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Redox flow batteries for energy storage: Recent advances in using organic active materials
Journal Pre-proof Redox flow batteries for energy storage: recent advances in using organic active materials Ruiyong Chen PII:
S2451-9103(20)30010-7
DOI:
https://doi.org/10.1016/j.coelec.2020.01.003
Reference:
COELEC 497
To appear in:
Current Opinion in Electrochemistry
Received Date: 14 December 2019 Accepted Date: 6 January 2020
Please cite this article as: Chen R, Redox flow batteries for energy storage: recent advances in using organic active materials, Current Opinion in Electrochemistry, https://doi.org/10.1016/ j.coelec.2020.01.003. 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. © 2020 Elsevier B.V. All rights reserved.
Redox flow batteries for energy storage: recent advances in using organic active materials Ruiyong Chen1,2,* 1
Transfercenter Sustainable Electrochemistry, Saarland University, 66125 Saarbrücken, Germany 2
KIST Europe, 66123 Saarbrücken, Germany
* Corresponding author: Chen, Ruiyong ([email protected])
Abstract The development of redox electrolytes using organic active materials as alternatives to metal-based species for redox flow batteries is booming recently. However, challenges and gaps remain towards commercialization. This review briefly discusses the most recent advances of utilizing electroactive organic materials. Strategies such as chemical modification through molecular engineering, new efforts towards energy-rich electrolytes and high power electrolytes are addressed. Furthermore, the limiting factors governing the cycling life are summarized.
Introduction Current tendency in the utilization of renewable energy such as wind and solar photovoltaic ignites demands for safe, low-cost and scalable stationary energy storage systems. Redox flow batteries (RFBs) with design flexibility and reliable long-term performance are promising technology that can be integrated into the smart-grid networks [1,2]. Redox electrolytes using organic materials show great advantages such as abundance, tailorable structure and properties [3-5]. High volumetric capacity of electrolytes will allow for the use of electrolyte reservoirs with reduced size and cost. The volumetric capacity and energy density of RFBs is often limited by the insufficient solubility of organic materials and a narrow redox potential gap between catholyte and anolyte. On the other hand, operation of RFBs at increased 1
concentrations suffers from challenges such as reduced utilization of electrolytes [6], increased electrolyte viscosity and high ohmic resistance. Consequently, it is more preferable to increase the cell voltage [7] and the number of electrons transferred for the redox organic molecules, rather than to use concentrated electrolytes. So far, high-power RFBs are mostly demonstrated in acid and alkaline electrolytes with high ionic conductivity [8••]. In addition, a trade-off between the high volumetric capacity and cycling stability is often observed. In general, calendar life of redox electrolytes should meet the need for decadal operation, corresponding to an ideal capacity fade rate below 10%/year. Critical factors that can unlock the great potential of organic materials for RFBs are summarized in Figure 1, ranging from the materials themselves to electrolyte and cell components. Here, new advances and emerging tendency in the past two years are discussed, with particular focus on some illuminating improvements of the key performance parameters and strategies for further research.
Figure 1. Representative organic electroactive materials used in anolyte (blue), catholyte (black), their structural variations with different -R groups, and defined factors to unlock the potential of organic molecule-based RFBs towards large-scale applications.
