Hydrogen storage in carbon nanotubes

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1 PERGAMON Carbon 39 (2001) Review article Hydrogen storage in carbon nanotubes * Hui-Ming Cheng, Quan-Hong Yang, Chang Liu Institute of Metal Research, Chinese Academy of Sciences, Shenyang , PR China Received 21 September 2000; accepted 26 November 2000 Abstract Hydrogen is the cleanest, sustainable and renewable energy carrier, and a hydrogen energy system is expected to progressively replace the existing fossil fuels in the future, the latter are being depleted very fast and causes severe environmental problems. In particular, one potential use of hydrogen lies in powering zero-emission vehicles via a proton exchange membrane fuel cell to reduce atmosphere pollution. To achieve this goal feasible onboard hydrogen storage systems have to be developed. The recent discovery of the high and reversible hydrogen storage capacity of carbon nanotubes makes such a system very promising. In this overview, theoretical predictions and experimental results on the hydrogen uptake of carbon nanotubes and nanofibers are summarized, and we point out that, in order to accelerate the development of carbon nanotubes and nanofibers as a practical hydrogen storage medium in fuel cell-driven vehicles, many efforts have to be made to reproduce and verify the results both theoretically and experimentally, and to investigate their volumetric capacity, cycling characteristics and release behavior Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon nanotubes; C. Adsorption; D. Gas storage 1. Importance of hydrogen as an clean energy source one of the best candidates. Hydrogen is an ideal fuel and versatile energy carrier, and its advantages are summarized The start of a new millennium presents us with countless as below [1,2]: opportunities and challenges. On one hand, the development of social economy and culture brings many of us (a) Easy to produce. high standards of living; on the other hand, our world is (b) Convenient fuel for transportation. facing a rapid depletion of natural resources and serious (c) Versatile, converts easily to other energy forms at the global environmental problems. All of these are connected user end. with the overuse of fossil fuels. Nowadays, public concern (d) High utilization efficiency. about the global environmental problem caused by the (e) Environmentally compatible (zero- or low-emission). utilization of fossil fuels and the over-reliance of the economy on them is increasing. There is, therefore, a Therefore, hydrogen provides the best route to a sustainsearch for possible alternative sources of energy to replace able energy for the transportation sector and some other fossil fuels. There are quite a number of primary energy uses, since it can be produced not only from the fossil sources available, such as thermonuclear energy, nuclear fuels, such as coal and natural gas, but also from wind, reactors, solar energy, wind energy, hydropower, geothersolar, thermal, hydroelectric, biomass or municipal solid mal energy, etc. In contrast to the fossil fuels, in most wastes with no consumption of non-renewable resources cases, these new primary energy sources cannot be used and no pollution of any kind. directly (e.g. used as fuels for transportation), and thus they Like conventional fossil fuels, the most important and must be converted into fuels, that is to say, a new energy the most urgent application of hydrogen is connected with carrier is needed. Among the many choices, hydrogen is transportation and vehicles. Hydrogen used as a fuel in vehicles is mainly divided into three kinds: In nickel *Corresponding author. Fax: hydride battery, in which hydrogen is combined as a metal address: cheng@imr.ac.cn (H.-M. Cheng). hydride; in a spark-ignition engine powered car; and in a / 01/ $ see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S (00)

2 1448 H.-M. Cheng et al. / Carbon 39 (2001) fuel cell. In the last case, hydrogen can be converted to hydrogen for a 500 km range [4]. Fig. 1 show that no electricity with emission of only water with very high storage technology is capable of meeting these goals to efficiency because the process is not subject to the date [4]. limitations of the Carnot Cycle. Thus true zero-emission In the concept vehicles proposed by automobile manuvehicles can be produced. The most significant news has facturers, three technologies for storing hydrogen fuel are come from the worldwide vehicle corporations such as considered: (1) Cryogenic liquid hydrogen (LH 2); (2) Daimler Chrysler, Ford, GM, Toyota, Honda, etc., that the compressed gas storage; (3) metal hydride storage technoldevelopment of proton-exchange membrane fuel cell ogy. However, these three technologies either cannot reach (PEMFC) electric vehicles has made great progress. Re- the benchmarks just mentioned, or have significant discently, the Daimler Chrysler Corp. announced that it would advantages: Liquefying hydrogen wastes at least 1/ 3 of the be the first automaker worldwide to offer fuel cell vehicles stored energy and LH2 storage suffers from potential on the market during the next several years [3]. hydrogen losses due to evaporation. The hydride-based However, several barriers have to be overcome before approach suffers from weight and cost concerns, and the hydrogen electric