Structure-adaptive single-atom nickel catalysts for pure hydrogen peroxide electrosynthesis at industrial current density

Domain: biology
Published: 2026-03-26
Editorial outcome: major_revision

Abstract
The generation of pure hydrogen peroxide (H O ) through two-electron oxygen reduction reaction
2 2
represents a promising avenue for sustainable chemical production. However, a fundamental understanding
of inevitable reconstruction of active sites during electrolysis remains elusive, hindering the development
of dualable electrocatalysts. Herein, we report a structure-adaptive electrocatalyst featuring self-regulating
capabilities under reaction. A series of single-atom Ni catalysts with B/N coordination (NiB N ) serve as a
x y
model system to explore structure flexibility. Under the applied potential, the structural evolution of Ni+0.98-
B N into Ni+0.98-B N occurs at initial stage. The Ni-B and Ni-N bond length in Ni+0.98-B N , as the genuine
2 2 1 2 1 2
active site, are self-regulated to redistribute interfacial electrons by B/N coordination and then boost both
intrinsic activity and stability of Ni site. When the potential is removed, the catalyst returns to its initial Ni-
B N configuration. The H O productivity is up to 9320 mmol g -1S h-1 with a continuous output of ~5
2 2 2 2 catalyst
wt% H O solution under industrial current density for over 300 h. This work elucidates the dynamic
2 2 E
reconstruction-activity enhancement for H O electrosynthesis.
2 2

Introduction
Hydrogen peroxide (H O ), a crucial green and efficient oxidant, is indispensable in diverse fields including
2 2
pulp bleaching, wastewater treatment, chemical synthesis, and medical sterilization1-3. The continuously
growing global market demand further highlights its strategic position as a basic chemical for modern
industry4. NeverthelessA, the conventional anthraquinone production process suffers from critical
environmental and energetic drawbacks, including high energy consumption, the use of toxic solvents, and
inherent safety risks in storage/transportation5-7. In contrast, electrocatalytic H O production via two-
2 2
electron oxygen reduction reaction (2e− ORR) has been regarded as a promising approach, which can
potentially address the issues of pollution, transportation, and storage in the traditional anthraquinone
method, especially powered by renewable electricity8, 9. This approach holds significant promise for
replacing anthraquinone-based methods and advancing decentralized on-site H O production10, 11. The
2 2
electrochemical ORR is a multi-electron transfer process involving several elementary steps, as well as the
disproportionation/decomposition of various oxygen-containing intermediates12, 13. Despite its potential,
achieving high activity, selectivity, and stability in catalysts during 2e− ORR, especially under industrial
current density, remains a significant challenge.

Recently, numerous transition metal (e.g., Fe, Co, Zn, Ni)-based compounds have been explored for
electrochemical synthesis of H O 12, 14-16. Among these, the Ni-based compounds are particularly interesting
2 2
owing to the intrinsically favorable thermodynamics toward the adsorption of the intermediates involved in
2e− ORR, leading to a high selectivity17. However, the previously reported Ni-based electrocatalysts still
suffer from poor ORR activity due to the weak-binding of metal sites toward adsorbates, thereby impeding
the efficient and stable production of pure H O 18. Single-atom catalysts (SACs) that incorporate atomically
2 2
dispersed Ni into a metal-nitrogen-carbon (M-N-C) structure provide a solution to this long-standing
weakness19. The strong σ/π-donation from the surrounding N ligands in SNi-N-C withdraws electron density
from the Ni 3d orbitals, upshifts the d band centre, and thereby strengthens the adsorption of *OOH
intermediates. Moreover, the exact NiN (x = 2-4) coordination pattern, including pyrrolic vs. pyridinic
x
configuration, planar vs. distorted geometry, and seco nd-shell heteroatom doping, can be synthetically
optimized. This offers an additional platform to Isimultaneously tune both the activity and selectivity of
catalytic reactions20.

A central question in the field of SACs is whether the single-atom active centers can maintain a stable and
well-embedded configuration under catalytic operating conditions21. When a reduction potential is applied
to the working electrode, the oxide-derived single-atom metal would often be reduced to a state resembling
metallic nanoclusters, steering the unavoidable reconstruction of catalysts22. Previous studies have revealed
that this reconstruction of SACs is a double-edged sword. It can enhance catalytic performance by
reforming morphology and structure, inducing in situ-generated active sites, and optimizing the reaction
microenvironment23-25. Conversely, uncontrolled reconstruction can lead to catalyst deactivation through
irreversible agglomeration of metal atoms, leaching of active sites, or the promotion of competitive
reactions21, 26, 27. The key to balancing this relationship lies in strategically guiding the reconstruction
process towards a favorable and stable configuration via rational catalyst design. In particular, there is a
lack of benchmark studies elucidating the correlation between the dynamic restructuring behavior and the
local coordination environment of Ni centers, which severely impedes the rational design of high-
performance Ni SACs with predictable stability for pure H O production.
2 2

Herein, we introduced a structure-adaptive Ni SAC electrocatalyst featuring self-adjusting structure
transformation during the reaction process. Precise engineering of the coordination element with
heteroatoms B in the first coordination shell of Ni-N moiety induces an asymmetric charge distribution
4
over the N atoms surrounding the metal centers. This can balance the stability of the initial structure and
preset a trigger to reconstruct into a more active structure during electrocatalysis. In situ X-ray absorption
fine structure (XAFS) revealed the potential-driven structure evolution of Ni+0.98-B N into Ni+0.98-B N
2 2 1 2
configuration as the genuine active site. Simultaneously, the Ni-EB and Ni-N bond length in Ni+0.98-B N is
1 2
self-regulated to redistribute interfacial electrons and simultaneously boosts both intrinsic 2e⁻ ORR
performance and stability. Consequently, the NiB 2NN
2
exhibits high 2e⁻ ORR activity, achieving 88-94%
H O selectivity and 92.5-95% Faraday efficiency (FE) at a wide controllable potential window (0.2-0.6 V
2 2
vs. RHE). Notably, the electro-synthesizLed H
2
O
2
by designed NiB
2
N
2
can realize 9320 mmol g
catalyst
-1 h-1
with continuously generated ~5 wt% pure H O solution for exceeding 300 h at 300 mA cm-2. This study
2 2
offers atomic-level insights into the in-situ reconstruction mechanism of asymmetric coordination Ni SACs
catalysts and offers guid A elines for the precise design of efficient 2e− ORR electrocatalysts.

Results
Fabrication and characterization of atomic Ni catalysis with B/N coordination.
The single-atom Ni catalysts with different B/N coordination environments (denoted as NiB N ) were
x y
synthesized through a pyrolysis strategy, as illustrated in Fig. 1a. High-temperature pyrolysis enables in
situ formation and stabilization of isolated metal sites on carbon matrices through strong metal-support
interactions, while also allowing adjustment of the local bonding environment by tuning the degree of
carbonization and nitrogen doping28. The facile modulation of pyrolysis temperature enables direct control
the Ni-N/B coordination motifs, allowing flexible and tunable adjustment of the coordination structure29.
In general, the physical confinement effect of zeolitic imidazolate framework-8 (ZIF-8) spatially confines
Ni2+ ions to its hierarchical porous structure, followed by the introduction of boric acid (H BO ) as a B
3 3
source for mechanical ball milling30. The obtained single-atom Ni catalyst at 700℃ was denoted as NiB N .
2 2
In addition, comparative analogs, including NiB N , NiN , and NiN , were also synthesized by modulating
1 3 3 4
pyrolysis conditions. As shown in Supplementary Table 1, the Ni content in NiB N was estimated to be
x y
0.43 wt% (NiN ), 0.40 wt % (NiB N ), 0.36 wt % (NiB N ), 0.22 wt % (NiN ), respectively, by inductively
4 2 2 1 3 3
coupled plasma optical emission spectroscopy (ICP-OES). (more eSxperimental details shown in the
methods).

Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images reveal similar
regular rhombic dodecahedral shapes of NiB N , with no metal nanoparticles observed on the graphite
x y
carbon (Fig. 1b and Supplementary Fig. 1-6). SimiIlarly, X-ray diffraction (XRD) patterns and N adsorption
2
curves are consistent with conclusions derived from TEM (Supplementary Fig. 7, 8). Evenly distributed
bright spots (circled by red dashed line) in the high-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) images of NiB N correspond to the atomic-level spatial distribution of Ni
2 2
atoms (Fig. 1c). FurthermAore, the pseudo-color surface plot derived from a yellow circle in Fig. 1d exhibits
a more vivid and unambiguous representation of the local surface environment. The isolated red peaks
indicate single Ni atoms, while the irregular blue regions represent the graphene framework. Energy-
dispersive X-ray spectroscopy (EDS) mapping confirms the homogeneous spatial distribution of Ni, B, N,
and C elements, with infrared spectroscopy (IR) spectra further corroborating the successful introduction
of B/N heteroatom (Fig. 1e and Supplementary Fig. 9).

To further elucidate the electronic configuration and local coordination environment of NiB N ,
the
x y
synchrotron radiation-based X-ray absorption near-edge structure (XANES) and extended X-ray absorption
fine structure (EXAFS) of the catalysts were obtained. The Ni K-edge XANES spectrum reveals that the
absorption edge of NiB N was located between Ni foil and NiO, suggesting that the valence state of Ni
x y
was between 0 and +2 (Fig. 1g). In addition, the quantitative analysis of Ni oxidation states showed that the
average valence state of Ni increased from +0.88 to +0.98 with the increase of Ni-B bonds (Supplementary
Fig. 10), which was attributed to the electronic transfer from Ni to B. The elevated white-line intensity
further confirms enhanced electron depletion at Ni sites via B-mediated charge transfer from Ni to B. This
conclusion is further supported by X-ray photoelectron spectroscopy (Supplementary Fig. 11-13). The Ni
2p peak shows a progressive positive shift across the NiB N series (NiN < NiN < NiB N < NiB N ),
3/2 x y 3 4 1 3 2 2
indicating the increase of Ni oxidation state, consistent with XANES. Concurrently, the B 1s peak in NiB N
2 2
shifts negatively (~0.2 eV vs. NiB N ), suggesting an increased electron density on B. These opposing shifts
1 3
collectively provide direct evidence for B-mediated charge transferS from Ni to B. To analyze the
coordination shell of the NiB N , the corresponding k -weighted Fourier transform EXAFS (FT-EXAFS)
x y 2 E
R
curves in R space are exhibited in Fig. 1h. The R-space plots exhibit strong peaks appearing between 1.40-
1.49 Å (Fig. 1h), which is mainly attributed to the sca ttering interaction between Ni atoms and the first
layer (Ni-N/B). There was no peak of Ni-Ni (2.17I Å) or Ni-O-Ni (2.56 Å) observed, excluding the presence
of metallic Ni or oxides. Consistent with the FT-EXAFS analysis, the WT contour plots for NiB N exhibit
x y
a single intensity maximum at 3.75 Å
C-1
ascribed to the Ni-B/N bond (Fig. 1f and Supplementary Fig. 14).

Subsequently, the least-squares EXAFS fitting reveals an asymmetric coordination structure with Ni-N and
Ni-B. As shown in Fig. 1Ai, Supplementary Fig. 15, and Table 2, the Ni K-edge EXAFS spectrum in R space
of NiB N can be well fitted by the Ni-N and Ni-B scattering paths with the fixed coordination numbers of
2 2
2.37 and 2.20. The possible configurations (trans- and cis-configuration) of NiB N were established by
2 2
calculating the formation energies (E). The trans-configuration has a lower formation energy (E = -5.96
f f
eV) compared to the cis-configuration (E = -4.91 eV), indicating that this configuration is
f
thermodynamically more favorable (Supplementary Fig.16). Additionally, the EXAFS tests of the
analogues, including NiN , NiB N , and NiN were also conducted (Supplementary Fig. 17-19 and Table
4 1 3 4,
2), which points out their specific and precise coordination structure for all the analogues.

Electrocatalytic 2e⁻ ORR performance of NiB N catalysts.
x y
The 2e⁻ ORR performance of NiB N and analogous catalysts was systematically evaluated using a rotating
2 2
ring-disk electrode (RRDE) technique within a standard three-electrode system. All applied potentials are
reported versus the reversible hydrogen electrode (RHE). As shown in Fig. 2a, polarization curves exhibit
distinct ring and disk currents, which are used to determine the 2e⁻ ORR selectivity, indicating variable
ORR performance among different NiB N catalysts. Obviously, NiN exhibited the lowest onset potential
x y 4
of 0.60 V and the low ring current density (0.07 mA cm-2 at 0 V), indicating its poor 2e⁻ ORR activity and
selectivity. In contrast, both NiB N and NiB N showed positively shifted onset potential and increased
2 2 1 3
ring current density after introducing B atoms, demonstrating an enhanScement in intrinsic activity and 2e⁻
ORR selectivity. NiB N exhibited the higher ring current density (0.18 mA cm-2 at 0 V), revealing the high
2 2 E
performance and the decisive role of B in enabling efficient electrosynthesis of H O . Additionally,
2 2
although NiN possessed the most positive onset potent ial of 0.76 V, the main pathway is the 4e⁻ ORR, as
3
the H O selectivity is < 50% within the whole teIsted potential range. Quantitative analysis of selectivity
2 2
and electron transfer number (n) was performed based on linear sweep voltammetry (LSV) data (Fig. 2b;
Supplementary Fig. 20). Notably, NiB N achieves 88–94% H O selectivity and the electron transfer
2 2 2 2
number of 2.2 in a wide potential window from 0.2 V to 0.6 V, which signifies the dominant 2e⁻ ORR
behavior. Meanwhile, boAth low and high-loading NiB
2
N
2
(L/H-NiB
2
N
2
) also exhibited a preference for the
2e⁻ ORR pathway (Supplementary Fig. 21-22). Upon the addition of SCN⁻, both the disk and ring currents
of NiB N decreased significantly, with the selectivity for H O dropping from 94% to approximately 45%,
2 2 2 2
demonstrating that Ni serves as the primary active site for ORR process (Supplementary Fig. 23). In
addition, Faradaic efficiency (FE) was measured in an H-cell electrolyzer, while the concentration of
generated H O was quantified via TiOSO spectrophotometry. Fig. 2c demonstrates that the FE of 2e⁻ ORR
2 2 4
over the NiB N achieves a maximum of 95% and consistently remains an approximate value of 92.5% in
2 2
a broad potential range of 0.2–0.6 V, which indicates the high energy utilization efficiency and practical
operational flexibility. In contrast, the FE of NiB N suffers efficiency loss at elevated potentials, with the
1 3
minimum value of 68%. This nearly constant H O selectivity and Faradaic efficiency, which remain
2 2
insensitive to potential variations, highlight the high performance of the B -Ni-N coordination
2 2
configuration in 2e⁻ ORR and suggest the presence of a highly adaptable Ni site (further experimental
analysis of this effect will be discussed in subsequent sections).

