Atomically Ru‐Doped Co₃O₄ With Asymmetric Ru–O–Co Sites for High‐Performance Chlorine Evolution

ABSTRACT

Chlorine production remains foundational to chemical manufacturing, yet its electrochemical generation is hindered by sluggish kinetics and unfavorable intermediate adsorption−desorption dynamics. Here, we fabricate atomically Ru‐doped Co₃O₄ featuring asymmetric Ru–O–Co bridge sites, in which isolated Ru atoms are incorporated into the Co₃O₄ spinel lattice to regulate the local coordination environment and electronic structure. In situ spectroscopic characterizations combined with first‐principles calculations reveal that Ru substitution disrupts the rigid symmetric Co–O–Co configuration and weakens the excessively strong Co–Cl interaction. The relatively weak Ru 4p–O 2p coupling endows Ru sites with abundant accessible vacant orbitals, favoring the formation of labile Ru–Cl species and enabling rapid and reversible Cl adsorption–desorption cycling while suppressing stable Co–Cl intermediates. Such asymmetric electronic modulation accelerates chlorine evolution kinetics and promotes a Ru‐dominant Volmer−Heyrovský pathway with reduced energy barriers. Benefiting from the optimized active‐site configuration, RuO_x‐Co₃O₄ with an ultralow Ru loading of only 1.64 wt.% delivers substantially higher CER activity than commercial dimensionally stable anodes (DSA), achieving a mass activity of 3049 A g_Ru ⁻¹ and a Cl₂ selectivity of 98.9% at 1.5 V. This work highlights asymmetric site engineering as an effective strategy for developing highly efficient and selective CER electrocatalysts.

Atomic Ru incorporation into Co₃O₄ breaks the intrinsic Co–O orbital confinement by constructing asymmetric Ru–O–Co bimetallic sites. Substitution of Ru for Co breaks the symmetric, strongly bound Cl adsorption at Co–O–Co sites, while simultaneously introducing weaker Ru 4p–O 2p coordination with abundant vacant orbitals. These unoccupied states enable the formation of labile Ru─Cl bonds with rapid adsorption‐desorption reversibility, thereby reestablishing highly active centers for Cl₂ evolution and delivering ultrahigh mass activity and outstanding Cl₂ selectivity.

1. Introduction

Chlorine (Cl₂) is a vital industrial chemical broadly applied in disinfection, pharmaceuticals, water treatment, and polymer manufacturing [1, 2, 3]. Its large‐scale production primarily relies on the chlor‐alkali process, where the chlorine evolution reaction (CER) at the anode plays a central role [4, 5, 6, 7, 8]. Although CER benefits from inherently faster kinetics due to its two‐electron nature, the concurrent oxygen evolution reaction (OER) often reduces Cl₂ selectivity, elevates energy consumption, and complicates gas separation [9, 10, 11, 12, 13, 14]. Thus, developing electrocatalysts with high activity and selectivity is critical for improving energy efficiency in Cl₂ production [15, 16, 17, 18, 19].

Industrial CER is currently dominated by dimensionally stable anodes (DSA), typically comprising Ti substrates coated with RuO₂‐based mixed metal oxides (MMOs) [20, 21, 22]. While DSA deliver robust activity and durability, they exhibit several intrinsic limitations: (i) High RuO₂ loading is necessary to sustain activity, leading to inefficient Ru utilization due to catalytically inactive bulk species [4, 23]; (ii) The high OER activity of RuO₂ compromises Cl₂ selectivity [24, 25]; and (iii) MMOs generally possess limited surface area and sluggish electron transport. Although doping and nanostructuring strategies have been explored, transformative improvements demand a fundamental reengineering of active site architectures [26, 27, 28, 29].

From a catalytic chemistry standpoint, most CER electrocatalysts are built upon either single metal active sites or symmetric bimetallic configurations [30, 31, 32]. For example, Wang et al. [30]. constructed a four‐coordinated Ir₁O₄ single‐atom catalyst that shows high mass activity and good stability for Cl₂ production. The unsaturated Ir atom acts as the active center for Cl⁻ adsorption and, more importantly, exhibits restrained ability for water oxidation. Similarly, a highly active O‐coordinated Ru single atom (Ru–O₄) catalyst exhibits high selectivity and excellent durability for Cl₂ synthesis in seawater‐like solutions [33]. Operando and theoretical investigations confirm that the unique Ru–O₄ motif facilitates the direct adsorption of Cl⁻ ions on the Ru atom that is different from the bulk RuO₂, accounting for the greatly reduced Gibbs free energy barrier for Cl₂ formation. While single‐site catalysts offer structural simplicity, they often lack the orbital adaptability required to optimize both adsorption and desorption of key intermediates [34]. In comparison, symmetric bimetallic sites exhibit high flexibility for intermediates. For instance, exposing the symmetric Ru–Ru bimetallic site at the MgO/Ru interface generates an “onion‐like” electron distribution, which moderates the adsorption of Cl and H intermediates and thereby enhances both Cl₂ and H₂ evolution activities [35]. Nevertheless, their rigid coordination environments typically lead to uniform adsorption behavior, which can hinder the release of intermediates, particularly in desorption‐limited steps such as Cl₂ evolution. To overcome these limitations, asymmetric bimetallic active sites, such as M₁–X–M₂ motifs (X = O, C, and N), provide a compelling alternative by integrating two electronically and chemically distinct metal centers [36, 37]. These architectures generate reorganized orbital coordination that promotes electron redistribution, enabling intermediate binding and facilitating rate‐limiting steps. While this strategy has proven effective in other electrocatalytic reactions, including OER [38, 39], H₂ evolution reaction [40, 41], and CO₂ reduction reaction [42, 43], its employment in the CER is currently limited.

