Mesoporous ruthenium titanium oxide solid solution with efficient three phase reaction interface for water electrolysis

Domain: physics
Published: 2026-03-10
Editorial outcome: major_revision

Abstract
The development of stable Ru-based anodes for acidic proton exchange membrane water electrolysis is promising, but strictly limited by Ru over-oxidation and structural collapse due to lattice oxygen participation under high current densities. Rational design of competitive Ru-based catalyst is, thereby, highly desired. Here, by exploring a customized self-assembly route, we report a type of mesoporous Ru-Ti-O solid solution catalyst delivering competitive performance (1 A cm-2 for over 450 h at 0.4 mg cm-2). Mechanistic investigations reveal that the enhanced performance arises from the integration of atomic-scale electronic structure tuning and mesoscopic triple phase interface engineering. The electron delocalization forms a conductive network and suppresses Ru overoxidation through electron donation. Atomically dispersed Ru-O-Ti motifs favor the oxygen pathway mechanism over the lattice oxygen mechanism, suppressing lattice oxygen release and enhancing structural stability. Simultaneously, the ordered mesoporous architecture and radially aligned nanorod bundles establish a robust, super-hydrophilic triple phase interface, enabling effective water and gas exchange and mitigating concentration overpotentials. This cross-scale design strategy offers a possible route to non-Ir catalysts with measurable activity and long-term durability for scalable acidic water electrolysis.

Introduction
Proton exchange membrane water electrolysis (PEMWE) stands as a important technology for next-generation green hydrogen production.1, 2 It provides several features such as high-purity H output, rapid response to fluctuating renewable power, and compact system integration.3-7 However, the sluggish four-electron transfer kinetics of the oxygen evolution reaction (OER) at the anode remain a major bottleneck for the commercialization of this technology, giving rise to significant energy losses.8-12 This necessitates the development of advanced electrocatalysts that are not only highly active but also durable under harsh oxidative and acidic conditions. Iridium (Ir)-based materials are currently the widely studied OER electrocatalysts,13-15 owing to their decent activity and durability under harsh oxidative and acidic environments. Yet, the extreme scarcity and escalating cost of Ir severely limit its large-scale deployment of PEMWE.16, 17 In this context, ruthenium dioxide (RuO )-based materials, as the most viable alternative to Ir, have been widely investigated due to their comparable acid stability and even higher intrinsic activity.2,17, 18 Nonetheless, this potential is offset by the poor durability under realistic operating conditions.19-21 Industrial PEMWE stacks typically operate above 1 A cm-2, where multiple polarizations, kinetic, ohmic, and mass transport, are drastically amplified. This elevation in cell voltage accelerates Ru over-oxidation into soluble high-valence species (e.g., RuO 2-),22-24 triggering irreversible dissolution, phase collapse, and loss of active surface area.

To mitigate this issue, extensive strategies have been devoted to enhancing the intrinsic stability of Ru catalysts, such as doping,25-30, 31, 32 defect engineering,33-35 or heterostructure construction,36-40 to optimize oxygen intermediate binding and reaction pathways. However, these investigations often overlook another important factor: the catalyst–electrolyte–gas three-phase interface (TPI), which becomes increasingly critical under water-fed, high-current PEMWE conditions. This interface governs rapid water wetting on catalyst surfaces, efficient adsorption/desorption of oxygenated species, and timely oxygen bubble removal. A poorly engineered TPI leads to interfacial flooding, concentration gradients, and mass transport limitations, all of which accelerate Ru-based catalyst degradation. Despite its importance, the rational design of TPI remains under explored and technically challenging so far. Therefore, the development of Ru-based catalysts that integrate both high intrinsic activity and an optimized three-phase reaction interface is a key to reducing overpotentials and suppressing degradation, yet remains a challenge in the field.41,42 This complexity arises from the need to reconcile competing structural demands across multiple length scales. At the atomic scale, the catalyst must resist over-oxidation and lattice collapse under harsh anodic potentials. Simultaneously, at the mesoscopic level, it must maintain fast water accessibility, effective desorption of oxygenated intermediates, and continuous gas evacuation across a dynamically evolving interface. Achieving these combined requirements within a single material system is far from trivial, it calls for a holistic structural manipulation through processable synthetic strategy.

Here, we report a customized self-assembly approach to construct a type of mesoporous rutile-phase Ru-Ti oxide solid solution featuring a distinctive nanorod-bundle-on-sphere architecture (termed meso-Ru Ti O ). The uniform synthesis involves the preformation of micelle hydrogels as a building block for subsequent oriented assembly and rutile lattice fusion. This catalyst system achieves multilevel optimization by coupling atomic-scale electronic tuning with mesostructural triple-phase interface engineering. Chain-like Ru incorporation into the TiO lattice forms a conductive Ru-O-Ti framework with electron delocalization, modulating electronic structure and suppressing lattice oxygen activation via an oxygen pathway mechanism (OPM). Meanwhile, the superhydrophilic radially aligned nanorods and gas-repelling mesopores significantly enhance mass transport of water and oxygen. As a result of these synergistic effects, the catalyst exhibits competitive acidic OER performance: a low overpotential of 257 mV at 10 mA cm-2 and 319 mV at 100 mA cm-2, with a Tafel slope of 59 mV dec-1. In full-cell PEMWE tests, it sustains 1 A cm-2 for over 450 h with a minimal Ru loading of 0.4 mg cm-2, exhibiting stable structural integrity and negligible performance decay. This work presents a robust Ir-free OER catalyst with competitive acidic durability, and highlights a generalizable strategy for TPI regulation, offering crucial insights toward scalable PEMWE technologies.

Results
Customized Self-Assembly

The uniform mesoporous Ru-Ti oxide solid solution nanospheres were synthesized through a customized micelle self-assembly strategy which involves three main steps (Fig. 1a, Fig. S1a). First, a homogeneous acidic precursor solution was prepared using triblock copolymer F127 as a template, tetrabutyl titanate (TBOT) and RuCl as the metal sources, and acetic acid as a chelating agent to regulate hydrolysis kinetics. This step mitigates uncontrolled nucleation and phase separation between the highly reactive Ti and Ru precursors, enabling a uniform metal-ligand coordination environment.43,44 Upon controlled tetrahydrofuran (THF) evaporation at 45 °C, discrete micellar hydrogels composed of block copolymer F127 and metal–oxo oligomers were formed as pre-assembled building blocks (~9.1 nm, Fig. 1b, Fig. S2a). This pre-micellization step, prior to cross-linking, allows control over spatial organization and ensures isotropic radial assembly. Notably, the low-humidity, acetic acid-assisted environment is essential for stabilizing Ti-Ru co-complexation, representing a key feature in directing cooperative metal precursor organization.

Subsequently, a hydrothermal treatment in autoclave induces in-situ polycondensation and crystallization of the metal-oxo species, giving rise to radially mesostructured frameworks. The presence of HCl facilitates the protonation of both TiO and RuO octahedra, guiding their edge-sharing polycondensation under slightly reducing, oxygen-deficient conditions, thus favoring the formation of rutile-type frameworks.45 The gradual crystallization feature from mini-crystallites during hydrothermal treatment also validates this phenomenon (Fig. S3). Importantly, the structural coherence between RuO and TiO , owing to their closely matched lattice parameters (Tab. S1), enables uniform substitutional doping at the atomic level rather than forming phase-separated Ru clusters, as corroborated by the comparable micelle intermediates in Ru-Ti and Ti-only systems (Fig. S2d). Note that the hydrothermal treatment primarily achieves the solidification of radial mesostructure and initiates Ru-O-Ti coordination, the complete incorporation of Ru into TiO lattice is predominantly occurred during calcination which simultaneously removes the template and promotes crystalline ordering (Fig. 1c), as evidenced by the phase evolution and thermogravimetric analysis results (Fig. S4). This stepwise, integrated synthesis route provides an approach for constructing hierarchically ordered, multicomponent mesoporous oxides with tailored interfaces for electrocatalytic applications.

Synthesis and Characterizations

The large-scale field-emission scanning electron microscopy (FESEM) image reveals a uniform branched nanosphere morphology with an average particle size at ~250 nm for the synthesized mesoporous Ru-Ti oxide solid solution (Fig. S5). Each nanosphere features multiple cylindrical mesoporous nano-bundles oriented from the core, forming a flower-like architecture (Fig. 2a, 2b). Transmission electron microscopy (TEM) image further evidences opened mesochannels with radial alignment, which facilitate mass and gas diffusion (Fig. 2c). Statistical analysis shows that each nanosphere typically contains 10-12 radial branches according to close inspections from different angles (Fig. 2d, 2e). High-resolution TEM (HRTEM) images disclose parallel lattice fringes with an interplanar spacing of 0.324 nm, corresponding to the (110) plane of rutile, which is identical to that of mesoporous TiO spheres synthesized through the same recipe (Fig. 2f, Fig. S6). The elongated diffraction spots with hexagonal symmetry observed in the selected area electron diffraction (SAED) pattern confirm the single-crystal-like characteristic of the mesopore walls and the absence of RuO derived from phase segregation (Fig. 2g). Note that the crystallographic orientation is partially at the branches while the entire nanospheres are still polymorphs (Fig. S7). Nitrogen sorption isotherms display the characteristic type IV curve at P/P = 0.5-0.9, indicative of the uniform mesoporous structure, as also evidenced by the pore size distribution centered at 9.0 nm (Fig. 2h, 2i). The broad peak at around 40 to 60 nm is ascribed to the interstices between branches and nanospheres. As a reference, the pure mesoporous TiO nanospheres with similar radial orientation were prepared (Fig. S8). The Brunauer-Emmett-Teller (BET) surface area and pore volume of the mesoporous solid solution are calculated to be 89 m2 g-1 and 0.44 cm3 g-1, comparable to that of the rutile mesoporous TiO (Tab. S2).

To provide atomic-scale insight into the structure, aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements were carried out. The presence of isolated bright atomic columns corresponding to Ru atoms embedded within TiO lattice without detectable nanocrystals verify the formation of homogeneous solid solution phase (Fig. 2j, 2k). Elemental mapping further validates the uniform distribution of Ru atoms across mesoporous nano-bundles (Fig. S9). Under high-energy electron beam irradiation, an in-situ aggregation phenomenon of Ru cluster in frameworks was occurred (Supplementary Video 1), further supporting the metastable Ru incorporation in TiO lattice. The electron energy loss spectroscopy (EELS) analysis identifies the characteristic Ti L -edge, Ru M -edge, and O K-edge, verifying the coexistence of Ru and Ti elements (Fig. 2l). The Ar+-sputtering X-ray photoelectron spectroscopy (XPS) profile further reveals the stoichiometric constitution of Ru Ti O . The decreasing Ru signal with sputtering depth suggests a surface-enriched Ru distribution, which may contribute to enhanced catalytic activity due to more accessible active sites (Fig. 2m). The energy-dispersive X-ray (EDX) analysis reveals Ru and Ti contents of 20.7 wt% and 42.2 wt% (Fig. S10a), corresponding to a Ru/Ti molar ratio of ~0.23 (Ti/Ru ≈ 4.3), which is consistent with the nominal composition of Ru Ti O and previous reports.

