**3. Results**

FE-SEM morphology of the grown films for the durations of 9 h, 18 h, and 27 h are shown in Figure 1. NiMoO4 nanostructures are evident for all the films grown for different durations, exhibiting a stacked structure with NiMoO4 nanograins. TEM analyses were carried out only on sample NMO-18 due to its superior electrochemical performance in comparison with NMO-9 and NMO-27. Figure 2a,b show typical TEM images for the 18 h sample, revealing crystalline NiMoO4 grain-like structure. From the observed twist dislocations in Figure 2b, it can be inferred that the crystals are composed of narrow platelets. Figure 2c shows a typical high resolution transmission electron microscope (HRTEM) image from a single nanograin. The interplanar distance (0.39 nm) corresponds to monoclinic NiMoO4 (021) crystal planes (JCPDS: 45-0142). Figure 2d shows the selected area diffraction (SAED) pattern, which confirms the polycrystalline nature. Figure 2e–h shows the HAADF-STEM electron, O, Ni, and Mo mappings, respectively, using the same scale is the TEM images. Ni, Mo, and O are relatively uniformly distributed, which is confirmed by the elemental line mapping, as shown in Figure 2i.

**Figure 1.** Typical field emission scanning electron microscope (FE-SEM) images for (**a**) NMO-9; (**b**) NMO-18; and (**c**) NMO-27 samples.

**Figure 2.** The transmission electron microscope (TEM) image for a single NiMoO4 nanograin grown for 18 h at (**a**) low and (**b**) high magnification; (**c**) The high resolution transmission electron microscope (HRTEM) image with d spacing noted; (**d**) The selected area diffraction (SAED) pattern of the same sample showing respective planes. High angle annular dark field imaging scanning transmission electron microscope (HAADF-STEM) elemental mapping for a selected area showing (**e**) the electron with line mapping; (**f**) oxygen; (**g**) nickel; (**h**) molybdenum; and (**i**) line mapping, showing the counts of Mo, Ni, and O.

The electrochemical characteristics of the electrode are mainly dependent on their dimensions and morphologies [20,21]. Electrochemical measurements of NiMoO4 nanostructure were performed in 2 M KOH electrolyte. Figure 3a shows the cyclic voltammograms (CV) for hydrothermally grown NiMoO4 at different durations (100 mV/s scan rate), over the potential range of 0–0.43 V (versus SCE). Each CV plot exhibits distinct redox peaks, indicating that the observed capacitance properties can be described by Faradic reactions [22,23]. The NMO-18 sample had the largest area under the CV curve compared with NMO-9 and NMO-27.

Figure 3b–d show NMO-9, NMO-18, and NMO-27 CVs, respectively, for various scan rates. The CV curve shapes are almost unchanged for the different scan rates, indicating near ideal capacitive characteristics. Voltage corresponding to the oxidation peak moved in the positive direction as the scan rate increased, whereas the reduction peak moved in the negative direction, which can be attributed to electrode's internal resistance [24].

The surface area accessible for electrochemical reactions can be estimated from the electrochemically active surface area (ECSA). Initially, the non-Faradic capacitive current was obtained from the linear region of the CV curves,

$$
\dot{n}\_{\rm DL} = \mathbb{C}\_{\rm DL} \times \nu \tag{1}
$$

where *i*DL is the capacitive current, *C*DL is the specific capacitance in the non-Faradic region, and *ν* is the scan rate. Electrochemical capacitance was estimated for each sample at the various scan rates, and ECSA was calculated from

$$\text{ECSA} = \mathbb{C}\_{\text{DL}} / \mathbb{C}\_{\text{s}} \tag{2}$$

where *C*s is the specific capacitance of the standard electrode in an alkaline electrolyte [25]. We chose the non-Faradic region as 0.21–0.23 V, as shown in Figure 4a. Figure 4b shows the *i*DL at 0.22 V for the scan rate, with a corresponding ECSA of 649, 874, and 342 cm<sup>−</sup><sup>2</sup> for NMO-9, NMO-18, and NMO-27, respectively. NMO-18 exhibited the highest ECSA, consistent with its observed maximum supercapacitance. Thus, hydrothermal growth duration is critical to obtain the largest surface area and, hence, the highest supercapacitance.

**Figure 3.** Cyclic voltammograms (CVs) for (**a**) NMO-9, NMO-18, and NMO-27 samples at 100 mV/s scan rate and various scan rates for (**b**) NMO-9; (**c**) NMO-18; and (**d**) NMO-27.

Figure 5a shows the galvanostatic charge-discharge profiles for the NiMoO4 nanostructure electrodes at a 1 A/g current density, with NMO-18 exhibiting the longest discharge time. Figure 5b–d show NMO-9, NMO-18, and NMO-27 charge-discharge profiles, respectively, at different current densities.

