Electrochemical hydrogen storage behavior of single-walled carbon nanotubes (SWCNTs) coated with Ni nanoparticles

international journal of hydrogen energy 35 (2010) 2336–2343

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Electrochemical hydrogen storage behavior of single-walled carbon nanotubes (SWCNTs) coated with Ni nanoparticles Chun-Chen Yang*, Yingjeng James Li, Wei-Huang Chen Department of Chemical Engineering, Mingchi University of Technology, Taipei Hsien 243, Taiwan, ROC

article info

abstract

Article history:

The electrochemical hydrogen storage properties of Ni nanoparticle coated SWCNT elec-

Received 20 August 2008

trodes were investigated. A surface modification technique enabled different amounts of

Received in revised form

Ni nanoparticles to be deposited on the SWCNT surface, which was first chemically

20 November 2009

oxidized by 6 N HNO3. The characteristic properties of the SWCNT samples coated with

Accepted 5 January 2010

4–12 wt.%Ni nanoparticles were examined using a scanning electron microscope with

Available online 27 January 2010

energy dispersive spectroscopy (SEM/EDX); micro-Raman spectroscopy; thermal analysis techniques consisting of both thermogravimetric analysis (TGA) and differential thermal

Keywords:

analysis (DTA), and Brunauer–Emmett–Teller (BET) measurements. It was found that all of

Single-walled carbon

the SWCNT samples coated with 4–12 wt.%Ni nanoparticles possessed a similar pore-size

nanotubes (SWCNTs)

distribution. According to the electrochemical test results, the highest electrochemical

Multi-walled carbon

discharge capacity of 1404 mA h g1 was obtained for the SWCNT electrode coated with

nanotubes (MWCNTs)

8 wt.%Ni nanoparticles, which corresponded to 5.27 wt.% hydrogen storage. This

Electrochemical hydrogen

enhancement of electrochemical hydrogen storage capacity was ascribed to the fact that

storage capacity

the Ni nanoparticles act as a redox site, thus leading to an improved electrochemical

Micro-Raman

hydrogen storage capacity. The results indicated that the SWCNT coated with Ni nanoparticles are a potential material for hydrogen storage. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Hydrogen is considered as an ideal fuel for many energy converters because of its low mass density, high energy density, and nonpolluting nature. Hydrogen can be directly used in fuel cells for transportation applications. The gravimetric and volumetric hydrogen storage targets set by US Department of Energy are 6.5 wt.% and 62 kg H2 m3, respectively. At the present time, it is still far away target. Moreover, hydrogen storage with a safe, effective and cheap system remains a main challenge before any automotive applications can be realized. With this advent, carbon nanotubes (CNTs) and activated carbons (ACs) have attracted much attention as candidates for a hydrogen storage media. Since the discovery

of the carbon nanotubes (CNTs) in 1991 [1], they have attracted great attention as one of the potential adsorbents for hydrogen due to their unique properties: the hydrogen storage in CNTs can be implemented by either chemisorption (atomic hydrogen bonded to CNTs) or physisorption (molecular hydrogen adsorption in CNTs through a Van der Waals interaction) [2]. However, improvement of the hydrogen storage capacity is crucial for achieving viable H2/O2 fuel cells or hydrogen-driven power source systems. The amount of hydrogen storage in CNTs has caused much controversy. Various studies of CNT hydrogen storage have been reported both in terms of the gas–solid medium and as electrochemical systems [3–8]. Some reports show that the microstructure and morphology of the carbon nanotube have

* Corresponding author. Tel.: þ886 29089899; fax: þ88 6 29041914. E-mail address: [email protected] (C.-C. Yang). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.01.007

