Near-100% site utilization of single atoms for efficient electrocatalysis

Domain: chemistry
Published: 2026-01-20
Editorial outcome: major_revision

Abstract
Despite that enhancing site utilization (U ) of single atoms (SAs) is crucial in electrocatalysis beyond increasing intrinsic activity and site density, numerous SAs are inaccessible during reaction due to the dense micropores and disordered stacking of carbon particles. Here, the enhancement for U of SAs is achieved through a surfactant-assisted freeze-casting (SAFC) strategy. The sodium dodecyl sulfate (SDS)-modified Fe-doped zeolitic imidazolate framework-8 (Fe/ZIF-8) particles assemble into single-layer two-dimensional (2D) superstructures along the ice-crystal. During pyrolysis, the dual-stress effect derived from SDS shell and tight alignment of particles induces Fe/ZIF-8 to transform into concave and mesopore-rich carbon. 2D FeNC with optimized structures at both macro- and micro-scales exhibits enhanced electron/mass transport capabilities, achieving near-100% U of SAs and a half-wave potential of 0.958 V for oxygen reduction in alkali. This SAFC strategy demonstrates its universality in improving the catalytic performance of various 2D MNCs across different electrocatalytic reactions.

Introduction
Carbon-supported single-atom catalysts, especially zeolitic imidazolate framework-8-derived metal-nitrogen-carbon (MNC), are viable candidates to replace precious metals for electrocatalysis due to their low cost and high efficiency1,2. Beyond the current primary methods to improve their catalytic performance, that is, elevating the intrinsic activity (turnover frequency (TOF) per active site) and/or mass density (SD , number of active sites per gram of catalyst)3–5, increasing the site utilization (U ) of single atoms (SAs) plays an equally pivotal role. Although SAs are theoretically predicted to achieve 100% atomic utilization, not all active sites are accessible due to the numerous embedded sites and dead volumes resulting from the dense micropores and disordered stacking of carbon particles, which critically hinder the mass/electron transfer (MT/ET) and the exposure of SAs. The regulation of the size, framework, or hollow structures through hard/soft templating or organic acid etching methods has been proposed to mitigate these issues6–9. Yet, their predominant emphasis on the morphology regulation of individual particles at the nanoscale usually overlooks the crucial impact of carbon particle stacking at larger scales on MT/ET and site accessibility. Moreover, the involvement of harsh experimental conditions and complex template removal processes poses significant challenges for large-scale production. To this end, enhancing the U of SAs by concurrently optimizing the micro- and macrostructures of nanocarbon particles through a simple and scalable strategy is greatly important but challenging.

Herein, we developed a surfactant-assisted freeze-casting (SAFC) strategy to reconfigure conventional MNCs into single-layer two-dimensional (2D) superstructures with abundant mesopores. During rapid freezing, sodium dodecyl sulfate (SDS)-modified Fe/ZIF-8 colloidal particles are confined within ice crystal channels. By precisely tuning SDS concentration, interparticle spacing and surface liquid film thickness can be well controlled to achieve the construction of single-layer 2D Fe/ZIF-8 superstructures. Upon pyrolysis, the synergistic dual-stress effects from the SDS shell and dense packing of Fe/ZIF-8 particles within the 2D planes induce their anisotropic contraction and micropore expansion, yielding concave carbon structures enriched with mesopores. Consequently, 2D FeNC exhibits enhanced MT/ET capability, achieving nearly 100% U of Fe SAs during oxygen reduction reaction (ORR). Furthermore, the tailored carbon matrix with rich defects and edges optimizes the local microenvironment of Fe SAs, enhancing their intrinsic catalytic activity. 2D FeNC demonstrates a half-wave potential (E ) of 0.958 V vs. a reversible hydrogen electrode (RHE) for ORR in alkaline media, much better than the SAFC strategy-free counterpart (3D FeNC, 0.914 V vs. RHE. The structural versatility of ZIF-8 toward diverse metal dopants and the controllability of the freeze casting collectively underscore the broad applicability of the proposed SAFC strategy across various 2D MNCs, demonstrating its universal effectiveness in boosting electrocatalytic performance.

Results
Structure and composition characterizations
Fe/ZIF-8 with an average size of ~50 nm was synthesized according to the previous report6. Then, Fe/ZIF-8 suspension was mixed with SDS aqueous solution to obtain the colloidal dispersion (Supplementary Fig. 1a). During rapid freezing in liquid nitrogen (N ), SDS-modified Fe/ZIF-8 particles are expelled by the advancing ice front, forming ordered arrays between ice crystals (Fig. 1a and Supplementary Fig. 1b)10. The ice is then removed in situ upon subsequent freeze-drying, yielding the 2D orderly arrangement of Fe/ZIF-8 superstructures (2D Fe/ZIF-8). Compared to the Fe/ZIF-8 obtained by direct drying (3D Fe/ZIF-8), 2D Fe/ZIF-8 with the equivalent mass exhibits a larger volume and distinct super-assembly characteristics (Supplementary Figs. 1-2). Scanning electron microscopy (SEM) images show that 2D Fe/ZIF-8 exhibit sheet-like morphology with particles assembled in a well-organized fashion. Then, 2D Fe/ZIF-8 was subjected to high-temperature pyrolysis at 1100 °C to obtain 2D FeNC. Low-magnification SEM images in Fig. 1b and Supplementary Fig. 3 show that millimeter-scale aligned carbon superstructures were retained during carbonization. Carbon particles are arranged in a single-layer feature with an interlayer spacing resulting from the ice template removal11. More importantly, periodic stripe-like patterns were observed in the 2D plane, which is attributed to the stress generated by ice crystal growth and particle shrinkage during annealing (Fig. 1c, d). Further observations through transmission electron microscopy (TEM) images reveal that individual particles contain abundant concave and hollow structures (Fig. 1e, f). This can be attributed to the dual outward shrinkage stresses from the close packing of Fe/ZIF-8 particles and rigid SDS-derived interfacial layer, which accelerate the micropore expansion and carbon particle deformation (Supplementary Fig. 4)12. Note that the interlayer spacing, periodic ‘strip’, and optimized carbon architecture provide hierarchical transport networks, enabling rapid MT/ET and improved active site accessibility during electrocatalysis.

Crystalline Fe species are excluded in 2D FeNC through high-angle annular dark-field scanning TEM (HAADF-STEM) images (Fig. 1g) and X-ray diffraction analysis (XRD) (Supplementary Fig. 5). Elemental mapping images (Fig. 1h) verify a homogeneous distribution of Fe, N, and C elements over 2D FeNC. Aberration-corrected (AC)-HAADF-STEM image combined with 3D Gaussian fitting analysis identifies the dominant presence of densely distributed Fe SAs (Fig. 1i). The Fe contents of both 2D FeNC and 3D FeNC were determined to be 1.01 wt% by inductively coupled plasma optical emission spectrometry (ICP-OES). Raman spectroscopy (Supplementary Fig. 6a) reveals a higher density of defects in 2D FeNC, further evidenced by its reduced contents of pyridinic N species because of the promoted micropore expansion and carbon decomposition (Supplementary Fig. 6b-f and Supplementary Note 1)13–15. Note that these defects and edges are anticipated to optimize the electronic structure of Fe SAs14.

Various methods were employed to elucidate the detailed structural characteristics of catalysts. Fourier-transformed (FT) extended X-ray absorption fine structure (EXAFS) analysis reveals that the dominant peak in both 2D and 3D FeNC corresponds to the Fe-N scattering path, with coordination numbers of approximately 4 based on the fitting results (Fig. 1j, k, Supplementary Figs. 7-8, Supplementary Table 1 and Supplementary Note 2). X-ray absorption near-edge structure (XANES) spectra (Supplementary Fig. 1l) show a positive shift in the Fe K-edge position for 2D FeNC relative to 3D FeNC. Given the identical FeN configuration, the lower charge density at the Fe sites in 2D FeNC compared to 3D FeNC highlight the impact of the defects and edges on the electronic structure of the FeN moieties16. By this token, the carbon substrate with optimized porous and concave structures facilitates electron delocalization at the Fe sites for 2D FeNC, which is expected to mitigate their excessive adsorption of oxygenated intermediates, thereby endowing 2D FeNC with enhanced catalytic activity17.

SAFC mechanism and condition optimization
To systematically evaluate the SAFC strategy’s effectiveness, we further investigated how surfactant concentration and type govern the structural evolution of both Fe/ZIF-8 precursors and derived FeNC (Fig. 2a). As shown in Supplementary Fig. 9, SDS-free assemblies (denoted as 2D Fe/ZIF-8_S , where the subscript represents relative SDS concentration to 2D Fe/ZIF-8) exhibit multilayered aggregates with reduced sizes. This aligns with the intrinsic difficulty in ice-templating small colloidal particles because their limited size prevents effective confinement within inter-ice channels18. Simultaneously, assembling them into large-scale 2D arrays remains problematic. The thermodynamic prerequisite for successful freeze-casting requires the free energy (γ) to satisfy Δγ = γ -(γ + γ ) > 0 (Fig. 2b, Supplementary Fig. 10 and Supplementary Note 3), where γ , γ and γ represent interface free energy between solid and particle, liquid and particle, and solid and liquid, respectively10. However, small colloidal particles with high specific surface area generally exhibit elevated γ , promoting random aggregation over directional alignment along ice crystals during freezing (Fig. 2c). Moreover, the equilibrium between these interfacial energies governs the repulsive (F = 2πrΔγ (α /d)n) and attractive (F = 6πηνr2/d) forces between the particles and ice front, where r is the particle radius, ν the ice-front velocity, η the liquid’s dynamic viscosity, α the average intermolecular distance in the liquid phase, d the liquid-layer thickness at the solid-liquid interface, and n an empirical correction factor (1 ≤ n ≤ 4). Surfactant-free Fe/ZIF-8 particles display small α and d values due to their low surface charge (Fig. 2d, Supplementary Fig. 11 and Supplementary Note 4), making them susceptible to be captured by ice crystals because of strong F (Fig. 2e). While introducing SDS (2D Fe/ZIF-8_S ) can elevate α and d through electrostatic repulsion, enabling particle alignment along ice-crystal interfaces (Supplementary Fig. 12). Increasing SDS concentration further optimizes α and d, which allows the construction of the expanded 2D sheet-like superstructures for 2D Fe/ZIF-819. Moreover, the strengthened electrostatic repulsion facilitates single-particle confinement within ice channels, producing single-layer 2D superstructures (Supplementary Fig. S1). Although higher SDS concentrations (2D Fe/ZIF-8_S) preserve the 2D morphology, the bilayer adsorption of SDS after its saturated adsorption on the Fe/ZIF-8 surface decreases α and d, ultimately causing multilayer stacking (Supplementary Fig. 13).

Furthermore, cationic cetyltrimethylammonium bromide (CTAB) and nonionic polyvinylpyrrolidone (PVP) were investigated as alternative surfactants (Supplementary Figs. S14-15). CTAB enhances Fe/ZIF-8 surface charge through hydrophobic interactions, enabling 2D Fe/ZIF-8_CTAB assembly. However, its lower critical micelle concentration (vs. SDS) induces premature micelle/bilayer formation at reduced concentrations (1.62 mM), decreasing α and d values and yielding multilayer architectures. In contrast, PVP with weaker charge modulation produces smaller, disordered arrays. These results unequivocally demonstrate the pivotal role of surfactant in controlling Fe/ZIF-8 alignment during freeze-casting.

The surfactant-mediated carbonization mechanism was further elucidated (Fig. 2f). With SDS integration, carbon particles evolve from well-defined rhombic dodecahedrons (2D FeNC_S ) to roof-like concave architectures (2D FeNC_S and 2D FeNC) (Fig. 2g-i and Supplementary Figs. 16-17). In SDS-free systems, ligand decomposition and gas release generate inward stresses that drive particle shrinkage20,21. Conversely, SDS-coated Fe/ZIF-8 undergoes anisotropic deformation through two concurrent outward stress effects. The first stress (F ) arises from the differential shrinkage coefficients between the SDS shell and Fe/ZIF-8 core, with vertex regions experiencing amplified stress due to force superposition from adjacent SDS layers. A secondary stress (F ) originates from interparticle compression within the 2D planes during freezing22. The synergistic action of F and F promotes anisotropic contraction and micropore expansion, ultimately yielding the characteristic concave mesoporous structures observed in 2D FeNC_S and 2D FeNC. While moderate SDS concentrations can optimize the morphology of 2D FeNC (Fig. 2i), excessive SDS (2D FeNC_S ) markedly augments F , accelerating ligand depletion and gas evolution. This triggers the structural collapse and Fe aggregation into nanoparticle (NP) in 2D FeNC_S (Fig. 2j and Supplementary Fig. 18). Similarly, the high decomposition temperatures of CTAB and PVP in comparison to SDS enable stronger and more prolonged F , also inducing structural collapse and NP formation (Supplementary Figs. 19-21 and Supplementary Note 5). Taken together, the microscopic morphology, porous structure, and macroscopical alignment of FeNC can be rationally governed through surfactant selection and concentration tuning, demonstrating the effectiveness of the SAFC strategy in the structural optimization of FeNC.

