Modulating the Electronic Structure of Ni/NiO Nanocomposite with High‐Valence Mo Doping for Energy‐Saving Hydrogen Production via Boosting Urea Oxidation Kinetics

Electrocatalytic urea oxidation reaction (UOR) has emerged as a promising alternative to the anodic oxygen evolution reaction (OER) in water electrolysis. However, UOR faces challenges like slow kinetics, high energy barriers, and a complex mechanism, necessitating the development of efficient electrocatalysts. Herein, a rapid method is proposed for synthesizing Mo‐doped Ni/NiO (Ni/MNO) nanocomposite as a highly effective UOR electrocatalyst. Mo doping oxidizes Ni2+ to Ni3+, creating abundant active sites for UOR. The Ni/MNO catalyst exhibits remarkable activity for both OER and UOR due to Mo doping, structural modulation, increased active sites, and the presence of Ni3+ ions. Optimized Ni/MNO‐10 shows a low OER overpotential of 280 mV and a UOR working potential of 1.37 V versus reversible hydrogen electrode at 10 mA cm−2, with exceptional stability over 12 h of continuous electrolysis. Notably, urea‐assisted water splitting requires only 1.45 V for 10 mA cm−2, significantly less than the overall water splitting voltage (1.65 V), indicating energy‐efficient hydrogen production. Moreover, the Ni/MNO catalyst exhibits outstanding long‐term stability. This work presents a rapid and effective approach to synthesizing cost‐effective and efficient electrocatalysts for clean energy production and wastewater treatment.

UOR catalysts are vital to strengthening the effect of intermediate species that interexchange to accelerate the slow reaction kinetics.Recently, various nonnoble metal catalysts have been used for UOR, mostly Ni-and Co-based compounds. [12,15]Inspired by urease with Ni metal sites that can effectively break down urea to CO 2 and NH 3 in origins, much research is being focused on Ni-based catalyst compounds for UOR, including Ni(OH) 2 , NiO, NiP, and Ni 3 N. [16][17][18][19] Among the several Ni-based catalysts investigated for UOR so far, NiO is one of the most attractive catalysts owing to its high-efficiency active site conversion into Ni 3þ (NiOOH), outstanding stability, affordable cost, exceptional catalytic performance, and unique e.g., orbitals. [20,21]In addition, NiO coupled with Ni nanoparticles (Ni/NiO) exhibited an excellent UOR performance owing to the quick charge transfer occurring for the electrochemical reactions from the strong synergistic effect of active NiO and Ni nanoparticles. [17,22]Nevertheless, these Ni/NiO catalysts still have poor performance and stability because of the urea desorption and adsorption during the reaction with the unfavorably strong binding of the intermediate COO* on the Ni 3þ active sites. [13,23]ost Ni-based catalysts produced with various structures and compositions involve binding the intermediate COO* on the Ni 3þ active sites, restricting the performance.Therefore, different strategies have been developed to improve the UOR performance of Ni-based catalysts, such as nanostructure engineering, heteroatom doping, defect introduction, and heterostructure assembly. [12,24]Among them, high-valence heteroatom doping has successfully enhanced the UOR intrinsic performance of the Ni/NiO catalysts by modifying the electronic structure of Ni 3þ active sites.Furthermore, the high-valence oxidation of transition elements, such as W, Mo, V, and Cr, has excellent capacities for modifying the electronic structure in Ni-based catalyst systems. [23,25]According to earlier studies, Mo is particularly fascinating because it effectively adjusts the electronic structure and enhances the electroconductivity.In addition, Mo has a high-valence oxidation state of þ6, which can facilitate straightforward oxidation of N 2þ to N 3þ . [16,26]For instance, Yu et al. developed a nanorod-like Ni-Mo-O composite and utilized it as an electrocatalyst for UOR.The Ni-Mo-O composite achieved a superior UOR activity with a low potential of 1.42 V versus reversible hydrogen electrode (RHE) at 100 mA cm À2 and good durability.These results imply that incorporating Mo 6þ into NiO can lead to substantial UOR improvement by promoting the oxidation of Ni 2þ into a higher oxidation state. [27]However, only few studies have inspected the effect of Mo doping on the catalytic performance of NiO for the UOR. [28]raditional methods, including hydrothermal or solvothermal, solid-state, and sol-gel processes, are commonly employed for the synthesis of nanomaterials, [21,29,30] even though the synthesized materials from these traditional methods have large particle sizes and irregular morphologies.In addition, it is necessary to utilize hazardous chemicals, surfactants, other reagents, and high temperature to achieve nanostructures using these traditional methods.Thus, a revolutionary pulsed laser ablation (PLA) method that has been considerably extended to advanced nanomaterial production can overcome the drawbacks of these traditional methods.[33] Recently, the number of studies in manufacturing nanostructured compounds using PLA has increased dramatically.][36] However, few reports are only available for the synthesis of Ni/NiO and NiO nanoparticles via the pulsed laser ablation (PLA) technique. [37,38]According to our knowledge, no reports are available, specifically on the Mo-doped Ni/NiO nanoparticles fabricated using the PLA-assisted technique and their UOR performances.This motivated us to synthesize a Mo-doped Ni/NiO nanocomposite using an advanced PLAL technique and examine its electrocatalytic activity for UOR performance.
