Microporous Carbon Derived from Biomass

Research Article pubs.acs.org/journal/ascecg

N,P-Codoped Meso-/Microporous Carbon Derived from Biomass Materials via a Dual-Activation Strategy as High-Performance Electrodes for Deionization Capacitors Dong Xu,†,‡ Ying Tong,†,‡ Tingting Yan,‡ Liyi Shi,‡ and Dengsong Zhang*,‡ †

School of Materials Science and Engineering, Jiangsu University, No. 301 Xuefu Road, Zhenjiang 212013, P. R. China Research Center of Nano Science and Technology, Shanghai University, No. 99 Shangda Road, BaoShan District, Shanghai 200444, P. R. China

‡

S Supporting Information *

ABSTRACT: Nitrogen and phosphorus codoped meso-/ microporous carbon derived from biomass materials has been designed and successfully fabricated as high-performance electrodes for deionization capacitors. The obtained materials were prepared by the pyrolysis of pomelo peel via a dualactivation strategy by using NH4H2PO4 and KHCO3. It is interesting that the as prepared nitrogen and phosphorus codoped meso-/microporous carbon possesses a high specific surface area of 2726 m2 g−1 with high percentage of mesopores of 52%. It has been demonstrated that the obtained electrode shows high specific capacitance, low inner resistance and good wettability. Consequently, the obtained electrode exhibits good deionization performance in a 300−1000 mg L−1 NaCl solution at 1.0−1.4 V with a flow rate of 20−60 mL min−1. The electrode reveals an ultrahigh deionization capacity of 20.78 mg g−1 at 1.4 V in a 1000 mg L−1 NaCl solution. The regeneration performance of the obtained electrode is good. The good performance is ascribed to the high specific surface area, superior micro-/mesoporous structure, and N,P-codoping. Hence, the nitrogen and phosphorus codoped meso-/microporous carbon should be a promising candidate as electrode material for deionization capacitors. The present work paves a way for the development of multiheteroatom codoped meso-/microporous carbon materials for electrochemical applications. KEYWORDS: Meso-/microporous carbon, Biomass, Deionization capacitors

■

INTRODUCTION The scarcity of water is a serious problem in the world.1,2 The requirement for freshwater will increase rapidly with population growth, industrial development, and climate change.3 Consequently, it is quite necessary to find an effective way to deal with this problem and produce freshwater. Seawater and brackish water desalination is a promising method to solve the lack of water resources.4−6 Reverse osmosis and distillation as traditional desalination technologies have many defects, such as high energy consumption as well as high cost.4,7 Recently, deionization capacitors have become an attractive technique for desalination, which relies on the use of electric double layer capacitors (EDLCs). Once the external voltage is applied, the salt ions can be electrostatically adsorbed by the oppositely charged electrodes. After removing the voltage, the salt ions will be desorbed and go back to the bulk solution.8−10 Moreover, the energy recovery can be realized during ion release without the second pollution. It is noted that the capacity of deionization capacitors is largely decided by the physical and chemical properties of the electrode materials.9,11 Ideally, the electrode materials should have rational porous structure, large surface area, high electrical © 2017 American Chemical Society

conductivity, good wettability, and long-term stability. Up to now, different types of carbon materials can be used to make electrodes for deionization capacitors, including activated carbon,12 graphene,13−15 carbon nanotubes,16−18 carbon nanofibers,19−21 carbon aerogels,22 mesoporous carbon,21,23 and their composites.24−27 Recently, activated carbon is always used as electrode materials for deionization capacitors due to its high specific surface area and environmental compatibility as well as low cost.28 Unfortunately, as we all know, the activated carbons mostly consist of micropores, which might hinder the salt ion diffusion and thus go against the deionization process.29 In contrast, an interconnected pore structure with the pore size range from 2 to 50 nm is expected to be favorable for fast ionic transport.30,31 However, mesoporous carbon which always possesses low specific surface area would hinder its further use in the deionization capacitors. To solve these problems, meso-/ microporous carbon has attracted more attention.32,33 To combine the advantages of different pores, meso-/microporous Received: February 21, 2017 Revised: May 10, 2017 Published: May 23, 2017 5810

DOI: 10.1021/acssuschemeng.7b00551 ACS Sustainable Chem. Eng. 2017, 5, 5810−5819

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration of the Preparation Process of N,P-Codoped Meso-/microporous Carbon

and mesopores.49,50 However, the majority of the physical activation and chemical activation methods can hardly obtain meso-/microporous carbon, especially with a high percentage of mesopores. Ismadji et al. prepared porous carbon derived from cassava peel, and the specific surface area is 1352 m2 g−1, which is mainly contributed by micropores.51 Yang et al. prepared a honeycomb-like porous carbon derived from pomelo peel with the percentage of mesopores of about 4%.52 It is quite challenging to fabricate a multiheteroatoms codoped meso-/microporous carbon with high specific surface area and high percentage of mesopores from biomass materials via a traditional activation method. In this work, we have synthesized a nitrogen and phosphorus codoped meso-/microporous carbon with high specific surface area of 2726 m2 g−1 and high percentage of mesopores of 52% derived from pomelo peel, a typical biomass material, via a dualactivation strategy by using NH4H2PO4 and KHCO3 (Scheme 1). The pomelo peel is composed of amino acids, phosphorus, magnesium, sodium, calcium, and other elements. As far as we know, no work about NH4H2PO4 as an activator is reported so far. It is interesting that the NH4H2PO4 will decompose into NH3 and H3PO4 with the temperature increasing. The NH3 will be released from the carbon precursor; and then a large number of pores are created. Meanwhile, NH4H2PO4 can provide nitrogen and phosphorus sources. The H3PO4 could promote the depolymerization of the constituents of the carbon precursor into smaller units.53,54 Besides, the KHCO3 can decompose with the temperature increase and release gas, including CO2 and CO, or react with carbon and cause a corrosion of the carbon wall, which could enlarge the size of a part of the micropores and render them to become mesopores.55 After the dual activation, the meso-/microporous carbon is expected to have a high surface area with high percentage of mesopores. The nitrogen and phosphorus codoped meso-/microporous carbon materials are proved to be a promising electrode material for deionization capacitors.

