Keywords

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

9.1 Introduction

The rapidly growing commercial markets in electric vehicle s and portable electronic products have created a seemingly insatiable demand for high-performance energy storage devices [1]. However, it seems hard to meet the increasingly higher standards by using the current energy conversation or storage devices alone, such as lithium -ion batteries or fuel cells. Supercapacitors (SCs), also called electrochemical capacitor s , store charges through either ion adsorption/desorption (electrochemical double-layer capacitors, EDLCs ) or fast surface redox reactions (pseudocapacitors), which can complement or replace batteries in electrical energy storage when high-power delivery or uptake is needed [2]. According to charge storage mechanisms, SCs may be placed into two distinctive classes: EDLCs and pseudocapacitors [3]. As is well known, SCs have the advantages of high power density, ultralong cycle life , and rapid recharge capability, devoting to meeting the ever-growing consumption demands and alleviating the energy crisis [4, 5]. The Ragone plot [2] in Fig. 9.1 (power density against energy density) shows that SCs shorten the gap between conventional capacitors and batteries.

Fig. 9.1
figure 1

Ragone plot (specific power against specific energy ) for various electrical energy storage devices (Reprinted from Ref. [2] with permission. Copyright 2008, Macmillan Publishers Limited)

Full size image

SCs were first reported in 1957 Becker’s patent [6] and then emerged in the commercial market in 1978 (gold capacitors from Panasonic/Matsushita) and 1980 (supercap from NEC Tokin) [7]. Generally, they can serve as load-leveling and uninterruptible power supplies, such as backup sources for memories and microcomputers, and have great potential applications in electric vehicle s or hybrid electric vehicles. Since the 1990s, electrochemical capacitor s ushered in its development with materials used as EDLCs mainly based on carbonaceous materials such as activated carbon [8, 9], carbon nanotube [10, 11], and graphene [12–14] which provide large surface areas for electrical charges to reside. Alternatively, pseudocapacitors utilizing transition metal oxides and conductive polymers combine electrosorption with fast surface redox reactions to store energy , where the amount of charge held is proportional to the applied voltage [7]. Researchers had no clear concept regarding pseudocapacitance until Trasatti and Buzzanca discovered the capacitive properties of RuO2 [15].

Note that electrode materials have become the core competence for supercapacitors and could be typically divided into three types including carbon materials, conducting polymers, and transition metal oxides . Carbon materials with low cost, extremely developed surface area (up to \( 3000\ {\mathrm{m}}^2{\mathrm{g}}^{-1} \)), and good electrical conductivity and chemical stability seem to be ideal materials for electrochemical double-layer capacitors, but it usually suffers from low specific capacitance (commonly lower than \( 400\ \mathrm{F}\ {\mathrm{g}}^{-1} \)) [16–18]. While conducting polymers have higher specific capacitance (usually lower than \( 530\ \mathrm{F}\ {\mathrm{g}}^{-1} \)) and excellent intrinsic conductivity, however, their cycle life is extremely poor because of substantial expansion and contraction during the charging–discharging process [19, 20]. Compared with these two types mentioned above, transition metal oxides usually possess multiple oxidation states that are in favor of fast redox reaction resulting in a much higher specific capacitance [21]. Impressively, specific capacitance of pseudocapacitors obtained from conducting polymers and transition metal oxides can be 10–100 times higher than EDLCs obtained from carbon materials [22].

While supercapacitors benefit from the large power capability (as high as 10 kW kg– 1), high current capability, ease of maintenance, and long-term cycling stability (\( > 1{0}^6 \) cycles) [23], their moderate energy density for the widespread deployment in everyday technology using clean and renewable energy media is still far away to meet the performances and cost requirements, in particular providing power boosts to start the engine or to assist acceleration. In order to make capacitors commercially viable and extensively used, the energy that they can store needs to increase significantly and to endow supercapacitors with sufficient energy for higher requirements in the future is still a challenge. Extensive efforts have been devoted to improving the specific capacitance of supercapacitors by introducing pseudocapacitive metal oxides . In this way, cheap transition metal oxides with high energy density are very attractive electrode materials for supercapacitors to realize commercialization quickly.

9.2 Energy Storage Mechanism

Transition metal oxides store its charges both on the surface and in the bulk near the surface of the solid electrode where the electrochemical processes occur. Moreover, it should be noted that the metal oxides possess not only double-layer capacitance but also high pseudocapacitance (typically \( 10\sim 100 \) times higher than double-layer capacitance), whereas the double-layer capacitances shown by CV curves for the electrodes are relatively too small to be observed [24]. In general, two kinds of electrolytes including alkaline solution and acid solution are used to analyze their corresponding electrochemical performances. Typically, RuO2 and MnO2 electrodes are measured in H2SO4 solution, while NiO, Co3O4, and NiCo2O4 use KOH as electrolyte , thus obtaining remarkable pseudocapacitances; their pseudocapacitive behaviors in the electrolyte can be described by the following equations [24, 25]:

$$ {\mathrm{RuO}}_2+{\updelta \mathrm{H}}^{+}+{\updelta \mathrm{e}}^{-}\leftrightarrow {\mathrm{RuO}}_{\left(2-\updelta \right)}{\left(\mathrm{O}\mathrm{H}\right)}_{\updelta} $$
(9.1)
$$ {\mathrm{MnO}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\leftrightarrow \mathrm{MnOOH} $$
(9.2)
$$ \mathrm{N}\mathrm{i}\mathrm{O} + {\mathrm{OH}}^{-}+{\mathrm{e}}^{-}\leftrightarrow \mathrm{Ni}\mathrm{O}\mathrm{O}\mathrm{H} $$
(9.3)
$$ \left\{\begin{array}{l}{\mathrm{Co}}_3{\mathrm{O}}_4 + {\mathrm{O}\mathrm{H}}^{-}+{\mathrm{H}}_2\mathrm{O}\ \leftrightarrow 3\mathrm{CoOOH} + {\mathrm{e}}^{-}\\ {}\mathrm{CoOOH} + {\mathrm{O}\mathrm{H}}^{-}\leftrightarrow {\mathrm{Co}\mathrm{O}}_2 + {\mathrm{H}}_2\mathrm{O} + {\mathrm{e}}^{-}\end{array}\right. $$
(9.4)
$$ \left\{\begin{array}{l}{\mathrm{NiCo}}_2{\mathrm{O}}_4 + {\mathrm{O}\mathrm{H}}^{-} + {\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{NiOOH} + 2\mathrm{CoOOH} + {\mathrm{e}}^{-}\\ {}\mathrm{CoOOH} + {\mathrm{O}\mathrm{H}}^{-}\rightleftharpoons {\mathrm{CoO}}_2 + {\mathrm{H}}_2\mathrm{O} + {\mathrm{e}}^{-}\end{array}\right. $$
(9.5)

