Mg(BH4)2-Based Hybrid Metal-Organic Borohydride System Exhibiting Enhanced Chemical Stability in Melt

The formation of a chemically stable and thermally cyclable Mg(BH4)2 melt is proven to be possible through the utilization of an organic borohydride salt additive. While extensive exploration of additives for lowering the melting point and modifying the chemical stability of Mg(BH4)2 has been reported, this is the first study to use the organic borohydride salt, tetramethylammonium borohydride (TMAB), to modify the melting behavior of Mg(BH4)2. Examination of a 5:1 molar mixture of Mg(BH4)2 and TMAB revealed a reversible melt between 180 and 195 °C, which was reproduced for five thermal cycles. The mixture melt exhibited an enhanced chemical stability compared to melts of the individual Mg(BH4)2 and TMAB species. It was observed that between room temperature and 250 °C (over 50 °C above the melting point), the mixture releases <0.1 wt % mass, consisting primarily of H2. The mixture also exhibits greatly reduced evolution of volatile boron containing compounds compared to either Mg(BH4)2 or TMAB. The use of TMAB to chemically stabilize a Mg(BH4)2-rich melt demonstrated in this work represents an exciting pathway to modification of Mg(BH4)2 relevant to both hydrogen-storage and magnesium battery fields.