Open access peer-reviewed chapter

Redox Mediated Electrolytes in Electrochemical Capacitors

Written By

Paulina Bujewska, Przemysław Galek, Elżbieta Frąckowiak and Krzysztof Fic

Submitted: 09 March 2022 Reviewed: 15 April 2022 Published: 03 June 2022

DOI: 10.5772/intechopen.104961

From the Edited Volume

Redox Chemistry - From Molecules to Energy Storage

Edited by Olivier Fontaine

Chapter metrics overview

257 Chapter Downloads

View Full Metrics

Abstract

Electrochemistry is strongly related to redox reactions. Charge transfer processes are used for the current generation in all electrochemical cells. Nowadays, redox reactions are still of evitable importance for energy storage/conversion technology. For instance, the charge and discharge of batteries exploit redox reactions. Moreover, these processes can also be used to improve the operating parameters of other energy storage devices like electrochemical capacitors. Although, in principle, the energy in electrochemical capacitors is stored in an electrostatic manner (by electrical double-layer formation), the redox reactions introduce an additional charge and improve the energy of these systems. This chapter presents the principles of electrochemical capacitors’ operation and provides comprehensive insights into this technology with special attention focused on hybrid systems, exploiting the redox activity of the electrolytic solution.

Keywords

  • energy storage devices
  • electrochemical capacitor
  • redox-active electrolytes
  • aqueous electrolytes
  • organic electrolytes
  • ionic liquids

1. Introduction

Growing demand for energy in all possible forms (mobility, heat, electricity) induced intensive research on various energy harvesting and storage systems. Today, it is clear that fossil fuels are no longer a reasonable choice for further society development. Various reasons, such as environmental pollution, depletion of natural resources, and remarkable climate changes, stimulated intensive research on sustainable solutions for energy harvesting and storage. In this context, numerous technologies are known for sourcing the energy in a “green” manner (like photovoltaics, wind turbines, flywheels); however, this energy must be somehow stored to be further used when needed.

Electrochemical energy conversion and storage systems are one of the most common solutions used every day by almost everyone—at home, at work, or in the car. This is true that well-known Li-ion batteries allowed the world “to move” and made our lives more “mobile”. Nevertheless, these are not the only ones that have been recently developed and used. Despite the high amount of energy stored in batteries, their power density is still not enough to eliminate other technologies. Furthermore, their typical redox-based charge storage mechanism makes their lifetime short (counted in thousands of cycles) and thus less resource-effective. Electrochemical capacitors, with their high-power density and moderate energy, cyclability counted very often in millions of cycles and much safer chemistry in the cell, appear to be an interesting technology that could serve as a standalone system or greatly accompany the battery.

Advertisement

2. Electrochemical capacitors: definition, construction, types

Conventional capacitors are composed of two flat, non-porous plates (electrodes) separated by a dielectric material. These devices are characterized by low energy density, limiting their application [1]. In 1957, a new group of capacitors, called electrochemical capacitors (ECs), super- or ultracapacitors, emerged. It must be here pointed out that only the “electrochemical capacitor” term should be used for scientific purposes, as other names (supercapacitors, ultracapacitors, etc.) refer to commercial products. Furthermore, “electrochemical capacitors” are often confused with “electric double-layer capacitors (EDLCs)”. In fact, EDLCs are always ECs; however, this term is reserved only for the systems exploiting the double-layer charging/discharging process, thus, the mechanism is entirely electrostatic, while ECs could also exploit redox-based processes in the charge storage (like hybrid systems).

Unlike conventional capacitors, in ECs, the electrodes of highly developed surfaces are used. Such electrodes allow higher capacitance to be reached and, in consequence, the energy accumulated increases while their superior power is maintained [2].

It is worth noting that besides ECs, there are many energy storage devices and their application depends on the performance parameters. Therefore, these properties, i.e., energy and power, are crucial from a practical point of view. The so-called Ragone plot (Figure 1) is the best way to compare various systems’ performance [4].

Figure 1.

Ragone plot presenting the performance parameters (energy and power) of different energy storage/conversion devices [3].

It can be noticed that the electrochemical capacitors demonstrate the properties between conventional capacitors and batteries—the specific power is very high, however, slightly lower than in the case of “dielectric” capacitors, and the specific energy is significantly higher—but still moderate if compared with the batteries (especially commonly used Ni/MH, Li-ion and Li-primary ones). Besides the tremendous power of ECs that allows them to be charged and discharged very quickly, these devices are getting more and more attention because of their long lifetime and safe/reliable use [5]. For these reasons, ECs are applied in the automotive industry—for instance, in regenerative braking, start-stop systems, or track control devices. Nonetheless, the energy density of these devices needs to be increased, as the volume or weight of the device must be reduced.

As already mentioned, ECs consist of two porous electrodes of highly developed surface area. The electrodes are very often made of carbon materials (especially activated carbons) due to their good conductive properties, high availability (abundance) as well as relatively low price [6]. During the ECs operation, the electrodes are polarized positively (+) and negatively (−). An insulator separates them to prevent short circuits. These components are immersed in an electrolyte, playing the role of ion source and carrier (Figure 2a). When charging the cell, positively charged ions (cations) are adsorbed on the (−) electrode surface, while negatively charged ions (anions) are adsorbed on the (+) electrode surface. An electrical double-layer is formed at the electrode/electrolyte interface during this process. For this reason, ECs are also called electric double-layer capacitors—EDLCs (Figure 2b inset). The opposite process (discharge) results in the desorption of ions from the electrode surface vicinity to the electrolyte volume (Figure 2c) [7, 8].

Figure 2.

Electrochemical capacitor: (a) construction, (b) types and (c) principle of operation.

Carbon materials can be enriched with surface functional groups, heteroatoms like oxygen or nitrogen, and transition metal oxides like MnO2. Moreover, carbon/electrically conductive polymer (e.g., PANI, PPy, PEDOT) composites can be synthesized and used as electrodes for ECs. These materials are classified as pseudocapacitive ones [9]. The charge storage mechanism in such devices can be described as quick, continuous faradaic reactions occurring with no phase change in the electrode material. The cells operating with these materials are very often called asymmetric or pseudocapacitance-based ECs (Figure 2b). One should restrain from using the “pseudocapacitor” term, as the pseudocapacitance concerns the electrode, not the system. If redox reactions occur on both electrodes, the system should rather be considered as a battery.

ECs incorporating pseudocapacitive materials may suffer from shorter cycle life, due to unstable behavior of the functional groups during long-term tests and chemical and mechanical composites degradation. Moreover, the cost of such materials exceeds the cost of non-modified activated carbon and impacts the final price of the cell. Thus, another solution was proposed to increase the capacitance, causing an increase in the energy of the ECs—i.e., electrolytes demonstrating redox activity (redox ECs, Figure 2b).

