A High-Current, Stable Nonaqueous Organic Redox Flow Battery

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Letter

A High-Current, Stable Nonaqueous Organic Redox Flow Battery Xiaoliang Wei, Wentao Duan, Jinhua Huang, Lu Zhang, Bin Li, David Reed, Wu Xu, Vincent L. Sprenkle, and Wei Wang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00255 • Publication Date (Web): 05 Sep 2016 Downloaded from http://pubs.acs.org on September 6, 2016

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A High-Current, Stable Nonaqueous Organic Redox Flow Battery Xiaoliang Wei,*,†,§ Wentao Duan,†,§ Jinhua Huang,‡,§ Lu Zhang,‡,§ Bin Li,† David Reed,† Wu Xu,† Vincent Sprenkle,† Wei Wang*,† †

Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland, WA 99352 (USA)

‡ Argonne National Laboratory, 9700 South Cass Ave, Argonne, IL 60439 (USA) § Joint Center for Energy Storage Research (JCESR)

Corresponding Author * Dr. Xiaoliang Wei: [email protected]. * Dr. Wei Wang: [email protected].

ABSTRACT: Nonaqueous redox flow batteries are promising in pursuit of high energy density storage systems owing to the broad voltage windows (>2 V), but currently are facing key challenges such as limited cyclability and rate performance. To address these technical hurdles, here we report a nonaqueous organic flow battery chemistry based on N-methylphthalimide

anolyte

and

2,5-di-tert-butyl-1-methoxy-4-[2’1

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methoxyethoxy]benzene catholyte, which harvests a theoretical cell voltage of 2.30 V. The flow cells exhibit excellent cycling stability under both cyclic voltammetry and flow cell tests upon repeated cycling. A series of Daramic and Celgard porous separators are evaluated in this organic flow battery, which enable the cells to be operated at greatly improved current densities as high as 50 mA cm-2 compared to those of other nonaqueous flow systems. The stable cyclability and high-current operations of the organic flow battery system represent a significant progress in the development of promising nonaqueous flow batteries.

TOC GRAPHICS

Redox flow batteries that utilize flowing liquid electrolytes to store energy have received growing attention because of their great potential in stationary energy storage applications to enable reliable integration of renewables and operative stabilization of the current and future power grid.1-3 The unique advantage of decoupled energy and power leads to high scalability and flexibility in design for the flow battery technology to be cost-effective for a wide range of applications in a diverse grid-energy market.

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Nonaqueous flow batteries, a departure from conventional aqueous systems, capitalize on the broader electrochemical window of nonaqueous electrolytes, which offer a promising alternative pathway in pursuit of high energy density.3-6 Organic electroactive compounds have the advantages of greater diversity, availability, and tunability in chemical structures, resources, and redox potentials, and therefore have been investigated in a variety of nonaqueous flow chemistries.7-11 High energy densities of >50 Wh L-1 have been achieved in a few nonaqueous flow systems, which exemplifies the great prospects of nonaqueous flow batteries.12-16 However, a number of existing key challenges still exist. In general, the ionic conductivity of nonaqueous electrolytes is intrinsically lower than their aqueous counterparts and becomes even less favorable at high concentrations of redox materials. Moreover, high-performance membranes for nonaqueous flow batteries have yet to be developed. Ion exchange membranes and ceramic separators usually result in high cell resistance in nonaqueous electrolytes, low cost-effectiveness, and undesirable materialmembrane interactions.7, 17-18 As a result, current reported nonaqueous flow batteries are often operated at the current density of 100 mA cm-2,19 more than two orders of magnitude higher. Low operational current resulted from low electrolyte conductivity and high membrane resistance has become a major bottleneck of the development of nonaqueous flow batteries. In addition, the irreversible performance degradation has also been recognized as another one of the most critical technical hurdles for nonaqueous flow batteries.20-22 A major cause is the decomposition of redox materials especially in the charged state. Deliberate structural engineering of redox materials has been attempted to


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improve their chemical stability, which however often involves extensive material synthesis and induce additional material cost.23-24 Therefore, it is highly desirable to develop stable nonaqueous flow chemistries with easily accessible redox materials. In the present work, we demonstrate a nonaqueous organic flow battery employing commercially available N-methylphthalimide (MePh) as the anode side and readily

synthesized

2,5-di-tert-butyl-1-methoxy-4-[2’-methoxyethoxy]benzene

(DBMMB) as the cathode side redox materials (hence donated as MePh|DBMMB). The electrochemistry of this flow battery at both anode and cathode sides is illustrated in Scheme 1, respectively. The cell chemistry involves the formation of radical ions for both redox materials. Following the use of porous separators in aqueous flow batteries,25-26 a series of Daramic® (a registered trademark of Daramic LLC) and Celgard® porous separators are tested to study the effect of key separator parameters (i.e. thickness and pore size) on flow cell performance. Due to the relatively large pore size, mixed-reactant electrolytes were used at both positive and negative half-cells to reduce the concentration gradients and mitigate the crossover of redox materials.27-28

Scheme 1. The cell reactions of the MePh|DBMMB organic flow battery.


