In Situ Electrochemical Dilatometry of Phosphate Anion

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Environmental Measurements Methods

In Situ Electrochemical Dilatometry of Phosphate Anion Electrosorption Daniel Moreno, Yousuf Z Bootwala, Wan-Yu Tsai, Qiang Gao, Fengyu Shen, Nina Balke, Kelsey Hatzell, and Marta C. Hatzell Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00542 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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Environmental Science & Technology Letters

In Situ Electrochemical Dilatometry of Phosphate Anion Electrosorption Daniel Moreno,† Yousuf Bootwala,† Wan-Yu Tsai,‡ Qiang Gao,¶ Fengyu Shen,§ Nina Balke,‡ Kelsey B. Hatzell,∗,§ and Marta C. Hatzell∗,† †George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, 1

Atlanta, Ga, 30313, USA ‡The Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA ¶The Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tn,37830, USA §Department of Mechanical Engineering, Vanderbilt University, Nashville, Tn, 37203, USA E-mail: *[email protected]; *[email protected] Phone: +1 404-385-4503

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Abstract

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Here we investigate the competitive electrosorption of mono- and divalent phos-

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phate anions through electrochemical desalination and dilatometry based experiments.

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Through in situ dilatometry, we monitor the strain at the electrode surface as anions

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and cations are electrosorbed. Strain measurements show that the presence of diva-

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lent ions promote a greater than anticipated electrode expansion during cation (Na+ )

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electrosorption. The expansion observed with Na+ equaled the expansion observed

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2− + with the HPO2− 4 . Since the ionic radius of Na is smaller than HPO4 , the symmet-

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ric expansion suggests that divalent anions do not completely desorb during electrode

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ACS Paragon Plus Environment


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regeneration, causing the adverse interactions with the cation during co-ion expulsion.

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This results in a decrease in desalination performance, indicated by decreased salt ad-

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sorption capacity. Conversely, an expected asymmetric expansion during anion and

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cation electrosorption occurs with monovalent phosphate anions (H2 PO− 4 ) indicating

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that monovalent ions are capable of being effectively replaced by the cation at the

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electrode surface.

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Introduction

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Ion separation processes are critical for many industrial, municipal, and environmental ap-

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plications. 1 Separation processes require the use of physical (membranes), or mass-transfer

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based operations (distillation, absorption, and adsorption). 2 Mass transport operations have

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a high-energy demand and typically require chemical based regeneration. Whereas physi-

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cal operations are low energy, but foul and are not intrinsically selective. Electrochemical

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driven separations (capacitive deionization, electrochemically modulated liquid chromatog-

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raphy, battery deionization) have demonstrated new avenues for achieving low energy ion

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removal and are electrochemically regenerated without the need for chemicals. 3 Furthermore,

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these approaches are beginning to achieve selectivity through the use of electrochemical

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redox-responsive receptors. 4,5

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Gaining molecular level insight into water|electrode interfaces is an important next step

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needed to answer questions related to the energetics and selectivity of electrosorption. 5–7

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Fundamental knowledge is needed regarding both the static and dynamic nature of the elec-

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tric double layer (EDL) and interface during separation. This is critical for understanding

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the rate limiting mechanism associated with electrosorption (adsorption and desorption)

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in environmentally relevant conditions. While in simple systems with model constituents

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(monovalent salts), adsorption and desorption occur at equal rates (symmetric), in systems

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with more complex constituents (salts with varying degrees of valence and hydration), com-

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petitive adsorption between species can alter the salt removal process. 8 One of the most 2


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common constituents which this applies to is phosphate anions, which are well known to ex-

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2− ist in a monovalent (H2 PO− ) state depending on the pH of the feed 4 ) and divalent (HPO4

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source. 9–12 Owing to the potential environmental and economic considerations associated

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with phosphorus recovery from waste streams, developing ways to recover and concentrate

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phosphate anions is growing in importance 13–15

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Here, we investigate the dynamics of phosphate anion electrosorption processes using elec-

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trochemical dilatometry, 16,17 desalination testing, 18,19 and through a theoretical analysis. 20

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Dilatometry experiments probe the phosphate anion interactions at the electrified interface

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through measuring changes in the electrode strain during cyclic voltammetry experiments.

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Strain measurements are also correlated with desalination and electrochemical based char-

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acterization. The results discussed here show increased strain in electrodes with divalent

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phosphate anions (HPO2− 4 ) during co-ion expulsion (negative polarization). Strain is associ-

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ated with an ions inability to adsorb to the electrode surface or transport through nonporous

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regions. In contrast, this mechanical strain is not observed for monovalent phosphate anions

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(H2 PO− 4 ). Ineffective desorption of bulky anions may be a contributing factor leading to

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lower desalination performance and salt adsorption capacity. These interactions may also

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promote a degree of selectivity associated with larger ionic radii and charged species. Prior

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results in batch desalination have suggested that divalent phosphate anions may accumulate

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more readily at the surface because of the energetic advantage associated with screening

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the surface charge. 21 This work presents an additional explanation related to the unfavor-

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able chemo-mechanics associated with the co-ion expulsion of divalent phosphate anions.

