Effects of heteroatom-doped hierarchical porous carbon on hydrogen storage properties of MgH2

In this paper, the nitrogen doped (N-HPC), nitrogen and phosphorus co-doped hierarchical porous carbon (NP-HPC) are prepared by cross-linking phytic acid and poly pyrrole/aniline precursor, respectively. They are mixed with MgH₂ by high-energy ball milling, and then their effects and mechanisms on the hydrogen absorption and desorption properties of MgH₂ are investigated. Meanwhile, the hydrogen storage properties of MgH₂ added with graphite (G) are also compared. The results show that the additions of NP-HPC, N-HPC, and G all exhibit the catalytic effect on the hydrogen absorption and desorption of MgH₂. As for the hydrogen desorption, the catalytic effect is enhanced in the order of N-HPC, G and NP-HPC. Compared with pure MgH₂, the hydrogen desorption temperature is reduced by 65.3 °C, 79.6 °C and 91.1 °C, respectively. Among them, the MgH₂ + NP-HPC system can release 5.17 wt% hydrogen at 300 °C within 30 min. First-principles calculations reveal that the P-doped and vacancy-containing carbon materials significantly reduce the H₂ recombination barrier from the surface of MgH₂ and distort the atomic structure of near-surface layer of MgH₂, which in turn weakens the Mg-H bond strength. This may be the intrinsic reason for the excellent catalytic effect of NP-HPC and vacancy-containing G on the hydrogen desorption performance of MgH₂.