Development Of Redox Flow Batteries Research Articles

Screening of First-Row Transition Metal Complexes for Aqueous Redox Flow Batteries: Experimental and Density Functional Theory Approaches

The development of redox flow batteries took new directions in the last decades. From initial metal-based systems, it evolved towards organic and hybrid approaches in aqueous media. Furthermore, redox targeting flow batteries are an emerging alternative to the traditional redox flow battery architecture which offer improved energy density via an added electroactive solid ‘booster’. Whatever the system, there is a need of soluble electroactive species with potentials allowing large cell potentials within the water stability window or imperative redox potentials matching between soluble redox mediator and the solid for redox targeting. Transition metal complexes are a promising class of redox electroactive centers due to the tunability of their solubility and electrochemical potential, and the ability to take into account sustainability1. Recent reports underlined for instance that either the modification of the ligand2 or using heteroleptic complexes3, can lead to iron complexes with solubilities higher than 1 M and potentials higher than 1 V vs SHE. Furthermore, reliable and time-efficient prediction tools for the redox potentials of transition metal complexes are needed to help the search for new potential systems. While density functional theory usually provides a good trade-off between computational cost and accuracy, calculations on transition metal complexes often result in large errors. We thus developed a procedure and compared different solvation methods and levels of theory using an initial experimental data set based on aqueous iron complexes with bidentate ligands (Figure 1).In the present contribution, we will illustrate this coordination approach through a screening of new complexes based on non-toxic and affordable transition metal complexes. In this perspective, electrochemical characterization at different pH, coupled with NMR and UV-Visible studies will be presented, to screen for new candidates as electrolyte in redox flow batteries. Finally, we will show how the calculations corrected using a simple linear regression using training data yields a good prediction of redox potentials (mean average error =0.09 V).

Electrochemical Behavior of Tris(2,2'-bipyridine)Iron Complex in 1-Butyl-1-Methylpyrrolidinium Bis(fluorosulfonyl)Amide Ionic Liquid in the Presence and Absence of Lithium Ion

Ionic liquids composed of bis(fluorosulfonyl)amide (FSA–) have been investigated as the promising electrolytes for rechargeable batteries because of the formation of favorable solid electrolyte interphase (SEI) for some alkali metal ions in addition to relatively low viscosity. The FSA–-based ionic liquids are also considered to be applied to redox flow batteries, which utilize the electrode reactions of soluble species on inert electrodes. However, few electrode reactions of dissolved species have been explored in the FSA–-based ionic liquids. On the other hand, ion-conductive membranes are necessary for development of redox flow batteries. An inorganic solid-state conductor, lithium ion conductive glass ceramics (LICGC, Li1+x+y Al x (Ti,Ge)2-x Si y P3-y O12), is one of the possible membranes usable in ionic liquids containing Li+. In the present study, the electrochemical behavior of tris(2,2'-bipyridine)iron complexes, [Fe(bpy)3]2+, was investigated in BMPFSA (BMP = 1-butyl-1-methylpyrrolidinium) in the presence and absence of Li+.The redox reactions of [Fe(bpy)3]3+/2+, [Fe(bpy)3]2+/+, [Fe(bpy)3]+/0, and [Fe(bpy)3]0/– were observed by cyclic voltammetry in the absence of Li+. However, these reactions were affected by the SEI formation at a negative potential region in the presence of Li+. The donor number of BMPFSA was estimated from the formal potential of [Fe(bpy)3]3+/2+. The diffusion coefficients of [Fe(bpy)3]3+ and [Fe(bpy)3]2+ were determined by chronoamperometry. The standard rate constant for [Fe(bpy)3]3+/2+ was evaluated by electrochemical impedance spectroscopy. The relationship between these kinetic parameters and physical properties was discussed.

