Redox Flow Battery Applications Research Articles

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

Abstract 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/cm2 and 6 mL/s, 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.

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.

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.

Scaling up Electrode Manufacturing with Non-Solvent Induced Phase Separation Technology

Grid-scale energy storage technologies must be deployed at a massive scale to balance the mismatch between energy generation using renewable technologies and demand. Redox flow batteries are an emerging technological option for large-scale and multi-hour electrochemical energy storage thanks to their ability to decouple energy and power1. Despite their intrinsic advantages, the current elevated costs of this technology hinders commercialization2. To improve cost competitiveness, research efforts have focused on developing novel electrolyte chemistries and optimized reactor concepts. To the latter, optimizing the porous electrode microstructure and surface chemistry offers a promising pathway to increase the performance of flow batteries. However, porous electrodes design is challenging as they must fulfil contradictory requirements such as providing high surface area for electrochemical reactions, facilitating mass transport of reactive species, and maintaining low operational pressure drop. Furthermore, state-of-the-art porous electrodes have been repurposed from other mature electrochemical technologies but have not been tailored for the unique requirements of redox flow batteries3.Non-solvent induced phase separation (NIPS) has been recently introduced as a versatile method to synthesize porous structures suitable for RFBs with unique properties, which are unattainable with current fibrous materials4,5. Drawing inspiration from membrane fabrication techniques, we reengineered the method for the synthesis of porous electrodes for use in redox flow batteries. The porous electrodes synthesized with NIPS were tested in a single-electrolyte cell configuration and then in an all-vanadium redox flow battery, showing promising electrochemical performance in comparison with existing commercial fibrous carbon materials. In this presentation, I will discuss our current efforts to scale up the NIPS method to manufacture electrodes at industrial scale. We believe that non-solvent induced phase separation is a cost-effective alternative to traditional carbon fibrous materials thanks to the few simpler steps required for the electrode preparation. Based on technoeconomic modeling, we estimate that the production cost of the NIPS electrodes can be less than half of the production costs of traditional carbon fibrous electrodes6. Moreover, the phase separated electrodes do not require post-manufacturing heat treatment to increase the surface area and wettability, which further translates into savings on energy costs. In this project, we aspire to scale-up the NIPS materials to a final size, after carbonization, of 30·30 cm2 while retaining similar microstructural and electrochemical performance properties as in laboratory scale. We perform a suite of microscopy, spectroscopic and flow cell diagnostics to correlate the electrode microstructure, surface chemistry and device performance. In this presentation, I will discuss the scalability of the method, reproducibility based on statistical analysis of sample properties and performance, and finally electrochemical performance on two flow battery chemistries.References Markard, J. (2018). The next phase of the energy transition and its implications for research and policy. Nature Energy, 3(8), 628-633.Darling, R. M. (2022). Techno-economic analyses of several redox flow batteries using levelized cost of energy storage. Current Opinion in Chemical Engineering, 37, 100855.Forner-Cuenca, A., and Brushett, F. R. (2019). Engineering porous electrodes for next-generation redox flow batteries: recent progress and opportunities. Current Opinion in Electrochemistry, 18, 113-122.Wan, C. T. C., Jacquemond, R. R., Chiang, Y. M., Nijmeijer, K., Brushett, F. R., & Forner‐Cuenca, A. (2021). Non‐Solvent Induced Phase Separation Enables Designer Redox Flow Battery Electrodes. Advanced Materials, 33(16), 2006716.Jacquemond, R. R., Wan, C. T. C., Chiang, Y. M., Borneman, Z., Brushett, F. R., Nijmeijer, K., & Forner-Cuenca, A. (2022). Microstructural engineering of high-power redox flow battery electrodes via non-solvent induced phase separation. Cell Reports Physical Science, 3(7), 100943.Minke, Christine, Ulrich Kunz, and Thomas Turek. "Carbon felt and carbon fiber-A techno-economic assessment of felt electrodes for redox flow battery applications." Journal of Power Sources342 (2017): 116-124.

(Invited) Understanding Capacity Fade Behavior in Aqueous Organic Redox Flow Batteries Via Zero-Dimensional Models and High-Throughput Cell Cycling

