Electrochemical capacitors utilising transition metal oxides: an update of recent developments

Abstract

Transition metal oxides receive considerable attention in the area of electrochemistry not only due to their beneficial reported structural, mechanical or electronic properties, but because of their capacitive properties ascribed to their multiple oxide states they exhibit pseudo capacitances which carbon counterparts generally cannot. Typically transition metal oxides may be classified as noble transition metal oxides which exhibit excellent capacitive properties but have the drawback of generally being relatively expensive. Alternatively base metal oxides may also be utilised which are considerably cheaper and more environment friendly than noble transition metals as well as exhibiting good capacitive properties. In considering that nanostructured materials can help ameliorate the electrochemical performances of transition metal oxides, this review summarizes the recent investigations of fundamental advances in understanding the electrochemical reactivity of transition metal oxides, thus leading to an improved capacitive performance, which is essential for their continual use in a plethora of supercapacitor applications.

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Introduction

The rapidly growing commercial markets providing portable electronic devices and other related electronics such as electric vehicles (EV) requires high-performance energy-storage systems.^1,2 There is of course global issues threatening the environmental resulting in academics and industry exploring clean alternative energies where electricity storage needs to be either stored as electricity or in other forms of energy to meet growing consumers needs.³ Electrochemical supercapacitors (ECs), also called supercapacitors or ultracapacitors, are electronic components that can be rapidly charged and discharged and relied upon to store energy reliably for long periods.⁴ Emerging as an ideal model, they have been touted as a solution to the mismatch between the fast growth in power required by devices and the inability of batteries in various applications which require transient but high/peak power pulses for the time-dependent usage.⁵

ECs were first reported in 1957 Becker's patent⁶ and then appeared on the commercial market in 1978 (Gold Capacitors from Panasonic/Matsushita) and in 1980 (Supercap from NEC/Tokin).⁷ They are often used as uninterruptible power supplies and load-leveling, such as backup sources for memories and microcomputers. They also have potential applications in electric vehicles and hybrid electric vehicles. It has been reported that the Airbus A380 has applied electrochemical double-layer capacitors (EDLCs) within its emergency doors.¹

Note that ECs possess high power density, fast power delivery or uptake, excellent reversibility, low equivalent series resistance (ESR) and long cycle life when compared with batteries. A direct comparison between batteries and ECs is shown in Table 1.⁸ It can be found that the Ragone plot in Fig.1 (specific power density against specific energy density) shows that ECs shorten the gap between conventional capacitors and batteries,¹ exhibiting much higher specific capacitances than conventional capacitors owing to their high specific area. It has been revealed that as for electrode-active materials, specific surface area and pore size are two of the crucial factors that can affect the specific capacitance of capacitors. It was once believed by scientists that it was advantageous to increase the pore size allowing for greater ion adsorption leading to higher capacitances. Interestingly, Gogotsi et al. demonstrated that the capacitance can be greatly enhanced with pore size down to the solvated ion size.⁹

Fig. 1 Ragone Plot. Reproduced from ref. 6 by permission of Nature Publishing Group.

Table 1 Comparative electrical characteristics of batteries and ECs

Battery	Supercapacitor
Ideally single-valued free energies of components	Continuous variation of free energy with degree of conversion of materials
Not capacitative, except in very general sense.	Capacitative Behaviour
Usual irreversible behaviour	High degree of reversibility is common
Response to linear modulation of potential gives irreversible i vs. V profile with nonconstant current.	Response to linear modulation of potential gives more or less constant charging current
Discharge at constant current arises at a more or less constant potential	Discharge at constant current gives linear decline of potential with times

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According to charge storage mechanisms, ECs may be placed into two distinctive classes: electrochemical double-layer capacitors (EDLCs) and pseudo-capacitors.^1,5 Since the 1990s, electrochemical supercapacitors ushered in its development with materials mostly used as EDLCs based on carbonaceous materials such as activated carbon,^9–12carbide-derived carbon,^9,12–15graphene^16–19 and carbon nanotubes^20–23 with a high surface area essential for the charge to reside. Alternatively pseudo-capacitors combine electrosorption with fast surface redox reactions to store energy, where the amount of charge held is proportional to the applied voltage.⁷ Transition metal oxides are often utilised or alternatively electronically conductive polymers may be used as electrode materials.^2,24–27 Impressively, their specific capacitance can be 10–100 times higher that those obtained from EDLCs.⁸

Researchers had no clear concept regarding pseudo-capacitance until Trasatti and Buzzanca discovered the capacitive properties of RuO₂.²⁸ While supercapacitors benefit from the highest known power capability (10 kW kg⁻¹), high current capability, ease of maintenance and long-term cycling stability (>10⁶ cycles),²⁹ their moderate energy density for the widespread deployment in everyday technology as clean and renewable energy media are still far away to meet the performances and cost requirements, in particular providing power boosts to start the engine or to assist acceleration.

