High pressure in situ diffraction studies of metal–hydrogen systems

Abstract

“Hybrid” hydrogen storage, where hydrogen is stored in both the solid material and as a high pressure gas in the void volume of the tank can improve overall system efficiency by up to 50% compared to either compressed hydrogen or solid materials alone. Thermodynamically, high equilibrium hydrogen pressures in metal–hydrogen systems correspond to low enthalpies of hydrogen absorption–desorption. This decreases the calorimetric effects of the hydride formation–decomposition processes which can assist in achieving high rates of heat exchange during hydrogen loading—removing the bottleneck in achieving low charging times and improving overall hydrogen storage efficiency of large hydrogen stores. Two systems with hydrogenation enthalpies close to −20 kJ/mol H 2 were studied to investigate the hydrogenation mechanism and kinetics: CeNi 5–D 2 and ZrFe 2− x Al x ( x = 0.02; 0.04; 0.20)–D 2. The structure of the intermetallics and their hydrides were studied by in situ neutron powder diffraction at pressures up to 1000 bar and complementary X-ray diffraction. The deuteration of the hexagonal CeNi 5 intermetallic resulted in CeNi 5D 6.3 with a volume expansion of 30.1%. Deuterium absorption filled three different types of interstices, Ce 2Ni 2 and Ni 4 tetrahedra, and Ce 2Ni 3 half-octahedra and was accompanied by a valence change for Ce. Significant hysteresis was observed between deuterium absorption and desorption which profoundly decreased on a second absorption cycle. For the Al-modified Laves-type C15 ZrFe 2− x Al x intermetallics, deuteration showed very fast kinetics of H/D exchange and resulted in a volume increase of the FCC unit cells of 23.5% for ZrFe 1.98Al 0.02D 2.9(1). Deuterium content, hysteresis of H/D uptake and release, unit cell expansion and stability of the hydrides systematically change with the amount of Al content. In the deuteride D atoms exclusively occupy the Zr 2(Fe,Al) 2 tetrahedra. Observed interatomic distances are Zr–D = 1.98–2.11; (Fe, Al)–D = 1.70–1.75 Å. Hydrogenation slightly increases the magnetic moment of the Fe atoms in ZrFe 1.98Al 0.02 and ZrFe 1.96Al 0.04 from 1.9 μ B at room temperature for the alloy to 2.2 μ B for its deuteride.