Dynamic hydroxyl mediated charge buffering stabilizes high valence ruthenium edge sites for acidic water oxidation

Rational design of cooperative host-guest architecture

To undermine the intrinsic paradox of hyperactive but unstable Ru(>IV) species, we conceptualized a cooperative host-guest architecture wherein isolated Ru atoms are embedded within an active and isostructural host lattice (Fig. 1a). Specifically, the host fuls two critical functions, including dynamically buffering Ru sites against oxidative dissolution and structurally inducing the formation of active high valence states. We employed a two-step density functional theory (DFT) screening to translate this conceptual framework into a material, first identifying the optimal host and subsequently defining the local Ru coordination environments.

An ideal host should have the capacity for both dynamic charge buffering and electronic stabilization of high valence Ru species. To ensure atomic-level integration and effective host-guest interactions with RuO₂, three isostructural rutile-phase oxides (TiO₂, SnO₂ and β-MnO₂) with thermodynamically stable (110) surfaces were selected as representative models (Supplementary Data 1)^16,17,18. Inspired by pseudocapacitance principles^19,20,21, we hypothesized that host surface hydroxyls could buffer oxidative equivalents via rapid and reversible dehydrogenation (M-OH → M = O + H⁺ + e⁻). This synchronous proton-electron release maintains local electroneutrality, thereby preventing the over-oxidation of active Ru centers. To validate this buffering capacity, we calculated surface Pourbaix diagrams to analyze potential-dependent protonation/deprotonation behaviors, using RuO₂ surface as a reference (Figs. 1b–d, S1 and S2)^22,23. As seen, RuO₂ exhibited a static deprotonated surface (1/2 μ₁-O) across the relevant potential window (1.20–1.70 V), demonstrating the lack of intrinsic buffering capability. Conversely, all three host candidates maintained hydroxylated surfaces. Notably, β-MnO₂ exhibited a two-step deprotonation initiated at a low potential of 1.32 V, indicating a highly responsive surface capable of mediating dynamic charge transfer.

While all candidates satisfied the buffering prerequisite, the decisive criterion was the host’s electronic modulation of the Ru dopant, governed by cation electronegativity (Fig. 1e). Mn⁴⁺ has the highest electronegativity (1.92 vs. 1.84 for Ru⁴⁺) and was predicted to withdraw electrons from Ru²⁴. Bader charge analysis quantified that the charge on a Ru atom embedded in β-MnO₂ increased to +1.89 |e| (compared to +1.79 |e| in bulk RuO₂), implying an elevated oxidation state primed for catalysis (Fig. S3). In contrast, TiO₂ and SnO₂ featured less electronegative cations and induced negligible electronic perturbation. Consequently, the synergy of dynamic buffering and electronic pre-oxidation capabilities established β-MnO₂ as the optimal host.

With β-MnO₂ selected, we evaluated the thermodynamic stability of Ru substitution at two dominant surface sites, including basal terraces and low-coordination edges. The calculations revealed a strong thermodynamic preference (0.63 eV) for Ru substitution at edge sites compared to terrace sites (1.78 eV), pointing to a self-organizing atomic arrangement (Figs. 1f, S4)²². While creating an initial Mn vacancy at the edge is energetically costly (7.44 eV), the substitution of a single Ru atom dramatically altered the local energetic landscape, lowering the energy required to remove an adjacent Mn atom to 5.03 eV (Figs. 1g, S5). This reduction in vacancy formation energy facilitates sequential Ru incorporation along the edge sites, thereby providing a thermodynamic driving force for the assembly of atomically precise Ru-O-Mn motifs. We further calculated the Ru dissolution energy to quantify the stability of this architecture. The value for a Ru atom in this confined and hydroxyl-covered edge site was 2.06 eV, which is substantially higher than the 1.67 eV for a Ru atom on RuO₂ (110) (Figs. 1h, S6). While these DFT insights are based on computational models that may not fully capture operational conditions, they consistently indicate that the specific Ru-O-Mn edge motifs have good potential to provide the necessary structure for the robust retention of active species.

