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A Large-Capacity Hydrogen-Absorbing Alloy


Dept. of Inorganic Materials Inorganic Synthesis Group
Yumiko Nakamura
Etsuo Akiba, Ph. D

to Japanese

1.The present state and the problems of hydrogen-absorbing alloy

@@Hydrogen-absorbing alloy is an alloy which absorbs and desorbs large amount ofhydrogen at room temperature, and is one of the ideal media for hydrogen storagebecause it is compact and safe. Today, this material is commercially used for the electrodes of Ni-MH(MH: Metal Hydride) batteries in emission free hybrid vehicles and in digital still cameras.
@@Recently, moreover, hydrogen-absorbing alloys have attracted great attentionas a storage medium for fuel tank of fuel cell vehicles which use hydrogen as fuel.Hydrogen-absorbing alloys enable to reduce hydrogen volume to a few hundredthsto one thousandths of gaseous condition. Moreover, one of the properties of thehydrogen is that when hydrogen, for some unknown reason, desorbs from a tankcontaining hydrogen-absorbing alloys, the temperature of the alloys go down andhydrogen desorption slows down. This demonstrates the safety of the alloys, theadvantage which is seen neither in high pressure gas nor in liquid hydrogen. However, regardless of this compactness and safety, on the premises that it is expectedto be on-board storage, hydrogen capacity per weight basis is not exactly appropriatefor application. For example, alloys (rare earth as a principal component) used for Ni-MH batteries have merely about 1 mass % of hydrogen storage capacity. In thisrespect, 'WE-NET' project, by Ministry of International Trade and Industry of Japanfor the development of technology of hydrogen energy, established the objective value of 3 mass % for storage capacity.

2.The results achieved by our research group

@@Our group started researches on hydrogen absorbing alloys around in 1972soon after the concept of the alloys was reported, and since then, has pioneered andbeen engaged actively in this research field.
@@In 1993, we proposed a new concept hydrogen-absorbing alloys "Laves phaserelated BCC solid solution." Moreover, we found that this new concept alloys showed approximately twice as much hydrogen capacity as the conventional alloys containing rare earth which had been used as hydrogen-absorbing alloys until then.On the basis of this concept, Toyota Motor Corporation developed hydrogen-absorbingalloy, and eventually manufactured a fuel cell vehicle with this alloy on-board. Thefirst such trial vehicle was manufactured and tested its practicality in 1996.
@@Subsequently, 'Laves phase related BCC solid solution' hydrogen absorbing alloyshave been studied to improve their properties by some universities and privateinstitutes, but neither new concept nor other performance of hydrogen absorption that exceed our research results have been discovered.

3. The latest research result

@@The actual example of 'Laves phase related BCC solid solution' hydrogen-absorbing alloys reported are Ti-V- Mn and Ti-V-Cr based systems. Ti-V-Crbased component alloys are being studied and improved by some of the academicinstitutes and industries widely, and consequently, hydrogen pressure compositionisotherms (P-C isotherms) are obtained as shown in Figure 1. Ti-V-Mn basedcomponent alloys, though, had not drawn the least attention after our proposal, due totheir low hydrogen capacity. Our group, on the other hand, by investigating Ti-V-Mnalloys in detail, discovered a hydride phase which has a new structure. Moreover, thepossibility of achieving higher hydrogen capacity has been suggested by the existenceof the hydride phase.
@@Figure 2 is the P-C isotherms of Ti-V-Mn based alloys. It shows three kinds ofhydride phases within the pressure area which can be measured at ambienttemperature. We named each phase in ascending order of their hydrogen content,BCC phase, deformed FCC phase and FCC phase. The 'deformed FCC' is the newly discovered phase, whereas the conventional BCC alloy presented in P-C isotherms in Figure 1 shows only two kinds of hydride phases. In this case,' deformed FCC' doesnot appear.
@@In general, BCC solid solution alloys take a BCC structure before absorbinghydrogen, and forms an FCC structure when filled with hydrogen. This structurechange is explained in the way that hydrogen penetrates into the BCC structure whichhas large interstice, and then forms the FCC structure with very high density.
@@Figure 3 shows the transformation of the Ti-V-Mn alloys undergoing from a BCC structure to an FCC structure together with the value of c'/a', a ratio of the lattice parameters. The transformation from the BCC structure to the FCC structure corresponds to the change of the c'/a' value approximately from 0.7 to 1. As for the Ti-V-Mn alloys, the structures of hydride phases with lower hydrogen content have a BCC structure (c'/a'=0.71) similar to the alloy, but subsequently forms a deformed FCC structure (c'/a'=0.96). In other words, transformation from BCC alloy¨BCC hydride¨deformed FCC hydride¨FCC hydride takes place.
@@The hydrogen capacity that can be utilized effectively increases due to the existence of these new hydrides . The effect is illustrated in Figure 4. Approximately mere 65% of the hydrogen absorbed has been utilized in conventional BCC alloys. Whereas in Ti-V-Mn alloys, by using the newly discovered hydride, the possibility of utilizing about 80% of the hydrogen absorbed is suggested. When hydrogen is absorbed fully in Ti-V-Mn alloys, entire absorption capacity is approximately 4 mass %. Thus, a theoretical explanation that by applying the result obtained here, hydrogen absorption capacity of 3 mass %, the object value of the 'WE-NET' project, can be achieved by the alloys with the BCC structure.

Acknowledgment

@@A part of this work has been supported by the 'WE-NET' project by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of International Trade and Industry, and a Grant-in-Aid for Scientific Research on Priority Area A of 'New Protium Function in Sub-Nano Lattice Mattersfrom the Ministry of Education, Science, Sports and Culture, Japan.

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Figure 1 Pressure-Composition (P-C) isotherms and the effect of heat treatment on hydrogen absorption in Ti-V-Cr alloys (conventional BCC solid solution alloys)

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Figure 2 P-C isotherms of Ti-V-Mn alloys

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Figure 3 Transformation from BCC structure to FCC structure due to the hydrogenation

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Figure 4 Effect of the increasing effective hydrogen capacity due to the formation of the new hydride phase

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