Reversible hydrogen sorption in NaBH4 at lower temperatures

Abstract

In the present study, a new hydrogen storage system, being able to reversibly absorb/desorb hydrogen at fairly low temperatures, was developed based on a 3NaBH₄–PrF₃ composite. It is shown that 3 wt% of reversible hydrogen sorption can be achieved in the 3NaBH₄–PrF₃ composite at 400 °C with fast kinetics. After the addition of 5 mol% VF₃, the dehydrogenation kinetics of the 3NaBH₄–PrF₃ composite can be significantly improved. The onset dehydriding temperature is lowered down to 46 °C in vacuum, and the dehydrogenation finishes in 2 min at 400 °C. Both the dehydrogenation enthalpy and activation energy of 3NaBH₄–PrF₃ can be lowered down through the addition of VF₃. In particular, the dehydrogenation products of the 3NaBH₄–PrF₃–5 mol% VF₃ composite can be rehydrogenated at a temperature as low as 48 °C with the regeneration of NaBH₄. At 84 °C, a reversible hydrogen sorption of about 1.2 wt% can be achieved in the 3NaBH₄–PrF₃–5 mol% VF₃ composite. The improvement in hydrogen sorption properties can be mainly attributed to the formation of the VB₂ phase during dehydrogenation as an efficient catalyst, which maintains well its catalytic effect in the re-/dehydrogenation cycles. Based on a series of controlled experiments and phase analyses, the de-/rehydrogenation mechanisms of the 3NaBH₄–PrF₃ composite without and with VF₃ addition are proposed and discussed in detail.