Trace Cobalt Doping and Defect Engineering of High Surface Area α‐Ni(OH)2 for Electrocatalytic Urea Oxidation

Owing to the intrinsically sluggish kinetics of urea oxidation reaction (UOR) involving a six‐electron transfer process, developing efficient UOR electrocatalyst is a great challenge remained to be overwhelmed. Herein, by taking advantage of 2‐Methylimidazole, of which is a kind of alkali in water and owns strong coordination ability to Co2+ in methanol, trace Co (1.0 mol%) addition was found to induce defect engineering on α‐Ni(OH)2 in a dual‐solvent system of water and methanol. Physical characterization results revealed that the synthesized electrocatalyst (WM‐Ni0.99Co0.01(OH)2) was a kind of defective nanosheet with thickness around 5–6 nm, attributing to the synergistic effect of Co doping and defect engineering, its electron structure was finely altered, and its specific surface area was tremendously enlarged from 68 to 172.3 m2 g−1. With all these merits, its overpotential to drive 10 mA cm−2 was reduced by 110 mV. Besides, the interfacial behavior of UOR was also well deciphered by operando electrochemical impedance spectroscopy.


Introduction
[3] Among them, modern urea-based technologies, including direct urea fuel cell (DUFC), [4] hydrogen production from urea electrolysis, and urine removal from domestic wastewater, [5] are considered to be very promising solutions for achieving sustainable energy utility and environmental protection. [1]Additionally, the urea oxidation reaction (UOR) is an excellent alternative to couple with other kinds of electrochemical reduction reactions, of which transform small molecules (CO 2 , N 2, and organic molecules) into value-added products, as its thermal-dynamic potential (0.37 V) is much lower than that of water oxidation (1.23 V), the required cell voltage will be tremendously reduced. [6]However, the practical use of them is still locked by its sluggish kinetics, in which the whole process (CO(NH 2 ) 2 + 6OH À N 2 + 5H 2 O + CO 2 + 6 e À ) is involved with a complicate six-electron transfer. [7]Thus, it is highly significant to develop high-performance electrocatalyst toward UOR.
Aiming at this, a series of catalysts based on noble and rare metals, [8] like Ru-TiO 2 , Ti/Pt, and Ti/Ta 2 O 5 -IrO 2 , have been reported to electrocatalyze UOR efficiently, but in terms of the costs and scale-up applications, UOR catalysts, derived from cheap and earth abundant metal, are in urgent need. [6,9,10]Ni-based electrocatalysts, including oxides, [11][12][13] hydroxides, [9,14] spinels, [10,15] sulfides, [7,16] and phosphides, [17] are considered to be potential candidates, whereas the improved performances of them are still not satisfying due to the high overpotential, low current density, and insufficient durability.Generally, for Ni-based catalyst toward UOR, widely accepted mechanism is proposed by Botte; [8] accordingly, low valence Ni 2+ is not catalytically active, under the anodic polarization potential; the transformed high valent Ni 3+ oxyhydroxide is the key to unlock UOR, which is also verified by many other researchers; [18,19] the urea then is reacted with Ni 3+ to form the product of N 2 , CO 2, and water; simultaneously, the Ni 2+ hydroxide is regenerated.Thus, principally, it is rational to lower the formation potential of Ni 3+ oxy-hydroxide and thereafter downsize the onset potential of UOR.To this end, tremendous efforts have been devoted to this field, including nanostructured LaNiO 3 perovskite, [11] S incorporated b-Ni(OH) 2 with metallic property, [20] atomically thick Ni(OH) 2 nanomeshes, [14] spinel Ni 1.5 Mn 1.5 O 4 particles, [15] and oxygen vacancies confined NiMo-oxide porous nanosheets. [12]Excellent works as they are, the synthesis of these kinds of catalysts is a complicate process involving with multiple steps, largely hindering their large-scale applications.The core designing principal of them is focused on the electronic structure tuning to improve intrinsic activity, and exposing more active sites of electrocatalyst, guided by this, it is rational and significant to explore facile strategy for optimizing the electron structure of Ni and increasing the specific surface area of catalyst into one pot.
