5.5 Survey on the research and development of technologies for hydrogen transportand storage by hydrogen absorbing alloys

5.5.1 R&D Goals

Subtask 5-5 aims at developing technologies of hydrogen transport and storage for localized consumers by using hydrogen absorbing alloys appropriate for the World Energy Network System using Hydrogen (WE-NET).

The wide use of hydrogen, which is produced by renewable energies, as an energy medium in society requires the accelerated introduction of the decentralized use of hydrogen in social life in addition to development of the transportation, storage and use of liquid hydrogen for the large-scale centralized use of hydrogen energy. In order to realize the technology for the decentralized use of hydrogen, the development of technologies to transport and store small amount of hydrogen in various public welfare is crucial to meet the diverse consumer demands for hydrogen energy in various aspects of social life. Particularly important is the development of high performance hydrogen absorbing alloys to achieve high density hydrogen storage for hydrogen-powered automobiles.

Because of the importance of hydrogen absorbing alloys, the following development targets have been established.
- Effective hydrogen absorbing capacity : 3 wt% or higher
- Hydrogen desorption temperature : 100°C or lower
- Hydrogen absorbing capacity after 5,000 cycles : 90% or higher of initial capacity

5.5.2 Results in Fiscal 1997

5.5.2.1 Concept of Intended Research and Development Work for Mg alloys

Based on the past R & D results, the work in fiscal 1996 examined the feasibility of various technologies to establish development guidelines for hydrogen absorbing alloys which would meet the above targets. For fiscal 1997, the following development concept and prospect were adopted.

The Mg-based hydrogen absorbing alloys can be crushed to fine crystal fragments and the structures of the alloys are changed to improve their hydrogen storage performance by grinding mechanically with a ball mill.

  1. The use of the mechanical grinding with a ball mill makes it possible to produce a new amorphous alloy phase with equal proportion of Mg and Ni. This new phase has an almost identical hydrogen reaction temperature to the existing Mg2Ni and has a hydrogen absorbing capacity of approximately 2 wt%.

  2. It is hoped that the addition of a third element to this newly discovered amorphous Mg-Ni phase will further change the structure of the alloy and increase the amount of absorbed hydrogen.

  3. It will be further hoped that by substituting Mg and Ni in this amorphous phase for other elements new phases or composite structures with more improved performances may be created
In fiscal 1998, WE-NET is focusing its R & D efforts on Mg-based alloys which have a potentially high hydrogen absorbing capacity. At present, stages 1) and 2) have been confirmed and development efforts are in progress to verify stages 3) and 4).

5.5.2.2 Amorphization by Mechanical Alloying Method

R & D work is in progress to increase the mass of absorbed hydrogen and to lower the reaction temperature using the recently discovered Mg-Ni-base amorphous alloys. MgNi0.9 X0.1 alloys (X signifies a transition metal) have been produced by the ball mill method which substitutes part of the Ni in MgNi amorphous alloys with another metal. It has been found that the hydrogen desorption temperature lowers to 150°C in the case of alloys where part of the Ni is substituted by Cr or Fe. This result implies that the hydrogen desorption temperature can be lowered as substitution of the element changes the distribution of the binding energy of the hydrogen and metal atoms. Further work is in progress, mainly with MgNi0.97 (Cr, Fe, Mn, CO)0.03-based alloys to analyze the mechanism by which the hydrogen desorption temperature is lowered.

Moreover, the relationship between the alloy structure and hydrogen desorption characteristics has been analyzed by means of differential scanning calorific value analysis and hydrogen quantity analysis. It has been found that the distribution of the potential energy of the interstitial hydrogen atoms spreads more broadly in the amorphous nano-chrystalline structure and that hydrogen desorption at a lower temperature becomes easier in such a phase.

5.5.2.3 Development of New Composite Hydrogen Absorbing Alloys and New Production Methods of them

The processing of a mixture of Mg2Ni and powdered metal by a ball mill often fails to produce excellent Mg-Ni-based alloys. Because of this, efforts are in progress to develop a method to use Mg2(Ni, X) alloys as raw materials and to process their mixture with powdered Ni by a ball mill with a view to producing alloys with wide ranging compositions as well as more homogeneous quality.