Synthesis and structural characterization

Guided by the theoretical prediction of thermodynamic preference for Ru substitution at edge sites, we synthesized a series of Ru_xMn_1-xO₂ catalysts (where x = 0.01, 0.03, 0.05, and 0.08) using a modified site-selective ion-exchange method²⁵ (Fig. 2a). Powder X-ray diffraction (XRD) patterns verified that all catalysts retained the pure β-MnO₂ phase. A systematic shift of the (110) peak to lower angles confirmed the isomorphous substitution of smaller Mn(IV) ions by larger Ru(IV) ions (Fig. 2b)²⁶. Crucially, the optimal Ru_0.03Mn_0.97O₂ catalyst exhibited no phase-separated RuO₂, corroborating the atomic-level incorporation of Ru. This substitution-induced local lattice distortion was further evidenced by systematic shifts in the Mn-O vibrational modes in Raman and FT-IR spectra (Fig. S7).

The overall morphology of the catalyst was first examined by scanning electron microscopy (SEM) (Fig. S8), revealing that the β-MnO₂ support possesses a characteristic nanorod structure with average dimensions of approximately 1.17 ± 0.2 μm in length and 100 ± 30 nm in diameter. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) provided direct visual evidence of the cooperative host-guest architecture. Atomically dispersed Ru atoms were clearly resolved as bright Z-contrast spots, preferentially populating the low-coordination edge sites of the β-MnO₂ nanorods to form atomically precise Ru-O-Mn edge chains (Figs. 2c, d, S9). The measured Ru-Ru spacings of 2.9 Å and 3.4 Å matched the Mn-Mn periodicities in the host lattice, confirming that Ru atoms directly replaced Mn within the one-dimensional octahedral chains (Fig. 2e). The selective substitution of Ru at the edge sites of β-MnO₂ nanorods is governed by a synergy of thermodynamic and electrostatic driving forces during the ion-exchange process. From a thermodynamic perspective, the edge regions of the nanorods possess a higher density of unsaturated metal sites compared to the basal planes. These sites exhibit higher chemical reactivity, confirmed by the lower energy barrier for Ru substitution (Fig. 1d, e)²⁷. More crucially, the electrostatic driving force plays a decisive role in this site-selectivity. As directly visualized by Kelvin probe force microscopy (KPFM) (Fig. S10), the edges of the β-MnO₂ nanorods exhibit a higher local surface potential compared to the basal surfaces. This intensified local electric field creates a strong electrostatic attraction for the positively charged Ru precursor cations, effectively guiding them to preferentially anchor and exchange at the edge positions²⁸. This synergistic mechanism ensures the robust and site-selective anchoring of Ru at the host edges, which is instrumental in forming the highly active Ru-O-Mn edge-chain architecture.

X-ray absorption spectroscopy (XAS) substantiated the formation of Ru-O-Mn coordination, the linkage within the edge chains (Figs. 2f, S11). The extended X-ray absorption fine structure (EXAFS) spectrum of Ru_0.03Mn_0.97O₂ was dominated by a Ru-O-Mn scattering path at 2.36 Å, while the characteristic Ru-O-Ru signal of RuO₂ located at 3.1 Å was absent^29,30. Such structural consistency, originating from the isostructural nature of β-MnO₂ and RuO₂, underscores the formation of a robust embedded Ru-O-Mn architecture that effectively anchors the Ru active sites against migration or detachment. This atomic arrangement enabled the predicted interfacial charge transfer, which was confirmed by both X-ray absorption near-edge structure (XANES) and X-ray photoelectron spectroscopy (XPS). Specifically, the strong inductive effect exerted by the electronegative Mn⁴⁺ host via Ru-O-Mn bridges leads to a significant reduction in electron density at the Ru centers. A positive shift of Ru K-edge and Ru 3p_3/2 binding energy (~0.9 eV) confirmed the structural induction of intrinsic high valence Ru(>IV) species³¹ (Figs. 2g, S12 and S13). Meanwhile, this electron withdrawal from Ru was balanced by a partial reduction of the Mn host to an average oxidation state of +3.14 (Figs. 2h, S14–S16). The accumulation of Mn(III) species promoted the formation of surface oxygen vacancies (Figs. S17, S18), which acted as Lewis acid sites to trigger water dissociation^32,33. Consequently, this process spontaneously anchored a dense protective hydroxyl layer, priming the surface for the subsequent dynamic charge buffering.

