Redox Flow Battery Applications Research Articles

Crosslinked sulfonated poly (arylene ether sulfone)/sulfonated poly (vinyl alcohol) membrane formed by in situ casting and reaction for vanadium redox flow battery application

The in situ crosslinked membranes for vanadium redox flow battery (VRFB) application, prepared from sulfonated poly (arylene ether sulfone) (SPAES) and poly (vinyl alcohol) (PVA) as well as sulfonated poly (vinyl alcohol) (SPVA), are detailedly evaluated in this paper. The results of scanning electron microscope (SEM) and X-ray diffraction (XRD) reveal that the cross reaction improved the compatibility between SPAES and SPVA, and reduced the crystallinity in the membrane. The highly homogeneous dispersed structures of the crosslinked membrane are found under the electronic microscope. The crosslinked structure effectively reduces the water uptake, swelling ratio, and vanadium ion permeability, and more importantly improved the mechanical performance and stability of the membranes. Due to the introduction of sulfonic groups into the PVA, the proton selectivity of SPAES crosslinked SPVA (SPAES-C-SPVA) reaches to 3.37×105 S min cm−3 which is 8 times higher than that of Nafion117. The VRFB single cell tests show that the cell assembled with SPAES-C-SPVA membrane significantly displays higher energy efficiency (70.1% vs 60.9% at 100 mA cm−2) and much longer self-discharge time than Nafion117 (98 h vs 41 h). After 100 charge-discharge cycling test, the crosslinked membrane still maintains excellent stability. The above results indicate the crosslinked membranes could be a promising candidate used in VRFB applications.

A Self-Trapping, Bipolar Viologen Bromide Electrolyte for Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs) have become increasing attractive for scalable energy storage. However, it remains challenging to develop high voltage, powerful AORFBs because of the lack of catholytes with high redox potential. Herein, we report methyl viologen dibromide ([MV]Br2) as a facile self-trapping, bipolar redox electrolyte material for pH neutral redox flow battery applications. The formation of the [MV](Br3)2 complex was computationally predicted and experimentally confirmed. The low solubility [MV](Br3)2 complex in the catholyte during the battery charge process not only mitigates the crossover of charged tribromide species (Br3-) and addresses the toxicity concern of volatile bromine simultaneously. A 1.53 V bipolar MV/Br AORFB delivered outstanding battery performance at pH neutral conditions, specifically, 100% total capacity retention, 133 mW/cm2 power density, and 60% energy efficiency at 40 mA/cm2.

Highly Soluble Imidazolium Ferrocene Bis(sulfonate) Salts for Redox Flow Battery Applications.

Redox flow batteries (RFBs) are scalable devices that employ solution-based redox active components for scalable energy storage. To maximize energy density, new highly soluble catholytes and anolytes need to be synthesized and evaluated for their electrochemical performance. To that end, we synthesized a series of imidazolium ferrocene bis(sulfonate) salts as highly soluble catholytes for RFB applications. Six salts with differing alkyl chain lengths on the imidazolium cation were synthesized, characterized, and electrochemically analyzed. While aqueous solubility was significantly improved, the reactivity of the imidazolium cation and the increased viscosities of the salt solutions in water (which increase with increasing imidazolium chain length) limit the applicability of these materials to RFB design.

Characterization of the Electrochemical Behavior of MnSO4 with and without TiOSO4 in H2SO4 Solution

An electrochemical characteristic evaluation of the Ti-Mn electrolyte system in sulfuric acid aqueous solution was conducted in different acid concentrations with and without the mixing of TiOSO4 and MnSO4 electrolyte system for a redox flow battery application. The reaction mechanism of Mn in sulfuric acid electrolyte was studied using the cyclic voltammetry and anodic voltage hold techniques. The overall anodic behavior of the Mn system can be described by the oxidation reaction of Mn2+ to MnO2 where Mn3+ is an intermediate species. As the acid concentration increases, less MnO2 is formed on the working electrode and more Mn3+ is formed in the solution; however, due to the instability of Mn3+ in an aqueous solution, Mn3+ undergoes a hydrolysis reaction to precipitate as MnOOH in the electrolyte solution. The addition of 1 M TiOSO4 to the Mn electrolyte is suggested to lower the H+ concentration which increases the amount of MnOOH in the solution and lowers the concentration of the aqueous Mn3+ species. The MnOOH formed by the Mn3+ hydrolysis reaction is found to be electrochemically active if it is deposited on the working electrode.

