J. Phys. Chem. C 2008, 112, 7385-7389
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Developing Ion Electroadsorption Stereoselectivity, by Pore Size Adjustment with Chemical Vapor Deposition onto Active Carbon Fiber Electrodes. Case of Ca2+/Na+ Separation in Water Capacitive Desalination Eran Avraham, Bouhadana Yaniv, Abraham Soffer,* and Doron Aurbach Department of Chemistry, Bar-Ilan UniVersity, Ramat-Gan, Israel ReceiVed: December 13, 2007
By using carbon chemical vapor deposition (CVD) onto active porous carbon fibers, we were able to reduce the pore size of the active carbon to obtain selective carbon molecular sieves (CMS) of variable properties. The pore size of these carbons was characterized by the adsorption of molecules of different dimensions (molecular probe adsorption) from the gas phase. By manipulating the several experimental parameters of the CVD process, the pore size of the CMS could be finely tuned to fall between sizes of alkaline earth cations (Ca2+; Mg2+, 6-7 Å in diameter), and the much smaller size of hydrated monovalent cations (4 Å in diameter). This was proven by voltammetric measurements of the pore-tailored carbon electrodes in the corresponding salt solutions, which also provided a measure of the differential capacity of the electrical double layer (EDL) vs the potential (e.g., at potentials negative to the potential of the zero charge, the EDL capacity was very small for CaCl2 and MgCl2 solutions, while it was high for NaCl solutions). The CVD process applied herein did not significantly reduce the specific surface area and the pore volume of the carbons. This implies that the carbon deposits are only superficial and that the pore blocking effect takes place only at the pore mouth. Hence, the adsorption processes related to these carbons involve surface barrier mechanisms. The importance of the selectivity thus obtained relates to a possible use of these carbon electrodes in electrochemical water desalination processes. By using these carbons as electrodes in capacitive water deionization processes, it is possible to selectively remove Na+ cations, leaving Ca2+ and Mg2+ ions (which are important nutrients) in the treated water.
Introduction Carbons are unique solids that can be activated, thus containing fine pores whose size can be adjusted to be in the nanometer range, in the order of molecular dimensions. The pores in such activated carbons may occupy a large portion (3080%) of their total volume. Since the internal surface area is on the order of hundreds of m2/g, the adsorption capacity is substantialsabove 20 mol/kg,1 thus enabling the use of activated carbons in separation and purification processes in gaseous2 and liquid3 phases. The main method of developing extensive porosity in carbons is actiVation, in which partial burnoff of the carbon is carried out using various oxidizers. Carbon dioxide and steam, at 800 to 1000 °C, are examples of such oxidative activating agents. An important consideration related to activated carbons is the difference between their average pore size and the size of the species whose absorption is required: pores that are smaller than critical molecule or ion size cannot adsorb these species. Thus, a mixture of species in fluid phases can be separated by pore-designed carbon molecular sieves (CMS).4 The electrical conductivity in most types of carbon (even when they are highly porous) enables their use in a wide variety of electrochemical applications. The electrochemistry of electrolyte solutions may be divided into two categories. (a) Electrostatic interactions, related to the electrical double layer (EDL), formed by the electronic charge at the electrode side of the interface, and an ionic layer at the solution side, separated by a dielectric layer of solvent molecules.5 Such
interactions enable the design of selective electroadsorption processes and the construction of EDL capacitors as fast and highly reversible devices for energy storage and conversion. When considering the large surface area of activated carbon electrodes, their electrical double layer can serve as a quantitative means for ion separation processes.6-8 (b) Electrochemical red-ox reactions, where an electronic charge crosses the interface, leading to chemical oxidation or the reduction of species in the solution side. Electrons crossing the interface in red-ox reactions are, in fact, leaks in the EDL capacitor. Fortunately, there are almost always experimental conditions at which the electrochemical red-ox leakage current is sufficiently small, so that electrochemical systems function as