Research Article pubs.acs.org/journal/ascecg
Electrodeposited Manganese Dioxide/Activated Carbon Composite As a High-Performance Electrode Material for Capacitive Deionization Yu-Hsuan Liu, Hsing-Cheng Hsi, Kung-Cheh Li, and Chia-Hung Hou* Graduate Institute of Environmental Engineering, National Taiwan University, No. 1, Sec. 4. Roosevelt Rd., Taipei 10617, Taiwan S Supporting Information *
ABSTRACT: Electrode materials are a crucial component for achieving high desalination performance via capacitive deionization (CDI). In the present work, we have successfully fabricated a manganese dioxide (MnO2)/activated carbon (AC) composite electrode using an anodic electrodeposition technique. Surface characterization confirms the presence of electrodeposited MnO2 on the AC surface with an amorphous structure and improved wetting behavior. Cyclic voltammetry and galvanostatic charge/discharge measurements indicate that the MnO2/AC composite electrode exhibits a high specific capacitance (77.6 F g−1 at 5 mV s−1), rate capability, and excellent cycling reversibility for capacitive charge storage. Furthermore, the salt electrosorption capacity is investigated using batch mode experiments at a working voltage of 1.0 V in a 0.01 M NaCl solution. The MnO2/AC composite electrode presents a superior electrosorption capacity of 9.3 mg g−1, which is approximately 1.6-fold higher than that of the pure AC electrode (5.7 mg g−1). This significant improvement can be attributed to the mixed capacitive-Faradaic process, corresponding to the combination of the double-layer charging of the high specific surface area (625 m2 g−1) and the pseudocapacitive redox reaction of MnO2. Therefore, the electrodeposited MnO2/AC composite is a potential electrode material for high-performance CDI. KEYWORDS: Manganese dioxide, Capacitive deionization, Electrosorption, Composite electrode, Pseudocapacitance
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INTRODUCTION Capacitive deionization (CDI) is an emerging separation technology for removing salt ions or ionic contaminants from aqueous solutions using an electrosorption process.1−4 Following the same working principle of electrochemical supercapacitors, CDI has several advantages, such as high energy efficiency, easy operation, low fouling potential, high water recovery, and environmental friendliness.5−7 During the electrosorption process, ionic species are attracted to the oppositely charged electrodes under an external electric field. When the electric field is removed, ionic species can be released back into the solution. In this case, CDI requires pairs of capacitor electrodes that have high capacitive charge storage to store ions during the charging step.8 In principle, the mechanism for the electrochemical storage of ions in capacitors is based on either the electrical double-layer (EDL) capacitance from a pure electrostatic interaction or the pseudocapacitance from fast and reversible Faradaic reactions.9 However, the salt adsorption capacity of current electrode materials is not sufficient for desalinating high-concentration salt water. Progress toward CDI technology can benefit from the development of capacitor electrode materials with improved capacitive charge storage and electrosorption capacity. © 2016 American Chemical Society
Various forms of carbon materials constitute the most common electrode materials used in CDI cells. Carbon aerogels,10,11 activated carbons,12−14 and activated carbon nanofibers15 represent the classic electrode materials for CDI applications. The latter materials, such as carbon nanotubes,16,17 graphene,18−20 mesoporous carbon,21,22 hierarchical porous carbon,23 and carbon−carbon composites,24−27 have been proposed as capacitor electrodes with enhanced salt adsorption capacity in CDI. Recently, intense interest has arisen in the development of novel three-dimensional (3D) graphenebased materials, such as 3D porous graphene,28 ion-selective 3D graphene,29 nitrogen-doped graphene composites,30 3D graphene architectures,31,32 and 3D graphene-based hierarchically porous carbon composites,33 as ultrahigh-performance electrodes for CDI. Note that activated carbons derived through the pyrolysis/activation of natural resources, waste, and resource-recovered biomass materials are recognized as the most cost-efficient CDI electrode materials because of their low cost, abundance, stability, and large specific surface areas.5,8 Received: May 5, 2016 Revised: July 6, 2016 Published: July 21, 2016 4762
