Amorphous Carbon Nanofibers and Their Activated Carbon ...

117 downloads 4627 Views 4MB Size Report
electrical conductivity are easily activated in KOH, using certain KOH/CNF weight ... alumina as a template, which leads to CNFs with marked ... Published on Web 05/17/2010 .... Figure 4b shows the pore size distributions (PSDs) obtained.
10302

J. Phys. Chem. C 2010, 114, 10302–10307

Amorphous Carbon Nanofibers and Their Activated Carbon Nanofibers as Supercapacitor Electrodes V. Barranco,† M. A. Lillo-Rodenas,‡ A. Linares-Solano,‡ A. Oya,§ F. Pico,| J. Iban˜ez,| F. Agullo-Rueda,† J. M. Amarilla,† and J. M. Rojo*,† Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de InVestigaciones Cientificas (CSIC), Sor Juan Ines de la Cruz 3, Cantoblanco, E-28049-Madrid, Spain, MCMA, Departamento de Quimica Inorganica, UniVersidad de Alicante, P.O. Box 99, E-03080-Alicante, Spain, Faculty of Engineering, Gunma UniVersity, Gunma 376, Japan, and Centro Nacional de InVestigaciones Metalurgicas (CENIM), CSIC, AVda. Gregorio del Amo 8, E-28040-Madrid, Spain ReceiVed: March 9, 2010; ReVised Manuscript ReceiVed: April 21, 2010

Carbon nanofibers (CNFs) show a high electrical conductivity but a reduced specific surface area that limits their use as electrode materials for supercapacitors. In this work, amorphous CNFs, with a relatively high electrical conductivity are easily activated in KOH, using certain KOH/CNF weight ratios. Activation does not produce any important change in the shape, surface roughness, diameter, graphene sheet size, and electrical conductivity of starting nanofibers. However, activation leads to new micropores and larger surface areas as well as a higher content of basic oxygen groups. They clearly enhanced the specific capacitance, attaining values higher than those reported for other activated CNFs. In this study, the effects of micropore size and oxygen content on the specific capacitance are discussed for three electrolytes: H2SO4, KOH, and (CH3CH2)4NBF4. Moreover, a good cycle life is found for the most activated CNFs. 1. Introduction Carbon nanofibers (CNFs) are carbon materials with a cylindrical shape like carbon nanotubes (CNTs) but with structural and textural characteristics different from CNTs. CNFs usually exhibit diameters in the range of 100-300 nm and lengths of a few micrometers (up to 200 µm). They can be classified in two kinds: (i) highly graphitic CNFs and (ii) lowly graphitic ones. The former can be obtained either by a catalytic vapor-grown procedure with a metal catalyst floating in the reaction media1-12 or by catalytic chemical vapor deposition with the metal catalyst on a support.13-19 In both cases, the CNF grows from the metal (Fe, Ni, Co...) particles chosen as catalysts, showing an ordered arrangement of graphene sheets and an internal hollow. These CNFs show a high electrical conductivity (on the order of 1 S cm-1 as measured from compacted pellets) and a low specific surface area (10-50 m2 g-1). Their specific capacitance is rather low (1-10 F g-1), in agreement with their low specific surface area. However, capacitance retention of CNFs at high currents is rather high, in accordance with its high electrical conductivity. The main drawback is the difficulty in activating this kind of CNF.5,20-22 The activation leads to a moderate increase in specific surface area (ca. 100 m2 g-1) and, hence, to a moderate increase in specific capacitance, achieving values below 100 F g-1.5 The other kind of CNF deals with nanofibers of a lowly graphitic character.23-35 In accordance with it, their electrical conductivity is at least 1 order of magnitude lower (ca. 0.1 S * To whom correspondence should be addressed. E-mail: jmrojo@ icmm.csic.es. † Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Cientificas (CSIC). ‡ Universidad de Alicante. § Gunma University. | Centro Nacional de Investigaciones Metalurgicas (CENIM), CSIC.

