Elucidating activity-stability trade-offs in nano-fingerprint carbon anchoring single atoms and clusters in oxygen reduction reaction

Domain: chemistry
Published: 2026-03-06
Editorial outcome: major_revision

Abstract
Hybrid sites (M ) that incorporate single atoms (M ) with clusters (M ) hold great
1+x 1 x
potential for overcoming linear scaling relations in electrocatalysis, but spatial decoupling of M
1
from M fundamentally hampers intercomponent synergy. Here, we report a composite
x
electrocatalyst (Zn Fe /f-NC) featuring Fe clusters encapsulated within nano-fingerprint carbon
1 1+x x
layers and surrounded by densely dispersed Fe/Zn-N atomic sites. Decoupling experiments
4
revealed that Fe /Zn atomic sites mainly initiate catalytic activity, while Fe clusters further boost
1 1 x
intrinsic activity, whereas nano-fingerprint carbon facilitates the confinement and stabilization of
M . The Zn Fe /f-NC demonstrates competitive oxygen reduction reaction performance,
1+x 1 1+x
achieving a half-wave potential of 0.93 V and stability over 50 hours. Zinc-air batteries with
Zn Fe /f-NC air cathode exhibit a peak power density of 263.82 mW·cm-2 and robust stability
1 1+x
exceeding 2200 hours at 10 mA cm-2. Density functional theory calculations reveal that the atomic
Zn sites, Fe clusters and nano-fingerprint carbon layers jointly enhance the catalytic activity and
x
stability by modulating the electronic structure of Fe-N sites and optimizing the adsorption energy
4
of the key intermediate, especially OH*.

Introduction
The energy crisis and the environmental pollution caused by using traditional fossil fuel promote
the development of other renewable energy technology such as the application of fuel cell and
metal-air battery.1, 2 However, the cathodic oxygen reduction reaction (ORR) slow sluggish
kinetics for multi-step electron and proton transfer process.3, 4, 5 The platinum-based catalysts are
the most commonly used and effective ORR catalysts today especially in proton exchange
membrane fuel cells,6 however, the inferior activity and stability of Pt in alkaline electrolytes limit
its application in anion-exchange membrane fuel cells and metal-air batteries.7, 8 Moreover, the
high cost and low storage capacity of Pt greatly limit large-scale commercialization, thereby low
cost, high efficiency and strong stability of non-noble metal-based ORR electrocatalyst with high
efficiency and stability is desirable to replace Pt-based catalysts.9, 10 Single atom catalysts (SAC)
containing metal-nitrogen sites (M-N-C) has demonstrated comparable or even favorable activity
to Pt-based catalysts especially in alkaline ORR,11, 12 which is benefited from its distinct
coordinative and electronic state.13 The M-N-C configuration has attracted attention because it has
an incomplete d orbital and is easier to accept electrons provided by ORR intermediates.14, 15, 16, 17
Among which atomically dispersed Fe-N-C catalysts with Fe-N sites are most widely studied and
4
considered as one of the most potential candidates due to their adjustable sites, convenience of
synthesis and low cost.18, 19

However, Fe-N has a highly symmetrical electronic structure distribution and the adsorption of
4
ORR intermediates (O* and OH*) on FeN
4
sites is still too strong compared to Pt,20, 21 which slows
down the reaction kinetics and limits the overall ORR activity. In addition, the migration and
aggregation of single atomic sites under continuous operating conditions, leading to a decline in
activity and stability, present a significant hurdle that must be overcome for their potential
commercialization.22 To this end, an effective method is to introduce exotic metal sites or form
metal clusters for synergistic catalysis.23 This method regulates the coordination and electronic
structure through pairing or long-range coupling of adjacent exotic metal sites and clusters, thereby
adjusts the adsorption intensity of Fe-N active sites for ORR intermediates.24 Another effective
4
approach is adjustment of support for carbon-hosted Fe-N sites,25 in which the spatial and
4
geometric structure of the supporting carbon framework of SAC is crucial in regulating the metal-
support interaction and the structure of metal sites.26 Compared with a flat surface without strain,
a curved carbon support facilitates the induction of intrinsic micro-strain, which affects the d-
orbitals and electronic structure of metal atoms.27 In addition, highly curved nanostructures can
regulate the electric field around the catalyst surface and adjust the microenvironment at the
reaction sites, thereby high activity and stability can be achieved simultaneously in ORR.28 The
above regulation means of electronic or orbital structure of Fe-N are usually separate, and the
4
cooperative regulation of more factors are rarely studied but instrumental for the intricate
environment of Fe-N embedded in carbon matrices.29, 30

Herein, a composite catalyst containing Fe clusters encapsulated within nano-fingerprint carbon
x
layers and surrounded by densely dispersed Fe/Zn-N sites (denoted as Zn Fe /f-NC) was
4 1 1+x
obtained. A series of decoupled control samples, including those with dual Fe/Zn single-atom sites
anchored on nano-fingerprint carbon (denoted as Zn Fe /f-NCS), dual Fe/Zn single-atom sites co-
1 1
anchored with Fe
x
sites on planar carbon (denoted as Zn Fe /NC), and dual Fe/Zn single-atom
1 1+x
sites on planar carbon (denoted as Zn Fe /NC), were also prepared. Decoupling experiments
1 1
revealed that Fe
x
clusters promoted the high activity of Fe/Zn-N
4
atomic sites, while the nano-
fingerprint carbon layers facilitate the confinement and stabilization of hybrid (M ) sites. In 0.1
1+x
M KOH electrolyte, the prepared Zn Fe /f-NC displayed competitive ORR performance with
1 1+x
E = 0.93 V, surpassing that of Zn Fe /f-NC (E = 0.904 V), Zn Fe /NC (E = 0.89 V),
1/2 1 1 1/2 1 1 1/2
Zn Fe /NC (E = 0.925 V) and Pt/C catalyst. After 50 hours of stability testing, Zn Fe /f-NC
1 1+x 1/2 1 1+x
and Zn Fe /f-NC exhibited less attenuation than Zn Fe /NC and Zn Fe /NC, demonstrating the
1 1 1 1+x 1 1
crucial role of nano-fingerprint carbon in enhancing stability The Zn Fe /f-NC catalyst was
1 1+x
further evaluated as the cathode by assembling zinc-air battery (ZAB). The Zn Fe /f-NC air
1 1+x
cathode showed a high power density (263.82 mW·cm-2) in ZAB, which is much higher than Pt/C
(145.22 mW·cm-2). The Zn Fe /f-NC-based ZAB exhibits high stability for over 2200 hours and
1 1+x
good resistance to deformation. Density functional theory (DFT) calculations have demonstrated
that introducing each site (single atom Zn, Fe clusters or nano-fingerprint carbon layers) in the
composite catalyst can optimize the adsorption of key intermediate OH* on Fe-N . Therefore,
4
single atom Zn/Fe, Fe clusters and nano-fingerprint carbon layer jointly enhanced the catalytic
x
performance in ORR through the multivariate regulation of electronic structure.

Results
Synthesis and Characterization
The utilization of ZIF-8 as a precursor for coordinating metal ions via high-temperature pyrolysis
has emerged as a prevalent approach for synthesizing nitrogen-doped carbon-anchored transition
metal catalysts. Typically, single-atom metal catalysts can be obtained by removing metal salts
that are weakly bound to ZIF-8. Figure 1a outlines the synthetic process of Zn Fe /f-NC and
1 1+x
Zn Fe /NC catalysts. Initially, the synthesized ZIF-8 undergoes stirring to adsorb Fe(acac) and
1 1 3
subsequent centrifugation without washing, resulting in supersaturated adsorption of Fe salts
(denoted as Fe(acac) @ZIF-8). After calcination at 1000 °C, Fe(acac) @ZIF-8 evolves into a
3 3
catalyst comprising Fe/Zn single-atom, and Fe clusters anchored on nano-fingerprint nitrogen-
x
doped carbon, which is labeled as Zn Fe /f-NC. The solid obtained by centrifugation and
1 1+x
washing after ZIF-8 adsorbing Fe(acac) (denoted as Fe(acac) -ZIF-8), and then calcined at
3 3
1000 °C, will yield a catalyst containing only single atom Fe and Zn without Fe clusters and
x
fingerprint carbon layers, which is labeled as Zn Fe /NC. It should be noted that based on the
1 1
synthetic pathways of these two fundamental catalysts, we also prepared two more comparative
samples for decoupling experiments, including Zn Fe /NC and Zn Fe /f-NC (where “M ”, “M ”,
1 1+x 1 1 1 x
and “f” represent “single-atom site”, “clusters”, and “fingerprint carbon layers”, respectively; more
details see the part of Methods).

The X-ray diffraction (XRD) patterns of the precursors Fe(acac) @ZIF-8 and Fe(acac) -ZIF-8
3 3
show obvious reduction in peaks intensity compared with ZIF-8 (Figure S1), indicating that
increasing the amount of Fe salt loaded onto ZIF-8 increasingly perturbs its crystallinity, while not
destroying the overall structure. The scanning electron microscopy (SEM) images of the precursors
Fe(acac) @ZIF-8 and Fe(acac) -ZIF-8 also prove that the introduction of Fe salt to ZIF-8 didn’t
3 3
change the morphology (Figure S2-S4). After pyrolysis at high temperature, the XRD patterns of
Zn Fe /f-NC and Zn Fe /NC show that there are no characteristic diffraction peaks of metal and
1 1+x 1 1
metal oxides except two broad peaks are observed at ≈24o and 44o corresponding to the (002) and
(010) crystal facets of graphitic carbon (Figure S5). The Zn Fe /f-NC and Zn Fe /NC is basically
1 1+x 1 1
maintaining a morphology similar to Fe(acac) @ZIF-8 and Fe(acac) -ZIF-8 with slightly smaller
3 3
size due to shrinkage (Figure 1b and S6). It was determined by inductively coupled plasma-optical
emission spectroscopy (ICP-OES) measurement that the Fe content in Zn Fe /f-NC was 0.90
1 1+x
wt%. The Brunauer-Emmett-Teller (BET) analysis revealed that the surface area of Zn Fe /f-NC
1 1+x
(958.7 m2 g-1) is slightly smaller than Zn Fe /NC (1140.6 m2 g-1) (Figure S7), and both achieved
1 1
a high surface area.

As shown by SEM and transmission electron microscope (TEM) images in Figure 1b, c, S6, and
S8, the morphology of Zn Fe /f-NC and Zn Fe /NC is similar to ZIF-8 with polyhedral size of
1 1+x 1 1
about 250 nm. The element mapping images of the relevant area shows that Fe, Zn, C and N
elements are uniformly distributed on Zn Fe /f-NC (Figure 1d). The high-resolution transmission
1 1+x
electron microscope (HRTEM) image reveals that an obvious multi-layer carbon like fingerprints
with a diameter of 8 nm were observed on the surface of the polyhedral particles in Zn Fe /f-NC
1 1+x
(Figure 1e), and this type of nano-fingerprint carbon layer is widely present (Figure S9). In
Zn Fe /NC (Figures S10-S12) and sample obtained by Fe(acac) @ZIF-8 annealed at 900 oC
1 1 3
(Figure S13 and S14), no multilayer carbon is observed. These indicate that supersaturated
adsorption of Fe(acac) and pyrolysis at 1000 oC are the key factors that induce the formation of
3
particular nano-fingerprint carbons layer structures. Recent studies have shown that adjusting the
concentration of iron metal salt can increase the curling degree of graphitic carbon layer.31, 32, 33
The high-angle annular bright-field scanning transmission electron microscope (HAABF-STEM)
image shows more clearly that the layer spacing of nano-fingerprint carbon layer is about 0.353
nm (Figure 1f), which is slightly larger than the spacing of some common nitrogen-doped
graphitized carbon layers.34 The high-angle annular dark-field scanning transmission electron
microscope (HAADF-STEM) image (Figure 1g) shown that a large number of atomically
dispersed bright spots can be clearly identified on the carbon support, which are the single Fe and
Zn atoms. The Fe clusters (marked by red circles) surrounded with Fe/Zn single-atoms (marked
by orange circles) were also observed as multiple bright spots gathered together on the nano-
fingerprint carbon layer, and the size of the Fe clusters are about 1 nm. It has been reported that
when the spatial distance between the cluster and the single atom is sub-nanometer, there is a
strong electronic coupling effect.35, 36, 37 In Figure S15, the number of Zn/Fe single atoms within
1.0 nm range around the cluster was counted, and calculated the average distance distribution from
single-atom to cluster. As shown in Figure S12, the HAADF-STEM image of Zn Fe /NC shows
1 1
that a large number of single atoms (marked by blue circles) are evenly distributed on the carbon
support without nano-fingerprint carbon layer and also lacking clusters. Subsequently, we
investigated the possible reasons for the generation of the nano-fingerprint carbon structure. Figure
S16 shows HRTEM image of the prepared “Zn /NC” catalyst from direct calcination of ZIF-8, but
1
no curled nano-fingerprint carbon layer appears. “Zn Fe /NC” catalyst was prepared by
1 1+x
impregnating the obtained Zn Fe /NC sample with an excess of Fe precursor, followed by
1 1
calcination. The resulting Zn Fe /NC catalyst also lacks the fingerprint-like carbon layer
1 1+x
structure, demonstrating that the formation of nano-fingerprint carbon layer is a self-catalyzed, in-
situ process requiring an appropriate amount of Fe precursor at a suitable calcination temperature.
In contrast, the nano-fingerprint carbon layers structure cannot be obtained by re-introducing Fe
precursors into the pre-graphitized carbon substrate (Figure S17).

The X-ray photoelectron spectroscopy (XPS) was used to characterize the electronic structure
of Zn Fe /f-NC and Zn Fe /NC. The measured full spectrum of Zn Fe /f-NC illustrates the
1 1+x 1 1 1 1+x
presence of Zn, Fe, C and N signals (Figure S18). The high-resolution N 1s spectrum of Zn Fe /f-
1 1+x
NC, Zn Fe /NC and Zn /NC (Figure S19a) can be deconvoluted into pyridinic-N (398.3 eV),
1 1 1
metal-N (M-N , 399.1 eV), pyrrolic-N (400.6 eV), graphitic-N (401.3 eV) and N-oxide- (403.2
x
eV), respectively. The peak area of M-N in Zn Fe /f-NC and Zn Fe /NC is larger than that in
x 1 1+x 1 1
Zn /NC, indicating that there are more pyridinic N coordinated with more single atom metal
1
species. The high-resolution C 1s spectrum (Figure S19b) of Zn /NC, Zn Fe /f-NC and
1 1 1+x
Zn Fe /NC can be deconvoluted into four peaks of C-C (284.8 eV), C-N (285.8 eV), C-O (287.0
1 1
eV) and C=O (289.7 eV), respectively. The Fe 2p fitting curves in Figure S20 show the presence
of positively charged iron species of Fe2+ and Fe3+. Table S1 shows the proportions of the two
valence states of Fe species in Zn Fe /f-NC (Fe2+ content 68 %, Fe3+ content 32 %) and
1 1+x
Zn Fe /NC (Fe2+ content 57 %, Fe3+ content 43 %). Mössbauer deconvolution of the transmission
1 1
profiles reveals that D1, D2 and D3 doublets are the dominant Fe sites in both Zn Fe /f-NC and
1 1+x
Zn Fe /NC (Figure 2a and 2b). D1 has been observed in a large number of Fe-N-C catalysts and
1 1
is usually attributed to low-spin Fe(II)-N or high-spin Fe(III)-N moiety (with O adsorbed),
4 4 2
which have been identified as highly active species for ORR in contrast to the D2 sites (D2 sites
represent medium-spin Fe(II)-N ).35, 38 On the contrary, the D1 sites exhibit less stable compared
4
to D2 sites.39 The percentages of D1 and D2 are 58.1% and 33.2% for Zn Fe /f-NC, respectively,
1 1+x
and 60.4% and 19.9% for Zn Fe /NC, respectively (Table S2). These findings indicate that
1 1
compared to Zn Fe /NC, Zn Fe /f-NC shows a better balance of activity and stability regulation.
1 1 1 1+x
In particular, D3 in Zn Fe /f-NC is a minor, high s-electron-density species interpreted as Fe-C
1 1+x
coupled (metal-like/carbide-like or encapsulated Fe) because of its negative IS and relatively small
QS.40 Overall, the Mössbauer spectroscopy show that nano-fingerprint carbon layers increases
local s-electron density at a subset of Fe sites and changes local symmetry.41 Raman spectra (Figure
S21) shows the I /I ratio of Zn Fe /NC and Zn /NC are 0.99, and the value of Zn Fe /f-NC is
D G 1 1 1 1 1+x
1.04, which indicates that the nano-fingerprint carbon layer structure destroys the graphitized
carbon lattice and thereby produce the defects synchronously.

As shown in the X-ray absorption near-edge structure (XANES) spectra (Figure 2c), the Fe K-
edge of Zn Fe /NC is located between Fe foil and Fe O , indicating that the oxidation state of Fe
1 1+x 2 3
is between 0 and +3. Compared with the Zn Fe /NC, the absorption edge energy of Zn Fe /f-NC
1 1 1 1+x
is slightly lower, which is because of the existence of Fe cluster. Figure 2d is the Fourier transform
extended X-ray absorption fine structure (FTI-EXAFS) spectra of the samples, Zn Fe /f-NC and
1 1+x
Zn Fe /NC have a prominent peak at about 1.5 Å, indicating Fe-N scattering. There is another
1 1
smaller peak at 2.3 Å, which is attributed to the Fe-Fe bonds in Fe clusters. As shown in Figure
S22, the valence state of Zn in Zn Fe /NC and Zn Fe /NC is between 0 and +2. The FT-EXAFS
1 1+x 1 1
spectra of Zn K-edge demonstrates that Zn Fe /f-NC and Zn Fe /NC only exist Zn-N bonds and
1 1+x 1 1
while no Zn-Zn or Zn-O bonds could be found (Figure 2e), indicating the atomically dispersed Zn
on the NC support. In the wavelet transform (WT) analysis (Figure 2f), Zn Fe /f-NC and
1 1+x
Zn Fe /NC have the highest intensity scattering path at about 4.5 Å, which can be inferred that the
1 1
Fe atoms of Zn Fe /f-NC and Zn Fe /NC are typical Fe-N coordination. In addition, Fe-Fe
1 1+x 1 1
scattering paths were observed at about 5.5 Å for Zn Fe /f-NC, indicating that the material
1 1+x
contains Fe clusters. In the WT analysis of Zn (Figure S23), there is only one intensity maximum
in both samples, proving that Zn is atomically dispersed in both Zn Fe /f-NC and Zn Fe /NC.
1 1+x 1 1
Figure 2g, 2h and Table S3 and S4 shows the best fitting curves for Zn Fe /f-NC, the fitting result
1 1+x
of Zn Fe /NC sample revealed that the Fe-N with Fe-N bond length of 1.85 Å is the main
1 1 4
structural features. The Zn is coordinated with four nitrogen atoms with Zn-N bond length of 1.88
Å for both Zn Fe /f-NC and Zn Fe /NC samples (Table S4).42, 43 Figure 2i shows the XANES
1 1+x 1 1
spectra of the C K- edge in Zn Fe /f-NC and Zn Fe /NC. The p* and s* intensities of Zn Fe /f-
1 1+x 1 1 1 1+x
NC are lower than those of Zn Fe /NC, which is due to the fact that the curved structure of the
1 1
nano-fingerprint carbon layer destroys the growth order of sp2 carbon. Although the Raman spectra
of Zn Fe /f-NC (I /I =1.04) indicate a high defect density, the low s* intensity indicates that the
1 1+x D G
defect type is edge-defect rather than sp3 hybrid carbon. This is consistent with the phenomenon
that most of the curved carbon layers appear at the edges of the support as shown in HRTEM.
Therefore, the curved structure of the nano-fingerprint carbon layer leads to uneven electron
distribution of the carbon matrix, which weakens the p* and s* peak intensities of Zn Fe /f-NC.44,
1 1+x
45 The three peaks at 286.1 eV (peak a), 289.2 eV (peak b1), and 293.1 eV (peak c) are assigned
to p*C=C, p*C-N-M, and s*C-C antibonding states in both Zn Fe /f-NC and Zn Fe /NC.
1 1+x 1 1
The peak at 287.5 eV (peak b2) represents the emergence of a new p*C-N-M type, which may be due
to the distinct interaction between the Fe clusters and the support differed to single atom metal
species.

