Mixed Isomer Chromium-Chelates for High Concentration Aqueous Flow Battery Electrolytes
Journal Article
Overview
abstract
Redox flow batteries have attracted attention for their potential for large-scale energy storage, largely due to their ability to decouple power and energy. However, their limited energy density is often cited as a major drawback preventing adoption at scale. Therefore, there is interest in research into increasing the solubility of the active material in these electrolytes. One such method is active materials with multiple redox events. Promising materials are, however, often poorly soluble, and computational studies showed that increasing differences in redox potential between e.g., two redox events will decrease the energy efficiency of the cell.1 Here, two Cr-chelate structural isomer complexes, with redox potentials 100 mV apart are characterized and compared electrochemically. The two chromium complexes are combined in mixed-anion electrolytes to generate an electrolyte of 2 M active species, corresponding to a capacity of 53.6 Ah L−1, significantly higher than the individual isomers could achieve. At 100 mA cm−2 the cell achieved average Coulombic efficiencies of 99.6% and an equilibrium cell potential of 1.65 V at 50% State-of-Charge. Due to high viscosity of the electrolyte, the voltaic efficiency was only 67%. At a 1 M concentration, the equimolar mixed-anion electrolyte had a comparable current efficiency to the individual Cr-isomers (within 0.2%) but lowered voltaic efficiency (75.8% compared to 77.8% and 78.6%). The lowered voltaic efficiency is attributed to inefficiencies caused by non-concerted multiple redox events. The high cell voltage and increased combined electrolyte capacity demonstrate that employing chirality, chelate geometry, and structural isomers in a mixed-anion approach can significantly increase overall active material concentrations, but that there are consequences for cell performance when using multiple molecules with disparate reduction potentials in an electrolyte. This highlights that concerted transitions should be prioritized in order to capitalize on multi-electron redox species.; References:; (1) Neyhouse, B. J.; Fenton, A. M.; Brushett, F. R. Too Much of a Good Thing? Assessing Performance Tradeoffs of Two-Electron Compounds for Redox Flow Batteries. J. Electrochem. Soc.; 2021, 168 (5), 050501. https://doi.org/10.1149/1945-7111/abeea3.
