Structural and electrochemical modification of graphitic carbons by vapor-phase iodine-incorporation

CARBON

4 8 ( 2 0 1 0 ) 4 1 7 8 –4 1 8 9

available at www.sciencedirect.com

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

Structural and electrochemical modification of graphitic carbons by vapor-phase iodine-incorporation P. Barpanda

a,* ,

K. Djellab a, R.K. Sadangi b, A.K. Sahu c, D. Roy d, K. Sun

e

a

Laboratoire de Reactivite et Chimie des Solides, Universite de Picardie Jules Verne, CNRS UMR6007, 33 rue Saint Leu, 80039 Amiens, France Department of Materials Science and Engineering, Rutgers University, 607, Taylor Road, Piscataway, NJ 08854-8065, USA c Department of Ceramic Engineering, Center of Relevance and Excellence (CORE), National Institute of Technology, Rourkela, 769008 Orissa, India d Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, CB23QZ, UK e Electron Microbeam Analysis Laboratory (EMAL), Department of Materials Science and Engineering, University of Michigan, 2300 Hayward Street, Ann Arbor, MI 48109-2136, USA b

A R T I C L E I N F O

A B S T R A C T

Article history:

Pristine and mechanically-milled graphitic carbons were chemically modified by vapor-

Received 23 June 2010

phase iodine-incorporation. The effectiveness of iodine uptake during vapor iodation

Accepted 21 July 2010

was gauged for pristine and mechanically-milled graphite. The doping of electronegative

Available online 25 July 2010

iodine, which is capable of triggering charge transfer reaction with carbon, was found to develop structural disordering, carbon–polyiodide covalent compounds (C–I3, C–I5), enhanced mesoporosity and reduced BET surface area in graphitic carbons. These intrinsic changes in iodine-modified graphite led to improved non-faradaic capacitance and development of faradaic pseudocapacitive reaction at 3.2 V versus Li. As a result, iodation develops manifold (100%) increment in gravimetric and volumetric capacity of precursor graphite, when tested versus Li. The effect of iodine-incorporation on physical and electrochemical properties of graphite is reported in detail. Ó 2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon exists in three main allotropic forms namely amorphous carbon, diamond and graphite [1], each having distinct physical and electrical properties. Additionally, carbon nanostructures like fullerene [2], carbon nanotubes [3] and graphene [4] have emerged as an attractive class of materials for scientific community. The versatility of structural and electronic features exhibited by carbon materials has paved way for applications encompassing display electronics [5], cutting tools and electrochemical energy storage [6–8] among others. Energy-storage devices are increasingly being used in consumer electronics, portable devices and hybrid vehicles in recent years. In the field of electrochemical energy-storage

devices, carbon materials find applications as conducting additives [9], rate-improving coatings [10], anode materials [11] in battery and positively and/or negatively charged electrodes in supercapacitors [12–14]. Particularly, the supercapacitor market is largely dominated by carbon materials like activated carbons, microporous carbons and recent carbon nanostructures [15,16]. Specially, activated carbons are attractive candidates for supercapacitor owing to its high surface area (1000–2500 m2/g), tunable morphology, economy/abundance, safe handling/storage and adequate electrochemical capacity [17]. The electrochemical performance of carbons can be improved by morphological development, inducing foreign element (e.g. nitrogen), developing surface functional groups

* Corresponding author: Fax: +33(0)3 22827590. E-mail address: [email protected] (P. Barpanda). 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.07.038

CARBON

4 8 ( 20 1 0 ) 4 1 7 8–41 8 9

(e.g. carboxyl) and suitable physicochemical modification [18,19]. One such physicochemical modification can be performed via iodine-incorporation into carbon. The electronegative iodine potentially prompts charge transfer reaction with host carbon modifying its physical and electrochemical properties. The resulting carbon–iodine nanocomposites delivers manifold increase in gravimetric and volumetric capacity owing to a synergistic development of non-faradaic and faradaic capacity. It has been observed and reported for various types of carbons with different degree of crystallinity, pore structure, surface morphology and net iodine-content [20–24]. The physical and electrochemical alteration of precursor carbon post iodine-incorporation can be linked to the effect of iodine species (atoms/compounds) on the local structure and chemical bonding of host carbon system. Further, the micro-scale electronic charge transfer involving iodine and the macro-scale interaction of guest iodine species with carbon (porous) morphology play major role in electrochemical modification of carbon. Invariably, all kinds of carbon comprise random mixture embedded graphitic domains inside an amorphous carbon matrix. While larger graphitic domains (with long structural ordering) gives graphitic carbons with low surface area, low porosity and high conductivity, the randomly-oriented smaller graphitic domains leads to activated carbons with high surface area, porous and low conductivity. In any case, the carbon–iodine interaction at a localized scale narrows down to the chemical reaction between nanoscale graphitic domains and iodine (guest) species. To shed light on the inherent properties of various carbon–iodine nanocomposites, the current paper attempts to investigate the effect of iodine on various properties of pristine synthetic graphite. In this regard, a vapor phase iodation technique was adopted to facilitate contamination-free, microscopic chemical interaction between graphite and iodine (vapor) species. To our knowledge, there has been no report on the electrochemical properties of iodine-doped graphite as electrode materials for supercapacitor. In light of this, we report the effectiveness of vapor iodation process and various structural and electrochemical developments in resulting graphite–iodine nanocomposites.

2.

Experimental

2.1.

