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Many efforts have been dedicated to the design of high-energy and power-based green energy storage systems. In this context, supercapacitors with tailored electrode and device architectures are found to be highly appropriate. Recent years have seen supercapacitors attracting worldwide interest due to their critical role in replacing conventional fuels in the transportation sector and also owing to their promising electrochemical characteristics like long cycle life, high power density, and low toxicity. Supercapacitors bridge the gap between conventional dielectric capacitors and primary or secondary Li-ion batteries in terms of their energy and power densities. However, the basic electrochemistry based on how different types of supercapacitors work is less established. Therefore, the underlying charge storage mechanisms, redox reactions, and processes may be confusing. A good supercapacitor electrode material should possess certain characteristics such as large specific surface area and porosity, good surface wettability, high electrical conductivity, tuning of textural parameters, and thermodynamic stability to deliver good electrochemical properties. This chapter discusses the fundamentals of supercapacitors, their classification, and storage mechanisms. This is followed by a brief discussion of various electrode materials used among the different supercapacitor types and their corresponding synthesis and electrochemical progress. Furthermore, the chapter also details the challenges and scope of each of the classifications.

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.

Figure 1.1

(a) Ragone plot, and charge storage behavior in (b and c) EDLCs, (d and e) pseudocapacitors, and (f and g) batteries.

Figure 1.1

(a) Ragone plot, and charge storage behavior in (b and c) EDLCs, (d and e) pseudocapacitors, and (f and g) batteries.

Close modal
Table 1.1

Classification of supercapacitors.

Type Electrode materials Storage mechanism Merits Demerits
Electrochemical double layer capacitor (EDLC)  Porous carbon,5  graphene,6  CNTs,7  MWCNTs,1  aerogels,1  etc Electrostatic or nonfaradaic 
  • Cost-effective

  • High stability

  • Good rate capability

 
  • Low specific capacitance

  • Low energy density

 
Pseudocapacitors or redox supercapacitors  Metal oxides,8  metal sulfides,9  MXenes,10  conducting polymers,11  composite, etc.   Electrochemical or faradaic 
  • High specific capacitance

  • High energy and power density compared to EDLC

 
  • Low-rate capability

  • Low cycle stability in comparison to EDLC

 
Hybrid supercapacitor 
  • Anode – pseudocapacitive & cathode – EDLC

  • Anode & cathode both different types of pseudocapacitive

  • Anode – lithium insertion material & cathode – carbon

 
  • Anode – electrochemical & cathode – electrostatic

  • Anode & cathode electrochemical

  • Anode – electrochemical & cathode – electrostatic

 
  • Large potential window

  • High specific capacitance/capacity

  • High power density compared to batteries

  • High energy density

 
  • Moderate cost

  • Requires charge balance between anode and cathode

 
Type Electrode materials Storage mechanism Merits Demerits
Electrochemical double layer capacitor (EDLC)  Porous carbon,5  graphene,6  CNTs,7  MWCNTs,1  aerogels,1  etc Electrostatic or nonfaradaic 
  • Cost-effective

  • High stability

  • Good rate capability

 
  • Low specific capacitance

  • Low energy density

 
Pseudocapacitors or redox supercapacitors  Metal oxides,8  metal sulfides,9  MXenes,10  conducting polymers,11  composite, etc.   Electrochemical or faradaic 
  • High specific capacitance

  • High energy and power density compared to EDLC

 
  • Low-rate capability

  • Low cycle stability in comparison to EDLC

 
Hybrid supercapacitor 
  • Anode – pseudocapacitive & cathode – EDLC

  • Anode & cathode both different types of pseudocapacitive

  • Anode – lithium insertion material & cathode – carbon

 
  • Anode – electrochemical & cathode – electrostatic

  • Anode & cathode electrochemical

  • Anode – electrochemical & cathode – electrostatic

 
  • Large potential window

  • High specific capacitance/capacity

  • High power density compared to batteries

  • High energy density

 
  • Moderate cost

  • Requires charge balance between anode and cathode

 

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 

On the cathode:
S C + A S C + A + e
(1.1)
On the anode:
S A + C + S A C + + e
(1.2)
Hence, the overall electrochemical reaction on both electrodes can be written as
S C + S A + A + C + S C + A + S A C +
(1.3)
SC and SA are the active surface areas of both the cathode and anode, C+ is the cation charge, and A is the anion charge.
The EDL formation on the surface of carbon materials during the charge storage is explained by many theories, such as those of Helmholtz, Gouy–Chapman, and Stern (Figure 1.2). The Helmholtz theory assumes a double layer consisting of firm charges separated by a fixed distance. According to the theory, the electrode holds a particular polarity of charges, which is balanced by equal but oppositely charged ions in the solution separated at a fixed distance, similar to a parallel plate capacitor. Accordingly, the variation of the electrode potential in the solution with distance is found to be linear, wherein the potential difference across the interface can be defined as:
d V = 4 π d ε d Q
(1.4)
where d is the distance between the two layers, dQ is the charge density on each layer, and ε is the dielectric constant.
Figure 1.2

EDLC models on a positively charged electrode surface: (a) the Helmholtz model, (b) the Gouy–Chapman model, and (c) the Stern model.

Figure 1.2

EDLC models on a positively charged electrode surface: (a) the Helmholtz model, (b) the Gouy–Chapman model, and (c) the Stern model.