Catalytic performance and quantitative analysis
Taking ORR as a model reaction, the electrocatalytic performance of the as-prepared catalysts was evaluated in 0.1 M KOH (aq) solutions. Notably, when the 2D FeNC is prepared as an ink, it retains a relatively intact 2D sheet-like structure, which not only facilitates MT and ET but also promotes the dispersion of ionomers on the catalyst surface (Supplementary Fig. 22). As illustrated in Figs. 3a, 3D FeNC demonstrates markedly improved ORR activity compared to metal-free NC, demonstrating the primary role of Fe SAs23. Notably, 2D FeNC achieves a E of 0.958 V vs. RHE for ORR, surpassing both 3D FeNC (0.914 V vs. RHE) and commercial Pt/C (0.906 V vs. RHE), further verified by its high kinetic current density (J , 66.2 mA/cm2 at 0.93 V vs. RHE) and low Tafel slope (45.6 mV/dec) (Supplementary Fig. 23). Rotating ring disk electrode (RRDE) and Koutecký-Levich analyses verify a dominant 4-electron ORR pathway for all catalysts (Supplementary Figs. 24-25). Among all 2D FeNC_S variants, 2D FeNC delivers the best performance, attributable to its ideal particle arrangement and porous architecture (Fig. 3b). Control experiments reveal that 3D FeNC/S derived from the physical mixture of Fe/ZIF-8 and SDS without freeze-casting fails to enhance ORR activity, underscoring the indispensable role of freeze-casting in structural optimization (Supplementary Fig. 26). Both 2D FeNC_CTAB and 2D FeNC_PVP exhibit degraded activity due to structural collapse and Fe NP formation. Remarkably, 2D FeNC shows low E loss (4 mV) after 30,000 cyclic voltammetry (CV) cycles and maintains structural integrity without Fe NP generation (Supplementary Figs. 27-29). These results collectively establish 2D FeNC as a top-tier ORR catalyst and make it competitive with most reported analogues in both activity and durability (Supplementary Tables 2-3).

In situ nitrite stripping tests were carried out to uncover the origin of the enhanced ORR performance, as detailed in the Experimental Section (Supplementary Fig. 30)24. Upon nitrite poisoning, all catalysts show reduced activity, with 2D FeNC exhibiting the most significant decline, reflecting its higher density of accessible active sites (Fig. 3c). Accordingly, 2D FeNC yields a 4-fold greater stripping charge (52.2 vs. 13.1 C/g for 3D FeNC). Based on the stripping charge, the SD can be calculated (Fig. 3d and Supplementary Figs. 31–33). Instead of the five-electron reduction of nitrosyl to ammonia as previously reported24, we assumed a three-electron pathway to hydroxylamine at FeN sites (Supplementary Note 5)5,25. This analysis reveals the SD of 6.52 × 1019 sites/g for 2D FeNC, four times that of 3D FeNC (1.63 × 1019 sites/g) (Fig. 3e). Given the identical Fe content across all catalysts, this difference demonstrates the higher accessibility of SAs in 2D FeNC during electrocatalysis. Importantly, the U is calculated to be 99.4% for 2D FeNC using Fe content determined by ICP-OES, demonstrating that 2D FeNC enables nearly all Fe SAs to participate in the catalytic process. Furthermore, the TOF is also estimated based on the nitrite stripping results. To exclude ORR contributions from non-Fe sites such as N-doped carbon, the J used for TOF calculation is defined as the difference between J values before and after nitrite poisoning (i.e., J unpoisoned-J poisoned)5. Fig. 3f shows that 2D FeNC also exhibits a significant enhancement in TOF values, which can be ascribed to the optimization of electronic structures for Fe SAs driven by defect and edge generation under dual-stress effects14. Leveraging the significantly improved U and intrinsic activity of 2D FeNC over 3D FeNC, we evaluated its ORR performance at reduced loadings (Supplementary Fig. 34). Even at one-fourth the loading of 3D FeNC, 2D FeNC (0.15 mg/cm2) still delivers superior performance. This is crucial for practical applications, as excessive loading of non-precious metal catalysts often leads to MT limitations26. The enhancement of U and intrinsic activity in 2D FeNC offers an effective solution to this challenge.

Scanning electrochemical microscopy (SECM) technique with low catalyst loading and minimized MT/ET barriers provides a highly sensitive signal feedback compared to the nitrite stripping method27. To corroborate our findings, we further employed in situ SECM and surface-interrogation (SI)-SECM to quantify SD and TOF (Fig. 3g). In tip-generation/substrate-collection (TG/SC) mode, oxygen generated at the Pt tip undergoes ORR at the substrate electrode, generating a feedback current28. Accordingly, 2D FeNC delivers high feedback currents compared to 3D FeNC, further confirming its superior ORR activity (Fig. 3h, i and Supplementary Figs. 35-37). Subsequently, the S and TOF were determined by the SI-SECM method, where the catalytic centers can be quantified based on the charge obtained from the Fe(II)/Fe(III) redox cycling between ferrocenemethanol (Fc) and Fe catalytic centers in catalysts (Supplementary Note 6)29. The tip current for 2D FeNC rises at a higher substrate E (1.05 V vs. RHE) than 3D FeNC (0.95 V vs. RHE), and attains a balance at E of ~0.5 V vs. RHE because the redox cycle at the catalytic site reaches saturation (Fig. 3j and Supplementary Figs. 38–40). Time-current integration obtains S values of 6.57 × 1019 (2D FeNC) and 1.64 × 1019 sites/g (3D FeNC), aligning with nitrite stripping results. Especially, 2D FeNC’s S from both methods matches its ICP-OES-derived Fe content, confirming near-100% U of Fe SAs. Besides, the TOF of both 2D FeNC (113.8 e site−1 s−1) and 3D FeNC (105.24 e site−1 s−1) was also obtained through the SI-SECM method, further suggesting the enhanced intrinsic activity of 2D FeNC compared to 3D FeNC. This explains its comparable ORR activity despite lower Fe loading to literature benchmarks (Supplementary Table 4, and Supplementary Note 7).

Origin exploration
Generally, both U and TOF critically depend on MT and ET efficiency during electrocatalysis (Fig. 4a). Brunauer-Emmett-Teller (BET) measurements reveal that 2D FeNC possesses the highest surface area at 1637.7 m2/g (Supplementary Figs. 41-42 and Supplementary Table 5). Non-local density functional theory (NLDFT) calculations show that 2D FeNC exhibits a significantly increased mesopore ratio of 80.3% compared to 3D FeNC (41.1%), 3D FeNC_S (65.4%), 2D FeNC_CTAB (59.1%) and 2D FeNC_PVP (65.2%) while its micropore ratio decreases to 15.4% from 46.8% (3D FeNC) and 24.3% (3D FeNC_S), respectively (Fig. 4b)30. This pore optimization induced by the dual outward stress effects overcomes the inherent MT/ET limitations of microporous ZIF-8-derived carbons, making it possible to expose more active sites to participate in the reaction26.

The electrochemical performance of the catalysts was evaluated after loading onto electrodes. Accordingly, the double-layer capacitances (C ) determined through electrochemical impedance spectroscopy (EIS) at open circuit potential (OCP) and CV curves in the non-Faradaic region (C ) correspond to the electrochemically wettable surface area (EWSA) and electrochemically active surface area (ECSA), respectively31. As displayed in Fig. 4c, 3D FeNC demonstrates limited EWSA and ECSA due to the poor pore structure and disordered stacking (Supplementary Figs. 43 and 44a). In contrast, 2D FeNC shows significantly improved C , C and a higher C /C ratio, confirming that its ordered architecture and tailored porosity simultaneously enhance electrolyte infiltration and electrochemical responsiveness. A progressive EWSA increase across 3D FeNC, 3D FeNC/S, 2D FeNC_S , and 2D FeNC directly correlates with structural evolution towards single-layer ordered arrays with multiple channels, highlighting their essential role in optimizing MT/ET. This is further supported by 2D FeNC’s superior hydroxide ion diffusion coefficient (D ) (Supplementary Fig. 44b and Supplementary Note 8)32.

Operando EIS analysis was conducted to investigate MT and ET processes during the ORR. By fitting an equivalent circuit model (Supplementary Fig. 45), we extracted potential-dependent charge transfer resistance (R ) and mass transport resistance (R ) across the ORR potential range (Fig. 4d and Supplementary Figs. 46–50)33. With increasing overpotential, the ORR process is progressively governed by R , mixed kinetic, and R . For 3D FeNC, mixed kinetic control initiates at 0.88 V vs. RHE, while R rebounds below 0.74 V vs. RHE, caused by the difficulty in the timely diffusion of products or intermediates on the active sites. In stark contrast, 2D FeNC exhibits consistently lower R and R . Its monotonically decreasing R indicates a rapid and sustained ET process, demonstrating the superiority of its single-layer 2D architecture in enhancing MT and ET.

The distribution of relaxation times (DRT) analysis serves as a model-free method to evaluate electrochemical processes34,35. As shown in Fig. 4e, both 2D and 3D FeNC exhibit three characteristic peaks in the high-, mid-, and low-frequency regions, corresponding to ion diffusion, ORR kinetics, and gas diffusion, respectively. Notably, ion diffusion impedance is much lower than that of ORR and gas diffusion. At 0.76 V vs. RHE, 2D FeNC reaches its lowest ORR kinetic resistance and maintains this level, in contrast to 3D FeNC, which exhibit persistently higher resistance. As the overpotential increased, the gas diffusion peaks of both catalysts moved toward lower frequencies, indicating faster O consumption and greater concentration gradients between the solution and electrode interface. Importantly, 2D FeNC demonstrates significantly reduced gas diffusion resistance compared to 3D FeNC, a result of its improved transport pathways. These findings were further corroborated by finite element method (FEM) simulations of O /OH- transport within the catalyst layers8. Supplementary Fig. 51 shows the models to simulate 2D FeNC and 3D FeNC. The time-dependent O /OH- concentration-time profiles reveal that 2D FeNC maintains significantly higher O /OH- concentration levels than 3D FeNC across both horizontal and vertical dimensions under identical time gradients (Fig. 4e and Supplementary Figs. 52–57). This is attributed to the large-scale ordered arrangement of carbon particles in 2D FeNC, which promotes efficient O /OH- transport, while the disordered stacking of 3D FeNC increases the O /OH- diffusion pathways, thereby hindering its transport.

In situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy measurements were performed to further investigate the ORR catalytic mechanism36–38. The four-electron ORR process proceeds via two fundamental pathways, as illustrated in Supplementary Fig. 58. The first is the associative mechanism, in which *O undergoes protonation to form the *OOH intermediate. The second is the dissociative mechanism, where *O-O* directly cleaves into two *OH species, followed by sequential protonation39–41. As shown in Fig. 4g, the peak at ~1050 cm−1 can be attributed to the *O-O* intermediate associated with the dissociative pathway, while the peak at ~1220 cm−1 corresponds to the *OOH intermediate from the associative pathway. Compared with 3D FeNC, the *OOH signal on 2D FeNC decreases significantly with decreasing potential, indicating its rapid consumption. In contrast, the *O-O* intermediate shows relatively minor changes. These results suggest that both catalysts preferentially follow the associative pathway, with 2D FeNC exhibiting higher ORR efficiency. Notably, the *OH adsorption peak emerges at more negative potentials for 2D FeNC, demonstrating weaker oxygenate binding on Fe SAs and highlighting its enhancement in intrinsic activity42.

To demonstrate practical applicability, we fabricated ZABs using 2D FeNC as the air cathode (Supplementary Figs. 59–60 and Supplementary Notes 9–10)43–45. In addition to an robust performance in liquid-based ZABs, the 2D FeNC + RuO -based flexible ZABs deliver a much higher discharge current and maximum power density than ZABs that use benchmark Pt/C + RuO as a catalyst (93.5 mA/cm2 and 48.4 mW/cm2 vs. 42.5 mA/cm2 and 21.0 mW/cm2, respectively). Furthermore, the assembled flexible ZABs successfully powered digital timers and smartphones, demonstrating their reliability and practical potential for real-world applications.