Acidic OER performance and PEMWE validation

We first evaluated the acidic OER performance using a rotating disk electrode (RDE) setup. Polarization curves revealed that the Ru_0.03Mn_0.97O₂ catalyst exhibited substantial activity, requiring an overpotential of only 232 mV to achieve 10 mA cm⁻² (Figs. 3a, S19 and S20), which compared favorably both β-MnO₂ and commercial RuO₂. The corresponding non-iR-corrected voltammograms are provided in Fig. S19b. The contribution from carbon black was confirmed to be negligible (Fig. S21). This performance is attributed to the high intrinsic activity, as evidenced by a turnover frequency (TOF) of 1.81 s⁻¹ and a mass activity of 6858 A g_Ru⁻¹ at 1.50 V vs. RHE. This represents a 223-fold enhancement over commercial RuO₂, demonstrating a competitive performance among recently reported acidic OER catalysts (Figs. S22, S23 and Table S1). The Faradaic efficiency (FE) for OER was further quantified via the rotating ring-disk electrode (RRDE) method^34,35, confirming that the observed anodic current originates almost entirely from oxygen evolution rather than side reactions (Fig. S24).

Crucially, this substantial enhancement in activity did not come at the expense of stability. In a 1750-h chronopotentiometry test at 20 mA cm⁻², it exhibited negligible activity degradation, whereas commercial RuO₂ failed within 70 h (Figs. 3d, S25). Post-test analysis detected minimal dissolution of Ru (0.43%) and Mn (1.02%). Consequently, the catalyst achieved a stability number (S-number) of 4.37 × 10⁸, exceeding the RuO₂ benchmark by over three orders of magnitude. Post-mortem HRTEM confirmed that the Ru-O-Mn edge chains remained well-preserved after prolonged operation (Figs. 3d, S26).

We further validated the practical viability of this architecture in a PEMWE assembled via the Catalyst Coated Substrate (CCS) method^36,37. Remarkably, our catalyst enabled the device to reach an industrial-scale current density of 3 A cm⁻² at a cell voltage of 1.96 V (80 °C). This performance surpassed commercial RuO₂ and IrO₂ benchmarks while reducing precious metal usage by 80% (Fig. 3e). Furthermore, the electrolyzer sustained continuous operation for over 1000 h at 1 A cm⁻² without significant voltage increase (Fig. 3f). This performance and stability are highly competitive compared to recently reported results (Table S2). This combination of robust durability, high mass activity, and minimal precious metal loading establishes a viable pathway toward cost-competitive green hydrogen production.

Mechanistic origin of the enhanced activity

To elucidate the mechanistic origin of the enhanced activity, we combined in situ spectroscopy with kinetic analysis. In situ Raman spectra revealed the formation of oxygenated intermediates on Ru_0.03Mn_0.97O₂. While the peak at 668 cm⁻¹ corresponds to the bulk metal-oxygen (M-O) vibration, the feature at 575 cm⁻¹ is specifically assigned to the Ru-O* intermediate (Fig. 4a)^38,39,40,41. This assignment is supported by the absence of the corresponding peak for β-MnO₂, which effectively rules out the possibility of Mn-O* species (Fig. S27). Furthermore, the feature is validated by a significant redshift from 575 to 542 cm⁻¹ upon ¹⁸O-labeling, whereas negligible shifts are observed in D-labeled electrolytes. Consistently, in situ differential electrochemical mass spectrometry (DEMS) performed on the ¹⁸O-labeled catalyst in normal electrolyte revealed negligible formation of ³⁴O₂ or ³⁶O₂, indicating that lattice oxygen does not participate in O₂ evolution (Fig. S28). Notably, unlike β-MnO₂, which exhibits a distinct MnO₄⁻ signal at ~839 cm⁻¹ at potential above 1.7 V vs. RHE, no such peak was detected for Ru_0.03Mn_0.97O₂ throughout the entire OER process. These results supported an adsorbate evolution mechanism (AEM), consistent with the structural robustness of Ru_0.03Mn_0.97O₂ during OER, as the exclusion of lattice oxygen participation effectively prevents the formation of oxygen vacancies that typically trigger structural reorganization and metal dissolution⁴².

