Synthesis and characterization of the nanocatalysts
TS-Sr0.1Ta0.1Ru0.8O2−x nanocatalysts were formed in the molten salts (NaCl + NaNO3) at 400 ℃ after preparing the precursor with 1/10 Sr, 1/10 Ta, and 8/10 molar ratio column feed. Due to the repulsion between cationic Na and Ru spreading the nucleation and crystallographic points, such a hot liquid bath allowed the uniform distribution of small-sized nanoparticles with higher crystal phases while eliminating the dopants-induced mutation of RuO2 − x morphology22,30. Thereafter, the liquid mixture was immediately quenched in excessive water (about 20 ℃), and crystallographic growth was abruptly stopped. Alkali metal salts therein were instantly and completely dissolved in cool water, which well-preserved the expanded lattice parameters and relaxed Ru-O interaction of high-temperature status, finally the TS was generated in RuO2 − x catalysts31,32,33. With the sole incorporation of Sr and Ta during a similar process, TS-Sr0.1Ru0.9O2−x and TS-Ta0.1Ru0.9O2−x were obtained respectively (Fig.S1). In contrast, the RuO2 − x and Sr0.1Ta0.1Ru0.8O2−x catalysts without TS were prepared via traditional natural cooling and followed a sonication-assisted water washing step. The intact nanoparticulate crystal with uniform 10–20 nm size and dominant (110) facet orientation of all RuO2 − x catalysts is shown in Transmission electron microscopy (TEM) and High-resolution TEM (HRTEM) images (Fig. 1a, Figs.S2-S6a,b).
High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and corresponding EDS elemental mapping images reflect the atomic homogenous distribution of Sr and Ta elements among bulk phase (Fig. 1b, Figs. S2-S6c). As observed from the HRTEM images, the average lattice spacing along (110) for RuO2 − x is 0.314 nm (Fig. 1c), which is rather close to the 0.315 nm of the theoretical value for commercial RuO2 (PDF 96-210-1931). The value is increased to 0.328 nm after TS modifications in TS-RuO2 − x (Fig. S3d) and further 0.33 nm in TS-Sr0.1Ta0.1Ru0.8O2−x (Fig. 1e). Meanwhile, the 0.333 nm and 0.328 nm are also displayed in the TS-Sr0.1Ru0.9O2−x (Fig. S4d) and TS-Ta0.1Ru0.9O2−x (Fig. S5d) nanocatalysts, respectively, in contrast to 0.318 nm of Sr0.1Ta0.1Ru0.8O2−x case (Fig. S6d). Serving our RuO2 − x as the benchmark quantifies the axial TS about 4.5% (TS-RuO2 − x), 5.1% (TS-Sr0.1Ta0.1Ru0.8O2−x), 5.1% (TS-Sr0.1Ru0.9O2−x), 4.5% (TS-Ta0.1Ru0.9O2−x), and 1.3% (Sr0.1Ta0.1Ru0.8O2−x) along (110) facets (Fig. 1g). The spatial TS distribution components (εxx perpendicular to (110) and εyy in the (110) plane associated with contraction/expansion of the respective lattice vectors and εxy shear strain) are mapped by the geometric-phase analysis (GPA)20, 22, 33. It displays that the values of εxx, εxy, and εyy are nearly zero on the surface of RuO2 − x (Fig. 1d), whereas those distinctly increase on tensile strained samples, especially the respective εyy maximum values of ~ 4.5% and ~ 5% are reached on the TS-RuO2 − x (Fig. S3e) and TS-Sr0.1Ta0.1Ru0.8O2−x (Fig. 1f), providing the visual evidence of the TS presence in the (110) plane. Lattice expansion also can be indicated by the (110) diffraction peak downshifting compared to those of RuO2 − x and Sr0.1Ta0.1Ru0.8O2−x in the X-ray diffraction (XRD) pattern (Fig. 1h, S7).
