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Hierarchically Porous Carbon Monoliths Comprising Ordered Mesoporous Nanorod Assemblies for High-Voltage Aqueous Supercapacitors George Hasegawa,*,† Kazuyoshi Kanamori,‡ Tsutomu Kiyomura,§ Hiroki Kurata,§ Takeshi Abe,∥ and Kazuki Nakanishi‡ †

The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka, Ibaraki 567-0047, Japan Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan § Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan ∥ Department of Energy & Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ‡

S Supporting Information *

ABSTRACT: This report demonstrates a facile one-pot synthesis of hierarchically porous resorcinol-formaldehyde (RF) gels comprising mesoporous nanorod assemblies with two-dimensional (2D) hexagonal ordering by combining a supramolecular self-assembly strategy in the nanometer scale and phase separation in the micrometer scale. The tailored multilevel pore system in the polymer scaffolds can be preserved through carbonization and thermal activation, yielding the multimodal porous carbon and activated carbon (AC) monoliths. The thin columnar macroframeworks are beneficial for electrode materials due to the short mass diffusion length through small pores (micro- and mesopores). By employing the nanostructured AC monolith as a binder-free electrode for supercapacitors, we have also explored the capability of “water-in-salt” electrolytes, aiming at high-voltage aqueous supercapacitors. Despite that the carbon electrode surface is supposed to be covered with salt-derived decomposition products that hinder the water reduction, the effective surface area contributing to electric double-layer capacitance in 5 M bis(trifluoromethane sulfonyl)imide (LiTFSI) is found to be comparable to that in a conventional neutral aqueous electrolyte. The expanded stability potential window of the superconcentrated electrolyte allows for a 2.4 V-class aqueous AC/AC symmetric supercapacitor with good cycle performance.

1. INTRODUCTION

Various synthetic strategies toward carbon monoliths with multilevel porous structures have been reported, most of which are related to the fabrication of porous polymers followed by carbonization.9−12 A family of phenolic resin is one of the most prominent examples where the deliberate design of porous morphology has been extensively examined because the polymer backbone involves −OH groups which allow the formation of supramolecular self-assemblies with poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO− PPO−PEO) type triblock copolymer surfactants, for example Pluronic F127, via hydrogen bonding.9−11,13−16 Consequently, the liquid crystal template of those nonionic surfactants affords ordered mesoporous polymer scaffolds. As for tailoring macroporous structures in monoliths, the sol−gel method accompanied by spinodal decomposition is a well-established

In modern civilization, carbonaceous species play a crucial role as ubiquitous and indispensable materials in diverse applications. Their good electrical conductivity and high surface area increase the significance of porous carbons as electrode materials in energy storage.1,2 As general carbon materials in powder form need to be immobilized with a polymeric binder on a current collector, carbon monoliths draw much interest as a binder-free electrode.3−8 In view of a macrosized monolithic form, nanoarchitectural design of porous carbons associated with control over pore sizes from subnanometer to micrometer scale is of great importance, because each pore size regime bestows multiple benefits. For instance, small pores offer high effective surface area for electrochemical reactions, whereas large pores facilitate electrolyte ingress throughout the structure. In particular, the importance of macropores is highlighted when employing an aqueous electrolyte because the hydrophobic surface of carbon electrodes inherently causes poor electrolyte penetration. © 2016 American Chemical Society

