J. Phys. Chem. C 2010, 114, 18745–18751
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Ion Transport Behavior in Triblock Copolymer-Templated Ordered Mesoporous Carbons with Different Pore Symmetries Gangwei Sun, Jitong Wang, Xiaojun Liu, Donghui Long,* Wenming Qiao, and Licheng Ling State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, China ReceiVed: July 5, 2010; ReVised Manuscript ReceiVed: September 2, 2010
Mesoporous carbons with 3-D body-centered cubic, 2-D hexagonal, and wormlike symmetries were synthesized via organic-organic self-assembly and further activated by CO2 to improve the pore connectivity. These samples have almost the same mesopore size but different pore symmetries and connectivity, which are used as the model materials to investigate the ion transport behavior in mesoporous channels by using galvanostatic charge-discharge, cyclic voltametry, and electrochemical impedance spectroscopy. Results show that 2-D hexagonally mesoporous carbon delivers the best capacitance retention and the lowest impedance to electrode kinetic processes. These indicate that the 2-D hexagonal pore symmetry is more favorable for ion diffusion than the isolated 3-D cubic and disordered wormlike pore characteristics. Compared to the pristine mesoporous carbons, the activated samples exhibit remarkable improvement in ion transport ability within the mesopores. The development of micropores makes the separated pore channels interconnect to each other. Such an improved interconnectivity of mesopores is favorable for the ion diffusion process through providing more entrances and shorter distances for electrolyte accessibility, consequently, improving the efficiency of ion transport. 1. Introduction In recent years, electric double-layer capacitors (EDLCs) have been extensively studied due to their promising properties in terms of energy storage and power supply, which could fill the gap between secondary batteries and conventional dielectric capacitors.1,2 A unit cell of EDLCs works on the principle of double-layer capacitance at the electrode/electrolyte interface where electrons are accumulated on the electrode surfaces and ions of opposite charge are arranged in the electrolyte side.3,4 Up until now, porous carbons were recognized as the most promising electrode materials due to their excellent physicochemical stability, good conductivity, and reasonable price.5-9 However, porous carbon-based EDLCs are known to suffer from electrode kinetic problems that are related to inner-pore ion transport and electrolyte diffusion.10 The large electrochemical impedance makes the high rate capability of EDLCs impossible, which is one of the advantages of EDLCs. The mechanism of ion transport within porous textures is very complex, because the pore properties, the nature of the electrolyte, as well as the solid-liquid interface all have to be considered.11,12 Among these factors, it is believed that the ion transfer process is critically dependent on the pore characteristics (pore size, pore size distribution, pore shape, tortuosity, connectivity, pore symmetry, etc.). As for physically or chemically activated carbons, their diverse and uncontrollable intrinsic porosity properties characterized by pore size ranging from ultramicropores (less than 1 nm in diameter) to macropores with pores randomly connected have made it impossible to precisely investigate the relationship between pore structure and ion diffusion behavior.13 Compared with a wide pore size distribution and tortuous pore channels of activated carbons, the order mesoporous carbons (OMCs) with tailorable mesopore size and regular mesoporous channel * To whom correspondence should be addressed. Tel.: (86) 21 64252934. Fax: (86) 21 64252914. E-mail:
[email protected] (D.L).
