Redox Flow Battery Applications Research Articles

Renewable and hydrophilic carbon quantum dots derived from human hair as the filler in Nafion composite membrane for vanadium redox flow battery application

Renewable and environmentally benign hydrophilic carbon quantum dots (CQDs) were synthesized using a hydrothermal method. Subsequently, composite membranes of Nafion/CQDs, demonstrating outstanding performance metrics for vanadium redox flow batteries (VRFBs), were developed. Employing a straightforward solution-casting technique, these membranes exhibited superior proton conductivity and reduced vanadium ion permeability. Tested at an elevated current density of 120 mA/cm2, the single cell VRFB integrated with the optimized Nafion/CQDs composite membrane outperformed its commercial counterparts, achieving a high coulombic efficiency of approximately 96.13 % and an energy efficiency of nearly 85.46 %. Moreover, it demonstrated significant cycle life, retaining about 17.1 % of its capacity after 100 cycles, compared to 27.2 % capacity retention for the N-recast membrane over the same period. These results position the Nafion/CQDs composite membrane as a formidable contender in the VRFB domain, emphasizing its potential for broader commercial applications.

Iron-vanadium redox flow batteries electrolytes: performance enhancement of aqueous deep eutectic solvent electrolytes via water-tuning

Deep eutectic solvents (DES) are being recognized as a highly promising electrolyte option for redox flow batteries. This study examines the impact of modifying the molar ratio of water to a DES consisting of urea and choline chloride on important measures of electrolyte performance, such as viscosity, cyclic voltammetry, and impedance spectroscopy. According to the findings, when the molar ratio of water to DES is 1:3, the viscosity of the electrolyte falls by 75 %, the peak current density increases by 5–9.5 times, the ionic conductivity increases by 5–10 times, and the ohmic resistance decreases by 60–90 %. This approach greatly enhances the conductivity and diffusion coefficient of the electrolyte, resulting in a novel, cost-effective, and highly efficient electrolyte for iron-vanadium redox flow battery applications. This study enhances the efficiency of DESs at the molecular level, establishing the foundation for future extensive implementations.

Advanced hybrid membrane constructed with silicon carbide, graphene oxide and functionalized silicon carbide for vanadium permeability, proton conductivity to battery and fuel cell applications

The energy sector is currently exploring eco-friendly energy conversion and storage devices, as an alternative to traditional fossil fuel devices. A unique hybrid proton exchange membrane is developed for proton exchange membrane fuel cells (PEMFC) and vanadium redox flow battery (VRFB) application. Polymeric nanocomposite membranes are effective for the selective and controlled transportation of ions between the electrodes. This study utilizes various modifiers such as GO, SGO, SPES, and SiC, to modify the PES polymer matrix. Analysis of the functional groups confirmed the presence of modifiers and hydroxyl groups on the nanocomposite membrane. Additionally, the use of Field Emission Scanning Electron Microscopy (FESEM) validated the existence of honeycomb and sponge-like voids as well as the surface morphology on the membrane surface. The incorporation of modifiers into the nanocomposite membrane matrix serves as a reliable barrier for the transport of vanadium ions, leading to a significant reduction in vanadium ion permeability across the membrane. The mechanical properties and ion exchange capacity of nanocomposite membranes are enhanced, while the permeability of VO2+ decreases compared to that of the bare PES membrane. The enhancement of proton conductivity is further achieved with the utilization of modifiers in PES polymer. As compared to bare PES membrane, PES/SPES + SGO (5%) exhibits less than half the permeability for vanadium, and PES/SiC shows 44.03 % elongation means twice of bare PES membrane.

