Mg-C Interaction Induced Hydrogen Uptake and Enhanced Hydrogen Release Kinetics in MgH2-rGO Nanocomposites

Hydrogen uptake at 250 degrees C, P-H2 > 15 bar and release at 320, 350 degrees C by MgH(2 )mixed with 10 wt % rGO alleviates the incubation period (slow kinetics) encountered during hydrogen release by pure MgH2. Ball milling establishes Mg-C interactions (similar to 283 eV) in these nano-composites through electron-transfer from Mg to pi* of C and weakens the C-C pi bond. These Mg-C interactions persist in the nanocomposites upon subsequent hydrogen uptake and release. These interactions change the hybridization of C from sp(2) to sp(3), aiding hydrogen uptake by C (C-H). On hydrogen release, H releases from C-H, and electrons are donated back from C to Mg. This electron back-donation weakens the Mg-H bond and enhances hydrogen release from MgH2. The persistent Mg-C interactions are crucial for alleviating the incubation period. For the present study, X-ray diffraction, Raman, X-ray photoelectron spectroscopy (C-1s core level, valence band), and Fourier transform infrared spectroscopy are used

On the parameters of Johnson-Mehl-Avrami-Kolmogorov equation for the hydride growth mechanisms: A case of MgH2

Kinetic parameter (k) and growth dimensionality (n) of Johnson-Mehl-Avrami-Kolmogorov equation are sensitive to phenomena controlling magnesium hydrogenation (210 degrees C, P-H2 = 1 MPa). Interfacial movement followed by H-atom diffusion through hydride controls hydride growth. During interfacial growth, k varies negligibly unlike n(> 0.50). Interfacial-to-diffusional transition is characterized by significantly varying k and negligibly varying n(< 0.50). Diffusional growth renders k and n(< 0.50) almost constant. Combined k - n analysis, supported by other kinetic and geometric parameters, can identify hydride growth mechanisms. (c) 2017 Published by Elsevier B.V

The dehydrogenation mechanism during the incubation period in nanocrystalline MgH2

The dehydrogenation mechanism during the incubation period in nanocrystalline MgH2 (low alpha: converted metal fraction and d alpha/dt) and the reasons for the occurrence of the incubation period at 320, 350, and 400 degrees C were investigated. Pre-existing Mg crystallites can enhance Mg nucleation during the incubation period, as suggested by the estimated activation energy for nucleation (12 +/- 2 kJ per mol H). The released H-atoms enter MgH2 as interstitials, as indicated by the MgH2 unit-cell contraction, resulting in increased equatorial Mg-H bond length, decreased charge-density distribution in the interstitial region, as observed from the charge-density maps, and decreased H-H distance in the {001} plane up to the midway of the incubation period. Eventually, hydrogen vacancies are created, as indicated by the red shift in the E-g and A(lg) peaks of Raman spectra. The high estimated activation energy for the growth of Mg (209 +/- 8 kJ per mol H) renders it difficult and explains the reason for the presence of an incubation period

Transition from interfacial to diffusional growth during hydrogenation of Mg

The transition from interfacial to diffusional growth during hydrogenation of Mg -> MgH2 (hydride) at 210 degrees C for 300 min is studied using Johnson-Mehl-Avrami-Kolmogorov equation (alpha = 1 - exp(- kt(n))). The growth dimensionality (n) decreases from 0.73 to 0.23. 1D (hydride/metal) interfacial growth occurs when n > 0.50, suggested by constant interface velocity (U). Diffusional growth at n <0.50 is confirmed by the core-shell (Mg-MgH2) structure, drop in U by similar to 2-orders and the diffusion coefficient (D) of H-atom through hydride. The transition from 1D interfacial to diffusional growth occurs at n approximate to 0.50. (C) 2015 Elsevier B.V. All rights reserved

Contributions of multiple phenomena towards hydrogenation: A case of Mg

Heterogeneous hydrogenation involves chemisorption (Chem), nucleation and growth by interfacial movement (NG) and diffusion (Diff). The slowest one of these phenomena is generally considered to control hydrogenation. However, the considered phenomenon cannot explain the hydrogenation in its entirety. Multiple phenomena can contribute to different extents towards hydrogenation. Contributions of multiple phenomena are explained by developing functions of the form xi(t) = Pi(i = Chem, NG, Diff) (j = Chem,) (i not equal j) f(j) (t)center dot(xi(ai(t))(i). xi(t) is the converted fraction of hydride. The indices a(i)(t) represent the extents to which the explicitly considered phenomena act (represented by xi(i)(t) from literature). Constraints Sigma(i)a(i)(t)6 = 1 and the condition f(j)(t)-> 1 ascertain the exhaustiveness of the phenomena considered. Mg-MgH2 is considered as a proof-of-concept system to apply the present approach. The (t) functions are applied to describe the hydrogenation behaviour of Mg (similar to 44 mu m) at 210 degrees C and P-H2 = 1 MPa. Present analysis shows that multiple phenomena can act simultaneously and the dominant one (highest value of index) controls the hydrogenation. This dominant phenomenon can change with time such that chemisorption followed by NG and finally diffusion contribute in controlling hydrogenation. The estimated activation energies for NG (42 kJ/mol H) and diffusion (97 kJ/mol H) compare well with literature. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved