# Why electrochemical capacitor electrolytes should not be ignored?

Sara Azmi, Amelia Klimek, Elzbieta Frackowiak

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Sara Azmi, Amelia Klimek, Elzbieta Frackowiak. Why electrochemical capacitor electrolytes should not be ignored?. Electrochimica Acta, 2023, 452, pp.142347. 10.1016/j.electacta.2023.142347. hal-05114305

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# Why electrochemical capacitor electrolytes should not be ignored?

Sara Azmi<sup>b, *</sup>, Amelia Klimek<sup>a</sup>, Elzbieta Frackowiak<sup>a, *</sup>

## ABSTRACT

Technological advances in various fields are frequently hampered by the insufficient capacity of energy storage devices. Electrochemical capacitors (ECs) emerge as high-power effective energy storage devices with a short charge/discharge time, long-term cycle life, and wide temperature range. The electrolyte is an important component influencing EC performance because of its physical and chemical properties. There are various types of electrolytes including organic, ionic liquids, water in salt and redox active electrolytes which offer diverse behavior to the ECs. Liquid state electrolyte as well as solid-state systems are described. This review emphasizes the significance of the electrolyte type and its influence on the EC performance. Different aspects are discussed starting from the specific ion effect at the electrode/electrolyte interface that affects the EC charge/discharge mechanism. Furthermore, emerging electrolytes and new directions in ECs are highlighted. Herein, the progress and challenges in water in salt (WIS) and deep eutectic solvent (DES) electrolytes are presented. Moreover, a discussion on the redox-active electrolyte for hybrid capacitors is carried out. ECs electrolytes are governing the crucial metrics of the device like the voltage window, the energy and power density as well as the long-term cyclability. Hence, as “a rule of thumb” the EC electrolyte should not be ignored.

## 1. Introduction

Nowadays, over 85% of the energy used in the world originates from fossil fuel sources (coal, oil, gas) or uranium that have been built up through the ages as a result of geological evolution [1]. However, the limited quantity of these reserves, the oil crisis in 1973 and the increase in demand for energy in all countries of the world have prompted the industrialized countries to seek and develop new supplies of energy. While the nuclear industry was already launched, its large-scale implementation can lead to dramatic environmental impacts, especially nuclear waste pollution and the possibility of accidents [2]. Scientists have developed another form of energy known as renewable energy. Those energies have all the tremendous advantages of being from a natural origin, inexhaustible and clean because of not emitting global warming gases [3]. One of these renewable energies is the electricity from solar energy by the photovoltaic effect. Photovoltaics has the potential to contribute to the transition to a sustainable energy supply system for the 21st century and to provide a significant part of electricity requirements.

Over the last few years, electricity production from renewable energies has been increasing rapidly worldwide. Therefore, the development of energy conversion and storage devices is necessary to efficiently exploit these intermittent energy sources. In this regard, batteries, electrochemical capacitors and redox-flow systems are considered to be the most important electrochemical energy storage (EES) devices [4]. Research efforts have concentrated mainly on electrochemical

capacitors (ECs) due to their satisfactory characteristics in high-power supply and storage of electricity and extended lifetime [5,6]. ECs are continuously developing energy devices that are designed for high energy supplies compared to batteries [7]. Nevertheless, EC still faces a major problem of low energy [8]. In the EC principle, the capacitance originates from the electric double-layer capacitance (EDLC) resulting from electrostatic attraction between the ions of the electrolyte and the electrode surface of opposite charge [9] whereas the pseudocapacitance is associated with the faradic reactions of the redox active species from electrolyte or the electrode material [10,11]. However, EDLC offers excellent cycle stability and performance, while EC with pseudocapacitance effects have a high energy density [10].

In the past decades, the scientific interest in various materials for the application of supercapacitors has gradually expanded [12–19,9, 20–22]. The EC performance depends heavily on charge storage processes and materials used as electrolytes, electrodes, and current collectors [23]. By selecting an appropriate combination of electrode materials and a suitable electrolyte, the EC capacitance performance and working voltage windows can be extended leading to increased power and energy densities. The typical electrode materials used for ECs are carbonaceous materials with a high specific surface area including carbon nanotubes [24,25], activated carbon (AC) [15,26], graphene [27, 28] and carbon nanofibers [29,30]. In recent decades, the scientific interest has been also focused on the use of various materials such as metal oxides [31,32], conducting polymers [33,34], MXenes [35,36],

<footnote><sup>a</sup> Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, Berdychowo 4, 60-965 Poznan, Poland</footnote>

<footnote><sup>b</sup> LMGC - Laboratoire de Mécanique et Génie Civil, CNRS-Université de Montpellier, 34090 Montpellier, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: sara.azmi.sa@gmail.com (S. Azmi), elzbieta.frackowiak@put.poznan.pl (E. Frackowiak).</footnote>
transition metal dichalcogenides [24,37] and metal organic framework [38,39] for applications in supercapacitors.

The electrolyte is an important component in EC. It plays a critical role in supplying the ionic conduction and affects the electrochemical performance of the device [40]. Furthermore, the manufacturing cost of the EC depends on the electrolyte type. The properties of the electrolyte such as a good electrochemical and chemical stability, a wide voltage window, appropriate conductivity, low toxicity, cost effectiveness, working temperature, safety should be taken into account for the high EC metrics (Fig. 1).

Various factors influence the electrochemical performance of ECs including the interaction between the electrode material and electrolyte, the pore and ion sizes, the thermal and chemical stabilities and the corrosion resistiveness [41]. These factors might reduce the EC performance by increasing the equivalent series resistance when the ion mobility and the conductivity are not sufficient; reducing the cycle life due to ageing phenomena resulting from corrosive electrolytes. Overall, the choice of electrolyte is the key to a safe and high-performance device. Several types of electrolytes have been developed including aqueous electrolytes [42], ionic liquid electrolytes [43], gel electrolytes [44] and solid electrolytes [45] as shown in Fig. 2.