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Moreover, significant research efforts have recently been devoted to the synthesis of metal oxide−carbon composite electrodes due to their synergistic effects.34 For instance, surface modification by titanium dioxide (TiO2) has been demonstrated to greatly enhance the wetting behavior and the specific capacitance.35,36 Zinc oxide (ZnO) nanorods on an activated carbon cloth electrode can increase the desalination efficiency for ion capture.37 In asymmetric CDI, the anode is coated with a nanoporous film of alumina oxide (Al2O3), and the cathode is coated with silicon dioxide (SiO2) to shift the surface zeta potential for a high efficiency range.38−41 Among the various transition metal oxides, manganese dioxide (MnO2) is a promising electrode material for supercapacitors due to its low cost, environmental benignity, and high theoretical pseudocapacitance value of 1370 F g−1.42 In general, MnO2 can be prepared through sol−gel-derived, chemical coprecipitation, and anodic electrodeposition methods to possess pseudocapacitive-like characteristics.43−45 However, MnO 2 typically suffers from poor electrical conductivity, which severely limits its capacitive charge storage. To overcome this drawback, research dedicated to the incorporation of MnO2 onto the surface of carbonaceous materials, such as carbon fibers,46,47 carbon nanotubes,48−50 and graphene-based materials,51,52 has attracted considerable attention for supercapacitors. Specifically, MnO2/activated carbon composites as electrode materials for supercapacitors can significantly enhance the specific capacitance through the combination of fast Faradaic reactions and double-layer charging.53,54 Notably, only a few researchers have investigated the modification of carbonaceous materials through the deposition of MnO2 for CDI applications. First, Yang et al.55 developed a novel MnO2/nanoporous carbon composite electrode for CDI using the coprecipitation method, in which the MnO2 film provided a high surface adsorption capability and an effective cation intercalation, suggesting a better desalination performance. Graphene wrapped MnO2 nanostructures have been exploited as effective and stable electrode materials for CDI application.56 Hu et al.57 later synthesized a MnO2/carbon fiber composite electrode using the electrosorption process for the removal of copper ions. Recently, Walker et al.58 reported the electrochemical deposition of MnO2 on carbon nanofoam and carbon fiber for improving the desalination performance in CDI. The electrochemical capacitive behaviors, however, may be susceptible to the specific surface area and deposition characteristics of MnO2. Notably, to the best of our knowledge, the MnO2/activated carbon composite as an electrode in CDI has not previously been reported. The objective of this study is to fabricate a MnO2/activated carbon (AC) composite electrode with the aim of improving desalination performance in CDI. The MnO2 was anodically electrodeposited onto highly porous activated carbon substrates. The resulting MnO2/AC composites were characterized by nitrogen adsorption/desorption isotherms, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) to understand their surface morphology and structure. The capacitive behaviors were investigated using electrochemical impedance spectroscopy (EIS), galvanostatic charge−discharge (GC) curves, and cyclic voltammetry (CV) measurements. Finally, batch-mode CDI experiments were conducted to determine the salt adsorption capacity of the composite electrode for capacitive water desalination.
Research Article
EXPERIMENTAL SECTION
