Electrochemical investigation of single-walled carbon nanotubes for hydrogen storage

Electrochemical investigation of single-walled carbon nanotubes for hydrogen storage

Electrochimica Acta 45 (2000) 4511 – 4515 www.elsevier.nl/locate/electacta Electrochemical investigation of single-walled carbon nanotubes for hydrog...

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Electrochimica Acta 45 (2000) 4511 – 4515 www.elsevier.nl/locate/electacta

Electrochemical investigation of single-walled carbon nanotubes for hydrogen storage N. Rajalakshmi a,*, K.S. Dhathathreyan a, A. Govindaraj b,c, B.C. Satishkumar c a

Centre for Electrochemical and Energy Research, SPIC Science Foundation, 111 Mount Road, Guindy, Madras 600032, India b Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 064, India c Jawaharlal Nehru Centre for Ad6anced Scientific Research, Jakkur, Bangalore 560 064, India Received 1 March 2000; received in revised form 26 April 2000

Abstract Electrodes made of purified and open single walled carbon nanotubes behave like metal hydride electrodes in Ni–MH batteries, showing high electrochemical reversible charging capacity up to 800 mAh g − 1 corresponding to a hydrogen storage capacity of 2.9 wt% compared to known AB5, AB2 metal hydride electrodes. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Electrochemical investigation; Carbon nanotubes; Hydrogen storage

1. Introduction The recent discovery [1] of the storage of hydrogen in carbon nanotubes and graphite nanofibres by gas phase measurements and electrochemical measurements has stimulated interest in the reversible electrochemical storage of hydrogen in single-walled carbon nanotubes. The safe storage of hydrogen is also critical to hydrogen–air fuel cells. The storage of hydrogen in metal hydrides has the drawback of relatively low weight density. To overcome this problem, there is considerable effort to study light metal hydrides and storage in light elements such as carbon. The capillarity of carbon nanotubes thus offers a good prospect for hydrogen storage. Single-walled nanotube (SWNTs) constitute the simplest tubular form of carbon wherein a single graphite sheet is rolled into a tube. While most methods of synthesis of the SWNTs do not generally produce a * Corresponding author. Tel.: +91-44-2352342 fax: +9144-2351504. E-mail address: [email protected] (N. Rajalakshmi).

monodisperse product, it has been possible to obtain relatively large quantities of SWNT’s of uniform pore diameter [1]. In recent times, a study has revealed that these are good microporous materials with relatively large total surface areas [2]. Dillon et al. [3] have reported that crystalline SWNT’s have hydrogen storage capacity of 5 – 10 wt% at pressures less than 1 bar near room temperature (r.t.) Such a hydrogen storage capacity would be a significant advance for the use of hydrogen as a fuel when high gravimetric density of hydrogen is a figure of merit. The best value of hydrogen absorption in carbon nanomaterials reported so far has been 5.3 wt% or 0.64 hydrogen – carbon (H – C) at a temperature of 77 K. Recently Chambers et al. [4] have claimed that graphite nanofibres have a capacity of 24 H – C at 300 K which is yet to be fully confirmed. Ye et al. [5] have shown recently that hydrogen absorption on crystalline ropes of single walled carbon nanotubes exceeds 8 wt%, which is the highest capacity obtained so far. These workers have calculated the cohesive energy and found it to be strongly affected by the quality of crystalline

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order in the ropes. These studies suggest the possible use of SWNTs for reversible electrochemical storage. A recent report of Nutzenadel et al. [6] shows that a commercial sample of carbon web produced by arc-discharge and containing some nanotubes, can electrochemically store relatively large amounts (110 mAh g − 1) of hydrogen, corresponding to a hydrogen storage capacity of 0.39 wt%. The reaction is reversible indicating that the nanotubes can be used to produce electrodes for rechargeable batteries. In metal hydride batteries, the hydrogen is stored reversibly in the interstitial sites of a host metal. The electric energy is produced by direct electrochemical conversion [7,8]. The decomposition of water molecules is an essential step in the charging mechanism and the charge–discharge processes occur at the electrode through solid

state charge transfer mechanisms [9]. We considered it most worthwhile to investigate the electrochemical hydrogen storage capacity of pure SWNTs and report the preliminary results of such a study in this communication.

2. Experimental SWNTs were produced by the dc arc discharge method using a composite graphite rod containing Y2O3 (1 at%) and Ni (4.2 at%) as anode and a graphite rod as cathode under a helium pressure of 660 torr with a current of 100 A and 30 V as reported earlier [1]. The mass obtained was heated at 300°C in air for about 24 h to remove amorphous carbonaceous materials. The

Fig. 1. High resolution electron microscope images of (a) as produced SWNT bundles, (b) purified SWNT bundles.

