NANO LETTERS
Electrochemical Hydrogen Storage Behavior of Ropes of Aligned Single-Walled Carbon Nanotubes
2002 Vol. 2, No. 5 503-506
Gui-Ping Dai, Chang Liu, Min Liu,* Mao-Zhang Wang, and Hui-Ming Cheng Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Received January 26, 2002; Revised Manuscript Received February 8, 2002
ABSTRACT Massive macroscopic ropes of well-aligned single-walled carbon nanotubes (SWNTs) with a larger mean diameter of about 1.72 nm, synthesized by a semicontinuous hydrogen arc discharge method, were employed for electrochemical hydrogen adsorption experiments. A high discharge capacity of 503 mAh/g, corresponding to 1.84 wt % hydrogen, was achieved reproducibly at 25 °C under normal atmosphere for about 200 mg SWNT sample. This result implies that the SWNTs are highly promising electrochemical hydrogen storage materials for rechargeable batteries.
Hydrogen, which can be produced from renewable energy resources while burning pollution-free, has emerged as one of the most promising candidates for the replacement of the current carbon-based energy services. Although hydrogen could satisfy all of the world’s vehicular energy demands,1 a major impediment to the development of this new technology is the difficulty to store it with a desired density. Hydrogen can be stored in metal hydrides, but there are still obstacles to overcome. High-capacity metal hydrides, such as magnesium metal2 (theoretically 7 wt % of hydrogen storage capacity), cannot release hydrogen unless it is heated to a moderately high temperature (573-623 K). Some commercialized metal hydrides, such as LaNi5, which can release hydrogen at room temperature, have low gravimetric storage densities and weigh too much to make commercial fuel cell vehicle applications feasible.3 In the past few years, more and more attention has been paid to elemental carbon. Particularly, single-walled nanotubes (SWNTs) seem to be a candidate for high-capacity hydrogen storage medium used in electric vehicles (EVs). Dillon and co-worker4 first investigated hydrogen adsorption properties of as-prepared soot containing only about 0.1-0.2 wt % single-walled carbon nanotubes (SWNTs), from which they extrapolated a hydrogen storage capacity of 5-10 wt % for pure SWNTs at room temperature. Ye et al. reported5 that hydrogen adsorption on the purified crystalline ropes of SWNTs was 8 wt % at about 60 atm and 80 K. High hydrogen storage capacities on a massive sample weight basis were subsequently demonstrated on SWNTs by Liu et al.,6 and 4.2 wt % was achieved at room temperature under 10 MPa. It has also been demonstrated7 * To whom correspondence should be addressed. E-mail:
[email protected]. 10.1021/nl020290c CCC: $22.00 Published on Web 03/20/2002
© 2002 American Chemical Society
that SWNTs could electrochemically store hydrogen, in which SWNT soot containing a few percent of 0.7-1.2 nm diameter SWNTs was mixed with either copper or gold as a compacting powder in a 1:4 ratio to form electrodes, with a capacity of 110 mAh/g, which corresponds to ∼0.39 wt % hydrogen. A very recent report8 showed that the electrochemical capacity of 800 mAh/g was obtained for SWNTs with a diameter of 1.4 nm, in which stable electrodes were formed by pressing the SWNTs with copper powder in a 1:3 ratio. Another recent study9 indicated that charge/ discharge capacities of 160 mAh/g were obtained for SWNTs, in which SWNT composite electrodes were fabricated by mixing with conducting Ni powders and an organic polytetrafluroethylene (PTFE) in a ratio of 40:50:10 (SWNT: Ni:PTFE). For these electrochemical studies, the weight percent of SWNTs in samples was very low, and in fact, no more than 10 mg net SWNTs were used in the experiments. In addition, compacting powders themselves were electrochemically active, so it is possible that compacting powder takes some role in the electrochemical capacity of SWNTs. On the other hand, there are also some recent studies10,11 showing negative results for hydrogen storage in carbon nanotubes, which suggested that more detailed and careful investigations on hydrogen uptake of carbon nanotubes should be performed. In this work, we measured the electrochemical chargedischarge hydrogen capacity of macroscopically long ropes (up to 100 mm in length) of well-aligned SWNTs with a larger mean diameter of about 1.72 nm synthesized by a hydrogen arc-discharge method,12 with a larger sample quantity (about 200 mg) at 25 °C under normal atmosphere. This synthesis method involves a direct current (dc) arc
Figure 2. Charge-discharge curves of the SWNT electrode at a constant current density of 25 mA/g.