Catalytic Activity Testing and Decoupling Experiments
Electrocatalytic ORR activities of the Zn Fe /f-NC and comparison samples were assessed by
1 1+x
the linear sweep voltammetry (LSV) curves in 0.1 M KOH solution, as shown in Figure 3a and
Figure S24. The activity of Zn /NC was the lowest, and while the Zn Fe /f-NC showed the
1 1 1 1+x
highest ORR activity, with the onset potential (E ) up to 1.05V and the half-wave potential (E )
onset 1/2
is 0.93 V. The Zn Fe /f-NC is also higher than catalysts obtained by pyrolysis of Fe(acac) @ZIF-
1 1+x 3
8 at 800 oC (E = 0.93 V, E = 0.83 V) and 900 oC (E = 0.95 V, E = 0.88 V) (Figure S25).
onset 1/2 onset 1/2
To determine the optimal loading amount of the Fe precursor, we also investigated the ORR
performance under different Fe contents before obtaining the Zn Fe / f-NC. As shown in Figure
1 1+x
S26a, when the precursor loading was 1.5 mmol (the sample of Zn Fe /f-NC), the half-wave
1 1+x
potential is the highest (E = 0.93 V), higher than at 0.5 mmol (E = 0.88 V), 1.0 mmol (E =
1/2 1/2 1/2
0.90 V) and 2.0 mmol (E = 0.92 V). Figures S26b-d show that the Tafel slope, kinetic current
1/2
densities and fitted double-layer capacitance (C ) increase with Fe precursor concentration up to
dl
1.5 mmol but decrease at 2.0 mmol (Figures S27), consistent with Fe cluster aggregation into zero-
valent nanoparticles and a reduced effective active area. The XPS results (Figure S28a) shown that
the relative proportion of Fe2+ gradually increased while that of Fe3+ decreased as the Fe precursor
loading increased from 0.5 mmol to 2.0 mmol (Table S5). A small zero-valent Fe peak (706.2 eV)
of Fe-NC can be seen in the Fe 2p spectrum when the Fe precursor reaching 2.0 mmol, and Fe
nanoparticles (diameter exceeding 2 nm) can also be observed (Figure S28b), which means the Fe
metal possibly have a large volume and break through the definition of clusters.46

Figure 3b shows the Tafel slopes of these catalysts, which can be used to evaluate the ORR
kinetics for these electrocatalysts. Compared with other samples, Zn Fe /f-NC has a smaller
1 1+x
Tafel slope (46.51 mV dec-1), which can prove the Zn Fe /f-NC has higher ORR kinetic character.
1 1+x
At 0.93 V, the kinetic current (j ) of Zn Fe /f-NC reaches 5.8 mA/cm-2, which was 4.8 times that
k 1 1+x
of commercial Pt/C (Figure 3c), and the j of Zn Fe /f-NC is as high as 43.7 mA/cm-2 at 0.90 V,
k 1 1+x
which was 16.2 times that of commercial Pt/C. From Figure S29 and S30, it can be found that
electrochemical double-layer C of Zn Fe /f-NC is 32.3 mF cm-2, which is larger than other
dl 1 1+x
contrast samples, indicating that Zn Fe /f-NC has a higher electrochemically active area.
1 1+x
According to the slope of Koutecky-Levich (K-L) diagram (Figure 3d), it can be known that the
ORR process of Zn Fe /f-NC followed a 4-electron pathway (Figure 3d and Figure S31).
1 1+x
Meanwhile, the H O selectivity is maintained at a low level within full potential range. In addition,
2 2
we also evaluated the catalytic performance of the Zn Fe /f-NC catalyst by using a flowing
1 1+x
reactor (configuration was shown in Figure S32). Figure S33 shows the flow-cell test results,
indicating that Zn Fe /f-NC does not produce H O at potentials above 0.4 V and generates only
1 1+x 2 2
trace amounts H O at potential below 0.3 V. Therefore, under typical device operating potentials
2 2
(which are generally higher), the Fenton reaction which causes iron dissolution and device
destabilization is almost unlikely to occur. The electron paramagnetic resonance (EPR)
spectroscopy was used to monitor the in-situ-generated OH* during ORR in real time at the gas-
diffusion-electrode level for Zn Fe /f-NC, employing 5,5-dimethyl-1-pyrroline N-oxide (DMPO)
1 1+x
as the OH* scavenger (quasi-in-situ testing). As shown in Figure S34, compared with the curve
without applied potential, the test results at 0.4 - 0.9 V are approximately linear, indicating that the
OH* radical is not detected. Weak free-radical signals appear at 0.3 V and 0.2 V, but the OH*
signal curve cannot be reliably fitted, indicating that the amount of OH* generated is small. The
resistance of Zn Fe /f-NC to methanol poisoning was tested (Figure S35). After injecting
1 1+x
methanol into the electrolyte (0.1 M KOH and 0.5 M CH OH), the current of Zn Fe /f-NC
3 1 1+x
changed slightly, while the current of commercial Pt/C fluctuated significantly. The LSV curve of
Zn Fe /f-NC changes slightly before and after injection methanol, while the Pt/C catalyst
1 1+x
generates a positive current peak after the addition of methanol, indicating that Zn Fe /f-NC has
1 1+x
high resistance to methanol poisoning. Electrochemical impedance spectroscopy was evaluated to
reveal the electron transfer characteristics of these electrocatalysts, and the results showed that
Zn Fe /f-NC had the smallest electrochemical transfer resistance (Figure S36).
1 1+x

To explore the main roles of single-atom sites, Fe clusters and nano-fingerprint carbon layers
x
in the composite catalysts, decoupling experiments were performed in depth. We prepared a total
of four decoupled samples, including Zn Fe /f-NC, Zn Fe /NC, Zn Fe /f-NC and Zn Fe /NC,
1 1+x 1 1+x 1 1 1 1
where “M ”, “M ”, and “f” represent “single-atom site”, “clusters”, and “fingerprint carbon layers”,
1 x
respectively. Firstly, we analyzed the appearance and species characteristics of the decoupled
samples. As shown in Figure S37, Zn Fe /f-NC exhibits nano-fingerprint carbon layers in the
1 1
absence of Fe clusters, whereas Zn Fe /NC possesses Fe clusters but lacks fingerprint carbon
1 1+x
structures. Furthermore, magnetic hysteresis loops of catalysts at room temperature (25 oC) in
Figure S38 provide additional validation, Zn Fe /NC exhibits clear S-shaped hysteresis and non-
1 1+x
zero remanent, indicating the presence of magnetic Fe clusters. Zn Fe /f-NC shows a similar, but
1 1+x
much weaker non-linear response, aroused by a smaller amount or smaller-size clusters. By
contrast, Zn Fe /f-NC and Zn Fe /NC display near-linear M-H behavior with negligible coercivity
1 1 1 1
and remanence, which is aroused by predominantly atomically dispersed metal sites. As shown in
Figure S39a, the BET surface area of the Zn Fe /f-NC is 976.5 m2 g-1, comparable to the surface
1 1
Zn Fe /f-NC, indicating that acid washing does not significantly affect the catalyst structure.
1 1+x
Figure S39b shows the Raman results for Zn Fe /f-NC, with an I /I value of 1.05 compared to
1 1 D G
with 1.04 for Zn Fe /f-NC and this means that the degree of graphitization does not change
1 1+x
significantly. After characterizing and confirming the species characteristics of the decoupled
samples, the next step is to decouple the activity and stability attribution of its species. LSV curves
indicated that compared with Zn Fe /f-NC, the half-wave potential of Zn Fe /f-NC decreased by
1 1+x 1 1
26 mV (E = 0.904 V, Figure 3a and 3e). The E of Zn Fe /NC is 0.925 V, which is close to
1/2 1/2 1 1+x
Zn Fe /f-NC. This result indicates that the activity is significantly impaired upon removal of Fe
1 1+x
clusters. As shown in Figure 3g, a comparison of the residual activity after 50 h i-t tests for
Zn Fe /f-NC (94%), Zn Fe /f-NC (93%), Zn Fe /NC (89%), and Zn Fe /NC (87%) reveals that
1 1+x 1 1 1 1+x 1 1
the nano-fingerprint carbon layers are mainly enhancing the catalyst stability. Specifically,
Zn Fe /f-NC manifests superior stability with negligible activity decay following 10,000
1 1+x
continuous potential cycles. HAADF-STEM analysis after the durability tests of Zn Fe /f-NC
1 1+x
(Figure S40a) and Zn Fe /f-NC (Figure S40b) shows that within regions protected by fingerprint
1 1
carbon layer, single-atom sites maintain better dispersion, and the clusters do not have obvious
aggregation. Whereas the single-atom sites in Zn Fe /NC (Figure S40c) show partial
1 1
agglomeration after long-term stability testing. This indicates that the fingerprint carbon layer can
prevent the large-scale aggregation of Zn/Fe active sites surrounding Fe clusters. In summary, the
decoupling experiments indicate that Fe clusters primarily promote activity, while the coiled,
nano-fingerprint carbon layer mainly contributes to enhance stability (Figure 3e). The ORR
activity of Zn Fe /f-NC in 0.1 M KOH is better than the other contrast samples and most non-
1 1+x
noble metal electrocatalysts reported in the literatures (Figure 3f and Table S6). As shown in the
Figure 3h, it can obtain gram-scale catalyst yields in a single batch. Simultaneously, we tested the
ORR performance of this batch of samples and found that its performance remains reliable and
reproducible (Figure 3h, E =0.93 V).
1/2

In view of the competitive ORR activity of Zn Fe /f-NC in 0.1 M KOH, we further studied its
1 1+x
ORR activity in the acidic electrolyte of 0.1 M HClO . As shown in Figure S41, Zn Fe /f-NC
4 1 1+x
(E = 0.91 V, E = 0.81 V) delivers an activity slightly inferior to that of commercial Pt/C
onset 1/2
(E of 0.99 V, E = 0.86 V), while is much higher than the Zn Fe /NC (E = 0.82 V, E =
onset 1/2 1 1 onset 1/2
0.67 V) and Zn /NC (E = 0.63 V, E = 0.43 V). It is worth noting that, as shown in Figure
1 onset 1/2
S42, the Tafel slope of Zn Fe /f-NC (61.42 mV dec-1) is lower than Pt/C (76.77 mV dec-1), which
1 1+x
indicated Zn Fe /f-NC has faster ORR kinetics. The C value of Zn Fe /f-NC is also the highest
1 1+x dl 1 1+x
among all comparison samples (Figure S43 and S44), which means that it has the largest
electrochemical active surface area. Rotating disk electrode (RDE) test result shows that the
electron transfer number of Zn Fe /f-NC is 3.85, which is close to the theoretical value of the 4-
1 1+x
electron transfer ORR pathway (Figure S45). In addition, Zn Fe /f-NC also shows high stability
1 1+x
in acidic electrolytes with a potential decay of approximately 33 mV after 10,000 repeated cycles
(Figure S46), and high tolerance to methanol in acidic electrolyte (Figure S47). In summary, Pt/C
is the state-of-the-art catalyst for ORR in acidic electrolytes own to its high dissociation efficiency,
low adsorption energy barriers, and favorable mass-transfer characteristics (Figure S48 and S49).
Nevertheless, the ORR activity of Zn Fe /f-NC in 0.1 M HClO is comparatively high among
1 1+x 4
reported acidic non-precious-metal catalysts (Table S7).

DFT Calculation
To clarify the distinct roles of carbon support curvature and co-catalytic effects in ORR
performance, we conducted systematic DFT calculations on FeN sites embedded in carbon
4
substrates with varying curvatures: graphene, (10,10)-CNT, (6,6)-CNT, and C . The ORR
60
mechanism involves sequential adsorption of *OOH, *O, and OH intermediates, modeled as the
thermodynamic rate-determining step under our DFT conditions (1.23 V). According to the
Sabatier principle, excessive adsorption of intermediates, particularly *OH, can hinder desorption
and reduce activity. Systematic calculations reveal that both curvature and co-catalytic effects
significantly enhance ORR activity. Increasing the curvature of the carbon support substantially
reduces the *OH desorption energy, from 1.40 eV in planar Fe /NC to 0.80 eV in Fe /(10,10)-CNT
1 1
and further to 0.66 eV in Fe /(6,6)-CNT (Figure 4a), this trend is consistent across other models
1
(Fe -Zn1/NC and Fe -Zn /(10,10)-CNT) (Figure S50-S53), consistent with the interpretation that
1 1 1
curvature-induced strain modifies the electronic structure of the Fe active site.47 Integrated crystal
orbital Hamilton populations (iCOHP) value below Fermi level for Fe-O bonds in Fe /NC,
1
Fe /(10,10)-CNT and Fe /(6,6)-CNT were -0.094, -0.081 and -0.062, respectively. The integration
1 1
of -COHP from the occupied states till Fermi level quantifies the total bonding contribution. A
larger integrated value corresponds to stronger overall bonding interaction, which is directly
correlated with Fe-O bonding strength (Figure S54).48, 49 Interestingly, when we investigated the
correlation between *OH desorption energy and polarization parameters of Fe-site in Fe /NC,
1
Fe /(10,10)-CNT and Fe /(6,6)-CNT, a strong negative correlation emerges between the Fe-site
1 1
total polarization (total spin-up electron minus total spin-down electron) and OH* desorption
energy (with a Pearson correlation coefficient of -0.988) (Figure S55)). As substrate curvature
increases, both polarization and adsorption energy decrease synchronously. This suggests that
curvature-induced strain reduces the spin polarization of Fe centers, promoting d-electron
delocalization and thereby weakening Fe–OH bonding. However, this trend is not observed for the
d-polarization parameters (total spin-up d orbital electron minus total spin-down d orbital electron),
indicating that the polarization changes also involve non-d electrons (s and p orbitals). It should
be noted that the statistical significance of this correlation is limited by the small sample size, but
the directional consistency supports the mechanistic interpretation that reduced spin polarization
facilitates OH* desorption in curved Fe-N₄ catalysts. Meanwhile, co-catalytic modifications also
improve performance: introducing a Zn single atom (Fe -Zn /NC) lowers the *OH desorption
1 1
energy to 0.89 eV, while adding an Fe cluster (Fe -Zn -Fe /NC) further reduces the barrier to
1 1 4
0.85 eV, suggesting possible electronic modulation and synergy (Figure 4b). Projected density of
states (PDOS) analysis reveals that the d-band centers of Fe, particularly the spin-down d and d
z2 xz
orbitals (d -down and d -down), correlate strongly with the reaction energy barrier (Figure 4d
z2 xz
and S56-S59). A regression equation (rds-height = 1.009 − 0.118 × d -down − 0.107 × d -down,
z2 xz
R2 = 0.9939) indicates that both curvature and co-catalysis may tune orbital energies to enhance
overlap with adsorbate orbitals. Curvature introduces intermediate states near the Fermi level
mainly from d -down, while Zn incorporation shifts both d and d centers downward (Table S8,
z2 xz
S9 and Figure S60). A regression equation (rds-height = 1.009 − 0.118 × d -down − 0.107 × d -
z2 xz
down, R2 = 0.9939) indicates that both curvature and co-catalysis could tune orbital energies to
enhance overlap with adsorbate orbitals. Electron density difference analysis further confirms that
curvature and Zn incorporation (Figure S61) efficiently redistributes electron density around the
Fe-N region (Figure 4c).47, 50, 51 Hence, both carbon support curvature and co-catalytic sites
4
optimize ORR performance by modulating the electronic structure of Fe active centers via d and
z2
d orbital regulation.52 Coupling nano-fingerprint carbon supports with dual single-atom sites and
xz
clusters may offer a promising design strategy.53, 54

Zn-Air Batteries Test
As shown in Figure S62, the practical application of the Zn Fe /f-NC material as ZAB cathode
1 1+x
catalysts was tested compared with Pt/C and Zn Fe /NC. This ZAB based on Zn Fe /f-NC
1 1 1 1+x
exhibits a peak density of 263.82 mW cm-2 (Figure 5a), outperforming that of Pt/C-based ZAB
(145.22 mW cm-2) and Zn Fe /NC-based ZAB (196.77 mW cm-2). In order to test the practical
1 1
application value of Zn Fe /f-NC electrocatalyst, a rechargeable and dischargeable ZAB was
1 1+x
prepared by using Zn Fe /f-NC to form a composite catalyst, and a long-term durability test was
1 1+x
carried out. The overall test uses constant current charge and discharge cycles, and each cycle lasts
for 60 min (30 min charging and 30 min discharging). The ZAB cycle test demonstrated robust
durability of over 2200 h under 10 mA cm-2 (Figure 5b). The stability of Pt/C+RuO significantly
2
declined after 200 h and Zn Fe /NC significantly declined after 100 h (Figure 5c and Figure S63).
1 1
Figure 5d and Table S10 and S11 show the competitive long-term stability compared to other
reported electrocatalysts.55, 56, 57, 58, 59, 60, 61, 62, 63, 64 The charge and discharge voltages of the
Zn Fe /f-NC-assembled ZAB after 50 hours of continuous operation were 1.93 V and 1.16 V,
1 1+x
which have a high round-trip efficiency. After 1000 hours of continuous operation, the charge and
discharge voltages were 1.94 V and 1.15 V, with a voltage difference of 0.79 V. Even after 2200
h of testing, the charge and discharge voltage difference remain almost unchanged from the value
before 2200 h of testing (insert Figure 5b, the charge and discharge voltages are 1.95 V and 1.14
V, and the voltage difference is 0.81 V). We further characterized the air cathode catalyst after the
long life cycling with HAADF-STEM. As evident from Figure S64, not only the nano-fingerprint
carbon structure remains clearly defined, but also the numerous atomically dispersed single atom
active sites onto clusters are also clearly visible. However, due to the prolonged stability testing, a
slight aggregation of some atomic sites forming particles approximately 2 nm in size. This is also
the reason for the minor decline in stability and activity after extended cycling. The test results
show that Zn Fe /f-NC catalyst not only has high stability in the RDE half-cell test, but also has
1 1+x
super strong stable cycle characteristics in practical applications. We also assembled button cells
and flexible solid-state ZABs. As shown in Figure 5e and 5f, the resulting button cell ZAB could
run stably for 1000 min. In addition, compared with quasi-solid-state battery shows good flexibility
and stability. As shown in Figure 5g, whether it’s flexible or flat, the discharge-charge curve hardly
changes, indicating its high anti-deformation performance. Further research into the potential
improvements for current rechargeable ZABs devices was conducted. Using button cells as a
model, we disassembled batteries before and after charge-discharge cycling and observed that the
electrolyte in the separator had almost evaporated after cycling, accompanied by the formation of
significant precipitate on both the separator and the zinc electrode (Figure S65). Based on these
findings, we postulate that in the future commercialization of the rechargeable ZABs, the
development of air cathode catalysts should be complemented by the creation of highly efficient
and durable electrolytes. This latter aspect represents a significant challenge and is a primary focus
of ongoing research efforts.65, 66

Methods
Materials
Zinc nitrate hexahydrate, sulfuric acid (96% ~ 98%), perchloric acid (70% ~ 72%), anhydrous
methanol, sodium hydroxide, and anhydrous ethanol were purchased from Sinopharm Chemical
Reagent Co., Ltd. (Shanghai, China). DMPO, 2-methylimidazole, and iron acetylacetonate were
purchased from Aladdin Co. Ltd. Pt/C (20 wt%) were supplied from Innochem Technology
(Beijing, China). Nafion solution (5 wt%) were supplied from Alfa Aesar. Nitrogen and Oxygen
(≥99.999%) was purchased from Liyuan Gas. The water used in all experiments was ultrapure
water (18.25 MΩ⸳cm-1). All reagents were used without further purification.

Catalyst synthesis
Synthesis of ZIF-8
In a typical synthesis, 11.9 g of Zn(NO) ⸳6H O were dissolved in 300 mL of methanol to form a
3 2
clear solution. It was subsequently introduced into 300 mL of containing 13.14 g of 2-
methylimidazole under vigorous stirring for 12 h at room temperature (25 oC). The as-obtained
precipitates were centrifuged, washed with ethanol, and dried in vacuum at 50 oC.

Synthesis of Zn Fe /f-NC
1 1+x
In a typical synthesis, 800 mg of as-synthesized ZIF-8 and 1.5 mmol of iron acetylacetonate were
dispersed in 60 ml of ethanol under vigorous stirring for 24 h at 60 oC in oil bath. Subsequently
the mixture was centrifugal separation of suspension and solid before being ground to give a fine
powder (Fe(acac) @ZIF-8). Finally, the product Zn Fe /f-NC was thermally treated at 1000 oC
3 1 1+x
under an argon atmosphere for 2 h with ramping rate of 3 oC min -1. The Fe loadings in the catalyst
were determined to be 0.90 wt %.

Synthesis of Zn Fe /NC
1 1
The synthetic method of Zn Fe /NC was similar to that of Zn Fe /f-NC, except mixture was
1 1 1 1+x
washed with ethanol for 3 times before being ground to give a fine powder (Fe(acac) -ZIF-8).
3
The Fe loading in the catalyst was measured to be 0.14 wt %.

Synthesis of Zn /NC
1
The synthetic method of Zn /NC was similar to that of Zn Fe /f-NC, except ZIF-8 was used
1 1 1 1+x
without the use of iron acetylacetonate.

Synthesis of Zn Fe /f-NC
1 1
Zn Fe /f-NC was prepared by treating 400 mg of Zn Fe /f-NC with dilute H SO under stirring
1 1 1 1+x 2 4
for 24 hours to remove the Fe clusters. The product was then collected by centrifugation, washed
thoroughly with deionized water until the supernatant reached neutral pH, and finally dried in a
vacuum oven at 60 oC.

Synthesis of Zn Fe /NC
1 1+x
Zn Fe /NC was prepared by loading Zn Fe /NC with Fe(acac) again, specifically by loading
1 1+x 1 1 3
400 mg of Zn Fe /NC with 1.5 mmol of iron acetylacetonate dispersed in 60 ml of ethanol under
1 1
vigorous stirring for 24 h at 60 oC in oil bath. Subsequently the mixture was centrifugal separation
of suspension and solid before being ground to give a fine powder (Fe(acac) @Zn Fe /NC),
3 1 1
Finally, the product Zn Fe /NC was thermally treated again at 1000 oC under an argon
1 1+x
atmosphere for 2 h with ramping rate of 3 oC min -1.

Electrochemical measurements
Electrochemical ORR measurements were implemented with a CHI 760E electrochemical
workstation (CH Instruments, Inc., Shanghai) connected to rotation-control equipment (Pine
Instrument Company, USA) with a standard three-electrode system in 0.1 M KOH and 0.1 M
HClO . Rotating-disk electrode (RDE, d = 5 mm, the geometric surface area of the RDE electrode
4
is 0.196 cm-2, Pine Instrument Company, USA) or rotating-disk electrode (RRDE, 5.61 mm in disk
outer diameter, the geometric surface area of the RDE electrode is 0.247 cm-2, Pine Instrument
Company, USA) was used as a working electrode. Ag/AgCl electrode and graphite rod were
applied as reference electrode and counter electrode, respectively. All potentials were referenced
to a reversible hydrogen electrode (RHE). All electrochemical measurements were conducted in a
thermostated water bath at 25 oC.

𝐸 (vs.RHE) = 𝐸 (vs.Ag⁄AgCl)+0.198+0.0591 × pH (1.)
10 mg of electrocatalyst was mixed with 500 μL of ethanol, 470 μL ultrapure water, and 30 μL of
Nafion solution (5 wt%) by ultrasonication to form a homogeneous catalyst ink. An aliquot of 10
μL of the as-obtained electrocatalyst ink was pipetted onto the working electrode surface with a
mass loading of 0.51 mg cm-2. The 0.1 M KOH electrolyte used for electrochemical measurements
was freshly prepared by dissolving 2.81 g of KOH in 500 mL of deionized water, and the solution
was maintained at 25 °C under dry, ventilated conditions prior to use. The actual pH of the 0.1 M
KOH electrolyte was measured to be 12.90 ± 0.15. The five-port electrolysis cell has a volume of
150 mL. Approximately 100 mL of 0.1 M KOH is used for each test. The 0.1 M HClO solution
4
used for electrochemical tests was prepared by dissolving 4.25 mL of concentrated perchloric acid
in 500 mL of deionized water, the actual pH was 1.1 ± 0.15. Approximately 100 mL of solution
was used per test. The electrolyte was purged by a specific gas for at least 30 min before the
measurements and the gas flow was maintained during the experiments. Before the ORR
measurements, cyclic voltammograms were performed at 50 mV s-1 for at least 20 cycles to clean
and stabilized the electrocatalyst surface. LSV measurements were recorded at a scan rate of 10
mV s-1. The double-layer capacitance (C ) of electrocatalyst was derived from CV curves in the
dl
non-Faradic potential region with different scan rates from 5 to 30 mV s-1. Electrochemical
impedance spectroscope (EIS) measurements were conducted in the frequency range from 106 Hz
to 1 Hz with a voltage amplitude of -3 mV. The solution resistance (R ) was obtained by
sol
measuring EIS at the open-circuit potential in the electrolyte (R = 35 ± 1.5 Ω in 0.1 M KOH and
sol
R = 16 ± 1.5 Ω in 0.1 M HClO ). The polarization curves of all studied catalysts (without iR-
sol 4
corrected) are shown in Figures S66 and S67.

The selectivity to methanol solution (0.5 M L-1) was conducted at 0.816 V vs RHE by
chronoamperometry. The selectivity of electrocatalyst during ORR was calculated by measuring
LSV curves on RDE with different rotating speeds (400, 900, 1225, 1600, and 2025 rpm). The
corresponding electron transfer number (n) was determined using Koutecky-Levich (K-L)
equation.
1 1 1 1 1
= + = + (2.)
𝐽 𝐽 𝐽 1 𝐽
L K Bω2 K
2 1
B = 0.62nFC (D )3v − 6 (3.)
0 0
Where J is the measured current density, J and J are diffusion-limiting current density and
L K
kinetic current density, respectively; ω is the angular velocity of the disk (2πN, N is the rotation
speed); n is the electron transfer number; F is the Faraday constant (96,485 C mol-1); D is the
0
diffusion coefficient of oxygen (1.9×10−5 cm−2 s−1); C is the bulk concentration of oxygen
0
(1.2×10−6 mol cm−3); v is the kinetic viscosity of the electrolyte (0.01 cm-2 s-1).

The electron transfer number (n) and hydrogen peroxide (H O ) yield were calculated from the
2 2
RRDE measurements via the following equations:
4×𝐼
d
n = (4.)
𝐼
𝐼 + r
d N
𝐼
r
N S
H O (%) = 200× (5.)
2 2 𝐼
𝐼 +
rS
d N
E
R
Where I and I are the disk and ring currents. N is the current collection efficiency of the Pt ring
d r
(N=0.37).

