Exploring the kinetics and thermodynamics of TiFe0.8CrxMn0.2-x hydrogen storage alloys

Abstract

The influence of Fe substitution by Cr and/or Mn on the hydrogenation properties of TiFe 0.8 Cr x Mn 0.2-x hydrogen storage alloys was explored for several substitution amounts (x = 0, 0.05, 0.1, 0.15 and 0.2). The synthesized alloys consisted of a dominant TiFe (cubic, CsCl-type) and a secondary C14 Laves phase (hexagonal, MgZn 2 -type). The increase in Cr content, and consequently in C14 Laves phase fraction, improved the first hydrogenation kinetics and the hydrogen intake after activation (1.60–1.70 wt% H 2 at 30 °C within 3 min), while substantially stabilizing the monohydride phase. The reaction enthalpy obtained from pressure-composition-temperature (PCT) curves hence increased along Cr substitution, ranging 24.35 ± 0.34 to 35.39 ± 1.00 kJmol -1 for the absorption, and 31.08 ± 1.39 to36.34 ± 1.99 kJmol −1 for the desorption (absolute values). These experimental findings were in good agreement with density functional theory (DFT) predictions performed across the explored composition space. The estimated entropy values on the other hand remained nearly constant, fluctuating between 84.28 ± 4.76 and 102.64 ± 3.12 J mol −1 H 2 K −1 for the absorption, and 100.46 ± 9.71 up to 105.45 ± 4.31 J mol −1 H 2 K −1 for the desorption. Solid-gas reaction modeling showed that, under practical service conditions, the hydrogenation of activated alloys was interface-controlled followed by diffusion-controlled mechanisms as the reaction proceeded. • Dual phase (TiFe + C14 Laves) TiFe 0.8 Cr x Mn 0.2-x alloys were prepared by arc-melting. • First hydrogenation kinetics increased along with Cr content hence with Laves phase. • Mn increase led to higher and flatter plateau pressure contrasting with Cr effect. • Consistent experimental and DFT estimations of the monohydride formation enthalpy. • Hydrogenation consisted in interface-followed by diffusion-controlled mechanisms.

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