A binder-free multi-walled carbon nanotube electrode containing oxygen functionalities for electrochemical capacitors

G Model EA 23319 No. of Pages 9

Electrochimica Acta xxx (2014) xxx–xxx

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A binder-free multi-walled carbon nanotube electrode containing oxygen functionalities for electrochemical capacitors M.A. McArthur a,b, *, N. Hordy a , S. Coulombe a , S. Omanovic b,1 a

Plasma Processing Laboratory, Department of Chemical Engineering, McGill University, 3610 University Street, Montréal, Québec, H3A 0C5, Canada Electrochemistry and Corrosion Laboratory, Department of Chemical Engineering, McGill University, 3610 University Street, Montréal, Québec, H3A 0C5, Canada b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 July 2014 Received in revised form 8 September 2014 Accepted 8 September 2014 Available online xxx

A method for the incorporation of immobile oxygen-containing functional groups onto multi-walled carbon nanotubes (MWCNTs) rooted in a metallic substrate for supercapacitor (SC) application is presented. Direct growth on the current collector (a 316 stainless steel (SS) mesh) offers the benefit of minimal MWCNT agglomeration, avoidance of using binder, and maintaining the 3-dimensional nanostructure. MWCNTs are grown by thermal-chemical vapour deposition (t-CVD) using SS as substrate and functionalized using an Ar/C2H6/O2 glow discharge. When used as a binder-free SC electrode, these functionalized MWCNTs (f-MWCNTs) show excellent surface stability and large specific capacitance values. At low charging/discharging current densities (0.2 mA cm
2 or 3 A g
1), a specific capacitance of 288  10 F g
1 is achieved. The material shows good rate capability up to charging/discharging current density of 10 mA cm
2 (37 A g
1) and displays excellent reversibility. Consequently, these new binderfree f-MWCNT SC electrodes are excellent candidates for future investigations of MWCNT-based SC electrodes. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: pseudocapacitance plasma functionalization thermal-chemical vapour deposition oxygen-containing functional groups

1. INTRODUCTION In today's society, energy is considered a currency on its own. With the finite supply of fossil fuels available, new energy sources are required to keep up with today's use and future needs. The development of new energy technologies requires a fine balance between output power and energy packed into a nominally small package. For example, in automobile applications, hydrogen fuel cells and Li-ion batteries dominate in terms of energy density. However, for high-power applications, these devices are lacking. Electrochemical capacitors, or supercapacitors (SCs), have been proposed to fill this role. Supercapacitors allow for large power densities with very short charge times and are beginning to see widespread integration into established commercial technologies currently on the market (cell phone camera flashes, electric vehicles, etc.). Carbon-based materials (graphite, glassy carbon, multi-walled carbon nanotubes (MWCNTs), etc.) show great promise as candidate supercapacitor electrodes due to their relatively large

* Corresponding author. Tel.: +15143985566; fax.: +15143986678 E-mail address: [email protected] (M.A. McArthur). ISE Member.

1

double-layer capacitance region (ca. 1 V or more for aqueous electrolytes), relative chemical inertness, global abundance, and low cost [1]. In a recent study by Borgohain et al., the electrochemical double-layer capacitance of a nano-onion carbon morphology was studied and a specific capacitance (Csp) of 27 F g
1 was obtained [2]. Activated carbons (ACs) have seen widespread use as SC electrode materials. Electrodes containing AC derived from coconut shells, waste-tea leaves, sunflower seeds, egg shell membranes to name but a few have been studied for their excellent electrochemical double-layer capacitor properties. Such electrodes, when measured in alkaline electrolyte, have achieved Csp's ranging from 175 to 330 F g
1 [3–7]. Similar studies were performed on graphite and graphene with resultant specific capacitances ranging from 62 to 341 F g
1 [8–12]. Further, MWCNTs have achieved specific capacitances ranging from 21 to 113 F g
1 [13–16]. Although these carbonaceous materials show good performance and stability in terms of their electrochemical double-layer capacitance, their Csp's are relatively low. This, in turn, leads to inadequate on-demand charge delivery and the requirement of having large/massive electrodes. An improvement in total specific capacitance can be achieved by developing hybrid supercapacitors, i.e., those which benefit from both: (i) electrochemical double-layer capacitance and (ii) pseudocapacitance (faradaic reactions). Nitrogen and carboxylic

http://dx.doi.org/10.1016/j.electacta.2014.09.019 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: M.A. McArthur, et al., A binder-free multi-walled carbon nanotube electrode containing oxygen functionalities for electrochemical capacitors, Electrochim. Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.09.019

