A Sulfonate Functionalized Viologen Enabling Neutral Cation

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A Sulfonate Functionalized Viologen Enabling Neutral Cation Exchange Aqueous Organic Redox Flow Batteries towards Renewable Energy Storage Camden Debruler, Bo Hu, Jared Moss, Jian Luo, and T. Leo Liu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01302 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

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ACS Energy Letters

A Sulfonate Functionalized Viologen Enabling Neutral Cation Exchange Aqueous Organic Redox Flow Batteries Towards Renewable Energy Storage Camden DeBruler, Bo Hu, Jared Moss, Jian Luo, and T. Leo Liu*

The Department of Chemistry and Biochemistry, Utah State University, Logan, UT *Corresponding Author: [email protected]

Abstract: Redox flow batteries using synthetically tunable and resource abundant organic molecules have gained increasing attention for large-scale energy storage. Herein we report a sulfonate functionalized viologen molecule, 1,1’-bis(3-sulfonatopropyl)-4,4’-bipyridinium, (SPr)2V, as an anolyte in neutral aqueous organic redox flow batteries (AORFBs) functioning through a cation charge transfer mechanism. Demonstrated (SPr)2V / KI AORFBs manifested high current performance from 40 to 100 mA/cm2 with up to 71% energy efficiency. In extended cycling studies, the (SPr)2V / KI redox flow battery delivered stable cycling performance at 60 mA/cm2, up to 67% energy efficiency and 99.99% capacity retention per cycle. DFT modeling of the electrostatic charge surface of (SPr)2V and its charged state, [(SPr)2V]-1, suggests charge repulsion and size exclusion enable their compatibility with a cation exchange membrane. The present findings expand the battery design of neutral viologen AORFBs and represent an attractive RFB technology for sustainable and benign renewable energy storage.

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Redox flow batteries (RFBs) represent an attractive electrochemical energy storage technology for grid-scale storage (up to MW/MWh) of renewable energy including solar and wind energy.1,2 The technical virtues of RFBs include decoupled energy and power, high current and high power performance, and non-flammable and low cost aqueous supporting electrolytes, empowering them as a well-suited option to overcome the intermittence of renewable energy and store it as reliable electricity.1,2 However, traditional aqueous inorganic redox batteries (AIRFBs) such as vanadium AIRFBs and Zn/Br2 AIRFBs, are difficult to implement for wide-spread energy storage applications due to a number of technical challenges including low abundance and high costs of redox-active metals, expensive separators, active material crossover, and corrosive and hazardous electrolytes. In order to achieve economical and environmentally benign electrochemical energy storage, there has been a rapid transition to use sustainable and tunable redox-active organic molecules to replace redox-active inorganic materials in RFBs.3-6

Figure 1. (A) Battery design of previously reported anion exchange viologen AORFBs, where R1 = -CH3 for methyl viologen (MV), R1 = -(CH2)3NMe3 for 4,4’-bis[3(trimethylammonio)propyl]-4,4’-bipyridinium tetrachloride ((NPr)2V), R2 = -OH for 4-OHTEMPO, R2 = -NMe3+ for 4-ammonium-TEMPO (NMe-TEMPO), R3 = -CH2NMe3+ and R4 = -H for (ferrocenylmethyl)trimethylammonium chloride (FcNCl), and R3 = R4 = -(CH2)3NMe3+ for bis((3-trimethylammonio)propyl)-ferrocene dichloride (BTMAP-Fc). (B) Battery design of cation exchange viologen AORFBs reported in this work.

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Both aqueous organic redox flow batteries (AORFBs)7-18 and non-aqueous redox flow batteries (NAORFBs)19-26 have emerged as potential alternatives to traditional AIRFBs. In addition to preserving the aforementioned technical strengths of traditional AIRFBs, AORFBs are characterized with resource sustainability and performance tunability of organic active materials, and thus hold a great promise for technical implementation.12 We and others have developed acidic, alkaline, and neutral AORFBs based on a variety of redox organic molecules/polymers.7-17 Particularly, we have contributed the development of neutral AORFBs employing water-soluble viologen (anolyte), TEMPO (catholyte), and ferrocene (catholyte) molecules.8,12,15-17 Neutral AORFBs8,11,12,14-17 are free of corrosion, more safe and environmentally benign than acidic and alkaline RFBs while achieving high current performance (up to 100 mA/cm2).9 We first reported a methyl viologen/4-OH-TEMPO AORFB operating in an anion charge transfer mechanism using an anion exchange membrane (Figure 1A).8 The battery chemistry of the anion exchange viologen AORFBs has been further advanced by us,12,1517

