Nanostructured Metal Oxides for Supercapacitor Applications

Abstract

The fundamental properties of supercapacitors (SCs) with descriptions restricted to the metal oxides systems and the effect on the electrochemical performance and synthesis are described in this chapter. Metal oxides such as manganese oxide (MnO), vanadium oxide (V₂O₅) and ruthenium oxide (RuO) have demonstrated great potential in the field of energy storage due to their structural as well as electrochemical properties, thus attracting huge attention in the past decade and in recent years. The major contributing factor to the electrochemical properties is their capability to achieve relatively high pseudocapacitive performance derived from their theoretical values resulting from their multiple valence state changes. The developments of the metal oxide (MO)-based electrode materials and their composites are being explored from the synthetic point of view as well as their emerging applications as energy storage materials. Therefore, the need to further exploration of MO-based electrodes is motivated by their considerably low-cost and environmentally friendly nature as compared to other supercapacitive electrode materials. This chapter accounts to the overview of various nanostructured metal oxide materials for application as energy storage materials in supercapacitors.

6.1 Introduction

Today’s society and particularly the developing countries rely heavily on fossil fuels as the primary source of energy since they are cheap and also exhibits high specific energy and power per weight (or per volume) output. The growing world population and increasing standard of life-style exerted much pressure on the deliverance of energy, and as a result, these have led to a rapidly increasing energy demand over a period of time (Poizot and Dolhem 2011). In addition, the fast-growing interest in portable electronic devices and electric vehicles has stimulated extensive research in high-performance energy storage devices. The above reasons are justified by the projections stated in the International Energy Outlook 2017 (IEO 2017 https://www.eia.gov/outlooks/ieo/) reference case, which shows that the total world energy consumption continues to rise from 575 quadrillion British thermal units (Btu) in 2015 to 736 quadrillion Btu in 2040, indicating an increase of 28% (IEA 2014). The incineration of fossil fuels results in severe environmental pollution and contributes to greenhouse effect, which is harmful to the environment while also placing many countries at disadvantage as the rate of coal depletion exceeds that of its production. The development of technology that is compatible with the resources provided by nature is essential in order to reduce the uprising malice to the environment by fossil fuel energy sources worldwide and also to meet the high energy demands resulting from the growth in civilization. Renewable energy has shown to be the escalating solution towards mitigating this energy crisis, and currently, they are the world’s fastest-growing energy source, with consumption increasing by an average 2.3%/year between 2015 and 2040 (Bang et al. 2000; UNEP 2013). As much as renewable energy resources (i.e. photovoltaics, wind turbines, geothermal heat, etc.) have displayed great potential to be employed as the alternative energy sources to fossil fuels globally, their success is still limited by their inability to store the harvested energy and a lack of a quick transmission of the same energy (Hatzell et al. 2015). Thus, the market for energy storage applications becomes an integrated critical component in clean energy development and sustainability. Electrochemical energy storage systems (EES) have emerged as an established, valuable approach for improving the reliability and overall use of the advantages of renewable energy resources (Béguin et al. 2014). An energy storage device is an apparatus used to store electrical energy when needed and releases the stored energy when required. The role of the technology involving energy storage device in the fields such as renewable energy generation and electric/hybrid electric automobile systems has become increasingly important in modern society as a measure of checking the ever-increasing global warming and other matters related to the energy crisis. Therefore, in consideration of reliable, stable and sustainable large-scale use of renewables, electrochemical capacitors (ECs) and batteries play a fundamental role in advanced and highly efficient energy storage and management. Lithium–ion batteries (LIBs) can store large amount of energy as high as 150–200 Wh kg⁻¹ but are confined to their low-power density (below 1000 W kg⁻¹) and poor cycle life (usually less than 1000 cycles) (Thackeray et al. 2012; Béguin et al. 2014). In comparison with batteries, electrochemical capacitors (ECs) vastly known as supercapacitors have demonstrated to be the competing measures with regards to storing energy output that can be delivered at the faster rate without the loss of the device efficiency (Burke et al. 2014). This electrochemical energy storage device has great potential in energy applications that require peak power pulses. The ability of the ECs to serve as a gap between conventional capacitors (that display great power output but limited energy storage output) and batteries (with great energy storage output but limited power deliverance) places this electrochemical energy storage device at the much added advantage. The advantages of ECs arise from their energy storage mechanism, which involves physical storage of energy through dielectric polarization or electronic double layer of ions and electrons (surface phenomenon). There are two types of fundamental storage mechanisms involved in ECs, viz. (a) ‘electrochemical double layer capacitor’ (EDLC) that uses nanocarbons as the main electrode materials for supercapacitors, and there are many abundant sources of nanocarbons with facile processes for the fabrication of cheap electrodes (Ghosh and Lee 2012) and (b) ‘pseudocapacitor’. The operation mechanism of the EDLC involves the non-faradaic separation of charges at the ‘double layer (i.e. electrode/electrolyte interface), while pseudocapacitor involves fast faradaic, redox reaction of electroactive materials at the interface (Béguin et al. 2014). A Ragone chart in Fig. 6.1 shows the power and energy densities of various energy storage and conversion devices.

Fig. 6.1

Ragone chart showing energy density as a function of power density of various capacitors and batteries

6.1.1 Electrochemical Capacitors as an Energy Storage Solution

The concept of the electrochemical double-layer capacitance dates back to a German physicist, Hermann von Helmholtz, in 1853 (Conway 1999). In 1957, General Electric Company filed the earliest ECs patented based on the double-layer capacitance structure. However, not until the 1990s did EC technology begin to draw some attention in the field of hybrid electric vehicles (Burke 2000). It was found that the main function of ECs could promote the electrochemical performance of a battery or fuel cell in a hybrid electric vehicle by providing necessary power for acceleration and recuperating brake energy (Toupin et al. 2004). Thus, the capacitor technology is regarded as a promising means for storing electricity (Jiang et al. 2013). The early electrochemical capacitors (ECs) were rated at a few volts and had capacitance values measured from fractions of farads up to several farads. Today’s trend is for cells with the size ranging from small millifarad-sized devices with exceptional pulse power performance up to devices rated at several kilo farads. The technology is experiencing increasingly broader use especially originating from the ‘humble’ beginning, both replacing batteries in some cases and in others complementing their performance (Miller and Burke 2008). Due to the intrinsic properties occurring at the electrode/electrolyte interface of an EC device, understanding of the materials used for fabricating the device is of great importance especially for the enhancement of the electrochemical performance of the EC device (Conway 1999; Béguin et al. 2009). Figure 6.2 shows the composition and working principle of an ideal parallel plate capacitor. In a charged capacitor, the metal plates are oppositely charged, and an electric field is formed in the dielectric medium (Thomas and Floyd 2009).

Fig. 6.2

Schematic diagrams of (a) the design of a parallel plate capacitor (with permission of Yuan et al. 2012) and (b) a charged parallel plate capacitor (With permission of Vol’fkovich et al. 2002)

Generally, to improve on the electrochemical performance of ECs, electroactive materials that provide supercapacitive effect contributing to the total double-layer capacitance are considered. Pseudocapacitance originates from electrochemical reduction-oxidation (Redox) reaction due to faradic transfer that occurs in the active electrode materials . This phenomenon is similar to the chemical reaction that occurs in many rechargeable batteries; therefore, the reaction must be reversible or quasi-reversible for optimum/efficient electrochemical performance. Taking into account a uniform distribution of the redox-based material on the electrodes, the redox process is governed by the reaction below (Yu et al. 2013a):

$$ {O}_X+{n_e^{-}}_X\leftrightarrow {R}_D $$

(6.1)

where n is the number of electrons transferred and O _X and R _D represent the oxidized and reduced species, respectively.

The electrochemical potential arising from Eq. 6.1 has been described by the Nernst equation:

$$ E={E}_{O_X/{R}_d}^0+\frac{\mathrm{RT}}{\mathrm{nF}}\ln \left(\frac{C_{O_X}}{C_{R_d}}\right) $$

(6.2)

The faradaic reaction in Eq. 6.1 corresponds to a reversible pseudocapacitance (per/vol) given by Eq. 6.3 for an ideal system.

$$ {C}_{pc}(E)=\frac{idt}{dE}=\frac{i}{v}=\frac{n^2{\mathrm{F}}^2}{RT}{dC}_{O_x}^0\frac{\exp \left(\frac{n\mathrm{F}}{\mathrm{RT}}\left({E}_{O_X/{R}_d}^0-E\right)\right)}{{\left[1+\exp \left(\frac{n\mathrm{F}}{\mathrm{RT}}\left({E}_{O_X/{R}_d}^0-E\right)\right)\right]}^2} $$

(6.3)

However, taking into account the irregularities in the matrix structure of the electrodes leads to uneven distribution of the redox centres and therefore leads to a quasi-reversible reaction and can be resolved by introducing a lateral interaction energy ($-g\frac{C_{O_X}}{C_{O_X}^0}$) term, and Eq. 6.3 can then be modified to Eq. 6.4 as shown below (Yu et al. 2013a).

$$ {\displaystyle \begin{array}{ll}& {C}_{\mathrm{pc}}(E)\\ {}& =\frac{n^2{F}^2}{\mathrm{RT}}{\mathrm{dC}}_{O_x}^0\frac{\exp \left(\frac{nF}{RT}\left({E}_{O_X/{R}_d}^0-E\right)+g\frac{C_{O_X}}{C_{O_X}^0}\right)}{{\left[1+\exp \left(\frac{nF}{RT}\left({E}_{O_X/{R}_d}^0-E\right)+g\frac{C_{O_X}}{C_{O_X}^0}\right)\right]}^2+g\exp \left(\frac{\mathrm{nF}}{RT}\left({E}_{O_X/{R}_d}^0-E\right)+g\frac{C_{O_X}}{C_{O_X}^0}\right)}\end{array}} $$

(6.4)

Equation 6.4 holds for a completely reversible redox reaction system when g = 0 and quasi-reversible reaction when g > 0. The capacitance defined by Eq. 6.4, which is a function of electrode potential, is known as the momentary capacitance. The intrinsic pseudocapacitance (C_sp, measured in F/g) of the redox reaction is expressed by Eq. 6.5:

$$ {\displaystyle \begin{array}{ll}{C}_{pc}& =\frac{{\mathrm{AM}}_{\mathrm{mw}}}{m}\sum \limits_{j=1\left({E}_{\mathrm{on}}\right)}^{j=n\left({E}_{\mathrm{off}}\right)}{C}_{pc}\left({E}_j\right)\\ {}& =\frac{n^2}{mRT}{\frac{F}{}}^2{\mathrm{AdM}}_{\mathrm{mw}}{C}_{O_X}^0\frac{\exp \left(\frac{nF}{RT}\left({E}_{O_X/{R}_d}^0-E\right)+g\frac{C_{O_X}}{C_{O_X}^0}\right)}{{\left[1+\exp \left(\frac{nF}{RT}\left({E}_{O_X/{R}_d}^0-E\right)+g\frac{C_{O_X}}{C_{O_X}^0}\right)\right]}^2+g\exp \left(\frac{nF}{RT}\left({E}_{O_X/{R}_d}^0-E\right)+g\frac{C_{O_X}}{C_{O_X}^0}\right)}\end{array}} $$

(6.5)

where A is the electrode geometric area (cm²), d is the thickness of the electrode layer, M_mw is the molecule weight of the redox material (g), m is the weight of redox material inside the entire electrode layer (g) and E _on and E _off are the onset and offset potentials.

