# Fiber-Based Sensors and Energy Systems for Wearable Electronics
## 4 Fiber-Based Energy-Storage Systems
### Fiber-Based Supercapacitor  
The energy-storage mechanism of supercapacitors is different from that of batteries. There are three types of supercapacitors; electrochemical double-layer capacitor (EDLC), pseudocapacitor, and hybrid capacitor. Since all of them store energy on the surface of electrodes, it is important to secure large surface areas of electrodes. Generally, supercapacitors can provide a higher power density, longer cycle life, and faster charging than batteries. Therefore, many researchers have proposed fiber-based supercapacitors with highly flexible, great mechanical, and electrical properties [157]. They generally used carbon-based materials, conductive polymers, metal oxide nanoparticles, and their composites as electrodes for supercapacitors (Table 4). Generally, carbon-based materials have excellent conductivity, chemical stability, long cycle life, relatively low mass density, and high stretchability. Furthermore, they are widely used to secure large surface areas of electrodes. However, the carbon-based materials are associated with limited capacitance and energy density, compared with pseudocapacitive materials, such as metal oxides. By contrast, metal oxides have high mass density and lower degrees of flexibility and stretchability. Conductive polymers have the advantages of high faradaic capacitance, conductivity, flexibility, and stretchability. Currently, efforts are being made to enhance the power density and energy density of conductive polymers. Fiber-based supercapacitors are composed of three main components (electrodes, electrolytes, and encapsulation layers) and are designed in three main types of configuration (parallel, twisted, coaxial). In the case of electrolytes, they are often gel or solid, rather than liquid, which has a risk of leakage and corrosion. Moreover, gel or solid electrolyte can act as a separator, as well as an ion-conducting medium and the manufacturing process can be relatively simple. Regardless of the type of configuration, it is very important to uniformly coat electrolytes without defects to avoid problems, such as an increase in resistance at the electrolyte-electrolyte or electrolyte-electrode interfaces. The most common gel electrolyte is made of PVA with non-volatile acids (e.g., H\({}_{2}\)SO\({}_{4}\)) as an ionic conductor. [PERSON] et al. proposed a unique all-graphene core-sheath fiber composed of graphene fiber core with a sheath of 3D graphene network by intertwining two graphene-based electrodes with sulfuric acid (H\({}_{2}\)SO\({}_{4}\))-PVA gel polyelectrolyte (Figure 8a) [146]. These graphene fibers had high mechanical strength and flexibility, electrical and thermal conductivities, and lightweight. Due to these characteristics, they achieved an efficient fiber-based supercapacitor. However, carbon-based materials generally have limited energy density because no redox reaction is involved. To enhance energy density, a pseudocapacitive mechanism using reversible Faradaic reactions through redox reactions or intercalations has been introduced by replacing carbon-based materials with metal oxides or conductive polymers, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Materials of Electrodes** & **Strengths** & **Weaknesses** \\ \hline \multirow{4}{*}{Carbon-based materials} & High conductivity, chemical & High faradaic capacitance, \\  & stability, long cycle life, low mass & high energy density, high \\  & density, good stretchability, large & industrial cost, low product \\  & specific surface area & throughputs, \\ \hline \multirow{2}{*}{Metal oxides} & High faradaic capacitance, high & High mass density, low \\  & energy density & flexibility, low stretchability \\ \hline \multirow{2}{*}{Conductive polymers} & High faradaic capacitance, high & Low power density, low \\  & conductivity, high flexibility, & energy density \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Characteristics of electrode materials for supercapacitors. as shown in Figure 8b [49].