As typical pseudocapacitor electrode materials, ruthenium oxides have a completely different mechanism with that of electrochemical double-layer capacitors and possess multiple redox states and good electrical conductivity. RuO2 has three distinct oxidation states accessible within 1.2 V, and its pseudocapacitive behavior in acidic solutions can be described by Eq. (9.1). Protons can participate in the redox reaction, where RuO2 acts as a proton condenser and the reaction is electrochemically reversible. Amorphous structures are the key parameter in determining the value of specific capacitances, as the proton can be easily intercalated into the bulk of RuO2 · xH2O but not into the crystalline phase [26, 27]. It is also proposed that hydrous ruthenium dioxide has an improved capacitive properties, better than those observed from anhydrous structures, as hydrous regions are more permeable to protons and proton conduction inside ruthenium dioxide is dominant when compared with the electron conduction [28]. The cyclic voltammetry (CV) curve and galvanostatic charge–discharge curve [29] are displayed in Fig. 9.2a and b. It shows a nearly rectangular shape and good coulombic efficiency as high as \( \sim 100\% \), which is closely related to the superlong cycle life and excellent rate capacity .

Fig. 9.2
figure 2

(a) The cyclic voltammetry (CV) curve and (b) galvanostatic charge–discharge curve of RuO2 (Reprinted from Ref. [29] with permission. Copyright 2014, The Royal Society of Chemistry)

Full size image

Very recently, spinel NiCo2O4 has sparked worldwide concern as an attractive pseudocapacitive electrode material for supercapacitor application due to its many intrinsic advantages. It possesses a ferrimagnetic cubic spinel structure with multiple redox states and good electrical conductivity as a P-type semiconductor with the band gap of 2.1 eV [30]. Note that NiCo2O4 adopts a spinel structure in which all the nickel cations occupy the octahedral interstices, whereas the cobalt cations are distributing among the tetrahedral and octahedral interstices [31]. The redox reactions offered by nickel cobaltite, including contributions from both nickel and cobalt ions, are richer than those of the monometallic nickel oxides and cobalt oxides as illustrated in Eq. (9.5). In addition, the electrical conductivity of nickel cobaltite is at least two orders of magnitude higher than that of pure Co3O4 and NiO, which facilities to acquire better cycling performance and fast charge–discharge capability [32, 33]. It is noteworthy that, in the charging–discharging process (Fig. 9.3), there exist valence state changes of \( {\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+} \) as well as \( {\mathrm{M}}^{2+}/{\mathrm{M}}^{3+} \) (\( \mathrm{M}=\mathrm{C}\mathrm{o} \) or Ni) on the surface of the electrode materials, where fast and reversible faradaic reactions occur [33, 34]. The electrochemical redox potentials of \( {\mathrm{M}}^{2+}/{\mathrm{M}}^{3+} \) and \( {\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+} \) transitions are so close that the redox peaks observed overlap as one. Intriguingly, the specific capacitance of NiCo2O4 is observed to rise after several hundred cycles in some work which attributed to their unique morphologies and process of activation (potential cycling). Other transition metal oxides have the same storage mechanism and are not covered here.

Fig. 9.3
figure 3

Schematic of charge storage mechanism for supercapacitors based on NiCo2O4 (Reprinted from Ref. [24] with permission. Copyright 2014, The Royal Society of Chemistry)

Full size image

9.3 Transition Metal Oxides for Supercapacitors

Transition metal oxides as the basis for supercapacitors can be typically divided into two types: noble metal oxides and base transition metal oxides. Noble metal oxides include RuO2, IrO2, etc., while base transition metal oxides for supercapacitors are developing from monometallic oxides (such as NiO, Co3O4, Fe3O4, and Mn3O4) to single-phase bimetallic oxides (such as NiCo2O4, MnCo2O4, NiMoO4, NiFe2O4, and CoMn2O4). The supercapacitor materials based on transition metal oxides are summarized and classified in Scheme 9.1.

Scheme 9.1
scheme 1

Classification of transition metal oxides utalised as supercapacitor materials

Full size image

9.3.1 Noble Metal Oxides

Nanostructured materials facilitate to help ameliorate the electrochemical performances of transition metal oxides , taking advantage of revealing the essentials during the charge–discharge process and grasping the key element in fabricating nanomaterials by transforming bulk transition metal oxides into nano-porous structures. Noble metal oxides such as RuO2, IrO2 and so forth were first found exhibiting good capacitive properties with remarkable electrochemical performances. In 1971, Trasatti and Buzzanca first reported that RuO2 films could form a rectangular shaped cyclic voltammogram which closely resembled the carbon-based electrochemical double-layer capacitor [15]. Since this pioneering report, researchers have explored the capacitive properties of RuO2 extensively since its theoretical value of specific capacitance could be as high as \( 1300 - 2200\ \mathrm{F}\ {\mathrm{g}}^{-1} \) [35]. RuO2 is a rare stoichiometric oxide which possesses wide potential window, remarkably high specific capacitance, high rate capability , long cycle life , thermal and chemical stability, and metallic-type conductivity [36]. The key methodology to boost the specific capacitance is their morphological and chemical composition design, as high surface-to-volume ratio with suitable pore sizes is desirable for the penetration of electrolytes and reactants into the whole electrode matrix and can promote the electrochemical double-layer capacitances and provide a large amount of superficial electroactive species for faradaic redox reactions. More specifically, the factors playing key roles in the electrochemical behavior of Ru oxides mainly include specific surface area, the combined water in RuOx, the crystallinity of RuO2, the size of RuO2. xH2O, and electrolytes used [37].