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heat-treated material was stirred with concentrated nitric acid at 60°C for about 12 h and washed with distilled water to remove the dissolved metal particles. This treatment has been used to open the carbon nanotubes [10,11]. The SWNT material so obtained was suspended in ethanol by using a ultrasonicator and filtered through a micropore filter paper (0.3 mm) from Millipore to remove polyhedral carbon nanoparticles

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present. The product was then dried at 150°C for about 12 h. The SWNT content of the product was found to be 80% by thermogravimetric analysis. High-resolution electron microscopic (HREM) examination showed that the SWNTs with an average diameter of 1.4 mm were present as bundles of 5 – 50 nanotubes. The electrochemical measurements were carried out using a EG&G galvanostat/potentiostat model 273, with the

Fig. 2. Charge–discharge hydrogen storage capacity of the SWNT electrode at 10 mA current.

Fig. 3. Comparison of the electrochemical hydrogen storage capacity of the two purified SWNT electrodes.

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help of a three electrode system using SWNTs as working electrode, Platinum as counter electrode and saturated calomel as reference electrode. Electrochemical measurements were carried out in a flooded electrolyte condition in open cells. The electrolyte 30% KOH, which is the same as that used in alkaline batteries, was prepared from reagent grade KOH and de-ionised water. Chronopotentiometry, controlled potential coulometry, and cyclic voltammetry were used to gather the electrochemical data reported here. Electrodes were prepared by mixing 10 mg of the SWNTs with Cu powder in the ratio of 1:3 with a polytetrafluoroethylene (PTFE) binder. The putty form of the mixture was mechanically pressed on to a current collector (Ni mesh) at r.t. Then the electrode was sintered at 200°C for about 1 h under vacuum. The geometric area of the electrode was about 2 cm2. The electrodes were tested for their charge–discharge characteristics, initial capacity and cycle life.

3. Results and discussion The web produced from the arc-discharge contained SWNT bundles, amorphous carbon along with metal encapsulated carbon particles as can be seen from the high resolution electron microscope (HREM) image in Fig. 1a. In Fig. 1b we show the HREM image of the SWNTs after purification. The electrodes made of the

as-produced web showed an electrochemical charging capacity of 87 mAh g − 1, comparable to the value of 110 mAh g − 1 reported by Nutzenadel et al. [6] for the commercial sample of the web. Electrodes made of the purified SWNTs, however showed a tremendous electrochemical hydrogen storage capacity of around 800 mAh g − 1 at a charging and discharging rate of 10 mA current. Fig. 2 shows the charge (lower curve) and discharge (upper curve) curve for pure SWNTs. The charging and discharging potential was around − 0.8 and − 0.6 V versus SCE, respectively. The coulombic efficiency was found to be 85%. The equilibrium curves were measured in the normal mode at a constant discharge current of 10 mA. The shape of the charge – discharge equilibrium curve is not much different from that of the metal hydride electrodes. In order to check for the reproducibility, a few electrodes made with different weights of SWNTs were examined and they gave nearly the same electrochemical capacity. Fig. 3 shows the charging curve of the electrodes after the eighth cycle, since first few cycles were used to activate the samples. Here, from the eighth cycle onwards, the electrodes started giving almost the same electrochemical capacity. The small difference in weight of the electrodes is reflected in the charging time. The electrodes were charged and discharged also at different current ratings from 10 to 100 mA. The variation in the charging capacity was found to be only 50 mAh g − 1 in the range studied as shown in Fig. 4.

Fig. 4. Variation of the electrochemical capacity of SWNT electrodes with respect to the charging current. Inset shows the electrochemical hydrogen storage capacity as a function of number of cycles.

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This behavior is in contrast to that of a metal hydride electrodes where the capacity decreases steeply at higher discharging currents. The maximum electrochemical hydrogen absorption capacity was found to occur after 20 cycles of charge and discharge, with the capacity remaining constant for almost 50 cycles (see inset of Fig. 4). It is to be noted that the samples investigated by Nutzenadel et al. [6] contained only a few percent of nanotubes, showing an electrochemical capacity of 0.39 wt%, but the sample investigated by us contains 80% of SWNTs giving eight times higher capacity (2.9 wt%). It is possible that the opened SWNTs facilitate absorption of hydrogen electrochemically. Hydrogen may be packed within the tubes at a higher density than assumed, if repulsive interactions between the hydrogen molecules were either screened or additional hydrogen is adsorbed by the exterior surfaces of SWNTs and/or the interstitial spaces between the bundled tubes. The present study reveals that SWNTs may indeed be useful in battery technology for the replacement of metal hydride electrodes. Further experiments are in progress for the study of the mechanism of charge–discharge process, cycle life and for the evaluation of diffusion coefficients of hydrogen in these nanotubes.

Acknowledgements We thank the management of SPIC Science founda-

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tion, for financial support. We thank Professor C.N.R. Rao for encouragement and guidance.

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