international journal of hydrogen energy 35 (2010) 2336–2343

a great influence on hydrogen storage [6]. Most reported hydrogen storage systems have required high pressure or low temperature, or both. Only a few studies are found reporting the hydrogen storage in CNT at ambient pressure and room temperature. For the electrochemical method, during the charge process, the KOH electrolyte dissociates around the CNT working electrode, atomic hydrogen is adsorbed on the electrode and OH is left in the solution. The amount of the adsorbed hydrogen can be calculated from the discharge (Q), which is equal to the discharge current (I ) multiplied by the time (t). Dillon et al. [9] studied the hydrogen adsorption properties of as-prepared soot containing 0.1–0.2 wt.% single-walled carbon nanotubes (SWCNTs), and they estimated a hydrogen storage capacity of 5–10 wt.% for pure SWCNTs. Rajalakshmi et al. [10] showed that the electrochemical capacity of 800 mA h g1 was obtained for SWCNTs with a diameter of 1.4 nm, in which the stable electrodes were prepared by pressing SWCNTs with copper powder fillers in a 1:3 ratio. Lee et al. [11] reported the charge/discharge capacity of 160 mA h g1 for SWCNT, in which the electrode was prepared by mixing SWCNT with Ni powder fillers and PTFE binders in a weight percent (i.e., SWCNT:Ni:PTFE) of 40:50:10. In this work, the characteristic properties of SWCNTs with and without metallic Ni nanoparticle coating were examined and compared using SEM/EDX, TGA/DTA, micro-Raman spectroscopy and BET measurements. The hydrogen storage capacities of single-walled carbon nanotubes (SWCNTs) coated with Ni nanoparticles were examined by the galvanostatic charge/discharge method. The performance of the electrodes comprised of MWCNT and SWCNT samples coated with 4–12 wt.%Ni nanoparticles was also studied and is discussed.

2.

Experimental

2.1.

Synthesis of Ni-coated CNTs

The multi-walled and single-walled CNTs were purchased from Aldrich. A chemical oxidation method was used to implant surface oxide groups onto the SWCNTs via acidtreating samples in a 6 N HNO3 solution at 60  C for 24 h. Following the oxidation treatment, the samples were washed with copious D.I. water until the pH was equal to 7. Then, these SWCNT samples were dried in a vacuum oven at 110  C for 12 h. To coat the oxidized nanotubes with Ni nanoparticles, an ionic adsorption method followed by heating treatment was used. The typical procedure utilized for coating the carbon nanotubes is as follows: about 0.5 g of oxidized SWCNTs were mixed with a 0.5–1.0 M Ni(NO3)2 solution and stirred under air atmosphere at 25  C for 12 h. This wet chemical process enables nickel ions to be bonded to the surface oxygen, forming a Ni ionic adsorption. Then, the Ni-adsorbed SWCNTs were separated from the Ni(NO3)2 solution using a filtration method. The heating process was performed at 300  C under an H2 atmosphere, ensuring a reduction of nickel ions to metallic Ni nanoparticles. The amount of Ni nanoparticles adsorbed onto the SWCNTs was controlled by varying the

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concentration of Ni(NO3)2, the resulting composition which was measured using Atomic Absorption Spectroscopy (AAS).

2.2.

Characterization of Ni-coated SWCNTs

All SWCNT samples were characterized using a Scanning Electron Microscope/EDX (SEM, Hitachi S-2600H). The thermal analyses were carried out using a TGA system (Mettler Toledo TGA/SDT 851e). Measurements were made by heating from 25 to 1000  C, under O2 atmosphere at a heating rate of 10  C min1 with about a 5–6 mg sample. The micro-Raman spectroscopy analyses were carried out using a Renishaw confocal-microscopy Raman spectroscopy system, which consisted of a microscope equipped with a 50 objective, and a charge-coupled device (CCD) detector. A micro-Raman excitation source was provided by a 633 nm laser beam, having a beam power of 17 mW and focused on the samples with a spot size of about 1 mm in a diameter. The Brunauer– Emmett–Teller (BET) surface area, the pore-size, and the poresize distribution of the SWCNT samples were measured by a Micromeritics. ASAP2010 system. The peak pore diameter, the mesopore area and the volume were estimated using the Barret–Joyner–Halenda’s (BJH) method.

