Metal oxide/hydroxide-based materials for supercapacitors

Abstract

Supercapacitors are promising energy storage and conversion devices with high power densities. However, their low energy densities limit their practical application. The electrode is a key component that determines the performance of supercapacitors. As electrode materials, transition metal oxides/hydroxides usually exhibit high capacitance, leading to high energy densities. This review discusses the advantages and disadvantages of different metal oxides/hydroxides, in order to synthesize high-performance electrode materials. Two main strategies to enhance the supercapacitive performance are proposed: developing composites and nanostructured materials.

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1. Introduction

In the past several decades, the global economy has developed rapidly, as has the world population, which has resulted in an increasingly strong demand for energy. Since global energy consumption is increasing continuously, the exhaustion of global energy may soon threaten human society. One research report suggests that the global energy needs will quickly double by the mid-century and even triple by 2100.¹ To date, fossil fuels have been used widely and largely in our society. However, fossil fuel reserves are limited, and will be depleted soon. Shafiee et al. have computed that the depletion time for oil, coal, and gas, are about 35, 107 and 37 years, respectively.² Thus, research into developing clean, reliable, and sustainable alternative energies, such as solar, tidal and wind, have been implemented worldwide to replace fossil fuels. An important intermediate step between these energy resources and versatile energy applications is energy storage, and this has attracted significant attention in academia and industry. Clearly, it is necessary to develop low-cost, efficient, and environmentally friendly energy storage devices that can satisfy the needs of modern life.

Among various energy storage systems, supercapacitors (also referred to as electrochemical capacitors) are considered to be one of the most promising future energy storage systems, due to their high power density (>10 kW kg⁻¹), high rate capability and long cycle life (>1 000 000 cycles).³ One of their unique advantages is that the power densities of supercapacitors are superior to batteries. In addition, their energy densities are several orders of magnitude higher than traditional capacitors, although they are still much lower than batteries and fuel cells.^4,5 Clearly, supercapacitors can play a role to bridge the power and energy gap between batteries/fuels and traditional dielectric capacitors. Since the first electrochemical capacitor patent was applied by Becker in 1957,⁶ considerable efforts have been put into researching efficient supercapacitors. Especially in recent years, to meet the ever-increasing demand of modern applications requiring high power density, such as energy back-up systems, portable electronic devices, and hybrid electric vehicles,^7–9 there has been an explosion in supercapacitors research, and researchers have made great progress in this field. Their main target has been to enhance the energy densities of supercapacitors without sacrificing their high power densities, with the aim that supercapacitors will be able to function as a primary power source, just like batteries.^10–12

2. Fundamentals of supercapacitors

2.1. Types of supercapacitors

Supercapacitors can be categorized into two types based on the energy storage mechanism: electrochemical double layer capacitor (EDLC) or a pseudocapacitor (Fig. 1).

Fig. 1 Schematic diagram of supercapacitors: (a) EDLC; (b) pseudocapacitor (M represents the metal atom. If anions in the electrolyte take part in the reversible redox reaction, they will move in the opposite direction to the cations).

In EDLCs, the energy storage and transport mechanism is similar to the two-plate conventional capacitors (Fig. 1a). There is a pure physical charge accumulation at the interface between the electrode and electrolyte.^6,13 During the charging process, the electrons move from the negative electrode to the positive electrode through the external loop, with anions moving towards the positive electrode, while the cations move towards the negative electrode in the electrolyte. During the discharging process, the electrons and ions move reversely.⁶ Because no charges transfer across the interface between the electrode and electrolyte, the energy storage mechanism is non-Faradaic and there are no redox reactions. Since only physical charge transfer occurs, there is almost no volume or morphology change of the electrode material, which results in the long cycle life of EDLC.

In contrast to EDLCs, pseudocapacitors involve faradaic redox reactions. When a potential is applied to a pseudocapacitor, fast and reversible faradaic reactions occur on the electrode materials, generating charges and resulting in the charge transfer across the double layers. This is similar to the charging and discharging processes that take place in batteries (Fig. 1b). There are three types of faradaic processes in the electrodes: reversible adsorption (e.g., the adsorption of hydrogen ions on the surface of the platinum electrode), redox reactions of transition metal oxides, and reversible electrochemical doping–dedoping process in the electrodes based on conductive polymers. Commonly, pseudocapacitors provide higher specific capacitance and energy density than EDLCs, because the faradaic processes can occur not only on the surface of pseudocapacitor electrodes but also in the interior of these electrodes.^7,8,13 However, since redox reactions are involved, pseudocapacitors often suffer from a lack of stability during cycling. The pseudocapacitors also have a lower power density than EDLCs because faradaic processes are slower than non-faradaic processes.⁶

2.2 Parameters for supercapacitors

Supercapacitors can be evaluated based on these following criteria:³ (1) a high specific capacitance (SC); (2) a high power density with a relatively high energy density (>10 W h kg⁻¹); (3) an excellent cyclability; (4) fast charging/discharging rates within seconds; (5) low self-discharging; (6) safe operation; (7) low cost. Like a battery, a full supercapacitor device consists of several parts: positive and negative electrodes, a separator, and an electrolyte. Some parameters of these parts affect the performance of the supercapacitor, such as the specific surface area, the pore size distribution of electroactive materials, the intrinsic properties of the electrolyte, the morphology, the electric conductivity of electroactive materials, and the interface between the electrode and electrolyte.¹⁴

A large specific surface area is a key point for electrode materials, and can facilitate the contact between the electrode and electrolyte, provide more active sites, and lead to an enhanced capacitance.⁶ Usually, porous structure materials possess large specific areas, which can offer more channels for the electrolyte as well, resulting in less electrochemical polarization. However, the capacitance does not linearly increase with the increasing specific surface area, since the pore size of electrode materials plays an important role and the materials must have desirable pore distribution.⁷ Macropores (>50 nm) do not contribute much to the specific surface area. They serve as ion-buffering reservoirs and conduits to deliver electrolytes to the interior. Micropores (<2 nm) are too small for ions to move in or out. Largeot et al. report that when the pore size of electrode materials is very close to the ion size, the capacitance reaches the maximum.⁴

The two significant parameters to evaluate the performance of a supercapacitor are energy density and power density, which can be obtained by the following eqn (1) and (2):

E = CV²/2m (1)

P = V²/4mR (2)

where C is the total capacitance of the cell (in Farads); V is the operating voltage (in volts), which is determined by the electroactive materials and the thermodynamic stability (potential window) of the electrolyte; m is the total mass of the supercapacitor; and R is the equivalent series resistance (ESR, in Ohms), which constitutes the intrinsic resistance of the electroactive materials and electrolyte solution, the contact resistance between the electroactive materials and the current collector, and the mass transfer resistance of the ions in the electrode and through the separator. Therefore, in order to obtain an excellent performance of the supercapacitor, it is necessary to possess a high SC and operating voltage and a minimum ESR, according to the above equations.

Usually, there are two methods to calculate the SC of the electrode materials. We can calculate the SC from the cyclic voltammetry (CV) curves according to eqn (3):

C = (∫IdV)/(vmV) (3)

or from the galvanostatic charge–discharge curves according to eqn (4):

C = IΔt/mΔV (4)

In eqn (3), C (F g⁻¹) is the specific capacitance, I(A) is the response current, V is the potential (V), v is the potential scan rate (V s⁻¹), and m (g) is the mass of the electroactive materials in the electrode. In eqn (4), I represents the discharge current, m represents the mass of active materials, ΔV represents the potential drop during the discharge and Δt represents the total discharge time.

Because of the high SC, pseudocapacitor materials intensively arouse researchers' interest. In this review, we provide a brief summary of the recent progress on transition metal oxide/hydroxide materials for high-performance supercapacitor electrodes.

3. Electrode materials of transition metal oxides/hydroxides

Among the parts of a full supercapacitor device, the electrode is considered to be a key component affecting the electrochemical performance of a supercapacitor. Therefore, choosing and designing efficient electrode materials is of great importance to fabricate high-performance supercapacitors.

Early studies in supercapacitors have focused on carbon materials due to their easy accessibility, large surface area, and relatively good electric conductivity. However, these carbon materials usually suffer from low SC. Hence, researchers have shifted their attention from pure carbon materials to pseudocapacitor materials (such as transition metal oxides/hydroxides and conducting polymers).^6,15–17 SCs of the latter type are 3–7 times larger than the former. Transition metal oxides/hydroxides are considered to be the best candidates. Among them, the most commonly used electroactive materials can be divided into two types: noble (RuO₂,¹⁸ IrO₂¹⁹) and cheap transition metal oxides/hydroxides (MnO₂,^20,21 Co₃O₄,^22–24 NiO,^25–27 Co(OH)₂^28,29 and Ni(OH)₂^30,31). However, the transition metal oxides/hydroxides usually have relatively low power density and poor cycling stability. The low power density can be ascribed to the poor electric conductivity of metal oxides/hydroxides, which limits the electron transfer rates. Moreover, the damage to the morphology caused by the swelling and shrinkage of the electrode materials during the charging and discharging processes leads to the lack of cycling stability. To solve these problems, the two main methods, involve combining metal oxides/hydroxides with conductive materials and developing porous nanostructures: the former can accelerate the reaction kinetics; the latter can buffer the stress from the swelling and shrinkage of the electrodes and provide more ion adsorption or active sites for the charge transfer reactions, as well as shorten the diffusion and transfer pathways of the electrolyte ions (Fig. 2).

