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 vapours under He/H2 atmosphere. The equilibrium pressures determined by in situ measurements at low temperature were slightly above those predicted using enthalpy H and entropy S of bulk magnesium. A single van 't Hoff fit over a range extended up to 550 K yields the thermodynamic parameters H = 68.1±0.9 kJ/molH2 and S = 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.

Interface Enthalpy-Entropy Competition in Nanoscale Metal Hydrides

“…Figure 4 shows sorption pseudo-kinetics obtained at 375 K. The hydrogen absorption and desorption rates were calculated from the initial slope of the kinetics, resulting in 0.28 wt % H 2 /min in absorption and 0.018 wt % H 2 /min. These values are roughly one order of magnitude lower than observed at 423 K [8], but are remarkable for MgH 2 at such low temperature and mild pressure, especially for hydrogen desorption. Figure 4 also suggests that the sorption rates rapidly decrease when p(H 2 )/p eq approaches unity because the thermodynamic driving force tends to zero in this limit.…”

“…For the NPs, the entropy is also less negative (by about 10%) compared to the bulk, whereas for the NDs, the difference is smaller (about 3%) and well within the uncertainty. In Figure 5, we also notice that the equilibrium pressure of the NDs (dotted black line) is larger than that of the NPs (solid black line) and of bulk Mg [8] by a factor of two. The NDs, therefore, realize the picture outlined Figure 1d, showing both an upward shift of the equilibrium pressure (thermodynamical bias) and a large extrinsic hysteresis.…”

“…The materials analyzed in this work are composite Mg-Ti-H nanoparticles (NPs) synthetized by gas phase condensation, as previously reported [8]. A broader description of the NPs growth technique is given in the Methods and Material section of this work.…”

“…The values obtained from the separate fits of absorption and desorption pressures reveal a discrepancy arising from hysteretic effects. Van't Hoff plot of sorption (red) and desorption (blue) equilibrium pressures (log scale) versus T (reciprocal scale) measured for NPs samples [8] and NDs (t = r = 30 nm) [5]. The black solid and dashed lines are the best linear fit on equilibrium pressure values for NPs and NDs respectively.…”

“…Figure 6 suggests, qualitatively, that the hysteresis rises with increasing dimensionality of the constraint (1D for thin films and 3D for NDs) and with decreasing confinement length (decreasing ND diameter). [8], confronted with hysteresis value for NDs of different diameter (60 nm and 320 nm) [5] and for a quasi-free 50 nm thick Mg film [7].…”