**Figure 4.** (**a**) Cyclic voltammograms (CV) for NMO-18 at various scan rates in the non-Faradic voltage region (0.21–0.23 V); (**b**) The measured current at 0.22 V as a function of the scan rate for NMO-9, NMO-18, and NMO-27.

**Figure 5.** Galvanostatic charge-discharge profile for (**a**) NMO-9, NMO-18, and NMO-27 at 1 A/g current density; (**b**) NMO-9; (**c**) NMO-18; and (**d**) NMO-27 at various current densities.

Each charge-discharge curve shows pseudo-capacitor characteristics, consistent with the CV data. There is a sharp voltage drop as the supercapacitor changes state from charge to discharge, which can be attributed to an IR drop. Each electrode exhibits nonlinear discharge followed by a plateau, which verifies faradaic reactions occurring on the electrode surface due to the redox reaction at the material/solution interface. Figure 6a represents the specific capacitance response for current densities for all samples. The performance of the specific capacitance decreases with increasing current density. The fall in the supercapacitance value at a high current density could be attributed to the lack of active material at the electrode/electrolyte interface during oxidation/reduction reactions and the slow diffusion of bulky OH ions [26,27].

**Figure 6.** (**a**) Specific capacitance as a function of current density for all the samples of NMO-9, NMO-18, and NMO-27; (**b**) The response of specific capacity to current density for NMO-9, NMO-18, and NMO-27.

Specific capacitance or *C*s was calculated from galvanostatic charge-discharge plots (Figure 5) from

$$C\_s = \frac{I \cdot \Delta t}{m \cdot \Delta V} \tag{3}$$

where *I* is the applied current density, Δ*V* is the potential window, and Δ*t* is the discharge time. The calculated specific capacitances at 1 A/g are 341, 619, and 219 F/g for NMO-9, NMO-18, and NMO-27, respectively. The observed specific capacitances are directly related to the available surface area for the electrochemical reaction in the NiMoO4 nanostructure electrode. The superior specific capacitance observed in NMO-18, in comparison with NMO-9 and NMO-27, can be attributed to the higher electrochemical surface area observed in NMO-18 from the ECSA estimation described above. Thus, NiMoO4 nanostructures showed comparable capacitance and rate capability with previously reported materials, such as NiO [26,28], Co3O4 [20], and MnO2 [29]. Table 1 shows the super capacitance values of this work along with other NiMoO4 reports on various nanostructures and different substrates. The specific capacity was also calculated due to the fact that CV-plots exhibit sharp redox and oxidation peaks arising from a Faradic battery-type mechanism [30,31]. Therefore, we also present specific capacity, *C* (mAh/g), which can be calculated from charge-discharge plots using,

$$\mathcal{C} = \frac{I \cdot \Delta t}{3600 \cdot m} \tag{4}$$

The specific capacity for all the samples at various current densities are shown in Figure 6b. The specific capacity for NMO-9, NMO-18, and NMO-27 are 33.9, 61.8, and 27.9 mAh/g, respectively, at a current density of 1 A/g.

**Table 1.** Data of previous NiMoO4 nanostructure-based reports compared with our work.


We also study electrochemical stability, an important characteristic for practical applications. Each NiMoO4 electrode was subjected to 3000 charge-discharge cycles. Figure 7a shows the electrode stability at 10 A/g current density. NMO-18 exhibited a steep capacitance loss during the initial several hundred cycles, and then stabilized to approximately 57% retention after 3000 cycles [32]. NMO-9 and NMO-27 showed 44% and 56.5% retention after 3000 cycles of charge-discharge operations, respectively. The lower retention level in NMO-9 may be attributed to a more rapid attrition of electrode material in relation to other two samples during charge-discharge cycles. The capacitance reduction can be explained by NiMoO4 material physical expansion as ionic transfer occurred and the partial dissolution of NiMoO4 material during charge-discharge operations [33].

Energy density (*E*) and power density (*P*) can be expressed as

$$E = \frac{1}{2} \mathbb{C}\_{\mathbb{S}} \cdot (\Delta V)^2 \tag{5}$$

(6)

*P* = *E*Δ*t*

**Figure 7.** (**a**) Response of specific capacitance over 3000 cycles of charge-discharge operations for NMO-9, NMO-18, and NMO-27; (**b**) Ragone plots with energy density versus power density.

Figure 7b shows the Ragone plots comparing energy and power density for all the electrodes. The peak energy density was observed for NMO-18 as 40 Wh/kg, and the highest power density of 7200 W/kg was associated with NMO-27. In a largely diffusion-controlled redox reaction between NiO and OH<sup>−</sup>, it is expected that energy density and charge-discharge rates will be inversely proportional [34–36]. It is notable that the NiMoO4 nanostructures exhibited high power density and energy density, consistent with past research reports [37,38]. Table 2 shows the specific capacitance and energy density found in the current work in relation to the previously published metal oxide reports.

**Table 2.** Electrochemical performance of this work compared with previously reported various metal oxide supercapacitor electrode materials.


and
