**4. Electrolyte-mediated operating voltage**

Electrolyte is one of the key components of SCs, basically, conveying ionic current and leading the formation of electrical double layer, more importantly, under certain circumstances of matched electrode materials and electrolytes, and facilitating reversible redox processes for larger amount of charges stored in the interfaces. In general, electrolytes used in SSCs can be sorted into two main categories: liquid electrolytes and solid/quasi-solid state electrolytes. Liquid electrolytes can be further sorted into three groups: aqueous electrolytes, organic electrolytes, and ionic liquids (ILs), while solid/quasi-solid state electrolytes can be divided into organic electrolytes and inorganic electrolytes [7]. There is no once-for-all solution for electrolyte selection, each electrolyte has its own advantages and disadvantages. For instance, in the group of liquid electrolytes, aqueous electrolytes exhibit very high ionic conductivity, low costs, and high safety, yet they can only deliver an operating potential window of about 1.0–1.3 V because the water splitting potential window is about 1.23 V. While the organic electrolytes and IL electrolytes can be operated at much higher voltages, typically organic electrolytes at 2.5–2.7 and IL electrolytes at 3.5–4.0 V. However, the expensive costs and potential risks for environment strongly hinder their practical applications. According to previous literatures, many aspects of electrolytes can have significant influence on the capacitive performance; here, in this short chapter, we focus on two main aspects: (a) the interaction between electrolyte and electrode materials and (b) the stable potential window. We will discuss the abovementioned two important aspects according to different types of liquid electrolytes (**Table 2**).

oxidation as example, H<sup>2</sup>

tron, the excessive electron in OH−

for HER and thus enabled a capacitor-like Li<sup>+</sup>

**Scheme 3.** Schematic depictions of (a) capacitive absorption and (b) water oxidation.

After complex processes of H<sup>+</sup>

OH−

tral Li<sup>2</sup>

SO<sup>4</sup>

O molecules and OH−

 serve as charge carriers to provide capacitance. Initially, these oxygen-containing species are strongly absorbed on electrode surface to supply EDL capacitance. Once the binding force driven by applied potential was large enough the dielectric layer becomes conductive for elec-

splitting at cathode is completed, as schematically depicted in **Scheme 3**. There seems to be one threshold for the absorption mode turning from capacitive absorption to water splitting, and this threshold is highly related with the toughness of absorption and the capability of electrode materials catalyzing the complex processes for water splitting and also the potential applied. In order to broaden the operating voltage window of SSCs, it is feasible through the suppression of splitting of electrolytes, for instance, in aqueous electrolytes, that can be done through the passivation of water splitting activity on the water splitting-inactive electrode surface. For instance, Zheng et al. developed a conductive polymer of poly(naphthalene four formyl ethylenediamine) (PNTE) and used them as anode materials for aqueous rechargeable Li-ion battery. The passivated hydrogen evolution activity of PNFE exhibited very large overpotential

Thereafter, Zhang et al. reported the fabrication of highly porous carbon materials with high carbon content (>93 at.%) and limited heteroatom doping. The as-synthesized carbon materials exhibited very sluggish water splitting performance, their stably operated voltage window for water splitting can be greatly broadened to 2.2 V or even higher on the carbon materials in neu-

hydrogen and oxygen at 1.3–1.8 V, depending on the types of aqueous electrolytes applied.

electrolyte, as shown in **Figure 3** [47]. Normally, water molecules can be split into

would be readily transferred from OH−

depletion, O=O bond formation, and desorption of O<sup>2</sup>

Toward High-Voltage/Energy Symmetric Supercapacitors via Interface Engineering

can both act as feeds for water oxidation while

http://dx.doi.org/10.5772/intechopen.73131

storage kinetics in aqueous electrolytes [46].

to current collector.

, the water

127

#### **4.1. Aqueous electrolytes**

There are many advantageous merits of aqueous electrolytes, for instance, high ionic conductivity (up to ~1 S cm−1) that can deliver very high power density, high safety that greatly simplifies the fabrication and assembly processes, and low costs that enables broad practical applications. Generally, aqueous electrolytes have three main classes: acidic electrolytes (H<sup>2</sup> SO<sup>4</sup> , H<sup>3</sup> PO<sup>4</sup> , etc.), basic electrolytes (KOH, NaOH, etc.), and neutral electrolytes (Li<sup>2</sup> SO<sup>4</sup> , Na<sup>2</sup> SO<sup>4</sup> , NaCl, etc.). Due to the narrow chemically stable window of water, the operable voltages for SSCs assembled using aqueous electrolytes are usually lying within 1 V. Consequently, the energy stored in single one SSCs is limited, which is far less competitive with organic and IL SSCs, not to mention batteries. Many efforts have been taken to broaden the safe working voltage of aqueous SSCs, which is mainly through the suppression of water splitting at electrode interfaces.

