Electrical energy is the smartest and safest form of energy ever discovered by human beings. But, still, in many developing and developed countries around the globe, electricity is produced by burning diesel, natural gas, and coal [1,2]. This biofuel consumption is severely impacting the global environment and threatening the healthy living assurance of human beings. As a consequence of that many efforts and technologies have been developed to produce clean and safe electrical energy, such as – nuclear fission reactor, windmill, hydroelectricity, etc. as the best alternatives of fossil fuel consumption [3,4,5]. Besides, the solar energy conversion to electrical energy added significant advancement in the energy harvesting sector. In the 21st century, the demand for electrical energy with the transformation rate of society has reached its zenith level. Now, electrical energy is not only limited to households or industrial applications but also transforming into portable heavy or light devices including vehicles. Rather, the incoming centuries will be solely dependent on the use of electrical energy just to overcome extensive global emission and warming problems raised by the burning of fossil fuels. However, producing the cleanest and safest electrical energy is not only a challenge to be solved in the modern era. Rather, storing the produced electricity and utilizing it efficiently for the required purposes is also considered a prime challenge. To serve this purpose, batteries are first introduced in the mid of 18th century. Since then, the advancement in electrical energy storage devices are continuously booming and in the 21st century, the advancements in Lithium-ion batteries have reached their zenith level [6]. Today’s many portable electronics are run by mainly Lithium-ion batteries and other conventional energy storage devices such as Nickel-Cadmium batteries, Nickel Metal Hydride batteries, Lead-acid batteries, etc. (shown in Figure 1.2).
batteries.
Besides battery devices, another type of electrical energy storage device is the capacitors. The basic mechanism of these batteries and capacitors are very similar [7]. It is important to understand the basic differences between capacitors and rechargeable (secondary) batteries in the way they function. Generally, a battery consists of a positive electrode (cathode), a negative electrode (anode), and an electrolyte that allows ions to move from anode to cathode during discharge and return during recharge [8]. The voltage developed across the battery terminals varies according to the type of battery and the chemistry involved between the electrodes and electrolytes. In Figure Besides battery devices, another type of electrical energy storage device is the capacitors. The basic mechanism of these batteries and capacitors are very similar [7]. It is important to understand the basic differences between capacitors and rechargeable (secondary) batteries in the way they function. Generally, a battery consists of a positive electrode (cathode), a negative electrode (anode), and an electrolyte that allows ions to move from anode to cathode during discharge and return during recharge [8]. The voltage developed across the battery terminals varies according to the type of battery and the chemistry involved between the electrodes and electrolytes. In Figure 1.3 (a), electrochemical reactions typical of rechargeable batteries and their operating voltages are shown. The fundamental difference between batteries and capacitors is that the former store’s energy in the bulk of chemical reactants is capable of generating charge, whereas the latter store’s energy is directly as surface charge. Battery discharge rates and therefore power performance is then limited by the reaction kinetics as well as the mass transport, while such limitations do not apply to capacitors, thereby allowing exceptionally high power capability during both discharge and charge [9].
power density.
characteristics of any energy storage device. Power density is simply defined as the amount of electrical charge released at a time from a device, and energy density is the storage capacity of charges in the devices [10]. As an example, If the energy storage device is used in an electric vehicle, power density shows how fast one can go, and the energy density shows how far one can go on a single charge. Capacitors put out enormous power, but store only tiny amounts of energy, similar to the behavior of the water cup in Figure 1.3 (b). On the contrary, batteries store a fair amount of charge but charge and discharge slowly, similar to the large water bottle in Figure 1.3 (b). Although the energy density of batteries is high enough, the power density is not optimum for supporting heavy equipment like- electric cars, electric trains as well grid-level storage systems. To ensure the utilization of electricity in electric vehicles or grid-level storage, a new class of energy storage devices with optimum power and energy density is required to meet the challenges of electrical transformation. Supercapacitor or ultra-capacitor is an innovative technology that fills the gap between batteries and capacitors. These energy storage devices behold significant characteristics which make them efficient and capable to combine the energy storage properties of batteries and the power discharge properties of conventional capacitors [11]. So, there are three classes of capacitors such as electrostatic, electrolytic, and electrochemical or supercapacitors [12]. In electrostatic capacitors, the electrical charge is stored through physical reactions, and hence they provide the lowest