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Increased strain is observed in the electrode during divalent desorption processes and may

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contribute to observed selectivity of divalent phosphate anions over mono-valent species.

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Understanding and measuring competitive electrosorption-based processes is imperative for

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separation of complex waste streams.

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Materials and Methods

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Preparation of the electrode

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The activated carbon electrode was prepared by dispersing YP-50F activated carbon (Sanwa

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Components International Inc), carbon black (Sigma-Aldrich) and polytetrafluoroethylene

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(60% weight dispersion, Sigma-Aldrich) in the ratio 90:5:5 by weight in ethanol (Fisher

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Scientific). The mixture was stirred constantly and dried on a hot plate stirrer (Fisher-

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Scientific), until a desired consistency was produced. After the mixture was dried the carbon

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slurry was rolled onto a graphite sheet (Alfa-Aesar) using a roller to a thickness of 0.4 mm.

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Two electrodes with diameters of 10mm were cut out and used for the tests as working and

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counter electrode. The weight of the electrode was 20 ± 2 mg.

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Electrochemical Dilatometry

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An ECD-nano-DL Dilatometer from EL-Cell (Germany) was used with gold current collec-

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tors was used for in-situ electrosorption experiments. Three-electrode electrochemical and

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dilatometric measurements were performed at 22±1◦ C with a Biologic VSP-300 potentiostat.

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The electrochemical cell contained a over-sized counter electrode with a mass at least 20×

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the mass of the working electrode. The vertical deformation during anodic and cathodic

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scans was measured with an accuracy of approximately 15 nm and was normalized to the

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electrode thickness at the point of zero strain.

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Desalination testing

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The desalination system was comprised of a flow cell with a projected area Acell = 25 cm2 .

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The feed chamber consisted of a polycarbonate flow channel with a thickness of 0.47 cm.

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Electrodes were placed on graphite sheets which were placed on gold-plated copper current

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collectors. The feed flows vertically through the center of the cell. A cation (CEM) and

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anion exchange membrane (AEM) separated the feed from electrode (Selemion AMV/CMV, 4


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Chiba, Japan).

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Results and Discussion

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The degree of desalination from a capacitive deionization cell was conducted in both con-

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tinuous (Figure 1a) and batch-mode (Figure S2) based experiments. In both series of ex-

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periments, phosphate aqueous solutions with a lower pH (pH ≈4) resulted in a faster rate

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of salt removal and total salt removed (∆C/C0 ) for each cycle. For the continuous mode

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desalination based testing the energy consumption for the pH 4 electrolyte was 0.03 kWh

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per mol, whereas the pH 9 phosphate electrolyte resulted in an increase in energy consump-

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tion to 0.09 kWh per mol. Due to the pKa of the solution, in testing conducted in pH 4

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electrolytes, H2 PO4 − was the dominant anion. In addition, in testing in pH 9 electrolytes

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the dominant anion was HPO4 2− . The increase in energy consumption ascribed to the pres-

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ence of divalent ions also resulted in a decrease in the energy normalized per adsorbed salt

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(ENAS) from 8.3 mol per joule (pH=4) to 3.2 mol per joule (pH=9). Note that the ENAS

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values for the phosphate anions is 2-4 times less than ENAS values reported with sodium

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chloride. 22 This is due to an increase in the ionic radius of the phosphate anion (≥ 0.302 nm

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hydrated ionic radius) compared to chloride ( 0.195 nm hydrated ionic radius). Due to the

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size of the desalination systems, batch mode experiments provided further insight into the

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potential for anion removal. Here, the energy consumption (kWh/mol) again was 2x greater

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for pH=9 solutions (divalent anions) compared with pH=4 (mono-valent). In all tests an

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increase in current was observed with the pH=9 solution, which was attributed to the higher

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conductivity (Figure S1 and S2).