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Modification of known molecular cores of organic molecules Current studies of organic active materials for RFBs are limited to a few earlier reported organic motifs, such as nitroxyl radical, hydroquinone, dialkoxybenzene, phenothiazine and alloxazine for catholyte, viologen, anthraquinone and phenazine for anolyte (Figure 1) [5,9•]. Organic synthetic chemistry by introducing functional groups (–R) has been used to tailor the solubility, redox potential and structural stability. For instance, anthraquinone derivatives have large potential variation between -0.68 V vs. SHE with electron-donating OH group, and 0.21 V vs. SHE with electron-withdrawing SO3H group [5]. A 0.2 V positive shift in redox potential for TEMPO derivatives were observed, by replacing the OH group with electron-withdrawing trimethylammonium group. Beh et al. [10] functionalized ferrocene and 4,4′-dipyridyl with quaternary ammonium groups, showing improved aqueous solubility (~2 M), and excellent capacity retention of 99.9%/day for 1.3 M electrolyte. The permeability of the grafted viologen through a Selemion® DSV membrane has been reduced to 6.7×10-10 cm2 s-1, compared to 3.4×10-9 cm2 s-1 for an unmodified viologen. A newly reported 4-[3-(trimethylammonio)propoxy]-TEMPO features a higher water solubility of 4.62 M, and reduced permeability of 6.4×10-10 cm2 s-1 through a Selemion® AMV membrane [11], compared to 2.1 M and 1.34×10-9 cm2 s-1 for 4-OH-TEMPO, respectively. However, the viscosity increases from 3.58 mPa s at 0.5 M to ~580 mPa s at 4.5 M. Thus, flow cell performance was only reported with 1.5 M electrolyte. From 0.5 to 1.5 M, the peak power density (pPD) at ~100% SOC was 0.134 and 0.11 W cm-2, respectively. By pairing with a bis-(trimethylammonio) propyl viologen anolyte, a capacity loss rate of 0.62%/day was observed, compared to 5.28%/day for 4-OH-TEMPO. By using triethylene glycol to chemically graft anthraquinone, Jin et al. [12] reported a high aqueous solubility of 1.5 M at pH 7, compared to 0.6 M in aqueous KOH for an earlier reported unmodified anthraquinone [13]. From 0.1 to 1.5 M, the viscosity increases from ~1 to ~90 mPa s at 37 °C [12]. The viscous electrolytes lead to a reduced utilization of the electrolyte. Compared to the performance at room temperature, flow cell tests at 45°C resulted in a 30% increase of the pPD (0.22 W cm-2 at ~300 mA cm-2) for the 1.5 M 3
electrolyte (Figure 2). Besides the functionalization, molecular engineering and theoretical screening are promising approaches for discovering new organic candidate materials [6,14,15].
Liquid organic compounds Solvent-free, liquid organic redox-active materials can largely increase the volumetric capacity of electrolytes [16]. Moreover, by using liquid redox-active oligomers, the unwanted cross-mixing can be largely reduced compared to small organic molecules. Baran et al. [17] reported that by using a branched oligoethylene oxide core motif, liquid oligomers with different redox centers and high electrochemical stability have been synthesized with a maximum concentration of 3.95 M. Moreover, compared to large polymers (diffusion coefficient of ~10-7 cm2 s-1), faster mass transport (~10-6 cm2 s-1) has been determined for the studied oligomers. In addition, the oligoethylene oxide core favors fast reaction kinetics, similar to the monomer counterparts. Most significantly, the application of oligomers allow for the use of low-cost size-exclusion membranes [18••]. However, RFB tests were demonstrated with a concentration of only 50 mM. Further feasibility tests with sufficient concentration is needed.
Eutectic electrolytes Eutectic electrolytes for several widely investigated organic materials have been reported with largely enhanced concentrations [19], such as 4.2 M viologen (mixed with ethylene glycol) [20], 3.5 M N-butylphthalimide (mixed with 1,1-dimethylferrocene, viscosity 4.5 mPa s) [21••], 4 M phthalimide (mixed with urea and LiTFSI, viscosity 0.13 Pa s) [22••]. The urea molecules may affect the coordination environment of the phthalimide and the local electron density distribution. Such intermolecular interactions facilitate the formation of eutectic mixture and also promote the redox reversibility and reaction kinetics [22••]. So far, it was found that these concentrated electrolytes are not suitable for high power output (typical operating current densities < 1 mA cm-2). However, they might be useful for high energy density systems, which demand small tank size, but do not require 4
charge/discharge in a short time (Figure 2). The ion conductivity of the eutectic mixtures can be increased by adding supporting salt and solvent [21••]. For instance, the 1 M biredox N-butylphthalimide electrolyte, with a good compromise between the viscosity (5.1 mPa s) and conductivity (18.5 mS cm-1), showed a high pPD of 0.19 W cm-2 at 190 mA cm-2. In order to reduce the pumping loss and to increase the system efficiency, new design of flow channel, use of catalysts and high operating temperatures are required (Figure 2).
Figure 2. Classification of reported organic molecules as high power density and high capacity materials, determined by the concentration, viscosity of electrolytes and operating current density.