carbon guarantees not only a large surface area for ion desorption, but also a shorter ion diffusion distance and smaller resistances for ion transport through the framework. To improve the electrical conductivity and wettability of porous carbon materials, it has been demonstrated that the incorporation of heteroatoms is quite necessary by us and other research groups.34,35 Pan et al. prepared nitrogen-doped carbon nanorods as electrodes for deionization capacitors, and the salt adsorption capacity achieved 17.62 mg g−1 in a 500 mg L−1 NaCl solution.36 In our previous work, our group also prepared a novel nitrogen-doped porous carbon as electrode for deionization capacitors and the salt adsorption capacity achieved 16.63 mg g−1 when the NaCl concentration is 500 mg L−1.37 It should be noted that most of the heteroatomsdoped carbon materials employed for deionization capacitors are nitrogen-doped carbon materials. Most recently, growing interest has emerged in P-doped carbon materials. P and N have the same number of valence electrons, which make Pdoped carbon materials also electron rich. Furthermore, the diameter of P is much larger than that of C, so P-doping results in more local structural distortion of the hexagonal carbon framework and P protrudes out of the graphene plane. Those are the reasons why P-doped carbon materials overcome the steric hindrance effects which are encountered in N-doped carbon materials.38 It is interesting that multiheteroatoms codoped carbon material can produce double functional group or diatomic synergies, which could bring improved electrical conductivity and enhanced wettability of carbon materials.39,40 However, to our knowledge, multiheteroatoms codoped carbon material used as electrodes for deionization capacitors has been rarely reported.39,41,42 Recently, sustainable biomass, such as eggplants,43 banana peel,44 bacterial fiber,45 soybeans,28 and so on, has been widely used as carbon sources for preparing electrode materials in energy applications due to its natural abundance and inexpensive and environmentally friendly features. Carbon materials derived from biomass materials always have large accessible specific surface area and good electrical conductivity. As far as we know, most of the biomass materials converted to carbon materials through an activation method. Previously, such activators, including KOH,46 H3PO4,47 K2CO3,48 and so on, have been developed to fabricate biomass carbon materials. All of them can promote a strong development of porosity. It has been demonstrated that KOH can only produce micropores, whereas K2CO3 and H3PO4 develop both micropores

■

EXPERIMENTAL SECTION

Preparation. The chemicals used during the whole experiment process were all purchased from Sinopharm Chemical Reagent Company, and all of them were not further purified. Deionized (DI) water was employed during the whole process. Pomelo peel was first cut into squares using a knife and then freezedried for 24 h. In a typical synthesis, 1 g of dried pomelo peel was immersed in a 50 mL NH4H2PO4 aqueous solution, and the ratio of peel to NH4H2PO4 is 1:1.24. Next, the mixture was evaporated at 60 5811


Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) SEM images, (b, c) TEM images, and (d) EDX elemental mapping of C, N, and P. °C in ambience and subsequently transferred to a freeze-drying oven to remove redundant water. Afterward, the dried mixture was heated at 800 °C with a heating rate of 5 °C min−1, and then the temperature was kept constant for 2 h; the whole process was performed under N2 atmosphere. Then the mixture was washed with a 2 mol L−1 HCl solution; the purpose of HCl elution is to remove the inorganic impurities. Then, the black powder was mixed with KHCO3, in 30 mL of deionized water and evaporated at 60 °C in ambience. Apparently, different activation conditions may have different effects on the specific surface area, the pore size distribution, and the ratio of mesopores to micropores.56,57 The ratio of carbon to KHCO3 is 1:8, which has been demonstrated in our previous work.58 Subsequently, the resultant mixture was placed into a horizontal tube furnace and heated at 800 °C with a heating rate of 5 °C min−1, and then the temperature was kept constant for 2 h; the whole process was performed under a N2 atmosphere. The mixture was then washed thoroughly with a 2 mol L−1 HCl solution and deionized water until the pH value reached 7. The samples were obtained after drying at 60 °C. The obtained black carbon materials were named as MMC. For comparison, K−C was prepared only activated by KHCO3 with a ratio of carbon to KHCO3 (1:8); and the other was only activated by NH4H2PO4 with a ratio of carbon to NH4H2PO4 (1:1.24) called P−C. D−C was prepared by calcining the pemole peel directly. The contrast samples were all washed with a 2 mol L−1 HCl solution and deionized water. Characterization. The materials were investigated by SEM, TEM, HRTEM, TG, XRD, Raman spectroscopy, XPS spectroscopy, nitrogen adsorption−desorption measurements, and dynamic contact angle. The details are given in the Supporting Information. Electrochemical Measurements. The cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) were measured with a CHI 660D (Chenhua, Shanghai). All the measurements used a three-electrode system. The working electrode is an MMC electrode; the counter electrode is a piece of graphite, and the reference electrode is a saturated calomel electrode. The tests were in a NaCl solution, and the concentration of the NaCl solution was always 0.5 M. The specific capacitance C can be calculated according to the following equation:

∫

C = ( I dV )/2vmΔV

Then the mixture was loaded on the graphite sheets and then transferred to a 120 °C oven for one night. The deionization performance of the electrodes was tested in a batch mode electrosorptive experiment with a continuous recycling system.59 The deionization capacitors included two sided electrodes, and an insulated spacer was put between the two electrodes. The NaCl aqueous solution was pumped into the deionization capacitors in a loop at 40 mL min−1; when voltage was added across the two opposite electrodes, the solution conductivity began to change. The solution conductivity was continuously monitored by a conductivity meter at the outlet, where the solution was released. The salt adsorption capacity (SAC) of the electrodes can be calculated using the following formula: SAC = (C0 − C)V /m

(2)

where C0, C, V, and m represent the initial concentration of the NaCl aqueous solution, the final concentration of the NaCl aqueous solution, the total volume, and the whole mass of the electrode material coated onto the graphite sheets, respectively. The charge efficiency was calculated using the following equation:

Λ = (Γ × F )/Σ

(3) −1

in which F and Γ represent the Faraday constant (96485 C mol ) and the SAC (mol g−1) and ∑ (charge, C g−1) are calculated through integrating the current.

■

RESULTS AND DISCUSSION Characteristics. Figure 1 presents the SEM and TEM images of MMC. It is clear that the MMC could be obtained after the pyrolysis process of pomelo peel under the atmosphere of N2 (Figure S1). As shown in the SEM images (Figure 1a), the sample exhibits a sheet-like shape and most of the sheets are about several micrometers in size. A highly porous structure was observed by TEM images. After the first activation, the resulting K−C (Figure S2a and S2b) and P−C (Figure S2c and S 2d) show richer porosity than that of D−C (Figure S2e and S 2f). A large number of micropores and a few mesopores can be identified from both K−C and P−C. After the process of dual activation, we can clearly see the mesopores from the MMC in Figure 1b, and the holes are homogeneously distributed. Figure 1c reveals abundant micropores of MMC. The effect of dual activation on the mesoporous is fully demonstrated by TEM images (Figure 1b). The MMC shows richer mesoporosity than that of K−C, P−C, and D−C.

