This work is aimed at improving the electrosorption capacity of carbon nanotube/reticulated vitreous carbon- (CNT/RVC-) based 3D electrodes and decreasing the duration of electrosorption-desorption cycles by facilitating the ions’ adsorption and desorption on the electrode surface. This was achieved by preparing composites of microwave-irradiated graphene oxide (mwGO) with CNT. All composite materials were coated on RVC by the dip-coating method. The highest loading level was 50 mg. This is because it exhibited the maximum electrosorption capacity when tested in terms of geometric volume. The results showed that the 9-CNT/mwGO/RVC electrode exhibited 100% capacitive deionization (CDI) cyclic stability within its 1st five cycles. Moreover, 27.78% time was saved for one adsorption-desorption cycle using this electrode compared to the CNT/RVC electrode. In addition, the ion removal capacity of NaCl by the 9-CNT/mwGO/RVC electrode with respect to the mass of the electrode (3.82 mg/g) has increased by 18.27% compared to the CNT/RVC electrode (3.23 mg/g) when measured at the optimum conditions. In a complete desalination process, the water production per day for the 9-CNT/mwGO/RVC electrode was increased by 67.78% compared to the CNT/RVC electrode when measured within the same CDI cell using NaCl solution of concentration less than 1 mg/L. When considered volume of 1 m3, this optimum 9-CNT/mwGO/RVC electrode produces water 29,958 L per day. The highest electrosorption capacity, when measured experimentally at 500 mg/L NaCl feed concentration, was 10.84 mg/g for this optimum electrode, whereas Langmuir isotherm gave the theoretically calculated highest value as 16.59 mg/g. The results for the 9-CNT/mwGO/RVC composite electrode demonstrate that it can be an important electrode material for desalination in CDI technology.
The electrosorption capacity and stability of an electrode depend on its pore structure, surface area, and electrical conductivity of electrode [
CNTs were accidentally discovered in 1991 by a Japanese scientist, Iijima, using an arc-discharge process [
Graphene has recently been attracted huge attention among the scientific communities because of its unique characteristics like large theoretical specific surface area (2630 m2/g), high intrinsic mobility (200,000 cm2/vs) [
The most common approach to graphite exfoliation is the use of strong oxidizing agents to produce graphene oxide. The first production of graphene oxide was demonstrated by Oxford chemist Brodie in 1859, who added a portion of potassium chlorate (KClO3) to a slurry of graphite in fuming nitric acid (HNO3) [
Graphene has become one of the most attractive subjects due to its several breakthroughs in fundamental research and some promising practical applications [
In this study, our aim is to prepare 3D electrode materials based on acid-functionalized single-walled carbon nanotubes (a-SWCNT) and mwGO using RVC as a substrate and check their performance in a CDI system using a feed stream flowing directly through the electrodes. The performance of the electrodes was tested at different working conditions like flow rate and bias potential, which were optimized. Furthermore, the electrosorption isotherms like Langmuir and Freundlich models were investigated to describe how ions interact with electrodes. The performance of electrodes was evaluated through the electrosorption dynamic study. All the characteristics are very important to develop electrode materials for using effectively in desalination technology.
The commercial SWCNT (Hipco-CCNI/Lot # p1001) and graphite powder were supplied by Carbon Nanotechnologies, Inc. (Houston, TX) and Bay Carbon, Inc., respectively, and those were used as received. The chemicals DMF, HNO3 (70%), KMnO4, C2H5OH, and NaCl were procured from Sigma-Aldrich. In addition, the chemicals like H2SO4 (98%,
In this study, the SWCNT was functionalized by treating with nitric acid (a-SWCNT) and graphene oxide (GO) was synthesized by the modified Hummers method as described by Marcano et al. [
The electrochemical characterizations of base materials and their composite electrodes were performed by cyclic voltammetry (CV). The measurement was done using the three-electrode system setup. The a-SWCNT/RVC, mwGO/RVC, or a-SWCNT/mwGO/RVC acted as the working electrode (WE) in the 1 M NaCl aqueous solution over the voltage range -0.2-1.0 V; RVC electrode and Ag/AgCl (3 M NaCl) acted as the counter electrode (CE) and reference electrode (RE), respectively. For the CDI characterization, Pt electrode was used as a CE to avoid any chance of limiting the performance of the other composite electrodes. The measurement was performed at the scan rate of 5, 10, 20, 50, 100, and 200 mV/s. A platinum wire was used to make contact between WE and CE.
