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11. Task11 DEVELOPMENT OF HYDROGEN ABSORBING ALLOYS FOR SMALL SCALE TRANSPORTATION AND STORAGE SYSTEM 11.1 R&D Goals For the early applying of hydrogen energy to the society, it is essential to success in the development of hydrogen storage technology for various needs of consumers. Particularly, development of sophisticated hydrogen absorbing alloys is required to realize a hydrogen car. For this purpose, following development targets are set for WE-NET Task 11 (development of hydrogen absorbing alloys for small scale transportation and storage system). - Effective hydrogen storage capacity : more than 3wt% - Temperature for hydrogen desorption : less than 100℃ - Duarability : hydrogen storage capacity more than 90% of the initial capacity after 5,000-cycle use These development targets are in view of its application to the fuel tank of hydrogen vehicles (range: 300km and longer, hydrogen desorption heat source: cooling water), however, if it succeeded, it would make way for another applications such as stationary storage systems. 11.2 Research and Development in the Project 11.2.1 Result in the first term As the result in the first term it becomes clear that it is very ddifficult to reach the R&D goals with conventionally improved metal hydrides because of the conflict trade-off relationship between decreasing desorption temperature and increasing absorption capacity. Mg-Ni alloys with 4wt% absorbing capacity attracted a great deal of interest. We tried to decrease the desorption temperature of it to less tha 373K keeping its high absorbing capacity in the WE-NET program. At the same time, we realized that new materials should be synthesized to break through for R&D goals. 11.2.2 Research and Development in the Second Term In the second term of WE-NET program (FY1999-2003), the following R&D are planned : (1) Early completion of Mg-Ni alloy (2) Creation and synthesis of new materials with innovative properties (3) Start the development of sodium aluminum hydride (4) Forward application for practical use (5) Start investigation of new hydrogen absorbing materials other than metal hydrides 11.2.3 Results in FY1999 The work content and result of Task11 in this fiscal year is summarized in the following. (1) The Definition of the effective Amount of Hydrogen Considering an application to a fuel cell, the amount of effective hydrogen in a hydrogen absorbing material is defined as follows: - absorption : at 293K, 1.1 and 3.0 MPa (2) The Mg-Ni alloys with Laves composition The possibility of low absorption-desorption temperature by hydrogen occupation in interstitial site was investigated for Mg-Ni Laves composition alloys prepared by ball milling. (A) The hydrogen properties of MgNi1.9M10.1(M1=Cr, Al, B, Cu, Si, Fe) alloys etc. were investigated using P-C-T and TG analysis. The results are as follows: (a) The amount of desorbed hydrogen of Mg-based alloys which have Laves Composi-tion is 0.2~0.35 wt% at 423K. (b) From TG analysis, the hydrogen desorption starts at 323K and ends at 423K. This temperature is the lowest among developed Mg-based alloys. (B) In order to investigate the state of hydrogen in metallic lattice of Mg-base alloys, Raman scattering analysis with microscope was done. The peak corresponding to Ni-H bond is observed in Mg2NiH4 hydride. However, in the case of Mg based hydride, which has Laves composition, the peak corresponding to Ni-H bond is not observed. Therefore, it is considered that the state of hydrogen in Mg based hydride, which has Laves composition, is different from that in Mg2NiH4. From the results of Crystal Mg2Ni・EMgNi2-based alloys and mechanical grinding treated Mg-Ni alloys, though the hydrogen capacity increased with increasing the ratio of Mg/Ni in Mg-Ni alloy system, high absorption-desorption temperature is needed. And though the absorption-desorption temperature lowers with decreasing the ratio of Mg/Ni in Mg-Ni alloy system, the hydrogen capacity decreases. (3) New Ca-Mg-X Multi component Hydrogen Storage Alloys For attaining to absorption of 3wt% hydrogen, the target of this project, the relationship between H/M (A) and the average atomic weight of alloy (B) must as follows. A/(A+B) ≧ 0.03B ≦ 32.3A If the H/M reaches up to 1.5, the average atomic weight of alloy should be 50 or less to achieve the target. Thus, Ca and Mg were selected as light constituent elements of alloy. In this research, new Ca-Mg-X ternary alloy with more unstable hydrogen site were investigated in order to decrease the hydrogen desorption temperature. Ni was firstly selected as X of the Ca-Mg-X alloy at current year. In XRD patterns of CaNi‚Q+xMg(x=0-2) made by reactive-sintering process, the peeks were identified as the mixture of CaNi2, CaMg2 and MgNi2. And no unknown peek was also observed in DSC measurement under H2 atmosphere, thus it supposed that no new compound exists in the triangle area consisting of CaNi2, CaMg2 and MgNi2. The theoretical calculation was executed as a preliminary investigation of Ca-Mg-Cu, which is planed for next alloy system. The bond-order ratio, BondCa-Cu/ (BondCa-Ca+BondCu-Cu) on Ca(A)-Cu(B) system was calculated with the DV-Xƒ¿ method based on the first-principle calculation. The result was compared with available data of hydrogen absorbing alloys. In case of Ca/Cu=1/1, the hydrogen absorption results in a disproportionation. To obtain a practicable hydrogen absorption characteristic, the ratio of Ca/Cu=1/3 or less is necessary. (4) Ternary Ca-Ni-Mg System Hydrogen absorbing Alloys In the fiscal 1999, we prepared wide-ranging compositions of ternary Mg-Ca-Ni alloys (39 samples) in order to develop sophisticated Mg-Ca-Ni systems hydrogen absorbing alloys and to make a breakthrough by discovering new ternary Mg-Ca-Ni intermetallic compound with new crystalline structure. The solidification structure, crystalline structures of each phase and chemical compositions of each phases were characterized by Scanning Electron Microscopy, Electron Probe Micro Analyzer and powder X-ray diffract meter. The absorption hydrogen contents and velocity were measured by applying 3MPa hydrogen at room temperature. The desorption hydrogen properties were characterized by conventional measurement of pressure composition isotherms and thermal analysis of hydrogenated samples. The transformation of crystalline structure because of hydrogen absorbing was characterized by comparison of x-ray diffraction profiles before and after hydrogen absorption. The major results are as follows: (A) Ternary Mg-Ca-Ni intermetallic compound with C15 type crystalline structure existed in many samples. (B) Some of the above intermetallic compounds transformed into amorphous structure owing to absorbing hydrogen and the others just expand with C15 type crystalline structure. (C) Some of the above intermetallic compounds transformed into C36 type crystalline structure owing to anneal (100hours, 673K) and the others just homogenized. (D) CaMg2 intermetallic compound transformed into amorphous structure owing to absorbing hydrogen, only in the case of coexistence with MgNi2 phase or Mg2Ni phase. (5) High Capacity Vanadium-based Hydrogen Storage Alloys Thorough our investigation, we have successfully developed a multi-component V-based alloy which has the largest effective capacity of hydrogen under the practical pressure and temperature ranges. The V-Ti-Cr-Mn alloy dissociates 2.64 mass% hydrogen from 273 K (0℃) and 3.3 MPa to 373 K (100℃) and 0.01 MPa. Moreover, we have also developed the V-Ti-Cr-Mn-Ni alloy. This alloy shows a very large hydrogen capacity without heat-treatment of the alloy. In the alloy, the density distribution of titanium and nickel, whose atomic radius are, respectively, the largest and smallest among constituent elements, are changed correlatively. As a result, this alloy has a small distribution of hydride stability and has a flat plateau on the isotherms unless the alloy is heat-treated. This alloy has a 2.47 mass% of an effective hydrogen capacity under the same condition mentioned above. We also have discovered adding copper to V-Ti-Ni alloy shows an anomalous effect on VH<1 stability. In this alloy, VH<1 would be made unstable, which is inconsistent of a larger unit cell length of a matrix phase than that of the alloy without copper. This V-Cu-Ni alloy consists of a matrix phase of a V solid solution and a secondary phase of a Cu solid solution. This matrix changes from BCC to FCC by hydriding like most vanadium solid solutions. However, this alloy does not show a 2-step plateau on its isotherms. (6) Development of Various BCC and B2 Alloys Ti-Cr-V alloy, which has BCC structure and wide effective hydrogen transfer area, was surveyed. This alloy has problems of large hysteresis and inferior durability. In this year, the hydrogenation property was investigated by changing the composition of alloy and/or the producing condition of Ti-Cr-V alloy. In addition, the effects of the additives and the durability were surveyed preliminary. As a result, the followings were found: (A) At Ti rich composition and Cr rich composition, effective hydrogen transfer area is narrow and plateau slope is steep. (B) In a wide V composition range from 5 to 50at%, effective hydrogen transfer of 2.0~2.4wt% was observed. (C) Some heat treatments bring about not only homogenizing but also segregation, which produce the phase not concerned with hydrogen absorption, so heat treatment for a short time at the high temperature is better. (D) Some additives especially Nb, Fe, Ni, Mn, Cu, Ca and Zn improve plateau property. (E) Durability test with pressure cycling (constant temperature) showed that the Nb added alloy improved 20% of effective hydrogen transfer increase after 1000 times of cyclic hydrogen absorption and desorption. (F) Ti-Cr-V alloy has various phases besides matrix phase. It was found that these phases caused degradation of hydrogenation property. (7) Study of Search for New Materials It is necessary to obtain a guiding principle for searching new materials. The following investigation and experiment were put into practice and a criterion to new hydrogen storage materials could be grasped. (A) Cubic or tetragonal compounds were extracted