stable EDL capacitors at wide potential ranges, thus allowing electroadsorption/desorption processes to be carried out. Such conditions prevail whenever the concentration of the reactants in the possible redox process is negligible, or when there is a high activation energy for charge transfer across the interface (hence, the electrodes are ideally polarized). Most of the water desalination methods are nonselective in the sense that they remove all the ions present in the feedwater. However, the alkaline earth ions, Ca2+ and Mg2+, are important nutrients in drinking water, and therefore have to be added to drinking water produced by standard desalination processes, e.g., reverse osmosis. The present method involves ion electroadsorption and desorption at microporous (7 Å.11 By tailoring the pore size of the carbon electrodes so as to fall between the dimensions of these ions, a molecular sieve carbon electrode is obtained, selective to the electroadsorption of monovalent ions.10 Tailoring the pore size of activated carbons can be carried out using either one or both of the following general methods: (1) A very careful activation process in which the partial burning off of the carbon is done in a controlled manner in which the oxidation periods of time and/or the temperatures are well-monitored, so that the pore openings can be finely controlled. (2) The chemical vapor deposition (CVD) of carbon onto the surface of highly activated carbon. By this method, organic molecules are pyrolyzed to smaller, chemically active fragments, some of which polymerize into thin layers of carbon, covering the mouths of the original pores.12-16 If this process takes place at optimal conditions, the original in-depth pore system is likely to be preserved, and hence, the adsorption capacity also.17 However, the pore openings or the pore dimensions at their outer side can be reduced by the thin, porous layer of the deposited carbon to create the desired electroadsorption stereoselectivity. In other words, by controlling the CVD parameters, the new pore size created by the deposited carbon layer can be adjusted to be between the size of the small species in the mixture that have to be removed and the size of the larger species that should remain in the fluid mixture. The method of the gradual pore opening of the carbon electrodes throughout the depth of their pores, employed for adjusting the pore size of active carbon electrodes to the range of Na+/Ca2+ selectivity,10 has already been nicely demonstrated. This method exhibits relatively slow adsorption kinetics, since the pore dimension of the entire pore system is reduced by this “bottom-up” approach. On the other hand, the CVD method of pore-opening adjustment (which reflects a “top-down” approach) is expected to lead to the faster insertion of adsorbed species of the same critical size, since the outer layer prepared by CVD, which controls the adsorption rate, is very thin, while the pores are very large. The CVD method for creating molecular sieve carbons is not new. It is commercially used in pressure-swing adsorption for oxygen/nitrogen separation from air.1,4,18 It is also a critical step in the pore adjustment of carbon molecular sieve membranes for other as separation processes.19 The innovation of this work is its application to the rejection of alkaline-earth ions in water desalination processes based on electroadsorption. In this paper, we report the use of CVD for adjusting the pore size of the negative electrode in water desalination cells toward the rejection of the alkali-earth metal cations in capacitive desalination. Further to the examination of the selectivity in cation electroadsorption, clear evidence is provided on the molecular sieving phenomenon of the CVDtreated carbons, based on the well-established and accurate criteria of gas molecular probe adsorption selectivity. The pore system of the carbon cloth electrodes studied in this work was solely microporous, without any contribution from mesoporosity. However, macroporosity is present as spaces between the cloth fibers and as gaps in the weaves of cloth. The contribution of these pores to the total surface area available for adsorption is negligible. However, their role as fast access channels of the adsorbates to the micropore systems is important. Experimental Section Equipment. Gas-Phase Operations. These were performed by means of a glass-constructed vacuum system composed of
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Figure 1. A scheme of the branch of the vacuum system used for the CVD process and related measurements.