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Cyclic voltammetry (CV) was recorded with repeated potential sweeping to identify the redox potentials and evaluate the electrochemical stability. Figure 1a shows the CV curves of a mixed-reactant electrolyte containing a 1:1 molar ratio of MePh and DBMMB at 0.1 M concentrations. Both redox materials exhibit well-defined redox peaks and their redox potentials can be calculated by averaging the oxidation and reduction peak potentials. With the redox potentials of -1.79 V for MePh and of 0.51 V for DBMMB versus Ag/Ag+, the MePh|DBMMB flow chemistry exhibits a cell voltage of 2.30 V. The single redox couple of the electrolytes containing only one redox material indicate clean electrochemistry for both of them in the full potential range (Figure 1b and 1c). Although the MePh has two redox pairs, inclusion of the more negative redox pair in the flow cell chemistry causes degradation to both redox materials under CV conditions (Figure S1 in the Supporting Information). Moreover, the CV curves during repeated scans (100 cycles) overlap with each other almost completely, suggesting the high chemical and electrochemical stabilities of both redox materials under CV conditions. In addition, both redox materials exhibit good solubilities in the DME solvent, with ~0.7 M for the MePh and high miscibility for the DBMMB (a liquid at room temperature).

Figure 1. Repeated CV scans on glassy carbon electrode at 100 mV s-1: (a) the mixture of 0.1 M MePh and 0.1 M DBMMB; (b) 0.1 M MePh; and (c) 0.1 M DBMMB. The 5 ACS Paragon Plus Environment

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supporting electrolyte was 1.0 M LiTFSI in DME. The CV curves at different cycles were almost completely overlapped, indicating the good stability of these redox materials under CV conditions.

Linear sweep voltammetry (LSV) was conducted using the rotating disk electrode (RDE) technique to investigate the electrochemical kinetics of both redox molecules. Figure 2a shows the LSV curves of DBMMB and MePh, respectively, measured in electrolyte solutions containing 1.0 mM MePh or 1.0 mM DBMMB in 1.0 M LiTFSI in DME. The rotation rate (ω) increases from 300 to 2100 rpm (i.e., from 10π to 70π rad s1

). These LSV curves display mass-transport controlled limiting currents (iL) with well-

defined plateaus. Figure 2b reveals linear relationships between the limiting currents (iL) and the square root of the rotation rate (ω1/2) for both MePh and DBMMB, which are consistent with the Levich equation (Equation 1)29: ݅୐ = 0.62݊‫ܦܣܨ‬ଶ/ଷ ߱ଵ/ଶ ߭ ିଵ/଺ ‫ܥ‬଴

(1)

where n is the number of electron transfer (n = 1); F is the Faradaic constant (F = 96485 C mol-1); A is the electrode area (0.2 cm2); D is the diffusion coefficient of the analyte; υ is the kinematic viscosity (υ = 0.0136 cm2 s-1 measured for 1.0 M LiTFSI in DME); C0 is the concentration of the analyte (Co = 1.0 mM). Using the slopes of the fitted linear Levich plots, the diffusion coefficients D are determined to be 8.38 × 10-6 for the MePh and 5.77 × 10-6 cm2/s for the DBMMB, respectively. Moreover, the electrochemical kinetics of the charge transfer reactions of MePh and DBMMB can be obtained based on the koutecky-Levich equation (Equation 2)29: ଵ ௜

=

ଵ ௜ೖ

+

ଵ ௜ಽ

=

ଵ ௜ೖ

+

ଵ ଴.଺ଶ௡ி஺஽మ/య ఠభ/మ జషభ/ల ஼బ

(2)


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where ik is the kinetics-controlled current. Figure 2c and 2d show the linear relationships between i-1 and ω-1/2 for MePh and DBMMB, respectively, at a series of overpotentials (η = 10, 20, 30, 40, 50, 75, and 100 mV). The ik obtained as the reciprocal of the Y-axis intercepts were then plotted as a function of the overpotential (Figure 2e). The exchange currents (io) were derived from the Y-axis intercepts (as Log i0) to be 4.68 × 10-5 A for MePh and 2.57 × 10-4 A for DBMMB, respectively. According to Equation 329: ݅଴ = ݊‫݇ܣܨ‬଴ ‫ܥ‬଴

(3)

the kinetic rate constants (ko) were calculated as 2.46 × 10-3 cm s-1 for MePh and 1.35 × 10-2 cm s-1 for DBMMB, respectively. The diffusion coefficients and rate constants of both compounds are comparable to, and even several orders of magnitude higher than, those of some redox–active organic materials used in aqueous flow batteries.30-34 The fast electrode kinetics of both MePh and DBMMB are expected to produce low activation polarization loss, which are favored for flow battery applications.

