From Chromonic Self-Assembly to Hollow Carbon Nanofibers: Efficient

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From Chromonic Self-Assembly to Hollow Carbon Nanofibers for Efficient Materials in Supercapacitor and Vapor Sensing Applications J. Rodrigo Magana, Yury V. Kolen'ko, Francis Leonard Deepak, Conxita Solans, Rekha Goswami Shrestha, Jonathan P Hill, Katsuhiko Ariga, Lok Kumar Shrestha, and Carlos Rodriguez-Abreu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09819 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

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ACS Applied Materials & Interfaces

From Chromonic Self-Assembly to Hollow Carbon Nanofibers for Efficient Materials in Supercapacitor and Vapor Sensing Applications J. Rodrigo Magana,† Yury V. Kolen’ko,‡ Francis Leonard Deepak,‡ Conxita Solans,*,† Rekha Goswami Shrestha,§ Jonathan P. Hill,§ Katsuhiko Ariga,§ Lok Kumar Shrestha,*,§ and Carlos Rodriguez-Abreu*,‡ †Instituto de Química Avanzada de Cataluña, Consejo Superior de Investigaciones Científicas (IQAC-CSIC) and CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Jordi Girona 18-26, 08034 Barcelona, Spain. ‡International Iberian Nanotechnology Laboratory, Av. Mestre José Veiga, 4715-330 Braga, Portugal. §World Premier International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Ibaraki, Tsukuba 305-0044, Japan. Keywords. Chromonics, Silica nanofibers, Carbon Nanofibers, Supercapacitors, Vapor sensing.

ABSTRACT: Carbon nanofibers (CNFs) with high surface area (820 m2/g) have been successfully prepared by a nanocasting approach using silica nanofibers obtained from chromonic liquid crystals as a template. CNFs with randomly oriented graphitic layers show outstanding electrochemical supercapacitance performance exhibiting a specific capacitance of 327 F/g at a scan rate of 5 mV/s with long life cycling capability. Approximately 95% capacitance retention is observed after 1000 charge-discharge cycles. Furthermore, about 80% of capacitance is retained at higher scan rates (up to 500 mV/s) and current densities (from 1 to 10 A/g). The high capacitance of CNFs comes from their porous structure, high pore volume and electrolyte-accessible high surface area. CNFs with ordered graphitic layers were also obtained upon heat treatment at high temperatures (> 1500 °C). Although it is expected that these graphitic CNFs have increased electrical conductivity, in the present case they exhibited lower capacitance values due to a loss in surface area during thermal treatment. High surface area CNFs can be used in sensing applications; in particular, they showed selective differential adsorption of volatile organic compounds such as pyridine and toluene. This behavior is attributed to free diffusion of these volatile aromatic molecules into the pores of CNFs accompanied by interactions with sp2 carbon structures and other chemical groups on the surface of the fibers.

1. INTRODUCTION Carbon based materials are of paramount importance in several technological applications, for which the design of pore structures and the optimization of surface areas and morphologies have proven to be key factors in improving their performance. Production of carbon nanostructures containing size-controlled and well-organized mesopores is of great importance in the fields of separation/adsorption, energy storage, sensing, catalysis and chromatography.1-5 Recently, extensive efforts have been given to fabricate mesoporous carbons templated using ordered mesoporous inorganic solids.6,7 The templating method is a powerful approach to fabricate mesoporous carbon materials with tunable pore sizes and large specific surface areas. Silica (SiO2) provides an ideal template for carbon since its pore distribution and surface area can be finely tuned.8-13 High surface area micro- and mesoporous carbons with controlled pore sizes can be obtained by embedding and carbonizing precursors in the silica nanochannels followed by removal of the silica by chemical etching. However, particle morphology is difficult to control using this approach since the most commonly used mesoporous silica templates do not usually have a well-defined particle shape. Recent studies have shown that by using positively charged chromonic molecules as templates for silica growth, morphology can also be controlled. Nanostructured mesoporous silica fibers can be obtained with nanochannels growing along the fibers’ long axis.14,15

