Application of Iron Oxide in Supercapacitor

Abstract

Iron oxide nanostructures have been considered very promising material as electrode in electrochemical energy storage devices because of their lower cost of synthesis and high theoretical charge storage capacity. Iron oxide nanoparticles and their nanocomposites have performed excellent in supercapacitor. Iron oxide as negative electrode has extended the working voltage window of a supercapacitor. The main problems associated with iron oxide based electrodes are their poor electrical conductivity and cycle stability. Therefore, a conductive carbon matrix has been added to the iron oxide based electrodes to improve the electrochemical performance. In this chapter, recent progress on iron oxide and its composite with different materials as electrode in supercapacitor is summarized. The various synergistic effects of nanocomposites and compositional engineering to enhance the electrochemical performance of iron oxide are also discussed.

1. Introduction

Serious environmental and climate problems have been arisen due to the continuous consumption and depletion of fossil fuels [1, 2, 3, 4]. This problem would be worst in the coming days as the demand of energy is going increased day by day. Consequently, there is a need of clean and sustainable energy resources such as solar cells and batteries. The solar cell always needs sun light to generate continuous electricity supply. This problem can be sort out by integrating an energy storage device to the solar cell. Batteries can be used for the above work however their replacement and maintenance are major issues [1, 3, 5, 6, 7]. Supercapacitors have drawn much attention in recent years to be used in several applications such as industrial power and energy management. Moreover, they are very efficient (fast charging/discharging and long life), low-cost and environmentally friendly [8, 9, 10, 11]. They have larger power density than batteries and capability to deliver the charge quickly [12, 13, 14, 15]. A supercapacitor is fabricated with two electrodes separated by a separator, and electrolyte (Figure 1). The electrode materials have decisive role in the charge storage capacity; therefore, different materials and their combinations have been used in supercapacitor [2, 16, 17, 18, 19]. Supercapacitor can store the energy via two process (1) formation of electric double layer (EDL) at the electrode/electrolyte interface [4, 19, 20] and (2) redox reactions (pseudocapacitor) [16, 21, 22] (Figure 2). Carbon and silicon based materials are generally used for EDL capacitor whereas pseudocapacitor consists of metal oxides and conducting polymers. A hybrid supercapacitor technology has also been developed by combining the properties of EDLC and pseudocapacitor [24, 25, 26, 27].

Figure 1.

Structure of (a) dielectric capacitor and (b) supercapacitor.

Figure 2.

(a) Ragone plot for different devices. (b) Schematic representation of charge storage at high surface area electrode. (c) Different models for EDL: Helmholtz model, Gouy–Chapman model and Gouy–Chapman–Stern model. Charge storage process via redox process (pseudocapacitor); (d) bulk redox and (e) surface redox. (Reprinted with permission from Ref. [23] Copyright (2020) American Chemical Society).

Carbon based electrodes can offer high power and long cycle stability. However, low energy density is major concern with carbon electrodes [28, 29]. In the past decade, major efforts have been made developing a hybrid supercapacitor in order to meet the demands of high energy and high power density [30, 31]. Nano-sized structure of metal oxides can offer higher surface area to the electrode for good electrode/electrolyte ions interaction and short ion-diffusion length. Among metals oxides, iron oxides (FeO_x) are being considered efficient electrodes as they have low cost of synthesis, high theoretical capacitance due to variable oxidation states [32, 33, 34]. Various nanostructures of Fe based materials such as nanorods, nanosheets, nanotubes and nanoparticles, nanowires, nanoflowers etc. have been employed to increase the surface area and shorten the diffusion length of ions [2, 21, 32, 35]. The above structures have shown excellent performance in supercapacitor. However, low conductivity and poor structural stability are major issues with iron oxides, which need to be addressed. Incorporating conductive materials to iron oxides can lead to improve the conductivity and stability as well. Therefore, different forms of carbon and polymers have been mixed with iron oxide to fabricate a hybrid electrode and this electrode has shown excellent performance in supercapacitor. In this chapter, the achievements and progress made towards developing iron oxides (Fe₂O₃ and Fe₃O₄) based electrodes have been reviewed.