The Ag/AgCl reference electrode was calibrated in H -saturated 0.1 M KOH electrolyte, the Pt
2
foil was used as both the counter and working electrodes.

Faradic efficiency (FE) test
To determine the actual FE of Zn Fe /f-NC, electrochemical ORR tests were performed in a flow
1 1+x
cell. with a three-electrode system controlled by a multichannel electrochemical workstation (Bio-
Logic VSP-3e), in which Ag/AgCl (filled with saturation KCl solution) served as the reference
electrode and Pt sheet served as the counter electrode. An anion exchange membrane (Fumasep,
FAA-3-PK-130) was adopted to separate the anolyte and catholyte chambers. The anion exchange
membrane has a thickness of 50 μm and dimensions of 20 mm × 20 mm. Prior to use, it is soaked
in 0.1 M KOH for at least 24 h and stored in 0.1 M KOH. The electrolytes were circulated into the
anode (30 mL) and cathode (30 mL) chambers at a constant flow rate of 10 mL min-1 using a
peristaltic pump. High-purity O (99.999%) gas was continuously purged into the gas chamber
2
behind the GDL (YLS-30T, yielding an effective catalytic area of 1×1 cm-2 and mass loading of
0.5 mg cm-2) with a feeding rate of 100 mL min-1 have been maintained under all test conditions.
The O products were collected during chronopotentiometry tests, which were performed at each
2
potential for 30 min. The FE was determined by spectrophotometry. Specifically, 30 μL of the
catholyte was added to 3 mL of Ce(SO ) solution and mixed thoroughly. The H O then reacted
4 2 2 2
with Ce4+, reducing it to colorless Ce3+.
2Ce4+ +H O → 2Ce3+ +2H+ +O (6.)
2 2 S2
The Ce4+ concentration before and after the reaction was measured using a UV–visible
spectrometer. As shown in Figure S33a, the full-wavelength UV–visible spectrum of the Ce(SO) 
4
standard solution indicates that the absorbance gradually increases with increasing Ce4+
concentration and reaches a peak at approximately 318 nm. A linear fit between Ce4+ concentration
and absorbance intensity was used to obtain a standard curve. The Ce4+ concentrations before and
after the reaction were then used to determine the H O content in the electrolyte, and the Faradaic
2 2
efficiency of Zn Fe /f-NC at applied voltages from 0.2 V to 0.9 V was calculated using the
1 1+x
following formula:
nF𝐶 𝑉 ∗10−3
FE (%) = H2O2
×100 % (7.)
Q
Where n is the electron-transfer number (2), F is the Faraday constant (96485 C mol-1), C is
H2O2
the molar concentration of hydrogen peroxide (mol/L), V is the volume of the electrolyte in the
flow cell (mL), and Q is the total charge passed during the i-t process (C).

Bulk electrolysis in flow cell
The cathode catalyst slurries were prepared by sonicating 20 mg catalyst, 1 mL ethanol, 1 mL H O
2
and 60 μL 5 wt% Nafion suspension in a bath sonicator for at least an hour. The ink was air-
brushed onto air-brushed onto a carbon paper until the cathode catalyst loading reached 0.5 mg
cm-2.

EPR measurements
The catalyst spray-coated onto carbon paper (prepared in the flow-cell tests) was used as the
cathode in the flow cell for EPR measurements to quantify OHS* generated during the ORR. The
catholyte (30 mL), containing 0.1 M KOH and 0.42 M DMPO, was continuously supplied to the
cathode liquid channel at 10 mL min⁻¹ using a peristaltic pump. O₂ was fed to the gas chamber at
30 mL min⁻¹. The anode was a commercial Pt foil (0.1 mm thick, 20 mm × 20 mm), and 0.1 M
KOH was supplied to the anode at the same flow rate. Tests were conducted at room temperature
(25 °C). The flow cell was operated under i-t conditions over the range 0.2 - 0.9 V, with each
potential held for 10 min. Immediately after each i-t measurement, a 10 μL aliquot was taken from
the catholyte reservoir; the sampling and analysis was completed within 3 min.

Zinc-Air Battery assembly and test
The home-made zinc-air battery was manufactured. A 0.2 mm thick zinc foil was pre-polished
with sandpaper as the anode. Catalysts were loaded on carbon fiber paper (1 cm2) as the air cathode.
The electrochemical cell was filled with 6.0 M KOH and 0.2 M zinc acetate solution as the
electrolyte. The Zn Fe /f-NC catalyst loading is 0.4 mg cm−2, and the Zn Fe /NC and commercial
1 1+x 1 1
catalysts’ loading are the same. Button-cell ZABs were assembled with a positive electrode, carbon
paper (diameter: 16 mm) loaded with catalyst ink, electrolyte, zinc plate, spring plate, and negative
electrode. The catalyst loading is 0.4 mg cm–2. The flexible solid-state ZABs were assembled with
a zinc plate, modified PVA hydrogel, and carbon cloth loaded with catalyst ink (catalyst loading:
0.4 mg cm−2). The measurements of ZABs (including home-made ZABs, Button-cell ZABs, and
flexible solid-state ZABs) were performed at room temperature with a battery test workstation
(Neware battery testing system). All the tests were under the air environment (the climate
temperature ranged from 20 oC to 30 oC).

Characterization
X-ray diffraction (XRD) characterization was performed on a Smart Lab 9 kW equipped with a
Cu Kα radiation source. X-ray photoelectron spectroscopy (XPS) was conducted on the Thermo
Scientific ESCALAB 250Xi. The BET surface area was measured using MicroActive for ASAP
2460 Version 2.02. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was
utilized to determine the Fe content of the catalyst (Agilent 5110). Raman spectra were collected
on an inVia (Renishaw) micro-Raman spectrometer using a 532 nm laser. UV-visible diffuse
reflection spectroscopy was carried out with a Shimadzu UV-2600, Scanning electron microscopy
(SEM) images were captured by using a Hitachi S8100 scanning electron microscope. TEM
images were collected using a Hitachi HT-7700. High-angle annular dark-field scanning TEM
(HADDF-STEM) was conducted on JEOL NEOARM 200F. High-resolution TEM (HRTEM)
images were obtained using a Jem-2100F and energy spectrum was Oxford X-MAN 65T.
XAFS characterization: The XAFS spectra are obtained at 1W1B station in BSRF (Beijing
Synchrotron Radiation Facility, P. R. China) operated at 2.5 GeV with a maximum current of 250
mA. All samples are pelletized as disks of 13 mm diameter with 1mm thickness using graphite
powder as a binder. Additionally, the C K-edge and N K-edge X-ray absorption spectra are
measured at beamline BL12B of National Synchrotron Radiation Laboratory (NSRL) in Hefei and
the samples are deposited onto double-sided carbon tap.
XAFS fitting analysis: The acquired EXAFS data are processed according to the standard
procedures using the Athena and Artemis implemented in the IFEFFIT software packages. The
fitting detail is described below: The acquired EXAFS data are processed according to the standard
procedures using the ATHENA module implemented in the IFEFFIT software packages. The
EXAFS spectra are obtained by subtracting the post-edge background from the overall absorption
and then normalizing with respect to the edge-jump step. Subsequently, the χ(k) data of are Fourier
transformed to real (R) space using a hanning windows (dk=1.0 Å-1) to separate the EXAFS
contributions from different coordination shells. To obtain the quantitative structural parameters
around central atoms, least-squares curve parameter fitting is performed using the ARTEMIS
module of IFEFFIT software packages.
57Fe Mössbauer spectroscopy measurements were performed using a Topologic Systems MFD-
500AV-02 spectrometer equipped with a 57Co(Rh) γ-ray source. The spectra were analyzed and
fitted by the Mosswinn software.