Received: March 29, 2016 Revised: May 10, 2016 Published: May 12, 2016 3944

DOI: 10.1021/acs.chemmater.6b01261 Chem. Mater. 2016, 28, 3944−3950

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Chemistry of Materials

calculated as d(10) = λ/(2 sin θ) and a0 = 2d(10)/√3 for a 2D hexagonal lattice. Mechanical properties of the specimens were measured by a material tester (EZGraph, Shimadzu Corp., Japan). For uniaxial compression tests, a columnar-shaped specimen (typical diameter × height was ϕ 10 mm × 10 mm) was compressed at a rate of 0.5 mm min−1. For short-beam three-point bending tests, a cylindrical sample with ∼3.3 mm in diameter and 40 mm length was put on a fixture with a 30 mm span and applied loading by a wedge-shaped crosshead with 60° and 0.3 mm diameter at the point at a rate of 0.5 mm min−1. The stress σ was calculated as σ = 8PL/πd3, where P, L, and d are the load, the span between the shores, and the sample diameter. 2.3. Electrochemical Measurements. All electrochemical tests were carried out on a VMP3 potentiostat (Bio Logic Science Instruments) at room temperature. The activated carbon (AC) monolithic electrode (200 μm thick, ca. 0.5 cm2) was tested in twoand three-electrode cells assembled with two AC electrodes of comparable mass as working and counter electrodes. In a threeelectrode cell, two AC monolithic electrodes were individually held by a tweezer (stainless steel (SUS)) with the distance of about 0.5 mm. In case of a two-electrode cell configuration, an aramid separator (60 μm) was used. The electrochemical measurements were performed in various aqueous electrolytes: 6 M KOH, 1 M Li2SO4, 2.5 M lithium bis(trifluoromethane sulfonyl)imide (LiTFSI), and 5 M LiTFSI. Before the electrochemical tests in each electrolyte, the AC monolithic electrodes were vacuum-impregnated with the corresponding electrolyte to ensure the electrolyte penetration through the pores. The reference electrodes of Hg/HgO (potential: 0.098 V vs Normal Hydrogen Electrode (NHE), Metrohm Autolab B.V.) and Ag/AgCl in sat. KCl (potential: 0.199 V vs NHE, BAS Inc.) were employed for the tests in basic and neutral aqueous electrolytes, respectively. The gravimetric capacitance values of the AC electrode, C (F g−1), were calculated from the cyclic voltammogram (CV) curves in the threeelectrode cells, according to C = Q/2mΔV(V1 − V0), where Q, m, ΔV and (V1 − V0) are voltammetric charges on positive and negative sweeps, the weight of a monolithic electrode, the potential sweep rate, and the sweep potential range of CV, respectively. The capacitance was also assessed from the galvanostatic experiments in the two-electrode cells by using the equation C = 2IΔt/mΔVd, where I, Δt, ΔVd are the current, the discharge time, and the voltage during the discharge. The energy density (E) and power density (P) of the AC/AC symmetric supercapacitor were calculated based on the mass of both electrodes (2m); E = ∫ VI dt/2m and P = E/Δt. As for the galvanostatic cycling test in 5 M LiTFSI, several CV scans (0−2.4 V) were carried out before the charge and discharge cycles to establish the passivation effects on both electrodes. Electrochemical impedance spectroscopy (EIS) measurements were conducted in the three-electrode configuration at −0.1 V (vs Ag/AgCl) with an AC amplitude of 10 mV in the 10 mHz−100 kHz frequency range.