Successful templating of these nanostructured fibers would produce high aspect ratio carbon nano-threads that could be of interest for different applications. Moreover, this is a one-pot, facile synthesis method quite suitable for scaling up. Energy storage is one of the most promising applications of carbon materials.2,3 The development of efficient supercapacitors (such as electrical double-layer capacitors; EDLCs) with high power density and long cycle life is essential to address the growing demand for portable electronic devices. Recent studies have shown that porous activated carbons,16-19 mesoporous carbon,20-24 carbon nanotubes,25-27 graphene,28-30 etc. are potential candidates for EDLCs. Compared to conventional batteries, EDLCs exhibit higher power densities (up to 10 kW/kg), short recharge times (in the order of milliseconds), long cycle lives (>10,000 cycles) and an environmentally friendly synthesis (no use of heavy metals). However, the energy density of EDLCs is rather low so that the improvement of this parameter is one of the main challenges of supercapacitor technology.31-36 Energy storage in EDLCs occurs due to the adsorption of electrolyte ions on the surface of the electrode material upon application of a potential difference between two electrodes. It is obvious that the quantity of energy stored should depend on the electrochemically accessible surface area and the amount of electrolyte that can be stored (total pore volume), although the actual mechanism of energy retention on porous surfaces is still a matter of discussion. In many cases, there is no clear correlation between surface area or pore volume and the value

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of capacitance obtained. This has been attributed to the poor accessibility of the electrolyte to the pores. A method to increase the energy density on EDLCs, and thus, to increase the capacitance, is by increasing the conductivity of the material, i.e., by graphitizing the carbon at high temperatures.27 This allows the active carbon material to easily transport electrons from the source into the solution. Nanoporous graphitic carbon has also received considerable attention for sensor technology particularly for sensing of volatile organic compounds (VOCs).37,38 Sensing toxic organic substances using mesoporous materials has also been the subject of intensive research.4 In sensing technology, accurate and dedicated sensors providing precise process control in manufacturing processes, industrial safety, security, environmental pollution monitoring, diagnosis of diseases, etc. are in high demand and thus has led to an acceleration in the development of excellent sensing materials for sensor technology.39-42 Mainstream research is focused to the development of outstanding sensing materials with newer structures or morphologies so that the overall sensitivity, selectivity and stability of the sensors can be improved.4,5,43-48 Again, it is known that a well-designed material architecture and functionalization can lead to high sensing performance. In this contribution we report the fabrication of carbon nanofibers (CNFs) using soft and hard nanotemplates, i.e., chromonic liquid crystals and their derived silica nanofibers (Scheme 1). CNFs having randomly oriented graphitic layers showed excellent super-capacitive electrochemical performance (specific capacitance was ca. 327 F/g at a scan rate of 5 mV/s) together with long life cycle. Approximately 95% of the capacitance value was retained after 1000 cycles demonstrating the potential of CNF materials as electrode materials in advanced supercapacitors. Furthermore, due to their porous structure with large pore volume and welldeveloped sp2 carbon structure, CNFs exhibited excellent vapor sensing properties selective towards aromatic solvent vapors such as benzene and pyridine. The selectivity of sensing among the solvent vapors studied decreased in the following order: pyridine > toluene> benzene > cyclohexane > hexane. The free diffusion of aromatic solvent vapors is promoted due to strong π-π interactions between solvent molecules and the sp2-bonded graphitic nanoporous carbon framework structure.

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Scheme 1. Schematic mechanism for SiO2NF and CNF-PiC synthesis: (a) Pinacyanol (PiC) aggregates (nematic liquid crystal), (b) SiO2/chromonic complex (hexagonal liquid crystal), (c) SiO2 nanofiber (SiO2NF), (d) Carbon nanofiber (CNF-PiC), (e) POM image of 3.5 wt% PiC in concentrated ammonia, (f) SEM image of SiO2NF, and (g) SEM image of CNFPiC.

