Professor

Department of Physics

TDPG College, VBS Purvanchal University

 Jaunpur, Uttar Pradesh, India 

Associate Professor

Department of Physics

TDPG College, VBS Purvanchal University

Jaunpur, Uttar Pradesh, India

Abstract

Super-capacitors, or SCs, are significant because of their unique characteristics, which include a long cycle life, high strength, and environmental friendliness. They also share fundamental equations with traditional capacitors, and in order to achieve high capacitance, SCs use electrode materials with high specific surface area and thinner dielectrics. Depending on the energy storage mechanism, all varieties of SCs have been described in the present chapter.

 Introduction

Energy has been one of the leading topics for debate and discussion at all levels and continues to be a key element to the worldwide development in current scenario [1]. It serves as the only universal impetus that drives virtually all social and individual activities, such as transportation, residential electricity generation, and several commercial applications [2]. Historically, energy supply has changed gradually since fuels have changed from wood to coal, oil, gas and now include biomass. Because of the rapid growth of the global population and human consumption, the energy demand is going to be continually increasing. By 2050, it is expected that the world population may reach to about 9.7 billion which may result in the increase of energy requirement to about 12.5 trillion watts [3]. Water, food, and energy are among the most fundamental to the well-being of society, and without safe, sustainable and economic energy, much of the contemporary civilization would nose-dive. In the contemporary society, the majority of energy consumers are mainly reliant on non-renewable energy sources. However, it is worth noting that dependence on primary fossil-based fuels such as petroleum, coal, and natural gas, afford more than 80% of the global energy [4]. Hence, an energy crisis is inevitable if we continue to consume fossil fuels unscrupulously. As a preliminary speculated, fossil fuels reserve will get out of stock shortly, petroleum (40 years), natural gas (60 years), and coal (156 years), for example [5]. This exquisite dependence on non-renewable energy sources has twofold consequences- the continuous depletion of energy sources at an alarming rate and the adverse health/ environmental impacts. The greenhouse gases and other pollutants released by the consumption of fossil-based fuel cause serious havoc to the plant, environment, and climate [6]. Until recently, the concept of a total energy revolution remained on the margins of world politics. Despite the idea of “carbon emissions should be cut” is now became an accepted policy across most of the developed world, still most of the current sustainable energy policies are aimed at more conservative targets than diverse and renewable energy policies [7]. So far, the trend is that the energy dependence on fossil fuels will continue to rise in a context of concerns about climate change and unpredictable energy prices caused by increased demand for energy. However, nature offers various renewable sources such as solar energy, wind energy, tidal energy, biomass energy, provide an alternative cleaner energy, and have vast potential to reduce the dependence on fossil fuels [8]. These energy sources have the benefits that they have a low environmental impact, are widely available, and produce no or little contamination. However, their widespread adoption is also constrained by geographical location, atmospheric conditions, and both economic and safety considerations [9]. They (i.e., sun power and wind power) have certain specific intermittent output and even peaks in generation that do not necessarily match with the demand for energy. This has led to a growing concern about the reliability and unpredictability in both supply and the demand for energy. Energy storage system is an important viable way to offer a solution to the rising demand in world energy generation and consumption. Essentially, energy from the different resources must be stored when an excess is produced and then released. There are other reasons why it is necessary to store energy [10]. Storing energy allows the electricity generated by renewables so as to match the fluctuating supply to the changing demand. Through such applications, it is also considered that energy storage can be multi-beneficial to both utilities and their customers in terms of (i) improved efficiency of operation of a system (ii) reduced primary fuel use by energy conservation (iii) provided security of energy supply (iv) decreased environmental impact.

Different energy storage technologies have been developed and their characteristics make them attractive to different applications. In general, energy storage systems can be described as either electrical or thermal [11]. Electrical energy storage includes a broad range of technologies, which either directly or indirectly provide electrical energy storage via an electrical input and output. The principal technologies are-

i. electrochemical systems (embracing batteries and flow cells)

ii. kinetic energy storage systems, more commonly referred to as flywheel energy storage

iii. potential energy storage in the form of either pumped hydro or compressed air storage

Among these technologies, electrochemical energy storage systems have gained much interest due to some advantageous factors. The period of the explosion demand for clean, sustainable, and renewable energies has already come and will continue to the next few years. In this pursuit, it is imperative to develop methods for generation and storage of renewable energy devices that can replace the conventional energy resources to meet the requirement of energy consumption.