Gas-liquid-solid triple-phase interface

To gain mechanistic insights into the solid-phase characteristics, we conducted control experiment with varied Ru contents at 10 wt% and 5 wt% for comparative analysis (Fig. S10). As shown in the powder X-ray diffraction (PXRD) patterns, the meso-TiO exhibits characteristic reflections corresponding to a pure rutile TiO phase (I4 /amd), while the positive shift to RuO phase with the increase of Ru content implies the formation of homogeneous solid-solution phase with no phase segregation occurrence (Fig. S11). The Rietveld refinement reveals the lattice parameters of meso-Ru Ti O (a = b = 4.58 Å, c = 2.97 Å, and V = 62.54 Å3), and the correlated crystal structure was constructed for calculations (Fig. S11c). In Raman spectra (Fig. S12), the meso-TiO exhibits typical bands at 236, 443, and 608 cm-1, corresponding to low-frequency Ti-O-Ti bending (t -O 2p π interactions), E bending (t -O 2p π overlap), and A symmetric Ti-O stretching (e -O 2p σ anti-bonding states) of rutile phase.46 These modes progressively weaken in the Ru–Ti solid solutions, reflecting the distortion of Ti-O-Ti linkages and altered orbital overlap caused by Ru-O-Ti bond. The Ru incorporation with modified local octahedral field around Ti4+ redistribute the t and e orbital energies and weaken the Ti-O vibrations, demonstrating the impact of Ru doping on both structural order and electronic configuration of TiO lattice. To further validate this phenomenon, XPS analyses were conducted. The full spectra exclude the presence of other compounds (Fig. S13a). In the high-resolution Ti 2p region, peaks at 464.0 and 458.3 eV correspond to Ti4+ in pristine meso-TiO . Upon Ru incorporation, a negative shift is observed, suggesting partial reduction of Ti4+ to Ti3+ due to lattice distortion and charge compensation (Fig. S13b). Electron paramagnetic resonance (EPR) spectra reveal an increased g-value of 1.97 (Fig. S13c), consistent with the formation of Ti³⁺ species and associated oxygen vacancy defects. In the Ru 3d region, deconvoluted peaks at 281.1 eV (Ru 3d ) and 285.3 eV (Ru 3d ) are assigned to Ru4+ state (Fig. S13d), confirming the oxidation state of Ru in lattice.13 Progressive positive shifts of Ru 3d and O 1s peaks as the Ru content increases suggest a modified local electronic environment and enhanced electron-withdrawing capability of the solid-solution lattice.39 The increased intensity at 531.5 eV further implies the generated oxygen vacancies in solid solution (Fig. S13e).

X-ray absorption near-edge structure (XANES) spectra at the Ru K-edge display the negative shift of absorption edge of meso-Ru Ti O compared to RuO , indicating a nominal Ru valence lower than +4 (Fig. 3a). Quantitative analysis based on the scaling relation between absorption energy and oxidation state confirms the Ru valence at +3.90 in meso-Ru Ti O (Fig. S14a), mainly attributed to strong Ru-O-Ti interaction for stabilized Ru sites within solid solution. The Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectrum of Ru reveals distinct Ru-O and Ru-Ti coordination peaks at 1.90 and 3.69 Å, further indicating a shorter Ru-O bond length compared to 1.94 Å in RuO (Inset of Fig. 3a, Tab. S3). This bond contraction, accompanied by a slightly elongated Ru–Ti (Ru-Ru) distance, suggests a lattice distortion in the Ru-Ti solid solution oxide compared to pristine RuO . The shortened Ru-O bond may arise from a reduced Ru coordination number, which locally decreases the Ru valence state and increases its ionic radius, while simultaneously enhancing Ru-O covalency and bond strength. Such strengthened Ru-O interactions are expected to improve the structural robustness of active sites under acidic OER conditions, contributing to the high durability. The application of EXAFS wavelet transform (WT) analysis also provides evidence for the coordinated environment of Ru in solid solution (Fig. 3b, Fig. S13b). In the density of state (DOS) plot of meso-TiO , a distinct gap between the valence and conduction band can be observed, demonstrating the semiconducting feature of TiO with wide band gap (Fig. 3c). In contrast, the band gap disappears in the Ru-Ti oxide solid solution, suggesting the transformation into a metallic conductor with electronic states near Fermi level. This observation is fully consistent with subsequent ultraviolet photoelectron spectroscopy (UPS) measurements (Fig. S15e), which provide detailed insight into the progressive modification of the material’s surface electronic structure upon Ru incorporation. To gain more insights into the electronic structure, the electron localization function (ELF) maps were performed, showing notably higher values around Ru-O bonds than Ti-O bonds (Fig. 3d, Fig. S15a,b). This enhanced electron localization in Ru-O regions suggests stronger covalent Ru-O bond or electronic interaction due to its higher electronegativity of Ru with d orbitals, which could offer favorable sites for facilitated adsorption and activation of reactants. Besides, the low-ELF regions surrounding Ru are more extensive than those around Ti in pure TiO , indicating a higher degree of electron delocalization in Ru-rich regions with enhanced electrical conductivity. UV-Vis absorption spectra and corresponding Tauc plots (Fig. S15c,d) indicate a progressive narrowing of the bandgap with increasing Ru content in the solid solutions, suggesting significant alterations in the electronic structure. This behavior further supports the transformation of the material from a wide-bandgap semiconductor to a conductive material. Analysis of the interfacial electronic structure of the catalyst via UPS (Fig. S15f) revealed that as the Ru doping concentration increased, the work function of the material continuously rose from 4.06 eV for pure TiO₂ to 4.83 eV (20% Ru doping). This change fundamentally reflects the significant modification of the surface electronic properties induced by Ru incorporation. The increase in work function indicates a downward shift of the Fermi level relative to the vacuum level, suggesting that Ru species act as electron acceptors, enhancing the material’s ability to capture and stabilize electrons. Since the material forms a Ru Ti O solid solution through substitution of Ti sites by Ru atoms, significant band bending is induced at the interface, and a built-in electric field directed from the TiO₂ matrix toward the Ru active centers is established. This reconstruction of the electronic structure provides an efficient electron transport pathway for catalytic reactions: on one hand, it improves interfacial charge separation efficiency, and on the other hand, it accelerates surface reaction kinetics.

We further carried out contact angle (CA) measurements to investigate the liquid-gas phase behavior during reactant transport. Notably, the water droplet rapidly spreads on the surface of meso-Ru Ti O , showing a enhanced hydrophilic nature compared to the commercial RuO with a contact angle of 105° (Fig. S16, Supplementary Video 2). This superhydrophilicity is attributed to the open, high-surface-area titania networks for mass diffusion. Additionally, gas bubble contact angle measurements exhibit a higher CA of 152° for the meso-Ru Ti O than commercial RuO (Fig. S17a, Supplementary Video 3), demonstrating the enhanced aero-phobicity. The combination of superhydrophilicity and aero-phobicity is expected to largely facilitate reactant diffusion and gas bubble detachment under high current densities in acidic OER. To further validate this hypothesis, we measured the adhesive force between oxygen bubbles and the catalyst surface. The meso-Ru Ti O exhibits a adhesive force of ~0 µN, enabling spontaneous and complete bubble detachment without morphological deformation, as also confirmed by the sequential images captured at different test stages (Fig. 3e, 3f, Supplementary Video 4). In contrast, the commercial RuO shows a significantly higher adhesive force of ~5 µN, which could hinder bubble release and potentially accelerate catalyst degradation due to mechanical disturbance (Fig. S17b). Overall, the integrated surface properties of meso-Ru Ti O , characterized by high wettability and minimal bubble adhesion, significantly enhance interfacial mass transport, indicating its potential applicability for high-performance, industrial-scale PEM water electrolysis systems.

OER and PEMWE Performances

The OER behavior of the mesoporous solid solution was first assessed in a standard three-electrode system using 0.5 H SO electrolyte, with commercial RuO as the benchmark (Fig. S18). In linear sweep voltammetry (LSV) curves (Fig. 4a), the meso-Ru Ti O demands an overpotential (η) of only 257 mV to reach a current density of 10 mA cm-2, which is lower than commercial RuO (302 mV) and enhanced to many RuO -based catalysts (Tab. S4). At a higher current density of 100 mA cm-2, the overpotential of meso-Ru Ti O remains at a moderate 319 mV, while commercial RuO fails to reach this current density within the same potential window, highlighting the enhanced activity attributed to high surface area. Tafel slope analysis (Fig. S19a) reveals a lower slope of 59 mV dec-1 for the meso-Ru Ti O compared to 92 mV dec-1 for the commercial RuO , indicating the accelerated reaction kinetics and efficiency in the formation of oxygenated intermediates under acidic condition. Moreover, electrochemical impedance spectroscopy (EIS) measurements (Fig. 4b) show that the meso-Ru Ti O exhibits the lowest charge transfer resistance (~20 Ω) compared to lower-Ru-content sample (~38 Ω) and commercial RuO (~24 Ω), suggesting charge transport characteristics in mesoporous structure (Fig. 4b). To determine the electrochemically active surface area (ECSA), double-layer capacitance (C ) tests were conducted (Fig. S19b). The C of meso-Ru Ti O reaches to 46.0 mF cm-2, significantly higher than that of lower-Ru-content samples, indicating an increase in ECSA upon Ru incorporation (Fig. S20). Besides, the mass activity, specific activity, and turnover frequency (TOF) values (Table S5) were further calculated at 1.53 V vs. RHE. The meso-Ru Ti O catalyst exhibits a mass activity of 921.6 A g-1 , a specific activity of 0.091 mA cm-2 , and a TOF of 0.2412 s-1, significantly higher than those of other samples, suggesting possible relevance for practical application.

