탄소나노튜브에 의한 수소저장의 전기화학적 거동

A. K. M. F. Kibria, 박기수, 모영환, 남기석

전북대학교 화학공학부

전북 전주시 덕진구 덕진동 전북대학교 화학공학부 561-756

Electrochemical behaviors of hydrogen storage in carbon nanotubes grown with different techniques

A. K. M. F. Kibria, K. S. Park, Y. H. Mo and K. S. Nahm

School of Chemical Engineering & Technology

Chonbuk National University, Chonbuk, Chonju 561-756, South Korea

Introduction

Recently, carbon nanotubes (CNTs) have been reported to be very promising candidates for large amount of hydrogen storage [1-3]. The low mass density and as well as chemical stability of CNTs are pointing it as an ultimate alternative material for portable electronics and moving vehicles. Till date, 4-10 wt.% hydrogen storage has been achieved by gas phase technique. In electrochemical method, the capacity about 110 mAh/g was obtained at low discharge currents, which corresponds to 0.39 wt.% of hydrogen [4]. It has been reported that lithium or potassium-doped CNTs can store 20 and 14 wt.% hydrogen at moderate or room temperatures under ambient pressure [5].

In this work, for the first time, we have investigated the hydrogen storage into alkali metal-doped CNTs electrochemically in KOH medium. CNTs used in this work were prepared by three different methods. The hydrogen storage behaviors have been measured for un-doped and doped CNTs. The charge and discharge phenomena of CNTs and the role of alkali metals in the hydrogen storage were also extensively discussed.

Experimental

The CNTs were synthesized by chemical vapor deposition (CVD) technique and arc-discharge (AD) method. Disk-type pellets were prepared by mixing fine Ni and Al2O3 powders with the ratio of 1:1, followed by exerting pressure. The pellet was then kept in a quartz boat and placed in a horizontal quartz tube reactor. Then, CNTs were grown at 600oC for 30 minutes by introducing a mixture of C2H2/H2 into the reactor at a flow rate of 10/100 sccm. In the case of AD method, CNTs were grown on the graphite cathode by applying a dc potential of 22 V between the electrodes at a current of 200 mA under 200 Torr helium gas atmosphere. The working electrode was prepared by mixing CNTs, acetylene black and PTFE in wt.% ratio of 4: 5: 1, respectively. We also purchased 90 wt.% SWNTs prepared by Laser ablation (LA) method and the electrode was prepared by following the same procedure. The doping of Li, Na and K into CNTs was done by solid-state reactions of CNTs and corresponding metal salts at 1000oC. The charge-discharge behaviors of un-doped and metal-doped CNTs were tested in a cell having a Ni counter electrode and 6M KOH electrolyte at a current density of 0.4 mA/cm2.

Results and discussion

The SEM images of CNTs synthesized by CVD and AD methods are shown in Fig.1(a) and (b), respectively. Each nanotube is terminated by a metal cap as seen as white spots, which indicates that the CNT were grown by tip growth mode [6]. The CVD and AD grown tubes were as usually multi-walled (MWNTs) and single-walled (SWNTs), respectively, and the purities were about 10 and 9 wt.%.

Shown in Fig. 2(a), (b) and (c) are the charge-discharge behaviors observed for the electrodes of CNTs obtained from CVD, AD and LA methods. It may be seen that the charge capacities of the electrodes remains unchanged with increasing cycle number. Among the investigated CNTs, the LA electrode showed the highest charge capacity whose value reaches 440 mAh/g. This value is about 15 and 17 folds higher than those of CVD and AD electrodes. It can be seen that the discharge capacities of AD and LA electrodes largely declined with increasing cycle number. Reversible capacity behavior, i.e., discharge capacity (Qd)/charge capacity (Qc) = 1, was observed only in the first cycle. The AD sample showed the highest fading rate of Qd. N□tzenadel et al. [4] have proposed that pure CNTs may show higher charge-discharge capacities than that of low content CNT electrodes. On the other hand, it is well known that the grown wt.%, size, type, diameter, etc., of CNTs largely varies depending on the growth method and conditions. In this sense, the appearance of low capacities for CVD and AD electrodes compared to that of LA electrode seems reasonable. The LA electrode consisted of 36 wt.% pure SWNTs. If we consider only the CNTs as the hydrogen absorber and take their parallel absorption capacity in each sample, it can be seen that the CNTs grown by CVD and AD methods only stored hydrogen with 2.45 and 2.13 wt.% capacities, respectively.