2.3. Preparation of the carbon electrodes and their electrochemical analyses The formation of the carbon electrodes was carried out by first preparing a paste mixture using a certain quantity (10–20 mg) of SWCNTs, with or without Ni-coating, Ni powders and PTFE binders (DuPont 30J) (SWCNT:Ni filler:PTFE ¼ 1:9:1). The resulting paste mixtures were spread onto the surface of Ni-foam current collectors, which had an area of 1 cm2 under a pressure of 100 kgf cm2. Then the electrodes were sintered at 350  C for 40 min. In order to remove the influence of Ni powders, a pure Ni-foam electrode was also made with only Ni powder and PTFE binders under the same conditions. The counter electrode was a Ni(OH)2/NiOOH electrode, having about three times the capacity compared to a pure SWCNT electrode. The electrolyte was a 6 M KOH aqueous solution. Galvanostatic charge/discharge measurements were used to measure hydrogen storage capacities of SWCNT electrodes. The galvanostatic charge/discharge measurements were performed using an Autolab PGSTAT30 electrochemical system with GPES 4.8 package software (Eco Chime, The Netherlands). All the electrochemical measurements were carried out at ambient temperature, pressure and atmosphere. The SWCNT electrodes were charged at about 40 mA cm2 for 2 h and then discharged at the same rate after a rest of 30 min. The discharge cell potential cutoff was set to 0.40 V.

3.

Results and discussion

3.1.

Characterization of Ni-coated SWCNTs

Fig. 1(a) and (b) show the SEM photographs of the SWCNT samples with about 8 wt.%Ni nanoparticles at: (1). 10 kx; (2). 20 kx, respectively. The SWCNT, with typical diameters of 1–10 nm and lengths of 1–10 mm, were highly curved and

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international journal of hydrogen energy 35 (2010) 2336–2343

Fig. 1 – SEM images for the SWCNTs with 8%Ni nanoparticles: (1). 10 kx; (2). At 20 kx

bunched together in a random and disordered shape. Fig. 2(a) shows the EDX results for the SWCNT sample coated with 8 wt.%Ni nanoparticles and Fig. 2(b) presents the EDX element mapping results for the Ni-coated SWCNT samples. It was found that elemental Ni indeed existed in these Ni-coated SWCNT samples uniformly distributed on the surface. The Au peak found in Fig. 2(b) is due to a thin layer of Au (10 nm) sputtered on the samples for SEM examination In Fig. 3 is shown the micro-Raman spectra of the asreceived and the acid-purified (in 6 N HNO3) SWCNT samples. The micro-Raman spectra of the SWCNT samples without Ni nanoparticles displayed two strong peaks located at 1324 and 1585 cm1. The band appearing at 1324 cm1 was due to a chemically induced disruption of the hexagonal carbon order in the carbon nanotube walls, so-called the D band (the disorder-induced or defect band). The band at 1585 cm1 was due to the tangential vibration of the carbon atom along the tube axis and along the circumferential direction, the socalled G band (the tangential graphitized carbon band). The intensity ratio of the IG/ID for the as-received SWCNT samples was found to be 3.27; however, it was about 11.89 for the acidtreated SWCNT samples. This ratio has been used as an index to assess the purity of the SWCNTs [12]. This high ratio near 12 indicates that the quality of acid-treated SWCNT samples was quite pure, also suggesting that the amorphous, or non-

