Currently, there is a growing demand for various energy storage devices, which drives research to develop energy storage systems with a high efficiency at lower costs ^[1][2][1,2]. Compared to other energy storage devices, supercapacitors can possess a fast charge/discharge feature, long cycling life, excellent reversibility, high power density, and wide temperature range operation. Due to these characteristics, supercapacitors are widely used in consumer electronics that require a burst of power density. It can also be utilized as power supplies for portable devices such as notebooks, smartphones, and computers ^[2][3][2,3].

Supercapacitors, known as electrochemical capacitors (ECs) or ultracapacitors, are a new electrochemical energy storage system that stores energy by charge accumulation [4]. Supercapacitors are classified into electric double-layer capacitors (EDLC) that are based on non-faradic processes—pseudocapacitors (PC), that are based on faradic processes, and hybrid capacitors (HC), which combines two types of the above charge storage mechanism. Supercapacitors can also be categorized into two types according to the arrangement of electrodes: symmetric capacitors have both anodes and cathodes identical to each other, and asymmetric capacitors are made of two different materials [5].

The choice of raw materials affects the supercapacitor type and the desired electrochemical characteristics.

Figure 1 below classifies the type of supercapacitor according to the electrode materials. Recently, scientists have paid more attention to obtaining electrode materials from non-conventional raw materials. The use of non-conventional raw materials can simultaneously reduce the cost and environmental pollution.

As shown in Figure 1, carbon materials, metal oxides, conductive polymers, and various composite materials are used as electrode materials. Different types of carbon materials (such as graphite, graphene, and carbon nanotube) have been applied as a supercapacitor electrode material due to its extraordinary properties, such as high electrochemical stability and high electrical conductivity ^[6][7][6,7]. However, these electrode materials have their disadvantages. For instance, graphene sheets have a high specific surface area, effective charge storage transport properties, and a wide potential window. They quickly form irreversible agglomerates and restack to their graphite structure, and the determination of intrinsic capacitance becomes very difficult [8]. Despite the superior mechanical and electrical properties of carbon nanotubes (CNTs), one of the main challenges in applications is their difficulty forming homogeneous dispersion, which weakens the electrode’s mechanical, electrical, and chemical properties. Applying these materials is hindered by the prevailing time-consuming and energy-wasting procedures. Because of this, it is preferable to use biowaste-derived carbon materials since they are easily accessible, renewable, and sustainable [7]. Among the above carbon materials, activated carbon (AC) is a popular commercial supercapacitor electrode material due to its low cost, high specific surface area, high electrochemical stability, and high electrical conductivity [6].

The specific capacitance and energy density of EDLCs cannot compete with pseudocapacitors, owing to their inherent electrostatic surface charging mechanism. Therefore, an effective way to prepare high-performance composite electrode materials is combining various functional carbon materials and typical metal oxides or conductive polymers [9]. To date, transition metal oxides have displayed pseudocapacitive behavior with high specific capacitance and multiple oxidation states that make them advantageous for supercapacitor applications [10]. Commonly used transition metal oxides are Fe₂O₃, MnO₂, ZnO, NiCo layered double hydroxides, CoAl layered double hydroxides, and Co₃O₄; in addition, their composites have been widely studied [11]. Manganese dioxide (MnO₂) is an excellent pseudocapacitive electrode material among other advanced transition metal oxides due to the electrochemical abilities, varied range of potentials, natural wide occurrence, environmentally benign property, and theoretical capacitance value (1380 F/g) ^[12][13][12,13]. MnO_x can have a higher capacitance than carbon materials because it possesses faradaic or pseudocapacitance, resulting from fast redox reactions occurring at the interface between metal oxides and electrolytes, in addition to the double-layer capacitance. However, the low electrical conductivity, strong agglomeration, and poor cycle life of metal oxides limit their use in high-capacity storage devices. The use of hybrid active materials consisting of a metal oxide and a carbon host is a promising approach to solve these problems [14].