Cross-Linked Anion Exchange Membranes for Non-Aqueous Redox Flow Battery

The development of redox flow batteries (RFBs) has been one of the primary responses to the need to store harvested renewable energy for improving grid reliability and utilization.1–6 In this study, we synthesized and characterized different Bromobutyl Norbornene backbone-based anion-exchange membranes (AEM) with different Tetramethyl hexadiamine crosslinker amounts and explored the performance of AEMs in non-aqueous redox flow batteries under ideal conditions. Performance was compared based on high stability in a non-aqueous solvent, high permeability to the charge-carrying ion, low electric cell resistance, low cross-over of the redox-active molecules, high mechanical property, and low cost. 100 cycles battery data indicates that Bromobutyl Norbornene-based AEM performance is far better than the commercial best membrane Fuma-375. The best membrane will be found which will show no cross-over of redox-active electrolytes after 1000 battery cycle experiments and exhibit low resistance and high structural/chemical stability in MeCN solvent. References (1) Mazumder, Md. M. R.; Burton, A.; Richburg, C. S.; Saha, S.; Cronin, B.; Duin, E.; Farnum, B. H. Controlling One-Electron vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate. Inorg. Chem. 2021, 60 (17), 13388–13399. https://doi.org/10.1021/acs.inorgchem.1c01699.(2) Mazumder, M. M. R.; Farnum, B. H. Two One Electron vs One Two-Electron Redox System of Nickel Diethyldithiocarbamate for Redox Flow Battery; ACS, 2021. https://doi.org/10.1021/scimeetings.1c00951.(3) Mazumder, M. M. R. Ni(IV/II) Redox Couple 2e- Reversibility Improvement for Redox Flow Battery. ECS Meet. Abstr. 2021, MA2021-02 (5), 1855–1855. https://doi.org/10.1149/ma2021-0251855mtgabs.(4) Islam, R.; Mazumder, M. M. R.; Farnum, B. H. Solvent Dependent Spectroscopic and Electrochemical Studies of Nickel (II) Diethyldithiocarbamate for Energy Storage. ECS Meet. Abstr. 2021, MA2021-01 (1), 54–54. https://doi.org/10.1149/ma2021-01154mtgabs.(5) Rahaman Mazumder, M. M.; Islam, R.; Khan, M. A. R.; Anis‐Ul‐Haque, K. M.; Rahman, M. M. Efficient AcFc‐[Fe III (Acac) 3 ] Redox Couple for Non‐aqueous Redox Flow Battery at Low Temperature. Chem. – Asian J. 2023, 18 (1). https://doi.org/10.1002/asia.202201025.(6) Mazumder, Md. M. R.; Dalpati, N.; Pokkuluri, P. R.; Farnum, B. H. Zinc-Catalyzed Two-Electron Nickel(IV/II) Redox Couple for Multi-Electron Storage in Redox Flow Batteries. Inorg. Chem. 2022, 61 (48), 19039–19048. https://doi.org/10.1021/acs.inorgchem.2c03124.

Cross-Linked Phenyl-Acrylate-Based Anion Exchange Membranes Synthesis, Characterization and Performance in Non-Aqueous Redox Flow Battery

The development of redox flow batteries (RFBs) has been one of the primary responses to the need to store harvested renewable energy for improving grid reliability and utilization.1–4 In this study, we synthesized and characterized seven types of Phenyl acrylate (PA) backbone-based anion-exchange membranes (AEM) and explored the performance of AEMs in non-aqueous redox flow batteries under ideal conditions. IR data (Figure 1 ) confirms the incorporation of ion carriers in the PA backbone. Methacroylcholine chloride (MACC) was used as an anion carrier, N, N-Methylenebis acrylamide (MBAA) was used as a cross-linker, and 1-Hydroxycyclohexyl phenyl ketone (HCPK) was used as a photoinitiator. Performance was compared based on high stability in a non-aqueous solvent, high permeability to the charge-carrying ion, low electric cell resistance, low cross-over of the redox-active molecules, high mechanical property, and low cost. The best membraned will be found which will show no cross-over of redox-active electrolytes after 1000 battery cycle experiments and exhibit low resistance and high structural/chemical stability in MeCN solvent. References (1) Mazumder, Md. M. R.; Burton, A.; Richburg, C. S.; Saha, S.; Cronin, B.; Duin, E.; Farnum, B. H. Controlling One-Electron vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate. Inorg. Chem. 2021, 60 (17), 13388–13399. https://doi.org/10.1021/acs.inorgchem.1c01699.(2) Mazumder, M. M. R.; Farnum, B. H. Two One Electron vs One Two-Electron Redox System of Nickel Diethyldithiocarbamate for Redox Flow Battery; ACS, 2021. https://doi.org/10.1021/scimeetings.1c00951.(3) Mazumder, M. M. R. Ni(IV/II) Redox Couple 2e- Reversibility Improvement for Redox Flow Battery. ECS Meet. Abstr. 2021, MA2021-02 (5), 1855–1855. https://doi.org/10.1149/ma2021-0251855mtgabs.(4) Islam, R.; Mazumder, M. M. R.; Farnum, B. H. Solvent Dependent Spectroscopic and Electrochemical Studies of Nickel (II) Diethyldithiocarbamate for Energy Storage. ECS Meet. Abstr. 2021, MA2021-01 (1), 54–54. https://doi.org/10.1149/ma2021-01154mtgabs. Figure 1