Behaviour of 3,4‐Dihydroxy‐9,10‐Anthraquinone‐2‐Sulfonic Acid in Alkaline Medium: Towards a Long‐Cycling Aqueous Organic Redox Flow Battery

AbstractThe performance of a redox compound in redox flow batteries (RFB) highly depends on the electrolytic medium and operating conditions. It is exemplified in this work with the commercially available and relatively low‐cost dye 3,4‐dihydroxy‐9,10‐anthraquinone‐2‐sulfonic acid (ARS), which was used as negolyte in basic medium. At high pH, the ARS behavior revealed interesting features for RFB applications, such as a low half‐wave potential of −0.99 V (vs Ag/AgCl), negatively shifted by phenolate groups, and an improved solubility compared with acidic medium depending on the nature of the cations. For the highly soluble ARS potassium salt (ARSK), a maximum power density of 117 mW cm−2 and a demonstrated energy density of 20 Wh L−1 were obtained with K4[Fe(CN)6] as posolyte. The capacity slightly decreased during cycling, reaching 90 % after 325 cycles. A long cycling of ARS sodium salt (ARSNa) over 11 operating months was demonstrated in this work. A slow chemical degradation was highlighted giving rise to the formation of 3‐hydroxy‐9,10‐anthraquinone‐2‐sulfonic acid (HAQS) as the main degradation product due to hydrodeoxygenation reaction. Interestingly, this compound exhibited high performance in RFB and a good stability with a loss of capacity of 0.29 % per day.

Comparative Study of Organic Radical Cation Stability and Coulombic Efficiency for Nonaqueous Redox Flow Battery Applications

Comparative Study of Organic Radical Cation Stability and Coulombic Efficiency for Nonaqueous Redox Flow Battery Applications

Novel, Stable Catholyte for Aqueous Organic Redox Flow Batteries: Symmetric Cell Study of Hydroquinones with High Accessible Capacity.

Owing to their broad range of redox potential, quinones/hydroquinones can be utilized for energy storage in redox flow batteries. In terms of stability, organic catholytes are more challenging than anolytes. The two-electron transfer feature adds value when building all-quinone flow battery systems. However, the dimerization of quinones/hydroquinones usually makes it difficult to achieve a full two-electron transfer in practical redox flow battery applications. In this work, we designed and synthesized four new hydroquinone derivatives bearing morpholinomethylene and/or methyl groups in different positions on the benzene ring to probe molecular stability upon battery cycling. The redox potential of the four molecules were investigated, followed by long-term stability tests using different supporting electrolytes and cell cycling methods in a symmetric flow cell. The derivative with two unoccupied ortho positions was found highly unstable, the cell of which exhibited a capacity decay rate of ~50% per day. Fully substituted hydroquinones turned out to be more stable. In particular, 2,6-dimethyl-3,5-bis(morpholinomethylene)benzene-1,4-diol (asym-O-5) displayed a capacity decay of only 0.45%/day with four-week potentiostatic cycling at 0.1 M in 1 M H3PO4. In addition, the three fully substituted hydroquinones displayed good accessible capacity of over 82%, much higher than those of conventional quinone derivatives.

High performance membranes based on pyridine containing poly (aryl ether ketone ketone) for vanadium redox flow battery applications

This reported study describes an effort to produce durable membranes for high performance vanadium redox flow batteries (VRFB). The reported novel, positively-charged membranes are derived from pyridine-containing poly(aryl ether ketone ketone) (PyPEKK) and are prepared using an environmentally-friendly method. Basic pyridine groups on the polymer backbone serve as proton acceptor sites and these electron-rich, polar sites establish proton transport channels, which provide PyPEKK membranes with an area resistance as low as 0.15 Ω cm2, which is comparable to Nafion 212 (0.13 Ω cm2). The novel PyPEKK membranes have low vanadium permeability due to the Donnan repulsion between the positively charged protonated pyridine groups and the vanadium ions. Consequently, the resulting dense structure of these PyPEKK membranes produce an impressive energy efficiency in VRFB (89.0% at 80 mA cm−2, and 80.1% at 180 mA cm−2). The PyPEKK membrane exhibit stable performance of 1000 cycles and low oxidation. These PyPEKK membranes are highly promising candidates for VRFB application.

