New Insights into the Relationship between Micropore Properties

Article pubs.acs.org/JPCC

New Insights into the Relationship between Micropore Properties, Ionic Sizes, and Electric Double-Layer Capacitance in Monolithic Carbon Electrodes George Hasegawa,*,†,‡ Kazuyoshi Kanamori,† Kazuki Nakanishi,† and Takeshi Abe‡ †

Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan Department of Energy & Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

‡

ABSTRACT: The effects of the pore properties and the ionic sizes on the electric double-layer capacitances have been investigated by using the monolithic carbon electrodes with different pore properties. The carbon monoliths with high surface areas which possess homogeneous pore properties in the whole monoliths were prepared from the bridgedpolysilsesquioxane gels via the nanophase extraction technique. The detailed investigations of the pore properties of the carbon monoliths were conducted by the nitrogen physisorption measurement as well as the mercury porosimetry. The electrochemical property of each monolithic carbon electrode was examined by the cyclic voltammetry in the different aqueous electrolytes in order to investigate the effects of the ionic sizes. These fundamental analyses have provided new insights into the efficient micropore sizes in each electrolyte for the superior electric double-layer capacitors.

1. INTRODUCTION On growing demands for energy storage, porous electrode materials are attracting increasing attention, and the pore properties are regarded as one of the most important factors for the electrode capability.1−8 In the case of Li-ion batteries, the pores in the electrode materials help the rapid diffusion of Li+ ions especially in the materials with poor ionic conductivities, such as olivine-type materials.9−11 The pores in the electrodes play a different role in the case of electric double-layer (EDL) capacitors,12−14 in which the electric charge is stored by a nonfaradaic mechanism in the double layer at the electrode/ electrolyte interface. Hence, the capacities of EDL capacitors are dependent on the accessibility of the electrolyte ions to the pores in the electrodes and therefore on the effective surface area. It indicates that the EDL capacitances depend not on the surface areas of the electrodes estimated by the gas adsorption techniques but on the relative difference between pore sizes and the ionic sizes in the electrolytes. In general, it is true that the electrodes with higher surface areas tend to exhibit the higher EDL capacitances, and this is why the activated carbons are mostly used as the electrodes for EDL capacitors.12−14 In addition, many researchers have discussed the EDL capacitances in relation to the estimated surface area, i.e., BET surface area, of the activated carbon electrodes. However, in some cases, the increase in the estimated surface area does not lead to the increase of the EDL capacitances because of the pore size effects.15−17 The design of the desired pore size in the electrodes is therefore highly important in order to develop the © 2012 American Chemical Society

superior EDL capacitors. From this motivation, the relation between pore size and ionic size and the effects on the EDL capacitances have been energetically studied in the past decade.15−25 In most of the studies, the electrodes of EDL capacitors were prepared as the composite electrodes of the activated carbons which are in powder form, and the relationships between the pore properties of the activated carbon powders and the EDL capacitances were discussed. However, the composite electrodes are prepared from the mixture (slurry) of activated carbons, conductive agent such as acetylene black, and binders which are typically polymers such as poly(vinylidene difluoride) (PVdF) and polytetrafluoroethylene (PTFE). This means that the pores of the activated carbons are possibly covered and filled with the conductive agent and the polymer binders, which prevents the precise investigation of the effects of the pore properties on the EDL capacitance. Monolithic electrodes26−29 are promising candidates as the substitute for the conventional composite electrodes. Since a monolithic material is binder-free and can be used for the electrode without any additional processes, it is possible to exploit the completely clear surface for the formation of EDL. It is therefore expected that the direct information on the effects of the pore properties of the electrode on the EDL capacitance Received: September 11, 2012 Revised: December 3, 2012 Published: December 4, 2012 26197