The charge-discharge behaviors observed for Li doped-CVD, AD and LA sample electrodes are presented in Fig. 3(x), (y) and (z), respectively. It may be seen that with increasing cycle number, the charge capacities showed the analogous trend of un-doped samples. The LA sample again showed the highest charge capacity of 580 mAh/g, which is more than 3.7 and 7.2 times higher than those of CVD and AD electrodes. The charge capacities of Li-doped CVD, AD and LA electrodes were almost 5.2, 3.1 and 1.3 folds higher than those of their un-doped electrodes. The Li-CVD electrode showed a unique discharge behavior during cycling. With increasing cycle number, the Qd value remained nearly constant. The AD electrode showed smaller Qd fading rate than the LA electrode. A unique capacity reversibility is observed for the Li-CVD electrode. The AD electrode showed that Qd/Qc ratio was about 1.9 in the first cycle, but decreased to 1.5 on the 6th cycle. Like the charge capacities, the Li-doped CNT electrodes showed remarkably higher discharge capacities over un-doped samples. During the first discharge process, the capacity ratios of Li-doped/un-doped CVD, AD and LA electrodes are 6.2, 5.8 and 1.4, respectively. Consequently, it was found that the doping of Li in CNT electrodes enhances the charge-discharge capacity remarkably.

In the cases of Na and K-doped CVD sample electrode, the metal doping did not show remarkable enhancement of capacities in contrast with the Li-doped electrode. The Na and K-doped AD sample electrodes showed unique capabilities in enhancing the charge-discharge capacities and these were quite similar to those observed for their Li-doped electrodes. In each case, the charge and discharge capacity enhancements are more than 3.1 and 6.2 times higher compared to those found for their un-doped electrodes.

In the present investigation, the doping of Li, Na and K in CNTs remarkably increased the hydrogen absorption capacities which showed a good agreement with the findings of Chen et al. [5] who stored hydrogen by the gas phase method. The mechanism for this storage capacity enhancement on metal-doping is not clear. However, it seems that the Li catalytic center at first generates an intermediate species LiHads by using part of its 2s electron. The next step is the formation of C-H bond with the most neighboring carbon of CNTs. The doped Na and K probably followed the similar route of Li for the formation of C-H bond as they have similar chemical properties.

References

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2. Y. Ye, C. C. Ahn, C. Witham, B. Fultz, J. Liu, A. G. Rinzler, D. Colbert,     K. A. Smith and R. E. Smalley, Appl. Phys. Lett., 74, 2307 (1999).

3. C. Liu, Y. Y. Fan, M. Liu, H. T. Cong, H. M. Cheng and M. S.            Dresselhaus, Science, 286, 1127 (1999).

4. C. N□tzenadel, A. Z□ttel, D. Chartouni and L. Schlapbach, Electrochem.       and Solid-State Lett., 2, 30 (1999).

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Figure captions

(a)	(b)

Fig. 1. SEM images for (a) Chemical vapor deposition (CVD) and (b) Arc-discharge (AD) grown carbon nanotubes (CNTs).

Fig. 2. Charge-discharge behaviors of: (a) CVD; (b) AD; and (c) Laser

ablation (LA) grown CNT electrodes in 6M KOH electrolyte.

Fig. 3. Charge-discharge behaviors of: (x) CVD; (y) AD; and (z) LA grown

Li doped-CNT electrodes in 6M KOH electrolyte.