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Designing Redox-Active Oligomers for Crossover-Free, NonAqueous Redox-Flow Batteries with High Volumetric Energy Density Miranda J. Baran, Miles N. Braten, Elena C. Montoto, Zachary T Gossage, Lin Ma, Etienne Chénard, Jeffrey S. Moore, Joaquín Rodríguez-López, and Brett A. Helms Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01318 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Chemistry of Materials

Designing Redox-Active Oligomers for Crossover-Free, NonAqueous Redox-Flow Batteries with High Volumetric Energy Density Miranda J. Baran1, Miles N. Braten2, Elena C. Montoto3, Zachary T. Gossage3, Lin Ma2, Etienne Chénard3, Jeffrey S. Moore3, Joaquín Rodríguez-López3, Brett A. Helms2,4,5* 1

Department of Chemistry, University of California, Berkeley, CA, 94720, USA Joint Center for Energy Storage Research, Lawrence Berkeley National Laboratory, Berkeley, California, 94720, USA 3 Department of Chemistry, University of Illinois Urbana-Champaign, Urbana, Illinois, 61801, USA 4 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, 94720, USA 5 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, 94720, USA 2

ABSTRACT: Here we show how to design organic redox-active solutions for use in redox-flow batteries, with an emphasis on attaining high volumetric capacity electrodes that minimize active-material crossover through the flow cell’s membrane. Specifically, we advance oligoethylene oxides as versatile core motifs that grant access to liquid redox-active oligomers having infinite miscibility with organic electrolytes. The resulting solutions exhibit order-of-magnitude increases in volumetric capacity and obviate deleterious effects on redox-stability. The design is broadly applicable, allowing both low potential and high potential redox-centers to be appended to these core motifs, as demonstrated by benzofurazan, nitrobenzene, 2,2,6,6-tetramethylpiperidin-1-yl)oxyl, and 2,5-di-tert-butyl-1-methoxy-4-(2’-methoxy)benzene pendants, whose reduction potentials range from –1.87 V to 0.76 V vs. Ag/Ag+ in acetonitrile. Notably, the oligoethylene oxide scaffold minimizes membrane crossover relative to redox-active small molecules, while also providing mass- and electron-transfer kinetic advantages over other macromolecular architectures. These characteristics collectively point toward new opportunities in grid-scale energy storage using all-organic redox-flow batteries.

Redox-flow batteries (RFBs) are versatile electrochemical devices that store energy—ideally from renewable, zeroemission sources—and in turn deliver reliable power on demand.1,2 During their operation, redox-active compounds, fluidized in electrolyte, are pumped from storage tanks into an electrochemical cell, where they undergo redox reactions, exchanging electrons with current collectors, which then flow through an external circuit and either drive the load on the system or charge it. While in residence, redox-active compounds should remain sequestered in their respective electrode compartments; this minimizes the likelihood of cross-annihilation, and maximizes the device’s energy efficiency. It is also desirable that they remain highly soluble in electrolyte at all states of charge, to maximize the accessible energy density (i.e., deep charge/discharge cycles). Achieving both high volumetric energy density and low crossover operation is critical to next-generation active material design for RFBs. Here we show that the molecular design of redox-active oligomers (RAOs) can now be tailored both to prevent crossover and to constitute low-viscosity, flowable electrodes with up to 106 Ah L–1 volumetric capacity, outpacing other chemistries.3,4 Key to our success is the implementation of oligoethylene oxide core motifs, which are redox-stable from –2.50 to 4.30 V (vs. Ag/Ag+) and allows for a welldefined number redox-active molecules to be appended

