Electrolytes in Metal Oxide Supercapacitors

Electrolytes in Metal Oxide Supercapacitors

3

Maria J. Carmezim 1, 2 , Catarina F. Santos 1, 2 1 ESTSetubal, Instituto Politécnico de Set
ubal, Set
ubal, Portugal; 2CQE-IST, Universidade de Lisboa, Lisboa, Portugal

3.1

Introduction

As other engineering systems that have reached an advanced stage of development, the electrochemical energy storage devices such as supercapacitors (SCs) put forward an urgent need of optimizing their performance. To achieve this upgrade, it is necessary not only to optimize the metal oxide material of the electrodes but also to promote an optimal device design. In the recent years, the performance of SCs has greatly improved, enabling more efficient storage mechanisms because of the development of new electrode materials at the nanoscale, development of new cell configurations, and the use of new electrolyte systems [1,2]. It is not uncommonly believed that the effect of electrolytes on the performance of the SCs is nowadays considered as one of the major challenges for researchers. Broadly, the SC devices are composed of an electrochemical supercapacitor (ES) of the electrolyte (Fig. 3.1), two electrodes, and a separator. The electrolyte in SC devices is maintained between the separator and active material layers to allow charge storage by several processes involving electrolyte ions on the electrodeeelectrolyte interface. The electrolytes to afford a high performance of SCs should guarantee wide voltage window, high electrochemical stability, high ionic conductivity, high wettability, a wide operational temperature range and low solvated ionic radius, low resistivity,

Figure 3.1 Schematic representation of a supercapacitor and the main electrolyte properties. Metal Oxides in Supercapacitors. http://dx.doi.org/10.1016/B978-0-12-810464-4.00003-6 Copyright © 2017 Elsevier Inc. All rights reserved.

50

Metal Oxides in Supercapacitors

low volatility, low toxicity, low cost, noninflammable property, and availability at high purity [3]. The most common electrolyte classification used in SCs includes three groups: the aqueous electrolytes, the organic electrolytes, and the liquid salts [usually called ionic liquids (ILs)] [3,4]. Although ILs have been extensively studied for carbon-based devices [4], usually they are scarcely applied in metal oxideebased SCs. The most relevant and advanced types of electrolytes for metal oxideebased SCs will be described in the following [5,6]. Aqueous electrolytes present many unquestionable advantages, namely, low price, low toxicity, environmental friendliness, safety, and more important they allow higher power capabilities [7e9]. However, they have intrinsic drawbacks such as low energy density and narrow operating voltage (w1 V) [4,9]. The aqueous electrolytes used in metal oxide SCs can be classified into three types depending on the solution pH: alkaline, acidic, and neutral. KOH is the most common alkaline electrolyte used in metal oxide SCs, whereas H2SO4 is the acid electrolyte usually applied. These alkaline and acidic electrolytes are able to minimize internal resistance and maximize power capability, as they have excellent ionic conductivity or lower resistivity [2]. The high ionic conductivity can be explained by the great number of proton (Hþ) and hydroxide (OH) involved in proton hopping or proton transport [2]. Nevertheless, the acid electrolytes are not so commonly used as the neutral electrolytes, despite their much higher conductivity, because they can cause corrosion of the device and/or dissolution of the electrodes. To mitigate the corrosive character of the acidic and alkaline electrolytes and to increase the working potential window and the cycle life, neutral electrolytes have been investigated [2,8]. The widely studied neutral electrolytes in metal oxide SCs are the compounds X2SO4, where X ¼ Li, Na or K, with concentration ranges between 0.1 and 2 M [1,8]. An electrolyte with high ionic conductivity is a critical requirement that prevails to other properties. However, in the recent years, there has been a tremendous effort focusing on the development of novel devices with both flexible electrodeeelectrolyte and optimal electrochemical and mechanical properties. Among which, polymer gel electrolytes encourage the design of flexible, thin, lightweight, and small-volume novel energy storage devices [10e12]. On the other hand, gel electrolytes have been demonstrated as an adequate substitute for aqueous electrolytes for solid-state SCs, being safer (liquid-leakage free and less nontoxic) and serving not only as ionic conducting media but also as electrode separator. It is well known that electrolyte composition plays a critical role in electrochemical performance of metal oxide electrodes. Accordingly, faradaic reactions at the electrodee electrolyte interface can be facilitated by introducing reversible redox species as additives or mediators (e.g., halide ions, phenylamide, and quinones) into conventional electrolytes. This electrolyte optimization allows enhancing redox activity and simultaneously improves ionic conductivity. As it will be shown in this chapter, the performance of the metal oxide SCs is limited by electrochemical processes that occur at the electrodeeelectrolyte interface and the transport path length for both electrons and ions. For this reason, it will also be unavoidable to point out the recent status of metal oxides in SCs.

Electrolytes in Metal Oxide Supercapacitors

3.2

51

Supercapacitors and Interaction With Electrolytes

Supercapacitors will be the next-generation centerpiece of power devices because of their excellent properties such as high power density, fast chargeedischarge rate, excellent reversibility, and long cycle life. For a long time, the SCs were named into different categories in light of their predominant electrochemical storage behavior: pseudocapacitance and electric double-layer capacitance. But recently the pseudocapacitance behavior has generated some controversy and fundamental discussion in the scientific community [5]. Charge storage in the electric double-layer SCs occurs at the electrodeeelectrolyte interface because of the formation of the Helmholtz double layer, and the storage mechanism is purely of electrostatic nature [13,14]. It means that the storage of electric charge and energy involves no chemical changes of the solid-phase electrode, usually called a nonfaradaic process [14,15]. In such case the electrolyte must be electrochemically inert. The electric double-layer SCs are considered as capacitor-type materials [13]. The cyclic voltammogram (CV) of double-layer SCs features a characteristic rectangular shape, with voltage-independent current, whereas the chargeedischarge galvanostatic plots display a linear triangular shape typical of a capacitive behavior and are completely reversible. Typically, the electrode materials that charged through a capacitive double-layer nonfaradaic process are carbon, carbon-based materials, and carbon allotropes, especially for their highly porous surface area. They are extensively studied because they have an extremely long cycle life, are cheap, are relatively environment friendly, and can operate over a wide range of temperatures. Nevertheless, they have low energy densities (5e10 W h L1) and they are not able to fully meet the various performances required nowadays in the market (20e30 W h L1) [9]. A completely different charge storage behavior called pseudocapacitance was introduced by B.E. Conway [13]. In the pseudocapacitance behavior the charge could be stored by a faradaic process, which means electrosoption of ions accompanied by surface redox reactions in which electron transfer occurs by crossing the interface of the current collector and active material [15]. According to B.E. Conway the pseudocapacitance behavior can result from mainly three mechanisms: (1) redox, (2) intercalation, and (3) underpotential deposition. Although the mentioned mechanisms are considered the classics, Qian-Long Lu and coworkers [16] found that mesoporous MoO3x electrodes in H2SO4 electrolyte comprise both redox and intercalation mechanisms, hence the name new redox/intercalation mechanism. Numerous reports ascribe the pseudocapacitance behavior to binary and ternary metal oxides but a timely paper has contributed to clarify this misleading topic [5]. Indeed, the pseudocapacitance behavior is only valid for capacitorlike electrochemical metal oxide electrodes, such as RuO2, Nb2O5, and MnO2 in mild aqueous electrolyte. Despite a faradaic process that occurs at the electrodeeelectrolyte interface, these oxide electrodes exhibit a linear dependence of charge storage with potential giving rise to an almost rectangular currentevoltage profile curve [13]. Chargeedischarge curves display a linear triangular shape (Fig. 3.2, left side) similar to that observed in carbon electrodes.

