Antecedentes. Fabricación y caracterización de un electrodo de carbono poroso para la desalinización de agua salobre

ANTECEDENTE 1

Fabricación y caracterización de un electrodo de carbono poroso para la desalinización de agua salobre

Fabrication and characterization of a porous carbon electrode for desalination of brackish water

Abstract

To increase the surface area of electrodes for electrosorption, porous carbon electrodes were fabricated using a wet phase inversion method. A carbon slurry consisting of a mixture of activated carbon powder (ACP) and polyvinylidene fluoride solution was cast directly on a graphite sheet. The cast film was then immersed in pure water for phase inversion. Scanning electron microscopy images verified that the electrodes were porous and the pore size was less than 100 nm. The electrochemical properties were characterized using cyclic voltammetry. The electrical capacitance ranged from 2.18 to 4.77 F/cm2 depending on the ACP content, and the electrical capacitance increased as the solvent content decreased. The solvent content was an important variable controlling the pore size and capacitance of the electrodes.

Keywords: Electrical double layer; Electrosorption; Desalination; Porous carbon electrode; Wet phase inversión.

1. Introduction

When an electrical potential is applied to a polarizable electrode, an electric double layer

(EDL) forms at the electrode surface. Electro- sorption is an efficient new process for removing

ionic substances from water by holding charged ions in the EDL. Once the electric field disappears,the ions are quickly released back into the bulk solution [1,2]. The recently proposed

electrosorption process has several advantages over conventional desalination technologies, such as ion exchange, reverse osmosis membranes, and electrodialysis. As it operates at low cell voltages,electrosorption is energy efficient process.

Electrosorption also has advantages over ion exchange because no acid, base, or salt solutions

are required to regenerate resins [3].

To achieve high capacitance in an EDL, the electrode material must have a large surface area.

Therefore, porous materials with a large surface area, such as activated carbon powder, carbon

cloth, and carbon aerogel, are used to make electrodes for electrosorption [4]. The capacitance of electrodes prepared from these materials depends on the properties of the material itself. Although the capacitance can be sufficiently high simply by using an electrode material with a high surface area, the surface area can be increased by modifying the surface morphology of the electrode.

We fabricated a new porous carbon electrode using a wet phase inversion method. To investigate pore formation according to the phase inversion conditions, carbon electrodes were

prepared with various activated carbon powder (ACP) and solvent contents. The physical and

electrochemical properties of the prepared electrodes were investigated using scanning electron

microscopy (SEM), porosimetry, and cyclic voltammetry.

2. Materials and methods

2.1. Electrode preparation

To fabricate porous carbon electrodes, a carbon slurry was prepared by mixing activated

carbon powder (Daedong AC Co., BP-15) and polyvinylidene fluoride (PVDF) as a polymer

binder in N-methyl-2-pyrrolidone (NMP). After stirring the mixture for 1 h to ensure homogeneity, it was cast directly on a graphite sheet with a thickness of 300 μm using a casting knife.

The cast film was then immersed in pure water as a non-solvent for 24 h where exchange occurred between the solvent (NMP) and non-solvent (water). To compare the pore formation and electrochemical properties according to the composition of the carbon slurry, various weight ratios of carbon slurry were used.

2.2. Surface morphology and pore size measurements

To characterize the physical structure of the prepared electrode, its surface morphology and

pore size were measured. The surface morphology was characterized by SEM (JSM-6335F;

JEOL) with an acceleration voltage of 10 kV. The pore size and pore size distribution of the

electrodes were analyzed using an automatic mercury intrusion porosimeter (AutoPore IV

9500; Micromeritics Instrument) at pressures of 30–33,000 psi.

2.3. Characterization of electrochemical properties

The electrochemical performance of the electrode was evaluated by cyclic voltammetry (CV)

using a potentiostat (PGSTAT30; AutoLab). A three-electrode cell was used with 0.1 M Na2SO4

solution as the electrolyte, Ag/AgCl as the reference electrode, and a platinum rod as the

counter electrode. On inserting the fabricated carbon electrodes as the working electrode, cyclic

voltammetry was performed at a scanning rate of 1.0 mV/s in potential windows of !0.6–0.2 V (vs. Ag/AgCl). The effective area of the electrode was 0.126 cm2. All of the electrochemical experiments were carried out at room temperature.

3. Results and discussion

3.1. Electrode surface structure

Fig. 1 shows SEM images of electrodes prepared with various NMP contents. As shown in

the figure, pores of various sizes formed uniformly on the electrode surface. These pores

resulted from the phase inversion of PVDF during the exchange of NMP and water. Moreover,

we observed different surface morphologies according to the NMP content. The pore size

tended to increase with the NMP content[pic 1].

Fig. 1. SEM images of porous carbon electrodes prepared with different NMP content.

3.2. Pore size distribution of the electrodes

To characterize the pores formed on the electrode surface, the pore size and size distribution

were analyzed using mercury intrusion porosimetry at pressures of 30 to 33,000 psi. Applying

Washburn’s equation to each measured pressure gave the pore size associated with each pressure.

The average pore sizes determined for the electrodes fabricated with various NMP contents

are summarized in Table 1. The pore sizes ranged from 64.2 to 82.4 nm and the size increased as the NMP content increased.

Fig. 2 shows the pore size distribution of electrodes prepared with different NMP contents.

In all cases, the pore size had a bimodal distribution, with the first peak at pore sizes of

300–500 nm and the second at 1,000–2,000 nm. Moreover, the number of small pores tended to

increase as the NMP content decreased.

[pic 2]

[pic 3]

3.3. Capacitance and electrical performance of the electrodes

To determine the capacitance of the carbon electrodes, cyclic voltammograms were obtained

at a scan rate of 1.0 mV/s. Fig. 3 shows the cyclic voltammograms of electrodes with different ACP contents obtained at a scan rate of 1.0 mV/s. All of the voltammograms showed behavior typical of an electrical double layer charging/discharging on the carbon surface. This means that ions are adsorbed in the electrical double layer due to Coulombic interaction [6]. As shown in Fig. 3, the current density increased with the ACP content.

The capacitance of an electrode (C) is calculated as C = i/v where i is the current density

and v is the potential scanning rate [5]. The capacitance of the porous carbon electrodes at

0.2 V (vs. Ag/AgCl) according to the ACP content is summarized in Table 1. The capacitance

increased with the ACP content and the values ranged from 2.18 F/cm2 for 50 wt% ACP to 4.77 F/cm2 for 90 wt% ACP. The specific resistances of the electrodes are also shown in Table 1. As expected, the resistance decreased exponentially as the ACP content increased.

Fig. 4 shows the changes in capacitance according to the potential of electrodes with various NMP contents. In the electrosorption process, no charge transfer reactions occur at the

electrode-solution interface. Instead, processes such as adsorption and desorption take place at

the electrode surface with changing potential. Thus the cyclic voltammograms shown in Fig. 4

are symmetric, indicating that the EDL charging process is reversible. Fig. 4 indicates that the capacitance decreases as the NMP content increases. As shown in Fig. 1, the pore size

increases as more NMP is added to the carbon slurry, decreasing the surface area of the electrode, which in turn decreases the capacitance. The solvent content was an important variable that controlled the pore size and capacitance of the electrodes.

4. Conclusions

To increase the electrode surface area, porous carbon electrodes were prepared using a wet

phase inversion method and their surface characteristics and electrochemical performance were

evaluated. SEM images showed that the electrodes were porous and the pore size was less

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