Nanoparticulate Dielectric Overlayer for Enhanced Electric Fields in a

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Nanoparticulate dielectric overlayer for enhanced electric fields in a capacitive deionization device Karthik Laxman, Daiki Kimoto, Armen Sahakyan, and Joydeep Dutta ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16540 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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ACS Applied Materials & Interfaces

Nanoparticulate dielectric overlayer for enhanced electric fields in a capacitive deionization device Karthik Laxman,1# Daiki Kimoto,1# Armen Sahakyan 2# and Joydeep Dutta 1*

1

Functional Materials Division, Department of Applied Physics, School of Engineering

Sciences, KTH Royal Institute of Technology, Isafjordsgatan 22, SE-164 40 Kista Stockholm, Sweden

2

Thomas Johann Seebeck Department of Electronics, Tallinn University of Technology,

Ehitajate tee 5, 19086 Tallinn, Estonia

# These authors contributed equally

*Corresponding Author: Prof. Joydeep Dutta, Functional Materials Division, Department of Applied Physics, School of Engineering Sciences, KTH Royal Institute of Technology, Isafjordsgatan 22, SE-16440, Kista Stockholm, Sweden Ph: +46-8-790 81 42; Mob:+46-73-765 21 86; Email: [email protected]

Keywords: Dielectric polarization, Capacitive Deionization, Electric Field, Zinc Oxide, Titanium Dioxide

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Abstract The magnitude and distribution of electric field between two conducting electrodes of a capacitive deionization (CDI) device plays an important role in governing its desalting capacity. A dielectric coating on these electrodes can polarize under an applied potential to modulate the net electric field and hence salt adsorption capacity of the device. Using finite element models we show the extent and nature of electric field modulation, associated with changes in the size, thickness and permittivity of commonly used nanostructured dielectric coatings like zinc oxide (ZnO) and titanium dioxide (TiO2). Experimental data pertaining to the simulation is obtained by coating activated carbon cloth (ACC) with nanoparticles (NP’s) of ZnO and TiO2 and using them as electrodes in a CDI device. The dielectric coated electrodes displayed faster desalting kinetics of 1.7 mg g-1min-1 and 1.55 mg g-1min-1 and a higher unsaturated specific salt adsorption capacity of 5.72 mg g-1 and 5.3 mg g-1 for ZnO and TiO2 respectively. In contrast, uncoated ACC had a salt adsorption rate and capacity of 1.05 mg g-1min-1 and 3.95 mg g-1 respectively. The desalting data is analyzed with respect to the electrical parameters of the electrodes extracted from cyclic voltammetry and impedance measurements. Additionally, the obtained results are correlated to the simulation data in order to ascertain the governing principles for the changes observed and advances that can be achieved through dielectric based electrode modifications for enhancing CDI device performance.


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Introduction Fresh water is a depleting resource worldwide and extended efforts are being made to increase domestic water resources via water desalination and reclamation.1 In the field of desalination, sea water is the prominent source with reverse osmosis (RO) being widely applied for large scale production of domestic water from sea water.2, 3 However for brackish water (salinity 1000 to 15000 ppm) and lower volumetric flows, other technologies present a viable alternative to RO in terms of capital cost, energy requirements and portability.4-8 One such technology is capacitive deionization (CDI), which is an upcoming disruptive technology for removing ions of salt from water instead of the usually practiced method of separating water from salt.9-11 CDI devices comprise of two high surface area conductive electrodes (usually activated carbon electrodes) separated by a non-conductive ion porous spacer, similar to the configuration of an electronic capacitor, sometimes with the inclusion of ion selective membranes.12-15 When potential is applied to the two electrodes (+tively & –tively polarized), anions are attracted to the anode (+tive) and cations to the cathode (-tive). CDI has been effectively used for inland ground water desalination along with the removal of specific contaminant ions like boron, arsenic etc.16-20 Similar to a supercapacitor, the ions are held at the electrode surfaces in the form of electrical double layers (EDL’s),21, 22 the capacity of which is determined by a complex interlink between the electric field present at the



