The main properties of SCs are low energy and high power density, fast charge and discharge, the termination of energy flow when fully charged, minimal internal resistance (ESR), long shelf life, and extended lifetime. The advantages of SCs make them superior to the other conventional storage devices in many ways. Comparing their distinctions and drawbacks, using SCs with other storage devices appears to be suitable [42]. SCs are low voltage components and require safe operation, where the voltage remains within specified limits. Standard SCs with aqueous electrolytes are rated within a voltage range of 2.1 to 2.3 V, and SCs with organic solvents are rated from 2.5 to 2.7 V [40]. For higher voltage requirements, the SC cells are connected in series. The rated capacitance value is between 1 F to 1000 F; for higher applications, capacitances are required to connect SC cells in parallel [3,95]. Figure 2 shows that the SCs can bridge batteries and capacitors [94,103,104]. The energy density of SCs is greater than in the conventional capacitors; however, the power density of capacitors is greater than in the SCs.

SCs can be classified depending on their manufacturing and construction details. SCs can be made in flat, cylindrical, or rectangular case styles [105,106]. The operation principle of SCs is based on energy storage and, depending on the energy storage method, SCs are divided into three main groups. SCs can be divided into EDLCs and pseudocapacitors (PCs) depending on the energy storage method. Charge storage occurs between the electrolyte and electrodes in EDLC, as shown in Figure 1b. PCs involve reversible and fast Faradaic redox reactions for charge in order to increase the capacitance of the SC, as shown in Figure 3a. A hybrid supercapacitor (HSC) stores the charges by matching the capacitive carbon electrode with a pseudocapacitive or lithium-insertion electrode, as shown in Figure 3b [107,108,109].

EDLCs consist of two carbon-based electrode materials, enough electrolytes, and a separator. EDLCs can either store charges electrostatically or via a non-Faradaic method, without the transfer of charge loads with the electrochemical double-layer storage principle [108,110,111]. The three main types of EDLCs have a specific condition of the carbon substance. Carbon nanotubes (CNTs), graphene, carbon aerogels and foams, carbide-derived carbon (CDC), and activated carbon are the main types of pseudocapacitors, as seen in Figure 4. [100]. PCs store charges via a Faradaic process involving the transfer of charge loads electrostatically [112]. When a voltage is applied to pseudocapacitors, reduction and oxidation occurs in the electrode material and Faradaic current passes through the SC cell. This Faradaic process leads to pseudocapacitors having higher energy densities than EDLCs. This type of capacitor includes metal oxides, metal-doped carbon, and conductive polymer electrode materials [113]. Conductive polymer types of SCs have a high capacitance, low ESR, and low cost compared to carbon-based EDLCs. However, pseudocapacitors also have a lower power density and a shorter life cycle, depending on the redox reactions in the SC [100,108]. A hybrid SC system offers a union of the energy source of a battery-like electrode with a power source of a capacitor-like electrode in the same cell [108,114]. This type of SC consists of polarisable electrodes, such as carbon, and non-polarisable electrodes, such as metal or conducting polymer. Faradaic and non-Faradaic processes obtain high energy storage through both electrodes [33,100,115]. SC researchers have focused on the three current types of hybrid SCs, distinguished by their electrode configurations: asymmetric, composite, and battery-type [28,108]. Hybrid SCs which primarily exhibit electrostatic and other electrochemical capacitance are called as a supercabatteries [106,116].

The electrolyte type and electrode material determine SC characteristics, and in recent years there have been some comprehensive studies in this area [106]. According to current reports, it is expected that materials will control SC performance and cost in the future. These reports include the percentage of new research on hierarchical and hexahedral electrodes [79,117]. SC materials are mainly investigated as electrode materials, electrolyte materials, separators, and collectors.

