Advances in the design and application of transition metal oxide-based supercapacitors

2.1 High-performance ruthenium oxide-based SCs

In particular, ruthenium oxide (RuO₂) is intensively investigated due to its high specific capacitance (theoretical capacitance, 1,300–2,200 F g⁻¹), high ionic conductivity, reversible redox reactions, high thermal stability, and long cyclic life [37]. Charge transfer on RuO₂ surface is caused by reversible storage of protons in interaction with the electrolyte, which is influenced by the type of crystalline phase that the oxide presents. RuO₂ can form two preferential crystalline structures, the tetragonal and cubic systems with space groups of P4₂/mnm and Pa 3 ¯ , respectively [41,42]. Besides, commonly a hydrated form of RuO₂·xH₂O is observed. The hydrated form affects the storage of electric charge, facilitating the diffusion of protons (H⁺) and electrons (e^–) [43]. The storage of protons, in its hydrated form, can be expressed according to reactions (1) and (2):

The pseudocapacitive behavior of RuO₂ involves different reactions in acidic and basic electrolyte solutions. In the presence of acid electrolytes, a reversible rapid electron transfer and electro-adsorption of protons occur at the surface, and a change in the oxidation state from Ru²⁺ to Ru⁴⁺ is observed. In the charge process, in the presence of alkaline electrolytes, RuO₂ is oxidized in various forms, from RuO 4 2 − and RuO 4 − to RuO₄, and in the discharge process, the oxide is reduced to RuO₂ [44]. This behavior makes RuO₂ a promising material for the development of SCs, but its toxicity, low abundance, and high cost make it necessary to search for alternative materials. Thus, TMOs such as MnO₂, NiO, Co₃O₄, and V₂O₅ supported on carbon, activated carbon, and polymers have been investigated since they present some advantages such as greater abundance, lower cost, and lower environmental impact than RuO₂ [16,45,46,47,48,49]. In addition, these oxides are used to increase the capacitance of the carbon electrodes, also increasing the working potential window [50]. Otherwise, new research has shown the use of mixed TMOs such as NiCo₂O₄, CoFe₂O₄, CuCr₂O₄, and TbMn₂O₅ [27,28,29,30].

2.2 Manganese oxide

Manganese oxide, MnO₂ (Mn with oxidation state +4), has received great attention due to its high theoretical specific capacitance (around 1,370 F g⁻¹), low cost, abundant availability, and environmentally friendly nature [38,51,52]. In this way, MnO₂ is widely used in energy storage applications such as electrodes for lithium batteries and SCs [45,53]. Charge storage mechanisms in MnO₂ are based on processes of intercalation and deintercalation of protons (H⁺) or electrolytic alkaline ions (X⁺ = Li⁺, K⁺, and Na⁺) adsorbed on the surface of MnO₂ and absorbed in the volume of its crystalline structures. These mechanisms are shown in reactions (3)–(5) [54]:

The second mechanism is based on the adsorption of metallic electrolyte cations on the surface of the MnO₂ electrode.

The pseudocapacitive properties of MnO₂ are influenced by its crystalline structure as MnO₂ has several crystalline phases such as α, β, γ, δ, and λ-MnO₂, where each crystalline structure has an arrangement of different atoms, leaving interatomic channels where small ions can diffuse (Figure 4) [55]. This diffusion process has an influence on the acceleration and/or deceleration of the intercalation–deintercalation processes of H⁺ and alkali ions that are dependent on the atomic channels of MnO₂ and grown crystals topography, which leads to a direct influence on their specific capacitance [55,56].

Figure 4

Differences in the sizes of interatomic channels of the δ-MnO₂ and α-MnO₂ phases, and possible diffusions of Na⁺ ions in these channels [55].