Material preparation

For the synthesis of iodine-treated graphite, synthetic polycrystalline graphite (trade name Timrex-SFG-44) was procured from Timcal Inc. [25]. The SFG graphites were doped with iodine by vapour-phase iodation route using a standard two-zone quartz tube furnace. The pre-weighed graphite was kept in an alumina crucible in the cold-zone (maintained at 50 °C). To produce iodated graphites with varying C–I stoichiometric composition, different amount of solid iodine powder (Aldrich-India) was kept in another alumina crucible in the hot-zone (maintained at 170 °C). The quartz tube was closed from both ends with openings for inlet/outlet pipes for steady flow of nitrogen to facilitate vapour movement inside tube. The quartz tube was heated to 170 °C (heating rate = 10 °C/min) and was kept at 170 °C for 2 h. In this

4179

process, iodine sublimes around 120 °C, filling the entire tube with iodine vapour. The continuous exposure of graphite (in cold-zone) with iodine vapour facilitates intimate iodineincorporation into graphite forming iodine-doped graphite (GI). The morphology and crystal structure of graphite can be easily modified with mechanical milling owing to its loosely connected graphene sheets. So as to study the effect of morphology and structure of the precursor graphite, pristine graphite was mechanically milled for different times (from 0 to 2 h). These mechanically-milled graphitic carbons were later subjected to vapour-phase iodation. Also, the carbon–iodine stoichiometry was varied by using 0–30 wt.% of iodine during the heating process. For simplicity of reference, the iodated graphite products were named as per the initial iodine feed (e.g. G-10% I means graphite iodated with 10 wt.% initial iodine feed). To avoid any possible external contamination or hydrolysis, all materials handling were performed inside a nitrogen-filled glove box.

2.2.

Material characterization

The X-ray diffraction (XRD) analysis of pristine and iodated graphite powders was conducted using a Phillips X-ray diffractometer (Model PW 1830) using Cu Ka radiation (30 kV/ 20 mA, k = 1.54056 ). The samples were scanned in the 2h range 10–60° at a scan rate of 0.5°/min. Silicon was used as an internal standard for all measurements. Since powder sample was used, the chance of texturing was kept to minimum. Subsequently, (0 0 2) and (1 0 0)/(1 0 1) peaks the diffractograms were analysed for calculating the apparent crystallite size along longitudinal direction (La) and lateral direction (Lc) respectively. The La and Lc parameters were calculated to an approximate value by using the Debye–Scherrer equations after due consideration of instrumental error and internal crystal strain [26] Lc ¼ ð0:91 cÞ=ðb cos hÞ; La ¼ ð1:13 cÞ=ðb cos hÞ

ð1Þ

where, k is the X-ray wavelength, h is the scattering angle (in radians) and b is the full width at half maxima (FWHM) (in radians of h) of the lines after correction of instrumental broadening. Further, the approximate degree of graphitization (q) was calculated to an approximate value by using the intensity of (0 0 2) and (1 0 0) peaks as per the formula, q ¼ ½ðI0 0 2 =I1 0 0 ÞX1 0 0=14:3

ð2Þ

It is worth noting the calculation of La, Lc and q can be quite tricky, especially for amorphous carbon showing very broad peaks. Hence, they have been calculated to an approximate value after proper base-line subtraction from the XRD patterns using embedded Phillips software. The surface morphology/porosity was analysed using a Quanta-chrome-BET (Brunauer–Emmett–Teller) surface area analyzer. Around 0.2 g powder sample was evacuated at 383 K and 104 Pa for 4 h prior to surface area analysis via surface adsorption of N2 adsorbant (at 77 K). Between each data collection point, ample time was allowed facilitate saturated nitrogen adsorption so as to have accurate BET data. As graphite is mesoporous in nature, as per classical thermodynamics,

4180

CARBON

4 8 ( 2 0 1 0 ) 4 1 7 8 –4 1 8 9

Barrett–Joyner–Halenda (BJH) theory was used to study the pore size distribution [27,28]. For Raman spectroscopy, a small amount (0.3 g) of powder sample was directly placed under the laser source to record the Raman spectra. The spectra were taken employing a Renishaw Ramascope-1000 multi-channel Raman spectrometer equipped with an infrared laser (k = 785 nm) and a CCD detector in a backscattering mode. To enhance the Raman signal coming from graphite samples, a confocal hole coupled with the microscope was adjusted to its minimum value. The laser beam was operated below 0.20 mW (exposure time = 900 s) to avoid any laser-induced damage. Subsequently, the Raman spectra were analysed for the D (1300 cm1) and G (1600 cm1) bands and the corresponding crystallite size was calculated using the formula: La ¼ 120ðIG =ID Þ

ð3Þ

X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Ultra DLD XPS instrument using a monochromatic Al source operating at 15 kV. The powder samples were placed on top of pure indium (In) foils in order to avoid any contamination from C and O. For high-resolution transmission electron microscopy (HRTEM) study, a few drops of powder samples mixed with acetone were deposited on a TEM holey (with carbon–copper grid). TEM images as well as selected area diffraction pattern (SAED) were collected using a FEI-Tecnai F20 S-Twin electron microscope operating at 200 kV. Special care was taken to minimise the exposure time so as to ensure minimal sample damage under the high-voltage beam.

Electrochemical measurement

For electrochemical study, positive electrodes were fabricated with iodated graphites (GI). The cathode composition was prepared by mixing 57 wt.% (non) iodated graphite (active material), 6 wt.% SuperP carbon black (conducting agent) and 15 wt.% poly(vinylidene diflouride) (hexafluoropropylene) copolymer (binder). Standard SwagelokTM type cells were prepared using the positive electrode powders containing active materials and Li metal foil as counter electrode separated by glass fibres. A 1 M LiBF4 dissolved in propylene carbonate (PC) was used as electrolyte solution. The cathode loading was 8–15 mg/cm2. The cells were assembled inside argon filled glove box (MBraun, Unilab, Germany). These cells were galvanostatically cycled in voltage range of 2.8–4 V (versus Li) using current rate of 0.20 mA cm2.

