standard 45-l automotive fuel tank) would require dangerously ... Encyclopedia of Inorganic Chemistry, Online © 2006â2009 John Wiley & Sons, Ltd. This article is © 2009 ..... Research into each of these classes of novel materials is poised to.
Nano/Microporous Materials: Hydrogen-Storage Materials David J. Collins and Hong-Cai Zhou Miami University, Oxford, OH, USA
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Introduction General Considerations Carbon Materials Inorganic Materials Hybrid Organic – Inorganic Materials Conclusions Related Articles Abbreviations and Acronyms References
1 INTRODUCTION The hydrogen-fuel-cell-powered vehicle offers the prospect of a clean, efficient, and renewable energy future. Besides the fuel cell itself, another barrier exists, however, on the road to the fuel-cell vehicle: namely, the problem of storing optimum amounts of hydrogen on board. To maintain a typical driving range of 400 – 500 km, it is estimated that about 5 kg of hydrogen would be needed.1 Hydrogen, unfortunately, has a density of only 0.09 g l−1 at room temperature and atmospheric pressure. It is this extremely low density that leads to the difficulty of onboard hydrogen storage. Either of the currently available storage technologies, high-pressure compression or liquefaction of hydrogen, would be difficult to implement in a typical small personal vehicle. Compression of 5 kg of hydrogen to a reasonable volume (a standard 45-l automotive fuel tank) would require dangerously high pressures, in excess of 1000 bar, and the tank itself would, by necessity, be quite heavy to withstand the pressure. Liquefaction requires extreme cooling (to 21 K) and efficient insulation; the hydrogen volume required would still be slightly larger than current automobile fuel tanks (before inclusion of the necessary insulation and refrigeration machinery). Hydrogen adsorption on porous materials is one of the alternative methods under consideration for onboard fuel storage for automotive applications. The US Department
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of Energy target for hydrogen-storage systems for 2010 is 6.0 wt% (60 g H2 /kg of storage system) and 45 g hydrogen/l; the 2015 target is 9.0 wt% and 81 g l−1 .2 Ideally, this storage would be at near-ambient temperatures (within the range of simple refrigeration) and reasonable pressures (less than 100 atm). In addition to improved tank technology, the bulk of research currently focuses on tank additives, which must store a greater volume of hydrogen than could be compressed into the space occupied by the additive itself to be advantageous. Proposed hydrogen-storage materials include chemical hydrides and metal hydrides, which chemically bind hydrogen, and porous adsorbents, which can hold hydrogen by physisorption via van der Waals forces. It is these nanoporous adsorbents and their hydrogenadsorption properties and prospects that are the focus of this article.
2 GENERAL CONSIDERATIONS Microporous materials are defined as materials with a regular organic or inorganic framework supporting a regular porous structure, with pore sizes between 0.2 and ˚ 3 Such materials include organic materials 2.0 nm (2 – 20 A). such as activated carbons (ACs) and carbon nanotubes, inorganic materials such as zeolites and other silicates, and
Encyclopedia of Inorganic Chemistry, Online © 2006–2009 John Wiley & Sons, Ltd. This article is © 2009 John Wiley & Sons, Ltd. This article was published in the Encyclopedia of Inorganic Chemistry in 2009 by John Wiley & Sons, Ltd. DOI: 10.1002/0470862106.ia379
NANOMATERIALS: INORGANIC AND BIOINORGANIC PERSPECTIVES
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Figure 1 H2 adsorption at 1 atm and 77 K for activated carbons (AC), single-walled carbon nanotubes (SWCNT), metal – organic frameworks (MOF), cyanometalates, and zeolites, as related to (a) specific surface area and (b) micropore volume. (Data from References 4 – 6 (activated carbons), 7 (zeolites and activated carbons), 8 (MOFs), 9 – 11 (cyanometalates), and 12 (SWCNTs))
