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Supercapacitor Energy Storage Device Using Biowastes: A Sustainable Approach to Green Energy

: The demand for renewable energy sources worldwide has gained tremendous research attention over the past decades. Technologies such as wind and solar have been widely researched and reported in the literature. However, economical use of these technologies has not been widespread due partly to cost and the inability for service during of-source periods. To make these technologies more competitive, research into energy storage systems has intensiﬁed over the last few decades. The idea is to devise an energy storage system that allows for storage of electricity during lean hours at a relatively cheaper value and delivery later. Energy storage and delivery technologies such as supercapacitors can store and deliver energy at a very fast rate, offering high current in a short duration. The past decade has witnessed a rapid growth in research and development in supercapacitor technology. Several electrochemical properties of the electrode material and electrolyte have been reported in the literature. Supercapacitor electrode materials such as carbon and carbon-based materials have received increasing attention because of their high speciﬁc surface area, good electrical conductivity and excellent stability in harsh environments etc. In recent years, there has been an increasing interest in biomass-derived activated carbons as an electrode material for supercapacitor applications. The development of an alternative supercapacitor electrode material from biowaste serves two main purposes: (1) It helps with waste disposal; converting waste to a useful product, and (2) it provides an economic argument for the substantiality of supercapacitor technology. This article reviews recent developments in carbon and carbon-based materials derived from biowaste for supercapacitor technology. A comparison between the various storage mechanisms and electrochemical performance of electrodes derived from biowaste is presented.


Introduction
The demand for energy worldwide is expected to double in about two decades [1,2]. Energy plays an important role in the quality of our lives, impacting our social and economic development [3]. Modern economies are driven by the availability of reliable energy sources. Conventional energy sources such as fossil fuels (coal, gas, and oil) are being depleted at a fast rate, accompanied by the destruction of ecosystems and habitats, the extinction of wildlife, and pollution of the environment [4][5][6][7][8]. A primary concern of harvesting energy from fossil fuels is that it is unsustainable in the long term, so this has driven researchers and the industry to adopt sustainable and renewable energy technologies. Over the past several decades, there has been a dramatic increase in research on technologies. Over the past several decades, there has been a dramatic increase in research on renewable energy sources such as solar energy, geothermal energy, wind energy, biofuels, etc., while electrochemical energy storage devices such as supercapacitors, rechargeable batteries, etc. have also attracted significant research [9][10][11]. It is not an overstatement to say that successful development of any renewable energy source (e.g., windmills and solar cells), hybrid and electric vehicles and smart grids depend significantly upon the availability of a suitable energy storage system. A considerable amount of literature has been published on the use of supercapacitors as a viable storage device for renewable energy. Over 20,000 articles, books etc. were published in 2017, a higher number of research work is projected for 2018 (data from google scholar). There has been a geometric increase in the research published since the year 2000. Since supercapacitors were first experimented in 1957 by engineers at General Electric, they have found commercial applications in portable electronics, transportation and aerospace industry [12,13]. These applications of supercapacitors have come about due to advances in materials and manufacturing technologies. The various components such as the anodes, cathodes, separators, binder, and electrolyte have received in-depth research activity leading to improved performance and reduction in the cost of manufacture [14][15][16].
Energy storage and delivery technologies such as supercapacitors can store and deliver energy at a very fast rate, offering high current in a short duration. Supercapacitors are categorized as an electrochemical storage device, sometimes called an ultracapacitor. They can store and deliver energy at a very fast rate offering high current in a burst. Hence, they have found applications in electric vehicles, uninterruptible power supplies (UPS), and memory backups in IT systems. They also have virtually unlimited cycle life, accompanied by high specific power. They also work better than batteries in extreme temperatures, offering excellent low-temperature charge and discharge performance. The Ragone plot [17][18][19] shows a comparison between various energy storage devices in terms of power and energy density ( Figure 1). Clearly, the plot gives a good overview of energy storage performance; however, the plot is silent on critical factors such as cycle life, cost, and safety. These factors have to be investigated on their own merits to develop a better understanding of a particular energy storage technology. Because of the charge storage mechanism, supercapacitors are capable of an extremely long cycling life. It is reported to have a cycle life of over 500,000 [21][22][23], which is considerably higher than other storage technologies. This is because supercapacitors store reversible electrostatic charges on the surface of electrodes, while other technologies such as batteries really on chemical reactions.