To further assess catalytic activity, the electrochemically active surface area (ECSA) was measured
(Supplementary Fig. 24-25). NiB N exhibited a higher ECSA (9.4 mF cm-2) compared to NiB N (8.1 mF
2 2 1 3
cm-2), NiN (2.3 mF cm-2), and NiN (4.1 mF cm-2), indicating more exposed active sites and faster mass
3 4
transfer31. Electrochemical impedance spectroscopy (EIS) also reveals the better charge transfer kinetics of
NiB N , evidenced by a lower R (Charge transfer resistance) of 42 Ω cm-2 (Supplementary Fig. 26).
2 2 ct
Stability is a crucial parameter for assessing the practical applicabilityS of electrocatalysts. The long-term
electrocatalytic stability of NiB N was evaluated through a 24-hour durability test at a fixed potential of
2 2 E
0.3 V (Fig. 2d). Concurrently, a stable H O selectivity of over 88.5% was maintained, confirming both the
2 2
structural durability of catalyst and its persistent prefer ence for the 2e⁻ ORR pathway32. Furthermore, the
statistical data of the reported transition metal singIle-atom electrocatalysts (Supplementary Table 3) further
highlighted the stable selectivity (88–94%) and high FE across a potential range (0.2–0.6 V). This stable
performance enables high adaptability with renewable energy systems (such as solar and wind power) while
significantly relaxing the stringe T nt requirement for precise potential control in practical applications33, 34.

Dynamic exploration of structure-adaptive performance in NiB N .
2 2
Understanding the nature of single-atom catalytic sites under working conditions is a prerequisite for the
rational design of target catalysts. The beneficial mechanisms can be validated through electrochemical
operando characterization techniques. Firstly, the operando XAFS measurements were performed to
monitor the evolution of the NiB N structure at different applied potentials from the open-circuit potential
2 2
(OCP) to 0.2 V vs. RHE (Fig. 3a-e and Supplementary Fig. 27). As depicted in Figure 3a, the magnitude of
the spectral change remains negligible under the applied potential and Ex-situ conditions, as further
confirmed by the differential Δμ spectra derived from subtracting the ex-situ signal from the XANES
spectra (inset of Fig. 3a). The observation verified that the Ni atoms maintain the stable oxidation state of
+0.98 during the cathodic ORR process.

It is intriguing to note that there are obvious potential-dependent changes in a series of FT-EXAFS spectra
collected at different applied potentials (Fig. 3b). The dominant scattering peak corresponding to Ni-B/N
coordination in the first shell changes from 1.50 Å to 1.41 Å, and then to 1.38 Å. Notably, its intensity
decreased with decreasing the applied potentials, indicating the variation in the Ni coordination
environment. Quantitative least-squares EXAFS curve-fitting analyses provided microscopic insights into
the evolution of dynamic coordination (Supplementary Fig. 28). The relSevant fitting parameters were listed
in Supplementary Table 4. The Ni−B and Ni−N bond lengths in the initial Ni−B N motif are 1.94 Å and
E 2 2
1.80 Å, respectively. Under cathodic applied potential, the Ni−B bond length was progressively reduced to
1.88 Å at 0.6 V vs. RHE, and then to 1.81 Å at 0.4 V, a nd finally to 1.76 Å at 0.2 V. In contrast, the Ni-N
bond length was progressively increased to 1.85 ÅI at 0.6 V vs. RHE, and then to 1.88 Å at 0.4 V, and finally
to 1.91 Å at 0.2 V (Fig. 3c). Correspondingly, the initial Ni−B N configuration would be reconstructed to
2 2
NiB N at 0.6 V, suggesting that the involvement of Ni in ORR initiates at this state. When the applied
1 2
potential was continuously decreased to 0.2 V, the Ni−B N configuration was retained with a coordination
1 2
number of one for Ni-BA and two for Ni−N (Fig. 3e)35. These findings are consistent with 2e⁻ ORR
performance shown in Fig. 2a-c, providing compelling evidence that the NiB N moiety was the genuine
1 2
active site and that the NiB N structure merely serves as a precursor for active site evolution. Under
2 2
working conditions, NiB N undergoes a dynamic structural evolution, as characterized by the contraction
2 2
of Ni−B bonds and the elongation of Ni−N bonds, while maintaining Ni with a stable oxidation state. This
synergistic variation in the Ni−B and Ni−N bonds directly reflects the redistribution of electron density
between the Ni atom and its surrounding coordination network36-38. Through such dynamic adjustment,
electronic self-equilibration within the first coordination shell is achieved, which effectively buffers
perturbations caused by electrochemical polarization and intermediate adsorption, thereby stabilizing the
apparent oxidation state of Ni. This structurally adaptive Ni active site is crucial for maintaining high
performance and stable 2e⁻ ORR. In addition, the nearly identical XANES spectra before and after the
reaction, together with the absence of detectable B dissolution, confirm that NiB N undergoes reversible
2 2
restructuring during reaction without the formation of Ni clusters or nanoparticles (Supplementary Fig. 29
and Table 4 and 5). In contrast, NiN undergoes irreversible reduction of the nickel atoms upon potential
4
application, indicating the absence of such self-regulating capability (Supplementary Fig. 30).

In situ Raman spectroscopy (Fig. 3f) was used to further identify reaction intermediates during ORR. As
the ORR proceeds, the peak at 1050 cm-1 due to the adsorbed *O was gradually enhanced as the potential
2
was shifted to more negative, implying that the enhanced ability of O adsorption and activation39.
2
Additionally, the increased intensity of another peak at approximately 91S7 cm-1, attributed to O-O stretching
vibration of *OOH intermediate formed on Ni atoms, indicated the accumulation of *OOH species on Ni
sites, which implies that the generation of *OOH is the rate-determining step in the ORR process40. More
importantly, when the potential was lowered to 0.4 V v s. RHE, the peak belonging to Ni-OOH shifted to
higher wavenumbers, evidencing a strengthening of *OOH adsorption energy on Ni sites. This potential-
dependent modulation of adsorption provides further direct evidence that the structure-adaptive Ni-B N is
1 2
the genuine active site for 2e⁻ ORR. As indicated in the entire reaction process shown in Fig. 3g, the Ni site
provided highly responsive reactivity for oxygen electrocatalysis, allowing the desired 2e⁻ ORR pathway
to take place. Once the aApplied potential was sufficiently low to initiate the 2e⁻ ORR, the Ni+0.98B
2
N
2
motif
was reconstructed into Ni+0.98B N as geneuis active sites. Moreover, the self-regulation of Ni-B/N bonds
1 2
continuously modulates the electronic distribution of the Ni center, stabilizing the adsorption strength of
the *OOH intermediate. Upon the removal of applied potential, the catalyst reverts to the initial Ni+0.98B N
2 2
configuration, demonstrating the reversibility and stability of NiB N .
2 2

The 2e− ORR activity and selectivity of NiB N could be performed through systematic density functional
x y
theory (DFT) calculations (Supplementary data 1-2). As shown in the activity-volcano plot (Fig 4a and
Supplementary Fig 31), NiN and NiN located at the right branch of the activity-volcano plot, possessed
4 3
weak *OOH adsorption, resulting in poor ORR activity. Upon the introduction of B atoms, NiB N and
1 3
NiB N show strengthened *OOH adsorption and shift to the left branch of the activity-volcano plot.
2 2
Notably, NiB N achieves a more moderate *OOH binding strength, with the optimized structure positioned
2 2
closest to the apex of the limiting potential volcano (U = 0.35 V, ∆*G =3.49 eV). This balanced
L OOH
adsorption energy facilitates efficient protonation of *OOH to H O , enabling high 2e⁻ ORR performance41,
2 2
42. The free energy diagram at U = 0.7 V further confirms the high 2e⁻ ORR selectivity of NiB N ,
2 2
which
exhibits the lowest overpotential (0.04 V) for the 2e− pathway (Fig 4b). Electronic structure analysis
attributes this optimized adsorption to an upshifted d-band center of Ni relative to NiN ,
4
which strengthens
the orbital coupling between Ni-3d and O-2p states of *OOH (Supplementary Fig 32-33). These
calculations suggest that the static NiB N configuration possesses an electronic structure potentially
2 2
favorable for the 2e⁻ ORR.

Moreover, the coordination environment undergoes dynamic reconstruction during operation. We further
investigated the dynamic reconstruction of the Ni site during operation, and the mechanism responsible for
the constant Ni oxidation state. As shown in Fig. 4c and Supplementary Fig.34 and Table 6-7, the potential-
determining step from *O to *OOH is exothermIic on both NiB N and NiB N , with the reaction energy
2 2 2 1 2
of -0.72 eV and -1.57 eV, respectively43, 44. The more exothermic nature on the reconstructed NiB N site
1 2
indicates that dynamic reconstruction creates a thermodynamically more favorable active center for the 2e⁻
ORR, thereby rationalizing the high performance observed experimentally. These computational results
point to a potential that tAhe dynamic reconstruction creates a more favorable active center (NiB
1
N
2
) for the
2e⁻ ORR, thereby rationalizing the experimental onset of high performance.

To elucidate the interaction between single atom Ni and *OOH during the reaction process, we calculated
the charge density distribution and the corresponding evolution of Ni-B/N bond lengths in the NiB N under
1 2
applied different cathodic potentials (0.2-0.6 V vs. RHE). The differential charge density and quantitative
Bader charge analyses reveal a pronounced charge redistribution within the first coordination shell of the
Ni single atom in response to the applied potential (Fig. 4 d-e, Supplementary Table 8). Upon application
of the potential, Ni−B bond cleavage leads to the structural transformation from NiB N to NiB N structure.
2 2 1 2
The dissociated B atom subsequently coordinates with OH⁻ in the electrolyte to form a B−OH species,
which remains stably anchored on the catalyst surface. The Bader charge analysis indicates that the charge
transferred from Ni atoms to the adsorbed *OOH intermediate gradually increases as the applied potential
decreases. Specifically, the transferred charge was 0.31, 0.39, and 0.49 e at 0.6, 0.4, and 0.2 V (vs. RHE),
respectively. Compared with the original NiB N structure (+1.56), the coordinated B atom exhibits electron
2 2
accumulation of +1.49, +1.46, and +1.42 e at 0.6, 0.4, and 0.2 V, respectively. At the same time, the
coordinated N atoms display a slight increase in electron density, with the charge varying from -1.13 to -
1.20 e. Unlike B and N atoms, the total charge density of the surrounding carbon framework decreases from
-1.38 to -0.55 e. Crucially, the charge on the Ni atom changes by less than 0.03 e, demonstrating the stability
of its apparent oxidation state. Accordingly, with the potential decreases from ex-situ to 0.2 V vs. RHE, the
calculated Ni-B bond length progressively contracts from 1.95 Å to .9S3 Å, 1.89 Å, and then to 1.85 Å,
whereas the Ni-N bond elongates from 1.97 Å to 1.97 Å, 1.99 Å, and then to 2.02 Å. The calculated Ni-B
and Ni-N bond lengths are consistent with the structural evolution observed in operando XANES analysis.
These results indicate that the observed bond-length ev olution represents the charge redistribution within
the first coordination shell.

To directly validate the proposed B/N-mediated electron-buffering mechanism, we employed operando
near-ambient pressure XPS (NAP-XPS) to monitor the evolution of the electronic states of B and N atoms
during the 2e⁻ ORR (Fig. 4f-g, Supplementary Fig. 35-36). Spectral deconvolution reveals that B
predominantly exists as AB–C bonds within the carbon framework, accompanied by minor contributions
from B–N and B–O species. The B 1s binding energy is centred at 190.5 eV under ex-situ and OCP
conditions. As the potential gradually varies from 0.6 to 0.2 V vs. RHE, this peak progressively shifts
toward lower binding energy by approximately 190.0 eV (0.2 V vs. RHE), indicative of the increased
electron density at the B sites. Concurrently, the proportion of B-O species increased from 15.3% to 22.0%
with the increasing potential, further indicating the single-nickel sites undergo reconfiguration during the
reaction process. Specifically, when one Ni–B bond cleaves, the liberated B atom coordinates with OH⁻ to
form a B–OH species anchored on the surface. Meanwhile, the reconstructed NiB N moiety serves as the
1 2
active site for O adsorption to drive the formation of the HOO–NiB N intermediate. Conversely, the N 1s
2 1 2
peak shifts toward lower binding energy by approximately 0.15 eV with increasingly negative potential,
signifying an increase in the electron density at N sites. These results agree well with the DFT predictions
and provide more solid evidence for potential-driven charge redistribution and buffering within the B/N
coordination network during the 2e⁻ ORR process.

Pure H O generation via solid-electrolyte cell configuration.
2 2
The catalyst was integrated into a solid-electrolyte cell to produce pure H O solution (schematic shown in
2 2
Fig. 5a). Specifically, a proton exchange membrane (PEM) and an anion exchange membrane (AEM) were
to separate the anode and cathode compartments, respectively, from Sthe central solid-electrolyte (SE),
thereby avoiding flooding issues and enabling selective ion transfer channels. O diffuses through the gas
2
diffusion layer (GDL) to the cathode and is reduced to HO - via 2e⁻ ORR, while H O at the anode can be
2 2
oxidized to form H+. The HO - and H+ will cross thro ugh AEM and PEM, respectively, into the solid-
2
electrolyte layer to form H O 45, 46.
2 2

The overall performance of the cell was investigated under galvanostatic electrolysis conditions (10-300
mA cm-2). The NiB N catalyst mainta C ined a high Faradaic efficiency (85%, up to 96% at 20 mA cm-2) with
2 2
a lower potential of 2.75 V. A hTigh production rate of 9320 mmol g-1 h-1 can be achieved at 300 mA cm-2,
surpassing the NiN
4
(225A2 mmol g-1 h-1) and NiB
1
N
3
(5545 mmol g-1 h-1). (Fig. 5b and Supplementary Fig.
37-38). In contrast, the Faradaic efficiency for NiN and NiB N significantly declined with increased
4 1 3
current density, dropping to as low as 31% and 43%, respectively. It is noteworthy that NiB N maintains
2 2
high FE (85%-96%) even under industrially relevant current densities, further highlighting the critical
advantage of its self-adaptive structure. In addition, the concentration of H O produced by NiB N
2 2 2 2
increased linearly over 10 h and accumulated to 1740 mM at 100 mA cm-2. This sustained linear production
not only demonstrates the capability of NiB₂N₂ for efficient H O accumulation but also confirms its high
2 2
electrochemical stability throughout prolonged operation (Fig. 5c).

