Development of Redox-Active Flow Electrodes for High-Performance

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Development of Redox-Active Flow-Electrodes for High-Performance Capacitive Deionization Jinxing Ma, Di He, Wangwang Tang, Peter Kovalsky, Calvin He, Changyong Zhang, and T. David Waite Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03424 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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Environmental Science & Technology

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Development of Redox-Active Flow-Electrodes for High-Performance

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Capacitive Deionization

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Jinxing Ma, Di He*, Wangwang Tang, Peter Kovalsky, Calvin He, Changyong Zhang and

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T. David Waite*

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School of Civil and Environmental Engineering, University of New South Wales, Sydney,

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NSW 2052, Australia

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Email addresses: [email protected] (Jinxing Ma); [email protected] (Di He);

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[email protected] (Wangwang Tang); [email protected] (Peter Kovalsky);

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[email protected] (Calvin He); [email protected] (Changyong Zhang);

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[email protected] (T. David Waite)

11 12 13 14


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(Re-submitted October 2016)

16 17 18 19 20 21

1 ACS Paragon Plus Environment


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ABSTRACT

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An innovative flow-electrode comprising redox-active quinones to enhance the effectiveness

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of water desalination using flow-electrode capacitive deionization (FCDI) is described in this

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study. Results show that, in addition to carbon particle contact, the presence of the aqueous

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hydroquinone (H2Q)/benzoquinone (Q) couple in a flowing suspension of carbon particles

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enhances charge transfer significantly as a result of reversible redox reactions of H2Q/Q. Ion

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migration through the micropores of the flow-electrodes was facilitated in particular with the

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desalination rate significantly enhanced. The cycling behavior of the quinoid mediators in the

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anode flow-electrode demonstrated relatively high stability at the low pH induced, suggesting

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that the mediator would be suitable for long-term operation.

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INTRODUCTION

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Provision of sufficient water of potable quality is generally acknowledged as one of the

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key challenges of the 21st century.1 Given the progressive salt water ingress to subsurface

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aquifers that occurs as a result of excessive groundwater extraction, economically viable

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technologies for the desalination of brackish groundwaters are required. In addition to the

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traditional desalination technologies such as reverse osmosis and multistage flash distillation,

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capacitive deionization (CDI) is an innovative alternative that is gradually gaining in

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popularity as a result of its relatively low energy consumption (0.5-1.5 kWh m−3), cost

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effectiveness and, if designed and operated appropriately, ease of maintenance.2-5

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In a CDI cell, the electrical charging of a pair of porous electrodes results in the

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capacitive storage of counterions in the electrical double layers (EDLs) at the

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electrode-solution interface with the salt electrosorption capacity related to the electrode

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surface area.3 For traditional solid CDI electrodes, total capacitance is largely constrained by

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the longitudinal dimensions of the electrodes as the electrode thickness has a very strong

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influence on both the salt electrosorption and charge transfer rate.6, 7 As a consequence, Jeon

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et al. pioneered the use of aqueous suspensions of carbon particles flowing through a flow

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channel carved on the current collector to improve the performance of CDI and termed this

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process flow-electrode capacitive deionization (FCDI).4 Recent studies suggest that FCDI

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systems not only exhibit excellent salt removal capacity (>20 mg NaCl g−1) in contrast to that

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achievable using traditional solid CDI electrodes (normally, 1-15 mg NaCl g−1), but also

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allow simultaneous desalination and concentration of a salt water stream combined with

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continuous regeneration of the electrodes.3, 8-12



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Despite these promising advantages, FCDI systems suffer from inefficient charge

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transfer between the current collectors and flow-electrodes as a result of the relatively low

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conductivity of flow electrodes compared to that of the solid electrodes. The effect of this low

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conductivity is that the carbon particles at distance from the current collector will be charged

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to a lower potential, with the potential drop inducing a further impact on the rate performance

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of the flow-electrodes under transient conditions, consequently reducing the carbon material

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utilization for ion adsorption.13 In addition to increasing the carbon content in the

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flow-electrodes to facilitate electron transport,8 recent innovation indicates that using an

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aqueous electrolyte with a high salt concentration (2.44 wt %) could be another approach to

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reduce the internal resistance of the electrode chambers.14 While enhancement in rate of ion

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adsorption by addition of salts is a possibility, concerns remain with regard to (i) the

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occurrence of significant concentration polarization at the membrane interface, especially

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when the feed solution has relatively low salinity and (ii) aggregation of particles at high salt

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concentration leading to their precipitation from suspension.13

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Consideration is given in this work to an alternative route of enhancing electron flow

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involving addition of an electron mediator, i.e., hydroquinone, to the flow-electrode with the

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possibility that the mediator may act as an electron shuttle as a result of the facile redox

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transformation

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electrode-electrolyte interface. While it has been reported that the addition of multi-electron

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organic redox molecules can increase the capacitance of carbon electrodes through

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pseudocapative effects,15, 16 there is no study available investigating the relevant impacts on

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desalination in a FCDI system. The purpose of this study is, therefore, to investigate the

between

hydroquinone

(H2Q)

and

benzoquinone


(Q)

at

the

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efficacy of use of aqueous H2Q/Q in facilitating both salt electrosorption and desorption

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processes with particular attention given to the role of Faradaic reactions involving the

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H2Q/Q couple in charge transfer. Consideration is also given to evaluating the long-term

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performance of the redox-active flow-electrodes.

