| Summary: | Complex metal hydrides typically have high enthalpies which lead to desorption temperatures
that are too high for practical use. Thermodynamic destabilization is one method used
to lower the enthalpy of decomposition and hence lower the temperature of desorption of
a complex metal hydride. A lower temperature (less than 100ºC) would enable waste heat
from a PEM(Polymer Electrolyte Membrane) fuel cell to drive the hydrogen desorption reaction.
NaBH₄ was destabilized by ball milling NaBH₄ and Mg₂NiH₄ in a 4:5 molar ratio,
respectively. Ball milling periods of up to 2 hours did not have an effect on the thermodynamics
or the kinetics of the system. Grain sizes of the two phases, NaBH₄ and Mg₂NiH₄,
were reduced during the first 30 minutes of ball milling. The decomposition enthalpy of the
system was measured and found to be 67 ± 4 kJ mol⁻¹
H₂
for the decomposition of Mg₂NiH₄
in the composite, 76 ± 5 kJ mol⁻¹
for the decomposition of NaBH₄ and 95 ± 7 kJ mol⁻¹
H₂
for the decomposition of NaH, which corresponds to measured desorptions at 275, 360 and
420 ºC respectively. The enthalpy of absorption corresponding to Mg₂NiH₄ in the composite
was 59 ± 4 kJ mol⁻¹
H₂
. During dehydrogenation of the NaBH₄ phase, the ternary
boride phase MgNi₂.₅B₂ is formed under a hydrogen back pressure of vacuum, 1 bar and 5
bar. The total capacity of the system is 5.1 wt%, and a capacity loss of 2.25 wt% hydrogen
was noted during cycling studies partially due to the formation of MgNi₂, which is a nonhydriding
phase, loss of Na from the sample holder, and the formation of large crystals of
Mg that could not be hydrogenated easily. Kinetic analysis was conducted and an activation
energy of 131 ± 24 kJ mol⁻¹ was determined for the decomposition of the Mg₂NiH₄ phase of the composite. XRD phase analysis showed that the Mg₂NiH₄ decomposed first starting
at about 275 ºC, followed by the decomposition of NaBH₄ at around 360 ºC. By 400 ºC,
XRD analysis showed that the MgNi₂.₅B₂ phase had formed. The effect of cycling on the
crystallographic phases showed a change from monoclinic to cubic for the Mg₂NiH₄ phase
of the composite as well as the formation of MgNi₂.
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