Subsequently, we evaluated the H O production performance of NiB N under an industrial current density
2 2 2 2
of 300 mA cm-2, confirming its potential for practical industrial applications. The NiB N exhibited high
2 2
operational stability, continuously producing ~5 wt% pure H O (pH=5.8) solution for over 300 hours
2 2
without any noticeable performance decay at a constant current density of 300 mA cm-2, with the voltage
maintained around 2.75 V (Fig. 5d and Supplementary Table 9). However, the NiN catalyst suffered a
4
complete degradation in performance after operating for 96 h at 200 mA cm-2. The pronounced difference
in stability underscores the high stability of the structure-adaptive NiB N , demonstrating great promise for
2 2
industrial electrocatalysis. The as-prepared pure H O solution exhibited pH = 5.8, further confirming that
2 2
NiB N maintains high 2e⁻ ORR performance under this pH condition (Supplementary Fig. 38). The ICP
2 2
analysis revealed that the produced pure H O solution was not contaminated by the anode electrolyte
2 2
(KOH), and the concentration of K+ ions was below 2 ppm (SupplemenStary Table 10). This negligible K+
level demonstrates that the membrane preserves high ionic selectivity under our operating conditions. These
results clearly demonstrate the NiB N competitive 2e⁻ ORR activity and stability relative to the reported
2 2
Co, Ni single-atom and carbon-based catalysts (Suppl ementary Table 11). To demonstrate the practical
application effect, we synthesized H O at a curreInt intensity of 300 mA cm-2 in the solid-state electrolysis
2 2
cell, and then flowed it into the heterogeneous Fenton reactor for the degradation of tetracycline (TC),
Ciprofloxacin (CIP), sulfamethoxazol C e (SMX), and phenol (PhoH)47,48. After a 5 min reaction, the detected
concentrations of the pollutants were all below the detection limit, and the removal rate was nearly 100%
(Supplementary Fig. 40)A. This high H
2
O
2
production and rapid degradation demonstrate that NiB
2
N
2
is a
promising candidate for scale-up electrochemical H O generation, and further highlights the significant
2 2
potential of its on-site produced H O for highly effective environmental remediation, validating the
2 2
advantages of self-adaptive structural catalysts for industrial-scale applications.

Methods
Chemicals and Reagents
All the chemicals and reagents, Dimethylimidazole (RG, CAS: 1739-84-0, Adamas), Zinc nitrate (AR, CAS:
10196-18-6, Sinopharm Group Chemical Reagent Co., Ltd.), Nickel nitrate (AR, CAS: 13478-00-7,
Sinopharm Group Chemical Reagent Co., Ltd.), Boric acid (GR, CAS: 10043-35-3, Greagent), Methyl
alcohol (HPLC, CAS: 67-56-1, Adamas), Dimethyl Formamide (RG, CAS: 68-12-2, Adamas), Titanyl
sulfate (RG, CAS: 13825-74-6, Aladdin), Potassium hydroxide (RG, CAS: 1310-58-3, Greagent), Sulfuric
acid (GR, 98%, CAS: 7664-93-9, Sinopharm Group Chemical Reagent Co., Ltd.) were employed directly
without any further purification. High-purity water for all solutions was prepared using a Millipore (USA)
Milli-Q water purification system.

Synthesis of Ni-ZIF-8
Typically, Zn(NO ) ·6H O (72 mmol) and Ni(NO ) ·6H O (7.2 mmol) with a Zn2+/Ni2+ ratio of 10:1 were
3 2 2 3 2 2
dissolved in 200 mL methanol to form solution A. Then, 2-methylimidazole (288 mmol) was ultrasonically
dissolved into 180 mL methanol to form solution B. The solution A was then added into B, and the mixture
was allowed to self-assemble at 60 ℃ for 12 h. The resulting precipitation was collected by centrifugation,
washed with dimethylformamide (DMF) and methanol, and then vacuum-dried at 60 °C. The final product
was named Ni-ZIF-8. The Ni-ZIF-8 with different Ni loadingRs were prepared by tuning the Zn2+/Ni2+ molar
ratio of prevusors (Zn(NO ) ·6H O and Ni(NO ) ·6H O) at 20:1, 10:1 and 5:1.
3 3 2 3 2 2

Synthesis of NiB N
x y
The as-prepared Ni-ZIF-8 (500 mg) precursor was uniformly mixed with H BO (50 mg) by ball milling
3 3
for 1 h. The resulting mixture was Ithen pyrolyzed at 700, 800, or 900°C with a ramp rate of 5°C min−1 for
2 h under an Ar atmosphereR. After naturally cooling to room temperature, the obtained power was treated
in 3 mol L−1 H SO at 80 °C for 4 h to thoroughly eliminate unstable metal species. The final product was
2 4
washed with deionized water until it reached neutral pH and was then dried at 60℃ overnight. These
products were denoted as NiB N (700°C), NiB N (800°C), and NiN (900°C). In addition, NiN catalysts
2 2 1 3 3 4
were prepared under the same process as NiB N but without the addition of H BO . These single-atom Ni
2 2 3 3
catalysts are collectively referred to as NiB N . The effect of Ni loading was investigated on the
x y
electrocatalytic performance of NiB N catalysts that were calcined at 700°C. The catalysts with different
2 2
Zn2+/Ni2+ ratios were prepared: L-NiB N (20:1), M-NiB N (10:1), and H-NiB N (5:1). Unless otherwise
2 2 2 2 2 2
specified, the optimized L/M/H-NiB N would be denoted simply as NiB N throughout the manuscript.
2 2 2 2

Preparation of electrodes.
Typically, 4 mg NiB N and 20μL of Nafion (31175-20-9, Admas) binder solution were mixed with 2mL
x y
of ethyl alcohol. After sonication in ice water for 30min, the obtained homogeneous ink was air-brushed
onto a 2×2 cm2 gas diffusion layer (GDL, SGL-39BB) electrode at room temperature. Then the prepared
electrode was dried in a vacuum at room temperature for 24h before use.

Material Characterization
The morphological features and crystalline phases of cathode materials were characterized using scanning
electron microscopy (SEM, S-4800, Hitachi, Japan). Transmission electron microscopy analysis was
performed on a field-emission instrument (TALOS-F200X, Thermo Fisher Scientific, USA) operated at
200 kV accelerating voltage. The Atomic-resolution strucRtural analysis was conducted via spherical
aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-
STEM, JEM-ARM200F, JEOL, Japan). Crystallographic information was obtained through X-ray
diffraction (XRD, D/max2550, Rigaku, JapEan) with Cu Kα radiation (λ = 1.5405 Å, 40 kV, 30 mA) at a
scanning rate of 5°/min in a 2θ range Cof 10°-90°. The infrared (IR) spectra were obtained on a Nicolet iS20
spectrometer. Specific surface areas were determined through N₂ adsorption-desorption isotherms using
Brunauer-Emmett-Teller methodology (TriStar II 2460, Micromeritics, USA). The XPS measurements
were implemented on a Kratos AXIS-HS equipped with a monochromatic Al Kα X-ray source (150 W, 15
kV, 10 mA). Near-edge X-ray absorption fine structure (NEXAFS) measurements at Ni K-edge were
acquired on beamline BL08U1A (Shanghai Synchrotron Radiation Facility, China) using Ni foil and NiO
as reference standards. Extended X-ray absorption fine structure (EXAFS) data processing, including
background subtraction, normalization, and Fourier transformation, was performed using IFEFFIT software
packages (Athena/Artemis modules).

RRDE measurement
Electrochemical measurements were carried out on a CHI 760E electrochemical workstation (CH
Instrument, Shanghai) using a standard three-electrode system. The setup comprised a rotating ring-disk
electrode (RRDE) serving as the working electrode (with a disk area of 0.2475 cm2 and a Pt ring area of
0.1866 cm2), a graphite rod as the counter electrode, and an Ag/AgCl electrode filled with saturated KCl
solution (CH Instruments) as the reference electrode49. The catalyst ink was prepared by adding 4 mg
catalyst powder to the mixture of 400 μL of deionized ethanol and 10 μL of 5 wt.% Nafion solution to form
a homogenous ink assisted by ultrasound. 10.0 μL of the catalyst ink was drop-casted on the disk electrode
for the RRDE measurement. The working electrode for rotating disk electrode (RDE, area: 0.1975 cm2) test
is similar to RRDE. For RRDE test, the linear sweep voltammetry (LSV) test was conducted with a rotation
rate of 1600 rpm in 0.1 M KOH with O -saturated (pH=13±0.2, elecStrolytes were prepared within two
2
weeks prior to use and stored at room temperature), the potential range was from 0 V to −1 V vs. Ag/AgCl
at a scan rate of 10 mV/s, with a constant potential of 0.3 V vs. RHE applied to the Pt ring. All potentials
measured against Ag/AgCl were converted to the RH E scale in this work using E (vs. RHE) = E (vs.
Ag/AgCl) + 0.197 V + 0.0591 × pH. The H O seIlectivity (H O %) and average electron transfer number
2 2 2 2
(n) were calculated based on (a) and (b):
𝐻 𝑂 % = 2𝐼𝑅 ⁄𝑁 ×100% (a)
2 2 𝐼𝐷+(𝐼𝑅 ⁄𝑁)
𝑛 = 4× 𝐼𝐷 ⁄ (𝐼𝐷+(𝐼𝑅 ⁄𝑁)) (b)
Where I and I are the background-corrected disk and ring currents, and N is the collection efficiency of
D R
the RRDE.

The electrochemically active surface area was measured by the double-layer capacitance (C ) method. CV
dl
scans were conducted in potential range from 0.965 to1.065 V vs. RHE with scan rates of 20-100 mV s−1.
Calibration of the Ag/AgCl reference electrode: The Ag/AgCl reference electrode was calibrated against
the reversible hydrogen electrode (RHE) by measuring the open-circuit potential between the two electrodes.
Both electrodes were immersed in a 0.5 M H SO solution (1 M H⁺), and the potential difference was
2 4
acquired using an electrochemical workstation. A measured open-circuit potential of 0.197 ± 0.005 V
indicates that the reference electrode is accurate. If the value falls outside this range, the internal electrolyte
of the Ag/AgCl electrode should be replaced with freshly prepared saturated KCl solution.

All electrochemical data were collected using a CHI 760E electrochemical workstation (CH Instruments).
The data were processed with the accompanying CHI 760e software and plotted using Origin.

Operando XAFS measurements.
Operando Cu K-edge absorption spectra were collected at different cathodic potentials on the BL11B
station in Shanghai Synchrotron Radiation Facility (SSRF). Firstly, the homogeneous catalyst ink was
prepared by ultrasonically mixing 5 mg of catalyst, 20 μL nafion and 980 μL of ethanol. Subsequently, the
catalyst ink was uniformly brushed on the one side of carbon paper covered on a heating plate with a setting
temperature of 50℃. Then, the prepared working electrode was mounted onto a home-built in-situ XAS
fluorescence cell with a three-electrode configuration in which Pt and Ag/AgCl electrodes (KCl-saturated,
CH Instruments) were used as the counter and referenc e electrodes, respectively. During operando XAS
measurements, O
2
was continuously bubbled witIh a flow rate of 50 mL/min to saturate the 0.1 M KOH
(pH=13±0.2) solution. All operando XAFS spectra were collected in a fluorescence mode at room
temperature. To monitor the spectralC changes during the ORR process, different potentials such as Open
Circuit Potential, 0.20 V, 0.40 VT, 0.60 V, 0.80 V vs. RHE were applied with a cathodic direction. Before
changing each potential, CV scanning for 10 cycles and then resting for another 5 minutes was allowed to
equilibrate the electrochemical system.

Electrochemical tests in solid electrolyte cell
The continuous electrosynthesis of H O was conducted using a solid electrolyte (SE) cell. The electrolyzer
2 2
cell consisted of two titanium backplates with a 4.0 cm2 serpentine flow field. The cathode side was supplied
with oxygen. The gas flow rate was controlled by a mass flow meter (MFC). In the middle chamber, the
Amberlyst15 ion exchange resin (Adams) cation conductor was employed as the SE. Deionized water
flowed into the SE layer controlled by a syringe pump. The anode side was circulated with 1 M KOH. The
catalyst was coated homogenously on the gas-diffusion layer (GDL, SGL-39BB, 2 × 2 cm2) with a loading
of 2 mg catalyst. The mass loading of the catalyst was determined as 0.5 mg cm-2 (normalized to the
geometric electrode area). For direct-current (DC) power supply tests, the anode and cathode were separated
by a pretreated proton/anion exchange membrane. A solid electrolyte layer was incorporated between the
anion and proton exchange membranes. 1 M KOH (pH=14±0.2) as electrolyte was cycled using pumps on
anode. The GDL coated with catalyst and a platinum gauze was used as the working electrode and counter
electrode, respectively. Pure O was purged at 50 mL/min through diffusion channel to the backside of the
2
GDL during the test. And the constant current test was performed for 1 h at different currents (10, 20, 50,
100, 150, and 300 mA cm-2). Quantification of H O in the cathodic compartment was performed employing
2 2
the titanium oxysulfate colorimetric method. A calibration plot was gSenerated by recording the UV-vis
absorbance at 406 nm for a series of standard H O solutions after complexation with an equal volume of
2 2
titanium oxysulfate (6 g L-1). The concentration of H O was subsequently interpolated from this calibration
2 2
curve based on their measured absorbance. The Far adaic efficiency (FE) for H O production was
2 2
determined from a single experiment using the eqIuation (c) below:
𝐹𝐸 (%) = 2𝑐𝑉𝐹 ⁄ 𝑄 ×100% (c)
where c is the H O concentration from the standard curve, V is the catholyte volume, F is the Faraday
2 2
constant (96485 C mol-1), and Q is the total charge passed49.

Activation of proton/anion exchange membrane. The proton exchange membrane (PEM, Nafion-117, 5 ×
5 cm2, ~0.18 mm thickness) was pre-treated with 5% (v/v) H O at 80°C for 1h, followed by immersion in
2 2
10% (v/v) H SO at 80°C for 1h. The anion exchange membrane (AEM, FAA-3-PK-130, 5 × 5 cm2, ~0.13
2 4
mm thickness) was pre-treated with 0.5 M NaOH solution at 80 ℃ for 2 h, after which it was rinsed with a
large amount of deionized water until the pH of the rinsing solution was close to neutral.

Fenton-based pollutant degradation using generated H O in a solid electrolyte cell.
2 2
A Fenton reaction was employed to degrade pollutants using electro-generated H O . Pollutant solutions
2 2
(10 ppm of tetracycline, ciprofloxacin, sulfamethoxazole, or phenol in 30 mL DI water (pH = 3) were
prepared. To each solution, 2 mg of FeSO was added to initiate the Fenton reaction (2 Fe2+ + H O → 2
4 2 2
Fe3+ + ·OH + OH⁻). The H O was produced in situ from the 2e⁻ ORR in the middle chamber of a solid
2 2
electrolyte cell (non-recirculating mode). The produced H O was then continuously introduced into the
2 2
pollutant-Fe2+ mixture to carry out degradation at room temperature and atmospheric pressure.

