Boosting Adsorption Isosteric Heat for Improved Gravimetric and Volumetric Hydrogen Uptake in Porous Carbon by N-Doping

Porous carbon materials (PCMs) hold great promise as hydrogen storage materials due to their high capacity but are limited by adsorbing H-2 at either cryogenic temperature or very high H-2 pressure due to their weak van der Waals forces with the H-2 molecules. In this study, N-doped hierarchical porous carbon (NHPC) materials were prepared by a simple one-step chemical activation method. Experimental results reveal that N-doping significantly enhances the interaction between H-2 and the PCMs, which is demonstrated by increased adsorption isosteric heat (Q(st)) and H-2 storage capacity per specific surface area (SSA). At lower H-2 coverage, the Q(st) increases from 7.45 kJ/mol (NHPC-0) to 7.95 kJ/mol (NHPC-2 and NHPC-3), which aligns with the enhanced gravimetric H-2 uptake per SSA. At higher H-2 coverage (77 K, 50 bar-H-2), there is a notable enhancement in the volumetric H-2 uptake per SSA for NHPC-3 (11.41 gL-1/m(2)g(-1)) compared to that for NHPC-0 (8.49 gL-1/m(2)g(-1)) as the N content increases. Furthermore, N-doping can increase the packing density, thereby improving the volumetric H-2 storage capacity of NHPC-x. The enhancement is strikingly demonstrated by NHPC-2, which achieves a volumetric H-2 uptake of 26.96 g/L (SSA = 2458.44 m(2)/g) at 77 K and 50 bar. This is almost the same as that for NHPC-0, despite a 21% reduction in SSA, which is 26.47 g/L (SSA = 3116.58 m(2)/g) at the same condition. This work contributes to a deeper understanding of the effect of heteroatom doping on the H-2 storage performance in PCMs.