Despite differences in the optimal metal species across catalytic systems, maximizing the accessibility and U of SAs remains a common demand for efficient electrocatalysis7,46. As shown in Supplementary Figs. 61-63, the ORR activities of 2D CoNC, 2D NiNC, 2D MnNC and 2D CuNC are superior to those of their 3D MNC counterparts. Furthermore, the SAFC strategy was extended to the synthesis of 2D IrNC and 2D PtNC, which were then applied to formic acid oxidation and methanol oxidation reaction systems, respectively (Supplementary Fig. 64). Despite their larger size compared to O , the optimized 2D MNCs significantly enhance MT/ET, thereby improving the accessibility of SAs and ultimately boosting catalytic activity.

Methods
Synthesis of 2D Fe/ZIF-8
Fe/ZIF-8 was prepared according to the reference6. Fe/ZIF-8 crystals were synthesized by mixing 300 mL methanol solution containing 3.94 g 2-methylimidazole and 300 mL methanol solution containing 3.39 g Zn(NO ) ·6H O and 100 mg Fe(NO ) ·9H O, followed by crystallization at 60 °C for 24 h. The obtained solids were collected by centrifugation, thoroughly washed with ethanol, and dried under vacuum for 30 min. ZIF-8 was synthesized with the same procedure as Fe/ZIF-8, except for the addition of Fe(NO ) ·9H O. The as-prepared Fe/ZIF-8 was dispersed into 1.62 mM SDS aqueous solutions and kept the concentration of Fe/ZIF-8 at 10 mg/mL, which was ultrasonicated for 10 min, followed by stirring for another 2 hours to obtain the colloidal solutions. Then, 30 mL colloidal solutions were frozen in liquid N for 3 min using paper cup molds, followed by 24 h freeze-drying (−50 °C condenser temperature, 20 Pa vacuum) to yield 2D Fe/ZIF-8 superstructures. By adjusting SDS concentrations to 0, 0.81, 1.62, and 3.24 mM, we obtained 2D Fe/ZIF-8_S , 2D Fe/ZIF-8_S , 2D Fe/ZIF-8, and 2D Fe/ZIF-8_S variants, respectively. Parallel experiments used CTAB (1.62 mM) or PVP (5 mg/mL) to prepare 2D Fe/ZIF-8_CTAB and 2D Fe/ZIF-8_PVP controls.

Synthesis of 2D FeNC
The as-prepared 2D Fe/ZIF-8 powders were transferred to a porcelain boat and subjected to thermal annealing at 1100 °C for 2 hours under N atmosphere with a heating rate of 5 °C/min to produce the final 2D FeNC series.

Synthesis of other 2D MNCs and 3D MNCs
To verify the universality of the SAFC strategy proposed in this work, we synthesized different types of 2D MNCs and evaluated their electrocatalytic performance. Meanwhile, to simplify the synthetic procedure and reduce the cost, we adopted a universal host-guest strategy reported in the literature to prepare the M/ZIF-8 precursors47. Although the M/ZIF-8 obtained by this strategy exhibited larger particle sizes compared with Fe/ZIF-8, the resulting 2D MNCs still demonstrated well-defined 2D array structures and excellent electrocatalytic performance.

Physical characterization
SEM analysis employed a Zeiss Gemini SEM 560 system operating at 2 kV acceleration voltage. TEM images were acquired using a JEM-2100 Electron Microscope, while AC-HAADF-STEM characterization was conducted on a Titan G2-600 instrument. XRD patterns were collected on a Bruker D8 ADVANCE diffractometer with Cu Kα radiation (λ=0.15406 nm) at 40 kV and 40 mA, scanning 2θ angles from 10° to 80°. Raman spectroscopy measurements utilized a Thermo Fisher Scientific DXR spectrometer with 780 nm laser excitation in backscattering geometry. The BET surface areas of all samples were determined by N adsorption-desorption measurements at 77 K using an AUTOSORB system. Prior to analysis, the samples were degassed at 200 °C for 12 h under vacuum. The pore size distribution was obtained using the NLDFT method. ICP-OES analysis was performed on a PerkinElmer Optima 8000 system for metal content quantification. XPS data were collected on a Thermo ESCALAB 250Xi instrument with Al Kα radiation, with all binding energies referenced to the C 1s peak at 284.6 eV after Shirley background subtraction and peak deconvolution using XPSPEAK software. EXAFS measurements were carried out at beamline 21A of the Taiwan Photon Source, National Synchrotron Radiation Research Center.

Electrochemical measurements and ORR performance evaluation
All RRDE measurements were performed using a three-electrode system with an ALS RRDE-3A rotating electrode setup at room temperature. An Ag/AgCl (KCl-saturated) electrode and a graphite rod were used as the reference and counter electrodes, respectively. The reference electrode was calibrated against RHE before testing. Catalyst inks were prepared by ultrasonically dispersing 4 mg of catalyst in 1 mL of water/ethanol/Nafion (49:50:1 volume ratio) solution for 2 h. A glassy carbon RRDE coated with catalyst ink with a controlled loading of 0.6 mg cm−2 was the working electrode. A commercial 20 wt% Pt/C catalyst (Alfa Aesar) with a loading of 20 μg /cm2 was used as a reference. ORR activity was recorded in O -saturated 0.1 M KOH (aq) solution by Linear Sweep Voltammetry (LSV, 1.0-0.2 V vs. RHE, 10 mV/s, 1600 rpm) curves. 4-electron selectivity and H O yield were determined by using RRDE with a constant Pt ring potential (1.20 V vs. RHE). The collection efficiency is determined to be 0.37. ORR results were background-corrected using N -saturated electrolyte measurements. E is defined as the middle point of a certain region of LSV curves from the reduction current that starts increasing sharply to the reduction current that gets saturated. During the accelerated durability testing, the catalyst was examined in N -saturated 0.1 M KOH (aq) electrolyte over a potential cycling window of 0.6 V to 1.0 V vs. RHE at a scan rate of 100 mV/s.

In situ nitrite stripping test
The nitrite stripping was conducted with 0.2 mg/cm2 catalyst loading. To ensure that nitrite can be adequately reduced, 0.5 M sodium acetate buffer (aq) with pH = 5.2 was chosen as an electrolyte25. The experimental process includes cleaning, measurement (unpoisoned), electrode poisoning, measurement (poisoned and stripped), and measurement (recovered), depicted in Supplementary Fig. 305,24. Based on the stripping charge derived from the CV curves, S could be calculated by the following equation:

S mass = (Q strip × N A) / (n strip × F)

where Q strip represents the nitrite reductive stripping charge derived from the stripping peak. N A and F are Avogadro’s constant (mol-1) and Faraday’s constant (C mol-1), respectively. n strip is the number of electrons for the nitrosyl ligand reduction. Then TOF can be calculated in the following equation:

TOF = (J kin,mass unpoisoned - J kin,mass poisoned) × N A / (SD mass × F)

where J kin,mass unpoisoned (A/g) and J kin,mass poisoned (A/g) represent the kinetic mass-specific activity obtained from in situ nitrite stripping test.

SECM and SI-SECM measurements
The SECM experiments were performed using a Model 920D system (CH Instruments, Inc.), equipped with a bi-potentiostat and a high-resolution 3D electrodepositioner. A Pt UME (CHI116, 25 μm tip diameter) was utilized as the probe, while a custom-made concave gold electrode (25 μm recessed microdisk, ~7 μm depth) served as the substrate27. The reference and counter electrodes were an Ag/AgCl and a Pt wire, respectively. The substrate was processed by inserting a 25 μm gold wire into a glass capillary sealed using an oxy-hydrogen flame. The capillary tip was polished with successive grit sandpapers, followed by electrochemical etching in a CaCl solution to create a cavity, the depth and sealing of which were verified using an optical microscope.

The catalyst was loaded by pressing the concave electrode onto the catalyst powder placed on a glass slide, ensuring flatness and thorough filling via optical microscopy. The positioning of the SECM probe along the z-axis was fine-tuned using the probe approach curve technique in an N -purged 0.1 M KOH (aq) solution with 0.5 mM Fc, with the probe biased at 0.50 V vs. Ag/AgCl for Fc oxidation. The platform was adjusted using the three-point fix method to prevent crashes and stabilize current readings. The SECM scanning was performed using 100 μm fine scan to locate the catalyst region. Images in the TG/SC mode were captured under various substrate potentials under the N -purged 0.1 M KOH (aq) solution.

Subsequent SI-SECM experiments involved measuring the C dl by CV at scan rates between 5 and 50 mV/s to determine the ECSA of the UME (ECSA ECSA). The resulting feedback current at the probe (I tip) was recorded, and the integrated charge (Q tip) was calculated after background subtraction. The SD mass was then determined based on the following equation:

SD mass = (Q tip × N A) / (F × ECSA substrate)

where F represents Faraday’s constant (96485 C mol-¹), and N A is Avogadro’s constant (6.02 × 1023 mol-¹). Then the TOF was determined using the following equation:

TOF = (N A × j m) / (SD mass × F)

where j m (A/g) represents the kinetic mass-specific activity obtained from prior electrochemical measurements, which were calculated as 1198.36 A/g and 276.62 A/g for 2D FeNC and 3D FeNC, respectively. While SD mass refers to the site density per gram of catalyst, which can be calculated using the following equation:

SD mass = (Q tip × N A) / (F × catalyst)

C dl test and operando EIS analysis
The C dl_CV was obtained through CV curves conducted within a non-Faradaic potential range (1.00-1.10 V vs. RHE) at scan rates varying between 1 and 50 mV/s. And ECSA is estimated using the following equation:

ECSA = C dl_CV / C s

where C s represents the specific capacitance of the total double layer (0.04 mF/cm2)48. Also, EIS was performed at OCP without rotating with an amplitude of 10 mV between 100 kHz and 10 mHz to obtain C dl_EIS. Specifically, C dl_EIS can be obtained from the slope of the linear relationship between the imaginary part of the impedance (Z'') and the reciprocal of the angular frequency (ω) in the low-frequency region31:

-Z'' = (1 / C dl) × (1 / ω)

R ct and R mt were calculated from operando EIS data using an SP-300 potentiostat (BioLogic Science Instruments, France) with an O -purged 0.1 M KOH (aq) solution. The impedance data were collected by varying the frequency range from 1 MHz to 10 mHz at a series of potentials with 11 data points recorded per decade, using a 5 mV sinusoidal AC amplitude. A single-sine mode was employed for all measurements. The spectra were first analyzed using complex non-linear least squares (CNLS) fitting using “Zview” software, which is a series circuit composed of solution resistance (R s) and the parallel circuits of R ct/CPE (Constant phase element) and R mass/CPE (Supplementary Fig. 45).

DRT analysis was then performed, which is a model-free analysis and visualization strategy designed for complex electrochemical systems34,35:

Z(f) = R∞ + R pol ∫ [g(τ) / (1 + i 2π fτ)] dτ

where R∞ represents the ohmic resistance, while R pol corresponds to the polarization resistance. The DRT function g characterizes the time relaxation behavior of the electrochemical system under investigation. τ and f denote the relaxation time and frequency, respectively. The DRT function of the measured spectra is obtained using a Tikhonov regularization approach. The regularization parameter λ, which balances the residual noise and the solution norm, is crucial for accurately extracting the pseudo-continuous DRT function from discrete impedance data. Based on Heinzmann’s study, a regularization parameter λ of 10-3 was selected, as it offers an optimal compromise between low residuals, high selectivity, and minimal oscillations. The DRT function was calculated using the Matlab® software DRT tools, developed by the Ciucci group35.

In situ ATR-FTIR tests
In situ ATR-FTIR was performed with a Thermo iS50 FT-IR spectrometer, equipped with a liquid N -cooled mercury-cadmium-telluride detector. A gold-plated silicon crystal modified with the catalyst served as the working electrode, while an Ag/AgCl (KCl-saturated) electrode and a graphite rod were used as the reference and counter electrodes, respectively. All measurements were performed in 0.1 M KOH (aq) solution, with the applied potential being stepped from 1.1 V to 0.2 V vs. RHE in 500 mV intervals.

FEM simulations
FEM simulations were conducted using the COMSOL Multiphysics finite-element solver22. Two modules were employed to develop an integrated chemistry and MT model for both 2D FeNC and 3D FeNC. This approach enables a detailed analysis of the transport phenomena and electrochemical reactions within the system. Based on the SEM images, the rhombic dodecahedron was simplified to a regular hexagon to simulate carbon particles in 3D FeNC catalysts, while an indented hexagon containing an internal hole structure was chosen for the carbon particles in 2D FeNC catalysts. Two parallel arrays of 20 × 20 particles were used to simulate 2D FeNC catalysts, where the blue regions represent solid particles and the gray area corresponds to the pore structure. For the 3D FeNC catalyst, the random packing of particles results in significant dead volume.