an outfit of high vacuum pumps, main vacuum line with vacuum gauges, gas manifold for the supply of different gases and vapors, and a branch illustrated in Figure 1, for performing adsorption kinetics and CVD. Both processes are operated at stationary conditions, namely, the CVD gas or vapor was admitted as a single dose into a volumetric system. While studying adsorption kinetics in a volumetric system is common, CVD is only rarely applied in a constant volume system. However, we preferred this system for the CVD as well, since the process can be readily monitored by measuring the pressure change in time. The known standard volume, Vs (Figure 1), is used once for calibrating the tubing volume, Vm, enclosed among valves 1, 2, and 3. This is used in turn for monitoring the CVD process and the adsorption kinetics by recording the pressure change in time. The branch of the system shown in Figure 1 also served for removing dissolved air from the organic volatile liquids. In this work, these were toluene, the vapor of which was used as the CVD precursor, and methyl tert-butyl ether (MTBE), which served as a molecular probe adsorbate of a large molecular size. These liquids were brought into the volume, Vs, by removing the plug of stopcock #3 and introducing the liquid with a syringe. Dissolved air was then removed from these liquids by a series of freeze-evacuate-thaw cycles. CVD or adsorption kinetics experiments were carried out by enclosing the corresponding gas (or vapor) into Vm, recording its pressure, and then connecting Vm to the adsorption cell via valve #2. From here on, the pressure was continuously monitored by the pressure transducer, digitized, and processed as required. The amount of gas adsorbed by the carbon, Mc, at any moment is given by
Mc ) [PmVm - Pt(Vm + Vc)]/RT
(1)
where Vc is the cell volume and Pt is the pressure at time, t. A typical CVD treatment employed pure toluene (without a carrier gas) at a pressure of about 1.2 kPa, which was introduced into the volume Vm, then admitted to the cell, which was preevacuated and preheated to 1050 °C. The course of the CVD process, as monitored by the pressure transducer attached to volume Vm (Figure 1), is demonstrated in Figure 2. As mentioned above, such results cannot be obtained from the CVD processes carried out at a continuous-flow regime. The moment of initiation of the CVD is shown as a sudden pressure drop due to the expansion of the CVD vapor from Vm to Vc. This is followed by a gradual pressure increase due to the cracking of toluene into smaller molecules. The increase in pressure slows down gradually, approaching the end of the process, but does not stop. Therefore, the CVD process was interrupted after 25 minutes by degassing the system and filling with nitrogen. The adsorption of nitrogen at liquid nitrogen temperature was carried out to estimate the BET surface area and the DFT pore
Developing Ion Electroadsorption Stereoselectivity
Figure 2. Response of the CVD process using toluene as the precursor. The pressure changes due to the fragmentation of toluene to smaller gaseous molecules during the CVD process.
Figure 3. Adsorption kinetics of N2 at 77 K and MTBE at RT into pristine activated carbon samples.
size distribution. For this purpose, we used a Gemini 2375 surface analyzer (Micrometeritics, Inc.) using a nitrogen adsorbate at 77 K. Electrochemical Measurements. A simple three-electrode cell was employed using a PGSTAT Autolab electrochemical measuring system from Ecco Chemie, Inc. (The Netherlands). Cyclic voltammograms for assessing the EDL capacitance of the various systems at various electrochemical potentials were measured at 1 mV/s. Materials. The carbon starting material was an activated carbon cloth, a product [Acc-507-15] of Nippon Kynol, Japan. Its measured BET surface area was 1440 m2/g. Analytical-grade calcium and sodium chlorides and toluene (>99%) were purchased from Frutarom, Israel. Magnesium chloride (>99%) and MTBE were obtained from Sigma-Aldrich. Nitrogen and helium, both 99.999%, were from the Oxygen and Argon Center, Rehovoth, Israel. Results and Discussion The changes in the properties of the carbon electrodes upon applying CVD modifications were compared with the properties of the untreated active carbon. At first, the molecular probe adsorption kinetics, which shows the onset of the molecular sieving property, is presented. This is followed by the presentation of the voltammetric behavior of solutions containing monovalent and bivalent ions with the carbon electrodes, before and after the application of the CVD process. Finally, a comparison between the nitrogen adsorption isotherms and the DFT pore-size distribution of the various carbon electrodes is presented. Figure 3 shows the adsorption kinetics, namely, the amount of nitrogen at a liquid nitrogen temperature and of MTBE at room temperature, adsorbed vs time into the as-received Kynol [Acc-507-15] active carbon cloth, calculated by eq 1. The initial
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Figure 4. Steady-state cyclic voltammogram (current density translated to specific capacity) of a pristine carbon electrode in a 0.1 M CaCl2 solution at 1 mV/s. The range of the potential of zero charge (PZC) is marked (around the minimum in the current of the CV).