Solid-state electrolyte in the form of gel exhibits a lower conductivity and ion mobility that diminishes the EC power, however, such system demonstrates smaller self-discharge and limits a gas evolution. It's worth mentioning that gel-type electrolytes are perfectly adapted for flexible and printed devices. Compared to solid-state electrolytes that are moderate ionic conductors, liquid electrolytes present better properties offering higher capacitive and energetic performance of the electrochemical device [46]. The liquid electrolytes are further classified as an aqueous electrolyte (alkaline, neutral and acid), a non-aqueous electrolyte (organic, ionic liquids) and water in salt electrolytes. Their characteristics are compared in Table 1. Aqueous electrolytes such as KOH, $H_2SO_4$, $Na_2SO_4$ are low cost and non-flammable, and highly conductive with a voltage range from 1.0 V to 1.6 V and a convenient assembly in air. Organic and ionic liquid electrolytes provide a wider voltage range (~3 V) but suffer from higher internal resistance and the need for an inert environment during assembly. Since the critical performance issues are environmental friendliness, efficiency and reliability, other classes of electrolytes appeared to overcome the drawbacks of the conventional electrolytes used in EC applications. Among them, the water in salt (WIS) electrolytes, in which the salt is more than the solvent in both mass and volume, are attracting several researchers [47–50] because of their properties being non-inflammable, cheap, with an acceptable ionic conductivity and moisture tolerance. Owing to their high concentration, WIS electrolytes effectively mitigate the problem of thermodynamic limitation of water and have a wide operating voltage range (2–3 V) that is much higher than that of traditional aqueous electrolytes (1.23 V) [47]. However, the charging/discharging mechanism of WIS based ECs is still poorly understood. Recently, another emerging class of electrolyte named deep eutectic solvents (DESs) has

been developed as a new type of wide stable voltage window electrolyte for ECs [51–53]. It presents numerous advantages being low-cost, easy to prepare, and environmentally friendly with the same characteristic as the expensive ionic liquids (ILs) [54,55]. However, these types of electrolytes suffer from low ionic conductivity and high viscosity. Although the considerations may differ for different electrodes, the key targets for the design of an ideal electrolyte remain the same for all technologies: to provide desirable processing steps, to ensure safety, cycling performance, as well as electrochemical stability and cost-effectiveness [56]. When all these conditions are fulfilled, the electrolyte system can be qualified as commercially viable. Generally, the electrolyte part should not be ignored in any electrochemical energy storage system, especially in electrochemical capacitors.

## 2. Specific ion interactions in EC electrolytes

A wide variety of physical and chemical processes in energy storage systems are deeply related to the properties of electrolyte solutions. In the EC application, the charging effect is dependent not only on the ion valence or concentrations but also on intrinsic ion properties [57]. Generally, ions are moving under the influence of forces resulting from a gradient of the chemical potential or the electrical field produced by the motion of oppositely charged ions [58]. Hence, the effect of the ion/ion and ion/solvent interactions is playing a crucial role in the charge transfer and mobility in the electrolytic system [59]. A different hypothesis has been revealed by the research community regarding the general ion properties. The first one was proposed by Hofmeister who linked the behavior of each ion to its capacity to adsorb water [57]. Fig. 3 illustrates the Hofmeister series for cations and anions.

According to Hofmeister, the influence on the hydrogen bond network depends on the ion and leads to a specific order of the ions [60]. On this basis, two categories were designed: the strongly hydrated small ions named the kosmotropes that are bringing order to the solution and the weakly hydrated big ions called the chaotropic that are disturbing the solution structure. The position of ions indicated in the Hofmeister series scale (Fig. 3) may change based on the system. To distinguish between various specific ion identities in Lewis acidity and basicity in water, Pearson developed in 1963 a hard/soft categorization parallel to that proposed by Hofmeister based on ion polarization [61]. In his classification, the hard Lewis acids (cations e.g. $H^+$, $Li^+$, $Fe^+$) and bases (anions i.e. $OH^-$, $CH_3COO^-$, $ClO_4^-$) are non-polarizable, while the soft acids (e.g. $Cu^+$, $Au^+$, $Ag^+$) and bases (e.g. $I^-$, $SCN^-$, $S_2O_3^{2-}$) are polarizable. This hard/soft ion sorting bears a resemblance to the kosmotropic/chaotropic classification in the Hofmeister series. Another classification has been reported by Collins and Washabaugh in their molecular interpretation of the Hofmeister series [62]. They have proposed a model based on the sequential alteration of three layers of water: a solute adjacent $L_1$, a transition layer $L_2$ that is supposedly free of salt, and a bulk-solute dominated $L_3$. They divided Hofmeister substances into three classes which interact differently with the $L_2$ transition layer

<figure><img src="image_4.png" /><figcaption>Fig. 1. Influence of the electrolyte properties on the EC performance.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 2. Classification of electrolytes for electrochemical capacitor applications.</figcaption></figure>

<table><caption>**Table 1**

Characteristics of electrolytes for electrochemical capacitor applications.</caption><thead><tr><th>Electrolyte</th><th>Aqueous media</th><th>Water in Salt</th><th>Organic media</th><th>Ionic liquid</th></tr></thead><tbody><tr><td>Conductivity (mS cm<sup>-1</sup>)</td><td>~ 800 mS cm<sup>-1</sup></td><td>~ 8–60 mS cm<sup>-1</sup></td><td>~ 20 mS cm<sup>-1</sup></td><td>~ 10 mS cm<sup>-1</sup></td></tr><tr><td>Voltage range (V)</td><td>1–1.6 V</td><td>2–3 V</td><td>2.5–3.7 V</td><td>> 3 V</td></tr><tr><td>Energy density (Wh kg<sup>-1</sup>)</td><td>~ 10 Wh kg<sup>-1</sup></td><td>~ 25 Wh kg<sup>-1</sup></td><td>~ 30 Wh kg<sup>-1</sup></td><td>~ 60 Wh kg<sup>-1</sup></td></tr><tr><td>Power density (kW kg<sup>-1</sup>)</td><td>~ 5 kW kg<sup>-1</sup></td><td>~ 10 kW kg<sup>-1</sup></td><td>~ 10 kW kg<sup>-1</sup></td><td>~ 20 kW kg<sup>-1</sup></td></tr><tr><td>Assembly</td><td>Air</td><td>Air</td><td>Inert</td><td>Inert</td></tr><tr><td>Safety</td><td>High</td><td>High</td><td>Low</td><td>High</td></tr><tr><td>Price</td><td>Low</td><td>Moderate</td><td>High</td><td>High</td></tr></tbody></table>

