Cerium driven active site relocation in spinel Co_3O_4 enables stable chlorine evolution in acidic media

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

Abstract
The chlorine evolution reaction underpins the chlor-alkali industry, yet its harsh acidic oxidative environment severely limits the stability of non-noble catalysts. Here we show that atomically dispersed Ce in spinel Co3O4 with a three-dimensional ordered macroporous nanostructure triggers active site relocation from lattice oxygen to cobalt, simultaneously enhancing activity and inhibiting lattice oxygen corrosion. Ce occupies octahedral Co sites, inducing polyhedral distortion and creating unsaturated Co centers that directly adsorb Cl⁻. In 4 M NaCl at pH = 2, the catalyst achieves overpotentials of 44 and 218 mV at 10 and 1000 mA cm–2, respectively, with ~99.1% chlorine selectivity and robust durability over 550 hours in a chlor-alkali cell at 2.5 kA m–2. In situ Raman, attenuated total reflection surface-enhanced infrared absorption spectroscopy and differential electrochemical mass spectrometry confirm the active-site transformation, while density functional theory calculations reveal optimized Cl adsorption free energy, suppressed oxygen-mediated degradation, and preserved structural integrity. This active-site engineering strategy offers a route to designing stable, high-current-density catalysts for chlorine production beyond noble metals.

Introduction
Chlorine (Cl2) is a cornerstone of the chemical industry, with global production exceeding 70 Mt annually for applications ranging from water disinfection to polyvinyl chloride and pharmaceuticals1-3. Industry Cl2 is predominantly produced via chlor-alkali electrolysis, which co-generates H2 and NaOH, consuming ~150 TWh of electricity each year4. Current anodes, dimensionally stable anode (DSA) based on Ru and Ir oxides, combine high activity with corrosion resistance,5 but their scarcity and high cost hinder sustainable scale-up and other applications. Moreover, their high oxygen evolution reaction (OER) activity compromises CER selectivity under industrial bias6-8. This motivates the search for low-cost, high-activity catalysts to ensure the sustainable advancement of the chlor-alkali industry and other chloride-related electrochemical treatment applications9,10.

3d transition-metal oxides have been proposed as promising alternatives due to their abundance, low cost, and tunable electronic structures11-15. However, the acidic, chloride-rich, and strongly oxidative environment of CER causes most non-precious oxides to dissolve rapidly16. Co-based oxides, including spinel Co3O4, have shown promise when modified via coordination tunning17,18, amorphization19,20, or heteroatom doping21-24, yet they still suffer rapid deactivation at higher current densities (>100 mA cm–2). This instability is widely linked to the dissolution of lattice oxygen-the dominant active site in many metal oxides during CER25,26. While oxygen sites are kinetically favorable for Cl⁻ oxidation, their susceptibility to oxidative etching leads to structural collapse. By contrast, lattice metal atoms are more resistant to oxidative etching27,28, suggesting that deliberately relocating the active site from oxygen to metal could reconcile activity with durability29.

Rare-earth (RE) elements, with large ionic radii, variable oxidation states, and strong 4f-3d coupling, can simultaneously modulate crystal and electronic structure. Although RE dopent such as Ce30,31, La32, and Nd33,34 have been used to enhance electrocatalysts, their role in controlling adsorption pathways during CER remain underexplored. Here, we show that Ce single-atom incorporation into octahedral sites of spinel Co3O4 with three-dimensional ordered macroporous nanostructures (3D Ce-Co3O4) induces active site relocation from oxygen to cobalt, enabling direct Cl⁻ adsorption and oxidation. The resulting 3D Ce-Co3O4 catalyst exhibits low overpotentials, high selectivity, and long-term stability under acidic industrial CER conditions. In situ spectroscopy and density functional theory (DFT) calculations reveal that Ce doping distorts Co polyhedra, optimizes Cl binding energy, and suppresses OER pathways, providing a robust design principle for corrosion-resistant, non-noble CER catalysts.

Results
Synthesis and characterization of 3D Ce-Co3O4

The preparation procedure of 3D Ce-Co3O4 is shown in Supplementary Fig. 1a. 35. A precursor solution containing cobalt nitrate, cerium nitrate, and 2-dimethylimidazole was infiltrated into a 3D ordered nanostructured template composted by polystyrene sphere (270 nm diameter) (Supplementary Fig. 1c) to form 3D Ce-ZIF67@PS (Supplementary Fig. 1d). The final oxide was obtained by a two-step calcination (Supplementary Fig. 2): 3D Ce-ZIF67@PS was first calcined in Ar to generate 3D Ce-Co@NC with an ordered macroporous carbon framework (Supplementary Fig. 1d), followed by calcination in air to yield 3D Ce-Co3O4 (Fig. 1a and Supplementary Fig. 1e). Raman spectra and elemental analysis confirm the removal of the carbon matrix and the formation of oxide frameworks (Supplementary Fig. 3). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reveal this 3D ordered macroporous architecture with interconnect pores in 3D Ce-Co3O4 (Supplementary Fig. 4). Additional morphology details are provided in Supplementary Figs. 5-7. Brunauer−Emmett−Teller (BET) nitrogen adsorption-desorption measurements show a high specific surface area of 68.6 m2 g−1 for 3D Ce-Co3O4, compared with 22.9 m2 g−1 for non-templated analogue (Ce-Co3O4, Supplementary Figs. 8 and 9), benefiting reactant mass transfer and gas release at high current densities36.

High-resolution transmission electron microscopy (HRTEM) images (Supplementary Fig. 10) of 3D Ce-Co3O4 reveal a slight expansion in the lattice spacing of the (311) and (111) crystal planes compared to that of 3D Co3O4, attributable to lattice distortion induced by Ce incorporation37. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images (Figs. 1b and c , Supplementary Figs. 11 and 12) confirm atomically Ce with detectable CeO2 nanoparticles or clusters. The enlarged HAADF-STEM (Fig. 1c ) view of the spinel (011) plane (Supplementary Fig. 13) shows coherent lattice matching and smooth interfacial transitions at the nanoscale, indicative of high crystallinity38. Elemental mapping of 3D Ce-Co3O4 (Fig. 1d) shows uniform distributions of Co, O and Ce. The inductively coupled plasma atomic emission spectrometry (ICP-AES) quantifies the Ce content at ~1.07 wt% (Supplementary Table 1), consistent with the X-ray photoelectron spectroscopy (XPS)-derived value of 1.39 wt% (Supplementary Fig. 14). Powder X-ray diffraction (XRD) patterns (Fig. 1e) further confirm the spinel Co3O4 phase (JCPDS Card: PDF#00-042-1467) with no Ce-related reflections observed39. In addition, Raman spectra (Fig. 1f), Ultraviolet-Vis spectroscopy (UV-Vis)19,40 (Supplementary Fig. 15), and Fourier transform infrared spectroscopy (FTIR)41 (Supplementary Fig. 16) are consistent with these finding, indicating that the Ce single atom are stably incorporated into the spinel lattice in 3D Ce-Co3O4.