Theoretical Calculations
Spin-polarized density functional theory calculations were carried out via the Quantum Espresso
suit67, 68, 69, 70, 71 in the framework of GGA-PBEsol.72 PBEsol functionals has been developed as a
modification of the widely used PBE73 functional to improve lattice constants and surface energies.
SSSP pseudopotential libraries (precision version)74 was selected as suggested by N. Marzari et
al.75 Kinetic energy and charge density cutoffs cutoff of 30 Ry and 360 Ry with a Marzari–
Vanderbilt cold smearing76 width of 0.002 eV were selected in the geometrical optimization. The
van der Waals effects were considered by using DFT-D3 scheme.77 According to experimental
data, models except for Fe /f-NC were constructed based on 2D graphene slab models. To prevent
1
erroneous interactions between neighboring periodic images, a vacuum layer of 20 Å along the z-
axis is added. The periodic cell is 17.089 × 17.265 × 20.000 Å3. Monkhorst–Pack k-point grids
were 2 × 2 × 1 in geometrical optimization and self-consistent field (SCF) calculations for flat
models. Isolated C in a periodic cell (15.000 × 15.000 × 15.000 Å3) was selected as one bent
60
carbon model. It seems that the curvature varies too drastic from flat carbon (graphene model) to
the bent carbon (C model) to investigate the general function of carrier materials. Hence, we
60
added two substrate models (single wall carbon nanotube with different curvatures) among them
to reveal the general role of carrier materials from flat to bent. Herein, (6,6)-CNT and (10,10)-
CNT were selected, while similar construction proposal was adopted as in graphene model to
embed FeN
4
moiety on them. The periodic cell is 31.483 × 31.483 × 14.757 Å3 and 36.907 ×
36.709 × 14.757 Å3 for (6,6)-CNT and (10,10)-CNT based models, respectively. Monkhorst–Pack
k-point grids were 1 × 1 × 2 in geometrical optimization and self-consistent field (SCF)
calculations for CNT based models. Geometry optimization was performed until the maximum
residual force was 0.03 eV/Å. The convergence requirements were chosen at 1 × 10−7 eV atom−1
for SCF iterations. For non-self-consistent field (NSCF) calculations k-point grids was 3 × 3 × 1
and 1 × 1 × 3. Considering the delocalization of Fe-3d and Zn-3d states, DFT+U approach78 was
employed to overcome its self-interaction error.79 U values of 3.29 and 4.12 eV for Fe and Zn80
3d
were adopted, respectively. The Gibbs energies were evaluated by including zero-point energy
correction and entropy contribution terms in the framework of computational hydrogen electrode
approach developed by Norskov and co-works:81, 82 ΔG = ΔH + ΔE – TΔS where ΔH is obtained
ZPE
from calculated DFT total energies, ΔE is the zero point energy correction which is evaluated
ZPE
in a harmonic fashion of intermediates. The entropic contribution term is the change of entropy
due to the reaction (Supplementary Data 1).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript successfully synthesized a mixed-site composite electrocatalyst (Zn1Fe1+x/f-NC) and tested its corresponding oxygen reduction reaction (ORR) catalytic activity. Experimental methods showed that Fe1/Zn1 atomic sites are primarily responsible for catalytic activity, while the presence of Fex clusters further enhances intrinsic activity, and curved carbon aids in the confinement and stabilization of M1+x. Density functional theory calculations further elucidate the synergistic enhancement mechanism from an electronic structure perspective. The data presented in this study are detailed, and the explanation of the synthesis process for Fe-MOFs with high defect density is thorough. However, the innovation of synthesizing Zn1Fe1+x/f-NC for ORR is limited, and the corresponding mechanism is of general applicability. I do not recommend this manuscript to publish in Nature Communications.\n1.The Zn1Fe1+x/f-NC structure proposed in the manuscript (Fex clusters encapsulated in curved carbon layers and surrounded by Fe/Zn-N4 single atoms) may have unique characteristics in terms of its specific implementation (e.g., curved carbon confinement). However, the work does not elaborate on its breakthrough innovative advantages. The nonvariations appear insufficiently prominent and convincing.\n2.Are there differences between the catalyst synthesized in the paper and the nitrogen-doped carbon substrate derived from MOFs reported in terms of their catalytic properties?\n3. The fingerprint carbon layer is the core innovation, stabilizing the M1+x sites, but this is attributed to “no washing → direct calcination” in the work. However, the influence of ZIF-8’s own thermal decomposition has not been ruled out. Does pure ZIF-8 calcination at the same temperature produce fingerprint carbon? The role of Fe has not been verified. Is graphitization catalyzed by excess Fe? Control experiments comparing “no washing but no Fe” and “washing followed by the addition of excess Fe” are needed.\n4. In ORR, carbon corrosion of carbon-based catalysts is a serious problem, even though the authors conducted stability tests before and after to prove the stability of the catalyst. However, a 10-hour test seems too short, and compared with the recently reported single-atom catalyst materials, this material does not seem to have any advantages.\n5. Please conduct additional experiments to determine the optimal ratio of the mixed-site composite electrocatalyst The Zn1Fe1+x/f-NC. It is insufficient and unconvincing to discuss only the synergistic effect between the dopant and the host.\n6. In terms of mechanism, the authors still propose the traditional adsorption mechanism without any innovation. Furthermore, the role of bent carbon is described as “contributing to the confinement and stabilization of M1+x,” but this seems to be a general function of carrier materials, and its unique mechanism of action in regulating electronic structure or promoting specific reaction pathways has not been thoroughly elucidated.", "reviewer_2": "Wang et al. report the development of a composite electrocatalyst (Zn1Fe1+x/f-NC) featuring Fe clusters encapsulated within curved carbon layers and surrounded by atomically dispersed Zn/Fe-N4 sites, aiming to enhance ORR activity and stability for Zn-air batteries. The authors claim that the cooperative modulation of electronic structure via multivariate active site engineering breaks the linear scaling relations in ORR catalysis, delivering a half-wave potential of 0.93 V and stable Zn-air battery operation over 1000 hours. While the findings are potentially interesting, the manuscript in its current form does not reach the depth and originality expected for publication in Nature Communications. My key concerns are outlined below:\n1. Hybrid Fe-based single-atom and cluster catalysts supported on N-doped carbon frameworks have been widely reported (Nat. Commun. 2018, 9, 5422; Adv. Mater. 2022, 34, 2200595). The proposed curved carbon support and Zn/Fe dual-site synergy are incremental extensions of established strategies. The manuscript would benefit from a clearer delineation of how this design fundamentally differs from previous multi-site SAC systems, and whether the observed performance truly surpasses state-of-the-art benchmarks.\n2. The role of curved carbon layers is primarily inferred from TEM morphology and Raman spectra. However, the manuscript lacks quantitative evidence linking curvature-induced strain to modulation of Fe d-band centers or ORR kinetics. Additional spectroscopic data or a more detailed theoretical analysis is needed to substantiate the claimed electronic effects.\n3. While decoupling experiments and DFT calculations are provided, they do not convincingly isolate the individual contributions of Zn single atoms, Fe clusters, and curved carbon.\n4. The reported half-wave potential (0.93 V) and peak power density (263.82 mW·cm-2) are competitive but not significantly higher than top-performing Fe-N-C. The authors should provide a more comprehensive benchmarking table and discuss what unique advantages their catalyst offers beyond incremental gains.\n5. The long-term cycling stability (>1000 h) of Zn–air batteries is presented as a key advantage. However, detailed post-mortem characterization of the catalyst after extended operation is missing. Evidence for atomic site retention and suppression of metal leaching or agglomeration would strengthen the durability claim.", "reviewer_3": "The work reported by Li and co-authors presents a well-structured study on a curved-carbon-supported Fe/Zn single-atom cluster catalyst (Zn1Fe1+x/f-NC) for the oxygen reduction reaction (ORR) and its application in Zn-air batteries. The synthetic strategy in this work is creative, and the combination of decoupling experiments with DFT calculations reveals how Zn single atoms, Fe clusters, and curved carbon collectively optimize the electronic structure of Fe–N4 sites. The fabricated catalyst shows good activity (E1/2 = 0.93 V in 0.1 M KOH), durability (>1 000 h), and high peak power density (263.8 mW/cm2). However, several issues need to be addressed to meet the standard of Nat Comm.\n(1) Could the authors provide high-resolution XPS data and Mössbauer data for Fe to unequivocally distinguish Fe0, Fe2+, and Fe3+ contributions from clusters versus single atoms. This is critical for validating the proposed synergy.\n(2) Include aberration-corrected HAADF-STEM images after acid leaching and long-term cycling to explicitly demonstrate that Fe clusters are removed and single atoms remain intact; current post-mortem images are not conclusive.\n(3) What is Faradaic efficiency of H2O2 generation? Quantify any Fenton-type OH radical formation during cycling. Trace Fe leaching could undermine the stability in real devices.\n(4) I suggest authors further considering which data shown in manuscript and which data shown in SI to improve readability.\n(5) I suggest authors adding a brief scalability discussion, can the protocol be scaled beyond the current 100 mg batch size?", "reviewer_4": "Reviewer #1\nThis manuscript successfully synthesized a mixed-site composite electrocatalyst\n(Zn1Fe1+x/f-NC) and tested its corresponding oxygen reduction reaction (ORR)\ncatalytic activity. Experimental methods showed that Fe1/Zn1 atomic sites are primarily\nresponsible for catalytic activity, while the presence of Fex clusters further enhances\nintrinsic activity, and curved carbon aids in the confinement and stabilization of M1+x.\nDensity functional theory calculations further elucidate the synergistic enhancement\nmechanism from an electronic structure perspective. The data presented in this study\nare detailed, and the explanation of the synthesis process for Fe-MOFs with high defect\ndensity is thorough. However, the innovation of synthesizing Zn1Fe1+x/f-NC for ORR\nis limited, and the corresponding mechanism is of general applicability. I do not\nrecommend this manuscript to publish in Nature Communications.\n\nReviewer #2\nWang et al. report the development of a composite electrocatalyst (Zn1Fe1+x/f-NC)\nfeaturing Fe clusters encapsulated within curved carbon layers and surrounded by\natomically dispersed Zn/Fe-N4 sites, aiming to enhance ORR activity and stability for\nZn-air batteries. The authors claim that the cooperative modulation of electronic\nstructure via multivariate active site engineering breaks the linear scaling relations in\nORR catalysis, delivering a half-wave potential of 0.93 V and stable Zn-air battery\noperation over 1000 hours. While the findings are potentially interesting, the\nmanuscript in its current form does not reach the depth and originality expected for\npublication in Nature Communications. My key concerns are outlined below:\n\nReviewer #3\nThe work reported by Li and co-authors presents a well-structured study on a curved-\ncarbon-supported Fe/Zn single-atom cluster catalyst (Zn1Fe1+x/f-NC) for the oxygen\nreduction reaction (ORR) and its application in Zn-air batteries. The synthetic strategy\nin this work is creative, and the combination of decoupling experiments with DFT\ncalculations reveals how Zn single atoms, Fe clusters, and curved carbon collectively\noptimize the electronic structure of Fe–N4 sites. The fabricated catalyst shows good\nactivity (E1/2 = 0.93 V in 0.1 M KOH), durability (>1 000 h), and high peak power\ndensity (263.8 mW/cm2). However, several issues need to be addressed to meet the\nstandard of Nat Comm.\n\nReviewer #4\nA composite catalyst containing Fe clusters encapsulated within curved carbon layer\nsand surrounded by densely dispersed Fe/Zn-N4 sites was obtained. A series of\ndecoupled control samples, including those with dual single-atom Fe and Znanchored\non curved carbon and on planar carbon were also prepared. Decoupling experimental\nrevealed that Fex clusters promoted. Single atom Zn, Fe clusters and fingerprint carbon\nlayer jointly enhanced the catalytic performance of Fe-N4 in ORR through the\nmultivariate regulation of electronic structure. The manuscript was well organized and\nwritten. I recommend its publication in Nature Communications after the following\nrevision:"}, "author_response": "Responses to Reviewers\nWe sincerely thank the reviewers for their valuable comments on our manuscript. Those comments are very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have carefully considered the reviewers’ comments and revised the manuscript accordingly. Here in this response letter, the reviewers’ comments are laid out below and their specific concerns are numbered in sequence. Revised portion are marked in yellow in the revised manuscript and revised supporting information. Below, the original comments are in black, and our point-to-point responses are in blue.\nReviewer #1\nThis manuscript successfully synthesized a mixed-site composite electrocatalyst (Zn1Fe1+x/f-NC) and tested its corresponding oxygen reduction reaction (ORR) catalytic activity. Experimental methods showed that Fe1/Zn1 atomic sites are primarily responsible for catalytic activity, while the presence of Fex clusters further enhances intrinsic activity, and curved carbon aids in the confinement and stabilization of M1+x. Density functional theory calculations further elucidate the synergistic enhancement mechanism from an electronic structure perspective. The data presented in this study are detailed, and the explanation of the synthesis process for Fe-MOFs with high defect density is thorough. However, the innovation of synthesizing Zn1Fe1+x/f-NC for ORR is limited, and the corresponding mechanism is of general applicability. I do not recommend this manuscript to publish in Nature Communications.\nResponse: We thank the reviewer for the constructive comments. Herein, we report a composite electrocatalyst in which Fe clusters are encapsulated within nano-fingerprint carbon layers and surrounded by densely dispersed Fe/Zn-N single-atom atomic sites. The obtained catalyst shows promising ORR activity (E = 0.93 V) and notable cycling life performance in zinc-air batteries (2200 h @ 10 mA cm-2). Although the synthetic procedure is straightforward, this is also a potential advantage for catalyst materials to scale up production (we have added relevant information of catalyst scaling up production in the revised manuscript as Figure 3h). More importantly, our key finding is that slight variations in synthetic conditions lead to systematic changes in both the chemical state of the metal sites and the morphology of the carbon support. And considering the complexity of our catalyst, we further conducted supplementary experiments on the decoupling experiment to distinguish in detail the contributions of curled fingerprint-like carbon layers, iron clusters and single atoms to catalytic activity and stability. The intriguing findings during this synthetic process, alongside the design of decoupling experiments for this complex catalytic system and the subsequent attribution of the roles of different species in catalytic activity or stability, may offer potential guidance for current M-N-C catalyst synthesis and the elucidation of structure-activity relationships.\nMany thanks for the Reviewer’s great input in reviewing the manuscript and in providing the valuable suggestions below, which are very helpful in improving the scientific merits and discussions of the manuscript. Therefore, we accept all these kind suggestions, and have supplemented substantial revisions by providing more experimental/analytical evidences and discussions. We hope the revisions would be satisfactory.\n1.The Zn1Fe1+x/f-NC structure proposed in the manuscript (Fex clusters encapsulated in curved carbon layers and surrounded by Fe/Zn-N4 single atoms) may have unique characteristics in terms of its specific implementation (e.g., curved carbon confinement). However, the work does not elaborate on its breakthrough innovative advantages. The nonvariations appear insufficiently prominent and convincing.\nResponse: We thank the reviewer for the valuable comment. We report a composite electrocatalyst (Zn Fe /f-NC) featuring Fe clusters encapsulated within curved carbon layers and surrounded by densely dispersed Fe/Zn-N atomic sites. We contend that the innovations and strengths of this work are primarily attributable to two aspects: (1) Evidence from decoupling experiments on stability and activity. (2) Performance advantages from dual guarantee of activity and stability. Motivated by the discerning insights of the reviewers, we have rigorously addressed these two aspects, augmenting the revised manuscript with a collection of more compelling evidence. The details are as follows:\n1. Evidence from decoupling experiments on stability and activity.\nConsidering the complexity of our working catalyst (including single atoms (M ), clusters (M ), and nano-fingerprint carbon layers (f-NC), we conducted decoupling experiments. In line with your insightful recommendations, we have prepared two more new contrast samples (see more details in question 3). This expands our experimental approach to include a total of decoupled contrast samples (including Zn Fe /NC, Zn Fe /f-NC and Zn Fe /NC), providing a direct comparison to our optimally performing main sample Zn Fe /f-NC. Based on the experimental results, we draw the following conclusions: (a) The inclusion of nano-fingerprint carbon layers in the decoupled contrast samples (Zn Fe /f-NC and Zn Fe /f-NC) primarily contributes to excellent ORR catalytic stability. (b) Further improvements in catalytic activity are observed when Fe clusters are incorporated into the decoupled contrast samples (Zn Fe /NC and Zn Fe /f-NC). The experiments indicate that it is difficult for a single component to maintain both high activity and stability. In Zn Fe /f-NC, in summary, decoupling experiments revealed that Fe clusters promoted the high activity of Fe/Zn-N atomic sites, while the nano-fingerprint carbon layers facilitate the confinement and stabilization of hybrid (M ) sites.\n2. Performance advantages: dual guarantee of activity and stability.\nIn our work, a composite catalyst (Zn Fe /f-NC) containing Fe clusters encapsulated within curved carbon layers and surrounded by densely dispersed Fe/Zn-N sites was obtained. The prepared Zn Fe /f-NC displayed promising ORR performance with a half-wave potential (E ) of 0.93 V, surpassing that of the four decoupled contrast samples, and most of the other reported catalysts (see question 2, Table R1). Inspired by your valuable suggestions (more details see Question 4), considering the lifespan of the machine under high-speed rotation (1600 rpm) and the potential risks associated with continuous oxygen supply, we extended the stability test to 50 hours (most reported stability tests ranging from 10-20 hours). The Zn Fe /f-NC catalyst was further evaluated as the cathode by assembling ZABs. Impressively, the ZAB cycle test demonstrated ultra-high durability of over 2200 h under the current density of 10 mA cm-2. HAADF-STEM characterization for the Zn Fe /f-NC air cathode after more than 2200 h charge-discharge stability operation demonstrates that Zn Fe /f-NC shows minimal aggregation at atomic sites and that the nano-fingerprint carbon layer structure remains intact. This demonstrates the advantageous long-term stability over other reported electrocatalysts (see more details in question 4).\nIn summary, we report a composite electrocatalyst (Zn Fe /f-NC) featuring Fe clusters encapsulated within curved carbon layers and surrounded by densely dispersed Fe/Zn-N atomic sites. Decoupling experimental revealed that Fe clusters promoted the high activity of Fe/Zn-N atomic sites, while the nano-fingerprint carbon layers facilitate the confinement and stabilization of hybrid (M ) sites. The Zn Fe /f-NC demonstrating superior ORR performance, and the ZAB with the Zn Fe /f-NC air-cathode showing outstanding stability.\n2. Are there differences between the catalyst synthesized in the paper and the nitrogen-doped carbon substrate derived from MOFs reported in terms of their catalytic properties?\nResponse: We thank the reviewer for the comments and suggestions. Below we provide a supplementary response to how our work differs from other MOF-derived N-doped carbon (M-N-C) catalysts. Recent years have witnessed tremendous progress in M-N-C catalyst research, with the ORR catalytic activity of these materials (particularly Fe-N-C materials) now rivaling that of commercial Pt/C catalyst. Our survey of the current research landscape reveals that single-atom sites may face challenges in simultaneously optimizing catalytic activity and stability. Moreover, disentangling the contributions of individual species to activity and stability in a hybrid active site systems necessitates further in-depth research.\nTo address these issues, we designed and investigated a composite catalyst architecture that concurrently incorporates M , clusters (M ), and nano-fingerprint carbon layers substrate. Our key finding is that slight variations in synthetic conditions lead to systematic changes in both the chemical states of the metal sites and the morphology of the carbon support. Based on systematic decoupling experiments, we dissected the functional roles of each component: the decoupling experiments show that Fe clusters significantly promote the high activity of Fe/Zn-N atomic sites, whereas the fingerprint-like nanocarbon layer primarily provides interfacial confinement and site stabilization, suppressing further cluster aggregation. Consequently, this composite catalyst exhibits clear advantages in achieving both high activity and long-term stability.\nFor direct comparison, we have revised and supplemented Tables S6, S9 and S10 to list key performance metrics for representative MOF-derived M-N-C catalysts from recent years (Table R1) from resent years. The table includes catalyst name, E , E and zinc–air battery cycling life, power density and testing current density, and full reference information. This compilation shows that, although a few reports outperform our work on individual metrics, Zn Fe /f-NC’s overall performance, particularly with respect to the activity-stability balance under long-term device operating conditions, is superior to most reported MOF-derived M-N-C catalysts. We have provided detailed comparison data and original spectra in the Supplementary Information for reviewers’ inspection.\nWe thank the reviewer for this suggestion; the additional comparative analysis helps to underscore the scientific significance of our work\nTable R1. The comparison of Zn Fe /f-NC with other reported ZIF-Derived M-N-C Catalysts in ORR and zinc-air battery.\nCatalyst E E Peak power Stability (t @ Reference\nonest 1/2\n(V vs. RHE) (V vs. RHE) density mA cm-2)\n(mW/cm-2)\nZn Fe /f-NC 1.05 V 0.93 V 263.82 2200 h @ 10 This work\n1 1+x mA cm-2\nZn Fe /NC 1.01 V 0.89 V 196.77 113 h @ 5 mA This work\n1 1 cm-2\nPt/C 1.00 V 0.87 V 145.22 270 h @ 5 mA This work\ncm-2\nCu-Co/NC 1.06 V 0.92 V 295.9 510 h @ 10 mA Adv. Mater. 2023, 35, 2300905.\ncm-2\nCR-Co/CINC 1.008 V 0.93 V 176.6 50 h @ 5 mA Nat. Com. 2024, 15,1675\ncm-2\nCo /Fe-N@CHC -[a] 0.915 V 232.4 240 h @ 10 mA Adv. Mater. 2021, 33, 2104718\n2 cm-2\nFe,Mn/NC - - 160.8 81 h @ 5 mA Nat. Commun. 2021, 12, 1734.\ncm-2\nFe / @HNC - - 171.5 135 h @ 5 mA Adv. Mater. 2024, 2400523\nSA AC cm-2\nCo@DMOF-900 - 0.866 V 158 200 h @ 5 mA Angew. Chem. Int. Ed. 2021, 60,\ncm-2 21685.\nFeCo-DACs/NC 0.984 V 0.877 V 175 240 h @ 10 mA Adv. Mater. 2022, 34, 2107421\ncm-2\nFe /d-CN - 0.95 V 144.0 20 h @ 1 mA Energy Environ. Sci., 2021, 14,\n1 cm-2 6455.\nFeNS/Fe C@CNS 1.02 V 0.91 V 173.6 50 h @ 2 mA Angew. Chem. Inter. Ed. 2024, 63,\n3 cm-2 e202313034\nSe-doped MOF CoS HSs 0.952 V 0.88 V 156.24 100 h @ 5 mA App. Catal. B: Environ., 2023,\n2 cm-2 330, 122523.\nCo SAs/3D GFs 1.032 V 0.901 V 200 160 h @ 5 mA Angew. Chem. Inter. Ed. 2020,\ncm-2 321, 122041.\nNi P-NP-C900 0.90 V 0.75 V - >70 h @ 10 mA App. Catal. B: Environ., 2023,\nx cm-2 330, 122523.\nFe-SAs/NPS-HC - 0.912 V 195.0 56 h @ 5 mA Nat. Commun. 2018, 9, 5422\ncm-2\nFe -N SO /NC - 0.91 V 282.4 900 Cycle J. Am. Chem. Soc., 2025, 147,\n1 4 2 number @ 10 6914.\nmA cm-2\nFe /Fe -NC 0.96 V 0.88 V 205.7 100 h @ 10 mA J. Energy Chem., 2025, 102, 413.\nSA AC cm-2\n[a] ‘-’ data is not provided in the original articles.\n3. The fingerprint carbon layer is the core innovation, stabilizing the M1+x sites, but this is attributed to “no washing → direct calcination” in the work. However, the influence of ZIF-8’s own thermal decomposition has not been ruled out. Does pure ZIF-8 calcination at the same temperature produce fingerprint carbon? The role of Fe has not been verified. Is graphitization catalyzed by excess Fe? Control experiments comparing “no washing but no Fe” and “washing followed by the addition of excess Fe” are needed.\nResponse: We thank the reviewer for the comment. We thank the reviewer for their insightful consideration of the origin of the fingerprint-like carbon layer. We fully agree with the reviewer’s suggestion and have prepared two new comparative samples.\n(1) Sample of “Zn1/NC” (the new comparative sample of “no washing but no Fe”). We calcined ZIF-8 at 1000 °C in a nitrogen atmosphere to obtain Zn /NC, as shown in Figure R1. After undergoing the same high-temperature calcination conditions, the Zn /NC sample did not exhibit a distinct nano-fingerprint carbon layer structure; instead, it presented a disordered structure of bent, graphitized carbon layers.