electrode-electrolyte interface,23-25 electrode surface charges,26-30 surface area of the electrodes, surface conductivity 31 and pore structure.15, 32-34 The surface charge/energy or surface area of the electrodes can be tailored by coating the electrodes with positively or negatively charged surface functional groups35-37 while surface area modifications have been achieved through the use of graphene, carbide derived carbons, aerogels, carbon nanotubes, hybrid electrodes, carbon beads etc.31, 38-44 along with pore size tuning of the electrodes.21, 45, 46 Indirect modification of the electrode surface by utilizing a reference electrode to control the electrical potential of the working electrodes and improve charge efficiency of CDI has also been reported.47 Another method for changing both surface area and energy of the electrodes is by coating the electrode surfaces with nanostructured dielectrics, wherein the isoelectric point of the dielectric modifies the surface energy of the electrode,37 while the morphology of the dielectric modulates the active surface area of the electrode has also been demonstrated.48-52 The nanostructured dielectrics also contribute to increasing the effective electric field between the CDI electrodes, leading to enhanced salt removal rates and capacities.23-25 The field enhancements are a result of polarization of the dielectric under an applied potential, with each dipole contributing to a charge unit, integration of which constitutes the net attractive/repulsive force (termed electric field, which is the force per unit charge) experienced by a charged ion in the vicinity.23-25 In essence, the


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dielectric consumes energy applied as electric potential and converts it to an associated electric field. Consequently the dielectric constant, thickness and density of the coating material play important roles in determining the electric field generated in such devices.25 The intricacies of this phenomenon within the context of CDI need to be further explored to gain a critical understanding of its dependencies on the material used and its morphology.

In this work we simulate the external and internal electric fields of zinc oxide (ZnO) and titanium dioxide (TiO2) nanostructured dielectrics coated on conducting electrodes using COMSOL. The electric field patterns as obtained from the simulations are correlated with experimental desalting results obtained by fabricating CDI devices with activated carbon cloth (ACC) electrodes coated with dielectric nanoparticles of ZnO and TiO2 of comparable sizes. Average salt adsorption rate (ASAR) and specific salt adsorption capacity (SSA) of the electrodes are also monitored over a length of time to study electrode stability with the dielectric coatings.



Experimental Finite element simulation Numerical simulations were conducted with COMSOL Multiphysics 5.2a. A three dimensional planar half-symmetric finite element model of a CDI cell (single electrode with dielectric coating excluding the spacer) was constructed in the electrostatics module. The MUMPS (Multifrontal Massively Parallel Sparse Direct solver) was used to characterize the distribution of the electric field in the model. Convergence of the numerical simulations was obtained by using a combination of 10 noded tetrahedral and triangular elements. The effect of nanoparticle coating on the interfacial electric field was studied using 20nm, 200 nm and 1 µm diameter zinc oxide nanoparticles (ZnO – dielectric constant 8.5) and Titanium dioxide nanoparticles (TiO2 – dielectric constant 60) using their bulk dielectric constant values for monolayer and multi-layer depositions. Effect of symmetric and asymmetric nanoparticle deposition on the electrode surface was also considered using a cubic structure of 4.5 µm3 with sodium chloride (NaCl) ionic solution as the environment surrounding the nanoparticles (Figure 1a). Two layers comprising a bottom layer with 4×4 array and a top layer with 3×3 array of nanoparticles having an inter-particle distance of 1.1 µm was constructed. For NaCl medium, the dielectric constant was considered to be equal to 80, which is the same as that of water. Activated carbon cloth (ACC) electrodes were considered as flat plate conductive


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materials with a real part of the dielectric constant as 1 and a conductivity of 3500 Sm-1 to simulate the conduction current described by the imaginary part of the dielectric constant. As ACC was considered to be a perfect conductor with no losses to the applied potential, the + 1.5 VDC potential was applied directly to the base of the dielectric nanoparticles in the case of coated electrodes. The top and lateral planes of the NaCl domain were set as the electrostatic ground to maintain homogeneous boundary conditions. Diagonal and vertical arcs were drawn and the electric field and potential distribution was quantified along the length of the arc as shown in Figure 1b and Figure 1c using the equation

D = ε 0ε r E , where D is electric displacement, E electric field, ε0 permittivity of free space,

εr relative permittivity (dielectric constant) of the coating material. Thus in summary, the following assumptions were made for the simulations: (1) the dielectric nanoparticles were perfect spheres without defects, (2) ACC electrode was perfect conductor without energy losses, (3) an electric potential of 1.5 V was applied to the base of nanoparticles, on the first layer and (4) the boundary conditions are defined by a ground potential along the 4 lateral sides and top surface of the cube.