Multielectron reactants Multielectron materials with a moderate concentration can help to tackle the sluggish kinetics caused by poor mass transport, meanwhile to maintain a high volumetric capacity. Luo et al. [23••] demonstrated thiazolo[5,4-d]thiazole as a structural extending framework for viologen and subsequent functionalization with hydrophilic ammonium groups, which enables a 5
solubility of 1.3 M in water and the use of 2e- reaction. Flow cell using 0.25 M anolyte showed an energy efficiency of 58% and a coulombic efficiency of 94% at 60 mA cm-2. In another work, DeBruler et al. [24] synthesized a 2e- viologen with a high aqueous solubility of 1.8 M. Flow cell was tested with a maximal anolyte concentration of 0.25 M, showing a pPD of 0.13 W cm-2 and an energy efficiency of 63% at 60 mA cm-2. Wang et al. [8••] reported a 2-hydroxy-3-carboxy-1,4-naphthoquinone (2,3-hcNQ), which has a solubility of 1.2 M in 1 M KOH and undergoes a 2H+/2e- reaction. Note that fast capacity fading rates of 3.4%/day and 6.4%/day were observed for the 0.5 M and 1 M anolyte, respectively. Metal coordination complexes [25,26] with redox organic ligands are promising for multi-electron reactions [27]. By using a mixture of water and imidazolium chloride, the metal phthalocyanines (with a solubility < 0.15 M) with low negative redox potentials (-0.2 to -1.6 V vs. Ag) have been successfully demonstrated in water-based electrolytes [25]. Further increase in solubility of those materials (> 0.5 M) is required to provide sufficient energy storage capability.
Towards high power performance by pairing with non-rate limiting electrolytes High power performance of aqueous electrolytes using concentrated organic materials has been demonstrated by pairing with non-rate limiting counter electrolytes (Figure 2). A high pPD of 0.255 W cm-2 at ~350 mA cm-2 at 100% SOC has been observed for 5 mL of 0.5 M 2,3-hcNQ anolyte (2e- reaction), when being paired with 15 mL of 0.4 M K4Fe(CN)6 catholyte in aqueous KOH [8••]. Similarly, phenothiazine-based catholyte (1 M, 2e- reaction) showed an energy efficiency of 66% at 200 mA cm-2 against anolyte of V2+ in H2SO4 [28••]. Even in pH neutral electrolytes, 1,1'-bis(3-sulfonatopropyl)-4,4'-bipyridinium ((SPr)2V) anolyte exhibited a pPD of 0.0725 W cm-2 (0.9 M (SPr)2V) at ~150 mA cm-2 by pairing with (NH4)4[Fe(CN)6] catholyte [29], and 0.227 W cm-2 (1.5 M (SPr)2V) at ~300 mA cm-2 by pairing with bromide catholyte, respectively [30]. In contrast, when the organic materials with fast reaction rates are paired with those of metal deposition/stripping with sluggish kinetics (such as Li+/Li [22••], Zn2+/Zn [31,32], Zn/[Zn(OH)4]2- [33]), the voltage efficiency and the power performance (typically < 10 mA 6
cm-2) may be largely restricted. These results indicate the importance of rationally pairing the catholyte and anolyte.
Impact of electrolyte environment on the cyclability The capacity fading is mainly governed by the stability of the organic materials [34,35], their compatibility with the electrolyte environment, pairing character with conducting ions [36], their crossover rate through the membrane, and the operating conditions such as cutoff voltage and SOC [37••]. Quinone derivatives, having carbonyl groups in the oxidized state, are more electron-deficient and susceptible to a nucleophilic attack from water. Tabor et al. [35] made a computational screening of quinone stability in aqueous media. The Michael addition with water molecules will cause chemical degradation. Similarly, water attack on the oxidized nitroxide radicals is thought to be responsible for the ring-opening of TEMPO derivatives [11]. Such findings of water-driven instability of organic materials provide important hints for the selection of suitable supporting electrolytes. Aqueous electrolytes with significantly reduced fraction of water [31,38], and with strong interactions between the organic solutes and the conducting ions might be feasible strategies to kinetically retard such nucleophilic attack [39]. Unmodified 2,6-dihydroxyanthraquione (2,6-DHAQ) showed a fast capacity fade of 5%/day, arising from a decomposition of a second γ-hydroxybutyrate moiety. The capacity fade rate has been significantly reduced to 0.014%/day by using a phosphonate-functionalized anthraquione (0.5 M) [40]. Goulet et al. [37••] found that the deep reduction of 2,6-DHAQ anolyte by setting a high charge cutoff voltage resulted in irreversible dimerization reactions. By restricting the SOC from 99.9% to 88%, the capacity fading rate can be reduced from 5.6% to 0.14%/day. Recently, a heteroaromatic phenothiazine derivative of methylene blue demonstrated excellent cycling stability [28••]. Flow cell tests using 1.2 and 1.5 M catholytes of methylene blue in H2SO4 showed slow capacity fade rates of 0.52% and 0.76%/day, respectively. In addition, redox behavior of organic materials is often concentration- and electrolyte environment-dependent (such as pH value, impurities and absorption of atmospheric CO2) 7
[41••]. Chemical decomposition through dimerization can be retarded through an increase in the coulombic repulsion of the species. The formation of dimers, trimers and polyaggregates through self-association of organic molecules at high concentration can affect the number of electrons transferred [41••]. Rational design of the electrolyte formulations and chemical structure of organic molecules by creating steric hindrance for the molecular agglomeration will ensure better utilization of the electroactive materials.