SC electrodes are generally thin sheets that are electrically connected to a conductive current collector. The environmentally friendly and low-cost electrodes must have good conductivity, low corrosion resistance, and long-time chemical stability [9,106]. The different types of carbon electrode materials commonly used in SCs include activated carbon (AC), carbon aerogel, graphene, graphite, and carbon nanotubes (CNTs) [32,33,34,35]. Activated carbon is enough for SC EDLC electrodes, although its electrical conductivity is much lower than metals. One of the most used electrode materials for SCs is a solid form activated carbon called consolidated amorphous carbon (CAC) [23,33]. Activated carbon fibres (ACF) have a diameter of about 10 µm and are derived from activated carbon [118]. Carbide derived carbon (CDC) is a family of tuneable and nanoporous carbon materials [119,120]. The other most widely used materials are random porous carbons, due to their advantages [121]. Graphene atoms, also called nanocomposite paper atoms, are arranged in a regular hexagonal pattern as seen in Figure 5a [122,123,124,125]. MnO₂ and RuO₂ electrode materials are also used for pseudocapacitors since they act as capacitive electrodes and exhibit Faradaic behaviour, as seen in Figure 5b. Pseudocapacitors occur within the active electrode materials created through Faradaic redox reactions and provide a high specific energy. All the commercial hybrid SCs are asymmetric, and they integrate an electrode [106,126].

Although most of the studies are focused on electrode materials of electrolytes, there are also significant studies on SC performance. The electrolyte consists of a solvent and dissolved chemicals that makes it electrically conductive and increases the quantity of ions in the electrolyte [9,106]. Electrolytes influence the operational voltage window of cells and their resistance [114]. Aqueous, organic, and ionic liquid electrolytes are currently available for SCs [94]. The electrolyte determines the SC’s operating characteristics [127]. Water is a perfect solvent for inorganic chemicals and aqueous electrolytes. Aqueous electrolytes are used in SCs with high specific power and low specific energy density, which have a 1.15 V dissociation voltage per electrode [106,128]. Electrolytes with organic solvents have a higher separation voltage and a temperature range; however, they are more expensive [106,128,129]. Ionic electrolytes consist of liquid salts, and they enable capacitor voltages above 3.5 V s. In addition, they have lower ionic conductivity than the other electrolytes [40].

Although much progress has been made in improving the electrode performance, in SCs, separators can negatively influence the performance of SCs to depend on the poorly designed dividers [9]. The separator can be very thin and must be very porous to minimise ESR. Developed polymer-based separators with low cost, high flexibility, and porosity lead the separator markets [41,94,106,130]. The majority of energy storage devices require collectors to connect the capacitor electrodes and supplement the performance of SCs, because of the active material’s insufficient conductivity. Additionally, they must carry high charge and discharge currents [94,106,131]. Sealing in cell mounting is very important to prevent performance loss in the SC. Aluminium metal should be used in collectors to prevent a corrosive galvanic cell housing [9,106]. A sealant material’s duty is to prevent foreign contaminants from entering the cell that can cause electrolyte disruption, surface oxidation, and the loss of life cycle [94].

3. Applications of SC

The SC has many advantages in applications with a high power density, and many charge/discharge cycles or a longer life are required. SCs are used in wind turbines, mobile base stations, electronic devices, and different industrial practices [135,136,137]. In addition, they have started to be used in UPS, electric vehicles, and various power electronics applications, thanks to their superiority over lead-acid batteries [138,139,140,141]. In recent years, SCs have been used as an energy storage device for voltage stability in renewable and hybrid energy storage systems to regulate the source and grid [3,10,141]. SCs can stabilise the power supply in applications with fluctuating loads [142]. SCs deliver power for flashes, which can be charged quickly [62,143], and portable speakers [144]. Reducing energy consumption and CO₂ emissions is a primary difficulty of all transportation systems, and braking energy recovery can reduce both. Many applications in all kinds of vehicles require elements that can rapidly store and deliver energy, and SCs fulfil these requirements. Some SC applications include consumer electronics [62,63], tools, power supply [64], voltage stabilisation [65], microgrid [66], renewable energy storage [3], energy harvesting [67,68], street lights [69], medical applications [70], military and automotive applications [71,72,73], and energy recovery [74,75,76,77]. Some of these examples are given in Figure 8.

Figure 8. Some examples of SC applications.

