English

• Hydrogen is not only a carbon-free renewable fuel, but also a crucial raw material for the chemical industry [1–3]. However, current hydrogen production is primarily driven by non-renewable energy sources [4,5]. The electrocatalytic splitting of water holds a fossil-free pathway for hydrogen production, which couples the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) [6–8]. Compared to HER, the kinetically sluggish OER serves as the rate-determining step in the overall water electrolysis [9–11]. Although substantial progress has been made in developing high-efficient electrocatalysts for OER overpotential reduction [12–15], the fundamental kinetic barriers persist. This makes the exploration of alternative kinetically advantaged electrocatalytic oxidation reactions a particularly compelling strategy.

  The chlorine evolution reaction (CER) holds promise as an alternative to the OER, owing to its fast two-electron transfer kinetics and high-value chlorine gas (Cl₂) production [16]. The CER is well-known to proceed through the Volmer-Heyrovský, Volmer-Krishtalik or Volmer-Tafel routes, involving chlorine (Cl) or chlorine-based intermediates (OCl) adsorbed on surface oxygen species [17]. Although RuO₂ and IrO₂ serve as the primary components of commercial CER electrodes, their scarcity and pronounced activity toward the competing OER fundamentally constrain the sustainable deployment for high-selectivity chlorine production [18–20]. Hence, the innovative development of non-precious metal catalysts featuring both suppressed OER activity and optimal Cl intermediate adsorption/desorption properties has emerged as the paramount objective in contemporary CER research.

  In recent years, extensive theoretical calculations and experimental efforts have yielded unprecedented opportunities for developing efficient non-precious CER catalysts [21–24]. In particular, cobalt-based spinel oxides has been verified to be highly attractive electrocatalysts for the CER owing to their tunable physicochemical properties and catalytic performance [25]. It is well known that two types of geometric cobalt sites with different oxidation states exist in cobalt-based spinel oxides: Co²⁺ in tetrahedral sites (Co_Td) and Co³⁺ in octahedral sites (Co_oh). Among them, the Co_Td site is identified as the locus for active cobalt oxyhydroxide (CoOOH) species formation during water oxidation, whereas the Co_oh site demonstrates greater affinity and stronger interaction with chlorine species. As a result, the Co_oh site is generally considered to be the optimal geometric site for CER in cobalt-based spinel oxides [26]. Sodium cobaltate (Na_xCoO₂) consists of alternating stacked layers of CoO₂ and Na⁺ ions, where the CoO₂ layers are composed of edge-sharing CoO₆ octahedra [27,28]. This distinctive structural features endow the Na_xCoO₂ with abundant Co_oh sites on the surface. The Na⁺ intercalation between layers substantially enhances the electrochemical stability [29]. But the intrinsic activity of NaCoO₂ still struggles to keep pace with that of noble metal CER catalysts. Notably, the tunable sodium density (x) in the intercalation layers can directly modulate the electronic structure of surface Co_oh sites, thereby contributing to enhanced electrochemical activity of Na_xCoO₂. However, this critical aspect remains largely unexplored to date.

  Herein, Na_xCoO₂ flakes with varying sodium densities (x = 0.6, 0.7, 0.9) were engineered as a platform for mechanistic studies and performance assessment of CER. It is revealed that the alteration of interlayer sodium density can adjust the d-band center level of surface Co atoms, thereby weakening the strength of Co-Cl bonds. This alteration optimizes interactions with catalytic intermediates, thereby enhancing CER activity. As a result, the Na_xCoO₂, particularly Na_0.7CoO₂, requires only 55.47 mV to achieve 10 mA/cm², significantly outperforming the benchmark RuO₂. Beyond its superior activity, Na_0.7CoO₂ confronts a substantial energy barrier (∆G_max) of 1.95 eV during the OER process, guaranteeing exceptional CER selectivity.