52

Metal Oxides in Supercapacitors

Figure 3.2 Metal oxide supercapacitor behavior and schematic electrochemical response.

Moreover, most binary and ternary metal oxide electrodes that also undergo a faradaic process at the electrodeeelectrolyte interface, i.e., redox reactions, exhibit a nonlinear dependence of charge storage with potential and a faradaic plateau in chargeedischarge curves, also called batterylike behavior (Fig. 3.2, right side). However, capacitorlike behavior can be the result of electrode design, i.e., nanoparticle size, thin film thickness, and not an intrinsic property of the material. In case of use of bulk LiCoO2 as positive electrode in lithium-ion batteries, a faradaic behavior is observed but a capacitorlike response occurs with LiCoO2 thin films when reaching a critical 6 nm film thickness [17]. Actually the validity of using capacitance to describe the charge storage performance of SCs where there is direct electron transfer between the oxide electrode material and the electrolyte redox species, originating a batterylike response, is convincingly brought into question [5]. In fact, the charge-to-potential ratio of an electrode (i.e., NiO2, Co2O3, and spinel oxide materials) in a redox electrolyte is significantly dependent on the applied voltage, resulting in an overestimated specific energy. Moreover, the noninclusion of the redox additive mass in the overall active mass could lead to overestimations of the specific charge and energy storage capacity of the device. The total mass of the redox species can be easily derived from the volume and concentration of the redox species in the electrolyte and if the redox species is ionic, the mass of the charge balancing the counterions should also be considered. Akinwolemiwa and coauthors [6] propose that true performance evaluation and comparison between batterylike SCs, with and without added redox electrolyte, should be based on energy capacity and power capability, instead of capacitance or pseudocapacitance.

Electrolytes in Metal Oxide Supercapacitors

3.3

53

Electrolytes for Metal Oxide Supercapacitors

To improve the performance of SCs, various types of electrolytes have been developed and reported in the literature. Despite the emergence of new electrolytes, an ideal electrolyte has not been developed yet, and all electrolytes have pros and cons. Taking into account the last published developments and applications, a possible categorization of electrolytes for metal oxide SCs is proposed: liquid, redox additive, and solid and/or gel state, as shown in Fig. 3.3. All liquid electrolytes are aqueous and can be subdivided into alkaline, acidic, and neutral depending on the pH. The solid can be divided into gel polymer and inorganic electrolytes and the redox additive into aqueous and polymer gel electrolytes. From the beginning, aqueous electrolytes have attracted special attention owing to their high conductivity (up to w1 S cm2) and capacitance, but their working voltage is limited (about 1.0 V) because of the narrow electrochemical stability window of water (1.23 V theoretical value), which further limits the energy stored in the device [4,18]. It is worthy to note that power is affected by the square value of the working voltage (P ¼ V2/4R). The specific energy (E) of SCs E ¼ ½CV2 can be enhanced by increasing the operating voltage window (V) and/or capacitance (C) [8,19,20]. It is known that the working voltage depends mainly on the stability of the electrolyte. Moreover, the specific capacitance is dependent on the electrolyte conductivity and the size of electrolyte ions that must penetrate into and out of the electrode pores [2,8,21]. Although it has been found that the working voltage of aqueous electrolytes

Figure 3.3 Classification of electrolytes used in metal oxideebased supercapacitors.

54

Metal Oxides in Supercapacitors

is restricted by the splitting of water, which could cause the rupture of the SC cells, the performance of a metal oxideebased SC with an aqueous electrolyte is consistently 40%e50% higher than that with the organic electrolytes [22]. Additionally, the aqueous electrolytes can be prepared and utilized without stringently controlling the preparing processes and conditions, which is an uncontested benefit, whereas the organic ones need controlled processes and conditions to get electrolytes with high purity [3]. The organic electrolytes can operate at higher voltages (w2.7e3.5 V) when compared with aqueous electrolytes (w1 V), which is an advantage and for this reason they are often an option [2,3]. Furthermore, the organic electrolytes allow the use of cheaper materials for the current collectors and packages [8], but they have low ionic conductivity, have higher cost, have smaller specific capacitance, are less environment friendly, and could have problems such as flammability, volatility, and toxicity [8,18]. The use of solid inorganic electrolytes is less common when compared with aqueous or organic electrolytes. Although solid inorganic electrolytes are usually mechanically robust, they can be produced in thin film form, which allows the miniaturization. For these reasons, inorganic electrolytes do not have problems with electrolyte leakage. Moreover, they are inflammable and thermally stable [8,23]. However, only one example of inorganic electrolytes used with metal oxide SCs as solid SCs can be found in the literature and correspond to a mixture of oxides (e.g., 0.4 LiClO4  0.6 Al2O3 [23]). This electrolyte operates at high temperature (100e300 C) [23], but usually the specific capacitance (29 F g1 at 0.05 A g1) is lower than that of liquid electrolytes [8,23].

3.3.1 3.3.1.1

Liquid Electrolytes Aqueous Electrolytes

3.3.1.1.1 Alkaline Electrolytes The alkaline electrolytes are the most widely reported aqueous electrolytes in the literature [1,4,8] for metal oxide SC. In many studies the selection of the alkaline electrolyte takes into account the performance of metal oxide SCs and the predominant storage mechanism. Among the numerous basic electrolytes, KOH has been the most extensively used because of its high ionic conductivity (Table 3.1); however, other basic electrolytes, such as NaOH and LiOH, have also been investigated. Several properties of electrolytes could affect the performance of metal oxide SCs, such as the operating temperature, ion nature, and concentration. For example, it was observed for NiO electrode that the concentration of alkaline electrolyte (2, 4, and 6 M KOH) can affect the value of specific capacitance, i.e., the specific capacitance increases with increasing concentration of electrolyte [24,25]. This is due to the increased conductivity of an electrolyte generally with the concentration in an aqueous solution. Thus, 6 M KOH will provide higher conductivity and higher concentration of OH species than 2 and 4 M solutions, which facilitates charge transfer in both bulk electrolyte and electrode [24]. Additionally, according to Zhao et al. [24] the cycling stability is excellent in high-concentration electrolyte (6 M KOH). Furthermore, using

Electrolytes in Metal Oxide Supercapacitors

Table 3.1

55

The Ion Size and Ionic Conductivity Values [8,19]

Ion

Ion Size (Å)

Hydrated Ion Size (Å)

Ionic Conductivity (S cm2 molL1)

Hþ

1.15

2.80

350.1

1.00

4.12

119

1.33

3.31

73.5

0.95

3.58

50.11

0.60

3.82

38.69

OH

1.76

3.00

198

SO4 2

2.90

3.79

160

Br

1.95

3.30

78.40

2.16

3.31

76.80

1.81

3.32

76.31

2.64

3.35

71.42

2þ

Ca

þ

K

þ

Na

þ

Li



I

 

C1

NO3



higher concentration of electrolyte the peaks in the CVs become broader and the peak area is larger in comparison with the sharp redox peak for low concentration, which is indicative of a better capacitive performance [20,24]. However, high concentration of electrolyte usually causes corrosion at the electrode surface and/or current collector, which affects the device performance. As mentioned earlier the operating temperature also has a strong effect on the efficiency of the metal oxide SCs. It is well known that an increase in the electrolyte temperature (0e60 C) can result in an increase in the specific capacitance and a decrease in the equivalent series resistance (ESR) of the device, as observed by R. Gupta et al. [26] for NiCo2O4 electrode material. A decrease in the ESR is usually attributed to the enhanced conductivity of the electrolyte due to the increase in the mobility of the ions [26,27]. Another important aspect is the cycle stability of the electrode after a repeated heating and cooling in the temperature. W. Li et al. [27] reported that the MnO2 nanobelt electrode had 91.3% of retention after 5000 cycles with repeating heating and cooling in the temperature range of 0e50 C, showing a good high-temperature-resistive long-term cycle stability. Another important issue to the overall metal oxide SC performance is the type of alkali metal ion of electrolytes. For example, I. Mismon et al. prepared d-MnO2 electrode [28] and R. Gupta et al. prepared NiCo2O4 electrode [26] and both observed that the specific capacity as well as the specific energy density increase with increase in the ion size (ion size: Liþ > Naþ > Kþ) [26,28]. The authors attributed this results to the Liþ ions in aqueous solution, which have the highest hydrated radius (Table 3.1). This contributes to reduce the mobility and consequently the redox current when compared

56

Metal Oxides in Supercapacitors

with Naþ and Kþ ions [26]. Similar results for other different metal oxide electrodes such as MnFe2O4 [29] and Bi2WO6 [30] have been reported by other researchers. As generally observed, for alkaline electrolytes the electrochemical reaction between the electrolyte and the electrode materials is well established, as demonstrated by the data present in Table 3.2.