vehicles can be put into large-scale crucial issue connected with compressed gas storage may practical utilization. One of the most severe challenges is be tank volume and safety. Most recently, tremendous the lack of a safe and efficient onboard storage technology, interests have been aroused by the discovery [4] and which may dramatically influence the vehicle s cost, range, reproduction [5 9] of high hydrogen adsorption capacity performance, and fuel economy, as well as shape the scale, in carbon nanotubes and other low-dimensional carbon investment requirements, energy use, and potential emis- materials. If these encouraging experimental results can be sions of a hydrogen-refueling infrastructure. That is to say, reproduced easily and the large-scale production of carbon the development of onboard storage technology will nanotubes made available in the near future, it will be directly determine the schedule of hydrogen-powered possible to reach the goals of the DOE hydrogen plan. vehicles into the market. The US Department of Energy (DOE) Hydrogen Plan has set a standard for this discussion by providing a 2. Prediction of hydrogen uptake in carbon commercially significant benchmark for the amount of nanotubes reversible hydrogen adsorption. The benchmark requires a system-weight efficiency (the ratio of stored hydrogen Due to its high surface area and abundant pore volume, weight to system weight) of 6.5 wt% hydrogen and a porous carbon is considered as good adsorbent. For 3 volumetric density of 62 kg H 2 / m, since a vehicle conventional porous carbon, the hydrogen uptake is propowered by a fuel cell would require more than 3.1 kg of portional to its surface area and pore volume, while, Fig. 1. Installed energy densities for several vehicular hydrogen storage technologies [4].

3 H.-M. Cheng et al. / Carbon 39 (2001) regretfully, a high hydrogen adsorption capacity (4 6 higher hydrogen pressure, (Fig. 2,bottom)] hydrogen upwt%) can be only obtained at very low temperatures such take for one layer of H2 adsorbed on a single graphene as liquid nitrogen temperature [10], consistent with theo- layer [11,12]. retical calculations. As for nanotubes, one important issue currently being In contrast, in spite of their relatively small surface area debated is whether hydrogen adsorption also occurs in the and pore volume, carbon nanotubes and carbon nanofibers interstitial channels between adjacent nanotubes in a rope show very surprising high hydrogen storage capacity. of SWNTs. Dresselhaus et al. [11] presented two geometri- Scientists employed different theoretical calculations and cal estimates for the filling of a rope (crystalline lattice) of deductions in search of a reasonable interpretation. The SWNTs. One assumes that hydrogen is a completely intent of this theoretical work is summarized in the deformable fluid that fills the space not occupied by the following points: carbon nanotubes, and the other is the packing of hydrogen molecules of kinetic diameter 0.29 nm on the inner walls (A) How do structural characteristics influence the and in the interstitial volume of the nanotubes, as shown in physical/ chemical process? Fig. 3. Using the geometrical model with close-packing of (B) Where does the adsorption occur, in inner hollow hydrogen molecules within the core of a (10,10) tube leads cavities and/ or other pore space (e.g. inter-tube space)? to 3.3 wt% hydrogen adsorption within the tube and 0.7 (C) In the adsorption of hydrogen onto carbon wt% adsorption within the interstitial space, or a total of nanotubes, what interaction, chemical or physical, 4.0 wt% hydrogen adsorption [11]. occurs between the hydrogen and the carbon? It is considered that under high-pressure conditions (e.g. (D) What are the adsorption mechanism and the maxiattractive 10 MPa), the high compressibility of hydrogen and the mum adsorption capacity? intermolecular interactions should lead to closer packing of hydrogen molecules [11], which is consistent with the detailed calculation by Stan et al. [14], yielding a 2.1. Simplistic geometric estimate and qualitative hydrogen density higher than the simple geometric picture. discussion Based on this, the hydrogen adsorption amount under high pressures would be higher than the simplistic geometric Since H2 molecules at elevated pressures on a solid estimate. surface are expected to form a close-packed configuration, Moreover, Dresselhaus thought that a hydrogen mole- Dresselhaus et al. [11] obtained a simple geometric cule adsorbed in the interstitial space undergoes much estimate for the close-packing capacity of hydrogen mole- stronger surface attraction than on a single planar graphene cules above a plane of graphite using purely geometric surface, since it is in close proximity to three graphene Œ] Œ] arguments, which yields 2.8 wt% [ 33 3 commensurate stacking, (Fig. 2, top)] or 4.1 wt% [dense triangular structure, incommensurate with the graphite, observed for Fig. 3. A typical configuration of H2 molecules adsorbed on a triangular array of carbon nanotubes. This configuration resulted Œ] Œ] Fig. 2. Relative density of a 33 3 commensurate (top) and an from a classical Monte Carlo calculation in which the simulated incommensurate (bottom) monolayer of H2 on a graphite surface storage pressure was 10 MPa and the simulated temperature was [12]. 50 K [11].

