Hydrogenation properties of KSi and NaSi Zintl phases

Abstract

The recently reported KSi–KSiH₃ 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 K₈Si₄₆, instead of forming the KSiH₃ 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 Na₈Si₄₆, indicating a very poorly reversible reaction.