Subject Number :S-17-MS-2011
Support Type : Common use (including technical support necessary for the training),
Proposal Title (English) : High Pressure STXM Cell
Username (English) : P. –A. Glans1, Y. S. Liu1, R. M. Qiao1, T. Ohigashi2,3, J. –H. Guo1,
Affiliation (English) : 1Advanced Light Source, Lawrence Berkeley Nat’l Lab. 1 Cyclotron Road, Berkeley, CA 94720, USA
2UVSOR Synchrotron, Institute for Molecular Science, Okazaki 444-8585, Japan
3School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan

Measurements of the electronic structure of materials in vacuum or even at atmospheric pressure are becoming more and more common. Some catalytic reactions, however, occur at, or create, higher pressures: gas evolution (such as water splitting), some catalytic processes are more efficient at higher pressures, some batteries operate at pressures higher than 1 atm, High pressures are also necessary to investigate phase transitions of gasses.
A high-pressure cell for the STXM has been designed and tested at the BL4U beamline at UVSOR, see Fig. 1. The body of the cell is made out of PEEK for maximum chemical compatibility. Windows similar to what is usually used at the STXM but with a custom window size were made at the Lawrence Berkeley Lab. The window size was 75m x 75m, set in frames of 5 mm x 5 mm and 10 mm x 10 mm. The cell was tested off-line and could reliably hold gauge pressures up to 10 bar. Liquid can flow through the cell through PEEK tubing. Once the system is filled, a valve on the drain end is closed and the system is pressurized by running the pump until required pressure has bee achieved.

Figure 1 Partially assembled high-pressure cell. The cell is mounted on the standard STXM sample holder. The back window (5 mm x 5 mm frame) is visible together with Teflon spacers and the outer o-ring. A larger window (10 mm x 10 mm frame) will be mounted on top and the lid – visible to the right in the photo – will finish the ‘sandwich.’ The large window is rotated 45 degrees with respect to the smaller one.

The cell was loaded with water or CaCl2 solutions with concentrations ranging from 0.5 M to 3.0 M. The pressure was controlled with a syringe pump and measured by a pressure gauge. In the STXM chamber we did not go past 2.0 bar gauge pressure for safety reasons. Figure 2 shows example data of the Ca L2,3 edge XAS from 1.5 M solution of CaCl2 in water at 0 bar gauge pressure and 2.0 bar gauge pressure.

Figure 2. STXM images of 1.5 M CaCl2 solution at a) 0 bar gauge pressure, and at b) 2.0 bar gauge pressure. c) Cl L2,3 XAS extracted from the images. Unfortunately, i) there was no access to an I0 channel to normalize the spectra, and ii) the optical density was too high at higher pressures due to bulging cells so the XAS is saturated at 2.0 bar.

The cell performed very well and we could collect spectra at elevated pressures. However, the next iteration needs a way to measure I0 and a solution to minimize the bulging of the windows. A future version of the cell will also include electrodes to allow for electrochemical reactions.

Subject Number :S-17-MS-2011-2
Support Type : Common use (including technical support necessary for the training),
Proposal Title (English) : Phase Evolution of Complex Metal Hydrides During De/Rehydrogenation
Username (English) : J. L. White1, 2, T. Ohigashi3, K. G. Ray2,4, Y.-S. Liu2,5, V. Stavila1,2, M. D. Allendorf1,2, and J. Guo2,5
Affiliation (English) : 1Sandia National Laboratories, Livermore, CA 94551, United States
2Hydrogen Materials–Advanced Research Consortium (HyMARC), Livermore, CA 94551, United States
3UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
4Lawrence Livermore National Laboratory, Livermore, CA 94550, United States
5Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States

Lightweight complex metal hydrides are of interest for use as energy-dense on-board vehicular hydrogen stores [1]. One material of particular interest, magnesium borohydride (Mg(BH4)2), has very high hydrogen capacity, at 14.9 wt.% H, but suffers from slow kinetics and the need for extreme conditions for both dehydrogenation and rehydrogenation from magnesium diboride (MgB2) [2]. In order to establish methods to improve the kinetic properties of this system, a greater understanding of the nucleation and growth of various solid phases is essential.
Several samples of partially dehydrogenated Mg(BH4)2 and partially hydrogenated MgB2 were examined by Scanning Transmission X-Ray Microscopy (STXM) at the boron K edge using the transfer system from a glovebox to BL4U to prevent oxidation upon exposure to air. The resulting series of X-ray absorption images were analyzed using computed spectra for several B containing species (Figure 1), since the experimental XAS spectra showed substantial amounts of oxidized boron.
The STXM maps revealed some intriguing phase propagation patterns not heretofore observed. MgB2 partially hydrogenated at 400 °C and either 200 or 700 bar H2 over the course of 72 h (Figure 2) showed that hydrogenation to Mg(BH4)2 began at the exterior of the particles and spread inward, with greater conversion evident at the higher pressure, as expected.
However, the Mg(BH4)2, partially dehydrogenated at 400 °C and H2 overpressures of 200 and 360 bar, showed a much more counterintuitive phase transformation (Figure 3). It has commonly been hypothesized that the dehydrogenated phase forms first on the outside of hydride particles as the hydride releases H2 from the surface [3]. Instead, our STXM results on Mg(BH4)2 show that the hydride remains on the exterior, whereas the interior becomes dehydrogenated first. The higher overpressure sample decomposed less, and consequently its interior retained more of the hydride phase. Therefore, nucleation of the dehydrogenated material begins in the bulk, and hydrogen atoms diffuse toward the surface where they can combine and desorb as H2 gas. Information regarding the phase evolution in in the Mg-B-H system provides valuable insights into the rate-limiting step of hydrogen desorption.

Fig. 1. Simulated B K edge XAS spectra of relevant chemical species used in STXM analyses.

Fig. 2. STXM maps of MgB2 hydrogenated for 72 h at 400 °C and (a) 200 or (b) 700 bar H2. Red indicates Mg(BH4)2, green MgB12H12, and blue B2O3.

Fig. 3. STXM maps of Mg(BH4)2 dehydrogenated for 72 h at 400 °C and H2 overpressures of (a) 200 and (b) 360 bar H2. Red indicates Mg(BH4)2, green MgB12H12, and blue B2O3.

[1] Orimo, S.-i., et al., Chem. Rev. 107 (2007) 4111.
[2] Ray, K. G., et al., Phys. Chem. Chem. Phys. 19 (2017) 22646.
[3] Wood, B. C., et al., Adv. Mater. Interfaces 4 (2017) 1600803.