Fig. 2 Strategies to enhance the performance of supercapacitors.

3.1. Noble metal oxides

The earliest studied transition metal oxide for a supercapacitor was RuO₂, and it is still recognized as one of the most promising electrode materials, because of the high theoretical SC (≈2000 F g⁻¹),³² long cycle life, wide potential window, high electric conductivity, high rate capability and good electrochemical reversibility.^15,33–36 The pseudocapacative behaviour of RuO₂ in an acidic electrolyte can be described as a fast and reversible faradaic reaction, according to eqn (5):⁷

RuO₂ + xH⁺ + xe⁻ ↔ RuO_2−x(OH)_x (5)

where 0 ≤ x ≤ 2. In the acidic system, the Ru oxidation states change from (II) to (IV). However, when the electrolyte is alkaline, the Ru oxidation states are different. It is reported that when the carbon/Ru composite is charged, RuO₂ in the electrode material will be oxidized to RuO₄²⁻, RuO₄⁻, and RuO₄, and when the electrode is discharged, these high valence state compounds will be reduced to RuO₂.³⁷

In fact, the SCs of RuO₂ electrode materials are usually smaller than the theoretical value because of the high crystalline and power constraints.³² The supercapacitive performance of RuO₂ depends on the amount of combined water, the crystallinity, annealing temperature and particle size.

It is known that the reaction of RuO₂ in the acidic system involves the insertion and extraction of H⁺ according to eqn (5), and that RuO₂·nH₂O is a good proton conductor.³⁸ So, the combined water in hydrous RuO₂ can accelerate the diffusion of H⁺ in the electrode. The hydrous Ru oxides exhibit high SCs of 861 F g⁻¹ and 900 F g⁻¹,^39,40 which are much larger than the anhydrous form. In addition, as the content of the combined water decreases from RuO₂·0.5H₂O to RuO₂·0.03H₂O, the SC decreases from 720 F g⁻¹ to 19.2 F g⁻¹.⁴¹

The crystallinity also affects the supercapacitive performance of RuO₂. The well-crystallized RuO₂ is so compact that the insertion and extraction of ions/electrons are difficult, which leads to an increase in the electrochemical impedance and a decrease in the supercapacitive performance. In contrast, the redox reactions of the amorphous RuO₂ take place not only on the surface but also in the bulk of the materials. Hence, there are many more articles reporting the superior performance of amorphous RuO₂ materials compared with crystallized ones.^41,42 The amorphous RuO₂ thin films anodically deposited on stainless steel substrates demonstrate stable electrochemical capacitor properties, with an ultrahigh maximum SC of 1190 F g⁻¹ in H₂SO₄.⁴³

The annealing temperature also has an important influence on the supercapacitive performance. When annealed at high temperature, RuO₂·nH₂O possesses a good crystallinity and low water content.⁴⁴ And a sharp decrease in capacitance is observed at an annealing temperature above 200 °C.⁴⁵ It is reported that when the annealing temperature is close to the crystallization temperature of Ru oxide, RuO₂·nH₂O can still remain amorphous and hydrous,^41,46 with the optimal annealing temperature being around 150 °C.^35,41

The particle size of RuO₂ is another important factor for the electrochemical performance. Small-sized particles have short diffusion and transport pathways of electrolyte ions, as well as high specific surface areas. As a result, the utilization of electroactive materials and the high rate charge/discharge capability of the supercapacitors will be improved. The ion diffusion time constant (τ) can be obtained by eqn (6):⁴⁷

τ = L²/2D (6)

where L is the transport length and D is the ion transport coefficient. As the size of particles decreases, the ion diffusion time decreases simultaneously. Thus, smaller particles provide a higher SC and utilization efficiency,^47,48 and this is an effective way to reduce the size of the RuO₂ particles to the nanoscale. When nanostructured RuO₂ are applied, the SC and ion conductivity are clearly improved. Since the capacitance is mainly derived from surface reactions and the nanostructured RuO₂ has a much higher specific surface area than the bulk one, which can provide more active sites for the fast and reversible redox reactions, many researchers have focused their efforts on developing RuO₂ nanostructures. Up till now, various nanostructured RuO₂ have been fabricated, such as nanorods,^49,50 nanoflowers,⁵¹ nanosheets,⁵² and nanotubes.^39,53 The RuO₂·nH₂O nanotubular array electrode shows an extremely high SC of 1300 F g⁻¹ with an energy density of 7.5 W h kg⁻¹ by using the anodic deposition technique.⁵³

Despite all the advantages, RuO₂ is still not suitable for commercial application with supercapacitors, due to its high cost and scarce source. To reduce the cost, two ways have been frequently proposed: composing RuO₂ with other cheap metal oxides to synthesize composites, such as MnO₂, Co₃O₄, NiO, TiO₂ and SnO,^54–59 or depositing RuO₂ on low-cost substrates to form composites, such as various kinds of carbons and conducting polymers.^{18,33,34,60–64} The RuO₂·nH₂O/TiO₂ nanocomposite synthesized by microwave-assisted hydrothermal synthesis exhibits a maximum SC of 992 F g⁻¹, with quite a high power density of 400 kW kg⁻¹.⁶⁵ In this composite, the TiO₂ crystallites are formed before RuO₂·nH₂O nucleation, thus enhancing the exposure and utilization efficiency of RuO₂·nH₂O. The PEDOT-PSS-RuO₂·nH₂O electrode formed by embedding hydrous RuO₂ particles into a conductive PEDOT-PSS matrix, which provides a 3D matrix for loading and stabilizing RuO₂·nH₂O, achieves a maximum SC of 653 F g⁻¹.⁶¹ The SC of using RuO₂·nH₂O/multi-walled carbon nanotubes/Ti as electrodes in H₂SO₄ aqueous solution can be up to 1652 F g⁻¹, which is attributed to the nanopore structure that allows fast and reversible faradaic processes to occur on the electrode surface.⁶⁶ The conductive substrates improve the conductivity of RuO₂, leading to a shorter ion transfer distance.³⁵

3.2. Cheap metal oxides/hydroxides

In order to reduce the cost of supercapacitor electrode materials, a feasible scheme is to develop cheap metal oxides/hydroxides as alternative candidates to replace RuO₂.

3.2.1. MnO₂. As an alternative to replace RuO₂, various forms of MnO₂ have been fabricated for its high theoretical SC (1370 F g⁻¹),⁶⁷ low cost, large abundance, and low toxicity. The capacitance of Mn oxides comes mainly from pseudocapacitance. There are two mechanisms proposed to explain the MnO₂ charge storage behavior (eqn (7) and (8)). The first one implies the insertion of electrolyte cations (C⁺ = H⁺, Li⁺, Na⁺ and K⁺) into the bulk of the electrode:^67,68

MnO₂ + C⁺ + e⁻ ↔ MnOOC (7)

The second one is based on the surface adsorption of electrolyte cations on the MnO₂ electrode:^67,68

(MnO₂)_surface + C⁺ + e⁻ ↔ (MnOOC)_surface (8)

Both the mechanisms involve a redox reaction between the III and IV oxidation states of Mn.

MnO₂ materials have various crystal structures (α-, β-, γ-, δ-, and λ-), which determine their electrochemical performance, especially when the size of the tunnels limit the intercalation of cations.⁶⁹ Birnessite δ-MnO₂, with a 2D tunnel structure doped with potassium, has the advantage of providing a relatively high SC (110 F g⁻¹) with a moderate BET surface area (17 m² g⁻¹). The SC followed is by λ-MnO₂ with a 3D tunnel structure. β- and γ-MnO₂, with 1D tunnel structures, just provide a pseudofaradaic surface capacitance that is correlated with the BET surface area of the crystalline materials.⁶⁹ Different synthesis conditions can lead to various MnO₂ structures. Gradually increasing the precursor acidity can result in a progression from layered birnessite δ-MnO₂, through α-MnO₂ with a relatively large tunnel structure, to a more compact and dense β-MnO₂ phase.⁷⁰

