Hydrogen plateau pressures

Hydrogen plateau pressures of LaNis with partial substitution of Ni with element affecting the (a) unit cell volumes and (b) stability of the LaNis [81]. 

Fig. 22. Hydrogen plateau pressures in various series of rare earth compounds.

The enthalpy of the phase conversion can be determined from Eq.(6) by plotting the log of the absorption or desorption plateau pressure, P lnleau, against the reciprocal temperature as indicated in Fig. 2. When the solubility of hydrogen in the metal (or) phase is small, then AHplM AH(, where AH( is essentially the enthalpy of forma... 

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. 

Increasing the hydrogen partial pressure initially causes an acceleration of the overall reaction rate imtil a plateau is reached beyond 7.5 bar. Further increase of the hydrogen partial pressure does not affect the overall reaction rate, which is in accordance with our results obtained in the batch reactor. 

Rozdzynska-Kielbik et al. [210] substituted a fraction of Ni in a LaNi compound by Zn. PCX tests from room temperature to 80°C showed decreasing plateau pressure, while XRD showed an increase of the unit cell volume. The hydrogen capacity slightly decreased with increasing Zn content. 

LaNij was cycled 1,000 times, the plateau pressures at 90, 110, and 130°C were only slightly changed, while a significant degradation of absorption kinetics was observed. The pulverization of the bulk alloys occurred, and the volume mean particle diameter was reduced from -35 pm before to -11 pm after 1,000 cycles. The changes in properties were explained by disproportionation and pulverization phenomena and the effect of impurities in the hydrogen used for charging. 

They found that the amounts of the hydrogen desorbed from the mixtures with = 6, 8, and 12 on a unit mass basis slightly decreased with increasing n ( 5.4, 5.1 and 4.5 wt%, respectively). However, the molar ratios of the desorbed hydrogen to the mixtures were almost equal and the PCT isotherms were similar to each other. The plateau pressure for desorption of the (3Mg(NH2)2 + 12LiH) mixture was equal to 8-10 MPa, 3.5 MPa and 2 MPa at 250°C, 225 and 200°C, respectively. The desorption/absorption PCT curve at 250°C exhibited only very small hysteresis which means that the plateau pressures at this temperature are nearly identical. The Li Mg(NH)2 and LiH phases were observed in XRD profiles of all the mixtures after PCT measurements. These results suggest that the dehydriding reaction of the... 

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. 

Figure 2 also shows a strong temperature dependence for the plateau pressure. This is an important consequence of the heat of reaotion, AH, associated with Eq, 1, Hydrogen absorption (- ) is exothermic and desorption (-<-) endothermic. The plateau pressure is related to the absolute temperature, T, by the familiar Van t Hoff equation ... 

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