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MATERIALS AND METHODS

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Experiment Setup. The FCDI cell used in this study (Figure 1a) consisted of a spacer

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made of a nylon sheet (thickness ~200 µm, 150 mm × 80 mm), two ion-exchange membranes

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(GE normal grade cation exchange membrane CR67HMR and anion exchange membrane

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AR204SZRA, each of thickness ~600 µm) and two graphite current collectors (150 mm × 80

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mm × 5 mm) with carved serpentine flow channels that were, individually, 3 mm wide, 3 mm

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deep and 830 mm long from the inlet to the outlet, resulting in an effective contact area

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between the ion-exchange membrane and the flow-electrode of 24.9 cm2. These components

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were held together with the use of perspex end plates.

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The redox-active flow-electrode was prepared by addition of 14 mM H2Q to a 1 wt%

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dispersion of 100-mesh DARCO® activated charcoal in Milli-Q water (the activated carbon

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suspension with no added H2Q will be referred to here as the “blank” flow electrode). As can

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be seen from the isotherm for adsorption of H2Q to activated charcoal shown in Figure S1 of

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Supplementary Information, this resulted in approximately 1 mM H2Q remaining in solution.

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It should be noted that the presence of aqueous H2Q did not significantly change the electrical

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conductivity of the supernatant (increasing by only 1.6 µS cm−1). To elucidate the particular

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functions of the quinone redox couple in the anode (or cathode) flow-electrode, comparative

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trials were conducted with H2Q added (together with the activated carbon suspension) to one



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of the electrodes (anode or cathode) while the other electrode (the “blank” flow-electrode)

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contained the activated carbon suspension only. Note that, in this study, the “anode” is the

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electrode that is positively charged during the charging process while the “cathode” is the

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electrode that is negatively charged during the charging process. All FCDI experiments were

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carried out in batch mode with the brackish stream requiring desalination (50 mL of a 2 g L−1

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NaCl solution) continuously cycled (using a peristaltic pump) through the spacer from a

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storage flask at a flow rate of 30 mL min−1.9 The electrical conductivity of this stream was

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continuously monitored using a conductivity meter (CON-BTA, Vernier, U.S.) connected to a

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data acquisition system (SensorDAQ, Vernier, U.S.). Flow-electrodes (100 mL each) were

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recirculated between the FCDI cell and two stirred conical flasks respectively using a

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peristaltic pump at a constant flow rate of 100 mL min−1. In one operation cycle,

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electrosorption was conducted at a constant charging voltage of 1.2 V using a DC power

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supply (MP3094, Powertech, Australia) followed immediately by reversed-voltage desorption

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(RVD).3 Given that the FCDI system is capable of generating a continuous stream of

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desalinated water with circulation of carbon slurries between two electrode compartments,9, 10

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consideration was therefore given to the effect of H2Q on the discharging process. For the

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ease of quantification of the transformation of H2Q in the experiments undertaken here, the

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adsorption and desorption processes were examined in individual electrode modules operated

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in conventional CDI operation mode. After regeneration of the electrodes, the polarity of the

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electrodes was reversed in order to initiate the next cycle (in which anions were again

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adsorbed to the activated carbon particles adjacent to the anode and cations were adsorbed to

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the activated carbon particles adjacent to the cathode).


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Analytical methods. Scanning electron microscope (SEM) images of the activated

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charcoal samples were obtained on an FEI Quanta 200 ESEM. Textural properties, such as

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BET surface area, pore size and pore volume, were determined from the adsorption and

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desorption isotherms of nitrogen on a Micrometric Tristar 3000 adsorption analyzer. The

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concentrations of hydroquinone remaining and benzoquinone formed on the oxidation of

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hydroquinone were determined spectrophotometrically by measuring the UV absorbance at

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289 and 247 nm using a Cary 60 spectrophotometer with baseline correction at 600 nm,

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respectively.17 Alternating current impedance spectroscopy was used to assess the internal

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resistance of the FCDI system using a potentiostat/galvanostat (CHI 650D). The impedance

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spectra were taken at the charging voltage (1.2 V) using a 5 mV amplitude at frequencies

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ranging from 104 Hz to 0.01 Hz with the anode of the FCDI cell used as the working

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electrode and the cathode as the counter and reference electrode respectively.

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The average salt removal rate (ASRR, µg cm−2 s−1)12 of the cell was calculated as follows (Eq. 1): ASRR=

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( C 0 − C t ) Vs At

(1)

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where C0 and Ct are the initial NaCl concentration and NaCl concentration at time t (g L−1),

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respectively. Vs, A and t represent the total volume of the salt solution (50 mL), the effective

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contact area between the ion-exchange membrane and the flow-electrode (24.9 cm2) and

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operation time (s), respectively.