Generally, there are two common methods to prepare ruthenium oxides for supercapacitor applications. One is the chemical vapor deposition that synthesizes nanomaterials by thermal decomposition and then oxidization of the precursor at high temperatures [38, 39]. The other is the sol–gel method which firstly produces a sol–gel of precursor followed by annealing at low temperatures. It was reported that amorphous ruthenium oxides obtained at low temperatures of 150 °C show specific capacitance as high as \( 720\ \mathrm{F}\ {\mathrm{g}}^{-1} \), while in the crystalline phase annealing with temperature beyond 175°C exhibited rapidly dropped specific capacitance [26].

Recently, it was reported that the group of Ji et al. developed a sol–gel method fabricating carbon spheres at RuO2 [29] and carbon quantum dots at RuO2 [40] hybrid for supercapacitors , demonstrating ultrahigh rate capability and excellent cycling stability. The excellent electrochemical performance was mainly attributed to the formation of the hybrid structure, which facilitates the fast charge transportation and ionic motion during the charge–discharge process and greatly improves the utilization of RuO2, leading to rapid redox reactions. The process of fabricating carbon quantum dots at RuO2 is elaborated in Fig. 9.4. In addition, Zhang et al. synthesized nanotubular ruthenium oxides by using manganite nanorods as a morphology sacrificial template and obtained a high specific capacitance of \( 860\ \mathrm{F}\ {\mathrm{g}}^{-1} \) with a good capacitive retention [41]. Extensive research has been undertaken by researchers in this field with the specific capacitance reported to reach more than \( 1300\ \mathrm{F}\ {\mathrm{g}}^{-1} \). For example, Hu et al. [35] used the membrane -templated synthesis route to obtain hydrous RuO2 (RuO2 · xH2O) nanotubular structures by means of an anodic deposition technique. The obtained ruthenium oxide exhibited an ultrahigh specific capacitance of \( 1300\ \mathrm{F}\ {\mathrm{g}}^{-1} \), excellent charge–discharge behavior at \( 1000\ \mathrm{mV}\ {\mathrm{s}}^{-1} \), and high-frequency (4.0–7.8 kHz) capacitive responses, which were much higher than any value ever reported before.

Fig. 9.4
figure 4

Preparation procedure for the RCQD/RuO2 hybrid (Reprinted from Ref. [40] with permission. Copyright 2013, The Royal Society of Chemistry)

Full size image

Iridium dioxide with its rutile structure is also a transition metal oxide for supercapacitors applications. In comparison with RuO2, the fact that IrO2 did not draw much attention for supercapacitor applications can mostly be owing to its relatively small value of specific capacitance [42]. For example, Chen et al. fabricated a composite of IrO2 and multiwalled carbon nanotubes, which showed a low specific capacitance of 69 F/g [43]. This was then extended to IrOx nanofoils with high surface area and then sputtered upon on multiwalled carbon nanotubes for supercapacitor application, exhibiting an improved specific capacitance of \( 370\ \mathrm{F}\ {\mathrm{g}}^{-1} \), which is still poor when compared with RuO2 [42].

9.3.2 Base Transition Metal Oxides

9.3.2.1 Monometallic Oxides

9.3.2.1.1 NiO

NiO is a very versatile material and has shown to exhibit good electrochemical activity based on the transformation process of NiO/NiOOH. Various morphologies such as flowers [44], hollow spheres [45, 46], and nanosheets [47] have been successfully prepared by different methodologies.

A number of chemical methods have been used to prepare nanostructured nickel oxides such as chemical precipitation, chemical bath deposition, microwave route, sol–gel process, and hydrothermal method [25]. In traditional chemical precipitation methods, Ni(OH)2 precursor is first obtained by adding precipitants into a soluble nickel salt such as Ni(CH3COO)2 and Ni(NO3)2, and then Ni(OH)2 is calcinated into NiO at high temperatures. The preparation method is facile, low cost, and suitable for large-scale production of NiO. For example,Zhang et al. [48] reported a facile hydrothermal process to grow various porous NiO nanostructures including nanoslices, nanoplates, and nanocolumns by calcinating \( \upbeta -\mathrm{N}\mathrm{i}{\left(\mathrm{O}\mathrm{H}\right)}_2 \). Consequently, the samples exhibited specific capacitances of 176, 285, and \( 390\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at a discharge current of \( 5\ \mathrm{A}\ {\mathrm{g}}^{-1} \), respectively.

Sol–gel process is another efficient way to obtain desired nanostructures . Typically, the first step is to prepare a sol–gel and the second step is to apply heat treatment. The sol–gel process seems very simple; however, in fact it is difficult to control. As reported, Yang et al. [49] obtained hexagonal and single crystalline NiO nanowires by a sol–gel process followed by calcination . In addition, NiOx xerogels were formed by a sol–gel method followed by annealing at 250 °C, obtaining a maximum specific capacitance of \( 696\ \mathrm{F}\ {\mathrm{g}}^{-1} \) [50].

There are some other methods such as chemical bath deposition [51], hydrothermal methods [52], and microwave routes [53] that are widely used in preparing NiO for supercapacitor applications. Nickel oxide is considered as an alternative electrode material for SC in alkaline electrolytes due to the advantages of high specific capacitance (theoretical specific capacitance of \( 3750\ \mathrm{F}\ {\mathrm{g}}^{-1} \)), environment friendliness, and low cost; however, it faces the challenges involving poor cycle performance and high resistivity [37]. The cycle performance and conductivity of nickel oxide could be improved by introducing cobalt ions into the nickel oxide matrix or combining with carbon materials [54, 55].

9.3.2.1.2 MnO2

Manganese oxide (MnO2) is advantageous to its low cost, no toxicity, easy accessibility, and large energy density and consequently has been considered as one of the most promising materials in application of SCs. MnO2 has not only a larger energy density but also is more economical than noble transition metal oxides . Over the past years, significant efforts have been directed toward the synthesis of nanomaterials with controlled sizes and ordered morphologies owing to the close relationship between the morphology and the capacitive performances.

