An overview of hydrogen storage for transportation application

Hydrogen storage is a key technology for realizing the hydrogen economy and has been one of the highest technical priorities of the DOE Office of Hydrogen, Fuel Cells and Infrastructure Technologies (HFCIT). Most challenging is to find onboard hydrogen storage solutions for transportation to meet the DOE FreedomCAR 2010 and 2015 goals. At present there are no materials that meet the operating requirements of weight, volume, durability, efficiency and cost. Composite gas tanks for vehicles are already available at 5,000 psi and recently up to 10,000 psi have been successfully tested at Quantum Technologies [1]. The new tanks are made of a lightweight carbon-fiber composite and are being developed in collaboration with Aceves, et al. at LLNL [2]. The volume efficiency is still poor and much energy is used in compressing the gas. Storage of liquid hydrogen in cryogenic containers [3] enables better volume efficiency, but about 1/3 of the energy stored has to be used for liquefaction and hydrogen will be lost due to evaporation. New light-weight, low-volume materials with low heat transfer and higher safety are required. With a future perspective, solid-state hydrogen storage materials are more likely to meet the requirements for onboard storage: appropriate thermodynamics, fast kinetics, high storage capacity (6wt%2at 100 degrees C and 0.1MPa before 2010), effective heat transfer, light weight, long cycle life, and safety. There are several candidates for hydrogen storage; most promising arc the nanostructured materials including metal hydrides, carbon nanotubes, inorganic nanotubes, as well as lithium nitride and metal-organic frameworks (MOF). Both reversible and irreversible hydrogen storage systems are currently being developed. Hydrogen can be irreversibly stored in, for example, NaBH4 and released by a catalyzed chemical reaction in water. Suda et al. [4] demonstrated a fuel cell powered by NaBH4. The byproduct NaBO2 can be recovered and reproduced back to NaBH4 but it is costly and timeconsuming. Millenium Cell is making progress in developing? an efficient low-cost off-board regeneration procedure [5]. Currently work is also being done to find an appropriate catalyst for obtaining higher hydrogen production at a productive rate at room temperature. Other chemical hydrides are also of interest. At Safe Hydrogen a type of magnesium hydride slurry has been developed that has shown some potential [6]. Researchers at Air Products and Chemical, Inc. developed a number of novel liquid-phase hydrogen carriers that have 5-7wt%2H-2 gravimetriccapacity [7]. It would be more practical to use a reversible solid hydrogen storage material once an appropriate system has been developed. An overview of the most interesting classes of such hydrogen storage materials follows: Among the class of metal hydrides, the complex type usually has higher hydrogen solubility? than the intermetallic type (AB, AB(2), A(2)B, etc). Most promising are the alanates and the borohydrides with anionic complexes of [AlH4] and [BH4](-) that are mainly stabilized with light metal cations. Theoretically both alanates and borohydrides have weight percent hydrogen far higher than required, but they decompose at high temperatures and are difficult to make reversible. So far, only NaAlH4 has been developed to show reversibility at reasonable temperatures by adding appropriate Ti-dopants, but practical storage capacity is only about 4wt%2[8,9, 10]. Hydrogen desorption kinetics has been investigate by Anton et al. [ I I]. Other alanates are currently under development and new practical synthesis routes need to be found, that are less complicated than wet-chemical processes. Among the borohydrides, LiBH4 was shown to have a reversible capacity of 13.3wt%2at higher temperatures by using a catalyst [12]. Some groups are also trying to discover new metal hydrides and there is a trend to use high-pressures for synthesis. An unusual ternary Mg-hydride, i.e. Mg7TiH16, was recently synthesized by using GPa high-pressures and has over 7wt%2hydrogen and a much lower desorption temperature than MgH2 [13]. Work is continuing to make this system practical by finding another synthesis route and designing a functional catalyst for