Unveiling proton-regulated Mo=O/Mo-O^- centers in reconstructed Ni₂Mo-NiOOH/MoO_x interfaces toward accelerated proton-coupled electron transfer for urea oxidation reaction

Chemicals and reagents

Nickel sulfate hexahydrate (NiSO₄·6H₂O), concentrated hydrochloric acid (HCl), and potassium hydroxide (KOH) were purchased from Tianjin Damao Chemical Reagent Factory with analytical reagent (AR) grade. Sodium molybdate dihydrate (Na₂MoO₄·2H₂O) and urea (CO(NH₂)₂) were obtained from Tianjin Guangfu Technology Development Co., Ltd. Sodium citrate (Na₃C₆H₅O₇) was supplied by Tianjin Aqiluopu Trading Co., Ltd., and anhydrous ethanol (C₂H₆O) was purchased from Tianjin Huihang Chemical Technology Co., Ltd. All the above reagents were used directly without further purification. Nickel foam (NF) with a thickness of approximately 1.5 mm was acquired from Tianjin Anuohe New Energy Technology Co., Ltd. and pretreated before use. In addition, sufficient deionized water was used throughout the entire experimental process.

Preparation of Ni₂Mo/NF

4 mmol NiSO₄·6H₂O, 2 mmol Na₂MoO₄·2H₂O and 4.3 mmol Na₃C₆H₅O₇ were separately dissolved in 50 mL deionized water, followed by magnetic stirring for 30 min. Afterward, the pH value of the mixed solution was adjusted to 9 using ammonia solution. Electrodeposition was performed at 40 °C for 10 min under a current density of -180 mA·cm^-2, with NF as the working electrode, graphite rod as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. The obtained electrode was rinsed three times with deionized water and then dried in a vacuum drying oven at 60 °C for 6 h. The as-prepared Ni₂Mo/NF catalytic electrode was finally sealed and stored for subsequent use.

Preparation of Ni₂Mo-NiOOH/MoO_x via in situ reconstruction

In the electrolyte containing 1 M KOH and 0.33 M urea, in situ reconstruction was conducted at a current density of 100 mA·cm^-2 for 60 min. Ni₂Mo/NF served as the working electrode, graphite rod as the counter electrode, and SCE as the reference electrode. After reconstruction, the sample was ultrasonically treated and dried in an oven to obtain the final product of Ni₂Mo-NiOOH/MoO_x.

Detailed information on material characterizations and electrochemical measurements is provided in the Experimental Section of the Supporting Information.

Morphological and composition analysis

To clarify the correlation between morphological evolution and phase structure of in situ reconstructed NiMo-based electrocatalysts, systematic scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations were conducted on the as-prepared catalytic electrodes. As shown in Figure 1A₁ and A₂, the control sample Ni-NiOOH exhibits a uniform nanoparticle array morphology with a compact and flat substrate surface. In comparison, the Ni₂Mo alloy [Figure 1B₁ and B₂] displays a denser particle stacking structure and presents typical alloy substrate features under low-magnification SEM. Further TEM characterization [Supplementary Figure 1] confirms its dense bulk morphology via bright-field TEM observation. The distinct lattice fringes with an interplanar spacing of 0.217 nm in high-resolution transmission electron microscopy (HRTEM) correspond to the (111) crystal plane of Ni₂Mo/NF, demonstrating the successful construction of highly crystalline Ni₂Mo/NF. The polycrystalline ring patterns in selected area electron diffraction (SAED) further validate the crystallinity of the precursor, which provides a fundamental structural basis for the subsequent anodic reconstruction. After anodic in situ reconstruction, SEM images of Ni₂Mo-NiOOH/MoO_x [Figure 1C₁ and C₂] show that the original granular morphology disappears and transforms into a rough, defect-enriched reconstructed layer. The inhomogeneous roughening of the substrate surface directly evidences the drastic surface reconstruction during anodic oxidation. Meanwhile, the introduction of Mo induces surface reconstruction and defect exposure, which not only enlarges the electrolyte contact area and increases the density of active sites, but also creates open channels for subsequent proton transfer.

Figure 1. Morphology and structural characterization of the as-prepared catalysts. (A₁ and A₂) SEM images of Ni-NiOOH; (B₁ and B₂) SEM images of Ni₂Mo/NF; (C₁ and C₂) SEM images of Ni₂Mo-NiOOH/MoO_x; (D) TEM image of Ni₂Mo-NiOOH/MoO_x; (E) HRTEM image of Ni₂Mo-NiOOH/MoO_x; (F) SAED pattern of Ni₂Mo-NiOOH/MoO_x; (G) EDS elemental mapping images of Ni₂Mo-NiOOH/MoO_x (Ni, Mo, O, N). SEM: Scanning electron microscopy; NF: nickel foam; HRTEM: high-resolution transmission electron microscopy; EDS: energy-dispersive X-ray spectroscopy; SAED: selected area electron diffraction.

TEM images of Ni₂Mo-NiOOH/MoO_x [Figure 1D] reveal that the overall morphology is well preserved after reconstruction, while a thin and translucent reconstructed shell layer emerges at the edge, which is identified as an amorphous/low-crystalline layer. This indicates that the reconstruction process only occurs on the material surface, and the inner core maintains the Ni₂Mo/NF phase. Such a core-shell structure provides rapid pathways for charge transport. HRTEM images [Figure 1E] clearly identify a triphase heterostructure, including the amorphous MoO_x phase (d = 0.374 nm), crystalline NiOOH active phase (d = 0.209 nm), and Ni₂Mo/NF core (d = 0.217 nm). This result directly confirms the formation of Ni₂Mo-NiOOH/MoO_x heterostructure after in situ reconstruction. The intimate contact of the triphase interfaces builds a favorable microenvironment for the synergistic transfer of electrons and protons. The coexistence of crystalline polycrystalline rings and amorphous halos in the SAED pattern [Figure 1F] further verifies the crystalline-amorphous composite structure. Such amorphous features are conducive to enhancing the dynamic surface reconstruction ability and regulating the adsorption behavior of reaction intermediates. Moreover, energy-dispersive X-ray spectroscopy (EDS) elemental mapping [Figure 1G] confirms the coexistence and distribution of Ni, Mo, O and N elements in the reconstructed catalyst. Although the signals of Mo and N are relatively weak due to their low content and surface-related distribution, they can still be detected throughout the material. The presence of Mo indicates the formation of MoO_x during the reconstruction process, while the N signal is mainly attributed to the adsorption of residual urea molecules and N-containing intermediates on the catalyst surface during UOR. This endows abundant Mo=O/Mo-O^- proton-capturing sites for UOR. The above characterization results verify that the in situ constructed NiOOH active sites are closely interfaced with MoO_x proton-capturing sites, laying a solid structural foundation for the electrocatalytic reaction.