(Invited) Innovating Sustainable, Cost-Effective, and Durable Redox Flow Battery Membranes

In the quest to integrate renewable energy sources like solar and wind into the electric grid efficiently, the development of long-duration energy storage (LDES) systems is paramount. The U.S. Department of Energy's Long Duration Storage Shot initiative is at the forefront of this endeavor, aiming to reduce the costs of grid-scale energy storage by 90% for systems with over 10 hours of operation. A key player in this mission is the non-aqueous redox flow battery (RFB), known for its use of earth-abundant materials and potential to meet these cost-reduction goals. However, the commercial viability of RFBs hinges on the development of high-performance, cost-effective membranes, which are crucial for their functionality and affordability. These membranes' ionic conductivity is vital for the power density of RFBs, while their ion selectivity and solvent uptake greatly influence the batteries' cycle life.Our team has made groundbreaking advancements in membrane technology for non-aqueous flow batteries. We've addressed the challenge of balancing ionic conductivity with mechanical strength in polymer electrolytes. Our findings reveal that enhanced solvent uptake while boosting membrane ionic conductivity, can compromise mechanical integrity. To counter this, we've employed two strategies: selective plasticization of the ion-conductive block in block copolymers and reinforcing the membrane's mechanical strength through hydrogen and ionic bonds with an inorganic scaffold. Significantly, we've also reduced the crossover of redox-active species in our membrane designs, notably decreasing polysulfide species crossover in a Na metal – polysulfide hybrid flow battery. This has led to improved capacity retention, Coulombic efficiency, and extended cycle life compared to traditional porous membranes.Our latest innovations include the development of hydrocarbon single-ion conductors, demonstrating enhanced conductivity and stability. We've also introduced crosslinking chemistry to control solvent uptake more effectively, thus improving ionic conductivity while preserving mechanical strength. Lastly, our pioneering tape casting method for producing ceramic-polymer composite membranes represents a major leap in scalable membrane production, paving the way for the practical application of non-aqueous redox flow batteries in long-duration energy storage. Acknowledgment This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) and is sponsored by the U.S. Department of Energy through the Energy Storage Program in the Office of Electricity. We acknowledge Dr. Jagjit Nanda SLAC National Accelerator Laboratory for the fruitful discussions.

Development of Modular Nitrenium Bipolar Electrolytes for Possible Applications in Symmetric Redox Flow Batteries.

Amid the escalating integration of renewable energy sources, the demand for grid energy storage solutions, including non-aqueous organic redox flow batteries (oRFBs), has become ever more pronounced. oRFBs face a primary challenge of irreversible capacity loss attributed to the crossover of redox-active materials between half-cells. A possible solution for the crossover challenge involves utilization of bipolar electrolytes that act as both the catholyte and anolyte. Identifying such molecules poses several challenges as it requires a delicate balance between the stability of both oxidation states and energy density, which is influenced by the separation between the two redox events. We report the development of a diaminotriazolium redox-active core capable of producing two electronically distinct persistent radical species with typically extreme reduction potentials (E1/2red < -2 V, E1/2ox > +1 V, vs Fc0/+) and up to 3.55 V separation between the two redox events. Structure-property optimization studies allowed us to identify factors responsible for fine-tuning of potentials for both redox events, as well as separation between them. Mechanistic studies revealed two primary decomposition pathways for the neutral radical charged species and one for the radical biscation. Additionally, statistical modeling provided evidence for the molecular descriptors to allow identification of the structural features responsible for stability of radical species and to propose more stable analogues.

Numerical Simulation of Flow Field Structure of Vanadium Redox Flow Battery and its Optimization on Mass Transfer Performance

The structural design of the flow channel of a redox flow battery directly affects ion transport efficiency, electrode overpotential, and stack performance during charge-discharge cycles. A tapered hierarchical interdigitated flow field design that has independent flow channel structures for different levels of flow was developed in this work. Especially, the secondary branch channels are the tapered type and the corresponding cross-sections are gradually reduced along the flow direction, which is beneficial for improving the flow rate at the end of channels and enhancing mass transfer. The performances of a vanadium redox flow battery with interdigitated flow field, hierarchical interdigitated flow field, and tapered hierarchical interdigitated flow field were evaluated through 3D numerical model. The results showed that at 240 mA cm−2 and 6 ml s−1, the pump-based efficiency of the hierarchical interdigitated flow field increased by 4%-7% compared with the interdigitated flow field. Furthermore, the pump-based efficiency with tapered hierarchical interdigitated flow field increased by 1.6%-3% compared with the hierarchical interdigitated flow field. This indicates that the tapered hierarchical interdigitated flow field shows further advantages in redox flow battery applications.