Electrochemical X-ray photoelectron spectroscopy
All electrochemical in-situ measurements were conducted at a commercial SPECS NAP-XPS system. The
system features a monochromatic small-spot Al Kα X-ray source (μFocus 600 NAP, 300 μm spot size), a
vertically oriented hemispherical energy analyzer (PHOBIAS 150 NAP), and a μ-metal main chamber for
shielding against external electric fields. It also features a separately pumped preparation chamber and a 4-
axis manipulator (x, y, z directions and polar rotation), equippRed with electrical feed-throughs for potential
control and thermocouples, as well as cooling pipes. The experiments were conducted in a home-made
PTFE-based three-electrode cell whose potential was controlled by using a CHI 760E electrochemical
workstation (CH Instrument, Shanghai). TEhe measurement sequence included the Ex-situ, open-circuit
potential (OCP), and applied potentiCals from 0.2 to 0.6 V vs. RHE in 0.2 steps. Thus, at each potential,
spectra for the B 1s and N 1s regions were acquired with a pass energy of 50 eV with an energy step size
of 0.01 eV. All experiments were carried out at a background water pressure of about 15 mbar.

Theoretical calculation details
First-principles calculations based on density functional theory were performed utilizing the Vienna ab
initio simulation package (VASP). The exchange-correlation energy was treated within the generalized
gradient approximation (GGA) as parameterized by the Perdew-Burke-Ernzerhof (PBE) functional. A
cutoff energy of 500 eV was employed following a series of tests. Ionic relaxation was performed until the
energy difference between consecutive steps was below 0.01 meV and the Hellmann-Feynman forces acting
on each atom were less than 0.01 eV·Å-1. The Brillouin zone was integrated using a Monkhorst-Pack
scheme with a 2 × 2 × 1 k-point mesh. Periodic boundary conditions in the z-direction were mitigated by
the inclusion of a 15 Å vacuum layer to prevent interaction between two neighboring surfaces.50,51.

To explicitly simulate electrochemical interfaces under realistic potentiostatic conditions, we
performed grand canonical Density Functional Theory (DFT) calculations using the Constant-Potential
VASP (CP-VASP) package50,51. This approach allows for the direct control of the electrode potential
relative to a reference electrode, rather than fixing the system's total charge. The implicit solvent model was
incorporated using the VASP sol method52,53 to account for the electrolyte environment. In this model, the
aqueous solution is represented as a continuum dielectric with a dielectric constant of ε = 78.