Generally, the redox processes in the batteries are attributed to the electrode material, ensuring high charge storage capacity. However, solid-state diffusion remarkably impacts the power capability. Shifting the redox processes to electrolytic solution remarkably diminishes the mass-transfer limitations and allows the power of electrostatic interactions to be almost maintained.

The operating potentials of each electrode in symmetric EDLCs are comparable. For instance, when ECs are investigated with cyclic voltammetry, the curves of rectangular shape are recorded (Figure 3; solid lines), since the capacitance does not depend on the potential.

Figure 3.

Comparison of the voltammetric responses of a positively (+) and negatively (−) polarized electrode of the electric double-layer capacitor (solid line) and a redox (hybrid) capacitor (dashed line) [10].

In the case of galvanostatic charge/discharge, the curves are triangular [11]. Obviously, it is possible to notice potential shifts (very often negligible) that originate from matching cations/anions with the pore diameter of the electrode material. The capacitance of the system (Ccell) can be calculated based on Eq. (1) because two electrodes that store the energy at the electrode/electrolyte interface are considered as two capacitors in series [10].

1Ccell=1C++1CE1

Assuming the capacitance values of both electrodes in symmetric cell are comparable (C+ ≈ C = Cele), Eq. (1) can be transformed to Eq. (2):

Ccell=Cele2E2

The specific energy for the EDLCs (EEDCL) can be calculated from Eq. (3):

EEDCL=0.5CV2E3

where C can be calculated from Eq. (4):

C=QmVE4

For accurate calculations, it is necessary to consider the ohmic drop for ΔV calculation [10].

In the case of ECs operating in redox-active electrolytes, the potential range of each electrode can significantly differ, as presented in Figure 3 (dashed lines). It is seen that one electrode demonstrates capacitive character, typical of EDL formation, with constant capacitive current recorded; at the same time, the positive electrode demonstrates a very high current response with a narrow potential range. This suggests high capacity, accumulated in a narrow potential range, typical of the redox process. In the galvanostatic charge/discharge technique, the redox activity is seen as a plateau on the E = f(t) plot [11].

For the cells’ performance characterization, the specific energy (E) should be calculated from the galvanostatic charge/discharge profile, with applied current I and change in the voltage (V) over the time (t), recalculated per active mass (m) of both electrodes:

E=13600·mVIdtE5

Power capability needs to be calculated as well for the full characterization of the investigated cells. It is directly related to the system’s energy, according to Eq. (6):

P=EtdischE6

where Δtdisch is the discharge time at which the energy is released.

For more detailed information and characterization techniques, comprehensive literature reports are published [9, 12, 13].

It must be clearly stated that the energy and power of the devices should be expressed per mass of the cell components and must not be calculated for the single electrode. However, on the laboratory scale, when the electrolyte is in great excess, only the mass of the electrolyte confined in the pores should be considered. The other possibility is to normalize these values per volume of the device’s components. All the presented methods of cells characterization is correct, but the author needs to comment on how the calculations were made [10, 11, 12].

Advertisement

3. Redox-mediated electrolytes

As mentioned, the redox-active electrolyte in EC allows the cell performance to be significantly improved. It is necessary to use the electrodes made of electrically conductive material to make the electron flow from the electrode to the electrolyte possible [14, 15, 16, 17].

There are many redox couples with well-defined and stable redox activity that can be used as additives for electrolytic solutions. The most popular ones, with their reduction potentials (expressed vs. normal hydrogen electrode; NHE), are presented in Figure 4 [18].

Figure 4.

Redox couples with their reduction potentials [18]. Redox couples marked in red are stable in acidic conditions, those marked in green are stable in neutral solutions and the one in blue is stable in alkaline electrolytes.

Depending on the cell construction, electrode material used, potential application, and expected operating performance, one can select which redox couple is suitable for EC that meets the requirements. In the case of aqueous-based systems, there are additional issues that need to be taken into account. First of all, at too high or too low potentials, water is decomposed, so oxygen and hydrogen evolution can be observed, respectively. These reactions are considered harmful for the cell because (i) the solvent should not be decomposed, (ii) evolving gases can block the electrode porosity, (iii) the highly active oxygen causes the irreversible electrode oxidation and its degradation, and (iv) corrosion of the current collectors remarkably affects the cell lifetime. Therefore, the potential of the chosen redox couple should preferably be between hydrogen (HEP) and oxygen (OEP) evolution potential.

The second issue is related to the electrolyte pH. Both HEP and OEP are pH-dependent—when the solution pH increases, these potentials are shifted toward lower potentials [19]. It is, thus, possible to slightly adjust the HEP and OEP by regulating the electrolyte pH. However, one should keep in mind that the potentials of some redox couples are also pH-dependent, so with the pH change, their potential will also change. Moreover, the stability of redox couples also depends on the solution pH.

Redox-active electrolytes are grouped in a way similar to the types of electrolytes. Hence, they can be divided into two main groups—aqueous and nonaqueous ones [11, 20].

3.1 Aqueous redox-active electrolytes

Aqueous solutions, despite their limited operating voltage related to the theoretical water decomposition above 1.23 V, are very attractive electrolytes for ECs due to their price lower than for nonaqueous electrolytes and the possibility of the cell manufacturing in an ambient atmosphere. Moreover, the impact of water-based solutions on the environment is rather negligible. These solutions are also characterized by high conductivity and low viscosity. The main drawback of the ECs operating in redox-active electrolytes is moderate cycle life related to the efficiency of the redox reactions and possible side reactions [21, 22].

In general, it seems beneficial to combine more than one redox additive in one electrolyte. If the ratio between different redox species is well-optimized, the energy reached in such cells is higher than reported for the single redox couple [23, 24].

Aqueous redox-active electrolytes can be divided into three groups: cationic, anionic, and neutral electrolytes, due to the charge of the redox-active ion. It is worth mentioning that the redox ions in cationic and anionic electrolytes contribute to the EDL formation, whereas in neutral electrolytes redox species quite often do not participate in this process [11].

3.1.1 Cationic aqueous redox-active electrolytes

Cationic redox electrolytes can be divided into three groups: lanthanides, transition metals, and organic species [18, 25, 26, 27, 28, 29, 30, 31]. The general requirement is that the solubility of these species should be possibly high and their standard potential should be close to HEP, as their activity is expected at the negative electrode [18].

Cerium, which belongs to lanthanides, was introduced to the acidic solution [11, 27]. However, standard redox potentials of lanthanides (∼+1.6 V vs. NHE of Ce3+/Ce4+ redox couple) being higher than OEP definitely limits their application.

