Influence of milling conditions on hydrogen storage capacities of activated carbon mechanically milled in an H2 atmosphere

We investigated whether the hydrogen storage capacity of activated carbon (AC) powder prepared by mechanical milling in an H2 atmosphere is influenced by the milling conditions. The hydrogen storage capacity of AC milled for more than 20 h using a 30 ml pot tended to be larger than that obtained when using a 45 ml pot. This suggests that the number of trapping sites for hydrogen storage increased as the nanostructure progressed because milling using a smaller pot might produce greater acceleration than that using a large one. In addition, the applied force exerted on powders on the mill wall by the mass of balls in the smaller pot might be stronger than that in the larger one because the ball filling fraction of the former is larger than that of the latter. The intensities of (0 0 2) diffraction of AC milled for more than 20 h using the 30 ml pot were weaker than those obtained when using the 45 ml pot. The hydrogen storage capacity increased as the initial H2 pressure in the milling pot increased. This indicates that physisorption might contribute to the hydrogen storage mechanism of AC mechanically milled in an H2 atmosphere. However, we assume that the physisorption capacity might be small because the hydrogen storage capacity showed little dependence on the specific surface area of the host AC. The hydrogen storage capacity might also be independent of the particle size of the host AC.