Hydrogen equilibrium pressure

Figure 7.12 Dependence of the hydrogen equilibrium pressure on the unit-cell volume of various LnNij-type compounds (Ln = rare earth). Open circles, LnCoj closed circles, LnNij open triangles, LaCo, (Following Buschow van...

For hydrogen equilibrium pressures below 100 Torr, an absorption technique was used. Thoroughly activated samples were outgassed at 500° C to a vacuum... 

The R(Ni,Fe,Al)5 hydrogen storage alloy prepared on the base of the commercial cerium ligature (R = Ce, La, Pr, Nd), lanthanum and nickel (both of technical purity grade) has been used in the unit. The composition of the alloy must provide a hydrogen equilibrium pressure of 10 bar over MH at room temperature and above 150 bar at elevated temperatures. 

At a given temperature T, this reaction has a hydrogen equilibrium pressure H. Consider now a solid solution of this metal with a non-hydride forming metal (B). The relative partial molar free energy of A in the alloy is AGa- If the resulting alloy AB reacts with hydrogen as... 

H/Pd atomic ratio (n) for a-phase equals 0.008, and the minimal n for P-phasc equals 0.607. The interval between these n values corresponds to the biphase field in which the increasing in n does not cause an increase in the hydrogen equilibrium pressure. With an increasing temperature, hydrogen solubility in palladium decreases and the biphase field cuts down. The mutual transformations of the two phases create strains in the material and may result in a splitting of the membrane. 

Caution. When stored in sealed containers, the hydrides must not be heated to a temperature at which their hydrogen equilibrium pressures exceed the rupture strength of the container. 

Self-discharge in nickel/metal hydride batteries is based on hydrogen oxidation as has been described for nickel/hydrogen batteries above. But, due to the low hydrogen equilibrium pressure, the rate of self-discharge is reduced so far that it only attains rates known for nickel/cadmium batteries (< 20% per month). 

Less noble metals are also able to store considerable amounts of hydrogen, and a great variety of elemental hydrides exists. But little practical use was possible, since only a few offer a reasonable hydrogen equilibrium pressure at room temperature. This situation has changed since intermetallic compounds have been developed that combine strong-hydride forming elements with those that form weak hydrides. An appropriate ratio between both components can be used to tailor metal hydrides of the desired decomposition pressure. 

In the equilibrium state, the hydrogen partial pressure P(H2) on the electrode can be equal to the hydrogen equilibrium pressure Peq(H) of the MH as follows ... 

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