30

30% I 25

Weight percent of Iodine intake (%)

2.3.

exposed to pristine and milled graphite for 2 h. The effect of graphite surface area (which varies upon milling as will be discussed later) on overall iodine intake ability is shown in Fig. 1. Even with the presence of very high iodine precursor (10–30 wt.%), pristine graphite (tM = 0 h) barely uptakes 1–3 wt.% of iodine. This poor iodine-uptake ability can be solely assigned to the inherently low surface area of graphite, in which case majority of carbon planes (hence atoms) are not exposed to iodine. However, mechanical milling (tM = 0.5–2 h) gradually opens up the graphitic platelets (Section 3.4), increasing the overall surface area and carbon species exposed to iodine vapours. As a result, the precursor graphite absorbs iodine vapour very effectively. The average iodine-uptake ability was calculated to be 96%, 94% and 90% for initial iodine feed of 10, 20 and 30 wt.% of graphite, respectively. With higher iodine feed, graphite gets saturated sooner. Further as discussed in Section 3.7, graphite has markedly different surface area and morphology corresponding to milling duration of 0.5, 1 and 2 h. However, interestingly the iodine-uptake ability seems to be independent of overall surface area/graphitic particulate arrangement. Assuming a monolayer of iodine formation on graphite surface, it was found the iodine-absorption by graphite is a bulk process rather than a surface process. This effective iodine intake by graphite leads to the formation of homogeneous graphite–iodine nanocomposites. A thorough study of graphite–iodine interaction and its effect on its structure and property is presented below.

20% I

20

15

10% I

10

5

3.

Results and discussion

3.1.

Effective iodine intake of graphite

Iodine was introduced into graphite by a standard two-zone tube furnace, where iodine was vaporized in hot-zone and was incorporated into graphite in the cold-zone by the interaction between graphite and iodine vapour. From an earlier study on activated carbon–iodine interaction [22], vapor-phase iodation was found to occur optimally at 170 °C for a soaking duration of 2 h. Thus, in the current study iodine (Tsublimation  120 °C) was vaporised at 170 °C and was

0

0

0.5

1

1.5

2

High-energy milling time (h) Fig. 1 – The effective iodine-intake by pristine and mechanochemically milled graphite for different amount of initial iodine feed. High-energy milled graphite very effectively incorporates iodine independent of its surface morphology. Iodation gradually saturates the graphite with higher iodine feed resulting in progressively lower iodineuptake.

CARBON

3.2.

4 8 ( 20 1 0 ) 4 1 7 8–41 8 9

4181

X-ray diffraction structural study of iodated graphite

First, X-ray diffraction (XRD) analysis of powder samples was carried out to examine the structural variation in graphite upon vapor-phase iodine-incorporation. The comparative XRD pattern for pristine carbon with/without iodine doping is shown in Fig. 2 and some corresponding TEM images are shown in Fig. 3. All XRD patterns showed reflections corresponding to graphitic (0 0 2), (1 0 0)/(1 0 1) and (0 0 4) planes at 2h values of 26°, 43° and 54°, respectively. The low surface area pristine graphite intakes very slight amount of iodine (Fig. 1), which barely affects the XRD pattern. When the precursor graphite is mechanically milled, the repeated fracture owing to impact of milling media gradually forms amorphous carbon. As shown in Fig. 2, longer milling duration destroys the (0 0 2), (1 0 0)/(1 0 1) and (0 0 4) peaks drastically forming small and broader peaks. TEM study corroborates this fact showing the transformation of crystalline, hexagonal graphite (Fig. 3a) to amorphous graphite (Fig. 3b). The corresponding SAED pattern (insets) shows the formation of more random diffraction pattern upon milling. The effect of iodine-incorporation on the structure of milled graphite (with high iodine-uptake ability) is shown in Fig. 4. While uniodated (0% I) specimen have sharp (0 0 2), (1 0 0) and (0 0 4) peaks, iodine-incorporation drastically destroys these peaks. Specially, the (1 0 0)/(1 0 1) peak around 43° is more severely destroyed. Even for 10% iodine introduced into graphite via a clean vapor phase exposure, the (1 0 0)/(1 0 1) peaks is significantly lowered. This peak at 43° characterizes the 2-dimensional in-

(002) Intensity (arb. unit)

(100/101)

(004)

G-0%I G-10%I G-20%I G-30%I G-M 30 G-M 60 G-M120 10

20

30

40

50

60

70

Diffraction angle (2θ)

Fig. 2 – Comparative X-ray diffractograms of graphite precursor modified with different amount of iodine-feed (10–30%) and duration of mechanical milling (0–120 min). The classical (0 0 2), (1 0 0/1 0 1) and (0 0 4) peaks are shown. Mechanical milling sharply disrupts the graphitic structure. In the figure, G and GM stands for Graphite and GraphiteMilled. The number following GM refers to milling duration in minutes.

Fig. 3 – TEM images and SAED patterns (insets) of (a) pristine graphite, (b) mechanically-milled graphite, and (c) mechanically iodated (30% I) graphite. A gradual disordering is marked upon milling and iodation.

plane symmetry along the graphene layers (c-axis). As shown in Fig. 3c, iodation leads to further disordering giving random diffraction pattern (inset) and developing elongated needleshaped particles. Overall, iodine-introduction into graphite

4182

CARBON

4 8 ( 2 0 1 0 ) 4 1 7 8 –4 1 8 9

disordering in graphite upon chemical iodation. Iodine and  related polyiodide compounds (I 3 =I5 as discussed later) forms along the edge carbons of graphitic rings and easily disrupts the 2D in-plane symmetry along graphene layers by easily breaking the weak van der Waals bonds along c-axis. Interestingly iodine leads to small longitudinal ordering (a-axis) and visible lateral disordering (c-axis). Using the (0 0 2) and (1 0 0)/(1 0 1) peaks, the degree of graphitization was calculated to an approximate degree and is shown in Fig. 5. Iodation leads to a slight increase in degree of graphitization owing to increase in the longitudinal graphitic dimension. The increase is more visible in case of pristine graphitic carbon than mechanically milled (amorphous) graphites. The effect of iodation on graphitic structure is further discussed using Raman spectroscopy in the following section.

