Insights into Hydrotropic Solubilization for Hybrid Ion Redox Flow

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Insights into Hydrotropic Solubilization for Hybrid Ion Redox Flow Batteries Yu Ding, Changkun Zhang, Leyuan Zhang, Hai-Yan Wei, Yafei Li, and Guihua Yu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01828 • Publication Date (Web): 03 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018

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ACS Energy Letters

Insights into Hydrotropic Solubilization for Hybrid Ion Redox Flow Batteries Yu Ding,†,§ Changkun Zhang, †,§ Leyuan Zhang, † Haiyan Wei,‡ Yafei Li, *,‡ and Guihua Yu*,† †

Materials Science and Engineering Program and Department of Mechanical Engineering, The

University of Texas at Austin, Austin, TX 78712, USA. ‡

Jiangsu Key Laboratory of New Power Batteries, School of Chemistry and Materials Science,

Nanjing Normal University, Nanjing 210023, China. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

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ABSTRACT

Inspired by the solubility enhancement in pharmaceutical research, we report a redox flow battery enabled by hydrotropic solubilization. An almost three-fold increase in the solubility of hydroquinone (H2BQ)-based catholyte can be achieved by employing urea as a hydrotropic agent, and the universal effect of urea on a variety of organic redox species has been demonstrated as well. By combining chemical characterization and computational modeling, the molecular interactions between solute, solvent and hydrotropic agent are elucidated to shed light on the hydrotropic mechanism. Moreover, the working potential of the flow battery can be improved by adopting deep eutectic solvents (DESs) as anolytes. The hydrotropic solubilization leads to an energy density of 114 Wh L−1 when pairing the catholyte with Li anode, and the energy density can still reach 25.3 Wh L−1 in the proof-of-concept hybrid ion flow battery.

TOC GRAPHICS

Amid different kinds of energy storage systems, redox flow batteries, which store chemical components in liquids and allow a decoupled control on energy and power, represent a promising technology toward stationary energy storage.1 It is noted that the widespread implementation of


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traditional redox flow batteries is mainly hindered by the use of electro-active metals with low abundance and environmental concerns.2,

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Recently, organic redox species have emerged as

novel electrode materials for redox flow battery applications.4-7 In addition to the advantages of potentially low cost and ease of scalability, the structural diversity of organic molecules allows for tailoring of their chemical and physical properties.8, 9 Nevertheless, current organic redox flow batteries fall short of meeting the energy density requirements due to insufficient solubility of redox molecules.10 Furthermore, the working voltage is not high enough since anode-active molecules with low redox potential are rare and currently remain limited to insoluble lithium salts.11 Therefore, realizing the vision of constructing a high performance organic redox flow battery necessitates the selection and modification of both a catholyte and an anolyte with high concentration and suitable redox potentials.12, 13 Similar to redox flow battery applications, solubility is also a critical parameter in pharmaceutical research.14 To get a desired pharmacological response, a specific concentration of drugs in the body’s internal circulation systems must be achieved. Currently, various techniques are used to improve the solubility of drugs in water, including chemical modification, solid dispersion, micellar solubilization and so on.14 Inspired by the fruitful results from pharmaceutical research, we report a high-concentration quinone-based redox flow battery using hydrotropic solubilization effect. As the principal redox-active moieties in bioelectrochemical systems, quinones represent an appealing class of biomimetic electroactive materials for redox flow battery applications.15 Historically, chemical functionalization with hydrophilic groups is normally required, especially for hydrophobic polycyclic quinones, given their insufficient solubility in water.11 Although the hydrotropic agent of nicotinamide has been adopted to enhance the solubility of flavin mononucleotide, a comprehensive study is still needed to