reversible hydrogen storage. Recently many materials other than the classical metal hydrides have appeared as promising competitors. The nanotubes are an interesting new class of materials, including carbon, boron nitride (BN), titanium sulfide (TiS2) and molybdenum sulfide (MoS2). Dillon et al. [14] first discovered the ability of carbon nanotubes to absorb hydrogen. Thereafter, promising publications reporting high hydrogen storage capacities in carbon nanotubes has appeared as summarized by Hirscher et al. [15]. However, the results have been difficult to reproduce. These H-2 uptake systems require either high pressure or very low temperature or both. One of the most reliable results was obtained by Ye et al. [ 16] who observed 8wt%2storage in SWNT at 80K. By using high-energy reactive ball milling under a hydrogen atmosphere graphite can be loaded with large amounts of hydrogen up to 7wt%2 but desorption temperature is 700K and re-loading in a hydrogen atmosphere is impossible [ 17]. Inorganic nanotubes, analogues to carbon nanotubes, have emerged and are still in its cradle stage and far from being optimized. Boron nitrides are isoelectronic and isostructural to carbon nanotubes. Ma et al. [18] demonstrated that multiwalled and bamboo type BN nanotubes can uptake 1.8 and 2.6wt%2hydrogen respectively under 10MPa at room temperature. BN powder itself only absorbs 0.2wt%2 It was also shown that when ball milling h-BN under hydrogen atmosphere for 80 hours, the sample absorbed 2.6wt%2hydrogen that could be partly desorbed at 570-1173K [19]. Tang et al. [20], received hydrogen absorption of 4.2wt%2for platinum-treated BN nanotubes. Collapsed surfaces of BN effectively increase the capacity due to higher specific surface area. MoS2 nanotubes are also analogues to carbon nanotubes and weak Van der Waals forces are responsible for the stacking of the S-Mo-S layers with room for hydrogen in between MoS2 nanotubes. Chen et al. [21] synthesized MoS2 by direct reaction of (NH4)(2)MoS4 and hydrogen and then sintered in hydrogen-thiophene (400 degrees C, 1 h). The electrochemical hydrogen-storage properties were investigated and the discharge capacity was 0.97wt%2 Gas-adsorption properties Of MoS2 nanotubes were also examined and a maximum capacity of 1.2wt%2was observed. Multiwalled TiS2 nanotubes were synthesized that reversibly absorb 2.5wt%2hydrogen at 25 degrees C and 4 MPa [21]. After 20 adsorption/dcsorption cycles different types of defects were formed that decreased the storage capacity, but increasing the kinetics of absorption/desorption??. Another competitor to metal hydrides are nitrides. It was recently discovered by Chen et al. [221 that lithium nitride (Li3N) can reversibly absorb hydrogen up to about 10wt%2in two steps below 300 degrees C. It was proposed that absorption leads to formation of lithium imide (Li2NH) + LiH and further to lithium amide (LiNH2) + LiH, theoretically 11.5wt%2in total. By mixing lithium imide with MgH2 the absorption/desorption properties were further improved to show reversibility for several cycles at 30bar and 220 degrees C [23]. The metallo-organic framework (MOF) materials are a new class of hydrogen absorbers. These are crystalline metal-organic frameworks with cubic cavities of uniform size and internal structure. Most microporous framework materials composed of metal oxides, such as zeolitcs, have poor hydrogen capacity. A new MOF-material reported by Rosi et al. [24], consisted of inorganic [OZn4](6+) groups joined to an octahedral array Of [O2C-C6H4-CO2](2)- (1,4benzenedicarboxylate) groups to from porous cubic framework and absorbed 4.5wt%2hydrogen at 78K. and moderate pressures. At room temperature only 1 wt%2was absorbed. Finally, we would like to emphasize that development of new practical hydrogen storage materials is a scientific and technical challenge and demands a joint effort by experimentalists and theorists of different fields to fully understand the bonding and diffusion paths of hydrogen. It will be crucial for future advancements in developing a practical hydrogen storage material to continue with fundamental research and detailed characterization of materials in order to fully understand the mechanism behind hydrogen uptake and release to be able to improve kinetics and thermodynamics.