Rechargeable liquid and flexible ZABs tests
Liquid ZABs were constructed in a two-electrode configuration using ~99.9% pure zinc foil as the anode and a catalyst-coated gas diffusion layer (TORAY-YLS 30T carbon paper) as the cathode49. Catalyst ink was dried at 80 °C, ensuring a catalyst loading of 1.0 mg cm−2. The electrolyte consisted of 6 M KOH and 0.2 M zinc acetate (aq), purged continuously with O at 40 mL min−1. Before testing, the electrolyte was purged with O for 30 min to ensure saturation. OCP, and discharge-charge polarization curves were recorded using a CHI760E electrochemical workstation. The rate performance of the assembled ZABs was assessed by monitoring the voltage profiles during galvanostatic discharge at varying current densities (0–50 mA cm−2). The constant current discharge-charge cycles and specific capacity were measured at room temperature using a Land 3001A battery test system. Each galvanostatic cycle involved 150 s discharge and 150 s charge periods at current densities of 10 mA cm−2. The specific capacity and energy density were calculated using the following equations:

Specific capacity (mAh g−1) = (I × t) / w Zn

Energy density (Wh kg−1) = (I × V × t) / w Zn

where I represents the applied current (A), t denotes the operation time (s), V is the average discharge voltage (V), and w Zn refers to the mass of zinc consumed (g).

