Electrochemical analysis of slurry electrodes for flow-electrode capacitive deionization

Accepted Manuscript Electrochemical analysis of slurry electrodes for flow-electrode capacitive deionization

Ko Yeon Choo, Chung Yul Yoo, Moon Hee Han, Dong Kook Kim PII: DOI: Reference:

S1572-6657(17)30748-8 doi:10.1016/j.jelechem.2017.10.040 JEAC 3600

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

12 July 2017 16 October 2017 17 October 2017

Please cite this article as: Ko Yeon Choo, Chung Yul Yoo, Moon Hee Han, Dong Kook Kim , Electrochemical analysis of slurry electrodes for flow-electrode capacitive deionization. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jeac(2017), doi:10.1016/j.jelechem.2017.10.040

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ACCEPTED MANUSCRIPT

Electrochemical analysis of slurry electrodes for flow-electrode capacitive deionization

Ko Yeon Chooa,b, Chung Yul Yooa, Moon Hee Hanb,*, Dong Kook Kima,**

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Separation and Conversion Materials Laboratory, Korea Institute of Energy Research, 152 Gajeong-

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ro, Yuseong-gu, Daejeon 34129, Republic of Korea Graduate School of Energy Science and Technology, Chungnam National University, 99 Daehak-ro,

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Yuseong-gu, Daejeon 34134, Republic of Korea

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* Corresponding author. Tel.: +82 42 821 8601; fax: +82 42 821 8839

** Corresponding author. Tel.: +82 42 860 3152; fax: +82 42 860 3133

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E-mail addresses: [email protected] (Moon Hee Han), [email protected] (Dong Kook Kim)

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ABSTRACT Due to recent advancements in electrochemical devices such as batteries, fuel cells, and supercapacitors, novel electrochemical processes for industrial plant scale including water treatment and desalination are being actively investigated. Slurry electrodes for flow-electrode capacitive deionization (FCDI) are representative process technology with continuous and easy scale-up characteristics. These characteristics are feasible as slurry electrodes can be flowed in microchannels,

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instead of stacking conventional electrodes fixed on plates. However, the electrochemical properties of slurry electrodes for electrochemical process engineering have not been clearly identified,

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compared to those of conventional fixed electrodes. In the present study, we investigated the electrochemical properties of capacitive slurry electrodes with changes in carbon content and

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electrolyte salt concentration using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and deionization/regeneration cycle tests with newly fabricated button-type cells. The CV

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patterns were rectangular, symmetrical, and reversible at a scan rate of 2 mV/s, indicating electrical double-layer capacitive behavior. The results of the EIS and cycle tests demonstrated that increasing the carbon content and electrolyte salt concentration in slurry electrodes improved the cell efficiency

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due to the higher capacitance and lower total resistance.

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Key words: slurry electrode, flow-electrode, capacitive deionization, electrochemical analysis,

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electrochemical impedance spectroscopy, cyclic voltammetry

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1. Introduction The main limitation of conventional electrochemical devices is their lack of energy-efficient and costeffective scalability [1]. The large-scale application of these devices, for instance capacitive deionization (CDI) and membrane capacitive deionization (MCDI), requires static devices to be stacked, resulting in significant increase in size and cost. Hence, flow-assisted electrochemical systems, such as redox flow batteries and electrochemical flow capacitors (EFCs), have been recently

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introduced to simplify large-scale implementation [2]. Flow-electrode capacitive deionization (FCDI) is a desalination technology based on a flow-assisted

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system. And flow-electrodes flow through the path between the current collector and spacer (or ion exchange membrane) in a FCDI cell [3], while fixed electrodes with a certain mass of carbon are

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placed between the current collector and spacer in a conventional (M)CDI cell. In order to increase the adsorption capacity of a fixed carbon electrode, basic cells have to be stacked leading to increase

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in dimensions of the system. In contrast, flow-electrodes can be incessantly supplied from the outside. FCDI needs no cyclic operation which repeats the deionization and regeneration step in turn and is undoubtedly needed in conventional (M)CDI cells with plate-type fixed electrodes, and enables

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continuous desalination with a single cell due to downstream electrode regeneration of the cell. In addition, it can have a larger surface area than a fixed carbon electrode and its surface area can be scaled regardless of the spacer area [4][5].

