Synchronous Defect and Interface Engineering of NiMoO4 Nanowire Arrays for High-Performance Supercapacitors

Developing high-performance electrode materials is in high demand for the development of supercapacitors. Herein, defect and interface engineering has been simultaneously realized in NiMoO4 nanowire arrays (NWAs) using a simple sucrose coating followed by an annealing process. The resultant hierarchical oxygen-deficient NiMoO4@C NWAs (denoted as “NiMoO4−x@C”) are grown directly on conductive ferronickel foam substrates. This composite affords direct electrical contact with the substrates and directional electron transport, as well as short ionic diffusion pathways. Furthermore, the coating of the amorphous carbon shell and the introduction of oxygen vacancies effectively enhance the electrical conductivity of NiMoO4. In addition, the coated carbon layer improves the structural stability of the NiMoO4 in the whole charging and discharging process, significantly enhancing the cycling stability of the electrode. Consequently, the NiMoO4−x@C electrode delivers a high areal capacitance of 2.24 F cm−2 (1720 F g−1) at a current density of 1 mA cm−2 and superior cycling stability of 84.5% retention after 6000 cycles at 20 mA cm−2. Furthermore, an asymmetric super-capacitor device (ASC) has been constructed with NiMoO4−x@C as the positive electrode and activated carbon (AC) as the negative electrode. The as-assembled ASC device shows excellent electrochemical performance with a high energy density of 51.6 W h kg−1 at a power density of 203.95 W kg−1. Moreover, the NiMoO4//AC ASC device manifests remarkable cyclability with 84.5% of capacitance retention over 6000 cycles. The results demonstrate that the NiMoO4−x@C composite is a promising material for electrochemical energy storage. This work can give new insights on the design and development of novel functional electrode materials via defect and interface engineering through simple yet effective chemical routes.


Introduction
Several alternative energy technologies have been under development globally in a great effort to mitigate the energy and environmental challenges faced and in accordance with the current "carbon neutral" policies. Supercapacitors (SCs), also known as electrochemical capacitors (ECs), have been considered as one of the most promising energy storage devices due to their unique characteristics of high power density (>10 kW/kg), fast charging and discharging capability (within a few seconds), long lifespan (over 100,000 cycles), and good operational safety. SCs have been widely applied in some important fields, including smart electric grids, memory back-ups, (hybrid) electric vehicles, and aerospace crafts. Although SCs have the advantages of high power density and very long calendar lives, their further application is still hindered by their limited energy density. of the NiMoO 4 electrodes via increased exposed surface for ion adsorption and insertion, shortened path distances for ion transport and diffusion, and improved electrolyte impregnation and permeation. Specifically, various low-dimensional NiMoO 4 nanostructures directly grown on conductive substrates (e.g., Ni/Cu foams [25,26], graphene [27], and carbon substrates [28,29]) are particularly preferred for directional electron transport with reduced charge carrier scattering at grain boundaries and easy integration into flexible devices with some specific applications.
However, monotonous strategy sometimes has a limited contribution for the overall electrochemical performance improvement of NiMoO 4 materials. In addition, some reported approaches for hybridization or doping of NiMoO 4 involve multiple and complex chemical and physical processes that are not economically or environmentally friendly. Thus, the rational design and the design of a NiMoO 4 -based composite electrode for high-performance supercapacitors remains a challenge.
In this work, we report the simultaneous defect and interface engineering of NiMoO 4 nanowires arrays (NWAs) using a simple and effective sucrose coating followed by a thermal treatment approach. In this process, an amorphous carbon shell was uniformly coated on the NiMoO 4 surface, effectively improving the electronic transport and structural integrity of the NiMoO 4 during electrochemical cycling. Additionally, oxygen-vacancy defects were incorporated into the NiMoO 4 during the carbonization process, further enhancing the electronic conductivity of NiMoO 4 and redox activity in the NiMoO 4 electrode surface. As expected, the resultant NiMoO 4−x @C composite exhibited a higher specific capacitance than that of the pristine NiMoO 4 NWAs. Furthermore, an asymmetric supercapacitor (ASC) was assembled with the NiMoO 4−x @C as positive electrode and activated carbon (AC) as negative electrode, delivering a remarkably high energy density of 51.6 W h kg −1 at a power density of 203 W kg −1 and an excellent cycling stability with a retention of 84.5% after 6000 cycles under a high current density of 10 A g −1 .