Schubert et al.,11 and Aziz et al. recently.14 So far, neutral viologen AORFBs have

demonstrated the most stable flow battery performance and stand for the state of the art for organic redox flow batteries.8,11,12,14-17 NAORFBs have also received rapid development with the use of different redox active molecules as catholyte and anolyte materials.19-26 In spite of numerus reported NOARFBs using a variety combinations of anolyte and catholyte materials, the battery performance of NOARFBs are still in in the proof-of-concept stage. The state-of-the-art NAORFBs only delivered limited cycling performance including low current performance from 10 to 35 mA/cm2 and low capacity retention less than 99.98% per cycle within 50 cycles.20,27 The major technical challenges encountered by NAORFBs include the lack of selective ion exchange membranes in organic electrolytes, low conductivities of organic electrolytes, and radical side reactions of active 3



materials. It can be anticipated that fundamental breakthroughs are critical to address these challenges in order to develop practical NAORFBs for electrochemical energy storage.12,15,16 As the electrochemical performance of viologen molecules demonstrated in anion exchange AORFBs is rather reliable,11,12,14 it is highly attractive to apply viologen molecules in cation exchange AORFBs. In addition, little has been done to modulate the chemical structure of viologen molecules for redox flow battery applications.14,15,17 Herein, we explore the synthesis of sulfonate functionalized viologen molecules to exhibit a neutral viologen AORFB operating in a cation charge transfer mechanism (Figure 1B). The new AORFB design is demonstrated with a sulfonate functionalized viologen, 1,1’-bis[3-sulfonatopropyl]-4,4’-bipyridinium, (SPr)2V, as anolyte, and enables the use of low cost inorganic redox active catholyte materials such as KI. The demonstrated (SPr)2V / KI AORFBs (13.4 Ah/L) were able to cycle from 40 to 100 mA/cm2 with up to 71% energy efficiency and nearly 100% coulombic efficiency. Particularly, the long cycling studies of the (SPr)2V / KI redox flow battery delivered excellent cycling performance at 60 mA/cm2, up to 67% energy efficiency and 99.99% capacity retention per cycle. The presented concept of negatively charged group functionalized viologen molecules expand the application of viologen compounds in redox flow batteries and offer new opportunities to develop low cost and high performance viologen redox flow batteries that promise sustainable and green storage of renewable energy. Synthesis of Sulfonate Functionalized Viologen Molecules. We envisioned that viologen molecules functionalized with flanged negatively charged groups such as sulfonate could be compatible with a cation exchange membrane in a cation exchange AORFB. To this end, we first attempted to synthesize 1,1’-bis[2-sulfonatoethyl]-4,4’-bipyridinium ((SEt)2V, Scheme 1A). No reaction was observed between 1:2 ratio of 4,4’-bipyridine and 2-sulfonatoethyl bromide in CH3CN or DMSO at room temperature. At an elevated temperature (140 °C) in DMSO, the 4


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reaction slowly took place. Even after 48 hours, (SEt)2V was only isolated in a yield of ca. 15% along with unreacted 4,4’-bipyridine. Due to the low yield, (SEt)2V was not pursued in further studies. We rationalized that 2-sulfonatoethyl bromide is not a reactive electrophile, and thus applied propane sultone as a more reactive sulfonate alkylation reactant (Scheme 1B). Alkylation of 4,4’-bipyridine with 2 equiv. propane sultone was conducted in refluxed toluene for 3 hr to give a yield of 98% of (SPr)2V. The reaction was scaled up to 20 g with a 100% yield. The compound was characterized by 1H NMR and elemental analysis (Figure S1). Even as a neutral compound, the solubility of (SPr)2V is 2.0 M in H2O (53.6 Ah/L charge capacity), a desired characteristic for redox flow battery applications. 2 equiv. N

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Fundamental electrochemical properties of (SPr)2V were examined by cyclic voltammetry (CV) and rotation disc electrode (RDE) linear sweep voltammetry (LSV). In its CV plot (Figure 2), (SPr)2V displayed a reversible one electron reduction at -0.43 V vs NHE. The reduction of (SPr)2V is slightly less negative than that of methyl viologen (-0.45 V)11 as the 3sulfonatopropyl substituent is a more electron-withdrawing group compared to the methyl group.

