Enhanced hydrogenation kinetics of magnesium diboride through carbon substitution into the boride layers

Abstract

It was recently found that hydrogenation of MgB₂ undergoes significant kinetic enhancement upon modification with graphene additives, a significant step towards its development as a practical hydrogen storage material. In order to gain a fundamental understanding of the kinetic activation of the modified material, it has been subjected to (S)TEM, synchrotron X-ray diffraction, neutron diffraction analyses, and high pressure hydrogenation studies. (S)TEM analyses of mechanically milled MgB₂/graphene shows no reduction in grain size, and thus the enhanced kinetics cannot be attributed to increased surface area. The lattice parameters obtained from Rietveld refinements of synchrotron X-ray diffraction data for the directly synthesized Mg_1+x/2B_2−xC_x and undoped MgB₂ show that the a lattice parameter (a = 3.05915(8) Å) is about 0.8% smaller than that of the undoped material (a = 3.08158(5) but the c lattice parameter is invariant. Refinements of powder neutron diffraction show a similar reduction of the a parameter in both the materials prepared by ball-milling and direct synthesis (∼3.0476(7) Å) compared to the undoped sample (a = 3.0758(2)) while the c lattice parameters remain constant. The finding of a significant increase in the c/a ratio for these materials indicates that the mechanical milling of MgB₂ with graphene represents an alternative method to prepare Mg_1+x/2B_2−xC_x. This conclusion is supported by the finding that both materials undergo major conversion to Mg(BH₄)₂ upon hydrogenation under identical conditions under which the mechanically milled material was found to undergo hydrogenation, establishing a correlation between carbon substitution into the bulk MgB₂ lattice and enhanced hydrogenation kinetics.