Cycling and engineering properties of highly compacted sodium alanate pellets

Sodium alanate powder comprised of NaH and Al was doped with 3 mol% titanium chloride (TiCl3) and pelletized into highly compacted cylindrical pellets. The pelletization process was performed to improve thermal conductivity and volumetric hydrogen capacity of the metal hydride, compared to loose or tapped powder, which are vital requirements for on-board hydrogen storage applications. The pelletization process was performed over a range of 69–345 MPa (10–50 kPSI) with a 95% increase in density and improvement in thermal conductivity 18 times greater compared to powder at the maximum pelletization pressure (1.60 g/cm3 and 0.82 g/cm3; 9.09 W/m K and 0.50 W/m K, respectively). Hydrogen cycling capacities and kinetics were not adversely affected by the pelletization process although 10 cycles are required to obtain full hydrogen capacity. Pellet cycling capacity maintained a stable 4 wt% H2 over 50 cycles. Ti-doped NaH + Al pellets exhibited similar thermal cycling expansion as with the loose powder; within 30 cycles there was a 50% loss in pellet density and by 50 cycles the loss in pellet structural integrity made handling problematic.

► Thermal conductivity of pelletized sodium alanate mixture increased by 230%. ► Pelletization of sodium alanate does not impair H2 capacity over long-term cycling. ► Pelletized sodium alanate showed a kinetic dependence on pellet dimension. ► Pellets expanded through 50 cycles with a 50% decrease in pellet density. ► Thermal conductivity decreased by 80% within 10 cycles due to pellet expansion.