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It appears the N-substituents have a small electronic impact on the reduction potential of the bispyridium moiety. LSV plots and a derived Levich plot for (SPr)2V are shown in Figure 2B-C. A linear Levich plot (Figure 2C) was constructed for the reduction of (SPr)2V using limiting currents (Figure 2B) and the square root of rotation speeds. The diffusion coefficient (D) of (SPr)2V was calculated as 3.26 x 10-6 cm2/s by the Levich equation (Equation S1 in the supporting information) using the slope of the Levich plot. The electron transfer rate constant was estimated using Nicholson’s method.28 Consistent with the reversible CV and ideal Levich plot, the electron transfer rate constant of (SPr)2V is very fast, > 0.28 cm/s (see the experimental information for detail). The fast diffusion coefficient and electron transfer rate constants of (SPr)2V are comparable with or even larger than most reported redox active compounds applied in AORFBs, and implies minimal mass transport and charge transfer overpotential in flow battery studies.

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Figure 2. (A) Cyclic voltammograms of (SPr)2V (-0.43 V) and KI (+0.57 V) in 0.5 M KCl aqueous electrolyte. Experiment conditions: 4.0 mM (SPr)2V or KI, 100 mV/s scan rate, glassy carbon working electrode, glassy carbon counter electrode, Ag/AgCl reference electrode. (B) LSV scans with rotating working electrode for the reduction of (SPr)2V. (C) Levich plot for the reduction of (SPr)2V. Flow Battery Studies of 1,1’-bis[2-sulfonatopropyl]-4,4’-bipyridinium, (SPr)2V. To demonstrate the proof of concept of the cation exchange viologen AORFBs, we investigated KI as catholyte with a positive redox potential (0.57 V vs NHE, Figure 2A), and a high solubility in H2O (ca. 8.0 M in H2O). It is noted that the I3-/I- catholyte has exhibited viable flow battery performance in Zn/I- aqueous redox flow batteries.29,30 Shown in equations 1-2 below are the half-cell reactions for the corresponding (SPr)2V / KI AORFB. The (SPr)2V / KI AORFB has a cell voltage of 1.0 V that is comparable with the anion exchange MV/ferrocene AORFB (1.05 V) previously reported by us.12 The voltage window of the AORFB is bracketed within the water splitting window (1.2 V vs. NHE for oxygen evolution and -1.5 V vs. NHE for hydrogen evolution) on a graphite electrode at neutral pH,12 thus water electrolysis side reactions are unlikely to occur. 0.5 M (SPr)2V (103.2 mS/cm) and 2.0 M KI (234.2 mS/cm) in 2.0 M KCl were used as anolyte and catholyte respectively. The flow battery (13.4 Ah/L and 6.7 Wh/L) was cycled between 1.25 V and 0.3 V from 40 to 100 mA/cm2 using a Nafion 212 membrane (Figure 3A), which mediates selective K+ cation charge transfer. Representative charge and discharge

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profiles are displayed in Figure 3B. Rate performance of the battery was studied from 40 mA/cm2 to 100 mA/cm2 with an increment of 20 mA/cm2. Representative charge and discharge profiles are shown in Figure 3B. At all tested current densities, the battery delivered nearly 100% coulombic efficiency. At pH neutral conditions, the battery recorded energy efficiencies of 71%, 58%, 48%, and 38% at 40, 60, 80 and 100 mA/cm2, respectively. The good rate performance and energy efficiencies are consistent with high conductivities of the anolyte (103.2 mS/cm) and catholyte (234.2 mS/cm).

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Figure 3. (A) Plot of battery capacity versus cycling numbers of the (SPr)2V / KI AORFB using Nafion 212 at current densities from 40 mA/cm2 to 100 mA/cm2; (B) Representative charge and discharge curves at current densities from 40 mA/cm2 to 100 mA/cm2 for the (SPr)2V / KI AORFB; (C) Plots of average coulombic efficiency, energy efficiency, and voltage efficiency at different operational current densities of the (SPr)2V / KI AORFB. Conditions: anolyte, 0.5 M (SPr)2V in 2.0 M KCl; catholyte, 2.0 M KI in 2.0 M KCl, Nafion 212 cation exchange membrane. To validate long cycling stability, the (SPr)2V / KI AORFB was further studied for 300 cycles at 60 mA/cm2 under the same conditions (Figure 4A). The battery delivered rather stable cycling performance, 94.1% total capacity retention or 99.99% capacity retention per cycle with an average energy efficiency of 58% and coulombic efficiency of nearly 100%. A polarization