2. EXPERIMENTAL SECTION 2.1 Materials. Pinacyanol Chloride (95%, across organics) was purified by washing the powder with water and recovering the insoluble fraction by filtration; this process was repeated three times before drying the insoluble fraction (90% yield). The corresponding acetate salt was prepared by adding silver acetate (1.1 equivalents) into a solution of pinacyanol chloride in EtOH. Prior removal of precipitated AgCl by filtration, the solvent was removed under nitrogen flow to obtain the solid pinacyanol acetate (PIC) in 95% yield. Furfuryl alcohol (96%, Sigma Aldrich) and ammonia solution (NH4OH 25% basis, Sigma Aldrich) were used as received without further purification. 2.2 Fiber Synthesis. The synthesis of silica nanofibers (SiO2NF) was carried out following our previous publications.14,15 A reaction mixture consisting of 3.5 g of pinacyanol acetate in 100 mL of concentrated ammonia (NH4OH) 25 wt% and 22 g of tetraethyl orthosilicate (TEOS) was stirred at room temperature for 2 h, followed by aging at 70 °C for 2 h. Samples were filtered and washed with water followed by calcination at 550 °C for 5 h to obtain SiO2NF (3 g). Temperature ramp was set at 1 °C/min during calcination. To improve the carbonization of furfuryl alcohol, SiO2NF (3 g) was stirred for 24 h in a solution of AlCl3 (0.5 g) in ethanol. The resultant aluminated silica was collected by filtration, washed with ethanol and dried at 80 °C for 12 h. Carbon nanofibers (CNF-PiC) were synthesized following a procedure adapted from the literature.9 SiO2NF were soaked with furfuryl alcohol by the incipient wetness technique. Furfuryl alcohol (2.1 mL) was added to SiO2NF (3 g, total pore volume 0.7 cc/g). The sample was then carbonized in nitrogen atmosphere at 150 °C for 3 h at a heating rate of 1 °C/min from 80-150 °C, followed by a temperature increase from 150 to 300 °C at a heating rate of 1 °C/min. Temperature was finally increased to 850 °C at a rate of 5

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°C/min and maintained at this temperature for 4 h. SiO2 was etched with a 1 M NaOH solution at 90 °C. The fibers were dried at 100 °C for 24 h. 2.3 Thermal Treatment. CNF-PiC were heated to 1100 °C, 1500 °C and 2000 °C (to obtain CNF-1100, CNF-1500 and CNF-2000 respectively) under high vacuum (7 × 10-3 Pa) in a FVHP-1-3, FTR-20-3VH furnace (Fujidempa Kogyo Co., Ltd, Osaka, Japan). 2.4 Material Characterization. Part of the characterization was performed in the Nanostructured Liquid Characterization Unit of the Spanish National Research Council (CSIC) and the Biomedical Networking Center (CIBER-BBN), located at IQAC-CSIC. Materials were characterized using powder X-ray diffraction (XRD, Rigaku RINT2000), Small-Angle X-ray Scattering (SAXS, Hecus S3MICRO), nitrogen adsorption/desorption (Quantachrome Instrument, Autosorb-1 USA), Raman spectroscopy (JobinYvon T64000), Scanning Electron Microscopy (SEM, Hitachi S-4800, operated at 10Kv), Transmission Electron Microscopy (TEM, JEOL Model JEM-2100F operated at 200 kV as well as a Titan ChemiSTEM 80-200 kV microscope operated at 200 kV) 2.5 X-ray Photoelectron Spectroscopy (XPS). A Theta Probe spectrometer (Thermo Electron Co. Germany) was used for the XPS measurements. The instrument is equipped with a monochromatic Al−Kα radiation source (photon energy 15 KeV, energy resolution ≤ 0.47 eV, space resolution ≤ 15 µm). C 1s and O 1s core level XPS spectra were recorded in 0.05 eV steps. A built-in electron flood gun prevented sample charging. 2.6 Ultra-Microtome Cuts. CNFs were dispersed in Spurr® epoxy resin and heated to 80 °C for 48 h. Nanometer thick cuts of the resin were obtained using a diamond cutter Ultra-microtome (ULTRACUT E). These films were deposited on a copper grid and observed by TEM (JEOL 1010 operating at 80 kV as well as Titan ChemiSTEM 80-20 kV microscope operating at 200 kV). 2.7 Thermal Analysis. Thermogravimetric analysis (TGA) was performed for the evaluation of thermal stability of carbon samples. Measurements were performed on the Hitachi HT-Seiko 6300 instrument heating samples from 30 to 1000 °C under argon atmosphere (temperature ramp 10 °C/min). 2.8 Preparation of Working Electrodes. CNFs (1 mg) were dispersed in H2O: EtOH (70:30, 1 mL) and stirred for 24 h to form a homogeneous dispersion. While stirring, 5 µL of the dispersion were taken and added to a glassy carbon (GC) electrode and dried at 80 °C in vacuum for 2 h. NAFION solution (3 µL of 0.5 wt% in ethanol) was then added onto the electrode surface and dried at 80 °C in vacuum for 2 h. The mass of materials loaded on the GC electrode was estimated using a quartz crystal microbalance (QCM, 9 MHz, AT-cut): 4.67 µg for CNF-PiC, 2.53 µg for CNF-1100, 2.06 µg for CNF-1500, and 2.63 µg for CNF-2000. 2.9 Electrochemical Studies. The electrochemical supercapacitive performances of the CNF materials were evaluated in a three-electrode system. Ag/AgCl wire immersed in saturated KCl solution, platinum wire, and CNF-coated GC were used as a reference, counter, and working electrode, respectively. Measurements were performed using Electrochemical Analyzer, model 850D, ALS/CH instrument