MnO₂ has a low electrical conductivity around 10⁻⁶ to 10⁻⁵ S cm⁻¹ and a low structural stability that generates a decrease in electronic transport. Furthermore, a low structural stability has been observed in extensive cycling processes of charge–discharge [38,53]. Depending on the crystalline structure, MnO₂ presents differences in its electrochemical properties, as reported by Li et al. [55]. In the case of δ- and α-MnO₂ phases deposited on gold, the current collector showed a specific capacitance of 922 and 617 F cm⁻³, respectively, at 5 mV s⁻¹ in Na₂SO₄ 1.0 mol L⁻¹. This difference is due to the crystalline structure, for example: in the case of δ-MnO₂ has an interlayer distance of 0.7 nm, whereas α-MnO2 presents channels of 0.46 nm × 0.46 nm and 0.23 nm × 0.23 nm in its structure. By theoretical calculations such as DFT, energy barrier of migration of electrons between δ and α-MnO₂ was calculated and thus obtaining a value of 0.56 and 2.5 eV, respectively. It means that there is greater resistance to ion diffusion in the α-MnO₂ structure compared to the δ-MnO₂ phase (Figure 5).

Figure 5

Comparison of the electrochemical behavior between the α-MnO₂ and δ-MnO₂ phases deposited on a gold current collector: (a) cyclic voltammetry, (b) charge and discharge profiles, (c) volumetric capacitance, and (d) specific capacitance [55].

To optimize the electrochemical properties of MnO₂, various modifications to its morphology have been proposed. For example, Qiu et al. [57] electrodeposited different morphologies of MnO₂ on paper fiber carbon, such as nanospheres, nanosheets, nanoflowers, and nanorods, showed a specific capacitance of 134, 226.3, 235.6, and 362.5 F g⁻¹, respectively, at 0.5 A g⁻¹ in Na₂SO₄ 0.5 mol L⁻¹. Xia et al. [58] synthesized nanosheets of δ-MnO₂ by hydrothermal method and showed a high specific capacitance of 411 F g⁻¹ at 5 mV s⁻¹ and 332.7 F g⁻¹ at 0.5 A g⁻¹, with a capacitance retention of 93% after 10,000 cycles. This high value was attributed to the porosity of the material, increasing the electroactive sites, creation of short distances for ions diffusion in δ-MnO₂, and decreasing the resistance of charge transfer. Conversely, the size of the oxides and their composition with other transition metals is also discussed. Feng et al. [59] synthesized MnO₂@SnO₂ spheres by the hydrothermal method, in which MnO₂ presented a pseudocapacitive behavior, while the SnO₂ intensified the electrical conductivity. Finally, the authors obtained a high specific capacitance of 541.6 F g⁻¹ at a current density of 1 A g⁻¹, with a retention of capacitance of 92% after 1,500 charge and discharge cycles, using KOH 6 mol L⁻¹ as an electrolyte.

Furthermore, the addition of other oxides such as Cu₂O contributes to the pseudocapacitance of MnO₂. For example, Purushothaman et al. [31] electrodeposited consecutively CuMnO₂ and Cu₂O (Cu₂O@CuMnO₂@Cu) and obtained a specific capacitance of 257 F g⁻¹ at the scan rate of 5 mV s⁻¹ in Na₂SO₄ 0.1 mol L⁻¹ as an electrolyte. In addition, hybrid systems using MnO₂ nanoparticles grown on highly porous carbonaceous materials (carbon nanotubes, graphene, activated carbon, and carbon fibers) show specific capacitances between 258 and 330 F g⁻¹ at a current density of 1.0 A g⁻¹ [60]. Likewise, 3D MnNiCoO₄@MnO₂ mesoporous ternary systems supported in carbon cloth (CC) have been reported, with a specific capacitance of 1,931 F g⁻¹ at a current density of 0.8 A g⁻¹ and a retention of capacitance of 91.2% at 6 A g⁻¹ after 6,000 charge–discharge cycles in KOH 6 mol L⁻¹. This high capacitance is due to the ternary composite in a 3D porous structure that intensifies electroactive area and improves ion transport [32]. Another ternary system composed of PANI/MnO₂/carbon nanofibers presented a stability of 91% after 1,000 charge–discharge cycles and a specific capacitance of 289 F g⁻¹[61]. Furthermore, α-MnO₂ nanoparticles deposited on platinum electrodes in the form of films have shown specific capacitances of 1,380 F g⁻¹, which is higher than the theoretical value (∼1,370 F g⁻¹) [23].