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Fixed carbon electrodes consist of interconnected carbon particles sticking to a current collector by a

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binder such as PVdF or PTFE, while flow-electrodes consist of flowable slurries of porous carbon particles suspended in an electrolyte [6]. The all principles of both electrodes are identical excepting the composition of each electrode. When an external electric field is imposed, ions in salty water are

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forced to move towards the oppositely charged electrodes, and the charge storage mechanism is based on the electrostatic adsorption of ions at the interface between the active material and electrolyte [7].

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As electrosorption of ions is an interfacial process [8], porous carbon materials with high surface area are commonly used as the electrode to maximize the contact area between the electrode and electrolyte. Therefore, porous carbon materials have been utilized as the electrode in a variety of forms, for examples carbon aerogels [9], activated carbon [10][11], carbon nanotubes [12] and mesoporous carbon [13]. There is a growing interest in carbon-based composite materials that take advantage of the physical and chemical properties of carbon and another one more different constituent materials [14]. 3

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It is important for flow-electrodes to use high surface area carbon, but they have a poor electron conductivity by the electrolyte which is another main component consisting of the flow-electrode, compared to fixed electrodes with interconnected carbon particles. In fixed carbon electrodes, electric charge takes place by transporting through continuous network among particles as carbon particles are interconnected with one another and in direct contact with a current collector. On the other hand, in the case of flow-electrodes composed of porous carbon particles, electrolyte and a conductive

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additive (if any) [7], the particles are neither in direct contact with an inert current collector nor with one another. Hence, when the particles touch the current collector to which an electrical voltage is applied, electrons are transferred between the collector and particles [4]. As flow-electrodes are

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inherently dynamic, the particles are constantly moving and are constantly making and breaking

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contact with each other [5]. The electronic conductivity through such a discontinuous network of particles has been reported to be of order 0.1 – 1 mS/cm, few orders lower than that achieved by fixed electrodes, which can limit the achievable salt removal rate and cell efficiency [15].

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In order to enhance the electronic conductivity of flow-electrodes, three types of methods may be applied. First, flow-electrodes are significantly affected by the weight percentage of carbon, the main

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constituent. Increase in the carbon percentage leads to more effective electronic charge percolation [15][16]. But, until now, the carbon weight percentage cannot exceed about 20 wt.%, and electrodes with the higher concentrations are difficult to flow [15][17]. Second, increasing salt concentration in

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an aqueous electrolyte, the other main constituent of flow-electrodes can reduce the internal

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resistance of the electrode and enhance ion adsorption rate [18]. There are problems that significant concentration polarization may occur at the membrane interface and precipitation can be caused due to aggregation of particles [19]. Lastly, the charge transfer can be enhanced by addition of a third constituent, for examples, adding conductive agents such as carbon black [20] or adding electron

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mediators such as H2Q/Q couple [19].

And the major resistive contributors of fixed electrodes using various experimental methods such as

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electrochemical impedance spectroscopy (EIS) were classified into (1) setup resistance defining as the ionic resistance of the solution in the separator(s), ionic exclusion membrane (if any) resistance(s), the electrical resistance of current collectors, and resistance of any wires, (2) contact resistance referring to the interfacial resistance between current collector and porous electrodes, and (3) the impedance of the porous electrodes using transmission line models [21]. As mentioned above, flow-electrodes are composed of carbon slurry and the composition of carbon slurry determines its rheological and electrochemical properties. The most basic composition of the 4

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slurry is carbon and an electrolyte. Not only the content, particle size and shape of carbon but also the fluid type and salt concentration of an electrolyte have an influence on the properties of slurry. In addition, such factors are expected to vary the contribution of major resistive contributors when carbon slurry is used as flow-electrodes. In the present study, we designed and fabricated three types of electrochemical cells suitable for characterizing the electrochemical properties of slurry electrodes in a static configuration.