Multiple variable loads, such as hybrid electrical vehicles (HEVs), electrical vehicle (EV) charge stations, electrical machines, and other power systems, cause current fluctuations and harmonics and power oscillations on the grid [63,64]. SCs can be used between the load and the grid as an interface to overcome these problems [145]. One wireless screwdriver with SCs is charged fully in 90 s, and it can retain 85% of its charged energy after three months left idle [146]. The backup power for actuators in wind turbine pitch systems is also provided by SCs [144]. Photovoltaic and wind energy systems act as a fluctuating supply induced by weather conditions. SCs can stabilise such as voltage fluctuations for power lines by acting as dampeners [66]. SCs can be used for microgrid storage, usually powered by renewable energy which cannot instantaneously match the demand to inject power when the demand increases and the production decreases temporarily [68,147]. SCs are suitable energy harvesting systems for temporary energy storage devices [3]. For example, in Japan’s Niigata Prefecture, Sado City, there are streetlights that combine stand-alone SCs with s power source for storage [69].

Hybrid SCs are also implemented in navigators, sensors, and communication devices based on batteries. The radar system, electromagnetic pulse weapons, torpedoes, etc., can also be operated using a suitable installation of hybrid SCs [148]. Many SC systems for military applications are manufactured by the Tecate Group corporation, as shown in Figure 9 [148]. The radar antenna, airbag exploitation power, avionics, GPS, and missiles are applications that require a high specific power [28].

Figure 9. SCs used for different defence applications.

SCs also fulfil the requirements for some transportation applications, which are given here. Toyota’s Yaris hybrid-R concept car and Peugeot Société Anonymes (PSAs) Peugeot Citroën both use an SC to increase the performance of the vehicles [149]. The Maxwell Technologies manufacturer corporation claimed that several hybrid buses use SC devices to improve acceleration [149]. Batteries can be supplemented with SCs in the starter systems of diesel railroad locomotives with hybrid transmissions [71]. Mobile hybrid rubber tyre gantry cranes use SCs to move stack containers [70]. One hybrid forklift primarily uses fuel cells and batteries, while SCs store the braking energy of buffer power peaks. In 2003, a light-rail vehicle prototype was developed with a roof-mounted SC unit to save braking energy and replace overhead lines in Mannheim [72]. The Paris T3 tram line and Geneva Public Transport tram were powered using SCs to recover the energy during braking in 2012 [150].

The first hybrid bus in Europa with SCs was the so-called “Ultracap Bus” tested in Nuremberg, Germany, in 2001. Then, an electric bus fleet was tested in Luzern, Switzerland, in 2002. After every transportation cycle, the SCs could be recharged within 3 to 4 min with a high-speed power charger [151]. A new type of electric bus using SCs, called the “Capabus”, that moves without power lines and fully charges at the last terminal was tested in Shanghai in 2005 [75]. A Toyota hybrid racing car used a hybrid drivetrain with SCs that was developed every year [152]. More researchers have explored hybrid electric vehicles (HEVs) [153,154]. The ability of SCs to charge much faster than batteries, their longer lifetime, stable electrical properties, and wide temperature range make them suitable for electric vehicles. However, SCs’ lower specific energy density makes them unsuitable for long-distance driving as a stand-alone energy source [77]. As of 2013, all EV or HEV automotive manufacturers have developed prototypes to improve driveline efficiency and store braking energy that use SCs instead of batteries [152,155]. Today’s HEV technology has used SCs to develop more topologies using power electronics converters to increase the efficiency of EVs, improve the environmental perspective, and lower cost. [156,157].

An HEV uses different energy sources, including batteries, SCs, and fuel cells (FCs), to power the electric drive system, as seen in Figure 10a. A fraction of the energy exchange capability of the SC can be used in a battery/SC configuration only. Therefore, a hybrid fuel cell/battery/SC configuration still provides the most extended lifetime of the batteries [158,159]. As a solution, a forklift truck project was carried out with a 16 kW power hybrid system, as shown in Figure 10b. An ‘Integrated Fuel Cell Hybrid Test Platform in Electric Forklift’ designed in the Technical Research Centre of Finland Ltd. (VTT) consisted of an 8 kW power proton exchange membrane (PEM) type fuel cell, which provided 72 kW of power to Maxwell BOOSTCAP^® (165F, 48 V) SCs and 300 Ah lead-acid batteries [160,161]. Another study reviewed energy systems for light-duty vehicles, and highlighted the main characteristics of electric and hybrid cars based on power train structure. Different topologies and energy management strategies for electric and hybrid vehicle powertrains have been investigated [162,163].

Figure 10. (a) A basic HEV drive system. (b) Hybrid forklift power source DC schema with two SC modules [160].

