J. Phys. Chem. C 2009, 113, 21319–21327
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The Rate-Determining Step of Electroadsorption Processes into Nanoporous Carbon Electrodes Related to Water Desalination M. Noked, E. Avraham, A. Soffer, and D. Aurbach* Department of Chemistry, Bar Ilan UniVersity, Ramat Gan 52900 Israel ReceiVed: June 26, 2009; ReVised Manuscript ReceiVed: October 1, 2009
Using potential perturbation techniques, we investigated the kinetics of electroadsorption processes of ions with different dimensions and different concentrations into activated carbon electrodes with various porous structures. We found that the ratio between the pore size and the ion size is of great significance in terms of electroadsorption kinetics. Microporous carbon fiber electrodes exhibited a lower charging rate as compared with mesoporous aerogel carbon. The use of activated aerogel electrodes with a fractal structure composed of both micro- and mesopores, was found to be the best for effective electroadsorption due to both high electrical double layer capacity obtained and high rates of electroadsorption. We also explored the kinetics of electroadsorption processes into molecular sieve carbon electrodes, demonstrating that, despite the hindered adsorption processes into these electrodes, it is possible to obtain effective deionization by using them for selective water desalination processes. Introduction Carbons are unique solids that can be partially burned off by oxidation in well-controlled procedures known as activation processes. These processes result in the formation of highly porous material. The size of the pores in activated carbons typically falls within the micropore range (2 nm) inside the pore system: AG > AAG > ACF > CMS.
Rate-Determining Step of Electroadsorption Processes
Figure 3. Specific cumulative pore volume of the four types of carbon studied, as a function of the pore width, calculated by the DFT model from N2 adsorption measurements data. These plots indicate the fraction of nanopores in the total pore volume and the extent to which the carbon porosity has an opened structure. From this figure, we can set the following order in terms of opened pore structure: AG > AAG > ACF > CMS.
Figure 4. Comparison among the typical steady-state cyclic voltammograms (current density translated to specific capacity) of the four carbon electrodes in 0.125 M NaCl at 1 mV/s.
Table 1 summarizes the pores’ net properties of those electrodes obtained by the adsorption isotherms of N2. Figure 4 shows the comparison of the steady-state cyclic voltammogrames (CVs) of the four carbon electrodes. The CVs in Figure 4 show the expected butterfly rectangular shape of their fully capacitive response, from which the specific electroadsorption capacity can be calculated. The order of the specific capacity of these carbons is: ACF > AAG ) CMS > AG. A comparison of the gas adsorption data in Table 1 and Figure 3 with the EDL capacity data presented in Figure 4 shows that there is no clear correlation between the total specific pore volume of the carbons and their specific electroadsorption capacity: the specific pore volume of the AAG carbon is 2-fold higher than the volume of the CMS carbon, although their specific capacity is similar. This mismatching is due to the use of N2 as the adsorbate in the adsorption isotherm, which makes pores with thinner entrance inaccessible to the gas molecule but still accessible to ions in the electrolyte; this is also correct for the BET calculation, according to the N2 adsorption data. Indeed, adsorption measurements of CO2 on the CMS electrode showed a larger surface area (DR > 1100 m2/g) and micropore volume (DR ) 0.58 cc/g), as presented in previous work.20 Until now, the measurements were all steady-state measurements that were made in order to define the adsorption properties and the porosity of the different electrodes. However, the kinetics description of those processes is not of less importance as the magnitude of the capacity of the carbon material. Figure 5 shows normalized chronocoulometric curves obtained with the four types of electrodes by their charging in 0.125 M NaCl solutions. The processes measured involved anion (Cl-) adsorption (anodic polarization). The relevant time
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Figure 5. Normalized chronocoulometric curves obtained by anodic polarization of the four types of carbon electrodes in 0.125 M NaCl solutions to 250 mV (vs Ag/AgCl/Cl- reference electrode), which exhibit different pore morphologies, as presented in Table 1 and Figure 1. The relevant time constants, calculated according to eq 3, are marked near each plot.
