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Next-generation aqueous flow battery chemistries
Journal Pre-proof Next Generation Aqueous Flow Battery Chemistries Sri R. Narayan, Archith Nirmalchandar, Advaith Murali, Bo Yang, Lena HooberBurkhardt, Sankarganesh Krishnamoorthy, G.K.Surya Prakash PII:
S2451-9103(19)30157-7
DOI:
https://doi.org/10.1016/j.coelec.2019.10.010
Reference:
COELEC 470
To appear in:
Current Opinion in Electrochemistry
Received Date: 2 July 2019 Revised Date:
13 September 2019
Accepted Date: 10 October 2019
Please cite this article as: Narayan SR, Nirmalchandar A, Murali A, Yang B, Hoober-Burkhardt L, Krishnamoorthy S, Prakash GKS, Next Generation Aqueous Flow Battery Chemistries, Current Opinion in Electrochemistry, https://doi.org/10.1016/j.coelec.2019.10.010. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Next Generation Aqueous Flow Battery Chemistries Sri R. Narayan*, Archith Nirmalchandar, Advaith Murali, Bo Yang, Lena Hoober-Burkhardt, Sankarganesh Krishnamoorthy and G. K. Surya Prakash Department of Chemistry and Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, CA 90089-1661, USA Abstract. The battery industry is seeking solutions for large-scale energy storage that are affordable, durable and safe. Aqueous redox flow batteries (RFBs) have the inherent properties to meet these requirements. While much has been learned over the past decade on the properties of redox materials, the focus of next generation systems must be primarily on lowering redox material cost and increasing durability. In this context, in addition to inexpensive materials such as iron salts, redox couples based on small organic molecules have shown significant promise. A considerable level of understanding has been gained on the factors affecting the durability of aqueous RFB systems, specifically relating to molecular stability and crossover. New molecular classes, substituent strategies, and cell configurations have been identified to enhance the durability of systems in the future. Next generation systems will also need to focus on designing molecules for achieving high energy efficiency and power density as well. Further, the application of computational methods for screening of chemical stability could accelerate discovery of new molecular architectures.
Background. The exponential growth in the deployment of renewable electricity generation systems demand the deployment of large-scale energy storage systems to deal with the fluctuations in supply. While there are many types of rechargeable batteries that can serve the energy storage function [1,2], applications such as energy time-shift and “firming” that involve daily charge and discharge from 4 to 12 hours, demand substantially higher durability and cycle life than offered by today’s lithium-ion batteries. The anticipated “mega” scale of deployment of such energy storage systems demands a paramount level of safety, almost unlimited reserves of critical materials, and avoidance of negative impacts on the environment [3]. Redox flow batteries (RFBs) based on aqueous chemistry have the potential to meet the demanding economic, environmental and technical requirements for large-scale energy storage [4]. The ability to separate the components that produce power and store the energy offers not only ease of scalability but also intrinsic safety. The charged electrolyte being restricted to separate tanks outside the power-producing element (the cell stack) precludes a runaway hazard, an advantage not often emphasized. Fire hazards are reduced by the use of water as a solvent. Further, the soluble redox couples avert degradation induced by phase changes commonly experienced during charge and discharge of solid active materials. However, “hybrid” RFBs that entail metal deposition at the negative electrode (typically elemental iron or zinc), cannot offer all the foregoing advantages. Among the mature aqueous RFBs are the all-vanadium and zinc-bromine systems that have been studied for over 25 years [5–7]. At a large scale the installed cost of these systems is projected to be in the range of $300- $500/kWh, while their projected lifetimes are as long as 20 years. Despite these long lifetimes the large-scale commercialization of vanadium and zinc-bromine
systems has been limited mainly by the high-cost of the redox materials and system components, and the hazards of handling relatively toxic active materials [8].
Yet from a commercial
standpoint these systems may be considered as a springboard for designing and deploying next generation systems. Thus, besides being durable and safe, we seek that the next generation systems use active materials that are low-cost, abundantly-available, and environmentallybenign. To this end, we will discuss specifically the basic requirements to be satisfied by next generation aqueous RFBs. Cost of Redox Active Materials.
The cost of active materials is one of the principal economic drivers for RFB systems operating for longer than 3 hours. The active materials cost can be 25-50% of the system cost depending on the power density of stack. At the active materials cost of $50/kWh, for 5 hours of charge/discharge at a cell power density of 1 kW/m2, the total system cost is estimated to be $200/kWh. Such a system when operated for 20 years with a capacity degradation rate of no more than 2.5 x10-3 % /day will meet the U.S. Department of Energy target for the levelized cost of energy storage (LCOS) of 5 cents/kWh for utility-scale systems [9].
Inorganic redox materials that meet this demanding cost criterion primarily include iron sulfate, iron chloride, manganese sulfate and zinc sulfate that are available at less than $1/kg (based on bulk pricing from Alibaba.com). Iron sulfate is particularly attractive as it is a waste product of the steel industry and is available at $ 0.1/kg. Potassium ferrocyanide is available at a bulk price of $2-$5/kg. Where inorganic redox couples are involved, acidic solutions are necessary to prevent hydrolysis and precipitation, zinc ions being an exception. An all-iron flow battery that
uses iron chloride is quite attractive from a materials cost standpoint, although other technical challenges with the operation of the battery could offset this cost advantage [10,11].