The dissociation energy obtained directly from DFT calculations for the adsorbate on the catalyst model
surface was calculated as: ΔE = E + E - E , where E is the total energy calculated
diss Adsorbate Surface Adsorbate/Surface
using DFT.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents the single-atom Ni catalysts for direct hydrogen peroxide production through two-electron oxygen reduction reaction (2e⁻ ORR) at industrial current density. Series single-atom Ni catalysts with B/N coordination (NiBxNy) was designed as a model system to exhibit remarkable structure flexibility under electrochemical working reactions. Through a combination of experimental characterization and theoretical calculations, the author demonstrates that the self-adjustment of Ni–B and Ni–N bonds stabilizes the active center and then achieves exceptional H2O2 selectivity and long-term stability. This work reveals a comprehensive understanding on structural self-adaptation mechanism for 2e⁻ ORR and offers a new design strategy for stable H2O2 production under practical industrial conditions. I suggest that this paper can be accepted pending several revisions. Detailed comments were listed as follows.\n1. The authors propose a trans-configuration of the B and N atoms in NiB2N2 coordination structure (Figure 1i, 2c, 2g). However, a cis-configuration of B and N is also possible. There is no evidence and explanation to rule out the possibility of cis-configuration. Additional DFT calculations are required to validate the trans-configuration structure for NiB2N2.\n2. The TEM image of Figures 1b-1d can be modified on the figure, with specific names and details labeled. The current visualization effect is not very strong. In addition, please provide details about the inset structures diagram in Figures 1 (a, i) and Figure 3 (c, g).\n3. The authors indicate that the single-atom Ni is the primary active site. However, it has been reported in the previous literatures that boron-doped carbon (B-C) can also be the active sites for electrosynthesis of H2O2. please provide additional experiments to explain this observation.\n4. The single-atom Ni catalysts with different coordination environments were obtained by varying pyrolysis temperature. Why did the author choose this strategy? How about other preparation methods to modulate the coordination environment of Ni atoms?\n5. The authors are suggested to measure the actual pH value of H2O2 solution produced in the middle chamber. Meanwhile, the catalytic performance of 2e⁻ ORR with NiB2N2 should also be measured by RRDE under this pH value, thereby demonstrating its effectiveness under real working conditions.\n6. Please check the bond length changes as indicated in Figure 3c and the data in the Supporting Information. In addition, please provide the changes of Gibbs free energy.\n7. In Figure 1e, the annotation of O/or C atom is not clear. Please check and make the corrections.\n8. The authors should provide the standard PDF card for ZIF-8 in Supplementary Figure 7.\n9. Regarding the literature comparison chart in Figure 5e, the target catalyst in this work is an atomically dispersed catalyst. Its H2O2 yield should be compared with the H2O2 production rates of the same type of atomically dispersed catalysts (such as Co, Mn, etc.). It is necessary to appropriately increase the types of comparative catalysts to highlight its own innovation.\n10. What is the guiding role of the structural adaptability of NiB2N2 in future practical production fields? Can some practical application effects be supplemented? To enrich the contents of this work, several related studies sharing a similar topic are suggested to be incorporated (Adv. Funct. Mater. 2025, 35, 2417090; Appl. Catal. B: Environ. 2024, 343, 123533).", "reviewer_2": "This manuscript presents a structure-adaptive nickel single-atom catalysts (Ni SACs) with a flexible coordination structure capable of varying coordination numbers and bond lengths. The designed Ni SACs have high selectivity and stability for pure H2O2 production via two-electron oxygen reduction reaction (2e⁻ ORR) at industrial current density. Through a combination of in-situ experimental characterizations and theoretical calculations, the study reveals that a potential-driven structure evolution of initial Ni+0.98-B2N2 configuration transforms into Ni+0.98-B1N2, which is identified as the genuine active site under working conditions. Overall, this manuscript is well organized and provides very interesting observation regarding the precise design of 2e⁻ ORR electrocatalysts. However, before publication, major revisions are required to address few issues in the manuscript. The detailed comments are listed as follows.\n1. Why the Ni+0.98-B1N2 configuration can be reversibly recovered to their initial electronic configuration after removing the cathodic bias? How can the authors confirm this variation, it seems no experimental evidence supporting this phenomenon. In addition, whether the traditional Ni-N4 configuration has the structure-adaptive performance under working conditions?\n2. The concentration of atomic Ni is not provided in this manuscript; please carry out the additional characterization, such as inductively coupled plasma mass spectrometry (ICP-MS) to confirm the content of Ni in the series of catalysts.\n3. The NiB2N2 catalyst demonstrates high selectivity and faraday efficiency of 2e− ORR across a wide potential range (0.2-0.6 V vs. RHE). How about the value of Faraday efficiency compared with previous studies? A comparison of Faraday efficiency with previous studies is required.\n4. As shown in Fig.5a, a proton exchange membrane (PEM) was employed between anode chamber and central solid-electrolyte (SE). Why the authors choose alkaline solution (0.1M KOH) as electrolyte in anode chamber. In addition, whether K+ ions migrate into the middle chamber? Please detect the impurity ions possibly existed in the middle chamber.\n5. I wonder whether the HER was existed under working conditions with NiB2N2 catalyst?\n6. A few spelling mistakes were found in lines 57 and 233. Please correct these errors and check the whole manuscript again to revise the spelling and grammar errors.", "reviewer_3": "This manuscript investigates an important catalytic system based on Ni–B–N coordination structures for enhancing oxygen reduction reaction (ORR) performance. The authors present a series of structural characterizations, including in situ Raman, XANES, EXAFS and DFT analyses, to support the proposed reconstruction mechanism from Ni–B₂N₂ to Ni–B₁N₂ during ORR operation. The manuscript aims to correlate this structural evolution with improved electrochemical activity through electronic modulation and charge redistribution around Ni sites. The study is timely and relevant, and the authors combine various advanced characterization techniques to understand the structure–activity relationship. While the manuscript provides interesting insights, several critical points require clarification to strengthen the validity and internal consistency of the proposed mechanism. Clarifying these issues will improve the reliability and scientific impact of the manuscript.\n1. In the Supplementary Information, the EDS elemental mapping in the TEM images does not include labels identifying each element. Please provide proper elemental annotations to ensure clarity and reproducibility.\n2. The manuscript claims that the Ni–B₂N₂ structure reconstructs into a Ni–B₁N₂ configuration during the ORR process. If one B atom is removed during this transformation, where does the detached B species migrate or terminate? Additionally, if B is partially lost from the coordination environment, how is the Ni oxidation state maintained at +0.98? A mechanistic explanation or supporting evidence is needed to reconcile this discrepancy.\n3. In situ Raman spectroscopy indicates the formation of OOH species on Ni sites during ORR, yet XANES analysis shows that the Ni oxidation state remains unchanged at +0.98. If OOH binds to Ni and participates in the reaction, some change in the Ni valence would generally be expected. Please clarify why the Ni oxidation state appears invariant despite the formation of OOH intermediates and provide additional justification for this interpretation.\n4. Although XPS data are presented in the Supplementary Information, the manuscript does not discuss these results. The XANES section states that the Ni oxidation state increases from +0.88 to +0.98 due to enhanced Ni–B bonding and B-mediated charge transfer (Supplementary Fig. S13). To substantiate this claim, please elaborate on the XPS results and describe how the XPS spectra support the proposed electronic redistribution from Ni to B."}, "author_response": "1 Response to the reviewers’ comments (Highlighted in blue)\n2\n4 production through two-electron oxygen reduction reaction (2e⁻ ORR) at industrial current density. Series\n5 single-atom Ni catalysts with B/N coordination (NiB N ) was designed as a model system to exhibit\nx y\n6 remarkable structure flexibility under electrochemical working reactions. Through a combination of\n7 experimental characterization and theoretical calculations, the author demonstrates that the self-adjustment\n8 of Ni–B and Ni–N bonds stabilizes the active center and then achieves exceptional H O selectivity and\n2 2\n9 long-term stability. This work reveals a comprehensive understanding on structural self-adaptation\n10 mechanism for 2e⁻ ORR and offers a new design strategy for stable H O production under practical\n2 2\n11 industrial conditions. I suggest that this paper can be accepted pending several revisions. Detailed\n12 comments were listed as follows.\n13 1. The authors propose a trans-configuration of the B and N atoms in NiB N coordination structure (Figure\n2 2\n14 1i, 2c, 2g). However, a cis-configuration of B and N is also possible. There is no evidence and explanation\n15 to rule out the possibility of cis-configuration. Additional DFT calculations are required to validate the\n16 trans-configuration structure for NiB N .\n2 2\n19 of different configurations within the NiB N coordination environment. As shown in Supplementary Fig.\n2 2\n20 16, we calculated the formation energies (E) for both the trans- and cis-configurations of the B and N atoms.\nf\nf\n22 eV) compared with the cis-configuration (E = -4.91 eV). The lower E of the trans-arrangement suggests\nf f\n23 that this configuration is thermodynamically favorable，thus more likely to be formed under the studied\n1\n24 conditions. Therefore, the trans-configuration was adopted in our subsequent analyses. The corresponding\n25 explanation was added in the revised manuscript (page 6).\n2 2\n27 the formation energies (E). The trans-configuration has a lower formation energy (E = -5.96 eV) compared\nf f\n28 to the cis-configuration (E = -4.91 eV), indicating that this configuration is thermodynamically more\nf\n29 favorable (Supplementary Fig. 16).\n30\n2 2 2 2\n32\n33 2. The TEM image of Figures 1b-1d can be modified on the figure, with specific names and details labeled.\n35 structures diagram in Figures 1 (a, i) and Figure 3 (c, g).\n37 we have revised the annotations of several Figures to improve the interpretability. Specifically:\n38 (1) In Figure 1b, the catalysts (NiB N ) are clearly labeled.\n2 2\n39 (2) In Figures 1c–d, the circled regions have been annotated to highlight the formation of isolated Fe\n40 single atoms.\n2\n41 (3) Additionally, detailed elemental legends have been added to Fig. 1 (a, i) and Fig. 3 (c, g), clarifying\n42 the atomic species represented by each color in the structural diagrams.\n44 is presented more clearly and accessibly.\nx y\n46\n47 a Synthetic route and possible scheme of the single atom NiB N catalyst. b TEM image of NiB N . c High-\n2 2 2 2\n48 resolution HAADF-STEM image of NiB N ; the red circles indicate the atomically dispersed Ni centers in\n2 2\n49 the carbon aerogel. d Pseudo-color surface plot corresponding to the yellow circle in HAADF-STEM\n50 image. Red shows the single Ni atoms, and blue is the carbon matrix. e TEM and corresponding element\n3\n51 mapping of NiB N . f WT-EXAFS of Ni K-edge for NiB N and Ni foil samples. g Ni K-edge XANES, h\n2 2 2 2\n2 2 2 2\n53 structure.\n54\n2 2\n56\n4\n57 a Operando Ni K-edge XANES spectra of NiB N at the OCP and different potentials. b Corresponding Ni\n2 2\n2 2\n59 diagram of the in-situ XAS device. e Fitting results of the coordination number with Ni sites. f In-situ\n2 2 2\n61 the catalysis mechanism and structure evolution for the NiB N in the 2e⁻ ORR.\n2 2\n62\n63 3. The authors indicate that the single-atom Ni is the primary active site. However, it has been reported in\n64 the previous literatures that boron-doped carbon (B-C) can also be the active sites for electrosynthesis of\n2 2\n67 single-atom Ni sites from that of boron-doped carbon (B-C), we conducted a control experiments in the\n68 revised manuscirpt.\n70 selectively block the single-atom Ni centers. Upon addition of SCN⁻, both the disk and ring currents in the\n2 2\n72 pronounced suppression in activity and selectivity indicates that the single-atom Ni sites dominate the\n73 catalytic process for the two-electron oxygen reduction reaction.\n2 2\n2 2\n77 selectivity for H O dropping from 94% to approximately 47%, demonstrated that Ni serves as the primary\n2 2\n78 active sites for the ORR process (Supplementary Fig.21).\n5\n79\n2 2 2 2\n81 detection currents at the ring electrode in 0.1 mol L-1 KOH with 10 mM KSCN. (b) LSV corresponding\n82 electron transfer number and H O selectivity.\n2 2\n83\n84 4. The single-atom Ni catalysts with different coordination environments were obtained by varying\n85 pyrolysis temperature. Why did the author choose this strategy? How about other preparation methods to\n86 modulate the coordination environment of Ni atoms?\n88 environment of single-atom catalysts (SACs) critically influences their electronic structure and,\n89 consequently, catalytic activity, selectivity, and stability. In addition, lot of alternative methods have been\n90 explored to tailor coordination environments beyond thermal treatment, such as solvothermal synthesis,\n91 wet-chemical routes, support functionalization, and electrodeposition. These approaches can offer finer\n6\n92 control over the first and second coordination shells and enable coordination motifs that may be difficult to\n93 access solely by pyrolysis. (Matter 2025, 8, 11, 102424)\n95 current system: (i) it enables straightforward tuning of coordination environments by a single parameter\n96 (temperature) without requiring complex precursor design or additional post-modification steps, and (ii) it\n97 effectively creates strong interactions between Ni atoms and doped carbon supports, enhancing stability\n98 under electrochemical conditions. Therefore, thermal pyrolysis remains a robust and practical strategy for\n99 modulating single-atom coordination environments. (Acc. Chem. Res. 2025, 58, 12, 1878–1892, Adv.\n102 environment of Ni single atoms. High-temperature pyrolysis enables in situ formation and stabilization of\n103 isolated metal sites on carbon matrices through strong metal-support interactions, while also allowing\n104 adjustment of the local bonding environment by tuning the degree of carbonization and nitrogen doping.\n4 x y\n107 been commonly adopted in the synthesis of SAC due to its simplicity, scalability, and ability.\n109 catalysts and the regulation of their coordination structures, the relevant description have been added to the\n110 revised manuscript (Page 5).\n112 carbon matrices through strong metal-support interactions, while also allowing adjustment of the local\n113 bonding environment by tuning the degree of carbonization and nitrogen doping. The facile modulation of\n114 pyrolysis temperature enables direct control the Ni-N/B coordination motifs, allowing flexible and tunable\n115 adjustment of the coordination structure.\n7\n116 5. The authors are suggested to measure the actual pH value of H O solution produced in the middle\n2 2\n117 chamber. Meanwhile, the catalytic performance of 2e⁻ ORR with NiB N should also be measured by RRDE\n2 2\n118 under this pH value, thereby demonstrating its effectiveness under real working conditions.\n120 the actual pH of the H O solution accumulated in the middle chamber during operation. As shown in\n2 2\n2 2\n2 2\n2 4\n124 chamber (Supplementary Fig. 35 and Table. 7). The results show that NiB N maintains a high H O\n2 2 2 2\n125 selectivity of 95% and an electron-transfer number close to 2.0 under this pH condition, which is\n126 comparable to its performance in the 0.1 M KOH.\n2 2 2 2\n128 electrosynthesis under the real pH conditions encountered in the middle-chamber configuration.\n2 2\nSample pH\nproduced H O solution 5.8\n2 2\n130\n8\n131\n2 2 2 2\n133 detection currents at the ring electrode in 0.1 mol L-1 K SO (pH=5.8). (b) LSV corresponding electron and\n2 4\n2 2\n135\n136 6. Please check the bond length changes as indicated in Figure 3c and the data in the Supporting\n139 comments, we have thoroughly rechecked the bond-length variations presented in Fig. 3c and the\n140 corresponding structure was listed in the Supplementary Information. The values have now been corrected\n141 and fully aligned between the main text and the Supplementary Information (please refer to the updated\n143 supplied the Gibbs free-energy changes (ΔG) for the key elementary steps involved in the 2e⁻ and 4e⁻ ORR\n144 pathway over the NiB N active site.\n2 2\n9\n2 2\n146\n147 a Operando Ni K-edge XANES spectra of NiB N at the OCP and different potentials. b Corresponding Ni\n2 2\n2 2\n149 diagram of the in-situ XAS device. e Fitting results of the coordination number with Ni sites. f In-situ\n10\n2 2 2\n151 the catalysis mechanism and structure evolution for the NiB N in the 2e⁻ ORR.\n2 2\n152\n2 2 4\nElectrocatalyst O *OOH *O *OH H O\n2 2\nNiB N 4.92 3.83 2.42 0.72 0\n2 2\nNiN 4.92 4.55 3.91 1.95 0\n4\n154\n2 2 4\nElectrocatalyst O *OOH H O\n2 2 2\nNiB N 4.92 3.83 3.39\n2 2\nNiN 4.92 4.55 3.09\n4\n156\n157 7. In Figure 1e, the annotation of O/or C atom is not clear. Please check and make the corrections.\n159 of the C element in the NiB N catalyst, with the annotations for the C element having been modified.\n2 2\nx y\n11\n161\n162 a Synthetic route and possible scheme of the single atom NiB N catalyst. b TEM image of NiB N . c High-\n2 2 2 2\n163 resolution HAADF-STEM image of NiB N ; the red circles indicate the atomically dispersed Ni centers in\n2 2\n164 the carbon aerogel. d Pseudo-color surface plot corresponding to the yellow circle in HAADF-STEM\n165 image. Red shows the single Ni atoms, and blue is the carbon matrix. e TEM and corresponding element\n166 mapping of NiB N . f WT-EXAFS of Ni K-edge for NiB N and Ni foil samples. g Ni K-edge XANES, h\n2 2 2 2\n2 2 2 2\n168 structure.\n169\n12\n170 8. The authors should provide the standard PDF card for ZIF-8 in Supplementary Figure 7.