- Mg-nanocarbon composites demonstrate fast reversible hydriding kinetics. The kinetics is strongly correlated with the structure of the added nanocarbon allotrope. We conducted a systematic exploration of the reversible hydriding kinetics of Mg upon addition of nanocarbons with different dimensionalities, namely, 1D carbon nanotubes, 2D graphene nanoplatelets and 3D activated carbon. Our findings showed that the Mg (de) hydriding rate could be tuned by the dimensionality and defect density of the nanocarbon. The resulting kinetic enhancement was explained in terms of the hydrogen spillover mechanism. The use of nanocarbons offers an alternative to the utilization of high concentrations of expensive transition-metal catalysts for hydrogen storage.