Conclusion and perspective Largely enhanced solubility of up to about 4 M has been recently reported for several representative organic materials through ionic grafting [11], using room-temperature liquid organic materials [16,17] and eutectic mixtures [20-22••]. However, flow cell tests are generally restricted to only low concentrations with consequently low volumetric capacities. Fundamental knowledge of the microscopic molecular coordination environment for the concentrated electrolytes remains less explored [42]. It is feasible to enhance the power performance by pairing concentrated electrolytes with non-rate limiting counter electrolytes. To date, the use of catalysts to boost the reaction turnover rates of concentrated electrolytes has rarely been reported [33]. Alternative strategies to develop new organic molecules, which can maximize the cell voltage and can exhibit multi-electron reactions are promising [43]. A targeted cycling stability over decades requires a low permeability of organic materials through the membranes, as well as an excellent (electro)chemical durability. The reported cycling stability still cannot meet such requirements. Knowledge about the decomposition mechanisms of organic components under different electrolyte environments and electrochemical reaction conditions can guide the design of the active materials and the electrolyte formulations, which, however, have not been well studied yet.
References Papers of particular interest, published within the period of review, have been highlighted as: •Paper of special interest. ••Paper of outstanding interest. 8
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S2451-9103(20)30010-7
DOI:
https://doi.org/10.1016/j.coelec.2020.01.003
Reference:
COELEC 497
To appear in:
Current Opinion in Electrochemistry
Received Date: 14 December 2019 Accepted Date: 6 January 2020
Please cite this article as: Chen R, Redox flow batteries for energy storage: recent advances in using organic active materials, Current Opinion in Electrochemistry, https://doi.org/10.1016/ j.coelec.2020.01.003. 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. © 2020 Elsevier B.V. All rights reserved.
Redox flow batteries for energy storage: recent advances in using organic active materials Ruiyong Chen1,2,* 1
Transfercenter Sustainable Electrochemistry, Saarland University, 66125 Saarbrücken, Germany 2
KIST Europe, 66123 Saarbrücken, Germany
* Corresponding author: Chen, Ruiyong ([email protected])
Abstract The development of redox electrolytes using organic active materials as alternatives to metal-based species for redox flow batteries is booming recently. However, challenges and gaps remain towards commercialization. This review briefly discusses the most recent advances of utilizing electroactive organic materials. Strategies such as chemical modification through molecular engineering, new efforts towards energy-rich electrolytes and high power electrolytes are addressed. Furthermore, the limiting factors governing the cycling life are summarized.
Introduction Current tendency in the utilization of renewable energy such as wind and solar photovoltaic ignites demands for safe, low-cost and scalable stationary energy storage systems. Redox flow batteries (RFBs) with design flexibility and reliable long-term performance are promising technology that can be integrated into the smart-grid networks [1,2]. Redox electrolytes using organic materials show great advantages such as abundance, tailorable structure and properties [3-5]. High volumetric capacity of electrolytes will allow for the use of electrolyte reservoirs with reduced size and cost. The volumetric capacity and energy density of RFBs is often limited by the insufficient solubility of organic materials and a narrow redox potential gap between catholyte and anolyte. On the other hand, operation of RFBs at increased 1
concentrations suffers from challenges such as reduced utilization of electrolytes [6], increased electrolyte viscosity and high ohmic resistance. Consequently, it is more preferable to increase the cell voltage [7] and the number of electrons transferred for the redox organic molecules, rather than to use concentrated electrolytes. So far, high-power RFBs are mostly demonstrated in acid and alkaline electrolytes with high ionic conductivity [8••]. In addition, a trade-off between the high volumetric capacity and cycling stability is often observed. In general, calendar life of redox electrolytes should meet the need for decadal operation, corresponding to an ideal capacity fade rate below 10%/year. Critical factors that can unlock the great potential of organic materials for RFBs are summarized in Figure 1, ranging from the materials themselves to electrolyte and cell components. Here, new advances and emerging tendency in the past two years are discussed, with particular focus on some illuminating improvements of the key performance parameters and strategies for further research.
