Excellent capacitive deionization performance of meso-carbon microbeads

Abstract

Capacitive deionization (CDI), which is based on the formation of electric double layers on the surface of electrode materials at low power supply, is one of the most promising technologies for efficient ion removal from brackish water or seawater. In this paper, meso-carbon microbeads (MCMB), which have an ultrahigh surface area and rich functional groups, exhibited excellent double layer performance with specific capacity of 300 F g⁻¹. In a batch-mode configuration, symmetrical MCMB electrodes reached the maximum electrosorption capacity of 17.7 mg g⁻¹ at 1.5 V in 25 ml of 5 mM NaCl aqueous solution. Furthermore, the electrosorption capacity of MCMB is two times larger than that of commercially used activated carbon.

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1. Introduction

The population expansion and industry development have constantly increased the demand for clean water.^1,2 In contrast, deteriorative water pollution and environmental changes have further aggregated the shortage of fresh water. To efficiently utilize the huge seawater resource is not only necessary, but also urgent.³ Conventional desalination methods, such as reverse osmosis,⁴ multi-effect distillation,⁵ and electrodialysis,⁶ are based on driving water out of feed water, leading to high energy-consumption and high salinity residual.⁷ To solve those issues, capacitive deionization (CDI) is being developed as a promising desalination technology.^8,9 In the CDI process, ions of Na⁺ and Cl⁻ are removed by double layers formed on the surface of porous materials.¹⁰ Such a direct ion-transfer process can be efficient even for a low-ion-concentration solution at low power supply and cost.^11–13 The electrochemical process of CDI looks similar with double-layer capacitors,^14–16 but the CDI process is conducted in a solution with a reduced ion concentration.¹³ As an important part of CDI configuration, an electrode material should possess the following properties:^17–20 (1) desirable conductivity for electron transfer, (2) large accessible surface areas for Na⁺ and Cl⁻ adsorption, (3) continuous structure networks and suitable porous structures to ensure ions transportation, and (4) excellent chemical and electrochemical stability for a long operation period. So far, carbon-based materials have attracted the most attention for CDI electrodes because of their diverse structures, controlled pore distribution, large surface area, excellent stability and good conductivity. Various types of carbon materials, such as activated carbon,²¹ carbon nanofibers,^22,23 carbon aerogel,²⁴ carbon nanotube,²⁵ activated carbon cloths,²⁶ mesoporous carbons,²⁷ and graphene,^28,29 have been exploited as electrode materials for CDI. However, searching more efficient electrode materials for deionization is still a central research effort.^30,31

Meso-carbon microbeads (MCMB) possess rough-surface and spherical structure.^32,33 Furthermore, they have been applied for batteries,^32,34 capacitors,³⁵ and catalysis³⁶ due to their abundant porosity and high surface areas. Those unique electrochemical and catalytic properties have encouraged us to explore them as electrode materials for capacitive deionization (CDI) in this work. It was demonstrated that the MCMB electrode exhibited excellent double-layer performance and high Na⁺ and Cl⁻ adsorption/desorption ability.

2. Experiments

Characterization of MCMBs

The MCMBs were obtained from Osaka Gas Co. Ltd, Japan. The microstructures of the MCMB sample was characterized by Hitachi-4700 field emission scanning electron microscope (FESEM) with energy dispersive spectroscopy (EDS) and JEOL JEM2010F transmission electron microscope (TEM). X-ray diffraction (XRD) was carried out using a Scintag XDS-2000 powder diffract meter with Cu Kα (λ = 1.5406 Å) radiation. Fourier transform infrared (FTIR) spectra of MCMBs were recorded with a Perkin Elmer spectrum one (KBr pellets method). An element analysis instrument (model 240XA, Control Equipment Corporation) was used to evaluate its composition. The pore size distribution and surface area of the MCMB were measured by N₂ adsorption at liquid nitrogen temperature (77 K) with ASAP 2000 instrument. Sheet resistances were determined by RM3 test unit.