Accessing a high‐voltage nonaqueous hybrid flow battery with a sodium‐methylphenothiazine chemistry and a sodium‐ion solid electrolyte

AbstractThe development of redox flow batteries (RFBs) with nonaqueous electrolytes offers the possibility of accessing a high cell‐operation voltage (no restrain of hydrogen evolution and oxygen evolution potentials) and a low operation temperature (can be operated below the freezing point of water). Therefore, nonaqueous RFBs have recently garnered increasing attention. However, the cross‐mixing of liquid electrode/electrolyte materials has been plaguing the progress of the nonaqueous RFBs. Herein, we present a crossover‐free, high voltage nonaqueous hybrid flow battery (HFB) with a novel sodium‐methylphenothiazine (MPT) chemistry and a single‐ion solid‐electrolyte separator. The Na‐MPT redox couple delivers a high voltage of ~2.6 V when the cell was operated at a medium current density. A NASICON‐type solid electrolyte membrane (Na3Zr2Si2PO12) could circumvent the crossover of the liquids between the positive and negative electrodes, and meanwhile could maintain a single‐ion (Na+‐ion) conduction between the two electrodes to sustain the electrochemical reactions. Under such an electrochemical mechanism, the nonaqueous Na‐MPT HFB shows remarkable cycling performance.

A Chemistry and Microstructure Perspective on Ion-Conducting Membranes for Redox Flow Batteries.

Redox flow batteries (RFBs) are among the most promising grid-scale energy storage technologies. However, the development of RFBs with high round-trip efficiency, high rate capability, and long cycle life for practical applications is highly restricted by the lack of appropriate ion-conducting membranes. Promising RFB membranes should separate positive and negative species completely and conduct balancing ions smoothly. Specific systems must meet additional requirements, such as high chemical stability in corrosive electrolytes, good resistance to organic solvents in nonaqueous systems, and excellent mechanical strength and flexibility. These rigorous requirements put high demands on the membrane design, essentially the chemistry and microstructure associated with ion transport channels. In this Review, we summarize the design rationale of recently reported RFB membranes at the molecular level, with an emphasis on new chemistry, novel microstructures, and innovative fabrication strategies. Future challenges and potential research opportunities within this field are also discussed.

A coupled machine learning and genetic algorithm approach to the design of porous electrodes for redox flow batteries

The design of porous electrodes with large specific surface area and high hydraulic permeability is a longstanding target for the development of redox flow batteries (RFBs), but traditional trial-and-error strategies are hindered by the heavy cost of collecting large amounts of data and the limitation of human intuition when multiple trade-offs are at play. In this work, a novel framework coupling machine learning and genetic algorithm is developed to identify the optimal electrode structures for RFBs. A custom-made dataset containing 2275 fibrous structures is first generated by adopting a combination of stochastic reconstruction method, morphological algorithm, and lattice Boltzmann method. Based on the dataset, our best machine learning models allow to achieve test errors of 1.91% and 11.48% for predicting specific surface area and hydraulic permeability, respectively. Combined with well-trained prediction models, the genetic algorithm is developed to screen more than 700 promising candidates with up to 80% larger specific surface area and up to 50% higher hydraulic permeability than the commercial graphite felt electrodes. Results show that the fiber diameter and electrode porosity of these promising candidates exhibit a triangle-like joint distribution, with a preference for fiber diameters of around 5 μm with aligned arrangements.

Pressure drop analysis on the positive half-cell of a cerium redox flow battery using computational fluid dynamics: Mathematical and modelling aspects of porous media

Description of electrolyte fluid dynamics in the electrode compartments by mathematical models can be a powerful tool in the development of redox flow batteries (RFBs) and other electrochemical reactors. In order to determine their predictive capability, turbulent Reynolds-averaged Navier-Stokes (RANS) and free flow plus porous media (Brinkman) models were applied to compute local fluid velocities taking place in a rectangular channel electrochemical flow cell used as the positive half-cell of a cerium-based RFB for laboratory studies. Two different platinized titanium electrodes were considered, a plate plus a turbulence promoter and an expanded metal mesh. Calculated pressure drop was validated against experimental data obtained with typical cerium electrolytes. It was found that the pressure drop values were better described by the RANS approach, whereas the validity of Brinkman equations was strongly dependent on porosity and permeability values of the porous media.