Electrochemical Energy Storage Devices

Besides, different types of existing electrochemical energy storage systems, number of energy storage devices have been developed so far like fuel cell, batteries, capacitors, solar cells etc. Amongst, fuel cell was the first energy storage device which produces large amount of energy, developed in the year 1839 by a British scientist William Grove [12]. National Aeronautics and Space Administration (NASA) introduced the first commercially used fuel cell in 1960, using used Grove’s approach to generate electricity . Afterward, a number of modifications have been made to improve its energy production ability. Furthermore, researchers have focused on the storage of electrical energy and developed energy storage devices such as battery and capacitor (supercapacitor). The first battery (lead-acid) was developed by Gaston Plante in 1859. However, Emile Alphonse Faure introduced lead-acid battery in 1880 [13]. In his development, he coated lead plates with a paste of lead powder and sulfuric acid, which helps in the storing of charges and improves the storage capacity of the battery. This process has widely been used by the other researchers leading to the improvement in storage capacity. Sellon has patented a process by using the Faure’s process and demonstrate a higher storage capacity. Later, in 1961 an electronic double layer capacitor (supercapacitors), were the first time established and patented by American Oil Company, Standard Oil of Ohio (SOHIO). Moreover, these devices have attracted great attention to the researchers and a remarkable improvement have been observed. Another, tremendous improvement in the field of energy storage was the development of solar cell devices, which have brought a new revolution in energy storage application. The concept of solar cell was first introduced by Becquerel in the year 1839 and developed first solar cell devices [14]. To improve further storage ability and stability of these devices, researchers have explored number of materials like carbon-based materials, metal oxides, composite, and hybrids etc. which can be used in the energy storage application. Some of the foremost ESDs, such as supercapacitors, batteries, fuel cells, etc., have been discussed in the following sections.

Conventional Capacitors

The conventional capacitors, as a passive electronic component, are composed of two adjacent conductors and an insulating medium between them. In 1745, the invention of the Leyden jar opened the door of capacitor technology. In 1876, the paper capacitors were constructed by placing a waxed paper among metal foils and firmly rolling it into the circular column. In 1896, the first electrolytic capacitor was patented by using a less impurity etching aluminium leaf with alumina as dielectric. Some prominent capacitors have also appeared in succession including mica dielectric capacitor (1909), polyethylene terephthalate–based capacitor (1941), and plastic dielectric capacitor (1959). At first, capacitors are mainly used in electrical and electronic commodities, but currently they are utilized in various domain involving vehicles, aircraft, aerospace, medical, and power grids based on their ultrahigh-power density, extremely rapid charge–discharge rates, and superior service life.

The application of a potential difference across the terminals results in the polarization of the dielectric with the most favorable dipole alignment i.e., each charge pole faces a plate of opposite polarity. Because of the charge storage mechanism being electrostatic, these capacitors are also called electrostatic capacitors. Fig.1 illustrates the design and function of the simplest dielectric (electrostatic) capacitor. In this case charge storage is voltage dependent (higher voltage, higher dielectric polarization thus greater charge storage) and we can define a quantity called capacitance (C) which is the charge storage capacity of metal plates through the following equation- 

                                                                    (1)

As the electric field developed between parallel plates of area A, having stored charge Q coulombs is given by-

                                                                 (2)

Thus, capacitance equation for a parallel plate capacitor transforms to

                                                                  (3)

Here, Q denotes the total charges at the plates V is the voltage across the plate, C_s is the capacitance ε refers to the permittivity of the medium and d is the separation between the plates.

Thus, the above equations confirm that, capacitance depends on the dimensions of the electrodes and not the material of the electrodes, however, capacitance can be enlarged by increasing the area of ​​the electrodes and reducing their separation. The other parameter that decides the superiority of a capacitor is the total energy stored at the surface of capacitor and is shown by a familiar Eqn.

                                                           (4)

Where E is the energy storage in a capacitor at the highest possible voltage V.

The energy distribution per unit time is called maximum power P_max. In general, capacitors are constructed with the help of series internal resistance, known as equivalent series resistance (ESR). Herein, it confirms all internal components of capacitors like current collectors, electrodes, and dielectric materials play a very significant role in ESR. Discharge current is also demonstrated with the help of ESR. The Eqn. (5) shows the relationship between ESR and P_max [15-18].

                                              (5)