3.3.1.1.2 Acidic Electrolytes For practical applications, the metal oxide SCs should exhibit many desirable properties, such as high power density, good pulse chargeedischarge characteristics, and superior cycling stability. Aqueous electrolytes, with acid groups, have also been studied in systems with metal oxideebased materials [49e51]. The most commonly used acidic electrolyte is H2SO4, mainly because of its very high ionic conductivity (0.8 S cm2 for 1 M at 25 C). The ionic conductivity of the aqueous acid electrolyte is strongly dependent on the concentration and has a strong influence on the specific capacitance [21]. Although the acidic electrolytes have demonstrated advantages (high specific capacity) when used in SCs, they have also caused problems, especially in the electrodes. In acidic aqueous electrolytes the metal oxide electrodes present poor stability, mainly because of their sensitivity to the type and pH of the electrolytes [35]. S.-K. Chang et al. [35] found that the stability of NiCo2O4 was poor in acidic electrolyte (1 M HCl) because of the dissolution of spinel oxide. To overcome this limitation, other electrolytes (alkaline or neutral) are being targeted as the final choice by most of the researchers. In the past few years, one of the widely studied electrode materials in acid aqueous electrolyte was RuO2. The reason for this interest is related to the high stability, chemical resistance, and specific capacitance obtained for RuO2. This high value of specific capacitance, which could reach 1000 F g1 for amorphous RuO2, is most of the times associated to the surface reaction between ruthenium ions present in electrode and Hþ ions existing in the electrolyte. The RuO2 is an expensive material and the sources of Ru are limited, which have restricted their commercial usage. To solve these issues, other alternative oxide electrodes, for example, WO3 [50], NiFe2O4 [51], and IrO2 [49], have been searched and tested in such acidic electrolytes (Table 3.3). Most recently, a new combination of stable nanometric oxide clusters also known as polyoxometalates (POMs) with reversible redox activities and working window in an extended voltage range (1.6 V) in acidic (H2SO4) aqueous electrolyte has been reported [52,53]. Most of the POMs are anchored to conducting polymers or carbon-based materials to increase the faradaic (POMs) and capacitive (polymer or carbon-based) charge storage in a hybrid nanocomposite electrode. Two examples of POMs, H3PW12O403$H2O [53] and H3PMO12O40 [52], are reported by Su
arezGuevara et al. and V. Ruiz et al. respectively. Table 3.3 also depicted the energy storage/delivery process between electrolyte and some recent metal oxideebased electrodes. There are also different acidic aqueous electrolytes, for instance, perchloric acid, hexafluorosilicic acid, and tetrafluoroboric acid, that are used in SCs. As far as known, until today, none of them were used in SCs with metal oxideebased electrodes.

Alkaline Aqueous Electrolyte Used in Metal Oxide Supercapacitors, the Reaction Between Electrolyte and Electrodes, and Electrochemical Supercapacitor Performance

Table 3.2

Aqueous Electrolyte/ Concentration

Electrode

Electrode Setup

Specific Capacitance (F gL1)

Potential Window (V)/ Reference 1

Cycling Stability

Power Density

Reaction Between Electrolyte and Electrode þ

References

KOH/0.5 M

SnO2 quantum dots

3

10 at 20 mV s

0.2 to 0.5

98% after 1000 cycles

e

SnO2 þ Ka þ e a 4 SnOOKa

[31]

KOH/1 M

Networked NiCo2O4/ MnO2

3

1891 at 100 mA cm2

0e0.6

98.4% after 3000 cycles

e

Faradaic redox reaction MeO/MeOeON, where M ¼ Mn, Co, Ni and N ¼ K or H ions

[32]

KOH/1 M

LaMnO3.09

2/3

400 at 10 mV s1

1.2 to 0

220.4 W kg1

i h La Mn2d 2þ ; Mnð12dÞ 3þ Oð3dÞ þ 2dOH

1

r-LaMnO2.91

KOH/1 M

KOH/1 M

Fe3O4

2/3

379.8 at 2 A g1

575.2 at 2 A g1

Co2AlO4@ MnO2

915.1 at 2 A g1

294.6 at 2 mV s1

3

4214 W kg

4 LaMn3þ O3 þ 2de þ dH2 O h LaMn3þ O3 þ 2dOH 4 La Mn2d 4þ ; i Mnð12dÞ 3þ Oð3dÞ þ 2de þ H2 O

800 W kg1

Fe0eFe3þ

1

500 at 10 mV s

Co2AlO4

MnO2 nanoflower

e

0e0.4

94.23% after 2000 cycles 93.3% after 2000 cycles

Co2þeCo4þ associated with OH

1 to 0

96.1% after 2000 cycles

MnO2  adsorption Kþ cations

0.45 to 0.45

74% after 2000 cycles

e

EDLC mechanism

[20]

[33]

[28]

  ðMnO2 Þsurface þ Cþ þ e 4 MnO2  Cþ surface Cþ ¼ Hþ, Liþ, Naþ, Kþ Pseudocapacitance mechanism (MnO2)surface þ Cþ þ e 4 (MnOOC)

Continued

Alkaline Aqueous Electrolyte Used in Metal Oxide Supercapacitors, the Reaction Between Electrolyte and Electrodes, and Electrochemical Supercapacitor Performancedcont’d

Table 3.2

Aqueous Electrolyte/ Concentration

Electrode

Electrode Setup

Specific Capacitance (F gL1)

Potential Window (V)/ Reference 1

KOH/1 M

RuO2

2/3

124.8 at 50 mV s

1 to 0

KOH/1 M

NiCo2O4

3

249.8 at 0.5 A g1

0.2 to 0.55

Cycling Stability e w100% after 1000 cycles

Power Density 1

Reaction Between Electrolyte and Electrode

References

900 W kg

Not shown

[34]

e

Reversible process:

[35]

OH bonded to Co2þ of Co3O4

Irreversible process: Co3O4 þ H2O þ OH 4 3CoOOH þ e Co(OH)2 þ OH 4 CoOOH þ H2O þ e CoOOH þ OH 4 CoO2 þ H2O þ e KOH/2 M

NiO nanospheres

3

612.5 at 0.5 A g1

0e0.7

90.1% after 1000 cycles

e

NiO þ OH 4 NiOOH þ e

[36]

KOH/2 M

Porous NiCo2O4 nanotubes

2/3

1647.6 at 1 A g1

0e0.41

93.6% after 3000 cycles

205 W kg1

Faradaic redox reaction MeO/MeOeOH, where M ¼ Co or Ni

[37]