Motivated by the high activity of the meso-Ru Ti O catalyst, we further investigated the performance of meso-Ru Ti O in a practical PEMWE device. A full-cell configuration was assembled using meso-Ru Ti O as the anode catalyst, commercial Pt/C as the cathode catalyst, and Nafion 115 as the proton exchange membrane. The meso-Ru Ti O || Pt/C achieves a current density of 1.0 A cm-2 at a low cell voltage of only 1.65 V (Fig. 4c), lower than that measured for commercial RuO₂ under the same conditions. Even under an ultralow Ru loading of 0.02 mg cm-2, the electrolyzer maintains a current density of 1.0 A cm-2 at 1.72 V, compareable to the DOE 2025 Ru target (1.0 mg cm-2, 1.72 V@1 A cm-2). Moreover, the meso-Ru Ti O can operate steadily at 1.0 A cm-2 for over 450 h with a minimal voltage decay rate of 21.1 μV h-1 (Fig. 4d). Post-reaction characterizations (Fig. S21) of the meso-Ru Ti O catalyst confirm the preservation of radial morphology without collapse or aggregation in characteristic rutile phase, further underscoring its verified structural stability during prolonged acidic electrolysis, which strongly verifies the desired Ti-Ru incorporation for suppressed Ru overoxidation. A comparative summary of state-of-the-art PEMWE devices indicates stable operation and performance of meso-Ru Ti O for scalable hydrogen production among RuO -based OER anode catalysts (Fig. S22),47-59 which benefits from the synergistic contribution of its mesoporous structure and homogeneous solid-solution composition.

In-situ measurements and theoretical insights

To elucidate the origin of the enhanced stability of the mesoporous solid solution, we adopted in-situ XANES spectroscopy to monitor the evolution of Ru electronic structure under OER operating conditions. As presented in Fig. 5a, Ru K-edge XANES spectra were recorded from the open circuit potential (OCP) to 1.23 V, and subsequently up to 1.83 V vs. RHE. With the increase of applied potential, the absorption edge progressively shifted to higher energies, indicating a continue oxidation of Ru species. Quantitative valence fitting reveals that the oxidation state of Ru in meso-Ru Ti O increases from +3.54 at OCP to +3.75 at 1.23 V, and further to +3.80 at 1.43 V vs. RHE. Remarkably, when the potential reaches 1.63 V vs. RHE, the Ru oxidation state stabilized at +3.80, and remains nearly unchanged even with further increase to 1.83 V, where it slightly increased to +3.90. Interestingly, the initial Ru valence at OCP is slightly lower than that of pristine RuO , which may be attributed to partial electron transfer from the TiO lattice to Ru centers. This lattice-mediated electronic modulation effectively stabilizes the Ru sites in a more reduced and active state at open circuit conditions. As the applied potential increased to 1.63 V vs. RHE, the Ru valence approached +3.80 and remained nearly constant despite the onset of vigorous OER, indicating that the Ru electronic structure is effectively buffered against overoxidation. Besides, the Ru oxidation state first decreases and then increases as the applied potential increases, accompanied by a corresponding evolution in the local Ru-O coordination environment (Fig. S23, Table S6). At low potential (~1.2 V), partial reduction of Ru leads to slightly elongated Ru-O bonds and a reduced effective coordination number, indicating a more relaxed local structure. Upon further increasing the potential, the Ru-O coordination number gradually increases toward octahedral coordination while the Ru-O bond lengths shorten, consistent with the formation of higher-valence Ru species and strengthened Ru-O bonding. This self-limiting oxidation behavior mitigates the formation of soluble high-valence Ru species responsible for catalytic degradation through dissolution. These results highlight that the electronic coupling between Ru and TiO host lattice plays a role in buffering Ru oxidation state to maintain a stable and catalytically favorable window, thereby ensuring long-term durability under high anodic potentials.

To further probe the underlying reaction mechanism, we performed in-situ differential electrochemical mass spectrometry (DEMS) with isotope-labeled electrolyte water H 18O as electrolyte (Fig. 5b). During OER operation, the evolved oxygen gas was analyzed to determine the contribution of lattice oxygen participation. The presence of 34O , formed by the combination of 16O from electrolyte and 18O from catalyst lattice, serve as a key indicator of the lattice oxygen mechanism (LOM). Quantitative analysis reveals that only a small fraction (8.1%) of the evolved oxygen is 34O , while the majority (91.9%) is 32O , which originates solely from the electrolyte. This result suggests that lattice oxygen participation is effectively suppressed on the meso-Ru Ti O catalyst, favoring a non-LOM pathway. Such pathways typically proceed via the adsorbate evolution mechanism (AEM) or oxide path mechanism (OPM), both of which are less prone to catalyst degradation and thus desirable for long-term operation in acidic media.15 This aligns with the latest findings that stabilizing lattice oxygen is critical for long-term stability, as strain-engineered Pt-RuO 31 and demonstrated in Co-RuO 32 systems. To elucidate the surface reaction intermediates during OER, in-situ attenuated total reflectance surface-enhanced infrared absorption spectra (ATR-SEIRAS) were conducted under varying applied potentials (Fig. 5c). A distinct vibrational band emerges at ~1060 cm-1 at potentials above 1.8 V vs. RHE, which is assigned to the O-O stretching mode in the *OOH intermediate, confirming the participation of AEM pathway. In addition, a broader signal centered around ~1170 cm-1 appears at higher potentials, corresponding to the vibrational signature of coupled *O-O*, characteristic of the OPM pathway. The simultaneous presence of both vibrational features indicates that the meso-Ru Ti O catalyst engages dual-site reaction pathways involving both AEM and OPM, which contributes to its stable oxygen evolution behavior under harsh acidic conditions. This mechanistic duality also highlights the structural and electronic adaptability of mesoporous solid solution capable of efficient and stable OER via complementary reaction routes.

To elucidate the underlying OER mechanism, we conducted first-principles calculations to determine the preferred two possible AEM and OPM pathways (Fig. 5d). On the RuO (110) surface, there are two distinct catalytic sites: the fivefold coordinatively unsaturated site (CUS) and sixfold-coordinated bridge site (BRI).60 In pristine RuO , the Ru atoms at CUS sites are recognized as the dominant active centers for OER, where the *OOH intermediate formation constitutes the rate-determining step (RDS) in the AEM pathway. This supports the widely accepted AEM dominance on commercial RuO .61 However, in meso-Ru Ti O , the spatial isolation of Ru centers due to Ti substitution disrupts the formation of adjacent Ru-CUS pairs, effectively suppressing potential bimetallic Ru–Ru interactions that are often involved in AEM catalysis (Table S3). Our calculations reveal that the free energy barrier for *O to *OOH step in AEM is 2.091 eV for the meso-Ru Ti O (Fig. 5f), significantly higher than in pristine RuO , indicating a thermodynamically disfavored AEM pathway due to Ti incorporation. In contrast, for the OPM pathway (Fig. 5e), where the RDS involves the coupling of surface-bound *O and *OH to form *O-O* intermediate, the calculated energy barrier is only 1.581 eV. Moreover, the overpotential (η) for AEM is estimated at 0.861 V, much higher than 0.361 V calculated for OPM, further confirming that the OPM pathway is thermodynamically favored in mesoporous solid solution (Supplementary Data 1). We note that the calculations employ idealized surface models under static conditions and therefore do not fully capture potential-dependent reconstruction, explicit solvent dynamics and defect distributions; the computed energetics are intended to indicate qualitative mechanistic trends. These results are in high agreement with the in-situ spectroscopic results and isotope-labeling experiments, jointly demonstrating that Ti-induced lattice modulation in meso-Ru Ti O redirects the OER pathway from AEM to OPM, contributing to its enhanced activity and durability under acidic conditions.

Methods
Materials

Pluronic F127 (PEO PPO PEO , Mw = 12,600 g mol-1) was sourced from Sigma-Aldrich. Tetrabutyl titanate (TBOT, 99.5%) was obtained from Sigma-Aldrich. Ruthenium trichloride hydrate (RuCl ·xH O) was sourced from Aladdin Reagent Co., Ltd. Tetrahydrofuran (THF, 99%), concentrated hydrochloric acid (HCl, 36-38 wt%), acetic acid (99%), perchloric acid (HClO ), and anhydrous ethanol were purchased from Shanghai Chemical Reagent Co., Ltd. All chemicals were used as received without further purification.

Synthesis of mesoporous Ru Ti O oxide solid solution

Specifically, 1.6 g of the triblock copolymer Pluronic F127 and 0.37 g of RuCl ·xH O were added to 30 mL of THF, followed by the addition of 2.4 mL of acetic acid as a chelating agent and 3.2 mL of concentrated HCl as a stabilizer to slow down the rapid hydrolysis of the titanium precursor. After stirring for 5 min, 3.0 mL of TBOT was introduced into the mixture. The resulting precursor solution was then stirred continuously for 10 min before being transferred into a 30 mm × 50 mm volumetric flask and placed in an oven at 45 °C for 24 h. This step allows for the slow evaporation of THF solvent, forming a black mono-micelle hydrogel. During the THF evaporation process, the relative humidity was maintained at approximately 40% to prevent excessive hydrolysis of TBOT, which could lead to the formation of unassembled TiO impurities. Next, the as-formed mono-micelle hydrogel was transferred into a 50-mL Teflon-lined autoclave, sealed, and heated at 120 °C for 24 h, resulting in a black gel. The gel was subsequently annealed in air at 400 °C for 3 h with a heating rate of 1 °C min-1 to obtain the final mesoporous Ru Ti O oxide product (denoted as meso-Ru Ti O ). By adjusting the added amount of RuCl ·xH O to 0.09 and 0.36 g, products with different Ru content could be synthesized for comparison.

Synthesis of mesoporous rutile TiO nanospheres

The 3D mesoporous TiO nanospheres (denoted as meso-TiO ) were fabricated through a reported hydrothermal treatment.45 For a typical synthesis, the white yellow mono-micelle hydrogel was prepared through THF evaporation at 45 ºC for 24 h using a similar recipe for the meso-Ru Ti O except for removing the addition of RuCl ·xH O. A flask of the prepared mono-micelle hydrogels (around 5 g) was then transferred into a 50-mL Teflon-lined autoclave, sealed, and heated at 120 °C for 24 h. After cooling down to room temperature naturally, the white viscous precipitates after discarding supernatant were collected into a porcelain crucible. The meso-TiO sample in rutile phase was finally obtained after calcination in air at 400 °C for 3 h with a slow heating rate of 1 °C min-1.

Materials Characterizations

Field-emission scanning electron microscopy (FESEM) images were acquired using a Hitachi S-4800 microscope operated at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and high-angle annular dark-field (HAADF) imaging were performed on a JEOL JEM-2100F microscope equipped with an energy-dispersive spectrometer (EDS) and operated at 200 kV. TEM samples were prepared by dispersing the materials in ethanol and drop-casting onto amorphous carbon-coated copper grids. Powder X-ray diffraction (PXRD) patterns were recorded using a Bruker D8 diffractometer (40 kV, 40 mA) with a lab-based X-ray source and a Cu Kα target (λ = 1.54056 Å). Nitrogen adsorption-desorption isotherms were measured at 77 K using a Micromeritics Tristar 3020 analyzer, with samples degassed under vacuum at 180 °C for 8 h prior to measurement. Specific surface areas were calculated using the Brunauer-Emmett-Teller (BET) method, while pore size distributions were determined using the Barrett-Joyner-Halenda (BJH) model. Raman spectra were collected at room temperature using a Horiba Scientific Raman spectrometer with a 532 nm excitation wavelength. X-ray photoelectron spectroscopy (XPS) was conducted on an Axis Ultra spectrometer using monochromatic Al Kα radiation in vacuum. The C 1s peak at 284.6 eV was used as a reference for calibrating the XPS spectra.