\nFigure R1. HRTEM images of Zn /NC (ZIF-8 no washing). (Figure R1 has been renumbered as Figure S16 in the revised Supporting Information)\n(2) Sample of “Zn Fe /NC” (the new comparative sample of “washing followed by the addition of excess Fe”): Based on your effective suggestion, when we prepared Zn Fe /NC sample, we washed away the excess Fe precursor before the first calcination, and then added the excess Fe precursor again after obtaining Zn Fe /NC sample, followed by a second calcination under the same conditions to obtain sample Zn Fe /NC. Figures R2a and R2b indicate that there is no distinct nano-fingerprint carbon layer structure (the structure of the graphitized carbon layers like the sample of Zn /NC). Figure R2c shows the presence of small nanoclusters. AC-HAADF-STEM in Figure R2d reveals that, except for the single atoms displayed at the bright spots, the carbon layer also contains nanoclusters. These experimental results demonstrate that the formation of nano-fingerprint carbon layer is a self-catalyzed, in-situ process requiring an appropriate amount of Fe precursor at a suitable calcination temperature. In contrast, the nano-fingerprint carbon structure cannot be obtained by re-introducing Fe precursors into the pre-graphitized carbon substrate.\nWe subsequently performed ORR testing on Zn Fe /NC sample, including its stability test. Figure R3a illustrates that Zn Fe /NC and Zn Fe /f-NC (the main sample with the best performance in the manuscript) exhibit similar activity, which is significantly higher than that of Zn Fe /NC and Zn Fe /f-NC (clusters were removed from Zn Fe /f-NC by using strong acid washing), demonstrating the indispensable contribution of Fe clusters to the activity. However, as revealed by the stability test (Figure R3b), Zn Fe /NC sample, lacking a fingerprint carbon substrate, shows notably inferior stability compared to Zn Fe /f-NC. This finding further corroborates our decoupled experimental results. Decoupling experimental revealed that Fe clusters promoted the high activity of Fe/Zn-N atomic sites, while the nano-fingerprint carbon layers facilitate the confinement and stabilization of hybrid (M ) sites. Your insightful suggestions have significantly enhanced the scientific rigor and completeness of our decoupling experiments.\nFigure R2. Zn Fe /NC (Zn Fe /NC washing followed by the addition of excess Fe): a, b) HRTEM images; c) HADDF-STEM image; d) AC-HAADF-STEM image. (Figure R2 has been renumbered as Figure S17 in the revised Supporting Information)\nFigure R3. a) ORR polarization curves measured in 0.1 M KOH solution; b) relative i-t curves for decoupling experiment catalyst; c) comparison of the E values and relative i-t retention curves of Zn Fe /f-NC and decoupled experimental samples in 0.1 M KOH. (Figure R3 has been renumbered as Figure 3a 3e and 3g in the revised manuscript)\n4. In ORR, carbon corrosion of carbon-based catalysts is a serious problem, even though the authors conducted stability tests before and after to prove the stability of the catalyst. However, a 10-hour test seems too short, and compared with the recently reported single-atom catalyst materials, this material does not seem to have any advantages.\nResponse: We thank the reviewer for the valuable comment. In ORR testing, we require an RDE to ensure stable oxygen mass transfer, because prolonged high-speed rotation at 1600 rpm can pose a potential risk of damage to such delicate instruments, and continuous O purging also carries certain risks, most stability tests reported in the literature do not exceed 50 h and are typically concentrated in the 10-20 h range (J. Am. Chem. Soc., 144, 18155 (2022); Angew. Chem. Int. Ed., 55, 14510 (2016)). The original 10-hour i-t test was intended to distinguish the stability differences between materials with and without a curled carbon layer, as well as commercial Pt/C. Considering the reviewer’s suggestion, we extended the test duration to 50 hours. As shown in Figure R4, the current decayed to 94% after 50 hours for Zn Fe /f-NC and to 93% for Zn Fe /f-NC. These stability values are superior to those for Zn Fe /NC (87%) and Zn Fe /NC (89%), demonstrating the role of the curled carbon nano-fingerprint carbon layers in catalyst stability. To demonstrate its stability in battery devices, the ZAB with Zn Fe /f-NC air cathode exhibited no significant degradation for over 2200 hours at the current density of 10 mA cm-2, which is more superior than most of the other reported ORR catalysts. (question 2, Table R1)\nFigure R4. relative retention i-t curves for a) Zn Fe /f-NC and Pt/C, b) Zn Fe /NC, Zn Fe /NC and Zn Fe /f-NC that in 0.1 M KOH. (Figure R4 has been renumbered as Figure 3g in the revised manuscript)\n5. Please conduct additional experiments to determine the optimal ratio of the mixed-site composite electrocatalyst The Zn1Fe1+x/f-NC. It is insufficient and unconvincing to discuss only the synergistic effect between the dopant and the host.\nResponse: We are grateful for the reviewer's constructive feedback and advice. Indeed, as part of our efforts to optimize the catalyst for peak performance, we explored a concentration gradient of Fe precursors. The significance of this gradient experimental data was not fully appreciated in the original submission. In response to the reviewer's suggestion, we have incorporated this gradient data into the revised manuscript, complemented by additional relevant characterization results for the respective gradient samples. The comparative ORR performance tests demonstrate a progressive increase in activity from 0.5 to 1.5 mmol of Fe precursor (the best performing Zn Fe /f-NC catalyst is obtained by selecting 1.5 mmol of Fe precursor). However, a decline in performance is observed at 2.0 mmol (Figure R5a). This phenomenon is likely due to the aggregation of Fe species into actual Fe particles, exceeding the cluster formation regime at this higher precursor concentration. The AC-HADDF-STEAM image further confirm the presence of these Fe particles (Figure R5c).\nFigure R5. a) LSV curves for evaluating the ORR catalytic activity of different Fe precursor concentrations; b) High-resolution XPS of Fe 2p spectra; c) AC-HAADF-STEM image of the contrast sample with a precursor Fe loading at 2.0 mmol. (Figure R5 has been renumbered as Figure S26 in the revised Supporting Information)\nXPS tests were performed on Zn Fe /f-NC and the optimized samples obtained by annealing precursors with different Fe precursor loading concentrations (Figure R5b). The XPS results showed that the relative proportion of Fe2+ gradually increased while that of Fe3+ decreased as the Fe(acac) loading increased from 0.5 mmol to 2.0 mmol. A small zero-valent Fe peak (706.2 eV) can be seen in the Fe 2p spectrum, which means the Fe metal possibly have a large volume and break through the definition of clusters (Adv. Mater. 35, 2305945 (2023)). This result indicates that Fe2+ plays a dominant role in catalytic activity. When the coverage exceeds 1.5 mmol beyond the Fe saturation point-Fe clusters aggregate into Fe nanoparticles, causing the activity to decrease gradually (Table R2 and Figure R5b).\nTable R2. The proportion of different iron types calculated from the peak area ratio in Fe 2p XPS spectra. (Table R2 has been renumbered as Table S5 in the revised Supporting Information)\nFe(acac) @ZIF-8 Content Fe2+ (%) Content Fe3+ (%) Content Fe0 (%)\n3\n0.5 mmol 58 42 0\n1.0 mmol 64 36 0\n1.5 mmol 68 32 0\n2.0 mmol 69 29 2\n6. In terms of mechanism, the authors still propose the traditional adsorption mechanism without any innovation. Furthermore, the role of bent carbon is described as “contributing to the confinement and stabilization of M1+x,” but this seems to be a general function of carrier materials, and its unique mechanism of action in regulating electronic structure or promoting specific reaction pathways has not been thoroughly elucidated.\nResponse: Sincerely thanks the reviewer for the insightful suggestions. In the manuscript, we selected C as a calculation model of bent carbon. It seems that the 60 curvature varies too drastic from flat carbon (graphene model) to the bent carbon (C 60 model) to investigate the general function of carrier materials. In the revised version, we added two models (single wall carbon nanotube with different curvatures) among them to reveal the general role of carrier materials from flat to bent. Herein, (6,6)-CNT and (10,10)-CNT were selected, while similar construction proposal was adopted as in graphene model to embed FeN moiety on them. These two models were named as 4 Fe /(6,6)-CNT and Fe /(10,10)-CNT, which contain 143 and 239 atoms in them, 1 1 respectively. Generally speaking, the curvature of C is similar as it of (5,5)-CNT (J. 60 Phys. Chem. B 112, 15442 (2008); J. Phys. Chem. C 115, 3220 (2011)). Hence, the curvature increases not too drastically as graphene < 10,10-CNT < (6,6)-CNT < (5,5)-CNT ≈ C , as shown in Figure R6a. 60\nFigure R6. (a) Side view for C and three armchair single-wall carbon nanotube with 60 different diameters. (b) ORR free energy diagrams of Fe /NC, Fe /f-NC, Fe /(6,6)-CNT 1 1 1 and Fe/(10,10)-CNT at U=1.23 V. (Figure R6 has been renumbered as Figure 4a and S50 in the revised manuscript)\nBased on their optimized structures together with the adsorption intermediates on them, we calculated their free energy profiles alone ORR reaction path. The free energy profiles of ORR were shown in Figure R6b. At a potential 1.23 V, the highest upslope free energy of the fourth step (OH*+ H+ + e- →*+H O) is the rate-determining step 2 (rds) for all considered models, in which the rds height on Fe /NC is 1.40 eV. When 1 curvature effect was introducing, for small curvature model such as Fe /(10,10)-CNT, 1 the desorption energy of OH* decreased to 0.80 eV. For large curvature model such as Fe /(6,6)-CNT, the desorption energy of OH* further decreased to 0.66 eV. Since rds 1 height on Fe /(6,6)-CNT is close to its counterair in Fe /C , this convergence implied 1 1 60 that curvature effect modulation FeN /carbon materials has its intrinsic upper limit. To 4 further validate the curvature effect for Fe -Zn /NC, we constructed Fe -Zn /(10,10)- 1 1 1 1 CNT model and also calculated its free energy profiles, as shown in Figure R7. For Fe - 1 Zn , its rds height decrease from 0.89 eV on Fe -Zn /NC to 0.84 eV Fe -Zn /(10,10)- 1 1 1 1 1 CNT.\nFigure R7. (a) Structures for Fe -Zn /NC and Fe -Zn /(10,10)-CNT. (b) ORR free 1 1 1 1 energy diagrams of Fe -Zn /NC and Fe -Zn /(10,10)-CNT at U=1.23 V. (Figure R7 has 1 1 1 1 been renumbered as Figure 4a and S51 in the revised manuscript)\nFor planar reaction sites, it is obviously that that position of d would affect the z2 adsorption strength for critical intermediates (OH* in this work). However, for curved reaction sites, the difference of sensitivities for particular m-components for 3d containing z-direction distribution (such as d and d besides d ) would be involved, xz yz z2 as we suggested in our previous work (ChemRxiv. 2025, doi:10.26434/chemrxiv-2025-s1xjc). In data mining, regression is a type of supervised learning tasked with predicting continuous numerical values. Here a regression equation was fitted:\nrds_height = 1.009 - 0.118 × d -down - 0.107 × d -down\nz2 xz\nwhere d -down and d -down are the d band center and d band center in their spin- z2 xz z2 xz down channel of reactive Fe site in each model except Fe /C . 1 60\nAs shown in Table R3, the relative errors for all samples are within ±5.5%, indicating that the prediction accuracy of this regression equation is extremely high (R2=0.9939). Although this regression equation is derived from a limited sample size and may exhibit some degree of overfitting, it demonstrates a chemically intuitive regularity that aligns well with the underlying physical principles.\nTable R3. Regression analysis based on the regression equation. The mean absolute error (MAE) of this model is 0.0086. (Table R3 has been renumbered as Table S8 in the revise d supporting information)\nsample dz2-down dxz-down Rds-calculated(eV) Rds-fitted(eV) Error Relative error(%)\nFe1/NC 0.8507 -1.2216 1.40 1.394 -0.006 -0.4%\nFe1-Fe1/NC 0.3997 -1.3678 0.99 0.991 +0.001 +0.1%\nFe1-Zn1/NC 0.5387 -1.4676 0.89 0.896 +0.006 +0.7%\nFe1-Zn1-Fex/NC 0.3884 -1.5083 0.85 0.896 +0.046 +5.4%\nFe1/(10,10)- CNT -1.6529 0.1903 0.80 0.803 +0.003 +0.4%\nFe1-Zn1/(10,10)- CNT -1.7075 -0.2466 0.84 0.837 -0.003 -0.4%\nFe1/(6,6)- CNT -0.1366 -2.0760 0.66 0.661 +0.001 +0.2%\nFigure R8. Projected density of states for Fe site in Fe /NC, Fe /(10,10)-CNT and 1 1 Fe /(6,6)-CNT. (Figure R8 has been renumbered as Figure 4d in the revised manuscript) 1\nIt should be noted that we did not select (5,5)-CNT as support model since in our previous study (ChemRxiv. 2025, doi:10.26434/chemrxiv-2025-s1xjc), we determined that in MN /(5,5)-CNT (M is the 3d, 4d, and 5d metal elements), reaction center MN 4 4 would suffer huge stain if the curvature of (5,5)-CNT is maintained. While for C , 60 although it has the similar dimeter as (5,5)-CNT, its five-membered ring and six-membered ring arrangements around the embedded FeN reaction center could release 4 the strain partially.\nIn Figure R8 we calculated the projected density of states (PDOS) for Fe /NC, 1 Fe /(10,10)-CNT and Fe /(6,6)-CNT to exhibit their corresponding d and d . For 1 1 z2 xz Fe /NC, a significant gap between d -down and d -down, which agrees the typical d- 1 xz z2 orbital splitting in D crystal field between e (d ) and a (d ). When curvature effect 4h g z2 1g xz is introduced, in Fe /(10,10)-CNT a clear intermediate state appears near Fermi level 1 which mainly contributed by d -down. While for Fe /(6,6)-CNT with higher curvature, xz 1 the intermediate states show a dispersion character, at the same time, main peak of d - z2 dwon is pushed above Fermi lever again. If the total d-band center is considered, the detailed variation would be omitted. While if only the d band center is considered, the z2 contribution from d due to curvature effect would be ignored. Based on the regression xz equation we fitted above, we supposed that it was the compromising between d -down and d -down regulating the critical electronic states to overlap the p orbital in xz z2 adsorption intermediates. This conclusion agrees well with our precious work based on partial carbon nanotube embedded MN models. Figure R9 shows the PDOS for Fe - 4 1 Zn /NC and Fe -Zn /(10,10)-CNT to exhibit their corresponding d and d . Similarly, 1 1 1 z2 xz for Fe -Zn /NC, a significant gap between d -down and d -down. When curvature 1 1 xz z2 effect is introduced, in Fe -Zn /(10,10)-CNT a clear intermediate state appears near 1 1 Fermi level which mainly contributed by d -down. Meanwhile, the relative position of xz main peaks for d -down and d -down reversed. The shapes of main peaks for d -down xz z2 xz and d -down in Fe /NC and Fe -Zn /NC are analogous, only the positions of d -down z2 1 1 1 xz and d -down centers were pushed down after introducing Zn in Fe /NC. Therefore, z2 1 1 for planar Fe /NC, the curvature effect at least alike in (10,10)-CNT introducing more 1 drastic electronic structure modulation than hybrid Zn sites. 1\nFigure R9. Projected density of states for Fe -Zn /NC and Fe -Zn /(10,10)-CNT. 1 1 1 1 (Figure R9 has been renumbered as Figure S52 in the revised Supporting Information)\nReviewer #2\nWang et al. report the development of a composite electrocatalyst (Zn1Fe1+x/f-NC) featuring Fe clusters encapsulated within curved carbon layers and surrounded by atomically dispersed Zn/Fe-N4 sites, aiming to enhance ORR activity and stability for Zn-air batteries. The authors claim that the cooperative modulation of electronic structure via multivariate active site engineering breaks the linear scaling relations in ORR catalysis, delivering a half-wave potential of 0.93 V and stable Zn-air battery operation over 1000 hours. While the findings are potentially interesting, the manuscript in its current form does not reach the depth and originality expected for publication in Nature Communications. My key concerns are outlined below:\nResponse: We greatly appreciate the reviewer for taking time to evaluate our work and providing constructive and valuable feedback. We have made a point-by-point response to your comments and carefully revised the manuscript as you suggested, and we believe that the quality of the revised manuscript has been significantly improved under your guidance. The specific response for each comment is attached below.\n1. Hybrid Fe-based single-atom and cluster catalysts supported on N-doped carbon frameworks have been widely reported (Nat. Commun. 2018, 9, 5422; Adv. Mater. 2022, 34, 2200595). The proposed curved carbon support and Zn/Fe dual-site synergy are incremental extensions of established strategies. The manuscript would benefit from a clearer delineation of how this design fundamentally differs from previous multi-site SAC systems, and whether the observed performance truly surpasses state-of-the-art benchmarks.\nResponse: We thank the reviewer for the valuable comment. Below we first provide a supplementary response to how our work differs from other metal-nitrogen-carbon (M-N-C) catalysts. Recent years have witnessed tremendous progress in M-N-C catalyst research, with the ORR catalytic activity of these materials (particularly Fe-N-C materials) now rivaling that of commercial Pt/C catalyst. For instance, reviewer-suggested Reference 1 (Nat. Commun. 2018, 9, 5422) delves into the mechanism by which single iron atomic sites supported on a nitrogen, phosphorus and sulfur co-doped hollow carbon polyhedron (Fe-SAs/NPS-HC) catalyst exhibits outstanding ORR activity. Reviewer-suggested Reference 2 (Adv. Mater. 2022, 34, 2200595), a comprehensive review, has also been thoroughly studied to understand the current state of research. We have cited both of these references in the introduction part of the revised manuscript to improve the comprehensiveness of our literature review. (see references 19 and 20 in the revised manuscript). Our survey of the current research landscape reveals that single-atom sites may face challenges in simultaneously optimizing catalytic activity and stability. Moreover, disentangling the contributions of individual species to activity and stability in a hybrid active site systems necessitates further in-depth research. Our key finding is that slight variations in synthesis conditions lead to systematic changes in both the chemical state of the active sites and the morphology of the carbon support. We contend that the innovations and strengths of this work are primarily attributable to two aspects: (1) Evidence from decoupling experiments on stability and activity. (2) Performance advantages from dual guarantee of activity and stability. The details are as follows:\n(1) Evidence from decoupling experiments on stability and activity. Considering the complexity of our working catalyst (including single atoms (M ), clusters(M ), and nano-fingerprint carbon layers), we conducted decoupling experiments. The experiments indicate that it is difficult for a single component to maintain both high activity and stability. In Zn Fe /f-NC, decoupling experimental revealed that Fe clusters promoted the high activity of Fe/Zn-N atomic sites, while the nano-fingerprint carbon layers facilitate the confinement and stabilization of hybrid (M ) sites. Moreover, in response to other valuable suggestions by other Reviewers, we performed more comprehensive decoupling experiments and successfully prepared the comparative samples: Zn Fe /f-NC and Zn Fe /NC (more details see Reviewer 1, Question 3). These experimental results demonstrate that the formation of nano-fingerprint carbon layer is a self-catalyzed, in-situ process requiring an appropriate amount of Fe precursor at a suitable calcination temperature. In contrast, the nano-fingerprint carbon structure cannot be obtained by re-introducing Fe precursors into the pre-graphitized carbon substrate.\n(2) Performance advantages from dual guarantee of activity and stability. In our work, a composite catalyst (Zn Fe /f-NC) containing Fe clusters encapsulated within curved carbon layers and surrounded by densely dispersed Fe/Zn-N sites was obtained. The prepared Zn Fe /f-NC displayed promising ORR performance with a half-wave potential (E ) of 0.93 V, surpassing that of Pt/C and most of the other reported catalysts (see Table R1 as below). The Zn Fe /f-NC catalyst was further evaluated as the cathode by assembling ZABs. Impressively, the ZAB cycle test demonstrated ultra-high durability of over 2200 h under the current density of 10 mA cm-2. HAADF-STEM characterization for the Zn Fe /f-NC air cathode after more than 2200 h charge-discharge stability operation demonstrates that Zn Fe /f-NC shows minimal aggregation at atomic sites and that the nano-fingerprint carbon layer structure remains intact (see Question 5). Table R1 showing the advantageous long-term stability over other reported electrocatalysts.\nTable R1. The Advantage of Zn Fe /f-NC over Prior M-N-C Catalysts for the ORR and ZABs.\nCatalyst E E Peak power Stability (t @ Reference\nonest 1/2\n(V vs. RHE) (V vs. RHE) density mA cm-2)\n(mW/cm-2)\nZn Fe /f-NC 1.05 V 0.93 V 263.82 2207 h @ 10 This work\n1 1+x mA cm-2\nZn Fe /NC 1.01 V 0.89 V 196.77 113 h @ 5 mA This work\n1 1 cm-2\nPt/C 1.00 V 0.87 V 145.22 270 h @ 5 mA This work\ncm-2\nCu-Co/NC 1.06 V 0.92 V 295.9 510 h @ 10 mA Adv. Mater. 2023, 35,\ncm-2 2300905.\nCR-Co/CINC 1.008 V 0.93 V 176.6 50 h @ 5 mA Nat. Com. 2024, 15,1675\ncm-2\nCo /Fe- -[a] 0.915 V 232.4 240 h @ 10 mA Adv. Mater. 2021, 33,\nFe,Mn/NC - - 160.8 81 h @ 5 mA Nat. Commun. 2021, 12,\ncm-2 1734.\nFe / @HNC - - 171.5 135 h @ 5 mA Adv. Mater. 2024,\nSA AC cm-2 2400523\nCo@DMOF- - 0.866 V 158 200 h @ 5 mA Angew. Chem. Int. Ed.\n900 cm-2 2021, 60, 21685.\nFeCo-DACs/NC 0.984 V 0.877 V 175 240 h @ 10 mA Adv. Mater. 2022, 34,\ncm-2 2107421\nFe /d-CN - 0.95 V 144.0 20 h @ 1 mA Energy Environ. Sci.,\n1 cm-2 2021, 14, 6455.\nFeNS/Fe C@C 1.02 V 0.91 V 173.6 50 h @ 2 mA Angew. Chem. Inter. Ed.\nSe-doped MOF 0.952 V 0.88 V 156.24 100 h @ 5 mA App. Catal. B: Environ.,\nCoS HSs cm-2 2023, 330, 122523.\ncm-2 2020, 321, 122041.\nNi P-NP-C900 0.90 V 0.75 V - >70 h @ 10 mA App. Catal. B: Environ.,\nx cm-2 2023, 330, 122523.\nFe-SAs/NPS- - 0.912 V 195.0 56 h @ 5 mA Nat. Commun. 2018, 9,\nHC cm-2 5422\nFe -N SO /NC - 0.91 V 282.4 900 Cycle J. Am. Chem. Soc., 2025,\n1 4 2 number @ 10 147, 6914.\nmA cm-2\nFe /Fe -NC 0.96 V 0.88 V 205.7 100 h @ 10 mA J. Energy Chem., 2025,\nSA AC cm-2 102, 413.\n[a] ‘-’ data is not provided in the original articles.\n2. The role of curved carbon layers is primarily inferred from TEM morphology and Raman spectra. However, the manuscript lacks quantitative evidence linking curvature-induced strain to modulation of Fe d-band centers or ORR kinetics. Additional spectroscopic data or a more detailed theoretical analysis is needed to substantiate the claimed electronic effects.\nResponse: Sincerely thanks the reviewer for the insightful suggestions. In the manuscript, we selected C as a calculation model of bent carbon. It seems that the 60 curvature varies too drastic from flat carbon (graphene model) to the bent carbon (C 60 model) to investigate the general function of carrier materials. In the revised version, we added two models (single wall carbon nanotube with different curvatures) among them to reveal the general role of carrier materials from flat to bent. Herein, (6,6)-CNT and (10,10)-CNT were selected, while similar construction proposal was adopted as in graphene model to embed FeN moiety on them. These two models were named as 4 Fe /(6,6)-CNT and Fe /(10,10)-CNT, which contain 143 and 239 atoms in them, 1 1 respectively. Generally speaking, the curvature of C is similar as it of (5,5)-CNT (J. 60 Phys. Chem. B 112, 15442 (2008); J. Phys. Chem. C 115, 3220 (2011)). Hence, the curvature increases not too drastically as graphene < (10,10)-CNT < (6,6)-CNT < (5,5)-CNT ≈ C , as shown in Figure R1a. 60\nFigure R1. (a) Side view for C and three armchair single-wall carbon nanotube with 60 different diameters. (b) ORR free energy diagrams of Fe /NC, Fe /f-NC, Fe /(6,6)-CNT 1 1 1 and Fe/(10,10)-CNT at U=1.23 V. (Figure R1 has been renumbered as Figure 4a and S50 in the revised manuscript)\nBased on their optimized structures together with the adsorption intermediates on them, we calculated their free energy profiles alone ORR reaction path. The free energy profiles of ORR were shown in Figure R1b. At a potential 1.23 V, the highest upslope free energy of the fourth step (OH*+ H+ + e- →*+H O) is the rate-determining step 2 (rds) for all considered models, in which the rds height on Fe /NC is 1.40 eV. When 1 curvature effect was introducing, for small curvature model such as Fe /(10,10)-CNT, 1 the desorption energy of OH* decreased to 0.80 eV. For large curvature model such as Fe /(6,6)-CNT, the desorption energy of OH* further decreased to 0.66 eV. 1 Since rds height on Fe /(6,6)-CNT is close to its counterair in Fe /C , this 1 1 60 convergence implied that curvature effect modulation FeN /carbon materials has its 4 intrinsic upper limit.\nTo further validate the curvature effect for FeN reaction center, we constructed 4 Fe -Zn /(10,10)-CNT model and also calculated their free energy profiles, as shown in 1 1 Figure R2. For Fe -Zn , its rds height decrease from 0.89 eV on Fe -Zn /NC to 0.84 eV 1 1 1 1 Fe -Zn /(10,10)-CNT. 1 1\nFigure R2. (a) Structures for Fe -Zn /NC and Fe -Zn /(10,10)-CNT. (b) ORR free 1 1 1 1 energy diagrams of Fe -Zn /NC and Fe -Zn /(10,10)-CNT at U=1.23 V. (Figure R2 has 1 1 1 1 been renumbered as Figure 4a and S51 in the revised manuscript)\nFor planar reaction sites, it is obviously that that position of d would affect the z2 adsorption strength for critical intermediates (OH* in this work). However, for curved reaction sites, the difference of sensitivities for particular m-components for 3d containing z-direction distribution (such as d and d besides d ) would be involved, xz yz z2 as we suggested in our previous work.5 In data mining, regression is a type of supervised learning tasked with predicting continuous numerical values. Here a regression equation was fitted:\nrds_height = 1.009 - 0.118 × d -down - 0.107 × d -down\nz2 xz\nwhere d -down and d -down are the d band center and d band center in their spin- z2 xz z2 xz down channel of reactive Fe site in each model except Fe /C . 1 60\nAs shown in Table R2, the relative errors for all samples are within ±5.5%, indicating that the prediction accuracy of this regression equation is extremely high (R2=0.9939). Although this regression equation is derived from a limited sample size and may exhibit some degree of overfitting, it demonstrates a chemically intuitive regularity that aligns well with the underlying physical principles.\nTable R2. Regression analysis based on the regression equation. The mean absolute error (MAE) of this model is 0.0086. (Table R2 has been renumbered as Table S8 in the revise d Supporting Information)\nsample dz2-down dxz-down Rds-calculated(eV) Rds-fitted(eV) Error Relative error(%)\nFe1/NC 0.8507 -1.2216 1.40 1.394 -0.006 -0.4%\nFe1-Fe1/NC 0.3997 -1.3678 0.99 0.991 +0.001 +0.1%\nFe1-Zn1/NC 0.5387 -1.4676 0.89 0.896 +0.006 +0.7%\nFe1-Zn1-Fex/NC 0.3884 -1.5083 0.85 0.896 +0.046 +5.4%\nFe1/(10,10)- CNT -1.6529 0.1903 0.80 0.803 +0.003 +0.4%\nFe1-Zn1/(10,10)- CNT -1.7075 -0.2466 0.84 0.837 -0.003 -0.4%\nFe1/(6,6)- CNT -0.1366 -2.0760 0.66 0.661 +0.001 +0.2%\nFigure R3. Projected density of states for Fe site in Fe /NC, Fe /(10,10)-CNT and 1 1 Fe /(6,6)-CNT. (Figure R3 has been renumbered as Figure 4d in the revised manuscript) 1\nIt should be noted that we did not select (5,5)-CNT as carrier model since in our previous study (ChemRxiv. 