Tailored heat transfer characteristics of pelletized LiNH2-MgH2 and NaAlH4 hydrogen storage materials

Complex hydrides are attractive candidates for solid-state hydrogen storage because of their high hydrogen storage capacities and moderate operation temperatures. However, the fast and efficient transport of reaction heat through the hydride bed is an unsolved problem due to the low intrinsic heat conductivities of complex hydrides. Here, we report on increasing the effective thermal conductivities of a NaAlH 4- and a LiNH 2-MgH 2-based material by admixing expanded natural graphite (ENG) up to 25 mass% and compaction with up to 400 MPa. Thermal conductivities in radial and axial direction, microstructure and phase fractions of these pellets were determined. With increasing ENG content the heat transfer characteristics of both systems were enhanced from less than 1 W m -1 K -1 up to 38 W m -1 K -1. The pelletized hydride-graphite composites can be processed easily and safely compared to loose powders. Further, they have increased volumetric storage capacities of up to 59 g-H 2 l -1 and 54 g-H 2 l -1 compared to the loose powders with 19 g-H 2 l -1 and 18 g-H 2 l -1 for the NaAlH 4- and a LiNH 2-MgH 2-based material, respectively, and they are very suitable for a tubular hydride tank design due to anisotropic heat transfer characteristics.