Dependence of structural evolution and hydrogen storage mechanism on milling parameters during controlled reactive milling of graphite in hydrogen
Graphite was mechano-chemically processed in hydrogen using controlled reactive ball milling (CRBM). As-milled and heat treated products were characterised using techniques of combustion analysis, X-ray diffractometry (XRD), selected area electron diffracton (SAED) and high resolution transmission electron microscopy (HRTEM). Both the mechanisms of hydrogen uptake and of hydrogen evolution during subsequent heat treatment were found to be dependent on the mode of milling. Samples milled under a ball-particle impact mode absorbed more hydrogen (2.7wt%) than the samples milled under shearing mode, but the fraction of hydrogen released during subsequent low temperature heat treatment (220-500ºC) was significantly less than for the samples milled under shear. XRD of the shear milled product showed increased, but varying c-axis lattice parameters while XRD of the impact milled product showed complete loss of (002) graphite and formation of peaks which could be associated with CnHx species. Low temperature heat treatment of the shear milled product resulted in significant disorder in the graphite structure, characterised by a loss of diffraction peak contrast and high background scattering in both XRD and selected area electron diffraction patterns. In contrast there was little change in XRD patterns from impact milled samples during low temperature heat treatment. For the shear milled samples which were heat treated for short times there was also an associated loss of contrast from graphene layers in HRTEM images, however, HRTEM contrast from samples shear milled and heat treated for longer times showed an increased fraction of graphene sheets in the product.
It was concluded that hydrogen uptake under impact mode milling was predominately via an irreversible mechanism involving destruction of graphene sheets and reaction to from product including CnHx -type molecules. However, hydrogen uptake under shearing mode milling occurred via a partially reversible mechanism involving absorption between graphite layers and expansion of the graphite inter-sheet spacing.