Mesoporous Transition Metal Oxides for Supercapacitors

Recently, transition metal oxides, such as ruthenium oxide (RuO2), manganese dioxide (MnO2), nickel oxides (NiO) and cobalt oxide (Co3O4), have been widely investigated as electrode materials for pseudo-capacitors. In particular, these metal oxides with mesoporous structures have become very hot nanomaterials in the field of supercapacitors owing to their large specific surface areas and suitable pore size distributions. The high specific capacities of these mesoporous metal oxides are resulted from the effective contacts between electrode materials and electrolytes as well as fast transportation of ions and electrons in the bulk of electrode and at the interface of electrode and electrolyte. During the past decade, many achievements on mesoporous transition metal oxides have been made. In this mini-review, we select several typical nanomaterials, such as RuO2, MnO2, NiO, Co3O4 and nickel cobaltite (NiCo2O4), and briefly summarize the recent research progress of these mesoporous transition metal oxides-based electrodes in the field of supercapacitors.


Introduction
Energy crisis and environmental pollution have triggered the development of clean and renewable energy storage systems. Supercapacitors, also called electrochemical capacitors, are a novel type of charge energy storage devices in between traditional capacitors and batteries [1,2]. Compared with traditional capacitors, supercapacitors possess higher specific capacity and specific energy. Meanwhile, they exhibit higher specific power, shorter charging time, more efficient discharging than batteries, and they cause no pollution in the environment [3]. Although their energy density is 10 to 50 times lower than lithium ion batteries now, supercapacitors with a near unlimited cycle life are useful in the fields of power system, memory storage, and vehicle assistant equipment ( Figure 1). For example, combining a supercapacitor and a battery in a single unit can create an electric vehicle with longer life, lower costs and more power. Based on the principle of energy storage, supercapacitors can be classified into electric double layer capacitors (EDLCs) and pseudo-capacitors [4]. EDLCs store charges using the very thin double layer structure formed at the interface between electrode and electrolyte, while pseudo-capacitors use fast and reversible redox reactions on the surface and bulk near the surface of electrodes for energy storage. Compared with EDLCs, pseudo-capacitors exhibit higher capacitance and higher energy density [4][5][6]. Nanoporous carbons are considered as one kind of ideal electrode materials for the EDLCs due to their large surface area, controlled pore structure, and high conductivity [7][8][9]. While for pseudo-capacitors, transition metal oxides as well as conducting polymers are common electrode materials [3]. Among them, transition metal oxides, such as ruthenium oxide (RuO2), manganese dioxide (MnO2), nickel oxides (NiO) and cobalt oxide (Co3O4), are investigated widely as electrodes materials [10][11][12]; both faradaic and non-faradaic mechanisms are involved in charge storage [13][14][15][16]. RuO2 involves faradaic charge-transfer reactions, and its cyclic voltammogram (CV) shape is quite broad, exhibiting a quasi-rectangular shape [17]. The good conductivity, rapid proton transport and larger surface area all contribute to rapid and reversible faradaic reactions with high capacitance [18]. Unfortunately, the high cost of RuO2 limits its large-scale applications. Therefore, significant efforts are being focused on finding a promising alternative to RuO2. MnO2, NiO, and Co3O4 are investigated as electrode materials to replace RuO2. MnO2 possesses low cost and rectangular voltammogram due to fast and reversible faradaic reactions [19,20], which is taken as one of the promising alternatives to RuO2. NiO, and Co3O4 are battery materials in the bulk state. When storage sites are limited on the surface of materials through nanostructure, significant pseudo-capacitance emerges [17]. Moreover, nickel cobaltite (NiCo2O4) also exhibits high capacitance values at short charge-discharge times recently, and redox behaviors of both nickel and cobalt are involved in energy storage [21,22]. Herein, RuO2, MnO2, NiO, Co3O4 and NiCo2O4 are specially selected, and their roles in supercapacitors will be reviewed.
