ARTICLE pubs.acs.org/JPCC
Thermodynamic and Kinetic Investigations on Pure and Doped NaBH4-MgH2 System Chiara Milanese,*,† Sebastiano Garroni,‡ Alessandro Girella,† Gabriele Mulas,§ Vittorio Berbenni,† Giovanna Bruni,† Santiago Suri~nach,‡ Maria Dolors Baro,‡ and Amedeo Marini† †
C.S.G.I. & Department of Chemistry, Physical Chemistry Section, University of Pavia, Viale Taramelli 16, 27100 Pavia, ITALY Departamento de Física, Universitat Autonoma de Barcelona, 08193 Bellaterra, Spain § Department of Chemistry, University of Sassari, Via Vienna 2, 07100 Sassari, Italy ‡
ABSTRACT: In this work, a thermodynamic picture of the dehydrogenation mechanism of NaBH4 in NaBH4-MgH2 mixtures with 2:1 and 1:2 molar ratio is drawn, for the first time in literature, thanks to coupled manometric-calorimetric measurements up to 580 °C. Such a new approach also allows, after the measurement of the borohydride melting enthalpy, the evaluation of the dehydrogenation enthalpy of the complex hydride in the mixtures. The thermodynamics of the 2:1 sample (where the borohydride decomposition takes place mainly in liquid state) is more favorable than that of the 1:2 mixture, where the process evolves fully in solid state. The kinetics of the systems is studied at 450 °C, the minimum temperature at which the borohydride decomposition takes place. In these conditions, the 1:2 system is kinetically favored. Several additives (fluorides; chlorides; hydroxides) have been tested as possible destabilizing/catalyzing agents. These substances react with the component hydrides upon discharging, forming stable binary and ternary compounds that do not change the macroscopic desorption pathway of the composites (separate decomposition of the component hydrides) but lead to variations in the desorption temperature and kinetics. In particular, MgF2 is found to improve the desorption kinetics of both the component hydrides and to reduce the decomposition temperature and enthalpy of NaBH4 in the 2:1 system. On the contrary, none of the tested dopants exerts any positive effect on the 1:2 system.
’ INTRODUCTION In the field of solid state hydrogen storage, particular attention has recently been paid to reactive hydride composites (RHC),1,2 where the combination of an alkaline borohydride with a metallic hydride,3,4 and/or another complex hydride5,6 induces a mutual thermodynamic destabilization and a decrease of hydrogen desorption temperatures with respect to the component phases, maintaining high gravimetric H2 capacities. The NaBH4-MgH2 system seems to be a good model for the study of the sorption mechanism in alkaline metal borohydridesmagnesium hydride RHC, due to the fact that only one species, i.e., the intermetallic compound MgB2, forms and acts as a destabilizing agent,7,8 while in the Li containing system both MgB2 and the mixed LixMgy alloys could play a role.9,10 Moreover NaBH4 is more stable upon exposure to air and, hence, easier to handle with respect to the other borohydrides.7 Finally, the overall system is relatively cheap (