Preparation of Manganese Dioxide/Activated Carbon Electrodes. Activated carbon (AC), Calgon F400, was obtained from Calgon Carbon Co., U.S.A. First, to prepare the AC electrode, AC powders were mixed with a polymer binder (polyvinylidene fluoride, PVDF, MW = 534 000, Sigma-Aldrich) in a weight ratio of 9:1. The details of manufacturing the carbon sheet-like electrodes for CDI are described in our previous work.48 Then, manganese dioxide (MnO2) was deposited on the AC electrode through an anodic electrodeposition method using a potentiostat (CHI 627D, CH Instruments, Inc.) with a three-electrode cell. This three-electrode system consisted of a working electrode (ACs), counter electrode (platinum wire), and Ag/AgCl reference electrode (BAS model RE-1, Bioanalytical Systems, Inc.). Nitrogen was bubbled through the solution for at least 15 min prior to the electrodeposition, and then it was used continually to protect the experimental environment. Prior to the electrodeposition, the AC electrodes were cycled and oxidized in a 0.1 M sulfuric acid (H2SO4) solution between 0.0 and 1.2 V for 20 cycles at a scan rate of 20 mV−1, and then a 0.1 M sodium chloride (NaCl) solution was used between −0.8 and 0.0 V for 40 cycles at a scan rate of 100 mV−1. In a typical electrodeposition synthesis procedure, the pretreated AC electrode was oxidized in 0.0125, 0.025, and 0.5 M manganese acetate ((CH3COO)2Mn·4H2O) solutions for different durations at a scan rate of 20 mV−1. Following the anodic electrodeposition, the MnO2/AC electrode was rinsed with deionized water and dried overnight in a vacuum oven at room temperature. Material Characterization. The specific surface areas and pore size distributions of the prepared electrode materials were measured using a physisorption analyzer (ASAP 2020M, Micromeritics Inc.) with the Brunauer−Emmett−Teller (BET) method from the nitrogen adsorption−desorption isotherm obtained at a relative pressure (P/P0) of 0.99. The surface morphologies of MnO2/AC were investigated using a scanning electron microscope (SEM, JSM-7600F, JEOL) equipped with an energy-dispersive spectrometer (EDS). X-ray photoelectron spectra (XPS) were recorded using a ULVAC PHI5000 VersaProbe spectrometer with monochromatic Al Kα radiation. X-ray powder diffraction (XRD) analyses were performed using an Ultima IV multipurpose XRD system (Rigaku Co., Sendagaya, Shibuya-Ku, Tokyo, Japan) with Cu Kα radiation (λ = 1.54051 Å). Step scanning was used with 2θ intervals from 10° to 80° with a residence time of 1 s. Thermogravimetric (TG) analysis was conducted using a PerkinElmer Diamond analyzer under a nitrogen atmosphere. The samples were heated from 20 to 800 °C at a rate of 10 °C min−1. A contact angle system (FTA125, First Tech Angstroms) was used to measure the wetting behavior of the electrodes. In this experiment, the volume of water droplets (10 μL) and the distance between the needle and electrode surface (2 ± 0.05 mm) were maintained constant in all measurements. The images were captured after a 5 s stabilization period. Electrochemical Measurements. The capacitive performances of the fabricated electrodes were characterized by electrochemical impedance spectra (EIS), cyclic voltammetry (CV), and galvanostatic charge/discharge (GC) tests performed using a potentiostat (CHI 627D, CH Instruments, Inc.). These experiments were performed in a three-electrode cell with a 1 M NaCl electrolyte solution. The working and counter electrodes were a small piece and a large piece of the prepared carbon materials, respectively, and a Ag/AgCl electrode (BAS model RE-1, Bioanalytical Systems, Inc.) was used as the reference electrode. EIS measurements were performed to determine the electrical conductivity of the carbon electrode. The amplitude of the alternating current complex impedance was 5 mV, and the data were collected in the frequency range from 0.01 to 100 kHz. The GC curves were obtained in the potential range of 0 to 1.0 V with current densities ranging from 0.1 to 0.5 A g−1. The changes in iR drop versus different current densities were also recorded. The CV test was executed in the potential window of 0.6 to −0.4 V at various scan rates: 5, 10, 20, 50, 100, and 200 mV s−1. The specific capacitance derived from the CV curve can be calculated using the following formula: 4763
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C=
∫V c I dV a
2mv(Vc − Va)
(1)
where C is the specific capacitance, I is the instant current, m is the mass of the electrode materials, and v is the potential scan rate. Vc and Va represent the high and low potential limits of the CV tests, respectively. Batch Mode CDI Experiments. The desalination performances of the MnO2/AC electrodes were evaluated in batch mode electrosorption experiments (Figure 1), in which the solution was Figure 2. N2 adsorption/desorption isotherms of the AC and MnO2/ AC electrode materials.