Aqueous Organic Redox Flow Batteries (AORFBs) have emerged as promising and potentially disruptive technologies for the storage of electrical energy from intermittent renewable sources for use over long discharge durations when the sun isn’t shining and the wind isn’t blowing. AORFBs could become preferred over Li-ion batteries for grid-scale stationary storage due to their potentially low-cost active materials made of Earth-abundant elements, their inherent non-flammability, and the intrinsic decoupling of energy and power capacities in the flow battery design. Our group has demonstrated that calendar life, rather than cycle life, limits molecular lifetimes in AORFBs due to various molecular instabilities that lead to side reactions, thus inhibiting performance [1]. To accurately determine molecular fade rates, we utilize potentiostatic cycling to avoid artifacts caused by drifts in internal resistance and employ volumetrically unbalanced compositionally symmetric cell configurations to distinguish molecular fade from membrane crossover or the two sides drifting out of balance.We have developed a high-throughput setup for AORFBs, capable of cycling cells at elevated temperatures, providing a new dimension in the flow battery characterization space to explore multiple cell parameters simultaneously. The recent incorporation of the decay of active material into zero-dimensional cell-cycling models for AORFBs [2,3] allows for exploration of the causes of various trends seen in flow cell temporal capacity behavior. Complemented by zero-dimensional modelling, we explore temporal capacity evolution in symmetric cells driven by different capacity fade mechanisms such as active species degradation [4], self-discharge [5], and membrane crossover. Collectively, these results highlight the relevance of electrochemical techniques to understand molecular degradation in AORFBs and expedite the screening process of candidate molecules for long lifetime AORFBs, which may enable massive grid penetration of intermittent renewable energy.[1] M.-A. Goulet and M. J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods,” Journal of The Electrochemical Society, 165, A1466 (2018).[2] S. Modak and D. G. Kwabi, “A Zero-Dimensional Model for Electrochemical Behavior and Capacity Retention in Organic Flow Cells,” Journal of the Electrochemical Society, 168, 080528 (2021).[3] B. J. Neyhouse, J. Lee, F. R. Brushett, “Connecting Material Properties and Redox Flow Cell Cycling Performance through Zero-Dimensional Models” Journal of The Electrochemical Society, 169, 090503 (2022).[4] D. G. Kwabi, Y. Ji, M. J. Aziz, “Electrolyte Lifetime in Aqueous Organic Redox Flow Batteries: A Critical Review,” Chemical Reviews, 120, 6467 (2020).[5] E. M. Fell, D. De Porcellinis, Y. Jing, V. Gutierrez-Venegas, R. G. Gordon, S. Granados-Focil, M. J. Aziz, “Long-term stability of ferri-/ferrocyanide as an electroactive component for redox flow battery applications: On the origin of apparent capacity fade,” ChemRxiv (2022). https://chemrxiv.org/engage/chemrxiv/article-details/62913087f89e5d4e6ee8828d

Fabrication and Performance of Symmetric Flow Battery Systems Incorporating Novel Pyridinium Anolytes

The push to bring more renewable energy sources onto the grid has spawned a flurry of work to develop viable options for long duration energy storage. Redox flow batteries (RFBs) represent one of the most attractive possibilities, and research toward employing redox-active organic materials to improve or replace the incumbent commercially available vanadium-based RFB technology is burgeoning. In particular, numerous pyridinium-based salts have been extensively studied as anolytes in both aqueous and nonaqueous RFBs. We have recently reported a series of readily synthetically accessible 2,6-dimethyl-4-arylpyridinium compounds that display quite negative reduction potentials (ca. -1.55 to -1.8 V vs Fc/Fc+), along with high solubility and excellent electrochemical kinetics. Some of these materials are also extremely persistent in the reduced state, suggesting that they may be good candidates for RFB applications.In this presentation, we will describe the preparation and properties of new pyridinium compounds from the corresponding pyrylium precursors, as well as the fabrication of symmetric RFB systems in which these materials are covalently linked to phenothiazine or ferrocene catholyte moieties. Synthesis, solubility, and electrochemical performance (including preliminary RFB cycling data) of these materials will also be discussed.

Highly ion selective sulfonated poly(ether ether ketone)/polyzwitterion functionalized graphene oxides hybrid membrane for vanadium redox flow battery

High-density zwitterionic groups functionalized graphene oxides (PSBMA@GO) were incorporated into sulfonated poly(ether ether ketone) (SPEEK) membranes to create continuous channels for efficient proton/vanadium ion selective conduction. The incorporation of zwitterionic polymers, specifically poly(sulfobetaine methacrylate) (PSBMA) combined with graphene oxide (GO) sheets, enhanced the compatibility of additives with the SPEEK matrix. Furthermore, the impact of PSBMA@GO content on the properties of S/PSBMA@GO hybrid membranes for vanadium redox flow battery (VRFB) application was investigated. S/PSBMA@GO-1 hybrid membrane exhibits ultrahigh ion selectivity 55.3×103 S min cm-3, and the cell with this membrane shows excellent coulombic efficiencies (98.2-99.1%), energy efficiencies (83.6-74.5%) at 100-180 mA cm-2, long cycle life and high capacity retention (35.8% after 100 cycles at 100 mA cm-2). This study demonstrates that PSBMA@GO have great application potential to solve the trade-off effect between proton conductivity and vanadium ion permeability of PEM.