A supercapacitor consists of electrode, electrolyte and separator, where electrode plays a key role for the performance of supercapacitor. It is a crucial task to explore electrode materials with excellent performance. According to the work principle of pseudo-capacitors, electrode materials and electrolyte should contact effectively, meanwhile, ions/charges should transport fast in the bulk of electrode and at the interface between electrode and electrolyte in order to achieve excellent properties. Therefore, transition metal oxides with mesopores (between 2 and 50 nm) are favorable for building supercapacitors with high performance due to their large specific surface area and suitable pore size distribution [23]. During the past decade, mesoporous RuO2, MnO2, NiO, Co3O4, and NiCo2O4 have been widely studied for supercapacitors. The simple and effective synthetic routes were developed, the nanomaterials with different morphologies were prepared, and the specific capacity and stability of devices were significantly improved. In this review, we briefly summarize the recent progress on these mesoporous transition metal oxide-based electrodes in the field of supercapacitors.

Ruthenium Oxide (RuO2) for Pseudo-Capacitors
RuO2 is known as the best electrode material due to its large specific capacitance (700 F·g −1 ), low resistivity, high chemical and thermal stability [18,24,25]. When RuO2 is used as an electrode material, a series of redox processes occur, resulting in the variation of oxidation state among Ru 4+ , Ru 3+ and Ru 2+ , where pseudo-capacitance mainly contributes to capacitance. Another feature is that these redox processes of RuO2 are reversible. These unique electrochemical features result in quasi-rectangular shape CV curve ( Figure 2). Due to these remarkable advantages, RuO2, especially mesoporous RuO2, has attracted much attention from scientific community and industry. Until now, several different routes have been developed for the synthesis of mesoporous RuO2. For example, Galizzioli and Rochefort synthesized RuO2 by thermal decomposition of RuCl3 on metallic supports in solutions of electrolytes [26,27]. Zheng et al. prepared hydrous RuO2 by a sol-gel process and obtained specific capacitance as high as 720 F·g −1 for a powder formed at 150 °C [18]. This is the highest specific capacitance for RuO2. In addition, Liu et al. prepared RuO2 films by thermal and electrochemical methods [28]. Dubala and coworkers reported a surfactant-less and binder-free chemical bath deposition method to synthesize RuO2 thin films [29]. Although RuO2 has large specific capacitance, it is difficult to use RuO2 in real application, considering its very high cost and environmental toxicity. These limitations can be partly overcome by hybridization of RuO2 with other common conductive materials. For instance, NiO/RuO2 composite materials were prepared and a maximum specific capacitance of 210 F·g −1 was obtained for composite electrode with 10 wt % RuO2 in the voltage range of −0.4 to 0.5 V in 1 M KOH solution [30]. SnO2/RuO2 composite films were prepared by chemical bath deposition method. The specific capacitance of 150 F·g −1 was obtained by optimizing synthetic conditions [31]. RuO2/polyaniline electrodes were prepared by electrodeposition, revealing a specific capacitance of 474 F·g −1 and a small charge transfer resistance of 2.24 Ω [32]. RuO2/TiO2 nanotubes composites were synthesized by loading various amounts of RuO2 on TiO2 nanotubes and a maximum specific capacitance of 1263 F·g −1 was obtained [33]. By using carbon fibre paper as a support for RuO2, a specific capacitance of 977 F·g −1 was also obtained in a supercapacitor with 1M H2SO4 as electrolyte [34]. The special capacitances of these hybrid materials are summarized in Table 1.

