**2. The categories of supercapacitors and main components of supercapacitors**

There are three types of categories (**Figure 1**), depending on the charge storage mechanism. (1) The first one is double-layer capacitors [35], which form capacitance using double large electrodes, consisting of a physical process. The sandwich structure forms capacitance, according to the following equation:

$$\mathbf{C} = \mathfrak{es} \wedge d,\tag{2}$$

where *C* is the capacitance and is determined by *ε, s,* and *d,* where *s* is the area of the two electrodes, *d* is the thickness between them, and *ε* is the permittivity of the electrolyte. The capacitance will also be different between capacitors and supercapacitors. The active materials consist of CNT, graphene, and a conductive polymer with a large area that is stable during the charge and discharge process. (2) For pseudo-capacitors [36], the energy storage in pseudo-capacitive comes from the surface redox reaction [2] between the metal oxide and conductive polymers, as they generally have many oxidation states that can absorb and emit electrons such as Ni oxide, Co oxide, Mn oxide, and Co oxide. (3) Hybrid supercapacitors, which mix double-layer capacitors and pseudo-capacitors, have more advantages than each individual type. They exhibit a high charge ability and an increase in the capacitance compared with double-layer capacitors, resulting in good performance with high cycle and charge abilities, with the same high capacitance as pseudocapacitors. Based on the discussion mentioned earlier, the hybrid supercapacitors are future trend to overcome the limitation of power density and applications. The supercapacitors can be divided into different categories, and they all have the same components, including collectors, separators, electrolytes, and active materials, with designed pore space for active materials.

#### **2.1 Collectors**

Conductive metal-organic frameworks operate as collectors and can be regarded as the supporting components for capacitors. To fabricate metal-organic frameworks, various approaches can be used, which can be divided into bare metal foam, sacrificial templates, micro/neon network structures with in situ deposition, and freestanding fiber films through electrospinning. Cellulose and self-doped multi-porous lignin-based biocarbon with a three-dimensional network structure are frameworks that can be used as the substrate of a solid-state flexible supercapacitor. The shape design of anodic aluminum oxide (AAO) and nanomesh template [37] have shown excellent nanostructures and mechanical properties, making them suitable for the fabrication of supercapacitors with many properties, exhibiting easy fabrication, versatility, and a high surface area. Sacrificial polystyrene colloidal particles [19] and cube sugar were shown to increase the power intensity with a large specific surface area per mass volume for the electrodes. The metal coating on the pre-electrospinning fiber film resulted in the formation of metal-framework networks, with a process to fabricate a larger area collector with auto-fabrication. Other approaches have also

#### **Figure 1.**

*Three categories of supercapacitors and they consist of collectors, active materials, separators and electrolytes.*

been used, such as wrinkled graphene on polydimethylsiloxane [7], as well as crackle and leaf templates, which have been used to fabricate metal frameworks with deposited conductive metals. The shape design of collectors is more suitable used in various fabrications and applications due to its higher energy density.