Supercapacitors can be classified as three main types according to the energy storage mechanism. Figure 2 shows the main categorization of supercapacitors and a further classification based on electrode materials [24,25]. The first type, which is the most common, is called the electrochemical double layer capacitor (EDLC). These commonly use KOH, H2SO4 etc. as a liquid Because of the charge storage mechanism, supercapacitors are capable of an extremely long cycling life. It is reported to have a cycle life of over 500,000 [21][22][23], which is considerably higher than other storage technologies. This is because supercapacitors store reversible electrostatic charges on the surface of electrodes, while other technologies such as batteries really on chemical reactions.
Supercapacitors can be classified as three main types according to the energy storage mechanism. Figure 2 shows the main categorization of supercapacitors and a further classification based on  [24,25]. The first type, which is the most common, is called the electrochemical double layer capacitor (EDLC). These commonly use KOH, H 2 SO 4 etc. as a liquid electrolyte between the electrodes. The formation of Helmholtz double layers on the interface between the conductive electrodes/electrolyte causes fast adsorption and desorption of the electrolyte ions at the interface, providing high power density through the non-faradaic process. The size of the electrode surface and Helmholtz layer thickness has an effect on the performance of the electrochemical double layer capacitor. Hence, highly porous carbon materials such as activated carbon (AC), carbon nanotube, and graphene are widely used as electrode material in the industry. Activated carbon has a large specific surface area of 500-3000 m 2 /g [26][27][28][29][30]. Figure 3 shows a schematic illustrating the basic architecture of an EDLC supercapacitor. electrolyte between the electrodes. The formation of Helmholtz double layers on the interface between the conductive electrodes/electrolyte causes fast adsorption and desorption of the electrolyte ions at the interface, providing high power density through the non-faradaic process. The size of the electrode surface and Helmholtz layer thickness has an effect on the performance of the electrochemical double layer capacitor. Hence, highly porous carbon materials such as activated carbon (AC), carbon nanotube, and graphene are widely used as electrode material in the industry. Activated carbon has a large specific surface area of 500-3000 m 2 /g [26][27][28][29][30]. Figure 3 shows a schematic illustrating the basic architecture of an EDLC supercapacitor.  The second type is called pseudocapacitors; these operate based on fast reversible Faradaic redox reaction that occurs at the electrode. Similar to EDLCs, reactions occur at the surface of the electrode producing a high energy density at a short cycle life and low rate capability [31]. They, however, operate at a lower voltage compared to EDLCs, thereby limiting their practical energy densities. Pseudocapacitors are typically made using metallic oxides/hydroxides/sulfides or conductive polymers or compounds with O and N functional groups. The third type is essentially a combination of an EDLC electrode and a pseudocapacitive electrode; thus, utilizing the high-power density of EDLC with the high energy of pseudocapacitors combined with good cyclic stability.
Li-ion hybrid capacitors (LIHCs) have emerged as a leader in this field [32], offering a bridge between supercapacitors and Li-ion batteries. This review focuses on providing a summary of the latest advances in electrode materials derived from biowaste for supercapacitors. Furthermore, we briefly discuss and compare the electrochemical performance (charge storage capacity, energy/power densities, cyclic stability) of supercapacitors. electrolyte between the electrodes. The formation of Helmholtz double layers on the interface between the conductive electrodes/electrolyte causes fast adsorption and desorption of the electrolyte ions at the interface, providing high power density through the non-faradaic process. The size of the electrode surface and Helmholtz layer thickness has an effect on the performance of the electrochemical double layer capacitor. Hence, highly porous carbon materials such as activated carbon (AC), carbon nanotube, and graphene are widely used as electrode material in the industry. Activated carbon has a large specific surface area of 500-3000 m 2 /g [26][27][28][29][30]. Figure 3 shows a schematic illustrating the basic architecture of an EDLC supercapacitor.  The second type is called pseudocapacitors; these operate based on fast reversible Faradaic redox reaction that occurs at the electrode. Similar to EDLCs, reactions occur at the surface of the electrode producing a high energy density at a short cycle life and low rate capability [31]. They, however, operate at a lower voltage compared to EDLCs, thereby limiting their practical energy densities. Pseudocapacitors are typically made using metallic oxides/hydroxides/sulfides or conductive polymers or compounds with O and N functional groups. The third type is essentially a combination of an EDLC electrode and a pseudocapacitive electrode; thus, utilizing the high-power density of EDLC with the high energy of pseudocapacitors combined with good cyclic stability.