are considered to be very promising materials for investigating the impact of pore structure on ion diffusion process.14,15 Some researches have referred to the ion transport behavior in ordered mesopores.16-21 Wang and coauthors investigated the effect of pore packing defects in 2-D OMCs on ion transport,10 and their results suggested the pore packing defect would obstruct electrolyte ion diffusion and ultimately deteriorate the EDLCs performance. Xia’s results showed that OMCs with short pore length delivered much better capacity retention than that of conventional OMCs.22 The superior performance of short pore length OMCs was ascribed to its having more entrances for electrolyte accessibility and a short pathway for rapid ion diffusion. However, there are few systematic and convincible studies on how the rate capability or frequency responses are influenced by different pore arrangements, symmetries, and connectivity. In this work, a series of ordered mesoporous carbons with different pore symmetries were synthesized via organic-organic self-assembly.23 Diversified architectures including 3-D cubic, 2-D hexagonal, and wormlike mesostructures were obtained by simply changing the surfactant concentration. The obtained samples were further activated by CO2 to improve the pore connectivity. The ion transport behaviors in these mesopores with different symmetries and connectivity have been evaluated by using galvanostatic charge-discharge, cyclic voltametry (CV), and electrochemical impedance spectroscopy (EIS). To our knowledge, it is the first time that effects of pore characteristics (such as pore symmetries, pore arrangements, pore connectivity, etc.) on the ion transport are clearly demonstrated. 2. Experimental Methods 2.1. Preparation of Samples. Mesoporous carbons were synthesized via a solvent evaporation-induced self-assembly using resorcinol and furfural as carbon sources and triblock copolymer Pluronic F127 as a pore-directing agent according
10.1021/jp106205n 2010 American Chemical Society Published on Web 10/11/2010
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to our previous report.23 In a typical synthesis, 1.75 g of resorcinol was dissolved in 45 g of ethanol with stirring at room temperature followed by addition of 2.25 g of furfural and 0.5 g of 0.01 mol/L hydrochloride ethanol solution. Then 0.2 g of hexamethylenetetramine and 2 g of F127 was added with stirring for 10 min to obtain a clear dark red solution. The solution was transferred into smooth glass plates to evaporate off ethanol at room temperature for about 6 h, and the resulting films were heated in an oven at 393 K for 24 h. The samples were then carbonized at 1173 K for 3 h under nitrogen flow to decompose the triblock copolymers and to obtain mesoporous carbons. In this work, the mass ratios of F127 to the resorcinol-furfural oligomer (F127/RF) were 0.5, 0.75, and 1.5, and resultant samples were 3-D cubic, 2-D hexagonal, and wormlike mesoporous carbons (denoted as 1F, 2F, and 3F), respectively. The CO2 activation was subsequently employed to tailor the texture of mesoporous carbons. In brief, the sample was placed in the center of a quartz tube in a tube furnace and heated (5 °C/min) to 950 °C under nitrogen, and then the gas was switched to CO2 for 3 h. After the activation, the samples were cooled to room temperature in nitrogen atmosphere. The activated samples thus were referred to as 1F-A, 2F-A, and 3F-A, respectively. 2.2. Characterization of Samples. The X-ray diffraction (XRD) patterns were obtained on a RigakuD/max2550 diffractometer operating at 40 KV and 20 mA using CuKR radiation (λ ) 1.5406 Å). The morphologies of mesoporous carbons were observed using a transmission electron microscope (TEM, JEOL 2100F) operated at 200 KV. N2 adsorption-desorption isotherms were carried out using a Micromeritics ASAP2020 analyzer at 77 K. Before the measurements, the samples were degassed in vacuum at 473 K for at least 6 h. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas. The total pore volume was estimated from the adsorbed amount at a relative pressure of P/P0 ) 0.985. The micropore surface was calculated by the t-plot method. The pore size distribution was derived from the desorption branch using the BarrettJoyner-Halenda (BJH) model. 2.3. Electrochemical Measurements. The mesoporous carbon powders were processed into capacitor electrodes by mixing them with poly(tetrafluoroethlyene) (PTFE, 5 wt %) and acetylene black (10 wt %) homogenized in a mortar and pestle, then rolled into a thin film of uniform thickness, and finally punched into pellets. Each electrode contained active materials of 10 mg and had a geometric surface area of about 1 cm2. Electrochemical experiments were carried out with Teflon Swagelok type two-electrode configuration, which was constructed with two facing carbon electrodes, sandwiched with a separator. H2SO4 (1 M) was employed as electrolyte. The electrochemical performances of samples were characterized by CV and EIS with a GAMRY instrument. CV was performed in the voltage range 0-0.9 V and EIS with the frequency in the range from 1 mHz to 100 kHz, ac amplitude, 5 mV. Galvanostatic charge-discharge tests were conducted to estimate the specific capacitance of samples on an ABIN BT2000 apparatus. The gravimetric capacitance of electrode material was calculated according to the equation C ) 2*I*∆t/ (∆V*m), where I is the discharge current, ∆t, the discharge time from 0.9 to 0 V, ∆V, the working voltage, and m, the mass of carbon on an electrode. The factor of 2 in this equation comes from the fact the overall capacitance measured from the twoelectrode system is the sum of two equivalent single-electrode capacitors in series.