cm-1 as measured in compacted pellets), their specific surface area is higher (100-500 m2 g-1), and they look amorphous (or less crystalline), as checked by the X-ray diffraction technique. The lowly graphitic CNFs have been obtained by certain procedures: (i) from blends of polymers in which a polymer acts as the carbon precursor and the other gives way to porosity, the latter being removed along the carbonization process,23-26 (ii) by electrospinning of precursors, which gives way to webs of CNFs after carbonization,27-31 (iii) by using an anodic alumina as a template, which leads to CNFs with marked mesoporosity,32,33 (iv) from electrochemical decomposition of chloroform,34 and (v) from flames of ethanol.35 The two last procedures lead to nanofibers with high oxygen contents. In all cases, the low crystallinity of the CNFs could be an advantage for subsequent activation, and thus, activated CNFs with a larger specific surface area and a higher specific capacitance can be obtained. Regarding the polymer blend-based CNFs, which is the carbonaceous material dealt with in this work, it is worth noting that, despite their amorphous character, they show a relatively high electrical conductivity (0.7 S cm-1). Moreover, their amorphous character seems to lead to (i) a special texture of deposited oxides,36 different from that observed when highly graphitic CNFs are chosen,37 and (ii) an easy activation of the as-prepared CNFs.26 Hence, these CNFs seem to be a promising material as electrode for supercapacitors. Nevertheless, neither the as-prepared carbon nanofibers nor the activated ones obtained from them have been studied as electrode materials for supercapacitors in current electrolytes, such as H2SO4, KOH, and Et4NBF4. In this work, we report a systematic study on the activation of polymer blend-based CNFs using KOH as an activating agent and certain KOH/CNF weight ratios. The apparent specific surface area (SBET), pore size distribution, and surface oxygen content are analyzed as a function of the KOH/CNF ratio. The

10.1021/jp1021278  2010 American Chemical Society Published on Web 05/17/2010

CNFs as Supercapacitor Electrodes activated CNF samples are studied as supercapacitor electrodes in cells having as electrolytes three solutions: aqueous KOH, aqueous H2SO4, and acetonitrilic Et4NBF4. The specific capacitance measured is discussed in terms of the SBET, pore size distribution, and surface oxygen content of the activated samples. The cycle life of the most activated CNFs is compared with that of the as-prepared CNFs. 2. Experimental Section Carbon nanofibers (CNFs) were prepared from a polymer blend of a Novolac-type phenolic resin (PR) and a high-density polyethylene (PE) pyrolyzing polymer in a PR/PE weight ratio of 3/7. The PR polymer acts as a carbon precursor and the PE polymer is chosen for inducing porosity along carbonization.23,24 Briefly, PR was dissolved in acetone and sprayed in a toluene solution of PE. After solvent evaporation under vacuum and then homogenization at 150 °C, the resulting polymer blend was spun continuously with a conventional spinning equipment at 120-130 °C, stabilized in acid solution, neutralized, washed, and dried under vacuum at room temperature. Finally, the stabilized blend polymer fibers were carbonized at 800 °C for 1 h under a N2 atmosphere. Activation of CNFs was conducted through a chemical procedure in which the activating agent was KOH. CNFs and KOH were mixed at room temperature, the KOH/CNF weight ratio being 0.5/1, 1/1, 2/1, and 3/1. The samples were then carbonized under nitrogen flow (500 mL min-1) in a horizontal cylindrical furnace (65 mm interior diameter). The heating rate was 5 °C min-1, and the maximum temperature reached, 750 °C, was held for 1 h. After the heat treatment, washing was carried out first with 5 M HCl solution and afterward with distilled water. In the first washing, the mineral matter was removed, and chlorine ions were eliminated with the distilled water. The activated samples are called, hereafter, 0.5/1, 1/1, 2/1, and 3/1 activated CNFs. Powder X-ray diffraction (XRD) patterns were recorded at room temperature by a D-8 Bruker diffractometer, with Cu KR radiation. The XRD patterns were obtained in the step scanning mode of 0.02° (2θ) and a 1 s/step counting time, within the range of 10 e 2θ e 70°. Images of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were taken by a Jeol JSM 6500F and 3000 FE microscopes, respectively. The samples were dispersed in n-butyl alcohol, and drops were deposited on carbon-coated copper grids. The SEM images were obtained in the secondary electron mode. Raman spectra were measured with a Renishaw Ramascope 2000 spectrometer coupled to an Olympus BH-2 optical microscope. An argon ion laser (514.5 nm wavelength) was used for excitation. To focus the light on the sample, a microscope objective with 10× magnification was employed, and the laser power density on the sample was kept below 30 W cm-2 to avoid any sample modification. Spectra were collected from different fibers of a given sample to check the sample homogeneity. Characterization of the porous texture of the starting and activated CNF samples was done using physical adsorption of N2 and CO2 at 77 and 273 K, respectively. The samples were previously outgassed at 250 °C for 4 h. Nitrogen adsorption/ desorption isotherms were measured at 77 K using an automatic adsorption system (ASAP2020, Micromeritics). From the nitrogen adsorption isotherms, the BET equation was applied and the apparent surface areas were calculated. Also, when the Dubinin-Radushkevich equation was applied to the nitrogen