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Electrochemical analyses were carried out using various techniques, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and deionization/regeneration cycle test, to investigate the influence of changes in carbon content and electrolyte salt concentration as the two main

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components of slurry electrodes on their electrochemical properties.

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2. Material and methods

2.1 Materials

A commercially available activated carbon powder (Maxsorb MSC-30; Kansai Coke & Chemicals Co.

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Ltd., Japan) was used as the active material for the slurry electrodes. Electrolyte was prepared by dissolving either 0.35 or 3.5 wt.% of reagent-grade NaCl in DI water. Slurry electrodes were prepared by dispersing a suitable amount of the AC powder in the electrolyte to 5, 10, or 12 wt.% carbon

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content. The slurries were stirred overnight to allow the pores of the AC particles to be filled with the

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electrolyte.

The surface morphology of the activated carbon was examined by scanning electron microscopy (SEM; S-4700, Hitachi, Japan) and is shown in Figure 1. The SEM images indicated that the activated carbon

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was comprised of highly porous carbon spheres with diameters of less than 10 μm. The physical properties of the activated carbon were measured in liquid nitrogen at 77.3 K using a Micromeritics ASAP 2420 system (Norcross, USA). The activated carbon had a high specific surface area of 3453

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m2/g, an average pore diameter of 2.1 nm and a total pore volume of 1.85 mL/g. Rheological analysis of slurries used in this study was performed upwardly and then downwardly at shear rates ranging from 19.42 to 233.01 1/s at 25 C using a viscometer (DV-II+Pro, Brookfield, USA). Figure 2 shows the slurries are non-Newtonian fluids with shear thinning that the apparent viscosity of the slurry reduces when the shear rate increases. This is similar to the trends observed for the suspensions which are used in flow-assisted systems [22]. The apparent viscosity of the slurry was increased with the increased carbon content and was decreased with the increased electrolyte 5

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salt concentration. Especially, the slurries used in this study have easier flowability than the slurries (the higher carbon content) which Presser et al. [23] used, due to the lower apparent viscosity.

2.2 Cell configuration Three types of symmetrical two-electrode cells were designed to examine the electrochemical properties of slurry electrodes in a static configuration. These cells have the same placement CDI unit

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cell with fixed electrodes. The basic cell (A1B1 cell) is comprised of two graphite current collectors, onto which the slurry was uniformly loaded into a chamber defined by a glassy fiber separator (Whatman Cat No 1821 090) to ensure polarization between the two slurry electrodes. The chamber

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was of 18 mm in diameter and 1 mm thick. Cylindroid ribs (1 mm in diameter and 1 mm high) were

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located on the collectors. The diameter of the cell was equivalent to that of a commercially available electrochemical cell (EL-CELL, Germany). The cell components were held together using polycarbonate housings and a fastener. Slurries were simultaneously injected into the two chambers

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with equal masses through small holes of the collectors using regulated syringe pumps (Fusion Touch 100, Chemyx, Stafford, USA). Schematic diagrams of the graphite current collector and cell are shown

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in Figure 3.

The second cell has two chambers of 18 mm in diameter and 2 mm thick for slurry loading and is named as A1B2 cell. The third cell forming chambers of 36 mm in diameter and 1 mm thick is named

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A2B1 cell.

2.3 Electrochemical analysis

Electrochemical analysis was performed using symmetrical two-electrode cells filled with slurry

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electrodes at ambient conditions with a ZIVE SP2 Electrochemical Workstation (WonATech, Seoul, Republic of Korea). CV was measured at scan rates of 2, 5, and 10 mV/s in a –1.2 to 1.2 V potential

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window. The electrode capacitance was calculated according to the following equation [24]:

Here, V/s and

is the specific volumetric capacitance in F/cm3, is the volume of the two electrodes in cm3. 6

is the current in A,

is the scan rate in

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All of the impedance tests were performed over a frequency range of 1 MHz to 5 mHz at a sinusoidal perturbation of the open circuit voltage (OCV) with a 5 mV amplitude. Because the slurry electrodes remained in the cell, rather than flowing through the cell, cycling tests were carried out by repeating a deionization step at a constant voltage of 1.2 V and a regeneration step at a constant current of -2 mA up to a cut-off voltage of 0 V. The transferred charge was

is the transferred charge during the deionization or regeneration step in C,

is the electrical

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Here,

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calculated using the following equation [18]:

current passing through the cell during each step in A, and

is the duration of each step in s. The

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coulombic efficiency was expressed as the ratio of the transferred charges during each cycle. The electrode capacitance and equivalent series resistance (ESR) were also calculated from the fifth

Here,

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galvanostatic regeneration step using the following equation [6][25]:

is the electrode capacitance in F,

is the slope of regeneration curve starting from the

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bottom of the IR drop and ending at the half of the high potential in V/s. the beginning of regeneration step in V.

3. Results and discussion

3.1 CV analysis 7

is the initial voltage at

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To evaluate the electrochemical behavior of slurry electrodes with different compositions, CV measurements were obtained at scan rates of 2, 5, and 10 mV/s with a voltage window from –1.2 to 1.2 V and the results are shown in Figure 4. The CV graphs were measured by applying an electrical potential difference over two symmetric electrodes which are composed of positively and negatively polarized electrodes, a kind of electric double-layer capacitor. The voltage window was chosen to confirm the reversibility of ion adsorption and desorption cycle of the surface of electrodes. The CV

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patterns obtained at a scan rate of 2 mV/s exhibit rectangular, symmetrical, and reversible polarizable characteristics without any redox peaks, which indicates electrical double-layer capacitive behavior. The CV patterns became narrower and exhibited an oval-like shape at higher scan rates, indicating

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increasing resistivity because of the limited diffusion of ions into pores and the contact resistance

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among particles [7]. At a scan rate of 2 mV/s, the capacitance was found to be 2.04 F/cm3 for slurry electrodes containing 10 wt.% carbon dispersed in 0.35 wt.% NaCl electrolyte in the A1B1 cell. The capacitance decreased to 1.68 F/cm3 at 5 mV/s and 1.51 F/cm3 at 10 mV/s; these values represent

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18 and 26% reductions in capacitance, respectively, compared to the value obtained at 2 mV/s. In the case of slurry electrodes containing 12 wt.% carbon in the same cell, the capacitance at 2 mV/s

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was 2.47 F/cm3, which decreased to 2.05 and 1.90 F/cm3 at 5 and 10 mV/s, respectively, representing losses of 17 and 23%. Thus, electrode slurries with higher carbon contents resulted in cells with higher capacitances. This can be attributed to the increased accessible surface area [7].

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Capacitance decay was observed at high scan rates.

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The capacitance of slurry electrodes containing 10 wt.% carbon in the A1B2 cell was 2.44, 2.06 and 1.62 F/cm3 at 2, 5 and 10 mV/s, respectively. As the A1B2 cell has twice as large chambers as the A1B1 cell, its slurry loading is increased. The capacitance decay was 16% at 5 mV/s and 34% at 10 mV/s, compared to the value obtained at 2 mV/s. More slurry loading also resulted in higher

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capacitances. The capacitance decay was more intensified in the A1B2 cell than in the A1B1 cell. This phenomenon was similar to that observed for fixed electrodes with increasing electrode film thickness,

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which hinders ion movement within the electrode, resulting in longer diffusion pathways and higher electrode resistivity [26]. Hence, the slurry electrodes behaved as if they were much thicker fixed electrodes. The slurry electrode resistivity can result from indirect particle-particle contacts in addition to diffusion limitations due to the thickness of slurry. The carbon utilization was improved in the cells with higher carbon content, as seen from the capacitances normalized to the mass of active materials at a scan rate of 2 mV/s. This implies that the higher the carbon content is, the more effective the electronic charge percolation through the 8

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electrode is [15].

