Abstract

Redox flow batteries (RFBs) are emerging as a robust solution for large-scale energy storage, crucial for meeting the growing demand for sustainable power sources. While conventional RFBs typically utilize metallic based electrolytes, aqueous organic redox flow batteries (AORFBs) are gaining momentum as a cost-effective and safer alternative. This study delves into the electrochemical characteristics of key redox couples, 1,2-dihydroxyanthraquinone (1,2-DHAQ) and ferrocyanide (FeCN) to assess their potential for AORFB applications. Employing cyclic voltammetry (CV) and rotating disk electrode (RDE) voltammetry, we evaluated their redox reversibility, pH stability, kinetic parameters, and diffusion coefficients. This study reveals that both 1,2-DHAQ and FeCN exhibit quasi-reversible redox behavior, with peak separations (ΔE) of 58 mV and 100 mV, respectively. The half-wave potentials (E1/2) versus an Ag/AgCl reference of 0.864 V for 1,2-DHAQ and 0.253 V for FeCN indicate their suitability as negolyte and posolyte, respectively. In addition, 1,2-DHAQ’s redox behavior is influenced by pH due to proton-coupled electron transfer (PCET) mechanisms, while FeCN’s redox potential remains stable across pH variations due to its coordination with cyanide ligands. Kinetic analyses showed that 1,2-DHAQ has a charge transfer coefficient (α) of 0.23 and a rate constant (k0) of 1.05 × 10⁻³ cm/s, compared to FeCN’s α of 0.7 and k0 of 1.2 × 10⁻³ cm/s. Diffusion coefficients, determined via the Randles-Sevcik equation, were 1.78 × 10⁻⁶ cm²/s for 1,2-DHAQ and 2.17 × 10⁻⁶ cm²/s for FeCN.
1,2-DHAQ electrolytes were cycled in both symmetric and full cell configurations and found to decrease in capacity over time. To understand the source of this capacity fade, aliquots of cycled electrolyte were analysed via CV and UV-Vis spectroscopy to track the concentration of 1,2-DHAQ over time. Although capacity fade was always associated with a decrease in 1,2-DHAQ concentration, the amounts were not consistently in quantitative agreement, suggesting that more work needs to be done to identify and quantify degradation mechanisms. Lastly, a combination of CV and open-circuit voltage (OCV) measurements were used to estimate the state of charge (SOC) of electrolytes. This method was found to be effective for quasi-reversible systems like 1,2-DHAQ and FeCN but less accurate for irreversible systems like the V4+/V5+ redox couple. The study also validated the use of a pseudo-reference electrode, which simplifies experimental setup and reduces contamination risks, thus enhancing the reliability of SOC measurements.