Novel, Stable Catholyte for Aqueous Organic Redox Flow Batteries: Symmetric Cell Study of Hydroquinones with High Accessible Capacity

Abstract

Owing to their broad range of redox potential, quinones/hydroquinones can be utilized for energy storage in redox flow batteries. In terms of stability, organic catholytes are more challenging than anolytes. The two-electron transfer feature adds value when building all-quinone flow battery systems. However, the dimerization of quinones/hydroquinones usually makes it difficult to achieve a full two-electron transfer in practical redox flow battery applications. In this work, we designed and synthesized four new hydroquinone derivatives bearing morpholinomethylene and/or methyl groups in different positions on the benzene ring to probe molecular stability upon battery cycling. The redox potential of the four molecules were investigated, followed by long-term stability tests using different supporting electrolytes and cell cycling methods in a symmetric flow cell. The derivative with two unoccupied ortho positions was found highly unstable, the cell of which exhibited a capacity decay rate of ~50% per day. Fully substituted hydroquinones turned out to be more stable. In particular, 2,6-dimethyl-3,5-bis(morpholinomethylene)benzene-1,4-diol (asym-O-5) displayed a capacity decay of only 0.45%/day with four-week potentiostatic cycling at 0.1 M in 1 M H3PO4. In addition, the three fully substituted hydroquinones displayed good accessible capacity of over 82%, much higher than those of conventional quinone derivatives.

Family Tree for Aqueous Organic Redox Couples for Redox Flow Battery Electrolytes: A Conceptual Review

“…Another strategy for improved stability is substituting p-quinone with protonated morpholino groups, which increases the solubility up to 2 mol/l (107 Ah/l) in water, and the formal redox potential to 0.89 V vs. SHE; however, the performance was found to be highly dependent on the pH and the electrolyte composition [142]. The further molecular tuning enabled the authors to enhance the electrochemical stability of the fully substituted derivates [143], see Figure 11.…”

Strategies for Improving Solubility of Redox‐Active Organic Species in Aqueous Redox Flow Batteries: A Review

“…The large solubility increase for 1,4-PEGAQ may be due to destruction of the molecular symmetry. [55] 1,8-PEGAQ has high solubility (1.2 M in 1 M KCl), excellent redox reversibility, and fast kinetics (diffusion of 2.6 × 10 À 6 cm/s 2 ). [56] In a parallel study, Jiang et al presented a series of AQ compounds with different PEG lengths: 3 (PEG3-AQ), 12 (PEG12-AQ), and 45 (PEG45-AQ) ethylene glycol units at the same substitution site (Figure 5c).…”

Redox Flow Batteries: Materials, Design and Prospects

“…X. Yang and colleagues published a work where they synthesized four new hydroquinone derivatives bearing morpholinomethylene and/or methyl groups in different positions on the benzene ring. 2,6-dimethyl-3,5-bis(morpholinomethylene)benzene-1,4-diol showed a capacity decay of 0.45% per day [156]. Some studies have the main objective of studying the performance of organic electrolytes in a cell configuration.…”