Preparation and dehydrogenation properties of two cobalt-based ammine borohydrides: CoCl_3·3NH_3/3LiBH_4 and CoCl_2 3NH_3/2LiBH_4

摘要

Two new cobalt-based ammine borohydrides were prepared via ball milling of LiBH_4 and CoCl_n-3NH_3 (n = 3, 2) with molar ratios of 3:1 and 2:1, respectively. X-ray diffraction (XRD) results revealed the as-prepared composites having amorphous state. Thermogravimetric analysis-mass spectrometry (TG-MS) measurements showed that the two composites mainly release H_2, concurrent with the evolution of a small amount of NH_3. Further results showed that the excessive addition of LiBH_4 can suppress the liberation of NH_3, resulting in the release of H_2 with a high purity (>99 mol.%). By combination with the temperature-programmed-desorption (TPD) results, the CoCl_3-3NH_3/4LiBH_4 and CoCl_2-3NH_3/3LiBH_4. composites can release 7.3 wt.% (4.2 wt.% including LiCl) and 4.2 wt.% (2.0 wt.% including LiCl) pure hydrogen, respectively, in the temperature range of 25-300 ℃. Isothermal dehydrogenation results reveal that CoCl_3-3NH_3/3LiBH_4 shows favorable dehydrogenation rate at low temperatures, releasing about 5.2 wt.% (2.9 wt.% including LiCl) of hydrogen within 45 min at 80 ℃.