The phase composition and crystal structure of the samples were systematically analyzed via X-ray diffraction (XRD) [Figure 2A]. The characteristic diffraction peaks of the Ni₂Mo/NF catalytic electrode located at 43.77°, 50.69°, and 73.65° correspond to the (111), (200), and (220) crystal planes, respectively, which are highly consistent with the standard card of NiMo (PDF#03-2714), confirming the successful preparation of Ni₂Mo/NF. The diffraction peaks of the control sample Ni-NiOOH match well with the standard card of β-NiOOH (PDF#06-0141), providing a benchmark for the phase identification of the reconstructed phase. After anodic in situ reconstruction, the XRD pattern of Ni₂Mo-NiOOH/MoO_x exhibits obvious phase evolution. The intensity of the characteristic peaks belonging to the Ni₂Mo precursor decreases significantly, indicating that partial reconstruction of the alloy phase occurs during anodic oxidation. Meanwhile, new characteristic peaks emerge at 37.28° and 66.76°, which are assigned to the (002) and (110) crystal planes of β-NiOOH and perfectly match PDF#06-0141. This result verifies the generation of crystalline NiOOH active phases during the reconstruction process. Combined with TEM analysis, the newly formed MoO_x presents an amorphous structure; thus, no crystalline diffraction peaks of MoO_x are detected. Fourier transform infrared spectroscopy (FT-IR) spectroscopy was employed to elucidate the evolution of chemical bonds and functional groups on the catalyst surface [Figure 2B]. Ni₂Mo/NF is dominated by the metallic state with weak signals of metal-oxygen bonds and hydroxyl functional groups. After anodic in situ reconstruction, Ni₂Mo-NiOOH/MoO_x shows remarkably enhanced characteristic absorption peaks in the range of 400-800 cm^-1, which are attributed to the Ni-O bonds of NiOOH as well as the Mo-O and Mo=O bonds of MoO_x species. This confirms the successful formation of NiOOH active phases and MoO_x components after reconstruction. Additionally, the stretching vibration intensity of hydroxyl O-H groups at 3,200-3,600 cm^-1 increases obviously, suggesting abundant enrichment of hydroxyl groups on the catalyst surface. Furthermore, the rich Mo=O/Mo-O^- functional groups in MoO_x can serve as efficient proton-capturing sites. Meanwhile, the hydrogen bond network constructed by surface *OH effectively connects proton transport pathways, accelerating the migration rate of H⁺ released from the dehydrogenation of *-NH₂ during urea oxidation and facilitating the PCET process. These results corroborate the rationality of the material structural design from the molecular functional group perspective.

Figure 2. Phase, surface chemical and valence state characterization of catalytic electrodes. (A) XRD patterns of Ni/NF, Ni-NiOOH, Ni₂Mo/NF and Ni₂Mo-NiOOH/MoO_x; (B) FT-IR spectra of the above catalytic electrodes; (C-E) XPS spectra of Ni₂Mo/NF and Ni₂Mo-NiOOH/MoO_x: (C) Ni 2p, (D) Mo 3d, (E) O 1s; (F) N 1s XPS spectrum of Ni₂Mo-NiOOH/MoO_x. XRD: X-ray diffraction; NF: nickel foam; FT-IR: Fourier transform infrared spectroscopy; XPS: X-ray photoelectron spectroscopy.

X-ray photoelectron spectroscopy (XPS) characterization was carried out to further investigate the electronic structure, elemental valence distribution and electron transfer behavior of the catalysts before and after in situ reconstruction. All binding energies were calibrated with the C 1s characteristic peak at 284.8 eV as the reference [Supplementary Figure 2]. The XPS survey spectra show that Ni₂Mo/NF mainly presents characteristic signals of Ni and Mo, while the response of O element is extremely weak. After anodic in situ reconstruction, the signal intensity of O element in Ni₂Mo-NiOOH/MoO_x is enhanced, and the characteristic peaks of Mo remain stable. It is confirmed that Mo species do not leach during the reconstruction process, but undergo synchronous oxidation and anchor on the catalyst surface, forming a composite structure with coexisting NiOOH and MoO_x phases. As shown in Figure 2C, in the Ni 2p spectra, Ni species in Ni₂Mo/NF are dominated by low-activity Ni²⁺. The proportion of Ni³⁺ in the Ni 2p_3/2 orbital is only 13.34%, and that in the Ni 2p_1/2 orbital is 7.28%. After anodic in situ reconstruction, the proportion of Ni³⁺ in Ni₂Mo-NiOOH/MoO_x increases remarkably to 17.76% in Ni 2p_3/2 and 7.98% in Ni 2p_1/2 [Supplementary Table 1]. The enrichment of high-valent Ni³⁺ species directly confirms the generation of crystalline NiOOH active phases. Meanwhile, the binding energy of the Ni 2p orbital shifts toward a higher value after reconstruction, which originates from the Mo-induced charge rearrangement effect. Mo has higher electronegativity than Ni, and the formation of MoO_x species during reconstruction induces electron transfer from Ni sites toward Mo. This reduces the electron density around Ni atoms, strengthens the constraint of atomic nuclei on electrons, increases the energy required for electron escape, and ultimately results in a positive shift of binding energy. Such electronic structure regulation promotes the stable formation of high-valent Ni³⁺ active sites, providing electronic structural guarantee for efficient UOR. As shown in Figure 2D and Supplementary Table 2, the Mo 3d XPS spectra reveal the coexistence of Mo species with different oxidation states in the Ni₂Mo/NF catalysts. After anodic electrochemical reconstruction, the relative contributions of different Mo oxidation-state components undergo noticeable changes, indicating a redistribution of the surface chemical states of Mo species, accompanied by the evolution of their local coordination environments. The changes in Mo oxidation states reflect the complex surface reconstruction process associated with the formation of MoO_x-related species. Meanwhile, the electron-enriched MoO_x species possess more coordinatively unsaturated sites, particularly Mo=O/Mo-O^- species. These negatively charged oxygen sites can serve as efficient proton-capturing sites, rapidly capturing H⁺ released during the dehydrogenation of *-NH₂ intermediates in UOR, thereby accelerating the PCET process, preventing proton accumulation on the catalyst surface, and lowering the energy barrier of the key reaction step. As shown in Figure 2E, the O 1s spectra exhibit two distinguishable oxygen-containing components. The component located at approximately 531.1 eV is associated with oxygen-containing coordination environments, whereas the higher-binding-energy component at approximately 532.9 eV is related to surface-adsorbed oxygen-containing species. These changes indicate the evolution of the surface oxygen chemical environment of the catalyst during electrochemical reconstruction. As presented in Figure 2F, the N 1s spectrum of Ni₂Mo-NiOOH/MoO_x exhibits distinct characteristic signals, which can be attributed to the residual adsorption signals of urea molecules and dehydrogenated *-NH₂ intermediates on the catalytic electrode after in situ reconstruction. This confirms the excellent capacity of the catalyst surface for effective adsorption and activation of reaction intermediates, providing direct electronic structural evidence for the UOR reaction pathway. In general, XPS characterization clarifies the regulatory effect of anodic in situ reconstruction on the electronic structure of catalysts from the perspectives of elemental valence state, electron density and charge rearrangement mechanisms. On the one hand, the reconstruction process promotes the transformation of Ni species into high-valent Ni³⁺, and the increase in the proportion of Ni³⁺ directly raises the number of UOR active sites. On the other hand, the Mo-induced charge rearrangement effect remodels the electronic environment of Ni and Mo sites, reducing the electron density of Ni active sites while increasing that of Mo sites. It not only optimizes the adsorption behavior of reaction intermediates, but also provides an electron-enriched structural basis for MoO_x proton-capturing sites. The synergistic optimization of valence state transition and electron cloud distribution not only realizes the synergistic regulation of NiOOH active sites and MoO_x proton-capturing sites, but also elucidates the intrinsic mechanism of Ni₂Mo-NiOOH/MoO_x accelerating UOR reaction kinetics from the electronic perspective. This work provides direct electronic structural evidence for the proposed in situ reconstruction synergistic catalytic strategy.