Graphite surface area = 283 m 2 /g (002)

Intensity (arb. unit)

(100/ 101)

(004)

0% I 10% I 20% I 30% I

10

20

30

40

50

60

3.3. Raman spectroscopy and XPS study of iodated graphite

Diffraction angle (2θ) Fig. 4 – X-ray diffraction patterns illustrating the effect of chemical iodation on the structural features of precursor graphite pre-milled for 30 min. While, pristine graphite has sharp (0 0 2), (0 0 4) and a broad (1 0 0/1 0 1) peaks, iodation drastically reduces these graphitic peaks. Specially, the (1 0 0/1 0 1) peak is completely diminished.

leads to structural disordering in graphite (both longitudinally and more so laterally). Using the Debye–Scherrer equation, the crystallite dimension was calculated for iodated carbons to approximate values. The longitudinal crystallite size (along a-axis) for various iodated graphite is listed in Table 1. Its worth noting, when precursor graphite (SBET = 8 m2/g) is exposed to iodine vapor, the La value gradually increases from 81.81 nm to 114.85 nm. Considering the a and c values for graphite to be 0.24614 nm and 0.6708 nm respectively [29], iodation increases the La dimension from 540 to 760 cell lengths. This increase in La values were also noted in case of mechanicallymilled graphite precursor with higher surface area (Table 1). In general, on a localized scale, iodation favours graphitic ordering along a-axis. Coming to the c-axis (lateral graphitic dimension) related to 43°, the Lc values could not be calculated due to very broad/shallow peaks. Thus, the lateral dimension (Lc) was studied simply by plotting the 43° (1 0 0)/ (1 0 1) peak intensity as a function of iodine content in graphite (Fig. S1). Upon iodation, there is a sharp drop in (1 0 0) peak intensity from 0 to 10 wt.% I. With further iodation, the (1 0 0) peak intensity decreases further. It indicates the structural

Iodine, being electronegative and chemically reactive, readily interacts with multiple iodine atoms forming polyiodides, having fascinating linear one-dimensional or three-dimen3 sional network structure with anion ranging from I 2 to I29 [30]. These polyiodides are formed by donor–acceptor interaction with iodine (I2, Lewis base acceptor) and I/I 3 (Lewis base donor) basic building blocks [31]. Their catenation is energetically favoured, hence forming linear chains ½I2 I 11 , quadratic non-linear networks ½I2 I 21 and cubic 3-dimensional network ½I2 I 31 using a generic reaction: MI2 þ nI ! In2mþn ðm and n integers > o; n ¼ 1  4Þ These octet-rule violating polyiodides are hypervalent and have very intriguing chemical bonding that involves electrostatic interaction, localized d-orbital covalent bonding, delocalized r-bonding. Here, the cation environment plays very vital role in final polyiodide structure. The exposure of iodine vapour to graphite and possible polyiodide formation was probed using Raman spectroscopy. Fig. 6a shows the Raman spectra (at low wavenumbers) of pristine graphite exposed to different degree of iodation. There is no Raman peak corresponding to molecular iodine (I2) (181 cm1), which negates the possibility of physical accumulation of molecular iodine inside graphite. However, two Raman peaks (a broad peak 117 cm1 followed by a sharp peak 154 cm1) are observed bearing the signature of polyiodide compound formation. The broad peak 117 cm1 arises as a convolution of three minor peaks in the range 107–140 cm1, which can be assigned to triiodide 1 proves the (I 3 ) formation [32]. The sharp peak 154 cm  formation of pentaiodide (I5 ) [33]. Thus, a simultaneous

Table 1 – The longitudinal graphitic dimension (La) as a function of iodine content in graphite. Iodine content (%)

Mechanical milling duration (in minute) 0

00 10 20 30

1.81 103.04 110.51 114.85

30

60

120

45.96 48.71 51.98 59.15

14.28 14.73 17.05 18.43

8.39 9.87 10.08 11.43

CARBON

4 8 ( 20 1 0 ) 4 1 7 8–41 8 9

60

Degree of graphitisation, q (%)

GM 30

50

GM 60

20

GM 120

10

0

10

20

30

Weight percentage of iodine inside graphite

Fig. 5 – Graph showing the relation between wt.% iodine inside graphite to the calculated degree of graphitization (q). Due to broad XRD peaks in these amorphous carbons, approximate q values are presented. Irrespective of milling duration and morphology, chemical iodine-incorporation gradually increases the local degree of graphitization. This trend is later confirmed by Raman spectroscopy results.