2.3 Vanadium oxide

Vanadium oxide has several oxidation states from 0 to +5 to form VO₂, V₂O₃, and V₂O₅ [62]. Depending on the physical and chemical conditions such as temperature, pressure, and medium (acidic or basic solution), it is possible to reduce V₂O₅ to VO₂ (oxidation state from +5 to +4) [63,64]. V₂O₅ is the most studied oxide and has been used in electrodes for ion batteries (Li⁺, Na⁺, and Mg²⁺) [65,66]. In addition, V₂O₅ has been proposed in the development of SCs due to its high specific capacitance and low acquisition cost. The orthorhombic phase of V₂O₅ makes it a promising material for energy storage applications due to its high ion storage capacitance. V₂O₅ has an n-type conductivity and is the most stable among all vanadium oxides, reaching a melting point of 685°C [64]. The pseudocapacitive nature of V₂O₅ is due to the processes of intercalation that occurs within its structure, inducing distortions in its morphology and surface chemistry, as shown in the following reaction [67]:

The pseudocapacitive behavior is mainly attributed to its structure in the form of 2D layers and its complex 3D structure where interconnected channels are formed. These channels facilitate a rapid diffusion of electrolytic ions. The pseudocapacitive process in V₂O₅ in aqueous and organic electrolytes is well known due to its structural stability in these media and the use of large windows potential [68]. In theory, a monolayer of V₂O₅, according to reaction (6), participates in the adsorption process of ions from the electrolytic solution, which leads to a maximum theoretical capacitance of 883.3 F g⁻¹ in a working potential range of 1.2 V [69]. Likewise, intercalation processes depend on porosity and oxide morphological structure. Zhu et al. [25] compared various structures of V₂O₅ crystalline nanoparticles in the form of bulk, stacked, and 3D (Figure 6a–c) and demonstrated higher electrochemical efficiency for complex 3D structures (Figure 6c). This increase in electrochemical efficiency (specific capacitance, power density, and energy density) is associated with the higher surface area (about 133 m²g⁻¹) than other structures. Furthermore, according to the diffraction results, a crystalline phase of the bulk structure with planes at (001), (003), (004), (005), (006), and (007) was observed. However, 3D structure only showed planes at (001), (003), and (004) with preferred orientation at (001). As a result, the high surface area for 3D V₂O₅ was obtained as a consequence of the channel formed by 2D thin sheets of V₂O₅. This structure improved the diffusion of ions, resulting in a capacitance of 451 F g⁻¹ higher than stacked V₂O₅ (314 F g⁻¹) and bulk V₂O₅ (108 F g⁻¹), all of them measured at a current density of 0.5 A g⁻¹ in Na₂SO₄ 1 mol L⁻¹ (Figure 6d–g) [25].

Figure 6

FESEM images of V₂O₅: (a) commercially available bulk, (b) staked films, and (c) 3D architecture constructed from (b). Electrochemical performance of 3D V₂O₅, staked films, and bulk for supercapacitors: (d) specific capacitances calculated from the cyclic voltammetry curves, (e) galvanostatic charge/discharge at a current density of 0.5 A g⁻¹ in the range of −1.0 to +1.0 V in Na₂SO₄ 1 mol L⁻¹ aqueous solution, (f) specific capacitances calculated from galvanostatic charge/discharge with various current densities and (g) power density and energy density of V₂O₅ 3D architecture, stacked film, and bulk electrodes [25].