In situ Raman analysis during UOR

To reveal the in situ phase evolution and proton-capturing mechanism of the Ni₂Mo-NiOOH/MoO_x catalytic electrode in the UOR, in situ Raman spectroscopy was adopted to real-time track the surface structure and species evolution of the catalyst during anodic polarization. Figure 3 compares Ni-NiOOH and Ni₂Mo-NiOOH/MoO_x in terms of spectral features in different wavenumber ranges and three-dimensional in situ Raman profiles under UOR conditions. The region of 440-600 cm^-1 is assigned to the bending and stretching vibration signals of Ni-O bonds in NiOOH. For the Ni-NiOOH catalytic electrode [Figure 3A], obvious characteristic Raman peaks of NiOOH can only be observed at a relatively high anodic polarization potential, and the peak intensity increases slowly with rising potential. This indicates that without amorphous MoO_x, the in situ reconstruction of Ni sites into the high-activity NiOOH phase exhibits a delayed process with a high phase-transition energy barrier, which is unfavorable for the rapid initiation of UOR. In contrast, Ni₂Mo-NiOOH/MoO_x presents distinct NiOOH characteristic signals even at a much lower applied potential [Figure 3D], and the peak intensity grows rapidly as the anodic potential increases. This confirms that the introduction of amorphous MoO_x effectively reduces the energy barrier for in situ reconstruction of the active phase. Meanwhile, amorphous MoO_x induces local charge rearrangement, modulates the electronic structure of Ni active centers, and optimizes the adsorption configuration and adsorption strength of UOR intermediates. Characteristic signals associated with MoO_x-related species can be further identified in the high wavenumber range of 980-1,020 cm^-1. Combined with the two-dimensional stacked in situ Raman spectra [Supplementary Figure 3], the Ni-NiOOH system shows no obvious characteristic response in this wavenumber region throughout the test [Supplementary Figure 3A]. By comparison, the Ni₂Mo-NiOOH/MoO_x sample [Supplementary Figure 3B] displays well-resolved characteristic Raman peaks that gradually strengthen with increasing potential. These signals are attributed to the stretching vibrations of terminal Mo=O double bonds and coordinatively unsaturated Mo-O^- species in the MoO_x phase, which directly proves that the anodic UOR environment can induce the in situ generation of MoO_x active components rich in Mo=O/Mo-O^- on the catalyst surface. Moreover, the characteristic peaks of carbonate intermediates derived from UOR evolve synchronously within 1,040-1,100 cm^-1 [Figure 3C and Figure 3F]. Comparative results demonstrate that Ni₂Mo-NiOOH/MoO_x possesses a lower onset potential and a more prominent signal response for intermediate peaks. It indicates that Mo=O/Mo-O^- sites can rapidly capture H⁺ released from the dehydrogenation of *-NH₂ during urea oxidation, timely alleviate interfacial proton accumulation, markedly lower the energy barrier of the key PCET step, and accelerate the dehydrogenation and intermediate transformation kinetics of UOR. The three-dimensional in situ Raman spectra in Figure 3B and Figure 3E further verify that the NiOOH active phase signals, MoO_x characteristic peaks at 980-1,020 cm^-1, and high-wavenumber intermediate peaks of the Ni₂Mo-NiOOH/MoO_x system follow synchronous potential-dependent evolution, with highly consistent onset polarization potentials. This confirms that MoO_x species are not inert substrates but synergistically participate in the UOR catalytic process together with NiOOH active sites. To distinguish the behavioral differences between UOR and OER and exclude the interference of structural evolution, in situ Raman tests of Ni₂Mo-NiOOH/MoO_x were conducted under OER conditions [Supplementary Figure 4]. The results show that only characteristic vibration signals of NiOOH can be detected during OER, while no obvious Mo=O/Mo-O^- characteristic response appears in the 980-1,020 cm^-1 region. This fully verifies that the MoO_x characteristic signals in this wavenumber range are specifically triggered by the dehydrogenation and proton-releasing process of *-NH₂ in UOR, which directly corresponds to proton-capturing behavior rather than simple potential-induced phase transition.

Figure 3. Operando Raman spectra of catalytic electrodes under UOR. (A-C) Operando Raman spectra of Ni-NiOOH at varied anodic potentials: (A) 2D contour plot in the range of 440-600 cm^-1; (B) corresponding 3D waterfall plot; (C) 2D contour plot in the range of 980-1,200 cm^-1; (D-F) Operando Raman spectra of Ni₂Mo-NiOOH/MoO_x at varied anodic potentials: (D) 2D contour plot in the range of 440-600 cm^-1; (E) corresponding 3D waterfall plot; (F) 2D contour plot in the range of 980-1,200 cm^-1. RHE: Reversible hydrogen electrode; UOR: urea oxidation reaction.

In summary, the in situ Raman results confirm that a Ni₂Mo-NiOOH/MoO_x composite active interface is in situ constructed on Ni₂Mo/NF under anodic UOR conditions. In this system, NiOOH acts as the core oxidative active site to drive urea molecule adsorption and bond dissociation, while Mo=O/Mo-O^- species in MoO_x characterized at 980-1,020 cm^-1 serve as efficient proton-capturing sites. These sites rapidly anchor and transfer protons released from *-NH₂ dehydrogenation, relieve interfacial proton accumulation, and accelerate the PCET process. Meanwhile, Mo-induced electronic rearrangement finely regulates the adsorption behavior of reaction intermediates and realizes the synergistic coupling of electron transfer and proton transport. Collectively, these effects enhance the UOR catalytic performance of the catalyst from both structural and kinetic perspectives.

