Professor

Department of Physics

TDPG College, VBS Purvanchal University

 Jaunpur, Uttar Pradesh, India 

Abstract

Supercapacitor is an intriguing electronic device because of its advanced electrochemical characteristics, which include high energy density, quick charge and discharge rates, great cyclic stability, and specific capacitance. Over the past few decades, a great deal of research has been focused on developing new electrode materials in order to improve the electrochemical performance of supercapacitors. This chapter describes the insights of the recent electrode materials such as carbon-based materials, metal oxide/hydroxide-based materials, conducting polymer-based materials, and 2D materials.

Introduction

Electrode materials in supercapacitors play a vital role in capacitance performance and can be categorized into subsections, including carbon-based materials, transition metal oxides, conducting polymers etc. Material selection is very important for supercapacitors. Various materials are used as an electrode (anode & cathode) and electrolyte in supercapacitors system. The properties of supercapacitors come from the interaction of their internal materials. Especially, the combination of electrode material and type of electrolyte determine the functionality and thermal and electrical characteristics of the capacitors.  Henceforth, the proper selection and design of electrode materials are vital to enhance supercapacitor performances [1].

Carbon‑Based Materials

The most commonly used electrode material for supercapacitors is carbon. For high availability, established industrial production processes and low cost, various carbon-based materials are widely used in many applications for supercapacitors [2]. Carbon electrodes can be manufactured in several forms of 1D to 3D structure such as foams, fibres, and nanotubes. One might expect the specific capacitance to be directly proportional to the carbon electrode’s surface area, however, this is not always the case. Often, a type of carbon with a lower surface area will have a higher specific capacitance than a type with a larger surface area [3]. These carbon-based materials, including activated carbon, carbon nanotubes, and graphene, possess desirable chemical and physical properties such as high surface areas, high porosity, low costs and varied forms (e.g., powders, nanotubes, aerogels).

 Activated Carbon Materials (ACs)

Activated carbon materials possess large surface areas, complex porous structures, and good electrical conductivities and are relatively inexpensive, making them attractive electrode materials for supercapacitors [4]. These materials are generally synthesized in two steps and involve the carbonization and activation of carbon-rich organic precursors such as coconut shells, wood, fossil fuels, coke, coal, or synthetic polymers [5]. In the carbonization process, carbon-rich precursors are treated at high temperatures (700 to 1200 °C) in inert atmosphere to form amorphous carbon and in the activation process, lower temperatures are applied (400 to 700 °C) in the presence of activating agents such as alkalis, carbonates, chlorides, or acids (e.g., KOH, NaOH, ZnCl), which can provide porous networks in the bulk of the carbon particles. And overall, these two processes can lead to tremendous increases in specific surface area and porosity in which the physiochemical property and specific surface area of ACs depend on precursor materials and activation methods. Here, a maximum achievable surface area of 3000 m²g⁻¹ and a usable surface area in the range of 1000 – 2000 m²g⁻¹ can be obtained along with the formation of porous structures in various sizes ranging from nanoporous (50 nm) based on the different activation processes used [6-7]. Despite these enhancements, however, researchers also report that increased specific surface areas can also lead to possible risks of electrolyte decomposition and dangling bond positions [8]. In addition to specific surface area and pore shapes, size and distribution, electrical conductivity and surface functionality are also important factors influencing the performance of activated carbon [9]. Here, the specific capacitance of AC as an electrode material is higher in aqueous electrolytes than in organic electrolytes due to the larger size of electrolyte ions in organic solutions [7]. And to address the major disadvantage of low density in AC for practical applications, Moreno-Fernandez et. al. reported that the synthesis of AC fiber monolith can result in three times higher densities in which a maximum specific capacitance of 200 Fg⁻¹ can be achieved [10]. In another study, Li et. al. reported that AC derived from lignite at various activation temperatures as an electrode material for SCs can exhibit a specific capacitance of 207.5 Fg⁻¹ and cyclic stability up to 3000 cycles at a current density of 0.5 Ag⁻¹ for an activation temperature of 650 °C [11]. Despite these performances, however, due to poor mechanical properties and the need for metal current collectors, activated carbon-based supercapacitors are generally only available in button cell or spiral-wound configurations [3]. Table 1 shows different biomass derived activated carbon-based electrode materials with their electrochemical performances. 

Table 1 Electrochemical performance of various biomass derived AC based electrodes.

 Carbon Nanotubes (CNT)

CNTs are also known as carbon allotropes like graphene. It can be disclosed by HR-TEM analysis Carbon soot obtained by arc discharge of graphite. In 1997, Niu et. al. first suggested that CNTs could be used in supercapacitors [18]. CNTs attracted significant attention as electrode materials for supercapacitors due to excellent characteristics, unique internal structures, fully accessible external high surface areas, low mass densities, excellent electrical conductivities, and exceptional mechanical, thermal, and chemical stability [19]. In addition, the high mechanical resilience and open tubular structure of CNTs can provide a port to active materials, and the mesoporous structure of CNT networks can allow for faster and easier diffusion of electrolytes, which can decrease ESR and maximize usable power [8, 20]. Carbon nanotubes are generally synthesized through the decomposition of certain hydrocarbons and by manipulating certain parameters in which crystalline orders can change to obtain different nanostructures such as single wall carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs). Here, the specific capacitance electrode is influenced by the purity and morphology of the obtained CNT material [21]. For example, Niu et. al. prepared MWCNT sheet electrodes from catalytically grown carbon nanotubes (Fig. 1) and reported that the synthesized CNT possessed a diameter of 80 Ǻ and a specific surface area of 430 m²g⁻¹, which is higher than that of CNT materials obtained from different methods in which a maximum specific capacitance of 104 Fg⁻¹ was obtained for a single-cell test device by using a H₂SO₄ electrolyte solution [22]. Table 2 shows different CNT based electrochemical performances

Table 2: Comparison of different composite of CNTS based materials.