dx.doi.org/10.1021/jp309010p | J. Phys. Chem. C 2012, 116, 26197−26203

The Journal of Physical Chemistry C

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obtained were washed with ethanol (EtOH) followed by the stepwise solvent exchange from EtOH to H2O. The resultant wet gels were subsequently subjected to the solvent exchange to 2-propanol followed by slow evaporative drying at 40 °C to obtain the bridged polysilsesquioxane xerogels. Some of the wet gels were hydrothermally treated in 1 M urea(aq) at 120 or 200 °C for 24 h before drying. After washing with H2O, the samples were subjected to 2-propanol and dried at 40 °C, resulting in the hydrothermally treated xerogels. The obtained xerogels were subsequently heat-treated at 1200 °C for 2 h with a heating rate of 4 °C min−1 under an argon flow at 1.0 L min−1. The resultant carbon/silica composites were immersed in 1 M NaOH(aq) at 60 °C for 12 h exchanging with fresh solvent three times to remove silica. The obtained carbon monoliths were washed with H2O at 60 °C for 4 h for three times followed by drying at 60 °C. In order to improve the electric conductivity, the obtained carbon monoliths were heat-treated at 1000 °C for 2 h with a heating rate of 4 °C min−1 under an argon flow at 1.0 L min−1. The samples are denoted as xHT. Here, x represents the hydrothermal treatment temperature (in the case of the sample without hydrothermal treatment, x is described as “wo”). 2.3. Characterization. The electric conductivities of the monolithic samples were measured by the four-probe method using a Physical Property Measurement System (PPMS, Quantum Design). Observation of the microstructures of the fractured surfaces of the samples was conducted by scanning electron microscopy (SEM) (JSM-6060S, JEOL, Japan). A mercury porosimeter (Pore Master 60-GT, Quantachrome Instruments) was used to characterize the macropores and bulk densities of the samples. Nitrogen adsorption−desorption (BELSORP-max, Bel Japan Inc., Japan) was employed to characterize the meso- and micropores of the samples. Before nitrogen adsorption−desorption measurements, the samples were degassed at 300 °C under vacuum for more than 6 h. 2.4. Electrochemical Measurements. All the electrochemical measurements were performed in a three-electrode PTFE cell at room temperature by an electrochemical analyzer (Model 660A, ALS Technology Co., Ltd., Japan). The carbon monolith shaped into ca. 8 mm square and 100 μm thick was attached to platinum plate with carbon paste (DOTITE, FC404CA, Fujikura Kasei Co., Ltd., Japan) to configure the monolithic carbon electrode. Before the electrochemical tests, the monolithic carbon electrodes were vacuum-impregnated with electrolyte to guarantee that the pores of the electrode were thoroughly filled with electrolyte. The counter electrode was a platinum plate. The electrochemical measurements were performed in the different aqueous electrolytes (6 M KOH(aq), 2 M H2SO4(aq), 0.2 M LiCl(aq), 0.2 M NaCl(aq), and 0.2 M KCl(aq)) using the appropriate reference electrodes (Hg/HgO (potential: 0.098 V vs normal hydrogen electrode (NHE), Metrohm Autolab B.V., Netherlands) for alkali chloride electrolyte, Hg/Hg2SO4 (potential: 0.615 V vs NHE, BAS Inc., Japan) for acid electrolyte, and Ag/AgCl in saturated KCl (potential: 0.199 V vs NHE, BAS Inc., Japan) for neutral electrolyte). Cyclic voltammetry (CV) was carried out at different sweep rates between −0.8 and 0.2 V (vs Hg/HgO) in 6 M KOH(aq), between −0.7 and 0.3 V (vs Hg/Hg2SO4) in 2 M H2SO4(aq), and between −0.5 and 0.5 V (vs Ag/AgCl) in the neutral electrolytes. The gravimetric capacitance, C (F g−1), was calculated as C = q/2mΔV, where q, m, and ΔV are voltammetric charges on positive and negative sweeps, the

