Observing Table 08-01 we can see that, for now, the battery manages to have a specific energy greater than the supercapacitor. In the other items there is a superiority on the part of the supercapacitor. We must emphasize the amount of charges and discharges, as well as the charge time that the supercapacitor presents, far surpassing the lithium-ion batteries. The thermal amplitude during the charge and discharge of a supercapacitor puts it in an extremely advantageous position in designs that need this feature. We can also highlight the efficiency and, mainly, the specific power. This feature allows the supercapacitor to deliver a large amount of power to the system in a short interval of time. And vice versa. It can also absorb large amounts of power from the system to return it in a subsequent moment.

This last feature (mentioned above) is being implemented in urban and suburban trains. So when the train is approaching a boarding/disembarkation station, when decreasing its speed, the train enters a braking regime regenerative, a regime that allows the engines to start operating as an energy generator. This energy is stored in supercapacitors. When the train departs towards the next station, the energy accumulated in the supercapacitors is used to assist in accelerating the train until it reaches a constant speed. This extra power supply stabilizes the voltage on the grid train power supply.

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A supercapacitor consists of two electrodes, an electrolyte and a separator that electrically isolates the two electrodes. The electrodes can be of the same material or of different materials. When the materials are the same, we say that the supercapacitor is symmetric. And when the materials are different, then they are called asymmetrical.

The electrode is the most important element of a supercapacitor. The performance of the supercapacitor is affected by electrochemical properties of the materials that compose it, the choice of electrolyte and the potential window. Therefore, to produce a supercapacitor with optimized properties it is necessary that it have a high potential gap between the cathode and anode, as this will provide greater energy density. Like this, a suitable combination of cathode and anode is of utmost importance for the fabrication of a supercapacitor with the properties mentioned above. For this reason, several nanostructured materials based on carbon, conductive polymers and metallic oxides are being implemented to manufacture the most diverse types of supercapacitors.

The electrolyte is a liquid that has a mixture of positive and negative ions usually from a non-toxic salt, solvated by a polar solvent, such as example, water. On each surface of the electrodes originates an area where the electrolyte liquid makes contact with the conductive metal surface of the electrode, forming a common interface between the two different phases of matter. Such an interface behaves like an insoluble coating of the electrode and the adjacent liquid electrolyte, a very special phenomenon that is the formation, or effect, of the double layer.

The separator is located between the two electrodes and its main characteristic is that it is permeable to ions. It is known as electric separator or electric separation membrane.

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Supercapacitors don't use solid dielectric like capacitors conventional electrolytics, instead use a solvated substance that forms a electrostatic double layer, also called electrical double layer. Supercapacitors make use of electrostatic capacitance and pseudocapacitance electrochemistry or a combination of both. In this way we can obtain three different types of supercapacitors. Let's study each of them separately.

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Electric double layer supercapacitors use activated carbon electrodes, or graphene (which is one of the allotropic forms of carbon) which increases the double-layer capacitance, as the specific area reaches hundreds of meters squares per gram, having much greater capacitance than the capacitors that use electrochemical principles. It should be noted that this type of supercapacitor uses only physical phenomena via electrostatics, no chemical change in the process is involved. loading and unloading process.

The high capacitance of double-layer supercapacitors electrical power is obtained due to the very small distance between the surface of the fixed reinforcement and (surface of activated carbon or graphene) and the electrostatic layer that is formed by ions surrounded by a thin solvent layer, whose thickness of the solvent layer is of the order of tenths of nanometers (typically 0.3 to 0.8 nm), thus making the thickness of the dielectric many times smaller than that of a conventional capacitor.

Supercapacitors utilize the capacitance resulting from charge separation. at the electrode-electrolyte interface. This arrangement of charges at the interface gives rise to the double layer electric. A polarizable electrode is one in which no charge transfer occurs when its potential is varied. Therefore, every increase in potential leads to an increase in electrical double layer capacitance. If the electrode area is increased through pores, large amounts of charge can be stored in a supercapacitor.

This type of supercapacitor has an electron-deficient positive electrode and a negative electrode. with excess electrons, both in contact with an electrolyte. An ion permeable separator is placed between the electrodes. During charging, the negative electrode attracts cations, while anions are accumulated by the positively charged electrode.

For the construction of an electric double layer capacitor, the material of the electrodes must be stable with respect to the electrolyte solution, therefore faradaic processes such as oxidation, corrosion and oxide film formation must not occur. For that, research is looking for non-corrosive electrode materials, such as activated carbon, which is widely used in electrochemical capacitors due to its low cost, high specific area and high stability for technological applications.