Electrode fabrication and electrochemical performance test

The MCMB electrode was fabricated as followings: MCMBs (51 mg), carbon black (3 mg), and poly(tetrafluoroethylene) (6 mg) were mixed in about 20 ml isopropyl alcohol to form a homogeneous slurry. The slurry was dropped on graphite foil (0.254 mm thick, Alfa Aesar) and its thickness was controlled by film casting knife (micrometer adjustable film applicator, 50 mm). The obtained MCMB electrode was dried at 80 °C for 24 hours before performance tests. The electrochemical performance test of the MCMB electrode was carried out with an electrochemical workstation (Princeton Potentiostat/Galvanostat Model 273A). The three-electrode configuration was utilized to get double-layer performance of Na⁺ and Cl⁻ in a high-concentration salt solution. The MCMB electrode (1 × 1 cm²) is used as a working electrode, Hg/Hg₂Cl₂ electrode with saturated KCl solution as a reference electrode, and Pt wire as a counter electrode. 0.1 M, 0.5 M, or 1 M NaCl aqueous solution was used as an electrolyte. Cyclic voltammetry (CV) and galvanostatic charge/discharge cycling were carried out in a potential range from −0.4 to 0.6 V. Electrochemical impedance spectroscopy (EIS) was tested at frequency of 0.01 to 10⁴ Hz. Furthermore, the symmetrical two electrodes cell was constructed, namely, both the cathode and anode were the MCMB electrode (3 × 3 cm²) with a separator (Millpore glass fiber filter, AP2005500) between them. The test was performed in 5 mM, 10 mM, or 20 mM NaCl solution with a potential range from 0 to 1 V.

Capacitive deionization test with batch-mode configuration

The flow-type CDI was tested with the symmetrical two electrodes cell in 25 ml NaCl solution (5 mM, 10 mM or 20 mM) at flow rate of 20 ml min⁻¹. A selected voltage was applied to the 3 × 5 cm² electrodes for 30 minutes and then the voltage reduced to 0 V for 30 minutes. The applied voltage was conducted and response current was collected by Model 273A Princeton Potentiostat/Galvanostat. The salt solution was propelled by Cole–Parmer Masterflex L/S 7520-40 with 7518-12 pump head. The conductivity change of the deionization solution was monitored by a conductivity meter (Horiba DS-71) with a flow-type electrode (Horiba 3574-10C).

3. Results and discussion

MCMB material has an extremely large surface area of 3187 m² g⁻¹ (of which 3030 m² g⁻¹ is from micropores),³⁶ which was determined by N₂ adsorption at 77 K. SEM image showed that MCMBs possess a morphology of homogeneously elliptical spheres with rough and granular surface (Fig. 1A). Their micropore structure was confirmed by TEM image (Fig. 1B). Furthermore, as shown in Fig. 1C one can see an obvious (001) peak located at 2θ = 14° in XRD pattern, which can be attributed to crystalline structure transformation with a wider d-space.^37,38 The interlayer space of MCMBs calculated from the XRD peak is around 0.63 nm, which is much larger than that of graphite due to oxygen functional groups. This was supported by FTIR spectra. Fig. 1D shows IR bands at 1100 and 1230 cm⁻¹, which can be attributed to C–O stretching. The stretching vibrations of C=C, C=O (of –COOH group), and C–H were also detected at 1600, 1730, and 2950 cm⁻¹, respectively.³⁸ EDS and element analysis were employed to determine the oxygen content of the MCMB material, which is 4.42 and 13.5 wt%, respectively. This clearly confirms the presence of oxygen functional groups. Those unique properties make MCMB an ideal material for electrical double layer adsorption. Besides, to determine the electrode fabrication influence for surface area, the MCMB materials scraped off from graphite foil to do the N₂ adsorption/desorption measurement and sheet resistance test.³⁹ The BET surface areas drop obviously from 3187 m² g⁻¹ to 2106 m² g⁻¹ due to the addition of PTFE and carbon black. However, the sheet resistance of MCMB keep almost unchanged from 105 ohm sq⁻¹ (MCMB materials) to 107 ohm sq⁻¹ (MCMB electrodes).