pressures of the gas and the vapor in the preparation volume, Vm, (see Figure 1) were 19.77 and 5.6 kPa for the two adsorbents, respectively, in the experiment related to Figure 3 (which is a typical one). The final pressures in this experiment were 5.35 and 1.8 kPa, respectively. This implies that a sufficient excess of gas remained in the system at the end of the adsorption process. Thus, the termination of the adsorption, as expressed by the plateau in Figure 3, is not the result of a lack of gas in the system, but rather due to the achievement of equilibrium. We may therefore consider the initial adsorption rate, measured when the adsorption process is far from equilibrium, as a qualitative measure of the rate of adsorption. As seen from Figure 3, the initial adsorption rate for both nitrogen and MTBE is high, and hence indicates that both molecules are free to enter the carbon pore system. The lower plateau of the adsorption vs time observed for MTBE, compared to N2, is not due to molecular sieving, but rather to the micropore volume filling at lower adsorption values for MTBE because of its greater molar volume. This effect is typical of microporous systems (the Gurvitsch rule20). In a previous work,11 we established a series of dimensions of several probe molecules and ions, from which it appeared that the size of the monovalent hydrated (aqueous) alkali metal and chloride ions falls between N2, 3.6 Å, and CF4, 4.2 Å. Similarly, the size of the aqueous alkali earth metal ions is larger than the size of MTBE, which is 5.8 Å. Thus, the fact that MTBE is quickly adsorbed into the porous carbon studied herein indicates that the pore size of the Kynol [Acc-507-15] carbon is greater than the size of this molecule. Thus, we may expect that this carbon will have pores wide enough to electroadsorb Ca2+ and Mg2+ ions. This is verified in Figure 4, presenting a steady-state cyclic voltammetry of a pristine carbon electrode in a CaCl2 solution, which shows that the as-received Kynol [Acc-507-15] carbon adsorbs Cl- and Ca2+ ions equally well. The potential of zero charge (PZC) of these systems (whose size is between the potential domains into which cations and anions are adsorbed) is indicated by the minimum observed in the current of the CV curves, as has been discussed previously.21 After the CVD treatment, the measurements of the adsorption rate of the N2 and MTBE molecular probes were repeated, as shown in Figure 5. It is clear that MTBE cannot be adsorbed anymore into the carbon’s pore system after the CVD treatment, while nitrogen adsorption remained fast and reached equilibrium values of ∼12 mmol/g, only about 20% less than the equilibrium value for pristine, nontreated carbon (Figure 3). A typical, steady-state cyclic voltammetry of the CVD-treated carbon electrodes is shown in Figure 6, together with a typical CV of a pristine carbon electrode, for comparison, in CaCl2 solutions. Judging from the significant drop in the EDL capacity of the
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Figure 5. Adsorption kinetics of N2 at 77 K and MTBE at RT into an activated carbon sample after a CVD process.
Figure 6. Comparison of steady-state cyclic voltammograms (current densities translated to specific capacity) measured with porous carbon electrodes before and after CVD treatment, in 0.1 M CaCl2 solution at 1 mV/s.
Figure 7. A typical steady-state cyclic voltammogram (current density translated to specific capacity) of a porous, CVD-treated electrode in a 0.1 M MgCl2 solution at 1 mV/s.
CVD-treated carbon electrode at the negative potentials, it is clear that the adsorption of the large cations to the carbon after the CVD treatment is no longer possible, the monovalent Clanion can be easily adsorbed into the treated carbon. This proves the electroadsorption stereoselectivity of the mono vs the bivalent ions. A molecular sieving effect similar to that seen for Ca2+ ions is shown in Figure 7 for Mg2+ with the CVDtreated carbon (reflected well by the asymmetric CV presented in Figure 7). Figure 8 shows a typical steady-state cyclic voltammogram of a CVD-treated carbon electrode in a NaCl solution. It shows that Na+ ions can be electroadsorbed into the CVD-treated carbon. This shows again that the stereoselectivity point of the CVD-treated carbon can be set between the sizes of the Na+ and Ca2+ ions. In Figure 9, the pore distribution functions of the carbon before and after CVD are plotted according to the DFT model.
