Kroll-carbons based on silica and alumina templates as high-rate electrode materials in electrochemical double-layer capacitors †

Inorganic Chemistry, TU Dresden, Bergs E-mail: stefan.kaskel@chemie.tu-dresden.de Department of Materials Science and Engi Atlanta, GA-30332, USA. E-mail: yushin@ga Physical Chemistry, TU Dresden, Bergstrass Institute for Chemical and Bioengineering, CH-8093 Zurich, Switzerland † Electronic supplementary information material characterization and electrochem Cite this: J. Mater. Chem. A, 2014, 2, 5131


Introduction
Porous carbon materials are of outstanding importance in various energy-and environmentally relevant elds such as gas adsorption, 1,2 catalysis, [3][4][5] or water treatment 6-8 because they combine large specic surface area with high chemical and thermal stability. Additionally, their high electrical conductivity makes them crucial components in various electrochemical energy storage devices. [9][10][11][12][13][14][15][16] Among them, electrochemical double-layer capacitors (EDLCs) stand out due to their power densities, cycle life, and charge-discharge times which are unachievable by lithium-based batteries due to the absence of time consuming redox reactions. 17 In EDLCs, the charge accumulation is of purely physical nature based on the formation of an electrochemical Helmholtz double-layer on the surface of the usually carbonaceous electrode material. However, fast transport and rapid access of the electrolyte ions to the electrode surface area are required to achieve high power densities, especially when electrolyte systems with low ionic mobility (e.g. ionic liquids) are used. 18 This can be ensured by lowering the carbon particle size down to the nanometer range 19,20 or by the introduction of internal transport pores. 18,21 While ion adsorption in narrow micropores results in high capacities, [22][23][24] mesopore channels can serve as ion highways allowing for enhanced ion diffusion kinetics within the electrode. [25][26][27][28][29] In the current literature, many different routes for the synthesis of high-capacity microporous carbon materials are reported. Chemical or physical activation of carbonized natural or synthetic precursors produces carbon materials with moderate specic capacitances of usually 100-120 F g À1 . The presence of bottle neck pores or torturous pores in the range of 0.3-4 nm 30,31 may slow down the ion transport within large activated carbon (AC) particles and therefore limits their power characteristics. 18,21,32 However, advanced synthesis strategies for ACs with impressively high specic capacities have been reported [33][34][35][36] and they will remain one of the most promising electrode materials for EDLCs due to their well-known manufacturing techniques, comparably low cost, and easy production of large quantities. Zeolite-templated carbons (ZTCs) display another interesting class of materials because they consist of a very uniform micropore system and can be designed in a well-aligned structure. [37][38][39][40] Alternatively, carbidederived carbons (CDCs) can be prepared by the selective etching of metal or semi-metal atoms from carbide precursors. [41][42][43] With a few exceptions, [44][45][46] the micropore size is narrow (usually #1 nm) and precisely controllable by the carbon distribution within the carbide precursor and the synthesis temperature. 41,42 CDCs offer highly uniform pores with advanced accessibility due to the absence of bottle-necks. Microporous CDCs show specic capacitances up to 160 F g À1 but the ion transport is relatively slow in the narrow micropores and therefore they suffer from a moderate rate of charge-discharge. 47 While microporous carbons can store large amounts of ions leading to high volumetric and gravimetric capacitances, purely mesoporous materials such as carbon onions 28 or carbon nanotubes 11 are favorable for applications where ultrafast charge-discharge is required. But electrodes produced from such mesoporous carbon nanomaterials suffer from a comparably low specic surface area limiting the achievable energy density. Therefore, many mesoporous materials have been equipped with additional micropores to increase the ion storage capacity. In most cases, templated mesoporous carbons undergo a post-synthesis activation procedure 48 or metal extraction for the insertion of micropores when the CDC process is applied to mesoporous carbide precursors. 18,21,42,49 However, the mesopore generation in such hierarchically structured materials by very complex templating approaches oen requires the use of extremely toxic hydrouoric acid solution for efficient template removal or surfactant-assisted structure evolution making commercialization more difficult. In contrast to such complex templating approaches, physical and chemical activation do not allow precise control of mesopore sizes and therefore an ultimate process for the generation of carbon materials with well-dened mesopores is still not available.