constants (calculated with eq 3 above) are marked near the plot, and it is clearly seen that the transient’s relaxation time constant decrease as the pore’s dimensions are larger. Hence, decreasing the micropore size has both positive (higher capacity) and negative (slower electroadsorption) effects on the EDL properties of these electrodes. We note that the higher electroadsorption time constants in the microporous carbons (as compared with the mesoporous ones) are not only due to their higher EDL capacitance (Figure 4) but also due to higher ionic resistance owing to the smaller pores. In fact, the capacity of the CMS and AAG electrodes is almost the same, but the RC is completely different. In addition to the four morphologically different carbon electrodes described above, another set of carbon CVD-treated (benzene precursor), ACF electrodes was prepared and examined. The carbon CVD treatment of the electrodes affects more the entries to the pores and less their bulk volume.19,20 The tunable parameter was the CVD process duration. Figure 6a shows impedance spectra (presented as Nyquist plots) measured from CVD-treated ACF electrodes (different from each other in the duration of the CVD treatment), in NaCl solutions. The details of these measurements are described in the Experimental Section. Note that we discuss these measurements at the qualitative level. These spectra show a clear trend of impedance increase, from a fully opened ACF carbon electrode to intensively treated electrodes. We attribute the straight inclined “Z” versus ‘Z’ response at the high-frequency domain to the solution resistance within the macropores. At low frequencies, the response becomes very steep, approaching a typical capacitive behavior. The semicircular shape of the plot at the moderate frequency domain may be due to the polarization resistance of the electrode. This is caused mainly by the ionic resistance in the micropores due to the narrow (by the CVD treatment) entrance of ions into the pore system. The size of this semicircle increases indeed monotonically as the CVD treatment of the ACF carbon was longer (i.e., producing a more serious barrier for ions’ entry to the pores). Figure 6b shows normalized chronocoulometric curves upon charging these electrodes in NaCl solutions, and the inserted numbers marked therein near each plot indicate the relevant time constant calculated from the logarithmic plots (eq 3 above). From the plots in Figure 6b, it is evident that, the longer the duration of the CVD treatment, the more the electroadsorption into the carbon is hindered, as clearly seen by the longer time constants obtained.
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Figure 6. Response of CMS electrodes that comprised ACF carbons that were treated by carbon CVD processes. The effect of the CVD process duration is demonstrated: (a) impedance spectra, presented as Nyquist plots, measured with symmetrical cells comprising two identical, CVD-treated ACF electrodes (the samples are different from each other in the duration of the CVD treatment) in 0.125 M NaCl solutions around the equilibrium potential of 0 V. Note the monotonic increase of the radii of the moderate frequencies semicircle as the CVD treatment of the ACF carbon is longer. (b) Normalized chronocoulometric curves measured with the carbon CVD-treated ACF electrodes due to anodic polarization to 250 mV (vs Ag/AgCl/Cl- reference electrode) in 0.125 M NaCl solutions. The relevant time constants are marked near each plot. As the CVD process duration was longer, the time constant obtained is higher.
Critical factors are the ratios between the ion radii and critical pore dimensions that are the average pores’ size and the width of their entries (Ri/Rp and Ri/Re, respectively). Those ratios determine the ionic resistance inside the pore system and, consequently, should determine also the electroadsorption time constant. Ions’ Dimension Effect. In the section describing the effect of the pores’ structure, the effect of the Ri/Rp and Ri/Re ratios on the electroadsorption kinetics was addressed by focusing on the pores’ morphology (for example, size and opening). Highly important is also the effect of the ions’ size. The kinetics of electroadsorption into the ACF, AG, AAG, and CMS carbon electrodes was measured in solutions containing MgSO4, LiClO4, and NaCl. The actual dimension of the ions that affects their electroadsorption is determined by their hydration shell. We focused on the anion adsorption processes (in response to anodic polarization), and hence, the choice of different cations had no influence. MgSO4 was chosen in order to remain with a 1:1 electrolyte also while measuring the adsorption of bivalent anions. We emphasize that the concentrations of the different solutions used for the chronocoulometric measurements were selected so as to have the same ionic conductivity. Therefore, there were no resistance differences in the bulk that could affect the kinetics measurement. Thus, the measurements reflect only to the ionic resistivities of the porous electrodes and to the response of the EDL kinetics to ions of different sizes, which consequently have specific interactions with the pores’ walls. Figure 7 shows normalized chronocoulometric curves related to adsorption of the three anions into ACF, AG, and AAG electrodes. The relevant time constants (calculated by eq 3 from the wide enough range of the exponential decay of the current) are marked in Table 2. The slowest electroadsorption was
Figure 7. Normalized chronocoulometric curves related to electroadsorption of the three anions Cl-, ClO4-, and SO42- into (a) ACF, (b) AG, and (c) AAG electrodes. The order of the ions’ effective dimensions Cl- < ClO4-< SO4- is reflected well by the electroadsorption rates to the ACF electrodes and more moderately by electroadsorption to the AG and AAG electrodes. Polarization to 250 mV vs SCE in salt solutions with the same ionic conductivity in the bulk. The relevant time constants (calculated by eq 3 from the wide enough range of the exponential decay of the current) are summarized in Table 2.
