Investigating possible kinetic limitations to MgB₂ hydrogenation

Journal Article · Fri Nov 29 00:00:00 EST 2019 · International Journal of Hydrogen Energy

DOI:https://doi.org/10.1016/j.ijhydene.2019.09.125· OSTI ID:1574810

Liu, Y. -S. ^[1]; Klebanoff, L. E. ^[2]; Wijeratne, P. ^[2]; Cowgill, D. F. ^[2]; Stavila, V. ^[2]; Heo, T. W. ^[3]; Kang, S. ^[3]; Baker, A. A. ^[3]; Lee, J. R. I. ^[3]; Mattox, T. M. ^[1]; Ray, K. G. ^[3]; Sugar, J. D. ^[2]; Wood, B. C. ^[3]

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1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
2. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
3. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

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 MgB₂, with the Ti and the MgB₂ 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. Finally, 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.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States); Sandia National Laboratories (SNL-CA), Livermore, CA (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE), Fuel Cell Technologies Office (EE-3F); USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
• Grant/Contract Number: AC02-05CH11231; AC04-94AL85000; AC52-07NA27344; NA0003525
• OSTI ID: 1574810
• Alternate ID(s): OSTI ID: 1577899; OSTI ID: 1597233
• Report Number(s): LLNL-JRNL--776812; SAND--2019-5900J; 675831
• Journal Information: International Journal of Hydrogen Energy, Journal Name: International Journal of Hydrogen Energy Journal Issue: 59 Vol. 44; ISSN 0360-3199
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

08 HYDROGEN
Energy - Hydrogen
H-D exchange
hydrogen storage
hydrogenation
kinetics
magnesium diboride
surface diffusion