  Fig. 1a illustrates the X-ray diffraction (XRD) patterns of the as-synthesized Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂. The diffraction peaks appear at 16.26°, 32.78°, 36.7°, 40.36°, 44.56°, 49.98°, 63.76° and 66.02°, which match the (002), (004), (100), (102), (103), (104), (106) and (110) facets of the typical P₂-phase hexagonal Na_xCoO₂ (PDF #87–0274). In the Raman spectra of all Na_xCoO₂ samples (Fig. 1b), the observed phonon vibration modes can be classified into three categories: The E_1g mode (O—Co-O twisting vibration) corresponding to the in-plane (ab-plane) vibration of oxygen atoms within the CoO₂ layer, the A_1g mode (Co-O symmetric vibration) associated with the out-of-plane (c-axis) vibration of oxygen atoms in the CoO₂ layer, and the E_2g mode arising from the cooperative vibrations of Na and O atoms. It can be seen that the increase in sodium density enhances Na-O interactions, leading to a redshift of the E_2g mode peaks. Simultaneously, the strengthened Na-O bonding induces varying degrees of O—Co-O distortion, as evidenced by the redshift of the E_1g mode peaks (185, 188, 192 cm^-1). The A_1g mode peaks of Na_0.6CoO₂ and Na_0.7CoO₂ show significant differences, with shifts observed around 670 cm^-1. In Na_0.9CoO₂, the presence of a high concentration of Na⁺ disrupts the symmetry of the Co-O bonds, resulting in enhanced intensity of the A_1g mode peaks and a frequency shift to higher wavenumbers, reaching 684 cm^-1.

  Figure 1

  

  Figure 1.  Phase and chemical states of Na_XCoO₂. (a) XRD spectra and (b) Raman spectra of Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂. XPS spectra of (c) O 1s, (d) Na 1s and (e) Co 2p (f) Co valence ratio of Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂.

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  The surface composition and chemical state of Na_xCoO₂ were analyzed using X-ray photoelectron spectroscopy (XPS). The full survey spectra of Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂ all clearly exhibit the expected elements of Na, Co, and O (Figs. S1a-c in Supporting information). The high-resolution O 1s spectra of all Na_xCoO₂ samples are shown in Fig. 1c, which can be deconvoluted into four components: oxidative oxygen (O₂²⁻/O^-), Co-O bonds, Na-O bonds and the Na KLL auger peaks [30–32]. The results demonstrate that as the sodium content (x) increases, the peak intensity of Na-O bonds enhances, whereas the Co-O bond peak intensity diminishes. The Na 1s spectra show a strong peak centered at the binding energy of 1071.6 eV (Fig. 1d), which can be fitted into four characteristic peaks: Na-OH, Na_e, Na_f, and Na-O-Co [33,34]. Among them, Na_e and Na_f correspond to sodium ions sharing edges or faces with CoO₆ octahedra.

  For the high-resolution Co 2p spectra of all Na_xCoO₂ samples (Fig. 1e), typical spin-orbit doublet comprising Co 2p_3/2 and Co 2p_1/2 were observed [35]. By employing the Shirley background, the Co 2p_3/2 and Co 2p_1/2 can be deconvoluted into contributions from Co²⁺ (797.05 and 782.05 eV), Co³⁺ (779.31 and 794.49 eV) and Co⁴⁺ (780.43 and 795.88 eV), indicating the coexistence of multiple cobalt oxidation states [36]. Table S1 (Supporting information) summarizes the peak intensities of Co²⁺, Co³⁺ and Co⁴⁺. As depicted in Fig. 1f, the Na_0.7CoO₂ sample exhibits the highest relative peak intensity of Co³⁺ (62%), followed sequentially by Na_0.6CoO₂ (57%) and Na_0.9CoO₂ (40%). Referring to the previous report, Co³⁺ species with partially filled d-orbitals demonstrate enhanced activity and stability during CER [37]. From this analytical perspective, Na_0.7CoO₂ shows advantages over both Na_0.6CoO₂ and Na_0.9CoO₂ as CER catalyst.