KOH/2 M

NiCo2O4@ NiCo2O4 nanocatus

3

1264 at 2 A g1

0e0.7

93.4% after 5000 cycles

e

NiCo2O4 þ OH þ H2O 4 NiOOH þ 2CoOOH þ e

[38]

KOH/2 M

NiCo2O4

3

1118.6 at 5.56 mA cm2

0.2 to 0.6

89.4% after 2000 cycles

e

Redox reaction related to MeO/MeOeOH, where M refers to Ni or Co

[39]

KOH/2 M

Ni0.61Co0.39

2/3

1523.0 at 2 A g1

0.05 to 0.7

95.3% after 1000 cycles

142 W kg1

Redox reaction related to MeO/MeOeOH, where M refers to Ni or Co

[40]

KOH/2 M

MnFe2O4

3

97.1 at 0.1 A g1

e

70% after 2000 cycles

e

Not shown

[29]

CoOOH þ OH 4 CoO2 þ H2O þ e

KOH/2 M

LiCoO2

3

814.5 at 1 A g1

0e0.6

96.8% after 2000 cycles

e

Not shown

[41]

KOH/3 M

NiOeIn2O3 foam

2/3

1096.8 at 5 A g1

0.2 to 0.6

79% after 50000 cycles

1752.8 W kg1

NiO þ OH 4 NiOOH þ e

[42]

The In3þ creates the oxidation of Ni2þ into Ni3þ, followed by holes migration based on the following reaction: ½O2 4O0 þ V0Ni þ h$ V0Ni refers to a singly ionized nickel vacancy, O0, to an oxygen ion on an oxygen site, and h$ to an electron hole.

KOH/6 M

Bi2O2.33 microspheres

3

893 at 0.1 A g1

1 to 0.2

96% after 1000 cycles

e

Not shown

LiOH/1 M

MnO2 nanoflower

3

363 at 2 mV s1

0.45 to 0.45

93% after 2000 cycles

e

EDLC mechanism

[43]

[28] 

ðMnO2 Þsurface þ Cþ þ e 4 MnO2  C

 þ

surface

Cþ ¼ Hþ, Liþ, Naþ, Kþ Pseudocapacitance mechanism (MnO2)surface þ Cþ þ e 4 (MnOOC) LiOH/1 M

Co3O4@MnO2

3

400 at 10 mA cm2

0.2 to 0.6

97.3% after 5000 cycles

e

LiOH/2 M

MnFe2O4

2/3

74.2 at 0.1 A g1

e

82% after 1000 cycles

NaOH/1 M

Ni(OH)2 @a-Fe2O3

2/3

908 at 21.8 A g1

0e0.6

85.7% after 5000 cycles

Reversible reactions of Co3þ/Co4þ associated with anions OH   ðMnO2 Þsurface þ Liþ þ e 4 MnO2  Liþ surface

[44]

14.5 kW kg1

Not shown

[29]

16.4 kW kg1

Reversible reaction of Ni2þ/Ni3þ associated with anions OH

[45]

Continued

Alkaline Aqueous Electrolyte Used in Metal Oxide Supercapacitors, the Reaction Between Electrolyte and Electrodes, and Electrochemical Supercapacitor Performancedcont’d

Table 3.2

Aqueous Electrolyte/ Concentration NaOH/1 M

Electrode MnO2 nanoflower

Electrode Setup 3

Specific Capacitance (F gL1)

Potential Window (V)/ Reference 1

312 at 2 mV s

0.45 to 0.45

Cycling Stability 93% after 2000 cycles

Power Density

Reaction Between Electrolyte and Electrode

References

e

EDLC mechanism

[28]

þ

ðMnO2 Þsurface þ C þ

e 4



 þ

MnO2 C

 surface

Cþ ¼ Hþ, Liþ, Naþ, Kþ Pseudocapacitance mechanism (MnO2)surface þ Cþ þ e 4 (MnOOC) NaOH/1 M

NaOH/2 M

CoO@NiOOH

MnMoO4/ CoMoO4

3

3

798.3 at w1.67 A g1

0.01e0.52

187.1 at 1 A g1

0.1e0.4

96.7% after 2000 cycles

e

98% after 1000 cycles

e

CoO þ OH 4 CoOOH þ e

[46]

CoOOH þ OH 4 CoO2 þ H2O þ e 2[Mn(OH)3] 4 Mn2O3 þ 3H2O þ 2e

[47]

3[Co(OH)3] 4 Co3O4 þ 4H2O þ OH þ 2e Co3O4 H2O þ OH 4 3CoOOH þ e Mn2O3 þ 2OH 4 2MnO2 þ H2O þ 2e CoOOH þ OH 4 CoO2 þ H2O þ e

NaOH/2 M

MnFe2O4

2

93.9 at 0.1 A g1

NaOH/5 M

3D CuO flowerlike

3

1462.8 at 5 mA cm2

e

60% after 2000 cycles

e

Not shown

[29]

0e0.6

79% after 10,000 cycles

e

CuO þ H2O þ 2e 4 Cu2O þ 2OH 

Cu2O þ H2O þ 2OH 4 2Cu(OH)2 þ 2e CuOH þ OH 4 2Cu(OH)2 þ e

EDLC, electric double-layer capacitor.

[48] 

Acid Aqueous Electrolyte Used in Metal Oxide Supercapacitors, the Reaction Between Electrolyte and Electrodes, and Electrochemical Supercapacitor Performance

Table 3.3

Aqueous Electrolyte/ Concentration H2SO4/0.5 M

Electrode

Electrode Setup

Specific Capacitance (F gL1)

Potential Window (V)/ Reference

Cycling Stability

Power Density

Ti/IrO2/WO3

3

46 at 50 mV s1

0.2e1.2

e

e

Reaction Between Electrolyte and Electrode WO3 þ xHþ þ xe 4 HxWO3 (0 < x < 1)

References [49]

IrO2 þ 2Hþ þ 2e 4 Ir2O3 þ H2O H2SO4/1 M

RuO2

3

120.4 at 50 mV s1 1

0e0.9

e

e

H2SO4/1 M

H3PMO12O40

3

160 at 2 A g  183 at 2 A g1

0e1

91% after 8000 cycles

e

H2SO4/1 M

H3PW12 O403.H2O

2/3

254 at 6 A g1

0e1.7

98% after 30,000 cycles

115 kW kg1

Not shown

[34]

PMoðVIÞO40 3 þ 6e þ 6Hþ 4H6 PMoðVÞ6 MoðVIÞ6 O4 3

[52]

PW12 O40 3 þ e 4PW12 O40 4

[53]

PW12 O40 4 þ e 4PW12 O40 5 PW12 O40 5 þ 2e þ Hþ 4PW12 O40 6

H2SO4/1 M

WO3 nanoflowers

2/3

196 at 10 mV s1

0.5e0

85% after 5000 cycles

229.3 mW cm3

Redox reactions from W6þ to W5þ

[50]

(W6þ þ e/W5þ) Hþ insertion/extraction WO3 þ e þ Hþ 4 HWO3

H2SO4/1 M

NiFe2O4

3

454 at 2.5 A g1

0e0.6

98.7% after 1000 cycles

HCl/1 M

NiCo2O4

3

3.8 at 0.5 A g1

0.35e1

w100% after 1000 cycles

e

Redox reactions (Ni2þ/3þ and/or Fe3þ/2þ)

[51]

e

Not shown

[35]

62

Metal Oxides in Supercapacitors

It may also be noted that the acid aqueous electrolytes are extremely corrosive for current collectors and packaging materials. To solve this problem, gold (Au) and indium tin oxide (ITO) have been used in current collectors [8]. However, this solution is less used in part due to the increased prices.