surface area and total pore volume of the pure AC electrode are measured to be 724 m2 g−1 and 0.41 cm3 g−1, respectively. There is, however, a trade-off between the mass loading of MnO2 and specific surface area for optimizing the capacitive behavior.53,58 As illustrated in Figure S1 in the Supporting Information, the MnO2/AC composite electrode prepared using the cyclic voltammetry method in 0.025 M (CH3 COO)2Mn·4H 2O for five cycles can achieve the maximum value of specific capacitance (79.9 F g−1), and thus, it was used in the following experiments for investigating the physicochemical characteristics, electrochemical properties, and desalination performance. As shown, the resulting MnO2/ AC composite electrode has a specific surface area of 625 m2 g−1 and a total pore volume of 0.36 cm3 g−1. This result indicates that when the MnO2 is anodically deposited onto the AC substrate, the pores of the composite electrode are partially blocked with a decrease in specific surface area. Note that this surface area can still provide a sufficient electrode/electrolyte interface for EDL formation and ion accumulation. Moreover, the composite electrode has a ratio of mesopores to total pore volume (Vmeso/Vtot) of 55.6%, suggesting the coexistence of mesopores and micropores. Previous studies have proven that a well-balanced ratio of mesopores to micropores is an important requirement for ensuring high-performance capacitive behavior.15,59 Figure 3a and b present representative SEM images of the MnO2/AC composite electrodes. It can be observed that the surface of AC was covered by MnO2 clusters featuring a petallike nanostructure. The EDS mapping reveals the distribution of C, O, and Mn elements for the MnO2/AC composite, as depicted in Figure 3c−e (see also Table 2). The surface composition of the electrodeposited MnO2/AC electrode is found to be C (80.4%), O (9.2%), and Mn (6.7%), whereas the pure AC electrode has 0% Mn (see Supporting Information, Figure S2). This finding confirms that the MnO2 could be successfully coated on the AC using the anodic electrodeposition technique. The successful incorporation of MnO2 in AC was further confirmed by TG analysis, as depicted in Figure 4. The weight loss below 250 °C corresponds to the evaporation of physically adsorbed and interlayer water. The following significant weight loss at temperatures between 250 and 400 °C is attributed to the combustion of AC, measuring approximately 73.6 and 68.0 wt % weight loss for the pure AC and MnO2/AC composite electrodes, respectively. According to the TG analysis after 400 °C, the MnO2 load was estimated to be 9.6 wt % in the composite electrode. Furthermore, the contact angle of a water droplet on the surface of the AC electrode was measured to be 123.9°, indicating a hydrophobic property. At the same time,
Figure 1. Schematic of the batch-mode CDI setup for water desalination. continuously circulated through the CDI cell by a pump (EYELA MP-1000) at a flow rate of 5 mL min−1 with a total solution volume of 50 mL. The CDI cell was composed of two-sided carbon electrodes separated by a spacer at a distance of 2 mm for solution flow. In each experiment, the effective surface area of the CDI electrodes was 4 cm × 4 cm with a mass of 0.4 to 0.5 g. The electrical potential difference between the two oppositely charged electrodes was operated by a CHI 627D potentiostat with a voltage of 1.0 V. The concentration change of the solution was measured using an online conductivity meter (Suntex SC-2300). The electrosorption capacity (qe) was determined by the amount of NaCl removed per unit mass of electrodes using the following equation:
qe =
(C0 − C t)V m
(2)
where C0 and Ct are the initial and equilibrated NaCl concentrations, respectively, V is the solution volume, and m is the total mass of the electrode materials. Charge efficiency (Λ), defined as the ratio of the amount of electroadsorbed ions to the equivalent total charge, was calculated using the following equation:28,58
Λ=
(C0 − C t)V Σ/F
(3)
where Σ is the charge obtained by integrating the corresponding current and F is Faraday’s constant (96 485 C/mol e−).
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RESULTS AND DISCUSSION Surface Morphology and Structural Characterization. The MnO2/AC composite electrodes were prepared using the cyclic voltammetric electrodeposition method, and their physicochemical characteristics were investigated in detail. Figure 2 presents the N2 adsorption−desorption isotherms of the AC and MnO2/AC composite electrodes. According to the IUPAC classification, typical type-I isotherms with a welldefined plateau can be observed for both of the electrodes, indicating the presence of micropores. Table 1 summarizes the pore characteristics of the carbon samples. Here, the specific 4764
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ACS Sustainable Chemistry & Engineering Table 1. Pore Characteristics of the AC and MnO2/AC Electrode Materials material
SBETa (m2 g−1)
Smicrob (m2 g−1)
Vtotc (cm3 g−1)
Vmicrod (cm3 g−1)
Vmesoe (cm3 g−1)
Vmeso/Vtot (%)
AC MnO2/AC
724 625
326 293
0.41 0.36
0.18 0.16
0.23 0.20
56.1 55.6
a BET specific surface area. bMicroporous surface area derived from the t-plot method. cTotal volume of pores. dVolume of micropores. eVolume of mesopores.
Figure 3. (a,b) SEM images of the MnO2/AC material and the corresponding EDS mapping analyses of (c) carbon, (d) oxygen, and (e) manganese.
