| Abstract |
“Renewable” hydrogen is being widely investigated as a future energy carrier in an effort to reduce greenhouse gas emissions and replace energy production from fossil fuels whose availability is projected to steadily decline in the next few decades. The development of safe and efficient hydrogen storage technologies for mainly vehicular applications is key for the establishment of a hydrogen-based economy. High pressure and liquid cryo-storage of hydrogen are not practical for mobile applications as a result of lower-than-required energy densities and safety considerations. Significant effort is being devoted to solid state hydrogen storage based on either sorbents or metal, chemical and complex hydrides. In particular complex hydrides (such as light metal aluminium hydrides and borohydrides). Much attention is given nowadays to borohydrides (or boranates) where a boron atom forms covalent bonds with four surrounding H atoms to form [BH4]- anions, the charge of which is compensated by metal cations M+ (e.g. Na, Li, Mg, Ca). Despite their high hydrogen content (usually >10wt% H), the application of complex metal hydrides in general has not so far led to technically viable solutions application-wise, as hydrogen delivery generally proceeds through complex, multi-step schemes at elevated temperatures with very slow kinetics, giving rise also to irreversibility effects upon cycling. The most effective strategies used to improve the behaviour of borohydrides' dehydrogenation are (a) the doping of a borohydride with a metal cation of a different electronegativity towards destabilized bimetallic compounds of intermediate thermodynamic stability, and (b) nanosizing, by infiltrating the borohydride into a porous inert scaffold. The latter approach has been theoretically and experimentally found to alter the thermodynamic properties of the bulk phase and/or boost the reaction kinetics, as it may prevent particle agglomeration during cycling as well as phase separation which would hinder cyclability. However the process has not been optimized yet and the role of the porous scaffold has not been clarified. The present work has aspired to investigate in a systematic way the nanoconfinement effect on the dehydrogenation/hydrogenation properties of borohydrides. Apart from the investigation of different compounds with emphasis on NaBH4, Ca(BH4)2, Mg(BH4)2 και LiBH4, significant effort was devoted to the study of the combined effect of destabilization and nanosizing through melt infiltration in different porous scaffolds of ball milled borohydride eutectic mixtures, namely LiBH4 / Ca(BH4)2 and LiBH4/ Mg(BH4)2. By mixing two light-metal borohydrides it is possible to induce their destabilization and at the same time preserve the gravimetric hydrogen capacity of the system as high as possible.
Different types of (mainly mesoporous) carbons, covering a wide range of pore sizes were prepared and used as scaffolds in an effort to investigate the effect of pore size on the thermal decomposition and cycling properties of the borohydrides. A large number of composite materials were prepared by both solvent impregnation and melt infiltration methods and systematically characterized with a wide range of advanced techniques aiming to elucidate their structural, pore and hydrogen storage properties. In summary, it is demonstrated that generally, the use of solvents for the infiltration is attached to several practical problems (such as complexation with the borohydrides, slow infiltration due to reduced diffusivity, strong adsorption of the solvent onto the scaffold and hence problematic removal, etc.). For example, the use of liquefied ammonia can be employed for the successful infiltration of borohydrides in carbon matrices with sufficiently high loadings while the hydrogen release temperatures are reduced relative to the bulk however, it is also shown that ammonia tends to form complexes with the borohydrides which leads to detrimental behaviors during dehydrogenation. Melt infiltration is preferable, where possible (the melting temperature should be lower than the decomposition temperature). For example, a sample of melt infiltrated LiBH4/Ca(BH4)2 in a porous matrix presented a much reduced H2 release temperature and a better cycling behavior of 5% over 7 cycles. The matrix pore size is very influential, as the best results are achieved with carbons with a pore size of ~5nm.
Finally, it is shown that carbon matrices act in a catalytic capacity resulting in the acceleration of dehydrogenation kinetics but with a possible impact on the rehydrogenation of eutectic borohydride mixtures.
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Ανάπτυξη, χαρακτηρισμός και αποτίμηση πορωδών σύνθετων υλικών για αποθήκευση υδρογόνου