The second group—transition metals like Zn, Sn, Mn, Fe, Ni, Cu, include a solid phase in the solution of neutral or acidic pH. The cations are reduced at relatively low potential, between −0.762 V and +0.337 V vs. NHE. However, still irreversible hydrogen evolution reaction can occur during the metal electrodeposition in aqueous solutions. Although in general hydrogen evolution reaction is considered parasitic or unwanted, it is possible to store hydrogen reversibly in the electrode porosity—it is necessary to use microporous electrodes for this purpose. Moreover, the addition of halide ions to the electrolytic solution can be beneficial—halide anions will block the carbon and its active sites preventing hydrogen reactions [32, 33]. Finally, metal electrodeposited on the electrode can affect the specific surface area of the electrode and worsen the performance stability of the system. It is also possible to avoid solid-state metal deposition on the electrode, by applying redox couples dissolved in the liquid state, like Fe2+/Fe3+, Cu+/Cu2+ [26, 34].

Viologen di-cations can be included in the organic cationic additives. Moreover, these species are characterized by fast redox kinetics and high reversibility [35, 36]. It was found that 1,10-dimethyl-4,40-bipyridinium cation (MV2+) is strongly attracted to the electrode surface. However, after reduction to MV+, the physical interaction between these species and the electrode can be even stronger [37]. This may be beneficial to reduce self-discharge, which is caused by redox shuttling.

As the cations are supposed to be attracted to the negatively polarized electrode, redox reactions originating from cationic additives are mostly at the negative side. However, the synthesis of carbon material exhibiting the affinity to cations and application of such an electrode as the positive one in ECs is also reported [26, 38, 39, 40]. It is worth noting that not only carbon materials can be functionalized—in fact, but various polymers can also be enriched with cationic (or anionic) functional groups.

The systems operating in redox-active electrolytes with transition metals as active species need to be assembled with ion-selective membranes as separators. These membranes can mitigate the self-discharge and leakage current which are relatively high for such systems [23, 39]. Nevertheless, the application of viologens (organic molecules) as a redox additive to the electrolytic solution can also decrease self-discharge without the necessity of ion-selective membrane employment. It is caused by viologens strong adsorption at the porous electrode surface [18, 28].

The main disadvantage of using viologens is their limited solubility and large size of the molecule that can negatively influence the ECs performance [32, 41], especially because of mass-transport issues.

3.1.2 Anionic aqueous redox-active electrolytes

The anionic redox-active electrolytes contain halides (iodide [14, 42, 43, 44], bromide [4, 45], pseudohalides (thiocyanate [41], selenocyanate [46]), organometallic complexes (ferricyanide and, ferrocyanide [21, 47, 48, 49, 50, 51, 52, 53]) and organic anion—like indigo carmine [54].

In the case of halide and pseudohalides-based electrolytes, a well-defined redox response is recorded at the positively polarized electrode. They are characterized by strong adsorption at the electrode surface. Hence, the self-discharge of the cell operating in such electrolytes is relatively low and the application of an ion-selective membrane is not needed. Moreover, halides can be coupled with metal ions deposition reaction, especially Zn/Zn2+, and viologen redox couple [30, 31, 55], however, such systems are no longer typical capacitors. To avoid metal dendrites formation, some additional components should be used, like dendrite suppression or nanoporous separators [56, 57, 58, 59].

The standard potentials of bromide and iodide reactions are similar; however, the bromides demonstrate slightly higher values [60]. It can be beneficial for reaching higher energy of the ECs, as the operating voltage might be shifted toward higher values. Nonetheless, bromide solutions are toxic, so for safety, it is favorable to use iodide-based solutions. Also due to the high standard potential of Br/Br2, close to oxygen evolution potential, the electrolyte decomposition can be difficult to control and corrosion on current collectors can be observed [31]. Iodide-based ECs are widely described in the literature. These systems are characterized by stable operation even during long-term experiments [32, 61, 62].

Pseudohalides solutions exhibit similar electrochemical behavior to halide solutions when used as electrolytes in ECs but self-discharge is definitely more pronounced. Thiocyanates-based solutions are especially interesting for ECs application due to their higher maximum operating voltage than selenocyanate-based electrolytes. Moreover, the energy and power of such systems are comparable to those reached in iodide-based electrolytes, but their lifespan is still limited [41].

Organometallic-based electrolytes (ferricyanide- or ferrocyanide-based solutions) ensure the promising performance of the ECs. The main drawback of these electrolytes is high self-discharge, seen as low efficiency, especially at low current loads. Therefore, ion-selective membranes are very often used to limit redox shuttling [48, 63].

3.1.3 Non-ionic aqueous redox-active electrolytes

Even in aqueous-based systems, organic redox-active additives can be used. For instance, hydroquinone (HQ), anthraquinone [64, 65, 66], catechol (an isomer of benzoquinone), rutin [67], p-phenylenediamine [68], and conducting polymers [69, 70] (if soluble in water) are popular neutral electroactive species added to the electrolytic solutions. To enable redox reaction with proton transfer, the use of supporting electrolytes is necessary. For this reason, acid solutions (H2SO4) are used as a source of protons. As a consequence, the maximum operating voltage of the ECs operating in an acidic medium is limited to ∼1 V, and, because of corrosion issues, the use of gold or other noble metal current collectors is necessary.

As the representative reaction, the reduction of benzoquinone to hydroquinone (Q/HQ) is presented in Eq. (7).

E7

Moreover, the conductivity of the electrolytic solutions with organic molecules can be diminished. Therefore, additional ionic species are very often introduced (like neutral salts—KNO3 or alkaline KOH [71]); hence, the formation of EDL can be more efficient. These systems are also characterized by considerable self-discharge related to the movement of neutral molecules between the polarized electrodes. To reduce self-discharge and increase the efficiency of the charging and discharging processes, the use of an expensive proton exchange membrane is recommended, which significantly increases the price of ECs [72]. The cells operating in the electrolytes with polymeric additives (i.e., sulfonated polyaniline or p-nitroaniline) also required the use of cheaper membranes. It is possible to use a semipermeable membrane that allows the movement of protons and supporting ions like SO42−. The drawback of such electrolytes is the solubility of the polymeric molecules—when the concentration of electroactive molecules is relatively low, the capacity of the cell is also limited [69]. Therefore, it is necessary to investigate the ECs with new polymer-based electrolytes to develop these systems and reach satisfactory operating parameters.