To determine the Ce occupation sites (octahedral vs tetrahedral), we combined Raman/FTIR spectra with DFT calculations. Spinel Co3O4 exhibits five Raman-active modes at 188.2 (F2g), 464.2 (Eg), 508.3 (F2g), 599.0 (F2g) and 667.1 cm–1 (A1g) (Fig. 1f), where A1g corresponds to octahedral Co-O vibrations and F2g to tetrahedra units42,43. After Ce doping, the A1g peak blue-shifts from 667.1 cm –1 to 671.7 cm–1, while the F2g mode remains essentially unchanged (188.2 cm –1 to 189.8 cm–1). In the FTIR spectroscopy (Supplementary Fig. 16), the octahedral Cooct-O stretching shifts from 559.2 cm–1 to 556.4 cm–1 44. These spectroscopic signatures indicate preferential Ce substitution at octahedral sites rather than tetrahedral ones. DFT calculations (Fig. 1g) further show that Ce substitution at octahedral positions is energetically more favorable than at tetrahedral positions (ΔE(Ceoct) = -3.78 eV vs. ΔE(Cetet) = -3.01 eV). Together, the experimental and computational results confirm that the Ce atoms selectively occupy octahedral Co sites as isolated single atoms in 3D Ce-Co3O4.

To resolve how Ce modifies the electronic and local coordination structure in 3D Ce-Co3O4, we performed the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Relative to pristine Co3O4, the Co K-edge XANES profiles of 3D Ce-Co3O4 exhibits a absorption edge shifts to the higher energy (Fig. 2a), indicating a modest increase in the average Co oxidation state, consistent with electron transfer from Co to Ce45,46. The concomitant enhancement of the white line further supports Co oxidation-state enrichment after Ce incorporation. EXAFS analysis evidences both the conserved spinel framework and subtle local distortions in 3D Ce-Co3O4. Fourier transformed k3-weighted EXAFS (FT-EXAFS) spectra (Fig. 2b) display three dominant contributions assignable to the Co-O first shell, Co-Co tet, and Co-Co oct scattering paths47,48. Quantitative fitting (Supplementary Figs. 17-19 and Supplementary Table 2) reveals a contracted path length for the Co-O tet scattering in 3D Ce-Co3O4 (R ≈ 3.34 Å) compared with 3D Co3O4 (3.37 Å), consistent with octahedral Ce substitution inducing distortions in adjacent Co tetrahedra. The k3-weighted wavelet-transformed EXAFS (WT-EXAFS) plots (Fig. 2c) show two high-intensity regions centered at k ≈ 4.71 and 6.31 Å, characteristic of Co–O and Co–O–Co contributions, respectively, confirming that Ce loading does not disrupt the bulk spinel coordination motif45. Ce L3-edge XANES (Supplementary Fig. 20a) reveals the Ce oxidation state. The Ce absorption edge energy of 3D Ce-Co3O4 is between Ce3+ of CeCl3 and Ce4+ in CeO2 and CeNO3, indicating relative decrease in Ce valence state and corroborating net electron withdrawal from Co species38. Ce L3-edge EXAFS (Supplementary Fig. 20b and Supplementary Table 3) shows two main peaks at R ≈ 2.23 Å (Ce-O) and 3.53 Å (Ce-O-Co)49, with no detectable Ce-Ce signal, consistent with atomically dispersed Ce anchored in the Co3O4 lattice and in agreement with the HAADF-STEM observations. Surface-sensitive XPS further supports these assignments. The Co 2p spectrum of 3D Ce-Co3O4 exhibits an increased Co3+/Co2+ ratio (2.24 to 2.32) relative to 3D Co3O4 (Fig. 2d)50. Meanwhile, the Ce 3d spectrum (Fig. 2e) indicates a mixed-valence of 3D Ce-Co3O4 and an increased Ce3+/Ce4+ ratio (0.88 to 0.89) relative to CeO2. This further indicates an increase in the oxidation state of Co, with electrons transferring from Co to Ce, which is consistent with the synchrotron radiation conclusion mentioned above. Notably, despite partial substitution at octahedral Co sites, the overall Co3+ fraction does not decrease (contrary to some report REF 42)42. We attribute this to Ce-induced lattice distortion (Supplementary Fig. 10) that stabilizes additional octahedral Co in 3D Ce-Co3O4. In the O 1s region (Fig. 2f), 3D Ce-Co3O4 shows an increased lattice oxygen (Olat) component and a reduced adsorbed oxygen (Oads) contribution, consistent with an octahedra-enriched surface.

To differentiate surface-electronic changes at Co and O site, we conducted soft X-ray adsorption spectroscopy (XAS) in total electron yield mode (TEY) at the Co L-edge and the O K-edge 50-52. Spinel Co3O4 comprises low-spin (LS) Co3+ in the octahedral sites (Cooct) and high-spin (HS) Co2+ in the tetrahedral sites (Cotet)52. The Co L-edge (Fig. 2g) feature a peak near 778.1 eV (Co tet t2g orbital) and a shoulder near 779.6 eV (Co oct eg orbital). After Ce doping, the eg/t2g intensity ratio increases from 1.78 (3D Co3O4) to 2.09 (3D Ce-Co3O4), indicating enrichment of octahedral Co3+ states at the surface. The above conclusions are further supported by the Magnetic-hysteresis (M-H) and Temperature-dependent magnetic susceptibility (M-T) measurements (see Supplementary Figs. 21 and 22 for details)53,54. It has been confirmed that LS Cooct can serve as a catalytic active site with higher reactivity in the reaction23,52, which would contribute to the enhancement of CER activity.

At the O K-edge spectrum (Fig. 2h), three features at 530.4-531.6, 537.2, and 542.5 eV, associated with O 2p states hybridized with Co 3d, Co 4s and Co 4p orbitals,51,55 are all intensified in 3D Ce-Co3O4. The enhancements point to the orbital coupling of 3d (Co)–2p (O)–4f (Ce), leading to the re-distribution of the Co electronic cloud and a strong interaction between Co and O, which may further result in the formation of distorted polyhedra on the surface. Taken together, these spectroscopic results show that Ce doping (i) increases the population of octahedral Co environments at the surface, (ii) induces measurable polyhedral distortions with out perturbing the bulk spinel framework, and (iii) reorganizes Co-O electronic states via Co–O–Ce hybridization. This confluence of structural and electronic effects provides the foundation for the diversified and metal-centered active-site landscape observed during CER in 3D Ce-Co3O4.

