Optimum Conditions for Adsorptive Storage Suresh K. Bhatia † DiVision of Chemical Engineering, The UniVersity of Queensland, Brisbane, QLD 4072 Australia Alan L. Myers* Department of Chemical and Biomolecular Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104 ReceiVed September 1, 2005. In Final Form: NoVember 23, 2005 The storage of gases in porous adsorbents, such as activated carbon and carbon nanotubes, is examined here thermodynamically from a systems viewpoint, considering the entire adsorption-desorption cycle. The results provide concrete objective criteria to guide the search for the “Holy Grail” adsorbent, for which the adsorptive delivery is maximized. It is shown that, for ambient temperature storage of hydrogen and delivery between 30 and 1.5 bar pressure, for the optimum adsorbent the adsorption enthalpy change is 15.1 kJ/mol. For carbons, for which the average enthalpy change is typically 5.8 kJ/mol, an optimum operating temperature of about 115 K is predicted. For methane, an optimum enthalpy change of 18.8 kJ/mol is found, with the optimum temperature for carbons being 254 K. It is also demonstrated that for maximum delivery of the gas the optimum adsorbent must be homogeneous, and that introduction of heterogeneity, such as by ball milling, irradiation, and other means, can only provide small increases in physisorption- related delivery for hydrogen. For methane, heterogeneity is always detrimental, at any value of average adsorption enthalpy change. These results are confirmed with the help of experimental data from the literature, as well as extensive Monte Carlo simulations conducted here using slit pore models of activated carbons as well as atomistic models of carbon nanotubes. The simulations also demonstrate that carbon nanotubes offer little or no advantage over activated carbons in terms of enhanced delivery, when used as storage media for either hydrogen or methane. Introduction In recent years, the increasing worldwide demand for energy has placed considerable strain on petroleum and other conven- tional sources such as coal. Combined with concerns about climate change arising from larger gas emissions associated with coal use, this has led to an acceleration of efforts to facilitate the development and utilization of technologies based on alternate sources such as natural gas and hydrogen. However, their application in the large mobile energy consumption sector, in conjunction with fuel cells or otherwise, has been impeded by the absence of safe and economical techniques for their on- board storage and this has been an area receiving much attention. Issues of safety and delivery pressure control preclude conven- tional ambient temperature storage as compressed gas, since pressures as high as 200-300 bar would be involved. Although the safety concern is mitigated by cryogenic storage or liquefaction (e.g. at 20 K for H 2 ), which involve substantially reduced pressures, this is not an economically viable option. Other options being considered are storage as chemisorbed hydrogen in hydrides, 1 of both hydrogen and methane in clathrate hydrates, 2 or as an adsorbed species within a suitable adsorbent. 3-5 For methane, the DOE storage target is that of 150 v/v at 35 bar, which represents the volume of stored methane at standard conditions (298 K and 1 bar) per unit volume of vessel, though recently this has been revised to 180 v/v to achieve the same energy density as compressed natural gas. For hydrogen, the target is set at 6.5 wt % of stored hydrogen and a volumetric density of 60 kg/m 3 , to be achieved by 2010, with more ambitious targets of 9 wt % and volumetric density of 80 kg/m 3 set for 2015. Although hydrides such as NaAlH 4 , Li 3 NH 4 , and LiBH 4 are readily able to meet the 6.5 wt % DOE target for hydrogen, the high temperature needed for desorption, the stability of the hydrides, and high costs remain key impediments. 3 In the case of hydrates, the targets have still not been achieved because of the prohibitively high pressures (in excess of 120 bar) needed for their formation. 2 Consequently, much effort has been devoted to investigating adsorptive storage as an alternative, 5-15 whereby significantly higher storage densities comparable to that of the bulk fluid can be achieved at more moderate pressures. Key to the success of adsorptive storage is the choice of suitable adsorbent and operating conditions. The early reports of over 60 wt % storage of hydrogen at ambient temperature and 112 bar pressure in carbon nanofibers 16 and of 14-20% in alkali doped carbon nanotubes at 1 bar pressure and temperatures from ambient to 673 K 7 have evoked much interest in carbons as the storage * To whom correspondence may be addressed. E-mail: amyers@ seas.upenn.edu. † E-mail: sureshb@cheque.uq.edu.au. (1) Hu, Y. H.; Yu, N. Y.; Ruckenstein, E. Ind. Eng. Chem. Res. 2004, 43, 4174. (2) Lee, H.; Lee, J.-W.; Kim, D. Y.; Park, J.; Seo, Y.-T.; Zeng, H.; Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. Nature 2005, 434, 743. (3) Schlapbach, L.; Zu ¨ttel, A. Nature 2005, 414, 353. (4) Orimo, S.; Zu ¨ttel, A.; Schlapbach, L.; Majer, G.; Fukunaga, T.; Fujii, H. J. Alloys Compd. 2003, 356-357, 716. (5) Matranga, K. R.; Myers, A. L.; Glandt, E. D. Chem. Eng. Sci. 1992, 47, 1569. (6) Chahine, R.; Bose, T. K. Int. J. Hydrogen Energy 1994, 19, 161. (7) Chen, P.; Wu, X.; Lin, J.; Tan, K. L. 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B 1998, 102, 4253. 10.1021/la0523816 CCC: $33.50 © xxxx American Chemical Society PAGE EST: 13 Published on Web 01/17/2006