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Characterizing the Impacts of Deposition Techniques on the Performance of MnO Cathodes for Sodium Electrosorption in Hybrid Capacitive Deionization 2

Steven Hand, and Roland D. Cusick Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03060 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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Environmental Science & Technology

Characterizing the Impacts of Deposition Techniques on the Performance of MnO2 Cathodes for Sodium Electrosorption in Hybrid Capacitive Deionization

Steven Hand1 and Roland D. Cusick1*

1

Department of Civil & Environmental Engineering

University of Illinois at Urbana-Champaign, Urbana, IL 61801-2352

*Corresponding author 3217 Newmark Civil Engineering Laboratory 205 North Mathews Avenue Urbana, IL 61801-2352 E-mail: [email protected]; Phone: +1 (217) 244-6727

ACS Paragon Plus Environment


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Abstract Capacitive deionization (CDI) is currently limited by poor ion-selectivity and low

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salt adsorption capacity of porous carbon electrodes. To enhance selectivity and

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capacity via sodium insertion reactions, carbon aerogel electrodes were modified by

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depositing amorphous manganese dioxide layers via cyclic voltammetry (CV) and

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electroless deposition (ED). MnO2-coated electrodes were evaluated in a hybrid

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capacitive deionization (HCDI) system to understand the relationship between oxide

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coating morphology, electrode capacitance, and sodium removal efficacy. Both

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deposition techniques increased electrode capacitance, but only ED electrodes

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improved desalination performance over bare aerogels. SEM imaging revealed ED

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deposition distributed MnO2 throughout the aerogel, while CV deposition created a

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discrete crust, indicating that CV electrodes were limited by diffusion. Sodium

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adsorption capacity of ED electrodes increased with MnO2 mass deposition, reaching a

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maximum of 0.77 mmol-Na+ per gram of cathode (2.29 mmol-Na+ g-MnO2-1), and peak

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charge efficiency of 0.95. The presence of MnO2 also positively shifted the electrode

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potential window of sodium removal, reducing parasitic oxygen reduction and inverting

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the desalination cycle so that energy discharge coincides with salt removal (1.96 kg-

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NaCl kWh-1). These results highlight the importance of deposition technique in

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improving desalination with MnO2-coated electrodes.


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1. Introduction Water stress has driven the desalination capacity of saline water sources, such

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as brackish aquifers, to increase six-fold in the past 20 years.1–4 While reverse osmosis

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is the primary method for brackish water desalination,4–7 activated carbon (AC)

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electrode capacitive deionization (CDI) is an attractive alternative technology due higher

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energy efficiency.6,8 In AC CDI, porous carbon electrodes are typically polarized at

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either constant voltage or constant current. The resultant electric field removes salt ions

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from solution and stores them within electric double layers (EDLs).9,10 However, the

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viability of AC CDI systems is limited by the low charge efficiency and salt adsorption

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capacity (SAC) of carbon electrodes.11,12 An additional limitation to efficient desalination

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with carbon electrodes are parasitic reactions, such as oxygen reduction, which occur

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as cathodes are polarized below 0 V vs. SHE.13–16 When the AC cathode is polarized

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below 0 V vs. SHE, dissolved oxygen in solution can be reduced to OH- via a two-step

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reaction (Equations 1 and 2). O + H O + 2e → HO + OH # 1 HO + H O + 2e → 3OH # 2

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When operated at low current density, parasitic reactions can significantly reduce

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charge efficiency, which increases desalination energy consumption.17

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CDI performance can be enhanced by modifying the chemical charge balance at

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the boundary between the electrode and flow channel to promoting counter-ion

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adsorption efficiency and prevent parasitic reactions. Membrane capacitive deionization

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(MCDI) exhibits increased charge efficiency because the inclusion of ion exchange

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membranes (IEMs) controls ionic flux out of the flow channel and prevents oxygen