Table 2. EDS Elemental Compositions of the AC and MnO2/AC Composite Materials material
C (%)
O (%)
F (%)
Mn (%)
AC MnO2/AC
76.7 80.4
9.4 9.2
13.9 3.7
6.7
Figure 5. XPS spectra of the AC and MnO2/AC composite electrodes: (a) survey XPS spectra, (b) C 1s peaks. (c) O 1s peaks, and (d) Mn 2p peaks.
the AC and MnO2/AC composite electrodes are shown oxygenated groups, namely, Mn−O−Mn (529.7 eV), Mn− O−H (531.8 eV), and H−O−H (532.0 eV), and thus, the main O 1s peak shifts to the right with its center at 531.5 eV. Notably, for the MnO2/AC composite electrode, two obvious peaks centered at 642.1 and 653.8 eV were observed in the Mn 2p core region, corresponding to Mn 2p3/2 and Mn 2p1/2, respectively (see Figure 5d). The values of the Mn 2p peaks with a spin-energy separation of 11.7 eV are consistent with the XPS data reported for Mn 2p3/2 and Mn 2p1/2 in MnO2,53,60 thereby confirming that the predominant oxidation state is +4 for the MnO2 nanostructure in the MnO2/AC composite electrode. Typical XRD patterns were recorded in the 2θ range of 10° to 80° to identify the structures of the pure AC and MnO2/AC composite electrodes, as shown in Figure 6. For the pure AC electrodes, a diffraction peak appears at approximately 2θ = 24.0°, corresponding to the (002) reflection arising from the crystal plane of graphite from AC. Similarly, the broad peak at
Figure 4. TGA analysis curves of the AC and MnO2/AC composite electrodes. The inset is the contact angle image.
the MnO2/AC composite electrode exhibited a contact angle of 96.4°. The improved wetting behavior is beneficial for the participation of the pore volume in the CDI process. Figure 5a presents the wide-scan survey XPS spectra of the pure AC and MnO2/AC composite electrodes. The C 1s and O 1s core-level regions of the AC electrode present two main peaks centered at approximately 284.5 and 532.0 eV, which are associated with sp2 carbon atoms and oxygenated groups of H− O−H, respectively. In the spectrum of the MnO2/AC composite electrode, the peaks centered in the C, O, and Mn core-level regions can be assigned to C 1s, O 1s, and Mn 2p, respectively. The curve fits of the C 1s spectra of the AC and MnO2/AC composite electrodes are presented in Figure 5b. The XPS spectra of the AC and MnO2/AC electrodes exhibit two main types of carbon bonds: CC/C−C (284.2 eV/284.7 eV) and C−O (285.8 eV). The curve fits of the O 1s spectra of
Figure 6. XRD pattern of the AC and MnO2/AC electrode materials. 4765
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relationship for the entire voltage range. Notably, the MnO2/ AC composite electrode displays a considerably longer discharge time than that of the AC electrode, indicating a higher capacitive charge storage feature. To further investigate the rate capability, the GC profiles of the MnO2/AC electrodes in 1 M NaCl at various current densities (0.1 to 0.5 A g−1) are shown in Figure 7b. As the current density increases, the GC curve has a shorter discharge time; this is a typical capacitive behavior for highly porous carbon electrode materials. A similar tendency can also be observed for the AC electrode (see Figure S4a). Note that the voltage drop, also known as iR drop, at the turning point of the GC curve originates from the inner resistance of ion diffusion into the pore network of the electrode. Figure 7c shows the variation of the iR drop against the current density of the AC and MnO2/AC composite electrodes. When the current density increases from 0.1 to 0.5 A g−1 in 1 M NaCl, the iR drop of both electrodes increases accordingly. Notably, no significant difference in iR drop between the AC and MnO2/AC composite can be observed at lower current densities. This result confirms that the introduction of MnO2 has less influence on the internal resistance for ion diffusion into the pore network of AC. Therefore, the composite still retains the good rate capability of the AC in the electrosorption process. Figure 7d shows the continuous charge−discharge behavior of the MnO2/AC composite electrode at a constant current density of 0.1 A g−1. Clearly, all of the GC profiles exhibit a nearly triangular and symmetric characteristic, indicative of the high reversibility of the composite electrode. Figure 8a presents the CV curves of the AC and MnO2/AC composite electrodes at a scan rate of 5 mV s−1 in a 1 M NaCl
2θ = 43.8° corresponding to the (101) reflection supports the higher degree of interlayer condensation of carbon. The dominant peak of AC is not obvious and sharp, indicating high disorder of individual AC pieces formed in the structure of the AC electrode. In the XRD pattern of MnO2/AC, two weak peaks are observed at 2θ = 50.1° and 68.3°, corresponding to the (411) and (541) reflections present in the diffraction pattern of α-MnO2 (JCPDS no. 44-0141).54 No obvious diffraction peak attributed to the crystal planes of MnO2 can be observed for MnO2/AC, reflecting its poor crystallinity and suggesting that the structure of MnO2 obtained via the anodic electrodeposition is amorphous. Briefly, the mass loading of MnO2 can be controlled by adjusting the duration of the cycling time and the concentration of the (CH3COO)2Mn· 4H2O solution during the