In situ Ru K-edge XAFS spectra were collected at the BL17B beamline of the Shanghai Synchrotron Radiation Facility (SSRF), operated in fluorescence mode at custom-designed electrolytic cell. The SSRF storage ring was maintained at 2.5 GeV with a maximum electron current of 250 mA. The incident beam was calibrated using a standard Ru foil. The working electrode by loading the catalyst onto carbon paper with a polyimide film backing was integrated into the cell. The acquired EXAFS data were processed using the ATHENA and ARTEMIS modules of Demeter software package. Raw absorption spectra were subjected to post-edge background subtraction and normalization relative to the edge-jump step. The resulting χ(k) functions were Fourier-transformed into R-space using a Hanning window (dk = 1.0 Å-1) to isolate contributions. Quantitative structural parameters were obtained through least-squares curve fitting in the ARTEMIS module based on the following equation:

𝜒(𝑘) = ∑ 𝑗 𝑁𝑆2𝐹(𝑘) 𝑗 ⋅exp⁡[−2𝑘2𝜎2]⋅exp⁡[ −2𝑅 𝑗 ]⋅sin⁡[2𝑘𝑅 +𝜙 (𝑘)]
𝑘𝑅2 𝑗 𝜆(𝑘) 𝑗 𝑗 𝑗

The theoretical scattering amplitudes (𝐹(𝑘)), phase shifts (𝜙 (𝑘)), and photoelectron mean free paths (𝜆) were calculated ab initio using the FEFF9 code. Structural refinement was conducted in R-space, incorporating phase corrections for the first coordination shell to align the R values with physical interatomic distances. The amplitude reduction factor (𝑆2) and coordination numbers (N) were fixed to values determined from reference models, while the interatomic distances (R), Debye-Waller factors (𝜎2), and edge-energy shifts (ΔE ) were allowed to vary freely to achieve the best fit. The Ru oxidation state was determined by the linear relationship between the Ru K-edge absorption energy (E , defined at the half-height of the normalized edge jump) and the formal valence of standard references (Ru foil for Ru0, RuO for Ru4+).

Electrochemical tests

All electrochemical measurements were performed using a CHI760e electrochemical workstation. A three-electrode configuration was employed, consisting of a glassy carbon electrode (GCE) or carbon paper as the working electrode (with an area of 0.196 cm2), a graphite rod (0.5 cm in diameter) as the counter electrode, and an Ag/AgCl electrode as the reference electrode. The electrolyte used was an O -saturated 0.5 M H SO solution. The applied potential was converted to the reversible hydrogen electrode (RHE) scale using the following equation: E = E + φ(Ag/AgCl) + 0.0591*pH. For the preparation of the GCE-based working electrode, 5.0 mg of catalyst sample was dispersed in a mixture of 10 μL Nafion and 490 μL ethanol to form homogeneous catalyst ink. Subsequently, 10 μL of the ink was drop-cast onto the GCE and dried at room temperature. Linear sweep voltammetry (LSV) was conducted at a scan rate of 5 mV s-1, with all LSV curves corrected for iR compensation. Electrochemical impedance spectroscopy (EIS) measurements were performed at a potential of 1.53 V vs. RHE, over a frequency range of 0.01 Hz to 100 kHz. For the determination of the electrochemically active surface area (ECSA), cyclic voltammetry (CV) was conducted at scan rates of 20, 40, 60, 80, and 100 mV s-1. The Nafion 115 membrane was sequentially treated in ultrapure water, 6 wt% H O , 0.5 M H SO , and ultrapure water at 80°C, with each treatment lasting 1 h. After cooling to room temperature, the treated membrane was stored in ultrapure water.

In situ DEMS measurements

For in situ differential electrochemical mass spectrometry (DEMS), meso-Ru Ti O was first loaded onto a porous Au film. The catalyst surface was labeled with ¹⁸O by performing five CV cycles between 1.20 and 1.55 V (vs. RHE) in 0.5 M H₂SO₄ prepared with H₂¹⁸O. The ¹⁸O-labeled catalyst was then thoroughly rinsed with a large amount of deionized water to remove adsorbed H₂¹⁸O, followed by three CV cycles in 0.5 M H₂SO₄ prepared with H₂¹⁶O. During each cycle, the gaseous oxygen products, including ³²O₂ (¹⁶O¹⁶O), ³⁴O₂ (¹⁶O¹⁸O), and ³⁶O₂ (¹⁸O¹⁸O), were monitored online by mass spectrometry.

PEMWE tests

The membrane electrode assembly (MEA) was prepared using Nafion 115 via the catalyst-coated membrane (CCM) method, with an active geometric area of 2 cm × 2 cm (4 cm2). A commercial Pt/C (60 wt%) catalyst was used as the cathode for the HER, while the mesoporous Ru Ti O served as the anode catalyst for the OER. The preparation of the anode catalyst ink involved dispersing 40 mg of mesoporous Ru Ti O powder in a mixture of 600 mg of isopropanol, 600 mg of water, and 200 mg of Nafion (5 wt%), followed by at least 2 hours ultrasonication to ensure uniform dispersion. The loading of the mesoporous Ru Ti O was 2 mg cm-2. The cathode catalyst ink was prepared by dispersing 20 mg of commercial Pt/C (60 wt%) powder in 300 mg of isopropanol, 300 mg of water, and 100 mg of Nafion (5 wt.%), followed by ultrasonication for 2 h. The loading of Pt/C catalyst was 0.3 mg cm-2. For comparison, a PEMWE system using the commercial RuO was also prepared, maintaining an identical loading of anode catalyst. The catalyst-coated membrane was hot-pressed at 135°C and 6 MPa for 2 min, then assembled with a gas diffusion layer (titanium felt) to form the MEA. The PEMWE performances were evaluated using a battery test system (BTS-4000, NEWARE, China). During the tests, both the anode and cathode bipolar plates were heated to 60 °C, while preheated deionized water (60 °C) was supplied to the anode side at a flow rate of 20 mL min-1. Linear sweep voltammetry (LSV) curves were performed with a scan rate of 5 mV s-1, typically in the voltage range of 1.2 to 2.5 V. Stability tests were conducted at a constant current density of 1 A cm-2 under the same operating conditions.