Herein, to address the inferior activity and stability of pristine Co₃O₄ for the CER arising from the sluggish adsorption−desorption kinetics of Cl‐containing intermediates, we fabricated atomically Ru‐doped Co₃O₄ (RuO_x‐Co₃O₄), where the notation “RuO_x‐Co₃O₄” denotes atomically dispersed Ruⁿ⁺ species incorporated within the Co₃O₄ lattice, rather than a multiphase RuO_x/Co₃O₄ heterostructure. Via a Co‐metal–organic framework (MOF)‐derived cation exchange approach, enabling the construction of asymmetric Ru–O–Co bimetallic active sites. The incorporation of isolated Ru atoms into the Co₃O₄ spinel lattice effectively optimizes the local coordination environment and electronic structure, thereby boosting intrinsic CER activity. Mechanistic investigations demonstrate that Ru substitution breaks the symmetric Co–O–Co configuration and suppresses excessively strong Cl adsorption on Co sites. Meanwhile, the relatively weak Ru 4p–O 2p interaction leaves abundant accessible vacant orbitals on Ru sites, which favor the formation of labile Ru–Cl species and enable fast, reversible Cl adsorption–desorption cycling. This favorable structural and electronic optimization directs the CER to a Ru‐dominant Volmer−Heyrovský pathway with accelerated reaction kinetics. Consequently, the as‐obtained RuO_x‐Co₃O₄ electrocatalyst exhibits an ultrahigh Ru mass activity of 3049 A g_Ru ⁻¹ at 1.5 V and an outstanding Cl₂ selectivity of 98.9%, significantly outperforming pure Co₃O₄ with symmetric Co–O–Co sites. This work demonstrates that asymmetric bimetallic bridging sites offer a universal strategy for highly efficient and selective CER electrocatalysts.

2. Results and Discussion

2.1. Structural and Electronic Characterization of RuO_x‐Co₃O₄

The RuO_x‐Co₃O₄ electrocatalyst was synthesized via a facile cation exchange strategy, in which pre‐anchored Co²⁺ ions in the MOF precursor are partially substituted by Ru³⁺ species at elevated temperature, driven by differences in ionic radii and lattice energies (details provided in Experimental Section, Figure 1a, and Figures S1 and S2). Scanning electron microscopy (SEM) image reveals that RuO_x‐Co₃O₄ retains the characteristic leaf‐like morphology of the Co‐MOF precursor, albeit with a visibly thinner structure due to the decomposition of organic ligands during thermal treatment (Figure 1b). Transmission electron microscopy (TEM) images further illustrate the formation of ultrathin and porous sheets composed of discrete nanocrystals (Figure 1c,d). Energy dispersive x‐ray (EDX) elemental mapping images reveal a homogeneous distribution of Ru throughout the nanosheets, with an overall Ru content of 1.64 wt% as determined by inductively coupled plasma‐optical emission spectrometer (ICP‐OES) (Figure 1e and Table S1). X‐ray diffraction (XRD) analysis shows that all diffraction peaks can be indexed to the spinel Co₃O₄ phase [44], with no detectable reflections corresponding to RuO₂ or metallic Ru (Figure S3). High‐resolution TEM (HRTEM) image displays clear lattice fringes with interplanar spacings of 0.47 and 0.24 nm, corresponding to the (111) and (311) planes of Co₃O₄, respectively (Figure 1f and Figure S4) [45, 46, 47]. The associated fast Fourier transform (FFT) patterns confirm the high crystalline of Co₃O₄. The aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) image further reveals isolated bright spots, assigned to individual Ru atoms, uniformly embedded in the Co₃O₄ matrix (Figure 1g). The highly dispersion of Ru is expected to modulate the local electronic structure of Co₃O₄, a hypothesis supported by Raman and x‐ray photoelectron spectroscopy (XPS) analyses. Both techniques reveal evident changes in vibrational modes and binding energies upon Ru incorporation, pointing to strong electronic interactions at the atomic scale (Figures S5 and S6).