3.1.4 Cationic-anionic electrolytes

As cationic additives exhibit redox activity at the negatively polarized electrode and anionic additives at the positively polarized one, they can be combined, giving significant performance improvement. These redox couples should be carefully selected because they must be stable and soluble under the same conditions. Otherwise, it would be necessary to use more expensive separators/membranes and the assembly process would be more complex [18]. ECs operating in the electrolyte containing viologen cation and halide anion were tested. In the case of the electrolyte with MV2+ and I redox-active species during cell charging, an irreversible capacitance loss was noticed. It was caused by precipitate formation (MX•+–I) [73]. When the iodide was replaced by bromide (the anion of higher standard potential) the processes were reversible, and higher energy was reached. However, because of the high potential needed for Br/Br3 activity, the signs of corrosion were observed. MVCl2/KBr-based cells suffer from a relatively high self-discharge, which was more pronounced than for halide-based electrolytes, suggesting that MV species are, mostly, responsible for this voltage loss. Therefore, other viologen was used—1,10-diheptyl-4,40-bipyridinium dibromide (HVBr2), resulting in lower self-discharge. Probably, not only stronger adsorption of HV2+ cation was the reason for the lower self-discharge but also these cations were immobilized due to the precipitate formation within the carbon electrode [74]. The optimization of redox-active species concentration, choice of the appropriate counter anion/cation for redox-active cation/anion, respectively, and experimental conditions optimization is definitely more complex and time-consuming than for one active component within the electrolytic solution. However, taking into account the significant improvement of the energy stored in the EC operating in the redox-active electrolytes, it is still worth discovering the potential of this field.

3.2 Redox-mediated nonaqueous electrolytes

Commercially used ECs very often employ nonaqueous electrolytes (organic ones) despite the fact, that they cannot be considered environmentally friendly solutions, because of the necessity of toxic solvents use—like acetonitrile or propylene carbonate. However, they have a few advantages that make them more attractive for ECs construction: wider electrochemical window (up to 3.8 V [75, 76]) which allows higher energy to be stored, and long cycle life. On the other hand, there are ionic liquids called “green solutions”, that can be also used in ECs but they are relatively expensive.

3.2.1 Organic electrolytes with redox activity

In the organic electrolytes, conductive salts like tetraethylammonium tetrafluoroborate (TEABF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI), lithium hexafluorophosphate (LiPF6), are dissolved in acetonitrile (ACN) or propylene carbonate (PC), which are the most popular solvents for ECs application [22, 77, 78, 79, 80, 81]. As already mentioned, organic electrolytes allow the ECs to operate at higher voltages than aqueous-based electrolytes do [82] and they provide higher power than the systems with ionic liquid (IL) electrolytes [11, 83] due to the higher ionic conductivity of organic electrolytes.

The maximum voltage of reported organic-based cells is 2.5 V, when IL (1-ethyl-3-methylimidazolium ferrocenylsulfonyl-(trifluoromethylsulfonyl)-imide, [EMIm][FcNTf]) in ACN [76] and p-phenylenediamine additive to lithium perchlorate LiClO4 in ACN [84] were used as electrolytes. The mixture of microporous carbon with carbon black and graphite was used as the electrode material. However, there are also other materials that can be used, for instance two-dimensional titanium carbide (MXene) [81].

Organic electrolytes exhibiting redox activity are not as popular as aqueous electrolytes. Therefore, there is a gap in this field of study as there are many possible redox additives that could be employed for organic electrolytes [85].

3.2.2 Ionic liquids

Ionic liquids (ILs) are compounds composed entirely of ions—bulky, usually asymmetric organic cation and anion (weakly coordinating) that can be both organic and inorganic [86, 87]. As they are ionic conductive, there is no need to use additional solvents. They are characterized by high electrochemical stability, ensuring a high voltage window (>3 V) and high thermal stability [88, 89]. It is possible to introduce redox additives to IL, for example by incorporating metal ions (Cu2+ added in the form of copper chloride to [EMIm][BF4] [40], neutral redox molecules [90, 91] (HQ added to [TEA][TFSI] [92]) or sulfates (SnSO4 and VOSO4 [23]).

However, ILs themselves can also exhibit redox activity if an anion of IL is electroactive. Hence, such an electrolyte can be called redox-active IL. To observe effective and beneficial redox contribution to ECs charge/discharge, a high concentration of electroactive species needs to be ensured. Electrolyte composed of two ILs—[EMIm][BF4] and [EMIm]Br, where the latter one is a redox additive (1 mol L−1) to the former one, was used in microporous electrodes-based EC. The operating parameters were significantly improved due to the bromide activity (the specific energy was almost twice higher if compared to the [EMIm]BF4-based system, where only EDL formation is assumed, and the Coulombic efficiency was ∼100%) [83]. Moreover, the leakage current was reduced, probably due to strong adsorption of halide on the positively charged carbon electrode, described for aqueous-based cells [18, 32].

Biredox ILs can also be used as electrolytes for ECs. The idea arises due to the potential balancing issue when additives, like metal ions, HQ, or redox-active anions, are introduced to the system operating with microporous carbon electrodes [75]. The cation of IL ([BMIm][TFSI]) was functionalized with AQ, whereas the anion was functionalized with 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) molecule. The energy density of such a system was definitely higher than for IL with redox additive as an electrolyte but the specific power and lifetime were rather moderate.

The application of redox-active IL as electrolytes in ECs is a promising strategy to increase the specific energy of the systems. However, one should take into account that the price of such devices is relatively high. Moreover, the power performance and the lifespan of the ECs operating in ILs should be improved.

3.2.3 Redox-mediated gel electrolytes

Gels are characterized by very good stability (both chemical and mechanical) and they can be made of eco-friendly materials [93]. They can be successfully applied as electrolytes (based on aqueous solutions or ILs) for ECs [61, 94, 95]. Gel electrolytes were introduced to ECs to reduce their self-discharge [96] and enable the development of flexible devices, where liquid electrolytes would expose the cells to leaks. Redox mediators can be introduced to the gel electrolytes and increase both ionic conductivity and capacitance of the ECs [97]. For instance, when indigo carmine was added to the gel electrolyte based on polyvinyl alcohol (PVA) and sulfuric acid, the ionic conductivity increased by almost 190% [98]. Moreover, the lifetime of the devices can be prolonged. Redox-active compounds, like 1-butyl-3-methylimidazolium iodide and bromide (BMImI, BMImBr) [99, 100], 1-anthraquinone sulfonic acid sodium [101], 1,4-naphthoquinone [102], including indigo carmine [98] and FeBr3 [103], can be incorporated into gel structure. BMImBr with Li2SO4 as an additive to the PVA-based gel electrolyte was reported as a perfect solution for lowering self-discharge, increasing energy, and lifetime of the EC [100]. Flexible capacitors based on gel electrolyte—poly(methyl methacrylate)-propylene carbonate-lithium perchlorate electrolyte with HQ as a neutral redox additive were also investigated [104].