In general, homogeneous amorphous oxides have a lower energy loss than crystalline oxides since they present channels that increase the ionic diffusion [70]. The optimization of V₂O₅ capacitance is dependent on the morphology. Qian et al. [71] reported the synthesis of different morphologies composed by crystalline V₂O₅ such as nanowire, flower-like flakes, and curly bundled nanowires. 1D V₂O₅ nanowires showed a high capacitance of 349 F g⁻¹ with a surface area of 123 m²g⁻¹; however, this material presented a poor capacitance retention of 27.6% after 200 charge and discharge cycles. Likewise, in the case of hydrated V₂O₅ curly bundled nanowires after cycling, there was an increase in 42 to 127 F g⁻¹ due to the presence of nanopores in its structure [71]. Furthermore, Zheng et al. [72] reported the synthesis of microstructures in the shape of a butterfly-like, rhombohedral, and a flower-like V₂O₅ with capacitances of 556, 641, and 609 F g⁻¹, respectively, at 0.5 A g⁻¹ in LiClO₄ 1.0 mol L⁻¹ (Figure 7a–d). Also, these microstructures were formed by a crystalline phase of V₂O₅ with an orthorhombic structure. Likewise, the high capacitance was mainly attributed to the V₂O₅ rhombohedral structure, explained by its simple morphology, which induces easy surface polarization at low electrolyte concentrations. Conversely, butterfly-like and flower-like V₂O₅ structures were polarized at higher electrolyte concentrations. In addition, electrochemical impedance spectroscopy showed resistivities of 1.22, 1.17, and 0.76 Ω for butterfly-like, flower-like, and rhombohedral structures, respectively. These results indicate a rapid diffusion of ions and an increase in capacitive values for V₂O₅ of the rhombohedral structure. Also, butterfly-like, flower-like, and rhombohedral structures showed a capacitive retention of 132.6, 70.4, and 119.8% after 2,000 charge and discharge cycles, respectively (Figure 7e). The increase in the capacitance during the cycling process is attributed to the porosity of the material [72].

Figure 7

SEM images of (a) butterfly-like, (b) rhombohedral, and (e) flower-like V₂O₅. Electrochemical performance of butterfly-like, rhombohedral, and flower-like V₂O₅: (d) specific capacitances of V₂O₅ with different morphologies calculated from GCD curves at different current densities and (e) cycling performance of V₂O₅ with different morphologies collected at 100 mV s⁻¹ [72].

Previous reports had shown that some V₂O₅-based materials present intrinsic limitations such as low conductivity, loss of mass due to the formation of soluble species during extensive charge and discharge processes, affecting its morphology and crystal structure and resulting in serious degradation problems [73]. Several strategies have been studied to improve the electrochemical properties of V₂O₅, one of these is to modify the structure by generating oxygen vacancies through the chemical reduction caused by the oxidative polymerization of conductive polymers [68], also improving its intrinsic conductivity and pseudocapacitive charge storage kinetics [74]. Bi et al. [74] used the vapor phase polymerization method to synthesize V₂O₅/poly(3,4-ethylenedioxythiophene) with oxygen vacancy gradient (G-V₂O₅/PEDOT). G-V₂O₅/PEDOT presented a capacitance of 614 F g⁻¹ at a current density of 0.5 A g⁻¹ using 1 mol L⁻¹ Na₂SO₄. Moreover, crystallographic results of VOx showed an amorphous structure that is attributed to the oxygen vacancies of V₂O₅ and PEDOT. These vacancies generate vanadium oxidation states of V⁴⁺ and V³⁺ on the material surface, facilitating charge transfer kinetics with a capacitance retention of 122% after 50,000 cycles. HRTEM microscopy revealed that the core nanoparticles corresponded to a crystalline phase.