Manganese Dioxide (MnO2) for Pseudo-Capacitors
MnO2 has been taken as a promising alternative to RuO2 because of its low cost and superior electrochemical performance [19,20,35,36]. Generally, capacitance properties of MnO2 are investigated in Na2SO4 electrolyte. In Na2SO4 electrolyte, reversible transitions of MnO2↔MnOONa occur, which is responsible for pseudo-capacitance properties [19,20]. The rectangular voltammogram implies fast and reversible capacitive behaviors, which, together with excellent cycle stability of MnO2, were demonstrated from the linearship of galvanostatic charge/discharge (GCD) profiles ( Figure 3). According to theoretical calculation, specific capacitance of MnO2 is up to 1100 F·g −1 over a potential window of 1.0 V. However, the specific capacitances from experiments were five or ten times lower than theoretical values [37][38][39][40][41]. Both crystallographic forms (such as α, β, γ, δ, λ, etc.) and morphological nature are contributed to the low specific capacitance [37]. In order to obtain MnO2 with high specific capacitance, much effort has been devoted to the synthesis of mesoporous MnO2. Template method, microemulsion method, hydrothermal method, sonochemical method and ultrasound irradiation have been utilized to synthesize mesoporous MnO2 [38][39][40][41]. For example, semicrystalline gyroidal mesoporous MnO2 was prepared by using mesoporous silica KIT-6 as hard template, and stable reversible electrochemical behavior with capacitance of 220 F·g −1 in a potential range of −0.1-0.55 V was observed [38]. Mesoporous MnO2 nanoparticles with 4-5 nm of pore size were synthesized by a soft template method, and specific capacitance of 297 F·g −1 was obtained at a high loading level of 1.55 mg·cm −2 [39]. Mesoporous MnO2 with 2-20 nm of average pore sizes were obtained in sonochemical method from KMnO4 by using a tri-block copolymer (P123) as a soft template, and a maximum specific capacitance of 265 F·g −1 was obtained (Figure 4a,b) [40]. Mesoporous MnO2 with specific surface area of 192 m 2 ·g −1 and 10 nm of pore distribution was synthesized through the reaction of potassium permanganate and ethanol under ultrasound irradiation (Figure 4c,d). The specific capacitance of 229 F·g −1 and the specific capacitance retention of 97.3% after 2000 cycles were obtained [19]. Birnessite-type mesoporous MnO2 nanospheres were synthesized by the microwave-hydrothermal method, and the electrochemical test showed that the specific capacitance was 210 F·g −1 at 200 mA·g −1 in 1.0 M Na2SO4 electrolyte with the specific capacitance retention and columbic efficiency of over 96% and 98%, respectively, after 300 cycles at 1.6 A·g −1 [41].

Nickel Oxide (NiO) for Pseudo-Capacitors
NiO is regarded as a promising pseudo-capacitor material due to its high theoretical capacitance value of 2573 F·g −1 , low cost and distinct redox [42]. For NiO, the variation of oxidation state of Ni is still not very clear. The reaction, NiO + OH ↔ NiOOH + e − , is usually believed to occur first. Then, the oxyhydroxide participates in the following reversible redox processes [17]. There are still some issues to be solved for the practical applications of NiO in supercapacitors. Among them, the significantly lower specific capacitance than theoretical value is one of the main issues. Due to the tight correlation between electrochemical performance of NiO and its porosity and surface area, most of the recent works are focused on the synthetic method and morphology study of materials. It has been known that mesoporous structured transition metal oxides are favorable for specific capacitance. On one hand, large specific surface is beneficial for electrolytes to access the electrochemically active sites. On the other hand, the mesoporous pore size is profitable for fast redox process. For the preparation of NiO with mesoporous structures, several methods have been employed. For example: (1) Ni(OH)2 → NiO: Wu et al. prepared highly porous NiO via a combination of sol-gel process with supercritical drying method [5]. They prepared aerogel-like Ni(OH)2 samples and then heated them to achieve aerogel-like NiO. The as-synthesized NiO samples exhibited 80%-90% of porosity and 180.5-325.6 m 2 ·g −1 of surface area. The average specific capacitance was observed to be ca. 75-125 F·g −1 between a potential window of 0-0.35 V vs. SCE [5]. Yuan and coworkers synthesized β-Ni(OH)2 microspheres by using coalescence and Ostwald-ripening mechanisms, and then these β-Ni(OH)2 microspheres were calcinated to produce hierarchical porous NiO microsphere (Figure 5a-d) [41]. Electrochemical data demonstrated that the hierarchical porous NiO nano/micro superstructures were capable of delivering a specific capacitance of 710 F·g −1 at 1 A·g −1 and offered a specific capacitance retention of ca. 98% after 2000 continuous charge-discharge cycles [43]. Lee et al. applied hexamethylenetetramine hydrolysis to synthesize Ni(OH)2 microstructures, followed by calcination to produce NiO microstructures at high temperature. The specific capacitance of fabricated NiO at 2, 5, 10, and 20 mV·s −1 scan rate was 718, 470, 420, and 403 F·g −1 , respectively. The NiO microstructures had good retention for more than 1000 cycles in a cycling test [44].