A validated dynamical model of a kW-class Vanadium Redox Flow Battery

The development of redox flow batteries depends on the research on new materials as well as on the technological development, but also on appropriate models which allow to simulate their performance in operative conditions. Very few investigations are reported in the literature concerning the technology, modeling and simulation of large-scale Vanadium Redox Flow Battery systems, built around multi-cell stacks. This paper regards the modeling of an industrial-sized 9 kW test facility. In particular, a complete dynamic model is presented, that takes into account all thermal effects occurring inside the stack, resulting in a complex non-linear coupled formulation, that allows to simulate the battery operation in any realistic conditions. The model is able to simulate the thermal behavior both in standby, i.e. without power and reactant flow, as well as in load operation, i.e. in charge and discharge. The numerical implementation of the model is described in detail. The model validation is also described, consisting in comparing computed data with experimental measurements taken on the available test facility.

Mechanistic Insights of V2+/V3+ Reaction on Glassy Carbon for Vanadium Redox Flow Batteries

Energy demand is increasing due to the world’s growing population and rising quality of life, accompanied by increasing CO2 emissions. A shift towards renewable energy sources like solar and wind is needed to sustainably meet our growing energy requirements. However, the intermittent nature of renewable resources creates the need for energy storage technologies, which would store excess energy and then supply this stored energy when it is needed. Pumped hydroelectric is the most used electrical energy storage method, but is limited to specific geographical regions, leading to a push towards development of redox flow batteries (RFBs). Despite the unique ability of RFBs to decouple energy and power density while having high current densities, their commercial viability is limited by high costs. In particular, All Vanadium Redox Flow Batteries (VRFBs, i.e., (VO2+/VO2 +//V2+/V3+)) have received much interest due to the use of the common element vanadium that removes the issue of cross-contamination, leading to higher Coulombic efficiencies. The sluggish kinetics of V2+/V3+ redox couple on the negative electrode (Reaction 1), however, is found to limit VRFBs efficiency.1 Thus, improving V2+/V3+ kinetics can lead to higher current densities, reducing the overall cost of energy storage. Studies indicate the V2+/V3+ reaction is an inner sphere electron transfer based on differences in activity on certain surfaces,2 but there is a lack of the mechanistic understanding needed to further improve rates. V2+→ V3+ + e‾ (Reaction 1) Eº = −0.255 V vs SHE Traditionally, VRFBs use porous graphite felts with sulfuric acid (H2SO4) as an electrolyte, often without accurate area estimates needed for obtaining normalized current densities. Recent studies have shown increased stability of V ions and higher energy capacities for mixed H2SO4 and HCl electrolytes,3 but the effect of electrolyte on kinetics is also not well understood. Here, we study the V2+/V3+ reaction kinetics on a controlled electrode surface in H2SO4 and HCl, and show that the kinetics increase with State of Charge (SoC) and in the presence of Cl-, where Cl- also changes the V3+ coordination shell as detected by UV-Vis. We use a polished glassy carbon disk as the reaction surface, with high conductivity and controlled electrochemical surface area as compared to graphite felts. We find the V2+/V3+ reaction rates are independent of the total vanadium ion concentration, but instead depend on SoC. SoC was estimated by deconvoluting the individual V2+ and V3+ concentrations from UV-Vis. Exchange current densities for oxidation (iO,oxidation ) decrease with decreasing SoC, whereas Tafel slopes for oxidation (boxidation ) remain constant with SoC. Rate measurements at varied temperature (23.3 – 40.0 °C) were conducted to evaluate effective activation energies (Ea ) for the V2+/V3+ reaction at different SoC by two independent measurements, a) iO,oxidation and b) Charge Transfer Resistance (Rct ). The two independent methods for determining Ea matched closely, and show that Ea increases with decreasing SoC (Figure 1). This dependence of Ea on SoC along with high magnitudes (42 – 44 kJ mol-1 for 50% SoC, as compared to ∼13 kJ mol-1 for the outer sphere ferri-ferrocyanide redox couple4) confirms V2+/V3+ is an inner sphere reaction. This variation of iO,oxidation and Ea with SoC is indicative of a mechanism involving an adsorbed V intermediate affecting reaction kinetics. We measure higher iO,oxidation in HCl compared to H2SO4, which we hypothesize is due to a change in the coordination shell of the reacting specie, leading to formation of a different transition state with lower Ea . V2+ and V3+ ions in pure water typically exist as [V(H2O)6]2+ and [V(H2O)6]3+ (Oh symmetry) respectively with six labile water molecules constituting their first coordination sphere.5 We use UV-Vis to determine the structure of the coordination shell with anions present, by a shift in peak locations (based on ligand field strength) and absolute absorbance (depicting changes in molecular symmetry). The V2+ UV-Vis spectra remained unchanged in H2SO4, HCl, and mixed acid electrolytes of varied concentrations, indicating [V(H2O)6]2+ was the dominant species, with no anions in the first coordination sphere. However, the UV-Vis spectra of V3+ had a shift in peak locations with reduced absolute absorbance in HCl relative to H2SO4, indicating formation of a different V3+ complex, which we hypothesize is related to the change in redox kinetics.

