Chapter 1: Introduction to Green Supercapacitors: Fundamentals, Design, Challenges, and Future Prospects

Rapid progress in the field of science and technology has led to a drastic increase in the energy consumption rate of the human race. Increased utilization of fossil fuels and their ever-soaring prices has raised concern about their irreversible depletion and greenhouse gas emissions.¹  Hence, the need for alternate energy sources is inevitable. Although energy production from solar and wind renewable sources is on the rise, the intermittent availability of these resources requires efficient energy storage systems that can store the generated energy during surplus and release it on demand.²  In this regard, rechargeable batteries, supercapacitors, etc., are considered prime high-performance energy storage systems complying with the needs of smart storage applications such as electric vehicles (EVs), start–stop systems, regenerative braking, etc. Batteries are attractive, considering their high energy densities, but they are limited by their high material as well as manufacturing costs, long charging times, low cyclability, scarce resources, and low power densities.²  Consequently, supercapacitors with 100–1000 times higher power densities, extremely long cycle lives (>100 000 cycles), fast charging times (10 s–10 min), high cycle efficiency (>90%), cryogenic applications (workable at −30 °C), and cost-effective raw materials have come into the limelight.³  While the energy storage capacity of a battery is high, supercapacitors can quickly transfer the energy. The Ragone plot in Figure 1.1a explains the energy and power density attributes of a few such energy storage systems. Figure 1.1b compares the cyclic voltammetry (CV), and charge–discharge (CD) profiles of EDLC supercapacitors, pseudocapacitors, and battery systems. Supercapacitors are essentially employed for applications involving high acceleration/braking, solar energy warning lights, beacon lights, offshore wind power generation or turbines, and high-speed transport systems.²  A generic architecture of a supercapacitor consists of two electrodes (positive and negative), separated by a thin film known as a separator immersed in an electrolyte that allows the electron and ion transport from the electrolyte to the electrodes. Upon application of a voltage, the charges become separated and adsorbed on the surface of the oppositely charged electrode resulting in the formation of the Helmholtz double-layer within which the energy is stored. In principle, supercapacitors are classified into three types, namely: electrochemical double layer capacitors (EDLCs), pseudocapacitors or redox supercapacitors, and hybrid capacitors (Figure 1.1). Table 1.1 shows the simplified classification with the major attributes of materials employed, charge storage mechanism, and the merits and demerits of each.

The electrochemical performance of supercapacitors entirely depends on the type of electrode materials employed. The main parameters affecting a supercapacitor device performance would be the conductivity of the active material, electrolyte, specific surface area (SSA) of the electrode, pore size distribution, and electrochemical reversibility/stability. Most commercial supercapacitors are made of carbon electrodes owing to their high electronic conductivity, tailored pore size/SSA, high stability, and ease of availability. However, due to the nonfaradaic charge storage mechanism, the device capacitance is limited to 100–200 F g⁻¹ along with a low energy density. Nevertheless, the pseudocapacitors employ metal oxides, MXenes, conducting polymers, etc., as active materials that are involved in the faradaic charge-storage process, thus resulting in much superior specific capacitance and energy densities to EDLCs. However, these are restricted by their inevitable agglomeration during cycling leading to capacitance fading, poor cycle stability, and rate capability. The third class of supercapacitors, namely, hybrid capacitors, combines the merits of both EDLCs and pseudocapacitors by a combination of faradaic and nonfaradaic means. They have the added advantages of large electrochemical windows, high specific capacitance/capacity, and thus surpass the batteries in power density and conventional supercapacitors in energy density parameters. Nevertheless, the charge balance between the anode and cathode or specific capacity matching is one of the challenging aspects of the design of these architectures.

The EDLCs are made of carbon-based electrode materials that store charge by electrostatic or nonfaradaic means. Application of a voltage results in charge separation leading to the double layer formation. The process of charging takes place by the transfer of electrons from the cathode to the anode via the external circuit, which results in the adsorption of the cations and anions on the electrodes of opposite polarity, respectively. The reverse takes place during the process of discharge, indicating no charge transfer. The mechanism of charge storage on both the electrodes can be explained based on the following equations:¹² 

Among the multitude of materials displaying the EDLC behavior, carbon has been the choice due to its low cost, ease of availability, and tailored electrical properties.¹⁴  The conversion of carbon to its activated form has opened up wide opportunities for energy storage with increased surface area, electrical conductivity, and varied pore size distribution that allows easy access to electrolyte ions. Conventionally, carbons can be divided into three forms, namely, diamond, graphite, and amorphous carbon.¹  Development of science and technology has seen new facets of graphitic carbons in multidimensions, including 0D fullerene, 1D carbon nanotubes (CNTs), 2D graphene, 3D graphene foam, etc. A brief description of a few carbons and their electrochemical performance is given below.