The desalination experiments were performed within a flow-through electrode system using a capacitive deionization (CDI) cell. In this measurement, the total volume and concentration of the NaCl solution were 70 mL and 75 mg/L, respectively. The distance between electrodes was 5 mm, and the solution temperature was maintained at 293 K. The total desalination processes, which involve the measuring of the amount of ion removal from the NaCl aqueous solution, the construction of a capacitive deionization (CDI) cell, measuring the effect of flow rate and voltage on ion removal efficiency, and the calculation of electrosorption capacity, are described within supplementary sections
The adsorption performance test was carried out at the optimum applied voltage 1.5 V (in this study, the ferricyanide solution was used to test the 3 electrode system. We observed the oxidation peak shift to 0.59 V, where the ideal oxidation peak was 0.29 V. Hence, the maximum/optimum applied voltage for our CDI system was 1.5 V.) and optimum flow rate 50 mL/min, as reported in our previous study for the CNT/RVC electrode [
(a) Adsorption behaviour and (b) the electrosorption capacity in terms of the mass of composite material loading and the geometric volume of the electrode of various 9-CNT/mwGO/RVC electrodes. Loadings (mg): 10, 30, and 50.
This study is based on the 50 mg 9-CNT/mwGO composite-coated RVC electrode because it showed the highest electrosorption capacity in terms of geometric volume. The optimization was carried out for electrical voltage and flow rate. The investigated cell voltages were 1.3 V and 1.5 V, and the flow rates were 25 mL/min, 50 mL/min, and 75 mL/min as shown in Figures
Conductivity variations of the NaCl solution with various (a) applied voltages and (b) applied flow rates, with respect to operating time, using the 9-CNT/mwGO/RVC electrode loaded with 50 mg composite material.
The CDI system was investigated with respect to the influence of increasing ratios of mwGO in the CNT/mwGO composite material-coated RVC electrodes on the ion removal performance. The ratio levels were 10 : 0, 9 : 1, 8 : 2, and 7 : 3 CNT : mwGO, respectively, and the mass of materials coated on all RVC electrodes was 50 mg. All experiments were performed with the same previous conditions at 1.5 V and 50 mL/min flow rate with 6 min adsorption processes. Figure
(a) Adsorption and release behaviour and (b) the electrosorption capacity in terms of mass of CNT/mwGO and the geometric volume of electrode of various ratios 10, 9, 8, and 7 CNT in CNT/mwGO/RVC electrodes.
Figure
Electrosorption of NaCl by the CNT/mwGO/RVC electrodes with various ratios of CNT and time of one desalination cycle (
Ratio of a-SWCNT in electrodes | Electrosorption | Time of one desalination cycle | |||
---|---|---|---|---|---|
mg/g | mg/cm2 | mg/cm3 | % | min | |
7 | 3.01 | 0.07 | 17 | ||
8 | 3.52 | 0.09 | 8.98 | 16 | |
9 | 3.82 | 0.10 | 18.27 | 13 | |
10 | 3.23 | 0.08 | 18 |
The performance of electrode adsorptions is evaluated by dynamics study, which describes the solute uptake rate, and evidently, this rate controls the residence time of adsorptive uptake at the solid-solution interface [
(a–d) Electrosorption and (e–h) pseudo-first-order adsorption kinetics of the NaCl electrosorption onto CNT/RVC, 9-CNT/mwGO/RVC, 8-CNT/mwGO/RVC, and 7-CNT/mwGO/RVC electrodes, respectively, at 1.5 V and 50 mL/min flow rate. Results have been derived from Figure 5.15 (a) (adsorption process).
The pseudo-first-order kinetics for all electrodes was studied within the first four minutes as shown in Figures
The comparison between the adsorption rate constant (
a-SWCNT : mwGO | Theoretical | Experimental | ||
---|---|---|---|---|
7 : 3 | 0.990 | 0.639 | 2.75 | 3.01 |
8 : 2 | 0.987 | 0.555 | 3.45 | 3.52 |
9 : 1 | 0.992 | 0.525 | 3.66 | 3.82 |
10 : 0 | 0.994 | 0.816 | 3.19 | 3.23 |
The regeneration of electrodes plays a significant role in their commercialization for using in CDI systems. To test reversibility, the 9-CNT/mwGO/RVC electrode was selected because it had the highest electrosorption capacity among all the electrodes. Several charging and discharging cycles for this electrode are presented in Figure
Multiple electrosorption-desorption cycles of the 75 mg/L NaCl solution for the 9-CNT/mwGO/RVC electrode measured at 50 mL/min flow rate through electrode upon polarization and depolarization at 1.5 V and 0 V, respectively.
