**5. Applications of supercapacitors**

As a power storage technology, there are still many challenges for the practical applications of supercapacitors, and the major drawback of supercapacitors is their low energy density. Significant efforts have been made to improve their performance. Unlike work on main power storage systems, supercapacitors have shown to be more important as battery complements, especially due to the many advantages of supercapacitors compared with lithium-ion batteries, such as a highly charge ability, longterm stability, and power density, as well as flexibility, transparency, microscale, and stretchability. Furthermore, the ions and electrolyte are both the key considerations at the electrode-electrolyte interface for various applications. The biocompatible electrolytes and ions are more suitable for solid-state and stretch ability supercapacitors using in wearable applications, while the highly diffusing rate of ions can be used in hybrid electric vehicles and solar cells with low impedance. The dynamic of the ion at the electrode-electrolyte interface endows electrochemical energy storage apparatuses. Finally, there are many applications, and we list some important samples, including portable electronics [43], solar cells, hybrid electric vehicles [46], and wearable devices [5, 47].

### **5.1 Vehicles**

Vehicles are common devices, as fossil fuels are no longer sustainable and result in significant pollution. To achieve efficient energy management systems [6], combination of batteries and supercapacitors has been proposed, as the supercapacitor can absorb energy from braking and provide energy when powered on, and these electrochemical energy storage apparatuses are based on the highly diffusing rate at the electrode-electrolyte interface with high energy or power density, long lifetime, and high safety insurance. This energy management system has been used in many electronic buses, and the same saving power technology has been used in elevators and trains.

#### **5.2 Integration of solar cells and supercapacitors**

Solar energy [48] is the main energy source for all plants and humans, with a major utilization of solar energy relying on photovoltaic technology; however, this process is unstable because solar radiation is intermittent and unstable [49], which can destroy the lifetime of solar cells and devices. Advanced approaches have been proposed using electric energy storage systems, where the integration of supercapacitors and solar cells [50] consisted of three parts, namely, dye-sensitized solar cells, perovskite solar cells, and organic solar cells. Overall, supercapacitors have shown the potential to be next-generation power sources [51], especially for providing power supply over a short amount of time and working as an energy buffer and integrating with other power storage systems.

#### **5.3 Supercapacitors for wearable devices**

Wearable systems offer a considerable amount of health information, such as heart rate, electrocardiogram, and activity data, and these devices have rapidly gained market approval. Driven by the rapid growth of portable and wearable electronics, significant research attention has been focused on the development of energy storage devices. Traditionally, energy is stored in Li-ion micro-batteries; however, their lifetime usually imposes the need for costly and impracticable maintenance, and the integrated devices and immune reactions restrict periodic part replacements. Furthermore, the hard shell and toxicity of the electrolyte in coin cells will be harmful *Supercapacitors: Fabrication Challenges and Trends DOI: http://dx.doi.org/10.5772/intechopen.107419*

#### **Figure 3.**

*Smart soft contact lens with wireless charge supercapacitor: (a) illustration of integrated contact lens, (b) photograph of the fully integrated device, scale bar 1 cm, (c) circuit diagram of supercapacitor recharge, (d) photograph of contact lens on an eye of a mannequin, scale bar 1 cm, (e) infrared radiation image of this contact lens on an eye of a mannequin, scale bar 1 cm, and (f) infrared radiation image and photograph (inset) during the discharging state on the eye of a live rabbit eye, scale bar 1 cm [52].*

to organisms. The need for power for wearable electronics has motivated the development of suitable replacements. Supercapacitors are promising candidates for portable and wearable devices [37], as they should be light in weight, compact in size, and high in energy density and have a lifetime of over 100,000 cycles with excellent recharge rate capabilities. Furthermore, the biocompatible electrolyte and mild reactions at the electrodes-electrolyte interface shows highly potential being the batteries complements and new generation power storages, including remote sensors [23], implantable biosensors [52], and nanorobots. As can be seen, **Figure 3** is a sample supercapacitor application with integrated soft, smart contact lens with wireless charge supercapacitors. In this system, the supercapacitor worked as a complementary storage system for the lens, and it could charge with an antenna from the power supply. Moreover, supercapacitors may be completely biodegradable and bioabsorbable, which have never been achieved in other power storage systems [42].

### **6. Conclusions**

Supercapacitors are devices for energy storage systems, which shown great potential as important complements to batteries. These devices consist of collectors, electrodes, active materials, separators, and electrolytes. We introduced the principle and components of supercapacitors, and then we compared the advantages of supercapacitors with other energy storage systems. In the third part, the approaches for supercapacitor fabrication were described. Finally, as complementary storage for batteries, supercapacitors are becoming increasingly important in certain applications. The applications of supercapacitors for electronic devices were described, indicating that supercapacitors may be more suitable for use in wearable and portable devices.