3.3.1.1.3 Neutral Electrolytes In addition to the acidic and alkaline electrolytes, neutral electrolytes have also been widely studied for metal oxide SCs (Table 3.4). This is because of their advantages, such as: larger working potential windows (up to 2 V), environmentally friendly, low cost and greater safety [4,54]. However, the neutral electrolytes also have disadvantages, the power performance of the device is still limited due to their relatively low ionic conductivity at room temperatures (Table 3.1) [55]. Moreover, devices with neutral electrolytes can suffer crevice corrosion, due to cell design, like in coin cell. Among the various neutral electrolytes, Na2SO4 is the most commonly used neutral electrolyte [56e63]. But there are various other neutral electrolytes, such as LiCl [64], Li2SO4 [65], K2SO4 [20] and LiNO3 [66], Na2SO3 [67], Ca(NO3)2 [68], and KCl salt [35], which could be used in metal oxideebased SCs. To date the MnO2 electrode material is the most studied in neutral electrolytes because neutral electrolyte ions play a significant role in the pseudocapacitive performance of the MnO2, as they are directly involved in the chargingedischarging process [69]. But the list of metal oxide electrode materials studied to optimize the performance of SCs in neutral electrolytes is already quite long (see Table 3.4). Recently, some efforts have been made to the use of neutral electrolytes in SCs. However, it should be emphasized that for neutral electrolytes the energy storage/delivery processes are not well understood as demonstrated by the absence of equations in the Table 3.4. This is due to the complexity of the electrochemical mechanisms, which are involved in the charge storage process. Like in alkaline or acidic electrolytes, various aspects of the neutral electrolytes, for instance, the types of cation and anion species, additives, and solution temperatures, have been found to have influence on the SC performance. Another weakness of the neutral electrolytes (e.g., K2SO4) is the inability to reach high molar concentration as the alkaline electrolytes at lower temperatures, which has a strong impact on the SC performance. For example, a study carried out by Li and coworkers [70] using comparatively three neutral electrolytes (Na2SO4, K2SO4, Li2SO4) on mesoporous MnO2 electrode has shown that the specific capacitance increases and the resultant energy and powder densities increase in the order Li2SO4 > Na2SO4 > K2SO4 for low scan rate, whereas at high scan rate the specific capacitance values overlap [70]. This trend was explained by the fact that in the charge storage, intercalation/ deintercalation of smaller alkaline metal cations occurs and the radius of the hydrated ions follows the order: Liþ > Naþ > SO4 þ [70]. Q. Qu et al. [71] also reported that V2O5$0.6H20 nanoribbons showed the largest specific capacitance value in the 0.5 M K2SO4 electrolyte compared with the two other neutral electrolytes (Li2SO4 and Na2SO4) with same concentration because of the most facile intercalation/ deintercalation of Kþ into/from the interlayers of V2O5$0.6H20. Results similar to

Neutral Aqueous Electrolytes Used in Metal Oxide Supercapacitors, the Reaction Between Electrolyte and Electrodes, and Electrochemical Supercapacitor Performance Table 3.4

Aqueous Electrolyte/ Concentration

Electrode

Electrode Setup

Specific Capacitance (F gL1) 1

Potential Window (V)/Reference

Cycling Stability

Powder Density

Reaction Between Electrolyte and Electrode

References

e

Not shown

[63]

Na2SO4/0.1 M

ZnO nanorod/NiO shell composite

3

305 at 10 mV s

0.1 to 0.5

w100% after 500 cycles

Na2SO4/0.5 M

Wo3x/MoO3x

2/3

190 at 1.5 A g1

0e1.9

75% after 10,000 cycles

0.73 W cm3

Not shown

[56]

Na2SO4/1 M

NiOx composite

2/3

150 at 1 A g1

1.2 to 1.0

85% after 2500 cycles

4 kW kg1

Not shown

[73]

Na2SO4/1 M

3D V2O5 architecture

2/3

521 at 5 mV s1

1 to 1

90% after 4000 cycles

9.4 kW kg1

Not shown

[58]

Na2SO4/1 M

MnO2 mesoporous

3

278.8 at 1 mV s

0e1

82.5% after 2000 cycles

e

Not shown

[70]

Na2SO4/1 M

Co3O4@Pt@MnO2 nanowires

2/3

497 at 10 mV s1

0e1

105.6% after 5000 cycles

19.6 kW kg1

Not shown

[59]

Na2SO4/1 M

ZnO@MoO3 nanocable

3

241 at 5 mV s1

1.3 to 0.2

89.7% after 1000 cycles

e

Not shown

[60]

Na2SO4/1 M

MnO2 nanotubes

3

245 at 10 A g1

0e0.8

81% after 2000 cycles

e

Not shown

[61]

Na2SO4/1 M

MnO2 nanopillars

2/3

603 at 5 mV s1

0e0.8

93% after 5000 cycles

3.57 kW kg1

Not shown

[62]

Na2SO4/1 M

RuO2

2/3

117.6 at 50 mV s

1 to 0.8

e

e

Not shown

[34]

Na2SO4/1 M

Bi2O3@MnO2

3

93.1 at 1 A g1

0.2 to 0.8

112% after 1000 cycles

e

Not shown

[74]

1

1

Continued

Neutral Aqueous Electrolytes Used in Metal Oxide Supercapacitors, the Reaction Between Electrolyte and Electrodes, and Electrochemical Supercapacitor Performancedcont’d

Table 3.4

Aqueous Electrolyte/ Concentration

Electrode

Electrode Setup

Specific Capacitance (F gL1)

Potential Window (V)/Reference

Cycling Stability

Powder Density

Reaction Between Electrolyte and Electrode

References

Na2SO4/1 M

CuO@MnO2

2/3

49.2 at 0.25 A g

0.2 to 0.8

92.1% after 1000 cycles

85.6 kW kg1

Not shown

[75]

Na2SO4/1 M

MoO3 microrods

2/3

194 at 5 mV s1

1.2 to 0.5

101% after 1000 cycles

1200 W kg1

Not shown

[76]

Na2SO4/1 M

Mn3O4 nanorods

3

258 at 5 mV s1

0e1

95.1% after 1000 cycles

e

Mn3O4 (spinel) 4 NadMnOxnH2O (birnessite)

[77]

1

NadMnOxnH2O þ yHþ þ zNaþ(y þ Z)e  4 Nad þ zMnOxnH2O Na2SO4/2 M

MnFe2O4

2

47.4 at 0.1 A g1

e

82% after 1000 cycles

e

Not shown

[29]

K2SO4/0.65 M

MnO2 mesoporous

3

224.9 at 1 mV s

0e1

87.7% after 2000 cycles

e

Not shown

[70]

Li2SO4/1 M

MnO2 mesoporous

3

284.24 at 1 mV s

0e1

82.9% after 2000 cycles

e

Not shown

[70]

Li2SO4/1 M

MnO2 amorphous

3

139 at 5 mV s1

0e0.9

e

e

MnO2 þ (x þ y) e þ yCþ 4 MnOOHxCy (C: cation from electrolyte)

[65]

1

1

1

MnO2 birnessite

200 at 5 mV s

MnO2 cryptomelane

102 at 5 mV s1

MnO2 spinel

78 at 5 mV s1

LiNO3/5 M

FeWO4

3

35 at 10 mV s1

0.6 to 0

95% after 10,000 cycles

e

Reversible redox reactions (Fe3þ/Fe2þ)

[66]

LiCl/5 M

V6O13x

3

1050 at 10 mV s

1 to 0

52.3% after 200 cycles

e

Reversible redox reactions of V3þ/V4þ and V5þ states

[64]