formation of triiodide and pentaiodide is triggered upon iodation of graphite. This kind of polyiodide formation has been marked for mesoporous/microporous activated carbons  [21,22]. No higher order polyiodides (I 7 , I9 etc.) can be detected. As shown in Fig. 1, pristine graphite uptakes very slight amount of iodine due to its low surface area. However, even with small iodine uptake, it starts forming polyiodide complexes. The well-detected polyiodide peaks in iodated graphite (even for 10 wt.% iodine) proves that polyiodides formation is energetically favoured from the very beginning of graphite–iodine interaction. With higher degree of iodation, the Raman peaks intensified indicating the formation of increasing amount of polyiodides. The polyiodide complex formation is further shown for mechanically modified graphite with higher surface area (Fig. 6b). Higher surface area facilitates improved iodineincorporation (Fig. 1), thus developing higher amount of polyiodides. The amount of polyiodide inside graphite can be roughly estimated using the surface area enclosed by the  I 3 =I5 peaks. Comparing Fig. 6a and b, milled graphite with high surface area observed to have higher amount of polyiodide compounds. Further, the broader and slightly up-shifted peaks indicate a disordered ensemble of polyiodides built-up inside graphite. In all cases, the doping of higher amount of iodine leads to higher intensity of pentaiodide, keeping the triiodide peak unmodified. In fact, pentaiodide (I 5 ) can be assumed to form by combining triiodide with molecular iodine (I 3 þ I2 ), so the initial triiodide acts as host-site for catenation to interact with new iodine species to formulate pentaiodide complex. Thus graphite becomes richer in pentaiodide content with increasing degree of iodation as shown in Fig. 6c.

4183

Following, the effect of polyiodide formation on inherent carbon structure was looked into. Graphite consists of planar sp2 with tetrahedral sp3 structure, having D6h symmetry with 2E2g, 2B2g, E1u and A2u vibrational modes [34]. While perfectly ordered graphite delivers a single peak i.e. G peak (1580 cm1, E2g mode), disordered graphite exhibit another disorder-induced peak known as D peak (1350 cm1, A1g mode) associated with double-resonant effect [35]. The Raman spectra of milled graphite with different degree of iodation are shown in Fig. 7. The D and G peaks are well detected along with a 2D (G 0 ) peak 2600 cm1. The D and G peak intensity (and hence the ID/IG ratio) remains unaffected by iodation. However, with increase in iodine/polyiodide content, the FWHM of D and G bands gradually decreases for graphite (Fig. 8 and Fig. S2). This small but gradual decrease in FWHM values indicates graphite ordering at a localized scale [14]. Additionally, a close look revealed that iodation forces the D and G peaks to move towards each other as shown in Fig. 9. The data points are average of three sets of experiments and the shifts values are higher than the instrumental resolution limit (2 cm1). This peak shift indicates the possible charge transfer reaction (CT) between host graphite and guest iodine species during polyiodide formation [36]. Though similar effect was earlier expected for iodine-treated amorphous mesoporous/microporous carbons [21–23], for the first time, it has been observed with the help of graphitic carbons here. XPS study was conducted to capture any possible evidence of CT reaction upon graphite–iodine interaction [37] and the resulting spectra is given in Fig. 10. No clear shift in the C1s peak (1202 eV) and hence evidence of significant CT reaction is detected. Similar results were earlier obtained for activated carbons with iodine and bromine modification [37]. This does not rule out CT reaction completely as the charge transfer between carbon and iodine is weak and isolated to the surface that make the shift too small to observe. Ex situ solid-state NMR and X-ray absorption spectroscopy may be very valuable to clearly observe the CT reaction upon iodation.

3.4.

Morphological analysis of iodated graphite

The inception of iodine and consequent formation of polyiodide species affects the inherent porosity and surface morphology of pristine and milled graphite. The effect of mechanical milling and vapor-phase iodation on graphite morphology is summarized in Fig. 11 and Table 2. When graphite is mechanically milled, the strong covalent planar sp2 (C–C) bonds and the weak van der Waals interplanar bonds are disrupted in a three-stage mechanism (namely division, fracture and agglomeration stages) [38–40] resulting in a final crumpled card-house type arrangement. A schematic diagram showing these three stages of mechanical milling of pristine and iodated graphitic carbon is shown in Fig. 12. While the first two stages (division, fracture) increases the surface by exposing more graphitic planes, the third stage causes the reverse effect by agglomeration of finer particles. Thus, the BET area follows an inverse V trend (Fig. 11). When different amount of iodine is introduced into graphite, the inherently small iodine species (vapor-phase) react homogeneously with carbon, forming monolayer/multilayer of polyiodide species inside available pores. Hence, increasing

4184

CARBON

4 8 ( 2 0 1 0 ) 4 1 7 8 –4 1 8 9

a

Pristine Graphite

Raman Intensity (arb unit)

triiodide

pentaiodide 30 wt% I

20 wt% I

10 wt% I

0

100

200

300

400

500

Raman shift (cm-1)

Raman Intensity (arb unit)

b

Milled Graphite

pentaiodide

triiodide

30 wt% I

20 wt% I

10 wt% I

0

100

200

300

Raman shift

400

500

(cm-1)

c Pentaiodide/ Triiodide (I5- /I3- ) Ratio

4

3

2

1

10

20

30

Weight percentage of Iodine in Graphite  Fig. 6 – Low wave-number Raman spectra showing the triiodide (I 3 ) and pentaiodide (I5 ) compounds with no molecular iodine (I2) in case of iodine doped (a) low surface area pristine graphite with minimal iodine-uptake ability and (b) pre-milled graphite with higher surface area and larger iodine-uptake ability. (c) A graph showing almost linear increase in the  polyiodide peak ratio (I 5 =I3 ) with higher degree of iodation.

CARBON

4185

4 8 ( 20 1 0 ) 4 1 7 8–41 8 9

1590

D (sp3)

G-band

2D (G/)

30 wt% I

20 wt% I

10 wt% I

1000

1500

2000

Raman shift

2500

Raman peak position (cm-1)

Raman Intensity (arb unit)

G (sp2)

1585

D-band 1310

1305

3000

10

(cm-1)

20

30

Weight percentage of iodine in graphite

Fig. 7 – High wave-number Raman spectra of iodated graphite (GIx) showing the D, G and 2D (G 0 ) peaks.