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functional groups have been shown to improve the specific capacitance of several carbonaceous electrodes, including graphene and MWCNTs [2,8,9,17–19]. For example, Park and coworkers have recently shown that with the addition of carboxyl groups to MWCNT surfaces, a four-fold increase in Csp can be realized over non-functionalized MWCNTs (nf-MWCNTs) [19]. However, in this case, a solutions-based approach was taken to graft functional groups to the MWCNT surfaces. In so doing, many environmentally unfriendly acid treatment steps are used. Further, it is difficult to assess the total functional group coverage in solution as the MWCNT powders inevitably agglomerate, thus leaving large portions of the nanotube walls completely barren (near the interior of the agglomerates). The activity of these uncovered interior regions is questionable when the functionalized MWCNTs (f-MWCNTs) are formed into an electrode. The Plasma Processing Laboratory (PPL) at McGill University has recently reported a fully scalable and repeatable technique to grow MWCNTs in a single-step, without complex precursors, directly onto commercially available stainless steel (SS) mesh using a thermal-chemical vapour deposition (t-CVD) technique [20,21]. In this process, the SS mesh acts as both a conductive support (current collector) and catalyst for MWCNT growth. The relevant defining feature of this catalysis-free t-CVD approach is the relatively large number of MWCNT defect sites combined with a relatively large MWCNT separation distances. These defect sites are proposed as excellent anchor points for functionalization and immobilization of, say, carboxylic and hydroxyl functional groups and nanoparticles, to name but a few.

Using a plasma functionalization approach, MWCNTs can be fully functionalized using a series of process gases, thus adding desired functional groups to the MWCNT surfaces (see, for example ref. [22]). Because the use of the SS mesh leads to the growth of an open 3D network of MWCNTs, a high degree of functionalization is possible as the MWCNTs are relatively separated. These f-MWCNTs solidly rooted into the grid bars of the SS mesh are herein proposed as SC electrode materials. In this study, we demonstrate a simple two-step process to fabricate f-MWCNT electrodes for use in SC applications using the t-CVD process combined with plasma functionalization that exhibit extraordinary specific capacitance values at the laboratory benchscale. These high specific capacitance values can be attributed to the enhanced wettability and pseudocapacitive properties of the f-MWCNT after treatment by plasma functionalization. 2. EXPERIMENTAL 2.1. Single-step MWCNT growth by t-CVD and plasma functionalization MWCNTs were grown in-house in a single step using the t-CVD technique similar to that described by Hordy et al. [21]. The details of the t-CVD growth are given in ref. [23]. The average mass density of the MWCNTs was 0.135 mg cm
2. Functionalization of the MWCNT-covered stainless steel disc electrodes (henceforth referred to as MWCNT electrodes) was done using a low-pressure capacitively-coupled radio-frequency (RF)

Fig. 1. Scanning electron (a-b) and transmission electron (c-d) micrograph images of f-MWCNT electrodes prior to electrochemical measurements.

Please cite this article in press as: M.A. McArthur, et al., A binder-free multi-walled carbon nanotube electrode containing oxygen functionalities for electrochemical capacitors, Electrochim. Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.09.019

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to detect any mass loss due to carbonaceous species. Surface chemistry measurements were carried out by X-ray photoelectron spectroscopy (surface functionalities; K-alpha XPS system, Thermo Scientific). A monochromatic Al Ka micro-focused monochromator photon source with a spot size of 400 mm was used for both the survey (200 eV pass energy, 1 eV step size) and high resolution (50 eV pass energy, 0.1 eV step size) scans. The Shirley method was used for background removal and any charging was corrected by calibrating the Au 4f7/2 peak to 84.0 eV. Raman spectroscopy was used to identify surface characteristics of the nf- and f-MWCNT SC electrodes, specifically, to quantify the defects present in the MWCNTs. Raman spectra were taken using a Bruker Senterra confocal Raman microscope (Bruker, Karlsruhe, Germany) using a 633 nm HeNe laser (20 mW power) and a 50x objective at a spectral resolution of 9–18 cm
1 (2 co-additions, 60 s integration time). Spectra were analyzed using the OPUS software package v. 7.2.; Bruker, Karlsruhe, Germany).

Total electrode mass / %

100.5

100.0

1.12% 99.5

99.0

98.5 150

250

350

450

550

650

Temperature / °C

750

Fig. 2. Thermogravimetric analysis (TGA) of f-MWCNTs used to determine active electrode mass.

glow discharge. The MWCNT electrodes were loaded onto a quartz plate covering the live stainless steel electrode (13 cm o.d.) and the plasma chamber evacuated to the system's base pressure (20 mTorr). An Ar/C2H6/O2:250/1/5 cm3 min
1 gas mixture was injected by mass flow controllers (Brooks 5850 E; Brooks, USA) and the pressure regulated to 1.25 Torr using an automatic butterfly valve (Intellisys Adaptive Pressure Controller, NorCal USA). The RF glow discharge was sustained at 20 W for 5 min using a continuous wave 13.56 MHz Advanced Energy RFX600 power supply. 2.2. Characterization of f-MWCNTs The morphology of the f-MWCNTs, used in this work as SC electrodes, was characterized by scanning electron microscopy (SEM) (Phenom Pro tabletop SEM; Phenom-World, NL) and transmission electron spectroscopy (TEM) (Phillips CM200; Phillips, USA). Physical characteristics of the f-MWCNT electrodes were determined by TGA (f-MWCNT mass; TA Q500, TA Instruments, USA). TGA measurements were performed over a 25–800  C temperature range in air at a constant heating rate of 50  C min
1