Batteries

Batteries are one of the essential devices for energy storage systems consisting of one or more electrochemical cells. It releases the electricity by discharging the potential energy stored in the elements of the battery. So far, batteries have become the most common power source for various applications in industrial and consumer electronics. A battery is a device that converts stored chemical energy into electrical energy through a redox reaction. A typical battery contains one or more electrochemical cells. Each electrochemical cell consists of two electrodes which are electrically connected by a conductive electrolyte with anions (negative charge ions) and cations (positive charge ions). The polarity of a cell is identified by the transport of the anions and cations. In the charging process, the electrode with anions transport is called the anode or negative electrode, while the other electrode with cations transport is called the cathode or positive electrode. Generally, batteries are powered by a redox reaction with a reduction of cations at the cathode and an oxidation of anions at the anode. Based on their charging capability, batteries can be divided into two types - disposable batteries and rechargeable batteries. Disposable batteries, for example the zinc-carbon batteries and alkaline batteries, are designed to irreversibly convert the chemical energy into electric energy. On the other hand, the rechargeable batteries can restore the original composition with charging process (e.g., lead-acid, nickel metal hydride (NiMH), nickel-zinc (NiZn), nickel-cadmium (NiCd), and lithium-ion cells). As a consequence, disposable batteries have higher energy density than rechargeable batteries. Currently lithium-ion batteries (Fig. 2) represent the best electrochemical cells with a high energy density of 120-170 Wh/kg, moderate weight and no memory effect as well [19]. In spite of their high specific energy, the lithium-ion batteries and their substitutes are suffering from a low charge-discharge rates or low power density. Generally, the anode of a commercial lithium-ion cell is made of graphite or other carbon materials. Typically, the cathode material is lithium intercalated compounds like iron phosphate, cobalt oxide, manganese oxide, and nickel oxides. The lithium ions can migrate into and away from both electrodes. During the charging process, the lithium ions move into the graphite anode. In the discharge process, when connected to an external load, the ions migrate back to the cathode. The lithium ion is highly reactive and can react with water in the electrolyte forming hydrogen gas and lithium hydroxide. Thus, the organic electrolyte and a well-sealed packaging method are employed in lithium ion batteries to minimize the possibility of dangerous reactions.

Fig. 2: Schematic diagram of a Li-ion battery.

There are various parameters which determine performances of the battery are summarized as follows-
Capacity (Q)

It is a measure of the total charge that can be stored reversibly in an electrode material (anode/cathode). The potential of an electrode material can be calculated by the using the Eqn. (6).
        (6)

Here, N is number of electrons involved in charge transfer and MW is molecular weight of electrode material.

Rate capacity

It is a measurement which involves the capacity measurement at different currents generally starting from lower currents to higher currents (of the range of 21 Ag^-1 - 20 Ag^-1) then again to the lower currents. A material is said to be rate stable if it retains the initial capacity after going through this cycle.

Cycle stability

It involves measuring the capacity in a particular current in hundreds of cycles. The measurement is done to test the stability of the material over those multiple cycles.

Operating Point (O.P.)

It is the voltage versus Na⁺/Na at which the reversible electrochemical uptake and release of Na occurs. An anode close to 0 V is preferred for the O.P. while a voltage higher than 0 V for the cathode is preferable to the cathode material. Electrolytic stability concerns Maximum 4.5 V (for cathode) and minimum (for anode) 0.01 V.

Energy Density (E.D)

It is the total energy that can be stored in the cell per unit electrode weight and is a simple product of the full cell's capacity and operating voltage.

  (7)

Here, Q'' is reduced capacity of a full cell device in mAhg^-1 and V is O.P. in Volts. Q” can further be defined as-

                              (8)

Here, Q_an is Anode capacitance and Q_ca represents cathode capacitance.

Fuel cells

Fuel cells (FCs) are considered as a revolutionary material for energy storage applications ranging from electronic devices to automobiles and power plants. The advantage of fuel cells is a high efficiency, load performance, and emits nontoxic by-products. Similar to batteries, fuel cells convert the chemical energy of a fuel into electric energy, shown in Fig. 3. However, fuel cells require no recharging and the by-product of the reaction generally is environmentally friendly like water and heat. As long as the fuel supply is adequate and consistent, the cell can work ideally without replacement with outstanding reliability. Compared with thermomechanical methods, fuel cells do not have combustion as an intermediate step, which results in a higher energy conversion efficiency of 40 %-60 % [20]. For these reasons, fuel cell technology has become a clean, economical and reliable solution for power sources. Among all the commercialized energy storage alternatives, fuel cells hold one of the highest energy densities, although their low power density remains an obstacle for use in high power applications.

Fig. 3: Schematic of a proton exchange membrane fuel cell [21].

Generally, a fuel cell consists of a cathode, an anode and an electrolyte. The major difference among various types of fuel cells is the electrolyte. Common electrolytes in both research and commercial devices are aqueous alkaline solution, polymer membrane and ceramic oxide.

The following operational parameters - Reactant flow rates, operating temperature, operating pressure, reactant humidity, and flow rates are the prime factors to determine the fuel cell`s performances.

To get optimal fuel cell performance, each parameter must be used under the proper operating conditions.

(a) The Working Pressure

A fuel cell can run under pressure or at room pressure. Pressure increases frequently result in better fuel cell performance. On the other hand, the system can become less effective due to the requirement for gas compression and storage. Fuel cell operating circumstances need to be examined from a systemic standpoint since pressurization of the fuel also modifies the water management in each cell.