FIGURE 1.

(a) Schematic illustration for the synthetic process of RuO_x‐Co₃O₄. (b) SEM image, (c, d) TEM images, (e) EDX elemental mapping images, (f) HRTEM images and FFT patterns, and (g) HAADF‐STEM image of RuO_x‐Co₃O₄.

X‐ray absorption spectroscopy (XAS) investigations were performed to elucidate the local coordination environment of the Ru atom embedded within the Co₃O₄ lattice. The Ru K‐edge x‐ray absorption near‐edge structure (XANES) spectrum of RuO_x‐Co₃O₄ exhibits a clear positive shift relative to Ru foil and a negative shift compared with RuO₂ (Figure 2a), indicating that the Ru species in RuO_x‐Co₃O₄ adopt an intermediate oxidation state between Ru⁰ and Ru⁴⁺. This edge shift is widely recognized as a robust indicator of the average oxidation state of Ru in XANES analysis. The absence of a metallic‐like pre‐edge feature further confirms that Ru atoms are not in a fully reduced state, consistent with atomically dispersed Ru coordinated with oxygen in the lattice. The extended x‐ray absorption fine structure (EXAFS) of Ru foil shows a prominent peak at 2.42 Å, corresponding to the characteristic Ru–Ru scattering path (Figure 2b), while RuO₂ displays two distinct peaks at 1.56 and 3.16 Å, attributable to Ru─O and Ru–O–Ru scattering paths, respectively [48, 49]. In stark contrast, RuO_x‐Co₃O₄ exhibits a markedly different coordination profile. The Ru–Ru and Ru–O–Ru peaks are absent, and instead, two new peaks emerge at 1.47 Å and 2.55 Å, corresponding to the Ru─O bond and the presence of Ru–Co scattering path in the second shell (representing Ru–O–Co in the crystal structure), respectively [50, 51]. The absence of Ru–Ru interaction confirms the atomic dispersion of Ru species, while the emergence of Ru–O–Co path provides direct evidence for heteronuclear interfacial bonding between isolated Ru atoms and the surrounding Co₃O₄ framework. Wavelet transformed (WT) analysis of the k ³‐weighted Fourier‐transformed (FT)‐EXAFS signals offers complementary insight into the bonding environment. The Ru–O–Co signal for RuO_x‐Co₃O₄ appears at a radial distance longer than that of Ru–Ru in Ru foil, and shorter than the Ru–O–Ru path in RuO₂ (Figure 2c–e). This intermediate bonding distance is consistent with a single‐atom Ru species embedded in an asymmetric coordination environment, further corroborating the formation of atomically dispersed Ru stabilized through Ru–O–Co site (Table S2) [52]. The absence of any high‐intensity Ru–Ru signals effectively rules out the formation of Ru clusters or nanoparticles, while the precise radial shift of the WT maxima confirms the formation of well‐defined interfacial coordination motifs. Furthermore, the Co K‐edge XANES spectrum of RuO_x‐Co₃O₄ shows a negative shift relative to pristine Co₃O₄ (Figure 2f), suggesting a lower average oxidation state of Co after Ru incorporation. This phenomenon reflects an electron redistribution process within the asymmetric Ru–O–Co bridge structure, where electron transfer from Ru to Co is induced by their different electronegativities and coordination environments. Such electronic modulation simultaneously renders Ru centers more electropositive, which is beneficial for facilitating Cl⁻ adsorption. Additionally, the Ru–O–Co configuration is consistently observed in both EXAFS and k ³‐weighted FT‐EXAFS spectra of RuO_x‐Co₃O₄ (Figure 2g and Figure S7), providing conclusive structural evidence for the successful integration of Ru atom into the Co₃O₄ lattice. Specifically, the signals appearing at approximately 1.44 Å and 2.59 Å for RuO_x‐Co₃O₄ are attributed to Co_Oh/Co_Td–O and Co_Oh─O─Co_Oh/Ru scattering paths, respectively (Table S3) [53]. Compared to the Co_Td─O─Co_Oh in Co₃O₄, the positively shifted peak at 3.44 Å represents the coordination of Co_Td─O─Co_Oh/Ru after Ru incorporation. The nearly unchanged Co_Oh─O─Co_Oh/Ru bond distance reflects the rigid and bulk‐stabilized octahedral coordination environment of Co_Oh within the CoO₆ framework, which is less sensitive to external perturbations. In contrast, the noticeable shift in the Co_Td─O─Co_Oh/Ru coordination arises from the more flexible and surface‐exposed tetrahedral Co_Td sites, which are more susceptible to local lattice distortion induced by Ru incorporation. This site‐dependent structural response indicates that Ru incorporation predominantly perturbs Co_Td‐associated regions, thereby facilitating the formation of asymmetric Ru–O–Co coordination motifs, which serve as the key active sites for the CER.