manner without using hard template.4,6−10 In the particular sol−gel systems, where the hydrolysis and polycondensation can be highly controlled and the formed oligomers contain abundant −OH groups, like alkoxysilane-based systems, the integration of periodically organized mesopores into macroporous frameworks is even possible due to the strong interaction between the PEO chain and the polymeric network.17,18 In the case of a phenolic resin, however, it is fairly challenging to govern both surfactant/oligomer selfassembly and phase separation during polycondensation because of insufficient −OH groups; polymerization of resorcinol and formaldehyde (RF) in aqueous media results in macroscopic biphasic separation by adding F127 in many cases.11 Hence, despite numerous trials, only a few reports have been published on the monolithic phenolic resins with welldefined macropores as well as periodically arranged mesopores, and thereby the same goes for carbon monoliths.9 Here, we demonstrate an one-pot synthesis of hierarchically porous RF gels with ordered mesopores embedded in macroporous cylindrical frameworks by incorporating the micellar templating in nanometer-scale and the phase separation in micrometer-scale. The subsequent carbonization of the RF gels yielded hierarchically porous carbon monoliths preserving the tailored nanostructures. In addition, the prepared carbon monolith was utilized as a binder-free electrode to explore high-voltage aqueous supercapacitors. In particular, the capability of a superconcentrated neutral aqueous electrolyte was investigated in detail.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Hierarchically Porous RF Gels and Carbon Monoliths. The preparation procedure was as follows: 1.5 g of triblock copolymer Pluronic F127 (EO106PO70EO106) and 1.1 g of resorcinol were added to a mixture of 15 mL of triethylene glycol (TEG), 1.5 mL of 1,3,5-trimethylbenzene (TMB), 1.5 mL of benzyl alcohol (BzOH), and 2.5 mL of 1 M HCl aqueous solution, followed by vigorous stirring until F127 and resorcinol were completely dissolved. Then, 1.5 mL of formaldehyde solution (37 wt %) was added to the resultant homogeneous solution. After stirring for 30 min at room temperature, the mixture was kept under static conditions at 60 °C for 48 h. The wet gel was subsequently treated in 1 M NH3 aq. at 80 °C for 24 h. After washing with H2O, the gel was dried at 60 °C followed by the heat treatment at 300 °C for 30 min under N2 atmosphere containing 10% of air to remove the remaining F127. Thus, the obtained sample was carbonized at 1000 or 1600 °C for 2 h with a heating rate of 4 °C min−1 under the stream of N2 gas (1 L min−1). The thermal activation of the carbon monoliths (carbonized at 1000 °C) was performed at 1000 °C for 2 h under flowing N2 (0.9 L min−1) and CO2 gas (0.1 L min−1). In order to ensure the activation inside of the monolith, the carbon monolith was shaped into a thin plate with a thickness less than 1 mm. 2.2. Characterization. The microstructures of the fractured surfaces of the samples were observed by scanning electron microscopy (SEM, JSM-6060S, JEOL), field emission scanning electron microscopy (FE-SEM) (JSM-6700F, JEOL), and transmission electron microscopy (TEM, JEM-2200FS, JEOL, equipped with a CEOS image corrector). A nitrogen adsorption−desorption apparatus (Belsorp Max, Bel Japan Inc.) was employed to characterize the mesoand micropores of the samples. The samples were degassed at 200 °C under vacuum prior to the measurement. The SAXS measurements of the monolithic specimens were carried out with a RINT system (RINT Ultima III, Rigaku Corp.) equipped with a Cu Kα X-ray generator (λ = 0.154 nm), a multilayer mirror, a two-slit collimator, a vacuum pass, and a scintillation counter. Scattering intensities from 0.3° to 2.5° were examined with an increment of 0.01°. The parameters, such as d-spacing d(10) and lattice constant a0, were

3. RESULTS AND DISCUSSION 3.1. Hierarchically Porous Polymer Scaffolds and Carbon Monoliths. Since the −OH groups on RF polymers interact with the hydrophilic portion of block copolymer via hydrogen bonding with the hydrophobic region facing outward, the F127/RF composite exhibits relatively hydrophobic nature, which poses a macroscopic biphasic separation in aqueous media. Such an extremely strong phase separation tendency precludes the introduction of spinodal decomposition, which is the key to construct a well-defined macroporous morphology,17,18 in the sol−gel system. In order to suppress the unfavorably strong phase separation tendency, we employed TEG as a predominant solvent in terms of the compatibility with the F127/RF oligomer assemblies.10 Unlike short chain alcohols (e.g., methanol and ethanol), a glycolic solvent is not significantly detrimental for the formation of lyotropic mesophases of surfactants.18,19 So as to ensure the micelle formation, BzOH and TMB were also added as a cosurfactant17 3945

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Figure 1. (a) SEM image and appearance (inset) of the porous carbon monolith calcined at 1600 °C. (b) FE-SEM images of the cross-section of the macropore skeleton of the carbon monolith. (c) TEM micrographs of the ordered mesoporous structure in the macropore skeleton. (d) SAX patterns of the samples calcined at different temperatures. (e) Nitrogen physisorption isotherms and (f) mesopore size distributions of the samples calcined at different temperatures.