while also yielding liquid RAO products that are infinitely miscible with non-aqueous electrolytes (e.g., those based on acetonitrile, glyme, carbonate, sulfoxide, and amide solvents). To highlight the generality of our approach, we prepared four new liquid RAOs in multi-gram quantities and high purity (Scheme 1): two were low-potential RAOs (i.e., anolytes/negolytes), based on either nitrobenzene or benzofurazan; and two high-potential RAOs (i.e., catholyte/posolytes), based on either (2,2,6,6tetramethylpiperidin-1-yl)oxyl (TEMPO) or 2,5-di-tertbutyl-1-methoxy-4-(2’-methoxy) benzene (DB3). High solubility was maintained at all states-of-charge during bulk electrolysis experiments, with nitrobenzene and TEMPO RAOs exhibiting superior performance with respect to reversible electrochemical cycling. We further evaluated the electrontransfer kinetics between RAOs and electrodes, k0, which were one or two orders of magnitude higher than redoxactive polymers, which allows RAO-based RFBs to be operated at high current densities on par with small molecules.5 Additionally, RAO crossover behavior was readily gated by size-sieving microporous polymer separators based on polymers of intrinsic microporosity, boding well for high roundtrip energy efficiency. Our success overcomes outstanding challenges in achieving high volumetric capacity (and ultimately, energy density for the RFB) with redox-active small

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molecules and polymers presently available,6–8 while also making possible cross-over free operation in a purely masstransfer limited kinetic regime, which is rare.

Figure 1. a) Schematic representation of a redox-flow battery incorporating redox-active oligomers sequestered in their respective electrode compartments by a size-sieving polymer membrane. b) Molecular design of redox-mers based on oligoethylene oxides, which are liquids with infinite miscibility with electrolyte. c) Anolyte (orange) and catholyte (blue) redox centers implemented in this work.

To advance the design of macromolecular active materials for RFBs, it is imperative to delineate the influence of any particular pairing of scaffold and redox-active molecule on the hybrid’s solubility, stability, kinetics, and crossover behavior. To address solubility, which is key to attaining high volumetric capacity electrodes, we implemented a branched oligoethylene oxide core motif in our design. This motif is a liquid at ambient temperature and is available in a variety of molecular weights (e.g., 200–1000 g mol–1) with a welldefined number of end groups (e.g., 2–6). With these motifs implemented, theoretical volumetric capacities are in the range of 160 Ah L–1, outpacing other strategies by more than an order of magnitude (Table S2).3,6–7 We approached the functionalization of branched trifunctional oligoethylene oxide core motifs using the strategy

outlined in Scheme 1. Briefly, the hydroxyl group at each terminus was first converted to the tosylate before reacting with either 4-nitrophenol, 5-hydroxybenzofurazan, 4hydroxy-TEMPO, or 2,5-di-tert-butyl-4-methoxyphenol under tailored etherification conditions to yield RAOs 3a–d, respectively, in 48–90% overall yield with >95% end-group fidelity. In all cases, the products were obtained on multigram scale as viscous oils, which were infinitely miscible with non-aqueous electrolytes. Trimeric RAOs 3a–d shown in Scheme 1 yielded flowable electrodes with ~ 106 Ah L–1 volumetric capacity (i.e., [redox center] = 3.95 M). Moreover, branched oligoethylene oxide core motifs are stable over a voltage range spanning –2.80 to 4.30 V (vs. Ag/Ag+), as evidenced by cyclic voltammetry of the tosylate precursor (Figure S1). This voltage range encompasses all known active materials presently in use in non-aqueous RFBs, including those advanced here, and thus may be considered a platform for future technology maturation.9–17 To understand the influence of the core motif on the redox chemistry of the appended centers, cyclic voltammetry (CV) was conducted at scan rates of 10–100 mV s–1 for 3a–d dissolved in acetonitrile (ACN) (5 mM) containing TBA-PF6 (500 mM) (Figure 2). Anolyte materials 3a and 3b incorporate benzofurazan and nitrobenzene redox centers, respectively. With a redox potential (E1/2) of at –1.865 V, 3a is around 200–300 mV lower than other benzofurazan derivatives, while 3b is comparable to other macro-architectures that incorporate nitrobenzene with E1/2 –1.556 V, around 150mV more positive than the monomer 4-nitroanisole (Figure S6).14–15 Catholyte compounds 3c and 3d incorporate TEMPO (whose monomeric redox potential occurs at 0.30 V) and DB3 (whose monomeric redox potential occurs at 0.51 V), respectively.13,16 Elaboration of the oligoethylene oxide core with TEMPO redox centers results in a 56 mV increase in redox potential compared to the small molecule (i.e., to 0.356 V). The oligoethylene oxide core had a larger effect on DB3 redox centers, with only a 245 mV increase in redox potential to 0.755 V observed. Small peak separations of 61, 67, 57, and 63mV (3 a–d respectively) at a 10 mV s–1 scan rate are preliminary indicators of fast (reversible) reaction kinetics. Among the RAOs tested, 3b and 3c appeared to be more stable, with anodic:cathodic peak-current ratios near unity – 0.99 and 1.01 respectively – indicating the oxidized and reduced forms do not undergo side reactions on the timescale of the measurement. By comparison, 3a and 3d are less stable, with anodic:cathodic peak-current ratios of 1.09 for 3a and 0.97 for 3d.


