Nanostructure, porosity and electrochemical performance of chromium carbide derived carbons

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Available at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/carbon

Nanostructure, porosity and electrochemical performance of chromium carbide derived carbons Pedro Gonza´lez-Garcı´a a,*, Adriana M. Navarro-Sua´rez b, Javier Carretero-Gonza´lez b, ´ vila-Brande d,*, Luis C. Otero-Dı´az d Esteban Urones-Garrote c, David A a

Centro de Investigacio´n en Micro y Nanotecnologı´a, Universidad Veracruzana, Calzada Ruiz Cortı´nes No. 455, 94294 Boca del Rı´o, Veracruz, Mexico b ´ lava, Spain CIC EnergiGUNE, Albert Einstein 48, 01510 Min˜ano, A c Centro Nacional de Microscopı´a Electro´nica, Universidad Complutense, E-28040 Madrid, Spain d Departamento de Quı´mica Inorga´nica, Facultad de Ciencias Quı´micas, Universidad Complutense, E-28040 Madrid, Spain

A R T I C L E I N F O

A B S T R A C T

Article history:

This paper presents the results from the investigation of the influence of the chlorination

Received 13 October 2014

temperature, the carbide crystal structure, the Cr/C ratio and physicochemical properties of

Accepted 18 December 2014

CrCl3 on the morphology, nanostructure, textural properties and electrochemical

Available online 24 December 2014

performance of CDCs. Electron microscopy and its analytical associated techniques reveal that these carbons, mainly composed by disordered graphene layers, evolve into graphitic nanostructures as a result of increasing the Cr/C content, the reaction temperature and the template effect of the etched CrCl3 halide. Their textural analysis indicates the formation of micro/mesoporous carbons with a pore width below 1.5 nm, surface area as high as 835 m2/g and exhibit capacitive behavior in aqueous electrolyte. Ó 2014 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbide-derived carbons (CDC) represent a group of materials intensively studied due to their interesting features such as shape conservation, variety of nanostructures, tunable texture and potential use in many demanding applications especially for energy storage devices [1,2]. Synthesis of CDC materials from metal carbides has been achieved by vacuum decomposition, selective etching by melts and leaching in supercritical water; nevertheless, selective etching of metal carbides at high temperatures in presence of halogens seems to be the most used method to produce them [3]. Here the metal carbide crystal structure is used as a template and during the synthesis the metal is extracted layer by layer from

the carbide precursor providing an atomic-level control over the properties of the final CDC. In this sense, numerous studies have shown their highly tuneable surface area and narrow pore size distribution (PSD) [4,5] as a result of several factors such as the chlorination temperature [6,7], carbide structure and stoichiometry [8,9]. In order to understand the relationship between the CDC nanostructure and their physicochemical properties, transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS) and physical adsorption of gases have become powerful observation and analysis techniques. By means of TEM studies the proposal of mechanisms for the formation and evolution of CDC has been possible [10]. In some carbides such as b-SiC [11], B4C [12], Cr3C2 [13], NbC [14], and WC [15],

* Corresponding authors. ´ vila-Brande). E-mail addresses: [email protected] (P. Gonza´lez-Garcı´a), [email protected] (D. A http://dx.doi.org/10.1016/j.carbon.2014.12.066 0008-6223/Ó 2014 Elsevier Ltd. All rights reserved.

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(400–800 °C), carbon coatings are transformed from the external surface of the metal carbide during the synthesis of the CDC at intermediate temperatures. With the increase of the reaction temperature (900–1200 °C), several carbon nanostructures have been observed, for instance nano onions (external diameter from 15 to 35 nm) from SiC [16], straight, curved or cross-linked ribbons from ZrC and TiC [17], multiwalled nanotubes and turbostatic graphite from Al4C3 [18]. It is well accepted that the crystal structure of the metal carbides strongly influences the textural properties of the CDCs, however bearing in mind that the chlorination involves the etching of the metals as metal halides, high volatile halides such as TiCl4 (bp: 135 °C) or SiCl4 (bp: 57.6 °C) are quickly eliminated, in the case of CrCl3 (bp: 1300 °C), the crystal structure of the metal halide can also influence the final nanostructure and/or porosity of the CDC. In fact, we have reported that the CDC obtained from Cr3C2 contains nanoplates of CrCl3 trapped inside the graphene layers even at 900 °C. The laminar crystal structure of CrCl3 could act as a template in the stacking of graphene layers yielding more graphitic materials compared with other CDC in similar conditions. In 2011 Thomberg et al. [19] studied the influence of the chlorination temperature and the chemical composition in the family of chromium carbides (Cr3C2, Cr7C3 and Cr23C6) in the textural properties of the CDCs, however a detailed nanostructural characterization to understand their different textural properties in comparison of other CDCs is not given. For this reason, in this work we analyse the effects of the chlorination temperature, the carbide crystal structure, the Cr/C ratio and crystal the structure of CrCl3 on the morphology, nanostructure, chemical composition and bonding of the final CDC mainly by TEM and associated techniques. Besides, their in-plane correlation length will be extracted from the Raman spectra. The nitrogen adsorption–desorption measurements will permit to evaluate their textural features, specially the presence of micro and mesoporosity and finally, their electrochemical performances in aqueous electrolyte are evaluated by cyclic voltammetry measurements at different scan rates.

