Effects of La fluoride and La hydride on the reversible hydrogen sorption behaviors of NaBH4: a comparative study
Abstract
In the present work, two new reversible hydrogen storage composites, NaBH4 + LaF3 and NaBH4 + LaH2, have been prepared through a mechanical milling method with the aim of comparatively studying the effects of La fluoride and La hydride on the hydrogen sorption behaviors of NaBH4. Experimental investigations have shown that both La fluoride and La hydride enable reversible hydrogen sorption in NaBH4. In particular, LaF3 exhibits a superior promoting effect than LaH2, which agrees well with theoretical predictions. Surprisingly, better hydrogen sorption properties can be achieved in both systems through undergoing de-/rehydriding cycles. The reversible hydrogen storage capacity reaches up to 3.0 wt% at 238 °C and 2.9 wt% at 326 °C in NaBH4 + LaF3 and NaBH4 + LaH2 systems after the 6th dehydrogenation, respectively. In both cases, the formation of La boride plays the major role in the reversible hydrogen sorption in NaBH4. The superior promoting effect of La fluoride than La hydride upon modifications of thermodynamics and kinetics of NaBH4 should be ascribed to the following factors: (i) the formation of a thermodynamically more stable compound NaF instead of NaH reduces the overall enthalpy changes of re/de-hydriding reactions in NaBH4 + LaF3 to −31.8 kJ mol−1 H2 and 72.5 kJ mol−1 H2, respectively; (ii) the ion exchange of F− for H− leads to the reduction of the onset dehydrogenation temperature of NaBH4 to 160 °C in the NaBH4 + LaF3 composite; (iii) the F− anion favors the formation of LaB6 while H− favors the formation of LaB4. The role of functional anions and cations, de-/rehydrogenation mechanisms and nucleation modes in the two reversible hydrogen storage composites have been proposed based on experimental and theoretical analyses. The comparison study carried out in this work helps to design and search for new metal borohydride based composites for reversible hydrogen storage.