Hydrogen uptake at 250 °C, P H2 > 15 bar and release at 320, 350 °C by MgH2 mixed with 10 wt % rGO alleviates the incubation period (slow kinetics) encountered during hydrogen release by pure MgH2. Ball milling establishes Mg–C interactions (∼283 eV) in these nanocomposites through electron-transfer from Mg to π* of C and weakens the C–C π bond. These Mg–C interactions persist in the nanocomposites upon subsequent hydrogen uptake and release. These interactions change the hybridization of C from sp2 to sp3, 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.

The dehydrogenation mechanism during the incubation period in nanocrystalline MgH (low α: converted metal fraction and dα/dt) and the reasons for the occurrence of the incubation period at 320, 350, and 400 °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 MgH as interstitials, as indicated by the MgH 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 and A 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.

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