(1)

where C represents the specific capacitance, I represents the response current density, dV is the potential window, v is the potential scan rate, and m is the total mass of the electrode materials. Batch Mode Deionization Experiments. For preparing electrodes for deionization capacitors, the active components (80 wt %), the binder (10 wt %), and conductive carbon black (10 wt %) were mixed in ethanol. The size of the electrodes is 70 mm × 60 mm × 0.2 mm. 5812


Research Article

ACS Sustainable Chemistry & Engineering Overall, compared with the K−C and P−C, the MMC has a higher specific surface area and more mesopores, which can guarantee sufficient adsorption sites for salt ions and can reduce the ion transport resistance. Besides, we find P and N elements are incorporated into the carbon materials (Figure S3 and Table S1). The elemental compositions of the carbon material after the first-step and second-step calcination of pomelo peel are shown in Table S1. After two-step calcination, there are no N and P elements in the materials. Energy filter TEM mapping images (Figure 1d) also reveal that P and N elements are homogeneously dispersed on the surface of MMC. Incorporation of heteroatoms can also enhance the electrical conductivity and wettability of carbon materials.35 Therefore, as a kind of electrode material, plenty of mesporoes and micropores in the MMC can offer a large accessible surface area for ion desorption, a shorter ion diffusion distance, and smaller resistances for ion transport through the framework. Meanwhile, the existence of nitrogen and phosphorus elements in the carbon networks can guarantee good conductivity and wettability. The MMC possesses a graphite-like structure with many defects, as demonstrated in XRD patterns (Figure S4a) and Raman spectra (Figure S4b). The N2 adsorption/desorption isotherms of all the samples are depicted in Figure 2. As can be seen, the isotherm of MMC

Table 1. Surface Texture Properties of MMC, K−C, P−C, and D−C Samples

SBET (m2 g−1)

Smicro (m2 g−1)

Sext (m2 g−1)

Vtotal (cm3 g−1)

Dave (nm)

Sext/ SBET

MMC K−C P−C D−C

2726 1740 1594 354

1315 1443 1151 316

1411 297 443 38

1.73 0.82 0.88 0.15

0.89 0.61 0.52 0.61

52% 17% 28% 12%

specific surface area is greatly increased. The MMC possesses a specific surface area of 2726 m2 g−1 and pore volume of 1.73 cm3 g−1, of which the specific surface area contributed by mesopores is about 1411 m2 g−1. The specific surface area of MMC is much higher than those of K−C (1740 m2 g−1 and 0.82 cm3 g−1) and P−C (1594 m2 g−1 and 0.88 cm3 g−1). We attribute the improvement in the specific surface area to the coeffects of NH4H2PO4 and KHCO3. After the first activation by NH4H2PO4, its specific surface area was increased. The formation of the pores could be ascribed to the gas release of volatile species such as NH3, CO, CO2, acetic acid, and so on.48,54,62,63 After the second activation of KHCO3, the number of mesopores was greatly enhanced. When the temperature increased, the decomposition rate would be quicker, and it would promote the enlarging of the pore size. In conclusion, the nitrogen and phosphorus codoped meso-/microporous structure can shorten the distance of ion diffusion and make ion transfer through the porous framework quick. To study the effect of heteroatoms and the meso-/ microporous structure on the wettability, as shown in Figure 3, the contact angle measurements were conducted for MMC, K−C, P−C, and D−C. The droplet on the surface of MMC disappeared quickly and completely by adsorption and/or spreading within 50 s. In contrast, the contact angle of K−C, P−C, and D−C only had a little bit of change with the time increasing. The speed of the disappearance of the droplet on the surface of electrodes can be influenced by two aspects: (1) The hydrophobicity/hydrophilicity of the nature of the carbon materials, which can be influenced by the micro-/mesoporous structure, specific surface area, and heteroatom doping. (2) The effect of porosity (like capillary effects) of the material. The richer porosity can be beneficial to the adsorption onto the porous materials. The MMC shows improved wettability because of its richer porosity, the meso-/microporous structure, and the existence of phosphorus and nitrogen elements. The content of the surface functional groups and the superior meso-/microporous structure of MMC can lead to the efficient access of the aqueous solution into the MMC electrode. Good wettability of the electrode will bring an easier accessible surface which is beneficial to the penetration of the solution. Thus, the MMC is expected to show good deionization performance.64 Electrochemical Performance. The electrosorption behavior of the electrode materials was studied by CV analysis. The tests on the samples were performed at room temperature. Figure 4a shows the CV curves of MMC, K−C, P−C, and D− C electrodes at 1 mV s−1 with 0.5 M NaCl aqueous solution, and the potential window is −0.5−0.5 V. All of the CV curves present nearly rectangular shape without any other oxidation and reduction peaks, indicating an ideal EDL formation rather than faradic reaction and efficient ion transport through the meso-/microporous framework of the materials.65,66 As expected, the specific capacitances of K−C, P−C, and D−C

Figure 2. Nitrogen sorption isotherms and pore size distributions of MMC, K−C, P−C, and D−C.

is a typical type-IV isotherm with the type H4 hysteresis loop, indicating the existence of a large amount of mesopores in modified nitrogen and phosphorus codoped meso-/microporous carbon via a dual-activation strategy by using NH4H2PO4 and KHCO3.60 The conclusion was also supported by the pore size distribution, in the inset of Figure 2. The pore size of MMC is distributed mainly with a range of 1.2−3.3 nm, implying the existence of both micropores and mesopores. The isotherm of K−C (Figure 2) is a type-I isotherm without an evident hysteresis loop, and the curve is very steep at relatively low pressure, showing that it is a typical microporous structure.61 The isotherm of P−C is also a type-IV isotherm with a few mesopores. The specific surface area and pore volume of all the samples are summarized in Table 1. It is obvious that after activation by NH4H2PO4 and KHCO3, the 5813


Research Article


Figure 3. Dynamic water contact angle analysis of MMC, K−C, P−C, and PC at different times.

Figure 4. (a) CV curves of all the electrodes at 1 mV s−1; (b) Nyquist profiles of the electrodes. All the curves were obtained in a 0.5 M NaCl aqueous solution.