In Figure 08-01 we see an internal schematic of a double-layer supercapacitor. We see the electrodes with their respective separator. Note that the electrodes are wound in a cylindrical shape, to the length determined by the design to fit together achieve the specified capacitance.

The electrochemical process takes place after the application of a difference of potential (ddp), where there is an accumulation of charge (polarization) in the electrodes. During the charging process, electrons migrate from the positive electrode to the negative electrode, with this, the ions present in the electrolyte migrate by diffusion to the pores on the surface of the electrodes in order to maintain the electrostatic balance of the system. To prevent recombination of ions at the electrodes, the double charge layer is formed. The double layer, combined with the specific surface area of ​​the electrodes, and with the decrease of the distance between the electrodes, allows the same reach high values ​​of energy density.

In this process, an electrode negatively charged, solvent molecules are observed on the surface of the electrode adsorbed by dipole-electrode interaction. At this stage there may also be adsorbed anions with large ionic radius and low solvation. The plan where where all adsorbed ions are centered is called the plane internal Helmholtz. The outer Helmholtz plane corresponds to maximum approximation of solvated ions that are free to move within of the electrolyte.

Adsorption is the adhesion of molecules from a fluid (called adsorbed) to a solid surface (called adsorbent); the degree of adsorption depends on the temperature, pressure and the surface area that the electrode it presents.

The first double layer model was proposed by Helmholtz who investigated the properties between a solid electronic conductor and an ionic liquid conductor. he modeled this interface using two interfacial charge distributions. The first is nature electronic, on the electrode side, and the second of ionic nature, of opposite sign, on the electrolyte, as illustrated in the diagram in Figure 08-02 a). It should be noted that in this diagram the loads electronics in the electrode are represented by the symbol “+”, and in the electrolyte by the symbol “-” for anion or “+” for cation, surrounded by circles indicating solvation. The model Helmholtz method does not take into account the dependence of capacitance on voltage.

Thinking about that, Gouy and Chapman introduced in 1,910 a model that adopts random thermal motion, and which considers the spatial distribution of ionic charges in the electrolyte. This distribution of loads became known as the diffuse layer, represented in Figure 08-0 b). In 1,924 Stern improved the model of Gouy and Chapman, introducing the dimensions of ions and molecules of the solvent, and dividing the space charge into two distinct zones, conform Figure 08-02 c): a layer compact, consisting of ions adsorbed on the surface of the electrode, and a diffuse layer as the one defined by Gouy and Chapman.

Generally, the thickness of the electrical double layer is of the order of 0.5 nm to 1 nm, which results in a high capacitance, especially when combined with porous electrodes, which produce a very large increase in the effective area of ​​contact. In Figure 08-03 sketches of a capacitor of double layer, highlighting its main elements:

Also illustrated in Figure 08-03 is a simplified electrical circuit for the double capacitor layer, being represented by two capacitors connected in series.

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The pseudocapacitance is a faradaic mechanism (transfer of electric charge between the active material and the electrolyte) charge storage based on the fast and reversible redox reactions that occur involving materials present in the electroactive material of the electrode, such as for example: metal oxides/sulfides and conductive polymers. A pseudocapacitance can occur throughout the electrode volume, allowing a absolute capacitance greater than the electrical double layer mechanism that occurs only on the surface of the electrode.

The pseudocapacitance of a material can be intrinsic or extrinsic. At the first case, the materials have pseudocapacitive behavior for different types of morphologies and particle sizes. The pseudocapacitance extrinsic only appears under severe conditions, such as, for example, materials with nanometric dimensions, while the same behavior is not observed in bulk type material.

When a potential is applied to a pseudocapacitor, the process of reduction and oxidation in the electrode material, involving the passage of charge through the double layer, resulting in the passage of faradic current through the cell of the supercapacitor. The faradic process involved in supercapacitors allows they reach a specific capacitance about 10 - 100 times greater than the values obtained for the electrical double layer devices.

There's a difference fundamental between the electrochemical behavior of these capacitors in relation to that of batteries. How a battery and an electrochemical capacitor store energy chemistry and converts it into electrical energy are different. At least ideally, the electrochemical energy storage in an electrochemical capacitor occurs locally in a two-dimensional region, at the electrode/electrolyte interface, through reactions fast and reversible, and therefore do not involve density changes within the electrolyte or electrode. In batteries, the storage of electrochemical energy involves a region in the three-dimensional volume, within the electrolyte and electrodes, where density changes through reversible but slower chemical reactions. Per occur in an interface and have rapid reversibility, resembling a electrostatic energy storage, electrochemical energy storage in electrochemical capacitors is called pseudocapacitance.

