Nanoconfined Alkali-metal borohydrides for Reversible Hydrogen Storage

Research output: ThesisDoctoral thesis 1 (Research UU / Graduation UU)

Abstract

Hydrogen has been identified as a promising energy carrier. Its combustion is not associated with pollution when generated from renewable energy sources like solar and wind. The large-scale use of hydrogen for intermittent energy storage and as a fuel for cars can contribute to the realization of a more sustainable energy system in the future. However several technical challenges have to be addressed, of which compact, safe and cost effective on-board hydrogen storage is a crucial one. A promising approach is the use of metal hydrides with high hydrogen content for reversible hydrogen storage. Alkali-metal borohydrides like LiBH4 and NaBH4 have high gravimetric and volumetric hydrogen densities. However due to unfavourable thermodynamics and poor kinetics, hydrogen release from these compounds occurs at elevated temperatures only, and is not readily reversible under practical conditions. This thesis describes a study on nanosizing and nanoconfinement of alkali-metal borohydrides in nanoporous scaffolds as a strategy to address these issues. The preparation and characterization of nanoconfined LiBH4 and NaBH4 are described in this thesis. Ordered mesoporous silica (SBA-15) and nanoporous carbon materials were used as scaffolds for the synthesis. The effects of physical confinement at the nanoscale and interaction with the nanoporous host material on the hydrogen sorption properties of these complex-metal hydrides are discussed. Furthermore the effect of combining nanoconfinement and the use of additives (Ni nanoparticles) on the hydrogen release and uptake kinetics is described. Nanoconfinement of alkali-metal borohydrides resulted in a significant decrease in their hydrogen release temperatures compared to the bulk. Support effects were clearly observed; the use of mesoporous silica as a template led to irreversible formation of stable silicates during hydrogen release from the nanocomposites, while the use of high purity nanoporous carbon materials resulted in remarkable increase in the reversibility of the hydrogen release. For LiBH4, combination of nanoconfinement in carbon and addition of Ni nanoparticles resulted in a further increase in the reversible capacity of the nanocomposites under moderate rehydrogenation condition(40 bar H2 and 320 °C). The equilibrium decomposition temperature (at 1 bar H2) of LiBH4 and NaBH4 confined in nanoporous carbon material was significantly lower than that of the bulk, showing that their stability and/or decomposition pathway were changed upon nanoconfinement in carbon material. The results discussed in this PhD thesis shows that nanoconfinement in porous carbon materials is an effective strategy to fundamentally change the kinetics, reversibility and the equilibrium of hydrogen uptake and release in complex metal hydrides. These changes are attributed to the effect of nanosizing which results in reduced mass transport distances(nanoscale); physical confinement at nanoscale which restricts segregation of the immobile dehydrogenated phases, and interaction with the nanoporous support. Our work established a new preparation technique for nanoconfined alkali borohydrides, and providesvaluable information on factors that influence the dehydrogenation and rehydrogenation rates, stability and most importantly on how to achieve full reversibility of the hydrogen release from alkali-metal borohydrides.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Utrecht University
Supervisors/Advisors
  • de Jong, Krijn, Primary supervisor
  • de Jongh, Petra, Co-supervisor
Award date16 Jan 2012
Publisher
Publication statusPublished - 16 Jan 2012

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