A Proposed Mechanism for the Cerium Electron Transfer for Use in RFB Applications

Redox flow batteries (RFBs) are a promising technology for large scale energy storage, including grid-scale storage of renewable energy, due to their longer lifecycles and easier scalability than other, more developed battery technologies, e.g., lithium-ion.1 Despite their promising nature, RFBs are currently too expensive for market deployment. For instance, the U.S. Department of Energy has reported a target capital cost for new energy storage technologies of 150 $/kWh,2 yet recent reports of the all-vanadium RFB indicate the current capital cost is at least 555 $/kWh.1 The large expense of RFBs is in part driven by inefficiencies in mass transport, ohmic, and kinetics, resulting in large losses in voltage at high current densities. One way to improve the efficiency of RFBs is to explore new chemistries that increase the voltage window of the battery, effectively reducing the significance of any incurred voltage losses. The cerium redox reaction between Ce3+ and Ce4+ is being considered for use at the positive electrode in an aqueous RFB, due to its high positive redox potential that ranges between 1.28 V vs SHE in HCl and 1.74 V vs SHE in HClO4.3 Despite being thermodynamically promising for RFB applications, the kinetics of the Ce3+/Ce4+ electron transfer have been reported to be a limiting performance factor in studies of cerium RFB systems.4 The cerium charge transfer mechanism, which is currently not known, must be elucidated to improve the kinetics. Identifying whether the Ce3+/Ce4+ electron transfer occurs via an inner- or outer-sphere mechanism would allow for advances in the kinetics because it would indicate whether the electrode (inner-sphere) or the electrolyte (outer-sphere) controls the rate of the electron transfer.Previous Ce3+/Ce4+ kinetic studies4 have demonstrated that the kinetic behavior varies significantly depending on the conditions used, e.g., electrode material and geometry, acid concentration, and total cerium, Ce3+ and Ce4+ concentration. The wide range of kinetic performance parameters reported in the literature makes it difficult to discern whether the electrode or electrolyte is controlling the rate of the reaction. In this study, to first identify the role of the electrode we analyze the Ce3+/Ce4+ electron transfer kinetics in sulfuric acid on three different electrode materials, platinum (Pt), glassy carbon (GC), and boron-doped diamond (BDD). We utilize a rotating disk electrode (RDE) configuration for each of these electrode materials to control for mass transfer effects, and we study the effect of Ce3+ and Ce4+ concentration on kinetic performance, quantified by exchange current density. The purpose of these experiments is two-fold; first, to determine whether there are significant differences in kinetic performance as a function of electrode material, which would suggest an inner-sphere electron transfer, and second, to identify a rate law for the Ce3+/Ce4+ electron transfer. This rate law can be derived using mechanistic theories such as Marcus Theory and fitted to the experimental data to identify key performance parameters such as reorganization energy (dependent on electrolyte). These efforts will help to identify the charge transfer mechanism, which as noted earlier, is critical to furthering the use of the cerium redox couple in RFB applications. We find that the exchange current densities as a function of Ce4+ concentration for each RDE material are within a factor of five of each other, which could suggest the electron transfer occurs via an outer-sphere mechanism. However, our Ce K-edge EXAFS analysis of the Ce ions in sulfuric acid, as well as our previous Ce L3-edge EXAFS and computational study5 on the Ce ion structures, demonstrates that an inner-sphere structural change occurs during the electron transfer, as Ce3+ prefers the [Ce(H2O)9]3+ structure, while Ce4+ is complexed by at least one anion. This indicates that the electron transfer cannot occur in a simple single outer-sphere step. Thus, to reconcile these seemingly contradictory kinetic and structural data, we propose a mechanism which is consistent with our kinetic data in which a quasi-equilibrated ligand exchange reaction occurs in addition to the rate-determining electron transfer step. Identification of a likely Ce3+/Ce4+ charge transfer mechanism will help to advance the eventual goal of using the cerium redox couple in RFB systems by helping to demonstrate which aspects are critical to performance, e.g., electrode or electrolyte effects. PNNL. Energy Storage Technology and Cost Characterization Report. (2019).U.S. DOE. Energy Storage Program Planning Document. (2011).Smith & Getz. Ind. Eng. Chem. Res. 10, 191–195 (1938).Walsh. et al. ChemPlusChem 80, 288–311 (2015).Buchanan. et al. Inorg. Chem. 59, 12552–12563 (2020).

