Dynamic ligand-vacancy engineering drives metal dimerization for efficient urea electrooxidation

Domain: biology
Published: 2026-03-23
Editorial outcome: major_revision

Abstract
The electrochemical urea oxidation reaction offers environmental benefits by enabling hydrogen generation and nitrogen recycling. However, catalyst instability caused by surface reconstruction remains a challenge. Here, we develop a heteronuclear vacancy-to-bond strategy that achieves both catalytic activation and structural preservation via atomic-level self-optimization. Using Fe-doped bimetallic frameworks, we construct a self-adaptive coordination microenvironment that dynamically generates controllable ligand vacancies while promoting metal dimerization, leading to shortened interatomic distances. The resulting ligand-vacancy-mediated stabilization delivers an low potential of 1.222 V @ 10 mA cm-2 (188 mV lower than IrO2) with 87.7% Faradaic efficiency for nitrogen oxides. Spectroscopic analysis and theoretical calculations reveal that ligand-deficient structure reduces the C–N cleavage energy from 1.33 eV to 0.75 eV and shifts the rate-determining step from chemical C–N cleavage to potential-dependent *NO oxygenation, lowering the overall energy requirement. In industrial-scale electrolyzers, the catalyst sustains 1 A cm-2 for 100 hours with negligible degradation, achieving 13% energy savings over conventional water splitting. This work investigates a dynamic vacancy-to-bond conversion mechanism, offering insights into the design of adaptive electrocatalysts for sustainable energy applications.

Introduction
Alkaline water electrolysis is widely regarded as a sustainable hydrogen production method, especially when integrated with renewable energy sources for carbon-neutral operation1,2. However, the sluggish kinetics of the oxygen evolution reaction (OER), which involves multiple proton-coupled electron transfer steps, requires high overpotentials and limits energy efficiency3,4. In contrast, the urea oxidation reaction (UOR) presents a promising alternative, owing to its much lower theoretical oxidation potential (0.37 V vs. RHE) compared to OER (1.23 V vs. RHE) 5,6,7. Urea is abundantly available in industrial wastewater, agricultural byproducts, and human or animal urine, offering advantages such as low toxicity, cost-effectiveness, and easy handling8,9. This dual functionality—combining wastewater treatment and hydrogen production—positions UOR as a model for the circular economy. As a result, UOR has gained significant attention as an energy-efficient route for hydrogen production10,11,12. Despite its advantages, current UOR catalysts based on noble metals face major challenges—including scarcity, high costs, and poor durability—that limit their industrial scalability. Moreover, a fundamental challenge for the UOR field as a whole is the high kinetic overpotential, which keeps the operating potentials of most catalysts, including state-of-the-art systems, significantly above the low thermodynamic potential of UOR.13,14,15.

To overcome these limitations, developing non-precious metal catalysts with high activity, long-term stability, and economic viability has become a key priority in electrocatalysis. Among various candidates, metal–organic frameworks (MOFs) are particularly promising due to their high surface area, tunable pore structures, and multiple active sites adaptable to specific reactions16,17,18. However, maintaining the structural integrity of MOFs under harsh electrochemical conditions remains a major challenge.19 Studies show that in alkaline media, coordination bonds between metal nodes and organic linkers degrade during activation, leading to irreversible transformation into metal oxyhydroxides20,21,22. This is mainly due to ligand leaching during surface reconstruction23,24. Previous studies attributed the enhanced activity of reconstructed MOFs solely to increased surface area or amorphous phase formation, failing to reveal their precise post-reconstruction atomic structure—a critical insight achievable only through a combined experimental-theoretical approach. However, the underlying structural evolution mechanisms, such as the nature of the active centers and the behavior of ligands during transformation, remain unclear25.

Here, we propose a heteronuclear vacancy-to-bond strategy to resolve the activity-stability trade-off in MOF electrocatalysts. Leveraging the distinct thermodynamic driving forces and kinetic stabilities of Fe and Co under high-potential, OH--rich alkaline conditions, Fe doping in cobalt single-metal frameworks (CoSMF) generates a self-adaptive dynamic reconstruction microenvironment. This architecture enables site-specific ligand vacancy generation while suppressing full ligand leaching and hydroxide transformation. Specifically, ligands remain anchored at Fe sites, whereas vacancies form selectively at Co positions. This spatial modulation promotes metal dimerization with strengthened metal-metal interactions at ligand-connected sites, facilitating direct bonding. Theoretical and experimental analyses confirm that ligand leaching during dynamic reconstruction critically impacts catalytic performance. Consequently, the optimized DR-FeCoBMF catalyst achieves competitive UOR performance, delivering 10 mA cm-2 at 1.222 V with <1% degradation over 60 hours of continuous operation. Furthermore, a urea-assisted AEM electrolyzer using FeCoBMF operates stably for 100 hours at 1 A cm-2, outperforming most reported UOR catalysts, including noble-metal references. This work highlights the crucial role of ligand vacancies in MOF dynamic reconstruction and introduces heteronuclear vacancy-to-bond strategy as a general design principle for robust electrocatalysts in sustainable energy applications.

Results
2.1. Theory predictions and Materials synthesis
To predict how ligand-vacancy-induced structural reconstruction governs UOR performance in aromatic dicarboxylate-based MOFs via a heteronuclear vacancy-to-bond strategy, we initiated density functional theory (DFT) calculations. As illustrated in Fig. 1a, this approach leverages two heterometals to form M1-O-M2 oxygen-bridge motifs, where M-atoms coordinate with organic linkers into layered frameworks. A secondary model incorporating ligand vacancies was constructed through spatially selective modulation—controlled ligand dissociation at some Co sites—while Fe sites anchored ligands to prevent leaching. DFT analysis of bimetallic FeCo-MOFs revealed that selective ligand removal induced significant coordination-environment alterations, shortening the intermetal distance (from 3.57 Å to 2.80 Å, Fig. 1b-c) and promoting direct metal–metal bonding, thereby driving dimeric restructuring. The underlying driving force for this process is the thermodynamic tendency to minimize system energy after ligand vacancy generation. The coordinatively unsaturated metal centers produced upon ligand removal are highly energetic and seek stabilization. Forming direct metal–metal bonds enables these metal centers to achieve coordination saturation, lower surface energy, and stabilize the electronic structure, ultimately resulting in the observed dimerization. This reconstructed bimetallic framework preserved the parent topology, suppressing irreversible phase transformation to metal oxyhydroxides. Furthermore, ligand vacancies and metal–metal bonds synergistically modulated the electronic structure of metal centers, evidenced by a downshifted d-band center (Fig. 1c), which weakens intermediate adsorption, avoiding excessive binding during the UOR.

In light of the above theoretical results, the catalysts were synthesized via a solvothermal method by immersing pretreated nickel foam (NF) into a homogeneous solution containing metal salts, aromatic dicarboxylic acid ligands, and a solvent mixture of H2O, ethanol, and DMF (Fig. S1 and S2). To investigate the influence of ligand loss during dynamic reconstruction on UOR catalytic performance, we employed FeSMF, FeCoBMF, and CoSMF as precursors, synthesized using metal salts with varying Lewis acidity. Scanning electron microscopy (SEM) revealed that the synthesized FeCoBMF exhibits a layered nanosheet morphology with vertical alignment on the NF substrate (Fig. S3). Energy-dispersive X-ray spectroscopy (EDX) elemental mapping confirmed uniform Fe and Co distribution, indicating successful doping (Fig. S4 and S5). High-resolution transmission electron microscopy (HR-TEM) showed clear lattice fringes with an interplanar spacing of 1.36 nm (Fig. S6), consistent with reported values for dicarboxylate-based MOFs, thereby confirming the retention of the parent MOF’s structural periodicity after heterometallic incorporation26,27. Powder X-ray diffraction (XRD) patterns demonstrated that FeCoBMF shares similar diffraction peaks with CoSMF and differs substantially from FeSMF (Fig. S7), indicating that Fe incorporation does not alter the original CoSMF crystal phase. These results suggest a successful partial substitution of Co sites by Fe without introducing phase impurities28.

