**Electrochemical Capacitor Performance: Influence of Aqueous Electrolytes Aqueous Electrolytes**

**Electrochemical Capacitor Performance: Influence of** 

DOI: 10.5772/intechopen.70694

Rajendran Ramachandran and Fei Wang Rajendran Ramachandran and Fei Wang Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

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

#### **Abstract**

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50 Supercapacitors - Theoretical and Practical Solutions

10.1016/j.jpowsour.2009.11.146

DOI: 10.1016/j.jpowsour.2016.03.044

Due to low energy characteristics such as energy density and cyclic life, it is mandatory to enhance the energy characteristics of the supercapacitors (ESs). Electrolytes have been recognized as the most prominent ingredients in electrochemical supercapacitor performance. Most commercially available ESs use organic electrolytes and have some advantage like wide operating voltage. However, compared with aqueous alternatives, organic electrolytes are expensive, flammable, and, in some cases, toxic. It is reliable to assert that even though aqueous electrolytes examined by a cramped working voltage, the ions present in them are yet capable of incredibly faster carrier rates than organic electrolytes and can achieve better performance of ESs. Thus, efforts turned toward enlarging the working voltage window of aqueous electrolytes to increase overall operating potential and energy density of supercapacitor devices. This book chapter comprises the latest accomplishments in this area and provides an insight into the aqueous electrolyte advancement.

**Keywords:** supercapacitors, energy density, aqueous electrolyte, operating voltage, electrolyte

#### **1. Introduction**

The dramatic global warming and the accessibility to fossil fuels in the earth require society to shift in the direction of renewable and sustainable resources. Due to this, the growth and ramp-up of sustainable, clean energy sources, as well as their associated technologies, are taken into account worldwide as a critical problem. The majority of the renewable and clean energy sources depend on the countrywide weather conditions. Among different energy storage systems, the electrochemical energy storage (EES) systems including batteries, fuel cells, as well as electrochemical capacitors or supercapacitors (ESs) are most efficient and frequently used in several applications [1]. The most common characteristic of these three

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

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 to seeking new electrode materials.

performance, aside from the operating voltage, the electrolytes have substantially influenced on the other parameters such as power density, cycling stability, operating temperature, equivalent series resistance, life time, and self-discharge rate of the capacitors [8]. The electrolyte ionic conductivity performs a serious role in the internal resistance of supercapacitors. It is highly crucial that the electrolyte ion size should be equal or less than that of the pore size of electrode material to possess a high capacitance and a high power density [9]. For a few cases, the freezing point and viscosity of electrolytes also affect the thermal stability of supercapaci-

Electrochemical Capacitor Performance: Influence of Aqueous Electrolytes

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

53

An ideal electrolyte of supercapacitor needs to have some fundamental requirements: (1) broad potential window, (2) a wide range of working temperature, (3) high ionic conductivity, (4) low viscosity, (5) high electrochemical stability, (6) environmentally friendly, (7) low cost, and (8) low flammability. Each electrolyte has its merits and drawbacks, and it is feasible to meet all the above specifications with one electrolyte. Nonstop and tremendous research efforts have been made in the present and will also keep going in the future for the electrolyte

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].

tor performance, and hence the working voltage range would be shifted [10, 11].

development investigation.

**2. Types of electrolyte**

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

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

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 the potential choice of 1.0–1.3 V due to H<sup>2</sup> /O2 evaluation reactions [7]. Since the interaction involving the electrode and electrolyte acts an essential function in the overall supercapacitor

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

performance, aside from the operating voltage, the electrolytes have substantially influenced on the other parameters such as power density, cycling stability, operating temperature, equivalent series resistance, life time, and self-discharge rate of the capacitors [8]. The electrolyte ionic conductivity performs a serious role in the internal resistance of supercapacitors. It is highly crucial that the electrolyte ion size should be equal or less than that of the pore size of electrode material to possess a high capacitance and a high power density [9]. For a few cases, the freezing point and viscosity of electrolytes also affect the thermal stability of supercapacitor performance, and hence the working voltage range would be shifted [10, 11].

An ideal electrolyte of supercapacitor needs to have some fundamental requirements: (1) broad potential window, (2) a wide range of working temperature, (3) high ionic conductivity, (4) low viscosity, (5) high electrochemical stability, (6) environmentally friendly, (7) low cost, and (8) low flammability. Each electrolyte has its merits and drawbacks, and it is feasible to meet all the above specifications with one electrolyte. Nonstop and tremendous research efforts have been made in the present and will also keep going in the future for the electrolyte development investigation.
