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Metal-organic frameworks-derived porous carbon polyhedrons for highly efficient capacitive deionization Received 00th January 20xx, Accepted 00th January 20xx
Yong Liu, Xingtao Xu, Wang Miao, Ting Lu, Zhuo Sun and Likun Pan*
DOI: 10.1039/x0xx00000x www.rsc.org/
Porous carbon polyhedrons (PCPs) were prepared through direct carbonization of zeolitic imidazolate framework-8 and used as electrode mateial for capacitive deionization. The results show that PCPs treated at 1200 °C exhibit the highest electrosorption capacity of 13.86 mg g-1 when the initial NaCl concentration is 500 mg l-1, due to their high accessible surface area and low charge transfer resistance.
Water, as the source of life, plays an indispensable role in people’s life on earth. However, with the rise in population rates and the expansion of industrial and agricultural activities water scarcity has th become the most serious problem in the 21 century. As most of earth’s water is saline water (sea water or brackish water) which is not suitable for direct consumption, desalination has emerged to be the most promising approach to generate fresh water. Conventional desalination techniques such as membrane separation and flash distillation require either high-pressure pumps, membranes or thermal heaters, which will result in high capital or operational 1 expenditure. Therefore, it is in great urgency to find a low-cost desalination technology. Capacitive deionization (CDI), also known as electrosorption, has emerged as a promising technique in desalination field due to its minimum energy consumption and environmental friendliness 2-4 compared to traditional desalination techniques. This emerging technology is proceeded by adsorbing ions into the double layer formed at the electrode surface when a low direct current potential (normally less than 2 V) is applied. 5 Since the concept of CDI was introduced by Caudle et al. in the 1960s, various carbonaceous materials including activated carbon 6, 7 8, 9 10 (AC), carbon nanofibers (CNFs), carbon aerogels (CAs), 11, 12 13 mesoporous carbon (MC), carbon nanotubes (CNTs) and 14 graphene has been used as CDI electrodes due to their high specific surface area, good conductivity and chemical stability. However, some carbon materials suffer problems such as high
Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China Normal University, Shanghai 200062, China. Fax: +86 21 62234321; Tel: +86 21 62234132; E-mail: [email protected]. †Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x
manufacturing cost and low adsorption capacity which limit CDI 15 from being used for scaling-up application. Therefore, further exploration of new strategy for carbonaceous material fabrication is necessary in current CDI research. Metal-organic frameworks (MOFs) as a new kind of crystalline porous materials have attracted great attention in many fields like 16 17 18 19 gas storage and separation, catalysis, drug delivery, chemical 20 sensors etc. due to their designable framework structures, adjustable pore and particle sizes as well as facile fabrication. Recently, direct carbonization of MOFs has been demonstrated to 21, 22 be a facile route to synthesize porous carbon materials with extremely high surface area which have shown some appealing performances in energy storage device such as lithium/sodium ion 23 24-26 24 battery and supercapacitor. For example, Yamauchi et al. prepared nanoporous carbon though thermal pyrolysis of zeolitic imidazolate framework-8 (ZIF-8) for supercapacitors, and a high -1 -1 specific capacitance of 214 F g at a scan rate of 5 mV s was 26 achieved. Later, Yamauchi et al. further used zeolitic imidazolate framework-67 (ZIF-67) as precursors to fabricate nanoporous -1 carbon with a specific capacitance of 238 F g at a scan rate of 20 -1 mV s . Due to their excellent capacitive performance, an obvious expectation can be made that MOFs-derived porous carbon materials should be very promising electrode materials for CDI. Unfortunately, to our best knowledge such an exploration on MOFs-derived porous carbon materials for CDI has not been reported in the literatures so far. In this work, MOFs-derived porous carbon polyhedrons (PCPs) were studied as electrode material for CDI for the first time and they exhibit excellent performance with an electrosorption capacity -1 -1 of 13.86 mg g in 500 mg l NaCl solutions that is among the 15, 27 highest values reported in the literatures. PCPs were prepared through direct carbonization of ZIF-8 at different temperatures. The experimental details are given in the † ESI. The temperatures of thermal treatment were set at 800, 1000 and 1200 °C and corresponding obtained PCPs were labelled as PCP800, PCP1000 and PCP1200, respectively. Fig. 1 (a)-(d) show the field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images of PCP1200 at different magnifications. The morphologies of PCP800 and PCP1000
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(not shown here) are similar to that of PCP1200. It can be seen that PCP1200 exhibits a uniform polyhedron structure that is composed of hexagons. The mean particle size of the polyhedrons is around 100 nm, as shown in Fig. 1 (d). Clearly, the polyhedron structure of 22 ZIF-8 precursors remains intact after carbonization. The inset of Fig. 1 (d) shows the selected area electron diffraction (SAED) pattern of PCP1200. The somber diffraction rings indicate the amorphous structure of PCP1200.