(2) Hydrothermal Method: Li et al. used hydrothermal route to synthesize Ni(OH)2, and then prepared various mesoporous NiO hierarchical microspheres via thermal decomposition of Ni(OH)2 in air [45]. Electrochemical data demonstrated that the mesoporous NiO network-like hierarchical microspheres could deliver a specific capacitance of 555 F·g −1 at 2 A·g −1 , and 390 F·g −1 even at a current density of 10 A·g −1 [45]. Yang et al. prepared mesoporous slit-structured NiO materials through a hydrothermal route with sodium dodecyl benzene sulfonate (SDBS) as an additive (Figure 5e-f). The as-prepared NiO samples presented specific capacitance of over 1700 F·g −1 in the potential range from 0.10 to 0.56 V at a constant current of 2 A·g −1 , and capacitance retention of 90% after 1000 continuous charge-discharge cycles [46].
(3) Template Method: Mesoporous NiO was synthesized by a hydrothermal homogeneous precipitation method using mixed anionic/non-ionic surfactants as template. The electrochemical results showed that the as-prepared mesoporous NiO sample had a specific capacitance of 268 F·g −1 [47]. A nanospherical porous NiO electrode material was prepared by using porous carbon nanospheres as a hard template (Figure 5g-j). The GCD measurements demonstrated that the optimal electrode possessed a specific capacitance of 1201 F·g −1 at a discharge current density of 0.5 A·g −1 and cycling stability of 70% capacity retention after 500 continuous charge/discharge cycles [48].
(4) Microwave Assisted Heating Method: Meher et al. prepared porous NiO by microwave assisted heating method under homogeneous precipitation conditions [49]. Compared with that from the traditional reflux method, the sample prepared from microwave method showed higher rate specific capacitance (370 F·g −1 ) in the charge-discharge measurements made at a discharge current of 2 A·g −1 [49].
(5) Sol-Gel Method: NiO nanostructures with three distinct morphologies were fabricated by a sol-gel method, and nanoflower-shaped NiO with a distinctive three-dimensional (3D) network showed good supercapacitor properties [50].
Another obstacle to specific capacitance of NiO is its poor conductivity, which results in low electron transport. To enhance electrochemical activity of the electrode, the dopants, such as transition and non-transition metal ions, have been introduced into the lattices of metal oxides. For instance, La 3+ doped NiO microspheres with porous structures were fabricated using colloidal carbon spheres as hard template via a hydrothermal method followed by a calcination process. As a result, 1.5 mol% La 3+ doped NiO showed a remarkable specific capacitance of 253 F·g −1 (two times higher than that of the pure NiO) and good cycling stability (34% capacity increase after 500 cycles) ( Figure 6) [51].  [43]. (e) SEM and (f) high resolution transmission electron microscopy (HRTEM) images of the NiO sample synthesized via a hydrothermal route with sodium dodecyl benzene sulfonate (SDBS) as an additive [46]. SEM images (g,h) at different magnifications, and TEM (i) and HRTEM (j) images of NiO sample prepared by using porous carbon nanospheres as a hard template [48]. There are also some reports on the synthesis of NiO/conductive materials. For example, monolithic NiO/Ni nanocomposite electrodes were fabricated recently, and the maximum capacitance was 910 F·g −1 due to the highly activated NiO surface layer and the conductive network of metal cores [52]. Flower-shaped NiO/α-Ni(OH)2 hybrid structures were synthesized by a solvothermal process. The hybrid composite demonstrates a solid capacitance value of 707 F·g −1 at 2 A·g −1 and 474 F·g −1 at a high discharge rate of 10 A·g −1 . In order to further improve the performance and the capacitance retention, conductive carbon nanomaterials were used as supports for the NiO/α-Ni(OH)2 hybrid and it was found that single-walled carbon nanotubes significantly enhanced the performance of composite to 810 F·g −1 at a high current discharge of 10 A·g −1 [42].