Close modal
However, the Helmholtz model suffers from a few limitations, as follows (a) it assumes a rigid layer of charges, however, due to the thermal motion of ions, it may not exist, (b) it predicts a constant capacity for the double layer interface that experimentally does not exist, (c) electrolyte and temperature-dependent variation of EDL properties are not taken into consideration13 
C = d Q d V = ε 4 π d
(1.5)
Furthermore, according to the Gouy–Chapman model, the EDL is not a firm array of charges but is a diffused layer. The electrode surface is assumed to be fixed by a layer of oppositely charged ions along with the diffused or mobile charges in the solution due to thermal agitation extending for some distance. The diffused layer thickness is partially found to be dependent on the kinetic energy of the ions. The variation in the electrode potential with distance is further found to be exponential, in contradiction to the Helmholtz model. It is understood that the Gouy–Chapman model fails for the highly charged double layers, and hence the Stern theory combines the essential aspects of both the above theories. According to Stern, the ions have a finite size. The electrode surface is attached with a layer of oppositely charged ions at a specific distance depending on the ionic radii forming an inner stern layer, or inner Helmholtz plane (IHP), and the other compensating charges are mobile in the solution known as the outer Helmholtz plane (OHP).

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 (Csp) 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 Csp 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 Csp 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 Csp of 533.26 F g−1 along with good cycle stability in a three-electrode system configuration.

Figure 1.3

(a and b) SEM, (c and d) TEM of hierarchical porous activated carbon, (e) CV, and (f) CD of symmetric supercapacitor. Reproduced from ref. 15 with permission from American Chemical Society, Copyright 2022.

Figure 1.3

(a and b) SEM, (c and d) TEM of hierarchical porous activated carbon, (e) CV, and (f) CD of symmetric supercapacitor. Reproduced from ref. 15 with permission from American Chemical Society, Copyright 2022.

Close modal

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 Csp 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 Csp 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 Csp 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 Csp of 280 F g−1 at 0.3 A g−1.

Figure 1.4

(a) Schematic illustration of the synthesis of jute sticks derived activated porous carbon and (b) the corresponding surface morphological analysis. Reproduced from ref. 21 with permission from American Chemical Society, Copyright 2019.

Figure 1.4

(a) Schematic illustration of the synthesis of jute sticks derived activated porous carbon and (b) the corresponding surface morphological analysis. Reproduced from ref. 21 with permission from American Chemical Society, Copyright 2019.

Close modal

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 Csp 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 Csp 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 Csp 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 Csp 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 applications41–43  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 Csp 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 Csp 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 Csp 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 Csp 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.

Pseudocapacitors involve surface or near surface faradaic reactions resulting in electrosorption or electrodesorption along with charge transfer phenomena.4  However, bulk phase transformations are not involved, differing from the conventional batteries. In comparison to the EDL charge storage that is based upon the accumulation of electrostatic charges with applied potential, pseudocapacitance is faradaic in nature.4  The electrochemistry of pseudocapacitors can be classified into three categories according to Conway et al.,4  namely (a) under potential deposition (UPD), (b) surface redox pseudocapacitance, and (c) intercalation pseudocapacitance in porous electrode materials, as shown in Figure 1.5. UPD takes place when an applied potential to a metal results in the formation of an adsorbed monolayer of another metal. The overall process takes place by lowering the equilibrium potential to more positive.49,50  The UPD process results from the strong attractive interaction between the depositing metal ions and the substrate. It is also a self-limiting process, indicating that the process stops upon a uniform monolayer formation.51  The redox pseudocapacitance involves the adsorption of electroactive ions on the surface of the electrode material along with the charge transfer from the surface or near-surface region. Typical examples of redox pseudocapacitance include metal oxides, metal hydroxides, and conducting polymers such as MnO2, RuO2, PANI, etc., which exhibit pseudocapacitance based on the rapid reversible redox reactions by the intercalation of protons (H+) or alkali metal cations (M+ = Na+, K+, etc.) from the surface or near the surface of the active electrode materials4 
MO 2 + H + + e MOOH
(1.6)
MO 2 + M + + e MOOM
(1.7)
The involved faradaic reaction does not result in any bulk chemical transformation and also, the electrode potential is linearly dependent on the charge stored and the area of the electrode covered by electroactive ions, thus differing it from the conventional batteries.4 
Figure 1.5

Charge storage mechanism in (a) EDLCs, (b) pseudocapacitors with UPD, (c) surface redox phenomena, and (d) intercalation pseudocapacitance.

Figure 1.5

Charge storage mechanism in (a) EDLCs, (b) pseudocapacitors with UPD, (c) surface redox phenomena, and (d) intercalation pseudocapacitance.

Close modal

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.

