(221a) Research and Development of Hydrogen Storage Materials for Lightweight Tanks

Fuel cell vehicles (FCV) feature 35-70 MPa hydrogen storage tanks and can travel 620 km (pressure: 35 MPa, inner volume:171 L) -780km (pressure: 70 MPa, inner volume:156 L) on full tanks. The tank size of the vehicle is large compared to those of gasoline vehicles. Hydrogen can be stored in many different forms as compressed or liquefied hydrogen in tanks, as hydrogen adsorbed carbon materials, as hydrogen-absorbing alloys, as metal hydrides with light elements such as NaBH4, NaAlH4, MgH2, LiBH4 and metal nitrides, or as organic hydrides (methylcyclohexane, decalin)[1]. In this study, we will review our experimental results on metal hydride with high dissociation pressure (Ti1.1CrMn), light weight Li-Mg-N-H system, NH3-based materials and NaH-NH3BH3 composites. We found that the effective hydrogen capacity of Ti1.1CrMn exhibited the maximum value of 1.8 mass% in the pressure range of 33 MPa and 0.1 MPa at 296 K (dissociation pressure: 11 MPa) [2]. At the low temperature of 233 K, the hydrogen desorption capacity at 0.1 MPa was 1.6 mass%. The standard enthalpy difference of Ti1.1CrMn was -22 kJ/molH2. The absolute value was about 10 kJ/molH2 smaller than those of LaNi5 and Ti-Cr-V. The mixtures of LiH and Mg(NH2)2 with molar ratios of 8 to 3 were milled for 2-20 h under 1 MPa H2 pressure to synthesize the Li-Mg-N-H system by a planetary ball milling [3]. It was found that milling time had a remarkable effect on improving kinetics and then an optimum milling time was 20 h. The H2 desorption capacity close to 6 mass% was obtained after keeping for 8 h at 423 K. For the dehydrided Li-Mg-N-H system (vacuum, 473K, 24h), ball milling treatment was performed at 423 K for 5 h under 15 MPa of H2 pressure, leading to the H2 absorption capacity up to 5.8 mass% . Alkali metal hydrides (LiH, NaH and KH) reacted with NH3 at room temperature by the exothermic reactions [4, 5]. In the hydrogen generating reactions, the activation treatment by ball milling drastically improved the kinetics for the Li, Na, K systems. After these H2 generations, alkali metal amides were able to store H2 at 373-573 K, forming NH3 and the corresponding alkali metal hydrides under 0.5 MPa H2 flow for 4 h. H2 of 8.1 [H2/LiH+NH3] , 4.9 [H2/NaH+NH3] and 3.5 mass% [H2/KH+NH3] can be reversibly stored by this reaction. Sodium hydride-ammonia borane composite systems were prepared by ball milling. The excess hydride (NaH/NH3BH3= 2-3 /mol/mol) suppressed the foaming and ammonia gas release. The dehydrogenation peak temperature decreased with increase in the NaH content. For the composite materials of 3NaH/ NH3BH3, it reaches the dehydrogenation peak at 345 K, which is much lower than the dehydrogenation temperature of Na NH2BH3 (357K). When we filled the tank (inner volume: 100 L) with the Ti1.1CrMn alloy of 50 kg, the gravimetric and the volumetric hydrogen densities at 35 MPa and 298 K were calculated as 6.0 mass% and 3.0 kg/100 L, respectively. The volumetric hydrogen density provides 30 % more capacity than compressed hydrogen at 35 MPa (volumetric hydrogen density: 2.3 kgH2/100L) and the gravimetric and the volumetric hydrogen densities are similar to those of the Li-Mg-N-H system (packing ratio: 50%, packing density : 0.6 g/cm3, gravimetric hydrogen density: 5.8 mass%, volumetric hydrogen density: 3.5 kgH2/100L). We have calculated the gravimetric and the volumetric H2 densities of the LiH-NH3 system. At 70 MPa, the volume of a tank (100L) for gaseous hydrogen allows storage density of 3.9 kgH2. We assumed that the density of LiH was 0.78 g/cm3. When we filled the tank (internal volume 62 L) with liquid NH3 (density: 0.61 g/cm3 ) of 38 kg and the tank (internal volume 46 L, packing ratio 50 %) with LiH of 18 kg (LiH/NH3=1mol/1mol), the gravimetric and the volumetric H2 densities are calculated as 8.1 mass% and 4.2 kg/100L, respectively. The volumetric H2 density provides 1.8 and 1.1 times of compressed hydrogen at 35 and 70 MPa (3.9 kgH2), respectively. References [1]?Hydrogen Storage?. In: J. Garche, C. Dyer, P. Moseley, Z. Ogumi, D. Rand, B. Scrosati, editors. Encyclopedia of Electrochemical Power Sources, Vol 3. Amsterdam: Elsevier; 2009. [2]Y. Kojima, Y. Kawai, S. Towata, T. Matsunaga, T. Shinozawa and M. Kimbara, J. Alloys Compd. 419 256 (2006). [3]Y. Wang, T. Ichikawa, S. Isobe, M. Tsubota, S. Hino, T. Nakagawa and Y. Kojima, Abstracts of MH2008, International Symposium on Metal Hydrogen Systems, June 24-28, Reykjavik, Iceland. [4]Y. Kojima, K. Tange, S. Hino, S. Isobe, M. Tsubota, K. Nakamura, M. Nakatake, H. Miyaoka, H. Yamamoto and T. Ichikawa, J. Mater. Res., 24, 2185 (2009). [5]H. Yamamoto, H. Miyaoka, S. Hino, H. Nakanishi, T. Ichikawa and Y. Kojima, Int. J. Hydrogen Energy, 34, 9760 (2009).