Although the theoretical SC of MnO₂ is pretty high, the practical SC of unmodified MnO₂ is usually lower than 350 F g⁻¹, which is not comparable to RuO₂.^20,71 The SC and rate capability of thick MnO₂ electrode are limited by its poor electric conductivity and electrochemical dissolution during cycling.^32,72 To improve the electric conductivity and charge storage capability, the incorporation of other metal elements into MnO₂ compounds or composing MnO₂ nanostructured composites with well-conductive materials are both feasible ways.^73–77 The former method can introduce more defects and charge carriers to enhance the conductivity.^78–80 A hybrid structure consisting of nanoporous gold and nanocrystalline MnO₂ with an enhanced electric conductivity had a high SC of 1145 F g⁻¹, which is close to the theoretical SC. The nanoporous gold acts not only as a double-layer capacitor, but also as a good electronic/ionic conductor to improve the pseudocapacitive performance of the MnO₂.⁷³ Moreover, Co doping can prevent the dissolution of MnO₂, V doping can inhibit the crystal growth of MnO₂, and Fe doping can improve the cycle stability due to reducing the concentration of unstable Mn³⁺ ions.⁸¹ Based on the latter method, various MnO₂ composites, such as MnO₂/carbon nanotubes,^72,82 MnO₂/graphene,⁸³ MnO₂/carbon nanofibers,⁸⁴ MnO₂/nanoporous gold,⁸⁵ and MnO₂/PANI,⁸⁶ were also fabricated. Nanowire arrays in which a gold core nanowire is encapsulated inside a hemicylindrical shell of MnO₂ show an ultrahigh SC of 1020 ± 100 F g⁻¹, with an extremely poor cycle stability.⁷⁴ However, when the nanowire core/shell arrays are transferred into an ultradry acetonitrile electrolyte, the cycle stability improved dramatically. Recently, Chen et al. fabricated a MnO₂/CNT/sponge composite hybrid electrode, which achieved a remarkable SC of 1230 F g⁻¹, with an energy density of 31 W h kg⁻¹ and a power density of 63 kW kg⁻¹, as well as excellent cycle stability—the SC only degraded about 4% after 10 000 cycles at a current density of 5 A g⁻¹.⁸⁷ Graphene, a unique two-dimensional carbon material, has attracted a lot of attention from researchers recently due to its large surface area, high electric conductivity, outstanding mechanical properties, and good chemical stability. It is considered as one of the best substrates to deposit MnO₂.^83,88 Several kinds of MnO₂/graphene with good capacitance have been prepared.^88–91 A 3D macroporous MnO₂/graphene composite, demonstrating a SC of 389 F g⁻¹, was fabricated by using a sacrificial template of polystyrene colloidal particles.⁹² MnO₂-nanoflowers-coated graphene with a SC of 328 F g⁻¹ was fabricated by electrodeposition.⁹¹ The large specific surface area of graphene facilitates rapid ionic transport within the electrodes, while the excellent conductivity accelerates the redox reactions.

During cycling, the partial dissolution of MnO₂ in even mildly acidic and near-neutral electrolyte, which can be described in eqn (9) and (10), is a serious problem confining the practical application of MnO₂ in supercapacitors.^20,93,94

MnO₂ + H⁺ + e⁻ → MnOOH (9)

MnOOH + 3H⁺ + e⁻ → Mn²⁺ + 2H₂O (10)

To solve this problem, preparing a protective polymer shell on MnO₂ is usually employed by researchers. Various conducting polymer shells (PANI, PPy, PTh, and their derivatives) have been fabricated, which can not only prevent the dissolution of MnO₂, but also improve the mechanical stability and flexibility.^95–99 The PANI/mesoporous carbon/MnO₂ ternary composite synthesized by chemical oxidative polymerization has an 88% capacitance retention after 1000 cycles, and the MnO₂/CNT/PEDOT-PSS composite exhibits an extremely long cycle life (99% retention of SC after 1000 cycles).^100,101 MnO₂/conducting polymer composites also show high SCs. For example, the hierarchical MnO₂/PPy@carbon nanofiber composite reaches a SC of 705 F g⁻¹, and the PANI/mesoporous carbon/MnO₂ ternary nanocomposite possesses a SC of 695 F g⁻¹.^99,100

3.2.2. NiO/Ni(OH)₂. NiO is a promising material for pseudocapacitor electrodes, due to its ultrahigh theoretical SC of 3750 F g⁻¹, low cost and environmental friendliness.^47,102,103 The supercapacitive mechanism of NiO and the oxidation state changes in NiO are controversial. There are two main theories: one considers that the energy storage process occurs between NiO and NiOOH (eqn (11) and (12)); the other one indicates that first NiO changes to Ni(OH)₂ in alkaline electrolyte, then the electrochemical reactions occurs between Ni(OH)₂ and NiOOH (eqn (13) and (14)).^{25–27,104–106} The reactions of these two theories can be expressed as follows:

NiO + OH⁻ ↔ NiOOH + e⁻ (11)

NiO + H₂O ↔ NiOOH + H⁺ + e⁻ (12)

or

Ni(OH)₂ ↔ NiOOH + H⁺ + e⁻ (13)

Ni(OH)₂ + OH⁻ ↔ NiOOH + H₂O + e⁻ (14)

The two theories reach a consensus that Ni²⁺ oxidizes to NiOOH through losing an electron, resulting in the supercapacitive reactions. Most researchers tend to favour the first theory, but the second one is reasonable as well, because NiO can combine with OH⁻ in the alkaline electrolyte to generate Ni(OH)₂, which will contribute to part of the capacitance.

There are two main issues for NiO electrode materials in supercapacitors: (1) NiO is a kind of p-type semiconductor with low electric conductivity; (2) the cycle stability is poor. To address these problems, fabricating nanostructured NiO and composing NiO with other materials are both feasible.

Nanostructured NiO materials can provide a large specific surface area, as well as short diffusion and transport pathways of ions and electrons, resulting in fast reaction kinetics.^27,107–113 Meanwhile, nanostructures can buffer the stress from swelling of the electrodes and inhibit pulverization, to enhance the cycling performance. Porous NiO with macropores from using electrophoresis and electrodeposition have much larger SC (351 F g⁻¹) than bare NiO film (140 F g⁻¹).¹¹⁴ Various NiO nanostructures, such as nanobelts, nanowires, nanorods, nanoplatelets, and nanoflowers, have been fabricated.^115–120 The hierarchically porous NiO films synthesized through different methods also show good electrochemical performance. Xia et al. developed chemical bath deposition with a colloidal crystal template to synthesize a hierarchically porous NiO film, which delivered a SC of 309 F g⁻¹ at 1 A g⁻¹, with a capacitance retention of 89% after 4000 cycles (Fig. 3).²⁷ The structure of the film consists of two parts: the understructure, which is a NiO monolayer hollow-sphere array, and the superstructure, which constitutes net-like NiO nanoflakes. The degradation mechanism of NiO capacitance is mainly attributed to two factors: the self-discharge, due to the partial dissolution of NiOOH and the bubbling effect of oxygen evolution. The enhanced electrochemical capacitive performance is mainly attributed to the large surface area and high porosity of this unique hierarchically porous architecture. Zhang et al. used a facile ammonia-evaporation method to synthesize a hierarchical structure consisting of triangular prism-like NiO and randomly porous NiO nanoflakes.²⁵ This hierarchically porous NiO film exhibits a SC of 232 F g⁻¹ at 2 A g⁻¹, with the SC increasing to 441 F g⁻¹ after around 1500 cycles.

Fig. 3 Morphological and structural characterizations of hierarchically porous NiO film prepared by a 1 mm PS sphere: (a) SEM images of top and side views (side view presented in inset); (b) an enlarged view. Reproduced from ref. 27.

Composing NiO with carbon materials or porous metals can efficiently enhance the electric conductivity and surface area, leading to a high utilization of active materials and an excellent rate capability.^{26,111,121–128} The NiO_x/CNT (carbon nanotube) electrodes prepared by electrochemical deposition show an ultrahigh SC of 1701 F g⁻¹ with 8.9 wt% NiO_x in the composite electrode.¹²⁹ However, when the weight percentage of NiO_x is increased to 36.6%, the SC drops significantly to 671 F g⁻¹. This decrease in SC can be attributed to the larger dead volume of the oxides, high equivalent series resistance for heavier deposits, and a more ineffective ionic transportation resulting from the reduced pore size and pore clogging.

Ni(OH)₂ is a hexagonal-layered structure and has two polymorphs, α- and β-Ni(OH)₂, corresponding to γ- and β-NiOOH after oxidizing.^130–133 α-Ni(OH)₂ is a hydroxyl-deficient phase with interlayered anions and water molecules. β-Ni(OH)₂ possesses a brucite structure without water molecules. The α-Ni(OH)₂/γ-NiOOH couple shows a higher SC than the β-Ni(OH)₂/β-NiOOH couple due to the greater change in valence. The transformation between Ni(OH)₂ and NiOOH is described in Fig. 4. Under the common charging condition, Ni(OH)₂ transforms between β-Ni(OH)₂ and β-NiOOH. When the material is overcharged, β-NiOOH will change to γ-NiOOH, and then α-Ni(OH)₂ after discharging. α-Ni(OH)₂ is not very stable, since it can transform to β-Ni(OH)₂ easily through aging in alkaline electrolyte.^134–136

Fig. 4 Transformation between Ni(OH)₂ and NiOOH.