Recently, we have reported the reductive carbochlorination (which is the key step of the Kroll-process for the production of elemental titanium) of TiO 2 nanoparticle templates in a dense carbon matrix for the generation of highly mesoporous carbon materials (designated as Kroll-carbons, KCs). This new synthesis route allows precise control over the mesopore size and simultaneous formation of micropores in analogy to physical activation procedures leads to KCs with specic surface areas up to 2000 m 2 g À1 coupled with total pore volumes as high as 3.1 cm 3 g À1 . The highly useful byproduct TiCl 4 can be removed by distillation leading to a scalable production scheme for hierarchically structured carbons with high performance in lithium-sulfur battery cathodes. 50 Because this method is highly versatile for producing porous carbon materials with different structures, it is of interest to extend it to a large number of different oxidic templates.
We present in the following that carbochlorination using commercially available fumed silica and alumina templates is a versatile alternative for the production of porous carbons at lowcost. Inltration with sucrose followed by carbonization and subsequent high-temperature chlorination leads to the formation of KCs with specic surface areas close to 2000 m 2 g À1 and total pore volumes as high as 3.2 cm 3 g À1 with contributions from both the micropore-and mesopore regions. Due to the formation of additional micropores during template removal by reductive carbochlorination, the specic surface areas of the carbons signicantly exceed the values achievable by classical hard-templating approaches using hydrouoric acid for the removal of template particles of comparable dimensions. 51 Despite the fact that chlorination at high temperatures is a dangerous and toxic process, the reductive carbochlorination is highly efficient because the corresponding metal chlorides can be obtained as very useful byproducts. SiCl 4 could be directly reused for the SiO 2 template synthesis by ame spray pyrolysis. Mesopore sizes are precisely controllable by the template particle dimensions while the applied synthesis temperature only has minor inuence on the properties of the resulting materials. KCs are tested as electrode materials in EDLCs based on aqueous and ionic liquid (IL) electrolytes. Due to the presence of the hierarchical pore structure, they combine high capacities with outstanding rate capability rendering them as advanced materials for EDLCs. Especially in the IL electrolyte, KCs show high stability and a wide operating window leading to advanced energy density, rendering reductive carbochlorination as a promising route for large-scale production of hierarchically structured carbon for use in high-power EDLCs.

Kroll-carbon synthesis
For the synthesis of Kroll-carbons based on Al 2 O 3 templates, 2.54 g of commercially available alumina nanoparticles (Aeroxide Alu 130; Degussa-Evonik, Germany; specic surface area of 130 AE 20 m 2 g À1 ) were dispersed in a solution of 5.0 g sucrose in 80 ml deionized water followed by the addition of 8 droplets ($160 mg) of concentrated sulfuric acid. For KCs based on SiO 2 templates, 2.0 g of commercially available silica nanoparticles (Aerosil 380 or Aerosil 90; Degussa-Evonik, Germany; specic surface areas of 380 AE 30 m 2 g À1 for Aerosil 380 and 90 AE 15 m 2 g À1 for Aerosil 90) were dispersed in a solution of 4.75 g sucrose in 10 ml deionized water followed by the addition of 2 droplets ($40 mg) of concentrated sulfuric acid. The mixture was transferred to a Petri dish and treated for 3 h at 100 C and for another 3 h at 160 C in air atmosphere. The resulting black composite was then carbonized under an argon ow of 150 ml min À1 at 900 or 1000 C for 1 h (heating rate 300 K h À1 ). Subsequently, the gas ow was changed to a mixture of 80 ml min À1 chlorine and 70 ml min À1 argon while the temperature was maintained for another 2 h. The resulting Kroll-carbon was cooled down to 600 C under 150 ml min À1 of owing argon and maintained at that temperature for 1 h followed by changing the gas ow to 80 ml min À1 hydrogen for another 1 h. Samples were then cooled down to room temperature under owing argon.