Fig. 9 – A graph showing the variation in D and G band position with chemical iodation. Higher degree of iodation leads to down-shift of G band and up-shift of D band, which indicates the possibility of charge transfer reaction between (host) graphite and (guest) iodine species.

Milling time 60 min

D Band

I3d

C1s Intensity (a.u.)

70

Intensity (a.u.)

Raman peaks FWHM (cm-1)

75

65

G Band

20% I

10% I 0% I 850

860

870

880

Kinetic Energy (eV)

60

20% I 10

20

30

10% I

Weight percentage iodine content

0% I Fig. 8 – FWHM of D and G bands plotted as a function of weight percentage of iodine inside graphite. Gradual simultaneous narrowing (i.e. smaller FWHM) of D and G bands with higher degree of iodation indicates localized structural ordering inside carbon. This phenomenon is shown for different precursor graphites (prepared with different milling duration, Fig. S2). This structural ordering is corroborated by X-ray diffraction results.

iodation continuously decrease the surface area by blocking pores. The comparative adsorption isotherms are shown in Fig. 13, illustrating the decrease in gas adsorption ability with higher degree of iodation while retaining the type-IV mesoporous nature. In many applications like catalysis and electrochemical energy storage, apart from surface area, the underlying pore structure of carbon plays a vital role for effective interfacial physicochemical reactivity. Using the BJH method, the

1190

1195

1200

1205

1210

1215

1220

Kinetic Energy (eV) Fig. 10 – XPS spectra of pristine and iodated graphite (for fixed milling duration of 1 h), showing no visible shift in C1s peak. The inset shows the XPS peaks corresponding to iodine (I3d3 and I3d5) present inside graphite, which grow with higher iodine content.

approximate pore size distribution (as a function of iodine content) is plotted in Fig. 14. Graphite is essentially mesoporous in nature with a peak around 2–4 nm (small mesopores) followed by a broad shoulder from 5 to 50 nm (large mesopores). Iodine was observed to fill up the small mesopores gradually. Calculation shows that small mesopores are big enough to fit multiple layers of triiodide/pentaiodide species easily. Presumably, polyiodide species are attracted to smaller mesopores to reduce the overall interfacial surface energy.

4186

CARBON

4 8 ( 2 0 1 0 ) 4 1 7 8 –4 1 8 9

Table 2 – BET surface area and Langmuir surface area of iodine-modified graphite.

0% I

300

BET Surface area (m2/g)

10% I 250

Iodine content (%)

20% I 30% I

30

200

100 50

0

30

60

120

Mechanical milling time (min) Fig. 11 – Histogram showing the variation in the BET surface area of mechanochemically modified graphite. The inverse ‘V’ nature of pristine graphite is a result of three-stage morphological modification (Fig. 12). Iodation decreases the BET surface area in all cases by filling up the available pores.

This kind of preferential filling of smaller pores has been observed in many types of activated carbons [21–24]. Consequently, iodation leaves the larger mesopores open, thus increasing the average pore size in iodated graphite (Fig. S3). These results follow the same trend as observed in case of iodated meso/microporous activated carbons [22,23].

3.5.

60

120

2

150

0

Mechanical milling duration (in minute)

Electrochemical characterization

Modifying the surface morphology, electronic conductivity and chemical composition can alter the electrochemical

Brunauer–Emmett–Teller surface area (m /g) 00 206.75 312.18 10 129.18 252.21 20 86.52 131.18 30 85.66 80.69

222.18 117.54 107.46 82.89

Langmuir surface area (m2/g) 00 282.51 10 179.40 20 120.53 30 119.86

423.10 347.74 182.56 113.38

300.81 162.43 148.95 115.62

Micropore area (m2/g) 00 10 20 30

98.75 24.60 4.53 2.43

87.74 14.87 13.75 4.37

51.2 3.58 4.40 3.17

properties of any electrode. In the current examination, pristine and milled graphite has been chemically modified via vapor-phase iodine-incorporation. As suggested by XRD and Raman analysis, iodation triggers visible structural modification via formation of polyiodide complexes. The possibility of charge (electron) transfer reaction between host carbon and guest iodine species can modify its electronic properties. All these factors consequently force the voltage profile of iodated graphite to deviate from its linear nature. Fig. 15 compares the charge–discharge voltage profiles of pristine and iodine-modified graphite. When tested versus Li as counter electrode, the presence of iodine (and related polyiodide) leads to twofold modification in the galvanostatic voltage profiles. First, in the linear non-faradaic region (4–3.2 V versus Li) involving

Fig. 12 – Schematic presentation of morphological development in graphite owing to mechanical milling and chemical iodation. Mechanical milling leads to a three-stage development (division, fracture, agglomeration [38–40]) indicated by step I, II, III, respectively. When introduced, iodine species subsequently fill the available pores.

CARBON

4187

4 8 ( 20 1 0 ) 4 1 7 8–41 8 9

200 4

0

10 30 20

0% I

150

Cycling voltage Vs Li/Li+

Volume of N2 adsorbed (cm3/g)

3.8

10% I

100

20% I

3.6 3.4 3.2 3

50

30

10

0 2.8

20

30% I 0

1000

2000

3000

Cycling duration (s) 0

0

0.2

0.4

0.6

0.8

1

Relative pressure (Po/P) Fig. 13 – Comparative adsorption isotherms showing the gradual reduction in surface area upon iodineincorporation. The IUPAC type-IV nature with hysteresis loop proves the mesoporous nature of graphite.

