Supporting Information. 13 Pages, 9 Figures. Mechanisms of Humic Acid Fouling on Capacitive and Insertion Electrodes for Electrochemical Desalination

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1 Supporting Information 13 Pages, 9 Figures Mechanisms of Humic Acid Fouling on Capacitive and Insertion Electrodes for Electrochemical Desalination Xitong Liu, 1 Jay F. Whitacre, 2,3,4 and Meagan S. Mauter 1,2,4* 1. Department of Civil & Environmental Engineering, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA, 15213, United States 2. Department of Engineering and Public Policy, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA, 15213, United States 3. Department of Material Science and Engineering, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA, 15213, United States 4. The Scott Institute for Energy Innovation, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA, 15213, United States *Author to Whom Correspondence Should be Addressed: M. S. Mauter: mauter@cmu.edu Supporting information includes the following sections and figures: Text S1: Calculation of diffusion coefficient of Na + in NMO. Text S2: Method for cleaning of HA-fouled NMO electrode using sodium dodecyl sulfate (SDS). Figure S1: FTIR spectra of fresh and HA-fouled electrodes. Figure S2: Adsorption of HA on electrodes in the absence and presence of electrical field. Figure S3: Cycling stability of carbon electrodes in the absence of HA. Figure S4: Influence of HA on cycling stability of Darco electrode under Argon sparging. Figure S5: Cyclic voltammetry (CV) of Darco electrode at 5 mm NaCl and ph 7, showing the potential of zero charge of Darco electrode before and after HA fouling. Figure S6: XRD patterns of NMO electrodes cycled in NaCl in the absence and presence of HA. Figure S7: Dissolution of NMO in the absence and presence of HA. Figure S8: EIS spectra of fresh and HAfouled NMO electrodes. Figure S9: Efficacy of SDS rinsing in restoring capacity of HA-fouled electrode. S1

2 Text S1: Calculation of diffusion coefficient of Na + in NMO using the Randles-Sevcik equation. The Randles-Sevcik equation describes the dependence of peak current on scan rate in the case of reversible redox reaction controlled by diffusion: 1 i! = ( ! )n!/! AD!!/! C! ν!/! where n is the number of electrons per species reaction (1 for the Mn 4+ /Mn 3+ redox couple), A is the electroactive area of the electrode (cm 2 ), D O is the apparent diffusion coefficient of Na + (cm 2 /s), and C! is the amount of Na + in unit volume of NMO particles (mol/cm 3 ). 2 The electroactive surface area of NMO is taken as the effective area of the ab plane since Na + diffusion takes place along the c direction, 3 and is one-third of the BET specific surface area of NMO (1.85 m 2 /g). 4 At the onset of the first Na + insertion peak shown in Figure 5a, the composition of NMO is approximately Na 0.22 MnO 2. 3, 5 Therefore, the amount of Na + in unit volume of NMO particles is mol/cm 3 = mol/cm 3 (note: mol/cm 3 is calculated from the molar weight of Na 0.22 MnO 2 which is 92 g/mol and the bulk density of NMO which is 4.1 g/cm 3 ). 6 S2

3 Text S2. Cleaning of HA-fouled NMO Electrode using sodium dodecyl sulfate (SDS). We tested the efficacy of surfactant rinsing in restoring the capacity loss of NMO after HA fouling. After NMO electrodes were cycled in 200 mm NaCl and 200 mg/l HA for 40 cycles, the HA-fouled electrode was rinsed with DI water, followed by immersion in a 30 mm SDS (ph 10) solution under agitation for 2 h. The SDS rinsed electrodes were gently rinsed with DI water and were subsequently placed in 200 mm NaCl for additional 40 CV cycles. The amount of HA adsorbed on NMO during CV cycling and desorbed during SDS rinsing was measured using UV-vis. S3

4 Figure S1. (A) FTIR spectrum of humic acid. FTIR spectra of fresh and HA-fouled (B) Darco, (C) YEC, and (D) NMO electrodes. S4

5 Figure S2. Comparison of HA adsorption on Darco, YEC, and NMO electrodes in the absence and presence of CV cycling normalized to mass of active materials (A) and BET surface area of two activated carbons (B). BET surface areas of Darco and YEC are 650 m 2 /g and m 2 /g, respectively. In panel B, the BET surface area of YEC was taken as 2500 m 2 /g. Potential windows for CV curves: 0 to +0.4 V for Darco and YEC; 0.15 to +0.7 V for NMO; total adsorption time 26 h. S5

6 Figure S3. Cycling stability of Darco and YEC electrodes in the absence of humic acid. [NaCl] = 200 mm, ph = 7.0 ± 0.5, cycling potential window: 0 to +0.4 V vs Ag/AgCl. S6

7 Figure S4. Cycling stability of Darco electrode in the absence and presence of 200 mg/l humic acid. [NaCl] = 200 mm, ph = 7.0 ± 0.5, cycling potential window: to V vs Ag/AgCl. Argon gas was constantly sparged into the electrolyte solution to minimize carbon electrode oxidization. Scan rate = 1 mv/s. Duration of experiment: 22 h. S7

8 Figure S5. Cyclic voltammetry (CV) of Darco electrode at 5 mm NaCl and ph 7 (ph maintained by 1 mm phosphate buffer) before and after HA adsorption. The electrolyte was constantly sparged with argon during CV measurements. S8

9 Figure S6. X-ray diffraction patterns for A) NMO powder, B) Ti mesh, C) NMO electrode on Ti mesh cycled in 200 mm NaCl, and D) NMO electrode on Ti mesh cycled in 200 mm NaCl and 200 mg/l HA. Scanning potential window is 0.15 to +0.7 V vs Ag/AgCl. A total of 40 CV cycles were performed. S9

10 Figure S7. Dissolution of NMO electrodes in 200 mm NaCl during CV cycles in the absence and presence of 200 mg/l HA. ph = 7.0 ± 0.5. Scanning potential window is 0.15 to +0.7 V vs Ag/AgCl. The duration of the dissolution experiment corresponds to 40 CV cycles. S10

11 Figure S8. (A) Nyquist and (B) Bode plots of fresh and HA-fouled NMO electrodes. EIS spectra of fresh electrodes were recorded in 200 mm NaCl at ph 7. EIS spectra of HA-fouled electrodes were recorded in 200 mm NaCl and 200 mg/l HA after 20-h stirring. All EIS spectra were recorded over the frequency range of 100 khz to 2 mhz at 0.4 V vs Ag/AgCl. (C). EIS spectra fitted to an equivalent circuit model comprising of electrolyte resistance (R1), charge transfer resistance (R2), a constant phase element representing interfacial impedance (Q2), and a Bisquert element representing anomalous diffusion (Mg3). S11

12 Figure S9. Efficacy of SDS (30 mm, ph 10) rinsing in recovering NMO capacity after being cycled in HA. (A) Square: Electrode was cycled in 200 mm NaCl for 40 cycles, rinsed with SDS, and cycled in 200 mm NaCl for additional 40 cycles. Circle: Electrode was cycled in 200 mm NaCl and 200 mg/l HA for 40 cycles, rinsed with SDS, and cycled in 200 mm NaCl for additional 40 cycles. SDS rinsing resulted in a 3% capacity loss for NMO electrodes. (B) Circle: reproduced from panel A. Triangle: Electrode was cycled in 200 mm NaCl and 200 mg/l HA for 40 cycles, rinsed with DI water, and cycled in 200 mm NaCl for additional 40 cycles. Compared to DI water rinsing, SDS rinsing did not help restore the capacity loss of NMO electrodes. S12

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