Results and Discussion
The synthesis route of oxygen-deficient NiMoO 4 @carbon nanowire arrays (NiMoO 4−x @C) is schematically shown in Figure 1. The preparation process mainly involved three critical steps. Firstly, a NiMoO 4 nanowire arrays (NWAs) precursor (NiMoO 4 ·xH 2 O NWAs, light green) was directly deposited on a ferronickel foam by a hydrothermal reaction process (Step 1, Figure 1). Secondly, the NiMoO 4 NWAs precursor was transferred into NiMoO 4 NWAs by annealing in Ar to remove crystal H 2 O and improve crystallinity (Step 2, Figure 1). Finally, the as-obtained NiMoO 4 NWAs were immersed in a sucrose solution, followed by drying and annealing in an Ar atmosphere to fabricate oxygen-deficient NiMoO 4 @carbon NWAs (NiMoO 4−x @C) (Step 3, Figure 1). In this work, we report the simultaneous defect and interface engineering of NiMoO4 nanowires arrays (NWAs) using a simple and effective sucrose coating followed by a thermal treatment approach. In this process, an amorphous carbon shell was uniformly coated on the NiMoO4 surface, effectively improving the electronic transport and structural integrity of the NiMoO4 during electrochemical cycling. Additionally, oxygen-vacancy defects were incorporated into the NiMoO4 during the carbonization process, further enhancing the electronic conductivity of NiMoO4 and redox activity in the NiMoO4 electrode surface. As expected, the resultant NiMoO4−x@C composite exhibited a higher specific capacitance than that of the pristine NiMoO4 NWAs. Furthermore, an asymmetric supercapacitor (ASC) was assembled with the NiMoO4−x@C as positive electrode and activated carbon (AC) as negative electrode, delivering a remarkably high energy density of 51.6 W h kg −1 at a power density of 203 W kg −1 and an excellent cycling stability with a retention of 84.5% after 6000 cycles under a high current density of 10 A g −1 .

Results and Discussion
The synthesis route of oxygen-deficient NiMoO4@carbon nanowire arrays (Ni-MoO4−x@C) is schematically shown in Figure 1. The preparation process mainly involved three critical steps. Firstly, a NiMoO4 nanowire arrays (NWAs) precursor (NiMoO4·xH2O NWAs, light green) was directly deposited on a ferronickel foam by a hydrothermal reaction process (Step 1, Figure 1). Secondly, the NiMoO4 NWAs precursor was transferred into NiMoO4 NWAs by annealing in Ar to remove crystal H2O and improve crystallinity (Step 2, Figure 1). Finally, the as-obtained NiMoO4 NWAs were immersed in a sucrose solution, followed by drying and annealing in an Ar atmosphere to fabricate oxygen-deficient NiMoO4@carbon NWAs (NiMoO4−x@C) (Step 3, Figure 1).
Step 1, growth of the NiMoO4 NWAs precursor directly on a ferronickel foam substrate using a hydrothermal process; Step 2, conversion of the NiMoO4 NWAs precursor into the NiMoO4 NWAs via annealing in Ar; Step 3, fabrication of Ni-MoO4−x@C composite by sucrose coating followed by annealing in Ar.
