**2. Types of electrolyte**

devices is that the energy-producing processes perform at electrode/electrolyte interfaces. In both academia and industry, supercapacitors have drawn much importance due to benefits of high power density and long cyclic life compared with batteries and fuel cells [2]. By their charge storage process, ES is divided into three categories: (1) electric double layer capacitors (EDLC), (2) pseudocapacitors, and (3) hybrid capacitors [3]. Due to the technical maturity of EDLCs, almost all of the commercially available supercapacitors are made up by using EDLC electrode materials such as activated carbon. The low energy density (~10 Wh kg−1 for commercial supercapacitors) is truly the most significant challenge for supercapacitors in comparison to rechargeable batteries and fuel cells [4]. For that reason, enormous research attempts were carried out aiming to enhance the energy density of supercapacitors [5]. The important characteristic to get extraordinary energy density of supercapacitors is shown in **Figure 1**. As seen from the figure, the energy density of supercapacitors is directly proportional to the capacitance and square of the working voltage. Therefore, enhancing the capacitance and improving the working potential are considered as promising approaches to further improve high energy density supercapacitors. The high energy density can be attained by choosing appropriate electrode material with a high specific capacitance and electrolyte with a large operating voltage. Considering that the energy density is directly proportional to the square of the voltage, increasing the working potential window could be a more efficient way to improve the energy density rather than to improve the specific capacitance. Therefore, developing a new electrolyte with a large potential window is the top priority effort in comparison

It is well recognized that the working potential of the supercapacitors is highly relying on the electrochemical stability of the electrolytes. For example, organic electrolytes and ionic liquid (IL)-derived supercapacitors can easily be handled at a large potential window of 2.5–2.7 and 3.5–4.0 V, respectively [6]. However, the electrodes are steady in aqueous electrolytes within

/O2

involving the electrode and electrolyte acts an essential function in the overall supercapacitor

evaluation reactions [7]. Since the interaction

to seeking new electrode materials.

52 Supercapacitors - Theoretical and Practical Solutions

the potential choice of 1.0–1.3 V due to H<sup>2</sup>

**1.1. Effect of the electrolyte on supercapacitor performance**

**Figure 1.** Important characteristic of high energy density supercapacitors.

Following the nature of electrolyte like the ion type, ion size, ion concentration, and the interplay among the ion and solvent, various electrolytes have been developed and examined currently. The electrolyte can be divided into three groups such as liquid, solid or semisolid, and redox-additive as shown in **Figure 2**. The liquid electrolyte can be further classified into aqueous electrolyte, nonaqueous electrolyte, and organic electrolyte [12].

**Figure 2.** Classification of supercapacitor electrolytes.

As mentioned previously, no single electrolyte can meet all the requirements. For example, both high capacitance and ionic conductivity can be achieved by aqueous electrolytes, but the narrow decomposition voltage made the lower operating voltage of aqueous electrolytes. Though organic and ionic liquid electrolytes provide a wide operating voltage, they normally suffered from the low conductivity (large internal resistance), high cost, and high flammability which cause continual trouble during application in supercapacitor device. In this regard, extensive efforts are committed to the successful development of the overall performance of electrolytes. Some strategies have already been strived to enhance the electrolyte performance, including "(1) developing an entirely new electrolyte as well as improving its efficiency concerning a wide range of functioning voltage, high ionic conductivity, large working temperature range, and so on [13]; (2) investigation of the effect of electrolytes on capacitance, energy and power densities, thermal stability, and self-discharge rate of supercapacitors [14]; and (3) initiate and becoming familiar with a fundamental concept of the electrolyte on supercapacitor performance over sophisticated characterization, modeling, and simulation utilities [15]."

The high conductivity of the aqueous electrolyte is propitious for reducing equivalent series

To evaluate the overall performance of aqueous electrolytes, some typical criteria should be taken into consideration such as the dimensions of hydrated and bare ions (**Table 2**), the flow

The aqueous electrolytes can be categorized into three groups such as alkaline, acid, and neu-

the electrolyte ionic conductivity is dependent on the concentrations, optimum molar concentrations have already been investigated to attain the highest possible ionic conductivities of a given electrolyte at particular temperature. So far, the electrolyte ionic conductivity can be quickly dropped when the concentration becomes extremely low or excessively high. So,

) is the most frequently used aqueous acid electrolyte in supercapacitor

Electrochemical Capacitor Performance: Influence of Aqueous Electrolytes

SO4

SO4

http://dx.doi.org/10.5772/intechopen.70694

SO4

**(S cm<sup>2</sup> mol−1)**

solution, especially for carbon-based

**Gibbs energy (kcal mol−1) Ion conductivity** 

, and Na<sup>2</sup>

SO4 , 55

) [16]. Since

resistance (ESR) which leads to significantly high power density supercapacitors.

of ions which alters the ionic conductivity, as well as the specific capacitance.

tral solutions. The most commonly used aqueous electrolytes are KOH, H<sup>2</sup>

simply because of its extremely high conductivity (~0.8 cm−1 at 25°C for 1 M H2

**Size of hydrated ions** 

H+ 1.15 2.80 350.1 Li+ 0.60 3.82 138.4 38.69 Na<sup>+</sup> 0.95 3.58 162.3 50.11 K+ 1.33 3.31 179.9 73.5

<sup>+</sup> 1.48 3.31 – 73.7

<sup>+</sup> 0.72 4.28 – 106.12

<sup>+</sup> 1.00 4.12 – 119

<sup>+</sup> 1.35 4.04 – 127.8 Cl<sup>−</sup> 1.81 3.32 – 76.31

<sup>−</sup> 2.64 3.35 – 71.42

2− 2.90 3.79 – 160.0 OH<sup>−</sup> 1.76 3.00 – 198

<sup>−</sup> 2.92 3.38 – 67.3

3− 2.23 3.39 – 207

2− 2.66 3.94 – 138.6

**Table 2.** Size of electrolytic bare and hydrated ions and their conductivity [10].

**(Å)**

respectively.

**3.1. Acid electrolytes**

SO4

the greater number of research studies uses 1 M H2

Sulfuric acid (H2

supercapacitors.

NH<sup>4</sup>

Mg2

Ca2

Ba2

NO<sup>3</sup>

SO4

ClO4

PO4

CO3

**Type of ion Size of bare** 

**ion (Å)**
