5.5 Survey on the research and development of technologies for hydrogen transport and storage by metal hydrides

5.5.1 R&D goals

• Mid-term Target:

  To brighten the prospects for developing hydrogen storage alloys such as those of light weight and high discharging rate of hydrogen (for motor vehicles), of compact size (for small storage units) and of light weight and air open type (for large storage units).

• Final Target:

  To develop hydrogen storage alloys practically applicable to hydrogen storage systems for dispersive consumers.

• Specific Targets:

  To develop hydrogen storage alloys with the following
  properties:
  Absorption temperature-room temperature
  Desorption temperature--100ｰC or lower
  Absorption capacity--1.1 wt%
  

  Durability--

  ability to retain 90% of the initial hydrogen absorption capacity after 2,000 cycles of hydrogen absorption and desorption.

5.5.2 Results in fiscal year 1995

(1) Surveys on research of hydrogen storage alloys for hydrogen transport and storage units

1. Production method of Mg-based hydrogen storage alloys
   For the purpose of producing Mg-based hydrogen storage alloys, particular melting methods and casting technologies fit for the relevant alloys are required because the melting point of Mg is low and its vapor pressure is high. A number of Mg-based binary alloys including Ni, Li, Al, Zn, Mn, Ca or rare earth elements were taken up for surveys and reviews over requirements such as equipment, materials, raw materials, procedures, operating conditions and caution items for their respective mass production in a specified chemical composition and homogeneous quality by melting and casting methods. In addition, for the purpose of providing information on adjusting alloying elements, examinations were also made of effects of heat treatment on metallographical conditions and hydriding characteristics of such alloys.

2. Composite hydrogen storage alloys
   Recently studies are on the increase, aimed at modifying hydriding characteristics and improving durability by intentionally changing chemical composition, structure and metallographic structure by means of making compounds of different kinds of metal hydrides such as LaNi₅-based and Mg-based ones. Our survey was focused on the status of research centering around Mg-based alloys and summarized in accordance with the classification by alloying methods such as powder sintering method, mutual diffusion method, disproportionation method, mechanical alloying method and mechanical grinding method. Good results have been reported of such studies in terms of enhancing initial activation and increasing hydrogen absorption capacity, indicating their promising progress in future.
   

3. Surface treatment of hydrogen storage alloys.
   Various surface treatments of hydrogen storage alloys have been tried to improve the function of surface film, which plays two roles of providing a site for reacting with hydrogen and of serving as protective film for the bulk phase. Such surface treatments were examined in terms of methods using acid, alkali and fluoride aqueous solutions. Although effects of such surface treatments on the surface film condition have not been fully made clear yet, hot alkaline treatment has been practically used for LaNi₅-based alloys for secondary batteries. This treatment can also be expected as a promising method to improve initial activation, hydrogen absorption-desorption rates and durability of hydrogen storage alloys.

(2) Analytical method for characteristics of hydrogen absorption and desorption, and present state of research on the characteristics

1. Rate of hydrogenation and dehydrogenation
   Survey and investigation were given on the methods and equipment which accurately determine the rate of hydrogen absorption and desorption of finely powdered hydrogen storage alloys, and on the analytical methods for correctly evaluating the rate. In addition, reaction rate characteristics and the controlling variables of the characteristics were reviewed for LaNi₅, Mg Ni, Ti, or Zr-based alloys. Generally, the rate of hydrogenation and dehydrogenation is controlled by three factors, namely chemical reaction on the surface of alloy, diffusion in the bulk phase, and transfer of reaction heat. Since the rate of supply and removal of reaction heat often controls the rate of hydrogen absorption and desorption, the measurement and analytical evaluation require the consideration for the effect of heat transfer. Accordingly, in manufacturing hydrogen transfer and storage vessels, reaction vessels with high heat exchange capacity must be completed.

2. Life of hydrogen absorption and desorption cycles
   Hydrogen storage alloys deteriorate with increased cycles of hydrogen absorption and desorption, and result in decreased hydrogen storage capacity and change in equilibrium hydrogen dissociation pressure. Status of research was surveyed focusing on the degradation of LaNi₅, TiFe, TiMn_1.5, Mg₂Ni and their multi-alloy system, in terms of behavior, mechanisms, preventive measures, evaluation methods. Factors of degradation are largely divided into the intrinsic factor which is triggered by a change in metallographic structure as a result of the cycles of hydrogenation and dehydrogenation, and the extrinsic factor which originates from the impurities in hydrogen. A major factor of intrinsic degradation is the decomposition of alloy phase via hydrides, and the degradation is accelerated with the increase of temperature and of hydrogen pressure. The extrinsic degradation is caused by the deactivation of surface layer resulted from poisoning and by the progress of reaction with impurities, and the external degradation becomes faster along with the progress of powdering. Multiplication of alloys is effective for the prevention of degradation. For example, LaNi_4.7Al_0.3 is more stable than LaNi₈. Since a durability test requires a long time, established efficient methods for an accelerated life test and for a reasonable durability evaluation are desired.