Inspired by the above reasons, herein, we developed a Mo-doped Ni/NiO (Ni/MNO) nanocomposite for the first time via a rapid and straightforward PLA method.The resultant nanocomposite was employed as an effective anode catalyst in urea-assisted water-splitting (UWS) for cleaning wastewater and producing green energy.X-Ray photoelectron spectroscopy (XPS) revealed that the Mo dopant in the Ni/NiO nanocomposite and the Ni atoms could promote the oxidation of Ni 2þ into Ni 3þ that act as surface active sites during the electrochemical reaction, resulting in an enhanced UOR performance.The influence of Mo doping in the Ni/NiO nanocomposite on electrocatalytic performance was investigated.The optimized Ni/MNO-10 catalyst exhibited superior electrocatalytic activity for OER and UOR, as demonstrated by the electrochemical studies, such as outstanding catalytic performance, rapid reaction kinetics, and improved durability.Remarkably, UWS was established using Ni/MNO-10(þ) as the anode and Pt/C(-) as the cathode.The Ni/MNO-10 electrocatalyst in UWS has shown promising results in conserving energy and improving wastewater treatment efficiency for hydrogen production.It attained a small cell voltage of 1.45 V at 10 mA cm À2 , lower than the overall water splitting (OWS) voltage of 1.65 V. Besides, it demonstrates the outstanding long-term stability of Ni/MNO-10 at 10 mA cm À2 for 12 h.This indicates the enormous potential of the Ni/MNO-10 nanocomposite in UWS for energy-saving hydrogen production and wastewater treatment.

Synthesis of Mo-Doped Ni/NiO Electrocatalyst
Figure 1a shows a detailed schematic illustration of Mo-doped Ni/NiO synthesis using pulsed laser ablation (PLA).The Ni target and sodium molybdate dihydrate (Na 2 MoO 4 •2H 2 O) synthesized the Mo-doped Ni/NiO samples.PLA used a monochromatic laser (neodymium-doped yttrium aluminum garnet, Nd:YAG, Surelite-II, 7 ns, and 10 Hz) with a pulse power of 100 mJ cm À2 with 1064 nm wavelength.The distance between the target and the lens was 30 mm.In a typical synthesis, a well-polished Ni target was soaked in deionized water (10 mL) containing 10 mM Na 2 MoO 4 •2H 2 O salt solution and loaded into a 20-mL glass vial.After that, the reaction mixture-focused laser beam was ablated into the Ni target to generate Ni ion species with a plasma plume for 10 min.In Ni, ion species reacted in the water medium to generate partially oxidized NiO.These Ni ions species react with the Mo salt solution, doping Mo ions in Ni/NiO.Next, the obtained dark brown colloidal solution (Figure S1, Supporting Information) was sonicated for 10 min and then stirred for 30 min to improve the particle dispersion.Subsequently, Mo-doped Ni/NiO was collected via centrifugation at 14 000 rpm for 10 min and washed three times with ethyl alcohol to remove impurities.Finally, the Mo-doped Ni/NiO samples were dried at 60 °C for 12 h.Herein, the Mo-doped Ni/NiO samples were synthesized by varying the Mo content: 5, 10, and 15 mM; then, the synthesized samples were denoted MNO-5, MNO-10, and MNO-15, respectively.Similarly, Ni/NiO nanoparticles were produced without Mo salt solution using the previous procedure.