4 1450 H.-M. Cheng et al. / Carbon 39 (2001) surfaces [11]. Therefore, the hydrogen adsorbed in the space would be expected to be denser than on the single graphene surface. In short, through a simple physical discussion, it is concluded that for SWNTs hydrogen is stored in both the pores formed by the inner tube cavities and the inter-tube space, and the storage density is possibly higher than that on a planar graphene surface. Accordingly, the hydrogen adsorption amount may be higher than 4 wt%, consistent with the experimental results Simulation for hydrogen adsorption onto carbon nanotubes Monte Carlo simulations [13,15 18] and other calculations [12,19,20] have been carried out to verify and predict the adsorption capacity of hydrogen in carbon nanotubes Fig. 4. Adsorption potentials for hydrogen in tube arrays and based on the assumption of physical adsorption. For all idealized split pores. The solid line denotes the idealized carbon split pore with a pore width of 9 A. these approaches, the most important factor in the simulapotential for a (9,9) tube array, and the dot-dashed line represents The dashed line is the tions is the choice of the intermolecular potential function a (18,18) tube array [18]. describing the molecular interaction between hydrogen and carbon atoms. In spite of the use of different calculation methods, the following conclusions can be reached. (a) Adsorption of hydrogen does possibly occur in sites of different binding energy in SWNTs. Williams et al. [13] array, adsorption is negligible due to the quantum effect. conducted quantitative examinations of the maximum The (18,18) interstices show a local enhancement of potential energies in a 7-SWNT rope at 10 MPa and 77 K. potential energy. The minimum energy in the interstice is They found that the strongest average attractive potential greater than that inside the (18,18) tube by a factor of energy is in the interstices (21443 K), followed by the nearly 2. Interstitial adsorption accounts for, at most, 14% endohedral sites along the inner (2758 K) and outer of the total adsorption for the (18,18) tube array at 77 K (2603 K) cylindrical surface of the nanotubes. Besides, [18]. their results show that the attractive potential energies (d) The packing geometry of the SWNTs plays an along the grooves (the wedge-shaped channel running important role in hydrogen adsorption. The simulation of along the outer surface of the rope where two nanotubes Williams et al. [13] suggests that the strong dependence of meet) is K, comparable to the energy in the the gravimetric adsorption on the packing geometry and interstitial sites. These estimates suggest that treatment of diameter of a SWNT rope correlates with computed values the outer surface of SWNT ropes in the simulation of of the specific surface area, and delaminating of nanotube physisorption is important not only for the obvious ropes should increase the gravimetric storage capacity. geometrical reasons (high surface area), but for energetic 21 (e) The calculated heat of adsorption (6.3 kj mol by reasons as well, since typical ropes possess many wedge Wang et al. [18], 1082 K by Stan et al. [19]) is much less channels on the their exterior surface. than the experimental value of Dillon et al. (19.63 kj (b) The inner-tube cavity has high adsorption potential 21 mol, 2360 K) [4]. for hydrogen, compared with the planar surface and slit- (f) Most of these simulations do not confirm the high pores of similar size (Fig. 4). SWNTs have a very narrow hydrogen uptake capacity obtained experimentally for diameter distribution, and have dimensions of the order of similar systems of SWNTs and graphitic nanofibers. the range of carbon attraction interaction [15]. Fig. 4 However, Williams et al. reported the results of Monte shows that the depth of the potential well for the (9,9) tube Carlo simulations for the physisorption of H2 in finite- (1.22 nm in diameter) array