2.

Experimental

2.1.

Sample preparation

Chromium carbides (99%), purchased from Alfa Aesar, were placed in a quartz vessel and heated in a tubular furnace at 700, 900 and 1050 °C as final temperatures, with a heating rate of 50 °C/min. under a continuous flow of high purity chlorine gas (25 cm3/min). When the final temperature was reached, the reaction time was set on 1 h. Elapsed the reaction time, chlorine gas flow was cut and replaced by argon (25 cm3/min) for removal of the rest of reacting chlorine and halides during the reactor cooling to room temperature by natural convection. The synthesis can be described by the next chemical reaction: x Crx Cy ðsÞ þ nCl2 ðgÞ ! yC ðsÞ þ xCrCln ðgÞ 2

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In order to identify the different samples of CDC, as result of their different Cr/C content and reaction temperature, a nomenclature type CrxCy-CDCT will be used where the subscripts x, y correspond to the number of carbon and chromium atoms in the precursor and T is the chlorination temperature.

2.2.

Sample characterization

X-ray powder diffraction (XRD) patterns were recorded on a ´ Pert-MDP diffractometer (CuKa1 k = 0.15406 nm), in Philips X the range 5° 6 2h 6 65°. Scanning electron microscopy (SEM) micrographs were obtained with a JSM 6335 F electron microscope operating at 10 kV and a working distance of 15 mm. Transmission electron microscopy (TEM) studies have been performed in a JEOL 3000 F (acceleration voltage of 300 kV) microscope (point resolution of 0.17 nm) equipped with an ENFINA spectrometer for Electron Energy Loss Spectroscopy (EELS) measurements. The EEL spectra were acquired in the same conditions as described in [13]. The Raman spectra of samples prepared at 900 and 1050 °C, were acquired with a micro-Raman spectrometer, using a Spectra-Physics solid state laser operating at 532.0 nm. The optical system is always calibrated with a standard neon discharge lamp, getting a resolution of the spectra close to 4 cm1. The surface area and pore structure characterization were made by means of N2 adsorption–desorption isotherms at 77 K using a surface area and porosity analyzer (Micromeritics ASAP 2020). The surface area (SBET) was calculated according to the BET method in the relative range of p/p0 = 0.05–0.25, in order to avoid over or underestimate this parameter [20]. The total pore volume was determined at the relative pressure p/p0 = 0.99. Taking into consideration the inconsistencies of the BET model along the evaluation of those materials with high microporous volume [21], as in most of CDC, in this work we report an evaluation of the adsorption–desorption data by using alternative models: Dubinin–Radushkevich approximations, the empirical as method and the density functional theory (DFT) [22–24]. On the other hand, the pore size distribution (PSD) calculations were made using the software for the Non-linear Density Functional Theory (NLDFT) model provided by Micromeritics considering slit-shape pore geometry. All the voltammetric measurements were made in a cavity micro-electrode (CME), in which the electrochemical interface area is around a fraction of mm2 and the ohmic drop coming from the bulk of the electrolyte can be neglected, allowing the use of high scan rates. All the samples were studied electrochemically in a 1 M sulfuric acid solution (H2SO4, Fisher Chemical, A.R.) in a 3-electrode configuration with a platinum wire as counter electrode and a Hg/HgSO4 as reference electrode. The microcavity was filled with active material by pressure of the carbon powders against a glass plate. The cavity was cleaned by immersing the electrode in ethanol in an ultrasonic bath between experiments. All the measurements were performed at ambient conditions with a multichannel potentiostat/galvanostat (Biologic VMP3, France).

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3.

Results and discussion

3.1.