Figure 1. Representative organic electroactive materials used in anolyte (blue), catholyte (black), their structural variations with different -R groups, and defined factors to unlock the potential of organic molecule-based RFBs towards large-scale applications.
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Modification of known molecular cores of organic molecules Current studies of organic active materials for RFBs are limited to a few earlier reported organic motifs, such as nitroxyl radical, hydroquinone, dialkoxybenzene, phenothiazine and alloxazine for catholyte, viologen, anthraquinone and phenazine for anolyte (Figure 1) [5,9•]. Organic synthetic chemistry by introducing functional groups (–R) has been used to tailor the solubility, redox potential and structural stability. For instance, anthraquinone derivatives have large potential variation between -0.68 V vs. SHE with electron-donating OH group, and 0.21 V vs. SHE with electron-withdrawing SO3H group [5]. A 0.2 V positive shift in redox potential for TEMPO derivatives were observed, by replacing the OH group with electron-withdrawing trimethylammonium group. Beh et al. [10] functionalized ferrocene and 4,4′-dipyridyl with quaternary ammonium groups, showing improved aqueous solubility (~2 M), and excellent capacity retention of 99.9%/day for 1.3 M electrolyte. The permeability of the grafted viologen through a Selemion® DSV membrane has been reduced to 6.7×10-10 cm2 s-1, compared to 3.4×10-9 cm2 s-1 for an unmodified viologen. A newly reported 4-[3-(trimethylammonio)propoxy]-TEMPO features a higher water solubility of 4.62 M, and reduced permeability of 6.4×10-10 cm2 s-1 through a Selemion® AMV membrane [11], compared to 2.1 M and 1.34×10-9 cm2 s-1 for 4-OH-TEMPO, respectively. However, the viscosity increases from 3.58 mPa s at 0.5 M to ~580 mPa s at 4.5 M. Thus, flow cell performance was only reported with 1.5 M electrolyte. From 0.5 to 1.5 M, the peak power density (pPD) at ~100% SOC was 0.134 and 0.11 W cm-2, respectively. By pairing with a bis-(trimethylammonio) propyl viologen anolyte, a capacity loss rate of 0.62%/day was observed, compared to 5.28%/day for 4-OH-TEMPO. By using triethylene glycol to chemically graft anthraquinone, Jin et al. [12] reported a high aqueous solubility of 1.5 M at pH 7, compared to 0.6 M in aqueous KOH for an earlier reported unmodified anthraquinone [13]. From 0.1 to 1.5 M, the viscosity increases from ~1 to ~90 mPa s at 37 °C [12]. The viscous electrolytes lead to a reduced utilization of the electrolyte. Compared to the performance at room temperature, flow cell tests at 45°C resulted in a 30% increase of the pPD (0.22 W cm-2 at ~300 mA cm-2) for the 1.5 M 3
electrolyte (Figure 2). Besides the functionalization, molecular engineering and theoretical screening are promising approaches for discovering new organic candidate materials [6,14,15].
Liquid organic compounds Solvent-free, liquid organic redox-active materials can largely increase the volumetric capacity of electrolytes [16]. Moreover, by using liquid redox-active oligomers, the unwanted cross-mixing can be largely reduced compared to small organic molecules. Baran et al. [17] reported that by using a branched oligoethylene oxide core motif, liquid oligomers with different redox centers and high electrochemical stability have been synthesized with a maximum concentration of 3.95 M. Moreover, compared to large polymers (diffusion coefficient of ~10-7 cm2 s-1), faster mass transport (~10-6 cm2 s-1) has been determined for the studied oligomers. In addition, the oligoethylene oxide core favors fast reaction kinetics, similar to the monomer counterparts. Most significantly, the application of oligomers allow for the use of low-cost size-exclusion membranes [18••]. However, RFB tests were demonstrated with a concentration of only 50 mM. Further feasibility tests with sufficient concentration is needed.
