Investigating possible kinetic limitations to MgB2 hydrogenation |
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| Affiliation: | 1. Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA;2. Sandia National Laboratories, Livermore, CA, 94551, USA;3. Lawrence Livermore National Laboratory, Livermore, CA, 94551, USA;1. NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899-6102, United States;2. Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742-2115, United States;3. National Renewable Energy Laboratory, Golden, CO 80401, United States;4. Energy Nanomaterials, Sandia National Laboratories, Livermore, CA 94551, United States;1. Department of Hydrogen and Heat Application Research and Development, Japan Atomic Energy Agency, 4002 Narita-cho, Oarai, 311-1393, Ibaraki, Japan;2. Department of Applied Chemistry, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, 135-8548, Tokyo, Japan;1. Moscow M.V. Lomonosov State University, Faculty of Physics, Moscow, Russia;2. Technological Institute for Superhard and Carbon Materials, Troitsk, Russia;3. Shmid’t Institute of the Physics of the Earth, Moscow, Russia;4. Institute of Solid State Chemistry and Mechanochemistry SB RAS, Novosibirsk, Russia;6. ESRF, Grenoble, France;1. Department of Experimental Physics, Faculty of Mathematics, Physics and Informatics, Comenius University in Bratislava, Slovakia;2. Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovakia;3. Department of Condensed Matter Physics, Institute of Physics, P.J. Safarik University, Košice, Slovakia;4. Institute of Physics, Slovak Academy of Sciences, Bratislava, Slovakia;5. Toyota Technological Institute, Nagoya, Japan;1. Advanced Energy Materials Research Institute, North China Electric Power University, No.2 Beinonglu Changping District, Beijing 102206, China;2. China Iron & Steel Research Institute Group, Advanced Technology & Materials Co., Ltd, No.76 Xueyuan nanlu, Haidian District, Beijing 100081, China |
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| Abstract: | An investigation is reported of possible kinetic limitations to MgB2 hydrogenation. The role of H–H bond breaking, a necessary first step in the hydrogenation process, is assessed for bulk MgB2, ball-milled MgB2, as well as MgB2 mixed with Pd, Fe and TiF3 additives. The Pd and Fe additives in the MgB2 material exist as dispersed metallic particles in the size range ~5–40 nm diameter. In contrast, TiF3 reacts with MgB2 to form Ti metal, elemental B and MgF2, with the Ti and the MgF2 phases proximate to each other and coating the MgB2 particulates with a film of thickness ~3 nm. Sieverts studies of hydrogenation kinetics are reported and compared to the rate of H–H bond breaking as measured by H-D exchange studies. The results show that H–H bond dissociation does not limit the rate of hydrogenation of MgB2 because H–H bond cleavage occurs rapidly compared to the initial MgB2 hydrogenation. The results also show that surface diffusion of hydrogen atoms cannot be a limiting factor for MgB2 hydrogenation. Instead, it is speculated that it is the intrinsic stability of the B–B extended hexagonal ring structure in MgB2 that hinders the hydrogenation of this material. This supposition is supported by B K-edge x-ray absorption measurements of the materials, which showed spectroscopically that the B–B ring was intact in the material systems studied. The TiF3/MgB2 system was examined further theoretically with reaction thermodynamics and phase nucleation kinetic calculations to better understand the production of Ti metal when TiB2 is thermodynamically favored. The results show that there exist physically reasonable ranges for which nucleation kinetics supersede thermodynamics in determining the reactive pathway for the TiF3/MgB2 system and perhaps for other additive systems as well. |
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| Keywords: | Hydrogen storage Magnesium diboride Hydrogenation Kinetics H-D exchange Surface diffusion |
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