(Invited) Development and Practical Verification of the Redox Flow Battery in SEI

More than 40 years have passed since the basic principle of the Redox Flow Battery (RFB) was proposed by NASA. Currently, with the widespread use of renewable energy, demand for energy storage batteries is increasing more and more. MW class RFBs are already in practical use in electric power systems. On the other hand, further improvement of performance and cost reduction are urgent issues for mass introduction of RFBs. A wide variety of research and development activities are being conducted all over the world. The development of RFBs in Japan began with AIST, a national laboratory aimed at the load-leveling of electric power companies in the 1970s. SEI began development as a collaborative research project with a power company in 1985 and has been developing first the Fe/Cr RFB followed by the V/V RFB for commercialization. Along with changes in the situation of the power industry for many years, SEI has delivered about 30 RFBs from large batteries to small ones according to customer's request. In particular, since the establishment of the FIT system in Japan in 2012, a large amount of PV systems has been introduced and the need for energy storage batteries is rapidly rising. With the support of METI (the Ministry of Economy, Trade and Industry), a 15 MW RFB was installed in 2015 and is currently operated by Hokkaido Electric Power Company. In this presentation I will introduce the power situation and the necessity of energy storage in Japan, the history of RFB development in SEI, and the operation experience of some systems. SEI is also developing a new electrolyte technology using titanium and manganese ions, which is one of our latest developments aimed at cost reduction. I would like to introduce these developments as well.

Development of Redox Flow Battery Based on Lithium Ion Secondary Battery System

Recently, in the context of concerning resource depletion and leveling electrical load, the demand for expanding stationary energy storages have been increasing to utilize natural resources and energy more effectively. Redox flow batteries are considered as promising ones other than lead-acid or sodium-sulfur ones for stationary use, while their energy densities should be raised for widespread use. The low energy densities of the conventional flow batteries are mainly due to the following two factors; one is that they employ aqueous electrolyte solutions with narrow potential window, leading to their low cell voltages. Another is that solubilities of active materials to the electrolytes are low, leading to their low specific capacities. As the solution of these problems, we developed a novel type of the redox flow batteries based on the original concept that can increase their energy densities dramatically. By using a nonaqueous electrolyte, the potential window becomes wider so that the cell voltage can be higher than 3V. Furthermore, the cell utilizes solid active material of Li-ion battery set in, not dissolved in, the electrolyte. Therefore, the energy density can be dramatically improved for general redox flow batteries whose capacity is limited by the solubility of the active material in the electrolyte. In our system, active materials are charged and discharged without direct contact with current collectors by introducing redox molecules in a non-aqueous electrolyte. There have been no reports of batteries having such a mechanism for both positive and negative electrodes and having cell voltages of 3V or higher. In this study, we report the results of the experiment that prove our concept of the new flow battery. We have to select redox molecule species, solid active materials, and a Li-ion conductive separator that can divides electrolytes in the positive electrode side and the negative one to demonstrate our concept. Furthermore, the redox molecule and the solid active material need to have different redox potential so that the redox molecule can mediate electron transfer between current-collector and active material. Such redox mediators are required to be dissolved to nonaqueous electrolyte and to be reduced and oxidized repeatedly. We focused on tetrathiafulvalene (TTF) as a positive redox mediator, which has been reported as an positive active material of organic batteries and can be expected to be durable for redox reactions because they have π-conjugated electron cloud on the molecular plane. Cyclic voltammetry confirmed the two-step reversible redox reactions（E1 = 3.3 V, E2 = 3.6 V vs. Li/Li+）. Since olivine type lithium iron phosphate has its equilibrium potential between the two-step redox potentials of TTF, we selected TTF and LiFePO4 as a positive-electrode system. On the other hand, in a negative electrode system has, the more negative potential enables the higher cell voltage. We employed biphenyl as a negative redox mediator, and graphite as a negative solid active material. This is because we found that biphenyl-Li solution has extremely low potential (E=0.03 V vs. Li/Li+), and can reduce (i.e. insert Li+ ions in) graphite at that potential. Note that it is known that an ether-based solution in which a certain aromatic compound is dissolved has the ability to dissolve Li metal and its redox potential is low. After confirming that the mediators reacted with the active materials in both electrode systems as designed, we prepared 1M LiBF4/propylene carbonate (PC) with 5mM TTF dissolved in and a LiFePO4 powder immersed in as the positive electrode electrolyte, and with 0.1M biphenyl, 1M LiPF6/2-methyltetrahydrofuran (2-MeTHF) that dissolved Li metal and with a graphite powder as a the negative electrode electrolyte. A garnet type LLZ solid electrolyte was used as the Li-ion conductive separator set between electrolytes in the positive and negative electrode systems. Charge-discharge tests for flow battery cells constructed as above showed that the charge/discharge capacities exceeding the capacity due to charge accumulation on mediator molecules. This suggests that the mediators reduced and oxidized the solid active materials, i.e., enabled charging and discharging of the solid active materials in the flow battery system. At this stage, it turned out that about 22% of the solid active materials was utilized. As a result, we demonstrated that it is possible to operate the new battery and that a basic part of our concept for the novel high-energy density redox flow battery. Figure 1