energy density compared to the other two classes. Electrolytic capacitors work by growing insolation on a rough metal surface like etched aluminum and this type of capacitor has 10 times higher energy density than electrostatic capacitors [13]. But, the electrochemical capacitors process electric double layer capacitance (EDLC) at the solid electrolyte interface and that ensures impressive capacitance (10000 times greater than electrolytic capacitors) [14]. The capacitor stores charge electrostatically in an electric field and the amount of charge is measured by its capacitance, C, which is a function of the area of the metal plates, A, and the spacing between them, d. For typical capacitors, d ≈ 1 µm and A < 1 m2 (see Figure 1.4). So, to increase the amount of charge stored, one needs to minimize d and maximize A and that is what a supercapacitor does. A supercapacitor consists of two electrodes separated by an ion-permeable separator to prevent short circuits between the electrodes and the cell is impregnated with a liquid electrolyte [14]. As the potential is applied to a supercapacitor, ions from the electrolyte diffuse into the pores of the electrode of opposite charge. Charge accumulates at the interface between the electrodes and the electrolyte, forming two charged layers (known as electric double layers) with an extremely small separation distance on the order of 1 nm. The current supercapacitor technology uses activated carbon as the active electrode material since this material is electrically conductive and exhibits an extremely large surface area on the order of 1000-2000 m2 /g, meaning that high capacitance values can be achieved in a small space. For instance, comparing a capacitor and a supercapacitor of similar volume shown in Figure: 1.4, the capacitor stores only 0.003 F, whereas the supercapacitor stores >10,000 times more charge – about 50 F.
Supercapacitors have several advantages over conventional energy storage devices which makes it feasible to be implemented in the next generation vehicles:
I. Supercapacitors perform mid-way between conventional capacitors and batteries. They offer higher energy density than conventional capacitors and higher power density than batteries,
II. Supercapacitors can be charged in seconds, compared to hours for batteries and milliseconds for capacitors.
III. Supercapacitors store charge in a highly reversible process. Typical rechargeable batteries last only for a few hundreds of charge/discharge cycles, whereas supercapacitors can be used up to a million cycles.
IV. Environmentally friendly—no heavy metals or disposal issues as is common for conventional batteries.
V. Supercapacitors can be operated under extreme working conditions. For example, supercapacitors can be used at temperatures as low as −40 °C, where batteries cannot function properly. VI. Maintenance-free—providing superior cost-over-life, and fit-and-forget.
of storage devices.
Based upon the storage mechanism, electrochemical capacitors can be categorized broadly into two groups namely double-layer capacitors and pseudocapacitors. In EDLC, electrical energy storage occurs at the phase boundary between an electrode (electronic conductor) and the electrolyte solution (liquid ionic conductor) with no involvement of charge transfer [15]. Moreover, the current generated in this type of capacitor is merely a displacement current due to charge rearrangement or better known as an ideally polarized electrode.
The layer formation on the electrodes is called the Helmholtz layer. On the opposite, fast faradaic redox reactions caused by redox-active species involving metal oxides and conducting polymers with electrolytes are responsible for the potential determining charge transfer reaction that induces the charge storage mechanism of pseudocapacitors [16]. The capacitance and power density of pseudocapacitors are satisfactory compared to EDLC based materials, but the cyclic stability of pseudocapacitors is so poor that alone pseudocapacitors cannot solve the commercial crucial demand of cyclic stability. The destructive redox reactions are responsible for the unstable cyclic stability because, during these reactions, the surface morphology changes its phases and blocks accessible surfaces for incoming electrolytes. Towards this solution, merging these two types (EDLC and pseudocapacitors), hybrid capacitors are introduced to the supercapacitors family. This new hybrid type of capacitors can eventually be able to fulfill the energy and power density demand at a time as it stores charges in both faradic and non-faradic ways. So, supercapacitors have a wide range of applications not only in electronics but in automobiles as well. These devices are designed with various capacitance values to cater to different industrial requirements. Electronics applications such as cell phones and digital cameras, medical devices, and uninterruptible power supplies depend on supercapacitors ranging from 1 F to 150 F. Meanwhile, power backup in industrial and Telecom based stations and renewable energy systems require supercapacitors with the capacitance value ranging from 300 F to 350 F. Higher capacitance supercapacitors (650 F to 3000 F) are designed for automotive subsystems, hybrid drive trains, rail system power, heavy transportation, and many other applications. Besides, this technology has overcome the environmental issues whereby discarded batteries that may cause serious disposal waste are controllable. Moreover, the market interests in supercapacitors are expected to grow due to their greater power and longer shelf life that may lead to a greener environment.
Insight 