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reduction leakage current,10,18 but IEMs incur a high capital cost ($100–200 m-2). Salt

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adsorption and charge efficiency can be enhanced without IEMs by fixing co-ions within

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carbon micropores to promote adsorption of counter ions.19–21 Parasitic reactions can be

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prevented by applying functional groups or oxides to the surface of the electrodes,22–24

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shifting the operating potential window away from oxygen reduction overpotentials. By

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reducing leakage current, the charge efficiency of the system can be increased. While

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these treatments improve charge efficiency, they do not significantly improve the Na+

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adsorption capacity of the electrodes.22–24

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An attractive alternative to MCDI and surface treated carbon CDI is the addition

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of redox active cation insertion materials, which shift electrode operating potentials

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while also increasing electro-adsorption capacity.25–27 Previous studies investigating

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redox cathodes for desalination have utilized bulk Na+-insertion materials such as

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sodium manganese oxide (NMO).28 Pairing NMO (Eqn. 3) with a capacitive AC anode

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as a hybrid capacitive deionization (HCDI) cell led to significantly improved SAC (0.54

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mmol g-1 vs. 0.23 mmol g-1 for MCDI).26 While yet unexplored, MnO2 (Eqn. 4) provides a

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low-cost material capable of storing Na+ in aqueous electrolytes at a higher

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stoichiometric capacitance than NMO (1:1 vs. 4:9).25,27,29–31 9MnO + 4Na + 4 ↔ Na Mn O # 3 MnO + Na + ↔ MnOONa # 4

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MnO2 has been shown to have a theoretical specific capacitance of over 1,300 F g-1 and

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observed bulk specific capacitances of over 290 F g-1 depending on the crystal

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structure.27,32–34 Comparatively, typical AC electrode specific capacitance in CDI is

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below 100 F g-1 when operated at brackish water concentrations.12,35,36


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Depositing thin layers of MnO2 onto carbon substrates may provide an efficient

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alternative to bulk phase sodium insertion electrodes. Thin layer deposition preserves

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the internal pore structure and conductivity of the carbon substrate while minimizing the

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Mn mass required to achieve increased capacity and cation selectivity. Amorphous

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MnO2 can be directly deposited onto a conductive carbon substrate by either oxidizing

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Mn2+ with cyclic voltammetry (CV)27 or through an electroless (ED)34,37 redox reaction

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between MnO4- and the carbon substrate. While both ED and CV coatings were found

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to increase electrode capacitance, neither technique has been evaluated for

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desalination of brackish water. High capacity flow-electrodes can be used to evaluate

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cathodes treatments without the capacity limitations of solid-state anodes.38,39 The

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objective of this study was to evaluate the efficacy of thin-film, MnO2 coatings on carbon

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aerogels (CA) to improve HCDI desalination in terms of charge efficiency, sodium

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adsorption capacity (NAC), rate of salt removal, and energy consumption. Links

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between deposition technique (CV and ED), mass deposition, resulting MnO2

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morphology, and performance were also explored to identify the most efficient use of

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sodium insertion MnO2.

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2. Materials and Methods

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2.1 Electrode fabrication

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MnO2 was deposited onto precursor carbon aerogels (Type I, Marketech

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International, USA) via CV and ED as previously described.27,34 CV deposited MnO2

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electrodes were fabricated in a standard three electrode setup (Pt wire counter

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electrode and Ag/AgCl reference) containing 0.1 M Na2SO4 and 0.1 M manganese (II)



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acetate (Mn(CH3COO)2). Using a potentiostat (Bio-logic VMP3, France), the CA voltage

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was swept from 0.3 V to 0.6 V vs Ag/AgCl, at a scan rate 0.25 mV s-1, to oxidize Mn(II)

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to Mn(IV) and deposit MnO2. Prior to CV deposition, precursor aerogels were vacuum

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infiltrated in the deposition electrolyte. After CV deposition, electrodes were dried