J. Phys. Chem. C, Vol. 114, No. 22, 2010 10303 adsorption data, the total volume of micropores (pore size smaller than 2 nm) was assessed. Nitrogen adsorption data were also used to calculate the pore size distributions of these samples using the DFT model supplied by the adsorption equipment. Carbon dioxide adsorption isotherms were measured at 273 K using an automatic adsorption system (Autosorb-6, Quantachrome). When the Dubinin-Radushkevich equation was applied to the carbon dioxide adsorption data, the volume of narrow micropores (pore size smaller than 0.7 nm) was determined. Temperature-programmed desorption (TPD) experiments were carried out in a DSC-TG equipment (TA Instruments, SDT 2960) coupled to a mass spectrometer (Thermostar, Balzers, GSD 300 T3). In these experiments, samples of 2-4 mg were heated to 900 °C (heating rate 20 °C/min) under a helium flow rate of 100 mL/min. Supercapacitor electrodes were processed as cylindrical pellets with a diameter of 13 mm and a height of ca. 1 mm. The asprepared and activated CNFs (ca. 25 mg) were mixed with the polymer PVDF (10 wt %) and compacted under a cold pressure of 2 ton cm-2 for 2 min. No carbon black or other good electrical conductor was added. Supercapacitors were assembled in twoelectrode cells in which a glassy microfiber paper (Whatman 934 AH) was used as a separator. Three electrolytes were chosen: 2 M aqueous H2SO4, 1 M aqueous KOH, and 1 M Et4NBF4/acetonitrile. The specific capacitance of as-prepared and activated CNFs was determined from discharge galvanostatic measurements (Figure 5a) at certain current densities and at room temperature by a PGSTAT30 Autolab potentiostat/ galvanostat. From the voltage drop E2, the specific capacitance of the as-prepared and activated CNFs was calculated as C ) 2 · I · td/E2 · mc, where I is the current applied, td is the time spent along the discharge, and mc is the carbon nanofiber mass in one electrode. From the voltage drop E1, the equivalent series resistance (ESR) of the supercapacitor cells can be calculated according to the equation E1 ) 2 · I · ESR. The electrical conductivity of parallelepiped electrodes, processed equally as the supercapacitor electrodes and compacted at the same pressure, was measured by a conventional four-probe method. Silver paint was used to get the four probes on the parallelepiped pellets. Current was applied at the two outer probes, and the voltage drop was measured at the two inner ones. The conductivity was calculated according to the equation σDC ) L/R · A, where L is the length between the two inner probes, R is the resistance measured, and A is the geometrical area defined by the probe length and the pellet thickness. 3. Results and Discussion 3.1. Structural, Microstructural, and Electrical Characterization of As-Prepared and Activated CNFs. X-ray diffraction patterns of the as-prepared CNF sample and the 3/1 activated one are shown, as examples, in Figure 1. No sharp peaks are observed in the patterns of the two samples, confirming the amorphous character of both the as-prepared CNFs and the resulting activated ones. SEM and TEM images of the as-prepared CNFs and the 3/1 activated ones are shown in Figure 2, left and right, respectively. The as-prepared CNFs (Figure 2a,b) and the 3/1 activated ones (Figure 2c,d) show compact fibers (i.e., without an internal hollow) with no crystalline structure (i.e., without ordered graphene planes), unlike highly graphitic CNFs.2,3,5,10,11,17,20,21 The as-prepared and activated nanofibers show some roughness at the surface. From measurements done on 84 as-prepared nanofibers and 80 activated ones, it was deduced that the average