Materials: Activated woven carbon cloth (Zorflex FM-100) of ca. 0.8 mm thickness with a specific surface area of 1200 m2/g 53 (as specified by manufacturer) was cleaned with 2M nitric



acid (HNO3) heated to 115 ˚C for 24 hours to remove contaminants. Subsequently, the ACC substrates were thoroughly rinsed with deionized water until the surface was neutralized (pH 7.5), dried in a vacuum oven at 150 °C for 12 hours and then stored in a desiccator until further use. Commercial 21 nm diameter zinc oxide (ZnO) and titanium dioxide (TiO2) nanoparticles were purchased from Aldrich chemicals and used as received. Titanium(IV) butoxide from Fluka and zinc acetate dihydrate from Sigma-Aldrich were also used as received.

Electrode modification Nanoparticle coated ACC surfaces were fabricated using a 0.5 weight % suspension of ZnO and TiO2 nanoparticles pre-dispersed in 25 mL isopropanol (C3H8O) with 0.5 mL of Tween80 added as a stabilizing agent. The suspension was stirred overnight at room temperature using a magnetic stirrer. Following this, the supernatant was collected and used for coating the ACC electrodes. The ACC electrodes were then dipped into the ZnO and TiO2 nanoparticle suspensions for approximately 180 minutes after which they were dried. Subsequently for the ZnO coated ACC, 10 mL of 10 mM of zinc acetate dihydrate (Zn (CH3COO)2.2H2O in DI water was sprayed onto the ZnO NP coated ACC substrates using spray pyrolysis (spray rate of 1 ml min-1at 250 oC from a distance of 15 cm) after which the substrates were annealed in air at 350 oC for 60 minutes. Similarly, 10 mL of 10


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mM of Titanium (IV) butoxide in ethanol was sprayed (same parameters as for ZnO-ACC) onto the TiO2 coated ACC surface and subsequently annealed in air at 350 oC for 60 minutes.

Capacitive deionization experiments The desalination/regeneration experiments were conducted using flow through capacitor model, wherein the saline water flows in through the four corners of a square CDI cell and flows out through a central outlet. The CDI cell consists of two symmetrical square shaped ACC electrodes having a geometrical area of 23 cm2 with a 0.6 mm graphite sheet placed on its outer side (connected to current collector). The two electrodes are separated by a pair of cellulose spacers having a combined thickness of 500 µm and the sandwiched structure is housed inside a reservoir with a volumetric capacity of 10 mL (Supporting Info. Figure S1). Graphite rods are used as the current collectors to electrically connect the ACC electrodes to the external power source and electronic control circuit.

A complete CDI cycle comprised of ion adsorption during desalination and desorption during regeneration. The desalination cycle was carried out using 1000 ppm sodium chloride (NaCl) solution with a flow rate of 6.5 ml/min under an applied potential of 1.5 VDC, while during regeneration a pulsing reverse potential was applied to hasten the



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electrode recovery process. A peristaltic pump (Cole Parmer) was used to maintain a constant flow rate of the saline water into the CDI cell. The total charging (desalination) time was maintained at four mins while discharging time (regeneration) was maintained at three minutes, giving a water recovery ratio of ~60%. A test bench consisting of a programmable DC power source, automated solenoid valve, data logging system, pH and conductivity measurements comprised the remaining test bench components.

The electrosorption capacity of the electrodes was estimated using equation 1:

SSA =

(mg)

m(g)

V(L)

(1)

where SSA is the specific salt adsorption capacity in milligram of salt adsorbed per gram of electrode. ‘Saltads’ is the total salt adsorbed on the electrodes in ‘mg’ which is calculated by integrating the area under the conductivity curve as a function of time, during desalination. Only the part of the curve below the 2 mScm-1 (taken as base line) is considered for the salt adsorption measurements, while the rest is ignored to avoid over-estimating the electrode capacity. ‘V’ is the volume of solution desalinated in liters (L) per desalination cycle and is calculated by multiplying the flow rate with respect to the duration of desalination and ‘m’ is the mass of both electrodes in ‘g’. The average salt adsorption ratio ‘ASAR’ is determined from the ratio of the SSA to the total duration of the desalination cycle (four minutes) as shown in equation 2.


