The triazine-based graphitic C3N4(g-C3N4)sheet,which has been recently synthesized in experiment,is promising in gas storage due to its porous structure and light mass.In this work,we have studied hydrogen storage of g-C3N4 doped with Li atoms under ambient thermodynamic conditions.Using first-principles calculations,we have found that the most stable binding site for Li atom on g-C3N4 substrate is the open-hollow site with the binding energy of 3.26eV.Based on the force field parameters derived from quantum mechanics calculations,we have further performed grand canonical Monte Carlo(GCMC)simulations to investigate the H2 adsorption isotherms on g-C3N4 sheet and found that the adsorption energy of H2 is 3.48kcal/mol,and the excess uptake of hydrogen is about 4.50wt%at 298K and 100 bar.