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overnight at 85 °C in air, in accordance with previous studies.27 Using the ED protocol

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developed by Fischer et al.,34 precursor aerogels were vacuum infiltrated with 0.1 M

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Na2SO4 then placed in a solution containing 0.1 M NaMnO4 and 0.1 M Na2SO4 under

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vacuum for 15, 60, 240, and 1440 minutes. MnO2 was deposited via reduction at the

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aerogel surface with the graphitic carbon serving as the electron donor. The electrodes

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were then rinsed with ultrapure water before drying under N2 at 50 °C for 8 h and then

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under vacuum for 12 h. After drying, total electrode mass was determined

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gravimetrically (Sartorius Entris 64i-1S, Germany).

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2.2 MnO2 Characterization To characterize the impact of deposition technique and exposure time on

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electrode composition, MnO2 aerogel content was determined via thermogravimetric

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analysis (TGA) (PerkinElmer Pyris 1, USA).40 Pore size distribution and volume for each

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electrode was measured using nitrogen adsorption at 77 K (Micrometrics ASAP 2020,

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USA).29 Differential pore volume was calculated using density functional theory (DFT).

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To investigate the resulting crystal structure of Mn-deposited electrodes, x-ray

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diffraction (XRD) was performed using a Cu Kα source (PANalytical / Philips X’pert

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MRD, The Netherlands).33 To characterize MnO2 deposit morphology and distribution,


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scanning electron microscopy was performed on a JEOL JSM-6060LV equipped with an

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Oxford Instruments ISIS energy-dispersive X-ray spectroscopy (EDS) system.

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2.3 Experimental setup and desalination experiments

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A flow-electrode, hybrid capacitive deionization (HCDI) cell was contracted with

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saline flow channel (L×W×H: 3 cm × 0.5 cm × 0.125 mm) sandwiched between a solid

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MnO2 deposited cathode and a carbon slurry flow-anode (Fig. 1). The MnO2 deposited

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cathode (L×W×H: 3 cm × 0.5 cm × 0.175 mm) was attached to graphite current

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collectors with a polyvinylidene difluoride (PVDF)/Carbon Black conductive binder slurry

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(10%/90% by weight). A flow-anode composed of 5% (w/w) powdered activated carbon

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(DARCO, Sigma-Aldrich) suspended in 100 mL of 0.1 M NaCl solution (5.26 g carbon +

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106 g solvent) was used to provide sufficient capacitance for the full characterization of

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MnO2 electrode NAC. The carbon flow anode was recirculated from a reservoir through

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a channel ( = 500 µm) carved into a graphite current collector at 1 mL min-1, and

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separated from the flow channel by an anion exchange membrane (AEM Type-I,

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Fujifilm, The Netherlands). The feed solution was set at 0.1M NaCl in order to evaluate

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desalination performance for brackish groundwater. The desalination flow channel feed

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solution was pumped through a polypropylene, mesh spacer ( = 125 µm) at 0.1 mL

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min-1 with a syringe pump (NE-1000, New Era, USA). The flow channel effluent

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conductivity was measured with an in-line conductivity flow cell (ED916, eDAQ,

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Australia). Effluent concentration was determined from a conductivity calibration curve

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for NaCl. A Ag/AgCl (3M NaCl) reference electode (BASi RE-5B, IN) was inserted into

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the NaCl influent line near the cathode current collector to measure and control



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electrode potential during desalination experiments. All desalination experiments were

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run in triplicate with data points taken from the last two cycles at each current density (6

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values per condition).

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The HCDI system was operated under constant current. In each cycle, the

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cathode was reduced at a fixed, negative current until a minimum potential limit (-0.1 V

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vs Ag/Cl) was reached. The direction of current was then reversed, and the cathode

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was oxidized until a maximum potential limit was reached (0.7 V vs Ag/Cl). For

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experiments measuring energy consumption during desalination, an aminated AC flow-

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anode was paired with MnO2 cathodes to increase the whole cell voltage window. The

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aminated carbon was prepared as previously described.41 Briefly, AC was mixed with

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nitric acid and refluxed at 373 K for 24 h. The resulting oxidized AC was then mixed 1:1

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by weight with n,n'-dicyclohexylcarbodiimide in ethylenediamine solvent for 48 h at 373

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K. The resulting aminated AC was rinsed with ethanol and dried under vacuum.

