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Nanoscale Structures and Hydrogen Storage Capacity of Fe-C-H Produced by Milling Graphite with Steel Balls in a Hydrogen Atmosphere

To elucidate the influence of Fe on the interaction between carbon and hydrogen in graphite nanocrystals, graphite nanocrystals were mechanically milled with steel balls in a hydrogen atmosphere, and the potential of the material produced to store hydrogen was evaluated. X-ray diffraction and Raman spectra revealed that milling reduced the graphene size and increased the average distance between graphene layers. Elemental analysis showed that milling increased both the H/C and Fe/C ratios in the material. After being milled in a hydrogen atmosphere, samples released hydrogen at a lower temperature than before milling. Thermal decomposition occurred in two stages—235 to 475°C and 692 to 749°C—and yielded a total of 1.0 wt% hydrogen. It is hypothesized that these two stages corresponded to hydrogen released from Fe3C structures and from the Fe-catalyzed graphitization reaction, respectively. Before milling, samples slowly released a total of 0.5 wt% hydrogen over a temperature range from about 300 to over 900°C. Fe-C-H materials are attractive for hydrogen storage because they are composed of carbon and iron, which are inexpensive and abundant elements on Earth, and they have a high hydrogen weight density of 11 wt%.