Enhancement of Alkaline Hydrogen Evolution in BaRuO₃ Thin Film via Surface Self-Reconstruction

Hydrogen production through water splitting presents a promising avenue for carbon-free energy generation. Transition metal oxides (TMOs) are pivotal electrocatalysts known for their remarkable activity and tunability. Among TMOs, perovskite Ruthenates stand out due to their significant electrocatalytic potential in driving the hydrogen evolution reaction (HER). However, the precise influence of the surface chemistry of Ruthenates in alkaline solutions on their catalytic performance in HER remains a subject of ongoing exploration.<br/>In this investigation, we concentrate on the dynamic chemical and structural transformations occurring on the surface of cubic perovskite BaRuO3 (3C BRO) during the HER cycle and their direct impact on HER activity. To facilitate our analysis, we utilized epitaxial thin films, meticulously crafting atomically precise crystalline surfaces. This approach allowed us to elucidate the fundamental role of the surface in catalytic activity.<br/>Remarkably, the HER activity of the 3C BRO epitaxial thin film experiences a substantial boost through cycling in an alkaline environment. Specifically, after the initial cycle, the HER overpotential decreased from 210 to 60 mV, reflecting an impressive ~70% improvement. With continued cycling, the initially high HER activity gradually wanes, ultimately reaching a saturation overpotential similar to that of RuO2 catalysts after approximately 50 cycles.<br/>Examination via X-ray photoelectron spectroscopy reveals that the highly active state of 3C BRO retains a similar bulk stoichiometry to the pristine 3C BRO within the thin film region. However, a noticeable reduction in the Ba/Ru ratio (~0.75) becomes apparent on the surface. Complementary observations using scanning transmission electron microscopy indicate the expansion of the Ba-deficient region across the entire film as the heightened activity diminishes. Our interpretation suggests that the initial significant activation arises from the presence of a highly active local Ba-deficient surface stabilized on the pure 3C BRO film. Subsequently, as the overall 3C BRO film loses Ba, the catalytic activity gradually diminishes. These findings offer fundamental insights into the underlying mechanisms governing the formation of a highly active catalytic surface and its stabilization strategy."