][23] Attributing to this, it can be easily deprotonated and transformed into active c-NiOOH toward UOR by a anodic bias potential. [24,25]Thus, it theoretically shows great potential to be excellent UOR electrocatalyst, although its application is restricted to the inherited low intrinsic activity, inadequate exposure of active sites, poor wettability, and electron conductivity. [24,26]Besides, the performance improvement of pure Ni-based electrocatalysts is always obstructed by the strong adsorption of reaction intermediates COO* on Ni sites. [13,27]It is reported that electronic structure regulation is an effective way to address the above issues such as modulating the surface chemical state, narrowing the band gap to enhance electron conductivity and optimizing the adsorption strength of intermediates. [13]Co doping [13,28,29] and defect engineering [12,30] have been demonstrated to efficiently regulate the electronic structure and catalyze the UOR; specifically, the Co doped into Ni-based material can not only optimize the electronic structure of Ni site but also promote the catalytic oxidation of CO* and alleviate catalyst poisoning. [16]Herein, using a-Ni(OH) 2 as model electrocatalyst for UOR, Co doping and defect engineering were proposed to synergistically boosting UOR performance in one step.
Inspired by the dual role property of 2-Methylimidazole (MIM), of which acts as alkali in water solvent due to the hydrolysis and owns strong coordination ability to Co 2+ rather than Ni 2+ in methanol solvent. [31]In this work, trace Co doping, for the first time, was employed to induce defect engineering on a-Ni(OH) 2 by using a dual-solvent system of water and methanol.All integrated into one pot; the Co doped a-Ni(OH) 2 (WM-Ni 1-x Co x (OH) 2 ) was facilely obtained.Physical characterizations indicate that, in contrast to the pristine one, rich defects were formed in the representative electrocatalyst (WM-Ni 0.99 Co 0.01 (OH) 2 ); the surface chemical state was finely altered and its band gap was largely narrowed; the specific surface area was tremendously increased from 68 to 172.3 m 2 g À1 .Consequently, the required overpotential for WM-Ni 0.99 Co 0.01 (OH) 2 to drive current density of 10 mA cm À2 was decreased by almost 110 mV, and robust operation durability up to 24 h was achieved; the origin of the boosted performance was further explored by using operando electrochemical impedance spectroscopy (EIS) to probe the interfacial behavior of UOR.This work provides unique perspective to design electrocatalyst and may promote the practical use of urea-based devices in the future.

Results and Discussion
As is illustrated in Scheme 1, the defective WM-Ni 0.99 Co 0.01 (OH) 2 was synthesized via a facile double-solvent method.By simply mixing Ni and Co salts with MIM in a double-solvent system of methanol and water, WM-Ni 0.99 Co 0.01 (OH) 2 with rich channels and defects was obtained.During the synthesis, the hydrolysis of MIM in water phase will produce numerous OH À , which preferentially combine with Ni 2+ and Co 2+ to form hydroxide precipitation.By taking the advantage of the feature that Co rather than Ni could easily coordinates with MIM, excess MIM will subsequently coordinate with the surface Co atoms of the formed hydroxide, leading to the etching of Co from the slabs, which was also confirmed by the inductive coupled plasma atomic emission spectrometer (ICP-AES) result, as its molar ratio of Ni and Co was 99.4:0.6,higher than that of 99:1.Thus, defective WM-Ni 0.99 Co 0.01 (OH) 2 nanosheets were successfully obtained.
According to the powder X-ray diffraction (PXRD) pattern (Figure 1a) of WM-Ni 0.99 Co 0.01 (OH) 2 , it shows typical characteristic diffraction peaks of a-Ni(OH) 2 (JCPDS No. 38-0715), and no other impurity phases are observed, implying that Co doping did not change the main crystalline structure of a-Ni(OH) 2 during the reaction.The Fourier transform infrared spectroscopy (FTIR; Figure S1, Supporting Information) confirmed the existence of intercalated NO 3 À and residue solvent including water and methanol molecules, while no signal of MIM was detected.In light of its scanning electron morphology (SEM) and corresponding energy dispersive spectroscopy (EDS) mapping of WM-Ni(OH) 2 (Figure S2, Supporting Information) and Ni 0.99 Co 0.01 (OH) 2 (Figure S3, Supporting Information), the trace Co atoms were uniformly doped into WM-Ni 0.99 Co 0.01 (OH) 2 .The micro-morphology was also characterized by transmission electron microscope (TEM; Figure 1b), and numerous nanosheets with ultrathin structure are presented.By contrast, for the sample without Co doping (WM-Ni(OH) 2 ), it also presents the typical diffraction peaks of a-Ni(OH) 2 (Figure S4a, Supporting Information).