Li-ion hybrid capacitors (LIHCs) have emerged as a leader in this field [32], offering a bridge between supercapacitors and Li-ion batteries. This review focuses on providing a summary of the latest advances in electrode materials derived from biowaste for supercapacitors. Furthermore, we briefly discuss and compare the electrochemical performance (charge storage capacity, energy/power densities, cyclic stability) of supercapacitors. The second type is called pseudocapacitors; these operate based on fast reversible Faradaic redox reaction that occurs at the electrode. Similar to EDLCs, reactions occur at the surface of the electrode producing a high energy density at a short cycle life and low rate capability [31]. They, however, operate at a lower voltage compared to EDLCs, thereby limiting their practical energy densities. Pseudocapacitors are typically made using metallic oxides/hydroxides/sulfides or conductive polymers or compounds with O and N functional groups. The third type is essentially a combination of an EDLC electrode and a pseudocapacitive electrode; thus, utilizing the high-power density of EDLC with the high energy of pseudocapacitors combined with good cyclic stability. Li-ion hybrid capacitors (LIHCs) have emerged as a leader in this field [32], offering a bridge between supercapacitors and Li-ion batteries. This review focuses on providing a summary of the latest advances in electrode materials derived from biowaste for supercapacitors. Furthermore, we briefly discuss and compare the electrochemical performance (charge storage capacity, energy/power densities, cyclic stability) of supercapacitors.

Type of Charge-Storage Mechanism
The performance of supercapacitors can be evaluated using similar criteria employed for energy storage systems. Generally, three main techniques, namely, cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) are used to evaluate parameters such as specific capacitance, energy density, power density, series resistance, cycling life, rate capability etc. [33][34][35][36][37][38][39][40]. These electrochemical properties are evaluated to characterize a supercapacitor (electrode material) based on specific parameters to be studied [41][42][43]. Collectively, these techniques complement each other, giving a broad understanding of the energy storage mechanism and the surface phenomena between the electrode and the electrolyte.
Together, these techniques provide important insights into the performance of the electrode; for example, the CV test provides information on the degradation process [44,45], specific capacitance [46,47] and can be used to distinguish between EDLC and pseudocapacitors [46]. Electrochemical properties are evaluated using applied voltage and current response. CV scans fall short of providing valuable thermodynamic properties; however, kinetic aspects are well covered [48]. Just like CV scans, a GCD scan is useful in predicting specific capacitance and differentiating between EDLC and pseudocapacitors. The stability of the supercapacitor can also be evaluated using GCD [49][50][51]. EIS is a non-destructive technique that is used to evaluate the capacitive performance of the electrode material, while differentiating between resistive and inductive behavior of the energy storage system [52,53].
The specific capacitance of a supercapacitor can be estimated using CV and GCD experiments. The specific capacitance, C g (F/g) of the electrodes can be evaluated using: where m is the loading, I is the applied current, and dV dt is the slope of the charge/discharge curve. The energy density, E (Wh/kg) and power density, P (kW/kg) can be calculated using Equations (3) and (4), respectively [36,54,55].
where V (V) is operational voltage window of the cell, ∆t (s) is the discharge time, P max (kW/kg) is the maximum power density, and R s (Ω) is the equivalent inner resistance of the supercapacitor device. As discussed in previous sections, the electrochemical double layer capacitor exhibits higher energy density compared to the conventional parallel plate capacitors because of the exposed surface area and comparatively small charge separation distance. The mechanism of the charge storage as proposed by Helmholtz in the 19th century involves the alignment of charges at the electrode/electrolyte interface and separated by an atomic distance [23]. Further modification of the Helmholtz model has been reported by Gouy and Chapman [56]. The charge storage characteristics depend heavily on the choice of electrode material and electrolyte used. Carbon and carbon-based materials that are derived from coconut shells, petroleum pitch and phenol resin together with porous carbon, carbon nanotubes and carbon nanofibers exhibit large specific area (due to the porous structure), good electronic conductivity and high chemical stability have found widespread application in electrochemical double layer capacitor. Also, electrolytes such as KOH, H 2 SO 4 , Na 2 CO 3 etc. have been reported, due to their wettability of the electrode material. EDLCs do have some major drawbacks, as listed below:

•
Limited lifespan due to the usage of an electrolyte.