Low pH Titanium Electrochemistry in the Presence of Sulfuric Acid and its Implications for Redox Flow Battery Applications

Titanium (Ti) is a promising elemental redox active species for redox flow batteries (RFBs) due to its 100x availability in the Earth crust, and 10x lower cost (compared to elemental vanadium). Furthermore, Ti salts are highly soluble in water and concentrations >5 M can be easily obtained. Seeking to harness the higher solubility (and hence energy density) of the Ti electrolyte for flow battery applications, the Ti4+/Ti3+ redox couple was investigated at high concentrations (up to 5 M) relevant to RFB applications. The behavior of Ti ions in H2SO4 supported electrolytes was investigated by varying the ratio of Ti redox active species to counterion. The electrochemical characteristics, transport properties, and redox kinetics of the Ti4+/Ti3+ redox couple were measured and the impact of the Tix+ to solvating ligand ratio was examined. The coordination structures around solvated Tix+ ions were spectroscopically determined and the effect of solvation structure on the Ti3+/Ti4+ redox rate constants were examined and correlated to the calculated solvation energy (hence distinguishing between inner- and outer-sphere processes) and the role of catalysts was addressed. The Ti electrolyte development guidelines presented herein will advance the development of Ti-based RFBs as a promising pathway towards cost effective, grid-scale energy storage.

Air Plasma Modification of Graphite-Based Electrode for Improved Performance of Aqueous Redox Flow Batteries

Graphite felt is widely utilized as a porous carbon electrode in aqueous redox flow batteries (RFBs). However, its inherent hydrophobic nature and limited electrochemical activity present challenges. While the correlation between RFB performance and electrode properties has been extensively studied for vanadium chemistry and other inorganic redox active materials, it remains scarce in literature for organic systems. In this study, we employ air plasma treatment, known for its controllability, solvent-free nature, and short treatment duration, to modify commercially available graphite felt for RFB applications. A comprehensive analysis is conducted to establish correlations between plasma treatment, physical properties, electrochemical characteristics, and overall cell performance in aqueous RFBs. Comparative evaluation reveals a significant enhancement, with treated graphite felt exhibiting an 85% increase in capacity at 140 mA cm−2 compared to its pristine counterpart. By intentionally utilizing authentic RFB electrodes and employing state-of-the-art ferrocyanide posolyte, this study underscores the crucial role of the interface, even for rapid (reversible) redox-active materials utilized in AORFBs.

Anion exchange membrane based on poly(arylene ether sulfone)s functionalized with quinuclidinium-piperidinium dual cations for vanadium redox flow battery applications

Vanadium redox flow batteries (VRFBs) have gained significant interest as a prospective energy storage solution due to their scalability and extended cycle durability. The efficient functioning of VRFBs relies significantly on anion exchange membranes (AEMs), which facilitate the selective transport of ions while preventing crossover. In this study, we present a novel AEM based on poly(arylene ether sulfone)s (PAES) functionalized with dual cations (quinuclidinium-piperidinium) (DC-PAES) bearing a butyl spacer, explicitly designed for VRFB applications. The DC-PAES membrane containing dual cations separated by a butyl spacer results in a balanced combination of hydrophilicity and hydrophobicity, which enables efficient ion transport across the membrane. The DC-PAES membrane showed hydroxide conductivity of 0.085 S/cm at 80 °C, having an ion exchange capacity (IEC) of 1.42 mmol/g. The resulting DC-PAES membrane displayed low permeability (2.1 × 10−7 cm2 min−1) and high selectivity (12.3 × 105 S min cm−3) to vanadium. The performance of the AEMs fabricated for VRFB application showed better Coulombic (97.1 %), energy (90.0 %), and voltage efficiency (92.63 %) at 40 mAcm−2 higher than Nafion 117. The reported AEMs revealed excellent stability due to the steric and ring constraints of quinuclidinium and piperidinium cation, hindering the attack of oxidative species. Furthermore, the DC-PAES membrane demonstrated improved battery cycling performance lasting up to 150 cycles under in-situ conditions, making it a promising separator material for VRFB applications.

