TEMPO/Phenazine Combi-Molecule: A RedoxActive Material for Symmetric Aqueous RedoxFlow Batteries Jan Winsberg,†,‡ Christian Stolze,†,‡ Simon Muench,†,‡ Ferenc Liedl,†,‡ Martin D. Hager,†,‡ and Ulrich S. Schubert*,†,‡ †
Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstrasse 10, 07743 Jena, Germany ‡ Center for Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany S Supporting Information *
ABSTRACT: The combination of 2,2,6,6-tetramethylpiperidinyl-N-oxyl and phenazine yields an organic redox-active material for redox-flow battery applications. This combined molecule (combi-molecule) features a redox voltage of 1.2 V and facilitates the utilization of aqueous electrolytes. It was synthesized from cost-efficient starting materials, electrochemically characterized and applied as charge-storage material in a symmetric aqueous redox-flow battery.
vanadium species dissolved in a sulfuric acid electrolyte.8,9 The expensive heavy metal and the highly corrosive electrolyte should be replaced by an organic charge-storage material in order to improve the environmental impact and to reduce the system capital costs.10−13 Several flow batteries utilizing all-organic or organic/inorganic redox-active charge-storage materials were reported recently,11,12,14−17 but only a few bipolar materials are known. These bipolar materials, such as nitronyl nitroxides,18 polythiophene,19 and boron-dipyrromethenes,20 feature a delocalized π-electron system and chemically reversible oxidation and reduction reactions. Unfortunately, the voltage between these two redox-reactions is often >1.6 V and, consequently, exceeds the water potential window; thus, an organic solvent is required. These solvents are accompanied by low-current ratings, elevated costs, and environmental drawbacks, which render them disadvantageous in comparison to aqueous electrolytes. A novel approach to obtain bipolar redoxactive materials within the potential window of water is the combination of a cathode active material (can be oxidized) and an anode active material (can be reduced) via covalent bonding. The advantage of this approach compared to the utilization of
R
edox-flow batteries (RFBs; see Figure 1) are considered to represent one of the best storage technologies for electrical energy that is obtained from renewable sources like wind power and solar energy.1−7 State of the art RFBs feature
Figure 1. Schematic representation of a symmetric aqueous RFB utilizing an organic bipolar combi-molecule as a charge-storage material. © XXXX American Chemical Society
Received: September 6, 2016 Accepted: October 10, 2016
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DOI: 10.1021/acsenergylett.6b00413 ACS Energy Lett. 2016, 1, 976−980
Letter
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Letter
ACS Energy Letters
Figure 2. Schematic representation of (a) the synthesis of compound 8 and (b) the redox mechanism of 8.
species is assumed as an explanation. This results in a theoretical cell voltage of 1.2 V for an aqueous TEMPO/phenazine battery. An ideal bipolar redox-active material features equal amounts of electron donating and electron accepting capabilities for the reduction and oxidation. Accordingly, an artificial combimolecule has to consist of two TEMPO subunits and one phenazine subunit. A well-suited derivative is dihydroxy phenazine functionalized with two hydroxy-TEMPO units via TEG linkers (compound 8, Figure 2a). The developed synthesis route is distinguished by low costs for the utilized raw materials and simple reaction management. The TEG linker was first tosylated to facilitate nucleophilic substitutions of TEMPO and phenazine alkoxides. 4-HOTEMPO was deprotonated with sodium hydride and added to an excess of TEG-ditosyl to yield 3. The dimethoxy phenazine can be synthesized via a Wohl−Aue reaction, and the crude product was afterward heated in hydrobromic acid to obtain dihydroxyphenazine 7. This compound can be easily deprotonated with potassium carbonate and etherified with 3, yielding the artificial bipolar target combi-molecule 8 (Figure 2).