•
Incorrect usage of the capacitor may result in electrolyte leakage. • These capacitors cannot be used in AC circuits because they have a relatively high internal resistance.

•
The temperature ranges at which they can be used is limited because of the organic solvents that may be volatile, toxic and inflammable.

•
Enhancing surface area of electrodes. • Relatively low energy density.
Pseudocapacitor is a hybrid between a battery and an electric double layer capacitor. They also consist of an electrode and electrolyte and charge storage mechanisms is through chemical and electrostatic means. Most reported electrode material includes transition metal oxides (such as ruthenium oxide, nickel cobaltite, vanadium oxide aerogels) and conducting polymers. However, pseudocapacitors' performance is reported to be lower than EDLC mainly because of the inherently slow faradaic charge/storage mechanism; this leads to poor cycle life, energy density, and mechanical stability [57]. Conducting polymers used for pseudocapacitors can also be unstable at the nanoscale and ruthenium oxide is extremely costly. Hybrid capacitors occupy the middle ground between batteries and capacitors. These exhibit high energy/power densities compared to EDLCs and pseudocapacitors. They also have a high operational temperature range of −55 • C to 125 • C. Materials mostly used include carbon-coated conducting polymers, metal oxides (such as nickel and manganese-based oxides) and graphene oxides.

Carbon from Bio-Wastes as an Excellent Material for EDLC
During the past decade, researchers have shown an increased interest in the use of porous carbon materials as electrodes in high capacity supercapacitors [23]. Several carbon-based materials have been investigated for use as electrodes in supercapacitors because of their outstanding electrical conductivity and high specific surface area. The most investigated carbon-based materials widely reported include activated carbons [58][59][60], carbon aerogels [61][62][63], graphene [64][65][66][67][68], carbon nanotubes [69][70][71][72][73][74], carbon nanofibers [75][76][77][78][79][80], and nano-sized carbons [81][82][83]. These materials are popular because of the ease of accessibility, processability, non-toxicity, high chemical stability, and wide temperature range. Activated carbon has received considerable critical attention because of its high porosity and surface area [84]. Table 1 shows a summary of different bio-waste and corresponding Brunauer-Emmett-Teller (BET) surface values for activated carbons. These properties effectively promote charge accumulation at the interface of the electrode and electrolyte, aiding with the formation of electrostatic adsorption of positive and negative charges.
In recent years, there has been an increasing interest in the production of activated carbon from bio-waste for sustainable development [85][86][87][88][89][90][91][92][93]. Several sources of bio-waste such as animal, mineral, plant, and vegetables etc. have been reported in the literature as base materials for activated carbon production for application as an electrode material for electrochemical energy systems [86,[94][95][96][97][98][99][100][101][102][103][104][105][106][107][108][109]. Several types of electrodes have been tried and the most common systems today are built on the electrochemical double-layer capacitor that is carbon-based, has an organic electrolyte, and is easy to manufacture. Juan Mi et al. [129] focused on the preparations of coconut-shell-based porous carbons with a tunable micro/mesopore ratio for high-performance supercapacitors applications. They used a one-step thermal treatment combined with pyrolysis and steam activation to produce porous carbon. The ratio of mesopore to total pore volume (V meso /V total ) was reported to be greater than 75%. Using a three-electrode setup, the cyclic voltammetry ( Figure 4a) shows a quasi-rectangular shape, which is perfect for the double layer energy storage mechanism. The specific capacitance values ranged from 209-228 F/g at 5 mV/s depending on the activation, water flow rate, and activation time. Galvanostatic charge/discharge cycling plots (Figure 4b) show negligible voltage drops, indicating that these carbons exhibit good electric conductivity. Juan Mi et al. [129] focused on the preparations of coconut-shell-based porous carbons with a tunable micro/mesopore ratio for high-performance supercapacitors applications. They used a onestep thermal