Electrolytes based on water-soluble polysulfides have been of interest as negative electrode materials because of their relatively low cost. However, the presence of various orders of polysulfides in solution at the same time and the oxidation of polysulfides to insoluble sulfur presents technical challenges to the operation and the cycle life that need to be addressed [12]. Similarly, manganese in +7(permanganate) and +6 (manganate) has shown promise in alkaline media as a cost-effective redox couple for the positive side of the cell [13]. The long-term cycleability of the manganese redox couple in conjunction with organic redox couples needs to be established. While $/kg is a comparison that is often used in the cost discussions relating to active materials, we emphasize that the actual deployed cost of material is governed by $/kWh that accounts for the practical values of utilization, cell voltage, and energy efficiency.
Another class of redox couples that have emerged for aqueous RFBs are small organic molecules that can be synthesized from inexpensive raw materials such as coal, tar sand, heavy fractions of crude, and asphalt [14–16]. Anthraquinone, benzoquinone, and their derivatives are examples of such redox couples that are obtainable at a bulk price of $1-$3/kg [17,18]. While molecules with a low molecular weight are desirable to achieve high specific charge storage capacity, substituents and extra ring moieties are often added to the organic molecules to increase solubility, increase chemical stability, reduce crossover, or achieve a desired electrode potential [19,20]. However, the addition of such substituent groups (often referred to as molecular engineering) not only increases the molecular mass but also the number of steps in the synthesis,
thereby raising the cost of manufacturing. Thus, the benefits of molecular engineering become a trade-off against material cost. The material cost must also factor the handling and disposal requirements for hazardous substances. For example, methyl viologen is a very efficient redox couple for the negative electrolyte but has been banned in the European Union since 2007 [21] and faces restrictions in its use in the United States because of acute health hazards. It is not clear if other molecules of the viologen family will face similar restrictions.
Chemical Durability Even if the active materials are inexpensive, durability is an overriding techno-economic driver, as the LCOS depends on the time elapsed before the need for replacement. Characterization of degradation rates during operation and idle stand are important in the selection of durable molecules. The negative electrolyte in an all-iron flow battery suffers precipitation by hydrolysis if the minute inefficiencies due to hydrogen evolution are not curtailed. Organic additives and complex formation have not resulted in complete suppression of hydrogen [11,22]. Interestingly, the rate of hydrogen evolution can be decreased by operating at higher temperatures, but the approach of recombination of the hydrogen with Fe(III) in a fuel cell like device is still the preferred approach to mitigate the effects [11,23]. This solution adds another reactor system that also needs to be durable and cost-effective. On the other hand, the positive electrolyte, iron(II)/iron(III) does not degrade, unequivocally satisfying the durability criterion for next generation systems.
For the zinc-bromine battery, the long-term stability of the bromine electrode continues to present a challenge [7]. Many new types of doped-carbon materials and oxidized carbon surfaces
have shown increased catalytic activity and
durability [24–26]. Further, the challenge of
uniformity of zinc electroplating also continues to be the focus.
Figure 1. a) Schematic of the Michael reaction on BQDS during charging. b) Mechanism of Michael reaction on BQDS showing nucleophilic addition of water. c) Quinone-methide formation: Reaction of methyl groups with the basic media. d) Mechanism of protodesulfonation. e) Electrochemical redox reaction of quinoxaline in acidic and neutral media, and the side reaction leading to the formation of an unstable hydroxy derivative.
While organic redox couples of a variety of molecular structures are possible, one of the major challenges is the propensity of these molecules to undergo degradation reactions. Quinone/Hydroquinone based redox couples have been widely studied for use in flow battery systems. Anthraquinone derivatives form a class of promising negative side materials. Anthraquinone disulfonic acid (AQDS) and anthraquinone monosulfonic acid (AQS) are stable in acidic media and have been widely used in flow battery research [14–16,27,28]. Dihydroxyanthraquinone and its derivatives have been explored for use as a negative material in basic media. Upon cycling, these molecules underwent a disproportionation reaction to yield an anthrone intermediate that decomposed irreversibly causing capacity loss. The rate of capacity fade can be reduced by avoiding higher states of charge or by adding oxygen to chemically oxidize the anthrone intermediate [29]. In recent studies, Bruschett et al noted that AQDS and its reduced form produce a “quinhydrone” type of dimer that increases the viscosity and reduces utilization. However, such type of molecular association is reversible and hence does not cause any permanent loss of capacity[30].
Benzoquinone derivatives used at the positive electrode are prone to nucleophilic addition reactions in aqueous medium. The greater the positive electrode potential of the molecule, the
greater is its tendency to react with nucleophiles [31]. In acidic and neutral solutions, the nucleophile is water while in basic solutions it is the hydroxide ion. While electron-withdrawing groups are desirable for both increasing the reversible electrode potential to more positive values and increasing solubility, the electron deficiency in the ring is simultaneously exacerbated rendering the molecule susceptible to nucleophilic attack. A case in point is that of 1,2dihydroxybenzene-3,5-disulfonic acid that reacts in the quinone form with water to form the 1,2,4,6-tetrahydroxybenzene-3,5-disulfonic acid. Such a reaction may be considered a Michael reaction (1,4-addition) of water to the quinone (Figure 1a and 1b) [16]. An example of mitigating this reactivity is demonstrated with 1,4-dihydroxy-2,6-dimethylbenzoquinone-3- sulfonic acid (DHDMBS) where we found electron-donating groups to beneficially alter the local electron density distributions [28]. Though having an electron-donating methyl group provides increased stability [32,33], the electrode potential of the molecule is lowered.
In basic media, these benzoquinones will undergo rapid Michael addition due to the hydroxide ion being a strong nucleophile. Here, the addition of methyl groups is not beneficial. In basic media the hydroxide ion deprotonates the methyl groups to form a quinone methide intermediate which will irreversibly decompose to other products (Figure 1c). Thus, in basic media, the literature abounds in the use of ferrocyanide/ferricyanide at the positive electrode [32,34,35]. While ferrocyanide/ferricyanide couple appears to be relatively stable in base, Liu et al have shown that such stability cannot be claimed for long duration of operation in base [36].