Cobalt Oxide (Co3O4) for Pseudo-Capacitors
Co3O4 has low environmental toxicity and high specific capacitance of 3560 F·g −1 in theory, and is of low cost [53,54]. During electrochemical processes, both pseudocapacitive behavior (CoOOH + OH − ↔ CoO2 + H2O + e − ) and battery-type behavior (Co3O4 + OH − + H2O ↔ 3MOOH + e − ) are contributed to capacitance, which results in high capacitance values (3560 F·g −1 in theory) [17,53,54]. However, the specific capacitances from real devices are much lower than that from theory. Electron effective transportation and ion fast diffusion would result in high performance of supercapacitors. Therefore, much effort has been devoted to synthesizing Co3O4 with appropriate nanostructures, particularly, Co3O4 nanomaterials with mesoporous structures, to improve the electron and ion transportation in electrodes and at the interface between electrode and electrolyte in supercapacitor devices [55,56]. For example, mesoporous nanocrystalline Co3O4 with a particle diameter of ca. 3 nm was synthesized based on strong chemical coordination interactions between Co 2+ in solution and amino groups in the polyacrylamide template, and capacitance of 401 F·g −1 was obtained from these prerequisites [57]. Mesoporous Co3O4 microspheres with crater-like morphology were obtained by utilizing the mesoporous silica material named MCM-41 as a template. This material provided a specific capacitance of 102 F·g −1 and capacity retention of 74% in 500 continuous cycles test at a sweep rate of 3 mV·s −1 [58]. Hierarchically porous Co3O4 film was prepared by electrodeposition via liquid crystalline template, and specific capacitances of these Co3O4 films were 443 F·g −1 at 2 A·g −1 and 334 F·g −1 at 40 A·g −1 , respectively [59]. Ultrafine nanosized Co3O4 materials with interconnected macroporous and mesoporous structure were synthesized through sol-gel method, followed by freeze-drying. The specific capacitance of the Co3O4 material was 742.3 F·g −1 at a scan rate of 5 mV·s −1 and the capacity retention was 86.2% after 2000 cycles [60]. With the assistance of mesoporous carbon nanorods, Co3O4 nanocubes with uniform diameter and high crystallinity were obtained. After calcination, mesoporous Co3O4 nanocubes were formed. Electrochemical tests revealed that the specific capacitance of Co3O4 nanocube electrode is ca. 350 F·g −1 at the current densities of 0.2 A·g −1 (Figure 7) [61]. In addition, meosoporous Co3O4 with other morphologies were also prepared. For example, tubular Co3O4 was fabricated by biomorphic synthesis route [62], nanosheets Co3O4 and microspheres Co3O4 were prepared by an ethanolamine-assisted solvothermal method and sequential thermal decomposition at mospheric pressure [63]. In order to simplify the synthetic route and improve the electrochemical performance, mesoporous Co3O4 was grown on some support materials to form hybrid structures. For example, ultrathin mesoporous Co3O4 nanosheet arrays were grown on Ni foam with robust adhesion, which endows fast ion and electron transport, large electroactive surface area, and good structural stability. As a result, superior pseudo-capacitive performance was achieved with ultrahigh specific capacitance in the range of 2735-1471 F·g −1 and excellent cycling stability up to 3000 cycles (Figure 8) [64]. Co3O4 was embedded into SBA-15 nanoparticles to form composites by wetness impregnation method (Figure 9). The composite structure improves charged ion transmission inside the channels and the electrochemical utilization of Co3O4 during the charge/discharge processes. A supercapacitor electrode material based on Co3O4(66%)@SBA-15 exhibited a maximum specific capacitance of 1086 F·g −1 in 6 M KOH solution. After 10 000 cycles, retention of 90% of the initial capacitance was observed (Figure 9) [65]. Co3O4 nanowire/nanoflower hybrid structure on carbon fibre cloth was prepared via a hydrothermal approach followed by thermal treatment in air ( Figure 10). A supercapacitor made from this hierarchical hybrid architecture showed a maximum specific capacitance of 4.8 mF·cm −2 at a constant density of 3 mA·cm −2 in organic electrolyte. In terms of energy and power, the supercapacitor conveyed an energy density of 4.2 mW·h·cm −3 with a power density of 1260·mW·cm −3 [66].