Material characterization
Nitrogen physisorption experiments were carried out at À196 C on an Autosorb 1C instrument (Quantachrome Instruments). Specic surface areas (SSA BET ) were calculated using the multipoint BET equation (p/p 0 ¼ 0.05-0.2). Total pore volumes (PV Total ) were determined at p/p 0 ¼ 0.99. Pore size distributions (PSDs) were calculated using the quenched-solid density functional theory (QSDFT) method (adsorption branch kernel) for nitrogen adsorbed on carbon with a slit/cylindrical/spherical pore shape at À196 C. Specic surface area values (SSA DFT ) and total pore volumes (PV DFT ) were also obtained from this kernel. Micropore volumes (PV Micro ) correspond to the cumulative pore volumes at a diameter of 2 nm. Furthermore, the t-plot method (p/p 0 ¼ 0.3-0.4) was used for the calculation of the micropore volume (PV t-plot ).
Transmission electron microscopy (TEM) investigations were carried out using a Tecnai G 2 F30 (FEI, Netherlands) operating at an accelerating voltage of 300 kV. Scanning electron microscopy (SEM) measurements of gold sputtered samples were performed using a DSM982 (Zeiss). The same instrument was used for energy-dispersive X-ray spectroscopy (EDS) analyses. Elemental analyses are an average value of 3-5 measurements at a magnication of 5000. Raman spectra were obtained on a Renishaw RM-2000 Raman microscope using a 532 nm laser (Gem532, Laser Quantum) as the excitation source. Thermal analyses were performed using a STA 409 PC LUXX (Netzsch) with a heating rate of 5 K min À1 under oxidizing conditions (synthetic air).

Electrode preparation and EDLC device assembly
The KCs were ground to ne powders in a mortar and $100 mg were suspended in ethanol under mild ultra-sonication. A suspension of a polytetrauoroethylene binder (PTFE, 60 wt% in water, Sigma Aldrich) was added and the resulting slurry of 5 wt% PTFE and 95 wt% of carbon was concentrated by slow evaporation of ethanol at 80 C under constant stirring. The highly viscous mixture was then dried on a glass plate and mixed with a spatula or razor blades. When the mass became dry with a rubberlike (clay) consistency, it was rolled to a thickness of $150 mm between aluminium foil sheets using a commercial roll mill. The resulting composites were dried overnight at 80 C under vacuum.
For the measurements in the aqueous electrolyte (1 M H 2 SO 4 solution), electrodes of $1 cm 2 (2-3 mg active material) were cut out and the device assembly took place under air atmosphere. A high-purity gold foil (Sigma Aldrich, USA) was used as the current collector and a commercially available Dreamweaver Silver separator (Dreamweaver International, Inc., USA) was placed between the electrodes. The sandwich was assembled in a beaker-type cell conguration and held together using Teon slabs and screws. Sufficient wetting of the porous carbon electrodes with sulfuric acid was ensured by adding an excess of electrolyte solution to the beaker followed by a treatment under vacuum at RT for 1 h.
For the measurements in 1-ethyl-3-methylimidazolium tet-rauoroborate (EMIBF 4 , >98%, IoLiTec Ionic Liquids Technologies GmbH, Germany) ionic liquid electrolytes the devices were assembled in a stainless steel coin cell conguration in an argon lled glovebox. Carbon coated aluminium foil was used as the current collector and the above mentioned Dreamweaver product as the separator. 4-5 droplets of the electrolyte were used for the wetting of the electrodes and the separator and the excess amount was removed during compression of the coin cell. For electrochemical measurements, aluminium contacts were xed to the coin cells with plastic clamps.

EDLC measurements
Electrochemical characterization of the KC-based EDLC devices was carried out using cyclic voltammetry (CV), galvanostatic charge-discharge (C-D), and electrochemical impedance spectroscopy (EIS) experiments. CV measurements were performed on a Solartron 1480A (AMETEK Advanced Measurement Technology) in a potential range from À0.6 V to +0.6 V (aqueous electrolyte) or À2.0 V to 2.0 V (ionic liquid electrolyte) at scan rates of 1-1000 mV s À1 . The gravimetric capacitance of each electrode at different scan rates was calculated from the CV data according to: where dU/dt is the scan rate, m is the mass of the active material in a single electrode, and I(U) is the total current. C-D experiments from À0.6 V to +0.6 V at charge/discharge current densities from 0.1-20 A g À1 (based on the mass of a single electrode) were carried out with an Arbin SCTS supercapacitor testing system (Arbin Instruments). The specic capacitance was calculated according to: where I is the total current, dU/dt is the slope of the discharge curve, and m is the mass of active material in a single electrode. EIS measurements were performed on a Gamry Potentiostat from 100 kHz-1 mHz with a 10 mV alternating current (AC) amplitude.