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As we have seen, electrical double-layer supercapacitors offer good cyclic stabilities and good energy performance. the supercapacitors pseudocapacitives offer high specific capacitance. In the case of a system hybrid, it offers a combination of both, i.e. combining the power source battery-type electrode, with a capacitor-type electrode power source in the same cell. With a correct combination of electrodes, it is possible to increase the cell potential, which leads to an improvement in energy and power densities of the system.

The key for the system to behave like a supercapacitor, independent of the mechanism in which it operates, consists of the nature of the material used as an electrode. Several materials are reported in the literature that can be used as electrodes such as: metallic oxides (RuO₂, NiO, MnO, Co₃O₄), metallic sulfides, carbon-based materials (Graphene, Nanotubes Carbon, Activated Charcoal, Mesopores Carbon), and nanocomposites. Of the materials used as electrodes, carbon-based materials, such as graphene, are the most used.

In Figure 08-04 we see a typical illustration of a hybrid capacitor using as one of the electrodes a compound formed by doping of lithium ions with graphite. This causes the supercapacitor voltage to equal the lithium battery voltage, significantly improving the energy density of the supercapacitor. The company EATON already manufactures supercapacitors with a 3.8 V working voltage, voltage compatible with lithium batteries. In addition, your working temperature may reach at + 85° C.

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The main characteristics that an electrode must have, for a double-layer supercapacitor, are a high surface area and good conductivity (or low resistivity). One of the first materials to be used in supercapacitor electrodes was Activated carbon is still the most used commercially due to the good characteristics mentioned above. In addition, it has Low cost compared to other materials. Let's look at some types of materials that can be used as electrodes.

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The activated carbon consists of particles with pores that provide high surface area and can reach 2,500 m²/g, area measured by nitrogen adsorption. Activated carbon has been produced from carbonaceous materials such as bamboo, coconut shell, wood, etc... The most used raw material for applications with high area superficial is the coconut shell. It is pyrolyzed by being heated in an inert atmosphere with a temperature ranging from 600° C up to 900° C, producing charcoal.

Next, the activation process is carried out by taking this charcoal to a high temperature furnace (600 - 1,200° C) in the controlled presence of water vapor or oxygen. As a result, pores open in the particles as the charcoal reacts with oxygen, followed by the production of CO₂. Most activated carbon pores have smaller openings than 2 nm, being classified as micropores. However, they also have a minority of mesopores with apertures from 2 nm to 50 nm. As activated carbon consists of loose particles, for the manufacture of electrodes it is necessary to use a binder that mechanically unites these particles. To increase the conductivity between particles and lowering the general resistance of the electrode is used in the mixture of carbon black particles. After preparing the mix it is deposited on a sheet of metal, known as the collector, which makes the electrical connection between the electrode and the world external.

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Porous silicon is a material obtained from monocrystalline silicon, composed of pores that resemble wells cylindrical rods extending perpendicular to the surface of the silicon substrate, the depth of which determines the thickness of the layer.

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The scientific and technological advances used in the study of compounds of carbon led to the discovery of an important allotropic form called carbon nanotubes (NTC). They were observed by Sumio Ijima in 1991, with the aid of the MIT technique (transmission electron microscopy). Since their discovery, carbon nanotubes have been object of intense investigation, both from the point of view of basic science and applications for the development of new technologies.

Carbon nanotubes are long cylinders made up of carbon atoms covalently bonded to three others, forming a hexagonal structure with sp² hybridization, similar to that observed in graphene sheets that make up graphite. From the point of view structural, there are two basic types of NTCs: the wall simple, that is, formed by winding a single sheet of graphene (known such as SWCNT) and multi-walled ones, in which several sheets of graphene are rolled up in concentric shape (known as MWCNT), which also includes the formation of double-walled carbon nanotubes (DWCNT). The MWCNT is formed by several cylinders of graphene wound along the central axis with separation interlayers of approximately 0.34 nm, indicative of the interplanar spacing of the graphite. Figure 08.05 shows the structure of single-walled and multi-walled nanotubes.

Carbon nanotubes can be thought of as a single sheet of graphene rolled along a characteristic cylinder-shaped axis. The axis in which sheets curl up is responsible for the chirality of carbon nanotubes. This axis is defined by vectors positioned in the unit cell of the nanotube and the winding can take place in different forms, giving rise to three possible classifications of chirality for carbon nanotubes: zigzag, armchair and chiral.