 Graphene

Graphene is an allotrope of carbon material consisting of a single layer (monolayer) of atoms arranged in unique two-dimensional honeycomb lattice nanostructure, which is the basic building block of all dimensional graphitic materials including the wrapped 0D buckyballs (fullerenes), the rolled 1D nanotubes and the stacked 3D graphite as shown in Fig. 2 [30]

It has obtained expeditiously growth consideration, because of its peerless structure. graphene has become valuable and useful nanomaterial due to high surface to volume ratio, good thermal and electrical conductivity, structural flexibility, highly tunable surface area (up to 2675 m²g⁻¹), short diffusion distance (due to its thickness), good chemical stability, strong mechanical strength (~ 1 Tpa), and wide potential window [32]. Apart from these, it also takes the advantage of having large surface area of monolayer (theoretically 2620 m²g⁻¹) and open pore system which helps in improving the transport kinetics phenomenon. These unique properties texture graphene is a benchmark candidate in all carbon allotropes for energy storage in supercapacitor application [33]. Basically, graphene is a non-metal substance but due to its unique properties, it behaves like a semi-conducting metal. In recent years, graphene sheet could be synthesised by various techniques: such as mechanical cleavage of graphite (exfoliation), chemical exfoliation of graphite [34], unzipping carbon nanotubes [35], epitaxial growth on SiC surface [36], thermal chemical vapour deposition (CVD) [37] and microwave synthesis. Among these synthesis methods, mechanical exfoliation using AFM cantilever was found capable of fabricating few-layer graphene and chemical method could impure the productivity of the graphene. Fig.3 schematically illustrates the chemical method to synthesise graphene [38]. Exfoliation of graphitic oxide, and heating nanodiamonds reveals high specific capacitance in H₂SO₄ electrolyte. The value could reach 117 F/g, that was homologous to that obtained using ACs as electrode materials in an aqueous electrolyte. However, Stoller et. al. fabricate graphene via modified chemical method and reported specific capacitances as 135 F/g in aqueous electrolyte and 99 F/g in organic electrolyte [39]. Furthermore, graphene proclaims good achievement at energy storage devices because of its outstanding electrical conductivity. In general graphene oxide behaves as an intermediate product to produce high quality graphene. As a monolayer of graphene oxide shows, a variety of oxygen contains functional groups at the edges which perform pseudo capacitance in supercapacitance application. Graphene oxide contains a huge amount of oxygen functional group compared to refined graphene, hence graphene oxide manifests higher specific capacitance, best rate of charge loss and cycle durability [40-41]. By virtue of better execution and low-cost production, graphene oxide plays a handsome material in supercapacitor applications [41]. Table 3 shows electrochemical performances of different graphene-based electrode materials.

 

Table 3 : Comparison of graphene-based supercapacitors.

Carbon aerogels and xerogel based supercapacitors

Another approach to increase the power densities of carbon-based supercapacitors is to use carbon aerogels. Carbon aerogels are a mesoporous, low-density and high surface area, amorphous carbon materials with 3 d interconnected clusters of carbon nanoparticles (diameter range 3-20 nm) with sp² bonding, which can be used as electrode materials without binder. It has been obtained by a super cooling process via the sol-gel method and controlled by catalysts and the mass ratio of reactants [47]. For example, the carbon aerogel has a density of 0.16 milligrams per cubic centimetre, about one-sixth the density of air.

Qin et. al. developed a carbon aerogel by a carbonisation of Resorcinol formaldehyde (RF) resin via a sol-gel route [48]. It was the first time to use the carbon aerogel as electrode material for supercapacitors. They observed high surface areas per unit volume (100-700 m²cm^-3, as measured by BET analysis) with bulk densities of 0.3 to 1.0 g cm^-3. Yong Kong et al. develop a high-performance CO₂ adsorbent by a simple, cost-effective and environmental-friendly route based on amine hybrid aerogel. A new amine hybrid titania/silsesquioxane composite aerogel (AHTSA) was prepared by a one-pot sol-gel process followed by supercritical drying (SCD) [49]. Fig. 4 depicts SEM image of carbon aerogel with activating agent CO₂.