Morphology

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In Fig. 1, SEM micrographs show the morphology of the precursors and the synthesized CDC. In the case of the Cr3C2 and their derived CDC, the micrographs show that they consist of conglomerated particles of non-uniform size distribution and after the corresponding thermal treatment there is not a remarkable modification in their morphology and the texture of the surface respecting to the carbide precursor. In contrast, the CDC produced from Cr7C3 and Cr23C6 present important changes in their morphology in comparison with their precursors. The CDC produced from Cr7C3 consists of highly textured particles, especially in Cr7C3-CDC1050, where some slits and large pores are detected. This situation could be originated from the amount of chromium atoms removed (Cr/C = 2.3) from the carbide precursor. On the other hand, the texture observed in the micrographs suggest that the elimination of

the chromium atoms from the crystalline Cr7C3 starts at 700 °C and it is continuous along the thermal treatments up to 1050 °C. The CDC obtained from Cr23C6 present a similar behavior, although the changes in their morphology are more remarkable in comparison with the previous carbides. Notice, that in some cases, the particles of Cr23C6-CDC700 seem to be disintegrated. This situation could be related to the high amount of chromium that is removed from the Cr23C6 precursor (Cr/C = 3.8) as CrCl3. As the produced halide is generated from the surface to the inner core of the carbide particles, the high amount of CrCl3 transported from the core increase the pressure inside the particle producing cracks (see Fig. 1) and allowing easier halide diffusion. The observed changes on the microstructure are connected with the amount of the metal halide removed during the chlorination process, in this sense, in the SEM micrographs corresponding to Cr23C6-CDC900 and Cr23C6-CDC1050 some larger pores and channels are found in comparison with the carbides with lower chromium content.

Cr23C6

Cr3C2

Cr7C3

Cr3C2-CDC 700

Cr7C3-CDC 700

Cr23C6-CDC 700

Cr3C2-CDC 900

Cr7C3-CDC 900

Cr23C6-CDC 900

Cr3C2-CDC 1050

Cr7C3-CDC 1050

Cr23C6-CDC 1050

Fig. 1 – SEM micrographs showing the morphology of the metal carbides used as precursors and the changes in the morphology and surface of the produced CDC. The yellow arrow in Cr7C3-CDC1050 mark slits and the green one in Cr7C3CDC1050 and Cr23C6-CDC1050 round pores. (A color version of this figure can be viewed online.)

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Fig. 2 – Characteristic XRD patterns for each crystalline carbide precursor and their derived CDC synthesized at the indicated chlorination temperatures.

3.2.

X ray powder diffraction analyses

The X-ray diffraction patterns displayed in Fig. 2 reflect the evolution of the metal carbide with the chlorination temperature. After the first thermal treatment (700 °C), the characteristic maxima of the crystalline carbide precursor disappear suggesting their total conversion into disordered materials. Only in the diffractogram of Cr7C3-CDC700, some intense and sharp maxima are observed, that can be identified as the reflections of residual CrCl3, either as an impurity or as an intercalated material among the carbon structure as it has been previously observed in the chlorination of Cr3C2 [13]. This situation is probably a result of the low volatility of CrCl3, due to its high boiling point (1300 °C) and low diffusion rate (0.31 cm2/s at 700 °C) [25]. In the case of the CDC produced at 900 °C, an incipient definition of the 002 graphite reflection (2h  26°) is observed indicating an increase in the graphitization degree. This maximum is slightly more intense and defined with the increase of the Cr/C ratio. Finally in the CDC synthesized at the highest temperature (1050 °C), the 002 graphite reflection appears well defined, especially in the Cr7C3-CDC1050 and Cr23C6-CDC1050 samples. Additionally a broader maximum near to 2h  43°

corresponding to the 100/101 graphite reflections is faintly visible, suggesting a higher graphitization degree in these CDC. The evolution of the graphitization of the CDC could be related with the content of chromium in the precursor. The CrCl3 presents the layered YCl3-type structure [26] and its high boiling point (1300 °C) produces a low elimination rate from the precursor acting as a template in the stacking of graphene layers yielding a more graphitic CDC.

3.3.

Raman spectroscopy studies

Raman spectroscopy has also been employed to investigate the bulk average structure of the CDC prepared at 900 and 1050 °C, providing information about ordering and the inplane correlated length (La). Due to the characteristics of these CDC, the analysis of the Raman spectra showed in Fig. 3 will be divided in two regions. At the low frequency region, commonly named as first-order Raman, the spectra display the two typical bands for disordered carbon materials: the so-called graphite (G) band at 1580–1600 cm1 and the disorder-induced (D) band at 1300–1350 cm1 [27,28].

Fig. 3 – Raman spectra for the produced CDC at 900 (a) and 1050 °C (b).