The incorporation or combination of electrodes having double layer and pseudocapacitance leads to a coupling effects between the two materials taking advantage of the synergy to enhance the total capacitance of the electrodes. Thus, for an ideal condition, the total capacitance (C _T) of an electrode must be the sum of electrochemical double-layer capacitance (C _dl) and pseudocapacitance (C _pc) which is expressed in Eq. 6.6:

$$ {C}_T={C}_{\mathrm{dl}}+{C}_{pc}(E)={C}_{\mathrm{dl}}+\frac{n^2{F}^2}{\mathrm{RT}}{dC}_{O_X}^0\frac{\exp \left(\frac{nF}{\mathrm{RT}}\left({E}_{O_X/{R}_d}^0-E\right)+g\frac{C_{O_X}}{C_{O_X}^0}\right)}{{\left[1+\exp \left(\frac{nF}{\mathrm{RT}}\left({E}_{O_X/{R}_d}^0-E\right)+g\frac{C_{O_X}}{C_{O_X}^0}\right)\right]}^2+\mathrm{gexp}\left(\frac{nF}{\mathrm{RT}}\left({E}_{O_X/{R}_d}^0-E\right)+g\frac{C_{O_X}}{C_{O_X}^0}\right)} $$

(6.6)

Figure 6.3 is a reversible cyclic voltammetry of an electrode showing the total computed capacitance against the potential. This capacitance clearly shows the double-layer capacitance and the huge contribution from the pseudocapacitive components, indicating that materials that exhibit pseudocapacitance would significantly contribute to the development of supercapacitor technology.

Fig. 6.3

(a) CV curves showing fundamental differences between static capacitance (rectangular) and pseudo-/faradaic capacitance (curved) and (b) shows the calculated total capacitance as a function of electrode potential reflecting the double-layer capacitance and the pseudocapacitance contribution (Yu et al. 2013a)

Different faradaic mechanisms can occur in electrochemical charge storage in pseudocapacitors, and these include electrosorption (underpotential deposition), reduction-oxidation reactions (redox pseudocapacitance) and intercalation (intercalation pseudocapacitance). These processes are illustrated in Fig. 6.4 (Augustyn et al. 2013).

Fig. 6.4

Schematic showing different types of reversible redox mechanisms that give rise to pseudocapacitance (With permission of Augustyn et al. 2014)

Underpotential deposition occurs when the onset of deposition of metal adatoms (adsorbed atom) at various types of substrates in potential ranges positive to the reversible equilibrium potential (also referred to as the Nernst potential). A classic example of underpotential deposition is the deposition of lead (Pb) on the surface of a gold (Au) electrode (Sudha and Sangaranarayanan 2002). While redox pseudocapacitance occurs when the electrochemical adsorption of ions near surface or onto the surface of a material with a concomitant faradaic charge transfer (Augustyn et al. 2014), e.g. surface redox reactions as in RuO_X (Augustyn et al. 2013). Intercalation pseudocapacitance occurs when ions intercalate into the pores or layers of a redox active material accompanied by a faradaic charge transfer with no crystallographic phase change (Augustyn et al. 2014). Underpotential deposition and redox pseudocapacitance represent charge storage on the surface; however, in intercalation pseudocapacitance, charge storage does not occur on the surface but in the bulk material (Augustyn et al. 2013). Figure 6.5 shows the schematics of the double capacitors, the pseudocapacitance and the working potential windows of many oxide materials in aqueous electrolyte with the average working potential of above 0 V vs. SCE.

Fig. 6.5

Schematic representation of the mechanism in (a) double capacitors, (b) pseudocapacitors, (With permission of Zhang et al. (2016a) and (c) the working potential windows of various pseudocapacitve-type materials in aqueous electrolyte (With permission of Qu et al. 2011))

6.1.2 Design and Synthesis of the Transition Metal-Oxides-Based Materials for Application in Supercapacitors (SCs)

In order to keep the effectiveness of the electrochemical energy storage technology to improve the advancement of renewable energy sources, several important requirements should be considered during the design and selection of the suitable electrode material. These desired requirements include (1) high stability in various types of electrolyte solution, (2) high resistance against electrochemical corrosion and (3) good electronic conductivity. Nanomaterials of about the length scale of less than 100 nm have received increasing interest, owing to their fundamental scientific significance as well as their potential applications that is derived from their fascinating electrical, magnetic and catalytic properties (Burda et al. 2005; Tan et al. 2011). As compared to the bulk active electrode materials, the corresponding nanomaterials possess outstanding electrochemical activity, such as higher capacitance, larger surface areas and relatively good conductivity; thereby, nanomaterials have vast potential application in the electrochemistry field. Many transition metal oxides (TMOs) and metal hydroxide (Johan E. ten Elshof et al. 2016) give rise to faradaic reaction that offers high specific capacity and high energy densities due to their rich electrochemistry and have been extensively explored as supercapacitive materials due to their high power and energy output through nanostructuring and materials engineering. Consequently, metal oxides (MOs) with various chemistries (i.e. simple, double or mixed metal oxides, nanostructures, composites, etc.) have been investigated as high-performance electrode materials for pseudocapacitive application. The studied MOs in this chapter include RuO₂ (Xia et al. 2012), MnO₂ (Bello et al. 2013b; Makgopa et al. 2015), SnO₂ (Yan et al. 2010), V₂O₅ (Wei et al. 2012), Fe₂O₃ (Zhou et al. 2012), Fe₃O₄ (Wu et al. 2003), Bi₂O₃ (Gujar et al. 2006), WO₃ (Deb 2008; Zheng et al. 2011) and In₂O₃ (Chang et al. 2008). Conducting polymers (CPs) have also attracted attention and are among the most promising pseudocapacitor materials for flexible supercapacitors, motivating the existing energy storage devices towards the future advanced flexible electronic applications due to their high redox active-specific capacitance and inherent elastic polymeric nature (Shown et al. 2015). Similarly, metal nitrides/oxynitrides have shown emerging properties compared to the metal oxide and in some cases show better and interesting result and therefore have attracted attention as an emerging supercapacitor electrode beyond oxide (Ghosh et al. 2018).

6.1.2.1 Ruthenium Oxide (RuO_x)

At the early stages of exploring MOs as suitable electrode material for supercapacitor applications, RuO₂ emerged as a promising and predominant candidate in energy storage due to its excellent electrical characteristics, thermal stability and other unique electrochemical properties such as high specific capacitance. Therefore, the above-mentioned properties made RuO₂ to be one of the most extensively studied electrode materials for supercapacitor applications. Ruthenium is a multivalence rare metal element, hard, brittle and light grey in colour (Zheng et al. 1995; Li et al. 2018). As a group of multifunctional material, Ru-based materials family include ruthenium dioxide (RuO₂), hydrous ruthenium dioxide (RuO₂·xH₂O), ruthenium oxide (RuO_x), ruthenium (Ru) and their composites (Vijayabala et al. 2018). There are two phases of RuO₂: a crystal phase (RuO₂, rutile phase) and an amorphous hydrous phase (RuO₂·xH₂O). The efficiency of Ru-based materials depends on variety of factors, but their overall performance strongly rests with the surface area that can result from the careful selection of the method of preparing the desired material (Wang et al. 2016a; Li and He 2018). The synthesis of Ru-based nanomaterials has been achieved via various synthetic routes such as wet chemical, vapour phase, hydrothermal and biological and also through various synthetic methods such as thermal decomposition, sol–gel process, electrostatic spray deposition and electrodeposition, among others (Hu et al. 2006; Yin et al. 2012; Park et al. 2015; Li et al. 2018; Vijayabala et al. 2018). From the above synthetic methods, the hydrothermal technique displayed several advantages, such as one-step synthesis method, low aggregation level and narrow crystallite size distribution, over the other synthetic methods (Yin et al. 2012). In addition, the high purity and excellent control over the morphology and size of the particle have been achieved by control of the nucleation and growth owing to its surfactant or protective agent (Park et al. 2015; Vijayabala et al. 2018). During this process, surfactants such as cetyltrimethylammonium bromide (CTAB), hexamethylenetetramine (HMT), polyethylene glycol (PEG), citric acid, citrate potassium, ethylenediamine (EDA) or polyethylene glycol (PEG) are used to control the morphology of the materials (Vijayabala et al. 2018). Several architectures of different morphologies of Ru-based material ranging from one-dimensional (1D) nanostructures, nanorods and nanosheets were synthesized in the presence and absence of the surfactant. The quest for other suitable and cheaper electrode materials with excellent electrochemical properties similar to RuO₂ has been explored. In addition, these RuO₂ capacitor-type materials are only suitable for aqueous electrolytes, thus limiting the nominal cell voltage to 1.0 V. In order to manipulate the morphology of RuO₂, the annealing temperature is a key factor affecting the morphology, which exists usually in the form of nanostructure such as nanoparticle , nanorod and nanofiber. High annealing temperature, for example, from 300 to 800 °C, leads to the crystalline phase of RuO₂ and less water content, resulting in the change of the number of active reaction sites, as well as the electron and proton conductivity (Park et al. 2015). Synthesis of the RuO₂ nanocatalyst is shown in Fig. 6.6. A schematic diagram for the fabrication of RuO₂ nanowires and 3D RuO₂ branched Au-TiO₂ nanostructure can be clearly observed in Fig. 6.6a, and the above results were supported by the top-view SEM images of RuO₂ nanowires and 3D branched nanostructures (Fig. 6.6b, c). A simple and cheap way of synthesising RuO₂ nanocatalyst with various morphologies can be obtained by following the route in Fig. 6.6d, e).

Fig. 6.6

(a) Schematic diagram for the fabrication of RuO₂ nanowires and 3D RuO₂ branched Au-TiO₂ nanostructure, (b, c) top-view SEM images of RuO₂ nanowires and 3D branched nanostructures (scale bars in the left and right images, 1 μm and 100 nm, respectively) (With permission of Park et al. 2015) and (d, e) synthesis of the RuO₂ nanocatalyst ((d), with permission of Li et al. 2018, and (e), with permission of Gustafson et al. 2017)

It is well known that RuO₂ is a good cathode material for electrochemical capacitors due to the fact that its hydrous form can reversibly store 150–260 F cm⁻² in aqueous electrolytes or approximately ten times the double-layer capacity of carbon (Zheng et al. 1995). This material stores charge by reversibly accepting and donating protons from an aqueous electrolyte governed by the potential dependent equilibrium of Eq. 6.7.

$$ {\mathrm{RuO}}_2+{\updelta \mathrm{H}}^{+}+{\updelta \mathrm{e}}^{-}\leftrightarrow {\mathrm{RuO}}_{2-\updelta}{\left(\mathrm{OH}\right)}_{\updelta};0\le \updelta \le 2 $$

(6.7)

Due to the lack of abundance and the expensive cost of the noble metallic element ruthenium (Ru), there are drawbacks for its commercial application in supercapacitors (SCs), leading to the reduced use of this electrode for SC applications. However, in order to improve the electrochemical properties of other materials that have limited application in energy storage , Ru-based nanomaterials are being explored resulting in Ru-based composites. Miller and co-workers (Miller 1997) have demonstrated the specific capacitance of ruthenium nanoparticles deposited on the surface of the carbon aerogels to be 330 F g⁻¹, while Zheng and co-workers (Zheng et al. 1995) have reported values as high as 768 F g⁻¹ in the early and mid-1990s. Other researchers explored various synthetic routes of making Ru-based nanocomposites by using functionalized carbon nanotubes as supports to these nanoparticles . The surface carboxyl groups from the acid-functionalized carbon nanotubes (CNTs) are able to protect against spontaneous reduction into disadvantageous Ru metal through blocking the contact between MWCNT surfaces and Ru ions by certain bond formation. The as-mentioned mechanism above is presented in schematic form in Fig. 6.7a. The electrochemical performance of the synthesized Ru-Carbon nanocomposite shown in Fig. 6.7b exhibits quite large difference in capacitance behaviour between RuO₂/P-MWNT and RuO₂/O-MWNT, with capacitance values of 182 and 304 F g⁻¹, respectively. The results obtained in this study are lower than other Ru-based results obtained from the literature. However, the ability to reduce the usage of Ru alone but substitute it with other cheap materials and yet obtain such promising capacitive performance emphasizes the promise that lies ahead in terms of developing cheap yet effective Ru-C composite SC devices.