Figure 2. RDE studies of MePh and DBMMB using electrolyte solutions containing 1.0 mM MePh or 1.0 mM DBMMB in 1.0 M LiTFSI in DME: (a) LSV curves at a scan rate of 5 mV s-1 with rotation rates from 300 to 2100 rpm; (b) fitted linear Levich plots of the 7 ACS Paragon Plus Environment

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limiting current (iL) versus the square root of rotation rates (ω1/2); (c) and (d) Linearly fitted koutecky-Levich plots of i-1 with respect to ω-1/2 for MePh and DBMMB, respectively; (e) Linearly fitted plots of Log ik as a function of the overpotential (η). One of the key challenges for nonaqueous flow batteries is the lack of suitable membranes, which has largely hampered reliable assessment of the feasibility of interested cell chemistries under real flow conditions. Here we performed a systematic study of three Daramic® and two Celgard® porous separators and evaluate their performance for applications in the MePh|DBMMB flow cells. The Daramic® separators have different thicknesses (175 µm, 450 µm and 800 µm, denoted as Daramic-175, Daramic-450, and Daramic-800, respectively) with the same polyethylene/silica composite formulation and pore structures (median pore size of 0.15 mm and porosity of 57%).35 The two Celgard® separators have varied thicknesses and pore sizes: 2325 (25 µm thick, 28 nm pore size), and 4560 (laminated film 110 µm + base film 25 µm, 64 nm pore size). The generally large open pores in these porous separators offer relatively low membrane resistance and favors high current operation in flow cells. Figure 3a shows the area-specific resistivity (ASR) of the MePh|DBMMB flow cells using 0.1 M mixedreactant electrolytes. Here the cell ASR is defined as the high-frequency ohmic resistance multiplied by the active area of the flow cell and was measured with electrochemical impedance spectroscopy (EIS). In general, the ASR of the flow cells using these Daramic and Celgard porous separators ranges between 3 – 18 Ω cm2. Especially, Daramic-175, Celgard 4560, and Celgard 2325 lead to the cell ASRs around the threshold value of 5 Ω cm2 required to meet cost-effective energy storage, as predicted by a recent technoeconomic analysis.36


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Figure 3. Electrochemical performance of the MePh|DBMMB flow cells with different porous separators: (a) flow cell ASR before cell cycling; (b) CE; (c) VE; and (d) EE. The redox electrolytes used were 0.1M MePh/0.1M DBMMB/1.0 M LiTFSI/DME.

The voltage curves of flow cells using these porous separators all display welldefined single plateaus (Figure S2a-e in Supporting Information) and no obvious changes were observed visually after cell cycling, indicating negligible redox material-separator interactions. Since the three Daramic® separators have the same pore structures, the thickness plays a significant role in controlling both cell resistance and redox material crossover. When the thickness decreases from 800 to 175 microns, the flow cell ASR decreases from 18 to 3.8 Ω cm2 (Figure 3a), which is reflected by greatly improved voltage efficiencies (VEs) of corresponding flow cells (Figure 3b). Meanwhile, thinner separators yield more materials crossover and self-discharge reactions, as demonstrated 9 ACS Paragon Plus Environment

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by the lower coulombic efficiencies (CEs) at tested currents (Figure 3c). Similarly, due to significantly reduced thickness, both Celgard separators produce lower CEs than Daramic separators. For the Daramic® separators, VE appears to be more affected than CE. Therefore, thinner separators (e.g., Daramic-175) afford better rate capability for the flow cells, i.e., higher energy efficiencies (EEs, see Figure 3d) and redox material utilization ratios (Figure S2f in Supporting Information) especially at current densities >10 mA cm-2. On the other hand, the pore size of porous separators is another important parameter that dominates cell performance. It is expected that smaller separator pores yield less redox materials crossover but higher membrane resistance. For example, even 7-fold thinner than Daramic-175, the much smaller pore size (28 nm) of Celgard 2325 still leads to a slightly higher cell ASR (5.1 Ω cm2) and lower VEs at tested currents (Figure 3a and 3c). To summarize, there indeed exists a tradeoff in the thickness and pore size of porous separators in order to achieve optimal EE and material utilization. Thin separators with large pores are generally favored for high current densities because of low membrane resistance, and vice versa. Among these separators, Daramic-175 almost always leads to the highest EEs and is especially advantageous for high-current operations. For example, the flow cell using this separator can operate at currents as high as 50 mA cm-2 while still delivering an EE of ~70% and a material utilization ration of ~80% (Figure 3d and S1f). Such operational current densities are around two orders of magnitude higher than those for many nonaqueous flow batteries developed in other research teams,10, 37-38 and are comparable to or even better than those for neutral electrolyte – based aqueous flow batteries.39-40