treatment combined with pyrolysis and steam activation to produce porous carbon. The ratio of mesopore to total pore volume (Vmeso/Vtotal) was reported to be greater than 75%. Using a three-electrode setup, the cyclic voltammetry ( Figure 4a) shows a quasi-rectangular shape, which is perfect for the double layer energy storage mechanism. The specific capacitance values ranged from 209-228 F/g at 5 mV/s depending on the activation, water flow rate, and activation time. Galvanostatic charge/discharge cycling plots (Figure 4b) show negligible voltage drops, indicating that these carbons exhibit good electric conductivity. In a similar work [130], coconut shell activated charcoal was synthesized using chemical activation method using KOH as an activating agent. The specific surface area (mesopores size of 3 nm) was reported to be 1640 m 2 /g. EDLCs fabricated using the samples as electrode material with polymer electrolyte exhibited energy and power density of 88.8 Wh/kg and 1.63 Kw/kg, respectively. In a similar work [130], coconut shell activated charcoal was synthesized using chemical activation method using KOH as an activating agent. The specific surface area (mesopores size of 3 nm) was reported to be 1640 m 2 /g. EDLCs fabricated using the samples as electrode material with polymer electrolyte exhibited energy and power density of 88.8 Wh/kg and 1.63 Kw/kg, respectively. Yin et al. [131] prepared activated carbon from coconut fibers with a multi-tubular hollow structure using KOH for activation. The sample with 4:1 mass ratio of KOH to carbonized coconut fibers exhibited a specific surface of 2898 m 2 /g with a pore volume of 1.59 cm 3 /g (30% mesopores). The prepared supercapacitor electrode (with 6 M KOH electrolyte) exhibited a specific capacitance of 266 F/g at a current of 0.1 A/g, maintaining 76% of its capacitance at 100 A/g. They found that the 3-dimensional hierarchical porous activated carbon electrode exhibited a high capacitance of 155 F/g at 0.1 A/g and 142 F/g at 10 A/g and achieved a high-energy density of 53 Wh/kg and a high-power density of 8224 W/kg.
A number of researchers have reported on agricultural crops and residues as a major source of carbon-based material. Malik Wahid et al. [132] employed hydrothermal pretreatment on sugarcane bagasse to prepare a three-dimensional (3D) interconnected, conducting, and high surface area carbon nanochannel. The hydrothermal preprocessing is depicted in Figure 5. Samples were prepared with different synthesis protocols (pyrolyzed temperature and atmosphere). Figure 6 shows FESEM images of different carbon forms synthesized from different synthesis protocols with the same basic sugarcane bagasse precursor at 800 • C pyrolysis. exhibited a specific surface of 2898 m 2 /g with a pore volume of 1.59 cm 3 /g (30% mesopores). The prepared supercapacitor electrode (with 6 M KOH electrolyte) exhibited a specific capacitance of 266 F/g at a current of 0.1 A/g, maintaining 76% of its capacitance at 100 A/g. They found that the 3dimensional hierarchical porous activated carbon electrode exhibited a high capacitance of 155 F/g at 0.1 A/g and 142 F/g at 10 A/g and achieved a high-energy density of 53 Wh/kg and a high-power density of 8224 W/kg. A number of researchers have reported on agricultural crops and residues as a major source of carbon-based material. Malik Wahid et al. [132] employed hydrothermal pretreatment on sugarcane bagasse to prepare a three-dimensional (3D) interconnected, conducting, and high surface area carbon nanochannel. The hydrothermal preprocessing is depicted in Figure 5. Samples were prepared with different synthesis protocols (pyrolyzed temperature and atmosphere). Figure 6 shows FESEM images of different carbon forms synthesized from different synthesis protocols with the same basic sugarcane bagasse precursor at 800 °C pyrolysis.  