Fig. 1 MCMB structures: (A) SEM image, (B) TEM image, (C) XRD pattern, and (D) FTIR spectrum.

The electrochemical performance of MCMB was evaluated by three-electrode configuration with loading area of 1 × 1 cm². 1 M NaCl aqueous solution was used as an electrolyte for the abundant Na⁺ and Cl⁻ ions. The cyclic voltammetry tests were carried out at scan rates of 1, 10, and 100 mV s⁻¹ with −0.4 to 0.6 V potential range. As shown in Fig. 2A, one can see that CV curves show an ideal rectangular shape without polarization (except the case with 100 mV s⁻¹ scan rate, which exhibited a slight polarization), indicating reversible adsorption/desorption of Na⁺ and Cl⁻ at electrical double-layers and easy-accessible surface area of MCMBs. Galvanostatic charge/discharge measurements at current densities of 0.2, 0.5, and 1 A g⁻¹ (Fig. 2B) also exhibited an excellent double layer performance, namely, the charge/discharge profile possesses a symmetric triangle shape without an observable IR drop, which indicates an excellent charge propagation in the electrode. The specific capacities from the charge/discharge profile were calculated with the following equation:

where C_s, I, V, m, and t are specific capacity of electrode (F g⁻¹), discharge current (A), potential range (V), mass of active materials on electrode (g), and discharge time (s), respectively. The calculation results showed that the capacity of MCMB reached up to 300 F g⁻¹ at the current density of 0.2 A g⁻¹, which is larger than those with other materials.^40,41 Even when the current density was increased to 0.5 and 1 A g⁻¹, the capacity still remained 87.5 and 80.7%, respectively. Therefore, the MCMB electrode showed excellent electrochemical performance with 1 M NaCl aqueous solution. To evaluate the effect of NaCl concentration, CV and charge/discharge measurements were performed at 0.1 M, 0.5 M, and 1 M concentrations of NaCl in its aqueous solution. At the scan rate of 100 mV s⁻¹, CV curves showed a rectangular shape with a slight polarization at three different concentration (Fig. 2C). When the concentration of Na⁺ and Cl⁻ decreased from 1 to 0.1 M, the obvious drop of CV areas occurred. This indicates that the decrease of ion concentration reduced the electrosorption. However, when the scan rate was small enough (1 mV s⁻¹) to provide enough time for ion adsorption/desorption, the CV curve exhibited an ideal rectangular shape and its area remained constant with decreasing ion concentration (Fig. 2D). This was further supported by galvanostatic charge/discharge measurements. As Na⁺ and Cl⁻ concentrations decreased, the triangle charge/discharge profile remained unchanged, but the discharge time markedly decreased (Fig. 2E). Furthermore, the capacity difference between different electrolyte concentrations was enhanced with increasing current density (Fig. 2F). The capacity reached 150 F g⁻¹ with 1 M NaCl solution at current density of 10 A g⁻¹. However, when the concentration of NaCl decreased from 1 to 0.5 and 0.1 M, the capacity decreased by 32.7% and 70%, respectively. This happened because the rapid charging/discharging process with a diluted electrolyte was determined by a low mass transfer rate of ions inside the micropores.⁴²

Fig. 2 Electrochemical performance of MCMB with three-electrode configuration, CV curves (A) at various scan rates and galvanostatic charge/discharge profiles (B) at various current densities in 1 M NaCl aqueous solution, CV curves at scan rate of 100 mV s⁻¹ (C), CV curves at scan rate of 1 mV s⁻¹ (D), galvanostatic charge/discharge profile (E) at current density of 0.5 A g⁻¹, and specific capacities vs. current densities (F) in 0.1 M, 0.5 M, and 1 M NaCl aqueous solutions.