Amorphous alloys with homogeneous quality have so far been successfully produced with X as a rare earth element, calcium, silicon or iron. When the substituting element X is calcium, the research results suggest that the temperature for commencement of the hydrogen desorption process may be 125°C with a level of hydrogen desorption reaching 2 wt%.

In the coming years, the work will proceed with a further search for new hydrogen absorbing alloys with multiple compositions and structures by enlarging the application scope of this new production method and also with detailed evaluation of the characteristics of new alloys.

5.5.2.4 Amorphization by Rapid Cooling Method

A method of producing amorphous alloys by means of a process suitable for mass production other than the ball mill method was investigated. A number of Mg-Ni alloys with different proportions of Mg and Ni were produced using the dissolving method. Using the molten of these alloys as raw materials, rapidly cooled alloys were produced using the melting and spinning method and their composition, structure and hydrogen absorbing performance were evaluated. As a result, the prospect of obtaining Mg-Ni alloys with varying proportions of Mg and Ni while suppressing the generation of NgNi2 which would adversely affect the hydrogen absorption was established. In the coming years, it is planned to further produce a Mg-Ni-M ternary amorphous phase with various compositions and new experimental alloys with a compound structure of such minute composition as Mg2Ni and to apply the melting and spinning method on a larger scale.

5.5.2.5 Search for Hydrogen Absorbing New Multi-System Alloys using Reaction Sintering Method

In addition to the research efforts described in 1.2 through 1.4 above, joint research with the National Research Institute is in progress to search for new advanced materials based on an entirely different concept.

As both Mg and rare earth metals have a large hydrogen absorption capacity, New hydrogen absorbing alloys of ternary or more systems based on these two kinds of elements have been explored. Single crystals of several new promising alloys have so far been successfully synthesized using the reaction sintering method.

The next stage will be analysis of the crystal structure of these synthesized alloys and their hydrides and evaluation of their hydrogen absorption/desorption characteristics.

5.5.2.6 Study on Latest Research Trends of Hydrogen Absorbing Alloys

The latest technical research trends have been studied reviewing some 200 papers published in Japan and abroad in the last three years, focusing on four categories, i.e. "nano-scale ultra fine structure", "amorphous structure", "composites" and "new materials". The main results are summarized below.
  1. By creating a nano-scale ultra fine structure, the amount of absorbed hydrogen and the hydrogen absorption/desorption temperature can be improved for such metals with a large hydrogenation enthalpy as Mg2Ni.

  2. An amorphous structure can lower the hydrogen desorption temperature for ionic binding-type or covalent binding-type compounds, such as Mg2Ni and Mg. There are reports that the amount of hydrogen absorption increases with solid solution-type alloys (Ni-Zr-based).

  3. The hydrogen desorption characteristic is improved with such micro composites as La2Mg17/LaNi5 and Mg/TiFe1.2. By a nano-scale composite of Mg2Ni/LaNi5 the hydrogen desorption temperature is lowered while the hydrogen absorbing amount is sustained. Both Ti-Mn-V alloys with the composite structure of solid solution BCC phase / Laves phase and solid solution BCC alloys with a nano-order spinodal decomposition structures are some actual examples of composite structure materials.

  4. Among new materials, some complex hydrides capable of reversible disproportionation show promising characteristics.
- Y5Mg22.5Ni1.5 -> YH2 + d + MgH2 + Mg2NiH4 < -> YH2 + Mg + Mg 2Ni +H2
:230°C / 3.7wt% desorption
- NaAIH4 <-> Na3AIH6 + AI + H2 <-> NaH + AI + H2
:210°C / 3~4wt% desorption

5.5.3 Research plan for fiscal year 1998

(1) In fiscal 1998, which is the final year of Phase I, the research work which has begun to produce positive results in controlling the compositions and structures of new Mg-based alloys will be further intensified with a view to developing new alloys with a high hydrogen absorption capacity and low absorption/desorption temperatures.

(2) New research work will commence to analyze the controlling factors of the hydrogen absorption/desorption temperatures as well as absorption/desorption quantities with non-Mg-based alloys and to establish design guidelines for new alloys as preparatory steps for the R&D in Phase II.

(3) Research will be conducted on reversible disproportionation-type new materials.

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