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The cycling stability is a critically important performance parameter for flow batteries that guarantees the system to deliver consistent, durable energy storage service over extended operations. Capacity loss caused by unbalanced redox material transport can be possibly restored in certain aqueous flow batteries such as all-vanadium through electrolyte re-mixing maintenance or pressure regulation.41 However, performance degradation originating from irreversible decomposition of redox species is more detrimental because of the unrecoverable capacity decay leading to shortened cycle life of the flow battery system. This case is commonly observed in nonaqueous flow batteries because of the chemically active organic redox species. In this regard, we demonstrate that the MePh|DBMMB system exhibits an excellent cycling stability even under batteryrelevant conditions. As the redox concentration increases, the viscosity and ionic resistance of the electrolytes increase, which leads to higher cell polarization. This explains the lower VEs of the 0.3 M MePh|DBMMB flow cell than that of the 0.1 M cell with the same Daramic-175 separator at all tested currents, as shown in Figure 4a. Also shown are the lower CEs due to longer charge/discharge durations and more selfdischarge. However, the EE of the 0.3 M flow cell still remains >60% even at the current as high as 50 mA cm-2. Figure 4b depicts the cycling capacity and efficiency of the 0.3 M MePh|DBMMB flow cell using Daramic-175 separator at 35 mA cm-2. The flow cell maintained constant capacities and efficiencies (CE 90%, VE 77%, and EE 69%) throughout the overall 50 cycles. The average charge and discharge capacities reached 85% and 77% of the theoretical capacity, indicating a high redox material utilization ratio. Capacity retention was also acquired in the 0.1 M MePh|DBMMB flow cell for 100 cycles with Daramic-800 separator (Figure S3 in Supporting Information). Stable cycling


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at two different redox material concentrations provides strong evidence for the excellent stability of the MePh|DBMMB system. Such an impressive capacity retention greatly exceeds other reported nonaqueous flow batteries that showed continuous capacity fading over even shorter cycling.20-22 Notably, the performance parameters achieved in this system are even comparable to the recently reported organics-based aqueous flow batteries (Table S1 in Supporting Information), in terms of theoretical energy density, operating current density, energy efficiency, and capacity retention.32, 42 Although longer cycling may need to be demonstrated, this MePh|DBMMB flow system represents a significant advance in nonaqueous flow battery development.

Figure 4. Electrochemical performance the 0.3 M MePh|DBMMB flow cells using Daramic-175 separator: (a) efficiencies at currents from 20 to 50 mA cm-2; (b) cycling efficiency and capacity at 35 mA cm-2. The theoretical capacity is 4.02 Ah L-1, calculated by (0.3 mol L-1) * (26.8 Ah mol-1) / (2 electrolyte volumes).

The good cycling stability of the MePh|DBMMB flow cells is closely associated with the high chemical stability of both radical ion species especially the MePh•− radical anion, which essentially arises from a combination of environmental and structural factors. This argument has been confirmed in our previous study of another all-organic nonaqueous flow battery chemistry.28 First, the DME solvent and LiTFSI salt structurally


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contain no electron-deficient functionalities, and thus are less susceptible to the nucleaphilic attacks from the MePh•− radical anion than other solvents such as nitriles and carbonates and salts such as tetrafluoroborates. Figure 5a shows possible parasitic side reactions of the MePh•− with other solvents and salts. This is supported by the comparison among the cycling capacities in the 0.1 M MePh|DBMMB flow cells employing a variety of solvents and salts, shown in Figure 5b. In contrast to LiTFSI/DME that afforded constant capacity, use of acetonitrile (MeCN), propylene carbonate (PC), or tetraethylammonium tetrafluoroborate (TEABF4) all led to substantial capacity fading. Second, the MePh•− structurally benefits from the high canonical resonance delocalizing the spin and charge from a single carbonyl bond into the fused bicyclic aromatic ring and the other carbonyl bond. Also, the electron-withdrawing imide (-C(=O)-N-C(=O)-) group decreases the electron density thus offering additional stabilization. For the DBMMB•+, in addition to the resonance-induced stabilization, the close proximity of the two bulky t-butyl groups provides good shielding to the unpaired electron and offers strong steric protection against incoming attacking species. These factors, coupled with two electron-donating alkoxyl substituents decreasing the charge density, function synergistically to stabilize the DBMMB•+.


