**1. Introduction**

The urgent need for renewable energy sources has resulted in concerted research efforts into electrochemical energy storage. Capacitors can maintain power for a long time, according to the following equation:

$$E = \mathbb{W}\,\mathrm{CV}^2\tag{1}$$

where *C* is the capacitance and *V* is the applied voltage [1]. However, even though capacitors have been known to scientists for many centuries, conventional capacitors still have values at pF and uF/cm2 , which are far from energy storage requirements. When applying capacitors to energy storage, the capacity of capacitors must be increased, which are known as supercapacitors. Following developments over the past decade, supercapacitors have shown great promise for next-generation energy storage devices [2]. Supercapacitors feature a high power density [3], fast charge and discharge capacity, environmental friendliness [4], safety, and a long life cycle [5], allowing them to be used as important complements to batteries [6]. To promote the application field of supercapacitors for researchers, in this chapter, we introduce the main characteristics of supercapacitors, including categories, components, advantages, fabrication, and applications. Moreover, we discuss future application trends.

In the first section, we introduce the types and main components of supercapacitors, which can be separated into three categories: (1) double-layer capacitors [7], (2) pseudo-capacitors, and (3) hybrid supercapacitors [8], which consist of active

materials, collectors, separators, and electrolytes. Conductive metal-organic frameworks operate as collectors, while the active materials are the main components that determine the power density of supercapacitors. Separators are composed of fibers and polymers, while ions are the main components of the electrolytes. Moreover, we illustrate the structure of supercapacitors and compare the power densities in different active materials. **Table 1** shows the various supercapacitors and their power densities. The MnO2 and NiO are the most popular oxide metals with the high energy density, and they usually worked with the graphene and carbon nanotubes (CNTs) to fabricate the hybrid supercapacitors. As can be seen, both the flexibility and stretch ability are common parameters in advanced supercapacitors; however, the high energy density hardly achieved in the supercapacitors compared to the Li-batteries.

Comparing with other energy storages, this chapter exhibits many advantages, including the long-term cycling stability, high safety, and power density, which make them promising candidates for energy storage in many applications.


*Supercapacitors: Fabrication Challenges and Trends DOI: http://dx.doi.org/10.5772/intechopen.107419*


#### **Table 1.**

*Parameters of various main supercapacitors.*

In this chapter, we present the advanced achievements of supercapacitors over the past 5 years, and we overview the technologies used for fabricating supercapacitors, including lithography, laser writing, inkjet printing, template sacrifice, and physical and chemical vapor deposition.

We also present an overview of applications where supercapacitors have been widely used such as in vehicles and other storage buffers. We describe the stretchable and flexible supercapacitors in wearable devices. We also present wirelessly rechargeable flexible supercapacitors used in soft and smart lenses. The later have shown potential applications for implantable and wearable medical devices. Moreover, we present supercapacitors that may be completely biodegradable and bioabsorbable, which has never been achieved in other power storage devices. As complementary storage for batteries, supercapacitors are becoming increasingly important in several applications.

PANI, polyaniline; GO, graphene Oxide; PPY, polypyrrole; CNTs, carbon nanotubes; PEDOT, poly(3,4-ethylenedioxythiophene); PSS, polystyrene sulfonate.
