Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A. This journal is © The Royal Society of Chemistry 2017 Supplementary information for: Differential pH as a method for increasing cell potential in organic aqueous flow batteries Amirreza Khataeea, Kristina Wedegea, Emil Draževića, and Anders Bentien*a aHangøvej 2, 8200 Aarhus (Denmark), Department of Engineering, Aarhus University. *Corresponding author. E-mail: [email protected] Table of Contents S1: Redox flow battery setup .............................................................................................................. 2 S2: Conductivity measurements .......................................................................................................... 3 S3: Aqueous disproportionation reactions of bromine ........................................................................... 3 S4: RDE studies of Na2AQDS at pH = 0,7 and 8 ................................................................................. 5 S5: pH titration and NMR spectra ........................................................................................................ 9 S6: Experimental capacity as function of current density .................................................................... 14 S7: Low concentration battery cycling ............................................................................................... 15 S8: Calculating cell potential of the Na2AQDS/Bromine battery .......................................................... 15 S9: pH variation on negative and positive side of battery ..................................................................... 16 References ....................................................................................................................................... 19 1 S1: Redox flow battery setup The RF cell charging/discharging experiments were conducted in a single flow (Fig. S1a). The RF cell is a homemade cell and consists of carbon paper electrodes, end plates, current collectors and a cation exchange membrane. In order to facilitate current collection, solid gold plated copper plates were employed. Graphite plates with interdigitated flow field were used as bipolar plates. Carbon papers were used as electrodes on both sides of the cell. Nafion-117 was adopted as a cation exchange membrane and Viton and Teflon gaskets were used for sealing the cell assembly. To assemble the cell, current collectors were screwed together by eight bolts with bipolar plates, electrodes and gaskets in between. For circulating the electrolytes through the cell, two Grundfos DDA pumps with Teflon tubing were utilized. The solutions were purged with nitrogen before use. Charge/discharge studies were carried out under constant current densities with upper limit of 1.6 V during charge and lower limit of 0.2 V during discharge, respectively. A battery testing system (BTS-5V3A) was used for charging and discharging the cell and data collecting. a b Fig. S1 a) Cell used for battery tests; b) Photo of the experimental setup of the Na2AQDS/Bromide redox flow. 2 S2: Conductivity measurements Concentration of Conductivity (mS Concentration of Conductivity (mS -1 -1 NH4Br (M) cm ) H2SO4 (M) cm ) 0.5 55 0.5 278 1 108 1 506 2 206 2 686 3 295 3 787 4 367 4 802 5 433 5 780 6 499 6 581 S3: Aqueous disproportionation reactions of bromine - - - - Bromine disproportionates in water giving the species Br2, Br , Br3 , HBrO, BrO , HBrO3, and BrO3 . The most important equilibria are1: Disproportionation to hypobromous acid (pKa = 8.69 [2]): + − Br2 + H2O ⇌ HBrO + H + Br −9 [3] Keq = 7.2 ∙ 10 3 Hypobromous acid further disproportionate to bromate (bromic acid pkA = -2 [2]): − − − 3BrO ⇌ 2Br + BrO3 15 [3] Keq = 10 Tribromide arises when bromide and bromide is present, and this equilibrium is responsible for the large solubility of bromine: − − Br + Br2 ⇌ Br3 4 Keq = 16 In a 0.3 M bromine-water solution, if all equilibria are taken into account, the pH of the solution should be 2.665 which fits well with the measured 2.69. From these the sum of inactivated bromine (in terms of the battery conditions) compounds can be calculated5. − − pH = −log([HBrO] + 2[BrO ]+ 5[HBrO3] + 6[BrO3 ]) = −log (∑[Disproportionation products]) → ∑[Disproportionation products] = 0.002 M The addition of 2 M NH4Br to the 0.3 M bromine-water solution a) causes the concentration of tribromide to increase, b) drives the equilibria away from bromate and hypobromous acid and as a consequence, the pH increases to 2.83. Before the battery test in Figure 6 of the main paper, the pH of the catholyte was decreased to 1.5 with HBr as a starting condition, and during 100 cycles the pH increases to 2.1. Calculating the sum of the concentration of disproportionation products from this gives: ∑[Disproportionation products] = 0.008 M So, there is a constant amount of disproportionation products in the catholyte, but it is too small (1.6% of 0.5 M Br2) to solely explain the capacity loss in the battery which was 10% over 200 cycles. 