Recently, activated carbons have been researched quite effectively for supercapacitors due to their high SSA, cost-effectiveness, and electrical conductivity.¹  Nevertheless, a large number of micropores (<2 nm pore size) hinders these properties due to the lower accessibility of electrolyte ions. Hence, more devoted research has been focused on tuning the properties of carbon materials with mesoporous structures (2–50 nm). For example, Liu et al.⁵  developed mesoporous carbon (MC) with tunable pore size in the range of 3.9–5.7 nm by coassembly of resorcinol formaldehyde. The MC exhibited a sheet-like morphology with a specific capacitance (C_sp) of 258 F g⁻¹ at 0.5 A g⁻¹ and good stability 88% for 10 000 cycles. It is understood that a higher content of mesopores in the developed active material can lead to higher SSA and charge storage. However, beyond a threshold limit, a further increase in mesopores is found to reduce the device capacitance due to lower packing density (Figure 1.3).¹⁵  Hence, the effect of pore size distribution is found to be more influential in comparison to the material surface area. A hierarchical distribution of mesopores and micropores is found to be optimum in maintaining both the charge storage and stability of supercapacitors.¹⁶  It is observed that the activation of MC can also help in micropore formation. Optimization of the concentrations of the activation agent and process can lead to the desired ratios of mesopores : micropores. For instance, petroleum coke; a byproduct from oil refineries with high carbon content is one of the mentioned sources of carbon. Zhang et al. showed the effect of KOH activation in improving the SSA and mesopore to micropore ratio to further enhance the charge storage properties.¹⁷  The optimized ratio of KOH activation agent has resulted in a high SSA of 1129 m² g⁻¹ with a C_sp of 261 F g⁻¹ at an ultrahigh current density of 50 A g⁻¹, respectively. Similarly, Xia et al. showed the synthesis of mesoporous carbon from CMK-3 precursor followed by CO₂ activation for 6 h at 950 °C for micropore formation.¹⁸  The hierarchical structure has shown a high SSA of 2749 m² g⁻¹, pore volume of 2.09 cm³ g⁻¹, and a C_sp of 227 F g⁻¹ at a 2 mV s⁻¹ scan rate. Recently, doping of MC is seen as an alternate strategy for improving the electrochemical storage properties. Sun et al.¹⁹  developed nitrogen-doped mesoporous carbon microspheres by a spray drying vapor deposition technique with high packing density and an SSA of 1528 m² g⁻¹. Benefitting from the combined mesopore and nitrogen doping, the active material achieved a C_sp of 533.26 F g⁻¹ along with good cycle stability in a three-electrode system configuration.

Biomass is the live organic matter obtained from living sources such as animals and plants. Various sources of biomass wastes include agro waste, municipal waste, industrial waste, etc. All of these have the potential to be converted into efficient cost-effective carbon sources to be employed for energy storage applications, although some are left as such leading to environmental pollution. Other applications of biomass waste include feedstock for boilers, organic fertilizers, etc.²⁰  Although the major constituent of biomass waste is carbon, it also includes a few alkali metal ions and the presence of nitrogen, oxygen, sulfur, etc. that can be used as such for improved electrochemical activity. Some reports also state the chemical activation of pristine carbon to modify the porosity and SSA resulting in enhanced charge storage properties. For instance, Nanaji et al. developed graphene-like structured porous carbon from jute sticks as biomass and carbon sources.²¹  The pristine carbon was activated in KOH to obtain the desired micropore to mesopore ratio and good electrochemical performance. The optimum amount of KOH activation agent has shown an increase in the SSA from 949 to 2396 m² g⁻¹, and pore volume from 0.69 to 1.6 cm³ g⁻¹. The increased SSA and optimum pore size have yielded a C_sp of 282 F g⁻¹ and 70% rate retention at a high current density of 50 A g⁻¹ (Figure 1.4). Similarly, Jiang et al.²²  developed high-performance porous carbon materials from waste tobacco straw as the precursor and a nano ZnO template by one-step carbonization-activation and ball milling. The obtained carbon had a high surface area of 1293.2 m² g⁻¹ and a C_sp of 220.7 F g⁻¹ at 1 A g⁻¹. Gur et al.²³  reported sugar beet pulp derived oxygen-rich porous carbon by using Mg(CH₃COOH)₂·4H₂O and ZnCl₂ as activation agents. The obtained carbon had in situ high content of oxygen resulting in good methylene blue adsorption and a C_sp of 263 F g⁻¹ in 6 M KOH electrolyte. In the same area, Pang et al.²⁴  derived porous carbon from bamboo cellulose fibers. A green cellulose solvent was employed as both activation agent and nitrogen source, resulting in a hierarchical porous conducting network with doping of 8.43 at% and a C_sp of 280 F g⁻¹ at 0.3 A g⁻¹.

Porous and activated carbons have attracted wide attention for various energy storage applications. Of late, MOF-derived porous carbons have come into the limelight due to their high SSA, hierarchical pore distribution, and controllable structural sizes.²⁵  MOFs are constituted by the coordination bonds between the metal ions and coordination ligands.²⁶  The organic ligands form the source of carbon, while the uniformly distributed metal ions influence the porous structure and crystallinity. The SSA of MOF-derived carbons can range between 1000 to 10 000 m² g⁻¹, with the pore size as high as 9.8 nm.²⁷  The MOF-derived carbons can have the desired morphology and in situ doping of metal atoms, depending on the type of precursors employed. However, the unstable spacing of the coordinated bonds results in the structural collapse during pyrolysis resulting in diminished active surface area.²⁶  To address this issue, Tang et al.²⁶  proposed an inner graphene quantum dot (QD) support strategy. The –COOH groups on the graphene QDs can distribute uniformly within the MOF-5 precursor and coordinate well with ligand groups as support during pyrolysis. The resulting porous carbon could achieve a high SSA of 1841 m² g⁻¹ along with a C_sp of 200 F g⁻¹ at 1 A g⁻¹. Similarly, Li et al.²⁵  reported the synthesis of nitrogen-doped porous carbon from UiO-66 MOF with different ratios of adenine and 1,4-benzene dicarboxylate coated CNT. Nitrogen doping in the MOF-derived carbon has shown a prominent effect in improved surface area and capacitance with 121.5 mF cm⁻² at a 5 mV s⁻¹ scan rate. Furthermore, the designed flexible solid-state supercapacitor has shown an areal capacitance of 43.2 mF cm⁻² along with a volumetric energy and power density of 53.7 μW h cm⁻³ and 2.4 mW cm⁻³ at 0.5 mA cm⁻². Although MOF synthesis has grabbed huge attention, it has a few challenges still to be addressed. Specifically, it is necessary to identify new MOF precursors apart from MOF-5, ZIF-8, and ZIF-67, to extend their potential applications.²⁸  Furthermore, the entire electrochemistry of carbon materials is dependent on SSA, pore size distribution, and crystallinity. Hence, development of new synthesis methods and pyrolysis strategies are crucial in modulating the electrochemical properties.²⁹ 