The theoretical SC of Ni(OH)₂ is about 3650 F g⁻¹, and its practical SC is much higher than NiO.^{132,133,136–138} The powder materials usually show SCs of 500–600 F g⁻¹, while the modified film can even reach an extremely high SC of 3000 F g⁻¹, which is very close to the theoretical SC (Fig. 5).¹³⁹ Porous α-Ni(OH)₂ electrodeposited on Ni foam exhibits a very high SC of 3152 F g⁻¹ at a current density of 4 A g⁻¹, but the SC drops quickly as the current density increases. The amazing SC can be attributed to its loosely crystal structure, which contributes to the easy insertion/extraction of ions and the hydratability of Ni(OH)₂ in favor of electrochemical reactions.¹⁴⁰ The terrible rate capability, due to the poor electric conductivity of Ni(OH)₂, and the poor cycle stability limit the application of Ni(OH)₂.¹⁴⁰ The Ni(OH)₂/carbon composites exhibit good rate capabilities and cycling performance. A Ni(OH)₂/graphene sheet electrode has demonstrated a high SC of 1335 F g⁻¹ at a current density of 2.8 A g⁻¹ and 953 F g⁻¹ at 45.7 A g⁻¹ with good rate capability, and there is no obvious capacitance drop at a current density of 28.6 A g⁻¹.¹⁴¹ α-Ni(OH)₂ nanosheets composited with low defect density CNTs have been fabricated by a one-step hydrothermal method.¹⁴² They delivered a higher SC of 1302.5 F g⁻¹ than their individual components (372.1 F g⁻¹ for Ni(OH)₂ and 101.4 F g⁻¹ for carbon nanotubes), demonstrating the synergistic effects of the hybrid materials to enhance electrochemical performance.

Fig. 5 (a) FESEM photographs of Ni(OH)₂ deposited on the nickel foam branch; (b) an enlarged view. Reproduced from ref. 132.

3.2.3. Co₃O₄/Co(OH)₂. Co₃O₄ is the AB₂O₄ spinel structure belonging to the cubic system. The theoretical SC of Co₃O₄ is approximately 3560 F g⁻¹, and it has better cycle stability than NiO, as well as good corrosion resistance.^22,143–150 The supercapacitive reactions of Co₃O₄ can be described as follows:^22,151

Co₃O₄ + OH⁻ + H₂O ↔ 3CoOOH + e⁻ (15)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ (16)

Co₃O₄ is a p-type semiconductor with low electronic and ionic conductivity, leading to poor rate capability. It has a large volume change, even with pulverization during the cycle process, resulting in a short cycle life.^22,144,152 The nanoscale Co₃O₄ shows a larger SC than bulk Co₃O₄. The Co₃O₄ nanosheet arrays on Ni foam exhibit a SC of 2735 F g⁻¹ by electrodepositing Co(OH)₂ and then undergoing a thermal transformation to Co₃O₄.¹⁵³ The binder-free Co₃O₄ nanowire arrays synthesized by a facial hydrothermal method deliver a SC of 1160 F g⁻¹ at 2 A g⁻¹, and the SC degradation is only 9.6% after 5000 cycles.¹⁵⁴ Xia et al. also synthesized a single-crystalline Co₃O₄ nanowire array through a hydrothermal method (Fig. 6). It shows a SC of 754 F g⁻¹ at 2 A g⁻¹ and a long cycle life, due to the unique 1D nanostructure, which can provide fast ionic diffusion paths, alleviate the expansion stress, and restrain the pulverization of Co₃O₄.²² Gao et al. prepared Co₃O₄ nanowire arrays, displaying a SC of 746 F g⁻¹ at 5 mA cm⁻² by a hydrothermal method.¹⁵⁵ However, the capacitance loss is about 14% after 500 cycles. The Co₃O₄ nanotubes were prepared by chemically depositing Co(OH)₂ into anodic aluminum oxide templates with heat treatment at 500 °C.¹⁵⁶ This unique architecture displays a good supercapacitive performance, with a SC of 574 F g⁻¹ at 0.1 A g⁻¹ and 95% capacitance retention after 1000 cycles. Composites can also significantly enhance supercapacitive performance.^157–162 Co₃O₄ nanowires fabricated on 3D graphene foam grown by the CVD method show a SC of ≈1100 F g⁻¹ at 10 A g⁻¹, and with good cycle stability.¹⁶³ A facile one-step hydrothermal treatment is employed to synthesize reduced graphene oxide (rGO)/Co₃O₄ composites. The specific surface area increases from 42.65 to 124.89 m² g⁻¹, and the SC also increases from 22.6 to 263.0 F g⁻¹ at 0.2 A g⁻¹ with the increment of the mass ratio of rGO/Co₃O₄ from 0 to 1 : 2.¹⁵⁷ Liang et al. incorporated Co₃O₄ into 1D nanoporous carbons through a controlled thermolysis of organometallic precursors.¹⁶⁴ The SC of this composite can reach as high as 1066 F g⁻¹.¹³⁷

Fig. 6 (a) SEM images of Co₃O₄ nanowire array grown on nickel foam (magnified top view presented in insets); (b) cycling performance of the Co₃O₄ nanowire array at a current density of 2 A g⁻¹. Reproduced from ref. 22.

Co(OH)₂ has similar properties and structure to Ni(OH)₂, which is also a hexagonal-layered structure. It can also be divided into α- and β-Co(OH)₂, where α-Co(OH)₂ has a better supercapacitive performance than β-Co(OH)₂. The reactions can be expressed by eqn (17) and then (16):^164–170

Co(OH)₂ + OH⁻ ↔ CoOOH + H₂O + e⁻ (17)

The theoretical SC of Co(OH)₂ is about 3600–3700 F g⁻¹, and also the practical SC can reach a high value.^171–173 Its shortcomings are still poor rate capability and cycle stability. β-Co(OH)₂ nanoflakes potentiodynamically deposited on stainless steel provide a SC of 890 F g⁻¹ and a capacitance retention of 84% after 10 000 cycles.¹⁷⁴ Material possessing one of the highest SC was obtained by Zhou et al.¹⁷⁵ They electrodeposited mesoporous Co(OH)₂ on Ni foam, which then exhibited a maximum SC of 2646 F g⁻¹. However, when they used Ti plate as the substrate to replace Ni foam, the film only demonstrated a SC of 1018 F g⁻¹ because of the smaller specific surface area than Ni foam. The Co(OH)₂ nanoflakes electrodeposited on 3D porous nano-Ni also exhibited a high SC of 2028 F g⁻¹ at 2 A g⁻¹.³⁰ It is important that the film also demonstrates an ultrahigh SC of 1920 F g⁻¹ at 40 A g⁻¹ and maintains 94.7% capacitance after 2000 cycles.

However, Co₃O₄/Co(OH)₂ and NiO/Ni(OH)₂ have a similar drawback, in that their potential window is low, which restricts their practical application.

3.2.4. Iron oxides (Fe₂O₃ and Fe₃O₄). Iron oxides have a relatively high electric conductivity (≈200 S cm⁻¹ for Fe₃O₄) compared to other metal oxides.^32,176,177 However, the low SC of iron oxides limits their practical application. Highly ordered α-Fe₂O₃ nanotube arrays show a SC of 138 F g⁻¹ with 89% capacitance retention after 500 cycles; and octadecahedron Fe₃O₄ thin film exhibits a SC of 118.2 F g⁻¹ with 88.75% capacitance retention after 500 cycles.^178,179 Their cycle stability is not very good yet. Wang et al. studied the capacitive mechanisms of Fe₃O₄ in Na₂SO₃, Na₂SO₄ and KOH aqueous solutions.¹⁸⁰ Strong specific adsorption of the anion species was observed in all solutions. In Na₂SO₃ solution, the capacitive current is from both the EDLC and the pseudocapacitance; in Na₂SO₄ solution, the capacitive current is entirely from EDLC; in KOH solution, the surface oxidation leads to the formation of an insulating layer, resulting in a significant low SC. Composing iron oxides with other materials is a promising way to improve the supercapacitor performance. A rGO/Fe₂O₃ composite delivers a SC of 480 F g⁻¹ and a graphene/Fe₂O₃/PANI composite delivers a SC of 638 F g⁻¹ with only 8% capacitance loss after 5000 cycles.^181,182 The CNT/Fe₃O₄ composite provides a SC of 165 F g⁻¹ and 85% capacitance retention after 1000 cycles.¹⁸³

However, iron oxides are not ideal materials for supercapacitors, until researchers can solve their poor capacitance and cycling stability.

3.2.5. Binary metal oxides. Recently, binary metal oxides with spinel structures have attracted a lot of attention (Fig. 7), such as NiCo₂O₄,¹⁸⁴ NiFe₂O₄,¹⁸⁵ CoFe₂O₄,¹⁸⁶ ZnMnO₄,^187,188 ZnCo₂O₄^189,190 and CoMn₂O₄.¹⁹¹ Some articles report that these binary metal oxides have higher electrical conductivity than unitary metal oxides and contain both components' contributions to the total capacitance, which result in better electrochemical performance than individual components.^192,193 The hierarchical NiCo₂O₄ @NiCo₂O₄ core/shell nanoflake arrays synthesized by a hydrothermal process and chemical bath deposition show a SC of 1115.6 F g⁻¹ and a 98.6% capacitance retention after 4000 cycles.¹⁹⁴ 3D flower-like NiCo₂O₄ hierarchitectures prepared by the solvothermal method exhibit a high SC of 1191.2 F g⁻¹ and 755.2 F g⁻¹ at current densities of 1 A g⁻¹ and 10 A g⁻¹.¹⁹⁵ However, they have a poor cycling performance, since the charge transfer resistance of the electrode materials increases during the cycling process. 1D MnCo₂O₄ nanowire arrays synthesized by a hydrothermal method demonstrate a high SC of 349.8 F g⁻¹ at 1 A g⁻¹, and with excellent cycle stability.¹⁹⁶ Kuo et al. reported that MnFe₂O₄ shows an unusually large capacitance, while the other ferrites (Fe, Co and Ni) do not.¹⁹⁷ In their report, the pseudocapacitance is observed only for the crystalline, instead of amorphous form.