Based on the TEM images (Figure S4b,c, Supporting Information) under different magnification, bulk and lamellar morphology of WM-Ni(OH) 2 is found, indicating that the introduction of Co significantly changed the morphology and benefited the formation of nanosheets.
The microstructure characterization of WM-Ni 0.99 Co 0.01 (OH) 2 was further conducted by high-resolution TEM (HR-TEM; Figure 1c); it is clearly found that the nanosheet is composed with rich meshes and channels, which are considered to be beneficial to increasing active sites and promoting mass transfer during the gas evolving reaction, [32,33] and the lattice spacing of (101) is 0.235 nm.While, for WM-Ni(OH) 2 (Figure S4d, Supporting Information), no obvious defects are found on the slab surface, the measured lattice spacing of (101) is 0.254 nm.
According to previous reports, [34,35] the prominent lattice spacing contraction from 0.254 to 0.235 nm should be ascribed to the existence of rich vacancy defects, which further confirms the successful defect engineering on a-Ni(OH) 2 by Co doping.In light of the selected area electron diffraction (SAED) patterns of WM-Ni 0.99 Co 0.01 (OH) 2 (Figure 1d) and WM-Ni(OH) 2 (Figure S4e, Supporting Information), diffraction rings could be observed in the both samples, suggesting the polycrystalline nature of them.
To explore the effect of solvent on the crystalline growth mechanism of hydroxide, pure Ni hydroxides without adding Co salt were synthesized in single solvent system.As depicted in Figure S5a, Supporting Information, the characteristic diffraction peaks of Ni hydroxide prepared in pure water phase (W-Ni(OH) 2 ) are different from that of the one obtained in methanol phase (M-Ni(OH) 2 ).The crystalline structure of W-Ni(OH) 2 is in consistent with that of b-Ni(OH) 2 (JCPDS No. 14-0117), while, for M-Ni(OH) 2 , typical peaks of a-Ni(OH) 2 are observed.Such discrepancy should be ascribed to that the hydrolysis of MIM in methanol system is far harder than that in water, as the MIM in methanol could only be hydrolyzed by the dissolved crystal water from Ni salt; the content of OH À may be not enough to form b-Ni(OH) 2 and the excess positive charge would be compensated by NO 3 À ; thus, a-Ni(OH) 2 was formed in methanol system (Figure S5b, Supporting Information).In addition to the crystalline structure, the morphology of them is also significantly affected by the solvent kind.From the SEM images of W-Ni(OH) 2 (Figure S6, Supporting Information) and M-Ni (OH) 2 (Figure S7, Supporting Information), closely stacked lamellar nanosheets are found for W-Ni(OH) 2 , while the obtained morphology for M-Ni(OH) 2 is a kind of hydrangea-like structure.To investigate the influence of Co doping amount on WM-Ni 1-x Co x (OH) 2 , PXRD characterization of them was also conducted.As is shown in Figure S8, Supporting Information, the main crystalline structure of WM-Ni 1-x Co x (OH) 2 was not changed even by increasing Co doping content up to 3%.With the increment of doping amount, the interlayer peak (003) shifts to lower diffraction angle, implying the enlarged interlayer space of WM-Ni 1-x Co x (OH) 2 , which may be caused by that more anions were intercalated into the interlayer space to coordinate and etch Co from the slabs.Based on the above analyses, the solvent composition significantly influences the crystalline growth of Ni hydroxide, and the Co doping is in favor for enlarging the interlayer space, but will not change its main crystalline structure.
