Aqueous organic redox flow batteries offer a safe and potentially inexpensive solution to the problem of storing massive amounts of electricity produced from intermittent renewables. However, molecular decomposition represents a major barrier to commercialization—and although structural modifications can improve stability, it comes at the expense of synthetic cost and molecular weight. Now, utilizing 2,6-dihydroxy-anthraquinone (DHAQ) without further structural modification, we demonstrate that the regeneration of the original molecule after decomposition represents a viable route to achieve low-cost, long-lifetime aqueous organic redox flow batteries. We used in situ (online) NMR and electron paramagnetic resonance, and complementary electrochemical analyses to show that the decomposition compound 2,6-dihydroxy-anthrone (DHA) and its tautomer, 2,6-dihydroxy-anthranol (DHAL) can be recomposed to DHAQ electrochemically through two steps: oxidation of DHA(L)2− to the dimer (DHA)24− by one-electron transfer followed by oxidation of (DHA)24− to DHAQ2− by three-electron transfer per DHAQ molecule. This electrochemical regeneration process also rejuvenates the positive electrolyte—rebalancing the states of charge of both electrolytes without introducing extra ions.
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Research at Harvard was supported by the US National Science Foundation through grant no. CBET-1914543, by a US DOE award no. DE-AC05-76RL01830 through PNNL subcontract no. 535264, and by a grant from the Massachusetts Clean Energy Center. Research at University of Cambridge was supported by Centre of Advanced Materials for Integrated Energy Systems (CAM-IES), via EPSRC grant no. EP/P007767/1 and Shell. E.W.Z acknowledges the STFC Futures Early Career Award, grant no. ST/R006873/1. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. We acknowledge P.A.A. Klusener from Shell for useful discussions. We thank D. A. Pollack, and K. Amini for useful discussions.
The authors declare no competing interests.
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a & b, Linear sweep voltammetry (LSV) and the 1st order differential of 10 mM DHA(L)2− in 1 M KOH supporting electrolyte indicating a peak for dimerization reaction around −260 mV vs. SHE and a second oxidation reaction starting around +300 mV vs. SHE, before being overshadowed by oxygen evolution at around +650 mV vs. SHE. We consider the potentials (−0.32 V, + 0.57 V) at the 1st order differential peak current as the approximations of oxidation potentials of [DHA(L)2− to (DHA)24−] and [(DHA)24− to DHAQ2−]. The 1st order differential was from the LSV curve obtained at 20 mV/s. c & d, Cyclic voltammograms of 10 mM pure DHA in 1 M KOH before and after a + 110 mV vs. SHE 10-minute potential hold with a highly porous Zoltek PXFB working electrode. New peaks match those of dimer observed in a previous study; cyclic voltammograms of 10 mM pure DHA in 1 M KOH before and after +410 mV vs. SHE 10-minute potential hold with a highly porous working electrode. New peaks match those of DHAQ2−. e, Potential hold of 0.1 M DHA(L)2− in 1.2 M KOH negolyte in full cell containing a 2X excess of 50 mM K4[Fe(CN)6] and 50 mM K3[Fe(CN)6] posolyte. The Y-axis represent the number of coulombs extracted from DHA(L)2− electrolyte over electrochemical oxidation. f, 1H NMR spectra of DHA(L)2− negolyte before and after 30-min hold at different cell voltages.
a, Repeated application of electrochemical regeneration of negolyte in battery composed of 6 mL 100 mM DHAQ2− in the negolyte and a mixture of 35 mL 100 mM K4[Fe(CN)6] and 50 mM K3[Fe(CN)6] in the posolyte. Galvanostatic cycling at ±50 mA/cm2 with potential holds at 1.0 and 1.5 V and current cutoff of ±1 mA/cm2 during normal cycles. Every 50 cycles an additional discharge step at −2 mA/cm2 until potential limit and a further hold until current decreases to −0.2 mA/cm2. b, Aggregate data for 3 cell replicates with varying potential limits for regeneration treatment indicating partial dependence of recovery ratio on recovery from prior treatment.
a, Integrated areas above and below 0 V of the cell discharge voltage profile with electrochemical regeneration in Fig. 4c. b, Integrated areas above and below 0 V of the cell discharge voltage profiles with 13 regeneration processes in Fig. 4a. The area ratio (%) represents the percentage of discharged electrical energy that is subsequently used for regeneration during a single discharge process, reflecting the energetic cost. It is, on average, 1.30%. Note that the electrochemical regeneration step is not performed in every cycle, but rather once every 51 cycles. Relative to the total electricity discharged during 51 cycles, the energetic cost for regeneration is only 0.025%.
a, Cycling of battery composed of 5 mL 100 mM DHAQ2− in the negolyte and 20 mL of a mixture of 100 mM K4[Fe(CN)6] and 50 mM K3[Fe(CN)6] in the posolyte. Galvanostatic cycling at ±50 mA/cm2 with potential holds at 1.0 V and 1.5 V and current cutoff of ±1 mA/cm2 during normal cycles. Three electrochemical oxidation treatments before an aliquot was removed from negolyte reservoir on day 10 for concentration measurement. b, Cyclic voltammograms of 20 mM DHAQ2− in 1 M KOH on glassy carbon working electrode. Pristine DHAQ2− electrolyte measured before and after dilution by 10% to verify resolution of measurement. Cycled DHAQ2− data refers to an aliquot removed from negolyte in part a, after ~25% capacity had been lost. No change in peak current suggests DHAQ2− concentration has not changed during cycling after regeneration treatment. c & d, Comparison of DHAQ2− NMR peak integral with respect to DMSO internal standard. Similarity of the peak integral of cycled electrolyte which had lost ~25% of capacity to that of pristine DHAQ2− does not indicate overall decrease in DHAQ2− concentration. An aliquot of the cycled DHAQ2− was taken for 1H NMR measurement when ~25% capacity was lost on day 10.5. e, LC−MS result of the cycled DHAQ2−. The peak at 10.8–11.1 min is from DHAQ2−, which is the sole compound detected. Other peaks are from background, which can be ignored.
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Jing, Y., Zhao, E.W., Goulet, MA. et al. In situ electrochemical recomposition of decomposed redox-active species in aqueous organic flow batteries. Nat. Chem. (2022). https://doi.org/10.1038/s41557-022-00967-4