Various transition metal oxides have been studied for their pseudocapacitive properties due to their ease of availability/synthesis and multiple oxidation states that contribute to the improved capacitance in comparison to EDLCs. However, their low electronic conductivity and inevitable agglomeration during cycling limit their overall performance. RuO2 is one of the early researched transition metal oxides for pseudocapacitor applications due to its high theoretical capacitance of 2000 F g−1, excellent electrochemical reversibility, wide electrochemical window, and stability.56  RuO2 is found to be more active in acidic electrolytes and the electrochemical reaction can be defined as:57 
RuO 2 + x H + + x e RuO 2 x ( OH ) x
(1.8)
where 0 ≤ x ≤ 2. In acidic electrolytes, the electrochemistry of RuO2 proceeds via the intercalation and deintercalation of H+ ions along with the change in oxidation states from 2+ to 3+. There are various other factors that affect the electrochemical performance of RuO2 such as crystallinity, annealing temperature, and particle size.57  Wang et al. stated that particle size reduction is a straightforward way for the improvement in electrochemical performance of RuO2.56  Thereby, an aqueous synthesis of ultrafine RuO2 under controlled pH on different carbon substrates has resulted in a high specific capacitance of 1099 F g−1 at 0.5 A g−1. Similarly, RuO2 quantum dots on rGO by microwave-assisted hydrothermal synthesis has yielded a high specific capacitance of 1120 F g−1 at 1 A g−1 with 89% retention after 10 000 cycles.58 
MnO2 is another transition metal oxide that is found to be a cost-effective alternative to RuO2 due to its high theoretical capacitance of 1370 F g−1, abundance, and low toxicity. It is also one of the first commercialized pseudocapacitor by NESCAP with a 2.3 V operating window and excellent stability. The charge storage mechanism is expected to proceed in two ways: first implying the participation of the bulk of the electrode by the electrolyte insertion and the second by the surface adsorption and desorption of electrolyte ions:
MnO 2 + M + + e MnOOM
(1.9)
MnO 2 surface + M + + e MnOOM surface
(1.10)
Both mechanisms involve oxidation and reduction between the 3+ and 4+ states of Mn. Various polymorphs of MnO2 have been studied for supercapacitor applications including α, β, γ, δ, and λ. The different tunnel structures limit the electrolyte ion penetration or intercalation and thereby restrict the electrochemical performance. Although the theoretical specific capacitance of MnO2 is quite high, the practical capacitance of pristine MnO2 is limited to less than 350 F g−1 owing to the nanostructured MnO2 agglomeration with cycling, Mn3+ dissolution, poor electronic conductivity, and surface limited redox reactions.57  To enhance the same, and to restrict the mild dissolution of Mn3+ ions in acidic and neutral electrolytes various metal doping such as Co, V, and Fe59,60  are used along with protective polymer coatings and carbon composites to increase the cycle life performance.61,62 
Co3O4 is a cubic spinel metal oxide with a theoretical specific capacity of 3560 F g−1. It is a p-type semiconductor with a band gap of 1.6 V and exists in two different oxidation states of 2+ and 3+.63  The electrochemical reactions during charge–discharge can be defined as:
Co 3 O 4 + OH + H 2 O 3 CoOOH + e
(1.11)
CoOOH + OH CoO 2 + H 2 O + e
(1.12)
Nevertheless, Co3O4 suffers from significant volume expansion during cycling resulting in pulverization of the active material. Co3O4 can be synthesized through many routes to nanoscale dimensions resulting in superior electrochemical performance. Co3O4 is also developed in composites of carbon and other metal oxides to improve the cycle life performance.64  On the other hand, Co(OH)2 is a hexagonal layered structured material similar to Ni(OH)2 existing in both α and β forms, with α-Co(OH)2 being more supercapacitor active than β-Co(OH)2.57  The theoretical specific capacitance can reach up to 3600–3700 F g−1. Redox reactions during charge–discharge can be expressed as:
Co ( OH ) 2 + OH CoOOH + H 2 O + e
(1.13)
However, both Co3O4 and Co(OH)2 have similar drawbacks in that they have narrow potential windows and poor cyclability, which restricts their overall device performance with limited energy density and cycle life.63 

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, MoOx, 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 Cu0.75Co0.25MnS,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 Mn+1AXn (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  Mo2CTx 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, Ti3C2Tx 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 Csp 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 

Figure 1.6

FESEM images of (A) Ti3AlC2-untreated MAX phase, (B) Ti3AlC2, (C) Ti2AlC after HF treatment, and (D) HRTEM of Ti3C2 formed after HF treatment of Ti3AlC2. Reproduced from ref. 84 with permission from American Chemical Society, Copyright 2012.

Figure 1.6

FESEM images of (A) Ti3AlC2-untreated MAX phase, (B) Ti3AlC2, (C) Ti2AlC after HF treatment, and (D) HRTEM of Ti3C2 formed after HF treatment of Ti3AlC2. Reproduced from ref. 84 with permission from American Chemical Society, Copyright 2012.

Close modal

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.

Figure 1.7

(a–g) Schematic illustration of vertically aligned and physically crosslinked PANI–H2SO4 hydrogel formation, and (h and i) FESEM images on the surface and cross section. Reproduced from ref. 92 with permission from American Chemical Society, Copyright 2020.

Figure 1.7

(a–g) Schematic illustration of vertically aligned and physically crosslinked PANI–H2SO4 hydrogel formation, and (h and i) FESEM images on the surface and cross section. Reproduced from ref. 92 with permission from American Chemical Society, Copyright 2020.

Close modal

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 Ti3C2Tx 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 Ti3C2Tx 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.

Figure 1.8

(a) Schematic of FeGa2O4 (FGO) synthesis, (b) FESEM surface morphology analysis, and (c) electrochemical performance analysis of Li-ion HC with carbon and FGO as cathode and anode, respectively. Reproduced from ref. 106 with permission from American Chemical Society, Copyright 2021.

Figure 1.8

(a) Schematic of FeGa2O4 (FGO) synthesis, (b) FESEM surface morphology analysis, and (c) electrochemical performance analysis of Li-ion HC with carbon and FGO as cathode and anode, respectively. Reproduced from ref. 106 with permission from American Chemical Society, Copyright 2021.

Close modal

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 

Figure 1.9

Schematic of organohydrogel with antifreezing and flexible supercapacitor application. Reproduced from ref. 116 with permission from American Chemical Society, Copyright 2021.

Figure 1.9

Schematic of organohydrogel with antifreezing and flexible supercapacitor application. Reproduced from ref. 116 with permission from American Chemical Society, Copyright 2021.