within the potential window of 0 to 1.0 V in 1 M H2SO4 solution. The specific capacitance, Cs, was calculated from cyclic voltammetry (CV) curves by: =

∗∗∆

∗

(1)

where m is the active electrode mass (i.e., CNFs mass), v is the scan rate and ∆V is the potential window. Galvanostatic charge/discharge (CD) measurements were also performed to estimate the specific capacitance and cyclic stability of the electrode. Capacitances from CD curves were calculated from following equation. =

∗ ∆

(2)

where Im, td and ∆V are the current density, discharge time and potential window, respectively. 2.10 Sensing Performance. Vapor sensing performance of CNF was studied by Quartz Crystal Micro-balance (QCM) technique (9 MHz AT-cut) by measuring the variation of frequency of the QCM gold resonator with time. QCM electrode was prepared as follows: 1 mg of CNF-PiC was dispersed in water/EtOH mixture (1 mL, 80:20) and stirred during 24 h to form a homogeneous dispersion. While stirring 3 µL of dispersion were taken and added to a QCM gold resonator followed by drying at 80 °C during 24 h under vacuum. The frequency shift was measured upon exposure of different vapors to the CNF-PiC modified QCM electrode in a sealed container. Between measurements, the solvent was desorbed from the electrode by exposure to air; desorption was assumed to be complete when the frequency got back to its original value (i.e. before exposure to vapor). The repeatability test was also performed by exposing and removing of the guest vapor molecules alternately. As the blank test, frequency shift of bare Au-resonator without loading any material was recorded upon exposing to different solvent vapors (hexane, cyclohexane, benzene, toluene and pyridine) and corrected from the frequency shift of the CNF-PiC modified QCM sensors. These vapors caused a small frequency shift of bare Au-resonator: hexane (-2.9 Hz), cyclohexane (-4.9 Hz), benzene (-12.0 Hz), toluene (-12.1) and pyridine (-20.5 Hz). For all QCM measurements the temperature was 25 °C.

3. RESULTS AND DISCUSSION 3.1 Synthesis and Characterization of Fibers Pinacyanol acetate is a cyanine dye known to form chromonic nematic and hexagonal liquid crystalline phases in water. SAXS suggests that both of these phases are composed of hollow pipes.15 Pinacyanol acetate also forms a nematic liquid crystal in concentrated ammonia (Scheme 1a and 1e). When a silica precursor, such as TEOS, is added to the dye liquid crystal, hydrolysis and condensation of silica species induces a hexagonal structure while maintaining the fiber-like morphology in the precipitated silica/pinacyanol composites (Scheme 1b, Scheme 1e), which upon calcination SiO2 nanofibers (SiO2NF) are obtained (Scheme 1c). Calcined SiO2NF have a BET surface area of 230 m2/g, a pore volume of 0.7 cm3/g and an average pore size of ~3.1 nm. SiO2 nanofibers have a width of around 50 nm and are several