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Figure 1. Small-angle XRD patterns of (left) mesoporous carbons of 1F, 2F, and 3F and (right) their activated samples of 1F-A, 2F-A, and 3F-A.
3. Results and Discussion 3.1. Structural Characterization of Samples. Ordered mesoporous carbons with different pore symmetries were synthesized via evaporation-induced organic-organic selfassembly. Three kinds of mesopore with 3-D cubic, 2-D hexagonal, and wormlike structures were defined by using XRD analysis (Figure 1, left) and TEM observations (Figure 2). The 1F sample (3-D cubic) has an intense diffraction peak at 2θ of 0.96° and two resolved peaks at the 2θ range of 1-2° in XRD pattern, which should be indexed to the [110], [200], and [211] reflections of body-centered cubic structure with Im3m symmetry.24 The characteristic TEM projections along the [100] and [111] directions (Figure 2a, b) manifest an ordered arrangement of spherical mesopores with large domains, providing further evidence for the highly ordered Im3m mesostructure. The 2-D hexagonal sample has well-resolved XRD peaks at the 2θ range of 0.6-2.5°, which can be assigned to 10, 11, and 20 diffractions of the 2-D hexagonal space group (p6m). The typical stripelike and hexagonally arranged TEM images (Figure 2c, d), recorded along the [10] and [01] directions, respectively, confirm that this sample possesses a highly ordered 2-D hexagonal p6m structure. The wormlike sample has only a broad diffraction peak at 2θ around 0.9° in the XRD pattern. TEM images reveal that its mesoporous structure is composed of short wormlike mesoporous channels interconnected in a disordered way, possibly resulting in some closed mesopores. The N2 adsorption-desorption isotherms and BJH pore size distributions of these pristine samples are shown in Figure 3. The detailed pore structure parameters are summarized in Table 1. All the isotherms are type-IV with a sharp capillary condensation step (hysteresis loop) at P/P0 of 0.4-0.6, responding to the ordered mesoporous characteristics. The samples deliver narrow mesopore size distribution with the mean size of 3.4 nm, in good agreement with the TEM observation. Except for the mesoporous characteristics, the pristine mesoporous carbons contain a considerable number of micropores, as indicated in the adsorption platform at low relative pressure. The data in Table 1 show that three types of carbons have very similar microporous and mesoporous structures, even though their pore symmetries are quite different. The micropores should be created due to the emission of small molecules from the carbon precursor and PEO blocks during the carbonization, whereas the mesopores result from the framework of triblock copolymer template.
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J. Phys. Chem. C, Vol. 114, No. 43, 2010 18747 TABLE 1: Structural Characteristic of Mesoporous Carbons and Activated Mesoporous Carbons sample
SBETa (m2/g)
Sextb (m2/g)
Smicc (m2/g)
Vtd (cm3/g)
DBJHe (nm)
1F 2F 3F 1F-A 2F-A 3F-A
637 714 677 825 977 907
306 386 296 257 228 198
331 328 381 568 749 708
0.46 0.49 0.43 0.50 0.57 0.53
3.4 3.4 3.4 3.2 3.3 3.4
a BET specific surface area. b External specific surface area calculated by t-plot method. c Micropore specific surface area calculated by t-plot method. d Total pore volume. e BJH desorption average pore diameter.
Figure 4. N2 adsorption-desorption isotherms (left) and resulting BJH pore size distribution curves (right) of the activated 1F-A, 2F-A, and 3F-A samples. Figure 2. TEM images of mesoporous carbons. (a, b) 1F sample with 3-D cubic structure, recorded from [100] and [111] directions of Im3m, respectively. (c, d) 2F sample with 2-D hexagonal structure, recorded along the [10] and [01] directions of p6m. (e, f) 3F sample with disordered wormlike structure.
Figure 3. N2 adsorption-desorption isotherms (left) and resulting BJH pore size distribution curves (right) of the pristine 1F, 2F, and 3F samples.