electrodeposition. Note that the amorphous MnO2 obtained from the anodic electrodeposition method is more favorable for capacitive charge storage due to the easy penetration of ions through the bulk of active materials.43,60 Electrochemical Properties for Capacitive Ion Storage. To investigate the electrochemical performance of MnO2/ AC composites as electrodes, EIS, GC, and CV measurements were conducted in a three-electrode system in NaCl aqueous solutions. To characterize the electrical conductivity, the EIS analysis of the MnO2/AC composite electrode presented as a Nyquist plot is shown in Figure S3. The Nyquist plot is derived from real (abscissa x axis, Z′) and imaginary data (ordinate y axis, Z″) at different frequencies (0.01 to 100 kHz). Clearly, the EIS spectra of the MnO2/AC composite electrode exhibits a linear trait at low frequency and a quasi-semicircle at high frequency. The intersection of the quasi-semicircle represents the equivalent series resistance (ESR) associated with the charge-transfer resistance of the electrode and solution interface. The Rct obtained from the diameter of the quasisemicircle is 2.92 Ω. Figure 7a presents the GC curves of the pure AC and MnO2/ AC composite electrodes tested in 1 M NaCl between 0 and 1.0 V with a current density of 0.1 A g−1. As shown, the GC curves of both electrodes show a nearly linear voltage/time
Figure 8. (a) CV curves of the AC and MnO2/AC composite electrodes with a scan rate of 5 mV s−1. (b) MnO2/AC electrode at various scan rates. (c) Comparison of the specific capacitance values between the AC and MnO2/AC electrodes at different scan rates. (d) Specific capacitance of the MnO2/AC electrode at 5 mV s−1 as a function of cycle number. All CV curves were obtained in 1 M NaCl aqueous solution.
solution. As shown, the CV curves of both electrodes exhibit a nearly quasi-rectangular shape in the entire potential window. Identical characteristics of CV profiles have been observed in other MnO2-coated carbon materials for supercapacitors and CDI applications.54−56,58,61−63 Specifically, for the MnO2/AC composite electrode, the current responses in the anodic and
Figure 7. (a) GC curves of the AC and MnO2/AC composite electrodes with a current density of 0.1 A g−1. (b) MnO2/AC electrode at various current densities. (c) iR drops of the electrodes as a function of current density. (d) Continuous GC curves of the MnO2/AC electrode with a current density of 0.1 A g−1. All GC curves were obtained in 1 M NaCl aqueous solution. 4766
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ACS Sustainable Chemistry & Engineering cathodic regions are mirror images, suggesting an ideal capacitive behavior.57 In CV analysis, the response of the current is a combination of capacitive electrical double-layer formation and the Faradaic reaction of redox-active MnO2. Compared to the pure AC electrode, the MnO2/AC composite electrode exhibits a larger CV enclosed area, representing a larger specific capacitance. As calculated, the specific capacitances of the AC and composite electrodes are 45.0 and 77.6 F g−1, respectively. It is estimated that 42% of the specific capacitance of the composite is related to the MnO2. Therefore, the incorporation of MnO2 into the AC significantly improved the specific capacitance, corresponding to better electrosorption performance. As the scan rate increases from 5 to 200 mV s−1, the CV curve tends to be distorted from a typical rectangular shape into a leaf-like shape due to the reduced diffusion time, as shown in Figure S4b and Figure 8b for the AC and composite electrodes, respectively. Figure 8c also shows the scan rate dependence of the specific capacitance for the electrodes. As shown, the specific capacitance decreases with increasing scan rate. This behavior occurs because the ions do not have sufficient time to access the electrode materials due to their pore size distributions; the large volume of micropores restricts the transport of ions into the pores for electrical double-layer formation.64 Note that the MnO2/AC composite electrode exhibits considerably higher specific capacitance than the pure AC electrode at the same scan rate. In other words, the specific capacitance of the MnO2/AC composite is less dependent on the scan rate, suggesting a better rate capability. The improved electrochemical performance is due to the presence of electrodeposited MnO2. Here, the charge storage mechanism of MnO2 is primarily attributed to the surface process of insertion/extraction of cations (i.e., Na+).42,62 Note that the electrodeposited MnO2 has a positive impact by contributing pseudocapacitance due to its fast Faradaic and reversible charge-transfer reaction on the external surface of the electrodes. Therefore, this tremendous improvement can be attributed to the Na+ ion intercalation of MnO2 according to the following electrochemical reaction:42,55,62,63
Figure 9. (a) Representative desalination performance of the AC and MnO2/AC electrodes in 10 mM NaCl with an applied voltage of 1.0 V. The inset is the electrosorption capacities of the AC and MnO2/AC electrodes. (b) Electrosorption capacities of the AC and MnO2/AC electrodes with different initial concentrations.