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Table 1 – Parameters calculated from Raman measurements. Sample

Cr3C2-CDC900 Cr3C2-CDC1050 Cr7C3-CDC900 Cr7C3-CDC1050 Cr23C6-CDC900 Cr23C6-CDC1050

Raman ID/IG

La (nm)

1.05 0.84 1.56 0.76 0.91 0.73

4.7 5.9 3.2 6.5 5.4 6.8

At the second-order region of the Raman spectra the 2D band (2700 cm1) is clearly observed. In graphitic carbon materials, this band is asymmetric and comprises at least two bands located, one below 2700 cm1 and another above 2700 cm1 [29]. In the CDC samples produced from Cr7C3 and Cr23C6 an additional band located at 3250 cm1 emerge, termed as 2D 0 [30]. Most of times, the presence of these bands is related to the three-dimensional stacking of the graphene layers in graphite and the graphite crystal size [29,30]. An additional feature is observed for the CDC obtained from Cr23C6 since

its spectra presents a weak band located at 2450 cm1. The presence of this band has been attributed to a non-dispersive overtone mode of an in-plane optical phonon produced during the double resonance Raman scattering in an intervalley around the K symmetry point in the Brillouin zone [30]. The ratio between the intensity of the D and G bands (ID/IG) is related to in-plane correlated length of the graphene-like layers, as it described by the Tuinstra–Koenig (T–K) relationship [31]. In the data displayed in Table 1, it is possible to observe that the intensity ratio of ID/IG increases inversely to La; hence, the T–K model can accurately represent the dimensionality of these CDC. This bulk technique suggest that the degree of graphitization in these materials increase with the chlorination temperature as it has been observed in other CDCs, but also the template effect of the CrCl3 seems to play an important role in the graphitization of these family of CDC. In order to confirm these hypotheses a detailed HRTEM study is performed.

3.4.

High-resolution TEM studies

A direct observation of the local nanostructure in the produced CDC has been obtained from the HRTEM images

Cr7C3-CDC 700

Cr3C2-CDC 700

5 nm

5 nm Cr3C2-CDC 900

Cr7C3-CDC 900

5 nm

5 nm

Cr7C3-CDC 1050

Cr3C2-CDC 1050

Cr23C6-CDC 700

5 nm Cr23C6-CDC 900

5 nm Cr23C6-CDC 1050

5 nm Fig. 4 – HRTEM images showing changes in the nanostructure of the produced CDC as a result of the Cr/C ratio and the chlorination temperature. The Fast Fourier Transform (FFT) inset in each image display confirms the evolution of the graphitization degree. (A color version of this figure can be viewed online.)

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displayed in Fig. 4. Here, the contrast of the images corresponding to Cr3C2-CDC700 and Cr3C2-CDC900 is the typical for highly disordered carbon materials, showing the stacking of a maximum four graphene-like layers in local areas. In the case of Cr3C2-CDC900 we can also detect some open graphene-like layers at the edge of the particle. The image of Cr3C2-CDC1050 shows significant changes since its structure seems to be formed by folded and interlocked graphitic ribbons up to 3–6 stacked graphene layers, as a clear indicative of the graphitization degree derived from the higher chlorination temperature. This behavior is also reflected in the Fast Fourier Transform (FFT) of each sample (see insets in Fig. 4) where at higher temperatures we observe the hkl-type graphite diffraction rings, with the presence of 00l-type reflections absent at 700 °C in Cr3C2-CDC700 and Cr7C3-CDC700. In Cr7C3-CDC700, the contrast of the image indicates a nanostructure composed by disordered graphene layers intercalated with thicker layers of CrCl3 identified by its FFT and the EDS analyses. In the CDC produced at higher thermal treatments, the CrCl3 is completely eliminated and as it is observed in the HRTEM images of Cr7C3-CDC900 and Cr7C3-CDC1050 and the number of the stacked graphene layers is as high as 19 layers and their length exceeds 6.5 nm. The high graphitization degree detected in Cr7C3-CDC900 and Cr7C3-CDC1050, could be related to the presence of the encapsulated CrCl3 up to 700 °C detected in Cr7C3-CDC700. As a result of the increase in the chlorination temperature from 700 to 900 and 1050 °C, the intercalated halide is slowly eliminated acting as a template promoting the ordering of the graphene layers into more graphitic structures. As observed at the bottom of Fig. 4, Cr23C6 as carbon precursor produces the most graphitic structures of this group of chromium carbides. Once again, the HRTEM images show that the graphitization degree increases with the chlorination temperature. Cr23C6-CDC700 presents local graphitic domains formed by 2–4 stacked graphene layers and pores of 5 nm in diameter. In Cr23C6-CDC900 the number of stacked graphene layers increases (4–14 layers) and some of them are curved. Finally in Cr23C6-CDC1050 the number of stacked graphene layers seems to be unaffected, in comparison with Cr23C6-CDC900. In these CDCs we notice the presence or larger pores with diameters around 19 nm. The chlorination kinetics seems to play an important role in the formation of these CDC, in addition to the reaction temperature and the Cr/C content in the carbide precursor. Cr3C2 has the highest melting point of chromium-carbon system (1810 °C); as consequence, the conversion of Cr3C2 into CDC takes place gradually and is controlled by the reaction temperature yielding, at 400 °C, core–shell materials formed by a crystalline Cr3C2 nucleus surrounded by coating of disordered carbon [13]. In our synthetic conditions, the continuous chlorine flow passing through the reactor while rising the temperature up to the selected thermal treatment (700, 900 or 1050 °C) permits the complete formation of the CDC and the almost complete elimination of the formed halide at 700 °C. In contrast, Cr7C3 and Cr23C6 do not present any sign of etching at T 6 700 °C; however at this temperature, they have been totally converted into CDC (see Fig. 2) suggesting that the halide is suddenly produced and eliminated.