Electrochemical CER performance

We assessed CER activity of 3D Ce-Co3O4, 3D Co3O4, and commercial RuO2 in 4.0 M NaCl (pH = 2) using steady state linear sweep voltammetry (LSV). 3D Ce-Co3O4 delivered the optimal performance, exhibiting an onset potential as low as 1.398 V vs RHE (Supplementary Fig. 23) and rapidly rising current density with increasing potential (Fig. 3a). The overpotentials to reach 10 and 100 mA cm–2 are only 44 and 115 mV, respectively (Fig. 3b), outperforming 3D Co3O4 (59 and 153 mV) and RuO2 (52 and 204 mV). At high current densities, 3D Ce-Co3O4 requires only 228 mV overpotential to reach 1000 mA cm–2, whereas 3D Co3O4 and RuO2 do not achieve this current density within the same potential window. We attributed this performance to 3D porous ordered porous architecture to some extent, which promotes rapid mass transport and gas disengagement, together with Ce-induced optimization of interfacial adsorption states. In stark contrast, Ce-Co3O4 synthesized without using a porous ordered PS template exhibits a lower CER current density (Supplementary Fig. 24). In addition, we further examined the influence of Ce content on CER performance, and detailed structural and electrochemical analyses are provided in Supplementary Fig. 25. Tafel slope analysis (Fig. 3c) is consistent with more favorable CER kinetics on 3D Ce-Co3O4 (58.2 mV dec–1).

The CER selectivity was quantified by rotating ring-disk electrode (RRDE) measurement at low currents, yielding 99.1% for 3D Ce-Co3O4, exceeding 3D Co3O4 (94.6%) and RuO2 (92.5%) (Fig. 3d and Supplementary Fig. 26)56. To probe CER selectivity under higher current densities, we employed in situ differential electrochemical mass spectrometry (DEMS) using a probe-cell configuration (Supplementary Fig. 27)57. At 100 mA, 3D Ce-Co3O4 produce clear Cl2 signals at m/z = 70 (35Cl35Cl), 72(35Cl37Cl), and 74(37Cl37Cl) with negligible O2 detected at m/z = 32 (Fig. 3e). In contrast, RuO2 generated a discernible O2 signal under comparable conditions(Supplementary Fig. 28), consistent with the RRDE results. In addition, DEMS measurement of 3D Ce-Co3O4 under LSV testing condition revealed Cl2 signal increases progressively with potential and follows the same trend as the anodic current, whereas the O2 signal does not increase; instead, it shows a slight decrease, which we attribute to displacement of pre-dissolved O2 in the electrolyte by the rapidly generated Cl2 (Supplementary Fig. 29). Furthermore, the onset potential of the Cl2 signal (~1.416 V) is very close to the onset of the electrochemical current (~1.398 V), further confirming the high CER selectivity of 3D Ce-Co3O4.

Catalyst CER durability was evaluated in a flow cell with the electrolyte maintained at 30°C. 3D Ce-Co3O4 operated stably for 600 h at 10 mA cm–2 (Fig. 3f), and sustained 100 mA cm–2 for 100 h without degradation (Supplementary Fig. 30). ICP-AES analysis during the 600-h test quantified cumulative Co dissolution at 1.47% (Fig. 3f and Supplementary Table 4). By comparison, pristine Co3O4 exhibited pronounced performance decay after ~26 h at 10 mA cm⁻2, accompanied by higher Co leaching (Supplementary Fig. 31 and Supplementary Table 5). A survey of recent CER reports (Supplementary Fig. 32 and Supplementary Table 6) highlights that 3D Ce-Co3O4 combines lower overpotential with markedly improved stability. It is competitive among the currently reported Co-based and even noble metal-based catalysts.

Mechanistic Study of CER

To identify the origin of the enhanced CER activity and elucidate the reaction pathway of 3D Ce-Co3O4, we first performed in situ Raman spectra (Supplementary Fig. 33) on 3D Ce-Co3O4, pristine 3D Co3O4, and other control samples over the CER potential window (1.2 to 1.6 V vs RHE). As shown in Fig. 4a and e, both catalysts display the five characteristic Co3O4 modes (A1g, Eg, and F2g) at the open-circuit potential (OCP)38,42,58. Remarkably, in 4 M Cl⁻, 3D Ce-Co3O4 develop a clear Co-Cl doublet at ~366.5 and ~410.3 cm–1 once the potential reaches 1.55 V vs RHE. Chloride-dependence control experiments confirm this assignment: no Co-Cl features appear in 0 M Cl⁻ (Fig. 4a, lower) and only weak Co-Cl bands are observed in 1 M Cl⁻ (Supplementary Fig. 34). This trend tracks the CER activity rise at higher [Cl⁻] (Supplementary Fig. 35). Additionally, the CeO2 control sample exhibits only the Ce-O vibration mode at 466.8 cm–1 without potential-induced new bands (Supplementary Fig. 36), excluding Ce-oxide artifacts in 3D Ce-Co3O4. By contrast, Co3O4 shows no Co-Cl vibrational bands in 4 M, 1 M, and 0 M Cl⁻ over CER potential window (Fig. 4e and Supplementary Figs. 37 and 38). Collectively, these in situ Raman results imply that Ce incorporation reroutes the adsorption pathway in 3D Ce-Co3O4 during the CER towards a Co-site, metal centered mechanism.

We next correlated adsorption behavior with surface polyhedral evolution using intensity ratios. In spinel, A1g reflects octahedral Co–O stretching, where F2g reports tetrahedral units46. The A1g/F2g ratio thus serves as a proxy for octahedral/tetrahedral population and surface reconstruction state59, allowing us to distinguish three regimes: original (<1.2 V), reconstruction (1.2-1.5 V), and CER process (>1.5 V). For 3D Ce-Co3O4 (Fig. 4b), A1g/F2g ratio remains nearly constant at low bias, increases during reconstruction, consistent with tet → oct conversion, and then decreases in the CER regime as Co-Cl bonds form, which attenuates the A1g intensity. The decline coincides with a rise in ICo-Cl/A1g ratio when the voltage was greater than 1.5 V, confirming metal-site Cl adsorption. For 3D Co3O4 catalyst (Fig. 4f), A1g/F2g ratio is essentially constant before gradually decreasing, indicating limited reconstruction and subsequent spectral damping from surface chlorine adsorption without Co-Cl bond formation. Together, the Raman data point to metal-site (Co) adsorption on 3D Ce-Co3O4, versus oxygen-site adsorption on 3D Co3O4 (schematized in Fig. 4c, g).