To understand the phenomenon occurring at the electrode/electrolyte interfaces, we need to look into the details of potential-driven water splitting at electrode interfaces. Water splitting basically requires three main steps: absorption, conversion, and desorption. Take water


**Table 2.** The comparative properties of different types of electrolytes.

oxidation as example, H<sup>2</sup> O molecules and OH− can both act as feeds for water oxidation while OH− serve as charge carriers to provide capacitance. Initially, these oxygen-containing species are strongly absorbed on electrode surface to supply EDL capacitance. Once the binding force driven by applied potential was large enough the dielectric layer becomes conductive for electron, the excessive electron in OH− would be readily transferred from OH− to current collector. After complex processes of H<sup>+</sup> depletion, O=O bond formation, and desorption of O<sup>2</sup> , the water splitting at cathode is completed, as schematically depicted in **Scheme 3**. There seems to be one threshold for the absorption mode turning from capacitive absorption to water splitting, and this threshold is highly related with the toughness of absorption and the capability of electrode materials catalyzing the complex processes for water splitting and also the potential applied.

instance, in the group of liquid electrolytes, aqueous electrolytes exhibit very high ionic conductivity, low costs, and high safety, yet they can only deliver an operating potential window of about 1.0–1.3 V because the water splitting potential window is about 1.23 V. While the organic electrolytes and IL electrolytes can be operated at much higher voltages, typically organic electrolytes at 2.5–2.7 and IL electrolytes at 3.5–4.0 V. However, the expensive costs and potential risks for environment strongly hinder their practical applications. According to previous literatures, many aspects of electrolytes can have significant influence on the capacitive performance; here, in this short chapter, we focus on two main aspects: (a) the interaction between electrolyte and electrode materials and (b) the stable potential window. We will discuss the abovementioned two important aspects according to different types of liquid

There are many advantageous merits of aqueous electrolytes, for instance, high ionic conductivity (up to ~1 S cm−1) that can deliver very high power density, high safety that greatly simplifies the fabrication and assembly processes, and low costs that enables broad practical applications.

to the narrow chemically stable window of water, the operable voltages for SSCs assembled using aqueous electrolytes are usually lying within 1 V. Consequently, the energy stored in single one SSCs is limited, which is far less competitive with organic and IL SSCs, not to mention batteries. Many efforts have been taken to broaden the safe working voltage of aqueous SSCs, which is mainly through the suppression of water splitting at electrode interfaces.

To understand the phenomenon occurring at the electrode/electrolyte interfaces, we need to look into the details of potential-driven water splitting at electrode interfaces. Water splitting basically requires three main steps: absorption, conversion, and desorption. Take water

**Advantage** High conductivity High voltage window High thermal and chemical

conductivity,

**Aqueous electrolyte Organic electrolyte Ionic liquid**

SO<sup>4</sup> , H<sup>3</sup> PO<sup>4</sup>

stability, wide voltage

window

High viscosity

SO<sup>4</sup> , Na<sup>2</sup> SO<sup>4</sup> , etc.),

, NaCl, etc.). Due

Generally, aqueous electrolytes have three main classes: acidic electrolytes (H<sup>2</sup>

**Operation voltage window (V)** ≤1.2 2.5–2.8 2–6 **Ionic conductivity**, **σ (ms cm−1)** High Low Very low **Viscosity**, **η** Low Medium/high High **Cost** Low Medium/high Very high **Work temperature** Narrow Wide Wide **Toxicity** Low Medium/high Low

**Disadvantage** Low voltage window Large electrolyte ions, low

**Table 2.** The comparative properties of different types of electrolytes.

basic electrolytes (KOH, NaOH, etc.), and neutral electrolytes (Li<sup>2</sup>

electrolytes (**Table 2**).

**4.1. Aqueous electrolytes**

126 Supercapacitors - Theoretical and Practical Solutions

In order to broaden the operating voltage window of SSCs, it is feasible through the suppression of splitting of electrolytes, for instance, in aqueous electrolytes, that can be done through the passivation of water splitting activity on the water splitting-inactive electrode surface. For instance, Zheng et al. developed a conductive polymer of poly(naphthalene four formyl ethylenediamine) (PNTE) and used them as anode materials for aqueous rechargeable Li-ion battery. The passivated hydrogen evolution activity of PNFE exhibited very large overpotential for HER and thus enabled a capacitor-like Li<sup>+</sup> storage kinetics in aqueous electrolytes [46]. Thereafter, Zhang et al. reported the fabrication of highly porous carbon materials with high carbon content (>93 at.%) and limited heteroatom doping. The as-synthesized carbon materials exhibited very sluggish water splitting performance, their stably operated voltage window for water splitting can be greatly broadened to 2.2 V or even higher on the carbon materials in neutral Li<sup>2</sup> SO<sup>4</sup> electrolyte, as shown in **Figure 3** [47]. Normally, water molecules can be split into hydrogen and oxygen at 1.3–1.8 V, depending on the types of aqueous electrolytes applied.

