Absorption desorption isotherms, hydrogen

Figure 2. Static hydrogen absorption/desorption isotherms for HY-STOR Alloy...

The Li-Mg-B-N-H structure possesses storage capacity of more than 10 wt.% at around 150-200 "C. However, the reversibility of the hydrogen sorption characteristics was determined using pressure-composition isotherms as shown in Figure 6. From this study, we found reversible hydrogen absorption-desorption behavior (-3-4 wt.%) of the new complex hydride Li-Mg-B-N-H. The improvement in temperature and reversible hydrogen storage capacity were unaffected even after ten... 

Figure 1. Ideal absorption and desorption isotherm for a metal-hydrogen system (1)...

As mentioned above for 245°C the hydrogen absorption isotherm was obtained (see Fig. 2 and Fig. 5). Comparing the absorption and desorption isotherms one could see that hysteresis of the enthalpy values is absent in practice even for the range of H concentrations 1.8reference data [8, 9] AHdes. > AHabs.. ... 

An example of the experimental set-up used for the measurements is shown in Fig. 18.26. Basically, this set-up is an adaptation of the original Sievert, volumetric technique (Fig. 18.27). It can be used to record experimental isotherms (thermodynamics) and collect kinetic data during permeation or sorption (absorption/desorption) experiments, by sequentially transferring known amounts of hydrogen from (to) the calibrated reference volume chamber (Chj) to (from) the reaction chamber (Ch2). A reservoir... 

Fig. 2. Example of a hysteresis loop on the isotherm pHj = /(H/Pd) obtained during absorption (upper curve) and desorption (lower curve) of hydrogen from palladium black at 100°C. After Sieverts and Danz (19).

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... 

Figure 1. Pressure-composition isotherms for the Dy2Co7-H system. Hydrogen concentration is expressed as atoms of H per formula unit of Dy2Co7. X designates absorption O designates desorption.

C. E. Buckley, E. Mac A. Gray and E. H. Kisi, Stability of the hydrogen absorption and desorption plateaux in LaNis-H Part 1 Hysteresis dynamics and location of the equilibrium isotherm, J. All. Compds. 215, (1994) 195-199. 

Pressure-composition isotherms may be constructed under an isochoric constraint by adding hydrogen aliquots to the system and measuring the hydrogen absorption or desorption stepwise, or under an isobaric constraint by actively controlling the pressure over the sample to a constant value while the sample absorbs or desorbs. 

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