Development of novel borohydrides as hydrogen storage materials .

摘要

High throughput screening studies of the dehydrogenation behavior of the first row transition metal borohydrides of Sc, Ti, V, Cr, Mn, Fe, Co and Ni as well as Zr was performed. The goal was to synthesize Group I salts of anionic transition metal borohydride in view of their reasonable stability and high gravimetric hydrogen densities (9-15%), suited for practical hydrogen storage applications. The high throughput screening indicated that borohydrides of Sc, Mn and Zr were the most promising, and were subjected to further study. Thermal analysis of the anionic zirconium borohydrides, MxZr(BH 4)4+x M = Li, Na or K, demonstrated that they are more stable than neutral Zr(BH4)4.;Optimization of the Sc system led to synthesis of new homoleptic borohydride complexes, LiSc(BH4)4 and NaSc(BH4) 4. Vibrational spectroscopy indicates tridentate coordination of BH 4- groups to the Sc3+ ion. XRD showed that LiSc(BH4)4 and NaSc(BH4)4 have P42c and Pmnc crystal structures, respectively. Solid state NMR indicated that the complexes have close 11B and 45Sc chemical shifts. An equivalent of 10.8 wt % hydrogen of a pure complex was desorbed from the NaSc(BH4)4 material.;Neutral Mn(BH4)2 and an anionic manganese borohydride salt of Na have been synthesized. FT-IR and DFT calculation indicate that the sodium manganese borohydride has bidentate coordination of BH4 - units to Mn2+ ion. IR and thermal analysis confirmed that these two complexes have different properties. Thermal studies indicate that both complexes have reasonable desorption kinetics at PEM fuel cell temperatures. Evolved gas analyses indicate that the manganese borohydride complexes evolve a small amount of diborane compared to other transition metal borohydrides, making them better candidates for hydrogen storage application.;During the course of the high pressure hydrogenation studies of metal borohydrides, it was found that mixtures of MgB2 can be directly hydrogenated at 350-400°C and 700-950 bars to Mg(BH4) 2 in at least 75% yield, thus demonstrating for the first time full reversibility in this system. Thermolysis of the product gave >11 wt % H 2. Analysis of the product by XRD and 11B MAS NMR confirmed it to be beta-Mg(BH4)2. Details of these results will be presented in depth.