graphitic carbon content is almost negligible. It was noted that the intensity of the lower energy side of the graphitic peak was enhanced due to the nanotube bundles typically forming when the CNTs were treated with the strong acid. The micro-Raman spectra are shown in Fig. 4(a) for the SWCNT samples containing 4%, 8%, and 12 wt.%Ni nanoparticles, in the range of 100–2000 cm1 using a 633 nm He–Ne laser. The peak at around 1324 cm1 (D band) is mainly due to defective and carbonaceous particles present in the samples; however, the peak at around 1585 cm1 corresponds to the optical mode E2g of two-dimensional graphite (G band). In addition, the peaks at around 100–400 cm1 correspond to the radial breathing mode (RBM) of SWCNT samples, which is associated with isolated and bundled SWCNTs. The range of micro-Raman spectra between 100 and 300 cm1 is highly sensitive to the difference in nanotubes’ chirality and diameters [13]. For the SWCNT samples containing 4%, 8% and 12 wt.%Ni nanoparticles, the micro-Raman spectra in the range of 100– 500 cm1 are shown in Fig. 4(b) corresponding to the RBM. In the low-frequency band range there were several RBM peaks for the SWCNT samples. The peaks were at 149, 163, 212, 252 and 333 cm1, which were highly dependent on the state of the SWCNTs’ bundling. The peak frequency varies as 1/dt, where dt is the SWCNT diameter in nm (using the equation: dt ¼ 223.5/lRBM, where lRBM is the observed RBM frequency in cm1) [13,14]. The diameter of the SWCNTs was determined by a micro-Raman spectrum to be 1.5 nm for the RBM peak at lRBM ¼ 149 cm1. Table 1 lists all results of the IG/ID ratio for the MWCNTs, SWCNTs, and the SWCNT samples containing different amounts of Ni nanoparticles. The IG/ID ratios for the SWCNT samples containing 4, 8, and 12 wt.%Ni nanoparticles were 18.57, 19.35 and 21.25, respectively. However, for comparison, the IG/ID ratios for the MWCNT samples with and without 8 wt.%Ni nanoparticles were 1.99 and 0.97, respectively. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) thermographs are shown in Fig. 5 for the as-received SWCNT and acid-treated SWCNT samples, from room temperature to 1000  C in an O2 atmosphere. Both types of samples exhibited only one weight loss step. The weight loss of the as-received SWCNT and acid-purified SWCNT samples at 600  C were about 96.7 and 99.1 wt.%, respectively. It was noted from the DTA results for both samples that the peak temperature was around 516–526  C [15], which was ascribed to oxidation of the carbon. The peak of the purified sample was more intense due to the removal of the impurities of metallic catalysts and amorphous carbon. This also revealed that the acid purification procedure was effective and provided samples over 99% pure. In Fig. 6 is shown the TGA thermographs for the SWCNT samples coated with different amounts of Ni nanoparticles (4–12 wt.%). These results demonstrated that the weight loss of the Ni-containing SWCNT samples with 4, 8, and 12 wt.%Ni nanoparticles was 95.7%, 92% and 87.5%, respectively. This remaining mass, 4.3%, 8% and 12.5%, indicates that the actual amounts of the Ni nanoparticles adsorbed onto the SWCNT samples were consistent with the nominal compositions. To obtain detailed information about the pore-size, the specific surface area, the mesopore volume and the pore-size

international journal of hydrogen energy 35 (2010) 2336–2343

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Fig. 2 – EDS spectrum (a) and mapping results (b) for the SWCNTs with 8 wt.%Ni. distribution, the N2 adsorption and desorption isotherms were performed on the pristine MWCNT and SWCNT samples, and the SWCNT samples with 4%, 8% and 12 wt.%Ni nanoparticles. The total surface area and pore volume were determined by using the BET equation and the single point method.

Fig. 7 shows the typical isotherms of N2 adsorption and desorption (Type IV) for MWCNT and SWCNT samples with 8 wt.%Ni nanoparticles. The isotherms exhibited obvious hysteresis loops at high relative pressure (P P1 o ), indicating that both CNT powders had mainly mesoporous structures.

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international journal of hydrogen energy 35 (2010) 2336–2343

1585 (G-band)

Table 1 – Results of the Raman spectra analyses of all SWCNT and MWCNT samples, displaying D bands and G bands, as well as the ratio of their intensities, IG/ID.

1912

Types

1678

1045

(2)

1324 (D-band)

Reative intensity/ a.u.

(1). As-received SWCNTs. (2). Acid-treated SWCNTs.

(1) 0 0

500

1000

1500

Raman shift/ cm

2000

-1

Fig. 3 – Micro-Raman spectra for the SWCNTs: (1). asreceived; (2). acid-treated.

G-band

a

D-band

Relative intensity/ a.u.