Metal Oxides/Hydroxides

Carbon-based electrode materials suffer from low specific capacitances; therefore, researchers are shifting their focus to alternative materials. And because the pseudo capacitance of pseudo capacitors is 3 to 10 times greater than that of EDLCs due to charge being stored in the double layer and fast and reversible redox reactions, pseudocapacitive materials are the most promising candidates in modern supercapacitors. Here, conducting polymers and transition metal oxides are the most common materials for pseudo capacitors and have attracted major attention from researchers due to high specific capacitances that originate from fast reversible redox reactions and long operation lifespans. Currently, several metal oxides/hydroxides are being investigated, including RuO₂, IrO₂, MnO₂, NiO, Co₃O₄, SnO₂, V₂O₅, CuO, Ni (OH)₂, and Co (OH)₂, in which the most studied materials are RuO₂, MnO₂, NiO and Ni(OH)₂ due to their higher theoretical specific capacitances. Table 5 shows metal oxide-based electrode materials electrochemical performances.

Table 5: Different electrochemical performance of metal oxide based supercapacitors.

 Ruthenium Oxide (RuO₂)

Ruthenium oxide is one of the most explored TMOs for supercapacitor electrodes because of its high theoretical capacitance (1200–2200 Fg⁻¹), high electric conductivity, reversible redox reaction, wide potential window (1.2 V), long cycle lifespan, good rate capability, and excellent stability [57]. In addition, ruthenium oxide possesses three oxidation states (Ru⁴⁺, Ru³⁺, and Ru²⁺) [58] and the fast faradaic redox reaction of RuO₂ can be represented as follows [59]. The presence of structural water can enhance proton conductivity but can also reduce electric conductivity. And in the case of RuO₂, super capacitive performance can be enhanced by optimizing the combined water (x = 0.5) content to maintain a balance in proton conductivity and electric conductivity [8] Furthermore, crystallinity, annealing temperature, and particle size can also affect the pseudocapacitive performance of RuO₂ [60] in which the high crystallinity of RuO₂ can cause low specific capacitances due to the compact nature of RuO₂, which can restrict the insertion/extraction of ions/electrons, leading to increased electrochemical impedance [8]. As for the charge storage mechanism of RuO₂, four stages are involved, including the hopping of electrons within RuOx·nH₂O, the hopping of electrons between particles, the hopping of electrons between electrodes and current collectors, and proton diffusion without RuOx·nH₂O particles [61]. As for the synthesis of RuO₂ for supercapacitor electrode materials, various synthesis methods have been used. For example, Zheng et. al. prepared hydrous ruthenium oxide using a sol–gel method at an annealing temperature of 150 °C and reported specific capacitance of 720 Fg⁻¹ for the prepared material. The energy density of 96 J/g (or 26.7 Wh/kg), based on electrode material only, was measured for the cell using hydrous ruthenium oxide electrodes. It was also found that hydrous ruthenium oxide is stable in H₂SO₄ electrolyte [62].

Manganese Oxide (MnO₂)

The high cost and rarity of RuO₂-based materials hinder application in supercapacitors, therefore, alternative materials are needed to replace RuO₂-based materials as electrode materials [63]. Here, MnO₂ has been reported to be a promising alternative due to its exceptional properties such as high natural abundance, a wide electrochemical potential window, high theoretical specific capacitance (1370 Fg⁻¹) as well as its low cost, low toxicity, and low environmental impact [64]. In addition, MnO₂ possesses various crystal structures (α, β, δ, ɤ, λ) that can influence super capacitive values through the size of tunnels and the control of cation intercalation in which α and δ crystal structures have been reported to exhibit the highest specific capacitances among other crystal structures [65]. Furthermore, the pseudo capacitance energy storage mechanism of MnO₂ can be attributed to the reversible redox reaction and its multioxidation states in which the charge storage mechanism can be described with the following equation-

Here C represents protons and alkali metal cations (K⁺, Na⁺, H⁺, and Li⁺) in the electrolyte and two mechanisms are responsible for charge storage including the insertion.

MnO₂ + C⁺ + e^- = MnOOC

reinsertion of cations into the bulk of the electrode and the second adsorption/desorption of electrolyte cations onto the electrode surface in which in both mechanisms, the reversible transition between III and IV oxidation states occurs [66]. As for limitations, MnO₂ suffers from low conductivity and slow proton and cation diffusion, leading to bulk redox reactions near the subsurface [67]. Furthermore, researchers report that chemical and physical factors have major influences on the electrochemical performance of MnO₂, such as cyclic stability being mainly dictated by the morphology of the prepared material and the specific capacitance being dependent on the chemical hydration state of MnO₂ [68].

Nickel Oxide (NiO)

NiO has recently attracted attention from researchers as a promising alternative to RuO₂ due to its high theoretical capacitance (2584 Fg⁻¹), environmentally friendly nature, high chemical stability, and its low cost and ease of availability [8]. As for the energy storage process of NiO, there are two main theories involving a single-step conversion between NiO and NiOOH as described in (Eqn. 1.17 and 1.18) and a two-step conversion with an initial conversion between NiO and Ni(OH)₂ and a subsequent conversion between Ni(OH)₂ and NiOOH (Eqns. 2 and 3) [69]. Here, both supercapacitive reactions are a result of the transformation of Ni²⁺ to NiOOH through the loss of electrons and can be represented in the following Eqn.