LiCl/5 M

MnO2/Fe2O3

2/3

1.2 F cm3 at 10 mV s1

0.8 to 0

95% after 10,000 cycles

0.1 W cm3

Not shown

[78]

Na2SO3/1 M

Fe3O4 nanoparticles

3

207.7 at 0.4 A g

0.9 to 0.1

100% after 2000 cycles

e

2SO3 2 þ 3H2 O þ 4e 4S2 O3 2 þ 6OH

[67]

1

1

S2 O3 2 þ 3H2 O þ 8e 42S2 þ 6OH Ca(NO3)2/2 M

(2D) MnO2

3

587.3 at 2 mV s

0e1

97% after 10,000 cycles

e

Not shown

KCl/1 M

NiCo2O4

3

41.9 at 0.5 A g1

0e1

92.9% after 1000 cycles

e

MOH2 þ þCl 4MOH2 þ Cl

1



þ

[68]



MO þ K 4 MO K

þ

[35]

66

Metal Oxides in Supercapacitors

that K2SO4 resulted in a fast chargeedischarge process and a superior power compared to the Li2SO4 and Na2SO4 electrolytes were reported for KxMnO2$nH2O electrodes [72]. The authors attributed this to slight structural expansion/contraction degree during the chargeedischarge process [72]. Some comparative studies between aqueous electrolytes have been done. Using the (LaMnO3.09) metal oxide electrode, Mefford et al. [20] have shown that the specific capacitance is lower in neutral electrolytes (Li2SO4, Na2SO4, K2SO4) compared to alkaline electrolytes (LiOH, NaOH, KOH) because of the lower ionic conductivity of the neutral electrolytes. Although the neutral electrolytes have been extensively studied in metal oxidee based SCs, they have lower Hþ and OH concentration compared with alkaline or acid electrolytes, which is in several systems a disadvantage. With this in mind, S.-K. Chang et al. [35] explored the influence of different types of electrolyte systems (acidic 1 M HCl, alkaline 1 M KOH, and neutral 1 M KCl) on pure-phase NiCo2O4 electrode performance. They showed that in acidic electrolyte (1 M HCl), there is dissolution of spinel oxide (NiCo2O4), whereas the nanosized NiCo2O4 electrode exhibits excellent discharge capability in alkaline (1 M KOH) and neutral (1 M KCl) electrolytes [35].

3.3.1.2

Nonaqueous Electrolytes

3.3.1.2.1 Organic Electrolytes Nowadays, organic electrolytes dominate the ES world market and are extensively applied in double-layer SCs mostly for their high operative cell voltage (2.5e2.8 V). But organic electrolytes have disadvantages already conveyed and require a special handling procedure to avoid impurities, such as water, that implies high cost compared to the aqueous electrolytes. Concomitantly, research reports of organic electrolytes in metal oxide SCs are scarce and typically the developed organic electrolytes contain salts with lithium ions to facilitate the mechanism of ion intercalation/deintercalation for Li battery applications. The most common salts found in the literature include LiClO4 [79] for MoO3 and LiPF6 [80e82] for electrode oxide materials such as SnO2 and TiO2. The most used organic solvents are propylene carbonate (PC) [79] and a mixture of different solvents such as ethylene carbonate (EC)edimethyl carbonate (DMC)eethyl methyl carbonate (EMC) [81] and ECeDMCediethyl carbonate (DEC) [80]. For instance, the 1 M LiPF6/PCeDMC electrolyte enables the MnO2 electrodes from reaching higher specific capacitance (455 F g1) when compared to that showed in the 1 M NaOH aqueous electrolyte (342 F g1) [83]. Otherwise, the Li-containing organic electrolytes such as LiClO4/PC and lithium bis(trifluoromethanesulfonyl)imide/acetonitrile (LiTFSI/ACN) have a wider application, for example, with V2O5, because they also permit higher specific capacitance than the aqueous electrolyte KCl [84,85]. Moreover, the 1 M LiClO4/PC electrolyte on an MnO2eCNT SC allows a specific energy density about six times higher than that obtained with 1 M KCl electrolyte because of the resulting larger potential window, but it shows a lower cycling stability than KCl electrolyte [86].

Electrolytes in Metal Oxide Supercapacitors

67

Organic electrolytes used for asymmetric ESs are also reported, such as in asymmetric carbon/MnO2 [86], carbon/V2O5 [85], and carbon/TiO2 [82], which exhibit high energy density because of the resulting large working potential window (up to 3e4 V).

3.3.1.2.2 Ionic Liquids ILs are known as low-temperature or room-temperature molten salts and those salts are composed solely of cations and anions. Normally the ILs present several advantages including high thermal, chemical, and electrochemical stability and low volatility [87]. However, they also present some drawbacks such as high viscosity, low ionic conductivity, and high cost, which limit their use in metal oxide SCs. As mentioned earlier, a few reports could be found in the literature that explore metal oxide SCs as electrodes and ILs as electrolyte. Nonetheless, Rochefort and Pont [88] evaluate the pseudocapacitance of RuO2 electrodes in two ILs. The authors concluded that RuO2 present pseudocapacitance in a protic IL composed of 2-methylpyridine (a-picoline) and trifluoroacetic acid (TFA), whereas no obvious pseudocapacitance was observed in 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) IL [88]. Another study reported by Mayrand-Provencher et al. [89] in which RuO2 was used as the electrode and different pyridinium-based protic ILs were used as electrolytes showed that the alkyl chain length of a cation’s substituent and the substituent position had an effect on the electrolyte conductivity and viscosity and the specific capacitance and cycling stability of the electrode material. Mn oxide electrode was also studied with IL electrolytes. It was reported that the Mn oxide has a pseudocapacitive performance in an aprotic 1-ethyl-3-methylimidazoliumdicyanamide IL electrolyte [90]. The same researchers, found that in the 1-butyl-1methylpyrrolidinium dicyanamide ([BMP][DCA]) IL electrolyte, smaller [DCA] anions, instead of [BMP]þ cations, could reversibly insert into or desert out of the MnO6 structure, compensating the Mn3þ/Mn4þ valency state variation during the chargee discharge process [91]. Other metal oxide SC-IL electrolyte systems, such as Ru-doped Cu oxide in 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM] [HSO4]) [92], TiO2 in [EMIM][TFSI] [93], and Fe2O3 in [EMIM][BF4] [94], have also been studied. One of the problems of ILs it their high viscosity. In order to reduce the viscosity and increase the conductivity, some organic solvents have been added and the mixtures were tested as electrolytes; however, just few were explored with metal oxide electrodes. Zhang et al. [95] found that the addition of N,N-dimethylformamide to 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) IL increased the capacitance and also decreased the internal resistance of the asymmetric AC/MnO2 SC when compared to pure IL. This improvement was attributed to the improved electrolyte penetration and ion mobility.

