(720f) Kinetic Mechanisms of Hydrogen Sorption in Nanocrystalline Magnesium (hydride) Conference: AIChE Annual MeetingYear: 2015Proceeding: 2015 AIChE Annual MeetingGroup: Transport and Energy ProcessesSession: Advances in Hydrogen Production and Storage Time: Thursday, November 12, 2015 - 4:40pm-4:57pm Authors: Shriniwasan, S., Indian Institute of Technology Bombay Gor, N., Indian Institute of Technology Bombay Tien, H. Y., University of Florida, FL, USA Tanniru, M., University of Florida Ebrahimi, F., University of Florida Tatiparti, S. S., Indian Institute of Technology Bombay Abstract Magnesium (hydride) is a very promising material for hydrogen storage with very high gravimetric capacity (7.6 wt.%). However, its slow kinetics of hydrogen absorption/desorption (sorption) hinders its application. Strategies such as particle size reduction, catalyst addition are employed to improve the kinetics of sorption. To device the strategies for improving the kinetics, it is essential to unravel the mechanisms of hydrogen sorption. The kinetics of hydrogen sorption in ball milled nanocrystalline magnesium (hydride) powders are investigated using the popular Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation (α = 1?exp(?ktn)) at different temperatures (180 °C-400 °C). The exponent n indicates the dimensionality of the growth of Mg (during desorption) or MgH2 (during absorption). n decreases with time from a high value (>1) to a negligible value (<0.5) suggesting that the growth of the phase (Mg or MgH2) decreases with time. The cross-sectional observation of these powders using SEM and TEM revealed that the depth of the growing phase from the particle surface increases with time rendering a core-shell structure during sorption. The estimated hydride/metal interface velocity (U) for growth of these phases shows two clear regimes of high and low velocities in the n>0.5 and n<0.5 regimes respectively. The former velocity is at least 2 orders of magnitude higher than the latter. The diffusion coefficients (D) are also estimated using Fick's 2nd law (transient) for hydrogen diffusion through the magnesium hydride phase. The estimated diffusion coefficients are at least 4 orders of magnitude higher in the n>0.5 regime than in those in the n<0.5 regime. Moreover, the diffusion coefficients in the n<0.5 regime are closer to the expected values from literature. The estimated activation energy for hydrogen diffusion through hydride is 91 kJ/mol H and is close to the literature reported value of 100±10 kJ/mol H. The combined n-U-D analysis supported by the microstructural examinations suggests that the growth of the magnesium or hydride is governed by hydride/metal interfacial movement in the n>0.5 regime and by hydrogen diffusion through hydride in the n<0.5 regime. The present analysis distinguishes growth by the interfacial velocity from that by diffusion during hydrogen sorption in magnesium (hydride). The present analysis contributes to the advancement of the understanding on the kinetic mechanisms of hydrogen sorption of MgH2.