The flexible solid-state ZABs comprise an air electrode, a polyacrylic acid (PAA) gel film as the solid electrolyte, and zinc foil as the anode. The air electrode consists of nickel foam as the current collector, a gas diffusion layer in the middle, and a carbon cloth coated with catalysts (1.0 mg cm−2) on the electrolyte-facing side. The PAA gel electrolyte is prepared by combining acrylic acid, KOH, and zinc acetate, followed by cross-linking with N,N-methylenediacrylamide, and initiated by K S O . Polarization curves were measured using LSV on a CHI 760E electrochemical workstation. ZABs underwent galvanostatic cycling with 150-second discharge and charge intervals at current densities of 1 mA cm−2.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, the authors presented a surfactant-assisted ice-templating strategy to construct 2D FeNC catalysts based on ZIF-8 and achieved near-100% site utilization of single atoms for efficient ORR. Unlike most works on single-atom catalysts that focus on enhancing intrinsic activity through electronic structure modulation and increasing site density, the authors tune the nano- and macrostructure of carbon particles simultaneously and thus improve the accessibility of single-atom sites under limited site density and intrinsic activity. The resultant 2D FeNC achieves near-100 % site utilization of Fe single atoms and remarkable oxygen reduction performance with a half-wave potential of 0.958 V in an alkaline solution. Overall, this work is interesting and offers a new approach to the design of advanced single-atom catalysts. I believe the manuscript is suitable for publication in Nature Communications after minor revisions.\n1. The authors claim that this strategy improves both the accessibility of catalytic sites and intrinsic activity. Please clarify which one dominates the enhanced performance would strengthen the mechanistic discussion.\n2. Additionally, the reported 4-fold increase in active-site utilization raises the question of whether reducing the loading of the best 2D FeNC catalyst by a quarter would yield performance comparable to the 3D counterpart.\n3. The observed differences in C-N content between 2D and 3D FeNC catalysts are attributed to stress-induced microporous expansion. However, it remains unclear whether this expansion also affects the density of FeN4 sites or Fe content, which is critical for evaluating true site utilization. The author should explain it and make it clear.\n4. Regarding the in-situ nitrite stripping experiments, the authors propose a 3-electron transfer process forming hydroxylamine (NH2OH), contrasting with the widely reported 5-electron reduction to ammonium (NH4+). A more detailed justification for this mechanistic divergence is needed.\n5. The preservation of the 2D structure after ink preparation and electrode fabrication should be verified by SEM characterization. Structural collapse during electrode preparation is unfavorable for the ORR performance.\n6. Minor grammatical and logical errors should be carefully corrected to enhance clarity and precision.", "reviewer_2": "In this manuscript, the authors successfully constructed a FeNC catalyst featuring a well-defined two-dimensional (2D) architecture at the micrometer scale. Through a detailed synthesis mechanism and comprehensive quantitative analysis, they demonstrated that the single-atom utilization efficiency of the 2D FeNC catalyst approaches 100%, significantly surpassing that of its three-dimensional (3D) counterpart. 2D FeNC also shows exceptional ORR performance in alkaline media. Mechanistic elucidation and quantitative active-site analysis were performed in detail. The synthetic strategy proposed is both innovative and broadly applicable, extending beyond single-atom catalysts to other material systems. Overall, this work meets the high standards required for publication in Nature Communications after minor revisions.\nHowever, the following points require further clarification:\n1. The interaction between SDS and Fe/ZIF-8 particles is straightforward, but the mechanism underlying CTAB’s role in regulating the structure of catalysts remains unclear.\n2. The authors attribute the distinct Fe-based nanoparticle formation induced by CTAB and PVP to their higher decomposition temperatures compared to SDS. Experimental evidence should be provided to substantiate this claim.\n3. In finite element simulations, the packing modes of particles vary, potentially affecting the number of particles within models of identical dimensions. Will this affect the results of the simulation?\n4. The in-situ nitrite stripping method, as reported in the literature, typically detects less than half of the active sites due to mass transport and electron-transfer limitations. Given this constraint, how do the authors justify their near-100% atomic utilization? A deeper discussion to explain this discrepancy is essential to validate the exceptional efficiency claimed.\n5. For the SECM measurements, the catalyst was loaded into concave electrodes. How was the catalyst mass precisely determined? Without accurate mass quantification, the reported SDmass and TOF values may lack reliability. The authors should clarify the methodology or provide additional experimental data.\n6. The interpretation of in-situ FTIR spectra should be checked carefully. For instance, the authors state: “As the potential decreases, 2D FeNC shows a much greater increase in *O2- peak intensity compared to 3D FeNC, reflecting stronger O2 adsorption and a more efficient first electron transfer (*O2 → *O2-)”, while the first electron transfer step in ORR should be *O2 → *OOH.\n7. The authors demonstrated the universality of the surfactant-assisted ice-templating strategy. Does this apply to all catalyst systems, or are there specific requirements for the precursors?", "reviewer_3": "The manuscript reports a new strategy to enhance the site utilization efficiency of FeNC single-atom catalysts by engineering an aggregation super-structure and the internal pore structure of MOF particles through freeze-drying the catalyst-dispersed solution followed by consecutive pyrolysis. The strategy is straightforward yet effective, and its potential for generalization to other catalytic systems is substantial. The authors used multiple techniques (in situ nitrite reduction and SI-SECM) to reliably measure the site density of the MOF-based FeNC catalyst and conducted extensive electron microscopic observations to elucidate the aggregation super-structure and internal pore structure of the FeNC single-atom catalyst. However, several key issues and clarity improvements are needed before publication. I recommend acceptance after the following revisions.\n1. MNC should be defined, presumably metal-nitrogen-carbon.\n2. There should be corresponding references for this statement: “The regulation of the size, framework, or hollow structures through hard/soft templating or organic acid etching methods has been proposed to mitigate these issues.”\n3. I would like to know whether the aggregation superstructure is still preserved even after electrode preparation from the catalysts. I think it is possible that the aggregation superstructure could be destroyed during the catalyst ink preparation process (such as sonication for catalyst dispersion) for the electrode coating in the half-cell or single-cell experiments.\n4. Fig. 1h: The EDS mapping results (in particular for Fe and N) are barely visible on screen and it becomes almost invisible in print. The author should modify the figures for better visibility.\n5. Fig. 1k: I had difficulty viewing the points and line for the real component of FT(k3χ(k)). Adopting a slightly more vivid colors would help.\n6. Fig. 1l: The authors used “Normalized χμ(E)” for the y-axis label, which is unfamiliar to me. I guess they intended to write “Normalized μ(E)” instead. Could the authors please check if the naming is correct. I am also interested in the energy difference of the edges for 2D and 3D FeNCs. Since the spectral shapes appear almost identical, I am concerned that the displayed energy difference may be due to instrumental error rather than a real effect. Therefore, I kindly ask the authors to reaffirm that the energy scales of the spectra were calibrated with the simultaneously collected reference spectra.\n7. Supplementary Fig. 2: It would be helpful to include a low-magnification SEM image of 3D Fe/ZIF-8 particles to compare their macrostructure of aggregated ZIF particles with that of 2D Fe/ZIF-8 particles, which is well depicted in Supplementary Fig. 1.\n8. Supplementary Fig. 4: I would like to see how different the 3D FeNC looks compared to 2D FeNC in TEM.\n9. Supplementary Fig. 6: Suggest changing “(b-c) C XPS spectra of (b) 3D FeNC and (c) 2D FeNC.” → “C 1s XP spectra of (b) 3D FeNC and (c) 2D FeNC.”\n10. Supplementary Fig. 6: I am not sure whether the XPS fitting has been done properly. The shape, binding energy position, and FWHM of the sub-peak are inconsistent between the 3D and 2D FeNCs. I am also uncertain whether C-N component has been actually reduced based on the C 1s XPS, therefore, I can’t agree with the main text stating “further evidenced by its reduced contents of C-N species ~”. I think it is necessary to compare the N 1s to C 1s intensity ratio in the XPS spectra of the two samples (2D and 3D FeNCs) to clarify this.\n11. Supplementary Fig. 7: The fitting results should be presented as lines rather than points.\n12. Fig. 2d: I noticed a break on the y-axis in the positive direction, which hampers a fair comparison.\n13. There should be a simple explanation on the physical meaning of the equation in the main text: ∆γ0=γparticle-solid-(γparticle-liquid+γsolid-liquid)>0.\n14. Suggested revision: ‘Supplementary Fig. S1’ → ‘Supplementary Fig. 1’\n15. Fig. 3a and b: The two graphs should be plotted on the same scale for proper comparison.\n16. Fig. 3e: The left y-axis is invisible in print.\n17. Fig. 3h and i: The graphs of the two samples should be plotted on the same scale for proper comparison.\n18. Supplementary Table 2: I think the authors should mention that the information in the table was plotted in Fig. 3k.\n19. Suggested modification: “Instead of the five-electron reduction of nitrosyl to ammonia as previously reported,” → “Instead of the five-electron reduction of nitrosyl to ammonia as previously reported,24” referring to “24 Malko, D., Kucernak, A. & Lopes, T. In situ electrochemical quantification of active sites in Fe–N/C non-precious metal catalysts. Nat. Commun. 7, 13285 (2016).”\n20. Suggested modification: “To corroborate our findings, we further employed in-situ SECM and surface-interrogation (SI)-SECM to quantify SDmass and TOF (Fig. 3g).” → “To corroborate our findings, we further employed in-situ SECM and surface-interrogation (SI)-SECM to quantify SDmass (Fig. 3g).”\nThe reason for the suggestion: I found the TOF value derived from the nitrite reduction test in Fig. 3f, but I could not find any discussion on the TOF values obtained with SI-SECM. I understand that it is self-evident that the TOF value from SI-SECM would be the same to the one from the nitrite reduction test, but the TOF has not been specifically mentioned and displayed. Therefore, I think it is more natural not to mention ‘TOF’ to avoid unnecessary misunderstanding.\n21. In the Methods section: I recommend that the authors clearly specify that the solution is aqueous, for example, by writing “KOH(aq) solution” rather than just “KOH solution”.\n22. In the Methods section: “The reference and counter electrodes were Ag/AgCl and Pt wire, respectively.” → “The reference and counter electrodes were an Ag/AgCl electrode and a Pt wire, respectively.”\n23. In the Methods section: I would like to know the unit of Jkin,mass.\n24. In the Methods section: “ECSA = Cdl/Cs”→ “ECSA = Cdl_CV/Cs”\n25. In the Methods section: A reference is necessary for this comment: ‘where Cs represents the specific capacitance of the total double layer (0.04 mF/cm²).’\n26. In ‘Cdl test and operando EIS analysis’: “, using a 5 mV sinus AC amplitude” → “, using a 5 mV sinusoidal AC amplitude”\n27. In ‘Cdl test and operando EIS analysis’: “Rct/CPEdl (Constant phase element) and Rmass/CPEmass (Supplementary Fig. 37).” → “Rct/CPEdl (Constant phase element) and Rmass/CPEmass (Supplementary Fig. 38).”\nIn Rct, the \"ct\" part is not subscripted, and the Supplementary Figure number should be revised.\n28. In ‘Cdl test and operando EIS analysis’: There has to be a reference for the sentence, “The DRT function was calculated using the Matlab software DRTtools, developed by the Ciucci group.”\n29. In ‘In situ ATR-FTIR tests’: The first sentence starts with “In-situ”, which is inconsistent with the title starting with “In situ”.\n30. In ‘Rechargeable liquid and flexible ZABs tests’: “Liquid ZABs were constructed in a two-electrode configuration using ~99.9% pure zinc foil as the anode and a carbon paper-based gas diffusion layer (TORAY-YLS 30T) as the cathode.” → “Liquid ZABs were constructed in a two-electrode configuration using ~99.9% pure zinc foil as the anode and a catalyst-coated gas diffusion layer (TORAY-YLS 30T carbon paper) as the cathode.”\n31. In ‘Rechargeable liquid and flexible ZABs tests’: “OCP, discharge, and charge polarization curves were recorded using a CHI760E electrochemical workstation.” → “OCP and discharge-charge polarization curves were recorded using a CHI760E electrochemical workstation.”", "reviewer_4": "This manuscript presents a novel Surfactant-Assisted Freeze-Casting (SAFC) approach to synthesise two-dimensional (2D) Fe–N–C single-atom catalysts (FeNC) with nearly 100% active site utilization (Usite). The authors demonstrate how the dual-stress effect from SDS surfactants and freeze alignment of Fe/ZIF-8 particles, leading to a hierarchical carbon framework with enhanced mesoporosity and electron/mass transport characteristics. The study provides convincing evidence for the enhanced oxygen reduction reaction (ORR) performance and generalises the method across various transition metal-based catalysts. However, there are some areas where authors need to be corrected.\n1.The authors present an elegant surfactant-assisted freeze-casting (SAFC) strategy to achieve 2D FeNC architectures with nearly 100% site utilization. While the morphological transformation is convincingly demonstrated with SEM, TEM, and HAADF-STEM in Figure 1, and EXAFS/XANES confirm atomic Fe N sites, a quantitative spatial mapping of site accessibility (e.g., via tomography or 3D HAADF reconstruction) would strengthen the claim of “nearly all Fe SAs being accessible.” Please consider adding or referencing a cross-sectional or depth-resolved image or simulation to rule out potential buried sites even in the 2D structure.\n2.How does the SDS concentration precisely control interparticle spacing and liquid-film thickness during ice-templating? Is there a theoretical model for the \"striped\" pattern formation in Fig. 1d?\n3.Figure 4 excellently combines BET, EIS, DRT, FEM simulation, and ATR-FTIR to link structure to function. The authors convincingly show superior mass transport and reaction kinetics in 2D FeNC. However, to assess reproducibility and statistical relevance, please provide standard deviation/error bars for key metrics like R_ct, R_mt, and C_dl across multiple batches (n ≥ 3). Also, clarify if the FEM models were based on real SEM data or idealized shapes, as the interpretation of diffusion pathways depends on such assumptions.\n4.The paper claims universality of the SAFC strategy for other metals (e.g., Co, Ni, Mn, Cu, Ir, Pt), mentioned briefly near the end and in Supplementary Figure 51. However, the main text lacks direct performance data or structural characterization for these 2D MNCs. For the strategy to be truly “universal,” consider showing at least one more detailed case study (e.g., 2D CoNC) with electrochemical data and corresponding figures. This would substantiate the generalizability claim more rigorously.\n5.Nitrite stripping assumes a 3e⁻ reduction pathway for hydroxylamine formation. How was this pathway verified, and could side reactions (e.g., 5e⁻ reduction to NH₃) inflate Usite estimates?\n6.The dual-stress mechanism (Fig. 2f) relies on SDS decomposition timing. Why does CTAB/PVP (higher decomposition T) cause structural collapse, while SDS succeeds? Is stress magnitude quantifiable? Can you provide quantitative comparisons for layer thickness or porosity?\n7.FEM simulations (Fig. 4f) model O₂ diffusion but neglect electrolyte/ion transport. How do concentrated ion flows impact performance?\n8.Flexible Zn-air batteries use 2D FeNC + RuO₂. Why is RuO₂ needed? Does FeNC alone suffice for bifunctional (ORR/OER) activity?\n9.Despite stability tests, did operando techniques (e.g., XAS) confirm Fe-SA integrity during ORR? Could mesopores accelerate metal leaching?"}, "author_response": "Response: Thank you very for your comments. We will carefully revise our manuscript according to your comments point by point.\n1. The authors claim that this strategy improves both the accessibility of catalytic sites and intrinsic activity. Please clarify which one dominates the enhanced performance would strengthen the mechanistic discussion.\nResponse: We demonstrate that the structural modulation of the catalyst at both micro- and macro-scales enhances the site utilization (U ) of Fe single atoms (SAs) during the ORR process. Moreover, the dual-stress effect induces abundant edge and defect sites, effectively tuning the intrinsic activity of the Fe centers. These findings are supported by the significantly increased mass site density (SD ) and turnover mass frequency (TOF) per active site of the 2D FeNC (6.57×1019 sites/g and 113.81 e site-1 s-1) compared to its 3D FeNC counterpart (1.64×1019 and 95.24 e site-1 s-1) quantified by the surface-interrogation (SI)-scanning electrochemical microscopy (SECM) test. Therefore, we believe that both the improved accessibility of catalytic sites and the enhanced intrinsic activity have contributed to the ORR performance. If it is necessary to determine which factor predominates in the performance enhancement, we suggest assessing this by comparing the respective fold changes of each factor relative to those of the 3D FeNC. As we can see, the enhancement factors of SD (4.11 times) and mass TOF (1.19 times) for 2D FeNC over 3D FeNC collectively indicate that improved U site is the primary contributor to the observed activity enhancement.\n2. Additionally, the reported 4-fold increase in active-site utilization raises the question of whether reducing the loading of the best 2D FeNC catalyst by a quarter would yield performance comparable to the 3D counterpart.\nResponse: According to your suggestion, we evaluated the ORR performance of 2D FeNC and 3D FeNC at various electrode loadings. Please check it in the revised manuscript, supporting information, and the following. We found that at the same loading, 2D FeNC consistently outperforms 3D FeNC (Supplementary Fig. 34a-c). Remarkably, even when the loading of 2D FeNC is only one-fourth that of 3D FeNC, it still exhibits superior ORR performance (Supplementary Fig. 34d). This enhancement is attributed not only to the significantly improved U of 2D FeNC (4.11 site times higher), but also to its increased intrinsic activity (1.19 times higher) compared to 3D FeNC.\n3. The observed differences in C-N content between 2D and 3D FeNC catalysts are attributed to stress-induced microporous expansion. However, it remains unclear whether this expansion also affects the density of FeN sites or Fe content, which is 4 critical for evaluating true site utilization. The author should explain it and make it clear.\nResponse: Firstly, to ensure the reliability of the Fe content determination, ICP measurements were performed on catalysts from different batches. The results show that the Fe contents in 2D and 3D FeNC are comparable (1.01 wt%). Moreover, XRD patterns (Supplementary Fig. 5) show no diffraction peaks except for carbon, indicating that the Fe species in both 2D and 3D FeNC are not formed as Fe-based nanoparticles, but exist in an atomically dispersed state. Importantly, we have now included high-resolution XPS spectra for C and N, along with quantitative analysis of the corresponding species (Supplementary Fig. 6). Please refer to the revised manuscript, supporting information, and the following. We found that the content of C- N species in 2D FeNC decreased from 27.9% to 22.4% compared to 3D FeNC. Further analysis of the N 1s spectra revealed that this reduction is primarily due to the loss of pyridinic N (from 27.6% to 21.9%). These observations are consistent with a previous reference (J. Am. Chem. Soc. 2018, 140, 11594-11598.), which reported that high-temperature pyrolysis selectively cleaves pyridinic N species adjacent to FeN sites, x thereby exposing FeN moieties to more defects and edge structures. Based on the x above results and relevant literature, we believe that stress-induced micropore expansion facilitates the cleavage of C-N species without damaging the FeN structure, x and these edge and defect structures induced by micropore expansion can effectively enhance both the U and the intrinsic activity of Fe single-atom sites.\n4. Regarding the in-situ nitrite stripping experiments, the authors propose a 3-electron transfer process forming hydroxylamine (NH OH), contrasting with the widely 2 reported 5-electron reduction to ammonium (NH +). A more detailed justification for 4 this mechanistic divergence is needed.\nResponse: Unlike CuN sites, NO - or NO - reduction at FeN more readily leads to 4 3 2 4 products involving fewer electron transfers rather than undergoing deep reduction to ammonia (NH ) (Angew. Chem. Int. Ed., 2023, 135(39), e202308044.). Accordingly, 3 Fe2+ species in FeN exhibit a preference for the reduction of NO (originating from 4 adsorbed NO - intermediates) to hydroxylamine (NH OH) rather than ammonia (NH ) 2 2 3 (Figure 1, copy from Nat. Commun., 2021, 12(1), 1856.). The reduction of NO on Fe sites is a pH-dependent Nernstian process, where proton-coupled electron transfer governs the formation of NH OH. This implies that at 2 lower pH, the formation of NH OH is favored, while at higher pH, a portion of NO is 2 instead reduced to N O. These findings indicate the average number of electrons 2 transferred per NO molecule is no more than three. Based on this, the site density calculated under the proposed 3e- mechanism should represent the theoretical lower limit, rather than exceeding the actual number of active sites as would be the case in a 5e- mechanism. More importantly, the number of active sites derived from the 3e- mechanism is in excellent agreement with the surface-interrogation (SI)-scanning electrochemical microscopy (SECM) quantification, further validating the rationality of using this mechanism in our site density estimation. Notably, this approach has also been applied and explained in previous reports (Nat. Mater., 2021, 20, 1385-1391).\n5. The preservation of the 2D structure after ink preparation and electrode fabrication should be verified by SEM characterization. Structural collapse during electrode preparation is unfavorable for the ORR performance.\nResponse: According to your suggestion, we have added SEM images of catalysts after ink preparation (2D FeNC/nafion/ink and 3D FeNC/nafion/ink). Please refer to the revised manuscript, Supporting Information, and the following. The images show that although the macroscopic array size in 2D FeNC/nafion/ink is reduced compared to original structure of 2D FeNC during ultrasonic dispersion, the well-defined array structures with a relatively smaller size are still clearly observed compared to 3D FeNC/nafion/ink. Notably, beyond mass transfer advantages, one important practical advantage of the macroscopic array in 2D FeNC lies in the abundant porous architecture within the electrode, which facilitates a more uniform ionomer distribution on the catalyst surface. Studies have shown that the pore morphology of the catalyst layer and the distribution of ionomers significantly influence electrode performance (Chem. Rev., 2015, 115 (9), 3433-3467.).\n6. Minor grammatical and logical errors should be carefully corrected to enhance clarity and precision.\nResponse: According to your suggestions, we have revised grammatical and logical errors. Please find them in the revised manuscript and Supporting Information.\nReviewer #2 (Remarks to the Author):\nIn this manuscript, the authors successfully constructed a FeNC catalyst featuring a well-defined two-dimensional (2D) architecture at the micrometer scale. Through a detailed synthesis mechanism and comprehensive quantitative analysis, they demonstrated that the single-atom utilization efficiency of the 2D FeNC catalyst approaches 100%, significantly surpassing that of its three-dimensional (3D) counterpart. 2D FeNC also shows exceptional ORR performance in alkaline media. Mechanistic elucidation and quantitative active-site analysis were performed in detail. The synthetic strategy proposed is both innovative and broadly applicable, extending beyond single-atom catalysts to other material systems. Overall, this work meets the high standards required for publication in Nature Communications after minor revisions.\nHowever, the following points require further clarification:\nResponse: Thank you very for your comments. We will carefully revise our manuscript according to your comments point by point.\n1. The interaction between SDS and Fe/ZIF-8 particles is straightforward, but the mechanism underlying CTAB’s role in regulating the structure of catalysts remains unclear.\nResponse: To further elucidate the interaction mechanism between surfactants and Fe/ZIF-8 in detail, we have added zeta potential measurements of Fe/ZIF-8 dispersed in various surfactant solutions, as well as thermogravimetric analysis (TGA) of 2D Fe/ZIF-8/surfactant. Please check them in the revised manuscript, supporting information, and the following. Zeta potential analysis indicates that Fe/ZIF-8 particles carry a positive surface charge. At low CTAB concentrations, the interaction between CTAB and Fe/ZIF-8 is primarily driven by hydrophobic interactions, where the C 16 alkyl chains of CTAB adsorb onto the hydrophobic domains of the Fe/ZIF-8 surface via hydrophobic aggregation. This increases the surface charge, thereby enhancing electrostatic repulsion and interparticle spacing. However, when the CTAB concentration exceeds its critical micelle concentration (CMC, ~0.9-1.0 mM), the formation of micelles or bilayer adsorption reduces the surface charge of Fe/ZIF-8, which in turn decreases the interparticle distance and results in the multilayered packing during freezing. In contrast, PVP with weaker charge modulation produces smaller, disordered arrays. These results unequivocally demonstrate the pivotal role of surfactant in controlling Fe/ZIF-8 alignment during freeze-casting.\nTGA reveals that compared with SDS, CTAB and PVP exhibit higher decomposition temperatures. When Fe/ZIF-8 is mixed with different surfactants, 2D Fe/ZIF-8_CTAB and 2D Fe/ZIF-8_PVP show a more prolonged weight-loss process and greater weight loss at the end of annealing (1100 °C) than 2D Fe/ZIF-8 and 2D Fe/ZIF-8_SDS. This behavior induces extended outward stress and continuous C-N bond cleavage during pyrolysis in both 2D Fe/ZIF-8_CTAB and 2D Fe/ZIF-8_PVP, ultimately leading to the formation of Fe-based nanoparticles. As a result, CTAB and PVP tend to promote the generation of multilayered array structures containing Fe-based nanoparticles, causing the loss of both Fe single-atom density and U , and consequently deteriorating the site ORR performance.\n2. The authors attribute the distinct Fe-based nanoparticle formation induced by CTAB and PVP to their higher decomposition temperatures compared to SDS. Experimental evidence should be provided to substantiate this claim.\nResponse: According to your suggestion, we have added thermogravimetric analysis (TGA) data for pure SDS, CTAB, and PVP, as well as their mixtures with Fe/ZIF-8 (i.e., 2D Fe/ZIF-8, 2D Fe/ZIF-8_CTAB, and 2D Fe/ZIF-8_PVP). Please check them in the revised manuscript, supporting information, and the following.\n3. In finite element simulations, the packing modes of particles vary, potentially affecting the number of particles within models of identical dimensions. Will this affect the results of the simulation?\nResponse: As you said, model construction in finite element simulations can be based on either a fixed number of particles or a fixed overall model size. To more intuitively illustrate the differences in mass transport between the two models, we chose to fix the model size and compare the O /OH- concentration at the same spatial positions over 2 time, as well as the time required for O /OH- to diffuse from the initial location to 2 another boundary. Under a fixed model size, the number of particles varies due to differences in packing structure, which, in electrode terms, corresponds to differences in catalyst loading at the same thickness. This motivated us to experimentally investigate the ORR performance of 2D FeNC with a reduced loading. The results show that even when the loading of 2D FeNC is only one-quarter that of 3D FeNC, it still exhibits superior ORR performance, which further demonstrates ability to increase the actual U of Fe single atoms. site\n4. The in-situ nitrite stripping method, as reported in the literature, typically detects less than half of the active sites due to mass transport and electron-transfer limitations. Given this constraint, how do the authors justify their near-100% atomic utilization? A deeper discussion to explain this discrepancy is essential to validate the exceptional efficiency claimed.\nResponse: As you said, the disordered stacking of particles and dense carbon structures in conventional catalysts generate substantial dead volume, which impedes mass and electron transport during catalysis. This is a key reason why the number of active sites measured by the in situ nitrite stripping method is often underestimated relative to the actual value. Notably, the 2D FeNC developed in this work features optimized architecture from macroscopic alignment to microscopic pore structure. A series of characterizations demonstrate its superior electron and mass transport properties compared to 3D FeNC. These advantages help overcome the limitations of conventional catalysts and raise the upper limit of active site quantification using the in situ nitrite stripping method. In contrast, surface-interrogation scanning electrochemical microscopy (SI-SECM), which is less affected by transport and conductivity constraints, is considered capable of measuring site numbers close to the actual values. We found that the number of active sites determined for 2D FeNC using the in situ nitrite stripping method is in good agreement with the SI-SECM results, further supporting the accuracy of this approach.\n5. For the SECM measurements, the catalyst was loaded into concave electrodes. How was the catalyst mass precisely determined? Without accurate mass quantification, the reported SD and TOF values may lack reliability. The authors should clarify the mass methodology or provide additional experimental data.\nResponse: The amount of catalyst loaded into the concave electrode is random and cannot be determined by weighing. Therefore, we estimated the catalyst mass on the concave electrode by comparing the electrochemically active surface areas (ECSA) obtained from rotating disk electrode (RDE) and concave electrode, respectively. Specifically, the catalyst mass on the RDE is known and fixed. By comparing the ECSAs derived from CV measurements in the non-Faradaic region on both the RDE and the concave electrode, the catalyst mass on the concave electrode can be calculated. For example, in the case of 2D FeNC, the mass-normalized ECSA measured on the RDE is 165.69 m2/g (obtained from Supplementary Fig. 40), and the ECSA on the concave electrode is 7.5 × 10-7 m2 (Supplementary Fig. 38). Thus, the catalyst mass on the concave electrode is calculated to be 4.52 ng.\n6. The interpretation of in-situ FTIR spectra should be checked carefully. For instance, the authors state: “As the potential decreases, 2D FeNC shows a much greater increase in *O - peak intensity compared to 3D FeNC, reflecting stronger O adsorption and a 2 2 more efficient first electron transfer (*O → *O -)”, while the first electron transfer step 2 2 in ORR should be *O → *OOH.\nThe four-electron ORR process proceeds via two fundamental pathways, as illustrated in Supplementary Fig. 58. The first is the associative mechanism, in which *O 2 undergoes protonation to form the *OOH intermediate. The second is the dissociative mechanism, where *O-O* directly cleaves into two *OH species, followed by sequential protonation. As shown in Fig. 4g, the peak at ~1050 cm-1 can be attributed to the *O-O* intermediate associated with the dissociative pathway, while the peak at ~1220 cm-1 corresponds to the *OOH intermediate from the associative pathway. Compared with 3D FeNC, the *OOH signal on 2D FeNC decreases significantly with decreasing potential, indicating its rapid consumption. In contrast, the *O-O* intermediate shows relatively minor changes. These results suggest that both catalysts preferentially follow the associative pathway, with 2D FeNC exhibiting higher ORR efficiency.\n7. The authors demonstrated the universality of the surfactant-assisted ice-templating strategy. Does this apply to all catalyst systems, or are there specific requirements for the precursors?\nResponse: The successful implementation of the ice-templating strategy, which is widely used for constructing hierarchical porous or ordered macroscopic structures, relies on the following key prerequisites (Nat. Rev. Methods Primers, 2024, 4(1), 28; Adv. Mater., 2020, 32, 1907176.): (1) Freezability of the system. The precursor system must be capable of freezing under controlled conditions. Typically, the precursors should be dispersible or soluble in water or another freezeable solvent, allowing the formation of an ice-crystal network that acts as a physical template. (2) Stable dispersion. A stable suspension of precursor particles in the freezing medium is essential to ensure uniform structure formation during the freezing process. Severe agglomeration or sedimentation may lead to structural defects or inhomogeneity. (3) Effective interaction with growing ice crystals. Precursor particles should be effectively excluded from the advancing ice front and enriched in the intercrystalline regions. This interaction is critical for generating ordered porous or aligned architectures. Surfactants can further assist by modifying surface properties and enhancing ice-particle interactions. (4) Controllability of the freezing process. The morphology and orientation of the final structure are determined by the freezing rate and direction. Slow directional freezing favors the formation of well-aligned macropores, while rapid freezing typically results in smaller, more disordered pores. (5) Structural integrity during post-treatment. During freeze-drying or pyrolysis, the precursor material must maintain its structural integrity to preserve the templated architecture. Materials with inherent mechanical robustness or those that undergo gradual carbonization (e.g., MOFs like ZIF-8) are preferable. In summary, the prerequisites for ice-templating include a precursor that is stably dispersed in a freezeable medium, exhibits appropriate interaction with ice crystals, and can retain the induced structure during subsequent drying or thermal treatment.\nReviewer #3 (Remarks to the Author):\nThe manuscript reports a new strategy to enhance the site utilization efficiency of FeNC single-atom catalysts by engineering an aggregation super-structure and the internal pore structure of MOF particles through freeze-drying the catalyst-dispersed solution followed by consecutive pyrolysis. The strategy is straightforward yet effective, and its potential for generalization to other catalytic systems is substantial. The authors used multiple techniques (in situ nitrite reduction and SI-SECM) to reliably measure the site density of the MOF-based FeNC catalyst and conducted extensive electron microscopic observations to elucidate the aggregation super-structure and internal pore structure of the FeNC single-atom catalyst. However, several key issues and clarity improvements are needed before publication. I recommend acceptance after the following revisions.\nResponse: Thank you very for your comments. We will carefully revise our manuscript according to your comments point by point.\n1. MNC should be defined, presumably metal-nitrogen-carbon.\nResponse: According to your suggestions, we have revised it. Please check it in the revised manuscript and the following.\nCarbon-supported single-atom catalysts, especially zeolitic imidazolate framework-8 derived metal-nitrogen-carbon (MNC), are viable candidates to replace precious metals for electrocatalysis due to their low cost and high efficiency.1,2\n2. There should be corresponding references for this statement: “The regulation of the size, framework, or hollow structures through hard/soft templating or organic acid etching methods has been proposed to mitigate these issues.”\nResponse: According to your suggestions, we have revised it. Please check it in the revised manuscript and the following.\nThe regulation of the size, framework, or hollow structures through hard/soft templating or organic acid etching methods has been proposed to mitigate these issues.6- 9\n3. I would like to know whether the aggregation superstructure is still preserved even after electrode preparation from the catalysts. I think it is possible that the aggregation superstructure could be destroyed during the catalyst ink preparation process (such as sonication for catalyst dispersion) for the electrode coating in the half-cell or single-cell experiments.\nResponse: According to your suggestion, we have added SEM images of catalysts after ink preparation (2D FeNC/nafion/ink and 3D FeNC/nafion/ink). Please check them in the revised manuscript, Supporting Information, and the following. The images show that although the macroscopic array size in 2D FeNC/nafion/ink is reduced compared to original structure of 2D FeNC during ultrasonic dispersion, the well-defined array structures with a relatively smaller size are still clearly observed compared to 3D FeNC/nafion/ink. Notably, beyond mass transfer advantages, one important practical advantage of the macroscopic array in 2D FeNC lies in the abundant porous architecture within the electrode, which facilitates a more uniform ionomer distribution on the catalyst surface. Studies have shown that the pore morphology of the catalyst layer and the distribution of ionomers significantly influence electrode performance (Chem. Rev., 2015, 115 (9), 3433-3467.).\n4. Fig. 1h: The EDS mapping results (in particular for Fe and N) are barely visible on screen and it becomes almost invisible in print. The author should modify the figures for better visibility.\nResponse: According to your suggestions, we have revised it. Please check it in the revised manuscript and the following.\n5. Fig. 1k: I had difficulty viewing the points and line for the real component of FT(k3χ(k)). Adopting a slightly more vivid colors would help.\nResponse: According to your suggestions, we have revised it. Please check it in the revised manuscript and the following.\n6. Fig. 1l: The authors used “Normalized χμ(E)” for the y-axis label, which is unfamiliar to me. I guess they intended to write “Normalized μ(E)” instead. Could the authors please check if the naming is correct. I am also interested in the energy difference of the edges for 2D and 3D FeNCs. Since the spectral shapes appear almost identical, I am concerned that the displayed energy difference may be due to instrumental error rather than a real effect. Therefore, I kindly ask the authors to reaffirm that the energy scales of the spectra were calibrated with the simultaneously collected reference spectra.\nResponse: According to your suggestions, we have revised it. Please check it in the revised manuscript and the following.\n7. Supplementary Fig. 2: It would be helpful to include a low-magnification SEM image of 3D Fe/ZIF-8 particles to compare their macrostructure of aggregated ZIF particles with that of 2D Fe/ZIF-8 particles, which is well depicted in Supplementary Fig. 1.\nResponse: According to your suggestions, we have added low-magnification SEM images of 3D Fe/ZIF-8. As can be seen, compared with 2D Fe/ZIF-8 (Supplementary Fig. 3), the Fe/ZIF-8 particles in 3D Fe/ZIF-8 exhibit a pronounced aggregated state with substantial dead volumes, which would render many active sites inaccessible during the reaction process.\n8. Supplementary Fig. 4: I would like to see how different the 3D FeNC looks compared to 2D FeNC in TEM.\nResponse: According to your suggestions, we have added comparative figures similar to that of 2D FeNC in Supplementary Fig. 4. Please check them in the revised Supporting Information and the following. Specifically, the differences between 3D FeNC and 2D FeNC observed in TEM can be summarized in two main aspects: (1) Individual carbon particles in the catalyst: The outward dual-stress effect induced by the SDS shell and the tight packing between particles leads to anisotropic shrinkage of the carbon particles, resulting in concave surfaces and expanded micropores in 2D FeNC. Therefore, in 3D FeNC, the carbon particles retain the intact dodecahedral structure inherited from Fe/ZIF-8, whereas the individual particles in 2D FeNC contain abundant hollow and concave features; (2) Particle packing within the catalyst: Due to the SAFC strategy that enables the formation of a single-layered 2D array of Fe/ZIF-8 particles, the derived 2D FeNC shows almost no particle overlap under TEM, while in contrast, 3D FeNC exhibits a large amount of particle stacking.\n9. Supplementary Fig. 6: Suggest changing “(b-c) C XPS spectra of (b) 3D FeNC and (c) 2D FeNC.” → “C 1s XP spectra of (b) 3D FeNC and (c) 2D FeNC.”\nResponse: According to your suggestions, we have revised all of them. Please check them in the revised manuscript, Supporting Information and the following.\n10. Supplementary Fig. 6: I am not sure whether the XPS fitting has been done properly. The shape, binding energy position, and FWHM of the sub-peak are inconsistent between the 3D and 2D FeNCs. I am also uncertain whether C-N component has been actually reduced based on the C 1s XPS, therefore, I can’t agree with the main text stating “further evidenced by its reduced contents of C-N species ~”. I think it is necessary to compare the N 1s to C 1s intensity ratio in the XPS spectra of the two samples (2D and 3D FeNCs) to clarify this.\nResponse: To ensure the accuracy of the results, we refitted the high-resolution C 1s XPS spectra while maintaining consistent peak shapes and full widths at half maximum (FWHM). Importantly, we have now included high-resolution XPS spectra for C and N, along with quantitative analysis of the corresponding species (Supplementary Fig. 6). Please refer to the revised manuscript, supporting information, and the following. We found that the content of C-N species in 2D FeNC decreased from 27.9% to 22.4% compared to 3D FeNC. Further analysis of the N 1s spectra revealed that this reduction is primarily due to the loss of pyridinic N (from 27.6% to 21.9%). These observations are consistent with a previous reference (J. Am. Chem. Soc. 2018, 140, 11594-11598.), which reported that high-temperature pyrolysis selectively cleaves pyridinic N species adjacent to FeN sites, x thereby exposing FeN moieties to more defects and edge x structures.\n11. Supplementary Fig. 7: The fitting results should be presented as lines rather than points.\nResponse: According to your suggestions, we have revised all of them. Please check them in the revised manuscript, Supporting Information and the following.\n12. Fig. 2d: I noticed a break on the y-axis in the positive direction, which hampers a fair comparison.\nResponse: According to your suggestions, we have revised it. Please check it in the revised manuscript, Supporting Information and the following.\n13. There should be a simple explanation on the physical meaning of the equation in the main text: ∆γ0=γparticle-solid-(γparticle-liquid+γsolid-liquid)>0.\nResponse: According to your suggestions, we have supplemented and clarified the above explanation. Please check them in the revised manuscript, Supporting Information, and the following for details.\n14. Suggested revision: ‘Supplementary Fig. S1’ → ‘Supplementary Fig. 1’\nResponse: According to your suggestions, we have revised it. Please check it in the revised Supporting Information and the following.\n15. Fig. 3a and b: The two graphs should be plotted on the same scale for proper comparison.\nResponse: According to your suggestions, we have revised it. Please check it in the revised manuscript and the following.\n16. Fig. 3e: The left y-axis is invisible in print.\nResponse: According to your suggestions, we have revised it. Please check it in the revised manuscript and the following.\n17. Fig. 3h and i: The graphs of the two samples should be plotted on the same scale for proper comparison.\nResponse: According to your suggestions, we have revised it. Please check it in the revised manuscript and the following.\n18. Supplementary Table 2: I think the authors should mention that the information in the table was plotted in Fig. 3k.\nResponse: In fact, the data presented in Fig. 3k are derived from Supplementary Table 4. According to your suggestion, we have added a clarification in the revised manuscript. Please check it in the revised manuscript and the following.\n19. Suggested modification: “Instead of the five-electron reduction of nitrosyl to ammonia as previously reported,” → “Instead of the five-electron reduction of nitrosyl to ammonia as previously reported,24” referring to “24 Malko, D., Kucernak, A. & Lopes, T. In situ electrochemical quantification of active sites in Fe–N/C non-precious metal catalysts. Nat. Commun. 7, 13285 (2016).”\nResponse: According to your suggestion, we have revised it in the manuscript. Please check it in the revised manuscript and the following.\n20. Suggested modification: “To corroborate our findings, we further employed in-situ SECM and surface-interrogation (SI)-SECM to quantify SDmass and TOF (Fig. 3g).” → “To corroborate our findings, we further employed in-situ SECM and surface-interrogation (SI)-SECM to quantify SD (Fig. 3g).”\nmass\nThe reason for the suggestion: I found the TOF value derived from the nitrite reduction test in Fig. 3f, but I could not find any discussion on the TOF values obtained with SI-SECM. I understand that it is self-evident that the TOF value from SI-SECM would be the same to the one from the nitrite reduction test, but the TOF has not been specifically mentioned and displayed. Therefore, I think it is more natural not to mention ‘TOF’ to avoid unnecessary misunderstanding.\nResponse: We sincerely apologize for not including the TOF data obtained from SI-SECM in the initial manuscript. However, we believe this data should be retained. The in situ nitrite stripping experiments were conducted in an acetate buffer solution at pH 5.2, while both the SI-SECM and RDE measurements were carried out in 0.1 M KOH. Due to the pH difference between in situ nitrite stripping experiments and RDE measurements, the ORR kinetics (particularly the kinetic current density) vary, leading to differences in the calculated TOF values. Given that this work focuses on the ORR performance of catalysts in alkaline media, we consider the TOF derived from SI-SECM measurements to be more relevant and comparable. It is also worth noting that the SD values obtained from both in situ nitrite stripping and SI-SECM are mass consistent, while the TOF values obtained from SI-SECM better reflect the intrinsic ORR activity differences between 2D FeNC and 3D FeNC under alkaline conditions.\nIn response to your suggestion, we have now included this data in the revised manuscript. Please check it in the revised manuscript and the following discussion for details.\n21. In the Methods section: I recommend that the authors clearly specify that the solution is aqueous, for example, by writing “KOH(aq) solution” rather than just “KOH solution”.\nResponse: According to your suggestion, we have revised it in the manuscript. Please check it in the revised manuscript and the following.\n22. In the Methods section: “The reference and counter electrodes were Ag/AgCl and Pt wire, respectively.” → “The reference and counter electrodes were an Ag/AgCl electrode and a Pt wire, respectively.”\nResponse: According to your suggestion, we have revised it in the manuscript. Please check it in the revised manuscript and the following.\n23. In the Methods section: I would like to know the unit of Jkin,mass.\nResponse: According to your suggestion, we have revised it in the manuscript. Please check it in the revised manuscript and the following.\n24. In the Methods section: “ECSA = Cdl/Cs”→ “ECSA = Cdl_CV/Cs”\nResponse: According to your suggestion, we have revised it in the manuscript. Please check it in the revised manuscript and the following.\n25. In the Methods section: A reference is necessary for this comment: ‘where Cs represents the specific capacitance of the total double layer (0.04 mF/cm²).’\nResponse: According to your suggestion, we have revised it in the manuscript. Please check it in the revised manuscript and the following.\n26. In ‘Cdl test and operando EIS analysis’: “, using a 5 mV sinus AC amplitude” → “, using a 5 mV sinusoidal AC amplitude”\nResponse: According to your suggestion, we have revised it in the manuscript. Please check it in the revised manuscript and the following.\n27. In ‘Cdl test and operando EIS analysis’: “Rct/CPEdl (Constant phase element) and Rmass/CPEmass (Supplementary Fig. 37).” → “Rct/CPEdl (Constant phase element) and Rmass/CPEmass (Supplementary Fig. 38).”\nIn Rct, the \"ct\" part is not subscripted, and the Supplementary Figure number should be revised.\nResponse: According to your suggestion, we have revised it in the manuscript. Please check it in the revised manuscript and the following.\n28. In ‘Cdl test and operando EIS analysis’: There has to be a reference for the sentence, “The DRT function was calculated using the Matlab software DRTtools, developed by the Ciucci group.”\nResponse: According to your suggestion, we have revised it in the manuscript. Please check it in the revised manuscript and the following.\n29. In ‘In situ ATR-FTIR tests’: The first sentence starts with “In-situ”, which is inconsistent with the title starting with “In situ”.\nResponse: According to your suggestion, we have revised it in the manuscript. Please check it in the revised manuscript and the following.\n30. In ‘Rechargeable liquid and flexible ZABs tests’: “Liquid ZABs were constructed in a two-electrode configuration using ~99.9% pure zinc foil as the anode and a carbon paper-based gas diffusion layer (TORAY-YLS 30T) as the cathode.” → “Liquid ZABs were constructed in a two-electrode configuration using ~99.9% pure zinc foil as the anode and a catalyst-coated gas diffusion layer (TORAY-YLS 30T carbon paper) as the cathode.”\nResponse: According to your suggestion, we have revised it in the manuscript. Please check it in the revised manuscript and the following.