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Scheme 1. Synthesis of aliphatic ether core RAOs from trimethylolpropane ethoxylate. Reagents and conditions: (i) TsCl, NaOH, THF, H2O, RT; (ii) 5-hydroxybenzofurazan (3a), 4-nitrophenol (3b), 2,5-di-tert-butyl-4-methoxyphenol (3d), K2CO3, 18-crown-6, acetone, reflux; and (iii) 4-hydroxy-TEMPO, NaH, THF, reflux.

Figure 2. Cyclic voltammograms 3a–d (5mM redox centers) at various scan rates in ACN with 0.5 M TBA-PF6 supporting salt.

Having demonstrated reversible redox chemistry during voltammetry experiments, we further explored the effect of the oligoethylene oxide core on the reactivity of the redox centers. Several spectroelectrochemical experiments were employed to explore the possibility of electron exchange between redox centers. The proximity of redox centers within each RAO core may lead to intramolecular effects that change the apparent number of electrons exchanged at a set potential. In conjugated molecules and highly interacting oligomeric structures, electrostatic Born sphere effects lead to stepwise electron transfer events with multiple CV waves separated by few-hundred mV.17–19 On the other hand, noninteracting redox centers display multiple electron transfer events with a single standard potential, E0.20 Ultramicroelectrode (UME) CVs with an expanded potential window (Figure S10) allowed us to observe a single redox process for each of the oligomers. Additionally, chronoamperometric analysis using the method of Denuault, Mirkin and Bard based on UME transients suggested that in all cases multiple electron transfer occurs for the redox process observed (Figure S11, Table S1).21 A further consideration with tethered

redox centers concerns the potential for the creation of intervalence states at intermediate states of charge. These states are conveniently measured using Near-IR (NIR) absorbance. For this, solutions of RAOs 3a–d were electrolized via bulk electrolysis (BE) to their oxidized (3c, 3d) or reduced (3a, 3b) state and studied via UV-Vis-NIR spectroscopy for the presence of intervalence peaks. In each case, no such intervalence charge transfer bands were observed in the RAOs (Figure S5).22 Altogether, this evidence strongly suggests that each observed redox wave consists of multiple electron transfers with non-interacting redox moities. In addition to granting access to soluble active materials with near-ideal redox-behavior, oligomeric designs are also hypothesized to provide advantages over larger, macroarchitectured active materials (e.g., redox-active polymers or colloids) with respect to diffusion and mass transfer as well as charge-transfer with an electrode.7,9,23 Delivery on those points has not yet been demonstrated in any depth. Furthermore, it is unclear whether oligomers are in the same range of desirable performance attributes typically seen using redox-active small molecules, thus motivating the present in-



vestigation. The ~450 g mol–1 three-arm oligoethyleneoxide core employed here was selected for its intermediate size (11.9, 12.3, 16.0, and 21.3 Å radii for 3a–d respectively), which is large enough to be excluded by a microporous membrane (PIM-1, 6–8 Å pores), but also diffuse fast in solution.3 Quantitative analysis of RAO diffusion coefficients (D) in ACN/TBA-PF6 electrolyte was achieved using UME-CV steady-state currents (Figure 3, initial curves). The steadystate current is used to calculate D using the relationship iss = 4FaDnC*, where F is Faraday’s constant and a is the radius of the electrode (12.5 µm). The quantity nC* considers