The nanosheet structure of WM-Ni 0.99 Co 0.01 (OH) 2 was confirmed by the atomic force morphology (AFM; Figure 2a); the thickness of these nanosheets was estimated to be around 5-6 nm (Figure 2b).The deep insight into the defect engineering induced by Co doping was further gained by nitrogen adsorptiondesorption tests.In light of nitrogen adsorptiondesorption isotherms curves (Figure 2c) of WM-Ni 0.99 Co 0.01 (OH) 2 , the clear H3 hysteresis loop shape in a type-IV isotherm is observed in the range of high P/P0 (0.6-1.0) area, which is characteristic feature of capillary condensation caused by mesoporous structure, [9,14] and the Brunauer-Emmett-Teller (BET)-specific surface area is as high as 172 m 2 g À1 .The pore structure of WM-Ni 0.99 Co 0.01 (OH) 2 was also confirmed by the pore size distribution curve (Figure 2d); it can be found that mesopores (width: 2-50 nm)  For M-Ni(OH) 2 , the BET specific surface area is 60 m 2 g À1 , the coexistence of mesopores and macropores (width: 50-1000 nm) is identified, as the hysteresis loop disappears at high P/P 0 (>0.9)(Figure S10a, Supporting Information), and the contribution of macropore to pore volume is not ignorable (Figure S10b, Supporting Information).The formation of macropore should be ascribed to the aperture caused by the stack of microspheres, which is in line with the observation in Figure S7, Supporting Information.Yet the BET specific surface area of W-Ni(OH) 2 is only 8 m 2 g À1 , no hysteresis loop, namely no signal of mesopores, is observed (Figure S11a,b, Supporting Information), because the slabs of W-Ni (OH) 2 are closely stacked, of which is typical feature of b-Ni(OH) 2 .Conclusively, the Co doping and double-solvent system are vital to the formation of mesopores and the large specific surface area.
In addition to the aforementioned characterizations of morphology and structure properties, the X-ray photoelectron spectroscopy (XPS) tests of W-Ni(OH) 2 , M-Ni (OH) 2 , WM-Ni(OH) 2, and WM-Ni 0.99 Co 0.01 (OH) 2 were conducted to probe their surface chemical state and composition variation.In light of Ni 2p spectra of W-Ni (OH) 2 , M-Ni(OH) 2 , WM-Ni(OH) 2, and WM-Ni 0.99 Co 0.01 (OH) 2 in Figure S12a, Supporting Information, the Ni 2p 3/2 peak of all the samples except for W-Ni(OH) 2 moves to the higher binding energy position; this should be explained by that the valence state of Ni in samples with a-Ni (OH) 2 structure is higher than that of W-Ni(OH) 2 with b-Ni(OH) 2 structure, [36] as the intercalated anions in between the slabs of a-Ni(OH) 2 own stronger electronegativity than that of OH -. [24] Besides, from their Ni 2p 3/2 regions (Figure S12b, Supporting Information), it can be clearly seen that the peak of WM-Ni 0.99 Co 0.01 (OH) 2 shifts to the leftmost position, of which binding energy is higher than that of the ones without Co doping, implying that the doping of Co promotes the oxidation of Ni 2+ .Likewise, as is revealed by their O 1 s spectra (Figure S12c, Supporting Information), the chemical state of the oxygen varies greatly.
To get deep understanding on the relationship between valence state variation and defect, the de-convolution of Ni 2p 3/2 and O 1s spectra was made.For WM-Ni(OH) 2 (Figure 3a,b), two peaks at 855.0 and 856.6 eV, representing Ni 2+ and Ni 3+ , respectively, [26] can be observed from the fitting results of Ni 2p 3/2 (Figure 3a); the area ratio of Ni 2+ and Ni 3+ was calculated to be 65.5: 34.5.The peak of chemical shift can be found at 855.8 eV, which is originating from tetrahedral Ni that is coordinated to intercalated anion with stronger electronegativity than that of OH À . [37]The O 1 s spectra of WM-Ni (OH) 2 can be fitted into three peaks centered at 530.5 eV (O1), 531.1 eV (O2), and 532.5 eV (O3), respectively.O1 is the typical signal of Ni-O bond, O2 is attributed to the oxygen atom adjacent to oxygen vacancy or on the edge of defect, and O3 should be ascribed to the oxygen from intercalated species including H 2 O and NO 3 À ; [14] the area ratio of them was determined as 39.1:39.9:21.0.By contrast, the obtained area ratio of Ni 2+ and Ni 3+ for WM-Ni 0.99 Co 0.01 (OH) 2 is Energy Environ.Mater.2024, 7, e12576 46.1:53.9(Figure 3c), signifying that the Co doping increased the proportion of Ni 3+ ; this may be explained by the existence of Ni cation defects. [31,38]The area ratio of O1, O2, and O3 for WM-Ni (OH) 2 is 17.0: 65.2: 17.83 (Figure 3d), for which the proportion of O1 is tremendously decreased, while the observation for O2 is on the opposite, indicating that numerous oxygen vacancies or defects were formed by the induction effect of Co doping.In addition, according to the fitting result of Co 2p 3/2 spectra (Figure 3e), it was found that the average valence state of Co is higher than +2, and the area ratio of Co 2+ and Co 3+ is determined to be 75.3f).