FIGURE 2.

(a) Ru K‐edge XANES spectra and (b) FT‐EXAFS spectra of RuO_x‐Co₃O₄, RuO₂, and Ru foil. WT contour plots of (c) RuO_x‐Co₃O₄, (d) Ru foil, and (e) RuO₂. (f) Co K‐edge XANES spectra and (g) FT‐EXAFS spectra of RuO_x‐Co₃O₄, Co₃O₄, and Co foil.

2.2. Electrocatalytic Performance of RuO_x‐Co₃O₄ for CER

The CER activity of RuO_x‐Co₃O₄ was evaluated in saturated NaCl solution (pH = 7), with Co₃O₄ and commercial DSA tested under identical conditions for comparison (Figures S2 and S8). As shown in the polarization curves (Figure 3a), both DSA and Co₃O₄ exhibit limited catalytic activity, achieving current densities of only 18.1 and 22.4 mA cm⁻² at 1.5 V, respectively. In stark contrast, RuO_x‐Co₃O₄ delivers a substantially enhanced current density of 161.0 mA cm⁻² at the same potential. Furthermore, the onset potential (defined as the potential required to achieve a 10 mA cm⁻² current density) is significantly reduced for RuO_x‐Co₃O₄ (1.23 V) compared to Co₃O₄ (1.37 V) and DSA (1.43 V), underscoring the superior intrinsic activity imparted by Ru incorporation. It is worth noting that the Ti substrate does not provide any active sites and does not influence the intrinsic catalytic performance of the material. Tafel slope analysis reveals a significantly lower value for RuO_x‐Co₃O₄ compared to Co₃O₄ and DSA (Figure S9), indicative of improved reaction kinetics [54, 55, 56]. Notably, RuO_x‐Co₃O₄ exhibits exceptional intrinsic activity, achieving an ultrahigh Ru mass activity of 3049 A g_Ru ⁻¹ and a large turnover frequency (TOF) of 5745 h⁻¹ at 1.5 V (Figure 3b,c). Such an outstanding mass activity ranks among the highest reported for CER electrocatalysts, highlighting the nearly perfect atomic utilization of Ru in the RuO_x‐Co₃O₄ system (Table S4). The reaction order of CER on RuO_x‐Co₃O₄ was experimentally determined by evaluating the catalytic activity as a function of Cl⁻ ion concentration. As shown in Figure 3d, the polarization curves exhibit a distinct [Cl⁻]‐dependent potential shift, suggesting that RuO_x‐Co₃O₄ possesses sufficient active sites to accommodate the increasing Cl⁻ concentration. This is further supported by the largest electrochemically active surface area (ECSA) of RuO_x‐Co₃O₄ (Figure S10). The ECSA‐normalized activity comparison demonstrates that RuO_x‐Co₃O₄ still exhibits higher intrinsic activity than pristine Co₃O₄ after eliminating the influence of surface area differences, indicating that the enhanced CER performance does not solely arise from an increased number of exposed active sites, but is also strongly associated with the intrinsically improved catalytic activity induced by the asymmetric Ru–O–Co bridge sites (Figure S11). Furthermore, when the logarithm of current density (logj) was plotted against log [Cl⁻], a nearly linear correlation was obtained over the potential range of 1.16–1.24 V, yielding reaction orders between 0.67 and 0.70 with an average value of 0.69 (Figure 3e). Such a reaction order indicates a moderate dependence of CER activity on Cl⁻ concentration rather than a first‐order behavior. This intermediate value is consistent with a mixed kinetic regime involving both Volmer‐type adsorption and Heyrovský‐type electrochemical desorption steps. Therefore, it is inferred that the CER on RuO_x‐Co₃O₄ proceeds via a Volmer–Heyrovský mixed pathway, without a single exclusive rate‐determining step.

FIGURE 3.

(a) Polarization curves of RuO_x‐Co₃O₄, DSA, Co₃O₄, and Ti plate. (b) Mass activity and (c) TOF of RuO_x‐Co₃O₄. (d) Polarization curves of RuO_x‐Co₃O₄ in NaCl solution with varying concentrations. (e) Log j vs. log [Cl⁻] plots of RuO_x‐Co₃O₄. (f) Cl₂ selectivity of RuO_x‐Co₃O₄, DSA, and Co₃O₄. (g) Comparison of Cl₂ selectivity between RuO_x‐Co₃O₄ and other CER electrocatalysts. (h) Stability test of RuO_x‐Co₃O₄ and Co₃O₄.