Nickel Cobaltite (NiCo2O4) for Pseudo-Capacitors
From the above discussion, one can see that NiO and Co3O4 have attracted wide interest recently in supercapacitor applications due to their high theoretical capacity, abundant structure, good stability and low cost. However, in real supercapacitor devices, the observed specific capacitances are still far lower than theoretical values, especially at high rates [67]. A novel compound, NiCo2O4, attracts researchers' attention. In 2010, Wei et al. obtained its specific capacitance as high as 1400 F·g −1 at a sweep rate of 25 mV·s −1 in a 1 M NaOH solution within a potential window of 0.04-0.52 V [68]. After that, NiCo2O4 has been extensively investigated for pseudo-capacitors. NiCo2O4 possesses good electrochemical activity and electrical conductivity at least two orders of magnitude higher than pure Co3O4 and NiO [69]. Both nickel and cobalt ions are involved in redox reactions, which is richer than pure nickel oxides and cobalt oxides. The energy storage occurs with reversible transformation of NiCo2O4 into nickel and cobalt oxyhydroxides as well as the reversible redox reaction of CoOOH + OH − ↔ CoO2 + H2O + e − . The theoretical capacity for NiCo2O4, including the pseudocapacitive and the battery-like faradaic contributions, is 1203 C·g −1 or 2005 F·g −1 assuming at 0.6 V potential window in an aqueous alkaline electrolyte [22]. Up to now, NiCo2O4 with different structures and morphology, different dimensions have been reported and their electrochemical performances were improved greatly. For example, one-dimensional (1D) ultralayered mesoporous NiCo2O4 nanowires were synthesized by a template-free strategy ( Figure 11). The ultralayered mesoporous nanowire electrode exhibited specific capacitance of 401 F·g −1 at 1 A·g −1 and excellent cycling stability (only ca. 10% loss after 5000 cycles) [70]. Mesoporous NiCo2O4 nanostructure was synthesized via a D-glucose-assisted solvothermal process. Electrochemical measurements showed that the spinel NiCo2O4 nanostructure heated at 300 °C exhibited maximum specific capacitances of 524 F·g −1 at 0.5 A·g −1 and 419 F·g −1 at 10 A·g −1 with good cycle stability and only ~9% of capacitance loss after 2500 cycles [71]. Hierarchical mesoporous spinel NiCo2O4 was synthesized by a hydrothermal method assisted by polyvinylpyrrolidone (PVP) and a post annealing treatment. Compared to conventional flower-like NiCo2O4, the hierarchical mesoporous structured NiCo2O4 exhibited better supercapacitance performance. The specific capacitance could reach 1619.1 F·g −1 at a current density of 2.0 A·g −1 . When the current density was increased to 10.0 A·g −1 , a specific capacitance of 571.4 F·g −1 could be obtained ( Figure 12) [72]. Spinel NiCo2O4 was synthesized through a thermal decomposition method and specific capacitance was 764 F·g −1 at 2 mV·s −1 [73]. 3D network-like mesoporous NiCo2O4 nanostructures were fabricated through a solvothermal route coupled with a post annealing treatment. The as-obtained NiCo2O4 manifested specific capacitance of 931 F·g −1 at 3 A·g −1 , capacity retention rate of 85.2 and 72.5% at 20 and 50 A·g −1 , respectively [74].  respectively; (f) GCD curves at various current densities; (g) Effects of current density on its specific capacitance; (h) Cycling performance over 1000 cycles at a current density of 6.0 A·g −1 . The inset shows the GCD curve at a current density of 6.0 A·g −1 [72].