positively charged Na+ ions react toward the cathode for charge storage, while negatively charged Cl− ions are electroadsorbed on the anode. Furthermore, with increasing operating time, the change in conductivity gradually reaches a pseudoequilibrium. As evidenced, the final conductivity of the MnO2/AC composite as a cathode in the CDI cell is 907 μS cm−1, which is considerably lower than that of the AC electrode (959 μS cm−1). When 0.0 V was applied (short-circuit regeneration mode),40 the solution conductivity returned to its initial value, indicating that the electroadsorbed ions can be rapidly released back to the water. Notably, the pH remains in a range from 7.63 to 8.03 in the CDI experiments. The phenomenon of pH-fluctuation is not observed for the desalting NaCl solution, demonstrating that the concentrations of H+ and OH− ions are maintained. In this case, the variation of solution conductivity is only related to the removal of Na+ and Cl− ions. Additionally, charge efficiency is an important parameter to identify the ratio of the amount of electroadsorbed ions from the bulk solution to the total charge transferred to the electrode surface. As calculated, the charge efficiency of the pure AC and MnO2/AC composite electrodes is 43.6% and 24.5%, respectively. The Faradaic reaction of MnO2, which involves the ions at the electrode surface and the electrode materials, might reduce the charge efficiency in CDI.65 Figure 9b shows the further CDI experiments performed with different initial concentrations of NaCl solution. The electrosorption capacity of the composite electrode at 0.002, 0.005, 0.01, and 0.015 M is 4.85, 6.99, 9.26, and 9.39 mg g−1, respectively. A higher NaCl concentration results in an increased electrosorption capacity of the electrode materials due to compression of the electrical double layer and reduction of the overlapping effect, which ensures more accessible sites for electrosorption of ions in the pore structure. Note that the salt electrosorption capacity is an important parameter in practical applications of the CDI process. According to the above CDI experiments, the electrosorption capacity of the MnO2/AC composted electrode with an initial
MnO2 + Na + + e− ↔ MnOONa
Figure 8d shows the electrochemical stability of the MnO2/AC composite electrode at 5 mV s−1 during 1000 cycles. It is observed that the composite has excellent cycling performance of ∼95% specific capacitance after 1000 cycles. Consequently, electrodeposited MnO2 on the AC has a great impact on enhancing the capacitive charge storage, in which the surface layer of MnO2 can actively participate in the fast Faradaic reactions.54 As an electrode material, the MnO2/AC composite has superior electrochemical performance with excellent cyclic stability and reversibility due to the synergistic effect of the high pseudocapacitance of MnO2 and the porous structure of AC. Electrosorption Performance in CDI. The desalination performances of the fabricated electrodes were investigated through batch mode CDI experiments at an applied voltage of 1.0 V. Figure 9a shows the electrosorption/desorption behaviors of the AC and MnO2/AC composite electrodes in a 0.01 M NaCl solution with an initial conductivity of 1060 μS cm−1. Clearly, once the electric field is imposed on a pair of working electrodes, a dramatic decrease in the solution conductivity appears at the early stage, suggesting the rapid removal of salt ions from aqueous solutions. Here, the 4767
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ACS Sustainable Chemistry & Engineering concentration of 0.01 M is 9.3 mg g−1, which is 1.6-fold higher than that of the pure AC electrode (5.7 mg g−1). This finding is in good agreement with the results of the electrochemical measurements. For comparison, Table 3 shows the electro-
The experiments were performed by operating the charging− discharging process 10 times: 1.0 V for 30 min in the charging step and 0.0 V for 30 min in the discharging step. The current variation was recorded simultaneously. Consequently, the same pattern of electrosorption−desorption curves was observed. By applying the voltage at 0.0 V, the ions can effectively come off to achieve the regeneration of the MnO2/AC composite electrode. This result indicates that the composite electrode has regeneration ability that is as good as that of the pure AC electrode (see Figure S5). This feature is essential for the CDI process, in which the removal of salt ions relies on a reversible process. Furthermore, the electrodeposited MnO2/AC composite electrode has superior salt electrosorption capacity compared to the classic CDI materials (e.g., carbon aerogels or activated carbons) due to the following reasons. By incorporating with AC, the fast, reversible redox-active MnO2 can ensure high capacitive behavior for intercalating the cations and make the best use of the large pseudocapacitive charge to improve the electrosorption capacity. The synergistic effect is contributed by the mixed capacitive-Faradaic processes, in which salt ions can be captured by the electrical double-layer capacitor of highly porous AC and the pseudocapacitive-charge storage reaction of MnO2.