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elucidate the chemistry of hydrotropic solubilization on a variety of organic redox species.16 In this work, the solubility of hydroquinone (benzene-1,4-diol: H2BQ) in water can be improved with urea added as a hydrotropic agent. Compared with complex molecular engineering methods, this hydrotropic method is facile and cost-effective, and the added agents can be recycled in the long term even after the battery dies. Furthermore a combined experimental and computational study was conducted to elucidate the nature and mechanisms of the hydrotropic process. In addition to concentration enhancement, the working voltage should also be broadened to increase the energy density of the redox flow battery. Herein, several deep eutectic solvents (DESs) with low redox potentials were prepared as anolytes. Compared to conventional anode-active redox species, Al or Zn-based DESs possess a lower voltage, much higher concentration, and a multielectron transfer process.17-20 More importantly, the dendrite issue faced in previous works using an alkaline metal anode can be circumvented.21, 22 The above-mentioned advantages show the promise of this next generation of environmentally benign and cost-effective redox flow batteries with high safety and large energy density. DES is a kind of eutectic mixture of Lewis acids and bases incorporating several ionic components.23 Typically, a DES contains asymmetric ions with delocalized charge, which results in reduced lattice energy and hence depression of freezing points.24 In our study, the Al-based eutectic is formed using a disproportionation process between AlCl3 and the hydrogen bond donor of urea.25 In comparison, the Zn-based DES is prepared by mixing choline chloride, in which a quaternary ammonium cation functions as the cationic component and chloride as a Lewis base, with ZnCl2 as a Lewis acid. The concentration of Al- and Zn-based DES can reach 5.6 M and 4.0 M, respectively, both of which exceed that of most reported electrode materials for


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ACS Energy Letters

anolytes.1 The obtained DESs are presented in Figure S1, and detailed information on the preparation process can be found in Experimental Methods. To demonstrate the reversibility of DES-based anolytes, cyclic voltammetry (CV) measurements were conducted as shown in Figure S2 and S3. In general, the deposition and stripping of metals can be clearly delineated, which corresponds to the reversible redox reaction of both Al- and Znbased anolyte. Notably, the current intensity of Al-based DES is larger and the polarization is less compared to Zn-based anolyte, which should be attributed to the effect of concentration, reaction kinetics, and viscosity. As discussed above, the speciation mechanism of Al-based DES has been investigated, and it consists of cations and anions via a disproportionation process: 2AlCl3

+ 2(urea) → [AlCl2·(urea)2]+ + [AlCl4]−,

(1)

where both anodic and cathodic metal complexes are produced to ensure conservation of mass and charge.25 Upon adding more AlCl3, [Al2Cl7]− would be formed from bonding to [AlCl4]−. Therefore, there are mainly two pathways in aluminum deposition:25 4[Al2Cl7]− + 3e−

→

Al + 7[AlCl4]−,

2[AlCl2·(urea)2]+ + 3e− → Al + [AlCl4]− + 4(urea),

(2)

(3)

As for the speciation of the Zn-based anolyte, a series of complex zinc anions are generated, including [ZnCl3]−, [Zn2Cl5]−, [Zn3Cl7]− as well as a number of higher clusters at very low intensities.26 The content of anion complexes depends on the ratio of Lewis acids and bases, and stripping/deposition of Zn involves production and consumption of certain anodic species.


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Figure 1. (a) Charge-discharge profile of the flow battery at 100 µA cm−2 with Al-based DES as anolyte, and the catholyte containing 0.1 M H2BQ. (b) Charge-discharge profile of the flow battery at 60 µA cm−2 with Zn-based DES as anolyte, and the catholyte containing 0.1 M H2BQ. (c and d) Cycling stability and efficiency of the flow battery at 100 µA cm−2 with Al-based DES as anolyte, and the catholyte contains 0.1 M H2BQ. To further test the feasibility of DES-based anolytes, a home-made redox flow battery was built. Schematic of the flow battery is presented in Figure S4, and details on the assembly process are discussed in Experimental Methods. Figure 1 shows the electrochemical performance of the H2BQ-based redox flow battery paired with a deep eutectic anolyte. The working voltage maintains around 1.5 V with Al-based anolyte, and it can still reach 1.3 V using a Zn-based DES. It is noted that more obvious polarization occurs when using a Zn-based anolyte, most likely as a result of high viscosity and slow reaction kinetics, and this also correlates with the CV curves in