can be acquired. Carbon monoliths with high surface area, which are used for monolithic electrodes of EDL capacitors, are usually obtained by the thermal activation of carbon monoliths with oxidative gases.28,30,31 However, since the increase in micropores takes place via the pyrolysis of carbons by oxidative gas, the micropores are inhomogeneously generated in the carbon monoliths; more micropores are generated in the outer part of the monolith which is well-exposed with oxidative gas than in the less-exposed inner part. Hence, it is impossible to estimate the real pore properties of the monolithic carbon electrode because the gas sorption analysis provides the information on the averaged pore properties of the whole sample. The carbon monoliths with homogeneous pore structure in the whole monoliths are therefore required for the valuable information on the pore properties. Recently, we have successfully synthesized porous carbon monoliths with high surface area from the porous carbon/silica nanocomposites, which are derived from pyrolysis of bridged polysilsesquioxanes, by the nanophase extraction technique.32,33 The enhancement of microporosity can be achieved by the sacrificial templating of the nanoscaled silica phase without any thermal activation process. The resultant carbon monoliths are expected to possess homogeneous pore properties in the whole monoliths. In the present paper, we report the investigation of the effects of pore properties on the EDL capacitance by utilizing the monolithic carbon electrodes prepared by this specific technique. The electrochemical measurements of the monolithic carbon electrodes with different pore properties in various aqueous electrolytes have been performed. The measured EDL capacitances of the monolithic carbon electrodes have been compared relative to their micropore size distributions. Most of the previous studies just compared the EDL capacitances based on the specific surface areas and the mean (or average) micropore widths of the activated carbons.15,19−25 However, since the micropore size distributions of the activated carbons are not narrow enough, discussions should not be based on the figures of mean micropore widths but on the micropore size distributions. The detailed analyses on the micropore size distributions of the monolithic carbon electrodes and the comparison of the measured EDL capacitances have suggested the new insights with respect to the effective micropore size for the formation of EDL.

2. EXPERIMENTAL SECTION 2.1. Chemicals. The precursor alkoxysilane, 4,4′-bis(triethoxysilyl)-1,1′-biphenyl (BTEBP), was purchased from Sigma-Aldrich Co. Aqueous solution of nitric acid (HNO3) in 65 wt % and urea were purchased from Hayashi Pure Chemical Industry Ltd. (Japan). Pluronic F127 (PEO106-PPO70-PEO106) was obtained from BASF Co. (Germany). All other reagents were purchased from Kishida Chemical Co., Ltd. (Japan). All reagents were used as received, and distilled water was used in all experiments. 2.2. Synthesis. The carbon monoliths were synthesized from the bridged polysilsesquioxane gels according to the previous reports.33 In a typical synthesis, 1.40 g of Pluronic F127 was dissolved in 8.0 mL of N,N-dimethylacetamide (DMAc), and 0.50 mL of 1 M HNO3(aq) was added. After the complete mixing, 2.0 mL of BTEBP was added to the obtained homogeneous solution, followed by stirring for 3 min at room temperature for hydrolysis. The resultant sol was then stood at 60 °C for 24 h for gelation and aging. The wet gels thus 26198



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weight of the monolithic carbon electrode, and the potential range of CV.

3. RESULTS AND DISCUSSION 3.1. Pore Properties of Monolithic Carbon Electrodes. The carbon monoliths were prepared from the bridgedpolysilsesquioxane monoliths without and with the hydrothermal treatments. The hydrothermal treatment of the asprepared bridged polysilsesquioxane gels, which are micro- and macroporous without mesopores, increases micropore size and tailors mesopores owing to Ostwald ripening.34 As a result, the resultant carbon monoliths prepared from the hydrothermally treated polysilsesquioxanes possess different pore properties compared to the carbon monoliths prepared from the polysilsesquioxane without hydrothermal treatment. In order to utilize the obtained carbon monoliths as monolithic electrodes, however, it was found that the electric conductivity of the carbon monolith after the extraction of nanophase silica was relatively poor (0.07 S cm−1), which was almost the same as that of the carbon/silica composite. Only a few effects on the conductivity were observed by the extraction of nanophase silica, which is an insulator. It indicates that the nanophase carbon parts in the carbon/silica nanocomposite possess poor conductivity. It is deduced that this is because the graphitic structures hardly develop in the restricted nanoscale regions which are constrained by nanophase silica domains. The carbon monoliths were therefore subsequently heat-treated at 1000 °C under an inert atmosphere to improve the electric conductivity. After the additional heat treatment, the electric conductivity was effectively improved to 0.9 S cm−1, which is comparable to the conductivities of other carbon materials.12,35 Hence, the investigations of the pore properties as well as the electrochemical measurements were conducted with the additionally heat-treated carbon monoliths. The interconnected macroporous structure, which was originally tailored by the sol−gel method accompanied by spinodal decomposition in the polysilsesquioxane gels, retained in the carbon monoliths after the additional heat treatment as shown in Figure 1a. Figure 1b shows the cross section and the appearance of the carbon monolith which was shaped into 100 μm thick plate for the monolithic carbon electrode. The mercury porosimetry result (Figure 1c) reveals that both macropore size and macropore volume increased when the samples were subjected to the hydrothermal treatment at higher temperature. This is because the shrinkage of the polysilsesquioxane gels during drying and heat treatment decreased by the hydrothermal treatment. The hydrothermal treatment in a weakly basic condition facilitates the polycondensation and strengthens the polysilsesquioxane gel skeletons.34,36 It is also found that all the carbon monoliths possess narrow macropore size distributions. Figures 1d and 1e show the nitrogen physisorption isotherms and the mesopore size distributions, respectively. Whereas there are no mesopores in woHT, the carbon monoliths with hydrothermal treatment possess mesopores, and the mesopore size and the mesopore volume increase as the hydrothermal treatment temperature increases due to the similar reason as described above. The meso- and macropore properties of the carbon monoliths basically reflect those of the precursor polysilsesquioxane gels.33 The pore properties of the carbon monoliths are summarized in Table 1. The specific surface areas of the obtained carbon monoliths are 1370−1490 m2 g−1; those of woHT and 120HT are almost the same, and that of 200HT is slightly lower than

