The replacement of N-2 with H-2 in sII clathrate hydrates was studied through ab initio calculations and molecular dynamics simulations. From the thermodynamic analysis, we suggest that the replacement is more likely to occur in the large cages than in the small cages. H-2 and N-2 can coexist in the same large cage or in the different cages, forming binary H-2-N-2 hydrates. Simulations show that the loading capacity of H-2 can reach up to similar to 4.4 wt% by the replacement method, indicating that the clathrate hydrates are a promising material for hydrogen storage under moderate conditions. (C) 2016 Elsevier B.V. All rights reserved.