The role of transition metal doping in enhancing hydrogen storage capacity in porous carbon materials

Porous carbon materials with high specific surface area are potential hydrogen storage materials. However, due to the weak van der Waals forces between the material and H2, hydrogen can generally be stored only at a cryogenic temperature under high hydrogen pressure. To solve this problem, in this work, ZIF-8-derived porous carbon material (PCM) modified with transition metals (PCM-TMs, TM: Mn, Fe, Ni) was prepared to investigate the role of TM doping. The developed PCM-TMs have a high specific surface area (up to 1363 m2/g) and pore volume (up to 0.77 cm3/g). Experimental results show that PCM-TMs exhibit higher hydrogen storage capacity, usable capacity, and hydrogen uptake capacity per specific surface area compared to PCM. In addition, PCM-TMs have a higher isosteric heat of adsorption (Qst) than PCM, which suggests that TM doping enhances the interaction between H2 and carbon materials. Theoretical calculations further indicate that this interaction is a nearly weak chemisorption force (-9.32 kJ/mol). Notably, Mn-doped porous carbon material (PCM-Mn) exhibits a hydrogen storage capacity of -3.45 wt% (20.44 g/L) at 77 K under 35 bar-H2. Even at 253 K under 50 bar-H2, the capacity can reach -1.41 wt% (8.34 g/L). The hydrogen uptake capacity per specific surface area for PCMMn reaches 1.03 x 10-3 wt% / m2 & sdot;g-1 at 253 K under 50 bar-H2, which is 1.5 times larger than that of PCM. Under proper operating conditions, such as loading at 77 K and 35 bar-H2, and discharging at 253 K and 2 bar-H2, PCMMn can achieve a usable capacity of at least 19.30 g/L. The hydrogenation kinetics curves reveal that PCM-Mn can store 1.39 wt%-H2 (8.25 g/L) under 12 MPa H2 at 298 K, while it is only 0.88 wt% (5.53 g/L) for PCM at the same condition. This work provides valuable insight into improving the hydrogen storage performance of hydrogen storage materials of physisorption.