Chirality is understood as the property of a molecule or ion of not being able to be superimposed to its mirror image by any combination of rotation, translation or some conformational change.

Single-walled carbon nanotubes have a diameter of the order of 1 nm and has attracted the attention of researchers due to its mechanical properties and Exceptional electrical. They can be metallic or semiconductor, have modulus Young's densities between 640 GPa and 1 TPa, tensile strength of 150-180 GPa and conductivity theoretical thermal of 6,000 W/mK.

In the field of storage, conversion and transmission of electrical energy, Carbon nanotubes are also being widely used. Multi-walled nanotubes are being used in lithium-ion batteries mixed with cathode and anode materials, increasing electrical conductivity and mechanical resistance, which implies a increase in battery cycle life. They have also been applied, both pure and in composites, in electrode materials for supercapacitors, including high-performance flexible devices.

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Graphene, another of carbon's allotropes, is a planar monolayer of carbon atoms hybridized in sp² and arranged in a hexagonal structure, that is, it can be understood as a single sheet of graphite separated from its structure three-dimensional. The term graphite should be used in reference to a structure of graphene sheets stacked in a three-dimensional arrangement.

Graphene has been considered as one of the most promising materials for electrodes in electrochemical devices. Several works in the literature have proposed the use of graphene for the manufacture of electrodes suggesting that this class of material can, in a short time, complement the technological advances achieved by carbon nanotubes.

A reproducible synthesis method for obtaining graphene is through mechanical exfoliation. This technique has been widely used, but many Efforts have been made to develop new processing routes that enable the efficient production of graphene in large quantities scale. Currently, three main methods of synthesis are used, one of which is physical, a chemical method and the third, by chemical vapor deposition.

The advantage of using carbon nanomaterials lies in the versatility that they present in structural and textural properties, in addition to the high electrical conductivity, low density values ​​and high compatibility with other materials.

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Electrolytes are ionic liquids consisting of a diverse group of salts that are found in the liquid phase at room temperature. They can be composed of a large number of cations and anions with an estimated number of possibilities on the order of 10¹⁸, which makes this class of compounds one of the most comprehensive known in Chemical. Several synonyms are found with reference to ionic liquids, such as room temperature molten salts, low temperature molten salts, salts molten organics and can therefore be used in the search for information on these substances in the literature. Its physicochemical properties are very similar to of molten salts at high temperature, but practical handling aspects are sufficient to maintain the merit of the distinction. The term ionic liquid is often extended to salts that have melting points below 100 °C and remain liquid in a considerable temperature range. This behavior is possible, since ionic liquids are formed by the combination of a bulky and asymmetric cation with a weakly coordinating anion, which causes a reduction in the interaction between them and on salt grid power.

These electrolytes have a set of characteristics very interesting, such as good chemical and thermal stability, vapor pressure negligible, high densities, are liquid over a wide temperature range, negligible flammability, low toxicity, high ionic conductivity and wide window potential electrochemistry. However, knowledge about their physical and chemical properties is still limited and the large number of different combinations of cations and anions further intensifies the problem.

We must have an electrolyte that is chemically inert in order not to chemically attack the supercapacitor materials, allowing long life and stability to electrical parameters of the supercapacitor. The electrolyte must also have a low viscosity to allow it to the ions permeate the pores of the electrode.

Since the energy contained in capacitors increases with the square of the potential, researchers are looking for a way to increase the value of the rupture potential of the electrolyte.

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The electrical separator, that is, the electrical separation membrane, provides a physical separation between electrodes to prevent short circuits due to direct contact between themselves. It must be very thin, on the order of a few thousandths of a millimeter and, naturally, it must have a large porosity to allow the easy passage of solvated ions and thus reduce the equivalent series resistance. Electric separators must also be chemically inert to preserve the stability of the electrodes and the condition of conductivity. To reduce manufacturing process costs, paper is used as electrical separator on some models of supercapacitors.

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    8.1   Capacitance

Supercapacitors have their capacitance given by eq. 08-01.

Where the variables are:

Thus, the so-called specific capacitance of a supercapacitor, which is defined as the capacitance per unit area or volume, it's hundreds of thousands of times greater than the specific capacitance of electrostatic capacitors and electrolytes. This is due to the existing separation between the charges in the supercapacitor, which is of the order of tenths of nanometers.

To calculate the capacitance, another approach is possible knowing that it depends on the charge and voltage on the capacitor. Thus, the eq. 08-02 presents this relationship.

This equation expresses the capacitance value knowing the charge current of the capacitor and the time elapsed until the voltage across the capacitor changes ΔV.