The crystal structures of NiMoO4−x@C and neat NiMoO4 NWAs samples were characterized by X-ray diffraction (XRD) analysis as depicted in Figure 2. The two strongest diffraction peaks, located at ca. 45° and 52°, were from the ferronickel foam substrate. The   [37]. Compared to the NiMoO 4 NWAs samples, the NiMoO 4−x @C sample exhibited a slightly lower diffraction peak intensity possibly due to the covering of carbon on the NiMoO 4 surface as well as the reduced crystallinity of NiMoO 4 with increased structural defects. In addition, no characteristic peaks for carbon phases were noted, suggesting the amorphous nature of the carbon species in the NiMoO 4−x @C sample. The formation of amorphous carbon could be partially ascribed to the lower low annealing temperature (400 • C herein). to the NiMoO4 NWAs samples, the NiMoO4−x@C sample exhibited a slightly lower diffraction peak intensity possibly due to the covering of carbon on the NiMoO4 surface as well as the reduced crystallinity of NiMoO4 with increased structural defects. In addition, no characteristic peaks for carbon phases were noted, suggesting the amorphous nature of the carbon species in the NiMoO4−x@C sample. The formation of amorphous carbon could be partially ascribed to the lower low annealing temperature (400 °C herein). Raman spectra of pristine NiMoO4 NWAs and NiMoO4−x@C samples are illustrated in Figure 3. The bands at 961 cm −1 and 913 cm −1 corresponded to the symmetric and asymmetric stretching modes of Mo=O bonds, while the band at 706 cm −1 could be ascribed to the stretching mode of Ni/Mo-O bonds of the orthorhombic α-NiMoO4 phase [53]. In addition, two bands ascribed to the presence of carbon species were identified. The band at around 1360 cm −1 could be attributed to the D band from defects and disorders in the amorphous carbon layers, while the other band at around 1590 cm −1 was related to the G band related to the vibration of sp 2 -bonded carbon atoms [54]. This result implied the successful deposition of amorphous carbon layer on the surface of the NiMoO4 NWAs.  Raman spectra of pristine NiMoO 4 NWAs and NiMoO 4−x @C samples are illustrated in Figure 3. The bands at 961 cm −1 and 913 cm −1 corresponded to the symmetric and asymmetric stretching modes of Mo=O bonds, while the band at 706 cm −1 could be ascribed to the stretching mode of Ni/Mo-O bonds of the orthorhombic α-NiMoO 4 phase [53]. In addition, two bands ascribed to the presence of carbon species were identified. The band at around 1360 cm −1 could be attributed to the D band from defects and disorders in the amorphous carbon layers, while the other band at around 1590 cm −1 was related to the G band related to the vibration of sp 2 -bonded carbon atoms [54]. This result implied the successful deposition of amorphous carbon layer on the surface of the NiMoO 4 NWAs. to the NiMoO4 NWAs samples, the NiMoO4−x@C sample exhibited a slightly lower diffraction peak intensity possibly due to the covering of carbon on the NiMoO4 surface as well as the reduced crystallinity of NiMoO4 with increased structural defects. In addition, no characteristic peaks for carbon phases were noted, suggesting the amorphous nature of the carbon species in the NiMoO4−x@C sample. The formation of amorphous carbon could be partially ascribed to the lower low annealing temperature (400 °C herein). Raman spectra of pristine NiMoO4 NWAs and NiMoO4−x@C samples are illustrated in Figure 3. The bands at 961 cm −1 and 913 cm −1 corresponded to the symmetric and asymmetric stretching modes of Mo=O bonds, while the band at 706 cm −1 could be ascribed to the stretching mode of Ni/Mo-O bonds of the orthorhombic α-NiMoO4 phase [53]. In addition, two bands ascribed to the presence of carbon species were identified. The band at around 1360 cm −1 could be attributed to the D band from defects and disorders in the amorphous carbon layers, while the other band at around 1590 cm −1 was related to the G band related to the vibration of sp 2 -bonded carbon atoms [54]. This result implied the successful deposition of amorphous carbon layer on the surface of the NiMoO4 NWAs.  composed of oriented nanowires (NWs) with a smooth surface. In addition, the NiMoO 4 NWAs have relatively uniform diameters of~300 nm, on average, and lengths of several micrometers. After the coating of the carbon, the surface of the NiMoO 4−x @C sample became obviously coarse as shown in Figure 4b. The element composition analyses using energy-dispersive X-ray spectra (EDS) analysis indicated the existence of Ni, Mo, O, C, Fe, and Al elements in the NiMoO 4−x @C sample ( Figure S1, Supporting Information). Note that the Fe and Al signals mainly stemmed from the ferronickel foam substrate and the sample