Aqueous 2,2,6,6-Tetramethylpiperidine-N-oxyl Catholytes for a High

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Aqueous TEMPO Catholytes for a High Capacity and High Current Density Oxygen-Insensitive Hybrid-Flow Battery Jan Winsberg, Christian Stolze, Almut M. Schwenke, Simon Muench, Martin D. Hager, and Ulrich S. Schubert ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00655 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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Aqueous TEMPO Catholytes for a High Capacity and High Current Density Oxygen-Insensitive Hybrid-Flow Battery Jan Winsberg,a,b Christian Stolze,a,b Almut Schwenke,a Simon Muench,a,b Martin D. Hager,a,b and Ulrich S. Schuberta,b * a.

Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstraße 10, 07743 Jena, Germany

b.

Center for Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany

CORRESPONDING AUTHOR * Prof. Dr. Ulrich S. Schubert, Friedrich Schiller University Jena, Humboldtstrasse 10, 07743 Jena, Germany; [email protected]

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ABSTRACT Hybrid-flow batteries are a suitable storage technology for “green” electricity generated by renewable sources like wind power and solar energy. Redox-active organic compounds have been investigated, recently, to improve the traditional metal and halogen-based technologies. Here we report the utilization of a 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) derivative that is in particular designed for application in semi-organic zinc hybrid-flow batteries. The TEMPO derivative is synthesized and electrochemically characterized via cyclic voltammetry and rotating disc electrode measurements. This derivative features a high solubility in aqueous electrolytes; thus, volumetric capacities above 20 Ah L-1 are achieved. The fabricated hybrid-flow batteries (HFB) feature over 1,100 consecutive charge/discharge cycles at constant capacity retention and current densities up to 80 mA cm-2 are applied.

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Promising storage technologies for the storage of renewable energy are redox-flow and hybridflow batteries.1-6 A panoramic overview of various possible storage technologies was presented for example by Skyllas-Kazacos et al.,7-8, Weber et al.9 and Winsberg et al.10 A common representative is the zinc hybrid-flow battery, which typically combines a Zn anode with a halogen cathode, e.g., polybromide and polyiodide.11-14 These batteries feature a safe and costefficient zinc anode but suffer from the critical cathode, which is toxic in the case of bromine and also hazardous, in particular for water organisms, in the case of iodine. This drawback restricts the application as “green” energy storage devices. Some studies proposed the utilization of quaternary species to complex the generated bromine.15-16 However, these additives are expensive and an evaporation of toxic bromine cannot be excluded reliably. Several organic redox-active materials, which comprise both polymers and small molecules, were investigated in recent years.17-28 An improvement of the Zn/Br2 technology can be achieved by the replacement of the halogen based cathode-material by an organic alternative. First attempts were conducted by Zhao et al., who developed a Zn/polyaniline HFB.29 Significant advancements of polymeric active materials were recently reported.24, 30-33 The utilization of TEMPO-containing polymers as cathode active material in polymer-based TEMPO/Zn HFBs was recently reported.34-35 These systems showed an elevated cell voltage of 1.7 V in aqueous electrolytes and expanded the in water applicable potential range up to 2 V. However, the capacity was limited to 2.4 Ah L−1 and applicable current densities were restricted to 20 mA cm−2. A significant improvement of these performance parameters can be achieved with the utilization of low molar mass compounds (i.e. small molecules). Liu et al. employed 4-HO-TEMPO and dimethyl viologen as charge-storage materials.19 However, the application of 4-HO-TEMPO is not optimal for RFB applications, due to its limited solubility of 0.5 M in aqueous electrolytes. Additionally, TEMPO is frequently


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used for the oxidation of alcohols to aldehydes and ketones. Hence, a self-oxidation is evident.36 The application of a better soluble and not hydroxyl-containing TEMPO derivative would lead to a significantly improved battery performance. Here we describe the synthesis of a novel TEMPO-containing redox-active material (TEMPO-4-sulfate potassium salt) as well as the subsequent utilization in TEMPO/zinc hybrid-flow batteries (Figure 1a).