(a)

O

300

1s

*

O

KLL

The electrochemical behaviour of f-MWCNT electrodes was characterized using cyclic voltammetry (CV), galvanostatic chronopotentiometric charge/discharge (GCD) cycling and electrochemical impedance spectroscopy (EIS). GCD was performed to accurately determine the specific capacitance of the f-MWCNT electrodes in conditions similar to those in which an actual device would operate. Electrochemical measurements were conducted in a standard 3-electrode cell in 4 M NaOH electrolyte (99.6% assay; Fisher Scientific, USA). NaOH was chosen for its high conductivity and compatibility with the SS mesh current collector (SS corrosion resistance is in NaOH). The f-MWCNT working electrode was held vertically using a PTFE holder (flat specimen holder, Princeton Appliced Research, USA) with a polished (1200 grit) 316 SS backing plate button (McMaster-Carr, USA) to add rigidity. The cross-sectional geometric area of the electrode exposed to electrolyte was 0.785 cm2. The graphite counter electrode was separated from the main body of the cell through a glass frit (Ace Glass, Inc. USA) in order to prevent any interference from possibly-evolved oxygen gas. A saturated calomel electrode (SCE; Accumet electrode, Fisher Scientific, USA) was used as the reference electrode. All electrochemical measurements were

Intensity / a.u.

C1s

2.3. Electrochemical characterization

nf−MWCNT f−MWCNT

(b)

C−OH C=O O−C=O

295

290

285

280

275

Binding energy / eV + +

*

(3) Intensity / a.u.

Intensity / a.u.

nf−MWCNT (1) f−MWCNT (2) f−MWCNT after GCD (3)

3

(2)

0 cycles 100 cycles 300 cycles

(c)

(1) 1000

800

600

400

Binding energy / eV

200

0

300

295

290

285

280

275

Binding energy / eV

Fig. 3. XPS data of various MWCNT electrodes. (a) survey scan comparing nf-MWCNT, f-MWCNT, and f-MWCNT after GCD cycling, (b) high-resolution C1s scan contrasting nfand f-MWCNTs, and (c) high-resolution C1s scan comparing f-MWCNTs before (0 cycles) and after 100 and 300 GCD cycles, respectively.

Please cite this article in press as: M.A. McArthur, et al., A binder-free multi-walled carbon nanotube electrode containing oxygen functionalities for electrochemical capacitors, Electrochim. Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.09.019

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Table 1 Oxygen level and relative carbon-to-oxygen ratio found on various MWCNT electrode surfaces from XPS survey scan data. Errors provided at a 95% confidence level. The ratios of the intensity of the Raman G-to-D peaks (IG/ID) are also shown. Sample

O (at.%)

nf-MWCNT f-MWCNT f-MWCNT-100 f-MWCNT-200 f-MWCNT-300 f-MWCNT-1200

0.4 9.2 7.8 8.8 9.2 7.7

*

     

0.1 0.9 1.2 1.4 1.0 0.7

C:O

IG/ID

221 9.9 11.7* 10.3* 9.7* 11.8*

0.62 0.59 0.58 0.61 0.55 0.61

Survey spectra after GCD cycling contained trace amounts of residual Na

performed employing a computer-controlled combination potentiostat/galvanostat/frequency response analyzer (Autolab PGSTAT30, Metrohm, NL) using the NOVA software package (v. 1.10; Metrohm, NL). 3. RESULTS AND DISCUSSION 3.1. Physical characterization of f-MWCNT electrodes Fig. 1 shows both scanning electron (a-b) and transmission electron (c-d) micrographs of a f-MWCNT electrode at various magnifications. MWCNTs (4 mm long, 55 nm in diameter) grown by t-CVD form relatively open matrices securely rooted into the underlying SS substrate. This enables excellent contact between the f-MWCNTs and the electrolyte for proper charge storage, maximizes the MWCNT/electrolyte interfacial area, and reduces agglomeration between nanotubes thus avoiding a decrease in active surface area. In addition, because the f-MWCNTs are physically rooted into the SS, a high electronic conductivity is achieved. From these high-resolution images, we see that the MWCNTs are riddled with defects (inter-wall stacking faults, encapsulated fullerenes, Y-junctions, kinks). As previously mentioned, it is hypothesized that these defect sites are excellent regions on which functional groups can adhere. This is in accordance with previous work done on MWCNTs as nanoparticle scaffolds, described in ref. [23]. Fig. 2 displays the mass loss results of a f-MWCNT electrode heated by TGA. The mass loss above 535  C is attributed to the loss of well-graphitized carbons (f-MWCNTs) [24], and it is consistent with previous work on MWNCTs [20,21]. Further, heating of the f-MWCNT electrode did not show any mass loss associated with amorphous carbon (mass loss expected below 450  C), indicating that the material is pure. From TGA, it was found that the mass of f-MWCNTs varied with the mass of the SS mesh. Specifically, the mass of f-MWCNTs was 11.2 mg per 1 g of SS mesh. Mass determination based on the mass of the SS mesh was necessary as the mass difference before and after growth by t-CVD does not correspond entirely to the actual MWCNT mass [21]. As a result, this rule was employed for the calculation of specific capacitance in later sections. Fig. 3 shows XPS results for nf- and f-MWCNTs for a variety of experimental conditions. Fig. 3a contrasts the XPS survey scans between nf-MWCNTs, f-MWCNTs, and f-MWCNTs after 1200 GCD cycles with appropriate peaks labeled. From the survey scan of the nf-MWCNT electrode, a single, large peak corresponding to the C1s excitation is observed near 285 eV. It can be concluded that the as-prepared nf-MWCNT electrode surface is free of metal impurities (from the SS mesh) prior to plasma functionalization (below the XPS detection limit). After plasma functionalization (f-MWCNT), a relatively large oxygen peak at 532 eV is present in the XPS spectra indicating that oxygen functionalities are clearly present on the MWCNT surfaces. These functional groups are also stably bound to the MWCNT surfaces. Even after 1200 GCD cycles (f-MWCNT after