2. Raw Materials

The raw material is essential for obtaining high-quality electrodes. To synthesize activated carbon, the carbon content must be high in the raw material because it will affect the yield of the product. It is also crucial to choose raw materials with low impurities or easily removable impurities. The raw material composition should not contain any toxic compounds for safety reasons. Two types of raw materials are used to obtain activated carbon: non-conventional and conventional activated carbon. Traditional raw materials include wood, peat, bituminous coal, lignite, petroleum coke, and pitch ^{[15][16][17][18][19][20][21][22]}[15,16,17,18,19,20,21,22]. Table 1 lists characteristics of some carbon-based supercapacitor electrodes from fossil fuels, such as, petroleum, coke, coal, and pitch. Despite the high quality of the obtained carbon, the non-conventional raw materials have some disadvantages, namely, the decreasing availability of fossil fuels, the growing global energy demand, and increased awareness of the environmental impacts of fossil fuel combustion [22]. Therefore, scientists have recently obtained electrode materials from renewable raw materials, mainly waste [23]. To not recycle household waste pollutes the environment and is still an actual problem. In addition to industrial pollution, the environment is also polluted by wood products [24]. To tackle this issue and save non-renewable resources, it is essential to use different types of waste. Recently, renewable biowaste-derived activated carbons have attracted significant attention due to their interesting characteristics of naturally porous or hierarchical structured and heteroatom doping [28]. This biowaste-derived activated carbon has reduced electrochemical properties compared to carbon fibers, commercial activated carbon, CNTs, etc. However, their major advantage is low cost, ease of production, environmental friendliness, and the presence of their inherent functional groups and heteroatoms, which makes them a suitable substitute for carbonaceous materials derived from fossil fuels. Biowaste-derived activated carbon has been identified as a suitable electrode for energy storage devices due to its required pore size, large specific surface area, low equivalent series resistance, and high stability [29]. Non-conventional raw materials are very diverse, and most of them could be easily found in daily lives, such as food waste (coconut shell [30], pecan nutshell waste [31], ginkgo shells [32], onion peel [33], pomelo peel [34], garlic peel [35], orange peel [36], etc.), animal waste (crab shell [37], pork bone [38], blackfish bone [38], eel bone [38], fish bladder [39], etc.), plant waste (Couroupita guianansis [40], Elm flower [41], kapok flower [42], hemp stem [43], pine cones [4], banana leaves [44], etc.). Figure 2 illustrates the types of biowaste-derived activated carbon. Notably, the raw material types increase yearly due to the demand for activated carbon-based supercapacitors. Currently, scientists also synthesize activated carbon for the supercapacitor not only from biowaste, for instance, Coca-Cola waste [45], tires [46], tobacco waste [47], plastic bags, and other industrial wastes [48]. Due to the pandemic, the amount of used masks has increased, and scientists in China synthesize carbon from the covers for the supercapacitor electrodes [49]. The properties of biowaste determine the properties of the derived activated carbon, and consequently, the supercapacitor electrode characteristics. Therefore, choosing the proper biowaste organ/tissues is also essential. Biowaste generally consists of organic substances (saccharides, vitamins, fatty acids) and other elements (H, O, N and S). Besides C, O, H, S, and N, some minerals, such as Ca, K, Mg, Na, and Si, may also be contained in the biowaste. The thermal pyrolysis or carbonization process decomposes large quantities of organic matter into H₂O and CO₂ and forms pores. Other elements can also be removed by thermal conversion to some extent. Therefore, activation is used to obtain porous carbon with a high carbon concentration [50]. The composition may also vary according to the raw material location (country, etc.). For instance, biowaste from saline–alkali land is rich in mineral elements such as, Ca, K, Mg, Na, and Si ^[6][50][6,50]. The raw material is usually pre-treated before carbonation. The biowaste pre-treatment depends on the type of raw material. Currently, there are several types of raw material pretreatment. It includes physical pretreatment (coarse size reduction, chipping, shredding, grinding, milling), biological pretreatment (the action of fungi capable of producing enzymes that can degrade lignin, hemicellulose, and polyphenols), chemical pretreatment (with acids, alkali, organic solvents, and ionic liquids), and physicochemical pretreatment (use of conditions and compounds that affect the physical and chemical properties of biowaste) [51]. In order to make homogeneous and reproducible carbons, the precursors need to be pre-treated through careful grinding and purification [23]. Table 2 lists characteristics of some carbon-based supercapacitor electrodes from biowaste. ^57][112]. By comparing Table 1 and Table 2, rwesearchers can observe that biowaste materials are excellent for a supercapacitor, and by using biowaste materials, researcherswe can even obtain a supercapacitor with a comparatively higher capacity than with fossil fuels that have obtained activated carbon ^{[25][26][27][55]}[25,26,27,55]. Depending on the method of obtaining them (such as temperature, activation time, activation type, activating agents etc.), different types of biocarbon behave differently as supercapacitors.

3. The Performance of Supercapacitors

The performance of supercapacitors is evaluated on the basis of acceptably high energy densities (>10 Wh kg), substantially greater power density, fast charge discharge process (within seconds), excellent cycle stability (more than 100 times of batteries), low self-discharging, safe operation, and low cost [2]. Advanced electrical double-layer capacitors (EDLCs) contain organic electrolytes that operate at 2.7 V, and reach energy densities around 5–8 Wh/kg or 7–10 Wh/L. A commercial corporation offers a 48 V ultra-capacitor module with 1,000,000 duty cycles or a ten-year DC life and 48 V DC working voltage, and the modules are engineered explicitly for hybrid bus and construction equipment to provide cost-effective solutions. Currently, the industry leaders in SC production are Cellergy, Ioxus, Maxwell Technologies, Murata Manufacturing, Nanoramic Laboratories, Nec Tokin, Nippon Chemi-Con, and Panasonic. Supercapacitor devices from these manufacturers have capacitance in the range of 1200–5000 F, low direct current (DC) equivalent series resistance (ESR) values of less than one mΩ, short-circuit current in the range of 600–2400 A, and per cell energy-storage capabilities in the range of 0.6–3 Wh ^[56][111]. Nippon Chemi-Con uses activated carbon with a large specific surface area of around 2000 m²/g for the electrodes, and the raw materials of the activated carbon include pitch, resin, and natural plant materials. RAfter thesearchers literature review, we can say that currently activated carbon composite supercapacitors are not yet widely used commercially, while activated carbon supercapacitors, including from biowaste, are in high demand in the commercial sector. Owing to the two different working mechanisms, EDLCs have a higher cyclic stability or performance. Since an electrode chemical reaction is involved in pseudocapacitors, irreversible components will accumulate during cycling, leading to deteriorating performance; therefore, the cycling stability of porous carbon will be higher than composites (Table 3) ^[ SCs have the following advantages: long cycle life, high specific power, quick charge and discharge, and a wide operating temperature range, but the disadvantages may include low energy density, high dielectric absorption, price, and high self-discharge ^[10][56][10,111]. Due to the advantages of supercapacitors, they are used in the public sector, in automobiles and transportation equipment, in the defense and military industries, in computers and memory backup chips, in medicine and industry, and in power grids for power quality control and battery monitoring ^[60][113].