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DOE OSTI2022

Membrane design for non-aqueous redox flow batteries: Current status and path forward

Redox flow batteries are promising technologies for large-scale, long-duration energy storage applications. Among them, non-aqueous redox flow batteries (NARFB) represent a transformative flow battery system since NARFBs potentially offer a higher energy density than aqueous flow batteries. However, many technical challenges remain for NARFBs, including the lack of high-performance membranes, low solubility of redox materials, and poor cycling efficiencies. Membranes serve a vital function in NARFBs, as they allow for selective ion transport while providing separation between the anolyte and catholyte. NARFB membrane development is an emerging research area, and this article reviews their design and critical factors that influence membrane properties, including solvent uptake, ion transport, and redox species permeability. A greater understanding of membrane behavior in non-aqueous solutions provides design principles for developing next-generation membranes for NARFB. In conclusion, we summarize the challenges, target metrics, and future perspectives for NARFBs.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗

DOE OSTI2021

Critical role of structural order in bipolar redox-active molecules for organic redox flow batteries

Bipolar redox-active molecules (BRMs) have been suggested as a means to address crossover related issues in all-organic redox flow batteries (RFBs). In such species, electron donors (anolytes) and electron acceptors (catholytes) are linked by a chain, and the same chemical composition is used in both anode and cathode compartments to reduce chemical gradients and unbalanced ion transport. The resultant RFBs resemble the aqueous vanadium RFBs, but offer greater design flexibility and potentially more favorable electrochemical and physicochemical properties. Yet the complex trade-offs in these properties have been a complication. Here we developed a fundamental rubric to uncover the likely origins in the performance metrics of these BRMs that are salient for their use in RFBs. Methylene linked phenothiazine and phthalimide moieties were employed as model BRMs and variations in their properties and molecular conformations were evaluated by systematically varying the methylene linker. Our results revealed that even minimalistic changes in the linker length resulted in dramatic oscillations in the solubility and stability. Using crystallography, quantum chemistry, and molecular dynamics, we clarify that the seemingly fluctuating behavior is due to (1) the inter- and intra-molecular charge transfer between the donor and acceptor through bond or/and through space; (2) the formation of distinctive packing/clustering motifs. Both interactions are strongly dependent on the molecular conformation. As these structural factors modify the electronic structure of both electroactive functional groups and do not change monotonically, the structure–property response in BRMs is often complex. Finally, accordingly, these behaviors need to be taken into account in developing BRMs and knowledge of these structural factors will allow the more rational design of BRMs for grid-scale energy storage.

25 ENERGY STORAGE↗

DOE OSTI2021

Redox flow battery

Disclosed herein is a redox flow battery (RFB). The battery generally includes: a positive electrolyte that is a first metal ion, a negative electrolyte that is a second metal ion, an ion exchange membrane positioned between the positive electrolyte and the negative electrolyte. The membrane is configured to restrict and/or prevent the passage of the first metal ion and/or the second metal ion therethrough, and is configured to maintain ionic conductivity between the positive electrolyte and the negative electrolyte.

Sankarasubramanian, Shrihari↗

DOE OSTI2024

Electrode-decoupled redox flow battery

Described herein is a novel electrode-decoupled redox flow battery, a novel reinforced electrode-decoupled redox flow battery, and methods of using same to store energy. Advantages of these novel electrode-decoupled redox flow batteries include long life, excellent rate capability, and stability.

Sankarasubramanian, Shrihari↗

DOE OSTI2024

Miniaturize the Redox Flow Battery for Accelerated Materials Discovery and Development

Redox flow batteries are a promising technology for grid-scale energy storage. The aqueous organic redox flow battery is of particular interest for its potentially low material cost and sustainability. Developing novel organic active material for flow battery electrolytes typically entails molecular engineering toward desired properties, necessitating organic synthesis. In a research laboratory setting, the synthesis of specifically designed organic molecules featuring targeted functional groups is time and resources intensive. In the past, synthesizing materials required for battery testing has often required gram-scale production, presenting considerable constraints on the pace of novel organic material discovery. In this report, we introduce a miniaturized cell design that mandates only milligram-scale material synthesis while yielding testing outcomes equivalent or superior to those reported with other commercially available or homemade flow cells in the literature. The test results under various pH conditions validate the scale-down strategy to accelerate the flow battery material discovery and development using the newly designed mini cell. This approach offers researchers an efficient means to notably reduce the time and resources required to develop novel materials for flow batteries.

25 ENERGY STORAGE↗

DOE OSTI2022

Microemulsions: Breakthrough Electrolytes for Redox Flow Batteries

Aqueous and non-aqueous redox flow batteries (RFBs) have limited energy and current densities, respectively, due to the nature of the electrolytes. New approaches to electrolyte design are needed to improve the performance of RFBs. In this work, we combined a highly conductive aqueous phase and an organic redox-active phase in a microemulsion to formulate a novel RFB electrolyte. As a proof-of-concept, we demonstrate an RFB using this microemulsion electrolyte with maximum current density of 17.5 mA·cm −2 with a 0.19 M posolyte and 0.09 M negolyte at a flow rate of only ∼2.5 ml·min −1 , comparable to early vanadium electrolyte RFBs at similar flow rates on a per molar basis. The novel active negolyte component is an inexpensive oil-soluble vitamin (K 3 ). By combining aqueous and organic phases, the solvent potential window and energy density may be increased without sacrificing current density and new redox couples may be accessed. Microemulsion electrolytes show great promise for improved performance and increased energy densities in aqueous RFBs but the path forward is complex. We end with discussion of areas that need work to achieve the potential of these electrolytes.