NiO + OH^- = NiOOH + e^-        (1)

NiO + H₂O = NiOOH +H⁺ + e^-       (2)

Ni (OH)₂ + OH^- = NiOOH +H₂O + e^-     (3)

Ni (OH)₂ + OH^- = NiOOH +H₂O + e^-  (4)

And among these two theories, the first theory is widely accepted whereas the second theory is also practical because NiO can combine with OH⁻ to produce Ni (OH)₂ in alkaline electrolytes and contribute to capacitance generation. As for challenges, NiO possesses low electrical conductivities, which can result in lower specific capacitances and poorer cyclability, restricting practical application as a supercapacitor electrode material. To address this issue, researchers have proposed solutions such as the direct deposition of NiO onto conductive materials or the formation of nanocomposites with conductive carbon-based materials to enhance conductivity [70]. For example, Gund et. al. synthesized hierarchical nanoflake structured NiO films on SS 304 through the use of a successive ionic layer adsorption and reaction method and reported that the prepared material delivered a specific capacitance of 674 Fg⁻¹ and good cyclic stability in which 72.5 of the specific capacitances remained after 2000 cycle [71].

Nickel Hydroxide (Ni (OH) ₂)

Among transition metal hydroxides, nickel hydroxide has attracted major attention due to its high specific capacitance (2082 Fg⁻¹), excellent stability in strong alkaline electrolytes, good rate capability, lower costs, and ease of availability [71] in which previous studies have demonstrated that the electrochemical performance of Ni(OH)₂ is mainly dependent on phase structure, morphology, porosity and surface area [72]. In addition, two pseudo-polymorphs of α and β exist for Ni(OH)₂ that possess special layered structures similar to layered double hydroxides in which the interplanar spacing for α-Ni(OH)₂ is larger due the intercalation of water molecules and anions [73]. As a result, α-Ni(OH)₂ can deliver higher specific capacitances than β-Ni(OH)₂ but is not as stable in alkaline electrolytes and will transform to the β phase [74]. In the late 1960s, Bode et. al. revealed a scheme for the redox behaviour of nickel hydroxides involving the oxidation of nickel hydroxide to nickel (III) oxyhydroxide and the subsequent reduction back to nickel hydroxide [75]. This scheme also involves two phases (α and β) for nickel hydroxide and two phases (β and ɤ) for the oxidized material. Lang et. al. synthesized loosely packed Ni(OH)₂ using a facile chemical precipitation method in which low crystallinity nanoflakes were formed and reported a maximum specific capacitance of 2055 Fg⁻¹ for the prepared sample [76]. Chai et. al. synthesized Ni(OH)₂/ graphene composites through a chemical precipitation route in which the graphene sheet can act as a highly conductive medium and provide larger surface areas to enhanced electrochemical performance [77].

Conducting Polymers (CPs)

Recently, CPs have also attracted attention from researchers due to their relatively high capacitance, high energy density, high voltage windows, adjustable redox activity through chemical modification and good conductivity in doped states as well as ease of fabrication, feasibility, low costs, and low environmental impact [78]. In addition, specific capacitances can arise in CPs due to fast and reversible redox reactions related to the π-conjugated polymer chains [79] in which during the oxidation process, ions transfer to the polymer backbone and are released back into the electrolyte during reduction [80]. Furthermore, CPs can store charge in its bulk because no structural changes such as phase changes occur during the charging/discharging process. As a result of all of this, CPs can provide higher capacitances due to redox storage capabilities and larger surface areas. And currently, p-dopable polymers are mostly studied by researchers because of stable performances against degradation as compared with non-dopable polymers [8]. However, despite all the advantages of CPs, low power densities due to slow ion diffusion rates in bulk materials hinder performance [8]. Currently, the most commonly used CPs for supercapacitor electrodes include polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), and various derivatives . Here, various methods are available for the synthesis of CPs in which the oxidation of monomers through chemical or electrochemical methods is the most common approach [81]. Wang et. al. fabricated PANi with a unique nanowire structure using an electrochemical polymerization method as the active material for a supercapacitor and reported a high specific capacitance of 950 Fg⁻¹ at a current density of 1 Ag⁻¹ [82].

Carbon-based nanomaterials with TMOs

TMOs have low electronic conductivity, poor Cs and low electrochemical stability. To improve its performance, nanostructured TMOs are mixed with the carbon material to make composites. This combination of MOs and carbon is useful for high-performance SCs. various hybrid materials such as Co₃O₄/graphene [83], Co₃O₄/CNTs [84], and Co₃O₄/CNFs [85] with improved electrical conductivity and a huge surface area have been prepared. Wang et. al. have reported a 3D NiO/GF composite, which exhibits an ultrahigh supercapacitive performance. Secondly, functionalized hierarchical porous nitrogen-doped carbon nanotubes (HPNCNTs) are prepared via a facile chemical activation route with polypyrrole (PPy) nanotubes as the precursor and KOH as the activating agent. The HPNCNTs exhibit a high specific capacitance (270 Fg^-1 at 2 Ag^-1 and 182 Fg^-1 at 60 Ag^-1) and a long cycle life (96% capacitance retention after 6000 cycles) [86]. Patil et. al. preparation of MnO₂-nanospheres and SnS–nanoflowers nanostructured binder-free electrodes for ASSCs directly on cost efficient, high mechanical strength, excellent flexible and stable in acidic electrolyte SS substrate. It shows specific capacitance of 994 Fg^-1 at 5 mVs^-1, energy density and power density of 83 Whkg^-1 and 10 kWkg^-1 and capacity retention of 94.3% over 3000 cycles [87-88].