Figure 1. (a) SEM image of the carbon monolith (200HT). (b) Cross section and appearance (inset) of the carbon monolith. (c) Macropore size distributions of the carbon monoliths measured by mercury porosimetry. (d) Nitrogen adsorption−desorption isotherms of the carbon monoliths. (e) Mesopore size distributions of the carbon monoliths obtained by the Barrett−Joyner−Halenda (BJH) method.

those. On the other hand, both the macropore sizes and volumes of the carbon monoliths are considerably different due to the difference in the degrees of the shrinkage during drying and heat treatment. The bulk densities of the carbon monoliths are therefore highly different, and the bulk density of 200HT is almost half of that of woHT. Figure 2a shows the nitrogen physisorption isotherms of the carbon monoliths in the low relative pressure region, and their micropore size distributions calculated by the Horváth− Kawazoe (HK) method37 are shown in Figure 2b,c. It is known that the HK method, which is based on the slit-pore model, is suitable for the micropore analysis of carbon materials, which are composed of graphene sheets. Note that the information on the micropores narrower than 0.4 nm is not reliable since the size of the nitrogen molecule is about 0.36 nm,38 and it is impossible to investigate micropores smaller than this size by the nitrogen adsorption−desorption method. The adsorption branch shifts to the higher relative pressure in the order of woHT, 120HT, and 200HT in Figure 2a. It is also found that the micropores of ∼0.5 nm decrease and the larger micropores increase in this order as shown in Figure 2c. Since a similar tendency is observed in the micropores of the polysilsesquioxane gels, which is caused by Ostwald ripening,34 this micropore properties of the carbon monoliths are partially derived from those of the polysilsesquioxanes. As mentioned above, the micropore size distributions of the samples can be changed by varying the hydrothermal treatment 26199



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Table 1. Pore Properties of the Carbon Monoliths Prepared from the Biphenylene-Bridged Polysilsesquioxanes without and with the Hydrothermal Treatment woHT 120HT 200HT

S(αs)a (m2 g−1)

S (t-plot)b (m2 g−1)

Vmicromesoc (cm3 g−1)

Vmicrod (cm3 g−1)

Dmacroe (μm)

Vmacrof (cm3 g−1)

ρbg (g cm−3)

1470 1490 1370

1460 1490 1360

0.579 0.799 1.06

0.526 0.594 0.535

1.93 2.19 2.96

1.54 2.22 3.59

0.362 0.275 0.185

Specific surface area calculated by the αs method. bSpecific surface area calculated by the t-plot method. cMicro- and mesopore volume estimated from the adsorption branch at p/p0 = 0.99. dMicropore volume calculated by the HK method. eMean macropore diameter obtained by Hg porosimetry. fMacropore volume obtained by Hg porosimetry. gBulk density obtained by Hg porosimetry. a