Structure and porosity
KCs were obtained by inltration of commercially available pyrogenic alumina or silica template particles with an aqueous solution of sucrose as the carbon precursor followed by carbonization under an inert atmosphere (ESI, Fig. S1 †). Subsequent template removal was performed by a reductive carbochlorination reaction under a hot chlorine atmosphere leading to the formation of volatile metal chloride species and carbon monoxide according to: At a chlorination temperature of 900 C, Aerosil Alu 130 as well as Aerosil 380 and 90 template particles are quantitatively removed from the oxide/carbon composites leading to the formation of highly pure Kroll-carbons. The carbon content of the samples measured with EDS is about 99 atom% (Table 1). Potential impurities of Si, Al, O and Cl are below the detection limit. Accordingly, thermal analyses of the KCs under oxidative conditions (air atmosphere) up to 1000 C show complete combustion of the carbon materials (ESI, Fig. S2 †). Due to the complete template removal, KCs offer a highly open and accessible mesopore structure as shown by TEM investigations (Fig. 1). The mesopore sizes increase from values signicantly below 30 nm in the KCs derived from Alu 130 and Aerosil 380 to diameters above 40 nm for the Aerosil 90 particles rendering the template removal by the reductive carbochlorination reaction as being highly conformal allowing precise control over the pore sizes of the resulting KCs.
One of the most important properties of porous carbon materials for use in EDLCs is the carbon microstructure because it strongly inuences the electrical conductivity, interactions with the electrolyte 27,52 and double-layer formation. 53 TEM images at higher magnications show the carbon microstructure as an arrangement of mostly disordered sp 2 carbon fringes with a low degree of graphitization as it is typical for carbon materials prepared at comparable temperatures (Fig. 1B). Raman spectroscopy (Fig. 2) is another useful tool for the investigation of the carbon microstructure at a larger scale. 54 In accordance with the TEM investigations, the spectra indicate a highly disordered carbon structure due to the presence of the characteristic D band at $1350 cm À1 . The shoulder D2 band present in all samples and the absence of sharp peaks in the range from 2400 cm À1 -3000 cm À1 also indicate a low graphitization in the KCs at the elevated synthesis temperature. However, the higher I D /I G ratio (Table 1) of the Alu 130-derived Kroll-carbon as well as the lower full width at half maximum (FWHM) of its D band (135 cm À1 ) compared to the KCs obtained from Aerosil 380 (136 cm À1 ) and Aerosil 90 (141 cm À1 ) templates indicates a slightly higher amount of graphitic ribbon structures in this sample.
The pore structure of the hierarchically structured KCs was investigated with nitrogen physisorption experiments at À196 C ( Fig. 3 and Table 1). High specic surface areas of 1867 m 2 g À1 and total pore volumes of more than 3 cm 3 g À1 are obtained by using alumina and silica templates, respectively. Because the adsorption isotherms in the case of the KCs derived from silica templates do not reach saturation at p/p 0 ¼ 0.99 these values must be regarded as minimum total pore volumes and the actual values might be even higher. The distinct uptake of nitrogen at low relative pressures (p/p 0 < 0.1) is associated with the lling of narrow micropores which are generated by the CO evolution during carbochlorination in analogy to physical   activation procedures and previously reported Kroll-carbons based on TiO 2 templates. QSDFT-PSDs (Fig. 4) show the presence of 1 nm sized micropores and small volumes of narrow mesopores of 2-6 nm in size depending on the template. The latter are a result of the presence of empty spaces between the template nanoparticles due to incomplete lling of their voids with precursor molecules during inltration as already reported for TiO 2 -derived KCs. 50 Due to the higher molar ratio of carbon etching and therefore the preferred introduction of micropores, the highest specic surface areas and micropore volumes are obtained for KCs from the alumina template (Table 1) being responsible for the higher amount of adsorbed gas in the low pressure region. The 1 nm sized micropores are large enough for relatively fast and effective electrosorption of ions and small enough to serve for high specic and volumetric surface area available for double layer formation. According to prior reports, ion solvation shells become distorted in such small pores and can even be partially removed, thus increasing the capacitance due to smaller charge separation distance between the ion centres and the pore walls. 