Enhanced hydrogen storage properties of ball-milled Mg with C₆₀ and Fe

Abstract

Mg-based materials with added C₆₀ and/or Fe, i.e., Mg-C₆₀, Mg-Fe and Mg-C₆₀-Fe, were prepared by ball milling. We also added stearic acid (SA) as a process control agent to Mg-Fe + SA and Mg-C₆₀-Fe + SA. We investigated the effects of C₆₀, Fe and stearic acid on the hydrogen storage properties of Mg by X-ray diffraction (XRD), morphology observation, differential scanning calorimetry and pressure-composition-temperature measurements using Sievert apparatus under lower hydrogen pressure (<1.0 MPa). Morphology observation revealed that the particles of Mg-Fe + SA, Mg-C₆₀-Fe and Mg-C₆₀-Fe + SA were less agglomerated than Mg-Fe, and also nanoparticles of C₆₀ were formed on the surface of the C₆₀-added samples. We observed MgO in the as-milled samples of Mg-Fe + SA and Mg-C₆₀-Fe + SA, and MgH₂ in all the Fe-added samples after hydrogenation at 280℃ under a hydrogen pressure of 0.99 MPa with XRD analysis. The Fe-added samples released hydrogen at temperatures 71-152℃ lower than commercial MgH₂ in DSC analysis. The hydrogen storage capacities of Mg-Fe + SA and Mg-C₆₀-Fe + SA were 2.93 wt.% and 2.42 wt.%, respectively, and they did not release hydrogen at 280℃ even the equilibrium pressure as below 0.1 MPa. On the other hand, Mg-Fe and Mg-C₆₀-Fe absorbed hydrogen up to 2.57 wt.% and 3.10 wt.%, respectively, and released hydrogen at 280℃ and about 0.1 MPa. The Fe had a catalytic effect on Mg, and stearic acid oxidized Mg during ball milling, although it refined the particles. In contrast, C₆₀ inhibited agglomeration without oxidizing Mg and dispersed Fe finely on it, and also increased specific surface area by the nanoparticles provided the more active sites for hydrogenation. The co-addition of C₆₀ and Fe exhibit the synergetic effects in enhancing the hydrogen storage properties of Mg.