In order to make capacitors commercially viable and become extensively used, the energy that they can store needs to increase significantly and to achieve supercapacitors with sufficient energy for higher requirements in the future is still a challenge. Extensive efforts have been devoted to improve the specific capacitance of supercapacitors by introducing pseudo-capacitive metal oxides. Compared with EDLC-based capacitors, pseudocapacitors based on transition metal oxide electrodes have attracted a large amount of attention since they can produce higher capacitances than double-layer carbonaceous materials. Despite their importance, efforts are still needed to further improve the practical use of metal oxide supercapacitors.

Transition metal oxides used as electrode active materials for ECs have developed at a high rate over the past decades and may be classed as either noble or based metal oxides. Noble metal oxides, for instance, RuO₂, IrO₂, etc exhibit good conductivities and excellent power densities. However, their relatively high cost and potential (reported) harmful nature to the environment have limited their widespread application in supercapacitors. In terms of base metal oxides such as MnO₂, NiO, Fe₃O₄, etc, their outstanding pseudo-capacitive behaviour, practical availability, environmental compatibility and lower cost have been demonstrated when compared to the state-of-the-art supercapacitor material RuO₂. The detailed maximum specific capacitance of these materials and energy density in aqueous solution is depicted in Table 2.

Table 2 Maximum specific capacitance and energy density of metal oxide

Materials	RuO2	NiO	Ni(OH)2	MnO2	Co(OH)2
Specific capacitance F g^−1	1580 (1 mv/s) ^128	1329 (84 A g^−1) ^129,130	3152 (4 A g^−1) ^131	698 (50 mv/s) ^90	3108 (10 mv/s) ^132
Theoretical energy density W h Kg^−1	342 ^128	46 ^130	109 ^133	78 ^90	276 ^132

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The aim of this review is to detail the recent investigations of using metal oxides as electrode materials for utilisation in electrochemical capacitors as well as reporting on the fundamental advances in understanding the electrochemical reactivity of metal oxides.

1. Noble metal oxides

Noble metal oxides such as RuO₂, IrO₂ and so forth were first found to exhibit good capacitive properties with rich electrochemical performances. Iridium dioxide is one kind of transition metal oxide with rutile structure.³⁰ It can be used as optical switching layers in electrochromic displays,³¹ ferroelectric capacitors for non-volatile memories³² and supercapacitors.^30,33,34 The fact that IrO₂ did not draw much attention in comparison to RuO₂ for use in supercapacitors can mostly be attributed to its relatively small value of mass-specific capacitance caused by its heavy relative atomic mass.³⁰ For example, Chen et al. fabricated a composite of IrO₂ and multiwalled carbon nanotubes, which showed a low specific capacitance of 69 F/g.³⁵ After that, the authors prepared IrOx nanofoils with high surface area and sputtered them on multi-wall carbon nanotubes for supercapacitor application.³⁰ The active materials exhibited an improved specific capacitance of 370 F/g, which is still poor when compared with RuO₂. Hence, we will focus on summarizing the electrochemical properties of Ruthenium oxide.

1.1 Synthesis methods of RuO₂

Trasatti and Buzzanca first reported that RuO₂ films could form a rectangular shaped cyclic voltammogram which closely resembled the carbon-based electric double-layer capacitor.²⁸ Since this pioneering report, researchers have explored the capacitive properties of RuO₂ as its theoretical value of specific capacitance is estimated to range from 1300–2200 F/g³⁶ and is now consequently playing an important role in the application of electro-catalysis^37–40 and power sources^36,41,42.

Nowadays nanostructured materials are used to help ameliorate the electrochemical performances of transition metal oxides. Transforming bulk transition metal oxides into nano-porous structures was thought to have the advantage of effectively alleviating the strain generated during the ion insertion/desertion process and leading to improved capacitive performance.⁴³ A key methodology to boost the specific capacitance is their morphological and/or chemical composition design at the nanometre-scale, because high surface-to-volume ratio with suitable pore sizes is desirable for the penetration of electrolytes and reactants into the whole electrode matrix, and can promote the electric double-layer capacitances and accommodate a large amount of superficial electroactive species to participate in Faradaic redox reactions.