3.3.2

Redox Additives

Actually the electrochemical behavior of metal oxide electrode materials is strongly dependent on the nature of electrolytes. Consequently, the foremost goal is that both electrolyte and electrode can contribute synergistically to an optimal pseudocapacitance

68

Metal Oxides in Supercapacitors

and\or faradaic response, driving redox reactions that can enhance the charge storage capacity of SCs. Redox aqueous electrolytes (e.g., H2SO4, Na2SO4, and KOH) are intrinsically able to undertake fast electron-transfer reactions at the electrodeeelectrolyte interface but can be successfully upgraded with redox additives, in which redox-active species are added to the electrolyte to improve electron-transfer reactions [96]. The actual developments in acid, neutral, and alkaline aqueous redox additive electrolytes benefit from the background of previous studies on redox-active species (e.g., KI) for activated carbonecarbon SC devices [97,98]. Nowadays an emergent strategy is reinforced because of intense research demonstrating the advantage of using redox additive aqueous electrolytes to improve the performance of metal oxide asymmetric supercapacitors (ASCs). As a matter of fact, the addition of K3Fe(CN)6 as redox additive electrolyte to 2 M KOH solution provokes about three times increment in the charge storage performance of binder-free CoMoO4 nanoplate array (NPA)/activated carbon ASC [99]. Similarly, addition of 0.02 M K3Fe(CN)6 redox species revealed a maximum areal capacity of 603.5 mA h cm2 at 2.5 mA cm2, with an extended operating voltage window of 1.5 V. A mechanism has been proposed based on the charging process, in which the presence of this redox additive accelerated the transition of Co(II)/Co(III) by the redox  reaction FeðCNÞ6 3 FeðCNÞ6 4 facilitated by the high electrochemical reversibility  nature of FeðCNÞ6 3 FeðCNÞ6 4 ions. The FeðCNÞ6 3 ion accepts the electron from the Co element of the ternary metal oxide and gives FeðCNÞ6 4 ion. When the electrode is discharged, the FeðCNÞ6 4 loses one electron (oxidation reaction) giving FeðCNÞ6 3 in hexacyanoferrate for the reduction of Co(III) to Co(II) in CoMoO4. In this circumstance, the FeðCNÞ6 4 ion acts as an electron donor and Co(III) is an electron acceptor. Enhancement in the redox reaction is attributed to hexacyanoferrate ions that play the role of “electron shuttle” in the chargeedischarge processes and accelerate the rate of the reaction, leading to the higher charge capacity. The CoMoO4 NPA/activated carbon ASC device exhibited a high energy density of 125 mW h cm2 at a power density of 1507 mW cm2 and excellent cycling stability even after 2000 continuous cycles. Research reveals that a redox electrolyte system consisting of sodium persulfate added conventional KOH electrolyte is adequate not only for metal oxide electrodes but also for metal sulfide, vanadate, and phosphate electrodes. Additionally, when Na2S2O8 is added into KOH electrolyte, fast charge and slow discharge is observed, which means that the redox-active electrolyte system can be applied in the batterytype SC [100]. Nickel oxide electrodes in Na2S2O8/KOH redox-active aqueous electrolyte exhibit a specific capacitance threefold higher than without the additive, reaching 6317.5 F g1 at 0.5 A g1. Moreover, the specific capacitance showed a decreasing trend, and the reason is when the Na2S2O8 concentration is lower, the contribution of the redox reaction from Na2S2O8 is relatively lower, resulting in low specific capacitance. If the electrolyte is a pure KOH aqueous solution, the chargeedischarge process corresponds to the reversible redox reaction of Ni(II)/Ni(III), and the chargeedischarge time is similar. When Na2S2O8 is added to the KOH electrolyte,

Electrolytes in Metal Oxide Supercapacitors

69

S2 O8 2 will take part in the reaction. During charging process, the Ni(II) is oxidized to Ni(III), though Ni(II) can also reacts with the S2 O8 2 due to the two steps reaction, and the charging time is shortened. During the discharging process, Ni(III) is reduced to Ni(II) and then it will be oxidized to Ni(III) by S2 O8 2 and the Ni(III) will repeat the backward reaction, thus the discharging time is extended. Furthermore, galvanostatic chargeedischarge curves show that the effect of addition of Na2S2O8 disappears for higher concentration (>0.05 M) probably because of the concentration polarization phenomenon. For temperatures above 60 C, sodium persulfate may decompose and generate sodium sulfate. The effect of various concentrations (0.010, 0.025, 0.050, 0.075, and 0.100 g) of p-phenylenediamine added to 1.78 M KOH has been investigated for MnO2 electrodes [101]. A maximum capacitance of 325.24 F g1 (1 A g1) was found for 0.050 g of p-phenylenediamine added to KOH. This maximum capacitance was nearly 6.25 fold higher than the value observed for pure KOH electrolyte and the energy density also increased from 1.29 to 10.12 W h kg1. These increases in capacitance and energy density are attributed to the redox reaction between p-phenylenediamine/ p-phenylenediimine without neglecting that p-phenylenediamine also modifies the conductivity mechanism of electrolyte. The system capacity retention attains 75% in redox additive electrolyte for 5000 cycles. Redox-active aqueous electrolyte is also explored to evaluate the electrochemical properties of a monoclinic WO3 thin film in 1 M H2SO4 and in a mixture of H2SO4e hydroquinone (HQ) redox electrolytes [102]. Various concentrations (from 0.1 to 0.4 M) of HQ are added to 1 M H2SO4 electrolyte to assess the ionic conductivity of the resulting redox electrolytes. The authors found that electrolyte ionic conductivity increases with the concentration of HQ from 0.1 to 0.2 M and decreases for higher HQ concentration in 1 M H2SO4. The redox electrolyte with 0.2 M HQ in H2SO4 attends a maximal ionic conductivity of 129.4 mS cm1. For higher HQ concentrations, there is interaction of HQ molecules resulting in aggregation of free ions and crystallization of HQ in H2SO4, with expected slow diffusion of ions that decreases the ionic conductivity. The specific capacitance and energy density of WO3 electrode in conventional H2SO4 electrolyte are 352 F g1 and 12.25 W h kg1, respectively, which increase to 725 F g1 and 25.18 W h kg1 in H2SO4 þ 0.2 M HQ redox electrolyte, at the same power density of 1166 W kg1. However, the WO3 film electrode shows low capacity retention (81%) in redox additive electrolyte as compared to pure H2SO4 electrolyte (89%) for 1000 cycles. Authors attribute the slight loss in capacitance to faster degradation of electrode material in the presence of redox electrolyte. KI redox additive has been proposed as an economically favorable and environment-friendly redox additive aqueous electrolyte contrasting with toxic CAN or expensive ILs usually applied in ASCs [103]. Neutral aqueous electrolyte of 1 M Li2SO4 with varying concentration of KI redox additive achieved a significant enhancement in the specific energy while maintaining the specific power for various transition metal oxide (TMO)/multiwalled carbon nanotube (MWCNT) composite ASCs. Indeed, along with w105% increase in specific energy at an optimized concentration (7.5 mmol KI concentration), iodine-based redox reactions can also ensure good cycling stability and high specific power of mesoporous MWCNT/ZrO2 (WO3) composite ASCs. The proposed charge storage mechanism for a system with

70

Metal Oxides in Supercapacitors

1 M Li2SO4 electrolyte was a convolution effect originating from the intercalation/ deintercalation and surface absorption/desorption of the lithium electrolyte cations. Moreover, the improved faradaic capacitance observed in 1 M Li2SO4 and KI mixture electrolyte is attributed to various redox reactions of the iodine/iodide redox pairs, which can form a variety of negatively charged polyiodides (In  ), such as I3  , I5  and IO3  through dissolved I2. The iodine/iodide pairs produce redox peaks in CVs and a faradaic plateau in chargeedischarge curves. In this study, for high KI concentrations (15, 30, 45, 75 mmol KI concentration) the galvanostatic chargeedischarge curves lose linearity and the stable potential window of the positive electrode show a trend toward narrowing, resulting in H2/O2 evolution at a potential lower than 2.2 V (stability window provided by the electrolyte). The literature proposes that oxidation/reduction reactions can occur at the electrodeeelectrolyte interface because of the presence of these iodide/iodine redox pairs [97,103].

