Engineering considerations in the use of catalyzed sodium alanates for hydrogen storage

Abstract

The hydrogen storage properties of catalyzed NaAlH 4 (and associated Na 3AlH 6) were studied in relation to various practical engineering considerations. Properties measured were cyclic capacity, charging and discharging rates, thermal effects, gaseous impurities, volume changes, low temperature plateau pressures and detailed isothermal desorption kinetics over the temperature range 23–180°C. Two materials were evaluated, one mechanically milled with the liquid alkoxides Ti(OBu n) 4 and Zr(OPr i) 4 and one milled with dry TiCl 3 as catalyst precursors. The alkoxide-catalyzed materials had low reversible capacities and released significant levels of hydrocarbon impurities during H 2 discharge. These problems were virtually eliminated with the inorganic TiCl 3 catalyst precursor. The NaAlH 4 and Na 3AlH 6 decomposition kinetics of TiCl 3-catalyzed Na-alanate conform to Arrhenius behavior with activation energies of 79.5 and 97 kJ/mol H 2, respectively. Measured absorption and desorption kinetics were surprisingly good and it is shown that 3–4.5 wt.% H 2 can be stored and recovered in reasonable times at 100–125°C. It may even be ultimately possible to use the NaAlH 4 decomposition reaction to provide 3 wt.% H 2 at room temperature for low-rate applications.