Close modal

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.

1
Chen
 
X.
Paul
 
R.
Dai
 
L.
Natl. Sci. Rev.
2017
, vol. 
4
 (pg. 
453
-
489
)
2
Dubal
 
D. P.
Kim
 
J. G.
Kim
 
Y.
Holze
 
R.
Lokhande
 
C. D.
Kim
 
W. B.
Energy Technol.
2014
, vol. 
2
 (pg. 
325
-
341
)
3
Li
 
B.
Zheng
 
M.
Xue
 
H.
Pang
 
H.
Inorg. Chem. Front.
2016
, vol. 
3
 (pg. 
175
-
202
)
4
Liu
 
J.
Wang
 
J.
Xu
 
C.
Jiang
 
H.
Li
 
C.
Zhang
 
L.
Lin
 
J.
Shen
 
Z. X.
Adv. Sci.
2018
, vol. 
5
 pg. 
1700322
 
5
Liu
 
B.
Liu
 
L.
Yu
 
Y.
Zhang
 
Y.
Chen
 
A.
New J. Chem.
2020
, vol. 
44
 (pg. 
1036
-
1044
)
6
Chen
 
J.
Chen
 
S.
Chen
 
B. J.
Cao
 
Y.
Chen
 
J. F.
Cheng
 
Y. L.
Chen
 
Z. M.
Fu
 
J. W.
Appl. Surf. Sci.
2022
, vol. 
590
 pg. 
153156
 
7
Avasthi
 
P.
Kumar
 
A.
Balakrishnan
 
V.
ACS Appl. Nano Mater.
2019
, vol. 
2
 (pg. 
1484
-
1495
)
8
Pappu
 
S.
Rao
 
T. N.
Martha
 
S. K.
Bulusu
 
S. V.
Energy
2022
, vol. 
243
 pg. 
122751
 
9
Zhu
 
X.
Liu
 
S.
J. Energy Storage
2022
, vol. 
51
 pg. 
104582
 
10
Zhu
 
Z.
Wang
 
Z.
Ba
 
Z.
Li
 
X.
Dong
 
J.
Fang
 
Y.
Zhang
 
Q.
Zhao
 
X.
J. Energy Storage
2022
, vol. 
47
 pg. 
103911
 
11
Alipoori
 
S.
Mazinani
 
S.
Aboutalebi
 
S. H.
Sharif
 
F.
J. Energy Storage
2020
, vol. 
27
 pg. 
101072
 
12
Wang
 
G.
Zhang
 
L.
Zhang
 
J.
Chem. Soc. Rev.
2012
, vol. 
41
 (pg. 
797
-
828
)
13
W.
Schmickler
,
Encyclopedia of Electrochemical Power Sources
,
2009
, pp.
8
13
.
14
Muzaffar
 