two separate redox-active materials is that the capacity of the battery is not irreversibly affected by the crossover of the active material into the opposite half-cell. The original state of the electrolytes can be reconstituted by a rebalancing procedure like in vanadium redox flow batteries (VRFBs). An appropriate linker to combine both redox-active subunits promotes simultaneously solubility in water. Thus, a triethylene glycol (TEG) unit is the linker moiety of choice. 2,2,6,6-Tetramethylpiperidinyl-N-oxyl (TEMPO) was chosen as a cathode active material because it features a high oxidation potential at the upper potential window of water, fast reaction kinetics, and an electrochemical reversibility of the redox reaction in neutral aqueous media. Phenazine was not investigated in RFBs so far but is known to undergo chemically reversible reduction reactions.21 In contrast to TEMPO, the redox potential of phenazine varies strongly depending on the utilized electrolyte. In organic aprotic solvents, for example, acetonitrile, a two-electron reduction reaction occurs at −1.5 V vs AgCl/Ag; in aqueous media (pH 7), the reduction is shifted to −0.6 V vs AgCl/Ag and increases further by reduction of the pH value.21 An equilibrium reaction (protonation) of the reduced phenazine 977
DOI: 10.1021/acsenergylett.6b00413 ACS Energy Lett. 2016, 1, 976−980
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ACS Energy Letters
Figure 3. (a) Cyclic voltammogram of 8 in 0.1 M NaCl/H2O, at varying scan rate. (b) Electrical performance determined by the application of different current densities and the resulting capacities and efficiencies. (c) Long-term battery cycling of a pumped RFB, 10 mM 8 in 0.5 M NaCl/ H2O and 10 vol % diglyme, 1851 charge−discharge cycles at 4 mA cm−2; (inset) exemplary charge/discharge potential curve at 1 mA cm−2. (d) Polarity reversal experiment; voltage and capacities vs time at a current density of 1 mA cm−2.
× 1017 mg−1, which indicates full activity of the TEMPO subunits (Figure S3, SI). An aqueous electrolyte containing 10 mM 8, 0.5 M NaCl as the supporting electrolyte, as well as 10 vol % diglyme as an additive was applied both as the catholyte and anolyte in all RFB measurements. Diglyme was used to homogenize the electrolyte solution. Various current densities were applied in a pumped flow battery setup at a flow rate of 20 mL min−1 to investigate the electrical performance of the flow battery (Figure 3b). Applicable current densities were in the range of 1−5 mA cm−2, and 60% of the theoretical capacity was achieved in this experiment. The accomplished charge/discharge capacities are almost identical for all applied current densities. The cell resistance was just 0.7 Ω as determined by electrochemical impedance spectroscopy, which corresponds to a theoretical IR drop of just 3.5 mV at a current density of 1 mA cm−2 (Figure S4, SI). However, a voltage drop of ∼570 mV was observed between the charging and discharging plateau (Figure 3c, inset) and is attributed to the hindered redox reaction of the phenazine subunit, as previously indicated by CV measurements. Because of this, the energy efficiencies are below 50% so far. A long-term charge/discharge test at a current density of 4 mA cm−2 was performed subsequently with the same battery and revealed an excellent chemical reversibility of the redox reactions of 8 (Figure 3c). The battery was charged/discharged for over 1800 consecutive cycles, with Coulombic efficiencies of up to 98.3% and constant capacity retention. However, the capacity is reduced in comparison to the current rating experiment performed before. This behavior is attributed to the establishment of an equilibrium. The steps in the capacity originate from daily maintenance of the flow battery (see the SI for a detailed explanation). The capability of 8 as a bipolar redox-active material was confirmed in a pole reversal
Compound 8 was characterized electrochemically via cyclic voltammetry (CV) measurements in H2O with 0.1 M NaCl (Figure 3a). The electrochemically reversible oxidation of TEMPO to the oxammonium cation and the subsequent rereduction occur at a formal potential of 0.6 V vs AgCl/Ag with a peak split of 59 mV using a glassy carbon working electrode and a coiled platinum wire as the counter electrode. With increasing scan rate, the peak potentials do not vary, while the peak currents increased linearly with the square root of the scan rate (Figure S1a). In contrast to this, the quasi-reversible redox reaction of phenazine occurs at −0.6 V vs AgCl/Ag with a peak split of 246 mV at a scan speed of 50 mV s−1 under the same experimental conditions. This phenomenon was observed in aqueous media for several phenazine derivatives and may be attributed to limited redox kinetics.22 CV measurements with a continuous scan of both redox reactions reveal equal behavior, and therefore, a suitability of 8 as bipolar redox-active material is concluded (Figure S1b). Rotating disk electrode (RDE) measurements with rotation rates in the range of 0 to 3600 rpm displayed welldefined voltammograms at a concentration of 1.86 mM 8 in aqueous 0.1 M NaCl with 10 vol % diglyme. Levich analysis revealed a mass-transport-controlled behavior of 8 for limiting currents at 0.607 V vs AgCl/Ag, and a diffusion coefficient of 2.35 × 10−6 cm2 s−1 was calculated from the obtained Levich plot (Figure S2, SI). Subsequent Koutecký-Levich analysis yields mass-transport-independent currents. The standard rate constant, k0, of 3.99 × 10−3 cm s−1 and a transfer coefficient, α, of 0.6 were obtained by Tafel analysis. An investigation of the reduction reaction of the phenazine subunit was not possible because limiting currents were not detectable. Electron paramagnetic resonance spectroscopy revealed a specific radical content of 7.48 978