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Figure S2 and S3. As displayed in Figure 1c and 1d, the cycling stability of the Al-based hybrid ion battery is outstanding, with capacity retention of 99.90% per cycle. Moreover, the stabilized Coulombic efficiency is ~ 99% and the energy efficiency is ca. 89%, both of which outperform the reported organic flow batteries.27, 28 Although the current density is not comparable with aqueous flow batteries limited by the high viscosity of DESs and the low conductivity of membranes, it can still compete with nonaqueous flow batteries, and further improvement can be expected with the development of advanced membranes.29 Moreover, the Al-based anolyte after battery cycling shows almost the same 1H NMR signals as that before battery cycling without any water signals detected (Figure S5), which further attests the reliability of such a hybrid aqueous-nonaqueous design. To prove the broad applicability of the deep eutectic system, a galvanostatic charge-discharge measurement was also conducted using a Zn-based anolyte (Figure S6), but the demonstrated stability and power density has yet to be improved. In light of Stokes’ law, lower solvent viscosity can give rise to higher ionic conductivity in electrolytes given the same supporting salt,30 hence improved battery performance can be expected by further rational screening of hydrogen bond donors in the formation of less viscous DESs. Since the energy density of flow batteries is dictated by the concentration of redox-active substances, the solubility of H2BQ in the cathode part is the bottleneck considering the highly concentrated eutectic anolyte. In selecting the suitable hydrotrope, cost, mass, and electrochemical stability should be taken into account. After rational screening, urea was selected as the hydrotropic agent considering its low cost, small molecular weight and high stability compared with other agents, such as caffeine and nicotinamide. Figure 2a shows the progressively increased solubility of H2BQ in the presence of the hydrotropic agent, and 4 M urea can lead to a nearly three-fold increase in solubility. The ionic conductivity of the


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hydrotrope-enhanced electrolytes is also tested as shown in Table S1, which is in the same level as that of conventional aqueous electrolytes. Meanwhile, the hydrotropic effect of urea on benzoquinone (BQ) is investigated (Figure S7). Notably, it is interesting to find that the hydrotropic effect of urea can be further extended to a variety of organic redox species, including anthraquinone-2-sulfonic acid sodium salt monohydrate (AQS), 1,2-dihydroxybenzene (1,2DHB), and riboflavin 5’-monophosphate sodium salt hydrate (RFMP) (Figure S8, S9, S10). All these merits promise the next-generation of high energy organic redox flow batteries enabled by the hydrotropic method.

Figure 2. (a) Solubility of H2BQ with different concentrations of urea. (b) UV-vis spectra of H2BQ, urea and the H2BQ solution with different concentrations of urea. (c) FTIR spectra of H2BQ, urea and urea-enhanced H2BQ solution. (d) 1H NMR spectra of H2BQ, urea and urea-enhanced H2BQ solution.


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To gain a deeper understanding of the hydrotropic mechanism, a comprehensive study encompassing detailed chemical characterization and advanced computational modeling was conducted. As shown in Figure 2b, the UV-vis absorptions at 288 nm and 245.5 nm correspond to H2BQ and dissociated H2BQ separately, but urea exhibits no light absorptions in this range of wavelengths.31 When comparing the absorptions of H2BQ in various hydrotrope concentrations, no variation of spectral band position was observed, suggesting that no strong interactions between H2BQ and urea exist to form complexes in this system. Moreover, the almost overlapped spectra indicate that no parasitic side reactions took place to change the activity of H2BQ, and similar conclusions can also be derived on the hydrotropic solution of BQ (Figure S11). Fourier transform infrared spectroscopy (FTIR) was then conducted to further interpret the mechanism of solubilization. In Figure 2c, the pure H2BQ solution shows a broad band at 3373 cm−1, which is ascribed to the hydrogen-bonded OH stretching vibration, and the wavenumber between 3300 and 3500 cm−1 of urea points out the presence of NH stretching mode.32 As indicated by the dotted line, the emergence of another minor peak at 3490 cm−1 in H2BQ and urea mixed solution suggests the environmental change of the H2BQ molecules. The band variation should arise from a manifestation of intermolecular interactions, and in this region corresponding to stretching vibration of O-H groups it is believed that hydrogen bonding is mainly responsible for the emergence of this minor band.33 FTIR was also conducted to compare BQ-based solutions (Figure S12). Despite that the bands of BQ were almost covered by that of water, it is still obvious to observe the band variations as indicated by the dot lines. Similar conclusions can also be gathered from Raman spectra in Figure S13, in which the bands in the range between 2900 cm−1 and 3700 cm−1 indicate the hydrogen bonds in water molecule clusters.34 The noticeable band variation of urea-enhanced H2BQ solution, including the peak


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shape (3237 cm−1, 3386 cm−1) and the minor shoulder (3500 cm−1), should be attributed to the change of the hydrogen-bonding environments in the hydrotropic solution.34 The nuclear magnetic resonance (NMR) measurements were conducted to elucidate the hydrotropic chemistry as well, and 1H NMR spectroscopy was performed in D2O for all three samples (Figure 2d). Due to the rapid exchange of protons with deuterons, no spectra corresponding to the hydroxyl or amine groups were seen, and the chemical shift at 4.65 ppm of all three solutions corresponds to water. It should be noted that the obvious signal shift of 1H-atoms on the aromatic ring, as labeled by the dotted line at about 6.6 ppm, further demonstrates the environmental variation around H2BQ molecules, which is consistent with the FTIR and Raman results.35 The 1