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Figure 1. Schematic representation of a) an aqueous TEMPO/Zn hybrid-flow battery. A cationexchange membrane is used as separator. R = solubility promoting substituent, redox mechanism of Zn2+/Zn0 and TEMPO+/TEMPO and b) of the synthesis of compound 3. The electroplated Zn0(s) typically tends to form dendritic structures, which grow towards the direction of the cathode. With continued growth, short circuits are likely. As a consequence,


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additional space between the anode and the membrane is required for the electroplated zinc deposit. A simple cellulose cloth was utilized as spacer material. The utilized HFB comprised a Zn anode with graphite felt to increase the surface area of the electrode and to minimize zinc dendrite formation.34,

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A fumasep® F-930-RFD cation exchange membrane was used as

separator, because anion exchange membranes are not usable in combination with zinc electrolytes (Figure S2). For this reason the recently reported TEMPTMA is not suitable for TEMPO/zinc hybrid flow batteries.38 The anolyte comprised ZnCl2 as active material and supporting electrolyte. NH4Cl was utilized as additive to prevent the formation of zinc hydroxide,39-40 as well as to increase the electrical performance.41 An ideal cathode-active material for the application in semi-organic zinc hybrid-flow batteries (HFB) would feature an elevated concentration of 1 M in aqueous electrolytes. In order to facilitate this, an anionic substituent has to be introduced to the TEMPO moiety (a detailed explanation is provided in the SI). The derivative TEMPO-4-sulfate potassium salt 3 was developed and straightforwardly synthesized in a one-step reaction. 4-HO-TEMPO 1 and concentrated sulfuric acid 2 were reacted in bulk at room temperature (Figure 1b). Despite the harsh reaction conditions, a degradation of 4-HO-TEMPO 1 was not observed, which is attributed to a chemically reversible disproportionation of TEMPO in concentrated sulfuric acid at room temperature.42 The esterification occurs immediately and requires minimal synthetic effort. The reaction mixture was neutralized with potassium hydrogen carbonate and the TEMPO-sulfate potassium salt was obtained. Similarly, the ammonium- and the zinc-salt can also be synthesized by using, for instance, NH4HCO3 or Zn(OH)2 for neutralization. However, the hydrodynamic radius of Zn2+ (600 pm) is larger in comparison to K+ (300 pm) and NH4+ (250 pm).43 Even a small excess of NH4HCO3 leads to the formation of NH3 in the purification procedure and, therefore, the


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potassium-salt represents the preferred species. Because of the simple synthesis an expansion of the synthetic scale is easily feasible and offers the possibility to utilize 3 as charge-storage material on large scale. A brief calculation of the active material costs can be found in the supplementary information. Electrochemical characterization 3 was electrochemically characterized via cyclic voltammetry (CV) measurements and a reversible redox behavior was detected at 0.61 V vs. Ag/AgCl, revealing a peak split of 63 mV and a cathodic to anodic peak current ratio of ~1 (Figure 2). The peak potentials of both oxidation and re-reduction do not vary with increasing scan rate and remain at 0.64 V and 0.57 V. The peak current follows a strict linear trend with the square root of the scan rate, for both the oxidation and re-reduction reaction (Figure 2, inset), which is an indication of an electrochemically reversible redox-reaction as well as a diffusion controlled process. Also electro plating of Zn0 on the glassy carbon electrode was observed for potentials lower than –1.16 V and a subsequent dissolution at potentials above –1.08 V vs. Ag/AgCl. The special shape of the voltammogram is typical for the electroplating of zinc on a glassy carbon electrode.12, 34-35, 44


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Figure 2. Cyclic voltammetry measurements. 1 mM 3 in aqueous 0.05 M ZnCl2 and 0.05 M NH4Cl at varied scan rates in the range of 25 to 500 mV s-1, inset: peak current vs. square root of scan rate. This leads to a potential cell voltage of 1.69 V in a subsequent battery application. 3 was further investigated via rotating disc electrode (RDE) measurements with rotation rates in the range of 0 to 3600 rpm (Figure S3a). Well-defined voltammograms were measured at a concentration of 0.014 M 3 in 0.05 M ZnCl2 and 0.05 M NH4Cl. Levich-analysis revealed a diffusion-controlled behavior of 3 for limiting currents at 0.95 V vs. Ag/AgCl and a diffusion coefficient of 2.98×10-6 cm2 s-1 was calculated from the obtained Levich-plot (Figure S3b). This is in the typical range of small molecule organic charge storage materials.25 Subsequent Koutecký-Levich analysis yields mass-transport-independent currents. The electron rate constant k0 of 1.91×10-3 cm s-1 and a transfer coefficient α of 0.68, which is close to the value 0.5 of an ideal reversible redox reaction, were obtained by Tafel analysis (Figure S3c and d). In particular worth mentioning is the high electron rate constant, which is one order of magnitude higher in comparison to other TEMPO derivatives.19 This is awarded to the negatively charged sulfategroup; an attractive behavior between this and the positively polarized electrode is assumed as an