3.2 EIS analysis EIS measurements were performed to investigate the electrochemical properties of slurry electrodes. The A1B1 cell was used unless stated otherwise. Figure 5 and Figure 6 show the Nyquist and Bode plots for the slurry electrodes and electrolytes used in the present study. Semi-ellipses were not

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observed in the Nyquist plots obtained for the electrolytes. The Nyquist plots exhibit similar patterns for all of the slurry electrodes. These patterns largely consist of a small and obvious semi-ellipse, a larger arc and a linear region. The semi-ellipse and arc correspond to two inflection points in the Bode

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plots, and the linear region suggests typical electrical double-layer capacitive behavior [27]. The

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intersection values with the real impedance axis in the high-frequency region of the electrolyte curves were left-shifted from 13.08 ohm to 1.97 ohm with increasing salt concentration, which indicates decreasing resistance. The values are related to the setup resistance including the ionic resistance of

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the electrolyte in the separator, the electrical resistance of the current collectors and the resistance of any wires [21]. Hence, the semi-ellipses and arcs present in the Nyquist plots were caused by the

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slurry electrodes. Increasing the electrolyte concentration of the slurry electrodes reduced the diameter of the first semi-ellipse. Increasing the carbon content of the slurry electrodes decreased the magnitude of the subsequent arc.

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Figure 5 shows the Nyquist plots and Bode plots for slurry electrodes with different carbon contents

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dispersed in 0.35 wt.% NaCl electrolyte. The setup resistance was 10.48, 10.92 and 9.92 ohm for 5 wt.%, 10 wt.% and 12 wt.% carbon content, respectively. The values were similar regardless of the carbon content, and slightly decreased compared to the value obtained for 0.35 wt.% NaCl electrolyte. The different carbon content in the same electrolyte had a significant influence on size of the arc,

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leading to changes in the total resistance. In the Bode plots against frequency for slurry electrodes, the initial impedance and the subsequent impedance increment were similar while the final

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impedance increment increased more steeply for slurry electrodes with less carbon content. The impedance with regard to the semi-ellipse was affected by the electrolyte salt concentration of slurry electrodes. As seen in Figure 6, the setup resistance was 2.26, 1.77 and 1.95 ohm for 5 wt.%, 10 wt.% and 12 wt.% carbon content, respectively, when the electrolyte salt concentration of 3.5 wt.% was used to prepare slurry electrodes. The resistance values were similar to the value obtained for 3.5 wt.% NaCl electrolyte. In addition, increasing electrolyte salt concentration of slurry electrodes decreased the size of the semi-ellipse. As a result, higher carbon content and electrolyte concentration in slurry 9

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electrodes led to lower real impedance, which in turn can improve the cell efficiency. Eventually, utilization of slurry electrodes led to higher capacitance and resistance, simultaneously. But, the electrolyte salt concentration of slurry electrodes can have an effect on the desalting efficiency in a FCDI cell. Based on the necessity of a moderate amount of salt in flow-electrodes [18], it is necessary to find the optimum salt concentration range according to the carbon content. The effect of separator thickness is shown in Figure 7. The Nyquist plots were right-shifted with

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increasing separator thickness. The setup resistance increased from 9.06 ohm for 0.5 mm separator to 22.07 ohm for 1.0 mm separator. As seen in the Bode plots, there was no significant difference in the increment tendency of the impedance against frequency, excluding the initial impedance related

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to the setup resistance. Decreasing separator thickness may be one method which can decrease the

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total resistance.