TABLE 2: Time Constants of the Different Anions’ Electroadsorption onto the Four Types of Carbon Electrodes (Calculated by eq 3 from the Wide Enough Range of the Exponential Decay of the Current)a ACF AG AAG CMS a
Cl-
ClO4-
SO42-
15 s 4.02 s 5.9 s 25.3 s
18.6 s 4.38 s 7.2 s 33.5 s
36.6 s 4.5 s 7.5 s
See Figure 8 for chronocoulometric plots.
measured with the CMS electrodes (the time constant related to Cl- and ClO4- adsorption were 25.3 and 33.5 s, respectively; SO42- was not adsorbed due to the sieving effect). It is evident that the kinetics of the EDL charging is significantly slowed down with the microporous ACF. This charging is even slower with the CMS electrodes when the Ri/Rp and Ri/Re ratios became higher, compared with the electrodes comprising the aerogel carbons, with the relatively opened structure. When the AG carbon electrodes were measured, their electroadsorption kinetics was only moderately affected by the ion dimension, as demonstrated in Figure 7b. Unexpectedly, as seen in Figure 7c, the kinetic behavior of the AAG electrode, although possessing a microporous structure, was more similar to the behavior of the pristine AG than to the
Rate-Determining Step of Electroadsorption Processes
Figure 8. Impedance spectra (presented as Nyquist plots), measured with symmetrical cells comprising two identical ACF electrodes in NaCl, LiClO4, and MgSO4 0.125 N solutions, as indicated, around 0 V (OCV). Note the wider medium frequency semicircle as the effective ion’s size is bigger.
behavior of the ACF electrode. This similarity suggests that the fractal configuration of the AAG carbon, which includes mesoporous channels leading to the interior microporous network within this active mass, diminishes the negative effect
J. Phys. Chem. C, Vol. 113, No. 51, 2009 21325 of the microporous structure on the kinetics of the electroadsorption into this carbon. Figure 8 shows impedance spectra measured with ACF electrodes in NaCl, LiClO4, and MgSO4 solutions, as indicated, relate to anions’ adsorption. The medium frequency semicircle that appears in these spectra increases as the effective diameter of the ion increases. This finding can be attributed to an increase in the Ri/Rp and Ri/Re ratios as the ions are larger, leading to higher ionic resistance within the pores. This seems to be a clear demonstration of the relation between the size of the adsorbed ion and the size of the pores within the electrode bulk, which has a crucial influence on the electroadsorption kinetics. Nevertheless, if the carbon has a structure that allows a relatively open approach of the ions to the interior of the microporous system, it may considerably reduce the effect of slow electroadsorption caused by the fine microporous structure, thus leading to overall “fast” EDL kinetics. This seems to be well reflected by the behavior of the AAG electrodes (Figures 5 and 7). The activation process forms a high surface area (1200 m2/g) due to micropores that are probably branched from the pristine mesopores. This fractal structure is highly desirable to achieve fast electroadsorption kinetics, as the mesoporous systems allow a fast approach of the ions to the micropores. Therefore, the EDL response is the main contributor to the overall specific capacity of these carbons. Electrolyte Concentration Effect. The concentration of electrolyte solutions in CDI reactors depends on several factors:
Figure 9. Normalized chronocoulometric curves related to anodic polarization of three carbon electrodes to 250 mV vs Ag/AgCl/Cl- reference electrode, in different solution concentrations, as indicated: (a) ACF electrode, (b) AG electrode, and (c) AAG electrode. The time constants (calculated by eq 3) for these charging processes are listed in Table 3.
