Nitrogen doped carbon nanotubes decorated with iron carbide nanoparticles and their electrochemical capacitance

Journal Pre-proof Nitrogen doped carbon nanotubes decorated with iron carbide nanoparticles and their electrochemical capacitance E. Tovar-Martinez, J.V. Cabrera-Salazar, D. Hernandez-Arriaga, M. ´ Reyes-Reyes, Luis F. Chazaro-Ruiz, R. Lopez-Sandoval

PII:

S2352-4928(19)30292-2

DOI:

https://doi.org/10.1016/j.mtcomm.2019.100667

Reference:

MTCOMM 100667

To appear in:

Materials Today Communications

Received Date:

19 June 2019

Revised Date:

6 September 2019

Accepted Date:

26 September 2019

Please cite this article as: Tovar-Martinez E, Cabrera-Salazar JV, Hernandez-Arriaga D, ´ Reyes-Reyes M, Chazaro-Ruiz LF, Lopez-Sandoval R, Nitrogen doped carbon nanotubes decorated with iron carbide nanoparticles and their electrochemical capacitance, Materials Today Communications (2019), doi: https://doi.org/10.1016/j.mtcomm.2019.100667

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Nitrogen doped carbon nanotubes decorated with iron carbide nanoparticles and their electrochemical capacitance E. Tovar-Martinez1, J. V. Cabrera-Salazar1, D. Hernandez-Arriaga2, M. Reyes-Reyes2*, Luis F. Chazaro-Ruiz3 and R. López-Sandoval1* 1

Advanced Materials Department, IPICYT, Camino a la Presa San José 2055, Col. Lomas 4a sección, San Luis Potosí 78216, Mexico. 2

Instituto de Investigación en Comunicación Óptica, Universidad Autónoma de San Luis Potosí, Álvaro Obregón 64, San Luis Potosí 78000, Mexico. 3

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Enviromental Science Department, IPICYT, Camino a la Presa San José 2055, Col. Lomas 4a sección, San Luis Potosí 78216, Mexico.

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*Corresponding author. E-mail: [email protected] (Marisol Reyes-Reyes)

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*Corresponding author. E-mail: [email protected] (Román López-Sandoval)

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Graphical abstract

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Abstract Different ethanol-benzylamine (EB) ratio (9/1 v/v and 1/1 v/v) reaction mixtures were prepared for the synthesis of nitrogen doped multiwalled carbon nanotubes (N-MWCNTs). The morphology of the synthesized sample depends of the used catalyst percentage, 1wt% or 3wt%, during the synthesis. In low ferrocene concentration, EB91-1wt % and EB11-1wt% samples are mainly composed by N-MWCNTs with iron-carbide nanoparticles at their tips, whereas at high ferrocene concentration, EB91-3wt % and EB11-3 wt% samples are composed by a mix of N-

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MWCNTs and carbon nano-onions (CNOs) with iron-carbide core. The way in which CNOs are distributed in EB91-3wt% and EB11-3wt% samples is related to the ethanol ratio used in the

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reaction mixture. In EB91-3wt% sample, CNOs are distributed more homogeneously on the surfaces of the N-MWCNTs, while EB11-3wt% sample, CNOs are segregated from N-

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MWCNTs. High ethanol ratio in the reaction mixture increases the probability of attaching oxygenated groups on N-MWCNTs surfaces, i.e. the surfaces functionalization is carried out during the synthesis, making possible CNOs anchoring on N-MWCNTs walls. This difference in

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morphology in the sample plays an important role for the high electrochemical capacitance in the

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Introduction

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EB91-3wt% samples compared with other samples.