Figure 8 shows the effect of cell type. The intersection value with the x-axis in the high frequency region, which represents the setup resistance, decreased in the order of A1B2, A1B1 and A2B1 cell.

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In the A1B2 cell with thicker slurry electrodes, the largest total resistance was observed. Thicker slurry electrodes seem to be lead to longer diffusion paths and higher electrode resistivity, like the CV

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results. To improve resistance characteristics of slurry electrodes, increasing the contact area between the current collector and electrode and decreasing the slurry thickness are advantageous.

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3.3 Deionization/regeneration cycle tests

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To investigate the electrosorption performance of the slurry electrodes, deionization/regeneration cycle tests were performed five consecutive times. The initial current at the first deionization step increased with increasing carbon content and electrolyte salt concentration, as shown in Table 1. This suggests that there was a corresponding decrease in the resistance of the cell. Yang et al. [18]

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presented a close connection between desalting efficiencies and electrical currents in a FCDI cell, where the desalting efficiency of the flow-electrode was proportional to the electrical current of the

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cell. Therefore, desalting efficiency of slurry electrodes in FCDI is somewhat predictable from the initial current of static slurry electrodes. Figure 9 shows the changes in coulombic efficiency with cycle number. Only 40-55% of the total charge transferred during the first deionization step was transferred during the first regeneration step. When the transferred charges between the deionization and regeneration step in each cycle were compared, they were significantly affected by the carbon content rather than the electrolyte salt concentration. In the first cycle, the difference between the charges is assumed to be caused by 10

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irreversible side reactions that can occur at the surface functional groups [28] on the carbon during deionization (charging). When the cycle tests were repeated (aging), the charges at the deionization and regeneration step became gradually stabilized. About 90% of the low columbic efficiency was attributed to the charge recombination [23] or the co-ion effect that ions with the same polarity as an electrode may be captured [20][29]. Increase in carbon content resulted in electrodes with higher coulombic efficiency. The similar trends were observed in 3.5 wt.% NaCl electrolytes.

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Table 2 shows the electrode capacitance and ESR calculated from the fifth regeneration step. The capacitance was directly related to the carbon content of slurry electrodes. The ESR decreased with increasing the carbon content and electrolyte salt concentration in slurry electrodes. Higher carbon

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content and electrolyte salt concentration in slurry electrodes can improve the cell efficiency due to

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the higher capacitance and the lower ESR, which was similar to the EIS results.

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4. Conclusions

Slurry electrodes were analyzed in novel cells that can maintain slurry electrodes in a static state. CV,

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EIS, and deionization/regeneration cycle tests were used to characterize their electrochemical properties. The CV patterns obtained at a scan rate of 2 mV/s were rectangular, symmetrical, and reversible polarizable characteristics, indicating electrical double-layer capacitive behavior. At high

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scan rates, these patterns became more resistive and exhibited capacitance decay. The slurry

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electrodes behaved as if they were much thicker fixed electrodes. EIS analysis confirmed that the resistance of the cell was significantly affected by the electrolyte and carbon concentrations of the

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slurry electrodes, in addition to the setup resistance including the ionic resistance of the electrolyte in the separator, the electrical resistance of the current collectors and the resistance of any wires. The

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results of the deionization/regeneration cycle tests confirmed that the total resistance of the cell decreased with increasing carbon content and electrolyte concentration. The electrochemical properties of slurry electrodes were improved by increasing their carbon content and electrolyte salt concentration (slurry composition), decreasing separator thickness, and increasing the contact area between the current collector and electrode and decreasing the slurry thickness (size of slurry electrode). However, further study is required to determine the optimal carbon content and electrolyte 11

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salt concentration, and the maximum efficiency of the slurry electrodes.