Fig. 6.7

(a) Schematic diagram of the different formation mechanisms of RuO₂ on P-MWNT and O-MWNT in the preparation process and their actual TEM images (scale bar = 20 nm) and (b) cyclic voltammograms in 1 M H₂SO₄ electrolyte with a scan rate of 20 mV s⁻¹ (With permission of Kim and Mitani 2006)

Hwang and co-workers (2015) demonstrated a simple one-step process for the synthesis and processing of laser-scribed graphene (LSG) and RuO₂ nanocomposites fabricated into electrodes that exhibit ultrahigh energy and power densities. The high voltage asymmetric electrochemical capacitors made-up of LSG/RuO₂ as positive electrode and AC as negative electrode showed improved electrochemical performance as shown in Fig. 6.8. The asymmetric electrochemical capacitor demonstrated extremely high energy density of 55 Wh kg⁻¹ at a power density of 12 kW kg⁻¹, placing it among the best-performing hybrid electrochemical capacitors.

Fig. 6.8

Electrochemical performance of an asymmetric LSG/RuO₂//AC SCs. (a) Illustration of the structure of asymmetric SCs assembled with LSG/RuO₂ +ve electrode and activated carbon −ve electrode. (b) CV curves comparing the electrochemical properties of the +ve and −ve electrodes in their stable operating voltage windows, obtained at 70 mV s⁻¹. (c) CV curves of the asymmetric SCs at increasing voltage window from 1.0 to 2.0 V, all acquired at 70 mV s⁻¹. (d) CV performance of the asymmetric supercapacitor at different scan rates tested at a maximum voltage of 1.8 V. (e) Galvanostatic charge and discharge curves of the asymmetric supercapacitor collected at different current densities. (f) Evolution of the capacitance of the cell stack (including the current collector , active material, electrolyte and separator) as a function of the applied specific current for symmetric LSG/RuO₂, asymmetric LSG/RuO₂//AC and LSG SCs (With permission of Hwang et al. 2015)

Recently, Jiang and co-worker (2018) have managed to integrate the Ru nanoparticles on the 2D transition metal carbides and nitrides , known as MXenes instead of the carbon nanomaterials employed in the literature and tested them for SC applications (Fig. 6.9). They have demonstrated that the hydrophilicity of MXenes combined with their metallic conductivity and surface redox reactions which is the key for high-rate pseudocapacitive energy storage in MXene electrodes can be used to support electrode materials that have good theoretical capacitive performance but yet limited in applications due to the poor conductivity. Their study utilized titanium carbide MXene (Ti₃C₂T_x) as negative electrode in acidic electrolyte while ruthenium oxide (RuO₂) as positive electrode. The asymmetric device operated at a voltage window of 1.5 V and managed to deliver an energy density of 37 μW h cm⁻² at a power density of 40 mW cm⁻², with 86% capacitance retention after 20,000 charge–discharge cycles.

Fig. 6.9

(a) Low- and (b) high-magnification FESEM images of RuO₂/CF. (c) Low-magnification SEM image showing uniform coating of Ti₃C₂T_x on carbon fibres. (d) Wrapped Ti₃C₂T_x flakes over an individual fibre of the carbon fabric. (e) CVs of RuO₂/CF, Ti₃C₂T_x/CF and the asymmetric device at a scan rate of 50 mV s⁻¹. (f) CVs of RuO₂//Ti₃C₂T_x asymmetric sandwich device at different scan rates, inset shows the digital photograph of the asymmetric device. (g) Galvanostatic charge–discharge curves at different current densities. (h) CVs of asymmetric in-plane device; inset shows optical image for the in-plane solid-state device attached to polypropylene substrate. (i) Ragone plot displays real energy and power densities of RuO₂//Ti₃C₂T_x device in comparison with the other state-of-the-art microsupercapacitors, square represents carbon-based microsupercapacitors, triangle represents pseudocapacitive asymmetric microsupercapacitors. (j) Cyclic voltammograms of the RuO₂//Ti₃C₂T_x tandem in-plane devices at scan rate of 10 mV s⁻¹ (inset shows that two devices connected in series can power up a green LED during the discharge state) (With permission of Jiang et al. 2018)

6.1.2.2 Manganese Oxide (Mn_xO_y)

Manganese oxide (Mn_xO_y)-based electrode materials (i.e. MnO₂, Mn₃O₄, Mn₂O₃ and MnO) and its derivatives have been studied as the alternative electrode material in supercapacitors due to their contrasting properties as compared to RuO₂. The Mn_xO_y-based electrode materials and their derivatives have demonstrated to be environmentally friendly while being relatively cheap and abundant in nature. Apart from the socio-economic benefits of the Mn_xO_y-based materials and their derivatives, these electrode materials are also characterized by their relatively high theoretical specific surface area (i.e. MnO₂, 1370 m² g⁻¹), multiple crystal phases, high theoretical capacitance and good electrochemical response (Subramanian et al. 2005; Yan et al. 2009; Wu et al. 2010; Wang et al. 2012c; Yu et al. 2013b; Jafta et al. 2013; Chidembo et al. 2014; Borgohain et al. 2014). Manganese oxide displays various oxidation states and phases, and the most often studied phases of Mn_xO_y-based materials are MnO₂ having an oxidation state of +4, Mn₂O₃ having an oxidation state of +3 and Mn₃O₄ having an oxidation state of both +2 and +3. The MnO₂ nanomaterials further exhibit various crystallographic forms depending on the types of structural phases (i.e. -, -, γ-, -, ε- and -types) that are dominant during synthesis, as shown in Fig. 6.10 (Xiao et al. 2009). They can also be identified in their various mineral forms such as birnessite, hausmannite, pyrolusite, manganite, manganosite and psilomelane (Potter and Rossman 1979; Fan and Yang 1999; Wei et al. 2011). Several structural forms of MnO₂ with different nanoarchitectures such as nanowires (Wang et al. 2014; Yin et al. 2014), nanorods (Wu et al. 2013), single crystal nanotubes (Xiao et al. 2009), nanourchins (Xu et al. 2007) and amorphous (Hatzell et al. 2014) have been synthesized using hydrothermal techniques. Some other techniques used for the synthesis of MnO₂ nanostructures include thermal decomposition, co-precipitation (Lee and Goodenough 1999; Brousse et al. 2006; Nam et al. 2010), simple reduction (Ni et al. 2009; Ragupathy et al. 2009), sol–gel (Wang et al. 2005, 2007), solid-state process and microwave process (Meher and Rao 2012; Ming et al. 2012; Bello et al. 2013a). The other rising Mn_xO_y material in the literature is manganese (II, III) oxide with the chemical formula Mn₃O₄. It is found in nature as the mineral hausmannite and is made of a spinel structure with tetragonal distortion elongated along the c-axis due to Jahn-Teller effect on the Mn³⁺ ion. Manganese ions occupy the octahedral B-site (Mn³⁺) and tetrahedral A-site (Mn²⁺) corresponding to a normal spinel structure (Fig. 6.10). Several studies have been conducted in order to improve Mn₃O₄ electrode materials in SC application. Subsequently, to improve the electrochemical capacitive behaviour of Mn₃O₄, the nanosized particles of Mn₃O₄ have been prepared by various methods such as successive ionic layer adsorption and reaction (SILAR) (Dubal and Holze 2013), hydrothermal (Yang et al. 2015), solution combustion (Gao et al. 2011), chemical bath deposition (Xu et al. 2006), sonochemical (Baykal et al. 2010) and microwave techniques (Zhou et al. 2011; Sankar et al. 2012). Different nanoarchitectures of Mn₃O₄ particles such as single crystals, nanocrystals, nanocubics, nanorods, microspheres and interlocked cubes have been successfully synthesized and tested for SC application (Dubal et al. 2009, 2010; Sankar et al. 2012; Wang et al. 2012; Lee et al. 2012; Dong et al. 2013). Manganese (ΙΙΙ) oxide, Mn₂O₃, and manganese (ΙΙ) oxide, MnO, are other types of the oxides of the Mn_xO_y-based derivatives. The Mn₂O₃ type exists in two recognizable forms, α-Mn₂O₃ and γ-Mn₂O₃. The α-Mn₂O₃ has the cubic bixbyite structure, while pure Mn₂O₃ has an orthorhombic structure, and γ-Mn₂O₃ has a structure related to the spinel structure of Mn₃O₄ with the oxide ions closely packed in cubic form (Wells 1984). A MnO, just like any monoxides, adopts the rock salt structure, with anions and cations being both octahedrally coordinated (Greenwood 1997). Just as much as any manganese oxide material, Mn₂O₃ nanospheres have been synthesized using hydrothermal and sonochemical techniques by Nathan et al. as an EC electrode material (Nathan et al. 2008). Chen et al. has successfully synthesized γ-Mn₂O₃ nanowire bundles (Chen et al. 2005). Recently, Li et al. synthesized nanocubic Mn₂O₃ using hydrothermal technique for the EC application (Li et al. 2015). Nano-sized manganese oxide (Mn₂O₃) was synthesized by Chiang et al. using a solvothermal technique (Chiang et al. 2006). There is a limited literature on the synthesis of Mn₂O₃ for EC application even though several of these materials have been widely synthesized using various techniques such as chemical oxidation (Chen et al. 1997), calcination of MnO₂ and hydrothermal (He et al. 2003).

Fig. 6.10

Different Mn_xO_y crystal structures With permission of Liu et al. 2015c)

Several studies on electrochemical behaviour of MnO₂-based materials and its derivatives in an aqueous electrolyte have been conducted in order to better understand their mechanism of charge storage on the surface of the electrode. Hydrated manganese oxides exhibit specific capacitances within the range of 100–200 F g⁻¹ in alkaline electrolyte. The surface reaction occurring at the MnO₂ electrode involves the adsorption of electrolyte cations (C+ = H+, Li+, Na+ and K+) on the MnO₂-based materials, whereas the bulk faradaic reaction relies on the intercalation or de-intercalation of electrolyte cations in the bulk of the manganese oxide and is illustrated by Eqs. 6.7 and 6.8, respectively, (Toupin et al. 2004; Yang et al. 2014):

$$ {\left({\mathrm{MnO}}_2\right)}_{\mathrm{surface}}+\mathrm{C}++{\mathrm{e}}^{-}\leftrightarrow {\left({{\mathrm{MnO}}_2}^{-}{\mathrm{C}}^{+}\right)}_{\mathrm{surface}} $$

(6.8)

$$ {\mathrm{MnO}}_2+{\mathrm{C}}^{+}+{\mathrm{e}}^{-}\leftrightarrow \mathrm{MnOOC} $$

(6.9)