10304

J. Phys. Chem. C, Vol. 114, No. 22, 2010

Figure 1. Powder X-ray diffraction patterns of the as-prepared CNFs and the 3/1 activated ones.

Figure 2. SEM (left) and TEM (right) images of the as-prepared CNFs (a, b) and the 3/1 activated CNFs (c, d). Magnifications of the square regions are shown in the insets of (b) and (d).

diameter is 145 nm for the as-prepared nanofibers and 142 nm for the activated ones with standard deviations of 79 and 76 nm for the as-prepared and activated nanofibers, respectively. Therefore, the activation does not produce any significant change in either surface roughness or diameter value of the starting carbon nanofibers. Raman spectra of the as-prepared CNFs and the 3/1 activated ones, both chosen as examples, show two broad bands at about 1350 and 1600 cm-1 (see Figure 3). These bands are, respectively, the so-called D and G bands, characteristic of carbon materials, and provide information about the nanostructure and bonding of these materials. The G band corresponds to phonons propagating along the graphene sheets. The position of this band in our samples (1602 cm-1) for an excitation laser wavelength of 514.5 nm corresponds to nanocrystalline graphite.38 The D band is defect-induced. Its intensity is zero for infinite and perfectly ordered graphite crystal but increases with disorder in the layers, either by a reduction in the in-plane size of the graphene sheets or by the introduction of imperfections within these sheets, which limit the phonon coherence length L. The peak height ratio I(D)/I(G) and L are related by the empirical relation I(D)/I(G) d C/L, where C ) 4.4 nm for an excitation length of 514.5 nm.39 For the as-prepared and all activated CNFs,

Barranco et al.

Figure 3. Raman spectra recorded for the as-prepared and 3/1 activated CNFs.

the shape and the intensity ratio of the two bands are nearly the same, and I(D)/I(G) ) 0.87. In all these samples, the nanofibers are formed by graphene sheets with a typical size of L ) 5 nm, indicating that the activation did not modify the graphene sheets’ size. Moreover, the amorphous character of the as-prepared CNFs agrees with the lower value of L as compared with that found for crystalline CNFs obtained by catalytic CVD (9 nm).18 Raman spectra collected from different parts of analyzed samples are identical. Because the laser spot has a diameter of a few micrometers, all the samples are homogeneous at least above the micrometer scale. The electrical conductivities of the as-prepared CNF and activated CNF samples are comparable (same order of magnitude): 0.7, 0.9, and 0.4 S cm-1 for the as-prepared, 1/1, and 3/1 activated CNFs, respectively. This similarity in electrical conductivity agrees with the similar in-plane size of the graphene sheets of the starting and activated nanofibers, as observed by Raman spectroscopy. From the above results, we can set up that the chemical activation does not change: (i) the amorphous character, surface roughness, and diameter value of the starting nanofibers and (ii) the in-plane size of the graphene sheets and the electrical conductivity. However, activation causes a significant change in surface properties and electrochemical behavior, as we will see below. 3.2. Pore Texture of the As-Prepared and Activated CNFs. Nitrogen adsorption/desorption isotherms of the asprepared and all the activated CNFs are shown in Figure 4a. All them show IUPAC type I isotherms, indicating that all samples are microporous. The results of Figure 4a confirm preliminary findings on NaOH and KOH activation of polymer blend-based CNFs26 where the surface area, the adsorption capacity, and porosity of CNFs were significantly increased by activation. Our present results, focused on KOH activation with certain KOH/CNF ratios, show a gradual development of the adsorption capacity with an increase in the KOH/CNF ratio from 0.5/1 to 3/1, the increase in porosity being almost limited to the low relative pressure range (