Critical Role of Supporting Electrolyte Selection on Flow Battery Cost: Combined Effects of Cell Materials and Operation

With commercial development of redox flow batteries (RFBs) underway and given the importance of capital cost, we analyze RFB supporting electrolyte charge carrier, membrane, open-circuit voltage (OCV), cell performance, and active species size as possible major cost drivers. Physics-based and techno-economic modeling includes a porous electrode model and accounts for chemical and material selection and amounts, stack and tank sizing, and flow-dependent pumping power.1 The 1D porous electrode model, validated in a prior study,2 couples mass transfer, charge transfer, and ohmic transport to estimate the electrode contribution to full cell area specific resistance (ASR). Primary findings for a 5-h discharge of an aqueous RFB are that present costs are in general $10 ~ 100/kWh lower with 1.4 V versus 1.0 V OCV, size selective separators (SSS) versus ion exchange membranes (IEM), as shown in Figure 1a. Large active species size alone would increase costs slightly due to higher viscosity and thus higher pumping cost, reduced mass transfer, and higher mass-based electrolyte cost versus lower molecular weight actives. However, large actives enable SSS use, and the slight incremental cost is more than offset by cost reductions from the high conductivity and inherent low cost of SSS membranes. In the future, the costs of different scenarios converge assuming significant cost reductions in the stack, membranes, and unit cost less materials. In Figure 1b, multiple options, led by H+/IEM with small active species and OCV > 1.3 V, can meet a Joint Center for Energy Storage Research target adjusted to $100/kWh for 5-h storage. Implications of the work here are: 1) recommendations on the path to enable low cost aqueous RFBs, and 2) a modeling tool and opportunity to quantify cost implications of additional factors held constant in the present study. One recommendation is the further exploration of IEMs using H+ or Na+ and SSS, as these membranes have relatively favorable likelihoods to enable system cost targets. We also suggest development work towards redox couples and conditions for high OCV, which enhances electrolyte energy content and stack power. Subsequently, engineering effort and adoption of RFBs is needed to enable significant reduction in cost through high volume manufacturing of stacks, membranes, and overall RFB systems. Lastly, one can use the present model to explore the effect of various additional parameters on cost. Such parameters include: the cell components (e.g., electrode properties, membrane thickness) affecting ASR, the operation (e.g., flow rate, V-I operating point) affecting ASR, and the application (e.g., discharge duration) affecting power-related cost (e.g., stack) versus energy-related cost (e.g., electrolyte volume, tank size). Acknowledgments This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy. JDM acknowledges additional support from the NSF Graduate Research Fellowship.

