We have performed DFT simulations to quest for an optimal material for onboard hydrogen (H-2) storage applications. Using first-principles calculations, we established that the selected transition metals (M: Sc, Ti, Ni, V) decorated two-dimensional (2D) g-C3N4 sheets as optimal materials with reversible and significantly high H-2 gravimetric densities. By effectively avoiding metal-metal (M-M) clustering effect in case of mono doping, up to four molecules of H-2 per dopant could be adsorbed with an average binding energy of around 0.30-0.6 eV/H-2, which is ideal for practical applications. Decorating the g-C3N4 sheet with (M-M) dimers, the systems are found to be even more efficient for H-2 binding than single dopant decoration. The stability of these M decorated g-C3N4 sheets have been confirmed with ab-initio molecular dynamics simulations. We have further calculated the H-2 desorption temperatures of metal decorated g-C3N4 sheets, which confirms the practical application of these metal decorated sheets at ambient working conditions. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.