Using a three-electrode cyclic voltammetry method (1 M H2SO4, platinum strip as the counter electrode and AgCl as the reference electrode), the researchers reported results of cycling the current density from 1 to 20 A/g a 72% capacitance retention. At a current density of 1 A/g and a scan rate of 5 mV/s, they recorded a capacitance of 280 F/g and 275 F/g, respectively. The material exhibited an energy density of 7 Wh/kg at a power density of 571 W/kg. Izan Izwan Misnon et al. synthesized carbon from oil palm kernel shell for high-performance supercapacitors [99]. The samples that were chemically activated showed a specific capacitance of 210 F/g in 1 M KOH electrolyte at 0.5 A/g, whereas the physically activated samples exhibited 50% lower specific capacitance. In addition, both samples showed similar quasi-rectangular shape at this scan rate region, indicating EDLC behavior in the charge storage mechanism. The electrodes maintained approximately 95-97% of capacitance exhibited a specific surface of 2898 m 2 /g with a pore volume of 1.59 cm 3 /g (30% mesopores). The prepared supercapacitor electrode (with 6 M KOH electrolyte) exhibited a specific capacitance of 266 F/g at a current of 0.1 A/g, maintaining 76% of its capacitance at 100 A/g. They found that the 3dimensional hierarchical porous activated carbon electrode exhibited a high capacitance of 155 F/g at 0.1 A/g and 142 F/g at 10 A/g and achieved a high-energy density of 53 Wh/kg and a high-power density of 8224 W/kg. A number of researchers have reported on agricultural crops and residues as a major source of carbon-based material. Malik Wahid et al. [132] employed hydrothermal pretreatment on sugarcane bagasse to prepare a three-dimensional (3D) interconnected, conducting, and high surface area carbon nanochannel. The hydrothermal preprocessing is depicted in Figure 5. Samples were prepared with different synthesis protocols (pyrolyzed temperature and atmosphere). Figure 6 shows FESEM images of different carbon forms synthesized from different synthesis protocols with the same basic sugarcane bagasse precursor at 800 °C pyrolysis.  Using a three-electrode cyclic voltammetry method (1 M H2SO4, platinum strip as the counter electrode and AgCl as the reference electrode), the researchers reported results of cycling the current density from 1 to 20 A/g a 72% capacitance retention. At a current density of 1 A/g and a scan rate of 5 mV/s, they recorded a capacitance of 280 F/g and 275 F/g, respectively. The material exhibited an energy density of 7 Wh/kg at a power density of 571 W/kg. Izan Izwan Misnon et al. synthesized carbon from oil palm kernel shell for high-performance supercapacitors [99]. The samples that were chemically activated showed a specific capacitance of 210 F/g in 1 M KOH electrolyte at 0.5 A/g, whereas the physically activated samples exhibited 50% lower specific capacitance. In addition, both samples showed similar quasi-rectangular shape at this scan rate region, indicating EDLC behavior in the charge storage mechanism. The electrodes maintained approximately 95-97% of capacitance after 1000 cycles. Table 2 shows the performance data for electrode materials from biowaste. Using a three-electrode cyclic voltammetry method (1 M H 2 SO 4 , platinum strip as the counter electrode and AgCl as the reference electrode), the researchers reported results of cycling the current density from 1 to 20 A/g a 72% capacitance retention. At a current density of 1 A/g and a scan rate of 5 mV/s, they recorded a capacitance of 280 F/g and 275 F/g, respectively. The material exhibited an energy density of 7 Wh/kg at a power density of 571 W/kg. Izan Izwan Misnon et al. synthesized carbon from oil palm kernel shell for high-performance supercapacitors [99]. The samples that were chemically activated showed a specific capacitance of 210 F/g in 1 M KOH electrolyte at 0.5 A/g, whereas the physically activated samples exhibited 50% lower specific capacitance. In addition, both samples showed similar quasi-rectangular shape at this scan rate region, indicating EDLC behavior in the charge storage mechanism. The electrodes maintained approximately 95-97% of capacitance after 1000 cycles. Table 2 shows the performance data for electrode materials from biowaste. Qu et al. [133] used corncob residue to prepare a porous carbon for supercapacitor electrodes, using a green and low-cost steam activation method. The carbon obtained at 850 • C, which was further treated with ash removal and acid soaking exhibited S BET of 1210 m 2 /g with a yield of 23.2 wt.%. They reported a capacitance of 314 F/g at a scan rate of 5 mV/s and a capacitance retention of 82%. The performance of the samples was attributed to the well-developed porosity and good conductivity. The same authors made use of an organic and 6 M KOH aqueous electrolyte to determine the electrochemical performance of corncob residue-derived carbon. They found an energy density of 5.3 Wh/kg at a power density of 8276 W/kg in 6 M KOH, while the organic electrolyte exhibited energy and power density of 15 Wh/kg and 2827 W/kg, respectively.