Electrochemical performance analysis for UOR

Electrochemical measurements were performed to verify the synergistic effect between the in situ reconstructed Ni₂Mo-NiOOH/MoO_x catalytic electrode and MoO_x proton-capturing sites from the perspectives of catalytic activity, reaction kinetics and reaction selectivity. A standard three-electrode configuration was adopted throughout the electrochemical tests. The UOR performance of the catalysts was evaluated in 1.0 M KOH solution containing 0.33 M urea, while the OER performance was measured in pure 1.0 M KOH electrolyte. Optimization experiments on the Ni/Mo precursor molar ratio [Supplementary Figure 5A] and in situ reconstruction duration of Ni₂Mo/NF [Supplementary Figure 5B] were systematically conducted. The results demonstrate that Ni₂Mo/NF with a Ni/Mo ratio of 2 exhibits the optimal UOR activity. This can be attributed to the balanced synergy between NiOOH active sites and MoO_x proton-capturing sites. An insufficient Mo content leads to the formation of fewer MoO_x species and limited proton-capturing capability, whereas excessive Mo incorporation reduces the proportion of NiOOH active sites, resulting in an unfavorable catalytic balance. Moreover, the Ni₂Mo-NiOOH/MoO_x electrode obtained after 60 min of in situ reconstruction delivers the best UOR performance, providing the basis for subsequent electrochemical studies.

The linear sweep voltammetry (LSV) curves in Figure 4A compare the current density of bare Ni/NF, Ni-NiOOH, Ni₂Mo/NF and in situ reconstructed Ni₂Mo-NiOOH/MoO_x at the same applied potential. It is clearly observed that Ni₂Mo-NiOOH/MoO_x with MoO_x proton-capturing sites induced by anodic reconstruction exhibits remarkably superior UOR performance to other control catalysts. It should be noted that the potential differences presented in the inset of Figure 4A were calculated by subtracting the theoretical equilibrium potential of OER [1.23 V vs. reversible hydrogen electrode (RHE)] from the measured UOR operating potentials. Here, the OER equilibrium potential is employed solely as a common reference benchmark to facilitate a comparative evaluation of the operating potentials required by different catalysts. The as-prepared Ni₂Mo-NiOOH/MoO_x exhibits outstanding UOR activity, requiring only 1.341 and 1.352 V vs. RHE to reach current densities of 100 and 200 mA·cm^-2, respectively, corresponding to only 111 and 122 mV higher potentials than the theoretical OER potential (1.23 V). These values are far lower than those of Ni₂Mo/NF (125 mV, 151 mV), Ni-NiOOH (183 mV, 252 mV) and Ni/NF (213 mV, 313 mV). Meanwhile, Ni₂Mo-NiOOH/MoO_x also shows better OER performance than other counterparts [Supplementary Figure 6A]. The enhanced current density at the same potential originates from the spontaneous reconstruction of Ni₂Mo/NF into high-activity NiOOH phases under anodic conditions, which provides abundant oxidative active sites for UOR. Meanwhile, the generated MoO_x is rich in Mo=O/Mo-O^- species, which act as efficient proton-capturing sites to rapidly trap H⁺ released during the dehydrogenation of *-NH₂ and alleviate interfacial proton accumulation. Furthermore, Mo-induced charge rearrangement further optimizes the adsorption-desorption behavior of UOR intermediates. The synergistic effect of the above two aspects greatly improves the catalytic activity and charge transport efficiency. Tafel slope analysis based on Figure 4B shows that Ni₂Mo-NiOOH/MoO_x possesses the smallest Tafel slope of 32.36 mV·dec^-1, which is markedly lower than those of Ni₂Mo/NF (64.76 mV·dec^-1), Ni-NiOOH (143.06 mV·dec^-1) and Ni/NF (166.72 mV·dec^-1). The Tafel slope directly reflects the energy barrier of the rate-determining step (RDS). This result confirms that the synergistic cooperation between NiOOH active sites and MoO_x proton-capturing sites effectively reduces the energy barrier of the PCET step during UOR. The proton-capturing effect of MoO_x species accelerates the rapid diversion of interfacial protons and avoids kinetic hysteresis caused by proton accumulation. In addition, Mo-induced local charge rearrangement optimizes the charge transport pathway and intermediate transformation process, thereby remarkably accelerating the reaction rate and achieving superior reaction kinetics. To further verify the product selectivity of the UOR process, the generated N₂ product was quantitatively analyzed, and the corresponding Faradaic efficiency (FE) was calculated. As shown in Supplementary Figure 6B, the amount of N₂ continuously increases with electrolysis time, exhibiting an approximately linear growth trend, which indicates the stable conversion of urea during the electrochemical oxidation process. Moreover, the FE for N₂ production remains at a high level of approximately 93%-95% throughout the electrolysis period, demonstrating the high selectivity of Ni₂Mo-NiOOH/MoO_x toward nitrogen conversion.

Figure 4. Electrocatalytic UOR performance characterization of various catalysts. (A) LSV curves of Ni₂Mo-NiOOH/MoO_x, Ni₂Mo/NF, Ni-NiOOH and Ni/NF (Inset: Comparison of potential differences relative to the OER equilibrium potential for different catalysts at specific current densities); (B) Corresponding Tafel slope curves (R² values above 0.95); (C) Nyquist impedance plots and equivalent circuit model; (D) Bode phase plots; (E) C_dl plots (R² values above 0.99); (F) ECSA-normalized LSV curves; (G) LSV curve comparison between UOR and OER of Ni₂Mo-NiOOH/MoO_x; (H) 300 h i-t test of Ni₂Mo-NiOOH/MoO_x at the current density of 100 mA·cm^-2, with the LSV curves before and after the stability test shown in the inset; (I) Comparison of the potential of Ni₂Mo-NiOOH/MoO_x and other UOR catalysts at a current density of 100 mA·cm^-2. NF: Nickel foam; LSV: linear sweep voltammetry; UOR: urea oxidation reaction; OER: oxygen evolution reaction; ECSA: electrochemically active surface area; C_dl: double-layer capacitance.

Electrochemical impedance spectroscopy (EIS), phase angle analysis, cyclic voltammetry (CV) and double-layer capacitance (C_dl) measurements were performed to clarify the synergistic enhancement mechanism of the in situ reconstructed Ni₂Mo-NiOOH/MoO_x composite structure and MoO_x proton-capturing sites toward UOR performance from the aspects of charge transport efficiency, active site exposure and intrinsic reaction kinetics.