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explanation. Electron paramagnetic resonance (EPR) spectroscopy was performed to investigate the radical content of 3 and revealed a spin count of 1.9×1018 mg-1, which equals full activity. This value is lower in comparison to 1 with a theoretical spin count of 3.5×1021 mg-1 because 3 has a higher molar mass. Nevertheless, this reduced gravimetric spin count is overcompensated by the better solubility of 3 within the catholyte. The EPR spectrum is depicted in Figure S4. The viscosity of the electrolyte has an important influence on the overall system-efficiency as well as the charge carrier mobility and was investigated via rheological measurements with sheer rates >100 and temperatures in the range of 5 to 40 °C. This is the relevant range concerning flow battery applications. An aqueous solution of 1 M 3 reveals a viscosity of 1.6 mPa s at 25 °C, which is only slightly increased compared to pure water (1 mPa s, Figure S5). Battery Tests Pumped battery tests were performed under ambient atmosphere and room temperature with three different catholytes (Table 1). Despite the low reduction potential of Zn of –1.16 V vs. Ag/AgCl the presence of oxygen does not lead to any side reactions. Therefore, an expensive inertization of the battery is not required. Concentrated catholytes consisting of 0.6 M 3, 2 M ZnCl2 and 2 M NH4Cl (catholyte A) were applied in flow battery tests. Table 1. Overview of the catholyte compositions used in this study. Theoretical capacity / Ah L-1 Supporting Ratio electrolyte (3:Cl-) Theoretical energy densitya) / Wh L-1 2 M ZnCl2, 1:12 16.0 / 14.4 A 2 M NH4Cl 1 M ZnCl2, 1.0 M 3 1:3 26.7 / 24.0 B 1 M NH4Cl 2 M ZnCl2, 1.0 M 3 1:5 26.7 / 24.0 C 1 M NH4Cl a) energy density calculated as product of capacity and OCV (Figure S6) and referred to the overall electrolyte volume (total volume of catholyte and anolyte).

Catholyte

Cathode active material 0.6 M 3


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Well-defined flat charge/discharge plateaus were achieved, with a mean charging voltage of 1.7 V and a mean discharging voltage of 1.5 V at a current density of 20 mA cm-2 (Figure 3a). 70% of this moderate voltage drop can be attributed to the ohmic overpotential induced by the limited conductivity of the membrane, current collectors, the electrolyte and the graphite felts. Effect of the current rating The electrical performance was initially investigated with a catholyte having a capacity of 16.0 Ah L-1. A capacity utilization of more than 88% was observed for current densities up to 30 mA cm-2 with a maximum of 96% at 10 mA cm-2. A decline in the active material utilization was observed for higher current densities (Figure 3b). The maximal applicable current density with recognizable capacity utilization was 80 mA cm-2. This exceeds the typical range of 5 to 40 mA cm-2 of Zn/Br2 HFBs by a factor of two.12-14, 41, 45 In particular, the high concentration of NH4+ facilitated this high current rating,41 in combination with a low cell-resistance of 1.39 Ω (6.95 Ω cm2). The coulombic efficiency was always above 98% with a maximum of 100% at 60 mA cm-2. However, the voltage efficiency decreases linearly with increasing current density (52% at 80 mA cm-2). Consequently, the energy efficiency values develop in the same manner.


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0.0 600

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Figure 3. Battery performance. a), Exemplary charge/discharge curves at a current density of 20 mA cm-2, the cycling was conducted in the range of 1 and 2 V. b), Electrical performance: Capacity, coulombic, voltage and energy efficiency depending on the applied current density, catholyte A, value at 80 mA cm-2 determined in a separate battery experiment. c), Long-term charge/discharge experiment in a non-pumped cell, at a constant current of 3 mA cm-2 and a capacity of 0.92 Ah L-1(35 mM 3), inset charge/discharge potential curves vs. capacity, 500th and 1,000th cycle are superposed (see Figure S7-S9 for additional pumped long-term chargedischarge test). The battery cycling was conducted in the range of 0.8 V to 2.1 V. Despite the high upper voltage limit, no water splitting or formation of chlorine was observed. Furthermore, the high coulombic efficiencies indicate stable and reversible chemical reactions. The application of catholytes with a higher capacity was investigated initially with catholyte B, which revealed also