  The morphology of Na_xCoO₂ with varying sodium content were characterized by scanning electron microscope (SEM) and transmission electron microscopy (TEM). The SEM images depicted in Figs. 2a-c reveal that all the as-synthesized Na_xCoO₂ exhibit a lamellar stacking structure. But their lateral dimensions and stacking thickness vary. Specifically, the stacking thicknesses of Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂ are approximately 1 µm (Fig. S2b in Supporting information), 0.7 µm (Fig. S2d in Supporting information), and 1.2 µm (Fig. 2c), respectively. For the Na_0.6CoO₂, an lamellar with lateral dimension of approximately 1–5 µm was obseverd (Fig. 2a and Fig. S2a in Supporting information). Compared with Na_0.6CoO₂, both Na_0.7CoO₂ and Na_0.9CoO₂ demonstrate more regular hexagonal layered stacking, with lateral dimensions of about 1–3 µm (Fig. 2b and Fig. S2c in Supporting information) and 1–2.7 µm (Fig. 2c and Fig. S2e in Supporting information), respectively. Apparently, the lateral dimensions of Na_xCoO₂ decrease with increasing sodium content. Notably, visible pore structures have formed on the surface of the Na_0.9CoO₂ sample, which may result from surface structure degradation induced by the higher sodium content. The elemental mapping images of Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂ demonstrate the uniformly distribution of Na, Co, and O (Fig. 2d, Figs. S3a and b in Supporting information). The TEM images reveal that Na_xCoO₂ presents a well-defined flake structure, consistent with the SEM characterization results (Figs. 2e-g). The high-resolution TEM image in Fig. 2h reveals lattice fringes with an interplanar spacing of 0.246 nm, matching well with the (100) plane of P₂-phase hexagonal Na_xCoO₂.

  Figure 2

  

  Figure 2.  Morphology of Na_XCoO₂. SEM images of (a) Na_0.6CoO₂, (b) Na_0.7CoO₂ and (c) Na_0.9CoO₂. (d) SEM image and corresponding elemental mapping of Na_0.7CoO₂. (e-g) TEM images of Na_0.7CoO₂. (h) The corresponding HR-TEM image of selected yellow line region labelled in (g).

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  The CER performance of the synthesized Na_xCoO₂ catalysts was evaluated using a standard three-electrode system at room temperature. The selectivity is the foremost parameter for evaluating the catalytic efficiency of CER catalysts [38]. Fig. 3a displays the stable linear sweep voltammetry (LSV) curves of Na_xCoO₂ in both HCl or HNO₃ electrolyte systems (pH 2) with varying Cl^- concentrations. In the electrolytes of HCl and HNO₃ without NaCl, negligible current density was observed in the potential region below 2.0 V. Upon NaCl addition to the electrolyte, the current density at identical potentials exhibits a gradual increase with increasing Cl^- concentration. Especially in 4 mol/L NaCl + HCl electrolyte, only 1.538 V is required to achieve the current density at 100 mA/cm², with the maximum current density reaching nearly 800 mA/cm². The results demonstrate that the CER predominates in 4 mol/L NaCl + HCl electrolyte at potentials below 2.0 V. For this reason, 4 mol/L NaCl + HCl was selected as the optimal electrolyte system for this investigation. Fig. 3b presents the iR-corrected polarization curves of Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂ obtained at a scan rate of 5 mV/s. The corresponding overpotentials at 10 mA/cm² (η₁₀) are indicated in Fig. 3c. Compared to Na_0.6CoO₂ (79.83 mV_η10) and Na_0.9CoO₂ (84.71 mV_η10), Na_0.7CoO₂ exhibits the lowest overpotential of 55.47 mV_η10, indicating its superior CER activity. The Tafel plots summarized in Fig. 3d further confirm this conclusion (Table S2 in Supporting information).

  Figure 3

  

  Figure 3.  Electrocatalytic CER performance of Na_xCoO₂. (a) Linear sweep voltammetry (LSV) curves in HCl or HNO₃ electrolytes (pH 2) with varying Cl^- concentrations. (b) Polarization curves. (c) Overpotential at 10 mA/cm² (η₁₀). (d) Tafel slopes. (e) TOF values. (f) Double-layer capacitance (C_dl) values. (g) Nyquist plots (EIS). (h) Chronopotentiometry measurements of Na_0.7CoO₂ at 10 mA/cm². (i) LSV curves of Na_0.7CoO₂ before and after 1000 cycles. (j) Comparison of the CER performance of Na_0.7CoO₂ with recently-reported electrocatalysts at 10 mA/cm².