In terms of charge transport, the Nyquist plots presented in Figure 4C and Supplementary Table 3 show the impedance values of Ni₂Mo-NiOOH/MoO_x, Ni₂Mo/NF, Ni-NiOOH and Ni/NF measured at an open-circuit potential of 0.23 V are 5.81, 10.43, 16.45 and 19.64 Ω, respectively. Among all samples, Ni₂Mo-NiOOH/MoO_x exhibits the smallest charge transfer resistance (R_ct), implying the lowest charge transfer barrier at the electrode/electrolyte interface. This favorable characteristic originates from the local charge rearrangement induced by Mo during in situ reconstruction, which optimizes the electron conduction pathway of the composite architecture and reduces the intrinsic electron transport resistance. Meanwhile, the MoO_x proton-capturing sites generated during UOR can rapidly guide interfacial proton migration and avoid interfacial polarization caused by proton accumulation from the dehydrogenation of *-NH₂, thereby greatly improving the charge transfer efficiency. Compared with the Nyquist plots under OER conditions [Supplementary Figure 7A], although Ni₂Mo-NiOOH/MoO_x still delivers a relatively low impedance value, its overall R_ct is remarkably higher than that under UOR, and the performance gap with other samples is narrowed. This demonstrates that its low-resistance interfacial property is a specific optimization targeted at the UOR process, rather than a general enhancement of electron conduction. The Bode phase angle plots [Figure 4D and Supplementary Figure 7B] further confirm that the phase angle peak of Ni₂Mo-NiOOH/MoO_x under UOR appears at a higher frequency region with a broader peak shape. This indicates faster high-frequency charge transfer and a stronger coupling effect between the low-frequency electric double layer and Faradaic reaction, which directly reflects that MoO_x proton-capturing sites accelerate interfacial proton delivery and realize efficient coupling of charge and proton transfer. In contrast, the position difference of phase angle peaks among various samples under OER is negligible, failing to embody the superiority of this synergistic effect.

CV curves recorded at various scan rates under UOR conditions [Supplementary Figure 8] show that Ni₂Mo-NiOOH/MoO_x possesses a much larger enclosed CV area than all control samples, indicative of a higher double-layer capacitance and enhanced electrochemically active surface area (ECSA). Under OER conditions [Supplementary Figure 9], although Ni₂Mo-NiOOH/MoO_x still exhibits a larger CV area, the overall enhancement degree is far lower than that under UOR, further demonstrating that its active sites are more sufficiently utilized during the UOR process. The C_dl values calculated from CV curves [Figure 4E, Supplementary Figure 10 and Supplementary Table 4] reveal that Ni₂Mo-NiOOH/MoO_x delivers a markedly higher C_dl than Ni₂Mo/NF, Ni-NiOOH and Ni/NF. Given the positive linear correlation between C_dl and ECSA, a larger electrochemically active surface area is confirmed for Ni₂Mo-NiOOH/MoO_x. This superiority originates from the structural optimization induced by in situ reconstruction. During the transformation of Ni₂Mo/NF into the Ni₂Mo-NiOOH/MoO_x composite structure, abundant active crystal facets and reaction sites are formed on the surface. Meanwhile, the introduction of Mo=O/Mo-O^- optimizes the surface hydrophilicity of the electrode, facilitates electrolyte infiltration, and further improves the accessibility of active sites, providing sufficient reaction interfaces for UOR. As displayed in the ECSA-normalized LSV curves [Figure 4F], Ni₂Mo-NiOOH/MoO_x achieves a higher current density per unit electrochemically active area than other samples. This result indicates that the excellent UOR performance is not merely attributed to the increased number of active sites, but essentially stems from the substantially enhanced intrinsic catalytic activity of individual active sites. The improvement of intrinsic activity strongly verifies the synergistic regulation mechanism between NiOOH active sites and MoO_x proton-capturing sites. NiOOH serves as the core oxidative site to drive the adsorption and bond dissociation of urea molecules, while Mo=O/Mo-O^- species rapidly capture the released H⁺ during dehydrogenation, alleviate interfacial proton accumulation, and lower the energy barrier of the PCET step. Moreover, Mo-induced local charge rearrangement modulates the electronic structure of Ni active centers and regulates the adsorption-desorption behavior of UOR intermediates, thereby remarkably boosting the reaction rate of individual active sites.

Figure 4G shows the LSV comparison between UOR and OER, which further reflects the specific performance advantages of the catalyst. At the same current density, Ni₂Mo-NiOOH/MoO_x exhibits remarkably higher activity in UOR than in OER, accompanied by a much lower overpotential. This superiority stems from the specific adaptation of MoO_x proton-capturing sites toward the UOR reaction. The Mo=O/Mo-O^- species in MoO_x can only accelerate the PCET process by capturing H⁺ released from the *-NH₂ dehydrogenation step during UOR, while they fail to exert the proton-capturing effect in OER lacking rapid proton release. This endows the catalyst with distinctive kinetic advantages toward UOR and achieves energy consumption reduction. A performance radar chart was adopted for comprehensive evaluation across multiple key indicators [Supplementary Figure 11], including Tafel slope, R_ct, low overpotential 1/E₁₀₀, as well as C_dl and ECSA closely related to active site density. The results reveal that Ni₂Mo-NiOOH/MoO_x delivers prominent superiority in all these parameters. A smaller Tafel slope represents favorable reaction kinetics to efficiently drive urea oxidation; a lower R_ct indicates ultrahigh charge transport efficiency and reduced energy loss; a higher 1/E₁₀₀ corresponds to a lower overpotential at 100 mA·cm^-2, reflecting outstanding energy-saving effect and lowering the energy input required for the reaction. Larger C_dl and ECSA values demonstrate abundant active sites on the electrode surface, providing sufficient catalytic interfaces for the reaction. These merits jointly construct an all-round optimized performance matrix, which strongly verifies that the synergistic cooperation between NiOOH active sites and MoO_x proton-capturing sites in the in situ reconstructed Ni₂Mo-NiOOH/MoO_x composite structure realizes the full-scale optimization of electrocatalytic performance.