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Figure 5. (a) Possible side reactions of the MePh•− radical anion with PC, MeCN, and BF4− (the proposed reaction between MePh•− and BF4− is inspired by those between the superoxide radical anion (O2•−) with BF4− in lithium-air batteries43); (b) cycling capacity retention of the 0.1 M MePh|DBMMB flow cells using different solvents and salts; in all cell tests, the separators were Daramic-800, and the current densities were 10 mA cm-2. In conclusion, we have developed an organic MePh|DBMMB nonaqueous flow battery chemistry that has a cell voltage of ~2.3 V. RDE-based LSV studies revealed high diffusivity and fast kinetics for both organic compounds that are desired for promising redox candidates. Because of favorable combination of pore structures and membrane thickness, selected Daramic® and Celgard® porous separators produced significantly low membrane resistance and flow cell ASRs of 70%) especially in high-current operations (e.g. 50 mA cm-2). In addition, these separators are highly cost-effective ($500 m-2),44-45 which leads to significant reduction in the membrane contribution to the overall system cost. More importantly, the MePh|DBMMB flow cells demonstrated excellent cycling stability both under CV conditions and in battery-relevant flow cell tests. Constant capacity and efficiency were maintained for 50 galvanostatic charge/discharge cycles at 0.3 M redox concentrations. Flow cell cycling with a series of solvents and salts indicates that the chemical compatibility between the charged species (organic radical ions) and the supporting electrolyte may dominate the cycling stability of MePh|DBMMB flow cells. The organic redox materials may also harvest good chemical durability from their structural characteristics, such as electron delocalization, steric protection, and electron-tuning functional groups.


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Indeed, the current flow battery is still facing challenges such as limited energy density compared to traditional aqueous systems (e.g. all-vanadium), redox materials crossover, and high electrolyte resistivity. In fact, these are general concerns for nonaqueous flow batteries. More engineering work is needed for future performance improvements to fully demonstrate the high potential of nonaqueous systems. For example, molecular engineering could introduce quaternary ammonium or oligo(glycol ether) functional groups to the MePh to increase its solubility. Highly selective membranes that minimize redox material crossover need to be developed to eliminate use of mixed-reactant electrolytes and increase effective redox concentrations. The flow cell design could be further optimized to mitigate the elevated electrolyte viscosity at high redox concentrations. With these implemented, nonaqueous flow batteries are expected to become more competitive for grid-attached energy storage applications.

ASSOCIATED CONTENT Supporting Information. The voltage curves and material utilizations of the MePh|DBMMB flow cells using Daramic® and Celgard® porous separators, and the cycling performance of the 0.1 M MePh|DBMMB flow cells using Daramic-800 are shown in Supporting Information.

AUTHOR INFORMATION Corresponding Authors * Dr. Xiaoliang Wei: [email protected]; * Dr. Wei Wang: [email protected]. 15 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was financially supported by the U.S. Department of Energy’s (DOE’s) Office of Electricity Delivery and Energy Reliability (OE) under Contract No. 57558 (flow chemistry development and electrochemistry study); and by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (synthesis of DBMMB). PNNL is a multi-program national laboratory operated by Battelle for DOE under Contract DE-AC05-76RL01830.

REFERENCES (1) Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928-935. (2) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111, 3577-3613. (3) Wang, W.; Luo, Q.; Li, B.; Wei, X.; Li, L.; Yang, Z. Recent Progress in Redox Flow Battery Research and Development. Adv. Funct. Mater. 2013, 23, 970-986. (4) Leung, P.; Li, X.; de Leon, C. P.; Berlouis, L.; Low, C. T. J.; Walsh, F. C. Progress in redox flow batteries, remaining challenges and their applications in energy storage. RSC Adv. 2012, 2, 10125-10156. (5) Noack, J.; Roznyatovskaya, N.; Herr, T.; Fischer, P. The Chemistry of Redox-Flow Batteries. Angew. Chem. Int. Ed. 2015, 54, 9775-9808. (6) Zhao, Y.; Ding, Y.; Li, Y.; Peng, L.; Byon, H. R.; Goodenough, J. B.; Yu, G. A chemistry and material perspective on lithium redox flow batteries towards high-density electrical energy storage. Chem. Soc. Rev. 2015, 44, 7968-7996. (7) Brushett, F. R.; Vaughey, J. T.; Jansen, A. N. An All-Organic Non-aqueous LithiumIon Redox Flow Battery. Adv. Energy Mater. 2012, 2, 1390-1396. (8) Huang, J.; Cheng, L.; Assary, R. S.; Wang, P.; Xue, Z.; Burrell, A. K.; Curtiss, L. A.; Zhang, L. Liquid Catholyte Molecules for Nonaqueous Redox Flow Batteries. Adv. Energy Mater. 2015, 5, 1401782, DOI: 10.1002/aenm.201401782. 16 ACS Paragon Plus Environment