A.
Ahamed
 
M. B.
Deshmukh
 
K.
Thirumalai
 
J.
Renew. Sustain. Energy Rev.
2019
, vol. 
101
 (pg. 
123
-
145
)
15
Abdelaal
 
M. M.
Hung
 
T.-C.
Mohamed
 
S. G.
Yang
 
C.-C.
Hung
 
T.-F.
ACS Sustain. Chem. Eng.
2022
, vol. 
10
 (pg. 
4717
-
4727
)
16
Li
 
W.
Liu
 
J.
Zhao
 
D.
Nat. Rev. Mater.
2016
, vol. 
1
 pg. 
16023
 
17
Zhang
 
Y.
Li
 
X.
Huang
 
J.
Xing
 
W.
Yan
 
Z.
Nanoscale Res. Lett.
2016
, vol. 
11
 pg. 
163
 
18
Xia
 
K.
Gao
 
Q.
Jiang
 
J.
Hu
 
J.
Carbon
2008
, vol. 
46
 (pg. 
1718
-
1726
)
19
Sun
 
X.
Kong
 
Y.
Liu
 
Y.
Zhou
 
L.
Nanjundan
 
A. K.
Huang
 
X.
Yu
 
C.
Front. Chem.
2020
, vol. 
8
 pg. 
592904
 
20
Shaker
 
M.
Ghazvini
 
A. A. S.
Cao
 
W.
Riahifar
 
R.
Ge
 
Q.
New Carbon Mate.
2021
, vol. 
36
 (pg. 
546
-
572
)
21
Nanaji
 
K.
Upadhyayula
 
V.
Rao
 
T. N.
Anandan
 
S.
ACS Sustain. Chem. Eng.
2019
, vol. 
7
 (pg. 
2516
-
2529
)
22
Jiang
 
B. X.
Cao
 
L.
Yuan
 
Q. H.
Ma
 
Z. W.
Huang
 
Z. R.
Lin
 
Z. D.
Zhang
 
P.
Materials
2022
, vol. 
15
 pg. 
15030924
 
23
Gur
 
E.
Semerci
 
T. G.
Semerci
 
F.
J. Energy Storage
2022
, vol. 
51
 pg. 
104363
 
24
Pang
 
X. N.
Cao
 
M.
Qin
 
J. H.
Li
 
X. J.
Yang
 
X.
J. Porous Mater.
2022
, vol. 
29
 (pg. 
559
-
569
)
25
Li
 
H.
Fu
 
D.
Zhang
 
X.-M.
R. Soc. Open Sci.
2022
, vol. 
5
 pg. 
171028
 
26
Tang
 
T.
Yuan
 
R.
Guo
 
N.
Zhu
 
J.
Gan
 
X.
Li
 
Q.
Qin
 
F.
Luo
 
W.
Wang
 
L.
Zhang
 
S.
Song
 
H.
Jia
 
D.
J. Colloid Interface Sci.
2022
, vol. 
623
 (pg. 
77
-
85
)
27
Zhao
 
Y.
Song
 
Z.
Li
 
X.
Sun
 
Q.
Cheng
 
N.
Lawes
 
S.
Sun
 
X.
Energy Storage Mater.
2016
, vol. 
2
 (pg. 
35
-
62
)
28
Chu
 
X.
Meng
 
F.
Deng
 
T.
Zhang
 
W.
Nanoscale
2021
, vol. 
13
 (pg. 
5570
-
5593
)
29
Zheng
 
S. Q.
Lim
 
S. S.
Foo
 
C. Y.
Haw
 
C. Y.
Chiu
 
W. S.
Chia
 
C. H.
Khiew
 
P. S.
Front. Mater.
2021
, vol. 
8
 pg. 
777149
 
30
Mathew
 
E. E.
Balachandran
 
M.
Carbon Lett.
2021
, vol. 
31
 (pg. 
537
-
555
)
31
Stoller
 
M. D.
Park
 
S.
Zhu
 
Y.
An
 
J.
Ruoff
 
R. S.
Nano Lett.
2008
, vol. 
8
 (pg. 
3498
-
3502
)
32
Lee
 
C.
Wei
 
X.
Kysar
 
J. W.
Hone
 
J.
Science
2008
, vol. 
80
 (pg. 
385
-
388
)
33
Morozov
 
S. V.
Novoselov
 
K. S.
Katsnelson
 
M. I.
Schedin
 
F.
Elias
 
D. C.
Jaszczak
 
J. A.
Geim
 
A. K.
Phys. Rev. Lett.
2008
, vol. 
100
 pg. 
16602
 
34
Brodie
 
B. C.
Philos. Trans. R. Soc. London
1859
, vol. 
149
 (pg. 
249
-
259
)
35
Mathew
 
E. E.
Balachandran
 
M.
Carbon Lett.
2021
, vol. 
31
 (pg. 
537
-
555
)
36
Tang
 
Q.
Li
 
L.
Guo
 
K.
Zhu
 
R.
Liu
 
M.
Chen
 
X.
Int. J. Energy Res.
2022
, vol. 
46
 (pg. 
14105
-
14115
)
37
Kumar
 
S.
Goswami
 
M.
Singh
 
N.
Soni
 
P.
Sathish
 
N.
Kumar
 
S.
Carbon Lett.
2022
, vol. 
32
 (pg. 
979
-
985
)
38
Chen
 
J.
Chen
 
S.
Chen
 
B.
Cao
 
Y.
Chen
 
J.
Cheng
 
Y.
Chen
 
Z.
Fu
 
J.
Appl. Surf. Sci.
2022
, vol. 
590
 pg. 
153156
 
39
Yu
 
S.
Sano
 
H.
Zheng
 
G.
Tanabe
 
S.
Chem. Lett.
2022
, vol. 
51
 (pg. 
264
-
268
)
40
Pan
 
H.
Li
 
J.
Feng
 
Y.
Nanoscale Res. Lett.
2010
, vol. 
5
 pg. 
654
 
41
Nguyen
 
Q. X.
Nguyen
 
T. T.
Pham
 
N. M.
Khong
 
T. T.
Cao
 
T. M.
Van
 
V.
Pham, Prog. Org. Coatings
2022
, vol. 
167
 pg. 
106838
 
42
Yang
 
P.
Wang
 
N.
Zhang
 
J.
Lei
 
Y.
Shu
 
B. P.
Mater. Res. Express
2022
, vol. 
9
 pg. 
036404
 
43
Zhang
 
C. M.
Gao
 
K. Y.
Zhu
 
H. B.
Liu
 
J. W.
Chen
 
J. L.
Xie
 
F. Z.
Xie
 
W. J.
Wang
 
X. F.
ChemCatChem
2022
, vol. 
14
 pg. 
e202200225
 
44
Zhang
 
Y.
Xie
 
E.
Dalt. Trans.
2021
, vol. 
50
 (pg. 
12982
-
12989
)
45
Zhang
 
Y.
Xie
 
E.
Dalt. Trans.
2021
, vol. 
50
 (pg. 
12982
-
12989
)
46
Aval
 
L. F.
Ghoranneviss
 
M.
Pour
 
G. B.
Heliyon
2018
, vol. 
4
 pg. 
e00862
 
47
Díez
 
N.
Botas
 
C.
Mysyk
 
R.
Goikolea
 
E.
Rojo
 
T.
Carriazo
 
D.
J. Mater. Chem. A
2018
, vol. 
6
 (pg. 
3667
-
3673
)
48
Conway
 
B. E.
J. Electrochem. Soc.
1991
, vol. 
138
 (pg. 
1539
-
1548
)
49
Conway
 
B. E.
Prog. Surf. Sci.
1984
, vol. 
16
 (pg. 
1
-
137
)
50
Conway
 
B. E.
Electrochim. Acta
1993
, vol. 
38
 (pg. 
1249
-
1258
)
51
K.
Jackowska
and
P.
Krysiński
,
Applied Electrochemistry
,
De Gruyter
, Berlin, Boston, 2020.
52
Liu
 