Because Raman spectroscopy is relatively insensitive to O-Cl vibrations, we employed in situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR–SEIRAS), which is highly responsive to surface functional groups, to search for O-centered intermediates27,28. For 3D Co3O4, an O-Cl band emerges at ~775.3 cm–1 with increasing potential (Fig. 4h-left). After 18O substitution of lattice oxygen, this band red-shifts to 736.7 cm-1 (Fig. 4h-right), validating its O-Cl assignment27,28. Additionally, the CeO2 control sample exhibits only the ClO4- vibration mode at 1012.2 cm-1 without potential-induced new bands (Supplementary Figs. 39 and 40), excluding Ce-oxide artifacts. We further verified the accuracy of in situ ATR–SEIRAS through DEMS (Supplementary Fig. 41). The catalysts labeled with 18O reacted in a 0.1 M HClO4 containing 16O, and both types of catalysts produced a certain amount of 18O in the product, indicating the successful introduction of the isotope (Supplementary Figs. 42 and 43). The 18O in lattice oxygen adsorbs Cl, causing a redshift in its O-Cl vibration peak (Supplementary Fig. 44), which is consistent with the conclusions of the above experiments60. Consistently, the 3D Co3O4 CV curve features a reduction peak at 1.20 V vs RHE (Supplementary Fig. 45), attributable to oxygen-mediated Cl2 reduction, i.e., an oxygen-site pathway28,29. By contrast, 3D Ce-Co3O4 shows a cathodic peak at 1.35 V vs RHE, close to the theoretical redox potential of Cl2/Cl⁻, indicating that the Co active sites directly undergo the transformation from Cl2 to Cl⁻. We also observe a transient O-Cl signal for 3D Ce-Co3O4 test (Fig. 4d-left). The peak position redshifts from 748.3 to 732.8 cm–1 after 18O substitution, and replace the intensities of the front and rear peaks both weaken and disappear by ~1.7 V. We attribute this to the high steady-state coverage of surface oxygen reported under CER conditions61, which initially permits some O-Cl formation. Upon completion of surface reconstruction, abundant unsaturated octahedra [CoO6-x] form, which favors direct Co-Cl adsorption, suppressing O-Cl signatures—consistent with the rise-then-fall of A1g in Raman spectra of 3D Ce-Co3O4. Notably, as shown in Fig. 4d, 3D Ce-Co3O4 exhibits a broad OH band around ~1600 cm–1 at low bias of 1.10 V (RHE) (relative to 1.60 V for 3D Co3O4), consistent its stronger water adsorption on Ce sites (Supplementary Fig. 46), indicating that the catalyst was more likely to bind O in water and undergo tetrahedral to unsaturated octahedral reconstitution.

Overall, in situ data establish two complementary roles for Ce. First, the oxygen affinity of Ce promotes water adsorption and O atoms delivery to the surface, populating unsaturated octahedra that act as high-activity Co sites62. Second, Ce-induced polyhedral distortion63,64, may also cause the catalyst to expose its metallic active sites and stabilize metal-centered adsorption pathways. Consequently, CER on 3D Ce-Co3O4 predominantly proceeds at Co sites, which are far less susceptible to oxidative etching than lattice oxygen, thereby suppressing structural degradation and underpinning the robust stability relative to Co3O4.

To discriminate adsorption pathways on 3D Co3O4 versus the Ce-modified surface that represents 3D Ce-Co3O4, we performed DFT calculations of CER. Guided by experimental results, we constructed a Co3O4 (001) slab (Supplementary Tables 7 and 8) with saturated top-oxygen termination19,61, and substituted a Co with Ce atoms (Fig. 5a and Supplementary Fig. 47). CER proceeds via a Volmer step (*/O* + 2Cl− → Cl*/ClO* + Cl− + e−) and a subsequent Heyrovsky step (Cl*/ClO* + Cl− → */O* + Cl2 + e−), which both require direct chloride absorption at the active site65. We surveyed candidate adsorption sites intrinsic to the spinel Co3O4: metal Co sites, on-top oxygen (Oot) at the tetrahedral (Cotet-Oot), octahedral (Cooct-Oot) vertex, and the bridging oxygen (Obri) connecting tetrahedral and octahedral sites. On pristine Co3O4, Cl adsorption is most favorable at Obri (Cotet-Obri-Cooct) with ΔGads-Cl = -0.23 eV 3,26 (Fig. 5b and Supplementary Fig. 48). It is worth noting that when Cl is initially placed on a Co site, it spontaneously relaxes to adjacent oxygen, indicating that oxygen rather than cobalt serve as active sites on Co3O4, consistent with our in situ ATR-SEIRAS assignment of O-Cl intermediates (Fig. 4h).

Ce substitution fundamentally reshapes the local geometry and electronics. In the Ce-modified slab, the O-Co-O angle of the tetrahedron opens from 135.6° to 144.3°, creating a quasi-square distortion (Fig. 5a, right) that can host a coordinatively unsaturated Cocus site. Free-energy analyses at UC = 0 and 1.36 V (Supplementary Fig. 49) show that Obri becomes less hospitable to Cl, for instance, the ΔG of Cl at (Ce-Obri-Cooct) increases to +0.77eV (Fig. 5b). Notably, an unsaturated Co site emerges as optimal with ΔGads-Cl = +0.12 eV (Fig. 5b and Supplementary Fig. 50). The projected density of states (PDOS) for representative Obri sites (Fig. 5c) reveal a downward shift in the p orbital energy level away from Fermi level (EF) after Ce substitution, weakening Cl binding at oxygen and steering adsorption toward cobalt, consistent with in situ Raman detection of Co-Cl (Fig. 4a-c).

Because Raman indicates an increased coordination number at distorted polyhedron Co sites, we further modeled the active center as a square-pyramidal [CoO5] unit (Fig. 5d and Supplementary Figs. 51 and 52). In this configuration, Ce substitution lowers the Cl adsorption Gibbs free energy from +0.59 eV (pristine [CoO5]) to +0.12 eV (Ce-incorporated Ce-[CoO5]) (Fig. 5d). This decrease is supported by the upward shift in the energy level of the Co d-orbital from -1.54 eV to -1.39 eV following Ce doping, as shown in Fig. 5e. The increased delocalization of electron states around Co near the EF (-0.3 to 0 eV, Fig. 5f) after Ce substitution promotes greater orbital overlap with Cl 2p lone pair, thereby facilitating Co-Cl bond formation—consistent with the experimentally observed metal-centered pathway.