\n173 one , indicating that ZIF-8 has been successfully prepared and the Ni doping has not altered its crystal\n174 structure.\n175\nx y\n177 catalysts.\n178\n179 9. Regarding the literature comparison chart in Figure 5e, the target catalyst in this work is an atomically\n180 dispersed catalyst. Its H O yield should be compared with the H O production rates of the same type of\n2 2 2 2\n181 atomically dispersed catalysts (such as Co, Mn, etc.). It is necessary to appropriately increase the types of\n182 comparative catalysts to highlight its own innovation.\n13\n184 comparison should include atomically dispersed catalysts reported for 2e⁻ ORR, particularly those based\n185 on transition-metal single atoms.\n187 this work. Specifically, the representative Co, Ni single-atom and carbon-based catalysts that reported in\n188 recent studies were selected. As shown in Fig. 5e, the updated comparison shows that the desinged NiB N\n2 2\n189 catalyst exhibited a competitive H O yield relative to these state-of-the-art SACs under comparable\n2 2\n190 operating conditions. This observation not only presents the highly catalytic reactivity of the NiB N , but\n2 2\n191 also demonstrates its unique self-regulating capability under reaction conditions. The corresponding\n192 description have been provided on page 17 of the revised manuscript.\n2 2\n194 device, surpassing almost reported Co, Ni single-atom and carbon-based catalysts in both 2e⁻ ORR activity\n195 and durability (Fig. 5e and Supplementary Table 9).\n196\n2 2 2 2\n198 electrocatalysts in a solid-electrolyte cell.\n199\n14\n2 2\nCurrent density Production rate\nNumber Catalyst Stability Ref.\n(mA cm-2) (mmol 𝐠−𝟏 𝒉−𝟏)\n𝐜𝐚𝐭𝐚𝐥𝐲𝐬𝐭\n20 814.9\n50 2006.1\n100 3971.1 300 h@300 This\n2 2\n150 4866.3 mA cm-2 work\n200 6267.8\n300 9320.0\n18 h@37.5 mA\ncm-2\n50 390.6\n100 718.3 10 h@200 mA\n200 1526.5 cm-2\n400 3125.0\n20 173.7\n40 347.4\n500 h@50 mA\ncm-2\n80 645.2\n100 744.5\n25 1000.0\n50 2000.0\n100 h@120 mA\ncm-2\n175 5600.0\n200 6500.0\n15\n20 1250.0\nBBL- 40 2200.0 200 h@530 mA\n6 5\nPcNi/CB 60 3700.0 cm-2\n100 5800.0\n100 1600.0\n150 2700.0\n200 h@100 mA\ncm-2\n250 4100.0\n300 4250.0\n10 600\n20 1300\n40 1575\n100 h@100 mA\nNiSA- 50 2375\n8 cm-2 7\nNiO/pCNs\n100 4250\n140 6250\n200 9250\n230 11000\n15 195 96 h@30 mA\n30 435 cm-2\n154 h@200 mA\ncm-2\n10 2000\nNiFeW 140 h@200 mA\n11 15 2800 10\nLDH/OCNTs cm-2\n30 3500\n16\n80 5199\n140 8799\n310 10580\n201\n203 1. Zhang, H. et al. Integrating H O generation from electrochemical oxygen reduction with the\n2 2\n204 selective oxidation of organics in a dual-membrane reactor. Chem. Eng. J. 428, 131534 (2022).\n205 2. Ruggiero, B. N. et al. Local reaction microenvironment impacts on H O electrosynthesis in a dual\n2 2\n206 membrane electrode assembly solid electrolyte electrolyzer. Chem. Eng. J. 486, 150246 (2024).\n207 3. Zhang, X. et al. Electrochemical oxygen reduction to hydrogen peroxide at practical rates in strong\n208 acidic media. Nat. Commun. 13, 2880 (2022).\n209 4. Xia, C. et al. Direct electrosynthesis of pure aqueous H O solutions up to 20% by weight using a\n2 2\n210 solid electrolyte. Science 366, 226-231 (2019).\n211 5. Zhang, M.-D. et al. Continuous electrosynthesis of pure H O solution with medical-grade\n2 2\n212 concentration by a conductive Ni-phthalocyanine-based covalent organic framework. J. Am. Chem.\n214 6. Long, Y. et al. Tailoring the atomic-local environment of carbon nanotube tips for selective H O\n2 2\n215 electrosynthesis at high current densities. Adv. Mater. 35, 2303905 (2023).\n216 7. Wang, J., et al. Catalyst-engineered proton transfer pathways for selective hydrogen peroxide\n217 electrosynthesis in solid-state electrolytes. Angew. Chem. Int. Ed. 64, e202510645 (2025).\n218 8. Zhao, Y., et al. On-site H O2 electro-generation process combined with ultraviolet: A promising\n2\n219 approach for odorous compounds purification in drinking water system. Chem. Eng. J. 430, 132829\n220 (2022).\n17\n221 9. Liu, C., et al. Substrate curvature enhances molecular electrocatalysts for the efficient production of\n222 hydrogen peroxide. Nano Lett. 25, 16013-16019 (2025).\n223 10. Lin, S., et al. Electrochemical pilot H O production by solid-state electrolyte reactor: Insights from a\n2 2\n224 hybrid catalyst for 2-electron oxygen reduction reaction. Angew. Chem. Int. Ed. 64, e202502144\n225 (2025).\n226\n227 10. What is the guiding role of the structural adaptability of NiB N in future practical production fields?\n2 2\n229 related studies sharing a similar topic are suggested to be incorporated (Adv. Funct. Mater. 2025, 35,\n230 2417090; Appl. Catal. B: Environ. 2024, 343, 123533).\n232 supplemented and the related studies were incorporated into the revised discussion (Adv. Funct. Mater.\n233 2025, 35, 2417090; Appl. Catal. B: Environ. 2024, 343, 123533).\n2 2\n235 with high performance and stability. This catalyst exhibited high stability to generate pure hydrogen\n236 peroxide (H O ) under industrial current density (300 mA cm-2), offering the guidelines for the distributed\n2 2\n237 and on-site electrochemical synthesis of hydrogen peroxide solutions. To demonstrate the practical\n238 application efficacy of this system, we evaluated the degradation of representative pollutants (\ntetracycline,\n239 sulfamethoxazole, and phenol) via Fenton reactions driven by the generated H O . Experimental details are\n2 2\n240 provided in the Supplementary Information. As shown in Supplementary Fig. 36, The results indicate that\n241 the on-site produced H O via our NiB N catalyst enables high removal efficiency (>99%, 5 min) for tested\n2 2 2 2\n242 pollutant species.\n244 manuscript (page 17).\n18\n2 2\n246 cm-2 in the solid-state electrolysis cell, and then flowed it into the heterogeneous Fenton reactor for the\n247 degradation of tetracycline (TC), Ciprofloxacin (CIP), sulfamethoxazole (SMX), and phenol (PhOH). After\n248 a 5 min reaction, the detected concentrations of the pollutants were all below the detection limit, and the\n249 removal rate was nearly 100% (Supplementary Fig. 36). This exceptional H O production and rapid\n2 2\n250 degradation demonstrate that NiB N is a promising candidate for scale-up electrochemical H O\n2 2 2 2\n251 generation, and further highlights the significant potential of its on-site produced H O for highly effective\n2 2\n252 environmental remediation, validating the unique advantages of self-adaptive structural catalysts for\n253 industrial-scale applications.\n254\n2 2\n257\n260 solutions (10 ppm of tetracycline, ciprofloxacin, sulfamethoxazole, or phenol in 30 mL DI water\n261 (pH = 3) were prepared. To each solution, 2 mg of FeSO 4 was added to initiate the Fenton reaction\n19\n262 (2 Fe2+ + H 2 O 2 → 2 Fe3+ + ·OH + OH⁻). The H 2 O 2 was produced in situ from the 2e⁻ ORR in the\n263 middle chamber of a solid electrolyte cell (non-recirculating mode). The produced H 2 O 2 was then\n264 continuously introduced into the pollutant-Fe2+ mixture to carry out degradation at room\n265 temperature and atmospheric pressure.\n266\n268 with a flexible coordination structure capable of varying coordination numbers and bond lengths. The\n269 designed Ni SACs have high selectivity and stability for pure H O production via two-electron oxygen\n2 2\n270 reduction reaction (2e⁻ ORR) at industrial current density. Through a combination of in-situ experimental\n271 characterizations and theoretical calculations, the study reveals that a potential-driven structure evolution\n272 of initial Ni+0.98-B N configuration transforms into Ni+0.98-B N , which is identified as the genuine active\n2 2 1 2\n273 site under working conditions. Overall, this manuscript is well organized and provides very interesting\n274 observation regarding the precise design of 2e⁻ ORR electrocatalysts. However, before publication, major\n275 revisions are required to address few issues in the manuscript. The detailed comments are listed as follows.\n276 1. Why the Ni+0.98-B N configuration can be reversibly recovered to their initial electronic configuration\n1 2\n277 after removing the cathodic bias? How can the authors confirm this variation, it seems no experimental\n278 evidence supporting this phenomenon. In addition, whether the traditional Ni-N configuration has the\n4\n279 structure-adaptive performance under working conditions?\n281 address these points, and the new data are included in the Supplementary Information.\n2 2\n283 applied potential, we have supplemented the XANES spectral, FT-EXAFS pectra, and cooresponding\n284 fitting results of the catalyst after potential removal in the operando XAFS measurements (labeled as\n2 2\n20\n286 catalyst is nearly identical to that of the pristine NiB N , the oxidation state of Ni remains stable and\n2 2\n287 unchanged. More importantly, detailed EXAFS fitting (Supplementary Table 4) reveals that the\n288 coordination numbers of Ni-B and Ni-N recover to approximately 2.31 and 2.49, respectively, closely\n289 matching the initial NiB N structure. These results provide direct experimental evidence that the NiB N\n2 2 2 2\n290 configuration undergoes a reversible reconstruction during electrocatalysis.\n4\n292 conducted operando XAFS measurements under varying applied potentials. The results, presented in the\n294 edge XANES spectra decreases progressively as the potential becomes more negative, indicating the Ni\n295 center was reduced during the operation process. This contrasts sharply with the NiB N , demonstrating\n2 2\n296 that the Ni-N lacks the dynamic and reversible structure flexibility.\n4\n2 2\n298 configuration and highlight the unique self-regulating capabilities that distinguishes it from conventional.\n4\n300 manuscript.\n2 2\n302 after reaction confirm its reversible structural reconfiguration under operando conditions (Supplementary\n4\n304 potential, demonstrating a static configuration lacking self-regulating capability (Supplementary Fig. 29).\n305\n21\n306\n2 2\n308 potential. (b) Corresponding Ni K-edge FT-EXAFS spectra of NiB N . (c) The corresponding in-situ\n2 2\n2 2\n310\n22\n311\n2 2\n313 potential.\n314\n2 2\n316 potentials.\nPotential\nCoordination Bond length R factor\n(V vs. Cathode Bond type\nnumber R (Å) (100%)\nRHE)\nNi-B 2.20 1.94\nEx-situ NiB N 0.013\n2 2\nNi-N 2.37 1.80\nNi-B 2.28 1.90\nOCP NiB N 0.017\n2 2\nNi-N 2.15 1.80\nNi-B 2.27 1.91\n0.8 NiB N 0.014\n2 2\nNi-N 2.05 1.81\nNi-B 1.10 1.88\n0.6 NiB N 0.016\n2 2\nNi-N 2.17 1.85\n0.4 NiB N Ni-B 1.10 1.81 0.014\n2 2\nNi-B 1.15 1.76\n0.2 NiB N 0.013\n2 2\nNi-N 2.18 1.91\nReoved Ni-B 2.31 1.97\nNiB N 0.013\n2 2\npotential Ni-N 2.49 1.83\n317\n318 2. The concentration of atomic Ni is not provided in this manuscript; please carry out the additional\n319 characterization, such as inductively coupled plasma mass spectrometry (ICP-MS) to confirm the content\n320 of Ni in the series of catalysts.\nx y\n324 temperatures were determined to be 0.43 wt% (NiN ), 0.40 wt % (NiB N ), 0.36 wt % (NiB N ), 0.22 wt %\n4 2 2 1 3\n325 (NiN ), respectively.\n3\n327 supported the conclusion that isolated Ni centers are responsible for the observed catalytic behavior.\nx y 4\n330 wt % (NiB N ), 0.36 wt % (NiB N ), 0.22 wt % (NiN ), respectively, by inductively coupled plasma optical\n2 2 1 3 3\n331 emission spectroscopy (ICP-OES).\nx y\nCatalysts NiN NiB N NiB N NiN\n4 2 2 1 3 3\nICP (wt%) 0.43 0.40 0.36 0.22\n24\n333\n334 3. The NiB N catalyst demonstrates high selectivity and faraday efficiency of 2e− ORR across a wide\n2 2\n335 potential range (0.2-0.6 V vs. RHE). How about the value of Faraday efficiency compared with previous\n336 studies? A comparison of Faraday efficiency with previous studies is required.\n338 with reported stuidies in recent literatuers to Supplementary Table 3. The NiB N demonstrates a Faraday\n2 2\n339 efficiency of 95%, surpassing most of catlysts reported in the recent studies.\n2 2\n341 transition metal single-atom catalysts.\nOnset potential\nSelectivity Faraday Efficiency\nNum. Catalyst Ref.\n(V vs. RHE) (100%) (100%)\n2 2\n3 2\n342\n25\n344 11. Ross, R. D. et al. Operando elucidation of electrocatalytic and redox mechanisms on a 2D metal\n345 organic framework catalyst for efficient electrosynthesis of hydrogen peroxide in neutral media. J.\n347 12. Wang, Y. et al. Tailoring the d-band center of Ni on a dual-single-atom Ni–ZnNC catalyst for\n348 efficient H O production. Mater. Today Energy 38, 101459 (2023).\n2 2\n349 13. Lin, Z. et al. Defect engineering induced high metal loading Co-single-atom catalyst on carbon dots\n350 for efficient H O electrosynthesis. Chem. Eng. J. 501, 157661 (2024).\n2 2\n351 14. Yuan, J. et al. Superior efficiency hydrogen peroxide production in acidic media through epoxy\n352 group adjacent to Co-O/C active centers on carbon black. Chem. Eng. J. 465, 142691 (2023).\n353 15. Lim, J. S. et al. Understanding the preparative chemistry of atomically dispersed nickel catalysts for\n354 achieving high-efficiency H O electrosynthesis. Chem. Sci. 15, 13807-13822 (2024).\n2 2\n355 16. Hu, J. et al. Uncovering dynamic edge-sites in atomic Co−N−C electrocatalyst for selective\n356 hydrogen peroxide production. Angew. Chem. Int. Ed. 62, e202304754 (2023).\n357 17. Jia, S. et al. Modulated nickel single-atom sites as highly active catalysts for the synthesis of neutral\n2 2\n359 18. Li, Y. et al. Single-atom iron catalyst with biomimetic active center to accelerate proton spillover for\n360 medical-level electrosynthesis of H O disinfectant. Angew. Chem. Int. Ed. 62, e202306491 (2023).\n2 2\n361 19. Liu, L. et al. Atomically dispersed asymmetric cobalt electrocatalyst for efficient hydrogen peroxide\n362 production in neutral media. Nat. Commun. 15, 4079 (2024).\n363\n364 4. As shown in Fig.5a, a proton exchange membrane (PEM) was employed between anode chamber and\n365 central solid-electrolyte (SE). Why the authors choose alkaline solution (0.1M KOH) as electrolyte in anode\n26\n366 chamber. In addition, whether K+ ions migrate into the middle chamber? Please detect the impurity ions\n367 possibly existed in the middle chamber.\n369 the potential migration of K⁺ ions were carefully considered in the design of our solid-electrolyte (SE)-\n370 based reactor.\n372 reaction (OER) kinetics and to maintain the chemical stability of the anode catalyst and bipolar membrane\n373 interface. As reported in previous PEM/anion-hybrid configurations, alkaline anolytes significantly reduce\n374 the overpotential of OER, thereby improving overall reactor durability during long-term H O\n2 2\n375 electrosynthesis.\n377 solution collected in the middle chamber. ICP-MS results (Supplementary Table 8) indicate that the\n378 concentration of K⁺ in the middle chamber after 10 hours of operation remains extremely low (2 ppm),\n379 confirming that the PEM effectively suppresses cation migration. This negligible K⁺ level demonstrates\n380 that the membrane preserves high ionic selectivity under our operating conditions. The relevant descriptions\n381 have been added into the revised manuscript (Page 17). The details are as follows:\n2 2\n383 electrolyte (KOH), and the concentration of K+ ions was below 2 ppm (Supplementary Table 8). This\n384 negligible K⁺ level demonstrates that the membrane preserves high ionic selectivity under our operating\n385 conditions.\n2 2\nTime (h) 0 2 4 6 8 10\n0 1.89 1.68 1.32 1.98 1.12\n(ppm)\n387\n388 5. I wonder whether the HER was existed under working conditions with NiB N catalyst?\n2 2\n390 to evaluate HER activity within the potential window used for 2e⁻ ORR. As shown in Supplementary Fig.\n391 26, the linear sweep voltammetry (LSV) curves collected in O -saturated electrolyte reveals that the\n2\n392 overpotential for the HER reaction at the NiB N electrode surface is -0.48V, indicating that the HER\n2 2\n393 activity of this catalyst is relatively poor, and the ORR reaction mainly occurs under the operating voltage\n394 range.\n2 2\n396 electrosynthesis with the NiB N catalyst, and the 2e⁻ ORR pathway is the dominant cathodic process.\n2 2\n397\n398\n2 2\n400\n28\n401 6. A few spelling mistakes were found in lines 57 and 233. Please correct these errors and check the whole\n402 manuscript again to revise the spelling and grammar errors.\n404 have been corrected. In addition, we have thoroughly re-examined the entire manuscript and revised all\n405 spelling, grammar, and typographical errors to ensure clarity and consistency throughout the text.We thank\n406 the reviewer again for this helpful suggestion, which has improved the quality of the manuscript.\n407\n409 structures for enhancing oxygen reduction reaction (ORR) performance. The authors present a series of\n410 structural characterizations, including in situ Raman, XANES, EXAFS and DFT analyses, to support the\n411 proposed reconstruction mechanism from Ni-B N to Ni-B N during ORR operation. The manuscript aims\n2 2 1 2\n412 to correlate this structural evolution with improved electrochemical activity through electronic modulation\n413 and charge redistribution around Ni sites. The study is timely and relevant, and the authors combine various\n414 advanced characterization techniques to understand the structure–activity relationship. While the\n415 manuscript provides interesting insights, several critical points require clarification to strengthen the\n416 validity and internal consistency of the proposed mechanism. Clarifying these issues will improve the\n417 reliability and scientific impact of the manuscript.\n418 1. In the Supplementary Information, the EDS elemental mapping in the TEM images does not include\n419 labels identifying each element. Please provide proper elemental annotations to ensure clarity and\n420 reproducibility.\n422 mapping images in the Supplementary Fig. 5 and Supplementary Fig. 6. Each map now includes clear,\n423 direct labels (e.g., “B,” “N,” “Ni,” “C”) on the figure panels themselves.\n424\n29\n425\n1 3\n427\n428 .\n3\n430\n431 2. The manuscript claims that the Ni-B N structure reconstructs into a Ni-B N configuration during the\n2 2 1 2\n30\n433 migrate or terminate? Additionally, if B is partially lost from the coordination environment, how is the Ni\n434 oxidation state maintained at +0.98? A mechanistic explanation or supporting evidence is needed to\n435 reconcile this discrepancy.\n437 of NiB N .\n2 2\n2 2 1 2\n439 configuration, which is a potential-driven and reversible coordination reconstruction. As we have already\n440 invesitaged in the manuscript that, upon the removal of applied potential, the catalyst reverts to the initial\n2 2\n442 transformation. Otherwise, this reversible structural evolution between NiB N and NiB N would not occur.\n2 2 1 2\n444 literatures during electrolysis 20-22, which confirms again that the B atom is not removed from the sinle\n445 atom catalysts.