A hybrid molecular simulation, as a combination of molecular dynamics and Grand Canonical Monte Carlo simulation, is performed to investigate the storage capacity of hydrogen in carbon nanostructure adsorbents. Pure graphene sheet, nitrogen-doped graphene sheet, palladium-decorated graphene sheet and nitrogen-doped graphene sheet decorated with palladium atoms are selected for this purpose. Palladium is added to the structure in atomic and nanoparticle forms. Initially, all selected systems are optimized using density functional theory (DFT). The atomic charges of various structures are incorporated in the hybrid simulation. Then, hybrid simulations of hydrogen adsorption in different structures are performed at a temperature of 300 K in the pressure range of 1-40 bar. Simulation results show that among various structures, the simultaneous doping of graphene sheet with nitrogen atom and decoration of sheets by palladium atoms could increase the storage capacity by about 437% in comparison to pure graphene. In addition, the atomic form of palladium is more efficient than its nanoparticle form. Finally, comparing the adsorption capacity of the proposed structure with the target set by the US Department of Energy for 2020 indicates that proposed nanostructure can improve this target for hydrogen storage in comparison with previous carbon structure materials. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.