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TABLE 3: Time Constants of the Chloride Anions’ Electroadsorption onto the Four Types of Carbon Electrodes in Different Concentrations of NaCl (Calculated by eq 3 from the Wide Enough Range of the Exponential Decay of the Current)a 1M 0.5 M 0.25 M 0.125 M 0.0625 M 0.03125 M a
ACF
AG
AAG
4.4 s 5.2 s 9s 16.5 s 22 s 30.5 s
1.5 s 1.8 s 3.5 s 4.3 s 8s 8s
1.65 s 2.2 s 3.6 s 5.9 s 11.08 s 11.12 s
See the chronocoulometric plots in Figure 9.
the concentration of the feed solution that flows into the reactor, the solution flow rate, the starting and end points of the applied potential steps, the product, and the waste concentrations in the course of the adsorption-desorption cycles. The feed concentration may vary from area to area, and even from day to day, according to the varying needs of production rate and purity. Hence, at any moment, a desalination reactor must cover a wide range of changing solution concentrations. To assess the influence of the solution concentration on the electroadsorption properties of the different carbon electrodes, the ACF, AG, and AAG electrodes were examined by measurements similar to those described in the previous sections in NaCl solutions of different concentrations. Figure 9a-c demonstrates the effect of solution concentration on the EDL charging rates of the different types of electrodes. The figure show the relevant normalized chronocoulometric curves related to chloride adsorption. The main time constants (eq 3) for these charging processes are marked in Table 3. These measurements show that the electroadsorption kinetics of all the electrodes is impeded as the solution concentration decreases (well expressed by longer time constants in Figure 9). This effect is expected because the higher ionic resistance is due to decreased concentration, thereby leading to higher RC constants. However, the negative effect of the low ionic concentration is more significant with the microporous ACF electrodes. We attribute this to an additional increase in the resistance for ionic transport within the micropores as the effective size of the ion increases because the size of hydrated ions is bigger as the ion concentration in solution is lower.35,36 Figure 10 shows impedance spectra (presented as Nyquist plots) measured with ACF electrodes in NaCl solutions of different salt concentrations and that relate to anion (Cl-) adsorption. As expected, Figure 10, as clearly demonstrated by the spectra, shows that, as the concentration of the electrolyte decreases, the bulk solution resistance increases. Therefore, ‘Z’ measured at the highest frequency and the inclined line in the high-frequency domain, which represents the solution resistance, increases as the solution concentration decreases. As seen in Figure 10, decreasing the salt concentration increases the diameter of the medium frequency semicircle in the Nyquist plots. This indicates the increase of the ionic resistance in the pores as the solution concentration decreased. Conclusions The kinetics of the EDL charging of porous carbon electrodes was examined and analyzed as a function of the electrode’s pores’ structure, the ion dimensions, and the ion concentration. The Ri/Rp and Ri/Re ratios were found to have a crucial effect on the kinetics of electroadsorption of ions into activated carbon
Figure 10. Impedance spectra (presented as Nyquist plots) measured in symmetrical cells comprising two identical ACF electrodes in NaCl solutions of different concentrations, as indicated, measured around 0 V. Both the solution resistance and the medium frequency semicircle are increased as the concentration of the solution is lower.
electrodes. When the ions’ radii approached the pore dimension, the EDL charging process is impeded, and hence, the time constants of the current and charge transients upon the electrodes polarization (that lead to their EDL charging) become longer. From these observations, we suggest that the use of microporous electrodes with large surface area, while being associated with high EDL capacity, is not necessarily the best choice for electroadsorption applications due to their relatively slow electroadsorption kinetics. A better choice may be the use of hierarchical multiporous electrodes with a fractal structure. This would include mesopores that are the entries to micropores, which play the main role in the electroadsorption processes, and serve as the main contributors for the electrodes’ capacity. Such electrodes demonstrate high electroadsorption rates. The AAG electrodes prepared in this work seem to fulfill this design: exhibiting both high capacity and high rates. The morphology of the AAG electrode is indeed fractal, containing mesoporous channels leading to the smaller micropores (Figure 1); this structure enables higher ionic mobility inside the pore system. In addition, we examined the possible use of CMS electrodes for selective electroadsorption. We prepared the carbon of these electrodes by a delicate CVD process that partially blocks the entries to the microporous system of the pristine carbon (ACF). We found that, due to the narrow pore size distribution, with the opening of the pore entrance close to the size of the ion, the electroadsorption is hindered (due to the CVD process that makes the carbon selective). Nevertheless, the time constant of the electroadsorption process for these electrodes was of the same order of magnitude as the magnitude related to the pristine (ACF) material. Thus, we conclude that the use of CMS electrodes for selective desalination, as suggested in previous work, should not be excluded. The concentration of the electrolyte inside the adsorption cell was also found to be a very important factor for the EDL capacity, the solution resistance (Rs), the charging rate, and also for the ionic resistance within the porous system of the electrodes. Supporting Information Available: Adsorption isotherms of N2 into the different carbon electrodes are presented, together with the pore size distribution of the microporous carbons and the relative pressure ranges used for the gas measurements and
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