References

[1] Balaji TE, Tanaya Das H, Maiyalagan T. Recent trends in bimetallic oxides and their composites as electrode materials for supercapacitor applications. ChemElectroChem. 2021; 8: 1723-46.
[2]ang Z, Tian J, Yin Z, Cui C, Qian W, Wei F. Carbon nanotube-and graphene-based nanomaterials and applications in high-voltage supercapacitor: A review. Carbon. 2019; 141: 467-80.
[3]Béguin F, Presser V, Balducci A, Frackowiak E. Supercapacitors: carbons and electrolytes for advanced supercapacitors. Advanced materials. 2014; 26: 2283.
[4]Pandolfo AG, Hollenkamp AF. Carbon properties and their role in supercapacitors. Journal of power sources. 2006; 157: 11-27.
[5Hassan MF, Sabri MA, Fazal H, Hafeez A, Shezad N, Hussain M. Recent trends in activated carbon fibers production from various precursors and applications—A comparative review. Journal of Analytical and Applied Pyrolysis. 2020; 145: 104715.
[6]Hassan N, Shahat A, El-Didamony A, El-Desouky M, El-Bindary A. Equilibrium, Kinetic and Thermodynamic studies of adsorption of cationic dyes from aqueous solution using ZIF-8. J Moroccan Journal of Chemistry. 2020; 8: 8-3.
[7]Huang J, Sumpter BG, Meunier V. A universal model for nanoporous carbon supercapacitors applicable to diverse pore regimes, carbon materials, and electrolytes. Chemistry–A European Journal. 2008;14: 6614-26.
[8]Lokhande PE, Chavan US, Pandey A. Materials and fabrication methods for electrochemical supercapacitors: overview. Electrochemical Energy Reviews. 2020; 3: 155-86.
[9]Redondo E, Carretero-González J, Goikolea E, Ségalini J, Mysyk R. Effect of pore texture on performance of activated carbon supercapacitor electrodes derived from olive pits. Electrochimica Acta. 2015; 160: 178-84.
[10]Tijerín-Triviño J, Moreno-Fernández D, Zavala MA, Astigarraga J, García M. Identifying forest structura types along an aridity gradient in peninsular Spain: Integrating low-density LiDAR, forest inventory, and aridity index. Remote Sensing. 2022; 14: 235.
[11]Li L, Wang X, Wang S, Cao Z, Wu Z, Wang H, Liu J. Activated carbon prepared from lignite as supercapacitor electrode materials. Electroanalysis. 2016; 28(1): 243-248.
[12]Vinayagam M, Babu RS, Sivasamy A, de Barros ALF. Biomass-derived porous activated carbon from Syzygium cumini fruit shells and Chrysopogon zizanioides roots for high-energy density symmetric supercpacitors. Biomass and Bioenergy. 2020; 143: 105838.
[13]Du J, Zhang Y, Lv H, Chen A. Silicate-assisted activation of biomass towards N-doped porous carbon sheets for supercapacitors. Journal of Alloys and Compounds. 2021; 853: 157091.
[14]Fasakin O, Dangbegnon JK, Momodu DY, Madito M, Oyedotun KO, Eleruja M, et al. Synthesis and characterization of porous carbon derived from activated banana peels with hierarchical porosity for improved electrochemical performance. Electrochimica Acta. 2018; 262: 187-96.
[15]Xie X, Zhang B, Wang Q, Zhao X, Wu D, Wu H, et al. Efficient microwave absorber and supercapacitors derived from puffed-rice-based biomass carbon: Effects of activating temperature. Journal of Colloid and Interface Science. 2021; 594: 290-303.
[16]He J, Zhang D, Han M, Liu X, Wang Y, Li Y, et al. One-step large-scale fabrication of nitrogen doped microporous carbon by self-activation of biomass for supercapacitors application. Journal of Energy Storag. 2019; 21: 94-104.
[17]Lu W, Cao X, Hao L, Zhou Y, Wang Y. Activated carbon derived from pitaya peel for superapacitor applications with high capacitance performance. Materials Letters. 2020; 264: 127339.
[18]Niu C, Sichel EK, Hoch R, Moy D, Tennent H High power electrochemical capacitor based on carbon nanotube electrodes. Appl. Phys. Lett. 1997; 10: 1063.
[19]Peng HJ, Huang JQ, Zhao MQ, Zhang Q, Cheng XB, Liu XY, et al. Nanoarchitectured graphene/CNT@ porous carbon with extraordinary electrical conductivity and interconnected micro/mesopores for lithium‐sulfur batteries. Advanced Functional Materials. 2014; 24: 2772-81.
[20]Lei Y, Tian Z, Sun H, Zhu Z, Liang W, Li A. Self-cleaning and flexible filters based on aminopyridine conjugated microporous polymers nanotubes for bacteria sterilization and efficient PM2.5 capture. Science of The Total Environment. 2021; 766: 142594.
