Fractal disperse hydrogen sorption kinetics in spark discharge generated Mg/NbOx and Mg/Pd nanocomposites

Abstract

Isothermal hydrogen desorption of spark discharge generated Mg/NbO x and Mg/Pd metal hydride nanocomposites is consistently described by a kinetic model based on multiple reaction rates, in contrast to the Johnson-Mehl-Avrami-Kolmogorov [M. Avrami, J. Phys. Chem. 9, 177 (1941); W. A. Johnson and R. F. Mehl, Trans. Am. Inst. Min., Metal. Eng. 135, 416 (1939); A. N. Kolmogorov, Izv. Akad. Nauk SSSR, Ser. Mat. 3, 355 (1937); F. Liu, F. Sommer, C. Bos, and E. J. Mittemeijer, Int. Mat. Rev. 52, 193 (2007)] model which is commonly applied to explain the kinetics of metal hydride transformations. The broad range of reaction rates arises from the disperse character of the particle size and the dendritic morphology of the samples. The model is expected to be generally applicable for metal hydrides which show a significant variation in particle sizes, in configuration and/or chemical composition of local surroundings of the reacting nanoparticles. In the context of hydrogen storage, MgH 2 remains an attractive candidate despite its relatively high stability and slow sorption kinetics. The latter can be substantially increased using small grain sizes 1 and adding transition metals or their oxides. In particular, niobium 2 and palladium 3,4 have a pronounced impact on the sorption kinetics. Previous studies [4][5][6] showed that spark discharge generation (SDG) is a versatile method for the production of metal nanoparticles. In particular, the onset of hydrogen desorption of spark discharge produced MgH 2 nanoparticles loaded from the gas phase occurs at a rather low temperature of $400 K. 5 The hydrogen desorption is characterized by a broad desorption profile, which could be explained in terms of multiple apparent activation energies.Here, we present a systematic study of the isothermal hydrogen desorption kinetics of spark discharge Mg/NbO x and Mg/Pd nanocomposites loaded from the gas phase. We found that a kinetic model based on multiple reaction rates within the sample can consistently describe the results from our studies, in contrast to the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, 7-10 applied to metal hydride transformation kinetics, and explains the transformation as a homogeneous nucleation and growth process. The broad range of reaction rates arises from the disperse character of the particle size distribution and the dendritic morphology of the SDG samples.A dual spark generator setup 5 was used for the production of Mg nanoparticles and for the intermixing with spark discharge generated Pd or Nb catalyst particles in-situ in the spark discharge reactor. The frequency of the 40 mJ Mg sparks is 300 Hz. In order to produce a targeted amount of a few at. % of catalyst, the second spark is operated with 14 mJ pulses at a frequency of 34 Hz. The samples were loaded in a hermetically closed sample holder in an Ar glovebox. Transmission electron microscopy (TEM) was performed on a FEI TECNAI TF20 monochromatic electron microscope at 200 kV. The x-ray diffraction (XRD) was performed using a Panalytical X'pert Pro ...

Reduced Enthalpy of Metal Hydride Formation for Mg–Ti Nanocomposites Produced by Spark Discharge Generation

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