Figure 5. (a) Plots of SAC vs deionization time and (b) Ragone plots of SAR vs SAC for MMC, K−C, P−C, and D−C electrodes. The curves were obtained in a 1000 mg L−1 NaCl aqueous solution at 1.4 V with a flow rate of 40 mL min−1.

abundant ion transport pathways with a low resistance, so it is beneficial to the transport of the ions in the bulk solution. Meanwhile, the existence of P and N elements can create more defects, which favors the ion diffusion.39 All of the above reasons can improve the capacitance. We also prepared the carbon material with the same porosity but fewer or no N and/ or P contents. As shown in Figure S7, the result proves that codoping has an obvious influence on the specific capacitances and high specific capacitances are beneficial to improve the deionization capacity.

electrodes are lower than that of MMC. The inherent resistivity of the salt solution and the electrode polarization make the CV curves exhibit leaf-like shape and deviate from the typical rectangular shape (Figure S6). In particular, the MMC exhibits the highest specific capacitance of 207 F g−1 when the scan rate is 1 mV s−1. At the same time, the specific capacitances of P−C, K−C, and D-C are 150 F g−1, 115 F g−1, and 109 F g−1 respectively. The higher capacitance of MMC can be attributed to its larger surface area, its meso-/microporous structure, as well as the existence of P and N elements. The larger surface area can provide more adsorption sites. Mesopores offer 5814


Research Article


Figure 6. (a) Plots of SAC vs deionization time and (b) Ragone plots of SAR vs SAC for MMC electrodes at 1.0−1.4 V in a 500 mg L−1 NaCl aqueous solution with a flow rate of 40 mL min−1. (c) Plots of SAC vs deionization time and (d) Ragone plots of SAR vs SAC for MMC electrodes in a 300−1000 mg L−1 NaCl aqueous solution at 1.4 V with a flow rate of 40 mL min−1. (e) Plots of SAC vs deionization time and (f) Ragone plots of SAR vs SAC for MMC electrodes at 20−40 mL min−1 in a 500 mg L−1 NaCl aqueous solutions at 1.4 V.

suggesting they have the same capacitive behavior. The EIS result demonstrates that the MMC electrodes with easy transport pathways for electron/ion are suitable for the deionization capacitors. Deionization Performance. The deionization behavior of the electrodes was carried out as shown in Figure 5. The MMC electrodes show better adsorption−desorption capability than other electrodes (Figure 5a). Apparently, the solution conductivity decreases rapidly at the initial 4 min when 1.4 V is applied and then plateaued after 60 min, indicating the deionization procedure was completed within 60 min (Figure S8). It is noted that there is no side-reaction occurring when the operation voltage is up to 1.4 V due to the existence of inner resistance, which is mainly from the weak adhesion between the current collector, electrode materials, and binder used in the fabrication of electrodes. During the whole process, the plots of the solution conductivity in the K−C, P−C, and D−C systems are always much lower than that in the MMC system, indicating that the MMC electrodes absorbed more salt ions than other electrodes. It is worth noting that the solution conductivity of P−C decreases slowly in the deionization capacitors than K−C electrodes and this will lead to low charge efficiency. This can be ascribed to the high heteroatom content of P−C electrodes, because high heteroatom content may result

The ion-transport behavior and electrical resistance of different electrodes were further evaluated by EIS, and the concentration of NaCl aqueous solution was 0.5 M and the frequency was 100 kHz to 0.01 Hz. As shown in Figure 4b, obviously, the Nyquist plots of MMC, K−C, P−C, and D−C are all composed of a semicircle and a slop line in a highfrequency range and low-frequency range, respectively. In a high-frequency area, the width of a semicircle relates to the charge transfer resistance of the ion; lower charge transfer resistance can result in a smaller semicircle.67 Clearly, all of the diameters are quite small, indicating that the charge transfer resistance of the electrodes can be ignored. The MMC shows a smaller x-intercept and the x-intercept presents the equivalent series resistance (ESR); the value of ESR corresponds to the inner resistance, including the charge transfer resistance, the electrodes resistance, and contact resistance between the current collector and the electrodes.68 The lower ESR can be attributed to the following aspects: First, the meso-/microporous structure is beneficial to the salt ion diffusion, which can decrease the contact resistance at the electrode/solution interface. Second, the heteroatom leads to a good electronic conductivity. At low frequency, the line of the electrode manifests the capacitive behaviors of electrodes.69 The plots of MMC, K−C, P−C, and D−C show a similar line shape, 5815


Research Article


shifts upward and right at 1.4 V, suggesting a higher SAC and SAR. The Coulombic interactions between the charged ions and electrode can be enhanced at a high voltage, and the electrical double layer (EDL) would be thick, which can lead to high SAC and high SAR.24 The effect of the initial NaCl concentration was also examined. As shown in Figure 6c and 6d, the result shows that a higher concentration leads to a higher SAC. The SAC increased from 16.84 mg g−1 to 20.78 mg g−1 when the initial concentration of NaCl solution is 300 mg L−1 to 1000 mg L−1. This could be explained by the higher concentration providing the formation of a more compact EDL, which could result in high SAC. Meanwhile, the high conductivity can accelerate the transport of ions.70,71 In addition, the ratio of adsorbed salt over charge was evaluated by charge efficiency.72 The current transient during the electrosorption process was tested in different concentrations of NaCl aqueous solution from 100 mg L−1 to 500 mg L−1 under the condition of 1.4 V and 40 mL min−1. When the initial concentrations are 100 mg L−1, 300 mg L−1, and 500 mg L−1, the corresponding charge efficiencies are 0.40, 0.39, and 0.36, respectively (Figure S5). The charge efficiency is far from the theoretical value of 1, because the existence of co-ions will repel each other from the electrodes and the adhesion between the electrode and substrate is weak, as well as the binder used in the fabrication of the electrodes. The deionization performance was also carried out at different flow rates, as shown in Figure 6e. At 40 mL min−1, the MMC electrodes own the highest SAC of 18.27 mg g−1 and the Ragone plot located at the upper and right region (Figure 6f), indicating the highest salt adsorption rate. When the flow rate decreases to 20 mL min−1 or increases to 60 mL min−1, the SAC of MMC is 12.25 mg g−1 and 16.30 mg g−1. Both of them are lower than the SAC at the flow rate of 40 mL min−1. As a result, electrodes can adsorb more salt ions from the solution at a high flow rate. However, it does not have enough retention time for mass transfer in the CDI cell under too high a flow rate, and the high flow rate does not synchronize with the adsorption rate of the active sites. Therefore, to increase the adsorption capacity and rate, a certain period of time for mass transfer is necessary. 40 mL min−1 is the best choice for deionization performance. To further investigate the regeneration performance of MMC electrodes, the consecutive deionization−regeneration experiments were conducted in a 100 mg L−1 NaCl solution (Figure 7), which has been widely employed in the literature.73−75 Once the voltage was applied, the solution conductivity dropped dramatically, and the solution con-