(b) Operating Temperature

Improved fuel cell efficiency is achieved at higher operating temperatures. The ideal temperature varies depending on the fuel cell architecture, and each fuel cell system requires a different operating temperature. When an electrochemical reaction occurs in a fuel cell, heat is produced as a byproduct that needs to be managed to keep the temperature where it is. A multitude of factors are impacted by the fuel cell's specified operating temperature. The waste heat is transferred to the latent heat of vaporization and less liquid water is forced out of the fuel cell at a higher operating temperature because more of the product water is vaporized. Also, faster kinetics and a voltage gain greater than the voltage loss resulting from the negative thermodynamic relationship between the open-circuit voltage and temperature are associated with higher temperatures.

(c) Reactants flow rate

The pace at which the reactants flow through the cell must match or exceed the rate at which they are consumed. The fuel cell system is filled with oxygen and hydrogen at the proper flow rate to provide the desired current. In order to maintain a consistent stoichiometry, a variable-flow system is necessary. A pressure that is somewhat above atmospheric is required to force the liquid water out of the system and push the gases through the flow fields, even in systems that use "atmospheric pressure."

(d) Reactant humidity

For high ionic conductivity of the membrane in PEM fuel cell stacks, the reactant gases must be humidified before to entering the cell. Fig. 4 displays the polarization curves for various operating air and hydrogen relative humidity levels. When hydrogen was at 70% humidity, the best results were achieved. Membrane dryness and reduced performance result from the transport from the anode by electro-osmotic drag being greater than the transport to the anode via back diffusion from the cathode at high current densities. Low humidity can make this impact worse by slowing down the cathode's rate of back diffusion. Moreover, enhancing the humidity of the anode gases increases  fuel cell efficiency.

Fig. 4: Polarization curves as function of feed gas humidity fuel cell [22].

Supercapacitors (SCs)

Recently, supercapacitor (SCs) has been attracted much as an energy storage device like a battery in design and manufacture. The SCs also called ultracapacitors or electrochemical capacitors, utilize high surface area electrode materials and thin dielectrics to achieve higher capacitance as compared to the conventional capacitors [23]. Batteries and capacitors seem similar as they both store and release electrical energy. However, in general, batteries provide higher energy density for storage, while capacitors have more rapid charge and discharge capabilities [24]. Supercapacitor, an upgrade version of the capacitor, can be successfully performed with large amounts of power for efficiency enhancement as energy storage technologies [25]. Due to their high-power capabilities and long cycle-life (> 100 times battery life), these have attracted significant attention, both on-board and stationary applications.

The theoretical investigation of the Electric Double layer by German Physicist Hermann L. F. von Helmholtz has formulated the base for super-capacitive electrochemical energy storage [26]. Since their accidental discovery in General Electric laboratories in 1950’s and their first commercial applications in the 1970's, real interest for supercapacitors have started to increase from the 1990's due to their ability to provide a high burst of power needed for acceleration in hybrid vehicles [27]. In 1957, Howard I. Becker designed an electrochemical capacitor using porous carbon [28] which he mistakenly called electrolytic capacitor. Almost a decade later (1966), engineers and scientists at Standard Oil of Ohio (SOHIO) developed another similar electrochemical capacitor. SOHIO sold the technology to NEC (Nippon Electric Company, Japan) who introduced its first commercial application as backup power for computer memory in 1971 [29]. NEC coined the electrochemical capacitor "Supercapacitor" because of its capacitance [30]. Since then, the commercialization of the supercapacitor started with the development of key technologies. These included improving the electrode materials, electrolyte and the manufacturing process. A kind of supercapacitor named “Gold capacitor”, used for memory backup applications, was developed by Panasonic in 1978. The Pinnacle Research Institute (PRI) developed the first high power double-layer capacitors named “PRI Ultracapacitors” by incorporating metal-oxide in the electrode in 1982. In late 70’s and 80’s, Conway et. al. made a great contribution to the supercapacitor research, using RuO2 as the electrode material which showed a high specific capacitance and a low internal resistance [31]. The US Department of Energy developed a study in hybrid electric vehicles, in which, the Ultracapacitor Development Program was developed by Maxwell Laboratories in 1992. Recently, supercapacitors are manufactured by a number of companies around the world, and widely used. The commercial supercapacitors are widely used as power sources for communication devices, digital cameras, mobile phones, electric hybrid vehicles, electric tools, pulse laser techniques, uninterruptible power supplies for computers, standby power for random access memory devices and storage of the energy generated by solar cells [32].

In the future, the SCs is thought to be used either in conjunction with batteries or replace batteries in the storage system (continuous power supply, and load levelling). Recently electrochemical double layer capacitors (EDLCs) were used in emergency doors on an airbus A-380 with providing the safety, performance and reliability issues [25].

