Interface engineering breaks both stability and activity limits of RuO₂ for sustainable water oxidation

The practical deployment of water electrolysers for green hydrogen requires electrocatalysts that are both highly active and durable. Although RuO2 is among the most active OER catalysts, Pourbaix diagrams and numerous studies indicate that RuO2 is thermodynamically unstable across the OER potential range, undergoing oxidation to high-valence Ru species and dissolution. Common stabilization strategies, such as mixing RuO2 with corrosion-resistant materials or controlling its dispersion to limit electrolyte contact, often trade activity for stability, producing a seesaw relation. Inspired by sacrificial protection (e.g., zinc-plated steel), the authors hypothesize that integrating RuO2 with a support that oxidizes preferentially could suppress Ru corrosion. Additionally, constructing an interface may generate new active sites to overcome the activity limits of ‘stable’ RuO2 and reduce Ru usage. The study aims to construct a RuO2/CoOx interface to decouple and simultaneously improve stability and activity in neutral and alkaline environments.

Prior work and theory show RuO2’s instability during OER, with RuIV transforming into higher valence, unstable species that dissolve, as predicted by Pourbaix analyses and validated experimentally. Stabilization strategies include forming composites with more corrosion-resistant phases and engineering dispersion or surface coverage to reduce exposure, but these approaches typically diminish activity, reinforcing a stability–activity trade-off. Theoretically, ‘stable’ RuO2 surfaces exhibit limited activity without generating high-valence Ru species. Interface engineering and alloying (e.g., with Ir or other oxides) can improve stability but may not overcome activity limitations. These insights motivate an interfacial design that offers sacrificial protection and creates new active motifs to enhance both properties concurrently.

Synthesis: RuO2/CoOx hybrids were prepared by depositing Ru nanoparticles onto CoO nanorod arrays followed by in situ electrochemical oxidation. CoO nanorods were grown on carbon fiber paper or FTO via cation exchange. A RuCl3 ethanol/water solution (30 mM) was used to load Ru onto CoO, aged 6 h, dried, and annealed under N2 at 400, 500, or 550 °C for 0.5 h to tune Ru particle sizes (2–4 nm). The Ru/CoO precursor was electrochemically oxidized by cyclic voltammetry between 0.80–1.50 V versus RHE to form RuO2/CoOx. A RuO2 reference catalyst was similarly prepared on carbon black. Typical loadings: RuO2 on CoOx, 10 µg cm−2; RuO2 on carbon black, 84 µg cm−2. Characterization: Morphology and structure were examined by SEM, TEM, and aberration-corrected HAADF-STEM; interfaces were probed by EELS across Ru-M23, O-K, and Co-L23 edges; FT-EXAFS and XRD were used for local/long-range structures. In situ/operando techniques included ambient-pressure XPS under applied potentials (1.0–2.0 V_RHE), in situ UV–Vis to monitor Co oxidation states, quantitative EPR (Co2+/Co3+/Co4+ analysis), and surface-enhanced in situ IR spectroscopy to track OER intermediates (e.g., *OOH). Ru retention was quantified by ICP-MS after potentiostatic tests. Electrochemistry: OER was evaluated in neutral 1.0 M PBS and 1.0 M KOH (alkaline) in a three-electrode setup with a rotating disk electrode (1600 rpm). Potentials were referenced to RHE (hydrogen calibration) with 75% iR correction; polarization curves were acquired at 5 mV s−1. Stability was assessed via long-term potentiostatic tests and Ru retention after 20 h at up to 1.80 V_RHE. Faradaic efficiency was measured by gas chromatography. Kinetic isotope effect (KIE) was probed via multicycle chronoamperometry in H2 16O and H2 18O to identify the rate-determining step. Computations: Spin-polarized DFT (VASP, PAW, PBE) with a 400 eV plane-wave cutoff, energy and force convergence of 1e−5 eV and 0.05 eV Å−1, respectively. A U = 3.7 eV was applied to Co 3d states. Interface supercells for RuO2/CoO, RuO2/Co3O4, RuO2/CoOOH, and RuO2/CoO2 were optimized. Free energies were computed as ΔG = ΔE + ΔZPE − TΔS − eU. Bader charge analyses evaluated interfacial charge redistribution and bonding (Ru–O–Co). Pourbaix stability analyses were used to assess interface stability across potentials in neutral/alkaline media.

