Decomposition Kinetics of Lithium Amide and Its Implications for Hydrogen Storage
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Decomposition Kinetics of Lithium Amide and Its Implications for Hydrogen Storage Frederick E. Pinkerton Materials and Processes Laboratory, MC 480-106-224, General Motors Research and Development Center, Warren, MI 48090-9055 ABSTRACT Kinetics of the lithium amide (LiNH2) decomposition reaction 2 LiNH2 Li2NH + NH3 were determined using thermogravimetric analysis (TGA). LiNH2 is a primary component of the hydrided state of Li3N- and Li2NH-based storage materials. Its decomposition by ammonia release, and the resulting degradation of hydrogen storage capacity, has important implications for the durability of Li-N-H storage systems. Fine powders of LiNH2 were prepared by ball milling for 20 min. Kinetic parameters were extracted from a set of TGA weight loss curves taken at different heating rates between 1 and 20ºC/min, and the activation energy Ea was determined to be 124 kJ/mole. Although decomposition occurs slowly below 300ºC, isothermal TGA measurements on Li3N demonstrate that its cumulative effect is large in real Li-N-H systems, where LiNH2-containing hydrided material is held at elevated temperature under dynamic gas flow.
INTRODUCTION The prospect of replacing fossil fuels with clean hydrogen fuel to alleviate pollution and reduce dependence on foreign petroleum has stimulated both the development of highperformance fuel cells for automotive applications and an intense search for new reversible hydride materials for on-vehicle hydrogen storage. Finding a practical means to store a sufficient supply of hydrogen on board the vehicle remains a significant challenge [1]. Recently Chen et al. [2] reported hydrogen storage based on compounds in the Li-N-H system. They found that lithium nitride (Li3N) absorbed hydrogen in two steps: Li3N + 2 H2 ↔ Li2NH + LiH + H2 ↔ LiNH2 + 2 LiH,
(1)
with a total theoretical hydrogen capacity of 10.4 wt%. In 300 kPa H2 gas, the first hydriding step required temperatures between 170ºC and 210ºC , while the second step occurred at about 255ºC. Although both stages were reversible, only the rightmost reaction released H2 gas under practical pressure and temperature conditions. Under dynamic vacuum (1 mPa), lithium amide (LiNH2) and lithium hydride (LiH) recombined to form lithium imide (Li2NH) and released H2 at temperatures near 200ºC. Temperatures above 320ºC, however, were required to regenerate Li3N, even in dynamic vacuum. The practical reversible capacity for reaction (1) is thus 5.2 wt%. Removing the excess LiH from the right-hand reaction in (1) improves the capacity to 6.5 wt% [3,4]: Li2NH + H2 ↔ LiNH2 + LiH.
(2)
The H2 plateau pressure for this system was quite low, ~10 kPa at 240ºC, and reached 100 kPa only at the relatively high temperature of about 280ºC [3]. Ichikawa et al. [4] improved the kinetics of hydrogen release near 200ºC by incorporating TiCl3 into mixed LiNH2 + LiH powders.
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In a competing reaction, LiNH2 decomposes by releasing ammonia [5]: 2 LiNH2 → Li2NH + NH3.
(3)
Reaction (3) has serious consequences for hydrogen storage applicati
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