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

In this work, the complex hydrogen sorption behaviors in a 3NaBH4/HoF3 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 HoF3 can react with NaBH4 during ball milling, leading to the formations of Na-Ho-F and Na-Ho-BH4 complex compounds. The first dehydriding of the 3NaBH4/HoF3 composite can be divided into 4 steps, including the ion exchange between H− and F−, the formation of NaHo(BH4)4, the decomposition of NaHo(BH4)4 and reaction of NaBH4 with Na-Ho-F compounds. The final products, HoB4, HoH3 and NaF, can be rehydrogenated to generate NaBH4 and NaHoF4 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 3NaBH4/HoF3 composite are determined to be 88.3 kJ mol−1 H2 and −27.1 kJ mol−1 H2, respectively.