Generally there are two common methods to prepare ruthenium oxides.⁴⁴ The first, chemical deposition, is to synthesize the materials by thermal decomposition and then oxidize the precursor at high temperatures. The other is a sol–gel synthesis methodology which firstly produces a sol–gel precursor which is then annealed at low temperatures. It was found that amorphous ruthenium oxides can be obtained at low temperatures while the crystalline phase were observed with higher temperatures; the resultant capacitance was found to be as high as 768 F/g.⁷

Zhang et al. synthesized nanotubular ruthenium oxides by using manganite nanorods as a morphology sacrificial template and obtained a high specific capacitance of 860 F/g with a good capacitive retention.⁴⁵ Extensive research has been undertaken by researchers in this field with the specific capacitance reported to reach more than 1300F/g.^{36,43,46–57} Hu et al. used the membrane-templated synthesis route to obtain hydrous RuO₂ (RuO₂·xH₂O) nanotubular structures by means of an anodic deposition technique.³⁶ They obtained ruthenium oxide which exhibited a ultrahigh specific capacitance (1300 F g⁻¹), excellent charge/discharge behaviour at 1000 mV s⁻¹, and high-frequency (4.0–7.8 kHz) capacitive responses, which were much higher than any value ever reported before. Some typical synthesis methods and synthesising processes of RuO₂ are given in Table 3 and Fig. 2.

Fig. 2 Typical synthesis processes of RuO₂. Reproduced from ref. 24, 73, 74 by permission of the American Chemical Society, the Electrochemical Society and Wiley.

Table 3 Methods to synthesis RuO₂

Method	Precursor	Solution	Specific capacitance
Chemical deposition	RuCl3	Water	210 F/g ^134
Sol–gel synthesis	RuCl3	Water and ethanol	570 F/g (38.3 wt% Ru loading)^47
Electrochemical deposition	RuCl3	Water	1300 F/g ^36

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1.2 Energy storage mechanism of RuO₂

As typical pseudo-capacitor electrode materials, ruthenium oxides have a completely different mechanism with that of electrochemical double-layer capacitors and have multiple redox states while demonstrating good electrical conductivity. Note that RuO₂ has three distinct oxidation states accessible within 1.2V, and its pseudo-capacitive behaviour in acidic solution can be described by the following equations:^1,43

RuO₂ + xH⁺ + xe⁺ ↔ RuO_2−x(OH)_x 0 ≤ x ≤ 2 (1)

or

RuO_x(OH)_y + δH⁺ + δe⁻ ↔ RuO_x−δ(OH)_y+δ (2)

As described in the equations above, protons can participate in the redox reaction, where RuO₂ acts as a proton condenser and the reaction is electrochemically reversible.^28,50,51 It should be noted that amorphous structures are the key parameter in determining the value of specific capacitances, as the proton can be easily intercalated into the bulk of RuO₂·xH₂O but not into the crystalline phase.^43,58 It is also proposed that hydrous ruthenium dioxide has improved capacitive properties than those observed from anhydrous structures as hydrous regions are more permeable to protons and proton conduction inside ruthenium dioxide is dominant when compared with the electron conduction.⁵⁹

2. Base transition metal oxides

Though RuO₂, IrO₂ and similar structures exhibit excellent capacitive performances, the drawback is their relative high cost and consequently scientists are exploring other low cost alternatives which still exhibit good capacitive properties. Attention has been focused on base transition metal oxides such as MnO₂, NiO and Fe₃O₄ for electrode active materials of ECs in recent years.

2.1 NiO

Nickel is cheap and abundant in the earth, and it is often used as a catalyst^60–62 and the active material of batteries,^63–65 especially in the form of nickel hydroxide. It is a very versatile material and has shown to exhibit good electrochemical activity based on the transformation of Ni(OH)₂/NiOOH.^53,55 Hierarchical micro-flowers, hollow microspheres and nanosheets have all been successfully prepared with various methodologies, followed by the paramount important steps of dehydration and re-crystallization of Ni(OH)₂ in preparing NiO.^66,67