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  To gain further insight into the intrinsic catalytic activity of each active site, the turnover frequency (TOF) of the corresponding catalysts was calculated (Fig. 3e). The TOF of Na_0.7CoO₂ (2.38 s^-1) is higher than that of Na_0.6CoO₂ (1.05 s^-1) and Na_0.9CoO₂ (1.19 s^-1). The double-layer capacitance (C_dl) measured in the non-faradaic region reflects the electrochemical active surface area (ECSA) of catalysts, representing a critical parameter for assessing their intrinsic catalytic activity. As shown in Fig. 3f and Figs. S4a-c (Supporting information), the C_dl values for Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂ are 34.84, 58.14 and 32.47 mF/cm², respectively. This consistent with their TOF results, underscoring the excellent electrochemical accessibility of Na_0.7CoO₂. Fig. 3g illustrates the electrochemical impedance spectroscopy (EIS) and the corresponding equivalent circuit model for all Na_xCoO₂ catalysts tested at fixed potential. Among these parameters, R_s represents the interfacial charge transfer resistance, serving as a crucial indicator for evaluating electron transfer kinetics during the CER process. The fitted R_s values are tabulated in Table S3 (Supporting information). Apparently, Na_0.7CoO₂ exhibits a lower R_s value, indicating faster charge transfer kinetics. Contact angle measurements provided further evidence supporting this result (Figs. S5a-c in Supporting information). In relative to Na_0.6CoO₂ (13°) and Na_0.9CoO₂ (22°), Na_0.7CoO₂ exhibits the smaller contact angle (6°).

  The above findings demonstrate that modulating sodium density (x) in the intercalation layers can significantly influence the CER activity of Na_xCoO₂. The optimal catalytic performance was achieved at x = 0.7. Fig. S6 (Supporting information) presents the LSV curves of Na_0.7CoO₂ and commercial RuO₂ in both 4 mol/L NaCl + HCl and HNO₃ electrolytes. Within chloride-containing electrolyte (4 mol/L NaCl + HCl), Na_0.7CoO₂ requires 1.412 V to attain η₁₀, superior to the 1.463 V of commercial RuO₂ (Fig. S6a in Supporting information). Conversely, in HNO₃ electrolyte (Fig. S6b in Supporting information), Na_0.7CoO₂ (2.035 V) requires substantially higher overpotentials than commercial RuO₂ (1.697 V) to achieve η₁₀. The results further indicate that Na_0.7CoO₂ exhibits excellent CER activity and low OER activity. In addition to high activity and selectivity, the stability also serves as a crucial parameter toward the practical evaluate of CER. To this end, the stability of the optimal Na_0.7CoO₂ catalyst was assessed by chronopotentiometry at constant current density and continuous cyclic voltammograms (CV) testing. As depicted in Fig. 3h, Na_0.7CoO₂ demonstrates excellent stability after 20 h of chronopotentiometry testing at a current density of 10 mA/cm². Fig. S7 (Supporting information) illustrate SEM, TEM, HADDF-TEM and element mapping images of Na_0.7CoO₂ after 20 h of chronopotentiometry testing. From SEM and TEM images (Figs. S7a and b), Na_0.7CoO₂ maintains a well-defined flake structure after the durability test. The HADDF-TEM and elemental mapping images demonstrate the uniformly distribution of Na, Co, and O (Figs. S7c-g). Evidently, the structure of Na_0.7CoO₂ remains unchanged during CER testing. Fig. 3i presents the LSV curves were measured at a scan rate of 50 mV/s before and after 1000 CV cycles. The overpotential shows merely a 28 mV increase after 1000 continuous CV cycles, further highlighting the exceptional stability of the Na_0.7CoO₂ catalyst. Additional, the electrocatalytic activity of Na_0.7CoO₂ outperforms most previously reported CER catalysts under comparable operating conditions (Fig. 3j and Table S4 in Supporting information).