Fu et al. [146] obtained multi-hierarchical porous carbon from a typical food waste, crab shell. The multi-hierarchical porous carbon exhibited a specific capacitance of 322.5 F/g and 223.4 F/g at current densities of 1 A/g and 10 A/g, respectively. The same authors reported that the crab shell-derived carbon/SrFe 12 O 19 composites showed 94.5% capacitance retention over 10,000 cycles, exhibiting a specific capacitance of 690.4 F/g at 1 A/g, and 401.3 F/g even at 10 A/g. The authors conclude that such a cheap, green and high-performance electrode composites based on crab shell waste provide good prospects in energy storage applications. In another novel work, activated carbon tubes were prepared from biomass waste cotonier strobili fibers [102]. They reported that the optimized material demonstrated a specific capacitance of 214.5 F/g at 50 A/g in the three-electrode setup. A fabricated supercapacitor exhibited 84.21% capacitance retention at a remarkable specific energy of 33.04 Wh/kg at 160 W/kg.
Ismanto et al. [147] used cassava peel waste as a precursor in the preparation of activated carbon-based electrodes. Proximate analysis results show that the cassava peel had high carbon of 28.7% and low as content of 0.4%, making it a promising precursor for preparation of activated carbon. Various carbon content from biowastes is given in Table 3. The activation of carbon was prepared through a combination of chemical and physical methods and the surface was modified with oxidizing chemical agents (H 2 SO 4 , HNO 3 and H 2 O 2 ). They reported a BET surface area of 1352 m 2 /g. Galvanostatic charge-discharge electrochemical testing was used to investigate the gravimetric specific capacitance. The specific capacitance of unmodified sample was 153 F/g, whereas the modified sample exhibited over 60% increase. Guo et al. obtained porous carbon material from soybean roots [161]. The roots were carbonized for 2 h at 500 • C (SRC) under nitrogen atmosphere and further functionalized for 2 h at 900 • C under a nitrogen atmosphere (SRPC-4K-900). Figure 7 depicts the sample preparation and SEM images. Samples were denoted by SRC (Soybean Root-Derived Carbons), and the activated Porous Carbons were named SRPC-nK, where n represents the KOH/char weight ratio. Using a symmetric two-electrode supercapacitor in 6 M KOH, they reported the existence of quasi-rectangular shapes for CV plots (Figure 8). They found that the sample SRPC-4K exhibited a specific capacitance of 276 F/g at 0.5 A/g and a capacitance retention of 98% after 10,000 cycles ( Figure 8). They assembled a supercapacitor using an ionic liquid electrolyte (EMIM BF 4 ) and reported an energy and power density of 100.5 Wh/kg and 4353 W/kg, respectively.
Ahmed et al. [162] in their studies reported successful preparation of nitrogen-doped activated carbon from orange peels. The investigated the properties of the fabricated capacitor cells using electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge-discharge. The electrochemical performance of the samples was tested in a two-electrode assembly using 6 M KOH as the electrolyte. They observed a surface area of 1577 m 2 /g for the activated carbon and established a specific capacitance of 167 F/g at 0.7 A/g. The samples exhibited specific energy and power densities of 23.3 Wh/kg and 2334.3 W/kg, respectively. Li Yin-Tao et al. [141] obtained porous active carbon from fallen leaves (activation process is shown in Figure 9). They employed KOH, K 2 CO 3 and mixed KOH, K 2 CO 3 for activation and observed that the mixed activation produced enlarged pore sizes for the activated carbon (S BET of 1078 g/cm), noting that the surface area and hierarchical pore structures were related to the mass ratio of two activators. They reported a high specific capacitance of up to 242 F/g (0.3 A/g, 6 M KOH) in a two-electrode system, maintaining a high retention rate.