Fig. 7 Spinel structure (M represents the metal atom).

3.2.6. Other metal oxides. In addition to the above-mentioned materials, materials such as SnO,^198,199 V₂O₅,^200,201 Bi₂O₃,^202,203 MoO₂^204,205 and TiO₂,^206,207 have also been extensively studied by researchers. Sb-doped SnO₂ nanocrystallites fabricated through a sol–gel method show a SC of only 16 F g⁻¹.¹⁹⁹ Electrodeposited Bi₂O₃ thin films grown on copper substrate exhibit a maximum SC of 98 F g⁻¹.²⁰² 1D MoO₂ nanorods synthesized by the thermal decomposition of tetrabutylammonium hexamolybdate (((C₄H₉)₄N)₂Mo₆O₁₉) in an inert atmosphere show a SC of 140 F g⁻¹.²⁰⁴ From the above results, we can see that these materials usually have low capacitance or some other drawbacks and are impractical nowadays. However, in developing these materials, pioneers are finding new ways to build supercapacitors. They may become promising supercapacitive materials if researchers make breakthroughs in solving their shortcomings.

4. Conclusions

As a kind of electrochemical energy storage device, supercapacitors play an important role in our society. The behaviour of supercapacitors mainly depends on the electrode materials. We have reviewed various transition metal oxides/hydroxides as electrode materials of supercapacitors, due to their high capacitance, as well as high energy density with faradaic reactions. Properties, such as a large surface area, excellent electric conductivity and short diffusion paths for ions and electrons, lead to high specific capacitance and good rate capability. Thus, developing nanostructured materials and composing metal oxides/hydroxides can efficiently enhance supercapacitive performance. Moreover, these two methods are able to alleviate the stress from the swelling and shrinkage of the electrodes during charge/discharge processes and reduce the partial dissolution of some metal oxides/hydroxides in the electrolyte, which together can improve the cycling stability. In addition to the excellent electrochemical performance, many other factors of electrode materials should be taken into consideration, such as price, resource accessibility, and impact on the environment. If we combine all the strategies above, it should be much easier to fabricate high-performance electrode materials for supercapacitors in practical applications.

Acknowledgements

This work was supported by the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037) and Key Science and Technology Innovation Team of Zhejiang Province (2010R50013).