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(9) Nagarjuna, G.; Hui, J.; Cheng, K. J.; Lichtenstein, T.; Shen, M.; Moore, J. S.; Rodriguez-Lopez, J. Impact of Redox-Active Polymer Molecular Weight on the Electrochemical Properties and Transport Across Porous Separators in Nonaqueous Solvents. J. Am. Chem. Soc. 2014, 136, 16309-16316. (10) Cappillino, P. J.; Pratt, H. D.; Hudak, N. S.; Tomson, N. C.; Anderson, T. M.; Anstey, M. R. Application of Redox Non-Innocent Ligands to Non-Aqueous Flow Battery Electrolytes. Adv. Energy Mater. 2014, 4, 1300566, DOI: 10.1002/aenm.201300566. (11) Liu, Q.; Sleightholme, A. E. S.; Shinkle, A. A.; Li, Y.; Thompson, L. T. Nonaqueous vanadium acetylacetonate electrolyte for redox flow batteries. Electrochem. Commun. 2009, 11, 2312-2315. (12) Wei, X.; Xu, W.; Vijayakumar, M.; Cosimbescu, L.; Liu, T.; Sprenkle, V.; Wang, W. TEMPO-Based Catholyte for High-Energy Density Nonaqueous Redox Flow Batteries. Adv. Mater. 2014, 26, 7649–7653. (13) Zhao, Y.; Byon, H. R. High-Performance Lithium-Iodine Flow Battery. Adv. Energy Mater. 2013, 3, 1630-1635. (14) Duduta, M.; Ho, B.; Wood, V. C.; Limthongkul, P.; Brunini, V. E.; Carter, W. C.; Chiang, Y. M. Semi-Solid Lithium Rechargeable Flow Battery. Adv. Energy Mater. 2011, 1, 511-516. (15) Wei, X.; Cosimbescu, L.; Xu, W.; Hu, J.; Vijayakumar, M.; Feng, J.; Hu, M. Y.; Deng, X.; Xiao, J.; Liu, J.; et al. Towards High-Performance Nonaqueous Redox Flow Electrolyte Via Ionic Modification of Active Species. Adv. Energy Mater. 2015, 5, 1400678, DOI: 10.1002/aenm.201400678. (16) Ding, Y.; Yu, G. A Bio-Inspired, Heavy-Metal-Free, Dual-Electrolyte Liquid Battery towards Sustainable Energy Storage. Angew. Chem. Int. Ed. 2016, 55, 47724776. (17) Zhao, Y.; Ding, Y.; Song, J.; Li, G.; Dong, G.; Goodenough, J. B.; Yu, G. Sustainable Electrical Energy Storage through the Ferrocene/Ferrocenium Redox Reaction in Aprotic Electrolyte. Angew. Chem. Int. Ed. 2014, 53, 11036-11040. (18) Park, M. S.; Lee, N. J.; Lee, S. W.; Kim, K. J.; Oh, D. J.; Kim, Y. J. High-Energy Redox-Flow Batteries with Hybrid Metal Foam Electrodes. ACS Appl. Mater. Interfaces 2014, 6, 10729-10735. (19) Reed, D.; Thomsen, E.; Wang, W.; Nie, Z.; Li, B.; Wei, X.; Koeppel, B.; Sprenkle, V. Performance of Naflon (R) N115, Naflon (R) NR-212, and Naflon (R) NR-211 in a 1 kW class all vanadium mixed acid redox flow battery. J. Power Sources 2015, 285, 425430. (20) Park, S. K.; Shim, J.; Yang, J.; Shin, K. H.; Jin, C. S.; Lee, B. S.; Lee, Y. S.; Jeon, J. D. Electrochemical properties of a non-aqueous redox battery with all-organic redox couples. Electrochem. Commun. 2015, 59, 68-71. (21) Escalante-Garcia, I. L.; Wainright, J. S.; Thompson, L. T.; Savinell, R. F. Performance of a Non-Aqueous Vanadium Acetylacetonate Prototype Redox Flow Battery: Examination of Separators and Capacity Decay. J. Electrochem. Soc. 2015, 162, A363-A372. (22) Duan, W.; Vemuri, R. S.; Milshtein, J. D.; Laramie, S.; Dmello, R. D.; Huang, J.; Zhang, L.; Hu, D.; Vijayakumar, M.; Wang, W.; et al. A symmetric organic-based