In addition to activity, RuO_x‐Co₃O₄ demonstrates high Cl₂ selectivity. Very low current is observed in Cl⁻‐free electrolytes such as 0.5 M H₂SO₄ and saturated NaNO₃ (Figure S12), suggesting that the current from RuO_x‐Co₃O₄ originates from predominant Cl⁻ oxidation rather than water oxidation. Iodometric titration reveals a high Cl₂ selectivity of up to 98.9% for RuO_x‐Co₃O₄, surpassing DSA (84.9%) and Co₃O₄ (89.2%) as well as numerous state‐of‐the‐art CER electrocatalysts (Figure 3f,g and Table S5). The rotating ring‐disk electrode measurements further validate the high Cl₂ selectivity of RuO_x‐Co₃O₄ (94.5%) than that of Co₃O₄ (88.5%) (Figure S13). The significantly enhanced selectivity compared with other representative CER catalysts indicates that Ru doping effectively modulates the electronic structure of Co₃O₄, thereby optimizing the adsorption–desorption kinetics of Cl intermediates during the CER process. Durability testing demonstrates that RuO_x‐Co₃O₄ maintains stable performance for 300 min with a minor potential increase of 70 mV (Figure 3h), far outperforming Co₃O₄, which rapidly deactivates under identical conditions. Post‐reaction characterizations reveal that the morphology, microstructure, and phase composition of RuO_x‐Co₃O₄ remain largely intact (Figures S14–S17). XRD pattern shows neither discernible peak shift nor broadening, indicating that the crystalline phase of RuO_x‐Co₃O₄ is well preserved (Figure S14). HAADF‐STEM images further display clear isolated Ru atoms and undistorted lattice fringes, confirming the structural robustness of RuO_x‐Co₃O₄ (Figure S16). XPS analysis indicates only a slight increase in the oxidation states of Ru, attributable to the strongly oxidizing Cl‐containing environment, while ICP‐OES results reveal negligible Ru leaching and significantly suppressed Co dissolution upon Ru incorporation (Figure S17 and Table S6).

2.3. In Situ Spectroelectrochemical Analyses

Previous studies have demonstrated that Co₃O₄‐based electrocatalysts exhibit outstanding OER activity in alkaline media [57, 58, 59]. Consistently, RuO_x‐Co₃O₄ shows excellent OER activity in 1.0 M KOH (Figure S18). However, when the electrolyte is switched to a neutral NaCl solution, the reaction pathway undergoes a significant transition, favoring CER over OER. Moreover, the Cl₂ selectivity progressively increases with increasing NaCl concentration (Figure S19), further confirming the critical role of Cl⁻ concentration in regulating the reaction pathway selection. To unravel the origin of the exceptionally high Cl₂ selectivity in RuO_x‐Co₃O₄, we performed in situ spectroelectrochemical investigations. In situ attenuated total reflection surface‐enhanced infrared absorption spectroscopy (ATR‐SEIRAS) reveals the absence of characteristic *O or *OOH vibrational signals, which are typical intermediates for OER, confirming that the OER pathway is significantly suppressed in Cl‐rich media (Figure 4a,b) [60]. In contrast, characteristic *OOH‐related signals can be clearly detected for RuO_x‐Co₃O₄ in 1.0 M KOH solution (Figure S20), further supporting the distinct reaction pathway difference between OER and CER conditions. Such suppression can be rationalized by two synergistic effects: (i) Cl⁻ ions at high concentration preferentially adsorb on the electrocatalyst surface, sterically blocking and electronically outcompeting the adsorption of OH⁻ species necessary for initiating OER; and (ii) CER involves a kinetically more favorable two‐electron process compared with the sluggish four‐electron OER pathway, granting CER a substantial kinetic advantage under competitive conditions [54]. Complementary mechanistic insights were obtained from in situ Raman spectroscopy, which detects distinct Ru–Cl vibrational feature (142 cm⁻¹) at anodic potentials for RuO_x‐Co₃O₄, confirming the formation of surface‐adsorbed Cl intermediates (Figure 4c,d) [61, 62]. In sharp contrast, bare Co₃O₄ shows no evidence of Cl adsorption, directly accounting for its inferior CER activity [33].

FIGURE 4.

In situ ATR‐SEIRAS spectra of (a) Co₃O₄ and (b) RuO_x‐Co₃O₄ in saturated NaCl. In situ Raman spectra of (c) Co₃O₄ and (d) RuO_x‐Co₃O₄ in saturated NaCl. In situ EIS spectra of (e) Co₃O₄ and (f) RuO_x‐Co₃O₄ in saturated NaCl. Corresponding Bode plots of (g) Co₃O₄ and (h) RuO_x‐Co₃O₄. (i) Phase peak variations of RuO_x‐Co₃O₄ and Co₃O₄ at different potentials.