Y.
Jiang
 
S. P.
Shao
 
Z.
Mater. Today Adv.
2020
, vol. 
7
 pg. 
100072
 
53
Cao
 
L. L.
Yu
 
B. Z.
Cheng
 
T.
Zheng
 
X. L.
Li
 
X. H.
Li
 
W. L.
Ren
 
Z. Y.
Fan
 
H. M.
Ceram. Int.
2017
, vol. 
43
 (pg. 
14897
-
14904
)
54
Yoo
 
H. D.
Li
 
Y. F.
Liang
 
Y. L.
Lan
 
Y. C.
Wang
 
F.
Yao
 
Y.
ChemNanoMat
2016
, vol. 
2
 (pg. 
688
-
691
)
55
Thalji
 
M. R.
Ali
 
G. A. M.
Algarni
 
H.
Chong
 
K. F.
J. Power Sources
2019
, vol. 
438
 pg. 
227028
 
56
Wang
 
P. F.
Liu
 
H.
Xu
 
Y. X.
Chen
 
Y. F.
Yang
 
J.
Tan
 
Q. Q.
Electrochim. Acta
2016
, vol. 
194
 (pg. 
211
-
218
)
57
Shi
 
F.
Li
 
L.
Wang
 
X.
Gu
 
C.
Tu
 
J.
RSC Adv.
2014
, vol. 
4
 (pg. 
41910
-
41921
)
58
Zhao
 
J.
Zhang
 
J. M.
Yin
 
H.
Zhao
 
Y. L.
Xu
 
G. X.
Yuan
 
J. S.
Mo
 
X. Y.
Tang
 
J.
Wang
 
F. Y.
Nanomaterials
2022
, vol. 
12
 pg. 
12071210
 
59
Yan
 
L. J.
Niu
 
L. Y.
Shen
 
C.
Zhang
 
Z. K.
Lin
 
J. H.
Shen
 
F. Y.
Gong
 
Y. Y.
Li
 
C.
Liu
 
X. J.
Xu
 
S. Q.
Electrochim. Acta
2019
, vol. 
306
 (pg. 
529
-
540
)
60
Gao
 
Q.
Wang
 
J. X.
Ke
 
B.
Wang
 
J. F.
Li
 
Y. Q.
Ceram. Int.
2018
, vol. 
44
 (pg. 
18770
-
18775
)
61
Yoo
 
H. N.
Park
 
D. H.
Hwang
 
S. J.
J. Power Sources
2008
, vol. 
185
 (pg. 
1374
-
1379
)
62
C.
Wen
,
F. K.
Cheng
,
L. B.
Peng
,
H. Y.
Chen
and
D.
Shu
,
Proc. 7th Natl. Conf. Chinese Funct. Mater. Appl.
,
2010
, vol.
1–3
, pp.
1750
1752
.
63
Hu
 