The general performance parameters of the SCs. It is illustrated in the term of specific capacitance (Cs) which is used to reflect the property of the active material on a single electrode. It is most reliably measured by the three-electrode cell but can also be derived from the two-electrode cell. For a commercial symmetrical supercapacitor,

Cs = 4 × Ccell / a% × Wcell          (9)

Where, Ccell is the cell capacitance, Wcell is the overall weight of the device, and a% is the percentage of active material mass over the overall cell weight. For the commercial asymmetrical cell, to calculate the specific capacitance of each electrode material, the masses of the active materials on both the positive electrode and negative electrode must be known with Ccell and Wcell.

As for the difference in material loading on the electrode, the value is usually high (> 1 mg/cm2 ) in commercial supercapacitors, whilst the reported data in literature was measured from electrode with mass loading being < 0.1 mg/cm2 which can produce over estimated specific capacitance due to contribution from the surface of the current collector [33]. Though the SCs exhibit greater capacitance than conventional capacitors yet SCs must meet the demands of batteries and fuel cell regarding energy density. Supercapacitors are used in applications requiring many rapid charge/discharges cycles rather than long term compact energy storage within cars, buses, trains, cranes and elevators, where they are used for regenerative braking, short-term energy storage or burst-mode power delivery. Supercapacitors can be classified into the following classes based on charge storage phenomenon. Fig. 5 exhibits different types of supercapacitors. The performance parameters of different electrochemical storages devices have been summarized in Table 1 as-

Supercapacitors can be classified into the following classes based on the charge storages phenomenon- Electrochemical double layer capacitors (EDLC), Pseudo-capacitors (PSc) and Hybrid capacitors (HSc).

Fig. 5: Taxonomy of supercapacitors.

Fig. 5 shows the classification of supercapacitor based on the electrode material, while Fig. 6 illustrates different types of SCs and the respective charge storage mechanism involved.

Fig. 6: Classification of supercapacitor based on electrode materials.

These supercapacitors are distinguished by their unique properties such as electrostatic charging/ discharging, reversible Faradaic redox, and a combination of both. Due to Oxidation–reduction reaction pseudocapacitors (PSc) show Faradaic nature in charge storage mechanism [34-36]. Energy is stored in PSc by a chemical mechanism, such as transition metal oxide-based supercapacitors, ELDCs represents non-Faradic nature, in which charges are held apart at the electrode/electrolyte interface by physical processes that do not involve the process of forming or breaking chemical bonds. Carbon-based material like Activated carbon, Carbon nanotubes (CNTs), graphene quantum dots etc. [37] are the best examples of ELDCs charge storage devices. The third type is hybrid capacitors (HSc), showing both (Faradaic and non-Faradic) types of nature. Carbon and transition metal oxide composite materials are the best example of hybrid supercapacitors. Table 2 summarizes the typical electrode material and operating principles of SCs.

Electrical Double Layer Capacitors (EDLCs)

An electrical device stores energy on their highly pore surface, formed by their electrode/electrolyte interface are known as Electrochemical double layer capacitors (EDLCs). Since the EDLCs are electrochemical variants of dielectric capacitors where electrolyte polarization instead of dielectric polarization (Fig. 7a) leads to charge storage without any faradaic reaction, long cycle life can be obtained in addition to high capacitance. The polarization of ions is mediated by surface adsorption of ions at oppositely charged interfaces. This type of charge storage is more efficient by virtue of the better mobility of ions in the electrolyte compared to dielectric charges [38]. Moreover, EDLCs gained superior power density on comparison against energy storage benchmark, Li-ion batteries. Hence these formidable properties have allowed EDLCs to be known as supercapacitor. The unpredictable size and huge amount of charge storage properties permit it to be called an ultracapacitor. These types of prepared supercapacitors permit 10 to 100 times more energy per unit volume or mass than ordinary conventional capacitors, and authorize many more charge/discharge cycles than rechargeable batteries. Electrochemical double layer supercapacitors contain two considerably pore and parallel electrodes. They are separated by ionically dielectric separator. This interface forms a common margin among two different phases of matter, such as solid electrode and liquid electrolyte. Fig. 7b shows the schematic of an EDLC super capacitor. Due to the dielectric separation at the interface, d being very small (molecular dimensions) the capacitance of such a device increases substantially.

Fig.7: (a) Dielectric capacitor with a dielectric medium between metal plates.

(b) Corresponding double layer capacitive charge storage in porous carbon material.
After applying a sufficient potential to the electrochemical double-layer capacitor, it provides two opposite polarity charges- one of the insoluble solid electrode surface and other an adjacent liquid electrolyte. One such class of electrode materials, which has indeed been a revelation due to their large surface area, is engineered activated carbon. Fig. 8 shows the working mechanism of super capacitive charge storage in large surface area material. This type of carbon has very large surface areas in the range 500–3000 m² g^-1, which can increase the capacitance by several powers of magnitude. Since the total capacitance of the device is material dependent, very little can be inferred about the superiority of the material from this capacitance value.