Fig. 1 (a), (b) FESEM and (c), (d) TEM images of PCP1200. Inset of (d) shows the SAED pattern of PCP1200.
corresponding to D line and G line, can be observed in the Raman spectra of all PCPs samples. The D line is attributed to the presence of defects in graphite sheets and the G line indicates the presence of hexagonal graphitic networks in the PCPs. The intensity ratio of the D peak to G peak (ID/IG) is related to the amount of defects in 28 the carbon materials. The ID/IG values of PCP800, PCP1000 and PCP1200 are 0.99, 1.02 and 1.05, respectively, indicating that the PCPs have a disorder structure and with the increase in the carbonization temperature more defects are generated in the PCPs. The presence of defects can generate more accessible surface area and cause an increase in ability for the accumulation of charges, which is beneficial for the charge transfer in the adsorption 29 process. Fig. 2 (c) shows the X-ray photoelectron spectroscopy (XPS) spectra of the PCPs. Carbon, nitrogen and oxygen elements were detected in all three samples, which originated from the ZIF-8 precursor. Obviously, the carbon content increases and oxygen content decreases when the temperature rises from 800 to 1200 °C, which may favor a high electrical conductivity. Fig. 2 (d) depicts the nitrogen adsorption-desorption isotherms of PCP800, PCP1000 and PCP1200. It is clearly observed that all isotherms show typical type IV behavior. The specific surface area, pore volume and mean pore diameter were determined by the Brunauer–Emmett–Teller method and listed in Table 1. It can be seen that PCP1200 has a 2 -1 3 largest surface area of 1187.8 m g and pore volume of 0.78 cm g 1 . Clearly, PCPs display highly porous structure, which is crucial for their capacitive behaviour. Table 1 The measured parameters of PCP800, PCP1000 and PCP1200.
S BET V total Mean pore Specific Rct (m2 g-1) (cm3 g-1) diameter(nm) capacity (F g-1) (Ω) PCP800 606.4 0.33 5.18 129.47 1.69 PCP1000 829.5 0.49 4.14 210.26 1.17 PCP1200 1187.8 0.78 3.50 275.69 0.84 Fig. 2 (e) shows the potential sweep cyclic voltammetry (CV) curves of PCP800, PCP1000 and PCP1200 electrodes at a potential sweep rate of 1 mV s-1 within a potential range of -0.5-0.5 V. All CV curves are nearly rectangular. The current increases and decreases steadily with the electric potential, indicating that no faradaic reaction happens and ions are adsorbed on the electrode surface by forming an electric double layer. The specific capacitances of PCP800, PCP1000 and PCP1200 electrodes are 129.47, 210.26 and 275.69 F g-1, respectively, as shown in Table 1. Obviously, the specific capacitance increases with the increase in the carbonization temperature. The enhanced capacitive performance is mainly due to the following reasons: (i) with the increase in the carbonization temperature the electrical conductivity of PCPs is improved and the charge transfer becomes easier, which can be proved by electrochemical impedance spectroscopy (EIS) measurement. Fig. 2 (f) shows the Nyquist plots of PCPs electrodes. The Nyquist plots are fitted and interpreted with the help of an equivalent electric circuit, as shown in the inset of Fig. 2 (f). The intersection of the impedance spectra with the real axis at the high-frequency end is the bulk resistance (Rs), which includes all contact resistances and the resistance of the electrolyte. The high frequency arc corresponds to the charge transfer limiting process and is ascribed to the doublelayer capacitance (Cdl) in parallel with the charge transfer resistance (Rct) at the contact interface between the electrode and electrolyte Sample
Fig. 2 (a) XRD patterns, (b) Raman spectra, (c) XPS spectra, (d) nitrogen adsorption-desorption isotherms, (e) CV curves and (f) Nyquist plots of PCP800, PCP1000 and PCP1200 electrodes. Inset of (f) shows the corresponding equivalent electric circuit.