Notes and references

1. D. G. Nocera, Chem. Soc. Rev., 2009, 38, 13–15.
2. S. Shafiee and E. Topal, Energy Policy, 2009, 37, 181–189.
3. L. L. Zhang and X. S. Zhao, Chem. Soc. Rev., 2009, 38, 2520–2531.
4. C. Largeot, C. Portet, J. Chmiola, P.-L. Taberna, Y. Gogotsi and P. Simon, J. Am. Chem. Soc., 2008, 130, 2730–2731.
5. S. G. Kandalkar, D. S. Dhawale, C.-K. Kim and C. D. Lokhande, Synth. Met., 2010, 160, 1299–1302.
6. G. P. Wang, L. Zhang and J. J. Zhang, Chem. Soc. Rev., 2012, 41, 797–828.
7. P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854.
8. J. R. Miller and P. Simon, Science, 2008, 321, 651–652.
9. D. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, P.-L. Taberna and P. Simon, Nat. Nanotechnol., 2010, 5, 651–654.
10. P.-C. Chen, G. Z. Shen, Y. Shi, H. T. Chen and C. W. Zhou, ACS Nano, 2010, 4, 4403–4411.
11. J. Yan, Z. J. Fan, W. Sun, G. Q. Ning, T. Wei, Q. Zhang, R. F. Zhang, L. J. Zhi and F. Wei, Adv. Funct. Mater., 2012, 22, 2632–2641.
12. X. H. Xia, D. L. Chao, Z. X. Fan, C. Guan, X. H. Cao, H. Zhang and H. J. Fan, Nano Lett., 2014, 14, 1651–1658.
13. B. E. Conway, Electrochemical supercapacitors: Scientific fundamentals and technological applications, Kluwer Academic/Plenum, New York, 1999.
14. A. G. Pandolfo and A. F. Hollenkamp, J. Power Sources, 2006, 157, 11–27.
15. Y. Zhang, H. Feng, X. B. Wu, L. Z. Wang, A. Q. Zhang, T. C. Xia, H. C. Dong, X. F. Li and L. S. Zhang, Int. J. Hydrogen Energy, 2009, 34, 4889–4899.
16. K. Wang, H. P. Wu, Y. N. Meng and Z. X. Wei, Small, 2014, 10, 14–31.
17. S. Liu, S. H. Sun and X. Z. You, Nanoscale, 2014, 6, 2037–2045.
18. N. Soin, S. S. Roy, S. K. Mitra, T. Thundat and J. A. McLaughlin, J. Mater. Chem., 2012, 22, 14944–14950.
19. Y. M. Chen, J. H. Cai, Y. S. Huang, K. Y. Lee and D. S. Tsai, Nanotechnology, 2011, 22, 115706.
20. W. F. Wei, X. W. Cui, W. X. Chen and D. G. Ivey, Chem. Soc. Rev., 2011, 40, 1697–1721.
21. Y. W. Cheng, S. T. Lu, H. B. Zhang, C. V. Varanasi and J. Liu, Nano Lett., 2012, 12, 4206–4211.
22. X. H. Xia, J. P. Tu, Y. Q. Zhang, Y. J. Mai, X. L. Wang, C. D. Gu and X. B. Zhao, RSC Adv., 2012, 2, 1835–1841.
23. X. H. Xia, J. P. Tu, X. L. Wang, C. D. Gu and X. B. Zhao, Chem. Commun., 2011, 47, 5786–5788.
24. X. H. Xia, J. P. Tu, Y. J. Mai, X. L. Wang, C. D. Gu and X. B. Zhao, J. Mater. Chem., 2011, 21, 9319–9325.
25. Y. Q. Zhang, X. H. Xia, J. P. Tu, Y. J. Mai, S. J. Shi, X. L. Wang and C. D. Gu, J. Power Sources, 2012, 199, 413–417.
26. X. H. Xia, J. P. Tu, Y. J. Mai, R. Chen, X. L. Wang, C. D. Gu and X. B. Zhao, Chem.–Eur. J., 2011, 17, 10898–10905.
27. X. H. Xia, J. P. Tu, X. L. Wang, C. D. Gu and X. B. Zhao, J. Mater. Chem., 2011, 21, 671–679.
28. T. Xue, X. Wang and J.-M. Lee, J. Power Sources, 2012, 201, 382–386.
29. J. Jiang, J. P. Liu, R. M. Ding, J. H. Zhu, Y. Y. Li, A. Z. Hu, X. Li and X. T. Huang, ACS Appl. Mater. Interfaces, 2011, 3, 99–103.
30. X. H. Xia, J. P. Tu, Y. Q. Zhang, Y. J. Mai, X. L. Wang, C. D. Gu and X. B. Zhao, J. Phys. Chem. C, 2011, 115, 22662–22668.
31. J. Yan, W. Sun, T. Wei, Q. Zhang, Z. J. Fan and F. Wei, J. Mater. Chem., 2012, 22, 11494–11502.
32. J. Yan, Q. Wang, T. Wei and Z. J. Fan, Adv. Energy Mater., 2014, 4, 1300816.
33. J. T. Zhang, J. W. Jiang, H. L. Li and X. S. Zhao, Energy Environ. Sci., 2011, 4, 4009–4015.
34. H. L. Wang, Y. Y. Liang, T. Mirfakhrai, Z. Chen, H. S. Casalongue and H. J. Dai, Nano Res., 2011, 4, 729–736.
35. Z.-S. Wu, D.-W. Wang, W. C. Ren, J. P. Zhao, G. M. Zhou, F. Li and H.-M. Cheng, Adv. Funct. Mater., 2010, 20, 3595–3602.
36. I. H. Kim and K. B. Kim, J. Electrochem. Soc., 2006, 153, A383–A389.
37. Y. F. Su, F. Wu, L. Y. Bao and Z. H. Yang, New Carbon Mater., 2007, 22, 53–58.
38. K. E. Swider, C. I. Merzbacher, P. L. Hagans and D. R. Rolison, Chem. Mater., 1997, 9, 1248–1255.
39. J. T. Zhang, J. Z. Ma, L. L. Zhang, P. Z. Guo, J. W. Jiang and X. S. Zhao, J. Phys. Chem. C, 2010, 114, 13608–13613.
40. J. W. Long, K. E. Swider, C. I. Merzbacher and D. R. Rolison, Langmuir, 1999, 15, 780–785.
41. J. P. Zheng, P. J. Cygan and T. R. Jow, J. Electrochem. Soc., 1995, 142, 2699–2703.
42. K. Kuratani, T. Kiyobayashi and N. Kuriyama, J. Power Sources, 2009, 189, 1284–1291.
43. V. D. Patake, S. M. Pawar, V. R. Shinde, T. P. Gujar and C. D. Lokhande, Curr. Appl. Phys., 2010, 10, 99–103.
44. Z. R. Cormier, H. A. Andrea and P. Zhang, J. Phys. Chem. C, 2011, 115, 19117–19128.
45. H. Kim and B. N. Popov, J. Power Sources, 2002, 104, 52–61.
46. M. K. Seo, A. Saouab and S. J. Park, Mater. Sci. Eng., B, 2010, 167, 65–69.
47. C. Liu, F. Li, L.-P. Ma and H.-M. Cheng, Adv. Mater., 2010, 22, E28–E62.
48. W. Sugimoto, K. Yokoshima, Y. Murakami and Y. Takasu, Electrochim. Acta, 2006, 52, 1742–1748.
49. A. Ananth, M. S. Gandhi and Y. S. Mok, J. Phys. D: Appl. Phys., 2013, 46, 155202.
50. Y. F. Ke, D. S. Tsai and Y. S. Huang, J. Mater. Chem., 2005, 15, 2122–2127.
51. J. Y. Kim, K. H. Kim, H. K. Kim, S. H. Park, K. Y. Chung and K. B. Kim, RSC Adv., 2014, 4, 16115–16120.
52. W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami and Y. Takasu, Angew. Chem., Int. Ed., 2003, 42, 4092–4096.
53. C.-C. Hu, K.-H. Chang, M.-C. Lin and Y.-T. Wu, Nano Lett., 2006, 6, 2690–2695.
54. T. S. Hyun, J. E. Kang, H. G. Kim, J. M. Hong and I. D. Kim, Electrochem. Solid State Lett., 2009, 12, A225–A228.
55. Y. H. Li, K. L. Huang, D. M. Zeng, S. Q. Liu and Z. F. Yao, J. Solid State Electrochem., 2010, 14, 1205–1211.
56. X. M. Liu and X. G. Zhang, Electrochim. Acta, 2004, 49, 229–232.
57. F. Pico, J. Ibanez, T. A. Centeno, C. Pecharroman, R. M. Rojas, J. M. Amarilla and J. M. Rojo, Electrochim. Acta, 2006, 51, 4693–4700.
58. Y. G. Wang and X. G. Zhang, Electrochim. Acta, 2004, 49, 1957–1962.
59. C. C. Hu, K. H. Chang and C. C. Wang, Electrochim. Acta, 2007, 52, 4411–4418.
60. C. C. Hu and W. C. Chen, Electrochim. Acta, 2004, 49, 3469–3477.
61. L.-M. Huang, H.-Z. Lin, T.-C. Wen and A. Gopalan, Electrochim. Acta, 2006, 52, 1058–1063.
62. C. J. Zhang, H. H. Zhou, X. Q. Yu, D. Shan, T. T. Ye, Z. Y. Huang and Y. F. Kuang, RSC Adv., 2014, 4, 11197–11205.
63. Z. Zhou, Y. R. Zhu, Z. B. Wu, F. Lu, M. J. Jing and X. B. Ji, RSC Adv., 2014, 4, 6927–6932.
64. B. K. Balan, H. D. Chaudhari, U. K. Kharul and S. Kurungot, RSC Adv., 2013, 3, 2428–2436.
65. C. C. Hu, Y. L. Yang and T. C. Lee, Electrochem. Solid State Lett., 2010, 13, A173–A176.
66. T.-F. Hsieh, C.-C. Chuang, W.-J. Chen, J.-H. Huang, W.-T. Chen and C.-M. Shu, Carbon, 2012, 50, 1740–1747.
67. M. Toupin, T. Brousse and D. Belanger, Chem. Mater., 2004, 16, 3184–3190.
68. J. X. Zhu, W. H. Shi, N. Xiao, X. H. Rui, H. T. Tan, X. H. Lu, H. H. Hng, J. Ma and Q. Y. Yan, ACS Appl. Mater. Interfaces, 2012, 4, 2769–2774.
69. T. Brousse, M. Toupin, R. Dugas, L. Athouel, O. Crosnier and D. Belanger, J. Electrochem. Soc., 2006, 153, A2171–A2180.
70. S. W. Donne, A. F. Hollenkamp and B. C. Jones, J. Power Sources, 2010, 195, 367–373.
71. J. F. Zang and X. D. Li, J. Mater. Chem., 2011, 21, 10965–10969.
72. J. M. Ko and K. M. Kim, Mater. Chem. Phys., 2009, 114, 837–841.