Graphene is one of the most exciting and promising energy storage materials. It belongs to the family of carbon materials made of a hexagonal lattice-shaped single layer of sp² hybridized carbon atoms. The interatomic layer between each graphene sheet is found to be 1.42 Å.³⁰  It is known for its exceptional properties of high SSA of 2630 m² g⁻¹,³¹  thermal conductivity of 5000 W m⁻¹ K⁻¹,³²  intrinsic charge carrier mobility of 200 000 cm² V⁻¹ s⁻¹,³³  etc. The first synthesis of graphene was initialized in 1859 when the British chemist Brodie used concentrated sulfuric and nitric acids to intercalate graphite to form graphite oxide.³⁴  The following invention led to many modifications and new synthesis techniques involving less concentrated and toxic chemicals such as the Hummer’s, CVD, plasma vapor deposition (PVD), chemical and mechanical exfoliation, electrochemical exfoliation, etc. To have a high charge storage performance, it is essential to have fully exfoliated high surface area graphene. One of the strategies involves having graphene with a crumpled nanosheet morphology that prevents restacking and accommodates a large number of ions.³⁵  To further improve the performance, they are doped by various ions such as nitrogen, phosphorous, sulfur, etc., which insert within the interlayers and contribute to additional pseudocapacitive charge storage. Tang et al.³⁶  developed reduced graphene oxide fibers by a one-step hydrothermal approach, which showed excellent stability of 90.80% capacitance retention after 100 000 cycles at 10 A g⁻¹ along with a C_sp of 246.01 F g⁻¹ at 1 A g⁻¹. Kumar et al.³⁷  reported the development of graphene ink 3D printed in interdigitated electrode format devoid of high-temperature processing and functional group requirements. The developed symmetric supercapacitor yielded a C_sp 137.50 F g⁻¹ at 0.5 A g⁻¹ along with an energy density of 12.23 W h kg⁻¹. Additionally, the printed device was found to be flexible and mechanically stable for 150 bending angles. Alternatively, Chen et al.³⁸  reported the novel catalysis strategy to develop ultrathin microporous carbon with a few-layer graphene heterostructure within the hollow carbon spheres. The micropores are expected to reduce the ion transport distance while the FLG contributes to the electronic conductivity. The developed heterostructure has exhibited a high C_sp of 192 F g⁻¹ at a 5 mV s⁻¹ scan rate. Yu et al.³⁹  reported rapid microwave exfoliation of graphene oxide for high voltage applications in a water-in-salt electrolyte. The exfoliated graphene oxide showed a thin sheet-like structure with a 2.5 V electrochemical window and an energy density of 22 W h kg⁻¹ at a power density of 12.5 kW kg⁻¹, respectively.

CNTs were first discovered in 1991,⁴⁰  and ever since have been widely explored for various applications^41–43  including energy storage due to their intriguing properties of high SSA, electrical conductivity, and mechanical stability. CNTs are formed by the curling up of the graphite sheet into cylindrical shapes. They are classified into single-walled (SWCNTs) and multiwalled (MWCNTs). Both types have been explored for supercapacitor application individually and together with other metal oxide composites. The CNTs have wide pore size distribution and high surface area accessibility, which promotes easier diffusion of ions and charge storage. Also, they accommodate the volume expansion in metal oxide composites during charge–discharge owing to their cylindrical open porous structure.⁴⁰  They are highly stable in both acidic and alkaline electrolytes. However, CNTs connect by the weak van der Waals forces, which leads to poor stability, high resistance, serious self-discharge, and peel-off issues. To avoid the same, they are grown directly over the conducting current collectors such as carbon paper, stainless steel foil, etc.⁴⁴  Then, they are either directly tested as supercapacitor electrodes that show EDLC behavior or are modified with various functional groups by acid treatment or metal doping, exhibiting a pseudocapacitance contribution too. It is noted that the electrochemical performance of CNTs is restricted by their closed tips and lower active sites, and hence to modify the same, Zhang et al.⁴⁵  employed the chemical acid etching technique to open the tips and introduce different functional groups. The electrochemical performance was analyzed in various cation-based aqueous electrolytes such as Li⁺, Na⁺, and Mg²⁺. The highest performance was observed with a Li⁺ based electrolyte delivering a C_sp of 542 mF cm⁻² at a scan rate of 10 mV s⁻¹ along with energy and power densities of 39 μW h cm⁻² and 10.2 mW cm⁻², respectively. Similarly, Aval et al. report the study of paper supercapacitors based on CNT in a PVA/H₃PO₄ gel electrolyte and BaTiO₃ film as the separator exhibiting a C_sp of 411 F g⁻¹ at 20 mV s⁻¹ scan rate.⁴⁶  Avasthi et al.⁷  reported the chemical vapor deposition (CVD) of CNT on SS mesh followed by atomic layer deposition of TiO₂ nanoparticles to improve the hydrophilicity. The composite material has shown a super hydrophilic nature with a C_sp of 16.24 mF cm⁻² at 1.67 mA cm⁻² current density. It also shows 99.7% capacitance retention for 5000 cycles and an energy density of 1.18 mW cm⁻². Diez et al.⁴⁷  showed the development of free-standing and binder-free partially reduced graphene oxide–CNT composite used for supercapacitor application. With only 2 wt% CNT, the composite has shown a C_sp of 250 F cm⁻³ at 1 A g⁻¹ with a superior mass loading of 12 mg cm⁻² in 6 M KOH electrolyte.