2

Precursor Graphite BET area = 206 m /g

0

Fig. 15 – Comparative voltage profiles of non-iodated and iodated graphites cycled between 2.8 and 4 V (versus Li+/ Li0). The numbers alongside the curves indicates the iodine content in graphite. Iodation improves the non-faradaic slope and gives birth to a faradaic plateau at 3.2 V. The red line at 3.2 V divides the entire voltage range into nonfaradaic zone (4–3.2 V) and faradaic zone (3.2–2.8 V). plateau around 3.2 V versus Li. This faradaic plateau (marked in both charging and discharging segment) can be related to the interfacial adsorption of Li+ ions and subsequent formation of Li–I or In compounds (conversion reaction) as follows: þ

Pore volume (cm3/g)

CI x þ x Li ! C þ x LiI

ð4Þ

This kind of faradaic plateau has been observed in microporous/mesoporous amorphous activated carbons containing polyiodides [21–24], with high degree of polarization.

10

20

14 30

1

10

100

Pore size (nm)

Fig. 14 – Relative pore volume is plotted against pore size of iodated graphites with a peak around 2–4 nm. The numbers along the graphs indicate the iodine weight percentage in graphite. Higher iodation leads to preferential filling of smaller mesopores.

Galvanostatic capacity (mAh/g)

12 10 8 6 4 2 0

ion adsorption/desorption, iodation improves the slope of voltage profiles (related to higher capacitance). Even though the surface area decreases with iodation, this increase in slope can be due to the improved electronic conductivity [41] of iodated carbons. A detail study on the effect of iodine content on the electronic properties of graphitic and amorphous carbons will be presented elsewhere. Secondly, chemical iodation facilitates the inception of faradaic (non-linear)

0 %I 10 %I 20 %I 30 %I

0

30

60

120

Mechanical milling time (min) Fig. 16 – Histogram summarizing the electrochemical capacity of various non-iodated and iodated graphites. In all cases, iodation sharply improves the capacity as a result of non-faradaic increment and development of faradaic plateau. In all cases, the cycling stability (retention) is excellent (>95% efficiency, not shown here).

4188

CARBON

4 8 ( 2 0 1 0 ) 4 1 7 8 –4 1 8 9

Following the galvanostatic electrochemical capacity of these vapor-iodated graphites was analyzed (Fig. 16). For pristine graphite, the net capacity comes solely due to electrolyte ionic adsorption/desorption, which is directly related to the existing surface area. Hence, it shows an inverse V trend following the trend of BET surface area. But, iodation induced charge transfer reaction and polyiodide formation develops higher degree of ionic adsorption/desorption ability as well as some faradaic capacity owing to the conversion reaction 3.2 V versus Li. As a result, for all pre-milled graphite, the electrochemical capacity gradually increases with net iodine content (Fig 16). It gives a unique combination of modified graphite with lower surface area and higher capacity. Though small in quantity, iodation definitely improves the electrochemical capacity with the help of battery type-faradaic reaction.

4.

Conclusions

Graphite was physico-chemically modified via vapor-phase iodine-incorporation. Mechanically-milled graphite (with high surface area) was found to be an efficient host to accommodate guest iodine species. Iodation breaks the lateral structural ordering, while slightly improving the longitudinal ordering. The lateral disordering comes due to the formation  of mono/multiple layers of polyiodide complexes (CI 3 ; CI5 ) preferably inside the smaller mesopores of graphite. The pore blockage by polyiodide results in reduced BET surface area with enhanced mesoporosity. From Raman and XPS study, there is a hint of charge transfer reaction between graphite and iodine. This physicochemical modification in graphite owing to iodation is reflected in electrochemical capacity improvement by non-faradaic development and faradaic capacity addition. This study on the effect of iodation on graphite shed light on carbon–iodine interaction, which is useful for developing (activated) carbon–halide nanocomposites electrodes.

Acknowledgements PB acknowledges ECS for H.H. Dow Student Achievement Award and NSF for an International Research Fellowship. The help of Dr. K. Yates and Dr. V. Adyam (Cambridge, UK) for Raman spectroscopy is greatly appreciated. The kind permission of Prof. T.W. Clyne for using micro-Raman facility at Gordon research laboratory (Cambridge, UK) is greatly acknowledged. The authors are grateful to Prof. S. Adak, S. Bhattacharya (NIT, Rourkela), Prof. R.K. Sinha (TRL, India) for technical/scientific help. PB thanks Prof. G. Fanchini and Prof. G.G. Amatucci for technical discussion and scientific suggestions.

R E F E R E N C E S

[1] Bundy FP. Pressure–temperature phase diagram of elemental carbon. Physica A 1989;156(1):169–78. [2] Kroto HW, Heath JR, O’Brien SC, Curl RF, Smalley RE. C60: Buckminsterfullerene. Nature 1985;318:162–3.