Upon increasing the electron density on the aromatic ring as in DHDMBS, we make these molecules susceptible to electrophilic attack. In acidic solutions the H+ electrophile reacts with
such electron-rich arene sulfonic acids causing a protodesulfonation reaction (Figure 1d). The sulfonic acid group which is responsible for the solubility of the molecule is lost to the medium and hence precipitation of the redox species occurs. We have shown that the rate of protodesulfonation can be controlled by adjusting the acidity of electrolyte [28]. Therefore, a delicate balance exists between avoiding the Michael addition reaction and the protodesulfonation reaction. AQDS and AQS are examples of molecules particularly resistant to Michael addition in acid or base media, or protodesulfonation in strongly acidic media. This resistance is because the HOMOs are unfavorably positioned with respect to the nucleophiles, and the lower electron density of the sulfonated rings prevents attack by electrophiles.
Pyrazine, quinoxaline and phenazine family of heterocycles are promising negative side materials. Quinoxalines undergo a one-step two-electron reduction reaction to yield dihydroquinooxalines. This process is reversible in basic media [37]. In acidic and neutral media, the dihydroquinoxalines can further undergo a one-electron irreversible reaction to form a hydroxy-derivative that isomerizes to a lactam-tautomer that can decompose further (Figure 1e) [38]. Pyrazines are similar to the quinoxalines and can be used only in basic media. Wei Wang et al have reported a seminal result of water-soluble phenazine derivative as stable negative material in basic media [34]. In phenazine, the radicals formed during the redox reaction are stabilized by electron delocalization due to the presence of two adjacent benzene rings to the pyrazine motif making it resistant to dimerization and side reactions.
The alloxazine-based molecules such as flavin mono-nucleotide (FMN) can be used as a negative material by exploiting a reversible one-electron redox process. Major drawbacks of the
alloxazine system are its low solubility due to its ߨ-stacking ability and its tendency to undergo hydrolysis to give redox-inactive species. Aziz et al synthesized a derivative of alloxazine with a carboxylic acid group to increase its solubility [39]. This derivative showed chemical stability in alkaline solutions for six weeks. Meng et al have used nicotinamide as an additive to increase the solubility of the sodium salt of FMN and found no unintended side reactions and attributed the chemical stability to electron delocalization [40].
Viologen and its derivatives are water-soluble negative electrolyte materials. Simple derivatives such as methyl viologen form a dimer that undergoes disproportionation yielding insoluble products. With the addition of two quaternary ammonium groups to the viologen backbone, Aziz et al have mitigated the formation of dimers [33]. Similarly, Liu et al have added alkyl sulfonate groups to achieve good stability [41]. Thus, the viologen derivatives are promising negative materials in neutral aqueous medium, provided the toxicity of these molecules is not an issue.
Water-soluble TEMPO derivatives with hydrophilic groups in the 4-position are attractive as positive side materials in neutral solutions [42–44]. While the 4-hydroxy derivative of TEMPO is unstable due to irreversible self-oxidation caused by hydroxyl radicals, other substituent groups such as sulfate and quaternary ammonium groups at the 4-position have resulted in very stable molecules [45]. Consequently, TEMPO derivatives are promising from the durability standpoint.
Given that a finite number of the pathways for degradation exist for the promising molecular classes, we recognize that we could expand the use of computational methods to rapidly screen for the formation of specific reactive intermediates in each molecular class. This approach will
accelerate the identification of potentially viable molecules for durable next generation redox electrolytes.
The durability of stack components is dependent on the redox chemistry and the pH of the solutions. Acidic solutions necessitate carbon-based corrosion-resistant materials for the current collectors and electrodes. Titanium plates may also be used as in the case of the negative electrode of the all-iron RFB. Other parts of the cell can be serviced by inexpensive polypropylene. Alkaline media permit stainless steel components to be used in the flow fields and connector plates, although nickel would be preferred for long-term stability. Electrode structures continue to be made of carbon felt or carbon fiber paper although nickel foam could be an option for alkaline media. Recent advances in iron-based substrates for alkaline electrolyzers can be adapted for RFBs [46]. Where oxidative stability of the membrane is not an issue, expensive perfluorinated membranes may be replaced with hydrocarbon membranes. Sulfonated PEEK and sulfonated PES are the most common and cost-effective alternatives. Today’s alkaline RFBs most commonly use Nafion as the alkali metal ion transporter. Next generation alkaline systems can benefit from aromatic backbone polymers based on biphenyl and terphenyl groups and those with polynorbornene backbones developed for alkaline fuel cells that are expected to be less expensive than their fluorinated counterparts [47–49]. These new anion exchange membranes can also serve as transporters for chloride, bromide and sulfate.
Crossover Effects In the conventional (asymmetric) configuration of RFBs where dissimilar materials are used on either side of the cell, capacity fade usually occurs by crossover of materials from one side to the other, eventually leading to a mixture of material on both sides of the cell. The rate of crossover is dependent on charge-based and size-based exclusion by the ion-exchange membrane. E750 and F1850 membranes from Fumatech have shown reduced permeability for organic redox couples compared to Nafion 212 [28]. AQDS is found not to crossover even over several months of testing, while DHDMBS was found to permeate the membrane quite readily [28]. Alternate ion-exchange membranes with sulfonated mesoporous structure have shown reduced permeability to ions [50].
However, even a small permeation rate will result in a significant
capacity fade, if allowed to persist over a long period of operation. While molecules with large pendant groups that increase the molecular size are expected to avoid crossover, it is important to ensure that the viscosity of the solutions and the cost of manufacturing these molecules do not reduce the overall benefit. In the case of the all-iron RFB, as in the case of the vanadium RFB, large levels of crossover may be tolerated without permanent loss of capacity because the same element is present on both sides of the cell.