H NMR spectroscopy was also conducted on the BQ solution (Figure S14), and the

environmental variation enabled by urea is also evidenced by the signal shift of 1H-atoms on BQ. We also conducted

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C NMR measurements to probe into the environmental change around

quinone molecules. Nevertheless, almost no chemical shift variations can be observed for all the samples (Figure S15). Generally H atoms are located on the end of a molecule, thus the chemical shift of 1H NMR is greatly affected by the neighboring molecules.36 On the other hand, carbon atoms are located at the backbone of a molecule, thus the chemical shift of

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C NMR is more

sensitive to intramolecular forces.37 Because the hydrotropic effect is attributed to the intermolecular interactions between solute, solvent and hydrotrope molecules, it is reasonable to see more obvious chemical shift changes from 1H NMR. Considering that the interaction between H2BQ and urea is not dominant, as proven by UV-vis spectroscopy, the environmental change should be ascribed to the attraction and association of solvent with solute molecules.


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Figure 3. (a and b) Radial distribution functions g(r) with respect to distance between H2BQ-H2BQ in 0.75 M H2BQ solution, 0.75 M H2BQ and 0.5 M urea solution, 1.4 M H2BQ solution, 1.4 M H2BQ and 4 M urea solution, respectively. (c and d) Molecular graphics represent the spatial distribution and hydrogen bonds (cyan dashed lines) around three adjacent H2BQ molecules (ball-and-stick models) within 5 Å in 1.4 M H2BQ and 4 M urea solution, 1.4 M H2BQ solution, respectively. For further insight into the origin of the solubilization mechanism, molecular dynamics (MD) simulation was employed to probe into solute interactions in a hydrotropic solution model system. In a typical solvation process, the solute molecules prefer to spread out and become surrounded by water molecules. In contrast, insoluble substances tend to maintain interactions


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between solute molecules instead of breaking apart.38 The radial distribution functions between H2BQ and water molecules in Figure S16 show sharp peaks at around 3 Å. By comparing the intensity of the peaks in the inset, it is inferred that adding urea can reinforce the intermolecular interactions between solute and solvent.39 The radial distribution functions between the solute and the hydrotropic agent were also calculated as present in Figure S17, and we can observe attenuated interactions when adding more urea into the system, affirming that the enhanced solubility is not contributed by the formation of molecular aggregation between H2BQ and urea molecules. The radial distribution functions between H2BQ molecules were calculated before and after adding urea (Figure 3a and 3b) as well, and it is evident that urea molecules play a critical role in reorganizing the H2BQ and water molecules. It is noted that there are different peaks indicating the solvation shells around H2BQ molecules, and the interactions between those molecules are mainly dominated in the first solvation shell. By comparing the first peaks in radial distribution functions, the general trend shows that the contact between H2BQ molecules becomes looser when hydrotropic agent of urea was added.40 In Figure 3b, we can see distinctly the subdued interaction between H2BQ molecules as highlighted in the dashed rectangle. Furthermore, we simulated the molecule distribution and hydrogen bonds around H2BQ molecules within 5 Å in 1.4 M H2BQ solution with or without urea, as shown in Figure 3c and 3d. It is observed that the randomly selected three adjacent H2BQ molecules are surrounded only by water and urea molecules in the urea-enhanced solution (Figure 3c). Nevertheless, seven H2BQ molecules are detected around the randomly selected three H2BQ molecules within the same distance of 5 Å without urea as hydrotropic agent (Figure 3d). This result also confirms the weakened contact between H2BQ molecules and the uniform distribution of solute molecules in


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ACS Energy Letters

the presence of hydrotrope agents. Meanwhile, more hydrogen bonds can be observed when urea was added, which should play a key role in reorganizing the solute and solvent molecules. Additionally, more intuitive evidence can be acquired from the snapshots of simulated systems. Without urea as a hydrotropic agent, minor stacking of H2BQ molecules can be detected in Figure S18a. The aggregation becomes more serious when increasing the concentration of H2BQ (Figure S18b and S18c), indicating phase separation and precipitation due to partitioning of the solute and water. In comparison, uniformly distributed H2BQ molecules can be observed in the presence of urea as a hydrotrope (Figure S18d and S18e), thus leading to enhanced solubility. The number of hydrogen bonds at different times in the hydrotropic solution as displayed in Table S2 shows the notably reinforced hydrogen bonding between the solute and solvent molecules and the inhibited association between solute molecules in the solvation process, which correlates well with experimental and computational results above. Given the facts above, we conclude that hydrogen bonding between the solute and solvent molecules facilitated by hydrotropes should be the driving force of the hydrotropic solubilization.