Carbon nanotubes (CNTs) are carbon allotropes, which arise from rolling a graphene sheet in a tubular form. In 1991, Iijima reported the first multi-walled carbon nanotubes (MWCNTs) obtained from soot using the arc discharge method [1]. Two years later, he reported the existence of single-walled carbon nanotubes (SWCNTs) [2]. There are several methods to synthesize

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CNTs, being the most populars laser ablation, electrolysis, arc discharge and chemical vapor deposition. Each of these methods has its advantages and disadvantages, which are related with different CNTs growth mechanisms, and this in turn, generate CNTs with different particular properties. Due to the possible applications of energy storage devices, such as in portable electronics, in power supply devices and in electric vehicles, supercapacitors based on carbon nanostructes have attracted significant interest for their research [3,4]. The operation of these devices is due to the formation of electric double layers (EDL), which gives a high electrochemical performance, as well as high power density, long life cycles and relatively low 2

costs [5-7]. In contrast to activated carbon (ACs) [8], MWCNTs have a relatively low specific surface area (SSA) and, consequently, a low density of energy storage [8, 9]. However, while the low SSA of MWCNTs limits their capacitance, their high electrical conductivity and open porosity allow a rapid ions transport and, in this way, they show good electrical power characteristics [8]. In general, for their use as EDLCs, MWCNTs are treated in acids to increase their SSA, thus their capacitance, as well as their processability [10-13]. Depending on the type of MWCNTs and the acid treatment, capacitances in the range of 4-80 F g -1 have been reported [13]. Moreover, it has been shown that the treatment of MWCNTs in concentrated nitric acid

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(69%) at 80 C for 1 h increases the amount of oxygenated groups on the surface as well as its

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SSA to 475 m2 g-1, giving rise to an increase in the capacitance of 137 F g-1 [13]. However, the oxygenated groups on the surface of the MWCNTs, which were responsible for the high capacitance, caused self discharge in the capacitor, thus, these highly oxidized MWCNTs will

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have a very limited application [13]. On the other hand, it has been fabricated nanocomposite electrodes using different kinds of carbon nanostructures and MWCNTs. In the case of Carbon-

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MWCNTs (C-MWCNTs) nanocomposite electrodes, after nitric acid treatment of MWCNTs, these were mixed with a phenol-formaldehyde resin and processed to a thin film, then, this was

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carbonized at 850 C [14]. This resulted in a C-MWCNT film with a porous composition, a BET SSA of 150 m2 g-1, a pore volume of 0.45 cm2 g-1 and with a capacitance of 90.8 F g-1 in a concentrated H2SO4 electrolyte (38 wt%) [14]. These porous carbon composite electrodes with

densities.

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high surface area and good electrical conductivity offer a combination of high energy and power

Doping in carbon nanomaterials, on the other hand, has attracted considerable research attention due to the specific chemical and electrical properties that can be generated with it, especially,

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doping with nitrogen atoms [15,16]. Nitrogen atoms can dope carbon nanostructures in various chemical configurations [15,16]. In addition, the amount of nitrogen atoms introduced and, specifically, the amount of nitrogen atoms in a specific chemical state determine certain physical and chemical properties [16-18]. For example, it has been observed that doping with nitrogen atoms in the graphitic state increases the electrical conductivity and electrical capacitance of carbon nanostructures due to the good wetting of these materials with electrolytes [19,20]. In contrast, doping with nitrogen atoms in pyrrole configuration decreases the electrical conductivity due to the increase in structural defects generated on the MWCNT walls [21]. Other 3

effect of using a nitrogen source in the mixture reaction is that the morphology of the synthesized MWCNTs can be different and depends on the type of nitrogen source used, the N/C ratio of the reagents used for the synthesis process, the synthesis temperature, the methodology, the catalyst type, the weight percentage of the catalyst, etc. [17, 18, 22-25]. In general, metal particles coming from the catalyst are found at the tips of the MWCNTs [22, 26], but in the case of synthesized MWCNTs using nitrogen-containing solvents, it has been found that a few quasispherical metal nanoparticles covered with carbon nanoshells can decorate the tube walls [17,

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25]. These carbon nanoshells are sometimes called carbon nano-onions (CNOs) [27, 28] and they present good electrical characteristics for their use in electrical double layer capacitors (EDLC) because they can be synthesized with small diameters <10 nm [29-32], which make them

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attractive for their use in EDLC; the smaller the CNOs diameters, the larger the exposed surface

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area and, thus, much larger the capacitance [28-32].

