Supporting Information

Transcription

1 Supporting Information A Low-Potential Pyridinium Anolyte for Aqueous Redox Flow Batteries Christo S. Sevov, a,b Koen. endriks, a,b and Melanie S. Sanford* a,b a. Department of Chemistry, University of Michigan, 93 N University Ave., Ann Arbor, Michigan, 4819, United States b. Joint Center for Energy Storage Research (JCESR) Materials Compounds 1 + and 1 were synthesized as described previously. 1 Sodium chloride and potassium hydroxide were obtained from Thermo Fisher Scientific and used as received. Electron Paramagnetic Resonance (EPR) Spectroscopy EPR spectra were collected at 13 C using a Bruker EMX ESR Spectrometer with a nitrogen-cooled cryostat. The samples of 1 were prepared in a nitrogen-filled glovebox and loaded into quartz tubes (3 mm outer diameter) as 1 mm solutions in degassed (45 min N 2 sparge) aqueous KO or aqueous Cl. Nuclear Magnetic Resonance (NMR) Spectroscopy All manipulations were performed inside an N 2 filled glovebox unless otherwise noted. NMR spectra were obtained on a Varian VNMR 7 ( Mz for 1 ; Mz for 13 C). 1 and 13 C NMR chemical shifts are reported in parts per million (ppm) relative to TMS, with the residual solvent peak used as an internal reference. p Measurements p Measurements were performed with a Mettler Toledo SevenMulti p Meter. p measurements were performed directly on the aqueous solutions from the cycling experiments. Cyclic Voltammetry Cyclic voltammetry was performed with a Biologic VSP multichannel potentiostat/galvanostat using a three electrode cell, consisting of a glassy carbon disk working electrode (.7 cm 2, BASi), a Ag/AgCl quasi-reference electrode (BASi) with 1 M aqueous NaCl, and a platinum wire counter electrode (23 cm, ALS). The glassy carbon disk electrode was polished in air using aluminum oxide polishing paper (9 micron and.3 micron, Fiber Instrument) and water. CVs were acquired under an atmosphere of nitrogen and with degassed aqueous solutions All experiments were run at a scan rate of 1 mv/s in the degassed electrolyte solutions containing 1 mm 1 +. S1

2 Charge/discharge cycling Figure S1. Photographs of RFB showing the flow cell, reservoirs, pump, and tubing (left). Photograph of the -cell (right). -Cell Cycling: Cycling experiments were carried out under an atmosphere of nitrogen on the benchtop using Schlenk techniques. Bulk electrolysis charge/discharge was measured with a Gamry Interface 1E potentiostat/galvanostat in a custom glass -cell using reticulated vitreous carbon electrodes (1 ppi, Duocell).2 A Ag/AgCl quasi-reference electrode (BASi) with 1 M NaCl was used on the working side of the -cell. A porous glass frit (P5, Adams and Chittenden) was used as the separator. The electrolyte contained 1 mm active species and 1-2 M supporting electrolyte. Both chambers of the -cell were loaded with 5 ml of the electrolyte solution and were stirred continuously during cycling at a rate of 6 ma to a coulombic cutoff of 75% of the theoretical capacity (1. mah, 3C rate). Flow-Cell Cycling: Cycling under flow conditions was performed with a zero-gap flow cell2 comprised of graphite charge-collecting plates containing an interdigitated flow field in combination with two layers of non-woven carbon felt electrodes (Sicracet 29AA) on each side. PTFE gaskets were used to achieve ~2% compression of the felt. An anion exchange membrane received from Tokuyama-America separated the two half cells, and the exposed area of the membrane in the gasket window was used as the active area (2.55 cm2). No pretreatment of the membrane or electrodes was performed. The cell, reservoirs, and tubing were assembled and brought into a nitrogen-filled glovebox. The anolyte chamber was charged with 1+ (56 mg,.25 mmol) and the catholyte chamber was charged with K4Fe(CN)6 (92 mg,.25 mmol). The assembly was sealed and removed from the glovebox. On the benchtop, a degassed solution of 2 M aqueous KO (5 ml per chamber) was added via syringe to dissolve the solids. A peristaltic pump (ColeParmer) with Tygon-C and PFA tubing was used to circulate the electrolyte solutions through the cell at 1 ml/min. Galvanostatic charge/discharge cycling was performed using a BioLogic VSP galvanostat with a coulombic cutoff of 5.25 mah during charge (75% SOC) and a voltaic cutoff of +.3 V during discharge. S2