DFT calculations

The DFT calculations were executed in the Vienna Ab initio Simulation Package (VASP) program. The PAW pseudopotential and PBE functional (GGA-PBE) were employed to describe electron interactions in DFT calculation. The DFT-D3 method with the Becke-Johnson damping method was used to describe the dispersion interaction within atoms. The solvent model method using the VASPsol codes was introduced to describe solute-solvent interactions. To ignore the effect of adjacent layer atoms, a 15 Å thick vacuum layer was placed on the surface in Z-direction. In the calculation, the cut-off energy was set to 400 eV, and the convergence thresholds were determined to 10-5 eV and -0.02 eV Å-1. The Bloch vectors sampled in Brillouin zone (k-points) was fixed to 3 × 3 × 1 in structural relaxation processes, while the Bloch vectors was set to 11 × 11 × 1 in electronic structure calculation. The densities of states (DOS) and entropy correction were completed based on VASP toolkit (vaspkit).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, Zhang et al. report an advanced assembly route to construct mesoporous RuTiO2 solid-solution with a three-phase interface and superior potential in acidic OER and PEMWE. The authors comprehensively discuss how to delicately establish a mesoscopic oxide material with both high activity and long-term durability in PEMWE, including super-hydrophilic triple phase, mesoscale pathways and atomically dispersed Ru-O-Ti motifs. Such an electrocatalyst delivers superior OER behaviour and industrial-level PEMWE performance, achieving 1 A cm-2 for 450 h at 0.4 mgRu cm-2, which is highly promising. The overall narrative is engaging and well-organised, providing valuable insights for a wide range of researchers interested in synthesising porous and nano-materials and electrocatalytic applications. I recommend its publication in Nature Communications after addressing the following suggestions.\n1.The authors use EDX results (Ru 20.7 wt%, Ti 42.2 wt%) to conclude the Ru/Ti molar ratio = 4:1. This is not identical with the stoichiometry Ru0.2Ti0.8O2 and requires further analysis or correction.\n2.Regarding the formation of the solid-solution structure, it is stated that RuO6/TiO6 octahedra already build up a Ru-Ti solid-solution framework during the hydrothermal process, whereas the solid-solution structure is mainly formed during calcination in Figure S4. These two descriptions are contradictory.\n3.Although the radially mesoporous architecture is similar for meso-TiO2 and meso-RuTiO2, I am curious about the reason for the large shrinkage in spherical diameter from ~2 µm to ~200 nm after incorporating Ru species.\n4.The authors state that the F127 template is completely removed after calcination, please provide evidence to support this claim and eliminate the effect of carbon in OER performance.\n5.Despite these key factors, how does hydrothermal temperature and time impact the final products?\n6.It is preferable to provide mass activity data (A gRu-1at a potential) to reflect Ru utilisation efficiency. A comparison with other Ru-based OER catalysts in terms of noble metal utilisation would better position the advantages of this work.\n7.In the Methods, the final calcination time (2 h) of the solid solution at 400 °C is inconsistent with that in Figure S5 (3 h).\n8.In the main text, measurements are performed in 0.5 M H₂SO₄, but in the Methods the electrolyte is 1.0 M HClO₄. This error should be corrected.\n9.To aid visualisation of the emphasised structural design, a conceptual scheme illustrating the triple-phase interface engineering for high-performance electrocatalysis is suggested.", "reviewer_2": "Review of “Customized Self-Assembly of Mesoporous Ruthenium-Titanium Oxide Solid Solution with Efficient Three-Phase Reaction Interface for Water Electrolysis” for Nature Communications\nRecommendation: Minor Revision\nComments to the Authors\nIn this manuscript, the authors report a mesoporous Ru-Ti-O rutile-phase solid-solution catalyst (meso-RuxTiyO2) prepared via a customized micelle self-assembly strategy, targeting durable Ru-based anodes for acidic proton exchange membrane water electrolysis (PEMWE). The work proposes that atomic-scale Ru-O-Ti motifs modulate the electronic structure to suppress Ru over-oxidation and reduce lattice oxygen participation, while the ordered mesoporous nanorod-bundle architecture improves the gas-liquid-solid three-phase interface by enhancing wettability and facilitating oxygen bubble detachment. The catalyst is shown to deliver strong acidic OER activity and long-term PEMWE operation at industrial current density with relatively low Ru loading. Overall, the study is well organized and the results are promising; the following minor points (clarity, labeling, and additional supporting characterization) should be addressed to improve readability, accuracy, and completeness.\n1. Table S1 is titled “Comparative crystallographic parameters of rutile-type TiO₂ and RuO₂,” yet the second column is labeled as rutile SnO₂. This appears to be a typographical error and should be corrected to rutile RuO₂. Please also verify that the lattice parameters reported in this column indeed correspond to RuO₂ (rather than SnO₂), since this table is used to support the lattice-matching/solid-solution rationale.\n2. In Figure S2, panels (a-c) correspond to the F127/TBOT/RuCl₃ micelle intermediate, whereas panels (d-f) correspond to the F127/TBOT (Ti-only) micelle intermediate; however, this distinction is not sufficiently clear from the current caption. Please explicitly label each set either (1) directly on the TEM panels (e.g., “+RuCl₃” vs “Ti-only”) and/or (2) by dividing the caption into two clearly separated parts. This will prevent confusion when comparing micelle sizes (~9.1 nm vs ~8.8 nm) and interpreting the effect of Ru on pre-micellization.\n3. Overstated statement regarding RuO₂ stability - The statement that “commercial RuO₂ fails to maintain stable operation” is ambiguous and potentially overstated. Please specify what instability is observed (e.g., inability to reach/hold 100 mA cm⁻², rapid degradation in chronoamperometry/chronopotentiometry, electrode delamination, fluctuating current/voltage). If stability at 100 mA cm⁻² is not directly demonstrated, the wording should be revised to a more precise and evidence-based description.\n4. Post-stability characterization - Given that long-term durability is a central claim of this work (e.g., extended PEMWE operation at industrial current density), the manuscript would be significantly strengthened by including post-test structural and compositional characterization of the catalyst/electrode. For example, SEM/TEM (morphology and mesostructured integrity), XRD/PXRD (phase stability), and surface/chemical analysis such as XPS or ICP (Ru oxidation state changes and potential Ru dissolution) after durability testing. These data would directly support the assertions of structural robustness and suppressed Ru over-oxidation under realistic operating conditions.\n5. Punctuation consistency and tone - Several sentences (e.g., around lines 44-45, 55, and 64) use em dashes in a way that reads stylistically inconsistent with the rest of the manuscript. Consider replacing them with commas or parentheses for a more formal and uniform tone throughout.\n6. Typographical and language corrections - The manuscript contains multiple typographical errors (e.g., “charateristics,” “ontorl experiments” and so on) and phrasing issues that require careful proofreading. A full language edit is recommended to ensure clarity and professionalism.", "reviewer_3": "Zhang et al. proposed a customized self-assembly strategy to synthesize mesoporous Ru-Ti solid-solution nanospheres. The nanosphere features multiple cylindrical mesoporous nano-bundles oriented from the core, forming an intriguing flower-like architecture. The meso-Ru0.2Ti0.8O2 shows an overpotential (η) of 257 mV at a current density of 10 mA cm-2. The assembled PEMWE device operation 1 A cm-2 for 450 h. There are several aspects that need to be addressed before the manuscript is suitable for publication in Nature Communications.\n(1) The Ru K-edge XANES spectra of the Meso-Ru0.2Ti0.8O2 sample exhibits fine structural features similar to those of the RuO2 reference and distinctly different from those of Ru foil, indicating the absence of metallic Ru particles or cluster species in the sample. However, to further exclude the possible presence of RuO2 species in Meso-Ru0.2Ti0.8O2, it is recommended to also provide the radial structure function of RuO2 and its corresponding fitting curves for comparison.\n(2) In Figure 3b, the Ru K-edge WT-EXAFS spectra of the Meso-Ru0.2Ti0.8O2 sample shows that the characteristic peak assigned to the Ru-O scattering path in R space range of 1-2 Å corresponds to k values higher than 10 and is close to those of the Ru–Ru (Ti) peak at 3-4 Å, which is confusing. Generally, the coordination of light elements such as O does not exhibit k values exceeding 10 in WT-EXAFS spectra.\n(3) The method used to determine the oxidation state of Ru in Figures 5a and S14a should be described in greater detail in the Experimental section.\n(4) The experimental conditions of XAS measurements, as well as the data processing and analysis methods, should be systematically and clearly described in the Experimental section.\n(5) In Table S3, two Ru-O scattering paths were used in the fitting of the RuO2 reference, but only a single disorder parameter (σ²) was reported. This point requires further clarification. In addition, it is recommended to provide the specific physical meaning and uncertainties of each fitting parameter.\n(6) It is suggested to supplement the in-situ EXAFS spectra in R space and to analyze the evolution of the catalyst’s local structure during the OER process based on these data.\n(7) In Figures 3b and S14b, it is recommended that the k-axis label in the WT-EXAFS plots be consistently formatted using lowercase letters.\n(8) The calculation of mass activity, specific activity, and TOF values should be added.\n(9) In spite of this success of Ru, Ti incorporation, I am wondering the possibility in fabricating other mesoporous oxide solid solutions in other elements."}, "author_response": "Point-to-point responses to Reviewers’ comments\nReviewer 1\nComments:\nIn this work, Zhang et al. report an advanced assembly route to construct mesoporous RuTiO solid-solution\n2\nwith a three-phase interface and superior potential in acidic OER and PEMWE. The authors\ncomprehensively discuss how to delicately establish a mesoscopic oxide material with both high activity\nand long-term durability in PEMWE, including super-hydrophilic triple phase, mesoscale pathways and\natomically dispersed Ru-O-Ti motifs. Such an electrocatalyst delivers superior OER behaviour and\nindustrial-level PEMWE performance, achieving 1 A cm-2 for 450 h at 0.4 mg cm-2, which is highly\nRu\npromising. The overall narrative is engaging and well-organised, providing valuable insights for a wide\nrange of researchers interested in synthesising porous and nano-materials and electrocatalytic applications.\nI recommend its publication in Nature Communications after addressing the following suggestions.\nResponse:\nWe are grateful for the reviewer’s encouraging summary and the recommendation to publish this work in\nNature Communications. We have addressed all the specific questions and suggestions raised to further\nstrengthen the manuscript. Our point-by-point responses are listed as follows.\nQuestion 1:\nThe authors use EDX results (Ru 20.7 wt%, Ti 42.2 wt%) to conclude the Ru/Ti molar ratio = 4:1. This\nis not identical with the stoichiometry Ru Ti O and requires further analysis or correction.\n0.2 0.8 2\nResponse:\nWe thank the reviewer for pointing out this issue. The Ru/Ti molar ratio was recalculated based on the EDX\nweight percentages (Ru: 20.7 wt%, Ti: 42.2 wt%). After conversion using the corresponding atomic weights,\nthe Ru/Ti molar ratio is determined to be ~0.23 (Ti/Ru ≈ 4.3), which is fully consistent with the nominal\ncomposition of Ru Ti O . We have corrected the related statement in the manuscript accordingly to avoid\n0.2 0.8 2\nambiguity and ensure consistency between the compositional analysis and the proposed solid-solution\nstoichiometry.