In 1 M KOH + 0.33 M urea solution, repeated linear sweep voltammetry tests were performed on FeSMF, FeCoBMF and CoSMF until the catalytic performance stabilized, as shown in Fig. 2a-c and S8a-c. This electrochemical activation triggered dynamic reconstruction of the electrode materials, we constructed three representative structural models with varying degrees of ligand loss, illustrated in Fig. 2d. One model retains the intact layered-pillared architecture of the parent MOF without detectable ligand leaching. The second features partial ligand leaching forming ligand vacancies while preserving the parent framework, where direct metal–metal bonding begins to form within the metal atoms initiates metal dimerization. The third model describes a complete collapse of the MOF framework into hydroxide or oxide phases as a result of full ligand removal. Notably, FeSMF reached stable performance after only 3 cycles, suggesting minimal structural alteration, with a 36 mV reduction in overpotential at 100 mA cm-2. CoSMF stabilized after 13 cycles, reducing its overpotential by 55 mV at 100 mA cm-2. In contrast, FeCoBMF exhibited significant performance enhancement after 12 cycles, lowering the overpotential at 100 mA cm-2 by 120 mV. These results demonstrate that the structure with ligand vacancies formed through partial ligand leaching delivers optimal catalytic performance and the most significant dynamic reconstruction enhancement29,30.

2.2. Investigation on the dynamic reconstruction
To gain deeper insights into the role of heteronuclear atomic pairs in improving catalytic performance and regulating ligand leaching during the dynamic reconstruction process, we conducted field-emission scanning electron microscopy (FESEM) to monitor the structural evolution of catalysts before and after electrocatalysis. As shown in Fig. 2e–g and Fig. S9–S11, FeSMF exhibits disordered nanosheets growing in random orientations, resembling spiky protrusions akin to a hedgehog’s quills. This morphology remains largely unchanged before and after dynamic reconstruction, suggesting minimal structural evolution. In contrast, CoSMF initially presents a densely packed lamellar morphology with narrow interlayer gaps, which collapses into a bulk-like agglomerate following dynamic reconstruction, reflecting severe structural degradation. Therefore, the Fe-carboxylate bonds typically exhibit higher kinetic inertness (more hydrolysis-resistant), while the Co-carboxylate bonds are relatively more susceptible to nucleophilic attack by OH- and hydrolytic cleavage31,32. As expected, FeCoBMF displays a loosely stacked layered structure with relatively large interlayer spacing in its pristine state, which becomes smoother and more compact after activation, indicating a moderate degree of surface reorganization without compromising structural integrity. High-resolution transmission electron microscopy (HRTEM) further supports these findings. As shown in Fig. 2h-i and Fig. S12, FeCoBMF retains a well-defined lattice fringe spacing of approximately 1.5 nm after dynamic reconstruction, closely matching its pre-reconstruction value of 1.36 nm. This consistency indicates that the layered MOF structure is well preserved throughout the reconstruction process. In contrast, CoSMF exhibits a significantly reduced fringe spacing of only 0.45 nm after dynamic reconstruction, markedly deviating from the characteristic spacing of layered metal–ligand frameworks. This suggests a complete collapse of the CoSMF structure, leading to the formation of disordered CoOx(OH)y species33. The core of this strategy lies in leveraging the intrinsic bond stability differences between Fe and Co ions and the organic ligands, rooted in fundamental coordination chemistry principles. Specifically, the less stable Co–O bonds, attributable to the borderline Lewis acidity of Co2+ and its lower ligand field stabilization energy in an octahedral field compared to high-spin Fe3+, are selectively cleaved under electrochemical conditions. This selective cleavage generates ligand vacancies. The coordinatively unsaturated metal centers adjacent to these vacancies then tend to form direct metal–metal bonds to achieve a lower-energy state. Concurrently, the highly stable Fe–O bonds, a result of the strong interaction between the hard acid Fe3+ and the hard base carboxylate, along with the high kinetic inertness of the Fe3+ octahedral site, act as structural "anchors." These anchors effectively suppress the complete collapse of the framework and its irreversible phase transformation into inactive metal hydroxides. This design enables a self-limiting reconstruction from partial ligand leaching to a stable metal-dimer architecture. Therefore, the local Fe sites in the MOFs may serve to some extent as a "host" or "stabilizing agent", while Co sites serve as targeted "precursor sites" where selective ligand leaching creates vacancies essential for metal dimerization.

To further probe the subtle structural evolution and evaluate potential ligand loss during dynamic reconstruction, Raman spectroscopy was employed to compare the materials before and after reconstruction. As shown in Fig. 2j, both FeSMF, FeCoBMF and CoSMF initially exhibit four characteristic peaks at 1614, 1414, 1133, and 858 cm-1, corresponding to C=C bonds in the aromatic ring, C–O bonds of the coordinated carboxylate groups, and C–H bonds in the aromatic dicarboxylic acid ligand34. After dynamic reconstruction, FeSMF and FeCoBMF retain distinct ligand-associated signals, indicating that their frameworks remain largely intact and ligand leaching is minimal. In contrast, CoSMF shows no detectable ligand-related signals, suggesting extensive ligand loss and, when combined with SEM and TEM results, confirming an irreversible structural transformation and collapse of the framework. Compared to FeSMF, the ligand peaks of FeCoBMF appear slightly weakened, suggesting that partial ligand leaching may occur during dynamic reconstruction. This implies the formation of ligand vacancies within the framework. To further elucidate the ligand behavior during dynamic reconstruction and ensure the rigor of this process, we performed in-situ Raman spectroscopy on both CoSMF and pre-reconstructed FeCoBMF under potentials slightly above the LSV test conditions (Fig. S8d-e). The results clearly show that the characteristic signals of the aromatic dicarboxylic acid ligand gradually weaken and eventually disappear with increasing potential in CoSMF. In contrast, distinct signals of the aromatic dicarboxylic acid ligand remain detectable in pre-reconstructed FeCoBMF even at a high potential of 1.8 V. This provides further evidence that only partial ligand leaching occurs in FeCoBMF. Combined with electron microscopy observations, which show no evident structural collapse, it is reasonable to infer that the layered pillar architecture of FeCoBMF is preserved despite the occurrence of partial reconstruction. Notably, after dynamic reconstruction, the in-plane bending vibration peak (δM-O) and the out-of-plane stretching vibration peak (νM-O) of the metal–oxygen bonds in the catalyst gradually exhibit pronounced splitting and sharpening. Comparative analysis shows that, in the 300–700 cm-1 characteristic vibration region, the three catalysts display distinct spectroscopic responses: the CoSMF-derived material presents a broadened multiplet peak structure, while the FeSMF and FeCoBMF systems exhibit asymmetrically sharpened double-peak features. These differences are closely related to the lattice dynamics arising from the distinct coordination environments of their respective metal sites, resulting from varying degrees of ligand loss and electrochemical oxidation. Controllable ligand-vacancy formation combined with metal-metal bonding synergistically induces structural distortion of the active M-O octahedral units.

X-ray photoelectron spectroscopy (XPS) analysis provides further evidence of electronic modulation induced by Fe doping. As shown in Fig. S13, the Co 2p3/2 peak in FeCoBMF shifts relative to that in pristine CoSMF, indicating a modified electronic environment around the Co centers due to Fe incorporation. This shift is attributed to altered coordination structures and the electron-withdrawing nature of Fe species35. Notably, after dynamic reconstruction, the Co 2p3/2 spectrum of FeCoBMF reveals three components: split Co2+ peaks at 779.8 eV and 782.3 eV, and a new peak at 780.9 eV assigned to Co3+ species36. The appearance of Co3+ is also observed in CoSMF after activation, indicating a general oxidation process. Collectively, these findings demonstrate that Fe incorporation effectively tunes the electronic structure of Co active sites. To further elucidate the effects of Fe doping on the local structure and chemical configuration of Co centers, we performed X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy on reference standards (CoO, Co3O4), pristine catalysts (FeCoBMF, CoSMF), and their post-dynamic reconstruction counterparts (DR-FeCoBMF, DR-CoSMF). In the Co K-edge XANES spectra (Fig. 3a), the absorption edges of FeCoBMF and CoSMF nearly overlap and align closely with that of CoO. Combined with the XPS analysis, this indicates that the cobalt species in both pristine catalysts are in an oxidation state close to +2. Conversely, the absorption edge of DR-CoSMF appears at a higher energy than Co3O4 and DR-FeCoBMF, revealing a higher Co oxidation state in DR-CoSMF37. Critically, the suppressed over-oxidation in DR-FeCoBMF is attributed to the key role of bridge oxygen and the electron pumping effect during dynamic reconstruction. This process moderates the Lewis acidity of the Co sites, thereby stabilizing the Co centers38.