and water in their three-layer model. First are the Lewis bases (e.g. ammonium sulphate) that are polar kosmotropes interacting more strongly with L₂ than water. The second category is the chaotropic Lewis acids (e.g. calcium thiocyanate) that interact less strongly than water with L₂. The third class is the non-polar kosmotropes (e.g. nonionic organic compounds) that interact more weakly. These correlations might not be similar when using solvents other than water as a result of the solvent varied environment, which has diverse properties and causes

distinct ion effects. Furthermore, the origin of these classifications was a subject of interest in numerous studies [62–65] and models leading to determining some properties linked to the ions that are responsible for the ion effect in an electrolyte. In his review [63], Leontidis hypothesized that the Hofmeister series are resulting from the combination of the ionic charge distribution, size, shape and hydrophobicity (Fig. 4). By analyzing the ion properties (Fig. 4), it can be noticed that the ion has its specific effect independently of the used solvent which makes the electrolyte part a very crucial component in all applications, especially in electrochemical energy storage systems where the electrolyte properties (ion mobility and diffusion) are reflecting directly the energetic performance of devices. Various interactions such as contact ion pair (CIP), solvent-ion pair (SIP), solvent separated ion pair (SSIP) should be considered. These ions effects can be changed and adapted by choosing the appropriate solvent, condition, and additives as well as the adequate porous electrode. In ECs, the energy storage processes are traditionally attributed to the electrosorption of ions (anions and cations) on micro-porous electrodes forming an electrical double layer. Nevertheless, during the charge-discharge cycles, the ion effects can influence electrode/electrolyte interface which reflects the performance of the device [66]. Frequently, numerical modelling has been widely used to investigate electrolyte mobility in EC systems [67].

Several advanced experimental techniques such as electrochemical

<figure><img src="image_6.png" /><figcaption>Fig. 3. The Hofmeister series of ions in water.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** The major ion properties (solid black box) linked to the effects studied widely (dashed black box) and sparsely (dotted grey box). Reproduced with permission from ref [58].</figcaption></figure>

quartz crystal microbalance (EQCM) [68], electrochemical dilatometry [69], atomic force microscopy [70] and NMR [71] performed in oper-
ando mode are efficiently used to understand the charge-discharge
mechanism of ECs. Griffin et al. investigated the in-pore ion behavior
using the EQCM and the *in-situ* NMR experiments during charging an
activated carbon-based EC performing in an organic electrolyte (tet-
raethylphosphonium tetrafluoroborate (PEt₄ BF₄/ACN) [71]. In their
analysis, they have been using ³¹P and ¹⁹F NMR in order to distinguish
the presence of PEt₄ cations and BF₄ anions. At 0 V the in-pore popu-
lation for the anions and cations is equal. By applying a polarization at a

potential ranging from -1.5 to +1.5 V, the in-pore cations population
increases for negative charging (Fig. 5(b)) and the in-pore anions pres-
ence increases for positive charging (Fig. 5(a)). A large number of
co-ions can be trapped within the micropores during charging in the
positive and negative voltage ranges leading to jumps in the recorded
capacitances. The authors concluded that while charging the system, the
cations are completely ejected from the positive electrode and the anions
continue to be adsorbed. Dilatometry studies have been also used in
order to report the volumetric changes of the electrodes during positive
and negative charging [69,72,73]. The carbon electrode expands by
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** a) and b) *In situ* $ ^{31}P $ and $ ^{19}F $ NMR spectra of individual supercapacitor electrodes at different states of charge. Spectra recorded in the range 0 to 1.5 V. Reprinted with permission from ref [71]. c) Electrochemical dilatometry studies of activated carbon-based ECs operating in 1 mol L$^{-1}$ $Li_2SO_4$ with different pH. Reprinted with permission from ref [73]. d) Mass change recorded during a cyclic voltammetry scan at 5 mV s$^{-1}$ for a wide potential range recorded in EQCM 3-electrode set-up for YP50F electrode operating in 0.1 mol L$^{-1}$ RbI, Reprinted with permission from ref [74].</figcaption></figure>

creating a dilatation while charging and relaxation during discharging
due to different reasons depending on the used electrolyte [72].

In ionic liquids, Hantel et al., reported the dilatometry investigations
of different activated carbon-based ECs working in the ionic liquid 1 mol
L⁻¹ 1-ethyl-3-methyl-imidazolium tetrafluoroborate ([EMIM][BF₄])
with and without the addition of acetonitrile (AN) [72]. In Fig. 5(c)
electrochemical dilatometry diagrams is recorded for activated carbon
ECs operating in 1 mol L⁻¹ Li₂SO₄ at different pH. It can be noticed that
the pH variation influences both the positive and negative electrodes
while charging/discharging processes. The operando techniques were
also used to explore the charging mechanism of an ECs operation in
redox electrolytes [74,75]. Fig. 5(d) displays an EQCM study for acti-
vated carbon-based ECs operating with an iodide redox system in order
to bring insight into the iodide ions and carbon-specific interactions
[74]. From this study different effects of ion properties listed by Leon-
tidis (Fig. 4) have been highlighted in the reported EQCM study to in-
fluence the ions mobility and the charging process in EC devices. Among
these parameters, there is the ions hydration shell, the pore size, and the
ion sizes that are influencing the ion flux trajectory. Overall, the per-
formance of ECs is significantly dependent on the device charging/di-
scharging mechanism and the electrolyte is an essential component. It
affects all processes owing to the ion origin and ion/solvent interactions.
With the large number of ions present within the carbon pores, various
mechanisms occur depending on the electrolyte type and property of
ions, solvent, specific ion sorption and charge distribution.