\nChanges in the Manuscript:\nIn the Results and Discussion section (Synthesis and Characterizations, near Fig. S10a), the original\nsentence: “The energy-dispersive X-ray (EDX) analysis points out the Ru and Ti contents to be 20.7 wt%\nand 42.2 wt% (Fig. S10a), which determines the Ru/Ti molar ratio of 4:1, consistent with previous results.”\nhas been revised to:\n“The energy-dispersive X-ray (EDX) analysis reveals Ru and Ti contents of 20.7 wt% and 42.2 wt% (Fig.\nS10a), corresponding to a Ru/Ti molar ratio of ~0.23 (Ti/Ru ≈ 4.3), which is consistent with the nominal\ncomposition of Ru Ti O and previous reports.”\n0.2 0.8 2\nQuestion 2:\nRegarding the formation of the solid-solution structure, it is stated that RuO /TiO octahedra already build\n6 6\nup a Ru-Ti solid-solution framework during the hydrothermal process, whereas the solid-solution structure\nis mainly formed during calcination in Figure S4. These two descriptions are contradictory.\nResponse:\nWe thank the reviewer for pointing out this inconsistency. We agree that the original descriptions of the\nsolid-solution formation process in the main text and the Supplementary Information were not sufficiently\nconsistent and may cause confusion.\nIn this work, the hydrothermal process mainly serves to construct the radially ordered mesostructured\nTi–O based frameworks and to initiate local Ru–O–Ti coordination through precursor co-condensation and\nshort-range structural mixing. However, the complete substitutional incorporation of Ru into the rutile TiO\n2\nlattice, leading to a homogeneous Ru–Ti solid solution, is predominantly achieved during the subsequent\ncalcination step. During calcination at 400 °C, template removal, lattice rearrangement, and atomic\ndiffusion collectively promote the formation of the rutile-phase solid-solution structure.\nWe have revised the relevant descriptions in both the main text and the Supplementary Information to\nclearly distinguish the respective roles of the hydrothermal and calcination steps, ensuring a consistent and\nphysically reasonable formation mechanism.\nChanges in the Manuscript:\nMain text (Results and Discussion, “Customized Self-Assembly” section), the original sentence:\n“Afterwards, calcination at 400 °C removes the template and promotes crystalline ordering with surface-\nrich Ru atoms while preserving the mesostructure (Fig. 1c, Fig. S4). The resulting nanospheres exhibit\nradially aligned mesoporous branches and a robust Ru–Ti–O solid solution framework.”\nhas been revised to:\n“Note that the hydrothermal treatment primarily achieves the solidification of radial mesostructure and\ninitiates Ru-O-Ti coordination, the complete incorporation of Ru into TiO lattice is predominantly occurred\n2\nduring calcination which simultaneously removes the template and promotes crystalline ordering (Fig. 1c),\nas evidenced by the phase evolution and thermogravimetric analysis results (Fig. S4).”\nSupplementary Information (Fig. S4), original text:\n“The calcination process plays a vital role for the Ru incorporation.”\nrevised text:\n“The calcination plays a vital role in driving lattice rearrangement and atomic diffusion, which promotes\nthe substitutional incorporation of Ru into the rutile TiO lattice and leads to the formation of the Ru–Ti\n2\nsolid-solution structure, whereas the hydrothermal step mainly defines the mesostructured frameworks.”\nQuestion 3:\nAlthough the radially mesoporous architecture is similar for meso-TiO and meso-RuTiO , I am curious\n2 2\nabout the reason for the large shrinkage in spherical diameter from ~2 μm to ~200 nm after incorporating\nRu species.\nResponse:\nWe thank the reviewer for this insightful and thoughtful question, which highlights an important structural\nevolution induced by Ru incorporation. The significant difference in the spherical diameter between meso-\nTiO (~2 μm) and meso-RuTiO (~200 nm), despite their similar radially mesoporous architecture, can be\n2 2\nattributed to several factors influencing the synthesis process:\n1. Evaporation rate and time: The addition of Ru species likely alters the volatility and surface tension of\nthe precursor solution. This can lead to differences in the evaporation rate during the 40 °C evaporation\nstep. A faster evaporation rate in the case of meso-RuTiO could lead to a more rapid supersaturation and\n2\nnucleation, resulting in smaller and more numerous spheres.\n2. Emulsion droplet size during hydrothermal treatment: The hydrothermal treatment step involves the\nformation of an emulsion. The presence of Ru species can modify the interfacial tension and stability of the\nemulsion droplets. If the Ru-containing precursor leads to the formation of smaller emulsion droplets, this\nwould directly result in smaller final spherical particles.\nIn summary, the interplay of these factors contributes to the observed difference in spherical diameter\nbetween meso-TiO and meso-RuTiO . We prefer not to add this explanation into the main text since it is\n2 2\nnot our emphasis.\nQuestion 4:\nThe authors state that the F127 template is completely removed after calcination, please provide evidence\nto support this claim and eliminate the effect of carbon in OER performance.\nResponse:\nWe thank the reviewer for this notification. To verify the removal of F127, we have performed\nThermogravimetric analysis (TGA) of the calcined sample and added as Fig. S4e, which shows negligible\nweight loss above 400 °C, confirming the complete removal of the template and ruling out carbon\ncontribution to the OER activity.\nFigure S4. (e) TGA curve of the as-made meso-Ru Ti O precursor carried out in air, indicating the\n0.2 0.8 2\ncomplete removal of the F127 template above 400 ºC.\nQuestion 5:\nDespite these key factors, how does hydrothermal temperature and time impact the final products?\nResponse:\nWe thank the reviewer for this important question regarding the synthetic mechanism. We have added a\ndetailed mechanistic discussion and Fig. S3e, 3f on the Supplementary Information, as shown below.\n“According to our experience, the effect of hydrothermal time and temperature can be explained as\nfollows: For the impact of hydrothermal time, results in Figure S3 clearly indicate the gradual\ntransformation of intermediate products from mini-crystallites at early stage (< 2 h), crystalline grains in\nthe middle (10 h) and the final branched nanospheres at final stage (after 24 h). Thus, a sufficient reaction\ntime of 24 h is required to achieve high crystallinity and fully developed radial structure. In terms of the\nhydrothermal temperature, a suitable temperature at 120 °C leads to the branched nanospheres in hundreds\nnanometer, a lower temperature of 70 °C results in micrometer-scale spheres, which suggests the driving\nforce for crystallization and growth is insufficient for generating more nucleates compared to that at high\nreaction rate. Besides, a higher temperature of 150 °C would cause the formation of irregular aggregates.\nThus, cooperative assembly kinetics is disrupted at excessively high temperature. A proper hydrothermal\ntemperature with sufficient reaction time is, therefore, necessary to drive the assembly of branched rutile\nnanospheres.”\nFigure S3. SEM images of products obtained after hydrothermal reactions at 70 ºC (e) and 150 ºC (f).\nQuestion 6:\nIt is preferable to provide mass activity data (A g -1at a potential) to reflect Ru utilization efficiency. A\nRu\ncomparison with other Ru-based OER catalysts in terms of noble metal utilization would better position the\nadvantages of this work.\nResponse:\nWe thank the reviewer for this valuable suggestion. In response, we have added the Ru mass activity (A\ng ⁻¹ at representative potentials), specific activity normalized by electrochemically active surface area, and\nRu\nthe corresponding turnover frequency (TOF) values to the Supplementary Information Table S5. These\nmetrics collectively demonstrate the efficient utilization of Ru in the present meso-Ru Ti O catalyst and\n0.2 0.8 2\nfurther highlight its advantage over representative Ru-based OER catalysts reported in the literature.\nRevised part in Main text and Supporting Information:\n“Besides, the mass activity, specific activity, and turnover frequency (TOF) values (Table S5) were further\ncalculated at 1.53 V vs. RHE. The meso-Ru Ti O catalyst exhibits a mass activity of 921.6 A g-1 , a\n0.2 0.8 2 Ru\nspecific activity of 0.091 mA cm-2 , and a TOF of 0.2412 s-1, significantly higher than those of other\nECSA\nsamples, demonstrating the huge potentials in industrial application.”\nTable S5. Comparison of J , C , mass activity, specific activity, and TOF values of different mesoporous\ngeo dl\nsolid oxide solutions.\nSamples J (mA cm-2) C Mass activitya Specific activityb TOFc\ngeo dl\n@1.53 V vs. RHE (mF cm-2) (A g-1 ) (mA cm-2 ) (s-1)\nRu ECSA\nRuO 12.97 4.65 33.5 0.111 0.0088\n2\nmeso-Ru Ti O 0.11 0.10 3.9 0.044 0.0010\n0.05 0.95 2\nmeso-Ru Ti O 4.34 13.60 77.5 0.013 0.0203\n0.10 0.90 2\nmeso-Ru Ti O 105.00 46.00 921.6 0.091 0.2412\n0.20 0.80 2\namass activities were calculated by normalizing the measured current to the mass of loaded Ru on the\nworking electrode based on the total catalyst loading and Ru molar fraction:\nMass activity = I/m\nRu Ru\nwhere I is the measured current (A) at a given potential, and m is the mass of Ru (g) loaded on electrode.\nRu\nbspecific activities were calculated by normalizing the current to the electrochemically active surface area\n(ECSA) estimated from the double-layer capacitance C . A specific capacitance value C of 40 μF cm-2\ndl s\nwas adopted for acidic electrolytes. The ECSA and specific activity were calculated as follows:\nECSA = C /C, Specific activity = I/ECSA\ndl s\ncturnover frequencies (TOF) were calculated based on the four-electron oxygen evolution reaction using\nthe following equation, assuming all Ru atoms are electrochemically accessible:\nTOF = I/4Fn\nRu\nwhere F is the Faraday constant (96485 C mol-1), n is the molar amount of Ru loaded on electrode.\nRu\nQuestion 7:\nIn the Methods, the final calcination time (2 h) of the solid solution at 400 °C is inconsistent with that in\nFigure S5 (3 h).\nResponse:\nWe sincerely apologize for this inconsistency. The correct calcination time for the mesoporous Ru-Ti solid\nsolution is 3 h, as accurately described in the Methods section. The mention of “2 h” in the caption of the\nfinal calcination time was a typographical error.\nRevision: We have corrected the caption of the final calcination time (and other relevant figures in the\nSupplementary Information) to “3 h” to ensure consistency in the whole main text.\nQuestion 8:\nIn the main text, measurements are performed in 0.5 M H SO , but in the Methods the electrolyte is 1.0 M\n2 4\nHClO . This error should be corrected.\nWe thank the reviewer for identifying this error. We confirm that the electrochemical measurements were\nperformed in 0.5 M H SO , and we have corrected the Methods accordingly. The electrolyte description is\n2 4\nnow consistent throughout the main text, Methods, and Supplementary Information.\nChanges made:\nMethods: revised “1.0 M HClO ” to “0.5 M H SO ”.\n4 2 4\nSI (e.g. Table S4 and any figure captions/text mentioning electrolyte): revised to 0.5 M H SO .\n2 4\nMain text: no change needed (already 0.5 M H SO ).\n2 4\nQuestion 9:\nTo aid visualization of the emphasized structural design, a conceptual scheme illustrating the triple-phase\ninterface engineering for high-performance electrocatalysis is suggested.\nResponse:\nWe appreciate the reviewer for this constructive suggestion. To visually elucidate the advantages of our\nstructural design, we have added a new conceptual scheme as Figure 5g in Discussion of the revised\nmanuscript. This diagram explicitly illustrates the structure-activity relationship at the three-phase interface.