22 All materials show a very distinct and narrow hysteresis loop at high relative pressures (p/p 0 > 0.7) indicating the presence of high volumes of uniformly sized mesopores. According to the template particle sizes, the largest mesopores are obtained from the Aerosil 90 templates indicated by the pore lling at the highest relative pressures while Aerosil 380 and Alu 130 particles result in smaller pore diameters. No blocking or cavitation effects are observed due to the highly accessible mesopore systems present in the KCs. QSDFT-PSDs in the large mesopore region show template-induced mesopores centered at 18 nm and 23 nm for the Alu 130 and Aerosil 380 templates, respectively. The majority of mesopores present in KCs derived from Aerosil 90 are too large to be analysed by the QSDFT model. The distinct mesopore systems of the KCs ensure a high accessibility of the surface area throughout the entire particle. The additional microporosity is mainly responsible for the high specic surface area, which is not achievable by classical hardtemplating approaches of oxidic template particles of comparable size. 51 SEM micrographs of the silica-based KCs (Fig. 5) show the additional presence of macropores already present in the templates and which are not inltrated with the precursor. They are responsible for the additional gas uptake in the nitrogen physisorption at high relative pressures and can serve as additional ion transport pathways and allow rapid access of the entire particle. As already indicated by the saturation of the nitrogen physisorption isotherm, such pores are not present in the alumina-based KC due to a rather dense particle structure and the absence of large meso-or macropores (Fig. S3 †). In the KCs derived from Aerosil 90 particles, the large mesopores are clearly observed and their size of 40-70 nm is signicantly higher as for the Aerosil 380-based sample in accordance with the nitrogen physisorption experiments again proving the precise control over the pore sizes provided by the reductive carbochlorination reaction.
We have also investigated the inuence of the synthesis temperature on the properties of the resulting KCs. An increase of the pyrolysis/chlorination temperature to 1000 C only slightly affects the properties independent of the used template particles (ESI, Fig. S4, S5 and Table S1 †). The I D /I G ratios are in   This journal is © The Royal Society of Chemistry 2014 the same range as for the materials prepared at 900 C indicating a comparable degree of graphitization. The hightemperature samples exhibit a similar pore structure as shown by the nitrogen physisorption measurements and SEM images. EDS measurements reveal complete template removal at 1000 C as well. From an ecological and economical viewpoint, the synthesis at 900 C is benecial because the advantages of the reductive carbochlorination route for the production of well-dened mesoporous carbon materials are fully achievable even at the lower temperature.

EDLC performance
The KCs derived from Alu 130 and Aerosil 90 templates (corresponding to the smallest and largest mesopores) were characterized as electrode materials in EDLCs (Fig. S6 †) based on aqueous (1 M H 2 SO 4 ) and ionic liquid (EMIBF 4 ) electrolytes (Table S2 †). While aqueous electrolytes offer the advantages of high ionic conductivity and low cost, they suffer from narrow stability (usually they operate below 1 V due to the decomposition of water) limiting the achievable energy density. 12 Ionic liquids exhibit large thermal-and higher voltage stability and therefore higher energy density but the mobility of ions within this electrolyte system is signicantly lower leading to slow charge/discharge limiting the achievable power density within this electrolyte system especially in the absence of transport pores. The ionic liquid used in this study, EMIBF 4 , has recently shown to be a promising ionic liquid electrolyte with a relatively high ionic conductivity (15.7 mS cm À1 ) and low viscosity (36.2 cP) at room temperature. 18,35 Aqueous electrolyte The CV curves of the KCs in a 1 M aqueous H 2 SO 4 electrolyte are of rectangular shape within the applied potential range, indicating the absence of a large number of surface functional groups and the high purity of the materials ( Fig. 6 and S7 †). Capacitances as high as 135 F g À1 are obtained at 1 mV s À1 for the alumina-derived KCs due to their higher specic surface areas and micropore volumes compared to the silica-derived analogues showing a 26% lower value. While these are high values, our initial expectations based on the well-developed porosity ( Fig. 3 and 4) were slightly higher. We hypothesize that the hydrophobic nature of the carbon surface may have prevented access of aqueous electrolytes to some of the smallest pores, an argument which in microporous carbons has been recently supported by small angle neutron scattering (SANS) analysis. 55 Moreover, the adsorption potential of ions within the large volumes of mesopores is lower compared to strictly microporous materials. 22 This also explains the moderate capacitance per surface area of the KCs in this electrolyte system (7.2 mF cm À2 for KCs prepared from Alu 130 templates and 6.6 mF cm À2 for KCs prepared from Aerosil 90 templates).
