Thermodynamic Changes in Mechanochemically Synthesized Magnesium Hydride Nanoparticles

Abstract

The thermodynamic properties of magnesium hydride nanoparticles have been investigated by hydrogen decomposition pressure measurements using the Sieverts technique. A mechanochemical method was used to synthesize MgH(2) nanoparticles (down to approximately 7 nm in size) embedded in a LiCl salt matrix. In comparison to bulk MgH(2), the mechanochemically produced MgH(2) with the smallest particle size showed a small but measurable decrease in the decomposition reaction enthalpy (DeltaH decrease of 2.84 kJ/mol H(2) from DeltaH(bulk) = 74.06 +/- 0.42 kJ/mol H(2) to DeltaH(nano) = 71.22 +/- 0.49 kJ/mol H(2)). The reduction in DeltaH matches theoretical predictions and was also coupled with a similar reduction in reaction entropy (DeltaS decrease of 3.8 J/mol H(2)/K from DeltaS(bulk) = 133.4 +/- 0.7 J/mol H(2)/K to DeltaS(nano) = 129.6 +/- 0.8 J/mol H(2)/K). The thermodynamic changes in the MgH(2) nanoparticle system correspond to a drop in the 1 bar hydrogen equilibrium temperature (T(1 bar)) by approximately 6 degrees C to 276.2 +/- 2.4 degrees C in contrast to the bulk MgH(2) system at 281.8 +/- 2.2 degrees C. The reduction in the desorption temperature is less than that expected from theoretical studies due to the decrease in DeltaS that acts to partially counteract the effect from the change in DeltaH.

Influence of Crystal Structure of Bulk Phase on the Stability of Nanoscale Phases: Investigation on MgH2 Derived Nanostructures

“…45 This MgH 2 −TiH 2 nanocomposite exhibits outstanding kinetic properties and cycling stability. 45 Nanocrystals of MgH 2 (∼7 nm in size) embedded in a matrix of LiCl were reported by Paskevicius et al 46 using mechanochemical metathesis reaction between MgCl 2 and LiH. 46 In this study the enthalpy (ΔH) was decreased by 2.84 kJ/mol H 2 , from 74.06 ± 0.42 kJ/mol H 2 for the bulk to 71.22 ± 0.49 kJ/mol H 2 for the 7 nm crystals.…”

“…45 Nanocrystals of MgH 2 (∼7 nm in size) embedded in a matrix of LiCl were reported by Paskevicius et al 46 using mechanochemical metathesis reaction between MgCl 2 and LiH. 46 In this study the enthalpy (ΔH) was decreased by 2.84 kJ/mol H 2 , from 74.06 ± 0.42 kJ/mol H 2 for the bulk to 71.22 ± 0.49 kJ/mol H 2 for the 7 nm crystals. The entropy (ΔS) also decreased from 133.4 ± 0.7 J/k mol H 2 to 129.6 ± 0.8 J/k mol H 2 .…”

Facile preparation of β-/γ-MgH2 nanocomposites under mild conditions and pathways to rapid dehydrogenation

“…The ensuing large decomposition enthalpy of 75 kJ mol -1 H 2 results in relatively high desorption temperatures (>300 °C) coupled with a slow hydrogen diffusion rate, limiting its practical applications. 5 Different approaches such as catalysis, 6,7 alloying, 8,9 composite formation 10,11 and nanostructuring [12][13][14] have been employed to improve the hydrogen uptake/release kinetics and thermodynamics of MgH 2 . One of the most widely used strategies is to reduce the MgH 2 particle size by mechanical ball milling, hence lowering the activation energy of desorption.…”

Powder diffraction methods for studies of borohydride-based energy storage materials

“…This may allowp ractical utilisation under more moderatec onditions. Experimental studies have shown that the thermodynamic properties are more favourable for nano particles ($5nm) (Paskevicius, Sheppard and Buckley, 2010) and nanoconfinement have been shown to improvet he reversibility,s tability,k inetics, and possibly also the thermodynamic properties of chemical reactions involved in reversible release and uptake of hydrogen (Nielsen et al,2009;Nielsen et al,2010). Bogdanović and Schwickardi, 1997 showed that NaAlH 4 can reversibles tore hydrogen at more moderate conditions by doping with titanium compounds.…”