holder, respectively. The morphologies of the NiMoO4 samples were firstly observed using scanning electron microscopy (SEM). From Figure 4a, the pristine NiMoO4 NWAs sample was composed of oriented nanowires (NWs) with a smooth surface. In addition, the NiMoO4 NWAs have relatively uniform diameters of ~300 nm, on average, and lengths of several micrometers. After the coating of the carbon, the surface of the NiMoO4−x@C sample became obviously coarse as shown in Figure 4b. The element composition analyses using energy-dispersive X-ray spectra (EDS) analysis indicated the existence of Ni, Mo, O, C, Fe, and Al elements in the NiMoO4−x@C sample ( Figure S1, Supporting Information, SI). Note that the Fe and Al signals mainly stemmed from the ferronickel foam substrate and the sample holder, respectively.  Figure 5a) taken from this nanowire depicted a clear two-dimensional dot pattern, suggesting its single-crystalline structure in nature. Two diffraction spots, as marked by white circles, could be indexed to the (220) and (−222) crystal facets of orthorhombic NiMoO4. From Figure 5b, the crystal plane with a lattice spacing of 2.73 Å in the HRTEM micrograph corresponded to the (−222) planes of NiMoO4 [49,52]. In contrast, the TEM image in Figure 5c indicated that a layer of amorphous carbon film with a thickness of ca. 20~50 nm had been coated on the NiMoO4 nanowire's surface, confirming the core-shell structure of the NiMoO4−x@C composite sample with different brightness contrasts of NiMoO4 and carbon. The deposition of amorphous carbon on the surface of NiMoO4 can be further confirmed by HRTEM micrograph as shown in Figure  5d.  [49,52]. In contrast, the TEM image in Figure 5c indicated that a layer of amorphous carbon film with a thickness of ca. 20~50 nm had been coated on the NiMoO 4 nanowire's surface, confirming the core-shell structure of the NiMoO 4−x @C composite sample with different brightness contrasts of NiMoO 4 and carbon. The deposition of amorphous carbon on the surface of NiMoO 4 can be further confirmed by HRTEM micrograph as shown in Figure 5d. Next, the chemical composition and valence states of element on the surface of Ni-MoO4 NWAs and NiMoO4−x@C samples were identified by X-ray photoelectron spectroscopy (XPS, Figure 6). In the high-resolution Ni 2p spectrum of the pristine NiMoO4 NWAs sample (Figure 6a), two main peaks were observed at binding energies (BEs) of 873.4 eV Next, the chemical composition and valence states of element on the surface of NiMoO 4 NWAs and NiMoO 4−x @C samples were identified by X-ray photoelectron spectroscopy (XPS, Figure 6). In the high-resolution Ni 2p spectrum of the pristine NiMoO 4 NWAs sample (Figure 6a), two main peaks were observed at binding energies (BEs) of 873.4 eV and 856.3 eV with a spin-orbital splitting energy of 17.1 eV, corresponding to the Ni 2p 1/2 and Ni 2p 3/2 of Ni 2+ in NiMoO 4 lattice [55]. In addition, two satellite peaks with BEs of 877.3 eV and 860.6 eV were noted for Ni 2+ . In Figure 6b Then, the effects of pyrolysis temperatures (from 200~800 °C) during the carbon coating of the morphologies and microstructures of the NiMoO4/carbon composites were investigated. It is noted that some aggregates of residual sucrose were observed after annealing at 200 °C ( Figure S2a and SI), suggesting the carbonization of sucrose was incom- Then, the effects of pyrolysis temperatures (from 200~800 • C) during the carbon coating of the morphologies and microstructures of the NiMoO 4 /carbon composites were investigated. It is noted that some aggregates of residual sucrose were observed after annealing at 200 • C ( Figure S2a), suggesting the carbonization of sucrose was incomplete under a lower temperature. This SEM result also coincides well with the thermogravimetric (TGA) and the differential scanning calorimetry (DSC) analyses ( Figure S3), where the thermal decomposition process of sucrose mainly occurs between 223 and 389 • C. With the increase of annealing temperature, sucrose was decomposed, and the carbonization process occurred accompanied by the release of some gases (e.g., CO, CO 2 ). At a higher temperature, the generated reductive gases (e.g., CO) reacted with NiMoO 4 and generated some oxygen vacancies on the NiMoO 4 surface via abstracting some surface oxygen atoms. In contrast, well-defined nanowires were obtained for the samples prepared after annealing at 400 and 600 • C, respectively ( Figure S2b,c). However, the nanowire structure was destroyed when the pyrolysis temperature was increased to 800 • C ( Figure S2d), which might have been caused by the large inner stain in the NiMoO 4 NWAs or at the NiMoO 4−x @C interface. Thus, the standard annealing temperature was chosen as 400 • C.