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good electrical performance characteristics but a precipitation of the oxidized species was observed during the battery cycling (see Figure S7a for electrical performance). Coulombic efficiencies were in the range of 96 to 100% and a material utilization of 60% was reached at a current density of 40 mA cm-2. In the charged state, 3 is a zwitterion (3+) and, thus, 3+ can act as counterion for 3. Hence, large aggregates can form and the active material precipitates as zwitterionic salt. This can be circumvented with an adequate amount of supporting electrolyte; chloride anions shield the oxoammonium cation from ion pairing with the sulfate moiety and the ionic surface groups of the cation exchange membrane. The precipitation of 3 was induced by the decreased ratio of active material 3 to Cl-, which was twelve for catholyte A and only three for catholyte B. As a consequence, the concentration of ZnCl2 was increased to 2 M, which leads to a ratio 3:Cl- of 1:5 (catholyte C). This surplus enabled a stable battery cycling and no precipitation of 3 was observed. High material utilizations were achieved with catholyte C for current densities of up to 20 mA cm-2 (Figure S7b). Furthermore, high energy densities of 20.4 Wh L-1 were reached, which are in the range of commercial all-vanadium redox-flow batteries,46-47 and are much higher in comparison to the highly considered anthraquinone organic/inorganic RFBs.20, 25 Long-term battery cycling A long-term charge/discharge experiment of a flow battery utilizing catholyte A was conducted in the voltage range of 0.9 to 2 V at a current of 40 mA cm-2. Constant capacity retention and coulombic efficiencies of 99.4% were achieved (Figure S8). The capacity increased slightly until it reached its maximum determined in the electrical performance measurement (~100 mAh at 40 mA cm-2). This is caused by a decreasing cell resistance during the battery cycling (Figure S14). The cycling of a battery comprising catholyte C showed a


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stable battery operation of an extended time period of 97.2 h (four days; Figure S7c). Similar results revealed a long-term charge/discharge experiment conducted in a non-pumped cell, which is ideally suited to study the chemical reversibility of TEMPO-derivatives and to investigate the tendency of irreversible side reactions. Therefore, a catholyte with a capacity of 0.92 Ah L-1 was utilized to facilitate a large number of charge/discharge cycles in a moderate period of time (212.1 h, nine days). A good long-term discharge capacity retention of 93.6% after 1,100 charge/discharge cycles (Figure 3c) was achieved. Only in the first 100 cycles a slight decrease of the discharge capacity was observed (see SI, section zinc deposit), but in the following 1,000 cycles a stable capacity retention was found and the coulombic efficiency reached 98.1% (see Figure S9 for voltage vs. time charge-discharge curves). The results of this experiment indicate a high chemical reversibility of the TEMPO-sulfate as well as a high selectivity of the cation exchange membrane against the cathode active material 3; no crossover of 3 into the anolyte was observed by eye and CV (Figure S10). A reference cell utilizing 4-HO-TEMPO as cathode active material featuring a capacity of 2 Ah L-1 revealed significantly worse results. A constant decrease of the capacity to 30% of its initial value was observed after 300 cycles, which can be ascribed to both crossover and self-oxidation (Figure S11). The flow battery experiments revealed excellent coulombic efficiencies, which is an indication for a high chemical reversibility of both 3+/3 and Zn2+/Zn0 in the applied electrolyte and a sufficient inhibition of dendrite growth. The utilized spacer between the anode graphite felt and the membrane, which was a simple twolayered cellulose cloth, facilitated these high coulombic efficiencies and allowed a stable battery cycling. Otherwise, an incipient short circuit would be indicated by reduced coulombic efficiencies and noise in the voltage curves. The combination of a fast flow rate (20 mL min-1) and a graphite felt electrode inhibited the typical tree-like dendrite growth and allowed to reduce


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the spacer thickness to a few µm. A detailed investigation of the zinc deposit is provided in the supporting information (Figure S12 and Figure S13). We demonstrated a notably advanced small molecule TEMPO/Zn hybrid-flow battery, which utilizes easily accessible, low-cost and robust cathode and anode active materials, in combination with a highly selective membrane. TEMPO-4-sulfate potassium salt 3 was synthesized with regard to the utilization in Zn hybrid-flow batteries. A straightforward purification procedure offers convenient upscaling possibilities and enables the application of 3 in large-scale low-cost flow batteries. Electrochemical investigations via cyclic voltammetry revealed a reversible redox reaction of 3+/3. Several battery tests were performed and confirmed the good redox properties of 3 and a high cell-voltage of 1.8 V. In comparison to the previously reported aqueous polymerbased TEMPO/Zn flow batteries, the capacity of the catholyte was raised considerably from 2.4 Ah L-1 to 26.7 Ah L-1.34 Moreover, the maximal applicable current density with notable capacity utilization was increased by the factor of 4 from 20 to 80 mA cm-2, which is higher compared to conventional laboratory Zn/Br2 hybrid-flow batteries.12-14, 41, 45 The achieved energy density of 20.4 Wh L-1 is also higher than in the best performing semi-organic flow batteries utilizing aqueous electrolytes,10,

20, 25

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flow batteries.46-47 A long-term battery cycling was possible for over 1,100 consecutive charge/discharge cycles with a discharge capacity retention of 94%. Further improvements can be performed by a reduction of the cell resistance as well as an optimization of the electrolyte, referring to an increase of the solubility of the oxidized TEMPO-derivative. Nevertheless, the reported small molecule TEMPO/zinc HFB is one of the most advanced aqueous semi-organic flow-batteries known so far.


