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

Abstract

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 NaAlH4- and a LiNH2–MgH2-based material by admixing expanded natural graphite (ENG) up to 25mass% and compaction with up to 400MPa. 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 1Wm−1K−1 up to 38Wm−1K−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 59g-H2l−1 and 54g-H2l−1 compared to the loose powders with 19g-H2l−1 and 18g-H2l−1 for the NaAlH4- and a LiNH2–MgH2-based material, respectively, and they are very suitable for a tubular hydride tank design due to anisotropic heat transfer characteristics.