X. R.
Wei
 
L. S.
Chen
 
R.
Wu
 
Q. S.
Li
 
J. F.
ChemistrySelect
2020
, vol. 
5
 (pg. 
5268
-
5288
)
64
Wang
 
X. Y.
Lu
 
S. X.
Xu
 
W. G.
Crystals
2022
, vol. 
12
 pg. 
12050664
 
65
Pappu
 
S.
Anandan
 
S.
Rao
 
T. N.
Martha
 
S. K.
Bulusu
 
S. V.
J. Energy Storage
2022
, vol. 
50
 pg. 
104598
 
66
Samiei
 
E.
Mohammadi
 
S.
Torkzadeh-Mahani
 
M.
Diam. Relat. Mater.
2022
, vol. 
127
 pg. 
109157
 
67
Ramachandran
 
T.
Mourad
 
A. H. I.
Raji
 
R. K.
Krishnapriya
 
R.
Cherupurakal
 
N.
Subhan
 
A.
Al-Douri
 
Y.
Int. J. Energy Res.
2022
68
Bi
 
W. C.
Deng
 
S. Y.
Tang
 
H. S.
Liu
 
Y.
Shen
 
J.
Gao
 
G. H.
Wu
 
G. M.
Atif
 
M.
AlSalhi
 
M. S.
Gao
 
G. Z.
Sci. China Mater.
2022
, vol. 
65
 (pg. 
1797
-
1804
)
69
Gao
 
W.
Li
 
Y. F.
Zhao
 
J. T.
Zhang
 
Z.
Tang
 
W. W.
Wang
 
J.
Wu
 
Z. Y.
Li
 
Z. Y.
Chem. Res. Chin. Univ.
2022
, vol. 
38
 (pg. 
1097
-
1104
)
70
Bai
 
Y. Y.
Ma
 
Y. T.
Zheng
 
S. Z.
Zhang
 
C. Q.
Hu
 
C. Y.
Liang
 
B. P.
Xu
 
Y. L.
Huang
 
G. P.
Yang
 
R. F.
Colloids Surf. A
2022
, vol. 
647
 pg. 
129064
 
71
Zakar
 
S.
Iqbal
 
M. Z.
Haider
 
S. S.
Int. J. Energy Res.
2022
, vol. 
4
 (pg. 
15696
-
15708
)
72
Tian
 
F.
Wang
 
H.
Li
 
H.
Liu
 
S.
Li
 
D.
J. Nanopart. Res.
2022
, vol. 
24
 pg. 
123
 
73
Chen
 
X.
Chang
 
P.
Zhang
 
S.
Guan
 
L.
Ren
 
G.
Tao
 
J.
Nanotechnology
2021
, vol. 
33
 pg. 
85403
 
74
Chen
 
X.
Chang
 
P.
Zhang
 
S.
Guan
 
L.
Ren
 
G.
Tao
 
J.
Nanotechnology
2021
, vol. 
33
 pg. 
85403
 
75
Li
 
C.
Wang
 
J.
Yan
 
Y.
Huo
 
P.
Wang
 
X.
Chem. Eng. J.
2022
, vol. 
446
 pg. 
137108
 
76
Zhou
 
Y.
Guo
 
W.
Li
 
T.
Ceram. Int.
2019
, vol. 
45
 (pg. 
21062
-
21076
)
77
Liu
 
T.-C.
Pell
 
W. G.
Conway
 
B. E.
Roberson
 
S. L.
J. Electrochem. Soc.
1998
, vol. 
145
 (pg. 
1882
-
1888
)
78
Zhou
 
Z.
Liang
 
Z.
Shao
 
G.
Liu
 
Q.
Chen
 
D.
Yang
 
W.
J. Electrochem. Soc.
2021
, vol. 
168
 pg. 
70529
 
79
Sun
 
N.
Zhou
 
D.
Liu
 
W.
Shi
 
S.
Tian
 
Z.
Liu
 
F.
Li
 
S.
Wang
 
J.
Ali
 
F.
ACS Sustain. Chem. Eng.
2020
, vol. 
8
 (pg. 
7869
-
7878
)
80
Lei
 
J.-C.
Zhang
 
X.
Zhou
 
Z.
Front. Phys.
2015
, vol. 
10
 (pg. 
276
-
286
)
81
Anasori
 
B.
Lukatskaya
 
M. R.
Gogotsi
 
Y.
Nat. Rev. Mater.
2017
, vol. 
2
 pg. 
16098
 
82
Meshkian
 
R.
Näslund
 
L.-Å.
Halim
 
J.
Lu
 
J.
Barsoum
 
M. W.
Rosen
 
J.
Scr. Mater.
2015
, vol. 
108
 (pg. 
147
-
150
)
83
(John) Zhang
 
C.
Anasori
 
B.
Seral-Ascaso
 
A.
Park
 
S.-H.
McEvoy
 
N.
Shmeliov
 
A.
Duesberg
 
G. S.
Coleman
 
J. N.
Gogotsi
 
Y.
Nicolosi
 
V.
Adv. Mater.
2017
, vol. 
29
 pg. 
1702678
 
84
Naguib
 
M.
Mashtalir
 
O.
Carle
 
J.
Presser
 
V.
Lu
 
J.
Hultman
 
L.
Gogotsi
 
Y.
Barsoum
 
M. W.
ACS Nano
2012
, vol. 
6
 (pg. 
1322
-
1331
)
85
Lin
 
Z.
Shao
 
H.
Xu
 
K.
Taberna
 
P.-L.
Simon
 
P.
Trends Chem.
2020
, vol. 
2
 (pg. 
654
-
664
)
86
Lukatskaya
 
M. R.
Mashtalir
 
O.
Ren
 
C. E.
Dall’Agnese
 
Y.
Rozier
 
P.
Taberna
 
P. L.
Naguib
 
M.
Simon
 
P.
Barsoum
 
M. W.
Gogotsi
 
Y.
Science
2013
, vol. 
341
 (pg. 
1502
-
1505
)
87
Fan
 
Z.
Wang
 
Y.
Xie
 
Z.
Xu
 
X.
Yuan
 
Y.
Cheng
 
Z.
Liu
 
Y.
Nanoscale
2018
, vol. 
10
 (pg. 
9642
-
9652
)
88
Sardana
 
S.
Gupta
 
A.
Singh
 
K.
Maan
 
A. S.
Ohlan
 
A.
J. Energy Storage
2022
, vol. 
45
 pg. 
103510
 
89
Dhandapani
 
E.
Thangarasu
 
S.
Ramesh
 
S.
Ramesh
 
K.
Vasudevan
 
R.
Duraisamy
 
N.
J. Energy Storage
2022
, vol. 
52
 pg. 
104937
 
90
Tomczykowa
 
M.
Plonska-Brzezinska
 
M. E.
Polymers
2019
, vol. 
11
 pg. 
11020350
 
91
Loganathan
 
N. N.