To evaluate the electrochemical performance of NiMoO 4 NWAs and NiMoO 4−x @C samples, electrochemical measurements were tested by a three-electrode system with 2 M KOH electrolyte (Figure 7). Figure 7a shows the CV curves of NiMoO 4 NWAs and NiMoO 4−x @C samples at a scan rate of 20 mV s −1 with a potential window of 0 to 0.7 V. Overall, the NiMoO 4−x @C sample had a larger integral area than that of the NiMoO 4 NWAs sample, indicating a significant increase of capacitance after carbon deposition and introduction of oxygen vacancies. Meanwhile, the CV curves of the two samples exhibited typical oxidation peaks, demonstrating typical pseudocapacitive charge storage characteristics. In addition, the CV curves of four different pyrolysis temperatures of NiMoO 4−x @C samples are revealed in Figure S4. The sample collected at 400 • C shows the highest capacitance which is consistent with the result of SEM in Figure S2. The sample collected at 800 • C exhibited an unsatisfactory performance due to its collapsed morphology. Figure 7b shows the GCD curves of the two samples. It revealed that the discharge time of the NiMoO 4−x @C sample was almost twice as much as that of the NiMoO 4 NWAs sample at a current density of 1 A cm −2 . Figure 7c shows the electrochemical impedance spectroscopy (EIS) of NiMoO 4 NWAs and NiMoO 4−x @C. The direct impedance and charge transfer resistance of the NiMoO 4−x @C sample was significantly lower than that of the NiMoO 4 NWAs sample. The remarkably reduced size of the semicircle for the NiMoO 4−x @C indicated an improved charge transfer kinetics due to enhanced electrical conductivity provided by the carbon shell and oxygen vacancy defects. In addition, the NiMoO 4−x @C exhibits the steepest slope in the low-frequency region, clearly indicating the lowest Warburg impedance and, hence, the highest K-ion diffusion capability at the interface between the electrode and electrolyte. Through the AC EIS, we added the corresponding equivalent circuit diagram in Figure 7c.
The true impedance of capacitor can be estimated using the following Equation (1): Figure 7d shows the capacitance of NiMoO 4 NWAs and NiMoO 4−x @C samples calculated from different current densities. After coating the carbon layer, the capacitance of the NiMoO 4−x @C sample was greatly increased. The areal capacitance can be calculated as high as 2.24 F cm −2 (1720 F g −1 ) at a current density of 1 mA cm −2 . In contrast, the NiMoO 4 NWAs electrode only demonstrated a specific capacitance of 1.206 F cm −2 (927 F g −1 ) at the same current density. The cycling performances of NiMoO 4 NWAs and NiMoO 4−x @C samples are presented in Figure 7e. The capacitance retention of NiMoO 4−x @C is 84.5% at 20 mA cm −2 after 6000 cycles, which is considerably better than that of the NiMoO 4 NWAs sample (63.1% after 6000 cycles). To illustrate the difference of the cycling process between NiMoO 4 NWAs and NiMoO 4−x @C samples, we also obtained the SEM results after cycling as shown in Figure S5. It is evident that the NiMoO 4−x @C sample still held some nanorod structures under the protection of amorphous carbon shell. Instead, NiMoO 4 NWAs were aggregated after 10,000 cycles, with unsatisfactory cycle abilities. In addition, we made a comparison of the C s and cycling stability of this work with some previously reported NiMoO 4 -based electrodes materials as summarized in Table S1. 800 °C exhibited an unsatisfactory performance due to its collapsed morphology. Figure  7b shows the GCD curves of the two samples. It revealed that the discharge time of the NiMoO4−x@C sample was almost twice as much as that of the NiMoO4 NWAs sample at a current density of 1 A cm −2 . Figure 7c shows the electrochemical impedance spectroscopy (EIS) of NiMoO4 NWAs and NiMoO4−x@C. The direct impedance and charge transfer resistance of the NiMoO4−x@C sample was significantly lower than that of the NiMoO4 NWAs sample. The remarkably reduced size of the semicircle for the NiMoO4−x@C indicated an improved charge transfer kinetics due to enhanced electrical conductivity provided by the carbon shell and oxygen vacancy defects. In addition, the NiMoO4−x@C exhibits the steepest slope in the low-frequency region, clearly indicating the lowest Warburg impedance and, hence, the highest K-ion diffusion capability at the interface between the electrode and electrolyte. Through the AC EIS, we added the corresponding equivalent circuit diagram in Figure 7c.  