- Thermodynamic stabilization beyond Pourbaix limits: Calculated Pourbaix diagrams indicate CoOx preferentially oxidizes (CoO → Co3O4 → CoOOH → CoO2) across OER potentials while RuO2 remains stabilized at the RuO2/CoOx interface via Ru–O–Co bonding and interfacial energy gain. - Interfacial electron redistribution: Bader analysis shows interfacial Ru gains electron density (e.g., in RuO2/CoOOH, average Ru charge increases from 6.3 e in bulk to 6.7 e at the interface), with O ions mediating charge transfer; Co shows minimal charge change at the interface. - Experimental validation of charge effects: EELS reveals a 0.3 eV low-energy shift at Ru-M23 at the interface (decreased Ru valence); O-K edge evolves from RuO2-like to CoO-like across the interface; Co-L23 shows negligible shift. - In situ stability under OER: In situ XPS shows Ru 3d at 280.9 eV with stable Ru3+/Ru4+ ratios from 1.0–2.0 V_RHE; ~9% Ru3+ persists at 2.0 V_RHE, consistent with ~15% interfacial Ru atoms for ~2 nm particles. In situ UV–Vis and quantitative EPR confirm progressive Co oxidation to Co3+/Co4+ without dissolution, supporting sacrificial oxidation. - Durability: ~100% Ru retention after 20 h at up to 1.80 V_RHE; stable operation at 10 mA cm−2 for >200 h; robust under dynamic current (10–100 mA cm−2). In contrast, RuO2 on carbon black shows severe dissolution/performance decay. - Activity in neutral media: Overpotential ~0.24 V to reach 10 mA cm−2; Tafel slope 70 mV dec−1 vs 109 mV dec−1 for RuO2; Faradaic efficiency ~98% at 10 mA cm−2; TOF 3.61 s−1 at 400 mV overpotential (≈10× higher than a prior Ru-based benchmark). With higher loading (1.5 mg cm−2 on Ni foam), 400 mA cm−2 at 1.92 V_RHE. - Kinetics and RDS: KIE on RuO2 gives KIE_O−O ≈ 1.03, identifying O–O bond formation as RDS; negligible isotope effect on RuO2/CoOx indicates a different RDS. In situ IR shows stronger *OOH bands and a red-shift on RuO2/CoOx, consistent with a hydrogen-bonded *OO–H…O configuration and stabilized *OOH. - DFT mechanism: On pristine RuO2, ΔG*OOH ≈ 1.12 eV limits activity, consistent with literature. At Ru/Co dual-atom sites exposed near the interface, *OH/*O/*OOH co-adsorb in a stabilized quadrilateral geometry; *OOH is stabilized via hydrogen bonding to CoOx, lowering the barrier and shifting the RDS to O2 desorption with an energy input of ~0.50 eV.

The work addresses the long-standing activity–stability trade-off of RuO2 by interface engineering with CoOx. Sacrificial oxidation of CoOx under OER conditions protects RuO2 from over-oxidation and dissolution, effectively surpassing the stability boundary predicted for bulk RuO2. Electron redistribution across Ru–O–Co bonds enriches interfacial Ru with electrons, reducing its effective valence and corrosion propensity. Concurrently, the interface exposes Ru/Co dual-atom sites that synergistically bind OER intermediates; hydrogen-bond-assisted stabilization of *OOH lowers its formation barrier, changing the RDS from O–O bond formation to O2 desorption. These interfacial geometric and electronic effects translate into exceptional durability and leading activity in neutral electrolyte, with strong performance also in alkaline media. The findings provide a clear mechanistic framework for using sacrificially oxidizable supports and dual-site motifs to decouple and simultaneously enhance activity and stability in precious-metal oxide OER catalysts.

A RuO2/CoOx hybrid catalyst was designed to break both stability and activity limits of RuO2 for OER. Preferential oxidation of CoOx and interfacial Ru–O–Co bonding stabilize RuO2 beyond its Pourbaix limit, while Ru/Co dual-atom sites at the interface optimize adsorption of OER intermediates, stabilize *OOH via hydrogen bonding, and shift the RDS to O2 desorption. The catalyst exhibits ultra-low overpotential at 10 mA cm−2, improved kinetics, high Faradaic efficiency, and exceptional long-term stability in neutral media, with strong alkaline performance. This interfacial strategy offers a generalizable pathway to engineer stable and active OER catalysts and may be extended to address RuO2 stability in acidic environments by selecting appropriate supports, as well as to other renewable energy systems coupled to OER.