DOI: 10.1021/acsenergylett.6b00413 ACS Energy Lett. 2016, 1, 976−980
ACS Energy Letters experiment. After the 1800 charge/discharge cycles of the longterm test, the test cell was charged/discharged for an additional five cycles; subsequently, the polarity was reversed and the battery was again charged/discharged for five cycles (Figure 3d). The capacity was 2 mAh in both polarities. However, the capacity increased further with continued pole reversal and reached 3 mAh (70% activity) after 20 reversals (200 additional charge/ discharge cycles, 2000 in total, Figure S5a, SI; see Figure S7, SI, for a photograph of the RFB setup). Furthermore, the voltage profile of the charge/discharge curves did not vary with continued cycling/polarity reversal, which confirms again the stability and chemical reversibility of the utilized redox-active material 8 (Figure S5b, SI). This increase of the capacity (material utilization) is attributed to a rebalancing of the whole battery system by continued pole reversals. A continual change of the pH value was not observed in any of the battery experiments; therefore, the protonation of the reduced phenazine subunit by water and a shift of the pH value to basic conditions did not occur on great extent. We reported the synthesis and electrochemical characterization of the first bipolar organic charge-storage material for symmetric aqueous RFBs as well as the first time utilization of a phenazine derivative as an anode active material in RFB applications. The combi-molecule contains the redox-active subunits TEMPO and phenazine, covalently bound via TEG linkers. CV measurements revealed electrochemically reversible and quasi-reversible redox reactions for the TEMPO and phenazine subunits, respectively, and indicated a cell voltage of 1.2 V for battery application. The combi-molecule was applied in symmetric RFBs and exhibited excellent electrochemical stability. No capacity degradation was observed after over 1800 consecutive charge−discharge cycles with Coulombic efficiencies of up to 98.3%. However, a voltage drop between charging and discharging limits energy efficiency to 50%. Nevertheless, the combi-molecule revealed its capability as a bipolar redox-active material in subsequent multiple polarity reversals without capacity loss (with 70% capacity utilization, 200 additional charge−discharge cycles) and can be seen as a pioneer on the pathway to a bipolar organic alternative to all-vanadium RFBs. Further development focuses on an increase of the capacity and energy efficiency as well as a reduction of the synthetic efforts.
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ACKNOWLEDGMENTS
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REFERENCES
The authors acknowledge the European Regional Development Fund (EFRE), the Thuringian Ministry for Economic Affairs, Science and Digital Society (TMWWdG), Federal Ministry for Economic Affairs and Energy (BMWi), and the Central Innovation Programme for SMEs (ZIM).
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00413. Chemicals and materials, characterization techniques, syntheses, cyclic voltammetry evaluation, rotating disk electrode voltammograms and evaluation, electron spin paramagnetic resonance measurement, electrochemical impedance spectroscopy, polarity reversal experiment, and a photograph of the battery setup (PDF)
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The authors declare no competing financial interest. 979
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ACS Energy Letters State of Charge Diagnostics by FTIR. J. Mater. Chem. A 2016, 4, 5448− 5456. (19) Oh, S. H.; Lee, C. W.; Chun, D. H.; Jeon, J. D.; Shim, J.; Shin, K. H.; Yang, J. H. A Metal-Free and All-Organic Redox Flow Battery with Polythiophene as the Electroactive Species. J. Mater. Chem. A 2014, 2, 19994−19998. (20) Winsberg, J.; Hagemann, T.; Muench, S.; Friebe, C.; Häupler, B.; Janoschka, T.; Morgenstern, S.; Hager, M. D.; Schubert, U. S. Poly(Boron-Dipyrromethene) A Redox-Active Polymer Class for Polymer Redox-Flow Batteries. Chem. Mater. 2016, 28, 3401−3405. (21) Wang, R.; Okajima, T.; Kitamura, F.; Kawauchi, S.; Matsumoto, N.; Thiemann, T.; Mataka, S.; Ohsaka, T. Catalytic Reduction of O2 by Pyrazine Derivatives. J. Phys. Chem. A 2004, 108, 1891−1899. (22) Lin, K.; Gómez-Bombarelli, R.; Beh, E. S.; Tong, L.; Chen, Q.; Valle, A.; Aspuru-Guzik, A.; Aziz, M. J.; Gordon, R. G. A Redox-Flow Battery with an Alloxazine-Based Organic Electrolyte. Nat. Energy 2016, 1, 16102.
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