Abstract
Introduction
Hydrogen separation membranes using vanadium can supply high-purity hydrogen compactly and at low cost. There is currently high demand for improved hydrogen permeation performance in vanadium membranes. In this study, two types of hydrogen visualization methods are used to observe hydrogen permeating through the vanadium membranes in situ. The first is the observation of hydrogen ions using an operando hydrogen microscope. The second is the observation of silver deposited by the silver decoration method. As a result, we confirmed that the hydrogen permeation characteristics depend on the crystal orientation of the vanadium membrane surface.
Experiment
We prepared a rolled vanadium (99.936%) plate material for the hydrogen permeation measurement. The grain size was controlled to 50-150 μm by heat treatment at 1173 K for 2 h. The structure and crystal orientation were confirmed by electron backscatter diffraction (EBSD). The specimen shape is the same for both experiments, a circular plate with a diameter of 16 mm and a thickness of 0.5 mm, with a mirror-polished surface. In the operando hydrogen microscope, we used the electron stimulated desorption (ESD) method to observe hydrogen ions desorbing from the metal surface in real time during the hydrogen gas permeation experiment. In the silver decoration experiment, the sample cell was placed on the stage of an optical microscope and observed in situ [1].
Result
First, we observed hydrogen permeation at a sample temperature of 573 K and a hydrogen supply pressure of 100 Pa using an operando hydrogen microscope. We found that grains that appear bright in SEM image (secondary electron image) have a relatively high concentration of hydrogen distribution [2]. Next, we observed the deposition of silver in a silver decoration experiment at a sample temperature of 296 K and a current density of -5.3 mA/cm2. As time passed, bright and dark areas appeared due to the deposition of black spots (Fig. 1 (a)). Elemental analysis using energy dispersive X-ray spectroscopy (EDX) confirmed that the deposited black spots corresponded to the distribution of silver (Fig. 1 (c)). Finally, EBSD analysis identified the crystal orientation of each grain. A comparison of the silver distribution in Fig. 1 (a) with the IPF map in (d) shows the anisotropy of the silver distribution on the surface. The amount of silver distribution is high in the crystal grains with (101) and (111) orientations, and low in the crystal grains with (001) orientation. The areas with high silver distribution correspond to areas with high hydrogen permeation flux. Furthermore, the SEM image of the same area in Fig. 1(b) shows that the crystal grains that appear dark in the SEM image correspond to the (001) orientation. These results show similar trends in both the operando hydrogen microscope experiment and the silver decoration experiment.
Conclusion
In this study, we succeeded in visualizing the behavior of hydrogen that permeated through a vanadium membranes by using two different methods. EBSD analysis established a clear correlation between the hydrogen permeation flux through the vanadium membranes and the crystal orientation of the surface. It is presumed that the difference in hydrogen permeation flux on the observed surface is due to the difference in diffusion coefficient due to crystal orientation affecting hydrogen diffusion. This study provides a new understanding of the mechanism of hydrogen permeation and clarified the effect of crystal orientation on hydrogen permeation efficiency.
References
[1] A.N. Itakura, T. Kusawake, T. Fujimaru, S. Miyai, Y. Matsumoto, Y. Murase, e-Journal of Surface Science and Nanotechnology 22, 174–178 (2024).
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