Fig. 8: The working mechanism of an EDLC capacitor illustrated here. Charging and discharging states can be distinguished on the basis of polarisation upon voltage application [39].

Moreover, two capacitances which will be characteristic of the material rather than the device. Specific capacitance (C_s) or gravimetric capacitance (C_g) can be defined as the capacitance per unit mass of the electrode material and can be represented from Eqn. 10.

                                                              (10)

Where C_g is gravimetric or specific capacitance, C is capacitance of individual electrode measured separately (in a 3-electrode cell) and m is mass loading of material on electrode.

The other capacitance is the areal capacitance (C_A) which may be defined from Eqn. 11 capacitance per unit electrode area.

                                                               (11)

Here, C_A represents real capacitance, C denotes capacitance of individual electrodes measured separately (in a 3-electrode cell) and A is the area of material coating on electrode [40].

Fig. 9: Cyclic voltammetry of EDLCs at different (a) scan rates and (b) current density.

Carbon-based materials, such as activated carbon, graphene, carbon nanotube, carbon nanofiber, templated porous carbon and carbide-derived carbon, are most usual common materials for supercapacitor electrode application due to large surface area (1000 ~ 2000 m²/g), high stability, low cost and environmentally friendly behaviour. Nowadays, a variety of carbon materials are extensively used to store charge in EDLCs electrodes, such as activated carbon (AC), carbon nanotubes, carbon aerogels, and graphene.

Different theories have been proposed to explain the double layer that forms when two charged conductors are in close proximity. (a) Helmholtz model, (b) the Gouy-Chapman model, (c) the Stern and Graham models, and (d) Bockris, Devanathan, and Muller (BDM) model.

(a) Helmholtz Model

The idea of an electrical double layer was first introduced by Helmholtz in 1853. When a charged conductor (electronic charges) is in contact with an electrolyte (ionic charges), an electrostatic charge separation between two layers of opposite charges occurs at the electrode/electrolyte interface. The two layers are apart by only the thickness of one molecule. In this case, there is no electron transfer between the layers [41]. Although the model developed by Helmholtz does not consider the effect of ions behind the first layer of directly adsorbed ions at the interface, it is widely used due to its simple image which explains the capacitance of an EDLC. It should be noted that the Helmholtz model works better in electrolytes with high ion concentration. This is mostly the case for EDLCs.

(b) Gouy and Chapman Model

Gouy (1910) and Chapman (1913) developed the model separately to describe the dynamics of ions close but not adsorbed to the interface (the diffuse layer) and considered the thermal motion of ions in the electrolyte (Fig. 7). The model known as the diffuse double layer showed that different parameters such as the applied potential, the concentration of ions in the electrolyte, and their thermal motion affect the capacitance of the double layer [42]. To find the total amount of the charge in the diffuse layer and the ion distribution in space, Poisson Eqn. (12) and Boltzmann equation at both positive and negative electrodes Eqn. (13 and 14) must be solved [43].

where ρ is the volumetric charge density, Φ the electric potential, and ε the dielectric constant; N⁺ (N^-) the concentration of ions in the diffuse layer, N⁺⁰ (N^-0) the concentration of ions in the bulk solution, n⁺ (n^-) the valence of the ions, and T the absolute temperature (e = 1.602 x 10⁻¹⁹ C, k =1.38x10⁻²³ J/K).

(c) Stern and Graham Model

Stern developed the model in 1923 which included both Helmholtz and Gouy-Chapman models. His model separated the double layer into two regions, a uniform region of ions close to the electrode (stern layer), and a diffuse layer reaching into the bulk of the electrolyte [44] (Fig. 10 c). Graham added in 1947 the specific adsorption of ions to the Stern model. The Stern Graham model include three layers- the inner Helmholtz plane (IHP), the outer Helmholtz plane (OHP), and the diffuse layer [51]. The capacitance of the Stern-Graham model can be found by applying the Langmuir theory [43]

Where C_s is the capacitance of the Stern layer, C_d the capacitance of the Gouy-Chapman diffuse layer, σ the surface charge density on the electrode, and σ_A the surface charge density of the specifically adsorbed ions.

(d) Bockris, Devanathan, and Muller Model (BDM)

The BDM model (1963) added to the Stern-Graham model the presence of solvents molecules. Using water as solvent, they showed that some of the water molecules were adsorbed at the interface inside the inner Helmholtz plane. The alignment of the dipole of water molecules would be different depending on their distance from the electrode. Molecules close to the electrodes would have fixed dipole alignment due to their proximity with the electronic charges in the electrode [43]. Fig. 10 shows the different double layer models as discussed above.