Fig. 2 (a) shows the X-ray diffraction (XRD) patterns of different PCPs samples. Typically, two broad peaks appear at ~22° and ~43° corresponding to the (002) and (100) diffraction modes of the graphitic structure, which is characteristic of disordered carbon materials. The Raman spectra of PCP800, PCP1000 and PCP1200 were shown in Fig. 2 (b). Two obvious peaks at 1344 and 1590 cm-1,
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solution. As shown in Table 2, the Rct values of PCP800, PCP1000 and PCP1200 are 1.69, 1.17 and 0.84 Ω, respectively. (ii) High carbonation temperature enhances the specific surface area of PCPs, which is favourable to their capacitive performance. Fig. 3 (a) shows the conductivity transients in NaCl solution with -1 an initial concentration of 100 mg l during batch-mode experiments using PCP800, PCP1000 and PCP1200 electrodes (the total weight of active material is ∼270 mg per electrode) at 1.2 V. It can be seen that once the voltage is imposed, ions are driven onto the electrodes and the conductivity decreases dramatically and reaches equilibrium after about 40 min. Then the electrodes were shorted for regeneration. The electrosorption capacities of PCP800, -1 PCP1000 and PCP1200 electrodes are 5.87, 7.00 and 7.71 mg g , respectively. As expected, PCP1200 possesses the best electrosorption performance.
Fig. 3 (a) Conductivity transients for PCP800, PCP1000 and PCP1200 electrodes at 1.2 V in NaCl solution with an initial concentration of 100 mg l1 ; (b) Electrosorption capacity and current transient for PCP1200 electrode over 40 minutes in NaCl solution with different initial concentrations; (c) Recycle electrosorption experiment for PCP1200 electrode.
To investigate the recycle of electrosorption process of the PCPs, repeating charge-discharge experiment using PCP1200 electrode was carried out. The initial NaCl concentration is 100 mg l-1. When the conductivity gets back to the initial value in the first discharge process the second charge process starts. Fig. 3 (c) shows the conductivity transient over 30 charge-discharge cycles. Obviously, the repeatability of electrosorption process can be realized in this unit cell. To get a better understanding of the electrosorption behavior of PCPs, the electrosorption experiments using PCP1200 electrode in NaCl solution with different initial concentrations at 1.2 V were carried out and the corresponding currents were monitored simultaneously, as shown in Fig. 3 (b). Charge efficiency (Λ) is a functional tool to gain insight into the double layer formed at the interface between the electrode and solution,15, 27, 30 as described according to the following equation: Γ×F Λ= (1) Σ -1 where F is the Faraday constant (96485 C mol ), Γ is the -1 -1 electrosorption capacity (mol g ) and Σ (charge, C g ) is obtained by integrating the corresponding current. The electrosorption capacity and charge efficiency of PCP1200 electrode are listed in Table 2. It can be seen that the electrosorption capacity increases with the