In this work, we have synthesized nitrogen doped MWCNTs (N-MWCNTs) using reaction mixtures with different ethanol-benzylamine (EB) volume ratio. Additionally, the concentration

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of the catalyst in the reaction mixture has been varied. We have found that the overall morphology of the synthesized nanostructures depends strongly on the EB volume ratio as well

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as the concentration of ferrocene used as a catalyst. In the case of high concentrations of ferrocene in the reaction mixture, synthesized N-MWCNTs are decorated with CNOs with iron carbide nanoparticles as their core. These synthesized nanostructures with different

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morphologies showed different electrical and electrochemical properties Experimental

Synthesis of nitrogen doped Carbon Nanotubes

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The N-MWCNTs samples were synthesized by spray pyrolysis method using a quartz reactor, heated by an electrical furnace at 850 °C. We have prepared reaction solutions of ethanol as source of carbon atoms and benzylamine as nitrogen dopant reactive; using different volume relationships of ethanol:benzylamine (9:1 v/v and 1:1 v/v). Besides, we have added ferrocene (1wt % or 3 wt%) acting as the catalyst. The reaction solutions were ultrasonicated during 20 minutes and transferred to furnace inside using Argon as a gas carrier (0.20 L/min). MWCNTs were deposited in the hot zone of the quartz reactor. At the end of the reaction, the furnace was cooled down to room temperature. This material was scraped off using a metal spatula. 4

Characterization of CNTs Samples for conventional and high-resolution transmission electron microscopy (TEM, HRTEM) were prepared by dissolving the powder in isopropanol using an ultrasound bath for 20 min. A solution drop was put on lacey-carbon grid and the isopropanol was allowed to evaporate. Grids were examined using a TECNAI F30 HRTEM microscope operated at 300 kV. Raman spectra were recorded at room temperature using an InVía Micro-Raman Renishaw system. The 514.5 nm (2.51 eV) laser excitation line was focused using the 50 magnification objective to a ~ 1 µm

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spot. The X-ray diffraction analysis was performed using a SmartLab RIGAKU diffractometer, which has a copper tube as an X-ray generator and a NaI scintillation detector. The XRD

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measurements were obtained at room temperature with a step of 0.01° in a range of 20-80°. The conductivities of the N-MWCNTs films were determined by four-point probe measurements.

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The carbon and nitrogen contents of each sample were determined by using an elementary combustion system CHNS-O ECS-4010. The Brunauer–Emmett–Teller (BET) specific surface

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area and average pore diameter were determined from nitrogen adsorption/desorption isotherms by using a surface area and porosimetry system Micromeritics ASAP 2020.

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Electrode preparation and electrochemical measurements

The working electrodes (WEs) were prepared using the following procedure: (1) a binder agent, poly(vinylidene fluoride) (PVDF), was dissolved in 5 mL of N,N-dimethylformamide (DMF);

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(2) activated carbon derived from coconut shells (AC-CS), as well as synthesized CNTs, used as conductive additive and active material respectively, were added to the PVDF dissolution. The weight ratios (wt%) were 22.13/67.87/10 of PVDF, AC-CS and CNTs, respectively. This mixture was stirred at 100 °C, allowing the complete solvent evaporation to obtain a paste, which was deposited on a predetermined active area of a glassy carbon electrode used as current

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collector. Finally, the electrode was set to dry at 80 °C. The mass loading of the electrodes ranged from 5 to 10 mg/cm2, which is a standard loading for evaluating the properties of an electrode for supercapacitors. In addition, WEs were prepared without the use of AC-CS and PVDF, eliminating the possible resistive and capacitive effects of these components. These WEs were prepared as follows: 100 L of N-MWCNTs/DMF suspension (1 mg/ml) was deposited on glassy carbon electrode and dried for 24 hours at 100 ° C. The electrochemical measurements were carried out in an electrochemical cell with a three electrodes arrangement containing 2M

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KOH aqueous solutions using a VMP3 Bio-Logic SAS potentiostat/galvanostat controlled by EC-Lab software. The Hg/HgO/NaOH 1M system was used as a reference electrode and platinum gauze as a counter electrode. The prepared electrodes were tested by cyclic voltammetry (CV) performed at different scan rates ranging from 1 mV s-1 up to 50 mV s-1. Results and discussion Fig. 1 shows twelve SEM images, using two different magnifications, corresponding to the four synthesized samples. In all the analyzed samples, iron nanoparticles covered with graphene

