Mitigation of degradation in the dehydrogenation behavior of air-exposed MgH2 catalyzed with NbF5

This paper addresses the dehydrogenation behavior of MgH2 exposed to air for up to 24 h at room temperature, focusing on the effect of the addition of a small amount of NbF5 and Nb2O5 as a catalytic additive. Although the dehydrogenation reaction of MgH2 with no catalytic additive and Nb2O5 is significantly retarded after air exposure compared with unexposed samples, the degradation in the dehydrogenation behavior of MgH2 with NbF5 due to air exposure is found to be effectively mitigated. Although differential scanning calorimetry of the air-exposed samples indicates that the activation energy for the dehydrogenation reaction significantly increases after air exposure, the activation energy for MgH2 with NbF5 remains the lowest. Scanning electron microscopy with energy dispersive X-ray spectroscopy shows that the amount of oxygen on the surfaces of air-exposed MgH2 particles catalyzed with NbF5 is much lower than that of air-exposed MgH2 with no catalytic additive and Nb2O5. This implies that the addition of NbF5 forms protective layers on the surfaces of MgH2 particles against oxygen and moisture in the air, which seems to be responsible for the positive effect of NbF5 on the degradation of the dehydrogenation behavior of air-exposed MgH2.