(1). 4 wt.%Ni (2). 8 wt.%Ni (3). 12 wt.%Ni

RBM (2) (3) (1)

0 200

400

600

800

1000

1200

1400

1600

1800

SWCNT (as-received) SWCNT (acid-treated) SWCNT þ 4 wt.%Ni SWCNT þ 8 wt.%Ni SWCNT þ 12 wt.%Ni MWCNT MWCNT þ 8 wt.%Ni

D band/cm1

G band/cm1

IG/ID

1323 1329 1324 1327 1322 1330 1334

1588 1581 1578 1579 1578 1595 1590

3.27 11.89 18.57 19.35 21.25 0.97 1.99

The surface and volume characteristics of MWCNT and SWCNT samples, calculated according to the isotherm data, are summarized in Table 2. In Fig. 8 is shown the isotherm of N2 adsorption and desorption (Type IV) for the SWCNT samples containing various amounts of Ni nanoparticles. The isotherms exhibited hysteresis loops at high relative pressure (P P1 o ), indicating that the SWCNT samples with Ni nanoparticles still maintained a mesoporous structure. The surface and volume characteristics of SWCNT samples coated with 4–12 wt.%Ni nanoparticles are also summarized in Table 2. A comparison of the BET results for the pristine MWCNT and SWCNT samples showed that SWCNT powders had a higher BET surface area and total pore volume of 584.8 m2 g1 and 1.068 cm3 g1, respectively. However, the pristine MWCNT powders, which had a diameter of 10–20 nm and a length of 1–5 mm, only showed a BET surface area of 152.5 m2 g1 and a total pore volume of 0.452 cm3 g1. The pore-size distribution for SWCNT samples coated with 4–12 wt.%Ni nanoparticles is shown in Fig. 9. All SWCNT samples coated with 4–12 wt.%Ni nanoparticles possessed similar pore diameter distributions of 2–100 nm, but different peak pore-sizes (3.8–4.94 nm). The SWCNT samples coated with 4 wt.%Ni nanoparticles showed much higher pore volume and a wider pore-size distribution, as compared with SWCNT samples coated with 8 and 12 wt.%Ni nanoparticles. This may be due to the blocking effect on the SWCNT samples;

2000

Raman shift/ cm-1 -0.040

120

b

(1). 4 wt.%Ni (2). 8 wt.%Ni (3). 12 wt.%Ni

(1). as-received (2). acid-treated

100

-0.032

163

(3)

252

(1)

-0.024 60 (2)

40

-0.016

(1)

dw/ dt/ mg s-1

149

(2)

Weight/ wt.%

212

Relative intensity/ a.u.

80

333

20 (1)

-0.008

0 (2)

0 0

100

200

300

400

500

Raman shift/ cm-1

Fig. 4 – Micro-Raman spectra for the SWCNTs D X wt.%Ni nanoparticles: (a). at a full range; (b). at a short wavelength range of 100–400 cmL1.

0.000

-20 0

100 200 300 400 500 600 700 800 900 1000

Temperature/ °C

Fig. 5 – TGA thermographs for the pure SWCNTs powders without and with acid (6 N HNO3) treatment.

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international journal of hydrogen energy 35 (2010) 2336–2343

120 (1). 12wt.%Ni (2). 8 wt.%Ni (3). 4 wt.%Ni

100

80

Types

-0.06 (3)

dw/ dt/ mg s-1

Weight/ wt.%

Table 2 – Results for N2 adsorption/desorption isotherms performed on MWCNT and SWCNT powders, including the SWCNT samples coated with varied amounts of Ni nanoparticles.

-0.08

(1)

60

-0.04 (2)

40

20

-0.02 (3)

0

(2)

(1)

Parameters BET surface area/m2 g1

MWCNT SWCNTs SWCNT þ 4 wt.%Ni SWCNT þ 8 wt.%Ni SWCNT þ 12 wt.%Ni

152.5 584.8 520.8 478.6 436.0

Total pore Avg. pore volume diameter/nm (BJH)/cm3 g1 0.452 1.068 1.263 1.183 1.031

7.46 11.85 9.63 9.70 9.80

0.00 0

200

400

600

800

1000

Temperature/ °C

Fig. 6 – TGA thermographs for the SWCNTs D X wt.%Ni nanoparticle samples.

some of the outer tube surfaces of the SWCNTs were blocked by further incorporation of the Ni nanoparticles beyond 4%. It was also found that the surface area and total pore volume of the SWCNT samples coated with 4%, 8% and 12 wt.%Ni nanoparticles were slightly lower than those of the pristine SWCNT samples, as displayed in Table 2. In fact, the pristine SWCNT samples exhibited a BET surface area of 584.8 m2 g1 and a total pore volume of 1.068 cm3 g1.

