Oral Presentation 24th Australian Conference on Microscopy and Microanalysis 2016

Hydrogen release mechanisms in bulk MgH2 by ultra-high voltage transmission electron microscopies (#117)

Kazuhiro Nogita 1 , Xuan Q Tran , Stuart D McDonald , Syo Matsumura 2
  1. Nihon Superior Centre for the Manufacture of Electronic Materials, School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD, Australia
  2. Department of Applied Quantum Physics and Nuclear Engineering and the Ultramicroscopy Research Center, Kyushu University, Fukuoka, Japan

Ultra-high voltage transmission electron microscopy (UHV-TEM) has significant advantages compared with conventional TEM including  (1) higher resolution due to the shorter wave length of incident electron beams, (2) the minimization of surface effects and the ability to image a thicker specimen, and (3) the inelastic incident beam interaction with the sample atoms is less than occurs at lower voltages and, as a result, less pronounced ‘electronic excitation’ occurs. Here we report on the real-time atomic level observation of hydrogen release behavior in MgH2 particles at a grain size of a few micrometers, which is at a scale applicable to real industrial hydrogen storage technologies. Techniques used included temperature controlled UHV-TEM with supporting synchrotron atmospheric and temperature controlled X-ray diffraction, X-ray absorption fine structure analysis and DSC. These observations help elucidate the fundamental mechanisms of hydrogen release and help with the development of high performance and safe hydrogen storage stations for fuel cell vehicles in the form of solid metal hydrides. This paper focuses on characterising and comparing the hydrogen release behavior in MgH2 particles using two different TEMs with acceleration voltages of 1,000kV (JEM-1000, EM-HSTH heating holder) and 200kV (JEM-2100HCLM, GATAN Model 652 double tilt heating holder). The differences of the obtained hydrogen release behaviors were, (1) the hydrogen release mechanism from bulk (2 μm) MgH2 particles observed at high voltage is based on the growth of multiple pre-existing Mg crystallites within the MgH2 matrix, present due to the difficulty of fully transforming all Mg during a hydrogenation cycle. In comparison in thin samples (2) analogous to nano-powders, dehydriding occurs by a ‘shrinking core’ mechanism. The sample thickness and electron beam-sample interactions might effect the observation results, and we believe the former case is the most realistic hydrogen release mechanism for industrially relevant bulk hydrogen storage systems.