In situ electrochemical impedance spectroscopy (EIS) was performed to probe the interfacial dynamics of electrocatalysts during CER. At low bias, both Co₃O₄ and RuO_x‐Co₃O₄ display semicircular Nyquist features typical of diffusion‐controlled processes at the electrode‐electrolyte interface (Figure 4e,f). However, RuO_x‐Co₃O₄ exhibits a distinctly stronger potential‐dependent frequency shift in the low‐frequency region, which reflects the gradual activation of surface reaction kinetics and improved electron‐transfer capability as the potential increases [63]. With increasing applied potential, the EIS response of Co₃O₄ transitions toward dominance by high‐frequency elements, indicating the emergence of bulk mass‐transport limitations (Figure 4g). In contrast, RuO_x‐Co₃O₄ shows a pronounced decrease in phase angle and a significant reduction in electron‐transfer resistance, suggesting more efficient electron transport across the Ru–O–Co site and accelerated kinetics of the CER (Figure 4h). The continuous frequency shift toward higher frequencies implies that the Heyrovský step is effectively accelerated by the electronic coupling between Ru and Co sites. Moreover, the transition‐phase peak associated with interfacial electron dynamics diminishes more rapidly for RuO_x‐Co₃O₄ than for Co₃O₄ (Figure 4i), consistent with faster electron consumption and enhanced surface redox reversibility [48]. These findings uncover the pivotal role of the Ru–O–Co interfacial bridge in simultaneously accelerating Cl⁻ adsorption−oxidation−desorption kinetics and suppressing the competing oxygen evolution, thereby endowing RuO_x‐Co₃O₄ with exceptional Cl₂ selectivity under harsh oxidative conditions.

2.4. Mechanistic Elucidation of the CER on RuO_x‐Co₃O₄

As established in previous studies [21, 64, 65], the CER may proceed via several elementary pathways involving the Volmer, Heyrovský, Krishtalik, or Tafel steps. In the Volmer step, a Cl⁻ ion undergoes electrochemical adsorption and discharge on the active site (*) to form *Cl. The Heyrovský step involves the electrochemical oxidation of *Cl through its reaction with another Cl⁻ from the electrolyte to form Cl₂. In the Krishtalik pathway, a *Cl⁺ intermediate is first formed and subsequently combines with one Cl⁻ to produce Cl₂. Alternatively, in the Tafel step, two adjacent *Cl species recombine directly to evolve Cl₂. As summarized in the following Equations ((1), (2), (3), (4), (5)):

$$\mathrm{Volmer}\mathrm{step}:\mathrm{C}{\mathrm{l}}^{-}{+}^{\ast }{\to }^{\ast }\mathrm{Cl}+{\mathrm{e}}^{-}$$ (1)

$$\mathrm{Heyrovsk}\acute{y}\mathrm{step}{:}^{\ast }\mathrm{Cl}+\mathrm{C}{\mathrm{l}}^{-}\to \mathrm{C}{\mathrm{l}}_2+{\mathrm{e}}^{-}+\ast$$ (2)

$$\mathrm{Krishtalik}\mathrm{step}{:}^{\ast }\mathrm{Cl}{\to }^{\ast }\mathrm{C}{\mathrm{l}}^{+}+{\mathrm{e}}^{-}$$ (3)

(4)

$$\mathrm{Tafel}\mathrm{step}{:}^{\ast }\mathrm{Cl}{+}^{\ast }\mathrm{Cl}\to \mathrm{C}{\mathrm{l}}_2+2^{\ast }$$ (5)

The Volmer−Tafel mechanism requires the spatial proximity of two *Cl intermediates to form Cl₂ via surface coupling. However, this pathway is structurally implausible for RuO_x‐Co₃O₄ because atomically dispersed Ru sites, anchored through Ru–O–Co sites, are well separated. Such isolated active sites suppress the *Cl–*Cl recombination essential for the Tafel route. From a kinetic perspective, the measured Tafel slope of 96 mV dec⁻¹ (Figure S9) deviates significantly from the theoretical value of 30 mV dec⁻¹ expected for a Tafel‐determining step, thereby ruling out this pathway [66]. Similarly, the Volmer−Krishtalik pathway, which involves lattice oxygen participation and high‐valent *OCl intermediates, would induce detectable structure degradation and oxidation‐state variations. Yet, in situ ATR‐SEIRAS detects no *OCl species at 727 cm⁻¹ (Figure S21) [33], while post‐electrolysis XRD, HAADF‐STEM, and XPS confirm preserved structural integrity and oxidation states, excluding oxygen lattice participation. In contrast, multiple lines of evidence suggest the dominance of the Volmer−Heyrovský mechanism. In situ Raman spectroscopy identifies a characteristic Ru–Cl vibration, validating the Volmer step involving Cl⁻ adsorption. The potential dependence of the *Cl signal further distinguishes between possible pathways. Within the Volmer−Heyrovský mechanism, increasing potential accelerates Cl₂ generation through the Heyrovský electrochemical desorption step, while *Cl coverage remains relatively stable or slightly increases, due to the concurrent acceleration of both *Cl formation and consumption processes [67]. In situ Raman measurements reveal that the Ru–Cl band intensity remains nearly constant with increasing potential, reflecting a dynamic equilibrium between *Cl formation and consumption, in agreement with the Volmer−Heyrovský mechanism. By combining structural stability, kinetic analysis, and in situ vibrational evidence, we conclude that CER on RuO_x‐Co₃O₄ follows a Volmer−Heyrovský mechanism, with sequential Cl⁻ adsorption and electrochemical desorption to Cl₂.