Elucidating the Iron‐Based Ionic Liquid [C4py][FeCl4]: Structural Insights and Potential for Nonaqueous Redox Flow Batteries

AbstractIn this study, a low‐melting organic‐inorganic crystalline ionic liquid compound, N‐butyl pyridinium tetrachlorido ferrate (III) is described. The material can easily be synthesized using a one‐pot approach in an ionic liquid medium. Single‐crystal X‐ray diffraction confirms that the basic inorganic block is [FeCl4]−, which is counterbalanced by an N‐butyl pyridinium cation. The compound exhibits a melting point of 37.6 °C by differential scanning calorimetry, which is among the lowest values for a pyridinium‐based metal‐containing ionic liquid. The material shows promising electrochemical behavior at room temperature in both aqueous and nonaqueous solvents, and at elevated temperatures in its pure liquid state. Given its appreciable solubility in both water and acetonitrile, the compound can act as a redox‐active species in a supporting electrolyte for redox flow battery applications. These classes of low‐melting ionic solids with long‐range order and interesting electrochemical applications are potential candidates for a range of green energy storage and harvesting systems.

Simple Air-Stable [3]Radialene Anion Radicals as Environmentally Switchable Catholytes in Water.

The hexacyano[3]radialene radical anion (1) is an attractive catholyte material for use in redox flow battery (RFB) applications. The substitution of cyano groups with ester moieties enhances solubility while maintaining redox reversibility and favorable redox potentials. Here we show that these ester-functionalized, hexasubstituted [3]radialene radical anions dimerize reversibly in water. The dimerization mode is dependent on the substitution pattern and can be switched in solution. Stimuli-responsive behavior is achieved by exploiting an unprecedented tristate switching mechanism, wherein the radical can be toggled between the free radical, a π-dimer, and a σ-dimer-each with dramatically different optical, magnetic, and redox properties-by changing the solvent environment, temperature, or salinity. The symmetric, triester-tricyano[3]radialene (3) forms a solvent-responsive, σ-dimer in water that converts to the radical anion with the addition of organic solvents or to a π-dimer in brine solutions. Diester-tetracyano[3]radialene (2) exists primarily as a π-dimer in aqueous solutions and a radical anion in organic solvents. The dimerization behavior of both 2 and 3 is temperature dependent in methanol solutions. Dimerization equilibrium has a direct impact on catholyte stability during galvanostatic charge-discharge cycling in static H-cells. Specifically, conditions that favor the free radical anion or π-dimer exhibit significantly enhanced cycling profiles.

Effects of N-functional groups on the electron transfer kinetics of VO2+/VO2+ at carbon: Decoupling morphology from chemical effects using model systems

Carbons and nanocarbons are important electrode materials for vanadium redox flow battery applications, however, the kinetics of vanadium species are often sluggish at these surfaces, thus prompting interest in functionalization strategies to improve performance. Herein, we investigate the effect of N-functionalities on the VO2+/VO2+ redox process at carbon electrodes. We fabricate thin film carbon disk electrodes that are metal-free, possess well-defined geometry and display smooth topography, while featuring different N-site distribution, thus enabling a mechanistic investigation of the intrinsic surface activity towards VO2+/VO2+. Voltammetry and electrochemical impedance spectroscopy show that N-functionalities improve performance, with pyridinic/pyrrolic-N imparting the most significant improvements in charge transfer rates and reversibility, compared to graphitic-N. This was further supported by voltammetry studies on nitrogen-free electrodes modified via aryldiazonium chemistry with molecular pyridyl adlayers. Computational modeling using an electrochemical-chemical mechanism indicates that introduction of surface pyridinic/pyrrolic-N can increase the heterogeneous rate constants by approximately two orders of magnitude relative to those observed at nitrogen-free carbon (k0 = 1.29 × 10−4 vs 9.34 × 10−7 cm/s). Simulations also suggest that these N-functionalities play a role in affecting reaction rates in the chemical step. Our results indicate that nitrogen incorporation via basic functional groups offers an interesting route to the design of advanced carbon electrodes for VRFB devices.

Numerical Investigation of Flow Hydrodynamics in a Flow Field and Porous Substrate Configuration for Redox Flow Battery Application

In the recent past, most of the literature reported that the electrolyte circulations in parallel flow field configurations exhibit severe non-uniformity with higher Pressure Drop (Δp). The present work proposes a three-dimensional computational design of flow field configurations to achieve a single-phase uniform flow with minimal pump power and flow dispersion over an active cell area of 131cm2 for All Iron Redox Flow Battery (AIRFB). Computational investigation of the Pressure Drop (Δp), electrolyte flow velocity and uniform flow distribution in the channels and through the graphite felt electrode under various flow conditions was conducted using the Computational Fluid Dynamics (CFD) tool. It is observed from the results that the Multi-Channel Serpentine Flow Field (MCSFF) has the least pressure drop among the other flow fields. However, the Cross-Split Serpentine Flow Field (CSSFF) resulted in better flow circulation and dispersion over the entire active cell area with a high uniformity index, operating at a wide range of flow rates with a reasonable Pressure Drop (Δp). The porous media permeability and a strong function of Compression Ratio (CR) were numerically validated from the well-known correlation existing in the literature. At CR 50% it was observed that the volume uniformity index of the felt was 69%, which would correspondingly enhance the rate of mass transfer and electro-kinetics at electrode felt and ion conductivity across the membrane. The CSSFF configuration is predominant in terms of uniform flow distribution and wettability at the defined operating conditions resulting in enhanced cell performance.