Figure 4H presents the i-t curve of Ni₂Mo-NiOOH/MoO_x for continuous steady-state operation over 300 h at 100 mA·cm^-2. The current density maintains stable output without obvious attenuation throughout the ultra-long cycling period. Among them, the inset shows the LSV curves before and after the stability test, which almost completely overlap, and no obvious shift of catalytic overpotential is observed, fully demonstrating the excellent electrochemical durability of the catalyst. Such long-term stability benefits from the synergistic superiority of the in situ constructed Ni₂Mo-NiOOH/MoO_x composite structure. Thermodynamically stable NiOOH serves as the core oxidative active framework, which effectively inhibits the dissolution loss and excessive oxidation of active components under high anodic potential. Meanwhile, the Mo=O/Mo-O^- species in MoO_x can continuously capture H⁺ released from *-NH₂ dehydrogenation during UOR via reversible proton adsorption-desorption cycles, timely relieve interfacial proton accumulation and weaken local polarization corrosion, thus achieving dynamic self-maintenance of proton-capturing sites. Microscopic characterizations were performed on Ni₂Mo-NiOOH/MoO_x after 300 h continuous operation. SEM images [Supplementary Figure 12] confirm that the original microscopic morphology is well preserved without particle aggregation, structural collapse or exfoliation of active phases. Elemental mapping verifies the homogeneous distribution of Ni, Mo and O elements on the material surface. Further TEM characterization [Supplementary Figure 13] confirms at the microscopic scale that the catalyst retains an intact lattice structure without phase separation or structural collapse, and Ni, Mo and O elements still maintain a highly uniform distribution, proving the long-term stability of the heterogeneous interface constructed by NiOOH and MoO_x. Multi-current step V-t test results [Supplementary Figure 14] show that Ni₂Mo-NiOOH/MoO_x can rapidly reach potential equilibrium at each current gradient within a wide current density range of 100-400 mA·cm^-2, without obvious potential drift or aggravated polarization. Even under harsh working conditions at high current densities, NiOOH active sites and MoO_x proton-capturing sites still maintain efficient synergistic coupling. By virtue of Mo-induced charge rearrangement to optimize the electron transport pathway and relying on MoO_x to rapidly dissipate enriched protons, the electrode interface always maintains an efficient and stable charge transfer environment under wide-load working conditions. The microscopic characterizations after long-term reaction confirm the intrinsic origin of catalytic stability from the structural perspective. Furthermore, as shown in Figure 4I and Supplementary Table 5, comparison with other modified materials confirms that Ni₂Mo-NiOOH/MoO_x delivers a particularly remarkable performance enhancement.

Ion dependence analysis

To directly verify the pivotal role of Mo=O/Mo-O^- species in the in situ reconstructed MoO_x as proton-capturing sites, control experiments were conducted by introducing extra MoO₄^2- and Ni²⁺ into the electrolyte, and the relevant results are displayed in Figure 5. In the MoO₄^2- addition experiment, the LSV curves of the Ni₂Mo-NiOOH/MoO_x catalyst only show a negligible response with the increase of MoO₄^2- concentration [Figure 5A], and the curves almost overlap with those obtained in the original electrolyte, presenting an obvious saturation effect. This is because abundant highly active Mo=O/Mo-O^- proton-capturing sites have been in situ generated on the electrode surface during anodic reconstruction. These sites are uniformly distributed on the surface of the NiOOH active phase, which can efficiently trap H⁺ released from *-NH₂ dehydrogenation during UOR, alleviate interfacial proton accumulation, and lower the energy barrier of the key PCET step. Therefore, the extra introduced solution-phase MoO₄^2- cannot form more effective proton-capturing sites on the pre-reconstructed electrode surface, resulting in a greatly limited performance improvement space. This directly confirms that the UOR performance of the catalyst mainly originates from the in situ generated MoO_x proton-capturing sites rather than free Mo species in the solution. In sharp contrast, the control catalytic electrodes including Ni₂Mo/NF, Ni-NiOOH and Ni/NF exhibit an obvious response effect, where the LSV curves shift toward the left and the current density increases remarkably with the rising MoO₄^2- concentration [Figure 5B-D]. This phenomenon arises from the lack of intrinsic MoO_x proton-capturing sites on the surface of these catalysts. The dissolved MoO₄^2- in the solution can be in situ converted into Mo=O/Mo-O^- contained MoO_x species on the electrode surface under anodic potential, providing additional proton-capturing capacity for UOR, accelerating PCET kinetics, and thereby substantially reducing the overpotential and boosting catalytic activity. This result indirectly corroborates the essential function of Mo=O/Mo-O^- as proton-capturing sites, and also demonstrates that the MoO_x sites derived from in situ reconstruction possess higher stability and efficiency than those formed by Mo species introduced from the bulk solution. In the Ni²⁺ addition tests [Figure 5E-H], the LSV curves of all catalysts remain nearly unchanged and their performance is barely affected by the variation of Ni ion concentration in the electrolyte. This excludes the interference of solution-phase Ni species on UOR activity and further confirms that the performance modulation is specifically attributed to the efficient proton-capturing behavior of Mo=O/Mo-O^- species in MoO_x. Such species can synergize with NiOOH active sites to realize the synergistic regulation of electron and proton transfer during UOR, thereby remarkably accelerating the reaction kinetics.

Figure 5. Effects of different concentrations of MoO₄^2- and Ni²⁺ in the electrolyte on the UOR electrocatalytic performance of various catalytic electrodes. (A-D) LSV curves of Ni₂Mo-NiOOH/MoO_x, Ni₂Mo/NF, Ni-NiOOH and Ni/NF catalytic electrodes with 0-5 mM MoO₄^2- added into the electrolyte containing 1 M KOH and 0.33 M CO(NH₂)₂; (E-H) LSV curves of the above catalytic electrodes with 0-50 μM Ni²⁺ added into the electrolyte. RHE: Reversible hydrogen electrode; KOH: potassium hydroxide; LSV: linear sweep voltammetry; UOR: urea oxidation reaction.

Electrolyte concentration-dependent kinetic analysis

To intrinsically reveal the modulation mechanism of Mo=O/Mo-O^- proton-capturing sites in MoO_x on UOR reaction kinetics, electrolyte concentration-dependent kinetic tests and redox charge capacity characterization were performed^[51]. Direct experimental evidence for the synergistic catalytic mechanism was provided from three aspects, including reaction order, interfacial proton transfer efficiency, and active site utilization efficiency.