2. Supercapacitor

A supercapacitor or ultracapacitor is an advanced technology which has much larger capacitance than a normal capacitor. Supercapacitors can store the energy about 10 times more than dielectric capacitors and return the energy back faster than batteries. They occupy the place between dielectric capacitors and rechargeable batteries in Ragone plot (Figure 2a). The charge storage process in supercapacitor is completely different than that of conventional capacitor. In electrostatic capacitor, two conducting electrodes are separated by a dielectric material. The charge is stored by the polarization process in the dielectric material. In case of supercapacitor, an electrolyte is filled between two electrodes of high surface area. During charging, opposite charge starts accumulating on the surface of electrodes. As the charge is stored physically with no chemical change, it is highly reversible and it shows stable behavior for a large number of cycles. Capacitance (C) is defined as the ratio of stored charge (Q) to the applied voltage (V)

$$\mathrm{C}=\frac{\mathrm{Q}}{\mathrm{V}}$$E1

C is directly proportional to the surface area (A) of each electrode and inversely proportional to the distance (d) between the electrodes

$$\mathrm{C}=\frac{\mathrm{\epsilon A}}{\mathrm{d}}$$E2

Where ε is the permittivity of the insulating material, A is the area of the electrode and d is the distance between the electrodes. The energy (E) of a capacitor is calculated by the following equation;

$$\mathrm{E}=\frac{1}{2}\mathrm{C}{\mathrm{V}}^2$$E3

Power (P) is the energy delivered by a device per unit time. The maximum power is limited by equivalent series resistance (ESR) with the following relation

$$\mathrm{P}=\frac{{\mathrm{V}}^2}{4\ast \mathrm{ESR}}$$E4

An EDLC has two electrodes which are immersed in an electrolyte and separated by a separator. Its charge storage is non faradaic in nature, there is no transfer of charge between electrode and electrolyte. Electric charge is accumulated electrostatically on electrode/electrolyte interface which is responsible for the formation of EDL (Figure 2b). The charging and discharging processes are highly reversible and there is no chemically change in the electrodes. This leads to the increased in cycle life, compared to batteries but less than conventional capacitor.

Pseudo means false so, as the name indicates this type of materials do not store charge like a conventional capacitor that is by forming EDL but work like a capacitor. The charge storage mechanism of such type of capacitor is faradaic in nature or the electrode reacts with the electrolyte to store charge (Figure 2d and e). Transition metal oxide and polymer show such kind of behavior. A reversible redox processes take place in which the valence electrons of electro active materials are transferred across the electrode/electrolyte interface, resulting in a potential-dependent capacitance (Figure 2e). The term pseudo-capacitance is used to distinguish this mechanism of charge storage from double layer. So we can say that that pseudo capacitance possess battery-like behavior with faradic reactions between electrodes and electrolyte. Table 1 shows the difference between EDLC and pseudocapacitor.

EDL capacitor	Pseudocapacitor
It stores charge by non-faradaic mechanism.	It stores charge by faradaic mechanism.
Capacitance is low as compared to pseudo capacitance.	Capacitance is 10 times higher than EDLC.
Large cycle stability	Poor cycle stability
The charging/discharging curve is highly reversible	Reversible but not as compared to EDLC

Table 1.

Comparison between EDLC and pseudocapacitor.

3. Fe₂O₃ based electrodes

Fe₂O₃ electrode stores the charge via reversible oxidation/reduction reactions at the electrode and electrolyte interface. Low conductivity and poor cycling stability hamper its application in supercapacitor. Shivakumara et al. [36] have used porous α-Fe₂O₃ as electrode in supercapacitor. Porous α-Fe₂O₃ synthesized by sol–gel route offered BET surface area of 386 m² g⁻¹. α-Fe₂O₃ demonstrated specific capacitance of 300 F g⁻¹ in 0.5 M Na₂SO₃ at a discharge rate of 1 A g⁻¹. However, the electrode could retain only 73% of the initial capacitance after 1000 cycles.

Flower-like α-Fe₂O₃ nanostructures prepared by ethylene glycol mediated self-assembly process exhibited capacitance value of 127 F g⁻¹ in 0.5 M Na₂SO₃ determined at a current density of 1 A g⁻¹ [37]. Flower-like α-Fe₂O₃ has retained 80% of the initial capacitance after 1000 cycles. Despite of good value of capacitance, Fe₂O₃ does not meet the requirement of long cycle life. Change in volume during multiple cycles causes large degradation in the capacitance. Therefore, maximum efforts have been made towards shorting the diffusion length of electrolyte ions and minimizing the volume change by combining Fe₂O₃ with conducting materials.