The most common method to prepare MnO2 is a hydrothermal synthesis in which \( {\mathrm{Mn}}^{2+} \) is often used as raw material, and the as-prepared mixture solution is placed in a Teflon-lined pressure vessel. It has been demonstrated that the morphology of MnO2 depends much on the dwell time of hydrothermal process, showing an evolution of crystallinity of the nanostructures as the hydrothermal dwell time increased from 1 to 18 h. Scanning electron microscopy (SEM ) images of nanostructure MnO2 exhibiting different morphologies are depicted in Fig. 9.5, and the MnO2 nanostructures prepared at 140 °C for 6 h showed the best performance [56]. Electrochemical deposition is another effective methodology to prepare nanostructured MnO2. For example, Tao et al. [57] have successfully prepared a solid-state supercapacitor based on a \( \mathrm{P}\mathrm{P}\mathrm{y}-{\mathrm{MnO}}_2 \) nanoflake-carbon fiber hybrid structure by electrochemical deposition process. The device exhibited a high-volume capacitance of \( 69.3\ \mathrm{F}\ {\mathrm{cm}}^{-3} \) at \( 0.1\ \mathrm{A}\ {\mathrm{cm}}^{-3} \) and a large energy density of \( 6.16\times 1{0}^{-3}\;\mathrm{Wh}\ {\mathrm{cm}}^{-3} \) at \( 0.04\ \mathrm{W}\ {\mathrm{m}}^{-3} \). Furthermore Li et al. [58] developed an electrospinning approach to fulfill scale and thickness requirements of nanostructured MnO2 electrodes. The MnO2 electrode for supercapacitors could also be fabricated by a sol–gel method. For instance, Pang et al. [59] fabricated an MnO2 film via dip coating or drop coating with manganese dioxide suspensions (sols) onto nickel foils, showing high specific capacitance of \( 698\ \mathrm{F}\ {\mathrm{g}}^{-1} \).

Fig. 9.5
figure 5

SEM images of MnO2 prepared at different hydrothermal dwell times (Reprinted from Ref. [56] with permission. Copyright 2005, American Chemical Society)

Full size image

Regarding MnO2 as SC materials, several challenges should be addressed, including partial dissolution problem of MnO2 in the acidic electrolyte and its poor electronic conductivity and ionic conductivity . Impressively, Kang et al. [60] improved the electronic conductivity of MnO2 by nonequilibrium Au doping changing the electronic structure of MnO2, demonstrating a high specific capacitance (\( 626\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 5\mathrm{mV}\ {\mathrm{s}}^{-1} \)) and excellent cycling stability (7 % increment after 15,000 cycles). Furthermore, Zhang et al. [61] investigated the electrochemical properties of MnO2 in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim] PF6)/N,N-dimethyformamide (DMF) electrolyte, fabricating a high-voltage (3 V) asymmetric supercapacitor with a maximum specific energy of \( 67.5\ \mathrm{Wh}\ {\mathrm{kg}}^{-1} \) and a maximum specific power of \( 20.4\ \mathrm{kW}\ {\mathrm{kg}}^{-1} \). The impressive results revealed that [Bmim] PF6/DMF can be a promising electrolyte for MnO2 -based supercapacitors .

Extensive work is ongoing to prepare a better modality of MnO2 achieving higher specific capacitances, and the whole electrochemical performances of asymmetric supercapacitors have also been improved by enhancing the electrochemical properties of the negative electrode and the voltage of the whole cell.

9.3.2.1.3 Co3O4

Co3O4 is an important transition metal oxide for supercapacitors with excellent reversible redox behavior, high conductivity, and good stability. Different methods have been performed to synthesize Co3O4 nanostructures with morphologies from one dimension to three dimensions. In general, these methodologies consist of two steps: formation of precursor and calcination . Great efforts have been made to improve the electrochemical performance of Co3O4 by constructing different morphologies and combining with other novel materials.

For example, Co3O4 nanocrystals with well-controlled shapes, including nanosheets and microspheres assembled from nanosheets, have been successfully prepared by a facile ethanolamine-directed solvothermal method and sequential thermal decomposition at atmospheric pressure [62]. Moreover, Xia et al. [63] developed a facile hydrothermal synthesis method to prepare self-supported hollow Co3O4 nanowire arrays on various conductive substrates, revealing high specific capacitances with \( 599\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 2\ \mathrm{A}\ {\mathrm{g}}^{-1} \) and \( 439\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 40\ \mathrm{A}\ {\mathrm{g}}^{-1} \), thus making it suitable for high-rate supercapacitor applications. The enhanced capacitive performance is due to its unique porous structure and hollow structure which could provide fast ion and electron transfer, good strain accommodation, and large reaction surface area. Chen et al. [64] reported a simple hydrothermal method through in situ growing Co3O4 nanowires on three-dimensional graphene foam grown by chemical vapor deposition, demonstrating high specific capacitance of \( \sim 1100\ \mathrm{F}\ {\mathrm{g}}^{\hbox{--} 1} \) at a current density of 10 A g– 1 with remarkable cycling stability. These recent studies indicate that appropriate morphology , microstructure , and chemical composition are important in achieving high-performance SC based on Co3O4.

9.3.2.2 Bimetallic Oxides

9.3.2.2.1 NiCo2O4

Nickel cobaltite (NiCo2O4) has become a new class of energy storage material for supercapacitors with outstanding electrochemical performance, delivering not only large power density but also high energy densities [65]. This material possesses richer electroactive sites and at least two magnitudes higher electrical conductivity than that of Co3O4 and NiO. Moreover, it shows comparable capacitive performances with noble metal oxides of RuO2, but with much lower cost and more abundant resources. It is believed that the redox reactions offered by nickel cobaltite, including contributions from both nickel and cobalt ions, are richer than those of the monometallic nickel oxides and cobalt oxides. More importantly, multiple oxidation states and various nanostructures can also enable rich redox reactions for spinel nickel cobaltite to store more charges [32, 33, 66]. Last but not least, there are abundant nickel and cobalt elements on the earth, and it seems to be a promising opportunity for nickel cobaltite to be widely applied and quickly industrialized as electrode material for supercapacitors due to its environmental friendliness and low cost [67]. To some extent, NiCo2O4 is playing an important role in complementing or replacing electrode materials based on Co3O4, NiO, and RuO2 in energy storage field. A Ragone plot, as shown in Fig. 9.6, is derived from NiCo2O4-based asymmetrical supercapacitors’ work which illustrates the performances in terms of energy density versus power density indicating this material is beneficial.