Although different types of carbon sources owing to their ease of availability, processibility, high stability, and electronic conductivity have been widely tested for supercapacitor applications they are still limited by low specific capacitance and energy density attributes that need to be addressed by electrode, electrolyte, and device architecture modifications.⁴⁸  The various carbons, depending on the source of origin exhibit different morphology and pore size distribution. A combination of mesopores and micropores is found to be optimum to extract the maximum charge storage performance. Fine tuning the activation process to attain these desired properties is of utmost importance. Simultaneously, the process cost-effectiveness, abundance, and scalability are to be taken into account.

Intercalation pseudocapacitance is a third class of charge storage mechanism observed in pseudocapacitors that involves intercalation of cations or anions into the layers of the redox active materials resulting in faradaic charge transfer without any crystallographic phase change during the charge–discharge.⁵²  Various metal oxides such as Nb₂O₅, MoO₃, V₂O₅, etc. have been explored for this charge storage phenomenon. Up to now, many cations such as Li⁺, Na⁺, K⁺, Mg²⁺, Al³⁺,^53–55  etc. have been studied as intercalation cations for improved charge storage. Recently, O²⁻ is also seen as a potential intercalation anion where O²⁻ can carry two negative charges per unit charge and can store twice the amount of pseudocapacitance in comparison to the monovalent Li⁺ intercalation process.⁵²  Numerous metal oxides, hydroxides, and polymeric composites have been studied for their pseudocapacitive properties, which are discussed in detail below.

Recently, binary metal oxides such as NiCo₂O₄,⁶⁵  ZnCo₂O₄,⁶⁶  CoMn₂O₄,⁶⁷  etc., with spinel structures have attracted wide attention owing to their multiple oxidation states, superior conductivity to individual oxides, and high reversible electrochemistry, which leads to higher specific capacitance and energy densities. The above-mentioned metal oxides and hydroxides are employed as positive electrodes for supercapacitors. Apart from these, a few metal oxides such as Fe₂O₃, V₂O₅, MoO_x, Ce(OH)₃,^68–70  etc., are also explored as negative electrode materials.

Recently, metal sulfides are attracting wider attention for supercapacitors due to their lower electronegativity (electronegativities of S 2.58 and O 3.44) and narrower bandgap resulting in higher electronic conductivity in comparison to metal oxides and hydroxides. The anionic polarizability of large-size S²⁻ also improves the ionic diffusibility leading to enhancement in the ionic conductivity and electrochemical activity. Recently, many binary and ternary metal sulfides like Cu_0.75Co_0.25MnS,⁷¹  CuNi₂S₄,⁹  Ni₃S₂,⁷²  etc. are being studied for their supercapacitor performances. Subsequent to metal sulfides, n-type semiconductors with transition metal phosphides have also attracted major attention during the past few years. The electronegativity of phosphorous is lower than oxygen and sulfur, making it superior in electronic conductivity and redox chemistry.⁷³  Chen et al. showed the synthesis of fluffy nano dendritic shaped CoP by hydrothermal and electrodeposition routes resulting in a specific capacity of 461 C g⁻¹ at 1 A g⁻¹ due to the unique structure and high electronic conductivity of phosphides over oxides and sulfides.⁷⁴  The assembled asymmetric supercapacitor with the developed phosphide has also shown a high energy density of 42.4 W h kg⁻¹ at a power density of 800 W kg⁻¹. Recently, metal–organic-framework (MOF) derived phosphides and sulfides have also been widely researched due to the reasonable synthesis methods, high surface area, and unique pore structure directed towards better electrochemical performance. For instance, Li et al. reported the design of 2D–3D NiZnCo-P from 3D porous ZnCo-MOF and 2D NiZnCo-LDH. Owing to the improved interfacial contact between the electrode and electrolyte, which facilitates the ion diffusion and electron transfer, the supercapacitor anode has resulted in a high specific capacitance of 2816 F g⁻¹ at 1 A g⁻¹ and 89% capacitance retention for 10 000 cycles.⁷⁵  Transition metal nitrides (TMNs) are yet another class of emerging supercapacitor materials. The bonding between the TMNs is a coagulation of metallic, covalent, and ionic, respectively.⁷⁶  Structurally, they are metallic with disorderly oriented nitrogen atoms at the interstitial sites, giving rise to unique physical and chemical properties. One of the first studied TMNs for supercapacitors is molybdenum nitride as a substitute for RuO₂ in 1998,⁷⁷  which led to the subsequent research on other nitrides including vanadium nitride, titanium nitride, their composites, and so on. In one recent report, Zhou et al. noted the synthesis of VN nanobelts by thermal nitridation of vanadium oxide nanobelt precursors resulting in simultaneous pore creation and nitridation with a specific capacitance superior to the oxide parts of 446 F g⁻¹ at 10 mV s⁻¹.⁷⁸  In another report, Sun et al. deposited TiN films by direct current sputtering that were then studied for onchip micro supercapacitor applications.⁷⁹  Due to the high electronic conductivity, density, and low internal resistance the nano/micrometer thick TiN yielded excellent energy and power densities of 23 mW h cm⁻³ and 7.4 W cm⁻³, respectively, along with promising cycling stability.

