**4.1 Construction and production of SC**

The construction of the laboratory prototype and zero series begins with the choice of material and type of supercapacitor (asymmetrical - polarized, or symmetrical nonpolarized). The variant with a solid active material, which the authors of this chapter have often used, is shown in **Figure 4**. The connection of the active material

*Construction of supercapacitor prototype [23].*

#### **Figure 5.**

with the power supply is achieved by using conductive silver glue. The role of a separator is provided by a Nafion foil soaked in a selected electrolyte.

Today, the active material is applied from a suspension, as well as the printing techniques that are also applied for micro SC (**Figures 5** and **6**).

#### **Figure 6.**

*(a–c) Schematic diagram of the fabrication process for micro-supercapacitors by laser scribing method. (d, e) Flexible micro-supercapacitors with high areal density [25].*

**Figure 7.** *Structure of Murata's supercapacitor (cross-section) [26].*

*Supercapacitors: The Innovation of Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.106705*

#### **Figure 8.** *Supercapacitor engine start module [27].*

After testing and optimization, a zero series is made, where the technological processes of production are elaborated (and often simplified) and only then the mass production could start. The appearance and structure of a commercial supercapacitor of low power and capacitance are shown in **Figure 7**, and for higher powers, voltages, and capacitances, it is shown in **Figure 8**.

Depending on the field of application and the set of parameters, the final structure, production technology, and housing are selected. If the supercapacitor is used as a replacement for a battery to power small consumers, in addition to the capacitance, it is very important that it has a low leakage current. In contrast, when powering larger consumers, it is much more important that the supercapacitor has a low series resistance due to losses at high currents. Thus, the type of supercapacitor is defined. As an example, in **Figure 9**. the types of supercapacitors are given for the different needs of power supply backup.

### **4.2 Modeling and characterization of SC**

In order to predict the behavior in different conditions, electrochemical processes are modeled and simulated on the computer [2]. Most often, an equivalent electric circuit with two or more RC branches is taken as a model (**Figure 10**) [29].

For many years, the authors of this chapter have done research in this area and have developed their models. The simplified model will be presented here for the case of sulfide minerals in an aqueous solution of sulfuric acid. The equivalent circuit, shown in **Figure 11**, was adopted. Capacitors represent double-layer and diffusion capacitance, and resistors correspond to electrolyte resistance, diffusion resistance, and leakage current.

**Figure 9.** *Performance for supercapacitor selection [28].*

#### **Figure 10.**

*EDLC models: A - ideal capacitor, B - series RC model, C - model B with added leakage resistor, D - model C with added high-frequency inductance component, and E - model D expanded with n-branch RC circuits and voltagedependent main capacitance [29].*

*Supercapacitors: The Innovation of Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.106705*

**Figure 11.** *The equivalent electric circuit.*

**Figure 12.** *The circuit excitation and the response.*

For the assumed equivalent electric circuit of the observed electrochemical system and a short voltage pulse (of the order of 0.1 s for such systems), the response of the system (current in this case) will be as shown in **Figure 12**. The following parameters are:

*<sup>I</sup>* <sup>¼</sup> *<sup>E</sup> <sup>R</sup>*0þ*R*<sup>123</sup> - quasi-stationary charging current. *<sup>R</sup>*<sup>123</sup> <sup>¼</sup> *<sup>R</sup>*1*R*2*R*<sup>3</sup> *<sup>R</sup>*1*R*2þ*R*1*R*3þ*R*2*R*<sup>3</sup> - eq. resistance of parallel connection R1, R2, and R3. *<sup>I</sup>*<sup>10</sup> <sup>¼</sup> �*UC*<sup>10</sup> *<sup>R</sup>*1þ*R*<sup>023</sup> - initial discharge current. *<sup>R</sup>*<sup>023</sup> <sup>¼</sup> *<sup>R</sup>*0*R*2*R*<sup>3</sup> *<sup>R</sup>*0*R*2þ*R*0*R*3þ*R*2*R*<sup>3</sup> - eq. resistance of parallel connection R0 , R2, and R3. *UC*<sup>10</sup> <sup>¼</sup> *<sup>I</sup>*2∙Δ*<sup>t</sup> C*1 *R*2 *<sup>R</sup>*1þ*R*<sup>2</sup> - initial discharge voltage of capacitor C1. *<sup>I</sup>*<sup>20</sup> <sup>¼</sup> �*UC*<sup>20</sup> *<sup>R</sup>*2þ*R*<sup>03</sup> - quasi-stationary discharge current. *<sup>R</sup>*<sup>03</sup> <sup>¼</sup> *<sup>R</sup>*0*R*<sup>3</sup> *<sup>R</sup>*0þ*R*<sup>3</sup> - eq. resistance of parallel connection R0 and R3. *UC*<sup>20</sup> <sup>¼</sup> *<sup>I</sup>*2Δ*<sup>t</sup> C*2 *R*1 *<sup>R</sup>*1þ*R*<sup>2</sup> - initial discharge voltage of capacitor C2. *τ*<sup>1</sup> ¼ ð Þ *R*<sup>1</sup> þ *R*<sup>023</sup> *C*<sup>1</sup> - time constant of the first discharge phase. *τ*<sup>2</sup> ¼ ð Þ *R*<sup>21</sup> þ *R*<sup>03</sup> *C*<sup>2</sup> - time constant of the second discharge phase.

Based on the given analysis, the parameters of the electric circuit can be determined, and therefore, in the same manner, the physical parameters of the electrochemical system [30].

In order to check the provided model, an electrochemical system was formed. Several experiments were performed using this method and it has been verified and determined that it can be applied for rapid characterization of electrochemical systems. The experiments were carried out using a system for electrochemical tests based on a PC and the LabVIEW software package (**Figure 13**) [31]. The system is designed to cover most electrochemical tests in wide ranges, both at the level of the electrochemical cell, and at the level of the completed supercapacitor (**Figure 14**) [32].

