Different aspects of the hydrogen storage properties of TMC2H2 (TM = Sc-Ni) were evaluated using density functional theory and ab initio molecular dynamics calculations. Hydrogen saturation conformations indicate that the hydrogen gravimetric density of TMC2H2 (TM = Sc-Ni) is 4.54-12.43%. The free energy profiles and first-principles molecular-dynamics (MD) simulations showed that when the first H-2 molecule attaches to TMC2H2 (TM = Sc, Ti), the H-2 molecule is first dissociated over Ti/Sc, and then, one of the H atoms goes to a carbon atom, forming a CH2 group. That is, there are two types of hydrogen bonding in TiC2H2 and ScTiC2H2: the first set of hydrogen binds to C, and the subsequent set of hydrogen binds to Ti/Sc. Importantly, TiC2H2 is able to bind five H2 molecules with the hydrogen gravimetric density of 12.00%, regardless of CH2 group formation. However, ScC2H3 H can bind only three H2 molecules, reducing the gravimetric uptake capacity of ScC2H2 from 12.43% to 10.13%. For TMC2H2 (TM = V, Cr, Mn), the first hydrogen molecule is dissociated, but there is no "spillover effect". For TMC2H2 (TM = Fe, Co, Ni), the first hydrogen molecule is bound quasi-molecularly. Most of our findings, such as those regarding H-2 dissociation and TMC2H3 H formation, should be valid for other TM decorated nanostructures. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.