Viologen Hydrothermal Synthesis and Structure–Property Relationships for Redox Flow Battery Optimization

Aqueous organic redox flow batteries (AORFBs) are an emerging technology for fire safe grid energy storage systems with sustainable material feedstocks. Yet, designing organic redox molecules with the desired solubility, viscosity, permeability, formal potential, kinetics, and stability while remaining synthetically scalable is challenging. Herein, the adaptability is demonstrated of a single‐step, high‐yield hydrothermal reaction for nine viologen chloride salts. New empirical insights are gleaned into fundamental structure–property relationships for multiobjective optimization. A new asymmetric Dex‐DiOH‐Vi derivative showcases an enhanced solubility of 2.7 m with minimal tradeoff in membrane permeability. With a record viologen cycling volumetric capacity (67 Ah L−1 anolyte theoretical), Dex‐DiOH‐Vi exhibits 14‐d of stable cycling performance in anolyte‐limiting AORFB with no crossover or chemical degradation. This work highlights the importance of designing efficient synthetic approaches of organic redox species for molecular engineering high‐performance flow battery electrolytes.


Introduction
Redox flow battery (RFB) is a promising technology for grid scale energy storage due to its long-duration scalability and adaptability. Aqueous organic redox flow battery (AORFB) uses redox active organic molecules soluble in water to replace inorganic molecules (e.g., vanadium) for improved resource availability and sustainability. However, the development of organic catholytes and anolytes are challenging since many factors need be concurrently considered, including solubility, stability, kinetics, and cost. [1][2][3][4][5][6][7] Viologen anolytes show promising overall performance DOI: 10.1002/aenm.202203919 in pH neutral conditions, which offers reduced electrolyte corrosivity for enhanced safety and device robustness. Yet, this class has been mainly limited to bis(trimethylammonium propyl) viologen (BTMAP-Vi) [8] in stable, nonsymmetric RFB demonstrations at high anolyte concentration (≥1.5 m). Other viologen derivatives have shown potential in AORFB application, [9][10][11][12] yet further improvements in viologen structural designs are needed to reach higher volumetric capacities while remaining chemically stable and membrane compatible.
Conventionally, viologen derivatives are produced through multistep reactions in organic solvent. [8,10,13,14] Although this results in simple purification as the charged product precipitates out of organic solution, so does the partially reacted, charged monosubstituted byproduct. Since chloro-terminating 4-position functionalization reagents have limited substitution activity within organic solvent temperature ranges (≈80°C), bromo-terminating reagents with heightened leaving group capability are often used to improve yield. [10,13] However, these reagents are more expensive with still limited yields below 50%, [13] and ion-exchange processes are required to obtain the preferred chloride salt forms for AORFB application. Bromide viologen salts have higher electrolyte viscosity and anion-exchange membrane (AEM) resistance than chloride due to its increased size, resulting in worsened energy density and power density performance. Also problematic is the lower formal potential (+1.07 V vs SHE) of bromide, [3] which results in undesired oxidation to bromine when paired with high-voltage cathodic species (i.e., TEMPO). Thus, the expansion of viologen AORFB derivatives in high concentration demonstration has been slowed by scalable syntheses of chloride salt forms.
We have developed a new method of viologen production through a high-yield, single-step hydrothermal reaction. By elevating temperature above the boiling point of water inside an autoclave reactor, low-cost chloro-terminated functionalization units can be activated to perform efficient S N 2 di-substitution with the 4,4′-dipyridyl redox core. This single step synthetic method is scalable while resulting in high-purity chloride salt viologen products. Lv et al. successfully demonstrated this sustainable synthesis at kilogram scale with the Dextrosil substituent to obtain a new viologen derivate anolyte, Dex-Vi, which displayed stable and high-energy density RFB performance as a pH neutral aqueous anolyte. [15] By expanding the scope of this reaction route and gaining insights into how structural features alter physicochemical and electrochemical properties, viologen can be tuned for optimal AORFB performance.
