Niobium fluoride-modified hydrogen evolution reaction of magnesium borohydride diammoniate

Abstract

Metal borohydride ammoniates have become one of the most promising hydrogen storage materials due to their ultrahigh capacities. However, their application is still restricted by the high temperature of hydrogen desorption and the release of ammonia. Here, to promote the dehydrogenation evolution and suppress the ammonia release, different amounts of NbF5 were introduced into Mg(BH4)2·2NH3. Compared to the pure Mg(BH4)2·2NH3, the Mg(BH4)2·2NH3-NbF5 composites exhibit lower onset dehydriding temperatures (53–57 °C) and higher dehydriding capacities (5.6 wt.%–8.2 wt.%) at below 200 °C, with the complete suppression of ammonia. In addition, 7.4 wt.% H2 could be released from Mg(BH4)2·2NH3–5 mol% NbF5 composite at 200 °C within 20 min and the apparent activation energy is calculated to be 60.28 kJ mol–1, which is much lower than that of pure Mg(BH4)2·2NH3 (92.04 kJ mol–1). Mg(BH4)2·2NH3 should mechanochemically react with NbF5, forming dual-metal (Mg, Nb) borohydride ammoniate and spherical MgF2. The introduction of electronegative Nb cation results in-situ formation of (Mg, Nb) borohydride ammoniate towards a lower dehydrogenation temperature and a higher hydrogen release purity. The increased phase boundaries among the Mg(BH4)2·2NH3, dual-metal (Mg, Nb) borohydride ammoniate, and MgF2 phases further facilitate the hydrogen diffusion during the dehydrogenation of the composites.