With a symmetric cell configuration in which a mixture of the positive and negative redox materials of the same concentration are used on both sides of the cell, irreversible capacity fade may be avoided. Coulombic inefficiency resulting from crossover cannot be avoided, but changes in composition can be reset by a “mix-and-split” operation. Such methods have already been used successfully with maintaining operation of iron/chromium cells [51]. Recently, AQDS/DHDMBS in a symmetric cell configuration was tested and found to be viable over
hundreds of cycles without noticeable loss of capacity [28]. One of the major advantages of the symmetric cell configuration is that water transport due to the osmotic pressure differences can be avoided. As this approach requires the use of twice the stoichiometric amount of material in the tanks, this option becomes acceptable only when the cost of materials is very low. One of the negative aspects of the symmetric design is the increased viscosity of the mixed electrolyte.
Figure 2. a) Positive and negative electrode reactions of bipolar molecule AQ-BQ-MS. b) Cyclic voltammogram of the sulfonated bipolar molecule on a glassy carbon working electrode. The concentration of the bipolar molecule was 1 mM in 1 M sulfuric acid. MSE reference electrode (E° = +0.650 V) and a platinum counter electrode were used. The scan rate was 50 mV/sec. [52,53]
In an alternate molecular design, instead of a mixture of molecules, a single molecule that incorporates both positive and negative electrode redox couples may be used. An example of such a “bipolar” molecule is AQ-BQ-MS (Figure 2a). The same molecule will be used on both sides of the cell [52,53]. Similarly, TEMPO/viologen derivatives, phenazine/TEMPO derivatives and anthraquinone derivatives have been successfully tested to demonstrate the bipolar function although molecular durability is still an issue [54–56]. To be cost-competitive, the synthesis of the bipolar molecule must not add to the complexity. Further, we must ensure that the diffusion coefficients and solubility of bipolar molecule do not become significantly lower than their monopolar counterparts.
Traditionally, sulfonic acid groups have been used to increase the aqueous solubility of the redox molecules. While sulfonic acid groups do provide a large increase in solubility, they affect stability through protodesulfonation. To overcome this problem other water-soluble groups such as tetraalkylammonium, imidazolium, and morpholinium are being attached to the redox active backbone to provide solubility in acidic and neutral media.[33,43,57]. These groups provide flexibility while designing the redox couples. Apart from increasing solubility such groups also make the molecule larger, thereby reducing crossover.
Energy Efficiency and Power Density. An important approach towards improving energy efficiency and power density of redox flow battery stacks is by the rational design of flow fields and electrode structures [58]. This approach is broadly applicable to RFBs. Redox couples that offer a high cell voltage have the advantage
of producing high power densities at a proportionately lower cell current density. Considering that organic molecules are susceptible to degradation by oxidation at high positive electrode potentials attaining the cell voltages as high as those seen in vanadium flow batteries can be a challenge. However, redox couples operating at a low cell voltage will also be viable if they can offer the required power density at high efficiency and minimal overpotential losses. With such a view, the gamut of molecules expands further.
Fast redox process with a standard heterogeneous electron transfer rate constant of 1x10-5 cm s-1 or higher are desirable to allow high efficiencies to be maintained. Poor kinetics are observed when intramolecular hydrogen bonding of the quinone group is possible with a sulfonic acid or hydroxyl group in the ortho- position. Otherwise, the inefficiencies are dominated by overpotential losses from mass transport and electrolyte (Ohmic) resistance. Under these conditions, the molecular properties of solubility, diffusion coefficient, viscosity, and degree of ionic dissociation become intertwined in determining the voltage losses and the power density. Inorganic redox electrolytes tend to be less viscous, more soluble, and have higher diffusion coefficients and degree of ion dissociation than their organic counterparts. With organic molecules, the properties of the oxidized form relative to that of the reduced form are often quite different and this could affect how the power density varies with the state-of-charge. For example, in the case of AQDS, the reduced form is considerably more viscous compared to the oxidized form because of hydrogen bonding and formation of stacked dimers leading to reduced utilization and power density [30]. Further, when using solutions of high concentration, it is important to ensure that solubility levels are not exceeded during charge or discharge because of the reduced solubility of the transformed species.
Aromatic sulfonic acids tend to have pKa values in the range of 0.5 to 2. Consequently, the degree of dissociation is small, and the solutions of these materials by themselves are not very conductive. Amino substituent groups tend to get protonated at higher pH values resulting in more conductive solutions. Thus, to increase the conductivity of the solutions, a supporting electrolyte such as sulfuric acid, hydrochloric acid, sodium hydroxide, or neutral salts is required to achieve low electrolyte resistance. Reducing the ohmic over-potential losses also require that the electrodes be thin, with high surface area and adequate porosity to support high flow rates. The use of elevated temperatures comes with the concern of side reactions being accelerated. Thus, improvements to durability would also beneficially impact power density by permitting higher temperatures of operation.
Conclusions.
Much of the effort in the last six years of research has served well to understand the technical requirements and challenges with various new types of RFBs. The inherent advantages and the interesting challenges of the water-soluble organic redox couples have been unveiled. These efforts indicate that a safe, affordable, sustainable, and robust long-duration energy storage system is quite promising with next generation RFBs. However, the future rests on addressing specific molecular designs that will simultaneously satisfy the primary techno-economic drivers of cost, durability, efficiency and power density. Computational studies could be extended to
examining the propensity of specific undesirable transformations such as the Michael reaction, dimer formation and proto-desulfonation, prior to synthesis and testing.
