthraquinone-based anolyte is presented. Based on symmetric and asymmetric cell cycling design, various performance metrics were evaluated. The roles of ion-exchange membranes and electrolyte concentration on performance of the cell are also presented.