increase in NaCl concentration and reaches a highest value of 13.86 -1 mg g . As seen, the Λ of PCP1200 is far from ideal value, which is 31 mainly caused by the co-ion effect, the weak adhesion between the porous electrode and substrate, and the blocking effect of the 4, 8 binder. Fortunately, an effective method has been proposed to solve this problem by introducing charge barrier membrane into 32-34 CDI. Table 2 Electrosorption capacity and charge efficiency of PCP1200 electrode in NaCl solution with different initial concentrations. -1
Initial concentration / mg l 100 250 500 Electrosorption capacity / mg g-1 7.71 10.10 13.86 Charge efficiency 0.67 0.72 0.72 In order to further evaluate the electrosorption performance of the PCPs, the electrosorption capacity of PCP1200 is compared with those of other carbon electrode materials reported in the 35 literatures and other water desalination technique, as shown in † ESI. Obviously, the electrosorption capacity of our PCP1200 electrode is among the highest values of the existing carbon materials measured in the similar experimental conditions. Unlike most conventional carbon materials, MOFs-derived PCPs enjoy both high specific surface area and low electrical resistance, which are both crucial factors for the electrosorption process. In summary, PCPs were successfully synthesized through thermal treatment of ZIF-8 at 800, 1000, and 1200 °C, and their electrochemical and electrosorption performances were investigated. The experimental results show that (i) the carbonization temperature plays an important role in the electrochemical performance of PCPs by influencing their specific surface area and charge transfer resistance; (ii) PCP1200 exhibits the best electrochemical performance among all the samples, with a specific capacitance of 275.69 F -1 -1 g ; (iii) a high electrosorption capacity of 13.86 mg g is achieved for PCP1200 when the initial NaCl concentration was -1 500 mg l ; (iii) no obvious declination in desalination performance has been observed during charge-discharge experiments, indicating a good regeneration ability of PCPs electrode; (iv) The MOFs derived PCPs should be a promising candidate as electrode material for CDI application. Financial support from National Natural Science Foundation of China (No. 21276087) is gratefully acknowledged.
Notes and references 1. 2. 3. 4. 5. 6. 7. 8.
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This article can be cited before page numbers have been issued, to do this please use: Y. Liu, X. Xu, M. Wang, T. Lu, Z. Sun and L. Pan, Chem. Commun., 2015, DOI: 10.1039/C5CC03999A.
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Metal-organic frameworks-derived porous carbon polyhedrons for highly efficient capacitive deionization Received 00th January 20xx, Accepted 00th January 20xx
Yong Liu, Xingtao Xu, Wang Miao, Ting Lu, Zhuo Sun and Likun Pan*
DOI: 10.1039/x0xx00000x www.rsc.org/
Porous carbon polyhedrons (PCPs) were prepared through direct carbonization of zeolitic imidazolate framework-8 and used as electrode mateial for capacitive deionization. The results show that PCPs treated at 1200 °C exhibit the highest electrosorption capacity of 13.86 mg g-1 when the initial NaCl concentration is 500 mg l-1, due to their high accessible surface area and low charge transfer resistance.
