Synthesis, Characterization and Chemical Stability Analysis of Tetrasubstituted Hydroquinones for Positive Electrolytes of Aqueous Organic Redox Flow Batteries

Broader adoption of renewable energy technologies will benefit from cost-effective and scalable energy storage methods to address the intermittency of renewable sources. The development of aqueous organic redox flow batteries (AORFBs), utilizing organic molecules composed of earth-abundant elements, represents one promising approach. Organic molecules may be readily functionalized to modulate chemical properties, including solubility and redox potential, and aqueous systems offer several advantages with respect to cost, conductivity, toxicity, and flammability. Organic species are, however, prone to decomposition, which may dramatically limit system lifetimes and practical implementation.<br/><br/>Chemical degradation represents a particularly significant challenge for positive electrolyte (posolyte) active species, as higher reduction potentials are typically associated with increased susceptibility to nucleophilic reactions with water. To address this problem, we designed tetrasubstituted hydroquinone posolyte active species candidates hypothesized to possess high stability, solubility, and redox potential. The synthesis of three such candidates, hydroquinonetetrasulfonic acid (HQTS), 2,5-dibromohydroquinone-3,6-disulfonic acid (HQDBDMS), and hydroquinonetetramethylsulfonic acid (HQTMS), will be reported. Although chemical stability studies revealed surprisingly rapid decomposition of the HQTS redox couple, the other two candidates demonstrated significantly greater stability. HQDBDMS, in particular, exhibited reversible redox kinetics and no observable decomposition after 15 days of cell cycling. Solubility, permeability, diffusivity, kinetic rate constants, and cell polarization performance will also be reported.<br/><br/>This presentation will highlight promising new hydroquinone AORFB posolyte active species candidates and will demonstrate how the synthesis of novel targets guided by chemical stability analysis can promote continued improvements in battery lifetime and performance.