Electronic structure characterizations
To explore the effects of TS and Sr-Ta dual dopants on the electronic structures and chemical states, X-ray photoelectron spectroscopy (XPS) was first carried out and carefully investigated. As shown in Fig. 2a, the M-OL (lattice oxygen) peak in O1s XPS slightly upshifts after TS modifications, but the extrinsic Sr-Ta atoms co-insertion brings significant alterations to the oxygen chemical state in terms of a rather noticeable shifting of the M-OL peak. For TS-Sr0.1Ru0.9O2−x and TS-Ta0.1Ru0.9O2−x catalysts (Fig. S8a), the sole Sr and Ta atoms substitution gives rise to an opposite change of M-OL peak position due to the large difference between the valent electrons configurations of Sr2+ and Ta5+. Thus, the presence of codoped Sr-Ta may optimize the electronic structure of O sites to some extent, which is favorable for the deprotonation process of *OH intermediate during the OER34,35,36. A similar conclusion also can be obtained from the Ru 3d XPS spectra, in which the electronic and valent modulations of Ru sites primarily are ascribed from the simultaneous Sr and Ta doping rather than the TS presence (Figs. 2b, S8). Convinced that the foreign atoms primarily result in moderate alterations in chemical state and electronic density on Ru sites, ensuring the optimal oxophilicity towards the species during OER36,37. The ratios of (oxygen vacancy) VO/ M-OL and Ru> 4+/Ru4+ are summarized in Figs. 2c and 3d, respectively, indicating that the TS gradually alleviates the formation rate of VO and unstable Ru composition, which solids the integrity of crystal RuO2 − x. While the independently cationic Sr2+ and Ta5+ affect those components' ratios dramatically, especially showing the opposite tendency in Sr2+ and Ta5+ cases. The balance between (VO)/ M-OL and Ru> 4+/Ru4+ occurs as a result of the synergistic interaction between Ru and codoped Sr/Ta atoms, rendering the preferable catalytic ability on both intrinsic O and Ru sites in TS-Sr0.1Ta0.1Ru0.8O2−x38,39.
In addition, the change of Ru valence states and Ru-O bond lengths are revealed by the Ru K-edge X-ray absorption near-edge spectroscopy (XANES) (Figs. 2e, S9a). The absorption edge of our RuO2 − x shifts to a higher energy position compared with those of references, including RuO2 (r-RuO2), RuCl3 (r-RuCl3), and Ru metal (r-Ru), indicating that the average oxidation state of Ru in RuO2 − x is higher than 4+, which is also consistent with the XPS conclusion about the abundant VO and Ru> 4+ presence. Introducing the lattice TS decreases the valence state reflected by the absorption edge negative shifting of TS-RuO2 − x. Like the analysis from the XPS, the moderate valence state of Ru can be achieved once co-modulated by the Sr and Ta atoms, while its Ru-K absorption edge is positioned between that of sole Sr and Ta cases (Figs. S9b,c).
Those suggest that more favorable electronic redistributions among active sites are facilely triggered by synergetic modulations of atomic Sr2+ and Ta5+ (details in supporting information). The extended X-ray absorption fine structure (EXAFS) spectra show a slight stretch of Ru-O bond in all TS samples (Figs. 2f, S9d). Interestingly, a strained effect also stretches the Ru-Ru and Ru-O-M lengths longer compared with those of RuO2 − x due to TS-induced variation of spatial lattice parameters, possibly corresponding to the strain existence along εxx, and εyy directions. Thus, it is predictable that the elongated Ru-O bonding and synergistic electronic interactions among Sr-Ru-Ta units can improve the stability-activity of RuO2 − x for acidic OER, respectively (Fig S10).
