WE-NET Home Page/Summary of Annual Reports 1998

5.5 Survey on the Research and Development of Technologies for Hydrogen Transport and Storage by Hydrogen Absorbing Alloys

5.5.1 R&D Goals

In order to settle global issues of energy and environment, the WE-NET project is promoted with the aim of realizing international clean energy network vision which does not depend on fossil energy at all. In addition to such long-term development of global, large-scale, effective utilization of hydrogen energy, short and medium-term development targets are set for incremental introduction of hydrogen energy to realize its use in society early. It has become an important challenge to establish elemental technology to achieve those targets.

In order to realize early introduction of hydrogen energy, it is essential to establish technology for decentralized transport of hydrogen to meet various needs of consumers in society. Particularly, development of sophisticated hydrogen absorbing alloys is required to realize hydrogen car.

In this context, following development targets are set for Subtask 5-5 (development of hydrogen absorbing alloys for decentralized transport and storage.)

Rechargeable hydrogen storage capacity : more than 3wt%

Temperature for hydrogen desorption : lower than 100°C

Duarability : hydrogen storage capacity being 90% and above of the initial capacity after 5000-cycle use

These development targets are in view of its application to the 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.

5.5.2 Results in FY 1998

5.5.2.1 Development of Amorphous Nanocrystal Structure Mg-Type Alloy

(1) Mg-type alloy has been considered to have the potential of achieving those development targets. Although crystalline Mg-type alloy has a disadvantage that it absorbs or desorbs hydrogen only at high temperature, amorphous MgNi_0.88Cr_0.03 alloy desorbs 0.4wt% of hydrogen (0.01 - 1MPa) at low temperature of 423K. Therefore, we experimentally examined hydrogen desorption mechanism of this type of alloy at low temperature through structure observance and thermal analysis. At the same time, we evaluated impact of the ratio of Mg to Ng to hydrogen desorption characteristics with the aim of increasing amount of hydrogen desorption. The results are as follows.

Desorption mechanism of MgNii_0.88M1_0.03 (M1=Cr, Fe, Mn, Co) alloy at low temperature is regarded as below.
1. In case of replacing M1 with Cr, energy distribution of hydrogen site is wider. Such a wide distribution enables hydrogen desorption at low temperature (423K.)
2. As for alloy whose M1 is replaced with Co or Mn, it does not desorb hydrogen at low temperature because of its sharp energy distribution.
3. In case of replacing M1 with Fe, energy distribution is between the case of Cr replacement and Co, Mn replacement.
It is presumed that the difference in energy distribution depends on amorphous forming ability.
With the increase in the ratio of Mg to Ni, absorption amount increased, however, desorption amount at low temperature decreased.
These results show that it is important to choose best replacing element for improving amorphous forming ability of Mg-type alloy and widen energy distribution of hydrogen site so as to design Mg-type alloy which can desorb hydrogen at low temperature.

(2) As another alloy being expected to be high capacitance alloy, the possibility of making V-type alloy high capacitate was being studied.

Among BCC-type alloys whose elemental composition is TiCrV, we could obtain an alloy with 2.2wt% of total absorption, 0.1MPa to 1MPa of pressure range and 1.9wt% of hydrogen desorption at 313k.

5.5.2.2 Development of Ternary Alloy of Mg-Ca-Ni

In 1997, impact of ternary addition (replacement) and amorphous materialization were studied with Mg-type hydrogen absorbing alloy (Mg₂Ni intermetallic compound) as initial material. Results of thermal analysis have shown that Mg_1.9Ca_0.1Ni_1.9 alloy absorbs or desorbs 2.1wt% of hydrogen and that desorption starts at around 125°C.

In 1998, we developed Mg-Ca-Ni₃ type alloy with wider compositional scope based on the results of 1997 study, assessed characteristics of hydrogen absorption and desorption, and explored unknown Mg-Ca-Ni₃ type intermetallic compound. Major results were as follows.