GCD), no statistically significant decrease in the oxygen peak is observed evidencing the excellent surface stabilization of the functionalities (at a 95% confidence level). The XPS survey spectra can also be used to quantitatively describe the relative amounts of carbon and oxygen present on the MWCNT surfaces. Table 1 summarizes the surface concentrations of O and the C:O ratio on the f-MWCNTs from XPS analysis for various MWCNT electrodes (after various GCD cycle numbers). For nf-MWCNTs, only trace amounts (< 0.5 at.%) of oxygen at the surface are detected. The oxygen concentration increases to over 9 at.% after plasma treatment and remains high even after prolonged electrochemical testing. Thus, any increase in specific capacitance for f-MWCNT electrodes over nf-MWCNT electrodes is likely due to the increased amount of oxygen-containing surface species present. Fig. 3b contrasts the high-resolution XPS spectra of the C1s peak for nf- and f-MWCNTs with contributions from oxygen functionalities clearly labeled. It is clear that the C1s peak for nf-MWCNTs is highly asymmetric. This peak broadening into the high-binding energy region is consistent with our own previous work on MWCNTs [20] and the work of others [25–28]. The functionalities of interest, primarily hydroxyl (C-OH) carbonyl, (CQO), and carboxyl (O-CQO) groups, appear in the region between 286–290 eV for the f-MWCNT surface. Unfortunately, a large overlap is observed between the functional groups and the C1s peak broadening for the f-MWCNTs. Due to this overlap, deconvolution of the functional groups from XPS data matching to determine relative amounts of relevant functional groups present on the MWCNT surface is extremely difficult (and may even be an incorrect treatment of the data). Suffice it to say, the typical peak locations for C-OH, CQO, and O-CQO groups can be examined on the f-MWCNT spectra to qualitatively describe the functional groups present (these near approximately 285.8, 286.8, and 288.9 eV, respectively [29,30]). In the 286–290 eV region, a marked change in surface chemistry is observed between the nf- and f-MWCNTs from the C1s spectra (Fig. 3b). This suggests that plasma functionalization adequately grafts the oxygen-containing functionalities identified above onto the MWCNTs. A more detailed examination of the surface chemistry of the f-MWCNTs can be found elsewhere (ref. [31]). Further confirmation of these functional groups is given in a recent study by Vandsburger et al. [32] who used a similar approach to functionalize MWCNTs.

Fig. 4. Raman spectra of a f-MWCNT electrode after 100 GCD cycles in 4 M NaOH at 0.2 mA cm
2 with appropriate deconvolution. Inset show a high-resolution TEM image of a f-MWCNT with graphene lattice defects circled. For complete details of deconvolution of Raman spectra for f-MWCNTs, the reader is referred to ref. [31].

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M.A. McArthur et al. / Electrochimica Acta xxx (2014) xxx–xxx Table 2 Results of NLLS fitting of the EEC to the experimental Nyquist data displayed in Fig. 7. Circuit element

Value

Rel (V) Cdl (F g
1) Rct (V) ZW (mV
1 s1/2) CF (F g
1)