Structural and Morphological Properties
X-Ray diffraction (XRD) measured the phase and crystallinity of the as-synthesized samples for intrinsic evaluation.The XRD patterns of the Ni/NiO and Mo-doped Ni/NiO (Ni/MNO) samples are displayed in Figure 1b.The diffraction patterns exhibit prominent peaks at 2θ = 44.8°,52.1°, and 76.7°, which attributed to the (1 1 1), (2 0 0), and (2 2 0) planes, respectively.These planes are characteristic of metallic cubic Ni (JCPDS card no.04-0850) in all samples. [39]Additionally, three weak diffraction peaks observed at 2θ = 37.4°, 44.6°, and 63.8°are attributed to the (1 1 1), (2 0 0), and (2 2 0) planes of NiO, respectively (JCPDS card no.47-1049). [40]The XRD pattern indicates the coexistence of Ni and NiO, confirming the successful preparation of Ni/NiO nanocomposites.Moreover, no diffraction peaks corresponding to MoO 3 and MoO 2 are observed in the Mo-doped Ni/NiO samples, indicating that Mo doping does not develop a new crystalline phase.After Mo doping, the NiO (1 1 1) peak slightly shifts toward the lower diffraction angles (Figure S2, Supporting Information), which can be attributed to the increased interlayer spacing caused by the larger atomic radius of Mo (1.36 Å) compared to that of Ni (1.25 Å). [41] However, the position of the Ni peak remains unchanged, indicating that Mo selectively dopes into the NiO phase without affecting the Ni phase.In addition, based on the XRD patterns, the Ni species content was quantified for both the Ni/NiO and optimized Ni/MNO-10 nanocomposite, yielding approximately 76.1% and 62.8%, respectively.It is noteworthy that in the case of the Ni/MNO-10 nanocomposite, the content of Ni species showed a slight decrease with the increase in Mo-doped NiO content.This decrease occurred during the ablation of the Ni target in a water medium in the presence of Mo salt.Consequently, a higher content of Mo-doped NiO can significantly enhance its participation in electrochemical oxidation reactions.Raman spectroscopy was performed to discover the structural information of the as-synthesized samples.The Raman peaks of Ni/NiO (Figure 1c) are found at %542.3, 733.4,905.7, and 1076.3 cm À1 , assigning to the first-order transverse optical mode (TO), longitudinal optical mode (LO), combined TO þ LO mode, and second-order transverse mode (2TO), respectively; these peak values agree with the previously reported values of the NiO phase. [42,43]After Mo doping, the Raman peak positions are shifted toward high vibrational mode, probably because highvalence Mo is essentially doped through the Ni/NiO lattice.
The field-emission scanning electron microscopy (FESEM) images of Ni/NiO presented in Figure 2a show that Ni formed a spherical-like structure, and the small NiO nanoparticles were attached to the surface of the spherical like Ni metal nanoparticles.Furthermore, the transmission electron microscopy (TEM) images of Ni/NiO (Figure 2b,c 2j) are presented in Figure 2k, in which the lattice fringes of 0.205 nm corresponding to the (1 1 1) crystal plane of metallic Ni and 0.236 nm ascribed to the (1 1 1) plane of NiO.Interestingly, the HRTEM image (Figure 2k) shows a small lattice distortion of 0.212 nm corresponding to the (2 0 0) planes of NiO in some parts of the Ni surface.These observations understated that lattice distortions may enhance the conductivity and create numerous electrocatalytic active sites. [44]Additionally, the FESEM of the Ni/MNO-5 and Ni/MNO-15 samples, presented in Figure S3 and S4, Supporting Information, respectively, also exhibited a similar surface morphology to that of the Ni/MNO-10 sample.After that, the compositional distribution of Ni/NiO and different percentages of Mo-doped Ni/NiO were confirmed using the elemental mapping analysis via energy-dispersive X-ray spectroscopy (EDS).The EDS mapping of Ni/NiO showed a higher detection of signals from the Ni element with a spherical shape and uniform dispersion of Ni and O elements across all samples (Figure 2f ).Similarly, Figure 2l [45] The second doublet peaks at 855.19 and 873.08 eV were related to the Ni 2þ 2p 3/2 and Ni 2þ 2p 1/2 of NiO nanoparticles.Similarly, the third doublet peak at 856.45 and 876.11 eV, which corresponds to the Ni 3þ 2p 3/2 and Ni 3þ 2p 1/2 states, respectively, indicates the formation of oxygen vacancies in the NiO structure. [46,47]The Ni 2þ and Ni 3þ states in NiO are associated with the four broad satellite peaks at 860.71 and 864.68 eV for Ni 2p 3/2 , and 