Synthesis of Alkoxy-TEMPO Aminoxyl Radicals and Electrochemical Characterization in Acetonitrile for Energy Storage Applications

In this paper, we describe the synthesis and characterization of alkoxylated TEMPO, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, radicals with potential application in organic non-aqueous redox flow batteries. The behavior of a series of TEMPO derivatives with varying lengths of alkoxy chain is analyzed in acetonitrile solutions using electrochemical techniques, electron paramagnetic resonance (EPR) spectroscopy, and measurements of permeability through three different membranes. Electrochemical redox potentials are only weakly dependent on the substituent, but, in contrast, exchange current densities derived from the data do depend on the substitution. EPR lends further insight into these properties via the determination of hyperfine splitting constant and rotational correlation time. There is a negligible effect of the substituents on those parameters among the modified TEMPO radicals. Finally, permeation rates of modified TEMPO derivatives through membranes depend significantly on both the membrane and the substitution of TEMPO, providing insights into capacity fade measurements in the literature.

Proton Exchange Membranes from Sulfonated Lignin Nanocomposites for Redox Flow Battery Applications.

Redox flow batteries (RFBs) are increasingly being considered for a wide range of energy storage applications, and such devices rely on proton exchange membranes (PEMs) to function. PEMs are high-cost, petroleum-derived polymers that often possess limited durability, variable electrochemical performance, and are linked to discharge of perfluorinated compounds. Alternative PEMs that utilize biobased materials, including lignin and sulfonated lignin (SL), low-cost byproducts of the wood pulping process, have struggled to balance electrochemical performance with dimensional stability. Herein, SL nanoparticles are demonstrated for use as a nature-derived, ion-conducting PEM material. SL nanoparticles (NanoSLs) can be synthesized for increased surface area, uniformity, and miscibility compared with macrosized lignin, improving proton conductivity. After addition of polyvinyl alcohol (PVOH) as a structural backbone, membranes with the highest NanoSL concentration demonstrated an ion exchange capacity of 1.26 meqg-1, above that of the commercial PEM Nafion 112 (0.98 meqg-1), along with a conductivity of 80.4 mScm-1 in situ, above that of many biocomposite PEMs, and a coulombic efficiency (CE), energy efficiency (EE) and voltage efficiency (VE) of 91%, 68% and 78%, respectively at 20mAcm-2. These nanocomposite PEMs demonstrate the potential for valorization of forest biomass waste streams for high value clean energy applications.

Designing the next generation of symmetrical organic redox flow batteries using helical carbocations

In recent years, non-aqueous fully organic Redox Flow Batteries (RFBs) have displayed potential in broadening the electrochemical window and enhancing energy density in RFBs by relying on redox-active organic molecules to provide improved sustainability in comparison to metal-based charge carriers. Of particular interest, systems that rely on a single bipolar redox molecule (BRM) for their operation, known as symmetrical organic RFBs, have gained momentum as the utilization of a BRM eliminates membrane crossover issues, thus extending the lifespan of electrical energy storage systems while reducing their cost. In this manuscript, we will present our contribution to this field through the design of tunable bipolar molecules within the helicene carbocation class. This particular type of BRM is synthetically very affordable and has proven to be highly modifiable and robust. Through the examination of 11 examples, we will demonstrate how an approach based on readily available electrochemical tools can be efficiently employed to generate and assess a library of compounds for future full flow RFB applications.

