Hydrogen storage materials based on nabh4/mgh2 reactive hydride composite. Study of the desorption mechanism and kinetics

  1. Sebastiano Garroni
Supervised by:
  1. Maria Dolores Baró Mariné Director
  2. Santiago Suriñach Director

Defence university: Universitat Autònoma de Barcelona

Year of defence: 2011

Committee:
  1. Juan Santiago Muñoz Domínguez Chair
  2. Yolanda Calventus Solé Secretary
  3. Chiara Milanese Committee member

Type: Thesis

Teseo: 301782 DIALNET

Abstract

On-board hydrogen storage for fuel cell vehicles is the major technological challenge for the successful commercialization and market acceptance of hydrogen-powdered vehicles. High-pressure and cryogenic hydrogen storage systems are impractical for vehicular applications due to safety concerns and volumetric constraints. Therefore, sufficient storage of hydrogen in a safe and economical way while meeting all consumer necessities is a major issue to be solved to improve hydrogen power systems. In this contest, enormous efforts have been devoted to the research and development of materials that can hold sufficient hydrogen in terms of gravimetric and volumetric densities, and, at the same time, with favourable thermodynamic and kinetic properties. In that framework, the present thesis provides a valuable contribution to elucidate the sorption properties of the NaBH4/MgH2 system. The choice seemed obvious in the light of the fact that the NaBH4/MgH2 system has a high gravimetric capacity, high volumetric hydrogen density and a rather low dehydrogenation temperature compared to the single compounds. Several techniques have been applied in order clarify which phases are involved in sorption reaction and the amount of hydrogen released or absorbed. The dehydrogenation path of the 2:1 mixture follows a multi-step reaction and is strongly dependent on pressure and temperature conditions. In particular, the formation of nanocrystalline MgB2 was proved. Regarding the opposite stoichiometry 1:2, complete desorption was achieved in 3 h, whereas for the 2:1 stoichiometry 16 h were needed. Finally, several additives were tested to check their influence on the kinetics and thermodynamics of NaBH4 decomposition. The majority of the tested additives reacted with the hydrides upon discharging, forming stable binary and ternary compounds that did not change the macroscopic desorption pathway of the composites.