Molecular Simulation of Hydrogen Physisorption and Chemisorption in Nanoporous Carbon Structures

The effect of pore morphology on the hydrogen-storage capacity of carbon materials at room temperature (298 K) has been studied using Grand Canonical Monte Carlo (GCMC) molecular simulation. Prototypical pore geometries such as slit pores and nanotubes were considered along with carbon foams and a random disordered carbon structure. Both physisorption and chemisorption models were considered in order to take into account the most favourable adsorption scenarios. Fluid fluid and fluid solid interactions were assumed to follow a Lennard-Jones-type potential. It was seen that physisorption alone cannot account for the adsorption of more than a few weight percentages, regardless of the pore morphology. Under physisorption conditions, the simulation results show that carbons with a slit-shaped pore geometry are more efficient than other geometries at storing hydrogen, particularly at pore distances which allow the formation of fluid layers commensurate with the pore geometry. Slit pores appear to store a maximum of 1.77 wt% hydrogen by physisorption as opposed to carbon foams (1.48 wt%), carbons with a random structure (1.04 wt%) and carbon nanotubes (0.31 wt%). Simulation with hypothetical chemisorption models where the solid fluid interaction is artificially enhanced showed that the storage capacity is higher in pore geometries with a larger accessible volume. Strong chemisorption gave loadings of up to 17 g/l (1.5 wt%), 91 g/l (13.8 wt%), 57.33 We (9.6 wt%) and 42.94 g/l (8.29 wt%) in nanotubes, foams, random structures and slit pores, respectively. Desorption is a problem in this scenario, as only ca. 1-3 wt% hydrogen can be delivered from these structures. Ultimately, the comparison of the different fluid solid potentials revealed that if solid fluid interactions could be enhanced beyond the classical dispersion, the more open structures could be ideal candidates for hydrogen-storage materials.