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micrometers long (Scheme 1f). Hexagonal arrangement of pores can be inferred by SAXS (Figure 1), presenting two bands with the relative positions of 1: √3 . The distance between pores (calculated from the first peak on SAXS pattern) is 4.8 nm. The wall thickness, calculated as the difference between the pore size and distance between pores, is 1.7 nm. The cooperative assembly of chromonic liquid crystals with silica species induces nanostructuring within the fibers during the synthesis. Experimental evidence suggests that negatively charged oligomeric silicate intermediates binds to the aggregate surface and neutralizes the long-range repulsion between them causing separation of silicatechromonic column complexes from excess water, which leads to the condensation into an intermediate hybrid silica/dye hexagonal liquid crystal during silica polymerization, followed by co-precipitation of the silicon oxide/dye composite (Scheme 1a-c).15 Ultra-microtome film observations by TEM revealed that SiO2NF are actually hollow, with a tube-like appearance (Figure S1 in Electronic Supporting Information (ESI)).

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3.2 Heat Treatment of Carbon Nanofibers Investigations on the pyrolysis of carbon-rich precursors in carbonaceous particles have demonstrated the fact that exists a close relation between the starting precursor material and the product.49 When using furfuryl alcohol as precursor, a high ratio of sp2 carbon is expected which could increase the carbon conductivity; a key factor in obtaining high capacitance values. However, carbon obtained at relatively low temperatures usually gives disordered graphite layers and, therefore, poor conductivity. Furthermore, they are also thermally less stable compared to samples heat-treated at higher temperatures (Figure S3). The graphitization process comprises of the ordering and stacking of graphite layers, which can be achieved by high-temperature heat treatment. Therefore, CNFPiC were treated at 1100 °C, 1500 °C and 2000 °C to obtain samples CNF-1100, CNF-1500 and CNF-2000, respectively. SEM and TEM imaging techniques were used to determine the surface morphology and structure of the heat-treated CNFs. CNF morphology was preserved regardless of treatment temperature (Figure S4 and Figure S5). HRTEM imaging (Figure 2a) of CNF-PiC shows randomly oriented graphite layers, while CNF-2000 shows stacking of graphite layers (up to 7 sheets: Figure 2d). This shows that graphitization was achieved by the heat treatment.

Figure 1. SAXS patterns for CNF-PiC and SiO2NF. The values indicate the q ratios with respect to the first peak.

SiO2NF were treated with AlCl3 in order to provide acidic sites to improve the polymerization of furfuryl alcohol (FA). FA was then added, polymerized and carbonized. After etching with NaOH (1 M) the resulting templated carbon (CNF-PiC) contained less than 5 wt% of SiO2. Up to 400 mg of carbon were obtained. SEM observations showed fiber-like structure (Scheme 1g). Closer inspection shows that the fiber cross section is deformed. Ultra-microtome films observed by TEM showed evidence that the carbon fibers are hollow, similar to their original SiO2NF templates, but with thinner walls (ca. 7 nm, Figure S1). The ordered nano-structure of the original template was also lost, as inferred by the lack of peaks in the SAXS pattern (Figure 1). The loss of ordering of the pores may be attributed to the lack of pore interconnections in the silica template, which prevents the formation of carbon bridges that may serve as structural supports. This phenomenon has been observed in other templated carbons such as MCM-41.13 In CNF-PiC the surface BET area increased to 830 m2/g and pore volume augmented to 2.7 cc/g compared to the original silica templates. The pore size distribution of CNF-PiC was sharp with a maximum at 4 nm (Figure S2), more than twice the SiO2NF wall thickness. This difference demonstrates that a structural transformation of the carbon frameworks takes place upon removal of the silica wall, which can be attributed to strain in the carbon frameworks formed inside the silica pores.11

Figure 2. Electron microscopy observations: (a) HR-TEM, (b) TEM, and (c) SEM image of CNF-PiC, (d) HR-TEM, (e) TEM, and (f) SEM image of CNF-2000.