Bipolar Diimide Based Molecule for Nonaqueous Symmetric Redox Flow Battery

Redox flow batteries (RFBs) allow decoupling of power and energy capacity, a unique feature that makes them a promising energy storage technology for large scale applications[1]. In recent years, there has been a massive development in aqueous organic RFBs. However, their commercialization has been hindered by low energy density owing to low solubility of redox active materials and narrow electrochemical stability window of water which limits the cell voltage[2] .Nonaqueous redox flow batteries based on organic redox active material are a regarded as an alternative to aqueous organic RFBs[3] . Apart from enjoying the advantages of organic redox active materials such as low cost, diverse molecular structure and tailorability, nonaqueous RFBs also benefits from using organic supporting electrolytes which provide a wider voltage window (2-5V) facilitating a high energy capacity and low-cost storage system[4, 5] .Herein, a diimide based molecule is designed, chemically modified, analysed and employed as bipolar redox active materials for a total organic symmetric redox flow battery. Cyclic voltammetry studies show three redox couples, two anodic one electron transfer and one cathodic two electrons transfer redox transitions which are electrochemically reversible. The potential difference between the cathodic peak and the inner and outer anodic peaks leads to a promising cell voltage of 1.51V and 2.22V respectively in redox flow battery application. This work suggests an alternative way to design a multielectron redox active materials for symmetric batteries.Figure 1: Cyclic voltammogram of 0.002M diimide based molecule in 0.1M tetrabutylammonium hexafluorophosphate in dimethylformamide recorded on a glassy carbon working electrode with a scan rate of 50 mV/s. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R) References [1] P. Alotto, M. Guarnieri, F. Moro, Renewable and Sustainable Energy Reviews. 29 (2014) 325-335.[2] J. Luo, B. Hu, M. Hu, Y. Zhao, T.L. Liu, ACS Energy Letters. 4 (2019) 2220-2240.[3] S. Shin, S. Yun, S. Moon, Rsc Advances. 3 (2013) 9095-9116.[4] W. Wang, V. Sprenkle, Nature Chemistry. 8 (2016) 204-206.[5] M. Hayyan, F.S. Mjalli, M.A. Hashim, I.M. AlNashef, T.X. Mei, Journal of Industrial and Engineering Chemistry. 19 (2013) 106-112. Figure 1

Assessing the Design and Operation of Redox Flow Batteries through Levelized Cost Analysis

Energy storage is expected to play an important role in enabling deep decarbonization of the electric sector by addressing the intermittencies of renewable power generation1. The redox flow battery (RFB) is a potential energy storage solution whose unique decoupling of energy and power make it increasingly competitive, on a capital cost basis, at longer discharge durations2. While the capital cost benefits to RFBs are well-described in the literature3, there are additional economic benefits associated with operation and maintenance of open systems (e.g., tune-ups, sparing strategies)4 as compared to closed systems like lithium-ion batteries. For example, crossover, undesirable species transport through the semi-permeable membrane that separates the positive and negative electrolytes, leads to comparatively rapid capacity fade in RFBs5 but can be remediated via electrolyte rebalancing, replacement, or other servicing, all of which can be performed without sacrificing or altering the reactor components. The costs necessary to maintain battery performance over time impact its economic viability, but are not captured in the conventional capital cost estimations6. This motivates the development of techno-economic models that consider the variable operating principles of different battery formats and chemistries.In this presentation, we describe a simple levelized cost of storage (LCOS) model for RFBs that captures long-term performance changes and maintenance costs by including capacity fade and recovery7. We use this model to assess the impact of different design and operational decisions on RFB cost. Specifically, we contemplate different chemistries (symmetric vs. asymmetric, finite lifetime vs. infinite lifetime), operating strategies (e.g., rebalancing schedule), performance improvements (e.g., reducing fade rates), design decisions (e.g., battery sizing), and investment approaches (e.g., electrolyte leasing). We find that there are tradeoffs in capital and operating expenses, and in many cases upfront investments pay off in long-term savings. We anticipate this analysis will provide new insights into the cost-drivers for RFBs and motivate further research efforts in the evaluation and development of new chemistries, component materials, and reactor configurations. Acknowledgements We gratefully acknowledge funding from the MIT Energy Initiative. References Intergovernmental Panel on Climate Change. Global Warming of 1.5 C. https://www.ipcc.ch/sr15/ (2018).Darling, R. M., Gallagher, K. G., Kowalski, J. A., Seungbum, H. & Brushett, F. R. Pathways to low-cost electrochemical energy storage: a comparison of aqueous and nonaqueous flow batteries. Energy Environ. Sci. 7, 3459–3477 (2014).Viswanathan, V. et al. Cost and performance model for redox flow batteries. J. Power Sources 247, 1040–1051 (2014).Yuan, X.-Z. et al. A review of all-vanadium redox flow battery durability: Degradation mechanisms and mitigation strategies. Int. J. Energy Res. 1–40 (2019) doi:10.1002/er.4607.Prifti, H., Parasuraman, A., Winardi, S., Lim, T. M. & Skyllas-Kazacos, M. Membranes for redox flow battery applications. Membranes vol. 2 275–306 (2012).US Department of Energy. Grid Energy Storage. https://www.energy.gov/sites/prod/files/2014/09/f18/Grid Energy Storage December 2013.pdf (2013).Rodby, K. E. et al. Assessing the levelized cost of vanadium redox flow batteries with capacity fade and rebalancing. J. Power Sources 460, 227958 (2020).