Two-dimensional (2D) materials have attracted significant attention in the past decade for their high potential to address society’s most pressing issues of high energy and power-based energy storage systems. One of the latest, and relatively large, families of 2D materials is transition-metal carbides and nitrides, called MXenes.⁸⁰  These are generally produced by the selective etching of atomic layers of the MAX phases. MAX phases are layered ternary metal carbides, nitrides, or carbonitrides, with a general formula of M_n+1AX_n (n = 1, 2, 3), where M, A, and X represent the early d-block transition metals, main-group elements (predominantly IIIA or IVA), and either or both C and N atoms, respectively. It is also reported that MXenes can be synthesized from non-MAX phase precursors.⁸¹  Mo₂CT_x is the first MXene of this kind that was synthesized by the selective etching of Ga layers from the Mo₂Ga₂C with two Ga layers separating the carbide.⁸²  MXenes are acclaimed with many exceptional properties like high electronic conductivity of 104 S cm⁻¹ comparable to that of multilayered graphene,^83,84  high surface area, and unique 2D structure with accordion-like morphology accommodating various cations for charge storage applications⁸⁵  (Figure 1.6). Specific to supercapacitors, Ti₃C₂T_x was the first studied MXene by the accommodation of various cations in both alkaline and neutral electrolytes resulting in a specific capacitance of 300 F cm⁻³ in 2013.⁸⁶  It is seen that free-standing MXenes hold great promise for supercapacitors. However, their self-restacking is an issue of concern. Therefore, Fan et al. showed the design of free-standing and flexible MXene with incorporated Fe(OH)₃ nanoparticles resulting in interconnected nanopore channels easing the transportation of ions. The resulting MXene shows a promising supercapacitor performance with a C_sp of 1142 F cm⁻³ at 0.5 A g⁻¹ in 3 M H₂SO₄ electrolyte. Retarding the problem of high mass loading, the developed MXene has also shown an excellent volumetric capacitance of 749 F cm⁻³ and good flexibility even at a high mass loading of 11.2 mg cm⁻².⁸⁷  Similarly, MXene hydrogels have also shown their promising electrochemical performance in supercapacitor applications. One recent report by Zhu et al. showed the development of MXene-holey graphene hydrogels prepared by simple hydrothermal treatment. The incorporation of graphene was expected to accelerate the ion transport by reducing the diffusion path lengths and improving the overall capacitance. The developed composite material has been shown to achieve a high gravimetric and volumetric capacitance of 415 F g⁻¹ and 597 F cm⁻³, respectively.¹⁰ 

Conducting polymers (CPs) are simply covalently bonded large monomer units deriving their conductivity from the presence of conjugated π-electrons.⁸⁸  They are being extensively studied for supercapacitors acclaimed for their good electronic properties, high electron affinity, environmental benignity, inherent flexibility, low cost, and ease of production.⁸⁹  The conductivity of polymers can be briefly understood by two mechanisms: (i) delocalization of electrons through the conjugated π-systems, and (ii) electron transfer in the redox polymers.⁹⁰  CPs are employed as both active (electrode materials, electrolyte) and passive (current collectors and separators) components of supercapacitors and can further be classified as synthetic polymers and biopolymers depending on their origin. Biopolymers are typically derived from biologic sources such as lignin, starch, etc., or bioorganisms, whereas synthetic polymers are artificially developed.⁹¹  Some of the most commonly developed CPs are polyaniline (PANI), polypyrrole (Ppy), polythiophene (PTh), etc. PANI is one of the widely researched electrode materials for energy storage due to its unique properties of ease of synthesis, stability, and doping–dedoping electrochemistry (Figure 1.7).⁹²  PANI can be synthesized into different shapes and structures by facile oxidation of aniline monomer by chemical or electrochemical routes. Inherently, they are known to exist in three different oxidation states of (i) Leucoemeraldine, (ii) Emeraldine, and (iii) Pernigraniline, contributing to the supercapacitor performance.⁹³  They are mostly employed as composite electrodes along with other carbons or metal oxide materials to improve the cycle stability performance. A recent report by Dadashi et al. stated the synthesis of PANI-g-MWCNT/vertically oriented graphene nanosheet (VOGN) for supercapacitors in H₂SO₄ electrolyte indicating a specific capacitance of 880 F g⁻¹ at 1.5 A g⁻¹ current density.⁹⁴  The developed electrode has also shown capacitance retention of 80% for 10 000 cycles. The obtained high electrochemical performance is attributed to the improved surface area from 14 m² g⁻¹ to 26 m² g⁻¹ by a VOGN layer and the synergistic pseudocapacitance from PANI–MWCNT. Another report by Yeasmin et al. stated the development of pencil-drawn paper-based multilayer graphene–PANI composite as a supercapacitor electrode.⁹⁵  When tested as an all-solid-state symmetric supercapacitor with PVA gel electrolyte, the developed composite has shown an areal capacitance of 93.64 mF cm⁻² along with 91% capacitance retention after 5000 cycles. Similar to PANI; Ppy, and PTh are a few among the widely researched CPs for supercapacitors. PTh exists in both p-doped and n-doped forms with the electrochemical performance and conductivity of the former being superior to the latter.⁸⁹  One recent report by Zhang et al. showed the electropolymerization of PTh on MWCNT in an oil-in-liquid microemulsion resulting in good electrochemical properties for supercapacitors with a specific capacitance of 216 F g⁻¹ in 1 M Na₂SO₄ electrolyte.⁹⁶  It is well known that a thick electrode with high mass loading can effectively address the energy density issue of supercapacitors. Hence, Liu et al. studied the synthesis of PPy/polydopamine on 3D carbon foam with an active mass loading of 8.5 mg cm⁻².⁹⁷  Owing to the good electronic and electrochemical properties of CPs, the resulting composite electrode has shown promising electrochemical performance in a symmetric configuration with a specific capacitance of 996 mF cm⁻² and an energy density of 0.12 mW h cm⁻² along with 100% capacitance retention after 10 000 cycles. Alongside the synthetic polymers, many biopolymers like keratin, lignin, starch, cellulose, pectin, etc., have also been studied for supercapacitor applications. Keratin is a natural polymer found in hair, skin, and nails. It has high wettability and the presence of cysteine and amino acids results in the presence of in situ sulfur atoms, making them suitable candidates for energy storage. In one recent work by Wu et al. a hetero atoms doped keratin derived porous carbon network was chosen for the fabrication of an all-solid-state supercapacitor.⁹⁸  The N, O, and S codoped carbon obtained from chicken feathers as keratin source has shown a high specific surface area of 2864 m² g⁻¹. When tested for supercapacitor applications, the formulated device showcased an energy density of 11.84 W h kg⁻¹ at 8525 W kg⁻¹ in neutral electrolyte along with exceptional flexibility and cycle stability. Not only as the active material, keratin fibers derived separator with high ionic conductivity and stability has also been chosen for supercapacitor applications.⁹⁹  Zhao et al. reported keratin-based nanofibers developed by an electrospinning and glutaraldehyde post strategy has shown a separator with a high ionic conductivity of 2.21 mS cm⁻¹ and mechanical strength of 7.87 MPa. The formulated supercapacitor with these separators has shown exceptional electrochemical performance with 98% stability after 8000 charge–discharge cycles. Similarly, lignin contains a large number of aromatic rings and is one of the widely available biomass sources, making it an ideal precursor of carbon generation. One report by Wang et al. showed the synthesis of all lignin-based carbon nanospheres by self-assembly, stabilization treatment, and carbonization, respectively.¹⁰⁰  The developed nanospheres could have a tunable size ranging from 256–416 nm with a specific surface area of 652–736 m² g⁻¹. When analyzed for supercapacitor applications, the material shows a specific capacitance of 147 F g⁻¹, ultralow relaxation time of 0.86 s and excellent cycle stability. Likewise, cellulose and starch are polysaccharide-based biopolymers mostly found in plant cell walls. They are used as starting precursors to produce carbon or in combination with other polymers to improve the supercapacitor performance.

