Nanoconfinement of complex hydrides in porous hosts for hydrogen storage applications.

The transition from a fossil fuel-dependent society to a cleaner, more sustainable society will not be possible without renewable energy sources. Hydrogen holds great potential as an energy carrier as an alternative to fossil fuels in such society. However, the compact and safe storage of hydrogen are still major challenges. Solid state hydrogen storage offers the possibility to store hydrogen in solids offering high volumetric and high gravimetric energy densities, while reducing the risks associated when handling hydrogen gas. Nevertheless, no single system has fully achieved the required properties for on-board mobile applications. Various approaches can be adopted with the aims of improving the kinetics and thermodynamics of hydrogen sorption. The nanostructuring of materials is one of the more promising strategies to achieve these aims. Reduction of the particle size of hydrides by nanoconfinement in forms of porous matrix leads to an increased surface area of the active material, and shorter diffusion distances for hydrogen atoms or ions to travel in the solid state. Kinetic barriers can be overcome and thermodynamics manipulated. An enhanced dehydrogenation rate and a reduced dehydrogenation temperature can be achieved by impregnating metal hydrides into porous scaffolds. Two complex hydrides are selected for study in this work; LiAlH4 and LiNH2. LiAlH4, is the lightest of the alanates, with a theoretical hydrogen storage capacity of 10.5 wt.%, and 7.9 wt. % H2 evolved below 220 °C. LiNH2 mixed with LiH, as part of the Li-N-H system, can reversibly desorb/uptake 6.5 wt. % H2 at 300 °C. When LiNH2 is heated alone, it releases ammonia (which is decomposed to N2 and H2 at higher temperatures > 400 °C). In this work, LiAlH4 has been impregnated in different types of commercial and synthesised porous carbon scaffolds for the first time. Nanoconfinement of the active material was achieved using solution impregnation with diethyl ether as a solvent. Analogously, the confinement of LiNH2 in porous carbon was achieved “in-situ” using lithium-ammonia solutions. Both confined composites showed lower dehydrogenation temperatures in comparison with the respective bulk materials. The influence of the design of the carbon scaffold (as manifested for example, by the surface area and the pore volume and pore size distribution) on the dehydrogenation behaviour of the impregnated complex hydrides is demonstrated. By judicious selection of an appropriate porous host, we show how it is possible to induce faster H2 desorption and substantially reduce the desorption temperature. The onset of hydrogen release for confined LiAlH4 decreased significantly in temperature, being reduced by 51 °C (in both porous hosts used, AX-21 and FDU-15) in comparison with as-received LiAlH4. The temperature at which the hydrogen release was maximised was also lowered (by 16 °C in FDU-15 and by 26 °C in AX-21) in comparison with as-received LiAlH4. The confined LiNH2 showed a much earlier release of hydrogen in comparison with as-received LiNH2. Normally LiNH2 would thermally decompose to Li2NH with ammonia evolution, but ammonia release was eliminated for the confined sample. Reaction with carbon led to irreversible Li2CN2 formation and hydrogen evolution. A set of experiments to establish the formation of Li2CN2 with physically mixed samples were performed. The physically mixed samples showed hydrogen release between 400 - 450 °C, producing a mixture of Li2NH and Li2CN2, suggesting two decomposition pathways were followed. In contrast, confined LiNH2 released hydrogen ca. 220 °C lower than the physically mixed sample, with no detectable trace of ammonia release