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From these results, it seems that the higher the Cr content the less effective is the elimination of CrCl3, yielding CDCs composed by graphitic ribbons. However the presence of large pores in the Cr23C6-CDC900 and Cr23C6-CDC1050 detected by SEM and HRTEM (see Figs. 1 and 4), could facilitate the elimination of CrCl3 and the graphitic ribbons are formed at lower temperatures than in the case of Cr7C3, where the more compact arrangement of the graphene layers prevents the effective elimination of CrCl3, remaining trapped inside the carbide structure until higher chlorination temperatures.

3.5.

Mass–density and bonding ratio calculations

The low–loss regions of the EEL spectra are collected in Fig. 5. Here, the spectra exhibit the p–p* and p + r plasmon peaks such as in graphite. Their intensity and positions do not follow any trend that can be related to the chlorination temperature or the Cr/C content in the carbide used as precursor. However, all the peaks have values below the graphite ones (7 and 27 eV respectively). Shifting of the p + r peak is related to changes in the mass density values as a result of the variations in the local nanostructure detected previously by HRTEM. As it is collected in Table 2, the mass density values are below the graphite used as reference (2.2 g/cm3). In the case of Cr7C3 the decrease of the density from 700 to 900 °C can be assigned to the already discussed less effective elimination of CrCl3, remaining then, almost constant, at 1050 °C. The evolution in the graphitization degree of the CDC can be also followed by using the high-loss energy region of the EEL spectra. In this context, the carbon–K ELNES of the CDC prepared at 700 °C display a round and featureless r* peak, indicative of the highly disordered structure of these materials, as the previous XRD diffractograms, Raman spectra and HRTEM studies suggested. The increase of the reaction temperature up to 900 °C is reflected in the slight sharpening of the C–K ELNES r* peak, as a result of the incipient formation of the graphitic domains along the CDC. In the highest thermal treatment (1050 °C) the shape of the C–K ELNES is graphite-like in all the spectra, as evidence of the high graphitization degree of the CDC. Additionally, it is possible to observe that the intensity of the p* peak increases with the reaction temperature becoming sharper. The calculated sp2/sp3 bonding content is almost 100% and does not vary significantly with the increase of the reaction temperature or the Cr/C in the carbide selected as precursor (see Table 2). These data suggest that in spite of the different graphitization degree the bonding type is almost unaffected, and the temperature only induces the arrangement of the sp2 graphene layers into graphitic structures.

3.6.

Assessment of the surface area

The textural parameters of the carbon materials prepared from the chromium carbides under study were obtained from the nitrogen adsorption–desorption isotherms at 77 K. In Fig. 6 we do not observe a significant evolution in the isotherms shapes related to the synthesis temperature, however

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Fig. 5 – EEL spectra collected from the produced CDC. Low-loss regions (a–c) and ELNES (d–f) of the carbons derived from Cr3C2, Cr7C3 and Cr23C6, respectively. Notice the evolution of the r* peak of the CDC into graphite-like shape as a result of the chlorination temperature. (A color version of this figure can be viewed online.) important variations in the curves are related to the different Cr/C content in the initial carbide. Attending to the IUPAC classification, the isotherm plots for the CDC derived from Cr3C2 (Fig. 6a), are practically Type I, characteristic of microporous materials. The presence of the thin hysteresis loop can be associated to the presence of some small mesopores. In the case of the

CDC produced from Cr7C3 (Fig. 6b), the isotherms vary significantly in comparison with the previous ones. The curves can be classified as Type I, however the wide hysteresis loops Type H4 indicate the presence of mesoporosity. Additionally, the considerable amount of nitrogen adsorbed at p/p0 P 0.8 suggests the presence of textural porosity or macropores.