in a low charge efficiency. During the salt removal process, the presence of heteroatom can cause a parasitic faradaic reaction, so every Coulomb of charge is not just taking part in the formation of an EDL; this can decrease the charge efficiency.35 The SAC values of MMC, K−C, P−C, and D−C are 20.78 mg g−1, 16.63 mg g−1, 15.20 mg g−1, and 9.95 mg g−1, respectively. Obviously, the MMC shows the highest electrosorption capacity because of its high specific surface area, high percentage of mesopores, and good electrical conductivity. The electrosorption capacity of MMC is much higher than that of various carbon electrode materials from the literature (Table S2). The existence of P and N elements can create more defects, which favors the ion diffusion and makes carbon materials electron rich. Compared to that of solely N-doped carbon materials,32,34,36,37 the codoping can produce a double functional group or diatomic synergies, which could improve the electrical conductivity and wettability of carbon materials.39,40 The K−C and P−C electrodes have lower desalination capacitance than MMC electrodes, although the surface area achieves around 1600 m2 g−1, but most of the pores are micropores, the microporous structure limits the mass transfer of ions, and the electrical conductivity is relatively poor. The D−C electrodes not only have low surface area but also show a bad electrical conductivity, so the SAC value is the lowest. In addition, the good wettability of MMC will contribute to adequate contact between the electrodes and saline solution, and it will facilitate the adsorption of ions. In a word, the nitrogen and phosphorus codoped meso-/microporous carbon obtained via a dual-activation strategy can present an improved deionization performance. Ragone plots of salt adsorption rate (SAR) vs SAC for the electrodes are a valid way to evaluate the deionization performance of a system, which give a visual representation of the deionization performance.70 As shown in Figure 5b. The plot of MMC electrodes is located at the upper and righter region, showing that the MMC electrodes have higher salt adsorption rate and a higher adsorption amount compared to K−C, P−C, and D−C. The increasing in the aspect of SAR and SAC can be attributed to a high specific surface area, superior micro-/mesoporous structure, and an appropriate content of heteroatom doping in carbon materials. It is noted that during the process of the desalination, at first, the adsorption of ions occurred in mesopores mostly, and then micropores play an important role in the last process of adsorption. In a word, the MMC electrodes exhibit a better adsorption ability. In addition, during the whole deionization process, the pH value does not have an obvious change (Figure S9), which indicates that no obvious side-reactions were observed in the CDI process. Figure 6a shows the SAC curves of MMC electrodes at different applied voltages. As can be seen, at first the capacity of all the electrodes increases; then it reaches the adsorption equilibrium after 60 min. When increasing the voltage, the downward trend of solution conductivity is more obvious, indicating better salt adsorption capacity under the voltage of 1.4 V. Generally, the high voltage can lead to strong electrostatic interaction, which is beneficial to enhance the deionization performance. However, the applied voltage is always under 1.4 V, because faradic current and electrolytic reactions could appear at too high voltage. The salt adsorption capacity of MMC electrodes is gradually increased from 11.16 mg g−1 to 18.27 mg g−1 when the voltage increased from 1.0 to 1.4 V. Figure 6b depicts the Ragone plots of SAR vs SAC for MMC electrodes at voltages of 1.0 to 1.4 V. Obviously, the plot

Figure 7. Deionization−regeneration curves of MMC electrodes in a 100 mg L−1 NaCl aqueous solution. 5816


ACS Sustainable Chemistry & Engineering ductivity could return to the initial value once the voltage was removed. The SAC of MMC electrodes almost had no obvious decline after ten regeneration processes, indicating that the regeneration performance is good. This can be ascribed to the following factors: (1) A large number of mesopores can make the ion transmission easier; (2) High specific surface area can provide sufficient active adsorption sites; (3) Heteroatoms doping can offer good electrical conductivity and good wettability, which can also facilitate the adsorption of ions. All of these demonstrate that the MMC is a promising material for deionization capacitors.