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=

(2)

(min)

Characterization Cyclic voltammetry (CV) measurements at scan rates of 50 mV sec-1, 10 mV sec-1 and 1 mV sec-1 were conducted using EG&G 263A Potentiostat (Princeton Applied Research, USA) with a 0.5 M NaCl electrolyte under constant stirring at room temperature. Specific capacitance of the electrodes was estimated by integrating the area within the CV current curves (between± 0.5 V scan) as shown in equation 3.

E2

CSC = ∫

E1

i( E )dE 2( E2 − E1 )mν

(3)

where i (mA) is electrode current and E potential, m the mass of electrode and v the sweeping speed of the potential (mVsec-1).

EIS measurements were conducted usingVMP2 (Bio-Logic Science Instruments, France). For EIS, the frequency range was set between 200 kHz to 0.01 Hz at 6 points per decade with a 10 mV p-p AC signal applied to a 1 cm2 surface area working electrode. Electrode surface was studied by ZEISS ULTRA55 field emission scanning electron microscope (FESEM) working at 3 kV. Particle size measurements were conducted using Beckman Coulter Delsa Nano C particle size analyzer.



Results and Discussion The electrostatic finite element model developed to quantify the electric field vector distribution within a NaCl medium in the vicinity of the nanoparticles deposited on ACC is shown in Figure 1. The model depicts the structure of one electrode within a CDI cell (comprises of two electrodes) with the assumption that the other electrode is symmetric. The model is developed for nanoparticles (NP’s) with diameters of 20 nm, 200 nm and 1 µm. Electric field (E-field) analysis along the horizontal cross sections of the NP’s (Figure 1b) shows that with decreasing NP diameter, higher E-fields are generated in the vicinity (Figure 2a).

Figure 1: (a) Geometric model of nanoparticles within a NaCl medium; Arc length along which (b) horizontal and (c) vertical electric field and potential distributions are mapped for the 2 layer NP coated conductive substrate.

Practically however it is difficult to obtain a mono-dispersed NP coating on a surface. Agglomeration and distributed surface energy generally lead to multi-layered coatings


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with non-uniform deposition density. To better accommodate these variations we extended the model with two layers of NP coating of varying thickness (number of particulate layers). Potential is applied to the base of the lower nanoparticle (as would be the case on a coated electrode), with the assumption that the potential magnitude at the boundary of the lower and upper nanoparticles would be the apparent potential at the base of the upper particle. Thus the loss of the electric potential magnitude along the volume of the first layer of nanoparticles was mapped along the z-axis (Figure 1c) to quantify the potential at the base of the second layer of nanoparticles. For 200 nm diameter particles of ZnO and TiO2, the potential loss within the 1st layer of NP’s was approximately 0.9 VDC, with the remaining 0.6 VDC being applied to the base of NP’s in the 2nd layer (Figure 2b). The resulting electric field vectors mapped along the diagonal cross-section (Figure 1b) of the 1st layer of 200 nm NP’s display a clear enhancement of the E-fields along the circumference and the immediate vicinity of the nanoparticles compared to bare ACC or bulk NaCl solution (trend is same for the 2nd layer) as depicted in Figure 2b.The field enhancement is similar for both ZnO and TiO2 nanoparticles, albeit slightly higher for the ZnO NP’s as observed in the inset of Figure 2c. The trends of the E-field can be understood by considering that the observed net E-field magnitude is a combination of fields generated from free charges within the material along with the charges induced due to the crystal polarization occurring within both ZnO and TiO2.25



Figure 2: Theoretical electric field and potential vectors extracted from the FEM’s (a) along the diagonal arc depicted in figure 1b for 20 nm, 200 nm and 1 um NP’s of ZnO (b) along the cross sectional line (figure 1c) for 200 nm NP’s and (c) along the diagonal arc depicted in figure 1b for 200 nm NP’s. L1 & L2 stand for Layer 1 and Layer 2. 14 ACS Paragon Plus Environment