In this work, we showcase the universal capability of this simple one-step hydrothermal method for symmetric and asymmetric viologens and elucidate fundamental structure-property relationships to guide rational design of organic anolytes. Nine derivatives have been successfully synthesized in high yield through the hydrothermal method to enrich the members of the viologen class. Each derivative was experimentally characterized to unveil physical and electrochemical property perturbations resulting from structural changes. Ultimately, we demonstrated the cycling performance of a new viologen anolyte, Dex-DiOH-Vi, with record viologen volumetric capacity (67 Ah L −1 anolyte theoretical). This anolyte-limiting AORFB with 2.5 m Dex-DiOH-Vi exhibited no apparent decay for 14 d of continuous cycling with 2.5 m of a TEMPO catholyte. Thus, through rationalization of a simple and universal synthetic method, new molecular design guidelines for energy dense and stable AORFB properties are gleaned.

Results and Discussion
To demonstrate the adaptability of the hydrothermal method towards viologen chloride salts, several substitution reagents were selected based on four main factors. First, the reagents are alkyl chains with terminal chloro leaving group for substitution reaction at the pyridyl nitrogen. Second, the reagents have additional hydrophilic functionality, including charged and alco-hol moieties, for enhanced water solubility and membrane exclusion. Third, positive, negative, and neutral reagents of varying alkyl length were selected for systematic evaluation. Finally, the reagents are readily available at low cost to be produced in bulk for experimental testing. In general, hydrothermal reactions mixed 4,4-dipyrdine solid with excess 4 m aqueous solution of chloroalkyl reagent to react in autoclave at 120°C for 48 h. The viologen product was then crashed out with addition of organic antisolvent. The hydrothermal approach was also adapted to synthesize asymmetric viologens with two different chloroalkyl reagents through a one-pot process, which permitted stepwise evaluation of viologen functionalization charge and size structureproperty effects. In total five symmetric and four asymmetric viologens were produced through the new hydrothermal approach (Figure 1).
The yield and purity of each viologen was confirmed through NMR (Figures S3-S11, Supporting Information), ranging from 65% to 89% yield. Although the yield of di-substituted product could be increased through tuning of chloroalkyl reagent ratio and concentration, reaction temperature, and reaction time, each viologen would require different optimal conditions. Thus, standard conditions of 1:3 dipyridyl to chloroalkyl reagent, at a concentration of 4 m chloroalkyl reagent, 120°C reaction temperature, and 48 h reaction were used for each to permit fair comparison of chloroalkyl yield in identical reaction. Furthermore, the yields are not solely an artifact of chloroalkyl substitution reactivity at the pyridyl site but also the purification efficiency. The disubstituted viologen products crash out of the aqueous reaction media first with addition of organic solvent (i.e., DMF) to the aqueous reaction media because they are more polar than the partially reacted monosubstituted form and unreacted chloroalkyl reagents. However, for viologens bearing neutral functionalization, including EtOH, DiOH, EtS-DiOH, SHOP-DiOH, and Dex-DiOH, their nonpolar solvent affinity increases and makes separation from the aqueous phase and monosubstituted forms more difficult, resulting in yields less than 70% for viologens containing at least one neutral functionalization. In contrast, viologens with two charged functionalization exhibited yields over 75% and as high as 89% due to ease of separation. Thus, all chloroalkyl reagents tested appear to be reactive in hydrothermal conditions and pure products were obtained for characterization product.
Upon showing the hydrothermal method is adaptable to various chloroalkyl substitution reagents, the resultant viologen products were experimentally screened for physiochemical and electrochemical properties to delineate structural effects. Ideally, a viologen suitable for RFBs should have high chemical stability and water solubility, while maintaining low membrane permeability and viscosity. However, many of these properties are interrelated, and tradeoffs often occur. For example, larger and highly charged viologens would expect reduced ion-exchange membrane (IEM) permeation due to increase size exclusion and charge repulsion but will also lead to higher viscosity at the same concentration with the increase in intermolecular forces, resulting in a tradeoff between cycling capacity retention and energy density. [2] Thus, it is critical to systematically evaluate the viologen derivatives to identify the key structural dynamics for multiobjective optimization.