Fig. 9.6
figure 6

Ragone plot of the current electrochemical performance for NiCo2O4-based asymmetric supercapacitors (Reprinted from Ref. [24] with permission. Copyright 2014, The Royal Society of Chemistry)

Full size image

The methods for preparing NiCo2O4 for supercapacitors can primarily be divided into three categories: (i) sol–gel method, (ii) hydrothermal synthesis , and (iii) electrodeposition process. In brief, the synthetic route of NiCo2O4 nanomaterial can be thought as the formation of precursor followed by an annealing process. These three methods are summarized in Fig. 9.7 and are discussed in detail below; other pertinent methods are also briefly mentioned.

Fig. 9.7
figure 7

Typical synthetic approaches of NiCo2O4 for supercapacitor application (Reprinted from Ref. [24] with permission. Copyright 2014, The Royal Society of Chemistry)

Full size image

In general, the sol–gel method can be briefly defined as the conversion of a precursor solution into an inorganic solid by a chemical means [68]. Herein, the precursor solution consists of either an inorganic metal salt or a metal organic species, while nickel and cobalt acetates, halides, and nitrates are typically employed here for preparing NiCo2O4 [69, 70]. It is well known that the sol–gel method is simple and cheap with products of high purity, homogeneity, and porosity, mainly including four steps: (a) preparation of the precursor solutions, (b) formation of the intermediates as sol, (c) transformation from sol to gel, and (d) calcination at high temperature. Note that solvents, surfactants, reaction time, and temperature are crucial regulatory factors for obtaining appropriate structures with outstanding electrochemical performances [68].

Hu and coworkers [71] first reported the preparation of NiCo2O4 aerogels via an epoxide-driven sol–gel process in 2010, which showed an ultrahigh specific capacitance of \( 1400\;\mathrm{F}\ {\mathrm{g}}^{-1} \) with a mass loading of \( 0.4\ \mathrm{mg}\ {\mathrm{cm}}^{-2} \). The majority of the pores were in the optimal sizes of 2–5 nm for supercapacitor applications, and the as-obtained aerogels were of excellent reversibility and cycle stability. Since this pioneering work, several researches have been devoted to this sol–gel approach. The typical preparation process is illustrated in Fig. 9.7a, and the mechanism of gelation can be explained by the following reaction where M represents nickel or cobalt ions and where \( {\mathrm{A}}^{-} \) denotes \( {\mathrm{Cl}}^{-} \) or \( {{\mathrm{NO}}_3}^{-} \) ions [72]:

figure a

Additionally, Kong et al. [73] developed a sol–gel process by adding different surfactants, such as hexadecyltrimethylammonium bromide (CTAB) and polyvinyl alcohol (PVA), to control the microstructure and surface morphology of NiCo2O4 nanoparticles. The product modified by CTAB displays the highest specific capacitance of \( 1440\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at a current density of \( 5\ \mathrm{m}\mathrm{A}\ {\mathrm{cm}}^{-2} \), while Wu et al. [74] pointed out that the initial molar concentration of reactants, reaction time, and solvent species involved are crucial for preparing the target products. Through the use of citric acid as the chelating ligand and H2O–DMF as solvent via a facile sol–gel process, the submicron-sized NiCo2O4 particles exhibited a specific capacitance of \( 217\ \mathrm{F}\ {\mathrm{g}}^{-1} \) with high mass loading of \( 5.6\ \mathrm{mg}\ {\mathrm{cm}}^{-2} \). Recently, Ji et al. [75] discussed the effect of three different chelating agents including citric acid, oxalic acid, and ethylenediaminetetraacetic acid on fabricating spinel NiCo2O4 via a facile sol–gel method, revealing that NiCo2O4 prepared by oxalic acid has the highest specific capacitance of \( 1254\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 2\ \mathrm{A}\ {\mathrm{g}}^{-1} \) due to the relatively higher specific surface area, while NiCo2O4 prepared by ethylenediaminetetraacetic acid exhibits the best rate capability and cycling stability owing to the relatively larger pore size. Extensive work is being carried out via sol–gel processes to prepare a better modality of NiCo2O4 so as to achieve a higher specific capacitance and a longer cycle life . It has been proven to be an effective way indeed to synthesize NiCo2O4 electrode materials with low cost and high electrochemical performances.

Hydrothermal /solvothermal synthesis refers to the heterogeneous chemical reactions in a sealed heated solution to dissolve and recrystallize materials above ambient temperature and pressure conditions [76]. Hydrothermal synthesis and solvothermal synthesis are similar synthetic strategies that are distinct from whether the solution is aqueous or not. For convenience and habit, we call this hydrothermal synthesis. In general, the precursors of NiCo2O4 are formed by a combination of reaction substances being heated in a sealed Teflon-lined stainless steel autoclave. As the most common method of synthesizing nickel cobaltite, hydrothermal method is advantageous to form highly monodispersed nanoparticles with a control over size and morphology .