878.95 and 882.30eV for Ni 2p 1/2 , respectively.After Mo doping, the binding energy of Ni 2p 3/2 and Ni 2p 1/2 peaks shows a sequential shift to a positive position in Ni/MNO-10 catalysts.This indicates that the Ni electronic structure has been altered due to the higher electronegativity of Mo (2.16) compared to that of Ni (1.91), which potentially leads to the transfer of Ni electrons to Mo. [48] Additionally, Figure 3c shows the presence of Mo 3 d with a high oxidation valence state of þ6 in Ni/MNO-10 catalyst.The doublet peaks located at 232.19 and 235.29 eV correspond to Mo 3d 5/2 and Mo 3d 3/2 , respectively. [49]This suggests that Ni atoms in the Ni/MNO-10 catalyst can promote the oxidation of Ni 2þ into Ni 3þ because Mo possesses a high oxidation valence state of þ6, which is expected to accelerate the electrochemical reaction kinetics. [27]Furthermore, three distinct peaks with binding energies of 529.28, 530.64, and 532.15 eV are exhibited in the highresolution O spectrum of Ni/NiO (Figure 3d), all indicative of Ni-O, oxygen vacancy, and adsorbed oxygen, respectively. [30]he presence of oxygen vacancy can be attributed to the doping of higher valence state Mo þ6 into NiO structure, which can provide unique active sites and reduce charge transfer resistance, thereby enhancing the electrocatalytic activity. [50]These XPS results strongly support the successful synthesis of the Mo-doped Ni/NiO nanocomposite using the PLA technique.

Electrocatalytic Oxygen Evolution Performance
The electrocatalytic OER activity of the as-synthesized Ni/NiO and various Mo-doped Ni/NiO (Ni/MNO) samples was investigated using a conventional three-electrode system (Figure .S5, Supporting Information) in 1.0 M KOH electrolyte at a scan rate of 5 mV s À1 .The iR (a potential drop owing to solution resistance) compensation of linear sweep voltammetry polarization (LSV) curves depicted in Figure 4a shows that Ni/MNO-10 exhibited an excellent OER performance, which requires only 283 and 374 mV to attain current densities of 10 and 100 mA cm À2 , respectively.These values are considerably lower than that of other as-synthesized samples and similar to the state-of-the-art IrO   [34] The Tafel slope and EIS characterizations results imply that the Mo doping can enhance the efficiency of the charge transfer process, which could ultimately lead to an effective OER process. [51]esides, the mass activity of each electrocatalyst was measured at various potential values (1.50-1.60 vs RHE) to demonstrate the intrinsic activity of catalysts during reactions (Figure 4e). [52]At every potential value studied, Ni/MNO-10 electrocatalyst exhibited the highest mass activity (current generated per gram of catalyst) compared to all other electrocatalysts, demonstrating its greater OER performance.The exposed electrocatalytic active surface area (ECSA) was inspected by estimating the double-layer capacitance (C dl ) to investigate the intrinsic OER performance.The C dl values were estimated using cyclic voltammetry (CV) at various scan rates (20-200 mV s À1 ) of the non-Faradaic region, as shown in Figure S8, Supporting Information.The Ni/MNO-10 electrocatalyst demonstrates a higher C dl value of 17.5 mF cm À2 (Figure 4f ) than that of Ni/NiO (5.4 mF cm À2 ), Ni/MNO-5 (8.2 mF cm À2 ), and Ni/MNO-15 (10.4 mF cm À2 ).The ECSA can be estimated using the following equation: ECSA = (C dl /C s ) Â A, where C s represents the specific capacitance of the CC (88 mF cm À2 ) and A denotes the active area of the electrode (cm 2 ). [11]The ECSA value of Ni/MNO-10 (0.198 cm 2 ) is higher than 0.061, 0.093, and 0.118 cm 2 of Ni/NiO, Ni/MNO-5, and Ni/MNO-15, respectively.The longterm durability of as-synthesized catalysts is also an essential parameter for assessing their electrocatalytic performance.Herein, we confirm the long-term stability of the Ni/MNO-10 electrocatalyst using the chronopotentiometry test.Figure S9, Supporting Information, demonstrates that under chronopotentiometry test over 12 h of OER measurement at 10 mA cm À2 ; the Ni/MNO-10 electrocatalyst showed stable potential behavior with only minor changes.The above-obtained OER results clearly demonstrate that the Mo-doped Ni/NiO nanocomposite exhibits exceptional OER performance.This can be attributed to its high conductivity, rapid electron transfer kinetics, large ECSA, and an increased number of surface active sites, which are a result of the synergistic coupling between the Ni and Mo metal sites.