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curve of the battery was collected at 100% state of charge, and the flow battery exhibited a peak power density of 67.5 mW/cm2 (Figure 4B). To further understand the observed stable cycling performance of the (SPr)2V / KI AORFB in the prior battery studies, post-cycle NMR and CV analyses were performed for the (SPr)2V / KI AORFB using Nafion 212 after 300 cycles. Consistent with the recorded stable capacity retention, no chemical degradation was observed (Figures S3A and S4). In addition, the NMR and CV results revealed no detectable (SPr)2V observed in the KI catholyte. Although the redox potentials measured in Figure 2A predicted a cell voltage of 1.0 V, the open circuit potential measured at 0.5 M observed in Figure 3B was reduced to 0.86 V. At higher concentrations of iodide, the redox potential for I-/I3- shifts from

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Figure 4. (A) and (C) Extended cycling data of the (SPr)2V / KI AORFB using the Nafion 212 and Selemion CSO cation exchange membranes, respectively, showing charge capacity, discharge capacity, and coulombic efficiency versus cycle number at 60 mA/cm2 current density.

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Inset: Representative charge and discharge curve from the experiment. (B) and (D) Polarization (primary axis) and power density data (secondary axis) collected at 100% state of charge for the AORFB using the Nafion 212 and Selemion CSO cation exchange membranes, respectively. Conditions: anolyte, 0.5 M (SPr)2V in 2.0 M KCl; catholyte, 2.0 M KI in 2.0 M KCl, Nafion 212 or Selemion CSO cation exchange membrane. Since the demonstrated (SPr)2V/KI AORFB operates at pH neutral conditions, it is feasible to utilize low-cost hydrocarbon cation exchange membranes that are less than expensive than Nafion membranes, and can further make the cation exchange AORFB technology more economically viable. To this end, we also studied a Selemion CSO hydrocarbon cation exchange membrane in the (SPr)2V / KI AORFB under the same conditions (Figure 4C). The CSO membrane (ca. $100/m2) is ca. 5 times cheaper than Nafion membranes (ca. $500 /m2). To our delight, the (SPr)2V / KI AORFB using the CSO membrane under the same testing conditions also delivered outstanding battery performance. At 60 mA/cm2, the (SPr)2V / KI AORFB using the CSO membrane exhibited 67% energy efficiency, which is even higher than that of the battery using Nafion 212 membrane and also higher than those of reported anion exchange neutral aqueous organic redox flow batteries.8,11,12,14-16 It is believed that the enhanced energy efficiency is attributed to the higher K+ ion conductivity of the CSO cation exchange membrane than the Nafion 212 cation exchange membrane. EIS analysis of the RFBs employing Nafion 212 and CSO showed significantly a lower high frequency area specific resistance from the CSO membrane (2.26 Ω·cm2 vs. 3.25 Ω·cm2 for 212) (Figure S6), indicated a higher K+ ion conductivity for the CSO membrane. In an extended 100 cycle study, the battery delivered 98.8% total capacity retention or 99.99% capacity retention per cycle. A much higher peak power density of 92.5 mW/cm2 was recorded for the battery at 100% SOC compared to 67.5 mW/cm2 for the battery using the Nafion 212 membrane, further indicting that the CSO membrane is more 10


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conductive than Nafion 212 membrane. To take advantage of the cost benefit and the promising battery performance, we will continue evaluating hydrocarbon cation exchange membranes for cation exchange viologen AORFBs.