ACKNOWLEDGMENTS

■

REFERENCES

(1) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M. Science and technology for water purification in the coming decades. Nature 2008, 452, 301−310. (2) Han, Y.; Xu, Z.; Gao, C. Ultrathin graphene nanofiltration membrane for water purification. Adv. Funct. Mater. 2013, 23, 3693− 3700. (3) Kim, S. J.; Ko, S. H.; Kang, K. H.; Han, J. Direct seawater desalination by ion concentration polarization. Nat. Nanotechnol. 2010, 5, 297−301. (4) Semiat, R. Energy issues in desalination processes. Environ. Sci. Technol. 2008, 42, 8193−8201. (5) Yin, H.; Zhao, S.; Wan, J.; Tang, H.; Chang, L.; He, L.; Zhao, H.; Gao, Y.; Tang, Z. Three-dimensional graphene/metal oxide nanoparticle hybrids for high-performance capacitive deionization of saline water. Adv. Mater. 2013, 25, 6270−6276. (6) Amiri, A.; Ahmadi, G.; Shanbedi, M.; Savari, M.; Kazi, S.; Chew, B. Microwave-assisted synthesis of highly-crumpled, few-layered graphene and nitrogen-doped graphene for use as high-performance electrodes in capacitive deionization. Sci. Rep. 2015, 5, 17503. (7) Yeh, C. L.; Hsi, H. C.; Li, K. C.; Hou, C. H. Improved performance in capacitive deionization of activated carbon electrodes with a tunable mesopore and micropore ratio. Desalination 2015, 367, 60−68. (8) Blair, J. W.; Murphy, G. W. Electrochemical demineralization of water with porous electrodes of large surface area; University of Oklahoma, 1960. (9) Porada, S.; Zhao, R.; Van Der Wal, A.; Presser, V.; Biesheuvel, P. Review on the science and technology of water desalination by capacitive deionization. Prog. Mater. Sci. 2013, 58, 1388−1442. (10) Suss, M.; Porada, S.; Sun, X.; Biesheuvel, P.; Yoon, J.; Presser, V. Water desalination via capacitive deionization: what is it and what can we expect from it? Energy Environ. Sci. 2015, 8, 2296−2319. (11) Liu, Y.; Nie, C.; Liu, X.; Xu, X.; Sun, Z.; Pan, L. Review on carbon-based composite materials for capacitive deionization. RSC Adv. 2015, 5, 15205−15225. (12) Wu, T.; Wang, G.; Zhan, F.; Dong, Q.; Ren, Q.; Wang, J.; Qiu, J. Surface-treated carbon electrodes with modified potential of zero charge for capacitive deionization. Water Res. 2016, 93, 30−7. (13) Jia, B.; Zou, L. Functionalized graphene as electrode material for capacitive deionization. Sci. Adv. Mater. 2013, 5, 1111−1116. (14) Yang, Z. Y.; Jin, L. J.; Lu, G. Q.; Xiao, Q. Q.; Zhang, Y. X.; Jing, L.; Zhang, X. X.; Yan, Y. M.; Sun, K. N. Sponge-templated preparation of high surface area graphene with ultrahigh capacitive deionization performance. Adv. Funct. Mater. 2014, 24, 3917−3925. (15) Wang, H.; Shi, L.; Yan, T.; Zhang, J.; Zhong, Q.; Zhang, D. Design of graphene-coated hollow mesoporous carbon spheres as high performance electrodes for capacitive deionization. J. Mater. Chem. A 2014, 2, 4739−4750. (16) Zhang, D.; Shi, L.; Fang, J.; Dai, K. Influence of diameter of carbon nanotubes mounted in flow-through capacitors on removal of NaCl from salt water. J. Mater. Sci. 2007, 42, 2471−2475. (17) Fan, L.; Feng, C.; Zhao, W.; Qian, L.; Wang, Y.; Li, Y. Directional neurite outgrowth on superaligned carbon nanotube yarn patterned substrate. Nano Lett. 2012, 12, 3668−3673. (18) Shi, K.; Ren, M.; Zhitomirsky, I. Activated carbon-coated carbon nanotubes for energy storage in supercapacitors and capacitive water purification. ACS Sustainable Chem. Eng. 2014, 2, 1289−1298. (19) El-Deen, A. G.; Barakat, N. A.; Khalil, K. A.; Kim, H. Y. Development of multi-channel carbon nanofibers as effective electrosorptive electrodes for a capacitive deionization process. J. Mater. Chem. A 2013, 1, 11001−11010.

CONCLUSION In summary, we successfully fabricated the nitrogen and phosphorus codoped meso-/microporous carbon derived from pomelo peel via a dual-activation strategy. The as prepared carbon materials possess meso-/microporous carbon with a specific high specific surface area of 2726 m2 g−1, high percentage of mesopores of 52%, and good electrochemical properties. As a result, the salt adsorption capacity of the as prepared carbon materials reaches a high value of 20.78 mg g−1. The high specific surface area, superior micro-/mesoporous structure, and N,P-codoping can improve the deionization performance of the obtained materials. In a word, the nitrogen and phosphorus codoped meso-/microporous carbon should be a promising candidate as electrode for deionization capacitors. The present work paves a way for the development of multiheteroatoms codoped meso-/microporous carbon materials for electrochemical applications. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00551. TG curves of the pomelo peel; TEM images of K−C, P− C, and D−C; C 1s, N 1s, and P 2p XPS spectra of MMC; XRD pattern and Raman spectra of MMC; current transient curves and charge efficiency for the electrodes in a 100−500 mg L−1 NaCl aqueous solution at 1.4 V; CV curves of different electrodes at a scan rate of 1−30 mV s−1; CV curves of the two electrodes at 10 mV s−1; electrosorption behavior of MMC, K−C, P−C, and D−C electrodes; plot of pH-fluctuation in the process of desalination; elemental compositions of MMC; elemental compositions of the carbon material after the first and second calcinations of pomelo peel; comparison of SAC of various carbon electrode materials from the literature (PDF)

■

■

The authors gratefully acknowledge the support of the National Natural Science Foundation of China (U1462110, 51572113).

■

■

Research Article

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]; Fax: +86-21-66136079; Tel: +86-21-66137152. ORCID