On the other hand, we know that the material used in the electrodes must have high porosity, formed by micropores, as if was a spongy material, since this characteristic provides high specific surface area, allowing to achieve high specific capacitance. As we saw earlier, activated carbon and graphene are the most used materials in manufacture of supercapacitors. Thus, we can conclude that if we want a supercapacitor with high energy density, we use material that has small micropores. Otherwise, that is, we want high power density, so we choose materials that have larger micropores, as this reduces the equivalent series resistance of the supercapacitor.

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    8.2   Power Density

The limiting parameter of the power that a supercapacitor can deliver to a circuit is the equivalent series resistance. We can calculate what this power is using the maximum power transfer theorem (remember this theorem in Maximum Power Transfer Theorem). Applying the theorem, we calculate that maximum power delivered is given by eq. 08-03.

In the electronic market we find supercapacitors of 220 F that have an internal resistance of 40 milliohms and a breakdown voltage of 2.7 V. In this case, using eq. 08-03, we calculate that the maximum power that the supercapacitor can supply the circuit is 45.5 W.

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    8.3   Charging and Discharging Feature in one

The double layer supercapacitors, when under the action of a discharge, present a continuous voltage drop of linear shape. This can lead to some problems in some applications. Suppose a device that needs a stable voltage to function correctly. A double-layer supercapacitor does not meet this specification. In this case, it would be necessary to use of a DC-DC converter. A DC-DC converter is a device that allows the input voltage to be varied within certain limits and keeps the output voltage stable.

In the case of a lithium-ion battery, it has the ability to maintain a stable voltage at its output under different consumption levels of the equipment it is supplying. This differential benefits the use of such batteries to the detriment of the supercapacitors. To make this characteristic clearer, Figure 08-06 illustrates the behavior of a supercapacitor of double layer and lithium batteries when under load.

Notice how the Li-Ion battery maintains an almost constant voltage while supplying power to the device. when there is the energy depletion, the voltage drops sharply. In this case, there is a need to recharge the battery.

However, this reality is a little different when it comes to hybrid supercapacitors. like supercapacitors Hybrids are a mixture of a double-layer supercapacitor and an electrochemical supercapacitor, it has the ability to maintain a stable voltage for a certain time while powering the circuit. Similar to lithium-ion batteries. The Figure 08-07 illustrates this behavior.

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After everything that has been seen so far, we are going to study how we can apply all the advantages that the supercapacitor offers. We must keep in mind that the best feature of the supercapacitor is its power density, in addition to its high voltage capacity store energy. Thus, various technologies can take advantage of these great features. Let's study some of them.

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    9.1   Electric Car Development

It is not new that there is a desire to develop a satisfactory technology so that cars can have an electric motor as a driving force. The energy source for electric motors is clean and inexhaustible, unlike fossil fuels that have an expiry date. Furthermore, there is a high expectation that fossil fuels will reach a price exorbitant when close to its exhaustion.

The answer to this dilemma lies precisely in technology. The possibility of producing electricity at an ever lower cost, clean and renewable, it has fantastic potential to lower the cost of fueling a vehicle. There is also an advantage due to the efficiency that the electric car has transforming electrical energy into mechanical energy. Virtually all available energy is converted into mechanical power.

To be able to manufacture electric cars with the same capacity and autonomy as fossil fuel cars, several approaches are being tested by companies in this time of transition. Today, using electric motors, there are four types of cars we can describe:

With the development of supercapacitors we can add the fifth model of electric car: the electric car with supercapacitors. The great advantage of using supercapacitors is their great power density, a very important characteristic during the starting of a vehicle when great power is needed to accelerate it.

In Figure 08-09 we see a prototype of an electric car using only supercapacitors idealized in the doctoral thesis at the University of Tokyo in 2008. A 1,000 F capacitor was used allowing the vehicle to be used for 20 minutes reaching a speed of 50 Km/h. The most interesting thing is the supercapacitor recharge time that needs just 20 seconds to acquire full charge.

Many car manufacturers are embracing the use of supercapacitors, not only for starting, but also for distributing various supercapacitors throughout the car body to meet the needs of headlights, turn signals, radios, etc...

One of these manufacturers is Lamborghini which launched its Sian model, as we can see in Figure 08-10. This is a hybrid model, however it does not use any type of battery. Only supercapacitors. And the trend, according to the company, is replace batteries with supercapacitors in all its models in a short period of time.

Thus, it is expected that with the advancement in supercapacitor manufacturing technology, we will be able to replace the supercapacitors in the near future batteries used in automobiles and other types of vehicles, by supercapacitors.