2.62  0.02 7.60  0.07 51  2 12.1  0.4 251  15

Thus, any change in capacitance over nf-MWCNTs can be ascribed to the presence of the oxygen-containing functionalization. Fig. 3c shows the high-resolution C1s spectra for f-MWCNTs before electrochemical testing (0 cycles), after 100 GCD cycles at 0.2 mA cm
2 (100 cycles), and after 1200 GCD cycles at various current densities (1200 cycles, after rate capability testing). The surface concentration of functional groups did not vary appreciably after GCD cycling. Thus, one can conclude that functionalization of MWCNTs by plasma treatment is a simple and efficient way to stably bond oxygen-containing groups to MWCNT surfaces and ensure their surface-presence even after intensive electrochemical testing, rinsing, and drying. Fig. 4 shows a Raman spectrum for an f-MWCNT electrode after 100 GCD cycles and associated deconvolution. No appreciable differences were observed in the Raman spectra between the nf-MWCNT and f-MWCNT electrodes after electrochemical testing (not shown). Raman spectroscopy is a useful tool for structural characterization of MWCNTs. The two most relevant characteristic stretches for MWCNTs appear as the defect (D) and graphitic (G) bands found about 1330 and 1580 cm
1, respectively (for a 633 nm excitation source). For the D-band to appear in the spectrum, defects must be present in the hexagonal sp2 carbon lattice of the graphene planes. Thus, the G-to-D peaks intensity ratio (IG/ID) yields information relating to the relative amount of defects or disorder present in the MWCNTs (this is true as the MWCNTs presented in this work are free of impurities (confirmed by XPS) and amorphous carbons). The inset to Fig. 4 provides a highresolution image of a MWCNT showing an example of what is meant here as a defect–a discontinuity in the graphene walls of the MWCNT. Other defects such as bond rotations are also present but do not result in a change in the nanotube's wall structure. The intensity ratio of G-to-D peaks is presented in Table 1 for MWCNT electrodes produced under various conditions. The fresh nf-MWCNTs contain a relatively large amount of defects with a

IG/ID ratio of 0.62 based on the conditions of t-CVD. After plasma functionalization, no appreciable change in the IG/ID ratio is observed, indicating that the plasma functionalization process introduces little-to-no further defect/disorder in the graphene lattice. Plasma functionalization of MWCNTs has been known to lead to a decrease in the IG/ID ratio [33]. It is likely that no appreciable decrease was observed here due to the highly defective nature of the MWCNTs prior to treatment. A more detailed analysis of the graphitic structure of the MWCNTs, including a complete deconvolution of all the Raman active bands before and after plasma functionalization, is provided elsewhere [31]. After GCD cycling for various durations (nomenclature follows f-MWCNT-x where x is the number of GCD cycles prior to the Raman measurement in Table 2), the IG/ID ratio remains relatively constant (0.59  0.02). Thus, GCD cycling does not influence the amount of defects within the f-MWCNT electrodes. These defect sites are believed to improve the amount of oxygen-containing functional groups grafted to the MWCNT surface [34]. Fig. 5 shows cyclic voltammograms (CVs) of an f-MWCNT electrode (solid curve) and a nf-MWCNT electrode (dashed curve) recorded in 4 M NaOH at a sweep rate of 200 mV s
1 between -0.7 and 0.3 V vs SCE. The CVs displays a semi-rectangular shape typical of supercapacitor electrodes. The symmetry of both CVs indicates that the capacitance of the electrodes is reversible in the anodic and cathodic directions. A current density increase is observed for the f-MWCNT electrode over the nf-MWCNT after plasma functionalization. This is indicative of an increase in surface charge and hence capacitance stored on the f-MWCNT electrode over its non-functionalized state. A very weak shoulder in the anodic sweep, at around 0 V, and a gradual increase in cathodic current negative of -0.3 V in the cathodic sweep, could be related to the pseudocapacitive effect of the oxygen-containing functional groups on the MWCNTs. The lack of well-defined redox peaks on the CV is typical of f-MWCNTs due to both the relatively slow redox kinetics of surface functional groups, compared to the charging kinetics of the electrochemical double-layer layer, and their small surface fraction [19]. 3.2. Electrochemical performance of f-MWCNT electrodes Fig. 6a shows the specific capacitance results with cycle number of a typical f-MWCNT electrode cycled 140 times at a current

500

0.2

/Fg

1

2

C

sp

1 0

400

E vs SCE / V

450

f−MWCNT nf−MWCNT

−2

0.3

(a)

(b)

0.1

3

j / mA cm

5

0 0.1 0.2 0.3 0.4 0.5

350

0.6 0.7 4000

−1

5000

6000

7000

8000

time / s

300

−2 −3 −0.7

250 0 −0.5

−0.3

−0.1

0.1

0.3

20

40

60

80

100

120

140

Cycle number

E vs SCE / V Fig. 5. Cyclic voltammogram of the f-MWCNT and nf-MWCNT electrodes recorded in 4 M NaOH at a sweep rate of 200 mV s
1.

Fig. 6. (a) Dependence of specific capacitance (Csp) on GCD cycle number for a fMWCNT SC electrode calculated form the curves in (b) recorded at a current density of 0.2 mA cm
2.

