Hydrogen Desorption Below 150 °C in MgH₂–TiH₂ Composite Nanoparticles: Equilibrium and Kinetic Properties

Abstract

Reversible hydrogen sorption coupled with the MgH2 ↔ Mg phase transformation was achieved in the remarkably low 340–425 K temperature range using MgH2–TiH2 composite nanoparticles obtained by reactive gas-phase condensation of Mg–Ti vapors under He/H2 atmosphere. The equilibrium pressures determined by in situ measurements at low temperature were slightly above those predicted using enthalpy ΔH and entropy ΔS of bulk magnesium. A single van’t Hoff fit over a range extended up to 550 K yields the thermodynamic parameters ΔH = 68.1 ± 0.9 kJ/molH2 and ΔS = 119 ± 2 J/(K·molH2) for hydride decomposition. A desorption rate of 0.18 wt % H2/min was measured at T = 423 K and p(H2) ≈ 1 mbar, i.e., close to equilibrium, without using a Pd catalysts. The nanoparticles displayed a small absorption–desorption pressure hysteresis even at low temperatures. We critically discuss the influence exerted by nanostructural features such as interface free energy, elastic clamping, and phase mixing at the single nanoparticle level on equilibrium and kinetic properties of hydrogen sorption.