3. Water in salt (WIS) as electrolytes for ECs

Commonly used electrolytes in ECs are organic or ILs mostly due to
their wide voltage window greater than 3 V. However, owing to the non-

environmental friendly and flammable character of the organic medium
as well as the expensive price of ILs, there is a need for an alternative
which exhibits similar electrochemical characteristics of conventional
EC electrolyte but as well fulfils postulates of green chemistry [42,76,
77]. On the other hand, the aqueous medium is optimistic in terms of
environmental and economic aspects, but due to its narrow electro-
chemical stability window restricted by the thermodynamic limit of
water electrolysis (1.23 V) is not enough adequate [78]. Recently, new
high-concentrated aqueous solutions of salts so-called water in salt
(WIS) have been developed. These electrolytes are prepared according
to the mass and volume ratios (msalt/mwater and Vsalt/Vwater) higher than
1 [79]. They exhibit several advantages having high chemical stability,
considerable ion transport, high safety, environmental friendliness, low
cost, moisture tolerance [80] and wide working temperature range [81].
Moreover, their remarkable benefit is the wide electrochemical stability
window (2–3 V) resulting from lowering the binding degree of water
molecules via the breaking of hydrogen bonds [82–84]. WIS was first
reported by Suo et al. in 2015 as an electrolyte for lithium ion batteries
[79]. They reached an electrochemical stable voltage of 3 V by using 21
mol kg⁻¹ lithium bis(trifluoromethane)sulfonimide (LiTFSI) as an elec-
trolyte. They attributed the wide voltage window to the high concen-
tration of LiTFSI (at 25 °C) that provides a stable electrode/electrolyte
interface preventing oxidation and reduction because of decreased
water activity and due to the limitation of inconvenient TFSI⁻ reduction.
Different studies have been reported on the use of 21 mol kg⁻¹ LiTFSI as
an electrolyte for different aqueous energy storage systems [50,83,
85–91]. Nevertheless, there is still a need of understanding the char-
ging/discharging mechanism of WIS electrolytes in ECs. For that pur-
pose, Yambou et al. studied the effect of salt concentration in LiTFSI
electrolytes on the performance of carbon-based ECs in order to
understand the working mechanism of the WIS [85]. Their thermogravimetric analysis (Fig. 6(a)) revealed the decrease in the sorption of water on the positive porous carbon electrodes due to the low hydration degree of Li⁺ cations in the 20 mol kg⁻¹ LiTFSI electrolyte. Consequently, the TFSI⁻ ions are adsorbed and stored deeply in the porosity of the positive electrode leading to a shift of the oxygen evolution potential.

As a result, a wider voltage window was recorded compared to the 1 mol kg⁻¹ LiTFSI electrolyte. Mahankali et al. performed an in-situ Raman spectroscopy study (Fig. 6(b)) to report the interfacial behavior of WIS electrolytes at porous electrodes with different specific surface area (activated carbon and activated graphene) [92]. The authors applied different positive and negative potentials during a potential sweep from -1.1 V to +1.5 V and recorded Raman spectra of the WIS electrolyte for both porous electrode interfaces. In the case of activated graphene, the Raman spectra Fig. 6(b) show LiTFSI vibrational bands and the typical D-band and G-band carbon peaks for all the applied potentials. The reported spectra of LiTFSI/electrodes interface in the charged and discharged state remain the same. Moreover, there is no shift in Raman peak intensity, suggesting that the ion-rich location does not change with the potential of activated graphene interface. It proves mainly the surface adsorption process but not ion diffusion into the pores. This is

exactly opposite situation to the spectra of LiTFSI/activated carbon interface where the changes of D-band peak intensity are observed. In the activated carbon, the WIS ions diffuse into the pore network and the charging potential is controlling the ion dynamics at the interface. The commercialization of imide-based WISs, such as LiTFSI electrolyte, still faces difficulties given a large amount of salt required, which increases the cost of the material [93]. Therefore, alternative WIS electrolytes with inexpensive salts, such as perchlorate, sulfate, chloride, and nitrate [84,94], have been investigated for ECs [95–97]. Among mentioned salts, sodium perchlorate (NaClO₄) is a promising WIS alternative with high solubility in water (17 mol kg⁻¹ at room temperature) and the same characteristics of water network structure in comparison to 21 mol kg⁻¹ LiTFSI [49]. However, this perchlorate salt presents the drawback of being not safe because of its possible explosive nature and it is difficult to be produced due to its high melting point and the strong oxidation property of ClO₄⁻. As displayed in Fig. 7(a) several water molecules were coordinated with the Na⁺ cation, although there were still some free water molecules which allow the creation of hydrogen bonds with water [96]. Compared to the LiTFSI, the 17 mol kg⁻¹ NaClO₄ electrolyte has significantly higher ionic conductivity (64.2 mS cm⁻¹) and lower kinematic viscosity (5.0 mm² s⁻¹). It was also reported that the ionic conductivity fades gradually at very high concentrations of NaClO₄, but