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The experiments are consistent with Gouy-Chapman-Stern theory, which predicts salt

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removal dynamics within an electric double layer (Figure 1b). 20,23 In aqueous solutions, the

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electric double layer (EDL) is comprised of the Stern layer which has oppositely charged

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counter-ions attracted to the surface and a diffuse layer which has a mix of counter-ions and

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co-ions. The length at which the diffuse layer disperses into the medium, otherwise known

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as the Debye length λD , varies inversely with the electron valance, resulting in less disperse

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EDLs when di- or tri-valent ions are present. The Debye length is important to determine

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the electric potential of the double layer and is incorporated into a transient model. Through

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the introduction of valance into the model, the result predicts a decrease in salt removal with

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an increase in valance (Figure 1b). (a )

1 .2 p H = 4 (z = 1 ) p H = 9 (z = 2 ) p H = 7 ( M ix tu r e )

C /C

0

1 .1

1 .0

0 .9

0 .8 0

6 0

1 2 0

1 8 0

2 4 0

1 8 0

2 4 0

T im e ( m in ) (b ) 1 .2 p H p H

= 4 (z = 1 ) = 9 (z = 2 )

C /C

0

1 .1

1 .0

0 .9

0 .8 0

6 0

1 2 0 T im e ( m in )

Figure 1: (a) Effect of initial pH on the effluent concentration in continuous mode and (b) theoretically predicted effect of valance on ion removal.

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During the dilatometry experiment, the whole cells voltage was varied through performing

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cyclic voltammograms (CV). The associated relative height change experienced by the cell,

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ranged from 0 to nearly 1% depending whether the anion or cation was being electrosorbed

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into the carbon film (Figure 2a-b). Each cyclic voltammogram demonstrates a pseudo-

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rectangular shape at 2 mV/s (slow scan rate) which is typical of electric double layer electrode 6


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0.05

0.6

0.00

0.4

-0.05

0.2

-0.10 -0.15

pH4 -0.4

Relative Height Change (%)

(c) 0.8

-0.2 0.0 0.2 Voltage (V) vs. Carbon

0.6

0.0 -0.4

-0.2 0.0 0.2 Voltage (V) vs. Carbon

0.4

pH9

-0.10

(d)

pH 4

0.2

-0.05

-0.6

0.2

0.4

0.00

0.0

0.4

0.6

0.05

0.4

2 mV/s 10 mV/s 20 mV/s

0.8

0.10


Current Density (A/g)

0.10

(b)


0.8

0.8 0.6


(a)

Current Density (A/g)

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-0.4 -0.2 0.0 0.2 Voltage (V) vs. Carbon

0.0

0.4


0.4 0.2

pH 9

0.0 -0.6

-0.4 -0.2 0.0 0.2 Voltage (V) vs. Carbon

0.4

Figure 2: Dilatometry experiments detailing the relative height change experienced by the electrode during a cyclic voltammegram conducted in a pH 4 (a) and pH 9 phosphate-based electrolytes (b). Relative height change recorded at various scan rates a pH 4 (c) and pH 9 phosphate-based electrolyte (d). 122

systems. The specific capacitance for the pH 4 and pH 9 electrolytes in a three electrode

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configuration are 38 and 40 F/g (Figure S3). The low capacitance is attributed to the

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transport limitations associated with phosphate anions in the micropores. Evidence of high

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tortuosity can be observed at the potential boundaries of the cyclic voltammagrams. In a

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highly accessible electrode the current is independent of voltage (vertical line) at the voltage

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boundaries. 19,20,24 However, in these electrolytes there is a slight voltage dependence on the

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current density at the voltage limits. Subsequently, as the potential swings into the positive

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region a large expansion is observed in both the pH 4 and pH 9 electrolytes systems. The

130

large expansion is associated with the bulky anion (monovalent and divalent phosphate)

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adsorption. The electrode expansion or strain is reversible for each successive cycle. Large 7



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expansions are observed while positively polarizing the electrode in the acidic electrolyte with

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a monovalent phosphate anion (H2 PO− 4 ) and less expansion is observed when the electrode

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is polarized in the negative region. The anisometric charge dependent swelling of the porous

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carbon is well documented for non-aqueous and ionic liquid electrolytes characterized by

136

bulky cations. 25

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The assymmetric expansion behavior indicates that the carbon experiences more strain

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during phosphate electroadsorption processes than Na+ (Figure 2c). In contrast, the expan-

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sion process is more symmetric between positive and negative polarization swings for the

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basic electrolyte which contains divalent phosphate anions (HPO2− 4 ) (Figure 2d). This result

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is counterintuitive because both electrolyte systems have the same cation which suggests

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that there is some interaction between the divalent phosphate anion (HPO2− 4 ) and cation

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during negative polarization. When the electrode switches from positive to negative polar-

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ization regimes the anion is expelled from the surface and replaced with a cation. During

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this switch, co-ion expulsion dynamics can play a role in the electric double layer structure

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and impact the efficiency of the electroadsorption processes. Furthermore, this interaction

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may be amplified for the divalent phosphate anion, because two sodium ions are required to

148

maintain charge neutrality at the surface during the potential reversal. Thus, the symmetric

149

expansion observed in the basic electrolyte may occur because of a complex desorption mech-

150

anisms associated with the dual charged phosphate anion. The electrode expansion process

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decreases with scan rate (Figure 2c-d) for both electrolytes (mono- and divalent anions). The

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decrease in electrode expansion is a signal that ions are not reaching adsorption equilibrium

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because of transport limitations in the microporous region of the electrode.