  To further ascertain the impact of interlayer sodium density (x) on the CER performance of Na_xCoO₂ catalysts, density functional theory (DFT) calculations were conducted. Fig. S8 (Supporting information) displays the optimized Na_xCoO₂ model. The projection density of state (PDOS) of the d-bands at the Co sites (Co_s) in the Na_xCoO₂ structure is illustrated in Fig. 4a. The corresponding d-band center is represented by the purple dashed line. It can be seen that the d-band center of Co_s is correlated with the interlayer sodium density. According to the d-band theory, the adsorption of intermediates on the metal active sites weakens as the d-band center shifts away from the Fermi level. Compared with Na_0.6CoO₂ and Na_0.9CoO₂, the d-band center of Co_s in Na_0.7CoO₂ is lower, indicating weaker binding of adsorbed Cl (Cl*) and Cl₂ on its surface. The weaker adsorption would facilitate the release of Cl₂. The optimized model of Cl* adsorbed on Co_s in Na_xCoO₂ (Na_xCoO₂/Cl) are shown in Fig. S9 (Supporting information). The bond lengths of Co_s-Cl* for Na_0.6CoO₂/Cl, Na_0.7CoO₂/Cl and Na_0.9CoO₂/Cl are calculated to be 2.131, 2.149, and 2.131 Å, respectively. This further demonstrates the rapid release of Cl* from the Na_0.7CoO₂ surface. Bader charge analysis and differential charge density were conducted to investigate the electronic interactions between Cl* and Na_xCoO₂. Bader charge analysis reveals that upon Cl* adsorption on the Co_s of Na_xCoO₂, the charge value of Co_s decreases while that of Cl* increases (Fig. 4b and Table S5 in Supporting information). Differential charge analysis demonstrates that electron accumulation localized around Cl* across all Na_xCoO₂/Cl systems (Fig. 4c). The results indicate that charges in Na_xCoO₂/Cl readily transfer from Co_s to Cl*, which facilitates the evolution of chlorine gas.

  Figure 4

  

  Figure 4.  Theoretical analysis of the electrocatalytic mechanism. (a) PDOS of Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂. (b) Bader charge and (c) differential charge density of Na_0.6CoO₂/Cl, Na_0.7CoO₂/Cl and Na_0.9CoO₂/Cl. (d) Gibbs free energy for CER over Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂. (e) Gibbs free energy for OER over Na_0.7CoO₂.

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  Ulteriorly, given that the Na_xCoO₂ surface exposes Co atomic layers, the Gibbs free energy (∆G) for the Cl*-mediated pathway on the Na_xCoO₂ was calculated (Fig. 4d). The closer ∆G_Cl* approaches zero, the more favorable the chlorine adsorption-desorption equilibrium becomes. Relative to Na_0.6CoO₂ (−0.334 eV) and Na_0.9CoO₂ (−0.587 eV), Na_0.7CoO₂ shows a significantly higher ∆G_Cl* value (−0.109 eV), which is closest to the thermodynamic equilibrium state. Furthermore, the OER activity of Na_0.7CoO₂ was calculated to assess its CER selectivity. The comparative adsorption energy reveals that Cl* (−1.410 eV) exhibits stronger binding to the Na_0.7CoO₂ surface than H₂O (−0.509 eV), as evidenced in Fig. S10 (Supporting information), which thermodynamically favors the chlorine activation pathway. Fig. 4e exhibits the Gibbs free energy of OER on Na_0.7CoO₂. The largest free energy change (∆G_max) in thermodynamics corresponds to the formation of OOH*. The formation of the OOH* intermediate encounters an energy barrier of 1.95 eV, indicating the inactivity of Na_0.7CoO₂ towards OER. Taken together, these theoretical calculations reveal that altering the interlayer sodium concentration endows Na_xCoO₂ with a unique electronic structure, enabling moderate adsorption and desorption of Cl species. As a result, CER performance has significantly improved.

  In summary, Na_xCoO₂ flakes with varying sodium densities (x = 0.6, 0.7, 0.9) has been constructed for efficient CER. The catalytic behavior was systematically investigated through an integrated experimental and theoretical calculations. It is demonstrated that modulating the interlayer sodium density endows Co atoms in Na_xCoO₂ with an optimally level of d-band center, enabling finely-tuned adsorption-desorption of Cl species (∆G_Cl* = −0.109 eV) and consequently boosting CER performance. As a result, the optimized Na_0.7CoO₂ demonstrates superior CER activity outperforming the benchmark RuO₂, with the overpotential of 55.47 mV at 10 mA/cm². More fascinatingly, Na_0.7CoO₂ confronts a substantial energy barrier (∆G_max) of 1.95 eV during the OER process, which conclusively confirms its essential inactivity toward the competing OER. These findings not only provide groundbreaking insights into the catalytic mechanism of Na_xCoO₂ modulated by interlayer sodium concentration, but also paves the way for designing high-activity and high-selectivity CER catalysts.

  CRediT authorship contribution statement

  Guanjun Chen: Writing – original draft, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization. Jiayi Yang: Writing – original draft, Investigation, Data curation. Zheming Huang: Investigation, Formal analysis. Long Chen: Investigation, Formal analysis. Wenyuan Duan: Validation, Supervision, Investigation, Data curation. Tong Wang: Methodology, Formal analysis. Xingang Kong: Supervision, Investigation, Funding acquisition. Haibo Yang: Writing – review & editing, Formal analysis, Conceptualization.