2025; doi:10.26434/chemrxiv-2025-s1xjc), we determined that in MN /(5,5)-CNT (M is the 3d, 4d, and 5d metal elements), reaction center MN 4 4 would suffer huge stain if the curvature of (5,5)-CNT is maintained. While for C60, although it has the similar dimeter as (5,5)-CNT, its five-membered ring and six-membered ring arrangements around the embedded FeN reaction center could release 4 the strain partially.\nIn Figure R3 we calculated the projected density of states (PDOS) for Fe /NC, 1 Fe /(10,10)-CNT and Fe /(6,6)-CNT to exhibit their corresponding d and d . For 1 1 z2 xz Fe1/NC, a significant gap between d -down and d -down, which agrees the a typical xz z2 d-orbital splitting in D crystal field between e (d ) and a (d ). When curvature 4h g z2 1g xz effect is introduced, in Fe /(10,10)-CNT a clear intermediate state appears near Fermi 1 level which mainly contributed by d -down. While for Fe /(6,6)-CNT with higher xz 1 curvature, the intermediate states show a dispersion character, at the same time, main peak of d -dwon is pushed above Fermi lever again. If the total d-band center is z2 considered, the detailed variation would be omitted. While if only the d band center z2 is considered, the contribution from d due to curvature effect would be ignored. Based xz on the regression equation we fitted above, we supposed that it was the compromising between d -down and d -down regulating the critical electronic states to overlap the p xz z2 orbital in adsorption intermediates. This conclusion agrees well with our precious work based on partial carbon nanotube embedded MN models. Figure R4 shows the PDOS 4 for Fe -Zn /NC and Fe -Zn /(10,10)-CNT to exhibit their corresponding d and d . 1 1 1 1 z2 xz Similarly, for Fe -Zn /NC, a significant gap between d -down and d -down. When 1 1 xz z2 curvature effect is introduced, in Fe -Zn /(10,10)-CNT a clear intermediate state 1 1 appears near Fermi level which mainly contributed by d -down. Meanwhile, the xz relative position of main peaks for d -down and d -down reversed. The shapes of main xz z2 peaks for d -down and d -down in Fe /NC and Fe -Zn /NC are analogous, only the xz z2 1 1 1 positions of d -down and d -down centers were pushed down after introducing Zn in xz z2 1 Fe /NC. Therefore, for planar Fe /NC, the curvature effect at least alike in (10,10)-CNT 1 1 introducing more drastic electronic structure modulation than hybrid Zn1 sites. 1\nFigure R4. Projected density of states for Fe -Zn /NC and Fe -Zn /(10,10)-CNT. 1 1 1 1 (Figure R4 has been renumbered as Figure S52 in the revised Supporting Information)\nMössbauer spectroscopy has been known as a useful technique to examine the chemical environment and spin state of Fe nuclei (Figure R5). The deconvolution of the transmission profiles reveals that D1, D2 and D3 doublets are the dominant Fe sites in both Zn Fe /f-NC and Zn Fe /NC. D1 has been observed in a large number of Fe- 1 1+x 1 1 N-C catalysts and is usually attributed to low-spin Fe(II)−N or high-spin Fe(III)−N 4 4 moiety (with O adsorbed), which have been identified as highly active species for ORR 2 in contrast to the D2 sites (D2 sites represent medium-spin Fe(II)-N ). (Nat. Cat., 4, 4 615-622 (2021); Nat. Mater., 14, 937-942 (2015)). On the contrary, the D1 sites exhibit less stable compared to D2 sites (Nat. Energy, 7, 652-663 (2022)). The percentages of D1 and D2 are 58.1% and 33.2% for Zn Fe /f-NC, respectively, and 60.4% and 19.9% 1 1+x for Zn Fe /NC, respectively. These findings indicate that compared to Zn Fe /NC, 1 1 1 1 Zn Fe /f-NC shows a better balance of activity and stability regulation. In particular, 1 1+x D3 (IS = −0.813, QS = 0.786, A = 8.7%) in Zn Fe /f-NC is a minor, high s-electron- 1 1+x density species interpreted as Fe-C coupled (metal-like/carbide-like or encapsulated Fe) because of its negative IS and relatively small QS (J. Am. Chem. Soc., 145, 15600 (2023)). Overall, the Mössbauer spectroscopy show that nano-fingerprint carbon increases local s-electron density at a subset of Fe sites and changes local symmetry, and Zn Fe /f-NC possessed superior regulation of activity-stability trade-offs. 1 1+x\nFigure R5. Room-temperature Mössbauer spectrum of a) Zn Fe /f-NC and b) 1 1+x Zn Fe /NC (the absorption in the y-axis is normalized with the integrated area). (Figure 1 1 R5 has been renumbered as Figure 2a and 2b in the revised manuscript)\nTable R3. Mössbauer parameters derived from fittings. (Table R3 has been renumbered as Table S2 in the revised Supporting Information)\nSamples Content IS (mm s-1) QS (mm s-1) LW (mm s-1) Area (%)\nD1 0.381 0.726 0.387 58.1\nZn Fe /f-NC D2 0.033 3.565 0.277 33.2\n1 1+x D3 -0.813 0.786 0.209 8.7\nD1 0.340 0.831 0.662 60.4\nZn Fe /NC D2 0.143 2.829 0.472 19.9\n1 1 D3 0.944 3.246 0.655 19.7\n3. While decoupling experiments and DFT calculations are provided, they do not convincingly isolate the individual contributions of Zn single atoms, Fe clusters, and curved carbon.\nResponse: We thank the reviewer for the insightful comment. Our response will be twofold, incorporating “supplementary decoupled experimental design” and “supplementary theoretical calculation evidence”:\n(1) Supplementary decoupled experimental design. In line with your insightful recommendations, we have prepared more new contrast sample. This expands our experimental approach to include a total of four decoupled contrast samples (including Zn /NC, Zn Fe /NC, Zn Fe /f-NC, Zn Fe /NC), providing a direct comparison to our 1 1 1 1 1 1 1+x optimally performing main sample Zn Fe /f-NC (Figure R6 and R7). Based on the 1 1+x experimental results, we draw the following conclusions: (a) The inclusion of nano-fingerprint carbon layers in the decoupled contrast samples (Zn Fe /f-NC and 1 1 Zn Fe /f-NC) primarily contributes to excellent ORR catalytic stability. (b) While 1 1+x decoupled contrast samples with only Zn single-atom sites (Zn /NC) perform poorly, 1 their activity is markedly enhanced by the co-existence of Fe and Zn single atoms (Zn Fe /NC). (c) Further improvements in catalytic activity are observed when Fe 1 1 clusters are incorporated into the decoupled contrast samples (Zn Fe /NC and 1 1+x Zn Fe /f-NC).\n1 1+x\nFigure R6. ORR performance. a) ORR polarization curves measured in 0.1 M KOH solution; b) the corresponding Tafel plots of electrocatalysts measured in 0.1 M KOH solution; c) comparison of j for samples in 0.1 M KOH; d) ORR polarization curves k of Zn Fe /f-NC at different rotating speeds (inset: K-L plots and electron-transfer 1 1+x number); e) comparison of the E values and relative i-t retention curves of Zn Fe /f- 1/2 1 1+x NC and decoupled experimental samples in 0.1 M KOH; f) comparison of E , E , 1/2 onset J at 0.93 V, J at 0.90 V and Tafel slope for Zn Fe /f-NC and selected reference k k 1 1+x samples in 0.1 M KOH; g) relative i-t retention curves of Zn Fe /f-NC and reference 1 1+x samples in 0.1 M KOH (inset: before and after 10,000 potential cycles (0.6-1.0 V vs RHE)); h) LSV curves of Zn Fe /f-NC (scale up) in 0.1 M KOH (inset: picture of 1 1+x Zn Fe /f-NC catalysts). (Figure R6 has been renumbered as Figure 3 in the revised 1 1+x manuscript)\nFigure R7. a). LSV curves and b) the corresponding Tafel plots of Zn /NC in 0.1 M 1 KOH. Figure R7 has been renumbered as Figure S24 in the revised Supporting Information)\n(2) Supplementary theoretical calculation evidence. Computational results indicate that based on a planar structure, the introduction of either Fe or Zn near an isolated 1 1 Fe /NC site can enhance the ORR performance (the rate-determining step (rds) changes 1 from 1.40 eV to 0.99 eV and 0.89 eV, respectively). Moreover, the performance of Fe - 1 Zn is superior to that of Fe -Fe (0.89 eV vs. 0.99 eV). Therefore, further introducing 1 1 1 an Fe N moiety to Fe -Zn leads to additional improvement in ORR performance (rds 4 6 1 1 decreases from 0.89 eV to 0.85 eV). Similarly, introducing Fe N to Fe -Fe also further 4 6 1 1 enhances the ORR performance (rds decreases from 0.99 eV to 0.89 eV). These calculations support the experimental decoupling conclusion.\nIn the revised version, we incorporated Fe /(10,10)-CNT and Fe /(6,6)-CNT, 1 1 which correspond to transitioning from a planar structure to bent substrates with small curvature (Fe /(10,10)-CNT) and large curvature (Fe /(6,6)-CNT), respectively. Free 1 1 energy calculations along the ORR pathway reveal a monotonic change in the rds (1.40 eV → 0.80 eV → 0.66 eV).\nWe also validated this curvature effect for Fe -Zn /NC by constructing Fe - 1 1 1 Zn /(10,10)-CNT. The results show that the rds decreases from 0.89 eV to 0.84 eV. 1\nProjected density of states for Fe /NC, Fe /(10,10)-CNT, Fe -Zn /NC and Fe - 1 1 1 1 1 Zn /(10,10)-CNT were show in Figure R8. For Fe /NC, a significant gap between d - 1 1 xz down and d -down, which agrees with the typical d-orbital splitting in D crystal field z2 4h between e (d ) and a (d ). When curvature effect is introduced, in Fe /(10,10)-CNT g z2 1g xz 1 a clear intermediate state appears near Fermi level which mainly contributed by d - xz down. Similarly, for Fe -Zn /NC, a significant gap between d -down and d -down. 1 1 xz z2 When curvature effect is introduced, in Fe -Zn /(10,10)-CNT a clear intermediate state 1 1 appears near Fermi level which mainly contributed by d -down. Meanwhile, the xz relative position of main peaks for d -down and d -down reversed. The shapes of main xz z2 peaks for d -down and d -down in Fe /NC and Fe -Zn /NC are analogous, only the xz z2 1 1 1 positions of d -down and d -down centers were pushed down after introducing Zn in xz z2 1 Fe /NC. Therefore, for planar Fe /NC, the curvature effect at least alike in (10,10)-CNT 1 1 introducing more drastic electronic structure modulation than hybrid Zn1 sites. 1\nDecoupling tests in theoretical way determined that the curvature effect is more dominant than hybrid Zn or Fe or Fe sites construction for planar Fe /NC in 1 1 x 1 modulating its electronic structures together with its ORR performance.\nFigure R8. Projected density of states for Fe /NC, Fe -Zn /NC and Fe /(10,10)-CNT 1 1 1 1 as well as Fe -Zn /(10,10)-CNT. (Figure R8 has been renumbered as Figure S53 in the 1 1 revised Supporting Information)\n4. The reported half-wave potential (0.93 V) and peak power density (263.82 mW·cm-2) are competitive but not significantly higher than top-performing Fe-N-C. The authors should provide a more comprehensive benchmarking table and discuss what unique advantages their catalyst offers beyond incremental gains.\nResponse: We thank the reviewer for the comment. This paper reports an Fe-based composite electrocatalyst in which Fe clusters are encapsulated within a nano-fingerprint carbon layer and surrounded with dispersed Fe/Zn-N single-atom sites. 4 This material exhibits excellent ORR activity and Zn-air battery stability. Although some published studies have demonstrated superior performance on individual metrics, our Zn Fe /f-NC catalyst exhibits exceptional balance between activity and stability. 1 1+x Consequently, our Zn Fe /f-NC catalyst is currently among the forefront of 1 1+x outstanding Fe-N-C family catalysts (Table R4). Furthermore, we contend that the unique advantages of our Zn Fe /f-NC catalyst are as follows: (1) Firstly, our catalyst 1 1+x is amenable to large-scale fabrication and demonstrates high reproducibility. We have further elaborated on the scale-up production in our revised manuscript (more details see Reviewer 3, Question 5). (2) More importantly, our key finding is that slight variations in synthetic conditions lead to systematic changes in both the chemical state of the active sites and the morphology of the carbon support. Decoupling experimental revealed that Fe clusters promoted the high activity of Fe/Zn-N atomic sites, while x 4 the nano-fingerprint carbon layers facilitate the confinement and stabilization of hybrid sites. (3) The prepared Zn Fe /f-NC displayed promising ORR performance with a 1 1+x half-wave potential (E ) of 0.93 V, surpassing that of Zn Fe /f-NC (E = 0.904 V), 1/2 1 1 1/2 Zn Fe /NC (E = 0.89 V), Pt/C, and most of the other reported catalysts. Impressively, 1 1 1/2 the ZAB with Zn Fe /f-NC air cathode demonstrates ultra-high durability of over 1 1+x 2200 h under the current density of 10 mA cm-2. Thus, the intriguing findings during this synthetic process, alongside the design of decoupling experiments for this complex catalytic system and the subsequent attribution of the roles of different species in catalytic activity or stability, may offer potential guidance for current M-N-C catalyst synthesis and the elucidation of structure-activity relationships.\nTable R4. The Advantage of Zn Fe /f-NC over Prior Fe-N-C Catalysts for the ORR and ZABs. 1 1+x (Table R4 has been renumbered as Table S10 in the revised Supporting Information)\nCatalyst E E Peak power Stability (t @ Reference\nonest 1/2\n(V vs. RHE) (V vs. RHE) density mA cm-2)\n(mW/cm-2)\nZn Fe /f-NC 1.05 V 0.93 V 263.82 2207 h @ 10 This work\n1 1+x mA cm-2\nZn Fe /NC 1.01 V 0.89 V 196.77 113 h @ 5 mA This work\n1 1 cm-2\nPt/C 1.00 V 0.87 V 145.22 270 h @ 5 mA This work\ncm-2\nFe,N,P-CNSs/Fe P 0.955 0.854 170 120 h @ 10 mA Nano Energy. 2023, 108,\n2 cm-2 108179.\nFe/OES -[a] 0.85 186.8 >130 h @ 5 mA Angew. Chem., Int. Ed.,\ncm-2 2020, 59, 7384.\nCo /Fe-N@CHC - 0.915 V 232.4 240 h @ 10 mA Adv. Mater. 2021, 33,\n2 cm-2 2104718\nFe,Mn/NC - - 160.8 81 h @ 5 mA Nat. Commun. 2021, 12,\ncm-2 1734.\nFe / @HNC - - 171.5 135 h @ 5 mA Adv. Mater. 2024,\nSA AC cm-2 2400523\nFeCo@CNTs 1.05 0.95 268.3 73 h @ 50 mA Appl. Catal. B. 2024, 341,\ncm-2 123346.\nFeCo-DACs/NC 0.984 V 0.877 V 175 240 h @ 10 mA Adv. Mater. 2022, 34,\ncm-2 2107421\nFe /d-CN - 0.95 V 144.0 20 h @ 1 mA Energy Environ. Sci.,\n1 cm-2 2021, 14, 6455.\nFeNS/Fe C@CNS 1.02 V 0.91 V 173.6 50 h @ 2 mA Angew. Chem. Inter. Ed.\n3 cm-2 2024, 63, e202313034\nFe-SAs/NPS-HC - 0.912 V 195.0 56 h @ 5 mA Nat. Commun. 2018, 9,\ncm-2 5422\nFe -N SO /NC - 0.91 V 282.4 900 Cycle J. Am. Chem. Soc., 2025,\n1 4 2 number @ 10 147, 6914.\nmA cm-2\nFe /Fe -NC 0.96 V 0.88 V 205.7 100 h @ 10 mA J. Energy Chem., 2025,\nSA AC cm-2 102, 413.\nD-Fe-N/C - 0.918 V 306.1 1200 h @ 20 Adv. Mater., 2025, 37,\nmA cm-2 2417711.\nP-FeN4@CNT - 0.904 V 236.3 > 400 h @ 5 Energy Stor. Mater., 2024,\nmA cm-2 69, 103421.\n[a] ‘-’ data is not provided in the original articles.\n5. The long-term cycling stability (>1000 h) of Zn–air batteries is presented as a key advantage. However, detailed post-mortem characterization of the catalyst after extended operation is missing. Evidence for atomic site retention and suppression of metal leaching or agglomeration would strengthen the durability claim.\nResponse: We are grateful to the reviewer for the insightful comments, which have been crucial in refining our assessment of the catalyst's stability. In the original manuscript, the zinc-air battery with the Zn Fe /f-NC air cathode exhibited a cycling 1 1+x lifespan of 1000 hours at a current density of 5 mA/cm2. At that time, we inadvertently discarded the air cathode post-cycling and neglected to characterize the air cathode catalyst samples after cycling. In the revised manuscript, we have re-tested the charge-discharge cycling performance of the zinc-air battery with the Zn Fe /f-NC air 1 1+x cathode. In Figure R9a, we achieved a cycling lifespan of 2200 hours at a current density of 10 mA/cm2, and further characterized the air cathode catalyst after the long life cycling with HAADF-STEM. As evident from the Figure R9b, the nano-fingerprint carbon structure remains clearly defined. The numerous atomically dispersed active sites are also clearly visible. However, due to the prolonged stability testing, a slight aggregation of some atomic sites, forming particles approximately 2 nm in size, can be observed. This is also the reason for the minor decline in stability and activity after extended cycling.\nFigure R9. a) Galvanostatic discharge/charge cycling performance of rechargeable ZAB based on Zn Fe /f-NC at a current density of 10 mA cm-2 (the yellow rectangular 1 1+x marks indicate the positions for replacing the zinc plate and electrolyte); and b) AC-HAADF-STEM image with bright and dark field of Zn Fe /f-NC after more than 2200 1 1+x h of cycling. (Figure R9 has been renumbered as Figure 5b and S58 in the revised manuscript)\nReviewer #3\nThe work reported by Li and co-authors presents a well-structured study on a curved-carbon-supported Fe/Zn single-atom cluster catalyst (Zn1Fe1+x/f-NC) for the oxygen reduction reaction (ORR) and its application in Zn-air batteries. The synthetic strategy in this work is creative, and the combination of decoupling experiments with DFT calculations reveals how Zn single atoms, Fe clusters, and curved carbon collectively optimize the electronic structure of Fe–N4 sites. The fabricated catalyst shows good activity (E1/2 = 0.93 V in 0.1 M KOH), durability (>1 000 h), and high peak power density (263.8 mW/cm2). However, several issues need to be addressed to meet the standard of Nat Comm.\nResponse: Thank you for your great efforts in reviewing our manuscript. We are very grateful for your interest and appreciation of this work as well as the confirmation of its creative. All the comments are very valuable for us to improve this work, which has been carefully addressed in the revised manuscript. We sincerely hope that the revised manuscript will satisfy your stringent criteria for publication.\n(1) Could the authors provide high-resolution XPS data and Mössbauer data for Fe to unequivocally distinguish Fe0, Fe2+, and Fe3+ contributions from clusters versus single atoms. This is critical for validating the proposed synergy.\nResponse: We thank the reviewer for the comment. In our initial draft, we performed high-resolution XPS analysis of Fe, but due to the low Fe loading the XPS spectral resolution was poor. Now we re-test the XPS analysis, increasing the measurement time and number of scans, and ultimately obtained a clearer XPS spectrum of Zn Fe /f-NC 1 1+x and Zn Fe /NC (Figure R1), Fe 2p could be deconvoluted into Fe2+ and Fe3+ peaks. 1 1+x The absence of a zero-valent peak indicates that the clusters are very small. The Fe2+ proportion in Zn Fe /f-NC is 68% (Table R1), higher than in Zn Fe /NC (57%), 1 1+x 1 1 indicating that the clusters are mainly contributed to Fe2+, whereas single-atom Fe sites exhibit both + 2 and + 3 valence states.\nFigure R1. High-resolution Fe 2p spectra of Zn Fe /f-NC and Zn Fe /NC. (Figure R1 1 1+x 1 1 has been renumbered as Figure S20 in the revised Supporting Information)\nTable R1. The Fe contents determined by XPS. (Table R1 has been renumbered as Table S1 in the revised Supporting Information)\nSamples Content Fe2+ (%) Content Fe3+ (%)\nZn Fe /f-NC 68 32\n1 1+x\nZn Fe /NC 57 43\n1 1\nFigure R2. Room-temperature Mössbauer spectrum of a) Zn Fe /f-NC and b) 1 1+x Zn Fe /NC (the absorption in the y-axis is normalized with the integrated area). (Figure 1 1 R2 has been renumbered as Figure 2a and 2b in the revised manuscript)\nMössbauer spectroscopy has been known as a useful technique to examine the chemical environment and spin state of Fe nuclei. The deconvolution of the transmission profiles reveals that D1, D2 and D3 doublets are the dominant Fe sites in both Zn Fe /f-NC 1 1+x and Zn Fe /NC. D1 has been observed in a large number of Fe-N-C catalysts and is 1 1 usually attributed to low-spin Fe(II)−N or high-spin Fe(III)−N moiety (with O 4 4 2 adsorbed), which have been identified as highly active species for ORR in contrast to the D2 sites (D2 sites represent medium-spin Fe(II)-N ). (Nat. Cat., 4, 615-622 (2021); 4 Nat. Mater., 14, 937-942 (2015)). On the contrary, the D1 sites exhibit less stable compared to D2 sites (Nat. Energy, 7, 652-663 (2022)). The percentages of D1 and D2 are 58.1% and 33.2% for Zn Fe /f-NC, respectively, and 60.4% and 19.9% for 1 1+x Zn Fe /NC, respectively. These findings indicate that compared to Zn Fe /NC, 1 1 1 1 Zn Fe /f-NC shows a better balance of activity and stability regulation. In particular, 1 1+x D3 (IS = −0.813, QS = 0.786, A = 8.7%) in Zn Fe /f-NC is a minor, high s-electron- 1 1+x density species interpreted as Fe-C coupled (metal-like/carbide-like or encapsulated Fe) because of its negative IS and relatively small QS (J. Am. Chem. Soc., 145, 15600 (2023)). Overall, the Mössbauer spectroscopy show that nano fingerprint carbon increases local s-electron density at a subset of Fe sites and changes local symmetry, and Zn Fe /f-NC possessed superior regulation of activity-stability trade-offs. 1 1+x\nTable R2. Mössbauer parameters derived from fittings. (Table R2 has been renumbered as Table S2 in the revised Supporting Information)\nSamples Content IS (mm s-1) QS (mm s-1) LW (mm s-1) Area (%)\nD1 0.381 0.726 0.387 58.1\nZn Fe /f-NC D2 0.033 3.565 0.277 33.2\n1 1+x D3 -0.813 0.786 0.209 8.7\nD1 0.340 0.831 0.662 60.4\nZn Fe /NC D2 0.143 2.829 0.472 19.9\n1 1 D3 0.944 3.246 0.655 19.7\n(2) Include aberration-corrected HAADF-STEM images after acid leaching and long-term cycling to explicitly demonstrate that Fe clusters are removed and single atoms remain intact; current post-mor tem images are not conclusive.\nResponse: We thank the reviewer for the comment. Taking the reviewers' suggestions into consideration, we adopted magnetic hysteresis loops of catalysts at room temperature measurements, which were performed to examine the aggregation state of Fe in all the decoupled samples. Firstly, we need to clarify that: the main catalyst in our manuscript is Zn Fe /f-NC, the contrast samples are Zn Fe /NC, Zn Fe /NC, 1 1+x 1 1 1 1+x Zn Fe /f-NC, and the “M ”, “M ”, and “f” represent “single-atom”, “cluster”, and 1 1 1 x “nano-fingerprint carbons”, respectively. As shown in Figure R3a and b, Zn Fe /NC 1 1+x exhibits a clear S-shaped hysteresis and non-zero remanent, indicating the presence of magnetic Fe clusters. Zn Fe /f-NC shows a similar, but much weaker non-linear 1 1+x response, consistent with a smaller amount or smaller-size clusters. By contrast, Zn Fe /f-NC (Zn Fe /f-NC with strong acid washing) and Zn Fe /NC display near- 1 1 1 1+x 1 1 linear M-H behaviour with negligible coercivity and remanence, which is consistent with predominantly atomically dispersed metal sites.\nFigure R3. Magnetic hysteresis loops of catalysts at room temperature a) Wide-field loops measured to ±20k oe. b) Low-field region (±500 oe). (Figure R3 has been renumbered as Figure S36 in the revised Supporting Information)\nFollowing the reviewer's recommendation, we have additionally supplied HAADF-STEM images of the corresponding samples. Figure R4 is the HAADF-STEM image of Zn Fe /f-NC. This result indicates that the Fe clusters are indeed removed 1 1 after washing with sulfuric acid, while the curled and fingerprint-like carbon layer is retained.\nFigure R4. HAADF-STEM image of Zn Fe /f-NC. (Figure R4 has been renumbered 1 1 as Figure S35 in the revised Supporting Information)\nIn the original manuscript, the zinc-air battery with the Zn Fe /f-NC air cathode 1 1+x exhibited a cycling lifespan of 1000 hours at a current density of 5 mA/cm2. At that time, we inadvertently discarded the air cathode post-cycling and neglected to characterize the air cathode catalyst samples after cycling. In the revised manuscript, we have re-tested the charge-discharge cycling performance of the zinc-air battery with the Zn Fe /f-NC air 1 1+x cathode. In Figure R5a, we achieved a cycling lifespan of 2200 hours at a current density of 10 mA/cm2, and further characterized the air cathode catalyst after the long life cycling with HAADF-STEM. As evident from the Figure R5b, the nano-fingerprint carbon structure remains clearly defined. The numerous atomically dispersed active sites are also clearly visible. However, due to the prolonged stability testing, a slight aggregation of some atomic sites, forming particles approximately 2 nm in size, can be observed. This is also the reason for the minor decline in stability and activity after extended cycling.\nFigure R5. a) Galvanostatic discharge/charge cycling performance of rechargeable ZAB based on Zn Fe /f-NC at a current density of 10 mA cm-2 (the yellow rectangular 1 1+x marks indicate the positions for replacing the zinc plate and electrolyte); and b) AC-HAADF-STEM image with bright and dark field of Zn Fe /f-NC after more than 2200 1 1+x h of cycling. (Figure R5 has been renumbered as Figure 5b and S58 in the revised manuscript)\nHAADF-STEM analysis for Zn Fe /f-NC (Figure R6a) and Zn Fe /f-NC (Figure 1 1+x 1 1 R6b) after the i-t (50 h) durability tests shows that within regions protected by fingerprint carbon layer, single-atom sites maintain better dispersion, the clusters do not have obvious aggregation. The Zn Fe /NC (Figure R6c) show agglomeration after 1 1 stability testing. This indicates that the fingerprint carbon layer can prevent the large-scale aggregation of Zn/Fe active sites surrounded Fe clusters.\nFigure R6. HAADF-STEM image after the i-t durability test of a) Zn Fe /f-NC; b) 1 1+x Zn Fe /NC and c) Zn Fe /f-NC. (Figure R6 has been renumbered as Figure S38 in the 1 1 1 1 revised Supporting Information)\n(3) What is Faradaic efficiency of H2O2 generation? Quantify any Fenton-type OH radical formation during cycling. Trace Fe leaching could undermine the stability in real devices.\nResponse: We thank the reviewer for the valuable suggestions. In our repeated RRDE measurements, the electron-transfer number for Zn Fe /f-NC is approximately 4 1 1+x (Figure R7), essentially the same four-electron pathway as Pt/C. Furthermore, the H O 2 2 selectivity is essentially zero at high potentials and remains very low at low potentials.\nFigure R7. H O selectivity derived from RRDE date. (Figure R7 has been renumbered 2 2 as Figure S29 in the revised Supporting Information)\nTo determine the actual Faradaic efficiency (FE) of Zn Fe /f-NC, 1 1+x electrochemical ORR tests were performed in a flow cell (Figure R8). with a three-electrode system controlled by a multichannel electrochemical workstation (Bio-Logic VSP-3e), in which Ag/AgCl (filled with saturation KCl solution) served as the reference electrode and Pt sheet served as the counter electrode. An anion exchange membrane (Fumasep, FAA-3-PK-130) was adopted to separate the anolyte and catholyte chambers. The electrolytes were circulated into the anode and cathode chambers at a constant flow rate of 10 mL min-1 using a peristaltic pump. High-purity O (99.999%) gas was 2 continuously purged into the gas chamber behind the GDL (YLS-30T, yielding an effective catalytic area of 1×1 cm-2 and mass loading of 0.5 mg cm-2) with a feeding rate of 100 mL min-1 have been maintained under all test conditions. The O products 2 were collected during chronopotentiometry tests, which were performed at each potentials for 30 min. The FE was determined by spectrophotometry. Specifically, 30 μL of the catholyte was added to 3 mL of Ce(SO ) solution and mixed thoroughly. The 4 2 H O then reacted with Ce4+, reducing it to colorless Ce3+. 