We further investigated the reaction kinetics of Ru_0.03Mn_0.97O₂ using isotope-dependent Tafel analysis, focusing on how H/D substitution influences the apparent Tafel slope in H₂O versus D₂O (Fig. 4b). We observed pronounced isotope sensitivity, where the Tafel slope shifted significantly from 42 to 118 mV dec⁻¹ upon replacing H₂O with D₂O (Figs. 4c, S29). Prior studies^43,44,45 have shown that such isotope-dependent Tafel slopes arise only when the rate-determining step (RDS) involves a concerted proton-electron transfer (CPET) due to the modification of transfer coefficient (α). Accordingly, our isotope-dependent Tafel behavior supported a CPET-type RDS, possibly the *O + H₂O → *OOH. This concerted pathway is kinetically advantageous as it couples proton and electron transfer to bypass high-energy charged intermediates, effectively lowering the activation barrier for rapid oxygen evolution⁴⁶. Unlike RuO₂, which typically shows near pH-invariant kinetics on the RHE scale⁴⁷, Ru_0.03Mn_0.97O₂ exhibited a fractional proton order of 0.68 (Figs. 4d, S30). Notably, the nearly constant ρ^RHE values (0.63-0.69) across the 1.46–1.50 V vs. RHE range confirm the mechanistic pathway remains invariant within this potential window. Given the concerted nature of RDS, the observed non-ideal pH dependence implies that the apparent kinetics were decoupled from the intrinsic CPET barrier. Instead, reaction rates were modulated by a pH-dependent surface pre-equilibrium, which regulated the hydroxyl coverage and reactivity of key oxygenated intermediates, including *OH, *O, and *OOH^48,49,50. To experimentally capture these active surface hydroxyls, methanol probe experiments were further conducted. As shown in Fig. S31, Ru_0.03Mn_0.97O₂ exhibits a significantly higher methanol oxidation (MOR) current density and a much earlier onset potential compared to RuO₂. Given that MOR in acidic media is primarily triggered by the presence of reactive surface *OH, this result provides direct kinetic evidence that our host-guest architecture promotes a higher coverage of active hydroxyl species at the interface. This enriched *OH environment not only facilitates the chemical probe reaction but also acts as the key regulatory factor in optimizing the OER pathway.

To identify the origin of this pre-equilibrium, we computed the surface Pourbaix diagram for Ru_0.03Mn_0.97O₂ using DFT calculations. Under OER conditions, a hydroxyl-rich surface termination (1/2 µ₁-O + 1/6 µ₂-OH) was predicted to be thermodynamically favored (Figs. 4e, S32 and S33). To show the dependence of activity trend with hydroxyl-rich termination, as shown in Fig. 4f, g, we compared the free-energy diagrams of three representative surfaces, including the hydroxylated and non-hydroxylated Ru_0.03Mn_0.97O₂, and RuO₂. Notably, all these surfaces exhibited the same linear scaling relation (LSR) between *OOH and *OH (ΔG_*OOH = ΔG_*OH + 2.9 eV), consistent with a hydrogen-bond-assisted reaction pathway^51,52,53,54. Indeed, in situ ATR-FTIR spectra detected the distinct shoulder features at 1055/1065 cm⁻¹ attributable to strengthened hydrogen-bonding interactions involving *OOH (Fig. S34). Crucially, this intercept value is significantly lower than the conventional universal scaling of ~3.2 eV, indicating that the conventional activity limitations are effectively circumvented. With the new intercept value fixed, the activity differences were primarily governed by the binding strength of *O. Specifically, the hydroxyl-rich termination tuned the binding strength of *O toward the Sabatier optimum, thereby balancing the two uphill steps (*OH → *O and *O → *OOH) and minimizing the theoretical overpotential to 0.25 V (Fig. 4h). Removing surface hydroxyls increased the overpotential to 0.52 V and shifted the RDS back to *OH deprotonation^48,51,55. Moreover, this hydroxyl-assisted surface was energetically more favorable than that of RuO₂ (0.68 V) and the alternative oxide pathway mechanism (OPM) (0.57–0.71 V, Fig. S35). Collectively, the surface hydroxyl species circumvent conventional LSR limitations by selectively stabilizing *OOH relative to *OH, while simultaneously tuning the O binding strength to the Sabatier optimum.