Hydrated eutectic electrolyte as catholyte enables high-performance redox flow batteries

Compared with conventional electrolytes, eutectic electrolytes are recognized as an ideal electrolyte for the next-generation electrochemical energy storage system characterized by facile synthesis and composition tunability. However, the eutectic electrolytes still suffer from sluggish kinetics, which inhibit their widespread application in redox flow batteries. Here, a well-designed hydrated eutectic electrolyte system (HEE) composed of iron chloride hexahydrate, urea and water with stoichiometric ratio at 2:1:6 is developed as catholyte to improve the cycling performance of hybrid redox flow batteries. The HEEs with precisely controlled water content could inhibit the water-induced side reactions, while expand the electrochemical stability windows. Moreover, the water molecules gradually replace the anions to participate in the solvation sheath of Fe3+ ions, forming a solvation structure of [FeClx(H2O)y]3−x, which result in an improved ion dissociation degree, ionic conductivity and redox kinetics. As a result, hybrid iron-based redox flow battery based on the optimal HEE exhibits a highly reversibly redox reaction of Fe3+/Fe2+ and reduced overpotentials. Furthermore, the battery could stably cycle over 120 cycles with a capacity retention of 87.75 % at a relatively high current density of 10 mA cm−2, delivering a maximum power density of ∼50 mW cm−2, which is much higher than reported eutectic-based flow batteries. This research offers new insights into rationally designing of advanced eutectic electrolytes for high-efficient redox flow batteries.

Ammonium Bifluoride‐Etched MXene Modified Electrode for the All−Vanadium Redox Flow Battery

AbstractThe development of electrodes with high performance and long‐term stability is crucial for commercial application of vanadium redox flow batteries (VRFBs). This study compared the performance of VRFB with thermal‐treated and MXene‐modified carbon paper. To prepare the MXene, a modified‐etching process with ammonium−bifluoride (NH4HF2) led to a mild and efficient conversion of the MAX‐phase to MXene compared to etching process with hydrofluoric‐acid (HF). Electron microscopy and X‐ray diffraction studies revealed that the etching process with NH4HF2 led to MXene nanostructures with a large interlayer spacing. The results show that at a current density of 60 mA cm−2, the energy efficiency increased by 25.5 % when using a NH4HF2 ‐etched MXene‐modified negative electrode, by 12.5 % with a thermal‐treated MXene‐modified electrode, and by 4 % with an HF‐etched MXene‐modified electrode, in comparison to the pristine electrode. The maximum power density of the battery was increased by more than 40 %. In long‐term cycling experiments the MXene modified electrode exhibited excellent stability over 1000 cycles of charge‐discharge, with 0.05 % discharge capacity decay per cycle, amongst the lowest values reported to date and four times lower than for thermally‐treated electrode. The superior performance was linked to the improved electrical conductivity and wettability, higher interlayer spacing, and lower charge transfer resistance for the V2+/V3+ redox reaction.

Realizing one-step two-electron transfer of naphthalene diimides via a regional charge buffering strategy for aqueous organic redox flow batteries.

Naphthalene diimide derivatives show great potential for application in neutral aqueous organic redox flow batteries (AORFBs) due to their highly conjugated molecular structure and stable two-electron storage capacity. However, the two-electron redox process of naphthalene diimides typically occurs via two separate steps with the transfer of one electron per step ("two-step two-electron" transfer process), which leads to an inevitable loss of voltage and energy. Herein, we report a novel regional charge buffering strategy that utilizes the core-substituted electron-donating group to adjust the redox properties of naphthalene diimides, realizing two electron transfer via a single-step redox process ("one-step two-electron" transfer process). The symmetrical battery testing of NDI-DEtOH revealed exceptional intrinsic stability lasting for 11 days with a daily decay rate of only 0.11%. Meanwhile, AORFBs with NDI-DMe/FcNCl and NDI-DEtOH/FcNCl exhibited a remarkable 40% improvement in peak power density at 50% state of charge (SOC) in comparison to NDI/FcNCl-based AORFBs. In addition, the battery's energy efficiency has increased by 24%, resulting in much more stable output power and significantly improved energy efficiency. These results are of great significance to practical applications of AORFBs.