CO2 activation causes the deterioration of long-range ordered pore arrangements, as indicated by the XRD patterns in Figure 1 (right). Compared to those of the pristine samples, the intensities of the major diffraction peaks of activated resultants dramatically decrease. Only a weak peak around 1° can be discerned for 1F-A and 2F-A, suggesting that these activated samples still possess a long-range periodic structure. No peaks
can be discerned for the activated wormlike sample 3F-A, indicating a totally disordered structure. The deterioration of ordered periodic structure should be due to the development of micropores within the carbon wall. As shown in N2 adsorptiondesorption isotherms (Figure 4, left), the activated samples possess higher nitrogen adsorbed volume at relative pressure below 0.1 than their mother samples, while their hysteresis loops remain similar. A uniform pore size distribution centered at about 3.3 nm without the existence of large pores is observed in Figure 4 (right). These results suggest that the CO2 activation only drills narrow micropores in the mesopore wall, leaving the integrity of the mesopore. After CO2 activation, the BET and micropore specific surface area increase significantly. The average pore size diameters decrease slightly, due to the presence of a more marked micropores content compared to that of their precursors. 3.2. Electrochemical Performances of Mesoporous Carbons. Galvanostatic charge/discharge between 0.0 and 0.9 V has been employed to estimate the electrochemical performance of samples, as shown in Figure 5. All charge/discharge curves display the regularly triangular shape. The feature indicates the good Coulombic efficiency and ideal capacitor behavior. Among three types of pristine mesoporous carbons, 2F with the highest SBET gives the highest gravimetric capacitance of 150 F/g at a current density of 0.1 A/g, which has a good agreement with the results of previous study.21 The CO2 activation cause notable enhancements in gravimetric capacitances. The capacitance of 184 F/g is obtained for 3F-A. In comparison, 1F-A possesses the relatively lower gravimetric capacitance of 157 F/g due to the relatively lower SBET. The results indicate that the gravimetric capacitances have a strong dependence on the SBET.
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Figure 5. Galvanostatic charge/discharge curves for mesoporous carbons and activated samples at current density of 0.1 A/g.
Figure 6. Retained specific surface capacitance of all samples with different current densities.
TABLE 2: Specific Surface Capacitance of As-Synthesized and Activated Mesoporous Carbons specific surface capacitance at different charge-discharge currentsa (µF/cm2) sample
1.5 mA
6 mA
30 mA
45 mA
90 mA
1F 2F 3F 1F-A 2F-A 3F-A
20.4 20.9 20.8 19.0 19.2 20.3
16.9 18.4 18.0 16.7 17.1 18.0
13.2 15.8 14.1 14.9 15.2 15.7
10.9 14.6 11.9 13.8 14.2 14.8
6.9 13.6 9.8 11.7 12.4 12.2
a
Specific surface capacitance (Cs) is calculated by the equation of Cs ) Cg/SBET, where Cg is the gravimetric capacitance.
To better understand the effect of pore structure on the capacitive behaviors, we exclude the effect of SBET through the normalized specific surface capacitance by dividing the SBET of the samples. The specific surface capacitances with different charge/discharge currents are listed in Table 2. All the samples exhibit nearly identical specific surface capacitances of 20 µF/ cm2 at the current of 1.5 mA, which is very close to the theoretical value of carbon materials.25 This result suggests that mostly surface area can be electrochemically accessed by the solvated ions at low current density. Notably, 2F has higher specific surface capacitance than other samples at all ranges of discharge current. This probably originates from the superior pore structure for ion transfer and electrolyte diffusion processes. Moreover, it is apparent that capacitance performances of astreated samples by CO2 are superior to those of their corresponding precursors, even though they possess incremental micropores content, especially for 3-D cubic and wormlike mesoporous carbons. To evaluate the rate capability of all samples, the ratio of retained specific surface capacitance versus the charge/discharge current density is plotted in Figure 6. The specific surface capacitances decrease with the current density increasing for all six samples investigated. This indicates that there is always less electrochemically active surface area of pores being utilized at higher rate occasions. However, 1F-A maintains 62% of its initial capacitance at high current density of 9 A/g, which is significantly higher than the ratio of 34% for its precursor of 1F. The similar trend is also observed from the samples of 3F-A and 3F. These results highlight a better rate capability of activated mesoporous carbons. Generally, the faster the penetration of electrolyte ions into electrochemically active surface, the better the capacitive behavior at large charge-discharge rate. The more convenient pore architecture for ion transfer and
Figure 7. Comparisons of the cyclic voltammograms at 10 mV/s for (a) 1F, 2F, and 3F, (b) 1F and 1F-A, (c) 2F and 2F-A, and (d) 3F and 3F-A in 1 M H2SO4 electrolyte.