hybrid materials such as metal – organic frameworks (MOFs). These materials exhibit type I gas-adsorption isotherms. For materials with this pore size, the potential fields of attraction between pore walls and adsorbate molecules overlap, increasing the attractive force acting on the adsorbate and, in turn, increasing adsorption. In some cases, the adsorbate molecules may pack nearly as closely as in the bulk liquid. However, for some adsorbate gases, the pores and passages may be small enough to render some portions of the interior volume inaccessible; obviously, this is more a problem for larger adsorbate molecules than for small molecules such as hydrogen.3 Microporous materials typically have an internal surface area on the order of tens to thousands of square meters per gram. The internal surface area of a microporous material is relatively easy to measure using the Brunauer – Emmett – Teller (BET) method; however, surface area does not necessarily correlate well with hydrogen uptake, as shown in Figure 1(a). A better (although, still rough) correlation is found when the hydrogen uptake is compared to the micropore volume, as shown in Figure 1(b). Other important parameters in a practical hydrogenstorage application include the kinetics and thermodynamics of recharging and release. Of primary consideration is the heat of formation (for species that bind hydrogen chemically) or heat of adsorption (for physisorbents), Hf or Hads . The large Hf of chemisorbents, from 50 to over 200 kJ mol−1 , necessitates operation considerably above ambient temperature to drive hydrogen release. In contrast, physisorbents interact with adsorbed hydrogen weakly, with Hads typically considerably less than 10 kJ mol−1 ; significant adsorption of hydrogen can only occur at cryogenic temperatures. For the binding of hydrogen on a homogeneous surface, Bhatia and Meyers have calculated the optimum
Hads at room temperature and 30 atm pressure to be ∼15 kJ mol−1 .13
3 CARBON MATERIALS
3.1 Activated Carbons Charcoal has been known as an effective adsorbent of gases since the late eighteenth century. Since then, much improvement has been made both in the synthesis and understanding of AC materials. In a typical ‘‘top-down’’ synthesis, solid organic materials such as coal, cellulosic materials (wood, coconut shells, etc.), or polymeric materials, are pyrolyzed in the absence of oxygen to prepare a charcoal, which is then activated either thermally or chemically to create surface functionalization. A ‘‘bottom-up’’ synthetic route can also be used, in which a template, such as porous silica or microporous zeolite, is suffused with a gaseous or liquid organic precursor, which is then carbonized. The template is then etched away, leaving a porous carbon black, which can then be activated as above.14 Generally speaking, the hydrogen uptake of ACs is directly related to the specific surface area and micropore volume of the material. The theoretical maximum surface area for carbon adsorbents, ∼2630 m2 g−1 , is derived from consideration of an infinite single graphene sheet. As the typical AC studied has a specific surface area of 500 – 1500 m2 g−1 , the overall uptake is generally limited to less than 4.5 wt% at 77 K.15 It is also difficult to synthesize an AC material with small pore sizes and minimal pore-size
Encyclopedia of Inorganic Chemistry, Online © 2006–2009 John Wiley & Sons, Ltd. This article is © 2009 John Wiley & Sons, Ltd. This article was published in the Encyclopedia of Inorganic Chemistry in 2009 by John Wiley & Sons, Ltd. DOI: 10.1002/0470862106.ia379
HYDROGEN-STORAGE MATERIALS
variation. Conventional thermal processing typically results in 50% or more of pore volume as macropores (greater than ˚ 16 a wide distribution of pore sizes, and a fraction of 40 A), carbon atoms that are inaccessible (i.e., not part of a surface). The synthesis of nanostructured (templated) carbon materials is one way of surmounting this problem. Carbon materials have a low affinity for the dihydrogen molecule. Between two ideal infinite parallel sheets of graphite with the appropriate spacing, the enthalpy of adsorption of dihydrogen has been calculated to be ∼10 kJ mol−1 ;17 actual materials have measured adsorption enthalpies considerably lower than this, with typical values 3 – 5 kJ mol−1 . The overall potential for AC as a