Hyperbranched TEMPO-based polymers as catholytes for redox flow battery applications.

Application of redox-active polymers (RAPs) in redox flow batteries (RFBs) can potentially reduce the stack cost through substitution of costly ion-exchange membranes by cheap size-exclusion membranes. However, intermolecular interactions of polymer molecules, i.e., entanglements, particularly in concentrated solutions, result in relatively high electrolyte viscosities. Furthermore, the large size and limited mobility of polymers lead to slow diffusion and more sluggish heterogeneous electron transfer rates compared to quickly diffusing small molecules. Although a number of RAPs with varying electrolyte viscosities have been reported in the literature, the relation between the RAP structure and the hydrodynamic properties has not been thoroughly investigated. Herein, hyperbranched 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO)-based polymers intended for application as low-viscosity catholytes for RFBs are presented and the influence of the structure and the molar mass distribution on the hydrodynamic properties is investigated. A new synthesis approach for TEMPO-based polymers is established based on step-growth polymerization of a TEMPO-containing monomer using an aza-Michael addition followed by a postpolymerization modification to improve solubility in aqueous solutions. The compact structure of hyperbranched polymers was demonstrated using size-exclusion chromatography (SEC) with viscometric detection and the optimum molar mass was found based on the results of viscometric and crossover investigations. The resulting RAP revealed a viscosity of around 21 mPas at a concentration corresponding to around 1 M TEMPO-containing units, according to the calculated mass of the repeating unit, showing potential for high capacity polymer-based catholytes for RFBs. Nevertheless, possible partial deactivation of TEMPO units lowered the active TEMPO concentration of the hyperbranched RAPs. A faster diffusion and higher charge transfer rate were observed for the hyperbranched polymer compared to the previously reported linear polymers. However, in RFB tests, a poor performance was observed, which is attributed to the side reactions of the oxidized TEMPO moieties. Finally, pathways for overcoming the main remaining challenges, i.e., high loss of material during dialysis as an indication of being prone to crossover, the partial deactivation of TEMPO moieties, and the subsequent side reactions under battery conditions, are suggested.

Reprint of “Characteristics and electrochemical performance of copper/graphite felt composite electrodes for vanadium redox flow battery”

BackgroundVanadium oxide is considered environmentally benign; however, its economic utilization is highly challenging due to the inability to control the size of etched holes. To address this limitation, it is crucial to regulate the depth of the hole by modifying the etching conditions. A controllable hole depth has the potential to enable its application in vanadium redox flow batteries (VRBs). MethodsIn this study, electrodeposition is used to deposit copper particles on the surface of graphite felt electrodes. The copper particles are then employed to etch the graphite felt surface under a reducing atmosphere, which produces porous copper/graphite felt composite electrodes that can be used in VRBs. The effect of etching temperature and time on the material characteristics and the electrochemical performance of this composite electrode were investigated. Significant FindingsThe holes on the graphite felt surface following a heat treatment are studied, especially as the temperature approaches 900 °C. The observed specific surface area is 16.59 m2/g. Upon grafting of oxygen functional groups, the O/C ratio of the composite electrodes reaches 0.12. After the annealing at 900 °C, the energy efficiency of the porous copper/graphite felt composite electrode is 67.40%, the voltage efficiency is 70.13%, and the current efficiency is 96.11%. Under long-term cyclic discharge, the composite electrode exhibits a good capacitance retention of 91% and a capacitance utilization of 63%.