In the established charge storage mechanisms, a redox reaction between the III and IV oxidation states of Mn ions occurs (Yang et al. 2014). Dedicated approach on the synthesis of MnO₂-based materials and the electrolyte type played a major role on the electrochemical outcome of its SC applications. An amorphous hydrous manganese oxide (α-MnO₂·nH₂O) was anodically deposited onto a graphite substrate showing an improved maximum specific capacitance of 330 F g⁻¹ and 320 F g⁻¹ achieved from cyclic voltammetry and galvanostatic charge-discharge measurements, respectively, using Na₂SO₄ (0.1 M) as electrolyte (Hu and Tsou 2002). However, due to the high resistivity of α-MnO₂·nH₂O, a conducting additive , such as carbon materials, CNTs or graphene and OLCs, is required for the realization of EC electrodes (Cottineau et al. 2006; Sharma et al. 2007). As a result, α-MnO₂ has been decorated on the SWCNT by a simple precipitation technique with good cycle power at the specific current of 2 A g⁻¹ (Subramanian et al. 2006). Ma et al. (2008) synthesized a birnessite-type MnO₂ coated uniformly on multi-walled carbon nanotubes (CNTs) with specific surface area of 200 m² g⁻¹ by using a spontaneous direct redox reaction between the multi-walled CNTs and permanganate ions (MnO₄ ⁻) with a high specific capacitance of the MnO₂/CNT nanocomposite in an organic electrolyte of ca. 250 F g⁻¹ at a high specific current of 1 A g⁻¹. Jafta et al. (2013) synthesized α-MnO₂ from the raw electrolytic manganese dioxide (EMD) using hydrothermal technique in the presence of a sodium dodecyl sulphate (SDS) surfactant and decorated them on the surface of the graphene oxide (GO). The electrochemical properties of nanostructured α-MnO₂/GO composite fabricated in an aqueous asymmetric electrochemical capacitor exhibited high energy density of 35 Whkg⁻¹ and specific capacitance of 280 F g⁻¹ at high voltage window of 1.8 V using 1 M Li₂SO₄ as electrolyte . The electrochemical behaviour of vertical graphene nanosheets (VGNs) and MnO₂ was also investigated; the VGNs provide controlled porous structure and high electrolyte accessible surface area for multiple pathways for ion transport. The charge storage behaviour of the MnO₂/VGNs was excellent exhibiting a high areal capacitance of 5.6 mF/cm², which is 110 times higher than that of VGNs (51.95 μF/cm²) at 50 mV s⁻¹. The enhanced capacitance is attributed to the highly conductive 3D network of VGNs which provide fast electron and ion transport and also a large pseudocapacitance from MnO2 coating (Ghosh et al. 2016). Makgopa et al. (2015) showed that the use of birnessite-type MnO₂ decorated on highly graphitized onion-like carbon (OLC) can improve the electrochemical performance of the MnO₂. The nanocomposite of OLC/MnO₂ shown by the FESEM images (Fig. 6.11a, b) displayed better interaction and was fabricated on a symmetrical two-electrode device using the Ni foam as a current collector (Fig. 6.11c, d,) and exhibited a large specific capacitance of 254 F g⁻¹ and a remarkably high power density of 74.8 kW kg⁻¹. The surface morphologies of OLC/MnO₂ nanohybrid are shown in Fig. 6.11a (FESEM) and in Fig. 6.11b (TEM) which indicates clear interactions of the precursor material with each other for good capacitive performance. The SAED pattern in the inset (Fig. 6.11b) shows lattice fringes for crystalline MnO₂ and circular lattice shells for OLC confirming the synergistic effect from the nanocomposites.

Fig. 6.11

(a) SEM (b) TEM images of the OLC/MnO₂ nanohybrid (the inset is the corresponding SAED pattern of (b)), (c) cyclic voltammograms at 5 mV s⁻¹ and (d) galvanostatic charge–discharge curves at 0.3 A g⁻¹ comparing OLC and the OLC/MnO₂ nanohybrid, (e) galvanostatic charge–discharge curves for the OLC/MnO₂ nanohybrid at various current densities and (f) C_sp vs. current densities of the OLC/MnO₂ nanohybrid. Three-electrode configuration with nickel foam as the current collector . Electrolyte: aqueous 1 M Na₂SO₄ (With permission of Makgopa et al. 2015)

Manganese (II, III) oxide (Mn₃O₄) as a potential electrode material for ECs showed up just after the researchers discovered MnO₂ as pseudocapacitive electrode material. In 2003, a thin film Mn₃O₄ electrode material was reported to have displayed specific capacitance of 92 F g⁻¹ in aqueous electrolyte and 58 F g⁻¹ in organic electrolyte from cyclic voltammetry at the scan rate of 20 mV s⁻¹ (Jiang and Kucernak 2002). Wu and Hu (2005) studied a mixture, consisting of Mn₃O₄ and MnOOH crystals and reported specific capacitance ranging between 45 and 71 F g⁻¹, and Djurfors et al. (Djurfors et al. 2005) showed that Mn₃O₄ film prepared by either thermal or electrochemical oxidation of Mn/MnO films has an effect on the capacitive performance of the electrode material. Nagarajan et al. (2006) later followed the principle of Djurfors et al. to form a spinel Mn₃O₄ phase at 300 °C with an improved specific capacitance value of 445 F g⁻¹ in Na₂SO₄ electrolyte. As a race in finding a stable and suitable electrode derived from the Mn₃O₄ material, researchers developed composites of this material from carbon templates. In 2008, Mn₃O₄/MWCNT composite was examined using cyclic voltammetry and obtained a maximum specific capacitance of 330 F g⁻¹ (An et al. 2008). Wang et al. (2010) later used graphene sheet as a template for decoration of Mn₃O₄ by mixing graphene suspension in ethylene glycol with MnO₂ organosol, followed by subsequent ultrasonication processing and heat treatment. The as-prepared Mn₃O₄/graphene nanocomposite gave a specific capacitance of 175 F g⁻¹ in Na₂SO₄ electrolyte and 256 F g⁻¹ in KOH electrolyte. The higher result obtained in the alkaline medium is due to the higher ionic conductivity of the electrolyte compared with the neutral electrolyte. As it has been shown from the literature, the hydrated ion size (3.31 Å for K⁺ and 3.58 Å for Na⁺) and ionic conductivity (73.5 S cm² mol⁻¹ for K⁺ compared to 50.11 S cm² mol⁻¹ for Na⁺) play a crucial role in electrochemical performance of carbon electrode materials (Zhong et al. 2015). Similarly, the ionic radius of the hydrated negatively charged anions contributes to the EDL behaviour via electroadsorption, and the sizes are in the following order OH⁻ (3.00 Å) < ${\mathrm{NO}}_3^{-}$ (3.35 Å) < ${\mathrm{SO}}_4^{2-}$ (5.33 Å) (Zhong et al. 2015). Hence, the alkaline electrolyte is expected to give the best electrochemical performance taking into account electrode that could easily accommodate the smaller size of K⁺ and the electroadsorption of the negatively charge anions (OH⁻), coupled with it better conductivity and ionic mobility. Qiao et al. (2015) synthesized micro/nanostructured Mn₃O₄ with an open 3D flower-like morphology by a facile solvothermal approach using hexadecyltrimethylammonium bromide as a surfactant and ethanol as a solvent. The Mn₃O₄ microspheres used exhibited electrochemical performance with a specific capacitance of 286 F g⁻¹ at a low specific current (0.5 A g⁻¹) and still retained 80% (230 F g⁻¹) and 73% (210 F g⁻¹) at higher current densities of 5 A g⁻¹ and 10 A g⁻¹, respectively. The enhanced capacitive performance of Mn₃O₄/graphene nanocomposites was attributed to the nanoparticulate nature of Mn₃O₄ nanoparticles and functional groups attached to graphene nanosheets. Recently, Makgopa et al. (2017) reported on the electrochemical performance of the tetragonal hausmannite Mn₃O₄, when embedded on various carbon materials to form OLC/Mn₃O₄, CNT/Mn₃O₄, GO/Mn₃O₄ and AC/Mn₃O₄ nanohybrids, and investigated as electrode material for symmetric, as shown in Fig. 6.12, and asymmetric pseudocapacitor device. The nanohybrid electrode materials demonstrated higher electrochemical performance (in terms of specific capacitance and rate capability as energy storage devices) compared to the pure Mn₃O₄. The OLC/Mn₃O₄ nanohybrid was shown to be the best-performing symmetric pseudocapacitor device, and it exhibited higher specific capacitance of 195 F g⁻¹, specific energy of 4.3 Wh kg⁻¹ and power density of 52 kW kg⁻¹ compared to other carbon nanohybrid materials studied. When it was tested as a high-voltage asymmetric pseudocapacitor device, the OLC/Mn₃O₄ nanohybrid displayed maximum energy and power densities of ca. 19 Wh kg⁻¹ (at 0.1 A g⁻¹) and 45 kW kg⁻¹ (at 10 A g⁻¹), respectively. The high performance of the OLC-based system compared to the other carbon systems is ascribed to the combined unique intrinsic properties of the OLC, high electrical conductivity, highly accessible outer surface and large interparticle pore volumes. The above properties have ensured OLC/Mn₃O₄ nanohybrid as a suitable candidate for the high-voltage asymmetric pseudocapacitor device.

Fig. 6.12

SEM images of (a) OLC/Mn₃O₄, (b) CNT/Mn₃O₄, (c) GO/Mn₃O₄ and (d) AC/Mn₃O₄ nanohybrids. Inset in (a–d) is the high-magnification SEM image of respective nanohybrids. Comparative (e) cyclic voltammograms at 5 mV s⁻¹, (f) galvanostatic charge-discharge curves at 0.3 A g⁻¹, (g) Csp vs. specific current plot and (h) energy vs. power densities, for OLC/Mn₃O₄, CNT/Mn₃O₄, GO/Mn₃O₄, AC/Mn₃O₄ nanohybrids and Mn₃O₄ in aqueous 1 M Na₂SO₄ (With permission of Makgopa et al. 2017)

To the best of our knowledge, there is no much literature on the use of Mn₂O₃ and MnO as SC applications. The study of Mn₂O₃ as electrode material for supercapacitors surfaced at around 2006, when Chiang et al. (2006) reported on the synthesis and the examination of the nanoparticles of Mn₂O₃ as a potential SC electrode material. The electrode material exhibited a maximum specific capacitance of about 197 F g⁻¹ from cyclic voltammetry at the scan rate of 10 mV s⁻¹. Yu et al. (2008) reported on the synthesis of Mn₂O₃ particles that were made of micropores having a specific capacitance of 350 F g⁻¹. The improved performance was attributed to the relatively high specific surface area of 283 m² g⁻¹. Zhang et al. (2009) reported on the nano-sized Mn₂O₃ which was homogenously incorporated on a mesoporous carbon template exhibiting an improved specific capacitance of 600 F g⁻¹. In 2011, Wang et al. (2011) reported a composite of Mn₂O₃ and carbon aerogel microbead (CAMB), a maximum specific capacitance of 368 F g⁻¹, and it was emphasized the fact that this type of manganese oxide material is well suited for EC applications. Li et al. (2015) developed porous Mn₂O₃ nanocubins through hydrothermal technique that was followed by calcination in the air which exhibited a specific capacitance of 191 F g⁻¹ at a specific current of 0.1 A g⁻¹. This electrode material showed relatively high rate capability at high specific current of 5.0 A g⁻¹ and excellent long-term cycle stability even after 3000 cycles. The enhanced rate capability of this material was associated with the improved porosity. Mn₂O₃-based electrode materials are potentially useful as supercapacitor electrode materials, albeit its growth or attention has been dramatically slow compared to other manganese oxide-based electrode such as MnO₂ and Mn₃O₄. To the best of our knowledge, there is no literature on the use of manganese mono-oxide (MnO) as an electrode material for SC application. For more information on this Mn_xO_y-based electrode material, the reader is referred to the book chapter by Makgopa et al. (2016).

6.1.2.3 Iron Oxide (Fe_xO_y)

Iron oxide (Fe₃O₄) is another cheap alternative to RuO_x and has attracted attention in several fields of application due to its favourable magnetic, electric and optical properties (Ozkaya et al. 2009) and has been chosen as a potential electrode material in energy storage due to its high theoretical capacity (924 mA h g⁻¹), natural abundance, low cost, good chemical stability, environmental friendliness and cost-effectiveness (Wang et al. 2008). Similar to the manganese discussed above, different crystal structures can easily lead to pathway for the synthesis of these materials. Figure 6.13 shows the various enigmatic structural, electronic and magnetic transformations at high pressures and high temperatures.