\nIt highlights how the radially aligned mesoporous architecture facilitates efficient electrolyte penetration\n(liquid phase) and accelerates oxygen bubble detachment (gas phase), while the solid-solution framework\nensures rapid electron transport (solid phase). This visualization clearly demonstrates the mechanism\nbehind the high-performance electrocatalysis.\nFigure 5. (g) Schematic illustration for the material design of high-efficiency mesoporous solid solution\nwith gas-liquid-solid triple-phase interface.\nReviewer 2\nComments:\nIn this manuscript, the authors report a mesoporous Ru-Ti-O rutile-phase solid-solution catalyst (meso-\nRu Ti O ) prepared via a customized micelle self-assembly strategy, targeting durable Ru-based anodes for\nx y 2\nacidic proton exchange membrane water electrolysis (PEMWE). The work proposes that atomic-scale Ru-\nO-Ti motifs modulate the electronic structure to suppress Ru over-oxidation and reduce lattice oxygen\nparticipation, while the ordered mesoporous nanorod-bundle architecture improves the gas-liquid-solid\nthree-phase interface by enhancing wettability and facilitating oxygen bubble detachment. The catalyst is\nshown to deliver strong acidic OER activity and long-term PEMWE operation at industrial current density\nwith relatively low Ru loading. Overall, the study is well organized and the results are promising; the\nfollowing minor points (clarity, labeling, and additional supporting characterization) should be addressed\nto improve readability, accuracy, and completeness.\nResponse:\nWe sincerely thank the reviewer for the positive evaluation and the accurate summary of our work regarding\nthe atomic-scale electronic modulation and mesoscopic interface engineering. We appreciate the\nconstructive suggestions to improve the readability and completeness of our manuscript. We have carefully\naddressed the points in the revised manuscript and Supplementary Information are listed below.\nQuestion 1:\nTable S1 is titled “Comparative crystallographic parameters of rutile-type TiO and RuO ,” yet the second\n2 2\ncolumn is labeled as rutile SnO . This appears to be a typographical error and should be corrected to rutile\n2 2\n(rather than SnO ), since this table is used to support the lattice-matching/solid-solution rationale.\nWe sincerely apologize for this typographical error. We have carefully verified the data and confirm that\nthe lattice parameters reported in the second column (a = 4.4990 Å, c = 3.1070 Å) indeed correspond to\nrutile RuO , which matches well with the standard values (JCPDS No. 40-1290). These values were used\n2\nto support the lattice-matching rationale with TiO (a = 4.5933 Å, c = 2.9592 Å).\nWe have corrected the column header in Table S1 from “rutile SnO ” to “rutile RuO ”.\n2 2\nQuestion 2:\nIn Figure S2, panels (a-c) correspond to the F127/TBOT/RuCl micelle intermediate, whereas panels (d-f)\n3\ncorrespond to the F127/TBOT (Ti-only) micelle intermediate; however, this distinction is not sufficiently\nclear from the current caption. Please explicitly label each set either (1) directly on the TEM panels (e.g.,\n“+RuCl ” vs “Ti-only”) and/or (2) by dividing the caption into two clearly separated parts. This will prevent\n3\nconfusion when comparing micelle sizes (~9.1 nm vs ~8.8 nm) and interpreting the effect of Ru on pre-\nmicellization.\nResponse:\nWe appreciate the reviewer’s suggestion to improve the figure clarity. We have revised Figure S2 and its\ncaption to explicitly distinguish between the two micelle systems:\nDirect Labeling: We have added distinct labels directly onto the TEM panels. Panels (a-c) are labeled as\n“+RuCl ” (Ru-Ti composite micelles), and panels (d-f) are labeled as “Ti-only” (pure Ti micelles).\nFigure S2. Comparison of micelle intermediates. TEM images with different magnifications (a, b) and size\nd size\ndistribution (c) for the F127/TBOT/RuCl micelle primitive (~9.1 nm). TEM images with different\n3\nmagnifications (d, e) and size distribution (f) for the F127/TBOT micelle primitive (~8.8 nm).\nQuestion 3:\nOverstated statement regarding RuO stability - The statement that “commercial RuO fails to maintain\n2 2\nstable operation” is ambiguous and potentially overstated. Please specify what instability is observed (e.g.,\ninability to reach/hold 100 mA cm-2, rapid degradation in chronoamperometry/chronopotentiometry,\nelectrode delamination, fluctuating current/voltage). If stability at 100 mA cm-2 is not directly demonstrated,\nthe wording should be revised to a more precise and evidence-based description.\nResponse:\nWe thank the reviewer for pointing out this ambiguity. We acknowledge that this description is inaccurate.\nOur intended meaning was related to the activity limitation observed in the Linear Sweep Voltammetry\n(LSV) measurements (Figure 4a), rather than a degradation over time. Specifically, within the applied\npotential window (up to 1.7 V vs. RHE), the commercial RuO could not reach the current density of 100\n2\nmA cm-2 due to its inferior catalytic kinetics, whereas the meso-Ru Ti O easily exceeded 100 mA cm-2.\n0.2 0.8 2\nThus, we have revised the sentence in the Results section to avoid confusion with stability testing.\nOriginal: “...while commercial RuO fails to maintain stable operation.”\nPost-stability characterization - Given that long-term durability is a central claim of this work (e.g.,\nextended PEMWE operation at industrial current density), the manuscript would be significantly\nstrengthened by including post-test structural and compositional characterization of the catalyst/electrode.\nFor example, SEM/TEM (morphology and mesostructured integrity), XRD/PXRD (phase stability), and\nsurface/chemical analysis such as XPS or ICP (Ru oxidation state changes and potential Ru dissolution)\nafter durability testing. These data would directly support the assertions of structural robustness and\nsuppressed Ru over-oxidation under realistic operating conditions.\nResponse:\nWe fully agree with the reviewer that post-mortem characterization is indispensable for substantiating the\ndurability claims. To thoroughly verify the structural and chemical stability of the catalyst under realistic\noperating conditions, we have performed a comprehensive set of analyses on the meso-Ru Ti O\n0.2 0.8 2\nelectrode after the 450-hour stability test. The provided data have been added as Figure S21 in the\nrevision. The results are summarized as follows:\nIn the “Electrocatalytic Performance” section, we added:\n“Post-reaction characterizations (Fig. S21) of the meso-Ru Ti O catalyst confirm the preservation of\n0.2 0.8 2\nradial morphology without collapse or aggregation in characteristic rutile phase, further underscoring its\nremarkable structural stability during prolonged acidic electrolysis, which strongly verifies the desired Ti-\nRu incorporation for suppressed Ru overoxidation.”\nFigure S21. Post-reaction structural characterization of the meso-Ru Ti O sample after 450-hours\n0.2 0.8 2\nstability test: (a) SEM image, (b) TEM image, (c) HRTEM image, (d) XRD pattern.\nQuestion 5:\nPunctuation consistency and tone - Several sentences (e.g., around lines 44-45, 55, and 64) use em dashes\nin a way that reads stylistically inconsistent with the rest of the manuscript. Consider replacing them with\ncommas or parentheses for a more formal and uniform tone throughout.\nResponse:\nWe appreciate the reviewer’s meticulous attention to the stylistic details of our manuscript. We have\ncarefully reviewed the text, particularly in the highlighted sections, and have replaced the em dashes (—)\nwith commas to ensure a more formal and uniform academic tone. We have also conducted a thorough\nproofreading of the entire manuscript to eliminate similar stylistic inconsistencies.\nQuestion 6:\nTypographical and language corrections - The manuscript contains multiple typographical errors (e.g.,\n“charateristics,” “ontorl experiments” and so on) and phrasing issues that require careful proofreading. A\nfull language edit is recommended to ensure clarity and professionalism.\nResponse:\nWe thank the reviewer for this suggestion. The manuscript has been thoroughly proofread and carefully\nrevised to correct typographical errors (including those pointed out by the reviewer) and to improve\nwording and phrasing throughout. A full language edit has been performed to enhance clarity, consistency,\nand overall professionalism of the manuscript.\nReviewer 3\nComments:\nZhang et al. proposed a customized self-assembly strategy to synthesize mesoporous Ru-Ti solid-solution\nnanospheres. The nanosphere features multiple cylindrical mesoporous nano-bundles oriented from the core,\nforming an intriguing flower-like architecture. The meso-Ru Ti O shows an overpotential (η) of 257\n0.2 0.8 2\nmV at a current density of 10 mA cm-2. The assembled PEMWE device operation 1 A cm-2 for 450 h. There\nare several aspects that need to be addressed before the manuscript is suitable for publication in Nature\nCommunications.\nResponse: We would like to thank the reviewer for the insightful comments and for recognizing the novelty\nof our work. We have carefully addressed each point, supplemented the experimental data, and revised the\nmanuscript accordingly.\nQuestion 1:\nThe Ru K-edge XANES spectra of the meso-Ru Ti O sample exhibits fine structural features similar to\n0.2 0.8 2\nthose of the RuO reference and distinctly different from those of Ru foil, indicating the absence of metallic\n2\nspecies in meso-Ru Ti O , it is recommended to also provide the radial structure function of RuO and\n0.2 0.8 2 2\nits corresponding fitting curves for comparison.\nResponse:\nWe appreciate the reviewer’s suggestion to further verify the phase purity of our samples. Regarding the\nstructural characterization, we have primarily employed Rietveld refinement of the XRD patterns to exclude\nthe presence of RuO species and confirm the formation of a solid solution, rather than relying solely on\n2\nradial structure function (RSF) fitting. This choice is based on the consideration that while RSF is a\npowerful tool for probing local coordination environments (short-range order), Rietveld refinement\nprovides a macroscopic and statistically significant evaluation of the entire crystalline lattice (long-range\norder). As shown in the revised Figure S11d, the diffraction pattern of meso-Ru Ti O can be perfectly\n0.2 0.8 2\nfitted using a single rutile model with a very low R factor (6.3%), providing more direct and definitive\nwp\nevidence of Ru4+ substitution into the Ti4+ sites. Therefore, we believe that the Rietveld refinement results\noffer more holistic and conclusive proof of the atomic-level integration of Ru into the TiO framework,\n2\nthereby unambiguously excluding the presence of secondary RuO phases.\n0.2 0.8 2\ncalcination in air at 400 ºC for 3 h.\nRevised part in Supporting information Fig S11:\n“To rigorously verify the phase homogeneity and exclude the presence of segregated RuO species, Rietveld\n2\nrefinement was performed on the PXRD patterns (Fig. S11d). Unlike local structure probes, this technique\nprovides a macroscopic and statistically significant assessment of the long-range crystalline order. The\ndiffraction pattern of meso-Ru Ti O can be perfectly fitted to a single tetragonal rutile model with a low\n0.2 0.8 2\nweighted profile R-factor (R ) of 6.3%. This excellent fitting quality offers definitive evidence for the\nwp\nsubstitutional incorporation of Ru4+ into the Ti4+ lattice sites, unambiguously confirming the formation of\na uniform solid solution without detectable secondary phases.”\nQuestion 2:\nIn Figure 3b, the Ru K-edge WT-EXAFS spectra of the Meso-Ru Ti O sample shows that the\n0.2 0.8 2\ncharacteristic peak assigned to the Ru-O scattering path in R space range of 1-2 Å corresponds to k values\nhigher than 10 and is close to those of the Ru-Ru (Ti) peak at 3-4 Å, which is confusing. Generally, the\ncoordination of light elements such as O does not exhibit k values exceeding 10 in WT-EXAFS spectra.