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 6^th 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.