Abstract
Growing energy demands necessitate optimized battery storage systems. Redox mediated flow batteries (RMFBs) provide a unique solution, combining the decoupled energy and power of a flow-battery with the high energy density of a solid-state battery. This work aims to synthesize and characterize a solid active material (SAM) that can be added to the battery system to boost the overall energy density via an indirect reduction-oxidation reaction. To this extent, metal hexacyanometalates (MHCM) are considered an optimal SAM due to their tunable reduction potential, stability, and accessibility. Four MHCMs - manganese, cobalt, nickel, and copper hexacyanoferrate - were synthesized and characterized via cyclic voltammetry, Fourier transform infrared spectroscopy, and scanning electron microscopy. As a SAM, the redox potential of MHCMs must be readily tuned to match the proposed mediator. Within an RMFB system, the mediator solution contains redox-active species that shuttle electrons throughout. Ensuring a match between the SAM and mediator optimizes the performance and capability of the RMFB. One method of tuning SAM redox potential is via electrolyte additives. The addition of crown ethers were determined to have a significant impact on the potential of the system, controlling the intercalation of cations to the MHCM lattice by forming a more stable complex with them in solution than their solvated state. Specifically, 18Crown-6 ether’s ability to form a stable complex with K+ enables the redox potential of the reaction to be further tuned towards a more negative reduction potential. The results of this work contribute towards building a library of metal hexacyanometalates, improving the overall understanding of these compounds and their role as a “booster” material in RMFBs.