functional groups, which also have unignorable influences on the EDL capacitances.12 On the other hand, although the functional groups remain in the carbon monoliths prepared in this study, the amounts of the functional groups in each sample should be similar because of the same precursor gels and the same heat-treatment conditions. From this viewpoint, the resultant carbon monoliths are suitable for the investigation of the pore size effects on the EDL capacitances. 3.2. Pore Size and Ionic Size Effects on EDL Capacitance. The electrochemical performances of the monolithic carbon electrodes of three carbon monoliths (woHT, 120HT, and 200HT) were measured in the five aqueous electrolytes: 6 M KOH(aq), 2 M H2SO4(aq), 0.2 M LiCl(aq), 0.2 M NaCl(aq), and 0.2 M KCl(aq). Figure 3 shows the CV curves of the monolithic carbon electrodes at different scan rates in 6 M KOH(aq). Even when the specific surface areas of three carbon monoliths are similar as shown in Table 1, the obtained CV curves of three electrodes are remarkably different. In particular, the monolithic carbon electrode of woHT did not exhibit significant capacitance. The electrochemical performances of the monolithic carbon electrodes in different aqueous electrolytes are summarized in Figure 4. The gravimetric capacitances increased in the order of woHT, 120HT, and 200HT in all the electrolytes except for 2 M H2SO4(aq). The gravimetric capacitances of woHT and 120HT in 2 M H2SO4(aq) were very small and almost the same. The capacitance of 200HT in 2 M H2SO4(aq) was also small but larger than the capacitances of woHT and 120HT. Taking the EDL into the consideration, it is deduced that the difference of the electrochemical properties of the monolithic carbon electrodes is mainly derived from the difference of the micropore sizes.18 Among three carbon electrodes, the mesoand macropore properties are considerably different as shown in Table 1. The meso- and macropores help the electrolyte to penetrate into the micropores and allow the efficient mass transport in the monolithic electrode. In this study, however, the monolithic carbon electrodes were vacuum-impregnated with electrolyte in order that the pores of the electrode were

Figure 2. (a) Semilogarithmic chart of nitrogen adsorption− desorption isotherms of the carbon monoliths. (b) Micropore size distributions of the carbon monoliths obtained by the HK method. (c) Magnified chart of (b).

conditions of the precursor gels. It means, unlike other studies, that the carbon monoliths with different micropore size distributions can be obtained even when the heat-treatment conditions are the same. In almost all the previous studies of the pore size effects on the EDL capacitances, the carbons with different pore size distributions were obtained by changing the activation conditions. Since the activation processes naturally introduce the oxygen-containing functional groups such as carbonyls to the carbon samples,22,39 the carbons with different pore size distributions should have different amounts of

Figure 3. CV curves of the monolithic carbon electrodes at different scan rates in 6 M KOH(aq): (a) woHT, (b) 120HT, and (c) 200HT. 26200



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micropore size larger with a broader size distribution. It indicates that the carbon materials which possess small micropore sizes (similar to the hydrated ionic size) with narrow size distributions are suitable for the investigation of the effects of pore size and ionic size on the EDL capacitance. Since the carbon monoliths obtained in this study satisfy these requirements, the effects of the hydrated ionic size on the EDL capacitances are clearly observed in Figure 4. In the case of the KOH aqueous electrolyte, the resultant capacitances are larger than those in the KCl(aq) with all the monolithic carbon electrodes. One reason is that the concentration of the electrolyte was remarkably different; the concentration of KOH(aq) was larger than that of KCl(aq). The higher concentration of the electrolyte leads to the higher conductivity and has more chance to enter micropores.47 The smaller size of hydrated OH− ions than that of hydrated Cl− ions is also regarded as another reason. On the other hand, all the monolithic carbon electrodes showed very low specific capacitances in 2 M H2SO4(aq). This is because of the considerably larger hydrated ionic size of SO42−, which is about 0.1 nm larger than the hydrated ionic sizes of other ions, as shown in Table 2. The gravimetric and volumetric capacitances of the monolithic carbon electrodes at 5 mV s−1 in various aqueous electrolytes are shown in Figures 5a and 5b, respectively. When

Figure 4. Gravimetric capacitances of the monolithic carbon electrodes at different scan rates in various aqueous electrolytes: (a) woHT, (b) 120HT, and (c) 200HT.

thoroughly filled with electrolyte. In addition, the influence of the meso- and macropore properties is negligible when the scan rate is low enough because the formation of EDL would be slow and the kinetics of ion transport is no longer a limiting factor. Hereafter, we discuss the electrochemical properties of the monolithic carbon electrodes based on the relationship between the micropore sizes and the sizes of the ions in the electrolytes. In an aqueous electrolyte, the ions are naturally hydrated.40 The reported hydrated ionic sizes of the aqueous electrolytes41,42 are shown in Table 2. In the case of proton, many Table 2. Simulated Hydrated Ionic Sizes40 in the Aqueous Electrolytes Used in This Study

a

cations

diam (nm)

anions

diam (nm)