Table 3. Comparison of Electrosorption Capacities of Various Carbon Electrodes Reported in the Literaturea initial concentration (mM)
applied voltage (V)
electrosorption capacity (mg g−1)
AC
2
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3.66
3D porous graphene 3D graphene 3D RGO/ MF MnO2/NPC MnO2/GNS MnO2/ MWCNTs MnO2/PSS/ CNT MnO2/AC
1.2
2
11.86
this work 28
8.6 3
1.6 2
13.7 21.93
29 30
0.5 1 0.5
1.2 1.2 1.8
0.99 5.01 6.65
55 56 61
2
1.2
4.70
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4.85
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9.26
this work this work
material
ref.
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CONCLUSIONS In the present study, the controlled deposition of MnO2 on an AC electrode has been achieved using an electrochemical deposition method to retain the sufficient conductivity and high porosity of the AC electrode. The physicochemical properties of the fabricated MnO2/AC composite were characterized in detail. Compared with the pure AC electrode, the composite electrode with a specific surface area of 625 m2 g−1 exhibits a higher specific capacitance (77.6 F g−1 at 5 mV s−1), better rate capability, and excellent cycling reversibility after 1000 cycles. Notably, the electrodeposited MnO2 with a pseudocapacitive charge-storage reaction plays a decisive role to ensure the highperformance capacitive behavior. Furthermore, the MnO2/AC composite electrode has an electrosorption capacity of 9.3 mg g−1, which is considerably higher than that of an AC electrode. This result demonstrates the successful combination of porous AC (electrical double-layer capacitor) and redox-active MnO2 (pseudocapacitor), reflecting a mixed capacitive-Faradaic process. In summary, the facile approach to incorporating MnO2 into porous carbon materials is effective and significant, with remarkably enhanced salt electrosorption capacity in CDI process. The findings of this work can provide useful information for success in the practical use of CDI technologies.
a
Note: RGO/MF, reduced graphene oxide/melamine formaldehyde; NPC, nanoporous carbon; GNS, graphene nanosheet; MWCNT, multiwalled carbon nanotubes; PSS, polystyrene sodium sulfate.
sorption capacities of other porous carbon-based materials for CDI reported in the literature. Graphene-based porous carbon materials exhibit superior desalination capacity in a range from 11.86 to 21.93 mg g−1.13,28,30 Thus, the electrosorption capacity of the MnO2/AC composite electrode is promising compared to other MnO2 carbon electrodes.55,56,61,66 Figure 10 shows consecutive electrosorption−desorption cycles of the MnO2/AC composite electrode in 0.005 M NaCl.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00974. Figures S1−S5 (PDF)
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AUTHOR INFORMATION
Corresponding Author
Figure 10. Electrosorption−desorption cycles of the MnO2/AC composite electrode in 5 mM NaCl (1.0 V for 30 min in the charging step, and 0.0 V for 30 min in the discharging step). (a) The change in solution conductivity, (b) current, and (c) voltage profiles during a 10cycle operation.
*Tel.: +886 2 33664400. Fax: +886 2 23928830. E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 4768
DOI: 10.1021/acssuschemeng.6b00974 ACS Sustainable Chem. Eng. 2016, 4, 4762−4770
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ACKNOWLEDGMENTS This research was funded by the National Taiwan University (105R7710) and the Ministry of Science and Technology, Taiwan (104-2628-E-002-004 -MY3).
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