\n31. In ‘Rechargeable liquid and flexible ZABs tests’: “OCP, discharge, and charge polarization curves were recorded using a CHI760E electrochemical workstation.” → “OCP and discharge-charge polarization curves were recorded using a CHI760E electrochemical workstation.”\nResponse: According to your suggestion, we have revised it in the manuscript. Please check it in the revised manuscript and the following.\nReviewer #4 (Remarks to the Author):\nThis manuscript presents a novel Surfactant-Assisted Freeze-Casting (SAFC) approach to synthesise two-dimensional (2D) Fe–N–C single-atom catalysts (FeNC) with nearly 100% active site utilization (Usite). The authors demonstrate how the dual-stress effect from SDS surfactants and freeze alignment of Fe/ZIF-8 particles, leading to a hierarchical carbon framework with enhanced mesoporosity and electron/mass transport characteristics. The study provides convincing evidence for the enhanced oxygen reduction reaction (ORR) performance and generalises the method across various transition metal-based catalysts. However, there are some areas where authors need to be corrected.\nResponse: Thank you very for your comments. We will carefully revise our manuscript according to your comments point by point.\n1.The authors present an elegant surfactant-assisted freeze-casting (SAFC) strategy to achieve 2D FeNC architectures with nearly 100% site utilization. While the morphological transformation is convincingly demonstrated with SEM, TEM, and HAADF-STEM in Figure 1, and EXAFS/XANES confirm atomic Fe N sites, a quantitative spatial mapping of site accessibility (e.g., via tomography or 3D HAADF reconstruction) would strengthen the claim of “nearly all Fe SAs being accessible.” Please consider adding or referencing a cross-sectional or depth-resolved image or simulation to rule out potential buried sites even in the 2D structure.\nResponse: We sincerely thank the reviewer for this valuable suggestion. Following the recommendation, we attempted X-ray tomography to analyze the 3D structure and pore network of catalysts. However, due to the highly porous, aerogel-like structure and low density of 2D FeNC, the X-ray absorption contrast between the material and the pores (air) was too weak, which made it impossible to obtain reliable simulation results. Nevertheless, this further confirms the highly porous architecture of 2D FeNC. To better compare the structural differences between 2D FeNC and 3D FeNC from different perspectives, as suggested by the reviewer, we conducted cross-sectional SEM imaging and SEM analysis after mixing with Nafion, which can better mimic the stacking states of the catalysts in practical electrodes. The cross-sectional SEM images (Figure 1) reveal that 2D FeNC maintains a clear single-layered structure, whereas 3D FeNC exhibits a densely packed arrangement of carbon particles. More importantly, when both samples were prepared into inks, the images show that although the macroscopic array size in 2D FeNC/nafion/ink is reduced compared to original structure of 2D FeNC during ultrasonic dispersion, the well-defined array structures with a relatively smaller size are still clearly observed compared to 3D FeNC/nafion/ink (Supplementary Fig. 22). Notably, beyond mass transfer advantages, one important practical advantage of the macroscopic array in 2D FeNC lies in the abundant porous architecture within the electrode, which facilitates a more uniform ionomer distribution on the catalyst surface. Studies have shown that the pore morphology of the catalyst layer and the distribution of ionomers significantly influence electrode performance (Chem. Rev., 2015, 115 (9), 3433-3467.).\n2.How does the SDS concentration precisely control interparticle spacing and liquid-film thickness during ice-templating? Is there a theoretical model for the \"striped\" pattern formation in Fig. 1d?\nResponse: The regulation of particle spacing and liquid film thickness during the ice-templating process by SDS mainly originates from the modulation of steric hindrance and surface charge. First, upon the introduction of SDS, the steric hindrance between particles increases, leading to a larger interparticle distance. Meanwhile, the zeta potential (negative charge) on the particle surface is enhanced, which enlarges the diffuse layer thickness and strengthens the interparticle electrostatic repulsion. As a result, the interparticle spacing increases and the liquid film becomes thicker.\nThe “stripe” morphology observed in Figure 1d is related to the following factors: (1) during rapid freezing, minor perturbations at the solidification front lead to the formation of “particle-rich” and “particle-poor” regions, in which the “particle-poor” regions correspond to the gaps between the stripes; (2) during freeze casting, the concentration of colloidal particles must be strictly controlled, as excessively high concentrations result in particle aggregation and the formation of multilayered structures. Therefore, at relatively low concentrations, the rapid freezing process prevents particles from occupying all gaps, giving rise to the stripe-like voids; (3) compared with the annealed 3D Fe/ZIF-8 (Supplementary Fig. 1), the 2D FeNC shown in Figure 1d exhibits more pronounced stripe features, which can be attributed to the particle shrinkage during the annealing process.\n3. Figure 4 excellently combines BET, EIS, DRT, FEM simulation, and ATR-FTIR to link structure to function. The authors convincingly show superior mass transport and reaction kinetics in 2D FeNC. However, to assess reproducibility and statistical relevance, please provide standard deviation/error bars for key metrics like R_ct, R_mt, and C_dl across multiple batches (n ≥ 3). Also, clarify if the FEM models were based on real SEM data or idealized shapes, as the interpretation of diffusion pathways depends on such assumptions.\nResponse: According to your suggestion, we have added error bars for R , R and C ct mt dl based on multiple experimental repetitions; please refer to the manuscript, Supporting Information, and the following for details. Meanwhile, the FEM simulation models were constructed based on the morphology of carbon particles observed in SEM and TEM, where the carbon particles in 2D FeNC exhibit concave structures, whereas retain the regular dodecahedral geometry in 3D FeNC. It is worth noting that, for simplicity, the simulations were performed only on a single plane. In this model, 2D FeNC exhibits a band-like structure formed by the 2D single-layered arrangement of a certain number of carbon particles within the plane, whereas 3D FeNC shows significant overlapping and aggregation of particles, which is consistent with the SEM observations.\n4.The paper claims universality of the SAFC strategy for other metals (e.g., Co, Ni, Mn, Cu, Ir, Pt), mentioned briefly near the end and in Supplementary Figure 51. However, the main text lacks direct performance data or structural characterization for these 2D MNCs. For the strategy to be truly “universal,” consider showing at least one more detailed case study (e.g., 2D CoNC) with electrochemical data and corresponding figures. This would substantiate the generalizability claim more rigorously.\nResponse: According to your suggestion, we have supplemented the detailed comparison of the material characterizations and ORR performance of 2D CuNC and 3D CuNC. Please refer to the revised manuscript, the supporting information, and the following for details. Notably, to simplify the synthetic procedure and reduce the cost, we adopted a general host-guest strategy reported in the literature to prepare the M/ZIF-8 precursors (Nature chemistry, 2020, 12(8): 764-772.). Although the M/ZIF-8 obtained by this strategy exhibits larger particle sizes compared with Fe/ZIF-8, the resulting 2D MNCs still demonstrate well-defined 2D array structures and excellent electrocatalytic performance.\n5.Nitrite stripping assumes a 3e⁻ reduction pathway for hydroxylamine formation. How was this pathway verified, and could side reactions (e.g., 5e⁻ reduction to NH₃) inflate Usite estimates?\nResponse: Unlike CuN sites which tend to favor ammonia (NH ) synthesis, NO - or 4 3 3 NO - reduction at FeN more readily leads to products involving fewer electron 2 4 transfers rather than undergoing deep reduction to NH (Angew. Chem. Int. Ed., 2023, 3 135(39), e202308044.). Accordingly, Fe2+ species in FeN exhibit a preference for the 4 reduction of NO (originating from adsorbed NO - intermediates) to hydroxylamine 2 (NH OH) rather than ammonia (NH ) (Figure 1, copy from Nat. Commun., 2021, 12(1), 2 3 1856.). The reduction of NO on Fe sites is a pH-dependent Nernstian process, where proton-coupled electron transfer governs the formation of NH OH. This implies that at 2 lower pH, the formation of NH OH is favored, while at higher pH, a portion of NO is 2 instead reduced to N O. These findings indicate the average number of electrons 2 transferred per NO molecule is no more than three. Based on this, the site density calculated under the proposed 3e- mechanism should represent the theoretical lower limit, rather than exceeding the actual number of active sites as would be the case in a 5e- mechanism. More importantly, the number of active sites derived from the 3e- mechanism is in excellent agreement with the surface-interrogation (SI)-scanning electrochemical microscopy (SECM) quantification, further validating the rationality of using this mechanism in our site density estimation. Notably, this approach has also been applied and explained in previous reports (Nat. Mater., 2021, 20, 1385-1391).\n6.The dual-stress mechanism (Fig. 2f) relies on SDS decomposition timing. Why does CTAB/PVP (higher decomposition T) cause structural collapse, while SDS succeeds? Is stress magnitude quantifiable? Can you provide quantitative comparisons for layer thickness or porosity?\nResponse: To further elucidate the interaction mechanism between surfactants and Fe/ZIF-8 during pyrolysis in detail, we have added thermogravimetric analysis (TGA) of 2D Fe/ZIF-8/surfactant. Please check them in the revised manuscript, supporting information, and the following. TGA reveals that compared with SDS, CTAB and PVP exhibit higher decomposition temperatures. When Fe/ZIF-8 is mixed with different surfactants, 2D Fe/ZIF-8_CTAB and 2D Fe/ZIF-8_PVP show a more prolonged weight-loss process and greater weight loss at the end of annealing (1100 °C) than 2D Fe/ZIF-8 and 2D Fe/ZIF-8_SDS. This behavior induces extended outward stress and continuous C-N bond cleavage during pyrolysis in both 2D Fe/ZIF-8_CTAB and 2D Fe/ZIF-8_PVP, ultimately leading to the formation of Fe-based nanoparticles. As a result, CTAB and PVP tend to promote the generation of multilayered array structures containing Fe-based nanoparticles, causing a dual loss of Fe single-atom density and U , and consequently deteriorating the site ORR performance. To quantitatively assess the distinct pore structures of the catalysts induced by different surfactants, we have supplemented the Brunauer-Emmett-Teller (BET) measurements of 2D FeNC_CTAB and 2D FeNC_PVP, and further obtained the distributions of micropores, mesopores, and macropores through non-local density functional theory (NLDFT) calculations. Please refer to the revised manuscript, the supporting information, and the following for details. Overall, similar to 2D FeNC, the expansion of micropores results in a relatively higher proportion of mesopores (Supplementary Fig. 42 and Supplementary Table S5). Meanwhile, due to the larger and more persistent stresses induced by CTAB and PVP, mesopores are further transformed into larger pore structures. However, the formation of Fe-based nanoparticles leads to partial blockage of mesopores, resulting in a significantly lower overall pore volume of 2D FeNC_CTAB and 2D FeNC_PVP compared to 2D FeNC.\n7. FEM simulations (Fig. 4f) model O₂ diffusion but neglect electrolyte/ion transport. How do concentrated ion flows impact performance?\nResponse: According to your suggestions, we have added FEM simulations of OH- transport performance in different models. Please refer to the revised manuscript, the supporting information, and the following for details.\n8. Flexible Zn-air batteries use 2D FeNC + RuO . Why is RuO₂ needed? Does FeNC 2 alone suffice for bifunctional (ORR/OER) activity?\nResponse: According to your suggestion, we tested the OER performance of 2D FeNC. Please check it in the following (Figure 2). It can be clearly observed that 2D FeNC exhibits almost no OER activity. Therefore, in order to achieve good catalytic performance of the Zn-air batteries, we employed commercial RuO with superior OER 2 performance as the catalyst for the OER process.\n9.Despite stability tests, did operando techniques (e.g., XAS) confirm Fe-SA integrity during ORR? Could mesopores accelerate metal leaching?\nResponse: We sincerely thank the reviewer for the insightful suggestion regarding operando characterization (e.g., XAS) to further monitor the stability of Fe SAs during the ORR process. While such advanced techniques would indeed provide additional mechanistic insights, the primary objective of this work is to explore structural modulation of the catalyst at both micro- and macro-scales to enhance the U of single- site atom catalysts during electrocatalysis. In fact, according to references, it is often the microporous structures that accelerate metal leaching during the electrocatalytic process (Energy Environ. Sci., 2018, 11, 365). Therefore, the expansion of micropores and the increased mesopore proportion in 2D FeNC can effectively alleviate metal dissolution. To address the reviewer’s concern, we have carried out systematic ex-situ analyses, including ICP and XPS, before and after durability tests. These results clearly demonstrate that the Fe sites are well preserved, with negligible leaching or structural change, thereby confirming the stability of the catalyst under ORR operating conditions. We believe these complementary studies provide strong and reliable evidence that supports the robustness of the active sites.\n\nReply to reviewers\nReviewer #3 (Remarks to the Author):\nI think the authors responded well to most comments. However, I found two comments remain unresolved:\nResponse: Thank you so much for your comments.\n- In the caption of Fig. S6, “C XPS” and “N XPS” are still used. I recommend changing these to \"C 1s XPS\" and \"N 1s XPS.\"\nResponse: According to your suggestions, we have revised this, please check it in the revised in the Supporting Information and the following.\n- In Comment 6, I mentioned that the authors should check the energy calibration for their Fe K EXAFS data for 2D and 3D FeNC, but this does not appear to have been done.\nResponse: According to your suggestion, we have provided the Fe K-edge EXAFS data calibrated using Fe foil from the same batch. Specifically, 3323-1_Fe and 3323-2_Fe correspond to the 3D FeNC and 2D FeNC samples in the manuscript, respectively. As can be seen, after verifying the energy calibration, the 2D FeNC exhibits a lower electron density around Fe than the 3D FeNC, which may be attributed to the optimization of the Fe single-atom electronic structure induced by defects and edge structures."}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors have addressed all my concerns and it can be published as is now.", "reviewer_3": "I think the authors responded well to most comments. However, I found two comments remain unresolved:\n- In the caption of Fig. S6, “C XPS” and “N XPS” are still used. I recommend changing these to \"C 1s XPS\" and \"N 1s XPS.\"\n- In Comment 6, I mentioned that the authors should check the energy calibration for their Fe K EXAFS data for 2D and 3D FeNC, but this does not appear to have been done.", "reviewer_4": "The authors have addressed all concerns from the reviewer. It is a good shape for the publication in Nature Communications."}, "author_response": "Response: Thank you so much for your comments.\n- In the caption of Fig. S6, “C XPS” and “N XPS” are still used. I recommend changing these to \"C 1s XPS\" and \"N 1s XPS.\"\nResponse: According to your suggestions, we have revised this, please check it in the revised in the Supporting Information and the following.\n- In Comment 6, I mentioned that the authors should check the energy calibration for their Fe K EXAFS data for 2D and 3D FeNC, but this does not appear to have been done.\nResponse: According to your suggestion, we have provided the Fe K-edge EXAFS data calibrated using Fe foil from the same batch. Specifically, 3323-1_Fe and 3323-2_Fe correspond to the 3D FeNC and 2D FeNC samples in the manuscript, respectively. As can be seen, after verifying the energy calibration, the 2D FeNC exhibits a lower electron density around Fe than the 3D FeNC, which may be attributed to the optimization of the Fe single-atom electronic structure induced by defects and edge structures."}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors replied well to my comments and questions. I think the manuscript can be accepted for publication."}, "author_response": ""}]

Decision letter