2.1.1 Methods to synthesize NiO. Various chemical methods have been used to prepare nanostructured nickel hydroxide such as: common chemical precipitation, sol–gel synthesis, chemical bath deposition, spray pyrolysis methods and electrochemical deposition. In traditional chemical precipitation methods, a soluble nickel salt is often used as precursor, such as Ni(NO₃)₂ and Ni(CH₃COO)₂, and then Ni(OH)₂ is obtained by a hydrothermal process.^{44,56,57,67–69} There is no need for any surfactant in the chemical precipitation processes and the fabrication protocol is simple, facile, low cost and suitable for large-scale of nanostructures of nickel hydroxide. For instance Zhang et al. reported a facile way to grow various porous NiO nanostructures including nanoslices, nanoplates and nanocolumns by calcinating β-Ni(OH)₂ synthesized by hydrothermal process.⁴⁴ Consequently, porous NiO nanocolumns exhibited a specific capacitance of 390 F/g at a discharge current of 5 A g⁻¹, while nanoslices exhibited 176 F/g and nanoplates 285 F/g.

Sol–gel process is another efficient way to obtain the desired nanostructures. Typically the first step is to prepare a sol–gel and the second step is to apply heat treatment. Though the sol–gel progress is very simple, it is difficult to control, especially when very fine nanostructures are required.^69–73 Yang et al. obtained hexagonal and single crystalline NiO nanowires by a sol–gel process and subsequent calcination, with the lateral size of the nanowires found to be ∼20–60 nm.⁷⁰

In terms of electrochemical deposition, a thin Ni film is deposited electrochemically onto a substrate. In comparison with the methods identified above, it is more controllable and relatively easy to control the mass and thickness of films by simply controlling the current, bath chemistry and temperature.^74–76 Porous nickel oxide films are directly deposited onto conducting indium tin oxide coated glass substrates by cyclic voltammetric (CV), galvanostatic, and potentiostatic strategies.⁷⁷ It was shown that the deposition potential influences the pore distribution of the film and with an optimized deposited potential at + 0.9 V, the obtained sample showed a high specific capacitance as 320 F/g.

There are many other methods such as chemical bath deposition^53,57,78 and the spray pyrolysis method;⁷⁹ typical methodologies and their processes to synthesize Ni(OH)₂ or NiO are reported in Table 4 and Fig. 3.

Fig. 3 Typical synthesis processes of Ni(OH)₂ or NiO. Reproduced from ref. 26, 50, 70 by permission of Springer, the Electrochemical Society, and Elsevier.

Table 4 Methods to synthesize Ni(OH)₂ or NiO

Method	Precursor	Solution	Specific capacitance
Chemical deposition	NiCl2·6H2O	Water	390 F/g (NiO)^44
Sol–gel synthesis	NiSO4·6H2O	Water	698 F/g ^137
Electrochemical deposition	Ni(NO3)2	Water	277 F/g (NiOx)^80

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2.1.2 Energy storage mechanism of Ni(OH)₂ and NiO. Nickel oxides or its hydroxides have high chemical/thermal stability, are readily available and are environmentally benign in-nature and have relatively lower costs in comparison to RuO₂. NiO is of particular interest owing to its high theoretical specific capacitance of 2573 F/g.^55,80Nickel hydroxide exists in a variety of forms⁸¹ and four polymorphs of nickel hydroxide are known: α-Ni(OH)₂, β-Ni(OH)₂, β -NiOOH, χ-NiOOH respectively. α-Ni(OH)₂, β -Ni(OH)₂ are the two most common forms. It is not difficult to see that these various forms of nickel hydroxide differ from each other in their chemical structure, degree of hydration and morphology.⁵³ It has been demonstrated that the redox reaction of the nickel electrode is usually considered to be a solid-to-solid transformation.⁸² Also note that for Ni(OH)₂ it is considered that the β (II)/β (III) system follows a single-phase (NiOOH and Ni(OH)₂ homogeneous mixture) redox mechanism throughout the electrochemical process.⁸³ When NiO is in a basic solution, it displays similar electrochemical behaviours with that of Ni(OH)₂. The mechanism can be described by the equations below:

2.2 MnO₂

Manganese oxide is a material that possesses the advantages of low cost, no toxicity, easy to obtain and high specific capacitances and consequently has been considered as the most promising material in application of ECs.

2.2.1 Methods to synthesize MnO₂. MnO₂ has not only a high specific capacitance but is more economically priced in comparison to noble transition metal oxides. Over the past several years, significant efforts have been directed toward the synthesis of nano-and micro-materials with controlled sizes and ordered morphologies due to the close relationship between the morphology and the capacitance performances.^84–86 The most common method to prepare MnO₂ is a hydrothermal synthesis in which Mn²⁺ is often used as precursor, and the as-prepared mixture solution is placed in a Teflon-lined pressure vessel.^20,87–92 It has been demonstrated that the morphology of MnO₂ depends on the dwell time of hydrothermal process with an increase in crystallinity observed as hydrothermal dwell time is increased.⁸⁷Scanning electron microscopy (SEM) images of nanostructure MnO₂ exhibiting different morphologies are depicted in Fig.4,^20,87–90 which have been found to exhibit specific capacitances from a–d: 168 F/g, 184 F/g, 120 F/g, and 167 F/g respectively.

