UBC Theses and Dissertations
Studies of hydrogen storage in the sodium borohydride and magnesium nickel hydride composite system Afonso, Louis Greg
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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