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Figure 4. (a) Viscosity of the catholyte at different concentrations of urea. (b) Volumetric capacity at different concentrations of H2BQ. The anolyte is Al-based DES with the catholyte composition: 0.1 M H2BQ at 100 µA cm−2; 0.5 M H2BQ, 1.5 M urea at 100 µA cm−2; 1 M H2BQ, 4 M urea at 200 µA cm−2. (c) Cycling stability of the redox flow battery at 200 µA cm−2. The anolyte is Al-based DES with the catholyte composition: 0.5 M H2BQ, 1.5 M urea. (d) Polarization curve of the flow battery at a flow rate of 4 mL min−1. The anolyte is Al-based DES with the catholyte composition: 0.5 M H2BQ, 1.5 M urea. To validate the feasibility of the hydrotropic catholyte, the influence of urea on the electrochemical properties of H2BQ should be taken into consideration. Figure S19 exhibits the CV curves of H2BQ with and without hydrotropic agent, and there is almost no distinction, suggesting that urea has no side effects on the redox reaction. Figure 4a gives the viscosity-


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temperature plot of the H2BQ-based catholyte, in which only slight variation of viscosity can be observed with different concentrations of urea. Notably, the solution’s viscosity even decreases when 1 M urea was added, which is beneficial in practical applications.30 The electrochemical impedance spectroscopy (EIS) measurements were carried out as shown in Figure S20, which also demonstrates that urea barely affects the conductivity and reaction kinetics of quinone-based catholyte. Moreover, rotating disk electrode (RDE) test was conducted as shown in Figure S21 and S25). The linear relationship in Figure S22 indicates a diffusion-controlled process, from which the diffusion coefficient of H2BQ was calculated to be 7.7×10−6 cm2/s, and even in the presence of urea, the diffusion coefficient can still reach 7.4×10−6 cm2/s (Figure S26).41 Whereafter, Koutecky–Levich plots at different overpotentials (Figure S23, Figure S27) were extrapolated to an infinite rotation rate to get the kinetic current density jk. The exchange current density can be obtained by fitting jk to the Tafel plot (Figure S24, Figure S28), from which the reaction rate constant of H2BQ was determined to be 1×10−3 cm/s and 8.1×10−4 cm/s with and without urea. In the proof-of-concept battery system, the catholyte was first paired with a Li anode to assess the electrochemical performance, and it demonstrates an energy density as high as 114 Wh L−1 (Figure S29). The high Coulombic efficiency of 92% for the first cycle also proves high reversibility of H2BQ molecules in the presence of large amounts of urea molecules. Figure 4b shows the performance of the flow battery employing Al-based DES as anolyte at different concentrations of H2BQ. At a high concentration of 1 M H2BQ, the practical energy density reaches 25.3 Wh L−1, which is also comparable with reported organic redox flow batteries.42-44 It is interesting to find that precipitation of BQ will not occur in the battery test due to the improved hydrotropic effect of urea in weak base electrolyte solution.45 The cycling stability of


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the hybrid ion flow battery is given in Figure 4c, in which little degradation can be found from the steady charge/discharge profiles for nearly 100 hours. No dendrites were observed on the surface of the Al anode after battery cycling (Figure S30 and Figure S31), showing promise of the novel anolyte alternatives for highly safe redox flow batteries. Furthermore, the chemical stability of the anoltye can be evidenced from the EDS patterns of the Al anode before and after battery cycling (Figure S32), which shows little composition change on the Al surface. It should be noted that H2BQ may undergo oxidation reactions with the formation of superoxide and hydrogen peroxide, and this degradation process can be mitigated greatly after purging inert gases through the electrolytes in advance.46 The UV-vis spectra of catholtye after cycling were presented in Figure S33, which mainly only contain the absorptions of H2BQ and BQ, evincing that degradation of the redox species may not be the principal cause of battery decay. To evaluate the battery performance in flow mode, the polarization curve at a flow rate of 4 mL min−1 was plotted in Figure 4d, and the power density of 1.5 mW cm−2 can be output at a current density of 2 mA cm−2. This power density is comparable with reported nonaqueous flow batteries but inferior to aqueous flow batteries.47 Improved performance can be realized through modification of the DESs, optimization of the cell design and exploitation of better membrane separators.48-51 Notably, the price of both positive and negative electroactive materials (H2BQ, AlCl3) is cheap, and urea as the hydrotropic agent as well as the deep eutectic solvent component is also very cost-effective.52 To further demonstrate the electrochemical performance of the hydrotrope-enhanced catholtye, an aqueous redox flow battery was built when paired with an anthraflavic acid-based anolyte, which delivers an output voltage of 1.1 V (Figure S34). By leveraging the knowledge of solubility enhancement techniques in pharmaceutical research, we developed a novel hybrid ion redox flow battery employing a hydrotrope-enhanced H2BQ