First-principles density functional theory (DFT) simulations were next adopted to further probe the structure-performance relationship of the NiMoO 4−x @C composite electrode in supercapacitors. The optimized geometry configurations of pristine and oxygen-deficient NiMoO 4 (110) surface slabs are shown in Figure S6. The pristine NiMoO 4 (110) surface was flat and composed of fivefold Ni and Mo atoms and twofold O atoms ( Figure S6a). The defective NiMoO 4 (110) surface can be produced after eliminating one surface O atom, leaving one threefold Ni and Mo atoms nearby ( Figure S6b). The resultant oxygen-deficient NiMoO 4 (110) plane retains flat. Next, the adsorption behavior of OH group on the pristine NiMoO 4 (110) surface was first investigated. As shown in Figure 8, the OH can be adsorbed on the top of either the surface of the Mo atom (Figure 8a,b) or the Ni atom (Figure 8c,d), yielding an adsorption energy (E ads ) of −0.50 and −5.97 eV, respectively. Evidently, the adsorption of OH on the Ni site was much stronger than that on the Mo site, which is consistent with the fact that the Ni in NiMoO 4 is electrochemically active for pseudocapacitive charge storage process based on Faradic reactions. The chemisorption of OH on Ni and Mo sites of the NiMoO 4 (110) surface was further verified by the interfacial charge transfer from the charge density difference contours ( Figure S7). In the following, the adsorption of OH adsorbed on the oxygen-deficient NiMoO4 (110) surface was evaluated. Specifically, the adsorption on the Ni and Mo sites with lower coordination due to the removal of surface O was considered. Interestingly, it is noted that the OH group was preferred to be adsorbed at the vicinity of the oxygen-vacancy position (Figure 9a), leading to an Eads of −3.63 eV and a charge transfer at the OH/NiMoO4 interface (Figure 9b). This result suggests that the presence of surface oxygen vacancies offers more active sites for OH adsorption, concentration, and subsequent redox reactions for enhanced pseudocapacitive charge storage. In the following, the adsorption of OH adsorbed on the oxygen-deficient NiMoO 4 (110) surface was evaluated. Specifically, the adsorption on the Ni and Mo sites with lower coordination due to the removal of surface O was considered. Interestingly, it is noted that the OH group was preferred to be adsorbed at the vicinity of the oxygen-vacancy position (Figure 9a), leading to an E ads of −3.63 eV and a charge transfer at the OH/NiMoO 4 interface (Figure 9b). This result suggests that the presence of surface oxygen vacancies offers more active sites for OH adsorption, concentration, and subsequent redox reactions for enhanced pseudocapacitive charge storage. In the following, the adsorption of OH adsorbed on the oxygen-deficient NiMoO4 (110) surface was evaluated. Specifically, the adsorption on the Ni and Mo sites with lower coordination due to the removal of surface O was considered. Interestingly, it is noted that the OH group was preferred to be adsorbed at the vicinity of the oxygen-vacancy position (Figure 9a), leading to an Eads of −3.63 eV and a charge transfer at the OH/NiMoO4 interface (Figure 9b). This result suggests that the presence of surface oxygen vacancies offers more active sites for OH adsorption, concentration, and subsequent redox reactions for enhanced pseudocapacitive charge storage. Based on the above experimental data and theoretical simulations, the significantly improved electrochemical performances of the NiMoO 4−x @C composite can be mainly attributed to the following points: (i) the deposition of amorphous carbon shell effectively enhances the electron transport of NiMoO 4 nanowires and charge transfer at NiMoO 4 /C heterointerface; (ii) the deposited carbon layer also improves the structural integrity of the NiMoO 4 nanowire arrays during long-term electrochemical cycling; (iii) the creation of oxygen vacancies in NiMoO 4 accompanied by the coating of the carbon further enhances the electronic conductivity of the NiMoO 4 electrode and creates more active sites for pseudocapacitive charge storage. Therefore, the synergy of defect and interface engineering of NiMoO 4 NWAs realized by carbon deposition effectively improves the overall electrochemical performance of the resultant NiMoO 4−x @C composite electrode in supercapacitors.