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The polarization for polycrystalline ZnO and TiO2 NP’s is predominantly observed in the direction of crystal poling (z-axis) along which the potential is applied and the density of this polarization is proportional to applied potential magnitude. It can be observed that along the z-axis, the applied potential loss slope is steeper for ZnO, but its internal E-field is comparable to that of TiO2 (Figure 2b). Considering that the number density of atoms is equal (comparable) for both ZnO and TiO2 NP’s, Clausius–Mossotti relation suggests that a material with a higher dielectric constant will have a higher polarizability. As mentioned above, in addition to the dielectric constant, the crystal symmetry also governs polarization characteristics. For a non-centrosymmetric crystal like ZnO, the net polarization within the crystal core is typically higher than that of a centrosymmetric crystal like TiO2, since in centrosymmetric crystals a more uniform dipole charge distribution leads to cancellation and hence reduction of the net field within the material (Figure 3).

Figure 3: Schematic representations of the (a) non-centrosymmetric crystal structure of ZnO and (b) centrosymmetric crystal structure of TiO2 15 ACS Paragon Plus Environment


However near the NP edges in the direction of crystal poling, TiO2 has a higher net E-field due to its higher dielectric constant. This is in contrast to the net interfacial E-field observed along the diagonal axis (45o to the x-y axis) which is marginally higher for ZnO (Figure 2c). The difference observed between the z-axis and diagonal axis is attributed to the non-centrosymmetric crystal structure of ZnO leading to a higher polarization density normal to the direction of crystal poling compared to TiO2. Thus it appears that the interfacial E-field magnitude is higher for NP’s with a higher dielectric constant,25 only if measured in the direction of the crystal poling. In other axes, crystal symmetry, size and applied potential loss within the NP core modulates the net E-field magnitudes, with the results suggesting that non-centrosymmetric materials are better suited for obtaining higher E-fields in this configuration.

Figure 4: Scanning electron spectroscopic image of (a) plain ACC; (b) TiO2 NP’s coated ACC and (c) ZnO NP’s coated ACC electrodes. Inset show the close-up of the electrodes.

The simulation results are validated by fabricating and characterizing ACC electrodes coated with ZnO and TiO2 NP’s with an average NP diameter of 21 nm. SEM


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micrographs of the coated electrodes indicate that the average diameter for the deposited particle aggregates for both ZnO and TiO2 is approximately ~200 nm (Figure 4) with ZnO deposited as a hexagonal wurtzite crystal and TiO2 having mixed anatase and rutile crystal phases (Supporting Info. Fig S2). In agreement with the finite element model, the deposition is non-uniform and multi-layered for both TiO2 and ZnO coated ACC electrodes (Figure 4). The dielectric NP coated electrodes are observed to have a higher specific capacitance (126 and 120 Fg-1 for ACC-ZnO and ACC-TiO2 at 1 mVs-1) compared to ACC (~76 Fg-1 at 1 mVs-1) at various applied potential scan rates (Table 1 & Supporting Info Figure S3). With increased scan rate, the % difference in specific capacitance between uncoated ACC and dielectric NP coated ACC increased (Table 1).

Table 1: Specific capacitance from CV measurements and electrical parameters of electrodes extracted after fitting the Nyquist plots Parameter

Uncoated ACC

ZnO Coated

TiO2 Coated

50mV

0.52

1.57

1.87

10mV

5.20

20.94

23.52

1mV

76.58

120.08

126.29

Coating capacitance (Cc) in Farads

2.0

6.1

5.6

Double layer capacitance (Cdl) in Farads

4.2

3.1

3.1

Charge transfer resistance (Rct) in Ohms

2.5

3.6

3.5

Pore resistance (Rpo) in Ohms

2.5

1.9

1.9

Series resistance (Rs) in Ohms

37.6

34.2

34.6

Specific capacitance (F/g)



Figure 5: Nyquist plots of the uncoated and nanoparticles-coated ACC. Inset of 5bshows the equivalent electrical circuit (modified rebar model)