A Mechanistic Investigation of Crossover-Induced Capacity Fade in Redox Flow Batteries with Non-Selective Separators

The independent control of power capability, which is affected by reactor design, and energy capacity, which is affected by stored electrolyte volume, enables redox flow batteries (RFBs) to be potential candidates for large-scale energy storage systems. Typical RFBs consist of two large tanks with positive and negative electrolyte pumped to an electrochemical reactor through a network of pipes, while the reactor consists of high surface area porous electrodes (such as carbon felt) separated by an ion exchange membrane (IEM) or a non-selective separator (NSS). Previous research has established inorganic (such as vanadium) and organic/semi-organic (such as TEMPO) molecules for use in RFBs with aqueous or non-aqueous electrolytes. IEMs are used to electronically insulate the two electrodes and reduce crossover of the redox species by charge selectivity. IEMs are expensive and can contribute as much as 20% to the total battery cost depending on the size of the reactor [1]. Reduction of RFB capital costs could incentivize the adoption of RFBs for grid-energy arbitrage [2], and, hence, the development of RFBs with inexpensive NSSs with minimal crossover could enhance RFB use. In this regard, redox active polymers (RAPs) have been developed in both aqueous and non-aqueous systems to replace IEMs with NSSs. Here, NSSs mitigate crossover by size exclusion of RAPs [3,4]. However, the viscosity of such polymer based redox molecules increases with concentration leading to higher pumping pressures required to maintain the flow rate, thus adding to the pumping costs. In addition, higher pumping pressures increase electrolyte crossover which induces RFB capacity fade from cycle-to-cycle. The objectives of the present work are (1) to model multi-component transport during simultaneous electrochemical reactions within RFBs using NSSs, (2) to assess the tradeoffs in rate capability and cycle life incurred when replacing IEMs with NSSs, and (3) to predict performance as a function NSS design (including pore size and thickness). In this context, we are developing a two-dimensional model using porous electrode theory that explicitly captures porous-media flow, electronic current conservation, and conservation of molecular species (typically four redox-active species and two supporting ionic species that are inert) with simultaneous electrochemical reactions. These processes produce coordinated molecular fluxes can result in crossover and shuttling of redox species. In this model, molecular fluxes arise due to (1) pressure differences across the membrane (advection) (2) concentration gradient between the two electrolytes in the reactor (diffusion) and (3) migration due to the gradient in the electrolyte potential. In addition, we observe electrolyte volume losses which arises due to bulk electrolyte flow through the porous membrane because of viscosity (and therefore pressure) difference between the two electrolytes. Each electrode experiences primary reactions due to the major redox couple in the electrolyte and secondary reactions arising from the species that crossover from the counter electrode. We find that the secondary reactions experience a much higher overpotential than the primary reactions. The shuttling process occurs when the products of the secondary reactions diffuse back into the parent electrode. This shuttling process is used in bio sensing devices to amplify and detect the amount of catechol in human nervous system [5] and for overcharge protection in Li-ion batteries [6]. In the context of RFBs, this must be controlled to minimize capacity fade. The present porous-electrode model and associated reduced-order models will enable mechanistic interpretation of experimental data from which performance will be correlated as a function of several non-dimensional parameters that will aid in the design of NSSs. Further, we will validate our porous electrode model with experiments using aqueous RFBs having interdigitated flow fields using various NSSs and IEMs. We gratefully acknowledge the financial support of the Joint Center for Energy Storage Research (JCESR). References Arora, P., Zhang, Z. M., Chem. Rev., 2004, 104, 4419−4462.Darling et al., Energy Environ. Sci., 7 ,2014.Gavvalapalli et al., Am. Chem. Soc., 136 (46),2014Janoschka et al., Nature 527, 78–81, 2015Bernhard Wolfrum et al., Analytical Chemistry, Vol. 80, No. 4, 2008Jun Chen et al., Solid-State Lett., volume 8, issue 1, A59-A62, 2005

In Situ Localized Current Distribution in a Vanadium Redox Flow Battery

Significant development of redox flow battery (RFB) technology has taken place recently to assess their viability as a large-scale energy storage system1. Such systems have been implemented in select instances to demonstrate their feasibility; however there is still a lack of fundamental understanding regarding these devices. Of the numerous possible redox couples, the all-vanadium chemistry is the most commonly studied RFB. With the extensive literature available regarding this chemistry, there is still a lack of available diagnostic techniques to provide distributed measurements for understanding fundamental mechanisms within these cells.The use of a printed circuit board in electrochemical cells provides a useful distributed diagnostic as demonstrated by its extensive use in fuel cells2. The application of this technology to measure in-plane current distribution in a RFB has been previously demonstrated as shown in Figure 1.3This work will build upon that foundation to gain further insight regarding the influence of cell architecture and operating conditions on distribution.