Table 1. Pore Properties of the Hierarchically Porous Polymer and Carbon Monoliths as-dried 300 °C 1000 °C 1600 °C

S (t-plot)a (m2 g−1)

Vmicromesob (cm3 g−1)

Vmesoc (cm3 g−1)

Dpd (nm)

d(10)e (nm)

a0e (nm)

Twallf (nm)

ρbulkg (g cm−3)

498 779 436

0.419 0.447 0.344

0.273 0.216 0.231

4.9 3.3 3.8

14.5 12.1 10.1 9.4

16.7 14.0 11.7 10.8

9.1 8.4 7.0

0.312 0.307 0.287 0.271

a

Specific surface area obtained by the t-plot method. bMicro- and mesopore volume obtained by nitrogen adsorption isotherms at p/p0 = 0.99. Mesopore volume obtained by the BJH method. dMean mesopore size calculated from the adsorption branches by the BJH method. eCalculated from SAXS measurements. fWall thickness calculated as Twall = a0 − Dp. gBulk density calculated as [weight]/[bulk volume]. c

and a swelling agent,20 respectively. Both additives are known to enhance the self-organization of F127; TMB also improves the interactions among the hydrophobic moieties of F127.17 With the aid of these additives, the cooperative self-assemblies of surfactant/RF oligomer are formed with enhanced durability. Under the specific condition where the periodically arranged self-assemblies are stably maintained over the polymerization and the concurrent spinodal decomposition, the macro/ mesoporous structure with periodic ordering of mesopores can be tailored in monolithic gels, as shown in Figure S1. Since the micellar-templated oligomers show stronger structural anisotropy arising from the high periodicity, the macroporous morphology is more governed by the crystallographic orientation of ordered aggregates as observed in the preceding studies on silica-based systems,17,18 resulting in the RF gel consisting of nanorod assemblies. After gelation, the RF gels were subjected to the treatment in NH3 aq. for reinforcing the

gel skeleton. Although this process is not necessarily required to preserve the ordered mesoporous structure in dried and heat-treated samples, the shrinkage during drying and heattreatment can be suppressed to some extent by this posttreatment, resulting in the larger mesopore size (see Figure S2). The carbon monolith derived from the porous RF gel is displayed in Figure 1a. Both crack-free monolithic shape and well-defined macroporous morphology were retained after carbonization at 1600 °C. The magnified images of the fractured macroframework shown in Figure 1b,c confirm that straight cylindrical mesopores were embedded in each nanorod. The SAXS profiles (Figure 1d) exhibit a strong diffraction peak and a weak broad band in the 2θ range of 0.7−1.0° and 1.2− 1.8°, respectively. In light of the microscopic observation, the first strong peak can be indexed as the (10) diffraction of 2D hexagonal symmetry and the second weak band can be attributed to the (11) and (20) diffractions.14 These results 3946

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solution (“water-in-salt” electrolyte) provides a remarkably wide potential window reaching ∼3 V.29 In this case, a LiF-rich passivation layer formed by the reductive decomposition of LiTFSI stably lies on the electrode surface, which blocks H2 evolution, in addition to the above-mentioned reducing effect of water activity at high concentration.29 In this study, the applicability of the water-in-salt electrolyte to EDLCs has been investigated by employing the obtained AC monolith with the nanostructural hierarchy as a binder-free electrode. Figure 2a shows the CV curves of the monolithic electrode in three electrode cells with various neutral aqueous electrolytes.

