Hydrogen Storage in Carbon Nanostructures Still a Long Road from Science to Commerce? Abstract: PACS-number: Text: Introduction

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1 Hydrogen Storage in Carbon Nanostructures Still a Long Road from Science to Commerce? Ulrich Bünger, Werner Zittel LB-Systemtechnik GmbH Daimlerstr. 1 D-821 Ottobrunn Fax: Phone: buenger@lbst.de; zittel@lbst.de November 2000 Abstract: Hydrogen storage in carbon nanostructures is still at a research level and not yet mature for industrial application. For the time being it is unfair to compare carbon nanostructures for hydrogen storage at the same level as metal hydrides or other established storage technologies, as not yet enough research is carried out. Nevertheless we compare carbon nanostructures with well established hydrogen storage technologies to develop a feeling of the needs and to identify where bottle necks might exist. We try to sketch the long way for carbon nanostructures to become a commercial product for hydrogen storage with focus on mobile applications. PACS-number: Text: Introduction The recent change of BP Amoco s logo to bp and the symbol of a sun with the company s own interpretation beyond petroleum (whatever the reasons for this change might be) signals a change of the fossil fuel dominated energy economy to non fossil alternatives. First choice as fuel in a post carbon based energy world would be hydrogen, but the crux is the storage problem in mobile applications. Recent euphoric press statements on already achieved storage densities of carbon adsorption beyond those of liquid hydrogen, feed the hope to overcome this problem. However, one has to keep in mind that hydrogen storage in carbon nanostructures is still at a research level and not yet mature for industrial application. This contribution tries to summarize the requirements for storage systems, to point out the situation of carbon adsorption storage with respect to other storage systems and to identify where bottle necks might exist. We try to sketch the long way for carbon nanostructures to become a commercial product for hydrogen storage with focus on mobile applications.

2 1 Hydrogen storage by carbon adsorption in competition with other hydrogen storage technologies First we sketch possible storage concepts with some key parameters in table 1. This is to develop a feeling of what we are talking about. Table 1: Storage weight and volume for different alternative hydrogen storage concepts. In all cases except gasoline and CH 4 which are listed for comparison kg of stored hydrogen are assumed, enough to run a car with 0.3 kwh/km consumption about 00 km. System Technical Parameters Volume incl. Containment (liter) Weight (kg) Gasoline liquid ~40 ~30 ~33 CGH 2 LH bar 00 bar 700 bar 208 Kelvin CH bar MeH Carbon adsorption 2 % -weight 3 % -weight % -weight 10 % -weight 20 % -weight 0 %-weight Incl. Containment Weight (kg) Basic assumptions for table 1: Today's gasoline fueled vehicle is taken as benchmark. For all others except methane a hydrogen storage capacity of kg is assumed. Only advanced technologies are assumed. (This should be enough to run an advanced fuel cell car about km). Methane (CH 4 ) was included as a reminder that this also represents some sort of chemical carbon storage of hydrogen with 2% storage capacity on a weight basis. The figures for methane are based on the assumption that this chemically bound hydrogen needs to be converted into pure hydrogen by an on-board reforming system. To account for losses, 60% conversion efficiency are assumed on an energy basis. Weight and volume of the necessary reformer are not taken into account here, but certainly, they must be included, not to mention the other operational deficiences of this system (e.g. cold start, system dynamics). Carbon nanotube storage is assumed at a density of 1 g/cm 3 of resulting material at best which might be achieved in a storage with all nanotubes alligned together to form a macroscopic bundle. This would account for a material with about 60% porosity - ordinary

