Analysis and performance of symmetric nonaqueous redox flow batteries

Abstract

Symmetric nonaqueous redox flow batteries (RFBs) use negative and positive battery solutions of the same solution composition to operate at high cell voltages. This research effort targets these systems since they offer performance improvements derived from using nonaqueous systems and symmetric active species. Nonaqueous solutions permit significantly higher cell voltages than state-of-the-art aqueous RFBs and symmetric active species chemistries reduce the required complexity of cell reactors. Both performance advantages correspond to significant cost improvements beyond already commercially competitive aqueous RFB chemistries. This document focuses on two classes of symmetric nonaqueous RFB chemistries: coordination complexes such as vanadium acetylacetonate [V(acac)3] or chromium acetylacetonate [Cr(acac)3], and organic active species such as 9,10-diphenylanthracene (DPA). V(acac)3 delivers reversible electrochemistry that supports a 2.2 V equilibrium cell potential, but there are some gaps in the understanding of its degradation mechanisms. Cr(acac)3 supports redox reactions that suggest cell potentials above 4 V, but shows signs of irreversibility in voltammetry experiments and is not yet well understood. Finally, the DPA system could be interesting because it does not use metal active species, and its voltammetry promises cell potentials above 3 V. Yet DPA suffers from low solubility in nonaqueous solvents that limit its practicality. These three systems show promise for symmetric nonaqueous RFBs and offer avenues for further improvement. Voltammetry and spectroelectrochemical electrolysis experiments on the metal coordination complexes clarify the mechanisms behind the voltammetry on these symmetric chemistries. Ligand dissociation causes the irreversible behavior observed in voltammetry on Cr(acac)3. The same experiments reaffirm the expected cyclability of V(acac)3. Chemical functionalization of the DPA center is performed to investigate the solubility and reactivity of various derivatives. Functionalizing DPA with ethylene glycol chains to form âDdPAâ significantly increases solubility limits from 0.6 mM and 44 mM for DPA in acetonitrile and 1,2-dimethoxyethane, respectively, to 12 mM and 0.21 M for DdPA in the same solvents. At the same time, DdPA retains redox activity that promises 3 V cell potentials. Ultimately, a custom, nonaqueous-compatible redox flow reactor was designed and used to test the performance of V(acac)3, DPA, and DdPA under various operating conditions. Contradicting previous reports, V(acac)3 delivers stable cycling over the 21- cycle experimental protocol. Exploration over a range of flow rates and current densities give energy and power densities up to 1.09 WhLâ1 and 0.16 Wcmâ2, respectively, for the battery solution compositions examined. These experiments further predict values up to 28 WhLâ1 and at least 0.22 Wcmâ2 for optimized V(acac)3 battery solutions. DPA and DdPA deliver the highest operating potential observed from organic nonaqueous RFBs, discharging at 3 V and 2.9 V, but require further work to understand degradation in the systems.</p