Electrochemical Hydrogenation of Nb: Unraveling Structural and Surface Dynamics of Hydride Formation

Niobium exhibits remarkable potential as a material for hydrogen storage due to its high hydrogen absorption capacity and stability. However, its practical application is compromised by challenges in the reversible release of hydrogen following hydride formation. This study investigates the electrochemical behavior of Nb in alkaline environments, with a focus on structural and surface transformations during the formation of niobium hydrides (β-NbH0.89 and δ-NbH2). A comprehensive suite of characterization techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM), was employed to elucidate the effects of Nb2O5 formation on the hydrogen evolution reaction (HER) and the hydrogenation-dehydrogenation processes of Nb electrodes. The study reveals that the surface oxide layer acts as a barrier to hydrogenation, requiring high cathodic overpotentials (η > 1 V) to induce hydride formation while maintaining Nb in its metallic state. Two hydride phases, β-NbH0.89 and δ-NbH2, were identified with XRD data. XPS data indicating modifications in Nb2O5 bonding due to Nb–H interactions. Although these hydrides formed, electrochemical desorption was found to be irreversible under the tested conditions, as confirmed by XRD analyses. Microstructural investigations suggest that the δ-NbH2 forms through a displacive transformation, surrounded by hydrogen-saturated β-NbH0.89, which contributes to the irreversibility of the hydrogenation process. Moreover, the oxide layer may act as a barrier to hydrogen desorption. These findings provide crucial insights into the electrochemical and structural dynamics of Nb, thereby enhancing its potential and identifying limitations for hydrogen storage applications.