  Acknowledgments

  This work was jointly supported by the Scientific Research Program Funded by Shaanxi Provincial Education Department (No. 24JR031), the Research Fund of Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials (No. SKL001), the National Natural Science Foundation of China (No. 52372288) and the Natural Science Basic Research Program of Shaanxi (No. 2022JQ-373). The authors would like to thank the Shiyanjia Lab (www.shiyanjia.com) for the support of the XPS test.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111662.

  
  
• 1. [1]

     W. Liu, A.Y. Wang, J.H. Zhang, et al., ACS Nano 10 (2025) 10038–10047. doi: 10.1021/acsnano.4c16678

  2. [2]

     Z. Li, Y.S. Wang, H. Liu, et al., Nat. Mater. 3 (2025) 1–9.

  3. [3]

     Y.C. Hou, Y.J. Li, M.R. Chai, Energy Mater. Adv. 6 (2025) 0177.

  4. [4]

     G.L. Gao, G.Z. Zhao, G. Zhu, et al., Chin. Chem. Lett. 36 (2025) 109557.

  5. [5]

     M. Aravindan, K.V. Madhan, V.S. Hariharan, et al., Renew. Sustain. Energy Rev. 188 (2023) 113791.

  6. [6]

     J.Z. Yang, T.Y. Lam, Z.X. Luo, et al., Renew. Sustain. Energy Rev. 218 (2025) 115804.

  7. [7]

     F. Meharban, C. Lin, X.T. Wu, et al., Adv. Energy Mater. 14 (2024) 2402886.

  8. [8]

     T.T. Liu, R.T. Lin, Y.Y. Chen, et al., Chin. Chem. Lett. (2025), doi: 10.1016/j.cclet.2025.111312.

  9. [9]

     J.S. Shaikh, M. Rittiruam, T. Saelee, et al., Mat. Sci. Eng. R. 160 (2024) 100813.

  10. [10]

      J.C. Zhang, E. Weatherspoon, A.S. Alsubaie, Adv. Compos. Hybrid Mater. 8 (2025) 72.

  11. [11]

      H.J.W. Li, Y. Lin, J.Y. Duan, et al., Chem. Soc. Rev. 53 (2024) 10709–10740. doi: 10.1039/d3cs00010a

  12. [12]

      J.W. Zhao, K.H. Yue, H. Zhang, et al., Nat. Commun. 15 (2024) 2928.

  13. [13]

      L. Magnier, G. Cossard, V. Martin, et al., Nat. Mater. 23 (2024) 252–261. doi: 10.1038/s41563-023-01744-5

  14. [14]

      B.H. Deng, G.Q. Yu, W. Zhao, et al., Energy Environ. Sci. 16 (2023) 5210–5219. doi: 10.1039/d3ee02360e

  15. [15]

      X.D. Zhu, W. Huang, L. Tan, et al., Chin. Chem. Lett. (2025), doi: 10.1016/j.cclet.2025.110852.

  16. [16]

      L. Quan, X.X. Chen, J.L. Liu, Adv. Funct. Mater. 33 (2023) 2307643.

  17. [17]

      J. Kim, M. Usama, K.S. Exner, S.H. Joo, Angew. Chem. Int. Ed. 64 (2024) 202417293.

  18. [18]

      F.Y. Guan, T.Q. Yu, Q. Liu, et al., Appl. Catal. B: Environ. Energy 366 (2025) 125043.

  19. [19]

      A.R. Jadhav, X.H. Liu, P. Silambarasan, et al., Appl. Catal. B: Environ. Energy 359 (2024) 124456.

  20. [20]

      L.Q. Wu, W.X. Hang, D.Y. Li, et al., Angew. Chem. Int. Ed. 64 (2025) 202413334.

  21. [21]

      T.Q. Gao, Y.Q. Zhou, X.J. Zhao, et al., Adv. Funct. Mater. 34 (2024) 2315949.

  22. [22]

      M.J. Xiao, Q.B. Wu, R.Q. Ku, et al., Nat. Commun. 14 (2023) 5356.