ACKNOWLEDGMENTS This work was conducted under framework of the Research and Development Program of the Korea

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solid lithium rechargeable flow battery, Adv. Energy Mater. 1 (2011) 511–516. doi:10.1002/aenm.201100152. [23]

V. Presser, C.R. Dennison, J. Campos, K.W. Knehr, E.C. Kumbur, Y. Gogotsi, The

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electrochemical flow capacitor: A new concept for rapid energy storage and recovery, Adv. Energy Mater. 2 (2012) 895–902. doi:10.1002/aenm.201100768. T. Kim, J. Yoon, Relationship between capacitance of activated carbon composite electrodes

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measured at a low electrolyte concentration and their desalination performance in capacitive deionization, J. Electroanal. Chem. 704 (2013) 169–174. doi:10.1016/j.jelechem.2013.07.003. [25]

J. Kang, J. Wen, S.H. Jayaram, A. Yu, X. Wang, Development of an equivalent circuit model for electrochemical double layer capacitors (EDLCs) with distinct electrolytes, Electrochim. Acta. 115 (2014) 587–598. doi:10.1016/j.electacta.2013.11.002. 14

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H. Yoon, H.J. Kim, J.J. Yoo, C.Y. Yoo, J.H. Park, Y.A. Lee, W.K. Cho, Y.K. Han, D.H. Kim, Pseudocapacitive slurry electrodes using redox-active quinone for high-performance flow capacitors: An atomic-level understanding of pore texture and capacitance enhancement, J. Mater. Chem. A. 3 (2015) 23323–23332. doi:10.1039/c5ta05403f.

[27]

W. Shi, H. Li, X. Cao, Z.Y. Leong, J. Zhang, T. Chen, H. Zhang, H.Y. Yang, Ultrahigh

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Performance of Novel Capacitive Deionization Electrodes based on A Three-Dimensional Graphene Architecture with Nanopores, Sci. Rep. 6 (2016) 18966. doi:10.1038/srep18966. P. Nativ, Y. Badash, Y. Gendel, New insights into the mechanism of flow-electrode capacitive

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Y. Bian, X. Yang, P. Liang, Y. Jiang, C. Zhang, X. Huang, Enhanced desalination performance

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of membrane capacitive deionization cells by packing the flow chamber with granular activated

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carbon, Water Res. 85 (2015) 371–376. doi:10.1016/j.watres.2015.08.058.

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deionization, Electrochem. Commun. 76 (2017) 24–28. doi:10.1016/j.elecom.2017.01.008.

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Figure 1. The surface morphology of the activated carbon powder

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Viscosity 10 wt.% 10 wt.% 12 wt.% Shear Stress 10 wt.% 10 wt.% 12 wt.%

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10 wt.% carbon in 0.35 wt.% NaCl electrolyte; A1B1 cell

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Figure 4. CV patterns for various slurry electrodes: (a) 10 wt.% carbon in 0.35 wt.% NaCl (A1B1 cell), (b) 12 wt.% carbon in 0.35 wt.% NaCl (A1B1 cell) and (c) 10 wt.% carbon in 0.35 wt.% NaCl (A1B2 cell) (d) capacitances normalized to the mass of active materials at a san rate of 2 mV/s

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0.023

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0.065

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Carbon content (wt.%)

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Table 2. The electrode capacitance and ESR calculated from the fifth regeneration step

Capacitance, F

ESR, ohm

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93.41

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60.23

12 wt.% AC in 0.35 wt.% NaCl

4.145

49.32

5 wt.% AC in 3.5 wt.% NaCl

0.868

82.20

10 wt.% AC in 3.5 wt.% NaCl

3.109

51.62

12 wt.% AC in 3.5 wt.% NaCl

4.642

18.28

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Highlights  Novel electrochemical cells suitable for slurry electrodes were developed.  The electrochemical properties of slurry electrodes were affected by their composition.  Increasing the carbon content of slurry electrodes led to a higher capacitance.

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 Increasing the electrolyte salt concentration of slurry electrodes improved the cell efficiency.

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