indicate that the 2D hexagonal channels are periodically oriented in the rod-like macropore skeleton. The d-spacing of the (10) peaks and the unit-cell parameter for the samples are summarized in Table 1. The cell parameter of the RF gel after removal of F127 is calculated as 14 nm. As predicted, the carbonization process reduced the cell parameter due to the shrinkage, ending up in a0 = 10.8 nm for the carbon calcined at 1600 °C, which is in good agreement with the microscopic observation. The nitrogen sorption isotherms (Figure 1e) and the corresponding mesopore size distributions (Figure 1f) evidence that the well-established mesopores with narrow distribution were also preserved after carbonization at 1600 °C. The pore properties of the samples are listed in Table 1. It should be stated that the mean mesopore size slightly increases despite the reduction of unit-cell parameter, when the carbonization temperature is raised from 1000 to 1600 °C. This is presumably because the thermal decomposition of carbon modestly took place on the mesopore surface, which decreased the thickness of mesopore wall to a larger extent than the decrement of unit cell by shrinkage. In fact, the bulk density became lower with reduced micropore volume with increasing the carbonization temperature, indicating that the gasification of carbon occurred more intensively at higher temperature. The microscopic geometry of the carbon monolith marked by the highly ramified mesoporous skeleton is reminiscent of a truss structure, which shows high mechanical properties. The compressive and flexural mechanical strengths of the carbon monolith (1600 °C) are shown in Figure S3. The carbon monolith can endure up to ca. 10% linear compression along with good fracture stress (∼28 MPa) and high Young’s modulus (∼300 MPa) compared to the RF-derived carbon aerogels with similar density.21,22 Against bending stress, the fracture stress and Young’s modulus of the carbon monolith were evaluated as around 22 and 1900 MPa, respectively. The thermal activation of the carbon monolith yielded the activated carbon (AC) monolith with the hierarchically porous structure, as presented in Figure S4. The specific surface area dramatically increased from 779 m2 g−1 to 2170 m2 g−1 (see Table S1). The micropore size distribution (Figure S4c) clearly demonstrates that micropores ranging from 0.5 to 1.5 nm were highly developed, which is suitable for an electric double-layer capacitor (EDLC) electrode in terms of the accessibility of hydrated ions.1,2 3.2. Challenge to High-Voltage Aqueous Supercapacitors. As aqueous energy storage systems have attracted considerable attention due to nonflammable, environmentally benign, highly conductive features of aqueous electrolytes, this work especially aims at developing high-voltage aqueous supercapacitors. Since the energy stored in a supercapacitor (E) relies on the capacitance (C) and voltage (V) according to the formula E = 0.5CV2, the enhancement of cell voltage is deemed as one of the most crucial points. In order to address the major issue of low cell voltage restricted by the potential window of H2O (∼1.23 V), recent efforts have largely focused on neutral aqueous electrolytes, some of which exhibit the extended potential window even beyond 2 V.23−28 For instance, in an alkali metal sulfate (e.g., Li2SO4) aqueous solution,24−28 the number of “free” H2O molecules (not participating in hydration) is assumed to be decreased due to the strong solvation of alkali metal cations and sulfate anion, which effectively reduces the activity of water.27 Very recently, Xu and co-workers reported that a superconcentrated LiTFSI aqueous

Figure 2. (a) Cyclic voltammograms of the AC monolithic electrode obtained in different aqueous electrolytes at 5 mV s−1. The vertical lines correspond to the potentials of water reduction and oxidation at pH 7. (b) CV profiles of the AC electrode at varied scan rates in 5 M LiTFSI (sweep range: ∼−1.3−1.2 V (vs Ag/AgCl)). (c) Comparison of the capacitance values as a function of scan rate in various electrolytes. The capacitances were evaluated from the CV curves in part b and Figure S5. (d) Nyquist impedance spectra of the AC electrode in different aqueous electrolytes at −0.1 V (vs Ag/AgCl) in the frequency range from 100 kHz to 10 mHz.