Figure 5. (A) Electrostatic charge surface of the optimized ground state structure of (SPr)2V; (B) electrostatic charge surface of the optimized ground state structure of [(SPr)2V]-; (C) Proposed interaction of (SPr)2V represented in a space-filling model adopting perpendicular orientation with the Nafion membrane; (D) Proposed interaction of (SPr)2V represented in a space-filling model adopting parallel orientation with the Nafion membrane. The blue double-headed arrows indicate the negative charge repulsion. Computational Modeling of the Cation Exchange Membrane Compatibility of 1,1’-bis[2sulfonatopropyl]-4,4’-bipyridinium, (SPr)2V. To understand the exceptional compatibility of the charge-neutral (SPr)2V with the Nafion cation exchange membrane whose ion channel structure was well understood,31 DFT modeling was applied to calculate the formal charge distribution and molecular size of the neutral (SPr)2V and its charged state, [(SPr)2V]-. Shown in Figure 5A and 5B are the optimized structures of (SPr)2V and [(SPr)2V]- encased with their electrostatic charge surface. The space-filling models of (SPr)2V and [(SPr)2V]- (Figure 5C and 5D) have a 3-dementional size ca. 0.6 × 0.8 × 2.2 nm3. For the neutral (SPr)2V, negative charge

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represented by red color is concentrated on the two terminal SO3- groups while the positive charge represented by blue color is delocalized throughout the bipyridine fragment. [(SPr)2V]displays a dominant negative charge surface while the positive charge density centered on the bipyridine fragment is significantly decreased. According to the parallel water channel model of Nafion membranes, ion transport nano-channels of hydrated Nafion membranes have an averaged size around 2.4 nm (Figure 5C and 5D).31 Regarding the probability of crossover, (SPr)2V can adopt two basic orientations when approaching the nano-channels of a Nafion cation exchange membrane, namely perpendicular orientation and parallel orientation, as displayed in Figure 5C and 5D, respectively. When (SPr)2V adopts the perpendicular orientation, the native charge repulsion between both SO3- groups of (SPr)2V and the SO3- groups of the cation exchange membrane applies, and also such orientation (2.2 nm) renders an unfavorable size match with the channel (2.4 nm) of the membrane to move in. When (SPr)2V adopts the parallel orientation, the native charge repulsion between the SO3- group of (SPr)2V and the SO3groups of the cation exchange membrane also disfavors the crossover of (SPr)2V. The parallel orientation (0.8 nm) can fit into the channel of the membrane and may have a higher chance than the perpendicular orientation to cross over. In addition, the irregularity of the ion channel size of the membrane can also minimize the crossover of the active species.31 Overall, the proposed charge repulsion and size exclusion explains the observed non-crossover of (SPr)2V during the long cycling battery testing. [(SPr)2V]- has one more net negative charge, the chance to crossover is even lower. Although the ion channel structure of the CSO is unknown, it is believed the membrane has the similar interactions with (SPr)2Vand [(SPr)2V]- to achieve the observed cycling stability. In comparison to the anion exchange methyl viologen /ammonium functionalized ferrocene AORFBs previously reported by us,12 the present cation exchange (SPr)2V / KI delivered 12


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comparable current performance, cycling stability, even a higher energy efficiency and power density. It can be predicted that introduction of additional negative charge functionalities such as -SO3- or -PO42- group can lead to new cation exchange viologen AORFBs. Furthermore, it is expected that the development of organic catholyte molecules with higher oxidation potential and higher solubility can further enlarge the cell voltage and energy density of the cation exchange viologen AORFBs. In conclusion, we demonstrated a new design of viologen AORFBs that employs a cation transfer mechanism. The cation exchange viologen AORFBs were demonstrated through 1,1’bis[3-sulfonatopropyl]-4,4’-bipyridinium, (SPr)2V, as anolyte, and KI as catholyte using Nafion 212 and low cost Selemion CSO cation exchange membranes. In terms of the tunable merit of viologen molecules, derivatives of (SPr)2V are anticipated to deliver further improved battery performance. The presented cation exchange viologen AORFBs further advance the application of viologen molecules in redox flow batteries and broadly expands the design of neutral viologen redox flow batteries by coupling with negatively charged catholytes. The reliable performance, tunability, sustainability, and benignity of the neutral cation exchange viologen AORFBs mark them as a promising RFB technology for electrochemical energy storage. Acknowledgement We thank Utah State University for providing faculty startup funds to the PI (T. Leo Liu) and the Utah Science Technology and Research initiative (USTAR) UTAG award for supporting this study. Camden DeBruler is grateful for his USU Presidential Doctoral Research Fellowship (PDRF) supported by USU. Bo Hu is grateful for China CSC Abroad Studying Fellowship and Utah Energy Triangle Student Award supported by the Office of Energy of the Utah State government, respectively. Computational resources from the Division of Research

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ACS Energy Letters

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A Sulfonate Functionalized Viologen Enabling Neutral Cation

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