As a result of hydrogen absorption at 100°C and 2MPa , Mg_1.925 C_0.075Ni_1.33 absorbed the largest amount (2.5wt%) of hydrogen,.
As for hydrogen desorption, equilibrium pressure was less than 1.0x 10-3MPa of measuring limit. Hydrogen desorption was confirmed at 140°C.
At that time, the sample remains to be amorphous, the same as the time of absorption,
PCT survey was conducted at 200°C. In the 5 cycle measurement, amount of hydrogen absorption and desorption declined cycle by cycle. At this time, hydride phase of crystalline Mg₂NiH₄ and CaH₂ precipitated due to disproportionation.
When heat treatment was conducted at the temperature lower than crystallizing temperature, composite construction of three-phase ( Mg₂Ni, MgNi₂, Mg₂Ca ) was confirmed.

5.5.2.3 Development of New Ternary Intermetallic Compound

In order to develop sophisticated hydrogen absorbing alloys, it is considered necessary to make a breakthrough by discovering new material with new crystalline structure. Therefore, we explored new alloy of AB₂C₉ type, close to AB₃ type which does not belong to traditional AB₂ type and AB₅ type.

AB₂C₉ type alloy of RMGxMy (R alkali earth element, rare earth element, M: transition element) was reported to have been synthesized experimentally. It is possible for this material to change the amount of Mg.

The material constitutes Laves phase R₂M₂ block and CaCu₅type RM₅ block stacking. It is thought that replacing A with B partially can make a series of new alloys such as A_1-xB_2-yC₉. It is also expected to have a new effect by replacing Cu with Al or Mg, whose atomic radius are different from that of Cu. Based on these theories, we aimed to develop new hydrogen absorbing alloys, which are low-cost and lightweight.

We used binary alloy as raw material and took solid reactant sintering process to experimentally synthesize and manufacture new ternary alloy and quarternary alloy of transition metal / alkali-earth metal / rare earth metal. Then, we studied structure and characteristics of those alloys and found that CaMg₂Ni₉ has 1.9% of hydrogen storage capacity and characteristics of absorbing and desorbing hydrogen reversibly at 0°C and a pressure of 1-10.

We will analyze crystalline structure of various synthesized materials and further study characteristics of absorbing and desorbing hydrogen from now on.

5.5.2.4 New Materials

Among new materials with reversible heterogeneous reaction that caught our attention in the study of 1997, we focused on NaAl hydride and made more specific research on it in 1998.

Although NaAlH₄ has 7.4wt% of hydrogen storage capacity, it has been regarded as difficult to be developed as hydrogen absorbing material because of high hydrogen desorption temperature, difficulty in hydrogenation and arduousness in repeating hydrogen absorption and desorption. However, since Bogdanovic and Schwickardi reported in 1997 that doping by titanium compound would facilitate hydrogenation and enable repetition of absorption and desorption, there is increasing expectation for its development.

NaAlH₄ will desorb 190°C, 3.6wt% of hydrogen by the reaction to Na₃AlH₆, then desorb 230°C, 1.8wt% of hydrogen by the reaction to NaH. Therefore, it may be possible to clear the target hydrogen amount of WE-NET in the first phase reaction. C. Jensen experimentally improved process of doping by titanium compound to lower the temperature at which hydrogen begins desorbing to around 100°C, and experimentally demonstrated that it is possible to desorb 4.5-5wt% of hydrogen by raising temperature from around 100°C to around 200°C.

In 1999, we will launch research and development of specific material based on the results.

5.5.2.5 Survey on New Technology

In 1998, a WG of top class researchers in universities was established to explore and discuss new technology. The WG has aimed to have research and proposals based on original ideas of the researchers, which is different from traditional research and study only compiling bibliographies and information.

In this year, 11 proposals for research and development were submitted to explore innovative new material. We will start real R&D activity in 1999 by analyzing and consolidating those proposals to seek for a breakthrough.

5.5.3 Research Plan for FY of 1999

(1) We will promote research of Mg type alloys explored in the first phase earlier and see development result of it.

(2) We will start exploration and R&D of new sophisticated alloys except for Mg type alloys.

(3) We will intensively investigate new compound and material which should be noted in 1999 and later, accurately grasp the trend and reality of new material, and determine the direction of WE-NET R & D.

(4) We will materialize cooperation system of related universities, laboratories and academies to find and foster germinating technology with potential as well as to create unknown, sophisticated new material