Metal-hydride systems

Compared with nickel-cadmium and nickel-metal hydride systems RAM cells exhibit very low self-discharge, making them ideal for intermittent or periodic use without the need to recharge before using, even in hot climates. Figure 6 shows a comparison of the temperature characteristics, for various battery systems in the form of Arrhenius diagrams. 

Zaluska, A., and L. Zaluski, New catalytic complexes for metal hydride systems, /. Alloys Compd., 404-406, 706-711,2005. 

Improvement in metal hydride hydrogen storage has been slow in achieving the targets needed for several applications. But slow, steady progress is foreseen as material processing in the nano-range improves the kinetics, thermodynamics, and capacity of the metal hydride systems so that they become acceptable for some applications perhaps for stationary systems. 

Vajo, J. J. Salguero, T. T. Gross, A. F. Skeith, S. L. Olson, G. L., Thermodynamic destabilization and reaction kinetics in light metal hydride systems. Journal of Alloys and Compounds 2007,446-447,409-414. 

As mentioned above, the safest method for hydrogen storage is the metal hydride system (Wilhamson and Edeskuty, 1986 Veziroglu, 1975). When hydrogen combines with the metal alloy, an exothermic reaction occurs. The gas is then stored in the metal particles until some applied heat releases the hydrogen and builds up pressure in the tanks. Heat is then emitted when a metal hydride absorbs hydrogen. 

The plateau region on the PCX cnrves in Fig. 2.43a is very flat, showing only a minimal slope (Sect. 1.4.1). However, each absorption-desorption pair of the PCX curves clearly exhibits a pressure hysteresis. This means that the pressure needed for absorption (hydride formation), is always greater than that of hydride decomposition, p. The cause of pressure hysteresis in metal hydride systems is not fnlly nnderstood. A number of models that attempted to explain this phenomenon... 

J.J. Vajo, T.T. Salguero, A.F. Gross, S.L. Skeith, G.L. Olsen, Thermodynamic destabilization and reaction kinetics in light metal hydride systems , J. Alloys Compd. 446-447 (2007) 409-414. 

TABLE 7.1. Theoretical Energies of Dihydrogen Bonding and H- H Distances in Some Transition Metal Hydride Systems... 

Table 7.1 lists energies of dihydrogen bonding and the H- H distances that have been calcnlated for transition metal hydride systems in the gas phase. As shown, the transition metal complexes calcnlated have different ligand environments and interact with different proton donors. 

Although the fundamental properties of metal hydride systems are still under intensive study, increased emphasis is being placed on developing the known applications for reversible metal hydrides, while the search for new applications continues. In this paper, we will summarize the current development status of a few selected applications. 

In conclusion, calculations for metal hydride systems have shown evidence of sizeable metal-hydrogen and hydrogen-hydrogen interactions. Stable hydride formation is favored by filling energetically low states. The more of these that are empty in the metal, the better. The added electrons should go into states associated with the hydrogen and not in states associated with the metal (at Ep—high and empty). We also have seen that detailed wavefunction information can be derived from such calculations, which can help put the results on a firmer basis. 

Prototype sealed button cells and larger prismatic cells have been fabricated and studied. As with the nickel-cadmium and nickel-metal hydride systems, an oxygen recombination route is necessary, but the use of membrane separators limits oxygen transport to the negative plate and... 

Besides Li-accumulators also Ni metal hydride systems and metal-air systems (like the zinc-air batteries already described ) come into consideration for electrotraction (Figure 10). Because metal air systems such as the Zn-air system, even though not proper secondary systems (nor are they fuel cells), can be chemically recharged, they are relevant for fleet operators (e.g taxi enterprises). 

There are also articles of generalizing character [4] where attempt to establish the total procedure of creation of systems of transformation of energy in pair metal hydride systems (including in HHP) is done. Alongside with questions of increase of efficiency of hydride devices are considered also questions of optimization of a hydride devices cycle. 

Lundin, C. E. Lynch, F. E. and Snape, E., The Development of Metal Hydrides Systems for Hydrogen Compressor Applications, Ergenics MPD Corp. and Denver Research Institute,... 

A systematic diffraction study was made with both neutrons and x-rays of metal- hydride systems in the composition range of 2 to 66.5 atomic % hydrogen of hafnium, titanium, and zirconium, and a nuclear null-matrix consisting of 62 atomic % titanium and 38 atomic % zirconium, with emphasis on the metal-rich regions. A nuclear null-matrix as defined here consists of two or more types of nuclei in which some of the nuclei scatter thermal neutrons 180° out of phase with others, such that the resultant structure factor is zero. 

Also, to provide completeness to this study a H2 storage medium is needed. It is beyond the scope of this study to evaluate all possible H2 storage systems. The incorporation of a metal hydride system provides one legitimate model for the assessment of downstream H2 distribution systems. By incorporating a MH storage and delivery system in this, two significant issues requiring additional research have been identified in the review process ... 

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