Exploring the Structure-Function-Performance Relationship of Carbon Electrodes Toward Rational Design of High-Performance Redox Flow Cells

Redox flow batteries (RFBs) are a promising grid-scale energy storage technology for the integration of intermittent renewable sources, such as wind and solar, into the electrical grid due to their modularity, flexible design, and cost-effectiveness in long-duration storage [1]. Unlike traditional batteries, for RFBs, electrolytes are stored in external tanks and are circulated through the flow cell. In the flow cell, electrical energy is stored via the electrochemical reactions of the redox active species dissolved in liquid electrolytes. While this new archetype provides unique benefits, the flow-assisted nature of RFBs presents many challenging issues, including but not limited to significant transport losses due to the poor electrode and cell design, and the related low power density [2-3]. It can be hypothesized that many of these challenges are primarily related to the concept of electrolyte utilization [4]. In this regard, porous electrode is the key component, which is responsible for multiple critical functions including delivery of liquid electrolytes as well as facilitating ion/charge transfer and providing sites for electrochemical reactions [4]. Despite its importance, the required fundamental knowledge on how to systematically design effective electrodes specifically tailored for RFB applications has not received much attention [5].In this talk, the recent efforts on establishing the structure-function-performance linkages for commercially available carbon electrodes will be presented. The results of the experiments utilizing an electrochemical protocol that combines symmetric flow cell cycling with electrochemical impedance spectroscopy (EIS) and polarization curve (PC) measurements will be discussed. The impedance data is fitted into a mathematical model, and key physicochemical properties of carbon electrodes and the correlated performance losses are quantified. Additionally, we will share our perspective on manufacturing high-performance carbon electrodes using a high-throughput screening platform.

A nonaqueous organic redox flow battery using multi-electron quinone molecules

Organic redox flow batteries are promising energy storage devices due to their moderately low-cost and scalability. This paper introduces a new multi-electron redox active material, tetra-aminoanthraquinone (DB-1) that is capable of forming cations with an oxidation state of 4+, yielding one of the highest electrode potentials (up to 4.4 V vs. Li) and the largest multi-electron transfer (ca. 4 e−). Computational calculations using density functional theory reveal that the DB-1 molecule has narrower HOMO-LUMO band gaps (<3 eV) than similar molecules (>3.9 eV). Stable charge-discharge cycling performance of this organic molecule is observed with high energy efficiencies (ca. 71%) at a relatively high current density of 20 mA cm−2 over 50 cycles, demonstrating the marked potential of DB-1 for future redox flow battery applications.

Evaluation of Electrochemical Stability of Sulfonated Anthraquinone-Based Acidic Electrolyte for Redox Flow Battery Application.