Electrocatalytic Urea Oxidation Performance
To further enhance the activity of the as-synthesized samples, the anode behavior of UOR was assessed using a three-electrode system in 1.0 M KOH electrolyte containing 0.5 M urea.As shown in Figure 5a, the UOR LSV polarization curves were attained at a scan rate of 5 mV s À1 for the as-synthesized catalyst.The potential of Ni/MNO-10 reaches 1.37 and 1.43 V versus RHE to attain a current density of 10 and 100 mA cm À2 , respectively.These values are not only lower than those of the as-synthesized Ni/NiO (1.40 and 1.52 V vs RHE), Ni/NMO-5 (1.39 and 1.48 V vs RHE), and Ni/NMO-15 (1.38 and 1.47 V vs RHE) electrocatalysts but also outperform the state-of-the-art IrO 2 catalyst (1.54 V vs RHE@10 mA cm À2 ) in terms of electrocatalytic performance.The steady-state LSV curves of Ni/MNO-10 in various electrolytes are shown in Figure 5b.Ni/MNO-10 exhibits negligible catalytic performance in 0.5 M urea electrolytes.However, after adding 1.0 M KOH into the urea electrolyte solution, the catalytic performance was improved.To attain a current density of 100 mA cm À2 of this electrocatalyst, UOR requires only 1.43 V versus RHE, which is %180 mV smaller than that of OER.These results demonstrate that UOR is more energy-efficient than OER for the Ni/MNO-10 electrocatalyst.Interestingly, no oxidation peak was observed for Ni/MNO-10 in the UOR curve, but an oxidation peak from Ni 2þ to Ni 3þ was observed at %1.36 V versus RHE in the OER curve. [27]This suggests that the oxidized state of Ni 3þ serves as the active site, participating in the urea oxidation process, which is consistent with the proposed indirect electrochemical-chemical mechanism. [53]As displayed in Figure 5c, the histogram of the driving overpotential attains various current densities at 10, 50, 100, and 200 mA cm À2 .Based on an intuitive observation, the overpotential required for UOR is smaller than that needed for OER at all current densities.The Tafel slopes were used to investigate the electrocatalytic reaction kinetics of UOR with the as-synthesized catalysts.As shown in Figure 5d, the Tafel slopes of the as-synthesized Ni/NiO, Ni/MNO-5, Ni/MNO-10, Ni/MNO-15, and IrO 2 catalysts are 78.3,64.5, 51.2, 62.9, and 146.3 mV dec À1 , respectively.This indicates that the Tafel slope of Ni/MNO-10 is the lowest among all the catalysts, suggesting better electrocatalytic performance and higher reaction kinetics compared to the other catalysts.These results indicate that the Ni/MNO-10 catalyst shows faster UOR kinetics than the other electrocatalysts.EIS was also evaluated at 1.5 V versus RHE to evaluate the electrocatalyst kinetics, as shown in Figure 5e.The R ct value of Ni/NiO exhibits a maximum of 8.6 Ω; however, the R ct value decreased after Mo doping to 5.6, 3.6, and 4.7 Ω for Ni/MNO-5, Ni/MNO-10, and Ni/MNO-15, respectively.The Ni/MNO-10 electrocatalyst exhibits the lowest R ct value, implying rapid charge transfer reaction kinetics.This observation is consistent with the finding of the smallest Tafel slope for Ni/MNO-10.The Bode plots obtained from EIS (Figure S10, Supporting Information) were used to calculate the electron lifetime (τ e ) by determining the maximum frequency peak ( f max ) at a steady potential of 1.5 V versus RHE for all samples.τ e was calculated using the equation τ e = 1/(2π f max ).The calculated τ e values of Ni/NiO, Ni/MNO-5, Ni/MNO-10, and Ni/MNO-15 were 0.073, 0.063, 0.019, and 0.068 s, respectively.The superior UOR performance of Ni/MNO-10 exhibited a lower lifetime than other as-synthesized catalysts.Furthermore, the optimal Ni/MNO-10 catalyst underwent analysis of its electrolyte/electrode kinetics of the charge transfer using EIS, with varying UOR potentials among 1.4 and 1.5 V versus RHE and implies an increase in potential leading to a decrease in the R ct value (Figure S11, Supporting Information).