Modulating Charge Percolation in Biocompatible Norbornene Redox-Active Polymers Obtained Via ROMP

Organic redox-active materials are actively being developed as next-generation energy storage systems. These materials have multiple advantages, including superior chemical and physical properties, and excellent electrochemical storage performance compared to metal-based inorganic materials. Specifically, redox-active and conductive polymers have been used over organic single molecules due to the advantages that polymeric materials offer such as good stability, excellent processability, and simple device fabrication. Furthermore, we can adjust the synthetic process to obtain a wide range of desirable properties and can incorporate conductive and redox-active polymers into biological entities to carry out bio-inspired functions such as shuttling electrons through wastewater mediated processes or sensing of heavy metals in wastewater. By leveraging natural processes, our goal is to engineer resistant and sustainable energy storage technologies using conductive and redox-active polymers. To that end, we present the synthesis and characterization, including material and electrochemical characterizations, of biocompatible norbornene redox-active polymers in which we modulated charge percolation by copolymerizing with a water-soluble polymer for potential applications in bio-inspired redox-flow batteries.

Micellar Solubilization for High-Energy-Density and Long-Cycle-Life Aqueous Organic Redox-Flow Batteries

Functional group control has been the most widely used strategy to enhance the solubility of redox-active organic materials (ROMs) in aqueous media for aqueous organic redox-flow batteries. However, this strategy inevitably requires complicated organic synthesis, typically involving 2–4 steps with high cost and under harsh conditions, reducing the full potential of ROMs. In this study, to achieve higher ROM solubility, we introduced surfactant molecules to provide a nonpolar microenvironment via micellization. The hexadecyltrimethylammonium chloride surfactant enabled micellar solubilization, resulting in an order-of-magnitude enhancement of the solubility of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). In addition, the micellar solubilization elevated the redox potential of TEMPO, increasing the energy density by more than 10 times compared with that in the bare electrolyte. Furthermore, the micellar solubilization improved the cycle stability due to not only mitigated crossover but also enhanced chemical stability of TEMPO+. This strategy can be applied to various ROMs and is expected to assist in achieving practical application of organic redox-flow batteries.

Selective Ion Conducting Polymers for Non-Aqueous Redox Flow Battery Applications

Providing sustainable supplies of clean energy is a critical global challenge for the future. As intermittent renewable energy sources are increasingly deployed, grid-scale energy storage solutions are increasingly needed. One approach to addressing this challenge is to use flow battery technology to store and deliver grid-scale amounts of energy. Non-aqueous redox flow batteries can be operated at higher voltages and energy densities compared to aqueous systems and, as such, may offer high volumetric energy density compared to other flow batteries. A significant challenge facing non-aqueous flow batteries, however, is the lack of selective membrane separators engineered for the unique challenges of non-aqueous electrochemical systems. Suitable membranes for non-aqueous redox flow battery applications must be stable in aggressive solvent environments, offer high conductivity, and provide selectivity to prevent cross-over of redox active molecules. Here, we report the synthesis and characterization of a series of negatively charged ion conductive polymeric membranes for non-aqueous flow battery applications. Fixed negative charges were added to a poly(phenylene oxide) backbone via a custom sulfonate group-based side chain. Lithium ion conductivity and ferrocene permeability properties were characterized to evaluate the selectivity of the membrane for ion transport relative to redox active molecule cross-over (as ferrocene is a representative redox active molecule). The polymers exhibited combinations of conductivity and selectivity that are favorable compared to other Nafion-based materials, and the materials appear to be dimensionally stable in a non-aqueous electrolyte over a period of several months. Redox active molecule cross-over was analyzed within a thermodynamic regular solution framework to highlight how thermodynamic factors contribute to cross-over properties. Ultimately, the data suggest a decoupling of ion and redox active molecule transport that could inform future efforts to develop advanced non-aqueous redox flow battery membrane separators. Altogether, this presentation discusses the transport properties and stability of these materials that show promise for non-aqueous flow battery applications.