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equivalent concentration of redox-units, with each molar equivalent of RAO contributing three redox-centers. All RAO diffusion coefficients were within the same order of magnitude: 5.2 × 10–6, 5.4 × 10–6, 4.0 × 10–6, 3.0 × 10–6 cm2 s–1 for 3a–3d, respectively. From these values, our RAOs fell into an intermediate diffusional range, with faster diffusion than large polymers. For example, TEMPO as a small molecule exhibits a diffusion coefficient of 1.1 × 10–5 cm2 s–1, which compares well to the obtained 4.0 × 10–6 cm2 s–1 for RAO 3c, while a TEMPO-containing polymer reported elsewhere has a diffusion coefficient of 1.65 × 10–7 cm2 s–1.23

Table 1. Parameters used for voltammetry simulations and kinetic results along with SECM-determined kinetics RAO

a c

nD (cm2 s–1)a

3b

5 × 10

3c 3d

–6

E0 (V)b

α

1–α

k0 (cm s–1) Simulated –3

k0 (cm s–1) SECM 1.3 × 10–3

–1.53

0.42

0.65

1 × 10

4 × 10–6

0.36

0.5

0.5

8 × 10–2

4 × 10–2

3 × 10–6

0.73

0.35

0.65

4 × 10–3

----c

b

Calculated from iss Taken as an average of the experimentally determined E1/2 from initial and oxidized/reduced UME-CVs (vs. Ag/Ag+) Not available, SECM approach curves lead to probe fouling


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transfer limitations of macroelectrodes (Figure 2), in both states of charge allowed us to estimate charge transfer kinetics via analysis of the cathodic and anodic branches. Experimental curves (Figure 3) were simulated using the calculated diffusion coefficients and observed E0 with the aid of DigiElch software. Full simulation parameters are found in Table 1. We found that TEMPO-based RAO 3c displayed the fastest kinetics with a standard electrochemical rate constant (k0) estimated at 0.08 cm s–1, similar to the k0 of 0.1 cm s–1 reported for the monomer, and a symmetry coefficient α of 0.5. RAOs 3b and 3d showed kinetics at 10–3 cm s–1.24 To further verify these differences in kinetics among RAOs, we turned to scanning electrochemical microscopy (SECM) feedback analysis via approach curves (Figures S7 & S8).25– 26 This analysis gave k0 values of 1.3 x 10–3 cm s–1 and 0.04 cm s–1 for 3b and 3c, respectively, in excellent agreement with UME-CV analysis. Using the same SECM method we evaluated the kinetics of the monomeric equivalent of 3b, 4-nitroanisole, in the presence and absence of equimolar tetraglyme to mirror the presence of the oligoethylene oxide core of the RAOs. From this experiment, 4-nitroanisole displayed a k0 of 2.2 x 10–3 cm s–1 in the absence of tetraglyme and 2.8 x 10–3 cm s–1 in the presence of tetraglyme (Figure S9). Hence, no significant change was observed upon the addition of an oligoethylene oxide in solution. Moreover, the k0 of 4-nitroanisole and RAO 3b were within the same order of magnitude. These results strongly suggest that the oligoethylene oxide core has little impact on the observed electrode kinetics, and that the rate constants observed for oligoethylene oxide core RAOs are suitable for use in RFBs.27 In order to fully understand the stability of the oligoethyleneoxide core RAOs over time, Galvanostatic bulk electrolysis was employed, measuring the ability of each compound to be quantitatively oxidized and reduced over multiple cycles. Compounds 3b and 3c, which appeared most stable by CV were successfully cycled while the less stable 3a and 3d underwent irreversible reactions upon oxidation and reduction respectively. Measurements for 3b and 3c over 25 cycles in ACN with TBA-PF6 supporting salt are presented in Figure 4. Both materials exhibited excellent stability over the period of cycling. The discharge capacity of 3b decays over time, resulting in a efficiency drop to 84% by the end of 25 cycles, indicating some degradation over time; on the other hand 3c maintains an efficiency of 98% over the full 25 cycles, indicating a high level of stability.

