Study on reversible hydrogen sorption behaviors of a 3NaBH_4/HoF_3 composite

摘要

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 H0F3 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, H0H3 and NaF, can be rehydrogenated to generate NaBH_4 and NaHoF_4 with an absorption capacity of 2.3 wt% obtained at 400 ℃. Based on the Pressure-Composition-Temperature measurements, the deVrehydrogenation 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.