The molar ratios of [LiTFSI]/[H2O] in 2.5 M (molality: 3.9 mol kg−1) and 5 M (18.7 mol kg−1) LiTFSI aq. are approximately 1/14 and 1/3, respectively. In terms of thermodynamics, the cathodic and anodic potential limits for water are −0.61 and 0.62 V (vs Ag/AgCl) at pH 7.29 In practice, the potential range available in an electrolyte is allowed to broaden beyond 1.23 V by the overpotentials for H2 and O2 evolution, which depend not only on the electrolyte pH but also on the material used as an electrode.27−29 The stability potential window of the AC monolithic electrode in 1 M Li2SO4 appeared to be around ∼1.8 V, which is lower than that reported previously27 probably due to the higher specific surface area. The higher overpotentials were observed for both cathodic and anodic water decomposition even in 2.5 M LiTFSI (salt-in-water electrolyte), which is assumed to contain a high fraction of “free” water according to the molecular dynamics simulation.29 Accompanied by the passivation effect of the LiF-based decomposition products of LiTFSI on water reduction as Xu et al. elucidated,29 it is put forward that upon positively polarizing the electrode the Helmholtz layer is mostly occupied by TFSI− anions playing as a role of barrier to hinder 3947

DOI: 10.1021/acs.chemmater.6b01261 Chem. Mater. 2016, 28, 3944−3950

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Chemistry of Materials water oxidation.30 In 2.5 M LiTFSI, however, the unignorable cathodic current derived from the reduction of LiTFSI was continuously observed below −0.4 V (vs Ag/AgCl) on cycling, which is unfavorable for a practical use. By contrast, the cathodic current relating to the decomposition of LiTFSI was less pronounced in 5 M LiTFSI. In addition, the stability window was further expanded toward both negative and positive ends reaching up to >2.5 V owing to the abovementioned effects. The onset potential for H2 evolution on the AC electrode was similar to that on the stainless steel electrode (around −1.3 V (vs Ag/AgCl)), while the stability against water oxidation on the high surface area AC was found to become substantially lower compared to the stainless steel because of the less passivation effect on the anodic reaction.29 Figure 2b shows the CV profiles of the AC electrode in 5 M LiTFSI as a function of scan rate. The quasi-rectangular CV curves were reversibly obtained in the potential range from −1.3 to 1.2 V (vs Ag/AgCl), indicative of 2.5 V for the sweep range. For reference, the CV curves in other electrolytes are also provided in Figure S5. The evaluated capacitances in various electrolytes are comprised in Figure 2c. Comparing the specific capacitance values in the different electrolytes, the AC electrode exhibited the highest capacitance in 6 M KOH, while the sweep range was limited to 1.0 V. One reason for the high capacitance is that the pseudocapacitive contribution of surface oxygenated groups such as quinone/hydroquinone redox couple is more enhanced in a basic electrolyte than under a neutral condition.24 Another reason lies in the presence of hydroxide ions as better mobile species with higher ionic diffusion rate in micropores than other ions. The neutral Li2SO4 electrolyte offered good capacitance of 202 F g−1 at 5 mV s−1 and 137 F g−1 at 100 mV s−1 with the extended sweep range of 1.8 V. It is noteworthy that the capacitances in 2.5 and 5 M LiTFSI at 5 mV s−1 showed similar values of 187 F g−1 and 183 F g−1, respectively, which are slightly lower than that in Li2SO4 electrolyte because of the bulky TFSI anion. In view of the formation of a passivation layer on the AC surface in LiTFSIbased electrolytes,29 one could assume that the capacitance might considerably drop because micropores were occluded by solid decomposition products and thus effective surface area was significantly decreased. However, this result indicates that the micropores can persist beneath the interfacial layer and contribute to the electric double-layer capacitance. It should be noted that the capacitance fade in 5 M LiTFSI was more susceptible to scan rate than in 2.5 M LiTFSI. To probe the ion diffusion in the porous AC electrode in the water-in-salt electrolyte, EIS measurements were carried out. Figure 2d represents the comparison of the Nyquist plots at −0.1 V (vs Ag/AgCl) in the neutral electrolytes, all of which consist of a small semicircle followed by a vertical line parallel to the imaginary axis characteristic of capacitive behavior. The impedance spectra in 1 M Li2SO4 and 2.5 M LiTFSI show almost identical profiles, whereas that in 5 M LiTFSI appears to be appreciably different. First, it is found that the resistance at high frequencies (100 kHz) mainly dominated by an electrolyte resistance outside pores becomes high in 5 M LiTFSI, which reflects the poor conductivity (∼13 mS cm−1) of the viscous water-in-salt electrolyte compared to 1 M Li2SO4 (∼60 mS cm−1)27 and 2.5 M LiTFSI (>50 mS cm−1).29 Second, the semicircle in the middle frequency range is larger in 5 M LiTFSI than in the other electrolytes. It is proposed that the arc portion is correlated with the contact resistance31 (between the