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ASSOCIATED CONTENT Supporting Information including: Experimental section, materials, cost calculation, EIS; EPR, rheological measurements, OCV vs. SOC, additional electrical performance measurements, SEM images and EDX spectroscopy. AUTHOR INFORMATION [email protected], http://www.schubert-group.de. ACKNOWLEDGEMENTS The authors acknowledge the European Regional Development Fund (EFRE), the Thuringian Ministry for Economic Affairs, Science and Digital Society (TMWWdG), the Federal Ministry for Economic Affairs and Energy (BMWi), and the Central Innovation Programme for SMEs (ZIM). The SEM facilities of the Jena Center for Soft Matter (JCSM) were established with a grant from the German Research Council (DFG) and the European Fonds for Regional Development (EFRE). REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928-935. 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. Banham-Hall, D. D.; Taylor, G. A.; Smith, C. A.; Irving, M. R. Flow Batteries for Enhancing Wind Power Integration. IEEE Trans. Power Syst. 2012, 27, 1690-1697. Soloveichik, G. L. Flow Batteries: Current Status and Trends. Chem. Rev. 2015, 115, 11533-11558. Alotto, P.; Guarnieri, M.; Moro, F. Redox Flow Batteries for the Storage of Renewable Energy: A Review. Renew. Sust. Energ. Rev. 2014, 29, 325-335. Noack, J.; Roznyatovskaya, N.; Herr, T.; Fischer, P. The Chemistry of Redox-Flow Batteries. Angew. Chem. Int. Ed. 2015, 54, 9776-9809. Skyllas-Kazacos, M.; Cao, L.; Kazacos, M.; Kausar, N.; Mousa, A. Vanadium Electrolyte Studies for the Vanadium Redox Battery—a Review. ChemSusChem 2016, 9, 1521-1543. Skyllas-Kazacos, M.; Chakrabarti, M. H.; Hajimolana, S. A.; Mjalli, F. S.; Saleem, M. Progress in Flow Battery Research and Development. J. Electrochem. Soc. 2011, 158, R55-R79. Weber, A.; Mench, M.; Meyers, J.; Ross, P.; Gostick, J.; Liu, Q. Redox Flow Batteries: A Review. J. Appl. Electrochem. 2011, 41, 1137-1164. Winsberg, J.; Hagemann, T.; Janoschka, T.; Hager, M. D.; Schubert, U. S. Redox-Flow Batteries: From Metals to Organic Redox-Active Materials. Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201604925. Bradley, C. S. Secondary Battery. US 312802, 1885.


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Li, B.; Nie, Z.; Vijayakumar, M.; Li, G.; Liu, J.; Sprenkle, V.; Wang, W. Ambipolar Zinc-Polyiodide Electrolyte for a High-Energy Density Aqueous Redox Flow Battery. Nat Commun 2015, 6, 6303. Suresh, S.; Kesavan, T.; Munaiah, Y.; Arulraj, I.; Dheenadayalan, S.; Ragupathy, P. Zinc-Bromine Hybrid Flow Battery: Effect of Zinc Utilization and Performance Characteristics. RSC Adv. 2014, 4, 37947-37953. Lai, Q.; Zhang, H.; Li, X.; Zhang, L.; Cheng, Y. A Novel Single Flow Zinc–Bromine Battery with Improved Energy Density. J. Power Sources 2013, 235, 1-4. Cathro, K. J.; Cedzynska, K.; Constable, D. C.; Hoobin, P. M. Selection of Quaternary Ammonium Bromides for Use in Zinc/Bromine Cells. J. Power Sources 1986, 18, 349-370. Jeon, J.-D.; Yang, H. S.; Shim, J.; Kim, H. S.; Yang, J. H. Dual Function of Quaternary Ammonium in Zn/Br Redox Flow Battery: Capturing the Bromine and Lowering the Charge Transfer Resistance. Electrochim. Acta 2014, 127, 397-402. 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Aqueous 2,2,6,6-Tetramethylpiperidine-N-oxyl Catholytes for a High

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