The electrochemical performances of these electrocatalysts were evaluated on glassy carbon electrode in aqueous electrolyte of 1.0 M KOH with or without 0.33 M urea.The scan rate plays a vital role on the electrochemical behavior of electrode, as depicted by Figure S16a, Supporting Information; the peak current density of UOR is enlarged gradually with the increment of scan rate, and the peak potential positively moved to higher potential; meanwhile, it is observed that the oxidation peak shape is getting less obvious; this should be mainly ascribed to the defective WM-Ni 0.99 Co 0.01 (OH) 2 with greatly enlarged specific surface area; thus, the thinner diffusion layer, faster diffusion rate, and a shorter time that will be required to reach certain potential under higher scan rate. [17]According to the linear relationship (Figure S16b, Supporting Information) between the square root of scan rate (m 1/2 ) and the current density of anodic peak (I P ), it can be determined that the UOR is a diffusioncontrolled process. [11]Aside from this, based on the linear relationship (Figure S16c, Supporting Information) between the logarithm of scan rate (log(m)) and the peak potential (E P ), it can be demonstrated that the UOR is also a kinetic limited process. [17]n light of linear sweep voltammetry (LSV) curves of WM-Ni 0.99 Co 0.01 (OH) 2 with (UOR) and without (OER) 0.33 M urea (Figure 4a), the onset potential for UOR is closely associated with the occurrence of the oxidation peak, namely the transformation of WM-Ni 99 Co 1 (OH) 2 to partially deprotonated WM-Ni 0.99 Co 0.01 OOH.In contrast to the OER, the required potential for UOR to achieve current density at 10 mA cm À2 is far smaller; almost 330 mV is downsized; the observed anodic peak should be attributed to the rapid consumption of urea by the highly active and full exposed catalytic sites on the catalyst surface.As demonstrated by the LSV curves (Figure 4b) of WM-Ni(OH) 2 and WM-Ni 0.99 Co 0.01 (OH) 2 , with the assistance of Co doping and defect engineering, the polarization potential for WM-Ni 0.99 Co 0.01 (OH) 2 toward UOR is tremendously reduced from 1.48 to 1.37 V, which is a very competitive performance in contrast to the previously reported UOR electrocatalysts (Table S1, Supporting Information).To further investigate the effects of Co doping contents on UOR performances of WM-Ni(OH) 2 , a series of WM-Ni 1-x Co x (OH) 2 (0 ≤ x ≤ 3%) were prepared by controlling the addition amount of Co salt during the synthesis, and their LSV curves for UOR (Figure S17a, Supporting Information) and OER (Figure S17b, Supporting Information) were collected, respectively.As depicted by Figure S17a, Supporting Information, the trace Co doping significantly enhanced UOR performance, while the excess Co content (1 < x ≤ 3%) will not promote the electrocatalytic activity.It is well accepted that UOR and OER are two competing reactions during the anodic oxidation process of urea, especially in the area where OER occurs.From the LSV curves (Figure S17b, Supporting Information) measured in 1.0 M KOH without urea, it is clearly observed that the introduction of Co suppresses the OER performance and promotes the oxidation peak of Ni 2+ to Ni 3+ negatively shifted, and the peak area is much smaller than that of pristine one, signifying the increased proportion of Ni 3+ in WM-Ni 0.99 Co 0.01 (OH) 2 arising from defect engineering, which is also in line with the previous fitting results of Ni 2p 3/2 (Figure 3a,c).Of note, different from the electrochemical behavior of WM-Ni 0.99 Co 0.01 (OH) 2 , the current density of WM-Ni(OH) 2 , ranging from 1.38 to 1.49 V (Figure S17c, Supporting Information) under UOR, is obviously smaller than that of OER, which may be mainly attributed to the insufficient expose of the active sites to urea in the electrolyte, and the absent anodic peak for UOR curve of WM-Ni(OH) 2 is caused by its lower activity and the competition of OER under high anodic potential.Additionally, the UOR performances of W-Ni(OH) 2 and M-Ni(OH) 2 were also evaluated to investigate the influence of solvent kind on UOR, and both of them are inferior to that of WM-Ni(OH) 2 (Figure S17d, Supporting Information), the W-Ni(OH) 2 with b-Ni(OH) 2 structure owns the worst performance, indicating the importance of dual-solvent system.To shed light on the role of Co content played in the UOR, the LSV curves of WM-Ni 1-x Co x (OH) 2 (5 ≤ x ≤ 40%) were collected under different scan rate (Figure S18, Supporting Information).With the increment of Co content from 5% to 40% (Figure S18a-e, Supporting Information), the current density shows a decreasing trend, while the onset potential negatively shifts to lower potential, revealing that the excess doped Co are not the true active sites.