Large efforts have been made to add a conductive phase to Fe₂O₃ electrodes to enhance overall capacitive performance. Abad et al. [38] have synthesized rGO-Fe₂O₃ nanocomposites by hydrothermal process for supercapacitor application. In the composite electrode, Fe₂O₃ nanoparticles have average size of 25 nm and are anchored on graphene sheets making good physical contact. The Fe₂O₃ nanoparticles prevent the restacking of rGO sheets which results in good accessibility of electrolyte ions to the electrode. rGO-Fe₂O₃ exhibited a specific capacitance of 291 F g⁻¹ at 1 A g⁻¹ in 1 M KOH aqueous solution. The performance was observed stable in the negative potential window of −1 to 0 V with three-electrode system. Fe₂O₃ worked well as negative electrode in asymmetric supercapacitor. A hybrid structure of porous Fe₂O₃ nanosheets on carbon fabric (CF-Fe₂O₃) was developed in order to overcome poor electrical conductivity and poor cycling stability of Fe₂O₃ [39]. Fe₂O₃ was deposited on CF via electrodeposition process. An asymmetric supercapacitor with CF-Fe₂O₃ as negative electrode and CF-Co₃O₄ as positive electrode was fabricated, which exhibited a good value of areal capacitance to be 842 mF cm⁻². The asymmetric supercapacitor demonstrated maximum volumetric energy density of 6.75 mWh cm⁻³ and power density of 104 mW cm⁻³. The device retained 93% capacitance after 4000 cycles.

Jiang et al. [40] have synthesized Fe₂O₃/porous carbon nanocomposites (Fe₂O₃/HAC) by hydrothermal method. The porous carbon prevents the agglomeration of Fe₂O₃ nanoparticles and minimizes the ion diffusion path on the electrode surface, providing more flow channels for the ions. Fe₂O₃/HAC demonstrated larger capacitance of 156 F g⁻¹ than that of individual components of HAC (145 F g⁻¹) and Fe₂O₃ (90 F g⁻¹). Redox peaks were observed in CV of both Fe₂O₃ and Fe₂O₃/HAC. The obtained CV consisted of a pair of oxidation peaks at −0.652 and −0.641 V and reduction peaks at −0.996 and −1.038 V, respectively. Fe₂O₃/HAC stores the charge via following mechanism;

$$\text{FeOOH}+{\mathrm{H}}_2\mathrm{O}+\mathrm{e}-\to \mathrm{Fe}{\left(\mathrm{OH}\right)}_2+{\mathrm{OH}}^{-1}$$E5

$$\mathrm{Fe}{\left(\mathrm{OH}\right)}_2+{\mathrm{OH}}^{-1}\to \text{FeOOH}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$E6

A specific capacitance of 970 F g⁻¹ was achieved from Fe₂O₃/rGO composite which was prepared by in situ synthesis and mechanical agitation methods [41]. This high value of capacitance is mainly due to the synergistic effect between Fe₂O₃ and rGO. rGO provides high electrical conductivity to the composite electrode and makes fast channels for the movement of charges. A degradation of 25% of capacitance was observed after 2000 cycles. A carbon shell layer has been coated on porous Fe₂O₃ nanowire arrays by hydrothermal route to improve the electrochemical properties of the electrode [42]. Fe₂O₃/C was used as the anode in solid state asymmetric supercapacitor with MnO₂ as the cathode. A wide voltage of 2 V was achieved with this configuration. The fabricated device could give a maximum energy density of 30.625 Wh kg⁻¹ and a maximum power density of 5000 W kg⁻¹. The device exhibited good stability about 82% retention of capacitance after 10,000 cycles. A α-Fe₂O₃/MnO_x and α-Fe₂O₃/C core-shell nanostructures were developed to design an asymmetric supercapacitor [43]. In the hybrid electrode, fast reversible redox-reactions occur on MnO_x and α-Fe₂O₃ NR core facilitates electron transfer between shell and the current collector. Similarly, α-Fe₂O₃/C core-shell works as negative electrode. The asymmetric device fabricated with α-Fe₂O₃/C//α-Fe₂O₃/MnO_x core-shell exhibited specific capacitance of 1.28 F cm⁻³ at a scan rate of 10 mV s⁻¹ with voltage window of 0–2 V. The supercapacitor was able to retain 78% capacitance at the scan rate of 400 mVs⁻¹ with a maximum energy-density of ∼0.64 mWh cm⁻³ and a maximum power-density of 155 mW cm⁻³. Zhang et al. [44] have developed Fe₂O₃@NiO nanorods on flexible carbon cloth by hydrothermal method. A specific capacitance of 800 mF cm⁻² at 10 mA cm⁻² was obtained with Fe₂O₃@NiO/CC electrode. Remarkable long cycle stability (96.8% capacitance retention after 16,000 cycles) was demonstrated by the electrode. This outstanding performance from Fe₂O₃@NiO/CC is due to the good electrical contact of active material with flexible carbon.