accumulation caused by the CO2 activation probably makes the results above convincing. Figure 7 shows the cyclic voltammograms of the mesoporous carbons in 1 M H2SO4 aqueous electrolyte, which are employed to evaluate the influence of pore structure on ion transport behavior.26-29 As is well-known, for an ideal double-layer capacitor where ion transfer kinetic process is not limited, the responding current keeps constant at a certain sweep rate in the CV measurements, demonstrating a rectangular-shaped current versus potential curve.30 As a consequence, the rectangular degree can be used to reflect the ion diffusion rate within a nanoporous carbon structure. The higher the rectangle degree, the faster the ion diffusion rate. Besides, the capacitive behavior can also be studied by changing the voltage sweep rates.31 At slow sweep rate, the electrolyte ions have enough time to penetrate into the interior surface area. The desired rectangular shape can be observed for all samples. For the purpose of comparisons, we only display the voltammogram curves at the voltage sweep rate of 10 mV/s, where the differences in CV results become obvious due to the difference of pore structure. Figure 7a compares the CV curves of three pristine mesoporous carbons. The area of CV curves can reflect the capacitance values, which is accordant with the results from galvanostatic charge/discharge. The sample 2F gives the best capacitive behaviors among the three samples as observed from the rectangular degree of their CV curves. The voltammograms of 3-D cubic and wormlike samples become slightly distorted,
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Figure 8. Complex-plane impedance plots (Nyquist plots) in the frequency rang of 1 mHz to 100 kHz for (a) 1F, 2F, and 3F, (b) 1F and 1F-A, (c) 2F and 2F-A, and (d) 3F and 3F-A in H2SO4 electrolyte.
Figure 9. Capacitance versus real part of the impedance (RC plot) for (a) 1F, 2F, and 3F, (b) 1F and 1F-A, (c) 2F and 2F-A, and (d) 3F and 3F-A in 1 M H2SO4 electrolyte.
indicative of an unacceptable capacitive performance. To account for the nearly same SBET, pore size distribution, and such huge differences capacitive behaviors, it is believed that the pore architecture must play a dominant role in determining the capacitive performances. From the above results, we can conclude that the ordered 2-D hexagonal symmetry of 2F is more favorable for fast ion transport, while the 3-D Im3m structure with the isolated spherical mesopores shows the worst capacitive performance. Figure 7 b-d shows comparisons of CV curves between mesoporous carbons and their activated resultants. The activated samples provide higher capacitance values and more satisfactory rectangular shape than their precursors. The situation is much more obvious for the 1F-A and 3F-A. The superior capacitive behavior of activated samples is unexpected because of their relatively higher micorpores content. However, activation could improve the pore connectivity, which provides not only more enhancement for the electrolyte to impregnate but also facilitates the electrolyte reaching the entire inner pore surface. Thus, activated samples give a better kinetic behavior at high currents. The electrochemical results of improved pore connectivity by CO2 activation is similar to the case in the literature, where the neighboring channels are interconnected by adding SiO2 nanoparticles.21 The effect of big interconnecting pores on rate capability is superior to that of generated micropores in our study. 3.3. Electrochemical Impedance Spectroscopy of Mesoporous Carbons. Although the CV method can be utilized to evaluate the ion transport behavior, it is still unable to precisely describe the actual electrochemical diffusion process. Hence, it is quite important to further investigate the influence of pore structure on ion transport based on EIS, which is considered to be a powerful method for obtaining the dynamic information of ion transport.32,33 The complex-plane impedance plots for all samples are illustrated in Figure 8. The “Randle circuit” is a suitable equivalent circuit model for describing EIS response. At the high-medium frequency region, all carbons exhibit a depressed semicircle, then straight lines nearly vertical to the realistic impedance axis when frequency is lower than the knee frequency. The knee frequency as indexed in the inset of Figure 8 is considered to be the critical frequency where EDLCs begin to exhibit capacitive behavior. The deviation from the vertical line is attributable to inner-mesopore diffusion resistance (named Warburg resistance) for electrolyte ions, which is determined