The reaction kinetics of WM-Ni 0.99 Co 0.01 (OH) 2 is also largely promoted, which is reflected by the Tafel plots (Figure 4c) and Nyquist plots (Figure 4d), in contrast to the pristine one; the Tafel slope is downsized from 93 mV dec À1 to 31 mV dec À1 , and the semicircle is far more small, and the charge transfer resistance (Rct) reduces from 145 Ω to 61 Ω at 1.45 V; this should be ascribed to its highly active sites and the defective structure that is advantageous to the mass transfer and gas evolution.As verified by the normalized LSV curves of WM-Ni(OH) 2 and WM-Ni 0.99 Co 0.01 (OH) 2 based on BET specific surface area (Figure 4d), the trace Co doping tremendously enhanced the intrinsic activity, namely the electrocatalytic activity of per unit area, increasing from 0.014 to 0.24 lA cm À2 at 1.37 V (Figure 4d), around 17 times that of the pristine one.Besides, the intrinsic activities were also evaluated based on electrochemical double-layer capacitance (C dl ).According to the CV curves collected under different scan rate (Figure S19a,b, Supporting Information), WM-Ni 0.99 Co 0.01 (OH) 2 owns typical rectangle-like CV curve shape, indicating its excellent electrochemical activity, while the CV shape for WM-Ni (OH) 2 is shuttle-like and the obtained C dl (Figure S19c, Supporting Information) for WM-Ni 0.99 Co 0.01 (OH) 2 and WM-Ni(OH) 2 is 0.43 mF cm À2 and 0.08 mF cm À2 , respectively.Likewise, from the normalized LSV curves based on C dl (Figure S19d, Supporting Information), the electrocatalytic current density of WM-Ni 0.99 Co 0.01 (OH) 2 (24.5 mA mF À1 ) is almost eight times that of WM-Ni(OH) 2 (3.1 mA mF À1 ) @ 1.37 V. Conclusively, the intrinsic activity difference (17 times) based on BET is larger than that of C dl (8 times); it is rational to infer that the Co doping and defect engineering reinforced the interfacial interaction between electrode and electrolyte solution.
In addition to the electrocatalytic activity, the electrochemical stability is another key factor to judge the UOR performance of catalyst.During the chronoamperometric test of WM-Ni 0.99 Co 0.01 (OH) 2 at 1.41 V (Figure 4f), the oxidation current increased steadily in the first 6 h, which may be caused by the activation of the electrocatalyst.As the WM-Ni 0.99 Co 0.01 (OH) 2 is a kind of layered structure materials (Figure 2a), with the reaction proceeds, more active sites within interlayers would be exposed due to the gas evolution and intercalation of small molecules.The current density remained around 20 mA cm À2 up to 24 h, demonstrating its excellent operational stability, which may be arising from the defects in Ni hydroxides; it is reported that such defective structure will offer buffering space to prevent the structure collapse. [32]Its structure stability is also confirmed by the XRD pattern (Figure S20a, Supporting Information) and TEM images (Figure S20b,c, Supporting Information) of WM-Ni 0.99 Co 0.01 (OH) 2 collected after durability test; the diffraction peaks and defective structure Energy Environ.Mater.2024, 7, e12576 are well maintained; the lattice spacing of (101) is measured to be 0.238 nm, which is almost the same with the one before durability test (Figure 1c).
