Synthesis, characterization, and chemical stability analysis of quinones for aqueous organic redox flow batteries

Abstract

Broader adoption of renewable energy technologies will require cost-effective and scalable energy storage methods to address the intermittency of renewable sources. Redox flow batteries (RFBs), in which dissolved redox active species are flowed between external tanks and an electrochemical cell, may enable low-cost energy storage at large scales and over long durations. The use of dissolved species as the active materials in RFBs enables the decoupling of the energy capacity, which is related to the tank volumes, from the power capacity, which is associated with the size of the cell. By scaling the two parameters independently, the system requirements may be met without incurring unnecessary additional costs. Aqueous organic redox flow batteries (AORFBs), utilizing organic molecules composed of earth-abundant elements, may address certain limitations of conventional vanadium-based RFBs in terms of cost and scalability. Organic molecules moreover may be readily functionalized to modulate solubility, redox potential, and other properties as desired. Aqueous systems also 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.

Substituted anthraquinones have been introduced as negative electrolyte (negolyte) active species for AORFBs but have suffered from limited battery lifetimes. In Chapter 2 of this dissertation, the chemical stabilities of several of these candidates were examined. Although 2,6-dibutanoate ether anthraquinone (2,6-DBEAQ) demonstrated much slower capacity fade than the previously reported 2,6-dihyroxyanthraquinone (2,6-DHAQ), elevated temperature chemical stability experiments revealed a decomposition pathway that is accelerated at high pH. This insight motivated the development of 2,6-di(3-phosphonic acid)propyl ether anthraquinone (2,6-DPPEAQ), which is soluble at lower pH and exhibited improved stability.

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. Hydroquinonetetrasulfonic acid (HQTS) was hypothesized to possess several promising characteristics, including high solubility, redox potential, and stability. Chapter 3 presents the synthesis of HQTS and of two related tetrasubstituted analogues, 2,5-dibromohydroquinone-3,6-disulfonic acid (HQDBDMS) and hydroquinonetetramethylsulfonic acid (HQTMS).

Chapter 4 discusses the properties of these novel hydroquinones. Chemical stability studies of the three hydroquinones and their corresponding quinones revealed several decomposition pathways with a high degree of variability among the molecules studied. Although the HQTS redox couple exhibited surprisingly rapid decomposition, the other redox couples were significantly more stable. Of the candidates studied, HQDBDMS exhibited the most reversible redox kinetics. Solubility, permeability, diffusion coefficient, and kinetic rate constant measurements are also reported. After 15 days of cell cycling, no observable decomposition of HQDBDMS was detected.

The results described in this dissertation highlight potentially promising AORFB active species candidates and demonstrate how the synthesis of novel targets guided by chemical stability analysis can promote continued improvements in battery lifetime and performance.