The solubility and viscosity are the determining factors for volumetric energy density of the anolyte solution. To date, BTMAP-Vi and Dex-Vi have demonstrated the highest viologen cycling concentration of 1.5 m with stable capacity retention in anolytelimiting, nonsymmetric configuration. [8,15] Although 1.0 m is often considered "high" volumetric capacity for AORFB, [2] it is much lower than the recently demonstrated i-TEMPOD catholyte with 4 m electron concentration. [16] To avoid this full cell AORFB capacity mismatch and improve the anolyte volumetric energy density, we hypothesized that viologens with neutral functionalization (i.e., alcohol moieties) will exhibit enhanced solubility and lower viscosity. This is because electrolyte with less charge will possess lower intermolecular interaction, resulting in reduced solution viscosity, while the smaller alcohol functionality permits hydrogen bonding network for water miscibility. [17] This hypothesis was confirmed through experimental determination of solubility of symmetric viologens (Figure 2a; Table S1, Supporting Information), with water solubilities of 2.9, 1.9, and 2.0 m for EtOH-Vi, DiOH-Vi, and Dex-Vi, respectively. In general, symmetric viologens with higher charge and larger size were more viscous (Figure 2b; Tables S2-S5, Supporting Information). Yet, it is worth noting that DiOH-Vi has significantly lower viscosity than Dex-Vi while having a similar water solubility, demonstrating the importance of molecular design on practical flowable concentration that considers pumping energy cost and internal pressure. We also hypothesized that asymmetric viologens would exhibit heightened solubility than what could be estimated from the average of the two corresponding symmetric viologens. This is because higher molecular symmetry typically increases the probability of orientating into a crystal structure, which maximizes the intermolecular interaction, excludes solvent interaction, and reduces the solubility. [18] The improved solubility of asymmetric derivatives was exemplified by Dex-DiOH-Vi with a measured solubility of 2.7 m, which is significantly higher than the average of DiOH-Vi (1.9 m) and Dex-Vi (2.0 m).
As we had previously demonstrated that the inclusion of secondary alcohol moieties to BTAMP-Vi to create Dex-Vi had minimal effect on solubility of the oxidized (uncharged) state, [15] we expected minimal difference in solubility between the newly synthesized EtS-Vi and SHOP-Vi derivatives from the reported solubility of propyl sulfonate viologen [(SPr) 2 Vi] of 2.0 m. [12] However, EtS-Vi only exhibited a solubility of 0.5 m despite its smaller molecular size, and SHOP-Vi of below 10 × 10 −3 m at room temperature despite the additional hydrophilic alcohol groups. Even asymmetric SHOP-DiOH-Vi demonstrated solubility below 50 × 10 −3 m at room temperature, and SHOP-Dex-Vi appeared to be initially soluble at 1.5 m but formed precipitates after sitting in neutral aqueous solution at room temperature for an hour ( Figure S14, Supporting Information). As has been previously reported, sulfonated viologens form an inner salt sigmoid structure between the terminal alkyl sulfonate anion and the redox core pyridinium cation. [19] We hypothesize that this inner salt bonding is increased with shorter alkyl chains (i.e., EtS-Vi) and the presence of a secondary alcohol (i.e., SHOP-Vi) that prevents water molecules from solvating the species. Despite these inner salt dynamics, the asymmetric strategy improves the solubility of EtS-DiOH-Vi to 1.3 m -the average solubility of EtS-Vi and DiOH-Vi is 1.2 m -and rotational confinement approaches similar to those reported could be employed to potentially further enhance room temperature solubility. [19] While selecting the viologen derivative with maximum solubility and minimum viscosity (i.e., EtOH-Vi) would be ideal for optimal volumetric capacity, low permeability is required for high AORFB coulombic efficiency and for capacity retention in nonsymmetric configuration. To observe the tradeoff between molecular viscosity and permeability, the crossover of the viologens only bearing positive charge was tested through high-power Selemion DSVN anion-exchange membrane (AEM) (Figure 2c; Table S6, Supporting Information). The positive viologen series was selected as they each exhibited solubilities over 1.0 m without complex inner salt dynamics, permitting elucidation of structure-property