Advertisement

4. Summary

Redox-active electrolytes can be successfully applied in electrochemical capacitors and these electrolytes remarkably improve the energy density. It is crucial to use redox additives with a well-reversible and well-defined redox response, as the efficiency of charging/discharging should not be affected by redox process.

A variety of redox couples can be selected depending on the user’s requirements: for the systems based on aqueous or nonaqueous electrolyte, with redox species supposed to be active at the positively or negatively polarized electrode, or which parameters are the most important—high energy, high power, or very long cycle life. Taking into account aqueous-based redox-active electrolytes, the most attractive from the practical point of view are cationic and anionic electroactive species—because of their good solubility in water ensuring high conductivity of the solution. Moreover, the cells operating in organic/polymer-based electrolytes are more expensive due to the proton/ions permeable membranes that have to be used.

There are also a few issues that need to be solved. Redox species cause higher self-discharge of the cell in comparison to ECs with pure EDL formation. Therefore, it would be beneficial to “trap” the redox species within the pores of the material to prevent their movement to the electrolyte bulk. Moreover, the lifetime of EC with redox-active electrolytes should be prolonged, because it is still significantly shorter than the lifetime of the cell operating in the typical capacitive electrolytes.

Nevertheless, redox-active electrolytes in electrochemical capacitors offer an interesting alternative to the solid-state compounds and composites with maintained power and improved charge/discharge efficiency.

Advertisement

Acknowledgments

European Research Council Starting Grant 2017 project “IMMOCAP” (GA 759603) is acknowledged for financial support covering The Open Access Publishing Fee.