**Scheme 3.** Schematic depictions of (a) capacitive absorption and (b) water oxidation.

First, proper organic solvents can be chosen based on the following rules: high solubility for electrolyte salts, low viscosity to facilitate ionic transport, no side reaction with other parts of supercapacitor (including active materials, current collector, and separator), wide work temperature, and environmental friendliness. Among all organic electrolytes, the most widely used solvents are acetonitrile (AN) and propylene carbonate (PC). AN is capable of dissolving large quantities of salts but it is toxic and risky to environment. PC, a green solvent, has been widely used, meanwhile, it also has very wide stable working temperature and good conductivity. Also, other electrolytes such as γ-butyrolactone (GBL), N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMA) are applicable electro-

Second, the most common used electrolyte salts are chain-like quaternary ammonium

ating voltage window for organic electrolyte is through the increasing of electrolyte salts concentration. On one hand, one can find the optimized salt concentration that has the best solubility and chemical stability; on the other hand, it is one feasible way to thicken the layer of charge carriers instead of solvent molecule layer at the electrode/electrolyte

solvent showed very high electrochemical stability on the interface due to its spiro rings

sodium salt are also applicable in carbon-based SSCs following the same rules for electro-

Low-temperature ionic liquids (ILs) are pure organic salts containing no solvents with melting points below 100°C. If the liquid state can maintain at ambient temperature, they are called room temperature ionic liquids (RTILs). RTIL are the type of ILs of broad interests to supercapacitors especially SSCs due to their unique properties including non-volatility, poor combustibility and high resistance to heat. However, the ionic conductivity of ILs usually fall in 0.1–15 mS cm−1, which is much lower than most of the commercial organic electrolytes. But ILs can show excellent conductivity at high temperature because of low viscosity. Many kinds of ILs have been widely used in supercapacitor; the two pairing ions of imidazole and pyrrole are most commonly studied. Generally speaking, the size and symmetry of cations strongly influence the melting points of ionic liquids. Normally, the imidazole possesses high conductivity with narrow potential window, alkylpyrrole shows wide operating voltage but high melting point. For instance, Chen et al. used a series of sponge-like carbon

tance of as-made SSCs can be improved to 445 F g−1 with a high discharge voltage of 4 V, very

as electrode and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF<sup>4</sup>

methylimidazolium hexafluorophosphate (BMIM-PF<sup>6</sup>

good rate capability, and cycling stability [52].

interfaces. For instance, 1 mol L−1 spiro-bipyrrolidinium tetrafluoroborate (SBP-BF<sup>4</sup>

) and triethylmethylammo-

http://dx.doi.org/10.5772/intechopen.73131

/PC shows higher conduc-

) and 1-Butyl-3-

) as electrolytes; the gravimetric capaci-

/PC [51]. Other metal salt such as lithium/

) in PC

129

) [49]. One feasible method to further enlarge the oper-

Toward High-Voltage/Energy Symmetric Supercapacitors via Interface Engineering

salts such as tetraethylammonium tetrafluoroborate (TEA-BF<sup>4</sup>

molecular structure [50]. Yu et al. reported 1.5 mol L−1 SBP-BF<sup>4</sup>

tivity of 17 mS cm−1 than 1.5 mol L−1 TEMA-BF<sup>4</sup>

lytes that have been widely studied.

nium tetrafluoroborate (TEMA-BF<sup>4</sup>

lyte picking up.

**4.3. Ionic liquid electrolytes**

**Figure 3.** Hydrogen evolution and oxygen evolution reaction performance of non-doped porous carbon materials (NDPC-6/7/8) measured in (a) 0.5 M H<sup>2</sup> SO<sup>4</sup> and (b) 0.1 M KOH, respectively. (c) CV profiles of NDPC-8 with different applied voltage windows, (d) GCD profiles of NDPC-8 with different current densities. Adapted with permission [47]. Copyright 2017, Royal Society of Chemistry.

Therefore, on the basis of large surface area, conductive electrode network, and proper electrolyte, the enlargement of operation voltage can be further realized through the application of electrode materials that have sluggish activity toward splitting of electrolytes. For example, it is feasible to use highly porous high-carbon content materials as electrode materials and Li<sup>2</sup> SO<sup>4</sup> solution as electrolyte for the assembly of SSCs with wide operating voltage window.

#### **4.2. Organic electrolytes**

The specific energy of aqueous SSCs was mainly limited by the water splitting voltage at ~1.23 V. Therefore, organic electrolytes with high conductivity, wide electrochemical voltage window, excellent chemical stability and acceptable cost become the mainstream electrolytes in practical supercapacitor market. Organic electrolytes consist of organic solvents and salts, usually have an operating voltage up to 2.7 V or higher, which makes it highly attractive for high-energy SSCs [48]. Since the organic electrolytes contain two parts, we will give separate illustrations.