were named SRPC-nK, where n represents the KOH/char weight ratio. Using a symmetric twoelectrode supercapacitor in 6 M KOH, they reported the existence of quasi-rectangular shapes for CV plots (Figure 8). They found that the sample SRPC-4K exhibited a specific capacitance of 276 F/g at 0.5 A/g and a capacitance retention of 98% after 10,000 cycles ( Figure 8). They assembled a supercapacitor using an ionic liquid electrolyte (EMIM BF4) and reported an energy and power density of 100.5 Wh/kg and 4353 W/kg, respectively.  Ahmed et al. [162] in their studies reported successful preparation of nitrogen-doped activated carbon from orange peels. The investigated the properties of the fabricated capacitor cells using electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge-discharge. The electrochemical performance of the samples was tested in a two-electrode assembly using 6 M KOH as the electrolyte. They observed a surface area of 1577 m 2 /g for the activated carbon and established a specific capacitance of 167 F/g at 0.7 A/g. The samples exhibited specific energy and power densities of 23.3 Wh/kg and 2334.3 W/kg, respectively. Li Yin-Tao et al. [141] obtained porous active carbon from fallen leaves (activation process is shown in Figure 9). They employed KOH, K2CO3 and mixed KOH, K2CO3 for activation and observed that the mixed activation produced enlarged pore sizes for the activated carbon (SBET of 1078 g/cm), noting that the surface area and hierarchical pore structures were related to the mass ratio of two activators. They reported a high specific capacitance of up to 242 F/g (0.3 A/g, 6 M KOH) in a two-electrode system, maintaining a In another work, porous starch was used as a precursor to produce porous carbon microspheres [142]. Samples were stabilized, carbonized and activated in KOH. They reported a high BET surface area of 3251 m 2 /g, observing specific capacitances of 304 F/g at a current density of 0.05 A/g and 197 F/g at a current density of 180 A/g in 6 M KOH. Samples exhibited a capacitance retention of 98% over 1000 cycles. active carbon from fallen leaves (activation process is shown in Figure 9). They employed KOH, K2CO3 and mixed KOH, K2CO3 for activation and observed that the mixed activation produced enlarged pore sizes for the activated carbon (SBET of 1078 g/cm), noting that the surface area and hierarchical pore structures were related to the mass ratio of two activators. They reported a high specific capacitance of up to 242 F/g (0.3 A/g, 6 M KOH) in a two-electrode system, maintaining a high retention rate.  Kishore et al. [136] recently developed carbonized milk-free coconut kernel pulp at low temperatures. They found that the surface area decreases with increasing temperature; at 600 • C, they observed a surface area of 1200 m 2 /g. The measured specific capacitance in 1 M H 2 SO 4 electrolyte was reported as 173 F/g for carbon sample prepared at 600 • C. Na et al. [95] used a novel broken eggshell and rice husks to fabricate a novel egg white gel polymer electrolyte and green solid-state supercapacitor ( Figure 10). On employing Green-S-SC based on this EW-GPE and RH-AC electrodes, they found that the specific capacitances decrease with increasing scan rate as expected. The sample exhibited good specific capacitance (214.3 F/g at 0.2 A/g), high flexibility and stable cycle performance. Various biowaste used for deriving activated carbon that finds application as an electrode material in supercapacitors are listed in Table 4. In another work, porous starch was used as a precursor to produce porous carbon microspheres [142]. Samples were stabilized, carbonized and activated in KOH. They reported a high BET surface area of 3251 m 2 /g, observing specific capacitances of 304 F/g at a current density of 0.05 A/g and 197 F/g at a current density of 180 A/g in 6 M KOH. Samples exhibited a capacitance retention of 98% over 1000 cycles.
Kishore et al. [136] recently developed carbonized milk-free coconut kernel pulp at low temperatures. They found that the surface area decreases with increasing temperature; at 600 °C, they observed a surface area of 1200 m 2 /g. The measured specific capacitance in 1 M H2SO4 electrolyte was reported as 173 F/g for carbon sample prepared at 600 °C. Na et al. [95] used a novel broken eggshell and rice husks to fabricate a novel egg white gel polymer electrolyte and green solid-state supercapacitor ( Figure 10). On employing Green-S-SC based on this EW-GPE and RH-AC electrodes, they found that the specific capacitances decrease with increasing scan rate as expected. The sample exhibited good specific capacitance (214.3 F/g at 0.2 A/g), high flexibility and stable cycle performance. Various biowaste used for deriving activated carbon that finds application as an electrode material in supercapacitors are listed in Table 4.

Summary/Future Prospects
The demand for renewable energy sources worldwide has gained tremendous research attention over the past decades. The development and optimization of novel materials towards energy storage is essential to the push to provide clean energy through renewable sources. Materials derived from waste and for that matter biowaste have continuously gained penetration into the field of supercapacitor technology. In this review, we gathered different activated carbons derived from biowaste for electrochemical energy storage systems, discussing the various performance parameters and storage mechanisms of the various types of supercapacitors.
In particular, we presented reports on the surface area and pore size effects on the performance of supercapacitors. Specific capacitance, energy/power densities, and cyclic stability have been reviewed, discussing the requirements for an application. We also reviewed the processing of electrode materials to optimize or maximize the performance of the supercapacitor. Supercapacitor electrode