α-Fe₂O₃ has shown good electrochemical performance with graphitic (g-C₃N₄) carbon nitride [45]. The as-prepared g-C₃N₄/α-Fe₂O₃ composites with large specific surface area exhibited specific capacitance of 580 F g⁻¹ determined at 1.0 A g⁻¹ in 1M KOH. The Fe₂O₃ nanoparticles accommodate the space between the layers of g-C₃N₄ avoiding restacking and increasing the probability of interaction between Fe₂O₃ and electrolyte. Zhong et al. [46] have fabricated a heterostructure of Fe₂O₃ nanospheres anchored on FeS₂ nanosheets by one-step hydrothermal approach for supercapacitor. The FeS₂@Fe₂O₃ hybrid electrode demonstrated a specific capacitance of 255 F g⁻¹ as well as good cycle stability, 90% capacitance retention after 5000 cycles. The excellent long cycle life could be ascribed to hybrid structure which can facilitate fast transportation of the electrolyte ions. A hybrid electrode of PEDOT coated onto Ti-doped Fe₂O₃ showed a remarkable capacitive performance [47]. A high value of areal capacitance of 1.15 F cm⁻² at 1 mA cm⁻² has been achieved with this strategy. Ti-Fe₂O₃@PEDOT demonstrated extraordinary cyclic stability of 96% retention in capacitance after 30,000 cycles, which is better than that of Ti-Fe₂O₃ (80.7%) and Fe₂O₃ (81.8%) electrodes. Ti-doping in the electrode enhances electrical conductivity and better utilization of Fe₂O₃. PEDOT has two important roles here, working as protective layer for Fe₂O₃ and relaxing the transfer of electrons.

A high specific capacitance of 1124 F g⁻¹ has been achieved form a ternary composite of polyaniline/Fe₂O₃ decorated graphene coated on carbon cloth (Figures 3 and 4) at a current of 0.25 A g⁻¹ in 1 M H₂SO₄ [48]. This large value of specific capacitance is due to the synergistic effects of individual component in the electrode. Graphene contributes as EDL capacitance and Fe₂O₃ and polyaniline as pseudocapacitance to the overall capacitance of hybrid electrode. In-situ polymerization of polyaniline also leads to improve the surface area of electrodes. Le et al. [50] have fabricated a hybrid electrode of polypyrrole-coated Fe₂O₃ nanotubes (Fe₂O₃@PPy) for high-performance electrode for aqueous asymmetric supercapacitor. A thin layer of polypyrrole was coated on Fe₂O₃ nanotubes by an in situ chemical oxidative polymerization method. Fe₂O₃@PPy could deliver a capacitance of 530 mF cm⁻² at 1 mA cm⁻². There is synergistic effect between PPy and Fe₂O₃, the shell of conducting PPy makes an ease transportation for charge carriers and improves stability of Fe₂O₃ nanotubes while charging/discharging process. The fabricated asymmetric supercapacitor with Fe₂O₃@PPy and MnO₂ nanotubes could deliver energy density of 51.2 Wh kg⁻¹ at a power density of 285.4 W kg⁻¹ operated at high voltage of 2.0 V. 83.5% capacitance retention was observed over 5000 cycles with the asymmetric supercapacitor.

Figure 3.