Perumal
 
V.
Pandian
 
B. R.
Atchudan
 
R.
Edison
 
T. N. J. I.
Ovinis
 
M.
J. Energy Storage
2022
, vol. 
49
 pg. 
104149
 
92
Li
 
W.
Li
 
X.
Zhang
 
X.
Wu
 
J.
Tian
 
X.
Zeng
 
M.-J.
Qu
 
J.
Yu
 
Z.-Z.
ACS Appl. Energy Mater.
2020
, vol. 
3
 (pg. 
9408
-
9416
)
93
Amarnath
 
C. A.
Kim
 
J.
Kim
 
K.
Choi
 
J.
Sohn
 
D.
Polymer
2008
, vol. 
49
 (pg. 
432
-
437
)
94
Dadashi
 
R.
Bahram
 
M.
Faraji
 
M.
J. Energy Storage
2022
, vol. 
52
 pg. 
104775
 
95
Yeasmin
 
S.
Talukdar
 
S.
Mahanta
 
D.
Electrochim. Acta
2021
, vol. 
389
 pg. 
138660
 
96
Zhang
 
H.
Hu
 
Z.
Li
 
M.
Hu
 
L.
Jiao
 
S.
J. Mater. Chem. A
2014
, vol. 
2
 (pg. 
17024
-
17030
)
97
Liu
 
J.
Wang
 
Z.
liu
 
Q.
Li
 
S.
Wang
 
D.
Zheng
 
Z.
Chem. Eng. J.
2022
, vol. 
447
 pg. 
137562
 
98
Wu
 
S. M.
Zhou
 
H.
Zhou
 
Y. H.
Wang
 
H.
Li
 
Y. H.
Liu
 
X. Q.
Zhou
 
Y. M.
J. Alloys Compd.
2021
, vol. 
859
 pg. 
157814
 
99
Zhao
 
C.
Niu
 
J. R.
Xiao
 
C. F.
Qin
 
Z. L.
Jin
 
X.
Wang
 
W. Y.
Zhu
 
Z. T.
Chem. Eng. J.
2022
, vol. 
444
 pg. 
136537
 
100
Wang
 
H.
Xiong
 
F. Q.
Yang
 
J. M.
Ma
 
B. L.
Qing
 
Y.
Chu
 
F. X.
Wu
 
Y. Q.
Ind. Crops Prod.
2022
, vol. 
179
 pg. 
114689
 
101
Xing
 
F.
Bi
 
Z.
Su
 
F.
Liu
 
F.
Wu
 
Z.-S.
Adv. Energy Mater.
2022
, vol. 
12
 pg. 
2200594
 
102
Le
 
Z. Y.
Liu
 
F.
Nie
 
P.
Li
 
X. R.
Liu
 
X. Y.
Bian
 
Z. F.
Chen
 
G.
Wu
 
H. B.
Lu
 
Y. F.
ACS Nano
2017
, vol. 
11
 (pg. 
2952
-
2960
)
103
Luo
 
J. M.
Zhang
 
W. K.
Yuan
 
H. D.
Jin
 
C. B.
Zhang
 
L. Y.
Huang
 
H.
Liang
 
C.
Xia
 
Y.
Zhang
 
J.
Gan
 
Y. P.
Tao
 
X. Y.
ACS Nano
2017
, vol. 
11
 (pg. 
2459
-
2469
)
104
Wang
 
R. T.
Wang
 
S. J.
Peng
 
X.
Zhang
 
Y. B.
Jin
 
D. D.
Chu
 
P. K.
Zhang
 
L.
ACS Appl. Mater. Interfaces
2017
, vol. 
9
 (pg. 
32745
-
32755
)
105
Li
 
C.
Zhang
 
X.
Wang
 
K.
Sun
 
X.
Ma
 
Y.
Chin. Chem. Lett.
2020
, vol. 
31
 (pg. 
1009
-
1013
)
106
He
 
Z. H.
Gao
 
J. F.
Kong
 
L. B.
Energy Fuels
2021
, vol. 
35
 (pg. 
8378
-
8386
)
107
Saito
 
M.
Takahashi
 
K.
Ueno
 
K.
Seki
 
S.
J. Electrochem. Soc.
2016
, vol. 
163
 (pg. 
A3140
-
A3145
)
108
Raj
 
C. J.
Manikandan
 
R.
Cho
 
W.-J.
Yu
 
K. H.
Kim
 
B. C.
Ceram. Int.
2020
, vol. 
46
 (pg. 
21736
-
21743
)
109
Wang
 
Y.
Wu
 
X.
Han
 
Y.
Li
 
T.
J. Energy Storage
2021
, vol. 
42
 pg. 
103053
 
110
Liu
 
Q.-C.
Xu
 
J.-J.
Xu
 
D.
Zhang
 
X.-B.
Nat. Commun.
2015
, vol. 
6
 pg. 
7892
 
111
Soram
 
B. S.
Dai
 
J. Y.
Thangjam
 
I. S.
Kim
 
N. H.
Lee
 
J. H.
J. Mater. Chem. A
2020
, vol. 
8
 (pg. 
24040
-
24052
)
112
Yang
 
C.
Yang
 
J.
Liang
 
C.
Zang
 
L.
Zhao
 
Z.
Li
 
H.
Bai
 
L.
J. Electroanal. Chem.
2014
, vol. 
894
 pg. 
115354
 
113
Zhao
 
D.
Chen
 
C.
Zhang
 
Q.
Chen
 
W.
Liu
 
S.
Wang
 
Q.
Liu
 
Y.
Li
 
J.
Yu
 
H.
Adv. Energy Mater.
2017
, vol. 
7
 pg. 
1700739
 
114
Na
 
R.
Huo
 
P.
Zhang
 
X.
Zhang
 
S.
Du
 
Y.
Zhu
 
K.
Lu
 
Y.
Zhang
 
M.
Luan
 
J.
Wang
 
G.
RSC Adv.
2016
, vol. 
6
 (pg. 
65186
-
65195
)
115
Zhu
 
K.
Han
 
X.
Ye
 
S.
Cui
 
P.
Dou
 
L.
Ma
 
W.
Heng-Sha
 , 
Tao
 
X.
Wei
 
X.
J. Energy Storage
2022
, vol. 
53
 pg. 
105096
 
116
Feng
 
E.
Li
 
J.
Zheng
 
G.
Yan
 
Z.
Li
 
X.
Gao
 
W.
Ma
 
X.
Yang
 
Z.
ACS Sustain. Chem. Eng.
2021
, vol. 
9
 (pg. 
7267
-
7276
)
117
Simon
 
P.
Gogotsi
 
Y.
Nat. Mater.
2008
, vol. 
7
 (pg. 
845
-
854
)
118
Miller
 
J. R.
Simon
 
P.
Science
2008
, vol. 
321
 (pg. 
651
-
652
)
119
Kötz
 
R.
Carlen
 
M.
Electrochim. Acta
2000
, vol. 
45
 (pg. 
2483
-
2498
)
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