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nonaqueous redox flow battery and its state of charge diagnostics by FTIR. J. Mater. Chem. A 2016, 4, 5448-5456. (23) Huang, J.; Su, L.; Kowalski, J. A.; Barton, J. L.; Ferrandon, M.; Burrell, A. K.; Brushett, F. R.; Zhang, L. A subtractive approach to molecular engineering of dimethoxybenzene-based redox materials for non-aqueous flow batteries. J. Mater. Chem. A 2015, 3, 14971-14976. (24) Carino, E. V.; Diesendruck, C. E.; Moore, J. S.; Curtiss, L. A.; Assary, R. S.; Brushett, F. R. BF3-promoted electrochemical properties of quinoxaline in propylene carbonate. RSC Adv. 2015, 5, 18822-18831. (25) Zhang, H.; Zhang, H.; Li, X.; Mai, Z.; Zhang, J. Nanofiltration (NF) membranes: the next generation separators for all vanadium redox flow batteries (VRBs)? Energy Environ. Sci. 2011, 4, 1676-1679. (26) Wei, X.; Nie, Z.; Luo, Q.; Li, B.; Chen, B. W.; Simmons, K.; Sprenkle, V.; Wang, W. Nanoporous Polytetrafl uoroethylene/Silica Composite Separator as a HighPerformance All-Vanadium Redox Flow Battery Membrane. Adv. Energy Mater. 2013, 3, 1215-1220. (27) Lopezatalaya, M.; Codina, G.; Perez, J. R.; Vazquez, J. L.; Aldaz, A. Optimization Studies on a Fe/Cr Redox Flow Battery. J. Power Sources 1992, 39, 147-154. (28) Wei, X.; Xu, W.; Huang, J.; Zhang, L.; Walter, E.; Lawrence, C.; Vijayakumar, M.; Henderson, W. A.; Liu, T.; Cosimbescu, L.; et al. Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All-Organic Redox Flow Battery. Angew. Chem. Int. Ed. 2015, 54, 8684-8687. (29) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications. 2nd ed., 2000, John Wiley & Sons Inc, New York, NY, USA. (30) 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. (31) Yang, B.; Hoober-Burkhardt, L.; Wang, F.; Prakash, G. K. S.; Narayanan, S. R. An Inexpensive Aqueous Flow Battery for Large-Scale Electrical Energy Storage Based on Water-Soluble Organic Redox Couples. J. Electrochem. Soc. 2014, 161, A1371-A1380. (32) Janoschka, T.; Martin, N.; Martin, U.; Friebe, C.; Morgenstern, S.; Hiller, H.; Hager, M. D.; Schubert, U. S. An aqueous, polymer-based redox-flow battery using noncorrosive, safe, and low-cost materials. Nature 2015, 527, 78–81. (33) Lin, K.; Chen, Q.; Gerhardt, M. R.; Tong, L.; Kim, S. B.; Eisenach, L.; Valle, A. W.; Hardee, D.; Gordon, R. G.; Aziz, M. J.; et al. Alkaline quinone flow battery. Science 2015, 349, 1529-1532. (34) Winsberg, J.; Janoschka, T.; Morgenstern, S.; Hagemann, T.; Muench, S.; Hauffman, G.; Gohy, J.-F.; Hager, M. D.; Schubert, U. S. Poly(TEMPO)/Zinc HybridFlow Battery: A Novel, “Green,” High Voltage, and Safe Energy Storage System. Adv. Mater. 2016, 28, 2238–2243. (35) Wei, X.; Li, L.; Luo, Q.; Nie, Z.; Wang, W.; Li, B.; Xia, G.; Miller, E.; Chambers, J.; Yang, Z. Microporous separators for Fe/V redox flow batteries. J. Power Sources 2012, 218, 39-45. (36) Darling, R. M.; Gallagher, K. G.; Kowalski, J. A.; Ha, S.; Brushett, F. R. Pathways to low-cost electrochemical energy storage: a comparison of aqueous and nonaqueous flow batteries. Energy Environ. Sci. 2014, 7, 3459-3477. 18 ACS Paragon Plus Environment

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(37) Saraidaridis, J. D.; Bartlett, B. M.; Monroe, C. W. Spectroelectrochemistry of Vanadium Acetylacetonate and Chromium Acetylacetonate for Symmetric Nonaqueous Flow Batteries. J. Electrochem. Soc. 2016, 163, A1239-A1246. (38) Kim, H. S.; Yoon, T.; Jang, J.; Mun, J.; Park, H.; Ryu, J. H.; Oh, S. M. A tetradentate Ni(II) complex cation as a single redox couple for non-aqueous flow batteries. J. Power Sources 2015, 283, 300-304. (39) Janoschka, T.; Martin, N.; Martin, U.; Friebe, C.; Morgenstern, S.; Hiller, H.; Hager, M. D.; Schubert, U. S. An aqueous, polymer-based redox-flow battery using noncorrosive, safe, and low-cost materials. Nature 2015, 527, 78-81. (40) Winsberg, J.; Janoschka, T.; Morgenstern, S.; Hagemann, T.; Muench, S.; Hauffman, G.; Gohy, J. F.; Hager, M. D.; Schubert, U. S. Poly(TEMPO)/Zinc HybridFlow Battery: A Novel, "Green," High Voltage, and Safe Energy Storage System. Adv. Mater. 2016, 28, 2238-2243. (41) Wei, X.; Li, B.; Wang, W. Porous Polymeric Composite Separators for Redox Flow Batteries. Polym. Rev. 2015, 55, 247-272. (42) Lin, K; Gómez-Bombarelli, R.; Beh, E. S.; Tong, L.; Chen, Q; Valle, A; AspuruGuzik, A.; Aziz, M. J.; Gordon, R. G. A redox-flow battery with an alloxazine-based organic electrolyte. Nat. Energy 2016, 16102, DOI: 10.1038/NENERGY.2016.102. (43) Nasybulin, E.; Xu, W.; Engelhard, M. H.; Nie, Z.; Burton, S. D.; Cosimbescu, L.; Gross, M. E.; Zhang, J. Effects of Electrolyte Salts on the Performance of Li-O-2 Batteries. J. Phys. Chem. C 2013, 117, 2635-2645. (44) Viswanathan, V.; Crawford, A.; Stephenson, D.; Kim, S.; Wang, W.; Li, B.; Coffey, G.; Thomsen, E.; Graff, G.; Balducci, P.; et al. Cost and performance model for redox flow batteries. J. Power Sources 2014, 247, 1040-1051. (45) Sakti, A.; Michalek, J. J.; Fuchs, E. R. H.; Whitacre, J. F. A techno-economic analysis and optimization of Li-ion batteries for light-duty passenger vehicle electrification. J. Power Sources 2015, 273, 966-980.