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Figure 8. Steady-state cyclic voltammogram (current density translated to specific capacity) measured with a CVD-treated porous carbon electrode in a 0.1 M NaCl solution at 1 mV/s.
Figure 9. Plots of specific pore volume vs pore width according to the DFT model (see text) for the pristine and CVD-treated carbon samples, as indicated.
Figure 10. Comparison between adsorption isotherms of N2 at 77 K, related to carbon samples before and after CVD treatment, as indicated.
The two curves are nearly the same, except for the few percent lower values over the entire pore size range obtained for the CVD-treated carbon. This indicates again that the interior of the carbon fibers is not affected by chemical vapor deposition. When comparing the nitrogen adsorption isotherms at 77 K for the carbons before and after CVD treatment (Figure 10), only an 8% lower adsorption of gas is apparent after CVD treatment. This is further evidence of the limited influence of CVD treatment on the depth of the pores of these carbon fibers. Interestingly, the N2 adsorption isotherm related to the CVDtreated carbon exhibits a hysteresis loop throughout the pressure range, as shown in Figure 11. This is attributed to the clogging of a minority of the pore openings at the fibers’ surfaces, leading to sites at which the adsorption/desorbtion processes are slow.22 The hysteresis loop is absent, as expected in the isotherm related to the pristine carbon, which possesses the open pore system, as seen in Figure 12. So far, we have observed three features that indicate the existence of surface barriers at the pore openings in the carbon
Developing Ion Electroadsorption Stereoselectivity
J. Phys. Chem. C, Vol. 112, No. 19, 2008 7389 other possibility is the formation of a thin layer on top of the fiber surface, as suggested in Figure 13b. Both possibilities are in line with the results thus presented. Conclusion
Figure 11. Demonstration of the hysteresis in the adsorptiondesorption isotherms of N2 at 77 K, related to the CVD-treated carbon samples.
Figure 12. Demonstration of the lack of hysteresis in the adsorptiondesorption isotherms of N2 at 77 K, related to pristine, porous carbon samples.
Figure 13. Illustration of pore opening and contracting by the carbon CVD process, in which (a) some deposited carbon penetrates the pores or (b) the CVD carbon accumulates only at the outer carbon surface.
fibers, resulting from the carbon CVD process. (i) There was only about an 8% drop in the nitrogen adsorption capacity after the application of the CVD process, implying that a major part of the pore volume is still available. (ii) The DFT plots of the carbon before and after CVD are very similar in shape. (iii) The cyclic voltammetric currents related to calcium and magnesium electroadsorption into the porous carbon electrode cations are remarkably low (at potentials negative to the PZC) after the application of the CVD process. We may thus conclude that the carbon layer deposited by the CVD process does not penetrate the pores to any significant depth. Thus, there may be two principal configurations for the CVD layer on top of the fiber faces. One possibility is the shallow penetration of carbon deposits into the pores, as shown in Figure 13a. The