[21]Oraon R, De Adhikari A, Tiwari S, Nayak GC. Enhanced specific capacitance of self-assembled three-dimensional carbon nanotube/layered silicate/polyaniline hybrid sandwiched nanocomposite for supercapacitor applications. ACS Sustainable chemistry & Engineering. 2016; 4: 1392-403.
[22]Niu C. Carbon nanotube transparent conducting films. MRS Bulletin. 2011;  36(10): 766-773.
[23]Kanninen P, Luong ND, Anoshkin IV, Tsapenko A, Seppälä J, Nasibulin AG, et al. Transparent and flexible high-performance supercapacitors based on single-walled carbon nanotube films. Nanotechnology. 2016; 27: 235403.
[24]Gilshteyn EP, Kallio T, Kanninen P, Fedorovskaya EO, Anisimov AS, Nasibulin AG. Stretchable and transparent supercapacitors based on aerosol synthesized single-walled carbon nanotube films. RSC Advances. 2016; 6: 93915-21.
[25]Niu Z, Zhou W, Chen J, Feng G, Li H, Hu Y, et al. A repeated halving approach to fabricate ultrathin single‐walled carbon nanotube films for transparent supercapacitors. Small. 2013; 9: 518-24.
[26]Yuksel R, Sarioba Z, Cirpan A, Hiralal P, Unalan HE. Transparent and flexible supercapacitors with single walled carbon nanotube thin film electrodes. ACS Applied Materials & Interfaces. 2014; 6: 15434-9.
[27]Zhang N, Zhou W, Zhang Q, Luan P, Cai L, Yang F, et al. Biaxially stretchable supercapacitors based on the buckled hybrid fiber electrode array. Nanoscale. 2015; 7: 12492-7.
[28]Ge J, Cheng G, Chen L. Transparent and flexible electrodes and supercapacitors using polyaniline/single-walled carbon nanotube composite thin films. Nanoscale. 2011; 3: 3084-8.
[29]Chen P-C, Shen G, Sukcharoenchoke S, Zhou C. Flexible and transparent supercapacitor based on In₂O₃ nanowire/carbon nanotube heterogeneous films. Applied Physics Letters. 2009; 94: 043113.
[30]Calvaresi M, Quintana M, Rudolf P, Zerbetto F, Prato M. Rolling up a graphene sheet. ChemPhysChem. 2013; 14: 3447-53.
[31]Xiao J, Han J, Zhang C, Ling G, Kang F, Yang QH. Dimensionality, function and performance of carbon materials in energy storage devices. Advanced Energy Materials. 2022; 12: 2100775.
[32]Le Ba T, Mahian O, Wongwises S, Szilágyi IM. Review on the recent progress in the preparation and stability of graphene-based nanofluids. Journal of Thermal Analysis and Calorimetry. 2020; 142: 1145-72.
[33]Zhang J, Terrones M, Park CR, Mukherjee R, Monthioux M, Koratkar N, et al. Carbon science in 2016: Status, challenges and perspectives. Carbon. 2016; 98: 708-32.
[34]Zhao W, Fang M, Wu F, Wu H, Wang L, Chen G. Preparation of graphene by exfoliation of graphite using wet ball milling. Journal of Materials Chemistry. 2010; 20: 5817-9.
[35]Jayasena B, Subbiah S. A novel mechanical cleavage method for synthesizing few-layer graphenes. Nanoscale Research Letters. 2011; 6: 1-7.
[36]Whitener Jr KE, Sheehan PE. Graphene synthesis. Diamond and related materials. 2014; 46: 25-34
[37]Chen Z, Ren W, Gao L, Liu B, Pei S, Cheng H-M. Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition. Nature Materials. 2011; 10: 424-8.
[38]Loh KP, Bao Q, Ang PK, Yang J. The chemistry of graphene. Journal of Materials Chemistry. 2010; 20: 2277-89.
[39]Zhu Y, Murali S, Stoller MD, Velamakanni A, Piner RD, Ruoff  RS. Microwave assisted exfoliation and reduction of graphite oxide for ultracapacitors. Carbon. 2010; 48(7): 2118-2122.
[40]Aliyev E, Filiz V, Khan MM, Lee YJ, Abetz C, Abetz V. Structural characterization of graphene oxide: Surface functional groups and fractionated oxidative debris. Nanomaterials. 2019; 9: 1180.
[41]Jia H, Wang Z, Li C, Si X, Zheng X, Cai Y, et al. Designing oxygen bonding between reduced graphene oxide and multishelled Mn₃O₄ hollow spheres for enhanced performance of supercapacitors. Journal of Materials Chemistry A. 2019; 7: 6686-94.
[42]Xu P, Kang J, Choi J-B, Suhr J, Yu J, Li F, et al. Laminated ultrathin chemical vapor deposition graphene films based stretchable and transparent high-rate supercapacitor. ACS Nano. 2014; 8: 9437-45.
[43]Gao Y, Zhou Y, Xiong W, Jiang L, Mahjouri-Samani M, Thirugnanam P, et al. Transparent, flexible, and solid-state supercapacitors based on graphene electrodes. Apl Materials. 2013; 1: 012101.
[44]Chen T, Xue Y, Roy AK, Dai L. Transparent and stretchable high-performance supercapacitors based on wrinkled graphene electrodes. ACS Nano. 2014; 8: 1039-46.