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sheets were observed. In general, these iron nanoparticles were observed at the tips of the NMWCNTs, but the synthesized samples using 3wt% of catalyst shown nanoparticles anchored to

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the N-MWCNTs walls. In particular, this is evident in images corresponding to EB91-3 wt% sample (Fig. 1a). Due to the contrast changes observed in figures, energy-dispersive X-ray

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spectroscopy (EDS) measurements were taken to obtain the elemental composition of the samples. Quantification of the EDS data obtained for the EB91-3wt% and EB91-1wt% can be

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found in the supplementary information (Fig S1 in supplementary information). From Fig. S1 and the quantification of the EDS data, we obtain that the EB91-3wt% (Fe 9.9 wt%) sample

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shows more iron nanoparticles covered with carbon shells than EB91-1wt% (Fe 1.3 wt%) sample.

Fig. 2 shows TEM and HRTEM images corresponding to N- EB91-1wt% sample whereas Figs.

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3 and S2 (in Supplementary information) show TEM and HRTEM images corresponding to EB91-3wt% sample. From figures, we observe that both samples show bamboo-type as well as compartment walls N-MWCNTs [17]. These different morphologies of the N-MWCNTs come from the doping of the nitrogen atoms in the graphite network. As has been reported in literature, the differences in the morphology can be related to the different experimental parameters used in

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the synthesis process [22-25]. In the case that benzylamine is used exclusively as the main source of carbon and nitrogen, the obtained morphology is mainly bamboo type [22, 25]. This implies that the use of ethanol play an important role for the N-MWCNT synthesis of the wall compartment type. These two figures, moreover, serve to show the differences in the overall morphology of the synthesized samples using different wt% of the catalyst in the reaction mixture. The largest difference between synthesized samples using 1 wt% catalyst (EB91, Fig. 2 and EB11, Fig. S3 in Supplementary information), with respect to those synthesized using 3wt%

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catalyst (EB91, Figs. 3, S2 and S4, and EB 11, Fig. S5 in Supplementary information), is that those synthesized using 1wt%, show very few quasi spherical iron nanoparticles covered with carbon nanoshells, while those synthesized using 3 wt% show samples that are a nanocomposite of CNOs and N-MWCNTs. Despite the fact that both 3wt% samples are conformed by a mixture of CNOs and N-MWCNTs, the way that these CNOs are distributed over the entire sample is completely different. In the EB91-3 wt% sample, the majority of CNOs are well dispersed and decorating the N-MWCNTs walls (Figs. 3, S2 and S4 in Supplementary information) while

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EB11-3 wt% sample is conformed by CNOs aggregates and N-MWCNTs (Fig. S5 in Supplementary information), only some N-MWCNTs show some CNOs anchored on their surfaces. These differences in their morphology have important consequences on their

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electrochemical properties, as we will show later. From these results, we can see that the great difference in samples is due to that those that were synthesized using the highest amount of

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ethanol in the reaction mixture, EB91 samples, have more oxygenated functional groups on the nanotube surfaces, which allows a greater number of CNOs to be anchored on N-MWCNT walls.

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This result is very important because, in general, in order to attach metal nanoparticles on the surface of MWCNTs is necessary to carry out its chemical activation and, then, an anchoring

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process of the nanoparticles using metallic precursors [33]. In our case, the whole process is carried out in one step; the functionalization process comes from the oxygen existing in the alcohol used in the reaction mixture and the CNOs with metal core come from the used catalyst.

