Hydrogen storage properties on mechanically milled graphite

We investigated hydrogen absorption/desorption and structural properties in mechanically milled graphite under hydrogen pressures up to 6 MPa to clarify catalytic and hydrogen pressure effects in the milling. The results indicate that a small amount of iron contamination during milling plays a quite important role as a catalyst for hydrogen absorption/desorption properties in graphite. Two-peak structure for hydrogen desorption in the TDS profile is due to existence of two different occupation sites for hydrogen, which is caused by the existence of high dispersing iron on graphite. From the experiment of high pressure milling, we clarified that the number of defects in graphite decreases with increasing the atmospheric hydrogen pressure during milling, leading to the decrease in the absorbed hydrogen content and the stabilization of hydrogen on the occupation sites. In addition, we confirmed the existence of physisorption-like reversible hydrogen at room temperature in the graphite prepared by milling under high pressure hydrogen atmosphere above 3 MPa. This unstable hydrogen might strongly be correlated with the existence of the lamella structure in nanometer scale, which only leaves in graphite prepared by high pressure milling.