Fig. 4 SEM images of nanostructure MnO₂ with different morphologies: (a) nanowire, (b) nanoflower, (c) hollow urchin, and (d) clews. Reproduced from Ref. 54 by permission of the American Chemical Society.

The active MnO₂ used in supercapacitors can also be prepared via a sol–gel synthesis. For example, Pang et al. fabricated a MnO₂ film viadip-coating or drop-coating with manganese dioxide suspensions (sols) onto nickel foils.⁹⁰ The resultant samples were found to exhibit good capacitive behaviours with specific capacitance values as high as 698 F/g. Electrochemical deposition is another effective methodology to prepare active MnO₂. Adelkahani et al. synthesized large-scale oriented nano-sheets of MnO₂ with thicknesses of less than 30 nm by pulse current electro-deposition.⁹³ The resultant MnO₂ nano-sheets exhibited high porosity, cation vacancy and water content producing a capacitance of 280 F/g at a scan rate of 5 mV s⁻¹ in a Na₂SO₄ electrolyte. Hu et al. designed composite nanowires consisting of MnO₂, Mn₃O₄ nanocrystallites and single-crystalline MnOOHvia a simple two-step electrochemical deposition process,⁹⁴ which exhibited a specific capacitance of 470 F/g in CaCl₂ and demonstrated excellent reversibility as well as outstanding stability. Extensive work is on-going on to prepare a better modality of MnO₂ in order to achieve higher specific capacitances; common methodologies are depicted in Table 5 and a typical synthesis process of each method is highlighted in Fig. 5.

Fig. 5 Typical synthesis processes of MnO₂. Reproduced from ref. 13, 61,75 by permission of the American Chemical Society, the Electrochemical Society, and Elsevier.

Table 5 Methods to synthesise MnO₂

Method	Precursor	Solution	Specific capacitance
Hydrothermal synthesis ^20,87–89	MnAc2·4H2O	Water	168F/g ^20
Sol–gel synthesis^90,135,136	Mn(ClO4)2	Water	698F/g ^90
Electrochemical deposition ^94,95	MnSO4·H2O	Water	310 F/g ^95

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2.2.2 Energy storage mechanism of MnO₂. It has been reported that the theoretical specific capacitance of MnO₂ is 1370 F/g,⁹⁵ and its observed high specific capacitance can be attributed to the redox oxidation of superficial oxy-cation species of various oxidation states.⁹⁶Fig. 6 shows a typical cyclic voltammogram of a single MnO₂ electrode recorded in mild aqueous electrolyte¹ exhibiting a roughly rectangular mirror image characteristic of double-layer capacitive behaviour. Currently there are two different views regarding the mechanism of MnO₂ charge storage behaviours. One view is that protons or/and alkali metal cations can intercalate the bulk material upon reduction and de-intercalation upon the electrochemical oxidation processes,^97,98 shown as the following:where C⁺ represents protons or alkali metal cations. This behaviour is similar to that observed by RuO₂. However, only a limited fraction of MnO₂ electrode materials is electrochemically active. The other viewpoint is that the redox process occurs just on the surface of MnO₂,⁹⁹ and the corresponding equation can be described as:

Fig. 6 CV of a single MnO₂ electrode Reproduced from ref. 6 by permission of Nature Publishing Group.

Though two points of view have some differences, they both indicate a redox reaction between the III and IV oxidation states of Manganese.

2.3 Other base metal oxides and composite materials

Other base metal oxides such as iron oxide,^100,101indium oxide,^102,103tin oxide,^104–106 and vanadium oxide^107–109 also have their applications in ECs. Transition metal oxides may be prepared as a powder and used to prepare composite electrodes or can be directly deposited onto a conducting substrate to overcome their poor conductive performances.^110–112 For example, Du et al. prepared nanoparticle Fe₃O₄/active carbon supercapacitors by a microwave fabrication methodology producing a resultant nano structure Fe₃O₄ with average particle sizes of 35 nm.¹⁰⁰ This AC–Fe₃O₄ hybrid supercapacitor was found to deliver a specific capacitance of 37.9 F/g and yet was reported to still keep 82% of initial capacity over 500 cycles in characteristic cyclic tests.