3.2. Electrochemical characterization of the Ni-coated CNT electrodes Hydrogen storage capacities were measured using a twoelectrode system in a 6 M KOH solution at 25  C under the atmospheric conditions. Fig. 10(a) shows the discharge (E vs. time) curves for the SWCNT electrodes with 4 wt.%, 8 wt.%, and 12 wt.%Ni nanoparticles and the blank electrode (without SWCNT) at a constant discharge current of 800 mA g1. It can clearly be seen that the discharge capacity of the blank

electrode is very small, considerably negligible. For comparison, Fig. 10(b) displays the discharge (E vs. capacity) curves for the SWCNT electrodes with various amounts of Ni nanoparticles (i.e., 4, 8, and 12 wt.%) at a constant current of 800 mA g1. The maximal electrochemical discharge capacity of 1404 mA h g1 was obtained for the SWCNT electrode coated with 8 wt.%Ni nanoparticles, which corresponds to 5.27 wt.% H2. Moreover, it was found that the electrochemical hydrogen storage capacity of the SWCNT electrodes with 4 wt.% and 12 wt.%Ni nanoparticles were 1.74 wt.% and 2.99 wt.%, respectively; the capacity of the 12 wt.%Ni nanoparticles electrode was lower than that with 8 wt.%. This may be due to the blocking effect or unevenly coated Ni nanoparticles on the surface of the SWCNT electrode, which causes the decrease of hydrogen storage capacity. As a matter of fact, the electrochemical discharge capacity of the Ni electrode without SWCNTs was only 4–5 mA h g1, which inferred that the added Ni fillers gave little capacity contribution to the SWCNT electrodes. Recently, Chen et al. [16] studied MWCNTs modified by a KOH impregnation method; they showed that optimum hydrogen storage capacity was achieved at about 4.47 wt.%. Rajalakshmi et al. [10] showed the electrochemical hydrogen storage capacity of 2.9 wt.% for the purified and open SWCNTs, equating to an electrochemical capacity of 800 mA h g1. 900

900 (1). SWCNT+ 8 wt.%Ni (adsorption) (2). SWCNT+ 8 wt.%Ni (desorption) (3). MWCNT+ 8 wt.%Ni (adsorption) (4). MWCNT+ 8 wt.%Ni (desorption)

700

(1). SWCNT+ 4wt.%Ni (adsorption) (2). SWCNT+ 4wt.%Ni (desorption) (3). SWCNT+ 8wt.%Ni (adsorption) (4). SWCNT+ 8wt.%Ni (desorption) (5). SWCNT+12wt.%Ni (adsorption) (6). SWCNT+12wt.%Ni (desorption)

800

Volume adsorbed/ cm3g-1 STP

Volume adsorbed/ cm3g-1 STP

800

600 500 400 300

SWCNTs

200 MWCNTs

100

700 600 500 400 300 200 100 0

0 0.0

0.2

0.4

0.6

0.8

1.0

-1

Relative pressure/ P Po

Fig. 7 – BET isotherm for the SWCNTs D 8 wt.%Ni and the MWCNTs D 8 wt.%Ni samples.

0.0

0.2

0.4

0.6

0.8 -1

Relative pressure/ P Po

Fig. 8 – BET isotherm for the SWCNT D X wt.%Ni nanoparticle samples.

1.0

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international journal of hydrogen energy 35 (2010) 2336–2343

0.10 (1). SWCNT+ 4 wt.%Ni (2). SWCNT+ 8 wt.%Ni (3). SWCNT+12wt.%Ni

4%

dV/dD/ cm2g-1nm-1

0.08

8%

Types

0.04

Maximum discharge capacity/ mAh g1

H2 storage/ wt.%

13.6 12.4 15.4 13.1 21.3

431.2 467.2 1404.0 800.6 362.2

1.61 1.74 5.27 2.99 1.35

SWCNT SWCNT þ 4 wt.%Ni SWCNT þ 8 wt.%Ni SWCNT þ 12 wt.%Ni MWCNT þ 8 wt.%Ni

1

10

100

Average diameter/ nm

Fig. 9 – The pore-size distribution (PSD) of the SWCNTs D X wt.%Ni samples.