Furthermore, the insertion of V₂O₅ nanostructures in carbonaceous matrices (such as nanowires, nanofibers, and nanotubes) generates a synergistic effect between pseudocapacitive properties and the electrochemical double layer [75]. For example, Panigrahi et al. [49] developed a composite material from activated carbon fabric (ACF) as a support and obtained a flexible network of V₂O₅@ACF. This composite showed a specific capacitance of 1,098 F g⁻¹ at 5 mV s⁻¹ in K₂SO₄ 0.5 mol L⁻¹. This high capacitance is due to the carbonaceous material porous structure, which promotes the dispersion and increment of electroactive sites of V₂O₅ nanoparticles. The diffraction pattern of this flexible 3D carbon network presented a broad peak at 25.75° and small peak at 43.42°, corresponding to a low crystalline order of hexagonal graphite structure. Meanwhile, V₂O₅ showed a highly crystalline diffraction planes at (110), corresponding to an orthorhombic structure. The composite showed a retention capacity of 92% after 10,000 galvanostatic charge–discharge cycles at 15 A g⁻¹ [49].

Another important tendency is the design of binary and ternary oxides with improved electrochemical properties, especially oxides with higher conductivity, stability, energy, and power density [76]. The binary and ternary oxides compounds are interesting due to the synergy of metal atoms with different oxidation states, improving the properties such as electronic conductivity, structure stability, ion diffusion rate, electrochemical activity, and high sites active [77]. Zhang et al. [33] synthesized 3D nanoroses of Co₃V₂O₈ and 2D nanoplates of NiCo₂V₂O₈ by the solvothermal method (Figure 8a and b) and obtained surface areas of 112.97 and 131.76 m² g⁻¹, respectively. Co₃V₂O₈ and NiCo₂V₂O₈ oxides showed an orthorhombic phase with a slight distortion in the unit cell of NiCo₂V₂O₈ due to Ni ion insertion within the Co₃V₂O₈. Moreover, when Ni is added an increment of the capacitive current is observed (Figure 8c). The specific capacitances obtained were 371 and 1,098.9 F g⁻¹, respectively, at 1 A g⁻¹ in KOH 2.0 mol L⁻¹ (Figure 8d). The rate capability shown an increment of the pseudocapacitance due to Ni presence (Figure 8e). Furthermore, both oxides demonstrated good capacitive retention of 89.6 and 68% after 7,000 charge and discharge cycles at 4 A g⁻¹ (Figure 8f). This decrease in capacitance retention of ternary oxide during cycling is influenced by the distortion caused by the substitution of Ni ion in the Co₃V₂O₈ structure, which shows greater structural instability.

Figure 8

Scanning electron microscopy (SEM) image of (a) Co₃V₂O₈ (CVO) with nanorose structures and (b) NiCo₂V₂O₈ (CNVO) with nanoplate structures. Electrochemical performance characterization of porous 3D Co₃V2O₈ nanoroses and 2D NiCo₂V₂O₈ nanoplates: (c) CV curves of Co₃V₂O₈ nanoroses and NiCo₂V₂O₈ nanoplates modified electrode at scan rate of 5 mV s⁻¹. (d) The corresponding galvanostatic charge–discharge curves at a current density of 1 A g⁻¹. (e) Capacitance retention rate as a function of current density. (f) Cycling performance of Co₃V₂O₈ and NiCo₂V₂O₈ at a constant current density of 4 A g⁻¹ for 7,000 cycles [33].

Other interesting results were obtained by adding carbonaceous materials in hybrid composites of binary or ternary oxides, improving the electrical conductivity and the mechanical properties, as is the case of the thin layers of Co₃V₂O₈/Ni₃V₂O₈ adsorbed on binder-free porous carbon nanofibers. This material presented a synergistic behavior among Co, V, Ni, and porous carbon, which improves the diffusion and migration of ions, and obtained a specific capacitance of 1,731 F g⁻¹ at a current density of 1.0 A g⁻¹ in 3.0 mol L⁻¹ KOH with 88.5% capacitance retention after 3,000 cycles [34]. The use of carbon cloth as support of Ni–V nanosheets also improves the flexibility and mechanical properties, showing a high specific capacitance of 2,790 F g⁻¹ at 1.0 A g⁻¹ in KOH 2.0 mol L⁻¹ [78].

