Realized potential as neutral pH flow batteries achieve high power densities

High power density operation of redox flow batteries (RFBs) is essential for lowering system costs, but until now, only acid-based chemistries have achieved such performance, primarily due to rapid membrane proton (H+) transport. Here, we report a neutral pH RFB using the highly reducing Cr-(1,3-propylenediaminetetraacetate) (CrPDTA) complex that achieves acid-like power performance while utilizing potassium ion (K+) transport. We investigate RFB resistance components and demonstrate the high and consistent K+ conductivity of the Fumasep E-620(K) membrane. When combined with a robust bismuth electrocatalyst, this membrane enables constant voltage efficiency operation of a CrPDTA|Fe(CN)6 RFB for 200 cycles. An optimized CrPDTA|Fe(CN)6 RFB, which combines a high cell potential with a low area-specific resistance (0.46 &Omega; cm2), demonstrates a maximum discharge power density of 1.63&nbsp;W cm&minus;2 and an average discharge power density over 500 mW cm&minus;2 while maintaining 80% round-trip energy efficiency cycling, which are records for non-acid-based RFBs. &nbsp;</p

Analysis and performance of symmetric nonaqueous redox flow batteries

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 ‘DdPA’ 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 WhL–1 and 0.16 Wcm–2, respectively, for the battery solution compositions examined. These experiments further predict values up to 28 WhL–1 and at least 0.22 Wcm–2 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

Analysis and performance of symmetric nonaqueous redox flow batteries

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

A method for quantifying crossover in redox flow cells through compositionally unbalanced symmetric cell cycling

Active species crossover continues to frustrate durational performance for redox flow batteries (RFBs), requiring thorough evaluation of membrane / separator properties. Characterization workflows typically employ a suite of ex situ experimental techniques, but these approaches do not capture the dynamic conditions (e.g., variable concentrations, alternating polarity) encountered in redox flow cells. Here, we report a facile method for assessing crossover directly in redox flow cells—compositionally unbalanced symmetric cell cycling (CUSCC). Based on conventional symmetric cell cycling, CUSCC imposes a concentration gradient between two chemically similar half-cells, inducing species crossover during galvanostatic cycling, which results in a characteristic “capacity gain” over time. We first develop a zero-dimensional model to describe fundamental processes that underpin the technique and examine the dependence of capacity gain on membrane / separator properties and operating conditions. Subsequently, we perform proof-of-principle experiments using FeCl2 / FeCl3 and Nafion 117 as a representative system and demonstrate results consistent with those predicted from simulations. Finally, we use model fits of the capacity gain data to extract membrane transport parameters, obtaining similar values to those measured from ex situ techniques. Overall, this work describes a promising new approach for characterizing species crossover and expands the RFB testing toolbox