4 S4: RDE studies of Na2AQDS at pH 0, 7 and 8 In order to compare diffusion coefficient and kinetics constant of Na2AQDS at pH 0, 7 and 8, linear sweep voltammetry (LSV) study of 1 mM Na2AQDS in 1 M H2SO4, 2 M NH4Br and 2 M NH4Br&0.05 M sodium carbonate was carried out at room temperature using a rotating disk electrode system as described in the main paper. LSV measurements were performed at constant potential sweep rate of 10 mV s −1 using a freshly polished 5 mm diameter glassy carbon disk electrode as a working (rotating) electrode, a platinum-wire counter electrode and Ag/AgCl reference electrode. The voltage was linearly swept while the rotating electrode was rotated at 200, 300, 400, 500, 700, 900, 1200, 1600, 2000 and 2500 r.p.m. -1 Fig. S4.I (a) Linear sweep voltammograms of 1 mM 푁푎2퐴푄퐷푆 in 1 M H2SO4 at scan rate of 10 mVs at pH=0 collected with the rotating glassy carbon disk electrode; (b) Levich plot using RDE – Variation of limiting current with square root of rotation rates for , 1/2 -1/2 -6 2 -1 푁푎2퐴푄퐷푆 the fitted line has a slope of 11 µA s rad , giving D= 3.48×10 cm s ; (c) Koutecky´–Levich plot using RDE and derived from six different Na2AQDS potentials; (d) Fit of Butler–Volmer equation. iK is the current extrapolated from the zero- intercept of Fig. S4.I(c). Best-fit line has the equation y = 69.25 (x + 3.95). This yields ɑ = 0.431 and k0= 5.8 × 10-3 cm s-1. 5 -1 Fig. S4.II (a) Linear sweep voltammograms of 1 mM 푁푎2퐴푄퐷푆 S in 2 M NH4Br at scan rate of 10 mVs at pH=7 collected with the rotating glassy carbon disk electrode; (b) Levich plot using RDE – Variation of limiting current with square root of rotation , 1/2 -1/2 -6 2 -1 rates for 푁푎2퐴푄퐷푆 the fitted line has a slope of 16 µA s rad , giving D= 6.49×10 cm s ; (c) Koutecky´–Levich plot using RDE and derived from six different Na2AQDS potentials; (d) Fit of Butler–Volmer equation. iK is the current extrapolated from the zero-intercept of Fig. S4.II(c). Best-fit line has the equation y = 69.29 (x + 3.36). This yields ɑ = 0.426 and k0= 2.1 × 10-2 cm s-1. 6 Fig. S4.III (a) Linear sweep voltammograms of 1 mM 푁푎2퐴푄퐷푆 S in 2 M NH4Br and 0.05 M Na2CO3 at scan rate of 10 mVs-1 at pH=8 collected with the rotating glassy carbon disk electrode; (b) Levich plot using RDE – Variation of limiting 1/2 -1/2 -6 current with square root of rotation rates for 푁푎2퐴푄퐷푆, the fitted line has a slope of 18.4 µA s rad , giving D= 7.5×10 2 -1 cm s ; (c) Koutecky´–Levich plot using RDE and derived from six different Na2AQDS potentials; (d) Fit of Butler–Volmer equation. iK is the current extrapolated from the zero-intercept of Fig. S4.III(c). Best-fit line has the equation y = 28.5 (x + 3.2). This yields ɑ = 0.405 and k0= 7.1 × 10-2 cm s-1. The number of electrons transferred during AQDS reduction was measured versus 1-eletron transfer 3- reference solution of 1 mM Fe(CN)6 as can be seen in Fig. S4.IV. The background capacitive current was evaluated by 3rd degree polynomial fitting as indicated with red dotted lined and subtracted the current before integration to give the apparent capacity in mC (blue numbers). More precisely, Fig. S4.VI shows that 1.44 electrons (72% of theoretical value which is 2 electrons) are transferred per AQDS. The 2 -1 -6 3- diffusion coefficients are similar (6.49×10-6 cm s for AQDS and 7.6×10 for Fe(CN)6 so the integrated areas provide a reasonable comparison of the electron/charge relationship. In order to confirm that the current densities at pH = 7 and 8 are higher than pH = 0, a comparison of LSVs at 3 different rotation rates is indicated in Fig. S4.V. 7 Fig. S4.IV Cyclic voltammograms at 25 mVs-1 on a glassy carbon electrode of 1 3- mM of Na2AQDS in 2 M NH4Br (left CV) and 1 mM Fe(CN)6 (right CV). The red dotted lines determine the background currents subtracted the curents before integration to give the apparent capacity (blue numbers) of reach oxidation or reduction process. Fig. S4.V a) Linear sweep voltammograms of 1 mM 푁푎2퐴푄퐷푆 in 1 M H2SO4, 2 M NH4Br and 2 M NH4Br/0.05 M -1 Na2CO3 at scan rate of 10 mVs and rotation rates of 400, 900 and 2500 rpm at pH = 0, 7 and 8, respectively. Collect ed with the rotating glassy carbon disk electrode. 8 RDE tests indicate that high at high pH values, Na2AQDS has higher standard heterogeneous electron transfer rate constant (k0). Another test was conducted to prove the results of RDE experiments. Therefore, to corroborate the enhanced reduction properties of Na2AQDS at pH 8 than in acidic, a CV peak separation analysis were made using 30 mM solutions in 1 M H2SO4 or 2 M NH4Br. The CVs were recorded at scan rates from 50-1000 mV/s on a 3 mm glassy carbon electrode polished identically with 0.05 μm alumina on a micro cloth.