To evaluate the CER selectivity of the catalyst, the Gibbs free energy of the key intermediates (*OH, *O, and *OOH) in the OER process was calculated. On the Ce-modified surface, formation of *O at the Co site (*OH → *SO + H+ + e−) is spontaneous, reflecting the transition from its tetrahedral to unsaturated octahedral form (Fig. 5g and Supplementary Figs. 53 and 54). Crucially, the *OOH formation (*O + H2O → *OOH + H+ + e−) remains energetically unfavorable (1.77 eV). For other possible OER active sites, the calculated free energies of corresponding rate-determining step are 2.23 eV (Cooct), 2.54 eV (Cotet), and 2.41 eV (Ce), also rendering OER thermodynamically disfavored relative to CER. These trends explain the near-unity Cl2 selectivity of 3D Ce-Co3O4 quantified by RRDE and DEMS. Therefore, DFT results support that Ce substitution converts an oxygen-centered CER on 3D Co3O4 into a Co-centered process by (i) geometrically generating [CoO5]/Cocus sites through polyhedral opening, and (ii) electronically promoting an upward shift of the d orbital energy level of Co sites to optimize Cl adsorption (ΔGads = +0.12 eV). This dual structural-electronic tuning underpins both the high activity and the suppression of oxygen-mediated degradation observed for 3D Ce-Co3O4.

Practical CER performance in a chlor-alkali electrolyser

Motivated by the strong three-electrode performance, we evaluated 3D Ce-Co3O4 in a practical chlor-alkali configuration (Fig. 6a). The anolyte was saturated NaCl adjusted to pH = 1, and the catholyte was 10 wt% NaOH,66,67 producing Cl2 at the anode and H2/NaOH at the cathode67. The anode consisted of the catalyst (3D Ce-Co3O4 or 3D-Co3O4) spray-coated onto a Pt-coated Ti felt or commercial DSA electrode (Supplementary Figs. 55 and 56), while Pt-coated Ti mesh served as the cathode, with a commercial cation exchange membrane separating the compartments. It is noteworthy that, even without the use of a membrane electrode structure, the resistance of the electrolytic cell remained very low, with a resistance of only 581.5 mΩ measured at the OCP (Fig. 6b). Uncompensated I–V curves of electrolyzers at room temperature in Fig. 6c show that 3D Ce-Co3O4 outperforms both 3D Co3O4 and DSA across the measured range. The thermodynamic cell voltage for chlor-alkali electrolysis is ~2.20 V; the 3D Ce-Co3O4 cell required only 2.64, 2.97, 3.28 and 3.61 V to reach a current density of 1.25, 2.50, 3.75 and 5.00 kA m−2, respectively (Fig. 6c, d). By contrast, 3D Co3O4 and DSA demanded higher voltages to attain the same current densities, underscoring the kinetic and transport advantages conferred by the 3D ordered porous architecture and Ce-induced interfacial adsorption optimization.

In such an acidic and oxidizing environment, non-noble electrodes rarely sustain long-term operation of chlor-alkali electrolyser, so stability is decisive.2 Fig. 6e shows that 3D Ce-Co3O4 operated continuously for 550 h at 2.50 kA m–2, with only a 103 mV increase in cell voltage (degradation 0.19 mV h–1) and retention of high performance after the run (Supplementary Fig. 57). By contrast, undoped 3D-Co3O4 lost activity within 45 h (Supplementary Fig. 58). At a current density of 5.00 kA m–2, 3D Ce-Co3O4 remained stable for 150 h (Supplementary Fig. 59). In addition, we conducted performance and stability tests of 3D Ce-Co3O4 anode in the electrolyser at 60°C (Supplementary Figs. 60 and 61), a typical temperature for industrial chlor-alkali processes. The cell requires only 3.35 V to reach 5.00 kA·m–2 and maintains stable operation for 300 h under these conditions. Among reported Co-based CER catalysts, this level of durability of 3D Ce-Co3O4 is competitive.

Discussion

We have developed a non-noble, 3D Ce-Co3O4 chlorine-evolution catalyst that couples ultralow overpotentials (e.g., 44 mV at 10 mA cm–2) with ~99.1% Cl2 selectivity and robust durability in a practical chlor-alkali cell (≥550 h at 2.50 kA cm–2). In situ Raman, ATR-SEIRAS and DEMS, together with DFT, reveal that Ce occupies octahedral Co sites, inducing polyhedral distortion, and generating unsaturated Co centers that directly adsorb Cl⁻. This metal-site pathway optimizes Cl binding free energy, suppresses lattice-oxygen participation (and hence oxygen-mediated corrosion), and preserves structural integrity under high current densities. The resulting active-site relocation from O-centered to Co-centered CER-provides a generalizable design principle for engineering stable, high-current-density anodes for the chlor-alkali process beyond noble metals.

Methods
Chemicals

Cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O), 2-methylimidazole, Cerium (II) nitrate hexahydrate (Ce(NO3)2·6H2O), sodium perchlorate monohydrate (NaClO4), sodium hydroxide (NaOH), styrene, polyvinylpyrrolidone (PVP), sodium chloride (NaCl), perchloric acid (HClO4), and hydrochloric acid (HCl) were purchased from Aladdin. Nafion solution (5 wt%) was purchased from Sigma-Aldrich. In all experiments, ultrapure deionized water (18.2 MΩ cm–1) was used to prepare electrolytes. All chemicals were used directly without further purification.

Catalysts synthesis

Synthesis of 3D ordered polystyrene (PS) template.

The PS template was prepared following the reported work35. In brief, 30 mL styrene was washed sequentially with 10 wt% NaOH and deionized (DI) water to remove inhibitor. The purified monomer was combined with 250 mL DI water and 0.3 g PVP in a round-bottom flask and stirred until homogeneous. After preheating under N2 at 75°C for 15 min, an aqueous initiator solution (25 mL, 0.1 g K2S2O8) was added. Polymerization proceeded under N₂ at 75°C and 450 rpm for 24 h to yield a monodisperse PS latex. The emulsion was centrifuged at 9000 rpm for 45 min, and the precipitate was vacuum-dried at 70°C overnight to form a close-packed, 3D-ordered PS opal template.

Synthesis of 3D Ce-Co3O4

A precursor solution was prepared by dissolving a certain amount cerium nitrate, 2.0 g cobalt nitrate, and 5.0 g 2-methylimidazole in 12 g methanol to form a homogeneous mixture. The 3D-ordered PS template was immersed in the precursor for 15 min, followed by vacuum degassing for 40 min to ensure complete infiltration of the PS pores. The infiltrated template was dried at 60°C, then aged in a methanol:ammonia (1:1 v/v) solution for 24 h to induce 3D Ce-ZIF-67@PS formation. The composite was calcined in Ar at 350°C (1 h) and 650°C (2 h) to obtain the carbide 3D Ce-Co@NC, then transferred to a muffle furnace and calcined in air at 350°C (2 h) to yield 3D Ce-Co3O4. Unless otherwise specified, the heating rate was 2°C min–1.

Synthesis of 3D Co3O4

3D Co3O4 was synthesized identically to 3D Ce-Co3O4, except that cerium nitrate was omitted from the precursor.