trend for viscosity-permeability tradeoff. As expected, viologens with more charge exhibited deterred crossover. For example, Dex-Vi (1.38 × 10 −11 cm 2 s −1 ) showed an order of magnitude lower crossover than DiOH-Vi (1.27 × 10 −10 cm 2 s −1 ) that has less positive charge and lower coulombic repulsion with the AEM quaternary ammonium chains while being similar in size. However, it is interesting to note that the asymmetric Dex-DiOH-Vi did not have an average permeability of Dex-Vi and DiOH-Vi (i.e., 8.36 × 10 −11 cm 2 s −1 ) and instead exhibited low permeation (2.76 × 10 −11 cm 2 s −1 ) closer to that of Dex-Vi. This provides key insight into optimization of the permeabilityviscosity relationship (Figure 2d) as it appears that adding charge to the redox molecules has diminishing returns on crossover with DSVN membrane if the size and geometry of the functionalization is similar. Similar trends can be studied for other redox species and membranes which may have differing conduction mechanism and tradeoff relationship influenced by membrane pore size, charge type, and charge density. [20,21] In addition to possessing ideal physiochemical properties, including solubility, viscosity, and permeability, AORFB anolytes must also exhibit reversible electrochemical activity, preferably at more negative voltages within the kinetic water hydrogen evolution (HER) electrochemical window. The viologen derivatives were characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) to identify structural trends in formal potential (E 0 ) (Figure 3a) and standard rate constant (k 0 ) (Figure 3b), respectively. SHOP-Vi was excluded from testing due to insolubility. All other viologens exhibited ideal electrochemical behavior following that of the Randles-Sevcik equation for the first-electron reduction (Figures S19-S34, Supporting Information). The E 0 of the viologens ranged from −0.320 to −0.383 V (vs SHE) with the general functionality trend of neutral < anionic < cationic (Table S7, Supporting Information). In contrast to solubility and permeability properties, the E 0 of asymmetric viologens is nearly the average of the symmetric counterparts. For example, the average of Dex-Vi (−0.320 V) and DiOH-Vi In general, all viologens displayed facile redox kinetics with k 0 greater than 10 −2 cm s −1 on glassy carbon electrode as determined by EIS (Table S9, Supporting Information). This is ideal for RFB application to ensure that the Faradaic resistance is not a significant contributor to the overall system polarization resistance, which will instead be dominated by the membrane area specific resistance even without electrode felt coating. For symmetric viologens, those of smaller functionalization size demonstrated more facile redox rate kinetics, which is hypothesized to be due to less shielding of the redox active aromatic core from the electrode surface. [22] Interestingly, the asymmetric derivatives displayed higher rate constants than the symmetric counterparts. Although electrode double layer dynamics are complex and require detailed theoretical and spectroscopic study, we hypothesize that the asymmetric geometry prevents tight, ordered packing between viologens and instead encourages increased entropy towards overcoming free energy activation barriers at the electrode surface. [23] Nevertheless, this empirical discovery provides a structural design path to improve the redox kinetics of organic redox species for reduced charge transfer impedance.
In addition to AORFB capacity decay arising from irreversible crossover of active material through the IEM in practical nonsymmetric configuration, capacity decay also occurs when the redox species undergoes chemical degradation in the electrolyte during charge-discharge cycling. Viologen derivatives are known to degrade in basic condition in which hydroxide is hypothesized to perform a nucleophilic elimination of the 4-position functionality in the oxidized state and result in loss of electrochemical reversibility. [24] Although viologens are applied in pH neutral AORFB starting conditions, hydroxide can form in the anolyte from HER or resulting pKa changes of the redox molecule itself in charge-discharge states. We hypothesized it is possible to deter this chemical degradation mechanism of the viologens through structural tuning while also exploring the relative stability of the pyridinium 4-position functionalization of the viologen series.