3 graphite has a density of about 2.3 g/cm 3. The upper limit points to a more realistic density of 0.33 g/cm 3 which might be achievable for many ropes each containing several tens of nanotubes under pressure. Uncompressed ropes might be spaced much more sparsely resulting in bulk densities of about 0.1 to 0.2 g/cm 3. The total weight including the containment is given in the last column of the table, while the change in volume due to the containment is neglected here, but included only for the case of liquid and gaseous hydrogen. At laboratory scale, NREL claimes to have achieved a storage density of more than 7% by weight in carbon nanotubes [1]. Therefore, it seems realistic that a technical product might be realized with at least % (wt) storage capacity of hydrogen. The US Department of Energy (DOE) has energy density goals for vehicular hydrogen storage of at least 6.% (wt) and 62 g H 2 /liter [2]. Even if the US goal will not be met, nanotubes have the potential to compete with metal hydride (MeH) being comparable in size and weight, but maybe much cheaper and environmentally more benign. Therefore, a niche market for portable small fuel cell systems with tiny nanotube-cartridges instead of a battery (or a metal hydride system) might be a welcome first test market for small scale production of nanotube-storage systems with reasonable return on invest and chances for continuous improvements along an industrial learning curve, almost from the beginning. A 10% (wt) storage capacity of nanotubes seems achievable in the mid-term or long-term future - provided that the many scientific and technical problems will be overcome satisfyingly. This would result in on-board fuel supply system geometric and gravimetric parameters almost comparable to today's vehicles. Certainly, this would imply the final breakthrough for hydrogen use in ground traffic on a broad level. Whether a goal of 0% (wt) or even beyond might be achieved seems less probable from today's knowledge, though some researchers already claimed such successes at laboratory scale. But beyond these figures one might speculate about a completely new fueling infrastructure now becoming possible: Instead of fueling the tank container in the vehicle, a new concept might arise where the empty cartridges are exchanged against hydrogen filled ones. These could be delivered in a logistics scheme completely independent of today's fueling: Either by parcel service or a similar service filled tanks might be delivered to the consumer and stored close to the car without any further need for specific fueling stations. Maybe the only component needed would be a small cartridge exchanger, which would have to be in reach of every car owner. On the other hand, one should always keep in mind that any form of stored energy poses a safety risk to the environment. Thus, although it might seem feasible at a first glance, the amount of energy stored in one location should be kept as small as possible by lowering the specific energy demand of the application first! Below that goal of 0%, according to our feeling, a system with exchangeable cartridges seems unrealistic. 2 Possible implications of hydrogen storage by carbon adsorption with respect to production volumes of carbon nanofibers or carbon nanotubes

4 Let us stress our imagination and summarize what it would really mean, if carbon storage was to become an ordinary every day technology. Besides hydrogen storage, carbon structures at nanoscale offer a wide field of possible future applications, starting from microelectronics, micro mechanics due to their mechanical stability they seem to be very good candidates for micro actuators Li-batteries and many others. Therefore, an impressive demand for these structures will arise. But, in contrast to the above mentioned fields of application, hydrogen storage or storage, separation and cleaning of any gas would be the only application, which would need large amounts of nanotubes at macro scale. This would be the driving market for future carbon nanotube producers. To pick out this major segment, table 2 lists today's market of products which might be substituted by nanotube hydrogen storage systems once they are available at reasonable quality, quantity and price. Table 2: Market assessment of possible carbon based hydrogen storage cartridges. The required storage capacity should be seen as an order of magnitude -figure Small applications - Camcorder - Minidisk - Digital video - Cellular phone - Notebook Large (Traffic) - passenger car - trucks Gas industry Market 1996 [3] 10, million 10 million 42 million 48 million 37 million 14 million Storage capacity Wh kwh 1 MWh H 2 (g) kg 30 kg Carbon content of a cartridge with 10% - % H g 1. 3 g 3 6 g g 9 18 g kg kg - trailer? 11 MWh 330 kg t Total carbon demand per year 1 30 kg t t t t mio t mio t For small applications, today's competing best technology are Li-batteries. These will provide energy densities of 0.3 kwh/l and 0.13 kwh/kg at best [4], resulting for instance in about 230 g per battery per notebook. Once, small fuel cells are implemented into these small applications, MeH storage is the competing technology, which must be beaten with better technology and/or price. This gives a rough estimate of 2 3% (wt) storage density of carbon,