Powder X-ray diffraction and Raman scattering were used to investigate the evolution of the crystalline nature of CNFs. XRD of CNFs treated at temperatures lower than 2000 °C shows two broad peaks, that can be ascribed to disordered graphite (Figure 3 a). In contrast to the fibers treated at lower temperatures, CNF-2000 shows a sharp peak at 2θ ~ 26°

4


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(0.344 nm) that is indexed to the (002) crystalline plane (interlaying spacing between graphite sheets). Raman scattering spectra from all the samples show two Raman bands at 1335 cm-1 and 1582 cm-1 corresponding to D and G bands of carbon materials (Figure 3b). The D band (also called defect-induced band) is ascribed to the existence of disordered carbon in the studied samples, while the G band is attributed to the stretching vibration mode of a graphitic structure. After heat treatment at 2000 °C, an additional Raman band (2D band) appears in the higher Raman frequency region indicating the formation of crystalline graphite.54

and 0.2% for CNF-2000, respectively. The four deconvoluted peaks of CNF-PiC (Figure 4b) at 284.4, 285.5, 288.0 and 290.0 eV correspond to sp2 carbon, sp3 carbon, C=O and π–π shake up, respectively. Closer inspection on the oxygen core level (Figure 4c) reveals the presence of C=O and OH groups on the surface of the carbon. XPS of C 1s and O 1s core level peaks with deconvoluted peaks for CNF-PiC, CNF-1500, and CNF-2000 are given in Figure S6.

Table 1. Surface area and pore volume for CNFs treated at different temperatures. Sample

BET Area (m2/g)

Total Pore volume (cc/g)

CNF-PiC

826

2.7

CNF-1100

847

2.8

CNF-1500

580

2.7

CNF-2000

271

2.3

BET surface area of CNFs slightly increased with the heat treatment at 1100 °C from 826 m2/g to 847 m2/g (Table 1) but as the treatment temperature is increased, pores are disrupted, and the specific surface area is therefore reduced considerably. This could be a result of material stress, shrinkage, or even sintering. Nitrogen physisorption isotherms and pore size distributions of CNFs are presented in Figure S2.

Figure 3. (a) XRD pattern and (b) Raman Spectra of CNF heat-treated at different temperatures. The asterisk-marked XRD peaks might be attributed to residual SiO2.

XPS spectra of CNFs reveal the presence of C 1s and O 1s core level peaks (Figure 4a). Minute observation reveals that the O 1s XPS peak intensity of CNF-1000, CNF-1500 and CNF-2000 is lower compared to CNF-PiC. This demonstrates that oxygen functional groups disappear after the high temperature treatment. The oxygen content is estimated to be 4.7% for CNF-PiC, 3.0% for CNF-1100, 2.2% for CNF-1500

Figure 4. (a) XPS survey spectra of CNFs, (b) XPS C 1s core level spectrum with deconvoluted peaks of CNF-1100 as representative example, and (c) the corresponding XPS O 1s core level spectrum with deconvoluted peaks.

3.3 Electrochemical Studies Our results below show that synthesized CNFs could be of great interest in applications in which high currents are needed. Electrochemical testing was carried out to study the supercapacitance performance of the as-synthesized CNFs. Cyclic voltammetry (CV) curves obtained for all samples exhibit approximately rectangular profiles (Figure 5a). A rectangular CV curve corresponds to the ideal response for pure electrical double layer supercapacitors (low internal resistance).2,3 When the scan rate is low, ions diffusion is slow, i.e., ions have sufficient time for diffusion into the inner surfaces of pores or pore channels. Therefore, ion penetration distance and easily accessible pore channels are less important at low scan rates. However, at higher scan rates ion penetration is possible only on the surface of large pores, i.e., less active surface area of the pores take part in the process.24 In general, the shape of CV curves deviates from the rectangular profile at increasing scan rate and, therefore, the capacitance decreases. Nevertheless, CV curves of CNFs maintained the rectangular shape even at high scan rates (Figure S7a). CNF-PiC shows a small deviation for the rectangular profile (i.e., a bump at 0.3 V vs Ag/AgCl), that might be caused by oxygen surface functional groups present in the carbon sample.55 The presence of mesopores and high pore volume (~2.7 cc/g) allows the fibers to easily adsorb guest electrolyte molecules even under high scan rates. CNFPiC and CNF-1100 showed outstanding capacitance (307 F/g and 327 F/g at 5 mV/s, respectively) (Figure 5b). CNFs