Despite intense research in the field of aqueous organic redox flow batteries, low molecular stability of electroactive compounds limits further commercialization. Additionally, currently used methods typically cannot differentiate between individual capacity fade mechanisms, such as degradation of electroactive compound and its cross-over through the membrane. We present a more complex method for in situ evaluation of (electro)chemical stability of electrolytes using a flow electrolyser and a double half-cell including permeation measurements of electrolyte cross-over through a membrane by a UV–VIS spectrometer. The method is employed to study (electro)chemical stability of acidic negolyte based on an anthraquinone sulfonation mixture containing mainly 2,6- and 2,7-anthraquinone disulfonic acid isomers, which can be directly used as an RFB negolyte. The effect of electrolyte state of charge (SoC), current load and operating temperature on electrolyte stability is tested. The results show enhanced capacity decay for fully charged electrolyte (0.9 and 2.45% per day at 20 °C and 40 °C, respectively) while very good stability is observed at 50% SoC and lower, even at 40 °C and under current load (0.02% per day). HPLC analysis conformed deep degradation of AQ derivatives connected with the loss of aromaticity. The developed method can be adopted for stability evaluation of electrolytes of various organic and inorganic RFB chemistries.

Electrochemical Characterization of Aromatic Molecules with 1,4-Diaza Groups for Flow Battery Applications

The aqueous redox flow battery is a promising technology for large-scale low cost energy storage. The rich possibilities for the tailoring of organic molecules and the possibility to discover active materials of lower cost and decreased environmental impact continue to drive research and development of organic compounds suitable for redox flow battery applications. In this work, we focus on the characterization of aromatic molecules with 1,4-diaza groups for flow battery applications. We examine the influence of electron-withdrawing and electron-donating substituents and the effect of the relative position of the substituent(s) on the molecule. We found that electron-withdrawing substituents increased the potential, while electron-donating decreased it, in agreement with expectations. The number of carboxy-groups on the pyrazinic ring was found to have a strong impact on the heterogeneous electron transfer kinetics, with the slowest kinetics observed for pyrazine-2,3,5,6-tetracarboxylic acid. The stability of quinoxaline was investigated by cyclic voltammetry and in a flow cell configuration. Substitution at the 2,3-positions in quinoxaline was found to decrease the capacity fade rate significantly. Furthermore, we demonstrated how molecular aggregation reduces the effective number of electrons involved in the redox process for quinoxalines. This translates to a significant reduction of the achievable volumetric capacity at higher concentrations, yielding values significantly lower than the theoretical capacity. Finally, we demonstrate that such capacity-limiting molecular aggregation may be reduced by introducing flexible side chains with bulky charged groups in order to increase electrostatic repulsion and steric hindrance.

Facile preparation of an oxygen-functionalized carbon felt electrode to improve VO2+/VO2+ redox chemistry in vanadium redox flow batteries

Herein, a new method for the facile introduction of oxygen-containing functional groups on a carbon felt electrode in a short treatment time has been proposed for application in a vanadium redox flow batteries (VRFBs). Benzoyl peroxide (BPO) can produce free radicals by attacking the CC bonds or CH bonds of the carbon felt. The generated unstable free radicals allow further reactions with oxygen to yield oxygen-containing functional groups on the surface of the carbon felt electrode. The electrochemical performance of the BPO-treated carbon felt electrode toward the VO2+/VO2+ redox reaction was superior to that of the pristine carbon felt electrode. Therefore, the VRFB cell employing the BPO-treated carbon felt electrode showed an outstanding energy efficiency of 75% at a current density of 100mAcm−2. This improvement is attributed to the well-known electrocatalytic effects of the oxygen-functionalized surface properties of the BPO-treated carbon felt electrode.

Chloride ions as an electrolyte additive for high performance vanadium redox flow batteries

Employing chloride ions as additive in electrolytes is proposed for all vanadium redox flow batteries. The influences of different concentrations of chloride ions in the electrolyte on the battery performance are investigated. It is found that the chloride ions improve the reaction activity of VO2+/VO2+ redox couples. At a current density of 200 mA cm−2, the battery with an optimum chloride ion concentration of 0.04 M attains an energy efficiency and vanadium utilization ratio of 82.5% and 86.3%, which are 2% and 4.1% higher than those without the additives. Further, at 400 mA cm−2, the battery with 0.04 M chloride ions shows a high rate capability by delivering the specific discharge capacity of 5.95 Ah L−1. Moreover, the battery with 0.04 M chloride ions exhibits the capacity retention rate of 83.1% after one hundred cycle test, which is nearly 12% higher than that of pristine battery. The results suggest that chloride ions, as low-cost additives in electrolytes, offer great promise for high-performance vanadium redox flow battery application.

