Synthesis and Characterization of Nanoscale Transition Metal Complex for Hydrogen Storage
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Synthesis and characterization of nanoscale transition metal complex for hydrogen storage Sesha S. Srinivasan1, Matthew T. Smith1, Deepak Deshpande2, Eias K. Stefanakos1, Yogi Goswami2, Michael Jurczyk1, Arun Kumar1, Ashok Kumar1 1 Clean Energy Research Center, College of Engineering, University of South Florida, Tampa, FL- 33620 2 Solar Energy and Energy Conversion Laboratory, College of Engineering, University of Florida, Gainesville, FL 32611 ABSTRACT The development of light weight hydrogen storage systems with high volumetric and gravimetric hydrogen densities is indeed essential for the on-board fuel cell vehicular applications [1]. Among the different hydrogen storage systems designed and developed so far, Ti- doped sodium aluminum hydrides exhibit potential promise of reversible hydrogen storage capacity (4-5 wt.%) at moderate temperatures [2,3]. However, the poor cyclic stability of these hydrides due to the partial reversibility of the two step reactions necessitates the development of exotic materials or tailoring the known hydride systems. On the other hand, transition metal complex hydrides, TMHx (T = Mg; M = Fe, Co, Ni) have also been identified as potential candidates for hydrogen storage [4-6] and/or analogous to alanates [7]. These hydrides especially Mg2FeH6, have shown excellent cyclic capacities (more than 500 cycles) even without a catalyst [8]. Besides, Mg2FeH6 possesses the highest volumetric and gravimetric hydrogen densities of 150 kg/m3 and 5.6 wt.% respectively [9]. However, at low temperatures, the rate of release of hydrogen and the effective reversible hydrogen capacity seems poor. Recent reports declared that the enhancement in the cycling kinetics and reduction in the operating temperature is very much possible by using a distorted nano-scale Mg structure [10, 11], doping the host lattice with Ti- species and/or lattice substitution [12]. Keeping these facts in view, the present investigation aims to improve the sorption kinetics and thermodynamics of Mg2FeH6, by 1) preparing nano-scale Mg-Fe-H system using mechano-chemical synthesis process, 2) surface localized catalyst (Ti- species) doping and 3) cationic substitution of Na+/Li+ for Mg2+ by incorporating NaH/LiH. The synergistic behavior of the tailored nano-scale transition metal complex for hydrogen storage is outlined. EXPERIMENTAL DETAILS The synthesis of nanostructured Mg2FeH6 was carried out by inexpensive mechano-chemical process (Fritsch Pulversette 6) under reactive gas (H2) milling. The nanoscale approach was adopted due to the following advantages; (i) minimizes the Hdiffusion path, (ii) enhances the rate of charging and discharging, (iii) optimizes the grain size, (iv) improves surface and bulk sorption and (v) improves cycle life performance. The appropriate stoicheometries of elemental magnesium (-325 mesh; 99.9% pure) and fine iron powder (-100 mesh; 99.8% pure) were taken in 2:1 mole ratio and milled under continuous flow of hydrogen (~2-3 atm). The milling parameters such as speed, mode of rotations, time
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