N-Doped Graphene-Rich Aerogels Decorated with Nickel and Cobalt Nanoparticles: Effect on Hydrogen Storage Properties of Nanoconfined LiBH4

Abstract

We present the decoration of N-doped graphene entangled in mesoporous carbon by nickel and/or cobalt nanoparticles. The textural properties of the mesoporous materials were characterized by nitrogen desorption isotherms. The materials presented metallic nanoparticles of at least 20 nm and diffraction peaks typical of cubic cobalt or nickel. Transmission microscopy revealed the presence of graphene layers, organized mostly in an onionlike fashion around the metallic nanoparticles, suggesting that these might promote the formation of extra graphene sheets from the surrounding amorphous carbon. The formation of a metallic alloy when nickel and cobalt were mixed is strongly suggested. Differential scanning calorimetry of these materials filled with LiBH4 demonstrated that the presence of nanoparticles displaced the typical transition, melting, and decomposition peaks of this hydride toward lower temperatures. Volumetric studies revealed that the liberation of hydrogen started at lower temperatures when nanoparticles were present, slowing down at higher temperatures. Cobalt-decorated materials liberated 8 wt % H2 at 325 °C during the first cycle (with 1 wt % released at 226 °C). After rehydrogenation at 400 °C, the materials decorated with nickel liberated 8 wt % H2, a 2 wt % excess compared to that of the nondecorated materials.