Eutectic electrolytes Eutectic electrolytes for several widely investigated organic materials have been reported with largely enhanced concentrations [19], such as 4.2 M viologen (mixed with ethylene glycol) [20], 3.5 M N-butylphthalimide (mixed with 1,1-dimethylferrocene, viscosity 4.5 mPa s) [21••], 4 M phthalimide (mixed with urea and LiTFSI, viscosity 0.13 Pa s) [22••]. The urea molecules may affect the coordination environment of the phthalimide and the local electron density distribution. Such intermolecular interactions facilitate the formation of eutectic mixture and also promote the redox reversibility and reaction kinetics [22••]. So far, it was found that these concentrated electrolytes are not suitable for high power output (typical operating current densities < 1 mA cm-2). However, they might be useful for high energy density systems, which demand small tank size, but do not require 4
charge/discharge in a short time (Figure 2). The ion conductivity of the eutectic mixtures can be increased by adding supporting salt and solvent [21••]. For instance, the 1 M biredox N-butylphthalimide electrolyte, with a good compromise between the viscosity (5.1 mPa s) and conductivity (18.5 mS cm-1), showed a high pPD of 0.19 W cm-2 at 190 mA cm-2. In order to reduce the pumping loss and to increase the system efficiency, new design of flow channel, use of catalysts and high operating temperatures are required (Figure 2).
Figure 2. Classification of reported organic molecules as high power density and high capacity materials, determined by the concentration, viscosity of electrolytes and operating current density.
Multielectron reactants Multielectron materials with a moderate concentration can help to tackle the sluggish kinetics caused by poor mass transport, meanwhile to maintain a high volumetric capacity. Luo et al. [23••] demonstrated thiazolo[5,4-d]thiazole as a structural extending framework for viologen and subsequent functionalization with hydrophilic ammonium groups, which enables a 5
solubility of 1.3 M in water and the use of 2e- reaction. Flow cell using 0.25 M anolyte showed an energy efficiency of 58% and a coulombic efficiency of 94% at 60 mA cm-2. In another work, DeBruler et al. [24] synthesized a 2e- viologen with a high aqueous solubility of 1.8 M. Flow cell was tested with a maximal anolyte concentration of 0.25 M, showing a pPD of 0.13 W cm-2 and an energy efficiency of 63% at 60 mA cm-2. Wang et al. [8••] reported a 2-hydroxy-3-carboxy-1,4-naphthoquinone (2,3-hcNQ), which has a solubility of 1.2 M in 1 M KOH and undergoes a 2H+/2e- reaction. Note that fast capacity fading rates of 3.4%/day and 6.4%/day were observed for the 0.5 M and 1 M anolyte, respectively. Metal coordination complexes [25,26] with redox organic ligands are promising for multi-electron reactions [27]. By using a mixture of water and imidazolium chloride, the metal phthalocyanines (with a solubility < 0.15 M) with low negative redox potentials (-0.2 to -1.6 V vs. Ag) have been successfully demonstrated in water-based electrolytes [25]. Further increase in solubility of those materials (> 0.5 M) is required to provide sufficient energy storage capability.
Towards high power performance by pairing with non-rate limiting electrolytes High power performance of aqueous electrolytes using concentrated organic materials has been demonstrated by pairing with non-rate limiting counter electrolytes (Figure 2). A high pPD of 0.255 W cm-2 at ~350 mA cm-2 at 100% SOC has been observed for 5 mL of 0.5 M 2,3-hcNQ anolyte (2e- reaction), when being paired with 15 mL of 0.4 M K4Fe(CN)6 catholyte in aqueous KOH [8••]. Similarly, phenothiazine-based catholyte (1 M, 2e- reaction) showed an energy efficiency of 66% at 200 mA cm-2 against anolyte of V2+ in H2SO4 [28••]. Even in pH neutral electrolytes, 1,1'-bis(3-sulfonatopropyl)-4,4'-bipyridinium ((SPr)2V) anolyte exhibited a pPD of 0.0725 W cm-2 (0.9 M (SPr)2V) at ~150 mA cm-2 by pairing with (NH4)4[Fe(CN)6] catholyte [29], and 0.227 W cm-2 (1.5 M (SPr)2V) at ~300 mA cm-2 by pairing with bromide catholyte, respectively [30]. In contrast, when the organic materials with fast reaction rates are paired with those of metal deposition/stripping with sluggish kinetics (such as Li+/Li [22••], Zn2+/Zn [31,32], Zn/[Zn(OH)4]2- [33]), the voltage efficiency and the power performance (typically < 10 mA 6
cm-2) may be largely restricted. These results indicate the importance of rationally pairing the catholyte and anolyte.
