Reversible Hydrogen Storage in Hydrogel Clathrate Hydrates By Fabing Su, Christopher L. Bray, Benjamin O. Carter, Gillian Overend, Catherine Cropper, Jonathan A. Iggo, Yaroslav Z. Khimyak, Andrew M. Fogg, and Andrew I. Cooper* The use of clathrate hydrates as potential hydrogen (H 2 )-storage materials [1] has attracted considerable attention recently. [2–4] Clathrate hydrates with cage-like, hydrogen-bonded structures comprising mostly water molecules might offer safe and environmentally friendly materials for H 2 storage under moderate conditions. However, there are three challenges to be addressed before practical applications can be achieved: the relatively low H 2 -storage capacity; the high pressures required for clathrate formation and stabilization; and the slow kinetics associated with gas enclathration. The work of Florusse et al. [5] has demonstrated that the pressure at which H 2 enclathration occurs can be significantly reduced from 200 to 7 MPa by adding a stabilizer (tetrahydrofuran, THF). Lee et al. [6] reported a H 2 -storage capacity around 4 wt% at 12 MPa and 270 K, and the composition of the clathrate hydrates was tuned to optimize gas-storage capacity, [6,7] although subsequent studies suggest lower storage capacities under similar conditions. [8–11] Other clathrates capable of retaining a significant amount of H 2 at moderate pressures and temperatures are being investi- gated. [12–15] The extremely slow kinetics of H 2 encapsulation into clathrate hydrate cages, [6,8,16,17] resulting from mass diffusion in a bulk solid phase, may be a generic limitation for the applicability of clathrates in the storage of H 2 and other gases. [2,4] It has been reported that mass diffusion can be enhanced by increasing the surface-to-volume ratio of clathrate hydrates using crushed small ice particles [8,16] or by dispersing the clathrate hydrate on silica-bead supports [6] or on a polymer substrate. [18] However, the use of small ice particles is inconvenient for multiple storage/ release cycles, since these revert to bulk water upon melting. Similarly, silica beads have relatively high densities and tend to reduce storage capacities considerably. [19] Thus, improving gas enclathration kinetics and obtaining good rechargeability/ recyclability are basic challenges in the development of clathrate hydrates as feasible storage materials for H 2 and other gases. In our previous work, [20] we demonstrated a method that drama- tically improved the kinetics and reusability of gas clathrates using an ultralow density, emulsion-templated polymer material as a support. However, the H 2 storage capacity was low under moderate conditions of pressure and temperature, possibly due to the highly hydrophobic nature of the polymer and inefficient wetting of the pore structure. We have also demonstrated that ‘‘dry water’’ [21] is effective in accelerating the formation of methane clathrates, [22] but this approach was unsuccessful for H 2 using THF–H 2 O mixtures, because the presence of THF destabilized the dry water inverse foam. Here, we investigate the possibility of using particulate hydrophilic water-swellable polymer networks as supports for improving the kinetics and recyclability of H 2 enclathration within clathrate hydrates. Superabsorbent polymer networks, such as lightly crosslinked poly(acrylic acid) sodium salt (PSA), are relatively inexpensive and have been used widely in sanitary applications (e.g., in diapers) because of their excellent water- absorption and -retention capabilities. Figure 1 illustrates schematically the clathration process using swollen PSA particles as a H 2 -storage medium. The polymer particles are first immersed in a THF–H 2 O solution and swell to form a hydrogel at room temperature. Upon cooling below a specific temperature, the THF–H 2 O solution within the hydrogel particles forms a clathrate hydrate that can encapsulate H 2 . Conversely, upon heating, the clathrate hydrate decomposes, releasing H 2 . Because the gels are superabsorbent, only small amounts are required to hold very substantial quantities of aqueous solution – this is the principle behind disposable diapers. The polymer-particle size and its surface chemistry can be easily tailored by synthesis conditions or post-treatment, and thus there is broad potential to optimize interactions between the support and the clathrate hydrate. We have found that it is possible to use hydrogels to enhance the encapsulation of H 2 in terms of kinetics, recyclability, and capacity. After dispersal in THF–H 2 O solution (20.0 g), dry PSA particles (1.0 g; Fig. 1b) were transformed into swollen and transparent hydrogel particles (Fig. 1c). The PSA particles readily absorbed all of the solution at this solution- support ratio. This produces a particulate gel material which, unlike ground/sieved ice particles, does not agglomerate or melt to form bulk water upon heating and gas release. Figure 1d shows the morphology of a THF–H 2 O hydrogel after five cycles of H 2 COMMUNICATION www.advmat.de [*] Prof. A. I. Cooper, Dr. F. Su, Dr. C. L. Bray, Dr. B. O. Carter, G. Overend, C. Cropper, Dr. J. A. Iggo, Dr. Y. Z. Khimyak, Dr. A. M. Fogg Department of Chemistry University of Liverpool Crown Street, Liverpool L69 7ZD (UK) E-mail: aicooper@liverpool.ac.uk Prof. A. I. Cooper, Dr. F. Su, Dr. C. L. Bray, Dr. B. O. Carter G. Overend, C. Cropper, Dr. J. A. Iggo, Dr. Y. Z. Khimyak, Dr. A. M. Fogg Centre for Materials Discovery University of Liverpool Crown Street, Liverpool L69 7ZD (UK) DOI: 10.1002/adma.200803402 2382 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 2382–2386