title: Hydrogenation properties of KSi and NaSi Zintl phases.
abstract: The recently reported KSi-KSiH(3) system can store 4.3 wt% of hydrogen reversibly with slow kinetics of several hours for complete absorption at 373 K and complete desorption at 473 K. From the kinetics measured at different temperatures, the Arrhenius plots give activation energies (E(a)) of 56.0 ± 5.7 kJ mol(-1) and 121 ± 17 kJ mol(-1) for the absorption and desorption processes, respectively. Ball-milling with 10 wt% of carbon strongly improves the kinetics of the system, i.e. specifically the initial rate of absorption becomes about one order of magnitude faster than that of pristine KSi. However, this fast absorption causes a disproportionation into KH and K(8)Si(46), instead of forming the KSiH(3) hydride from a slow absorption. This disproportionation, due to the formation of stable KH, leads to a total loss of reversibility. In a similar situation, when the pristine Zintl NaSi phase absorbs hydrogen, it likewise disproportionates into NaH and Na(8)Si(46), indicating a very poorly reversible reaction.
authors.affiliation: Laboratoire de Réactivité et Chimie des Solides, UMR 7314 CNRS Université de Picardie Jules Verne, 33 rue St Leu, 80039 Amiens, France.
authors.forename: Wan Si
authors.initials: WS
authors.lastname: Tang
authors.forename: Jean-Noël
authors.initials: JN
authors.lastname: Chotard
authors.forename: Pascal
authors.initials: P
authors.lastname: Raybaud
authors.forename: Raphaël
authors.initials: R
authors.lastname: Janot