25 ENERGY STORAGE↗

DOE OSTI2022

Redox flow battery with improved efficiency

A method for a redox flow battery includes using a cell of a redox flow battery to store electrical energy and discharge the stored electrical energy. The using includes circulating a first electrolyte solution through a first circulation loop in fluid connection with the first electrode of the cell; circulating a second electrolyte solution through a second circulation loop in fluid connection with the second electrode of the cell; and at least one of a first element from the first electrolyte solution in the first electrode permeates through the separator layer and precipitates as a first solid product in the second electrode and a second element from the second electrolyte solution permeates through the separator layer and precipitates a second solid product in the first electrode. The method also includes removing at least a portion of the first solid product or the second solid product from the first electrode and the second electrode, respectively.

25 ENERGY STORAGE↗

DOE OSTI2023

Methods and systems for redox flow battery electrolyte hydration

Methods and systems are provided for transporting and hydrating a redox flow battery system with a portable field hydration system. In one example, the redox flow battery system may be hydrated with the portable field hydration system in a dry state, in the absence of liquids. In this way, a redox flow battery system may be assembled and transported from a battery manufacturing facility to an end-use location off-site while the redox flow battery system is in the dry state, thereby reducing shipping costs, design complexities, as well as logistical and environmental concerns.

Song, Yang↗

DOE OSTI2023

Colocalized Raman spectroscopy – scanning electrochemical microscopy investigation of redox flow battery dialkoxybenzene redoxmer degradation pathways

Non-aqueous redox flow batteries offer high voltages for grid-level energy storage technologies. However, decomposition of the redoxmers - redox-active molecules that make up the anolyte and catholyte in the negative and positive cell compartments - is an important challenge to overcome for long-term storage. Here, we present a spectroelectrochemical study of the catholyte candidate 2,3-dimethyl-1,4-dialkoxybenzene (C7) and its decomposition mechanisms in the presence of a model nucleophilic base, pyridine. We utilize colocalized Raman microscopy and scanning electrochemical microscopy (Raman-SECM) to quantify the chemical rates of decom-position and qualitatively identify the reaction intermediates and products. A detailed study on how the Raman-SECM parameters (electrode distance, substrate electrode, and laser focal height) influence the Raman signal of the charged catholyte C7 ∙+ is presented. Using optimized conditions, we monitored the deprotonation of C7 ∙+ via Raman spectroscopy and the subsequent hydrogen abstraction from solvent molecules to regenerate C7 via electrochemistry. Finite element modeling was used to fit electrochemical and spectroscopic data, quantifying the deprotonation rates as k dep = 2000 and 700 L mol -1 s -1 and abstraction rates as k abs = 0.5 and 0.2 s - 1 in propylene carbonate and acetonitrile solvents, respectively. Our results show the value of spatiotemporal reso-lution in evaluating the chemical and electrochemical behavior of materials for redox flow batteries.

25 ENERGY STORAGE↗

DOE OSTI2020

In Situ Reliability Investigation of All-Vanadium Redox Flow Batteries by a Stable Reference Electrode

Redox flow batteries (RFBs) have been studied over the past several decades as a promising candidate for stationary energy storage applications. It is therefore important to understand the reliability of RFBs and the mechanisms that cause degradation with time. Contributions from individual electrodes (cathode or anode) are difficult to separate especially for long-term cycle testing, due to the lack of a stable reference electrode. In our work, the reliability and degradation mechanisms of an all-vanadium RFB were investigated by a stable reference electrode based on the dynamic hydrogen electrode (DHE). With factor exploration and design optimization, the newly developed DHE reference electrode demonstrated high accuracy and long-term stability that enables in-situ monitoring of individual electrode signals (potentials and impedances) over hundreds of cycles in a vanadium RFB. This approach enables the full cell degradation to be separated and the degradation attributed to contributions from the cathode and anode. The cathode and anode were found to play quite different roles in the increase in overpotential of the vanadium RFB during long-term cycling. The anode reaction limited both the charge and discharge capacity over 100 cycles. In addition, the negative side also appeared to be the rate limiting factor throughout cycling as determined by EIS measurement. The cathode contributed to the performance degradation as cycling exceeded 50 cycles.

Huang, Qian↗

DOE OSTI2022

All-vanadium sulfate acid redox flow battery system

All-vanadium sulfate redox flow battery systems have a catholyte and an anolyte comprising an aqueous supporting solution including chloride ions and phosphate ions. The aqueous supporting solution stabilizes and increases the solubility of vanadium species in the electrolyte, allowing an increased vanadium concentration over a desired operating temperature range. According to one example, the chloride ions are provided by MgCl 2 , and the phosphate ions are provided by (NH 4 ) 2 HPO 4 .