is larger than that for a slit diameter ropes of parallel SWNTs (considering the pore 0.9 nm in diameter because the curvature of the tube importance of outer surface of ropes), is consistent with the increases the number of nearest neighbor carbon atoms experimental results obtained by Ye et al. at 77 K. Their [18]. results show that the maximum gravimetric adsorption (c) Interstitial adsorption constitutes a significant frac- capacity of hydrogen onto an isolated (10,10) nanotube tion of the total amount adsorbed for a tube of larger can reach 9.6 wt% under cryogenic temperatures (77 K) diameter such as the (18,18) (2.44 nm) tube array. By and a pressure of 10 MPa. Although their result cannot yet comparison, in the smaller interstices of the (9,9) tube explain the hydrogen adsorption at room temperature, it

5 H.-M. Cheng et al. / Carbon 39 (2001) may be enough to provide scientists with the confidence to 3. Experimental investigations of hydrogen uptake in narrow the discrepancy between observed hydrogen storage carbon nanotubes capacities in SWNTs and the rather large body of physisorption simulation [13]. In 1997, Dillon et al. first claimed that SWNTs have a high reversible hydrogen storage capacity [4]. Thereafter, many research groups started to carry out hydrogen storage 2.3. Physisorption or chemisorption experiments and have made some noticeable progress. Most of the current experimental results of hydrogen Most recently, Lee et al. [21] have reported results of storage in carbon nanotubes are summarized in Table 1. calculation for hydrogen storage behavior in SWNTs by In their pioneering work, Dillon et al. [4] showed that density functional calculations, and proposed that the hydrogen can condense to high density (estimated to 5 10 adsorption of hydrogen in SWNTs is a chemisorption wt%) inside narrow SWNTs of 12 A, and predicted that process. Their calculation predicts that the hydrogen SWNTs with diameters of 16.3 and 20 A would come storage capacity in (10,10) nanotubes can exceed 14 wt% close to the target H2 uptake density of 6.5 wt%. The 3 (160 kg H 2 / m ), and the value is higher than the ex- adsorption of H2 in SWNT soots was probed with temperimental value. perature programmed desorption (TPD) spectroscopy and In general, it is quite difficult to reach a common the TPD experiment suggested that physi-adsorption of conclusion for maximum adsorption capacity and explain hydrogen mainly occurred within the cavities of SWNTs. the experimental observations from the results obtained by The activation energy for hydrogen desorption was found the different theoretical calculations and predictions. How- to be 19.6 kj/ mol, which is much higher than the ever, much information obtained from the calculations will theoretical predicted value discussed previously, or apbe helpful for experimental investigation. Meanwhile, deep proximately five times higher than that for a planar understanding into the pore structure and adsorption graphite surface, thereby promoting hydrogen storage process of carbon nanotubes will help us to choose the capacity at higher temperature. They have recently deoptimum intermolecular potential function and modify our veloped a method to produce samples with a high concalculations to direct the development of carbon nanotube- centration of short SWNTs with open ends that are based hydrogen storage systems. accessible to the entry of hydrogen molecules, and these Table 1 Summary of experimentally reported hydrogen storage capacities in carbon nanotubes Material Gravimetric Storage temp. (K) Storage Ref. hydrogen storage pressure (Mpa) amount (wt%) SWNTs (low purity) [4] SWNTs (high purity) [22] GNFs (tubular) [5] GNFs (herring bone) [5] GNFs (platelet) [5] Li MWNTs [6] K MWNTs 14.0, [6] SWNTs (high purity) [7] SWNTs (50% purity) [9] CNFs [8] CNFs [24] Li MWNTs [28] K MWNTs 1.8, [28] Li/ K GNTs(SWNT) [29] GNFs [29] GNFs [30] MWNTs [31] SWNTs [32] MWNTs (electro chemical),1 [33] Nano-structured graphite [34] SWNT (50 70% 2 [35,36] purity, electro chemical)