A quantitative evaluation of computational methods to accelerate the study of alloxazine-derived electroactive compounds for energy storage

Alloxazines are a promising class of organic electroactive compounds for application in aqueous redox flow batteries (ARFBs), whose redox properties need to be tuned further for higher performance. High-throughput computational screening (HTCS) enables rational and time-efficient study of energy storage compounds. We compared the performance of computational chemistry methods, including the force field based molecular mechanics, semi-empirical quantum mechanics, density functional tight binding, and density functional theory, on the basis of their accuracy and computational cost in predicting the redox potentials of alloxazines. Various energy-based descriptors, including the redox reaction energies and the frontier orbital energies of the reactant and product molecules, were considered. We found that the lowest unoccupied molecular orbital (LUMO) energy of the reactant molecules is the best performing chemical descriptor for alloxazines, which is in contrast to other classes of energy storage compounds, such as quinones that we reported earlier. Notably, we present a flexible in silico approach to accelerate both the singly and the HTCS studies, therewithal considering the level of accuracy versus measured electrochemical data, which is readily applicable for the discovery of alloxazine-derived organic compounds for energy storage in ARFBs.

Novel amphoteric ion exchange membranes by blending sulfonated poly(ether ether ketone) with ammonium polyphosphate for vanadium redox flow battery applications

AbstractA novel amphoteric ion exchange membrane for vanadium redox flow battery (VRFB) was explored by blending sulfonated poly(ether ether ketone) (SPEEK) and ammonium polyphosphate (APP). The high‐stability flame retardant of cross‐linked APP with a large number of NH4+ groups was first introduced into SPEEK membrane. It was observed that the addition of APP with special structure could achieve a good balance between proton conductivity and vanadium ions permeability. The abundant NH4+ in APP could block the penetration of vanadium ions by Donnan/Manning exclusion effect and ionic crossing networks due to the ionic bonds between cation and anion groups, and specially a small amount of APP within 5% could remarkably improve the proton conductivity of pristine SPEEK membrane might be ascribed to the unique fast proton transport channels formed by hydrogen bond networks and particular micro‐phase separation as a result of interaction between SPEEK and APP. When 5% APP was blended, the SPEEK/APP‐5% (S/APP‐5%) amphoteric membrane showed a higher selectivity of 20.87 × 104 S min/cm3 (with a good proton conductivity of 0.075 S/cm and a lower VO2+ permeability of 3.45 × 10−7 cm2/ min) and presented better thermal and chemical stability compared to Nafion115 and SPEEK membranes. The VRFB single cell assembled with S/APP‐5% amphoteric membrane exhibited more excellent performance than that of Nafion115 and pristine SPEEK membranes, which revealed a higher coulombic efficiency of 96.3%–98.3%, comparable voltage efficiency of 88.4%–78.7% and higher energy efficiency of 85.1%–77.4% from 40 to 80 mA/cm2, respectively, and showed relatively good stability of the efficiency up to 50 cycles at 60 mA/cm2. The results demonstrated that the designed S/APP amphiprotic membrane of outstanding selectivity, high battery efficiency, and good durability is a prospected VRFB separator.

Iron-iminopyridine complexes as charge carriers for non-aqueous redox flow battery applications

Non-aqueous redox flow batteries (RFBs) have been gaining increased attention in the energy storage arena. Some of their attractive features include the promise for high energy densities, wider voltage windows compared to aqueous systems, as well as wider operating temperature ranges. One of the major challenges in the development of these systems is the lack of electroactive materials that can undergo reversible redox events within the larger potential window of organic solvents. Herein, we present the design, synthesis, and measurement of electrochemical properties of some iron-based imine- and iminopyridine complexes as promising candidates for RFB applications. Synthesized complexes afforded three reversible redox couples over a ~3 V range. The redox events were also computationally explored and are in good agreement with experimental data. Theoretical calculations show that the redox event at the positive potential can be characterized as metal-centered event corresponding to Fe3+→ Fe2+ reduction, while the two redox events at negative potentials are associated with the consecutive reductions of iminopyridine or diimine moieties. This work has led to the identification of a promising anolyte material, [tris(imino)pyridine)Fe][OTf]2 (1), which is soluble in acetonitrile and is synthesized in two simple steps. This species shows outstanding performance when cycled between 1.0 V and 2.35 V, with 93% coulombic efficiency, 84% energy efficiency, and a capacity decay rate of 0.61% per cycle at 5 mA cm−2. Further modifications to this kind of charge carrier may lead to the development of high energy density materials for grid scale energy storage applications.