5 at least, to become competitive. Again this hints at these applications as small but early niche markets for CNF storage systems, in order to start with, to improve them by learning by doing and, last but not least, to receive some return on invest and market feed-back at an early stage. These figures also point out the great potential market for carbon-based storage systems, but, on the other hand, also exhibit the long road to market that still has to be pursued. Today's methods of choice to produce carbon nanotubes, which mostly are made of single wall nano tubes (SWNT see article by M. Heben), are laser growth and electric arc discharge. State-of-the-art is to produce about g/day of SWNTs. This again implies to start with these small niche markets. These production methods might be applied for laboratory scale quantities, but will never be applicable to large scale quantities as might be seen from the following rough estimates: The g/day production rates are achieved by the laser vaporization method. Based on data from Smalley et al. [] we assume an average laser power of about 60 W. Assuming 1.2% efficiency of electricity-to-laser conversion (Nd:YAG-laser), we end up with an electricity consumption of at least 120 kwh/day or 120 kwh/ g of SWNTs. The production of one storage tank with a capacity of kg hydrogen and 10% (wt) hydrogen adsorption of the carbon would result in an electricity consumption of about 1.2 GWh el per storage (besides other efforts during the production process). In other words, the production of the storage system would consume as much energy as the total vehicle would consume to run for about 3.6 million kilometers. This is twenty times its life span and at least about a factor of 400 above an acceptable effort. The drawbacks of current production technology are underlined by the fact that it would take about 27 years to produce the carbon material for one single storage tank, if produced with one production line. The energy balance for electric arc discharge might be better, but to the cost of much lower yield, which makes it even worse in that respect. E.g., Bernier at Montpellier university [6] first introduced the electric arc technique for SWNT fabrication. Consuming an average supply power of about 2 kw in the electric arc discharge about g/day of raw material are produced, with about 20% converted into SWNTs. Adding these figures up results in an overall energy consumption of about 2. GWh el per storage tank, which is even worse in terms of energy consumption than the laser method, and a factor of 1,000 above an acceptable level for mass production. In addition, the material produced with the arc discharge technique is said to be of poorer quality and to require considerably more purging efforts. With today's technology, more than hundred years would be required for the production of the carbon material for one storage tank. From today's point of view, only two possible production processes seem to possess the potential for industrial upscaling: The first one is growth of the SWNTs from vapor by means of chemical vapor deposition (CVD). Here, some experience already exists, which has to be improved [7]. The second method is completely different: All methods described above start at nano scale production and add the material up to achieve macro scale quantities. A different way could be to start with a macroscopic material which might be processed (at macro scale) in some

6 way to give it the desired properties. According to our knowledge, ideas and some experimental results exist which indicate that this method would immediately allow for upscaling to large quantities. The drawback to date is that so far it could not be shown that the resulting bulk material has the desired physico-chemical properties to store enough hydrogen. Nevertheless, indirect measurements indicate that it should be possible to improve the results. Large porosities of the bulk material have been achieved with more than 0% porosity and large inner surfaces of more than 2,000 m 2 /g have been demonstrated in a nitrogen atmosphere. 3 Economic Aspects At present it is not possible to seriously scale up cost figures and to estimate future cost reductions for large scale production. Therefore, we briefly sketch some figures from today's laboratories. These again exhibit that large breakthroughs are necessary for the economic production of large quantities. Today single wall nanotubes are commercially available at cost figures of about $60/g [8]. This would end up with storage cost of about $ 3 million per vehicle. Neglecting other process steps and investment costs of the laboratories, the electricity consumption for a laser grown storage tank adds up to a cost figure of about $60,000 per storage tank assuming electricity costs of $0.0/kWh. The economics of arc discharge grown nanofibers sound even worse: About $12,000 are the electricity costs per storage tank under equal economic conditions as above. In addition, the apparatus with 1g/day SWNT-production capacity costs about $10,000. Assuming a five year lifetime, this adds, at least, another $270,000 for the equipment. Even in cost, a factor of 100 to 1,000 in cost reduction is required for becoming economic. Most promising from an economic point of view could be the synthesis of bulk material as briefly sketched above by chemical vapor deposition or the macroscopic method. Since primary resources are very cheap like petrochemical gases and very abundant available natural minerals and heat is the most important input during processing, these could lead to very cheap storage systems once their ability to store hydrogen in large amounts can be proven. Only a few months ago, Prof. Smalley predicted that within the next ten years, an economic process to produce SWNT in ton quantities from the gas phase will be discovered....it seems most probable that it is only a matter of time before nanotubes are produced cheaply and in quantity for around the same cost as carbon fibers and probably less [9]. For a rough cost extrapolation it seems justifiable to use today's costs of ordinary carbon fibers grown by CVD, which are around $ 10 per kg (at sales prices of between 1 30 $/kg). Taking the lower figure, this sums up to about $20 00 per storage tank provided a hydrogen uptake of between 10% is state-of-the-art. In these rough calculations further costs for purging, cutting etc. are not included. Therefore we believe that it might be hard to achieve these cost figures even though they seem to be realistic. These figures should already include the effect of economies of scale since they are derived from today's carbon fibers which are already produced at large scale.