In addition, to evaluate the intrinsic performance of UOR, which is similar to the OER activity, ECSA and mass activity were analyzed.The mass activity of the as-synthesized UOR catalysts was estimated under different potentials from 1.40 to 1.50 V versus RHE (Figure 5f ).Among them, Ni/MNO-10 exhibited the highest mass activity of 193.To get a deeper understanding of the structure, morphology stability, and active sties of the Ni/MNO-10 catalyst, we conducted XRD, SEM, and Raman spectroscopy measurements after performing long-term stability tests for both OER and UOR over 12 h.Figure S13, Supporting Information, shows that there are no noticeable changes in the XRD patterns of Ni/MNO-10 catalyst before and after the stability tests for OER and UOR, indicating its good structural stability.The SEM images presented in Figure S14, Supporting Information, reveal a slight agglomeration of the Mo-doped NiO nanoparticles in the Ni/MNO-10 catalyst after the stability test, indicating some surface modification occurred during the continuous measurements.Furthermore, Figure S15, Supporting Information, shows the presence of new Raman peaks at %475 and 565 cm À1 for the Ni/MNO-10 catalyst after the stability tests, which were attributed to the Ni 3þ -O bending and stretching vibrations of NiOOH. [54]This observation suggests the formation of Ni 3þ as the actual active sites on the surface of the Ni/MNO-10 catalyst for both OER and UOR performance.Additionally, a new peak appeared at about 1082 cm À1 for the Ni/MNO-10 catalyst after the UOR stability test, corresponding to the symmetric stretching signal of C-N from the surface-absorbed urea molecules. [55]Based on these observations, we propose the possible reaction pathways and mechanism of the Ni/MNO-10 catalyst during the OER and UOR in Figure S16 and S17, Supporting Information, respectively.These findings contribute to a better understanding of the catalyst's performance and stability in urea oxidation and OERs.

Urea-Assisted Water Splitting Performance
Considering the outstanding electrocatalytic OER and UOR performance of the Ni/MNO-10 electrode, its OWS and UWS efficiency was further investigated.To evaluate this, we assembled a device using Ni/MNO-10(þ) as the anode and Pt/C(-) as the cathode, achieving OWS in 1.0 M KOH and UWS in a mixture of 1.0 M KOH with 0.5 M urea at ambient temperature (Figure 6a).Additionally, we evaluated the OWS and UWS performances of commercial state-of-the-art IrO 2 catalyst using the IrO 2 (þ)‖Pt/C(-) couple for comparison.As shown in Figure 6b, the Ni/MNO-10(þ)‖Pt/C(-) couple demonstrated highly beneficial catalytic performance toward UWS, requiring a cell voltage of only 1.45 V to achieve a current density of 10 mA cm À2 , which is significantly lower than that of the IrO 2 (þ)‖Pt/C(-) couple (1.68 V at 10 mA cm À2 ).Conversely, for OWS, a higher cell voltage of 1.65 V is required for the Ni/MNO-10(þ)‖Pt/C(-) couple to achieve the same current density, which is nearly 200 mV greater than that of UWS.In addition, the OWS performance of the Ni/MNO-10(þ)‖Pt/C(-) couple is also almost comparable to that of the IrO 2 (þ)‖Pt/C(-) couple (1.60 V at 10 mA cm À2 ).To achieve current densities of 10, 20, 30, 40, and 50 mA cm À2 , UWS require a cell voltage of 1.45, 1.53, 1.61, 1.70, and 1.81 V, respectively, whereas OWS require substantially greater values of 1.65, 1.71, 1.78, 1.84, and 1.90 V (Figure 6c).Thus, the UWS has superior cell voltage compared to OWS.These results indicate that the electrolyzer performance at the UWS has extensively improved, and replacing the OER with UOR has increased hydrogen production efficiency.To further confirm the potential of this system for efficient hydrogen production via electrolyzer, the long-term endurance of the Ni/MNO-10(þ)‖Pt/C(-) electrode was examined at 1.45 V of UWS and 1.65 V of OWS (current density of 10 mA cm À2 ) (Figure 6d).During the chronopotentiometry study for 12 h, only slight changes in cell voltage were observed, indicating the robustness of both the OWS and UWS systems.The LSV polarization curves analyzed before and after the chronopotentiometry stability test was identical, with a slight change in the original (Figure S18, Supporting Information).Furthermore, to assess its stability, Ni/MNO-10 was subjected to continuous cycling of 2000 CV scans at a scan rate of 100 mV s À1 .As displayed in Figure S19, Supporting Information, after 2000 CV cycles, the LSV polarization curves for OWS and UWS exhibited only a minor shift from the initial curve.These results confirm that the Ni/ MNO-10(þ)‖Pt/C(-) electrolyzer system has exceptional performance and durability, among the greatest ever reported for urea electrolysis systems (Figure 6e and Table S1, Supporting Information).As a result, this observation indicates that the pulsed laser-produced Ni/MNO-10 holds significant promise as a catalyst for the UOR, contributing to energy-saving hydrogen production.