In the electrolyte concentration-dependent experiments [Figure 6], the RDS of UOR corresponds to the dehydrogenation of urea molecules. The H⁺ released in this process will accumulate at the electrode interface if not removed timely, inhibiting the forward progression of the reaction and inducing kinetic hysteresis. As shown in Figure 6A and C, the LSV curves of Ni₂Mo-NiOOH/MoO_x and Ni₂Mo/NF were obtained at different urea concentrations. The half-wave current density (j_1/2) exhibits a significant linear positive correlation with urea concentration, yielding a higher slope of 616.64 for Ni₂Mo-NiOOH/MoO_x [Figure 6B], which is much higher than the value of 280.48 for Ni₂Mo/NF [Figure 6D]. The stronger concentration dependence indicates that Ni₂Mo-NiOOH/MoO_x presents a more sensitive response to urea concentration and a remarkably enhanced urea conversion efficiency. The fundamental origin lies in the fact that the in situ generated Mo=O/Mo-O^- species can rapidly trap H⁺ released during dehydrogenation, effectively alleviate interfacial proton accumulation, and drive the forward progression of the PCET process. Accordingly, more urea molecules can be efficiently converted with the increase of urea concentration, and the increment of reaction rate is far larger than that of the precursor catalyst without efficient proton-capturing sites. Kinetic tests targeting KOH concentration further confirm the superiority of MoO_x proton-capturing sites. OH^- in the solution acts as both the reactant of UOR and the final proton acceptor. As shown in Figure 6E and G, the LSV curves of Ni₂Mo-NiOOH/MoO_x and Ni₂Mo/NF were obtained at different KOH concentrations. The slope calculated from the Log[j] vs. Log[KOH] fitting for Ni₂Mo-NiOOH/MoO_x is 0.546 [Figure 6F], slightly lower than 0.581 for Ni₂Mo/NF [Figure 6H], indicating that this catalyst has a weaker dependence on the OH^- concentration in the electrolyte. This difference originates from the reversible proton capture cycle of Mo=O/Mo-O^-. Mo=O can in situ capture H⁺ to form Mo-OH, and subsequently regenerate Mo=O by rapidly reacting with OH^-, thus constructing a closed loop of “capture-transfer-regeneration”. Efficient proton conduction can be maintained even at low OH^- concentrations. In contrast, Ni₂Mo/NF lacks such surface proton-capturing sites and has to completely depend on OH^- in the solution to accept H⁺, rendering it more sensitive to the variation of OH^- concentration and a higher reaction order. Redox charge capacity characterization [Supplementary Figure 15] further verifies the Mo-induced synergistic enhancement effect. The integrated charge (Q) calculated from the LSV curve of Ni₂Mo-NiOOH/MoO_x reaches up to 7,335.64 mC [Supplementary Figure 15A], nearly twice that of Ni₂Mo/NF (3,735.64 mC) [Supplementary Figure 15B], and far higher than those of Ni-NiOOH (2,735.64 mC) [Supplementary Figure 15C] and Ni/NF (1,601.64 mC) [Supplementary Figure 15D]. Such a remarkable difference stems from the dual regulatory effect of Mo. On the one hand, Mo-induced local charge rearrangement during in situ reconstruction optimizes the electronic structure of Ni sites, reduces the oxidation potential for the transformation from Ni²⁺ to Ni³⁺/Ni⁴⁺, facilitates the formation of high-activity NiOOH phase, and increases the number of active sites participating in UOR. On the other hand, the proton-capturing effect of MoO_x effectively relieves interfacial proton accumulation and local polarization, avoids the excessive oxidation or dissolution of Ni active sites at high potentials, and substantially improves the utilization efficiency of active sites. Meanwhile, the higher Q value also reflects the reversible redox contribution of Mo sites during proton capture. Mo=O/Mo-O^- undergoes reversible valence state variation in the process of capturing and transferring H⁺, further increasing the overall charge transfer capacity. This represents a unique synergistic effect that cannot be achieved by MoO_x-free catalytic electrodes.

Figure 6. Concentration-dependent kinetic analysis of Ni₂Mo-NiOOH/MoO_x and Ni₂Mo/NF catalysts. (A and C) LSV curves of Ni₂Mo-NiOOH/MoO_x and Ni₂Mo/NF at different urea concentrations (0.0125-0.33 M); (B and D) Linear fitting relationships between j_1/2 and urea concentration for Ni₂Mo-NiOOH/MoO_x and Ni₂Mo/NF, respectively, reflecting the concentration dependence toward urea oxidation; (E and G) LSV curves of Ni₂Mo-NiOOH/MoO_x and Ni₂Mo/NF at different KOH concentrations (0.1-1 M); (F and H) Log(j)-Log[KOH] fitting curves for Ni₂Mo-NiOOH/MoO_x and Ni₂Mo/NF, respectively, used to determine the apparent reaction order with respect to OH^- concentration. RHE: Reversible hydrogen electrode; NF: nickel foam; KOH: potassium hydroxide; LSV: linear sweep voltammetry; j_1/2: half-wave current density.

In summary, electrolyte concentration-dependent kinetic measurements and redox charge capacity characterization validate the core role of MoO_x proton-capturing sites in Ni₂Mo-NiOOH/MoO_x. By rapidly trapping H⁺ released in the dehydrogenation step of UOR, these sites alleviate interfacial proton accumulation and lower the energy barrier of the PCET step. Meanwhile, they optimize the electronic structure and utilization efficiency of Ni active sites, realizing the synergistic regulation of electron and proton transfer. This work provides solid experimental support for its superior UOR kinetic performance.

In situ electrochemical analysis

To reveal the dynamic evolutionary kinetics of interfacial charge transfer and proton transport at the electrode interface during UOR, in situ electrochemical EIS measurements were conducted at different potentials. Three-dimensional waterfall plots of Nyquist impedance spectra and Bode phase diagrams were utilized to real-time track the potential-dependent evolution of interfacial reaction behavior, providing in situ kinetic evidence for the regulatory effect of MoO_x proton-capturing sites.

As presented in Figure 7A, the R_ct of Ni₂Mo-NiOOH/MoO_x decreases remarkably with the increase of potential and shows an obvious decline even in the low-potential region, reaching the minimum value among all catalysts at 0.65 V. This phenomenon directly confirms the crucial role of in situ generated MoO_x species. During UOR, the dehydrogenation of urea-derived *-NH₂ intermediates releases a large quantity of H⁺. If these protons cannot be removed promptly, they will accumulate at the electrode interface, induce interfacial polarization, and substantially increase the R_ct. In contrast, the Mo=O/Mo-O^- species on the surface of Ni₂Mo-NiOOH/MoO_x can rapidly capture H⁺ released from the dehydrogenation process, construct interfacial proton transfer pathways, and effectively alleviate interfacial proton accumulation and polarization effects. Consequently, the R_ct is greatly reduced, establishing a low-resistance interfacial environment for the PCET process. In comparison, the R_ct of Ni₂Mo/NF [Figure 7B] varies slowly in the low-potential range and remains distinctly higher than that of Ni₂Mo-NiOOH/MoO_x, indicating that stable and efficient MoO_x proton-capturing sites are not formed and the capacity of interfacial proton conduction is insufficient. Furthermore, the R_ct variation of the Ni-NiOOH catalyst [Figure 7C] exhibits a more delayed trend, with only a limited decline at high potentials. This further verifies that the lack of proton-capturing sites leads to blocked charge transfer caused by interfacial proton accumulation and severe retardation of reaction kinetics. Bode phase diagrams [Figure 7D-F] further clarify the kinetic characteristics of interfacial reactions. The phase angle peak of Ni₂Mo-NiOOH/MoO_x shifts toward the high-frequency direction and continuously increases in intensity as the potential rises, implying a greatly shortened time constant of interfacial reactions and a markedly enhanced Faradaic reaction rate. The high-frequency shift of the phase angle peak directly reflects the acceleration effect of MoO_x proton-capturing sites on the PCET process. The Mo=O/Mo-O^- mediated rapid proton transfer effectively reduces the coupling resistance between charge transfer and proton transport, thereby lowering the kinetic time constant of UOR Faradaic reactions. Meanwhile, the continuous enhancement of peak intensity suggests that the Faradaic contribution of UOR increases gradually with elevated potential, enabling the catalyst to efficiently drive the reaction even at low applied potentials. On the contrary, the amplitude of the high-frequency shift of the phase angle peak for Ni₂Mo/NF is much smaller than that of Ni₂Mo-NiOOH/MoO_x, with a broadened peak shape and low intensity at low potentials, which reflects its limited interfacial proton transport efficiency and inadequate acceleration of the PCET process. The phase angle peak of Ni-NiOOH barely shifts within the entire potential range and maintains a consistently low intensity, further confirming that interfacial proton accumulation seriously retards reaction kinetics without the participation of proton-capturing sites.