by the detailed mesoporous structure of the different samples. The diameter of the semicircle reflects charge transfer resistance which is strongly dependent on the ion transfer, and electron conduction abilities can be used to estimate the formation rate of the double layer. Figure 8a presents the Nyquist plots of three pristine mesoporous carbons. The charge transfer resistances decrease in the sequence of 1F > 3F > 2F as revealed by the semicircle, in good agreement with CV results. The lower charge transfer resistance of 2F is believed to be associated with its advantageous pore symmetry over those of other samples. Both the 3-D cubic structure with isolated mesopores of 1F and the disordered wormlike texture of 3F will block the ions transfer and, consequently, lead to the high hindrance for charge transfer. The variations of impedance characteristics of mesoporous carbons and their corresponding activated resultants are demonstrated in Figure 8 b-d. For all activated samples, the increase of the diameter of the semicircle in the high-medium frequency region indicates the worsening charge transfer ability. This may be related to the impaired electron conduction network after CO2 activation. However, it is noteworthy that there is a decline in the Warburg resistance for the electrolyte diffusion process after the activation, even though micropores content increases significantly. The improvement with respect to the electrolyte diffusion can be understood as the consequence of the development of pore connectivity. Large amounts of micropores in the pore wall produced by activation do not hamper the electrolyte diffusion into the inner pore. On a contrary, the presence of micropores makes the isolated pores interconnect to each other, thus providing more entrances for electrolyte diffusion and also shortening the diffusion distance. Consequently, a significant decrement in Warburg resistance can be found after CO2 activation, especially for 1F-A and 3F-A, as presented in Figure 8b and d. Pore texture, such as pore symmetry and pore connectivity, on the electrode kinetic process can be further revealed by the plot of capacitance versus the real part of the impedance (RC plot shown in Figure 9). According to a theoretical treatise on the impedance response of porous electrodes, the ac signal is increasingly propagating into inner pore sites of the electrode with decreasing frequencies. At the high-frequency region, the electrolyte ions transport into the nanopores is inhibited, and, thus, charge aggregation only occurs on the surface of the carbon electrode in contact with the bulk electrolyte, leading to negligible capacitance values. After that, with decreasing
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Figure 10. Simplified schematic graph of the pore structure of mesoporous carbons and their corresponding activation samples: (a) 1F, (b) 2F, (c) 3F, (d) 1F-A, (e) 2F-A, and (f) 3F-A.
frequency the electrolyte ions begin to diffuse into the mesopores and subsequently into the micropores, resulting in an increasing capacitance and real part of the impedance. It is known that the impedance to electrolyte diffusion within different pore dimensions (e.g., mesopore and micropore) can be obtained according to the slope of the RC plot.34 Therefore, two distinct slopes at the medium-frequency (low resistance, high C/R-ratio) and the low-frequency region (high resistance, low C/R) can be ascribed to the ion diffusion inside the mesopores and micropores, respectively. As observed from Figure 9, the major differences with respect to the RC plot mainly occur in the medium frequency, which is identified as ion transport in the mesoporous channel. For the 2F, its mesopore with 2-D cylindrical geometry possesses advantages over the disordered wormlike and 3-D cubic isolated pore shape for ion transport inside it. Consequently, 2F exhibits the lowest impedance (5.03 Ω) to ion transport inside the mesopores as revealed by Figure 9a. After CO2 activation, electrolyte diffusion resistances inside mesopores of activated samples experience a significant reduction (shown in Figure 9 b-d). The resistances decrease from 9.93, 5.03, and 13.63 Ω to 2.95, 2.05, and 4.39 Ω for 3-D cubic mesoporous carbon, 2-D hexagonal mesoporous carbon, and wormlike mesoporous carbon, respectively. This can be explained by the larger amount of micropores after activation which improves the pore connectivity between the neighboring channels to a certain extent. As a consequence, ion transport in the mesopores channels is accelerated through many more electrolyte diffusion pathways, and thus activated samples have much lower impedance to electrolyte diffusion within the interconnected channels than their precursors within the unconnected channel. Besides, a rise in total capacitance at 3.16 mHz can be observed after the activation. The higher surface area of the activated electrode accounts for additionally created micropores, which increase the capacitance. 