The Effect of Metal Oxides Additives on the Absorption/Desorption of MgH2 for Hydrogen Storage Applications

Magnesium is considered as one of the more promising candidates for hydrogen storage, primarily because of its abundance and the high hydrogen capacity of MgH2, magnesium dihydride (7.6 wt%). Unfortunately, practical applications of MgH2 are limited by poor hydrogen sorption kinetics and high thermodynamic stability, resulting in slow charging and the need for high temperatures to release the hydrogen. Methods to improve the sorption kinetics of MgH2, through ball-milling, alloying and introducing small amounts of additives are currently under investigation. The aim of this work was to investigate the effect of different transition metal oxides, as well as other catalysts, on the hydrogen sorption kinetics, hydrogen release temperature and cycling stability of MgH2. The rate of MgH2 absorption is determined by the physisorption and dissociation of molecular hydrogen, diffusion through the hydride layer and then nucleation of the hydride. Whereas desorption is determined by nucleation of metal phase, diffusion of atomic hydrogen through the metal and hydride, and recombination to form molecular hydrogen at the surface before dissociation of the molecule form the surface. The addition of small amounts of additives (< 10 mol%) to MgH2 during milling have been shown to have a significant effect on the kinetics of absorption and desorption of hydrogen. In addition, the effect of oxygen as a component of these additives has been extensively studied. Although a surface layer of MgO is known to slow the diffusion of hydrogen into metal, niobium oxide, Nb2O5, is one of the best additives for kinetic improvement of MgH2. It has been suggested that higher valance oxides have a greater effect on the kinetics; also recent studies have indicated that the formation of magnesium-niobium ternary oxide compound may be responsible for the enhancement, however the exact mechanism by which Nb2O5 enhances the kinetics is still unclear. Various transition metal oxides have proved to be effective additives for hydrogen absorption/desorption of Mg-based hydrides e.g., Nb2O5 and TiO2. In this work organometallic additives based on transition metal oxides (Ti and V) and halides have been chosen, some of which are liquid at room temperature. These additives were chosen to extend the oxide valency to higher values, provide a comparison between liquid and solid oxide additives, and compare a non-oxide transition metal additive (Ti-based chloride). Nb2O5 was used as the benchmark for comparisons. A range of different amounts of transition metal (Ti and V) oxides and other additives, both liquid and solid, were ball milled with pre-milled MgH2 under different gas environments and the hydrogen sorption behaviour of the ball milled composite samples was investigated using a TPD/TDS Sieverts apparatus. A key difference from previous work in this area was the use of organo-metallic compounds, some of which were liquid at room temperature. Samples were characterized using X-Ray Diffraction, and, where feasible, Scanning Electron Microscopy and Raman Spectroscopy before and after recording the hydrogen sorption behaviour. A Sievert apparatus was improved, then modified to include TPD and TDS capability to determine the hydrogen uptake and release and to perform all these measurements in one consistent environment. With the aim of understanding what effect the ball milling gas environment might have on the MgH2 sorption kinetics, two different gases, Ar and H2, were used. MgH2, with and without additives (C60, TTIP (Titanium TetraIsoPropoxide) and Nb2O5) were milled under the two different gases. In most cases the milling gas had little to no effect on desorption kinetics, but a noticeable effect in the absorption uptake was observed - by up to 2 wt% - depending on the gas used. To understand the role of an organo-metallic oxide liquid additive on MgH2 sorption kinetics, a systematic survey was performed by milling up to 2 mol% of TTIP with MgH2 and the results compared to composite samples with Nb2O5 as the additive. TTIP was found to be equally as effective as Nb2O5 with superior hydrogen capacity, and, just as for Nb2O5, only a small amount of additive, 0.5 mol% of TTIP was found to be sufficient for kinetics enhancement. Interestingly, 0.5 mol% TTIP hand-mixed with the pre-milled MgH2 was also found to be effective for desorption, but not for the absorption kinetics. To further investigate the effect of organo-metallic liquid oxides, particularly transition metal oxides, on the enhancement of the sorption kinetics of MgH2, a range of liquid oxides (Ti-based and V-based oxides) were milled with MgH2 and compared to powder oxides. Ti-based oxides were found to have superior desorption enhancement, with the liquids performing better than the powders. The V-based oxides (all liquids) showed faster absorption and higher uptake when compared to Ti-based oxides. The Ti-chloride based organo-metallic additive was investigated to compare a non-oxide transition metal additive to the oxide additives. It was found that a small amount of 1 mol% of additive milled for short time (1 h) had the best desorption of all the additives investigated in this work - but with quite poor absorption. Overall, this project established that the use of transition metal oxides as additives has a great impact on improving the sorption kinetics of MgH2. The TTIP additive produced results at least as good as the benchmark additive Nb2O5, but with the significant advantage of being able to be mixed with MgH2 without ball-milling. The Ti and V based oxides additives were also shown to be effective, Ti-based achieving better desorption enhancement, whereas V-based oxide samples had faster absorption and higher uptake. It was also determined the ball milling gas environment can have a significant effect on the sorption kinetics, but the effect depended on the additive. The difference in the effect of additives on desorption and absorption cycles confirms the need to study combinations of additives for optimal overall benefit.