Thermodynamic and Kinetic Properties of La20.5MgNi78.5 and La15.5Mg6Ni78.5 Hydrogen Storage Alloys: The Theoretical Models and Their Verifications

Xue-Hui An,Yuepeng Pang,Jie-Yu Zhang,Kuo-Chih Chou,Qin Li,Qian Li

doi:10.15344/2455-2372/2016/124

Abstract

In this paper, we present a statistic thermodynamic model quantitatively describing the pressurecomposition- isotherms (PCI) curve, which consists of hydrogen storage capacity, temperature, and equilibrium pressure in the hydriding and dehydriding (H/D) reactions, and a theoretical kinetic model to clarify the H/D kinetic mechanism of hydrogen storage alloys. The results of the calculations are well agreed with the experiments performed with La20.5MgNi78.5 and La15.5Mg6Ni78.5 alloys, their maximum hydrogen storage capacities and the hydride formation enthalpies at 303~333 K are 1.41 wt.% H2 and -31.64 kJ/mol H2 for La20.5MgNi78.5 and 1.31 wt.% H2 and -27.23 kJ/mol H2 for La15.5Mg6Ni78.5, respectively. A new diffusion kinetic model is proposed with the consideration of Pilling–Bedworth Ratio, i.e. hydrogen-induced volume change used for studying the hydrogen absorption reaction kinetics. The activation energies are calculated to be 31.25 kJ/mol H2 for La20.5MgNi78.5 and 24.24 kJ/mol H2 for La15.5Mg6Ni78.5, respectively.

Highlights

Due to their potential for practical application, hydrogen storage properties of alloys in the Ni-rich corner of the La–Mg–Ni ternary system were extensively studied, such as CaCu5–type LaNi5 (AB5), PuNi3-type (La,Mg)Ni3, Ce2Ni7-type (La,Mg)2Ni7, and Pr5Co19type (La,Mg)5Ni19 [1,2,3]
On the basis of the more realistic physical assumptions, a model for PCI curves from the view of thermodynamic statistic as well as a kinetic model with the consideration of hydrogen-induced volume change is proposed for describing the hydrogen storage properties
M β nβ ρm ρβ M mnm where η is Pilling–Bedworth Ratio; Mβ and ρβ are the amount and the density of the hydride; Vhydride is the volume of the hydride produced by the reaction; Vunreacted metal is the volume of the unreacted hydrogen storage alloy particles

Summary

Introduction

Due to their potential for practical application, hydrogen storage properties of alloys in the Ni-rich corner of the La–Mg–Ni ternary system were extensively studied, such as CaCu5–type LaNi5 (AB5), PuNi3-type (La,Mg)Ni3, Ce2Ni7-type (La,Mg)2Ni7, and Pr5Co19type (La,Mg)5Ni19 [1,2,3]. On the basis of the more realistic physical assumptions, a model for PCI curves from the view of thermodynamic statistic as well as a kinetic model with the consideration of hydrogen-induced volume change is proposed for describing the hydrogen storage properties.

Results

Conclusion