According to the aforementioned studies, a strong relationship has been observed between parameters such as crystallite sizes, morphology of the nanostructures, and their obtained capacitances. These results are mainly due to the direct influence of the capacitance with surface area and electroactive sites at the electrode–electrolyte interface. Apart from that, binary and ternary oxides showed a synergy for the metals present in the structure, generating several redox centers. The evidence from this study suggests a certain influence of crystallinity on the structure stability during charge and discharge processes. In addition, the substitution of a metal in the structure alters the crystal lattice decreasing the stability after the charge–discharge process.

2.4 Cobalt oxide

Co₃O₄ has a cubic structure formed by two oxidation states of cobalt, and Co²⁺ and Co³⁺ present tetrahedral and octahedral coordination with the oxygen atoms, respectively (Figure 9) [79,80]. Likewise, Co₃O₄ has a large theoretical specific capacitance of about 3,560 F g⁻¹ [26]. In addition, cobalt oxide is harmless, is eco-friendly, and has a low acquisition cost [46,72,81]. Reactions produced between Co₃O₄ electrodes and alkaline electrolytes such as KOH can be expressed as shown in equations (7) and (8):

Figure 9

Cubic structure of Co₃O₄, showing cobalt ions Co²⁺ and Co³⁺ that shows tetrahedral and octahedral coordinates with oxygen ions, respectively [80].

The oxidation mechanism of different cobalt compounds presents intermediate reactions involving cobalt hydroxides, oxyhydroxides, and others. These intermediate reactions are crucial factors to know the electrochemical and thermal stability of Co₃O₄ electrodes [26]. However, the capacitance and the stability while cycling are limited by the structural rigidity and low conductivity of these electrodes [57]. To overcome these limitations, various studies aim the control of Co₃O₄ morphology to increase the surface area. Different morphologies have been studied, such as nanoparticles, nanorods [82,83], nanowires [84], nanowires [85], nanosheets [86], hollow nanosheets [87], and nanoflowers [88]. For example, Co₃O₄ nanowires on nickel foam reach a specific capacitance of 1,160 F g⁻¹ at 2.0 A g⁻¹ and a retention of 90.6% after 5,000 cycles (performed at 8 A g⁻¹) [89].

Wang et al. synthesized structured nanonetworks of crystalline Co₃O₄ in 3D via heterogeneous precipitation method, which reported a specific capacitance of 820 F g⁻¹ at 5 mV s⁻¹ using KOH 6 mol L⁻¹ showing a capacitance retention of 90.2% after 1,000 of charge and discharge cycles [90]. Liu et al. [91] synthesized Co₃O₄ nanorods in the 1D via hydrothermal method showing a capacitance of 655 F g⁻¹ at 0.5 A g⁻¹ in KOH 1.0 mol L⁻¹. This increase in capacitance is related to the high crystallinity rod-like structure of Co₃O₄ nanorods [92]. Liu et al. [91] obtained core-shell type mesoporous nanospheres of crystalline Co₃O₄ by the solvothermal method, showing values of 837.7 F g⁻¹ at 1.0 A g⁻¹ and a good capacitance retention of 87% after 2,000 cycles at 5.0 A g⁻¹. The authors attribute the high capacitance to the type of mesoporous core-shell structure. Similarly, Co₃O₄@CoS composites with core shell morphology present a specific capacitance of 1,658 F g⁻¹ at 1.0 A g⁻¹ and a capacitive retention of 92% after 10,000 cycles. A crystalline cubic phase of Co₃O₄ was observed by X-ray diffraction. The authors did not detect a CoS crystalline phase due to its lower concentration localized on the core shell structure [93].