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Table 2 – Summary of the data extracted from EELS of the different CDC. Sample

p + r (eV)

q ± 0.1 (g/cm3)

sp2 ± 3 (%)

Cr3C2-CDC700 Cr3C2-CDC900 Cr3C2-CDC1050

24.1 24.2 25.7

1.8 1.8 2.0

93 100 100

Cr7C3-CDC700 Cr7C3-CDC900 Cr7C3-CDC1050

24.1 22.7 21.2

1.8 1.6 1.4

100 96 100

Cr23C6-CDC700 Cr23C6-CDC900 Cr23C6-CDC1050

22.4 23.6 23.8

1.6 1.7 1.7

99 100 100

Finally, in the CDC prepared from Cr23C6 (the highest Cr/C ratio) the isotherms change totally and follow the Type 3 (Fig. 6c). Here, we notice the smallest quantity of nitrogen adsorbed at p/p0 6 0.2 suggesting the smaller amount of micropores in all the CDC described before. In addition, the highest quantity of nitrogen adsorbed at p/p0 P 0.8 reflects their macroporosity. A clearer evidence of the development of microporosity is obtained from the low relative pressure isotherms (Fig. 6d). Notice the high amount of nitrogen adsorbed even at p/p0 = 0.00001 especially in those materials prepared at 900 °C. Table 3 summarizes the results obtained with the different models proposed in this work in order to

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evaluate the surface area values. The data follow the behavior observed in the experimental SBET: an important increase in the surface area values from 700 to 900 °C (up to 56 times higher in the case of Cr7C3-CDC900 in comparison with Cr7C3-CDC700 due to the strong elimination of CrCl3 in this temperature range) and a moderate decrease from 900 to 1050 °C (up to six times smaller in the case of Cr23C6-CDC1050 in comparison with Cr23C6-CDC900). This behavior has been previously observed in most of CDC [32–34] as result of the increase in the graphitization degree in the nanostructure of the CDC with the reaction temperature. This situation in also reflected in the Sav values (see Table 3) suggesting a good correlation among the results obtained from the different approaches and the observations derived from the microstructural analyses. Exceptionally, the CDC obtained from Cr23C6 present high surface area values at 700 °C almost ten times larger than the other carbide precursors, this behavior corresponds with the different microstructural features found between the precursors at this temperature, the TEM images shows highly porous particles in Cr23C6-CDC700 (see Fig. 4) compared to the other materials, and the presence of high amounts of CrCl3, strongly affects the surface area of Cr7C3-CDC700 yielding even a lower value compared with Cr3C2-CDC700 (see Table 3). Valuable information is obtained from the Smi and SDFT (<2 nm) values. Notice how these very different approximations indicate the presence of important microporous

Fig. 6 – Standard nitrogen adsorption–desorption isotherms at 77 K for the CDC derived from Cr3C2 (a), Cr7C3 (b) and Cr23C6 (c) at the indicated chlorination temperatures. High resolution N2 isotherms, in logarithmic scale, for the CDC having adsorption at p/p0 6 0.001 (d).

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Table 3 – Comparison of the surface area values (m2/g) for the CDC derived from Cr3C2, Cr7C3 and Cr23C6 at the indicated chlorination temperatures. Sample

Cr3C2-CDC700 Cr3C2-CDC900 Cr3C2-CDC1050 Cr7C3-CDC700 Cr7C3-CDC900 Cr7C3-CDC1050 Cr23C6-CDC700 Cr23C6-CDC900 Cr23C6-CDC1050

Se

3 6 3 5 36 28 125 105 37

Smi

12 694 124 10 496 128 126 286 34

SDR

15 700 127 15 532 156 251 391 71

surfaces in almost all the CDC. These data express more convincingly the presence of microporosity in the present materials as it was previously suggested by the analysis of their respective isotherm plots (see Fig. 6).

3.7.

Pore size distribution analyses

The pore size distribution (PSD) plots, derived from the NLDFT method are represented in Fig. 7. Here, the presence of micro and mesopores is observed in all the samples, except for Cr3C2-CDC700 and Cr23C6-CDC700 which present only a few mesopores. These plots confirm the development of wide (pore width 0.7–2 nm) and narrow (pore width < 0.7 nm) microporosity, according to the IUPAC definitions. In this sense, mesoporosity in these materials can be attributed to pores formed between graphitic ribbons, especially in the CDC produced from Cr7C3 and Cr23C6, as suggested by the HRTEM images (see Fig. 4). On the other hand, imperfections in graphitic ribbons as well as lingering regions of disordered graphene layers evolve towards micropore formation. The presence of these nanostructural features in the CDC particles contribute to the obtained PSD, typical of nanoporous carbon materials, as it has been previously identified in most of CDC [1,5,33–35]. The results collected in Table 4, together with the PSD plots, exhibit the complex pore structure developed in these CDC. Notice the evolution in the pore volume, only Cr7C3-CDC700 and Cr23C6-CDC700 present an incipient pore formation, then all the CDC achieve the highest pore volume