Fig. 10: Schematic of the double layer models. (a) Helmholtz model, (b) Gouy Chapman model, (c) Stern model, and (d) Bockris, Devanathan, and Muller (BDM) model (IHP=Inner Helmholtz plane and OHP=Outer Helmholtz plane).

Pseudocapacitors

The name “pseudocapacitors” involves electrochemical systems in which the capacity of the electric double layer is supplemented by the electrode-electrolyte distribution surface by charge transfer processes across this boundary [45]. Pseudo capacitor can achieve Pseudo capacitance by fast and reversible faradaic reactions (redox reactions), electro sorption or intercalation processes on the surface of suitable electrodes. Fig.11 demonstrate three basic surface redox mechanism responsible for charge storage. Since the charge accumulation process involves both the surface and volume of the electrode, PSc show almost twice the specific energy when compared to common SCs. The first PSc samples included Ru and Ir hydrated oxides as an active electrode material, although there were attempts to use oxides of other transition metals [46]. The redox transition Ru III/Ru IV is associated with rapid proton exchange between oxide and hydroxide sites. High capacity (up to 500 F/g based on active mass) and high values of velocity/current were obtained for the charge-discharge processes due to reversible reactions on the electrode surface. However, these systems have not been widely used due to low availability and high cost of the most efficient Ru and Ir-based materials. Over the last two decades, polymers with high electronic conductivity, such as polyaniline, polypyrrole, and polythiophene have also been studied as PSc electrodes [47]. On the other hand, Barsukov et. al. proposed a “capacitor” concept with polyaniline-based electrode with the possibility of obtaining ultra-high specific capacity (up to 500-600 F/g) [45]. However, these studies reported the gradual formation of a nonconductive layer in polyaniline near the boundary of the current collector. Developers of PSc based on electrically conductive polymers are faced with this effect, which may be at least one of the reasons for rapid degradation of the system and reduction in the number of charge-discharge cycles, thus preventing their entry into the energy storage market. Recently, the group of Gogotsi and Barsoum synthesized a new class of materials under the general name MXene, in particular, two-dimensional carbides or nitrides of transition metals [48]. The resulting materials, such as Ti₃C₂, form a layered structure with a layer thickness equal to several atoms. The material has high electronic conductivity and at the same time, the intercalation of various ions into an interlayer space is possible. Various possible applications of MXenes are offered, in particular, as electrode materials for PSc with high specific capacity, however, these concepts have not gone beyond laboratory studies yet.

Fig. 11: (a) Shows the under potential deposition of Pb from solution on Au surface. (b) Shows the redox pseudo capacitance of RuO, involving reversible surface redox reaction in an acidic medium. (c) Shows the intercalation super capacitance of layered Nb^{- 0}, in a Li ^- ion electrolyte [49].

Metal oxides like MnO₂, RuO₂, NiO, Co₃O₄, NiCo₂O₄, NiCo₂S₄ and other similar oxides and sulphides have shown a remarkably good performance in aqueous solution [50]. RuO₂, NiO and NiCo₂O₄ shows these reactions given below.

RuO₂ + xH⁺ + xe ^–↔ RuO2_–x (OH)_x

NiO + OH ^- ↔ NiOOH+ e ^–

NiCo₂O₄ + 3OH ^- ↔ NiOOH + 2CoO₂ + 2H₂O + 3 e ^–

The theoretical capacitance can be express as follows-

Where N is number of electrons involved in redox reaction, F is faradays constant, ΔV is voltage window and M.W is molecular weight. Fig. 12 represent cyclic voltammogram and charge-discharge curves of a pseudo capacitor electrode material.

 

Fig. 12: Shows Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) graph of MnO₂.

Hybrid Capacitors

Hybrid supercapacitor Instead of using graphene, metal oxides and polymers separately, combining each other we can get low cost, good electrical conductivity, mechanical flexibility and good chemical stability as hybrid electrode materials for supercapacitors. Hybrid electrode configurations show considerable potential, consisting of two different electrodes made of different materials. Composite electrodes consist of one type of material incorporated into another within the same electrode. During research into polymer electrodes at the University of Bologna it was found that a sufficiently high polymer concentration could not be realized in the negative electrode. The positive polymer electrode was successfully constructed, however, and activated carbon was used as the negative electrode. This hybrid configuration resulted in a supercapacitor that outperformed a cell comprised of two carbon electrodes [51]. Also, of interest are the results of experiments into depositing polymers onto carbon substrates to form composite electrodes. Fig. 13 (d) & (e) presents various commercial hybrid supercapacitors.

