Study on reversible hydrogen sorption behaviors of a 3NaBH4/HoF3 composite

In this work, the complex hydrogen sorption behaviors in a 3NaBH 4 /HoF 3 composite prepared through mechanical milling were carefully investigated, including the reactions occurred during ball milling and de-/rehydrogenation processes. Different from other rear earth fluorides, the HoF 3 can react with NaBH 4 during ball milling, leading to the formations of Na–Ho–F and Na–Ho–BH 4 complex compounds. The first dehydriding of the 3NaBH 4 /HoF 3 composite can be divided into 4 steps, including the ion exchange between H − and F − , the formation of NaHo(BH 4 ) 4 , the decomposition of NaHo(BH 4 ) 4 and reaction of NaBH 4 with Na–Ho–F compounds. The final products, HoB 4 , HoH 3 and NaF, can be rehydrogenated to generate NaBH 4 and NaHoF 4 with an absorption capacity of 2.3 wt% obtained at 400 °C. Based on the Pressure–Composition–Temperature measurements, the de-/rehydrogenation enthalpies of the 3NaBH 4 /HoF 3 composite are determined to be 88.3 kJ mol −1 H 2 and −27.1 kJ mol −1 H 2 , respectively.