Additionally, after 50 cycles, the SWCNT electrodes still maintained more than 80% of their nominal discharge capacity. The relatively higher electrochemical hydrogen storage capacity for the Ni-coated SWCNT electrodes may be (1). Blank electrode (without SWCNT) (2). SWCNT+ 4wt.%Ni (3). SWCNT+ 8wt.%Ni (4). SWCNT+ 12wt.%Ni

a 1.0

Cell potential/ V

CNTs weight/mg 12%

0.00

0.8

0.6

0.4 (2)

(1) 0.0

0.5

(4)

1.0

(3)

1.5

2.0

Time/ h

related to the high purity (about 99%) and larger mean diameter of our treated SWCNT samples. It was observed that those SWCNT samples with a larger mean pore diameter attain greater electrochemical hydrogen storage. According to the results of our BET analyses, the average pore diameters for SWCNT samples coated with Ni nanoparticles were about 9.6–11.8 nm; however, it was only about 7.46 nm for the Nicoated MWCNT sample that were not treated. Moreover, the specific surface areas of SWCNT samples coated with 4–12 wt.%Ni were about 436–584 m2 g1, which is much higher than that of MWCNT samples having 8 wt.%Ni; their specific area was about 152 m2 g1 as shown in Table 2. In general, the carbon nanotubes with a larger diameter and a higher surface area can store hydrogen more effectively. These characteristics make them more favorable, demonstrating both enhanced gas phase adsorption and electrochemical (charge/ discharge) properties. Table 3 lists the results of the electrochemical hydrogen capacities of SWCNT samples coated with various amounts of Ni nanoparticles. In addition, the result of the 8 wt.%Ni-coated MWCNT electrode is listed for comparison, which only achieved around 1.35 wt.% hydrogen storage. It was obvious that the electrochemical hydrogen storage capabilities of the Nicoated SWCNT electrodes were much higher than that of the Ni-coated MWCNT electrodes. In summary, it was found that the 8 wt.%Ni-coated SWCNT electrode exhibited the highest electrochemical hydrogen storage capacity.

1.2

4.

1.0

Cell Potential/ V

Parameters

0.06

0.02

b

Table 3 – The discharge and H2 storage capacities of MWCNT and SWCNT samples coated with various amounts of Ni nanoparticles.

0.8

0.6

0.4 (1) 0.2

(2)

(3)

(1). SWCNT+ 4wt.%Ni (2). SWCNT+ 8wt.%Ni (3). SWCNT+12wt.%Ni

0.0 0

300

600

900

1200

1500

Capacity/ mAh g-1

Fig. 10 – The discharge curves of the carbon electrodes: (a). E vs.t; (b). E vs. mA h gL1.

Conclusions

In this work, SWCNT powders were coated with Ni nanoparticles to enhance their hydrogen storage capacity. The electrochemical hydrogen storage properties of Ni-coated SWCNT electrodes with various content (4–12 wt.%) of Ni nanoparticles were investigated. The characteristic properties of the SWCNT samples coated with 4–12 wt.%Ni nanoparticles were examined by utilizing SEM/EDX, micro-Raman spectroscopy, TGA/DTA, and BET measurements. It was found that the highest electrochemical discharge capacity was 1404 mA h g1 for the SWCNT electrode containing 8 wt.%Ni nanoparticles, which corresponds to 5.27 wt.% hydrogen storage. The enhanced properties accompanying the addition of Ni was attributed to the fact that Ni nanoparticles act as a redox site for hydrogen storage, thus leading to a greater

international journal of hydrogen energy 35 (2010) 2336–2343

electrochemical hydrogen storage capacity. The results indicate that SWCNT powders coated with Ni nanoparticles with high specific surface area appear to be a viable material for hydrogen storage application.

Acknowledgements

[8]

[9]

Financial support from the Taiwan Power Company, Taiwan (Project No: T546-9610031) is gratefully acknowledged.

[10]

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