6 1452 H.-M. Cheng et al. / Carbon 39 (2001) purified cut SWNTs adsorbed wt% hydrogen fraction of the adsorbed hydrogen may fill the inter-tube under ambient conditions in several minutes [22]. spacing. In other words, it is more open tubes (providing Ye et al. [7] reported that a ratio of H to C atoms of inner surface) with large diameter (making the interstitial about 1.0 was obtained for crystalline ropes of SWNTs at space accessible) that lead to the high hydrogen adsorption 80 K and pressures.12 MPa. At a pressure of 4 MPa, a capacity. sudden increase in the adsorption capacities of the SWNT A striking experimental result about hydrogen storage in samples was reported, and they suspect that a structural graphite nanofibers (GNFs) was reported by Chambers et phase transition is responsible for this effect. al. [5]. They claimed that tubular, platelet, and herringbone Liu et al. have developed a semi-continuous hydrogen forms of GNFs were capable of adsorbing in excess of 11, arc discharge method to prepare SWNTs in a large scale 45, and 67 wt% H 2, respectively, at room temperature [23]. Both HRTEM observations and resonant Raman under a pressure of about 12 MPa. Their recent paper [27] measurements proved the existence of larger diameter reports further results about the interaction of hydrogen SWNTs ( nm) [23,24]. Meanwhile, High-resolution with GNFs, and proposed that the GNFs possess special adsorption measurements showed that the sample has structural conformation, which produces a material comabundant micropores of diameter similar to tube diameter, posed entirely of nanopores that accommodate small-sized indicating much more open tubes [25]. According to adsorbate molecules such as hydrogen, and the nonrigid Dillon [4] and Cheng s [26] prediction, this kind of SWNT pore walls can expand to accommodate hydrogen molemay be promising as high hydrogen storage carriers. Fig. 5 cules in a multiplayer conformation. They also pointed out [9] shows the change of hydrogen pressures as a function that the pretreatment before hydrogen storage is very of time for SWNTs under an initial hydrogen pressure of important and that ambient humidity is deadly harmful to 10 MPa in the first adsorption cycle at room temperature. the hydrogen uptake performance. The hydrogen uptake is complete within a few hours. It Chen et al. [6] reported in their TPD experiments that a was shown that even as-synthesized SWNTs have hydro- high H2 uptake of 20 and 14 wt% can be achieved in gen uptake capacity [9], possibly resulting from the open milligram quantities of Li-doped and K-doped multitubes in the sample. Meanwhile, it has been proved that walled carbon nanotubes (MWNTs), respectively, under notable changes occur for pore structure in the course of ambient pressure. The K-doped MWNTs can adsorb H2 at hydrogen uptake in SWNTs. All the above facts indicate room temperature, but they are chemically unstable, that the inner hollow cavity takes part in the hydrogen whereas the Li-doped MWNTs are chemically stable, but adsorption. In Section 2.2, it was emphasized that intersti- require elevated temperatures (473 to 673 K) for maximum tial adsorption constitutes a significant fraction of the total adsorption and desorption of H 2. Recently, Yang repeated amount adsorbed for larger diameter tube arrays, thus for the above experiment for dry hydrogen, and he showed this kind of SWNTs with large diameter (about 1.8 nm), a that K-doped MWNTs can only adsorb nearly 2 wt% Fig. 5. The amount of H2 in weight for SWNT samples, and the pressure change versus the adsorption time. Sample 1 was used as synthesized. Sample 2 was soaked in 37% HCI acid for 48 h, rinsed with deionized water, and dried at 423 K. Sample 3 was pretreated in the same way as sample 2, the vacuum heat-treated for 2 h at 773 K [9].