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** a) Thermogravimetric analysis (mass loss and derivative (DTG)) on aged positive (dotted lines), negative (dashed lines) and soaked (full lines) activated carbon cloth electrodes in 20 mol kg⁻¹ LiTFSI and 1 mol kg⁻¹. Reproduced with permission from ref [85]. b) In-situ Raman experiment at graphene EC electrodes: LiTFSI anion and cation responses with respect to the applied potential (range from -1.1 V to 1.5 V). Schematic illustration of surface adsorption of LiTFSI ions at Gr EC electrodes and cyclic voltammetric response of Gr electrodes at different scan rates (range from 50 mV s⁻¹ to 1000 mV s⁻¹). Reproduced with permission from ref [92].</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** a) 17 mol kg⁻¹ NaClO₄ electrolyte during DFT-MD simulations. Atom colors: Na, green; O, red; H, pink; Cl, yellow; C, brown; N, blue. b) Comparison of electrochemical performance of ECs using identical YP-50F carbon electrodes and different electrolytes. (a,b) reproduced with permission from ref [96]. c) Nyquist plots and (inset) the data in high-frequency ranges for assembled carbon-based SCs using 17 mol kg⁻¹ NaClO₄, 21 mol kg⁻¹ LiTFSI/H₂O, 1.2 mol kg⁻¹ Et₄NBF₄/ACN, 0.8 mol kg⁻¹ Et₄NBF₄/PC electrolytes. d) Ragone plot comparisons. e) Another type of Ragone plot, in which the energy and power densities were normalized by the electrolyte prices. (c,d,e) reproduced from ref [97] with permission from the Royal Society of Chemistry.</figcaption></figure>

remains superior until 17 mol kg⁻¹ [49,97]. In addition to the physicochemical properties, NaClO₄ exhibits an interesting electrochemical characteristic such as a wide stable voltage window (2 V) comparable to the one obtained with some organic electrolytes (Fig. 7(b)). Bu et al. reported the effectiveness of WIS electrolytes as a good alternative to the organic electrolyte in EC devices [97]. The authors compared the WIS electrolytes to the organic ones using the same activated carbon electrode. The Nyquist plot shown in Fig. 7(c) highlights that the smallest equivalent series resistance was obtained with EC based on 17 mol kg⁻¹ NaClO₄ due to the relatively small ion sizes (Na⁺ and ClO₄⁻) as well as exceptional conductivity [97]. By comparing the energy and power performance of such WIS with organic electrolytes, the energy and power of the EC is overlapping each other as seen in Fig. 7(d).

However, by taking into consideration the cost of the materials, the normalized Ragone plot shows a great increase of the energy and power density of NaClO₄ based ECs (Fig. 7(e)). Further attractive WIS electrolyte based on nitrate anion has been proposed by Guo et al. [94]. They performed investigation of ECs based on a 12 mol kg⁻¹ NaNO₃ WIS electrolyte which has the lowest concentration, additional benefit of this

WIS electrolyte is the cost of the salt [94,98]. The EC based on such electrolyte exhibited satisfactory energy characteristics at a relatively high, stable operating voltage of 2.1 V. Contrary to the other WIS, the 12 mol kg⁻¹ NaNO₃ electrolyte demonstrated a high conductivity (198 mS cm⁻¹) at 25 °C, compared to the commonly used 21 mol kg⁻¹ LiTFSI and 17 mol kg⁻¹ NaClO₄. As listed in Table 2, the EC operating in different WIS electrolytes have good capacitive performance as well as long-term stability. Overall, WIS electrolytes are promising candidates for use in energy storage devices such as ECs. They present attractive properties, compared to organic medium and ionic liquids such as higher conductivity, and lower viscosity as well as environmentally friendly character. Nevertheless, achieving high power and energy performances at low-cost needs a deep understanding and investigation of charging/discharging mechanism using advanced techniques.

## 4. Deep eutectic solvents (DES) as electrolytes for ECs

Deep eutectic solvents are new green electrolytes with similar properties to that of ILs [102]. DES are mixtures of at least two

<table><caption>**Table 2**

Characteristics of WIS electrolytes for electrochemical capacitor applications.</caption><thead><tr><th>Electrode materials</th><th>Electrolyte</th><th>Specific capacitance (Fg<sup>-1</sup>)</th><th>Energy density (Wh kg<sup>-1</sup>)</th><th>Power density (W kg<sup>-1</sup>)</th><th>Ref.</th></tr></thead><tbody><tr><td>AC (YP80F)</td><td>20 mol kg<sup>-1</sup> LiTFSI</td><td>94 at 0.2 A g<sup>-1</sup></td><td>-</td><td>-</td><td>[85]</td></tr><tr><td>AC</td><td>26.4 mol kg<sup>-1</sup> CH<sub>3</sub>COONH<sub>4</sub></td><td>50 at 0.2 A g<sup>-1</sup></td><td>6</td><td>~400</td><td>[99]</td></tr><tr><td>AC</td><td>31.3 mol kg<sup>-1</sup> LiTFSI</td><td>117 at 0.8 A g<sup>-1</sup></td><td>5.6</td><td>1680</td><td>[50]</td></tr><tr><td>AC (YP50F)</td><td>8 mol kg<sup>-1</sup> NaClO<sub>4</sub>/(H<sub>2</sub>O)<sub>1.5</sub>(AN)<sub>2.4</sub></td><td>31.9 at 1 A g<sup>-1</sup></td><td>-</td><td>-</td><td>[96]</td></tr><tr><td>CaMoO<sub>4</sub></td><td>7 mol kg<sup>-1</sup> NaOH</td><td>72 at 1 A g<sup>-1</sup></td><td>7</td><td>1010</td><td>[100]</td></tr><tr><td>AC (YP50F)</td><td>12 mol kg<sup>-1</sup> NaNO<sub>3</sub></td><td>32.7 at 1 A g<sup>-1</sup></td><td>20.5</td><td>~1000</td><td>[94]</td></tr><tr><td>AC (YP50F)</td><td>17 mol kg<sup>-1</sup> NaClO<sub>4</sub></td><td>33 at 1 A g<sup>-1</sup></td><td>23.7</td><td>~1000</td><td>[97]</td></tr><tr><td>AC (YP50F)</td><td>14 mol kg<sup>-1</sup> HCOOK</td><td>24 at 2 A g<sup>-1</sup></td><td>10.6</td><td>471</td><td>[101]</td></tr></tbody></table>
components A and B in a given molar ratio with significantly lower melting point compared to the starting one of A and B [54]. The proposal of these solvents was chosen to differentiate them from ILs formed by discrete anions [103]. This eutectic mixture includes anions and cations from different species listed in Fig. 8 and it can be described by the formula:

$$ \mathrm{Cat}^{+} \mathrm{X}^{-} \mathrm{zY} \qquad (1) $$

Four different categories of DES have been classified by the Abbott et al. group depending on the A and B origins [104]. Type I is formed by non-hydrated metal halides (MClx) and quaternary ammonium salts. Since the MClx number is limited, a type II of DES appeared to replace the MClx with hydrated metal halides coupled with organic salts as their inherent/air moisture insensitivity [105]. The type III of DES is consisting of mixtures of organic salts with hydrogen bond donor (HBD) (such as urea, and acetamide) and the last type IV of DES are composed of metal salt hydrate and HBD. Among these categories, the type III and type IV DESs have been the most widely used materials because of their

attractive properties similar to ILs with the privilege of being easy to prepare, biodegradable, environmentally friendly character, hence, suitable for different applications [53,106]. DESs are a promising candidate for ECs comparable to ILs but cost attractive. Compared to the organic commercial electrolytes such as acetonitrile or N, N-dimethylformamide, their cost is comparable to the one of DESs like choline chloride (ChCl) and urea or ChCl and glycerol, without being toxic [107]. In electrochemical energy storage systems, the working temperature range is an important characteristic that distinguishes the electrolytes, and most ECs are limited in their possibility to work at very low and very high temperatures. As seen in Fig. 9(a), the ILs-based ECs have a limited range of applications at a temperature below room temperature, and the conventional aqueous electrolytes are the only candidate performing at low and high temperatures. However, their drawbacks limit their commercialization and industrial use. The DES are the new electrolytes that appear with attractive properties in a wide range of temperature [40]. Viola et al. used an aqueous eutectic mixture of LiCl at 24.8 wt% and water as an electrolyte for ECs [108].

<figure><img src="image_11.png" /><figcaption>Fig. 8. Components of deep eutectic solvents. Reproduced with permission from ref [105].</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** a) Comparison of the temperature windows of operation for the different electrolytes used in ECs, b) Cyclic voltammetry (CV) of ECs operation in DES (LiCl 24.8 wt%) electrolyte at different recorded at 5 mV s⁻¹ at temperature. Reprinted with permission from ref [108] Copyright © 2021 American Chemical Society. c) The Arrhenius plot concerning the ionic conductivity of the Reline DES electrolyte. Reproduced from ref [51] with permission from the Royal Society of Chemistry. d) Walden plot under ambient temperature for: ●1 Choline Chloride (ChCl):2Urea (Reline) this work; ■ ChCl:Butanediol; ▲ ChCl:Ethylene glycol (EG); ▲ ChCl: Glycerine; ◆ Glutaronitrile (GLN): Lithium bis[(trifluoromethyl)-sulfonyl]imide (LiTFSI); ★ 2-methylglutaronitrile (MGLN):LiTFSI; ■ LiTFSI:N-methylacetamide (NMAc); ● (lithium nitrate) LiNO₃:NMAc; ★ lithium bis(fluorosulfonyl)-imide(LiFSI):Formamide (FMD); ♦ Tetraethylammonium chloride (TEAC):EG. The blue line is the “ideal” Walden product line fixed with 1 mol L⁻¹ aqueous KCl solution. Reproduced from ref [51] with permission from the Royal Society of Chemistry.</figcaption></figure>

The authors studied the behavior of the eutectic solvent-based ECs at different negative and positive temperatures and they compared them with organic-based EC (Fig. 9(a)). Based on their reported cyclic voltammograms (Fig. 9(b)), the LiCl based ECs display good capacitive behavior in a wide range of temperature to -70 °C while the organic-based ECs lost their capacitive behavior down to -40 °C. Their findings highlight the possible electrochemical energy storage of ECs also at high temperatures (80 °C) when using DES. Cyclic voltammograms recorded at the different temperatures display a rectangular shape revealing the capacitive behavior of the device. However, their electrochemical stable window is limited to 0.8 V because of the presence of water which influences directly the energy of the device. Other DESs have proved to be air and moisture stable and their electrochemistry is not harshly affected by the ambient atmosphere assembly and the presence of the controlled amount of water does not disturb the intermolecular peculiar interactions [109,110].

Among DES, low-cost, anti-corrosive and environmentally friendly DES named Reline has been recently applied as an electrolyte in ECs with remarkably improved capacitive and energetic properties compared to those of ILs [51,55]. The Reline DES is a mixture of choline chloride and urea at a 1:2 molar ratio [102]. It suffers from low conductivity due to its high viscosity which can be improved by increasing the temperature as can be noticed from the Arrhenius plot Fig. 9(c) or by the addition of water as displayed in the Walden plot Fig. 9(d). The cell

assembly was performed at room temperature without the need for inert environment conditions and the Karl-Fischer coulometry has shown the presence of 1900 ppm residual water [51]. EC was operating with a wide electrochemical stable window (2.2 V as displayed in Fig. 10(a). Additionally, the EC displayed superb capacitive and energetic performances Fig. 10(c) comparable to the organic electrolyte based ECs and better than the aqueous electrolyte based ECs. Furthermore, the long-term characteristics of the ECs operation in Reline and Reline+water electrolytes reveal high stability (>50 000 cycles) as seen in Fig. 10(b).