To explore the structure–activity relationship between surface species evolution and electrochemical behavior, operando electrochemical impedance spectroscopy (EIS) was conducted (Fig. 3b and Fig. S14). In the potential window of 1.05–1.4 V (vs. RHE) in 1 M KOH with 0.33 M urea, operando EIS revealed distinct time constants associated with different interfacial processes. Nyquist plot and the Bode magnitude (|Z|) and phase angle plots identified two electrochemical events: in the high-frequency region (log f > 1 Hz), impedance reflects electron transfer between the catalyst and its support5, while in the low-frequency region (log f < 1 Hz) 39,40, it corresponds to electron transfer from UOR intermediates to active sites. At lower potentials (1.05–1.15 V), oxidation of the catalyst is evidenced by a large semicircle in the Nyquist plot and a rapid decline in the high-frequency phase angle in the Bode plot39, suggesting the transition of low-conductivity Co2+ into highly conductive Co3+. As the potential increases, the further drop in the high-frequency phase angle highlights the facile electron transfer to Co3+ species, confirming a low activation barrier40. Upon reaching 1.25 V, the phase angle in the low-frequency region decreases markedly, indicating the onset of UOR-related interfacial reactions5. Meanwhile, the high-frequency region remains stable, suggesting that catalyst oxidation is complete and that UOR proceeds predominantly via direct electron transfer. Additionally, staircase voltammetry (SCV) was employed to probe the intrinsic UOR current by minimizing non-steady-state contributions from capacitive charging and transient intermediate oxidation processes (Fig. 3c and Fig. S15). The SCV profiles show a clear hysteresis between the forward and reverse scans, during the forward potential sweep, low-conductivity metal species are progressively oxidized to form highly conductive metal species, demonstrating an oxidation process within the catalyst with a transition from a low- to a high-valent state.40. In contrast, during the reverse scan, the system transitions from a high-conductivity to a low-conductivity state. Since the reverse scan originates from a highly conductive state, it exhibits improved performance relative to the forward scan. the observed electrochemical behavior is highly consistent with the cobalt oxidation state changes identified by our XPS and XAS analysis. These results clearly demonstrate that Co2+ is oxidized to Co3+ during UOR, and that Co3+ species serve as the actual catalytically active centers. To further support this conclusion, quantitative EXAFS fitting analysis was conducted on the activated CoSMF sample (Fig. S16 and Supplementary Table 1). The resulting coordination parameters closely resemble those of CoOOH, indicating the structural transformation of CoSMF into a CoOOH-like phase upon activation.

To further investigate ligand behavior during dynamic reconstruction, we employed in situ electrochemical Fourier-transform infrared spectroscopy (FTIR). Fig. 3d shows three distinct absorption bands emerging at 3407, 1645, and 1380 cm-1. These bands correspond to coordinated carboxylate groups and the C–H region of the aromatic ring in the dicarboxylic acid ligand41, confirming partial ligand leaching from FeCoBMF. This leaching generates ligand vacancies while preserving the overall structural integrity. Fourier-transformed EXAFS spectra (Fig. 3e) reveal changes in local coordination. Both pristine FeCoBMF and CoSMF exhibit first-shell Co–O peaks at 1.50 Å. After dynamic reconstruction, DR-FeCoBMF and DR-CoSMF show shifted first-shell peaks to 1.38 Å. Notably, second-shell Co–O–Co/Fe features differ significantly: FeCoBMF's peak shifts from 2.42 Å to 2.23 Å, while CoSMF's shifts from 2.36 Å to 2.42 Å. For reference, Co-foil shows a Co–Co peak at 2.17 Å, while CoO and Co3O4 exhibit second-shell peaks at 2.57 Å and 2.45 Å, respectively. The observed shortening of the Co–Co/Fe distance in DR-FeCoBMF indicates the formation of metal dimers. We have further performed quantitative EXAFS fitting for FeCoBMF and DR‑FeCoBMF to account for phase‑shift effects (Fig. S16b–c and Supplementary Table 1). The results confirm the pronounced shortening of the second‑shell distance and provide strong evidence for the formation of Fe–Co dimers. Combined with Raman and in situ FTIR data, which reveal partial ligand leaching and the creation of ligand vacancies. We infer that during the reconstruction of FeCoBMF, these vacancies generate coordinatively unsaturated metal centers at Co sites. These unsaturated sites subsequently engage with adjacent metal atoms, driving metal dimerization with enhanced metal–metal interactions and promoting direct bonding. This process allows the metal centers to achieve coordination saturation and stabilizes the electronic structure. This effect manifests in R-space as a second-shell peak shift toward the Co–Co coordination distance42,43,44. Furthermore, we calculated the electron localization function (ELF) distribution on the surfaces of FeCoBMF and DR-FeCoBMF, as shown in Fig. S37. In the region between the Co and Fe atoms on the FeCoBMF surface, the ELF value is relatively high (~0.35). In contrast, on the DR-FeCoBMF surface with ligand vacancies, where the Co–Fe distance is shorter, the ELF value in the interatomic region drops to approximately 0.10. This decrease in ELF corresponds to increased electron localization, supporting the transition from a classical covalent interaction toward a direct metal–metal bond, consistent with the structural evolution indicated by EXAFS. Collectively, these results further demonstrate that ligand vacancies generated by partial ligand leaching in DR-FeCoBMF, which then promote the formation of metal dimer motifs while preserving structural integrity.

2.3. UOR performances in alkaline media and application
The FeCoBMF catalyst grown in situ on NF was dynamically reconstructed to yield DR-FeCoBMF@NF, which served as the working electrode for evaluating UOR performance. Given that the urea concentration in human urine is approximately 0.33 M, we selected 1 M KOH + 0.33 M urea as the electrolyte45,46. For comparison, DR-FeSMF, DR-CoSMF, and FeCo LDH catalysts grown on NF were also tested. To exclude any contribution from the conductive substrate, bare nickel foam (NF) without any catalyst was evaluated as a control under identical conditions. Furthermore, the performance of DR-FeCoBMF was also tested in an electrolyte containing an additional 0.1 mM Ni(NO3)2, with the comparative results presented in Fig. S17. As shown in the linear sweep voltammetry (LSV) curves (Fig. 4a), the onset potentials follow a decreasing order from DR-FeSMF to DR-CoSMF to DR-FeCoBMF. Remarkably, DR-FeCoBMF achieves a current density of 10 mA cm-2 at 1.222 V, which is significantly lower than those required by DR-CoSMF (1.261 V) and DR-FeSSMF (1.389 V). Furthermore, DR-FeCoBMF reaches 100 mA cm-2 and 500 mA cm-2 at 1.297 V and 1.383 V, respectively (Fig. 4b), and is also capable of delivering 1 A cm-2 and 1.5 A cm-2 at 1.468 V and 1.545 V, respectively (Fig. S18). These results demonstrate that Fe-doped DR-FeCoBMF maintains its layered-pillared architecture while enabling controlled ligand-vacancy formation, which subsequently induces direct metal-metal bonding and thereby enhances UOR activity relative to pristine CoSMF.

