Pressure, plateau

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. 

Cycle-life plots for the La, tCexB5 electrodes are illustrated in Fig. 11. The decreased charge capacity in all La, tCerB5 alloys with x > 0.35 conforms with the shorter and higher plateau pressure of the isotherms depicted in Fig. 10. The extremely low electrochemical capacity of CeB, is a consequence of the... 

This behavior can be explained if we resort to the imaginary desorption PCT cnrve at 275°C in Fig. 1.33. The eqnilibrinm plateau pressure at 275°C is higher than 0.1 MPa at which desorption is carried ont and at this temperature MgH can desorb at atmospheric pressnre. However, the kinetics of desorption will depend on the driving force (as shown in Fig. 1.26). Since a larger mass of hydride will produce... 

Fig. 2.11 (a) PCX desorption curves at various temperatures for the activated commercial MgH Tego Magnan powder numbers indicate the average mid-plateau pressure, (b) The Van t Hoff plot for finding the enthalpy and entropy of decomposition, which is equal to -71 kJ/mol and-134 J/ mol K, respectively. Note excellent coefficient of fit = 0.991 (p - pressure)... 

As shown in Fig. 2.43b, the enthalpy of absorption and desorption calculated from the Van t Hoff plots using the mid-plateau pressures of PCT curves in Fig. 2.43a, which are listed in Table 2.18, is equal to -72 and 83 kJ/mol, respectively. The value of entropy is 138 and 151 J/mol K for absorption and desorption, respectively. The enthalpy value for absorption is very close to the values found in the literature for MgHj as discussed in Sect. 2.1.2 and 2.1.3. Surprisingly, however, the enthalpy of desorption at 83 kJ/mol is much greater than the former and also greater than the enthalpy of desorption of the as-received and activated MgH as shown in Fig. 2.11. The coefficients of fit are excellent and give good credibility to the obtained values. 

For the sake of clarity, it must be mentioned that Huot et al. [24, 35] reported that at 350°C the absorption/desorption PCT plateau pressure hysteresis of the unmilled MgH was quite substantial, while the hysteresis of the same material milled for 20 h was very small. They argued that the plateau pressure difference observed for the uiimilled material was due to a very slow desorption kinetics which did not allow reaching equilibrium. In turn, ball milling increased desorption kinetics, which allowed reaching equilibrium and eliminated hysteresis. However, it must be noticed that the situation reported by Huot et al. is completely opposite to what is observed in Fig. 2.43, in which MgH synthesized by reactive mechanical milling shows a pressure hysteresis. 

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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