Influence of transient operating conditions on pressure-concentration isotherms and storage characteristics of hydriding alloys

Pressure-concentration isotherms (PCI) are measured by both static and dynamic methods for mischmetal based alloys, MmNi3.9Co0.6Al0.5MmNi3.9Co0.6Al0.5 and MmNi4AlMmNi4Al. The effects of hydrogen flow rate on plateau pressure, enthalpy of formation and entropy of formation are studied. For MmNi3.9Co0.6Al0.5MmNi3.9Co0.6Al0.5 both shape of the PCI and plateau pressure are dependent on the flow rate. The effect on plateau pressure is not significant in the case of MmNi4AlMmNi4Al. Both materials showed significant variations in enthalpy of formation with flow rate due to variation in reaction rate. The effect of flow rate on desorption isotherm is negligible as the desorption equilibrium pressure is much lower compared to absorption equilibrium pressure due to large hysteresis in these materials. While the basic nature of variation of pressure with concentration is same for both static and dynamic PCIs, the thermodynamic properties estimated based on these data vary significantly.