Conclusions
Mo-doped Ni/NiO nanocomposite was developed via a fast and simple PLA method and proved to be an outstanding electrocatalyst with enhanced durability for ecological and energy-relevant urea-assisted water electrolysis.The Mo-doped Ni/NiO nanocomposite catalyst performs exceptionally well owing to the high-valence Mo doping, enhancing the catalytic activity of Ni atoms, facilitating the oxidation of Ni 2þ into Ni 3þ , and providing numerous active sites for the electrochemical reaction.Efficiently doping Mo into the Ni/NiO nanocomposite enhances its catalytic activity toward OER and UOR owing to the strong influence of Mo doping on the Ni/NiO nanocomposite, its unique structural characteristics, increased active sites, and the presence of essential Ni 3þ ions, all of which distinguish it from Ni/NiO alone.The exceptional catalytic capacities, such as essential catalytic efficiency, fast reaction kinetics, and long durability, were demonstrated for the electrocatalytic OER and UOR.As a result, the optimal content of Mo-doped Ni/NiO showed a low overpotential of 280 mV for the OER and also, an ultralow working potential of 1.37 V versus RHE for the UOR at 10 mA cm À2 with a superb catalytic durability.In addition, the Ni/MNO-10-enabled urea-assisted water electrolysis required a cell voltage of 1.45 V at 10 mA cm À2 , which is 200 mV less than regular water electrolysis (1.65 V), resulting in energy savings.Simultaneously, better structural/chemical stability resulted in excellent long-term durability.This work provides a simple and feasible approach to developing an inexpensive and high-efficiency catalyst for energy and the environment.
) also confirm that the spherical Ni metal nanoparticles were formed, and the NiO nanoparticles were anchored on the surface of the spherical Ni structure, which is consistent with the FESEM images.The lattice fringes with the d-spacing of the Ni/NiO sample were investigated using the high-resolution TEM (HRTEM) image shown in Figure 2d,e.The lattice spacings of 0.209 nm correspond to the (1 1 1) plane of metallic Ni, while the lattice spacings of 0.237 observed in the Ni metallic surface correspond to the (1 1 1) crystal plane of NiO.Meanwhile, the FESEM image in Figure 2g confirmed the ultrasmall Mo-doped NiO nanoparticles were attached to the spherical-shaped Ni nanoparticles in the Ni/MNO-10 sample.In addition, the HRTEM images of the Ni/MNO-10 sample in Figure 2h,i show the agglomeration of ultrasmall Mo-doped NiO nanoparticles on the spherical-like Ni nanoparticles.The selected red areas in the HRTEM image of Ni/MNO-10 (Figure demonstrates the presence of more signals from the Ni element with a spherical shape and well-distributed Ni, Mo, and O elements throughout the observed area of the Ni/MNO-10 sample, indicating the effective doping of Mo into NiO structure with Ni nanoparticles.Furthermore, the XRD pattern did not show Mo-containing crystal phases.EDS analysis showed that the Mo dopant content increased in Ni/NiO with an increased weight percentage of Mo.XPS measurements were conducted on the interaction among Ni/NiO nanocomposite and Mo dopant to investigate the chemical compositions and electronic interactions of surface elements.The Ni, Mo, and O elements in the Ni/NiO and Ni/MNO-10 catalysts are verified using the XPS full survey spectrum shown in Figure 3a, consistent with the EDX mapping results (Figure 2f,l).The high-resolution Ni 2p spectra in Figure 3b deconvoluted into three pairs of doublet peaks with satellite signals.The first doublet peaks at 853.46 and 871.25 eV attributed to the Ni 0 2p 3/2 and Ni 0 2p 1/2 of Ni nanoparticles.
