Alkali alanates, in particular sodium alanate NaAlH4, combine high, reversible hydrogen capacity (up to 5.6 weight %) with low cost and commonly available components. They operate at relatively low temperatures, i.e. between 60 and 130oC.
At present, sodium alanates represent one of the best combination of hydrogen capacity and temperature of operation, when combined with an appropriate catalyst.
In practice: storage capacities of
- 5% hydrogen content by weight at temperatures above 120oC
- 3% below 100oC
Improvement in kinetics of decomposition caused by catalysis: a. non-catalyzed; b. catalyzed NaAlH4
The most advantageous property of this material is a direct synthesis method starting from NaH and Al - inexpensive and abundant components.
Synthesis of Catalyzed, Modified NaAlH4
NaH + Al + Catalyst + Hydrogen ⇒ NaAlH4
First absorption of the catalyzed mixture of NaH and Al powders
→ complete formation of NaAlH4
First desorption of the hydrogenated, catalyzed mixture of Na and Al
X-ray diffraction patterns of the stages of hydrogenation and dehydrogenation
Example of absorption under 91 bars - more than 3 wt.% is absorbed within first 10 min.
Higher pressure e.g. 120 bars gives complete absorption within 10 min.
Desorption rates at various temperatures
Catalyzed alanates can desorb at temperatures < 100oC which is an important parameter for fuel cell applications.
After 100 cycles the material shows no signs of deterioration
First Reaction: 3 NaAlH4 → Na3AlH6 + 2Al + 3H2
Reversible capacity of 3.0 – 3.2% achieved
Absorption rate < 10 min.
Desorption at 70oC –100oC
Both Reactions NaAlH4 → NaH + Al + 3/2H2
- Reversible capacity of 4.5 % achieved
- Absorption rate < 10 min.
- Desorption at 120oC – 130oC
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