Synthesis of Ce-Co3O4

A Ce-ZIF-67 precursor was prepared by dissolving 0.48 mg Ce(NO3)3, 30 mg Co(NO3)2·6H2O and 0.5 g 2-methylimidazole in 10 mL water/methanol (1:1 v/v). After 24 h at room temperature, the Ce-ZIF-67 was collected by centrifugation and washed three times with water and methanol, then dried overnight at 50°C. The powder was calcined in air at 350°C for 2 h (heating rate 5 °C min⁻1) to obtain Ce-Co3O4.

Characterizations

Powder X-ray diffraction (XRD) patterns were tested on a Philips X-Pert Pro X-ray diffractometer with Cu Kα radiation (1.5418 Å). The scanning electron microscope (SEM) images were acquired on a Hitachi SU8020. Transmission electron microscopy (TEM) images were collected on a JEM-ARM 200F microscope. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was performed on a JEOL-NEOARM at 200 kV; elemental analysis used an Oxford Instruments EDX detector. X-ray photoelectron spectroscopy (XPS) was carried out on a ESCALAB 250 X-ray photoelectron spectrometer (Thermo, USA) equipped with Al Kα1,2 radiation at 486.6 eV X-ray source. Fourier transform-infrared (FTIR) measurements were conducted using a Nicolet Nexus FTIR spectrometer. The synchrotron-based XANES and EXAFS measurements were performed at Shanghai Synchrotron Radiation Facility (SSRF). The acquired EXAFS data were extracted and processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages. Inductively coupled plasma atomic emission spectrometry (ICP-AES) results were obtained on an ICP6300 (ThermoFisher Scientific, USA). The magnetic properties of the samples were tested by superconducting quantum interference device (SQUID) magnetometer (MPMS XL5, Quantum Design, America). M-H measurements were carried out at room temperature in the field range of -10 kOe to 10 kOe. M-T measurements were performed under H = 500 Oe in a field-cooling (FC) protocol.

Electrochemical tests

Electrochemical tests were performed on a CHI760E workstation (Shanghai Chenhua) using a three-electrode configuration in 4.0 M NaCl (pH = 2, adjusted with HClO4). A glassy carbon electrode (GCE, 3 mm diameter) coated with catalyst served as the working electrode, Pt mesh as the counter electrode, and Ag/AgCl (sat. KCl) as the reference. Catalyst inks are prepared by ultrasonication for 1 h. 4 mg catalyst and carbon black (Vulcan XC-72, Cabot) mixed in ethanol/DI water/Nafion (5 wt%) with volumes 850 μL/100 μL/50 μL, yielding a homogeneous suspension. Unless noted, a 1:1 mass ratio of catalyst: carbon black was used. An aliquot of 6 μL ink was drop-cast on the GCE and dried to give a loading of ~0.35 mg cm–2.

Polarization curves were collected by linear sweep voltammetry (LSV) from 1.2 to 2.2 V vs. RHE at 5 mV s–1. All LSVs were corrected with 95% iR compensation. Tafel slopes were obtained from linear regions of the LSV-derived overpotential vs log (current density) plots. Chronoamperometry was used to assess stability at fixed potentials or currents, as specified in the main text. Potentials were converted to the reversible hydrogen electrode (RHE) scale using:

E (vs RHE) = E (vs Ag/AgCl) + 0.059 V × pH + 0.197 V (1)

The equilibrium potential for CER (ECER) was evaluated with the Nernst relation (activities α used).

ECER = E0CER - (RT/F)lnα(Cl-) + (RT/2F)lnα(Cl2) + (RTln10/F)pH (2)

Where T, R, and F are the temperature (K), gas constant (8.314 J K−1 mol−1), and Faraday constant (96,485.3 C mol−1), respectively. E0 = 1.358 V (vs SHE). Using α(Cl2) = 0.01, and α(Cl⁻) = 4 under our experimental yield EθCER =1.383 V vs RHE27,68.

The Cl2 selectivity was measured by RRDE methods69. For RRDE measurements, the disk potential was set to give 10 mA cm–2 for 500 s at 1600 rpm, while the ring was held at 0.95 V to detect Cl2. Selectivity was computed as:

Cl2 selectivity(%)=100× (2×|ir|/N) / (id + |ir|/N) (3)

where ir, N and id denote the ring current, collection efficiency (0.37) and disk current.

In situ Raman measurements

In situ Raman spectra were collected on a HORIBA Xplora Plus confocal Raman microscope. Catalyst ink was drop-cast onto a custom glassy-carbon electrode. A quartz-window flow cell maintained a micron-scale electrode window gap to reduce laser scattering. Electrolyte was circulated at 20 mL min–1 by peristaltic pump. The excitation wavelength was 532 nm; laser power was set to 3.5%; a 50× objective was used. Raman shifts were calibrated to the Si band (520.7 cm–1) before each experiment. Spectra were acquired with 20 s integration and 5 accumulations. Each sample was tested independently three times to provide data with error bars.

In situ ATR-SEIRAS measurements

In situ ATR-SEIRAS measurements were carried out at the infrared beamline BL01B of National Synchrotron Radiation Laboratory (Hefei) through a homemade cell reflection IR setup with a CaF2 crystal as the infrared transmission window. Each infrared absorption spectrum was acquired by averaging 128 scans at are solution of 2 cm−1.

In situ differential electrochemical mass spectrometry (SDEMS) measurements.

DEMS measurements were performed on a differential electrochemical mass spectrometer (Linglu Instruments, Shanghai) in two configurations. For selectivity tests, a probe-cell setup was used; the electrolyte was Ar-purged to remove dissolved O2, and the current was held at 100 mA while monitoring m/z = 70, 72, and 74 (Cl2 isotopologues) and m/z = 32 (O2) (minor m/z = 34 and 36 were not quantified due to low intensities). For isotope-labeling experiments, a classic cell with a porous PTFE separator was employed; gases evolved from 18O-labeled catalysts in 0.1 M HClO4 were collected during constant-current operation at 5 mA.

Chlor-alkali electrolyser tests

A corrosion-resistant Pt-coated Ti felt served as the anode substrate; a dense catalyst layer was deposited by spraying the catalyst ink (geometric area 4 cm2, loading 2 mg cm–2). The cathode was a commercial Pt-coated Ti mesh. A cation-exchange membrane (SIN-D117, 260 μm) separated anolyte and catholyte. The cation exchange membrane is normally stored in a 0.5 M NaCl solution and requires no treatment before use. The anolyte (saturated NaCl, pH = 1) and catholyte (10 wt% NaOH) were delivered at 100 mL min–1 using peristaltic pumps. Electrochemical impedance spectroscopy (EIS) measurement is performed under open-circuit potential (OCP) conditions at frequencies ranging from 0.01 to 100,000 Hz on a CHI760E workstation (Shanghai Chenhua). Electrolyser performance tests are conducted on a DC power supply (SPPS-C3010), with three independent measurements taken to obtain data with error bars. All data are not iR corrected.