Figure 1. SEM images of the SWNT ropes. (a) A low-magnification image, showing that a SWNT rope is made up of many parallel thinner threads. (b) High-magnification image, showing that the thin threads are made up of numerous well-aligned and tightly packed SWNT bundles.
process, in which the cathode is a graphite rod, whereas the anode is made up of holes filled by evenly dispersed graphite powder and a suitable amount of catalysts (2.5 at. % Ni, 1.0 at. % Co) and growth promoter (0.8 at. % sulfur). The two electrodes are not aligned but make an oblique angle (3050°), which is beneficial to the formation of a plasma and is critical to the synthesis of the SWNT ropes. Hydrogen gas and argon gas were selected as the buffer gas. Two barshaped collectors were placed in the chamber for collecting the SWNT ropes. The electric arc was typically operated using 150 A dc under an atmosphere of 150 Torr H2 and 50 Torr Ar, with a distance of about 2 mm maintained between the two electrodes. About 1 min after the electric arc was initiated, we could see through the observation window that ropes began to form on the collectors inside the reaction chamber. The SWNTs ropes grew rapidly and had a roughly uniform orientation along the direction of the plasma flow. Typically, the synthesis procedure lasts about 5 min. Long well-aligned SWNTs ropes up to 100 mm are lightweight and free-standing and tend to adhere strongly to tweezers. In Figure 1, we show scanning electron microscopy (SEM) images of the well-aligned SWNTs ropes. Figure 1a shows that the SWNT rope (about 50 µm in diameter) is made of many thinner threads, which are roughly parallel with each 504
other. It is clearly observed (Figure 1b) that the thin threads are made of numerous tightly packed, soundly aligned, long, and straight nanotube bundles. The diameters of the SWNT bundles are estimated to be in the range 20-50 nm, and high-resolution transmission electron microscope (HRTEM) observations indicated that the mean diameter of our SWNTs is 1.72 nm.12 The results from thermogravimetric measurements showed that the purity of the well-aligned SWNTs in our products is in the range 60-70 wt %. The electrode for the electrochemical measurements was prepared as follows: without any compacting powder, the electrode was formed by directly pressing the 200 mg SWNT materials to a sheet of nickel foam at 50 MPa. Owing to the self-stickiness of SWNT ropes, we obtained a stable SWNT electrode disk, and the apparent surface area of the electrode disk was about 4 cm2. The relatively larger quantity of SWNTs used may ensure the accuracy of the measurements. The experiments were carried out with a three-electrode system in 6 M KOH electrolyte at 25 °C under normal atmosphere, in which the SWNT disk was used as a working electrode, Ni(OH)2/NiOOH as a counter electrode, and Hg/ HgO as a reference electrode. The SWNT electrode was charged for 2 h at a current density of 25 mA/g and discharged at the same current density after a 5 min rest. The cutoff voltage is -0.4 V (vs Hg/HgO). All electrochemical experiments were performed using an Arbin BT2000 system. Figure 2 shows the charge-discharge voltage change of SWNTs as a function of time at a constant current density of 25 mA/g, where the time axis was converted into the charge-discharge capacity. The plateau of the discharge potential was observed around -0.6 V (vs Hg/HgO) and a discharge capacity of 503 mAh/g was obtained, which corresponds to a hydrogen storage capacity of 1.84 wt % in SWNTs. This implies that the SWNTs could be used as a material for electrochemical hydrogen storage. To check the reproducibility, we measured three samples with similar weights of SWNTs. Table 1 shows that almost the same maximum discharge capacity was obtained for the three samples at the same current charge-discharge density. An important requirement for a rechargeable electrode material is the capacity retainability after certain chargeNano Lett., Vol. 2, No. 5, 2002
Table 1. Maximum Discharge Capacity for Three SWNT Samples
sample
SWNTs weight (mg)
maximum discharge capacity| (mAh/g)
1 2 3
188.3 192.6 200.0
501.3 502.1 503.4
