Catalytic Influence of Various Cerium Precursors on the Hydrogen Sorption Properties of NaAlH4

Sodium alanate (NaAlH4) is one of the metal complex hydrides most often investigated for use as a hydrogen-storage material. Doped with transition or rare earth metal compounds, NaAlH4 can absorb and release hydrogen in low and medium temperature ranges with excellent reversibility and cycling stability. The properties of NaAlH4 doped with CeCl3 differ from materials with other dopants, with faster sorption kinetics and a more stable capacity. In this paper, various precursors of Ce are applied to investigate their catalytic effects on the sorption performance of this material. The re-hydrogenation is found to be completed in approximately 10 min. Although all the Ce precursors investigated in this work result in reversible hydrogen storage materials, desorption kinetics are enhanced upon formation of cerium aluminide (CeAl4) in the composites. While the use of CeAl4 instead of CeCl3 can increase the hydrogen capacity by bypassing the formation of the ineffective NaCl, the highest capacity of 4.9 wt%close to the theoretical valueis obtained from NaAlH4 doped directly with metallic cerium. Furthermore, dehydriding under back pressures is also investigated to evaluate the H2 desorption rates under practical conditions. At 3 bar H2 pressure, the second desorption step of NaAlH4 is fully suppressed at 150 degrees C and only 2.5 wt% H was released, whereas at 180 degrees C the capacity is not much affected, which is interesting for combination in a system with a high-temperature PEM fuel cell.