Decomposition kinetics of lithium amide for hydrogen storage materials

Abstract

The kinetic behavior of LiNH 2 decomposition by ammonia release was quantified using thermogravimetric analysis (TGA). While not itself a hydrogen storage material, LiNH 2 is a primary component of the hydrided state in Li–N–H storage materials based on Li 3N or Li 2NH. Its decomposition by ammonia release, and consequent degradation of the hydrogen storage capacity, has important implications for the durability of Li–N–H storage systems. LiNH 2 from two commercial lots and one batch prepared at GM R&D were ball milled for 10 or 20 min to obtain fine LiNH 2 powders. Kinetic parameters were extracted from sets of TGA weight loss curves taken at different heating rates. The activation energy for the decomposition reaction was determined to be about 128 kJ/mole, virtually independent of the source and purity of the LiNH 2, its stoichiometry, ball milling time, and TGA sample size. The reaction rate was found to depend on the sample size as a consequence of the very low NH 3–LiNH 2 equilibrium vapor pressure at temperatures below 300 °C. Larger samples produce a local concentration of NH 3 high enough to inhibit further reaction. Direct isothermal measurements of the initial reaction rates at temperatures between 200 and 300 °C agree well with the values calculated from the heating rate-derived kinetic parameters. The durability of Li–N–H for hydrogen storage was estimated from the measured kinetic parameters by calculating the time required to decompose 20% of the initial LiNH 2, as a function of the operating temperature. The predicted lifetime falls below 10 5 min for operating temperatures in excess of ∼160 °C.