Acknowledgements.
Support by the Loker Hydrocarbon Research Institute and the Department of Chemistry, University of Southern California is gratefully acknowledged.
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S2451-9103(19)30157-7
DOI:
https://doi.org/10.1016/j.coelec.2019.10.010
Reference:
COELEC 470
To appear in:
Current Opinion in Electrochemistry
Received Date: 2 July 2019 Revised Date:
13 September 2019
Accepted Date: 10 October 2019
Please cite this article as: Narayan SR, Nirmalchandar A, Murali A, Yang B, Hoober-Burkhardt L, Krishnamoorthy S, Prakash GKS, Next Generation Aqueous Flow Battery Chemistries, Current Opinion in Electrochemistry, https://doi.org/10.1016/j.coelec.2019.10.010. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Next Generation Aqueous Flow Battery Chemistries Sri R. Narayan*, Archith Nirmalchandar, Advaith Murali, Bo Yang, Lena Hoober-Burkhardt, Sankarganesh Krishnamoorthy and G. K. Surya Prakash Department of Chemistry and Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, CA 90089-1661, USA Abstract. The battery industry is seeking solutions for large-scale energy storage that are affordable, durable and safe. Aqueous redox flow batteries (RFBs) have the inherent properties to meet these requirements. While much has been learned over the past decade on the properties of redox materials, the focus of next generation systems must be primarily on lowering redox material cost and increasing durability. In this context, in addition to inexpensive materials such as iron salts, redox couples based on small organic molecules have shown significant promise. A considerable level of understanding has been gained on the factors affecting the durability of aqueous RFB systems, specifically relating to molecular stability and crossover. New molecular classes, substituent strategies, and cell configurations have been identified to enhance the durability of systems in the future. Next generation systems will also need to focus on designing molecules for achieving high energy efficiency and power density as well. Further, the application of computational methods for screening of chemical stability could accelerate discovery of new molecular architectures.
Background. The exponential growth in the deployment of renewable electricity generation systems demand the deployment of large-scale energy storage systems to deal with the fluctuations in supply. While there are many types of rechargeable batteries that can serve the energy storage function [1,2], applications such as energy time-shift and “firming” that involve daily charge and discharge from 4 to 12 hours, demand substantially higher durability and cycle life than offered by today’s lithium-ion batteries. The anticipated “mega” scale of deployment of such energy storage systems demands a paramount level of safety, almost unlimited reserves of critical materials, and avoidance of negative impacts on the environment [3]. Redox flow batteries (RFBs) based on aqueous chemistry have the potential to meet the demanding economic, environmental and technical requirements for large-scale energy storage [4]. The ability to separate the components that produce power and store the energy offers not only ease of scalability but also intrinsic safety. The charged electrolyte being restricted to separate tanks outside the power-producing element (the cell stack) precludes a runaway hazard, an advantage not often emphasized. Fire hazards are reduced by the use of water as a solvent. Further, the soluble redox couples avert degradation induced by phase changes commonly experienced during charge and discharge of solid active materials. However, “hybrid” RFBs that entail metal deposition at the negative electrode (typically elemental iron or zinc), cannot offer all the foregoing advantages. Among the mature aqueous RFBs are the all-vanadium and zinc-bromine systems that have been studied for over 25 years [5–7]. At a large scale the installed cost of these systems is projected to be in the range of $300- $500/kWh, while their projected lifetimes are as long as 20 years. Despite these long lifetimes the large-scale commercialization of vanadium and zinc-bromine
systems has been limited mainly by the high-cost of the redox materials and system components, and the hazards of handling relatively toxic active materials [8].
Yet from a commercial
standpoint these systems may be considered as a springboard for designing and deploying next generation systems. Thus, besides being durable and safe, we seek that the next generation systems use active materials that are low-cost, abundantly-available, and environmentallybenign. To this end, we will discuss specifically the basic requirements to be satisfied by next generation aqueous RFBs. Cost of Redox Active Materials.
The cost of active materials is one of the principal economic drivers for RFB systems operating for longer than 3 hours. The active materials cost can be 25-50% of the system cost depending on the power density of stack. At the active materials cost of $50/kWh, for 5 hours of charge/discharge at a cell power density of 1 kW/m2, the total system cost is estimated to be $200/kWh. Such a system when operated for 20 years with a capacity degradation rate of no more than 2.5 x10-3 % /day will meet the U.S. Department of Energy target for the levelized cost of energy storage (LCOS) of 5 cents/kWh for utility-scale systems [9].
Inorganic redox materials that meet this demanding cost criterion primarily include iron sulfate, iron chloride, manganese sulfate and zinc sulfate that are available at less than $1/kg (based on bulk pricing from Alibaba.com). Iron sulfate is particularly attractive as it is a waste product of the steel industry and is available at $ 0.1/kg. Potassium ferrocyanide is available at a bulk price of $2-$5/kg. Where inorganic redox couples are involved, acidic solutions are necessary to prevent hydrolysis and precipitation, zinc ions being an exception. An all-iron flow battery that
uses iron chloride is quite attractive from a materials cost standpoint, although other technical challenges with the operation of the battery could offset this cost advantage [10,11].
Electrolytes based on water-soluble polysulfides have been of interest as negative electrode materials because of their relatively low cost. However, the presence of various orders of polysulfides in solution at the same time and the oxidation of polysulfides to insoluble sulfur presents technical challenges to the operation and the cycle life that need to be addressed [12]. Similarly, manganese in +7(permanganate) and +6 (manganate) has shown promise in alkaline media as a cost-effective redox couple for the positive side of the cell [13]. The long-term cycleability of the manganese redox couple in conjunction with organic redox couples needs to be established. While $/kg is a comparison that is often used in the cost discussions relating to active materials, we emphasize that the actual deployed cost of material is governed by $/kWh that accounts for the practical values of utilization, cell voltage, and energy efficiency.