[45]Fan X, Chen T, Dai L. Graphene networks for high-performance flexible and transparent supercapacitors. RSC Advances. 2014; 4: 36996-7002.
[46]Lee K, Lee H, Shin Y, Yoon Y, Kim D, Lee H. Highly transparent and flexible supercapacitors using graphene-graphene quantum dots chelate. Nano Energy. 2016; 26: 746-54.
[47]Tamon H, Ishizaka H, Mikami M, Okazaki M. Porous structure of organic and carbon aerogels synthesized by sol-gel polycondensation of resorcinol with formaldehyde. Carbon. 1997; 35: 791-6.
[48]Qin G, Guo S. Preparation of RF organic aerogels and carbon aerogels by alcoholic sol-gel process. Carbon. 2001; 12(39): 1935-1937.
[49]Kong Y, Jiang G, Wu Y, Cui S, Shen X. Amine hybrid aerogel for high-efficiency CO₂ capture: Effect of amine loading and CO₂ concentration. Chemical Engineering Journal. 2016; 306: 362-8.
[50]Huang X, Zhang H, Li N. Symmetric transparent and flexible supercapacitor based on bio-inspired graphene-wrapped Fe₂O₃ nanowire networks. Nanotechnology. 2017; 28: 075402.
[51]Borysiewicz M, Wzorek M, Ekielski M, Kaczmarski J, Wojciechowski T. Tuning transparent supercapacitor performance by controlling the morphology of its ZnO electrodes. Acta Physica Polonica A. 2017; 131: 1550-3.
[52]Borysiewicz MA, Wzorek M, Myśliwiec M, Kaczmarski J, Ekielski M. MnO₂ ultrathin films deposited by means of magnetron sputtering: relationships between process conditions, structural properties and performance in transparent supercapacitors. Superlattices and Microstructures. 2016; 100: 1213-20.
[53]Borysiewicz M, Ekielski M, Ogorzałek Z, Wzorek M, Kaczmarski J, Wojciechowski T. Highly transparent supercapacitors based on ZnO/MnO₂ nanostructures. Nanoscale. 2017; 9: 7577-87.
[54]Zhang CJ, Higgins TM, Park S-H, O'Brien SE, Long D, Coleman JN, et al. Highly flexible and transparent solid-state supercapacitors based on RuO₂/PEDOT: PSS conductive ultrathin films. Nano Energy. 2016; 28: 495-505.
[55]Li N, Huang X, Zhang H. High energy density transparent and flexible asymmetric supercapacitor based on a transparent metal hydroxides@ graphene micro-structured film via a scalable gas-liquid diffusion method. Journal of Alloys and Compounds. 2017; 712: 194-203.
[56]Lee H, Hong S, Lee J, Suh YD, Kwon J, Moon H, et al. Highly stretchable and transparent supercapacitor by Ag–Au core–shell nanowire network with high electrochemical stability. ACS Applied Materials & Interfaces. 2016; 8: 15449-58.
[57]Delbari SA, Ghadimi LS, Hadi R, Farhoudian S, Nedaei M, Babapoor A, et al. Transition metal oxide-based electrode materials for flexible supercapacitors: A review. Journal of Alloys and Compounds. 2021; 857: 158281.
[58]Atmaramani R, Chakraborty B, Rihani RT, Usoro J, Hammack A, Abbott J, et al. Ruthenium oxide based microelectrode arrays for in vitro and in vivo neural recording and stimulation. Acta Biomaterialia. 2020; 101: 565-74.
[59]Wen S, Lee J-W, Yeo I-H, Park J, Mho S-i. The role of cations of the electrolyte for the pseudocapacitive behavior of metal oxide electrodes, MnO₂ and RuO_2. Electrochimica Acta. 2004; 50: 849-55.
[60]Chang K-H, Hu C-C, Chou C-Y. Textural and pseudocapacitive characteristics of sol–gel derived RuO₂· xH₂O: Hydrothermal annealing vs. annealing in air. Electrochimica Acta. 2009; 54: 978-83.
[61]Sinha P, Banerjee S, Kar KK. Transition metal oxide/activated carbon-based composites as electrode materials for supercapacitors. Handbook of Nanocomposite Supercapacitor Materials II: Springer; 2020. p. 145-78.
[62]Zheng J, Cygan P, Jow T. Hydrous ruthenium oxide as an electrode material for electrochemical capacitors. Journal of the Electrochemical Society. 1995; 142: 2699.
[63]Xu C, Kang F, Li B, Du H. Recent progress on manganese dioxide based supercapacitors. Journal of Materials Research. 2010; 25: 1421-32.
[64]Jeong JM, Park SH, Park HJ, Jin SB, Son SG, Moon JM. Alternative‐Ultrathin Assembling of Exfoliated Manganese Dioxide and Nitrogen‐Doped Carbon Layers for High‐Mass‐Loading Supercapacitors with Outstanding Capacitance and Impressive Rate Capability. Advanced Functional Materials. 2021; 31: 2009632.
[65]Devaraj S, Munichandraiah N. Effect of crystallographic structure of MnO₂ on its electrochemical capacitance properties. The Journal of Physical Chemistry C. 2008; 112: 4406-17.