[3] Dresselhaus MS, Dresselhaus G. Carbon nanotubes: synthesis, structures, properties and applications, topics in advanced physics. Berlin: Springer; 2001. [4] Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich VV, Morozov SV, Gein AK. Two-dimensional atomic crystals. Proc Nat Acad Sci 2005;102(30):10451–3. [5] http://www.samsung.com. [6] Kawasaki S, Iwai Y, Hirose M. Electrochemical lithium ion storage properties of single-walled carbon nanotubes containing organic molecules. Carbon 2009;47(4):1081–6. [7] Moreno-Castill C, Maldonado-Hodar FJ. Carbon aerogels for catalysis applications: an overview. Carbon 2005;43(3):455–65. [8] Amatucci GG, Badway F, DuPasquier A, Zheng T. An asymmetric hybrid nonaqueous energy storage cell. J Electrochem Soc 2001;148(8):A930–9. [9] Lu W, Chung DDL. A comparative study of carbons for use as an electrically conducting additive in the manganese dioxide cathode of an electrochemical cell. Carbon 2002;40(3):447–9. [10] Armand M, Gauthier M, Magnan J, Ravet N. World Patent WO 02/27823 A1 2002. [11] Takada K, Inada T, Kajiyama A, Sasaki H, Kondo S, Watanabe M, Murayama M, Konno R. Solid-state lithium battery with graphite anode. Solid State Ion 2003;158(3–4):269–74. [12] Conway BE. Electrochemical supercapacitors: scientific fundamentals to technological applications. New York: Kluwer-Plenum; 1999. [13] Pandolfo AG, Hollenkamp AF. Carbon properties and their role in supercapacitors. J Power Sources 2006;157(1):11–27. [14] Ja¨nes A, Kurig H, Lust E. Characterisation of activated nanoporous carbon for supercapacitor electrode materials. Carbon 2007;45:1226–33. [15] Frackowiak E, Beguin F. Electrochemical storage of energy in carbon nanotubes and nanostructured carbons. Carbon 2002;40(10):1775–87. [16] Futaba DN, Hata K, Yamada T, Hiraoka T, Hayamizu Y, Kakudate Y, Tanaike O, et al. Shape-engineerable and highly densely packed single-walled carbon nanotubes and their application as supercapacitor electrodes. Nat Mater 2006;5:987–94. [17] Marsh H, Rodriguez-Reinoso F. Activated carbon. Amsterdam: Elsevier Pub; 2006. [18] Hulicova D, Yamashita J, Soneda Y, Hatori H, Kodama M. Supercapacitors prepared from melamine-based carbon. Chem Mater 2005;17(5):1241–7. [19] Boehm HP. Some aspects of the surface chemistry of carbon blacks and other carbons. Carbon 1994;32(5):759–69. [20] Barpanda P. Physical and electrochemical study of halidemodified activated carbons. PhD thesis, Piscataway, NJ, USA: Rutgers University; 2009. [21] Barpanda P, Fanchini G, Amatucci GG. Physical and electrochemical study of iodated activated carbons. J Electrochem Soc 2007;154(5):A467–76. [22] Barpanda P, Fanchini G, Amatucci GG. The physical and electrochemical characterization of vapor phase iodated activated carbons. Electrochim Acta 2007;52(24):7136–47. [23] Barpanda P, Li Y, Cosandey F, Rangan S, Bartynski RA, Amatucci GG. Fabrication, physical and electrochemical investigation of microporous carbon–polyiodide nanocomposites. J Electrochem Soc 2009;156(11):A873–85. [24] Barpanda P, Fanchini G, Amatucci GG. Faradaic and nonfaradaic reaction mechanisms in carbon–iodine nanocomposites electrodes for asymmetric hybrid supercapacitors. ECS Trans 2008;13(17):13–9. [25] http://www.timcal.com. [26] Iwashita N, Park CR, Fujimoto H, Shiraishi M, Inagaki M. Specification for a standard procedure of X-ray diffraction measurements on carbon materials. Carbon 2004;42(4):701–14.

CARBON

4 8 ( 20 1 0 ) 4 1 7 8–41 8 9

[27] Brunnauer S, Emmett PH, Teller E. Adsorption of gases in multimolecular layers. J Am Chem Soc 1938;60:309–16. [28] Barrett EP, Joyner LG, Halenda PP. The determination of pore volume and area distribution in porous substances: computation from nitrogen isotherms. J Am Chem Soc 1951;73:373–80. [29] Babu VS, Seehra MS. Modeling of disorder and X-ray diffraction in coal-based graphitic carbons. Carbon 1996;34:1259–65. [30] Svensson PH, Kloo L. Synthesis, structure and bonding in polyiodide and metal iodine–iodide systems. Chem Rev 2002;103(5):1649–84. [31] Blake AJ, Devillanova FA, Gould RO, Li W-S, Loppolis V, Parsons S, et al. Template self-assembly of polyiodide networks. Chem Soc Rev 1998;27:195–206. [32] Parrett FW, Taylor NJ. Spectroscopic studies on some polyhalide ions. J Inorg Nucl Chem 1970;32:2458–61. [33] Nour EM, Chen LH, Laane J. Far-infrared and Raman spectroscopic studies of polyiodides. J Phys Chem 1986;90:2841–6. [34] Kawashima Y, Katagiri G. Fundamentals, overtones and combinations in the Raman spectrum of graphite. Phys Rev B 1995;52(14):10053–9. [35] Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous graphite. Phys Rev B 2000;61:14095–107.

4189

[36] Dresselhaus MS, Dresselhaus G. Intercalation compounds of graphite. Adv Phys 1981;30(2):139–326. [37] Jung Y, Hwang S-J, Kim S-J. Spectroscopic evidence of weak electron transfer from intercalated iodine molecules to single-walled carbon nanotubes. J Phys Chem C 2007;111:10181–4. [38] Salver-Disma F, Aymard L, Dupont L, Tarascon J-M. Effect of mechanical grinding on the lithium intercalation process in graphites and soft carbons. J Electrochem Soc 1996;143(12):3959–72. [39] Salver-Disma F, Lenain C, Beaudoin B, Aymard L, Tarascon JM. Unique effect of mechanical milling on the lithium intercalation properties of different carbons. Solid State Ion 1997;98:145–58. [40] Salver-Disma F, DuPasquier A, Tarascon J-M, Lassegues J-C, Rouzaud J-N. Physical characterization of carbonaceous materials prepared by mechanical grinding. J Power Sources 1999;81–82:291–5. [41] Hahn M, Baertschi M, Barbieri O, Sauter J-C, Kotz R, Gallay R. Interfacial capacitance and electronic conductance of activated carbon double-layer electrodes. Electrochem SolidState Lett 2004;7(2):A33–6.