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density j = 0.2 mA cm
2. Fig. 6b shows the behaviour of the electrode potential with time for 6 cycles between 4000–8000 s. The data in Fig. 6b were used to compute the capacitance displayed in Fig. 6a using the following equation:

where Csp is the total specific capacitance [F g
1], I is the current applied for the corresponding half-cycle [A], Dt is the charge/ discharge time [s], m is the active mass of f-MWCNTs [g], and DE is the potential window [V]. Specific capacitance values were calculated by averaging the charge and discharge specific capacitances. An extraordinarily large initial capacitance is observed for each f-MWCNT electrode during the first 5–10 cycles (600 F g
1 at the beginning) and thereafter, it stabilizes quickly to ca. 290 F g
1 (Fig. 6a). After 140 GCD cycles, the specific capacitance reached 288  10 F g
1. This large initial capacitance is apparently ascribed to a change in the wetting behaviour of the nanotubes with the electrolyte. It is believed that initially, the nanotubes are completely and uniformly covered by electrolyte. After several cycles with an applied constant current, however, the deep pores of the f-MWCNT electrode apparently dry, thus leading to a reduced surface area and hence a decrease in specific capacitance. This effect is reversible–the capacitance follows the same initially large value after stopping the current, allowing the electrode to equilibrate at open circuit, and again cycled (data not shown). A similar stabilizing behaviour was observed in a study by Frackowiak et al. whereby MWCNTs were functionalized in boiling nitric acid [35]. Their MWCNT slurry electrodes showed an increased capacitance on the first few GCD cycles after surface stabilization. The specific capacitance of as-prepared nf-MWCNTs was determined to be 43  5 F g
1 by GCD cycling at 0.2 mA cm
2 (data not shown) which is in good agreement to what others have observed for pristine MWCNTs [16,17,19,36–38]. The increase in specific capacitance over nf-MWCNTs, observed in Fig. 6, is due to the pseudocapacitive contribution of the oxygen-containing surface functionalities [1,17,19,22]. It is believed that the pseudocapacitive redox reactions occurring from the presence of oxygen functional groups can be generally expressed as [1,39]: iCx O þ Hþ þ e
?iCx OH

400

(1)

(2)

where H+ is a proton and e
is an electron. Eq. (2) reflects the general reversible reaction between oxygen containing functional groups in the presence of protons and the effects of Eq. (2) can be readily observed on the CV in Fig. 5, and in Fig. 6b. In Fig. 6, the voltage profile of the f-MWCNT electrodes deviate from the triangular GCD cycling curves expected from a singularly electrochemical double-layer capacitor [1]; a slight curvature is observed near the upper and lower potential limits due to the pseudocapacitive nature of the f-MWCNTs, which is also in accordance with the behavior in Fig. 5. These pseudocapacitive effects contribute to additional charge storage/use due to the faradaic nature of the reversible surface reactions (Eq. (2)) over traditional electrochemical double-layer capacitor charging (kinetically fast). EIS (between 10 mHz-100 MHz, 10 mV amplitude) was performed to better understand the effect of the capacitive elements of the electrode materials. Fig. 7 shows the Nyquist plot (experimental points are presented as symbols) of a f-MWCNT (circles) and a nf-MWCNT (triangles) electrodes at the bottom of a discharge cycle (E = -0.7 V vs SCE) recorded after 500 GCD cycles, along with the corresponding non-linear least squares (NLLS) fit (solid curve). The inset to Fig. 7 shows the equivalent electrochemical circuit (EEC) used to model the Nyquist data. The EEC used was a modified Randles circuit consisting of the Randles circuit [40] in series with a Faradaic capacitance, CF, which has

300

Z’’ /

C sp ¼ ðIDtÞ=ðmDEÞ

500

Cdl CF

200 Rel

Rct ZW f MWCNT experimental nf MWCNT experimental model fit

100

0 0

100

200

300 400 Z’ /

500

Fig. 7. Nyquist plot from EIS measurements of f-MWCNT (circles) and nf-MWCNT (triangles) SC electrodes recorded at the bottom of discharge (-0.7 V vs SCE) after 500 cycles and the corresponding fit (line) to the EEC displayed in the inset.

been used successfully to match pseudocapacitive carbon nanotube composite electrodes and others [17,41–43]. Table 2 summarizes the parameters used in the EEC for the f-MWCNT. There are three frequency domains in Fig. 7 for the f-MWCNT electrode which must be considered for proper analysis of the impedance spectra. In the Nyquist plot, a large developed semicircle is observed in the high-frequency region. This semicircle is associated with a charge-transfer process at the electrode/ electrolyte interface [44] and is represented in the EEC as the charge-transfer resistance, Rct, in parallel with the double-layer capacitance, Cdl. At the potential considered (-0.7 V), the majority of Rct represents the kinetics of the pseudocapacitive redox reactions occurring in Eq. (2). The double-layer capacitance due to charge migration near the MWCNT walls was measured to be 7.60  0.07 F g
1 which is consistent with other work on MWCNTs [45,46]. The EIS response at intermediate frequencies in Fig. 7 for the f-MWCNT electrode can be associated with transport of ions to the electrode surface to compensate the charge and/or surface diffusion related to pseudocapacitance associated with the redox reactions and can be accounted for in the EEC by the Warburg impedance element, ZW. The Warburg element was required in the EEC as without it, the model match of the experimental data was poor. From EEC modeling of the impedance spectra in Fig. 7, the Warburg element yielded a value of 12.1  0.4 mV
1 s1/2. The use of the Warburg element is deemed correct as the match between the EEC and the impedance spectrum is very good, as evidenced by the agreement between the solid line and symbols in Fig. 7. A near-linear region is observed in the Nyquist plot at low frequencies. This linear behaviour is typical of pseudocapacitors [1,47]. Here, the effects of the electrochemical double-layer and pseudocapacitance are combined to yield the Faradaic capacitance, CF, which can be used to compare with the specific capacitance calculated from GCD cycling. Of the EIS parameters considered, the Faradaic capacitance is the most useful for determining the specific capacitance of the f-MWCNT electrode. In an ideal SC electrode, the CF would trace a vertical line parallel to the imaginary axis in a Nyquist plot. In Fig. 7, some deviation from vertical is observed. These deviations are thought to be due to “leakage” current associated with the diffusive resistance of electrolytes in electrodes [1,17]. From the EEC, a limiting