7 If the method of macro scale production from bulk material proves to become feasible, it may become the method of choice with achievable cost figures even lower than the $20 00 per storage tank calculated above. Rodriguez and Baker [10] estimated from their experiments that cost figures as low as $1/kg should be possible. If proven, this would yield to about $0 per vehicle at 10 % hydrogen adsorption. 4 Technical Aspects Given the early stage of research, only a few general aspects might be sketched here. Independent of whether carbon nanotubes or some other carbon modification will represent the best storage material, loading and unloading mechanisms will be similar. At present understanding, the degree of adsorption and desorption might be influenced by properly changing pressure, temperature or electric charge. The latter method might the best for application to nanotubes with metallic character. The storage device presumably would contain the nanotubes in a loose mixture. Since these are very tough structures no special care for their stability is required. However, the outer containment and especially the interfaces to the user and the filling system must prevent losses of the material. This could be achieved by proper filters. In addition, some space factor must be taken into account in order to allow a certain breathing of the tube mixture during loading and unloading. Thus, rigid carbon structures may be advantageous from this point of view. It is not yet clear which atmosphere is required in order to prevent the adsorption of competing molecules, thus reducing the storage factor. During fueling an eventual drop of temperature and/or pressure must be monitored. Also, a special treatment of the tank at the beginning of the loading process to prevent ageing effects could be a must in order to reach full storage capacity. Health aspects Though carbon is a harmless untoxic chemical substance its use might affect mens health. Generally speaking, even untoxic materials maybe harmfull when breathing them into the lung. Here a rule of thomb holds: The smaller the particle size the more dominates the mechanical interaction provoking a certain risk for lung cancer. In addition, with decreasing particle size the effectiveness of filters avoiding lung contact reduces considerably. Furthermore, the production processes still possess health risks resulting from the toxicity of the materials used for the preparation or from gases. This might be another factor demanding to produce and handle the nanoporous carbon as a monolithic bulk material. 6 Published storage densities Hydrogen is adsorbed in various carbon modifications, where two classes are reported with encouraging results: graphite nanofibers (GNF) and nanotubes. Both classes are made of carbon ropes of nm in diameter and up to 100 µm in length. In the first class, the ropes are made of planar graphite sheets, so-called platelets which may be oriented in

8 different angles relative to the rope axis. Also a tubular arrangement of the graphite sheets around the rope axis belongs to this class. In the second class, the ropes are made of bundles of nanotubes. Though remarkable results are reported for graphite nanofibers, from today's understanding it seems that hydrogen uptake might be best on the (inner and outer) surface of single wall carbon nanotubes. The following table 3, which is an updated version of the table given in [11] presents reported hydrogen uptake by the leading research groups. It should be noted here that nearly all results are not (yet) confirmed or reproduced by other researchers. Moreover, the huge hydrogen uptake of 20% with lithium graphite nanofibers was recently identified by [12] as being attributable mainly to moisture impurities of the hydrogen and related hydrate formation (LiOH H 2 O). We want to stress that hydrogen adsorption measurements on small samples in the order of several milligrams (as usual in present experiments) need very sophisticated experimental conditions and measurement procedures. Very easily parasitic effects may erroneously be attributed to hydrogen uptake. Therefore, nearly all described experiments should be interpreted very cautiously. But nevertheless, the already performed experiments give some indications and must be seen as an intermediate step within an advanced learning process. Table 3: Reported results of hydrogen adsoprtion in carbon modifications Material Max. %-wt H 2 T (K) P (MPa) Reference SWNT [13] SWNT ~7 ~300 *) Ambient [1] SWNT [14] SWNT [1] MWNT 0.2 ~ Ambient [16] ~GNF (tubular) ~ [17] GNF (heringbone) ~ [17] GNF (heringbone) ~ 6 ~300 ~12 [21] GNF (platelet) ~ [17] GNF (Li-doped) ~20 ~ [18] GNF (Li-doped) ~2 ~ [12] GNF (K doped) ~14 < [18] GNF ~ [19] GNF ~10 ~300 ~120 atm [20] *) The experiment was done at 300 K absorption temperature, but cooling down to liquid nitrogen temperature before the evacuation of the probe