To further assess the practical application potential of a NiMoO 4−x @C sample, an asymmetric supercapacitor device (NiMoO 4−x @C//AC) was assembled with the NiMoO 4−x @C as a positive electrode and activated carbon (AC) as a negative electrode. Before testing the ASC device, we performed the CV measurements of NiMoO 4−x @C and AC electrodes in a three-electrode system at a scan rate of 5 mV s −1 to estimate the suitable operating voltage range (Figure 10a). The maximum operating voltage of NiMoO 4−x @C//AC ASC was determined to be 1.6 V. The CV curves of NiMoO 4−x @C//AC ASC at different scan rates are shown in Figure 10b. All the curves display obvious redox peaks, indicating the main contribution from pseudocapacitance. An increased separation of redox peak position can be noted along with the increase of scan rates due to the increased polarization. Figure 10c shows the GCD curves of the NiMoO 4−x @C//AC ASC at different current densities while Figure 10d shows the specific capacitance calculated from different current densities. The overall capacitance of the ASC was calculated to be 1.01 F cm −2 (156.25 F g −1 ) at 1 mA cm −2 . Furthermore, the NiMoO 4−x @C//AC ASC device has demonstrated a good capacitance retention of 83.6% after 6000 cycles at 20 mA cm −2 (Figure 10e). As a result, the ASC device can power a yellow LED (inset of Figure 10e), showing its potential in practical applications. In addition, the electrochemical performances of our NiMoO 4−x @C//AC ASC device are also superior or comparable to some recently reported NiMoO 4 -based electrode materials for ASCs as summarized in Table S1.

Conclusions
In summary, synchronous defect and interface engineering was implemented in Ni-MoO4 material via the formation of oxygen vacancies and the coating of the carbon on

Conclusions
In summary, synchronous defect and interface engineering was implemented in NiMoO 4 material via the formation of oxygen vacancies and the coating of the carbon on NiMoO 4 nanowire arrays through a simple hydrothermal method paired with sucrose pyrolysis. During this process, an amorphous carbon layer was homogeneously deposited on the surface of NiMoO 4 nanowires and oxygen vacancies were created on the NiMoO 4 surface during the carbonization of sucrose. The deposited carbon layer and formed oxygen vacancies in NiMoO 4 boosted the electronic conductivity of NiMoO 4 nanowires. In addition, the coated carbon layer also improved the structural integrity of the NiMoO 4 electrode during long-term operation in supercapacitors. Consequently, the resultant NiMoO 4−x @C heterostructure electrode achieved a high specific capacitance of 2.24 F cm −2 (1720 F g −1 ) and maintained a good capacitance retention of about 83.6% after 6000 cycles at 20 mA cm −2 . In addition, the as-assembled NiMoO 4−x @C//activated carbon asymmetric supercapacitor device manifested a high energy density of 51.6 W h kg −1 at a high power density of 203.95 W kg −1 , indicating that NiMoO 4−x @C composite is a suitable electrode material for supercapacitor applications. The proposed synergistic defect and interface engineering strategy herein can be extended for the design and development of other novel composite electrode materials for applications in electrochemical energy storage and conversion.