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capacitance of 251 15 F g
1 was measured. Thus, the total specific capacitance from EIS is 259  15 F g
1 which is in very good agreement with the results from GCD cycling at 0.2 mA cm
2 (288  10 F g
1,Fig. 6). The excellent match between the model and the data, in addition to the excellent agreement with GCD, lead us to believe that the proposed EEC is correct and valid for modeling our f-MWCNT electrodes. The Nyquist curve for the nf-MWCNT electrode is given in Fig. 7 for comparison with that for the f-MWCNTs. The curve for the nf-MWCNT displays an EIS behavior typical of that for pure doublelayer capacitors, i.e. it increases smoothly as a straight line, indicating the absence of the resistive component of the electronic circuit, and the dominance of the pure capacitive behaviour. Compared to the Nyquist curve for the f-MWCNT electrode, the lack of a semicircle at high frequencies for the nf-MWCNT electrode thus indicates that there are no redox reactions associated with charge transfer. As discussed above, this region is the cause of the large pseudocapacitance increase observed for the f-MWCNT electrode over the nf-MWCNT electrode. Fig. 8 shows the rate capability results for a f-MWCNT electrode after GCD cycling at various current densities for 100 cycles each. Numbers along the top of Fig. 8 indicate the magnitude of the current density (in mA cm
2) applied to the f-MWCNT electrode. As expected, at low current density (j = 0.2 mA cm
2), the specific capacitance of the electrode is very large (265  5 F g
1 for this f-MWCNT electrode). As larger current densities are applied, the specific capacitance of the electrode decreases to a minimum of 50 F g
1 at j = 10 mA cm
2. This decrease is 77% of the initial specific capacitance of the f-MWCNT first cycled at 0.2 mA cm
2. Relatively large drops in capacitance during rate capability measurements of MWCNT-based electrodes have also been reported by others [19,41,48]. However, one should note that these current densities are very large (approaching 40 A g
1). Even so, when compared to f-MWCNT materials previously reported in the literature, 50 F g
1 is an acceptable value [49,50] even for modest current densities. Fig. 8 also displays the excellent reversibility of the f-MWCNT electrodes. After various selected charging/discharging current densities (1, 5, 10 mA cm
2), a low current density (0.2 mA cm
2) was re-applied in an attempt to recover the original high specific capacitance. Indeed, after a very stressful application of a high current density, the original high specific capacitance was recovered. This indicates that the functional groups are securely

7

grafted onto the MWCNT surfaces without loss even at high charging/discharging currents (10 mA cm
2 or 37.4 A g
1) and is supported by XPS measurements (Fig. 3). In a recent study by Park and coworkers [19], a similar study on f-MWCNTs as SC electrodes was done in nonaqueous electrolyte. The authors theorized that long-term cycling stability of f-MWCNT electrodes was strongly affected by the degradation of the functional groups after interacting with other electrolyte species. Further, it was concluded that oxygen-containing functional groups improve the long-term cycling stability of f-MWCNTs in comparison to nitrogen- and sulfur-containing functional groups. Thus, it is unsurprising that the original large Csp is recovered after cycling at very high current densities (Fig. 8). Fig. 9 complements Fig. 8 by summarizing the rate capability data and shows the specific capacitance as a function of current density (expressed as both mA cm
2 and A g
1) applied during GCD cycling. As mentioned above, a large drop in Csp with an increase in current density is observed. However, this drop is acceptable when considering the current density increase and is in accordance with the behaviour reported by others on various SC materials [51–53]. The drop in capacitance with an increase in current density can be, in our case, related to the electron-transfer kinetics and/or transport processes involving surface functional groups (Eq. (2)), i.e. to the fact that at higher current densities a larger overpotential is needed to overcome barriers related to the two processes. Given that the GCD cycling in the current work was done in a fixed potential region (Fig. 6b), one can indeed expect to see a decrease in the resulting capacitance at higher current densities, to the point corresponding to the situation when the contribution of surface functional groups to the capacitance almost diminishes, and the resulting capacitance becomes equal to that of nf-MWCNTs. Indeed, this is the case in Fig. 9, where the apparent limiting capacitance (near 50 F g
1) reaches what is expected for nf-MWCNT electrodes. Based on the previous discussion, the presented thermal CVD + plasma functionalization method is an extraordinary technique used to grow an open 3D MWCNTs forest onto the porous (mesh) SS current collector and then securely immobilize functional groups onto the MWCNT surface. Typically, when one functionalizes a bulk carbon nanotube powder, agglomeration inevitably takes place. Thus, the stability of the functional groups is always in question(only the outside of the agglomerated carbon nanotubes are