References

  1. 1. Barber P, Balasubramanian S, Anguchamy Y, Gong S, Wibowo A, Gao H, et al. Polymer composite and nanocomposite dielectric materials for pulse power energy storage. Materials. 2009;2(4):1697-1733
  2. 2. Ho J, Jow TR, Boggs S. Historical introduction to capacitor technology. IEEE Electrical Insulation Magazine. 2010;26(1):20-25
  3. 3. Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nature Materials. 2008;7(11):845-854
  4. 4. Maher M, Hassan S, Shoueir K, Yousif B, Abo-Elsoud MEA. Activated carbon electrode with promising specific capacitance based on potassium bromide redox additive electrolyte for supercapacitor application. Journal of Materials Research and Technology. 2021;11:1232-1244
  5. 5. Kötz R, Carlen M. Principles and applications of electrochemical capacitors. Electrochimica Acta. 2000;45(15–16):2483-2498
  6. 6. Beguin F, Frackowiak E, editors. Carbons for Electrochemical Energy Storage and Conversion Systems 1st ed. CRC Press; 2009
  7. 7. Béguin F, Frackowiak E. Supercapacitors: Materials, Systems, and Applications. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA; 2013
  8. 8. Simon P, Gogotsi Y. Perspectives for electrochemical capacitors and related devices. Nature Materials. 2020;19(11):1151-1163
  9. 9. Brousse T, Belanger D, Long JW. To be or not to be pseudocapacitive? Journal of the Electrochemical Society. 2015;162(5):A5185-A5189
  10. 10. Gorska B, Frackowiak E, Beguin F. Redox active electrolytes in carbon/carbon electrochemical capacitors. Current Opinion in Electrochemistry. 2018;9:95-105
  11. 11. Lee J, Srimuk P, Fleischmann S, Su X, Hatton TA, Presser V. Redox-electrolytes for non-flow electrochemical energy storage: A critical review and best practice. Progress in Materials Science. 2019;101:46-89
  12. 12. Balducci A, Belanger D, Brousse T, Long JW, Sugimoto W. A guideline for reporting performance metrics with electrochemical capacitors: From electrode materials to full devices. Journal of the Electrochemical Society. 2017;164(7):A1487-A14A8
  13. 13. Laheäär A, Przygocki P, Abbas Q, Béguin F. Appropriate methods for evaluating the efficiency and capacitive behavior of different types of supercapacitors. Electrochemistry Communications. 2015;60:21-25
  14. 14. Sankar KV, Kalai SR. Improved electrochemical performances of reduced graphene oxide based supercapacitor using redox additive electrolyte. Carbon (New York). 2015;90:260-273
  15. 15. Sun K, Feng E, Peng H, Ma G, Wu Y, Wang H, et al. A simple and high-performance supercapacitor based on nitrogen-doped porous carbon in redox-mediated sodium molybdate electrolyte. Electrochimica Acta. 2015;158:361-367
  16. 16. Fan L-Q, Zhong J, Zhang C-Y, Wu J-H, Wei Y-L. Improving the energy density of quasi-solid-state supercapacitors by assembling two redox-active gel electrolytes. International Journal of Hydrogen Energy. 2016;41(13):5725-5732
  17. 17. Wang C, Xi Y, Wang M, Zhang C, Wang X, Yang Q, et al. Carbon-modified Na2Ti3O7·2H2O nanobelts as redox active materials for high-performance supercapacitor. Nano Energy. 2016;28:115-123
  18. 18. Chen S-E, Evanko B, Wang X, Vonlanthen D, Ji X, Stucky GD, et al. Design of aqueous redox-enhanced electrochemical capacitors with high specific energies and slow self-discharge. Nature Communications. 2015;6(7818):1-10
  19. 19. Pourbaix M. Atlas of Electrochemical Equilibria in Aqueous Solutions/By Marcel Pourbaix; Translated from the French by James A. Franklin (Except Sections I, III 5, and III 6, which were Originally Written in English). 2d English ed. Houston, Tex: National Association of Corrosion Engineers; 1974
  20. 20. Zhang L, Yang S, Chang J, Zhao D, Wang J, Yang C, et al. A review of redox electrolytes for supercapacitors. Frontiers in Chemistry. 2020;8:413
  21. 21. Chodankar NR, Dubal DP, Lokhande AC, Patil AM, Kim JH, Lokhande CD. An innovative concept of use of redox-active electrolyte in asymmetric capacitor based on MWCNTs/MnO2 and Fe2O3 thin films. Scientific Reports. 2016;6(1):39205
  22. 22. Singh A, Chandra A. Enhancing specific energy and power in asymmetric supercapacitors—A synergetic strategy based on the use of redox additive electrolytes. Scientific Reports. 2016;6(1):25793
  23. 23. Lee J, Krüner B, Tolosa A, Sathyamoorthi S, Kim D, Choudhury S, et al. Tin/vanadium redox electrolyte for battery-like energy storage capacity combined with supercapacitor-like power handling. Energy & Environmental Science. 2016;9(11):3392-3398
  24. 24. Teng Y, Liu E, Ding R, Liu K, Liu R, Wang L, et al. Bean dregs-based activated carbon/copper ion supercapacitors. Electrochimica Acta. 2016;194:394-404
  25. 25. Ren L, Zhang G, Yan Z, Kang L, Xu H, Shi F, et al. High capacitive property for supercapacitor using Fe3+/Fe2+ redox couple additive electrolyte. Electrochimica Acta. 2017;231:705-712
  26. 26. Sun XN, Hu W, Xu D, Chen XY, Cui P. Integration of redox additive in H2SO4 solution and the adjustment of potential windows for improving the capacitive performances of supercapacitors. Industrial & Engineering Chemistry Research. 2017;56(9):2433-2443
  27. 27. Díaz P, González Z, Santamaría R, Granda M, Menéndez R, Blanco C. Enhanced energy density of carbon-based supercapacitors using cerium (III) sulphate as inorganic redox electrolyte. Electrochimica Acta. 2015;168:277-284
  28. 28. Roldán S, Granda M, Menéndez R, Santamaría R, Blanco C. Supercapacitor modified with methylene blue as redox active electrolyte. Electrochimica Acta. 2012;83:241-246
  29. 29. Evanko B, Boettcher SW, Yoo SJ, Stucky GD. Redox-enhanced electrochemical capacitors: Status, opportunity, and best practices for performance evaluation. ACS Energy Letters. 2017;2(11):2581-2590
  30. 30. Sathyamoorthi S, Kanagaraj M, Kathiresan M, Suryanarayanan V, Velayutham D. Ethyl viologen dibromide as a novel dual redox shuttle for supercapacitors. Journal of Materials Chemistry A, Materials for Energy and Sustainability. 2016;4(12):4562-4569
  31. 31. Yoo SJ, Evanko B, Wang X, Romelczyk M, Taylor A, Ji X, et al. Fundamentally addressing bromine storage through reversible solid-state confinement in porous carbon electrodes: Design of a high-performance dual-redox electrochemical capacitor. Journal of the American Chemical Society. 2017;139(29):9985-9993
  32. 32. Lee J, Srimuk P, Fleischmann S, Ridder A, Zeiger M, Presser V. Nanoconfinement of redox reactions enables rapid zinc iodide energy storage with high efficiency. Journal of Materials Chemistry A. 2017;5(24):12520-12527
  33. 33. Fic K, Meller M, Frackowiak E. Interfacial redox phenomena for enhanced aqueous supercapacitors. Journal of the Electrochemical Society. 2015;162(5):A5140-A51A7
  34. 34. Mai L-Q, Minhas-Khan A, Tian X, Hercule KM, Zhao Y-L, Lin X, et al. Synergistic interaction between redox-active electrolyte and binder-free functionalized carbon for ultrahigh supercapacitor performance. Nature Communications. 2013;4(1):2923
  35. 35. Monk P, Mortimer R, Rosseinsky D. Electrochromism and Electrochromic Devices/Paul Monk, Roger Mortimer, David Rosseinsky. Cambridge: Cambridge University Press; 2007
  36. 36. Michaelis L, Hill ES. The viologen indicators. The Journal of General Physiology. 1933;16(6):859-873