To probe the electrochemically active surface area (ECSA), cyclic voltammetry (CV) measurements were conducted at various scan rates within the non-Faradaic region47,48. The calculated double-layer capacitance (Cdl) reveals that DR-CoSMF has the largest Cdl (4.38 mF) compared to DR-FeCoBMF (3.34 mF) and DR-FeSMF (2.95 mF), suggesting DR-CoSMF has the highest degree of active-site exposure, with an ECSA of 109.5 cm2 (Fig. 4c and Fig. S19a-d). Moreover, DR-FeCoBMF exhibits favorable kinetic barrier, as evidenced by its smaller Tafel slope of 83 mV dec-1, which is lower than that of DR-CoSMF (108 mV dec-1) (Fig. 4d and Fig. S20). Although DR-FeSMF shows an even smaller Tafel slope of 40 mV dec-1, it requires significantly higher potentials to achieve comparable current densities, coupled with the lowest exchange current density, indicating poor overall catalytic efficiency (Fig. S20)49. Moreover, despite its ECSA being lower than that of DR-CoSMF, DR-FeCoBMF exhibits good reaction kinetics, strongly indicating higher efficiency per active site and consequently good intrinsic activity. As summarized in Fig. S17c and Supplementary Table 4, the DR-FeCoBMF catalyst achieves competitive voltages at both 10 mA cm-2 and 100 mA cm-2, respectively, compared to most NF-based catalysts. Electrochemical impedance spectroscopy (EIS) measurements further support the enhanced electron transfer properties of DR-FeCoBMF, which shows a lower charge transfer resistance (Rct) compared to DR-CoSMF (Fig.S19e and Supplementary Table 2). More importantly, considering the critical role of stability in determining a catalyst’s hydrogen production efficiency and its potential for industrial deployment, we performed durability tests on the materials at a current density of 50 mA cm-2. To minimize interference from the counter electrode during long-term electrolysis, a graphite rod was employed as the counter electrode. As shown in Fig. 4e, DR-FeCoBMF exhibited competitive stability, maintaining its performance without noticeable degradation for at least 14 hours. In contrast, DR-CoSMF experienced a rapid increase in UOR voltage, rising from 1.4 V to 1.59 V, which indicates a sharp decline in performance likely due to surface reconstruction. To further assess the long-term operational durability of DR-FeCoBMF under UOR conditions, extended stability tests were conducted at a current density of 10 mA cm-2. As shown in Fig. S21, DR-FeCoBMF retained over 99% of its initial activity after 60 hours of continuous operation, confirming its competitive stability and resilience.

To assess the generalizability of the heteronuclear vacancy-to-bond strategy, a series of validation experiments were conducted using various metals, including Fe, Co, Mn, Cu, and Mo. LSV screening of monometallic MOFs revealed that CoSMF exhibited the highest UOR activity (Fig. S22a), and was thus selected as the primary metal center (M1). Notably, the heterometallic substitution strategy proved broadly applicable. Targeted substitution was performed by replacing 10–50 at.% of Co with electron-withdrawing metals (Fe, Ni, Mn, and Cu). Electrochemical measurements identified an optimal atomic ratio of Fe:Co ≈ 1:2, which delivered the best UOR performance among all tested compositions (Fig. S22b and Supplementary Table 3). Both NiCoBMF and MnCoBMF maintained well-defined layered structures and exhibited enhanced UOR activity compared to pristine CoSMF (Fig. 4f and Fig. S23-24). These findings confirm that the heteronuclear vacancy-to-bond strategy during dynamic reconstruction is crucial for performance enhancement. Overall, the results validate the universality of the heteronuclear vacancy-to-bond strategy and highlight its potential as a scalable design principle for high-performance electrocatalysts.

2.4. Product analysis and device applications
Furthermore, as shown in Fig. 5a-b, DR-FeCoBMF requires only 1.297 V to reach a UOR current of 100 mA cm⁻², whereas the same current density for the OER demands 1.509 V, resulting in a voltage gap exceeding 212 mV. This highlights the advantage of UOR over OER in reducing energy consumption and further demonstrates the catalytic advantages of DR-FeCoBMF in urea electrolysis. Given the abundance and low oxidation potential of urea, urea-assisted water electrolysis not only enables energy-efficient hydrogen production but also generates valuable nitrogen-containing byproducts, as illustrated in Fig. 5b. We performed ion chromatography analysis of the reaction products at 1.45 V (Fig. 5c), comparing the DR-FeCoBMF catalyst with NF. The results showed that while NF exhibited a modest 51% selectivity toward NO2- and NO3-, FeCoBMF achieved a significantly higher Faradaic efficiency (FE) of 79.5% for NO2-, with a total nitrogen oxide FE of up to 87.7%. These findings demonstrate that DR-FeCoBMF not only enables efficient hydrogen production via urea-assisted water electrolysis, but also promotes the formation of valuable nitrogen-containing products such as nitrite (NO2-) and nitrate (NO3-). Moreover, the identification of these oxidation products helps elucidate the preferred reaction pathway during DR-FeCoBMF-catalyzed UOR45,50,51. From the perspectives of both energy efficiency and economic viability, electrochemical urea oxidation to yield peroxide-type nitrogen species (e.g., NO2-), alongside hydrogen evolution, presents a more favorable route than nitrogen gas formation. The oxidation of a single urea molecule to N2 involves the transfer of six electrons, while deep oxidation to two NO2- molecules involves twelve electrons, thereby enhancing the energy output and improving hydrogen production. Rationally designing catalysts to favor NO2- formation thus offers a promising strategy for improving overall system efficiency, which not only enables highly efficient hydrogen production but also removes urea from wastewater while simultaneously synthesizing value-added nitrogenous chemicals via electrochemical means. This results in a win-win-win outcome: energy-saving hydrogen production, wastewater purification, and the co-production of useful chemicals.

To explore the practical applicability of FeCoBMF, a two-electrode electrolyzer was assembled, using FeCoBMF as the anode and commercial Pt/C as the cathode. As shown in Fig. 5d, the device achieved a cell voltage of 1.75 V at 1 A cm-2, which is substantially lower than that typically required for conventional water splitting. Remarkably, the system maintained stable operation with negligible performance loss over 100 hours of continuous electrolysis at 1 A cm-2. Notably, XPS analysis of Co and HRTEM images revealed no significant changes in the elemental profiles and structural changes before and after the stability test at 1 A cm-2 (Fig. S25). This represents a 13% energy saving compared to conventional water splitting using the same catalyst for the OER (Fig. S26), highlighting its competitive durability and great promise for industrial-scale urea electrolysis applications52.

2.5. Mechanistic studies
To better understand the UOR mechanism at metal active sites, density functional theory (DFT) calculations were carried out, and because CoSMF is prone to phase transformation into CoOOH under dynamic reconstruction, FeCoLDH was used as a reference material, DFT models of FeCoLDH, FeCoBMF, and DR-FeCoBMF, which correspond to fully ligand-deficient, intact, and partially ligand-deficient states respectively, were constructed to compare their structural features (Fig. S27–S29), with explicit inclusion of urea intermediate adsorption configurations (Fig. S30-S32) based on experimental insights. The UOR process proceeds via four major steps: urea adsorption, dehydrogenation, C–N bond cleavage, and nitrogen species oxidation53. The first step—urea adsorption—was analyzed in detail (Fig. 6a). DFT calculations indicate that, the adsorption energy of urea on DR-FeCoBMF is -0.79 eV, which is higher in magnitude than that on FeCoBMF (-0.64 eV) and FeCoLSDH (-0.47 eV), indicating that the dynamically reconstructed layered MOF provides a favorable adsorption environment for urea. Meanwhile, for DR-FeCoBMF, which exhibits the strongest urea adsorption, the Bader charge of the adsorbed CO(NH2)2* was calculated, yielding a net charge of -0.16 e- for the entire urea molecule. The adsorption energy and Bader charge together demonstrate that urea undergoes relatively weak chemical adsorption on the catalyst. Such weak chemical adsorption prevents adsorbate-induced reconstruction of the catalyst, helping to maintain the structural integrity of active sites and promote urea diffusion54,55.

DFT calculations further reveal that the key C–N bond cleavage step (*CONNH2 → *NH2 + *CON), a chemical (non-electrochemical) step, is the rate-determining step (RDS) for FeCoLDH and FeCoBMF in the UOR pathway. As shown in Fig. 6b, the corresponding reaction energy for FeCoBMF is 1.33 eV, which is 0.15 eV lower than that of FeCoLDH (1.48 eV). This reduction in the reaction energy is attributed to the synergistic electronic effects between the Co and Fe bimetallic sites within the MOF structure. For DR-FeCoBMF, the energy required for this step is halved to 0.75 eV, indicating that ligand-vacancy induced metal–metal bonding significantly lowers the energetic cost of C–N bond cleavage.