Electrocatalytic activity and stability in acidic electrolyte
To unravel how the dominant contribution of TS and electronic modulations works in balancing the stability-activity trade-off, the electrocatalytic OER activity was measured with a three-electrode system in 0.5 M H2SO4, wherein our RuO2 − x catalysts, Pt wire, and Hg/Hg2SO4 were working, counter, and reference electrode respectively (Fig. S10). The linear sweep voltammetry (LSV) curves of all prepared nanocatalysts are shown in Figs. 3a,b. It can be noticed that TS and co-doped simultaneously improve the OER activity to various degrees. The overpotentials at 10 mA cm− 2 achieved on RuO2 − x and TS-RuO2 − x are about 243 mV and 210 mV, close to the most reported values (Supplementary Table S1). Whereas only 183 mV and 166 mV overpotentials are required on Sr0.1Ta0.1Ru0.8O2−x and TS-Sr0.1Ta0.1Ru0.8O2−x electrodes, respectively, outperforming the many latest excellent catalysts. Meanwhile, the Tafel slope values are significantly reduced from 154.5 mV dec− 1 of RuO2 − x to 67.9 mV dec− 1 of Sr0.1Ta0.1Ru0.8O2−x and 56.6 mV dec− 1 of TS-Sr0.1Ta0.1Ru0.8O2−x (Fig. 3c, Figs. S12a,b). Electronic modulation promotion is maximized under the co-presence of Sr and Ta. No distinct changes are observed from LSV curves obtained on the TS-RuO2 − x, TS-Sr0.1Ru0.9O2−x, and TS-Ta0.1Ru0.9O2−x electrodes. Moreover, the intrinsic activity of Ru sites was examined by the electrochemically active surface area (ECSA) normalized
LSV curves (Fig. S13)40,41. Figure 3d presents the approximately close ECSA-current densities on Sr0.1Ta0.1Ru0.8O2−x and TS-Sr0.1Ta0.1Ru0.8O2−x with the increase of potentials, and inconspicuous disparity can be noticed between RuO2 − x and TS-RuO2 − x. And the emergence of the Sr-Ru-Ta unit enables the optimized intrinsic activity of Ru sites, resulting in the superior ECSA-normalized LSV curves to those of TS-Sr0.1Ru0.9O2−x, and TS-Ta0.1Ru0.9O2−x (Fig. 3e). Similar phenomena are also reflected by the Ru mass-normalized LSV curves (Figs. S12c,d). These clues demonstrate the fact that the synergy of Sr and Ta electronic modulators primarily contributes to the enhanced activity of Ru sites than TS modification.
Keeping strong stability in acidic conditions while achieving outstanding OER activity always remains a great challenge for RuO2 catalysts. Thus the chronopotentiometry curves (V-T) were first obtained at 10 mA cm− 2 (Fig. 3f). Noticed that without lattice expansion, RuO2 − x and Sr0.1Ta0.1Ru0.8O2−x electrodes display acceptable resistance to corrosion within the front 300 h, respectively possessing the decay rate of 0.25 mV/h and 0.39 mV/h. Impressively, a dozen times smaller decay rates are present on the TS-RuO2 − x (0.03 mV/h) and TS-Sr0.1Ta0.1Ru0.8O2−x (0.02 mV/h) even during the continuous operation up to 1000 h. These illustrate that due to the decreased Ru-O covalency, the TS majorly functions on efficiently strengthening the RuO2 − x nanocrystal integrity and avoiding the LOM pathway19,42. The stability superiority of TS-Sr0.1Ta0.1Ru0.8O2−x also can be evidenced by the more harsh electrocatalysis (200 mA cm− 2 within 200 h) (Fig. 3g) and careful comparisons of overpotential at 10 mA cm− 2-measured stability period with abundant references catalysts (Fig. 3h, Supplementary Table S1). The homemade water electrolyzer was assembled by TS-Sr0.1Ta0.1Ru0.8O2−x (cathode) and commercial Pt/C (anode), delivering the 10 mA cm− 2 and 50 mAcm− 2 only at 1.45 V and 1.53 V in 0.5 M H2SO4, respectively, much lower than 1.55 V and 1.78 V of commercial RuO2 (c-RuO2) and Pt/C benchmark (Fig. 3i). Thanks to the merits of TS-Sr0.1Ta0.1Ru0.8O2−x, our water electrolyzer can sustainably work over 500 h and 100 h at 10 mA cm− 2 and 50 mAcm− 2 without apparent degradation, presenting the promising potential for large-power applications (Figs. 3j and k).