\n2 2\n447 the obtained results were shown in Supplementary Fig. 28. As expected, the XAFS spectral features and\n448 fitted structural parameters of NiB N before and after reaction remained almost the same, demonstrating\n2 2\n449 again that the reconstructed NiB N site can reversibly recover to the initial NiB N configuration upon the\n1 2 2 2\n450 removal of applied potential. The above results indicate that during the electrochemical reconfiguration\n451 process, one of the Ni-B bonds in NiB N was broken, leaving the B atom on the original carbon substrate,\n2 2\n452 while this bond will be regenerated when the potential was withdrawn.\n454 cleavage of metal-ligand bond and/or adsorption of oxygen-containing intermediates are generally\n455 accompanied with the changes in oxidation state of active sites. However, operando XANES spectra show\n456 that the Ni absorption edge position and white-line intensity remain essentially unchanged over the\n457 investigated potential range (Fig. 3a), indicating that the single-atom Ni exhibits a stable oxidation value\n31\n458 (+0.98) throughout the whole reaction process. The stability of the Ni oxidation state duing electrolysis\n459 does not mean a static active site. This evolution of bond lengths suggests pronounced local electronic\n460 rearrangement at the Ni center during electrocatalysis. Operando EXAFS analysis clearly reveals\n461 continuous reconstruction of the first coordination shell of Ni atom. Upon application of potential, NiB N\n2 2\n462 reconstructs into the NiB N configuration, and with progressively application of more negative potentials,\n1 2\n463 the Ni-B bond in NiB N continuously shortens, while the Ni-N bonds undergo concomitant elongation.\n1 2\n465 electronegative B to withdraw electron density effectively from the Ni center. Concurrently, the elongated\n468 shortening of the Ni-B and the elongation of the Ni-N caused by potential energy can reasonably be\n469 interpreted as an enhancement or weakening of the electron coupling between Ni and B/N. This reflects the\n470 redistribution of electron density along the Ni-B/N coordination network. In this case, the dynamic\n471 modulation of the Ni-B/N coordination can effectively buffer the local electronic perturbations caused by\n472 electrochemical polarization and intermediate adsorption, thereby stabilizing the apparent oxidation state\n473 of nickel.\n475 redox-active or electron-flexible components have been reported in recent studies. For example, Yu et al.\n476 demonstrated that CeO supports with reversible Ce3+/Ce4+ redox couples and highly mobile oxygen\n2\n477 vacancies can act as electronic reservoirs, enabling Rh clusters maintained in a stable +2 state with a well-\n478 defined electronic distribution23. Likewise, Xiang et al. constructed Fe–Ru dual single-atom catalysts,\n479 where directional electron transfer between adjacent Ru and Fe atoms effectively mitigated electronic\n480 polarization at the Fe site during catalysis, thereby markedly enhancing the stability of the Fe-N\n481 coordination environment24. These studies confirm that regulating local electronic distribution via electron-\n32\n482 buffering is an effective strategy to stabilize both the oxidation state and coordination structure of metal\n483 centers under electrocatalytic conditions.\n485 coordination reconstruction of the metal sites. These results indicate that the NiB N catalyst can adapt its\n2 2\n486 local coordination environment to changes in the reaction microenvironment during electrocatalysis, which\n487 is consistent with recent advances in adaptive-structure single-atom catalysts. In order to further explain\n488 the reason why the valence state of Ni in NiB N remains unchanged, the corresponding explanation has\n2 2\n489 been added to the revised manuscript (Page 11-12).\n2 2\n491 the reversible cleavage and reformation of a Ni-B bond, which transiently generates the active sites of\n1 2 2 2\n493 the Ni-B and elongation of the Ni-N, reflecting electron‑density redistribution within the first coordination\n494 shell without a net change in the nickel oxidation state. In this coordination environment, when the Ni site\n495 experiences transient electron gain or loss induced by electrode polarization or *OOH adsorption,\n496 electron-deficient B can reversibly accept or donate electron density through enhanced Ni-B coupling,\n497 thereby modulating fluctuations in the Ni 3d electron population. Meanwhile, the concurrent contraction\n498 of Ni-B and the elongation of Ni-N bonds facilitated the partial delocalization of electron density from the\n501 electrochemical polarization and intermediate adsorption, thereby stabilizing the apparent oxidation state\n502 of Ni.\n33\n503\n2 2\n505 potential. (b) Corresponding Ni K-edge FT-EXAFS spectra of NiB N . (c) The corresponding in-situ\n2 2\n2 2\n508 20. Wei, X., et al. Dynamic Reconstitution Between Copper Single Atoms and Clusters for\n510 21. Wu, J., et al. Electric bias-induced reversible configuration of single and heteronuclear dual-atom\n511 catalysts on 1Tʹ-MoS2. Nat. Nanotechnol. 20, 1043-1051 (2025).\n512 22. Yang, J., et al. Potential-Driven Restructuring of Cu Single Atoms to Nanoparticles for Boosting\n513 the Electrochemical Reduction of Nitrate to Ammonia. J. Am. Chem. Soc. 144, 12062-12071\n514 (2022).\n34\n515 23. Yu, Z.-K., Jiang, M., Dai, S., Zhan, W., Wang, Z.-Q., Gong, X.-Q. Valence restrictive metal-\n516 support interaction for boosting catalytic activity of Rh/CeO in CO hydrogenation. Nat.\n2 2\n518 24. Yu, H., Li, C., Lei, Y., Xiang, Z. Strategic secondary coordination implantation towards efficient\n519 and stable Fe-N-C electrocatalysts for the oxygen reduction reaction in PEMFCs. Angew. Chem.\n521\n522 3. In situ Raman spectroscopy indicates the formation of OOH species on Ni sites during ORR, yet XANES\n523 analysis shows that the Ni oxidation state remains unchanged at +0.98. If OOH binds to Ni and participates\n524 in the reaction, some change in the Ni valence would generally be expected. Please clarify why the Ni\n525 oxidation state appears invariant despite the formation of OOH intermediates and provide additional\n526 justification for this interpretation.\n528 sites would result in the change of Ni valence. However, XANES analysis in this work indicates that the\n530 deficient electron density at the Ni center can be redistributed through the Ni–B/N coordination network,\n531 rather than being localized on the Ni atom itself. Operando EXAFS provides direct evidence for this process,\n532 showing continuous contraction of the Ni-B bond under working potentials, which reflects enhanced\n533 electronic coupling and charge redistribution between Ni and B. This structural response indicates that the\n535 stabilizing the Ni oxidation state. For comparison, we conducted an in-situ XANES test on the NiN catalyst.\n4\n537 became more negative. The test results further indicated that B atoms could enable the rapid transfer of\n538 charges through the Ni-B bond during the formation of the Ni-B N -OOH intermediate, thus maintaining\n1 2\n539 the relatively stable oxidation state of the monatomic Ni. Similar mechanisms have been reported in recent\n35\n540 dual-atom catalyst systems. Xiang et al. constructed a Fe-Ru dual-single-atom catalyst in which cooperative\n541 electron transfer between adjacent Ru and Fe atoms effectively buffers the electron polarization induced by\n542 the formation of ORR intermediates (*OOH, *O, *OH) at Fe sites. This electron-buffering effect suppresses\n543 the passive elongation of Fe-N bonds, stabilizes the Fe-N coordination environment, and simultaneously\n4\n544 optimizes the electronic structure of Fe, leading to enhanced intrinsic ORR activity24.\n545\n547 24. Yu, H., Li, C., Lei, Y., Xiang, Z. Strategic secondary coordination implantation towards efficient\n548 and stable Fe-N-C electrocatalysts for the oxygen reduction reaction in PEMFCs. Angew. Chem.\n550\n551 4. Although XPS data are presented in the Supplementary Information, the manuscript does not discuss\n552 these results. The XANES section states that the Ni oxidation state increases from +0.88 to +0.98 due to\n553 enhanced Ni–B bonding and B-mediated charge transfer (Supplementary Fig. S13). To substantiate this\n554 claim, please elaborate on the XPS results and describe how the XPS spectra support the proposed\n555 electronic redistribution from Ni to B.\n557 incorporating a discussion of the XPS results in the revised manuscript (page 6). The corresponding\n558 description directly elaborates how the spectral features substantiate the electronic redistribution from Ni\n559 to B, thereby strengthening the evidence for the B-mediated charge transfer as concluded from XANES.\n3 4 1 3\n2 2\n36\n563 peak in NiB N shifts negatively (~0.2 eV vs. NiB N ), suggesting an increased electron density on B.\n2 2 1 3\n565\n566\nx y\nx y\n569\nx y\n37"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have well addressed the concerns. The revised version of this manuscript could be considered for publication.", "reviewer_2": "The revised manuscript addresses the comments well, and it can be accepted as it is.", "reviewer_3": "I have carefully read the revised manuscript and the authors’ rebuttal. My overall assessment is that, while the text has been expanded and additional discussion/data have been added, the core mechanistic deficiencies raised in my previous report remain unresolved. In particular, several central conclusions are still based on inference rather than direct evidence, and the new arguments do not satisfactorily reconcile the apparent inconsistency between adsorbate formation (in situ Raman) and the reported invariant Ni oxidation state (in situ XANES). Therefore, I do not believe the current revision adequately supports the key claims.\n1. The revised response still does not provide convincing evidence that B is actually located on/in the carbon substrate (or exists as a carbon-associated B species). At present, the revision mainly establishes that B is present in the catalyst, but it does not unambiguously distinguish whether B is bonded to carbon (B–C), bonded to Ni (Ni–B), present as boron–oxygen species (B–O/borate-like), or retained as residual precursor-derived species. Although the authors argue that the reversible restoration of the NiB2N2 structure upon potential removal implies B is retained, this indirect inference acts only as circumstantial evidence. The fact that the structure recovers does not prove that B was anchored to the carbon substrate during the reaction; it essentially assumes the conclusion it tries to prove. Because the proposed role of B in tuning/stabilizing the active site depends critically on B’s true chemical environment, the statement that B exists on the carbon substrate remains an assumption rather than a demonstrated conclusion.\n2. In the revision, the authors attribute the essentially unchanged in situ XANES-derived oxidation state to electronic stabilization from Ni–B and Ni–N bonding. Specifically, they claim that operando EXAFS shows a continuous shortening of the Ni-B bond and elongation of the Ni-N bond under negative potentials, which acts as an 'electron-buffering' mechanism. However, the evidence provided still does not convincingly demonstrate that this specific coordination change is the reason the XANES oxidation state does not vary. The current argument remains largely interpretive; mere bond length changes do not automatically prove an oxidation state buffer. The authors have not provided any quantitative analysis or simulation connecting the specific bond length values to the magnitude of charge transfer required to offset the potential bias. Thus, invoking Ni–B/Ni–N structural evolution as the sole explanation lacks sufficient direct proof.\n3. The central inconsistency I raised persists: the revision maintains that in situ Raman supports OOH adsorption on Ni sites, yet the in situ XANES analysis continues to report an essentially invariant Ni oxidation state (≈ +0.98) during reaction. The authors attempt to resolve this by comparing the system with a NiN4 control sample, stating that NiN4 shows a decrease in oxidation state while NiB2N2 remains stable due to B-mediated charge transfer. However, this additional in situ NiN4 dataset is not an appropriate support for this claim. It represents a different coordination system and does not directly validate why the Ni–B/Ni–N catalyst behaves differently. Comparing two different systems does not explain the physical anomaly of the NiB2N2 system itself—specifically, why the strong oxophilic nature of the OOH intermediate does not withdraw electrons from Ni, especially when operando XANES white-line intensity remains unchanged. Therefore, the authors have not provided a convincing mechanistic reconciliation for why Ni can bind/host OOH while showing no detectable redox response, and the contradiction remains unresolved."}, "author_response": "Response to the reviewer’s comments (Revisions in the main text highlighted in blue)\nI have carefully read the revised manuscript and the authors’ rebuttal. My overall assessment is that,\nwhile the text has been expanded and additional discussion/data have been added, the core\nmechanistic deficiencies raised in my previous report remain unresolved. In particular, several\ncentral conclusions are still based on inference rather than direct evidence, and the new arguments\ndo not satisfactorily reconcile the apparent inconsistency between adsorbate formation (in situ\nRaman) and the reported invariant Ni oxidation state (in situ XANES). Therefore, I do not believe\nthe current revision adequately supports the key claims.\nResponse:\nWe sincerely appreciate the careful evaluation of our revised manuscript and the rigious,\ninsightful comments provided. These valuable suggestions have helpled us further clarify our\ninterpretations and strengthen the evidence supporting the proposed mechanism.\nIn response to your assessment that the key mechanistic issues remain insufficiently resolved,\nwe carried out the potential-dependent DFT calculation and operando near-ambient pressure X-ray\nphotoelectron spectroscopy (NAP-XPS) during re-revision. Specifically, the additional DFT\nsimulations under different applied potentials were conducted to quantitatively analyze the charge\nredistribution and coordination evolution within the first coordination shell. These results help us to\nbetter understand the correlation among bond-length variation, charge transfer, and the preservation\nof the Ni oxidation state under working conditions. Moreover, we performed operando NAP-XPS\nto experimentally validate the proposed B/N-mediated electron-buffering mechanism. The\npotential-dependent shifts in B 1s and N 1s binding energies, together with quantitative analysis of\nB/N coordination network during the 2e⁻ ORR. We hope that the combination of potential-\ndependent DFT analysis and operando NAP-XPS measurements can offer more direct and\ncomplementary evidences to better address your concerns and strengthen the mechanistic\nfoundation of our conclusions.\n1. The revised response still does not provide convincing evidence that B is actually located\non/in the carbon substrate (or exists as a carbon-associated B species). At present, the revision\nmainly establishes that B is present in the catalyst, but it does not unambiguously distinguish\nwhether B is bonded to carbon (B–C), bonded to Ni (Ni–B), present as boron–oxygen\nspecies (B–O/borate-like), or retained as residual precursor-derived species. Although the authors\nargue that the reversible restoration of the NiB N structure upon potential removal implies B\n2 2\nis retained, this indirect inference acts only as circumstantial evidence. The fact that the\nstructure recovers does not prove that B was anchored to the carbon substrate during the\nreaction; it essentially assumes the conclusion it tries to prove. Because the proposed role of B\nin tuning/stabilizing the active site depends critically on B’s true chemical environment, the\nstatement that B exists on the carbon substrate remains an assumption rather than a\ndemonstrated conclusion.\nResponse:\nMany thanks for your valuable comments regarding the chemical environment and stability of B\nunder reaction conditions. Following your comments, we carried out additional experiments to\naddress these concerns in the revised manuscript.\n(1) To further clarify the chemical state of B under electrochemical polarization, we conducted\n2\noperando near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). This technique\nallows direct probing of B 1s electronic states under working conditions. The experimental\ntechnologies were introduced in detail in the revised Supplementary Information (Page 5-6, SI),\nand\nthe corresponding results are shown in Figure 4f, 4g, and Figure S34. Spectral deconvolution of the\nB 1s region shows that B predominantly exists as B–C species within the carbon framework,\naccompanied by minor B–N and B–O contributions. Under applied cathodic potentials, the B 1s\nbinding energy progressively shifts to lower values (from 190.5 eV under ex-situ/OCP to 190.0 eV\nat 0.2 V vs. RHE). This negative shift indicates an increase in electron density at the B sites during\nthe 2e⁻ ORR, which demonstrates that B actively participates in charge redistribution during reaction\nand functions as an electron-accepting buffer. Importantly, the N 1s peak exhibits a similar shift\ntoward lower binding energy with increasingly negative potential, further confirming the dynamic\ncharge redistribution within the B/N coordination network.\nFurthermore, quantitative peak fitting of the B 1s spectra reveals that the fraction of B–O\nspecies increases from 15.3% to 22.0% under increasingly negative potentials. The increase in B–\nO content indicates that Ni–B bond cleaves during electrochemical reconstruction. The dissociated\nB atom subsequently coordinates with OH⁻ from the electrolyte to form a surface-stabilized B–OH\nspecies. Concurrently, the reconstructed NiB N moiety serves as the genuine active site for O\n1 2 2\nadsorption and HOO–NiB N intermediate formation. We think that the operando NAP-XPS data\n1 2\nnot only confirm the retention of B within the catalyst but also directly reveal its dynamic chemical\nevolution and participation in the reconstruction process.\n(2) To determine whether B leaches into the electrolyte during operation, we performed inductively\ncoupled plasma optical emission spectroscopy (ICP–OES) analysis of the electrolyte before and\n3\nafter 4 hours of continuous electrolysis. The obtained results are shown in Supplementary Table 8\n(Page 14, SI). As expected, almost no changes of aqueous B concentration was detected after the\nORR. This result demonstrates that B is not released into the electrolyte during the reaction and\nexcludes the possibility that B exists predominantly as soluble borate-like species. The absence of\nleached B in solution supports that B remains associated with the catalyst framework throughout\nthe reaction.