Charge density difference maps for *NH2 + *CON adsorption on FeCoLDH, FeCoBMF, and DR-FeCoBMF (Fig. 6c-d and Fig. S33) demonstrate significantly more pronounced charge transfer in the vacancy-bonded structure versus hydroxide and pristine MOF, confirming enhanced adsorption of the *NH2 + *CON configuration. To further investigate the UOR mechanism on DR-FeCoBMF, electronic structure analysis based on the density of states (DOS) revealed that distinct post-reconstruction structures modulates the electronic environment of the Co active sites (Fig. 6e). Typically, a d-band center closer to the Fermi level correlates with stronger adsorption of reaction intermediates. Compared to FeCoLDH, Co d-band centers in FeCoBMF are shifted to higher energies, suggesting stronger intermediate interactions due to the rigid and stable MOF framework, thus improving catalytic performance56. In DR-FeCoBMF, owing to the presence of ligand vacancies and metal–metal bonding, the Co d-band center shifts downward relative to FeCoBMF yet remains higher than that of FeCoLDH. Meanwhile, the Fe d-band center shifts upward relative to FeCoLDH but downward compared to FeCoBMF, resulting in both d-band centers lying between those of FeCoLDH and FeCoBMF. According to the Sabatier principle, this indicates that DR-FeCoBMF exhibits intermediate adsorption strength that is neither too strong nor too weak, thereby achieving an optimal intermediate adsorption environment among all the structures examined.

As shown in Fig. 6f and Fig. S34, for DR-FeCoBMF, the chemical C–N bond cleavage step is no longer the RDS, and potential-dependent step *NO → *NOOH emerges as the potential‑determining step (PDS) with a substantially reduced reaction energy of 0.8 eV—0.68 eV lower than FeCoLDH and 0.53 eV lower than pristine FeCoBMF. This demonstrates that metal dimerization successfully shifts the RDS from a chemically challenging, non‑electrochemical C–N cleavage (with reaction energies of 1.48 eV and 1.33 eV, which are prohibitively high at room temperature) to a more facile electrochemical step with a reaction energy of only 0.8 eV. Furthermore, we also calculated the free‑energy profile for the reaction pathway leading to NO3- as the product, as shown in Fig. S35. The results confirm that for both FeCoBMF and DR‑FeCoBMF, the formation of NO3- is thermodynamically more favorable than that of NO2-, which explains why NO2- is the major product for DR‑FeCoBMF observed experimentally, while the yield of NO3- remains low, consistent with the product analysis.

Collectively, the rigid coordination structure of the MOF preserved during dynamic reconstruction optimizes the d-band center, enhancing *NH2 + *CON adsorption while weakening C–N bond strength to accelerate cleavage kinetics. These findings confirm that the rational design of the FeCoBMF structure effectively reduces the RDS barrier relative to FeCoLDH and improves C–N cleavage efficiency, promoting intermediate formation and bond breaking. Stabilizing the MOF framework thus proves critical for avoiding phase transformations and ensuring effective UOR catalysis. Crucially, DR-FeCoBMF maintains the parent layered-pillared architecture while controllably generating ligand vacancies and inducing metal-metal bonding. This configuration modulates the d-band center, enabling the optimal intermediate adsorption. By breaking the limitation of the non‑electrochemical C–N cleavage as the RDS, it switches the rate-determining step to the electrochemical step *NO → *NOOH and slashes the reaction energy by 0.68 eV, achieving improved urea oxidation efficiency57.

Methods
Materials
Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, > 98%), potassium hydroxide (KOH, > 95%) were all purchased from Sigma-Aldrich; Ferrous chloride tetrahydrate (FeCl2·4H2O , >98%), 1,4-dicarboxybenzene acid (BDC, > 99%), urea (> 99.9%) were purchased from Macklin Biochemical Co., Ltd; Nickel foam (NF) was purchased from Kunshan Shengshijing new new material Co., Ltd; Ethanol absolute (≥99.7%) and N, N-dimethylformamide (DMF, > 99.5%) were bought from Sinopharm Chemical Reagent Co., Ltd. The water used was deionized (DI) water. All the chemicals were used as obtained without further purification.

Pretreatment of nickel foam
Nickel foam (1 cm × 3 cm) was sequentially sonicated in acetone for 15 minutes to remove organic impurities, followed by 10 wt% HCl for 15 minutes to eliminate surface oxides and activate the metallic substrate, and finally in ethanol for 15 minutes to ensure complete removal of residual contaminants. After each sonication step, the nickel foam was thoroughly rinsed with deionized water to neutralize chemical residues. The cleaned substrate was then immersed in fresh ethanol for 10 minutes to enhance hydrophilicity and vacuum-dried at 60°C for 12 hours to obtain a pristine, oxide-free surface with optimized adhesion properties for subsequent catalyst growth.

Synthesis of FeCoBMF
The FeCo bimetallic MOF was grown on nickel foam via a solvothermal process. In a typical synthesis, Co(NO3)2·6H2O (101.8 mg), FeCl2·4H2O (29.8 mg), and BDC (83 mg) were dissolved in a mixture of deionized water, ethanol, and DMF (1:1:16 mL by volume) under magnetic stirring for 30 minutes to obtain a homogeneous solution. The resulting solution was transferred to a Teflon-lined stainless-steel autoclave containing a piece of pretreated NF (1 cm × 3 cm) and heated at 150°C for 3 hours. After the autoclave was allowed to cool naturally to room temperature, the NF substrate with the grown product was collected, thoroughly rinsed with DMF, ethanol, and deionized water to remove unreacted precursors, and finally dried at 60°C in an oven.

Synthesis of CoSMF
The Co-based single-metal MOF was synthesized following a similar procedure. Co(NO3)2·6H2O (145.5 mg) and BDC (83 mg) were dissolved in a mixed solvent of water, ethanol, and DMF (1:1:16 mL) with stirring for 30 minutes. The homogeneous solution was transferred into an autoclave containing pretreated NF (1 cm × 3 cm) and heated at 150°C for 3 hours. After cooling, the NF with the as-grown CoSMF was washed sequentially with DMF, ethanol, and deionized water, and then dried at 60°C (adapted from ref. 34).

Synthesis of FeSMF
The Fe-based single-metal MOF was synthesized following a similar procedure. FeCl2·4H2O (99.4 mg) and BDC (83 mg) were dissolved in a mixed solvent of water, ethanol, and DMF (1:1:16 mL) with stirring for 30 minutes. The homogeneous solution was transferred into an autoclave containing pretreated NF (1 cm × 3 cm) and heated at 150°C for 3 hours. After cooling, the NF with the as-grown CoSMF was washed sequentially with DMF, ethanol, and deionized water, and then dried at 60°C(adapted from ref. 34)

Synthesis of DR-FeCoSMF, DR-CoSMF and DR-FeSMF
Using FeCoBMF, CoSMF and FeSMF as the working electrode, a Hg/HgO electrode (1 M KOH) as the reference electrode, and a graphite rod as the counter electrode, continuous linear sweep voltammetry (LSV) was conducted in 1.0 M KOH + 0.33 M urea electrolyte. The potential was swept between 1.2 and 1.7 V vs. RHE at a scan rate of 5.0 mV s-1. Reconstruction cycles were repeated until electrochemical performance stabilized. The resulting stable electrocatalyst was designated as DR-FeCoBMF, DR-CoSMF and DR-FeSMF.

Characterizations
The morphology and microstructure of the as-prepared catalysts were examined using field-emission scanning electron microscopy (FESEM, Gemini Sigma 360) and high-resolution transmission electron microscopy (HRTEM, Jem-2100F) equipped with an energy-dispersive X-ray spectroscopy (EDX) detector (Oxford, X-MAX 65T). Crystal structures were analyzed by X-ray diffraction (XRD, MiniFlex600) with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS, ESCALAB Xi+) with Al Kα radiation was employed to probe the surface chemical states, and all spectra were calibrated using the C 1s peak at 284.8 eV. Raman spectra were recorded on a Horiba/LabRAM Odyssey spectrometer with a 532 nm excitation laser, using samples prepared by the KBr-disk method. X-ray absorption fine structure (XAFS) measurements were conducted on a Rapid-XAFS IM system (Anhui Absorption Spectroscopy Analysis Instrument Co., Ltd.) in transmission mode at 25 kV and 40 mA (characterization methods adapted from ref. 34).

