Open access peer-reviewed chapter

Redox Mediated Electrolytes in Electrochemical Capacitors

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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

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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.


  • 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.


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].


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


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


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


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:


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):


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].


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).


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].


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.



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


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Written By

Paulina Bujewska, Przemysław Galek, Elżbieta Frąckowiak and Krzysztof Fic

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