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solution as the catholyte and an Al-based DES as the anolyte. Compared with chemical functionalization to enhance the solubility of organic redox species, the hydrotropic solubilization technique represents a sustainable and cost-effective approach to the design of grid-scale energy storage systems.16,

28, 53

A comprehensive study unifying chemical

characterizations and MD simulations reveals that the hydrotropic mechanism arises from the favorable solvation process stemming from strong solvent-solute interactions. The integrated study in this work not only spans different disciplines but can be further extended to other solubility enhancement techniques inspired by pharmaceutics. Meanwhile, biodegradable and non-flammable DESs were explored to serve as high concentration anolytes with low working voltage. It is noted that the power performance in this work is not comparable with those aqueous redox flow batteries, and future attention can be geared towards development of advanced membranes and optimization of the cell structures. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:. Experimental Methods and additional characterization results AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]


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Author Contributions §

Y. Ding and C. Zhang contributed equally to this work

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT G.Y. acknowledges the financial support from the National Science Foundation award (NSFCMMI-1537894), Alfred P. Sloan Research Fellowship, and Camille Dreyfus Teacher-Scholar Award. Y. L. acknowledges the support in China by Natural Science Foundation of China (No.21403115, 21522305). The computational resources utilized in this research were provided by Shanghai Supercomputer Center. REFERENCES (1) Soloveichik, G. L. Flow Batteries: Current Status and Trends. Chem Rev 2015, 115, 11533-11558. (2) Park, M.; Ryu, J.; Wang, W.; Cho, J. Material Design and Engineering of NextGeneration Flow-Battery Technologies. Nat Rev Mater 2016, 2, 16080. (3) Zhang, C.; Zhang, L.; Ding, Y.; Peng, S.; Guo, X.; Zhao, Y.; He, G.; Yu, G. Progress and Prospects of Next Generation Redox Flow Batteries. Energy Storage Mater. 2018, 15, 324-350. (4) Wei, X.; Pan, W.; Duan, W.; Hollas, A.; Yang, Z.; Li, B.; Nie, Z.; Liu, J.; Reed, D.; Wang, W.; et al. Materials and Systems for Organic Redox Flow Batteries: Status and Challenges. ACS Energy Lett. 2017, 2, 2187-2204. (5) Huskinson, B.; Marshak, M. P.; Suh, C.; Er, S.; Gerhardt, M. R.; Galvin, C. J.; Chen, X.; Aspuru-Guzik, A.; Gordon, R. G.; Aziz, M. J. A Metal-Free Organic-Inorganic Aqueous Flow Battery. Nature 2014, 505, 195-198. (6) Ding, Y.; Yu, G. Molecular Engineering Enables Better Organic Flow Batteries. Chem 2017, 3, 917-919. (7) Chen, H.; Zhou, Y.; Lu, Y.-C. Lithium–Organic Nanocomposite Suspension for HighEnergy-Density Redox Flow Batteries. ACS Energy Lett. 2018, 3, 1991-1997. (8) Jian, L.; Bo, H.; Camden, D.; Leo, L. T. A π-Conjugation Extended Viologen as a TwoElectron Storage Anolyte for Total Organic Aqueous Redox Flow Batteries. Angew Chem Int Ed 2018, 57, 231-235. (9) DeBruler, C.; Hu, B.; Moss, J.; Luo, J.; Liu, T. L. A Sulfonate-Functionalized Viologen Enabling Neutral Cation Exchange, Aqueous Organic Redox Flow Batteries toward Renewable Energy Storage. ACS Energy Lett. 2018, 3, 663-668.


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