A large number of groups have made attempts to optimize the reaction conditions and form special morphologies so as to enhance the electrochemical performances of NiCo2O4 electrodes [77–84]. For example, Chen et al. [77] reported that a 3D hierarchical NiCo2O4 nanosheet –nanowire cluster arrays were manufactured via a facile hydrothermal method, exhibiting an ultrahigh specific capacitance of \( 2000\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 10\ \mathrm{A}\ {\mathrm{g}}^{-1} \) with 93.8 % retention (more than 10,000 cycles) and a high power density of \( 26.1\ \mathrm{kW}\ {\mathrm{kg}}^{-1} \) at a current density of \( 80\ \mathrm{A}\ {\mathrm{g}}^{-1} \). As illustrated in Fig. 9.7b, the process is based on Ni–Co nitrates induced by NH4F and urea being heated in a sealed Teflon-lined stainless steel autoclave at 100 ℃ for 5 h. Zou et al. [79] also reported a facile hydrothermal method that successfully grew 3D NiCo2O4 microspheres constructed by radial chain-like NiCo2O4 nanowires with different exposed crystal planes, demonstrating high specific capacitance \( \left(1284\ \mathrm{F}\ {\mathrm{g}}^{-1}\right. \) at \( \left.2\ \mathrm{A}\ {\mathrm{g}}^{-1}\right) \), good rate capability , and excellent cycling stability (only 2.5 % loss after 3000 cycles). It was proposed that the chain-like nanowires with different exposed crystal planes exhibited higher electronic conductivity than other mesoporous NiCo2O4 nanostructures . In comparison, solvothermal method just occurs in nonaqueous solvents such as ethanol and ethylene glycol. Very recently, the 3D flowerlike NiCo2O4 hierarchical architectures have been fabricated by An et al. [78] via a solvothermal method using polyvinylpyrrolidone (PVP) as the structure-directing agent in a polytetrafluoroethylene (PTFE) reaction environment followed by a simple thermal annealing treatment, displaying a large surface specific area of \( 212.6\ {\mathrm{m}}^2{\mathrm{g}}^{-1} \) and a high specific capacitance of \( 1191.2\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at current densities of \( 1\ \mathrm{A}\ {\mathrm{g}}^{-1} \); however, it suffered from a relatively poor cycling performance due to the electric isolation caused by collapse of the nanosheets . Meanwhile, Ji et al. [84] successfully fabricated 3D network-like mesoporous NiCo2O4 nanostructures through a solvothermal route coupled with a post-annealing treatment. Benefiting from the large specific surface area (\( 170.6\ {\mathrm{m}}^2{\mathrm{g}}^{-1} \)) and abundant mesoporosity (5–10 nm), the as-obtained NiCo2O4 manifests high specific capacitance of \( 931\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 3\ \mathrm{A}\ {\mathrm{g}}^{-1} \).

The electrodeposition process occurs in the electrolysis of plating solutions where micron- or submicron-sized particles are suspended, and variable amounts of these particles are embedded in the electrochemically produced solid phase with special properties [85]. As illustrated in Fig. 9.7c, the process can be described in three steps: (1) formation of precursor solution, (2) co-electrodeposition of nanoparticles, and (3) thermal decomposition. Due to cooperative formation mechanism of the composite electrode , as a general rule, ultrahigh specific capacitances and good cycling performances will be achieved through this novel method.

For instance, Du et al. [86] electrodeposited NiCo2O4 nanosheet arrays onto flexible carbon fabric successfully, which displayed high specific capacitance of \( 2658\ \mathrm{F}\ {\mathrm{g}}^{-1} \) (\( 2\ \mathrm{A}\ {\mathrm{g}}^{-1} \)), good rate property, and superior cycling life. Note that carbon fabric (CF) is an ideal conductive substrate for directly growing active materials owing to its superior electrical conductivity, good flexibility, and high mechanical strength, through which we can fabricate some distinctive flexible supercapacitors or lithium -ion batteries . Moreover, Lou and his coworkers [87] reported the synthesis involving the co-electrodeposition of a bimetallic (Ni, Co) hydroxide precursor on an Ni foam and subsequently thermal transformation to spinel mesoporous NiCo2O4, and the electrode exhibited an ultrahigh specific capacitance of \( 1450\ \mathrm{F}\ {\mathrm{g}}^{-1} \) even at a very high current density of \( 20\ \mathrm{A}\ {\mathrm{g}}^{-1} \). The sizes of interparticle mesopores ranged from 2 to 5 nm, which were important indeed to facilitate the mass transport of electrolytes within the electrodes for fast redox reactions and double-layer charging–discharging. Composite electrodes such as Ni(OH)2 @ NiCo2O4 [88] and Co3O4 @ NiCo2O4 [89] via the electrodeposition method were also reported in the literature. And the formation process of the nanoforest of hierarchical Co3O4 @ NiCo2O4 [89] nanowire arrays is elaborated in Fig. 9.8 as a typical example.

Fig. 9.8
figure 8

Illustration of the formation process of Co3O4@NiCo2O4 NWFA hierarchical heterostructures. (a) Ni foam substrate; (b) hydrothermal synthesis of aligned Co3O4 nanowire arrays; (c) formation of hierarchical Co3O4@NiCo2O4 nanoflake arrays via a co-electrodeposition process (Reprinted from Ref. [89] with permission. Copyright 2013, Elsevier Ltd)

Full size image

The electrodeposition process is widely used to fabricate composite electrodes with different nanomaterials . It is very important to choose the befitting materials that should possess good electrical conductivity or unique nanostructure to assist the target material in pursuing for better electrochemical performances. However, the ambiguous relationship between the composite structures and electrochemical performances has not been clarified yet.

As elaborated above, all of the three major methods are distinctive. The sol–gel method tends to acquire nanoparticles of high purity, homogeneity, and porosity. And the hydrothermal synthesis is controllable of size and nanostructure by adjusting temperature parameters or types and concentration of auxiliaries. By contrast, the electrodeposition process has some distinct advantages in depositing NiCo2O4 onto various conductive substrates such as nickel foams and carbon textiles and combining NiCo2O4 with other excellent materials that show intriguing synergistic effect, which can be manipulated through adjusting solution concentration, solution pH value, and applied overpotential or current density in the electrodeposition procedure. In addition, other methods, such as coprecipitation method, template approach, and microwave technique, are also simply mentioned below, which are not widely used or just assistant measures for fabricating NiCo2O4 material, and the following examples are only constructed for the purpose of comprehensively illustrating the diversity and novelty of processes as reported.

Recently, coprecipitation method is getting popular owing to its simple and facile process [90–92]. For example, Ji et al. [92] reported a facile coprecipitation way using NaHCO3 as precipitant, and the as-obtained uniform porous NiCo2O4 nanoparticles showed high specific capacitance of \( 726.8\ \mathrm{F}\ {\mathrm{g}}^{-1} \) (\( 1\ \mathrm{A}\ {\mathrm{g}}^{-1} \)) and good cycle stability of 72.7 % retention at a current density of \( 5\ \mathrm{A}\ {\mathrm{g}}^{-1} \) after 2000 cycles. In addition, it has been reported that microwave technique has been used to help prepare NiCo2O4 electrodes for supercapacitors . What is more, the microwave-assisted process has been demonstrated to be advantageous to extremely fast kinetics of crystallization, very rapid heating to the required temperature, and possible formation of new metastable phases, which facilitate to tune the crystal size and phase transformation degree of binary Ni–Co precursors [93, 94]. As a flexible and efficient approach, the template approach, using silica spheres [95] and sodium dodecyl sulfate [96] as templates, has also been used to synthesize hierarchical mesoporous hollow NiCo2O4 sub-microspheres and hexagonal mesoporous NiCo2O4 for supercapacitor electrodes, respectively. Both of the electrodes show remarkable electrochemical performances. Besides, sonochemical precipitation technique [97], chemical bath deposition [98], reflux synthesis [99], room-temperature solid-state reaction [100], and single-spinneret electrospinning technique [101] have also been utilized to synthesize NiCo2O4 nanostructures for supercapacitors. These approaches deserve a mention and readers are directed to these for further details.