Schematic illustration of synthesis process of (a) Fe₂O₃ decorated graphene and (b) ternary Polyaniline/Graphene/Fe₂O₃ (PGF) composite on carbon cloth. (Reprinted with permission from Ref. [48]. Copyright (2020) American Chemical Society).

Figure 4.

Electrochemical behavior of the GF-CNT@Fe₂O₃//GF-CoMoO₄ supercapacitor. (a) Full-cell structure. (b) CV curves, (c) rate capability, (d) cycling life, (e) Ragone plot of the asymmetric supercapacitor. Digital photos of the single electrode and full cell are depicted in insets of (c). (Reprinted with permission from Ref. [49]. Copyright (2015) American Chemical Society).

Guan et al. [49] have developed an ultrahigh-energy and long-life supercapacitor anode material by coating a thin layer of Fe₂O₃ on graphite foam-carbon nanotube framework GF-CNT@Fe₂O₃. GF-CNT@Fe₂O₃ as anode was also integrated into a full supercapacitor cell with GF-CoMoO₄ as cathode. A high energy density of ∼74.7 Wh kg⁻¹ with power density of∼1400 W kg⁻¹ has been obtained from the full device. The device also exhibited exceptional cycle stability, retention of about 95.4% capacitance after 50,000 cycles. These values make it a promising candidate for high performance supercapacitor. Improvement in the electrochemical performance of Fe₂O₃ based electrode may also be possible by introducing nitrogen in the electrode. In this regard, Zhao et al. [51] have synthesized nitrogen-doped graphene and Fe₂O₃ composites (NGFeCs) by hydrothermal method. NGFeCs exhibited improved electrochemical performance than GFeCs. The specific capacitance of NGFeCs electrode was determined to be 260.1 F g⁻¹ (150.4 Fg⁻¹ for GFeCs electrode) with current density of 2 A g⁻¹. In addition to above, 82.5% retention in capacitance was observed after 1000 cycles. The improvement in the performance is due to the good electronic conductivity and increased active sites.

Self-assembled α-Fe₂O₃ mesocrystals/graphene nanohybrids demonstrated a good value of specific capacitance of 306.9 F g⁻¹ at 3 A g⁻¹ in an aqueous electrolyte and in voltage window of 1 V [33]. Porous structure of α-Fe₂O₃ mesocrystals/graphene nanohybrids with high electrical conductivity can provide fast transportation for electrolyte ions even at high discharge current densities. 2D α-Fe₂O₃/rGO nanocomposites fabricated by one-pot solvothermal approach exhibited specific capacitance value to be 903 F g⁻¹ calculated at a current density of 1 A g⁻¹ [52]. This value of capacitance was observed superior than α-Fe₂O₃ nanoplates. In the hybrid composite, α-Fe₂O₃ nanoplates were encapsulated in rGO nanosheets to form a porous structure. The α-Fe₂O₃/rGO nanocomposites with high electrical conductivity and 2D nanostructure promote the charge transportation between electrode and electrolyte, hence enhancing the electrochemical performance of the electrode.

4. Fe₃O₄ based electrodes

Transition metal oxides (RuO₂, MnO₂, NiO, Fe₂O₃, Co₃O₄, etc.) have been used as electrode materials for supercapacitor application because they use rapid reversible redox reactions at the surface of active materials to offer high power density [9, 31, 53, 54, 55, 56, 57, 58]. Amongst the metal oxides, Fe₃O₄ is a better alternative electrode material for supercapacitor because of its low cost, natural abundance, environmental friendliness, high theoretical specific capacitance (2299 F g⁻¹), large potential window and different valence states [59, 60, 61]. It has been seen in literature that Fe₃O₄ with various dimensions and morphology shows high value of specific capacitance [62].

The charge storage mechanism of Fe₃O₄ was investigated in various aqueous electrolytes such as sodium chloride, potassium hydroxide, sodium sulphate, sodium sulphite and sodium phosphate. Fe₃O₄ in the Na₂SO₃ electrolyte demonstrated a higher capacitance of 510 F g⁻¹ with an operating potential of 1.2 V and this value was greater than those of other electrolytes [63]. Two pseudocapacitive mechanisms were purposed for Fe₃O₄ in the Na₂SO₃ electrolyte, which are given below

$$\mathrm{FeO}+\mathrm{S}{\mathrm{O}}_3^{2-}\leftrightarrow \mathrm{FeS}{\mathrm{O}}_4+2{\mathrm{e}}^{-}$$E7

$$2{\mathrm{Fe}}^{\mathrm{II}}\mathrm{O}+\mathrm{S}{\mathrm{O}}_3^{2-}\leftrightarrow \left({\mathrm{Fe}}^{\mathrm{III}}\mathrm{O}\right)+\mathrm{S}{\mathrm{O}}_3^{2-}\left({\mathrm{Fe}}^{\mathrm{III}}\mathrm{O}\right)+2{\mathrm{e}}^{-}$$E8