Water, as the source of life, plays an indispensable role in people’s life on earth. However, with the rise in population rates and the expansion of industrial and agricultural activities water scarcity has th become the most serious problem in the 21 century. As most of earth’s water is saline water (sea water or brackish water) which is not suitable for direct consumption, desalination has emerged to be the most promising approach to generate fresh water. Conventional desalination techniques such as membrane separation and flash distillation require either high-pressure pumps, membranes or thermal heaters, which will result in high capital or operational 1 expenditure. Therefore, it is in great urgency to find a low-cost desalination technology. Capacitive deionization (CDI), also known as electrosorption, has emerged as a promising technique in desalination field due to its minimum energy consumption and environmental friendliness 2-4 compared to traditional desalination techniques. This emerging technology is proceeded by adsorbing ions into the double layer formed at the electrode surface when a low direct current potential (normally less than 2 V) is applied. 5 Since the concept of CDI was introduced by Caudle et al. in the 1960s, various carbonaceous materials including activated carbon 6, 7 8, 9 10 (AC), carbon nanofibers (CNFs), carbon aerogels (CAs), 11, 12 13 mesoporous carbon (MC), carbon nanotubes (CNTs) and 14 graphene has been used as CDI electrodes due to their high specific surface area, good conductivity and chemical stability. However, some carbon materials suffer problems such as high
Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China Normal University, Shanghai 200062, China. Fax: +86 21 62234321; Tel: +86 21 62234132; E-mail: [email protected]. †Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x
manufacturing cost and low adsorption capacity which limit CDI 15 from being used for scaling-up application. Therefore, further exploration of new strategy for carbonaceous material fabrication is necessary in current CDI research. Metal-organic frameworks (MOFs) as a new kind of crystalline porous materials have attracted great attention in many fields like 16 17 18 19 gas storage and separation, catalysis, drug delivery, chemical 20 sensors etc. due to their designable framework structures, adjustable pore and particle sizes as well as facile fabrication. Recently, direct carbonization of MOFs has been demonstrated to 21, 22 be a facile route to synthesize porous carbon materials with extremely high surface area which have shown some appealing performances in energy storage device such as lithium/sodium ion 23 24-26 24 battery and supercapacitor. For example, Yamauchi et al. prepared nanoporous carbon though thermal pyrolysis of zeolitic imidazolate framework-8 (ZIF-8) for supercapacitors, and a high -1 -1 specific capacitance of 214 F g at a scan rate of 5 mV s was 26 achieved. Later, Yamauchi et al. further used zeolitic imidazolate framework-67 (ZIF-67) as precursors to fabricate nanoporous -1 carbon with a specific capacitance of 238 F g at a scan rate of 20 -1 mV s . Due to their excellent capacitive performance, an obvious expectation can be made that MOFs-derived porous carbon materials should be very promising electrode materials for CDI. Unfortunately, to our best knowledge such an exploration on MOFs-derived porous carbon materials for CDI has not been reported in the literatures so far. In this work, MOFs-derived porous carbon polyhedrons (PCPs) were studied as electrode material for CDI for the first time and they exhibit excellent performance with an electrosorption capacity -1 -1 of 13.86 mg g in 500 mg l NaCl solutions that is among the 15, 27 highest values reported in the literatures. PCPs were prepared through direct carbonization of ZIF-8 at different temperatures. The experimental details are given in the † ESI. The temperatures of thermal treatment were set at 800, 1000 and 1200 °C and corresponding obtained PCPs were labelled as PCP800, PCP1000 and PCP1200, respectively. Fig. 1 (a)-(d) show the field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images of PCP1200 at different magnifications. The morphologies of PCP800 and PCP1000
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(not shown here) are similar to that of PCP1200. It can be seen that PCP1200 exhibits a uniform polyhedron structure that is composed of hexagons. The mean particle size of the polyhedrons is around 100 nm, as shown in Fig. 1 (d). Clearly, the polyhedron structure of 22 ZIF-8 precursors remains intact after carbonization. The inset of Fig. 1 (d) shows the selected area electron diffraction (SAED) pattern of PCP1200. The somber diffraction rings indicate the amorphous structure of PCP1200.
Fig. 1 (a), (b) FESEM and (c), (d) TEM images of PCP1200. Inset of (d) shows the SAED pattern of PCP1200.
