  In the quest of new absorbent for hydrogen storage, we investigate the
capacities of slit pores formed by two BC3 sheets decorated with Li atoms.
Their hydrogen storage capacities are determined using density-functional
theory in conjunction with a quantum-thermodynamic model that allows to
simulate real operating conditions, i.e., finite temperatures and different
loading and depletion pressures applied to the adsorbent in the charge-delivery
cycles. We show that the capacities of the adsorbed hydrogen phase of
Li-decorated BC3 slit pores are larger than those reported recently for
graphene and Li-decorated borophene slit pores. On the other hand, the usable
volumetric and gravimetric capacities of Li-decorated BC3 slit pores can meet
the targets stipulated by the U.S. Department of Energy (DOE) for onboard
hydrogen storage at moderate temperatures and loading pressures well below
those used in the tanks employed in current technology. In particular, the
usable volumetric capacity for pore widths of about 10 {\AA} meets the DOE
target at a loading pressure of 6.6 MPa when depleting at ambient pressure. Our
results highlight the important role played by the rotational degree of freedom
of the H2 molecule in determining the confining potential within the slip pores
and their hydrogen storage capacities.