The nearly rectangular shape of the CV curves at high sweep rates indicates very rapid ion diffusion in the materials due to their distinctive mesopore system. Specic capacitances up to 90 F g À1 can be utilized at a scan rate as high as 500 mV s À1 , signicantly exceeding the values of purely microporous carbon materials with curved pores that usually exhibit signicant capacitance fading at higher scan rates. 18 Only zeolite-templated strictly microporous carbons with straight pores offer comparable capacitance retention. 39 The values determined by CV measurements are in good agreement with galvanostatic charge-discharge experiments ( Fig. 6C and S8 †) showing specic capacitances up to 134 F g À1 and 101 F g À1 for the KCs derived from Alu 130 and Aerosil 90 templates, respectively (at 0.1 A g À1 ). C-D measurements are most accurate to determine the performance of EDLCs in real applications. In principle, an (ideal) EDLC must deliver the same amount of energy independent of the current density. However, in practical experiments a more or less distinct decrease of the available capacitance is observed due to the resistance of ions during transportation through the pore channels and their interaction with carbon functional groups and defects. C-D investigations independently prove the outstanding electron transport properties and therefore impressive high-power characteristics of KCs. 88% and 85% of the initial capacitance (at 0.1 A g À1 ) can be retained at high current densities of 20 A g À1 . Such high capacitance retentions are not achievable with most commercial activated carbons optimized for EDLC devices. The commercial activated carbons, while showing comparable specic surface area usually retain only 50% of their initial capacitance at high current densities in the same electrolyte system. 18 Nyquist plots of the KCs show a nearly vertical line at low frequencies, where the contribution becomes almost exclusively capacitive (Fig. 7, inset). The EIS measurements allow for estimation of the capacitance changes with the operating frequency (Fig. 7). At low frequencies, the capacitance shows saturation in both materials indicating that they reach the equilibrium in ion adsorption. The maximum operating frequency (f max ) is approximately the frequency at which the capacitance decreases by not more than 50%. The Aerosil 90derived KCs can operate at higher frequency (>1 Hz) compared to the Alu 130-based sample (>0.2 Hz) due to its larger mesopores leading to enhanced ion diffusion. The equivalent series resistance (ESR) is a very important characteristic of an EDLC device including the electrical resistance of the electrodes and the current collector interfaces as well as the portion of the ionic resistance related to the ion transport outside of the carbon pore channels. It can be determined at very high frequencies when the imaginary component of the complex impedance becomes zero (intersection of the Nyquist plot with the x-axis). We see that the microstructure of Aerosil 90-derived KC architecture allows smaller equivalent series resistance to be attained, likely originating from the smaller current collector/ electrode contact resistance. The f max of the silica-based sample is comparable to previously reported hierarchical materials and signicantly surpassing the performance of activated carbons in the same electrolyte system. 18

Ionic liquid electrolyte
The CV curves of the KC-based electrodes in the IL electrolyte show the presence of pseudocapacitance contributions at $1.7 V in the symmetrical EDLC ( Fig. 8 and Fig. S9 †). In accordance with prior work, these peaks may either result from impurities present in the EMIBF 4 or from reactions of the IL with functional groups on the carbon surface. 18,35 However, those Faradaic processes do not negatively affect the cycle stability of the EDLC showing complete retention of the initial capacitance aer 10 000 galvanostatic charge/discharge cycles (À2.0 V to +2.0 V, ESI, Fig. S10 †). High specic capacitance values of 141 F g À1 (7.6 mF cm À2 related to KC surface area) for KCs prepared from Alu 130 templates and 124 F g À1 (8.1 mF cm À2 ) for KCs prepared from Aerosil 90 templates are determined from the CV measurements at low scan rates. When the rate increases up to 100 or even 500 mV s À1 , high specic capacitances can be retained and the CV curves exhibit a nearly rectangular shape as it is typical for pure EDLCs with low electrolyte diffusion limitations. At scan rates of 50 mV s À1 and higher, redox reactions are too slow to contribute to the capacitance leading to the disappearance of the redox peaks.