[66]Liu R, Zhou A, Zhang X, Mu J, Che H, Wang Y, et al. Fundamentals, advances and challenges of transition metal compounds-based supercapacitors. Chemical Engineering Journal. 2021; 412: 128611.
[67]Cao J, Li X, Wang Y, Walsh FC, Ouyang J-H, Jia D, et al. Materials and fabrication of electrode scaffolds for deposition of MnO₂ and their true performance in supercapacitors. Journal of Power Sources. 2015; 293: 657-74.
[68]Majumdar D. Review on current progress of MnO_2‐based ternary nanocomposites for supercapacitor applications. Chem Electro Chem. 2021; 8: 291-336.
[69]Wang L, Chen H, Daniel Q, Duan L, Philippe B, Yang Y. Promoting the water oxidation catalysis by synergistic interactions between Ni(OH)₂ and carbon nanotubes. Advanced Energy Materials. 2016; 6: 1600516.
[70]Abbasi H, Antunes M, Velasco JI. Recent advances in carbon-based polymer nanocomposites for electromagnetic interference shielding. Progress in Materials Science. 2019; 103: 319-73.
[71]Gund GS, Lokhande CD, Park HS. Controlled synthesis of hierarchical nanoflake structure of NiO thin film for supercapacitor application. Journal of Alloys and Compounds. 2018; 741: 549-556.
[72]Dubal D, Fulari V, Lokhande C. Effect of morphology on supercapacitive properties of chemically grown β-Ni(OH)₂ thin films. Microporous and Mesoporous Materials. 2012; 151: 511-6.
[73]Wang H, Gao J, Li Z, Ge Y, Kan K, Shi K. One-step synthesis of hierarchical α-Ni (OH)₂ flowerlike architectures and their gas sensing properties for NO_x at room temperature. Cryst Eng Comm. 2012; 14: 6843-52.
[74]Lang J-W, Kong L-B, Liu M, Luo Y-C, Kang L. Asymmetric supercapacitors based on stabilized α-Ni (OH)₂ and activated carbon. Journal of Solid State Electrochemistry. 2010; 14: 1533-9.
[75]Bode H, Dehmelt K, Witte J. Zur kenntnis der nickelhydroxidelektrode—I. Über das nickel (II)-hydroxidhydrat. Electrochimica Acta. 1966; 11(8): 1079-1087.
[76]Lang JW, Kong LB, Wu WJ, Liu  M, Luo YC, Kang L. A facile approach to the preparation of loose-packed Ni (OH)₂ nanoflake materials for electrochemical capacitors. Journal of Solid State Electrochemistry. 2009; 13(2): 333-340.
[77]Chai H, Peng X, Liu T, Su X, Jia D, Zhou W. High-performance supercapacitors based on conductive graphene combined with Ni(OH)₂ nanoflakes. RSC Advances. 2017;  7(58): 36617-36622.
[78]Sun J, Wu C, Sun X, Hu H, Zhi C, Hou L, et al. Recent progresses in high-energy-density all pseudocapacitive-electrode-materials-based asymmetric supercapacitors. Journal of Materials Chemistry A. 2017; 5: 9443-64.
[79]Uke SJ, Mardikar SP, Kumar A, Kumar Y, Gupta M, Kumar Y. A review of π-conjugated polymer-based nanocomposites for metal-ion batteries and supercapacitors. Royal Society Open Science. 2021; 8: 210567.
[80]Bay L, Jacobsen T, Skaarup S, West K. Mechanism of actuation in conducting polymers: Osmoti expansion. The Journal of Physical Chemistry B. 2001; 105: 8492-7.
[81]Magu T, Agobi A, Hitler L, Dass P. A review on conducting polymers-based composites for energ storage application. Journal of Chemical Reviews. 2019; 1: 19-34.
[82]Wang K, Wu H, Meng Y, Wei Z. Conducting polymer nanowire arrays for high performance supercapacitors. Small. 2014; 10: 14-31.
[83]Sahoo PK, Tseng C-A, Huang Y-J, Lee C-P. Carbon-based nanocomposite materials for high-performance supercapacitors. Novel Nanomaterials. 2021.
[84]Li Y, Fu Y, Liu W, Song Y, Wang L. Hollow Co-Co₃O₄@ CNTs derived from ZIF-67 for lithium ion batteries. Journal of Alloys and Compounds. 2019; 784: 439-46.
[85]Ramakrishnan V, Unnathpadi R, Pullithadathil B. p-Co₃O₄ supported heterojunction Carbon Nanofibers for Ammonia gas sensor applications. Journal of Materials Nano Science. 2022; 9: 61-7.
[86]Wang H, Yi H, Chen X, Wang X. Asymmetric supercapacitors based on nano-architectured nickel oxide/graphene foam and hierarchical porous nitrogen-doped carbon nanotubes with ultrahigh-rate performance. Journal of Materials Chemistry A. 204;  2(9): 3223-3230
[87]Patil AM, Lokhande VC, Patil UM, Shinde PA, Lokhande CD. High performance all-solid-state asymmetric supercapacitor device based on 3D nanospheres of β-MnO₂ and nanoflowers of O-SnS. ACS Sustainable Chemistry & Engineering. 2018;  6(1): 787-802.
[88]Zhong C, Deng Y, Hu W, Qiao J, Zhang L, Zhang J. A review of electrolyte materials and compositions for electrochemical supercapacitors. Chemical Society Reviews. 2015; 44: 7484-539.