Stot

28 794 239 19 942 239 257 539 83

SBET

28 799 223 19 950 231 257 535 66

SDFT

Sav

<2 nm

Total

3 772 81 3 867 133 235 425 33

11 818 106 15 1031 209 363 592 68

18 771 157 16 835 201 290 507 74

at the chlorination temperature of 900 °C and finally, at 1050 °C the total pore volume decreases. This kind of bellshape behavior in the textural parameters (surface area, pore volume and width) has been frequently observed in other CDC as a result of the increase in the reaction temperature [1,5,19,32–34]. In addition, the structural porosity and the micropore volume (W0) of the present CDC also follow this trend. Two important features detected in these CDC are the textural porosity, ascribed to the interparticle voids among the carbon particles, and the presence of macroporosity which can be mainly detected in the SEM micrographs of Cr23C6CDC700 and Cr23C6-CDC900 (see Fig. 1). This external porosity is later reflected in the high Se values displayed in Table 3. According to the results presented in Table 4, this parameter is clearly affected by the Cr/C ratio in the carbide selected as precursor, representing a good agreement with the texture detected by SEM and HRTEM. The comparison of the present CDC to those reported by Thomberg et al. [19], allow us to find significant differences in the PDS plots and the pore structure. First, the reported PSD plots do no present evidence of pore formation bellow 2 nm, indicating that these CDC materials are only mesoporous. Secondly, these CDC present a bimodal PSD function with a first maximum in the region from 2.5 to 3.5 nm and the second one from 6 to 8 nm. Our PSD present almost the same relative maxima, however, they show other features such as a third maximum from 7 to 12 nm and the presence of micropore volumes in most of samples. In addition, the

Fig. 7 – PSD for the CDC derived from Cr3C2 (a), Cr7C3 (b) and Cr23C6 (c) at the indicated chlorination temperatures.

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Table 4 – Pore volumes and width for the CDC derived from Cr3C2, Cr7C3 and Cr23C6 at the indicated chlorination temperatures. Sample

Cr3C2-CDC700 Cr3C2-CDC900 Cr3C2-CDC1050 Cr7C3-CDC700 Cr7C3-CDC900 Cr7C3-CDC1050 Cr23C6-CDC700 Cr23C6-CDC900 Cr23C6-CDC1050

Pore volume at p/p0 (cm3/g) (0.99–0.7) Textural

(0.7) Structural

E0 (kJ/mol)

W0 (cm3/g)

L0 DR

L0 DFT

0.04 0.46 0.16 0.04 0.71 0.45 0.78 1.22 0.42

0.02 0.04 0.03 0.03 0.15 0.28 0.55 0.84 0.36

0.02 0.42 0.13 0.01 0.56 0.17 0.23 0.38 0.06

15.4 22.1 18.5 15.6 18.6 19.2 17.7 19.8 16.5

– 0.35 0.08 0.01 0.34 0.08 0.09 0.19 0.03

2.36 1.01 1.31 1.51 1.36 1.25 1.36 1.29 1.45

2.73 1.03

reported total pore volume for the CDC produced from Cr23C6 at 1100 °C showed values as high as 0.719 cm3/g, what is 41% smaller than the same parameter of Cr23C6-CDC900 (1.22 cm3/g) produced in this work (see Table 4).

3.8.

Cyclic voltammetry data

The standard nitrogen adsorption–desorption isotherms depicted in Fig. 6 have showed that the highest surface area, for each of the three chromium carbide stoichiometries studied in this work, is developed at 900 °C. Therefore, these carbide-derived nanoporous carbons could be used as electrode materials in electrochemical double layer capacitors. In order to make a direct comparison between the different scan rates, all current values were normalized to current values measured in the pure capacitive behavior region (at 0.1 V/Ref). The samples Cr3C2-CDC900 and Cr7C3-CDC900 showed pure capacitive behavior as it can be observed from the perfect symmetry of the CVs (Fig. 8a and b). The increasing of the ohmic drop in the bulk electrolyte with increasing the scan rate produced a small distortion in the CVs. Fig. 8c depicts the capacitive current (i.e. the current value at 0.1 V) versus the scan rate, a perfect linear plot is observed for both CDC samples which prove that the currents measured from the CV experiments were purely capacitive following equation: I ¼ Cd Av

Average pore width (nm)