Typically an increased scan rate leads to lower time for ion diffusion, in which case the externally protruding dielectric NP coated surface with strong surrounding E-field can lead to a lower resistance for ion transport and adsorption (non-Faradaic reactions) compared to the time required for the ion transport into the micro-pores on uncoated ACC.23 Thus the possibility of forming overlapped double layers blocking the micropores is more prevalent for uncoated ACC that could explain the lower capacitance observed for increased scan rates. Since commercial power supplies are approximated by extremely high scan rates, the dielectric NP coated ACC electrodes constitute a better architecture for ion adsorption applications in real world environments. In order to 18 ACS Paragon Plus Environment

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ascertain the electrical changes brought about on the ACC after the dielectric coating, the electrical equivalent circuit of the electrodes was obtained by electrochemical impedance spectroscopy. The equivalent circuit parameters were extracted by fitting the Nyquist impedance plot of the electrodes with a modified rebar circuit (Figure 5 inset), wherein the depressed semi circles (Figure 5) are representative of multiple RC time constants attributed to non-uniform deposition density and sizes of the dielectric NP’s on the ACC. The double layer capacitance ‘Cdl’ in the rebar model constitutes the capacitive component of the electrode due to ion adsorption directly on the ACC surface in the form of electrical double layers, while the coating capacitance ‘Cc’ constitutes the capacitance arising due to the capacitive configuration formed due to the presence of an additional dielectric coating separating two conductive plates (2nd plate being water), with the value of the dielectric constant, coating thickness and geometrical area of the electrodes influencing the Cc magnitude. The double layer capacitance (Cdl) and coating capacitance (Cc) along with charge transfer resistance (Rct), pore resistance (Rpo) and series resistance (Rs) of the electrodes are extracted from the modified rebar circuit (Table 1). Dielectric coated electrodes show a higher combined capacitance (Cdl + Cc) than uncoated ACC (~6.2 F), with ZnO coated electrodes (~9.2 F) showing slightly higher total capacitance than TiO2 (~8.7 F). The Cdl component of the ZnO and TiO2 coated substrates was lower than uncoated ACC, as coating the porous ACC surface reduces the surface area available



for Cdl formation.23, 49 However the additional coating capacitance component of the dielectric coated electrodes (CcZnO > CcTiO2) led to an increase in their total effective capacitance compared to uncoated ACC electrodes. The dielectric coatings also increased the charge transfer resistance ‘Rct’ due to inherent resistance provided by the dielectric for electrons to escape into the electrolyte. A higher Rct increases the barrier towards Faradaic reactions occurring within the system and contributes to lowering the energy losses due to leakage currents as is also indicated by open circuit potential (OCP) (Supporting Info Figure S4). Additionally the dielectric NP coatings contribute to lowering the pore resistance ‘Rpo’, which is the resistive component arising from the differences in electrolyte concentration between the pores and the bulk. Smaller pores of uncoated ACC lead to larger concentration variations and hence a larger pore resistance compared to the large inter-particle pores of the dielectric NP coated ACC, where electrolyte exchange is not hindered. The dielectric NP’s may also block a large proportion of the micropores on ACC, negating their influence on Rpo. What was interesting to observe were the changes in the series resistance ‘Rs’ of the equivalent circuit, where both ZnO and TiO2 coated electrodes showed a lower uncompensated series resistance compared to uncoated ACC (Figure 5). Typically Rs is a combination of electrolyte resistance along with resistance associated with the current flow path within the measurement cell. Since the electrolyte resistance was kept uniform for all measurements, it appears that by coating the ACC


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with dielectric NP’s, there are changes induced in the current flow path which lead to lower overall resistance. However further focused experiments would need to be carried out to understand the causes for the changes observed in Rs. The electrochemical results above are directly related to the electric field simulation data considering that the capacitance or ion adsorption is mediated by the interfacial electric field strength and distribution at the electrode-water interface. As the magnitude of E-field is more for TiO2 in the vertical axis and for ZnO in the horizontal axis, there could be a net equalization effect which essentially generates comparable coating capacitance results. The moderately lower Cc values for TiO2 can probably be attributed to the thicker coating, as Cc is inversely proportional to coating thickness, which can also lower the E-field in the vertical axis due to higher potential loss. Nonetheless, the CV and Nyquist measurements indicate that by coating the ACC with dielectric NP’s, the electrochemical properties suited for adsorption of charged species can be enhanced.