Eq. (7) represents the surface redox reaction of sulphur in the form of sulphate and sulphite anions while Eq. (8) represents the oxidation/reduction reaction between Fe(II) and Fe(III). It was observed that due to the addition of both the EDLC and pseudocapacitor, the capacitance of Fe₃O₄ in the Na₂SO₃ electrolyte rises and involves the reduction/oxidation of specially adsorbed sulphite anions as given by the following equations [63].

$$2\mathrm{S}{\mathrm{O}}_3^{2-}+3{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\leftrightarrow {\mathrm{S}}_2{\mathrm{O}}_3^{2-}+6\mathrm{O}{\mathrm{H}}^{-}$$E9

$${\mathrm{S}}_2{\mathrm{O}}_3^{2-}+3{\mathrm{H}}_2\mathrm{O}+8{\mathrm{e}}^{-}\leftrightarrow 2{\mathrm{S}}^{2-}+6\mathrm{O}{\mathrm{H}}^{-}$$E10

Many methods have been used to fabricate the Fe₃O₄ materials for supercapacitor application such as hydrothermal method [64, 65, 66, 67], Electroplating [63], sol-gel method [68], Chemical precipitation method [69], Hydrolysis and annealing process [70], solvothermal method [71], Electrospinning [72], Aerosol method [73]. Reports on different synthesis processes of Fe₃O₄ based electrodes and their specific capacitance values are given in Table 2.

Electrode	Synthesis process	Electrolyte	Capacitance	Ref.
Fe2O3/porous carbon	Hydrothermal	6 M KOH	256 F g^−1 at 1 A g^−1	[40]
FeS2 nanosheet@Fe2O3 nanosphere	Hydrothermal	1 M Li2SO4	255 F g^−1 at 1 A g^−1	[46]
α-Fe2O3 nanostructures	Sol–gel route	0.5 M Na2SO3	300 F g^−1 at 1 A g^−1	[36]
Polyaniline/graphene/Fe2O3 composites hydrogel	Microwave irradiation and in-situ polymerization	1 M H2SO4	1124 F g^−1 at 0.25 A g^−1	[48]
α-Fe2O3/rGO	One-pot solvothermal approach	1 M KOH	903 F g^−1 at 1 A g^−1	[52]
Polypyrrole-coated Fe2O3 nanotubes	In situ chemical oxidative polymerization method	1 M Na2SO4	530 mF cm^−2 at 1 mA cm^−2	[50]
Porous-Fe2O3@C nanowire	Hydrothermal route	1 M Na2SO4	280 F g^−1 at 1 Ag^−1 and 241.3 mF cm^−2 at 1 mA cm^−2	[42]
α-Fe2O3/rGO composites	In situ synthesis and mechanical agitation methods	6 M KOH	970 F g^−1 at 1 A g^−1	[41]
Porous flowerlikeα-Fe2O3	Ethylene glycol mediated self-assembly process	0.5 M Na2SO3	127 F g^−1 at 1 A g^−1	[37]
Core-branch Fe2O3@NiO nanorods	Hydrothermal method	3 M KOH	800 mF cm^−2 at 10 mA cm^−2	[44]

Table 2.

Electrochemical capacitance of some Fe₂O₃ based electrodes.

Fe₃O₄@carbon nanosheet composite synthesized using ammonium ferric citrate precursor and graphene oxide as the structure-directing agent under a hydrothermal process was used as electrode in supercapacitor [59]. Fe₃O₄@carbon nanosheet composite exhibited a specific capacitance of 586 F g⁻¹ at 0.5 A g⁻¹ in KOH/PVA gel electrolyte. This composite showed excellent energy density of 18.3 Wh kg⁻¹ and power density of 351 W kg⁻¹.

