Graphenic C$_3$N$_4$: A New Template for Metal Decoration and Hydrogen Adsorption*

From density functional theory calculations we identify a graphenic C$_3$N$_4$ (g-C$_3$N$_4$) structure as an excellent template for stable and well dispersed decoration of alkali and transition metal atoms which, in turn, exhibits a high capacity for hydrogen adsorption with binding energies (a few tenths of eV) suitable for mobile applications. The unique porous micro-structural sites of g- C$_3$N$_4$ accommodate the excessive N lone-pair electrons and promote strong hybridization between the orbitals of N and metal atoms. It plays a key role in overcoming the tendency of metal-atom clustering that has plagued other proposed hydrogen storage media. These metal decorated g- C$_3$N$_4$ may also prove useful in a variety of catalytic and sensing applications.