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2.4 Calculations Capacitance was measured from the slope of the cathode charging voltage

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profile at 15 A m-2 (Fig. S1). This operating current was selected for comparison

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between techniques because parasitic reactions distorted CDI electrode potential

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profiles at lower current densities (Fig. S2). The applied current was divided by the

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measured slope to determine cathode capacitance in farads. Measured capacitance

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was then normalized to both total cathode mass (carbon substrate + MnO2) and

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deposited mass of MnO2 (Table 1). To measure the contribution of MnO2, it was

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assumed that the total capacitance is divided between double-layer and faradaic


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storage corresponding to the mass fraction of carbon and MnO2, respectively.34,37 Salt

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adsorption was determined via integration of the effluent concentration profile for each

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charge half-cycle. The adsorbed salt mass was converted to moles of salt adsorbed

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which was assumed to be equivalent to moles of Na+ adsorbed based on 1:1 NaCl

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stoichiometry. Cathode NAC was normalized to total cathode mass as well as to MnO2

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mass to determine MnO2-specific NAC. Charge efficiency was calculated as the total

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charge passed normalized to the charge equivalents of salt removed during

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desalination.10 Energy-Normalized absorbed salt (ENAS) was calculated as the amount

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of salt removed during desalination normalized to the total charging half-cycle energy

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consumption.17

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3. Results and Discussion

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3.1 Electrode crystal structure and morphology

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While both ED and CV techniques were found to deposit MnO2 onto aerogel

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electrodes, the treatment techniques resulted significant morphological differences (Fig.

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2). Our characterization suggests that a discrete crust of MnO2 forms on the surface of

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CV electrodes, blocking the carbon signal below (Fig. 2g and h). ED electrodes exhibit

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greater penetration and distribution of MnO2 throughout the carbon aerogel electrode

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(Fig. 2e and f). ED electrodes displayed no observable difference in surface structure

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from bare carbon aerogels regardless of deposition time. Despite minimal visual

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differences, the elemental composition of ED electrodes revealed a high degree Mn/C

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heterogeneity, with MnO2 content increasing with deposition time (Fig. S3). Conversely,

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CV deposition initially produces an elementally homogeneous, dendritic crust of MnO2



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on the surface of the electrode (Figs. 2g and k), which transitioned to a crust composed

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of rounded, thicker structures as the number of deposition cycles increased from three

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to ten (Figs. 2h and l).

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XRD was used to understand the relationship between deposition technique and

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relative crystallinity.33 Both techniques deposited relatively amorphous MnO2 (Fig. S4).

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While peaks associated with α(m)-, β-, γ-, δ-, λ-MnO2 were observed in for all deposited

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electrodes (37° and 65°), none were sufficient to identify a distinct crystal structure.

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Additionally, the high crystallinity of the carbon substrate obfuscates potentially

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overlapping MnO2 peaks. For all depositions except ED1440, MnO2 deposition

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increased BET surface area and micropore volume (Table 1). The decrease in pore

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volume and surface area for the ED1440 electrode suggests that prolonged deposition

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fills or covers micropores. After deposition, there was a significant increase in pore

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volume at 7 Å (Fig. S4), which closely aligns with the interlayer distance of δ-MnO2.33

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However, the deposition solution compositions and measured capacitance suggest that

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both ED and CV coatings could be δ- or α(m)-MnO2.33 Extended deposition samples

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(i.e. ED240 and CV10) disrupted the prominent carbon peaks at 26° and 44° to a

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greater degree than lower MnO2 content samples (ED15 and CV3). Additionally, CV

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samples display a greater carbon peak disruption than ED electrodes with comparable

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MnO2 content, which aligns with the morphological differences observed with SEM-EDS

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(Fig. 2).