−1

Specific current / A g 300

0.2 0.3 0.4 0.5 1.0 0.2 2

3

0.0 300

5 0.2 10 0.2

250

Csp / F g−1

−1

1

2

200

150

C

sp

/Fg

7.5 11.2 15.0 18.7 22.5 26.2 30.0 33.7 37.4 41.2

250

200

100

150

100

50 0 0

3.7

50

200

400

600

800

1000

1200

Cycle number Fig. 8. Specific capacitance (Csp) data from SC rate capability experiments at various current densities (listed in bold on top of the figure between the dashed vertical lines). For each current density, 100 GCD cycles were performed.

0 0

3

4

5

6

7

8

9

10

−2

Current density / mA cm

Fig. 9. Summary of the rate capability experiments of Fig. 8 done by plotting the average Csp with applied current density. Standard deviation bars are small, and thus not clearly visible. Dashed lines are a guide to the eye only.

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functionalized). Also, f-MWCNTelectrodes produced by others show a decrease in Csp with time as the functional groups degrade [19], which is not the case with our f-MWCNTs (Fig. 9). Not only do the oxygen-containing functional groups help to increase the capacitance of the f-MWCNT electrode by faradaic reactions, but also by modifying the hydrophobicity of the MWCNTs themselves. By their very nature, MWCNTs grown by t-CVD appear hydrophobic (their (super) hydrophobicity comes from trapped air in the MWCNT forest) with a water contact angle of 103.4 [32]. After plasma functionalization, f-MWCNTs become hydrophilic (approaching 0 water contact angle [32]). It is believed that these hydrophilic f-MWCNTs show improved wettability in the electrochemical cell, offering more accessible sites along the electrolyte/MWCNT interface and increases ionic conductivity of the f-MWCNT films [17]. These combined effects increase the specific capacitance attained by the material. 4. CONCLUSIONS A method to prepare binder-free MWCNT-based electrodes for supercapacitor applications was presented. The plasma functionalization of immobile oxygen-containing groups onto MWCNTs allowed for non-agglomerating, hydrophilic MWCNTs increasing the electrode area exposed to the electrolyte. It is likely that the plasma functionalization enhanced the wettability of the MWCNTs improving electrolyte/electrode contact thus allowing facile movement of charged species to/from the electrode. Most interestingly, the functional groups improved the capacitance of the electrode by apparently introducing faradaic reactions (pseudocapacitance). Taking this plasma functionalization approach combined with t-CVD, we eliminated the effects of non-uniform distributions of functional groups along the MWCNTs. Further, our electrodes could potentially be used “as-is” without the need of complicated inactive binder, leading to a cost and volume reduction in an actual electrochemical device. These f-MWCNTs, when used as a binder-free supercapacitor electrode, show excellent surface stability and large specific capacitance values in a half-cell configuration. At low current densities (j = 0.2 mA cm
2), a specific capacitance of 288  10 F g
1 is achieved after 140 GCD cycles. The material shows good rate capability up to 10 mA cm
2 (37 A g
1) and displays excellent reversibility. To the best of the authors' knowledge, these binderfree SC electrodes display the largest specific capacitance reported to-date for MWCNTs containing oxygenated functional groups and are excellent candidates for future supercapacitor investigations. ACKNOWLEGMENTS The authors gratefully acknowledge the financial support provided by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fonds de recherche du Québec– Nature et technologies (FRQNT), and McGill University through the McGill Engineering Doctoral Award (MEDA) program. References [1] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Springer (1999) . [2] R. Borgohain, J. Li, J.P. Selegue, Y.-T. Cheng, Electrochemical Study of Functionalized Carbon Nano-Onions for High-Performance Supercapacitor Electrodes, J. Phys. Chem. C. 116 (2012) 15068–15075, doi:http://dx.doi.org/ 10.1021/jp301642s. [3] X. Li, W. Xing, S. Zhuo, J. Zhou, F. Li, S.-Z. Qiao, et al., Preparation of capacitor's electrode from sunflower seed shell, Bioresour. Technol. 102 (2011) 1118–1123, doi:http://dx.doi.org/10.1016/j.biortech.2010.08.110. [4] L. Sun, C. Tian, M. Li, X. Meng, L. Wang, R. Wang, et al., From coconut shell to porous graphene-like nanosheets for high-power supercapacitors, J. Mater. Chem. A. 1 (2013) 6462–6470, doi:http://dx.doi.org/10.1039/C3TA10897J.

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