Capacitive Deionization An assessment of the desalting capacity of the uncoated and dielectric coated ACC electrodes was carried within a CDI device. Over a period of approximately three hours (170 minutes, 25 desalination/regeneration cycles), the desalination efficiency and salt adsorption performance of the plain ACC electrodes was found to be lower than the



dielectric coated ones (Figure 6).

The average salt adsorption rate (ASAR) of ACC varied from 1.05 to 0.91 mg g-1min-1, while for TiO2 and ZnO coated ACC it ranged from 1.55 - 1.43 mg g-1min-1 and 1.72 – 1.62 mg g-1min-1 respectively. As the ASAR is evaluated for the same electrode dimensions and desalination durations, any observed changes are directly proportional to the nature of electrode porosity, wherein lower resistance to ion transport translates to faster adsorption rates. As explained previously, the predominantly microporous nature of ACC (pore sizes < 2 nm) can lead to severe overlapping of double layers and block the pore entrances reducing the desalting rate, while the protruding surface and apparent pores of the dielectric NP coated electrodes has lower ion transport resistance and negligible probabilities for double layer overlapping. Faster adsorption rates are important for practical applications of CDI as it can reduce the desalting time and contribute to lower device energy requirements.

Similarly, the specific salt adsorption capacity (SSA) which is a measure of the surface area actively contributing to salt adsorption, was also lower for ACC and varied from 3.95 to 3.32 mg g-1, while those for TiO2 and ZnO coated ACC varied from 5.29 – 5.14 mg g-1 and 5.72 – 5.43 mg g-1 respectively. The reported SSA and ASAR observed for this


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work are lower compared to values reported in literature,9, 54 because the electrodes were not driven to saturation. Prior reports from our group have shown that plain ACC has a SSA of ~ 10 mg g-1 and is effective for both synthetic and well water desalination,55 while dielectric coated ACC was measured to have a peak SSA of ~ 15 mg g-1 (Supporting Info Figure S5). However for practical applications, electrodes are rarely driven to saturation as the energy to salt adsorption ratio is low after the point of minimum conductivity has been reached.56 While only 3 hours of data is shown here, the electrodes were tested for up to 7 hours of continuous operation with similar desalting trends (Supporting Info Figure S6). This improved and consistent performance is a combination of increased interfacial E-fields along with reduction in Faradaic reactions at the electrode surface57 which can lead to lowering of the desalting capacity with time.

Conclusion In conclusion, a finite model to study the effect of dielectric nanoparticles of zinc oxide (ZnO) and titanium dioxide (TiO2) on the interfacial electric fields generated on polarized conductive electrodes is developed and verified. The simulation indicates that the NP dielectric oxides can substantially improve the electric field magnitudes due to localized permittivity dependent dipole density formation within the NP’s. The increased



interfacial fields were experimentally verified using voltammetry and impedance measurements to lead to stronger double layer capacitance behavior of the electrodes and were successfully incorporated within a capacitive deionization (CDI) device to improve its desalting performance. It was observed that after dielectric coating, the CDI electrodes were more stable and displayed improved salt adsorption rate (ASAR) and salt adsorption capacity (SSA).

Figure 6: (a) Desalination performance of the NP coated and plain ACC electrodes and 24 ACS Paragon Plus Environment

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(b) average salt adsorption rate (ASAR) and specific salt adsorption (SSA) capacity of the electrodes with time

Supporting Information Supporting Information for the manuscript contains information and data on: a.

CDI cell construction scheme

b.

Cyclic voltammaograms of the dielectric coated and plain ACC electrodes at 10 and 50 mVs-1

c.

Open circuit potential (OCP) of electrodes

d.

Specific salt adsorption (SSA) of electrodes when driven to saturation

e.

Long term desalting data

Acknowledgements The authors would like to thank MISTRA Terraclean project for funding the work. The authors also thank Dr. Fei Ye for assisting with the SEM images, Mikael Karlsson for setting up the CV measurements and Dr. Karin Törne for helping with the EIS measurements.

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