2 2\n2Ce4+ +H O → 2Ce3+ +2H+ +O 2 2 2\nFigure R8. The photograph of the Flow cell device. (Figure R8 has been renumbered as Figure S30 in the revised Supporting Information)\nThe Ce4+ concentration before and after the reaction was measured using a UV–visible spectrometer. As shown in Figure R9a, the full-wavelength UV–visible spectrum of the Ce(SO) standard solution indicates that the absorbance gradually 4 increases with increasing Ce4+ concentration and reaches a peak at approximately 318 nm. A linear fit between Ce4+ concentration and absorbance intensity was used to obtain a standard curve (inset of Figure R9a). The Ce4+ concentrations before and after the reaction were then used to determine the H O content in the electrolyte, and the 2 2 Faradaic efficiency of Zn Fe /f-NC at applied voltages from 0.2 V to 0.9 V was 1 1+x calculated using the following formula:\nnFC V∗10−3\nFE(%) = H2O2 ×100%\nQ\nWhere n is the electron-transfer number (2), F is the Faraday constant (96485 C mol-1), C is the molar concentration of hydrogen peroxide (mol/L), V is the volume of the H2O2 electrolyte in the flow cell (mL), and Q is the total charge passed during the i-t process (C). Figure R9b shows the flow-cell test results. At higher potentials (0.4 - 0.9 V), no H O production was detected (FE = 0). At 0.3 V the FE was 1.0%, and at 0.2 V it was 2 2 1.7%. These results indicate that Zn Fe /f-NC does not produce H O at potentials 1 1+x 2 2 above 0.4 V and generates only trace amounts at 0.3 V and below. Therefore, under typical device operating potentials (which are generally higher in the zinc-air battery), the Fenton reaction which causes iron dissolution and device destabilization is unlikely to occur.\nFigure R9. a) The UV-Vis spectra of Ce4+ solutions with different concentrations. inset: The linear calibration relationship between the Ce4+ concentration and corresponding maximum absorbance intensity at around 318 nm. The Ce3+ concentration was pre-calibrated by UV-Vis and the test results were calculated from the average of three independent measurements. b) H O FE and productivity for Zn Fe /f-NC obtained 2 2 1 1+x under different operational voltage (0.2 V-0.9 V). (Figure R9 has been renumbered as Figure S31 in the revised Supporting Information)\nFigure R10. The EPR monitoring of catalytic process (from 0.2 to 0.9 V vs. RHE). (Figure R10 has been renumbered as Figure S32 in the revised Supporting Information)\nWe used electron paramagnetic resonance (EPR) spectroscopy to real-time monitor the in situ-generated OH* during ORR at the gas diffusion electrode level for Zn Fe /f-NC, incorporating 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the OH* 1 1+x scavenger. As shown in Figure R10, compared with the curve without applied potential, the test results at 0.4 - 0.9 V are approximately linear, indicating that the OH* radical is not detected. Weak free-radical signals appear at 0.3 V and 0.2 V, but the OH* signal curve cannot be reliably fitted, indicating that the amount of OH* generated is extremely small.\nPrevious reports indicate that this curled shell-like carbon layer functions to protect Fe sites and suppress the Fenton reaction. In addition, Zn Fe /f-NC exhibits a 1 1+x stronger 4e- ORR pathway than Zn Fe /NC, which further reduces the OH* 1 1 concentration under the same conditions (Nature, 644, 668 (2025)).\n(4) I suggest authors further considering which data shown in manuscript and which data shown in SI to improve readability.\nResponse: We thank the reviewers for their thoughtful suggestions. We have adopted them and reorganized the manuscript: secondary control data, full original spectra, detailed tables, and experimental/characterization protocols have been moved to the Supplementary Information (SI), while only key figures and tables, main results, mechanistic discussion, and conclusions remain in the main text. Clear headings and cross-references are used in the revised manuscript to clarify the distribution of content between the main text and the SI and to improve readability and logical coherence.\n(5) I suggest authors adding a brief scalability discussion, can the protocol be scaled beyond the current 100 mg batch size?\nResponse: We thank the reviewer for the constructive suggestion, which are highly important for the completeness of our manuscript. We have scaled up the Zn Fe /f- 1 1+x NC catalyst synthesis reaction proportionally to investigate the scalability of the catalyst synthesis and the reliability of its performance. As shown in the Figure R11a below, we are able to obtain gram-scale catalyst yields in a single batch. Simultaneously, we tested the ORR performance of this batch of samples and found that its performance remains reliable and reproducible (Figure R11b).\nFigure R11. LSV curves of Zn Fe /f-NC (scale up) in 0.1 M KOH (inset: picture of 1 1+x Zn Fe /f-NC catalysts). (Figure R11 has been renumbered as Figure 3h in the revised 1 1+x manuscript)\nReviewer #4\nA composite catalyst containing Fe clusters encapsulated within curved carbon layer sand surrounded by densely dispersed Fe/Zn-N4 sites was obtained. A series of decoupled control samples, including those with dual single-atom Fe and Znanchored on curved carbon and on planar carbon were also prepared. Decoupling experimental revealed that Fex clusters promoted. Single atom Zn, Fe clusters and fingerprint carbon layer jointly enhanced the catalytic performance of Fe-N4 in ORR through the multivariate regulation of electronic structure. The manuscript was well organized and written. I recommend its publication in Nature Communications after the following revision:\nResponse: We are very grateful to the reviewer’s positive feedback. In the revised manuscript, we have carefully revised the content to enhance the overall integrity of this work based on your constructive suggestions. Below, we provide a point-to-point response to your concerns.\n1. The manuscript proposes that the curved carbon layers regulate the electronic structure of Fe–N sites via strain effects, yet the supporting evidence mainly relies on 4 electrochemical comparisons and limited DFT calculations. There is a lack of direct characterization (e.g., Raman stress analysis, local valence band analysis) to substantiate the claim of “curvature-induced charge redistribution.”\nResponse: We thank the reviewer for the comment. In our initial draft, we performed high-resolution XPS analysis of Fe, but due to the low Fe loading the XPS spectral resolution was poor. Now we reran the XPS analysis, increasing the measurement time and number of scans, and ultimately obtained a clearer XPS spectrum of Zn Fe /f-NC 1 1+x and Zn Fe /NC (Figure R1), Fe 2p splits into Fe2+ and Fe3+ peaks. The fitting results 1 1 indicate that the Fe2+ proportion in Zn Fe /f-NC reaches 68 %, higher than in 1 1+x Zn Fe /NC (57%), indicating that the Fe clusters mainly contributed to +2 state. 1 1\nFigure R1. High-resolution Fe 2p spectra of Zn Fe /f-NC and Zn Fe /NC. (Figure R1 1 1+x 1 1 has been renumbered as Figure S20 in the revised Supporting information)\nTable R1. The Fe contents determined by XPS. Table R1 has been renumbered as Table S1 in the revised Supporting information)\nSamples Content Fe2+ (%) Content Fe3+ (%)\nZn Fe /f-NC 68 32\n1 1+x\nZn Fe /NC 57 43\n1 1\nMössbauer spectroscopy has been known as a useful technique to examine the chemical environment and spin state of Fe nuclei. The deconvolution of the transmission profiles reveals that D1, D2 and D3 doublets are the dominant Fe sites in both Zn Fe /f-NC and Zn Fe /NC. D1 has been observed in a large number of Fe-N- 1 1+x 1 1 C catalysts and is usually attributed to low-spin Fe(II)−N or high-spin Fe(III)−N 4 4 moiety (with O adsorbed), which have been identified as highly active species for ORR 2 in contrast to the D2 sites (D2 sites represent medium-spin Fe(II)-N ). (Nat. Cat., 4, 4 615-622 (2021); Nat. Mater., 14, 937-942 (2015)). On the contrary, the D1 sites exhibit less stable compared to D2 sites (Nat. Energy, 7, 652-663 (2022)). The percentages of D1 and D2 are 58.1% and 33.2% for Zn Fe /f-NC, respectively, and 60.4% and 19.9% 1 1+x for Zn Fe /NC, respectively. These findings indicate that compared to Zn Fe /NC, 1 1 1 1 Zn Fe /f-NC shows a better balance of activity and stability regulation. In particular, 1 1+x D3 (IS = −0.813, QS = 0.786, A = 8.7%) in Zn Fe /f-NC is a minor, high s-electron- 1 1+x density species interpreted as Fe-C coupled (metal-like/carbide-like or encapsulated Fe) because of its negative IS and relatively small QS (J. Am. Chem. Soc., 145, 15600 (2023)). Overall, the Mössbauer spectroscopy show that nano fingerprint carbon increases local s-electron density at a subset of Fe sites and changes local symmetry, and Zn Fe /f-NC possessed superior regulation of activity-stability trade-offs. 1 1+x\nFigure R2. Room-temperature Mössbauer spectrum of a) Zn Fe /f-NC and b) 1 1+x Zn Fe /NC (the absorption in the y-axis is normalized with the integrated area). (Figure 1 1 R2 has been renumbered as Figure 2a and 2b in the revised manuscript)\nTable R2. Mössbauer parameters derived from fittings. (Table R2 has been renumbered as Table S2 in the revised Supporting Information)\nSamples Content IS (mm s-1) QS (mm s-1) LW (mm s-1) Area (%)\nD1 0.381 0.726 0.387 58.1\nZn Fe /f-NC D2 0.033 3.565 0.277 33.2\n1 1+x D3 -0.813 0.786 0.209 8.7\nD1 0.340 0.831 0.662 60.4\nZn Fe /NC D2 0.143 2.829 0.472 19.9\n1 1 D3 0.944 3.246 0.655 19.7\n2. The authors show that removing Fe clusters decreases E1/2 by 26 mV; however, this difference may also stem from changes in carbon defects or pore structure. Moreover, the manuscript lacks quantitative statistics of the spatial relationship between clusters and single-atom sites (e.g., average cluster single atom spacing).\nResponse: We thank the reviewer for the valuable comment. To investigate whether the decrease in E is due to carbon defects or pore structure, we performed BET and 1/2 Raman measurements on the acid-washed Zn Fe /f-NC. The results, shown in Figure 1 1 R3a, indicate that the BET surface area of the Zn Fe /f-NC is 976.5 m2 g-1, comparable 1 1 to the surface areas of 958.7 m2 g-1 for Zn Fe /f-NC, indicating that acid washing does 1 1+x not significantly affect the pore structure. Figure R3b shows the Raman results for Zn Fe /f-NC, with an I /I value of 1.05 compared with 1.04 for Zn Fe /f-NC. This 1 1 D G 1 1+x indicates a slight decrease in the degree of graphitization, presumably due to the effect of acid washing on the carbon substrate. However, the degree of graphitization does not change significantly, suggesting that the decrease in half-wave potential is not due to major changes in carbon defects or pore structure.\nFigure R3. Zn Fe /f-NC of a) N adsorption/desorption isotherms and b) Raman spectra. 1 1 2 (Figure R3 has been renumbered as Figure S37 in the revised Supporting information)\nNumerous studies have examined the effect of interatomic separation on synergistic interactions between metal sites (J. Am. Chem. Soc., 146, 29084 (2024); Nat. Catal., 4, 615 (2021); Nat. Commun., 14, 5108 (2023)). Strongest synergy is typically observed at nearest-neighbor distances of 0.3 < 1.0 nm (3-10 Å) (ACS nano, 17, 8622 (2023); Nat. Commun., 13, 2963 (2022); Nat. Commun., 14, 6936 (2023)), When two Fe centers are separated by less than 0.3 nm (< 3.0 Å), they effectively fall into the metal–metal–bonded/cluster regime; most studies of single-atom–cluster coupling do not treat separations < 3.0 Å as distances between independent sites (Nati. Sci. Rev., 11, nwad327 (2024); Nat. Commun., 13, 2963 (2022)). Where electronic coupling is significant. By contrast, when separations exceed roughly 1.0-2.0 nm, electronic (tunneling) coupling decays rapidly and site to site interactions become negligible. To quantify this for our samples, we extracted per-atom positions from AC-HAADF-STEM images of Zn Fe /f-NC (Figure R4) and calculated the average nearest- 1 1+x neighbor Fe/Zn distance to clusters. Within the sub-nanometer spacing range of 0.3-1.0 nm, we identified Fe/Zn single atoms and calculated the average distance distribution from single-atom to cluster.\nFigure R4. Analysis and statistics were performed on the Zn/Fe single atoms in the sub-nanometer range surrounding the Fe clusters of Zn Fe /f-NC. (Figure R4 has been 1 1+x renumbered as Figure S15 in the revised Supporting information)\n3. The manuscript reports an E1/2 of 0.93 V (alkaline), claiming superiority over most reported catalysts; however, the supporting comparison table (Table S3) lacks details such as catalyst loading, electrode configuration, and testing parameters (rotation speed, O concentration). In acidic media, E1/2 is only 0.81 V, which is significantly lower 2 than Pt/C, but the manuscript does not discuss the mechanistic origin of this gap.\nResponse: We thank the reviewer for the comment. The details you mentioned have been provided in Tables S6 (Original Table S3) and S7 of the Supporting Information.\nTable R3. Comparison of ORR performance between catalysts in this work and other previously reported non-precious metal electrocatalysts in 0.1 M KOH, all tests were performed using a three-electrode system at 1600 rpm in an O -saturated electrolyte. (Table R3 has been renumbered as 2 Table S6 in the revised Supporting Information)\nCatalyst E E Loading Reference\nonest 1/2\n(V vs. RHE) (V vs. RHE) (mg cm-2)\nZn Fe /f-NC 1.05 0.93 0.51 This work\n1 1+x\nPt/C 1.00 0.87 0.51 This work\nZn Fe /NC 1.01 0.89 0.51 This work\n1 1\nZn /NC 0.92 0.79 0.51 This work\n1 1+x\nZn Fe /NC-800 oC 0.96 0.81 0.51 This work\n1 1+x\nFeNS/Fe C@CNS 1.02 0.91 - Angew. Chem. 2024, 63, e202313034.\nMn-SAS 0.99 0.85 - Angew. Chem. 2023, 62, e202314933.\nFe/Ni-N /OC 1.03 0.938 0.285 Adv. Mater. 2020, 32, 2004670.\nx\nFeCo@CNTs 1.05 0.95 1.4 Appl. Catal. B. 2024, 341, 123346.\nCu-S N /Cu 1.04 0.90 - Adv. Funct. Mater. 2023, 2214425.\n1 3 x\nFe-N-C/Fe C-op -[a] 0.911 - Adv. Sci. 2023, 2301656\nSA\nSA-Fe-HPC - 0.89 0.1 Angew. Chem., Int. Ed., 2018, 57, 9038.\nFe/Meso-NC-1000 0.885 0.34 Adv. Mater., 2022, 34, 2107291.\nFe-Zn@SNC 0.99 0.86 0.51 Angew. Chem., Int. Ed., 2023, 62,\ne202301833.\nP/Fe−N−C - 0.90 0.6 J. Am. Chem. Soc., 2023, 145, 3647.\nMn-SAS 0.99 0.85 - Angew. Chem., Int. Ed., 2023, 62,\ne202314933.\nFeN OS 1.01 0.874 0.4 Angew. Chem., Int. Ed., 2021, 60, 25296.\nFe SAs-N/C-20 - 0.909 0.408 or J. Am. Chem. Soc., 2018, 140, 11594.\n0.612\nFe -N SO /NC - 0.91 - J. Am. Chem. Soc., 2025, 147, 6914-6924.\n1 4 2\n[a] ‘-’ data is not provided in the original articles.\nTable R4. Comparison of ORR performance between Zn Fe /f-NC and other previously reported 1 1+x non-precious metal electrocatalysts in 0.1 M HClO or 0.5 M H SO , all tests were performed using 4 2 4 a three-electrode system at 1600 rpm in an O -saturated electrolyte. (Table R4 has been renumbered 2 as Table S7 in the revised Supporting Information)\nCatalyst E E Loading Reference\nonest 1/2\n(V vs. RHE) (V vs. RHE) (mg cm-2)\nZn Fe /f-NC 0.91 0.81 0.51 This work\n1 1+x\nPt/C 0.99 0.86 0.51 This work\nZn Fe /NC 0.82 0.7 0.51 This work\n1 1\nZn /NC 0.63 0.46 0.51 This work\nSA/AC\nSpa-S-Fe,Co/NC 0.98 0.846 0.6 Sci. Adv. 2023, 9, eadg0366.\nFe -N-C - 0.80 0.28 Nat. Commun., 2020, 11, 2831.\nSA\nSA-Fe-HPC - 0.81 0.2 Angew. Chem., Int. Ed., 2018, 57, 9038.\n(CM+PANI)-Fe-C - 0.80 0.6 Science 2017, 357, 479-484.\nFe-SNC - 0.77 0.6 Angew. Chem. Int. Ed. 2017, 56, 13800-13804.\np-Fe-N-CNFs 0.85 0.74 0.6 Energy Environ. Sci. 2018, 11, 2208-2215.\nFe-N-C-950 0.92 0.78 0.4 ACS Catal. 2018, 8, 2824-2832.\nSA-Fe-N NS - 0.81 0.6 Adv. Energy Mater. 2018, 8, 1801226.\nFe-C-N950 - 0.80 0.8 J. Am. Chem. Soc. 2020, 142, 1417-1423.\nFe-N-C-SP - 0.82 0.6 ACS Catal. 2022, 12, 6409-6417.\nHP FeN 0.95 0.80 0.6 Energy Environ. Sci. 2020, 13, 111-118.\n(1) On the Pt surface, the O–O bond is directly cleaved via an efficient dissociation mechanism with terminal adsorption, enabling a complete 4-electron oxygen reduction reaction (ORR) pathway to H O (Chem. Rev., 123, 9265-9326 (2023)). 2\n(2) The Fe-N 4 sites of Zn 1 Fe 1+x /f-NC are embedded in a coiled graphitized carbon layer. Our experiments indicate that this carbon layer protects the Fe-N sites and regulates 4 OH* adsorption. Nevertheless, the reaction in acidic medium is limited by the diffusion of H O+ from the bulk electrolyte to the active sites, as Fe-N itself lacks efficient proton 3 4 capture and transfer capabilities.\nIt is important to emphasize that the acidic ORR activity of Zn Fe /f-NC (E = 1 1+x 1/2 0.81 V) is in fact quite satisfactory A half-wave potential greater than 0.80 V is widely recognized as highly competitive for Fe-N catalysts, signifying a high level of intrinsic 4 activity (Nat. Commun., 13, 2963 (2022)). The remaining performance gap with Pt/C originates from the mechanistic limitations discussed previously (e.g., optimization of proton transfer and coordination structure of active centers in acidic environments). In response to the comments, we have correspondingly expanded our discussion in the manuscript to clarify the origin of this difference.\n4. Although the manuscript demonstrates over 1000 h cycling stability, only macroscopic current and voltage gap data are provided. There is no microscopic structural comparison before and after cycling, making it unclear how curved carbon and Fe clusters evolve during long-term operation.\nResponse: We are grateful to the reviewer for the insightful comments, which have been crucial in refining our assessment of the catalyst's stability. In the original manuscript, the zinc-air battery with the Zn Fe /f-NC air cathode exhibited a cycling 1 1+x lifespan of 1000 hours at a current density of 5 mA/cm2. At that time, we inadvertently discarded the air cathode post-cycling and neglected to characterize the air cathode catalyst samples after cycling. In the revised manuscript, we have re-tested the charge-discharge cycling performance of the zinc-air battery with the Zn Fe /f-NC air 1 1+x cathode. In Figure R5a, we achieved a cycling lifespan of 2200 hours at a current density of 10 mA/cm2, and further characterized the air cathode catalyst after the long life cycling with HAADF-STEM. As evident from the Figure R5b, the nano-fingerprint carbon structure remains clearly defined. The numerous atomically dispersed active sites are also clearly visible. However, due to the prolonged stability testing, a slight aggregation of some atomic sites, forming particles approximately 2 nm in size, can be observed. This is also the reason for the minor decline in stability and activity after extended cycling.\nFigure R5. a) Galvanostatic discharge/charge cycling performance of rechargeable ZAB based on Zn Fe /f-NC at a current density of 10 mA cm-2 (the yellow rectangular 1 1+x marks indicate the positions for replacing the zinc plate and electrolyte); and b) AC-HAADF-STEM image with bright and dark field of Zn Fe /f-NC after more than 2200 1 1+x h of cycling. (Figure R5 has been renumbered as Figure 5b and S58 in the revised manuscript)\n5. In DFT simulations, Fe /f-NC is modeled using a C₆₀ molecule, while other 1 models use graphene slabs, which may influence electronic state comparability. The choice of exchange-correlation functional (e.g., GGA/PBE) and its suitability for capturing strain effects on d-orbital regulation is not discussed.\nResponse: Sincerely thanks the reviewer for the insightful suggestions. In the manuscript, we selected C as a calculation model of bent carbon. It seems that the 60 curvature varies too drastic from flat carbon (graphene model) to the bent carbon (C 60 model) to investigate the general function of carrier materials. In the revised version, we added two models (single wall carbon nanotube with different curvatures) among them to reveal the general role of carrier materials from flat to bent. Herein, (6,6)-CNT and (10,10)-CNT were selected, while similar construction proposal was adopted as in graphene model to embed FeN moiety on them. These two models were named as 4 Fe /(6,6)-CNT and Fe /(10,10)-CNT, which contain 143 and 239 atoms in them, 1 1 respectively. Generally speaking, the curvature of C is similar as it of (5,5)-CNT (J. 60 Phys. Chem. B, 112, 15442 (2008); J. Phys. Chem. C 115, 3220 (2011)). Hence, the curvature increases not too drastically as graphene < (10,10)-CNT < (6,6)-CNT < (5,5)-CNT ≈ C , as shown in Figure R6a. 60\nFigure R6. (a) Side view for C60 and three armchair single-wall carbon nanotube with different diameters. (b) ORR free energy diagrams of Fe /NC, Fe /f-NC, Fe /(6,6)-CNT 1 1 1 and Fe /(10,10)-CNT at U=1.23 V. (Figure R6 has been renumbered as Figure 4a and 1 S50 in the revised manuscript)\nBased on their optimized structures together with the adsorption intermediates on them, we calculated their free energy profiles alone ORR reaction path. The free energy profiles of ORR were shown in Figure R6b. At a potential 1.23 V, the highest upslope free energy of the fourth step (OH*+ H+ + e- →*+H O) is the rate-determining step 2 (rds) for all considered models, in which the rds height on Fe /NC is 1.40 eV. When 1 curvature effect was introducing, for small curvature model such as Fe /(10,10)-CNT, 1 the desorption energy of OH* decreased to 0.80 eV. For large curvature model such as Fe /(6,6)-CNT, the desorption energy of OH* further decreased to 0.66 eV. 1 Since rds height on Fe /(6,6)-CNT is close to its counterair in Fe /C , this 1 1 60 convergence implied that curvature effect modulation FeN /carbon materials has its 4 intrinsic upper limit.\nTo further validate the curvature effect for Fe -Zn /NC reaction center, we 1 1 constructed Fe -Zn /(10,10)-CNT model and also calculated its free energy profiles, as 1 1 shown in Figure R7. For Fe -Zn , its rds height decrease from 0.89 eV on Fe -Zn /NC 1 1 1 1 to 0.84 eV Fe -Zn /(10,10)-CNT. 1 1\nFigure R7. (a) Structures for Fe -Zn /NC and Fe -Zn /(10,10)-CNT. (b) ORR free 1 1 1 1 energy diagrams of Fe -Zn /NC and Fe -Zn /(10,10)-CNT at U=1.23 V. (Figure R7 has 1 1 1 1 been renumbered as Figure 4a and S51 in the revised manuscript)\nIn the revised version, Fe /C60 model was selected as a curvature extremity of 1 carbon materials could embed FeN reaction site. While, the ORR performance as well 4 as the electronic properties were compared between Fe /NC, Fe -Zn /NC and 1 1 1 Fe /(10,10)-CNT, Fe -Zn /(10,10)-CNT as shown in Figure R8. 1 1 1\nFigure R8. Projected density of states for Fe /NC, Fe -Zn /NC and Fe /(10,10)-CNT 1 1 1 1 as well asFe -Zn /(10,10)-CNT. (Figure R8 has been renumbered as Figure S52 and 1 1 S53 in the revised Supporting Information)\nTo address the reviewer's concerns regarding the computational methodology, we have implemented the following improvements.\nIn this work, the exchange-correlation functional is PBEsol, which was developed as a modification of the widely used PBE functional to improve lattice constants and surface energies. The combination of PBESOL and SSSP pseudopotential libraries (precision version) was selected as suggested by N. Marzari et al (Science, 351, aad3000 (2016)). In our previous work (ChemRxiv. 2025; doi:10.26434/chemrxiv-2025-s1xjc), we have determined that this combination is suitable for the electronic properties’ calculation for MN type substrates. To validate its robust in d-orbital description, we constructed 4 two types of FeN under strain effects: Fe /CNT-(6,6) and Fe -CNT(10,10). For their 4 1 1 optimized structures, we calculated self-consistent field (SCF), non-SCF (NSCF) and projected density of states (PDOS) to investigated the suitability for capturing strain effects on d-orbital regulation. During the computational procedure, the van der Waals effects were considered by using DFT-D3 scheme (except for BEEF-vdw), while DFT+U approach was employed to overcome its self-interaction error U = 3.29 eV, 3d for all considered functionals.\nIn the FeN /CNT system, the Fe atom is in a coordination environment with 4 distorted D symmetry. Different functionals may affect the electronic structure 4h through variations in exchange-correlation potentials, thereby altering the energy levels and splitting of d-orbitals. Generally, PBE and PW91 as typical generalized gradient approximation (GGA) functionals that tend to underestimate electron localization in strongly correlated systems, potentially leading to smaller d-orbital splitting. PBESOL as an improved version of PBE, more suitable for solids, likely providing more reasonable crystal field splitting. BEEF (Bayesian Error Estimation Functional) includes van der Waals corrections and a more flexible exchange-correlation form, potentially offering a more accurate description of metal-ligand interactions and thus more reliable d-orbital energy levels. In the manuscript, we adopted PBESOL+U approach to penalize electron delocalization. This is the minimum requirement and most common method for handling strongly correlated transition metal involved models. We know that Hybrid Functionals (such as HSE06) could mix a portion of exact Hartree-Fock exchange energy, significantly improving band gap and electronic structure predictions. Accuracy is high, but computational cost is extremely expensive, making it difficult to use for large systems or complex surfaces.\nFor the five considered functionals, under the premise of introducing dispersion correction and Hubbard U parameter, the order of splitting energy magnitude is similar, while d-orbital energy level order remain consistent across all functional. The test indicates that including Hubbard U could partially cancel the electron's Coulomb interaction with itself (self-interaction error) in strong correlation systems. Thus, PBESOL+U with DFT-D3 could achieve robust d-orbital distribution and reveal the curvature effect in this work.\nFigure R9. Projected density of states for Fe /(6,6)-CNT achieved by different 1 functionals. The side and top views for tested Fe /(6,6)-CNT model were also exhibited. 1 (Figure R9 has been renumbered as Figure S54 in the revised Supporting Information)\nFigure R10. Projected density of states for Fe /(10,10)-CNT achieved by different 1 functionals. The side and top views for tested Fe /(10,10)-CNT model were also 1 exhibited. (Figure R10 has been renumbered as Figure S55 in the revised Supporting Information)\n6. Several figures (e.g., Figures 3 and 5) have small fonts and low color contrast, making them difficult to read. Some explanations in the Supporting Information repeat content already in the main text. Linguistically, some sentences are long or awkward (e.g., “which indicating that the more Fe salt added to ZIF-8 has a greater impact on its crystallinity although the overall structure has not been damaged”).