Nie, Zimin↗

DOE OSTI2022

In operando, non-invasive state-of-charge monitoring for redox flow batteries

This document describes techniques and systems for in operando, non-invasive SOC monitoring of redox flow batteries. The described techniques and systems allow for accurate, inexpensive, portable, and real-time methods to measure the SOC of redox flow batteries. System operators can monitor the SOC by measuring an acoustic attenuation coefficient of the electrolyte in the redox flow battery. The acoustic attenuation coefficient is measured using an ultrasonic transducer attached to a probing cell, which is connected to an electrolyte flow of a redox flow battery. The acoustic attenuation coefficient provides an accurate, real-time SOC measurement that is generally insensitive to varying operational temperatures of the electrolyte solution.

25 ENERGY STORAGE↗

DOE OSTI2021

Method and system for rebalancing electrolytes in a redox flow battery system

A method of rebalancing electrolytes in a redox flow battery system comprises directing hydrogen gas generated on the negative side of the redox flow battery system to a catalyst surface, and fluidly contacting the hydrogen gas with an electrolyte comprising a metal ion at the catalyst surface, wherein the metal ion is chemically reduced by the hydrogen gas at the catalyst surface, and a state of charge of the electrolyte and pH of the electrolyte remain substantially balanced.

Song, Yang↗

DOE OSTI2022

Method and system for rebalancing electrolytes in a redox flow battery system

Song, Yang↗

DOE OSTI2024

Methods and system for rebalancing electrolytes in a redox flow battery system

Song, Yang↗

DOE OSTI2025

Monitoring the long-term performance of organic redox flow battery by a distribution of relaxation time analysis

Organic redox flow batteries hold great promise as an energy storage technology, but their intricate chemistry makes them vulnerable to various degradation mechanisms. Monitoring this degradation is essential for identifying the limiting processes within the cells. Electrochemical impedance spectroscopy (EIS) offers a straightforward, in-situ method for measuring the total resistance of an operating cell. However, to pinpoint the limiting processes during long-term cycling, EIS data must be complemented by other techniques. Distribution of relaxation time (DRT) analysis is particularly effective for differentiating resistance components. Here, in this study, we perform a comprehensive analysis of resistance evolution and the separation of anode and cathode contributions during long-term cycling of a full cell employing 7,8-dihydroxyphenazine-2-sulfonic acid (DHPS) as the anolyte. Separate analyses of the DHPS anolyte and ferri-/ferrocyanide catholyte were conducted using a symmetric cell setup. The relaxation times derived from symmetric cells facilitate the identification of peaks in the DRT profiles from the full cell. Importantly, the DRT profiles indicate a correlation between the evolution of charge transfer resistance and the chemical degradation of DHPS. The methodologies and results outlined in this study offer significant insights for developing diagnostic tools applicable to other types of redox flow batteries.

Distribution of relaxation time↗

DOE OSTI2025

Spin‐State and Clustering Effects in Fe‐Complex Negolytes for Near‐Neutral Aqueous Redox Flow Batteries

Cost-effective redox-active materials are essential for advancing redox flow batteries (RFBs). Iron, with its abundance and suitability as a redox couple, is a promising candidate; however, achieving stable and fast redox reactions in aqueous RFBs remains a challenge. Here, this study presents an Fe-based negolyte stabilized by a hexadentate ligand, where Fe–ligand bonds are enhanced through intermolecular interactions. The sulfonate-substituted Fe complex exhibits a formal potential of −0.44 V versus Ag/AgCl and an exceptionally high rate constant of 0.69 cm s −1 . Near-neutral RFBs incorporating 0.5 M Fe complex show excellent cycling stability, with no discernible capacity fading over 300 cycles. This performance is attributed to intermolecular hydrogen bonds that reinforce Fe–ligand coordination and promote the formation of stable trimeric clusters. Operando electrochemical Raman spectroscopy and density functional theory reveal that π-backdonation from Fe(II) to the imino-phenolate moiety further stabilizes the complex after reduction. In contrast, the hydroxyl-substituted complex exhibits inferior stability due to weaker hydrogen bonding and less pronounced π-backdonation. These findings underscore the importance of ligand design and intermolecular interactions in developing cost-effective, high-performance redox-active materials for aqueous RFBs.

36 MATERIALS SCIENCE↗

DOE OSTI2021

High capacity organic radical mediated phosphorous anode for redox flow batteries

A battery includes a redox flow anode chamber coupled to an anode current collector, a separator, and an external container in fluid connection with the redox flow anode chamber. The external container has therein a solid phosphorus material. A first redox-active mediator and the second redox-active mediator are circulated through the half-cell electrode chamber and the external container. During a charging cycle the first redox-active mediator is reduced at the current collector electrode and the reduced first mediator reduces the phosphorus material, and wherein during a discharging cycle the second redox-active mediator is oxidized at the anode current collector electrode, and the second redox-active mediator is then reduced by the reduced phosphorus material. A method of operating a battery and a method of making a battery are also discussed.

Delnick, Frank M.↗