7 H.-M. Cheng et al. / Carbon 39 (2001) hydrogen, but the experiment for wet hydrogen gave a In all, hydrogen fuel is clean, versatile, efficient and value of 21 wt%. Therefore, Yang considered that it was safe, and is the best fuel for transportation. Hydrogen moisture in hydrogen that drastically increased the weight energy will play an important role in the future world gain by reactions with (or adsorption on) the alkali species energy structure. Preliminary experimental results and on carbon [28]. some of the theoretical predictions indicate that carbon Fan et al. found that carbon nanofibers (CNFs) of 100 nanotubes and carbon nanofibers can be a promising nm in diameter possess high hydrogen storage capacity candidate for hydrogen storage, which may accelerate the (5 10 wt%) at room temperature under moderate high development of hydrogen fuel cell-driven vehicles. Neverpressure [8,24]. Recently, their experimental results theless, many efforts have to be made to reproduce and showed that MWNTs (3 20 nm in diameter) also have a verify the hydrogen storage capacity of carbon nanotubes high gravimetric hydrogen storage capacity. It has been both theoretically and experimentally, to investigate their found that various post-treatment methods can modify the volumetric capacity, cycling characteristics and release pore structure and surface microstructure to enhance the behavior, to elucidate the adsorption/ desorption mechahydrogen adsorption capacity to a considerable extent. It is nism, and finally to clarify the feasibility of carbon therefore believed that pore structure and surface micro- nanotubes as a practical onboard hydrogen-storage materistructure exert a large influence on the hydrogen storage al. performance of CNFs, MWNTs and SWNTs. Acknowledgements 4. Future research topics and remarks This work was supported by National Natural Science Foundation of China ( and ) and the More and more experimental and theoretical results Special Funds for Major States Basic Research Projects of continue to appear, and more and more reproducible MOST, China (G ). evidence proves that carbon nanotubes are a potential hydrogen storage carrier, although there are a few negative results reported as well. Comparative investigations at different research labs on the same sample or comparative References investigations with the same measurement method at the same lab on different samples from different research labs [1] Veziroglu TN. Hydrogen energy system as a permanent solution to global energy environmental problems. Chem are highly expected. Ind 1999;53: In order to use carbon nanotubes as a practical hydrogen [2] Veziroglu TN, Barbir F. Hydrogen: The wonder fuel. Int J storage medium, the mass production and utilization of Hydrogen Energy 1992;17: carbon nanotubes still have a long way to go. Scientists [3] have to work on the following important points: [4] Dillon AC, Jones KM, Bekkedahl TA, Kiang CH, Bethune DS, Heben MJ. Storage of hydrogen in single-walled carbon 1. Mass production of SWNTs and other nano-carbons nanotubes. Nature 1997;386: with a controlled microstructure at a reasonable cost. [5] Chambers A, Park C, Baker RTK, Rodriguez NM. Hydrogen 2. Purification and surface functionization of carbon storage in graphite nanofibers. J Phys Chem B nanotubes, development and optimization of pretreat- 1998;102: [6] Chen P, Wu X, Lin J, Tan KL. High H2 uptake by alkaliment methods for opening the caps at the tube ends to doped carbon nanotubes under ambient pressure and moderimprove their hydrogen storage capacity. ate temperatures. Science 1999;285: Elucidation of the microstructure of nanotubes, espe- [7] Ye Y, Ahn CC, Witham C, Fultz B, Liu J, Rinzler AG, cially pore structure and surface microstructure in the Colbert D, Smith KA, Smalley RE. Hydrogen adsorption and viewpoint of hydrogen adsorption/ desorption. cohesive energy of single-walled carbon nanotubes. Appl 4. Elucidation of volume storage capacity and how to Phys Lett 1999;74(16): improve it. [8] Fan YY, Liao B, Liu M, Wei YL, Lu MQ, Cheng HM. 5. A thorough investigation of adsorption/ desorption pro- Hydrogen uptake in vapor-grown carbon nanofibers. Carbon cess, thermodynamics, kinetics and cycling behaviors of 1999;37: carbon nanotubes. [9] Liu C, Fan YY, Liu M, Cong HT, Cheng HM, Dresselhaus MS. Hydrogen storage in single-walled carbon nanotubes at 6. Interpretation of the hydrogen adsorption mechanism room temperature. Science 1999;286: and theoretical understanding to assist in designing a [10] Agarwal RK, Noh JS, Schwarz JA. Effect of surface acidity carbon nanotube-based hydrogen storage material with of activated carbon on hydrogen storage. Carbon excellent performance. 1987;25: Development of practical hydrogen storage systems for [11] Dresselhaus MS, Williams KA, Eklund PC. Hydrogen ad- PEMFC vehicles and other applications. sorption in carbon Materials. MRS Bull 1999;24:45 50.

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