  37. 37. Nakamura T, Kawasaki N, Ogawa H, Tanada S, Kogirima M, Imaki M. Adsorption removal of paraquat and diquat onto activated carbon at different adsorption temperature. Toxicological and Environmental Chemistry. 1999;70(3–4):275-280
  38. 38. Akinwolemiwa B, Peng C, Chen GZ. Redox electrolytes in supercapacitors. Journal of the Electrochemical Society. 2015;162(5):A5054-A5059
  39. 39. Frackowiak E, Fic K, Meller M, Lota G. Electrochemistry serving people and nature: High-energy ecocapacitors based on redox-active electrolytes. ChemSusChem. 2012;5(7):1181-1185
  40. 40. Li Q, Li K, Sun C, Li Y. An investigation of Cu2+ and Fe2+ ions as active materials for electrochemical redox supercapacitors. Journal of Electroanalytical Chemistry (Lausanne, Switzerland). 2007;611(1–2):43-50
  41. 41. Gorska B, Bujewska P, Fic K. Thiocyanates as attractive redox-active electrolytes for high-energy and environmentally-friendly electrochemical capacitors. Physical Chemistry Chemical Physics. 2017;19(11):7923-7935
  42. 42. Zhang Y, Zu L, Lian H, Hu Z, Jiang Y, Liu Y, et al. An ultrahigh performance supercapacitors based on simultaneous redox in both electrode and electrolyte. Journal of Alloys and Compounds. 2017;694:136-144
  43. 43. Gao Z, Zhang L, Chang J, Wang Z, Wu D, Xu F, et al. Catalytic electrode-redox electrolyte supercapacitor system with enhanced capacitive performance. Chemical Engineering Journal. 2018;335:590-599
  44. 44. Senthilkumar ST, Selvan RK, Lee YS, Melo JS. Electric double layer capacitor and its improved specific capacitance using redox additive electrolyte. Journal of Materials Chemistry A. 2013;1(4):1086-1095
  45. 45. Yamazaki S, Ito T, Murakumo Y, Naitou M, Shimooka T, Yamagata M, et al. Hybrid capacitors utilizing halogen-based redox reactions at interface between carbon positive electrode and aqueous electrolytes. Journal of Power Sources. 2016;326:580-586
  46. 46. Bujewska P, Gorska B, Fic K. Redox activity of selenocyanate anion in electrochemical capacitor application. Synthetic Metals. 2019;253:62-72
  47. 47. Iamprasertkun P, Ejigu A, Dryfe RAW. Understanding the electrochemistry of “water-in-salt” electrolytes: Basal plane highly ordered pyrolytic graphite as a model system. Chemical Science (Cambridge). 2020;11(27):6978-6989
  48. 48. Lee J, Choudhury S, Weingarth D, Kim D, Presser V. High Performance Hybrid Energy Storage with Potassium Ferricyanide Redox Electrolyte. ACS Appl Mater Interfaces. 2016;8(36):23676-23687
  49. 49. Veerasubramani GK, Krishnamoorthy K, Kim SJ. Improved electrochemical performances of binder-free CoMoO4 nanoplate arrays@Ni foam electrode using redox additive electrolyte. Journal of Power Sources. 2016;306:378-386
  50. 50. Lamiel C, Lee YR, Cho MH, Tuma D, Shim J-J. Enhanced electrochemical performance of nickel-cobalt-oxide@reduced graphene oxide//activated carbon asymmetric supercapacitors by the addition of a redox-active electrolyte. Journal of Colloid and Interface Science. 2017;507:300-309
  51. 51. Cha SM, Nagaraju G, Chandra Sekhar S, Yu JS. A facile drop-casting approach to nanostructured copper oxide-painted conductive woven textile as binder-free electrode for improved energy storage performance in redox-additive electrolyte. Journal of Materials Chemistry A, Materials for Energy and Sustainability. 2017;5(5):2224-2234
  52. 52. Shanmugavani A, Kaviselvi S, Sankar KV, Selvan RK. Enhanced electrochemical performances of PANI using redox additive of K4[Fe(CN)6] in aqueous electrolyte for symmetric supercapacitors. Materials Research Bulletin. 2015;62:161-167
  53. 53. Su LH, Zhang XG, Mi CH, Gao B, Liu Y. Improvement of the capacitive performances for Co-Al layered double hydroxide by adding hexacyanoferrate into the electrolyte. Physical Chemistry Chemical Physics: PCCP. 2009;11(13):2195-2202
  54. 54. Roldán S, González Z, Blanco C, Granda M, Menéndez R, Santamaría R. Redox-active electrolyte for carbon nanotube-based electric double layer capacitors. Electrochimica Acta. 2011;56(9):3401-3405
  55. 55. Yamamoto T, Kanbara T. Porous and electrically conducting clay-carbon composite as positive electrodes of zinc-oxygen primary cells and zinc-iodine secondary cells. Inorganica Chimica Acta. 1988;142(2):191-193
  56. 56. Banik SJ, Akolkar R. Suppressing dendrite growth during zinc electrodeposition by PEG-200 additive. Journal of the Electrochemical Society. 2013;160(11):D519-D523
  57. 57. Cheng X-B, Zhao M-Q, Chen C, Pentecost A, Maleski K, Mathis T, et al. Nanodiamonds suppress the growth of lithium dendrites. Nature Communications. 2017;8(1):336-339
  58. 58. Li B, Nie Z, Vijayakumar M, Li G, Liu J, Sprenkle V, et al. Ambipolar zinc-polyiodide electrolyte for a high-energy density aqueous redox flow battery. Nature Communications. 2015;6(1):6303
  59. 59. Bai P, Li J, Brushett FR, Bazant MZ. Transition of lithium growth mechanisms in liquid electrolytes. Energy & Environmental Science. 2016;9(10):3221-3229
  60. 60. Fic K, Morimoto S, Frackowiak E, Ishikawa M. Redox activity of bromides in carbon-based electrochemical capacitors. Batteries & Supercaps. 2020;3(10):1080-1090
  61. 61. Senthilkumar ST, Selvan RK, Ponpandian N, Melo JS. Redox additive aqueous polymer gel electrolyte for an electric double layer capacitor. RSC Advances. 2012;2(24):8937-8940
  62. 62. Lota G, Frackowiak E. Striking capacitance of carbon/iodide interface. Electrochemistry Communications. 2009;11(1):87-90
  63. 63. Yu S, Yang N, Zhuang H, Mandal S, Williams OA, Yang B, et al. Battery-like supercapacitors from diamond networks and water-soluble redox electrolytes. Journal of Materials Chemistry A. 2017;5(4):1778-1785
  64. 64. Guin PS, Das S, Mandal PC. Electrochemical reduction of quinones in different media: A review. International Journal of Electrochemistry. 2010;2011:1-22
  65. 65. Gastol D, Walkowiak J, Fic K, Frackowiak E. Enhancement of the carbon electrode capacitance by brominated hydroquinones. Journal of Power Sources. 2016;326:587-594
  66. 66. Huskinson B, Marshak MP, Suh C, Er S, Gerhardt MR, Galvin CJ, et al. A metal-free organic-inorganic aqueous flow battery. Nature (London). 2014;505(7482):195-198
  67. 67. Nie YF, Wang Q, Chen XY, Zhang ZJ. Nitrogen and oxygen functionalized hollow carbon materials: The capacitive enhancement by simply incorporating novel redox additives into H2SO4 electrolyte. Journal of Power Sources. 2016;320:140-152
  68. 68. Zhang ZJ, Zhu YQ, Chen XY, Cao Y. Pronounced improvement of supercapacitor capacitance by using redox active electrolyte of p-phenylenediamine. Electrochimica Acta. 2015;176:941-948
  69. 69. Chen L, Chen Y, Wu J, Wang J, Bai H, Li L. Electrochemical supercapacitor with polymeric active electrolyte. Journal of Materials Chemistry A, Materials for Energy and Sustainability. 2014;2(27):10526-10531
  70. 70. Nie YF, Wang Q, Chen XY, Zhang ZJ. Synergistic effect of novel redox additives of p-nitroaniline and dimethylglyoxime for highly improving the supercapacitor performances. Physical Chemistry Chemical Physics. 2016;18(4):2718-2729
  71. 71. Wu J, Yu H, Fan L, Luo G, Lin J, Huang M. A simple and high-effective electrolyte mediated with p-phenylenediamine for supercapacitor. Journal of Materials Chemistry. 2012;22(36):1925-1193