The comparison of SCs with the other energy storage devices has been investigated for PV–battery–SC systems in the literature, and has been shown that SCs have some advantages [14,42,43,46,47,48]. In addition, PV–battery–SCs or fuel cell combinations as HESSs are suggested as an alternative solution [49,50]. The HESS topologies include passive, active, and semi-active types. A passive HESS consists of the different energy storage devices connected directly to the DC bus without a DC–DC converter. If one side is combined with a DC–DC converter to a DC bus, it is called a semi-active HESS. If two sides of the energy storage devices are connected with converters, it is called an active HESS. The connection topology of HESSs is given in Figure 11a [3,12,43]. The active and passive HESSs were simulated and compared in a case study. While the SC semi-active HESSs performed with lower than 33% battery life, passive HESSs performed with lower than 9% battery life, only in the case of price function results during a day [3,12,43]. In another study, solar irradiance and temperature data were used for a solar farm model in MATLAB/Simulink from four diverse days from the 2017 simulation to define the annual storage cost, and the results showed that the battery + SC HESS cost was 25% cheaper annually [51]. In another study, a passive HESS was proposed for a wind and solar energy stand-alone system and the operation was tested via theoretical simulation and experimentally. The HESS (battery–supercapacitor) for the wind and solar energy-fed basic structure is shown in Figure 11b [164].

Figure 11. (a) A stand-alone active HESS with SC. (b) A HESS for a wind-solar fed system.

Several studies have presented comprehensive reviews of HESS control strategies for power quality improvement in microgrids in the last decade [132]. The increasing use of renewable energy sources and the interruption of the power generated has caused stability, reliability, and power quality problems in the primary electrical grid [165]. The microgrid is very sensitive to load or generation changes, as it is a weak electrical grid, and HESSs are used to decrease the effect of these variations [166]. Battery–supercapacitor HESSs in stand-alone DC microgrids have been reviewed, and a stand-alone photovoltaic-based microgrid with an HESS was presented as a case study [12]. A survey of a battery–supercapacitor HESS for a stand-alone PV power system in rural electrification was presented in a study [167]. A design and performance analysis of a stand-alone PV system with an HESS is given in another survey for a rural area of India. Bidirectional DC to DC converters are also used in controlling, with a fuzzy logic controller as a new control algorithm, as shown in Figure 12 [168].

Figure 12. A sample PV-HESS microgrid system structure for domestic application [168].

5. Modelling and Performance Tests for SCs

The applied voltage to the poles is linearly proportional to the amount of electric charge stored in an SC, which has units of Farads. The voltage distribution among the SC and its simplified equivalent DC circuit model is seen in Figure 13a, as a functionality illustration of an SC. The electrode’s equivalent circuit depends on the porous structure’s capacitance behaviour, and is defined with series and parallel connected RC elements. The voltage behaviour of SCs and batteries differs during charge and discharge intervals, as seen in Figure 13b. The energy is stored in a static electric field in conventional capacitors consisting of two electrodes. The total power increases linearly related to the potential between the plates and the accumulated charges. In contrast, SCs consists of two electrodes separated by a separator and the energy stored inside the double layers of both electrodes. The storage of electrostatic and electrochemical energy in SCs is linear concerning the stored energy charge similarly [106].

Figure 13. (a) The voltage distribution among the SC between simplified equivalent DC circuits. (b) Comparison of SC and battery voltage behaviour during the charge and discharge time.

The SC modelling and charge–discharge characteristics must be investigated differently from conventional storage devices. The capacitance value of an SC can be defined with RC components and time constants, depending on the frequency. The measurement characteristic for measuring capacitance is shown in Figure 14a. The rated voltage has to be applied to charge the capacitor for measurement firstly, and the SC is charged for 30 min. Next, the SC is discharged with a constant discharge current (I_discharge) [169]. Then, for the voltage drop from 80% (V₁) to 40% (V₂) of the rated voltage at the t₁ and t₂ time values is measured, and the capacitance value of the SC is calculated from this Equation (C=IΔtΔv) [106,170]. The internal DC resistance (R_i) of an SC can be calculated with the voltage drop (ΔV₂) obtained from the intersection of the auxiliary line extended from the straight part and from the time base at the time of discharge start, as shown in Figure 14b [29,169].

Figure 14. The charge–discharge characteristics for (a) measuring the capacitance and (b) internal DC resistance measurement conditions.