2.5. Theoretical Insights Into Asymmetric Site Engineering

Density functional theoretical (DFT) calculations were performed to uncover the atomic‐scale origin of the enhanced CER activity upon Ru incorporation. Guided by TEM and XRD results, the (111) and (311) planes of Co₃O₄ were identified as the dominant facets for Cl adsorption. As shown in Figures S22 and S23, three representative Co sites on the (111) plane exhibit relatively weak Cl adsorption energies of −0.60, −0.20, and −0.72 eV, respectively, indicating limited interaction strength between Cl and surface Co atoms. In sharp contrast, the (311) plane displays considerably stronger adsorption behavior, particularly at the symmetric Co–O–Co (Co₃–O–Co₂) bridge site with an adsorption energy of −1.66 eV, significantly lower than those of the atop‐Co₂ (−1.35 eV) and atop‐Co₁ (−1.21 eV) configurations (Figure 5a,b and Figures S24–S27). This suggests that the (311) plane provides more favorable adsorption and activation sites for Cl species, consistent with its dominant exposure and reported catalytic activity. However, although the symmetric Co–O–Co site stabilizes Cl adsorption, it concurrently impedes Cl desorption, thereby constraining Cl₂ evolution. The three surface Co sites were subsequently systematically substituted with Ru atoms. Ru preferentially occupies the Co₃ position with the lowest formation energy, generating an asymmetric Ru–O–Co (Ru₃–O–Co₂) bimetallic site (Figure 5c and Figure S28). By offering moderately weaker Cl binding than the symmetric Co–O–Co site, the asymmetric Ru–O–Co site optimizes the adsorption‐desorption balance and thereby decreases the energy barrier for Cl release (Figure 5a and Figure S29). It should be highlighted that the symmetric Cl adsorption on Co was destabilized upon Ru introduction. The elongation of the Cl─Co distance beyond the bonding range deactivates the Co site for Cl adsorption, redirecting Cl binding to the Ru atom, in agreement with the in situ Raman observations. Electron localization function (ELF) mappings reveal a higher electron density localized around Ru in RuO_x‐Co₃O₄ than around Co in Co₃O₄ (Figure 5d,e), indicative of enhanced electron transfer capability within the asymmetric Ru–O–Co bimetallic site. Hall coefficient measurements further support the increased electron density in Co₃O₄ (6.58 × 10⁻⁴ m³/C) relative to RuO_x‐Co₃O₄ (1.26 × 10⁻⁴ m³/C), corresponding to a 5.22‐fold enhancement (Figure 5f). The femtosecond transient absorption spectroscopy (Fs‐TAS) analyses confirm accelerated electron transfer dynamics in RuO_x‐Co₃O₄ [68, 69, 70]. RuO_x‐Co₃O₄ exhibits shorter average electron lifetimes (τ _ave = 699.4 ps) compared to Co₃O₄ (τ _ave = 895.4 ps) (Figure 5g–i and Figure S30), reflecting faster electron transfer and utilization in catalytic turnover. An additional intermediate decay component emerges in RuO_x‐Co₃O₄, indicative of an emerged pathway associated with electron transfer through the asymmetric Ru–O–Co bimetallic site [71].

FIGURE 5.

(a) Calculated adsorption energy of Cl atom at different adsorption sites on Co₃O₄ and RuO_x‐Co₃O₄. Structural models of (b) symmetric Co–O–Co bimetallic site in Co₃O₄ and (c) asymmetric Ru–O–Co bimetallic site in RuO_x‐Co₃O₄. ELF mappings of (d) Co₃O₄ and (e) RuO_x‐Co₃O₄. (f) Hall coefficient of Co₃O₄ and RuO_x‐Co₃O₄. Fs‐TAS pseudocolor maps of (g) Co₃O₄ and (h) RuO_x‐Co₃O₄. (i) Corresponding decay curves of Co₃O₄ and RuO_x‐Co₃O₄.