This is due to their anticorrosive character towards stainless steel current collectors as can be noticed from the Tafel and Raman plots in Fig. 10(d–f) [55]. It is well illustrated that stainless steel samples treated by Reline DES do not show any corrosive products. By following the ecofriendly and low-cost strategies, Reline DES is a promising candidate for the green low-cost and highly stable ECs. Many DESs with a wide voltage window (2.4 V) are hygroscopic, and their application in ECs requires the use of an inert environment while assembling. The widely investigated DES in EC application are based on tetraethylammonium bromide (TEAB) and lithium bis[(trifluoromethyl)sulfonyl]imide (LiTFSI) mixed with HBD [111–114]. Zaidi et al. were able to expand the electrochemical stable voltage window to 1.8 V at high temperature (80 °C) by using a DES based on N-methylacetamide and lithium salt as electrolyte for activated carbon-based supercapacitors [114]. Zhong et al. studied ECs based on choline chloride and ethylene glycol DES made
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** a) The CV curves recorded at 5 mV s⁻¹ for BP2000-based EC operating in Reline DES. b) Long-term cycling (1 A g⁻¹) of the EC based on 3DG using the pure Reline DES at 2.2 V. c) Ragone plots of 3DG based supercapacitors operating in DES, DES + 1% water and 1 M Li₂SO₄. (a,b,c) reproduced from ref [51] with permission from the Royal Society of Chemistry. d) Tafel plot of stainless steel 316 L disc electrode in Reline DES electrolyte. e) and f) Schematic presentation of anticorrosive property of Reline DES in EC adapted with permission from ref [55].</figcaption></figure>

with different molar ratios [41]. Their results show remarkable thermal stability of the tested devices with an improved energy density at a wide temperature range varying from RT to 115 °C with a maximum voltage window of 2 V (Fig. 11).

Overall, DESs have many common characteristics with ILs making

their application attractive electrolytes for ECs and more beneficial due to their cost. However, numerous mixtures have been presented as DES without any exact characterization of their phase diagrams. In addition, some researchers poorly characterized the electrochemistry behavior of their DES-based ECs [115].

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** The DES-1-1/DES-1-2/DES-1-4 samples under different temperatures from RT to 115 °C: (a,d,g) Specific capacitance; (b,e,h) Ragone plots; (c,f,i) Cycling stability. Reproduced with permission from ref [41].</figcaption></figure>
## 5. Hybrid electrochemical capacitors

Generally, the electrolyte nature of the activated carbon based ECs is influencing their energetic performance as well as their manufacturing cost [116]. As reported in this review, by changing the electrolyte type, the ECs are behaving differently with remarkably improved characteristics when using DES or WIS electrolytes. However, for industrial applications, the thermal stability of the electrolyte is an important issue to deal with. Aqueous-based electrolytes offer great thermal stability in addition to low assembling cost, sustainability and reasonable potential window [42]. Through the common ability of water to dissolve salts, several approaches and additives have been combined easily to develop aqueous electrolytes with novel properties leading to improve EC performances [75,117–119]. The first report on hybrid capacitors was proposed by Conway in 1991 using metal oxide-based redox electrodes combined with nanoporous EDL carbon electrodes [120]. Redox-active species can be added to aqueous electrolytes supplying rapid redox reactions to provide high ionic conductivity and enhance the energy density of the devices. Consequently, a new battery-like behavior is created in EC due to the occurred faradic redox reactions (Fig. 12(a)) which led to a hybrid system with a larger energy. Particular interest has been given to iodide-based hybrid capacitors due to their special electrochemical properties [74]. Lota et al. have been the first group to show faradic reactions by using iodide as an electrolyte [121]. Apart of good ionic conductivity, the iodide electrolyte provides a remarkable increase in capacitance owing to its redox activity as shown in Fig. 12(b). Beguin et al. reported a doubling of the capacitance of carbon-based EC operation in 1 mol L⁻¹ Li₂SO₄ when 0.5 mol L⁻¹ KI is added Fig. 12(c) [122]. They reported a long-term study of their iodide-based hybrid capacitor using floating testing at 1.6 V that showed a stable capacitance for more than 120 h. After its great advancement in the DES electrolytes, iodide choline salts Fig. 12(d) were also used as an electrolyte for hybrid capacitor electrolytes operation at low temperatures [118].

In aqueous-based ECs, few studies reported the application of these ECs in low temperatures by adding organic additives such as methanol [123] and ethylene glycol [124]. On the other hand, Przygocki et al.

proposed a choline aqueous-based electrolyte operating in negative temperatures without any organic additives [118]. Their hybrid system consisted of a mixture of non-corrosive choline ions (choline nitrate) with a redox-active component (choline iodide). Their electrochemical CV Fig. 12(e) and GCD Fig. 12(f) testing confirm the effectiveness of their choline electrolyte and proving a bi-functional property of the system leading to the implementation of a hybrid capacitor operating in a wide range of temperatures down to –40 °C with good performance using costless materials.

## 6. Challenges in developing electrochemical capacitors

Despite considerable progress in the field of EC electrolytes, several major challenges need to be considered for further EC development and its commercialization. The main objective is to elaborate a safe and low-cost device offering high capacitive and power performance with a long-term cycle life and an environmentally friendly character. Hence, several challenges may be faced towards the achievement of this purpose (Fig. 13). The most reported articles are focused on the electrolyte/ electrode interaction. However, it is necessary to consider the possible interaction between the electrolyte and the inactive components including the current collectors, separators and binders. Generally, the choice of a current collector is based on its chemical and electrochemical stability towards the used electrolyte. The certain electrolyte can strongly affect the EC behavior by provoking current collector corrosion [55]. In an acidic electrolyte, the used current collector is based on corrosion-resistant gold. To adopt a costless process, several cheap alternatives such as carbon-based material [125], indium tin oxide (ITO) [126], and stainless steel [55] have been used. Lately, a new free-standing electrodes have been developed and applied to flexible devices, avoiding the use of additional current collectors [127]. Additionally, binders can also interact with the electrolyte which influences the EC performance. Polytetrafluoroethylene (PTFE) is the most often used binder. Due to its nature, excessive use of PTFE would make the electrode film too hydrophobic, which would affect the wettability of the electrode and block the electrolyte impregnation process [128]. The

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** a) Schematic illustration of the typical CV curves characteristics of a hybrid capacitor. b) Voltammetry curve at 5 mV s⁻¹ for carbon-based ECs operation in 1 mol L⁻¹ KI solution. Reproduced with permission from ref [121]. c) CVs recorded at 2 mV s⁻¹ of activated carbon cells operating in 1 mol L⁻¹ Li₂SO₄ + 0.5 mol L⁻¹ KI (solid line) and 1 mol L⁻¹ Li₂SO₄ (dashed line). Reproduced with permission from ref [122]. d) Chemical structure of choline cation. e) CVs and f) GCD curves of carbon-based EC operating in 5 mol kg⁻¹ ChNO₃ + 0.5 mol kg⁻¹ ChI from room temperature to –40 °C. (e,f) reproduced with permission from ref [118].</figcaption></figure>
Challenges of EC Electrolytes

<figure><img src="image_16.png" /><figcaption>Fig. 13. Challenges in developing electrochemical capacitors.</figcaption></figure>

separator is another inactive component that can react with the elec-
trolyte and affect the EC performance, especially in redox active and
hybrid devices that need good mechanic, chemical and electrochemical
stability while long-term performing. The choice of inappropriate sep-
arators may lead to a self-discharge process [129] and change the in-
ternal EC resistance [130]. Depending on the electrolyte nature, the
optimization of the EC inactive components should be carefully
considered.

