High‐Energy‐Density Chelated Chromium Flow Battery Electrolyte at Neutral pH

Abstract

High-concentration operation of redox flow batteries (RFBs) is essential for increasing their energy-storage capacity, but non-acidic electrolytes struggle to achieve the high concentrations of metal ions dissolved in acid, limiting the development of energy-dense neutral pH electrolytes. We report neutral pH RFB operation of chromium 1,3-propylenediaminetetraacetate (CrPDTA) at concentrations of 1.2 M at room temperature and 1.6 M at 40 °C, demonstrating 60% higher negolyte capacity, up to 42.9 Ah L À 1 , than previously reported for non-additive-utilizing solutions of this promising material. With extended full cell cycling, we demonstrate the importance of buffer selection and pH when using the Fumasep E-620(K) membrane. Finally, we expand the pH operation range of CrPDTA to pH 7, which when cycled at 100 mA cm À 2 against a ferrocyanide posolyte demonstrated excellent coulombic efficiencies > 99.7% and energy efficiencies > 87%, while operating at almost 700 mV more negative than the thermodynamic hydrogen evolution window.

Sulfonated Diels-Alder Poly(phenylene) Membrane for Efficient Ion-Selective Transport in Aqueous Metalorganic and Organic Redox Flow Batteries

“…To further enhance the power density enabled by the S membrane, cell ASR tot was lowered through the following strategies proven for a CrPDTA|Fe(CN) 6 RFB: thinner membrane, elevated temperature operation, and a bismuth electrocatalyst to improve CrPDTA 1−/2− redox kinetics. 19,35,38 An RFB was assembled using a 12 μm S membrane, cell heating to 40 °C, bismuth-plated negolyte electrodes, and the previously described CrPDTA|Fe(CN) 6 RFB electrolytes. This enhanced S CrPDTA|Fe(CN) 6 RFB demonstrated an initial ASR tot of 0.52 Ω cm 2 .…”

“…[11][12][13][14] Organic reactants with projected decadal lifetime in batteries have been demonstrated, and a strategy of coordinating metal cations with ligands has been successful for enabling high voltage and high concentration aqueous RFBs. 12,[15][16][17][18][19][20][21] Organic and metalorganic reactants offer drastically lower crossover rates than metal cations due to their increased size and by taking advantage of charge exclusion by, for example, pairing anionic species with CEMs. 5,22,23 However, many of these chemistries Require neutral or alkaline solutions where protons cannot carry the required ionic current.…”

“…33,34 The potassium salt (KCrPDTA) exhibits a high water solubility (1.32 M) and, when paired against Fe(CN) 6 4−/3− in an RFB (CrPDTA|Fe(CN) 6 RFB) at a pH of 7, a high voltage (1.62 V) and coulombic efficiency (CE) (>99.5%) is observed. 19,33 To represent organic-based negolytes, we select the anthraquinone derivative 4,4'-((9,10-anthraquinone-2,6diyl)dioxy)dibutyrate (2,6-DBEAQ 2−/4− , hereafter DBEAQ), which has a demonstrated solubility of up to 0.6 M at pH 12 for the potassium salt (K 2 (DBEAQ)), open circuit voltage of 1.05 V paired against Fe(CN) 6 4−/3− (DBEAQ|Fe(CN) 6 RFB), and extremely stable cycling with a capacity fade rate as low as 0.01%/day. 23 Despite their promising features, each of these chemistries would benefit from improved potassium conductivity in the CEM, without compromising on crossover suppression, to increase voltage efficiency (VE) and prevent capacity fade.…”

Redox Flow Batteries: Electrolyte Chemistries Unlock the Thermodynamic Limits

“…Different from the EDTA-chelated Cr with one water molecule bound to the Cr center, the PDTA could complex the Cr ions in an octahedral geometry in electrolytes and exclude completely water molecules from the primary coordination sphere (Figure 5a). [85,91] Accordingly, the homogeneous HER in bulk electrolytes in the RFB has been successfully suppressed for the PDTAchelated Cr. At the surface of carbon electrode, the heterogeneous HER is kinetically inhibited.…”

Predicting the Conductivity–Selectivity Trade-Off and Upper Bound in Ion-Exchange Membranes

“…− counter-ions while suppressing the transport of vanadium co-ions, 23 and CEMs in iron/chromium flow batteries enable the migration of K + counter-ions while preventing the passage of anionic chelation complexes. 24 In the context of CO 2 electrolysis for carbon capture, 25,26 the transport of co-ions across AEMs influences the hydration and stability of the membrane, 27 the activity and selectivity of the catalyst, 28−31 electrodes. 27,31,32 Effectively managing co-ion selectivity while preserving counter-ion conductivity is crucial for optimizing product output and prolonging the life of the membrane− electrode assembly.…”