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Ergenics Solid State Hydrogen Energy Solutions |
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How Hydrides Work Metal hydrides begin as intermetallic compounds produced in much the same way as any other metal alloy. They exhibit one important difference. When exposed to hydrogen at certain pressures and temperatures, they absorb large quantities of the gas and form metal hydride compounds. When this happens, hydrogen is distributed compactly throughout the metal lattice. Metal hydrides represent an exciting method for storing hydrogen. They are inherently safer than compressed gas or liquid hydrogen and have a higher volumetric hydrogen storage capacity. Some hydrides can actually store twice the amount of hydrogen than can be stored in the same volume of liquid hydrogen. Many metals will form metal hydrides. The key to practical use of metal hydrides is their ability to both absorb and release the same quantity of hydrogen many times without deterioration. In chemical shorthand, a typical reaction can be expressed as: |
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where M represents the metal and H is hydrogen. The reaction is reversible and its direction is determined by the pressure of the hydrogen gas. If the pressure is above a certain level (the equilibrium pressure), the reaction proceeds to the right to form a metal hydride; if below the equilibrium pressure, hydrogen is liberated and the metal returns to its original state. The equilibrium pressure, itself, depends upon temperature; it increases with increasing temperature and vice versa. The hydrogen sorbing behavior of metal hydride alloys is characterized using equilibrium pressure-temperature-composition (PTC) data. This data is determined by keeping an alloy sample at constant temperature while precisely measuring the quantity of hydrogen sorbed and the pressure at which sorption occurs. The quantity of hydrogen sorbed is expressed in terms of alloy composition, either as an atomic ratio of hydrogen atoms to the number of atoms in the base metal alloy, or as the capacity of hydrogen in the alloy on a weight percent basis. As can be seen from Figure 1, most of the hydrogen is absorbed in a region where there is little pressure change. This region of near-constant pressure is known as the plateau pressure. Metal hydride formation is also accompanied by hysteresis, which appears as the difference between the upper absorption curve and the lower desorption curve in Figure 1. |
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Figure 1 - Pressure-Temperature-Composition Absorption and Desorption Isotherms for Ergenics' Hy-Stor® 207 alloy at 25°C. When filled to capacity, Hy-Stor 207 alloy (LaNi4.7Al0.3) holds 1 hydrogen atom for each metal atom to become LaNi4.7Al0.3H6 when fully hydrided. |
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M + H2 MH2 |