In recent two years, the growth of NiCo2O4 on a conductive substrate attracts more and more attention. By this means, not only the conductivity of electrodes but also the specific surface areas is enhanced greatly. These unique composite electrodes may become ideal materials to improve the performance of pseudocapacitors. Several works have demonstrated the advantages of the composite electrodes. For example, mesoporous NiCo2O4 nanoneedles were grown on three-dimensional (3D) graphene-nickel foam and were then used to construct supercapacitor. The specific capacitance of NiCo2O4 nanoneedles was 1588 F·g −1 at 1 A·g −1 and the power density and energy density was 33.88 Wh·kg −1 at 5 kW·kg −1 , respectively [75]. Hierarchical porous NiCo2O4 films composed of nanowalls on nickel foam were synthesized via a facile hydrothermal method. A capacity of 130 mA·h·g −1 was achieved at 2 A·g −1 with 97% capacity maintained after 2000 cycles [76]. NiCo2O4 multiple hierarchical structures composed of 1D nanowires and 2D nanosheets were grown on Ni foam. NiCo2O4 exhibited a specific capacitance of even up to 2623.3 F·g −1 , scaled to the active mass of NiCo2O4 sample at a current density of 1 A·g −1 . A nearly constant rate performance of 68% was achieved at a current density ranging from 1 to 40 A·g −1 , and the sample retained approximately 94% of its maximum capacitance even after 3000 continuous charge-discharge cycles at a consistently high current density of 10 A·g −1 [77]. The ultrathin NiCo2O4 nanosheets supported on nickel foam were prepared via a two-step process, electrodeposition method followed with thermal treatment. The as-prepared samples were directly fabricated as electrodes for supercapacitors, and the outstanding electrochemical performance of 2517 F·g −1 was achieved at 1 A·g −1 and could still maintain 1200 F·g −1 at current density of 15 A·g −1 . The cyclic stability was also tested under current density of 8 A·g −1 , and 64% of initial capacitance was still maintained after 800 cycles [78].
Although transition metal oxides, such as RuO2, MnO2, NiO and Co3O4, have been widely investigated for pseudo-capacitors, their practical applications are still limited due to their poor stability and low electrical conductivity [79]. While active carbon materials possess good conductivity and high stability. Thus, combining transition metal oxides with carbon materials may enhance the electric conductivity, and improve electrochemical performance of supercapacitors. Design and synthesis of transition metal oxides/active carbon materials were also reported [4,[80][81][82][83][84][85]. For example, RuO2/ordered mesoporous carbon materials were prepared by impregnating an ordered mesoporous carbon CMK-3 with RuCl3·xH2O solution followed by annealing in nitrogen atmosphere. The annealing temperature and the RuO2 content had great influence on the specific capacitance of composites. The highest specific capacitance reached 633 F·g −1 by adjusting synthetic conditions [4]. Metal oxide nanorods (MnO2, SnO2, NiO) inside mesoporous silica supported carbon nanomembranes (denoted as SS-CNM) were applied for electrodes to fabricate symmetrical supercapacitors. Owing to the high electrical conductivity of SS-CNM and the intimate contact between the carbon membrane and well-ordered metal oxide nanorods, specific capacitances of MnO2 nanorods/SS-CNM, SnO2 nanorods/SS-CNM, and NiO nanorods/SS-CNM were achieved to be 964, 745, and 620 F·g −1 , respectively. In addition, less than 10% of capacitance decays over 10 000 circles and energy density was 33.5, 25.7, and 21.6 Wh·kg −1 for MnO2 nanorods/SS-CNM, SnO2 nanorods/SS-CNM, and NiO nanorods/SS-CNM, respectively ( Figure 13) [79]. Mesoporous NiO/reduced graphene oxide composites were synthesized by a hydrothermal route. Because the 3D graphene conductive network and the mesoporous structure were favorable for charge transportation and electrolyte diffusion, NiO/RGO composites exhibited high specific capacitance (1016.6 F·g −1 ) and good cycling stability (94.9% capacitance retention after 5000 cycles) [85].

Conclusions
Some recent achievements on mesoporous transition metal oxides (RuO2, MnO2, NiO, Co3O4 and NiCo2O4) for applications in supercapacitors were selected and reviewed. From this research, one can see that much progress has been made in the past decade. High specific capacity and excellent stability of nanomaterials have been observed from some mesoporous materials. In order to make the electrochemical performance of each material chosen in this mini-review more clear, their highest specific capacitance values and synthetic methods from literature mentioned in this paper are listed in Table 2. In the following research, simple synthetic methods that can produce metal oxide nanomaterials with appropriate morphology and are suitable for large-scale production should be a continuous concern for the purpose of real supercapacitor devices. In addition, attention should also focused on the design and synthesis of composite materials, such as transition metal oxides/metal, and transition metal oxides/active carbon, even on the combining of ionic liquids with mesoporous transition metal oxides. By doing so, the electrochemical performance of mesoporous transition metal oxide-based pseudo-capacitance could be envisioned to be improved substantially attributing to the synergistic effect of individual constituents. Especially, composite materials with 3D structures may provide more efficient and more rapid transportation for ions and electrons and thus result in higher electrochemical performance.