9 Apart from reported experimental results theoretical calculations on hydrogen uptake have been investigated at various places. State of the art results indicate that 4 14 %-weight of hydrogen storage should be possible in carbon nanotubes [22]. Where some researchers claim that storage densities above % are hardly to imagine [23]. 7 Conclusions Hydrogen storage in carbon nanostructures is still at a research level and not yet mature for industrial application. For the time being it is unfair to compare carbon nanostructures for hydrogen storage at the same level as metal hydrides or other established storage technologies, as not yet enough research is carried out. Besides the basic research of proper physical and chemical conditions for optimum hydrogen storage, open questions concerning large scale application still remain or are even untouched: large scale production methods, containment and storage design and layout, systems integration, operation (fueling/unloading) and costs. Today only vague ideas exist, at best, how to solve these problems. Nevertheless, hydrogen storage in carbon nanostructures would have very attractive features which justify the efforts undertaken to verify the reported results and to improve their understanding. In this contribution we tried to widen the view for these possible applications and to draw the attention from the pure research interest to the changing focus of industrial application and its basic needs. Small applications as starter market (cost and scalability) are quite probable coming sooner due to the reduced requirements for these applications. Once these are proven under every day conditions there might exist enough expertise along the industrial learning curve that carbon nanostructures eventually might be used as vehicle fuel tanks. From today s point of view, at least for the first generation of direct hydrogen applications (such as FC vehicles, battery replacement), other alternatives such as compressed or liquid storage are required. References: 1. M. Heben: Minutes of Meeting of the Hydrogen Technical Advisory Panel, February 28-29, 2000, see at 2. For instance, see S. Hynek, W. Fuller, J. Bentley: Int. Journal Hydrogen Energy 22 (1997), p Data for small scale applications taken from Ref. [4] 4. M. Endo, C. Kim, K. Nishimura, T. Fujino, K. Miyashita: Recent development of carbon materials for Li ion batteries, Carbon 38 (2000) A.G. Rinzler, J. Liu, H. Dai, P. Nikolaev, C.B. Huffman. F.J. Rodriguez-Macias, P.J. Boul, A.H. Lu, D. Heymann, D.T. Colbert, R.S. Lee, J.E. Fischer, A.M. Rao, P.C. Eklund, R.E. Smalley: Large-scale

10 purification of single-wall carbon nanotubes: process, product and characterization, Appl. Phys. A67 (1998), P. Bernier: What about the Price, Discussion topic at Nanotube-99 workshop, see at 7. J.-F. Colomer, C. Stephan, S. Lefrant, G. van Tendeloo, I. Willems, Z. Kónya, A. Fonseca, Ch. Laurant, J.B. Nagy: Large-scale synthesis of single wall carbon nanotubes by catalytic chemical vapor deposition (CCVD) method, Chem. Phys. Lett. 317 (2000) For instance, see at 9. R. Andrews, D. Jacques, A.M. Rao, F. Derbyshire, D. Qian, X. Fan, E.C. Dickey: Continous production of aligned carbon nanotubers: a step closer to commercial realization,, J. Chen, Chem. Phys. Lett. 303 (1999) R.T.K. Baker: Synthesis, Properties and Applications of Graphite Nanofibers, see at M.S. Dresselhaus, K.A. Williams, P.C. Eklund: Hydrogen adsorption in carbon materials, MRS Bulletin November 1999, Seite R.T. Yang: Hydrogen storage by alkali-doped carbon nanotubes revisited, Carbon 38 (2000), A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, and M.J. Heben: Nature 386 (1997), 377ff 14. Y. Ye, C.C. Ahn, C. Witham, B. Fultz, J. Liu, A.G. Rinzler, D. Colbert, K.A. Smith, R.E. Smalley: Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes, Appl. Phys. Lett. 74 (1999), S C. Liu, Y.Y. Fan, M. Liu, H.T. Cong, H.M. Cheng, M.S. Dresselhaus: Hydrogen storage in Single- Wall Carbon Nanotubes at Room Temperature, Science, 286 (1999) X.B. Wu, P. Chen, J. Lin, K.L. Tan: Hydrogen uptake by carbon nanotubes, Int. J. of Hydrogen Energy 2 (2000), A. Chambers, C. Park, R.T.K. Baker, N.M. Rodriguez: J. Phys. Chem. B102 (1998) 423ff 18. P. Chen, X. Wu, J. Lin, K.L. Tan: High H 2 uptake by alkali doped carbon nanotubes under ambient pressure and moderate temperature, Phys. Rev. Lett., 82, (1999), Seite 282ff, 19. Y.-Y. Fan, B. Liu, M. Liu, Y.-L. Wie, M.-Q. Lu, H.-M. Cheng: Hydrogen uptake in vapor-grown carbon nanofibers, Carbon 37 (1999) B.K. Gupta, O.N. Srivastav: Synthesis and hydrogenation behaviour of graphitic nanofibres, Int. Journal of Hydrogen Energy 2 (2000) J.W. Patrick, presentation at Nanotec 1999, 8 10 th September 1999, Sussex 22. Seung Mi Lee, Ki Soo Park, Young Chul Choi, Young Soo Park, Jin Moon Bok, Dong Jae Bae, Kee Suk Nahm, Yong Gak Choi, Soo Chang Yu, Nam-gyun Kim, Thomas Frauenheim, Young Hee Lee: Hydrogen adsorption and storage in carbon nanotubes, Synthetic Metals 113 (2000)

11 23. V.V. Simonyan, P. Diep, J.K. Johnson: Molecular simulation of hydrogen adsorption in charged single-walled carbon nanotubes, Journal of Chemical Physics, Vol. 111, No. 21 (1999)