Lithium-ion batteries (LIBs) are by far the most explored energy storage system. However, their low ionic diffusion in the bulk electrode limits the commercially available energy density to 80–270 W h kg⁻¹ with a power density <1000 W kg⁻¹,¹⁰¹  thus restricting their high charge and power delivery. In contrast, supercapacitors can be charged within a few seconds and deliver high power densities of >10 kW kg⁻¹, but are limited with 1–2 orders of magnitude lower energy density than batteries. To overcome the above issue, hybrid capacitors (HCs) have come into the picture with an aim to achieve superior energy densities to conventional supercapacitors and higher power density of batteries by coagulation of one supercapacitor and one battery electrode. The key problems concerning the HCs are (i) charge/capacity balance between the anode and cathode, (ii) high conductivity of battery materials to address the lower ionic diffusion in the bulk of the electrode, (iii) robust electrolyte interface formation to retard capacity fading, columbic efficiency, and cycle stability, (iv) side reactions leading to excessive electrolyte consumption and reducing the active lithium, hence the design of new prelithiation strategies, and (v) design of new functional electrolytes for high voltage and chemical stability at both the electrodes. Various types of electrode materials have been researched for HC applications including insertion type, conversion type, and alloying type electrode materials. The insertion type electrode materials store charge by electrochemical processes involving the insertion of electrolyte ions within the layers of the electrode materials. The electrochemical process is primarily diffusion controlled and the diffusion path length can be minimized by nano structuring of the active materials. Different insertion type electrode materials have been studied including graphene,¹⁰²  MXenes,¹⁰³  MoS₂,¹⁰⁴  etc. Li et al. stated the synthesis of accordion like Ti₃C₂T_x MXene with an open porous structure and high crystallinity as anode for Li-ion HC.¹⁰⁵  Owing to the highly improved ion diffusion and electron transportation in Ti₃C₂T_x MXene the formulated HC could achieve a high energy density of 106 W h kg⁻¹ and could still exhibit a high energy density of 79 W h kg⁻¹ at a power density of 5.2 kW kg⁻¹. Similar to the insertion type electrodes, many conversion type electrode materials have also been researched for HCs. Nevertheless, the conversion type electrodes although having higher capacity than insertion electrodes, suffer from poor cycling and stability due to large volume expansions during cycling. These can be mitigated to an extent by nanosizing of active materials and conductive coatings to improve the electronic conductivity and cycle stability. In one of the reports He et al. studied inverse spinel type FeGa₂O₄ used as an anode for Li-ion HC (Figure 1.8).¹⁰⁶  The developed binary metal oxide has shown a stable capacity of 500 mA h g⁻¹ at 0.1 A g⁻¹ in a half cell and high energy and power densities of 107 W h kg⁻¹ and 4126 W kg⁻¹ within a wide potential window of 0.4–4.0 V, respectively. Alloying-type electrode materials are yet another class of HC electrode systems. Unstable SEI formation resulting in pulverization of active material is one of the crucial limitations of these electrodes. Hence, Saito et al. stated the use of Li-predoped Si negative electrodes for HCs wherein the effect of vinylene carbonate or fluoroethylene carbonate additives in Li pre doping were examined to improve the electrochemical characteristics.¹⁰⁷  The combination of additives was found to yield a discharge capacity of 700 mA h g⁻¹ per weight of Si along with a high energy density of 114 W h kg⁻¹, which is much superior to the EDLCs. Along with the use of various types of electrodes for HCs it is also essential to carry out interface engineering between the electrode material/current collector, and electrode/electrolyte to strengthen the interactions and electron transport carrying out the electrochemical performances.¹⁰¹  In this respect, conductive carbon coating and increasing the contact area are studied as the main strategies.

Flexible supercapacitor technologies are becoming research hotspots owing to their unique advantages of being applied as flexible devices, curved and foldable portable electronics, smart robotics, electronic skin, motion recognition devices, and implantable medical systems, etc.¹⁰⁸  Their added advantages of low weight, mechanical flexibility, unwavering performance under mechanical deformations, and enhanced safety make them more promising candidates for energy storage applications.¹⁰⁹  However, they are required to exhibit good conductivity along with high energy density and cycle stability.¹¹⁰  A flexible supercapacitor consists of flexible electrodes with good electrochemical performance, suitable electrolyte formulation, and a separator in a flexible assembly. The active electrode materials include EDLC-based carbon materials, pseudocapacitors, or a combination of both. Carbon materials with high surface area, electronic conductivity, and low specific capacitance result in lower device energy densities, while the pseudocapacitors involving metal oxides, carbides, etc. have low electronic conductivity and result in inevitable agglomeration during cycling. Hence, a combination of these two types can be an ideal flexible electrode material. The design of flexible electrodes is also another challenge. Most of the powdered active materials are mixed with suitable binders such as polyvinylidene difluoride (PVDF), carboxymethyl cellulose (CMC), etc., and coated onto flexible electrodes such as Cu foil, Al foil, carbon cloth, Ni foam, etc. Nevertheless, the insulating nature of the binder restricts the electronic and ionic mobility within and on the surface of the electrodes and thereby only adding to the weight of the supercapacitor alone. Therefore, the modern strategies involved the design of binder-free flexible supercapacitors. Electrodeposition, vacuum filtration, 3D printing, inkjet printing, electrospinning, etc. are chosen as the synthesis routes for the development of binder-free flexible supercapacitors. One recent work by Soram et al. reported electrodeposited MoS₂ on Ni mesh as a transparent and flexible negative electrode for supercapacitor.¹¹¹  The developed core–shell was found to yield a specific capacitance of 7.31 mF cm⁻² at 10 mV s⁻¹ with 80% capacitance retention for 5000 cycles. Another report stated that the precise control over the volume of rGO and tannin mixture on nylon filter membrane substrate not only prevents the aggregation of rGO sheets but also results in added pseudocapacitance and improved electrochemical properties.¹¹²  Owing to the 3D architecture of rGO sheets and the mechanical stability of both the electrode and substrate, the device exhibited good electrochemical properties at different mechanical deformations along with a specific capacitance of 40.37 mF cm⁻². Similarly, a 3D printed graphene ink-based planar supercapacitor has demonstrated a high specific capacitance of 137.5 F g⁻¹ at 0.5 A g⁻¹ along with robust flexibility to 150° bending angles.³⁷  The work provides a way for customizable shaped energy storage systems with high flexibility. Electrolytes are yet another major aspect in determining the safety, operational window, and lifetime of flexible supercapacitors. Most of the liquid or organic electrolytes are prone to leakage, flammability, toxicity, and corrosivity that limit the device’s performance. In contrast, solid-state electrolytes involving polymers are known for their high mechanical stability, flexibility, and durable performance. Research on various polymer-based gel electrolytes has been carried out including polyvinyl alcohol host with alkaline or acidic ion dopants,¹¹  KOH saturated cellular membrane,¹¹³  poly(aryl ether ketone)–poly(ethylene glycol) copolymer, and LiClO₄, resulting in good electrochemical properties.¹¹⁴  Nevertheless, some irreversible damage and electrolyte dryness degrade device performance. Hence, the demand for self-healing hydrogel electrolytes is on the rise. Zhu et al. showed the development of PVA/glycerin (gly)/H₂SO₄ self-healing and antifreezing gel electrolyte by freezing–thawing cycling.¹¹⁵  The developed electrolyte displayed remarkable self-healing properties due to synergistic hydrogen bonding between gly and PVA along with a specific capacitance of 476 mF cm⁻² at 0.2 mA cm⁻² and excellent antifreezing properties of 53% capacitance retention at −20 °C. Such electrolytes display broad applications in flexible energy storage systems. Recently, hydrogels are also attracting wide interest due to their flexible and wearable properties. One of the recent reports show the development of an organohydrogel in H₂O ethanol mixture by the addition of PVA, alizarin red, and H₂SO₄ (Figure 1.9). The developed organogel has shown high tolerance to freezing temperatures of −37 °C. Furthermore, incorporation of alizarin red has resulted in a good specific capacitance of 240 F g⁻¹ and impressive cycle stability.¹¹⁶ 

In view of the varied applications of flexible supercapacitors, new, cost-effective, and advanced methodologies are yet to be discovered to achieve high device energy and power densities along with good stability. Research on different electrode composites, electrolyte compositions and fabrication strategies are to be explored to attain improved electrochemical performance and efficiencies.

With the increasing dependency of humans on portable electronics and the recent transformation of electric mobility, the need for new and efficient energy storage systems is on the rise. Although the entire community is dominated by lithium-ion battery (LIB) technology, the limiting power density and high charging times have paved the way for the upcoming supercapacitor systems. With their high PD and cycle life, these systems can be used as buffer sources whenever high-power uptake or delivery is required. Small supercapacitors with a few farads of charge have many applications, from memory backups in cameras, toys, cell phones, and so on.¹¹⁷  They can also be used in cordless electronics such as electric cutters, drills, and screwdrivers that can be charged and discharged in less than 2 min, with the lifetime of these supercapacitors being much superior to the tool in use. The well-known Airbus A380 jumbo airlines also employ EDLC modules of 100 F, 2.7 V connected in series or parallel and directly integrated into the system for the emergency door opening and shutting.¹¹⁷  Also, symmetric supercapacitor systems with an average energy density of 5 W h kg⁻¹ and response time of 1 s are widely employed to efficiently store and release the regenerative breaking energy in both electric vehicles and industrial equipment.¹¹⁸  Supercapacitors are also found to be usable in seaport gantry cranes to store energy during load lowering, which has reported almost 40% lower energy usage.¹¹⁸  Supercapacitors are also utilized as day–night alternating power sources where during the day time, the load is supplied by the utility grid, e.g., solar cells, while also charging the supercapacitor. During the night, the power is dissipated by the supercapacitors. The major applications of such supercapacitors include solar watches, solar lanterns, etc.¹¹⁹  The supercapacitors also find application in e-mobility, especially in electric vehicles (EVs) and hybrid electric vehicles (HEVs), where they store the energy during regenerative braking and use the same for the next boosted acceleration.¹¹⁹  They can also be employed similarly in small vehicles at amusement parks, hospitals, wheelchairs, etc., where small masses help in fast acceleration. The system can also be charged on and offboard.

Although supercapacitor systems are in the limelight due to their wide range of applications mentioned above, they also have many challenges to be addressed. The low energy density of supercapacitors majorly hinders the vast commercialization and complete replacement of batteries. For instance, the Li-ion batteries used in the mobile phone have an approximate energy density of about 150–200 W h kg⁻¹. A supercapacitor of a similar dimension can store a maximum of 5–10 W h kg⁻¹. Subsequently, supercapacitor manufacturing and production costs are also high, indicating the need for cost-effective raw materials and modified device engineering requirements. Supercapacitor performance metrics depend mainly on the electrode materials, the electrolyte employed, and the device architecture. Since the energy density of a supercapacitor depends on the specific capacitance and square of the voltage, it is essential to develop new electrode materials with high charge storage capability and modify the device architecture to attain wide electrochemical windows. Hence, the current research is mainly focused on synthesizing new electrode materials involving various metal oxides, nitrides, sulfides, conducting polymers, etc., which by their inherent variable oxidation states and surface redox phenomena, lead to more charge storage than EDLCs. Also, modification of electrolyte properties by redox additive addition or the use of saturated electrolytes known as “water-in-salt” have been proposed, which widens the electrochemical window, leading to higher energy densities by overcoming the thermodynamic limit of aqueous systems.

Additionally, the design of an asymmetric supercapacitor device has gained prominence that employs two different materials as positive and negative electrodes with extended electrochemical windows and high energy densities. However, the developed metal oxides/nitrides/sulfides have low electrical conductivity and the possibility of agglomeration when synthesized in nanosize, restricting their high-rate performance and poor cycle stability. Hence, to meet future energy storage demands, there is still broad scope for developing new electrode materials and device strategies with improved electronic conductivity and electrochemical performance.

Supercapacitors are no doubt the next-generation energy storage systems for several applications. However, the full potentiality of different types of electrode materials is not yet realized. Based on the type of electrode material, they can be classified as EDLCs, pseudocapacitors, and hybrid capacitors. Each of these types has certain bottlenecks that need to be addressed to achieve the best-performing supercapacitor. Although EDLCs have high stability, ease of availability, and processibility they are limited by the low specific capacitance and energy densities. In particular, although mesoporous carbons with high surface area and structural defects exhibit good specific capacitance, they have very low packing densities, which ultimately limit the volumetric or power density of supercapacitors. Recently, pseudocapacitors using metal oxides, nitrides, sulfides, and MXenes as new 2D materials have shown promising energy storage properties. Being cost-effective and exhibiting much superior capacitance to EDLCs, pseudocapacitors have attracted wide interest. Nevertheless, they are limited by their low electronic conductivity, poor rate capability, and agglomeration at the nano level leading to capacitance fading. The above limitations are to be addressed by the incorporation of conductive skeletons leading to new composite developments. Hybrid capacitors and flexible supercapacitors are amongst the emerging trends, where a hybrid capacitor couples a battery electrode and a supercapacitor electrode with a higher energy density than supercapacitors and a comparable power density. The hybrid system exhibits the challenge of proper electrochemistry and charge match between the two types of energy storage systems. Addressing this issue requires the development of new materials that works in collaboration with both charge storage chemistries. A few such materials have been discussed above. The optimal choice of electrode material and the charge storage process can lead to a cost-effective and best performing green supercapacitor with commercial viability in the near future.

The authors acknowledge the financial support received from the ARCI-technical research center {ref. AI/1/65/ARCI/2014 (c)} funded by the Department of Science and Technology (DST), Government of India.