2 electrocatalyst (289 and 378 mV at 10 and 100 mA cm À2 , respectively).The results indicate a great improvement in the OER activity of Ni/MNO nanocomposites after doping Mo into the NiO lattice, compared to the Ni/NiO nanocomposite.This suggests that the presence of the high-valence Mo dopant enhances the OER performance of the Ni/NiO nanocomposite. Figure 4b exhibits the overpotential of all these electrocatalysts at current densities of 10 and 100 mA cm À2 , indicating that the bare samples of carbon cloth (CC), IrO 2, Ni/NiO, and different Mo-doped Ni/NiO nanocomposite considerably enhance the OER activity.The corresponding OER LSV polarization curves of the as-synthesized electrocatalyst without iR compensation are also provided in Figure S6, Supporting Information.Then, the influence of various Mo doping of Ni/NiO on the electrocatalytic performance of OER was examined.The superior OER activity of the Ni/MNO-10 demonstrates the robust synergistic effect between the Ni and Mo atom sites in NiO structure, resulting in an enhancement in the electrical conductivity and an increase in surface active sites.On the other hand, the decreased OER performance observed in Ni/MNO-15 indicates that higher Mo content may potentially block the active sites of the OER catalyst, leading to reduced activity.Tafel slope and electrochemical impedance spectroscopy (EIS) are essential techniques for investigating the kinetics of electrocatalytic OER.As shown in Figure 4c, the corresponding Tafel

Figure 2 .
Figure 2. a) SEM image, b-e) HRTEM images, and f ) EDS mapping images of Ni/NiO and g) SEM image, h-k) HRTEM images, and l) EDS mapping images of Ni/MNO-10.

Figure 4 .
Figure 4.The electrocatalytic OER performance of the as-synthesized Ni/NiO and various Mo-doped Ni/NiO catalysts in 1.0 M KOH electrolyte solution: a) LSV polarization curves, b) overpotential obtained at 10 and 100 mA cm À2 , c) Tafel plots, d) Nyquist plots at 1.60 V versus RHE, e) mass activity, and f ) calculated C dl values in ΔJ/2 versus scan rate plot.

Figure 5 .
Figure 5.The electrocatalytic UOR performance of the as-synthesized Ni/NiO and different Ni/MNO nanocomposites in 1.0 M KOH containing 0.5 M urea as the electrolyte: a) iR-corrected LSV polarization curves, b) comparison of LSV polarization curves of optimized the Ni/MNO-10 catalyst using the different electrolytes, c) comparison of attained OER and UOR potentials of Ni/NMO-10 catalyst at different current densities (10-200 mA cm À2 ), d) Tafel plots, e) Nyquist plot at 1.50 V versus RHE, f ) mass activity, g) the plots of ΔJ/2 versus scan rates for calculate C dl values, and h) long-term catalytic UOR durability of the optimized Ni/MNO-10 at 10 mA cm À2 for 12 h.
2 A g À1 at a constant potential of 1.50 V versus RHE than those of Ni/NiO (85.4A g À1 ), Ni/MNO-5 (126.3A g À1 ), and Ni/MNO-15 (147.1 A g À1 ).As a result, Mo doping improves the electrocatalytic activity of the intrinsic UOR performance.The ECSA values are calculated from the C dl , estimated from the CV curves obtained at different scan rates of 20-200 mV s À1 (Figure S12, Supporting Information).As displayed in Figure 5g, the derived C dl values for Ni/NiO, Ni/MNO-5, Ni/MNO-10, and Ni/MNO-15 are 4.6, 4.8, 7.7, and 6.4 mF cm À2 , which correlate to ECSA values of 0.052, 0.054, 0.087, 0.072 cm 2 , respectively.The Ni/MNO-10 electrocatalyst exhibited the highest C dl and ECSA values, indicating great exposure of active sites on its surface and enhanced UOR electrocatalytic performance.The long-term electrocatalytic durability is a critical parameter for evaluating catalytic performance.Therefore, the long-term durability of the Ni/NMO-10 electrocatalyst was assessed using the chronopotentiometry process at 10 mA cm À2 over12 h (Figure 5h).Interestingly, the Ni/MNO-10 electrocatalyst demonstrated a consistent UOR performance during the continuous chronopotentiometry test with only a slight change in overpotential after 12 h.