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Additionally, it is necessary to study the effect of the different experimental parameters on the size of the graphite crystals of the nanostructures, where an important quality that is sought in the electrodes manufactured using carbon nanostructure is a high conductivity. Raman spectroscopy is a powerful method used for determining the degree of structural ordering in graphitic materials. Fig. 4 presents the Raman spectra for the synthesized MWCNTs. The spectra show

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two main peaks around 1358 and 1590 cm-1, corresponding to the D and G bands, respectively [34, 35]. Effects of finite size, the existence of vacancies or other topological defects in the plane break the symmetry of the graphene sheet causing the D mode to become Raman active [34]. Band G is related to the vibration in the plane of a carbon structure linked in sp2 form. Using both bands, the average size of nanocrystals in the graphene sheet (La) can be calculated [34, 35]. For this, it is necessary a good definition of the D and G bands. Thus, other bands related with D and G bands should be considered [36]. The band at 1180 cm-1 (D4) has been assigned to sp3

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carbon or to impurities, the band at ~1500 cm-1 (D3) has been associated to the amorphous carbon, while the band at ~1620 cm-1 (D´) is related to the graphene layers at the surface of the graphite nanocrystals [36, 37]. Additionally, we observed four peaks in the 2400–3300 cm-1 region related to the second-order Raman spectra in samples with long range graphitic order. The 2D band is the first harmonic of the D band; it is mainly used to infer the long-range order in graphitic structure [34, 35]. The other bands in this region are harmonics of first order bands (2D´) or come from the combination of first order Raman bands (D+D” and D+G). The ID/IG,

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I2D/IG and I2D/ID ratios are generally used as an indication of the quality of the carbon materials [35, 37]. As been previously discussed, the ID/IG ratio is associated with the number of structural defects in the graphene sheets. On the other hand, I2D/ID ratio is a good indicator of the overall

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crystalline quality of the graphitic network. This can be understood in the following way: I2D band comes from a process involving two phonons; for this band to be Raman active, it is

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necessary long range graphitic order for the existence of the phonon coupling effects. Therefore, the intensities I2D/ID ratio is quite sensitive to the overall crystalline quality of the sample, i.e. it

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also considers the average size (Lc) and the type of stacking (AA, AB or BB) of the graphene sheets [34,35]. Table 1 summarizes the ratios of the peak intensities as well as their integrated

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intensities of the D band to the G band, the 2D band to the D band and the 2D band to the G band. In the case of the ratio of band D to band G, we observe that the EB91-1wt% sample presents the smallest ratios of their intensities (ID/IG) as well as their areas (AD/AG). This

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indicates that this sample has the largest average size of nanocrystals of the graphene sheet (La), i.e. it has a smaller amount of defects in their graphene sheets. This sample also shows the best overall crystalline quality, i.e. the carbon nanostructures also present a good average size of Lc crystals, related to the stacking of the graphene sheets. The EB11-1wt% sample, on the other hand, shows the smaller average size of graphene sheet (La). However, this sample has a good

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overall crystalline quality, indicating a good average crystalline size Lc. Note that in both cases the nanostructures were synthesized using 1wt% of ferrocene. Synthesized samples using 3wt% of ferrocene, EB91-3wt% and EB11-3wt%, show similar average size of graphene sheet La as well as overall crystalline quality, no matter the difference in the benzylamine concentration with respect to the ethanol used in the reaction mixture. In Fig. 5, the X-ray diffraction patterns of the samples are shown. From the X-ray diffractograms, we observe that the FHWM (β) of the 2θ~26° peak, corresponding to the (002)

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graphite plane, for the different samples follows the relation EB91-1wt%  EB91-3wt% < EB111wt% EB11-1wt% > EB11-3wt%. The results obtained with Raman spectroscopy, in general, are in good agreement with those obtained using X-rays diffractograms; the sample with the highest crystalline quality is the EB91-1wt% and the sample with the worst

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crystalline quality is the EB11-3wt%. Thus, the effect of nitrogen doping in the graphite nanocrystal as well as their differences in sample morphologies can be observed in the intensity

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differences of the 2D Raman band, the positions and their semi-widths of the (002) graphite peak in the X-rays spectra. Additionally, we can observe from the X-ray spectra other peaks, the crystalline planes corresponding to iron carbide from the metallic nanoparticles. The X-ray peaks

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in the diffractograms, associated with the iron carbide phase, are better defined in synthesized samples using a larger amount of catalyst in the reaction mixture, which is an expected result.