(0.99) Total

2.11 1.63 0.95 1.65

where, I is the current, Cd the capacitance (F m2), A the surface area (m2) and t the scan rate (V s1) [36]. Because of the solvated ion sizes of the cation (H+/H3O+; 0.90 nm [37]) and the anion (SO2 4 ; 0.76 nm [38]) being smaller than the average pore size in both Cr3C2 (1.03 nm calculated by DFT) and Cr7C3 (1.63 nm calculated by DFT) samples, most of the surface area of the CrxCy-CDC900 were accessible to these ions, therefore, exhibiting a capacitive behavior. Two additional current peaks were clearly distinguished in the other samples; peak 1 was observed during the anodic scan while peak 2 during the cathodic one. In Fig. 9, the results for the Cr23C6-CDC900 sample are depicted as example. These peaks appear between 0.05 and 0.4 V/Ref, range where the OCV is included and the charge storage mechanism changes from cation to anion adsorption. Results shown in Fig. 9a, are similar to the data shown by Aurbach et al. [39] and Lin et al. [40], indicating that both the cation and anion are being partially trapped into small pores while desolvated. Fig. 9c shows the change of the logarithm of the peak current versus the logarithm of the scan rate. The slope of the plot is 0.84 and 0.81 for peak 1 and 2 respectively; therefore, these peaks seem to be part of a capacitive charge storage mechanism in agreement with Eq. (1) and confirming the trapping of the H+/H3O+ and SO2 4 ions. The proposed mechanism consists of desolvated ions that can be partially trapped into small pores of comparable diameter or larger after overcoming an activation barrier associated with the partial ion

Fig. 8 – (a) and (b) Normalized Cyclic voltammetry of the samples Cr3C2-CDC900 and Cr7C3-CDC900 in 1 M H2SO4 electrolyte versus Hg/HgSO4 at scan rates from 50 to 1000 mV s1. (c) Current versus scan rate plot. Currents were measured in the middle of the plateau at approximately 0.1 V. (A color version of this figure can be viewed online.)

48

CARBON

8 5 ( 2 0 1 5 ) 3 8 –4 9

Fig. 9 – (a) Normalized Cyclic voltammetry of the sample Cr23C6-CDC900 in 1 M H2SO4 electrolyte versus Hg/HgSO4 at scan rate 200 mV s1; (b) cyclic voltammetry at scan rates from 50 to 1000 mV s1 and (c) logarithm of peak current versus logarithm of scan rate. (A color version of this figure can be viewed online.) dessolvation and the re-organization of the solvent molecules inside the pores [40,41]. The electrochemical results of the samples are in agreement with their respective isotherms (Fig. 6), as Cr3C2-CDC900 and Cr7C3-CDC900 have the largest amount of nitrogen adsorbed at p/p0 6 0.2 their behavior is purely capacitive while the smaller amount of micropores in the other samples enhances the partial trapping of the desolvated ions. Although the presence of certain mesoporosity in the samples would promote the ion diffusion into the pores, improving the wettability of the carbon particles by the electrolyte and therefore the rate capability, the principal parameter seems to be the interaction between ions and pores. Tuning the pore size in the carbon material for each electrolyte seems to be crucial in order to get profit of the most optimum pore size-ion size interaction. However, other parameters like tortuosity and pore shape could also be playing a crucial role in the final performance of the capacitor cell.

4.

Summary and conclusions

In this work we present the detailed structure-textural-electrochemical properties relationship in CDC synthetized from chromium carbides with different Cr/C content (Cr3C2, Cr7C3, Cr23C6). The differences in reactivity are influenced not only by the chromium content in the precursor but also in the crystal structure, being of orthorhombic symmetry for Cr3C2 and Cr7C3 and cubic in the case of Cr23C6. Although a faster and effective elimination of Cr as CrCl3 seems reasonable, when the content of chromium is lower (as it is observed when comparing the results obtained for Cr3C2, and Cr7C3) in Cr23C6 we should take into account that the highest symmetry of this crystal structure probably allows a more effective chlorine etching yielding a faster elimination than Cr7C3, where the lower symmetry can induce an anisotropic etching hindering the elimination of CrCl3 through determined crystallographic planes. Although, the chlorination temperature and the crystal structure of the precursor strongly influence the final nanostructure and porosity in CDCs, here we demonstrate that the low volatility and the layered structure of CrCl3 templates the graphitization of this family of CDCs, which is not observed in CDCs produced from carbides forming high volatile halides such us TiC or SiC.

Since EELS studies suggest similar values of sp2/sp3 ratio for the prepared CDC, the modifications observed in the Raman spectra are due to bond ordering and reorganization of the disordered graphene layers into graphitic ribbons as it is directly reflected on the HRTEM images. The textural analysis of the CDC indicates the presence of meso and microporosity, yielding the better surface area values at 900°. Then, the evolution of the microstructure into a more graphitic material is responsible of the collapse and loss of the microporosity at 1050 °C. The pore size distributions show a complex micro and mesopore texture of the prepared CDC. Finally the voltammetry measurements in CDC-CrxCy 900 °C shows a pure capacitive behavior, showing deviations from the ideal shape in Cr23C6 900 °C and the rest of CDC ascribed to the partial trapping of the desolvated ions.

Acknowledgements The authors would like to thank the financial support through the projects with references S-2009/PPQ-1626 and MAT201344964-R. P. Gonza´lez-Garcı´a would like to thank CONACYT (Mexico) for a Ph.D. grant. We thank Dr. Elena del Corro for Raman measurements. R E F E R E N C E S

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