Figure 3. Steady-state ultramicroelectrode voltammetry of RAOs 3b–3d in uncharged (initial) and charged states as compared to simulated fits. All data were acquired using 0.5 M TBAPF6 in acetonitrile as the electrolyte and a 12.5 µm-radius electrode. See Table 1 for simulation parameters. 3a not shown since a charged-state UME-CV could not be obtained.

We next turned to the evaluation of the active materials’ redox kinetics, which showed that RAOs exhibit comparable rate constants to monomeric redox centers (i.e., ROMs). Experimental curves of RAOs 3b, 3c, and 3d were taken in both their uncharged (initial) and charged (i.e., reduced or oxidized) states. Using UME-CV, to circumvent the mass



design of RAOs. These principles can be applied to a variety of oligoethylene oxide core sizes, number of appendages, and redox-centers with little impact on active-material stability. Looking forward, the development of more stable redoxcenters and electrolyte pairings are needed for practical application of this design.11,29 Emerging successes here are being aided considerably by materials genomics screening to identify new low- and high-potential redox-centers as well as QSAR studies for understanding active-material stability as substituents are varied on the central redox-active motifs.30–32

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website: materials and methods, characterization of compounds, electrochemical instrumentation, and additional data (PDF).

AUTHOR INFORMATION Corresponding Author * [email protected].

Author Contributions Figure 4. Charge capacity, discharge capacity, and current efficiency determined by Galvanostatic bulk electrolysis of a) 3b and b) 3c (5 mM by redox center) in ACN containing 0.5 M TBA-PF6 with 0–50% state-of-charge cycles at 1 C. Charge and discharge capacity (theoretical value 6.7 mA h L–1) are normalized to their respective maximum values.

Finally, our understanding of active-material crossover is fairly mature in that macromolecular redox-mers in a variety of architectures are effectively excluded from the pores of mesoporous polyolefin separators. Smaller oligomers require specialized but increasingly available microporous separators; in the context of non-aqueous RFBs, small molecule crossover is prevalent, hence the emphasis on macromolecular design.6–7,9,28 We found that the branched oligoethylene oxide core based RAOs behaved similarly to mesitylenebased RAOs previously reported (Figures S13 & S14), with a 325-fold reduction in the intrinsic effective diffusion coefficient through a microporous membrane relative to a macroporous membrane.3 The implementation of oligoethylene oxides presented in this work demonstrates a general approach to design liquid RAO electrolytes, which allow for unprecedented volumetric energy capacity. Redox-centers with high and low potentials can be appended to the core to produce stable RAOs in a variety of solvents. UME-CV, chronoamperometry, and NIR measurements reveal that redox centers behave independently within each RAO at the electrode/electrolyte interface. Mass transfer of the RAO were found to be more than one order of magnitude faster than those observed in polymers with similar redox-centers, but one or fewer order of magnitude slower than the monomer. Redox kinetics of RAOs were evaluated using UME-CV and SECM and showed similar k0 values to their monomer counterparts, showing a negligible impact from the use of the oligoethylene oxide core. In addition to favorable kinetics, this core maintains the low-crossover behavior intended by the macromolecular

The manuscript was written through contributions of all authors.

ACKNOWLEDGMENT This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Portions of this work, including active material synthesis, characterization, and crossover measurements were carried out as a user project at the Molecular Foundry, which is supported by the Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy under contract no. DEAC02-05CH11231. E. C. M. acknowledges support by the Ford Foundation Fellowship Program. J.R.L. acknowledges additional support from a Sloan Research Fellowship.

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