Origin of improved stability
Understanding which reaction pathway dominates is the premise before clarifying the origin of enhanced performance. Since the non-concerted proton-electron transfer step causes the typical pH-dependent OER behavior for the LOM mechanism, pH-dependent activity was measured on all catalysts (Fig. S14)43. As shown in Figs. 4a and b, pH-dependent phenomena of RuO2 − x are reversed after TS introduction due to the slopes of pH-overpotentials (5, 10, 20 mA cm− 2) tending to be parallel, and the same feature also can be found in other strained catalysts (Fig. S14). This confirms that the lattice oxygen release was significantly alleviated to enable higher stability due to the TS presence. Isotopic oxygen (18O) labeling experiments were further conducted in well-closed two-separated chamber cells to certify the suppressed LOM process induced by TS (Fig. S15)42,44. The gas products at the constant current density and period were collected to analyze in gas chromatography-mass spectrometry (GC-MS) (Fig. 4c, Figs. S16, S17). Noticed that the O2 (18O16O) relative intensity on c-RuO2, RuO2 − x, and Sr0.1Ta0.1Ru0.8O2−x surface achieve 3.9%, 2.9%, and 1.7%, while the TS-RuO2 − x and TS-Sr0.1Ta0.1Ru0.8O2−x is reduced to 0.6% and 0.4%, respectively (Fig. 4d). These clues unambiguously corroborate the less participation of lattice oxygen on strained samples, enabling these prefer to the AEM mechanism. Thereafter, the in-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) was conducted to validate the exclusive AEM process (Fig. S18). As shown in Fig. 4e, an absorption band (~ 1142 cm− 1) corresponding to the key water oxidation intermediate *OOH (AEM) can be observed and its intensity starts to gradually increase as the the potential is higher than 1.3 V45. An isotopic shift when the solvent is switched from H2O to D2O indicates that the vibrational mode of the species involves hydrogen46, excluding the direct O-O intramolecular coupling pathway and further verifying the AEM process (Fig. 4f)47. It is noteworthy that TS-Sr0.1Ta0.1Ru0.8O2−x displays rather higher vibration bands (*OOH and *OOD) intensity compared to those of RuO2 − x and TS-RuO2 − x (Figs. 4g, S18b), indicating that the Sr and Ta incorporation greatly promotes the rate-determined reaction step of acidic OER.
Due to the AEM predominance, the Ru dissolution rate from TS-RuO2 − x and TS-Sr0.1Ta0.1Ru0.8O2−x catalysts can be efficiently alleviated, as reflected by the ICP-OES measurements results (Figs. 5a, S19a). Lattice expansion preserves the Ru atoms from corrosion, and dissolved Ru ions tend to a constant value after 50 min, in sharp contrast to the samples without TS. This promotion also can be demonstrated by that an order of magnitude higher S-number is achieved on TS-Sr0.1Ta0.1Ru0.8O2−x (Figs. 5b, S19b-S19e)9, 48. Thus, acceptable alterations of chemical state and constituents emerge on the strained catalysts' surface even after 3000 CV cycles (Fig. S20, Supplementary Table S2). A rather higher portion of OL remains in O 1s XPS of post-TS-RuO2 − x and post-TS-Sr0.1Ta0.1Ru0.8O2−x catalysts, whereas almost all OL depletion in cases of post-RuO2 − x and post-Sr0.1Ta0.1Ru0.8O2−x (Figs. 5c, S20a). Likewise, in Ru 3d XPS spectra of post-reaction samples, a slight increase of Ru> 4+/Ru4+ ratio is observed after the reaction due to the TS presence (Figs. 5d, S20b), corresponding to the higher valence state observed in XANES. Those are in stark contrast to the 100% Ru> 4+ species that remained on the surface of post-RuO2 − x and post-Sr0.1Ta0.1Ru0.8O2−x. In addition, the rigidity of lattice TS itself is also praiseworthy, reflected by the unchanged bonding of Ru-O and Ru-O-M in EXAFS (Fig. S21), as well as the stable d-spacing distance (110) and positive strain component values of GPA (Figs. 5e, 5f, and Fig. S22).