Another class of redox couples that have emerged for aqueous RFBs are small organic molecules that can be synthesized from inexpensive raw materials such as coal, tar sand, heavy fractions of crude, and asphalt [14–16]. Anthraquinone, benzoquinone, and their derivatives are examples of such redox couples that are obtainable at a bulk price of $1-$3/kg [17,18]. While molecules with a low molecular weight are desirable to achieve high specific charge storage capacity, substituents and extra ring moieties are often added to the organic molecules to increase solubility, increase chemical stability, reduce crossover, or achieve a desired electrode potential [19,20]. However, the addition of such substituent groups (often referred to as molecular engineering) not only increases the molecular mass but also the number of steps in the synthesis,
thereby raising the cost of manufacturing. Thus, the benefits of molecular engineering become a trade-off against material cost. The material cost must also factor the handling and disposal requirements for hazardous substances. For example, methyl viologen is a very efficient redox couple for the negative electrolyte but has been banned in the European Union since 2007 [21] and faces restrictions in its use in the United States because of acute health hazards. It is not clear if other molecules of the viologen family will face similar restrictions.
Chemical Durability Even if the active materials are inexpensive, durability is an overriding techno-economic driver, as the LCOS depends on the time elapsed before the need for replacement. Characterization of degradation rates during operation and idle stand are important in the selection of durable molecules. The negative electrolyte in an all-iron flow battery suffers precipitation by hydrolysis if the minute inefficiencies due to hydrogen evolution are not curtailed. Organic additives and complex formation have not resulted in complete suppression of hydrogen [11,22]. Interestingly, the rate of hydrogen evolution can be decreased by operating at higher temperatures, but the approach of recombination of the hydrogen with Fe(III) in a fuel cell like device is still the preferred approach to mitigate the effects [11,23]. This solution adds another reactor system that also needs to be durable and cost-effective. On the other hand, the positive electrolyte, iron(II)/iron(III) does not degrade, unequivocally satisfying the durability criterion for next generation systems.
For the zinc-bromine battery, the long-term stability of the bromine electrode continues to present a challenge [7]. Many new types of doped-carbon materials and oxidized carbon surfaces
have shown increased catalytic activity and
durability [24–26]. Further, the challenge of
uniformity of zinc electroplating also continues to be the focus.
Figure 1. a) Schematic of the Michael reaction on BQDS during charging. b) Mechanism of Michael reaction on BQDS showing nucleophilic addition of water. c) Quinone-methide formation: Reaction of methyl groups with the basic media. d) Mechanism of protodesulfonation. e) Electrochemical redox reaction of quinoxaline in acidic and neutral media, and the side reaction leading to the formation of an unstable hydroxy derivative.
While organic redox couples of a variety of molecular structures are possible, one of the major challenges is the propensity of these molecules to undergo degradation reactions. Quinone/Hydroquinone based redox couples have been widely studied for use in flow battery systems. Anthraquinone derivatives form a class of promising negative side materials. Anthraquinone disulfonic acid (AQDS) and anthraquinone monosulfonic acid (AQS) are stable in acidic media and have been widely used in flow battery research [14–16,27,28]. Dihydroxyanthraquinone and its derivatives have been explored for use as a negative material in basic media. Upon cycling, these molecules underwent a disproportionation reaction to yield an anthrone intermediate that decomposed irreversibly causing capacity loss. The rate of capacity fade can be reduced by avoiding higher states of charge or by adding oxygen to chemically oxidize the anthrone intermediate [29]. In recent studies, Bruschett et al noted that AQDS and its reduced form produce a “quinhydrone” type of dimer that increases the viscosity and reduces utilization. However, such type of molecular association is reversible and hence does not cause any permanent loss of capacity[30].
Benzoquinone derivatives used at the positive electrode are prone to nucleophilic addition reactions in aqueous medium. The greater the positive electrode potential of the molecule, the
greater is its tendency to react with nucleophiles [31]. In acidic and neutral solutions, the nucleophile is water while in basic solutions it is the hydroxide ion. While electron-withdrawing groups are desirable for both increasing the reversible electrode potential to more positive values and increasing solubility, the electron deficiency in the ring is simultaneously exacerbated rendering the molecule susceptible to nucleophilic attack. A case in point is that of 1,2dihydroxybenzene-3,5-disulfonic acid that reacts in the quinone form with water to form the 1,2,4,6-tetrahydroxybenzene-3,5-disulfonic acid. Such a reaction may be considered a Michael reaction (1,4-addition) of water to the quinone (Figure 1a and 1b) [16]. An example of mitigating this reactivity is demonstrated with 1,4-dihydroxy-2,6-dimethylbenzoquinone-3- sulfonic acid (DHDMBS) where we found electron-donating groups to beneficially alter the local electron density distributions [28]. Though having an electron-donating methyl group provides increased stability [32,33], the electrode potential of the molecule is lowered.
In basic media, these benzoquinones will undergo rapid Michael addition due to the hydroxide ion being a strong nucleophile. Here, the addition of methyl groups is not beneficial. In basic media the hydroxide ion deprotonates the methyl groups to form a quinone methide intermediate which will irreversibly decompose to other products (Figure 1c). Thus, in basic media, the literature abounds in the use of ferrocyanide/ferricyanide at the positive electrode [32,34,35]. While ferrocyanide/ferricyanide couple appears to be relatively stable in base, Liu et al have shown that such stability cannot be claimed for long duration of operation in base [36].