Natural bonding orbital (NBO) and crystal orbital Hamilton population (COHP) analyses further clarify the bonding evolution at the orbital level [72]. In the symmetric Co–O–Co site (Figure 6a, up), O 2p and 2s orbital electrons transfer to empty Co 3p (p*) and 4s (s*) orbitals with conjugation energy of 47.13 and 20.58 kcal/mol, respectively, while lone‐pair (LP) electrons also populate unoccupied Co 3p orbitals (12.27 kcal/mol). Upon Ru substitution (Figure 6a, down), O 2p orbital and LP electrons transfer to the empty Ru 4p (p*) orbital with conjugation energy of 17.72 and 30.92 kcal/mol, respectively. These indicate that fewer Co empty orbitals are available due to stronger Co─O bonding. After Cl adsorption, significant orbital reorganization occurs. In the asymmetric Ru–O–Co bimetallic site (Figure 6b, down), Cl 3p (p*) and 3s (s*) orbital electrons are transferred to the vacant Ru 4p orbitals with conjugation energies of 16.31 and 4.09 kcal/mol, respectively, resulting in a total stabilization higher than that of the symmetric Co–O–Co site, where Cl 3p orbital electrons populate the unoccupied Co 3p (p*) and 4s (s*) orbitals with energies of 10.92 and 1.22 kcal/mol (Figure 6b, up). This suggests the weaker Ru 4p─O 2p bonding, which leaves more empty orbitals available for Cl binding on Ru while simultaneously weakening Co─Cl bond. Partial density of states (PDOS) analysis further shows a new Ru–Cl orbital overlap upon Ru incorporation, disrupting and weakening the original symmetric Co–Cl hybridization (Figure S31). Furthermore, the integrated crystal orbital hamilton population (ICOHP) value of the Ru─Cl bond (−0.25 eV) is substantially greater than for Co─Cl bond in both Co₃O₄ (−0.032 eV) and RuO_x‐Co₃O₄ (0.0016 eV) (Figures 6c,d and Figure S32), highlighting the thermodynamic preference for Cl adsorption at Ru site. Ru incorporation disrupts the intrinsic Co–O–Co symmetry of Co₃O₄ and generates asymmetric Ru–O–Co sites with reconfigured electronic properties. The strengthened Co─O bonds suppress Cl coordination at Co atoms by reducing orbital availability, whereas Ru, characterized by weaker Ru–O interactions and abundant empty orbitals, serves as the preferred site for both Cl adsorption and desorption. This asymmetric orbital modulation optimizes the Cl binding strength, enabling labile Ru–Cl interactions that accelerate adsorption‐desorption dynamics. Meanwhile, the redistribution of electron density at the Ru−O−Co bimetallic site facilitates electron delocalization and promotes efficient electron transfer, thereby lowering the free‐energy barrier for Cl₂ evolution. Collectively, the orbital‐level regulation breaks the limitation of symmetrical adsorption of Cl on the Co atom and constructs a CER‐favored local electronic microenvironment around the Ru atom, accounting for the markedly enhanced Cl₂ evolution activity and selectivity of RuO_x‐Co₃O₄ (Figure 6e).

FIGURE 6.

NBO and second‐order perturbation theory analysis of donor–acceptor (bond‐antibond) interactions and the schematic illustration of the corresponding NBO isosurface map in (a) Co₃O₄ and RuO_x‐Co₃O₄ and (b) Cl adsorbed Co₃O₄ and RuO_x‐Co₃O₄ (Isovalue = 0.05). ICOHP analysis of (c) Co─Cl bond in RuO_x‐Co₃O₄ and (d) Ru─Cl bond in RuO_x‐Co₃O₄ (Negative values represent anti‐bonding effect and positive values represent bonding effect.). (e) Optimized adsorption configurations of Cl on Co–O–Co and Ru–O–Co bimetallic sites.

3. Conclusions

In summary, we present a rational strategy to overcome the intrinsic limitations of Co₃O₄ in CER by incorporating atomic Ru via a MOF‐derived route. The substitution of Ru for Co at the symmetric Co–O–Co sites breaks structural symmetry and reconfigures orbital occupancy and bonding interactions at the active centers. This substitution strengthens Co─O bonds while generating weaker Ru 4p–O 2p coordination with abundant vacant orbitals that act as favorable sites for Cl hybridization and reversible adsorption−desorption. Such orbital‐level modulation breaks the confinement of symmetric Co sites, reconstructing the local electronic microenvironment around Ru into a CER‐favorable configuration. As a result, the RuO_x‐Co₃O₄ electrocatalyst achieves an ultrahigh Ru mass activity (3049 A g_Ru ⁻¹) and a remarkable Cl₂ selectivity (98.9%) at 1.5 V. This work not only maximizes Ru atom utilization efficiency but also establishes asymmetric bimetallic site engineering as an effective strategy for developing next‐generation high‐selective CER electrocatalysts.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll73962‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.