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 H₂ at either cryogenic temperature or very high H₂ pressure due to their weak van der Waals forces with the H₂ 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 H₂ and the PCMs, which is demonstrated by increased adsorption isosteric heat (Q_st) and H₂ storage capacity per specific surface area (SSA). At lower H₂ 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 H₂ uptake per SSA. At higher H₂ coverage (77 K, 50 bar-H₂), there is a notable enhancement in the volumetric H₂ uptake per SSA for NHPC-3 (11.41 g·L^–1/m²·g^–1) compared to that for NHPC-0 (8.49 g·L^–1/m²·g^–1) as the N content increases. Furthermore, N-doping can increase the packing density, thereby improving the volumetric H₂ storage capacity of NHPC-x. The enhancement is strikingly demonstrated by NHPC-2, which achieves a volumetric H₂ uptake of 26.96 g/L (SSA = 2458.44 m²/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 m²/g) at the same condition. This work contributes to a deeper understanding of the effect of heteroatom doping on the H₂ storage performance in PCMs.