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However, the novelty in this work is that the majority of these iron nanoparticles covered with graphene layers are decorating the walls of the EB91-3wt% nanotubes.

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In addition, we have investigated whether the crystallinity of the synthesized nanostructures correlates with their conductivity. In Table 3, the results of the conductivity measurements are

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shown. From Table, we observe that generally the samples that show better overall crystalline quality show a better electrical conductivity. This is an expected result. However, the EB111wt% sample has a better electrical conductivity than the EB91-1wt% sample, even though this last one presents a better overall graphitic quality. This result can be understood if the EB111wt% sample has a greater amount of nitrogen atoms doping the graphitic network than EB91-

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1wt% sample, because nitrogen atoms can contribute with an electron more to the π band [38]. The effect of a larger doping of nitrogen atoms in the graphite network of the EB11-1wt% sample compared with the EB91-1wt% sample can be observed in the intensity differences of the 2D Raman bands of both sample as well as in the semi-width differences of the 2θ = 26 ° peaks of the X-rays spectra. This was also corroborated measuring the nitrogen contents of all synthesized samples using an elementary combustion system CHNS-O ECS-4010 (Table 4). From table, we observe that samples containing the highest amount of nitrogen atoms are those

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that were synthesized using a higher benzylamine volume ratio in the reaction mixture, i.e EB11 samples. As previously mentioned, MWCNTs have been used for the manufacture of supercapacitors and various routes have been explored to increase their capacitance, such as the chemical activation of the surface, the doping of the graphitic structures with heteroatoms, mainly with nitrogen atoms, anchoring metallic nanoparticles on the surface of the MWCNTs with redox properties. In a recent work, Ornela et al. [39] have reported extremely high capacitance [C ~ 160 F/g using a

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50 mV s-1 scan rate] of N-MWCNTs synthesized in a one-step process, i.e. the synthesized NMWCNTs did not need post processes such as chemical activation of their surface and/or the

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doping of graphitic structures with nitrogen atoms using annealing of nitrogen-rich precursors. Similar to Ornela et al. [39], we have been able to synthesize N-MWCNTs using a one-step CVD

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process. The structural property of our synthesized samples depends on the used experimental parameters. In order to study their capacitances, we have performed CV measurements of the different samples, which are shown in Fig. 6 and in Table 3. From figure and table, we observe

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that the sample showing the larger capacitance is the EB91-3 wt% sample (100.9 F/g), followed by the EB11-3wt% sample (32.1 F/g), the EB11-1wt% sample (12.5 F/g) and, finally, the EB91-

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1wt% sample (6.6 F/g). This is an expected result, because the EB91-3wt% sample is a composite formed by N-MWCNTs whose walls are decorated by CNOs. It has been shown in the literature that CNOs have a larger surface area than MWCNTs, therefore the combination of both

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structures increases the overall area of the sample and, consequently, its capacitance, which was corroborated using measurements of the specific surface area of the samples showing the best and the worst electrochemical capacitance (see Table 5). This is an important result that opens the possibility of generating this composite, adding and increasing the quantity of nitrogenous groups on the surface of the carbon nanostructures in the way implemented by Ornelas et al. [39]

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In this way, the capacitance of EB91-3wt% sample could be increased due to pseudo-capacitive processes coming from electrochemical reaction of these nitrogenous groups Additionally, WEs were prepared with EB91-3wt% and EB91-1wt% samples, the samples showing the best and worst capacitance, respectively, without the use of AC-CS and PVDF, which allows to remove their possible resistive and capacitive effects. The results of the different CV measurements are shown in Figures 7 and 8 as well as in Table S1 in supplementary

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information. In Figure 7 and Table S1, the capacitances of the samples are shown as a function of the scan rate. From figure and table, we observe that the capacitance decreases for both samples as the scan rate increases. In the case of sample EB91-3 wt%, capacitances were obtained ranging from 278.9 F g-1 (1 mV s-1) to 80 F g-1 (50 mV s-1), while in sample EB911wt%, capacitances are obtained ranging from 109.1 F g-1 (1 mV s-1) to 17.8 F g-1 (50 mV s-1). Note that the capacitances obtained at low scan rate (1 mV s-1) are much larger than those obtained for electrodes fabricated using AC-CS and PVDF at the same scan rate. (Figure 6 and