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3.2 Impacts of deposition on electrochemical capacitance


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Regardless of technique, increasing deposition time/cycles resulted in higher

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MnO2 content and electrode capacitance (Table 1). For both ED and CV deposition,

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electrode specific capacitance increased with MnO2, from 36.3 F g-1 in the base carbon

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to 101 F g-1 for ED1440. After subtracting the capacitance associated with double-layer

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charge storage, the maximum MnO2-associated capacitance ranged from 164–285 F g-

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MnO2-1 (Table 1). While below the theoretical 1,370 F g-1 expected in ideal thin-

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films,25,42 our maximum values are comparable to the 297 F g-1 measured by Devaraj

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and Munichandraiah33 for α(m)-MnO2, and well above ~150 F g-1 which is commonly

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reported for other bulk MnO2 crystal structures.25,32 ED and CV electrodes displayed

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specific capacitance values at comparable MnO2 content (ED15/CV3 and ED240/CV10,

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Table 1), indicating that the two techniques generated similar crystal structures that

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differed in morphology. Discharge capacity increased with deposition time for MnO2

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electrodes (Fig. S5), but rapidly declined with current density. However, at low current

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density (2–5 A m-2), the discharge capacity for CV electrodes did not increase as

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significantly as ED electrodes (Fig. S5).

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3.3 Impacts of deposition on desalination cycle behavior, Na+ adsorption capacity and

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rate

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At comparable mass contents, ED MnO2-coated electrodes displayed longer

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cycle times, greater adsorption capacity and removal rates. With increasing deposition

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time, dynamic steady state desalination cycles increased in length and depth of

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desalination for both ED and CV electrodes (Figs. 3 and S6). At ~7% MnO2, ED15

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electrodes have a peak desalination of 6.0 mM, compared to 1.5 mM for CV3



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electrodes. In comparison, uncoated aerogel displayed peak desalination of 4.2 mM. As

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the MnO2 content increases to ~22%, ED240 electrodes further improve over CA

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electrode with peak desalination of 8.6 mM, and CV10 electrodes approach CDI

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performance (4.1 mM desalination). In addition to increased peak desalination,

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increases in MnO2 content doubled the total cycle length for both ED and CV electrodes

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(Fig. 3). The higher removal of ED electrodes suggests that the elementally

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heterogeneous morphology associated with ED is not as limited by ionic or solid state

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diffusion as CV electrodes. ED electrodes displayed an elementally heterogeneous

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structure (Fig. 2e and f), which appears to have promote greater distribution of sodium

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adsorption selectivity throughout the electrode. Both CDI and ED240 electrodes rapidly

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shifted from peak desalination to peak brine generation than CV10 electrodes,

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indicating that preserving the aerogel macrostructure benefited desalination kinetics. In

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addition to limiting ion diffusion, the distinct MnO2 layer found on CV electrode may limit

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the internal conductivity of the cathode. This would result in a spatially uneven reduction

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of deposited MnO2, initially occurring nearest the current collector, which is furthest from

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the influent solution.

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Our findings suggest that coating aerogels with MnO2 is viable means to prevent

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parasitic cathode reactions. However, only ED electrodes displayed enhanced sodium

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removal capacity and rate. A useful means of visualizing desalination performance in

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terms of capacity and rate for different electrode depositions is through a Kim-Yoon

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(KY) diagram (Fig. 4a).43 All MnO2-coated electrodes exhibited at least a twofold

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increase in NAC when operated at 5 vs 10 A m-2 (Fig. 4b). However, when the CA cell

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was operated at 5 A m-2, there was no increase in NAC over operation at 10 A m-2 (Fig.