Impact of electrolyte environment on the cyclability The capacity fading is mainly governed by the stability of the organic materials [34,35], their compatibility with the electrolyte environment, pairing character with conducting ions [36], their crossover rate through the membrane, and the operating conditions such as cutoff voltage and SOC [37••]. Quinone derivatives, having carbonyl groups in the oxidized state, are more electron-deficient and susceptible to a nucleophilic attack from water. Tabor et al. [35] made a computational screening of quinone stability in aqueous media. The Michael addition with water molecules will cause chemical degradation. Similarly, water attack on the oxidized nitroxide radicals is thought to be responsible for the ring-opening of TEMPO derivatives [11]. Such findings of water-driven instability of organic materials provide important hints for the selection of suitable supporting electrolytes. Aqueous electrolytes with significantly reduced fraction of water [31,38], and with strong interactions between the organic solutes and the conducting ions might be feasible strategies to kinetically retard such nucleophilic attack [39]. Unmodified 2,6-dihydroxyanthraquione (2,6-DHAQ) showed a fast capacity fade of 5%/day, arising from a decomposition of a second γ-hydroxybutyrate moiety. The capacity fade rate has been significantly reduced to 0.014%/day by using a phosphonate-functionalized anthraquione (0.5 M) [40]. Goulet et al. [37••] found that the deep reduction of 2,6-DHAQ anolyte by setting a high charge cutoff voltage resulted in irreversible dimerization reactions. By restricting the SOC from 99.9% to 88%, the capacity fading rate can be reduced from 5.6% to 0.14%/day. Recently, a heteroaromatic phenothiazine derivative of methylene blue demonstrated excellent cycling stability [28••]. Flow cell tests using 1.2 and 1.5 M catholytes of methylene blue in H2SO4 showed slow capacity fade rates of 0.52% and 0.76%/day, respectively. In addition, redox behavior of organic materials is often concentration- and electrolyte environment-dependent (such as pH value, impurities and absorption of atmospheric CO2) 7
[41••]. Chemical decomposition through dimerization can be retarded through an increase in the coulombic repulsion of the species. The formation of dimers, trimers and polyaggregates through self-association of organic molecules at high concentration can affect the number of electrons transferred [41••]. Rational design of the electrolyte formulations and chemical structure of organic molecules by creating steric hindrance for the molecular agglomeration will ensure better utilization of the electroactive materials.
Conclusion and perspective Largely enhanced solubility of up to about 4 M has been recently reported for several representative organic materials through ionic grafting [11], using room-temperature liquid organic materials [16,17] and eutectic mixtures [20-22••]. However, flow cell tests are generally restricted to only low concentrations with consequently low volumetric capacities. Fundamental knowledge of the microscopic molecular coordination environment for the concentrated electrolytes remains less explored [42]. It is feasible to enhance the power performance by pairing concentrated electrolytes with non-rate limiting counter electrolytes. To date, the use of catalysts to boost the reaction turnover rates of concentrated electrolytes has rarely been reported [33]. Alternative strategies to develop new organic molecules, which can maximize the cell voltage and can exhibit multi-electron reactions are promising [43]. A targeted cycling stability over decades requires a low permeability of organic materials through the membranes, as well as an excellent (electro)chemical durability. The reported cycling stability still cannot meet such requirements. Knowledge about the decomposition mechanisms of organic components under different electrolyte environments and electrochemical reaction conditions can guide the design of the active materials and the electrolyte formulations, which, however, have not been well studied yet.
References Papers of particular interest, published within the period of review, have been highlighted as: •Paper of special interest. ••Paper of outstanding interest. 8
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: