In Situ NMR Study on Hydrogen Incorporation in Electrochromic Oxides WO3 and TiO2
Authors: Oikawa Itaru , Sugawara Yu , Ishii Akihiro , Takamura Hitoshi 
Published: December 2024

Transition metal oxides such as WO3 and TiO2 are well-known electrochromic materials showing color change by ion insertion or extraction [1, 2]. Owing to their electrochromic properties, these oxides are widely investigated as a switchable glazing for a variety of energy-saving and optical switching applications. Recently, application of electrochromic oxides to hydrogen monitoring devices has been proposed to detect hydrogens in steel materials which cause hydrogen embrittlement [3]. In these devises, electrochromic oxides are deposited on steel materials and detect the incorporated hydrogens by their color change. For this application, elucidation of fundamental hydrogen incorporation behavior is necessary to determine suitable materials for hydrogen detection. This study focuses on hydrogen incorporation in well-known electrochromic materials, WO3 and TiO2. Their hydrogen incorporation behavior is analyzed by in situ nuclear magnetic resonance (NMR) spectroscopy. NMR spectroscopy is a powerful tool to observe difference in chemical environment around probing nuclei. Among NMR active nuclei, 1H shows the highest sensitivity and even a small amount of hydrogen species in oxides can be detected. Furthermore, 1H NMR experiments are carried out in situ under H2 atmosphere to clarify the hydrogen incorporation behavior. The purpose of this study is to clarify the hydrogen incorporation behavior of WO3 and TiO2 by in situ
1H NMR and evaluate capability for hydrogen monitoring applications.
Powders of WO3 and TiO2 were coated with Pd to ensure hydrogen incorporation in H2 atmosphere and pressed into f4 mm pellets. In situ
1H NMR experiments were carried out using a NMR probe and atmosphere control system specially designed for in situ measurements [4]. 1H NMR spectra were collected from room temperature to 300°C under H2 flow atmosphere. In the 1H NMR spectra of WO3, peaks from adsorbed hydrogen (H) is observed at temperature 100°C and below. This peak is originated from adsorbed H on the Pd coating because this peak is also observed in 1H spectra of Pd powder in H2 atmosphere. At 150°C, a new peak is observed and assigned to H in the WO3 lattice (H
x
WO3). This peak is separated into two peaks by increasing temperature. These results indicate hydrogen incorporation into WO3 occurs at temperature above 100°C and two chemically distinct H are exist in the WO3 lattice at higher temperature. At lowering temperature, two peaks from H
x
WO3 become one broad peak and a peak from adsorbed H appears again at 100°C. When atmosphere is changed from H2 flow to vacuum, the adsorbed H peak disappears and only the H
x
WO3 peak remains indicating incorporated H is stable at room temperature under vacuum. The same procedure is carried out for TiO2. Unlike WO3, a peak assigned to H in the TiO2 lattice is observed even at room temperature under H2 flow. By increasing temperature, the intensity of this peak decreases indicating desorption of H. This temperature dependence is almost reversible at lowering temperature. The peak from H in TiO2 almost disappears when atmosphere is changed from H2 flow to vacuum indicating H in TiO2 is unstable and removed from the lattice by evacuating. The present study reveals quite different hydrogen incorporation behavior of WO3 and TiO2. Using hydrogen incorporation behavior of WO3 as a benchmark, TiO2 is expected to show faster incorporation of hydrogen but the stability of hydrogen in the lattice can be a drawback for hydrogen detecting applications.

References

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[2] N. N. Dinh et al., Thin Solid Films, 423 (2003) 70–76.
[3] Y. Sugawara et al., ISIJ Int., 58 (2018) 1860-1867.
[4] S. Hashimoto et al., Rev. Sci. Instrum., 81 (2010) 104101.