73. X. Lang, A. Hirata, T. Fujita and M. W. Chen, Nat. Nanotechnol., 2011, 6, 232–236.
74. W. B. Yan, J. Y. Kim, W. D. Xing, K. C. Donavan, T. Ayvazian and R. M. Penner, Chem. Mater., 2012, 24, 2382–2390.
75. S. Hassan, M. Suzuki, S. Mori and A. Abd El-Moneim, RSC Adv., 2014, 4, 20479–20488.
76. G. Q. Han, Y. Liu, E. J. Kan, J. Tang, L. L. Zhang, H. H. Wang and W. H. Tang, RSC Adv., 2014, 4, 9898–9904.
77. Y. Gao, Y. S. Zhou, M. Qian, H. M. Li, J. Redepenning, L. S. Fan, X. N. He, W. Xiong, X. Huang, M. Majhouri-Samani, L. Jiang and Y. F. Lu, RSC Adv., 2013, 3, 20613–20618.
78. J. Yan, E. Khoo, A. Sumboja and P. S. Lee, ACS Nano, 2010, 4, 4247–4255.
79. J. P. Liu, J. Jiang, C. W. Cheng, H. X. Li, J. X. Zhang, H. Gong and H. J. Fan, Adv. Mater., 2011, 23, 2076–2081.
80. D. F. Yang, J. Power Sources, 2012, 198, 416–422.
81. H. N. Yoo, D. H. Park and S.-J. Hwang, J. Power Sources, 2008, 185, 1374–1379.
82. A. L. M. Reddy, M. M. Shaijumon, S. R. Gowda and P. M. Ajayan, J. Phys. Chem. C, 2010, 114, 658–663.
83. L. J. Deng, G. Zhu, J. F. Wang, L. P. Kang, Z. H. Liu, Z. P. Yang and Z. L. Wang, J. Power Sources, 2011, 196, 10782–10787.
84. J. G. Wang, Y. Yang, Z. H. Huang and F. Y. Kang, Electrochim. Acta, 2011, 56, 9240–9247.
85. X. Y. Lang, H. T. Yuan, Y. Iwasa and M. W. Chen, Scr. Mater., 2011, 64, 923–926.
86. Jaidev, R. I. Jafri, A. K. Mishra and S. Ramaprabhu, J. Mater. Chem., 2011, 21, 17601–17605.
87. W. Chen, R. B. Rakhi, L. B. Hu, X. Xie, Y. Cui and H. N. Alshareef, Nano Lett., 2011, 11, 5165–5172.
88. J. Yan, Z. J. Fan, T. Wei, W. Z. Qian, M. L. Zhang and F. Wei, Carbon, 2010, 48, 3825–3833.
89. Z. P. Li, J. Q. Wang, S. Liu, X. H. Liu and S. R. Yang, J. Power Sources, 2011, 196, 8160–8165.
90. Z. S. Wu, W. C. Ren, D. W. Wang, F. Li, B. L. Liu and H. M. Cheng, ACS Nano, 2010, 4, 5835–5842.
91. Q. Cheng, J. Tang, J. Ma, H. Zhang, N. Shinya and L. C. Qin, Carbon, 2011, 49, 2917–2925.
92. B. G. Choi, M. Yang, W. H. Hong, J. W. Choi and Y. S. Huh, ACS Nano, 2012, 6, 4020–4028.
93. S. Bakardjieva, P. Bezdicka, T. Grygar and P. Vorm, J. Solid State Electrochem., 2000, 4, 306–313.
94. S. Nijjer, J. Thonstad and G. M. Haarberg, Electrochim. Acta, 2000, 46, 395–399.
95. Q. Lu and Y. K. Zhou, Funct. Mater. Lett., 2011, 4, 31–36.
96. Q. Lu and Y. K. Zhou, J. Power Sources, 2011, 196, 4088–4094.
97. G. H. Yu, L. B. Hu, N. Liu, H. L. Wang, M. Vosgueritchian, Y. Yang, Y. Cui and Z. N. Bao, Nano Lett., 2011, 11, 4438–4442.
98. H. Jiang, J. Ma and C. Z. Li, J. Mater. Chem., 2012, 22, 16939–16942.
99. J.-G. Wang, Y. Yang, Z.-H. Huang and F. Kang, J. Mater. Chem., 2012, 22, 16943–16949.
100. Y. F. Yan, Q. L. Cheng, V. Pavlinek, P. Saha and C. Z. Li, Electrochim. Acta, 2012, 71, 27–32.
101. Y. Hou, Y. W. Cheng, T. Hobson and J. Liu, Nano Lett., 2010, 10, 2727–2733.
102. F. B. Zhang, Y. K. Zhou and H. L. Li, Mater. Chem. Phys., 2004, 83, 260–264.
103. M. S. Wu, Y. A. Huang, J. J. Jow, W. D. Yang, C. Y. Hsieh and H. M. Tsai, Int. J. Hydrogen Energy, 2008, 33, 2921–2926.
104. V. Srinivasan and J. W. Weidner, J. Electrochem. Soc., 1997, 144, L210–L213.
105. V. Srinivasan and J. W. Weidner, J. Electrochem. Soc., 2000, 147, 880–885.
106. X. H. Xia, J. P. Tu, J. Zhang, X. L. Wang, W. K. Zhang and H. Huang, Sol. Energy Mater. Sol. Cells, 2008, 92, 628–633.
107. Y. Zhang, Y. G. Gui, X. B. Wu, H. Feng, A. Q. Zhang, L. Z. Wang and T. C. Xia, Int. J. Hydrogen Energy, 2009, 34, 2467–2470.
108. X. M. Liu, X. G. Zhang and S. Y. Fu, Mater. Res. Bull., 2006, 41, 620–627.
109. K. R. Prasad and N. Miura, Appl. Phys. Lett., 2004, 85, 4199–4201.
110. B. R. Tao, J. A. Zhang, F. J. Miao, S. C. Hui and L. J. Wan, Electrochim. Acta, 2010, 55, 5258–5262.
111. J. B. Wu, Z. G. Li and Y. Lin, Electrochim. Acta, 2011, 56, 2116–2121.
112. X. Ge, C. D. Gu, Y. Lu, X. L. Wang and J. P. Tu, J. Mater. Chem. A, 2013, 1, 13454–13461.
113. M. Khairy and S. A. El-Safty, RSC Adv., 2013, 3, 23801–23809.
114. M. S. Wu, M. J. Wang and J. J. Jow, J. Power Sources, 2010, 195, 3950–3955.
115. B. Wang, J. S. Chen, Z. Wang, S. Madhavi and X. W. Lou, Adv. Energy Mater., 2012, 2, 1188–1192.
116. H. Pang, Q. Y. Lu, Y. Z. Zhang, Y. C. Li and F. Gao, Nanoscale, 2010, 2, 920–922.
117. Z. Y. Lu, Z. Chang, J. F. Liu and X. M. Sun, Nano Res., 2011, 4, 658–665.
118. C. Z. Yuan, J. Y. Li, L. R. Hou, L. Yang, L. F. Shen and X. G. Zhang, Electrochim. Acta, 2012, 78, 532–538.
119. C.-Y. Cao, W. Guo, Z.-M. Cui, W.-G. Song and W. Cai, J. Mater. Chem., 2011, 21, 3204–3209.
120. X. H. Xia, J. P. Tu, J. Zhang, X. L. Wang, W. K. Zhang and H. Huang, Electrochim. Acta, 2008, 53, 5721–5724.
121. Q. Lu, M. W. Lattanzi, Y. Chen, X. Kou, W. Li, X. Fan, K. M. Unruh, J. G. Chen and J. Q. Xiao, Angew. Chem., Int. Ed., 2011, 50, 6847–6850.
122. W. Lv, F. Sun, D.-M. Tang, H.-T. Fang, C. Liu, Q.-H. Yang and H.-M. Cheng, J. Mater. Chem., 2011, 21, 9014–9019.
123. X. H. Cao, Y. M. Shi, W. H. Shi, G. Lu, X. Huang, Q. Y. Yan, Q. C. Zhang and H. Zhang, Small, 2011, 7, 3163–3168.
124. B. Gao, C. Z. Yuan, L. H. Su, S. Y. Chen and X. G. Zhang, Electrochim. Acta, 2009, 54, 3561–3567.
125. M. L. Huang, C. D. Gu, X. Ge, X. L. Wang and J. P. Tu, J. Power Sources, 2014, 259, 98–105.
126. Z. Y. Ji, X. P. Shen, Y. L. Xu, H. Zhou, S. Bai and G. X. Zhu, RSC Adv., 2014, 4, 13601–13609.
127. M. E. Plonska-Brzezinska, D. M. Brus, A. Molina-Ontoria and L. Echegoyen, RSC Adv., 2013, 3, 25891–25901.
128. Y. G. Zhu, G. S. Cao, C. Y. Sun, J. Xie, S. Y. Liu, T. J. Zhu, X. B. Zhao and H. Y. Yang, RSC Adv., 2013, 3, 19409–19415.
129. K. W. Nam, K. H. Kim, E. S. Lee, W. S. Yoon, X. Q. Yang and K. B. Ki, J. Power Sources, 2008, 182, 642–652.
130. R. S. Jayashree and P. V. Kamath, J. Appl. Electrochem., 2001, 31, 1315–1320.
131. J. T. Li, W. Zhao, F. Q. Huang, A. Manivannan and N. Q. Wu, Nanoscale, 2011, 3, 5103–5109.
132. Q. L. Xian and J. A. Li, J. Inorg. Mater., 2010, 25, 1268–1272.
133. U. M. Patil, K. V. Gurav, V. J. Fulari, C. D. Lokhande and O. S. Joo, J. Power Sources, 2009, 188, 338–342.
134. Y. Ren and L. A. Gao, J. Am. Ceram. Soc., 2010, 93, 3560–3564.
135. M. C. Bernard, P. Bernard, M. Keddam, S. Senyarich and H. Takenouti, Electrochim. Acta, 1996, 41, 91–93.
136. H. M. Du, L. F. Jiao, K. Z. Cao, Y. J. Wang and H. T. Yuan, ACS Appl. Mater. Interfaces, 2013, 5, 6643–6648.
137. J. W. Lang, L. B. Kong, M. Liu, Y. C. Luo and L. Kang, J. Solid State Electrochem., 2010, 14, 1533–1539.
138. A. K. Mondal, D. W. Su, S. Q. Chen, B. Sun, K. F. Li and G. X. Wang, RSC Adv., 2014, 4, 19476–19481.
139. G. W. Yang, C. L. Xu and H. L. Li, Chem. Commun., 2008, 6537–6539.
140. H. Jiang, C. Z. Li, T. Sun and J. Ma, Chem. Commun., 2012, 48, 2606–2608.
141. H. L. Wang, H. S. Casalongue, Y. Y. Liang and H. J. Dai, J. Am. Chem. Soc., 2010, 132, 7472–7477.
142. S. Chen, J. W. Zhu, H. Zhou and X. Wang, RSC Adv., 2011, 1, 484–489.
143. H. Cheng, Z. G. Lu, J. Q. Deng, C. Y. Chung, K. L. Zhang and Y. Y. Li, Nano Res., 2010, 3, 895–901.
144. Q. Yang, Z. Y. Lu, Z. Chang, W. Zhu, J. Q. Sun, J. F. Liu, X. M. Sun and X. Duan, RSC Adv., 2012, 2, 1663–1668.