Electrochemical measurements
An electrochemical analyzer (CHI 760E) was used to perform all electrochemical experiments in a standard three-electrode configuration.A Hg/HgO electrode filled with 1 M KOH and a graphite rod were used as a reference and counter electrode, respectively. The potential was converted to reversible hydrogen electrode (RHE) via a Nernst equation (ERHE = EHg/HgO + 0.059 × pH + 0.098). The pH of the prepared solution was measured repeatedly and the average value was 13.6. The scan rate of linear sweep voltammetry (LSV) was set to 5.0 mV s−1, with potentials ranging from 1.2 to 1.6 V vs. RHE in both 1.0 M KOH and 1.0 M KOH + 0.33 M urea to investigate the OER and UOR activities of the as-prepared catalysts. EIS tests were conducted at intervals of 0.05 V from 1.05 V to 1.40 V vs. RHE with a frequency range from 105 to 0.01 Hz. All polarization curves were calibrated with the iR correction. CV cycles (CVs) with the scan rates from 20 to 100 mV s−1 were applied to analyze ECSA. The Tafel slopes were derived from the polarization curves after iR correction, according to the Tafel equation: η = b log (j/j0) = b log(j) - b log(j0), which can be expressed as η = a + b log (j) where η is the overpotential, j is the exchange current density, a is the Tafel constant (V) and serving as a direct indicator of intrinsic activity, b is the Tafel slope, and j is the current density. The Tafel slopes were obtained by fitting the linear regions of the Tafel plots (log j ≈ 0–1.5). Staircase voltammetry (SCV) was performed with the following parameters: a linear potential sweep from 1.2 to 1.5 V vs. RHE was applied using a potential increment of 5 mV per step, with each potential step maintained for 60 s.

In-situ ATR-FTIR experiments
In-situ attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was performed on an INVENIO-S spectrometer equipped with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector. A ZnSe hemispherical prism served as both the conductive substrate for catalyst loading and the IR reflection element. A graphite rod and a Hg/HgO electrode (filled with 1 M KOH) were used as the counter and reference electrodes, respectively. All spectra were collected in absorbance mode and are presented as Abs = −log(R/R0), where R represents the reflectance of the sample spectrum and R0 is the background spectrum (experimental setup adapted from ref. 34).

XAFS analysis
All Co K-edge data were analyzed using Athena. We applied a k-weight of 3 and Fourier transformed from k = 3.0-12.0 Å-1 (using a Hanning window) to obtain R-space spectra. The R-space fitting was performed using Artemis over a range of 1–3 Å, based on k3-weighted k-space. DR-CoSMF utilized the CoOOH model (mp-27913) for fitting; FeCoBMF utilized the CoO (mp-22408) and Co3O4 (mp-18748) for fitting; DR- FeCoBMF utilized the CoO (mp-22408) and FeCo intermetallic (mp-2090) for fitting.

Determination of Anions by Ion Chromatography
Anion analysis was performed using ion chromatography (IC, ICS1600 system) equipped with a DionexTM IonPacTM AG11-H RFICTM analytical column (4 × 50 mm). The mobile phase consisted of 20 mmol/L potassium hydroxide (KOH) eluent delivered at a constant flow rate of 1.0 mL min-1. The column compartment was maintained at 35°C, and a suppressor current of 50 mA was applied to enhance detection sensitivity. A conductivity detector was employed for signal acquisition under isocratic conditions. The total analysis time was set to 12 minutes per sample.