The electrosorption isotherm is generally used to describe how ions interact with carbon electrodes. The Langmuir and Freundlich isotherms are the two most common isotherms, and they were employed for simulating the ion adsorption on the 9-CNT/mwGO/RVC electrode. The electrosorption isotherms of NaCl onto the 9-CNT/mwGO/RVC electrode were evaluated, and their results were compared with the results of the CNT/RVC electrode. This experiment was performed using the different concentrations of NaCl as presented in Figure
The electrosorption isotherms for 9-CNT/mwGO/RVC and CNT/RVC electrodes at 1.5 V and 50 mL/min flow rate using different initial concentrations of the NaCl solutions.
Table
The parameters of Langmuir and Freundlich isotherms for the NaCl electrosorption using the 9-CNT/mwGO/RVC and CNT/RVC electrodes.
Isotherm | Parameter | Value | Value |
---|---|---|---|
Langmuir | 16.59 | 13.08 | |
0.01 | 0.01 | ||
R2 | 0.995 | 0.997 | |
Freundlich | 0.32 | 0.28 | |
1.74 | 1.74 | ||
0.981 | 0.989 |
The water production experiment and calculation were carried out at the NaCl feed solution concentration 75 mg/L. It has been shown earlier that 1 g of the 9:CNT/mwGO composite and CNT coated on 43.20 cm3 RVC electrode adsorbed 3.82 mg and 3.23 mg NaCl during 13 mins and 18 mins, respectively. Hence, the solution concentration was reduced from 75 mg/L to 71.18 mg/L and 71.77 mg/L for 9:CNT/mwGO/RVC and CNT/RVC electrodes, respectively, after 1 desalination cycle. Moreover, it has also been shown that the electrosorption capacity varied with the increase in solution concentration and exhibits a linear relationship below the concentration of 100 mg/L (Figure
(a) The variation of electrosorption with respect to feed concentration and (b) the variation of feed concentration with respect to desalination cycles.
For the 9:CNT/mwGO/RVC composite electrode,
For the CNT/RVC composite electrode,
From these equations, the variation of concentration can be known after each desalination cycle. Figure
The CNT/mwGO composites at their different ratios were successfully coated on the RVC electrode to prepare 3D electrodes and used in the CDI cell. The results showed that the optimal electrode had very high CDI cyclic stability, maintaining an electrochemical cycling stability of 100% when measured up to five cycles. Moreover, the time saving of one electrosorption-desorption cycle with the 9-CNT/mwGO/RVC electrode was 27.78%, compared with the CNT/RVC electrode, which required 18 min. In addition, the electrosorption removal of NaCl by the 9-CNT/mwGO/RVC electrode in terms of mass of the electrode (3.82 mg/g) increased 18.27% compared to the CNT/RVC electrode (3.23 mg/g) when measured at the optimum condition. The optimum electrode, 9-CNT/mwGO/RVC composite, showed a 67.78% increment per day in the desalinated water production compared to the CNT/RVC electrode at their same testing condition. The optimum electrode performed the highest 29,958 L production of water per day when using an electrode size of 1 m3. Moreover, the highest electrosorption capacity has resulted from the same electrode that is 10.84 mg/g at the solution feed concentration 500 mg/L, whereas the theoretically calculated value through the Langmuir isotherm showed the maximum electrosorption capacity value of 16.59 mg/g. The results for the 9-CNT/mwGO/RVC composite electrode demonstrate that it can be a promising electrode material in CDI technology.
The data can be found upon request to the corresponding author.
There is no competing financial interest among the authors.
The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through the research group (RG 1438-038).
S1: the functionalization of CNTs. S2: synthesis of GO. S3: the exfoliation and reduction of GO using microwave irradiation. S4: the dispersion of mwGO and a-SWCNT. S5: preparation of the a-SWCNT/mwGO composite coating solution. S6: the pretreatment of the RVC electrode. S7: the optimization of RVC electrodes coated with a-SWCNT. S8: a-SWCNT, mwGO, and a-SWCNT/mwGO composite dip-coated RVC electrodes. S9: the measurement and calculation of ion removal from the NaCl aqueous solution. S10: the construction of a capacitive deionization cell and desalination experiments. S11: the measurement of the effect of flow rate and voltage on the ion removal efficiency. S12: the calculation of the electrosorption capacity. S13: pseudo-first-order equation. S14: Langmuir and Freundlich isotherm.