\nResponse:\nWe apologize for the confusion caused by the initial WT-EXAFS plots. The reviewer is correct that light\nelements like Oxygen typically exhibit maximum intensity at a lower k-values (usually k< 6 Å-1) in the\nWavelet Transform. We have re-processed the WT-EXAFS data using optimized wavelet parameters. The\nrevised Figure 3b now clearly shows the Ru-O scattering path intensity centered at a lower k-range,\ndistinctly separated from the heavy-atom (Ru/Ti) scattering at higher k-values. We have updated the figures\nand the corresponding discussion in the manuscript.\nRevised part in manuscript:\nFigure 3b. WT-EXAFS curves of the meso-Ru Ti O .\n0.2 0.8 2\nFigure S14b. WT-EXAFS curve of the commercial RuO .\nThe method used to determine the oxidation state of Ru in Figures 5a and S14a should be described in\ngreater detail in the Experimental section.\nResponse:\nThank you for your valuable feedback on our manuscript. We have expanded the Methods to provide a\ndetailed protocol for oxidation state determination.\nRevised part in Material Characterizations:\n“The Ru oxidation state was determined by the linear relationship between the Ru K-edge absorption energy\n(E , defined at the half-height of the normalized edge jump) and the formal valence of standard references\n0\n(Ru foil for Ru0, RuO for Ru4+).”\nThe experimental conditions of XAS measurements, as well as the data processing and analysis methods,\nshould be systematically and clearly described in the Experimental section.\nResponse:\nWe thank the reviewer for this suggestion. Following your suggestion, we have significantly improved the\nexperimental section. We have added details regarding the synchrotron beamline parameters, measurement\nmode, and specific software environments used.\nRevised part in Material Characterizations:\n“In situ Ru K-edge XAFS spectra were collected at the BL17B beamline of the Shanghai Synchrotron\nRadiation Facility (SSRF), operated in fluorescence mode at custom-designed electrolytic cell. The SSRF\nstorage ring was maintained at 2.5 GeV with a maximum electron current of 250 mA. The incident beam\nwas calibrated using a standard Ru foil. The working electrode by loading the catalyst onto carbon paper\nwith a polyimide film backing was integrated into the cell. The acquired EXAFS data were processed using\nthe ATHENA and ARTEMIS modules of Demeter software package. Raw absorption spectra were subjected\nto post-edge background subtraction and normalization relative to the edge-jump step. The resulting χ(k)\nfunctions were Fourier-transformed into R-space using a Hanning window (dk = 1.0 Å-1) to isolate\ncontributions. Quantitative structural parameters were obtained through least-squares curve fitting in the\nARTEMIS module based on the following equation:\nNS2F(k) −2R\nχ(k)=∑ j 0 j ⋅exp⁡[−2k2σ2]⋅exp⁡[ j ]⋅sin⁡[2kR +ϕ(k)]\nkR2 ⁡j λ(k) j j\nj\nj\nThe theoretical scattering amplitudes (𝐹(𝑘)), phase shifts (𝜙 (𝑘)), and photoelectron mean free paths (𝜆)\n𝑗 𝑗\nwere calculated ab initio using the FEFF9 code. Structural refinement was conducted in R-space,\nincorporating phase corrections for the first coordination shell to align the R values with physical\ninteratomic distances. The amplitude reduction factor (𝑆2) and coordination numbers (N) were fixed to\n0\nvalues determined from reference models, while the interatomic distances (R), Debye-Waller factors (𝜎2),\nand edge-energy shifts (ΔE ) were allowed to vary freely to achieve the best fit. The Ru oxidation state was\n0\ndetermined by the linear relationship between the Ru K-edge absorption energy (E , defined at the half-\n0\nheight of the normalized edge jump) and the formal valence of standard references (Ru foil for Ru0, RuO\n2\nfor Ru4+).\nQuestion 5:\nIn Table S3, two Ru–O scattering paths were used in the fitting of the RuO reference, but only a single\n2\ndisorder parameter (σ²) was reported. This point requires further clarification. In addition, it is\nrecommended to provide the specific physical meaning and uncertainties of each fitting parameter.\nResponse:\nThank you for your insightful comment. In the EXAFS fitting of the RuO reference, two distinct Ru–O\n2\nscattering paths (corresponding to the short and long Ru–O bonds in rutile RuO ) were indeed included. To\n2\nreduce the number of free parameters and avoid overfitting in the reference fitting, the Debye–Waller factors\n(σ2) of these two Ru–O paths were constrained to be identical. This treatment is commonly applied to well-\ndefined crystalline references with minimal structural disorder. We will explicitly clarify this constraint in\nthe revised manuscript and Supplementary Table S3 to avoid misunderstanding.\nRevised part in Supporting Information:\nTable S3. EXAFS fitting parameters at the Ru K-edge of various samples (S 2=0.75±0.07).\nRu foil Ru-Ru 12 2.68±0.01 3.70±0.35 -5.3±0.8 0.1%\nRu-O1 1.6±0.3 1.90±0.01 1.43±0.40\nmeso-\nRu-O2 3.4±0.5 1.95±0.01 1.43±0.40 -7.0±1.2 1.9%\nRu Ti O\n0.2 0.8 2 Ru-Ru/Ti 7.2±1.2 3.69±0.02 2.42±0.85\n2.0 1.94±0.01\nRuO Ru-O 0.82±0.25f -0.5±0.6 0.9%\n2 4.0 1.99±0.01\naC. N.: coordination numbers. bR: bond distance. cσ2: Debye-Waller factors. dΔE : the inner potential\n0\ncorrection. eR factor: goodness of fit. fσ2values for Ru–O (short) and Ru–O (long) were constrained to be\nidentical in the fitting.\nQuestion 6:\nIt is suggested to supplement the in-situ EXAFS spectra in R space and to analyze the evolution of the\ncatalyst’s local structure during the OER process based on these data.\nResponse:\nWe thank the reviewer for this insightful suggestion. We have now included the R-space in-situ EXAFS\nspectra at different potentials in the revised manuscript (Figure S22 and Table S6) and provided a detailed\nanalysis of the catalyst’s local structural evolution during the OER process.\nRevised part in main text (In-situe measurements):\n“Besides, the Ru oxidation state first decreases and then increases as the applied potential increases,\naccompanied by a corresponding evolution in the local Ru-O coordination environment (Fig. S22, Table\nS6). At low potential (~1.2 V), partial reduction of Ru leads to slightly elongated Ru-O bonds and a reduced\neffective coordination number, indicating a more relaxed local structure. Upon further increasing the\npotential, the Ru-O coordination number gradually increases toward octahedral coordination while the Ru-\nO bond lengths shorten, consistent with the formation of higher-valence Ru species and strengthened Ru-\nO bonding.”\nFigure S22. (a) Changes of coordination number for the Ru–O coordination shells. Fourier-transformed\nmagnitudes for meso-Ru Ti O under at representative potentials. (b) Under open-circuit voltage. (c) At\n0.2 0.8 2\n1.23 V vs. RHE. (d) At 1.43 V vs. RHE. (e) At 1.63 V vs. RHE. (f) At 1.83 V vs. RHE.\nTable S6. EXAFS fitting parameters for the Ru K-edge of Meso-Ru Ti O (S 2=0.75±0.07).\n0.2 0.8 2 0\nSample path C.N. a R(Å)b σ2(10-3Å2)c E(eV)d R factore\nRu-O1 1.5±0.3 1.90±0.02 4.61±0.24\nOCP -5.3±0.7 1.8%\nRu-O2 3.3±0.4 1.95±0.05 2.34±0.10\nRu-O1 1.4±0.3 1.92±0.03 3.57±0.10\n1.23 V 3.5±1.8 0.9%\nRu-O2 3.1±0.6 1.98±0.02 3.04±0.05\nRu-O1 1.5±0.3 1.91±0.04 4.76±0.09\n1.43 V 2.7±0.5 1.8%\nRu-O2 3.3±0.4 1.97±0.04 1.97±0.02\nRu-O1 1.5±0.3 1.89±0.01 4.61±0.13\n1.63 V -1.5±1.2 2.0%\nRu-O2 3.6±0.5 1.94±0.03 1.38±0.09\nRu-O1 1.8±0.2 1.88±0.03 3.82±0.07\n1.83 V -2.4±1.1 1.1%\nRu-O2 3.9±0.4 1.93±0.02 5.32±0.11\naC. N.: coordination numbers. bR: bond distance. cσ2: Debye-Waller factors. dΔE : the inner potential\n0\ncorrection. eR factor: goodness of fit.\nQuestion 7:\nIn Figures 3b and S14b, it is recommended that the k-axis label in the WT-EXAFS plots be consistently\nformatted using lowercase letters.\nResponse:\nWe appreciate the reviewer’s careful observation. The k-axis labels in Figures 3b, S14b have been updated\nto lowercase “k” for consistency.\nFigure 3b. WT-EXAFS curves of the meso-Ru Ti O .\n0.2 0.8 2\nFigure S14b. WT-EXAFS curve of the commercial RuO .\nThe calculation of mass activity, specific activity, and TOF values should be added.\nResponse:\nWe thank the reviewer for this constructive suggestion. In response, the mass activity, specific activity, and\nturnover frequency (TOF) values were calculated and added for all catalysts at 1.53 V vs. RHE. The\ncorresponding results are summarized in Table S5. As shown in Table S5, meso-Ru Ti O exhibits\n0.2 0.8 2\nsignificantly enhanced mass activity, specific activity and TOF, highlighting the improved Ru utilization\nefficiency and intrinsic activity enabled by the mesoporous solid-solution structure.\nRevised part in Main text and Supporting Information:\n“Besides, the mass activity, specific activity, and turnover frequency (TOF) values (Table S5) were further\ncalculated at 1.53 V vs. RHE. The meso-Ru Ti O catalyst exhibits a mass activity of 921.6 A g-1 , a\n0.2 0.8 2 Ru\nspecific activity of 0.091 mA cm-2 , and a TOF of 0.2412 s-1, significantly higher than those of other\nECSA\nsamples, demonstrating the huge potentials in industrial application.”\nTable S5. Comparison of J , C , mass activity, specific activity, and TOF values of different mesoporous\ngeo dl\nsolid oxide solutions.\nSamples J (mA cm-2) C Mass activitya Specific activityb TOFc\ngeo dl\n@1.53 V vs. RHE (mF cm-2) (A g-1 ) (mA cm-2 ) (s-1)\nRu ECSA\nRuO 12.97 4.65 33.5 0.111 0.0088\n2\nmeso-Ru Ti O 0.11 0.10 3.9 0.044 0.0010\n0.05 0.95 2\nmeso-Ru Ti O 4.34 13.60 77.5 0.013 0.0203\n0.10 0.90 2\nmeso-Ru Ti O 105.00 46.00 921.6 0.091 0.2412\n0.20 0.80 2\namass activities were calculated by normalizing the measured current to the mass of loaded Ru on the\nworking electrode based on the total catalyst loading and Ru molar fraction:\nMass activity = I/m\nRu Ru\nwhere I is the measured current (A) at a given potential, and m is the mass of Ru (g) loaded on electrode.\nRu\nbspecific activities were calculated by normalizing the current to the electrochemically active surface area\n(ECSA) estimated from the double-layer capacitance C . A specific capacitance value C of 40 μF cm-2\ndl s\nwas adopted for acidic electrolytes. The ECSA and specific activity were calculated as follows:\nECSA = C /C, Specific activity = I/ECSA\ndl s\ncturnover frequencies (TOF) were calculated based on the four-electron oxygen evolution reaction using\nthe following equation, assuming all Ru atoms are electrochemically accessible:\nTOF = I/4Fn\nRu\nwhere F is the Faraday constant (96485 C mol-1), n is the molar amount of Ru loaded on electrode.\nRu\nQuestion 9:\nIn spite of this success of Ru, Ti incorporation, I am wondering the possibility in fabricating other\nmesoporous oxide solid solutions in other elements.\nResponse:\nWe appreciate the reviewer’s insightful comment. Beyond the successful incorporation of Ru and Ti, we\nagree that extending this strategy to other elemental combinations is an important and interesting direction,\nwhich we are very interested in. In fact, we are currently investigating the fabrication of mesoporous oxide\nsolid solutions based on the other bi-metal system, such as Ir-Ti, Sn-Ti, In-Ti oxides. Preliminary results\nindicate that Ir-Ti mesoporous oxide solid solutions are feasible, and a systematic study is now underway.\nBesides, the successful construction of mesoporous oxide solid solution requires a compatible lattice match\nof two metals according to our experiences. For example, a few oxides in rutile phase like RuO , IrO , or\n2 2\nSnO are available for ideal incorporation. Since our experiments on the extension of other solid solutions\n2\nare primary, we plan to report these results on our follow-up work in future."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The revised manuscript has addressed the reviewers' comments. The latest relevant studies should be compared and discussed in the main manuscript, such as Nature Catalysis, 2026,doi.org/10.1038/s41929-025-01456-w, Nature Commun. 2025, 16, 6217", "reviewer_2": "The authors addressed all the issues raised properly in the revision. Therefore, I recommend the publication of the manuscript in Nature Communications in the current form.", "reviewer_3": "The authors have fully addressed the comments and this manuscript is recommended for publication now."}, "author_response": ""}]

Decision letter