from the Binary Alloy Phase Diagrams edited by T.B.Massalski (ASM International, USA, 1990). (B) Structure, lattice constants and chemical unit number Z in unit cell were extracted from ICDD (International Center for Diffraction Data) for the above compounds. (C) An average space size per atom about each compound was calculated. This value can be used as an index of the lattice space of searched material. (D) The average space size of several conventional hydrogen storage alloys ranges from 0.013nm3 to 0.015nm3. So, some compounds of which average space sizes distribute larger than 0.015nm3, between 0.013 and 0.015nm3 and less than 0.013nm3 were prepared by melting method and hydrogenation characteristics of them were measured with PCT. Consequently, we realize that materials with large absorption / desorption capacity should possess both the average space size of 0.013~0.015nm3 and the higher A/B ratio. We can clarify a guiding principle and a useful criterion for searching new hydrogen storage compounds. (8) Development of Catalytically Enhanced Complex Aluminum Hydrides (A) Synthesis of ammonium and phosphonium aluminum hydrides A new class of hydrides, organic cation-substituted aluminum hydrides have been attempted to prepare. Tetraphenyl phosphonium chloride, and bis(triphenylphosphine)iminium chloride (PPNCl) were found to react with LiAlH4 in THF solution to produce red compounds that have not been fully characterized. The dehydriding of the phosphonium compound was attempted but no hydrogen evolution was observed upon heating a sample of the compound to 473K. On the other hand, the PPN compound was seen to evolve gas immediately upon its formation at room temperature. The starting halides of finally class of compounds we plan to explore, N-substituted phosphazenium salts, are presently being synthesized in our laboratory as they are not commercially available. (B) Determination of plateau pressures Recent studies of the dehydriding of catalytically enhanced NaAlH4 to Na3AlH6 have shown that the △H of this reaction in the solid state is very different than those previously determined for the molten hydride (Bogdanovic et al). A value of △H = 37 kJ/mol has been determined for the solid state reaction and a plateau pressure of about 8 atm is predicted at 353K. Thus earlier concerns that the first dehydriding plateau pressure of NaAlH4 might be too high in the 353-423K temperature range have been eliminated. Catalytically enhanced NaAlH4 therefore meets WE-NET goals of a hydrogen storage material that contains >3.0 weight percent available hydrogen and a plateau pressure lower than 1 MPa (9.87 atm). However, it has not been demonstrated that the rates of hydrogen desorbsion below 353K are adequate. (9) Hydrogen Storage Character of new Carbon Material Recent reports of very high, reversible adsorption of hydrogen in some new carbon materials have aroused tremendous interest in the research and industrial community worldwide. In this fiscal year, we review and summarize new carbon materials such as graphite, carbon nano-tubes and graphite nano-fibers. The amount of hydrogen adsorption achieved and reported in many studies seems to be 4-7wt% in graphite nano-fibers, 5-8wt% in carbon nano-tubes and about 5wt% in fulleren and graphite. The theoretical calculation leads the same range of adsorption amount. Some problems such as a measurement accuracy of hydrogen adsorption amount and purity or a characterization of materials are found out in many reports. We should seize an accurate and undoubted data in order to judge and decide how WE-NET should deal with carbon materials as a hydrogen storage material in the future program. (10) Hydrogen Storage with Sodium Hydride and its Hydrolysis Hydrogen can be generated with hydrolysis reaction of NaH or NaBH4. The reaction of NaBH4 is milder and it has been used as a generating method of hydrogen in a laboratory. The amount of hydrogen from these reactions is 4.7 wt% or 7.2wt% for each material. Therefore many ideas have been proposed recently as a hydrogen supplying method for a fuel cell. These ideas arouse our tremendous interest, however we should evaluate it carefully with a strict analysis of chemical thermodynamics. Because, this hydrolysis process produce a very stable NaOH or NaH2BO3, the exothermic energy in this hydrolysis is very large and so that a very large energy should be added to store in a recycle produced sodium hydride. Therefore the energy efficiency in this system may become very low. 11.3 Subject in future (1) In the early half of the second term of WE-NET, we will get prospects for new promising hydrogen absorbing alloys with performance of WE-NET goal. (2) In the latter half of the second term, we will start research for the new generation alloys with more excellent performance. (3) We will develop three categories of hydrogen absorbing materials : hydrogen absorbing alloys to WE-NET goal performance of more than 3wt% capacity, sodium aluminum hydride to more than 5wt% capacity and new carbon materials to more than 8wt% capacity.
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