145. X. X. Qing, S. Q. Liu, K. L. Huang, K. Z. Lv, Y. P. Yang, Z. G. Lu, D. Fang and X. X. Liang, Electrochim. Acta, 2011, 56, 4985–4991.
146. H.-K. Kim, T.-Y. Seong, J.-H. Lim, W. I. Cho and Y. S. Yoon, J. Power Sources, 2001, 102, 167–171.
147. V. Srinivasan and J. W. Weidner, J. Power Sources, 2002, 108, 15–20.
148. P. J. Kulesza, S. Zamponi, M. A. Malik, M. Berrettoni, A. Wolkiewicz and R. Marassi, Electrochim. Acta, 1997, 43, 919–923.
149. Z. Y. Xun, C. X. Cai, W. Xing and T. H. Lu, J. Electroanal. Chem., 2003, 545, 19–27.
150. Y. Q. Zhang, L. Li, S. J. Shi, Q. Q. Xiong, X. Y. Zhao, X. L. Wang, C. D. Gu and J. P. Tu, J. Power Sources, 2014, 256, 200–205.
151. Y. Shan and L. Gao, Mater. Chem. Phys., 2007, 103, 206–210.
152. H. Wang, C. M. Holt, Z. Li, X. Tan, B. S. Amirkhiz, Z. Xu, B. C. Olsen, T. Stephenson and D. Mitlin, Nano Res., 2012, 5, 605–617.
153. C. Z. Yuan, L. Yang, L. R. Hou, L. F. Shen, X. G. Zhang and X. W. Lou, Energy Environ. Sci., 2012, 5, 7883–7887.
154. F. Zhang, C. Z. Yuan, X. J. Lu, L. J. Zhang, Q. Che and X. G. Zhang, J. Power Sources, 2012, 203, 250–256.
155. Y. Y. Gao, S. L. Chen, D. X. Cao, G. L. Wang and J. L. Yin, J. Power Sources, 2010, 195, 1757–1760.
156. J. Xu, L. Gao, J. Y. Cao, W. C. Wang and Z. D. Chen, Electrochim. Acta, 2010, 56, 732–736.
157. G. J. Liu, L. Q. Fan, F. D. Yu, J. H. Wu, L. Liu, Z. Y. Qiu and Q. Liu, J. Mater. Sci., 2013, 48, 8463–8470.
158. R. T. Wang, X. B. Yan, J. W. Lang, Z. M. Zheng and P. Zhang, J. Mater. Chem. A, 2014, 2, 12724–12732.
159. L. Yang, S. Cheng, Y. Ding, X. B. Zhu, Z. L. Wang and M. L. Liu, Nano Lett., 2012, 12, 321–325.
160. C. Z. Yuan, L. Yang, L. R. Hou, J. Y. Li, Y. X. Sun, X. G. Zhang, L. F. Shen, X. J. Lu, S. L. Xiong and X. W. Lou, Adv. Funct. Mater., 2012, 22, 2560–2566.
161. J. W. Lang, X. B. Yan and Q. J. Xue, J. Power Sources, 2011, 196, 7841–7846.
162. J. Yan, T. Wei, W. M. Qiao, B. Shao, Q. K. Zhao, L. J. Zhang and Z. J. Fan, Electrochim. Acta, 2010, 55, 6973–6978.
163. X.-C. Dong, H. Xu, X.-W. Wang, Y.-X. Huang, M. B. Chan-Park, H. Zhang, L.-H. Wang, W. Huang and P. Chen, ACS Nano, 2012, 6, 3206–3213.
164. Y. Liang, M. G. Schwab, L. Zhi, E. Mugnaioli, U. Kolb, X. Feng and K. Muellen, J. Am. Chem. Soc., 2010, 132, 15030–15037.
165. D. T. Dam, X. Wang and J.-M. Lee, RSC Adv., 2012, 2, 10512–10518.
166. Z. J. Yu, Y. Dai and W. Chen, J. Chin. Chem. Soc., 2010, 57, 423–428.
167. V. Gupta, T. Kusahara, H. Toyama, S. Gupta and N. Miura, Electrochem. Commun., 2007, 9, 2315–2319.
168. X. Ge, C. D. Gu, X. L. Wang and J. P. Tu, J. Phys. Chem. C, 2014, 118, 911–923.
169. X. H. Xia, J. P. Tu, Y. Q. Zhang, J. Chen, X. L. Wang, C. D. Gu, C. Guan, J. S. Luo and H. J. Fan, Chem. Mater., 2012, 24, 3793–3799.
170. Y. Q. Zhang, X. H. Xia, J. Kang and J. P. Tu, Chinese Sci. Bullet., 2012, 57, 4215–4219.
171. J. A. Jiang, J. P. Liu, R. M. Ding, J. H. Zhu, Y. Y. Li, A. Z. Hu, X. Li and X. T. Huang, ACS Appl. Mater. Interfaces, 2011, 3, 99–103.
172. Y. L. Chen, Z. A. Hu, Y. Q. Chang, H. W. Wang, G. R. Fu, X. Q. Jin and L. J. Xie, Chin. J. Chem., 2011, 2011(29), 2257–2262.
173. L. J. Xie, X. Q. Jin, G. R. Fu, Y. L. Xie, Y. X. Wang and Z. A. Hu, Chem. J. Chin. Univ., 2010, 31, 353–356.
174. A. D. Jagadale, V. S. Jamadade, S. N. Pusawale and C. D. Lokhande, Electrochim. Acta, 2012, 78, 92–97.
175. W.-J. Zhou, M.-W. Xu, D.-D. Zhao, C.-L. Xu and H.-L. Li, Microporous Mesoporous Mater., 2009, 117, 55–60.
176. X. H. Xia, Y. Q. Zhang, D. L. Chao, G. Cao, Y. J. Zhang, L. Li, X. Ge, I. M. Bacho, J. P. Tu and H. J. Fan, Nanoscale, 2014, 6, 5008–5048.
177. Q. Q. Xiong, J. P. Tu, Y. Lu, J. Chen, Y. X. Yu, X. L. Wang and C. D. Gu, J. Mater. Chem., 2012, 22, 18639–18645.
178. K. Y. Xie, J. Li, Y. Q. Lai, W. Lu, Z. A. Zhang, Y. X. Liu, L. M. Zhou and H. T. Huang, Electrochem. Commun., 2011, 13, 657–660.
179. J. Chen, K. L. Huang and S. Q. Liu, Electrochim. Acta, 2009, 55, 1–5.
180. S.-Y. Wang, K.-C. Ho, S.-L. Kuo and N.-L. Wu, J. Electrochem. Soc., 2006, 153, A75–A80.
181. W. H. Shi, J. X. Zhu, D. H. Sim, Y. Y. Tay, Z. Y. Lu, X. Zhang, Y. Sharma, M. Srinivasan, H. Zhang, H. H. Hng and Q. Y. Yan, J. Mater. Chem., 2011, 21, 3422–3427.
182. X. F. Xia, Q. L. Hao, W. Lei, W. J. Wang, D. P. Sun and X. Wang, J. Mater. Chem., 2012, 22, 16844–16850.
183. Y.-H. Kim and S.-J. Park, Curr. Appl. Phys., 2011, 11, 462–466.
184. X. Y. Liu, Y. Q. Zhang, X. H. Xia, S. J. Shi, Y. Lu, X. L. Wang, C. D. Gu and J. P. Tu, J. Power Sources, 2013, 239, 157–163.
185. Z. Wang, X. Zhang, Y. Li, Z. T. Liu and Z. P. Hao, J. Mater. Chem. A, 2013, 1, 6393–6399.
186. Q. Q. Xiong, J. P. Tu, S. J. Shi, X. Y. Liu, X. L. Wang and C. D. Gu, J. Power Sources, 2014, 256, 153–159.
187. S. W. Kim, H. W. Lee, P. Muralidharan, D. H. Seo, W. S. Yoon, D. K. Kim and K. Kang, Nano Res., 2011, 4, 505–510.
188. G. Q. Zhang, L. Yu, H. B. Wu, H. E. Hoster and X. W. Lou, Adv. Mater., 2012, 24, 4609–4613.
189. K. Karthikeyan, D. Kalpana and N. Renganathan, Ionics, 2009, 15, 107–110.
190. B. Liu, B. Y. Liu, Q. F. Wang, X. F. Wang, Q. Y. Xiang, D. Chen and G. Z. Shen, ACS Appl. Mater. Interfaces, 2013, 5, 10011–10017.
191. L. Yu, L. Zhang, H. B. Wu, G. Q. Zhang and X. W. Lou, Energy Environ. Sci., 2013, 6, 2664–2671.
192. L. Huang, D. C. Chen, Y. Ding, S. Feng, Z. L. Wang and M. L. Liu, Nano Lett., 2013, 13, 3135–3139.
193. W. L. Yang, Z. Gao, J. Ma, X. M. Zhang, J. Wang and J. Y. Liu, J. Mater. Chem. A, 2014, 2, 1448–1457.
194. X. Y. Liu, S. J. Shi, Q. Q. Xiong, L. Li, Y. J. Zhang, H. Tang, C. D. Gu, X. L. Wang and J. P. Tu, ACS Appl. Mater. Interfaces, 2013, 5, 8790–8795.
195. C. H. An, Y. J. Wang, Y. A. Huang, Y. A. Xu, C. C. Xu, L. F. Jiao and H. T. Yuan, CrystEngComm, 2014, 16, 385–392.
196. L. Li, Y. Q. Zhang, X. Y. Liu, S. J. Shi, X. Y. Zhao, H. Zhang, X. Ge, G. F. Cai, C. D. Gu, X. L. Wang and J. P. Tu, Electrochim. Acta, 2014, 116, 467–474.
197. S. L. Kuo and N. L. Wu, Electrochem. Solid-State Lett., 2005, 8, A495–A499.
198. S. N. Pusawale, P. R. Deshmukh and C. D. Lokhande, Bull. Mater. Sci., 2011, 34, 1179–1183.
199. N.-L. Wu, Mater. Chem. Phys., 2002, 75, 6–11.
200. M. Sathiya, A. S. Prakash, K. Ramesha, J. M. Tarascon and A. K. Shukla, J. Am. Chem. Soc., 2011, 133, 16291–16299.
201. Q. H. Do, C. C. Zeng, C. Zhang, B. Wang and J. Zheng, Nanotechnology, 2011, 22, 365402.
202. T. Gujar, V. Shinde, C. Lokhande and S.-H. Han, J. Power Sources, 2006, 161, 1479–1485.
203. F.-L. Zheng, G.-R. Li, Y.-N. Ou, Z.-L. Wang, C.-Y. Su and Y.-X. Tong, Chem. Commun., 2010, 46, 5021–5023.
204. J. Rajeswari, P. S. Kishore, B. Viswanathan and T. K. Varadarajan, Electrochem. Commun., 2009, 11, 572–575.
205. L. Zheng, Y. Xu, D. Jin and Y. Xie, J. Mater. Chem., 2010, 20, 7135–7143.
206. X. H. Xia, Z. Y. Zeng, X. L. Li, Y. Q. Zhang, J. P. Tu, N. C. Fan, H. Zhang and H. J. Fan, Nanoscale, 2013, 5, 6040–6047.
207. A. Ramadoss, G.-S. Kim and S. J. Kim, CrystEngComm, 2013, 15, 10222–10229.

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