To shed light on the mechanism of electrode interfacial kinetics, operando EIS was employed to probe the interfacial behavior of electrocatalyst toward UOR with respect to anodic polarization potential.According to the Nyquist plots of WM-Ni(OH) 2 (Figure 5a) and WM-Ni 0.99 Co 0.01 (OH) 2 (Figure 5b), both of them present a typical capacitive behavior when the applied potential is under 1.30 V, in contrast to that of open circuit potential (OCP), the curve, accompanying with the increment of applied potential, gradually deviates from the imaginary part, indicating the changed interfacial environment by polarization potential due to the electrostatic adsorption.When the potential is further increased over 1.30 V, a sharp transition of interfacial behavior is observed for WM-Ni 0.99 Co 0.01 (OH) 2 , the first semicircle representing the charge transfer process appears at 1.35 V, which signifies the occurrence of electrooxidation behavior, and the Rct is only 124 Ω at 1.40 V.As sharp contrast, the semicircle cannot be observed until the applied potential is increased to 1.40 V, the Rct is up to 145 Ω even under the high potential of 1.45 V, indicating the much slow charge transfer kinetics for WM-Ni(OH) 2 than that of WM-Ni 0.99 Co 0.01 (OH) 2 .The interfacial properties are also unveiled by the corresponding Bode-phase plots (Figure 5c,d).The signal of phase angle peak (Ø peak ) is closely related to the charge transfer process of the interfacial reaction in low-middle frequency (<1000 Hz) or catalyst electrooxidation in high-frequency area, [39] only one Ø peak shows up for both WM-Ni(OH) 2 and WM-Ni 0.99 Co 0.01 (OH) 2 over the low-middle-frequency region, no signal of catalyst electrooxidation is found, of which also strongly support that the oxidized Ni 2+ can be rapidly reduced by the high concentration of urea.For WM-Ni(OH) 2 , the Ø peak appears only when the applied potential is increased to 1.35 V, and the Ø peak is gradually getting smaller and shifts to higher frequency region with the increment of potential.While, for WM-Ni 0.99 Co 0.01 (OH) 2, a sharp decrease in Ø peak is observed between 1.30 and 1.35 V, and the Ø peak is only changed from 54 to 47 degree by further increasing the anodic potential to 1.45 V, signifying that the trace Co doping and defect engineering greatly benefits the formation of high active species under lower potential and the full expose of active sites due to the tremendously enlarged surface area.

Conclusion
In summary, the defect engineering induced by trace Co doping was demonstrated to be an efficient way to boost the UOR performance of a-Ni(OH) 2 , and the dual role of Co doping played in WM-Ni 0.99 Co 0.01 (OH) 2 toward UOR was the key.By taking advantage of the strong coordination ability between MIM and Co rather than Ni, the doped Co in a-Ni(OH) 2 was partially etched and numerous defects were formed.The doped Co and the existence of defects tuned the electronic structure well and promoted the formation of Ni 3+ , which is vital to reduce the overpotential to catalyze UOR.Consequently, both the apparent and intrinsic activities of the representative WM-Ni 0.99 Co 0.01 (OH) 2 were greatly enhanced, and excellent stability was also obtained.In a word, this work presented a facile method to integrate heteroatom doping and defect engineering into one pot, which may broaden the avenue toward large-scale preparation of electrocatalyst for small molecules oxidation.

Experimental Section
Detailed information related to the synthesis of active electrodes, physicochemical characterization, and electrochemical evaluation of bifunctional electrodes towards UOR and supercapacitor application is provided in Supporting Information.
3:24.7, which is different from that of Ni, implying the coordination discrepancy between Ni and Co atoms.The importance of the role of Co doping acted in the defect engineering is also confirmed by the fitting results of M-Ni(OH) 2 (Figure S13, Supporting Information) and W-Ni(OH) 2 (Figure S14, Supporting Information).The observation for M-Ni (OH) 2 is similar to that of WM-Ni (OH) 2 , including the valence state of Ni and O2 peak.However, it is totally abnormal for that of W-Ni(OH) 2 , no peak of chemical shift is collected; peaks of Ni 2+ and O1s are the predominant peaks; this should be ascribed to its nature of b-Ni(OH) 2 .Conclusively, the Co doping is vital to inducing defect engineering on a-Ni(OH) 2 and obviously changed the chemical state.Moreover, based on the plots of the transformed Kubelka-Munk function derived from the UV-Vis spectra (Figure S15, Supporting Information) of W-Ni(OH) 2 , M-Ni(OH) 2 , WM-Ni(OH) 2, and WM-Ni 0.99 Co 0.01 (OH) 2 , Co doping effectively ameliorated the electronic conductivity of a-Ni (OH) 2 , which is confirmed by its minimal band gap (Figure