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The contribution of the quinoid couple to charge transfer was evaluated by calculating a

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dynamic charge efficiency (Λdyn) based around the approach described by Zhao et al.18 As can

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be seen from Figure 1b, the measured current (IQ) in the FCDI cell consisting of the 7 ACS Paragon Plus Environment


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redox-active (i.e., hydroquinone-containing) flow anode and blank flow cathode should be

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comprised of (i) the charge transfer between activated carbon particles and current collector

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per unit time (ΛbIQ), (ii) the current response attributed to the redox transformation of H2Q/Q

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(ΛredIQ), (iii) the quinoid couple-mediated charge transfer between the carbon particles and

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current collector per unit time (ΛcIQ) and (iv) current leakage (ΛlIQ) possibly due to

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unidentified side Faradaic reactions. Specifically, ΛbIQ indicates the conventional charge

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transfer for desalination in FCDIs via the direct and/or indirect contact of activated carbon

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particles with the current collector, with this value determined from the desalination rate in

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the control experiment in which blank flow-electrodes were used as both anode and cathode.

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ΛredIQ was calculated from the temporal variation of the concentrations of H2Q and Q (in

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moles per second). Λc represents the contribution of the quinoid couple-mediated charge

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transfer per unit time to the measured current in the FCDI cell using redox-active

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flow-electrodes, with ΛcIQ estimated from the improvement in desalination rate in the

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presence compared to the absence of H2Q. ΛlIQ was estimated from the remainder by

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subtracting the above-mentioned values (ΛbIQ, ΛredIQ and ΛcIQ) from the total current IQ.

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Moreover, the quantitative energy requirements (E, kT per ion removed) of the blank and

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redox-active FCDI were calculated as follows (Eq. 2):18 E=

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∫I

Vdt

Q

2 × ( C0 − Ct ) Vs × RT 58.5

(2)

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where V is the charging voltage (1.2 V), 2 indicates the energy requirement to remove an ion

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rather than a salt molecule, 58.5 is the molar mass (g mol−1) and RT (= 2.48 kJ mol−1 at room

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temperature) is a factor resulting in the energy in the unit of “kT per ion removed”.


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RESULTS AND DISCUSSION

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Effect of H2Q on deionization. The effect of addition of H2Q to both electrode

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compartments on the performance of the FCDI cell is shown in Figure 2a. It can be observed

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that following 1.2 V charging, more rapid salt removal occurred in the redox-active FCDI

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containing H2Q compared to that obtained when using blank flow-electrodes. Specifically,

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the ASRR in the charging process was increased by ~131% as a result of the incorporation of

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H2Q, with a similar positive effect found during discharging on polarity reversal (Figure 2a).

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These results are in agreement with previous studies of supercapacitors in which it has been

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found that the use of multi-electron organic redox molecules (such as H2Q) can increase the

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capacitance of carbon electrodes through pseudo-capacitive effects.15 By prolonging the

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electrosorption stage to 5 h, the salt concentration could be reduced to 400 mg L−1 though the

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system still did not reach equilibrium (Figure S2). A plausible explanation for the ongoing

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salt removal even after 5 h of adsorption could be that the activated carbon particles that are

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being recycled through the FCDI stacks and recycling tanks might undergo slow discharge

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when leaving the flow channel. There also exists a possibility that non-ideal membrane

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behavior could lead to ions spontaneously moving from the feed solution chamber towards

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the flow-electrode compartments when the blank and redox-active flow-electrodes were

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prepared with Milli-Q. However, preliminary experiments indicated that this spontaneous

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transport only accounted for ~1.5% of the salt removal in the control FCDI using blank

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flow-electrodes during charging at 1.2 V. While use of an aqueous electrolyte containing

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higher salt concentrations (1000 and 2000 mg L−1) in the flow-electrode compartments can



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improve the desalination performance (Figure S3) due to the decrease in the FCDI internal

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resistance,14 this approach appears to be relatively inefficient considering that the ASRR

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increased by only 21.9% and 19.3% (at 1000 and 2000 mg L−1 NaCl, respectively) compared

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to the blank flow-electrode FCDI (Figure S3).

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It has been reported in recent studies of supercapacitors that the direct deposition of

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quinones on carbon spheres under high carbon loading (23 wt%) could enhance the

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performance of flowable electrodes for energy storage.15, 19 However, in this study, we find

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that the ASRR of the FCDI using flow-electrodes comprising 1 wt% activated charcoal is

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likely dependent on the concentrations of aqueous H2Q in the flow-electrodes rather than that

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adsorbed on the carbon surface (Figure 2b). This suggests the presence of H2Q in solution is

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critical to salt removal in redox-active FCDI. Although it is expected that adsorbed H2Q can

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induce pseudocapacitive effects that contribute to the enhancement of electron storage in

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supercapacitors,15, 20, 21 its inefficiency in FCDI may be related to the concomitant change of

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micropore size following the deposition of H2Q; according to the report of Yoon et al.,19 the

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coverage of H2Q within the pores increases significantly as the pore diameter decreases. As

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can be seen from Figure S4 and Table S1, although the overall morphology of the activated

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charcoal did not change significantly, the BET surface area, average pore size and total pore

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volume of pores with

Development of Redox-Active Flow Electrodes for High-Performance

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