We have additionally investigated the effect of an extension of the CV voltage range to À2.5 V to +2.5 V for the Alu 130derived hierarchical KC (Fig. S11 †). The specic capacitance reaches even higher values of 174 F g À1 because the observed surface reactions now fully contribute to the capacitance values. However, at higher scan rates the CV still becomes rectangular indicating a reasonable stability. In spite of redox contributions present, it is known that the EMIBF 4 IL can operate in a wide voltage window leading to high energy density. 35,56 C-D measurements (À2.0 V to +2.0 V) at a current density of 0.1 A g À1 and 20 C conrm the values observed in CV investigations showing high specic capacitances of 135 F g À1 and 124 F g À1 for KCs from Alu 130 and Aerosil 90 templates, respectively ( Fig. 8C and S12 †). As for the 1 M H 2 SO 4 electrolyte, remarkable capacitance retentions of 75% are determined at ultrahigh current densities of 20 A g À1 . The relative capacitance retentions of KCs signicantly surpass those observed for highperformance polypyrrole-derived activated carbons, 35 and other tuned mesoporous materials. Moreover, we have also performed C-D measurements with the Alu 130 KC-based EDLC at a high temperature of 70 C (Fig. 8C and S13 †). A $10% increase of the specic capacitance is observed over the entire current density range due to the reduction of the viscosity of the electrolyte and an increase of its ionic conductivity. Compared to  previously reported high-performance activated carbons showing a $20% increase of the capacitance even at 60 C, 35 the enhancement of the KC performance is relatively moderate indicating their outstanding electron transport performance even at room temperature.
In contrast to the aqueous electrolyte system, EIS measurements of the IL-based EDLCs show the presence of the typical 45 segment due to the resistance of ions during diffusion into the bulk of electrode particles (Fig. 9, inset). The ESRs of the KCs are relatively equal due to comparable particle sizes (Fig. S6 †) and their high purity and similar surface chemistry.
Regarding the values of f max , both of the KCs show promising behaviour in the IL electrolyte system (Fig. 9). They can operate at frequencies as high as 0.15 Hz (KC from Alu 130) and 0.1 Hz (KC from Aerosil 90) being comparable with the high values of ordered mesoporous CDCs 18 and signicantly exceeding those of many activated carbons 16,57 even in cases where small mesopores are present. 35

Conclusions
When tested for a new templating concept, silica and alumina nanoparticles conned within dense sucrose-derived carbon shells were found to be sufficiently reactive to be completely leached out during carbochlorination. As a result, the use of such particles as templates allow for the synthesis of micro-and mesoporous carbon materials with tunable mesopore volume. In our experiments, the resulting Kroll-carbons offer specic surface areas close to 2000 m 2 g À1 and total pore volumes exceeding 3 cm 3 g À1 . The synthesis process is highly versatile for tailoring the pore morphology and diameter in a wide range by using various template particles. As the rst example of a potential application, KCs were tested as electrode materials in two electrode EDLCs and show high specic capacitance values of 135 F g À1 in an aqueous electrolyte (1 M sulfuric acid) and 174 F g À1 in ionic liquid (1-ethyl-3-methylimidazolium tetra-uoroborate). High capacitance retentions (90%) at high current densities can be achieved as a result of the hierarchical pore structure. Due to their efficient synthesis route as well as their promising electrochemical properties, KCs offer great potential for large-scale material production for high-power EDLCs (Fig. S14 †). Moreover, they are hopeful candidates for other applications where mesoporous carbon materials are required (e.g. blood ltration, 45,58 Li-S battery cathodes 11,42,50,59,60 ) or for academic research.