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The dissimilar strain or expansion properties between the mono-valent and di-valent phos-

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phate anions is more obvious when multiple cycles are presented with respect to normalized

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time (Figure 3a-b). The asymmetric expansion profile for the mono-valent phosphate anion

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is readily apparent at 2 mV/s through the presence of a repeating units of small and large

158

peaks. However, little to no expansion is observed at scan rates above 10 mV/s (small peaks

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pH 4 0

20 40 60 80 Normalized Time (%)

2mV/s

2.5

10mV/s

(b)

100

0

20mV/s

2.0 1.5 1.0 0.5 0.0 0

20

40 60 80 Normalized Time (%)

pH 9

0.25 0.00 -0.25 -0.50

Voltage (V)

0.50 0.25 0.00 -0.25 -0.50

Voltage (V)

(a)


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2mV/s

2.5

10mV/s

100

20mV/s

2.0 1.5 1.0 0.5 0.0 0

100

20 40 60 80 Normalized Time (%)

20

40 60 80 Normalized Time (%)

100

Figure 3: Electrode relative high change in pH 4 (a) and 9 phosphate electrolyte as a function of normalized time (b). 159

are completely suppressed). This is not surprising, as at higher scan rates, ionic access to

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the interface is limited. In contrast, significant expansion occurs in the electrolyte with the

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divalent phosphate anion at scan rates above 20 mV/s. The symmetry in expansion does

162

decrease with increasing scan rate (divalent phosphate anion), but is still readily apparent

163

indicating that the co-ion expulsion mechanism is not dependent on the kinetics of the charg-

164

ing process. Insight into the chemo-mechanics of these complex anions provides a potential

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explanation for recent experimental results by Huang et al., which demonstrated preferential

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2− − 9 electroadsorption selectivity of Cl− over H2 PO− 4 and HPO4 over Cl . Prior results have

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suggested that there is a preferential adsorption and accumulation of divalent phosphate

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anions in the double layers because there is an energetic advantage of screening the surface

169

charge with a negative two charge anion. 21 Our work extends this finding by demonstrat-

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ing that there is also chemo-mechanical considerations at play during the co-ion expulsion

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processes of divalent phosphate anions that do not exist for mono-valent phosphate anions.

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Molecular insight into fundamental interactions that occur at relevant solid|water in-

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terfaces for water treatment, remediation and desalination applications is important for

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engineering next generation materials. Electric field and electrochemical systems are being

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increasingly investigated in water technologies as a means to mitigate degradation (fouling),

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inform system performance (sensors), and to improve selectivity. This work investigates the

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fundamental nature of electroadsorption processes for relevant water contaminants and pro-

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vides insight into the origin of decreased ion separation properties for dual charged phosphate

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anions in comparison to single charged phosphate anions. Designing materials that can ex-

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ploit these mechanisms may provide a means for engineering future systems with selectivity.

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Here we highlight the complexity associated with field-assisted separations in water applica-

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tions where species of varying degrees of hydration and valance are present. Furthermore, we

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highlight how competitive electrosorption can be captured in various dynamic environments

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using dilatometry and theory to ultimately explain results obtained through desalination

185

based testing.

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Supporting Information Available

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Experimental procedures and calculations are provided in the supporting information: for

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the ex situ characterization, materials and methods, and theoretical framework.

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This material is available free of charge via the Internet at http://pubs.acs.org/.

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Acknowledgement

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This material is based upon work supported by the National Science Foundation under

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Grant No. (1706290) for MCH and Grant No. (1706956) for KBH. In situ electrochemical

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dilatometry and support for NB were conducted at the Center for Nanophase Materials

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Sciences, which is a DOE Office of Science User Facility. WYT and QG were supported as

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part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy

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Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office

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of Basic Energy Sciences. This manuscript has been authored by UT-Battelle, LLC, under 10


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Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States

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Government retains and the publisher, by accepting the article for publication, acknowledges

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that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide

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license to publish or reproduce the published form of this manuscript, or allow others to

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do so, for the United States Government purposes. The Department of Energy will provide

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public access to these results of federally sponsored research in accordance with the DOE

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Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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Graphical TOC Entry H2PO4-

-

+

278

-

Na+

+

-

-

2-

HPO4

-

+ +

-


0.8 +

0.6


0.4 0.2 0.0 -0.6

pH9 -0.4 -0.2 0.0 0.2 Voltage (V) vs. Carbon

15


0.4

In Situ Electrochemical Dilatometry of Phosphate Anion

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