Computational method

Spin-polarized density functional theory (DFT) calculations were performed using the Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation (GGA)71, as implemented in the Vienna Ab initio Simulation Package (VASP)71. The projector augmented wave (PAW)72 method was employed with a plane-wave cutoff energy of 450 eV. A vacuum layer of ~ 11 Å was introduced along the z-direction in all slab models. Atomic positions were fully relaxed until the Hellmann-Feynman forces on each atom were below 0.01 eV/Å, with an energy convergence threshold of 1×10–6 eV. The CoOx surface model, constructed based on previous studies19, consisting of about six Co and O layers. consisted of approximately six atomic layers of Co and O. Surface Co atoms exhibited mixed tetrahedral and octahedral coordination environments. For Ce-doped CoOx, one octahedral Co site was replaced by Ce, consistent with experimental observations.

The DFT-D3 dispersion correction method by Grimme was included to properly account for van der Waals interactions73. During geometry optimization, the bottom three atomic layers were fixed while the remaining atoms were allowed to relax. Notably, we examined the effect of Hubbard U correction on the structure. With U correction, the calculated lattice parameters for bulk Co3O4 (a = b = 8.52 Å, c = 8.05 Å) showed significant deviation from experimental values (a = b = c = 8.08 Å). Without U correction, the lattice constants (a = b = c = 8.05 Å) agreed well with experiments. Therefore, no U correction was applied to Co or Ce atoms in our calculations49,74. Due to computational constraints from large unit cells, only the Γ-point was used for Brillouin zone sampling.

The chlorine adsorption energy (Eabs) was calculated as:

Eabs = ECl@cat. - Ecat - ECl2/2 (4)

where ECl@cat. is the total energy of the chlorine-adsorbed catalyst surface, Ecat is the clean surface energy, and ECl2 is half the energy of a gas-phase Cl molecule. The adsorption Gibbs free energy (ΔGCl) at standard conditions was approximated as75.

ΔGCl = Eabs + 0.365 eV (5)

The thermodynamic overpotential for chlorine evolution reaction (CER) was defined as:

ηCER = | ΔGCl/e + 1.36 V| (6)

For the oxygen evolution reaction (OER), the reaction free energies were calculated as:

ΔG1 = ΔG(*OH) - ΔG(*) (7)

ΔG2 = ΔG(*O) - ΔG(*OH) (8)

ΔG3 = ΔG(*OOH) - ΔG(*O) (9)

ΔG4 = 4.92 eV - ΔG(*OOH) (10)

Each free energy included zero-point energy (ZPE) and entropy corrections at 298.15 K:

ΔGi = ΔEi + ΔZPEi - TΔSi (11)

where ΔEi represents energy differences calculated with respect to H2O and H2, and ΔSi denotes vibrational entropy changes.

The OER thermodynamic overpotential was calculated as:

ηOER = max (ΔG1, ΔG2, ΔG3, ΔG4)/e – 1.23V (12)