corresponding to D line and G line, can be observed in the Raman spectra of all PCPs samples. The D line is attributed to the presence of defects in graphite sheets and the G line indicates the presence of hexagonal graphitic networks in the PCPs. The intensity ratio of the D peak to G peak (ID/IG) is related to the amount of defects in 28 the carbon materials. The ID/IG values of PCP800, PCP1000 and PCP1200 are 0.99, 1.02 and 1.05, respectively, indicating that the PCPs have a disorder structure and with the increase in the carbonization temperature more defects are generated in the PCPs. The presence of defects can generate more accessible surface area and cause an increase in ability for the accumulation of charges, which is beneficial for the charge transfer in the adsorption 29 process. Fig. 2 (c) shows the X-ray photoelectron spectroscopy (XPS) spectra of the PCPs. Carbon, nitrogen and oxygen elements were detected in all three samples, which originated from the ZIF-8 precursor. Obviously, the carbon content increases and oxygen content decreases when the temperature rises from 800 to 1200 °C, which may favor a high electrical conductivity. Fig. 2 (d) depicts the nitrogen adsorption-desorption isotherms of PCP800, PCP1000 and PCP1200. It is clearly observed that all isotherms show typical type IV behavior. The specific surface area, pore volume and mean pore diameter were determined by the Brunauer–Emmett–Teller method and listed in Table 1. It can be seen that PCP1200 has a 2 -1 3 largest surface area of 1187.8 m g and pore volume of 0.78 cm g 1 . Clearly, PCPs display highly porous structure, which is crucial for their capacitive behaviour. Table 1 The measured parameters of PCP800, PCP1000 and PCP1200.
S BET V total Mean pore Specific Rct (m2 g-1) (cm3 g-1) diameter(nm) capacity (F g-1) (Ω) PCP800 606.4 0.33 5.18 129.47 1.69 PCP1000 829.5 0.49 4.14 210.26 1.17 PCP1200 1187.8 0.78 3.50 275.69 0.84 Fig. 2 (e) shows the potential sweep cyclic voltammetry (CV) curves of PCP800, PCP1000 and PCP1200 electrodes at a potential sweep rate of 1 mV s-1 within a potential range of -0.5-0.5 V. All CV curves are nearly rectangular. The current increases and decreases steadily with the electric potential, indicating that no faradaic reaction happens and ions are adsorbed on the electrode surface by forming an electric double layer. The specific capacitances of PCP800, PCP1000 and PCP1200 electrodes are 129.47, 210.26 and 275.69 F g-1, respectively, as shown in Table 1. Obviously, the specific capacitance increases with the increase in the carbonization temperature. The enhanced capacitive performance is mainly due to the following reasons: (i) with the increase in the carbonization temperature the electrical conductivity of PCPs is improved and the charge transfer becomes easier, which can be proved by electrochemical impedance spectroscopy (EIS) measurement. Fig. 2 (f) shows the Nyquist plots of PCPs electrodes. The Nyquist plots are fitted and interpreted with the help of an equivalent electric circuit, as shown in the inset of Fig. 2 (f). The intersection of the impedance spectra with the real axis at the high-frequency end is the bulk resistance (Rs), which includes all contact resistances and the resistance of the electrolyte. The high frequency arc corresponds to the charge transfer limiting process and is ascribed to the doublelayer capacitance (Cdl) in parallel with the charge transfer resistance (Rct) at the contact interface between the electrode and electrolyte Sample
Fig. 2 (a) XRD patterns, (b) Raman spectra, (c) XPS spectra, (d) nitrogen adsorption-desorption isotherms, (e) CV curves and (f) Nyquist plots of PCP800, PCP1000 and PCP1200 electrodes. Inset of (f) shows the corresponding equivalent electric circuit.
Fig. 2 (a) shows the X-ray diffraction (XRD) patterns of different PCPs samples. Typically, two broad peaks appear at ~22° and ~43° corresponding to the (002) and (100) diffraction modes of the graphitic structure, which is characteristic of disordered carbon materials. The Raman spectra of PCP800, PCP1000 and PCP1200 were shown in Fig. 2 (b). Two obvious peaks at 1344 and 1590 cm-1,
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solution. As shown in Table 2, the Rct values of PCP800, PCP1000 and PCP1200 are 1.69, 1.17 and 0.84 Ω, respectively. (ii) High carbonation temperature enhances the specific surface area of PCPs, which is favourable to their capacitive performance. Fig. 3 (a) shows the conductivity transients in NaCl solution with -1 an initial concentration of 100 mg l during batch-mode experiments using PCP800, PCP1000 and PCP1200 electrodes (the total weight of active material is ∼270 mg per electrode) at 1.2 V. It can be seen that once the voltage is imposed, ions are driven onto the electrodes and the conductivity decreases dramatically and reaches equilibrium after about 40 min. Then the electrodes were shorted for regeneration. The electrosorption capacities of PCP800, -1 PCP1000 and PCP1200 electrodes are 5.87, 7.00 and 7.71 mg g , respectively. As expected, PCP1200 possesses the best electrosorption performance.