9.3.2.2.2 MnCo2O4

As a potential candidate for supercapacitors , spinel MnCo2O4 has been restricted by the poor ionic conductivity and partial dissolution in alkaline electrolytes [102]. Despite some disappointing physicochemical properties of MnCo2O4, it is believed that MnCo2O4 -based supercapacitors can be modified by combining with certain carbonaceous materials which possess both light weight and good electronic conductivity [103]. It is fascinating that the faradaic redox reactions of MnCo2O4 occurring in the charge–discharge process are speculated to be both \( {\mathrm{Co}}^{2+}/{\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+} \) and \( {\mathrm{Mn}}^{2+}/{\mathrm{Mn}}^{3+}/{\mathrm{Mn}}^{4+} \), which facilitate to achieve much higher energy density with multiple valence changes [104].

As reported by Kong et al. [103], the spinel MnCo2O4 fabricated by a facile sol–gel method displayed high specific capacitance of \( 405\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 5\ \mathrm{m}\mathrm{A}\ {\mathrm{cm}}^{-2} \), remarkable rate capability of 67.9 % capacity retention at \( 40\ \mathrm{m}\mathrm{A}\ {\mathrm{cm}}^{-2} \), and good cycle stability of 4.9 % loss after 1000 cycles, owing to the porous structure and fairly stable spinel structure. In addition, Li et al. [105] developed a new strategy to prepare a flowerlike C @ MnCo2O4 hybrid composite using a facile hydrothermal method combined with a post-calcination process. Due to the unique hollow structure, high specific surface area, and suitable pore channels, the C @ MnCo2O4 composite exhibits high specific capacitance, low charge transfer resistance, and superior capacitance retention. A high specific capacitance of \( 728.4\ \mathrm{F}\ {\mathrm{g}}^{-1} \) has been achieved at a current density of \( 1\ \mathrm{A}\ {\mathrm{g}}^{-1} \), and 95.9 % of the initial capacitance is still retained after 1000 cycles. Recently, Xu et al. [106] successfully prepared porous MnCo2O4 nanowires by hydrothermal method followed by a calcinating process, showing excellent capacitance and desirable rate performance (\( 1342\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 1\ \mathrm{A}\ {\mathrm{g}}^{-1} \) and \( 988\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 20\ \mathrm{A}\ {\mathrm{g}}^{-1} \)).

9.3.2.2.3 NiFe2O4

Spinel NiFe2O4 can be a promising candidate for use as supercapacitor materials due to its low cost and environmental friendliness. However, it exhibits low electrical conductivity, small energy density, and poor cycling performance.

For example, Wang et al. [107] reported a facile synthesis of RGO– NiFe2O4 composites by a hydrothermal process. The synthesized electrodes were measured in a 1 M Na2SO4 aqueous solution, showing the best capacitive properties of \( 459.6\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 1\ \mathrm{A}\ {\mathrm{g}}^{-1} \) due to the suitable particle size and good dispersion property of the active material. Senthilkumar et al. [108] reported the PANI–NiFe2O4 composites prepared by an in situ chemical oxidative polymerization method, which provided high specific capacitance of \( 448\ \mathrm{F}\ {\mathrm{g}}^{-1} \) and good capacitance retention of 80 % after 1000 cycles. In addition, Sen et al. [109] developed a sol–gel method to prepare nanocrystalline nickel ferrites (5–20 nm), exhibiting good electrochemical performance.

9.3.2.2.4 NiMoO4

NiMoO4 has drawn significant research attention because of its excellent electrical conductivity and outstanding electrochemical performance resulting from the high electrochemical activity of the Ni element and the good electrical conductivity of the Mo element [110]. The crystalline structure of NiMoO4 is depicted in Fig. 9.9, and it displays that NiMoO4 occupies the monoclinic structure with molybdenum atoms and nickel atoms adopting the tetrahedral and octahedral sites, respectively. Note that the edge-sharing NiO6 octahedra interconnected by MoO4 tetrahedra facilitate to constitute a 3D network structure with open channels, which are beneficial for ion diffusion [111]. Different morphologies of NiMoO4, such as nanosheets [112, 113], nanorods [114], nanowires [115–117], and nanotubes [118], have been developed for pseudocapacitive application, and typical approaches [110, 115, 118] for synthesizing NiMoO4 electrode materials are described in Fig. 9.10.

Fig. 9.9
figure 9

Crystal structure of NiMoO4 (Reprinted from Ref. [111] with permission. Copyright 2014, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Full size image
Fig. 9.10
figure 10

Schematic illustration of the preparation process of NiMoO4. (a) Hydrothermal method; (b) electrodeposition process; (c) hydrothermal method (Reprinted from Ref. [110] with permission. Copyright 2015, The Royal Society of Chemistry. Reprinted from Ref. [115] with permission. Copyright 2013, The Royal Society of Chemistry. Reprinted from Ref. [118] with permission. Copyright 2015, The Royal Society of Chemistry)

Full size image

For example, Wang and his coworkers [110] have reported that amorphous honeycomb-like NiMoO4 nanosheets can be prepared by electrodeposition process followed by thermal treatment. The as-prepared NiMoO4 nanosheets with interconnecting pore channels and an ultrathin structure showed a high specific capacitance (\( 1694\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 1\ \mathrm{A}\ {\mathrm{g}}^{-1} \) and \( 1220\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 50\ \mathrm{A}\ {\mathrm{g}}^{-1} \)) and excellent cycle performance (92.7 % after 9000 cycles). Peng et al. [111] developed a facile hydrothermal method to grow hierarchical NiMoO4 architectures assembled from well-aligned nanosheets or nanorods on various conductive substrates for supercapacitor applications. It was found that NiMoO4 nanosheets manifest higher specific capacitance and better cycling stability than NiMoO4 nanorods and the assembled asymmetric supercapacitors based on NiMoO4 nanosheets and activated carbon delivered a high energy density of \( 60.9\ \mathrm{Wh}\ {\mathrm{kg}}^{-1} \) at a power density of \( 850\ \mathrm{W}\ {\mathrm{kg}}^{-1} \). Wang and his coworkers [114] synthesized NiMoO4 nanospheres by a facile hydrothermal method, demonstrating high specific capacitance of \( 974.4\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 1\ \mathrm{A}\ {\mathrm{g}}^{-1} \).