Supplementrary Data 1 provides the atomic coordinates of the initial and final configurations of all the structures.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript reports atomically dispersed Ce in spinel Co3O4 with a three-dimensional ordered macroporous nanostructure (3D Ce-Co3O4). The 3D Ce-Co3O4 catalyst superior activity (44 mV @ 10 mA cm−2) and stability (550 h @ 2.5 kA m–2) for the chlorine evolution reaction (CER), compared to undoped 3D Co3O4. The enhanced CER performance is attributed to atomically dispersed Ce, which induces polyhedral distortion and creates unsaturated Co centers that directly adsorb Cl−. However, despite the promising CER activity and stability, weaknesses in the structural analyses and numerous typos undermine this work. Hence, this reviewer cannot recommend the publication of this paper in Nat. Commun.\n1. EXAFS and XANES results have several issues.\n1) The FT-EXAFS spectra of 3D Ce-Co3O4 shown in Fig. 2 and Supplementary Fig. 18 appear inconsistent. They should present the same dataset.\n2) In Supplementary Table 2, coordination numbers (CN) of 3D Co3O4 and 3D Ce-Co3O4 are missing. While the authors mentioned that the CN of the Co foil was fixed during fitting, the CNs for other samples should also be provided.\n3) The discussion on the Ce oxidation state based on the Ce L3-edge XANES is unclear. What is the criterion for determining the oxidation state? Absorption edge or white line intensity? In addition, regardless of the chosen criterion, appropriate reference samples with similar structural environments should be used, since the XANES spectra can vary with the local structure (J. Am. Chem. Soc. 2025, 147, 36266.).\n2. The Co 2p XPS spectrum of 3D Ce-Co3O4 exhibited a negative shift compared with that of 3D Co3O4, indicating a reduction of Co species, whereas the increased Co3+/Co2+ ratio and Co K-edge XANES spectra indicate overall oxidation of Co. The authors should clarify these seemingly contradictory results.\n3. Supplementary Table 1 listed three different catalysts with differing contents. However, other relevant data, such as structural characterization or CER performance, are not provided. The authors should discuss how the variation in Ce content affects the catalysts’ structure and activity.\n4. All TGA measurements were performed under Ar gas, while the synthesis involves calcination in air. It would be helpful if the authors could provide additional TGA data under air and perform elemental analysis to confirm complete carbon removal.\n5. Providing the Ce L3-edge EXAFS fitting results would clarify the structural assignment and strengthen the claim of octahedral Ce substitution.\n6. While the pore-size distribution of the non-templated sample was presented, that of 3D Ce-Co3O4 was not provided.\n7. The impact of 3D structuring on the CER activity and selectivity should be provided; While the CER activity and selectivity of 3D colloid-templated Ce-Co3O4 and Co3O4 were measured, no data for untemplated samples were provided.\n8. There are numerous typos throughout the manuscript. For instance,\n1) In the Methods section, the synthesis of “3D Co3O4” is mislabeled as “3D Ce-Co3O4”.\n2) In page 8, Fig. 2c should be corrected to Fig. 2d.\n3) The term “KA” is repeatedly used instead of “kA”.\n4) Random capital letters appear within the main text.\n5) The capitalization style in the reference titles is inconsistent.\nThe manuscript should be thoroughly edited by an expert.", "reviewer_2": "This manuscript reports a Ce-modified spinel Co3O4 catalyst that delivers durable chlorine evolution in acidic media. The introduction of Ce drives a relocation of the active centers toward metal sites, thereby mitigating the instability associated with lattice oxygen participation. A series of in situ spectroscopies traces the evolution of the active centers, revealing that metal-site modification significantly enhances the catalyst’s robustness. Impressively, the catalyst maintains stable performance for more than 550 hours at an industrially relevant current density of 2.5 kA m-2. To the best of my knowledge, this represents the most stable non-precious metal CER catalyst reported so far and shows strong potential for long-term sustainable chlor-alkali operation. Given the corrosive and strongly acidic environment of CER, where many catalysts deactivate or dissolve, and industry often relies on Ru and Ir, this Co-based strategy is very timely, innovative and important. The study is thoughtfully designed and the DFT analysis strengthens the mechanistic picture. Beyond chlor-alkali electrolysis, the concept may also inspire seawater electrolysis integrated with clean power sources.\nI recommend its publication in Nature Communications after the authors address the points below.\n1. Chlor alkali plants typically operate at temperatures of 60 °C or higher, where the corrosivity of chlorine rises steeply. Performance and durability tests under such elevated temperatures would more accurately reflect practical viability.\n2. Ce incorporation likely introduces lattice distortion. Please examine whether oxygen vacancies are generated in Co3O4 and whether the reported exposure of Co sites correlates with vacancy formation, as this could contribute to the CER enhancement.\n3. In the Raman spectra in Figure 4, a band near 920 cm-1 appears in the electrolyte without chloride but disappears after adding chloride for both catalysts. Please provide an assignment for this band and a rationale for its disappearance.\n4. In the infrared data, the O-Cl vibration shows a sizable shift between the two materials. Given the DEMS results indicating limited 18O exchange (3.1% for Co3O4 and 0.6% for Ce-Co3O4), please discuss how such modest exchange could drive a large spectral shift. Could transient reconstruction in the electrolyte containing 18O contribute to this behavior?\n5. For the magnetic measurements in Supplementary Figs. 20 and 21 used to infer Co3+ to Co2+ ratios via spin states. Please provide full testing conditions and discuss how they influence the extracted parameters. It would also help to consider the possible spin configurations of Co3+ in Co3O4 and their implications for the analysis.\n6. For full cell electrolyzer tests, please include chlorine selectivity under operating conditions. Local environments can differ from three-electrode setups, and OH- back diffusion may trigger parasitic oxygen evolution.\n7. In the DEMS selectivity measurements, reporting gas signal evolution as a function of potential would help compare CER and OER onset behaviors between catalysts.\n8. In the DFT section, please clarify model selection, including the choice of the 001 surface and provide key structural parameters so readers can assess generality and reproducibility.\n9. Rare-earth dopants possess unique 4f characteristics. A short discussion on whether other rare earth elements beyond Ce might also promote CER on Co3O4 would broaden the impact.\n10. Some recently published related articles are suggested to be cited, for example, Nat Commun 14, 5356 (2023); Nature 617, 519–523 (2023); Electrochem. Energy Rev. 7, 19 (2024); Electrochem. Energy Rev. 2024, 7, 10; Nano Energy, 2024, 127, 109769; Angew. Chem. Int. Ed. 2025, 64 (5), e202414021; Nano Energy, 2024, 124, 109428; etc.\n11. The manuscript suggests water participates in reconstruction. In the in situ infrared data, the water-related band shows a trend change before and after isotopic labeling. Please explain this behavior. Also, while calculations indicate that Ce sites adsorb water, this alone does not pinpoint the oxygen source during reconstruction. Additional evidence, such as isotope-resolved operando spectra or surface oxygen accounting, would solidify the claim.\nOverall, this is an important and well-executed study that advances stable PGM lean CER catalysis. Addressing the above points will further reinforce its rigor and practical relevance."}, "author_response": "We sincerely thank you for the opportunity to revise our manuscript, and we are grateful for the valuable comments and suggestions provided by the reviewers. We believe that the revisions made in response to these professional opinions have substantially improved the quality and clarity of the paper. Below, we provide our responses and detailed explanations addressing each of the reviewers’ comments. All revisions made in the manuscript have been highlighted in yellow.\nResponses to Reviewers:\nTo Reviewer #1: ……………………………………………….…….……..............2-19\nTo Reviewer #2: ……………………………………………….…….……............20-37\nList of Revisions……………………………………...…….…….……….……….38-48"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have thoroughly revised the manuscript in response to the reviewers’ comments, leading to a substantial improvement in quality. Overall, the revised manuscript appears suitable for publication in Nature Communications, provided that the following points are adequately addressed.\n1. On page 10, the authors state: “High-resolution transmission electron microscopy (HRTEM) images (Supplementary Fig. 10) of 3D Ce–Co₃O₄ reveal a slight contraction of the (311) lattice spacing from 0.244 to 0.236 nm compared to 3D Co₃O₄, attributable to lattice distortion induced by Ce incorporation.” However, Supplementary Fig. 10b indicates that the (311) lattice spacing of 3D Ce–Co₃O₄ is 0.246 nm. Please clarify the correct (311) lattice spacings of 3D Co₃O₄ and 3D Ce–Co₃O₄, and reconcile the apparent inconsistency between the text and the figure.\nIn addition, considering that cerium has a larger ionic radius than cobalt, the reported lattice contraction in 3D Ce–Co₃O₄ relative to 3D Co₃O₄ appears counterintuitive. The authors are encouraged to provide a clear explanation or supporting evidence for this observation.\n2. Lines 181–183 state: “The Ce absorption edge energy of 3D Ce–Co₃O₄ is between Ce³⁺ of CeCl₃ and Ce⁴⁺ in CeO₂ and NH₄CeNO₃, indicating a relative decrease in Ce valence state, corroborating net electron withdrawal from Co species.” In contrast, lines 188–189 report that “The Co 2p spectrum of 3D Ce–Co₃O₄ exhibits a small binding energy shift (~0.2 eV) relative to 3D Co₃O₄ (Fig. 2d).”\nThe former statement implies electron transfer from Co to Ce in 3D Ce–Co₃O₄, which would be expected to result in a higher binding energy for the electron-deficient Co species in XPS. However, the reported XPS result suggests the opposite trend.\nPlease clarify and reconcile this apparent discrepancy.\n3. Line 176: “wavelet-transformed of EXAFS” should be revised to “wavelet-transformed EXAFS.”", "reviewer_2": "I am satisfied with the revised manuscript and recommend its acceptance as is."}, "author_response": "We sincerely thank you for the opportunity to revise our manuscript, and we are grateful for the valuable comments and suggestions provided by the reviewers. We believe that the revisions made in response to these professional opinions have substantially improved the quality and clarity of the paper. Below, we provide our responses and detailed explanations addressing each of the reviewers’ comments. All revisions made in the manuscript have been highlighted in yellow.\nResponses to Reviewers:\nTo Reviewer #1: ……………………………………………….…….……....................2-5\nTo Reviewer #2: ……………………………………………….…….…….......................6\nList of Revisions……………………………………...…….…….……….…….....………7"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have addressed the comments well, and the revised manuscript now appears suitable for publication Nature Commun."}, "author_response": ""}]

Decision letter