\nThis clarification has been added in the revised manuscript (Page 17, Lines 334–349). We hope that\nthese additional data and clarifications could better address the concerns.\nTo directly validate the proposed B/N-mediated electron-buffering mechanism, we employed\noperando near-ambient pressure XPS (NAP-XPS) to monitor the evolution of the electronic states\nof B and N atoms during the 2e⁻ ORR (Fig. 4f-g, Supplementary Fig. 34). Spectral deconvolution\nreveals that B predominantly exists as B–C bond within the carbon framework, accompanied by\nminor contributions from B–N and B–O species. The B 1s binding energy is centered at 190.5 eV\nunder ex-situ and OCP conditions. As the potential gradually varies from 0.6 to 0.2 V vs. RHE, this\npeak progressively shifts toward lower binding energy of approximately 190.0 eV (0.2 V vs. RHE),\nindicative of the increased electron density at the B sites. Concurrently, the proportion of B-O\nspecies increased from 15.3% to 22.0% with the increasing potential, further indicating the single-\nnickel sites undergo reconfiguration during the reaction process. Specifically, when one Ni–B bond\ncleaves, the liberated B atom coordinates with OH⁻ to form a B–OH species anchored on the surface.\nMeanwhile, the reconstructed NiB N moiety serves as the active site for O adsorption to drive the\n1 2 2\nformation of the HOO–NiB N intermediate. Conversely, the N 1s peak shifts toward lower binding\n1 2\nenergy by approximately 0.15 eV with increasingly negative potential, signifying an increase in the\n4\nelectron density at N sites. These results agree well with the DFT predictions and provide more solid\nevidence for potential-driven charge redistribution and buffering within the B/N coordination\nnetwork during the 2e⁻ ORR process.\nFigure 4 f B 1s high-resolution NAP-XPS spectra showing distinct shifts in binding energy with\naltered potential. g N 1s high-resolution NAP-XPS spectra showing distinct shifts in binding energy\nwith altered potential.\n2. In the revision, the authors attribute the essentially unchanged in situ XANES-derived\noxidation state to electronic stabilization from Ni–B and Ni–N bonding. Specifically, they claim\nthat operando EXAFS shows a continuous shortening of the Ni-B bond and elongation of the\nNi-N bond under negative potentials, which acts as an 'electron-buffering' mechanism.\nHowever, the evidence provided still does not convincingly demonstrate that this specific\ncoordination change is the reason the XANES oxidation state does not vary. The current\nargument remains largely interpretive; mere bond length changes do not automatically prove\n5\nan oxidation state buffer. The authors have not provided any quantitative analysis or simulation\nconnecting the specific bond length values to the magnitude of charge transfer required to offset\nthe potential bias. Thus, invoking Ni–B/Ni–N structural evolution as the sole explanation lacks\nsufficient direct proof.\nResponse:\nWe sincerely thank for your insightful comment regarding the relationship between coordination\nevolution and the invariant Ni oxidation state observed in operando XANES. Really, bond-length\nvariations alone are insufficient to establish an oxidation-state buffering mechanism, and we\nappreciate the opportunity to clarify this point with additional quantitative analysis. During the new-\nround revision, we strengthened this section by incorporating further thermodynamic and electronic\nstructure analyses, as detailed below.\n(1) First, to evaluate whether the reconstructed structure is energetically justified under reaction\nconditions, we calculated the reaction free energy of the potential-determining step (*O → *OOH)\n2\nfor both NiB N and reconstructed NiB N configurations under applied potentials. As shown in the\n2 2 1 2\nrevised Fig. 4c and Supplementary Tables 5–6 (Page 15, Lines 305–307), the formation of *OOH\nis exothermic on both structures; however, the reaction energy on NiB N (−1.57 eV) is more\n1 2\nfavorable than that on NiB N (−0.72 eV). This substantial thermodynamic advantage indicates that\n2 2\nelectrochemical reconstruction generates a more energetically favorable active centre for H O\n2 2\nproduction. Therefore, the structural evolution is not merely speculative but thermodynamically\ndriven and directly linked to improved catalytic performance.\n(2) More importantly, to address your concern regarding the connection between coordination\nevolution and oxidation-state stability, we performed potential-dependent differential charge density\n6\nand Bader charge analyses for the reconstructed NiB N configuration under applied cathodic\n1 2\npotentials from 0.6 to 0.2 V vs. RHE (Fig. 4d–e, Supplementary Table 7). The results reveal the\npronounced charge redistribution within the first coordination shell, while the charge on the Ni atom\nitself varies by less than 0.03 e across the entire potential range. In contrast, significant electron\naccumulation occurs on the coordinated ligands. The Bader charge on the B site evolves from +1.49\ne at 0.6 V to +1.42 e at 0.2 V, while the coordinated N atoms show an increase in electron density\nfrom -1.13 e to -1.20 e. Simultaneously, the surrounding carbon framework donates charge under\npolarization. These quantitative results clearly demonstrate that potential-induced electrons are\npredominantly accommodated by the B/N coordination network rather than localized on the Ni\ncentre, thereby explaining why the apparent Ni oxidation state remains unchanged in operando\nXANES analysis.\n(3) The calculated Ni-B and Ni-N bond lengths are in agreement with the structural evolution\nobserved in operando XANES analysis. As the potential decreases, the Ni–B bond contracts\nprogressively from 1.95 Å (ex situ) to 1.85 Å at 0.2 V, whereas the Ni–N bond elongates from 1.97\nÅ to 2.02 Å. Interestingly, the potential-dependent DFT and operando NAP-XPS analysis indicated\nan increase in the charge densities of both B and N sites, which seems inconsistent with the bond-\nlength evolutions. This phenomenon can be explained by the distinct nature of their frontier orbitals,\nwhich may exhibit bonding character for the Ni–B interaction but anti-bonding character for the Ni–\nN interaction. Thus, electronic redistribution leads to differential structural responses in these two\ncoordination paths.\nTaken together, the thermodynamic analysis (−1.57 eV vs. −0.72 eV), quantitative Bader\ncharge data (<0.03 e variation on Ni), and orbital-level interpretation could establish a link between\n7\nstructural evolution and electronic self-equilibration within the first coordination shell. The\ninvariant Ni oxidation state observed in operando XANES is not contradictory to coordination\nchange; rather, it reflects a ligand-assisted electron-buffering mechanism in which the B/N network\ndynamically accommodates potential-induced charge flux while preserving the electronic stability\nof the Ni centre.\nThese additional quantitative analyses and mechanistic clarifications have been incorporated\nin the revised manuscript (Page 15–16, Lines 290–333). We hope that this expanded and data-\nsupported discussion could better address your concern.\nThe 2e− ORR activity and selectivity of NiB N were elucidated through systematic density\nx y\nfunctional theory (DFT) calculations. As shown in the activity-volcano plot (Fig 4a and\nSupplementary Fig 30), NiN and NiN located at the right branch of the activity-volcano plot\n4 3\npossessed weak *OOH adsorption, resulting in poor ORR activity. Upon the introduction of B atoms,\nNiB N andNiB N show strengthened *OOH adsorption and shift to the left branch of the activity-\n1 3 2 2\nvolcano plot. Notably, NiB N achieves a more moderate *OOH binding strength, with the\n2 2\noptimized structure positioned closest to the apex of the limiting potential volcano (U = 0.35 V,\nL\n∆*G = 3.49 eV). This balanced adsorption energy facilitates efficient protonation of *OOH to\nOOH\nH O , enabling superior 2e⁻ ORR performance41, 42. The free energy diagram at U = 0.7 V further\n2 2\nconfirms the high 2e⁻ ORR selectivity of NiB N which exhibits the lowest overpotential (0.04 V)\n2 2，\nfor the 2e− pathway (Fig 4b). Electronic structure analysis attributes this optimized adsorption to an\nupshifted d-band center of Ni relative to NiN , which strengthens the orbital coupling between Ni-\n4\n3d and O-2p states of *OOH (Supplementary Fig 31-32). These calculations demonstrated that the\nstatic NiB N configuration possesses an optimized electronic structure for the 2e⁻ ORR.\n2 2\nWe further investigated the dynamic reconstruction of *OOH at the Ni site during operation, and\nelucidated the mechanism responsible for the constant Ni oxidation state to remain invariant. As\nshown in Fig. 4c and Supplementary Table 5-6, the potential-determining step from *O to *OOH\n2\nis exothermic on both NiB N and NiB N , with the reaction energy of -0.72 eV and -1.57 eV,\n2 2 1 2\nrespectively43, 44. The more exothermic nature on the reconstructed NiB N site indicates that\n1 2\ndynamic reconstruction creates a thermodynamically superior active center for the 2e⁻ ORR,\nthereby\nrationalizing the high performance observed experimentally. These results indicate that the dynamic\nreconstruction creates a superior active center (NiB N ) for the 2e⁻ ORR, thereby rationalizing the\n1 2\nexperimental onset of high performance.\nTo elucidate the interaction between single atom Ni and *OOH during the reaction process, we\ncalculated the charge density distribution and the corresponding evolution of Ni-B/N bond lengths\nin the NiB N under applied different cathodic potentials (0.2-0.6 V vs. RHE). The differential\n1 2\ncharge density and quantitative Bader charge analyses reveal a pronounced charge redistribution\nwithin the first coordination shell of the Ni single atom in response to the applied potential (Fig. 4d-\ne, Supplementary Table 7). Upon application of the potential, Ni−B bond cleavage leads to the\nstructural transformation from NiB N to NiB N structure. The dissociated B atom subsequently\n2 2 1 2\ncoordinates with OH⁻ in the electrolyte to form a B−OH species, which remains stably anchored on\nthe catalyst surface. The Bader charge analysis indicates that the charge transferred from Ni atoms\nto the adsorbed *OOH intermediate gradually increases as the applied potential decreases. Specially,\nthe transferred charge was 0.31, 0.39, and 0.49 e at 0.6, 0.4, and 0.2 V (vs. RHE), respectively.\nCompared with the original NiB N structure (+1.56), the coordinated B atom exhibits electron\n2 2\naccumulation of +1.49, +1.46, and +1.42 e at 0.6, 0.4, and 0.2 V, respectively. At the same time, the\ncoordinated N atoms display a slight increase in electron density, with the charge varying from -\n1.13 to -1.20 e. Unlike B and N atoms, the total charge density of the surrounding carbon framework\ndecreases from -1.38 to -0.55 e. Crucially, the charge on the Ni atom changes by less than 0.03 e,\ndemonstrating the remarkable stability of its apparent oxidation state. Accordingly, with the\npotential increases from ex-situ to 0.2 V vs. RHE, the calculated Ni-B bond length progressively\ncontracts from 1.95 Å to 1.93 Å, 1.89 Å and then to 1.85 Å, whereas the Ni-N bond elongates from\n1.97 Å to 1.97 Å, 1.99 Å, and then to 2.02 Å. The calculated Ni-B and Ni-N bond lengths are in\nagreement with the structural evolution observed in operando XANES analysis. These results\nindicate that the observed bond-length evolution represents the charge redistribution within the first\ncoordination shell. The B/N ligand network dynamically offer electron-buffering effect, thereby\nmaintaining the unchanged Ni oxidation state.\nFig. 4. Catalytic enhancement mechanism of NiB N .\n2 2\n10\na 2e− ORR activity volcano plot of NiB N and analogues with different coordination environments.\n2 2\nb Free energy diagram of 2e- ORR at 0.70 V of NiBxNy. c Free energy diagram of 2e-/4e- ORR of\nNiB N and NiB N . d The charge difference density of NiB N under different applied potential.\n2 2 1 2 1 2\nThe blue and yellow areasrepresent the decrease and increase in charge density respectively. e\nElectron gain/loss and bond length change of NiB N under 2e − ORR with different applied\n1 2\npotential. f B 1s high-resolution NAP-XPS spectra showing distinct shifts in binding energy with\naltered potential. g N 1s high-resolution NAP-XPS spectra showing distinct shifts in binding energy\nwith altered potential.\n3. The central inconsistency I raised persists: the revision maintains that in situ Raman\nsupports OOH adsorption on Ni sites, yet the in situ XANES analysis continues to report an\nessentially invariant Ni oxidation state (≈ +0.98) during reaction. The authors attempt to\n11\nresolve this by comparing the system with a NiN4 control sample, stating that NiN shows a\n4\ndecrease in oxidation state while NiB N remains stable due to B-mediated charge transfer.\n2 2\nHowever, this additional in situ NiN dataset is not an appropriate support for this claim. It\n4\nrepresents a different coordination system and does not directly validate why the Ni–B/Ni–N\ncatalyst behaves differently. Comparing two different systems does not explain the physical\nanomaly of the NiB N system itself—specifically, why the strong oxophilic nature of the OOH\n2 2\nintermediate does not withdraw electrons from Ni, especially when operando XANES white-\nline intensity remains unchanged. Therefore, the authors have not provided a convincing\nmechanistic reconciliation for why Ni can bind/host OOH while showing no detectable redox\nresponse, and the contradiction remains unresolved.\nResponse:\nWe sincerely appreciate your insightful comments. We agree that a convincing mechanistic\nreconciliation requires direct structural and electronic evidence under working conditions. In this\nnew-round revision, we therefore performed a comprehensive analysis combining operando EXAFS,\npotential-dependent DFT calculations, and operando near-ambient pressure XPS (NAP-XPS)\nmeasurements to provide more solid evidence for the proposed mechanism.\nAs is well known, operando Ni K-edge XAFS measurements have been widely applied to\nreveal the coordination reconstruction processes during the 2e⁻ ORR. As shown in Fig. 3a-3b, the\nXANES white-line peak for Ni K-edge remains essentially unchanged and indicates a stable Ni\noxidation state of approximately +0.98 under cathodic polarization, with the corresponding FT-\nEXAFS spectra showing pronounced potential-dependent structural evolution. Specifically, the Ni–\nB bond length decreases progressively from 1.94 Å (ex situ) to 1.88 Å at 0.6 V, 1.81 Å at 0.4 V, and\n12\n1.76 Å at 0.2 V vs. RHE. In contrast, the Ni–N bond length increases from 1.80 Å to 1.85 Å, 1.88\nÅ, and finally 1.91 Å over the same potential range. Simultaneously, the coordination number\nevolves from the initial NiB N configuration to NiB N under cathodic conditions. These data\n2 2 1 2\ndemonstrate that a structural reconstruction indeed occurs during reaction, even though the apparent\nNi oxidation state remains nearly invariant. A key mechanistic question lies in the coexistence of\ndynamic coordination evolution of the single-atom Ni sites with the oxidation-state stability.\nTo understand this reconstruction pathway, we established potential-dependent DFT models to\nreflect the operando EXAFS observations. The calculations show that under applied cathodic\npotential, one Ni–B bond cleaves within the NiB N motif. The dissociated B atom subsequently\n2 2\ncoordinates with OH⁻ from the electrolyte to form a surface-stabilized B–OH species, while the\nremaining Ni center adopts a NiB N configuration that serves as the genuine active site for O\n1 2 2\nadsorption. The reconstructed NiB N could stabilize the HOO–NiB N intermediate, consistent\n1 2 1 2\nwith the in-situ Raman observation of *OOH formation. Importantly, the calculated bond-length\nevolution closely matches the operando EXAFS trends. The Ni–B bond contracts from 1.95 Å to\n1.85 Å, whereas the Ni–N bond elongates from 1.97 Å to 2.02 Å as the potential decreases to 0.2 V.\nQuantitative Bader charge analysis further shows that the charge on Ni changes by less than 0.03 e\nunder polarization, whereas electron density accumulates predominantly on the B and N ligands.\nFor example, the coordinated B atom exhibits electron accumulation values of +1.49, +1.46, and\n+1.42 e at 0.6, 0.4, and 0.2 V, respectively, compared to +1.56 in the original NiB N structure.\n2 2\nMeanwhile, the coordinated N atoms show increased electron density from −1.13 to −1.20 e, and\nthe surrounding carbon framework displays corresponding charge redistribution. These results\nindicate that electron transfer during OOH adsorption is primarily buffered within the B/N\n13\ncoordination network rather than stored on the Ni center itself.\nTo experimentally verify this reconstruction and ligand-assisted charge redistribution\nmechanism, we performed operando NAP-XPS measurements. The B 1s peak, initially centered at\n190.5 eV under ex situ and OCP conditions, progressively shifts toward lower binding energy\n(approximately 190.0 eV at 0.2 V vs. RHE), indicating increased electron density at B sites under\ncathodic polarization. Concurrently, the proportion of B–O species increases from 15.3% to 22.0%\nwith decreasing potential. This increased B-O concentration supports the formation of surface B–\nOH species, consistent with the DFT observation. In parallel, the N 1s peak shifts by approximately\n0.15 eV toward lower binding energy under increasingly negative potential, confirming electron\naccumulation on N ligands. These operando spectroscopic results provide experimental evidence\nthat B and N dynamically accommodate potential-induced charge flux and participate in structural\nreconstruction during the reaction.\nTaken together, we believe that operando EXAFS establishes coordination reconstruction\n(NiB N → NiB N ), DFT quantitatively reproduces the bond-length evolution and demonstrates\n2 2 1 2\nminimal Ni charge variation (<0.03 e-), and operando NAP-XPS directly confirms B–OH formation\nand ligand-based electron accumulation. During the ORR reaction, although *OOH adsorbs at the\nNi center, the electron transfer required for its formation is mediated and buffered by the surrounding\nB/N coordination network. Consequently, the Ni oxidation state remains nearly invariant despite\nactive intermediate formation.\nThese clarifications and additional analyses have been incorporated into the revised manuscript\n(see Pages 12–17, Figs. 3 and 4). We respectfully hope that the combined operando and theoretical\nevidence can provide a more solid explanation on your concerns.\n14"}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors have intensively revised and corrected the manuscript according to the comments. The authors added new reference papers, experiment results and detail explanation about each section. In this way, this work is suitable for publication in."}, "author_response": ""}]

Decision letter