Wang et al. [60] developed a simple route to fabricate the hybrid electrode of Fe₃O₄-doped porous carbon nanorods (Fe₃O₄-DCN) supported by three dimensional (3D) kenaf stem-derived macroporous carbon (KSPC) for supercapacitor. The 3D-KSPC/Fe₃O₄-DCN showed excellent specific capacitance of 285.4 F g⁻¹ at the current density of 1 A g⁻¹ and excellent cyclic stability in 2.0 M KOH electrolyte (Figure 5). Kumar et al. [61] prepared a hybrid electrode of 3D rGO NSs containing Fe₃O₄ NPs using one-pot microwave approach. The experimental studies showed that the as-synthesized Fe₃O₄/rGO hybrids were made of faceted Fe₃O₄ NPs with interconnected network of rGO NSs. The schematic formation mechanism is shown in Figure 6. This material as electrode in supercapacitor exhibited a specific capacitance of 455 F g⁻¹ at the scan rate of 8 mV s⁻¹.

Figure 5.

(a) Schematic illustration of the formation process of carbon nanosheet embedded Fe₃O₄. (Reprinted with permission from Ref. [59]. Copyright (2016) American Chemical Society.) (b) Schematic illustration of the formation process of 3D-KSPC/Fe₃O₄-DCN nanocomposites. (Reprinted with permission from Ref. [60]. Copyright (2016) American Chemical Society).

Figure 6.

Schematic formation mechanism of 3D Fe₃O₄/rGO hybrids. (Reprinted with permission from Ref. [61]. Copyright (2017) American Chemical Society).

Chen et al. [74] fabricated a Fe₃O₄ film with regularly edge-affected cubic morphology on stainless foil using hydrothermal method and this film was used as electrode for supercapacitor. This film exhibited a specific capacitance of 118.2 F g⁻¹ at the current of 6 mA in Na₂SO₃ electrolyte and good cycle stability about 88.75% capacitance retention after 500 cycles. Wang et al. [63] prepared the Fe₃O₄ film by an electroplating method to be used in supercapacitor as electrode. Fe₃O₄ film demonstrated a specific capacitance of 170 F g⁻¹ in 1 M Na₂SO₃ electrolyte. Fe₃O₄ nanoparticles synthesized by sol gel method exhibited a specific capacitance of 185 F g⁻¹ at the current of 1 mA in 3 M KOH electrolyte [68]. Brousse et al. [69] prepared Fe₃O₄ by chemical precipitation, which demonstrated a specific capacitance of 75 F g⁻¹ in aqueous electrolyte of 0.1 M K₂SO₄. Liu et al. [64] synthesized Fe₃O₄ nanorods and carbon coated Fe₃O₄ nanorods via hydrothermal reaction and subsequent sintering procedure. Fe₃O₄ nanorods showed a specific capacitance of 208.6 F g⁻¹ while carbon coated Fe₃O₄ nanorods exhibited a specific capacitance of 275.9 F g⁻¹ at a current density of 0.5 A g⁻¹ in 1 M Na₂SO₃ aqueous solution.

Guan et al. [65] synthesized carbon nanotube/Fe₃O₄ (CNT/Fe₃O₄) nanocomposite by an easy and efficient hydrothermal method. CNT/Fe₃O₄ nanocomposite showed enhanced specific capacitances of 117.2 F g⁻¹ (which is three times than that of pure Fe₃O₄) at current density of 10 mA cm⁻² in 6 M KOH electrolyte. It also exhibited superior cyclic stability and energy density of 16.2 Wh kg⁻¹. Lin et al. [66] synthesized Fe₃O₄/GO by using hydrothermal method, which showed a specific capacitance of 661 F g⁻¹ at current density of 1 A g⁻¹ in 1 M KOH electrolyte. Qi et al. [70] prepared the Fe₃O₄/rGO composites using hydrolysis route and subsequent annealing process for supercapacitor application. The Fe₃O₄/rGO composite showed specific capacitance of 350.6 F g⁻¹ at 1 mV s⁻¹ in 6 M KOH electrolyte (Table 3).

