Development of Hydrogen Absorbing Alloy with High Dissociation Pressure
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Development of Hydrogen Absorbing Alloy with High Dissociation Pressure 1
Yoshitsugu Kojima, 1Yasuaki Kawai, 1Shin-ichi Towata, 2Tomoya Matsunaga, 2Tamio Shinozawa and 3Masahiko Kimbara 1 Toyota Central R&D Labs., Inc., Nagakute, Aichi, JAPAN 2 Material Engineering Div.3, Toyota Motor Corporation, Susono, Shizuoka, JAPAN 3 Corporate Technical Center, Toyota Industries Corporation, Obu, Aichi, JAPAN ABSTRACT The effective hydrogen capacity of TixCr2-yMny [x≅1.1 (1.08≤x≤1.16), y≅1.0 (0.96≤y≤1.08)] exhibited the maximum value of 1.8 wt% in the pressure range of 33 MPa and 0.1 MPa at 296K (dissociation pressure: 5-11 MPa), and the alloy provided over 10% more capacity than conventional Ti-Cr-Mn (Ti1.2CrMn: 1.6 wt%, Ti1.2Cr1.9Mn0.1: 1.3 wt%). At the low temperature of 233 K, the alloy absorbed 2.0 wt% of hydrogen and the hydrogen desorption capacity at 0.1 MPa was 1.6 wt%. The dissociation pressure decreased with the Ti and the Mn contents and was explained by the function of the bulk modulus and the cell volume. According to the van’t Hoff plots, the standard enthalpy differences (heat of formation) of the Ti1.16Cr0.92Mn1.08 and Ti1.08Cr1.04Mn0.96 hydrides were -21 and -22 kJ/molH2, respectively. These absolute values were about 10 kJ/molH2 smaller than those of LaNi5 and Ti-Cr-V. The alloy had sufficient hydriding and dehydriding kinetics. In the pressure range of 33 MPa and 0.1 MPa at 296 K, the alloy absorbed and desorbed 1.8 wt% of hydrogen in 60 sec and 300 sec, respectively. The hydrogen capacity changed gradually over many cycles and that after 1000 cycles was 94 % of the initial capacity. Thus Ti1.1CrMn can be utilized for a high- pressure MH tank which contains a hydrogen absorbing alloy with high dissociation pressure and compressed hydrogen. INTRODUCTION A fuel cell is a device that continuously converts the chemical energy of hydrogen (H2) and oxygen (O2) into electrical energy. Since the fuel cell has efficiency much higher than that of conventional combustion engines, a fuel cell vehicle (FCV) is expected to have high efficiency [1]. A polymer electrolyte fuel cell (PEFC, PEM fuel cell) is the prime power source for FCV. One of the most widely envisioned sources of fuel for FCV is H2. Therefore, it is necessary to have a storage tank of H2 to start the system on demand. The first FCVs were delivered on December 2, 2002. FCVs feature a 35 MPa H2 storage tank and can travel 300-355 km on a full tank. The driving ranges of the vehicles are small compared to those of gasoline vehicles. It is the biggest hurdle to FCVs and the improvement of the range is required for a new H2 storage system. Hydrogen (H2) can be stored in tanks as compressed [1] or liquefied H2 [1] or by adsorption on carbon materials [1-4]. It can also be stored in hydrogen absorbing alloys [5, 6] or as a chemical hydride, such as NaBH4 [7-12], LiBH4 [13,14 ], NaAlH4 [15-18], metal nitrides [19-23] or MgH2 [24-27] as well as in an organic hydride, such as methylcyclohexane or decalin [28, 29]. Among these methods, the hydrogen absorb
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