Fig. 6.13

Crystal structures of iron oxide phases. Building blocks are octahedral (brown) and trigonal prisms (blue) (With permission of Bykova et al. 2016)

They have been predominantly investigated as anode in Li-ion batteries , showing high- performance capability when the morphologically or compositionally is properly engineered (Luo et al. 2013). For SC applications, they are suitable as negative electrodes for hybrid devices due to their broad potential window of operation (1–1.2 V) in aqueous solutions (Qu et al. 2011). The preliminary evaluations revealed that the relatively low electrical conductivity of these oxides requires the introduction of conductive additive such as CB to increase its conductivity and boost its capacitance. In addition, it has been already demonstrated in several studies that combining these oxide materials with carbonaceous materials leads to high values of conductivity and can improve the overall electrochemical behaviour due to synergistic effects between the oxide and the carbon material. The capacitive nature of Fe₃O₄ has been investigated by Wu et al. (2003) in various electrolytes, and it was shown with a high capacitance value of 510 F g⁻¹. Similarly, it has been shown that Fe₂O₃ can operate in a wide negative potential window of 0 to −1.35 V; such an excellent supercapacitive performance makes it a perfect anode electrode material (Tang et al. 2015). The reaction governing the performance of aqueous electrolytes such as Na₂SO₃, Na₂SO₄ and KOH (Wang et al. 2006) was established using an electrochemical quartz crystal microbalance and X-ray photoelectron spectroscopy. It was found that the capacitance of Fe₃O₄ in the Na₂SO₃ electrolyte arises due to the combination of both the EDLC and pseudocapacitor involving the reduction/oxidation of specifically adsorbed sulphite anions given by the equations below. In the Na₂SO₄ solution, the performance was due to the formation of a double layer, and in the KOH, the performance arises due to partial surface oxidization of the oxide electrode.

$$ 2{\mathrm{S}\mathrm{O}}_3^2+3{\mathrm{H}}_2\mathrm{O}+4e\leftrightarrow {\mathrm{S}}_2{\mathrm{O}}_3^2+6\mathrm{OH} $$

(6.10)

$$ 2{\mathrm{S}\mathrm{O}}_3^2+3{\mathrm{H}}_2\mathrm{O}+8\mathrm{e}\leftrightarrow 2{\mathrm{S}}^2+6\mathrm{OH} $$

(6.11)

Quite a number of Fe₃O₄ electrodes have been reported, for example, Fe₃O₄ films made by electroplating technique exhibited a specific capacitance of 170 F g⁻¹ in Na₂SO₃ (Wang et al. 2006), and Fe₃O₄ nanoparticles with a specific capacitance of 185 F g⁻¹ was reported via a sol–gel technique (Mitchell et al. 2014). However, the capacitance reported is far below the theoretical value, which is because of low electronic conductivity, and morphology collapse, which often leads to low electrochemical performance and poor cycling performance. To build on the exhibiting and maximize the performance of SCs, it is necessary to develop hybrid electrode materials, combining electric double-layer capacitive materials and pseudocapacitive materials such as Fe₃O₄. Thus, Fe₃O₄ has been included in many carbon matrices to improve the overall capacitance of the system because of the good electrical conductivity and excellent electrochemical stability (Sevilla and Mokaya 2014; Meng et al. 2014). These Fe₃O₄–carbon composites showed improved electrochemical performance compared to the individual components due to a positive synergy between Fe₃O₄ and carbon materials, and their charge storage arises from both the double-layer and faradaic reactions. For example, cubic Fe₃O₄ and graphene layer assembled into a symmetric supercapacitor device with the specific capacitance ~782 F g⁻¹ at a specific current of 3 A g⁻¹ (Xu et al. 2008). An Fe₂O₃ decorated with single-walled carbon nanotubes (SWCNT) as flexible anodes in LIBs with a discharge capacity of 801 mA h g⁻¹ after 90 cycles was reported by Zhou et al. (2012). The excellent result obtained was attributed to the conductivity of the SWCNT. Min et al. (2017) reported a facile and efficient technique to synthesize porous iron oxide coated with graphene as was tested as electrode materials for lithium–ion batteries and supercapacitors. The encapsulation of porous Fe₂O₃ and Fe₃O₄ nanorods by graphene is directly carried out from FeOOH/GO colloids by taking advantage of an electrostatic self-assembly method, owing to the positively charged surface of FeOOH and the negatively charged surface of GO . This combination leads to multifunctional features of the electrode materials as follows: (1) An enhanced electrical conductivity makes the electrodes serve as the current collectors ; (2) reinforced softness of the electrodes accommodates the large volume changes during charge–discharge cycles; (3) improved high specific surface area of the electrodes increases the accessibility of the active electrode materials to electrolyte; (4) the pores formed by graphene and iron oxide particles facilitate ion transportation; and (5) iron oxide particles separate graphene and prevent their restacking or agglomeration and vice versa, thus improving the immersion and splitting of electrolyte into and out of the electroactive material. Consequently, the porous iron oxide/graphene hybrid nanocomposites deliver a good performance in the electrochemical energy storage . As expected, the incorporation of Fe₃O₄ in carbon matrix can greatly improve the electrochemical performance by inducing some vigorous redox reactions. Similarly, Kim et al. reported that composites of multi-walled carbon nanotubes and Fe₃O₄ exhibited specific capacitances of 165 F g⁻¹ at a specific current of 0.2 A g⁻¹ (Kim and Park 2011). The Fe₃O₄ and CNT together formed an interpenetrating network that provided a high specific capacitance and better cycling stability. An ultrathin nanoporous Fe₃O₄/carbon nanosheets with surface areas of 229 m² g⁻¹ was reported by Liu et al.; the composite exhibited specific capacities of 163.4 F g⁻¹ at a specific current of 1 A g⁻¹. An optimum Fe₃O₄ content in a Fe₃O₄/SiCF feeding ratio of 1.5:1 (SiCF/Fe₃O₄(1.5)) exhibited a high charge storage capacity, showing a specific capacitance of 423.2 F g⁻¹ at a scan rate of 5 mV s⁻¹ with a 81.8% rate performance from 5 to 500 mV s⁻¹ in 1 M KOH aqueous electrolyte (Kim and Kim 2017). These results indicate the potential and advantages of iron-based materials for use as a high-energy and high power density supercapacitor electrode. Furthermore, an optimized electrochemical performance of electrode structures with proper control of the metal oxide/carbon was produced as shown in the scheme in Fig. 6.14. The morphology shows a hierarchical graphite foam–carbon nanotube framework with thin layer of iron oxide coating on the surface to form (GF–CNT@Fe2O3) composite. Anode cells fabricated based on this material displayed a high energy of ∼74.7 Wh/kg at a power of ∼1400 W/kg and ∼95.4% capacitance retention after 50,000 cycles of charge–discharge. This performance is superior to the results obtained by the GF-CNT alone and to many different forms of metal oxide-based supercapacitors, making it a promising candidate for the next generation of high-performance electrochemical energy storage .

Fig. 6.14

(a) Schematic representation of the growth procedure of GF-CNT@Fe₂O₃ starting from graphite foam. (b) SEM and TEM characterization of GF-CNT@Fe₂O₃, (a–c) SEM and (d, e) HRTEM images of GF-CNT@Fe₂O₃, (f) HAADF-STEM image of a single CNT@Fe₂O₃ and (g–i) the corresponding STEM element mapping. The scale bar in (f) is 10 nm. (c) Electrochemical analysis, (a) CV curves and (b) charge-discharge curves of GF-CNT@400Fe₂O₃ and GF-CNT. (c) Rate properties and (d) EIS of four samples (With permission of Guan et al. 2015)

Similarly, the spray deposition method was used for the fabrication of a flexible supercapacitor based on thin film of multi-walled carbon nanotube (MWNT) cathodes and hematite (α-Fe₂O₃)/MWNT composite anodes. The device fabricated provided a very high specific energy density of 50 Wh kg⁻¹ at a power density of 1000 W kg⁻¹ with potential window of 0–2.8 V, this energy value was eight times bigger than that fabricated for symmetric supercapacitors based on MWNT electrodes under the same conditions. The excellent performance of the hybrid devices was attributed to the incorporation of MWNTs into the α-Fe₂O₃ anodes, which leads to a decrease of internal resistance and an improvement in both the ion diffusion behaviour and the integrity of the α-Fe₂O₃ containing films (Zhao et al. 2009) (Fig. 6.15).

Fig. 6.15

(a) an optical image showing that a LED was lit brightly even after 300 cycles of the MWNT-α-Fe₂O₃/MWNT, (b) an optical image, (c, d) SEM images of the top surface of a α-Fe₂O₃/MWNT composite anode after 600 cycles and then bending, (e) galvanostatic charging/discharging behaviour of α-Fe₂O₃ and α-Fe₂O₃/MWNT composite as anode (MWNT as cathode ) and (f) Ragone plot of MWNT cathode and various anode arrangements, together with other energy storage technologies (With permission of Zhao et al. 2009)

6.1.2.4 Vanadium Oxide (V₂O₅)

Vanadium oxide is gaining considerable research interest as suitable electrodes for supercapacitor owing to their low cost, layered structure and its ability to exist in variable oxidation states (e.g. V₂O₅, V₂O₃ and V₄O₇) (Wei et al. 2012; Saravanakumar et al. 2012; Li et al. 2013; Cao et al. 2014). The electronic structure, charge density, transport properties and phase transitions of different vanadium oxides are expected to vary with the composition. Vanadium pentoxide, V₂O₅, possesses the unique advantages of high-energy density (Yang et al. 2011b), and wide potential window arising from its variable oxidation states (Choi et al. 2006) makes it suitable as electrode material for supercapacitors. Different nanostructures of V₂O₅ such as nanowires (Wang et al. 2015), nanotubes (Yu et al. 2012), nanorods (Tien and Chen 2013), nanofibers (Wee et al. 2010), hollow spheres (Yang et al. 2013) and V₂O₅ nanosheets constructing 3D architectures (Xiong et al. 2008) have been synthesized using techniques such as hydrothermal (Xiong et al. 2008), electrospining (Wee et al. 2010; Yu et al. 2011), template-assisted growth (Cheng et al. 2008) and various other methods are used to produce these unique nanostructures. For example, Zhang et al. (Zhang et al. 2016b) reported three different structures of V₂O₅ (nanobelts, nanoparticles and microspheres) by a simple hydrothermal route, and the combination of calcination and the electrochemical results obtained indicate that microspheres lead to a significant improvement of storage capacity with a specific capacitance of 308 F g⁻¹ in 1.0 M LiNO₃ electrolyte. Figure 6.16 shows the processing of bulk V₂O₅ into nanosheets of V₂O₅ of 4 nm and lateral dimension of up to micrometres via hydrothermal system and characterized with the various state-of-the-art facilities such as TEM, SEM and STEM (Fig. 6.16b). The active electrode material displayed excellent specific capacitance of 451 F g⁻¹ in a neutral aqueous Na₂SO₄ electrolyte, as the 3D architectures are utilized for energy storage , with capacitance retention after 4000 cycles is more than 90%, and the energy density is up to 107 W·h·kg⁻¹ at a high power density of 9.4 kW kg⁻¹ (Zhu et al. 2013). The performance was due to the unique 3D V₂O₅ architecture that provides high surface area for enhanced electrolyte/electrode interaction and also reduces the diffusion path both for electrons and ions.

Fig. 6.16

(a) Schematics of the fabrication steps of the 3D V₂O₅ architectures: (1) producing thin and flexible V₂O₅ nanosheets via the hydrothermal treatment of V₂O₅ powder with H₂O₂ at 190 °C; (2) removal of water from the as-prepared V₂O₅ nanosheet samples via freeze-drying. (B) Characterization of the 3D V₂O₅ constructs, (a, b) FESEM; (c, d) TEM images (FFT, inset); (e−g) STEM image and corresponding elemental mapping of (f) oxygen and (g) vanadium. (c) Electrochemical performance of 3D V₂O₅ (a) Specific capacitances from CV. (b) Galvanostatic charge/discharge (GCD) at a specific current of 0.5 A·g⁻¹. (c) Specific capacitances from GCD (With permission of Zhu et al. 2013)

Nanoporous V₂O₅ network was prepared via a capping agent-assisted technique and examined the effect of annealing on the morphology, electrochemical and structural properties (Saravanakumar et al. 2012). The electrode fabricated exhibited a specific capacitance of 316 F g⁻¹, which is attributed to the creation of facile nanochannels for ion diffusion and facilitates the easy accessibility of ions. Nevertheless, despite the high capacitance values recorded for V₂O₅ nanostructures , their poor electronic conductivity and high dissolution issues in liquid electrolyte due to phase changes upon cycling, which gradually erodes the quantity of electrochemically active materials, are detrimental to high-rate, long-term cycling and mechanical stability (Qu et al. 2012). Thus, the combination of V₂O₅ with carbon-based material or doping of the V₂O₅ has been demonstrated to be an effective strategy to improve electronic transport (Chen et al. 2009, 2011b). As such, various composites have been investigated, for example, W-doped V₂O₅ nanobelts were prepared and exhibited specific capacitance of 407 F g⁻¹ at 0.5 A g⁻¹. Similarly, V₂O₅/reduced graphene oxide (rGO) hybrid nanostructures are fabricated and showed a higher specific capacitance (484 F g⁻¹ at 0.5 A g⁻¹), rate capacity (251 Fg⁻¹ at 10 A g⁻¹) and cyclic stability (83% up to 1000 cycles) (Saravanakumar et al. 2016). The improved capacitive performance was due to the high conductivity of rGO and electrochemical activity of V₂O₅. Similarly, an effective facile electrochemical oxidation method was reported to significantly boost the durability and capacitance of VO_x through tuning the valence state of vanadium (Yu et al. 2015). Figure 6.17 shows the SEM, TEM and the electrochemical performance of V₂O₃ nanorods. The valence state of vanadium is optimized through a very facile electrochemical oxidation method, with morphological characterization revealing porous tubular structures (Fig. 6.17a, b). A superior electrochemical performance with rectangular cyclic voltammograms (Fig. 6.17c) and a long stability after 100,000 cycles (Fig. 6.17h) for the prepared electrodes was demonstrated with an in-depth study on the variation for the valence state of vanadium during the oxidation process, and the cyclic stability indicates that the long cyclic stability has an important relationship with the distribution of the valence state of vanadium (Yu et al. 2015).