Modeling can also be done on a physical level. For example, a finite element model for charge transport in conjugated polymers has been developed, but it is still being refined [33].

**Figure 13.** *System for electrochemical testing.*

*Supercapacitors: The Innovation of Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.106705*

**Figure 14.**

*Block diagram of the system for supercapacitor prototype characterization.*

### **5. Use of supercapacitors**

From the user's point of view, the most environmentally friendly form of energy is electricity. However, if we take into account the way in which this energy was obtained, then it fully retains its ecological advantage only if it originates from solar energy, wind, and wave energy, and to a considerable extent, from hydropower. In the latter case, namely, significant harmful ecological effects may occur due to disturbance of water regimes, such as surface waters. However, even when the electricity comes from the burning of fossil fuels or from nuclear reactions, problems related to the negative effects of by-products can be solved much more efficiently in one place than, for example, in all vehicles that move using the appropriate energy. One of the conditions for the use of electric energy in vehicles is the existence of such a device that would have high specific energy but at the same time a high specific power, which standard electric devices could not provide. The advent of supercapacitors has made this application much more realistic.

In the case of supplying consumers with energy that comes from solar or wind energy, it is necessary to have an appropriate method of energy storage for the period when there is no sun or wind. In this case, supercapacitors have an advantage over standard batteries because they can withstand a much greater number of charging and discharging cycles.

From a very large number of possible applications of supercapacitors, current examples will be listed where their characteristics are irreplaceable.

#### **5.1 Energy harvesting**

Monitoring of environmental parameters requires the installation of systems in inaccessible and dangerous terrains. After installation, the system is expected to have a lifetime as long as possible with minimal maintenance. In addition, energy consumption is directly related to the lifetime of a wireless sensor network (WSN). It is similar to other scattered and/or remote systems. Primary (non-rechargeable) batteries, despite the application of modern energy management algorithms, have the greatest impact on the limited lifetime of a wireless sensor node. Also, regular technical interventions in the field, primarily battery replacement, drastically increase the cost of maintenance. With the aim of increasing the life span and reducing maintenance costs, current research studies involve the use of secondary (rechargeable)

**Figure 15.** *Environmental energy sources [35].*

batteries and the so-called collecting energy from the environment, that is, "energy harvesting" (EH), which contributes to WSM getting the self-powered prefix. Due to the characteristics of secondary batteries that degrade over time, the increase in lifetime is insufficient for multi-year monitoring of certain environmental parameters. This is the reason why, instead of rechargeable batteries, capacitors with very large capacities— supercapacitors—can be used to power the node. They represent reversible electrochemical systems, and they are increasingly used to power sensor nodes. For several reasons, supercapacitors are favorable for power supply, one of them being the exceptional scalability that allows increasing capacity and performance with increasing dimensions and weight. The characteristics of supercapacitors, such as high-power density, fast charging, large number of charging cycles, temperature stability, small equivalent series resistance, and low leakage current, favor the operation mode of most wireless sensor nodes. However, the lower energy density compared to batteries contributes to the fact that they are discharged relatively quickly and require frequent recharging. That is why it is necessary to provide a constant or at least intermittent source of energy in the natural environment. It can be a solar panel, piezo vibration transducer, thermoelectric generator, antenna, etc. [34]. Possible sources of environmental energy are shown in **Figure 15**.

#### *5.1.1 Piezoelectric effect*

Piezoelectric materials have the property of converting energy through the direct piezoelectric effect, the energy of mechanical deformations of the piezoelectric structure into the electric field, that is, voltage [36]. Two modes, that is, mode 33 and mode 31, are used in most developed EH applications (**Figure 16**). In both of these modes, the electric field, and thus, the generated voltage on the electrodes, are oriented along the direction of polarization 3, while external forces cause stresses in a single direction. In mode 33, this is the same direction as the stress (3), while in mode 31 the stresses are along the normal (1) [37].

*Supercapacitors: The Innovation of Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.106705*

**Figure 16.** *Operational modes of piezoelectric material for EH applications [37].*

#### *5.1.2 Electromagnetic conversion*

Electromagnetic induction, described by Faraday's law, is the creation of electromotive force (EMF), that is, voltage on an electric conductor in a changing magnetic field, a phenomenon that forms the basis of electric generators. The induced EMF is proportional to the strength of the magnetic field, the speed of the relative movement, and the number of turns of the conductor. If a conductor is connected to an electrical load, the current will flow, thus generating electricity. This system is often used as an effective tool for the realization of kinetic EH systems, where the relative displacement of the permanent magnet in relation to the coil is caused by the vibrations of the energy generation base [38].

#### *5.1.3 Comparison of piezoelectric and EH devices with electromagnetic vibrations*

The results of a thorough comparison of electromagnetic and piezoelectric vibration EH systems, with identical volumes, seismic masses, natural frequencies, quality factors, and excitation conditions, are given in [39]. Appropriate mathematical models of both types of vibration EH devices were used for the calculation of output voltages and powers (**Figure 17**) and depending on the intensity and frequency of harmonic dynamic excitation, the recommended configuration of the most efficient vibration EH systems [39].

#### *5.1.4 Magneto-strictive effect*

Physically in a way similar to the piezoelectric effect. A characteristic property of magneto-strictiveness is that the magneto-elastic coupling induces mechanical

#### **Figure 17.** *Model of electromagnetic (a) and piezoelectric (b) vibration EH systems for output power analysis [39].*

elongations if they are subjected to a magnetic field, while conversely, their magnetization will change due to changes in the applied mechanical stresses. This effect can be used in EH devices, however, an additional coil is required to obtain electrical energy [35].