Dengsong Zhang: 0000-0003-4280-0068 Notes

The authors declare no competing financial interest. 5817


Research Article

ACS Sustainable Chemistry & Engineering (20) Wang, G.; Pan, C.; Wang, L.; Dong, Q.; Yu, C.; Zhao, Z.; Qiu, J. Activated carbon nanofiber webs made by electrospinning for capacitive deionization. Electrochim. Acta 2012, 69, 65−70. (21) El-Deen, A. G.; Barakat, N. A.; Khalil, K. A.; Kim, H. Y. Hollow carbon nanofibers as an effective electrode for brackish water desalination using the capacitive deionization process. New J. Chem. 2014, 38, 198−205. (22) Gabelich, C. J.; Tran, T. D.; Suffet, I. M. Electrosorption of inorganic salts from aqueous solution using carbon aerogels. Environ. Sci. Technol. 2002, 36, 3010−3019. (23) Oschatz, M.; Borchardt, L.; Thommes, M.; Cychosz, K. A.; Senkovska, I.; Klein, N.; Frind, R.; Leistner, M.; Presser, V.; Gogotsi, Y. Carbide-Derived Carbon Monoliths with Hierarchical Pore Architectures. Angew. Chem., Int. Ed. 2012, 51, 7577−7580. (24) Li, H.; Pan, L.; Nie, C.; Liu, Y.; Sun, Z. Reduced graphene oxide and activated carbon composites for capacitive deionization. J. Mater. Chem. 2012, 22, 15556−15561. (25) Dong, Q.; Wang, G.; Qian, B.; Hu, C.; Wang, Y.; Qiu, J. Electrospun composites made of reduced graphene oxide and activated carbon nanofibers for capacitive deionization. Electrochim. Acta 2014, 137, 388−394. (26) Wimalasiri, Y.; Zou, L. Carbon nanotube/graphene composite for enhanced capacitive deionization performance. Carbon 2013, 59, 464−471. (27) Li, H.; Liang, S.; Li, J.; He, L. The capacitive deionization behaviour of a carbon nanotube and reduced graphene oxide composite. J. Mater. Chem. A 2013, 1, 6335−6341. (28) Long, C.; Jiang, L.; Wu, X.; Jiang, Y.; Yang, D.; Wang, C.; Wei, T.; Fan, Z. Facile synthesis of functionalized porous carbon with threedimensional interconnected pore structure for high volumetric performance supercapacitors. Carbon 2015, 93, 412−420. (29) Xia, K.; Gao, Q.; Jiang, J.; Hu, J. Hierarchical porous carbons with controlled micropores and mesopores for supercapacitor electrode materials. Carbon 2008, 46, 1718−1726. (30) Gu, X.; Hu, M.; Du, Z.; Huang, J.; Wang, C. Fabrication of mesoporous graphene electrodes with enhanced capacitive deionization. Electrochim. Acta 2015, 182, 183−191. (31) Rose, M.; Korenblit, Y.; Kockrick, E.; Borchardt, L.; Oschatz, M.; Kaskel, S.; Yushin, G. Hierarchical Micro-and Mesoporous Carbide-Derived Carbon as a High-Performance Electrode Material in Supercapacitors. Small 2011, 7, 1108−1117. (32) Deng, X.; Zhao, B.; Zhu, L.; Shao, Z. Molten salt synthesis of nitrogen-doped carbon with hierarchical pore structures for use as high-performance electrodes in supercapacitors. Carbon 2015, 93, 48− 58. (33) Qie, L.; Chen, W.; Xu, H.; Xiong, X.; Jiang, Y.; Zou, F.; Hu, X.; Xin, Y.; Zhang, Z.; Huang, Y. Synthesis of functionalized 3D hierarchical porous carbon for high-performance supercapacitors. Energy Environ. Sci. 2013, 6, 2497−2504. (34) Liu, Y.; Xu, X.; Lu, T.; Sun, Z.; Chua, D. H. C.; Pan, L. Nitrogen-doped electrospun reduced graphene oxide−carbon nanofiber composite for capacitive deionization. RSC Adv. 2015, 5, 34117− 34124. (35) Porada, S.; Schipper, F.; Aslan, M.; Antonietti, M.; Presser, V.; Fellinger, T. P. Capacitive Deionization using Biomass-based Microporous Salt-Templated Heteroatom-Doped Carbons. ChemSusChem 2015, 8, 1867−1874. (36) Liu, Y.; Xu, X.; Wang, M.; Lu, T.; Sun, Z.; Pan, L. Nitrogendoped carbon nanorods with excellent capacitive deionization ability. J. Mater. Chem. A 2015, 3, 17304−17311. (37) Wang, Z.; Yan, T.; Fang, J.; Shi, L.; Zhang, D. Nitrogen-doped porous carbon derived from a bimetallic metal−organic framework as highly efficient electrodes for flow-through deionization capacitors. J. Mater. Chem. A 2016, 4, 10858−10868. (38) Patel, M. A.; Luo, F.; Khoshi, M. R.; Rabie, E.; Zhang, Q.; Flach, C. R.; Mendelsohn, R.; Garfunkel, E.; Szostak, M.; He, H. P-Doped Porous Carbon as Metal Free Catalysts for Selective Aerobic Oxidation with an Unexpected Mechanism(48). ACS Nano 2016, 10, 2305−2315.

(39) Zhao, X.; Zhang, Q.; Zhang, B.; Chen, C.-M.; Wang, A.; Zhang, T.; Su, D. S. Dual-heteroatom-modified ordered mesoporous carbon: Hydrothermal functionalization, structure, and its electrochemical performance. J. Mater. Chem. 2012, 22, 4963−4969. (40) Zhang, J.; Qu, L.; Shi, G.; Liu, J.; Chen, J.; Dai, L. N,P-Codoped Carbon Networks as Efficient Metal-free Bifunctional Catalysts for Oxygen Reduction and Hydrogen Evolution Reactions. Angew. Chem., Int. Ed. 2016, 55, 2230−2235. (41) Ling, Z.; Wang, Z.; Zhang, M.; Yu, C.; Wang, G.; Dong, Y.; Liu, S.; Wang, Y.; Qiu, J. Sustainable Synthesis and Assembly of BiomassDerived B/N Co-Doped Carbon Nanosheets with Ultrahigh Aspect Ratio for High-Performance Supercapacitors. Adv. Funct. Mater. 2016, 26, 111−119. (42) Kotal, M.; Kim, J.; Kim, K. J.; Oh, I. K. Sulfur and Nitrogen CoDoped Graphene Electrodes for High-Performance Ionic Artificial Muscles. Adv. Mater. 2016, 28, 1610−1615. (43) Li, B.; Geng, D.; Lee, X. S.; Ge, X.; Chai, J.; Wang, Z.; Zhang, J.; Liu, Z.; Hor, T. S.; Zong, Y. Eggplant-derived microporous carbon sheets: towards mass production of efficient bifunctional oxygen electrocatalysts at low cost for rechargeable Zn-air batteries. Chem. Commun. 2015, 51, 8841−8844. (44) Lv, Y.; Gan, L.; Liu, M.; Xiong, W.; Xu, Z.; Zhu, D.; Wright, D. S. A self-template synthesis of hierarchical porous carbon foams based on banana peel for supercapacitor electrodes. J. Power Sources 2012, 209, 152−157. (45) Chen, L.; Huang, Z.; Liang, H.; Gao, H.; Yu, S. H. ThreeDimensional Heteroatom-Doped Carbon Nanofiber Networks Derived from Bacterial Cellulose for Supercapacitors. Adv. Funct. Mater. 2014, 24, 5104−5111. (46) Elmouwahidi, A.; Zapata Benabithe, Z.; Carrasco Marin, F.; Moreno Castilla, C. Activated carbons from KOH-activation of argan (Argania spinosa) seed shells as supercapacitor electrodes. Bioresour. Technol. 2012, 111, 185−190. (47) Chen, M.; Kang, X.; Wumaier, T.; Dou, J.; Gao, B.; Han, Y.; Xu, G.; Liu, Z.; Zhang, L. Preparation of activated carbon from cotton stalk and its application in supercapacitor. J. Solid State Electrochem. 2013, 17, 1005−1012. (48) Jin, X.-J.; Yu, Z.-M.; Wu, Y. Preparation of activated carbon from lignin obtained by straw pulping by KOH and K2CO3 Chemical activation. Cellul. Chem. Technol. 2012, 46, 79−85. (49) Molina-Sabio, M.; Rodríguez-Reinoso, F. Role of chemical activation in the development of carbon porosity. Colloids Surf., A 2004, 241, 15−25. (50) Adinata, D.; Wan Daud, W. M.; Aroua, M. K. Preparation and characterization of activated carbon from palm shell by chemical activation with K2CO3. Bioresour. Technol. 2007, 98, 145−149. (51) Ismanto, A. E.; Wang, S.; Soetaredjo, F. E.; Ismadji, S. Preparation of capacitor’s electrode from cassava peel waste. Bioresour. Technol. 2010, 101, 3534−3540. (52) Liang, Q.; Ye, L.; Huang, Z. H.; Xu, Q.; Bai, Y.; Kang, F.; Yang, Q. H. A honeycomb-like porous carbon derived from pomelo peel for use in high-performance supercapacitors. Nanoscale 2014, 6, 13831− 13837. (53) Lopez, M.; Labady, M.; Laine, J. Preparation of activated carbon from wood monolith. Carbon 1996, 34, 825−827. (54) Jagtoyen, M.; Derbyshire, F. Activated carbons from yellow poplar and white oak by H3PO4 activation. Carbon 1998, 36, 1085− 1097. (55) Zhang, C.; Geng, Z.; Cai, M.; Zhang, J.; Liu, X.; Xin, H.; Ma, J. Microstructure regulation of super activated carbon from biomass source corncob with enhanced hydrogen uptake. Int. J. Hydrogen Energy 2013, 38, 9243−9250. (56) Tseng, R. L.; Tseng, S. K. Characterization and use of high surface area activated carbons prepared from cane pith for liquid-phase adsorption. J. Hazard. Mater. 2006, 136, 671−680. (57) Yeh, C.-L.; Hsi, H.-C.; Li, K.-C.; Hou, C.-H. Improved performance in capacitive deionization of activated carbon electrodes with a tunable mesopore and micropore ratio. Desalination 2015, 367, 60−68. 5818


