Among the two-dimensional materials of the post-graphene era, borophene has raised an enormous interest due to its unprecedented diversity of structures and the wide variety of potential applications, including its ability for hydrogen storage. In the present paper we use van der Waals-corrected density functional theory in conjunction with a quantum-thermodynamic model to investigate the hydrogen storage capacity of confining Li-decorated borophene sheets in its most stable Pmmn8 configuration. Our theoretical approach surpasses the standard density functional theory calculations only valid at zero temperature and no pressure, thus providing the gravimetric and volumetric capacities as well as the isotherms in real conditions. We show that narrow Li-decorated slit pores of borophene have a good volumetric hydrogen storage capacity particularly at low temperature. Accordingly, nanoporous boron frameworks could be optimal for hydrogen storage in applications at low temperature. We compare the results with those corresponding to pristine graphene slit pores.