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Table 3). This is mainly related to the amount of active material used for the fabrication of the electrodes and slightly related to the use of PVDF and AC-CS [40, 41]. As the amount of active material deposited on the current collector surface is smaller, the ions have easier access to the

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surface of the carbon nanostructures and, consequently, the specific capacitance is increased. After performing the CV cycles by varying the scan rate, these same electrodes were used to

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study the stability of the electrochemical capacitance as a function of CV cycles using at scan rate of 5 mV s-1 (Figure 8). From figure, we observe that in the first CV cycle, the capacitance

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for both electrodes was larger than that obtained at the same scan rate, when the CV study was performed as a function of the scan rate (Figure 7, Table S1). In addition, the figure shows that

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the capacitance of the electrodes increases as a function of CV cycles. In the case of the sample EB91-3wt%, it increases ~150%, from ~ 230 F g-1 in the first CV cycle to ~360 F g-1 in the 500 CV cycle, while the sample EB91-1wt% increases ~163%, passing from 110 F g-1 in the first CV

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cycle to 180 F g-1 in the 500 CV cycle. This effect has been previously reported in electrodes of reduced graphene oxide and has been associated to the effects of reduction and/or oxidation of oxygenated functional groups anchored on carbon nanostructures [42]. When these residual oxygenated groups anchored on the surfaces of the carbon nanostructures are stable, in our case on the surfaces of the N-MWCNTs, these result in a desirable property since the electrochemical

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capacitance is continuously increased during the lifetime of the supercapacitor. In the synthesized samples reported in this work, this is observed at least for a few hundreds of CV cycles.

Conclusions Varying the concentrations of ethanol-benzylamine in the reaction mixture, as well as the concentration of the catalyst, carbon nanostructures have been synthesized. The synthesized

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samples show different morphologies, which depend mainly on the catalyst concentration. At low ferrocene concentrations, the synthesized samples, EB91-1wt% and EB11-1wt%, are NMWCNTs with iron carbide nanoparticles at their tips. In high ferrocene concentrations, the synthesized samples, EB91-3wt% and EB11-3wt%, are a mixture of N-MWCNTs and CNOs. However, the way in which the CNOs are distributed over the sample depends strongly on the amount of ethanol in the reaction mixture. In high ethanol concentrations, EB91-3wt% sample, the CNOs are decorating the walls of the N-MWCMTs, while in low ethanol concentration,

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EB11-3wt% sample, they are in general segregated from the N-MWCNTs. In high concentrations of ethanol, the probability of having oxygenated functional groups at the surface of the nanotubes is increased, and the existence of these oxygenated functional groups allows the

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anchoring of the CNOs. This anchoring process of the CNOs is carried out during the synthesis process, i.e. a single step, and does not require a post-process of chemical functionalization. The

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synthesized samples showed different conductivities, which depend mainly on their graphitization, and different electrochemical properties, which depend mainly on the overall

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morphology of the sample. The EB91-3 wt% sample showed the highest capacitance, which can be related to that CNOs are distributed more homogeneously on the N-MWCNTs walls, thus

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increasing the total surface area of the sample. Moreover, the electrochemical capacitance increases as a function of CV cycles due to redox reaction of oxygenated groups anchored on the N-MWCNTs. These results open the possibility of synthesizing carbon nanostructure mixture

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and adding more nitrogen groups, in a single step, using CVD technique. This kind of samples could present different mechanisms of energy storage, that of pseudo capacitance, due to redox reactions in the oxygenated group as well as the nitrogenous groups, and the increase in surface area, due to mixtures of two types of carbon nanostructures, thus increasing the capacitance in carbonaceous structures.

