The development of stable and active electrocatalysts is crucial for the practical application of water electrolyzers in generating sustainable green hydrogen energy. RuO₂ stands out as the most active electrocatalyst for the anodic oxygen evolution reaction (OER) in water electrolysis. However, Pourbaix diagrams and experimental evidence indicate that RuO₂ is thermodynamically unstable under OER conditions across the entire pH range. The OER process leads to the conversion of stable Ru⁴⁺ to unstable higher-valence Ru species, resulting in catalyst dissolution and deactivation. Current strategies to improve RuO₂ stability, such as mixing it with more corrosion-resistant materials or controlling its dispersion, often compromise its activity. This creates a seesaw effect where improving one property negatively impacts the other. The challenge lies in developing a strategy to simultaneously enhance both stability and activity in Ru-based catalysts. This research explores the concept of using a sacrificial component to protect the target material, inspired by the zinc-plated steel example where zinc preferentially oxidizes to protect the steel. The researchers hypothesized that creating a stable interface between RuO₂ and a suitable material could stabilize the RuO₂ catalyst. Furthermore, interface construction might create new active sites, potentially overcoming the activity limitations of 'stable' RuO₂. The use of cost-effective support materials could also contribute to sustainable water electrolysis by reducing precious metal usage. This study focuses on constructing a RuO₂/CoOₓ hybrid catalyst to break the stability-activity seesaw relationship in RuO₂ catalysts.

Existing literature extensively documents the instability of RuO₂ under OER conditions. Pourbaix diagrams consistently show its thermodynamic instability across a wide pH range. Experimental studies corroborate this, demonstrating the transformation of Ru⁴⁺ to unstable, higher-valence Ru species during OER, leading to dissolution and catalyst deactivation. Various approaches have been explored to enhance RuO₂ stability, including mixing it with more stable materials and controlling its dispersion to minimize electrolyte contact. However, these methods generally improve stability at the cost of reduced activity, highlighting the inherent trade-off between these two crucial properties. Previous works by Nørskov et al. suggested that even 'stable' RuO₂ exhibits unsatisfactory catalytic activity due to a lack of unstable, high-valence Ru species. The current research aims to address this longstanding challenge by utilizing interface engineering to decouple the stability and activity.

The researchers employed a multi-pronged approach combining theoretical calculations, in situ spectroscopic techniques, and electrochemical measurements to investigate the RuO₂/CoOₓ hybrid catalyst. **Theoretical Calculations:** Density Functional Theory (DFT) calculations were used to predict the stability and activity of the RuO₂/CoOₓ interface, including Pourbaix diagrams and Bader charge analysis to understand the electronic interactions at the interface. **Catalyst Synthesis:** The RuO₂/CoOₓ hybrid catalyst was synthesized by depositing Ru nanoparticles on CoO nanorods followed by electrochemical oxidation to convert Ru to RuO₂ in situ, forming the RuO₂/CoOₓ interface. **Material Characterization:** A suite of techniques, including Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), Electron Energy-Loss Spectroscopy (EELS), Fourier Transform Extended X-ray Absorption Fine Structure (FT-EXAFS), X-ray Photoelectron Spectroscopy (XPS), and X-ray Diffraction (XRD) were employed to characterize the morphology, structure, and chemical composition of the synthesized catalyst. **In Situ Spectroscopic Characterizations:** In situ XPS, UV-Vis spectroscopy, and attenuated total reflectance surface-enhanced IR spectroscopy were used to monitor the changes in the oxidation states of Ru and Co during OER. Electron Paramagnetic Resonance (EPR) analysis was also performed to quantify the proportions of Co²⁺, Co³⁺, and Co⁴⁺ species. **Electrochemical Measurements:** Electrochemical measurements including cyclic voltammetry and chronoamperometry were conducted to evaluate the OER activity and stability of the RuO₂/CoOₓ catalyst in both neutral and alkaline environments. Kinetic Isotope Effect (KIE) measurements were used to determine the rate-determining step (RDS) of the OER. The study employed various control experiments using RuO₂ and CoOₓ catalysts alone to isolate the effects of the RuO₂/CoOₓ interface.