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4a). Poor performance at lower current densities for CA was because more time was

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spent at overpotentials necessary for oxygen reduction and other parasitic reactions

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(Fig. S2).17 It has been well documented that MnO2 is capable of catalyzing oxygen

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reduction reactions in alkaline and neutral media at potentials below 0.0 V vs. SHE,44,45

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however this is below the MnO2 operating range used in this study (-0.1–0.7 V vs.

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Ag/AgCl).

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Operating the HCDI cell with a carbon flow-anode enabled full characterization of

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the storage capacity of MnO2 cathodes. Because AC flow-electrodes can exhibit much

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higher total capacity than solid-state electrodes, the faradaic cathodes were not limited

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by anode capacity. When normalized to total cathode mass, maximum NAC for ED1440

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(0.77 ± 0.21 mmol g-1) was far above reported values for AC CDI (0.3 mmol g-cathode-1)

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and MCDI (0.4 mmol g-cathode-1).9 If adsorption capacity is normalized to only cathode

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MnO2 mass, ED1440 have a MnO2-specific NAC of 2.3 ± 0.6 mmol g-MnO2-1 (Fig. 4c).

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This is nearly 3 times the value reported by Lee et al. for pure NMO electrodes (0.8

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mmol g-NMO-1).26 While NMO has been found to exhibit high capacitance, 300 F g-1,

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the anode used in Lee et al.’s study (~120 F g-1)35 was not balanced to the cathode

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capacitance which may have limited NAC. When compared to the stoichiometric

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adsorption limit (mol-Na+ mol-MnO2-1, Eqn 3), ED electrodes ranged from 39%

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maximum capacity for ED15 at 2 A m-2 to 1.5% for ED1440 at 15 A m-2. This suggests

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that at greater deposition times, less of the total deposited MnO2 is available for Na+

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storage. Likewise, as the current density is increased, the available voltage window for

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desalination is constrained by resistance and less MnO2 is utilized for Na+ storage.

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When operated under CV, the NMO cathode operated by Lee et al. reach 26% of



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stoichiometric maximum sodium adsorption capacity (mol-Na+ mol-MnO2-1, Eqn 4).

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Unlike Lee et al., this study utilized a flow-anode to observe the performance of MnO2-

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coated electrodes without the capacity limitations of a solid carbon anode. Lee et al.’s

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carbon anode was approximately twice the mass of the NMO cathode, but the reported

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SAC might still be limited by anode capacity. While flow-electrodes can provide high

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capacity, they are more resistive than traditional solid-state electrodes.39 Further

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improvements to NMO and MnO2 system capacity and performance may require the

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development of Cl-specific storage materials. Across comparative conditions, ED and CA electrodes kinetically outperformed

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CV electrodes (Fig. 4). However, ED MnO2 peak salt removal rates are lower than those

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reported by Lee et al.26 (~1.2 10-3 mmol g-1 s-1) for a NMO cathode operated at

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constant voltage. Under constant voltage operation a much higher peak current and

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corresponding rate of removal are expected. In fact, Lee et al. report peak current

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densities of approximately 150 A m-2, a full order of magnitude above our maximum 20

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A m-2. Since MnO2 is far less electrically conductive than carbon (10-5–10-6 vs ~50 S cm-

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1 46,47

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collector and aerogel. For all deposition techniques, the working potential window for

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desalination rapidly decreased with current density (Figs. S4 and S7). Additionally,

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unlike previous studies utilizing NMO electrodes, neither ED nor CV electrodes were roll

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pressed.26 The energetic losses attributable to the cathode could be reduced through

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fabrication methods which minimize the contact and internal resistances of the cathode.

),

coating MnO2 on carbon substrate creates a resistive barrier between the current

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3.4 Optimization conflicts between current density and charge efficiency


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Electrode resistance negatively impacted charge efficiency by reducing

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charge/discharge cycle length as current increased. While we report low charge

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efficiency for CA electrodes (

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