Electrochemical Regeneration of Anthraquinones for Lifetime Extension in Flow Batteries

Abstract

Aqueous organic redox flow batteries (AORFBs) offer a safe and potentially inexpensive solution to the problem of storing massive amounts of electricity produced from intermittent renewables. However, molecular decomposition is the major barrier preventing AORFBs from being commercialized. Structural modifications can improve molecular stability at the expense of increased synthetic cost and molecular weight. Utilizing 2,6-dihydroxy-anthraquinone (DHAQ), without further structural modification, we demonstrate that electrochemical regeneration could be a viable route to achieve low-cost, long-lifetime AORFBs. In situ (online) NMR and EPR and complementary electrochemical analyses reveal that decomposition compounds i.e., 2,6-dihydroxy-anthrone (DHA) and its tautomer, 2,6-dihydroxy-anthranol (DHAL), can be converted back to DHAQ in two steps: first DHA(L)2− are oxidized to the dimer (DHA)24− at − 0.32 V vs. SHE by one-electron transfer; subsequently, the (DHA)24− is oxidized to DHAQ2− at +0.57 V vs. SHE by three-electron transfer. Electrochemical regeneration rejuvenates not only DHAQ2−, but also the positive electrolyte – rebalancing the states of charge of both electrolytes without introducing extra ions. We demonstrate the repeated capacity recovery with DHAQ | potassium ferro-/ferricyanide flow battery in basic conditions, and show the approach is also effective for anthraquinone-2,7-disulfonate in acid. Electrochemical regeneration strategies may extend the useful lifetime of many water-soluble organic molecules with anthraquinone core structures in electrochemical cells.

A new hydroxyanthraquinone derivative with a low and reversible capacity fading process as negolyte in alkaline aqueous redox flow batteries

“…This led us to think that the observed capacity fading was not only a calendar phenomenon but was also correlated to the duration of the CV step at 1.3 V performed at the end of each charge, which increased along with the current density. This result tends to show that the 2,3-DHAQ degradation kinetic increases as it is longer subjected to more negative potential, as previously observed [59].…”

“…However, the oxidation performed during the deep discharge induced the recovery of the initial 2,3-DHAQ compound and of its corresponding capacity. This hypothesis was recently confirmed by the work of Jing et al [59].…”

Anthraquinone Flow Battery Reactants with Nonhydrolyzable Water-Solubilizing Chains Introduced via a Generic Cross-Coupling Method

“…To further confirm the formation of anthrone derivatives, we chemically synthesized diethylsulfonated-2,6-anthrone (anthranol) (ADES) and found that its 1 H NMR chemical shifts and peak splitting are indeed in line with those in the cycled and thermally treated AQDES spectra (Figure d). We previously found that anthrone derivatives can be chemically oxidized to AQs when exposed to air or other oxidants ,, and electrochemically oxidized back to AQs when proper potentials are applied . We built an electrochemical cell of 5 mL of 0.05 M ADES paired with 15 mL of 0.1 M AQDS (anthraquinone-2,7-disulfonate) in 1 M H 2 SO 4 for ADES electrochemical oxidation.…”

Bipolar Verdazyl Radicals for Symmetrical Batteries: Properties and Stability in All States of Charge

“…Ultimately, this provides opportunities for molecular engineering to minimize reactions that are detrimental to battery performance, or even suggest ways to revert decomposition and rejuvenate the electrolyte. [23] In this contribution, we present a detailed study of the molecular stability of bipolar Kuhn-type verdazyls in the context of symmetrical RFBs based on these materials. The structure and reactivity of radical 1 (Scheme 1) across the three relevant states-of-charge is reported, which provides key chemical insight into the factors that contribute to molecular decomposition and thus battery capacity fade.…”