discharge cycles. Figure 3 displays the cycle life performance of the SWNT electrode. It was obvious that the SWNT electrode showed a high capacity retention rate over many cycles. After 100 cycles, the electrode still retained more than 80% of the maximal capacity. The capacity loss is believed to be due to the mechanical instability of the electrode, which was also observed by other investigators.7 It is possible that hydrogen adsorption can result in a disintegration of the electrode. In fact, a black deposit can be found on the bottom of the cell after cycling, which was proved to be SWNTs by TEM. An improved electrode design may need to be developed to obtain a better stability. After 100 charge-discharge cycles, the discharge capacity of the SWNT electrode was evaluated at different chargedischarge current densities. In Figure 4, the discharge capacity is shown as a function of discharge current. The increase in the capacity for lower current densities is clearly observed, which is consistent with the results obtained by Nutzenadel et al.7 When comparing our result with the result7 of 110 mAh/g obtained by Nutzenadel, we think that the relatively higher electrochemical capacity of our SWNTs may be related to the purity and larger mean diameter of our SWNTs. From the Raman scattering spectrum of our samples, a main peak at 125 cm-1 with several shoulders can be seen in the lowfrequency band (100-200 cm-1), and this peak indicates a larger mean diameter of 1.79 nm.12 According to theoretical estimates,4,13 SWNTs with a larger mean diameter will allow greater electrochemical hydrogen storage. Cryo-nitrogen adsorption measurement was carried out to analyze the pore size distribution of SWNTs using a volumetric adsorption apparatus (ASAP 2010M, Micrometritics Instrument Corp.). To analyze the inner cavity in SWNTs, we analyzed the adsorption isotherm (Figure 5a) in the ultralow partial pressure range (less than P/P0 ) 0.01) by means of DubininAstakhov equation14 and obtained the micropore size distribution curve as shown in Figure 5b. The SWNTs show an evenly distributed micropore, and the average micropore diameter is 1.80 ( 0.05 nm from the adsorption result, in line with the high-resolution transmission electron microscope (HRTEM) direct observation (average diameter of the inside channel is 1.72 ( 0.02 nm) and the Raman characterization for outer diameter (1.79 nm). In other words, the SWNTs with larger diameters have larger micropores and can adsorb effectively, and it is considered to be more favorable to hydrogen storage by both gas phase adsorption and electrochemical charge-discharge. The analyses of the adsorption isotherm by means of Barrett-Joyner-Halenda Nano Lett., Vol. 2, No. 5, 2002
Figure 3. Cycle life performance of the SWNT electrode. After 100 cycles the electrode still delivers more than 80% of the maximal capacity.
Figure 4. Discharge capacity of SWNTs as a function of the discharge current of the SWNT electrode after 100 cycles.
equation can also reach the specific surface area, and the surface area of our SWNTs was measured to be 95 m2/g. Second, the good alignment and high purity may also contribute to the good hydrogen storage performance of our SWNTs samples. The alignment and straight growth of our SWNTs are important to achieve the high efficiency adsorption due to the higher exposure surface area of the SWNTs and better electronic conductivity. Third, and possibly most important, because compacting powder was not used in our experiments, the higher percentage of SWNTs in unit volume in the SWNT electrode will increase the total electrochemical capacity. It will be very attractive for practical battery industry application that the electrode can be directly pressed without purification and compacting powder. In fact, for purified SWNTs, it is a little difficult to be pressed to a stable SWNT electrode without compacting powders.8,9 Also purification of SWNTs may damage the alignment of SWNTs and increase the cost for future applications. It is known that, in a metal hydride electrode, the hydrogen is stored reversibly in the interstitial sites of the host metal. Usually, the charge transfer is the rate-controlling step. However, currently little is known about the electrochemical process in SWNT electrode. It is still a subject of investiga505
chemical hydrogen storage capacity is as high as 503 mAh/ g, which corresponds to a hydrogen storage of 1.84 wt %. After 100 charge-discharge cycles more than 80% of the maximal capacity can still be obtained. Hence, aligned SWNT ropes are very attractive for electrochemical hydrogen storage. Acknowledgment. This work was supported by the Special Funds for Major State Basic Research Projects (No. G2000026403) and the NSFC (Nos. 50032020 and 50025204). Note Added after ASAP Posting. This manuscript was originally published on the web on 03/20/2002 with a few minor errors. The corrected version was posted on 03/26/ 2002. References
Figure 5. (a) Nitrogen cryo-adsorption isotherm of SWNTs. (b) Micropore size distribution of SWNTs.
tion to find out which step is the rate-controlling step in the electrochemical charge-discharge process of SWNTs, and the detailed electrochemical hydrogen storage mechanism is worth deep investigation. In summary, we have measured the electrochemical charge-discharge capacity of well-aligned SWNT ropes with a mean diameter of 1.72 nm and found that the electro-
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NL020290C
Nano Lett., Vol. 2, No. 5, 2002