Li+ Na+ K+

0.482 0.436 0.424

OH− Cl− SO42−

0.424 0.448 0.533,a 0.556

Reference 41.

theoretical studies suggest that protons tend to be hydrated due to the strong hydrogen-bonding interactions with water molecules.43−46 Since the geometry of H+(H2O)n clusters remains an issue to be discussed, we will not take in as an important factor in this study. With respect to the alkali cations, the hydrated ionic sizes are K+ < Na+ < Li+ because the charge density of the alkali cations increases in this order, which leads to the increase in the size of the hydration shell. This agrees with the results in Figure 4, in which the specific capacitances increase in the order of LiCl(aq) < NaCl(aq) < KCl(aq) in all the electrodes. It has been reported that the alkali ions enter the pores in a hydrated form.19 Hence, the hydrated ions with the smaller size can enter the smaller micropores, making higher contributions to the EDL capacity. However, Salitra et al. reported that the less-activated carbon exhibited a large difference in the capacitances in different alkali chloride electrolytes whereas the more-activated carbon exhibited no difference.19 The thermal activation process makes the

Figure 5. Specific capacitances of the monolithic carbon electrodes (left: woHT; middle: 120HT; right: 200HT) obtained by the CV curves at 5 mV s−1 in various aqueous electrolytes: (a) gravimetric capacitance; (b) volumetric capacitance calculated by multiplying the gravimetric capacitance by the bulk density (ρb).

comparing the capacitances of 120HT and 200HT, the gravimetric capacitances of 200HT are larger than those of 120HT in all electrolytes. However, the volumetric capacitances of 200HT resulted in the similar values of those of 120HT in the electrolytes except for 2 M H2SO4(aq). Note here that we have to employ volumetric capacitance rather than gravimetric capacitance because the difference in bulk density among samples should be taken into consideration. Hence, it can be regarded that the EDL capacitances of 120HT and 200HT are almost the same in the electrolytes except for 2 M H2SO4(aq). 26201



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In the electrolytes of KOH(aq) and KCl(aq), the volumetric capacitances of 120HT are slightly but even larger than those of 200HT. This is because the efficient surface area of 120HT is larger than that of 200HT due to the smaller hydrated ionic size of K+ ion compared to Na+ and Li+ ions. On the other hand, the volumetric capacitance of 200HT is larger than 120HT in 2 M H2SO4(aq) due to the large hydrated SO42− ionic size as shown in Table 2. The capacitances of woHT measured in all electrolytes are remarkably small even at the low scan rate. The capacitances dramatically increase with 120HT in the electrolytes except for 2 M H2SO4(aq). The micropores which are larger than the hydrated ionic size of the electrolytes are of importance for the EDL capacitance as mentioned above. On comparing the micropore size distributions of the carbon monoliths in Figure 2c, 120HT possesses more micropores larger than 0.56 nm compared to woHT; two distribution curves (woHT and 120HT) intersect at w = 0.56. It is deduced that the large increase in the capacitance is based on this difference in micropores. When considering the case of the LiCl aqueous electrolyte (the largest hydrated ionic size among four electrolytes), it is estimated that the micropores whose sizes are about 16% larger than the simulated hydrated ionic size can contribute to the EDL capacitance. In the case of the H2SO4 aqueous electrolyte, on the other hand, the volumetric capacitances of woHT and 120HT are almost the same, and the increase in the capacitance was observed with the electrode of 200HT though the increasing ratio was relatively small compared to the aforementioned cases of other electrolytes. The difference in micropore size between 120HT and 200HT is the micropores whose pore widths were larger than 0.68 nm in Figure 2c; two distribution curves (120HT and 200HT) intersect at w = 0.68. Hence, it is estimated that the micropores whose sizes are at least 22−27% larger than the simulated size of the hydrated SO42− ion can contribute to the EDL capacitance in the H2SO4 aqueous electrolyte. In fact, Lin et al. suggested that pores less than about 0.8 nm in width do not contribute to the EDL capacitance in the H2SO4 aqueous electrolyte in the previous report.18 The simulated hydrated ionic sizes are calculated on the assumption of the spherical shape of hydrated ions, and it is natural that the actual hydrated ions are not real sphere. In addition, they thermally vibrate at room temperature. These are the reasons why the micropores whose sizes are about 20% larger than the hydrated ionic sizes of the electrolytes are required for the efficient formation of the EDL. It is likely that the effective micropore size on the basis of the simulated hydrated ionic size is different between SO42− and the other ions; the micropores whose sizes are much larger than the SO42− ion are required for the improvement of the EDL capacitances in 2 M H2SO4(aq) compared to the other electrolytes. This is presumably because SO42− ion has the more complicated shape with projections,41 which needs the larger pores to enter. The effects of the scan rate on the volumetric capacitance in various alkali chloride electrolytes with the same concentration have been investigated, as shown in Figure 6. The volumetric capacitances of 120HT are slightly larger than those of 200HT at 5 mV s−1 as described above, and the capacitance naturally decreases with increasing scan rate. Besides, the reduction rate of the capacitance with increasing scan rate is obviously different; the volumetric capacitance of 120HT more sharply decreases compared to that of 200HT for all electrolytes. In