For SC systems, modelling is necessary for system dimension monitoring conditions. Chemical, mathematical, and electrical characteristics, ageing, artificial intelligence, and the dynamic structure of SC models are available in the literature [80,81,82,83,84,85,86,90]. To describe the behaviour of SCs, a simple electrical model of SCs has also been given in the literature [91]. The module simulation was based on a two-branched SC circuit model [89,90,91,92]. This circuit was simplified for the SC module and is given in Equation (4). The SC module voltage and currents are U_SC and I_SC, and the primary voltage and currents are v_sc and i_sc, respectively [89].

$$U_{SC}=N_{S\_SC}{v}_{SC}=N_{S\_SC}(v_1+R_1.i_{SC})=N_{S\_SC}(v_1+R_1\frac{I_{sc}}{N_{P\_SC}})$$

(4)

Moreover, it considered the equivalent electric circuit with two RC branches proposed by Zubieta and Bonert [90] and Rafik et al. [91]. The calculation used to obtain the relationship between voltage (v₁) and capacitor charge (Q₁) is seen in Equation (5), and can be combined with Equation (4) as in Equation (6).

$$v_1=\frac{-C_0+\sqrt{C_0^2+2C_V{Q}_1}}{C_V}$$

(5)

$$U_{SC}=N_{S\_SC}{v}_{SC}=N_{S\_SC}(v_1+R_1.i_{SC})=N_{S\_SC}(\frac{-C_0+\sqrt{C_0^2+2C_V{Q}_1}}{C_V}+R_1\frac{I_{sc}}{N_{P\_SC}})$$

(6)

An SC module model was designed in MATLAB/Simulink using these equations, as seen in Figure 15a. Equations (5) and (6) were revised for 310 F SC parameters obtained and calculated in experimental result values and datasheets from previous studies in the simulation [52]. The SC model was simulated for cycle life, and Capacitor ESR measurement waveforms in the datasheet were checked; the results are presented in Figure 15b [102,169].

Figure 15. (a) SC module model in MATLAB/Simulink. (b) CAP/ESR waveforms of SC [169].

Higher source voltages are required when connecting SCs in series. Each component has a slight difference in capacitance value and ESR. Therefore, it is needed to actively or passively balance the SC to balance the applied voltage. Although passive balancing in SCs is supplied with parallel resistors, active balancing includes electronic voltage management that varies the current above a threshold. Active techniques have many advantages over passive methods. In contrast, passive strategies are straightforward, and despite their shortcomings, they are still prevalent, as shown in Figure 16 [171]. However, passive balancing with resistors improves voltage distribution in SCs, because extra currents passing through balancing resistors reduces the energy efficiency of SCs in their application as energy storage devices. The use of circuits has been proposed for active voltage balancing in SCs [61]. Although the load current and cycle stability of SCs are higher than that of rechargeable batteries, the SC life and the number of cycles increase with the lower load current [165].

Figure 16. SC voltage balance (a) resistor, (b) switched resistor, (c) Zener diode circuits.

It is possible to find some products on the market for special rates and offers, and the customers can mount them to reduce the prices. An active voltage balancing SC-assisted surge absorber (SCASA) was developed using a recently patented technique [172]. This patent-pending technique uses an SC-assisted temperature modification apparatus (SCATMA), as shown in Figure 17a. This technique is based on the availability of large EDLC devices with capacitances in the range of 1200–5000 F, shallow ESR values, short circuit current capability in the field of 600–2400 A, and per cell energy storage capabilities in the capacity of 0.6–3 Wh [30]. A six-string supercapacitor protection board used for the module design is shown in Figure 17b [173]. A circuit diagram of the protection board for four capacitors’ balanced storage is shown in Figure 17c. Each SC can have a single metal oxide semiconductor field effect transistor (MOSFET) or two devices in parallel, connected depending on the selected board. An equivalent MOSFET gives twice the output current and twice the sensitivity to voltage change, connecting two devices in parallel [60]. An example commercial product for the SC module for a modular solution with UPS and advanced technology extended (ATX) power modules is seen in Figure 17d [174].

Figure 17. (a) Implementation of the SCATMA technique. (b) A six-string supercapacitor protection board [173]. (c) A circuit diagram of the protection board [60]. (d) An example commercial product for the SC module [174].