  72. 72. Chen L, Bai H, Huang Z, Li L. Mechanism investigation and suppression of self-discharge in active electrolyte enhanced supercapacitors. Energy & Environmental Science. 2014;7(5):1750-1759
  73. 73. Lezna RO, Centeno SA. Spectroelectrochemistry of methyl viologen/iodide solutions at mercury film electrodes. Langmuir. 1996;12(20):4905-4908
  74. 74. Andreas HA, Lussier K, Oickle AM. Effect of Fe-contamination on rate of self-discharge in carbon-based aqueous electrochemical capacitors. Journal of Power Sources. 2009;187(1):275-283
  75. 75. Mourad E, Coustan L, Lannelongue P, Zigah D, Mehdi A, Vioux A, et al. Biredox ionic liquids with solid-like redox density in the liquid state for high-energy supercapacitors. Nature Materials. 2017;16(4):446-453
  76. 76. Xie HJ, Gélinas B, Rochefort D. Redox-active electrolyte supercapacitors using electroactive ionic liquids. Electrochemistry Communications. 2016;66:42-45
  77. 77. Dall’Agnese Y, Rozier P, Taberna P-L, Gogotsi Y, Simon P. Capacitance of two-dimensional titanium carbide (MXene) and MXene/carbon nanotube composites in organic electrolytes. Journal of Power Sources. 2016;306:510-515
  78. 78. Jäckel N, Weingarth D, Schreiber A, Krüner B, Zeiger M, Tolosa A, et al. Performance evaluation of conductive additives for activated carbon supercapacitors in organic electrolyte. Electrochimica Acta. 2016;191:284-298
  79. 79. Salunkhe RR, Young C, Tang J, Takei T, Ide Y, Kobayashi N, et al. A high-performance supercapacitor cell based on ZIF-8-derived nanoporous carbon using an organic electrolyte. Chemical Communications (Cambridge, England). 2016;52(26):4764-4767
  80. 80. Yang W, Yang W, Song A, Gao L, Su L, Shao G. Supercapacitance of nitrogen-sulfur-oxygen co-doped 3D hierarchical porous carbon in aqueous and organic electrolyte. Journal of Power Sources. 2017;359:556-567
  81. 81. Xie L, Sun G, Su F, Guo X, Kong Q, Li X, et al. Hierarchical porous carbon microtubes derived from willow catkins for supercapacitor applications. Journal of Materials Chemistry A, Materials for Energy and Sustainability. 2016;4(5):1637-1646
  82. 82. Zhao C, Zheng W. A review for aqueous electrochemical supercapacitors. Frontiers in Energy Research. 2015;3:23
  83. 83. Yamazaki S, Ito T, Yamagata M, Ishikawa M, editors. Nonaqueous Electrochemical Capacitor Utilizing Electrolytic Redox Reactions of Bromide Species in Ionic Liquid. 2012
  84. 84. Yu H, Wu J, Fan L, Hao S, Lin J, Huang M. An efficient redox-mediated organic electrolyte for high-energy supercapacitor. Journal of Power Sources. 2014;248:1123-1126
  85. 85. Gong K, Fang Q, Gu S, Li SFY, Yan Y. Nonaqueous redox-flow batteries: Organic solvents, supporting electrolytes, and redox pairs. Energy & Environmental Science. 2015;8(12):3515-3530
  86. 86. Austen Angell C, Ansari Y, Zhao Z. Ionic liquids: Past, present and future. Faraday Discussions. 2011;154:9-27
  87. 87. Mousavi MPS, Wilson BE, Kashefolgheta S, Anderson EL, He S, Bühlmann P, et al. Ionic liquids as electrolytes for electrochemical double-layer capacitors: Structures that optimize specific energy. ACS Applied Materials & Interfaces. 2016;8(5):3396-3406
  88. 88. Brandt A, Pohlmann S, Varzi A, Balducci A, Passerini S. Ionic liquids in supercapacitors. MRS Bulletin. 2013;38(7):554-559
  89. 89. Tsai W-Y, Lin R, Murali S, Li Zhang L, McDonough JK, Ruoff RS, et al. Outstanding performance of activated graphene based supercapacitors in ionic liquid electrolyte from −50 to 80 °C. Nano Energy. 2013;2(3):403-411
  90. 90. Dubal DP, Suarez-Guevara J, Tonti D, Enciso E, Gomez-Romero P. A high voltage solid state symmetric supercapacitor based on graphene–polyoxometalate hybrid electrodes with a hydroquinone doped hybrid gel-electrolyte. Journal of Materials Chemistry A, Materials for Energy and Sustainability. 2015;3(46):23483-23492
  91. 91. Navalpotro P, Palma J, Anderson M, Marcilla R. High performance hybrid supercapacitors by using para-benzoquinone ionic liquid redox electrolyte. Journal of Power Sources. 2016;306:711-717
  92. 92. Sathyamoorthi S, Suryanarayanan V, Velayutham D. Organo-redox shuttle promoted protic ionic liquid electrolyte for supercapacitor. Journal of Power Sources. 2015;274:1135-1139
  93. 93. Armelin E, Pérez-Madrigal MM, Alemán C, Díaz DD. Current status and challenges of biohydrogels for applications as supercapacitors and secondary batteries. Journal of Materials Chemistry A, Materials for Energy and Sustainability. 2016;4(23):8952-8968
  94. 94. Yamagata M, Soeda K, Ikebe S, Yamazaki S, Ishikawa M. Chitosan-based gel electrolyte containing an ionic liquid for high-performance nonaqueous supercapacitors. Electrochimica Acta. 2013;100:275-280
  95. 95. Menzel J, Frąckowiak E, Fic K. Agar-based aqueous electrolytes for electrochemical capacitors with reduced self-discharge. Electrochimica Acta. 2020;332:135435
  96. 96. Niu J, Conway BE, Pell WG. Comparative studies of self-discharge by potential decay and float-current measurements at C double-layer capacitor and battery electrodes. Journal of Power Sources. 2004;135(1–2):332-343
  97. 97. Alipoori S, Mazinani S, Aboutalebi SH, Sharif F. Review of PVA-based gel polymer electrolytes in flexible solid-state supercapacitors: Opportunities and challenges. Journal of Energy Storage. 2020;27:101072
  98. 98. Ma G, Dong M, Sun K, Feng E, Peng H, Lei Z. A redox mediator doped gel polymer as an electrolyte and separator for a high performance solid state supercapacitor. Journal of Materials Chemistry A, Materials for Energy and Sustainability. 2015;3(7):435-441
  99. 99. Tu QM, Fan LQ, Pan F, Huang JL, Gu Y, Lin JM, et al. Design of a novel redox-active gel polymer electrolyte with a dual-role ionic liquid for flexible supercapacitors. Electrochimica Acta. 2018;268:562-568
  100. 100. Fan L-Q, Tu Q-M, Geng C-L, Huang J-L, Gu Y, Lin J-M, et al. High energy density and low self-discharge of a quasi-solid-state supercapacitor with carbon nanotubes incorporated redox-active ionic liquid-based gel polymer electrolyte. Electrochimica Acta. 2020;331:135425
  101. 101. Feng E, Ma G, Sun K, Yang Q, Peng H, Lei Z. Toughened redox-active hydrogel as flexible electrolyte and separator applying supercapacitors with superior performance. RSC Advances. 2016;6(79):75896-77594
  102. 102. Hashemi M, Rahmanifar MS, El-Kady MF, Noori A, Mousavi MF, Kaner RB. The use of an electrocatalytic redox electrolyte for pushing the energy density boundary of a flexible polyaniline electrode to a new limit. Nano Energy. 2018;44:489-498
  103. 103. Wang Y, Chang Z, Qian M, Zhang Z, Lin J, Huang F. Enhanced specific capacitance by a new dual redox-active electrolyte in activated carbon-based supercapacitors. Carbon (New York). 2019;143:300-308
  104. 104. Kim D, Lee G, Kim D, Yun J, Lee S-S, Ha JS. High performance flexible double-sided micro-supercapacitors with an organic gel electrolyte containing a redox-active additive. Nanoscale. 2016;8(34):15611-15620

Written By

Paulina Bujewska, Przemysław Galek, Elżbieta Frąckowiak and Krzysztof Fic

Submitted: 09 March 2022 Reviewed: 15 April 2022 Published: 03 June 2022