Besides, many authors report that the insertion of carbonaceous materials such as graphene in composites improves conductivity. Because of that, Naveen et al. [94] synthesized Co₃O₄ spinels structure on graphene nanospheres and thereby obtained a specific capacitance of 650 F g⁻¹ at 5 mV s⁻¹ in ¹ KOH 1.0 mol L⁻¹ and a capacitance retention of 92% after 1,000 cycles. Development of binary oxide electrodes formed by crystalline NiCo₂O₄ (cubic structure with Fd3m space group) composites on activated carbon also increases notably the capacitance and conductivity due to the redox pairs of Co²⁺/Co³⁺ and Ni³⁺/Ni²⁺. Xu et al. [35] showed that these composites reach specific capacitances of 273.5 F g⁻¹ at 1.0 A g⁻¹ in KOH 6.0 mol L⁻¹ giving a capacitance retention of 96% after 3,000 cycles. The influence of the mesoporous structure of Co₃O₄ in electrochemical performance has also been evaluated. Qiu et al. [57] used a thermal decomposition method, which demonstrated a favorable growth of mesoporous nanocrystalline Co₃O₄ on nanosheets of reduced graphene oxide (NOGR). These materials showed a capacitance of 1,085.6 F g⁻¹ at 2.0 A g⁻¹ in 2 mol L⁻¹ KOH, resulting in a capacitance retention of 82.7% after 10,000 cycles. Sun et al. used biomass-derived activated carbon to obtain carbon–Co₃O₄ composites, and in this study, an increase in capacitance and conductivity of the material was observed. The authors used the hydrothermal synthesis method, followed by a calcination at 400°C where octahedral nanostructures composed by crystalline cubic phase of Co₃O₄ were supported on cotton-derived mesoporous carbon microtubes (Figure 10a–f). These composites showed a capacitance of 284.2 F g⁻¹ at 1.0 A g⁻¹ in KOH 6.0 mol L⁻¹, and a capacitance retention of 91.95% was also observed after 3,000 cycles [95]. Obodo et al. [96] used the hydrothermal method to synthesize crystalline nanoparticles of Co₃O₄ and MnO₂ (30 nm of crystallite size) supported in graphene oxide (Co₃O₄/MnO₂@GO). The composite had a flake-like structure. The electrodes produced by these composites reached specific capacitance of 1,518 F g⁻¹ at 10 mV s⁻¹ in Na₂SO₄ 1 mol L⁻¹. Besides, Table 1 summarizes the most used TMO for the manufacture of electrodes in supercapacitor systems.

Figure 10

SEM images of Co₃O₄ nanostructures: (a) cotton fibers, preparation of mesoporous carbon microtubes from cotton fibers previously calcined at T = 400°C for (b) 2 h and (c) 3 h; (d) unsupported Co₃O₄ nanostructures. TEM images: (e) in low magnification and (f) in high resolution (HRTEM), showing the composite formed by the Co₃O₄ nanostructures supported in the mesoporous carbon nanotubes previously calcined at T = 400°C for 2 h [99].

Finally, all of these works show excellent capacitive results using nanoparticles composed of unique crystalline phases of Co₃O₄ with the FCC structure (space group Fd3m) [97]. For example, Liu et al. [91] showed that a crystalline structure of Co₃O₄ in core-shell shape produced a higher specific capacitance than other structures already discussed. Likewise, if these Co₃O₄ core-shell structures are covered by small amounts of CoS, the specific capacitance obtained is enhanced, as shown by Lu et al. [93]. In addition, Co₃O₄ crystalline phase also improved the electrochemical stability after charge–discharge cycles as shown by Zhang et al. [89]. Therefore, the FCC crystalline phase of Co₃O₄ is a promising material for the development of high-capacitive and low-cost electrodes for energy storage.