Calculation method
All the spin-polarized DFT+U calculations were performed by the Vienna ab initio simulation package (VASP). The Perdew-Burke-Ernzerh (PBE) in the generalized gradient approximation (GGA) with the on-site Coulomb Repulsion U term was applied to describe the exchange-correlation function. In this work, U(Co-d) = 6.6 eV, and U(Fe-d) = 3.5 eV for all the calculations. The electron-ion interactions were described by using projector augmented wave (PAW) method. The pseudopotential of Fe (ver. 06sep2000, d7s1), Co (ver. 02Aug2007, d8s1), C (ver. 08Apr2002, s 2p2), N (ver. 08Apr2002, s2p3), O (ver. 08Apr2002, s2p4), and H (ver. 15Jun2001, s1) were used in our calculations. The VASPsol code was used to consider the effect of water on the charged intermediates through an implicit solvation model. By our convergence tests as shown in Figure S35, the cut off energy was set to 400 eV for all calculations, and the Brillouin zone was sampled on mesh with a k-point spacing of 0.04 Å-1. The van der Waals interactions were considered using the empirical correction in Grimme’s scheme (DFT-D3). The energy and force convergence criteria for the iteration in the self-consistent field were to be 1.0 × 10-5 eV and 0.02 eV/Å, respectively. A Gaussian smearing of 0.05 eV was applied to speed up electronic convergence. To avoid the interactions between two periodic images, a vacuum distance of 15 Å was adopted in the z direction. All the atomic and electronic structures were illustrated with VESTA software.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript entitled “Dynamic Ligand-Vacancy Engineering Drives Metal Dimerization for Efficient Urea Electrooxidation” reports a pioneering heteronuclear vacancy-to-bond strategy. The authors, through electrochemical reconstruction, dynamically and controllably form ligand vacancies, which maintain the framework from collapsing while further controllably forming metal dimers. This unique metal dimerization meanwhile preserves the framework structure, leading to a change of the rate-determining step, and shows excellent performance in urea oxidation. Moreover, in urea-assisted water electrolysis device tests, it can stably operate at 1 A cm-2, showing practical application significance, providing a new design paradigm for future catalyst development. Therefore, I would like to recommend its publication after minor revision:\n1. The authors state that “selective ligand removal promotes the formation of metal dimers” (Page 4), but the underlying reason for this phenomenon is not explained. Please complete.\n2. The authors state that the dynamic reconstruction of FeCoBMF stabilizes at a “partial ligand leaching” state, rather than complete collapse as observed for CoSMF (Page 5). What is the underlying mechanism that governs this self-limiting or termination behavior during reconstruction?\n3. The authors describe the role of Fe as both a “stabilizing agent” and an “electron pump” (Page 6). During the reconstruction process, does the valence state and coordination environment of Fe undergo any changes? If so, how do these changes facilitate its anchoring function and electron transfer? If not, how does Fe remain stable in a dynamic oxidative environment?\n4. The authors describe that “FeCoBMF's peak shifts from 2.42 Å to 2.23 Å” (Page 9). Considering that the interatomic distance of 2.23 Å is still larger than that in Co foil (2.17 Å), could this be indicative of a very strong metal–metal interaction rather than a classical σ-bond?\n5. The heteronuclear vacancy-to-bond strategy designed by the authors is quite novel, and related studies on ligand vacancies and metal dimers are limited. It would be helpful to cite a few additional relevant references. Also, recently reported works on the MOF design and OER/UOR Catalysis are suggested to be discussed: Nano Research, 2025, 18(5): 94907305; Chemical Engineering Journal, 2024, 491: 152023; Nature Communications, 2023, 14:5842.", "reviewer_2": "The manuscript reports an Fe-doped bimetallic framework catalyst for the urea oxidation reaction, highlighting dynamic ligand vacancy formation and metal dimerization as key features leading to enhanced performance. The topic is timely and of potential interest to the community. However, several critical aspects of the mechanistic claims and data presentation require further clarification and experimental validation before the work can be considered for publication.\n1.Dynamic reconstruction: The proposed dynamic evolution of coordination and structure requires more rigorous evidence. In situ Raman or synchrotron-based measurements are strongly recommended to validate the reconstruction process.\n2.Tafel slope: The higher Tafel slope of DR-FeCoBMF compared to DR-FeSMF raises questions about reaction kinetics. The authors should clarify whether this reflects intrinsically slower kinetics or another mechanistic factor.\n3.Stability evaluation: While 100-hour durability is reported, the voltage profile under constant-current operation is not clearly presented. Such data are essential to substantiate long-term stability claims.\n4.Active sites: The catalytic active centers remain undefined. Additional characterization is needed to unambiguously identify the active sites.\n5.Lattice spacing: The inset of Figure 2h does not specify lattice spacings. These values should be added for clarity and completeness.\n6.Energy diagram: The stepwise energy diagram (Figure 6) omits the product desorption step. This step should be included, and the accuracy of the current calculations re-examined.\n7.Adsorption description: Line 307 states that urea adsorption is weak and non-charge-transfer, whereas the energy diagram depicts it as exothermic. This inconsistency must be resolved.\n8.Site distribution: Figures S28–S30 show intermediates adsorbing only on Co for DR-FeCoBMF, in contrast to dual Fe/Co sites in other catalysts. The implications of this change for UOR activity should be discussed.\n9.Figure clarity: The atomic color scheme in Figure S30 is inconsistent with other figures and should be revised for uniformity.", "reviewer_3": "This manuscript, which details the mechanism of structural change in bimetallic frameworks (FeCoBMF) for urea electrooxidation, investigates a complex and relevant phenomenon in electrocatalysis which is catalyst stabilization through self-optimization. The central hypothesis that an electrochemically generated ligand vacancy induces Fe-Co metal dimerization is a valuable mechanistic proposal with significant practical implications for designing stable, active catalysts. The combination of experimental analysis and theoretical modeling provides a necessary framework for understanding this challenging system.\nHowever, the Density Functional Theory (DFT) calculations suffer from key methodological deficiencies that introduce substantial uncertainty into the calculated energies.\n1. The choice of the PBE functional without a GGA+U correction for a highly correlated 3d transition metal oxide (Fe/Co framework) is an issue. PBE is insufficient in accurately modeling the electronic band structure, charge localization, and the critical redox states necessary for the UOR mechanism.\n2. Were convergence tests performed for the chosen Ecut and the k-point mesh (3×3×1)? Please provide the convergence curves in the SI showing how the total energy changes with increasing these parameters.\n3. Please provide the calculated Fe-Co interatomic distance in both the initial (monomer/vacancy) structure and the final (dimerized) structure in the SI, to numerically substantiate the claimed bond shortening.\n4. Was any model (like an implicit solvent model) used to account for the effect of water on the charged intermediates (OH∗ and O∗)? Neglecting the aqueous environment can significantly change the calculated stability of these species.\n5. Which type of pseudopotential was used for the Fe, Co, O, and H atoms in the calculations?\n6. What was the thickness of the vacuum layer used in the supercell, and was this sufficient to minimize spurious interactions between periodic images of the slab?\n7. How does the calculated activation energy for the RDS on the dimer structure quantitatively compare (in eV) to the activation energy for the same step on the monomer/vacancy structure?", "reviewer_4": "Wu et al report a restructured MOF as an electrocatalyst for the urea oxidation reaction (UOR). Electrochemical measurements are complemented by X-ray absorption spectroscopy (XAS), Raman spectroscopy, DFT and electron microscopy.\nThe composite electrode performs well and operates at 1 A/cm2 for 100 h, which makes it interesting. Recent other reports report around 1.35 V at 100 mA/cm2 while the authors catalyst operates at around 1.3 V. However, the main products of the optimized catalyst are NOx, which are toxic. Synthesis of a valuable chemical is claimed but also not supported by discussion.\nSurface restructuring is common under UOR conditions and reconstruction of MOF surfaces a general issue for electrocatalysts. The authors address it using ex situ XAS and Raman spectroscopy to study metal-metal coordination and the organic linker, respectively. Loss of the linker signal is well supported. The XAS data and analysis do not support direct metal-metal bonds (as in a metal I presume) and there is generally too little discussion of this claim to support it. This also affects the interesting DFT analysis of the reaction pathway as it is unclear whether the correct surface is modeled.\nThe performance of the catalyst is potentially interesting. In the current state, the study should not be published. Significant revision is necessary to make it publishable by clarifying the novelty and technical issues.\nPlease address the following comments\n1) The introduction states the key challenge that MOFs reconstruct to metal oxyhydroxides and lose their organic linkers. While the loss of the linker could be somewhat controlled, conversion to metal oxyhydroxide is exactly what happens. Please perform EXAFS analysis of all samples in Fig. 3e. Clearly the FT peaks above 3 A are lost and a prominent peak at ~2.5 A appears, which supports the formation of an oxyhydroxide for DR-FeCoBMF. Co-Co distances in the metal are further separated, which is also visible in Fig. 3e. The shown data needs more analysis and discussion. It clearly contradicts the claim of “direct metal-metal bonds”. This lowers novelty of the work.\nThere are no details of XAS data analysis in the experimental. They need to be added. What range was Fourier transformed and what window function used? What range was fitted and in which space was the fit performed? What software was used? What model was used to generate the files for fitting?\nThe DFT models also may need to be revised. Discussion how well they model the experimental structure is necessary\nThe statement “suppressing […] hydroxide transformation” (P3 middle) is contradictory to the XAS data.\n2) Have other strategies reported (for any oxidative electrocatalytic reaction) to control linker leaching? Please include discussion to support novelty of this point. Specifically, was the role of Co known for linker “vacancy” formation? P6 “Co-carboxylate bonds are relatively more susceptible to nucleophilic attack by OH- and hydrolytic cleavage.” This statement needs a citation for support. It could be the nucleus for a more extensive discussion of the role of Co.\n3) NO2 and NO3 are postulated as valuable chemicals. A quantitative discussion is needed to support this. More valuable than O2 from OER? NOx are toxic and not high-value chemicals with trade values around $0.4/kg, which is about the same as the reactant urea with $0.4/kg. The main purpose of UOR seems to convert urea to N2 to clean water, rather than making a high value chemical. This catalyst has very low yield for N2\n4) Section 2.2 starts without a clear definition of the electrochemical protocol used for reconstruction. Please add it. Were the potentials for reconstruction high enough to support the oxygen evolution reaction (OER)? If so, is this reasonable for device operation?