AC monolith and the SUS tweezer holding the monolith in this case) and the electrolyte resistance within pores.32,33 Since the former should be the same in the three systems, the latter is responsible for the difference in semicircle size, which stems from the different conductivities as mentioned above. Third, the slight deviation from the vertical line at the low frequency region is detected in 5 M LiTFSI, indicating the slower ion diffusion in micropores as well.33 Although the poorer conductivity of the water-in-salt electrolyte incurs the lower rate performance as shown in Figure 2c, it seems that the decomposition products of LiTFSI are not detrimental to the electrode performance, which is corroborated by the negligible change of the Nyquist plots before and after the formation of decomposition products in 5 M LiTFSI (Figure S6). In the case of practical supercapacitors, the maximum operational cell voltage often becomes lower than the stability potential window because the electrode potential cannot be regulated in a two-electrode configuration.23−28 On increasing the cell voltage, the potential of either negative or positive electrode goes beyond the cathodic or anodic potential limit ahead of reaching the stability window value. We therefore evaluated the capability of 5 M LiTFSI electrolyte with a symmetric capacitor using the AC monolithic electrodes. In Figure 3a, the CV profiles of the symmetric capacitor in 2.5 and 5 M LiTFSI are compared with different maximal cell voltages. Compared to the correspondent CV curves in 1 M

Figure 3. (a) Comparison of the CV profiles in 2.5 M (broken line) and 5 M LiTFSI (solid line) performed in a two-electrode cell configuration at 5 mV s−1 with stepwise shifting of the maximum voltage of 0.2 V. (b) Potential change of the positive (E+, open triangles) and negative (E−, closed triangles) AC electrodes in the symmetric capacitor in 5 M LiTFSI applied at different voltage values. The potential E0 (open circles) corresponds to the potential when the working voltage became 0 V after each voltage charging. The upper and lower horizontal lines show the anodic and cathodic potential limits where the exponential current increase was observed in a threeelectrode cell configuration. (c) Capacitance retention of the symmetric capacitor in 5 M LiTFSI with a maximum operating voltage of 2.4 V during the galvanostatic cycling at 5 A g−1. (d) Ragone plots for the symmetric AC/AC EDLCs in different aqueous electrolytes. The energy and power densities were calculated based on the mass of both AC monolithic electrodes. 3948

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delivered high capacitance at low scan rate in 5 M LiTFSI though the rate performance was impaired by the lower conductivity compared to conventional (salt-in-water) aqueous electrolytes. The practical symmetric AC/AC supercapacitor involving 5 M LiTFSI allowed a maximum operating voltage of 2.4 V without significant capacitance decay during 10 000 cycles, which paves a pathway for high-voltage aqueous supercapacitors. Since the stability on the positive side of a capacitor constitutes a limiting factor of maximum cell voltage, further optimization of the AC electrode (e.g., engineering surface functional groups) is still required.