\nResponse: We sincerely thank you for these insightful and constructive comments. We have thoroughly revised our manuscript to address all the points raised. Please find our point-by-point responses below and the corresponding revisions in the marked-up manuscript.\n(1) Several figures (e.g., Figures 3 and 5) have small fonts and low color contrast, making them difficult to read. We agree with this comment and apologize for the oversight. We have now regenerated all relevant figures (including Figures 3, 5, and others throughout the manuscript) with significantly larger font sizes and optimized color schemes to ensure high contrast and clarity, both for on-screen viewing and in print.\n(2) Some explanations in the Supporting Information repeat content already in the main text. We thank the reviewer for highlighting this. We have carefully reviewed the Supporting Information and streamlined the content by removing all redundant explanatory text that was already present in the main manuscript. The Supporting Information has been revised to focus solely on providing supplementary data and experimental details that support, rather than repeat, the main conclusions.\n(3) Linguistically, some sentences are long or awkward. We thank you for your valuable feedback. To ensure the linguistic quality and clarity of the manuscript, the text has been thoroughly revised in multiple sections. Specifically, the example sentence and other similar lengthy or awkward constructions have been rewritten for conciseness and fluency.\nSimilar comprehensive language polishing has been applied throughout the manuscript. We believe these revisions have significantly improved the clarity and overall quality of our work. Thank you again for your time and valuable suggestions.\n7. Some related references are recommended to be considered for citing:\nNatl. Sci. Rev., 2022, 9, nwab197.\nAdv. Mater. 2024, 36, 2408317.\nAdv. Mater. 2024. 36 (32), 2406094.\nAdv. Mater. 2023. 35 (48), 2307003.\nResponse: We thank the reviewer for this suggestion. We have carefully reviewed the recommended references and found them highly relevant and instructive. Accordingly, we have now cited these important works in the revised manuscript to better situate our study within the existing literature. (see references 42-45 in the revised manuscript)\nPoint-by-point response to the reviewers’ comments\nWe sincerely thank the reviewers for their invaluable time and meticulous efforts. We also greatly appreciate the reviewers’ positive assessment of our revised manuscript, which serves as a significant encouragement for our work. Given the extensive revisions made in our previous submission, we recognize that some oversights or areas for improvement may have remained. The reviewer's high-value suggestions, diligently provided, have been instrumental in enhancing the completeness and scientific rigor of our manuscript. We have carefully considered each of the reviewer's comments and have provided detailed responses and supplementary experimental data accordingly. Here in this response letter, the reviewers’ comments are laid out below and their specific concerns are numbered in sequence. Revised portion are marked in yellow in the revised manuscript and revised supporting information. Below, the original comments are in black, and our point-to-point responses are in blue.\nReviewer #1 (Remarks to the Author):\nThe author has effectively supplemented and refined the data from previous proposals, significantly enhancing the rigor and completeness of the research. This study demonstrates innovation and clear functional delineation in the integrated design of nano-fingerprint carbon layers, single-atom sites, and cluster structures, with outstanding performance data and a well-defined research logic. Consequently, this paper holds high publication value. However, certain details could be further optimized to better support the conclusions.\nResponse: We thank the reviewer for the thorough and encouraging evaluation. We appreciate the recognition that the revised data and analyses substantially strengthen the manuscript, and that the integrated design of nano-fingerprint carbon layers, single-atom sites, and cluster structures demonstrates clear innovation, functional delineation, and exceptional performance. Given the extensive revisions in our previous manuscript, we recognize that some oversights or areas for improvement may have inevitably remained. We are sincerely grateful for the reviewer’s valuable time and insightful suggestions, which have been crucial in further enhancing the completeness and scientific rigor of our work. Below, we will respond to each of your valuable suggestions point-by-point and provide supplementary experimental data in the revised manuscript or supporting information accordingly.\n1. Density functional theory calculations provide robust theoretical support for the experimental results. Authors are advised to further detail computational parameters and boundary conditions to enhance transparency.\nResponse: Thank you for your positive comments on the theoretical support provided by our DFT calculations and for your constructive suggestion. We agree that detailing the computational parameters enhances the transparency and reproducibility of our work.\nIn response to your suggestion, we have now added a dedicated subsection (e.g., \"Computational Details\") in the revised manuscript. This subsection comprehensively lists all key parameters, including:\n The periodic boundary condition for C model\n60\n The periodic boundary condition for (6,6)-CNT model\n The periodic boundary condition for (10,10)-CNT model\n Monkhorst–Pack k-point grids for the models during optimization cacluation, self-consistent field (SCF) and non- self-consistent field (NSCF) calculations\nWe believe these additions significantly improve the clarity of our methodology, especially the models selected. The corresponding revisions are marked in the manuscript. Thank you again for your valuable feedback.\n2. The discussion on electronic structure regulation and OH* adsorption energy optimization is a major highlight of the paper. Consider expanding this section to provide deeper mechanistic insights.\nRespond: Thank you for your positive evaluation of our work and for highlighting the discussion on electronic structure regulation and OH* adsorption energy optimization as a major strength. We agree that providing deeper mechanistic insights will significantly enhance the impact of the paper.\nIn the revised manuscript, we have substantially expanded this section. Our deeper analysis now includes:\n1. Crystal orbital Hamilton population (COHP) calculations were carried out to access the bonding strength between Fe-sites in Fe /NC, Fe /(10,10)-CNT and Fe /(6,6)-CNT 1 1 1 substrates and O-site in OH adsorbates. Integrated (-iCOHP) from the occupied states till Fermi level quantifies the total bonding contribution. A larger integrated value corresponds to a stronger overall bonding interaction, which is directly correlated with Fe-O bonding strength. (Figure R1)\nFigure R1. Correlation between -iCOHP for Fe-O bonds and *OH desorption energies in Fe /NC, Fe /(10,10)-CNT and Fe /(6,6)-CNT. (Figure R1 has been renumbered as 1 1 1 Figure S54 in the revised Supporting Information)\n2. We also try to analysis the correlation between *OH desorption energy and polarization parameters of Fe-site in in Fe /NC, Fe /(10,10)-CNT and Fe /(6,6)-CNT to 1 1 1 get a deeper insight for *OH desorption energy under curvature effect. Interestingly, a strong negative correlation emerges between the Fe-site total polarization (total spin-up electron minus total spin-down electron) and OH* desorption energy (with a Pearson correlation coefficient of -0.988) (Figure R2a). As substrate curvature increases, both polarization and adsorption energy decrease synchronously. This suggests that curvature-induced strain reduces the spin polarization of Fe centers, promoting d-electron delocalization and thereby weakening Fe–OH bonding. However, this trend is not observed for the d-polarization parameters (total spin-up d orbital electron minus total spin-down d orbital electron) (Figure R2b and R2c), indicating that the polarization changes also involve non-d electrons (s and p orbitals). It should be noted that the statistical significance of this correlation is limited by the small sample size, but the directional consistency supports the mechanistic interpretation that reduced spin polarization facilitates OH* desorption in curved Fe-N catalysts.\nWe believe these additions offer the deeper mechanistic insights you suggested and strengthen the core narrative of the paper. The revised text can be found in the Discussion section.\n3. The extended discussion on iron precursor concentration gradient data is lacking: The authors determined 1.5 mmol as the optimal condition through experiments with a 0.5-2.0 mmol Fe(acac)3 concentration gradient, observing Fe species agglomeration into particles at 2.0 mmol. It is recommended to supplement the ORR kinetic parameters (e.g., Tafel slope) of the catalyst at different concentrations, analyzing the correlation between Fe2+ proportion changes and catalytic kinetics. Concurrently, combine this with the intensity variation of the zero-valent Fe peak in XPS to quantify the influence of Fe species agglomeration on activity.\nRespond: We thank the reviewer for the valuable suggestions. We supplemented the study with electrochemical data (Tafel slope analysis, kinetic current density, and double-layer capacitance) for samples prepared using Fe precursor loadings from 0.5 mmol to 2.0 mmol. As shown in Figure R3a, as the Fe precursor concentration increases from 0.5 mmol to 1.5 mmol, the catalytic activity gradually improves (the best performing Zn Fe /f-NC catalyst is obtained with 1.5 mmol Fe precursor), but 1 1+x performance decreases at 2.0 mmol. Figure R3b shows the Tafel-slope results for the different concentrations, the ORR kinetics are enhanced as the Fe precursor concentration increases up to 1.5 mmol; at 2.0 mmol the Tafel slope decreases, indicating a slowdown in ORR kinetics. Figure R3c presents the kinetic current densities at 0.93 V and 0.90 V, which also show an increase when the concentration rises to 1.5 mmol and a decrease at 2.0 mmol. Figure R3d displays the fitted double-layer capacitance (C ) results, the C for the 2.0 mmol coverage begins to decrease dl dl relative to 1.5 mmol, consistent with Fe clusters aggregating into zero-valent nanoparticles and a consequent reduction in the effective active area (Figure R4a and Table R1).\nFigure R3. (a) LSV curves for evaluating the ORR catalytic activity of different Fe precursor concentrations. (b) the corresponding Tafel plots of electrocatalysts measured. (c) comparison of j for samples. (d) Calculated C values. (Figure R3 has been k dl renumbered as Figure S26 in the revised Supporting Information)\nThe XPS results showed that the relative proportion of Fe2+ gradually increased while that of Fe3+ decreased as the Fe(acac) loading increased from 0.5 mmol to 2.0 mmol 3 (Figure R4a). A small zero-valent Fe peak (706.2 eV) can be seen in the Fe 2p spectrum, which means the Fe metal possibly have a certain volume and break through the definition of clusters (Adv. Mater. 35, 2305945 (2023)). This result indicates that Fe2+ plays a dominant role in catalytic activity. When the coverage exceeds 1.5 mmol beyond the Fe saturation point-Fe clusters aggregate into Fe nanoparticles, causing the activity to decrease gradually (Figure R4b).\nFigure R4. (a) High-resolution XPS of Fe 2p spectra; (b) AC-HAADF-STEM image of the contrast sample with a precursor Fe loading at 2.0 mmol. (Figure R4 has been renumbered as Figure S28 in the revised Supporting Information)\nTable R1. The proportion of different iron types calculated from the peak area ratio in Fe 2p XPS spectra. (Table R1 has been renumbered as Table S5 in the revised Supporting Information)\nFe(acac) @ZIF-8 Content Fe2+ (%) Content Fe3+ (%) Content Fe0 (%)\n3\n0.5 mmol 58 42 0\n1.0 mmol 64 36 0\n1.5 mmol 68 32 0\n2.0 mmol 69 29 2\n4. This work mentions Zn participation in forming Fe/Zn-N4 sites but does not clarify Zn's specific role (e.g., electron regulation, structural stabilization, altering reaction pathways).\nRespond: Thank you for this excellent and insightful question. We agree that clarifying the specific role of Zn is crucial for a deeper understanding of the Fe/Zn-N sites. In the 4 original manuscript, our discussion on this point was indeed insufficient. We sincerely appreciate you raising it.\nBased on our combined experimental and theoretical analyses, we have now performed a more in-depth investigation and clarified the roles of Zn as follows, which have been added to the revised manuscript:\nElectronic Regulation (Primary Role): DFT calculations reveal that the incorporation of Zn adjacent to the Fe-N center significantly modulates the electronic structure of 4 the Fe site, especially for spin-down channel. The electron density difference graph for Fe -Zn /NC indicates a net electron transfer from the Zn-N moiety to the Fe-N center, 1 1 4 4 making the Fe less electrophilic (Figure R6). The downshift of the Fe d and d -band z2 xz center, as projected from the PDOS, weakens the adsorption strength of key oxygen intermediates (e.g., *OH) (Figure R5), thereby optimizing the adsorption-free energy and enhancing the intrinsic ORR activity.\nTable R2. D-orbital occupation numbers for Fe-site in Fe /NC and Fe -Zn /NC after projecting 1 1 1 wavefunctions onto orthogonalized atomic wavefunctions. (Table R2 has been renumbered as Table S9 in the revised Supporting Information)\nSpin-up Spin-down\nd d d d d d d d d d\nz2 xz yz x2-y2 xy z2 xz yz x2-y2 xy\nFe /NC 0.963 0.961 0.980 0.568 0.845 0.144 0.728 0.247 0.464 0.731\nFigure R6. Electron density difference plots for Fe -Zn /NC, blue represents an 1 1 electron-loss region and yellow represents an electron-accumulation region; The isosurface level is 0.01 e/Å3. To highlight the role of Zn atom, the electron-accumulation region was exhibited separately. (Figure R6 has been renumbered as Figure S61 in the revised Supporting Information)\nStructural Stabilization: The smaller ionic radius of Zn2+ compared to Fe2+ and its stable d10 electronic configuration contribute to a more rigid and stable local coordination environment. This structural rigidity may help in stabilizing the FeN moiety during the 4 catalytic cycle, potentially inhibiting the demetallation or structural collapse of the active site, which aligns with our observed excellent durability.\nInfluence on Reaction Pathway: While the primary reaction pathway remains the typical 4e- associative mechanism on the Fe site, the modified electronic environment induced by Zn subtly alters the energetics of the potential-determining step. Our computational free-energy diagrams show that the presence of Zn reduces the energy barrier for the *OOH dissociation step, facilitating the overall reaction kinetics.\nIn summary, Zn plays a dual role: primarily as an electronic modulator to optimize the adsorption energetics on Fe, and secondarily as a structural stabilizer. The revised discussion now centers on this clear mechanistic picture.\nThank you again for prompting this significant improvement to our work.\n5. The Zn1Fe1+x/f-NC-based air cathode exhibits ultra-long stability and high peak power density in zinc-air batteries. This result holds significant application potential. It is recommended to thoroughly discuss its practical application fields and further optimization strategies in the Discussion section.\nRespond: We appreciate the reviewer's forward-looking insights regarding the future development of zinc-air batteries (ZABs). In this work, we developed an Zn Fe /f-NC 1 1+x composite catalyst that demonstrated exceptional oxygen reduction reaction (ORR) activity and stability. Encouraged by these results, we further assembled home-made ZABs using Zn Fe /f-NC as the air cathode, which exhibited remarkable charge- 1 1+x discharge cycling stability and power density. Building on this, we also fabricated more commercially viable button cells and flexible solid-state ZABs. Pleasingly, these devices showed promising potential for further commercialization. However, we also identified limitations in the current technology. Despite significant advances in ORR catalysts—where the reported state-of-the-art Fe–N–C based catalysts now rival the activity of commercial Pt/C—their practical application in button cells or flexible solid-state ZABs faces obstacles beyond the air cathode catalyst. A notable weakness lies in the electrolyte. Current aqueous alkaline electrolytes (6.0 M KOH and 0.2 M zinc acetate solution) suffer from two major issues:\n(1) The alkaline solution tends to react with atmospheric CO , forming insoluble 2 carbonates that cover active sites and block mass transport pathways;\n(2) The aqueous solvent is prone to evaporation and alkali salts will precipitate, especially problematic in button cells with limited electrolyte volume.\nOur experiments based on button cells also confirmed these issues: although the voltage of charge and discharge during initial stage of testing was satisfactory, post-test analysis revealed that performance degradation was not caused by air cathode catalyst but primarily due to the formation of deposits on the separator and the near-complete evaporation of the electrolyte (Figure R7). Therefore, addressing the shortcomings of current electrolytes is urgently needed. Furthermore, our thorough review of recent advances in metal-air batteries indicates that researchers are actively seeking solutions to the challenges posed by conventional strong alkaline electrolytes. For instance, a study by Martin Winter et al. on ZABs using a non-alkaline electrolyte has reinvigorated the potential for commercialization of rechargeable ZABs, marking the first “Science” paper in this field in decades (Science, 371, 46–51 (2021)). Their work employed an aqueous solution of 1 M Zn(OTf) as the electrolyte, investigating charge- 2 discharge processes and products in a non-alkaline environment. They made a pioneering discovery that the anode reaction involves the conversion between zinc and zinc peroxide. In their subsequent study, they also tested a non-alkaline 1 M zinc acetate (Zn(OAc) ) electrolyte, where the reversible formation/dissolution of 2 Zn (OH) (CH CO ) ·2H O outperformed that in strong alkaline systems (Angew. Chem. 5 8 3 2 2 2 Int. Ed., 61, e202207353 (2022)). Additionally, Huisheng Peng et al. reported a calcium-oxygen (Ca–O ) battery capable of being recharged up to 700 times at room 2 temperature. By employing a durable ionic liquid electrolyte, this system circumvented the volatility issue associated with aqueous solvents and achieved reversible two-electron redox reactions at room temperature (Nature, 626, 313-318 (2024)).\nThese cutting-edge studies point toward promising directions for the commercialization of metal-air batteries. While considerable progress has been made in air cathode catalysts, the development of electrolytes compatible with practical device configurations still requires further effort. This will be also a focus of our subsequent works. In line with the reviewer’s constructive suggestions, we have expanded the discussion in the ZABs section of this manuscript to include the limitations of the current system and prospects for future development.\nFigure R7. Photos before and after charge-discharge cycling of button cell with Zn Fe /f-NC as air cathode catalyst. (a) Before the charge-discharge cycling, the 1 1+x separator contains wet electrolyte. (b) After the charge-discharge cycling, substantial amounts of precipitate are generated, and the electrolyte in the separator is almost volatilized. Additionally, the separator is difficult to detach from the carbon paper due to the adhesion caused by the copious precipitate formation. (Figure R7 has been renumbered as Figure S65 in the revised Supporting Information)\nReviewer #2 (Remarks to the Author):\nThe issues have been resolved and the work can be accepted.\nResponse: Thank you for your previous constructive criticisms and suggestions, following which the quality of this work has been significantly improved. And we greatly appreciate your approval of our revised manuscript for publication now.\nReviewer #3 (Remarks to the Author):\nThe revised version have well addressed my concerns. The current manuscript can be considered for publication.\nResponse: Thank you for your previous in-depth and helpful comments, which significantly improve the quality of our manuscript. And we greatly appreciate your approval of our revised manuscript for publication now.\nReviewer #4 (Remarks to the Author):\nThe revised version of the manuscript is good enough to be published. The authors addressed each comment in detail and made appropriate changes. I recommend accepting it as soon as possible.\nResponse: We extend our sincere gratitude to you for your meticulous evaluation of our revised manuscript, which has yielded a favorable assessment. The insightful comments and suggestions provided by you have significantly enhanced the comprehensiveness, rigor, and persuasiveness of our research.\nA composite catalyst containing Fe clusters encapsulated within curved carbon layersand surrounded by densely dispersed Fe/Zn-N sites was obtained. A 4 series of decoupled control samples, including those with dual single-atom Fe and Znanchored on curved carbon and on planar carbon were also prepared. Decoupling experimental revealed that Fex clusters promoted. Single atom Zn, Fe clusters and fingerprint carbon layer jointly enhanced the catalytic performance of Fe-N in ORR through the multivariate regulation of electronic 4 structure. The manuscript was well organized and written. I recommend its publication in Nature Communications after the following revision:\n1. The manuscript proposes that the curved carbon layers regulate the electronic structure of Fe–N₄ sites via strain effects, yet the supporting evidence mainly relies on electrochemical comparisons and limited DFT calculations. There is a lack of direct characterization (e.g., Raman stress analysis, local valence band analysis) to substantiate the claim of “curvature-induced charge redistribution.”\n2. The authors show that removing Fe clusters decreases E1/2 by 26 mV; however, this difference may also stem from changes in carbon defects or pore structure. Moreover, the manuscript lacks quantitative statistics of the spatial relationship between clusters and single-atom sites (e.g., average cluster-single atom spacing).\n3. The manuscript reports an E1/2 of 0.93 V (alkaline), claiming superiority over most reported catalysts; however, the supporting comparison table (Table S3) lacks details such as catalyst loading, electrode configuration, and testing parameters (rotation speed, O₂ concentration). In acidic media, E1/2 is only 0.81 V, which is significantly lower than Pt/C, but the manuscript does not discuss the mechanistic origin of this gap.\n4. Although the manuscript demonstrates over 1000 h cycling stability, only macroscopic current and voltage gap data are provided. There is no microscopic structural comparison before and after cycling, making it unclear how curved carbon and Fe clusters evolve during long-term operation.\n5. In DFT simulations, Fe₁/f-NC is modeled using a C₆₀ molecule, while other models use graphene slabs, which may influence electronic state comparability. The choice of exchange-correlation functional (e.g., GGA/PBE) and its suitability for capturing strain effects on d-orbital regulation is not discussed.\n6. Several figures (e.g., Figures 3 and 5) have small fonts and low color contrast, making them difficult to read. Some explanations in the Supporting Information repeat content already in the main text. Linguistically, some sentences are long or awkward (e.g., “which indicating that the more Fe salt added to ZIF-8 has a greater impact on its crystallinity although the overall structure has not been damaged”).\n7. Some related references are recommended to be considered for citing:\nNatl. Sci. Rev., 2022, 9, nwab197.\nAdv. Mater. 2024, 36, 2408317.\nAdv. Mater. 2024. 36 (32), 2406094.\nAdv. Mater. 2023. 35 (48), 2307003."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The author has effectively supplemented and refined the data from previous proposals, significantly enhancing the rigor and completeness of the research. This study demonstrates innovation and clear functional delineation in the integrated design of nano-fingerprint carbon layers, single-atom sites, and cluster structures, with outstanding performance data and a well-defined research logic. Consequently, this paper holds high publication value. However, certain details could be further optimized to better support the conclusions.\n1. Density functional theory calculations provide robust theoretical support for the experimental results. Authors are advised to further detail computational parameters and boundary conditions to enhance transparency.\n2. The discussion on electronic structure regulation and OH* adsorption energy optimization is a major highlight of the paper. Consider expanding this section to provide deeper mechanistic insights.\n3. The extended discussion on iron precursor concentration gradient data is lacking: The authors determined 1.5 mmol as the optimal condition through experiments with a 0.5-2.0 mmol Fe(acac)3 concentration gradient, observing Fe species agglomeration into particles at 2.0 mmol. It is recommended to supplement the ORR kinetic parameters (e.g., Tafel slope) of the catalyst at different concentrations, analyzing the correlation between Fe2+ proportion changes and catalytic kinetics. Concurrently, combine this with the intensity variation of the zero-valent Fe peak in XPS to quantify the influence of Fe species agglomeration on activity.\n4. This work mentions Zn participation in forming Fe/Zn-N4 sites but does not clarify Zn's specific role (e.g., electron regulation, structural stabilization, altering reaction pathways).\n5. The Zn1Fe1+x/f-NC-based air cathode exhibits ultra-long stability and high peak power density in zinc-air batteries. This result holds significant application potential. It is recommended to thoroughly discuss its practical application fields and further optimization strategies in the Discussion section.", "reviewer_2": "The issues have been resolved and the work can be accepted.", "reviewer_3": "The revised version have well addressed my concerns. The current manuscript can be considered for publication.", "reviewer_4": "The revised version of the manuscript is good enough to be published. The authors addressed each comment in detail and made appropriate changes.I recommend accepting it as soon as possible."}, "author_response": ""}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have made proper revision and now the manuscript can be accepted."}, "author_response": ""}]

Decision letter