Disordered boron nitride - graphite composite for hydrogen storage

Molecular hydrogen can be stored as physisorbed on low weight materials such as metal-organic frameworks, and carbon-based materials in the form of nanotubes, activated carbon and other carbon nanostructures. A fundamental requisite for every-day applications is the adsorption reversibility at practical temperature and pressure conditions. From thermodynamic calculations, this could be achieved when the molecular hydrogen binding energy to the adsorbent surface is around 300-400 meV/molecule. Actually, for a graphitic surface the binding energy results to be a factor 3-4 below. A possible solution consists in allowing the contemporary interaction of the hydrogen molecule with two or more surfaces or, in other words, to sandwich H2 between graphite layers. Hexagonal BN and graphite share very similar structural properties. Taking advantage from the possibility of exfoliating hexagonal structures, a boron nitride – graphite (BN-C) intercalated composite material with statistically distributed slit-like porosities was obtained. For the purpose, pure compounds were mixed (1:1 by weight) and subjected to a mechano-chemical treatment by high energy Ball Milling (BM). Structural and morphological characteristics were studied as a function of the milling time by using X-ray diffraction (XRD), electron microscopy (SEM) and nitrogen adsorption techniques. Milling treatment produced slit-like pores (7 Å pore diameter) even at short milling times, increasing the surface area up to 10 h milling (SA=340 m2/g). At prolonged milling times, materials reactivity induce pores collapse as evidenced by the decreasing in pore volumes and surface area at constant pore size. The material was tested with respect to its hydrogen storage capacity by adsorption measurements at 77 K as reported in the figure. Work is in progress to enhance the composite porosity by decreasing components reactivity.