Upon increasing the electron density on the aromatic ring as in DHDMBS, we make these molecules susceptible to electrophilic attack. In acidic solutions the H+ electrophile reacts with
such electron-rich arene sulfonic acids causing a protodesulfonation reaction (Figure 1d). The sulfonic acid group which is responsible for the solubility of the molecule is lost to the medium and hence precipitation of the redox species occurs. We have shown that the rate of protodesulfonation can be controlled by adjusting the acidity of electrolyte [28]. Therefore, a delicate balance exists between avoiding the Michael addition reaction and the protodesulfonation reaction. AQDS and AQS are examples of molecules particularly resistant to Michael addition in acid or base media, or protodesulfonation in strongly acidic media. This resistance is because the HOMOs are unfavorably positioned with respect to the nucleophiles, and the lower electron density of the sulfonated rings prevents attack by electrophiles.
Pyrazine, quinoxaline and phenazine family of heterocycles are promising negative side materials. Quinoxalines undergo a one-step two-electron reduction reaction to yield dihydroquinooxalines. This process is reversible in basic media [37]. In acidic and neutral media, the dihydroquinoxalines can further undergo a one-electron irreversible reaction to form a hydroxy-derivative that isomerizes to a lactam-tautomer that can decompose further (Figure 1e) [38]. Pyrazines are similar to the quinoxalines and can be used only in basic media. Wei Wang et al have reported a seminal result of water-soluble phenazine derivative as stable negative material in basic media [34]. In phenazine, the radicals formed during the redox reaction are stabilized by electron delocalization due to the presence of two adjacent benzene rings to the pyrazine motif making it resistant to dimerization and side reactions.
The alloxazine-based molecules such as flavin mono-nucleotide (FMN) can be used as a negative material by exploiting a reversible one-electron redox process. Major drawbacks of the
alloxazine system are its low solubility due to its ߨ-stacking ability and its tendency to undergo hydrolysis to give redox-inactive species. Aziz et al synthesized a derivative of alloxazine with a carboxylic acid group to increase its solubility [39]. This derivative showed chemical stability in alkaline solutions for six weeks. Meng et al have used nicotinamide as an additive to increase the solubility of the sodium salt of FMN and found no unintended side reactions and attributed the chemical stability to electron delocalization [40].
Viologen and its derivatives are water-soluble negative electrolyte materials. Simple derivatives such as methyl viologen form a dimer that undergoes disproportionation yielding insoluble products. With the addition of two quaternary ammonium groups to the viologen backbone, Aziz et al have mitigated the formation of dimers [33]. Similarly, Liu et al have added alkyl sulfonate groups to achieve good stability [41]. Thus, the viologen derivatives are promising negative materials in neutral aqueous medium, provided the toxicity of these molecules is not an issue.
Water-soluble TEMPO derivatives with hydrophilic groups in the 4-position are attractive as positive side materials in neutral solutions [42–44]. While the 4-hydroxy derivative of TEMPO is unstable due to irreversible self-oxidation caused by hydroxyl radicals, other substituent groups such as sulfate and quaternary ammonium groups at the 4-position have resulted in very stable molecules [45]. Consequently, TEMPO derivatives are promising from the durability standpoint.
Given that a finite number of the pathways for degradation exist for the promising molecular classes, we recognize that we could expand the use of computational methods to rapidly screen for the formation of specific reactive intermediates in each molecular class. This approach will
accelerate the identification of potentially viable molecules for durable next generation redox electrolytes.
The durability of stack components is dependent on the redox chemistry and the pH of the solutions. Acidic solutions necessitate carbon-based corrosion-resistant materials for the current collectors and electrodes. Titanium plates may also be used as in the case of the negative electrode of the all-iron RFB. Other parts of the cell can be serviced by inexpensive polypropylene. Alkaline media permit stainless steel components to be used in the flow fields and connector plates, although nickel would be preferred for long-term stability. Electrode structures continue to be made of carbon felt or carbon fiber paper although nickel foam could be an option for alkaline media. Recent advances in iron-based substrates for alkaline electrolyzers can be adapted for RFBs [46]. Where oxidative stability of the membrane is not an issue, expensive perfluorinated membranes may be replaced with hydrocarbon membranes. Sulfonated PEEK and sulfonated PES are the most common and cost-effective alternatives. Today’s alkaline RFBs most commonly use Nafion as the alkali metal ion transporter. Next generation alkaline systems can benefit from aromatic backbone polymers based on biphenyl and terphenyl groups and those with polynorbornene backbones developed for alkaline fuel cells that are expected to be less expensive than their fluorinated counterparts [47–49]. These new anion exchange membranes can also serve as transporters for chloride, bromide and sulfate.
Crossover Effects In the conventional (asymmetric) configuration of RFBs where dissimilar materials are used on either side of the cell, capacity fade usually occurs by crossover of materials from one side to the other, eventually leading to a mixture of material on both sides of the cell. The rate of crossover is dependent on charge-based and size-based exclusion by the ion-exchange membrane. E750 and F1850 membranes from Fumatech have shown reduced permeability for organic redox couples compared to Nafion 212 [28]. AQDS is found not to crossover even over several months of testing, while DHDMBS was found to permeate the membrane quite readily [28]. Alternate ion-exchange membranes with sulfonated mesoporous structure have shown reduced permeability to ions [50].
However, even a small permeation rate will result in a significant
capacity fade, if allowed to persist over a long period of operation. While molecules with large pendant groups that increase the molecular size are expected to avoid crossover, it is important to ensure that the viscosity of the solutions and the cost of manufacturing these molecules do not reduce the overall benefit. In the case of the all-iron RFB, as in the case of the vanadium RFB, large levels of crossover may be tolerated without permanent loss of capacity because the same element is present on both sides of the cell.