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Scheme 1. The cell reactions of the MePh|DBMMB organic flow battery. 80x53mm (300 x 300 DPI)


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Figure 1. Repeated CV scans on glassy carbon electrode at 100 mV s-1: (a) the mixture of 0.1 M MePh and 0.1 M DBMMB; (b) 0.1 M MePh; and (c) 0.1 M DBMMB. The supporting electrolyte was 1.0 M LiTFSI in DME. The CV curves at different cycles were almost completely overlapped, indicating the good stability of these redox materials under CV conditions. 80x55mm (300 x 300 DPI)


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Figure 2. RDE studies of MePh and DBMMB using electrolyte solutions containing 1.0 mM MePh or 1.0 mM DBMMB in 1.0 M LiTFSI in DME: (a) LSV curves at a scan rate of 5 mV s-1 with rotation rates from 300 to 2100 rpm; (b) fitted linear Levich plots of the limiting current (iL) versus the square root of rotation rates (ω1/2); (c) and (d) Linearly fitted koutecky-Levich plots of i-1 with respect to ω-1/2 for MePh and DBMMB, respectively; (e) Linearly fitted plots of Log ik as a function of the overpotential (η). 150x78mm (300 x 300 DPI)


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Figure 3. Electrochemical performance of the MePh|DBMMB flow cells with different porous separators: (a) flow cell ASR before cell cycling; (b) CE; (c) VE; and (d) EE. The redox electrolytes used were 0.1M MePh/0.1M DBMMB/1.0 M LiTFSI/DME. 150x108mm (300 x 300 DPI)


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Figure 4. Electrochemical performance the 0.3 M MePh|DBMMB flow cells using Daramic-175 separator: (a) efficiencies at currents from 20 to 50 mA cm-2; (b) cycling efficiency and capacity at 35 mA cm-2. The theoretical capacity is 4.02 Ah L-1. 157x62mm (300 x 300 DPI)


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Figure 5. (a) Possible side reactions of the MePh•− radical anion with PC, MeCN, and BF4− (the proposed reaction between MePh•− and BF4− is inspired by those between the superoxide radical anion (O2•−) with BF4− in lithium-air batteries39); (b) cycling capacity retention of the 0.1 M MePh|DBMMB flow cells using different solvents and salts; in all cell tests, the separators were Daramic-800, and the current densities were 10 mA cm-2. 150x61mm (300 x 300 DPI)


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TOC graphics 56x50mm (299 x 299 DPI)


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76x50mm (299 x 299 DPI)


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Figure S1. Repeated CV curves of a mixed-reactant electrolyte of 0.1 M MePh/0.1 M DBMMB/1.0 M TEABF4/MeCN: (a) in a narrow voltage range; and (b) in a broader voltage range to include the second redox pair of the MePh. This clearly indicates that using both redox pairs of the MePh led to continuous degradation of both redox materials. 150x60mm (300 x 300 DPI)


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Figure S2. Voltage curves of the 0.1 M MePh|DBMMB flow cells at different current densities using Daramic® and Celgard® porous separators: (a) Daramic-800; (b) Daramic-450; (c) Daramic-175; (d) Celgard 2325; (e) Celgard 4560; (f) material utilization (defined as the percentage of discharge capacity to theoretical capacity) with these separators. These were the second cycle at each current density (as the legends in each figure, unit: mA cm-2). The theoretical capacity is 1.34 Ah L-1. Voltage cutoff limits were carefully controlled to avoid occurrence of the second electron transfer plateau. 150x84mm (300 x 300 DPI)


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Figure S3. Cycling performance of the 0.1 M MePh|DBMMB flow cell using a Daramic-800 separator at 10 mA cm-2. 80x60mm (300 x 300 DPI)


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A High-Current, Stable Nonaqueous Organic Redox Flow Battery

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