Fig. 6.17

SEM images of as-prepared (a) V₂O₃ and (b) VO_x samples. (c) CV curves collected at 100 mV s⁻¹ of VO_x electrode after different cycles. (d) TEM image, (e) SAED pattern, and (f) HRTEM images of a V₂O₃ nanorods. (g) TEM image and corresponding SAED pattern of a VO_x sample. (h) The concentration of V element in the electrolyte and the mass loading of active materials remained in electrode after different cycles (With permission of Yu et al. 2015)

The electrochemical co-deposition of vanadium oxide and polypyrrole was also explored for high-performance supercapacitor devices as shown in Fig. 6.18. The V₂O₅–PPy composite exhibited fibre-like morphology (Fig. 6.18a), and due to the organic-inorganic synergistic effect, the composite exhibits good charge storage properties with a large potential window from −1.4 to 0.6 V vs SCE in a three-electrode configuration, exhibiting a specific capacitance of 412 F g⁻¹ at 4.5 mA cm⁻² (Fig. 6.18b) with good capacitance retention as shown in Fig. 6.18c. The device assembled based on this V₂O₅–PPy composite displayed a high operating voltage of 2 V and a high energy density of 82 Wh kg⁻¹ (at the power density of 800 W kg⁻¹) (Bai et al. 2014). The results demonstrate the development of vanadium-based electrode materials as a low-cost strategy towards the advancement of high-performance supercapacitors.

Fig. 6.18

(a) SEM images of VPy-3 deposited on carbon cloth substrate, (b) galvanostatic charge−discharge profiles of VPy-3, V₂O₅ and PPy collected at a specific current of 4.5 mA cm⁻², (c) cycling stability of VPy-3//VPy-3 collected by galvanostatic charge−discharge experiments at a specific current of 9 mA cm⁻² and (d) Ragone plots of VPy-3//VPy-3, the values reported for other supercapacitors are included for comparison (With permission of Bai et al. 2014)

Similarly, VO and VO₂ have also been explored and have demonstrated excellent results, for example, VO coupled with amorphous vanadium covalent bonds (VO_x) was incorporated into carbon nanofibres (VO/VOx/CNF) by electrospinning and heat treatment. The VO/VOx/CNF electrode displayed specific capacitance of 325.7 F g⁻¹ at a specific current of 1 A g⁻¹ and maintaining 92% of its initial capacitance after 5000 cycles at a specific current of 4 A g⁻¹ in a 6 M KOH. The superior electrochemical performance of VO/VOx/CNF was attributed for two reasons, namely, the enhanced conductivity brought upon the incorporation of quasi-metallic VO (∼ 102 Ω⁻¹ cm⁻¹) and the network of nanowire and secondly the rapid ion transfer rate caused by the rich vanadium redox couples VO/VOx as well as the developed pore structure (Tang et al. 2016). VO₂ has also been studied as a potential electrode material for supercapacitors, and they have shown a great promise for potential application (Yan et al. 2016), for example, Rakhi et al. reported VO₂ nanosheet electrodes in organic gel electrolyte for supercapacitor applications by hydrothermal method, and the electrodes exhibited a specific capacitance of 405 F g⁻¹ at a specific current of 1 A g⁻¹ (Rakhi et al. 2016). Also, Deng et al. prepared hybrid of graphene/VO₂ (RG/VO₂) for supercapacitors by hydrothermal reduction method, and the material exhibited a specific capacitance of 225 F g⁻¹ at a specific current of 0.25 A g⁻¹ in 0.5 MK₂SO₄ aqueous solution (Deng et al. 2013).

6.1.2.5 Tin Oxide (SnO₂)

Tin oxide, SnO₂, is an n-type wide band gap semiconducting material not only with the direct band gap energy of 3.5–3.9 eV (Spence 1967; Arlinghaus 1974) but also with the indirect band gap energy of around 2.6 eV (Kawasaki et al. 1970). The SnO₂ possess the advantages of being inexpensive, efficient semiconducting in nature, environmentally safe, high electrical and optical properties and high chemical stability. These properties have made SnO₂ to be widely explored in many applications such as electrochemical sensors, transistors, catalysts , energy storage and conversions (Yan et al. 2010; Chen et al. 2011a; Zhang et al. 2011; Hou et al. 2013). The interest in the electrochemical energy storage such as SCs and batteries is due to its relative abundance, high theoretical specific capacity (782 mAh g⁻¹) and chemical stability (Tang et al. 2013).

The nanostructures of SnO₂ can also be synthesized via different techniques such as pulsed laser deposition (Pham et al. 2014), co-precipitation (Du et al. 2012), SILAR (Yıldırım et al. 2012), chemical bath deposition (Pusawale et al. 2013), electrodeposition (Yang et al. 2011a), hydrothermal (Hui-Chi Chiu 2007) and thermal evaporation (Kim et al. 2006). The SnO₂ nanoparticles synthesized by co-precipitation technique were evaluated and tested for SC application and displayed a maximum specific capacitance of 122 F g⁻¹ at a scan rate of 2 mV s⁻¹ (Manikandan et al. 2016). Hierarchical SnO₂ nanostructures (HTNs) prepared by hydrothermal method and characterized revealed a crystalline tetragonal rutile structure of SnO₂ as shown in Fig. 6.18 (Liu et al. 2014). The morphologies of the prepared SnO₂ revealed hierarchical flower shapes in very high density (Fig. 6.18b). The typical diameter of a single hierarchical flower-shaped morphology was ∼3 ± 1 μm. It was also observed that the hierarchical flower-shaped structures are made of ultrathin nanosheets of ∼20 nm in size, which intermingled with each other leading to triangular nanostructures. The HRTEM images (Fig. 6.18e, f) show well defined lattice fringes with lattice spacing of 0.337 and 0.265 nm, corresponding to the (110) and (101) lattice planes of SnO₂, respectively. The SAED pattern shown as inset confirmed the prepared SnO₂ nanosheet crystalline. The electrochemical performance of the SnO₂ structures was evaluated in three-electrode configuration using 1 M Na₂SO₄ aqueous electrolyte. A specific capacitance of 187.7 F g⁻¹ was obtained at a specific current of 1 A g⁻¹ and was attributed to the hierarchical nature and thin thickness of the nanosheets. Semiconductor quantum dots (QDs) play a vital role in technological and biological applications; SnO₂ QDs exhibit a typical behaviour in comparison with their bulk counterparts due to high surface to volume ratio and excitonic confinement effects (Chen et al. 2005, 2011b). Thus, it is important to understand this distinct nature of QDs such that it can be used more effectively in applications. Bonu et al. carried out electrochemical supercapacitor performance of SnO₂ quantum dots, and it was shown that (QDs) can significantly improve the understanding on the reasons behind the better performance, and it will also help in using QDs or smaller size nanoparticles (NPs) more efficiently in supercapacitors (Bonu et al. 2016). The specific capacitance of the stable SnO₂ QDs showed only 9% loss while increasing the scan rate from 20 to 500 mV/S, and the capacitance loss for the QDs is less than 2% after 1000 cycles of charging/discharging, whereas for the 25 nm SnO₂ NPs, the capacitance loss is 8% after 1000 cycles. The new findings about the SnO₂ QDs can be helpful supercapacitors (Fig. 6.19).

Fig. 6.19

XRD patterns of the prepared SnO₂ nanostructures before (red line) and after calcination (black line); (b−d) FESEM images, (e) low-magnification TEM image and (f) high-resolution TEM (HRTEM) image of the prepared hierarchical SnO₂ nanostructures. Inset in (f) exhibits the typical corresponding SAED pattern (With permission of Liu et al. 2014)

Velmurugan et al. (2016) further studied graphene-doped SnO₂ and showed the overall specific capacitance of the composite electrode (SnO₂/graphene) corresponds to the combined contribution from electrochemical double-layer charging and pseudocapacitive behaviour of the composite. The resultant specific capacitance was 50 F g⁻¹ for the pristine SnO₂ and 100 F g⁻¹ for SnO₂/graphene composite at a high scan rate of 50 mV s⁻¹. The specific capacitance of 470 and 818 F g⁻¹ was obtained at low scan rates. A pulse microwave-assisted deposition technique was also explored to grow SnO₂ crystals with homogeneously distribution on rGO sheets to form a composite of SnO₂/rGO composites as shown in Fig. 6.20.

Fig. 6.20

Schematic diagram for growing SnO₂/rGO composites through the following steps: (i) GO sheets by the Hummers’ method, (ii) SnO₂ nanocrystals by microwave heating technique and (iii) homogenizing dispersion and thermal reduction. The bottom right photo shows as-prepared flexible SnO₂/rGO CP electrode sheets (With permission of Hsieh et al. 2014)

The specific capacitance of the devices fabricated from the composite exhibited specific capacitance of 348 F g⁻¹ at a specific current of 50 mA g⁻¹, maximum energy density of 32.2 Wh kg⁻¹ with a power density of 1000 W kg⁻¹. The improved capacitance and energy values were attributed to the fact that the SnO₂ crystals not only serve as spacers to create more active sites on the rGO surface but also maximized the fraction of hydrophilic surface, which is more accessible for the formation of the electric double layer (Hsieh et al. 2014). Similarly, SnO₂ was also decorated onto Ti₃C₂ by an efficient and simple microwave irradiation method to form SnO₂-Ti₃C₂ nanocomposite . Figure 6.21a–c showed the layered structure of the Ti₃C₂ with the SnO₂ intercalated into the layer of the Ti₃C₂ to prevent stacking of the layers. The composite exhibited an ideal capacitive behaviour from the CV at different scan rates and GCD at different specific current densities presented in Fig. 6.21d, e. The high specific capacitance of 126 F g⁻¹ at 1 A g⁻¹ and 100 F g⁻¹ at 10 A g⁻¹ specific current was obtained for the composite. The composite material showed an improved electrochemical performance compared to its precursors (Fig. 6.21f). The composites retained almost 82% of its initial capacitance after 8000 cycles at a specific current of 3 A g⁻¹ (Fig. 6.21g). The improved performance observed was due to the intercalation of the Sn²⁺ into the layers of the Ti₃C₂ and also prevent the stacking of Ti₃C₂ layer (Zheng et al. 2017).