The electrochemical stability window, ionic conductivity, chemical and thermal stability as well as operating temperature range significantly contribute to the EC performance and affect their commercialization. A wide voltage range influences the EC energy and power density. This might be possible through the use of organic or ionic liquid electrolytes owing to the water-splitting limitations of aqueous electrolytes. However, this increased cell voltage may lead to the deterioration of EC metrics. When replacing the water-based electrolyte with ILs, the cell voltage can reach 4 V but with poor ionic conductivity caused by the high viscosity, lower specific capacity and high cost. Furthermore, the lower conductivity of ILs and organic electrolytes can increase the EC internal resistance leading to the power decrease. One further aspect of IL and organic electrolytes is their purification and synthesis process which may influence the EC performance if traces of water remain in the solution. Such traces could influence the self-discharge rate and the electrochemical stable window, causing a drop in energy and power densities [131]. With the development of new electrolytes used in ECs (such as solid-state electrolytes, ILs, and DESs), there remains an ongoing challenge to evaluate and understand the charge-discharge mechanism of these new systems. Different in situ and operando experimental advancements have been established. However, there are no proper and standardized characterization methods to evaluate the performance of these new EC electrolytes to provide a comparison with the already existing electrolytes. The performance of ECs reported in the literature has been identified under different conditions. Generally, the specific capacitance and energy density values are calculated as a function of the mass of the active material. However, the electrolyte is also contributing to the total mass as well as to the cost of the EC. Therefore, it makes it ambiguous to select the best electrolyte for the ECs.

Overall, it is a challenge to develop EC with improved energy density without sacrificing other advantages such as high-power density and durability. This would be achieved through the optimized matching between the electrolyte and EC components and the fundamental understanding of the charging–discharging mechanism. By overcoming these electrolyte technical challenges, the EC may open up to new features and opportunities following the market requirement [7]. The tremendous potential market will also provide unlimited opportunities

for EC development. With the rapid progress of portable and wearable electronics and flexible energy storage devices, the design of the conventional ECs restricts their implementation in such devices. For this purpose, most of the reported research is focused on the development of flexible electrodes. By proposing a novel solid state and flexible electrolyte some new applications can be achieved matching the market requirements for the next-generation flexible storage devices. The hybridization of the electrochemical energy storage system is an effective way to solve the problem of low energy density in ECs. Introducing some new electrolytes with redox activities will contribute to the increase of the EC energy density through the redox reactions resulting from the electrolyte and occurring on the electrode surfaces [20]. Furthermore, the achievement of the future commercialization of EC is directly linked to the material cost. Improving product performance and reducing production cost is the most important criterion considered by industry. In this context, by developing new low-cost raw materials for electrolytes and electrodes, as well as improving the production processes, the manufacturing cost will be significantly reduced leading to an effective EC progress.

7. Conclusions

This review summarizes the importance of electrolyte choice in electrochemical capacitors. Indeed, the electrolytes are found to be a pivotal component of EC responsible for the device performance. It influences the vital parameters of an EC including the voltage window, long-term cyclability, power and energy density, as well as the device cost and environmental sustainability. The characteristics of the electrolytes such as ionic conductivity, viscosity, and thermal and chemical stability are the major parameters influencing the operation of ECs. Hence, it is necessary to choose electrolytes with the appropriate physicochemical properties. Furthermore, understanding the charging/ discharging mechanism by studying the ions population in the electrode/electrolyte interface depending on the polarity, ion mobility, ion-solvent interactions, specific ion sorption, charge distribution leads to the improvement of the EC performance. For that purpose, several operando and in-situ techniques have been developed. Moreover, to overcome the expensive ILs, and the toxic organic electrolytes, several alternatives have been reported including the modification of the previously existing aqueous electrolyte by changing their properties. The highly concentrated water in the salt solutions are showing attractive properties as electrolytes for ECs with a wide voltage range (2–3 V) and high energy density. This is due to the bonding of water molecules in the solvation sheath of the salt cations, that gives extension of EC electrochemical window. Another new candidate, i.e., DES has properties such as the IL. Considering the significant economic and environmental
strategy, low cost and sustainability are key parameters for the large scale EC development. The non-flammable, moisture tolerant Reline DES fulfils conditions for EC operating with wide voltage range. However, the further attention should be focused on the EC operation at low and high temperatures. Furthermore, hybrid systems which combine EDL and faradaic reactions (e.g. iodide/iodine pair) offer better performance than conventional devices. This review gives an overall insight into the importance of the electrolyte in EC performance. Yet, the further efforts must be implemented in searching for possible alternatives to overcome some electrolyte drawbacks. With growing popularity of new energy vehicles and smart wearable devices, the development and commercialization of ECs can be expected.

## CRediT authorship contribution statement

Sara Azmi: Conceptualization, Writing – original draft. Amelia Klimek: Writing – original draft. Elzbieta Frackowiak: Conceptualization, Writing – review & editing, Supervision, Funding acquisition.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

## Acknowledgments

The authors would like to acknowledge OPUS project financed by the National Science Centre, Poland (project no. 2018/31/B/ST4/01852) and partially (0911/SBAD/2301) for the financial support.

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