바나듐 레독스 흐름전지용 Poly(arylene ether sulfone) 막의 특성

최근에 대용량 에너지 저장 시스템으로 레독스 흐름전지(Redox Flow Battery, RFB)가 활발히 연구 개발되고 있다. 불소계막을 대신할 저가의 탄화수소막이 RFB막으로 주목받고 있다. 본 연구에서는 Poly(arylene ether sulfone) (PAES) 막을 사용해 고가의 불소계막과 그 특성을 바나듐 레독스 흐름전지(VRB, Vanadium Redox Flow Battery)조건에서 비교하였다. 바나듐 이온투과도, 이온 교환 용량, OCV 변화, 팽윤, 충 방전 곡선, 에너지 효율 등을 측정했다. PAES 막은 Nafion 117막에 비해 바나듐 이온투과도가 낮고, 이온교환용량은 커서 Nafion 117을 사용한 RFB보다 에너지 효율이 높았다. Recently, there are many efforts focused on development of Redox Flow Battery (RFB) for large energy storage system. Economical hydrocarbon membranes alternative to fluorinated membranes for RFB membrane are receiving attention. In this study, characteristics of poly(arylene ether sulfone) (PAES) were compared with expensive fluorinated membrane at VRB (Vanadium Redox Flow Battery) operation condition. Permeability of vanadium ion through membrane, ion exchange capacity (IEC), change of OCV, swelling, charge-discharge curves and energy efficiency were measured. PAES membrane showed lower permeability of vanadium ion, higher IEC and then higher energy efficiency compared with Nafion 117 membranes.

Capital Cost Sensitivity Analysis of an All-Vanadium Redox-Flow Battery

Interest in the development of redox-flow batteries (RFBs) for large-scale grid storage is growing, and considerable investments have been made into the research and development of RFBs over the past few decades. Unfortunately, practical implementation has been hampered by various cost and performance issues typical of an immature state of development. One critical factor for the competitiveness of this technology is the installed cost. In this work, we incorporate recent developments in all-vanadium RFBs research and present an analysis of the associated cost factors. The major components of a RFB that affect installed cost are identified and used as variables to create a capital cost function. The function is then used to calculate the rate of change of the capital costs with respect to the major components. The capital costs are also calculated for a range of component values and plotted. Key findings include a high sensitivity of system capital cost to purity of vanadium and substantial fractions of the cost associated with perflurorosulfonic acid membranes currently used for proton transport.

Capital Cost Sensitivity Analysis of an All-Vanadium Redox-Flow Battery

Interest in the development of redox-flow batteries (RFBs) for large-scale grid storage is growing, and considerable investments have been made into the research and development of RFBs over the past few decades. Unfortunately, practical implementation has been hampered by various cost and performance issues typical of an immature state of development. One critical factor for the competitiveness of this technology is the installed cost. In this work, we incorporate recent developments in all-vanadium RFB research and present an analysis of the associated cost factors. The major components of a RFB that affect installed cost are identified and used as variables to create a capital cost function. The function is then used to calculate the rate of change of the capital costs with respect to the major components. The capital costs are also calculated for a range of component values and plotted. Key findings include a high sensitivity of system capital cost to purity of vanadium and substantial fractions of the cost associated with perflurorosulfonic acid membranes currently used for proton transport.

Development of Redox Flow Battery for Utility Load Leveling

A redox flow battery, one of the advanced batteries, has been in development since 1985 for the load-leveling of electrical power demand. A 10 kW class battery was successfully constructed and evaluated in 1986. At present, a 60 kW class battery system is under construction. To improve the system energy efficiency of this battery, several developing efforts have been done such as the scale up of the electrode area, membrane resistance reduction, a suitable design for piping and pumping system, etc. Preliminary test results have shown a high battery efficiency of 83% to 84%, and thus, a good system performance will be expected. This facility is to be installed at a sub-station of the Kansai Electric Power Company, Inc., and connected to a commercial power line for 500 cycle life test.

Development of redox flow batteries. A historical bibliography

Redox-flow battery systems have been investigated for sixteen years. Initially developed in the U.S.A., such batteries have since interested many researchers worldwide. It seems that development is not yet exhausted and, in recent years, many industries (practically all in Japan) have made great efforts to produce practical cells. In this paper, following an initial brief summary of the redox principle, the main reports and the activity they describe have been listed, with one or two minor exceptions, historically.