Research Article

ACS Sustainable Chemistry & Engineering (58) Zhao, S.; Yan, T.; Wang, Z.; Zhang, J.; Shi, L.; Zhang, D. Removal of NaCl from saltwater solutions using micro/mesoporous carbon sheets derived from watermelon peel via deionization capacitors. RSC Adv. 2017, 7, 4297−4305. (59) Zhang, D.; Wen, X.; Shi, L.; Yan, T.; Zhang, J. Enhanced capacitive deionization of graphene/mesoporous carbon composites. Nanoscale 2012, 4, 5440−5446. (60) Wu, Z. S.; Sun, Y.; Tan, Y. Z.; Yang, S.; Feng, X.; Müllen, K. Three-dimensional graphene-based macro-and mesoporous frameworks for high-performance electrochemical capacitive energy storage. J. Am. Chem. Soc. 2012, 134, 19532−19535. (61) Thommes, M.; Kaneko, K.; Neimark, A. V.; Olivier, J. P.; Rodriguez-Reinoso, F.; Rouquerol, J.; Sing, K. S. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl. Chem. 2015, 87, 1051−1069. (62) Foo, K.; Hameed, B. Utilization of rice husks as a feedstock for preparation of activated carbon by microwave induced KOH and K2CO3 activation. Bioresour. Technol. 2011, 102, 9814−9817. (63) Foo, K.; Hameed, B. Porous structure and adsorptive properties of pineapple peel based activated carbons prepared via microwave assisted KOH and K2CO3 activation. Microporous Mesoporous Mater. 2012, 148, 191−195. (64) Wang, D.; Li, C.; Guo, J.; Li, T. Carbon electrode modified by KOH solution to improve performance of capacitive desalination. Desalin. Water Treat. 2016, 57, 17731−17737. (65) Xu, B.; Wu, F.; Chen, R.; Cao, G.; Chen, S.; Wang, G.; Yang, Y. Room temperature molten salt as electrolyte for carbon nanotubebased electric double layer capacitors. J. Power Sources 2006, 158, 773− 778. (66) El-Deen, A. G.; Boom, R. M.; Kim, H. Y.; Duan, H.; Chan-Park, M. B.; Choi, J. H. Flexible 3D Nanoporous Graphene for Desalination and Bio-decontamination of Brackish Water via Asymmetric Capacitive Deionization. ACS Appl. Mater. Interfaces 2016, 8, 25313−25325. (67) Liu, Y.; Ma, J.; Lu, T.; Pan, L. Electrospun carbon nanofibers reinforced 3D porous carbon polyhedra network derived from metalorganic frameworks for capacitive deionization. Sci. Rep. 2016, 6, 32784. (68) Choi, B. G.; Chang, S.-J.; Lee, Y. B.; Bae, J. S.; Kim, H. J.; Huh, Y. S. 3D heterostructured architectures of Co3O4 nanoparticles deposited on porous graphene surfaces for high performance of lithium ion batteries. Nanoscale 2012, 4, 5924−5930. (69) Yan, J.; Wei, T.; Shao, B.; Ma, F.; Fan, Z.; Zhang, M.; Zheng, C.; Shang, Y.; Qian, W.; Wei, F. Electrochemical properties of graphene nanosheet/carbon black composites as electrodes for supercapacitors. Carbon 2010, 48, 1731−1737. (70) Kim, T.; Yoon, J. CDI ragone plot as a functional tool to evaluate desalination performance in capacitive deionization. RSC Adv. 2015, 5, 1456−1461. (71) Wimalasiri, Y.; Mossad, M.; Zou, L. Thermodynamics and kinetics of adsorption of ammonium ions by graphene laminate electrodes in capacitive deionization. Desalination 2015, 357, 178−188. (72) Zhao, R.; Biesheuvel, P.; Miedema, H.; Bruning, H.; Van der Wal, A. Charge efficiency: a functional tool to probe the double-layer structure inside of porous electrodes and application in the modeling of capacitive deionization. J. Phys. Chem. Lett. 2010, 1, 205−210. (73) Liu, Y.; Chen, T.; Lu, T.; Sun, Z.; Chua, D. H. C.; Pan, L. Nitrogen-doped porous carbon spheres for highly efficient capacitive deionization. Electrochim. Acta 2015, 158, 403−409. (74) Zhao, S.; Yan, T.; Wang, H.; Zhang, J.; Shi, L.; Zhang, D. Creating 3D Hierarchical Carbon Architectures with Micro-, Meso-, and Macropores via a Simple Self-Blowing Strategy for a Flow-through Deionization Capacitor. ACS Appl. Mater. Interfaces 2016, 8, 18027− 18035. (75) Li, Y.; Hussain, I.; Qi, J.; Liu, C.; Li, J.; Shen, J.; Sun, X.; Han, W.; Wang, L. N-doped hierarchical porous carbon derived from hypercrosslinked diblock copolymer for capacitive deionization. Sep. Purif. Technol. 2016, 165, 190−198. 5819


Microporous Carbon Derived from Biomass

Recommend Documents