3.3.3

Solid and/or Gel State

Solid-state SCs have emerged because of new challenges in transportation field (space and automotive), modern electronics, and bioimplantable systems that require high performance in extreme environmental conditions and most of all, electrolyte nonleakage. Adequate solid- or gel-state electrolytes have been proposed to address those requirements in new configuration devices as well as in microscale energy storage devices [8,104]. Accordingly, this chapter mainly focuses on the latest new developments of gel electrolytes applied on metal oxide SCs and hybrid, symmetric, and asymmetric configuration devices. The most commonly used ionic conducting hydrated gel electrolyte is poly(vinyl alcohol) (PVA) based, which is prepared by blending the polymer with common alkaline, acid, and neutral aqueous solutions to satisfy a specific condition. Gel electrolytes such as PVA/H3PO4, PVA/H2SO4, PVA/KOH, PVA/NaNO3, and PVA/LiCl are examples of solid electrolytes used in macro- and micro-SCs. The alkaline PVA/gel electrolyte (e.g., PVA/KOH, PVA/NaOH, and PVA/LiOH) has been extensively employed with the pseudocapacitive electrode material MnO2 [105], other TMOs such as spinel NiCo2O4 [106], and polyporous MoO3@CuO composite [107]. PVAeLiOH solid-state gel was used both as electrolyte and separator to assemble NiCo2O4/carbon cloth//porous graphene paper ASC that exhibits a maximum energy density (60.9 W h kg1), maximum power density (11.36 kW kg1), and cycling stability (96.8% capacitance retention ratio after 5000 cycles under mechanical bending) and an exceptionally high operating potential of 1.8 V [108]. On the other hand, acid PVA-based gel electrolytes (e.g., PVA/H2SO4 and PVA/ H3PO4) have been frequently used with the traditional pseudocapacitive electrode materials RuO2 [109] and MnO2 [110]. A solid-state symmetric SC is also fabricated using PVA/H2SO4 as gel electrolyte and one-dimensional (1D) nanofiber network of spinel CoMn2O4, having interconnected nanoparticles [111]. It reached a high energy density of 75 W h kg1 at a power density of 2 kW kg1, which was better than that of many battery systems (Pb-acid and Ni-MH batteries). The energy density and 2 V output voltage of this device was also confirmed by red light-emitting diode light at

Electrolytes in Metal Oxide Supercapacitors

71

1.8 V and 20 mA for 5 min. This high performance is attributed to 1D nanofibers consisting of voids/gaps with minimum interparticle resistance that facilitates smoother transportation of electrons/ions. The authors propose that these voids/gaps can act as intercalation/deintercalation sites for electrolyte. Mainly, the neutral PVA-based gel electrolytes (e.g., PVA/LiCl, PVA/Na2SO4, and PVA/NaNO3) have been studied with electrode materials such as MnO2 [112], Mn3O4 [113], V2O5 [10], and ZnO [114]. Generally, the neutral PVA/LiCl electrolyte has a wider application, namely, with V2O5, an amphoteric material that can dissolve in both basic and acidic media. Moreover, LiCl can be dissolved in PVA gel without crystallization during the gel drying process and preserves excellent electrochemical and mechanical properties without the need for periodical wetting. A study replaces an aqueous electrolyte that provokes vanadium oxide dissolution and poor structural stability during chargingedischarging cycling, with neutral PVA/LiCl gel electrolyte allowing a vanadium oxide nanowire SC to achieve a maximum areal capacitance of 236 mF cm2 at a current density of 0.2 mA cm2 and an excellent capacitance retention rate of more than 85% after chargeedischarge cycling of 5000 cycles [115]. Another trend in recent research explores symmetric and asymmetric neutral hydrogel-based devices to fabric new flexible solid-state SCs in an attempt to combine high power and high energy density. For instance, a PVA/Na2SO4 gel electrolyte permits the assembling of a flexible and lightweight all-solid-state asymmetric supercapacitors (AASCs) using 3D coated MnOx nanosheets on nanoporous current collectors (3D MnOx@Ni@CC) and a chemically converted graphene as the negative electrode [116].The polymer gel electrolyte under a pressure of w1 MPa and during 10 min penetrates into each electrode and also acts as a thin separator. A bending test was performed to evaluate the flexibility performance of the device and the influence of the bending-induced mechanical stress on specific capacitance. The flexible AASC exhibits minimal change in the chargeedischarge behavior on bending to various angles, being the change in capacitance with increasing bending angle (0 degree to 180 degrees) less than 5%. It also presents an excellent performance with energy density of 1.16 mW h cm3 at 1 mA cm2, with 81.5% capacitance retention after 10,000 cycles of charginge discharging at 2 mA cm2 and 85.7% capacitance retention after 200 bending cycles. Carboxymethyl celluloseeNa2SO4 (CMC-Na2SO4) gel electrolyte was used to fabricate flexible all-solid-state thin-film symmetric supercapacitors (FASSTF-SSCs) of identical electrodes MnO2 thin films over stainless steel substrate. The 0.7-mmthick FASSTF cell exhibits a specific capacitance of 145 F g1 with specific energy of 16 W h kg1 and excellent cycling stability after 2500 cycles [117]. The same inexpensive polymer gel CMC-1 M Na2SO4 that acts simultaneously as separator and electrolyte allows the assembling of a thin and flexible symmetric MnO2/ MnO2 SC solid device. Also an asymmetric SC device based on compiled MnO2 nanosheets as positive electrode and Fe2O3 nanoparticles as negative electrode was developed. The asymmetric configuration exhibits twofold high energy density compared to symmetric SC, a higher window voltage of 2 V and excellent mechanical flexibility associated to good cycling stability [118].

72

Metal Oxides in Supercapacitors

Three ionic conducting polymer gel electrolytes were prepared using PVA, CMC, and polyethylene oxide with lithium perchlorate (LiClO4) to fabric a solid-state MnO2 SC device [119]. Among others, the device of (PVA)elithium perchlorate (LiClO4) gel electrolyte assembled with MnO2 thin film electrodes on flexible stainless substrate provided better performance, with an operating potential window of 1.2 V, a specific capacitance of 112 F g1, and an energy density of 15 W h kg1 with extended cycling stability up to 2500 CV cycles. The better electrochemical performance of PVA gel electrolyteebased SC device is attributed to the electrolyte’s higher ionic conductivity (48 mS cm1) and higher wettability that allows maximum interfacial contact with electrodes.

3.4

Conclusions and Outlooks

In the past years, many efforts have been focused on low-cost aqueous electrolytes with and without redox additives for application in binary and ternary metal oxide SCs. Nevertheless, it is imperative to increase the working potential window and the corrosion resistance of those SC devices as well as to control the high risk of electrolyte leakage. Concomitantly, the state of the art reveals a tremendous effort focusing on developing novel electrolytes used in metal oxide SCs that allow production of miniature, flexible, safe, and lightweight energy storage devices. As a result, the next generation of SCs based on metal oxides demands the development of innovative electrolytes and electrode materials, as well as novel electrodee electrolyte configurations, to overcome their disadvantages and simultaneously raise their limited energy density. To improve the energy storage of SCs, future studies should explore novel electrolytes that guarantee adequate properties that fit the specificities of emergent metal oxideebased electrodes. The new strategies focus on the development of solid-electrolyte systems, namely, the solid-state gel electrolytes. These electrolytes must meet the requirements of high conductivity to increase the operating voltage, consequently leading to an increase in energy density and high chemical and physical stability to provide excellent cycling stability and longer-lasting cells. Another critical factor in the further development of electrolytes is to establish the charge storage mechanisms of neutral electrolyte on the electrodeemetal oxide interface and intensify theoretical kinetic studies on redox reactions. To obviate electrolyte degradation, it is also important to reach the fundamental understanding of degradation mechanisms.

Acknowledgments The authors would like to thank Fundaç~ao para a Ciência e Tecnologia (FCT) for the funding under the contract CQE (UID/QUI/00100/2013).

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