By virtue of the synergistic effect, the composite electrode based on NiMoO4 has been extensively investigated. Composite electrodes for supercapacitors , including Co3O4 @ NiMoO4 [119–121], MnO2 @ NiMoO4 [122], NiCo2O4 @ NiMoO4 [123], Co(OH)2 @ NiMoO4 [124], CoMoO4 @ NiMoO4 [125, 126], and graphene @ NiMoO4 [117], have been reported recently. For example, Liu et al. [125] designed CoMoO4–NiMoO4 bundle composite material for supercapacitors by a chemical coprecipitation method, and the composite electrode showed a maximum specific capacitance of \( 1039\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 2.5\ \mathrm{m}\mathrm{A}\ {\mathrm{cm}}^{-2} \) and an excellent rate capability ascribing to its unique microstructure , large specific surface area, multiple oxidation states, and synergistic effects between NiMoO4 and CoMoO4. In addition, Zhang et al. [115] developed a 3D sea urchin-like NiCo2O4 @ NiMoO4 core–shell structure constructed by interconnected ultrathin nanosheets via a facile two-step hydrothermal method using a post-annealing procedure. The NiCo2O4 @ NiMoO4 material revealed a high specific capacitance of \( 2474\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 1\ \mathrm{A}\ {\mathrm{g}}^{-1} \) and \( 2080\ \mathrm{F}\ {\mathrm{g}}^{-1} \) at \( 20\ \mathrm{A}\ {\mathrm{g}}^{-1} \) and maintained 95.0 % of the initial capacitance after 1000 cycles at \( 10\ \mathrm{A}\ {\mathrm{g}}^{-1} \), which can be attributed to its unique 3D core–shell and hierarchical mesoporous structures.

Considering the promising electrochemical performance of NiMoO4 and easy fabrication process, it can be one of the most potential electrode materials applied for supercapacitor in the future energy storage /conversion fields.

9.4 Summary and Outlook

Significant progress has clearly been made in electrochemical supercapacitors which has sparked worldwide attention due to the rapid development of supercapacitive electrode materials, which is of great significance to fast industrialize this important energy storage /conversion device. Since the first report of supercapacitors in 1957, electrode materials have developed from carbon materials to pseudocapacitive materials, while transition metal oxides for supercapacitors have developed from noble transition metal oxides to base transition metal oxides, from monometallic oxides to single-phase bimetallic oxides, and from mono-component materials to multicomponent composites. Note that bimetallic transition metal oxides usually have great advantage over monometallic transition oxides, including higher electrical conductivity, multiple valence changes, and potential synthetic effects. In addition, multicomponent composites such as transition metal oxides at carbon materials utilizing as supercapacitive electrode material have the advantages in obtaining more remarkable rate capability and much better cycle performance. It tends to combine the different virtues together, thus enhancing the electrochemical performances of supercapacitors.

Inspiringly, transition metal oxides have already been demonstrated to display striking performance as electrode materials for electrochemical supercapacitors . However, the majority of the reports regarding the applications of transition metal oxides for supercapacitors are still retained in the laboratory, with few solutions yet for a mass market. Most researches only focus on preparing various nanostructures for transition metal oxides, while little studies are devoted to improving the whole production process of the whole device of supercapacitor. Optimizing the manufacturing process and reducing the production cost are also vital to realize large-scale production for the industrial applications, and designing products based on market demand can more effectively and healthily promote the development of supercapacitor. Meanwhile, a comprehensive understanding of the relationship between the property/performance and the structure/composition of these transition metal oxides for supercapacitors should be systematically established. Developing reliable and efficient methods and criteria to solve this problem is urgently needed. The establishment of theoretical simulation and mathematical modeling is also highly anticipated to investigate the detailed energy storage mechanism and the rational design of electrode materials, directing the purposeful design of facile, low-cost, and large-scale preparation of transition metal oxides with excellent electrochemical performance.

At present, electrochemical supercapacitors based on transition metal oxides should provide improved energy density on the premise of keeping high power density. However, there is still considerable distance to meet the demand of commercial application. Narrow stable potential window of aqueous electrolytes greatly restricts the enhancement of energy density, and limited mechanism study cannot effectively guide the direction of investigation. To maximize the operating voltage output, some nonaqueous electrolytes such as ionic liquids and organic electrolytes that provide a two or three times wider working voltage window than the aqueous ones have been studied as desirable candidates. It is of high importance to explore nonaqueous electrolytes to obtain higher energy/power density, which will greatly pave the way for the practical production of electrochemical supercapacitors. In order to fully exploit the potential of transition metal oxide-based electrode materials for supercapacitors, it is imperative to optimize both synthesis parameters and material properties. The engineering factors of electrodes, including current collectors, electrolytes, membrane separators, counter electrodes, and packaging, still need a deeper investigation from fundamental investigation in the future.

Despite the challenges ahead, transition metal oxides utilizing as supercapacitive electrode material will be promising candidates for energy storage /conversation in the future. On one hand, systematical work should be performed on the current remarkable transition metal oxides to improve the electrochemical performance to the utmost. On the other hand, novel ideas such as discovering new metal oxide that is of excellent electrochemical performances are also urgently needed. In-depth understanding of the mechanisms of transition metal oxides for supercapacitors will greatly promote the whole industrial development of the supercapacitors. All in all, with sustained and dedicated research efforts, these fascinating transition metal oxides nanomaterials will offer a new avenue to make the attractive electrochemical supercapacitors commercially viable.