Li2SO4 illustrated in Figure S7a, it is obvious that LiTFSI-based electrolytes show higher stability against water decomposition, implying the superior capability for a high-voltage cell. In addition, the exponential increase in current at the elevated cell voltage was more suppressed in the water-in-salt electrolyte (5 M LiTFSI). In order to monitor the real potentials of the positive and negative electrodes on applying high voltage in 5 M LiTFSI, the potential of each electrode was separately determined by incorporating a Ag/AgCl reference electrode, which is plotted in Figure 3b. In analogy with the cell in 1 M Li2SO4 (Figure S7b and also reported previously26−28), the maximum voltage of the system in 5 M LiTFSI tends to be dominated by the positive electrode; the potential of the positive electrode reached beyond the practical potential limit at a voltage of ∼2.1 V, while that of the negative side remained higher than the negative limit even at 2.6 V. Béguin and co-workers have reported that the charge/ discharge cycling with high maximum voltage where the potential of the positive electrode slightly exceeds the limit causes a slight capacitance decay during the first thousands cycle due to the surface oxidation of AC, which in turn pushes the maximum potential downward resulting in the subsequent stable cycling.28 In this study, we have investigated the symmetric AC/AC supercapacitor in 5 M LiTFSI with a maximum voltage of 2.4 V. The capacitance values evaluated from the galvanostatic charge/discharge results (Figure S8) were 143 F g−1 at 2 A g−1 and 104 F g−1 at 10 A g−1. The longterm cycling in Figure 3c reveals that the operation with a maximal cell voltage of 2.4 V did not considerably deteriorate the AC electrode though the slight capacitance fade was observed after the sharp drop in the first several hundred cycles. The capacitance retention after 10 000 cycles was ca. 81%. The correlation between specific power and energy of the symmetric EDLCs configured with different aqueous electrolytes is shown in Figure 3d. The Ragone plots verify that the symmetric capacitor in 5 M LiTFSI operated at 2.4 V delivered the highest overall performance of 24 Wh kg−1 at 0.48 kW kg−1 and 10 Wh kg−1 at 7.6 kW kg−1 based on the mass of both AC monolithic electrodes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01261. SEM images of the RF gels with different starting compositions; nitrogen sorption isotherms and mesopore size distributions of the calcined samples with different post-treatments after gelation; mechanical properties of the carbon monolith; SEM image, nitrogen sorption isotherms, pore size distributions, and summary of pore properties of the AC monolith; CV curves of the AC monolithic electrode in various electrolytes; Nyquist plots in 5 M LiTFSI before and after forming decomposition products; CV curves and change of electrode potentials for the AC/AC symmetric capacitor in 1 M Li2SO4; and charge/discharge curves of the AC/ AC symmetric capacitor in 5 M LiTFSI (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(G.H.) E-mail: [email protected]. Tel.: +81 6 6879 8452. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS In summary, macroporous RF scaffolds comprising cylindrical mesopores with 2D hexagonal periodicity have been successfully synthesized via a facile one-pot sol−gel process accompanied by the cooperative self-assembly of amphiphilic surfactant and RF polymer. Carbonization and subsequent thermal activation provide hierarchically porous carbon monoliths and AC monoliths with the multimodal pore system. Carbon monoliths with ordered mesopores integrated into homogeneously accessible macropore frameworks will be exploited in a broad range of application fields such as catalysis and energy storage. This study exemplifies the application to high-voltage aqueous EDLCs. In particular, the capability of water-in-salt electrolytes has been explored by employing the AC monolith as a binder-free monolithic electrode. The superconcentrated (5 M) LiTFSI electrolyte offers highly broadened stability potential window of ∼2.5 V on the AC electrode with high surface area. The onset potential for H2 evolution on the AC electrode was comparable with that on a stainless steel, whereas the increase of the onset potential for water oxidation on the AC was found to be limited around 1.2 V (vs Ag/AgCl). Despite the formation of a passivation layer, the AC electrode

ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (Grant Number 26810123 for G.H.).

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