Mechanistic origin of stability

To elucidate the electronic origin of the catalyst’s resilience against oxidative stress, we tracked the surface state evolution using quasi in situ XPS techniques⁵⁶. For the Ru_0.03Mn_0.97O₂ catalyst, we monitored the spectral changes during a potential cycle from OCP to 1.5 V and back to OCP. The Ru 3 d peak remained invariant throughout this reversible cycle (Figs. 5a, S36a), indicating that the Ru centers maintained their stable valence state regardless of the potential stress. In contrast, the Mn host exhibited a dynamic and synchronized response involving both cation and anion sub-lattices (Fig. 5b, c). As the applied potential increased from OCP to 1.5 V, the Mn 2p peak progressively shifted to higher binding energies (Figs. 5b, S36b), reflecting the absorption of oxidative charge by the Mn host. This phenomenon identifies the MnO₂ support as a redox-active charge reservoir, where the Mn sites undergo self-adaptive oxidation to sacrificially dissipate the mounting oxidative charge. Meanwhile, the O 1s spectra showed a synchronous positive shift in the lattice oxygen peak (Fig. 5c), mirroring the increased oxidation state of the metal centers. Crucially, despite this high oxidative stress, the characteristic peak of adsorbed hydroxyls not only remained prominent but exhibited a distinct potential-driven enrichment. According to quasi in situ XPS quantification, the hydroxyl content increased significantly from 21.59% at OCP to 32.75% at 1.3 V and reached a maximum of 36.97% at 1.5 V (Fig. S37). Even after returning to OCP, the hydroxyl coverage remained as high as 36.31%, confirming the robust retention of the protective hydroxyl-rich termination. Upon returning the potential to OCP, both the Mn 2p and O 1s peaks shifted back to their initial positions. This behavior was distinct from that of commercial RuO₂ (Figs. 5d, S36c and S38). Under identical conditions, RuO₂ showed a prominent and irreversible shift of the Ru 3d and 3p peaks to higher binding energies, demonstrating the rapid accumulation of unstable high valence Ru species that act as precursors to oxidative dissolution. Post-stability characterization further corroborated these in situ observations and revealed that both the Ru valence state and surface hydroxyl content in our catalyst remained virtually unchanged after prolonged operation (Figs. S39–S41). Specifically, the potential-driven enrichment ensures a high coverage of surface hydroxyl species anchored on the MnO₂ host, which serves as the fundamental chemical reservoir for the catalyst’s stability. Crucially, the deprotonation of these host-carried hydroxyls acts as the primary mechanism for dynamically buffering the oxidative charge. Consequently, during the OER process, the MnO₂ host adaptively absorbs the mounting oxidative charge through a synergistic dual-pathway. In addition to the self-adaptive valence state transitions of the Mn sites, the dynamic evolution of host-carried surface hydroxyls effectively shields the adjacent Ru active sites from detrimental over-oxidation and subsequent leaching.

To elucidate the mechanistic link between the observed surface hydroxyls and the suppression of Ru oxidative dissolution, we employed DFT calculations to map the dissolution pathway and energetics (Fig. 5e). The dissolution of Ru-based oxides is typically governed by a multi-step process initiated by surface over-oxidation, followed by a critical structural reorganization of local RuO₆ octahedron⁵⁷. As the non-electrochemical distortion represents the kinetic bottleneck in the degradation pathway, we calculated the energy barrier for this specific event. For commercial RuO₂, the barrier was moderate at 1.28 eV (Figs. 5e, S42). However, the barrier increased significantly to 2.78 eV for the hydroxylated Ru_0.03Mn_0.97O₂ catalyst. Furthermore, we performed a control calculation on a model of our catalyst stripped of surface hydroxyls to isolate the specific contribution of hydroxyl-mediated charge buffering. This resulted in a barrier drop to only 1.19 eV. Crucially, the surface hydroxyls firmly coordinate to the Ru centers in Ru_0.03Mn_0.97O₂, restricting the geometric distortion required for the release of soluble RuO₄ species. As a result, the energy barrier for RuO₄ dissolution in hydroxylated Ru_0.03Mn_0.97O₂ is significantly elevated, exceeding that of its unhydroxylated counterpart by 1.04 eV (Fig. 5e). These computational insights indicate that static Ru-O-Mn motifs alone were insufficient to impart the observed stability, and instead, the dynamic charge buffering mediated by the surface hydroxyls effectively prevented the structural distortion and oxidative dissolution for our Ru_0.03Mn_0.97O₂ catalyst.