Fig. 6.21

SEM images for (a) Ti₃C₂, (b) Ti₃C₂-MW and (c) SnO₂-Ti₃C₂, (d) CV curves of SnO₂-Ti₃C₂ electrode at various scan rates, (e) GCD curves of SnO₂-Ti₃C₂ electrodes, (f) the plots of the specific capacitance of the three electrodes under different specific current and (g) capacitance retention (the inset shows charge-discharge curves before and after 8000 cycles), of SnO₂-Ti₃C₂ electrode after 8000 GCD cycles at 3 A g⁻¹ (With permission of Zheng et al. 2017)

6.1.2.6 Tungsten Oxide (W_xO_y)

Tungsten trioxide (WO₃) semiconductor having an indirect band gap of 2.5–2.8 eV has also emerged as a promising material for electrochemical applications due to its low cost, high electrical conductivity, good chemical stability in acidic aqueous solution and high theoretical capacity (Zheng et al. 2011). The interest for this material in electrochemical energy storage is due to its efficient charge storage/delivery dynamic and its reversible valence change (i.e. change of oxidation state between W⁶⁺ and W⁵⁺ centres) (Deb 2008; Zheng et al. 2011). Pseudocapacitive charge storage behaviour in highly conductive ordered mesoporous tungsten oxide electrodes was reported by Jo et al. (2011). They observed a voltage rise of the m-WO_3-X electrode, which is similar to the EDLC, displayed a high rate capability, which was attributed to its fast proton diffusion within the metal oxide wall and the short path of electrolyte transport through the well-ordered mesopores. Thus, the proposed Fig. 6.22 explained the possible mechanism in the m-WO_3-X electrode.

Fig. 6.22

Schematic illustration of the pseudocapacitive charge storage mechanism in the m-WO_3-X electrode. Upper part is for the schematic transport of electron and proton within m-WO_3-X. Bottom part indicates a schematic change of the oxidation state during pseudocapacitive charging/discharging. Note that transports of electron (e⁻) and proton (H⁺) within the interphase at 0 V became sluggish at 0.6 V vs Ag/AgCl (With permission of Jo et al. 2011)

As shown in Fig. 6.22, the three phases within the WO_3-X wall were separated. The tungsten metal has two oxidation states of W⁵⁺ and W⁶⁺, and the proton is obtained from the electrolyte phase (Jo et al. 2011). An interphase is assumed to exist between electrode and electrolyte, where the faradaic reaction takes place. Using a simple calculation to estimate the total charge (q*t = 67 C g⁻¹) in cyclic voltammetry (CV) measurement, leads to a charge utilization of 15,533 C mol⁻¹(67 C g⁻¹ X 231.84 g mol⁻¹), which indicated that only 16% of W atoms of the m-WO_3-X electrode. When the electrode potential increases to 0.6 V, most of the W⁵⁺ species oxidizes to the W⁶⁺ state. This oxidation state leads to a decrease in the conductivity, and the charge transfer resistance (R_ct) becomes larger. When the potential is lowered to 0 V, the state of the interphase is reduced to W⁵⁺ and also leads to an increase in the electrical conductivity (lower resistance) of the electrodes. However, WO₃ is suitable as a negative electrode material, and recently, Qiu’s group reported the flower-like nanostructures of WO₃ as negative electrode in solid-state asymmetric supercapacitors and exhibited a specific capacitance of 196 F g⁻¹ at a scan rate of 10 mV s⁻¹ (Qiu et al. 2016). Upadhyay et al. synthesized nanochannel layers of WO₃ by electrochemical anodization in orthophosphoric acid (o-H₃PO₄) (Upadhyay et al. 2017). The cross-sectional SEM images (Fig. 6.23a–c) show nanochannel layers of WO₃.

Fig. 6.23

(a–c) Top-view (left) and cross-sectional(right) SEM images of WO3 nanochannel layers formed by self-organizing electrochemical anodization of W foils in pure hot H₃PO₄ (100 C 5 V) for different times: (a) 1 h, (b) 2 h and (c) 4 h, (d–e) CV and galvanostatic charge-discharge plot of 2 h. anodized sample at different scan rates and current densities, respectively, (f) specific capacitance vs. current densities for all the samples and (g) capacitance retention up to 3500 cycles at 10 A cm⁻³ (inset: charge discharge plots over time). All the experiments were performed with annealed sample (450 C, air, 1 h) (With permission of Upadhyay et al. 2017)

The top surface of the layers is homogeneous and shows the open pores, with inner diameter of ca. 10 nm. The cross-sectional SEM images (right) show clearly that the anodic WO₃ has a nanochannel structure, with aligned vertically oriented channels that extend from the top surface of the anodic film to the oxide-metal interface. These channels are open at the top surface and remain open (with inner diameter of ca. 10 nm) across the entire channel length, regardless of the anodic oxide thickness. These layers show remarkable pseudocapacitive behaviour within the potential window of −0.8–0.5 V in 1 M Na₂SO₄ as shown in Fig. 6.23d–g.

A higher capacitance was obtained for the WO₃ structures grown for 2 h when compared to the other samples, i.e. 397 F cm⁻³ at 2 A cm⁻³ in a potential range of 1.3 V, and displayed an excellent capacitance retention up to 3500 cycles at 10 A cm⁻³ as shown in Fig. 6.23g. The cyclic stability showed that the returned 27% of the total capacitance originated from surface adsorption /desorption process, while the remaining 73% originated from a diffusion-controlled process. The WO₃ nanotube bundles made by Wu and co-workers (Wu and Yao 2017) and used as a negative electrode in an asymmetric device against PANI as a positive electrode. As shown in Fig. 6.24a (a–d), the SEM and TEM images revealed the bundles of the WO₃ nanotubes, and the electrochemical performance of the assembled pseudocapacitor device (Fig. 6.24b (a–e) showed that this electrode material could be a potential candidate in energy storage application.

Fig. 6.24

(a) SEM and TEM images, (a)–(c) SEM images of the as-prepared product of WO₃ nanotube bundle, (d) the corresponding HRTEM image. (b) Electrochemical analysis: (a) CV curves in different potential windows at 60 mV s⁻¹, (b) CV curves of the as-fabricated ASC device at various scan rates from 5 to 60 mV/s in 1.8 V potential window, (c) galvanostatic charging/discharging curves of ASC device at different current densities, (d) Ragone plots of ASC device and (e) cycling performance at constant specific current of 5 A/g (With permission from Wu and Yao 2017)

An area capacitance of 2575.3 mF cm⁻² at a specific current of 3 mA cm⁻² and a specific capacitance of 615.7 F g⁻¹ at a specific current of 1 A g⁻¹ were obtained in a three-electrode configuration, respectively. The asymmetrical device fabricated displayed a wide voltage window of 1.8 V from the CV and GCD presented in Fig. 6.24b (c). A high energy densities of 80.1 W h kg⁻¹ and 49.5 W h kg⁻¹ were obtained at power densities of 3240 W kg⁻¹ and 16,200 W kg⁻¹, respectively.

6.1.2.7 Bismuth Oxide (Bi_xO_y)

Bismuth oxide, Bi₂O₃, also exhibits intrinsic properties such as refractive index, energy bandgap (2.8 eV), dielectric constant and photoconductivity similar to many semiconductors. This metal oxide is suitable for large-range applications such as sensors, catalysis, energy storage and conversion. Recently, this metal oxide has emerged as good electrode material for energy storage applications in SCs due to its high theoretical capacity (690 mAh g⁻¹ and 6280 mAh cm⁻³) and electrochemical stability (Gujar et al. 2006; Liu et al. 2012, 2015b). The structure of the Bi₂O₃ can be described as a sequence of alternating layers of bismuth atoms parallel to the (100) plane of the cell and oxide ions in the c-axis direction (Michel Drache et al. 2007). The electrode made up of Bi₂O₃ thin film grown on copper substrates at room temperature via electrodeposition from an aqueous alkaline nitrate has been fabricated and tested for supercapacitor application by Gujar et al. (2006). The supercapacitor device made from this electrode displayed good electrochemical performance and durability, with specific capacitance of 98 F g⁻¹ reported. Similarly, electrode potential oscillations have been used for the synthesis of hierarchical rippled Bi₂O₃ nanobelts, and the electrodes fabricated exhibited excellent stability and electrochemical performance (Zheng et al. 2010). Based on the reported electrochemical energy storage capabilities of Bi₂O₃, there is no doubt that this material has potential for application in SCs. Nevertheless, this metal oxide still suffers from low abundance and poor conductivity and rate capability, thus limiting its application. Due to the mentioned challenges, researchers have explored the use of carbon-based materials with Bi₂O₃ either in composite form or in asymmetric configuration to improve the electrochemical performance of the SC device taking advantage of the properties offered by both materials. Senthilkumar et al. (2014) fabricated asymmetric device based on Bi₂O₃ and activated carbon (AC) in redox additive aqueous electrolyte and achieved remarkable energy density of 35.4 Wh kg⁻¹ (Senthilkumar et al. 2014). Ng and co-workers studied Bi₂O₃ temperature effects in a bismuth oxide/manganese oxide (Bi₂O₃/MnO₂) SCs. An improved electrochemical performance was achieved judging from the CV curves shown in Fig. 6.25 for the Bi₂O₃ measured at 60 °C displaying larger area compared to those at 30 and 0 °C, which corresponds to a high specific capacitance of 150.2 F g⁻¹ at a scan rate of 2 mV s⁻¹. The increase in capacitance was attributed to the enhanced ion mobility at elevated temperature, which prompted greater charge transfer and storage capability. Furthermore, the evaporation of electrolyte at a high temperature could induce the physisorption of electrolyte ions and give rise to faradaic currents, which enhanced the capacitance performance. The GCD also shown in Fig. 6.25 displayed evidence that the application of temperature stimulated electrochemical activation, leading to high capacitance performance compared to that at 0 °C. The linear voltage−time profiles at 60 °C compared to those at 30 and 0 °C from the prolonged discharge time indicated a good capacitive behaviour and better charge storage capability at elevated temperature. An energy and power densities of 9.5 Wh kg⁻¹ and 102.6 W kg⁻¹ were obtained at 60 °C, respectively, which shows a great improvement to values obtained at 0 °C and 30 °C. The Nyquist plot (Fig. 6.25c) shows a slightly higher resistance at 0 °C than that at 30 and lower resistance values at 60 °C which is believed to be due to the expansion of the matrix, which facilitated the diffusion of electrolyte ions. Moreover, the SC device achieved energy densities of 4.9 and 6.9 Wh kg⁻¹ and power densities of 53.8 and 74.8 W kg⁻¹ at 0 and 30 °C, respectively. The Bi₂O₃/MnO₂ composite exhibited a superior stability retaining 95% of the original capacitance at 30 °C and > 75% at the high temperature of 60 °C (Ng et al. 2018).

Fig. 6.25

(a) Schematic representation of the plausible growth mechanism of Bi₂O₃/MnO₂ on nickel foam substrate during hydrothermal process. (b) Electrochemical analysis: (a) Cyclic voltammogram, (b) galvanostatic charge/discharge, (c) Nyquist plot and (d) life cycle of Bi₂O₃/MnO₂ supercapacitors at different temperatures. Inset shows the magnified EIS spectra of Bi₂O₃/MnO₂ supercapacitors at different temperatures (With permission of Ng et al. 2018)

The synergistic effect between the Bi₂O₃ and MnO₂ materials enhanced the electrochemical performances of the composite electrodes and dramatically improved the cyclic reversibility, sustainability and stability of the Bi₂O₃/MnO₂ supercapacitor, regardless of the temperature influence. These results showed that the device with a longer defrosting period possessed better electrolyte ion diffusivity, which therefore leads to enhanced capacitive performances and show that the Bi₂O₃ is suitable for use in the automotive sector.

