Graphite and elemental magnesium (Mg) powders, whose chemical compositions were C100-xMgx (0 ≤ x ≤ 40 at.%), were mechanically alloyed (milled) in an argon gas atmosphere, and these powders were loaded with hydrogen in a high-pressure vessel at a temperature of 308 K. The initial hydrogen gas pressure for hydrogenation experiment was 4 MPa. The periodicity of the c-axis of the graphite crystal structure was destroyed completely for C100 powders and partly for the powder containing Mg at an early stage of mechanical alloying (MA), indicating the formation of a turbostratic structure (graphene) for all powders. Although elemental Mg still remained in Mg-rich powders (C60Mg40) even after longer MA (80 h), other powders reached to mostly the turbostratic structure with no sign of crystalline Mg. The maximum hydrogen concentration levels after hydrogenation for C100 powder were 0.1 wt.% after MA for 25 h and 0.4 wt.% after MA for 80 h, suggesting that nanostructured graphite uptakes more hydrogen. On the other hand, hydrogen concentration level for C90Mg10 powders was less than 0.1 wt.% after MA for 15 h, but it reached to about 1 wt.% after MA for 25 h, and dropped down slightly from 1 wt.% after MA for 80 h. Furthermore, hydrogen absorption occurred smoothly and quickly for C90Mg10 powders, although longer induction time was required for C100 powders. Further addition of Mg to graphite reduced the maximum hydrogen concentration level in the powders, and the powders containing more than 30 at.% Mg did not absorb hydrogen at all at 308 K.
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