The supercapacitor is mainly made of two electrodes, electrolytes (an aqueous or an organic system), a separator that allows the migration of electrolyte ions while electrically separating the two electrodes, fuses and packaging materials, and so on. The EDLCs are built much similar to a battery in which there are two electrodes soaked in an electrolyte, with an ion-permeable separator located between the electrodes.

In 1853, Hermann von Helmholtz explained the concept of the doublelayer capacitance (C) for the first time stipulating that charge separations happen on polarization at the interface of electrode-electrolyte and form two layers, which are separated by an atomic distance as presented in Fig. S2a (Supporting information). The model is similar to that of two-plate conventional capacitors [7-9]. The Helmholtz doublelayer (DL) can be considered as an electrical capacitor of capacitance C_H from the following equation:

where E_r is electrolyte dielectric constant; E₀ is the dielectric constant of the vacuum; d is the effective thickness of the double layer (charge separation distance); A represents the electrode surface area. Regarding the high specific surface area of porous carbon electrodes (up to 3000 m²/g) and Debye length in the range of < 1 nm, the resulting capacitance of DL's will be much larger than for flat plate capacitors.

In 1910, this simple model which considers the diffuse layer was modified by Gouy and Chapman, as shown in Fig. S2b (Supporting information), formed by electrolyte ions as a result of thermal motion wherein the potential decreases exponentially from the electrode surface to the fluid bulk [10]. However, the Gouy-chapman model leads to an overestimation of the EDLC capacitance. In 1924, Stern combined the Helmholtz model with the Gouy-Chapman model to explicitly recognize two regions of ions distribution, an inner region called the compact layer or Stern layer and a diffuse layer as shown in Fig. S2c (Supporting information) [11, 32]. The inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP) are used to distinguishing the two types of adsorbed ions.

When an electric potential difference is applied between the electrodes, the negative charge carriers, electrons, in the negatively polarized electrode are balanced by an equal number of positive cations at the electrode/electrolyte interface, while the holes stored at the positively polarized electrode are electrically balanced by anions. Hence, a supercapacitor consisting of two electrodes is equivalent to two capacitors in series and the resulting capacitance (C) can then be expressed according to 1/C = 1/C₊+1/C-, where C₊, C- and C are the capacitance (F = C/V) of the positive electrode, the negative electrode, and of the resulting device, respectively. The doublelayer capacitance is between 5 μF/cm² and 20 μF/cm² relying on the type of electrolyte [11]. Generally, EDLCs can be operated at a very high charge and discharge rate with a lifetime of over a million cycles. However, the energy stored in the EDLCs is lower than that in batteries.

LICs are electrochemical energy storage devices that combine the advantages of the high power density of a supercapacitor and the high energy density of a Liion battery. LICs are usually composed of a battery-type electrode with the insertion/extraction of Li ions and a capacitor-type electrode with pseudo-capacitance or adsorption/desorption of ions [12-14]. Since the battery-type electrode can not only act as an anode but also serve as a cathode, the LICs can be divided into two types: (1) The battery-type electrode acts as the anode, and the capacitor-type electrode serves as the cathode like a Li₄Ti₅O₁₂//AC system. Typically, during the charging process, anions are absorbed on the porous surface or defects of the cathode, while Li⁺ ions are inserted into the active material of the anode. During discharging, the adsorbed anions are released from the cathode, and the Li ions are extracted from the anode [15]; (2) The capacitor-type electrode acts as the anode, and the battery-type electrode serves as the cathode like an AC// LiFePO₄ system. Typically, during the charging process, Li⁺ de-intercalates from the cathode material and enters the electrolyte. At the same time, Li⁺ in the electrolyte migrates and adsorbs on the anode. The discharge process will be the reverse case as presented, and another example was also offered for illustration of the electrostatic and electrochemical charge and discharge of the double layer and intercalation function in the fabricated hybrid supercapacitor consisting of rGO/LTO and graphite (Fig. S3 in Supporting information) [16].

The supercapacitors can be classified into several classes owing to the energy storage mechanism and the active materials utilized. The first one is EDLCs, and the second one is pseudocapacitors, but there are other classes such as hybrid capacitors and battery-type capacitors (Fig. 1A).

The EDLCs can store the charge electrostatically employing reversible adsorption of ions of the electrolyte on the electroactive material which is electrochemically stable and possesses great accessible specific surface area. The high surface area of the EDLCs electrodes coupled with a small thickness of the double layer results in a specific and volumetric capacitance two orders of magnitude larger than that of the electrolytic capacitors. The electrolytic capacitors provide excellent power density features at the expense of lower energy density. Also, EDLCs can deliver ultrahigh power density and an excellent lifespan due to the non-degrative processes between the electrode and the electrolyte.

Typical voltage profiles for an EDLC cell and an LIC cell; schematics of the EDLC with activated carbon electrode as both positive and negative electrodes; conventional hybrid capacitor with a battery negative electrode and activated carbon (AC) positive electrode and nanohybrid capacitor with a pseudocapa-citive or an ultrafast nano-structured battery negative electrode combined with an AC positive electrode were developed as presented in Fig. 1B [17, 18].

The pseudocapacitors, where the fast and reversible redox reactions between the electrolyte and electroactive species on the electrode surface. Pseudocapacitors are, thus, suitable in applications where high capacitance is needed. The energy densities of pseudocapacitors are higher than EDLCs, but the phase changes within the electrode due to the faradaic reaction limit their lifetime and power density. The operating potential window (OPW) of pseudocapacitor is low (< 1.2 V), which is still affecting their energy density mostly needed for its widespread applications. The pseudocapacitors use the materials possessing redoxactive transitions metals with different oxidations states like metal oxides, RuO₂, Fe₃O₄, NiO and MnO₂, electronically conducting polymers such as polyanilines (PANIs), polypyrrole (Ppys), poly-thiophenes [19, 20] and heteroatoms-doped carbon materials. To address the phase changes issues, carbonaceous materials like carbon nanotubes and graphene with high electrical conductivity, ductility have been used to synthesize composites with pseudo-capacitive materials [21-23]. It is worth noting that due to the low cost, quick rate, longer lifespan, and low-self discharge, EDLCs-type supercapacitors currently dominate the market while the recent market fraction for the pseudocapacitors is so small.

The third category is hybrid supercapacitors with asymmetrical electrode configuration, where one electrode consists of electrostatic absorbed carbon materials while the other consists of faradaic electroactive materials (pseudocapacitance). The hybrid supercapacitors are classified into LICs, which can work in voltage ranged from 2.2 V to 3.8 V, and the nanohybrid capacitors (NHCs) can work and stable in electrochemical voltage window between 1.5 V and 2.8 V. These asymmetric configurations characterize an increased working voltage with a high energy density and a high power density. Therefore, it is necessary to design and develop the hybrid system to address the energy density limitations of the present generation of carbon-based supercapacitors because they use both the system of a battery-type (redox) and a capacitor-type (doublelayer) electrode, which will allow achieving a wide operating voltage and capacitance. The two various technique to hybrid supercapacitor has appeared (1) pseudocapacitive metal oxides with a capacitive carbon electrode, and (2) lithium-insertion electrodes with a capacitive carbon electrode [2]. Furthermore, asymmetric configurations demonstrate enhanced specific energy at the expense of rate performance because the battery behavior principally results from faradic redox reactions with the crystalline framework of the active materials [24].

LICs are the most interesting hybrid device due to their high OPW increased from 3.8 V to 4.3 V with high energy density up to 25-40 Wh/kg. Also, the LIC is a hybrid of a LIB negative electrode and EDLC positive electrode. Li⁺ insertion-extraction occurs in the graphite electrode with shallower state of charge (SOC, less than 50%) compared with the LIB system, while adsorption-desorption of anions, most typically BF₄^- or PF₆^-, occurs on the AC electrode, as in the EDLC. The overall process is a cation-and anion-consuming reaction [6]. The LIC has a specific energy and energy density of around 30 Wh/kg and 20 Wh/L, respectively. These values are more than three times higher than the values of a conventional EDLCs. Later, Kim et al. proposed a concept for LIC, Li₂MoO₃ is integrated into the cathode as a lithium-doping agent, which Li⁺ can be reversible extracted, and then inserted into the anode [25, 26]. In dual carbon lithiumion capacitor, the socalled dual carbon type of LICs use a high surface area AC material as the positive electrode and a lithiumion intercalating carbon material such as graphite or coke as a negative electrode. They can store about five times more energy than conventional EDLCs while keeping good power and long lifespan characteristics [26]. During charge/discharge, lithi-umion insertion/extraction occurs with the bulk of the negative electrode, whereas anion adsorption/desorption occurs on the surface of the AC positive electrode. As the latter process on the positive electrode is non-faradic and relatively rapid in comparison with the lithiumion exchange process at the negative electrode, the power capability of the LIC will generally be determined or limited by the rate capability of the negative electrode.

NHC is the second class of hybrid supercapacitors. In 2001, G.G. Amatucci and co-workers first suggested the nanohybrid capacitor, where Li₄Ti₅O₁₂ (LTO) was used as a negative electrode combined with AC as a positive electrode in acetonitrile electrolyte [27] to reach 3 V high voltage, energy density of 20 Wh/kg and cyclic stability for 4000 potential cycles. In 2009, Naoi and co-workers developed a hybrid supercapacitor system that attained a high energy-power density and high stability, whereas keeping safe. The socalled "nanohybrid capacitor employs an ultrafast anode electrode based on nanoscale Li₄Ti₅O₁₂ crystals, which were highly-dispersed and entangled within a nano-carbon framework. The authors revealed that LTO is stable and safe redox materials able to raise the energy density of hybrid supercapacitors without sacrificing the interfacial characteristics. LTO can work at a voltage of 1.55 V (vs. Li/Li⁺), which is outside of the voltage range where the electrolytes decompose. Others benefits of LTO in nanohybrid capacitors are (i) high coulombic efficiency (> 95% at 1 C) very close to a theoretical capacity of 175 mAh/g, (ii) zero-strain insertion that provides little volume change during charge-discharge, (iii) little electrolyte decomposition (little SEI formation and little gas evolution) and (iv) cheap raw material [28]. For example, Katsuhiko Noi et al. have used ultrafast materials to assemble the nanohybrid capacitor, which consisted of a faradaic Li-intercalating LTO electrode and a non-faradaic AC electrode using an anion (typically BF₄^-, anion from 1 mol/L LiBF₄ dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC)) adsorption-desorption process [25].

The assembled nanohybrid cell has exhibited remarkable energy, power, and ability performance as an electrochemical capacitor. Furthermore, the nanohybrid technology produced more than triple the energy density of a conventional electrochemical capacitor.

There is a large selection of electrolytes for NHC because of its narrower voltage window (2.7-3.0 V) compared with the LIC system (4.0-4.3 V). Acetonitrile (AN), ionic liquids (ILs) and linear carbonates, such as DMC or diethyl carbonate (DEC), can be utilized in NHCs. The selection of electrolytes is very important to obtain an even better power performance. There is another class of excellent supercapacitors configuration is micro-supercapacitors [29], an important prerequisite for the fast development of miniaturized, wearable, and portable electronics have been developed micro-power sources on chips. A comparative study of LIC and NHC on working mechanism was performed (Fig. S4 in Supporting information) [6].

The battery-type capacitor, where rechargeable battery electrodes influenced the development of electrodes for new hybrid-type supercapacitor electrodes as for LICs and battery-type capacitors. Together with a carbon EDLC electrode in an asymmetric construction offers higher specific energy than typical super-capacitors with higher specific power, longer cycle life, and faster charging and recharging times than batteries. The benefit of the battery-type capacitor is their higher voltage ranged from 1.5 V to 4.2 V and corresponding to their higher specific energy up to 10-20 Wh/kg. Research group of A. Yoshino, T. Tsubata et al. [30] have used a composite electrode prepared by heat treatment on an AC with pitch as the negative electrode in their device.

The Li⁺ intercalation/deintercalation of the negative was shown to occur in a potential range similar to that of graphite but with enhanced kinetics. The the-assembled device worked in the voltage window of 2.0-4.0 V, and its power 2.2 kW/L and energy density of 20 Wh/L has been claimed to be two to three times higher than EDLCs.

Physical activation usually happened directly after the pyrolysis step at elevated temperatures up to 1200 ℃ in a steam or carbon dioxide (CO₂) atmosphere [43, 44] or their combinations. The physical activation is generally performed in a gas such as air, steam, or CO₂. In the physical activation process, a carbon source is first heated in an inert atmosphere between 400 ℃ and 900 ℃ to remove bulky volatile materials-subsequently, partial gasification employing an oxidizing gas at temperature ranged from 350 ℃ to 1000 ℃. Firstly, the active oxygen in the activating agent burns away the tarry pyrolysis off-products embedded in the pores, conducting to the opening of some clogged pores. Secondly, the microporous structure is developed as the oxidizing agent burns away more reactive areas in the carbon framework resulting in CO and CO₂. The extent of annealing is relying on the use of gas and activation temperature. The CO₂, air, and steam are employed as activating agents [45-47]. The steam is an abundant activating agent employed for turning industrial raw material owing to its low value and no subsequent activation process to eliminate useless products. The steam is usually mixed with pyrolysis as one-pot. It can produce rich surface oxygen-functionalities, which enhance the wettability and capacitance of the produced carbon materials.

The chemical activation is performed in the presence of chemical activating agents at a temperature ranged from 450 ℃ to 900 ℃ but combining the carbon source with alkalis, carbonates, chlorides, acids, or same agents (KOH, NaOH, K₂CO₃, ZnCl₂, FeCl_3, H₃PO₄ and H₂SO₄)[44]. The chemical activation is greatly carried out at the laboratory level and provides benefits in pore size distribution control. Also, the chemical activation is a one-pot process, where an activating agent such as an acid, strong base, or salt is embedded into carbon precursor before pyrolysis at a temperature ranged from 450 ℃ to 900 ℃. The chemical activation has downsides such as much water required for washing away the impurities produced during activation and the management of polluted water [48]. The alkalis like KOH is mostly employed for activating the industrial raw materials-derived active materials for supercapacitors applications [49]. The chemical activation of various carbon sources with KOH activating agent is very promising method due to its lower activation temperature, short activation time, higher yields, and well-defined micropore size distribution, and as well as the ultrahigh specific surface area of the resulting porous carbons that are very important in charge storage. In the KOH activation method, different reactions mechanisms have been proposed for clarifying the preparation of activated carbon materials for supercapacitors. Those mechanisms are as follows:

Linares-Solano and co-authors reported that the evolution of CO₂, CO and H₂ was observed during KOH activation via the temperature-programmed reaction (TPR) experiments [50, 51]. The prominent global reaction stoichiometrically happening between carbon and KOH is suggested as presented in the following equation:

During KOH activation, K₂CO₃ produces at about 400 ℃ [52]. At about 600 ℃, KOH is totally used up. The as-produced K₂CO₃ in reactions (4) and (5), were significantly transformed into CO₂ and K₂O at temperatures higher than 700 ℃ (reaction (6)), totally vanishes at around 800 ℃. Furthermore, the resulting CO₂ can be further reduced by carbon to form CO at high temperatures (reaction (7)). The K₂O and K₂CO₃ also can be reduced by carbon to produce metallic K at temperatures over 700 ℃ (reactions (8) and (9)). Based on the results presented in literature [53, 54], there are three main activation mechanisms for KOH activation of carbon which are: (i) Etching the carbon framework by the redox reactions between various potassium compounds as chemical activating reagents with carbon as shown in reactions (5), (8) and (9), called chemical activation, is responsible for generating the pore network; (ii) The formation of H₂O (reaction (1)) and CO₂ (reactions (3) and (6)) in the activation system positively contributes to the further development of the porosity through the gasification of carbon, namely physical activation (reactions (2) and (7)) [55]; (iii) The as-formed metallic K (reactions (5), (8) and (9)), efficiently intercalating into the carbon lattices of the carbon matrix during the activation, results in the expansion of the carbon lattices [56]. After the removal of the intercalated metallic K and other K compounds by washing, the expanded carbon lattices cannot return to their previous nonporous structure and thus the high microporosity that is necessary for large specific surface area, and pore volume (PV)/micropore volume (MV) is created. For a given carbon precursor, experimental variables of KOH activation include the mass ratio of KOH/ carbon precursor, heating rate (3-10 ℃/min), activation temper-ature and time (1-4 h). It is worth noting that excess activation can cause a larger pore volume, which further makes the density and conductivity of the activated carbon decrease. Therefore, a lower volumetric energy density and loss of power capability are produced. So finely tailoring the porous microstructure and surface chemistry of porous carbons by KOH activation is crucial for obtaining high-performance supercapacitors while balancing the gravimetric and volumetric capacitances. The substantial efforts have been used for synthesizing and tailoring carbon microstructures materials for energy storage. For instance, Wang and Kaskel have reported the KOH Activation of carbon-based materials for energy storage [57]. From their deep analysis, they have concluded that: (i) The large micropore of the activated carbon is formed when the mass ratio of KOH-carbon and activation temperature raise; (ii) The integration of micropores, and small mesopores into the different carbon nanostructured framework by KOH activation highly increases the interconnected pore networks and porosity, while keeping the original textural properties; (iii) The carbon structure, pore size, and surface functionalities are among key factors which can affect the supercapacitor performance. For instance, the normalized capac-itance decreases with the increase of micropore size. Moreover, the introduction of heteroatoms by using nitrogen/oxygen-riched precursors can enhance the specific capacitance via the pseudo-capacitance effect. It is worth noting that long activation time or elevated temperature conducts to larger average pore size. The synergism of physical and chemical processes is also feasible. There are great advantages of using KOH such development of a high surface area and high porosity because of synergism and comprehensive actions including chemical activation, physical activation and carbon lattice expansion by the metallic K intercalation. The higher specific surface area plays a crucial role in EDLC performance, as the capacitance increases linearly with it approved by Helmholtz [8]. For instance, He and Qiu's group have prepared mesoporous carbons (MCs) for supercapacitors by using coal tar pitch throughout a microwave-assisted one-step process coupling the KOH activation and MgO template at only 30 min of heating [58]. The-obtained MC displayed a specific surface area of 1394 m²/g and a pore volume of 0.83 cm³/g and showed a high specific capacitance of 224 F/g in 6 mol/L KOH electrolyte after 1000 cycles. In comparison, the two-step activation approach is frequently applied for synthesizing plastic waste derived carbon electrode materials for supercapacitor applications. He et al. have used low-density polyethylene (LDPE) to synthesize a hierarchical porous carbon (HPC) through autogenic pressure carbonization at 600 ℃ at 10 ℃/min in a tube furnace followed by KOH activation at 700 ℃ at a rate of 5 ℃/min under nitrogen atmosphere [59]. The as-prepared HPC exhibited a large specific surface area of 3059 m²/g, a pore volume of 1.73 cm³/g, abundant surface functional groups, and used as an electrode material for supercapacitors.

ZnCl₂ is another commonly used chemical activating agent for turning industrial wastes into activated porous carbon materials for supercapacitors. ZnCl₂ plays the role of a dehydrating agent during the activation process, and it also has a deoxygenation effect at high temperatures by removing oxygen in the form of water as well as by carbothermal reduction. Some activated carbons with high performance derived from industrial waste have been reported throughout a simple one-step process and ZnCl₂-activating agent. In a research performed by Chang and Yang's research group, the hierarchical activated porous carbon (APC) was synthesized through a chemical activation route using ZnCl₂ and recycled waste filter paper (FP) as the carbon precursor [60]. The optimal sample, APC-7-4, was activated at 700 ℃ with a ZnCl₂/carbon precursor mass ratio of 4:1 and presented a large surface area of 2170 m²/g, a suitable pore size of 1.5 cm³/g. Significantly, when the as-prepared APC-7-4 material used as the electrode material for supercapacitor, APC-7-4 demonstrated an outstanding charge storage capacity with a satisfactory specific capacitance of 302.3 F/g in 6 mol/L KOH at 1 A/g.

Phosphoric acid (H₃PO₄) can also serve as a chemical activating agent. Compared with KOH and ZnCl₂, H₃PO₄-activated carbon usually possesses a relatively low specific surface area and small pore volume. For example, the waste-tires were used as carbon precursors for preparing activated carbon and H₃PO₄ as an activating reagent, after activation with H₃PO₄/char ratio of 5 at 900 ℃ for 3.5 h under nitrogen atmosphere, the obtained waste tire activated carbon (WTAC) displayed a high specific surface area of 563 m²/g and pore volume of 0.2 cm³/g, when used as electrode material for supercapacitor showed a specific capacitance of 106.4 F/g [42].

Over and above KOH, ZnCl₂, H₃PO₄, and others like ammonia (NH₃) are also applicable for preparing activated carbon materials derived from plastic bag waste and other industrial waste for supercapacitor application. In 2019, Yang's research group has designed an ingenious method for converting low-cost polyeth-ylene plastic waste into a mesoporous carbon material with a remarkable high surface area of 1219 m²/g and pore volume of 2.0 cm³/g via carbonization at 900 ℃ in N₂ for 1 h and further activated in ammonia for 2 h with different temperature, where the optimized one was 900 ℃ [61]. The-obtained mesoporous carbon materials displayed a specific capacitance of 244 F/g at 0.2 A/g, and still 125 F/g even at 50 A/g in 6 mol/L KOH electrolyte, again confirming the good rate capability. The prepared meso-porous carbon was used in assembling symmetrical super-capacitor with EMIMBF₄ electrolyte delivered a maximum energy density of 43 Wh/kg. Another method to synthesize the hierarchical porous carbon with mesopores and a high surface area of 1381 m²/g was also developed by Yang et al. [62] by using ionic liquid confined in the metal-organic framework by an in-situ ionothermal synthesis throughout carbonization and further activated in ammonia. Their method is also applicable for preparing the hierarchical porous carbon with mesopores for supercapacitor applications.

The hard template-based methods used hard particles, such as SiO₂ and polymer colloids, as a sacrificial template to build the macroporous structure. Typically, the empty spaces between colloidal particles in the templates are infiltrated by a fluid-like carbon precursor that penetrates the templates and can be turned into a solid. Next, under pyrolysis treatment, solid fillers are turned into carbons under an inert atmosphere, and the templates are eliminated by pyrolysis or chemical etching. The solid carbon framework is formed after removing templating particles that surround the air holes (macropores) left in the original locations of the particles [63]. If the KOH activation process is subsequently performed, hence hierarchically porous carbon structure is produced. Besides, when heteroatoms-rich compound (Boron, N-containing ionic liquid, melamine, P-, S-based compounds) is combined with a precursor; therefore, the heteroatoms doped-hierarchical porous carbon structure can be synthesized.

The soft template-based methods present some quite appealing benefits like its ability to prepare carbon materials with different morphologies and less harsh experimental conditions. We can also use the heteroatom rich compound for synthesizing the doped hierarchical porous carbon material for supercapacitor electrodes. For preparing the hierarchical porous carbon materials with the coexistence of macropores and mesopores, which are beneficial to good electrochemical performances of supercapacitors, it is better to mix soft-hard-templating method. In a typical run, hard particles and the micelles of triblock copolymers (F127) can be used as the macroporous and mesoporous templates, respectively. Owing to the little space of hard templates, the soft templates occupy the interstices between hard colloids with relatively large size. The benefits of macropores-based hierarchically porous carbon electrode contain not only facilitating electrolyte access and ions transport but also enhancing the loading and dispersion of active materials, thus fostering capacitance generated by the supercapacitor.

Many efforts have been used for synthesizing porous activated carbon material derived from carbon precursors coupled with various templates for supercapacitor applications, but also the hierarchical porous activated carbon materials can be prepared without introducing any templates. The template-free methods hold the following advantages, such as short-time, simple and affordable prices compared with template-based methods.

For example, Li and Zhao et al. have first designed an effective and simple self-assembly method without adding templates to prepare layer-stacked hierarchical porous activated carbons (LHPCs) from coal tar pitch [64]. The as-prepared LHPCs showed a super large specific surface area of 3114 m²/g, a large pore volume of 2.0 cm³/g, and exhibited excellent electrochemical perform-ances with a high specific capacitance of 356.8 F/g at 0.5 A/g in 6 mol/L KOH electrolyte.

Also, symmetrical all-solid-state supercapacitor was assembled using LHPCs achieved excellent flexibility and outstanding cycling stability up to 100, 000 cycles, as well as a high energy density of 10.25 Wh/kg at a power density of 496 W/kg. There is a new way to the controllable synthesis of activated carbons from coal tar pitch and would also be beneficial to the industrial development of carbon materials from low-cost value-added byproducts from coal. Another example, Chen, Mijowska and co-workers have demon-strated the preparation of interconnected nanoporous carbon (NPC) material from direct annealing of ultra-small Al-based metal-organic complex (Al-MOC) [65]. The-obtained intercon-nected NPC exhibited the specific surface area of was 1593 m²/g, the total pore volume was 2.49 cm³/g, good electrical conductivity, led to a large specific capacitance of 205 F/g, with a voltage range from 0 to 1.2 V, in symmetric supercapacitor, and a large energy density of 10.25 Wh/kg, in an aqueous electrolyte, suggesting a large potential in supercapacitors.

As the days go on, the production of polyethylene (PE) waste increases and tough to recycle. Several years ago, PE was suggested as a cheap (1 ＄/kg) carbon precursor with a high carbon content (86%). Harnessing polyethylene plastic waste bags into active materials for supercapacitors is a great achievement and cost-effective strategy for environmental protection and development of renewable energy sources particularly. However, porous carbon is mostly used in supercapacitors as effective active materials owing to their high surface area, excellent conductivity, low price, tailorable size and good chemical and physical stability combined with introducing heteroatoms into the structure.

Recently, Yang et al. have proposed a cost-effective strategy for turning polyethylene waste bag into hierarchical porous activated carbon materials by using polyethylene bag waste mixed with basic magnesium carbonate pentahydrate as fire retardants and subsequently activated in ammonia (denoted as PE-C-900NH₃) (Fig. 3b) and resulting materials were used for supercapacitor [61]. The PE-C-900NH₃ exhibited a high specific surface area of 1219 m²/g, a large mesopore of 1.97 cm³/g, high purity, and a small ratio of O/N. The PE-C-900NH₃ showed the high specific capacitance of 244 F/g at 0.2 A/g and cycling stability around 97.1% retention of its initial capacity after 10, 000 potential cycles at 2 A/g. A high energy density of 43 Wh/kg was achieved for the PE-C-900NH₃ symmetrical supercapacitor at a high voltage of 4 V in EMIMBF₄ electrolyte.

Later, Yang and co-workers have developed more green cost-effective technology and powerful strategy than conventional method for preparing graphene/mesoporous carbon (denoted as G@PE40-MC700) electrode materials derived from polyethylene (PE) plastic waste combined with graphene oxide (GO) additive and flame retardant via low-temperature carbonization at 700 ℃ without adding any activating agent as depicted in Fig. S8a (Supporting information) [78]. The G@PE40-MC700 presented a high surface area of 1175 m²/g and a huge amount of mesopores of 2.3 m³/g. The obtained G@PE40-MC700 exhibited the higher energy density of 63.3 Wh/kg in a high-voltage (4.0 V) symmetric supercapacitors in EMIMBF₄ electrolyte with enhanced cycling stability of 89.3% after 5000 cycles (Fig. S8b in Supporting information). Their study reveals that the high capacitance and rate capability of G@PE40-MC700 were due to the synergistic effect of graphene and the mesoporous carbon composites.

Another contribution was also made by He and co-workers, where the hierarchical porous carbon (HPC) spheres were manufactured by using LDPE waste through autogenic pressure carbonization followed by KOH activation. The as-synthesized HPC presented a micrometer-scale carbon sphere, a large specific surface area of 3059 m²/g, and more functionalities groups. The as-prepared HPC showed excellent performance of 355 F/g at 0.2 A/g in 6 mol/L KOH electrolyte, a high energy density of 9.81 Wh/kg at a power density of 450 W/kg, and outstanding cycling stability [67].

The plastic waste containing halogens like fluorine and chlorine, polyvinylidene fluoride (PVDF), polyvinylchloride (PVC) and polytetrafluoroethylene (PTFE) are also promising carbon precursors for synthesizing carbon materials for supercapacitors. PVC plastics play a great role in numerous areas, such as healthcare, packaging, and construction materials. After their use, a large amount of PVC wastes is greatly scattered in the environment and causing environmental problems. The landfills or incineration of PVC wastes are not satisfactory technology because they release other byproducts such as chlorine, hydrogen chloride, and organochlorine, which are harmful to human life and environments, hard to tackle [79, 80].

Sun and co-workers have suggested an effective and green method to convert PVC plastics into carbonaceous materials via KOH-assisted room temperature dehalogenation, followed by the formation of clean byproducts of potassium chloride and water as presented in Fig. S9a (Supporting information). The as-prepared carbon materials derived from PVC plastics waste were applied for supercapacitor. The porous carbon material derived from plastic wrap (PW-C) exhibited an excellent performance of 399 and 363 F/g at 1.0 A/g in 6.0 mol/L KOH and 1.0 mol/L H₂SO₄ electrolytes, respectively, (Fig. S9b in Supporting information) [81]. They concluded that the developed method is capable of treating PVC plastic wastes safely and efficiently and converting them into valued-added porous carbon electrode materials. The research group of Chang has designed a facile method of converting polyvinyl dichloride (PVDC) plastic waste into multiple doped carbon materials [82]. They have prepared N, S-doped carbon materials by employing the room-temperature dehaloge-nation of PVDC promoted by KOH combined with the different organic dopants. The prepared electrode material delivered an excellent specific capacitance of 427 F/g at 1.0 A/g in an acidic electrolyte and also maintained around 60% of capacitance at a high current density of 100.0 A/g. Zhang and co-workers [83] transformed the PTFE plastic waste into nanoporous carbon throughout the direct carbonization in the presence of zinc powder as a hard template. The obtained nanoporous carbon exhibited a large BET surface area of 800 m²/g and a high total pore volume of 1.6 cm³/g, also provided an excellent specific capacitance of 313.7 F/g at 0.5 A/g. In addition, it presented superior cycling stability with high capacitance retention of 93.10% after 5000 cycles. The nanoporous graphitic carbon materials were also prepared by using PVDF waste and Ni(NO₃)₂·6H₂O as the graphitic catalyst [84]. From deep investigation, the authors found that the carbonization temperature plays a crucial role in determining the pore structures as well as their electrochemical performances. They concluded that the increase of the carbonization temperature from 800 ℃ to 1200 ℃, the corresponding porosity has slightly decreased, accompanied by an increase of graphitization degree. However, the air-pollution induced by carbonization of plastic waste-based fluorine and chloride should also be concerned.

Another plastic waste, polystyrene has caused serious environmental problems due to its overuse and unsatisfactory method to recycle effectively. Converting it into functional carbon materials for supercapacitor applications is one of the cost-effective ways to recycle polystyrene and other waste plastics. A straightforward and efficient method was developed for the preparation of three-dimensional (3D) network structure PC by using Friedel-Crafts reaction.

In 2018, Chen and co-workers demonstrated a facile way to fabricate nitrogen-doped porous carbon nanosheets (NPCNs) from polystyrene waste [41]. In the preparation procedure, Zn and Co bimetallic zeolitic imidazolate framework (CoZn-ZIF) nanocrystals grown on magnesium hydroxide [Mg(OH)₂] sheets as a template (named Mg(OH)₂@CoZn-ZIFs). At high-temperature treatment, Mg(OH)₂ converts to magnesia (MgO), which helps to convert the gas-phase carbon precursors derived from PS decomposition into carbon framework. The as-prepared NPCNs with porous structure and large specific surface area were obtained. The N-PCNs exhibited a specific capacitance of 149 F/g at 0.5 A/g in 6 mol/L KOH electrolyte and excellent cycling stability of 97.6% even after 5000 cycles. Another report was done by Chen et al. where polystyrene waste foam was used for preparing a three-dimen-sional (3D) network structure PC with a facile and efficient method [85]. The prepared porous carbon displayed an excellent electrochemical capacitance of around 208 F/g at 1 A/g, and a superior energy density of 22.5 Wh/kg was achieved at a power density of 1024.4 W/kg. Meanwhile, it also displayed excellent capacitance retention at 94.3% over 5000 cycles at 5 A/g.

Very recently, another study on upcycling plastic waste dumped in the environment was presented by Min, Chen et al., where mesoporous carbon nanosheets (CNS) were prepared by mixing polystyrene waste and MgO as a template and subse-quently carbonized at 700 ℃ in an argon atmosphere for 1 h [86]. The pore structure of the-synthesized CNS was further tuned by KOH activation; the resulted hierarchical porous carbon sheets presented a specific surface area of 2650 m²/g and a pore volume of 2.43 cm³/g. The electrochemical measurement was performed in a three-electrode system; the hierarchical porous carbon sheets exhibited a specific capacitance of 323 F/g at 0.5 A/g and 222 F/g at 20 A/g in 6 mol/L KOH electrolyte, good rate capability and cycle stability (92.6% of capacitance retention after 10, 000 cycles). More importantly, an energy density of 44.1 Wh/kg was also displayed with a power density of 757.1 W/kg in an organic electrolyte. More consideration, their developed strategy presents a facile approach for turning plastic waste into high value-added products, which will potentially pave the way for the treatment of plastic waste in the future.

Later, Wen, Chen and co-workers [87] have prepared porous carbon sheets (PCSs) derived from polystyrene waste through the MgO template combined with KOH activation at 800 ℃ for 2 h under argon atmosphere. The as-prepared PCSs displayed surface area of 1163 m²/g and pore volume of 2.6 cm³/g after KOH activation, and specific capacitances of 135 and 97 F/g at 1 mV/s and at 1 A/g in 1 mol/L H₂SO₄ electrolyte and excellent rate performance of 82.9% retention at 20 A/g in symmetric supercapacitors were achieved. Moreover, the energy density of 3.4 Wh/kg with a high-power density of 250 W/kg in the aqueous electrolyte was achieved. Remarkably, PCS showed excellent capacitance retention of 92.4% even after 10, 000 cycles, clearly presenting the strong long-term stability.

Chen, Zhao, Tang and co-members [88] have successfully synthesized porous carbon flakes (PCFs) with the large specific surface area of 1087 m²/g, a pore volume of 4.42 cm³/g and high conductivity by direct pyrolysis of polystyrene waste through template method. After, manganese dioxide (MnO₂) nanosheets were selectively deposited on the surface of resultant PCFs to form hybrid PCF-MnO₂ materials. Due to the outstanding features of the prepared PCFs, native high specific capacity of MnO₂, and positive synergistic interaction between PCF and MnO₂, the PCF-MnO₂ materials showed an ultrahigh capacitance of 308 F/g at 1 mV/s and 247 F/g at 1 A/g in LiCl electrolyte, and excellent cycle stability of 93.4% capacitance retention over 10, 000 cycles at 10 A/g in symmetric supercapacitor device.

Their work demonstrates a convenient strategy for designing cost-effective technology and high-performance electrode mate-rial for electric capacitors. More importantly, it reveals a potential way to recycle polystyrene waste into high-valued products on large-scale with disposing of polymeric waste to alleviate environmental concerns.

The treatment of PET bottles waste accumulated in the environment and ocean is a big challenge. Chen et al. have turned the PET waste into high-valuable electrode materials for electrical double supercapacitors [89]. They have established an easy strategy to efficiently convert PET beverage bottles waste into porous carbon nanosheet (PCNS) through catalytic carbonization process and KOH activation. The PCNS features an ultrahigh specific surface area of 2236 m²/g, hierarchically porous architec-ture, and a large pore volume of 3.0 cm³/g. Benefit from these physicochemical properties, the outstanding super-capacitive performance of 169 F/g in 6 mol/L KOH electrolyte and 135 F/g in 1 mol/L Na₂SO₄ electrolyte was reached.

Also, PCNS displayed a high specific capacitance of 121 F/g and an energy density of 30.6 Wh/kg at 0.2 A/g in 1 mol/L TEATFB/PC electrolyte. When the current density increases to 10 A/g, the capacitance remains at 95 F/g, indicating the extraordinary rate capability. Another strategy was developed to tackle PET waste, Noha A. Elessawy and co-workers have prepared the 3D sponge nitrogen-doped graphene (NG) by mixing PET waste with urea in one simple step and industrial, scalable green synthesis [47]. The as-prepared NG exhibited an outstanding performance with the specific capacitance of 405 F/g at 1 A/g, and an energy density of 68.1 Wh/kg and a high maximum power density of 558.5 W/kg in 6 mol/L KOH electrolytes were achieved from the optimized sample. The NG samples presented appropriate cyclic stability with capacitance retention of 87.7% after 5000 cycles at 4 A/g with high charge /discharge duration.

Considerable attention was also paid to the utilization of plastics waste-derived carbon in energy storage. The team of Chen, Gong, and Tang [90] have produced porous carbon nanosheets (PCNSs) under catalytic carbonization of "real-world" mixed waste plastics on organically-modified montmorillonite (OMMT) and subsequently KOH activation. The-obtained PCNSs showed a high specific surface area of 2198 m²/g and a huge pore volume of 3.0 cm³/g. The PCNSs exhibited a superior performance with high specific capacitances approaching 207, 137 and 120 F/g at 0.2 A/g in 6 mol/L KOH, 1 mol/L Na₂SO₄ and 1 mol/L TEATFB/PC electrolyte, respectively. The results from their work not only provides a promising method to upcycle mixed waste plastics but also puts forward an easy cost-effective strategy for the large-scale production of porous carbon nanosheets (PCNSs) potential candidate materials for supercapacitors. The research team directed by Tang has prepared carbon nanotubes with high yields and of good quality from the mixture of PP, PE, PS and their blends carbonized via the catalyst of nanosized carbon black and Ni₂O₃. The as-prepared the CNT and the assembled supercapacitor exhibited a high specific capacitance as compared to super-capacitors using commercial carbon nanotubes (CNTs) and carbon black (CB) [91]. In 2014, research group of Chen and Tang have developed a novel, easy, and cost-effective strategy to convert mixed plastics of polypropylene, polyethylene, polystyrene, poly (ethylene terephthalate), and polyvinyl chloride on organically modified montmorillonite into high value-added porous carbon nanosheets (PCNS) Fig. 3c [92] through carbonization and further activated with KOH. The PCNS exhibited a high specific surface area of 1734 m²/g and a large pore volume of 2.4 cm³/g with high purity (more than 99.5%). More importantly, PCNS showed a high performance in the uptake of carbon dioxide and the storage of hydrogen. Also, the degradation products of mixed plastics such as hydrogen, propylene, and benzene during the growth of CNS could be used as important chemicals and fuels. The results from their work demonstrate that not only pave the way for large-scale utilization of mixed waste plastics but also advance the sustainable production of valuable PCNS for different applications such as energy storage.

To date, many tires are unavoidably discarded in the environment because vehicle industries develop quickly, and therefore, their treatment is a global concern. The waste tires are not degraded easily and resist to decompose them under chemically and strong physical conditions due to its interconnected structure and possession of different additives, and long life [93]. The most waste tires are widely dumped in the environment and accumu-lated in landfills, which are not cost-effective technology to dispose of them. The sustainable solution which is economical and ecological would be to upcycle waste tires and employ them as carbon precursors for high value-added materials with potential applications as active materials in renewable energy sources and energy storage device.

Later, Wu et al. have designed an effective synthetic method for turning waste tires into activated carbon via the pyrolysis and chemical activation techniques as shown in Fig. 3a [42]. They explained that to control the activation parameters can tune multiple physical properties of the prepared activated carbon, which may influence the performance of the activated carbon electrode. From their investigation, they conclude that ion diffusion in the liquid electrolyte is the rate-limiting factor for the rate capability of activated carbon electrode. Finally, the best EDLC-type supercapacitor electrode displayed a specific capaci-tance of 106 F/g as depicted in Fig. S10 (Supporting information). In 2015, Cui and Liu contributed to the management of hydrolyzing waste tires, which causes the world to increase environmental and economic challenges. They have prepared ACs derived from the industrial pyrolytic tire char (PTC) by using a steam activation method and evaluated as promising active electrode materials for supercapacitor [58]. For the as-prepared AC-800-4 electrode presented a high specific capacitance of 190 F/g in the initial cycle and has a large decline in the first 100 cycles, which may be caused by the electrochemical oxidation of functional groups and the consumption of electrolyte. However, after 100 cycles, the specific capacitance remains almost constant (100 F/g). The AC-850-2 electrode shows similar behavior with 210 F/g at the first cycle and 90 F/g after 100 cycles. They found that activation temperature and time have a great effect on yield and structural properties. The higher content of oxygen-functionalities groups has enhanced the capacitance and boost the carbon and electrode wettability. Li and Sun's research group have treated the waste tires and for the first time, achieved to directly turn them into 3D graphene by using low-cost chemical agents and simple setup and alkaline-assisted one-step pyrolysis process [48]. The 3D graphene structure presented a high conductivity of 18.2 S/cm, and favorable hierarchical with rich porosity, and can be used as a decent energy storage material. For a supercapacitor electrode, it exhibited a high capacitance of 324.9 F/g at 0.2 A/g and excellent cyclic life (retention of 95.9% after 10, 000 cycles). Their work not only suggests a new approach for high-value reuse of waste tires but also provides a potential low-cost feedstock for large scale production of graphene.

The research group directed by M.P. Paranthaman and Y. Gogotsi has used waste tires and conducting polymer to manufacture supercapacitors [94]. They synthesized a three-dimensional meso-/microporous network with a specific surface area of 1625 m²/g. The narrow pore-size distribution and high surface area led to good charge storage capacity, especially when used as a three-dimensional nano scaffold to polymerize polyaniline (PANI). The composite paper was highly flexible, conductive, and showed capacitance of 480 F/g at 1 mV/s with excellent capacitance retention of up to 98% after 10, 000 charges/ discharge cycles. The high capacitance and long cycle life were attributed to the short diffusional paths, uniform PANI coating, and tight confinement of the PANI in the inner pores of the waste tire-derived carbon through π-π interactions, which minimized the degradation of the PANI upon cycling. They concluded that PANI/TC or a combination of TC with other redoxactive polymers and metal oxides might find its application in the large-scale energy-storage systems, where materials with moderate cost and high energy density are critically needed.

The fast-growing of electronic wastes (e-wastes) produces approximately 20-50 million tons per year, and their increase rate is between 4-5 percent annually [95, 96]. Particularly, the printed circuit boards (PCBs) are found in almost every electrical and electronic gadget. PCB is a complex structure to assemble and also to disassemble; the PCBs are built by using metals (such as Cu, Al, Fe, Sn, Sb, Pb) and non-metals (for example thermosetting resins, reinforcing materials, brominated flame retardants (BFRs) [97]. PCBs are categorized into single/double-sided and multi-layered PCB. The PCBs in computers and communication equipment are made from glass fiber reinforced epoxy resin (referred to FR-4 type), but televisions, monitors, and home electronics predomi-nantly use PCBs made of phenolic based polymeric material (FR-2 type). Additionally, in PCBs, the arrangement of polymers (polyethylene, propylene, polyesters, etc.), ceramics (mainly, SiO_2, Al₂O₃) and metals assembled and manufactured in one substrate. Waste PCB of a monitor from an end of life computer is considered to be polymer-rich based material (FR-2) produces carbon-rich nonmetallic residue after the pyrolysis process. Therefore, this alternative secondary waste non-metallic fraction (NMF) can be considered for replacing the conventional sources to produce activated carbon, which helps in maintaining much-needed sustainability. For instance, Rajarao et al. reported the activated carbons materials derived from non-metallic fractions from PCBs waste throughout physical activation after pyrolysis processes as shown in Fig. S11a (supporting information) [47]. The prepared activated carbon materials exhibited the highest surface area of 700 m²/g and a higher volume of 0.022 cm³/g annealed at 850 ℃ for a 5 h and was electrochemically evaluated (Fig. S11b in Supporting information). The synthesized activated carbons showed a specific capacitance of 220 F/g at 30 mV/s and 156 F/g, even at 100 mV/s with the retention value of 98% over 1000 cycles. They have proposed an effective approach for producing value-added activated carbon materials from PCB waste demonstrating electrochemical behavior promising for the energy storage realm.

For laboratory experiments, the filter papers are thrown away or kept in the trash can or discarded in the environment or sometimes burned after filtration, which generates hazards compounds. For safety and environment issues, there is a need for a sustainable approach to dispose of waste filter paper. The research group of Chang and Yang has prepared hierarchical activated porous carbon (APC) from recycled waste filter paper via convenient chemical activation with ZnCl₂ (Fig. 4a) [60]. The produced activated carbon presented a large surface area of 2170 m²/g, suitable pore size, and prominent porosity. The as-synthesized APC showed a specific capacitance of 302.3 F/g in 6 mol/L KOH at 1 A/g. Furthermore, the APC electrode exhibited good long-term cycling stability, and 95.4% of its initial specific capacitance at 5 A/g and was maintained even after 10, 000 cycles. From the presented results, the suggested method offers a new view of exploring waste products to develop high-performance activated carbon electrode materials for ideal applications in electric doublelayer capacitors.

The compact discs (CDs) or digital versatile discs (DVDs) are portable storage for recording, storing, and playing audio, video or other digital data. In 2003, accounting for the production of CDs and DVDs worldwide was 12 billion pieces [98] and has highly grown. Waste CDs have contributed highly to dump because of the complex nature of the manufacturing process (approximately 10% of all made CDs are disqualified), postconsumer CDs, and destruction of CDs due to the artist's right issue (unsold copies). In 2018, Rifat Farzana and team-mates have prepared the microporous activated carbon from compact discs waste via physical activation method for supercapacitor applications [99]. The as-synthesized activated carbon showed the highest specific surface area of 1214 m²/g achieved at 900 ℃ and 8 h for activation. The produced activated carbon displayed good specific capaci-tance, cycle stability and good rate ability where the specific capacitance 51 F/g at the current density of 10 mV/s. The energy density of 21.43 Wh/kg at power density 0.7 W/kg and 9.68 Wh/kg at power density 2.2 W/kg were also achieved in aqueous electrolytes. Therefore, this innovative strategy of exploiting outdated CD activated carbon as supercapacitor electrode mini-mizes the environmental issues of waste CDs and offer an alternative active electrode material for supercapacitor application.

Coal tar is a low-cost and plentiful waste produced during the coke production process. The annual output of coal tar is around 20.0 million tons in China only. Coal tar is rich in polycyclic aromatic hydrocarbons that are active and linked by the single chain or bridge bonds. Compared with the carbon connection in graphene, the polycyclic aromatic hydrocarbons in coal tar are facile aromatized to make large sheet-like films that can be further converted into graphene-like nanosheets. Coal tar pitch (CTP), the main residue from the distillation of coal tar (more than 55%), is cheap and abundant with plasticity characteristics. There are many polycyclic aromatic hydrocarbon molecules in CTP, which are connected by the single chain or bridge bonds. Taking into account these chemical compositions; thus, CTP is regarded as carbon precursors for preparing carbon materials for supercapacitor. In 2014, He and co-authors developed 3D hollow porous graphene balls (HPGBs) from the coal tar pitch for the first time by a simple nano-MgO template strategy coupled with KOH activation (Fig. 4b) [100]. The as-made HPGBs feature a 3D spherical architecture with a thin porous shell that has a high specific surface area and consists of macropores, mesopores, and micropores in a well-balanced ratio.

The as-synthesized HPGBs electrode for supercapacitor dem-onstrated a high specific capacitance of 321 F/g at 0.05 A/g, an excellent rate performance of 244 F/g at 20 A/g, and good cycle stability over 94.4% capacitance retention after 1000 cycles. The good electrochemical performance of the prepared materials is due to the networks with a balanced pore distribution in the shells of HPGBs support to enhance the conductivity of electrons, and to decrease the resistance to ion transfer owing to the more porous channels and active sites in the thin carbon shells permit for storage and fast transportation of electrolyte ions. Their work results may provide a new approach for effective and mass production of affordable price spherical graphene, aromatic hydrocarbon sources such as coal tar and heavy oils, for super-capacitors applications. He and co-workers have developed a layered-hydroxide-template strategy combined with in-situ chemical activation to prepare porous carbon nanosheets (PCNSs) from coal tar [101]. The as-synthesized hierarchical PCNSs constituted by large sheets possess high electric conductivity and plentiful of short pores for rapid ion transport and adsorption. The as-prepared PCNS electrodes exhibited a high capacitance of 296.2 F/g at 0.05 A/g in 6 mol/L KOH electrolyte, excellent rate performance with capacitance remaining at 220.7 F/g at 20 A/g and cycle stability with more than 97.2% of initial capacitance after 11, 000 cycles.

The solid leather wastes are byproducts produced during the leather crusting process. The solid leather wastes are susceptible to serve as the porous carbons precursor material due to their abundance in the environment and low-cost. To convert the solid leather wastes into porous carbon materials for supercapacitor applications is a great achievement on the industrial scale and economy of the country in particular.

Kennedy and team-workers synthesized porous carbons from solid leather wastes (SLW) by using carbonization and chemical activation and were served as the active electrode material for supercapacitors in H₂SO₄ medium as shown in Fig. 4c [102]. Electrochemical measurements reveal that porous carbon activated at 900 ℃ demonstrated a maximum capacitance of 1833 F/g at 1 mV/s in 1 mol/L H₂SO₄ electrolyte. Besides, the produced porous carbon electrodes exhibited excellent cycling stability after 500 cycles at 20 A/g. To conclude, the results from their work proposed that prepared carbon active materials from Solid Leather Wastes can serve as desirable candidate materials for supercapacitors.

Zhang and team-members from the Research Institute of Chemical Defense (Beijing, China) have successfully synthesized an interconnected hierarchical porous carbon from sodium lignosul-fonate industrial waste via direct carbonization without adding templating and activating agents [103]. The as-synthesized carbon electrode material presented a moderate specific surface area of 903 m²/g and high contents of 8.11 at% oxygen and 1.76 at% nitrogen, which could enrich the electrolyte-affinitive surface area in an aqueous electrolyte. The electrochemical evaluation of the prepared carbon electrode material for symmetric supercapacitor showed a high gravimetric capacitance of 247 F/g, a volumetric capacitance of 240 F/cm³, and areal capacitance of 27.4 μF/cm² at 0.05 A/ in 7 mol/L KOH electrolytes. Furthermore, a superior energy density of 8.4 Wh/L at 13.9 W/L and a power density of 5573.1 W/L at 3.5 Wh/L, as well as excellent cycling stability after 20, 000 cycles at two various current densities were reached for the assembled supercapacitors. All results of their work suggested a powerful and environment-begin method favorable for mass-production of nanoporous carbons by employing plentiful and sustainable industry waste sodium lignosulfonate as carbon materials precursors, demonstrating potential applications in high-performance super capacitors. Wang and co-workers have synthesized the nitrogen-doped porous carbon derived from sodium alginate and urea via one-pot annealing at 700 ℃. The as-synthesized materials showed a mesoporous and macroporous structure and delivered the specific capacitance of 180.2 F/g at 1.0 A/g with excellent cycling life with 96% capacitance retention after 5000 potential cycles in 6 mol/L KOH electrolyte [104]. The result from their work shows the potential application of industrial waste for preparing electrode materials for supercapacitor applications.

In 2019, Magdalena Titirici and team-members have reported the preparation of the ordered mesoporous carbons (OMCs) produced from lignin via the evaporation induced self-assembly (EISA) method. They have proved that it is possible to substitute half of the phloroglucinol (a well-known carbon precursor currently not derived from bio-precursors) by hardwood organosolv lignin while obtaining a highly ordered pore structure. Notably, they have also used glyoxal instead of formaldehyde as a crosslinker, which makes the synthesis route even "greener" concerning the toxicity of the precursors and their renewable sourcing. The as-prepared bio-based material attains a volumetric energy density of 3 Wh/L at 1 kW/L, which could be easily improved by adapting the microporous structure to the electrolyte ion size [105]. Many efforts were devoted to turning the industrials waste, plastic waste and other waste into electrode materials for supercapacitors application, but we can say it is the beginning of a journey. Thus more research is needed to contribute to shaping and protecting the environment without losing any materials.