Electrode	Method	Electrolyte	Specific capacitance	Reference
Fe3O4 thin film	Electroplating	1 M Na2SO3	170 F g^−1	[63]
Fe3O4 nanoparticles	Sol–gel	3 M KOH	185 F g^−1	[68]
Fe3O4	Chemical precipitation method	0.1 M K2SO4	75 F g^−1	[69]
Fe3O4 nanorods	Hydrothermal	1 M Na2SO3	208.6 F g^−1	[64]
Fe3O4/graphene	Hydrothermal	1 M KOH	661 F g^−1	[66]
Fe3O4/reduced graphene oxide	Hydrolysis and annealing process	6 M KOH	350.6 F g^−1	[70]
Fe3O4/reduced graphene oxide	Solvothermal process	1 M KOH	480 F g^−1	[75]
Fe3O4–carbon nanosheets	Hydrothermal and heat treatment	1 M Na2SO3	163.4 F g^−1	[67]
Fe3O4 doped double-shelled hollow carbon spheres	Aerosol, spray and in-situ polymerization methods	6 M KOH	1153 F g^−1	[76]

Table 3.

Capacitance of Fe₃O₄ based electrodes synthesized by different process.

It is seen that iron oxides have demonstrated excellent performance in supercapacitor and asymmetric supercapacitor as well [71, 72, 73, 77, 78, 79]. Shi et al. [75] prepared the nanocomposites of Fe₃O₄ NPs connected to rGO sheets by using a solvothermal process. Fe₃O₄/rGO nanocomposites were utilized to prepare thin film supercapacitor electrodes by using a spray deposition technique without the addition of insulating binders. It was observed that the Fe₃O₄/rGO nanocomposite exhibited higher specific capacitance, 480 F g⁻¹ at a discharge current density of 5 A g⁻¹, which is larger than that of pure rGO or pure Fe₃O₄ NPs. Fe₃O₄/rGO nanocomposite exhibited energy density of 67 W h kg⁻¹ and power density of 5506 W kg⁻¹. Ultrathin nanoporous Fe₃O₄–carbon nanosheets synthesized by hydrothermal method demonstrated a specific capacitance of 163.4 F g⁻¹ in 1 M Na₂SO₃ electrolyte [67].

Mu et al. [80] synthesized Fe₃O₄ nanosheets on one-dimensional (1D) carbon nanofibers (CNFs) using the electrospinning technique and solvent-thermal process. The Fe₃O₄/CNFs nanocomposite exhibited a specific capacitance of 135 F g⁻¹ in 1 M Na₂SO₃ electrolyte. The high capacitance may be due to the improved electrical conductivity of the composite by adding the CNFs in Fe₃O₄. Li et al. [76] prepared the double-shelled hollow carbon spheres with conductive graphite structure and Fe₃O₄ species (C-C-Fe₃O₄) using aerosol, spray, and in-situ polymerization methods. The C-C-Fe₃O₄ showed remarkable value of specific capacitance to be 1153 F g⁻¹ at current density of 2 A g⁻¹. C-C-Fe₃O₄ also demonstrated good rate capability, 514 F g⁻¹ at 100 A g⁻¹. The electrode exhibited only 3.3% degradation in the capacitance after 8000 cycles. Energy density of 17–45 W h kg⁻¹ and powder density of 400–8000 W kg⁻¹ were achieved from the C-C-Fe₃O₄ electrode.

5. Conclusion

Recent progress on supercapacitor is to increase its specific capacitance, rate capability, cycle life and operating voltage by developing low cost and eco-friendly electrode materials. In this regard, iron oxide may be a suitable candidate with higher value of capacitance as negative electrode for asymmetric supercapacitor to be operated at wide voltage window. Therefore, a lot of work has been carried out on iron oxide based electrode for supercapacitor application. The electrochemical performance of iron oxide based electrodes has been reviewed and discussed in this chapter. It is observed that the synthesis process and morphology of iron oxide play important role in supercapacitor performance. Efforts have been made towards preparing a composite of iron oxide with high conductive materials in order to overcome its poor electrical conductivity. Nanostructure of iron oxide such as nanoparticles could enhance the active sites for electrochemical reactions. Iron oxide nanostructure with carbon can further increase the specific capacitance and energy density of supercapacitor. Synthesis of iron oxide nanoparticles/graphene nanocomposite seems to be more effective approach for high performance supercapacitor.

Conflict of interest

The authors declare no conflict of interest.