Accordingly, degradation tests were performed by dissolving 50 × 10 −3 m viologen derivative into 1 m NaOH aqueous solution and monitoring the viologen redox active concentration via CV over time. Excess base was used to ensure the reaction is favored in flooded conditions, and each experiment displayed pseudo first order kinetics ( Figures S35-S41, Supporting Information). Viologens lacking secondary alcohols, such as EtS-Vi and EtOH-Vi, showed increased vulnerability in basic condition, possibly due to steric effects; short alkyl chains leave the 4-position pyridinium open to nucleophilic hydroxide attacks while secondary alcohols may prevent direct interaction. Although asymmetric viologens containing secondary alcohols still displayed greatly deterred base degradation kinetics, they exhibited lower stability than their symmetric counterparts as exemplified by Dex-DiOH-Vi ( Figures S44-S47, Supporting Information). It is hypothesized that the asymmetric geometry with less intermolecular packing leaves the pyridinium core more sterically vulnerable for nucle-ophilic attack. [18] These findings are a step towards understanding viologen structural influences on chemical degradation and enabling design of electrochemically stable viologens (Figure 4).
With these systematic experiments, the AORFB properties of an ideal viologen were evaluated. Ultimately, Dex-DiOH-Vi was selected as the champion species as it possesses extreme water solubility (2.7 m) without a significant tradeoff in viscositypermeability relationship. Additionally, the asymmetric nature provides more facile redox kinetics and a more negative formal potential than previously demonstrated Dex-Vi, while the presence of secondary alcohols on both functionalization units provides adequate chemical stability for one-electron cycling. Equally important, Dex-DiOH-Vi can be produced at scale in high yield and purity through the newly developed one-pot hydrothermal method for concentrated AORFB demonstration.
Accordingly, an anolyte-limiting AORFB of 2.5 m Dex-DiOH-Vi (67 Ah L −1 anolyte theoretical) was tested in a capacitybalanced, nonsymmetric configuration with 2.5 m of a new TEMPO catholyte, methyl morpholine amide TEMPO (MMA-TEMPO) (Figure 4a). MMA-TEMPO can also be produced efficiently in large scale and high purity ( Figure S48, Supporting Information) and shows reversible CV behavior at a formal potential of +0.833 V versus SHE ( Figure S53, Supporting Information). This Dex-DiOH-Vi AROFB is the highest demonstrated viologen anolyte concentration (2.5 m) in AORFB cycling [8,11,15] with an open-circuit voltage (OCV) 1.09 V at 50% state-of-charge (SOC) ( Figure S58, Supporting Information). After 14 d of continuous charge discharge cycling in capacity-balanced configuration at 89.9% capacity utilization, no apparent capacity decay is observed (Figure 4b,c). Cell resistance remained unchanged after extended cycling ( Figure S59, Supporting Information). Post cycling CV and NMR ( Figures S60-S62, Supporting Information) show no signs of electrolyte crossover or chemical degradation. No reported viologens have demonstrated such stability in nonsymmetric viologen AORFB cycling at concentration over 1.5 m (Table S10, Supporting Information).

Conclusion
Viologens are a promising anolyte material for AORFBs, the nextgeneration grid scale long-duration energy storage devices to ensure an economical and secure energy transition. In this work, we introduced a new hydrothermal synthetic method for universal applicability toward water-soluble viologen derivatives, directly producing the preferred chloride salt forms of symmetric and asymmetric structures in one-step reaction. The hydrothermal method is truly a scalable "green" synthesis [25] and permitted the systematic screening of nine viologen derivates to glean new structure-property insight for electrolyte optimization. Ultimately, this study resulted in the design of a Dex-DiOH-Vi anolyte with no apparent capacity decay when used in AORFB cycling for 14 d and an anolyte theoretical volumetric capacity of 67 Ah L −1 . The experimentally unveiled design principles spurred optimization of viologen physiochemical and electrochemical electrolyte properties. With energy dense and stable one-electron viologen performance, pH neutral anolyte is now comparable to alkaline anolyte, [1][2][3][4][5][6][7] while permitting pairing with high voltage, waster miscible, and electrochemically stable TEMPO catholytes ( Figures S53-S57, Supporting Information). [16,[26][27][28][29][30][31] This highlights the importance of developing high-throughput synthetic methods to empirically screen complex structure-property relationships of organic redox molecules. Such insights will permit molecular engineering of ideal AORFB electrolyte for safe, sustainable, and reliable long-duration grid energy storage.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.