The requirement of using energy-storage devices for multifunctional applications is currently a hot research topic. Technological advancements ranging from electric vehicles to miniature portable electronics have considerably increased the demand for energy storage devices. Electrochemical energy storage is the most effective, cheap, and convenient method for meeting this demand ^[1][2][1,2]. Owing to their low costs and versatile performance, batteries and electrochemical capacitors (ECs) are two important categories of electrochemical energy-storage devices [3]. Although batteries are well known for exhibiting high energy densities, their lifespans are short and they exhibit low power densities ^[4][5][6][7][4,5,6,7]. In contrast, supercapacitors (SCs) are the most promising electrochemical energy storage devices that bridge the gap between conventional capacitors and batteries. SCs are notable for their unique high power densities, wide working-temperature ranges, extended cycling lives, improved energy densities compared with traditional capacitors, low maintenance, low leakage currents, long service lives, and high efficiencies ^[8][9][10][8,9,10]. Hence, SCs are employed in a wide range of applications, such as portable electronics, regenerative power-braking systems, hybrid electric vehicles, spacecrafts, aircrafts, military devices, biomedical equipment, and wind-energy power generators ^[11][12][11,12]. As a result, SCs have attracted considerable attention from both research and technological perspectives. However, SCs exhibit low energy densities, which are major drawbacks that limit their use in applications requiring both high energy and power densities. SCs have been broadly categorized as electric double-layer capacitors (EDLCs) and pseudocapacitors (PCs) according to their charge storage mechanisms, as shown in Figure 1 [13].

EDLCs are so named because they physically store an electrostatic charge in an electric field between an adjacent electrode and electrolyte, and the charge rapidly accumulates through electrostatic attraction without any chemical reactions, which generates a high device-power density ^[13][14][15][13,14,15]. In contrast, PCs operate by faradaically storing a charge at the electrode–electrolyte interface, where the charge is actually transferred between the electrode and the electrolyte. Through reversible redox reactions involving the highly reversible formation of chemical species, PCs exhibit higher energy densities than EDLCs ^[16][17][18][16,17,18]. The main components required to fabricate a hybrid-operating SC device include electrode materials, electrolytes, separators, and current collectors [19]. Although each component is important, selecting an electrode’s material and fabricating the electrode are the most crucial aspects. Carbon-based materials are the best EDLC electrode materials because their charge-storage mechanism mainly relies on the electrode-material surface area accessible to electrolyte ions. Activated carbon (AC), graphene and its derivatives, carbon nanotubes (CNTs), and carbon aerogels have been used as EDLC electrode materials ^[20][21][22][20,21,22] because they are inexpensive, are simply fabricated, and exhibit large surface areas and wide ranges of pore-size distributions. However, owing to their low specific capacitances, carbonaceous materials do not sufficiently contribute to the high SC energy density [23]. High-capacitance electrode materials are preferentially used to overcome the low energy density. Owing to their rapid reversible surface-redox reactions, conducting polymers and metal oxides (MOs) are pseudocapacitive electrode materials that provide high energy densities to SCs ^[24][25][26][24,25,26]. Compared to MOs, conducting polymers exhibit poor structural stabilities during continuous charge–discharge cycling. Therefore, various low-cost MOs and their composite electrode materials have been identified as promising alternatives to carbonaceous materials and conducting polymers ^[27][28][29][27,28,29]. Because of their exceptional physicochemical properties, MOs have been used in several applications (Scheme 1) and are usually applied as electrode materials because they can be synthesized using a wide range of methods and because of their high theoretical specific capacitances, natural abundance, excellent pseudocapacitive behaviors, high thermal and electrochemical stabilities, and reliabilities ^{[30][31][32][33]}[30,31,32,33].

MO-based SC electrodes have been synthesized using various chemical and vapor-phase-based techniques, including hydrothermal synthesis [34], sol-gel processing [35], electrochemical/electropolymerization synthesis [36], chemical bath deposition (CBD) [37], solvothermal synthesis [38], microwave-assisted synthesis [39], atomic-layer deposition (ALD) [40], and ball milling [41] (Scheme 2). However, not^[41] all these techniques can be applied to industrially produce high-quality low-cost MO-based SC electrodes. Therefore, an easy, efficient, economic, nontoxic, and reliable technique that can be used to produce numerous high-purity materials under mild reaction conditions should be developed for fabricating MO-based SC electrodes ^{[42][43][44][45]}[42,43,44,45].