Figure 7. Operando EIS characterization of various catalytic electrodes (potential range: 0.15-0.65 V vs. SCE). (A-C) 3D waterfall plots of Nyquist impedance spectra; (D-F) 3D waterfall plots of Bode phase diagrams; (A and D) Ni₂Mo-NiOOH/MoO_x; (B and E) Ni₂Mo/NF; (C and F) Ni-NiOOH. EIS: Electrochemical impedance spectroscopy; SCE: saturated calomel electrode; NF: nickel foam.

In summary, the dynamic characterization results of in situ EIS three-dimensional waterfall plots fully reveal the pivotal regulatory role of MoO_x proton-capturing sites in Ni₂Mo-NiOOH/MoO_x. By rapidly capturing H⁺ released during UOR dehydrogenation and constructing interfacial proton transfer pathways, these sites effectively mitigate interfacial polarization and proton accumulation, and synergistically optimize the kinetics of charge transfer and proton transport together with NiOOH active sites.

Purification performance of high-concentration urea wastewater

To evaluate the practical application potential of the Ni₂Mo-NiOOH/MoO_x electrocatalyst in urea-containing wastewater treatment and coupled urea oxidation systems, the diacetyl monoxime-antipyrine method (Chinese national standard method) was employed to measure the urea degradation efficiency^[52]. The urea degradation performance of various catalysts was systematically investigated through ultraviolet–visible spectroscopy (UV-Vis) absorption spectroscopy, solution color variation and real-time monitoring of urea concentration [Figure 8].

Figure 8. Urea degradation performance characterization of different catalytic electrodes. (A, D, G) UV-Vis absorption spectra at different reaction times; (B, E, H) Photographs of solution color evolution during the reaction; (C, F, I) Variation curves of relative urea concentration C/C₀ as a function of reaction time; where (A)-(C), (D)-(F), and (G)-(I) correspond to the test results of Ni₂Mo-NiOOH/MoO_x, Ni₂Mo/NF, and Ni-NiOOH catalysts, respectively. NF: Nickel foam.

As shown in Figure 8A, the characteristic absorption peak corresponding to urea degradation in the Ni₂Mo-NiOOH/MoO_x system decays rapidly with prolonged reaction time. After 420 min, the absorption intensity drops to only 2.79% of the initial value, corresponding to an ultrahigh urea removal efficiency of 97.21%. This value is remarkably superior to those of Ni₂Mo/NF (87.96%) [Figure 8D] and Ni-NiOOH (81.16%) [Figure 8G]. The solution color changes intuitively confirm such performance differences [Figure 8B, E and H]. The yellow color of the solution in the Ni₂Mo-NiOOH/MoO_x system fades rapidly as the reaction proceeds, while obvious color residue can still be observed in the control catalyst systems, directly reflecting Ni₂Mo-NiOOH/MoO_x system outstanding urea degradation capability. The time-dependent relative urea concentration curves [Figure 8C, F and I] further quantitatively reveal the kinetic differences. Ni₂Mo-NiOOH/MoO_x exhibits a much faster decline rate of urea concentration, with the C/C₀ value decreasing to 2.79% within 420 min. By contrast, the C/C₀ values of Ni₂Mo/NF and Ni-NiOOH remain as high as 12.04% and 18.84%, respectively, demonstrating the optimal urea conversion efficiency and reaction rate of Ni₂Mo-NiOOH/MoO_x. Such prominent superiority fundamentally originates from the synergistic regulation between NiOOH active sites and MoO_x proton-capturing sites. As the oxidative active center, NiOOH efficiently drives the adsorption and bond dissociation of urea molecules. Meanwhile, the in situ generated Mo=O/Mo-O^- species can rapidly trap H⁺ released during the *-NH₂ dehydrogenation step, effectively alleviate interfacial proton accumulation, markedly lower the energy barrier of the key PCET step, and accelerate UOR kinetics, thus achieving efficient urea degradation under practical working conditions. In comparison, Ni₂Mo/NF fails to form stable and efficient MoO_x proton-capturing sites, resulting in insufficient interfacial proton dissipation and restricted reaction kinetics. Ni-NiOOH without MoO_x proton-capturing sites lacks the proton capture mechanism; the accumulated interfacial protons retard the UOR process and greatly reduce the urea degradation efficiency.

From the perspective of practical application, Ni₂Mo-NiOOH/MoO_x achieves nearly complete urea removal within 420 min, showing great application potential in urea-containing wastewater treatment, direct urea fuel cells, and urea-assisted hydrogen production coupling systems. This work provides a feasible practical strategy for developing low-cost and high-activity electrocatalysts toward urea oxidation, and further verifies the reliability and superiority of this synergistic catalytic strategy under practical operating conditions.

Figure 9 illustrates the catalytic electrode with dynamically reconstructed Ni₂Mo-NiOOH/MoO_x, which possesses a heterogeneous interface co-constructed by NiOOH active phases and MoO_x species. The NiOOH phase functions as the core oxidative active center, providing reactive sites for the adsorption and stepwise dehydrogenation of urea molecules and driving the forward progression of UOR. Meanwhile, the synchronously generated MoO_x is rich in Mo=O/Mo-O^- species, which can act as efficient proton-capturing centers. These sites rapidly trap protons released from the *-NH₂ dehydrogenation step, effectively alleviate the polarization effect caused by interfacial proton accumulation, and markedly lower the reaction energy barrier of the key PCET step. Consequently, the synergistic regulation of oxidative catalysis and proton conduction is achieved, which substantially accelerates the reaction kinetics of UOR.

Figure 9. Schematic mechanism of the Ni₂Mo-NiOOH/MoO_x catalytic electrode during UOR. PCET: Proton-coupled electron transfer; UOR: urea oxidation reaction.