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showed great capacitance retention to increasing scan rates and current densities among them, CNF-PiC showed superior capacitance retention (up to 79 % retention from 5 mV/s to 500 mV/s). This retention behavior could be attributed to the surface functional groups of CNF-PiC. Polar groups on the carbon surface increase wettability and could make the carbon surface more accessible to aqueous electrolytes,19 allowing CNF-PiC to retain high capacitance even at high scan rates or currents. It is noteworthy that many methods exist for adding oxygen functionalization to carbon, such as chemical oxidation or plasma,46 but in the present case oxygencontaining groups result from the synthesis process without the need of an additional surface-treatment. As expected from the decrease in surface area, CNF-1500 and CNF-2000 capacitance was considerably lower, nevertheless, there is no linear correlation between capacitance and surface area. Ordered graphitic carbon nanofibers are expected to have higher conductivity, which would lead to higher capacitance values. However, we have observed the opposite results. This demonstrates that the loss of surface area has greater influence on the capacitance value than the conductivity.

Figure 5. (a) CV curves of CNFs at fixed scan rate of 50 mV/s, (b) Specific capacitance values for CNFs as a function of scan rate. (c) CD curves for CNFs at fixed current density of 1 A/g. (d) Cyclic stability of CNFs up to 1000 cycles at current density of 10 A/g.

It is worth mentioning that the BET surface area of CNFs is rather low (~840 m2/g) compared to other mesoporous carbon materials or commercially available conductive carbon black (1000-1500 m2/g), so the high capacitance value must arise from other properties such as pore structure, surface chemistry or particle morphology. It should be noted that the electrode material design is one of the main parameters to enhance the capacitive properties of the EDLCs. A material with high porosity and that allows the electrolyte ions to easily access most of the surface is the suitable electrode material for high energy storage.50 In fact, other templated carbons such as CMK-3 shows lower capacitance (~200 F/g at 1 mV/s) despite its high surface area (1200 m2/g).51 This suggests that some of the CMK-3 surface area is not accessible by the electrolyte. The hollow morphology of CNF could allow most of the area

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to be in full contact with the electrolyte solution. It is to be noted that many carbon materials with anisotropic shapes (such as fibers or CNTs) have shown better capacitive behavior per surface area compared to other carbons with isotropic morphologies.20,50,51 Charge-discharge (CD) curves showed typical symmetric response (Figures 5c and S7b) corresponding to EDLC behavior. CNFs particularly CNF-PiC and CNF-1100 sustained high capacitance even at higher current density of 10 A/g (Figure S7c) and displayed excellent cyclic stability. After 1000 cycles of charge discharge at 10 A/g, fibers showed high stability of 95 % retention for CNF-1100 and ~92 % retention for CNF-PiC (Figure 5d). CNF-1500 and CNF-2000 samples also showed higher cyclic stability (Figure S7d). CNFs have a capacitance similar to commercially available activated carbon that is already being used for supercapacitor electrodes (e.g. Maxsorb® with a capacitance of 330 F/g at 5 mV/s), 24 but additionally, CNFs capacitance retention at increasing scan rates (79 % retention from 5 to 500 mV/s) is considerable higher than that of activated carbon (< 20 % from 5 to 500 mV s–1). Thus, the present CNFs could be of special interest for applications in which high currents are needed (i.e., faster charging). 3.4 Vapor Sensing Our results show that CNF-PiC can be used as sensor material of industrial interest for sensing VOCs, such as pyridine, benzene, and toluene. Vapors of these compounds are toxic. For example, pyridine is an organic compound used for the synthesis of many common products (i.e. agrochemicals, pharmaceutics, dyes, adhesives, etc.). However, pyridine is a hazardous chemical; it is reported that reduces male fertility and is carcinogenic. Some foods emit this compound when grilled, so techniques that allow easy detection are of interest.53 Vapor sensing properties of CNF-PiC were evaluated by the QCM technique. QCM frequency shift of bare Auresonator without loading CNF-PiC was first recorded as blank test and corrected from the frequency shift of the CNFPiC modified QCM sensors. Frequency shift of CNF-PiC modified QCM sensor was very quick upon exposure of solvent vapors. Adsorption of vapors of aromatic compounds such as benzene (-77 Hz), toluene (-82.7 Hz) and pyridine (186 Hz) caused larger frequency shift compared to aliphatic hydrocarbons hexane (-5 Hz), and cyclohexane (-51 Hz) with almost similar molecular size and molecular weight (Figure 6a). The sensing selectivity to VOCs decreases as follows: pyridine > toluene> benzene > cyclohexane > hexane. The higher sensing selectivity towards aromatic solvent vapors could be explained by constrain-free diffusion into the pores trough strong π–π interaction between aromatic guest molecules and sp2 carbon on the CNFs surface.5, 43, 46, 48,52 Note that although the vapor pressure, molecular weight and structure of pyridine, benzene and toluene are more or less similar, pyridine caused a frequency shift more than twice in magnitude compared to benzene or toluene. This might be attributed to the basic nature of pyridine interacting with protonic Brønsted acid surface sites on CNF-PiC surface (i.e., OH, COOH as confirmed by XPS). In fact, the increase of acidic functionalities on carbon surfaces is known to improve the sensing capabilities towards specific basic molecules such

