**1. Introduction**

Supercapacitors have been widely studied as candidates for energy storage systems due to their large power density, rapid charge-discharge, excellent rate capabilities, and good endurance [1–3]. They have been used in tandem with rechargeable storage devices and fuel cells, and they have been broadly applied for electric vehicles, electrical grid buffers, space applications, and memory system power back-up. In the recent past, there has been significant surge of research in transition metal oxides-based supercapacitors due to their multiple oxidation states and reversible redox reaction capabilities [4–6]. However, progress in supercapacitor electrode materials has been constrained by high cost (e.g., RuO2) or environmental impact (e.g., metal sulfides) [7–9].

Metal oxides with two transition metals have also been studied to take advantage of their variable oxidation states, excellent electrical conduction, and enhanced pseudo-capacitance characteristics, such as CoMoO4 [10,11], NiCo2O4 [6], ZnWO4 [12], and NiMoO4 [13]. NiMoO4 is a promising binary metal oxide due to its potential for high specific capacitance arising from the excellent electrochemical nature of the Ni ion, although practical electrode use is deterred by its poor electrical conductivity. Intricate characteristics of the molybdate also complicate NiMoO4 nanostructure growth [14,15]. Hence, it is important to identify easier preparation routes for NiMoO4 nanostructure thin films with distinct morphologies and superior supercapacitor characteristics.

There is a promising route to grow the nanostructures directly on conducting substrates [16], countering inherent low metal oxide electrical conductivity and, hence, considerably enhancing electrochemical performance. There are many reports of NiMoO4 supercapacitor performance on Ni foam substrates [14–18]. However, the NiMoO4 on Ni foam (usually coated with a binder as a slurry) is often dominated by the Ni foam contribution to overall electrochemical performance, making it difficult to identify the exact supercapacitance value of the desired electrode material. Only NiMoO4 nanostructure synthesis on nonreactive substrates has been reported previously, such as stainless steel (SS) and carbon [16,19], without exploring its electrochemical characteristics. However, there are no reports available detailing the electrochemical performance of NiMoO4 thin film nanostructures on nonreactive substrate such as stainless steel. Here we intend to exclusively evaluate the electrochemical performance of NiMoO4 thin film nanostructures, without the hindrance of substrate contribution.

This study reports on the binder-free growth of NiMoO4 nanostructures on SS substrate using a facile, single-step hydrothermal technique. The optimal growth time was identified by studying NiMoO4 thin films grown for 9, 18, and 27 h, keeping other deposition conditions constant. Nanostructured NiMoO4 grown on SS substrate exhibited high specific capacitance, good cycling stability, and enhanced rate capability. The proposed technique offers a promising, environmentally friendly, and relatively low cost direct route to obtain high supercapacitance NiMoO4 nanostructures.

#### **2. Materials and Methods**

#### *2.1. NiMoO4 Nanostructure Growth Process*

Molybdenum chloride (1.5 mmol) was mixed with methanol (50 mL) and stirred for 10 min. Nickel chloride solution (1.5 mmol) was added and stirred for a further 10 min, then EDTA (1.5 mmol) was added and stirred for 1 h. After forming a clear solution, 1 mL H2O2 and 1 mL HNO3 were added and stirred for 10 min. The solution was transferred to a Teflon container that already contained a pre-cleaned SS substrate with 1 cm × 2 cm exposed area. The complete setup was placed in a stainless steel autoclave at 180 ◦C. In order to identify optimal growth time, experiments were conducted with wide ranging growth durations from 9 to 36 h. Three samples were identified with 9, 18, and 27 h growth duration and labeled as NMO-9, NMO-18, and NMO-27, respectively, due to their superior electrochemical performance over the other samples. The grown films are then harvested, washed with deionized water, and dried with N2 gas.

#### *2.2. Materials Characterization*

The obtained thin films were characterized using a field emission scanning electron microscope (FE-SEM, Hitachi-S-4800, Huntington Beach, CA, USA), transmission electron microscope (TEM, JEM-2100F, JEOL, Akishima, Tokyo, Japan), and high angle annular dark field imaging (HAADF) scanning transmission electron microscope (STEM, JEM-2100F, JEOL, Akishima, Tokyo, Japan). Electrochemical measurements were performed in a 2 M KOH aqueous solution using a standard three-electrode electrochemical cell in Versa-stat-3. NiMoO4 served as the working electrode, with a saturated calomel electrode (SCE) and graphite rod as reference and counter electrodes, respectively.