Figure 6. Volumetric capacitances of the monolithic carbon electrodes (left: 120HT; right: 200HT) at different scan rates in various alkali chloride aqueous electrolytes: (a) 0.2 M LiCl(aq), (b) 0.2 M NaCl(aq), and (c) 0.2 M KCl(aq).

addition, the reduction rate depends also on the electrolytes. For example, the capacitances of 120HT become lower than those of 200HT at the scan rates higher than 10 mV s−1 in 0.2 M LiCl(aq) as shown in part a. On the other hand, the capacitance of 120HT is still larger than that of 200HT at 10 mV s−1 in 0.2 M NaCl(aq) and 0.2 M KCl(aq) (parts b and c). In particular, the capacitance of 120HT is higher than that of 200HT even at 20 mV s−1 in 0.2 M KCl(aq). These behaviors may also derive from the relative size difference between the micropore and the hydrated ions. There might be some effects of meso- and macropores on the reducing rates of the capacitances; however, we believe that the contribution of micropores is dominant because the hydrated ions are remarkably small compared to meso- and macropores. At higher scan rates, only the smaller number of hydrated ions can enter the micropores. This effect becomes more significant for smaller micropores. Since the monolithic carbon electrode of 120HT possesses the larger amount of smaller micropores than that of 200HT, the number of the hydrated ions which can enter the micropores of 120HT more strongly depends on the scan rate. Similarly, the number of the hydrated ions that can enter the micropores strongly depends on the hydrated ionic size at higher scan rates. This is why the reduction rate of the capacitances with increasing scan rates are in the order of LiCl(aq) > NaCl(aq) > KCl(aq).

4. CONCLUSIONS The new insights into the pore size effects on the EDL formation have been acquired by using the monolithic carbon electrodes with different pore properties which were prepared from the porous biphenylene-bridged polysilsesquioxane monoliths via the nanophase extraction technique. The hydrothermal treatment of the polysilsesquioxane gels enables to change not only meso- and macropore properties but also micropore properties, especially micropore size distributions. 26202



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The electrochemical studies with the monolithic carbon electrodes clearly show the effects of the micropore sizes and the ionic sizes of the electrolytes on the EDL capacitances owing to the small micropore sizes and the relatively narrow distributions. It has been suggested that the micropores whose pore sizes are about 20% larger than the simulated hydrated ionic sizes of the electrolytes can effectively contribute to the EDL capacitances. It is deduced that the effective micropore size on the basis of the hydrated ionic size also depends on the hydrated ion shape; the larger marginal pore space is needed for complex-shaped ions such as SO42− rather than simple-shaped ions such as alkali ions. It is also found that the capacitance decreases in the larger extent with increasing scan rate in the electrolyte composed of larger hydrated ions. It is therefore suggested that KOH(aq) and KCl(aq) are more suitable than H2SO4(aq) for the electrolyte of EDL capacitors, which require high performance at a high scan rate. The obtained insights are of great importance for the improvement of EDL capacitor capabilities, which can be achieved by the efficient pore design of the electrodes.

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AUTHOR INFORMATION

Corresponding Author

*Tel +81 753 832 126; Fax +81 753 832 488; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was supported by the Grant-in-Aid for JSPS Fellows (No. 24·31 for G.H.) from the Japan Society for the Promotion of Science (JSPS).

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