Abstract
Electrode-decoupled redox flow batteries (ED-RFBs) employ disparate cationic actives in the anolyte and catholyte, significantly expanding the chemical design space for flow batteries. Crossover induced capacity fade in such systems can be mitigated or eliminated by the use of highly permselective anion exchange membranes (AEMs) as separators. Herein, we show that tailoring the solvation shell around the cationic actives aids in significantly reducing crossover and hence capacity fade. A model V-Ce ED-RFB was developed utilizing this paradigm and demonstrated a 30% enhancement in practical volumetric capacity1. By substituting sulfuric acid with methanesulfonic acid as the supporting electrolyte for V and Ce in conjunction with a highly permselective polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) triblock copolymer AEM2 enabled a >95% reduction in capacity fade compared to previous sulfuric acid systems. Employing the Marcus-Hush kinetic formulation to understand the solvation effects3 4, the presence of strongly solvated cations was shown to be the reason for the significantly improved performance. The ED-RFB maintained nearly 100% coulombic efficiency (CE) and ca. 70% energy efficiency (EE) (at a 50 mA.cm-2 galvanostatic charge/discharge current) over 100 cycles. Thus, we demonstrate that judicious selection of the electrolyte composition leads to ED-RFBs with long life, excellent rate capability and stability.
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