A CVD process of carbon, using a fixed initial amount of the precursor, toluene, was applied on top of the outer surface of highly activated fibrous carbon. By the fine control of the process parameters, a stereoselective carbon was obtained that could electroadsorb only monovalent ions, but not bivalent ions. Such selectivity can be very important in water desalination processes based on ion electroadsorption into active carbon electrodes. This study demonstrates that, by the use of such CVD-treated porous carbon electrodes, it is possible to separate alkaline ions such as Na+, which can be removed by electroadsorption, and alkaline earth cations such as Mg2+ and Ca2+, which remain in the treated water. The latter cations are important nutrients whose presence in fresh water is very desirable. To the best of our knowledge, such selectivity in water desalination processes can only be achieved when electroadsorption-based methods are applied. Hence, the ability to prepare selective, highly porous carbon electrodes with high electroadsorption capacity by a relatively simple process is one step further in the development of the method of water desalination by electroadsorption, and should stimulate the development of water desalination processes based on electroadsorption. References and Notes (1) Endo, M.; Kim, Y. J.; Maeda, T.; Koshiba, K.; Katayam, K.; Dresselhaus, M. S. Morphological effect on the electrochemical behavior of electric double-layer capacitors. J. Mater. Res. 2001, 16 (12). (2) Ju¨ntgen, H. Carbon 1977, 15, 273. (3) Namasivayam, C.; Kavita, D. Dyes Pigm. 2002, 54, 47-58. (4) Juntgen, H.; Knoblauch, K.; Munzner, H. Chem. Ing. Tech. 1973, 45, 533-7. (5) Grahame, D. C. Chem. ReV. 1947, 41, 441. (6) Farmer, J. C.; Fix, D. V.; Mack, G. V.; Pekala, R. W.; Poco, J. F. J. Appl. Electrochem. 1996, 26, 1007-1028. (7) Farmer, J. C.; Fix, D. V.; Mack, G. V.; Pekala, R. W.; Poco, J. F. Capacitive deionization of NaCl and NaNO3 solutions with carbon aerogel electrodes. J. Electrochem. Soc. 1996, 143, 159-169. (8) Oren, Y.; Soffer, A. Water Desalting by Means of Electrochemical Parametric Pumping; II. Separation Properties of a Multistage Column. J. Appl. Electrochem. 1983, 13, 489. (9) Pure Appl. Chem. 1985, 57, 603-619. (10) Salitra, G.; Soffer, A.; Eliad, L.; Cohen, Y.; Aurbach, D. Carbon Electrodes for Double layer Capacitors, I. Relations Between Ion and Pore Dimensions. J. Electrochem. Soc. 2000, 147 (7), 2486. (11) Eliad, L.; Salitra, G.; Soffer, A.; Aurbach, D. Ion Sieving Effects in the Electrical Double Layer of Porous Carbon Electrodes: Estimating Effective Ion Size in Electrolytic solutions. (12) Braymer, T. A.; Coe, C. G.; Farris, T. S.; Gaffney, T. R.; Schork, J. M.; Armor, J. N. Carbon 1994, 32, 445. (13) Cabrera, A. L.; Zenher, J. E.; Coe, C. G.; Gaffney, T. R.; Farris, T. S.; Armor, J. N. Carbon 1993, 31, 969. (14) Kawabuchi, Y.; Oka, H.; Kawano, S.; Mochida, I.; Yoshizawa, N. Carbon 1998, 36, 377. (15) Kawabuchi, Y.; Kishino, M.; Kawano, S.; Whitehurst, D. D.; Mochida, I. Langmuir 1996, 12, 4281. (16) Kawabuchi, Y.; Chiaki, S.; Kishino, M.; Kawano, S.; Whitehurst, D. D.; Mochida, I. Langmuir 1997, 13, 2314. (17) Freitas, M. M. A.; Figueiredo, J. L. Fuel 2001, 80, 1. (18) Kapoor, A.; Yang, R. T. Chem. Eng. Sci. 1989, 44, 1723. (19) Koresh, J. E.; Soffer, A. Molecular Sieve Carbon Permselective Membrane. Part I. Presentation of a New Device for Gas Mixture Separation. (20) Gurvitsch, L. J. Phys. Chem. Soc. Russ. 1915, 47, 805. (21) Kastening, B.; Hahn, M.; Rabanus, B.; Heins, M.; zum Felde, U. Electronic properties and double layer of activated carbon. Electrochim. Acta 1997, 42, 2789. (22) Villar-Rodil, S.; Navarrete, R.; Denyel, R.; Albiniak, A.; Paredes, J .I.; Mart’inez-Alonso, A.; Tascon, J. M. D. Microporous Mesoporous Mater. 2005, 77, 109-118.