  23. [23]

      K. Sood, R. Wadhwa, A. Arora, et al., ACS Appl. Energy Mater. 7 (2024) 9902–9910. doi: 10.1021/acsaem.4c01956

  24. [24]

      X.D. Shao, A. Maibam, F.L. Cao, et al., Angew. Chem. Int. Ed. 63 (2024) e202406273.

  25. [25]

      X.N. Zhang, D.X. Wu, X.J. Liu, et al., Appl. Catal. B: Environ. 330 (2023) 122594.

  26. [26]

      L.K. Cai, Y. Liu, J.F. Zhang, et al., J. Energy Chem. 92 (2024) 95–103. doi: 10.2528/pierc24091703

  27. [27]

      N. Voronina, K. Köster, J.H. Yu, et al., Adv. Energy Mater. 13 (2023) 2302017.

  28. [28]

      B. Xiong, T.X. Fu, Q.P. Huang, et al., Adv. Sci. 11 (2024) 2305959.

  29. [29]

      L. Sun, Z.F. Dai, L.X. Zhong, et al., Appl. Catal. B: Environ. 297 (2021) 120477.

  30. [30]

      Y. Li, X.X. Liu, T.Y. Ji, et al., Chin. Chem. Lett. 35 (2024) 109551.

  31. [31]

      T.Y. He, Y.Y. Ding, H. Zhang, et al., ACS Sustain. Chem. Eng. 10 (2025) 4093–4107. doi: 10.1021/acssuschemeng.4c10154

  32. [32]

      N. Xi, Q.W. Li, Y. Chen, et al., Chem. Eng. J. 512 (2025) 162222.

  33. [33]

      W. Si, Z.Z. Liu, Z.H. Zhang, et al., Environ. Sci-Nano 8 (2021) 657. doi: 10.1039/d0en01091j

  34. [34]

      B.B. Wang, S.L. Li, Z. Tian, et al., Energy Storage Mater. 78 (2025) 104278.

  35. [35]

      L. Dai, Y.X. Rena, S. Li, et al., Chin. Chem. Lett. 36 (2025) 109774.

  36. [36]

      Y.J. Li, C.Q. Qu, S.Q. Meng, et al., Chin. Chem. Lett. 36 (2025) 109872.

  37. [37]

      L. Liu, J. Xu, X.H. Yang, et al., J. Colloid Interface Sci. 682 (2025) 528–539.

  38. [38]

      S. Li, X. Guo, X.F. Liu, J.L. Jiang, ACS Catal. 14 (2024) 1962–1969. doi: 10.1021/acscatal.3c05738

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  Figure 1  Phase and chemical states of Na_XCoO₂. (a) XRD spectra and (b) Raman spectra of Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂. XPS spectra of (c) O 1s, (d) Na 1s and (e) Co 2p (f) Co valence ratio of Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂.

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  Figure 2  Morphology of Na_XCoO₂. SEM images of (a) Na_0.6CoO₂, (b) Na_0.7CoO₂ and (c) Na_0.9CoO₂. (d) SEM image and corresponding elemental mapping of Na_0.7CoO₂. (e-g) TEM images of Na_0.7CoO₂. (h) The corresponding HR-TEM image of selected yellow line region labelled in (g).

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  Figure 3  Electrocatalytic CER performance of Na_xCoO₂. (a) Linear sweep voltammetry (LSV) curves in HCl or HNO₃ electrolytes (pH 2) with varying Cl^- concentrations. (b) Polarization curves. (c) Overpotential at 10 mA/cm² (η₁₀). (d) Tafel slopes. (e) TOF values. (f) Double-layer capacitance (C_dl) values. (g) Nyquist plots (EIS). (h) Chronopotentiometry measurements of Na_0.7CoO₂ at 10 mA/cm². (i) LSV curves of Na_0.7CoO₂ before and after 1000 cycles. (j) Comparison of the CER performance of Na_0.7CoO₂ with recently-reported electrocatalysts at 10 mA/cm².

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  Figure 4  Theoretical analysis of the electrocatalytic mechanism. (a) PDOS of Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂. (b) Bader charge and (c) differential charge density of Na_0.6CoO₂/Cl, Na_0.7CoO₂/Cl and Na_0.9CoO₂/Cl. (d) Gibbs free energy for CER over Na_0.6CoO₂, Na_0.7CoO₂ and Na_0.9CoO₂. (e) Gibbs free energy for OER over Na_0.7CoO₂.

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