Fig. 13: Commercial supercapacitors as (a) EDLCs, (b) & (c) Pseudocapacitors, (d) & (e) Hybrid supercapacitors.

In the past few years, research focus is directed toward the development of hybrid electrochemical capacitors (HSc), which asymmetrically and simultaneously store charges by surface ion adsorption/ desorption on the cathode and by lithium/sodium-de/intercalation in the anode. Fig. 14 shows the various characteristics of a hybrid supercapacitor.

Hybrid supercapacitors attempt to exploit the relative advantages and mitigate the relative disadvantages of EDLCs and pseudocapacitors to realize better performance characteristics. Utilizing both Faradaic and non-Faradaic processes to store charge, hybrid capacitors have achieved energy and power densities greater than EDLCs without the sacrifices in cycling stability and affordability that have limited the success of pseudocapacitors. Research has focused on three different types of hybrid capacitors, distinguished by their electrode configuration [52].

1. Asymmetric hybrids
2. Battery-type hybrids
3. Composite hybrids

Asymmetric hybrids combine Faradaic and non-Faradaic processes by coupling an EDLC electrode with a pseudocapacitive electrode [52, 53]. In principle, as a negative electrode usually carbon-based material is used, and as a positive some pseudocapacitive materials. The combination of a negative carbon electrode with a conducting polymer or metal oxide positive electrode received a great deal of attention [54, 55]. Asymmetric hybrid capacitors that couple these two electrodes reduce the extent of this trade-off to achieve higher energy and power densities than comparable EDLCs. Also, they have better cycling stability than comparable symmetric pseudo capacitors [52].

Similar to asymmetric hybrids, the battery-type hybrids couple two different electrodes, however, the battery-type hybrids are unique in the combination of a supercapacitor electrode with a battery electrode. This configuration replicates the demand for higher energy supercapacitors and higher power batteries, combining the energy characteristics of batteries with the power, cycle life, and recharging times of supercapacitors [52].

Composite electrodes integrate carbon-based materials with either conducting polymer or metal oxide materials and incorporate both physical and chemical charge storage mechanisms together in a single electrode. The carbon-based materials facilitate a capacitive double layer of charge and provide a high-surface-area backbone that increases the contact between the deposited pseudocapacitive materials and electrolyte. The pseudocapacitive materials can further increase the capacitance of the composite electrode through Faradaic reactions [52]. The synergetic mechanism could improve corrosion stability, increased the specific capacitance and the operating potential windows. Many different materials have been investigated, mostly exotic and very expensive (starting materials and preparation procedures) for asymmetric composite supercapacitors [56]. Fig. 14 presents the possible combinations of all hybrid supercapacitors.

Fig. 14: Illustrating hybrid supercapacitor as a subset of other types of SCs.

Fig. 15 compares the power density, energy density, as well as response time of each type of SCs with Ragone Plot. Several researchers have compared the characteristics of different SCs based on state-of-the- art academic research, indicating the difference between their future developments [57]. This table 3 compares the performance of different commercial SCs, from the perspective of the application.

Fig. 15: Comparison of SCs in Ragone plot framework.

 Components of a super-capacitor

A typical supercapacitor consists of electrode, electrolyte and a separator as shown in Fig. 16. The electrode consists of an electrode material coated on a carbon coated aluminum foil, separated by a polymer or paper separator. Porous polymers such as polyethylene, polypropylene, PAN fiber and porous cellulosic paper have performed efficiently. The electrolyte is a very important component of a supercapacitor because it presents mobile charged ions that are polarized by the application of voltage. The electrolyte should have the properties of a wide voltage window and a high dielectric constant. In this regard various ionic salts dissolved in water or polar organic solvents such as acetonitrile, Ethylene carbonate (EC), (Dimethyl Carbonate) DMC, (Diethyl carbonate) DEC, (Propylene Carbonate) PC and their mixtures have emerged as good solvents for supercapacitor devices. Different aqueous electrolytes employed are 1 M H₂SO₄, 6 M KOH, 1 M Na₂SO₄, 1 M Li₂SO₄, 1 M NaClO₄ etc. Organic electrolytes include 1 M of LiClO₄, LiPF₆, and NaClO₄ in EC, DMC, PC or their mixtures [58].

Fig. 16: Dissection of a commercial super - capacitor showing different components.

Electrodes for any energy storage and conversion system must have good conductivity, high-temperature stability, long-term chemical stability, high corrosion resistance, high surface areas per unit volume and mass, environmental friendliness and low cost. Supercapacitor electrodes are generally thin coatings applied and electrically connected to a conductive, metallic current collector. These are typically made of a porous, spongy material with an extraordinarily high specific surface area. Additionally, the ability of the electrode material to perform faradaic charge transfers enhances the total capacitance. Electrode material fabrication is very important to get high-performance supercapacitor [59].

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