6.1.2.8 Indium Oxide (In₂O₃)

Nanostructured In₂O_3, a transparent conductive oxide with a wide bandgap (Eg∼3.65 eV), has recently attracted attention due to its high electrical conductivity, high optical transmittance and excellent luminescence. Because of these fascinating properties, In₂O₃ has been used for applications such as antireflection coatings (Wang et al. 2016b), gas sensors (Zheng et al. 2015), lithium–ion batteries (Osiak et al. 2013) and supercapacitors (Bastakoti et al. 2013) and has been produced using different methods such as spray pyrolysis (Korotcenkov et al. 2002), chemical vapour deposition (CVD) (Tuzluca et al. 2017), pulsed laser deposition (PLD), thermal evaporation (Liu et al. 2015a), carbothermal methods (Huang et al. 2007) and hydrothermal synthesis (Padmanathan et al. 2016). The interest in electrochemical capacitors is based on the reversible reaction below (equation xx) which gives rise to high theoretical specific capacity ∼580 mAh g⁻¹.

$$ \mathrm{In}+\mathrm{x}{\mathrm{Li}}^{+}+\mathrm{x}{\mathrm{e}}^{-}\leftrightarrow {\mathrm{Li}}_x\mathrm{In}\ \left(0\le x\le 3\right) $$

(6.12)

The In₂O₃ nanostructures can exhibit different morphologies, which are suitable as electrodes for materials for supercapacitors. For example, 1-D single crystal In₂O₃ nanotowers, nanobouquets, nanocones and nanowires were synthesized using the CVD system over different temperature regions (1000, 900, 800, 700 and 600 °C) as shown in Fig. 6.26. The interests of this work displayed the relationship between the size and shape of In₂O₃ nanostructures to the electrochemical properties. The synthesized materials were explored as electrodes for electrochemical capacitors, and the In₂O₃ nanowires displayed the highest areal capacitance of 16.6 mF cm⁻² with cycle stability of 66.8% after 10,000 cycles (Tuzluca et al. 2018). The morphology of these various shapes of In₂O₃ nanostructures indicated on FESEM image and line scanning and point EDS analyses of the sample, attesting that nanobouquets had In and O atoms at their main trunks and tips, are shown in Fig. 6.26. As it was also shown, the absence of Au at the tips of nanobouquets designates that the growth mechanism of these nanobouquets is via the growth path rather than the VLS growth pathway.

Fig. 6.26

(a) Low- and high-magnification FESEM images of In₂O₃ nanotowers grown in sample A and their backscattered electron image; (b) low- and high-magnification FESEM images of In₂O₃ nanotowers grown in sample B and their line scanning EDS analysis; (c) FESEM images of In₂O₃ nanobouquets grown in sample C and their line scanning and point EDS analyses; (d) FESEM images of In₂O₃ nanocones with complete and incomplete growth in sample D and their EDS analysis; (e) low- and high-magnification FESEM images of In₂O₃ nanowires grown in sample E and their EDS analysis (With permission of Tuzluca et al. 2018)

The electrochemical performance of the In₂O₃ nanostructure presented in Fig. 6.27 displayed the highest areal capacitance value obtained from In₂O₃ nanowires due to the fact that the nanoscale wires have high surface area to volume ratio compared to the nanostructures. The observed result is because electrode materials with nanowire morphology have a higher capacitance and amount of charge storage. It is known that the increase in surface area of an electrode makes ionic diffusion paths to become shorter during reactions due to the synergistic effect arising from both the electrode and electrolyte. Thus, electrode materials with nanowire morphology can increase the electrochemical performance of supercapacitors. The CV and GCD curves (Fig. 6.27b, c) taken in the potential range of 0–0.8 V and 0 to (−0.6) V, respectively, indicated the electrochemical behaviour similar to the ones observed in the literature . Figure 6.27b shows the CV curves of the sample and the bare Si substrate at 10 mV s⁻¹ scan rate. An areal capacitance values of 10.1, 5.7, 4.2 and 3.7 mF cm⁻² were obtained for sample A (nanotowers); 6.7, 4.5, 3.5 and 3 mF cm⁻² for sample B (nanotowers); 12.5, 10.8, 9.6 and 8.2 mF cm⁻² for sample C (nanobouquets); 4.9, 4.1, 3 and 2.7 mF cm⁻² for sample D (nanocones); and 16.6, 14, 12.3 and 11.3 mF cm⁻² for sample E (nanowires) (Fig. 6.27d) at a specific current of 0.04, 0.06, 0.08 and 0.1 mA cm⁻². As noted, nanostructured and three-dimensional nanorod of In₂O₃ deposited on stainless steel (SS) by a novel potentiodynamic method at a high scan rate of 200 mV s⁻¹ exhibited high specific capacitance, high power density and long-term stability when used as active electrode materials for redox supercapacitors (Kalakodimi Rajendra Prasad et al. 2004). In₂O₃ nanowires exhibited better electrochemical performance. It is also worth stating that an increase in the surface area of an electrode leads to decrease in the ionic diffusion paths; thus, electrode materials with nanowire morphology can increase the electrochemical performance of supercapacitors.

Fig. 6.27

(a) The schematic indicating the fabrication process of samples for electrochemical analyses; (b) Si substrate material and CV curves of all samples obtained at 10 mV s⁻¹; (c) GCD curves of all samples at 0.04 mA cm⁻²; (d) Areal capacitance values of all samples at the current densities of 0.04, 0.06, 0.08, and 0.1 mA cm⁻² (With permission of Tuzluca et al. 2018)

Carbon-based materials were also used to improve on the electrochemical performance of In₂O₃-based electrodes and have shown great improvements. Fine In₂O₃ nanoparticles were successfully synthesized within the mesopores by thermal decomposition of a metal source. The obtained metal oxide-impregnated mesoporous carbon was effectively utilized for the enhancement of capacitive performance through a combination of both faradaic and non-faradaic processes, and the composite exhibited a high capacitance of 275 F g⁻¹ in 1 M Na₂SO₄ (Bastakoti et al. 2013), demonstrating an addition of carbon materials as a strategy to improving energy density of supercapacitors. Table 6.1 summarizes the electrochemical performances of some transition metal oxide-based materials for application in supercapacitors (SCs) obtained from the literature.

Table 6.1 Summary of electrochemical performances of some transition metal oxide-based materials for application in supercapacitors (SCs)

6.2 Summary and Future Prospects

It is evident that significant progress has been made in SC applications, which has generated worldwide attention in these energy storage /conversion devices due to the need to resolve high demands for clean and sustainable energy followed by the fact that fossil fuels have shown potential threat to the environment and thereby deemed unreliable. The advantages of SC systems account to their high power density, high efficiency and long-life expectancy. However, the development of these energy storage devices depends upon the innovative ways of fine-tuning their electrode materials. Metal oxide nanostructured materials with their interesting properties such as low cost, good electrochemical response, various oxidation states and availability have proven themselves as attractive electrode materials for energy storage applications since they can be easily synthesized from various synthetic routes even though their synthesis and application are still restricted to the lab-scale production. Much of the limitation of the use of metal-oxides in SCs could be attributed to the low conductivity and agglomeration upon rapid cycling, which limits their electrochemical performance to match their theoretical estimation, thus hindering their commercial applications. Intensive work has been devoted in mitigating the above limitation by incorporation of highly conductive materials such as carbon allotropes and recently MXENES to make composite materials with an improved electrochemical performance and stability upon long-time cycling. The approach of discovering new materials that boosts the properties of metal oxides has drastically improved the rapid use of these nanostructured materials as EC electrode materials. Therefore, the innovation that further improves the existing metal oxide electrodes while also promoting their novel composites that results in stable fabrication in order to meet the market demand of EC technology is of great importance. From the literature and data presented in this work, it is very clear that the performance of metal-based oxides for supercapacitor applications has increased exponentially over the last decade and continues to improve. Specific capacitance, energy and power density values continue to improve, while the storage mechanisms of most of the oxide-based supercapacitors continue to be widely studied and understood. Efforts to produce SCs from renewable materials also encourage optimism that such an environmentally friendly option may soon be feasible for use on a larger scale. Currently, supercapacitors are being used for some small applications, and with the rapid improvement of their energy densities, it is only a matter of time before their industrial applications become common. The future of supercapacitor research is endlessly promising; however, there are certain things that should be taken into account in order to ensure that all future research can be used fully. One of the challenges is that results are communicated in multiple different ways, and this can make it difficult to compare different studies. Most studies choose to express their results in terms of specific capacitance, F·g⁻¹, measured at different scan rates, mV·s⁻¹ and/ or specific current, A·g⁻¹. Some papers, however, choose to express specific current in mA·cm⁻², and specific capacitance in mF·cm⁻², making it difficult to compare the performance of the subject material to that of others in the literature when additional information, such as the density of the material, is not provided. However, F/cm² is justified while the mass of electrode materials is beyond the measurable limit and as well for micro-supercapacitor. Therefore, it is recommended that for future studies, a standard be developed for such aspects of the research so that all materials may be accurately compared. The recent upsurge in the quantity of research carried out on the various oxide-based supercapacitors, as well as the quality of SCs being produced, has well-established their position in the field of energy storage . In order to fully succeed on the generation of the reliable, efficient and effective technology that can mitigate the global energy demand , more conductive materials obtained from different sources and various synthetic routes should be explored and be incorporated on these metal oxides. Another major drawback in the development of supercapacitor technology is the relatively high cost associated with it when compared to other energy devices. Thus, future research should be invested towards the development of fascinating transition metal oxide nanomaterials exhibiting a high charge capacity while demonstrating a minimum electronic and electrolytic resistance in a very cost-effective manner. As research advances, and improvements continue to be made, it is certain that the future held for supercapacitors is an auspicious one.

Appendix

Potentiostat – A potentiostat is the electronic hardware required to control electrochemical cell and run most electroanalytical experiments.

Cell potential – The overall electrical potential of an electrochemical cell. It is the sum of the reduction potential of the cathode and the oxidation potential of the anode.

Electrolytic cell – A cell that consumes electrical energy to drive a non-spontaneous redox reaction.

Charge (Q) – The quantity of unbalanced electricity in a body such as an electron or an ion.

Charge density (q) – The measure of charge Q and electrode area A, i.e. q = Q/A.

Chronoamperometry – The techniques and methodology of studying current as a function of time.

Current (I) – The rate of charge flow or passage, i.e. I = (dQ/dt).

Specific current – The measure of current I and electrode mass g, i.e. i = I/g.

Cyclic voltammogram (CV) – A plot of current on the y-axis against potential on the x-axis during a voltammetric experiment in which the potential is ramped twice, once forward to the switch potential and then back again.

Electrochemical area – The area of an electrode; the area that an electrode is ‘perceived’ to have.

Electrochemical cell – An electrochemical cell typically consists of two electronic conductors (also called electrodes) – An ionic conductor (called an electrolyte).

Electrode – A conductor employed either to determine an electrode potential (at zero current, i.e. for potentiometric experiments) or to determine current during a dynamic electroanalytical measurement. The electronic conductivity of most electrodes is metallic.

A reaction is classified as oxidation or reduction depending on the direction of electron transfer. There are two fundamental types of half-cell reactions: – Oxidation reactions – Reduction reactions

Oxidation – Involves the loss of an electron and involves the transfer of electrons from the species to the electrode R = O + ne

Reduction – Involves the gain of an electron and involves the transfer of electrons from the electrode to the species O + ne = R.

Electrode at which the oxidation reaction occurs is called the anode.

Electrode at which the reduction reaction occurs is called the cathode.

Anodic current – An anodic current is the flow of electrical charge (usually carried by electrons) into a working electrode from a second phase (usually an electrolyte solution) as a result of the oxidation of one or more species in the second phase.

Cathodic current – A cathodic current is the flow of electrical charge (usually carried by electrons) out of a working electrode into a second phase (usually an electrolyte solution) leading to the reduction of one or more species in the second phase.

The electrode at which the reaction of interest occurs is called the working electrode.

The electrode at which the other (coupled) reaction occurs is called the counter electrode.

A third electrode, called the reference electrode, may also be used. An ideal reference electrode is one that maintains a constant potential irrespective of the amount of current (if any) that is passed through it.

Electrode potential – The electrode potential for a reaction is derived directly from the free energy change for that reaction ΔG = −NFE. The standard oxidation potential is equal in magnitude but opposite in sign to the std. reduction potential.

Electrolyte – An ionic salt to be dissolved in a solvent or the solution formed by dissolving an ionic salt in a solvent.

Electrolytic processes – Reactions in which chemical changes occur on the passage of an electrical current.

Galvanic or voltaic processes – Chemical reactions that result in the production of electrical energy.