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Solid State Studies of [ReH9]-2 Complexes from Ambient to Extreme Conditions: High Volumetric Capacity Hydrogen Storage
Chemical and Materials Engineering
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The implementation of new alternative energy concepts is essential considering the increased pollution and exploitation of fossil energy resources. As the simplest and most abundant element, hydrogen has the potential to energy system. Solid state transition metal hydrides based on Re and transition metals are very attractive materials for hydrogen and energy storage due to their large volumetric capacity, providing a safe and efficient way of storing hydrogen. The [ReH9]-2 ion continues to captivate researchers because of its stability, unusually high oxidation state of rhenium metal (VII), D3h symmetry (tricapped trigonal prism); with 9 hydrogen atoms coordinating to Re atom. In this study, the effect of pressure on the metal-hydrogen bonding and the consequence on the hydrogen absorption and desorption are investigated for M[ReH9]-2, where M= Na, K, and Ba. Understanding the chemical and physical properties of metal hydrides at ambient as well as elevated pressure and temperature conditions will in sequence assist in the design of suitable storage materials with optimum thermodynamic parameters.The BaReH9 complex was synthesized and evaluated under ambient to extreme environments. For this compound, we report bulk moduli for lower and higher pressures obtained by fitting of pressure and volume data using obtained using 3rd order Birch-Murnaghan equation of state. The results exhibited a possible orientation disorder in the crystal without any phase transformation in the studied pressure, which is a divergence from computational studies. Grüneisen parameters were also calculated and reported here. High pressure studies of Na2ReH9 and K2ReH9 were also performed, report the assignment of vibrational spectroscopy at ambient condition. The structural behavior of Na2ReH9 and K2ReH9 performed at high pressure, using in-situ Raman spectroscopic and high resolution synchrotron x-ray diffraction, studies revealed new phase transformations above 9 GPa and 16 GPa for both Na2ReH9 and K2ReH9 due to compression. We deduced that the phase transformations occur to relieve this repulsion due to H-H interaction of two [ReH9]-2 ions as volume of the unit cell reduces due to pressure increases.Two new deuterated transition metal hydrides (Na2ReD9 and BaReD9) were successfully synthesized by reaction via metathesis reaction in deuterated ethanol. Raman and IR spectroscopy showed isotopic shift with (H/D) of ~1.4. The crystal structure of BaReD9 has been determined by synchrotron powder diffraction and the results were consistent with the P63/mmc (#194) space group, with lattice parameters a = 5.2924 (±0.0006) Å and c = 9.3286 (±0.0014) Å. In summary, the effect of pressure on the metal-hydrogen bonding of BaReH9, Na2ReH9, and K2ReH9 were investigated using synchrotron x-ray, Raman and IR spectroscopy. BaReH9 is stable up to the studied pressure while Na2ReH9 and K2ReH9 exhibit phase transformation at higher pressures. Two new deuterated transition metal hydrides (Na2ReD9 and BaReD9) were successfully synthesized and characterized.