3 Additional Figures a. b. O O O NEt 2 CN P(O)(OC 3 ) 2 SO 2 C 3 O O CD 3 D 2 O 1 cycle + Aldol products irreversible Figure S2. (a) Evaluated anolyte compounds by CV. (b) -cell cycling of the acyl analogue of 1 +. Rapid and irreversible decomposition was observed with this pyridinium due to enolization of the acyl group and subsequent addition/condensation reactions. Cycling experiments in D 2 O resulted in >9% deuterium incorporation into the acyl group as determined by LC-MS. 1-1 NMR COSY Bz Bz Bz + 1:1 ratio observed N 1 equiv Cl Cl resonances of phenyl protons 4.5 O Ph (d) (c) (b) C 3(a) Bz chemical shift (d/c) 8. (b/c) (b/a) chemical shift S3

4 Figure S3. 1D and 2D NMR spectroscopy was performed on a sample of 1 (13 mm,.78 mmol) in degassed CD 3 CN (.6 ml) with added 12 M Cl (11 µl,.13 mmol). 1-1 NMR COSY is plotted with highlighted correlations for C NMR SQC Bz Bz Bz + 1:1 ratio observed N 1 equiv Cl Cl resonances of phenyl protons 1 : 6.6 ppm 13 C: 73. ppm O Ph C chemical shift chemical shift Figure S4. 1D and 2D NMR spectroscopy was performed on a sample of 1 (13 mm,.78 mmol) in degassed CD 3 CN (.6 ml) with added 12 M Cl (11 µl,.13 mmol) C NMR SQC spectrum is plotted, which highlights the short-range correlation between the C atom at the 4-position and the attached proton. S4

5 8 6 Before Cycling After Cycling Z'' (Ω cm 2 ) Z' (Ω cm 2 ) Figure S5. EIS spectrum of the RFB before and after cycling at % SOC. The high frequency intercept with the x-axis is attributed to the ohmic resistance of the electrodes, solution and membrane. The (partial) high frequency semicircle is attributed to a capacitance element (RC) originating from the porous carbon electrodes/solution interface, which is followed by a straight line in the mid/low frequency domain, originating from diffusion limited processes (Warburg element). It shows that before cycling a low resistance of 2.4 Ohm/cm 2 is measured, which is slightly increased after cycling to 6.8 Ohm/cm 2. The semicircles are of identical diameter before and after cycling, and the slope at low frequency does not change. These results suggest that resistance of the membrane is slightly increased over the course of the experiment (assuming electrode and solution resistance do not change significantly during this time). This increase is not sufficiently large to impede battery cycling. a. capacity (mah) charge discharge Anolyte O Ph 1 + Cl Catholyte K 4 [Fe(CN) 6 ] cycle number Figure S6. RFB cycling data to failure. Cell failure is attributed to a decomposition of the anolyte due to reaction of the radical 1 with adventitious oxygen during the course of cycling coulombic efficiency (%) S5

6 2 M KO 1 M KO NaCl catalytic current from proton reduction 1 mm 1 + in 2 M KO current (A x 1 4 ) V vs Ag/AgCl Figure S7. (top) CV of 1 + through the first redox event under the standard conditions (1 mm anolyte, 1 mv/s, GCE) in various aqueous electrolytes. (bottom) CV of 1 + through the first and second redox events under the standard conditions (1 mm anolyte, 1 mv/s, GCE) but with 2 M KO potential (V vs. Ag/AgCl) discharge charge cycle 1 cycle time (h) Figure S8. Initial cycling data (cycles 1-2) for the -cell studies on anolyte solutions of 1 mm 1 + and 2 M KO. References (1) Sevov, C. S.; ickey, D. P.; Cook, M. E.; Robinson, S. G.; Barnett, S.; Minteer, S. D.; Sigman, M. S.; Sanford, M. S., Physical Organic Approach to Persistent, Cyclable, Low-Potential Electrolytes for Flow Battery Applications. J. Am. Chem. Soc. 217, 139, (2) Milshtein, J. D.; Kaur, A. P.; Casselman, M. D.; Kowalski, J. A.; Modekrutti, S.; Zhang, P. L.; arsha Attanayake, N.; Elliott, C. F.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A., igh current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries. Energy Environ. Sci. 216, 9, S6