3. Mixed metal oxides

Mixed metal oxides are used as active materials of supercapacitor because the additive metal oxide can affect the structure and improve the electrochemical properties of the composites to a certain extent.^113,114 Some work has been performed in pursuing better ingredients of mixed metal oxides, such as Ni–Co, V–Sn, Ru–Ti and Ru–Co composites, etc.^113–117 It is reported that the presence of cobalt oxide can help to increase the electronic and redox properties of metal oxides composites, the additive of Co into the lattice of nickel hydroxide can not only efficiently increase its conductivity but shift the redox peaks of Ni(OH)₂ to less positive potential.^115,118,119 Electrochemical characterization of a SnO₂–V₂O₅ composite indicates superior properties than the corresponding V₂O₅.¹¹⁷ The presence of SnO₂ affects the structure of V₂O₅, which enhances the use ratio of V to a different dimension actually. Kim et al. also prepared a Ru–Co mixed oxide. The Ru–Co mixed oxide was co-deposition on single-walled carbon nanotube (SWNT)¹¹⁴ which exhibited a specific capacitance of 570 F/g at a scan rate 500 mV s⁻¹, much better than the corresponding RuO₂ electrode (475 F/g), owing to the enhanced electronic conduction for which Co provided structural support through the active Ru species.

4. Asymmetrical supercapacitors

It is well known that the energy density of a supercapacitor is proportional to the potential window which can effectively enhance the energy density.^120,121 Asymmetrical supercapacitors (ASC) can utilise ionic liquids or environmentally harmful organic reagents as electrolytes to improve cell voltage,^20,29 and will often use appropriate electrode materials that allow working in wide potential windows which can significantly broaden the cell voltage of supercapacitor in order to achieve high energy density.^121–127 In an ASC, the active material in one electrode is not the same as that used in the other electrode in a cell system. For instance, Quet al. fabricated an ASC in which active carbon (AC) was used as anode and MnO₂ as cathode.¹²⁴ It was reported to exhibit an energy density of 17 Wh/kg at 2 kW kg⁻¹ with a voltage 1.8 V in 0.5 mol L⁻¹K₂SO₄ aqueous solution, which was much higher than its symmetrical counterparts (AC/K₂SO₄/AC). Ganesh et al. obtained similar conclusion that ASC provide higher capacitance value than symmetrical one when comparing asymmetrical cell (NiO/KOH/AC) with their corresponding symmetrical cell (NiO/KOH/NiO).¹²³

Considering the compromises of power density causing by poor conductivity of metal oxides, scientists have mixed metal oxide with materials of good conductivity like active carbon, graphene, carbon nanotube to fabricate electrodes forward to an improvement of electrochemical properties. Reduced graphene oxide sheets (RGO) and its modified ruthenium oxide (RGO–RuO₂) have been used as anode and cathode,¹²⁷ leading to an energy density of 26.3 Wh/kg

5. Summary and outlook

We have summarized the current literature regarding the relatively new progress of transition metal oxides and explored their applications in electrochemical supercapacitors. Their development can be summed as:

1. A continued exploration into new materials in order to enhance capacitance properties with a particular emphasis on finding materials those are relatively low in cost and environmentally friendly. Since RuO₂ was first found to have a capacitive property, many transition metal oxides have been explored as electrode materials in ECs. The morphology of electrode materials exerts a wide influence over the speed of ion transfer and the ability to synthesize materials with excellent morphology (pore size, surface area and so on) in order to achieve good capacitive performances with adequate electrolyte wetting is crucial; to meet these criteria is a challenge for scientists to design better materials, or composite materials in order to further improve capacitances.

2. Improvement of the electrochemical properties of known oxides. It is known that ions transfer in the surface or bulk of transition metal oxides when redox reaction occurs and the majority of the transition metal oxides suffer a disadvantage of relatively bad conductivity.

Although the energy and power density of ECs can be enhanced to a level to achieve high energy densities, using hybrid systems might be a final solution via combining double layer and pseudo-behaviour electrodes. In summary, for ECs to become more commercially viable, the materials at the heart of the capacitor will need to become significantly cheaper and more efficient.

Acknowledgements

Financial support from the National Natural Science Foundation of China (No.21050110115), International Joint Project from The Royal Society (No. JP090644), and Hunan Province Foundation of Natural Science (10JJ6026) are greatly appreciated.

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