Origin of enhanced activity
Once the AEM mechanism is confirmed, the OER activity is mainly controlled by the key species transfer behavior. In-situ electrochemical impedance spectroscopy (in-situ EIS) was measured from programmed 1.0 V to 1.5 V (vs. RHE) within the 102-105 Hz frequency range in 0.5 M H2SO4 (Figs. 6a, S23, and S24). Corresponding equivalent circuit models show three tandem components, wherein the Rs represent solution resistance, and the other two tandem components respectively reflect the charge-transfer kinetics in electrical double-layer (Q1/R1) and intermediates transfer among the active site (Q2/R2) (Fig. 6b)49,50. Then related species transfer numbers and related resistance were carefully simulated (Supplementary Tables S3-S9, Fig. S25). It is noteworthy that the lowest Q1 values of RuO2 − x are increased after Sr and Ta co-presence and the R1 on these two samples tends to approximate as the potential reaches about 1.35 V (close to OER onset potentials) (Fig. 6c). Meanwhile, the TS-Sr0.1Ta0.1Ru0.8O2−x is featured by the much higher Q1 and lower R1, manifesting the enlarged active surface area and facilitated charge transfer mainly induced by the electronic modulators. Thus, the almost same Q2 and relatively similar R2 values are present on the co-doped catalyst surface (Fig. 6d). The Bode plot demonstrates that the 1.4 V onset potential of RuO2 − x (Fig. S23d) is reduced to 1.35 V of TS-Sr0.1Ta0.1Ru0.8O2−x (Figs. 6e, S24h). Unlike the electrode evolution process undergone by c-RuO2 (Fig. S23b), associated peaks are absent in high-frequency regions in our catalysts, providing veritable catalytic phases51. Also noticed that only a negligible difference in phase angle peaks at 1.5 V is found after TS modification, whereas Sr and Ta introduction results in the rather lower phase angle peaks, especially the 10.051 θ and 9.557 θ observed on Sr0.1Ta0.1Ru0.8O2−x and TS-Sr0.1Ta0.1Ru0.8O2−x (Fig. 6e). Those indicate that TS weakly contributes to the facilitated migration kinetics of active species on Ru and O sites, but the Sr-Ru-Ta unit significantly does.
Theoretical investigation
After building the configurations including RuO2, TS-RuO2 (5% TS), Sr0.1Ta0.1Ru0.8O2, and TS-Sr0.1Ta0.1Ru0.8O2 (5% TS) to represent corresponding experiments models (Figs. S26a-d), density functional theory (DFT) calculations were carefully conducted. It reveals that the energy barrier of Vo formation and OER pathway can be primarily tuned fluctuation of εp and εd (d band center). This synergy prefers to optimize the absorption ability of active sites since too weak or too strong affinity for oxygen and proton intermediates can be avoided. Like conclusions from experiments, the electronic structural alterations are induced by the co-doping effect more efficiently23. Then both AEM and LOM free energy of OER coordinates was calculated (Figs. S27-S35). The theoretical overpotentials (η) at U = 1.23 V of LOM and AEM on RuO2 − x surface are respective 1.95 eV and 0.8 eV (Fig. 6g). As high as η = 2.19 eV is required for the LOM pathway on the TS-modified model, certifying that the TS mainly constrains the LOM participation during OER (Fig. S27a). Extrinsic Sr and Ta modulators accelerate AEM and LOM kinetics simultaneously due to the respectively smaller η = 1.61 eV and η = 0.56 eV, which may result in the instability of Sr0.1Ta0.1Ru0.8O2−x (Fig. S27b), whereas LOM of η = 2.08 eV and AEM of η = 0.52 eV are concurrent (Fig. 6h). It is noteworthy that only suitable movement of εp and εd finally minimizes the AEM energy barrier through making the simultaneous improvement of deprotonation on O and key intermediates sorption on Ru sites (Fig. 6i)52. These results jointly support the fact that a balanced stability-activity trade-off is experimentally achieved on our TS-Sr0.1Ta0.1Ru0.8O2−x.