Hydrogen equilibrium plateau pressure

In order to fully understand the electrochemical behaviour of AB, hydrides, a knowledge of their chemical properties is required. Van Vucht et al. [25] were the first to prepare LaNi5 hydride and it is arguably the most thoroughly investigated H—storage compound. It reacts rapidly with hydrogen at room temperature at a pressure of several atmospheres above the equilibrium plateau pressure. PC isotherms for this system are shown in Fig. 3. 

Starting from point 1, a small amount of hydrogen goes into solution in the metal phase as the H2 pressure increases. At point 2, the hydriding reaction begins (Eq, l) and H2 is absorbed at nearly constant pressure. This pressure Pp is termed the "plateau pressure" and corresponds to a two-phase mixture of metal. Me, and metal hydride, MeHx. At point 3, the metal has been completely converted to the hydride phase. Further increases in H2 pressure (point h) result in only a small addition of hydrogen in solution in the hydride phase. In principle this curve is reversible. Extraction of H2 from the gas phase results in the dissociation of the hydride phase in an attempt to maintain the equilibrium plateau pressure. 

This is demonstrated in Fig. 3 by DSC measurements of MgH2. A fast cycle leads to a kinetically destabilized MgH2 [9]. By annealing the sample in hydrogen, the equilibrium state is reached. Accordingly, the onset temperatures defining the equilibrium plateau pressures differ significantly. 

Also, in case of the Pd substitution for Ni atoms, the weak Pd-H interaction shown in Fig.3(a) will give an additional effect on the increasing hydrogen pressure, as explained earlier. In fact, the agreement is further improved by taking into account the M-H interaction. Here, the equilibrium plateau pressure of hydrogen is assumed to be expressed as,... 

The hydrogen atom is found to make a strong chemical bond with the Ni atoms rather than the La or the Mg atoms in pure LaNi and in pure Mg Ni hydride. For LaNij system, the La(M)-Ni and Ni(M)-Ni bond strengths in the small octahedron containing a hydrogen atom play the most dominant role to determine the hydride stability, where M is the alloying element substituted for the Ni or the La atoms in the compound. In fact, the ratio of the La(M)-Ni bond order to the Ni(M)-Ni bond order correlates well with the experimental data of the equilibrium plateau pressures... 

In case of the ZrMn, system (21), as shown in Fig.8(b), a hydrogen atom occupies the central site of a small tetrahedron and interacts strongly with Mn atoms. In this case, the ratios of the Zr-Mn bond order to the Mn-Mn bond order are calculated and the results are shown in Fig.8(b). It is evident that the calculated bond order ratios correlate well with the experimental results of the equilibrium plateau pressure of hydrogen for ZrMn, jM, alloys. 

Therefore, the hydrogen pressure as a function of the hydrogen concentration under the isothermal condition is kept at constant pressure in the PCI measurement, which is generally called the equilibrium (plateau-) pressure Peq. From the Peq obtained at various temperatures. 

Thermodynamically, one would expect a higher hydrogen content, since the equilibrium Y-YH2 plateau pressure is >10 Pa at room temperature [65]. At room temperature the polycrystalline films all tend to have a [0001]hcp or [lll]fcc texture, that is the closed-packed layers are oriented parallel to the substrate plane. 

The invariant plateau pressure is the equilibrium absorption (dissociation) pressure of the hydride at the temperature of the isotherm. Reaction (2) is complete at point x and the hydrogen pressure again increases as the nonstoichiometric hydride absorbs hydrogen according to equation (3) ... 

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