Correlation between Hydrogen Diffusion and Hydrogen-Induced Structural Phase Evolution in β-MoO3 Thin Films

Abstract

Hydrogen incorporation strongly alters the electronic, optical, and structural properties of transition-metal oxides. In MoO3, hydrogen intercalation forms HyMoO3 bronzes that exhibit reversible color changes, crystal structure distortions, and notably higher hydrogen diffusion coefficients. Although these effects are central to electrochromic, catalytic, sensing, and energy storage applications, the mechanisms of hydrogen transport remain poorly understood, especially in the metastable β-phase. Here, we show that hydrogen diffusion in β-MoO3 is controlled by two distinct, structurally driven increases of the diffusion coefficient, each amplifying it by nearly one order of magnitude as hydrogen concentration rises. To prove this, we perform electrochemical hydrogen insertion into β-MoO3 thin films covered by a PMMA layer, only allowing incorporation via a small, well-defined uncovered stripe. Hydrogen entering through this gap diffuses laterally beneath the PMMA in a semi-infinite space. Using the electrochromic response of MoO3 in combination with the Beer–Lambert law, we monitor the hydrogen concentration in situ. These diffusion profiles are then analyzed with a deep-learning algorithm to extract concentration-dependent diffusion coefficients. In contrast to the previously implicit assumption of a single effective diffusivity, our results reveal a piecewise concentration dependence that is directly linked to structural phase transformations, as confirmed by complementary in situ Raman spectroscopy during both intercalation and deintercalation. These findings establish a quantitative connection between hydrogen-induced structural evolution and ionic transport kinetics in β-MoO3. The combination of electrochromic profiling with machine-learning-based diffusion analysis provides a generalizable experimental framework applicable to related material systems, with implications for the design of electrochromic devices and hydrogen storage systems.