Fig. 3 (a) Conductivity transients for PCP800, PCP1000 and PCP1200 electrodes at 1.2 V in NaCl solution with an initial concentration of 100 mg l1 ; (b) Electrosorption capacity and current transient for PCP1200 electrode over 40 minutes in NaCl solution with different initial concentrations; (c) Recycle electrosorption experiment for PCP1200 electrode.
To investigate the recycle of electrosorption process of the PCPs, repeating charge-discharge experiment using PCP1200 electrode was carried out. The initial NaCl concentration is 100 mg l-1. When the conductivity gets back to the initial value in the first discharge process the second charge process starts. Fig. 3 (c) shows the conductivity transient over 30 charge-discharge cycles. Obviously, the repeatability of electrosorption process can be realized in this unit cell. To get a better understanding of the electrosorption behavior of PCPs, the electrosorption experiments using PCP1200 electrode in NaCl solution with different initial concentrations at 1.2 V were carried out and the corresponding currents were monitored simultaneously, as shown in Fig. 3 (b). Charge efficiency (Λ) is a functional tool to gain insight into the double layer formed at the interface between the electrode and solution,15, 27, 30 as described according to the following equation: Γ×F Λ= (1) Σ -1 where F is the Faraday constant (96485 C mol ), Γ is the -1 -1 electrosorption capacity (mol g ) and Σ (charge, C g ) is obtained by integrating the corresponding current. The electrosorption capacity and charge efficiency of PCP1200 electrode are listed in Table 2. It can be seen that the electrosorption capacity increases with the
increase in NaCl concentration and reaches a highest value of 13.86 -1 mg g . As seen, the Λ of PCP1200 is far from ideal value, which is 31 mainly caused by the co-ion effect, the weak adhesion between the porous electrode and substrate, and the blocking effect of the 4, 8 binder. Fortunately, an effective method has been proposed to solve this problem by introducing charge barrier membrane into 32-34 CDI. Table 2 Electrosorption capacity and charge efficiency of PCP1200 electrode in NaCl solution with different initial concentrations. -1
Initial concentration / mg l 100 250 500 Electrosorption capacity / mg g-1 7.71 10.10 13.86 Charge efficiency 0.67 0.72 0.72 In order to further evaluate the electrosorption performance of the PCPs, the electrosorption capacity of PCP1200 is compared with those of other carbon electrode materials reported in the 35 literatures and other water desalination technique, as shown in † ESI. Obviously, the electrosorption capacity of our PCP1200 electrode is among the highest values of the existing carbon materials measured in the similar experimental conditions. Unlike most conventional carbon materials, MOFs-derived PCPs enjoy both high specific surface area and low electrical resistance, which are both crucial factors for the electrosorption process. In summary, PCPs were successfully synthesized through thermal treatment of ZIF-8 at 800, 1000, and 1200 °C, and their electrochemical and electrosorption performances were investigated. The experimental results show that (i) the carbonization temperature plays an important role in the electrochemical performance of PCPs by influencing their specific surface area and charge transfer resistance; (ii) PCP1200 exhibits the best electrochemical performance among all the samples, with a specific capacitance of 275.69 F -1 -1 g ; (iii) a high electrosorption capacity of 13.86 mg g is achieved for PCP1200 when the initial NaCl concentration was -1 500 mg l ; (iii) no obvious declination in desalination performance has been observed during charge-discharge experiments, indicating a good regeneration ability of PCPs electrode; (iv) The MOFs derived PCPs should be a promising candidate as electrode material for CDI application. Financial support from National Natural Science Foundation of China (No. 21276087) is gratefully acknowledged.
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