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Acknowledgments

The authors acknowledge M. Sc. Beatriz A. Rivera, M. Sc. Ana I. Peña, Ing. Francisco RamírezJacobo, Dr. Gladis Labrada, Dr. Elizabeth Isaacs-Páez and Dr. Hector G. Silva-Pereyra for technical assistance as well as to LINAN at IPICyT for providing access to its facilities. This work was supported by CONACYT through grants No. CB-2015-01-256484 (R.L.S), Atención a problemas nacionales No. 789 (L.F.C.R) and for scholarships (E.T.M, and J.V.C.S.).

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Fig. 1. SEM images of the different synthesized N-MWCNTs using different ethanol benzylamine concentrations and different wt% of catalyst in the mixture reaction. The images corresponding to a), b) and c) are from EB91-3wt% sample, d), e) and f) from EB91-1wt% sample, g), h) and i) from EB11-3wt% sample, j), k) and l) from EB11-1wt% sample.

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of ro -p re lP ur na Jo Fig. 2. TEM and HRTEM images of EB91-1wt% sample.

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of ro -p re lP ur na Jo Fig. 3. TEM and HRTEM images of EB91-3wt% sample.

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Fig. 4. Raman spectra from all samples and their fitting curves.

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Fig. 5. X-ray difffractograms of (a) EB91-1wt% and EB91-3wt% and (b) EB11-1wt% and

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EB11-3wt%

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Fig. 6. Cyclic voltammograms of electrodes fabricated with (a) EB91-3wt%, (b) EB11-3wt%, (c)

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EB91-1wt% and (d) EB11-1wt% samples, in aqueous KOH 2 M at a scan rate of 1 mVs-1.

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Fig. 7. Cyclic voltammograms and the electrochemical capacitances of electrodes fabricated with EB91-3wt%, (a) and (b), and EB91-1wt%, (c) and (d), samples in aqueous KOH 2 M using

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different voltage scan rates.

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Fig. 8. Cycling performances and electrochemical capacitances of EB91-3wt%, (a) and (b), and

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EB91-1wt%, (c) and (d), samples in aqueous KOH 2 M at a scan rate of 5 mVs-1.

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Sample EB91-3wt% EB91-1wt% EB11-3wt% EB11-1wt%

ID/IG 0.74 0.49 0.69 0.97

AD/AG 0.96 0.82 0.97 1.26

I2D/ID 0.72 1.71 0.71 0.75

A2D/AD 0.97 2.08 0.83 0.98

I2D/IG 0.53 0.84 0.49 0.44

A2D/AG 0.93 1.7 0.80 1.23

Table 1. Intensity ratio of Raman peaks as well as their integrated areas obtained after fitting

Lc (nm) 7.16 7.16 3.91 4.26

d (nm) 0.34 0.34 0.34 0.34

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FWHM (°) 1.19 1.19 2.18 2.00

Lc/d 21.1 21.2 11.5 12.5

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EB91-3wt% EB91-1wt% EB11-3wt% EB11-1wt%

(002) Peak Peak center (°) 26.24 26.34 26.17 26.05

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Sample

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curves.

Table 2. Main parameter of fitting curve of 002 graphite peaks for all synthesized sample, interplanar distance d beween graphene sheets, obtained using Bragg's Law, as well as the

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crystalline size Lc, obtained using the Scherrer equation.

Capacitance (F/g) 100.9 6.6 32.1 12.5

Conductivity (S/cm) 2.1 6.1 2.7 12.3

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Sample EB91-3wt% EB91-1wt% EB11-3wt% EB11-1wt%

Table 3. Electrical conductivity and electrochemical capacitance at a scan rate of 1 mVs-1 of synthesized samples.

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Sample EB91-3wt% EB91-1wt% EB11-3wt% EB11-1wt%

N (Wt %) 0.50 0.11 3.41 2.53

C (Wt %) 82.33 93.96 94.57 91.96

SBET (m2/g) 42.7 36.6

Vpore (cm3/g) 0.14 0.08

dpore (nm) 13.26 8.5

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Sample EB91 3 wt% EB91 1 wt%

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Table 4. Determination of carbon and nitrogen contents of all synthesized samples.

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Table 5. BET specific surface area, pore volume and pore diameter of samples showing the

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highest (EB91-3wt%) and lowest (EB91-1wt%) electrochemical capacitance.

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