First, proper organic solvents can be chosen based on the following rules: high solubility for electrolyte salts, low viscosity to facilitate ionic transport, no side reaction with other parts of supercapacitor (including active materials, current collector, and separator), wide work temperature, and environmental friendliness. Among all organic electrolytes, the most widely used solvents are acetonitrile (AN) and propylene carbonate (PC). AN is capable of dissolving large quantities of salts but it is toxic and risky to environment. PC, a green solvent, has been widely used, meanwhile, it also has very wide stable working temperature and good conductivity. Also, other electrolytes such as γ-butyrolactone (GBL), N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMA) are applicable electrolytes that have been widely studied.

Second, the most common used electrolyte salts are chain-like quaternary ammonium salts such as tetraethylammonium tetrafluoroborate (TEA-BF<sup>4</sup> ) and triethylmethylammonium tetrafluoroborate (TEMA-BF<sup>4</sup> ) [49]. One feasible method to further enlarge the operating voltage window for organic electrolyte is through the increasing of electrolyte salts concentration. On one hand, one can find the optimized salt concentration that has the best solubility and chemical stability; on the other hand, it is one feasible way to thicken the layer of charge carriers instead of solvent molecule layer at the electrode/electrolyte interfaces. For instance, 1 mol L−1 spiro-bipyrrolidinium tetrafluoroborate (SBP-BF<sup>4</sup> ) in PC solvent showed very high electrochemical stability on the interface due to its spiro rings molecular structure [50]. Yu et al. reported 1.5 mol L−1 SBP-BF<sup>4</sup> /PC shows higher conductivity of 17 mS cm−1 than 1.5 mol L−1 TEMA-BF<sup>4</sup> /PC [51]. Other metal salt such as lithium/ sodium salt are also applicable in carbon-based SSCs following the same rules for electrolyte picking up.

#### **4.3. Ionic liquid electrolytes**

Therefore, on the basis of large surface area, conductive electrode network, and proper electrolyte, the enlargement of operation voltage can be further realized through the application of electrode materials that have sluggish activity toward splitting of electrolytes. For example, it is feasible to use highly porous high-carbon content materials as electrode materials and Li<sup>2</sup>

**Figure 3.** Hydrogen evolution and oxygen evolution reaction performance of non-doped porous carbon materials

applied voltage windows, (d) GCD profiles of NDPC-8 with different current densities. Adapted with permission [47].

and (b) 0.1 M KOH, respectively. (c) CV profiles of NDPC-8 with different

The specific energy of aqueous SSCs was mainly limited by the water splitting voltage at ~1.23 V. Therefore, organic electrolytes with high conductivity, wide electrochemical voltage window, excellent chemical stability and acceptable cost become the mainstream electrolytes in practical supercapacitor market. Organic electrolytes consist of organic solvents and salts, usually have an operating voltage up to 2.7 V or higher, which makes it highly attractive for high-energy SSCs [48]. Since the organic electrolytes contain two parts, we will

solution as electrolyte for the assembly of SSCs with wide operating voltage window.

SO<sup>4</sup>

**4.2. Organic electrolytes**

(NDPC-6/7/8) measured in (a) 0.5 M H<sup>2</sup>

Copyright 2017, Royal Society of Chemistry.

128 Supercapacitors - Theoretical and Practical Solutions

give separate illustrations.

SO<sup>4</sup>

Low-temperature ionic liquids (ILs) are pure organic salts containing no solvents with melting points below 100°C. If the liquid state can maintain at ambient temperature, they are called room temperature ionic liquids (RTILs). RTIL are the type of ILs of broad interests to supercapacitors especially SSCs due to their unique properties including non-volatility, poor combustibility and high resistance to heat. However, the ionic conductivity of ILs usually fall in 0.1–15 mS cm−1, which is much lower than most of the commercial organic electrolytes. But ILs can show excellent conductivity at high temperature because of low viscosity. Many kinds of ILs have been widely used in supercapacitor; the two pairing ions of imidazole and pyrrole are most commonly studied. Generally speaking, the size and symmetry of cations strongly influence the melting points of ionic liquids. Normally, the imidazole possesses high conductivity with narrow potential window, alkylpyrrole shows wide operating voltage but high melting point. For instance, Chen et al. used a series of sponge-like carbon as electrode and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF<sup>4</sup> ) and 1-Butyl-3 methylimidazolium hexafluorophosphate (BMIM-PF<sup>6</sup> ) as electrolytes; the gravimetric capacitance of as-made SSCs can be improved to 445 F g−1 with a high discharge voltage of 4 V, very good rate capability, and cycling stability [52].