practical fuel tank adsorbent was investigated by Hynek et al. in 1997.18 Results of this study indicated that (of the sorbents studied at the time) none were more than marginally able to increase the volumetric density of hydrogen in a realistic compressed-hydrogen storage system. Furthermore, at liquidnitrogen temperatures, none could provide better storage than an unaugmented cryogenic pressure cylinder. More recently, research has continued toward more highly structured carbon materials,2 with the intention of increasing their affinity for hydrogen by reducing pore size and modifying pore shape; uptakes of ∼5 wt% have been reported at 77 K for these materials.6,15 Of particular interest are nanostructured carbon materials with regular, well-defined pore shapes — especially ‘‘slitpores’’ with widths approximately twice the diameter of the hydrogen molecule. Grand canonical Monte Carlo (GCMC) simulations indicate that an optimal material with such pores could store up to 5.5 wt% hydrogen at 77 K.19 Another class of carbon materials, carbon aerogels, has also been shown to have very high surface area (>3000 m2 g−1 ) and adsorbs up to 5.3 wt% H2 at 77 K and 40 bar.20 3.2 Carbon Nanotubes Carbon nanotubes, and specifically single-walled carbon nanotubes (SWCNTs), are seemingly ideal material for the storage of hydrogen: such tubes contain internal pores of well-controlled size and distribution, and could conceivably be arrayed in a close-packed solid with minimization of macropore volume. Hydrogen would be adsorbed both into the interior of the nanotube and into the interstitial spaces between tubes. However, the production of nanotubes often results in soot with a very small fraction of SWCNTs; moreover, SWCNTs are often closed or capped, with the interior volume inaccessible, and a significant fraction of metal catalyst is also often present as an impurity. (See Inorganic Semiconductor Nanomaterials for High-Performance Flexible Electronics; Carbon Nanotubes and Nanocomposites for Electrical and Thermal Applications; Carbon Nanotubes, Single-Walled: Functionalization by Intercalation.) Early reports on hydrogen adsorption by SWCNTs indicated that high-density condensation of hydrogen was
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possible, with hydrogen adsorption in the range of 5 – 10 wt% at 77 K, and reported Hads of nearly 20 kJ mol−1 .21 Since this initial report, controversy has ensued over whether these values were misstated or miscalculated; a majority of the hydrogen adsorption may have arisen from metal impurities, either residual from synthesis or from the sonication used to break open closed tubes. At present, the general consensus, based on work with pure, nearly metal-free nanotubes and on simulation, indicates that unmodified SWCNTs do not make good candidates for a hydrogen-storage material. The seeming success of early, metal-contaminated nanotubes, however, indicates that carbon SWCNTs modified or doped with metals or alloys in a controlled manner to make metal – carbon hybrid nanotube materials may yet be worthy of continued investigation.2 Simulation and preliminary investigation also indicates that novel arrangements22 and doping23 or surface modification24,25 of SWCNTs may yet yield higher H2 adsorption values as well.
4 INORGANIC MATERIALS 4.1 Zeolites, Aluminates, and Silicates The utility of zeolite materials as sorbents and catalysts are well-known; however, it has only been in the last 10 years that the hydrogen-storage properties of zeolites have been intensively investigated. In 2001, Nijkamp et al. performed a survey of the hydrogen-adsorption properties of various carbon-based and silicate-based materials at low temperatures.7 Many of the commercially available materials investigated were mesoporous, with pore sizes ˚ these materials, as a group, considerably larger than 20 A; have relatively low surface areas and generally low hydrogen adsorption at 77 K and 1 bar. Those zeolitic materials with a large micropore volume were found to have the larger hydrogen-adsorption values, as seen in Figure 1. (See Nano/Microporous Materials: Hydrothermal Synthesis of Zeolites.) Zeolites typically have lower surface areas (