The most important feature of the CDI technique is its effective deionization at a low ion concentration via direct adsorption of Na⁺ and Cl⁻ on the electrode surface. To verify the possibility of the MCMB material for the CDI cell, its deionization performance was evaluated in static 5 mM NaCl aqueous solution. CV and galvanostatic charge/discharge curves were obtained using a symmetrical two-electrode configuration at potential range from 0 to 1 V. Fig. 3A shows that all CV curves have a good rectangular shape even at a high scan rate of 100 mV s⁻¹. Although the CV area remarkably decreased with increasing scan rate from 1 to 100 mV s⁻¹, no polarization was observed, indicating the excellent double-layer performance of the MCMB electrode. This was further supported by the charge/discharge measurements (Fig. 3B). The triangle profiles and undetectable IR drops demonstrate the excellent capacitive property of the MCMB. As the concentration of NaCl increased from 5 to 20 mM, the shape and area of the CV curve remained unchanged at scan rate of 5 mV s⁻¹ (Fig. 3C). Furthermore, from the charge/discharge curves (Fig. 3B), the specific capacities were calculated with the following equation:

where C_s, I, V, m, and t are specific capacity of electrode (F g⁻¹), discharge current (A), potential range (V), mass of active material on electrode (g), and discharge time (s). As shown in Fig. 3D, when the current density was increased by 50 times, the specific capacity of MCMB dropped from 168 to 41 F g⁻¹ at 5 mM NaCl. However, even at a high current density of 5 mA cm⁻² for the charge/discharge process, the specific capacity of MCMB at 5 mM NaCl is only slightly smaller than those at 10 and 20 mM NaCl. This demonstrates the excellent electrosorption performance of MCMB, which can be explained by the unique structure of MCMB. The ultra large surface area and some oxygen functional groups of MCMB provide a large number of active sites for the electrical double-layer formation. Furthermore, the large micropore surface area (95% of total surface area) of MCMB would also have a positive impact, because the surface area of micropores (especially for the pore size equal to or less than 1 nm) is proportional to ion-adsorption ability.^43,44 Compared to the electrode of activated carbon (AC) (purchased from Aldrich with surface area of 844 m² g⁻¹ and its other structure information included in the ESI†), MCMB electrode showed two times larger CV areas at scan rate of 5 mV s⁻¹ (Fig. 4A). The specific capacity of activated carbon electrode at 0.1 mA cm⁻² is 51.8 F g⁻¹, which is only one-third of MCMB electrode. When current density increased by 20 times, the specific capacity is still much higher for MCMB electrode than for activated carbon electrode (Fig. 4B). Furthermore, Nyquist plot of MCMBs displays a straight line with a greater slope than that of AC at the low frequency region (Fig. 4C), indicating that the diffusion of Na⁺ or Cl⁻ ions into MCMBs are faster than into AC.²⁸

Fig. 3 Electrochemical performance of MCMBs with two-electrode configuration in 5 mM, 10 mM and 20 mM NaCl solution: (A) CV curves with 5 mM NaCl at different scan rates, (B) galvanostatic charge/discharge profile with 5 mM NaCl at different current densities, (C) CV curves at scan rate of 5 mV s⁻¹ with different NaCl concentrations, and (D) specific capacities vs. current densities with different NaCl concentrations.

Fig. 4 Comparison between AC and MCMBs: (A) CV curves at scan rate of 5 mV s⁻¹, (B) specific capacities vs. current densities in 5 mM NaCl aqueous solution, and (C) Nyquist plots in 1 M NaCl aqueous solution.

To further confirm the excellent deionization performance of MCMB under practical conditions, a flow-type CDI cell with batch-mode configuration was conducted in 25 ml of 5 mM NaCl aqueous solution at 20 ml min⁻¹ flow rate. The test was carried out by applying a voltage to the cell for 30 min and then eliminating the voltage for 30 min. The conductivity and response current change were separately recorded. As shown in Fig. 5A and B, the application of voltage of 1.5 V on the CDI cell resulted in large initial current and conductivity. However, the current and conductivity rapidly decreased. This indicates that a large amount of ions were absorbed on the electrode surface. After the electrosorption of electrode materials had reached the maximum, the conductivity remained almost unchanged. The electrosorption capacity and charge efficiency were calculated using the following equations:

Fig. 5 CDI performances of MCMB in batch-mode configuration at flow rate of 20 ml min⁻¹: the applied voltage and response current (A), conductivity change (B), and electrosorption capacity vs. time (C) at 1.5 V for 30 min following with 0 V for 30 min; electrosorption capacities of activated carbon and MCMB at 1.0 V, 1.2 V and 1.5 V for 30 min in 25 ml of 5 mM NaCl aqueous solution (D); electrosorption capacity of MCMB at 1.5 V for 30 min in 5, 10, and 20 mM NaCl aqueous solution (E); CDI cycles of MCMB at 1.2 V and 0 V for 30 min in 5 mM NaCl aqueous solution (F).

Γ is the electrosorption capacity (mg g⁻¹), C₀ and C_e are the initial and final NaCl concentrations (mg l⁻¹), S is the volume of NaCl solution (l), m is the total mass of the active materials on the electrode (g), Λ is charge efficiency, F is the faradic constant (96 485 Coulomb per mol) and Σ (Coulomb per g) the integration of response current. Fig. 5C shows that the accumulated electrosorption capacity of symmetrical MCMB electrodes at the applied voltage of 1.5 V reached 17.7 mg g⁻¹ with charge efficiency of 0.46 in 30 minutes, which is comparable to the results reported for other materials.^40,41,45 After the applied voltage was removed for another 30 min, the electrosorption capacity became almost zero, indicating that the electrodes could be completely recovered. Furthermore, as shown in Fig. 5D, the electrosorption capacity of MCMBs is two times larger than that of commercially used activated-carbon. Namely, the electrosorption capacity of MCMBs is 11.9 mg g⁻¹ (1.0 V), 14.2 mg g⁻¹ (1.2 V), and 17.7 mg g⁻¹ (1.5 V), respectively, while corresponding results of AC are 3.1, 5.6, and 7.5 mg g⁻¹, respectively. The excellent electrosorption capacity of MCMBs contributes to larger surface areas, proper pore size and higher Na⁺ and Cl⁻ transfer rate compared with AC. To obtain the saturated (maximum) electrosorption capacity of MCMBs, different concentrations of NaCl aqueous solution (5, 10, and 20 mM) were employed (Fig. 5E). The saturated electrosorption capacities are 17.7, 19.7, and 20.1 mg g⁻¹ for 5, 10, and 20 mM NaCl solutions, respectively. The time of reaching the saturated electrosorption is also dependent on the concentration of NaCl, namely, the higher the concentrated NaCl solution, the shorter the time to reach the saturated electrosorption (Fig. 5E). This happened because a higher concentration of NaCl possesses a larger conductivity, leading to a higher ion-transfer rate and thus an enhancement in electrosorption rate. Finally, the stability of the MCMB-based CDI cells was evaluated by applying 1.2 V for 30 min of ion-adsorption and then eliminating voltage for 30 min of ion desorption. Fig. 5F, the CDI exhibited an excellent reversibility for 6 adsorption–desorption cycles.

4. Conclusion

Meso-carbon microbead (MCMB) electrode exhibited a high specific capacity of 300 F g⁻¹ at a current density of 0.2 A g⁻¹ in a three-electrode cell. Furthermore, in both the static two-electrode system and the flow-type capacitive deionization (CDI) system, the electrosorption capacity on MCMB material is two times larger than commercial used activated carbon. The electrosorption capacity of the MCMB-based CDI cell with batch-mode reached 17.7 mg g⁻¹ with charge efficiency of 0.46 in 25 ml of 5 mM NaCl aqueous solution, indicating that MCMB would be an ideal electrode material for CDI. The MCMB material would be an excellent candidate to replace the commercially used activated carbon for industrial CDI technologies.

Acknowledgements

This work was supported by the ACS Petroleum Research Fund (PRF-51799-ND10). Hu also thanks Charles and Carroll McArthur for their great support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08362e

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