The study's key findings demonstrate that the RuO₂/CoOₓ interface successfully breaks the stability-activity seesaw for RuO₂ catalysts. **Enhanced Stability:** The CoOₓ support is preferentially oxidized during OER, preventing RuO₂ dissolution and exceeding the Pourbaix stability limit of bulk RuO₂. In situ XPS analysis showed negligible changes in Ru 3d XPS peaks with increasing applied potential, indicating remarkable stability. Even at 2.0 VRHE, significant amounts of Ru³⁺ remained, attributed to the stabilizing interface. Long-term stability tests (over 200 h) showed minimal Ru loss in the RuO₂/CoOₓ hybrid, in stark contrast to the rapid degradation of the pristine RuO₂ catalyst. **Enhanced Activity:** The RuO₂/CoOₓ catalyst exhibited significantly higher OER activity compared to both RuO₂ and CoOₓ. It achieved an ultra-low overpotential of 0.24 V to drive a current density of 10 mA cm⁻², placing it among the most active OER catalysts reported under neutral conditions. The turnover frequency (TOF) was significantly enhanced (10-fold) compared to previously reported Ru-based catalysts. The high Faradaic efficiency (~98%) further highlighted its superior performance. **Mechanism Elucidation:** DFT calculations and HADDF-STEM imaging revealed the presence of highly active Ru/Co dual-atom sites at the interface. These sites promote co-adsorption of oxygen intermediates (*OH, *O, *OOH), optimizing reaction thermodynamics and kinetics, and shifting the RDS away from O-O bond formation. In situ IR spectroscopy supported this by showing distinct *OOH bands in the RuO₂/CoOₓ catalyst compared to RuO₂. The KIE measurements confirmed the change in RDS, providing further evidence for the interface's role in enhanced activity.

The results demonstrate a successful strategy for enhancing both the stability and activity of RuO₂ for OER through interface engineering. The preferential oxidation of CoOₓ and the electronic interactions at the Ru-O-Co interface contribute to significantly enhanced stability. The creation of highly active Ru/Co dual-atom sites around the interface is responsible for the substantial improvement in activity, changing the rate-determining step of the reaction. This integrated approach successfully addresses the long-standing challenge of balancing stability and activity in RuO₂-based OER catalysts, opening avenues for developing more efficient and durable electrocatalysts for green hydrogen production. The findings provide an atomic-level understanding of how interfacial engineering can be employed to simultaneously improve stability and activity, potentially applicable to other renewable energy technologies coupled with OER in neutral environments.

This study successfully constructed a RuO₂/CoOₓ hybrid catalyst that breaks the stability and activity limits of RuO₂ for OER by decoupling these two properties. The preferential oxidation of CoOₓ and electronic interactions at the interface enhance stability, while the exposed Ru/Co dual-atom sites boost activity by altering the rate-determining step. The high stability and excellent activity demonstrated in both neutral and alkaline environments highlight the potential of this approach for sustainable water electrolysis and other relevant energy applications. Future research could explore extending this interfacial engineering strategy to optimize RuO₂ performance in acidic environments by choosing suitable support materials and investigating the application of this approach to other electrocatalytic reactions.

While the study demonstrates remarkable improvements in RuO₂ stability and activity, there are some limitations to consider. The synthesis method may be complex and require optimization for large-scale production. The long-term stability tests were performed under specific conditions; further studies may be needed to assess the catalyst’s durability under broader operational parameters. The study primarily focuses on neutral and alkaline environments; extending the findings to acidic conditions requires further investigation. Finally, the cost-effectiveness of the hybrid catalyst compared to other state-of-the-art OER catalysts needs to be thoroughly evaluated for large-scale industrial applications.