\n5) The XANES edge position depends on the oxidation state but also coordination and identity of the ligand (e.g. https://doi.org/10.1002/anie.202211949). The assignment of Co2+ based on oxides is correct only if an oxyhydroxide forms (contradicting such claims). This point needs more discussion and a clean distinction between observation “edge shift” and interpretation (“oxidation state of Co”)\n6) P8 “Nyquist, Bode, and phase angle plots identified” Fig. 3 only shows a phase angle plot. The information of the Nyquist plot is completely contained in a Bode plot, so it may not be necessary to show the Nyquist plot.\nHow are the electrochemical events assigned to catalyst-support interactions and catalyst-electrolyte interactions? What equivalent circuit is assumed?\nWhy is a larger semi-circle (= larger resistance) indicative of catalyst oxidation? The bulk conductivity is commonly found as an offset of the semi-circles. The “size” of the semi-circle corresponds to the charge transfer resistance of the interface to which it is assigned.\n7) P9. The SCV results support that something is oxidized. It is not element specific and there is no direct evidence that Co oxidizes.\n8) P9 “FeCoBMF's peak […]. Combined with Raman and in situ FTIR data” Raman and FTIR are sensitive to bond vibrations and can be used to study the linkers. EXAFS is a method local around metal sites. It cannot be used to study the linkers.\n9) Wavelet transformations (WT) make colorful pictures that are hard (Fig. 3f) or impossible (Fig. S17) to interpret. Details of the WT construction are also not given in the experimental. Why is the peak in k-space not directly discussed in k-space? The WT is not necessary and may introduce artefacts. Why should a shift toward lower k values reflect “ligand vacancy” to which EXAFS is not sensitive (point 7) and modified metal-metal bonding? The latter point is probably correct but to unspecific to be useful. I suggest EXAFS fits to investigate this point properly and removal of the WT unless they provide any new information relative to k-space and R-space data.\n10) P10. “results demonstrate that Fe-doped DR-FeCoBMF maintains its layered-pillared architecture” Is there any experimental proof of it from microscopy or spectroscopy? I have also been wondering about the post-mortem analysis of the catalyst after 1 A/cm2 for 100 h. Please provide and discuss it.\n11) The CVs used to estimate the double layer capacitance are not box-like. Please follow the recommendation here for experimental planning and analysis: https://iopscience.iop.org/article/10.1088/2515-7655/abee33\nECSA has units of area. It is not clearly defined. The data with more box-like CVs can be compared within this study but not with values in literature.\n12) in which potential range was the Tafel slope obtained? What is the rational for the value of 88 mV/dec and 100 mV/dec. Please provide error bars to support “substantially lower”. The Fe sample has a much lower Tafel slope and apparently also lower exchange current density. At some potential, it lower Tafel slope will result in lower potential compared to the other samples. What is the crossover potential? Discuss in the context with competition by OER\nAre the experimental Tafel slopes compatible with the barriers predicted by DFT? Specifically on P15, there is a change in RDS predicted by DFT. Is this also found in the experimental Tafel slopes?\n13) P12 “Electrochemical measurements identified an optimal atomic ratio of Fe:Co ≈ 1:2, which delivered the best UOR performance among all tested compositions (Supplementary Table 3). ” Data to support this statement is not provided\n14) P13 “As shown in Fig. 5d, the device achieved a cell voltage of 1.75 V at 1 A cm-2, which is substantially lower than that typically required for conventional water splitting.” Did water splitting occur or not?\n15) What is the origin and the support for the proposed mechanistic sequence used in DFT calculations?\n16) Citations often do not support the statements, e.g. P5 “structure with ligand vacancies formed through partial ligand leaching delivers optimal catalytic performance and the most significant dynamic reconstruction enhancement13”. Ref 13 discusses spinel oxides not MOFs. It is unclear why the results of the reference should apply or support the statement.\n17) The theoretical overpotential of UOR is given as 0.37 V vs RHE. Currents attributed to UOR are detected at around 1 V vs RHE, giving it a rather high overpotential. This deserves some discussion. Isn’t it a key challenge to operate closer to the thermodynamic potential?\n18) Please provide a citation to support the urea concentration of 0.33 M in human urine.\n19) caption of figure 5 has panel b defined twice\n20) The catalyst is deposited on Ni foam. Does some Ni enter the catalyst after long experiments?\n21) How reproducible are the electrochemical results? How many electrodes were tested? At least the determined potentials in Fig. 4e should have error bars"}, "author_response": "We sincerely thank the reviewers for their thoughtful evaluation and constructive feedback on our manuscript. Their insightful comments have been invaluable in enhancing the clarity, depth, and overall quality of our work. All revisions have been carefully made and are highlighted in yellow for ease of review. We have thoroughly addressed each suggestion and provided detailed responses below, outlining the corresponding changes implemented in the manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_2": "The supplementary experiments and data provided by the authors address our concerns effectively, so we consider the current manuscript suitable for acceptance in Nature Communications.", "reviewer_3": "The authors have significantly improved the manuscript. In its present form, the manuscript may be accepted.", "reviewer_4": "Wu et al. have revised their manuscript with new experimental and theoretical data, new analyses and text revisions. The revision has not improved the support for the claims that a valuable chemical is produced and for metal-metal dimers. Many of my comments were addressed partially. Further revision is necessary for the work to become publishable.\n1. The assignment based on the peak of the Fourier transform is not valid as it is modified by a phase contribution. By “EXAFS analysis” in my previous comment, I meant fit the data. There is only the fit of CoSMF in Suppl. Table 1. Its Co-Co distance of 2.88 A perfectly matches that of the used hydroxide (mp-27913). I conclude CoSMF is CoOOH. No statements can be made about CoFeBMF as there are no EXAFS fits to support discussion of M-M distances. Missing experimental information was added. Since the point of hydroxide vs. claimed metal-dimer was not addressed, they could also not be considered properly in revised DFT calculations.\n2. I do not see any text changes. Please include your reply to me in the revised text or as a Supplemental Note to help the non-chemist readers understand your work\n3. The text change does not improve the support of the claim. The reply to me to lower the cell potential for H2 production is much stronger than changing “high value” to “valuable”.\n4. No further comments\n5. No further comments\n6. Please add the explanations of the assignment of the frequency ranges to the revised manuscript and support the assignment with citations or own data.\n7. No further comments\n8. The statements in the revision require the EXAFS fits that I requested for support\n9. Part of the reply is incorrect: “our material was loaded onto a Ni foam substrate, this introduces interference and oscillations in the Co k-space signal” The Co-K edge is at 7709 eV, the Ni-K edge is at 8333 eV. While the specific edges vary for MOFs and oxides, the difference is around 620 eV. You analyze to k=12 A-1. That is 548 eV < 620 eV. The Ni edge CANNOT affect your data. Even 12.5 A-1 is 600 eV < 620 eV.\nThe experimental states that WT was performed up to k=15 A-1 but there is no data to support it and it makes no sense as it should not include the Ni-K edge\nThe WT does not really help to mitigate the effects of interference even if it has other origins than the Ni-K edge\nMy comment remains that the WT does not add anything.\nEXAFS fitting is the method of choice to rigorously support “coordination changes”\n10. Please clarify in the revised manuscript which microscopy and spectroscopy data was conducted on the sample treated for 100 h at 1 A/cm2\n11. The question has only been addressed partially. The Cvs now look more box-like. ECSA still has the wrong units in the revised manuscript. The shift in Suppl. Fig. 20 is not acceptable and will not give reliable data. If it is due to oxygen, then purge with Ar (or N2 is you must and repeat. If ORR occurs, then the currents are not due to physisorption and do not indicate the surface area. When you repeat follow the recommendations in the work that I recommended: larger window, higher sweep speeds and more appropriate allometric function. You also use a non-ideal capacitor in your EIS analysis, then you cannot use an ideal one to model the same interface when CV is used.\n12. No text changes have been made. They are necessary. There is no error bar to support that 88 mV/dec is indeed significantly lower than 100 mV/dec. The equation in the reply to me is called the “Tafel equation”. If Fig. 4d show the fit range to obtain the Tafel slope, please document it in the experimental. None of the fits cover 2 orders of magnitude (being delta_log =2). If you extrapolate the slope of DR-FeSMF to eta=0, then it will be much lower than that of the other samples so the exchange current density is lower. 36 mV/dec for DR-FeSMF is the lowest in the plot; it also has the lowest estimated exchange current density. Using the Tafel slope alone is not a sufficient description of the formula in your reply. We need low Tafel slope and high exchange current density for a good catalyst. One can estimate the Tafel slope from DFT, e.g. https://chemistry-europe.onlinelibrary.wiley.com/doi/full/10.1002/elsa.202100213. My question was of qualitative nature: 88 and also 100 mV/dec indicate an electrochemically-limiting step with transfer coefficient larger than 0.5. Is an electrochemical step the PDS in the DFT calculations?\n13. No further comments\n14. How is the 19% energy saving calculated? Please add a supporting note\n15. No further comments\n16. No further comments\n17. No further comments\n18. No further comments\n19. No further comments\n20. No further comments\n21. No further comments"}, "author_response": "We sincerely thank the reviewers for their thoughtful evaluation and constructive feedback on our manuscript. Their insightful comments have been invaluable in enhancing the clarity, depth, and overall quality of our work. All revisions have been carefully made and are highlighted in yellow for ease of review. We have thoroughly addressed each suggestion and provided detailed responses below, outlining the corresponding changes implemented in the manuscript."}, {"round": 3, "reviewer_comments": {"reviewer_4": "Wu et al. have further revised their work. My concerns have been addressed very well and I have no further questions or comments. I recommend publication of the work in Nature Communications.\nOverall, the authors present an interesting study of urea oxidation using a coordination polymer that shows good performance for the reaction. They highlight the role of ligand vacancies to optimize bond formation, which is an attractive strategy to optimize future catalysts. It is an interesting and relevant insight for optimizing catalysis and for modifying materials by reconstruction."}, "author_response": ""}]

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