With a symmetric cell configuration in which a mixture of the positive and negative redox materials of the same concentration are used on both sides of the cell, irreversible capacity fade may be avoided. Coulombic inefficiency resulting from crossover cannot be avoided, but changes in composition can be reset by a “mix-and-split” operation. Such methods have already been used successfully with maintaining operation of iron/chromium cells [51]. Recently, AQDS/DHDMBS in a symmetric cell configuration was tested and found to be viable over
hundreds of cycles without noticeable loss of capacity [28]. One of the major advantages of the symmetric cell configuration is that water transport due to the osmotic pressure differences can be avoided. As this approach requires the use of twice the stoichiometric amount of material in the tanks, this option becomes acceptable only when the cost of materials is very low. One of the negative aspects of the symmetric design is the increased viscosity of the mixed electrolyte.
Figure 2. a) Positive and negative electrode reactions of bipolar molecule AQ-BQ-MS. b) Cyclic voltammogram of the sulfonated bipolar molecule on a glassy carbon working electrode. The concentration of the bipolar molecule was 1 mM in 1 M sulfuric acid. MSE reference electrode (E° = +0.650 V) and a platinum counter electrode were used. The scan rate was 50 mV/sec. [52,53]
In an alternate molecular design, instead of a mixture of molecules, a single molecule that incorporates both positive and negative electrode redox couples may be used. An example of such a “bipolar” molecule is AQ-BQ-MS (Figure 2a). The same molecule will be used on both sides of the cell [52,53]. Similarly, TEMPO/viologen derivatives, phenazine/TEMPO derivatives and anthraquinone derivatives have been successfully tested to demonstrate the bipolar function although molecular durability is still an issue [54–56]. To be cost-competitive, the synthesis of the bipolar molecule must not add to the complexity. Further, we must ensure that the diffusion coefficients and solubility of bipolar molecule do not become significantly lower than their monopolar counterparts.
Traditionally, sulfonic acid groups have been used to increase the aqueous solubility of the redox molecules. While sulfonic acid groups do provide a large increase in solubility, they affect stability through protodesulfonation. To overcome this problem other water-soluble groups such as tetraalkylammonium, imidazolium, and morpholinium are being attached to the redox active backbone to provide solubility in acidic and neutral media.[33,43,57]. These groups provide flexibility while designing the redox couples. Apart from increasing solubility such groups also make the molecule larger, thereby reducing crossover.
Energy Efficiency and Power Density. An important approach towards improving energy efficiency and power density of redox flow battery stacks is by the rational design of flow fields and electrode structures [58]. This approach is broadly applicable to RFBs. Redox couples that offer a high cell voltage have the advantage
of producing high power densities at a proportionately lower cell current density. Considering that organic molecules are susceptible to degradation by oxidation at high positive electrode potentials attaining the cell voltages as high as those seen in vanadium flow batteries can be a challenge. However, redox couples operating at a low cell voltage will also be viable if they can offer the required power density at high efficiency and minimal overpotential losses. With such a view, the gamut of molecules expands further.
Fast redox process with a standard heterogeneous electron transfer rate constant of 1x10-5 cm s-1 or higher are desirable to allow high efficiencies to be maintained. Poor kinetics are observed when intramolecular hydrogen bonding of the quinone group is possible with a sulfonic acid or hydroxyl group in the ortho- position. Otherwise, the inefficiencies are dominated by overpotential losses from mass transport and electrolyte (Ohmic) resistance. Under these conditions, the molecular properties of solubility, diffusion coefficient, viscosity, and degree of ionic dissociation become intertwined in determining the voltage losses and the power density. Inorganic redox electrolytes tend to be less viscous, more soluble, and have higher diffusion coefficients and degree of ion dissociation than their organic counterparts. With organic molecules, the properties of the oxidized form relative to that of the reduced form are often quite different and this could affect how the power density varies with the state-of-charge. For example, in the case of AQDS, the reduced form is considerably more viscous compared to the oxidized form because of hydrogen bonding and formation of stacked dimers leading to reduced utilization and power density [30]. Further, when using solutions of high concentration, it is important to ensure that solubility levels are not exceeded during charge or discharge because of the reduced solubility of the transformed species.
Aromatic sulfonic acids tend to have pKa values in the range of 0.5 to 2. Consequently, the degree of dissociation is small, and the solutions of these materials by themselves are not very conductive. Amino substituent groups tend to get protonated at higher pH values resulting in more conductive solutions. Thus, to increase the conductivity of the solutions, a supporting electrolyte such as sulfuric acid, hydrochloric acid, sodium hydroxide, or neutral salts is required to achieve low electrolyte resistance. Reducing the ohmic over-potential losses also require that the electrodes be thin, with high surface area and adequate porosity to support high flow rates. The use of elevated temperatures comes with the concern of side reactions being accelerated. Thus, improvements to durability would also beneficially impact power density by permitting higher temperatures of operation.
Conclusions.
Much of the effort in the last six years of research has served well to understand the technical requirements and challenges with various new types of RFBs. The inherent advantages and the interesting challenges of the water-soluble organic redox couples have been unveiled. These efforts indicate that a safe, affordable, sustainable, and robust long-duration energy storage system is quite promising with next generation RFBs. However, the future rests on addressing specific molecular designs that will simultaneously satisfy the primary techno-economic drivers of cost, durability, efficiency and power density. Computational studies could be extended to
examining the propensity of specific undesirable transformations such as the Michael reaction, dimer formation and proto-desulfonation, prior to synthesis and testing.
Acknowledgements.
Support by the Loker Hydrocarbon Research Institute and the Department of Chemistry, University of Southern California is gratefully acknowledged.
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