Hydrogen absorption process into graphite and carbon materials

In order to estimate bulk hydrogen retention and recycling in graphite and carbon materials, molecular hydrogen absorption has been studied. Hydrogen absorption rates significantly depend on samples which arise from grain size, trap concentration and so on. Absorption rate constants differ between the cases of low and high pressures. Trapping has a strong influence, especially in the low pressure range. Oxidation of graphite reduces hydrogen retention and enhances the absorption rate. This suggests that oxygen in graphite does not behave as trapping sites for hydrogen. Activation energies for apparent diffusion for H2 and D2 are determined to be 153 and 158 kJ/mol. They are smaller than those energies determined from desorption measurements.