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as aniline or amonia.46 CNF-PiC also showed good cyclic stability against adsorption/desorption of aromatic solvent vapors (Figure 6b).

CNF-PiC, CNF-1500, and CNF-2000, additional CV and chronopotentiometry data for CNFs, cyclic stability plots for CNF-1500 and CNF-2000. “This material is available free of charge via the Internet at http://pubs.acs.org.”

ACKNOWLEDGMENT

Figure 6. (a) QCM frequency shifts upon exposure to different solvent vapors; (b) Repeatability test of CNF-PiC upon exposure and removal of benzene and pyridine vapors.

J.R.M. acknowledges funding from the Ministerio de Economia y Competitividad, Spain (Grant: EEBB-I-15-10167 and project: CTQ2011-29336-C03-01) and the National Institute for Materials Science (NIMS) Tskuba, Japan for internship program. C. R-A. acknowledges the World Premier International Center for Materials Nanoarchitectonics (WPI-MANA) for the MANA Short Term Invitation Program. This work was partly supported by World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan.

4. CONCLUSIONS Carbon nanofibers (CNF) with high surface area have been successfully synthesized using a hard templating approach with meso/macroporous SiO2 nanofibers as templates. Graphitized CNFs were successfully obtained at 2000 °C. A small increase in surface area is observed in samples heated to 1100 °C. However, the pore structure collapses as a result of the ordering of graphite layers at high temperatures leading to a reduction in the surface area. CNF-PiC and CNF-1100 showed excellent electrochemical performance demonstrating their potential use as electrode materials for supercapacitor applications. The observed specific capacitance (327 F/g at a scan rate of 5 mV/s) is higher than the capacitance observed with many high surface area activated carbons. Furthermore, high capacitance retention at high current density and long cycle life makes our material a suitable candidate for use in advanced supercapacitor electrode designs. The low surface area, compared to other mesoporous or activated carbons, suggests that the high capacitance value, current and cycle stability might be a consequence of the morphology, pore structure and surface chemistry. This allows the electrolytes to be in full contact with the entire available surface calculated by N2 sorption measurements. Sensing measurements on CNF-PiC revealed selectivity for aromatic molecules, especially pyridine. This can be attributed to acidic oxygen groups on the surface of CNF.

AUTHOR INFORMATION Corresponding Author *[email protected] [email protected] [email protected] Author Contributions The manuscript is written through contributions of all authors. All the authors have approved its submission. The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information. TEM images of ultramicrotome cuts for SiO2NF and CNF-PiC, thermogravimetric analysis data, additional SEM images of CNFs, additional TEM and HR-TEM images of CNFs, XPS C 1s and O 1s spectra of

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Table of Contents Artwork

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From Chromonic Self-Assembly to Hollow Carbon Nanofibers: Efficient

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