(3) Case study on applicability of hydrogen storage alloys

1. Hydrogen transport system
   First, the transport technology of hydrogen gas and liquid hydrogen using pipe line and transport means was surveyed. The result showed that the hydrogen transport for dispersed consumers will continue to use containers, trailers, freight trains, and barges in the future, each of which has a liquid hydrogen capacity of from 1 to 1,000 kl (10 wt%) or more. Next, the applicability of hydrogen storage alloys to both the stationary small lot transport and the transfer large-quantity transport. The greatest merit of both the applications of hydrogen storage alloys is safety. The applicability is higher in the former than the latter. Even in the application to the former, however, considerable reduction of weight of the transport vessel is required.

2. Stationary hydrogen storage facilities for dispersive consumers
   For the hydrogen storage for dispersive consumers, liquid hydrogen is suitable in terms of energy density and economy. Liquid hydrogen, however, is difficult in controlling the quantity of vaporization responding to intermittent and variable mode of hydrogen consumption at the equipment. Accordingly, a buffer tank is necessary to add to ensure a stable supply of hydrogen. Thus, a hybrid hydrogen storage system using a combination of liquid hydrogen and hydrogen storage alloy was proposed, where the hydrogen storage alloy functions as a buffer and as an absorber of flash hydrogen and boil-off hydrogen generated during transshipment and storage of liquid hydrogen, respectively. A condominium of 100 houses where electric power and heat are supplied by fuel cells was assumed as an example of application of the storage system and a case study was given to the condominium for the case of liquid hydrogen transport at an interval of 15 days to evaluate the quantity of hydrogen storage and the economy. The estimated hydrogen storage capacity for the liquid hydrogen and the hydrogen storage alloy was 19,000 Nm³ and 2,100 Nm³, respectively. The calculated floor area for the facility as approximately 10 m x 20 m including the area for fuel cells and heat-storage tanks. The estimation revealed that the system showed high applicability in terms of facility space and safety, and a large economical effect of recovery and re-use of flash-hydrogen, and that this recovery of hydrogen alone can repay investments in hydrogen storage alloy unit within 10 years. Analysis of variables affecting the economy was made, and at the same time the target and procedures for the development of applicable alloys were considered.

3. Hydrogen fuel tanks for motor vehicles
   During fiscal 1993 and 1994, the status of test production of a hydrogen-fueled automobile was grasped, and the applicability of MH fuel tanks to motor vehicles was studied in terms of legal regulation, convenience, and economy. During the present fiscal year, we selected a hydrogen-fueled rotary engine automobile of Mazda Motor Corp. which has begun its actual driving performance evaluation tests on public roads. Detail investigation on the automobile was carried out centering on the target of development, development of hydrogen storage alloys and fuel tanks, major specifications of the vehicle, driving performance, and the hydrogen fuel supply system for securing performance and safety, and checking exhaust gas concentration. This hydrogen-fueled automobile is practically applicable enough in terms of driving performance, safety, and exhaust gas emission standards, and it raises no problem of driving on the public roads. The automobile has, however, disadvantages of short running distance per hydrogen charge, and of high cost of a fuel tank compared with ordinary gasoline-fueled vehicles.

   The first practical use of such vehicle is considered to be possibly realized under WE-NET. Nevertheless, for the spread of hydrogen-fueled motor vehicles it is indispensable to develop light-weight and inexpensive hydrogen storage alloys.

4. Treatment of hydrogen gas generated from liquid hydrogen storage - hydrogen combustion turbine power plant
   The applicability of hydrogen storage alloys to the treatment of boil-off hydrogen gas (BOG) and of flash-out hydrogen gas (FOG) generated from a liquid hydrogen tanker and a hydrogen combustion turbine power plant was studied from the points of economy, technology, response, safety, and environmental resources, on the basis of the WE-NET total system flow, compared with other methods. As a result, it was judged that the optimum application is to install hydrogen storage alloy tanks which function as the BOG treatment and the safety measures against leaked liquid hydrogen, based on the storage of about 150,000 Nm³ of FOG which is generated unsteadily during the initial period of transshipment to liquid hydrogen tanks. In this case, the quantity of applied alloys is about 750 tons (1.8 wt% of effective hydrogen storage), and the economical feasibility possibly increases if the alloy price is lowered to 1,000 yen/kg or less.

(4) Survey on development status of hydrogen storage alloys in the USA and Canada

Hawaii Natural Energy Institute:	Hydrogen storage by organic metal media, Solar photo-water decomposition, water electrolysis, Hydrogen production from biomass
Windro University:	Basic study on hydrogen storage alloys such as BCC solid solution alloys
West Chester University:	Reaction rate of AB5 system hydrogen storage alloy
Savannah Rever National Institute:	Basic study on hydrogen-fueled vehicles and on metal hydride heat pumps
Molycorp, Inc.:	Production of rare earth metals

The reports presented to both academic meetings were focused on the material characteristics of hydrogen storage alloys (metallographic structure, hydrogenation characteristics) and on the application to nickel-hydride batteries. General impression in the congresses was that the researchers on hydrogen storage alloys are oriented to basic studies such as development of alloys and analysis of characteristics rather than the development of application systems. In the U.S., however, much interest is taken in hydrogen energy, and for example, researches in hydrogen storage alloys are in progress under the Hydrogen Program of DOE. Therefore, it was thought that if only development environments are established, such as approval of budget and feasibility to practical application, then the relevant R&D in application technology will get more active not only in the field of hydride batteries but also in other fields in the U.S.

5.5.3 Research plan for the fiscal year 1996