3.4. Ion Transport in the Confined Pore Structure. Both the CV and EIS have clearly demonstrated the significant influence of pore architecture on the ion transport and electrolyte diffusion under almost identical pore dimension. Figure 10 demonstrates the simplified schematic graphs of the pore architectures for the samples investigated in this work. The mesostructures experience a transition from the ordered cubic to hexagonal phases and then to disordered wormlike phase as
demonstrated in Figure 10 a-c. Huge differences in electrochemical results of three mesoporous carbons with similar pore size and distribution suggest that ion transport is strongly dependent on the actual pore architectures. For 1F, it provides the body-centered cubic symmetry with isolated spherical pore. This distributed pore will severely constrict the ions transfer within carbon matrix. When the concentration of triblock copolymer increases (2F), the isolated pores connect together and finally form a hexagonally cylindrical pore which allows a facile mass transport along the mesoporous channel without any obstacles. In comparison, the pore arrangement of 3F becomes disordered, causing the close and isolation of partial pores. Consequently, this structure will lead to a confined effect on the ion transport and enhance the diffusion distance, which, in turn, result in the poor capacitive behavior of 3F. After activation, the introduction of micropores in the pores wall will make the mesopores channels three-dimensionally continuous as illustrated in Figure 10d, e. For the 2F, the ion transport pathway in the cylindrical pore direction is straight. However, the channels are not connected to each other. The ions diffuse into its channels only through the two entrances at the top and bottom of every channel. This will block the transfer of ions between the neighboring channels. In contrast, for 2FA, besides the above two types of entrances, ion diffusion in 2F-A is accelerated through the many more ion diffusion routes and much shorter electrolyte diffusion distances, and, thus, 2F-A has much lower impedances to ion transport. The similar interpretation can be applied for 1F and 3F. 4. Conclusion The capacitive behaviors of three types of mesoporous carbons and their activated resultant samples are investigated through galvanostatic charge-discharge, CV, and EIS. For 2-D hexagonally mesoporous carbons, the capacitive performance is superior to that of 3-D cubic and wormlike mesoporous carbons. The reason is believed to originate from different pore symmetry. 2-D cylindrical mesopores facilitate the faster ion transfer along their smooth channels than within the isolated pores of 3-D cubic mesopores and the tortuous pathway of disordered wormlike mesopores. Compared to their precursors, the activated samples exhibit remarkable improvement in ion diffusion process. The results can be related to their increment
Ion Transport in Mesoporous Carbon in pore connectivity which provides more entrances for electrolytes to impregnate. Acknowledgment. The authors gratefully acknowledge the financial support from the Natural Science Foundation of China (No. 50730003) and the National High-Tech Research and Development Program (No. 2007AA05Z311). References and Notes (1) Winter, M.; Brodd, R. J. Chem. ReV. 2004, 104, 4245. (2) Chen, H. S.; Cong, T. N.; Yang, W.; Tan, C. Q.; Li, Y. L.; Ding, Y. L. Prog. Nat. Sci. 2009, 19, 291. (3) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications; Kluwer Academic/ Plenum: New York, 1999. (4) Huang, J. S.; Sumpter, B. G.; Meunier, V. Chem.sEur. J. 2008, 14, 6614. (5) Ania, C. O.; Khomenko, V.; Raymundo-Pin˜ero, E.; Parra, J. B.; Beguin, F. AdV. Funct. Mater. 2007, 17, 1826. (6) Xia, K. S.; Gao, Q. M.; Jiang, J. H.; Hu, J. Carbon 2008, 46, 1718. (7) Wang, D. W.; Li, F.; Liu, M.; Lu, G. Q.; Cheng, H. M. Angew. Chem., Int. Ed. 2008, 47, 373. (8) Frackowia, E. Phys. Chem. Chem. Phys. 2007, 9, 1774. (9) Pandolfo, A. G.; Hollenkamp, A. F. J. Power Sources 2006, 157, 11. (10) Wang, D. W.; Li, F.; Fang, H. T.; Liu, M.; Lu, G. Q.; Cheng, H. M. J. Phys. Chem. B 2006, 110, 8570. (11) Rolison, D. R. Science 2003, 299, 1698. (12) Lee, G.; Pyun, S. Langmuir 2006, 22, 10659. (13) Qu, D. J. Power Sources 2002, 109, 403. (14) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169. (15) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712.
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